If you have ever eaten a donut, you will appreciate the problem. You want the whole of that sugar-laden doughy delight to enter your mouth. If you wanted half of it to end up on your face, then you would rub it over your cheeks, nose, and forehead. But as a sensible person you decide that the only place you want the donut is in your mouth, and so you put it there. Mmmmmm, that does taste good. But your face doesn't feel quite right for some reason, so you take a look in the mirror. Sure enough, your cheeks are covered in sugar and your nose has a splodge of icing on it. You have just eaten a Fabry-Perot donut.
The aim of a distributed feedback (DFB) laser is to sharpen up the output of regular Fabry-Perot lasers. From standard Fabry-Perot lasers you get not just one wavelength emitted but also a few others very close by, at slightly lower powers — just as a regular donut gives you unwanted sugar at the sides of your mouth. If you were eating a lot of donuts, you would prefer to have them all enter your mouth and stay well away from the rest of your face. If you wish to send lots of different wavelengths down the same optical fiber in a wavelength-division multiplexed (WDM) system, then any stray wavelengths can cause problems by interfering with other signals. What you therefore need is a laser that will emit almost entirely at one wavelength — you need a DFB laser.
The key to DFBs is the addition of a corrugated structure above the active layer of the laser. This is a zig-zaggy composition, and the combination of its refractive index and the spacing of the corrugations serves to reflect only a specific wavelength of light. Light traveling in the active layer is going in all directions, and so will eventually strike the upper surface of the active layer and hit this zig-zag. In this way the corrugations act as a grating, reflecting only a specific wavelength back into the cavity but allowing others to pass through. They are feeding back the desired wavelength into the cavity, and this is taking place over the whole length of the laser. Therefore the feedback is “distributed,” and we have a distributed feedback laser. The only laser light that builds up within the active layer is light of this specific wavelength, and so that is the only wavelength emitted from the laser.
Due to the extra complexity of this grating being added to the semiconductor laser, DFBs are naturally more expensive. In fact, a DFB laser may be up to 1000 times more expensive than a basic Fabry-Perot laser. However, in modern optical networks where more signals need to be crammed into smaller ranges of wavelengths, the improved performance makes DFBs well worth the extra money.
- Fabry-Perot output is lots of different wavelengths; DFB gives just one
- DFB is similar to Fabry-Perot cavity, but with grating above active layer
- Grating is a zig-zag structure, and its different refractive index causes reflection
- Exact wavelength reflected depends on refractive index change and zig-zag spacing
- Only desired wavelength reflected back into laser cavity and emitted
- Much more expensive than Fabry-Perot lasers due to increased complexity
Vertical Cavity Surface Emitting Lasers (VCSELs), Tunable Lasers