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Abstract
In this paper, a single-frequency quasi-continuous-wave partially pumped slab (Innoslab) amplifier at 1064 nm is reported. The 4.4-W single-frequency seed laser was amplified to 148.1 W with optical-optical efficiency of 30.4%. The output pulse duration was 141.4 μs at the repetition of 500 Hz. The beam quality factors of M2 were 1.41 and 1.37 in the horizontal and vertical direction respectively. The experimental results match well with the numerical simulation. To the best of our knowledge, this is the first report on single-frequency Nd:YAG Innoslab amplifier with such a high output power and efficiency.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1 Introduction
Narrow-linewidth lasers at 1064 nm with high output power and good beam quality, are commonly used as one of the fundamental lasers to generate the sodium beacon lasers in adaptive optics system [1]. To get enough photons return and avoid temporal overlap with backwards Rayleigh scattering light, the sodium beacon lasers need to operate in pulses with several hundred milli-joule pulse energy, 100-μs level pulse width and the linewidth of less than several hundred mega-hertz [1–3]. Narrow linewidth 1064-nm lasers with high beam quality can be obtained directly by a laser oscillator. But it usually needs further amplification to get high power output. That requires high pump intensity throughout the laser crystal which will lead to serious thermal problems. Various constructions of laser amplifiers have been used to reduce the thermal effects like rod, planar waveguide and slab [4–7].
Partially pumped slab (Innoslab) laser and amplifier were proposed by Du in 1990s [8,9]. It has been extensively applied in cw-, μs-, ns-, ps-, and fs-amplifiers [10–15]. An Innoslab amplifier usually consists of a slab-shaped laser crystal, a partially end-pumping system and a hybrid cavity for multi-pass amplification. It utilizes the two large faces of the slab for cooling. The heat removal is highly efficient because of the short distance between the two large faces [16]. The optical surfaces are partially end-pumped by diode laser stacks on a high aspect ratio. The heat distribution of the slab is nearly one-dimensional. The multi-pass cavity of Innoslab amplifier consists of two directions. In one direction, there is a stable cavity sustained by the thermal lens of the slab crystal. In the other direction, the multi-pass cavity which consists of two confocal mirrors is unstable. The seed laser beam is folded by the two confocal cylindrical mirrors and goes straight through the crystal several times in the amplifier. The laser beam is expanded by the two confocal mirrors in the unstable direction which leads to a uniform margin of laser intensity inside the slab to avoid crystal damage [9]. Because of the folded beam path, the overall gain of the amplifier and the overlap between the seed laser and pumped volume can be very high. And the good thermal management is to the benefit of beam quality. Those innovative arrangements of the Innoslab amplifiers decrease the possibility of parasitic oscillation, guarantee the high power, high beam quality laser output with compact and reliable stability [17]. There have been some reports on Innoslab amplifiers at 1064 nm. In 2016, a continuous-wave single-frequency Nd:YVO4 Innoslab amplifier at 1064 nm was reported [10]. The output power was 60 W at the pump power of 245 W with optical-optical efficiency of 24.5%. The beam quality M2 in the horizontal and vertical direction was measured to be 1.4 and 1.3, respectively. In 2017, a two-stage Nd:YAG Innoslab amplifier was reported by Strotkamp [18]. More than 50 W of average power at 1064 nm was generated after two-stage amplification with beam quality factors of 1.8 and 1.4. The pulse duration was about 30 ns at the repetition of 100 Hz. The optical-optical efficiency was about 23%. In 2020, Javed et al. reported a high power Nd:YAG Innoslab amplifier at 1064 nm [19]. A 40-W seed laser was amplified to 210 W of average power at repetition of 100 kHz. The pulse duration was 11.6 ns and the optical conversion efficiency was 19.5%. The beam quality M2 was 2 and 1.77.
In this paper, we give a study on the high-power, single-frequency, long-pulse Nd:YAG Innoslab amplifier at 1064 nm. The 4.4-W seed laser was amplified to 148.1 W with optical-optical efficiency of 30.4% by five-pass amplification. The beam quality factors of M2 were 1.41 and 1.37 in the horizontal and vertical direction respectively. The output power and optical-optical efficiency can be further improved with higher overlapping efficiency between the pump volume and the seed laser.
2 Experimental setup
The experimental setup of the 1064-nm Nd:YAG Innoslab amplifier is shown schematically in Fig. 1. The laser amplification system comprised of the single-frequency seed laser, the multi-pass cavity, Nd:YAG slab crystal and the quasi-continuous wave (QCW) pumping system at 808 nm.
The seed laser contains a single-frequency, 30-mW CW oscillator and a QCW pulsed fiber amplifier. In the experiment, the central wavelength of the linearly polarized seed laser was set to 1064.6257 nm with the line-width of 0.19 MHz. The pulse width of the seed laser was ∼141.0 μs at the repetition of 500 Hz, as shown in Fig. 2. The maximum average power of the seed laser was 4.4 W. The beam quality factors of M2 were measured to be 1.17 and 1.21 in the horizontal and vertical direction respectively. An optical isolator was inserted to prevent the optical feedback from the amplifier. The seed laser was shaped into an elliptical beam by a beam shaping system for mode matching. The beam size after shaping was 0.93 mm and 0.47 mm in the horizontal and vertical direction respectively. The seed laser was reflected into the Nd:YAG slab by 45° reflector mirrors M1 and M2, and reflected out of the multi-pass cavity by 45° reflector mirror M3. M1, M2 and M3 were high-reflective coated at 1064nm and anti-reflective coated at 808 nm.
The length of the Nd:YAG slab was 15 mm, the width was 10 mm, and the height was 2 mm. The Nd:YAG crystal was 0.8 at.% doped and mounted between two copper heat sinks with two large faces (15 mm×10 mm). The heat sinks of the crystal were cooled by circulating water at the temperature of 25 ℃. The optical faces (15 mm×2 mm) were anti-reflective coated at 1064 nm and 808 nm. A concave cylindrical mirror CM1 with curvature radius of 600 mm and a convex cylindrical mirror CM2 with curvature radius of 300 mm were used as cavity mirrors. CM1 and CM2 were high-reflective coated at 1064 nm. And the multi-pass cavity length between CM1 and CM2 was 150 mm. In the horizontal direction, the cavity mirrors were curved. In the vertical direction, the cavity mirrors were plane. The Nd:YAG slab was in the middle of the multi-pass cavity and end-pumped by two laser diode stacks.
The QCW pump source was two vertical laser diode stacks on either side of the Nd:YAG slab. Each laser diode stack had ten collimated diode bars. The central wavelength of the laser diodes was fixed at 808 nm by adjusting the temperature of the cooling water. The pump emission from each stack was focused into a planar waveguide for homogenization in the slow axis. The pump beam exited from the waveguide and imaged into the Nd:YAG slab. Two polarizers (P1 and P2) and a half-wave plate (HWP) were inserted to protect the laser diodes against the radiation of the laser diodes from the other side. The pump beam from each stack was shaped to a 15 mm×0.6 mm homogenous line-shape beam inside the Nd:YAG slab. The pulse width of the pump source was set to ∼170 μs at the repetition of 500 Hz and the duty cycle was ∼8.5%. The maximum average pump power was ∼470 W with the absorption efficiency of ∼92.7%. The maximum peak pump power of the QCW pump source was as high as ∼5.5 kW.
After synchronizing the seed laser and the QCW pump source, a five-pass amplification was carried out. The experiment results are presented and discussed below.
3 Experimental results and analysis
In the experiment, the highest output power of 148.1 W was obtained at the maximum pump power of 472.3 W. The optical-optical efficiency was as high as 30.4%. Such a high optical-optical efficiency was thanks to the high peak pump intensity provided by the QCW pump source. Though the stimulated emission cross-section of the Nd:YAG material at 1064 nm is much lower than that of Nd:YVO4 [20], the high peak pump intensity in the experiment makes a considerable gain in the Nd:YAG crystal, which results in the high efficiency.
As known, the multi-pass cavity is unstable in the horizontal direction and the seed beam is expanded by the concave and convex mirrors, i.e., CM1 and CM2. In the vertical direction, the cavity mirrors are plane and the stability is sustained by the thermal lens of the Nd:YAG slab. At the maximum average pump power, the thermal focal length was measured to be ∼75 mm which was essentially in agreement with the theoretical calculation of 72.4 mm [9]. The temperature on the surface of the crystal at the maximum pump power was measured to be 78℃ which was basically consistent with the simulation result of 81.7℃. At the maximum pump power, the two-dimensional optical intensity distribution of the seed laser in the Nd:YAG slab was simulated and shown as Fig. 3. The overlapping efficiency (η) between the seed laser and the pump volume was about 66.6% in this circumstance.
Fig. 3. (a) Two-dimensional optical intensity distribution of the seed laser in Nd:YAG slab and (b) normalized optical intensity distribution along the length of the Nd:YAG slab. The parameters of the seed beam and slab size are consistent with the experimental setup.
According to the rate equation for laser amplification in Ref. 22, the output power is simulated and compared with the experiment results. The simulation has taken into account 92.7% pump absorption efficiency and 66.6% overlapping efficiency. The rate equations for simulation are shown as Eq. (1) [20].
The experiment and simulation results are shown as Fig. 4. When the pump power is lower than the maximum pump power, the thermal focal length is longer than 75 mm. Therefore, the overlapping efficiency is lower than 66.6% and the simulation results are higher than the experiment results. As pump power increasing, the overlapping efficiency is close to 66.6% and the simulation results matches the experiment results well. With higher overlapping efficiency, the output power and optical-optical efficiency can be further improved.
The pulse duration of the output laser was 141.4 μs at the repetition of 500 Hz. As depicted in Fig. 5, the pulse waveform was smooth and approximately rectangular which indicated that the power extraction was sufficient and there was no self-excitation in the amplifier. The beam quality factors of M2 were 1.41 and 1.37 in the horizontal and vertical direction respectively (see Fig. 6). The power stability in 15 minutes has been measured to be 0.43% (RMS). The degree of linear polarization was measured to be 15 dB after the amplification, and the linear polarization degree of the seed laser was 20 dB.
4 Conclusion
An average output power of 148.1 W single-frequency laser at 1064 nm was realized by a five-pass Nd:YAG Innoslab amplifier. The beam quality factors of M2 were 1.41 and 1.37. The pulse duration was 141.4 μs at the repetition of 500 Hz. Because of the high pump power density provided by the QCW pump source, the optical-optical efficiency was up to 30.4%. With higher overlapping efficiency between the seed laser and the pump volume, the output power and optical-optical efficiency can be further improved.
Funding
Foundation of President of China Academy of Engineering Physics (YZJJLX2019015); Key Laboratory of Science and Technology on High Energy Laser (C-2020-HEL04-1); National Natural Science Foundation of China (61705208).
Disclosures
The authors declare no conflicts of interest.
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