Tenglong Li

An improved method for stripping cladding light in high power fiber lasers

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 study of the thermally expanded core technique in end-pumped (N+1)×1 type combiner

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.

Narrow Spectrum Kilowatt-Level MOPA Seeded By Yb-Doped Random Fiber Laser

We proposed and experimentally demonstrated an approach to achieving a narrow spectrum kilowatt-level fiber master oscillator power amplifier (MOPA). The MOPA is seeded by a narrow spectrum random laser with Yb-doped fiber gain and has the spectral broadening-free feature. The narrow spectrum property is sustained during power scaling due to the suppression of nonlinear process. The frequency and time domain of the random laser are well characterized. The proposed one-stage scheme eventually produces 1.105- and 1.073-kW output at 1064 and 1067 nm, and the 3-dB bandwidth are 0.40 and 0.47 nm, corresponding to 0.38- and 0.37-nm random laser seed, respectively. The beam quality of

Numerical analysis to four-wave mixing induced spectral broadening in high power fiber lasers

\text {M}^{2}\approx 1.4

Research of high brightness 1018nm ytterbium doped fiber laser

is achieved at 1.105-kW power level, and the proposed MOPA has the potential of wavelength tunability by simply tuning the fiber Bragg grating of random laser seed.

The design and experiment research of high brightness all-fiberized ytterbium doped laser operating near 980 nm

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.

Self-pulsing in a 2 km single-mode fiber with the seed source broadened via WNS phase modulation

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.

Self-pulsing in kilowatt level narrow-linewidth fiber amplifier with WNS phase-modulation

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%.

Estimation of mode instability threshold based on local thermal load

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.

Scalable hybrid beam combining of kilowatt fiber amplifiers into a 5-kW beam

The self-pulsing phenomenon in kilowatt level narrow-linewidth fiber amplifiers with white noise source (WNS) phase-modulation is observed experimentally. It possesses the obvious threshold of the pump power and prevents the narrow-linewidth fiber lasers from further power scaling. The experimental study shows that known explanations are not applicable here and indicates that occurrence of self-pulsing is closely related to Stimulated Brillouin Scattering (SBS) process. The theoretical discussion reveals that the spikes in the modulated spectrum are the critical factor that SBS threshold is lower than the theoretical estimation. The 1+1 dimensional SBS model analysis predicts that self-pulsing originates from forward second order Stokes pulses, which is in good qualitative agreement with the experimental data.