Page 400 - Fiber Bragg Gratings

8.5 The distributed feedback fiber laser 377

Figure 8.18: The loop mirror cavity for single-mode operation, incorporating
a DFB band-pass filter and an erbium-doped fiber amplifier (after Ref. [77]).

DFB fiber Bragg grating (see Chapter 6), also inside the loop, enforces
single-frequency operation. The laser may be tuned by applying compres-
sive or extensive strain on the intracavity DFB grating or, in the absence
of the DFB grating, the external grating. Single-frequency operation of
such a laser with a 0.075-nm band-pass DFB grating shows a linewidth
of ~2 kHz, with side-mode suppression of 50 dB. However, the long cavity
with a mode spacing of 11.4 MHz mode hops and requires the use of
stabilizing elements [77].
Replacing the DFB band-pass mtra-cavity filter by an acousto-optic
tunable filter (AOTF) may extend the principle of this type of a laser.
Ramping the AOTF sweeps the output frequency of the laser with <0.1-
nm linewidth [78].
To improve the strength of the grating, a specially deposited photosen-
sitive ring cladding can be fabricated with a rare-earth-doped core. This
scheme allows the core to have a high dopant concentration, while preserv-
ing the fiber's photosensitivity and allowing short, high-strength grating
to be inscribed for laser fabrication [79,80].

8.5 The distributed feedback fiber laser

The principle of the distributed-feedback (DFB) structure, generally
applied to semiconductor lasers [81], is easily translated to doped fiber