Vertical-cavity laser with a novel grating mirror

Hybrid III-V on silicon (Si) ‘vertical cavity lasers’ (hybrid VCLs), which can emit light laterally into a Si waveguide, are fabricated and investigated. The Si-integrated hybrid VCL consists of a top dielectric Bragg reflector (DBR), a III-V active layer, and a bottom high contrast grating (HCG) mirror formed in the Si layer of a Si-on-insulator (SOI) wafer.

The hybrid VCLs have a promising potential for very high-speed operation and low energy consumption, which is ideal for optical interconnects as well as large data center applications. For the experimental demonstration of hybrid VCLs, CMOS-compatible fabrication processes are designed and developed.

These include a low-temperature direct wafer bonding process for integrating III-V layers onto a SOI wafer, as well as two types of DBR formation processes: a lift-off process and an etch-back process. Based on these, two versions of optically-pumped hybrid VCLs have been fabricated. The first version of hybrid VCL is designed for demonstrating in-plane emission into a Si waveguide.

The in-plane emission is enabled by the bottom HCG abutting the Si waveguide, which not only functions as a highly reflective mirror but also routes the light from the vertical cavity laterally into the Si waveguide. The measured inplane emission proves the lasing action with a side-mode suppression ratio (SMSR) of 27.5 dB at a peak wavelength of 1486 nm.

The threshold pumping power corresponds to a current injection of 1.1 mA. A signature of highly anisotropic cavity dispersion has been observed and discussed, which is unique for HCG-based vertical cavities. The second version proves the potential for high-speed operation of hybrid VCL structure. In the hybrid VCL structure, the effective cavity length is substantially reduced by using a dielectric DBR and a TM-HCG with a very short evanescent tail.

This reduces the photon lifetime of the laser cavity significantly without reducing the mirror reflectivity, leading to a very high intrinsic speed. A 3 dB frequency of 27.2 GHz was measured at a pumping power corresponding to a current injection of 0.7 mA. Since the pumping power was limited by the setup, the 3 dB frequency could be even higher.

At this pumping level, the SMSR was about 49 dB and the lasing wavelength was 1541 nm. It was noteworthy that a modulation current efficiency factor (MCEF) of 42.1 GHz/mA1/2 , which is 3 times greater than the cutting edge 850 nm VCSEL. Besides, this large MCEF is desirable for significantly lowering the injection current at a given target speed, which implies the amount of heat generation can potentially be reduced by 2 orders of magnitude than the 850 nm VCSELs.

Last, a new type of grating reflector, referred to as hybrid grating (HG) is analyzed and demonstrated, which may improve the heat dissipation efficiency of HCG-based hybrid VCL structures. The HG mirror consisting of a bottom grating and a high-refractive-index cap layer integrated on the grating can provide a stop band even broader than HCG.

The interaction between the cap and the bottom grating results in strong Fabry-Perot (FP) resonance as well as weak guided mode (GM) resonance. Most of the reflected power come from the FP resonance while the GM resonance performs a crucial role in achieving a reflectance of almost 100% as well as broadening the stopband as wide as 300 nm.