Pu Zhou

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

We present theoretical and experimental research on coherent beam combining of fiber amplifiers using stochastic parallel gradient descent (SPGD) algorithm. The feasibility of coherent beam combining using SPGD algorithm is detailed analytically. Numerical simulation is accomplished to explore the scaling potential to higher number of laser beams. Experimental investigation on coherent beam combining of two and three W-level fiber amplifiers is demonstrated. Several application fields, i.e., atmosphere distortion compensating, beam steering, and beam shaping based on coherent beam combining using SPGD algorithm are proposed.

Coherent beam combination of 1.08 kW fiber amplifier array using single frequency dithering technique.

Coherent beam combination of a 1.08 kW fiber amplifier array has been demonstrated for the first time, to our knowledge. In the experiment, nine fiber amplifiers are tiled into a 3×3 array, and the output power of each amplifier is approximately 120 W. A single frequency dithering algorithm is used to compensate the phase noises between the elements, which runs on a signal processor based on field programmable gate array for phase control on the fiber amplifiers. When the phase control system goes into closed loop, the fringe contrast of the far-field intensity pattern is improved to more than 85%, and the residual phase error is less than λ/15.

Efficient Raman fiber laser based on random Rayleigh distributed feedback with record high power

We report a high-power Raman fiber laser based on randomly extremely weak Rayleigh feedback. Based on a mirrorless configuration, a total output power of 73.7 W is achieved with an optical efficiency of 74.7%. The cavity length of the Raman fiber laser is only 300 m. The Stokes wave is centered at 1184 nm with 3 dB bandwidth of about 4 nm at the maximal power. The spectrum and time-domain behavior at the Raman Stokes wave generation threshold is different from previous random distributed feedback Raman fiber lasers.

Average spreading of a Gaussian beam array in non-Kolmogorov turbulence

By using the rms beam width as the parameter to characterize the spreading property, the average spreading of Gaussian beam array propagates in non-Kolmogorov turbulence is studied. Numerical examples reveal that the beam width depends on the exponent value alpha, and the beam width in non-Kolmogorov turbulence is different from that in Kolmogorov turbulence with alpha=3.667. The beam width also depends on the distance between adjacent beamlets, the inner and outer scale parameters of the non-Kolmogorov turbulence. The beam array spreads more significantly with smaller inner scale and larger outer scale parameters.

Coherent beam combination of three two-tone fiber amplifiers using stochastic parallel gradient descent algorithm

Multitone radiation is a promising technique to mitigate stimulated Brillouin scattering effects in narrow-linewidth fiber amplifiers. We demonstrate coherent beam combination of three two-tone fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm. Phase control on the fiber amplifiers are performed by running the SPGD algorithm on a digital signal processor. The contrast of far-field intensity pattern of a coherently combined beam is more than 85%. Experimental results validate that a single-frequency seed laser is not indispensable for coherent beam combination in master oscillator power amplifier configuration.

Coherent beam combination with single frequency dithering technique

The single frequency dithering technique for coherent beam combination with one single modulation frequency and without a reference beam is presented for the first time to our knowledge. Coherent beam combination of four fiber amplifiers is successfully demonstrated by using the single-dithering technique. When the phase control system is in the closed loop, the fringe contrast of far-field intensity pattern is improved by more than 85% from 8% in open loop, and the residual phase error is less than lambda/20. Experimental results reveal that the single-dithering technique has great potential in scaling to a large number of beamlets with a simple phase control system and low cost.

Review on recent progress on mid-infrared fiber lasers

We present review on recent progress on mid-infrared fiber lasers. The relevant glass fiber host materials and different kinds of doped rare-earth ions are introduced. Lasers operating from 1.9 to 3 μm in the rare-earth ions Tm3+, Ho3+, Er3+, and Dy3+ in the recent two years are discussed. Three interesting research issues, i.e., supercontinuum generation based on mid-infrared fiber laser, mid-infrared Raman fiber laser, and coherent beam combining in the mid-infrared wavelength band are classified and analyzed. Some discussions and prospective predictions are proposed at the end of this paper.

Short cavity-length random fiber laser with record power and ultrahigh efficiency.

We report the result of achieving a random fiber laser (RFL) with record 200-W-level power output. The highest output power is realized by a simple 120 m long cavity at a working wavelength of 1173 nm while pumping at 1120 nm. The maximum observed optical-to-optical efficiency reaches ∼89%, which is believed to be the highest value ever reported for RFLs. In addition, numerical calculations on different order Raman Stokes wave thresholds based on the theoretical model are carried out for comparison with the experimental data. The presented work effectively advances the power scalability, and the numerical model well describes the lasing thresholds in such short cavity RFLs.

Tunable, multiwavelength Tm-doped fiber laser based on polarization rotation and four-wave-mixing effect

We propose and demonstrate a tunable multiwavelength fiber laser employing polarization-maintaining Tm-doped fiber based on polarization rotation and four-wave-mixing effect. Polarization-maintaining Tm-doped fiber and polarization controllers were employed to manipulate the polarization modes in the laser, and 400 m long single-mode passive fiber was used to enhance the four-wave-mixing effect and suppress the polarization mode competition. Stable fiber laser operation of 1-6 wavelengths around 1.9 μm was achieved at room temperatures. The wavelengths can be tuned through adjusting the polarization controllers. The optical signal-to-noise ratio of the laser is more than 31 dB. The wavelength shift is less than 0.05 nm and the peak fluctuation of each wavelength is analyzed. For most of the wavelengths the peak fluctuations are less than 3 dB and the peak fluctuations of wavelengths with more stability are below 1.5 dB.

Hundred-watt-level high power random distributed feedback Raman fiber laser at 1150 nm and its application in mid-infrared laser generation.

Two kinds of hundred-watt-level random distributed feedback Raman fiber have been demonstrated. The optical efficiency can reach to as high as 84.8%. The reported power and efficiency of the random laser is the highest one as we know. We have also demonstrated that the developed random laser can be further used to pump a Ho-doped fiber laser for mid-infrared laser generation. Finally, 23 W 2050 nm laser is achieved. The presented laser can obtain high power output efficiently and conveniently and opens a new direction for high power laser sources at designed wavelength.