Rüdiger Paschotta, Lukas Krainer and Ursula Kelle, Swiss Federal Institute of Technology, Zurich
Ultrashort pulse generation was totally dominated by lasers that only highly skilled researchers could operate in well-equipped laboratories. The equipment was not suitable for industrial applications: Short-lived components such as carcinogenic laser dyes and repeated fine-tuning of sophisticated photonic systems to maintain the performance are not acceptable outside the domain of academic research.
The advent of ultrafast solid-state lasers started to profoundly change this situation. Scientists made fast progress with the Ti:sapphire laser, which now generates pulses of less than 6-fs duration. The laser is fairly robust and efficient but still relies on a bulky and inefficient argon-ion pump laser (like dye lasers) or on relatively expensive all-solid-state green pump lasers.
In the 1980s, the development of powerful laser diodes led to great progress toward powerful, efficient, compact and reliable all-solid-state lasers in the continuous-wave regime. Still, technical problems delayed ultrashort pulse generation with these devices.
First, the directly diode-pumpable gain media did not have enough gain bandwidth to support pulses as short as Ti:sapphire can achieve. Second, researchers had difficulty obtaining stable passive mode locking, where a short pulse circulates in the laser cavity with constant pulse energy.
Semiconductor saturable absorber mirrors (SESAMs) enabled the use of intracavity absorb-ers to obtain stable passive mode locking with a great variety of solid-state materials, in the picosecond and femtosecond domain and in many wavelength regions.
This development has also allowed ultrashort lasers to leave the laser lab and find applications in other areas of research and industry.