A GaN surface emitting laser (SEL) based on angular-symmetry-breaking concentric-ring surface grating (ASB-CRSG) is proposed in this paper. The second-order CRSG located in the p-contact and p-cladding of an EPI wafer of GaN FP laser is adopted to select the radial mode and couple the optical power vertically out of the laser cavity. As the zero-order azimuthal CRSG with a two-lobe far field has the lowest mode loss in the angular-symmetric CRSG, the first-order ASB is adopted by the removal of two circular sections of GaN epitaxial layers to break the angular symmetry of the lasing modes. The simulation results show that degenerate modes in angular-symmetric CRSG have different mode losses with the help of the first-order ASB and the bigger breaking angles of CRSG results to higher loss difference between the first-order and other azimuthal modes. The loss and divergence angle decrease with the increasing area of CRSG, and the deeper CRSG results to the higher out-plane coupling. The first-order azimuthal mode has the lowest mode loss whose value is ~ 84% of that of the second-lowest-loss mode. A single-lobe far-field with a divergence angle of 1.33° in the wavelength of 450nm will be realized by an ASB-CRSG with the diameter of 10.6μm, the breaking angles of 12° and the depth of 325nm. Therefore, the single-mode operation of the first-order azimuthal mode which has a single-lobe far field is expected with the combination of the second-order CRSG and the first-order ASB.
The distributed feedback (DFB) laser is a key component for fiber communication due to its single-mode performance, but it usually requires complex and expensive regrowth after grating definition. The laterally-coupled distributed feedback (LC-DFB) laser has the advantage of a simple fabrication process without epitaxial regrowth, but the LC-DFB laser usually has a low coupling coefficient as the optical feedback is provided by the evanescent field and Fabry-Parot (FP) longitudinal modes arise from the pair of parallel cleaved facets. In this work, a triangular prism etched facet is proposed to suppress the FP longitudinal modes from cleaved facets of a 1.3 μm LC-DFB laser. The length-width ratio of a triangular prism facet is optimized on the compromise between the reflection and length by finite difference time domain (FDTD) method. The vertical etched facet with depth of 4 μm and tip curvature of 100 nm and the lateral gratings with depth of 1.8 μm and gap of 200 nm are fabricated by inductively coupled plasma (ICP) etching with the gas mixtures of Cl2/CH4/Ar and CH4/H2/Ar, respectively. The FP longitudinal modes of the etched-facet laterally-coupled distributed feedback (EF-LC-DFB) laser are effectively suppressed compared to the counterpart of cleaved facets, and the stable single-mode operation of EF-LC-DFB is demonstrated with the side mode suppression ratio (SMSR) of 54.35 dB.
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