定义：
辐射类似于激光光束的中红外光的光源。

本词条描述的是辐射类似于激光光束的中红外光的光源。这些光源通常包含一个中红外激光器或者更短波长的激光器和非线性频率转换的器件。中红外光谱区域包括从3μm 到8μm 的波长。有些人认为中红外区域包含更短的波长，例如从2μm 开始，因此极大的拓宽了中红外光源。 

中红外光源的典型应用包括气体光谱学和军事上飞机干扰红外线循迹武器。 


量子级联激光器 
量子级联激光器是半导体激光器领域中进来发展较快的激光器。早先的中红外半导体激光器利用的是带间跃迁原理，而量子级联激光器采用的是子带间跃迁。通过改变半导体多层结构，跃迁的光子能量（波长）可在较大范围内变化。

即使结构固定，若采用外腔器件波长调谐的范围也可以很大（有时调谐量大于中心波长的10%）。 
很多量子级联激光器可以工作在室温下，甚至是连续工作，尽管其最佳工作环境是在低温冷却下。这种激光器可以产生脉冲长度远小于1ns的短脉冲，然而峰值功率非常有限。 

铅盐激光器 
在量子级联激光器发展之前，主要是由各种类型的铅盐激光器得到中红外光谱中大部分波长的光。它们通常采用三元铅化合物，例如PbxSn1−xTe或者四元化合物PbxEu1−xSeyTe1−y。决定辐射波长的带隙能量很小，低于0.5eV。 

铅盐激光器需要在低温下工作（通常低于200K，尤其是波长更长时）。它们产生的光功率较低（通常在1mW量级），并且与短波长激光二极管相比，它们的电光转换效率很低。通过改变器件温度，通常可以得到几个纳米的波长调谐范围。 

掺杂绝缘激光器 
只有少数种类的掺杂绝缘固态激光器可以辐射中红外光谱区域的光。例如： 

1. Cr2+:ZnSe（掺铬硒化锌）激光器（以及其它采用类似材料的激光器）可以辐射将近3.5μm波长的光。它们可以在很大范围内调谐，并且输出功率可达几百毫瓦。 
2. Fe2+:ZnSe激光器可以辐射3.7-5.1μm的光。 
3. 基于掺铒光纤或者掺钬的氟化物光纤的光纤激光器可以辐射3μm波长的光。 

选取激光器晶体和玻璃需要具有较低光子能量的材料，否则激光跃迁会由于多光子跃迁的存在而发生淬灭。 

气体和化学激光器 
只有很少的气体激光器会辐射中红外波长的光。例如氦氖激光器辐射波长为3.391μm。 
氟化氘化学激光器辐射波长在3.8μm附近。它们通常用于军事用途。 

基于差频产生原理的光源 
利用非线性晶体中的差频产生（difference frequency generation, DFG）过程可以得到很大范围的中红外光，信号光为两个近红外光束。例如，可以采用1064 nm的Nd:YAG 激光器和波长可调谐的1.5μm掺铒光纤激光器，然后将二者的输出光在周期性极化的铌酸锂（LiNbO3）晶体中混合。
当光纤激光器调谐到1530nm和1580nm之间时，中红外输出光波长范围在3258nm和3993nm之间。（该范围与光纤激光器的变化范围相对应，但是长波长时需要激光器具有更大的波长变化。） 
对于连续光工作的激光器，非线性转化效率较低，并且产生的输出功率通常低于1 mW， 然而该功率足够用于光谱学中。采用脉冲光束（例如调Q激光器），可以得到高很多的输出功率，当然这也需要两激光更精确的混合。 
近年来，已经可以制备定向砷化镓晶体（GaAs），因此可以实现差频产生中的准相位匹配得到很宽的输出波长范围。 

光参量振荡器，放大器和发生器 
另一种非线性频率转换是先采用一个近红外激光器来泵浦光参量振荡器（OPO），放大器（OPA）或者发生器（OPG）。产生的闲散光就位于中红外光谱区域。例如： 

1. 锁模的1064nm皮秒Nd:YVO4激光器可通过晶体泵浦OPO，输出的闲散光波长为4 μm甚至4.5 μm，受限于当波长边长时闲散光的吸收也变大。这种OPO通常具有一个共振的信号波长，在该处闲散光之间在非线性晶体后耦合出去。 
2. 调Q激光器通常用来泵浦纳秒OPO得到中红外光。常用的晶体材料为磷化锌锗（ZGeP, ZnGeP2），硫镓银和硒镓银（AgGaS2, AgGaSe2），硒化镓（GaSe）和硒化镉（CdSe）。由于这些材料在1μm区域都不是透明的，通常需要采用串联OPO：第一个OPO将1μm的激光转化成更长的波长，通常用来泵浦中红外OPO。信号光和闲散光都在中红外区域。 

这一器件很容易得到能量为几十毫焦的脉冲。输出波长可在几百纳米范围内调谐。 

其它光源 
其它一些不常用的中红外光源为： 

1. 自由电子激光器 
2. 倍频CO2激光器

Definition: light sources emitting mid-infrared radiation in the form of a laser-like beam

More general terms: laser sources, mid-infrared light sources

This article discusses those sources of mid-infrared light which emit laser-like beams. They may either contain a mid-infrared (mid-IR) laser or some shorter-wavelength laser combined with means for nonlinear frequency conversion. The mid-infrared spectral range according to ISO 20473:2007 is the wavelengths range from 3 μm to 50 μm.

Typical Applications

A typical application of mid-infrared sources is in laser absorption spectroscopy of trace gases (e.g. medical diagnostics and remote sensing in environmental monitoring). Here, one utilizes the strong and characteristic absorption bands of many molecules (serving as “molecular fingerprints”) in the mid-infrared spectral region. While one can also address some of them molecules via overtone absorption lines in the near infrared, where laser sources are easier to make, it is advantageous to use the strong fundamental absorption lines in the mid-infrared for maximum sensitivity.

There are also applications in infrared imaging, where one typically exploits the advantage that mid-infrared light can penetrate deeper into materials and generally exhibits much less scattering. Imaging may also be realized in the form of hyperspectral imaging, providing for spectral information for each pixel (or voxel).

Applications in non-metal laser material processing also become more and more practical due to the ongoing development of more powerful mid-IR laser sources, e.g. in the form of fiber lasers (see below). Typically, one exploits strong infrared absorption of certain materials, for example polymeric films, for selectively removing materials. For example, transparent conducting films made of indium tin oxide (ITO), as used for electrodes in electronic and optoelectronic devices, need to be structured by selective laser ablation. Another example is the very controlled stripping of coatings from optical fibers. The required power levels are often much lower than those e.g. for laser cutting.

High-power mid-IR sources are also used by the military for directional infrared countermeasures against heat-seeking missiles. Besides rather high output powers, which are suitable for blinding infrared cameras, broad spectral coverage within the atmospheric transmission bands (around 3–4 μm and 8–13 μm) is required, preventing the protection of infrared detectors with simple notch filters..

The mentioned atmospheric transmission windows can also be utilized for free-space optical communications with directed beams, e.g. from quantum cascade lasers.

In some cases, ultrashort pulses in the mid-IR are required, as can be generated with mode-locked lasers. For example, one may use mid-IR frequency combs in laser spectroscopy, or utilize High Peak intensities of ultra short pulses for laser micromachining.

Note that very different types of mid-infrared lasers exist, which are suitable for very different types of applications. For example, some can generate high output powers, which makes them suitable for laser material processing, while others have low output powers but wavelength-tunable narrow-linewidth output, which is suitable for applications in spectroscopy.

Types of Mid-infrared Lasers

Quantum Cascade Lasers

Quantum cascade lasers represent a relatively recent development in the area of semiconductor lasers. Whereas earlier mid-infrared semiconductor lasers were based on interband transitions, quantum cascade lasers utilize intersubband transitions. The photon energy (and thus the wavelength) of transitions can be varied in a wide range by engineering the details of the semiconductor layer structure. Even for a fixed design, some significant range for wavelength tuning (sometimes more than 10% of the center wavelength) can be covered with external-cavity devices.

Many quantum cascade lasers can be operated at room temperature, even continuously, although the best performance values are achieved for cryogenic cooling. The generation of short pulses with durations far below 1 ns is possible, although with fairly limited peak powers.

The main application area of quantum cascade lasers is in optical spectroscopy, for example in the form of laser absorption spectroscopy with the purpose of trace gas detection. Due to the very wide spectral coverage in combination with a relatively narrow linewidth, one can make sensitive instruments for the detection of a wide range of molecules. That is relevant in areas like environmental monitoring and medical diagnostics (e.g. for breath analysis).

It is also possible to use quantum cascade lasers for free-space optical communications with directed beams.

For more details, see the article on quantum cascade lasers.

Lead Salt Lasers

Before quantum cascade lasers were developed, large parts of the mid-infrared spectrum were accessed with various types of lead salt lasers. These are typically based on ternary lead compounds such as Pb_xSn_1−xTe or with quaternary compounds like Pb_xEu_1−xSe_yTe_1−y. The band gap energy, which determines the emission wavelength, is fairly small – below 0.5 eV – as required for long-wavelength emission.

Unfortunately, the small bandgap energy also leads a substantial formal excitation of carriers at room temperature. Therefore, lead salt lasers generally need to be operated at cryogenic temperatures (normally well below 200 K, particularly for the longer wavelengths). They produce only low power levels (typically of the order of 1 mW), and their wall-plug efficiency is very low compared with that of shorter-wavelength laser diodes. Wavelength tuning over a few nanometers is normally possible via the device temperature.

Nowadays, lead salt lasers have largely been replaced with other types of lasers, for example quantum cascade lasers.

Doped Insulator Bulk Lasers

Only a few types of doped insulator solid-state lasers emit in the mid-infrared spectral region. Some examples are:

• Cr²⁺:ZnSe (chromium-doped zinc selenide) lasers (and some lasers with similar materials) can emit up to roughly 3.5 μm. They are broadly tunable.
• Fe²⁺:ZnSe lasers can emit at 3.7–5.1 μm.

The choice of laser crystals and glasses is limited to those with fairly low phonon energies, because otherwise the laser transition would be quenched by multi-phonon transitions. The output powers are tentatively lower than those for common near-infrared lasers, but more than 1 W is possible with careful optimization.

Depending on the spectral region, such lasers often cannot work with ordinary air in the laser resonator. One may have to use dry nitrogen, for example, or some other guys, in order to avoid the relevant absorption lines, which can be fairly strong in the mid-IR.

Fiber Lasers

Fiber lasers based on erbium-doped fluoride fibers (or other doped mid-infrared fibers) can emit at wavelengths e.g. around 2.8–2.9 μm. Similarly, holmium-doped fiber lasers have been developed which emit around 3 μm [4, 9] and at 3.9 μm [3, 25], in addition to the more common region of 2–2.2 μm. Besides, one can using dysprosium-doped fluoride fiber for laser emission at 3.24 μm [30]. In some cases, it is necessary to use additional rare-earth dopants which serve to depopulate the lower laser level via energy transfers, because the used laser transition would otherwise be self-terminating.

The common silica fibers cannot be used in the mid-infrared. First of all, they exhibit strong absorption at such long wavelengths. Second, the high phonon energy would allow strong multi-phonon transitions to bypass the laser transitions. Both problems can be solved by using suitable other glasses – usually, fluoride glasses or chalcogenide glasses. See the article on mid-infrared fibers for more details.

In comparison with bulk lasers (see above), fiber lasers can be more easily operated on “difficult” laser transitions, e.g. requiring high populations of metastable states. They are also less prone to thermal effects such as thermal lensing. Therefore, higher average output powers are possible – often multiple watts or even well above 10 W when using double-clad fibers. For industrial use, such devices should be realized in all-fiber technology [19], i.e., without free-space sections in the resonator which introduce sensitive alignment and the risk of problems with dust particles. Nevertheless, commercial mid-IR fiber lasers have been developed which can be used in practical applications like laser material processing. Here, one exploits the high absorption of various materials in the mid-infrared.

It is also possible to shift light to longer wavelengths with a Raman laser based on a mid-IR fiber [14, 16, 17].

Many mid-IR fiber lasers operate in continuous-wave operation. Besides, one can realize Q-switched lasers or use fiber amplifiers for nanosecond light pulses with substantial pulse energy in the mid-IR. Also, it has become possible to construct mode-locked fiber lasers in the mid-infrared, e.g. based on Er³⁺-doped fluoride glass fiber [20, 21].

Another possibility to reach the mid-infrared with fiber lasers is to use near-IR fiber lasers in conjunction with difference frequency generation (see below).

Gas and Chemical Lasers

Only few gas lasers emit in the mid-infrared region. An example is the helium–neon laser emitting at 3.391 μm with relatively low output power. Much higher powers are available from CO₂ lasers, typically at 10.6 μm, but also at various other wavelengths such as 10.25 μm and 9.3 μm.

Deuterium fluoride chemical lasers can emit very high powers around 3.8 μm wavelength. They are used for some military purposes.

Sources Based on Difference Frequency Generation

A wide wavelength range in the mid-infrared region can be covered by difference frequency generation (DFG) in a nonlinear crystal, starting with two near-infrared beams. For example, one may use a 1064-nm Nd:YAG laser and wavelength-tunable 1.5-μm erbium-doped fiber laser and mix their outputs in a periodically poled lithium niobate (LiNbO₃) crystal. When the fiber laser is tuned between 1530 nm and 1580 nm, for example, the mid-infrared output covers the range from 3493 nm to 3258 nm. (That range corresponds to the same variation of optical frequency as that of the fiber laser, but at long wavelengths this corresponds to a larger wavelength range.)

For continuously operating lasers, the nonlinear conversion efficiency is typically quite low, and the generated output power is often even below 1 mW, which however is often sufficient for spectroscopic investigations. Much higher outputs are possible with pulsed beams, e.g., from Q-switched lasers, which of course need to be synchronized precisely.

Recently, it has become possible to fabricate orientation-patterned gallium arsenide (GaAs), which allows one to obtain quasi-phase matching for difference frequency generation with a very wide range of output wavelengths.

Optical Parametric Oscillators, Amplifiers and Generators

Another option for nonlinear frequency conversion is to start with a single near-infrared laser and pump an optical parametric oscillator (OPO), amplifier (OPA) or generator (OPG). The generated idler wave can then be in the mid-infrared spectral region. Some examples:

• Q-switched lasers are often used for pumping nanosecond OPOs reaching far into the mid-infrared region. Common crystal materials for such applications are zinc germanium diphosphide (ZGP, ZnGeP₂), silver gallium sulfide and selenide (AgGaS₂, AgGaSe₂), gallium selenide (GaSe), and cadmium selenide (CdSe). As many of these materials are not transparent in the 1-μm region, one often has to use tandem OPOs: a first OPO converts the 1-μm laser radiation to a longer wavelength which is then used to pump the actual mid-infrared OPO. Both signal and idler of the latter can be in the mid-infrared spectral region.
• A mode-locked picosecond Nd:YVO₄ laser at 1064 nm can be used for synchronous pumping of an OPO with a LiNbO₃ crystal, allowing idler outputs up to 4 μm or even 4.5 μm, with the limit set by the increasing idler absorption at long wavelengths. Such an OPO will usually have a resonant signal wavelength, whereas the idler wave is directly coupled out after the nonlinear crystal.

Such devices can easily generate pulses with energies of tens of millijoules. The output wavelength may be tuned over hundreds of nanometers.

Supercontinuum Sources

There are sources based on supercontinuum generation, spanning a substantial part of the mid-IR region. Such sources can be based on certain mid-infrared fibers, through which intense light pulses are sent, such that strong nonlinear interactions occur.

If some tunable narrowband light is needed, one may extract the wanted spectral components from the broadband output with a tunable bandpass filter. In other cases, one utilizes the full optical spectrum. An example is optical coherence tomography (OCT). This is often done in shorter wavelength regions, but mid-IR light gives one the advantage of deeper penetration into materials with less scattering.

Miniature Mid-IR Sources

There are various attempts to develop photonic integrated circuits for mid-IR applications, e.g. based on the silicon photonics platform [26]. Unfortunately, it is not easy to realize mid-IR sources on chips, but various possible routes are investigated. For example, one may integrate light sources based on other semiconductors, although it is technically difficult, e.g. involving flip-chip bonding techniques. Another possibility is to integrate black body emitters (→ thermal radiation) or luminescent materials, although that does not lead to spatially coherent radiation.

Other approaches are based on nonlinear frequency conversion, utilizing the Kerr nonlinearity for four-wave mixing or stimulated Raman scattering. With micro-resonators, one can also generate frequency combs.

Other Sources

Some less frequently used mid-infrared source are:

• free electron lasers
• frequency-doubled CO₂ lasers

Required Optics

Mid-infrared laser sources require special optical elements working at the relevant long wavelengths; see the article on infrared optics.

Bibliography