⇦Chinese Physics Letters, 2020, Vol. 37, No. 6, Article code 064203 A 117-W 1.66-Times Diffraction Limited Continuous-Wave Nd:YVO$_{4}$ Zigzag Slab Laser with Multilayer Amplified-Spontaneous-Emission Absorbing Coatings * Zhi-Feng Zhang (张志峰)1,2, Shuai Li (李帅)3, Yang Li (李阳)2, Yang Kou (寇洋)2, Ke Liu (刘可)2, Yan-Yong Lin (林延勇)2, Lei Yuan (袁磊)2, Yi-Ting Xu (徐一汀)2**, Qin-Jun Peng (彭钦军)2, Zu-Yan Xu (许祖彦)2 Affiliations 1University of Chinese Academy of Sciences, Beijing 100190, China 2Key Laboratory of Solid-State Laser, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China 3Beijing Aerocim Technology Co., Ltd, Beijing 100043, China Received 10 March 2020, online 26 May 2020 *Supported by the National Key R&D of China (Grant No. 2016YFB0402103), the National Natural Science Foundation of China (Grant No. 61875208), and the Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. GJJSTD20180004).
**Corresponding author. Email: ytxu@mail.ipc.ac.cn Citation Text: Zhang Z F, Li S, Li Y, Kou Y and Liu K et al 2020 Chin. Phys. Lett. 37 064203    Abstract We report a continuous-wave end-pumped Nd:YVO$_{4}$ zigzag slab laser with multilayer amplified spontaneous emission (ASE) absorbing coatings. The coatings are deposited on the slab faces. A five-layer structure consists of SiO$_{2}$-Ti-SiO$_{2}$-Ti-Au, and the thicknesses are 2520 nm, 10 nm, 160 nm, 24 nm and 200 nm, respectively. The designed coatings show good performance for the ASE control in the experimental tests. A stable-unstable hybrid laser oscillator along orthogonal directions in the slab aperture is further configured, achieving the 117 W output at a pump of 328 W. The beam quality factors $M^{2}$ in the unstable direction and stable direction are 1.57 and 1.66, respectively. DOI:10.1088/0256-307X/37/6/064203 PACS:42.55.Xi, 42.60.Da, 42.60.Pk © 2020 Chinese Physics Society Article Text Many applications, such as optical communication, navigation systems, biotechnology, and laser medicine, demand compact, high power, and high beam quality polarized lasers.[1–3] Laser diode (LD) end-pumped rod solid-state laser has been a focus of development in the past decade. However, the output power is limited by thermally induced lensing and fracture damage. To further achieve high power, high beam quality polarized output, the InnoSlab and other multi-pass structures were developed by different groups. Among widely used neodymium-doped crystals, the Nd:YVO$_{4}$ has a stimulated emission cross section five times larger than that of Nd:YAG, and its large birefringence is favorable to polarized laser oscillation. Nevertheless, vanadate's relatively poor thermal and mechanical characteristic limits the pump power. To reduce the thermal load of the crystal, the efficient "four-level laser" pump scheme with direct pumping to the upper lasing level was investigated.[2,4,5] However, it only improves laser performance slightly. Therefore, the laser configurations with better thermal dissipation capability are demanded. It is well-known that the conduction-cooled end-pumped zigzag slab configuration is featured by the uniform pump, the high cooling capability, and the zigzag optical path that averages the thermo-optic effect.[6–8] Unfortunately, ASE is a critical factor that limits the performance of the Nd:YVO$_{4}$ zigzag slab lasers.[6,9–11] The ASE issue is severe due to the large stimulated emission cross section of Nd:YVO$_{4}$ and the long optical path of zigzag propagation. The severe ASE in Nd:YVO$_{4}$ zigzag slab hinders the laser from getting higher power. Although the ASE could be utilized in some applications such as superluminescent diodes (SLDs) and superfluorescent source (SFS),[12,13] the ASE in zigzag slab lasers is undesirable. The management of ASE in zigzag slab lasers is of significant importance for high power, high beam quality output. Fortunately, the ASE could be managed efficiently by special designed absorbing coatings. Dobrowolski's group proved that using metal/dielectric films could make broadband absorption coatings.[14,15] In this Letter, as for the zigzag structure that Nd:YVO$_{4}$ slab lasers rarely adopt because of the severe ASE, we solve the problem with designed absorbing coatings. A conduction-cooled double-end-pumped Nd:YVO$_{4}$ zigzag slab laser with a hybrid resonator is proposed. The proposed scheme has both the features of the high overlapping efficiency and excellent cooling capability.[16,17] The 888 nm pumping light is used to reduce the heat load of the slab further. The ASE effect is successfully managed with specially designed absorbing coatings. The experimental scheme discussed would support the development of high-power Nd:YVO$_{4}$ zigzag slab lasers technically. The Nd:YVO$_{4}$ slab used in our experiments has dimensions of 45 mm $\times$ 12 mm $\times$ 1.2 mm, as shown in Fig. 1(a). The slab was a-cut in the length direction and c-cut along the width direction. The slab contains a doped region that diffusely bonded to un-doped end caps. The doped region of Nd:YVO$_{4}$ slab crystal was 0.5 at.% doped concentration, and the un-doped end caps were cut into a trapezoidal sample. The two cut edge facets formed an angle of 45$^{\circ}$ with respect to the bottom surface, and the facet coatings were anti-reflective (AR) at 1064 nm and high-reflective (HR) at 888 nm.
cpl-37-6-064203-fig1.png
Fig. 1. (a) Two-dimensional view of the slab. (b) The measured surface shape of the two large surfaces from the Zygo interferometer.
cpl-37-6-064203-fig2.png
Fig. 2. (a) The five-layer coating structure, (b) four-layer coating structure, and (c) three-layer coating structure.
The pump was absorbed along the length of the doped region and the large surfaces of the slab provided the zigzag propagation with total internal reflection (TIR) for the 1064 nm laser beam. The laser beam was incident at a near-normal angle to the end faces. The shapes of the two large surfaces were measured by the Zygo interferometer, and the results are shown in Fig. 1(b). The PV and RMS of the two surfaces were 3.76$\lambda$, 1.10$\lambda$, 3.33$\lambda$, and 1.00$\lambda$ (@632.8 nm), respectively. The detailed designs of coating structures are presented in Fig. 2. At the bottom, a 2520-nm-thick SiO$_{2}$ coating was deposited on the large surfaces. The SiO$_{2}$ coating has three distinct and independent functions. Firstly, it ensured the TIR zigzag reflections of the laser. Secondly, it achieved an AR coating function for the pumping light wavelength when the end-pumped light passed through the bottom surface, as shown in Fig. 3. Last but not least, it made the ASE light AR and incident to the absorbing coatings. The absorbing coatings were deposited on the 2520 nm SiO$_{2}$ film and composed by Ti and SiO$_{2}$ films of different thicknesses. The refractive index of the Ti films is a complex quantity when the thickness is smaller than the wavelength of incident radiation. Thus, the stray light will be absorbed by Ti films when it entered the absorbing coatings, and the SiO$_{2}$ film acted as an AR coating. The 200 nm Au film was made to match with the indium solder. Then two large surfaces of the crystal were soldered to heat sinks with indium solder carefully to maintain high stability and thermal conductivity of the slab module. The thickness of the film was optimized by the Essential Macleod Program.
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Fig. 3. Experimental setup to test different ASE absorbing coatings.
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Fig. 4. Dependence of the reflectivity for 1064 nm ordinary light and extraordinary light as a function of the incidence angle of stray light from slab crystal to coatings: (a) five layers, (b) four layers, (c) three layers.
Because $\theta_{\rm ce}$ and $\theta_{\rm co}$ are different, by adjusting the incident angle, we can achieve that only e light oscillates in the resonator. Therefore, the incident angle $\theta$ of the laser beam inside the slab was designed between $\theta_{\rm ce} =42.1^{\circ}$ and $\theta_{\rm co} =47.9^{\circ}$ with respect to the normal vector of the reflection surface. Take Fig. 2(a) as an example, for the e light, the reflectivity did not exceed 10% when the incident angle ranged from 0$^{\circ}$ to 30$^{\circ}$. When the incident angle ranged from 30$^{\circ}$ to $\theta_{\rm ce} =42.1^{\circ}$, the reflectivity exceeded 50% only at an incident angle around 41.4$^{\circ}$. The other two cases, however, have higher reflection (both o light and e light) even with very a small incident angle, as shown in Figs. 4(b) and 4(c). The coating structure in Fig. 2(a) may have the best performance concluded from Fig. 4. To test the design of the ASE absorbing coatings shown in Fig. 2, a stable plane concave resonator with $R_1=700$ mm and $T=40{\%}$ was configured as schematically shown in Fig. 3. Three Nd:YVO$_{4}$ slab modules were developed with ASE absorbing coatings corresponding to Fig. 2.
cpl-37-6-064203-fig5.png
Fig. 5. (a) Pump light intensity distribution at slab entrance; (b) and (c) the calculated temperature distribution of the heat sink.
The pumping unit consisted of two stacks, and each stack had two laser diode bars. The central output wavelength of the pumping unit was 888 nm, and the spectral width of the laser diode stack was about 3 nm. The LD was 11.5 mm width in the slow axis direction with a divergence angle of 10$^{\circ}$. In the fast axis direction, the radiation from each laser diode bar was collimated with micro-lens and the divergence angle can be significantly decreased to 0.1$^{\circ}$. The collimated beam was focused into the rectangular waveguide by a cylindrical lens with a focal length of 300 mm in the fast axis direction. The planar waveguides were 300 mm long and 11.5 mm $\times 11.5$ mm in the fast and slow axial directions. The divergence angle was maintained within 10$^{\circ}$ at the output end of the planar waveguide in the slow axial direction. With this pumping unit, we obtained a homogeneous pumping line with dimensions of $\sim $1.1 mm $\times 11.5$ mm at each end of Nd:YVO$_{4}$ slab, as shown in Fig. 5(a). The Nd:YVO$_{4}$ slab module was conduction-cooled and the slab was soldered between two gold plating heat sinks with indium. The thickness of the indium was 0.4 mm, and the structure was optimized by the finite element method (FEM), as shown in Figs. 5(b) and 5(c). The thermal load was set to be 95 W in simulations. It could be seen that the temperature was uniformly distributed, as shown in Fig. 5(b), and the temperature difference was within 0.2 $^{\circ}\!$C, as shown in Figs. 5(b) and 5(c). The temperature difference was within 0.4 $^{\circ}\!$C in experiments monitored by the infrared radiation thermal sensor. The output power versus the absorbed pump power of different coating structures in short-cavity experiments is illustrated in Fig. 6(a). The curves marked with a, b, and c correspond to the designed absorbing coatings in Fig. 2, respectively. In situation c (the blue curve), it has the lowest efficiency and the output power decreased when the pump beyond 200 W, as shown in Fig. 6(a). In situation b (the red curve), the absorbing coatings were composed of Ti and Ni films of different thicknesses, and the output power was simply 65 W at the pump level of 238 W, corresponding to the optical-to-optical (O-O) efficiency of 27.3%. Further increasing pump led to the temperature rise in the slab. In situation a (the black curve), the O-O efficiency was 47.5%, and the slope efficiency was as high as 68%. In addition, the output curve shows no evidence of saturation. The test results show that the coating structure designed in Fig. 2(a) was efficient in managing the ASE in Nd:YVO$_{4}$ slabs. Thus the coating structure in (a) was adopted in the last. The experiment results are in accordance with the coating designs shown in Fig. 4. The wavefront of the Nd:YVO$_{4}$ slab without the pump and with 322 W pump (corresponding to 153 W output) are shown in Figs. 6(b) and 6(c). The slight variations in RMS and PV indicate that the slab has good thermal management.
cpl-37-6-064203-fig6.png
Fig. 6. (a) Output power versus the absorbed pump power of different coating structures in short-cavity experiments. (b) Wavefront without the pump. (c) Wavefront with 153 W output.
cpl-37-6-064203-fig7.png
Fig. 7. Schematic diagram of the unstable-stable hybrid resonator.
A hybrid resonator, stable direction in thickness and unstable direction in width, was configured experimentally to obtain high power, high beam quality laser. A schematic diagram of the positive branch unstable-stable hybrid resonator is shown in Fig. 7. Two highly reflective cylindrical mirrors, M1 ($R_1=-1000$ mm) and M2 ($R_2=650$ mm), were used as the resonator mirrors. The cavity length was about 175 mm, and the distances between $R_1/R_2$ to the end face were 20 mm/125 mm. A positive-branch confocal off-axis unstable resonator with magnification $M=1.5$ was formed in the $y$–$z$ plane, corresponding to an output coupling $T=35{\%}$. A flat-flat cavity was formed in the $x$–$z$ plane. Figure 8(a) shows the output power as a function of the pump power of the Nd:YVO$_{4}$ slab laser hybrid resonator. Because of the diffraction loss of fundamental mode and the machining error of resonator mirrors, a maximum output power of 117 W continuous-wave 1064 nm laser was obtained at a pump power of 328 W and showed no saturation. The corresponding O-O efficiency was 35% and the slope efficiency was 51%. The output beam has a size of about 1.1 mm $\times 4.0$ mm. The beam quality at the full pump power of 328 W was measured by a beam quality analyzer (M2-200S, Spiricon Inc.), as shown in Fig. 8(b). The laser beam quality factors were measured to be 1.57 in the stable direction and 1.66 in the unstable direction. The upper inset in Fig. 8(b) shows the two-dimensional spatial intensity profile of the laser beam. The central wavelength and spectral width of output beam had almost no change after the ASE management.
cpl-37-6-064203-fig8.png
Fig. 8. (a) Output power versus the pump power. (b) The measured beam quality of the laser at the full pump. Upper inset: 2D spatial intensity profile of the laser beam.
Table 1. Typical Nd-doped homology crystal slab laser experiment results.
Crystal Wavelength Output power $M^{2}$ Scheme References
Nd:YVO$_{4}$ 1064 nm 170 W 1.9 1.3 End-pumped [18]
Nd:YVO$_{4}$ 1064 nm 300 W 2.1 3.4 Innoslab [3]
Nd:YVO$_{4}$ 1342 nm 22 W 1.2 1.3 End-pumped [19]
Nd:GdVO$_{4}$ 1064 nm 120 W 2.3 2.3 Innoslab [20]
In conclusion, to achieve the ASE management of the slab, we performed short-cavity experiments using different absorbing coatings. Three kinds of coatings completed 43, 65, and 153 W maximum output power, representing three management levels of ASE, respectively. Through changing the design of the coating structure, the thickness of the film, and the sequence of films, we can achieve the flexible change of coating absorptivity to manage the ASE in different situations. With a hybrid resonator and absorbing coatings, a 117 W continuous-wave 1064 nm laser was obtained with an absorbed pump power of 328 W at 888 nm and showed no saturation. The optical-to-optical efficiency and slope efficiency were 35% and 51%, respectively. The beam quality factors were measured to be 1.57 in the stable direction and 1.66 in the unstable direction. References High-efficiency 60 W TEM_00 Nd:YVO_4 oscillator pumped at 888 nmHigh efficiency 165 W near-diffraction-limited Nd:YVO_4 slab oscillator pumped at 880 nm340 W Nd:YVO 4 Innoslab laser oscillator with direct pumping into the lasing levelSpatial dynamic thermal iteration model for 888 nm end-pumped Nd:YVO 4 solid-state laser oscillators and amplifiersThermal lensing in Nd:YVO4 laser with in-band pumping at 914 nmZigzag slabs for solid-state laser amplifiers: batch fabrication and parasitic oscillation suppression82 kW high beam quality quasi-continuous-wave face-pumped Nd:YAG slab amplifierCoherent combination of high-power, zigzag slab lasersAmplified Spontaneous Emission and External Signal Amplification in an Inverted MediumFabrication of broadband absorbing coatings for amplified spontaneous emission suppressionSuppression of parasitic modes in a YAG:Nd slab laser using selective coatingA stripe-geometry double-heterostructure amplified-spontaneous-emission (superluminescent) diodeHigh-stability Er/sup 3+/-doped superfluorescent fiber sourcesMetal/dielectric transmission interference filters with low reflectance 1 DesignMetal/dielectric transmission interference filters with low reflectance 2 Experimental resultsDouble-end-pumped Nd:YVO 4 slab laser at 1064 nmEfficient high-power diode-end-pumped TEM_00 Nd:YVO_4 laser with a planar cavityAn 880-nm Laser-Diode End-Pumped Nd:YVO 4 Slab Laser with a Hybrid ResonatorLaser Diode Pumped 1342 nm Nd:YVO 4 Slab Laser with a Compact Hybrid ResonatorHigh-Power Nd:GdVO 4 Innoslab Continuous-Wave Laser under Direct 880 nm Pumping
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