Thermal-induced wavefront aberration in sapphire-cooled Nd:glass slab

Abstract

and good beam quality, which can be achieved with diodepumped solid-state lasers (DPSSLs) [1]. DPSSLs can also provide good overlap efficiency between pumps and laser beams simultaneously. Current DPSSL development projects include mercury, with an output energy and repetition rate of 62 J and 10 Hz, respectively [2], LUCIA, with 6.6 J and 2 Hz [3], HALNA, with 8.4 J and 10 Hz [4], and dipole, with 6 J and 10 Hz [5].For efficient diode-pumped operation, the gain medium requires a long fluorescence lifetime in order to minimize the cost, few quantum defects to minimize the heat generation, and a high-gain cross section to extract energy efficiently. Nd:glass is a suitable material for high-energy DPSSL amplifiers as it exhibits long fluorescence lifetimes, acceptable gain cross sections, and few quantum defects. Furthermore, Nd:glass is available in large size with good optical quality, able to handle high pulse energies. However, phosphate glass has lower thermal conductivity and lower thermal shock resistance than other host materials, such as yttrium aluminum garnet (YAG). When operating at high repetition rates, the thermal problem is the main challenge for Nd:glass laser amplifier design, including thermal-induced wavefront aberration and depolarization loss; worst of all, it will lead to thermal fracture in the gain medium.Slab geometry can provide a larger cooling surface, although rod geometry is more frequently used for Nd:glass now [6,7]. Efficient heat removal is a key task for Nd:glass slab amplifier design. Current slab amplifiers with high repetition rates mainly use gas or liquid as a cooling medium [8,9]. However, these cooling systems may have negative effects on laser quality. As laser beams need to transmit through the cooling medium, solid materials may be a stable promising cooling strategy [10]. A team at the US Army Research Laboratory [11] and a Abstract We demonstrate for the first time a sapphirecooled Nd:glass composite assembly based on optical bonding of two thin sapphire plates to a Nd:glass slab for efficient heat removal. The distributions of temperature, stress, depolarization loss, and wavefront aberration were obtained by finite element analysis. The simulation results were verified experimentally. Although the heat generation rate was 4.5 W/cm 3 , the temperature increase was within 5.7 °C at the center of the sapphire surface, and the whole wavefront aberration was 1.21 λ (λ = 1053 nm). This demonstration opens up a viable path toward novel repetition rate Nd:glass laser amplifier designs with efficient doublesided room-temperature heat sinking on both sides of the slab.

Numerical Simulation of Thermo-Optic Effects in an Nd: Glass Slab with Low Thermally Induced Wavefront Distortion

“…Furthermore, the HAPLS project notably achieved an output of 97 J at 3.3 Hz in 2017, employing Nd: glass as the gain medium in the laser system [18,19]. The thermo-optic effects, including wavefront distortion [20], thermal depolarization [21], and thermal lensing effects [22], are inevitable in the operation of the laser amplifier. For a multislab gas-cooled laser amplifier, the temperature gradient is mainly concentrated around the pump area, and the strong temperature field leads to wavefront distortion, which is difficult to compensate.…”