Qingsong Gao

1-J 500-Hz MOPA solid state laser pumped by diodes

A diodes pumped high average power Nd:YAG rod E-O Q-switched MOPA system was designed, fabricated and tested. The laser achieved a maximum average power of 600W in a near top-hat beam with the repetition rate of 500Hz, pulse width of 15 ns. Three different type diode pump laser module with the pump power of 500W, 6.5kW and 12kW were successfully developed for master-oscillator and amplifiers. The modules have an efficient absorption of the pumped light and homogeneous pump beam distribution in the laser medium.

High-efficiency, high-average-power, CW Yb:YAG zigzag slab master oscillator power amplifier at room temperature.

We demonstrate a high-efficiency, high-average-power, CW master oscillator power amplifier based on a conduction-cooled, end-pumped Yb:YAG slab architecture at room temperature (RT). Firstly, the CW amplification property is theoretically analyzed based on the kinetics model for Yb:YAG. To realize high-efficiency laser amplification extraction for RT Yb:YAG, not only intense pump but also a high-power seed laser is of great importance. Experimentally, a composite Yb:YAG crystal slab with three doped and two un-doped segments symmetrically is employed as the gain medium, which is end-pumped by two high-power, 940-nm diode lasers. A high-power, narrow-spectral-width, 1030-nm fiber seed laser then double passes the composite slab to realize efficient power amplification. For 0.8-kW seed input, maximum output power of 3.54 kW is obtained at 6.7 kW of pump power, with the optical conversion efficiency of 41% and the highest slope efficiency of 59%. To the best of our knowledge, this is the highest power and efficiency reported for Yb:YAG lasing at RT except thin-disk lasers.

Performance of large aperture tapered fiber phase conjugate mirror with high pulse energy and 1-kHz repetition rate

A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M(2) factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.

Quasi-continuous-wave, laser-diode-end-pumped Yb:YAG zigzag slab oscillator with high brightness at room temperature

An experimental demonstration of a quasi-continuous-wave, laser-diode (LD)-end-pumped Yb:YAG zigzag slab oscillator with a high efficiency and power at room temperature has been presented. An image-inverting resonator was used in the slab laser to avoid beam excursion to the slab end and laser power instability. The emitted laser had an average power of 330 W at 1030 nm when the average pump power was 712 W at 940 nm with a repetition rate of 10 Hz and a pulse duration of 10 ms, which corresponded to an optical conversion efficiency of 46.3% and a slope efficiency of 56.0%.

944 mJ Nd:YAG planar waveguide laser amplifier with the optical to optical efficiency of 52%

We demonstrated a quasi-continuous wave (QCW) Nd:YAG planar waveguide laser amplifier with the single pulse output energy of 944 mJ. At the pulse repetition frequency (PRF) of 100 Hz, 1064 nm seed laser was coupled into the planar waveguide laser amplifier with the single pulse energy of 128 mJ and the pulse duration of 192 μs. With the total pump energy of 1582 mJ, the extracted energy of 816 mJ was obtained, leading to an optical to optical efficiency of 52% and an electrical to optical efficiency of 28%. To the best our knowledge, it is up to now the highest efficiency and output energy in Nd:YAG planar waveguide laser amplifier reported. At the maximum output, the beam quality factors M2 were measured to be 3.44 and 4.81 in the guided direction and unguided direction respectively, and the polarization degree was 98.6%. Higher average power could be obtained with the better performance of seeder oscillator.

7kW direct-liquid-cooled side-pumped Nd:YAG multi-disk laser resonator

A direct-liquid-cooled side-pumped Nd:YAG multi-disk QCW laser resonator is presented, in which the oscillating laser propagates through multiple thin disks and cooling flow layers in Brewster angle. Twenty Nd:YAG thin disks side-pumped by LD arrays are directly cooled by flowing deuteroxide at the end surfaces. A laser output with the highest pulse energy of 17.04 J is obtained at the pulse width of 250 μs and repetition rate of 25 Hz, corresponding to an optical-optical efficiency of 34.1% and a slope efficiency of 44.5%. The maximum average output power of 7.48 kW is achieved at the repetition rate of 500 Hz. Due to thermal effects, the corresponding optical-optical efficiency decreases to 30%. Under the 12.5 kW pumping condition while not oscillating, the wavefront of a He-Ne probe passing through the gain module is as low as 0.256 μm (RMS) with the defocus and tetrafoil subtracted.

Nd:YAG thin-disk laser with large dynamic range unstable resonance

In this paper, based on the self-reproduction condition of laser wavefront curvature, the influences of disk defocus on laser parameters can be calculated. The laser-pumping overlapping efficiency will decrease by 9%; the magnification will rise to 2.3, and the intra-cavity loss will be high to 30% due to a laser beam size mismatch when each disk has focal length of -100 m in a positive-branch confocal unstable resonator containing four disks with magnification of 1.8. Therefore, the optical conversion efficiency and stability will be reduced significantly. Several methods defocus compensation of are compared, it can be found that inserting variable-focus lens in resonant is useful in large dynamic range. In experiment, the defocus values are measured in different pumping power. A lens group, used for compensate components according to the single pass probe, is carefully designed. Under this compensation, the pulse energy can be maintained in 10 J from 1 Hz to 100 Hz. The output power can be improved 2.33 times compared to non-compensation condition.

Study on the amplifier experiment of end-pumped long pulse slab laser

The amplifier experiment research of end-pumped long pulse slab laser is developed, the results of out-put energy, optical-optical efficiency and pulse waveform are obtained at different experiment conditions, such as peak pumped power, amplifier power and pumped pulse width. The seed laser is CW fundamental transverse-mode operation fiber laser, the laser medium is composited Nd:YAG slab. Under end-pumped and the 2 passes, the laser obtain 7.65J out-put energy and 43.1% optical-optical efficiency with 45kW peak-pumped power and 386μs pump pulse width. The experimental results provide the basic for the optimization design to high frequency, high energy and high beam-quality slab lasers.

High Power Continuous Wave Yb:YAG Composite Crystal Zigzag Slab Amplifier at Room Temperature

A 7.07-kW Yb:YAG composite crystal zigzag slab amplifier at room temperature is developed and demonstrated, the optical-to-optical efficiency is about 29.5% and the slope efficiency is 39.5%, respectively, with the pumping power of 19.98 kW and the incident seed laser power of 1.18 kW. The influence of the pump coupling efficiency, the intensity of the seed laser, and the temperature to the extracted power from the Yb:YAG slab are analyzed by simulation. The fluorescence distribution at the end face and the transmission wavefront of the Yb:YAG slab are measured, the nonuniformity of the fluorescence distribution is less than 6%, and the P-V value of the transmission wave front distortion is less than 1.6 μm.

Experimental design of double-cladding planar waveguide laser amplifier

An end-pumped laser amplifier with high efficiency and compactness was designed. The thermal stress of symmetrical single-cladding and double-cladding planar waveguide with Nd:YAG as its core were analyzed theoretically, and the maximum thermal load for each were also obtained according to the stress fracture limit. For different inner cladding thickness, the maximum pump power that could be absorbed was calculated. The laser medium was chosen to be a symmetrical double-cladding planar waveguide with Nd:YAG as its core, the dimensions of the gain area were 50mm×12mm×100μm, and the waveguide were 60mm×12mm×2mm. The single thickness of inner cladding YAG and the outer cladding sapphire were 250μm and 700μm respectively. The size of LDA pump light was 18.9mm and 10mm respectively in fast axis and slow axis. The seeder was coupled into the waveguide from one end, and the outer cladding were welded with two heat sink for heat transfer. By theoretical calculation, if the seeder into the waveguide is 0.1W, the outputting power of 1651W could be obtained when the pump power from the LD array was 3600W, with the optical-optical efficiency of 46%.