Recently, narrow linewidth fiber lasers are widely applied in coherent detection and wavelength beam combining. In high-power linearly polarized narrow linewidth fiber lasers, the effect of mode instability (TMI) is one of the main factors limiting its power increase. In this paper, the influence of TMI effect on the output power of high-power linearly polarized narrow-linewidth fiber laser is analyzed, and the suppression method of TMI effect is proposed. Long-wave pumping technologies are used in this article. A single frequency laser with an output power of 100mW is used as the seed source. And the phase modulator broadens the linewidth of the seed source to 23GHz. After three stage amplification, the linewidth of 23GHz, power of 2.2kW, and center wavelength of 1064nm are finally realized. Linearly polarized narrow linewidth fiber laser output with extinction ratio of 98% is achieved. Beam quality is M2x=1.2 and M2y=1.21. The influence of the pump wavelength on the TMI effect is analyzed. Due to the small core diameter of the fiber (20μm), a high absorption coefficient of the gain fiber for the pump light (1.8dB/m@976nm), the core temperature is high. And the heat introduced by the pump photo quantum defect, causes the refractive index of the fiber core to change. Finally, the TMI effect occurs at lower power. When the pump wavelength is shifted to the long wavelength, the quantum defect of the pump light and the pump absorption coefficient are both reduced. The heat distribution on the entire length of the fiber or on the unit length is reduced. The TMI threshold is increased. And the output power of the linearly polarized narrow linewidth fiber laser is improved.
Recently, narrow linewidth fiber lasers are wide applied in coherent detection and wavelength beam combining. In high power narrow linewidth fiber laser, Stimulated Brillouin Scattering (SBS) is the major power limitation factor. Through increasing the frequency number and frequency spacing, SBS threshold power multiplied. In this paper, tow stage white noise phase modulation technology is used to control the frequency number and frequency spacing. Under tow stage white noise phase modulation and three stage fiber amplified technology, the laser linewidth of 13GHz,power of 2.7kW and central wavelength of 1064.4nm, are achieved respectively. And the beam quality is M2x = 1.31 and M2y = 1.3.
Recently, narrow linewidth fiber lasers are wide applied in coherent detection and wavelength beam combining. The influence of fiber grating oscillating longitudinal mode distribution and the amplifier gain fiber characteristics on the output laser spectrum width are explored. The influence of the optical field distribution and the spatial hole burning effect on oscillating longitudinal modes number is analyzed. And the influence of four-wave mixing effect (FWM) in the amplified laser on the spectral width of the output laser is analyzed. The main oscillation power amplification technology of one-stage oscillation and one-stage amplification is used. Finally, the influence of the reflection bandwidth of the resonant cavity, the longitudinal mode number of the main oscillation stage and the length and core diameter of the gain fiber of the amplifier stage on the output laser spectrum width is studied. Spectral width of 0.29nm, power of 2kW and center wavelength of 1064.4nm is realized. The beam quality is Mx2=1.2,My2=1.3.
The scheme of N×1 multi-beam combination with single gradient-index lens (GRIN) was proposed and the coupling model was simulated with ZEMAX software. One end of the GRIN was fabricated to polyhedral convex cone with N surfaces and taken as output port, and multiple beams of light converge after passing through this plane. The influences of length of GRIN lens, angle of inclined surface and distance between of fiber end and GRIN lens on coupling efficiency were analyzed. The results show that this kind of combiner has very high efficiency for multi-beam coupling, and the coupling efficiency of N×1 fiber pump combiner reaches 97.64%. When the length of GRIN lens was between 7.36mm and 7.96mm, the coupling efficiency of the beam combiner can be maintained above 95%. When the slope angle φ is 40.995︒ <φ< 43.095︒, the coupling efficiency of the combined beams were greater than 97%.
In this paper, limitation factor of high power narrow linewidth fiber laser is analyzed. The influence of Stimulated Brillouin Scattering and its suppressing theory are discussed. Influences of frequency number and frequency spacing to Stimulated Brillouin Scattering are analyzed respectively by simulation. The results indicate that increasing frequency number and controlling frequency spacing can reduce the power spectrum density in fiber. Then the Stimulated Brillouin Scattering threshold is increased. Finally, the fiber laser output power is increased a lot.
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.
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