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 laser 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 suitable for particularly short pulse durations below 10 fs, in extreme cases down to approximately 5 fs, and with substantial average powers – often of the order of 1 W, sometimes several watts.

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 between a few milliwatts and tens of 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 – in particular, the handling of the strong optical nonlinearities. The handling of strong nonlinear effects requires relatively complicated operation principles for high performance, which implies that device optimization is substantially more sophisticated than for bulk lasers.

Dye lasers dominated the field of ultrashort pulse generation before the advent of titanium–sapphire lasers in the late 1980s. 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 and the limited dye lifetime, femtosecond dye lasers are no longer frequently used – mostly in spectral regions which can otherwise be accessed only with optical parametric oscillators and the like.

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. Mode-locked semiconductor lasers are also suitable for very high pulse repetition rates, e.g. tens or even hundreds of gigahertz. In most cases, however, the pulse energy is severely limited to the picojoule region.

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. Due to the large mode area and small propagation length in the semiconductor, their pulse energies can be far higher than for edge-emitting diode lasers, but still much lower than for solid-state bulk lasers in particular, because the gain saturation energy is rather small.

Some femtosecond laser devices are strictly speaking not just a femtosecond laser, because they contain essential additional components such as an optical amplifier or means for nonlinear frequency conversion in order to get into other wavelength regions. For example, some devices contain a synchronously pumped optical parametric oscillator or a optical parametric generator, which allows for the generation of widely wavelength-tunable radiation.

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.

The pulse duration (usually specified as the full width at half maximum (FWHM)) is in most cases fixed, e.g. a 100 fs or 25 fs. In some cases, however, it is tunable in a certain range.

Femtosecond lasers with different center wavelengths are available. Frequently, the center wavelength is between 1 μm and 1.1 μm, where most powerful laser sources can be made. However, amplified sources can also be quite powerful e.g. in the 1.5-μm or 2-μm region.

In some cases, nonlinear frequency conversion is used to reach other wavelength regions, e.g. visible or ultraviolet light with frequency doubling.

There is a lower limit for the optical bandwidth which scales with the inverse pulse duration; if that limit is reached, one has so-called bandwidth-limited pulses. For 100-fs pulses around 1 μm wavelength, for example, that limit is around 10 nm. For very short pulses with e.g. 10 fs duration, the optical bandwidth becomes very large.

The pulse quality includes additional aspects such as details of the temporal and spectral pulse shape, such as the presence of temporal or spectral pedestals or side lobes, and the stability of pulse parameters. In such respects, different femtosecond lasers can differ a lot.

The pulse repetition rate from the laser is in most cases fixed, typically between some tens and hundreds of megahertz, sometimes several gigahertz. If it is tunable, then usually only in a small range.

The output pulse repetition rate may be strongly reduced with a pulse picker, e.g. down to 10 kHz or even less. Here, one essentially transmits only every N^th pulse, and by varying the number N one can change the resulting repetition rate in very wide ranges (but not continuously).

Some sources can produce powerful bursts of pulses with a rather high pulse repetition rate within a burst. That can be advantageous for certain applications, e.g. in laser material processing, including laser micromachining. Ideally, at least some parameters of the burst (e.g. the number of pulses) can be flexibly adjusted.

For more details, see the article on burst mode lasers.

Assuming a steady sequence of pulses with the same properties (which is usually the case for such lasers), the pulse energy is simply the average output power divided by the pulse repetition rate.

The laser output can be delivered into free space (usually as a collimated beam), e.g. through some optical window in the housing. Other devices have a fiber connector for plugging in a fiber cable.

Generally, fiber delivery of femtosecond pulses is considered as problematic due to the substantial chromatic dispersion and particularly the fiber nonlinearities. However, solutions for those problems have been developed, in particular hollow-core fibers which allow transmission with a minimum of nonlinear effects and possibly in addition with tailored dispersion properties. One may also apply dispersion compensation before or after a fiber cable.

There are various additional aspects which can be important for applications:

• Many femtosecond lasers offer a stable linear polarization of the output, whereas others emit with an undefined polarization state. If emission is polarized, it is also possible to transform this into other polarization states, e.g. to achieve radial polarization, using suitable optics.
• 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 different 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.
• 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.

Femtosecond laser systems have important applications in laser material processing. Picosecond and femtosecond pulses have substantially higher peak powers than nanosecond pulses of the same pulse energy. Therefore, the material can be evaporated even more quickly, which gives a potential for further improved processing quality in various situations. However, femtosecond pulses are not necessarily better suited than because picosecond pulses, particularly if the duration of those is already below the electron–phonon coupling time.

Another aspect is that the extremely high optical intensities achievable with femtosecond pulses lead to nonlinear effects which can also be utilized. In particular, such laser radiation can be absorbed even in actually transparent materials such as glasses or crystals, because multiphoton absorption (followed by avalanche ionization) becomes sufficiently strong: such materials are then no longer transparent for the laser radiation. In this domain, femtosecond pulse durations can be preferable or even indispensable.

Femtosecond lasers can thus be applied to a particularly wide range of materials to be processed, including metals, polymers (plastics), glasses and crystalline dielectrics (even diamond), ceramics and semiconductors. Often, even the same laser apparatus can be used to process very different materials.

Femtosecond lasers are also used in medical application areas, mainly for laser surgery. For example, it is now common to use femtosecond pulses for eye surgery (vision correction), e.g. in the form of femto-LASIK or cataract surgery. It is another area where the extremely short pulse durations are advantageous.

Other medical applications involve femtosecond lasers for diagnostic purposes. Methods of laser microscopy (see below) are particularly relevant in this area.

Femtosecond lasers have also become quite important for laser microscopy, e.g. in the form of fluorescence microscopy. Here, one frequently utilizes multiphoton excitation (based on multiphoton absorption), where very short pulse durations are quite advantageous. It is also possible to use stimulated Raman scattering (SRS spectroscopy).

Femtosecond laser pulses are useful for a very wide range of measurements. For example, they are essential for modern optical clocks, serving both as a highly stable frequency reference and as an optical clockwork creating a phase-coherent link between many different optical frequencies and microwave frequencies.

There are also very different measurement applications such as distance measurements with LIDAR, in interferometry and in pump–probe measurements. The latter method allows one to investigate ultrafast processes, for example in chemistry, including biochemistry.

In the area of optical fiber communications [4], femtosecond lasers can be used in different ways. For example, it is possible to realize dense wavelength division multiplexing (DWDM) with a very large channel count (sometimes >1000) by spectral slicing of broadband femtosecond pulses. By applying time division multiplexing in addition, one can achieve extremely high bit rates of >1 Tbit/s.

(Suggest additional literature!)