102 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.17: Longitudinal-mode selectivity in a coupled-cavity laser. Phase shift in the external
cavity makes the effective mirror reflectivity wavelength dependent and results in a periodic loss
profile for the laser cavity.
A holographic technique is often used to form a grating with a∼0.2-μm periodic-
ity. It works by forming a fringe pattern on a photoresist (deposited on the wafer sur-
face) through interference between two optical beams. In the alternative electron-beam
lithographic technique, an electron beam writes the desired pattern on the electron-
beam resist. Both methods use chemical etching to form grating corrugations, with the
patterned resist acting as a mask. Once the grating has been etched onto the substrate,
multiple layers are grown by using an epitaxial growth technique. A second epitaxial
regrowth is needed to make a BH device such as that shown in Fig. 3.14(b). Despite
the technological complexities, DFB lasers are routinely produced commercially. They
are used in nearly all 1.55-μm optical communication systems operating at bit rates of
2.5 Gb/s or more. DFB lasers are reliable enough that they have been used since 1992
in all transoceanic lightwave systems.
3.4.2 Coupled-Cavity Semiconductor Lasers
In acoupled-cavitysemiconductor laser [2], the SLM operation is realized by coupling
the light to an external cavity (see Fig. 3.17). A portion of the reflected light is fed
back into the laser cavity. The feedback from the external cavity is not necessarily in