High-brightness 100-kHz source of ultrashort pulses tunable in 200-1050 nm spectral range
Valentina Shumakova,¹,* Thomas Braatz,¹ Luke Maindment,¹ Alice Autori,¹ Bastian Manschwetus,¹ Philipp Merkl,¹ Hossein Goudarzi,¹ Sebastian Starosielec,¹ Mark Prandolini,¹ Michael Schultz,¹ Robert Riedel¹ and Jan Heye Buss¹
¹ Class 5 Photonics Gmbh, Notkestraße 85, 22607 Hamburg
* valentina.shumakova@class5photonics.com
Abstract: We present a multi-channel 100-kHz tunable table-top laser source of sub-50 fs pulses, based on an optical parametric chirped pulse amplification (OPCPA) and a multi-pass cell (MPC) post-compression technologies. © 2023 The Author(s)
1. Introduction
Scaling up the repetition rate and average power of the laser systems, while keeping high peak power level, remained challenging until the recent advancement of Yb-based technology. Nowadays, industrial-grade lasers with average power above 100 W, few-mJ pulse energies and sub-ps pulse duration are available. However, most of the potential applications, such as pump-probe spectroscopy, micro-machining and generation of coherent X-ray light require ultrashort pulses (<100 fs) and/or possibility to tune the radiation wavelength. Recent advancements in MPC post-compression technique [1], as well as well-established OPCPA technology [2] pave the way for tunable high-repetition rate table-top sources of ultrashort pulses.
2. System Overview
Here we demonstrate a multi-channel laser system, consisting of:
- High-power OPCPA with a central wavelength of 800 nm, pulse duration of 20 fs, and average power >25 W;
- Tunable OPCPA, operating between 700 and 900 nm, with a pulse duration below 30-fs and an average power >15 W at the central wavelength;
- Second-harmonic generation (SHG) module, tunable between 350 and 450nm with the pulse duration below 50-fs and the average power >2.5 W at the central wavelength;
- Third-harmonic generation (THG) module, tunable between 260 and 300 nm with the pulse duration below 50-fs and the average power >0.5 W at the central wavelength;
- Forth-harmonic generation (FHG) module, tunable between 200 and 225 nm with bandwidth, supporting transform-limited pulse duration of 40 fs and the average power of 1-10 mW;
- Nonlinear MPC-based post-compression module, delivering 45-fs pulses with average power above 220 W at central wavelength of 1030 nm.
The channel 1 can run in parallel with either channel 2, or 3, or 4. The timing jitter between the channels was measured with a single short SH cross-correlator and is equal to 1.85 fs at rms-level over a 5-minute measurement time. All channels are pumped by a 100-kHz 500-fs 300-W Yb-laser, which is based on InnoSlab technology.
Both, high-power and tunable outputs of the system are pumped by the second harmonic of the pump laser, centered at 515 nm, and use BBO-crystals as nonlinear media. The outputs share a white-light seeder and non-collinear broadband pre-amplifier, providing 1 W of power and a smooth broadband spectrum between 680 and 950 nm. After the pre-amplifier the signal is split between the tunable and the high-power channels, what insures a low timing jitter between the pulses.
The tunable output is based on a single non-collinear BBO-based OPCPA stage, a tunable stretcher and a switchable compressor, based on high-dispersive mirrors (HDM). For each central wavelength the stretcher is optimized to provide maximum amplification while keeping the spectral bandwidth broadband enough to support sub-30 fs pulse duration. The non-collinear geometry of OPCPA stage provides a broadband phase-matching and ease the thermal management of the system. The average power is above 10 W in the whole tuning range with a peak (≈15 W) between 750 and 820 nm. The pulse duration is 27 fs at the center of the tuning region, and increases to 45 fs at the edges.
The high-power channel consists of a stretcher, 2 non-collinear broadband OPCPA stages and HDM- compressor. Here the stretcher is optimized to provide a maximum amplification for a spectrum supporting 17 fs transform limited (TL) pulse duration. The first stage amplifies the signal to ≈6 W, the second stage gives ≈27 W. After the HDM-compressor the pulses have 20 fs pulse duration and energy above 250 μJ.
Both channels have similar beam quality with M2x <1.5 and M2y <1.15. The SHG module is pumped by the tunable OPCPA-output and based on the BBO-crystal. The crystal thickness is chosen to maximize the conversion efficiency and spectral acceptance. The module is tunable between 350 and 450 nm with a maximum output power of 2.9 W at 410 nm. The duration of SH-pulses was measured with transient-grating FROG apparatus and two-photon absorption autocorrelator and was found to be between 37 and 50 fs within the tuning range.
The THG module is based of the collinear sum-frequency generation between fundamental wave and SH in the BBO-crystal. The module has a dispersion pre-compensation unit, based on the a pair of CaF² prisms, providing on-target pulse duration of 50 fs within the whole tuning range (260-300 nm). The maximum power after the dispersion pre-compensation unit is above 500 mW.
To generate radiation between 200 and 225 nm we non-collinearly mixed a small fraction of fundamental radiation and its third harmonic in BBO-crystal. The pulses have a bandwidth of around 1.5 nm, corresponding to sub-40 fs TL-pulse duration and a power of 1-10 mW within the tuning range.
For the post-compression of the pump pulses we build a low-pressure MPC and a compressor, based on HDM. The original pulses has a spectral bandwidth of around 4 nm FWHM-level and are non-linearly broadened in argon to support transform-limited pulse duration below 50 fs. The duration of the pulses after the compressor was measured with SH-autocorrelator and is equal to 46-fs. The output beam has a close-to-Gaussian spatial intensity profile and an excellent focusabilty with M² <1.1. The system has energy throughput of 90%.
All the modules have excellent power stability (0.2-0.6% rms) over more than 12 hours of continuous work and support hand-free staring and operating regime. The high power output is used to drive high-harmonics generation in gases and provides a flux above 1012 photons/s/eV in 22-42 eV spectral region.
References
1. Anne-Lise Viotti, Marcus Seidel, Esmerando Escoto, Supriya Rajhans, Wim P. Leemans, Ingmar Hartl, and Christoph M. Heyl, ”Multi-pass cells for post-compression of ultrashort laser pulses,” Optica 9, 197-216 (2022)
2. Dubietis A, Matijosius A. ”Table-top optical parametric chirped pulse amplifiers: past and present”, Opto-Electron. Adv. 6, 220046 (2023).
This article was originally published in the Proceedings of High-Brightness Sources and Light-Driven Interactions Congress Technical Digest Series 2024 by Optica Publishing Group. © Optica Publishing Group 2024.
