About Fiber Laser

Fiber lasers generally use fiber Bragg grating to form the laser cavity.

Advantages:
1. Fiber lasers can produce high power light at excellence beam quality.
2. Fiber lasers use rare-earth doped glass fibers, which has larger gain bandwidth compared with the rare-earth doped bulk crystals. Fiber lasers thus can have broad tuning range and can generate short pulses via mode-locking.
3. Fiber lasers have large surface-to-volume ratio and are easier for thermal management.
4. Fiber lasers are very mechanically stable against vibrations.

Difficulties:
1. Efficiently coupling pump diodes into the fiber has tight alignment tolerance.
2. Fibers have birefringence and polarization control is difficult.

Fiber lasers use double-clad fiber. The gain medium is in the center core, where the lasing mode propagates; whereas the inner cladding layer contains the pump light.

To generate single-mode laser beam at high power, and to decrease nonlinearity, etc, "large mode area single-mode fiber" is now used for fiber laser [2]. Since it has large mode area, to ensure single-mode propagation, the fiber must have a low NA (keep V-number low). For example, a Yb-doped fiber laser has a 20um core (NA=0.06) and a 400um clad (NA=0.46).

Fiber lasers often are broad-band lasing to eliminate Brillouin scattering.

References:
[1] http://en.wikipedia.org/wiki/Fiber_laser
[2] http://www.nufern.com/whitepaper_detail.php/30