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. Widely tunable lasers 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: 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. 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 Corporation. Tunable VCSEL: One of the two mirror stacks is movable. To achieve sufficient output power out of a VCSEL structure, lasers in the nm domain are usually either optically pumped or have an additional optical amplifier built into the device. there is no widely tunable VCSEL commercially available any more 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. 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 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