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Tunable laser

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, CO₂ lasers, dye lasers (liquid and solid state), transition metal solid-state lasers, semiconductor diode lasers, and free electron lasers[1]. Tunable lasers find applications in spectroscopy, photochemistry 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 1064 nm wavelength transition of a Nd:YAG laser has a linewidth of approximately 120 GHz, corresponding to a 0.45 nm wavelength range[2]). 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 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[3]. 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.

Broadband tuning

Some types of laser have an inherently large linewidth, and thus can be continuously tuned over a significant wavelength range by modification of the laser's cavity.

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, thermal expansion 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 8 nm, as the laser temperature is changed over ~50 K. Such lasers are commonly used in optical communications applications such as DWDM-systems to allow adjustment of the signal wavelength.

The first true broadly tunable laser was the dye laser [4] [5]. Dye lasers and some vibronic solid-state lasers have extremely large linewidths, allowing tuning over a range of tens to hundreds of nanometres[6]. 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[7].