Narrow spectral linewidth, high output power operation and FM response characteristics in 1.5 /spl mu/m butt-jointed DBR lasers

Abstract

Narrow spectral linewidth(600kHz). high Output power(>20mW) operation and a flat and large FM response(which is not distorted by tuning wavelength) are presented with the newly fabricated two types of 1.5~rm butt-jointed tunable DBR lasers. In the coherent optical communication systems, the narrow spectral linewidth, high output power and tunable laser diode is required as a light source. FM response characteristics are also important for a transmitter LD in FSK systems. In this paper, we present two types of Df3R LDs which show suitable characteristics for a local oscillator(TypeA) and for a transmitter(Type6). Device structure Fig.1 shows the schematic diagram of 1.5pm DBR lasers which were fabricated by a 5-steps LPE growth method. An active layer and a passive guide layer are butt-jointed to obtain high couding efficiency. To form a high coupling structure, we used two growth techniques, which were island like selective etching and two phase solution growth. Fig.2 shows an SEM photograph around butt-joint region. The coupling efficiency between the two layers was estimated about 80-90%. Two types of DBR lasers were fabricated, one is for a local osciIlator(TypeA) and another is for a transmitter(Type6). In TypeB chips, an active layer was separated into two sections to obtain a flat FM response[l]. Furthermore, TypeB chips were mesa-etched to reduce the parasi tic capacitance. Narrow spectral linewidth and high output power operation (TypeA) Fig. 3 shows an example of outputpower-current characteristics and sDectraI linewidth. The threshold current was 18mA and lasing wavelength was 1:52pm. Output power exceed 20mW without mode hopping. Spectral linewidth decrease with increasing output power. The minimum linewidth of 6OOkHz was observed at the maximum output power. Fig.4 is the characteristics of spectral linewidth versus lasing wavelength for each mode. Total tuning range of 4.5nm was obtained. Continuous tuning range of 1.4nm(below 12MHz) and 0.4nm(below 5MHz) are also obtained.