Vertical external cavity surface emitting semiconductor lasers

Vertical external cavity semiconductor lasers have emerged as an interesting technology based on current vertical cavity semiconductor laser knowledge. High power output into a single transverse mode has attracted companies requiring good fibre coupling for telecommunications systems. The structure comprises of a grown semiconductor Bragg reflector topped with a multiple quantum well gain region. This is then included in an external cavity. This device is then optically pumped to promote laser action. Theoretical modelling of AIGaAs based VECSEL structures was undertaken, showing the effect of device design on laser characteristics. A simple 3-mirror cavity was constructed to assess the static characteristics of the structure. Up to 153 mW of output power was achieved in a single transverse mode. Thermal heating of the active region limited the maximum output of the laser device, Modelocked operation was undertaken using Acousto-Optic (AO) modelocking and synchronous pumping. AO pulse length varied from 100-200 ps with a maximum output power of 12 mW. Synchronous pumping produced up to 300 ps pulses with 25 mW laser output (-4 dBm) modulation. If the modulation was increased to 0 dBm then ∼150 pS pulses could be produce with a maximum laser output of 12 mW. Single longitudinal mode operation of the VECSEL showed up to 42 mW of power within a single mode. Reduction in spatial hole burning within the structure enables stable, single mode operation in a linear cavity. Active stabilisation showed a relative locked linewidth of ∼3 kHz. Coarse tuning over 7 nm was achieved using a 3-plate birefingent filter plate while fine-tuning using cavity length change allowed tuning over 250 MHz. (author)