Haris Apriyanto

Olivier D Bernal

Member, IEEE Michel Cattoen

Françoise Lizion

James Sharp

Frédéric Surre

SeniorMember, IEEE Valérie Chavagnac

Han Cheng Seat

Frederic Sharp

Surre

Analysis and Design of a Hybrid Optical Fiber Refractometer for Large Dynamic Range Measurements

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1 Abstract-A fiber refractometer with large dynamic range from 1.316 to 1.61 RIU has been realized using a hybrid configuration of a single-mode fiber (SMF) coupled to a stripped-cladding multimode fiber (MMF) as sensing element. An extended analysis of the diffraction principle of a Gaussian beam is specifically developed for this sensor configuration to determine the injected power density into the MMF which, when subsequently combined with ray optics, analytical wave optics and Fresnel equations, enables the sensor response to be comprehensively estimated. Simulation results have been experimentally corroborated to very high agreement for a 2-cm and a 5-cm decladded section of multimode fiber used as the sensing element. The results show, for the shorter sensor (2 cm), a very high sensitivity of ~ -250 a.u./RIU being achieved in the Zone II operating regime, i.e. for indices between the cladding and core indices together with a resolution of 2.76×10 -6 RIU being attained. In addition, the developed models have been used to accurately predict the response of sensing elements of various lengths, hence demonstrating the potential capability of this research to be exploited for optimizing bespoke design of fiber refractometers of any arbitrary sensing lengths or dimensions. As an example, we present the design of a refractometer achieving a maximum sensitivity of 300 a.u./RIU with a potential resolution of 2.26 ×10 -6 RIU.

Index Terms-Fresnel equation, Gaussian beam, ray optics, wave optics, hybrid sensor design, modelling, multimode fiber, single-mode fiber, fiber refractometer, refractive index measurement.

I. INTRODUCTION

LARGE variety of refractive index (RI) sensors or refractometers exploiting different measurement techniques have been reported since the end of the 19th century [START_REF] Jensen | Refractometers[END_REF] due to their broad applications in chemistry and physics, in environmental monitoring, as well as across the biochemical, pharmaceutical, and the food and beverage industries [START_REF] Norris | Optical fiber chemical sensors: fundamentals and applications[END_REF]. Amongst these RI sensors, a multitude of optical fiber refractometers (OFRs) have also been developed for their intrinsic advantages, such as their robustness to relatively harsh ambient conditions, explosion-free security since they require no on-probe electrical power, rapid real-time in situ measurement for remote sensing, multiplexing potential, and immunity to electromagnetic interference (EMI) [START_REF] Grattan | Fiber optic sensor technology: an overview[END_REF][START_REF] Kersey | A review of recent developments in fiber optic sensor technology[END_REF][START_REF] Lee | Review of the present status of optical fiber sensors[END_REF], to name a few.

Interferometric refractometers have recently been reported with suitable capabilities for RI measurement. A hybrid device exploiting a capillary fiber sandwiched between two SMFs in a Fabry-Perot configuration and interrogated by a swept wavelength laser has been reported for wide RI sensing from 1.3924 -1.5882 RIU with a precision of 7x10 -4 RIU [START_REF] Rodríguez-Quiroz | Hybrid fiber Fabry-Perot interferometer with improved refractometric response[END_REF]. Fukano et al. reported on a refractometer employing multimode interference in a hybrid sensor comprising an MMF sandwiched between two SMFs with detection performed using an optical spectrum analyzer [START_REF] Fukano | High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core singlemode fiber for biosensing[END_REF]. Another refractometric technique that is frequently used is that based on surface plasmon resonance, in particular, for biosensing [8].

Nevertheless, one RI measurement method which is widely exploited is the optical intensity-based refractometer [START_REF] Villatoro | High resolution refractive index sensing with cladded multimode tapered optical fibre[END_REF][START_REF] Zibaii | Measuring bacterial growth by refractive index tapered fiberopticbiosensor[END_REF][START_REF] Rahman | Tapered plastic multimode fiber sensor for salinity detection[END_REF][START_REF] Kim | Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter[END_REF][START_REF] Nath | All-fiber optic sensor for measurement of liquid refractive index[END_REF][START_REF] Xu | Simple fiber-optic refractive index sensor based on Fresnel reflection and optical switch[END_REF][START_REF] Chaudharia | Multi-wavelength optical fiber liquid refractometry based on intensity modulation[END_REF][START_REF] Muthuraju | Low cost fiber optic sensing of sugar solution[END_REF][START_REF] Feng | D-shaped plastic optical fiber sensor for testing refractive index[END_REF][START_REF] Liu | Evanescent wave analysis and experimental realization of refractive index sensor based on D-shaped plastic optical fiber[END_REF][START_REF] Arie | Measurement and analysis of light transmission through a modified cladding optical fiber with applications to sensors[END_REF][START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF][START_REF] Cole | Optical fiber refractometer for measuring the dispersion of turbid fluids[END_REF][START_REF] Mukherjee | Characterization of a fiber optic liquid refractive index sensor[END_REF][START_REF] Apriyanto | A multimode fiber refractive index sensor[END_REF][START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] due to its simplicity and cost effectiveness. In such a scheme, a variation in the RI value will induce a variation in the optical power or intensity to be measured. OFR techniques based on optical intensity modulation have been realized using tapered MMFs [START_REF] Villatoro | High resolution refractive index sensing with cladded multimode tapered optical fibre[END_REF][START_REF] Zibaii | Measuring bacterial growth by refractive index tapered fiberopticbiosensor[END_REF][START_REF] Rahman | Tapered plastic multimode fiber sensor for salinity detection[END_REF], back-reflection fiber core tips [START_REF] Kim | Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter[END_REF][START_REF] Nath | All-fiber optic sensor for measurement of liquid refractive index[END_REF][START_REF] Xu | Simple fiber-optic refractive index sensor based on Fresnel reflection and optical switch[END_REF], dual MMF set-ups similar to the fiber bundle proximity sensor [START_REF] Chaudharia | Multi-wavelength optical fiber liquid refractometry based on intensity modulation[END_REF][START_REF] Muthuraju | Low cost fiber optic sensing of sugar solution[END_REF], D-shaped multimode plastic optical fibers [START_REF] Feng | D-shaped plastic optical fiber sensor for testing refractive index[END_REF][START_REF] Liu | Evanescent wave analysis and experimental realization of refractive index sensor based on D-shaped plastic optical fiber[END_REF], and stripped-cladding MMFs [START_REF] Arie | Measurement and analysis of light transmission through a modified cladding optical fiber with applications to sensors[END_REF][START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF][START_REF] Cole | Optical fiber refractometer for measuring the dispersion of turbid fluids[END_REF][START_REF] Mukherjee | Characterization of a fiber optic liquid refractive index sensor[END_REF][START_REF] Apriyanto | A multimode fiber refractive index sensor[END_REF][START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF].

Development of the stripped-cladding MMF refractometer has been reported since the 1980's [START_REF] Arie | Measurement and analysis of light transmission through a modified cladding optical fiber with applications to sensors[END_REF][START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF][START_REF] Cole | Optical fiber refractometer for measuring the dispersion of turbid fluids[END_REF][START_REF] Mukherjee | Characterization of a fiber optic liquid refractive index sensor[END_REF][START_REF] Apriyanto | A multimode fiber refractive index sensor[END_REF][START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], inspired by the dielectric rod refractometer immersed in a liquid used as the equivalent cladding [START_REF] Kapany | Fiber Optics. Part IV. A Photorefractometer[END_REF][START_REF] Kapany | Photorefractometer I. Extension of Sensitivity and Range[END_REF][START_REF] Kapany | Photorefractometer II. Measurement of N and K[END_REF]. Various models of stripped-2 cladding MMF refractometer have been proposed [START_REF] Arie | Measurement and analysis of light transmission through a modified cladding optical fiber with applications to sensors[END_REF][START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF][START_REF] Cole | Optical fiber refractometer for measuring the dispersion of turbid fluids[END_REF] to estimate its optical power response measured against RI variation with respect to the core index. In these early refractometers, Snell's law and Fresnel's reflection law were employed to interpret the sensor response for RIs lower than that of the core, while only Fresnel reflections were employed when the indices were higher than the core index. Although previous models [START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF] attempted to determine and account for the power distribution density of the incident beam, the influence of evanescent wave absorption (EWA) was, however, not considered. Nevertheless, EWA exists since the beams or rays are essentially guided by total internal reflection (TIR) and will have to be accounted for to design an accurate fiber refractometer. In addition, the resulting approximation from these existing models was sufficiently precise only when applied to a beam launched through a lens system [START_REF] Arie | Measurement and analysis of a modified cladding optical fiber with various input illuminations[END_REF][START_REF] Arie | Light transmission through a modified cladding optical fiber -inverse problem analysis[END_REF].

We shall here build on an alternative Gaussian beam analysis method previously proposed [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] in order to accurately estimate the optical power distribution density injected from a singlemode (SM) laser beam into an MMF that is used as the sensing element. Although experimentally validated for beam injection via a microscope objective (MO), this method can also be extended to account for any launch conditions, including that from a lead-in SMF spliced to an MMF. Using this improved model subsequently allows the analysis of the impact of both launch conditions and sensing length on the sensor sensitivity in each of the three previously identified RI zones.

As a result, a compact hybrid all-fiber set-up is proposed using an SMF and MMF combination without the inconvenience of opto-mechanical elements which could introduce mechanical perturbations to the sensor as in [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF].

II. OPERATING PRINCIPLE

A. Determination of power input into the MMF

An SMF is fusion-spliced to a plastic-clad silica MMF (PCS MMF) with a core radius (a2) of 100 µm and a numerical aperture (NA2) of 0.50, as shown in Fig. 1(a). The subsequent lightwave will then be diffracted according to the principles illustrated by Fig. 1(b).

Here, it is supposed that the guided beam output from the SMF has similar characteristics to a collimated beam exhibiting a Gaussian distribution. The SMF's mode field diameter (MFDSMF) is then defined as being at 1/e² of the beam intensity with a spot radius of w0 = 0.5MFDSMF. Thus, 99% of the beam intensity in the SMF is contained within a diameter D0 = πw0 [START_REF] Siegman | Lasers[END_REF]. Furthermore, since the laser beam is launched from the lead-in SMF into the MMF with incident beam angle θb = 90°βb (see Fig. 1(b)), βb can be calculated in terms of both the SMF's numerical aperture (NA1) and the core index of the MMF (nco2) as

𝛽 𝑏 = 𝑠𝑖𝑛 -1 ( 𝑁𝐴 1 𝑛 𝑐𝑜2 ) (1)
This Gaussian beam is then diffracted into the MMF until distance zi [START_REF] Kowalevicz | Beam Divergence from an SMF-28 Optical Fiber[END_REF] (see Fig. 1(b)) and will first be incident at the core-cladding interface before being guided in its core. Hence, the true optical power distribution propagated along the MMF should then originate from the power distribution at zi. Consequently, according to the principle previously reported [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], the diffraction from the SMF to the MMF core at distance zi for a given operating wavelength λ will result in a beam spot radius (wz) of [31]

𝑤 𝑧 (𝑧 𝑖 ) = 𝑤 0 √1 + ( 𝜆𝑧 𝑖 𝑛 𝑐𝑜2 𝜋𝑤 0 2 ) 2 (2)
where distance zi can be obtained using a trigonometric function in terms of a2 and βb in the form

𝑧 𝑖 = 𝑎 2 𝑐𝑜𝑡 𝛽 𝑏 (3)
The optical intensity distribution of this Gaussian beam in the MMF at distance zi (Iz) can then be expressed by [31] 𝐼 𝑧 (𝑎) = 𝐼 0 𝑒 (-

2𝑎 2 𝑤 𝑧 2 ) (4)
Here, a varies from the center axis of the fiber to its outer radius a2 due to the symmetry of revolution. Further, by integrating the optical intensity in (4) over the transverse plane of the beam, followed by normalizing by I0 = πwz²/2, the normalized input optical power into the MMF can be obtained as [START_REF] Siegman | Lasers[END_REF]31] ( )

        - - = 2 2 2 1 z w a z e a P (5)
The input beam is thus injected from the SMF with NA1 = 0.16 at λ = 1550 nm into the MMF (NA2 = 0.50) with its middle section stripped of the cladding or buffer layer. The incident beam angle (θb) in the MMF is thus always higher than its critical angle (θc). Therefore, this beam will be entirely propagated by TIR without any part of it being refracted into the cladding.

B. Sensing mechanisms

As previously reported [START_REF] Apriyanto | A multimode fiber refractive index sensor[END_REF][START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], the sensing mechanisms in the stripped-cladding MMF-based refractometer are classed according to 3 conditions or RI zones. Zone I is defined for the condition when the light is fully guided by TIR in the sensing region and, therefore, only EWA occurs as the unique sensing mechanism. This is illustrated in Fig. 2.

Zone I is then defined when b > c, which corresponds to nsm < nb < ncl, where nb is expressed as The guided optical power, (𝑃 𝐿 ) 𝐼 , along the MMF for a sensing region of length L, can then be estimated by integrating the incident optical power for each ray (i) from b to 90° through [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] ( ) ( ) ( )

( ) i L T N i I L d e P P b i i        - = 90 0
) ( [START_REF] Fukano | High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core singlemode fiber for biosensing[END_REF] where P0 represents the input angular power density into the MMF while N(θi) is the number of reflections per unit length for an incident ray with angle θi and T is the transmission coefficient of the evanescent waves (EWs) penetrating into the external sensing medium, respectively. These two parameters can be given by [START_REF] Keller | Waveguiding properties of fiber-optic capillaries for chemical sensing applications[END_REF] ( )

2 2 cot a N i i   = (8) ( ) i csm csm sm i co i n n T        2 2 2 2 2 cos cos cos cos - = (9)
Here,  is defined as the absorption coefficient of the sensing medium, and csm the equivalent critical angle at the interface core-sensing medium (θcsm = sin -1 (nsm/nco2)). Note that N(θi)L is the total number of reflections and is hence rounded off to the nearest integer number during computing.

A clearer and more intuitive illustration of the EWA sensing mechanism in Zone I is shown in Fig. 3. The results physically demonstrate the evolution of EWs as a function of RI values. In this work, 2D wave optics simulation in the COMSOL Multiphysics environment is used since simulation in 3D would require excessively high computational load due to extremely high number of meshes to be generated. In addition, 3D simulation is not required due to the cylindrical symmetry of the MMF structure. During simulation, the core diameter used is proportionally reduced to 60 µm instead of 200 µm to avoid unwanted overloading during the computation. Further, as illustrated in Figs. 3(a) and 3(b), where for b > csm, the beam from the lead-in SMF is propagated entirely in the MMF by TIR. The penetration of EWs into the functional cladding (FC) representing the sensing medium thus increases with increasing nsm as illustrated in Fig. 3(c). This can be clearly understood from [START_REF] Villatoro | High resolution refractive index sensing with cladded multimode tapered optical fibre[END_REF], as, for a given θi, the square root term of the denominator decreases towards zero when csm increases, subsequently leading to an increase in T. For a sensing medium (or FC) index nsm of 1.435 RIU (inset in Fig. 3(b)), more EW power (higher-intensity blue shade in FC) is observed in the FC area compared to that for a lower nsm of 1.385 RIU (see inset of Fig. 3(a)). This will consequently contribute to decreasing optical power being guided along the fiber since more EW power is absorbed in the FC. In the case of the second condition or Zone II, where nsm > nb but nsm < nco2, a different sensing mechanism occurs. Here, in parallel to EWA, there is an additional contribution from mode losses due to the increase in critical angle in the MMF sensing region (see Fig. 4). This is proportional to an increase in nsm and will induce a larger number of modes being transmitted or refracted into the external medium, hence leading to increasing mode losses. Nevertheless, there are also Fresnel reflections from rays with individual incident ray angle i < csm which are no longer guided by TIR. Subsequently, the model for Zone II can be described by

( ) ( ) ( ) ( ) ( ) ( ) ( ) i L N i i i L T N i II L d R P d e P P csm b i csm i i              + =  - 0 90 0 ) ( (10)
The first term on the right of ( 10) is the usual EWA contribution from the TIR guided modes which still exist, while the second term corresponds to the influence of Fresnel reflections for rays from b to csm which are refracted to the exterior. R represents the reflectivity and can be calculated as a function of the reflectance under both p-polarization (rp) and s-polarization (rs), respectively, as [START_REF] Saleh | Fundamental of Photonics[END_REF] ( ) © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.

( ) ( ) ) ( 2 1 2 2 i s i p i r r R    + = (11)

4

for which the reflectance under the respective polarizations is given by ( )

t co i sm t co i sm i p n n n n r      cos cos cos cos 2 2 + - = ; ( ) t sm i co t sm i co i s n n n n r      cos cos cos cos 2 2 + - = (12)
However, the contribution from Fresnel reflections in Zone II is very weak (of the order of ~10 -6 to 10 -5 ) since the index contrast between nco2 and nsm is relatively small (nco2nb ≌ 0.0089 RIU). Hence, these reflections can be neglected. Subsequently, the model for Zone II in [START_REF] Zibaii | Measuring bacterial growth by refractive index tapered fiberopticbiosensor[END_REF] can be simplified to a combination of EWA for rays which are still guided by TIR, similar to Zone I, and mode losses for rays from b to csm which no longer exist in the integral given in ( 13) below. This is illustrated in Fig. 4. The input rays subject to EWA are now thus considered from csm to 90°, and can be expressed by

( ) ( ) ( ) ( ) i L T N i II L d e P P csm i i        - = 90 0
) ( (13) Finally, for Zone III, in which the sensing region of the refractometer is employed for measuring nsm > nco2, propagation by TIR is no longer supported. Nevertheless, some optical power will still be guided by the phenomenon of external reflection while the majority of the rays are refracted and transmitted into the external medium (or FC). This can be explained using Fresnel equations to describe the interaction of the optical rays at the interface of two media with different RI values, as illustrated in Fig. 5. When nsm > nco2, the optical power from the core will principally be transmitted to the external region and lost. There is, however, a small amount of power being reflected back into the core through Fresnel reflections in ( 11) and ( 12), whose guided power can be calculated via

( ) ( ) ( ) i L N i i III L d R P P b i        = 90 0 ) ( (14)

C. Experimental investigation and validation

The refractometer operates on the basis of ratiometric intensity measurements through a differential probe configuration, with one SMF-MMF serving as the sensing arm, and the other the reference arm to compensate common-mode noises, as shown in Fig. 6. The emitted beam from a 1550 nm SM fiber-pigtailed DFB laser diode is transmitted via a 50/50 SM fiber coupler to obtain two equal interrogating beams. Since SMF is directly spliced to MMF (Thorlabs FP200ERT) [35] with NA2 = 0.50, the injected beam will be diffracted at the SMF-MMF interface, then guided into and along the latter fiber. Two identical Ge-type Thorlabs PDA50B photodetectors are used for measuring the guided optical powers at the MMF outputs. The measured data are next recorded by a dedicated 1.25 MHz National Instrument data acquisition system (DAQ) for further processing. A combination of water-glycerol mixture with different concentrations is employed as the sensing medium to obtain an RI range from 1.3164 to 1.4571 RIU at 1550 nm, similar to [START_REF] Saunders | The refractive index of common solvents and solutions at 1550 nm[END_REF].

The corresponding absorption coefficient sm is dependent on the experimentally determined extinction coefficient, calculated according to the procedures in [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF][START_REF] Saunders | The refractive index of common solvents and solutions at 1550 nm[END_REF][START_REF] Riziq | Optical properties of absorbing and non-absorbing aerosols retrieved by cavity ting down (CRD) spectroscopy[END_REF][START_REF] Jenkins | Fundamental of Optics[END_REF]. For inducing RI values beyond those of glycerol (i.e. for RI > 1.4571), calibrated oils from Cargille Laboratories ranging from 1.468 to 1.608 RIU are employed.

III. RESULTS AND DISCUSSION

FEM-based simulation cannot be used to support 3D MMF simulation, since its dimension with respect to the operating wavelength will contribute to a very high mesh number and, thus, computational load. Hence, the most effective simulation technique to estimate the response of this refractometer is by combining the models through Gaussian beam analysis, ray optic principles, and analytical wave optics.

Equation ( 1) is first used to calculate b (b = 90°-βb = 83.6°), before obtaining the zi-distance through (3), giving zi = 896.94 µm. wz is next calculated using (2) for the SMF's w0 (w0 = 4.85 µm), given as 0.5MFDSMF. Subsequently, using (4) and ( 5), the optical intensity and power at zi can be obtained.

Inferring from the sensing principles, the boundary between Zone I and Zone II will be at nb, an index corresponding to θb, due to NA2 (MMF) being higher than NA1 (SMF). From ( 6), this boundary is determined to be at nb ~1.4351 RIU, and, consequently, the beam will be entirely guided by TIR for nsm < nb. Additionally, to estimate the remaining guided power by 5 TIR in Zone I, as expressed by [START_REF] Fukano | High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core singlemode fiber for biosensing[END_REF], the input angular power density distribution P0(i) in Fig. 7 for each angle has to be known. This is obtained using the method proposed in earlier work [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], for P0(i) from b to 90°. Hence, by substituting the values of P0(i) for all incident rays (from b to 90°) into ( 7)-( 9), the guided power under any perturbation in Zone I (i.e. nsm < 1.4351 (or nb)) can thus be accurately estimated. On the other hand, for nsm > 1.4351 and up to 1.444 (or nco2), the Zone II sensing response is obtained by numerically integrating P0(i) from csm to 90° with [START_REF] Nath | All-fiber optic sensor for measurement of liquid refractive index[END_REF].

Finally, for Zone III, the guided power can be estimated by employing [START_REF] Rahman | Tapered plastic multimode fiber sensor for salinity detection[END_REF], [START_REF] Kim | Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter[END_REF], and ( 14) for all incident rays from b to 90°.

The complete simulation results of the RI response across the 3 zones are plotted and corroborated experimentally in Fig. 8 for two sensing lengths of 2 cm and 5 cm 0.1 cm. The response curves show that, in comparison with previous results [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], the dynamic range of Zone I is wider in this work (from 1.3164 to 1.4351 RIU), contrary to Zone II, due to the injected b from the lead-in SMF into the MMF being higher (close to 90°).

For Zone I response, a higher power loss is achieved for the longer sensor (5 cm), and the measured optical power decreases for increasing RI due to increasing EWA as predicted by [START_REF] Fukano | High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core singlemode fiber for biosensing[END_REF]. Higher power losses are also achieved by the 5-cm sensor in Zone II since in this zone, there is still EWA which additionally contributes to the response. Furthermore, the sharp decrease in the Zone II response for both the 2-cm and the 5-cm sensors results from the mode losses due to the modification of the critical angle for increasing RI values. This leads to a higher number of modes being continuously lost through refraction. The dynamic range for Zone II response undergoes no change compared with previous results [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] since the characteristics of the MMF core are identical, i.e. PCS MMF with a silica core. However, the power response significantly increases comparatively [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF], in particular for the shorter 2-cm sensor since the incident beam angle from the lead-in SMF into the MMF, b, is closer to 90°. Hence, the total number of reflections along the sensor decreases, resulting in more power being guided along the MMF, according to [START_REF] Xu | Simple fiber-optic refractive index sensor based on Fresnel reflection and optical switch[END_REF].

The power response curves are next derived based on the models, as a function of RI across all 3 zones, to obtain the sensitivities for both sensors which are plotted in Fig. 9. Very high sensitivities are achieved in Zone II for both sensors, with the best sensitivity (-248 a.u./RIU, which is equivalent to 2480 V/RIU, depending on the photodetector system employed) achieved by the 2-cm stripped-cladding MMF. This can be justified by the sharp decrease in the optical power response in Fig. 8 as the shorter sensor is subject to the least EWA. Consequently, it suffers higher losses through the modification of the critical angle (mode loss mechanism) as previously explained in [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF]. As the measured index approaches nco2 at the end of Zone II, the sensitivity lessens. The optical power (Fig. 8) then exhibits a flat response at the exact boundary between Zone II and Zone III, consequently resulting in a zerosensitivity value. The sensitivity at the beginning of Zone I, on the contrary, is very low (-0.016 a.u./RIU and -0.040 a.u./RIU for the 2-cm and 5-cm sensors, respectively) since the slow gradient of the power response in this zone is affected by the low penetration of EWs into the cladding and a lower total number of reflections (NL) along the decladded area. This is due to θb in MMF being close to 90° (~83.6°). Consequently, a higher sensitivity for Zone I is achieved by the longer sensing region (5 cm) due to stronger EWA from the contribution of higher NL in this sensor. Further, the sensitivity (S) curves in Zone III (Fig. 9) describe a higher sensitivity being achieved for the 2-cm sensor. However, this sensitivity then decreases for further increasing RI. Nevertheless, the sensitivities for both the 2-cm and 5-cm sensors in Zone III are higher (11.82 a.u./RIU and 4.99 a.u./RIU, respectively) compared to previous results [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] and even in comparison with the shortest (1 cm) of the previous sensors where a sensitivity of only 1.20 a.u./RIU was obtained for this zone. This is due to a lower number of reflections, which contribute to higher transmitted power as well as a higher gradient which represent the sensor sensitivity. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.

7

The refractometer resolution, Res, is next determined using the sensitivity curves in Fig. 9 by taking 6 times the RMS measurement noise, σ, corresponding to ~99.7% confidence level, divided by the sensitivity, i.e. Res = 6σ/S [START_REF] Fleming | Review of nanometer resolution position sensors: Operation and performance[END_REF]. Results from this work are first compared against our previous benchmark in [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF]. The best resolution obtained here is 2.76×10 -6 RIU, achieved by the 2-cm sensor in Zone II. This is an 8-time improvement over the previous refractometer where the best resolution was 2.245×10 -5 RIU for the 1-cm sensor. A higher resolution is achieved in this work, due to a narrower input angular power density (from 83.6° to 90°) that thus allows to achieve both a higher sensitivity and a sharper response in Zone II. Further, the highest resolution obtained for Zone III in this work is 5.30×10 -5 RIU, also achieved by the 2-cm sensor, as compared to 1.39×10 -4 RIU for the 5-cm sensor. These values are at least one order of magnitude better than that obtained (1.45×10 -3 RIU) previously for the shortest sensor (1 cm) in Zone III, in direct accordance with a higher sensitivity due to a lower number of reflections obtained in the sensing region. However, the resolution achieved at the beginning of Zone I is limited to 1.63×10 -2 RIU and 3.39×10 -2 RIU for the 2-cm and 5-cm sensors, respectively. These are relatively weak compared to the best resolution (8.34×10 -3 RIU) obtained from the previous 4-cm long refractometer due to less EWA being achieved by the sensing region in this work.

Although the models developed in this work are principally used to accurately estimate the performance (in terms of response, resolution/sensitivity, RI delimitation, etc) of the hybrid SMF-MMF refractometers, the capability of these analytical models can also be extended to optimize the future design of other similar fiber refractometers. For example, in © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works. 8 Fig. 10, we investigate the influence of the sensing length on the response of the refractometers for lengths ranging from 1 cm to 5 cm. The simulation results in Fig. 10 demonstrate that the initial response of the refractometers under study (from 1 cm to 5 cm) in Zones I and II are relatively difficult to distinguish at low RI values. Nevertheless, in Zone I, the respective sensor response then separates with increasing RI since more optical power is exponentially absorbed via EWA with increasing sensing length. With regard to Zone II, the response of the refractometers subsequently converges when the measured RI increases. This is due to the decreasing influence of EWs since more modes are exponentially lost (or refracted) into the sensing medium. On the other hand, the response of each refractometer in Zone III can be clearly distinguished since the optical power responses as a power of L and N as described by [START_REF] Xu | Simple fiber-optic refractive index sensor based on Fresnel reflection and optical switch[END_REF].

Figure 11 displays the sensitivity values for refractometers with 1 cm to 5 cm of sensing length. The sensitivity curves of these sensors in Zone I converge with increasing RI to a crossing point in Zone II. Before this crossing point, the longer sensor has the largest sensitivity as we have seen and explained in Fig. 9. After the crossing point, the sensitivity of the longest sensor decreases in sensitivity while that of the shortest sensor becomes increasingly sensitive. This is due to the sharper response of the shorter sensing elements after the 1.4410 RIU point. In Zone III, the sensitivity curves decrease significantly and very rapidly beyond 1.445 RIU. However, the sensitivities of the shorter sensing elements are always larger than those of the longer sensors.

As shown earlier in this paper, the present model is highly accurate and, hence, can additionally be used to design a hybrid SMF-MMF RI sensor with the desired sensitivity. For instance, using Fig. 11 as the baseline (more detailed abacus generated with our model could also be used), the maximum length required to design a sensor with a sensitivity of at least 300 a.u./RIU can be determined. In this example, the maximum length of the sensing region must be 1 cm. Nonetheless, the model, which is relatively fast to run, could be coupled with an optimization method to automatically analyze the sensor parameters and find the correct length of the sensing region.

IV. CONCLUSION

Two hybrid SMF-MMF fusion splice-coupled refractometers have been realized to significantly improve the sensitivity in the Zone II and Zone III RI regimes by a factor of 8 to 10 compared to previous work [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF] while concurrently reducing parasite mechanical perturbations advantageously. Since the injected beam angle from the lead-in SMF corresponds to its NA, this then determines the equivalent angle to the RI value at the Zone I and Zone II boundary. Accordingly, the ray power distribution density which propagates along the MMF can theoretically be obtained by using Gaussian beam analysis for incident beam angles from θb (83.6°) to 90°, which are obtained from the value of NA1. Subsequently, the optical power response for both the 2-cm and 5-cm refractometers can be estimated by integrating the ray power distribution density into the models for Zone I and Zone II through ray optic principles and the analytical equation for evanescent waves. The model for Zone III, on the other hand, employs the ray power distribution density combined with Fresnel equations.

The results for the three sensing zones are then corroborated experimentally by decladding two MMFs over 2 cm and 5 cm in length, respectively, demonstrating very high agreement with the simulation results. In addition, since the incident beam angle (b) from the lead-in SMF is close to 90°, the dynamic range of Zone II is consequently very narrow (from 1.4351 to 1.444 RIU), compared to the relatively wider range obtained in Zone I. In terms of sensitivity, the results achieved with these hybrid SMF-MMF splice-coupled refractometers show significantly improved sensing performance by more than 8 times, in particular for the shorter 2-cm sensor over Zones II and III, respectively, compared to our previous results [START_REF] Apriyanto | Comprehensive modeling of multimode fiber sensors for refractive index measurement and experimental validation[END_REF]. On the other hand, the sensitivity of these sensors in Zone I, in particular at the beginning of this zone, is very low (-0.016 a.u./RIU and -0.034 a.u./RIU for the 2-cm and 5-cm sensors, respectively) when compared to -0.392 a.u./RIU previously achieved. This, however, increases exponentially with increasing RI toward the boundary between Zone I and Zone II, in accordance with the model for Zone I described in [START_REF] Fukano | High-sensitivity fiber-optic refractive index sensor based on multimode interference using small-core singlemode fiber for biosensing[END_REF].

Using the proposed models enables us to estimate the performance of MMF refractometers with different sensing lengths with relative ease since the simulation time for each sensing length is less than 20 seconds. Hence, various lengths can be efficiently and accurately simulated to obtain the desired sensor performance.

Fig. 1 :

 1 Fig. 1: Coupling between SMF and MMF: (a) photo image of fusion splicing region between SMF and MMF, and (b) illustration of the principles of lightwave injection and diffraction at the interface of both fibers. Incoming single-mode laser beam is injected through lead-in SMF with radiated beam angle βb into MMF at incident beam angle b.

Fig. 3 :

 3 Fig. 3: COMSOL 2D wave optics simulation of light propagation for Zone I for (a) nsm = 1.385, (b) nsm = 1.435, and (c) evanescent waves penetrating into FC via exponential decay of the electric fields without any mode loss to FC or external medium.

Fig 2 :

 2 Fig 2: Sensing mechanism in Zone I based uniquely on EWA.

Fig. 4 :

 4 Fig. 4: Illustration of EWA and mode losses represented by the yellow rays due to modification of csm with respect to nsm in Zone II.

Fig. 5 :

 5 Fig. 5: Illustration of sensing mechanism in Zone III by Fresnel reflection.

Fig. 6 :

 6 Fig. 6: Experimental schematic of hybrid differential SMF-MMF refractometer for RI measurement with common-mode noise compensation.

Fig. 7 :

 7 Fig. 7: Input angular power density distribution P0(i) injected into MMF for incident angles from b to 90°.

6

 6

Fig. 8 :

 8 Fig. 8: Response of 2-cm and 5-cm refractometers based on hybrid coupling of SMF and MMF for varying RI values.

Fig. 9 :

 9 Fig. 9: Sensitivity curves of 2-cm and 5-cm SMF-MMF hybrid refractometers, (a) in Zone I, (b) in Zone II, and (c) in Zone III.

Fig 1 :

 1 Fig 1: Simulation results of hybrid MMF refractometers for sensing length from 1 cm to 5 cm, with (a) response for Zones I, II and III, (b) zoom for Zone I, and (c) zoom for Zone II.

Fig. 11 :

 11 Fig. 11: Simulation results of hybrid MMF refractometers for sensing length from 1 cm to 5 cm, with (a) response for Zones I, II and III, (b) zoom for Zone I, and (c) zoom for Zone II.

Fig. 10 :

 10 Fig. 10: Sensor sensitivities for, (a) Zone I, (b) Zone II, and (c) Zone III.

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.

ACKNOWLEDGMENT

H. Apriyanto gratefully acknowledges support from the Indonesia Endowment Fund for Education (LPDP), Ministry of Finance, Republic of Indonesia, in the form of a PhD scholarship. H. C. Seat also acknowledges support from Aquitaine Electronique through an industrial secondment. The authors would like to thank C. Tronche and F. Jayat for technical support during the experimental set-up. This work was part-supported by the Agence Nationale de la Recherche (grant number ANR-13-IS06-0001-04).


Our vision of an HNV farming-friendly common agricultural policy

This policy paper builds upon the work carried out in the framework of HNV-Link (H2020 Project, 2016-2019, www.hnvlink.eu), a thematic multi-actor network on High Nature Value (HNV) Farming involving 13 partners from 10 European countries. The goal of this network is to support HNV farming systems by inspiring and sharing innovations/practices that improve their socio-economic viability while preserving their ecological value and the public services they provide.

HNV-Link informs policymakers and authorities at the European and national levels of the main policy stakes around HNV farming, and to recommend adjustments of the Common Agricultural Policy (CAP) and other policies in order to adequately support HNV farming, the territories in which they are embedded, and the communities that depend on them.

In Europe, farmers operate within a complex and constraining environment and policy/regulatory framework, including income support and rural development measures of the CAP, but also the numerous regulations related to agriculture, food hygiene/safety, animal health/welfare, environment protection, and climate change. This framework can provide farms with incentives or on the contrary, hinder their development, and it has consequently a major influence on their economic viability and the survival of the communities depending on farming. This institutional framework was designed to deal mainly with the problems that intensive farms face. Far less weight has been placed on designing and implementing policies adapted to the needs of HNV farms, i.e. those lowintensity farms which rely on and safeguard a rich biodiversity and associated ecosystem services made up of a variety of habitats and landscapes elements. Hence, there is a need for a creative yet thoughtful design and implementation of adapted policy measures.

We acknowledge that, in some aspects, the CAP has evolved positively, with an attempt to redress progressively the historic bias against less competitive but more sustainable agricultural production modes, such as HNV farming. The CAP already includes several measures which potentially can be supportive of HNV farming, such as direct support to areas with natural and other specific constraints (ANC), Agri-Environment-Climate Measures (AECM), Natura 2000 payments, and Operational Groups (OGs), and the ambition of its revised version (2021-2027 period) looks stronger in terms of both socio-economic and environmental objectives, which is encouraging.

The new CAP foresees increased subsidiarity for Member States (MS): while the EU will set basic policy parameters (e.g. the nine specific objectives of the CAP which cover economic, social and environmental dimensions) as well as the different types of intervention, Member States will be responsible for drawing up their CAP Strategic Plans (2021-2027) and translating the EU framework into support arrangements for beneficiaries (at the national or regional levels), to achieve production but also environmental, climate and socio-economic goals.

CAP Strategic Plans will be financed by the European Agricultural Guarantee Fund (EAGF) and the European Agricultural Fund for Rural Development (EAFRD). This increased subsidiarity could be double-edged: on the one hand, it will probably allow to better adapt regulations to regional and local contexts, but on the other, it raises concerns with regards to a possible renationalisation of definitions, priorities and environmental ambition of the CAP. In terms of environmental achievements, past experience (e.g. cross compliance implementation before the 2003 mid-term review) suggests that it could result in lesser efforts from Member States and a greater heterogeneity when it comes to reaching environmental objectives.

HNV-Link found that some countries/regions make a very positive use of a range of current CAP options and measures to support HNV farming, while others do very little for it and even implement the CAP in a way that is weighted against it. The crucial question therefore is how to design an EU policy framework and governance system that steer all Member States towards a highest common denominator in terms of HNV farming support.

It is hence a concern that, although HNV farming systems meet all the objectives of the new proposed CAP regulation as set out in articles 5 and 6 (Title II), the proposed text makes no explicit mention of these systems as being essential for fulfilling biodiversity conservation and sustainable development objectives. The CAP regulation puts much more emphasis on correcting the environmental impacts of intensive farming (by improving techniques and reducing chemical inputs) than on supporting existing farming models that deliver high levels of public benefit. Moreover, the fact that HNV farmland and farming concepts have been written out of the new CAP proposals is worrying. The logic of excluding a concept that has been the focus of previous CAP cycles is unusual given the apparently higher environmental ambition of the new proposals. Much work has been completed and public money spent in the Member States to characterise/maintain HNV farmland, since it was included as both a context and an impact indicator in the last cycle, and as a priority for rural development programmes (RDPs) since 2005.

HNV farmland covers over 25% of the EU agricultural land, including nearly all Natura 2000 farmland but also extending beyond these protected sites. Supporting and monitoring HNV farming systems inside and outside Natura 2000 is an essential complement to the policy of protecting habitats and species. Furthermore, a clear signal needs to be given that HNV farming areas of similar character to farmed Natura 2000 areas should be supported and not just fall by the wayside based on a line on a map. • Accompany the definition of permanent grasslands/pastures with EU and national guidelines that ensure the full eligibility of all areas that are effectively grazed or produce fodder, whatever the vegetation type.

• Improve CAP Pillar 1 and its system of rights and payments to reduce the bias against extensive farming and better reward the provision of public goods by HNV farming (e.g. eco-schemes to support HNV systems and semi-natural pastures).

• Improve CAP Pillar 2, adjusting the overall budget balance in favour of Pillar 2 with explicit measures for locally-led HNV conservation projects and results-based incentive schemes (transfers between Pillar 1 and 2, climate/environment schemes, etc.).

• Adjust the Performance Monitoring & Evaluation Framework (PMEF) to improve the characterisation/monitoring of HNV farming systems/territories (both HNV farmland quality and extent) and the evaluation of CAP measures, including an income indicator for HNV farms.

Recommendations to accompany HNV farming at national, regional, local levels:

• Do a full assessment of the HNV systems in the Member States (values, practices, challenges, etc.) and design ambitious programmes of CAP/RDP measures to ensure their maintenance, with indicators to monitor their condition.

• Strengthen Agricultural Knowledge and Innovation Systems (AKIS) with targeted approaches to boost specifically HNV farming innovation, including a facilitation and advisory role for NGOs, professional organisations, natural area managers and local authorities.

• Enhance joined-up policies and institutions for integrated HNV land management and sustainable development and develop the framework for participatory models, processes and institutions ruling the governance of these areas to allow HNV farmers to play a role in decision making.

• Improve land access/stewardship, management & monitoring, to support the installation of new HNV farmers, longer-term investments in HNV farming, more sustainable land planning, and the assessment of environmental results/services.

• Adapt the implementation of animal health and welfare regulations (e.g. TB eradication campaigns) to fit better HNV farming conditions.

• Develop sustainable food policy in urban/rural areas and promote innovative models of urban-rural relationships, to foster the local production of quality food and the reconnection between consumers and the farmers who supply sustainably food and public services.

• Adapt the implementation of food production, processing and marketing regulations, to support the creation of added-value and outlets for HNV farming products, including smallscale processing and direct sales.

What is High Nature Value (HNV) Farming?

The concept of High Nature Value (HNV) farming emerged in the 1990s and refers to those farming systems and farmlands that support a high diversity of wildlife species and habitats and/or species of conservation concern. It comprises mainly lowintensity livestock farming relying on permanent and wooded pastures and hay meadows, and in some areas includes lowintensity crop systems, traditional orchards and olive groves.

HNV farming maintains a diversity of land cover, including seminatural vegetation, and a high density of features such as hedges, stone walls, terraces and ponds that enhance landscape structure and connectivity. It occurs most frequently in areas where natural constraints (e.g. poorer land, steep slopes) hinder intensive production, but it is far from being marginal, as it covers over 25% of the European farmland.

HNV farms are multi-functional systems that, on top of producing quality food and conserving biodiversity, habitats and landscapes, supply a range of public goods and services: they contribute to water and soil protection, carbon storage, fire and climate change mitigation, employment, and are part of our cultural heritage. As such, they contribute to the sustainability of agrifood systems.

Owing to its relevance to fulfil the UN Sustainable Development Goals (SDGs) and EU Biodiversity Strategy, the HNV farming concept was integrated into the CAP as a rural development priority from 2005, and used in the Common Monitoring and Evaluation Framework (CMEF) as an impact and a context indicator for RDPs.

HNV farming systems face environmental and economic pressures, and are often neglected by public policies and not suitably rewarded for their public benefits. This may lead to HNV farmland reconversion or to its abandonment/encroachment, with subsequent irreversible biodiversity loss. The challenge is thus to increase the socioeconomic viability of HNV farms while maintaining HNV farmlands' natural values. Clearly, agricultural and rural development objectives cannot be fulfilled without adequately supporting HNV farming, and for this, a more innovative and HNV farming friendly policy framework is needed at all levels, as well as the commitment of all the stakeholders. HNV farming's specificities must be accounted for when designing and applying policies (e.g. agriculture, environment, land planning, rural development, food, energy, education) at the European and national/regional scales:

High

• HNV farming is an extensive form of agriculture often practised in areas where natural constraints hinder food production capacity. It sustains, e.g. through traditional style grazing and mowing, semi-natural pastures and meadows, with the biodiversity and landscapes connected to them. Those habitats constitute most of the farmland habitats listed in the EU Habitat Directive, and are also the most threatened according to EU data.

• HNV farming demands a strong and motivated labour force with specific skills and know-how allowing to reconcile production and environmental objectives. Thus, training and employment incentives must be given to HNV farmers to perpetuate this craft and make it more attractive for newcomers. In addition, access to land is critical, and therefore, land tenure policy must be improved to support farmer installations and farm succession.

• HNV farmers apply low quantities of inputs, a prerequisite for biodiversity conservation and self-sufficiency. They take advantage of spaces and resources (e.g. semi-natural grasslands, scrublands, forests) unused by more intensive systems and are therefore more resourceefficient. They also slow down scrub encroachment in open cultural landscapes, which is important in the context of climate change and increased fire risk. Thus, grazing land with shrubs or trees grazed directly or fed to livestock, should be eligible for CAP payments and other forms of support.

• HNV farms are multi-functional systems that, on top of producing quality food and conserving biodiversity, habitats and landscapes, also supply a range of additional public goods/services and contribute to structuring vibrant territories. They contribute to water regulation, soil protection, carbon storage, fire prevention, climate change mitigation, rural employment and social cohesion, and are part of our cultural heritage (e.g. pastoralism, transhumance, silvo-pastoralism). These public goods must be rewarded through suitable financial and other market incentives (e.g. Agri-environment payment schemes, eco-schemes, products' added value through appropriate labelling and/or certification).

• HNV farming may be considered less competitive than more intensive forms of farming when a broader range of ecosystem services (not just production) are not adequately rewarded. This "disadvantage" in the current policy and market context must therefore be addressed if we are to realise a multifunctional model of agriculture. When policy also incentivises and rewards biodiversity conservation, landscape maintenance, food quality/sustainability and rural economy and society's welfare then this "disadvantage" becomes an opportunity.

• HNV farms may be exposed to rigid control regimes for animal diseases (e.g. tuberculosis) and to predators (e.g. wolf, bear), which can affect their viability. Animal health regulations and control planning must be adapted to the realities of extensive grazing systems, and greater support given to mitigate predation impacts and reward livestock farmers who operate alongside large carnivores.

• HNV farms may be poorly served by infrastructures, regulations are stringent and hinder local/artisanal forms of food production and processing, and market access can be costly. Consequently, embracing the flexibility of food hygiene regulations or adjusting them, and enhancing the marketing opportunities would enhance the creation of added value to HNV farming products and thus the viability of HNV farms.

• HNV farms need a mix of synergistic initiatives and innovations (social/institutional, policy/regulations, technology/management, market/products) that promote sustainable infrastructures, practices, systems and territories overall, and help to ensure their viability while maintaining their ecological characteristics in the long term. So institutional and policy frameworks must be sufficiently innovative and enabling.

• Overall, HNV farmers' interests are insufficiently represented, both by mainstream farming unions (for whom profitability and competitiveness are priorities) and by conservationists (whose interests may be mainly environment-oriented). Their networks are often not enough professionalised and organised, fragmented, and lack resources. Supporting the empowerment, organisation, advocacy and cooperation of HNV farmers is thus critical to ensure that their interests are reflected in EU and national policies.

• Where there is sustained innovation in support of HNV farming at a local level, this has usually been facilitated by a local project of some sort, typically by NGOs, professional organisations and specialized advisors, natural areas managers and local authorities. Motivated "HNV facilitators" working closely with HNV farmers and institutions have a crucial pro-active animation role that is different from normal extension/advisory services. Such projects are needed in all HNV areas, on a longterm basis -it takes time to build momentum and for innovation to develop across themes, so continuity of projects over several years is critical, as is continuity of institutional cooperation and support. LAs gather farmers, practitioners, advisers, NGOs, authorities, and education/research institutes that work hand in hand to support the viability of HNV farms and the range of environmental and socioeconomic benefits they provide. These territories may have contrasting contexts and dynamics, but they share challenges. Each one has experience to share in terms of actors' organisation and collaboration, enabling policy, successful projects, innovative approaches to adding value to farm products, etc.

HNV-Link

LAs have formulated their own visions of sustainable HNV farming development pathways, identified the barriers to those and the opportunities. But more importantly, they have analysed, shared and applied a range of solutions to achieve their goals, combining innovations in the social/institutional, regulations/policy, farming techniques/management, and products/market fields.

We have identified a wealth of innovations suitable for HNV farming, and sharing/discussing these across diverse social and geographical contexts enabled us to inform how widely applicable each one is. The Project highlighted the urgent need to actively support HNV farming, by spreading innovation and implementing appropriate CAP Strategic Plans and measures focused on realising the potential of these areas in an integrated, targeted and results-focused policy framework. All reference to HNV farming has been written out of the new CAP proposals, which is unexpected and questionable given the apparently higher environmental ambition of the revised CAP, and considering that much work has been completed in most Member States to develop and implement the HNV farmland context and impact indicators in previous CAP cycles.

HNV farming is an overarching European concept and the only one linking explicitly biodiversity conservation on agricultural land to the maintenance of specific (extensive) farming practices. It has gained considerable momentum as the basis for targeted support measures in Pillar 2, and it should certainly remain in the CAP as a clear signal that this form of agriculture, which covers over 25% of the European farmland, must be suitably supported if biodiversity is to be preserved at large enough scale, in and beyond Natura 2000 areas. • Accompany the definition of permanent grasslands/pastures with EU and national guidelines that ensure the full eligibility of all areas that are effectively grazed or produce fodder, whatever the vegetation type.

As HNV farms rely on and maintain a range of valuable semi-natural herbaceous and nonherbaceous areas/resources (meadows, woody pastures, heathland, scrubland, forests, silvopastoral systems, etc.), these must be recognised when they constitute a component of the farming system (e.g. as a fodder supply or free-range space), and must be considered for financial support (e.g. through direct payments under Pillar 1, and through ANC, AECM, and organic production under Pillar 2).

Efforts were made in that direction with the Regulation (EU) No 2017/2393 (referred to as "Omnibus regulation") which recognizes the eligibility of woody pastures/grazed areas, and this regulation should be applied more consistently across Member States. To reinforce/complement this regulation, the concept and definition of permanent pastures/semi-natural vegetation should be adjusted and agreed upon at the EU level, and the terms "permanent grassland" and "permanent pasture" should always be mentioned together. All types of permanent vegetation contributing to the farming system that be included in the definition, while regularly reseeded pastures, which currently fit the EU Commission's definition of permanent pasture, should not be considered as permanent.

The term "permanent grassland/pasture" should refer to "land used to grow grasses or other forage (self-seeded or sown) and that has not been ploughed or reseeded for 5 years or longer" (as set by EFNCP since 2010), and "may include shrubs and/or trees that contribute directly or indirectly to the farming system, or do not impede farming activity" (Omnibus regulation, 2017).

• Improve CAP Pillar 1 and its system of rights and payments to reduce the bias against extensive farming and better reward the provision of public goods by HNV farming (e.g. eco-schemes to support HNV systems and seminatural pastures).

CAP overall payment scheme:

The CAP should ensure a fairer redistribution of direct payments between farmers to reduce the bias against extensive farming. Currently, direct payments in some Member States are still linked to historic production and may not alleviate negative externalities. In some countries, there are huge areas with no historic rights, and payments are very low on extensive grazing land. Direct payments should centrally support the provision of public goods/services by farming and should not be about "distribution". All Member States should move to a flat-rate system with no link to historic rights (all eligible land in active farming use should have payment rights regardless of historic claims). When applying CAP Pillar I payment system, to incentivise the provision of public goods/services, Member States should:

-set a maximum amount of aid per hectare per farm per worker ; -adapt the capping/calculation of payments per ha and to local conditions, e.g. for heterogeneous surfaces (under art18 of the proposed new Cap regulation), and ensure the recognition of all the pastoral areas (including silvopastoral systems) in the capping classes; -disconnect the capping from the calculation of the support measures from the 2nd Pillar; -apply degressive amounts per hectare to favour the first ha and reduce the pressure on land; -Give coupled livestock payments only for grazing livestock (permanent pastures included) up to a max LU/ha, or with payments weighted in favour of more extensive systems. -pay particular attention to the control methods (timing, simplicity, reliability, trust) taking into account the silvo-pastoralism characteristics; -ensure that the level of payments allows fair socio-economic living for HNV farmers.

Eco-Schemes:

Targeted measures should be implemented to support HNV farming at large scale, e.g. through the eco-scheme under article 20 of the proposed new CAP regulation. Eco-schemes represent an opportunity to provide a bonus payment/incentive to support already sustainable practices, including the sustainable management and conservation of semi-natural permanent pastures and possibly of other low-intensity systems (e.g. traditional olive groves and orchards, extensive crop systems). The bonus could be linked to a points/resultsbased system in order to favour progressive commitments for HNV conservation (avoiding a dual system). For the eco-scheme to be effective, it could consider an area-based top-up for extensive use of permanent grassland.

Areas facing natural or other specific constraints (ANC):

Generally, ANC payments should count towards the percentage expenditure on environmental measures only when they include mechanisms to target payments to particular farming systems of environmental value, e.g. HNV, extensive grazing up to a maximum LU/ha, traditional low-intensity olive groves. ANC payments must not pay irrigated systems.

Predation prevention/mitigation and compensation measures and payments:

Opportunities for dialogue, higher levels of participation, appropriate financial support and technical means must be given to HNV livestock farmers operating in the presence of large predators (e.g. wolf, bear), in order to support best participatory coexistence mechanisms that reduce attacks on livestock, and to mitigate and compensate for predation impacts on the farms viability and farmers' health. Predation prevention costs (as compensation ones) should preferably be covered by the European Regional Development Fund (ERDF, environment part), rather than by the European Agricultural Regional Development Fund (EARDF), since they support an environmental objective. That would increase the budget available for other farm investments that support herd management improvements, land access, etc., and that are essential to sustain HNV farming systems.

Agri-Environment-Climate Measures (AECM):

AECM should be designed involving different parties representing different fields of expertise, and covering notably ecological and farming system sciences. They should support the integrated management of HNV farming systems (i.e. focusing less on the surface area), including a diagnosis and definition of common objectives (e.g. by naturalists, pastoralists, breeders and other HNV farmers) targeted at one or more management units, with a follow-up over the duration of the contract (e.g. as the method used in the Life+ Mil'Ouv Project).

Within AECM, CAP regulations must give an explicit option for Results-Based Agri-Environment payment schemes (RBAPS). The notion of result should be adapted to the nature of the environmental output aimed at and should favour results on habitats and landscapes rather than results on species (whose achievement is more uncertain).

Investments and Innovation/Complementary Supports:

The CAP should provide incentives (fiscal, social, others) for young/new farmers to practice HNV farming, by this removing a stigma of HNV farming being "old-fashioned" and without future.

Non-productive investments should include individual/collective investments in HNV farming. Experience in many Member States has shown that HNV farmers participating in AECM (including results-based schemes) often need financial support for actions not covered by the annual payments but essential for maintaining the farming system, for example scrub clearance, repair of stone walls and hedges, and new water points for livestock.

The CAP should provide support for HNV farming producer groups and collective infrastructures, farm diversification, on farm processing and direct sales, considering local contexts (e.g. Art. 68investments, Art. 71-co-operation, Art. 72-knowledge exchange and information; LEADER if 100% agricultural). When considering the PMEF, the EU Commission's proposal is not for a system of payments linked to delivery of results at farm level (as the text of the proposals implies), rather it is a more conventional system of indicators to attempt to monitor results against broad objectives. In essence we might know if something is working or not but there is no signal to Member States that something needs to be done to adapt implementation as a result of the monitoring findings.

The indicators in Annex I should be further developed and the correspondence between Impact/Result/Output indicators improved. Indicators should be informed by expertise in DG Environment and DG Agriculture, especially those linked to the protection of biodiversity, enhancement of ecosystem services and preservation of habitats and landscapes (e.g. Indicators I.10, I.14, I.19).

The PMEF should make use of and improve the HNV farming results/impact indicators developed previously, to help Member States with the characterisation, mapping and monitoring/evaluation of HNV farming and measures, and to assess whether the measures implemented by Member States answer the needs of HNV systems.

HNV-Link has focused on the crucial question of HNV farming viability, and it is essential for authorities to gather data on the income trends specifically in these farming systems, given the tendency towards abandonment with subsequent environmental losses. The CAP therefore should introduce an Impact indicator under the group for "viable farm incomes and resilience" (where there is already an indicator for incomes in ANC), e.g. Contributing to the viability of HNV farming systems: evolution of incomes in HNV farming systems, by farming sector. The corresponding Result indicator would be, e.g. Enhancing support to high nature value farming systems: increase in support relative to non-HNV farming. HNV farming incomes can be monitored through a sample survey method, targeting areas identified as having a high proportion of HNV farming.

Recommendations to accompany HNV farming at national, regional, local levels

• Do a full assessment of HNV farming systems in Member States (values, practices, challenges, etc.) and design ambitious programmes of CAP/RDP measures to ensure their maintenance, with indicators to monitor their condition.

The overarching problem at present is not so much the current CAP at EU level, it is also the way that Member States implement existing policy options, and the failure of EU governance to ensure that enough effort is put into tackling issues such as the decline of HNV farming in and outside Natura 2000 areas. In particular, certain Member States give very little attention to their HNV farming when designing Pillar 1 regimes and RDPs, with the result that these systems do not receive the support that they need.

The new Strategic Plans are an invaluable opportunity for authorities to take a new approach based on an assessment of HNV farming and its challenges and needs on the ground.

Member States must give options to implement Results-based agrienvironment payment schemes (RBAPS), ensuring their continuity over the long term.

• Strengthen Agricultural Knowledge and Innovation Systems (AKIS) with targeted approaches to boost HNV farming innovation, including a facilitation and advisory role for NGOs, professional organisations, natural area managers and local authorities.

The CAP should contribute to enhance AKIS across the EU, promoting a clear understanding of HNV farming stakes and of HNV farmers' needs. Strategic AKIS plan should be considered as a compulsory component of CAP strategic Plans, and they should reflect the importance of HNV farming in education, research, advisory sector. They should contain elements to improve in the short, medium and long term the cooperation between stakeholders, the production and flow of knowledge, the identification of the farmers' needs, and the development and transfer of suitable/useful solutions to face current and future challenges. These measures should be supported by long-term investments in human resources that ensure the animation of the necessarily participatory processes.

Knowledge and understanding of HNV farming:

Member States should improve the mapping of HNV farmland and HNV farm characterisation, as well as their knowledge of HNV farming economies and income trends.

Farmers representation:

The empowerment, organisation of HNV farmers and their associations should be strengthened, to help them improve their representation at the national and EU levels, and better defend their interests.

Advisory services and locally-led facilitation:

Advisory services' awareness of the benefits of HNV farming for biodiversity/natural resource conservation should be raised, and farm advisers should be properly trained to deal with HNV farming specificities in a multiactor setting, following a "knowledge/innovation broker approach" rather than using the adviser-advised paradigm.

Advisers should promote good practices/innovations that enhance the productivity/viability of HNV farms while maintaining their ecological value.

The crucial role of local projects public/private actors (e.g. NGOs, local development agencies, professional organisations, specialised advisors, natural area managers, and local authorities) in supporting HNV farming must be acknowledged and strengthened, as those are key to sustaining innovation and development. Motivated "HNV facilitators" working closely with HNV farmers and institutions play an essential pro-active animation role that is different from normal extension/advisory services. Such projects are needed in all HNV areas, on a long-term basis, to build momentum and for innovation to develop across themes and institutional levels.

Funding must be made available to ensure the continuity of the projects over several years, and the continuity of institutional cooperation and support must be a priority.

Education & public awareness:

Several countries incorporate in their education systems and curricula teaching on HNV farming (pastoralism, agropastoralism, agroecology), but teaching must be strengthened and proposed at different academic levels, from primary school to higher education and post-graduate studies, with as much as possible practical field and landscape visits. Researchers, practitioners and authorities must foster publications and actions related to HNV farming constraints, opportunities, benefits, embedding HNV farming in the food, agriculture, and environment nexus (e.g. explaining that the insect collapse is linked to HNV farming decline to a large extent). Continuous training programmes should be designed and implemented in this regard.

• Enhance joined-up policies and institutions for integrated land management and sustainable development, and develop the framework for participatory models, processes and institutions ruling the governance of HNV areas to allow HNV farmers to play a role in decision making.

The CAP should drive an integrated approach to land use policy and management across the EU and the coordination between sectoral policies (agriculture, rural development, environment, animal health, food hygiene) should be improved to address in a more strategic, integrated, and sustainable way common issues related to agriculture, environment protection, energy, food, socio-economic development, etc. The UN Sustainable Development Goals should serve as overarching framework.

HNV farming is an overarching European concept that integrates the conservation of the most valued farmland biodiversity and the maintenance of the essential farming systems. Within this framework, CAP/RDP should be used to promote Natura 2000 objectives for habitats maintained by HNV farming.

Integration is also needed at the local level. Governance of HNV farming areas demand real participation of farmers involved. For this participation to be effective there is a need for training, facilitation, capacity building and collaborative institutions actually committed to sustainable development. The role of skilled facilitators is key for developing these infrastructures.

• Improve land access/stewardship, management & monitoring, to support the installation of new HNV farmers, longer-term investments in HNV farming, more sustainable land planning, and the assessment of environmental results/services.

Access to land is the largest barrier to new entrants to farming in Europe, as highlighted by several actors and studies (see for example EIP-Agri Focus Group on New Entrants to Farming), and the CAP should create opportunities for existing HNV farms to thrive, and for new ones to start. In many regions, the availability of land for sale or rent is low, and the competition for different uses is high (farming, residential, tourism, etc.). In addition, the current CAP tends to disincentivise land sales and rental, and exacerbate land speculation.

The rules for the allocation of and access to municipal grasslands/pastures should be adapted to support longer-term access and investments by livestock farmers, and fit the needs of extensive farming systems (e.g. pastoralism).

The CAP system of rights and direct payments should incentivise land sale/rental to farmers and ensure the best use of the land. National and regional rules, e.g. for grazing and management of municipal pastures should be adapted, as the rules for allocating municipal grasslands/pastures to livestock farmers, to distinguish livestock kept in-door/grass-fed only, plain/mountain (e.g. National legislation for allocation of municipal grasslands to local livestock farmers without tender procedure).

Long-term contracts for use of municipal land (adapting the law on land use and municipal regulations) and long-term pacts between owners, producers and other stakeholders should be promoted, to ensure stability, investment and social support to grazing, and other HNV activities.

Sustainability criteria in state-owned land should be implemented, for the monitoring of land given for concession, to preserve agricultural landscape and biodiversity.

• Adapt the implementation of animal health and welfare regulations (e.g. TB eradication campaigns) to fit better HNV farming conditions.

Animal health and welfare regulations should be adapted and implemented based on risk, and risk management should be addressed across the relevant policies. Member States must promote cooperation between national/regional authorities, farmers/herders and experts to design animal health regulations, campaigns, programmes adapted to HNV farming systems, notably on common pastures with wild fauna vectors (e.g. TB eradication in Spain).

Overall, governments must encourage practices and infrastructures that enhance animal welfare (e.g. free range husbandry, local or mobile abattoirs). The effective participation of HNV farmers and their organisations in committees, strategies and planning affecting health and welfare regulations must be guaranteed.

• Adapt the implementation of food production, processing and marketing regulations, to support the creation of added-value and outlets for HNV farming products, including small-scale processing and direct sales.

Member States should embrace the flexibility of EU food hygiene/sanitary legislation and rules and allow flexible application to encourage small-scale and on-farm processing and marketing of HNV farm products (e.g. on-farm slaughtering, artisan/raw milk cheese production and direct sale, mobile abattoirs, mobile cheese factories…).

The marketing of HNV products through direct sale and certification should be supported to increase their added value, based on guarantee systems made of criteria/indicators compatible with national certification systems. The link between the product and ecological characteristics of the HNV farm is key.

• Develop sustainable food policy in urban/rural areas and promote innovative models of urbanrural relationships, to foster the local production of quality food and the reconnection between consumers and the farmers who supply sustainably food and public services.

Cities and towns, which host many of the food consumers, must implement sustainable food policy/measures to promote environmentally sustainable farming systems: encouraging the consumption (e.g. in public school restaurants, public health infrastructures) of products originating from environmentally sustainable farms (e.g. HNV or organic); supporting the organisation/cooperation of agri-food value chain actors to multiply marketing options and allow a fairer redistribution of the products' added-value; fostering "short-circuit retail" which can decrease intermediary costs and result in higher profit for the farmers; encouraging the mobilisation of urban/peri-urban land for the installation of HNV farmers.

All these local food actions should entail explicit biodiversity and landscape criteria/goals, giving more positive meaning to local farming rather than simply "efficient" or "non-polluting" farming. The relationship/connection between HNV farms/territories and rural/urban consumers should be strengthened through collaborative tools such as land pacts with the participation of consumers, urban and rural inhabitants and all stakeholders involved.

Conclusion

Biodiversity and landscapes in particular, and natural resources in general, are being lost in the EU and worldwide at an unacceptable rate. Their conservation across sufficiently large scales, beyond nature reserves and sanctuaries, calls for the continuation of "High Nature Value farming", i.e. low intensity/input/impact farming, in areas threatened by land abandonment/encroachment or intensification and irreversible degradation.

The CAP, as the overarching EU Agricultural Policy framework, forms the basis of the EU common sustainable development vision, that all Member States should back and concretise. As such, it must clearly and effectively support the recognition and socio-economic viability of environmentally beneficial (including HNV) farming systems in the long run, as much as it supports the greening of the more intensive forms of agriculture. As biodiversity and landscapes are one of the most crucial and irreplaceable heritage for all citizens, the EU dimension of HNV conservation should be reaffirmed. The conservation of HNV farming is the responsibility of all levels of intervention: from the European to the local level.

The EU should promote effective and targeted support for HNV farming, e.g. through financial incentives from CAP Pillar 1 (direct payments, eco-scheme, etc.) and Pillar 2 (AECM, local HNV farming support and facilitation, etc.), and using other instruments that boost agricultural innovation and farming viability (e.g. Operational groups of the EIP-Agri). In addition, real efforts are needed to improve AKIS and promote joined-up policies and institutions, to develop sustainable food policy and food systems, to facilitate land access/stewardship, and to adapt nationally/regionally the implementation of animal health/welfare and food production/processing/marketing regulations.

While there should be some flexibility for Member States and regions to design locally-suited measures in support of HNV-farming (e.g. eco-schemes, AECM), the CAP should propose a strong and convincing framework ensuring that EU biodiversity conservation is a top priority for all Member States at all times, and that existing forms of agriculture that already support exceptional biodiversity are rewarded across the EU through results-based incentives. Food production cannot be EU's agriculture unique driver anymore, and the ongoing shift of paradigm and practices towards "agroecology" must continue if agroecosystems are to be managed sustainably and if our societies want to keep on enjoying the range of ecosystem services that HNV farming produces.

  Source: www.ec.europa.eu Source: ETC-SIA 2012, adapted from EEA/JRC 2007 Sustaining HNV farming & biodiversity in the EU: Our recommendations at a glance Recommendations to improve the Common Agricultural Policy: • Integrate HNV farming in the CAP vision, objectives and Strategic Plans, as a diversity of farming systems irreplaceable for achieving EU biodiversity conservation objectives and UN Sustainable Development Goals.

  Nature Value farming is an essential component of sustainable agri-food systems and territories and it must be rewarded as such! How can we enhance the viability of HNV farms while keeping their unique ecological characteristics and the public goods & services they provide? Why does High Nature Value (HNV) farming need policy change?

  : multi-actor network to support HNV farming HNV-Link (High Nature Value Farming: Learning, Innovation and Knowledge, H2020, 2016-2019) is a multi-actor thematic network driving a peer-learning process between 13 partners and 10 Learning Areas (LAs) across Europe. It aims at sharing best practices and innovations that support HNV farming systems and communities by simultaneously improving their socio-economic viability and environmental sustainability. It builds upon the Focus Group on HNV Farming of the European Innovation Partnership for Agricultural Productivity and Sustainability (EIP-AGRI).

  The 10 Learning Areas: Western Stara Planina (Bulgaria), Dalmatian Islands (Croatia), Thessalia (Greece), Causses & Cévennes (France), The Burren (Ireland), Sítio de Monfurado (Portugal), Eastern Hills of Cluj (Romania), La Vera (Spain), Västra Götaland (Sweden), Dartmoor (UK) Multi-actor networks such as HNV-Link drive innovation & policy change, as they connect research, policy and practice and foster co-innovation. Our recommendations to support EU HNV farming & biodiversity Recommendations to improve the Common Agricultural Policy (CAP) • Integrate HNV farming in the CAP vision, objectives and Strategic Plans, as a diversity of farming systems irreplaceable for achieving EU biodiversity conservation objectives and UN Sustainable Development Goals.

  Considering the increased subsidiarity to Member States, the sustainability dimension of the EU agricultural development vision and policy framework must be strengthened to prevent the renationalisation of definitions (e.g. permanent pasture), environmental targets, and monitoring/evaluation indicators. This will help to ensure non-discriminatory treatment for all farmers throughout the EU territory, to prevent distortion of competition, and to drive emulation towards environmental conservation, so as to fulfil EU CAP specific objectives and sustainable development goals. Strengthening the CAP sustainability calls for the recognition of HNV farming and farmland and an adjustment of the reward mechanisms for the public services provided.The CAP should reaffirm not only the need to mitigate the negative externalities of the more intensive farming systems, but also the need to give greater support to those nature-friendly farming systems (including HNV farms) that already contribute to environmental objectives, by supporting their economic and social viability. It should refer explicitly to HNV farming in the objective on biodiversity and ecosystem services (Title II article 6 1.f) as key for conserving EU farmland biodiversity, beside Natura 2000 areas ("Natura 2000 and farmland of similar HNV character ").Member States should continue their effort to define and characterise HNV farming and farmland, to size and adjust the support given to this form of agriculture, and the CAP should include a requirement for CAP Strategic Plans to analyse/assess HNV farming needs & opportunities (challenges, supply of ecosystem goods & services, options for support and innovation, development pathways, etc.), in order to develop and implement adequate support and monitoring measures.

  Source: EU Commission, 2019 (Presented at HNV-Link Final Conference, 31/01/2019)

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  Improve CAP Pillar 2, adjusting the overall budget balance in favour of Pillar 2 with explicit measures for locally-led HNV conservation projects and results-based incentive schemes (transfers between Pillar 1 and 2, climate/environment scheme, etc.). Budget balance between Pillar 1 and 2: Member States should have the possibility to transfer part of their CAP allocations between direct payments and rural development and vice-versa to ensure that their priorities and measures can be funded (The CAP allows a transfer of up to 15% -Art 90 of the new CAP regulation). Pillar 2 should receive greater amount of funding targeted to environmental objectives, and explicit mention should be made of the need to support HNV farming at a large scale and a more efficient use of Pillar 2 measures is needed to enhance the impact of AECM; the surface of eligible areas under Pillar 2 should increase. CAP regulation should make explicit mention of and foster locally-led projects to support HNV farming and boost innovation, supported by the Cooperation article of the rural development regulation. HNV farming should also be targeted for support from agri-environment-climate payments, non-productive investments, capacity building of farmers and extension services, etc. Since 2005, certain Member States have responded to explicit prioritisation of HNV farming in Pillar 2 by implementing targeted HNV farming measures, e.g. AECM in Bulgaria and Romania, locally-led projects in Ireland. The EU Commission should encourage other Member States to follow these examples.

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  Adjust the Performance Monitoring and Evaluation Framework (PMEF) to improve the characterisation and monitoring of HNV farming systems/territories (assessing both HNV farmland quality and extent) and the evaluation of CAP measures, including an income indicator for HNV farms.

  For example, specific HNV indicators should be designed to support the assessment/funding of Local Development Strategies developed by the Local Action Groups that operate in recognised HNV areas in the framework of the Leader Programme (2021 -2027).

This project has received funding from the European Union Horizon 2020 Research and Innovation Programme under Grant Agreement No. 696391