Настраиваемые лазеры: типы, принципы и применение.

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 lasers, gas lasers (such as CO2 and He Ne lasers), dye lasers (liquid and solid state), transition metal solid state lasers, semiconductor crystal and diode lasers, and free electron lasers. Tunable lasers find applications in spectroscopy, photochemistry, atomic vapor laser isotope separation, and optical communications.

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.

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 ion lasers, allowing tuning of the laser to a number of lines from the ultraviolet and blue through to green wavelengths.

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 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 or Nippon Telegraph and Telephone (NTT Corporation) contain an array of such lasers on a single chip and concatenate the tuning ranges.

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 wavelength 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 nanoparticles and single walled carbon nanotube 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. Tunable source can be a powerful tool for reflection and transmission spectroscopy, photobiology, detector calibration, hyperspectral imaging and steady state pump probe experiment to name only a few.

The first true broadly tunable laser was the dye laser in 1966. 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. 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. Multiple prism grating arrangements, in several configurations, as described by Duarte, are used in diode, dye, gas, and other tunable lasers.