Distributed Feedback Laser

Distributed feedback laser (English, German laser with distributed feedback ), usually just called the German DFB lasers, laser diodes, in which the active material is periodically structured. The structures of varying refractive index form a one-dimensional interference grating or interference filter ( Bragg mirror ). The interference results in a wavelength selective reflection, and provides the optical feedback of the laser.

Properties

DFB laser diodes have a lower threshold current limit and better beam quality than conventional, working according to the principle of the Fabry- Perot laser diodes, whose end faces function as a Fabry -Perot interferometer. Thus, the arising during Fabry -Perot laser side modes which result in the fiber optic cable transmission to dispersion effects are minimized.

While swinging conventional laser diodes on multiple longitudinal modes, work DFB lasers on a single longitudinal mode.

The spectral bandwidth of the DFB laser is very small. The deviations from the set wavelength are less than 10-7. At a wavelength of 2 microns, corresponding to a frequency of about 150 THz, they will be approximately 0.2 pm ( 0.0002 nm), corresponding to 20 MHz. Values ​​of 2 MHz are achievable in the laboratory (For comparison with conventional laser diodes, the spectral bandwidth is about 1 to 4 nm).

DBR laser and Bragg mirror (of English. Distributed Bragg reflector laser ) provide an inexpensive alternative to wavelength selection process outside of the laser crystal (external cavity diode laser ECDL ) is, however, not reach their even greater stability ( less than 1 MHz ). A frequency-stabilized DFB laser, there are now also in the wavelength range of infrared.

While in the DFB laser diode, the Bragg structure in the active region ( the gain region ) is, it is disposed at the DBR laser outside the active zone, but in a built-in to on-chip waveguide. Both principles can be applied to fiber lasers.

Both DFB and DBR laser can be detuned by temperature change. This requires on the one hand for high wavelength stability precise thermostating, but allows the other hand, a change or adjustment of the wavelength within a wide range. To regulate the temperature while heat dissipation thermoelectric coolers are used, the - can also heat - at reversal.

DFB and DBR lasers because of its accuracy in DWDM systems (of English. Dense wavelength division multiplexing ) for accurate length measurement in optical spectroscopy (Raman ) spectroscopy for the detection of trace gases (excitation of atomic and molecular resonances ) and used for testing / measuring of glass fibers.