Abstract
1. Introduction
Efficient diode pumping of solid-state lasers (DPSSL) has enabled lasers to reach CW output powers in the region of 100 kW[
In this paper, an overview of the HiLASE activities, including laser development and laser applications, will be presented.
2. Kilowatt-class thin disk laser system
For efficient generation of EUV and mid-IR light, a laser producing several mJ per pulse at a repetition rate of 1–100 kHz is required. For industrial applications, it is important to realize a robust, compact, and low-cost alternative to Ti:sapphire-based pulsed laser systems. Thin-disk lasers with their feature of a high pulse energy in the sub-picosecond region are one of the best devices suited for this application.
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A thin-disk laser is based on an amplifier concept[
On the other hand, the thinness of the disk causes minimal pump light absorption and laser light gain. Therefore, the number of passes of both pump and laser light must be high. The pump light is sequentially reflected back to the laser disk by a parabolic reflector and roof prisms (Figure
Within the HiLASE project, three thin-disk-based kW-class laser beamlines are being developed, each delivering different output parameters. Beamline A will deliver a 750 mJ pulse energy at a 1.75 kHz repetition rate. This beamline is subcontracted to Dausinger and Giesen GmbH in order to reduce the overall project risk associated with the high demands. The HiLASE research group is developing Beamlines B and C with output parameters of 500 mJ at a 1 kHz repetition rate and 5 mJ at a 100 kHz repetition rate, respectively. All beamlines will provide a pulse duration of 1–3 ps. The output of Beamline B could be diverted into a 10 Hz repetition rate cryogenic amplifier that would later be upgraded to multi-joule output at repetition rates up to 120 Hz. Figure
2.1. Beamline A
Beamline A consists of a fiber front-end that includes a pulse stretcher, pulse picker, and optical isolator. The front-end produces laser pulses with an energy of at a repetition rate of 1.75 kHz. These pulses are further amplified in a regenerative amplifier to an energy around 150 mJ, then in a linear amplifier to an energy around 0.9 J. The amplified pulses are compressed in a grating pulse compressor to below 3 ps.
2.2. Beamline B
Beamline B[
In order to reduce thermally induced stress between the heatsink and the gain media, CuW is adopted as the heatsink material, because it has similar linear expansion coefficient to YAG. The crystal, doped to 7 at.%, has thickness of 0.22 mm and is soldered on the CuW heatsink using gold-tin solder. The mounted disk is pumped by a fiber-coupled diode laser module delivering an optical power of up to 1 kW at a wavelength of 969 nm. The pump spot size on the disk was set to 4.8 mm to achieve an output of 45 mJ, and the amplifier cavity was designed so that the cavity mode was matched to the designed pump spot size. The optical-to-optical efficiency was close to 20%. The amplified laser pulses had a bandwidth of 1.5 nm, so they could be compressed down to 1 ps.
The laser cavity will later be upgraded with a second thin-disk head to reach an output energy of 100 mJ. Additionally, the Martinez-type stretcher will be replaced with a fiber-chirped Bragg grating stretcher that allows better control of dispersion and is more stable and compact. Finally, the amplified pulses will be directed to a second regenerative amplifier that will be constructed in 2014.
2.3. Beamline C
Beamline C[
The observed output energy was at a 100 kHz repetition rate. The low pulse energy enabled compression in a highly efficient CVBG compressor. The compressed pulse energy and the efficiency of the CVBG were and 88%, respectively. The output pulse had a spectral bandwidth of 1.2 nm and was compressed only to 4 ps pulse duration because the CVBG did not account for the dispersion of material in the path of the beam. By adding an additional diffraction grating compressor, the duration of the compressed pulses was decreased below 2 ps. The output energy will be increased by using more intense pump light and by modifying the thin-disk head and the cavity. Technical difficulties connected to further development of all the mentioned thin-disk beamlines are connected mostly to thermal management of the thin-disk modules and the availability of pump modules at a wavelength of 969 nm.
Laser system | Beamline A | Beamline B | Beamline C | Cryogenic beamline |
---|---|---|---|---|
Completed | Front-end | Regenerative amplifier with one thin-disk head | All, except high power pump modules | None |
Under development | Regenerative amplifier (May 2014) | Add second thin-disk head into regenerative amplifier | Add high power pump modules | 10 Hz concept amplifier |
Achieved energy | 45 mJ | 0.8 mJ | NA | |
Next milestone energy | 150 mJ | 100 mJ | 2 mJ | 1 J (10 Hz) |
Final energy | 750 mJ | 500 mJ | 5 mJ | 1 J (100 Hz) |
Operational | Q2 2015 | Q2 2015 | Q2 2015 | 2016 |
Table 1. Status of kW-class Thin-disk Beamlines.
The current status of thin-disk beamlines is indicated in Table
3. Kilowatt-class multi-slab laser system
To generate high energy pulses at low/moderate repetition rates, it is necessary to adopt an effective cooling mechanism and geometry. One of the solutions is to use an active medium in a slab geometry with active cooling of the slab faces, called a multi-slab, firstly adopted on the Mercury laser at Lawrence Livermore National Laboratory [
The system incorporates a low-energy, fiber-based front end oscillator (nJ), followed by a regenerative amplifier that increases the output energy to the mJ level and a thin-disk multi-pass booster amplifier to raise the output to 100 mJ. Two diode-pumped, helium-gas-cooled large-aperture power amplifiers then increase the output energy to between 7 and 10 J (Main Pre-amplifier) and finally to 100 J (Power Amplifier). The schematic of the system is shown in Figure
3.1. Front end
The front end starts with a temperature-stabilized tunable CW fiber oscillator. The wavelength of the oscillator is matched to the peak of the gain curve of the cryogenically cooled amplifiers. The CW beam is then temporally shaped in an acousto-optic (A-O) modulator to limit the repetition rate to 10 kHz and subsequently shaped by an electro-optic (E-O) modulator to produce 2–10 ns pulses with a semi-triangular shape. The temporal resolution of the shaper is below 200 ps. The pulses are further phase modulated by 2 and 4 GHz modulators to increase the bandwidth of the pulses and prevent stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in the amplifier chain. Then the repetition of the pulses is decreased by a pulse picker and pulses are amplified in a thin regenerative disk amplifier to 1 mJ. The Gaussian beam coming from the regenerative amplifier is then spatially shaped to a square cross-section super-Gaussian profile in a beam shaper. Then it is further amplified to 100 mJ in a multi-pass booster amplifier. The booster amplifier preserves the square super-Gaussian beam profile that is injected into the 10 J main pre-amplifier.
3.2. 10 J main pre-amplifer
The 10 J main pre-amplifier is based on a multi-slab design. It consists of four circular slabs with two doping levels of (1.1, 2.0 at.%). The different doping levels are needed to uniformly divide the heat load among the slabs. The volume of each circular slab is diameter in 45 mm with a thickness of 5 mm and the pumped area is square . The pump beam is homogenized light from diode stacks operating at 939 nm and producing long laser pulses at a repetition rate of 10 Hz. The is clad with a 5 mm absorber (absorption coefficient ) that prevents amplified spontaneous emission (ASE) and parasitic oscillations. The amplifier is cooled by forced helium gas flow and operates between 150 and 170 K.
The extraction scheme of the multi-pass amplifier is shown in Figure
3.3. 100 J power amplifier
The 100 J power amplifier is also based on the multi-slab design. It consists of six square slabs with three doping levels of (0.4, 0.6, 1.0 at.%). The volume of each slab is and the square pumped area is around . The parameters of the pump light are similar to the 10 J amplifier. The is clad with a 10 mm wide absorber (absorption coefficient ) that prevents ASE and parasitic oscillations.
The extraction scheme of the multi-pass amplifier is shown in Figure
The current status of the multi-slab laser system is indicated in Table
Laser system | Beamline A |
---|---|
Completed | 10 J main pre-amplifier |
Under development | 100 J power amplifier |
Achieved energy | 10 J |
Next milestone energy | 50 J |
Final energy | 100 J |
Operational | Q3 2015 |
Table 2. Status of kW-class Multi-slab Beamline.
3.4. Numerical modeling
The HiLASE team has undertaken extensive energetics, thermal and fluid-mechanical modeling in order to optimize various amplifier parameters.
For energetics modeling, we have developed a MATLAB code[
The wavelength-resolved absorption and emission cross-sections and lifetime on the upper laser level for a given temperature were obtained experimentally [
A three-dimensional finite-element method (FEM) using Comsol Multiphysics software was chosen to model the thermal and stress effects in the amplifiers. The sources of heat were calculated in the ASE code. The lateral surfaces of the slabs are assumed to be cooled by flowing helium gas at 160 K. The spatially resolved heat transfer coefficient was derived from a two-dimensional model of a turbulent flow of helium gas at 160 K using the standard model[
A beam propagation model of the 100 J power amplifier was created in MIRÓ using a Fresnel diffraction integral for propagation and the Frantz–Nodvik equation for amplification. The model was used to estimate beam aberrations, taking into consideration only the thermal OPD. The results of the beam intensity and OPD are shown in Figure
The numerical model for wavefront correction calculates influence functions from a plate equation describing the bending of the thin facesheet for each individual actuator of the deformable mirror. The deformable mirror consists of a continuous gold facesheet (size of , thickness of 1 mm) on which lateral forces are applied by piezoelectric stack actuators. The actuators form an equidistantly spaced rectangular array of actuators and are capable of push/pull operation. The deformation of the mirror is computed as a superposition of the influence functions and the algorithm minimizes the rms OPD value.
The OPD after subtraction of defocus and tilt and the OPD corrected by the deformable mirror are shown in Figure
4. Applications
One of the long-term objectives of HiLASE is the identification of new and promising industrial applications and technologies using the DPSSL systems that were described above. Once commissioned in the HiLASE center these advanced DPSSL systems will enable, for example, research relevant to the testing of new dielectric optical components with high damage thresholds, prototyping new pump lasers for OPCPA (Optical Parametric Chirped Pulse Amplification) systems, driving high yield secondary photon and particle sources, and industrial applications related to efficient processing of materials (ablative removal of thin layers, cutting of optically transparent materials, laser peening, surface structuring and modifications, etc.). An overview of the HiLASE laser application program is shown in Figure
4.1. Laser-induced damage threshold testing
First, the laser-induced damage threshold (LIDT) automated experimental station would be introduced. The station design allows one to measure the LIDT under a wide range of laser parameters: from the irradiation of small spots with 1–2 ps laser pulses at various wavelengths and a 1 kHz repetition rate to the irradiation of large spots with 2–10 ns laser pulses at 1030 nm with a 10 Hz repetition rate. The main advantage of this station is real-time monitoring of laser damage with an acquisition frequency of up to 1 kHz. This station will allow the determination of the damage occurrence, as well as following the damage growth and damage threshold variation under repetitive irradiation. A schematic of the station is shown in Figure
4.2. Mid-IR optical parametric generator
For the investigation of laser–material interactions and processing, as well as the thin-disk and multi-slab systems, a mid-IR pulse source with a high repetition rate and an average power of 10 W [
4.3. EUV light generation
For EUV generation, powerful lasers are used to evaporate tin (Sn) droplets to generate a plasma that emits light at 13.5 nm. The laser provides a much higher average power and higher conversion efficiency to UV light, but the laser footprint and plasma size are large[
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