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Abstract
We report on the generation of high energy, high power pulses in a tabletop diode-pumped Tm:YLF-based laser system, which delivers amplified pulse energies up to 108 J, as well as GW peak power performance when seeded with nanosecond duration pulses. Furthermore, the high power and efficiency capabilities of operating Tm:YLF in the multi-pulse extraction (MPE) regime were explored by seeding the experimental setup with a multi-kHz burst of pulses exhibiting a low individual pulse fluence, resulting in a 3.6 kW average power train of multi-joule-level pulses with an optical-to-optical efficiency of 19%.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Energetic short pulse laser systems are presently capable of driving intense laser-matter interactions under extreme conditions to produce, e.g., high flux charged particle bunches [1,2], inertial confinement fusion [3], and extreme ultraviolet (EUV) photon sources for high volume integrated circuit manufacturing [4,5]. While the peak powers of state-of-the-art petawatt-class laser systems [6] have proved sufficient to generate high yield laser-plasma interactions, significant laser technology and architecture enhancements are still required [7,8] to operate at average powers far surpassing the kilowatt regime with orders-of-magnitude improvements in the overall system efficiency.
To address the rising demand for such next-generation laser systems, we have explored novel laser pulse amplification techniques such as multi-pulse extraction (MPE [9]), which allows for laser operation that generally scales in efficiency with increasing repetition rate. Here, a laser-active material exhibiting a long radiative lifetime and a high saturation fluence is pumped continuously at high energy densities. Simultaneously, the stored energy is extracted by an incident pulse train with an interpulse spacing much shorter than the radiative lifetime, such that multiple pulses are amplified within this time frame (e.g., 10 kHz extraction in Yb-doped thin-disks [10] exhibiting few-ms duration lifetimes). Efficient extraction is possible when the average extraction intensity is close to or exceeds the saturation intensity of the laser-active material. Therefore, in the MPE regime, where the repetition rate significantly exceeds the inverse of the radiative lifetime, the average extraction intensity required for efficient pulsed operation can be achieved within high saturation fluence materials at a non-damaging fluence (i.e., with an individual pulse fluence well below the saturation fluence). Furthermore, both energy storage and extraction are conducted in a steady-state manner analogous to continuous wave laser operation, resulting in a constant amplification along the full high repetition rate laser pulse train.
The laser-active material Tm:YLF [11] possesses characteristics that are well-suited for high peak and average power MPE operation: a broadband emission spectrum [12] near 1.9 µm, a long radiative lifetime of 15 ms, and an absorption spectrum peaked near 790 nm that overlaps with the emission ranges of high power laser diodes. Additionally, Tm:YLF exhibits a cross-relaxation interaction [13] that can excite two Tm$^{3+}$ ions to the upper lasing level per pump photon, thereby allowing for a low quantum defect despite the large difference in laser and pump wavelengths. The Big Aperture Thulium (BAT [14,15]) laser concept is designed to utilize these advantageous material properties to achieve high peak and average power performance, up to the petawatt-class and multi-hundred-kilowatt-class, respectively, with an optical-to-optical efficiency exceeding 20%. The realization of such high power, efficient, pulsed laser operation using Tm:YLF, however, has not yet been achieved.
Prior to our investigations, energy extraction demonstrations in diode-pumped Tm:YLF lasers resulted in maximum pulse energies of 300 mJ in 450 µs duration pulses [16]. To the best of our knowledge, the highest energies reported from pulsed lasers operating near 2 µm are 1.1 J in a diode-pumped Ho:Tm:LuLF laser [17] and 4.2 J in a free-running, flash-lamp-pumped Cr:Tm:YAG laser [18], which also delivered 0.81 J in Q-switched mode with 135 ns duration pulses. To uncover the potential performance limitations of upcoming Tm:YLF-based MPE lasers and enable a surpassing of the state-of-the-art, we have investigated high energy density storage and extraction in Tm:YLF, recently achieving joule-level amplification [19] in a directly diode-pumped Tm:YLF amplifier. In this paper, we present a further, substantial increase in Tm:YLF laser performance, building upon previous results to demonstrate nanosecond duration pulse amplification above 1 GW peak power and energy extraction in long-pulse mode exceeding 100 J. The significant enhancements to the Tm:YLF laser system architecture detailed here include an upgrade of the amplifier to a far-field multiplexed, relay-imaging, aperture-scalable configuration, as well as improvements in both the pump homogeneity, through the use of microlens arrays, and delivered pump power. Furthermore, we have conducted a proof-of-principle demonstration of high optical-to-optical efficiency, multi-pulse extraction operation using Tm:YLF for the first time, employing the experimental system to amplify a multi-kHz burst to kW-level average powers.
2. Experimental setup
The compact, Tm:YLF-based setup constructed for the following high energy, high power amplification demonstrations is depicted in Fig. 1 and consists of an oscillator followed by a single multi-pass amplifier. The oscillator, further described in [19] along with the output spectrum and temporal profile, utilizes a diode-pumped, water-cooled, 6%-at. doped Tm:YLF crystal placed in a folded 3 m path length optical cavity. Lasing occurs at $\lambda$ = 1.88 µm, with the intracavity polarization state aligned to the $\pi$-axis of the Brewster-cut Tm:YLF crystal for maximum gain. The oscillator is capable of exhibiting laser operation either in active Q-switched cavity-dumped mode or in a passive free-running mode for short pulse and long pulse amplification, respectively.
Fig. 1. Layout of the far-field multiplexed Tm:YLF amplifier. The seed pulse from a Tm:YLF oscillator is directed through the 4-pass amplifier using mirrors M1-M7 and dichroic mirrors DM, which reflect the seed pulse (1.88 µm) and transmit the pump pulse (793 nm). Lenses L1 - L6 are utilized to magnify and relay-image the seed, which is then amplified in a 6%-at. doped Tm:YLF crystal that is pumped on both sides by 20 kW laser diode arrays (LDA). The pump beam profiles were homogenized using a pair of microlens arrays (MLA) along with lens L7. The amplified pulse is ejected from the setup by passing through mirrors M2 and M5, which were placed in vacuum, and is directed to diagnostics using mirrors M8 and M9. Mirrors M1-A through M1-D (in place of mirror M1) form a bypass that adds two additional passes for the long pulse amplification demonstration. An image of the homogenized pump profile and the pumped Tm:YLF crystal are shown in the inset.
To demonstrate multi-joule, GW-level Tm:YLF laser amplifier operation, the oscillator was actively Q-switched and cavity-dumped, producing stable 4.0 mJ $\pm$ 1.4% RMS, 20 ns FWHM seed pulses at 1 Hz repetition rate. Following ejection from the oscillator cavity, the beam size is expanded using lenses L1 and L2, with radii of curvature (ROC) of −40 mm and 125 mm, respectively, and is subsequently directed into a far-field multiplexed Tm:YLF amplifier using mirror M1. The beam is relay-imaged through 4 passes using lenses L3 - L6 (L3 = L5 = 125 mm ROC; L4 = L6 = 400 mm ROC), with the image planes located at the Tm:YLF crystal and at mirror M7. The amplifier telescope configuration enables both mode-matching with the pump beam at the Tm:YLF crystal as well as near-focus ejection through mirrors M2 and M5, which were placed inside vacuum (10$^{-6}$ Torr) to mitigate the impact of ionization-induced air breakdown on the amplifier performance.
The power amplifier employs a 100 mm diameter, 35 mm thick, 6%-at. doped Tm:YLF crystal as the laser gain medium, which is end-pumped on both sides using 793 nm, 20 kW laser diode arrays (LDA). The beam profile of each LDA is reformatted from a grid of 4$\times$50 laser lines into a hexagonal profile using an imaging homogenizer setup. Here, a set of cylindrical lenses image the beam from the source onto a microlens array (MLA) pair, each containing a 2" $\times$ 2" grid of hexagonal-shaped lenslets (1.75 mm pitch, 5.25 mm ROC). The MLA pair separates the beam into multiple beamlets, which are then overlapped and reimaged onto the Tm:YLF crystal surface using the final spherical lens L7 (103 mm ROC), forming the homogeneous profile shown in Fig. 1. The output pump beam size is 22 mm FWHM and can be scaled by replacing L7 and adjusting the working distance appropriately. The measured transmission of the pump delivery system was 96.5%, resulting in a total pump power of 38.6 kW at the Tm:YLF crystal with spatial amplitude fluctuations of $\pm$ 1.2% RMS. Emphasis was not placed on optimizing the amplified laser beam quality, long-term energy stability, nor on efficiently cooling the material, and although a similar amplifier architecture employing a gas-cooled, multi-slab geometry would allow for high repetition rate operation [20–23], the crystal is instead water-cooled radially and pumped at a repetition rate of up to one shot every 20 seconds for the proof-of-principle results presented here.
3. High peak power and 100-joule-level amplification in Tm:YLF
Direct measurements of the full output pulse energy after four passes through the amplifier were conducted using a pyroelectric detector (maximum single-shot energy 85 J, input pulse width <700 µs) located after mirror M9, with the results given in Fig. 2. The 4-pass pulse energy for the unpumped amplifier was determined using the measured $\pi$-axis Tm:YLF transmission of 20% per pass, resulting in a total transmission of $\sim$0.15%. Due to the quasi-3-level behavior in Tm:YLF, reabsorption of the seed photons occurs until a sufficient number of Tm$^{3+}$ ions have been excited to the upper lasing level via pumping and, ultimately, the two-for-one cross-relaxation interaction. The pump pulse duration was then set to 40 ms, and the pump was ramped up to 73.5% (28.4 kW) of the total delivered pump power to achieve an amplified pulse energy of 21.7 J within the 20 ns duration pulses. Energy measurements conducted while blocking the seed entrance to the amplifier revealed no measurable amplified spontaneous emission (ASE), which was passively suppressed by the low angular acceptance of the multi-pass amplifier configuration. For this pump pulse duration, the full pump power was not employed to avoid amplifying the seed pulse to a fluence level that may damage the optical components within the setup. A further characterization of the amplifier performance was subsequently conducted with the pump pulse duration adjusted from 10 ms to 40 ms – on the order of the 15 ms Tm:YLF radiative lifetime – and the total delivered pump power scaled towards the full 38.6 kW for the shorter pump pulse durations. The results, plotted on a logarithmic scale with respect to the amplified pulse energy, detail the exponential increase in pulse energy over multiple orders of magnitude as a function of input pump power, indicating non-saturated amplifier operation nearly up to the maximum extraction of 21.7 J (6.9 J/cm$^2$ average fluence), which corresponds to a net 4-pass gain of 5400 and an output peak power of 1.1 GW for this compact system.
Fig. 2. Measurements and corresponding exponential fits of the 4-pass output pulse energies for the Tm:YLF amplifier when seeded with 4 mJ, 20 ns pulses. A maximum pulse energy of 21.7 J in 20 ns pulses (1.1 GW peak power) was demonstrated when pumped with 28.4 kW, 40 ms pulses. Results are displayed on a linear scale (top), with a solid line used to guide the eye, and on a logarithmic scale (bottom) with the total delivered pump power increased up to 38.6 kW over pulse durations of 10 ms to 40 ms.
High peak power laser systems based on Tm:YLF are anticipated to deliver pulse energies up to the 100 J-level [2]. To experimentally rule out potential energy extraction limitations in Tm:YLF, the oscillator was set to operate in a free-running mode before seeding the power amplifier. In this configuration, the oscillator produces 3 ms duration pulses (a small fraction of the Tm:YLF radiative lifetime), which allows for an increase in the extracted pulse energy at non-damaging laser intensities. To further improve the energy extracted from the pumped Tm:YLF crystal, a bypass was inserted using mirrors M1-A through M1-D in place of mirror M1, as shown in Fig. 1, between the oscillator and the power amplifier to introduce two extra passes before the seed is coupled into the amplifier. The seed pulse was then temporally aligned to the end of the 40 ms pump pulse, and measurements of the 6-pass amplified pulse energy were conducted using a thermopile-based detector (maximum pulse energy 500 J, input pulse width <500 ms). As seen in Fig. 3, the amplifier rapidly experiences saturation and produces amplified pulse energies of 21.2 J at 23.6 kW pumping, nearly matching the maximum output pulse energy achieved for the 20 ns duration seed pulses. A further ramping of the total delivered pump power up to 36.6 kW safely resulted in amplified pulse energies of 108 J (34.4 J/cm$^2$ average fluence), along with an additional 1.7 J attributed to amplified spontaneous emission, which was measured with the seed entrance to the amplifier blocked. At pump powers below 36.6 kW, the energy of the ASE light was too low to be measured by the employed detector. Transverse parasitic lasing, which can substantially reduce the amplifier gain, was passively mitigated by the large unpumped region of the 100 mm diameter Tm:YLF crystal that, due to the quasi-3-level nature of this active material, absorbs the incident transverse ASE. At peak extraction, with the total deposited pump pulse energy of 1464 J ($\sim$100 J/cm$^3$ extractable energy storage density), the optical-to-optical efficiency of this tabletop, 100 J-level Tm:YLF laser is 7.4%. These results demonstrate, to the best of our knowledge, the highest pulse energies obtained for any laser system operating near 2 µm worldwide, and conclusively reveal that there are no gain physics phenomena preventing high energy density storage and extraction in Tm:YLF.
Fig. 3. Six-pass amplified pulse energy measurements in the Tm:YLF power amplifier, pumped with a total power up to 36.6 kW in 40 ms pulses. The 3 ms long seed pulses were injected at the end of the pump pulse and amplified up to 108 J.
4. Efficient burst-mode operation through multi-pulse extraction
As described in [14], a full BAT-like laser system is designed to operate not only at high pulse energies and peak powers, but simultaneously as an efficient average power device using the MPE technique with CW laser diode pumping and effectively continuous extraction. As a proof-of-principle experiment, efficient MPE operation in Tm:YLF was explored and demonstrated utilizing the power amplifier in a 6-pass configuration. The Tm:YLF crystal is pumped with an input power up to 18.2 kW and a pulse duration of 25 ms to achieve a steady state, in which pumping, extraction, and spontaneous emission are balanced to result in a constant population inversion within the crystal throughout amplification. For this experiment, a 1.88 µm, $\sim$6.8 kHz pulse train burst generated from a self-Q-switched Tm:YLF laser was employed as the seed source. The self-Q-switching process is a result of the saturable ground-state reabsorption [24] in Tm:YLF, and the output slope, pulse duration, and repetition rate can be tuned by varying, e.g., the pump power and crystal temperature. The temporal profile of the seed is adjusted to produce a nearly flat burst of pulses, with $\pm$ 6.4% RMS pulse-to-pulse amplitude fluctuations, and is then injected into the amplifier with a temporal delay, such that the end of the seed pulse train and pump pulse are aligned. Following amplification, the output pulse power is compared to the total pump power to determine the achievable optical-to-optical efficiency for this diode-pumped Tm:YLF system under MPE operation.
The results of this experiment are given in Fig. 4(a), which displays the output power of the amplified pulse train burst and the corresponding optical-to-optical efficiency. The temporal profile of the amplified burst in Fig. 4(b) shows, at lower pump powers, a steady increase in the slope over the full duration as the amplifier operates in a non-saturated state with a low optical-to-optical efficiency of 1.7%. Here, the average intensity of the amplified burst of 230 W/cm$^2$ is substantially lower than the Tm:YLF saturation intensity of 1.4 kW/cm$^2$ at 1.88 µm ($\pi$-axis, room temperature). However, as seen in Figs. 4(c)-d, when the pump power is increased up to 18.2 kW, the front of the burst experiences gain high enough to deplete the inversion within the pumped Tm:YLF crystal, and the slope achieves an approximately constant amplitude in a quasi-steady state through the remainder of burst duration. In this case, the average intensity of the amplified burst is approximately 4.2 kW/cm$^2$, far surpassing the saturation intensity and resulting in an amplified pulse power of 3.6 kW and an optical-to-optical efficiency of 19%. The pump energy necessary to generate the initial gain within the Tm:YLF crystal before equilibrium was not included in the efficiency calculations for this experiment on multi-pulse extraction in Tm:YLF, as this contribution takes places outside of steady-state operation and would be negligible following the intended, long-term pumping and seeding (i.e., as the pulse train duration is extended) of the amplifier. A further increase in the amplified pulse power and optical-to-optical efficiency can be accomplished following an optimization of the amplifier design and Tm:YLF crystal parameters, including, e.g., the doping density and material thickness. Although the individual fluence of each pulse within the burst (nearly 0.6 J/cm$^2$) is 36$\times$ lower than the Tm:YLF saturation fluence of 21.6 J/cm$^2$ at the gain peak, this high repetition rate, multi-joule-level Tm:YLF laser operates efficiently and in a saturated steady-state regime through the multi-pulse extraction technique.
Fig. 4. a) Amplified pulse power (black) and corresponding optical-to-optical efficiency (blue) as a function of input pump power (up to 18.2 kW at 25 ms) for quasi-steady-state Tm:YLF laser operation in the multi-pulse extraction (MPE) regime. b-c) Temporal profiles of the amplified pulses, which show the transition through saturation and towards constant gain for each pulse within the pulse train burst as the pump power is increased. d) Envelope profiles for b) and c) shown on a logarithmic scale for clarity.
5. Conclusion
In conclusion, we have demonstrated high power, efficient laser performance in Tm:YLF at high energy storage and extraction densities. Here, the fully diode-pumped, tabletop Tm:YLF system was employed to amplify 4 mJ, 20 ns pulses up to 21.7 J (1.1 GW peak power) in a 4-pass, far-field multiplexed power amplifier. Higher energy extraction representative of full BAT laser designs was subsequently achieved by seeding the amplifier with the free-running oscillator and introducing two additional passes, producing pulse energies of 108 J at 1.88 µm – the highest extracted pulse energy from any laser operating near 2 µm to-date – with an optical-to-optical efficiency of 7.4%. Finally, high power multi-pulse extraction was demonstrated in Tm:YLF for the first time, with the amplification of a 6.8 kHz pulse train burst up to 3.6 kW average power. In characteristic MPE fashion, the system exhibits a high optical-to-optical efficiency of 19% in steady-state operation, while maintaining an individual pulse fluence 36$\times$ below the Tm:YLF saturation fluence. These results indicate that Tm:YLF is capable of achieving high energy, high power, efficient operation, and is, therefore, a suitable and promising candidate for next-generation MPE laser systems.
Acknowledgments
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project Numbers 19-DR-009 and 20-ERD-016, as well as with funding from the Defense Advanced Research Projects Agency (DARPA) through the Defense Sciences Office.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time, but may be obtained from the authors upon reasonable request.
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