Charles Lhuillier

Pierre Brequigny

Francesco Contino

Christine Mounaïm- Rousselle

Experimental investigation on

Keywords: Reciprocating IC Engines (alternate: Turbulent Flames Ammonia, Hydrogen, Methane, SI engine, Turbulent flame, Nomenclature cp: isobaric specific heat capacity Karlovitz number. Lb: burnt-side Markstein length flame surface area equivalent radius longitudinal turbulence intensity. V: flame propagation velocity. x: mole fraction flame thickness 𝜆: gas thermal conductivity. 𝜙: equivalence ratio. 𝜌: density

Figure 1 96 Figure 2 282 Figure 3 231 Figure 4 236 Figure 5 181 Figure 6 170 Figure 7 182 Figure 8 237 Figure 9 147 Figure 10 150 Table 1 182 Total(excl. abstract) 6106

Supplemental material is available. Color figures only in the online version.

Introduction

Ammonia (NH3) is recognized as a carbon-free sustainable electro-fuel, useful to transport and store excess energy from variable renewable sources in complement to hydrogen (H2) [1]. Ammonia combustion in gas turbines or internal combustion engines is a meaningful approach to retrieve the stored energy, especially in hard-to-electrify applications such as maritime transport. However, current lack of understanding makes it the subject of several studies as underlined in [START_REF] Kobayashi | Science and technology of ammonia combustion[END_REF][START_REF] Valera-Medina | Ammonia for power[END_REF][START_REF] Dimitriou | A review of ammonia as a compression ignition engine fuel[END_REF].

In Spark-Ignition (SI) engines, the assessment of laminar combustion properties remains a meaningful first-order approach to describe the behavior of air-fuel mixtures, since it correlates to the initial flame kernel growth that governs the combustion resilience to cycle-to-cycle variability [START_REF] Szybist | Effects of Fuel Composition on EGR Dilution Tolerance in Spark Ignited Engines[END_REF]. However, experimental laminar burning velocity (LBV) data for NH3-based mixtures are still sparse in the literature, especially at SI engine-relevant conditions, i.e. high temperature and pressure. For example, Hayakawa et al. measured the LBV and Markstein length of NH3/air flames at various equivalence ratios (𝜙), at 298K and up to 0.5MPa [START_REF] Hayakawa | Laminar burning velocity and Markstein length of ammonia/air premixed flames at various pressures[END_REF]. Ichikawa et al. carried out similar measurements under the same initial conditions for stoichiometric NH3/H2/air flames with 0-100vol.%H2 [START_REF] Ichikawa | Laminar burning velocity and Markstein length of ammonia/hydrogen/air premixed flames at elevated pressures[END_REF]. Okafor et al.

provided another dataset for ammonia blended with methane (CH4) in air (up to 53 vol.%NH3 in the fuel) at various 𝜙, 298K and up to 0.5MPa [START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism[END_REF]. Recently, Lhuillier et al. attempted to partially fill in that gap with an extensive experimental LBV study by means of the outwardly propagating spherical flame (OPSF) method covering a wide range of 𝜙 (0.8 to 1.4), fuel hydrogen fractions (0 to 60 vol.%) and initial temperatures (298 to 473 K) at atmospheric pressure [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF]. Kinetic models of ammonia oxidation have been improved recently but still lack comprehensive experimental data to be validated over the range relevant for SI engine conditions [START_REF] Konnov | Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism[END_REF][START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry[END_REF][START_REF] Nakamura | Kinetic modeling of ammonia/air weak flames in a micro flow reactor with a controlled temperature profile[END_REF][START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF][START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF][START_REF] Okafor | Experimental and numerical study of the laminar burning velocity of CH 4 -NH 3 -air premixed flames[END_REF][START_REF] Li | Chemical mechanism development and reduction for combustion of NH3/H2/CH4 mixtures[END_REF].

Recently, we presented premixed NH3/air fueling of a modern SI engine with satisfactory stability and performances [START_REF] Lhuillier | Combustion Characteristics of Ammonia in a Modern Spark-Ignition Engine[END_REF][START_REF] Lhuillier | Performance and Emissions of an Ammonia-Fueled SI Engine with Hydrogen Enrichment[END_REF][START_REF] Lhuillier | Experimental study on ammonia/hydrogen/air combustion in spark ignition engine conditions[END_REF]. Spark-ignition timing (SIT) advance, intake pressure increase, and H2 enrichment of the fuel widened the stability range and improved performances. An analysis based on the LBV was not sufficient to fully elucidate the behavior observed in terms of combustion stability and cyclic variation when varying the latter parameters, especially for lean H2-enriched mixtures.

Therefore, since ammonia mixtures cause operation in unusual turbulent combustion regimes, it is necessary to investigate the combustion of NH3-based mixtures in controlled turbulent conditions, in order to bring deeper understanding, as well as experimental data for the validation of future adapted models [START_REF] Nilsson | Structures of turbulent premixed flames in the high Karlovitz number regime -DNS analysis[END_REF]. Ichikawa et al. performed ammonia/methane/air turbulent flames experiments on a nozzle burner at 0.5MPa and 298K for 𝜙=0.9 and 0-39vol.%NH3 in fuel [START_REF] Ichikawa | Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure[END_REF]. They reported a decreasing turbulent-to-laminar velocity ratio when increasing the ammonia content, mainly explained by the decrease of the flame surface density. Ichimura et al. conducted NH3/air turbulent flames experiments in a fan-stirred constant-volume vessel at 298K and 0.1MPa for 𝜙=0.6-1.3 and average turbulence intensities u'=0.32-1.61 [START_REF] Ichimura | Extinction limits of an ammonia/air flame propagating in a turbulent field[END_REF]. The longitudinal integral length scale reported (LT=20.9mm) is one order of magnitude higher than typical SI engines values and most of the studied flames are in the thin reaction zones regime. Extinction-free flame propagation could be achieved at the highest u' for 𝜙=0.9, and lean mixtures showed successful propagation over a wider Karlovitz number (Ka) range than rich ones, indicating a better resistance to turbulence-induced extinction. This may result from thermodiffusive accelerating effects, as inferred by the authors from lower-than-unity Lewis numbers (Le) and near-zero Markstein numbers. Such effects are well identified in the literature [START_REF] Abdel-Gayed | Lewis number effects on turbulent burning velocity[END_REF][START_REF] Lipatnikov | Molecular transport effects on turbulent flame propagation and structure[END_REF][START_REF] Brequigny | Lewis number and Markstein length effects on turbulent expanding flames in a spherical vessel[END_REF] and also invoked to explain the higher turbulent-to-laminar velocity ratios of lean ammonia mixtures in oxygenenriched air condition and identical configuration in [START_REF] Xia | Turbulent burning velocity of ammonia/oxygen/nitrogen premixed flame in O2-enriched air condition[END_REF].

The objective of the present study is to partially elucidate the phenomena responsible for successful stable ammonia combustion with or without help of combustion promoters in modern SI engines, by combining experimental data obtained in two complementary setups, namely a single-cylinder SI engine and a constant-volume combustion vessel in laminar and turbulent configurations. Since hydrogen combustion is strongly influenced by preferential transport phenomena due to its high molecular diffusivity, it appears meaningful to assess the turbulent flame propagation properties of NH3/H2/air in comparison with NH3/CH4/air and CH4/H2/air mixtures.

Experimental and numerical methods

SI engine set-up and conditions

Experiments were conducted in a four-stroke pentroof chamber single-cylinder all-metal SI engine (already described in [START_REF] Lhuillier | Combustion Characteristics of Ammonia in a Modern Spark-Ignition Engine[END_REF][START_REF] Lhuillier | Performance and Emissions of an Ammonia-Fueled SI Engine with Hydrogen Enrichment[END_REF]) with a 10.

Laminar flames set-up and conditions

The constant-volume spherical vessel used in the present study was fully described in [START_REF] Galmiche | Effects of high pressure, high temperature and dilution on laminar burning velocities and Markstein lengths of iso-octane/air mixtures[END_REF], along with the experimental procedure that was updated with ammonia flames specificities in [START_REF] Lhuillier | Experimental investigation on laminar burning velocities of ammonia/hydrogen/air mixtures at elevated temperatures[END_REF]. The fuel blends and 𝜙 are identical to those of the engine study (Table 1) but CH4/H2 blends were limited to 5vol.%H2.

The in-cylinder engine conditions at SIT were chosen for the unburned gas (445K, 0.54MPa). A minimum of three identical tests were performed for each condition. Experimental LBVs and Markstein lengths were extracted from the Schlieren-based flames propagation history by means of non-linear zero-stretch extrapolation. Even though sparse cracks typical for cellular instability could be observed under those thermo-diffusively and hydro-dynamically unstable conditions, the reproducibility of the measurement was very good with standard deviations below 10%.

Turbulent flames set-up and conditions

In this part of the study, the constant-volume vessel in Sec.2.2 was used with six identical fans continuously stirring the mixture during the combustion, as in [START_REF] Brequigny | Lewis number and Markstein length effects on turbulent expanding flames in a spherical vessel[END_REF][START_REF] Brequigny | Improvement of Turbulent Burning Velocity Measurements by Schlieren Technique, for High Pressure Isooctane-Air Premixed Flames[END_REF]. A region of mean homogeneous and isotropic turbulence was identified in a radius of 20mm around the vessel center [START_REF] Galmiche | Turbulence characterization of a highpressure high-temperature fan-stirred combustion vessel using LDV, PIV and TR-PIV measurements[END_REF] with a longitudinal integral length scale of LT=3.4mm. For this study, two values of mean turbulence intensities, u', induced by different fan rotation speeds were chosen (1.04m.s -1 and 1.73m.s -1 ). The unburned gas conditions were identical to the laminar experiments with the same fuel blends (Table 1) but only at 𝜙=0.9. Each experiment was repeated at least ten times. Error bars reported with the results are standard deviations. Examples of turbulent flames images obtained from the Schlieren technique are showed in Fig. 1 and in the SM. structures, affecting the global spherical shape as seen in Fig. 1. This results in a limited range of observable radii and in the fact that no steady propagation regime could be reached presently.

However, the observations made at small radii are still very valuable, since combustion propagation and stability are mainly driven by the initial flame kernel growth in SI engines [START_REF] Brequigny | Fuel performances in Spark-Ignition (SI) engines: Impact of flame stretch[END_REF]. Thus, most of comparisons are made at constant radius Rs=10mm in the following.

Kinetics simulation

LBVs and flame temperature profiles were estimated by means of the 1D premixed freely propagating adiabatic flame model in Cantera 2.4.0, as well as other mixture properties, such as the burned-tounburned density ratio thanks to equilibrium calculations [START_REF] Goodwin | Cantera: An object-oriented software toolkit for chemical kinetics, thermodynamics, and transport processes[END_REF]. Different kinetic models (Nakamura et al. [START_REF] Nakamura | Kinetic modeling of ammonia/air weak flames in a micro flow reactor with a controlled temperature profile[END_REF], Shrestha et al. [START_REF] Shrestha | Detailed Kinetic Mechanism for the Oxidation of Ammonia Including the Formation and Reduction of Nitrogen Oxides[END_REF], Otomo et al. [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF] and Okafor et al. [START_REF] Okafor | Experimental and numerical study of the laminar burning velocity of CH 4 -NH 3 -air premixed flames[END_REF]) were used. The first three were chosen for their relatively good accuracy with respect to NH3/H2/air LBVs, while the latter was mainly

Results and discussion

NH3-fueled SI engine

Figure 2 shows the averaged HRR during the combustion of various NH3 and CH4-based fuels in the engine cylinder at 𝜙=0.9 and constant SIT. Supplementary data, including coefficients of variation of the indicated mean effective pressure are available in the Supplemental Material. Hydrogen enrichment causes earlier, faster and more intensive heat release, as seen in Fig. 2a and 2c for the NH3based and CH4-based fuels, respectively. Unexpectedly, CH4 blending in NH3 results in opposite trends, as showed in Fig. 2b: increasing the CH4 fraction in the NH3-based fuel causes a delay, loss of intensity and widening of the HRR peak, even if the LBV with CH4 is higher than pure NH3. Increasing 𝜙 up to 1.1 results in an increase of the maximum HRR for all mixtures, but does not change qualitatively the previously observed trends for the hydrogen-enriched mixtures as seen in Figs.S1 to S3. However, the combustion duration of NH3/CH4 mixtures is modified with increasing 𝜙, as observed in Fig. 3. For hydrogen-enriched mixtures, the previously observed trend of combustion acceleration with increasing H2 fraction and 𝜙 is highlighted as the duration of the first propagation phase CA50-SIT decreases accordingly. For instance, a 15.5CAD decrease is observed for NH3/H2 blend at 𝜙=0.9 from 0 to 15vol.%H2 in the fuel. For the NH3/CH4, increasing the CH4 fuel fraction affects differently the CA50-SIT as a function of 𝜙: delayed at 𝜙=0.9 (Fig. 3a), not affected at 𝜙=1.0 (Fig. 3b) and slightly accelerated at 𝜙=1.1 (Fig. 3c). This unexpected behavior indicates different flame development and propagation properties and requires further investigations.

Fig. 3. Hydrogen significantly affects the duration of the flame propagation at all equivalence ratios, whereas methane enrichment of ammonia has little influence (SIT=42CAD bTDC).

Laminar flames results

Figure 4 shows experimental LBVs along with kinetic modelling predictions for NH3-based blends.

Some experimental values for CH4/H2 blends can be found in the Supplemental Material. As expected, the magnitudes of the measured LBVs for the ammonia-based blends are very low, about 5 to 6 times smaller than those of methane. Nevertheless, it is interesting to notice that adding H2 or CH4 to the NH3 fuel induces the same LBV increase at 𝜙=0.9 (Fig. 4a), and similar effects at 𝜙=1.0 and 1.1 (Figs. 4b and4c). Thus, the LBV does not fully explain the significantly different impacts between hydrogen and methane enrichment observed in the SI engine in Fig. 2 and Fig. 3. The comparison with the kinetic modeling indicates well predicted trends, but without quantitative agreement and a certain scatter between the mechanisms for NH3/H2, up to 23% discrepancy.

Turbulent flames investigations

All conditions studied in the turbulent constant-volume vessel and in the SI engine are reported in the Peters-Borghi diagram [START_REF] Borghi | Mise au point sur la structure des flammes turbulentes[END_REF][START_REF] Peters | Laminar flamelet concepts in turbulent combustion[END_REF] in Fig. 5. The engine conditions for NH3-based fuels belong to the thin reaction zone regime, located closer to the upper-right corner of the diagram than usual conventional fuel engine operation due to the very low LBVs present mixtures. Such conditions, out of the flamelet regime, may be characterized by a distributed combustion, strongly affected by turbulence. This is especially true for very-low-LBV mixtures such as lean NH3/air mixture. Please note that similar regimes were found in [START_REF] Ichimura | Extinction limits of an ammonia/air flame propagating in a turbulent field[END_REF], while the flames in [START_REF] Ichikawa | Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure[END_REF] belonged to the flamelet regime. The spherical vessel conditions are close to the engine ones, even though slightly shifted towards the bottom-right corner of the diagram because of higher LT and smaller u'. Thus, the present turbulent flame study is relevant to investigate the differences observed during the engine study in Sec.3.1. No significant difference is observed in Fig. 5 between NH3/H2 and NH3/CH4 fuels, indicating similar combustion regimes. Figure 6 also illustrates the difference in turbulence response between hydrogen and methane enrichment: adding hydrogen or methane in the NH3-based fuel causes respectively an increase or a decrease of the turbulent-to-laminar velocity ratio in comparison to pure NH3. Moreover, it can be observed that the turbulent-to-laminar velocity ratio of NH3/H2 blends is about twice higher than the one of CH4/H2 blends and that this difference increases with increasing hydrogen fraction and flame radius. Specifically, a small H2-enrichment causes steep increase of the turbulent-to-laminar velocity ratio versus Rs for the NH3-based fuel but not for the CH4-based fuel. Thus, the turbulent response of NH3 fuels is enhanced by slight hydrogen enrichment, while it remains relatively unchanged for CH4based mixtures. The turbulent-to-laminar velocity ratio with VT value considered at Rs = 10 mm, representative of the turbulence-flame interactions during early flame development [START_REF] Brequigny | Improvement of Turbulent Burning Velocity Measurements by Schlieren Technique, for High Pressure Isooctane-Air Premixed Flames[END_REF] is plotted as a function of u'/𝑆 L 0 in Fig. 7. First, as expected, increasing the induces higher turbulent-to-laminar velocity ratios in all cases. Furthermore, the respective enhancing or inhibiting effects of enriching ammonia with H2 or CH4 on the flame turbulence response observed in Fig. 6 are confirmed at increased turbulent intensity and various additive fractions. This observation agrees well with the SI engine results in Sec.3.1, where adding hydrogen caused earlier and stronger heat release, while adding methane caused the opposite (Fig. 2). Since no significant LBV gap appeared in Fig. 4 between H2 and CH4 enrichment, their opposite effects on the engine combustion phasing are likely related to the observed turbulenceflame interactions.

However, the unexpected tilt in the turbulent-to-laminar velocity ratio between 10 and 15vol.%H2 enrichment observed in Fig. 6 is again observed at higher turbulent intensity, suggesting a "bending" effect of higher 𝑥 H 2 on the turbulent burning velocity. Analogous effects were previously observed experimentally [START_REF] Filatyev | Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities[END_REF] and numerically [START_REF] Nilsson | Structures of turbulent premixed flames in the high Karlovitz number regime -DNS analysis[END_REF], and explained by the merging of flame regions strongly affected by turbulence thus reducing the total flame surface area. Presently, a more likely cause is a modification in the flame response to stretch. Furthermore, it is worth noticing anew that CH4-based mixtures exhibit velocity ratios about twice smaller than NH3-based mixtures, in spite of the factor five to six between their respective LBVs in favor of methane. This is likely due to significant differences in the combustion regimes of the two kind of blends, and highlights the limitations of LBV considerations to predict combustion processes in SI engines as underlined in [START_REF] Brequigny | Fuel performances in Spark-Ignition (SI) engines: Impact of flame stretch[END_REF]. evaluator of the turbulent flame-stretch interaction as evidenced in [START_REF] Brequigny | Fuel performances in Spark-Ignition (SI) engines: Impact of flame stretch[END_REF] and probably explains the bending effect observed in Fig. 6 and Fig. 7 between 10 and 15%H2 in the NH3/H2 blend. More sophisticated correlations are available in the literature for the turbulent burning velocity [START_REF] Brequigny | Improvement of Turbulent Burning Velocity Measurements by Schlieren Technique, for High Pressure Isooctane-Air Premixed Flames[END_REF][START_REF] Reddy Muppala | Development of an algebraic reaction rate closure for the numerical calculation of turbulent premixed methane, ethylene, and propane/air flames for pressures up to 1.0 MPa[END_REF]. Since thermo-diffusive and stretch effects show an influence on the ammonia flame response to turbulence, it appears meaningful to include those in a correlation. In Figure 9, experimental turbulent-to-laminar velocity ratios correlate very well with an updated form of the correlation proposed in [START_REF] Reddy Muppala | Development of an algebraic reaction rate closure for the numerical calculation of turbulent premixed methane, ethylene, and propane/air flames for pressures up to 1.0 MPa[END_REF]:

𝜌 b 𝜌 u 𝑉 T (𝑅 s = 10𝑚𝑚) 𝑆 𝐿 0 = |𝑀𝑎| A B 𝐿𝑒 eff 𝑅𝑒 T 0.25 ( 𝑢 ′ 𝑆 L 0 ) 0.3 ( 𝑝 𝑝 ref ) 0.2 + C (1)
with A,B and C constants (A=0.06 in Fig. 9), 𝑀𝑎 = 𝐿 b 𝛿 L ⁄ the Markstein number of the mixture, and

𝑅𝑒 T = (𝑢′𝐿 T ) (𝑆 L 0 𝛿 𝐿 ) ⁄
the turbulent Reynolds number. Therefore, thermo-diffusive properties and flame-stretch interactions appear to have a prominent role in the turbulent combustion of ammonia mixtures. Fig. 9. Experimental turbulent-to-laminar velocity ratios correlate well with a Ma-corrected variation of the correlation proposed by Muppala et al. [START_REF] Reddy Muppala | Development of an algebraic reaction rate closure for the numerical calculation of turbulent premixed methane, ethylene, and propane/air flames for pressures up to 1.0 MPa[END_REF] for all present conditions at 𝜙=0.9.

As turbulence-flame interactions strongly influence the propagation characteristics of the flames, as a function of combustion regimes delimited by characteristic numbers, it appears interesting to suggest a correlation between the turbulent-to-laminar velocity ratio and these characteristic numbers incorporating the various turbulent and chemical scales. Indeed, a very good correlation is found in Fig. 10 between the present velocity data corrected with the associated Damköhler (Da) and Karlovitz numbers for all fuel blends, and expressed as:

Conclusions

This work aimed at extending the understanding of NH3 combustion in SI engine conditions. New laminar burning velocity measurements under engine-relevant conditions are presented but do not sufficiently explain the engine combustion behavior. Unprecedented turbulent expanding flame measurements with hydrogen and methane enrichment showed that such combustion occurs in unusual combustion regimes. Enhancing and inhibiting effects of H2 and CH4 enrichment on the flame turbulent response were highlighted and partially linked to the different thermo-diffusive properties and flame stretch response of the fuels. Present experimental results correlate well with the characteristic quantities of turbulent premixed combustion and could explain trends observed in real SI engine operation. Thus, further experiments and modelling efforts including direct numerical simulation would be very valuable to gain further understanding on ammonia turbulent combustion and optimize practical combustion systems. Fig. 9. Experimental turbulent-to-laminar velocity ratios correlate well with a Ma-corrected variation of the correlation proposed by Muppala et al. [START_REF] Reddy Muppala | Development of an algebraic reaction rate closure for the numerical calculation of turbulent premixed methane, ethylene, and propane/air flames for pressures up to 1.0 MPa[END_REF] for all present conditions at 𝜙=0.9. Fig. 10. A new correlation of Da-corrected turbulent-to-laminar velocity ratios with Ka for all conditions of mixtures and turbulence.

List of

Fig. 1 .

 1 Fig.1. Example of double Schlieren images of a NH3/air turbulent flame at u'=1.73m.s -1 .

  validated for CH4-containing blends. The laminar flame thickness 𝛿 𝐿 was computed as 𝛿 𝐿 = 𝜆 u 𝜌 u 𝑐 p 𝑆 L 0 ⁄ , with 𝜆u and 𝑐 p the unburned gas thermal conductivity and specific heat capacity at constant pressure, respectively, estimated at the temperature of maximum flame temperature gradient, and 𝑆 L 0 the measured LBV of the mixture. Effective Lewis numbers Leeff were calculated following recommendations by Lapalme et al. for multi-component fuels, considering successively individual diffusion of each reactive species in the global remaining mixture, volume-based mixing rule and fuel/oxidizer combination [32].

Fig. 2 .

 2 Fig.2. Hydrogen enrichment in ammonia or methane causes earlier, more intense engine heat releases,

Fig. 4 .

 4 Fig.4. Effect of hydrogen and methane enrichment on the LBV at ignition timing conditions (445K,

Fig. 5 .Figure 6

 56 Fig.5. All conditions on Peters-Borghi's diagram. Blue: engine conditions; grey: spherical vessel

Fig. 6 .

 6 Fig.6. The turbulent-to-laminar velocity ratio increases with the flame surface equivalent radius and is

Fig. 7 .

 7 Fig.7. At Rs=10mm, the turbulent-to-laminar velocity ratio correlates well with the normalized

Figure 8a shows the

  Figure8ashows the dependence of the turbulent-to-laminar velocity ratio on the effective Lewis

Fig. 8 .

 8 Fig.8. Turbulent-to-laminar velocity ratio varies almost monotonically at Rs=10mm with Leeff and Lb

  with a=0.1843 and b=1.1694.

Fig. 10 .

 10 Fig.10. A new correlation of Da-corrected turbulent-to-laminar velocity ratios with Ka for all

Fig. 1 .Fig. 2 .

 12 Fig.1. Example of double Schlieren images of a NH3/air turbulent flame at u'=1.73m.s -1 .

Fig. 3 .

 3 Fig.3. Hydrogen significantly affects the duration of the flame propagation at all equivalence ratios,

Fig. 4 .

 4 Fig.4. Effect of hydrogen and methane enrichment on the LBV at ignition timing conditions (445K,

Fig. 5 .

 5 Fig.5. All conditions on Peters-Borghi's diagram. Blue: engine conditions; grey: spherical vessel

Fig. 6 .

 6 Fig.6. The turbulent-to-laminar velocity ratio increases with the flame surface equivalent radius and

Fig. 7 .

 7 Fig.7. At Rs=10mm, the turbulent-to-laminar velocity ratio correlates well with the normalized

Fig. 8 .

 8 Fig.8. Turbulent-to-laminar velocity ratio varies almost monotonically at Rs=10mm with Leeff and Lb

Table 1

 1 Investigated fuel blends.

	5:1 compression ratio fueled with premixed gaseous mixtures

Acknowledgements

This research has received funding from the French Government's "Investissement d'Avenir" program: "Laboratoire d'Excellence CAPRYSSES" (Grant N°ANR-11-LABX-0006-01). The authors would also like to acknowledge Amanda Alves Barbosa for her implication in the flame experiments.

List of supplemental material

TurbulentFlame_85NH3_15CH4_ER0-9_up1-73ms_edit.mp4 TurbulentFlame_85NH3_15H2_ER0-9_up1-73ms_edit.mp4 TurbulentFlame_90NH3_10H2_ER0-9_up1-73ms_edit.mp4 TurbulentFlame_CH4_ER0-9_up1-73ms_edit.mp4 TurbulentFlame_NH3_ER0-9_up1-73ms_edit.mp4


Coralie Fourcade-Dutin

Paul Robert

Romain Dauliat

Raphaël Jamier

Hector Munõz-Marco

Pere Pérez-Millan

John M Dudley

Hervé Maillotte

Philippe Roy

Damien Bigourd
email: damien.bigourd@u-bordeaux.fr

Raphael Jamier

Spectral Distributions of Chirped Pulsed Four-Wave Mixing in a Photonic Crystal Fiber Measured by Dispersive Fourier Transform Method

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Four-wave mixing (FWM) in optical fibers has been extensively investigated in order to amplify ultra-short pulses since it provides large gain-bandwidth [1,2], high gain [2] and very wide tunability. When a chirped pulse pumps a fiber, the instantaneous frequencies of the pump are temporally dispersed and the spontaneous photons are generated with temporal and spectral distributions according to phase-matched FWM relation [3]. In this case, we demonstrated that this configuration allows getting a very large gain bandwidth [2] that enables ultra-short pulse amplification [1]. However, the injected signal requires chirp optimization to match the temporal distribution of the parametric gain spectrum, which depends on both the pump and fiber properties [1]. Therefore, this parametric gain distribution needs to be firstly characterized before being used in a broadband amplifier. In this work, we present a simple approach that allows the spectral and temporal distribution of the instantaneous parametric process to be retrieved using the dispersive Fourier transformation (DFT) method [4]. This powerful technique is significantly simpler to implement compared to pump-probe techniques [5] since it provides the desired information without the need to inject a suitable signal. In our case, we use DFT to measure the spectral correlation between the side bands, which allows us to determine the particular portion of the pump spectrum responsible for generating the FWM.

Experiment and method

The pump pulse is generated from a mode-locked oscillator (Flint, LightConversion) delivering a train of pulses at 76 MHz with a duration of 80 fs centered at 1030 nm. A part of the oscillator output (around 750 mW) seeds an amplifier composed of a volume Bragg grating (VBG) that stretches the pulses to a duration of ~50 ps, an acoustooptic modulator to decrease the repetition rate to 1 MHz and cascaded ytterbium doped fiber amplifiers to increase the average power up to 1 W. The spectral bandwidth at the output of the amplifier is ~9 nm and is modulated due to self-phase modulation (SPM). The pump pulse is then injected in a 5-meter-long PCF with a zero-dispersion wavelength at 1028 nm. Two FWM lobes are observed and located around 1001 and 1071 nm for an average power of 18 mW (Fig. 1.a). Single shot spectra are then recorded by a dispersive Fourier transform set-up with a 11.5 km long fiber (SMF28) to stretch the output with an approximate chirp value of ~-2.54 nm/ns, a fast photodiode and an oscilloscope with a maximum bandwidth of 12 GHz. Figure 1.a displays the superposition of 500 single shot spectra and the averaged curve. Good agreement is obtained with the curve recorded by the optical spectrum analyzer (OSA). From the single shot spectra, the spectral correlation  has been calculated between any two wavelengthsand according to

1 2 1 2 1 2 2 2 2 2 1 1 2 2 ( ) ( ) ( ) ( ) ( , ) ( ) ( ) ( ) ( ) I I I I I I I I                 (1)
where the bracket represents the average over the ensemble. The correlation  varies from -1 to +1 indicating intensity fluctuations in the opposite or same directions respectively. For =0, no correlation exists between the two wavelengths.

Results and discussion

Figure 1.b shows the spectral correlation map between 980 and 1090 nm for an average power of 24 mW together with the average spectra. The pump spectrum is positively correlated with itself. Similarly, the correlation between two identical FWM bands corresponds to a positive yellow line with = 1. The signal and idler photons are degenerated with respect to the pump and therefore an anti-correlation between the pump and the two side lobes are observed (black dashed squares) [6]. A zoom view of the spectral correlations between two side bands (black square in Fig. 1.b) is also shown in Fig. 1.c. The black lines are plotted from the photon energy conservation law for several pump wavelengths p; ie p1+2. A positive correlation is observed with a specific shape and is mainly between 1070-1075 nm and 995-1005 nm. From the black line, we deduce that these parts of the bands are generated from the pump spectrum at 1030-1035 nm. A negative correlation value (=-0.15) at around 1082 nm is also observed. This is attributed to SPM from the pump that decreases the correlation value. According to the FWM relation, it originates from the pump spectrum at >1035 nm and correspond to the spectral hole in the pump observed at 1037 nm (Fig. 1.b-black circle).

Conclusion

We have shown that the DFT method can be used to infer the origin of the FWM within the pump from the spectral correlation between side bands. This technique brings significant new information such as the spectral distribution of parametric gain, which cannot be obtained from a standard spectrometer. We believe that it will be of prime interest for the development of fiber amplifiers supporting a very broad bandwidth [1].

Acknowledgments

ANR projects (ANR-16-CE2460009, ANR-17-EURE-0002, ANR-10-IDEX-03-02) and Bourgogne-Franche-Comté Council (SUM Project)

Fig. 1 .

 1 Fig. 1. a) FWM bands detected with an OSA (black line) and with the DFT method (red and blue lines). The averaged spectrum (red line) of 500 single shots (blue lines) are displayed. b) Spectral correlation map of the full spectrum. The black dashed squares correspond to the correlation between the pump and the FWM bands while the black square shows the correlation between the two bands. c) Spectral correlation between the bands. The black lines are plotted from the photon energy conservation law for several pump wavelengths.


Paul Robert

Coralie Fourcade-Dutin

Romain Dauliat

Raphaël Jamier

Hector Muñoz-Marco

Pere Perez-Millan

John M Dudley

Philippe Roy

Herve Maillotte

Damien Bigourd
email: damien.bigourd@u-bordeaux.fr

Raphael Jamier

Spectral correlation of

Pulse amplification through four wave mixing (FWM) in optical fiber has been extensively investigated for important applications linked with its large gain-bandwidth [1], high gain [2] and very wide tunability [3]. When the pulses involved in the process have a high peak power, the pulses are initially stretched before being injected in the fiber [4,5,6] such that the FWM process can be achieved in a fiber with a silica core, even for a pulse energy in the microjoule level [7,8,9]. For a chirped pump pulse, the instantaneous frequencies are temporally dispersed and the spontaneous photons are generated with temporal and spectral distributions according to phasematched FWM relation [10]. Significantly, we have shown that this configuration yields a very large gain bandwidth [2] that enables ultra-short pulse amplification [1]. However, the injected signal requires chirp optimization to match the temporal distribution of the parametric gain spectrum, which depends on both the pump and fiber properties [1]. Therefore, this parametric gain distribution needs to be understood and characterized to efficiently design a broadband amplifier.

Although some previous studies have used a pump-probe technique to study the gain distribution, this method is experimentally complex, which limits it broad use in studying chirped-pulse FWM over wider parameter ranges [11]. In this work, we present a much simpler approach that allows the spectral and temporal distribution of the instantaneous parametric process to be studied using the dispersive Fourier transformation (DFT) method [12]. This powerful technique is significantly simpler to implement since it provides the desired information without the need to inject a suitable signal, and also readily allows statistical correlation data to be extracted. The DFT method exploits time to frequency mapping to allow real time measurement of single-shot spectral fluctuations. DFT has been widely used to study a range of physical processes in fiber such as rogue wave dynamics, continuum generation and spontaneous FWM [13][14][15][16][17][18]. In our case, we use DFT to measure the spectral correlation between the FWM side bands generated from a chirped pump pulse, representing a new application of this method, and one which allows us to determine physical insights into the particular portion of the pump spectrum responsible for generating the FWM. In particular, for degenerate FWM, two pump photons are converted to signal and idler photons, with spontaneous generation achieved from different possibilities depending on phase matching criteria. This leads to an asymmetry in the FWM band in a single shot spectrum measurement [16]. Once the shots are averaged, the shape of the spontaneous FWM spectrum should represent the gain profile of the parametric amplification in the unsaturated regime. In the following, we exploit the DFT statistical measurements over many single shot spectra to correlate the signal and idler photons and to infer the origin of the FWM within the pump. We firstly illustrate the principle of this method using numerical simulations based on integrating the nonlinear Schrödinger equation using the split step method. The parameters used were as follows. The input chirped pump pulse has duration of t=50 ps and spectral bandwidth  of 6 nm centered at 1030 nm. This corresponds to a chirp of -8.3 ps/nm. The pulse is injected in a 5-meter-long photonic crystal fiber (PCF) which has a nonlinear coefficient of 11 W -1 .km -1 and a zero-dispersion wavelength (ZDW) at 1028 nm. This means that only part of the pump spectrum lies in the anomalous dispersion regime. 500 simulations have been performed with random noise initial condition by adding half photon per temporal mode. Figure 1 shows a spectrogram of the average electric field at the fiber output for input peak power Pp=120 W. The red line corresponds to the pump pulse. As it is chirped, the instantaneous pump wavelengths are linearly spread in the temporal envelope. The temporal and spectral axis (top and bottom) are therefore linked by the chirp value; i.e . For each delay, the pump has a particular instantaneous wavelength and a pump power that creates a FWM with specific properties [2]. Indeed, two specific FWM bands are generated with different spectral locations and bandwidth. For example, at the pump wavelengths of 1030 nm (t=0 ps) and 1032 nm (-16.6 ps), the two lobes are located at ~1078/986 nm and 1061/998nm, respectively. To analyse the shot to shot fluctuations statistically, the spectral correlation ) has been calculated between any two wavelengthsand  according to

1 2 1 2 1 2 2 2 2 2 1 1 2 2 ( ) ( ) ( ) ( ) ( , ) ( ) ( ) ( ) ( )
I I I I I I I I                
(1) where the angle brackets represent an ensemble average. The correlation  varies from -1 to +1 indicating intensity fluctuations in the opposite or same directions respectively. For =0, no correlation exists between 1 and. The spectral correlation between the lower and higher parts of the spectrum is shown in Fig. 2 together with the average spectra. The red line (top-right) corresponds to the positive correlation of the pump spectrum with itself; i.e the auto-correlation. Since the FWM process is originated from a depletion of the pump, the correlation between the side bands and the pump is negatively correlated (~-0.3).

As the two axes represent the wavelength of the signal s and idler i, the theoretical relationship of FWM from energy conservation (ps+i) can be also plotted. The black curves in Fig. 2 represent three cases for a selection of pump wavelengths p, and we can see from these results how it is possible to infer the particular pump component, which generates the FWM bands. For example, when the fiber is pumped by a chirped pulse with Pp=120 W (Fig. 2(a)), the bands at ~980 nm and 1000 nm are mostly generated from the pump part at 1030 nm lying in the anomalous dispersion regime. The photons in the range of 930-980 nm and 1090-1140 nm are generated from the pump between 1025-1030 nm. These observations are in very good agreement with those from the spectrogram (Fig. 1). We have also checked that the Raman effect does not affect this distribution. To gain further insight, we performed the simulations under the same conditions but with a monochromatic pump (Fig. 2(b)). Compared to the previous case, the bandwidths are lower since the FWM photons are generated from the pump with a narrow spectrum. Indeed, the spectral correlation is along the black line plotted from the energy conservation law for p=1030 nm. The numerical results for a chirped pulse with Pp=220 W (Fig. 2(c)) show that the FWM bands increase and the pump is affected by self phase modulation (SPM). In this case, the value decreases for the pump wavelength higher than 1030 nm and corresponds to the hole in the pump spectrum.

Our experimental setup is shown in Fig. 3a. The pump pulse is generated from a mode-locked oscillator (Flint, LightConversion) delivering a train of pulses at 76 MHz with a duration of 80 fs at FWHM centered at 1030 nm. The total average power is 1.5 W. A part of the oscillator output (around 750 mW) seeds the pump chain via a stretching stage composed of a volume Bragg grating (VBG) that stretches the pulses to a duration of ~50 ps (FWHM), an acoustooptic modulator (AOM) to decrease the repetition rate to 1 MHz and several ytterbium doped fiber amplifiers (YDFA) to progressively increase the average power up to 1 W. The pulse bandwidth (FWHM) at the output of the amplifier is ~ 9 nm and is modulated due to SPM. The pump pulse is then injected in a 5meter-long PCF which has the same dispersion profile as the one used in the simulations. Two FWM lobes are generated in the PCF and are located at 1001 and 1071 nm for an average power of 18 mW (Fig. 3(b)). In order to record single shot spectra, a dispersive Fourier transform set-up has been implemented at the PCF output. The beam is injected in an 11.5 km long fiber (SMF28) to stretch the output with approximate chirp value of ~-2.54 nm/ns. Thus, the delay between the two sidebands is ~27 ns. The output is detected with a fast photodiode and an oscilloscope with a maximum bandwidth of 12 GHz. For the following, we use a limited number of points to 5 × 106 for the 500 pulses corresponding to a spectral resolution of ~0.25 nm. Figure 3(b) displays the superposition of 500 single shot spectra (blue lines) and the averaged curve. To accurately reconstruct the DFT time-frequency mapping, we used both second and third order dispersion coefficients of the SMF (2=17 ps 2 /km, 3=-0.0134 ps 3 /km) in the analysis of the experimental data [12]. From Figure 3(b) (blue lines), we can see the large shot to shot fluctuation of the sideband but a good agreement is obtained between the average spectrum (red line) and the curve measured by an optical spectrum analyzer (black line). Figure 4 shows the spectral correlation map between 980 and 1090 nm for an average power of 24 mW. As expected, the pump spectrum is positively correlated with itself and the correlation between two identical FWM bands corresponds to a positive line with a maximum value equal to 1. From Eq. 1, the correlation map is symmetric about the correlated positive diagonal. As the Stokes and anti-Stokes photons are degenerate from the depletion of the pump pulse, an anti-correlation between the pump and the two side lobes are observed (black dashed squares) [13]. In the following, we will focus on the correlation between the two FWM bands (black squares). Fig. 4. Spectral correlation map of the full spectrum for a power of 24 mW. The black dashed squares correspond to the correlation between the pump and the FWM bands while the black squares show the correlation between the two bands.

The spectral correlations between the two side bands (black squares in Fig. 4) are also shown in Fig. 5 for an average power of 18 mW and 24 mW. The black lines are plotted from the photon energy conservation law for the pump wavelength of 1025, 1030 and 1035 nm. The correlation spectral maps have a different shape from the one predicted by the numerical simulations (Fig. 2), attributed to structure on the pump pulse used in the experiments or uncertainties in the fiber dispersion curve. From a pump-probe experiment [11], we have also observed similar behavior with another PCF. At a power of 18 mW, a positive value is observed with a specific shape. The maximum at ~0.4 corresponds to the correlation between the two parts 1068-1071nm/990-997nm. These parts of the bands are mainly generated from the pump spectrum around 1030 nm. Another positive  value (~0.25) located between 1073-1075/997-1002 is also obtained and these FWM bands are mostly generated from the pump spectrum at ~1035 nm. Surprisingly, the correlated bands are not symmetric around the pump spectrum; i.e the furthest part in one side is correlated to the closest part in the other side. This information cannot be obtained directly from a standard spectrometer. At higher pump power, the side bands are shifted away from the pump and the positive correlation is mainly between 1070-1075 nm and 995-1005 nm. A negative correlation value (=-0.15) at around 1082 nm is also observed. This is attributed to SPM accumulated by the pump that decreases the correlation value.

(b) (a)

From the black line (Fig. 5(b)), we can deduce that it originates from the pump spectrum at >1035 nm. Indeed, a spectral hole in the pump is observed at 1037 nm (Fig. 4) and is consistent with the signature of the decrease in correlation as expected from the simulations. Fig. 5. Spectral correlation between the Stokes and Anti-Stokes for a power of 18 mW (a) and 24 mW (b). The average spectrum are displayed for each cases. The black lines are plotted from the photon energy conservation law for several pump wavelengths.

In conclusion, we have shown that the DFT method can be implemented to measure the spectral correlation between side bands. In the case of a chirped pump pulse, the spectral correlation map can be used to infer the pump part that generated the FWM. This method is relatively straightforward since it requires only one pump pulse without any seed injection. In addition, the statistical analysis provides significant new information and therefore the DFT method can be used as a highly complementary diagnostic to standard spectrometers. We believe that it will be of prime interest for the development of fiber amplifier supporting a very broad bandwidth [1].

Fig. 1 .

 1 Fig. 1. Spectrogram of the averaged electric field at the PCF output for Pp=120 W.

Fig. 2 .

 2 Fig. 2. Spectral correlation between the Stokes and Anti-Stokes for a) Pp=120W and a chirped pump pulse, b) for Pp=120W and a monochromatic pump c) for Pp=220W and a chirped pump pulse. The average spectra are displayed for all cases as top and left inserts. The black lines are plotted from the photon energy conservation law for several pump wavelengths.

Fig. 3 .

 3 Fig. 3. Experimental set-up. VBG, Volume Bragg Grating; AOM, acoustooptic modulator; PCF Photonic Crystal Fiber; OSA, Optical Spectrum Analyzer et b)FWM bands detected with an OSA (black line) and with the DFT set-up. The averaged spectrum (red line) of 500 single shots (blue lines) are displayed.

Funding.

  ANR (ANR-16-CE2460009, ANR-17-EURE-0002, ANR-10-IDEX-03-02), Bourgogne Franche-Comté councils (SUM Project).

Acknowledgment. We thank C. Billet for helpful advice and T. Sylvestre for loan of the fiber spool used in the DFT experiments.

Disclosures. The authors declare no conflicts of interest.


Olivia Zurita-Miranda
email: olivia.zurita-miranda@u-bordeaux.fr

Coralie Fourcade-Dutin

Pierre Béjot

Frederic Fauquet

Jean-Paul Guillet

Frederic Darracq

Patrick Mounaix

Hervé Maillotte

Damien Bigourd

Tunable source of infrared pulses in gas-filled hollow core capillary

We report a tunable source that generates pulses in the infrared from an optical parametric amplification in a gas-filled hollow core capillary based on four-wave mixing process, in which the phase matching strongly depends on the gas pressure and the pump. In our case, we have generated pulses from 1 to 1.6 𝜇m in the sub-µJ level together with a parametric amplification in the visible.

Introduction

Since these recent years, an interest is growing for new generator in the ultra-short pulse regime in the near-infrared with the capability to reach the mid-infrared for demanding applications as life and molecular sciences or lasermatter interaction experiment [START_REF] Wolter | Strong-field physics with Mid-IR fields[END_REF]. These pulses are often generated from a laser emission or nonlinear photonic processes to generate a down converted frequency. However, the energy of the incoming laser pulse can reach the limit imposed by an optical damage, thus strongly compromise the energy scalability. In addition, the spectral window and the tunability are limited by the transparency material [START_REF] Travers | Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers[END_REF]. Gas-filled hollow core capillary (HCC) brings several benefits from solid materials as a higher damage threshold. In addition, some configurations are very interesting to get low losses in the fundamental mode or to excite high-order modes to reach other phase matched processes. The gas pressure can be used as a tunable parameter since it modifies directly the nonlinearity and the group velocity dispersion. Therefore, four wave mixing (FWM) based parametric process in a HCC is a key process to enable the creation of ultra-fast sources with a very large tunability. For example, a source in the visible has been implemented at a level of 10 µJ [START_REF] Ciriolo | Generation of ultrashort pulses by four wave mixing in a gas-filled hollow core fiber[END_REF] and in the near infrared at 1.4 µm with an energy of 5 µJ and a duration of 45 fs [START_REF] Faccio | Optical amplification in the near-infrared in gas-filled hollow-core fibers[END_REF]. On the other hand, deep-ultraviolet ultrashort pulses have been generated using four-wave mixing in gas-filled Kagomé photonic crystal fiber with a conversion efficiency of 38% [START_REF] Belli | Highly efficient deep UV generation by four-wave mixing in gas-filled hollow-core photonic crystal fiber[END_REF]. Kida at al. [START_REF] Kida | Optical parametric amplification of a supercontinuum in a gas[END_REF] have been used FWM in combination with (OPA) in HCC to amplify a supercontinuum signal, with a amplification gain up to 100 times. In this work, we highlight the tunability of the FWM based source in the infrared from 1 µm to 1.6 µm together with a parametric amplification of a continuum in the visible.

Experimental arrangement and condition

The degenerated FWM is a process involving two optical fields at different frequencies to produce a third wave in a third-order nonlinearity medium. In the following, we consider one pump pulse and a signal (at the angular frequency 𝜔 1 and 𝜔 2 respectively), that co-propagate in the HCC. When a phase matching condition is achieved, part of the pump energy is transferred to the signal and create a new component at 𝜔 3 ; the idler, according to the energy conservation relation, 𝜔 3 = 2𝜔 1 -𝜔 2 . We firstly calculated the linear phase matching, given by 2𝛽(𝜔 1 ) -𝛽(𝜔 2 ) -𝛽(𝜔 3 ) with  is the wave vector that depends on the waveguide and the gas for our experimental condition; i.e a Argon filled capillary with a core diameter of 150 µm. Fig. 1.a shows that a phase matching condition can be obtained in the near infrared and the visible when the pressure is adjusted between 1 and 5 bar for a pump wavelength of 800 nm. The experimental set-up is displayed in Fig. 1. The pump pulses are directly provided by a 1 kHz Ti:Sappire (Ti:Sa) laser source (800 nm, 120 fs) in combination with a continuum signal (420-600 nm). This signal is generated from the remaining part of the pump pulse focused into a 5-mm thick sapphire plate. The two collinear beams were coupled in the 30 cm long HCC. Their relative delay was controlled by an optical delay line endowed with a translation stage. The HCC is place inside a chamber with two 5-mm-thick windows and filled it with Argon. At the output chamber, the beam was divided in the visible and infrared, and detected by specific spectrometers simultaneously. a) b)

Results

When the seed is injected together with the pump pulse in the Argon filled HCC for a pump energy of 128 µJ and a pressure of 4 bar, the visible spectrum is amplified with a gain of 80 (Fig. 2.a). Due to the chirp imposed by the continuum, the tunability is achieved with the delay stage to overlap the targeted part of the seed with the pump. In addition to the seed amplification, pulses in the near infrared are generated and tuned from 1 to 1.6 µm (Fig. In conclusion, we implemented a tunable source in the infrared driven by a FWM based parametric process in a Argon filled HCC. We demonstrated a wide tunability from 1 to 1.6 µm together with the visible amplification. We are currently working to adjust the optimal conditions, which allow a best conversion efficiency and a wider tunability toward the mid-infrared by adjusting the pump pulse as [START_REF] Vanvincq | Ultrabroadband fiber optical parametric amplifiers pumped by chirped pulses. Part 1: analytical model[END_REF] and gas characteristic.

Fig. 1 a

 1 Fig. 1 a) Linear phase matching in HCC core diameter of 150 µm filled argon, pump energy 150 µJ at 120 fs. b) Experimental configuration. BS, beam splitter; MR mirror; ND neutral density filter; L lens; CM, Cold mirror cutoff 700 nm; SP sapphire plate; SF short-pass filter, DM dichroic mirror, SP-VIS Visible Optics Spectrometer, SP-IR Infra-red Spectrometer.

Fig. 2 a

 2 Fig. 2 a) Amplified signal spectra and b) generated infrared idler spectra at different time delays.