Silicon Photonic crystal low-noise Raman laser – PHLORA

1) Modeling and design of low noise photonic crystal Raman lasers in silicon
2) Fabrication of the PhC structures and optimization of the processes
3) Characterization of the PhC lasers

High-frequency self-induced oscillations in a silicon photonic crystal nanocavity [Cazier2012].

High quality factor in SOI cavities [Han2011]

New classes of high spectral purity emitters based on silicon and pumped by standard low cost semiconductor laser diodes can be envisioned.

[Han 2011] Zheng Han, Xavier Checoury, Laurent-Daniel Haret, and Philippe Boucaud. High quality factor in a two-dimensional photonic crystal cavity on silicon-on-insulator. Opt. Lett., 36(10), pp1749–1751, May 2011.

[Cazier 2012] N. Cazier, X. Checoury, L.-D. Haret, P. Boucaud, High-frequency self-induced oscillations in a silicon nanocavity, submitted

[Cazier 2012b] N. Cazier, X. Checoury, P. Boucaud, Doubly resonant photonic crystal cavities for enhanced Raman scattering, en préparation

Despite the recent advances in the field of silicon photonics, the fabrication of active optical functions and, in particular, the fabrication of lasers remains one of the most difficult tasks. Silicon is an inefficient light emitter because of its indirect bandgap. Until now, except for the approaches that use III-V materials, only the use of a nonlinear effect, the stimulated Raman scattering, has allowed to obtain a laser effect in a rib waveguide etched on a silicon-on-insulator (SOI) substrate. One of the limitations of these reported studies on the stimulated Raman scattering is that they require long cavities (> 1 cm) or high-Q factor ring cavities with large area (~ 1 cm^2) to achieve the gain required by the laser effect. To overcome this limitation, that is incompatible with the on-chip integration of these components, photonic crystals (PhCs) appear as ultra-compact alternatives. The PHLORA project aims at demonstrating continuous wave photonic crystal Raman lasers on SOI with enhanced modulation bandwidth and noise properties compared to classical semiconductor laser diodes in the near infrared range. During this project, we will fabricate the first laser entirely in silicon with dimensions (typically 10 µm by 50 µm) compatible with on-chip large-scale integration. It is expected that the spectral purity of Raman silicon laser is superior to other conventional III-V semiconductor lasers due to the absence of the linewidth enhancement effect resulting from the symmetry of Raman gain spectrum in Si. Moreover, some of the schemes proposed for the reduction of the relative intensity noise (RIN) in the case of fiber Raman lasers can be transposed into very compact ones due to the properties of PhCs. Finally, because photonic crystals are efficient to achieve high quality factor cavities with very small mode volume, i.e. with large spontaneous emission coupling factor b, very large modulation bandwidths are expected for micro-sized Raman laser. Because the Raman gain spectrum in Si is symmetric, no frequency chirping will occur during the modulation of the Raman gain. All these properties make these lasers suitable for direct modulation in high speed telecommunication systems but also for applications like sensing, RF-photonics, metrology, and general research.
During the project a particular effort will be put on the modeling of PhC Raman laser. Based on our previous results on the Purcell effect in the case of spontaneous Raman scattering, the model developed during the project will be able to accurately predict the threshold and efficiency of the laser, its noise characteristics and its dynamics properties. From preliminary results, a 10 mW pump power with a quality factor of 10^6 at the Stokes wavelength can be enough to achieve lasing in a cavity made in a W1 photonic crystal waveguide. Hence, we will put the emphasis on the optimization of the quality factor of the cavities, on the reproducibility of the fabricated structures and on the efficient coupling of light in the PhC structures, that are the key points in the successful achievement of a PhC Raman laser. In particular, we expect to demonstrate cavities with quality factors above the 2 millions, present state-of-the-art at IEF. The lasers will be fabricated in both the suspended membrane approach and the silicon on insulator approach, i.e. with the oxide cladding remaining below the photonic crystal, since this approach provides a better mechanical stability and thermal dissipation. One of great strength of this project is that the IEF has all the necessary knowledge and equipment to achieve the whole project: modeling, fabrication and characterization.