Martin Smrž

Suppression of nonlinear phonon relaxation in Yb:YAG thin disk via zero phonon line pumping

A quantitative comparison of conventional absorption line (940 nm) pumping and zero phonon line (ZPL) (969 nm) pumping of a Yb:YAG thin disk laser is reported. Characteristics of an output beam profile, surface temperature, and deformation of a thin disk under the different pump wavelengths are evaluated. We found that a nonlinear phonon relaxation (NPR) of the excited state in Yb:YAG, which induces nonlinear temperature rise and large aspheric deformation, did not appear in the case of a ZPL pumped Yb:YAG thin disk. This means that the advantage of ZPL pumping is not only the reduction of quantum defect but also the suppression of NPR. The latter effect is more important for high power lasers.

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.

Lasing and thermal characteristics of Yb:YAG/YAG composite with atomic diffusion bonding

We demonstrated the laser performance of an Yb:YAG/YAG composite ceramic laser medium mounted on an aluminium heatsink via atomic diffusion bonding (ADB) technique using nanocrystalline metal films at room temperature in air. The surface temperature rise of the ADB bonded laser medium was linear with 57 °C lower than that of the commercially available soldered Yb:YAG thin disk at the pump power of 280 W. Moreover, the ADB disk was pumped 1.5 times higher (7.3 kW cm−2) than the typical damage threshold of the soldered disk without any sign of damage. The undoped capping may be effective for the suppression of ASE heating; however, according to the in situ OPD measurement it induces strong thermal lensing. The CW laser output power of 177 W was obtained at the pump power of 450 W with the optical-to-optical efficiency of 40% using V-shape cavity.

Amplification of picosecond pulses to 100 W by an Yb:YAG thin-disk with CVBG compressor

High average power picosecond lasers have become an import tool in many fields of science and industry. We report on progress in development of 100 kHz, 100 W picosecond Yb:YAG thin disk laser amplifier with fundamental spatial mode at the HiLASE laser center. More efficient direct pumping to an upper laser level has been employed in order to suppress thermal loading of the thin disk active medium and to increase system stability. We also carefully analyzed and described all benefits of this so called zero phonon line pumping (ZPL) for fundamental spatial mode cavity design and successfully increased extraction efficiency of the amplifier to > 28 %. A novel approach of high-power picosecond pulse compression using a space saving and easy-to-align chirped-volume Bragg grating (CVBG) with high dispersion and high net efficiency approaching 88 % allowed us to build a robust and highly compact pulse compressor. A 100 kHz train of sub-1-milijoule pulses compressed bellow 2 ps (FWHM) in almost diffraction limited Gaussian beam has been successfully generated from this highly compact (900 x 1200 mm) thin-disk-based Yb:YAG regenerative amplifier.

Progress in kW-class picosecond thin-disk lasers development at the HiLASE

High average power picosecond Yb:YAG thin-disk lasers are being developed at Hilase. A compact 1 mJ/100 kHz and 4 mJ/100 kHz zero-phonon-line-pumped regenerative amplifiers PERLA C with a CVBG compressor provide <2 ps long pulses in a nearly diffraction-limited beam. The output was successfully converted to 2nd and 4th harmonic frequency with high conversion efficiency. High energy, QCW-pumped beamline PERLA B is operated at 45mJ/1kHz in fundamental spatial mode and pulse length < 2ps. Its second stage amplifier is being assembled and 1.8 J was extracted. The latest development status of all thin-disk beamlines at the Hilase center is reported.

50-mJ, 1-kHz Yb:YAG thin-disk regenerative amplifier with 969-nm pulsed pumping

We are developing a 100-mJ Yb:YAG thin-disk regenerative amplifier operating at 1-kHz repetition rate pumped at zero-phonon-line (968.825-nm1) and delivering 1-2 ps pulses for EUV plasma sources applicable in science and industry. Recently we achieved the output energy of nearly 50-mJ from a single laser-head cavity with good beam quality (M2<1.2) as well as stable beam-pointing (<4μrad). Applying pulsed pumping with the pulse duration shorter than the upper state lifetime of Yb:YAG helps to reduce the ASE and thermal loading of the thin-disk.

Tunable mid-IR parametric conversion system pumped by a high-average-power picosecond Yb:YAG thin-disk laser

The mid-IR wavelength range has gained increased interest due to its applications in gas sensing, medicine, defense, and others. Optical parametric devices play an important role in the generation of radiation in the mid-IR. Low thermal load of nonlinear crystals promises high average power outputs if powerful pump laser is available. We have developed 75-W average power pump laser operating at 100 kHz repetition rate. The pulses of Yb-fiber laser oscillator at 1030-nm wavelength are stretched by a chirped volume Bragg grating from 5 ps to 180 ps and inserted into a cavity of regenerative amplifier with an Yb:YAG thin-disk. The amplified pulses are compressed by a chirped volume Bragg grating with an 88% efficiency. We have proposed a wavelength conversion system generating picosecond pulses tunable between 2 and 3 μm. The seed signal radiation is acquired by the optical parametric generation in the first nonlinear crystal. Signal pulse energy is increased in the subsequent optical parametric amplifiers. Each amplification stage consists of a crystal pair in the walkoff compensating arrangement. The wavelength of the signal beam is tunable between 1.6 and 2.1 μm. The 2.1 - 3 μm tunable source will be the idler beam taken from the last amplification stage. Calculations show the output power of ten watt can be achieved for 100 W pump. The results of preliminary experiments with seeded optical parametric generation and subsequent amplification are presented and discussed.

In-situ optical phase distortion measurement of Yb:YAG thin disk in high average power regenerative amplifier

We are developing one kilohertz picosecond Yb:YAG thin disk regenerative amplifier with 500-W average power for medical and industrial applications. In case of high energy pulse amplification, a large area mode matching in gain media, which is drastically degenerated by the optical phase distortion, is required to avoid optical damage. We designed in-situ thin disk deformation measurement based on the combination of a precise wavefront sensor and a single mode probe beam. In contrast to a conventional interferometric measurement, this measurement is compact, easy-to-align, and is less affected by mechanical vibrations.

Picosesond pulses in deep ultraviolet produced by a 100 kHz solid-state thin disk laser

We report on the generation of 100 kHz 0.1mJ-level deep ultraviolet pulses based on frequency-quadrupled (257.5 nm) beam of a diode pumped Yb:YAG thin disk laser at the HiLASE Centre. The 100-kHz beamline used for the generation of the harmonic frequencies is operated at an average output power of 100 W level and 2 picosecond duration of pulses. The amplification of the oscillator beam is performed in a regenerative amplifier where the thin disk serves as an active mirror. The CPA technique is used for achieving high average output power of the whole system. The outcoming laser beam at 1030 nm wavelength is frequency-doubled in an LBO crystal and then frequency-quadrupled in BBO crystal, conversion efficiencies being 40% and 19%, resp. The basic characteristics of the harmonics generation in both crystals are given.

Timing jitter measurement and stabilization of a mode-locked ytterbium fiber laser

We present recent progress in development of low-jitter all-normal dispersion (ANDi) Yb-doped fiber laser. We developed a timing jitter measurement scheme employing a broadband lock-in amplifier working up to 600 MHz operational frequency which allowed us to measure single side band (SSB) phase noise and long-term drift in laser repetition rate. Following the measurement, we were able to determine timing jitter of our oscillator in the most critical frequency range (10 Hz – 10 kHz) to 1.6 ps. Long-term drift has been successfully suppressed by applying a piezo motor-based active feedback in the cavity.