Distributed feedback laser

A distributed feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured as a diffraction grating. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback for the laser.

DFB laser diodes do not use discrete mirrors to form the optical cavity (as they are used in conventional laser designs). The grating acts as the wavelength selective element and the facets of the laser diode form the resonator. 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 wavelength selection of the grating structure and thus the wavelength of the laser output, producing a wavelength tunable laser or TDL (Tunable Diode Laser). The tuning range is usually of the order of 6 nm for a ~50 K change in temperature, while their linewidth is in the order of a few MHz. Altering of the current powering the laser will also tune the device, as a current change causes a temperature change inside the device. Integrated DFB lasers are often used in optical communication applications, such as DWDM where a tunable laser signal is desired as well as in sensing where extreme narrow line width is required, or in gas sensing applications, where the signal of the absorbing gas is detected while wavelength tuning the DFB laser. DFB lasers are a cost effective alternative, whenever wavelength selection or tuning is required. One example for designing a DFB laser diode is shown in the picture.

In DFB fibre lasers the Bragg grating (which in this case forms also the cavity of the laser) has a phase-shift centered in the reflection band akin to a single very narrow transmission notch of a Fabry-Perot interferometer. When configured properly, these lasers operate on a single longitudinal mode with coherence lengths in excess of tens of kilometres, essentially limited by the temporal noise induced by the self-heterodyne coherence detection technique used to measure the coherence. These DFB fibre lasers are often used in sensing applications where extreme narrow line width is required.

External links

• Distributed feedback fiber lasers RP Photonics Encyclopedia of Laser Physics and Technology
• NKT Photonics, one of the leading commercial suppliers of single frequency DFB fiber lasers
• nanoplus, one of the leading commercial suppliers of single frequency DFB laser diodes

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