A high-power 1064 nm single-frequency polarization-maintained fiber amplifier based on an all-fiber master oscillator power amplifier configuration is demonstrated. To mitigate the stimulated Brillouin scattering (SBS) and the mode instability (MI) effect, a polarization-maintained Yb-doped fiber with a high dopant concentration and a 25 μm core diameter is adopted in the main amplifier stage; in addition, step-distributed longitudinal strain is imposed on the active fiber to broaden its effective SBS gain spectrum and further increase the SBS threshold. As a result, a pump-limited 414 W single-frequency fiber laser is obtained without signs of SBS and MI. Experimental results show that the SBS threshold is increased by at least two times. The slope efficiency of the main amplifier is about 80%. The polarization degree is higher than 98% at all the power levels. The beam quality is measured with a $M^2$ of 1.34. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on an all-fiber structure.

A non-null annular subaperture stitching interferometry (NASSI), combining the subaperture stitching ideal and non-null test method, is proposed for steep aspheric testing. Compared with standard annular subaperture stitching interferometry (ASSI), a partial null lens (PNL) is employed as an alternative to the transmission sphere, to generate different aspherical wavefronts as the references. The coverage subaperture number would be reduced greatly for the better performance of aspherical wavefronts in matching the local slope of aspheric surfaces. In this way, relatively large overlapping areas can be obtained for adjustment errors correction while the error accumulation would be decreased. With the reverse optimization reconstruction (ROR) method for retrace error correction, the figure error of each subaperture can be retrieved accurately. Therefore, the testing accuracy and efficiency are thus increased. The dynamic test range is extended as well. A numerical simulation exhibits the comparison of the performance of the NASSI and standard ASSI, which demonstrates the high accuracy of the NASSI in testing steep aspheric. Experimental results of NASSI are shown to be in good agreement with those of the Zygo interferometer.

The high absorption confined-doped ytterbium fiber with 40/250 μm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 μm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.

Random fiber laser, as a kind of novel fiber laser that utilizes random distributed feedback as well as Raman gain, has become a research focus owing to its advantages of wavelength flexibility, modeless property and output stability. Herein, a tunable optical parametric oscillator (OPO) enabled by a random fiber laser is reported for the first time. By exploiting a tunable random fiber laser to pump the OPO, the central wavelength of idler light can be continuously tuned from 3977.34 to 4059.65 nm with stable temporal average output power. The maximal output power achieved is 2.07 W. So far as we know, this is the first demonstration of a continuous-wave tunable OPO pumped by a tunable random fiber laser, which could not only provide a new approach for achieving tunable mid-infrared (MIR) emission, but also extend the application scenarios of random fiber lasers.

In this work, the impact of fiber bending and mode content on transverse mode instability (TMI) is investigated. Based on a modified stimulated thermal Rayleigh scattering (STRS) model considering the gain competition between transverse modes, we theoretically detailed the TMI threshold under various mode content and bending conditions in few-mode fibers. Our theoretical calculations demonstrate that larger bending diameters increase the high order mode (HOM) components in the amplifier, which in turn reduces the frequency-shifted Stokes LP_11o mode due to the inter-mode gain competition mechanism, thus improving the TMI threshold of few-mode amplifiers. The experimental results agree with the simulation. Finally, by optimizing the bending, an 8.38 kW output tandem pumped fiber amplifier is obtained with a beam quality M² of 1.8. Both TMI and stimulated Raman scattering (SRS) are well suppressed at the maximum power. This work provides a comprehensive analysis of the TMI in few-mode amplifiers and offers a practical method to realize high-power high-brightness fiber lasers.