There is a growing demand for MHz-repetition rate industrial ultrafast laser sources that operate from the ultraviolet to the near IR. This paper discusses third harmonic generation (THG) of a high repetition-rate, femtosecond dual function Yb:YAG thin slab amplifier. Both pre-amplifier and power amplifier functions take place in a single crystal and homogeneous pumping of the thin slab is achieved by means of novel diode bar imaging. Fundamental output powers of <130 W at a wavelength of 1030 nm are maintained across a range of pulse repetition frequencies from 1 MHz to 40 MHz with a pulse duration of 900 ± 100 fs and M2 ~ 1.2. Second harmonic generation (SHG) at 515 nm with a conversion efficiency of 76% with respect to the incident fundamental is achieved using type-1 critically phase-matched LBO resulting in average powers of <90 W, a pulse energy of <90 µJ, with an M2< 1.2 and pulse durations of 800 ± 100 fs. The fundamental and second harmonic outputs were frequency mixed in a critically phase matched LBO crystal to produce third harmonic generation (THG) at 343 nm. Several lengths of type-2 THG LBO were investigated alongside type-1 THG LBO, with <50 W of UV produced at single-pass conversion efficiencies of <40% with respect to incident fundamental power at the SHG stage, corresponding to pulse energies of <50 µJ. Beam quality and pulse durations at 343 nm were investigated across 1–40 MHz.
Industrial applications increasingly demand femtosecond and picosecond pulse duration laser systems with a range of wavelengths from the near infrared to the ultraviolet. This paper reports a dual function thin slab femtosecond amplifier system operating at a range of wavelengths via frequency conversion. A Yb:YAG thin slab amplifier was demonstrated that incorporates both the pre-amplifier and power amplifier functions in a single crystal. To ensure efficient, homogenous pumping of the thin slab a novel diode bar imaging technique is used. The combination of the homogenous pumping and the unique amplifier architecture enables average output powers < 200 W to be maintained across a range of pulse repetition frequencies from 1 MHz to 40 MHz with a pulse duration of 50 ps at a wavelength of 1030 nm and M2 ~ 1.2. The pulse was compressed to a duration of 900 ± 100 fs with a compressor efficiency of < 90 %. A modified LXR100 system which is based on this technology was used as the fundamental 1030 nm source, with output powers up to 130 W, from 1 MHz – 10 MHz and pulse durations in the 900 fs regime. Second harmonic generation of 515 nm was achieved by coupling the beam into a type-1 critically phase matched LBO crystal to enable efficient conversion whilst avoiding damage. Second harmonic generation of 515 nm to < 95 W at a conversion efficiency of ~74 % and pulse energy of < 96 μJ and M2 < 1.3 was achieved. Further wavelength conversion was undertaken by coupling the 515 nm and unconverted 1030 nm output into a second LBO crystal for third harmonic generation. Type-1 and type-2 phasematching configurations for third harmonic generation were investigated, with an output power of < 20 W and pulse energy of < 20 μJ achieved at 343 nm with a conversion efficiency with respect to incident 1030 nm of < 21 % for both configurations.
We present an energy-scalable ultrafast Yb:YAG MOPA system for material processing applications. The system consists of a dual-side-pumped Yb:YAG planar waveguide (PWG) amplification stage that is seeded by a commercial laser, pre-amplified by a dual-end-pumped Yb:YAG single crystal fibre (SCF). The SCF is pumped by two 80 W fibre-coupled laser diodes and amplified the seed (344 fs pulses, 10 MHz, 140 nJ) to 1.4 µJ in a single-pass configuration. Thermal lensing, astigmatism and depolarisation within the SCF was analysed and compensated for by careful selection of beam-shaping optics. The 12 mm wide by 13 mm long PWG crystal has a 150 µm core of 2 at.% Yb:YAG, bonded top and bottom to sapphire cladding of 1 mm thickness. The core is side-pumped using two 540 W phase-corrected diode stacks such that a uniformly distributed high gain was achieved. The advanced crystal design suppresses intra-crystal parasitic oscillations and the PWG geometry significantly alleviates thermal lensing. The seed is multi-passed through the PWG crystal and the mirror parameters were carefully chosen to optimise gain extraction. The seed path is scalable up to 7 passes through the crystal for which 8.7 µJ per pulse was achieved at a pump power of 860 W for sub-ps pulses at 10 MHz. Current investigations include suppression of unwanted parasitic oscillations between the multi-pass mirrors to improve the output beam quality. Future work is aimed at the inclusion of kHz burst modes of sub-ps pulses at 1MHz repetition frequency.
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally
associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a
significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in
a localised (zonal) response and enables the device to be scalable up to large aperture, multi-1000 element devices.
Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the
assembly of a high density 'tweeter' deformable mirror (DM) onto a lower density, high dynamic range 'woofer' DM to
generate an integrated, dual-stage deformable mirror which can deliver both high resolution and high dynamic range
simultaneously. Such a device has the potential to significantly simplify the design of astronomical adaptive optics (AO)
systems. We present the progress made on the development of the ZBDM as part of a collaborative project funded by the
newly formed UK Science and Technology Facilities Council.
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