Status of the High Average Power Diode-Pumped Solid State Laser Development at HiLASE

Highly Regular LIPSS on Thin Molybdenum Films: Optimization and Generic Criteria

A systematic experimental study was performed to determine laser irradiation conditions for the large-area fabrication of highly regular laser-induced periodic surface structures (HR-LIPSS) on a 220 nm thick Mo film deposited on fused silica. The LIPSS were fabricated by scanning a linearly polarized, spatially Gaussian laser beam at 1030 nm wavelength and 1.4 ps pulse duration over the sample surface at 1 kHz repetition rate. Scanning electron microscope images of the produced structures were analyzed using the criterion of the dispersion of the LIPSS orientation angle (DLOA). Favorable conditions, in terms of laser fluence and beam scanning overlaps, were identified for achieving DLOA values <10∘. To gain insight into the material behavior under these irradiation conditions, a theoretical analysis of the film heating was performed, and surface plasmon polariton excitation is discussed. A possible effect of the film dewetting from the dielectric substrate is deliberated.

Experimental study on compression of 216-W laser pulses below 2 ps at 1030 nm with chirped volume Bragg grating

We report on the characterization of a high-power, chirped volume Bragg grating (CVBG) pulse compressor. It includes measurements of the CVBG's diffraction efficiency, beam profile, beam quality (M² parameter), pulse spectrum, the CVBG's temperature, and the thermal lens. These parameters were monitored for a wide range of input laser powers and with different clamping forces applied on the CVBG. This analysis was performed with a CPA-based Yb:YAG thin-disk laser system operating at a wavelength of 1030 nm, a 92 kHz repetition rate, 2 ps pulse duration, and an average output power after compression of 216 W (270 W uncompressed), which is, to the best of our knowledge, the highest value reported to date using this pulse compression technique.

New observations on DUV radiation at 257 nm and 206 nm produced by a picosecond diode pumped thin-disk laser

We report new observations on picosecond deep ultraviolet coherent beams generated in a CLBO as the fourth and fifth harmonics of a diode pumped high average power Yb:YAG thin disk laser operating at 77 kHz repetition rate at 1030 nm. The effects of the two-photon absorption were observed, e.g. the modification of phase matching conditions, lowering of the conversion efficiency. The fifth harmonic generation (4ω+1ω) was studied for different time delays between both pump beams and for the case of excess input power of the fundamental. The latter effect suggests a possibility of increasing DUV output at short crystals. The highest output power obtained at 257 nm was 7.6 W and 1 W at 206 nm. To our knowledge these DUV output powers rank among the highest for picosecond pulses at the repetition rate of the order of magnitude of 100 kHz.

High power picosecond parametric mid-IR source tunable between 1.7 and 2.6 μm

A high-average-power wavelength-tunable picosecond mid-IR source based on parametric downconversion has been developed. The conversion system consists of two stages, optical parametric generator and optical parametric amplifier (OPA), which are pumped by an Yb:YAG thin-disk laser operated at 77 kHz repetition rate, 1030 nm wavelength, and pulse duration down to 1.3 ps. The signal beam is amplified up to 9.2 W and the idler up to 5.4 W at OPA pumping of 43 W. Tunability between 1.70 and 1.95 μm for the signal and between 2.2 and 2.6 μm for the idler has been achieved. The system is rather simple and power scalable.

Fast stabilization of a high-energy ultrafast OPA with adaptive lenses

The use of fast closed-loop adaptive optics has improved the performance of optical systems since its first application. Here we demonstrate the amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier devoted to Attosecond Science exploiting two high speed adaptive optical systems for the correction of static and dynamic instabilities. The exploitation of multi actuator adaptive lenses allowed for a minimal impact on the optical setup.

Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE

The development of kW-class diode-pumped picosecond laser sources emitting at various wavelengths started at the HiLASE Center four years ago. A 500-W Perla C thin-disk laser with a diffraction limited beam and repetition rate of 50–100 kHz, a frequency conversion to mid-infrared (mid-IR), and second to fifth harmonic frequencies was demonstrated. We present an updated review on the progress in the development of compact picosecond and femtosecond high average power radiation sources covering the ultraviolet (UV) to mid-IR spectral range at the HiLASE Center. We also report on thin-disk manufacturing by atomic diffusion bonding, which is a crucial technology for future high-power laser development.

Picosecond green and deep ultraviolet pulses generated by a high-power 100 kHz thin-disk laser

We report on the generation of the second (515 nm) and fourth (257.5 nm) harmonics from a 100 kHz diode-pumped solid-state laser operating at a wavelength of 1030 nm which uses one Yb:YAG thin disk in the regenerative amplifier and delivers 60 W of the average output power in pulses of 4 ps duration. Thirty-five W in green light and 6 W in deep ultraviolet (DUV) were achieved. The sensitivity of the second harmonic generation efficiency toward the lithium triborate crystal temperature is demonstrated in experiment. The overall conversion efficiency from NIR to DUV of 10% was achieved. The β-barium borate and cesium lithium borate crystals were used as green to DUV convertors and compared regarding the efficiency and spectral bandwidths. The achieved output power is unique for DUV picosecond pulses.

Optimal coating solution for a compact resonating cavity working at Brewster angle

In a compact Nd:Glass resonator, the laser that enters the gain medium at Brewster angle can work either for P-polarization or S-polarization, in which polarization the optical coatings possess higher laser-induced damage threshold (LIDT) was investigated. For the P-polarized configuration, only one high reflection (HR) coating on the rear surface of the Nd:Glass substrate is needed, and the laser-induced damage occurred near the substrate-coating interface at a fluence of 10 ± 2 J/cm² (1064nm 10ns). Although S-polarized configuration needs two coatings, one HR coating and one anti-reflection (AR) coating on the rear and front surface of the Nd:Glass substrate respectively, its overall LIDT was about 1.8 times higher than that of the P-polarized configuration. The laser-induced damage occurred at the interface between the S-polarized AR coating and the Nd:Glass substrate. The observed interfacial damage behaviors were interpreted using a phenomenological model that took the nano-sized absorbers, electric-field intensity (EFI) distribution and coating thickness into consideration.

Precompensation of the thermal-induced refractive index changes into a fully aperiodic LPF for heat load resilience

In this paper, a strategy consisting of precompensating the thermal-induced transverse refractive index changes is undertaken to push further the appearance threshold of a multimode regime. First, a standard air-silica large pitch fiber (LPF) and a fully aperiodic large pitch fiber are confronted in regard to their heat load resilience and capabilities for single-mode emission. Thereafter, slight refractive index depressions are judiciously introduced into the active core area. This approach enhances the delocalization of the high-order modes even under severe heat load levels. This combination of aperiodic cladding microstructuration and index-precompensation theoretically increases the multimode regime threshold while preserving large mode field areas. This investigation is performed at 1.03 and 2 μm operating wavelengths.

100 J-level nanosecond pulsed diode pumped solid state laser

We report on the successful demonstration of a 100 J-level, diode pumped solid state laser based on cryogenic gas cooled, multi-slab ceramic Yb:YAG amplifier technology. When operated at 175 K, the system delivered a pulse energy of 107 J at a 1 Hz repetition rate and 10 ns pulse duration, pumped by 506 J of diode energy at 940 nm, corresponding to an optical-to-optical efficiency of 21%. To the best of our knowledge, this represents the highest energy obtained from a nanosecond pulsed diode pumped solid state laser. This demonstration confirms the energy scalability of the diode pumped optical laser for experiments laser architecture.

Photothermal method for absorption measurements in anisotropic crystals

A measurement system for quantitative determination of both surface and bulk contributions to the photo-thermal absorption has been extended to anisotropic optical media. It bases upon a highly sensitive Hartmann-Shack wavefront sensor, accomplishing precise on-line monitoring of wavefront deformations of a collimated test beam transmitted perpendicularly through the laser-irradiated side of a cuboid sample. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of the test beam is distorted. Sign and magnitude depend on index change and expansion. By comparison with thermal theory, a calibration of the measurement is possible, yielding a quantitative absolute measure of bulk and surface absorption losses from the transient wavefront distortion. Results for KTP and BBO single crystals are presented.