Page 398 - Fiber Bragg Gratings

8.4 Erbium-doped fiber lasers 375

Mach-Zehnder modulators. Upto 60 mW of single-frequency power has
been achieved using a MOPA configuration [68].
Other configurations for single-frequency operation use long gratings
with a bandwidth less than the cavity mode-spacing [69,70], and operation
at 1 micron using Nd:doped fiber has been shown with intracore gratings
[71].

Composite cavity lasers
There are several methods for achieving single- and multifrequency opera-
tion of EDFGLs. As outlined already, short lasers with narrowband reflec-
tors are simple candidates; however, a composite cavity topology can
enforce stable single-frequency operation by longitudinal mode control,
adapted from semiconductor lasers [72-75]. The principle relies on a small
additional feedback element in the form of a short Fabry-Perot, which
modulates the gain spectrum of the main fiber laser cavity. Figure 8.16
shows the linear cavity configuration. The basic laser cavity gratings have
reflectivities of 0.9 and 0.8 with a weaker reflection of 0.1 as the external
reflector. The gain of the composite cavity is modulated, increasing the
discrimination between the modes. Since the lasing mode is influenced
by the composite cavity, a single mode tunes with temperature changes
but does not exhibit mode hops [53]. The 10-mm-long high erbium dopant
concentration fiber (120 dB/m absorption at 1530 nm) is spliced to fiber
gratings, forming a composite cavity ~7 cm long; 980-nm pumping with
a Ti:sapphire laser showed a threshold of 50 mW. The linewidth of this
laser is —40 kHz using conventional heterodyne techniques. The gain

Figure 8.16: A schematic of the composite cavity single-frequency EDFGL
[53].