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Abstract
A single-frequency quasi-continuous-wave partially end-pumped slab (Innoslab) laser amplifier at 1319 nm was demonstrated. The 3-W single-frequency all-fiber seed laser was amplified to a maximum average power of 80.1 W and the power stability was 0.52% in 10 minutes. The corresponding optical-optical efficiency was 16.1% under absorbed pump power of 478 W. The output pulse width was 131 µs at the repetition of 500 Hz. The beam quality factors of M2 were 1.3 in both the vertical and horizontal directions. To the best of our knowledge, this is the first report on single-frequency Nd:YAG Innoslab amplifier at 1319 nm with such high output power and efficiency.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1 Introduction
1319-nm laser plays a crucial role in extensive applications such as sodium guide star, image display, laser medical treatment, oceanic lidar, etc [1–6]. 1319-nm laser is commonly produced by stimulated emission of 4F3/2→4I13/2 in Nd:YAG and shares the same upper level with 1064-nm and 946-nm laser [7]. However, the stimulated emission cross-section at 1319 nm in Nd:YAG is about one third of that at 1064 nm. These cause low gain, high quantum defect, serious thermal effects in 1319-nm laser oscillators or amplifiers and may lead to baneful ASE (amplified spontaneous emission) or even parasitic oscillation at 1064 nm [8–14]. Therefore, it is quite difficult to get high-power 1319-nm laser output with high optical conversion efficiency and excellent beam quality.
In astronomical adaptive optics, since the atom column density of sodium layer in ∼90-km mesosphere changes dramatically throughout the year, QCW (quasi-continuous wave) µs-pulse sodium-guide-star laser with average power of more than one hundred watts is required to get enough photons return and avoid temporal overlap with backwards Rayleigh scattering light [1–3]. Thus, researchers have paid much attention to high-power QCW laser at 1319 nm with both good beam quality and narrow line-width. Until now, various constructions have been adopted and achieved significant progresses, such as rod, zigzag slab and Innoslab [1–4,11–14]. In 2019, Yanhua Lu et al. reported a master oscillator power amplifier (MOPA) system at 1319 nm [2]. It composed of a seed laser with line-width of 0.3 GHz and six Nd:YAG rod modules for power amplification. The average output power was 115 W with optical-optical efficiency of 9.8% and the pulse width was 120 µs at the repetition of 250 Hz. In 2017, Chuan Guo et al. employed a Nd:YAG zigzag slab for 1319-nm laser amplification [13]. A QCW µs-pulse 1319-nm oscillator with average power of 20.5 W was used as the seed source. The amplifier operated at the repetition of 500 Hz. A maximum output power of 51.5 W was obtained under the absorbed pump power of 217.8 W. The pulse width was 105 µs and the optical-optical efficiency was 14.2%. The beam quality factor of M2 was measured to be 1.61 and 1.81 in the vertical and horizontal direction, respectively.
Partially end-pumped slab (Innoslab) lasers which are characterized by the stable-unstable hybrid resonator can effectively alleviate thermal effects, and thus obtain high-power output with excellent beam quality [15–22]. In 2013, a high-power high beam-quality Nd:YAG Innoslab amplifier at 1319 nm was reported [14]. The seed source provided narrow line-width 1319-nm laser with average power of 10 W and the five-pass amplification yielded a 42.3-W output with the optical-optical efficiency of 6.5%. The beam quality factors were 1.13 and 2.16 in the orthogonal directions. The pulse width was 75 µs at the repetition of 1 kHz. We believe the output power and optical-optical efficiency can be boost by improving the overlapping efficiency between the pump volume and the seed beam [4].
In this paper, a seven-pass 1319-nm Innoslab amplifier was demonstrated. The 1319-nm single-frequency seed laser passed through the Nd:YAG slab seven times. The overlapping efficiency between the pump volume and the multi-pass laser beam was optimized. A maximum average output power of 80.1 W was reached with the magnification of ∼26.7. Under the absorbed pump power of 478 W, corresponding optical-optical efficiency of 16.1% was realized. The output pulse width was 131 µs at the repetition of 500 Hz. The beam quality factors of M2 were 1.3 in both the vertical and horizontal directions. The experimental data and numerical results were basically matched.
2 Experimental setup
The experimental setup of the QCW 1319-nm Innoslab amplifier is shown schematically in Fig. 1. It comprises of the QCW single-frequency all-fiber seed laser at 1319 nm, the QCW LD dual-end-pumping system at 808 nm, the multi-pass hybrid cavity and the Nd:YAG slab crystal.
The all-fiber seed laser contains a single-frequency, 15-mW CW oscillator and a QCW pulsed Raman fiber amplifier. In the experiment, the central wavelength of the linearly polarized seed laser was set to 1319.3672 nm with the line-width of ∼0.6 MHz. The pulse width of the seed laser was ∼150 µs (FWHM) at the repetition of 500 Hz, as shown in Fig. 2(a). For QCW laser amplification with pulse width over 100µs, the pulse waveform was nearly square.
The maximum average power of the seed laser was ∼3 W, and the corresponding peak power was ∼40 W. The beam quality factors of M2 were both 1.1 in the orthogonal directions. An optical isolator was inserted to prevent the optical feedback from the Innoslab amplifier. The seed laser was shaped into an elliptical beam by a beam shaping system for mode matching with the pump volume. The beam was shaped to 0.84 mm and 0.48 mm in the horizontal (x) and vertical (y) direction respectively (see Fig. 2(b)). Thus, the input peak intensity was ∼1.8 times of the saturation intensity of 1319-nm laser in Nd:YAG which ensured high extraction efficiency in the multi-pass amplification [7]. The seed laser was reflected into the Nd:YAG slab by 45° reflector mirrors M1 and M2, and reflected out of the cavity by 45° reflector mirror M3. M1, M2 and M3 were high-reflective coated at 1319 nm and anti-reflective coated at 808 nm.
The width (W) of the Nd:YAG slab was 15 mm, the length (L) was 10 mm, and the height (H) was 1.5 mm. The Nd:YAG crystal was 1.1 at.% doped and its two large faces (15 mm × 10 mm) were welded on two copper heat sinks for high-efficiency heat exchanging. The heat sinks were cooled by circulating water at the temperature of 25°C. The optical faces (15 mm × 1.5 mm) were anti-reflective coated at 1064 nm, 1319 nm and 808 nm. The Nd:YAG slab was in the middle of the multi-pass hybrid cavity and dual end-pumped by two commercial QCW LD stacks at 808 nm. Each LD stack was vertically encapsulated by ten diode bars which were collimated by microlens in the fast-axis (i.e. y-axis) direction. The central wavelengths of the two LD stacks were fixed at ∼808 nm by adjusting the temperature of the cooling water. The pump radiation from each stack was focused into the Nd:YAG slab by the coupling system. The planar waveguide in the coupling system was used for homogenization. The pump beam was shaped to a 14 mm (x) × 0.65 mm (y) line-shape beam inside the Nd:YAG slab, shown as Fig. 3(a). The intensity distribution was nearly homogeneous in x diretion and Gaussian in y direction. Two polarizers (P1 and P2) and a half-wave plate (HWP) were inserted to protect the LDs against the remnant radiation from the other side. The two QCW LD stacks provided a maximum average pump power of 590 W (corresponding peak power of ∼7.4 kW). The pulse width was ∼160 µs at the repetition of 500 Hz. The transparency of the coupling system and the absorption efficiency of the Nd:YAG crystal were both 90%. The maximum absorbed average pump power by the Nd:YAG slab was 478 W. The temperature of crystal rose to 75.4°C (see Fig. 3(b)) under the maximum pump power.
Fig. 3. (a) Line-shape pump beam and (b) the temperature of Nd:YAG slab under the maximum pump power.
Typically, a stable-unstable hybrid cavity is adopted in Innoslab laser amplifiers [15–18]. Due to the one dimensional heat flow and the homogenized line-shaped pumping, there is barely thermal lens in the width direction of the Nd:YAG slab. In the height direction of the Nd:YAG slab, the thermal focal length can be quite short under high pump power. A concave cylindrical mirror CM1 (with curvature radius of 600 mm) and a convex cylindrical mirror CM2 (with curvature radius of 400 mm) were employed as the cavity mirrors in the experiment. CM1 and CM2 were high-reflective coated at 1319 nm and anti-reflective coated at 1064 nm. The cavity mirrors were curved in x direction and plane in y direction. The seed beam was expanded in each round-trip with magnification of |600 mm/400 mm|=1.5 in x direction. In y direction, the cavity was stable sustained by the plane mirrors and the thermal lens of the crystal. In the experiment, the thermal focal length of the crystal was measured to be fth = 60 mm under absorbed average pump power of 478 W. The distance from CM1 to CM2 along the optical path (i.e. cavity length) was ∼100 mm.
In order to achieve efficient amplification, the pulse front of the seed laser has a delay relative to that of the pump source. However, a long-time delay can cause self-excitation and a short-time delay may result in a sharp instead of square wave pulse waveform. The delay time was finally set to 60 µs. The seed laser and the QCW pump source were temporally synchronized, and a seven-pass amplification was carried out. The experimental results are presented and discussed below.
3 Results and discussion
The two-dimensional intensity distribution of the seed laser at Nd:YAG slab, CM1 and CM2 were simulated and shown as Fig. 4(a).
Fig. 4. (a) Two-dimensional intensity distribution of the multi-pass laser and (b) overlapping efficiency as the function of thermal focal length.
The hybrid cavity was unstable in x direction, and the seven-pass amplification was carried out for making the best use of the width of the Nd:YAG gain medium. The overlapping efficiency (η) between the seed laser and the pump volume was about 74% under the maximum pump power (corresponding thermal focal length of fth = 60 mm).
According to the rate equation for laser amplification in Refs. [7] and [21], the average output power was simulated and compared with the experimental results. The experiment and simulation results are shown as Fig. 5. In the experiment, the highest average output power of 80.1 W was obtained under absorbed average pump power of 478 W. The magnification was ∼26.7 and the corresponding optical-optical efficiency was about 16.1%. Such a high optical-optical efficiency was thanks to the high peak pump intensity provided by the QCW pump source, the high input peak intensity of the seed laser and the high overlapping efficiency. Though the stimulated emission cross-section at 1319 nm in Nd:YAG is about one third of that at 1064 nm, the high peak pump intensity has led to a considerable gain at 1319 nm [7]. The input peak intensity which was ∼1.8 times of the saturation intensity contributed to the high extraction efficiency in the multi-pass Innoslab amplifier. The overlapping efficiency was lower than 74% while the absorbed average pump power was less than 478 W, depicted in Fig. 4(b). As the pump power increasing, the thermal focal length decreased and the overlapping efficiency rose to 74%. Therefore, the output power was lower than the simulation results when the pump power was low. As pump power increasing, the experimental results matched the simulation results gradually. The experimental results were lower than the simulation results because the ASE effect at the main emission line of 1064 nm was not taken account in the simulation. And the slop efficiency slightly reduced under the absorbed pump power of 478 W because of the serious ASE effect at 1064 nm. The parasitic oscillation at 1064 nm appeared as the pump power further increasing, even though the two optical end-faces were anti-reflective coated at 1064 nm. As the thermal focal length reducing to 50 mm, the overlapping efficiency can be improved to ∼85% and higher output power and efficiency can be realized theoretically. That requires further promotion of the pump power to reduce the thermal focal length. Meanwhile, ASE and parasitic oscillation at 1064 nm should be taken into account cautiously.
The beam quality factors of M2 were both 1.3 in the orthogonal directions (see Fig. 6). The output laser of Innoslab amplifier can avoid exiting from the cavity mirror and the crystal edges. There is no diffraction under the circumstances and it has no sidelobes at far field. As depicted in Fig. 7, the pulse width of the output laser was 131 µs (FWHM) at the repetition of 500 Hz. The pulse width of the seed laser and the pump source was 150 µs and 160 µs, respectively. The pulse front of the seed laser has a delay relative to that of the pump source and the delay time is 60 µs. That causes the pulse width to narrow.
The peak power was above 1.2 kW. The power stability in 10 minutes was measured to be 0.52% (RMS). And the degree of linear polarization was ∼15 dB after the amplification. The emission spectrum of Nd:YAG crystal at 1319 nm is sub-nanometer (>50 GHz) which is much larger than the FWHM of the seed laser spectrum (∼0.6 MHz). So, the FWHM of the amplified laser spectrum was 607.9 kHz which was barely narrowed.
4 Conclusion
An average output power of 80.1 W, QCW single-frequency laser at 1319 nm was realized by a seven-pass Innoslab amplifier. The beam quality factors of M2 were 1.3 in the orthogonal directions. The output pulse width was 131 µs at the repetition of 500 Hz. Because of the high overlapping efficiency, high input peak intensity of the seed laser and high pump intensity provided by the QCW pump source, the optical-optical efficiency was up to 16.1%. With higher overlapping efficiency between the seed laser and the pump volume, the output power and optical-optical efficiency can be further improved.
Funding
China Academy of Engineering Physics (2022-ZLJJ-03, YZJJLX2019015).
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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