Fiber lasers with the distributed feedback (DF) offer unique properties. Distributed Feedback laser is a laser where the whole resonator consists of a periodic structure, which acts as a distributed reflector in the wavelength range of laser action, and contains a gain medium. Prevalently, the periodic structure is made with a phase shift in its middle. This structure is essentially the series coupling of two Bragg gratings with optical gain within the gratings. The device has multiple axial resonator modes, but there is one mode which is used in case of losses. Consequently, the single-frequency operation is often easily achieved, despite spatial hole burning due to the standing-wave pattern in the gain medium. Due to the large free spectral range, wavelength tuning without mode hops may be possible over a range of several nanometers. However, the tuning range may not be as large as for a distributed Bragg reflector laser.
Fig. 1. Resonator reflection spectrum of the DFB laser for the relative dephased modulation of the FBG reflectivity: 0, π/2 or π.
Most distributed feedback lasers are either fiber lasers or semiconductor lasers, operating in a single resonator mode. In the case of a fiber laser, the distributed reflection occurs in a fiber Bragg grating, typically with a length of a few millimeters or centimeters. Efficient pump absorption can be achieved only with a high doping concentration of the fiber, and unfortunately, it is often not easy to write Bragg gratings into fibers with a composition which permits for a high doping concentration. DFB single-frequency fiber laser is very simple and compact. It is transportable and ruggedized that leads to a low intensity and phase noise level, i.e., also a low linewidth.
The typical output power of DFB lasers ranges from several milliwatts to tens of milliwatts. At the present day, Optromix single-mode distributed feedback fiber laser have output power up to 15 watts.
DFB play an important role in the optic world because DFB lasers are now using for optical communication, sensing, remote measurements and etc. The advantage of using DFB is better dynamic-single stability and low noise operation and distributed feedback laser are fast to transmit data in the optical communication world. Based on using Distributed Feedback lasers, there are a lot of advantages like tunable vertical cavity surface emitting laser , tunable distributed Bragg reflector , and grating assisted codirectional coupler with sampled rear reflector because this advantage has high power outputs, wave stability, reliability, and manufacturing seem to be complexity. Distributed Feedback fiber lasers find the application in spectroscopy and other scientific experiments, for the effective second harmonic generation , in a laser location, in the fiber sensors acting as a radiations sources or sensitive elements.
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