High operating temperature of passively mode-locked InGaAsP/InP semiconductor lasers

摘要

This thesis is concerned with the design and fabrication of MLLDs and assessment of the possibility of their uncooled operation. Fundamental mode locked laser diodes (MLLDs) were fabricated on both semi- insulating InP substrates and n-doped InP substrates. Devices were fabricated on semi-insulating InP substrates to allow for efficient direct modulation of the saturable absorber so that the output pulses can be synchronised to an external clock for communication applications. The devices fabricated from the semi-insulating substrate however only lase at operating temperatures of ≤20 °C and can only be mode locked at temperatures of ≤ 10 °C. Devices fabricated from epilayer grown on n-doped substrate could lase at temperatures of > 80 °C. For cavity lengths of 1.2 mm with absorber lengths of 40 mum these devices can be mode locked at temperatures of up to 65°C. The microwave spectra showed peaks at around 36 GHz with more than 35 dB above the noise floor. The full width half maximum (FWHM) of the spectra were 300 kHz across all the temperatures tested (15 °C to 65 °C), limited by the resolution bandwidth of 300 kHz. A positive shift of about 55 MHz was observed in the mode locked frequency between the operating temperatures of 15 to 65 °C. Passive mode locking was achieved at 75 using devices with absorber lengths of 20 mum. These devices however did not mode lock at lower temperatures. Devices with adaptable absorbers could be mode locked at temperatures from 15°C to 75°C These devices had three contact pads such that at temperatures of ≤ 65 °C, two of the shorter contact pads were biased as absorbers, while at temperatures of > 65 °C, only one of the shorter contact pads was biased as absorber. Microwave spectra showed peaks at around 37 GHz with more than 35 dB above noise floor across all the temperatures tested (15 °C to 75 °C). The tuning range, achieved via varying of absorber bias voltages, was discovered to vary with operating temperature, increasing from a tuning range of about 12 MHz at 15 °C to a tuning range of about 25 MHz at 55°C. Pilsewidth measurement was carried out using second harmonic generation autocorrelation. Due to power limitations, pulsewidths could only be measured up to at operating temperature of 35 °C. The pulsewidths varied between about 4 ps to alout 5.5 ps for absorber bias voltages of -0.3 V to +0.2 V. No relationship could be observed between the operating temperatures and the optical pulsewidths.