Modelling of Thermal Stress in Yb:YAG to Quantify Depolarisation in a Nanosecond 10 J, 100 Hz Laser

G. Quinn1, M. De Vido1
1Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, United Kingdom
Published in 2023

The high heat loads intrinsically associated with high-energy, high repetition rate laser systems require sophisticated thermal management analyses to minimise the impact of thermal effects on optical performance. Non-uniform heat deposition in optical elements can lead to the onset of thermally-induced stress birefringence which reduces the efficiency of lasers using polarisation-sensitive components. As a result, a model has been developed to simulate thermal effects to optimise the design of a 1030 nm laser based on the DiPOLE concept operating at 10 J, 100 Hz.

The 5 mm thick gain medium slabs are comprised of a central Yb:YAG region, 45 mm in diameter, surrounded by a Cr:YAG cladding with a width of 5 mm. The system will have six slabs in the amplifier which are face-pumped on both sides by two 940 nm diode pump sources (21 J, 0.5 ms pulses at 100 Hz from each pump source). The Yb-doping concentration across the slab set is graded to equalise the amount of pump power absorbed by each slab.

Heating within the slab arises via two processes: the quantum defect depositing heat within the pumped volume, and absorption of fluorescence emitted from the pumped volume in the cladding. Heat is extracted by a flow of 14.5 bar helium gas at 140 K and a mass flow rate of 200 g/s. This results in a non-uniform temperature gradient across the slab.

The simulation takes place in three stages: • Ray tracing to simulate light absorbed in the cladding as a result of fluorescence; • Heat transfer to simulate the quantum defect, heating from the previous step and effect of cooling; • Structural mechanics to simulate stresses as a result of temperature gradients in slab.

In the Ray Optics Module, rays are released from a domain representing the pumped volume with a spherical ray direction vector with a power based on the pumping parameters. A ray tracing study then takes place over 0.5 ns in 0.1 ns increments to allow the rays to propagate.

In the Heat Transfer Module, a convective heat flux is applied to both slab faces with an externally calculated heat transfer coefficient at 140 K to simulate the flow of the cooling gas. A heat source is applied to the pumped volume domain with an externally calculated power to simulate the quantum defect. A stationary study then calculates a temperature map of the slab based on the heat loads from the Heat Transfer Module and the Ray Optics Module.

Finally, the stresses in the slab are calculated using the temperature map by applying rigid motion suppression to the cladding domain in the Solid Mechanics interface and running a final stationary study. Data from these simulations in the form of temperature and stress maps can be extracted and used in a MATLAB® model to quantify depolarisation of the seed beam. The commissioning of the 10 J, 100 Hz system is at an advanced stage and this model will be used to predict the effect of different cooling scenarios on depolarisation and to optimise future laser systems.