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Abstract
We report on a high-energy and passive-cooled Nd:glass regenerative amplifier based on Innoslab laser technology. At a total pump power of 4.5 kW at 802 nm and pulse energies up to 27.2 mJ are generated with a pump pulse duration of 450 µs. The energy stability at 16.3 mJ is 1.5% (rms) over 2.5 hours at a 1 Hz repetition rate with beam propagation factors M2 in the horizontal direction and the vertical direction of 1.12 and 1.14, respectively. To the best of our knowledge, this is the first Innoslab-based Nd:glass regenerative amplifier without active cooling.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High gain and high energy rod Nd:glass regenerative amplifier which is quite compact and can provide high-optical signal-to-noise ratio and excellent stability, has been actively developed for the large-scale laser facility dedicated to inertial confinement fusion, such as the National Ignition Facility (NIF) [1,2], SGII series [3,4], and Inertial Fusion Energy (IFE) [5]. However, the thermal effect inside the Nd:glass rod limits the repetition rate below 1 Hz even with active water cooling [6].
In the last few years, among technologies such as fiber, slab, or thin disk, the Innoslab has demonstrated strong potential for the development of high repetition rate and high energy laser systems [7–9]. In this scheme, it is similar to disk in the sense that waste heat is extracted efficiently through the large surface which leads in very low thermal-optical distortion of the laser beam. However, unlike in the end-pumped disk laser, in which the laser medium of high doping concentration such as Yb doped crystals are required [10,11], in the Innoslab design, the slab is longitudinal pumped by a line-shape pump beam in combination with an appropriate beam path, so that the slab can get efficient absorption of the pump beam, making the Innoslab amplifier design extremely suitable for Nd:glass medium. The technique has been successfully implemented to high average power laser amplification. In 2010, Russbueldt et al. attained 1.1 kW femtosecond amplified laser output with high beam quality by cascading two Innoslab amplifiers [12]. Recently, Bruno E. Schmidt reported an yb-innoalsb laser towards high pulse energies of 54 mJ combined with high average power exceeding half a kW at 10 kHz repetition rate [13].
In this letter, we propose and demonstrate for the first time a Innoslab-based passive-cooled Nd:glass regenerative amplifier. Benefit from the excellent thermal management of the Innoslab amplifier design, a 27 mJ, 5 ns near diffraction limited laser output at 1 Hz without active cooling was achieved.
2. Design and amplifier setup
The whole amplifier system consisted of an all-fiber front-end seeder and an Innoslab-based regenerative amplifier, as shown in Fig. 1. The oscillator for the entire amplifier is a distributed feedback (DFB) laser with a bandwidth less than 100 kHz at 1053 nm. The acousto-optic (AO) modulator and electro-optic (EO) modulator driven by an arbitrary waveform generator (AWG) can control the repetition rate and temporal shape of the CW signal from the output of the oscillator. Phase modulation is applied to suppress buildup of stimulated brillouin scattering (SBS) in the integrated fiber optics and smoothing by spectral dispersion (SSD), which are both required in the process of inertial confinement fusion using lasers. Here, the laser pulse spectrum is broadened to 0.3 nm by using the phase modulator. The threshold of SBS is related to the bandwidth of the pump laser; therefore, it is improved by nearly 100 times compared to the original laser. Finally, a 5 ns temporally shaped signal pulse of energy 270 pJ at 1 Hz repetition rate is collimated by an aspherical lens into the regenerative amplifier.
Fig. 1. Experimental schematic of the passive-cooled Innoslab-based Nd:glass regenerative amplifier system. DFB, distributed feedback laser; AOM, acousto-optic modulator; AMP, single mode fiber amplifier; PM, phase modulator; EOM, electro-optic modulator; AWG, arbitrary waveform generator; PC, polarization controller; ISO, high power isolator; PBS, polarizing beam splitter; HWP, half-wave plate; QWP, quarter-wave plate; DM, dichroic mirror; LDA, laser diode array; f1, f2, aspherical lens.
The regenerative amplifier consists of a dual stage optical isolator to protect the seed laser system from the much higher energy regenerative output leakage which naturally propagates in the reverse direction. An 0.5 wt.% doped Nd:glass with dimensions of 15 mm×15 mm× 5 mm was used as the gain medium for the amplifier. The two large surfaces were mounted tightly with the copper heat sinks wrapped by liquid metal, allowing efficient heat removal. Both end faces of each slab were polished and antireflection (AR) coated at 1053 nm and 802 nm. The laser cavity of the amplifier resonator is a folded, linear one with a whole cavity length of 3 m, which accommodates 2 flat mirror, a convex lens with a focal length of f = 3.5 m and a KD*P-Pockels cell (Fastpulse Technology, Inc), inducing the λ/4 phase shift required by pulse picking. During the amplification, the quarter -wave voltage is added to the Pockels cell to convert the s-polarized light to p-polarized light or vice versa. The Pockels cell is opened immediately when the signal laser pass through. After a certain number of round-trips, the amplified laser pulse transmits outside the regenerative amplifier as the Pockels cell is closed.
Two laser diode array (LDA) were applied as the pumping source. The center wavelength was fixed at 802 nm by adjusting the temperature of the cooling water. With the fast axis collimated by cylindrical micro-lenses, each LDA emitted a maximum output power of 20 kW. The optical coupling system consisted of a homogenizer, four cylindrical lenses, and two spherical lenses. The pump light was shaped to be nearly a homogenous rectangular shape with a size of 13 mm × 5 mm on the pump face of the Nd:glass by the coupling system implying a pump intensity of 6.92 kW∕cm2 at a pump power level of 4.5 kW. The total pump absorption efficiency of all the slabs was about 99%. Two plane dichroic pump mirrors separate the pump light at 802 nm and the laser light at 1053 nm.
The amplifier was designed to propagated through different pump regions of the Nd:glass slab with three passages by using two fold mirror. As shown in Fig. 2, The resonator mode was calculated by using ABCD matrix formalism and is designed to keep power densities in the Nd:glass slab and KD*P crystal media in a range below the damage threshold. Furthermore, ensures a nearly similar mode size on the slab during the laser beam propagated through the slab three times.
Fig. 2. Calculated eigenmode of the resonator configuration. The vertical lines denote positions of Nd:glass slab (blue lines) and Pockels cell (green line)
3. Results and discussion
Figure 3 shows the output energy from the amplifier and round-trips as a function of total pump peak power. Upon increasing the pump power, the pulse energy increases monotonously. An output pulse energy of 27.2 mJ is obtained with 13 round trips at a pump power of 4.5 kW. The pump pulse duration is 450 µs. The output energy is measured using a pyroelectric joulemeter (Coherent EnergyMax).
Fig. 3. Maximum output pulse energy (red line) and number of round-trips (blue line) needed to reach the highest output energy versus total pump peak power
Over 10 millijoule output energy is enough for the regenerative amplifier used in the laser-driven inertial confinement fusion, so the long term output energy stability measurement of 16.3 mJ at 1 Hz was carried out over 2.5 hours with an energy meter. Figure 4 shows the output energy (7.36% sampling) of regenerative amplifier with a 1.5% rms overall energy. No damage or beam deterioration was observed during the long-term operation of the laser amplifier. Energy stability will improve if the laboratory environmental disturbance is eliminated.
The output beam profile and quality values were measured with the wavefront sensor SID4 (Phasics) including a function of beam quality analysis. As shown in Fig. 5, the M2 beam quality values measured along the horizontal and vertical directions are 1.12 and 1.14, respectively, corresponding to an average value of M2 to be 1.13, which indicates an almost single mode output beam. The inset at upper shows a symmetric, Gaussian-shaped profile. Its profile does not change significantly over the three hours without active cooling while the energy stability did not decrease. The excellent beam quality is a key factor for application in the front end of a large-scale facility. The nearly single-mode beam will improve the performance of the main amplifier in the facility.
Fig. 5. M2 of the output beam measured at 16.3 mJ without activing cooling. The inset shows the beam shape. The measured values are Mx2=1.12 and My2=1.14.
After amplification, the pulse approaches a deep saturated state, and it is reasonable that the gain saturation seriously influences the temporal profile. Square pulse distortion (SPD) of 1.43 has been measured at the maximum output energy as shown in Fig. 6.
4. Conclusion
In conclusion, we demonstrate a innoslab-based passive-cooled Nd:glass regenerative amplifier with more than 27 mJ output energy at 1 Hz without active cooling. The energy stability is approximately 1.5% (rms) at the energy of 16.3 mJ, while still maintaining an excellent beam shape and M2 =1.13. Further pulse energy scaling can be realized by enlarging the mode diameter in the Nd:glass gain medium. The Innoslab-based Nd:glass regenerative amplifier designed for application in a front end can replace several stages of an amplifier and simultaneously improve beam quality, energy stability, and temporal profile stability and fidelity. The Innoslab-based Nd:glass regenerative amplifier shows an alternative method for PAM in large-scale laser facility.
Funding
National Natural Science Foundation of China (NSFC) (61705242); Shanghai Science and Technology Foundation for Young Scholars (17YF1429600); Program of Shanghai Academic/Technology Research Leader (19XD1404000).
References
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