Yunfeng Qi

High power coherent beam combination from two fiber lasers

Phase locking of two fiber lasers is demonstrated experimentally by the use of a self-imaging resonator with a spatial filter. The high-contrast interference strips of the coherent beam profile are observed. The coherent output power of the fiber array exceeds 12W and the efficiency of coherent power combination is 88% with pump power of 60W. The whole system operates quite stably and, for the spatial filter, no thermal effects have been observed, which means that we can increase the coherent output power further by this method.

Nd:YAG ceramic laser obtained high slope-efficiency of 62% in high power applications

By using quite uniformly nine-stacks side-around arranged compact pumping system, a high power Nd:YAG ceramic quasi-CW laser with high slope efficiency of 62% has been demonstrated. With 450 W quasi-CW stacked laser diode bars pumping at 808 nm, performance of the Nd:YAG ceramic laser with different output coupling mirrors has been investigated. Optimum output power of 236 W at 1064 nm was obtained and corresponding optical-to-optical conversion efficiency was as high as 52.5%. The laser system operated quite stably and no saturation phenomena have been observed, which means higher output laser power could be obtained if injecting higher pumping power. The still-evolving Nd:YAG ceramics are potential super excellent media for high power practical laser applications.

Kilowatt Ytterbium-Raman fiber laser.

A kilowatt-level Raman fiber laser is demonstrated with an integrated Ytterbium-Raman fiber amplifier architecture. A high power Ytterbium-doped fiber master oscillator power amplifier at 1080 nm is seeded with a 1120 nm fiber laser at the same time. By this way, a kilowatt-level Raman pump laser at 1080 nm and signal laser at 1120 nm is combined in the fiber core. The subsequent power conversion from 1080 nm to 1120 nm is accomplished in a 70 m long passive fiber. A 1.28 kW all-fiber Raman amplifier at 1120 nm with an optical efficiency of 70% is demonstrated, limited only by the available pump power. To the best of our knowledge, this is the first report of Raman fiber laser with over one kilowatt output.

52% Optical-to-Optical Conversion Efficiency in a Compact 1.5 W 532 nm Second Harmonic Generation Laser with Intracavity Periodically-Poled MgO:LiNbO3

Intracavity second-harmonic generation of 1.56 and 1.52 W continuous-wave 532 nm green laser radiation was obtained by quasi-phase matching in periodically-poled MgO:LiNbO3 (MgO:PPLN) crystals with lengths of 2 and 1 mm, respectively. The maximum optical-to-optical efficiency achieved was 52%. The intracavity temperature bandwidth was 15 and 12°C for 1 mm crystal and 2 mm crystal, respectively.

High-energy LDA side-pumped electro-optical Q-switched Nd:YAG ceramic laser

By employing a uniformly compact side-pumping system, a high-energy electro-optical Q-switched Nd:YAG ceramic laser has been demonstrated. With 420 W quasi-cw laser-diode-array pumping at 808 nm and a 100 Hz modulating repetition rate, 50 mJ output energy at 1064 nm was obtained with 10 ns pulse width, 5 W average output power, and 5 MW peak power. Its corresponding slope efficiency was 29.8%. The laser system operated quite stably and no saturation phenomena have been observed, which means higher output energy could be expected. Laser parameters between ceramic and single-crystal Nd:YAG lasers have been compared, and pulse characteristics of Nd:YAG ceramic with different repetition rate have been investigated in detail. The still-evolving Nd:YAG ceramics are potential super excellent media for high-energy laser applications.

Experimental study of Ti : sapphire laser end-pumped Nd : YAG ceramic laser Q-switched by Cr4+: YAG saturable absorber

By employing a continuous-wave (CW) Ti:sapphire tunable laser as a pumping source and a Cr4+:YAG single crystal as the saturable absorber (SA), a passively Q-switched Nd:YAG ceramic laser has been demonstrated at room temperature. With an absorbed pumping power of 541 mW at 808 nm, an average output power of 61 mW at 1064 nm has been obtained with 3.5 mu J pulse energy, 15 ns pulse width and 18.18 kHz repetition rate, and the corresponding slope-efficiency is 15%. The relationships between the pulse width, repetition rate, average output power, pulse energy, and peak power on the absorbed pumping power for different initial transmission of the Cr4+:YAG SA are discussed separately. The Nd:YAG ceramic is one of the most promising laser materials for compact, efficient, all-solid-state pulsed lasers.

Mode controlling study on narrow-linewidth and high power all-fiber amplifier

In this paper, we demonstrate an ytterbium-doped all-fiber master-oscillator power amplifier (MOPA) system which uses a narrow-linewidth seed source, generating narrow-linewidth and high power continuous-wave output power at 1064nm. Our MOPA configuration system consist of three amplifier stages. We use single-mode Yb-doped fiber as the gain fiber in the first and second pre-amplifier stages, so it can keep good beam quality before entering the main amplifier stage. In order to raise the threshold of nonlinear effects, such as SBS and SRS, and to relieve heat effect, our high power system choose large mode area (LMA) fiber as the gain fiber in the main amplifier stage. For the sake of suppressing high-order modes in LMA fiber, we design novel watering cooling plates of different sizes, and using them in our main amplifier stage. By optimizing its structure, we get very good laser beam pattern on CCD at high power output. The beam quality factor (M2) was about 1.4 at 1.31 kW.

Dammann-grating-based passive phase locking by an all-optical feedback loop

A Dammann grating is used as a spatial filter for a passive coherent beam combination (CBC) of three Yb-doped fiber amplifiers with an all-optical feedback loop. Using this diffractive-optics-based spatial filtering technique, we demonstrate CBC with 20 W output power, and the visibility of the far-field interference pattern is up to 88.7%. Measurements suggest that this approach is robust with respect to laboratory environment perturbations, and it can scale to high powers and large arrays.

Impact of Phase Perturbation on Passive Phase-Locking Coherent Beam Combination

The impact of phase perturbation on passive phase- locking coherent beam combination is studied. A ring cavity passive phase-locking coherent beam combination system of an array of two fiber lasers is built. The phase perturbation is experimentally simulated with a phase modulator. Experimental results show that the contrast ratio of central fringe in the far- field beam patterns decreases with an increase of the perturbation frequency or amplitude. Theoretical analysis is given.

Single-frequency linearly polarized master-oscillator fiber power amplifier system and its application in high fill factor coherent beam combining

In this paper we combine the master-oscillator power fiber amplifier (MOPFA), active phase-compensation, and beam-tilting techniques to demonstrate high fill factor coherent beam combining. First, we optimize a single-frequency, linearly polarized MOPFA system with high scalability and flexibility based on compact, high efficiency Yb-doped fiber amplifier chains. Second, we demonstrate high fill factor coherent beam combining of these MOPFA arrays at a 50 W level in the far field successfully. Last, the interference matrix of eight element arrays under an opened loop condition is investigated. Scaling the system to higher power can be expected by increasing the power per fiber chain and adding the number of laser channels.