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Tunable Lasers

Definition: lasers the output wavelengths of which can be tuned

Alternative term: wavelength-tunable lasers

More specific term: wavelength-swept lasers

German: abstimmbare Laser

Category: laser devices and laser physics

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A tunable laser (alternative spelling: tuneable laser) is a laser the emission wavelength of which can be tuned (i.e. adjusted) (→ wavelength tuning). That tuning is usually possible during operation, i.e., it does not only mean that a certain wavelength can be permanently set in the factory. Very wide tuning ranges (hundreds of nanometers) are achieved in some cases, while in other cases tuning is possible only over a fraction of a nanometer. Lasers are sometimes called wavelength agile or frequency agile when the tuning can be done with high speed.

Tunable lasers are usually operating in a continuous fashion with a small emission bandwidth, although some Q-switched and mode-locked lasers can also be wavelength-tuned.

The tuning characteristics can be of substantially different kinds:

  • There are single-frequency lasers, where the emission linewidth is very narrow, corresponding to a very well defined wavelength. Some of these lasers can be continuously tuned only over a small range of optical frequencies, while others can be tuned over a frequency range which is much larger than the free spectral range of the laser resonator. Only with relatively sophisticated technology, one can achieve tuning over a large range without mode hopping, i.e., a discontinuous evolution of the optical frequency.
  • Other lasers operate on multiple resonator modes simultaneously, so that their optical spectrum exhibits several or even many spectral lines. In such cases, wavelength tuning usually just means that the envelope of the optical spectrum can be shifted, but with no control of the individual line frequencies.

There are certain lasers which are optimized such that the output wavelength can be periodically and rapidly swept through a substantial range. They are called wavelength-swept lasers and discussed in a separate encyclopedia article. Some of those lasers are not suitable for arbitrary tuning, but only for the mentioned periodic mode.

Note that there are also other kinds of wavelength-tunable light sources such as optical parametric oscillators and sources based on supercontinuum generation. The latter are much more limited in terms of radiance and particularly spectral radiance, but can cover a very wide spectral range.

Widely Tunable Lasers

Some types of lasers offer particularly broad wavelength tuning ranges:

tunable solid-state laser
Figure 1: Setup of a tunable solid-state bulk laser, realized e.g. with a Ti:sapphire laser crystal. The prism pair spatially disperses the different wavelength components, so that the movable slit can be used to shift the wavelength away from that of maximum gain.

Other types of lasers offer tuning ranges spanning a few nanometers to some tens of nanometers:

Some fine tuning, often continuously without mode hops, is possible for other lasers:

For wideband tuning in various spectral regions, optical parametric oscillators (OPOs) can be used. These are actually not lasers, but OPO sources are nevertheless sometimes included with the term tunable laser sources.

Applications of Tunable Lasers

Wavelength-tunable laser sources have many applications, some examples of which are:

  • In laser absorption spectroscopy, a wavelength-tunable laser with narrow optical bandwidth can be used for recording absorption spectra with very high frequency resolution. In a LIDAR system, a laser may be tuned to a wavelength which is specific to a certain substance to be monitored.
  • Various methods of laser cooling require a laser wavelength to be adjusted very precisely at or near some atomic resonance.
  • Tuning to atomic resonances is also used in laser isotope separation. The laser is then tuned to a particular isotope in order to ionize these atoms and subsequently deflect them with an electric field.
  • A tunable laser can be used for device characterization, e.g. of photonic integrated circuits.
  • In optical fiber communications with wavelength division multiplexing, a tunable laser can serve as a spare in the case that one of the fixed-wavelength lasers for the particular channels fails. Even though the cost for a tunable laser is higher, its use can be economical as a single spare laser can work on any transmission channel where it is needed. As the cost of a tunable lasers may not be much higher than for non-tunable ones, tunable lasers are sometimes even used throughout.
  • In optical frequency metrology, it is often necessary to stabilize the wavelength of a laser to a certain reference standard (e.g. a multipass gas cell or an optical reference cavity). This can be accomplished e.g. with an electronic feedback system, which automatically adjusts the laser wavelength.
  • Some interferometers and fiber-optic sensors profit from a wavelength-tunable laser source, e.g. if this makes it possible to remove an ambiguity or to avoid mechanical scanning of an optical path length.


The RP Photonics Buyer's Guide contains 88 suppliers for tunable lasers. Among them:

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[12]F. Mollenauer, J. C. White, and C. R. Pollack, Tunable Lasers, Springer, Berlin (1993)
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(Suggest additional literature!)

See also: wavelength tuning, wavelength-swept lasers, wavelength-tunable light sources, titanium–sapphire lasers, vibronic lasers, dye lasers, optical parametric oscillators, distributed Bragg reflector lasers, external-cavity diode lasers, mode hopping, laser spectroscopy, The Photonics Spotlight 2008-10-03
and other articles in the category laser devices and laser physics


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