Ultrafast vertical-external-cavity surface-emitting semiconductor lasers

We describe recent advances in the development of optically pumped passively mode-locked semiconductor lasers; ultrashort pulse sources that begin to offer levels of pulse duration, beam quality, and average power that formerly belonged only to diode-pumped solid-state lasers (DPSSLs) based on impurity-doped dielectric gain media. Unlike dielectric gain media, however, III–V semiconductors exhibit immense spectral versatility, with alloy compositions allowing emission wavelengths spanning the spectrum from visible through to the mid-infrared. Within the past few years, it has been shown that strained InGaAs/GaAs quantum well lasers operating around 1 µm are capable of generating transform-limited pulse durations of 100 fs or less; and moreover, that sub-400-fs pulses with > 300 W peak power, and 1.5-ps pulses with ~ 500 W peak power can be generated. Very recently, material systems other than InGaAs quantum wells have been used to demonstrate femtosecond mode locking, with results reported for a self-assembled quantum dot laser, a 2-µm antimonide laser and a 1.5-µm indium phosphide device. The vertical-external-cavity surface-emitting semiconductor laser, or VECSEL, mode-locked under the influence of a semiconductor saturable absorber mirror in the external cavity, is thus capable of bridging the gap in performance between mode-locked edge-emitting diodes and DPSSLs. A particular advantage of VECSELs is that they operate easily at repetition frequencies in the 1–20 GHz range, where dielectric lasers tend toward Q-switching instability, whereas monolithic diodes become inconveniently large – the range addressed both by electronics, and by the optical resolution of simple grating devices