In this paper, based on a linear polarized, white noise signal (WNS) phase modulated all fiber amplifier, the self-pulsing characteristics of polarization maintaining (PM) amplifier at the different phase modulation parameters with the same linewidth are analyzed experimentally. It is demonstrated that the self-pulsing threshold is closely related to the details of the modulation spectrum affected by the frequency and the signal power of the WNS, and it is near impossible to calculate by the linewidth. Besides, by suppressing the self-pulsing effect, we obtain an output of 2009 W with a linewidth of 22 GHz. The polarization extinction ratio (PER) is larger than 15 dB, and the M2 is lower than 1.2.
In this paper, we demonstrate 47 GHz linear polarized fiber amplifier injected by a simple fiber oscillator laser seed source with narrow linewidth and near diffraction-limited beam quality. Output powers of 419 W, 778 W, and 1107 W are achieved with 3dB linewidth of 26.5 GHz, 41 GHz, and 47 GHz, respectively. The M2 is 1.25 in the x-direction and 1.23 in the y-direction at the maximum laser power, respectively. The measured PER is large than 96% during the power scaling process. However, The SRS is observed when the laser power is 1107, the SNR is about 47dB, which means that the SRS effect has become a serious limitation for further power scaling of such PM-amplifier seeded by the fiber oscillator laser seed source.
The seed source with spectral linewidth broadening via phase modulation is potential to achieve the higher output power with effective SBS suppression. However, self-pulsing from the amplifier output is harmful. In this work, we study the self-pulsing characteristics in a long single-mode fiber with lower self-pulsing threshold instead of the high power amplifier. We provide a powerful experimental support for the self-pulsing mechanism in high-power narrow-linewidth fiber lasers, which is important for further output power scaling.
The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.
The output properties of the ∼980-nm Yb-doped fiber laser versus pump power and core–cladding ratio of gain fiber, also the amplified spontaneous emission (ASE) at different wavelengths of seed laser, are investigated theoretically. An all-fiber amplifier based on different wavelengths of seed laser at 974.4, 977, and 981.7 nm brings the studies on parasitic oscillation and ASE in the ∼980-nm Yb-doped fiber amplifier. Through the theoretical and experimental research, we found that the controlling of three-level ASE around ∼980-nm is pivotal for obtaining a high-power 980-nm Yb-doped fiber amplifier.
The effect of the pump source and the gain fiber on the output properties of ytterbium doped fiber laser (YDFL) work near 980 nm are analyzed theoretically. Base on this analysis, we design a high power all fiberized ytterbium doped laser operating near 980 nm with a core/cladding of 80/130 μm. A 980 nm YDFL experimental setup was constructed, 16.7W 980 nm fiber laser was achieved with an optical-optical conversion efficiency of 32.4%.
For powers exceeding a threshold the spectral broadening in fiber amplifiers becomes a significant challenge for the development of high power narrow bandwidth fiber lasers. In this letter, we show that the spectral broadening can be partly caused by four-wave mixing(FWM) process in which the power of the central wavelength would transfer to the side ones. A practical FWM induced spectral broadening theory has been derived from the early works. A numerical model of fiber amplifier has been established and FWM process has been added to the model. During the simulation process, we find that when a 10 GHz, several watts narrow bandwidth laser is seeded into a few modes fiber laser amplifier, the FWM induced spectral broadening effect might continually increase the FWHM of the spectra of the continuum laser to 100 GHz within the amplification process to several hundred watts which has been convinced by our experiments. Some other results have also been analyzed in this paper to complete the four-wave mixing induced spectral broadening theory in fiber amplifiers.
Tapering will raise the signal loss in an end-pumped (N+1)×1 type combiner. In this paper, the Thermally Expanded Core (TEC) technique is used in the signal loss optimization experiment with the tapering ratio of the pump combiner is 0.6. The experimental results indicate that the coupling efficiency of the 1.55μm signal light increases from 81.1% to 86.6%, after being heated 10 minutes at the homo-waist region of the tapered signal fiber with an 8mm wide hydroxygen flame. Detail analysis shows that the TEC technique can both reduce the loss of the LP01 mode and the LP11 mode in the signal fiber.
In order to ensure the high power all-fiber laser reliability and excellent beam quality, it is necessary to strip the unwanted cladding light. The common method for stripping cladding light is to recoat the double cladding fiber with a high index gel, but localized heating and low thermal conductivity of the recoating gel are the prime factors limiting the power-handling capability of the cladding power stripper(CPS). An improved fabrication technique to manufacture the CPS is presented. Light stripping section of the fiber is fused with a transparent quartz tube, by applying different amount of etchant along the quartz tube, frosted surface is created and uniformly removal of the cladding light is achieved. The quartz tube is joined to water-cooled thermal enclosure tightly without the gel to avoid heat aggregation. The power-handling capability of the device is tested under 200W of cladding light, and attenuation of 20 dB is achieved.
The effect of the gain fiber parameters for the design of a short wavelength ytterbium doped fiber laser (YDFL) is analyzed theoretically, and based on these analyses, we find viable approaches to improve the laser performances. High power 1018 nm YDFL system was constructed, output characteristics of 1018 nm laser with the core/cladding, doping concentration and gain fiber length are experimentally investigated. It is found that increasing the core/cladding area ratio, reducing the doping concentration and shortening the fiber length is effective way to suppress the amplified spontaneous emission (ASE) gain and ensure the laser operation at 1018 nm. A record output power of 154 W at 1018 nm is obtained, with a brightness of 5.7GW/cm2 -sr, optical-optical conversion efficiency is 71% when the laser operated at maximal power. Meanwhile, ASE and spurious oscillation are suppressed efficiently.
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