Martin Smrz

High-power, picosecond pulse thin-disk lasers in the Hilase project

Development of high-power, picosecond laser sources is a desirable venture for both industry and research. Within the Hilase project, we are conducting research on both 500-mJ, 1-kHz and 5-mJ, 100-kHz picosecond laser sources based on the Yb:YAG thin-disk technology. We have developed a prototype thin-disk regenerative amplifier operating up to 10- kHz repetition rate pumped by the 940-nm fiber-coupled laser diodes. We achieved 5-mJ pulse energy at 10-kHz operation and 29.5-mJ at 1-kHz. Afterwards, we developed the high-energy regenerative amplifier operating at fixed repetition rate of 1-kHz and the pulse energy was achieved up to 40-mJ. Simultaneously, we elaborated the highrepetition rate regenerative amplifier operating at 100-kHz with pulse energy of 220-μJ. The amplified pulse was compressed with the efficiency of 88% using chirped volume Bragg grating.

Development of a variable repetition rate, kW-level, picosecond ring regenerative amplifier

In this contribution, we report on development of a high-repetition-rate, kW-level picosecond Yb:YAG thin-disk laser system PERLA C [1]. Its design utilizes the CPA (Chirped Pulse Amplification) technique, comprising a low-energy Yb-fiber oscillator and preamplifiers, two thin-disk regenerative amplifiers and a chirped volume Bragg grating (CVBG) for both pulse stretcher and compressor. The target parameters of this system are 5-mJ pulse energy at 100 kHz repetition rate (500 W average power) and pulse duration below 2 ps, with possibility to adjust the repetition rate of the system from 50 kHz to 1 MHz while maintaining approximately the average power. Prospectively, we plan to upgrade the system to 1-kW average power level.

Modeling and optimization of thin disk structure for high power sub-joule laser

We analyzed the transient response characteristics of Yb:YAG thin disk in to clarify the experimentally obtained advantages of pulsed pumping in 1-kHz repetition rate reported in ref. 2. We applied commercial 2D FEA software which can calculate transient response of thermal effects. The temperature distributions of thin disk in both the CW power of 125-W and the average power of pulsed 125-W have been calculated. Even the net heat power were same in both CW and pulsed pumping, the temperature distribution was lower in pulsed pumping which can provide higher O-O efficiency and smaller beam distortion. The time evolution of OPD in the pulsed pumping has been analyzed, too.

Ten-watt level picosecond parametric mid-IR source broadly tunable in wavelength

Mid-IR wavelength range (between 2 and 8 μm) offers perspective applications, such as minimally-invasive neurosurgery, gas sensing, or plastic and polymer processing. Maturity of high average power near-IR lasers is beneficial for powerful mid-IR generation by optical parametric conversion. We utilize in-house developed Yb:YAG thin-disk laser of 100 W average power at 77 kHz repetition rate, wavelength of 1030 nm, and about 2 ps pulse width for pumping of a ten-watt level picosecond mid-IR source. Seed beam is obtained by optical parametric generation in a double-pass 10 mm long PPLN crystal pumped by a part of the fundamental near-IR beam. Tunability of the signal wavelength between 1.46 μm and 1.95 μm was achieved with power of several tens of miliwatts. Main part of the fundamental beam pumps an optical parametric amplification stage, which includes a walk-off compensating pair of 10 mm long KTP crystals. We already demonstrated the OPA output signal and idler beam tunability between 1.70-1.95 μm and 2.18-2.62 μm, respectively. The signal and idler beams were amplified up to 8.5 W and 5 W, respectively, at 42 W pump without evidence of strong saturation. Thus, increase in signal and idler output power is expected for pump power increase.

Femtosecond Yb:YGAG ceramic regenerative amplifier

Ytterbium-doped gain media are excellent materials for high power ultrashort pulse lasers. Especially Yb:YAG is very well established in picosecond lasers systems with kW power level due to its excellent optomechanical properties. However, limiting factor for amplification of high energy sub-picosecond and femtosecond pulses in Yb:YAG laser amplifiers is limited bandwidth of spectral gain and, as a consequence, gain narrowing in power amplifiers. Majority of existing Yb:YAG systems cannot generate pulses shorter than approx. 0.9 ps. One approach how to overcome the limitation is to introduce nonlinear effect (self-phase modulation) to increase spectral bandwidth [1]. Other approach is to use gain medium with broader spectral bandwidth. One of the best candidates to exchange the Yb:YAG is Yb:YGAG (Y3Ga2Al3O12).

High-power mid-IR picosecond pulses tunable between 2.2 and 2.4 μm

Mid-infrared (mid-IR) wavelength range is utilized in applications such as gas sensing, medicine, defence, and material processing. When using picosecond pulses high peak power is reached at moderate pulse energy and the heat affected zone is low. A wavelength tunable mid-IR source would enable optimization of the laser-matter interaction in dependence on wavelength. Increase in processing speed requires sources of higher average power. Parametric devices offer both broad wavelength tunability and power scalability [1]. We are developing a ten watt level wavelength tunable picosecond mid-IR source based on an optical parametric generator (OPG) followed by an optical parametric amplifier (OPA).

Wavefront aberration measurement in a cryogenically cooled Yb:YAG slab using a wavefront sensor

We investigated wavefront aberrations in a cryogenically cooled Yb:YAG slab with a wavefront sensor using a probe beam technique under non-lasing condition. To analyze the pump-induced phase aberrations created in the crystal, the measured wavefronts were fitted with orthonormal Zernike polynomials. The Yb:YAG crystal of 2 mm thickness, 10 mm diameter, and 3 at.% doping concentration was mounted in a copper holder in a closed-loop pulse tube cryostat with cooling capacity of 12 W at 100 K. The gain medium was single-end pumped by a fiber-coupled laser diode at pumping intensity of ~6.5 kW/cm2 with a maximum repetition rate of 100 Hz, pulse duration of 1 ms, and pump spot diameter of 2.5 mm. The time resolved measurement revealed that defocus, which was the main wavefront aberration, represents not only a thermal lensing effect but also an electronic lensing effect. The thermally induced defocus is more dominant at high repetition rate than the electronically induced defocus. We also measured wavefront aberrations of amplified beams in a cryogenically cooled Yb:YAG slab. A room temperature operated thin-disk regenerative amplifier was used as a seed laser. The seed beam was amplified in the cryogenically cooled crystal at 160 K in a double pass configuration. The wavefront measurement was conducted at semi-saturated conditions, at three different repetition rates: 10 Hz, 20 Hz and 40 Hz, and at five different pump intensities in the range between 6.5 kW/cm2 and 14.8 kW/cm2. Under lasing condition, only defocus aberration were induced. Due to opposite signs of the defocus aberration of the seed beam and pumped induced in the Yb:YAG crystal, wavefront of the amplified beam had smaller PtV (Peak to Valley) and RMS values than the seed beam.

A practical model of thin disk regenerative amplifier based on analytical expression of ASE lifetime

In this paper, a practical model of a thin disk regenerative amplifier has been developed based on an analytical approach, in which Drew A. Copeland [1] had evaluated the loss rate of the upper state laser level due to ASE and derived the analytical expression of the effective life-time of the upper-state laser level by taking the Lorentzian stimulated emission line-shape and total internal reflection into account. By adopting the analytical expression of effective life-time in the rate equations, we have developed a less numerically intensive model for predicting and analyzing the performance of a thin disk regenerative amplifier. Thanks to the model, optimized combination of various parameters can be obtained to avoid saturation, period-doubling bifurcation or first pulse suppression prior to experiments. The effective life-time due to ASE is also analyzed against various parameters. The simulated results fit well with experimental data. By fitting more experimental results with numerical model, we can improve the parameters of the model, such as reflective factor which is used to determine the weight of boundary reflection within the influence of ASE. This practical model will be used to explore the scaling limits imposed by ASE of the thin disk regenerative amplifier being developed in HiLASE Centre.

Single shot M2 measurement for near infrared high energy laser pulses

Since the principle of M2 measurement is to scan the beam discretely along with propagation direction, measurement time of several minutes is required which is not suitable for pulsed lasers. Several single-shot techniques have been proposed to measure M2 by using diffraction gratings and wavefront sensor, but were shown to be more complex and yield inaccurate results for multimode beams. Another approach to measure the M2 uses Rayleigh scattering from gas or liquid-filled cell. The scattered image by laser light in the cell, however, contains lots of speckle patterns which degrade the accuracy of M2 measurement. We developed a single shot M2 measurement based on a photosensitive glass. The measurement system consists of the photosensitive glass plate and the imaging camera with macro lens. When the pulsed laser beam focused into the cross-sectional direction of photosensitive glass plate, the visible fluorescence of the glass plate indicates the focusing property of laser beam. Then the visualized beam propagation in the glass is imaged precisely to measure the beam diameters around beam waist. Since the coherent laser beam is converted to the incoherent fluorescence, the beam propagation image is free from speckle patterns. The M2 can be calculated from the image within less than a second. This simple technique allows the possibility of the real time monitoring of the beam quality. We obtained M2=1.10 from a fiber coupled diode laser that is close to the actual value of M2=1.18 using the standard scanning method.

Development of a kW-level Picosecond Light Source Using Two-Stage Thin-Disk Regenerative Amplifier

We report on a 100-kHz, 5-mJ picosecond system based on a two-stage thin-disk regenerative amplifier. With a compact ring cavity, we obtained 565 W with 46.7% optical-to-optical efficiency in continuous-wave operation without any sign of roll-over.