Abstract
1 Introduction
Over the recent decades, diode-pumped solid-state lasers (DPSSLs) have attracted much attention due to their many advantages, which include high overall efficiency, low thermal load, compactness. However, in order to achieve high energy and high repetition rate, the choices of amplifier geometry and thermal management are important aspects of DPSSLs to consider. Gas-cooled multislab configurations, where slab surfaces are cooled directly by a high velocity stream of gas flowing between the slabs, and transversely to the direction of propagation, offer a promising solution by providing a large gain length while maintaining a high surface-to-volume ratio, efficiently removing residual heat[
Gas-cooled multislab designs are widely used both in modern and older high-energy DPSSL systems, including HAPLS and Mercury at the Lawrence Livermore National Laboratory in USA[
In this work, we demonstrated a laser-diode-pumped gas-cooled multislab laser amplifier. We selected Nd:phosphate glass as the gain medium owing to its sufficient storage lifetime , high saturation fluence to store energy, and high quality. Compared to Yb-doped materials, Nd:phosphate glass also meets the requirements for efficient high-average-power operation, while possessing an easy-to-realize large aperture[
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2 Design and experimental setup
The experimental setup of the diode-pumped gas-cooled multislab Nd:glass laser amplifier system presented in this work is shown in Figure
The main amplifier head consisted of four square Nd:phosphate glass slabs (NAP2), as shown in Figure
Performance | Value |
---|---|
Refractive index | 1.542 |
Fluorescence lifetime | 360 |
Temp. coeff. refractive index () () | 8.7 |
Thermal expansion coeff. () () | 82 |
Thermal conductivity () | 0.83 |
Density () | 2.76 |
Elastic modulus () | 58 |
Poisson’s ratio | 0.232 |
Table 1. Optical and thermal properties of Nd:glass slabs (NAP2).
The gain medium module was longitudinally pumped from a single side by two laser diode arrays as pump sources. Each array contained 60 LD bars with an emitting area of . With the fast axis collimated by cylindrical micro-lenses, each array emitted a maximum output power of 20 kW at a wavelength of about 802 nm with a coolant temperature of . A fringe mirror placed at was used to couple the output beams from the two arrays, as shown in Figure
A multipass architecture was used in the main amplifier for efficient extraction of the stored energy. This means a pair of 4f image-relaying Keplerian vacuum telescopes was incorporated per pass, one on each side of the amplifier head. Plano-convex fused silica singlet lenses with an effective focal length of 750 mm were used in each 1:1 image-relaying telescope, ensuring beam quality was maintained. To filter high spatial frequencies and prevent propagation of stray light, flat-plate pinholes were installed at the focal plane of each telescopes. Each pinhole plate has a circular aperture diameter of 3 mm, about 30 times the diffraction limit of the beam. KD*P-Pockels cell, plates, and a polarizer were used for seed pulse trapping and amplified pulse dumping. When the seed pulse passed through the Pockels cell, the quarter-wave voltage was applied to the Pockels cell, allowing the pulse to be trapped and amplified by the main amplifier head. A Faraday rotator was used to precautionary compensate for the thermally induced birefringence in the Nd:glass slabs, where the beam polarization will rotate by after two passes through the Faraday rotator. In order to maintain linear polarization when the laser beam propagates through a polarization beam splitter (PBS), another Faraday rotator was set up before the PBS.
3 Experimental results and discussion
The experiment of helium gas-cooled Nd:glass multislab laser amplifier was carried out using the setup described in Section
The transmitted single-pass gain distribution, shown in Figure
To assess the level of thermally induced wavefront aberration within the amplifier head at a repetition rate of 0.2 Hz for a pump energy of 7.3 J, single-pass wavefront distortion was measured experimentally. The lens positions in the spatial filters were adjusted to minimize defocus in the system before measurement. Numerical predictions were compared with experimentally measured data, as shown in Figure
To identify the main types of aberration observed on the square beam propagated through the amplifier, the measured wavefronts were decomposed into 2D-Legendre polynomials. Analytical results for the first 21 polynomial coefficients are shown in Figure
After wavefront assessment, a pulse from the front end was injected into the main amplifier; the beam was propagated through the amplifier repeatedly until reaching the maximum output energy. Figure
4 Conclusion
We have reported on the results obtained from a LD-pumped gas-cooled multislab Nd:glass laser amplifier. At a repetition rate of 0.2 Hz, the amplifier generated 0.5 J of energy at 1053 nm for a pump energy of 7.3 J at 802 nm, while at 0.5 Hz an output energy of 0.43 J was obtained. Numerical models predicted that an energy output of 1.2 J could be obtained from the amplifier for a gain of 2.6. After shaping and homogenizing, the pump intensity distribution reached up to 85%. We also experimentally measured the single-pass thermally induced wavefront aberrations under the former conditions, which was approximately 96 nm P–V. This confirms the viability of the proposed multislab Nd:glass amplifier concept, which is scalable for larger apertures and higher energy levels. Further increases in output energies greater than 10 J are expected for design apertures of .
References
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