Gain spectral narrowing of semiconductor laser based on dual-core vertical coupler structure

Abstract

Semiconductor FP ridge waveguide lasers based on dual-core vertical coupler structure with different ridge height were fabricated. Their modal gain of these devices around the threshold current were extracted and investigated using the Hakki–Paoli (H–P) method. It is observed that the gain spectrum profile and its 3 dB bandwidth vary with the ridge height. The narrower gain bandwidth of 12 nm was achieved when the ridge height was 4.59 μm, which is smaller than a quarter of that for the laser with the ridge height of 1.6 μm. This phenomenon is attributed to the narrowed gain spectrum induced by wavelength filtering effect of the dual-core vertical coupler (DCVC) structure induced by their co-directional coupling, which further shrinks its amplified spontaneous emission (ASE) spectrum. In addition, further simulation results show that the coupling length also affects the bandwidth of the DCVC structure. Within a single coupling length, the longer the coupling length, the narrower the bandwidth, and the larger the coupling efficiency. Such narrow-spectrum gain characteristic of DCVC structure will enable one to fabricate single longitudinal mode lasers with no need of fabricating gratings.