Our common lasers are FB laser diodes, so do you know what DFB laser diodes are? How does it work? Let's take a look at it together

DFB laser, in fact, is called distributed feedback laser. This name sounds a bit awkward, but the principle is actually quite interesting. Many newcomers may only know that "DFB can emit a single wavelength for communication" in their minds, but to really understand why it can do this, we must start with the structure.

Ordinary laser diodes, such as FP lasers, rely on two mirrors to reflect light back and forth in the cavity to form optical feedback. The problem is that this method is easy to produce multiple longitudinal modes-that is to say, it can emit many different wavelengths of light, which looks quite high in power, but the spectrum is wide and the signal is easy to drift, which is troublesome in communication systems.

The DFB laser is much smarter in structure. Instead of using two mirrors, it engraves a layer of grating in the active area (that is, the optical amplification area), which is like burying a "selector" inside the chip. Its function is to make only light with a specific wavelength form positive feedback and be amplified, while light with other wavelengths is suppressed. The resulting light is a single longitudinal mode output with stable wavelength and narrow spectral line, which is very suitable for precise scenes.

The structure of DFB laser generally includes an active region, a grating layer, a cladding, and two end faces. Grating can be made in the active layer or in the cladding. In order to further improve the single-mode performance, some DFB lasers will also adopt λ/4 phase shift design, and add a phase shift zone in the middle of the grating to make the laser modes more concentrated and reduce the competition between modes. This light is not only pure in wavelength, but also narrow in linewidth to MHz level, which is especially suitable for high-precision sensing, spectral analysis and coherent communication.

A layer of "grating filter" is made inside the chip of DFB laser, so that only the desired wavelength can survive in it. It is precisely because of this structure that it has stable, pure and single-mode output characteristics.