In this paper, the numerical steady-state model of mode competition in high power ytterbium-doped fiber amplifiers (YDFA) which considering the distributed photodarkening (PD) loss has been studied and developed for the first time. According to the relationship between PD loss and the upper-state population fraction, the formula of saturated PD loss which can distinguish different modes is established. Based on the rate-equation model considering transverse spatial-hole burning, we calculate the longitudinal distribution and quantitative results of each mode’s PD loss for the first time to the best of our knowledge. The dependence of parameters such as pumping direction, fiber length, and fiber doping composition on YDFA is also investigated and discussed. According to our simulation results, the output power of YDFA is strongly affected by PD loss. Meanwhile, PD increases the loss of higher-order mode (HOM), as well as enhances the ratio between fundamental mode (FM) and HOM. Thus, HOM suppression and improvement of beam quality can be expected within PD effect. In our work, the mode competition of high power YDFA under varying degrees of PD loss has been simulated. Obviously the output power will decrease with increasing the PD loss. However the ratio between FM and HOM has no evident changes with varying the PD loss, which providing guideline on appropriately using PD loss to regulate the mode competition between FM and HOM aiming at effective single-mode output of YDFA.
Numerical modal decomposition (MD) is an effective approach to reveal modal characteristics in high power fiber lasers. The main challenge is to find a suitable multi-dimensional optimization algorithm to reveal exact superposition of eigenmodes, especially for multimode fiber. A novel hybrid genetic global optimization algorithm, named GA-SPGD, which combines the advantages of genetic algorithm (GA) and stochastic parallel gradient descent (SPGD) algorithm, is firstly proposed to reduce local minima possibilities from sensitivity initial values. Firstly, GA is applied to search the rough global optimization position based on near-far-field intensity distribution with high accuracy. Upon those initial values, SPGD algorithm is afterwards used to find the exact optimization values based on near-field intensity distribution with fast convergence speed. Numerical simulations validate the feasibility and reliability.
A high power single-frequency fiber amplifier with linear polarization is demonstrated based on the master oscillator power amplifier configuration, consisting of a single-frequency seed laser at 1064.1 nm and three-stage amplifiers. To suppress stimulated Brillouin scattering, a short piece of polarization-maintaining active fiber with large core diameter of 25 μm and high pump absorption coefficient is adopted in the main amplifier. Besides, step-distributed longitudinal strain is intentionally imposed on the active fiber to broaden the effective SBS gain spectrum and correspondingly improve the SBS threshold. As a result, a pump-limited single-frequency output of 414 W is obtained with no sign of SBS and mode instability, experimentally showing that the SBS threshold is improved by at least two times through introducing extra strain.. The slope efficiency of the main amplifier is about 80 %. The polarization degree is better than 98 % at all the power levels. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on all-fiber structure.
We present an annular laser guide star (LGS) concept for large ground-based telescopes in this paper. The more stable annular LGS is generated by turbulence-resisted vortex beam. In the uplink, a vortex beam tends to wander more slightly than a Gaussian beam does in atmospheric turbulence. This may enable an annular LGS to wander more slightly than a traditional Gaussian beam generated LGS does, which would ease the burden of uplink tip-tilt mirror and benefit a dynamical closed-loop adaptive optics system. We conducted numerical simulation to validate the feasibility of this concept. And we have gotten 31% reduced variance of spot wandering of annular LGS. Besides, we set up a spatial light modulator based laser guide star simulator for beam propagation in turbulent atmosphere to experimentally test the annular LGS concept. Preliminary experimental results are given. To the best of our knowledge, it is the first time this concept is formulated.
Sodium laser guide star (LGS) is the key for the success of modern adaptive optics (AO) supported large ground based telescopes, however, for many field applications, Sodium LGS’s brightness is still a limited factor. Large amounts of theoretical efforts have been paid to optimize Sodium LGS exciting parameters, that is, to fully discover potential of harsh environment surrounding mesospheric extreme thin sodium atoms under resonant excitation, whether quantum or Monte Carlo based. But till to now, only limited proposals are demonstrated with on-sky test due to the high cost and engineering complexities. To bridge the gap between theoretical modeling and on-sky test, we built a magnetic field controllable sodium cell based lab-bench, which includes a small scale sum-frequency single mode 589nm laser, with added amplitude, polarization, and phase modulators. We could perform quantitative resonant fluorescence study under single, multi-frequency, side-band optical re-pumping exciting with different polarization, also we could perform optical field modulation to study Larmor precession which is considered as one of devils of Sodium LGS, and we have the ability to generate beams contain orbital angular moment. Our preliminary sodium cell based optical re-pumping experiments have shown excellent consistence with Bloch equation predicted results, other experimental results will also be presented in the report, and these results will give a direct support that sodium cell based lab-bench study could help a Sodium LGS scientists a lot before their on-sky test.
There is always a challenge on large aperture medium power laser’s resonator design, stable resonator would supports significant higher order transverse modes, folded and telescope stable resonator are too complex and not preferred by engineers, unstable resonator need rather large round trip gain to compensate its high geometric out-coupling, which is difficult for this kind of laser since its gain length is limited due to the power level and large aperture. Partial feedback unstable resonator had been proposed to tackle this difficulty since the early days of laser development, however, the debates of its effect never stopped even with those distinguished optical resonator scientists such as Siegman, Anan’ev, and Weber. Recently integrated partial feedback unstable resonator design had been successfully demonstrated on a medium size chemical oxygen iodine laser. In this paper, we carry this resonator configuration on a small scale discharge driven supersonic nozzle array Hydrogen Fluoride chemical laser, a typical large aperture short gain length device. With magnification equals 4/3, we successfully get ten Watts level ring beam output.
Under cavity loss modulation, enormous dynamic behaviors will come up in B type cascade laser. In this paper, we built a two-wavelength cascade transition balanced equation model for CW HF chemical laser, 2P(5) and 1P(6) line were taken as cascade pair. We carried out cavity loss modulation experiment on a small scale supersonic discharge driven hydrogen fluoride chemical laser. From the view of phase space, we explained the characteristics by nonlinear time series analysis tools.