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In coherent beam combining of fiber lasers, the combined beam's quality is strongly affected by not only the piston phase differences between each beamlet, but also the tilt-tip errors of each beamlet. In this paper, we first presented a numerical model of tilt-tip control, then performed the numerical simulation to investigate the tilt-tip wavefront control in coherent beam combining using stochastic parallel gradient descent (SPGD) algorithm. It is demonstrated that the tilt-tip errors can be compensated effectively using SPGD algorithm. Results show that the increase in both the mean square error (MSE) of tilt-tip errors and the number of lasers degrade the control effect. It is difficult to compensate the tilt-tip errors when the MSE is larger than 75% of each beamlet's divergence angle. When the iteration rate of the SPGD algorithm is 100 Hz, the simulated control bandwidth is 16.50 Hz with tilt-tip errors' MSE of 5% of the divergence angle, and 1.90 Hz with MSE of 8% of the divergence angle.

A pair of fiber lasers with high output power and low quantum-defect at 1018 nm was demonstrated. These two 1018 nm fiber lasers were used as the source of Michelson cavity to show experimentally that a coherent beam combination of two Yb-doped double-cladding fiber lasers using all-fiber Michelson cavity technique was achieved. We obtained an output power of 55 W with a coherent combination efficiency of 90.2%, which is the highest output power that single-mode 1018 nm fiber lasers have obtained so far. Moreover, we proved that coherent beam combination by using Michelson cavity can achieve high power single-mode output of fiber laser.

High peak power, single frequency nanosecond fiber lasers have aroused the intense interest in their applications such as nonlinear frequency generation, LIDAR, and remote sensing. However, self-phase modulation (SPM) will induce a temporally dependent phase shift φNL (L, t)=|Ap (0, t)|2γLeff, where Ap is the amplitude of pump wave, γ is the nonlinear parameter, and Leff is the effective fiber length. The nonlinear phase shift will broaden the spectral linewidth of pulsed laser, which degrades the coherence of the laser and influences the performance of the laser. In order to obtain laser pulses with narrower linewidth, we can phase-modulate the pulsed laser with a value of-φNL(L,t). Thus, the SPM induced the nonlinear phase shift can be eliminated, and the spectra of pulsed laser can remain during the amplification and transmission in the fiber. Stimulated Brillouin scattering (SBS) has very low threshold and should be taken into consideration in narrow linewidth fiber lasers. The SBS threshold, which is dependent on the linewidth of laser, will be changed at the same time when the SPM is pre-compensated for. Because the SPM pre-compensation will change the linewidth of the pulsed laser. According to three coupled amplitude equations, we numerically analyze the influence of SPM pre-compensation on SBS threshold and spectral characteristics. The stimulation results show that in a master oscillator power amplifier structured fiber laser system, when SPM is completely compensated for (φM(t)=φNL(L,t)), the spectrum of the output pulsed laser can be maintained as that of the laser seed, but the SBS threshold usually decreases. When the SPM is compensated for incompletely (φM(t) φNL(L,t)), the spectral linewidth of the output laser cannot be compressed to that of the laser seed, and the SBS threshold in this situation is lower than the SBS threshold obtained when φM(t)=φNL(L,t). When the SPM is overcompensated for (φM(t) > φNL(L, t)), the spectral linewidth of the output laser cannot be compressed to that of the laser seed either, but the the SBS threshold in this situation is higher than the SBS threshold when φM(t)=φNL(L,t). We also build an experimental setup to verify the feasibility of SPM compensation. In our experiment, the linewidth of the pulsed laser is reduced from 1.4 GHz to 120 MHz when SPM is compensated for by phase modulation. The SBS threshold of the system are measured before and after SPM pre-compensation, and correctness of theoretical simulation is experimentally verified. This analysis method can provide the design guidelines for narrow-linewidth pulsed fiber laser systems.

Narrow-linewidth nanosecond pulsed Raman fiber amplifiers possess many applications such as in nonlinear frequency generation, remote sensing and quantum information. By considering nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), self-phase modulation (SPM) and cross-phase modulation (XPM), we build a nonlinear dynamical model of narrow-linewidth nanosecond pulsed Raman fiber amplifier. A numerical simulation model is also built and the simulation is carried out based on the parallelizable bidirectional finite difference time-domain method. The pulse evolution processes in time and spectral domain are simulated. The influences of pump pulse width, fiber length and signal laser power are studied in detail. It is found that SRS peak power threshold is not influenced by pump pulse width, however, pump pulse width will affect SBS threshold and output linewidth. When the pump pulse width is 800 ns, tens of MHz narrow linewidth can be obtained, but the SBS occurs as the increasing of pump energy, which limits the power scaling of the narrow-linewidth laser pulses. When the pump pulse width is 80 ns, the SBS is effectively suppressed and the peak power can be further increased, but the linewidth of output laser is easily broadened to hundreds of MHz. The simulation results also show that lower SRS threshold and higher efficiency can be obtained by using longer passive fiber, however, if shorter passive fiber is used, SPM and XPM can be weakened and narrower linewidth can be obtained. We build an experimental setup to study the influence of fiber length. In our experiment, a polarization-maintained passive fiber with a core diameter of 10 m and core numerical aperture of 0.08 is used as the Raman gain fiber. The signal laser is a 1120 nm single frequency continuous wave fiber laser with an average power of 20 mW, and the pump laser is a 1064 nm pulsed laser with a pulse width of~40 ns and repetition rate of 500 kHz. When the fiber lengths are 100 m and 80 m, the efficiencies of the pulsed Raman amplifier are, respectively, 51.5% and 38.2% at a pump power of 6.8 W. It can also be found that increasing signal power can increase the efficiency of the amplifier, but it will reduce the SBS threshold at the same time. Therefore, in order to balance the different nonlinear effects in the arrow-linewidth nanosecond pulsed Raman fiber amplifier, we should take laser power, linewidth and efficiency into consideration, and choose the suitable system parameters such as pump pulse width, fiber length and signal power. These analyses can serve as design guidelines for narrow-linewidth nanosecond pulsed fiber Raman amplifiers.

We report an experiment on active phase locking of four all-fiber Yb-doped amplifiers with a fiber combiner. With active phasing, the far field intensity pattern of output beam is locked stable, and beam quality improves.

A two-stage all-fiber single-frequency MOPA is demonstrated. It provides 80W output power with a slope efficiency of 70% at 1064 nm. The phase noise of the fiber amplifiers measured by using multi-dithering technique is less than 50Hz.

Owing to damage, thermal issues, and nonlinear optical effects, the output power of fiber laser has been proven to be limited. Beam combining techniques are the attractive solutions in order to achieve high-power high-brightness fiber laser output. Designing such a high-power laser system relies on coherent and incoherent combination of radiation from multiple laser channels into a single beam with enhanced brightness. Spectral beam combination is a promising alternative way that allows each array to be overlapped in near-and far-field without spatial interference, thus relaxing the requirements for linewidth controlling and phase locking of individual array and practically allowing power and brightness to be scaled with the potential to combine a large number of channels. Spectral beam combination implementations can be divided into two subsets: serial and parallel, based on the combining elements. For scaling high power, we pursue spectral beam combining with parallel subsets as an alternative to other beam combination implementation. In the spectral beam combining system based on multi-layer dielectric grating, the combined beam suffers the degradation in beam quality, which is caused by the optical dispersion, and also by the random error due to the misalignment of arrays or the thermal-optic effect of grating in the experimental system. In this paper, we strictly derive the equation of M2 variation caused by the optical dispersion in both single-grating structure and dual-grating structure. And also, we discuss how the laser linewidth, beam size, spectral separation of two adjacent channels, distance between two adjacent channels and the period of grating influence the desired beam quality in detail, separately, in the single-grating structure and the dual-grating structure. The results show that with the value of M2 fixed, the finite beam size gives rise to a laser bandwidth decreasing in single-grating structure combination, whereas the beam size induces a laser bandwidth to increase in dual-grating structure combination. If M2 1.2, the laser bandwidth of dual-grating system can be over several sub-nanometers, rather than several tens of pm as in the single grating design.

A narrow-linewidth high power Yb-doped fiber laser in all-fiber format is set up. The seed laser has an output power of more than 40 mW with linewidth narrower than 100 MHz. The fiber laser employs three-stage master oscillator power amplifier(MOPA) configuration. It outputs 334 W narrow-linewidth laser when pumped with a total power of 405 W in the main amplifier. The optical-to-optical conversion efficiency is 82.4%. Presently, the output power is limited by the pump power. It can be predicted that the laser power can be further scaled with increased available pump power.

Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degrading beam quality and restricting the enhancement of power output for diffraction-limited high power fiber laser. The experimental study on MI in all-fiberized master oscillator power amplifer (MOPA) is presented in detail for the first time. It is revealed that MI in all-fiberized MOPA results in reducing the slope efficiency. The theoretical threshold power of MI for 20/400 step-index large-mode-area double-cladding Yb-doped fiber is about 1 kW and MI is rooted mainly from the thermal effect.

We present an experimental study on tilt-tip (TT) and phase-locking (PL) control in a coherent beam combination (CBC) system of adaptive fiber laser array. The TT control is performed using the adaptive fiber-optics collimator (AFOC), and the PL control is realized by the phase modulator (PM). Cascaded and simultaneous controls of TT and PL using stochastic parallel gradient descent (SPGD) algorithm are investigated in this paper. Two-fiber-laser-, four-fiber-laser-, and six-fiber-laser-arrays are employed to study the TT and PL control. In the cascaded control system, only one high-speed CMOS camera is used to collect beam data and a computer is used as the controller. In a simultaneous control system one high-speed CMOS camera and one photonic detector (PD) are employed, and a computer and a control circuit based on field programmable gate array (FPGA) are used as the controllers. Experimental results reveal that both cascaded and simultaneous controls of TT using AFOC and PL using PM in fiber laser array are feasible and effective. Cascaded control is more effective in static control situation and simultaneous control can be applied to the dynamic control system directly The control signals of simultaneous PL and TT disturb each other obviously and TT and PL control may compete with each other, so the control effect is limited.