All-polarization-maintaining (PM) fiber oscillator based on nonlinear amplifying loop mirror (NALM) modelocking has important applications for chirped pulse amplification systems. In this paper, we report an all-PM-fiberintegrated femtosecond chirped pulse amplification (CPA) system operating at 1030 nm, which consists of a NALM oscillator, an ytterbium-doped amplifier and a chirped volume Bragg grating (CVBG) compressor. The laser produces linearly polarized, linearly chirped pulses that can be recompressed down to 711 fs, corresponding to 95% compression efficiency. All-PM NALM fiber oscillator combined with CVBG compressor is a promising method for CPA system due to its extremely compact and robust architecture.
Fiber Bragg Gratings (FBG) have great significance in the development of high-power fiber laser systems. Fabrication sources, fibers, and fabrication technology would affect the performance of FBGs directly. In this research, a scanning exposure method for fabricating FBGs in different types of photosensitive fibers with a 213nm solid-state laser source was proposed. The Q-switched Nd: YVO4 laser fifth harmonic source was compared with the traditional excimer lasers and the impacts of lengths, center wavelengths, titled angles, and apodization on the reflection and transmission spectra of FBGs were discussed. The scanning fabrication system, which can realize the tilting of phase mask and the adjustment and monitoring of tension, was built according to the characteristics that the output spot diameter of the fabrication source was sub-millimeter. While the high quality of fabrication was guaranteed, FBGs with lengths of 3mm~20mm, center wavelengths shift of 0nm~4.2nm, and tilted angles of 0~21° were obtained, and all the parameters were simultaneously adjusted and controlled, the deepest transmission loss of about -70dB. FBGs fabricated were of good stability and repeatability, especially in hydrogen fibers whose stability was independent of high-temperature annealing (1500°C) which was different from the fabrication results of excimer laser sources. Then the unconformity between experimental results and theoretical simulation was discussed and the optimization schemes were proposed. The experimental results and analysis provided a better experimental scheme for UV laser fabricating FBGs and the experimental basis for further optimizing the fabrication technology and further analyzing the microscopic mechanism of optical fiber photosensitivity.
In order to reduce the influence of thermal lens effect on beam quality of the multi-pass laser amplifier, a new method of spherical-aberration self-compensation based on ZEMAX physical optical simulation is proposed. Firstly, the change of quality of gaussian beam after passing the thermal lens is simulated according to the principle of geometric optics. And the simulation results show that if two identical thermal lenses are placed symmetrically near the focal point of the laser beam, the degradation of the beam quality caused by the first thermal lens can be compensated by the second thermal lens. Secondly, a four-pass laser amplifier based on the spherical-aberration self-compensation theory is designed by the sequence mode of ZEMAX software. Finally, according to the theoretical model, we design a picosecond fiber-solid hybrid laser amplifier which is seeded by an all polarization-maintaining (PM) fiber laser. The output power of the all PM fiber laser is 2 W. After the solid-state amplifier, the final output laser power reaches 8.5 W with M2 factor of 1.2. The beam quality is well preserved by the four-pass amplification structure which is favorable to the spherical-aberration compensation. This system, which combines the advantages of the all PM fiber amplifier and the solid-state laser amplifier, enables high repetition rate and good beam quality with high gain picosecond pulses. It makes significant contributions to many applications such as material micro-processing, laser ranging and laser detection.
In this paper, the physical model of the coherent polarization beam combination (CPBC) system is established by using Jones-matrix mathematics, and then the coherent-adding mechanisms between the sub-beams in temporal, spatial and spectral domains are analyzed. The intrinsic relationship between optical-field coherence decay and beam misalignment is established from temporal domain (optical-path deviation, phase-locking residual), spatial domain (beam-pointing deviation, spot-overlapping deviation, spot-width error) and spectral domain (B-integral imbalance, dispersion imbalance, central-wavelength drift, spectrum-width error) respectively. Furthermore, the dependence between the combining efficiency and the individual experimentally-measurable misalignment is quantified using numerical simulation, and the error tolerance of the each factor is calibrated separately for efficient combination, which provides quantitative guidance for the practical system. The results of this paper can be applied to the analysis of cascade extended array CPBC system after proper mathematical extension. In addition, this study also provides a feasible quantitative analysis method for the degradation of optical-field coherence between coherent beams, which can be applied to other research fields that require optical coherence management.
We propose a fiber-solid hybrid amplifier system which consists of a semiconductor saturable absorber mirror (SESAM) mode-locked fiber seed with pulse width of 10.6 ps and repetition rate of 17.6 MHz, a two-stage fiber pre-amplifier and a Nd: YAG regenerative amplifier. This structure not only endures high peak power, but also reduces the influence of fiber nonlinear effects. In the regenerative amplifier, a Pockels cell made up of BBO crystal is used as the electro-optical Q switch. It can effectively amplify the fiber seed source while keeping the beam quality constant. Besides, the pulse frequency, round trip time and thermal lens effect of the regenerative amplifier are considered and the stability of the high repetition rate regenerative cavity is further improved. The system achieves average powers of 1.5, 2.5, 3.4 W at the repetition rates of 1, 3, 5 kHz, corresponding to single pulse energies of 1.5, 0.83, 0.68 mJ respectively. And the beam quality factor M2 reaches 1.5 at the output power of 3.4 W. This system will make significant contributions to many fields such as material micro-processing, laser ranging, and laser detection. To make it more systematizing and engineering, we design an engineering prototype of the fiber-solid hybrid amplifier system.
Through simulation calculations and experiments, the two popular methods of generating nanosecond (ns) pulsed laser were studied respectively. A new method for generating a ns pulsed laser with high power and ultra-high repetition frequency by using dispersive fiber after SESAM mode-locked oscillator was proposed. In the simulation calculation of the mode-locked ns pulsed laser, the time-frequency evolution characteristics of the laser formation process were analyzed. The variation process of the stimulated Raman scattering (SRS) effect was obtained at different injection powers. In the simulation calculation of the extracavity modulation ns pulsed laser, the threshold of the stimulated Brillouin scattering (SBS) effect was obtained in the 4-stage fiber amplification experiment. Then, the nonlinear dynamics model of the ns pulsed fiber laser was established by considering various nonlinear effects. The results showed the theoretical models were consistent with the experimental investigations. This provided a new way to further optimize the parameter of ns pulsed fiber lasers and also contribute to solve nonlinear damage problems in such lasers.
We report on a nanosecond all-fiber MOPA with ultra-high repetition rate (tens of MHz), which is seeded by a semiconductor saturable absorber mirror (SESAM) mode-locking fiber oscillator at 1064 nm. Ultra-high repetition rate of tens of MHz is realized by reducing fiber length in laser cavity. By using single-mode fiber stretcher with total length of 16 km, pulse width of mode locking laser pulse can be broadened from 28 ps to 1~7 ns. The maximum average power is achieved to be 54 W with pulse duration of 1.3 ns and repetition rate of 27.7 MHz. To the best of our knowledge, this is the first demonstration on an ultra-high repetition-rate (tens of MHz) nanosecond all-fiber MOPA based on a SESAM passively mode-locking fiber oscillator.
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