Page 401 - Fiber Bragg Gratings

378 Chapter 8 Fiber Grating Lasers and Amplifiers

lasers. Figure 8.19 shows a schematic of the laser, which is identical
to a Fabry-Perot laser, except that the two gratings are separated by
only a quarter-wavelength (see Chapter 6). The phase shift generates
a gap in the reflection spectrum, allowing narrow single-frequency
operation.
With the possibility of writing gratings directly into erbium-doped
fibers (see Chapter 2), DFB lasers may be fabricated with ease, especially
since long gratings (—100 mm) require modest refractive index modula-
tions to produce a high reflectivity. The first reported DFB laser operated
at 1 micron and was fabricated in ytterbium-doped fiber [82]. It has been
successfully demonstrated in erbium-doped fibers for 1500-nm operation
[83,84] and extended by cascading five DFB lasers separated in wave-
lengths by ~1 nm each, pumped by a single 1489-nm diode laser [85].
Simple modeling of a DFB laser may be achieved by including a gain
(complex term) in K dc in [Eqs. (4.3.5) and (4.3.6)], so that A/3 is modified
by iy, where 7 is the gain per meter. Numerical simulation of the transfer
function in Chapter 4 [Eq. (4.8.22)] of the DFB is shown in Fig. 8.20 as
a function of increasing gain. As the gain increases, the side modes of the
DFB structure begin to lase.
There are two possibilities for inducing a phase shift in the grating:
one which is localized, or by distributing the phase shift along the length,
as discussed in Chapter 6. The model takes account of the variation in
the gain as a result of pump depletion. It has been shown for a 5-cm-long
DFB laser that positioning the discrete TT phase-shift away from the center
of the laser, at ~0.6L DFB, increases the output power by as much as 60%.
For a phase shift distributed over 1 cm of the same laser, the optimum
value acquires a slightly larger value of 1.3 rr radians [86].
The position of the phase shift changes as a direct result of spatial
hole burning, since the intensity is highest at the center of the laser for
a symmetrically positioned phase shift. The distributed phase-shift laser
is as efficient as the discrete off-center phase-shifted laser.

Figure 8.19: The EDFGL distributed feedback laser (EDFGL-DFB). The
phase shift of 77/2 forms a band pass in transmission.