Tunable laser
Encyclopedia
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 laser
s, CO2 lasers, dye laser
s (liquid and solid state), transition metal solid-state laser
s, semiconductor crystal
and diode lasers, and free electron laser
s.
Tunable lasers find applications in spectroscopy
, photochemistry
, atomic vapor laser isotope separation, and optical communications.
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.
s. 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 laser
s, allowing tuning of the laser to a number of lines from the ultraviolet
and blue
through to green
wavelengths.
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.
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:
As of December 2008 there is no widely tunable VCSEL commercially available anymore 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.
. 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.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
whose wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of operation can be altered in a controlled manner. While all laser gain media
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...
allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
s, CO2 lasers, dye laser
Dye laser
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...
s (liquid and solid state), transition metal solid-state laser
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
s, semiconductor crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
and diode lasers, and free electron laser
Free electron laser
A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
s.
Tunable lasers find applications in spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, photochemistry
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
, atomic vapor laser isotope separation, and optical communications.
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 laserNd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
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
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
, to provide selection of a particular longitudinal mode
Longitudinal mode
For the longitudinal mode of conduction of electric currents, see Common modeA longitudinal mode of a resonant cavity is a particular standing wave pattern formed by waves confined in the cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive...
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 mirrorDielectric mirror
A dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
s. If a dispersive
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
element, such as a prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
, 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
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
-ion laser
Ion laser
An ion laser is a gas laser which uses an ionized gas as its lasing medium.Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions...
s, allowing tuning of the laser to a number of lines from the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
through to green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
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
Distributed feedback laser
A distributed feedback laser is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured as a diffraction grating...
(DFB) semiconductor lasers and vertical cavity surface emitting lasers (VCSELs) use periodic distributed Bragg reflector
Distributed Bragg reflector
A distributed Bragg reflector is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation...
(DBR) structures to form the mirrors of the optical cavity. If the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
. 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
Santur Corporation
Santur Corporation was founded during the telecom bubble in November 2000 in order to develop and commercialize tunable lasers for fiber optic telecommunications...
or Nippon Telegraph and Telephone
Nippon Telegraph and Telephone
, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
(NTT Corporation) contain an array of such lasers on a single chip and concatenate the tuning ranges.
Widely tunable lasers
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 IolonIolonIolon Inc. was a manufacturer and designer of tunable lasers and optical devices. Its headquarters were in San Jose, California. Its products included the Apollo line of lasers, as well as optical switches, polarization controllers, tunable filters, spectral monitors, and universal transponders...
. - 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 CorporationSantur CorporationSantur Corporation was founded during the telecom bubble in November 2000 in order to develop and commercialize tunable lasers for fiber optic telecommunications...
- 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.
As of December 2008 there is no widely tunable VCSEL commercially available anymore 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.
History
The first true broadly tunable laser was the dye laserDye laser
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...
. Hänsch introduced the first narrow-linewidth tunable laser.
Dye lasers and some vibronic
Vibronic transition
A vibronic transition denotes the simultaneous change of vibrational and electronic quantum number in a molecule. According to the separability of electronic and nuclear motion in the Born-Oppenheimer approximation, the vibrational transition and electronic transition may be described separately...
solid-state lasers have extremely large bandwidths, allowing tuning over a range of tens to hundreds of nanometres. Titanium-doped sapphire
Ti-sapphire laser
Ti:sapphire lasers are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses...
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
Lyot filter
A Lyot filter, named for its inventor Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths...
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
Multiple-prism dispersion theory
The first description of multiple-prism arrays, and multiple-prism dispersion, was given by Newton in his book Opticks. Prism pair expanders were introduced by Brewster in 1813. A modern mathematical description of the single-prism dispersion was given by Born and Wolf in 1959...
, in several configurations, as described by Duarte
F. J. Duarte
F. J. Duarte is a laser physicist and author/editor of several well-known books on tunable lasers. He introduced the generalized multiple-prism dispersion theory and has discovered various multiple-prism grating oscillator laser configurations...
, are used in diode, dye, gas, and other tunable lasers.
See also
- Diode lasers
- Dye lasers
- Free electron laserFree electron laserA free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
s - Liquid crystal laserLiquid crystal laserA liquid crystal laser is a laser that uses a liquid crystal as the resonator cavity, allowing selection of emission wavelength and polarization from the active laser medium. The lasing medium is usually a dye doped into the liquid crystal...
s - ModelockingModelockingMode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds or femtoseconds ....
- Multiple-prism grating laser oscillators
- Optical parametric amplifierOptical parametric amplifierAn optical parametric amplifier, abbreviated OPA, is a laser light source that emits light of variable wavelengths by an optical parametric amplification process.-Optical parametric generation :...