Liangjin Huang

High-power 1018 nm ytterbium-doped fiber laser and its application in tandem pump.

In this paper, we present our experimental results of a high-power 1018 nm fiber laser and its usage in tandem pump. A record output power of 476 W 1018 nm fiber laser is obtained with an efficiency of 78.2%. Utilizing a specially designed gain fiber, a one-stage high-power monolithic fiber amplifier tandem pumped by six 1018 nm fiber lasers is assembled. A 110 W 1090 nm seed is amplified to 2140 W, and the efficiency is as high as 86.9%. The beam quality factor M2 is measured to be 1.9. Limitations and possible solutions for purchasing higher output power are discussed.

1.01 kW superfluorescent source in all-fiberized MOPA configuration

We present a high power all-fiberized master oscillator power amplifier (MOPA) structured superfluorescent source based on dual-cladding ytterbium-doped fiber. The seed source is a 0.814 W homemade amplified spontaneous emission (ASE) source. Two-stage high power fiber amplifier is utilized to boost the seed power to 1.01 kW with a beam quality of M(x)(2) = 1.688 and M(y)(2) = 1.728. The central wavelength of the output light is 1074.4 nm, and the full width at half maximum (FWHM) linewidth is about 8.1 nm. No self pulsing or relaxation oscillation effect is observed and the power fluctuation is less than 2% in 100 seconds continuous operating. In additional, spectral evolution effects of central-wavelength-drifting and linewidth-narrowing of broadband amplification in high power superfluorescent source system are investigated. To the best of our knowledge, this is the first demonstration of an all-fiberized superfluorescent source with output power exceeding kilowatt.

Real-time mode decomposition for few-mode fiber based on numerical method

Today a specific attention has been paid to look into the modal characteristics of the high-power laser beam. And the instantaneous monitoring and analyzing on modal content via the mode decomposition technique will provide a novel route. We implement the first-ever experimental investigation on the real-time mode decomposition technique for few-mode laser beam based on stochastic parallel gradient descent algorithm. It will reduce the cost and the complexity of the mode decomposition system. We have succeeded to decompose the mode spectra as well as calculating the beam quality factor at about 9 Hz monitoring rate, while the high agreement between the measured and reconstructed intensity profiles in each frame indicating the high accuracy and stability during the process. By employing a fiber-squeezing-based polarization controller, the modal content under test can be time-varying automatically.

Analysis and simulation of the phenomenon of secondary spots of the TDI CCD camera irradiated by CW laser

The phenomenon of secondary spots is observed in the experiment of TDI CCD camera irradiated by CW He-Ne laser. It is considered to be related to the scattering of the slit in front of the sensor and the reflection of the window of the TDI CCD chip. Additional experiments and ray tracing simulation are performed to study the mechanism of the secondary spots. The experimental and simulated results demonstrated that the scattering of the side walls of the slit is the main source of the secondary spots. Furthermore, the operation mode of rotary scanning provides the chance of scattering incident beam to the side wall of the slit. This paper will provide a preliminary hint to the optimum design of slit in camera to reduce the effects of the secondary spots to the image quality.

Impact of high-order-mode loss on high-power fiber amplifiers

In this paper, the impact of the loss of high-order mode (HOM) on the performances of high-power fiber amplifiers (HPFA) has been numerically investigated by using the model considering transverse spatial-hole burning. The HPFA is modeled with various parameters such as pump wavelength, pumping direction, and fiber length. The fact that the output power of HOM will decrease with increasing loss of HOM is explained by the numerical analysis for the first time to the best of our knowledge. Moreover, the results show that the extracted efficiency of the amplifier will decrease dramatically with increasing HOM loss when the loss is low. The efficiency will increase smoothly and finally converge, as the growth of the HOM loss in the regime where the efficiency crosses the minimum. The evolution of the output power of HOM and the efficiency can be attributed to the interaction between the loss and the nonuniformly distributed local gain, which will be clearly illustrated by simulations. We believe the results will be beneficial to understanding the impact of the loss of HOM on the amplifier performances and providing guidelines on designing large-mode-area fiber and optimizing the HPFA aiming at effective single-mode output based on differential loss between the HOMs and the fundamental mode.

Adaptive mode control of a few-mode fiber by real-time mode decomposition

A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated. We stress the fiber through an electric-controlled polarization controller, whose driven voltage depends on the current and target modal content difference obtained with the real-time mode decomposition. We have achieved selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor, which may play crucial roles for high-power fiber lasers, fiber based telecommunication systems and other fundamental researches and applications.

Modal Analysis of Fiber Laser Beam by Using Stochastic Parallel Gradient Descent Algorithm

The ability of modal decomposition based on the analysis of the near-field intensity profile using the stochastic parallel gradient descent algorithm is experimentally characterized in detail for the first time. The results show that the decomposition method proposed in this letter turns to be highly accurate and rapid. Further, this method is applied to analyze the mode behavior in dependence on the pump power in a high-power fiber amplifier with different initial mode compositions.

Mode instability dynamics in high-power low-numerical-aperture step-index fiber amplifier

The study on mode instability (MI) in the large-mode-area fiber is generating great interest regarding the high-power applications of fiber lasers. To the best of our knowledge, we have investigated for the first time the dynamics of the output beam from a kilowatt-level all-fiber amplifier based on the low-numerical-aperture (<0.04) step-index (SI) fiber before and after the onset of the MI, including the temporal dynamics and mode evolution. The temporal power fluctuations indicate three evolution stages apart from the onset threshold of the MI, defined as stable, transition, and chaotic regions. In addition, the mode decomposition technique is utilized to accurately observe and investigate the mode evolution and relevant modal content corresponding to the transition and chaotic regions in the SI fiber laser for the first time. According to the mode decomposition results, the reduction of the extracted power can be explained by the high bending loss of the high-order mode excited in the MI process. Finally, the difference of MI dynamics between the fiber lasers based on the SI fiber and rod-type photonic crystal fiber is discussed.

736 W Average Power All-fiber Nanosecond MOPA Based on Ultra-low NA Ytterbium Doped Fiber

We report an all-fiberized high average power pulsed fiber amplifier based on ultra-low NA step-index Yb-doped double-clad fiber. An output power of 736W with M2~1.35 is obtained, which corresponds to 64kW peak-power and 0.39mJ pulse-energy.

Modal decomposition for large-mode-area fibers using stochastic parallel gradient descent algorithm

We have experimentally demonstrated an intensity-based mode decomposition method by using stochastic parallel gradient descent algorithm. Fast and accurate decomposition is performed on a large-mode-area fiber, which can be potentially used in fiber laser.