定义：
发射的脉冲的时间间隔在几个飞秒和几百飞秒之间的激光器。

飞秒激光器是可以发射光脉冲持续时间小于1ps（→超短脉冲）光脉冲的激光器，也就是说是在飞秒时间域内（1 fs = 10−15 s）。
因此这种激光器也被归于超快激光器或超短脉冲激光器的分类中。对于产生这种短脉冲，常常用到一种叫做被动模式锁定的技术。 

目录

1. 飞秒激光器的类型 

1.1 固体激光器
1.2 光纤激光器
1.3 染料激光器
1.4 半导体激光器
1.5 其他类型

1. 飞秒激光器的一些重要特性

飞秒激光器的类型 
固体激光器 
被动锁模的固体激光器能发出高质量的超短脉冲激光，拥有典型的持续时间30 fs和30 ps。各种二极管抽运激光器，如基于掺钕或掺镱的增益媒介，在这种体系中，典型的平均输出功率在100 mW和1 w之间。
应用先进的色散补偿的钛蓝宝石激光器甚至可适用于持续时间小于10 fs的脉冲，极限情况下可以达到约5 fs。脉冲重复率在大多数情况下为50 MHz到500 MHz，对于较大的脉冲功率，重复率会变小一些，大概几兆赫，而一些微型激光器则有几十兆赫。 

光纤激光器 
各种类型的超快光纤激光器，在大多数情况下也是被动锁模的，提供的典型脉冲持续时间在50到500 fs，重复率10到100 MHz，平均功率为几个毫瓦。
大体上，拥有更高的平均功率和脉冲能量的激光器是可行的，如与展宽脉冲光纤激光器或自相似脉冲激光器，或与一个光纤放大器结合。
全光纤的解决方案在大规模生产中是相当划算的，当然，所需的努力是开发出一种光纤产品，高性能和高运行可靠性应能承受各种技术挑战。 

染料激光器 
钛蓝宝石激光器出现之前，超短脉冲产生领域由染料激光器主导。染料激光器的增益带宽允许脉冲持续时间为10 fs级别，不同的激光染料适用于各种发射波长，且常在可见光谱的范围内。由于染料激光器一些处理中的缺点，飞秒染料激光器不再经常使用。 

半导体激光器 
一些锁模二极管激光器可以产生飞秒级别的脉冲。在激光器的直接输出端，脉冲持续时间通常至少几百飞秒，但利用外部脉冲压缩，可以获得持续时间更短的脉冲。
被动锁模的垂直外腔面发射激光器(VECSELs)也是可能实现的；这种类型的激光器受关注，主要是因为它可以提供一种结合的特点，短脉冲持续时间，脉冲重复率高，时而有很高的平均输出功率，但是它不适合于很高的脉冲能量。 

其他类型 
更多特别类型的飞秒激光器是色心激光器和自由电子激光器。后者可以发射飞秒脉冲甚至X-射线形式的脉冲。 

飞秒激光器的一些重要特性 
飞秒激光器的一些主要性能指标如下： 

• 脉冲持续时间（一定情况下在特定范围内可调） 
• 脉冲重复率（在多数情况下是固定的，或者只在很小范围内可调） 
• 平均输出功率和脉冲能量 

还有另外一些很重要的方面： 

• 时间带宽积（TBP）表征对于给定脉冲持续时间，光谱带宽是否大于所需带宽。脉冲品质包含额外的方面，如时间和频率上脉冲波形的细节，再如，时间或光谱的旁瓣。 
• 一些飞秒激光器提供了一个稳定的线性偏振输出，而其它的则发射不确定的偏振状态。 
• 不同类型和模式的飞秒激光器，其噪声特性会有很大区别。这包括噪声脉冲的时间(→定时抖动),脉冲能量(→强度噪音)，以及各种类型的相位噪声。检测脉冲参数的稳定也很重要，包括对外部影响的敏感性，如机械振动或光学反馈。 
• 一些激光器内置对应外部参考的脉冲重复率稳定设置，或用于调谐输出波长。 
• 激光输出可以输送到到自由空间，如通过房子的玻璃窗，或通过光纤连接器。 
• 内置监测输出功率，波长和脉冲持续时间的特性，会很方便。 
• 其它潜在特点方面，如住房大小，电力消费，冷却需求，接口同步或计算机控制。

Definition: lasers emitting light pulses with durations between a few femtoseconds and hundreds of femtoseconds

Alternative term: ultrafast lasers

More general term: mode-locked lasers

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 (which also include picosecond lasers).

The generation of such short (sub-picosecond) light pulses is nearly always achieved with the technique of passive mode locking. That leads to pulse trains with high pulse repetition rates in the megahertz or gigahertz region. This combined with the limited average output power leads to relatively small pulse energies (often in the nanojoule region). Far higher pulse energies (at lower repetition rates) – often higher by many orders of magnitude – are possible by using some kind of optical amplifiers system (→ ultrafast amplifiers) in addition to a femtosecond laser.

Types of Femtosecond Lasers

Femtosecond pulses can be generated with very different kinds of lasers, which are explained in the following. Some of these lasers are industrial lasers, while others are scientific lasers.

Solid-state Bulk 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.

Fiber Lasers

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

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.

Semiconductor Lasers

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.

Frequency-converted Sources

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.

Other Types

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.

Important Parameters of Femtosecond Lasers

The key performance figures of femtosecond lasers are the following:

Pulse Duration

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.

Center Wavelength

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.

Pulse Bandwidth

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.

Pulse Quality

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.

Pulse Repetition Rate

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).

Burst Mode

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.

Average Power and Pulse Energy

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.

Output Type

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.

Other Aspects

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.

Applications of Femtosecond Lasers

Is a very wide range of applications of femtosecond lasers, exploiting quite different properties of the pulses. In the following, we give some typical examples.

Laser Material Processing

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.

Medical Applications

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.

Laser Microscopy

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).

Measurements

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.

Telecommunications

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.

Bibliography