Mid-infrared Laser 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.
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..
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 PbxSn1−xTe or with quaternary compounds like PbxEu1−xSeyTe1−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
- 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 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  or somewhat longer wavelengths . 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 , 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.
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 . 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.
Mid-infrared laser sources require special optical elements working at the relevant long wavelengths; see the article on infrared optics.
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See also: infrared light, quantum cascade lasers, nonlinear frequency conversion, sum and difference frequency generation, orientation-patterned semiconductors, laser absorption spectroscopy, infrared optics
and other articles in the categories nonlinear optics, photonic devices, laser devices and laser physics