A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.
There are many types and categories of tunable lasers. They exist in the gas, liquid, and solid state. Among the types of tunable lasers are excimer lasers, CO2 lasers, dye lasers (liquid and solid state), transition metal solid-state lasers, semiconductor crystal and diode lasers, and free electron lasers. Tunable lasers find applications in spectroscopy, photochemistry, atomic vapor laser isotope separation, and optical communications.
Types of tunability 
Single line tuning 
Since no real laser is truly monochromatic, all lasers can emit light over some range of frequencies, known as the linewidth of the laser transition. In most lasers, this linewidth is quite narrow (for example, the 1064 -nm wavelength transition of a Nd:YAG laser has a linewidth of approximately 120 GHz, corresponding to a 0.45 -nm wavelength range). Tuning of the laser output across this range can be achieved by placing wavelength-selective optical elements (such as an etalon) into the laser's optical cavity, to provide selection of a particular longitudinal mode of the cavity.
Multi-line tuning 
Most laser gain media have a number of transition wavelengths on which laser operation can be achieved. For example, as well as the principal 1064 nm output line, Nd:YAG has weaker transitions at wavelengths of 1052 nm, 1074 nm, 1112 nm, 1319 nm, and a number of other lines. Usually, these lines do not operate unless the gain of the strongest transition is suppressed, e.g., by use of wavelength-selective dielectric mirrors. If a dispersive element, such as a prism, is introduced into the optical cavity, tilting of the cavity's mirrors can cause tuning of the laser as it "hops" between different laser lines. Such schemes are common in argon-ion lasers, allowing tuning of the laser to a number of lines from the ultraviolet and blue through to green wavelengths.
Narrowband tuning 
For some types of lasers the laser's cavity length can be modified, and thus they can be continuously tuned over a significant wavelength range. Distributed feedback (DFB) semiconductor lasers and vertical cavity surface emitting lasers (VCSELs) use periodic distributed Bragg reflector (DBR) structures to form the mirrors of the optical cavity. If the temperature of the laser is changed, the index change of the DBR structure causes a shift in its peak reflective wavelength and thus the wavelength of the laser. The tuning range of such lasers is typically a few nanometres, up to a maximum of approximately 4 nm, as the laser temperature is changed over ~50 K. As a rule of thumb the wavelength is tuned by 0.08 nm/°C for DFB lasers operating in the 1550 nm wavelength regime. Such lasers are commonly used in optical communications applications such as DWDM-systems to allow adjustment of the signal wavelength. To get wideband tuning using this technique, some such as Santur Corporation or Nippon Telegraph and Telephone (NTT Corporation)  contain an array of such lasers on a single chip and concatenate the tuning ranges.
Widely tunable lasers 
Sample Grating Distributed Bragg Reflector lasers (SG-DBR) have a much larger tunable range, by the use of vernier tunable Bragg mirrors and a phase section, a single mode output range of >50 nm can be selected. Other technologies to achieve wide tuning ranges for DWDM-systems  are:
- External Cavity Lasers using a MEMS structure for tuning the cavity length, such as devices commercialized by Iolon.
- External Cavity Lasers using multiple-prism grating arrangements for wide-range tunability.
- DFB Laser Arrays based on several thermal tuned DFB lasers: Coarse tuning is achieved by selecting the correct laser bar. Fine tuning is then done thermally, such as devices commercialized by Santur Corporation
- Tunable VCSEL: One of the two mirror stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the 1550 nm domain are usually either optically pumped or have an additional optical amplifier built into the device.
It is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser.
The first true broadly tunable laser was the dye laser. Hänsch introduced the first narrow-linewidth tunable laser. Dye lasers and some vibronic solid-state lasers have extremely large bandwidths, allowing tuning over a range of tens to hundreds of nanometres. Titanium-doped sapphire is the most common tunable solid-state laser, capable of laser operation from 670 nm to 1100 nm wavelength. Typically these laser systems incorporate a Lyot filter into the laser cavity, which is rotated to tune the laser. Other tuning techniques involve diffraction gratings, prisms, etalons, and combinations of these. Multiple-prism grating arrangements, in several configurations, as described by Duarte, are used in diode, dye, gas, and other tunable lasers.
See also 
- Diode lasers
- Dye lasers
- Free electron lasers
- Liquid crystal lasers
- Multiple-prism grating laser oscillators
- Optical parametric amplifier
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- Koechner, §2.3.1, p53.
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- See photograph 3 at http://spie.org/x39922.xml
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- F. J. Duarte and L. W. Hillman (eds.), Dye Laser Principles (Academic, 1990) Chapter 4
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