定义：
基于光的受激辐射，可以产生可见光或者不可见光的器件。

激光器（Laser）是受激辐射被放大的光（Light Amplification by Stimulated Emission of Radiation）的简称，它是在1957年由Gordon Gould发明的。
尽管该词最初是为了表示一种工作原理（利用激发原子或离子的受激辐射），但是现在主要用来表示基于激光器原理产生光的器件。有些特定的情况下，也指激光振荡器，或者指包含激光放大器的器件。 
第一个激光器装置是脉冲红宝石激光器，是1960年Theodore Maiman制作的。同年，实现了第一台气体激光器（氦氖激光器）和第一个激光二极管。
在该实验工作之前，Arthur Schawlow, Charles Hard Townes, Nikolay Basov和Alexander Prokhorov发表了关于激光器工作原理的开创性的理论工作，而在1953年Townes的小组就发表了关于微波放大器和振荡器的工作。最初是采用了镭射（Optical maser, microwave amplification by stimulated amplification of radiation）一词，之后由激光器（laser）取代。 
激光器技术是光子学领域的核心技术，主要是由于激光具有很多特殊的性质： 

1. 它通常是以激光光束的形式被辐射出来，发射角很小，并且可以聚焦成很小的点。 
2. 它的光学带宽很窄，而大多数照射灯辐射非常宽光谱的光。 
3. 它可能是连续光的形式，或者是短脉冲或超短脉冲，长度可以从毫秒降低到几飞秒。 

激光的这些性质使其有很广泛的应用，是激光辐射很高相干性的结果。更多细节可参阅词条激光和激光器应用。 

目录

1. 激光器工作原理
2. 激光器类型
3. 广泛意义上的激光光源
4. 激光安全

激光器工作原理 
激光振荡器通常包含一个光学谐振腔（激光谐振腔，激光腔），光可以在其中往返运动（例如，在两个反射镜之间），谐振腔中存在增益介质（例如，激光晶体），是用来放大光的。如果不存在增益介质，那么光每往返一周后都会变得越来越弱，因为它会受到一些损耗，例如来自于反射镜反射引起的损耗。
但是，由于增益介质可以放大往返的光，因此如果增益足够高可以补偿损耗。增益介质需要外加的泵浦能量，例如通过注入光（光学泵浦）或者电流（电泵浦，参阅半导体激光器）。激光放大的原理是受激辐射。 

图1：一种简单的光学泵浦激光器的示意图。激光器谐振腔是有很高反射率的弯曲反射镜和部分透射的平的反射镜，以及输出耦合器组成，后者将往返的激光提取出来一部分作为有用的输出。增益介质为激光器晶体，它是边泵浦的，例如来自于闪光灯发出的光。 
如果增益小于谐振腔损耗，那么无法实现激光产生，这一装置就是工作在激光阈值以下，只能得到发光现象。只有当泵浦功率大于激光阈值才会有较大的功率输出，这时激光器增益大于损耗。
如果增益大于损耗，激光谐振腔中的光功率从很低的荧光强度量级迅速变大。由于高的激光功率会使增益饱和，激光功率会达到一个稳态，在该态下饱和增益等于谐振腔损耗（参阅增益钳制）。阈值泵浦功率是指小信号增益足够产生激光时的泵浦功率。 
在谐振腔中往返光的一部分光功率通过一个部分透明反射镜后发生透射，该反射镜称为输出耦合反射镜。产生的光束包含激光器的有用输出部分。输出耦合反射镜的透射可以进行优化得到最大的输出功率（参阅斜度效率）。 
有些激光器是连续光工作，而其他的激光器则产生脉冲，脉冲可能非常强。有很多种方法使激光器得到脉冲产生，可以得到长度为毫秒，纳秒，皮秒甚至几个飞秒的脉冲（参阅来自锁模激光器的超短脉冲）。 
当只有一个谐振腔模式可以发生振荡时（参阅单频工作），连续光激光器的光学带宽（或线宽）非常小。而在其它情况下，尤其是锁模激光器，带宽非常大，极限情况下可以拓展到一倍频程。
激光辐射的中心频率通常在峰值增益频率附近，但是如果谐振腔损耗与频率有关的话，激光波长可以在增益足够大的一个范围内进行调谐。有些宽带增益介质，例如钛蓝宝石和铬硒化砷能够实现波长在几百个纳米范围内调谐。 
由于受到各种影响，激光器的输出中包含很多噪声，例如在输出功率或者光学相位中。 

激光器类型 
常见的激光器类型包括： 

1. 半导体激光器（大多数为激光二极管），是由电泵浦的（有时是光泵浦的），可以有效的产生很高的输出功率（通常光束质量较差），或者具有很好空间特性的低功率的光（例如，应用到CD和DVD播放器中），或者产生很高脉冲重复速率的脉冲（例如，用于通信应用）。一些特殊的类型包括量子级联激光器（中红外光）和表面发射半导体激光器（VCSELs和VESELs），后者适宜于得到高功率的脉冲。 
2. 基于离子掺杂晶体或玻璃的固态激光器（掺杂绝缘体激光器），是由放电灯或激光二极管泵浦的，得到很高的输出功率，或者具有很高光束质量、空间纯度和稳定性的低功率光（用于测量），或者长度为皮秒或飞秒的超短脉冲。常见的增益介质为Nd:YAG, Nd:YVO4, Nd:YLF, Nd:glass, Yb:YAG, Yb:glass, 钛蓝宝石，Cr:YAG和Cr:LiSAF。 
3. 光纤激光器，利用光纤纤芯中掺杂的一些激光活性离子。光纤激光器可以得到具有很高光束质量、很高输出功率的光（几千瓦），可以进行很宽的波长调谐，窄线宽工作等。 
4. 气体激光器（例如，氦氖激光器，二氧化碳激光器和氩离子激光器）和准分子激光器，通常利用在放电情况下气体被激发的原理。常用的气体包括CO2，氩气，氪气和一些气体混合物，例如氦氖。而常见的准分子包括ArF, KrF, XeF和F2。 

一些较不常见的激光器包括化学和核泵浦激光器，自由电子激光器和X射线激光器。 

广泛意义上的激光光源 
有些光源并不严格是激光，但是也被称为激光光源： 

1. 有些情况下，是指不需要入射就有辐射光的放大装置（种子放大器除外）。例如X射线激光器，通常是超发光光源，是自发辐射产生之后被单级放大得到的。它不包含激光谐振腔。光学参量产生器中的情况类似，但是放大过程不是基于受激辐射。这种装置产生的光具有激光性质，例如辐射具有很强的方向性和有限的光学带宽。 
2. 有些情况下，激光光源仅仅指光源中包含一个激光器，还有一些其它的器件。激光器和放大器组合在一起就是这种情况（主振荡功率放大器），还有非线性频率转换得到的激光辐射，例如，利用倍频器或者光参量振荡器。 

激光安全 
利用激光器工作会产生安全问题。有些直接与激光有关，尤其是产生很高的光强时，还有一些与激光光源相关。更多细节可参阅词条激光安全。

Definition: devices generating visible or invisible light, based on stimulated emission of light

More general term: light sources

More specific terms: solid-state lasers, diode lasers, gas lasers, excimer lasers, radiation-balanced lasers, cryogenic lasers, visible lasers, eye-safe lasers, infrared lasers, ultraviolet lasers, X-ray lasers, bulk lasers, fiber lasers, dye lasers, upconversion lasers, free electron lasers, Raman lasers, high-power lasers, narrow-linewidth lasers, tunable lasers, pulsed lasers, ultrafast lasers, industrial lasers, scientific lasers, alignment lasers, medical lasers

“Laser” (rarely written as l.a.s.e.r.) is an acronym for “Light Amplification by Stimulated Emission of Radiation”, coined in 1957 by the laser pioneer Gordon Gould. Although this original meaning denotes an principle of operation (exploiting stimulated emission from excited atoms or ions), the term is now mostly used for devices generating light based on the laser principle. More specifically, one usually means laser oscillators, but sometimes also includes devices with laser amplifiers, called master oscillator power amplifier (MOPA). An even wider interpretation includes nonlinear devices like optical parametric oscillators and Raman lasers, which also produce laser-like light beams and are usually pumped with a laser, but are strictly speaking not lasers themselves.

Laser technology is at the core of the wider area of photonics, essentially because laser light has a number of very special properties:

• It is usually emitted as a well directed laser beam which due to its high spatial coherence can propagate over long lengths without much divergence (often limited only by diffraction) and can be focused to very small spots, where a high intensity is achieved.
• It often has a very narrow optical bandwidth (high temporal coherence), whereas e.g. most lamps emit light with a very broad optical spectrum. However, there are also broadband lasers, particularly among ultrafast lasers.
• Laser light may be emitted continuously, or alternatively in the form of short or ultrashort pulses, with pulse durations from microseconds down to a few femtoseconds. The temporal concentration of pulse energy – in addition to the potential of strong spatial confinement in a beam focus – allows for even far higher intensities to be generated. Particularly extreme intensity values are used in high-intensity physics.

These properties, which make laser light very interesting for a range of applications, are to a large extent the consequences of the very high degree of spatial and/or temporal coherence of laser radiation. The articles on laser light and laser applications give more details.

The first laser was a pulsed lamp-pumped ruby laser (a kind of solid-state laser), demonstrated by Theodore Maiman in 1960 [2, 3]. In the same year, the first gas laser (a helium–neon laser [5]) and the first laser diode were made. Before this experimental work, Arthur Schawlow, Charles Hard Townes, Nikolay Basov and Alexander Prokhorov had published ground-breaking theoretical work on the operation principles of lasers, and a microwave amplifier and oscillator (maser) had been developed by Townes' group in 1953. The term “optical maser” (MASER = microwave amplification by stimulated amplification of radiation) was initially used, but later replaced with “laser”.

In laser technology, a wide range of optical components such as laser crystals, laser mirrors, polarizers, Faraday isolators and tunable optical filters are used; see the article on laser optics.

How a Laser Works

Basic Principle

A laser oscillator usually comprises an optical resonator (laser resonator, laser cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip, because it experiences some losses, e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy – it needs to be “pumped”, e.g. by injecting light (optical pumping) or an electric current (electrical pumping → semiconductor lasers). The principle of laser amplification is stimulated emission.

Figure 1: Setup of a simple optically pumped solid-state laser. The laser resonator is made of a highly reflecting curved mirror and a partially transmissive flat mirror, the output coupler, which extracts some of the circulating laser light as the useful output. The gain medium is a laser crystal or rod, which is side-pumped, e.g. with light from laser diodes or a flash lamp.

A laser can not operate if the gain is smaller than the resonator losses; the device is then below the so-called laser threshold and only emits some weak luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can reach (or temporarily exceed) the level of the resonator losses.

The exponential growth of optical power in a laser can be extremely fast, and lead to a very high peak power.

If the gain is larger than the losses, the power of the light in the laser resonator rises very rapidly, starting e.g. with low levels of light from fluorescence. Note that the resonator round-trip time is usually very small (e.g. a few nanoseconds, for compact laser types even much less), so that even a small net round-trip gain implies rapid exponential growth of the intracavity power. As high laser powers saturate the gain by extracting energy from the gain medium, the laser power will in the steady state reach a level so that the saturated gain just equals the resonator losses (→ gain clamping). Before reaching this steady state, a laser often undergoes relaxation oscillations (just one aspect of laser dynamics). The threshold pump power is the pump power where the small-signal gain is just sufficient for lasing.

Some fraction of the light power circulating in the resonator is usually transmitted by a partially transparent mirror, the so-called output coupler mirror. The resulting beam constitutes the useful output of the laser. The transmission of the output coupler mirror can be optimized for maximum output power (see also: slope efficiency). In most cases, one has only a single output coupler.

Spatial Coherence of Laser Radiation

How can laser light have such a high degree of spatial coherence?

A high degree of spatial coherence of the laser radiation can be achieved, essentially because the light emission is triggered (stimulated) by the intracavity radiation (i.e., the light circulating in the laser resonator) itself, rather than occurring spontaneously in an uncoordinated fashion. In the stimulated emission process, the laser-active ions are made to emit light in the direction of already existing light, and also with the same optical phase. In effect, the circulating laser light serves to strongly coordinate the emission of many atoms or ions. The resulting amplitude and phase profile of the laser beam is largely determined by the properties of the laser resonator, not usually by the laser gain medium.

As explained above, the spatial coherence is the physical basis for the potential of forming very directed laser beams with low divergence, and for focusing light to very small spots.

Temporal Coherence

Temporal coherence is a different issue, and it has completely different origins. Some laser gain media can emit light only in a narrow spectral range. However, even if that is not the case, a laser often (particularly in continuous-wave operation) emits light only at a precisely defined wavelength or frequency, because the conditions are such that a net zero round-trip gain is possible only for that wavelength, and other wavelengths exhibit a negative net round-trip gain. A laser may be tuned to the exact wanted wavelength (within the emission region of the gain medium) e.g. by using a tunable intracavity bandpass filter, such as a Lyot filter. Again, the mechanism of stimulated emission is of crucial importance: the laser-active ions can be made to emit at exactly the optical frequency of already existing light. The smaller the emission linewidth (i.e., the narrower the optical spectrum of emitted light), the higher the degree of temporal coherence.

In extreme cases, the linewidth of a laser can be forced to values below 1 Hz (with certain means of laser stabilization). This is many orders of magnitude below the mean frequency (hundreds of terahertz). Optical clocks involve such highly stabilized lasers.

Interestingly, even ultrashort pulses can exhibit a very high degree of temporal coherence, in that case involving coherence between subsequent pulses in a regular pulse train. This is related to the formation of a frequency comb as the optical spectrum. While the optical spectrum can then overall be very wide, each comb line can be extremely narrow and well defined in frequency.

Generation of Light Pulses

Some lasers are operated in a continuous fashion, whereas others generate pulses, which can be particularly intense. There are various (very different) methods for pulse generation with lasers, allowing the generation of pulses with durations of microseconds, nanoseconds, picoseconds, or even down a few femtoseconds (→ ultrashort pulses from mode-locked lasers). Often, a laser medium can accumulate some amount of energy over some “pumping” time in order to then release it within a much shorter time.

The optical bandwidth (or linewidth) of a continuously operating laser may be very small when only a single resonator mode can oscillate (→ single-frequency operation). In other cases, particularly for mode-locked lasers, the bandwidth can be very large – in extreme cases, it can span about a full octave. The center frequency of the laser radiation is typically near the frequency of maximum gain, but if the resonator losses are made frequency-dependent, the laser wavelength can be tuned within the range where sufficient gain is available. Some broadband gain media such as Ti:sapphire and Cr:ZnSe allow wavelength tuning over hundreds of nanometers.

Laser Noise

Due to various influences, the output of lasers always contains some noise in properties such as the output power or phase. For pulsed lasers, additional quantities can come into play, for example the timing jitter. For more details, see the article on laser noise.

Types of Lasers

Laser technology is a rather diverse field, utilizing a wide range of very different kinds of laser gain media, optical elements and techniques. Common types of lasers are:

• Semiconductor lasers (mostly laser diodes), electrically (or sometimes optically) pumped, efficiently generating very high output powers (but typically with poor beam quality), or low powers with good spatial properties (e.g. for application in CD and DVD players), or pulses (e.g. for telecom applications) with very high pulse repetition rates. Special types include quantum cascade lasers (for mid-infrared light) and surface-emitting semiconductor lasers (VCSELs, VECSELs and PCSELs). Some of those are also suitable for pulse generation with high powers.
• Solid-state lasers based on ion-doped crystals or glasses (doped insulator lasers), pumped with discharge lamps or laser diodes, generating high output powers, or lower powers with very high beam quality, spectral purity and/or stability (e.g. for measurement purposes), or ultrashort pulses with picosecond or femtosecond durations. Common gain media are Nd:YAG, Nd:YVO₄, Nd:YLF, Nd:glass, Yb:YAG, Yb:glass, Ti:sapphire, Cr:YAG and Cr:LiSAF. A special type of ion-doped glass lasers are:
• Fiber lasers, based on optical glass fibers which are doped with some laser-active ions in the fiber core. Fiber lasers can achieve extremely high output powers (up to kilowatts) with high beam quality, allow for widely wavelength-tunable operation, narrow linewidth operation, etc.
• Gas lasers (e.g. helium–neon lasers, CO₂ lasers, argon ion lasers and excimer lasers), based on gases which are typically excited with electrical discharges. Frequently used gases include CO₂, argon, krypton, and gas mixtures such as helium–neon. Common excimers are ArF, KrF, XeF, and F₂. As far as gas molecules are involved in the laser process, such lasers are also called molecular lasers.

Not very common are chemical and nuclear pumped lasers, free electron lasers and X-ray lasers.

Laser Sources in a Wider Sense

There are some light sources which are not strictly lasers, but are nevertheless often called laser sources:

• In some cases, the term is used for amplifying devices emitting light without an input (excluding seeded amplifiers). An example are X-ray lasers, which are usually superluminescent sources, based on spontaneous emission followed by single-pass amplification. There is then no laser resonator.
• A similar situation occurs for optical parametric generators, where the amplification, however, is not based on stimulated emission; it is parametric amplification based on optical nonlinearities.
• Raman lasers utilize amplification based on stimulated Raman scattering.

Light from such devices can have laser-like properties, such as strongly directional emission, high spatial and temporal coherence and a narrow optical bandwidth.

In other cases, the term laser sources is justified by the fact that the source contains a laser, among other components. This is the case for combinations of lasers and amplifiers (→ master oscillator power amplifier), and also for sources based on nonlinear frequency conversion of laser radiation, e.g. with frequency doublers or optical parametric oscillators.

Safety Aspects

The work with lasers can raise serious safety issues. Some of those are directly related to the laser light, in particular to the high optical intensities achievable, but there are also various other hazards related to laser sources.

See the article on laser safety for details.

Laser Applications

There is an enormously wide range of applications for a great variety of different laser devices. They are largely based on various special properties of laser light, many of which cannot be achieved with any other kind of light sources. Particularly important application areas are laser material processing, optical data transmission and storage and optical metrology. See the article on laser applications for an overview.

Still, many potential laser applications cannot be practically realized so far because lasers are relatively expensive to make – or more precisely, because they are so far mostly made with relatively expensive methods. Most lasers are fabricated in relatively small volumes and with a limited degree of automation. Another aspect is that lasers are relatively sensitive in various respects, for example concerning the precise alignment of optical components, mechanical vibrations and dust particles. Therefore, there is ongoing research and development for finding more cost-effective and robust solutions.

For business success, it is often vital not just to develop lasers with high performance and low cost, but also to identify the best suited applications, or develop lasers which are best suited for particular applications. Also, the knowledge of the application details can be very important. For example, in laser material processing it is vital to know the exact requirements in terms of laser wavelength, beam quality, pulse energy, pulse duration etc. for optimum processing results.

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