A. The concept of ultrafast lasers

Ultrafast lasers usually refer to mode-locked lasers used to emit ultra-short pulses, for example, pulses of femtosecond or picosecond duration. A more accurate name would be ultrashort pulse laser. Ultrashort pulse lasers are almost mode-locked lasers, but the gain switching effect can also produce ultrashort pulses.

B. The type of ultrafast laser

1. Ti-sapphire lasers, usually Kerr lens mode-locked, can produce pulses as short as about 5 fs in duration. Their average output power is typically a few hundred milliwatts, with pulse repetition rates of, say, 80MHz and tens of femtoseconds or less, and pulse durations of tens of femtoseconds or less, resulting in an extremely high peak power. But titanium-sapphire lasers require pumping light from some green-light lasers, which makes them more complex and expensive.

2. There are various diode-pumped lasers based on, for example, ytterbium-doped (crystal or glass) or chromium-doped laser crystals, which usually use SESAM passive mode-locking. Although the pulse duration of diode-pumped lasers is not as short as the pulse duration of titanium-sapphire lasers, diode-pumped lasers can cover a wide parameter region in terms of pulse duration, pulse repetition rate, and average power (see below).

3. Fiber lasers based on glass fibers doped with rare earth elements may also be passively mode-locked, for example, using nonlinear polarization rotation or SESAM. They are more limited than bulk lasers in terms of average power, especially peak power, but can be conveniently combined with fiber amplifiers. The article on mode-locked fiber lasers gives more details.

(4) mode-locked diode lasers can be integral devices or external cavity diode lasers, and can be active, passive or mixed mode-locked. Typically, mode-locked diode lasers operate at a high (several thousand megahertz) pulse repetition rate at a moderate pulse energy.

Ultrafast laser oscillators can be part of ultrafast laser systems, which can also include an ultrafast amplifier (such as a fiber optic amplifier) to increase peak power and average output power.