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The realization of mode-locked lasers capable of generating short optical pulses in the near-infrared is a key enabler for various applications including high data rate optical interconnects. The related technology mostly consists of bulky and stand-alone devices, which tends to hinder their widespread use. In addition, the integration of such devices onto chip-based platforms could bring advantages in terms of robustness and stability, while offering some prospects for the realization of advanced hybrid photonic-electronic architectures. There has been recently some progress regarding the miniaturization of fast pulsed lasers [1], but the underlying cavity typically remains longer than a few millimetres and their integration on a chip is still a challenge [2]. Here, we aim at realizing a compact integrated pulsed micro-laser using an innovative photonic crystal (PhC) cavity based on low dispersion slow light modes.
Most of the characteristics of mode-locked lasers are tightly linked to the product of the cavity length L and the group index ng of the modes traveling in the cavity. While miniaturization thus tends to degrade the quality of the generated optical pulses, we counterbalance this effect through exploiting slow light (high ng) modes in PhC structures. Dispersion engineering techniques have been successfully developed in passive Si/air suspended PhC cavities for decreasing the otherwise typically high dispersion of these modes [3]. We adapted these techniques to our asymmetric InP/silica structures so as to provide a frequency-equidistant comb of modes - a pre-requisite to laser mode-locking. While our geometry well improves laser heat dissipation with respect to PhC cavities suspended in air, it poses additional constraints and could increase the cavity mode optical losses.
Using 3D FDTD simulations, we demonstrated the validity of this approach and designed, as an example, a 30 μm long, linear dispersion PhC cavity, with a group index of 29, providing at least 9 equally-spaced modes over an 11.5 nm bandwidth, which can afford the generation of sub-picosecond optical pulses. Despite the structure asymmetry, quality factors were all above 10,000, i.e. sufficient for reaching laser operation. Compared with a standard strip-waveguide-based laser cavity, this design provides a length reduction factor of almost one order of magnitude, as given by the ratio of the group index of each structure. Various designs enabled us to achieve a range of low dispersion bands with different group index (15-40), offering a relevant trade-off between device miniaturization and short pulse duration. Based on these designs, PhC active structures including InAsP/ InP quantum wells or InAs/InP quantum dashes were fabricated using molecular beam epitaxy, III-V/silica bonding, e-beam lithography and reactive ion etching. Near-field optical microscopy was used to identify both the spectral and spatial signatures of the cavity modes, which were in good agreement with the simulations. We also probed our cavities by microphotoluminescence. These cavities provide a first step towards the realization of miniaturized integrated mode-locked lasers. Their robust hybrid InP/silica geometry offers the potential for integration with silicon photonic devices, while enabling the integration of a saturable absorber, such as graphene, for achieving mode-locked laser operation.
References
[1] Joshi, S. et al. (2014). Quantum dash based single section mode locked lasers for photonic integrated circuits. Optics Express, 22(9), 11254.
[2] Latkowski, S., et al. (2015). Monolithically integrated 25 GHz extended cavity mode-locked ring laser with intracavity phase modulators. Optics Letters, 40(1), 77
[3] Li, J. et al. (2008). Systematic design of flat band slow light in photonic crystal waveguides. Optics Express, 16(9), 6227–6232.
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