# Mid-IR supercontinuum generation in a birefringent SiGe waveguide with selectable dispersion regime

Adam Biegański<fnref n="1" />,<fnref n="2" />*, Alberto Della Torre<fnref n="1" />, Rémi Armand<fnref n="1" />, Marko Perestjuk<fnref n="1" />,<fnref n="2" />, Vincent Reboud<fnref n="3" />, Jean-Michel Hartmann<fnref n="3" />, Arnan Mitchell<fnref n="2" />, Thach Nguyen<fnref n="2" />, Sébastien Cueff<fnref n="1" />, Christelle Monat<fnref n="1" /> and Christian Grillet<fnref n="1" />

**Abstract:** We showcase pulse spectral broadening by pumping a birefringent SiGe-on-Si waveguide at 3.9 µm wavelength. By changing the pump polarization, we alter the waveguide's dispersion regime and therefore the properties of generated spectra. © 2023 The Authors

## 1. Introduction

Strong and well separated fundamental absorption lines of numerous gas molecules are present in the mid-IR spectral range, which is known as the “molecular fingerprints” region [1]. Mid-IR spectroscopy can be thus successfully employed for detecting various important gases such as carbon monoxide (CO) and methane (CH₄). The simultaneous detection of multiple gas species requires broadband mid-IR light sources such as a supercontinuum (SC), but commercial mid-IR technology is still relatively bulky and expensive. On the contrary, SC generation in integrated group IV waveguides can leverage the well-developed microfabrication Si technologies for low cost and mass production of broadband spectroscopy devices.

A SC can be generated either in anomalous or all-normal dispersion regime. In the former case, it is possible to generate ultra-broad spectra, but with poor flatness and phase coherence. In the latter the generated spectra exhibit a narrower bandwidth, but superior properties in terms of flatness and coherence. Our group demonstrated SC generation in SiGe-on-Si waveguides both in the anomalous and all-normal dispersion regime [2-4]. Depending on the spectroscopy technique (e.g. dual-comb spectroscopy or coherent Raman spectroscopy), one or the other may be more desirable. However, the waveguide dispersion profile is usually fixed and cannot easily be modified after being fabricated. A convenient way to control SC properties is exploiting birefringent waveguides that operate in different dispersion regimes for different pump pulse polarizations. Extreme polarization-dependent SC generation has been already demonstrated in chalcogenide [5] and SiN [6] waveguides, but their practical use for large scale spectroscopy devices would be limited by difficult mass production for the former, and limited spectral coverage in the mid-IR for the latter. Here, we demonstrate mid-IR supercontinuum generation in a birefringent SiGe-on-Si waveguide, and show that, by changing the pump polarization, we can simultaneously control the spectral properties of the generated supercontinuum.

## 2. Waveguide design and supercontinuum generation

We designed a 2.2 cm long, 3.5 µm wide, and 2.7 µm thick air-clad SiGe-on-Si waveguide (Fig. 1a) to achieve all-normal dispersion in TM polarization and a low anomalous region between ~3.8 and ~4.6 µm in TE polarization (Fig. 1b). This waveguide design also results in single mode operation beyond 3.5 µm and low mode effective area below 10 µm<fnref n="2" />, with mode confinement in the waveguide core greater than 98.5% between 2.5 and 5.5 µm in both polarizations. The photonic chip was fabricated at CEA-Leti using standard epitaxy and lithography [2].

<figure><img src="image_1.png" /><figcaption>Fig 1. (a) Waveguide's cross-section and (b) dispersion profile of the TE (red) and TM (blue) polarization modes. The arrow indicates the desired pump wavelength</figcaption></figure>

<footnote n="1">¹Univ Lyon, CNRS, ECL, INSA Lyon, UCBL, CPE, INL UMR5270, 69134 Ecully, France</footnote>

<footnote n="2">²Integrated Photonics and Application Centre (InPAC), School of Engineering, RMIT University, Melbourne, VIC 3001, Australia</footnote>

<footnote n="3">³Université Grenoble Alpes, CEA-Leti, 38054 Grenoble, France</footnote>

<footnote>*adam.bieganski@ec-lyon.fr</footnote>
<figure><img src="image_2.png" /><figcaption>Fig 2. Measured SC in (a) TE and (c) TM polarization for increasing pump power (coupled peak power and coupled average power indicated on the left and right side of the plot, respectively) with corresponding simulated SC in (b) TE and (d) TM polarization. Simulations were performed by solving the nonlinear Schrödinger equation, as described in [2].</figcaption></figure>

We pumped the waveguide with a pulsed OPA laser source (MIROPA-fs, Hotlight Systems) operating at a wavelength of 3.9 µm with a repetition rate of 63 MHz and ~200 fs pulse duration. Fig. 2a and 2c show the measured spectra at the waveguide output for increasing average coupled pump powers between 0.5mW and 20mW in TE and TM polarizations, respectively. In TE polarization, the -10 dB bandwidth of generated SC extends over 1.5 µm, but exhibits low spectral flatness. In TM polarization, the spectrum covers a narrower band (-10 dB bandwidth below 1.0 µm), but the spectral flatness is improved. Dips at around 4.3 µm are due to the absorption from CO₂ in the atmosphere. Measured spectra match well with the simulated ones (fig. 2b and 2d).

**3. Conclusion**

We experimentally demonstrated mid-IR SC generation in a birefringent SiGe-on-Si waveguide. The spectra exhibit different properties for different input pulse polarizations. Our results show the potential of birefringent waveguides for flexible and small footprint spectroscopy devices.

**Acknowledgements**

We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), the Agence Nationale de la Recherche (MIRSI COMB, ANR-17-CE24-0028; MetaOnDemand, ANR-20-CE24-0013 and MIRThFUL, ANR-21-CE24-0005), the Horizon 2020 research and innovation programme under the MSCA (REDI, grant agreement No 101034328).

**References**

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5. Y. Yu et al. "Experimental demonstration of linearly polarized 2–10 µm supercontinuum generation in a chalcogenide rib waveguide," *Opt. Lett.* **41**, 958 (2016).

6. E. Tagkoudi et al. "Extreme polarization-dependent supercontinuum generation in an uncladded silicon nitride waveguide", *Opt Express* **29**, 21348-21357 (2021).