Jisi Qiu

High beam quality 5 J, 200 Hz Nd:YAG laser system

A high beam quality, all-solid-state Nd:YAG laser system of high-repetition frequency has been built for Thomson scattering diagnosis. A 1.7 times diffraction limited output beam at a pulse energy of 5 J at 1064 nm is achieved for the first time with a pulse duration of 6.6 ns (FWHM) and a repetition rate of 200 Hz; the output energy stability is 4.9% peak-to-valley over 6000 shots. A novel pulsed laser system (Supplementary Fig. S1) with high average power and high beam quality has recently been built by Dr Zhong-Wei Fans group at the Academy of Opto-Electronics, Chinese Academy of Sciences. Both the laser diode side-pumped rod and slab crystals are integrated into the amplifier (AMP) system. A 1.7 times diffraction-limited output beam at a pulse energy of 5 J at 1064 nm is achieved for the first time with a pulse duration of 6.6 ns (FWHM) and a repetition rate of 200 Hz; the output energy stability is 4.9% peak-to-valley over 6000 shots. The test results are shown in Figure 1a and 1b. The laser system is constructed in a master oscillator power amplifier (MOPA) configuration, as shown in Figure 1c, with four components: a single-frequency seed laser, pre-amplifier unit, beam control unit, and post-amplifier unit. The pre-amplifier consists of a three-stage, side-pumped rod amplifier. The dimensions of the rod crystals are φ3 mm× 67 mm with a Nd3+ concentration of 0.8% for AMP1 and AMP2, and φ6.35 mm× 140 mm with a Nd3+ concentration of 0.6% for AMP3 and AMP4. The techniques of the stimulated Brillouin scattering phase-conjugate mirror (SBSPCM) and adaptive optics are implemented in the beam control unit to correct the wavefront distortion dynamically. The postamplifier unit is composed of a three-stage, large slab amplifier. The dimensions of the slab crystals are 138 mm (L) × 35 mm (W)× 7 mm (D) with a Nd3+ concentration of 0.6% for AMP5, AMP6 and AMP7. The single-frequency seed laser produces an output power of 8.58 μJ with a pulse duration of 33.9 ns (FWHM) at a 200-Hz repetition rate. The root-mean-square (RMS) fluctuation in pulse energy is smaller than 1% and the beam quality is better than 1.12 times diffraction limited. The seed pulses first pass through the pre-amplifier and then through the control unit, at which the beam shaping is applied. The pulse energy is amplified to 300 mJ. The pulse duration is 30.5 ns and the beam quality is better than 1.4 times diffraction-limited. After passing through the post-amplifier, the pulse energy reaches 5 J with 3.2 times diffraction limited beam quality. An adaptive optics system is applied for wavefront correction, and the beam quality is improved to 1.7 times diffraction limited. The core parts of the laser system include the single-frequency laser source, slab amplifier module with high-energy storage efficiency, and the phase-conjugated, stimulated Brillouin scattering mirror. The single-frequency source is an active Q-switching laser utilizing acousto-optic modulation. Single-longitudinalmode operation is achieved by applying a Fabry-Pérot (FP) etalon. The length of the resonant cavity can be accurately controlled using piezoelectric ceramics, while sampling precision can be improved using a smoothing algorithm. To compensate for the influence of the environment on the length of the resonant cavity, the fuzzy proportional-integral-derivative (PID) control algorithm is implemented. With the above setup, single-frequency pulses of high stability can be achieved at the nanosecond (ns) level. The single slab amplifier can store as much as 2.5 J, with a small signal gain 45, depolarization loss o2%, pumping homogeneity better than 90%, and single-pass wavefront distortion better than 0.15 λ (RMS) when fully loaded. In the SBS-PCM, FC-770 is chosen as the SBS medium. Attributed to the specific cleaning and fine purification during the preparation processes, the load capacity of the SBS-PCM is thus improved. When pumping energy reaches 1.1 J (220 W, 200 Hz), optical breakdown is prevented and a phase-conjugate reflectivity higher than 98% is achieved.

Two-beam combined 3.36 J, 100 Hz diode-pumped high beam quality Nd:YAG laser system

In this paper, we develop a diode-pumped all-solid-state high-energy and high beam quality Nd:YAG laser system. A master oscillator power amplifier structure is used to provide a high pulse energy laser output with a high repetition rate. In order to decrease the amplifier working current so as to reduce the impact of the thermal effect on the beam quality, a beam splitting-amplifying-combining scheme is adopted. The energy extraction efficiency of the laser system is 50.68%. We achieve 3.36 J pulse energy at a 100 Hz repetition rate with a pulse duration of 7.1 ns, a far-field beam spot 1.71 times the diffraction limit, and 1.07% energy stability (RMS).

High efficiency and good beam quality of electro-optic, cavity-dumped and double-end pumped Nd:YLF laser

In this paper, we describe a Nd:YLF laser based on high-speed RTP electro-optical cavity dumping technique. Two home-made 150 W fiber pump modules are used from both sides to pump Nd:YLF crystal. Coupling systems are the key elements in end-pumped solid-state lasers, the aberrations of which greatly affect the efficiency of the lasers. In order to get high efficient and good quality laser output, the optical software ZEMAX is used to design a four-piece coupling system. When the pumped energy is 32 mJ at the repetition rate of 1 Hz, the output energy is 6.5 mJ with 2.5 ns pulse width. When the pumped energy is 13.1 W at the repetition rate of 200 Hz, the output energy is 2.2 W with small M2 factor where Mx2 is 1.04, and My2 is 1.05, and the light-light conversion efficiency is up to 16.8%.

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.

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.