YAG Thin-disk Laser Research Articles

Diamond - silicon carbide composite “Skeleton” as a promising heatsink material for Yb:YAG thin-disk lasers

Diamond - silicon carbide composite “Skeleton” as a promising heatsink material for Yb:YAG thin-disk lasers

Electro-optic Q-switched Yb:YAG thin disk laser with widely continuously tunable pulse width at a stable output of over 80 W

We report an electro-optic Q-switched Yb:YAG thin disk laser with widely continuously tunable pulse width ranging from 16.4 to 432.2 ns. The tunability was achieved through a simple and convenient method by rotating both the Pockels cell and the quarter-wave plate at either identical or slightly different angles. Based on the 72-pass pump module and 224.4 W laser diode pumping source, a pulsed output of over 80 W was attained at a repetition rate of 100 kHz. The output power and output pulse train remained stable in the whole tuning range. The corresponding output power was 82.7 W and 84.4 W for the minimum and maximum pulse width, respectively. A model which was exceedingly more accurate than previous models describing the tunability of the pulse width were proposed and the simulation results of this model exhibited strong agreement with the experimental measurements.

High power single crystal KTA optical parametric amplifier for efficient 1.4–3.5 µm mid-IR radiation generation

In this paper, we present a single crystal, KTA (potassium titanyl-arsenate, KTiOAsO4) based picosecond optical parametric amplifier pumped by an in-house built 1030 nm Yb:YAG thin-disk laser, capable of tunability from 1.46 to 3.5 µm, operating at 90 kHz, with high average power in the signal and idler beams. The highest output power of 8.9 W was reached for the 1750 nm signal beam with 19% conversion efficiency and the respective 2500 nm idler beam power was 6.2 W with 13% efficiency. The highest combined signal and idler mid-infrared power was 17 W at the 2060 nm wavelength degeneracy point.

Cpa-free Yb:YAG thin-disk regenerative amplifier laser with hundreds of watts, hundreds of kHz, and hundreds of picoseconds

This paper introduces a CPA-free Yb:YAG thin-disk regenerative amplifier laser system. The seed pulse is generated by a passive mode-locked picosecond oscillator based on a semiconductor saturable absorber mirror, with a center wavelength of 1030.7 nm, a pulse width of 7.7 ps, and a repetition frequency of 26.3 MHz. After stretched and pre-amplified by an all-fiber front stage, the seed with a pulse width of 201.3 ps and a pulse energy of 15.2 nJ was obtained, then injected into the thin-disk regenerative amplifier. Yb:YAG crystal was chosen as the material for the thin-disk, with 100 μm thickness, 9 mm diameter and 7 at.% doping rate. In order to increase the one-way round-trip gain of the thin-disk regenerative amplifier, a symmetrical dual-pass resonant cavity was designed to double the number of passing through the crystal on a round trip in the regenerative resonant cavity to reduce the times of cycles, intra-cavity loss and improve optical efficiency. We used a 969 nm fiber-coupled semiconductor laser as the pump source to pump the Yb:YAG thin-disk crystal. With zero-phonon line pumping technology, the quantum loss and thermal effect were reduced. In the end, we obtained a regenerative amplification output with an average power of 104.5 W, a repetition frequency of 200 kHz, a pulse width of 143.9 ps, and a spectrum width of 0.39 nm. The amplified pulse had quite good beam quality with Mx2 = 1.09 and My2 = 1.14.

Efficient Resonant Waveguide Grating for High-Power Lasers in the 2-µm Wavelength Range

We present resonant waveguide-grating (RWG) tailored for lasers operating at the 2-µm wavelength range. Utilizing the RWG as a cavity mirror, generation of tunable narrow-bandwidth and linearly-polarized radiation from a Ho:YAG thin-disk laser was demonstrated.

High Efficiency Yb∶YAG Thin Disk Laser Based on Zero Phonon Line Pumping

High Efficiency Yb∶YAG Thin Disk Laser Based on Zero Phonon Line Pumping

Sapphire-based resonant waveguide-grating mirrors: advancing their intra-cavity power density capability

We report on the design, fabrication, and implementation of a single-layer resonant waveguide-grating (RWG) mirror on a sapphire substrate. Our goal is to enhance these optics capability to withstand high intra-cavity power densities by exploiting the superior thermal properties of sapphire. The RWG was implemented as an intra-cavity folding mirror in an Yb:YAG thin-disk laser to generate linearly polarized and spectrally stabilized radiation. A linearly polarized output power of 191 W with an optical efficiency of 39% was obtained in multi-mode operation. This corresponds to a power density of 52 kW/cm2 on the RWG, for which the increase of its surface temperature was measured to be 12 K, which resulted in a 46-fold reduction of the surface temperature rise dependence on the intra-cavity power density with respect to what has been reported for a RWG on a fused silica substrate. In near fundamental-mode operation, a linearly polarized emission with an output power of 90 W, an optical efficiency of 30%, and a spectral bandwidth of 28 pm FWHM was obtained.

High-power vortex beams generated from a Yb:YAG thin-disk laser with spot-defect mirrors

We report the generation of high-power and high-order vortex beams based on a Yb:YAG thin-disk laser oscillator with a spot-defect mirror. High-quality vortex beams were directly emitted by the oscillator with an output power of 80 W, which is the highest output power of any vortex laser oscillator. Tunable high-order Hermite-Gaussian modes from HG1,1 to HG5,1 were also generated, and subsequently converted to Laguerre-Gaussian beams by a cylindrical-lens mode converter. The vortex characteristics of the output were confirmed with a Mach-Zehnder interferometer. This work demonstrates an effective way to generate high-quality, high-power and high-order optical vortex beams.

Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method.

We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm, stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to obtain an error signal free from Trojan locking points, which allowed robust re-locking of the laser and reliable long-term operation. The single-frequency pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam quality of M2 < 1.1 and an adjustable length of 55-110 ns. Heterodyne measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse build-up time (850 ns) makes this laser suitable for laser spectroscopy of muonic hydrogen, pursued by the CREMA collaboration.

Introducing a 2V-resonator for the improvement of pulse stability in a high-gain Q-switched Yb:YAG thin-disk laser.

Pulse instability in Q-switched solid-state lasers at enough high repetition rates is a significant problem for getting high powers. This issue is more critical for Thin-Disk-Lasers (TDLs) due to the smallness of round-trip gain in the thin active media. The main idea of this work is that increasing the round-trip gain of a TDL makes it possible to diminish its pulse instability at high repetition rates. Accordingly, a novel 2V-resonator is introduced to overcome the low gain of TDLs, in which the passage of the laser beam from the active media is twice that of the standard V-resonator. The experiment and simulation results indicate that the threshold of laser instability considerably improves for the 2V-resonator relative to the traditional V-resonator. This improvement is well seen for various time windows of the Q-switching gate and different pump powers. By choosing appropriate Q-switching time and pump power, the laser was stably run at 18 kHz, a recorded repetition rate for Q-switched TDLs.

KHz, 10s TW, Femtosecond Source Based on Yb:YAG Thin Disk Laser Pumped OPCPA of Low Quantum Defect

We propose to obtain kHz, 10s TW, femtosecond sources through optical parametric chirped pulse amplification (OPCPA) pumped by Yb:YAG thin disk lasers. The final amplifiers of the OPCPA are based on LGS (LiGaS2) crystals with wide transparent range. To suppress the quantum defect for high efficiency, the final amplifiers are designed such that the wavelength of the signal is set very close to 1.03 μm, while the idler spectra span from 4–8 μm. Multiple crystals with different phase-matching configuration can be employed for the amplification of different spectral regions to support broadband pulse amplification. According to the numerical simulations, the pulse duration from Yb:YAG lasers can be shortened to 20–30 fs pulse with efficiency beyond 60%. This technique is energy scalable with the size of the LGS crystal size and can support a 26 TW pulse with current available LGS. The output pulses are ideal drivers for secondary light and particle source generation.

Realization of thermally-near-unstable resonator thin-disk laser and compensation of air wedge effect

This paper reports on a Yb: YAG thin-disk laser based on a thermally-near-unstable resonator. The cavity length of the resonator is 3 m, which is 3.5 m less than the cavity length of a stable resonator, simplifying the oscillator structure and reducing the optical loss of the resonator cavity. The disk laser based on a thermally-near-unstable resonator achieves a laser output power of 38.3 W with beam quality approaching the diffraction limit. The laser center wavelength is 1.03 μm. The resonator becomes misaligned during the increase in pump power due to air convection, reducing laser output power. In this paper, one end mirror of the thermally-near-unstable resonator is replaced with a retro-reflector to compensate for the misalignment of the resonator. The laser output power of the resonator with retro-reflector increased to 81 W. The beam qualityM2 factors in the × and y directions are 1.03 and 1.14, respectively. retro-reflector significantly improves the operational stability of the laser.

Double-pass optical parametric generator pumped by Yb thin-disk laser for efficient 1.4–2.9 µm mid-IR radiation generation

Many different fields benefit from the usage of light sources emitting in the mid-infrared wavelength range (2–10 µm). A rising need for precise and fast sources in the mid infrared (mid-IR) is reflected in the development of a high-power, picosecond mid-IR source capable of generation at high repetition rates. In this work, we present the optimization of an optical parametric generator, pumped by a 3 W portion of total power of the Yb:YAG thin-disk laser (1.3 ps, 90 kHz, 90 W) by comparing a single-pass and double-pass arrangement output parameters in terms of output power dependences on input power, efficiency, beam profiles, stability, and spectra. The output tunability of both arrangements spanned from 1459 nm to 2891 nm, with the upper limit being influenced by the limited transmission of the dichroic components used in the setup above 2700 nm. It was shown that the double-pass arrangement increases the output power, from 17 mW in the single-pass arrangement to 193 mW in the double-pass arrangement at 1459 nm, resulting in over ten-fold output power increase.

Pound-Drever-Hall locking scheme free from Trojan operating points.

The Pound-Drever-Hall (PDH) technique is a popular method for stabilizing the frequency of a laser to a stable optical resonator or, vice versa, the length of a resonator to the frequency of a stable laser. We propose a refinement of the technique yielding an "infinite" dynamic (capture) range so that a resonator is correctly locked to the seed frequency, even after large perturbations. The stable but off-resonant lock points (also called Trojan operating points), present in conventional PDH error signals, are removed by phase modulating the seed laser at a frequency corresponding to half the free spectral range of the resonator. We verify the robustness of our scheme experimentally by realizing an injection-seeded Yb:YAG thin-disk laser. We also give an analytical formulation of the PDH error signal for arbitrary modulation frequencies and discuss the parameter range for which our PDH locking scheme guarantees correct locking. Our scheme is simple as it does not require additional electronics apart from the standard PDH setup and is particularly suited to realize injection-seeded lasers and injection-seeded optical parametric oscillators.

Resonators with a continuously variable output coupling rate to enhance output performance of Yb:YAG thin-disk lasers.

Beam quality and average output power are two long-sought parameters of continuous-wave lasers. The operating characteristics of a Yb:YAG thin-disk laser based on a 72-pass pump module using output couplers with continuously variable coupling rates are reported. When the pump power is 450 W, the average power of more than 210 W is obtained, and the corresponding optical-optical efficiency is about 46.67%. The output beam is near diffraction limited with M2=1.20 and 1.18 on the horizontal and vertical directions respectively. The thin-disk laser maintains the beam quality near diffraction limited within the whole pump power range by continuously changing the coupling rate of the output coupler (the combination of thin film polarizer and quarter-wave plate) according to the incident pump power. The experimental results show that by continuously changing the coupling rate, combined with the dynamic change of thermal lens effect, the laser beam quality factor M2 can be reduced from 2.0 to 1.2, and the beam quality can be improved by about 40%. The improvement of and increase of average output power confirm that resonators with continuously variable coupling rate are a promising method for enhancing output performance of thin-disk lasers.

400 W average power Q-switched Yb:YAG thin-disk-laser

We report on producing up to 403 W average power directly from an acousto-optically Q-switched Yb:YAG thin-disk laser (TDL). To achieve this power, it has theoretically and experimentally been shown that the laser stability border could be shifted toward higher repetition rates by engineering of the output coupler transmittance. This allows for stable operation of the laser at higher frequencies and a further increase in the power extraction from the active medium. Using an output coupler with 93% reflectivity, a maximum average power of 403 W at the repetition rate of 12.0 kHz has been recorded under the pump power of 1220 W. Furthermore, the maximum pulse energy of 57 mJ was produced at the repetition rate of 1.00 kHz and the pump power of 520 W. The characteristics of the laser at various Q-switching rates and the pump powers have been investigated. In addition, a numerical study for supporting the experimental results has been proposed here. To the best of our knowledge, the achieved average power and the pulse energy are the highest values reported to date from a Q-switched Yb:YAG TDL. The results pave the way to further power scaling of solid-state Q-switched oscillators.

Efficient XUV-light out-coupling of intra-cavity high harmonics by a coated grazing-incidence plate.

We experimentally demonstrate an efficient and broadband extreme-ultraviolet light (XUV) out-coupling mechanism of intra-cavity generated high harmonics. The mechanism is based on a coated grazing-incidence plate (GIP), which utilizes the enhanced reflectivity of s-polarized light in comparison to p-polarized light for large angles of incidence (AoI). We design and produce a 60°-AoI coated GIP, tailored specifically for the high demands inside a sub-50-fs Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator in which high harmonic generation (HHG) is driven at ∼450 MW peak power and 17 MHz repetition rate. The coated GIP features an XUV out-coupling efficiency of >25% for photon energies ranging from 10 eV to 60 eV while being anti-reflective for the driving laser field. The XUV spectra reach up to 52 eV in argon and 30 eV in xenon. In a single harmonic, we out-couple 1.3 µW of XUV average power at 37 eV in argon and 5.4 µW at 25 eV in xenon. The combination of an improved HHG driving laser performance and the out-coupling via the coated GIP enabled us to increase the out-coupled XUV average power in a single harmonic by a factor of 20 compared to previous HHG inside ultrafast laser oscillators. Our source approaches the state-of-the-art out-coupled XUV power levels per harmonic of femtosecond enhancement cavities operating at comparable photon energies.

50-W average power Ho:YAG SESAM-modelocked thin-disk oscillator at 2.1 µm.

Ultrafast laser systems operating with high-average power in the wavelength range from 1.9 µm to 3 µm are of interest for a wide range of applications for example in spectroscopy, material processing and as drivers for secondary sources in the XUV spectral region. In this area, laser systems based on holmium-doped gain materials directly emitting at 2.1 µm have made significant progress over the past years, however so far only very few results were demonstrated in power-scalable high-power laser geometries. In particular, the thin-disk geometry is promising for directly modelocked oscillators with high average power levels that are comparable to amplifier systems at MHz repetition rate. In this paper, we demonstrate semiconductor saturable absorber mirror (SESAM) modelocked Ho:YAG thin-disk lasers (TDLs) emitting at 2.1-µm wavelength with record-holding performance levels. In our highest average power configuration, we reach 50 W of average power, with 1.13-ps pulses, 2.11 µJ of pulse energy and ∼1.9 MW of peak power. To the best of our knowledge, this represents the highest average power, as well as the highest output pulse energy so far demonstrated from a modelocked laser in the 2-µm wavelength region. This record performance level was enabled by the recent development of high-power GaSb-based SESAMs with low loss, adapted for high intracavity power and pulse energy. We also explore the limitations in terms of reaching shorter pulse durations at high power with this gain material in the disk geometry and using SESAM modelocking, and present first steps in this direction, with the demonstration of 30 W of output power, with 692-fs pulses in another laser configuration. In the near future, with the development of a next generation of SESAM samples for this wavelength region, we believe higher pulse energy approaching the 10-µJ regime, and sub-500-fs pulses should be straightforward to reach using SESAM modelocking.

High-efficiency, high-repetition-rate cavity-dumped Q-switched Yb:YAG thin-disk laser based on a 72-pass pump module.

We demonstrated a compact cavity-dumped Q-switched Yb:YAG laser based on a 72-pass pump module. A highly stable nanosecond pulse laser with a duration of 18.38ns was achieved at a repetition rate of 100 kHz, which is pumped by multimode laser diode stacks at 969 nm. The average output power of more than 150 W is delivered in a good output beam characterized by M2 = 1.53. The result shows that the pulse duration derived from simulation agrees well with the one retrieved from the experimental measurements. To the best of our knowledge, the 43.5% optical-optical efficiency is the highest reported to date from a cavity-dumped Q-switched Yb:YAG TDL (thin-disk laser).

Thermal analysis of side-pumped direct-liquid-cooled Nd: YAG thin-disk lasers

Thermal effects are the main obstacle for side-pumped direct-liquid-cooled Nd: YAG thin-disk lasers (SDNTDLs) to achieve high-power near-diffraction-limited laser output. A multiphysics model is established and several basic understandings are obtained. The temperature distribution of the disk under lateral non-uniform pumping shows that the total reflection transmission of the pump lights in the disk does not result in severe temperature fluctuations. The thickness of the thermal boundary layer is much less than half the thickness of the fluid, indicating that there is no thermal interaction between two adjacent disks. The component analysis of the thermally-induced wavefront aberration (TIWA) reveals that the influence of the fluid’s thermo-optic effect (TOE) which is awful in liquid lasers is not necessarily dominant in SDNTDLS due to the thin thermal boundary layer. When cooled by D2O, the wavefront aberration induced by the disk is much larger than that induced by the fluid. As a result, the TIWA exhibits a distribution of high at both sides and low at the center along the transverse direction of the disk, which is consistent with the experimental results. The wavefront aberration induced by the fluid’s TOE and that induced by the disk’s both TOE and deformation compensate for each other, indicating that the beam quality can be significantly improved by means of design optimization.