Understanding Lasers

Chapter 9.6.2: Distributed Feedback Lasers

9.6.2 Distributed Feedback Lasers

Applications such as high-speed data transmission in fiber optics require limiting laser emission to a narrower range of wavelengths than possible with a Fabry Perot cavity. This requires adding a

mechanism that limits the range of wavelengths emitted by the laser. The leading approach is called a distributed feedback or DFB laser, shown in Figure 9-15A, in which a diffraction grating is fabricated in the base of the active layer. The regularly spaced grooves scatter light back into the active layer at a narrow range of wavelengths, so only those wavelengths receive the feedback needed for laser oscillation.

The spacing of the grating D selects the oscillating wavelength λ according to a formula that also depends on refractive index n and an integer m (in practice, 1 or 2) that denotes how light is being scattered by the grating:

Plugging in the numbers for a 1550-nm DFB laser made of In-GaAsP (n = 3.4), we find that grating spacing D is 228 nm for m = 1

and 456 nm for m = 2. Although oscillation wavelength shifts slightly because refractive index depends on temperature, DFB resonators maintain the laser in a stable single longitudinal mode, limiting the range of emitted wavelengths and preventing modehopping. Both are critical for high-speed fiber-optic transmission.

An important variation on the DFB laser is placing the grating in a part of the active layer where there is no laser gain, as shown in Figure 9-15B. This has the same effect, but the physics differ in detail. The result is called a distributed Bragg reflection or DBR laser.

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