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Abstract
High beam quality 588 nm radiation was realized based on a frequency-doubled crystalline Raman laser. The bonding crystal of YVO4/Nd:YVO4/YVO4 was used as the laser gain medium, which can accelerate the thermal diffusion. The intracavity Raman conversion and the second harmonic generation were realized by a YVO4 crystal and an LBO crystal, respectively. Under an incident pump power of 49.2 W and a pulse repetition frequency of 50 kHz, the 588 nm power of 2.85 W was obtained with a pulse duration of 3 ns, corresponding to a diode-to-yellow laser conversion efficiency of 5.75% and a slope efficiency of 7.6%. Meanwhile, a single pulse's pulse energy and peak power were 57 µJ and 19 kW, respectively. The severe thermal effects of the self-Raman structure were overcome in the V-shaped cavity, which has excellent mode matching, and combined with the self-cleaning effect of `Raman scattering, the beam quality factor M2 was effectively improved, which was measured optimally to be Mx2 = 1.207, and My2 = 1.200, with the incident pump power being 49.2 W.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 550-600 nm band yellow laser sources have numbers of significant applications in biomedicine [1–3], Bose-Einstein condensation [4], sodium guide stars [5], atmospheric lidar detection [6], and other fields. Especially, a 550-600 nm yellow laser source with pulse energy of nanojoules level, pulse duration of a nanosecond, and beam quality of near-diffraction-limited are necessary for applications of leading-edge biophotonics, e.g., optical-resolution photoacoustic microscopy [7] and stimulated emission depletion microscopy [8]. The classical method for generating yellow laser in the early days was dye laser [9,10]. Although this laser can directly generate yellow laser, its gain medium is usually liquid dye, which is complicated to maintain and handle and has poor beam stability [11]. All-solid-state lasers have gradually become the main method for generating yellow lasers in recent decades due to their stable performance and compact structure. In recent years, it has been reported that some crystals doped with rare-earth ions can generate yellow laser directly [12–14]. Besides, the laser source in the yellow band can also be provided by the optical parametric oscillator (OPO) combined with the frequency doubling technology, but due to its unique dual-wavelength, it has higher requirements for the coating of the mirrors [15,16]. These two all-solid-state yellow lasers are generally continuous-wave (CW) operation and their diode-to-yellow conversion efficiency and output power are relatively low. The mainstream high-power all-solid-state yellow lasers are generated by the sum-frequency generation of lasers in the 1.06 µm and 1.3 µm bands [17,18]. Most of these methods require two separate laser systems to generate two different wavelengths of laser light and then combine to output yellow laser light. So, the system is enormous, the structure is complex, the cost is high, and the beam quality is relatively poor. Stimulated Raman scattering (SRS) combined with the frequency doubling effect proved to be an effective method to incubate yellow laser light, which have become a research hotspot in recent years due to their simple structure, low cost, and good stability [19,20]. Though the externally driven acousto-optic Q-switched (AOS) method costs slightly higher than the passive Q-switched (PQS), its output power is higher.
According to former studies, radiation yellow lasers based on SRS and second-order nonlinear effects usually possess a better beam quality (M2 < 2) under a low-power operation. For example, in 2009, T. Omatsu et al. achieved a yellow output at 588 nm with a beam quality factor of M2 = 1.7 and an output power of 264 mW using Nd:YVO4 crystals [19]. Using the same laser gain medium material, X. Li et al. bred 587.5 nm yellow radiation with an output power of 135 mW, M2 = 1.22 in 2011 [21]. The beam quality gradually deteriorates with increasing output power, and the beam quality factor M2 was hardly less than 2 at the watt level output power. In 2010, A. G. Lee et al. obtained a 590 nm output with a beam quality M2 factor of 2.5 at an output power of 2.9 W using a laser crystal Nd:GdVO4 and a Raman crystal SrWO4. [22] The output power and beam quality of self-Raman yellow laser are affected by high thermal loading (combined effect of waste pumping, quantum defects, and Raman heating), for example, in 2011, Y. Guo et al. pumped self-Raman crystal Nd:YVO4 to obtain a 587 nm yellow laser with Mx2 = 12.11 and My2 = 12.66 at 8.05 W [23]. In 2020, the Nd:YVO4 was coated to collect the reverse-propagating yellow laser, reduce the lens insertion loss and further compress the cavity length, thus Y. F. Chen et al. obtained a 588 nm yellow laser with a beam quality factor of M2 = 2-3 when the output power was 8.8 W, and the corresponding pulse width and single pulse energy were about 8 ns and 44 µJ [24]. Since the insertion of Raman crystals increases the length of the straight resonant cavity resulting in poor stability, self-Raman technology is usually chosen to compress the length of the resonator. Yet, the self-Raman technology would also bring serious thermal effects and lead to poor beam quality.
In this paper, a V-shaped cavity active Q-switched Raman frequency-doubled yellow laser (VAR) was designed, which can operate stably relying on the resonator is long enough to insert the Raman crystal. Double-end-bonded Nd:YVO4 crystal was used to further alleviate the thermal effect, combined with the self-cleaning effect of pure Raman crystal YVO4 to improve the beam quality [25]. When the incident pump power was 49.2 W, the 588 nm yellow laser emission with the optimal beam quality factor Mx2 = 1.207, My2 = 1.200 was obtained. The maximum output yellow laser power was 2.85 W corresponding to the pump source power of 49.2 W, and the pulse width and single pulse energy were 3 ns and 57 µJ, respectively.
2. Experimental setup
We designed and simulated a V-shaped cavity in Rezonator [26] with a folding angle of 18°. The schematic structure is illustrated in Fig. 1. A fiber coupled diode laser with the center wavelength of 878.4 nm (core diameter:200 µm, N.A. = 0.22, maximum output power = 65 W) was used as the pump source. A pair of plano-convex coupling lenses (1:2 magnification) re-imaged pump laser as a spot of diameter 400 µm on the laser gain medium. A 30 mm long double-end diffusion-bonded a-cut composite crystal consisted of a 20 mm long Nd:YVO4 (0.5% Nd-doped) and two 10 mm long pure YVO4 bonded at both facts, and Its absorption efficiency for the pump source was about 92% under non-lasing situations. The composite crystal was selected as a laser gain medium to effectively reduce thermal lensing effects. The Q-switched device was a 46 mm acoustic-optic modulator (Gooch&Housego QS27-4S-B) that was driven by a 50 kHz ultrasonic frequency radio-frequency generator operated at 100 W radio-frequency power. The AOS could convert CW to pulse-wave. A 4*4*30 mm pure a-cut YVO4 crystal was used as Raman gain medium, and it could frequency shift the fundamental laser from 1064 nm to 1176 nm. During the whole experiment, Nd:YVO4 and YVO4 were wrapped with indium foil and clamped in the copper block to be cooled by a closed-circuit water-cooled device at 17 °C. The 1176 nm laser was frequency-doubled to 588 nm yellow laser by using a class I phase-difference matching LBO (θ=90°, Ф=4°) crystal, which was cooled at 20°C in the same way as above. The fundamental frequency resonant cavity was composed of input mirror M1, folding mirror M2, and output mirror M5. So its total length was 393 mm (from M1 to M2) + 164 mm (from M2 to M5) = 557 mm. Raman resonator cavity was composed of M3 and M5. M3 collected back-propagating Stokes light to improve conversion efficiency. The role of M4 was to stop the back-propagation of the yellow laser. The lens and crystal coating parameters used in this experiment were listed in Table 1 and Table 2, respectively.
Fig. 1. Experimental schematic of the V-shaped cavity actively Q-switched intracavity frequency-doubled composite Nd:YVO4 Raman laser.
Table 1. Lens (coating) parameters
Table 2. Crystal (coating) parameters
We simulated the distribution of the beam mode in the fundamental resonator as shown in Fig. 2. The beam incident obliquely on the folded mirror M2 can cause the beam convergence points of the tangential plane (T) and the sagittal plane (S) to be misaligned, resulting in astigmatism. Here the T curve had a high degree of coincidence with the S curve, indicating that excellent mode matching had been achieved. According to experience, the pump beam cannot be directly incident on the surface of Nd:YVO4, which will cause the coating layer to be burned. Therefore, the design made the pump light-converging point about 10 mm inside the crystal, and the beam radius here was about 180 µm, which corresponded to the size of the pump light 200 µm on it. The folded mirror M2 reflected the light to provide a smaller beam radius, about 133 µm to 180 µm, for the Raman crystal YVO4. Stokes light is evolved from noise and amplified by the SRS process. The initially generated Stokes light will propagate in a low-order mode. Stokes light is the product of regeneration by consuming the fundamental light, so even if the fundamental light beam quality is poor or is Multimode is also not inherited to Stokes light. This process is called the self-cleaning effect of SRS [25], which can improve beam quality.
For comparison, in the case where the laser crystal and the frequency-doubling crystal were the same as the VAR, we followed the structure in [23] to design and simulate the straight cavity active Q-switched self-Raman yellow laser (SASR), which has a geometric cavity length of 118 mm. According to the simulation results, the beam radius of the fundamental wave on the Nd:YVO4 surface reaches about 270 µm, and the conversion efficiency is low. Figure 3 shows the simulation results of the thermal lens effect on Nd:YVO4 of the two. The thermal lens focal length limit (∼100 µm) of the VAR is lower than that of the SASR (∼220 µm). It is obvious, VAR had better thermal management and therefore expected to further improve the beam quality.
Fig. 3. Fundamental mode radius at the centre of Nd:YVO4 as a function of thermal focal lengths for 1064 nm operation in the SASR (a) and the VAR (b).
3. Results and discussions
The yellow light power leaked by the output mirror M5 was measured by a power meter (Physcience Opto-Electronics, LP-3C), and it was observed that the output yellow laser became extremely unstable with the increase of the pump power. We initially analyzed that the output yellow laser may contain stray light in other bands, the pump light (878.4 nm), fundamental frequency light (1064 nm, 1341 nm), and Raman light (1176 nm) were excluded due to the limitation of lens coating. In this way, the stray light could only come from high-valent Raman excitation. It was found that about 60% of the power was contributed by the second-order Stokes light (1314 nm) by monitoring with a spectrometer. We analyzed that this may be caused by the following two aspects: i) The LBO crystal generated heat during the frequency doubling process, so its actual temperature was higher than the temperature set by the water-cooled device. ii) The angle-matched LBO crystal required fine adjustment, and the adjustment error would cause most of the first-order Stokes to not be transformed. By improving these two aspects, the proportion of the excitation power at 1314 nm was reduced to less than 10%, a mirror M6 was set to filter out the 1314 nm laser, which was coated to be highly transmissive at 1314 nm (T > 96.8%) as well as highly reflective at 588 nm (R > 99.9%), about 100% of the yellow laser was reflected. If the measured laser power transmitted by M6 was X and the laser power reflected by M6 was Y, the output power of the pure yellow laser was Y - (X÷96.8% - X).
As shown in Fig. 4(a) the output power of the yellow laser was approximately proportional to the incident pump power. The oscillator reached the threshold at 22.8 W. When the incident pump power was 49.2 W, the output yellow laser power reached a maximum of 2.85 W, and the corresponding optical conversion efficiency was 5.75%. Limited by the low transmittance of the output mirror to 588 nm (T = 76%), the higher yellow output power obtained here was mainly due to higher pump power rather than higher diode-to-yellow conversion efficiency, which could be further increased by improving the coating technology. Under the maximum output power, the power stability was measured to be about 3% within one hour. This stable laser output was mainly attributed to two reasons. On the one hand, the reasonable resonator design and effective thermal management by using the double-end diffusion-bonded structure. On the other hand, the 878.4 nm (17%) pump source had lower quantum defects than the 808 nm (24%) pump source, and thus it had a smaller thermal lens. As the incident pump power continued to increase, when it exceeded 49.2W, the output yellow laser power began to decrease, and the oscillator appeared inversion in Fig. 4(a). We noticed that the thermal management of the diffusion-bonded structure had a specific limit, it was obvious that it had exceeded here. In addition, the actual temperature of the LBO crystal was difficult to maintain at 20°, which was mainly caused by the residual Stokes light heating the crystal oven. If the Raman cavity length was further reduced by compressing the size of micro-channel water-cooled copper holders, the 588 nm radiation power and conversion efficiency were expected to be improved. An optical spectrum analyzer (Yokogawa, AQ6374) with a resolution of 0.05 nm monitored the spectrum of the laser. Referring to Fig. 4(b) under the condition of maximum output, the central wavelength of the yellow laser measured was 587.9 nm, and the spectral linewidth was 0.05 nm (43.4 GHz) full width at half maximum (FWHM), which was slightly smaller than the linewidth of the fundamental frequency light. It is remarkable that the actual linewidths of these light lines are likely to be narrower than the measured values because of the resolution limitation of the current optical spectrum analyzer.
Fig. 4. (a) Average output power of the yellow laser with respect to the incident pump power. (b) The yellow laser spectra under the full incident pump power of 49.2 W.
At the maximum incident pump power of 49.2 W, the pulse duration was 3 ns. Meanwhile, we also recorded the pulse time behavior of the yellow laser leaked from M5, as visualized in Fig. 6(b) It can be seen that the pulse duration of the yellow laser was very narrow compared with the fundamental frequency laser (∼ 40 ns), which was caused by the pulse duration shortening effect of the SRS process [25]. Narrow pulse width yellow lasers allowed higher peak power and pulse energy at lower average output power, which had broad application prospects in medicine and photonic biology. According to the known conditions, the pulse energy and peak power of the yellow laser can be calculated. Both of them showed an upward trend with the increase of incident pump power, which were shown in Fig. 5(a) and Fig. 5(b), respectively. Under the maximum incident pump power, the pulse energy and peak power of the yellow laser were 57 µJ and 19 kW respectively.
Fig. 5. (a) Pulse energy of the yellow laser with respect to the incident pump power. (b) Pulse duration and peak power of the yellow laser with respect to the incident pump power.
Fig. 6. Pulse train and temporal pulse profile of the yellow laser under the PRF of 50 kHz and the incident pump power of 49.2 W.
Additionally, the pulse behavior in the time dimension was monitored in real-time by an Agilent digital oscilloscope (DSO90604A, 6 GHz bandwidth) connected to a fast photodiode (DET08CL/M, 5 GHz bandwidth). The measured pulse train and single pulse profile of the yellow laser were plotted in Fig. 6. Figure 6(a) indicated that the pulse train was relatively stable, and the measured value of pulse intensity fluctuation was better than ± 10%. It was preliminarily inferred that this was due to the thermal instability and mechanical instability of the laser system. Figure 6(b) demonstrated that the single pulse was comparatively smooth.
The spatial dimension information of the yellow laser was monitored by a laser analyzer (Spiricon,Inc. M2-200s). In Fig. 7(a), when the incident pump power was 49.2 W, the best beam quality factor of X-axis and Y-axis were measured as Mx2 = 1.207 and My2 = 1.200, respectively, corresponding to a standard Gaussian beam profile. The result matched the conclusion of the simulation. It was markedly improved compared with the self-Raman yellow laser [23]. However, there are a few points that don’t coincide with the fitted curve, which may be caused by the fact that the yellow radiation was mixed with a small amount of second-order Stokes light that was not fully transmitted by M6. Based on the above analysis and measurement errors, we believe that the actual beam quality is better (Mx2 < 1.207 and My2 < 1.200). Combining the experimental results and the simulation conclusions, although the insertion of the YVO4 crystal increased the length of the resonator, the higher beam quality obtained could be mainly attributed to 4 aspects: i)878.4 nm pump source has lower quantum defect (17%) than traditional 808 nm pump source (24%), i.e. generates less waste heat; ii)Nd: YVO4 was only used as a laser gain medium, which had better thermal effect than that used as both laser gain medium and Raman gain medium, and the doble-end bonded Nd:YVO4 crystal configured a cooling device to further accelerate thermal diffusion; iii)The pump beam spot and the fundamental beam spot achieved suitable mode matching on the gain medium and low sensitivity of the laser system to the thermal lens, and provided two beam waist positions on both Nd:YVO4 and YVO4, which came from the careful design of the V-shaped cavity; iiii)The self-cleaning effect of the Raman process further improved the beam quality, and its thermal effect could be ignored [25].
Fig. 7. The beam quality factor M2 in the horizontal and vertical directions and the corresponding beam profile at the pump power of 49.2 W in the VAR (a), and the pump power of 17.5 W in the SASR (b).
For comparison, we also replicated the above straight cavity active Q-switched self-Raman yellow laser system, and the experimental results are shown in Fig. 7(b), which depicted that the optimal beam quality factor in the horizontal and vertical directions was Mx2 = 12.85 and My2 = 10.57 respectively at 17.5 W incident pump power, and the beam profile of the yellow output was severely deformed, which was the consequence of the high-order mode oscillation caused by the severe thermal lens effect, and it was consistent with the simulation conclusion above. In comparison, the advantages of our laser system are quite clear.
4. Conclusion
In summary, a near-diffraction-limited beam quality at 587.9 nm was obtained by designing the optimized resonator shape and utilizing a-cut composite crystal Nd:YVO4 and pure Raman crystal YVO4. The maximum yellow laser output power was 2.85 W at incident pump power and PRF of 49.2 W and 50 kHz, respectively, with a corresponding diode-to-yellow conversion efficiency of 5.75%. The measured pulse width was 3 ns, and the corresponding pulse energy and peak power were 57 µJ and 19 kW. Under the high pump power of 49.2 W, the optimal beam quality factor in horizontal and vertical directions was measured to be Mx2 = 1.207 and My2 = 1.200, respectively, which was significantly improved compared to the previous self-Raman pulsed yellow laser.
Funding
Guangzhou Science and Technology Project (201904010294); Research and Development Program in Key Areas of Guangdong Province (2020B090922006); National Natural Science Foundation of China (51872307, 51972149, 61935010, 62175091, 62175093).
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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