A distributed feedback laser (DFB) is a type of laser diode where the active region of the device is structured as a diffraction grating. The grating, known as a distributed Bragg reflector, provides optical feedback for the laser due to Bragg scattering from the structure.Since the grating provides feedback, DFB lasers do not use discrete mirrors to form the optical cavity (as are used in conventional laser designs). The grating is constructed so as to reflect only a narrow band of wavelengths, and thus produce a narrow linewidth of laser output.Altering the temperature of the device causes the pitch of the grating to change due to thermal expansion. This alters the reflection wavelength of the grating structure and thus the wavelength of the laser output, producing a tunable laser. The tuning range is usually of the order of 6 nm for a ~50 K change in temperature. Altering of the modulation rate of the current powering the laser will also tune the device.
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