104 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.18: Coupled-cavity laser structures: (a) external-cavity laser; (b) cleaved-coupled-
cavity laser; (c) multisection DBR laser.
trolling the currents in the phase and Bragg sections. By 1997, such lasers exhibited a
tuning range of 17 nm and output powers of up to 100 mW with high reliability [51].
Several other designs of tunable DFB lasers have been developed in recent years. In
one scheme, the built-in grating inside a DBR laser is chirped by varying the grating pe-
riodΛor the mode index ̄nalong the cavity length. As seen from Eq. (3.4.2), the Bragg
wavelength itself then changes along the cavity length. Since the laser wavelength is
determined by the Bragg condition, such a laser can be tuned over a wavelength range
determined by the grating chirp. In a simple implementation of the basic idea, the grat-
ing period remains uniform, but the waveguide is bent to change the effective mode
index ̄n. Such multisection DFB lasers can be tuned over 5–6 nm while maintaining a
single longitudinal mode with high side-mode suppression [47].
In another scheme, asuperstructure gratingis used for the DBR section of a mul-
tisection laser [48]–[50]. A superstructure grating consists of an array of gratings (uni-
form or chirped) separated by a constant distance. As a result, its reflectivity peaks at
several wavelengths whose interval is determined by the spacing between the individ-
ual gratings forming the array. Such multisection DBR lasers can be tuned discretely