Femtosecond Lasers

Passively mode-locked solid-state bulk lasers can emit high-quality ultrashort pulses with typical durations between 30 fs and 30 ps. Various diode-pumped lasers, e.g. based on neodymium-doped or ytterbium-doped gain media, operate in this regime, with typical average output powers between ≈ 100 mW and 1 W. Titanium–sapphire lasers with advanced dispersion compensation are even suitable for pulse durations below 10 fs, in extreme cases down to approximately 5 fs. The pulse repetition rate is in most cases between 50 MHz and 500 MHz, even though there are low repetition rate versions with a few megahertz for higher pulse energies, and also miniature lasers with tens of gigahertz.

Various types of ultrafast fiber lasers, which are also in most cases passively mode-locked, typically offer pulse durations between 50 and 500 fs, repetition rates between 10 and 100 MHz, and average powers of a few milliwatts. Substantially higher average powers and pulse energies are possible, e.g. with stretched-pulse fiber lasers or with similariton lasers, or in combination with a fiber amplifier. All-fiber solutions can be fairly cost-effective in mass production, although the effort required for development of a product with high performance and reliable operation can be substantial due to various technical challenges.

Dye lasers dominated the field of ultrashort pulse generation before the advent of titanium–sapphire lasers. Their gain bandwidth allows for pulse durations of the order of 10 fs, and different laser dyes are suitable for emission at various wavelengths, often in the visible spectral range. Mainly due to the disadvantages associated with handling a laser dye, femtosecond dye lasers are no longer frequently used.

Some mode-locked diode lasers can generate pulses with femtosecond durations. Directly at the laser output, the pulses durations are usually at least several hundred femtoseconds, but with external pulse compression, much shorter pulse durations can be achieved.

It is also possible to passively mode-lock vertical external-cavity surface-emitting lasers (VECSELs); these are interesting particularly because they can deliver a combination of short pulse durations, high pulse repetition rates, and sometimes high average output power, whereas they are not suitable for high pulse energies.

More exotic types of femtosecond lasers are color center lasers and free electron lasers. The latter can be made to emit femtosecond pulses even in the form of X-rays.