Milan Sinobad

Christelle Monat

Barry Luther-Davies

Pan Ma

Stephen Madden

David J Moss

Arnan Mitchell

Regis Orobtchouk

Alberto Della

Salim Boutami

Jean-Michel Hartmann

Jean-Marc Fedeli

Christian Grillet

Coherent Supercontinuum Generation in a Silicon-Germanium Waveguide in the Mid Infrared

Keywords:

Photonics in the mid infrared (mid-IRat 3-20μm) has a large number of applications based on the optical detection of molecules that can be readily identified through measuring their strong fundamental absorption lines in this band [START_REF] Soref | Mid-infrared photonics in silicon and germanium[END_REF][START_REF] Zhang | Nonlinear group IV photonics based on silicon and germanium: from nearinfrared to mid-infrared[END_REF]. To perform this measurement, supercontinuum (SC) sources are particularly well-suited, since they allow for reliable molecule detection by accessing in parallel their distinct absorption lines. Broad mid-IR supercontinuum is traditionally generated via soliton fission, as obtained by pumping a nonlinear waveguide with an anomalous dispersion, close to the zero dispersion wavelength (ZDW) [START_REF] Yu | Experimental demonstration of linearly polarized 2-10 mum supercontinuumgeneration in a chalcogenide rib waveguide[END_REF][START_REF] Petersen | Mid-infrared supercontinuum covering the 1.4-13.3 μm molecular fingerprint region using ultra-high NA chalcogenide step-index fibre[END_REF]. Although these sources have a large spectral power density across a wide wavelength band and a relatively good power budget, the soliton fission process is triggered by noise seeded modulation instability, which degrades the coherence of the related source. However, many applications such as optical coherence tomography, coherent Raman and anti-stokes Raman spectroscopy, broadband time-resolved pump-probe spectroscopy or ultra-short pulse generation, require a broadband phase coherent source [START_REF] Heidt | Pulse preserving flat-top supercontinuum generation in all-normal dispersion photonic crystal fiber[END_REF][START_REF] Heidt | Coherent octave spanning near-infrared and visible supercontinuum generation in all-normal dispersion photonic crystal fibers[END_REF][START_REF] Hartung | Nanoscale all-normal dispersion optical fibers for coherent supercontinuum generation at ultraviolet wavelengths[END_REF].

Stable and coherent SC generation can be achieved by exploiting coherent self-phase modulation and optical wave breaking effects in a waveguide with low all-normal dispersion [START_REF] Heidt | Pulse preserving flat-top supercontinuum generation in all-normal dispersion photonic crystal fiber[END_REF]. The required pump power however tends to be larger in this dispersion regime, which has precluded its widespread use in applications. In this work, we report SC in the mid-IR spanning from 2.78 -5.05µm by launching 18.3mW pump power into a dispersion-engineered lowloss "air clad" Si0.6Ge0.4 waveguide with all-normal group velocity dispersion (Fig1binset). The waveguide consists of a 2.7μm thick and 3.5μm wide Si0.6Ge0.4 ridge on a silicon substrate. It was engineered to have singlemode operation and low normal dispersion with low effective area and strong mode confinement (>94% at 3.8μm, see Fig1binset). Although being air-clad, a propagation loss as low as 0.4dB/cm in the broad range from 3.8 to 5.0µm, has been reported in these waveguides [START_REF] Sinobad | Broadband mid-infrared supercontinuum generation in low loss dispersion engineered silicon-germanium waveguide[END_REF]. Supercontinuum generation is achieved by pumping the waveguide with ≈200fs TM polarized pulses from a midinfrared MIROPA-fs optical parametric amplifier at a repetition rate of 63MHz. Using a 2cm long waveguide pumped at 3.8µm in the normal dispersion regime we obtain the spectra shown in Fig 1a . The supercontinuum spectra achieved here are smoother than that traditionally obtained in the anomalous dispersion regime, and the remaining fluctuations observed on Fig. 1a are typical from self-phase modulation. When pumped with 18.3mW coupled average pump power, a 2.27µm bandwidth at -30dB (2.78-5.05µm) and a 1.84µm bandwidth at -10dB level are achieved. The lower propagation losses achieved here compared with previous results [START_REF] Carletti | Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared[END_REF][START_REF] Carletti | Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses[END_REF], enabled us to generate SC at much reduced pump intensity, thereby limiting the impact of high-order nonlinear absorption (Fig 1b ), resulting in an increased SC on-chip power (>5mW).

In summary, we report the first supercontinuum generated in all-normal dispersion regime in any CMOS compatible platform. Supercontinuum is spanning across the relevant mid-IR wavelength range from 3 -5µm, where most molecules have fingerprints, enabling coherent spectroscopy applications. The generated average mid-IR power exceeds that produced so far by other Si-based platforms. This establishes silicon germanium-on-silicon as a promising platform for integrated nonlinear photonics in the mid-IR.

Figure 1 a

 1 Figure 1 a) Output spectra measured at different coupled power, b) Measured on-chip SC power versus coupled power. Calculated group velocity dispersion (solid blue line) and mode confinement (dashed blue line) are show in the inset.

Acknowledgments: Agence Nationale de la Recherche (ANR) MIRSiCOMB (ANR-17-CE24-0028), European ERC grant GRAPHICS (648546); We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), CM acknowledges the support of the Institut Universitaire de France.


Milan Sinobad

Christelle Monat

Barry Luther-Davies

Pan Ma

Stephen Madden

David J Moss

Arnan Mitchell

Regis Orobtchouk

Salim Boutami

Jean-Michel Hartmann

Jean-Marc Fedeli

Christian Grillet

Mid-wavelength Infrared Supercontinuum Generation Spanning 1.4 Octaves in a Silicon-Germanium Waveguide

Keywords: Supercontinuum generation, (190.4390) Nonlinear optics, integrated optics, (140.3070) Infrared

published or not. The documents may come

In the mid-wavelength infrared band (MWIRat 3 -8µm), most chemical and biological compounds exhibit strong and unique absorption features allowing for molecular detection applications [START_REF] Zhang | Nonlinear group IV photonics based on silicon and germanium: from nearinfrared to mid-infrared[END_REF]. Reliable molecular detection requires a MWIR laser source with adequate output power that is either widely tunable or yields a broadband emission to cover the whole MWIR band of interest. Supercontinuum generation in highly nonlinear waveguides is an attractive solution as it allows, in contrast to QCLs [START_REF] Yao | Mid-infrared quantum cascade lasers[END_REF], for reliable molecule detection by assessing simultaneously distinct molecular absorption lines. The challenge is to achieve a supercontinuum MWIR source on a CMOS compatible chip in order to leverage the associated mature fabrication process towards mass and cheap production. To date, supercontinuum generation in the MWIR has been achieved in CMOS platforms such as Si/SiO 2 [START_REF] Lau | Octave-spanning mid-infrared supercontinuum generation in silicon nanowaveguides[END_REF] or SiGe/SiO 2 [START_REF] Ettabib | Broadband telecom to mid-infrared supercontinuum generation in a dispersion-engineered silicon germanium waveguide[END_REF] from 1.5 to 3.7μm and in Si/Al 2 O 3 [START_REF] Singh | Midinfrared supercontinuum generation from 2 to 6 um in a silicon nanowire[END_REF] from 1.9 to 5.5μm, but these are all intrinsically limited, on the long wavelength side, by absorption in the silica and sapphire substrate, respectively.

Here, we demonstrate the generation of 1.4 octave spanning supercontinuum in the MWIR from 2.6 to 7.3μm with a useful on-chip average power greater than 7mW by pumping dispersion engineered silicon germanium waveguide [START_REF] Carletti | Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared[END_REF][START_REF] Carletti | Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses[END_REF]. Our waveguide consists of a 4.2μm thick and 4.5μm wide Si 0.6 Ge 0.4 ridge with a top air cladding that seats on a silicon substrate (see inset Fig. 1b). Although the waveguide is multimode at 4.15μm, the air clad design with a large SiGe cross-section (effective area of ~10.5µm 2 for both TE and TM modes) results in low anomalous dispersion (in TE and TM) across a large bandwidth (see figure 1a), a cut-off wavelength of 7.7 and 8.2µm for TE and TM polarization respectively, a strong mode confinement in the waveguide core material (>98 % at 4.15µm) and a reduction of the contribution from surface roughness to the propagation loss. The waveguide propagation loss was first measured using a cut-back method, by probing the waveguide under relatively low average power (<1mW) with 7.5ps pulses at 20MHz repetition rate. The negligible effect of the field interaction with the waveguide surface roughness at the air/SiGe interface is confirmed by the low (~0.4dB/cm) propagation loss measured across a wide wavelength range between 3.8 -5.0µm. The minimum value is 0.32dB/cm at 5µm for the TM mode. In summary, we report supercontinuum generation from a silicon germanium-on-silicon platform covering almost the whole mid-wavelength infrared, where most molecules in atmosphere have relevant fingerprints. The generated on-chip supercontinuum MWIR power and bandwidth exceed that produced so far by other Si-based platforms that are intrinsically limited by the silica or sapphire substrate absorption. This establishes silicon germanium-on-silicon as a promising platform for integrated nonlinear photonics in MWIR, with the potential to extend the operating range beyond 8µm.

Figure 1 a

 1 Figure 1 a) Calculated TE (blue) and TM mode (red) group velocity dispersion (GVD) and confinement of the mode field in the waveguide core (dashed lines), black arrow indicate pump wavelength, blue and red arrows indicate the cut-off wavelengths of TE and TM mode, respectively, b) Measured propagation loss for TE (blue) and TM mode (red), schematic of an air clad SiGe waveguide as an insert.

Figure 2

 2 Figure 2 Output spectra measured at different average coupled power for TE mode a) and TM mode b) Coupled average power is calculated assuming 4dB and 4.5dB coupling loss for TE and TM mode, respectively Supercontinuum was then achieved by pumping the waveguide with ~200fs pulses delivered from a mid-infrared MIROPA-fs optical parametric amplifier at a repetition rate of 63MHz. Using a 7cm long waveguide pumped at 4.15μm close to the first zero-dispersion wavelength, we obtained the spectra shown in Fig 2a (TE polarization) and 2b (TM polarization) for average coupled powers increasing from ~1mW to 30mW. At 27.1mW, we measured a supercontinuum spanning 1.4 octaves, from 2.59 to 7.26μm at -30 dB level for the TE polarization. Assuming ~4 dB coupling loss per facet, the on-chip generated supercontinuum power is 7.5±1mW for 27.1mW coupled average power. When pumping the TM mode, where dispersion is lower and the cut-off wavelength is slightly shifted toward longer wavelengths, a supercontinuum with a bandwidth exceeding 2.7 to 7.9μm at the -30 dB level was achieved. Besides, the output spectrum measured for the TM mode is relatively smooth with a large -10dB bandwidth from 3.0 -7.4μm.

Acknowledgments: Agence Nationale de la Recherche (ANR) MIRSiCOMB (ANR-17-CE24-0028), H2020 EU framework, Marie Skłodowska Curie Action MSCA grant (PCIGA-2013-631543), European ERC grant GRAPHICS (648546); The Australian Research Council (ARC), Discovery Projects Program (DP150104327). We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), CM acknowledges the support of the Institut Universitaire de France.


Milan Sinobad
email: milan.sinobad@student.rmit.edu.au

Alberto Della

Barry Luther-Davies

Pan Ma

Stephen Madden

David J Moss

Arnan Mitchell

Regis Orobtchouk

Salim Boutami

Jean-Michel Hartmann

Jean-Marc Fedeli

Christelle Monat

Christian Grillet

Bright on-chip mid-IR supercontinuum generation to 7.7µm in silicon germanium-on-silicon platform

Keywords: Supercontinuum generation, (190.4390) Nonlinear optics, Integrated optics, (140.3070) Infrared

We report mid-IR supercontinuum generation, from 2.9 to 7.7µm, in a CMOS compatible silicon-germanium waveguide. This 1.3 octave bright supercontinuum has been achieved in a low loss dispersion engineered air-clad Si0.6Ge0.4/Si waveguide.

Introduction

The generation in a CMOS compatible platform of a spectrally bright supercontinuum covering the mid-wavelength infrared band (MWIRat 3 -8µm) is a technological challenge that is promising to have a strong impact on molecular detection, as most chemical and biological compounds exhibit strong and unique absorption features in this spectral region [START_REF] Zhang | Nonlinear group IV photonics based on silicon and germanium: from nearinfrared to mid-infrared[END_REF]. The miniaturization of broadband octave-spanning sources (SCG) in the mid-IR has been already demonstrated on a chalcogenide chip [START_REF] Yu | Experimental demonstration of linearly polarized 2-10 mum supercontinuum generation in a chalcogenide rib waveguide[END_REF]. Yet, these materials do not allow exploiting the advantages of the CMOS fabrication technologies in terms of reliability, mass manufacturing and cost. On the other hand, a mid-IR supercontinuum generation in group IV materials based platforms remains challenging. Although supercontinuum generation has been demonstrated in CMOS compatible platforms like silicon nitride-on-insulator [START_REF] Chavez Boggio | Dispersion engineered silicon nitride waveguides by geometrical and refractive-index optimization[END_REF][START_REF] Johnson | Octave-spanning coherent supercontinuum generation in a silicon nitride waveguide[END_REF], silicon-oninsulator [START_REF] Kuyken | Mid-infrared to telecom-band supercontinuum generation in highly nonlinear silicon-on-insulator wire waveguides[END_REF][START_REF] Lau | Octave-spanning mid-infrared supercontinuum generation in silicon nanowaveguides[END_REF], silicon-germanium-on-insulator [START_REF] Ettabib | Broadband telecom to mid-infrared supercontinuum generation in a dispersion-engineered silicon germanium waveguide[END_REF] and silicon-on sapphire [START_REF] Singh | Midinfrared supercontinuum generation from 2 to 6 um in a silicon nanowire[END_REF], these sources are limited to 3.5µm and 6µm, due to the absorption in the silica and sapphire substrates, respectively. By taking advantage of the wider transparency window of the silicon substrate, the recently proposed silicon germanium-on-silicon platform is promising to overcome these limitations [START_REF] Carletti | Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared[END_REF][START_REF] Carletti | Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses[END_REF][START_REF] Sinobad | Mid-infrared octave spanning supercontinuum generation to 8.5µm in silicon-germanium waveguides[END_REF], potentially extending the spectral region of integrated sources at least up to 8µm [START_REF] Ramirez | Low-loss Ge-rich Si0.2Ge0.8 waveguides for midinfrared photonics[END_REF].

Waveguide design and experiments

We just reported the generation of an octave spanning supercontinuum from 3 to 8.5µm in TE polarization in a silicon germanium-on-silicon waveguide [START_REF] Sinobad | Mid-infrared octave spanning supercontinuum generation to 8.5µm in silicon-germanium waveguides[END_REF]. Here, we demonstrate the generation of 1.3 octave spanning supercontinuum in the MWIR from 2.9 to 7.7μm in TM polarization, with a useful on-chip average power greater than 7mW by pumping dispersion engineered silicon germanium-on-silicon waveguide [START_REF] Carletti | Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared[END_REF][START_REF] Carletti | Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses[END_REF]. Our waveguide consists of a 4.2μm thick and 4.5μm wide Si0.6Ge0.4 ridge with a top air cladding that seats on a silicon substrate (see inset Fig. 1a). The air clad design with a large SiGe cross-section (effective area of ~10.5µm 2 ) results in low anomalous dispersion across a large bandwidth (see figure 1a), a cut-off wavelength of 8.2µm, a strong mode confinement in the waveguide core material (>98 % at 4.15µm) and a reduction of the contribution from surface roughness to the propagation loss.

Waveguide propagation losses were measured to be between 0.3 and 0.4dB/cm in the 4 -5µm wavelength range with the minimum value being 0.32dB/cm at 5µm. Supercontinuum was then achieved by pumping the waveguide with ~200fs pulses delivered from a mid-infrared MIROPA-fs optical parametric amplifier at a repetition rate of 63MHz. Using a 7cm long waveguide pumped at 4.15μm close to the first zero-dispersion wavelength, we obtained the spectra shown in Fig. 1b for average coupled powers increasing from ~1mW to 21mW. At 21.1mW coupled average power (~3kW coupled peak power), we measured a supercontinuum spanning 1.3 octaves, from 2.9 to 7.7μm at the -30 dB level was achieved. Besides, the output spectrum is relatively smooth with a large -10dB bandwidth from 3.0 -7.4μm.

Conclusion

In summary, we report supercontinuum generation in TM polarization from a silicon germanium-on-silicon platform covering almost the whole mid-wavelength infrared, where most molecules in atmosphere have relevant fingerprints. The generated on-chip supercontinuum MWIR power and bandwidth exceed that produced so far by other Si-based platforms that are intrinsically limited by the silica or sapphire substrate absorption. This establishes silicon germanium-on-silicon as a promising platform for integrated nonlinear photonics in MWIR, with the potential to the operating range beyond 8µm.

Figure 1 a

 1 Figure 1 a) Calculated group velocity dispersion (GVD) and confinement of the mode field in the waveguide core (dashed lines), black arrow indicate pump wavelength, blue arrow indicates the cut-off wavelength, schematic of an air clad SiGe waveguide as an insert, b) Output spectra measured at different average coupled power. Coupled average power is calculated assuming 4.5dB coupling loss.

Acknowledgments: Agence Nationale de la Recherche (ANR) MIRSiCOMB (ANR-17-CE24-0028), H2020 EU framework, Marie Skłodowska Curie Action MSCA grant (PCIGA-2013-631543), European ERC grant GRAPHICS (648546); The Australian Research Council (ARC), Discovery Projects Program (DP150104327). We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), CM acknowledges the support of the Institut Universitaire de France.


Steve Joumessi-Demeffo

Stephanie Sahuguede

Anne Julien-Vergonjanne
email: anne.julien-vergonjanne@unilim.fr

Pierre Combeau
email: pierre.combeau@univ-poitiers.fr

Lilian Aveneau

Hervé Boeglen

Damien Sauveron
email: damien.sauveron@unilim.fr

Power and Delay performance of Optical Wireless Communication inside Aircraft Cockpit

Keywords: Li-Fi, optical wireless communication, channel modeling, channel access control

come

Introduction

In the context of increasing bandwidth requirements, technologies are migrating to high frequencies such as millimeter and THz communications, but also optical wavelengths, which have the advantage of not being regulated. [START_REF] Dimitrov | Principles of LED Light Communications Towards Networked Li-Fi[END_REF]. In addition, optical wireless communication (OWC) provides security, energy efficiency and highspeed rates. Depending on the use case, different OWC technologies can be used such as Visible Light Communications, Free-Space Optics, Li-Fi [START_REF] Dimitrov | Principles of LED Light Communications Towards Networked Li-Fi[END_REF]. Standardization efforts on this technology are subject to different developments based on IEEE 802.15 (.7 or .13) and more recently 802.11bb.

As part of the Aircraft Light Communication (ALC) European project [2], the studied use-case involves the transmission of audio and pilot physiological data. We focus on bidirectional infrared communications between the pilots' headset and an access point in the cockpit taking into account pilot movements. Indeed, the brightness inside the cockpit must be greatly reduced during the critical phases of flight. Therefore, the visible band is not appropriate. In addition, the system should have an autonomy adapted to long-haul flights, typically 12 hours without any interruption. One of the main goal is to ensure global coverage, considering that significant changes in the settings due to pilot mobility can greatly affect the channel. To analyse this effect, we propose channel simulation results using a ray-tracing software developed at XLIM [START_REF] Behlouli | MCMC Methods for Realistic Indoor Wireless Optical Channels Simulation[END_REF]. Moreover, the multi-user communication is based on 802.11 medium access control mechanism that is DCF (Distributed Coordination Function) algorithm with RTS/CTS (Request to Send /Clear to Send) method. In addition, to manage consumption constraints, we consider a power-efficient modulation scheme, which is PPM (Pulse Position Modulation). Our objective is to highlight the trade-off between emitted power and communication delay.

Results and discussions

The studied environment is a A350 cockpit as illustrated in Fig. 1. Transmitters are located on the top of the headsets and on the AP at about 1m from the pilot's heads.

Channel simulation

We have developed a detailed 3D geometric model of the cockpit (see Fig. 1) including the pilots' bodies and transceiver orientation and position changes resulting from movements. Consequently, the Line-Of-Sight (LOS) link condition cannot always be fulfilled. As a result, Non-LOS links evaluated by the ray-tracing software [START_REF] Behlouli | MCMC Methods for Realistic Indoor Wireless Optical Channels Simulation[END_REF], contribute to the channel gain. The channel analysis has been performed in term of optical gain determining the achievable SNR for a fixed transmitter power. As an example, we present in Table 1 the minimum DC gain values required to reach 100% reliability, considering a worst mobility scenario. The reflectivity coefficient of the environment is set to 0.5, the half-power angle of the headset emitter is 60°, respectively 40° for the access point, the receiver Field of View is 60° and physical detection area is 7 mm².

Delay and power analysis

Based on PHY and MAC analysis, optical emitted power and communication delay have been determined. They are plotted in Fig. 2 for PPM-4, a maximal size of MPDU (2500 bytes) and for a packet error rate PER=10 -3 . As expected, the delay diminishes with the data rate, while the required power increases, highlighting the trade-off. Considering a maximal allowed power, we can deduce the optimal data rate to reach the minimal delay. Performance can be enhanced by increasing the spectral efficiency of PPM but at the cost of complexity. When a very low delay is required, adding diversity on the headset can be investigated as an alternative.

Figure 1 :

 1 Figure 1: AIRBUS A350 cockpit and pilot position configuration

Table 1 :

 1 Minimum DC gain Values (dB)

	Figure 2: Emitted power and DCF delay versus data rate for uplink and downlink; co-pilot rear position,
	contention window = 63 slots, collision probability p=0%