A distributed feedback (DFB) laser uses a periodic structure along its waveguide or gain region to provide optical feedback. That structure acts as a distributed reflector, helping select a wavelength or optical mode within the laser’s gain range.
How does a DFB laser work?
In a DFB laser, feedback comes from a repeating structure along the cavity, rather than solely from separate mirrors at its ends. The structure’s periodicity produces Bragg reflection, reinforcing light at wavelengths supported by the grating and the laser’s gain. This favors selected modes over neighboring ones.
The periodic structure can work by modulating the waveguide’s refractive index, its optical loss, or both. For example, the University of Cambridge Semiconductor Physics Group describes a terahertz quantum-cascade laser in which a metal grating modulates waveguide loss. That is one implementation, not a requirement for every DFB laser.
What does “distributed feedback” mean?
“Distributed” describes where the optical feedback is supplied: by a periodic structure extending along the waveguide or gain region. The grating acts as a reflector throughout that portion of the cavity, and its optical effect helps determine which modes are amplified. The Cambridge group defines a DFB laser as one whose waveguide has a periodic structure acting as a distributed reflector at a wavelength range within the gain envelope (source).
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Is a phase shift required?
No. A phase shift, often placed near the center of the grating, is a common design feature that can help favor single-mode operation. It is not part of the general definition of a DFB laser; designs differ in whether and how they use one. RP Photonics describes a central phase shift as typical.
DFB versus DBR lasers
Both names refer to laser designs that use gratings for optical feedback, but the placement of the grating relative to the gain region distinguishes them in the cited semiconductor-laser comparison:
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| Feature | DFB laser | DBR laser |
|---|---|---|
| Grating location | Distributed along the active medium. | Outside the active region. |
| Feedback arrangement | The periodic structure supplies feedback along the gain region. | The grating supplies feedback from a separate region outside the gain medium. |
This distinction describes the cited comparison, not a claim that every device in either category has identical construction or mode behavior. See the DFB and DBR comparison.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where are DFB structures used?
DFB structures are used in semiconductor lasers, including quantum-cascade lasers (QCLs). The Cambridge group discusses DFB QCLs for terahertz operation, and RP Photonics also identifies QCLs as an application. These examples illustrate the scope of the technology; they are not an exhaustive list of DFB laser types.
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