In high power fiber laser, the limiting factor of narrow linewidth output is analyzed. The
influence of Stimulated Brillouin scattering effect and sinusoidal phase modulation suppressing
method are studied. The linewidth of a single frequency laser is broadened from 1MHz to 2.9 GHz by sinusoidal phase modulation technology. The output power of single frequency laser is 50mW.
And through three stage fiber amplified, the central wavelength of 1064.34nm, linewidth of 2.9 GHz
and power of 780W are achieved respectively. The optical-optical efficiency is 79%. And the beam
quality is [see manuscript] and [see manuscript]. The distributing characteristic of longitudinal mode, under
every modulating coefficient, is measured. And the result is the same as theoretical result. The
increasing of longitudinal mode number, controlling of longitudinal mode spacing and reducing
power spectrum density by sinusoidal phase modulation are proved to be viable. Then Stimulated
Brillouin Scattering threshold is increased. Finally, the output power is increased a lot. The output
power of this laser is only limited by pump power. If the pump power is increased, the higher power
of narrow linewidth fiber laser will be achieved.
S-band fiber lasers, one type of ytterbium doped fiber lasers(YDFLs) which emitting in the spectral region of 1010 nm ~
1050 nm, have drawn more and more attention especially after the proposition of tandem pump fiber lasers. The S-band
laser plays an important role as the pump in a tandem pumping scheme. However, high power S-band laser output is
quite difficult because of severe re-absorption. Therefore, the optical properties of S-band fiber lasers are studied and
methods to achieve high power S-band laser output are presented. An S-band laser emission model based on gain
comparison is built for analyzing the mechanism of laser oscillation and amplification. The model is composed of rate
equations and gain comparison of several wavelengths in ASE spectrum. The gain differential between laser wavelength
and ASE peak wavelength is compared to the additional gain imposed by cavity mirrors in an oscillator or the seed in an
amplifier. The comparison results help in concluding the feasibility of S-band laser emission. Based on the model, the
influences of fiber properties (including core doping level, core -clad ratio, and fiber length), pump power and laser
power on the operation of S-band fiber lasers or amplifiers are researched. The analysis shows that fibers with lower
doping level, larger core-clad ratio and shorter length, seed with higher power, are more helpful in realizing high power
S-band laser output. Also, the profile of oscillator is found more suitable in building high power S-band lasers than that
of amplifier.
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