Page 403 - Fiber Bragg Gratings

380 Chapter 8 Fiber Grating Lasers and Amplifiers

Figure 8.21: The coupled-cavity dual-frequency source (from: Chernikov
S. V., Kashyap R., McKee RE., and Taylor J. R., "Dual frequency all fiber grating
laser source," Electron. Lett. 29(12), 1089, 1993. © IEEE 1993, Ref. [52]).

Multifrequency operation in ring lasers has been reported with the
use of a grating with a multiple wavelength reflection spectrum. Up to
8 wavelengths have been shown to lase simultaneously. While single-
frequency operation (~10 kHz linewidth) has been demonstrated, in such
long lasers, mode hopping is common, and special attention must be paid
to stabilize the operation [87].

8.5.2 Tunable single-frequency sources

The gain bandwidth of doped fibers is several tens of nanometers. Tunabil-
ity is possible with fiber Bragg gratings by strain or temperature tuning
(see Chapter 3). Ball and Morey [88] stretch-tuned a pair of Bragg gratings
of a 100-mm-long erbium fiber laser and showed 9 GHz mode-hop-free
operation. Since the gratings and the fiber length tune together, the laser
remains stable. Compression tuning can extend the tuning range, and a
tuning range of 32 nm has been reported for a short EDFGL (3 cm) in a
MOPA configuration. Continuous single-frequency tuning was observed
with the laser producing 3 mW of output. A feedback loop is necessary
to stabilize the operation of the laser to reduce relaxation oscillations
[89]. Compression tuning of a Yb/Er laser using a mirror and Bragg
grating configuration has shown sub milliwatt thresholds and both strain
and compression tuning ranges of up to 25 nm [90].

8.6 Bragg grating based pulsed sources

We have seen that the FGL can be used as a pulsed source for optical
fiber transmission, either directly modulated or mode-locked [15,17,