Editing Tunable laser (section)

==Types of tunability==

===Single line tuning===
No real laser is truly [[monochromatic#In physics|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 {{formatnum:1064}}&nbsp;nm wavelength transition of a [[Nd:YAG laser]] has a linewidth of approximately 120&nbsp;GHz, or 0.45&nbsp;nm<ref>Koechner, §2.3.1, p49.</ref>). 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 {{formatnum:1064}}&nbsp;nm output line, Nd:YAG has weaker transitions at wavelengths of {{formatnum:1052}}&nbsp;nm, {{formatnum:1074}}&nbsp;nm, {{formatnum:1112}}&nbsp;nm, {{formatnum:1319}}&nbsp;nm, and a number of other lines.<ref>Koechner, §2.3.1, p53.</ref> Usually, these lines do not operate unless the gain of the strongest transition is suppressed, such as by use of wavelength-selective [[dielectric mirror]]s. If a [[dispersion (optics)|dispersive]] element, such as a [[prism (optics)|prism]], is introduced into the optical cavity, tilting the cavity's mirrors can cause tuning of the laser as it "hops" between different laser lines. Such schemes are common in [[argon]]-[[ion laser]]s, 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 laser|Distributed feedback]] (DFB) [[semiconductor laser]]s and [[vertical cavity surface emitting laser|vertical-cavity surface-emitting laser]]s (VCSELs) use periodic [[distributed Bragg reflector]] (DBR) structures to form the mirrors of the optical cavity. If the [[temperature]] of the laser is changed, then 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&nbsp;nm, as the laser temperature is changed over ~50 [[Kelvin|K]]. As a rule of thumb, the wavelength is tuned by 0.08&nbsp;nm/K for DFB lasers operating in the 1,550&nbsp;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)<ref>{{cite journal | last1 = Tsuzuki | first1 = K. | last2 = Shibata | first2 = Y. | last3 = Kikuchi | first3 = N. | last4 = Ishikawa | first4 = M. | last5 = Yasui | first5 = T. | last6 = Ishii | first6 = H. | last7 = Yasaka | first7 = H. | year = 2009 | title =  Full C-Band Tunable DFB Laser Array Copackaged with InP Mach–Zehnder Modulator for DWDM Optical Communication Systems| journal =   IEEE Journal of Selected Topics in Quantum Electronics| volume = 15 | issue = 3 | pages = 521–527 | doi=10.1109/jstqe.2009.2013972| bibcode = 2009IJSTQ..15..521T | s2cid = 27207596 }}</ref> contain an array of such lasers on a single chip and concatenate the tuning ranges.

===Widely tunable lasers===
[[File:Diode laser.jpg|thumb|right|250px|A typical laser diode. When mounted with external optics, these lasers can be tuned mainly in the red and near-infrared.]]
Sample Grating Distributed Bragg Reflector lasers (SG-DBR) have a much larger tunable range; by the use of vernier-tunable [[Bragg mirror]]s and a phase section, a single-mode output range of > 50&nbsp;nm can be selected. Other technologies to achieve wide tuning ranges for [[DWDM]]-systems<ref>[http://www.lightreading.com/document.asp?doc_id=26332 Tunable Lasers at Lightreading]</ref> 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.<ref>P. Zorabedian, Tunable external-cavity semiconductor lasers, in ''Tunable Lasers Handbook'', F. J. Duarte, Ed. (Academic, New York, 1995) Chapter 8.</ref>
*DFB laser arrays based on several thermal tuned DFB lasers, in which coarse tuning is achieved by selecting the correct laser bar. Fine tuning is then done thermally, such as in devices commercialized by [[Santur Corporation]].
*Tunable VCSELs, in which one of the two mirror stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the {{formatnum:1550}}&nbsp;nm domain are usually either optically pumped or have an additional optical amplifier built into the device.

Rather than placing the resonator mirrors at the edges of the device, the mirrors in a VCSEL are located on the top and bottom of the semiconductor material. Somewhat confusingly, these mirrors are typically DBR devices. This arrangement causes light to "bounce" vertically in a laser chip, so that the light emerges through the top of the device, rather than through the edge. As a result, VCSELs produce beams of a more circular nature than their cousins and beams that do not diverge as rapidly.<ref>{{Cite web |title=Optoelectronics, Frequency Changing |url=https://studedu.org/s1266t1.html |access-date=2024-03-07 |website=studedu.org}}</ref>

{{As of|2008|12}}{{Update inline|date=November 2024}}, there is no widely tunable VCSEL commercially available any more for [[DWDM]]-system application.{{Citation needed|date=September 2009}}

It is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser.<ref>[http://spie.org/x39922.xml See photograph 3 at http://spie.org/x39922.xml]</ref>

===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<ref>[http://www.photonetc.com/tunable-laser-source PhotonEtc: Tunable Laser Source from 400nm to 2300nm].</ref><ref>{{usurped|1=[https://web.archive.org/web/20100816065812/http://www.leukos-systems.com/spip.php?rubrique23 Leukos : White light compact supercontinuum systems]}}.</ref><ref>[http://www.fianium.com/supercontinuum.htm Fianium : Powerful WhiteLase Supercontinuum Sources].</ref> with a spectral resolution that can go from 4&nbsp;nm to 0.3&nbsp;nm, depending on the [[wavelength]] range. With a good enough isolation (>OD4), tunable sources can be used for basic [[absorption (electromagnetic radiation)|absorption]] and [[photoluminescence]] studies. They can be used for solar cells characterisation in a light-beam-induced current (LBIC) experiment, from which the [[external quantum efficiency]] (EQE) of a device can be mapped.<ref>{{cite journal |author=L. Lombez|title= Micrometric investigation of external quantum efficiency in microcrystalline CuInGa(S,Se)<sub>2</sub> solar cells |journal=Thin Solid Films |pages=32–36 |date=2014 |doi=10.1016/j.tsf.2014.06.041 |bibcode= 2014TSF...565...32L |display-authors=etal |volume=565}}</ref> They can also be used for the characterisation of gold [[nanoparticle]]s<ref>{{cite journal |author=S. Patskovsky|title= Wide-field hyperspectral 3D imaging of functionalized gold nanoparticles targeting cancer cells by reflected light microscopy|journal=Journal of Biophotonics |pages= 401–407|date=2014 |doi= 10.1002/jbio.201400025|display-authors=etal |volume=8|issue= 5 |pmid=24961507|s2cid= 6797985}}</ref> and single-walled [[carbon nanotube]] [[thermopile|thermopiles]],<ref>{{cite journal |vauthors=St-Antoine B, etal |title= Single-Walled Carbon Nanotube Thermopile For Broadband Light Detection |journal=Nano Letters |volume= 11 |issue= 2 |pages=609–613 |date=2011 |doi= 10.1021/nl1036947|bibcode= 2011NanoL..11..609S |pmid=21189022}}</ref> where a wide tunable range from 400&nbsp;nm to {{formatnum:1000}}&nbsp;nm is essential. Tunable sources were recently{{When|date=November 2024}} used for the development of [[hyperspectral imaging]] for early detection of retinal diseases where a wide range of wavelengths, a small bandwidth, and outstanding isolation is needed to achieve efficient illumination of the entire [[retina]].<ref>{{cite journal |vauthors=Shahidi AM, etal |title= Regional variation in human retinal vessel oxygen saturation |journal=Exp Eye Res |pages=143–7 |date=2013 |doi=10.1016/j.exer.2013.06.001 |volume=113 |pmid=23791637}}</ref><ref>[http://www.photonetc.com/retinal-imaging Tunable Lasers For Retinal Imaging].</ref> Tunable sources can be a powerful tool for [[reflectivity|reflection]] and [[transmission spectroscopy]], [[photobiology]], detector calibration, hyperspectral imaging, and [[steady-state]] pump probe experiments, to name only a few.

===History===
The first true broadly tunable laser was the [[dye laser]] in 1966.<ref name=schafer90>[[F. P. Schäfer]] (ed.), ''Dye Lasers'' (Springer, 1990)</ref><ref>F. J. Duarte and L. W. Hillman (eds.), ''Dye Laser Principles'' (Academic, 1990)</ref> [[Theodor W. Hänsch|Hänsch]] introduced the first narrow-linewidth tunable laser in 1972.<ref name=hansch72>{{cite journal |last1= Hänsch |first1= T. W. |year= 1972 |title= Repetitively Pulsed Tunable Dye Laser for High Resolution Spectroscopy |journal= Appl. Opt. |volume= 11 |issue= 4|pages= 895–898 |doi=10.1364/ao.11.000895 |pmid= 20119064 |bibcode= 1972ApOpt..11..895H}}</ref> Dye lasers and some [[vibronic spectroscopy|vibronic]] solid-state lasers have extremely large bandwidths, allowing tuning over a range of tens to hundreds of nanometres.<ref>Koechner, §2.5, pp66–78.</ref> [[Ti-sapphire laser|Titanium-doped sapphire]] is the most common tunable solid-state laser, capable of laser operation from 670&nbsp;nm to {{formatnum:1100}}&nbsp;nm wavelengths.<ref name="steele91">{{cite journal |doi=10.1364/OL.16.000399|title=Broadly tunable high-power operation of an all-solid-state titanium-doped sapphire laser system |year=1991 |last1=Steele |first1=T. R. |last2=Gerstenberger |first2=D. C. |last3=Drobshoff |first3=A. |last4=Wallace |first4=R. W. |journal=Optics Letters |volume=16 |issue=6 |pages=399–401 |pmid=19773946 |bibcode=1991OptL...16..399S }}</ref> 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.<ref name=duarte90>F. J. Duarte and L. W. Hillman (eds.), ''Dye Laser Principles'' (Academic, 1990) Chapter 4</ref> [[Multiple-prism dispersion theory|Multiple-prism grating arrangements]], in several configurations, as described by [[F. J. Duarte|Duarte]], are used in diode, dye, gas, and other tunable lasers.<ref name=duarte15>[http://www.tunablelaseroptics.com F. J. Duarte, ''Tunable Laser Optics'', 2nd Ed. (CRC, New York, 2015) Chapter 7.]</ref>