TUNABLE LASER AND ITS APPLICATIONS

What are Tunable Lasers?
A tunable laser (alternative spelling: tuneable laser) is a laser the output wavelength of which can be tuned (i.e. adjusted) (? wavelength tuning).Tunable lasers are different to traditional lasers because they can continuously change their emission wavelength, or color, in a given spectral range.
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. In the latter case, it is possible to shift either the envelope of the frequency comb or the lines in the optical spectrum.
DESIGNING A TUNABLE LASER:
In order to make a tunable laser the first element needed is a laser gain medium with a broad gain profile. Once broadband laser emission is achieved in a simple mirror-mirror cavity, lasing must be restricted to a single-transverse mode (TEM00) using a suitable intracavity aperture. Once lasing is restricted to a TEM00 mode then one can proceed to limit emission to a single longitudinal mode (SLM). This is done via the introduction of suitable dispersive elements, such as prisms, gratings, or a combination of these. The dispersive resonator assemblies can provide a very narrow window for intracavity transmission in the frequency domain. A well designed multiple-prism grating assembly can restrict oscillation to a SLM. Wavelength tuning is generally achieved by rotation of either a grating or a mirror.
There are 4 ways of producing a Tunable Lasers:
1)The distributed feedback laser (DFB),
2)The external cavity diode laser,
3)The vertical cavity diode laser
4)The micro electro mechanical system (MEMS) technology.


WHY TUNABLE LASERS?
The primary purpose for manufacturing tunable laser solutions is in reduced manufacturing and operational costs deriving from inventory reduction,there are still a number of potential applications in the telecom area that might be important in the near future.The most important use is in reconfigurable optical-add-drop multiplexers (ROADMs).
The ROADM can vastly reduce the cost of dropping and/or adding a relatively small amount of information from or to the fiber.
SOME OF TUNABLE LASERS USED AND THEIR RANGES
1)Dye lasers: tunable from ~ 320 nm to ~ 1200 nm using various dyes
2)A few solid-state bulk lasers, in particular titanium–sapphire lasers and Cr:ZnSe and Cr:ZnS lasers allow tuning over hundreds of nanometers in the near- and mid-infrared spectral region.
3)Alexandrite lasers: tunable from ~ 701 nm to 818 nm
4)Fiber lasers: tunable in the near infrared depending on the dopant
5)II-VI semiconductor lasers: tunable around 400 nm
6)III-V semiconductor lasers: tunable from the red end of the visible spectrum towards the near infrared using varius semiconductors
7)Quantum cascade lasers (QCLs are semiconductor lasers): tunable from the mid infrared to the far infrared
8)Some free electron lasers can cover enormously broad wavelength ranges, and often in extreme spectral regions.
9)Some compact solid-state bulk lasers such as nonplanar ring oscillators (NPROs, MISERs) allow continuous tuning within their free spectral range of several gigahertz. Tuning may be accomplished by applying stress to the laser crystal via a piezo, or by varying the crystal temperature.

APPLICATIONS
1)In spectroscopy, a wavelength-tunable laser with narrow optical bandwidth can be used for recording transmission or absorption spectra with very high frequency resolution.
2)Various methods of laser cooling require a laser wavelength to be adjusted very precisely at or near some atomic resonance.
3)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.
4)A tunable laser can be used for device characterization, e.g. of photonic integrated circuits.
5)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.