Bragg Grating-Based Pulsed Sources 369 8.5.2 Tunable Single-Frequency Sources The gain bandwidth of doped fibers is several tens of nanometers. Tunability is possible with fiber Bragg gratings by strain or temperature tuning (see Chap- ter 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. Compres- sion 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. Continu- ous 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 [15,17,18,24,27,32]. We have also seen that the EDFGL is an excellent candidate as a source [50].