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A femtosecond laser is a laser which emits optical pulses with a duration well below 1 ps (→ ultrashort pulses), i.e., in the domain of femtoseconds (1 fs = 10−15 s). It thus also belongs to the category of ultrafast lasers or ultrashort pulse lasers. The generation of such short pulses is nearly always achieved with the technique of passive mode locking.
Types of 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.
Important Parameters of Femtosecond Lasers
The key performance figures of femtosecond lasers are the following:
- the pulse duration (which is in some cases tunable in a certain range)
- the pulse repetition rate (which is in most cases fixed, or tunable only within a small range)
- the average output power and pulse energy
There are, however, various additional aspects which can be important:
- The time–bandwidth product (TBP) shows whether the spectral width is larger than necessary for the given pulse duration. The pulse quality includes additional aspects such as details of the temporal and spectral pulse shape, such as the presence of temporal or spectral side lobes.
- Some femtosecond lasers offer a stable linear polarization of the output, whereas others emit with an undefined polarization state.
- The noise properties can differ strongly between different types and models of femtosecond lasers. This includes noise of the pulse timing (→ timing jitter), the pulse energy (→ intensity noise), and various types of phase noise. It may also be important to check the stability of pulse parameters, including the sensitivity of external influences such as mechanical vibrations or optical feedback.
- Some lasers have built-in means for stabilizing the pulse repetition rate to an external reference, or for tuning the output wavelength.
- The laser output can be delivered into free space e.g. through some glass window in the housing, or via a fiber connector.
- Built-in features for monitoring the output power, wavelength, or pulse duration, can be convenient.
- Other aspects of potential interest are the size of the housing, the electrical power consumption, the cooling requirements, and interfaces for synchronization or computer control.
Apart from these aspects of the laser itself, the quality of the documentation material, such as product specifications, user manual, etc., can be of interest.
See also: mode-locked lasers, ultrafast lasers, mode-locked diode lasers, titanium–sapphire lasers, solid-state lasers, picosecond lasers, passive mode locking, mode locking, ultrashort pulses
and other articles in the categories lasers, light pulses
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