Page 397 - Fiber Bragg Gratings

374 Chapter 8 Fiber Grating Lasers and Amplifiers

Equation (8.4.7) states that the loss of the second mode must be greater
than a third of the single pass gain plus the cavity roundtrip loss for the
first mode. The reflectivity of the grating may be computed by the methods
presented in Chapter 4 and used to determine the reflectivity for a given
length of doped fiber with an unsaturated gain of a 0 per meter.
With the gratings written in highly doped erbium-doped fiber, the
additional gain within the gratings must also be taken into account, since
the effective reflectivity is increased [62]. The best region for single-mode
operation is when the free-spectral range is twice the expected shift in
the lasing frequency induced by perturbations and cavity length changes.
This is greatly assisted by making the cavity as short as possible.

8.4.1 Single-frequency erbium-doped fiber lasers

Erbium-doped-fiber grating lasers (EDFGLs) offer a simple and elegant
solution for wavelength selection, providing a very narrow linewidth as
well as a high degree of wavelength stability. They are also compatible
with optical fiber systems and can be easily integrated with other fiber
components, such as WDM couplers and fiber isolators.
Tests performed on an externally modulated single-frequency
EDFGL have confirmed its robust suitability for error-free high-speed
application in transmission systems [50]. The laser is fabricated with
600 ppm GeO 2:Al 2O 3 doped silica fiber with a refractive index difference
of 0.023 (core diameter of 2.6 /mi) and cutoff at 880 nm. The tight
confinement ensures high efficiency, and aluminum reduces the concen-
tration-quenching effects that reduce the lifetime of the upper laser
level, also leading to instabilities [63]. The gain of this fiber was
reported to be —10 dB/m, with an overall length of the laser of 4.4
cm, including gratings (98% O/P coupler and —100% broadband high
reflector), using the linear cavity design shown in Fig. 8.15. Pumped
at 1480 nm, the laser is operated close to threshold to ensure single-
mode operation and amplified in a MOPA configuration through an
optical isolator. The packaged laser is sensitive to vibration that drives
relaxation oscillation at a frequency of ~150 kHz. These oscillations
can be actively controlled by using a feedback scheme to control the
diode-pump laser and reduced to insignificant levels [50,64,65]. The
error-free performance of several EDFGLs in transmission experiments
at 2.5 and 5 Gb/sec been have reported [66,50,67], using external