A tunable laser is a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

whose

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

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, gas lasers (such as CO₂ and He-Ne lasers), dye lasers (liquid and solid state), transition metal solid-state lasers, semiconductor

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...

and diode lasers, and free electron lasers. Tunable lasers find applications in

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...

, photochemistry,

atomic vapor laser isotope separation Atomic vapor laser isotope separation, or AVLIS, is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions. A similar technology, using molecules instead of atoms, is ...

, 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 nm wavelength transition of a Nd:YAG laser has a linewidth of approximately 120 GHz, or 0.45 nm). 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 nm output line, Nd:YAG has weaker transitions at wavelengths of nm, nm, nm, 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 Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as a ...

- ion lasers, allowing tuning of the laser to a number of lines from the

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...

and

blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...

through to

green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 Nanometre, nm. In subtractive color systems, used in painting and color printing, it is created by ...

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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

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 6 nm, as the laser temperature is changed over ~50 K. As a rule of thumb the wavelength is tuned by 0.08 nm/K for DFB lasers operating in the 1,550 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 Santur Corporation developed, manufactured and commercialized tunable lasers as well as parallel array devices and photonic integrated circuits for the telecommunications industry. It was established in November 2000 in Fremont, California initia ...

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

. *Tunable VCSEL: One of the two mirror stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the nm domain are usually either optically pumped or have an additional optical amplifier built into the device. there is no widely tunable VCSEL commercially available any more for DWDM-system application. It is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser.

Applications

The range of applications of tunable lasers is extremely wide. When coupled to the right filter, a tunable source can be tuned over a few hundreds of nanometers with a spectral resolution that can go from 4 nm to 0,3 nm, depending on the

range. With a good enough isolation (>OD4) tunable source can be used for basic absorption and photoluminescence study. It can be used for solar cells characterisation in a light beam induced current (LBIC) experiment from which external quantum efficiency (EQE) of a device can be mapped. It can also be used for the characterisation of gold

nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 10 ...

s and single-walled

carbon nanotube A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon nan ...

thermopile where a wide tunable range from 400 nm to nm is essential. Tunable sources were recently used for the development of hyperspectral imaging for early detection of retinal diseases where a wide range of wavelength, a small bandwidth and outstanding isolation is crucial in order to achieve an efficient illumination of the entire

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. Tunable source can be a powerful tool for reflection and

transmission spectroscopy Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...

, photobiology, detector calibration, hyperspectral imaging and steady-state pump probe experiment to name only a few.

History

The first true broadly tunable laser was the dye laser in 1966.

F. P. Schäfer F is the sixth letter of the Latin alphabet. F may also refer to: Science and technology Mathematics * F or f, the number 15 in hexadecimal and higher positional systems * ''p'F'q'', the hypergeometric function * F-distribution, a con ...

(ed.), ''Dye Lasers'' (Springer, 1990) Hänsch introduced the first narrow-linewidth tunable laser in 1972. Dye lasers and some vibronic solid-state lasers have extremely large bandwidths, allowing tuning over a range of tens to hundreds of nanometres.Koechner, §2.5, pp66–78. Titanium-doped sapphire is the most common tunable solid-state laser, capable of laser operation from 670 nm to 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.F. J. Duarte and L. W. Hillman (eds.), ''Dye Laser Principles'' (Academic, 1990) Chapter 4 Multiple-prism grating arrangements, in several configurations, as described by Duarte, are used in diode, dye, gas, and other tunable lasers.F. J. Duarte, ''Tunable Laser Optics'', 2nd Ed. (CRC, New York, 2015) Chapter 7.
/ref>