Conference Agenda

Future laser fusion drivers require not only high pulse energy, repetition rate, and wall-plug efficiency, but also sufficient usable gain bandwidth to support advanced scaling concepts and mitigate laser-plasma instabilities. In this context, cryogenic Yb:YLF is particularly attractive because it combines a low quantum defect and favorable thermo-optical behavior with retained gain bandwidth at cryogenic temperature. This talk will review recent progress in cryogenic Yb:YLF laser development at DESY, including operation from the few-hundred-watt to kilowatt-class regime, pulse energies up to 100 mJ, and pulse compression to 361 fs. Particular attention will be given to the spectral properties of Yb:YLF and their role in efficient amplification, bandwidth preservation, and future energy scaling. A 100 J cryogenic Yb:YLF module will also be discussed as a scalable building block for next-generation fusion drivers, with emphasis on tunable seeding and incoherent wavelength combining for scaling to higher energies while preserving bandwidth.

A spun tapered double-clad fiber with an Yb-doped ring-shaped core (sT-RCF) is presented as a robust gain medium for amplifying orbital angular momentum (OAM) modes while preserving their phase and polarization integrity. OAM beams with topological charges ℓ = 1 and ℓ = 2, carrying 60 ps pulses at 15 MHz repetition rate at 1030 nm, are amplified to average powers exceeding 1.2 W with modal purity exceeding 95%.

Laser pulse duration and temporal contrast are critical for petawatt (PW) applications such as fusion and laser-wakefield acceleration, which require sub-100 fs pulses with very high contrast (10^10-10^13). Existing contrast enhancement methods, including OPCPA seeding, stretcher-compressor optimization, and post-compression, often suffer from low efficiency and high complexity. Here, we address both high-dynamic-range pulse contrast measurements and efficient contrast improvement. Spectral filtering of shifted components improves temporal contrast by up to two orders of magnitude near the main pulse and ~1.5 orders on longer timescales. This approach provides a robust and energy-efficient method for generating high-contrast ultrashort laser pulses.

Spatial amplitude modulation is essential for achieving optimal per- formance in high-power laser systems, where spatial light modulators (SLMs) must operate over large apertures while withstanding high fluence. Conven- tional liquid-crystal SLMs are limited in both area and damage threshold due to their reliance on electrodes or photoconductive layers. We present an electrode- free Thermo-Optically Addressed SLM (TOA-SLM) based on the thermotropic response of liquid crystals, where a writing beam induces local temperature changes to control birefringence. A 40 mm aperture prototype demonstrates spatial amplitude modulation of high-energy near-infrared beams, achieving a high extinction ratio, 512 gray levels, and a nanosecond damage threshold ex- ceeding 1 J/cm2.