What are DFB Lasers?

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- GoPhotonics

Nov 4, 2022

A Distributed Feedback (DFB) laser is a laser device whose active medium consists of a repeating corrugated structure. The corrugated structure is a periodic variation of the refractive index and thus acts as a diffraction grating, which provides optical feedback throughout the structure. As a result, only a single wavelength is emitted from such lasers and so these lasers are capable of producing a very narrow spectral width output, usually less than 0.1 nm to several femtoseconds. The DFB lasers collectively emit wavelengths in the range of 760 nm - 3 µm; e.g., the Erbium-doped DFB laser covers a wavelength range from 1530 nm to 1615 nm.

Unlike other laser systems, end mirrors are not necessary to form a laser cavity in a DFB laser. The distributed optical feedback due to the partial reflection at each interface in the corrugation of the refractive index along with amplification allows the laser field to oscillate inside the gain medium and hence develop it to the required output power. This in turn performs the role of the laser cavity. The ends of the DFB laser cavity are coated with an anti-reflection coating on one side and a reflective coating on the other side. The Reflection coating acts as the cavity mirror which reflects back the diffracted light and the anti-reflection coating provides the laser output. DFB lasers are ideal for high-resolution spectroscopy, optical communication, and precision sensing applications.

Working

When the device is turned on, the laser field starts to grow in the gain medium and spreads into the corrugated structure as it is placed along the gain medium. The refractive index variation here will cause partial reflections and hence optical feedback are formed throughout the laser cavity. These feedbacks undergo repeated amplification and partial reflections. Hence the laser field is developed in a DFB laser.

In the DFB laser, the laser field is fed into the Bragg grating along the entire length of the gain medium. The partially reflected waves do experience optical gain and have different phase changes. The leftward moving waves are partially reflected and amplified to form the rightward traveling waves and vice versa. These traveling waves can coherently couple to form a mode if their frequencies are related to the spatial corrugation period. These modes are given by DFB laser wavelength,

 Where is the Bragg wavelength corresponding to the refractive index corrugation and 

are the effective refraction index and effective length and period of the corrugated structure respectively; q is the diffraction order and m = 0, 1, 2, … is the mode integer. Usually, the effective length, L of the laser gain medium is so large that the DFB laser wavelength is very close to the Bragg wavelength.

Due to a very high threshold gain requirement, the laser gain required to overcome the losses in the cavity, for the higher order modes (i.e., m=1, 2, …) is very large. So, only the m = 0 mode produces laser output effectively.

Types of DFB Lasers

There are mainly two types of DFB lasers. Namely, DFB semiconductor laser and DFB fiber laser.

DFB Fiber lasers


In a DFB fiber laser, a certain length of the doped core region acts as the gain medium. A fiber Bragg grating inscribed on the fiber core in this region acts as the corrugated structure that provides the Distributed optical feedback. This periodic corrugation of the core refractive index is phase adjusted by π radians near the center to ensure single-mode operation. DFB fiber laser is optically pumped at a shorter wavelength. 

Semiconductor DFB Laser


A simple schematic for the Semiconductor DFB Laser structure is shown in the figure above. It has a laser gain medium layer and a guiding layer which are sandwiched between p-type and n-type semiconductors. The guiding layer contains the corrugated structure responsible for providing the distributed optical feedback. The p-type and n-type layers are metal coated to provide electrical current input which supplies the pumping energy and hence supports lasing action in the system.

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