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Acronym: DFB lasers
Definition: lasers where the whole laser resonator consists of a periodic structure, in which Bragg reflection occurs
A 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. Typically, the periodic structure is made with a phase shift in its middle. This structure is essentially the direct concatenation of two Bragg gratings with internal optical gain. It has multiple axial resonator modes, but there is typically one mode which is favored in terms of losses. (This property is related to the above-mentioned phase shift.) Therefore, 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.
Figure 1: DFB fiber laser, containing a fiber Bragg grating with a phase change in the middle, directly written into a rare-earth-doped fiber.
Most distributed-feedback lasers are either fiber lasers or semiconductor lasers, operating on a single resonator mode (→ single-frequency operation). 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 (e.g. phosphate glass) which allows for a high doping concentration. Therefore, the output power is usually fairly limited (e.g. to a few tens of milliwatts). However, this kind of single-frequency fiber laser is very simple and compact.
Semiconductor DFB lasers can be built with an integrated grating structure, e.g. a corrugated waveguide. Such devices are available in a wide spectral range at least between 0.8 μm and 2.8 μm. Typical output powers are some tens of milliwatts. The linewidth is typically a few hundred MHz, and wavelength tuning is often possible over several nanometers. Temperature-stabilized devices, as used e.g. in DWDM systems, can exhibit a high wavelength stability.
DFB lasers should not be confused with DBR lasers = distributed Bragg reflector lasers.
Bibliography
| [1] | H. Kogelnik and C. V. Shank, “Coupled-wave theory of distributed feedback lasers”, J. Appl. Phys. 43 (5), 2327 (1972) |
| [2] | A. Yariv, “Coupled-mode theory for guided-wave optics”, IEEE J. Quantum Electron. QE-9, 919 (1973) |
| [3] | H. W. Yen et al., “Optically pumped GaAs waveguide lasers with a fundamental 0.11 μ corrugated feedback”, Opt. Commun. 9, 35 (1973) |
| [4] | K. H. Ylä-Jarkko and A. B. Grudinin, “Performance limitations of high-power DFB fiber lasers”, IEEE Photon. Technol. Lett. 15 (2), 191 (2003) |
| [5] | B. K. Das et al., “Distributed feedback-distributed Bragg reflector (DFB-DBR) coupled cavity laser with Ti:(Fe:)Er:LiNbO3 waveguide”, Opt. Lett. 29 (2), 165 (2004) |
| [6] | K. Sato, “Chirp characteristics of 40-Gb/s directly modulated distributed-feedback laser diodes”, IEEE J. Lightwave Technol. 23 (11), 3790 (2005) |
| [7] | A. Schülzgen et al., “Distributed feedback fiber laser pumped by multimode laser diodes”, Opt. Lett. 33 (6), 614 (2008) |
See also: single-frequency operation, single-frequency lasers, distributed Bragg reflector lasers, fiber Bragg gratings, fiber lasers, semiconductor lasers
Categories: lasers, resonators
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
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