As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced self-pulsing in polarization maintaining (PM) fiber amplifiers is well characterized and analyzed in this paper by comparing different white noise signal (WNS) phase-modulated modes in experiments. It is found that the self-pulsing effect is not observed in the PM-amplifier with single-frequency laser seed injection, and cascaded WNS modulation provides superior self-pulsing suppression than single WNS modulation with similar output linewidth. Moreover, the experimental results indicate that the self-pulsing threshold can hardly be predicted only by the output linewidth or the defined SBS threshold in a WNS phase modulated fiber amplifier system. As self-pulsing is originated from the spectral spikes in WNS modulated system, we theoretically analyzed characteristics of these spikes in different phase-modulation modes. It indicates the spectral peak intensity can be reduced by cascaded modulation, for which self-pulsing can be suppressed. The theoretical predictions agree well with the experimental results. At the same time, in order to suppress the mode instability effect, a plum blossom shaped bending mode selection device is used in this high-power narrow linewidth fiber amplifier system. Finally, a 32 GHz cascaded WNSs modulated, over than 2.5 kW linearly polarized all-fiber amplifier with a slope efficiency of 86.7% is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M2 maintains lower than 1.3 in the power scaling process.

For the inter-modal four-wave mixing (IMFWM) in high-power fiber lasers, the phase-matching frequency shift and coherence length are calculated to determine the fiber mode combinations corresponding to IMFWM peaks; then, the parameters of the laser are optimized accordingly to suppress the IMFWM. On the basis of laser parameter optimization, the fiber coiling method is applied to further suppress the IMFWM. To validate this method, a master oscillator power amplifier was configured with 20/400 fiber to produce IMFWM peaks at 1108 and 1071.6 nm in the output spectrum, and then those peaks were removed by reducing the bending radius of the fiber.

A complete complex amplitude reconstruction method for the determination of the laser beam quality factor $M^2$ based on the multiple exposure of a quadriwave lateral shearing interferometer (QWLSI) is presented. The theoretical analysis and simulation of the influence of the information in the small signal area on the calculation of the $M^2$ factor is provided. The experimental results demonstrate that the new method can be an accurate means to measure the $M^2$ factor. The proposed method can avoid the influence of phase inaccuracy in the small signal area of the interferogram, during the measurement of the $M^2$ factor.

Recently, the growing interest in few-mode fibers in telecommunications and high-power lasers has stimulated the demand for fiber mode decomposition (MD). Here we present a fast fiber MD method with a lensless fiber-point-diffraction interferometer. The complex amplitude at the fiber end is achieved by the polarization phase-shifting technique and the lensless imaging technique. Then, the eigenmode coefficients are determined by the mode orthogonal operations of the complex amplitude. In the experiment, the SMF-28e fiber containing 10 linear polarized modes at the wavelength of 632.8 nm is studied for MD. The decomposition of the 50 * 50 pixels interferograms takes only 0.0168 s. The similarity of the intensity patterns of the testing light is larger than 97% before and after the MD. This new, to the best of our knowledge, method can achieve fast and accurate 10-mode MD without using any imaging systems.

Isolated defects on the surface of the inertial confinement fusion (ICF) capsule reduce the probability of ignition. Here, to the best of our knowledge, we present the first null interferometric microscope (NIM) for direct and large-field surface defects detection on ICF capsules. The planar reference mirror in conventional interferometric microscopes is replaced by a spherical reference mirror to achieve null interference in the full field of view. Further, via the use of a short-coherence light source system, parasitic fringes are avoided. The feasibility of the NIM is verified via experiments on a 0.7 mm diameter capsule. A 1 mm diameter ICF capsule is also tested by the NIM to prove that the NIM has the ability to measure capsules with different diameters.

In this paper, Stokes light induced modulation instability (MI) in high power continuous wave (CW) fiber amplifiers is observed. The investigation shows that the Stokes light generated by inter-modal four wave mixing (IMFWM) and stimulated Raman scattering (SRS) in high power fiber amplifiers can be modulated by the signal light through XPM and cause MI. Then, a sideband will be generated around the second-order Raman frequency shift, which is amplified by SRS and shown as a train of pulses in time domain. It is shown that the frequency shift of the sideband will be influenced by IMFWM and SRS. In addition, the sideband was found to be blue-shifted with the increase of the power, which indicates that the frequency shift of the sideband is mainly depended on MI, while SRS plays the role of amplification.