(A) study on efficient output coupling of ring laser diode

Abstract

Bending loss and scattering loss of ring laser was calculated using simple ray model and ring mode from characteristic equation.From the total loss calculation as a function of ring radius,waveguide width and index step,it was shown that low loss cavity can be obtained for the width and radius at which ring mode changes from whispering gallery mode to normal guiding mode. GaAs circular ring laser diode was fabricated and the threshold current of half ring was 50mA and the threshold current density was 458A/㎠. The threshold current of full ring was 75mA and the threshold current density was 511A/㎠. To design efficient optical coupler, intensity pattern of multimode interfernce coupler was calculated. The idea of linear MMI coupler could not be applied to curved waveguide. Square ring laser diode that cavity itself is MMI 3dB coupler was proposed and GaAs/AlGaAs square ring laser was fabracated using RIBE and polyimide planarization. Threshold current of 200mA and 250mA were measured for 283㎛ and 509㎛ length cavity, respectively. Square ring laser diode with reflection coupler at MMI 3dB coupling length was proposed and fabricated. From the measurement of ring laser which has two output coupler for clockwise and counterclockwise rotating wave, the threshold current was 537mA (current density of 10.6kA/㎠). A ring laser which has one output coupler for clockwise rotating wave showed 362mA threshold current(current density of 8.4A/㎠). The output waveguide was 10˚ tilt from cleaved facet, so lasing from ring resonator was confirmed. We aimed to improve external quantum efficiency using MMI 3dB coupler, but the low efficiency of 1% was measured. One of the reason for low efficiency is low internal quantum efficiency of used epitaxial layer, because the external quantum efficiency of linear Fabry -Perot laser was as low as 2%. To verify MMI 3dB coupling experimentally, measurement of various cavity length laser is required. Improved efficiency is expected when ring l...