Tianzhuo Zhao

Beam shaping and compensation for high-gain Nd:glass amplification

Beam shaping and compensation are reported for a high Nd:glass amplification system based on the liquid crystal spatial light modulation technique. As a typical application, a signal Gaussian beam of 1 nJ, with a pulse width of 3 ns, was amplified to 5 J with a flat-top intensity distribution. A spot modulation of 1.41 on the dimension of 50 mm × 50 mm was obtained. The minor beam distortion was further eliminated by applying a closed loop control. Most parameters including pulse energy, frequency, spot intensity distribution, etc., can be measured and calculated simultaneously via this proposed system. Moreover, the structure of image transfer was confirmed to be capable of maintaining high-quality image transmission in the amplification process.

High quality beam shaping by square soft-edge diaphragm combined with liquid crystal spatial light modulator

The square soft-edge diaphragm with round angle is designed by Matlab, and is sent to a liquid crystal spatial light modulator by the computer. In order to obtain precompensation for the following laser system, local diaphragm transmission can be adjusted by feedback signals of surface-channel charge-coupled device (SCCD). This method can reduce the diffraction effect and realize no modulation, high stability, high homogeneity, and large scale laser beam. Several parameters of soft-edge diaphragms which affect the laser beam quality are studied systematically, and the optimized values are obtained. The method can avoid the serious modulation of hard edges and provide soft-edge diaphragms of different shapes in a fast and convenient way for the large scale laser beam system.

Realizing Gaussian to flat-top beam shaping in traveling-wave amplification

We implement Gaussian to flat-top beam shaping in a master oscillator power amplifier (MOPA) laser system by directing a Gaussian seed laser beam into a side-pumped laser amplifier via traveling-wave amplification. In theory, one can modulate the cross-sectional gain distribution of the working material in a laser amplifier by controlling its absorption coefficient and the distance between its center and a laser diode bar. In this work, this idea is realized using a side-pumped amplifier with a 15-mm-diameter Nd:YAG rod as the working material and 15 laser diode bars arranged around the rod as the pump. With this apparatus, a near-Gaussian signal laser beam, after being subjected to dual-pass amplification, was shaped to an eighth-order super-Gaussian flat-top distribution beam, while simultaneously amplifying the signal laser power from 10.7 mJ to 72.3 mJ.

Simulation and experimental implement of beam-shaping in a side-pumped Nd:YAG amplifier

A side-pumped Nd:YAG amplifier that can realize amplification and beam shaping simultaneously is reported. As a typical application, a Gaussian intensity profile of signal laser was amplified and converted into a flat-top distribution. The main parameters satisfying this requirement involved structural designation and optical parameters adjustment of the pumping laser diode LD, to obtain a specific gain distribution on cross section of the working material. Among them the key considerations include central wavelength of LDs and pumping radius to the centre of Nd:YAG rod. Take an example of 15mm-dameter Nd:YAG side-pumped rod amplifier, where 13.5kW laser diode bars were used, we simulated an uniform flat top laser profile by ray-tracing method. The following experiment shows a good agreement with the simulation. Moreover, gradual absorption coefficient of the working material could also be well compensated while realizing flat-top beam to flat-top beam amplification.

Comparative study of Nd:KGW lasers pumped at 808 nm and 877 nm

The laser performance and thermal analysis of Nd:KGW laser continuously pumped by 808 nm and 877 nm are comparatively investigated. Output power of 670 mW and 1587 mW, with nearly TEM00 mode, are achieved respectively at 808 nm pump and 877 nm pump. Meanwhile, a high-power passively Q-switched Nd:KGW/Cr4+:YAG laser pumped at 877 nm is demonstrated. An average output power of 1495 mW is obtained at pump power of 5.22 W while the laser is operating at repetition of 53.17 kHz. We demonstrate that 877 nm diode laser is a more potential pump source for Nd:KGW lasers.

High Energy, High Efficiency Nd: Glass Regenerative Amplifier

To obtain high energy, high efficiency Nd: glass preamplifier used in certain host device, a Nd:glass regenerative amplifier system has been designed. By adjusting the single pass gain and carefully optimizing the cavity mode, the small-scale self-focusing effect in the gain medium was effectively controlled. Maximum pulse energy of 21mJ, pulse width of 2.6ns was obtained at the repetition frequency of 1Hz, corresponding to a high optical to optical conversion efficiency of 11% and amplification ratio of 108. The pulse to pulse energy stability was < 2% rms during 2 h continous operation. The laser has a good beam quality of M2=1.5. The spectrum was measured to be at center wavelength of 1052.915nm.

Active beam shaping in multi-levels amplification system

Using Liquid Crystal Spatial Light Modulator (LC-SLM) as a beam shaping device to improve beam quality in high-gain amplification system is reported. 1.6 nJ injected small-size signal Gaussian beam can be amplified to 5 J by 4 stages amplification, and finally output beam is a 50mm×50mm square spot with flat-top intensity distribution. In the amplification system we designed, LC-SLM is placed after the second level of amplifier, where the signal laser energy is about 20mJ, and beam size is 10mm×10mm. The structure of Fourier image transfer is also implemented in this amplifications system to be capable of maintaining high-quality image transmission in the amplification process. The LC-SLM as an object, is imaged by beam expand lenses and spatial filters lenses in the amplifications system to get good quality of imaging. By catching output spot and making a feed-back, transmission efficiency of each pixel on LC-SLM is modulated, high energy density area can be decreased to realize flat-top intensity distribution. A spot modulation function is defined as, using the maximum grey value on spot area divided by the average grey value of the image after background correction. By this, amplified laser obtains the spot modulation of 1.24 on central 90% area of the spot. Furthermore, un-uniform distribution on the full spot, soften effects of spot edge, and output beam shape can also be optimized by the LC-SLM shaping scheme in the amplification system.

Thermal effect of large-diameter laser rod in multi-direction side-pump high power pulsed Nd:YAG laser amplifier

In this paper, a 15-direction ring laser diode array is chosen as pumping source in order to get uniform pump in laser medium. The diameter of laser rod is 15mm for obtaining high output laser energy. A numerical model of the side-pump pulsed Nd:YAG laser amplifier is set up. The finite element method using Ansys software is adopted to analyze the time-varying thermal effect. In order to find the temperature influence of the pump light’s distribution, the temperature distributions in laser rod loaded by 15-direction Gaussian beam and simplified uniform beam are calculated and the results are comparatively analyzed. Despite the highest temperature in laser rod is different, the whole variation trend is similar which indicates time-varying characteristic. The thermal lens effect is also calculated and the results indicate that the temperature gradient in the medium plays the most important role. This study could provide a simulation tool to evaluate the thermal effect of the laser amplifier.

Uniform pump scheme for a LD face-pumped Nd:YAG slab amplifier

A new pump-shaping scheme for a LD face-pumped Nd:YAG slab laser amplifier is proposed, aiming to achieve uniform pump distributions. Plano-concave cylindrical mirror arrays are used to homogenize the pump distributions in the LD slow axes, and meanwhile optical-waveguide structures are used for the LD fast axes. Simulations based on ray tracing method indicate that the scheme effectively realizes uniform pump intensity distributions. The fluorescence distributions and small signal gains at different locations both verify the pump uniformity reaches higher than 90%.

Study on influence of pumping spectrum on stable uniform pumping in a side-pumped Nd:YAG amplifier

A new method to realize temperature-stable uniform pumping was presented. Concept of effective absorption coefficient is introduced at first, which is used to indicate actual absorption coefficient of Nd:YAG working materials that concerned the influence of laser diode pumping spectrum characters. After this, flattop and Gaussian as a typical pumping spectrum shaping are discussed, and experimental measured laser diode spectrum is also used to calculate effective absorption coefficient as a comparison. Next, experimental results of pump laser diode deviating from absorption peak of neodymium ion are also numerical analysed, and these results are used to guide pumping central wavelength control. Finally, an optimized laser diode pumping spectrum shape is put forward. With such a pumping spectrum shape, effective absorption spectrum can be optimized to flattop shape, and temperature-stable uniform pumping can be realized.