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Mid-infrared Laser Sources

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

German: Laserquellen im mittleren Infrarot

Categories: lasers, nonlinear optics, photonic devices

How to cite the article; suggest additional literature

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 is understood to include wavelengths from 3 μm to 8 μm. Note that some authors consider the mid-infrared region to begin at shorter wavelengths such as 2 μm, thus greatly expanding the range of mid-infrared sources.

Typical applications of mid-infrared sources are the spectroscopy of trace gases and military countermeasures for airplanes against heat-seeking missiles.

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.

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 bandgap energy, which determines the emission wavelength, is fairly small – below 0.5 eV.

Lead salt lasers 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.

Doped Insulator Lasers

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

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.

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.

Deuterium fluoride chemical lasers can emit 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:

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

Other Sources

Some less frequently used mid-infrared source are:

Bibliography

[1]J. F. Butler et al., “Properties of the PbSe diode laser”, IEEE J. Quantum Electron. 1 (1), 4 (1965)
[2]J. Chen et al., “Single spatial mode room temperature operated 3.15μm diode lasers”, Electron. Lett. 46 (5), 367 (2010)
 [3]S. Mirov et al., “Progress in Cr2+ and Fe2+ doped mid-IR laser materials”, Laser & Photon. Rev. 4 (1), 21 (2010)
[4]D. G. Winters et al., “Subpicosecond fiber-based soliton-tuned mid-infrared source in the 9.7–14.9 μm wavelength region”, Opt. Lett. 35 (13), 2179 (2010)
[5]E. Lippert et al., “A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator”, Opt. Express 18 (25), 26475 (2010)
[6]J. Li et al., “High-power diode-pumped fiber laser operating at 3 μm”, Opt. Lett. 36 (18), 3642 (2011)
[7]T. W. Neely et al., “High-power broadband laser source tunable from 3.0 μm to 4.4 μm based on a femtosecond Yb:fiber oscillator”, Opt. Lett. 36 (20), 4020 (2011)
[8]M. Ebrahim-Zadeh and K. Vodopyanov, “Mid-infrared coherent sources and applications: introduction”, J. Opt. Soc. Am. B 33 (11), MIC1 (2016)

(Suggest additional literature!)

See also: infrared light, quantum cascade lasers, nonlinear frequency conversion, sum and difference frequency generation, orientation-patterned semiconductors, spectroscopy
and other articles in the categories lasers, nonlinear optics, photonic devices

In the RP Photonics Buyer's Guide, 48 suppliers for mid-infrared laser sources are listed.

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