Mid-infrared Laser Sources

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.

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.

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.

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 and can easily produce hundreds of milliwatts of output power.
• Fe²⁺:ZnSe lasers can emit at 3.7–5.1 μm.
• Fiber lasers based on erbium-doped or holmium-doped fluoride fibers (or other doped mid-infrared fibers) can emit at wavelengths e.g. around 3 μm. In other case, near-IR fiber lasers are used in conjunction with difference frequency generation (see below).

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.

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.

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.

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:

• 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.
• 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.

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 where the mid-IR region is spanned. This can be based on certain mid-infrared fibers, through which intense light pulses are sent, such that strong nonlinear interactions occur. One may then extract the wanted spectral components from the broadband output – for example, with a tunable bandpass filter to obtain a broadly tunable source.

Some less frequently used mid-infrared source are:

• free electron lasers
• frequency-doubled CO₂ lasers

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

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