Rumao Tao

1.89 kW all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality.

In this manuscript, we demonstrate high power, all-fiberized and polarization-maintained amplifiers with narrow linewidth and near-diffraction-limited beam quality by simultaneously suppressing detrimental stimulated Brillouin scattering (SBS) and mode instability (MI) effects. Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using three-stage cascaded phase modulation systems. Output powers of 477 W, 1040 W, and 1890 W are achieved with full widths at half maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The MI threshold is increased from ~738 W to 1890 W by coiling the active fiber in the main amplifier. Both the polarization extinction ratio (PER) and beam quality (M2 factor) are maintained well during the power scaling process. To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and near-diffraction-limited beam quality at 2 kW power-level.

Mitigating of modal instabilities in linearly-polarized fiber amplifiers by shifting pump wavelength

We investigated the effect of pump wavelength on the modal instabilities (MI) in high-power linearly polarized Yb-doped fiber amplifiers. We built a novel semi-analytical model to determine the frequency coupling characteristics and power threshold of MI, which indicates promising MI suppression through pumping at an appropriate wavelength. By pumping at 915 nm, the threshold can be enhanced by a factor of 2.1 as compared to that pumped at 976 nm. Based on a high-power linearly polarized fiber amplifier platform, we studied the influence of pump wavelength experimentally. A maximal enhancement factor of 1.9 has been achieved when pumped at 915 nm, which agrees with the theoretical calculation and verified our theoretical model. Furthermore, we show that MI suppression by detuning the pump wavelength is weakened for fiber with a large core-to-cladding ratio.

1.5-kW Yb-Raman Combined Nonlinear Fiber Amplifier at 1120 nm

A high power Yb-Raman combined nonlinear fiber amplifier employing dual-wavelength seed is demonstrated. The amplifier is seeded by 1070- and 1120-nm signal lasers simultaneously. The mechanism of power evolution of the two signals along the gain fiber is analyzed. By careful design and optimization of the amplifier, we make sure the 1120-nm signal extract majority of the gain in the amplifier. In the end, 1.52-kW 1120-nm laser is obtained with an optical efficiency of 75.6%, which is the highest power ever reported near this wavelength. The compact and efficient amplifier scheme proposed in this letter can be employed for high power amplification of 1100-1200-nm lasers.

Study of Wavelength Dependence of Mode Instability Based on a Semi-Analytical Model

We present theoretical study of wavelength dependence of mode instability (MI) in high power fiber lasers, which employs an improved semi-analytical theoretical model. The influence of pump/seed wavelength and photodarkening on threshold has been studied. The results indicate promising MI suppression through pumping or seeding at an appropriate wavelength. Small amounts of photodarkening can lead to a significant impact on MI.

High average/peak power linearly polarized all-fiber picosecond MOPA seeded by mode-locked noise-like pulses

The characteristics of mode-locked noise-like pulses generated from a passively mode-locked fiber oscillator are experimentally investigated. By carefully adjusting the two polarization controllers, stable mode-locked noise-like pulse emission with a high radio frequency signal/noise ratio of >55 dB is successfully achieved, ensuring the safety and possibility of high power amplification. To investigate the amplification characteristics of such pulses, one all-fiber master oscillator power amplifier (MOPA) is built to boost the power and energy of such pulses. Amplified noise-like pulses with average output power of 423 W, repetition rate of 18.71 MHz, pulse energy of 22.61 μJ, pulse duration of 72.1 ps and peak power of 314 kW are obtained. Near diffraction-limited beam is also demonstrated with M2 factor measured at full power operation of ~1.2 in the X and Y directions. The polarization extinction ratio at output power of 183 W is measured to be ~13 dB. To the best of our knowledge, this is the first demonstration of high-power amplification of noise-like pulses and the highest peak power ever reported in all-fiber picosecond MOPAs. The temporal self-compression process of such pulses and high peak power when amplified make it an ideal pump source for generation of high-power supercontinuum. Other potential applications, such as material processing and optical coherent tomography, could also be foreseen.

Coherent polarization beam combination of four mode-locked fiber MOPAs in picosecond regime

In this manuscript, we report on coherent polarization beam combination (CPBC) of a four-channel pulsed amplifier array in the picosecond regime by using single frequency dithering technique. By employing a photo-detector with low-pass bandwidth (8.5 MHz at 10 dB gain) to filter the intensity fluctuation and obtain phase errors for feedback, a combined laser pulse with~480 ps pulse width at~60 MHz repetition rate is obtained with an average power of 88 W. By adjusting the optical path differences (OPDs) and controlling the pump power to ensure the synchronizations and alleviate the influence of nonlinear phases among each channel, more than 90% combining efficiency is achieved with excellent beam quality (M(2)~1.1). Finally, the efficiency loss of the system along with the power scaling process is discussed.

High-Power Narrow-Band and Polarization-Maintained All Fiber Superfluorescent Source

In this letter, we experimentally investigate the near-diffraction-limited power scaling ability of a high-power narrow-band and polarization-maintained all fiber amplifier chain fed by selecting a small portion of a broadband superfluorescent source. By amplifying the filtered laser based on master oscillator power amplifier configuration, a record power of 800 W is achieved in this type of monolithic amplifier with a slope efficiency as high as 87.7% in the final amplifier and M2 factor of ~1.3. The polarization extinction ratio (PER) and 3-dB bandwidth (FWHM) are measured to be >12 dB and 0.2 ± 0.02 nm, respectively. Further power scaling the amplifier chain, we show that the mode instability effect is occurred, which causes sharply deteriorations of the beam quality and PER. Besides, we show that the influence of self-phase modulation effect and four-wave-mixing effect should also be carefully considered to ensure high portion of narrow-band energy when further enhances this kind of narrow-band architecture.

High-power coherent beam polarization combination of fiber lasers: progress and prospect [Invited]

Coherent polarization beam combining (CPBC) of fiber lasers has the potential to scale the output total power while simultaneously maintaining good beam quality. In this paper, we will present the very recent technical advance in a single-channel coherently combinable linearly polarized narrow-linewidth fiber amplifier and high-power CPBC system. The noise property of a recently developed near 2 kW fiber amplifier and its feasibility in a CPBC system, CPBC of four 500-W-level fiber amplifiers and two kilowatt fiber amplifiers are demonstrated for the first time, which is also the first result for a 2 kW CPBC system. We have also performed numerical analysis on the performance scaling of CPBC, and deduced handy design guidelines for CPBC of a 64-element high-power system.

Average spreading and beam quality evolution of Gaussian array beams propagating through oceanic turbulence

The propagation properties of a radial Gaussian beam array through oceanic turbulence are studied analytically. The analytical expressions for the average intensity and the beam quality (power-in-the-bucket (PIB) and M 2-factor) of a radial beam array in a turbulent ocean are derived based on an account of statistical optics methods, the extended Huygens-Fresnel principle, and the second order moments of the Wigner distribution function. The influences of w, e, and χ T on the average intensity are investigated. The array divergence increases and the laser beam spreads as the salinity-induced dominant, e decreased, and χ T increased. Further, the analytical expression of PIB and the M 2-factor in the target plane is obtained. The changes of PIB and the M 2-factor with three oceanic turbulence parameters indicate that the stronger turbulence with a larger w, smaller e, and larger χ T results in the value of PIB decreasing, the value of the M 2-factor increasing, and the beam quality degrading.

1.5 kW, near-diffraction-limited, high-efficiency, single-end-pumped all-fiber-integrated laser oscillator

We report a monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. The laser scheme design and fiber parameter selection are based on our theoretical analysis through one rate equation model consisting of changing pump wavelengths. The laser oscillator is pumped by 49 fiber-pigtailed 915 nm laser diodes with power of 50 W each through two stage combiners. The measured laser power is 1520 W at the pump power of 2054 W, and the corresponding optical-to-optical efficiency is 74%, in agreement with numerical simulation. Stimulated Raman scattering is observed when the laser power reaches 1030 W and the power ratio of Raman power is 4% at maximum output power. The experimental results show that the beam qualities represented by the M(2) factor are less than 1.2 at all pump power levels.