⇦Chinese Physics Letters, 2019, Vol. 36, No. 1, Article code 014205 First-Stokes Wavelengths at 1175.8 and 1177.1 nm Generated in a Diode End-Pumped Nd:YVO$_{4}$/LuVO$_{4}$ Raman Laser * Qing-Qing Zhou (周青青)1, Shen-Cheng Shi (施沈城)1, Si-Meng Chen (陈思梦)1, Yan-Min Duan (段延敏)1,2**, Xi-Mei Zhang (张喜梅)1, Jing Guo (郭锦)1, Bin Zhao (赵斌)3, Hai-Yong Zhu (朱海永)1** Affiliations 1College of Mathematics, Physics and Electronic Information Engineering, Wenzhou University, Wenzhou 325035 2International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060 3College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108 Received 19 November 2018, online 25 December 2018 *Supported by the Zhejiang Provincial Natural Science Foundation of China under Grant No LY19F050012, the National Natural Science Foundation of China under Grant No 61505147, the Laboratory Open Project of Wenzhou University under Grant No 18SK31, and the Research Funds of College Student Innovation of Zhejiang Province under Grant No 2018R42901.
**Corresponding author. Email: ymduan12@gmail.com; hyzhu@wzu.edu.cn Citation Text: Zhou Q Q, Shi C C, Chen S M, Duan Y M and Zhang X M et al 2019 Chin. Phys. Lett. 36 014205    Abstract A diode end-pumped acousto-optic Q-switched Nd:YVO$_{4}$/LuVO$_{4}$ Raman laser is demonstrated. Both YVO$_{4}$ and LuVO$_{4}$ can work as Raman gain, and slightly different active vibration modes of both crystals can result in different first-Stokes wavelengths. The output characteristic as the Raman competition between YVO$_{4}$ and LuVO$_{4}$ crystals for the laser systems with both shared cavity and coupled cavity is experimentally investigated. For the shared cavity, simultaneous Raman conversion in both YVO$_{4}$ and LuVO$_{4}$ crystals is achieved with dual-wavelength emission at 1175.8 and 1177.1 nm. The maximum output power of 1.03 W and the conversion efficiency of 10.3% are obtained. The 0.84 W single first Stokes wavelength at 1177.1 nm with LuVO$_{4}$ Raman conversion is achieved with the coupled cavity. The results show that the coupled cavity with short Raman cavity can obtain a narrow pulse width. The separated laser crystal and Raman gain media with different vanadates in shared cavity have advantages in achieving dual-wavelength lasers with small frequency intervals. DOI:10.1088/0256-307X/36/1/014205 PACS:42.55.Ye, 42.55.Rz, 42.55.Xi © 2019 Chinese Physics Society Article Text Stimulated Raman scattering has attracted much attention as an efficient frequency conversion technology.[1-4] Many kinds of crystals have been recognized as solid-state Raman gain materials.[5-8] Vanadate crystal is a typical Raman material with high Raman gain for its symmetric A1g optical vibration modes of tetrahedral VO$_{4}^{3-}$ ionic groups. In 2001, the Raman properties of YVO$_{4}$ and GdVO$_{4}$ crystals were introduced by Kaminskii et al.[9] Chen realized the first self-Raman laser output in 2004.[10] Then, the Raman lasers based on vanadate crystals have been well studied.[11-16] In 2008, Kaminskii et al. also introduced the $\chi(3)$-nonlinear-laser manifestations of LuVO$_{4}$ crystal,[17] and reported its self-Raman laser output in the next year.[18] The 120 mW first-Stokes light has been obtained with the conversion efficiency of about 5%. Recently, the high quality LuVO$_{4}$ crystal growth and its Raman laser investigation has also attracted increasing attention.[19-22] Lu et al. and Tan et al. reported the intracavity frequency-doubled and sum-frequency mixing in a self-Raman Nd:LuVO$_{4}$ laser, separately.[23,24] In addition to vanadate self-Raman laser, the Raman operation with separated laser crystal and Raman gain media also has advantage of simultaneously optimizing the fundamental laser generation and Raman operation.[25-29] Su et al. also reported a power scale-up Nd:YVO$_{4}$ Raman laser with a pure YVO$_{4}$ crystal as a Raman shifter by optimizing Raman crystal position.[30] Up to date, most LuVO$_{4}$ Raman laser reports were focused on Nd:LuVO$_{4}$ self-Raman operation. In this Letter, a pure LuVO$_{4}$ crystal Raman laser derived by an acousto-optic Q-switched Nd:YVO$_{4}$ crystal laser is investigated. The Raman competition between YVO$_{4}$ crystal and LuVO$_{4}$ crystal for both shared cavity and coupled cavity is studied. The slightly different active vibration modes of both crystals result in different first-Stokes wavelengths. Dual wavelengths at 1175.8 nm and 1177.1 nm are achieved in the shared cavity, and single wavelength at 1177.1 nm is obtained in the coupled cavity. The dual-wavelength laser with two closely spaced Stokes wavelengths may have potential applications in laser Doppler velocimeters[31,32] and terahertz generation.[33] Both YVO$_{4}$ and LuVO$_{4}$ crystals are excellent vanadate Raman crystals with strong stimulated Raman scattering active vibration modes around 890 and 900 cm$^{-1}$. Therefore, their first-Stokes wavelength converted from the 1.06 µm fundamental wavelength of Nd$^{3+}$ doped crystal laser will be located at 1.18 µm with slight difference on spectrum. Kaminskii et al. reported their Raman gain coefficient greater than 4.5 cm/GW[9] and 3.2 cm/GW.[17] The experimental setup of the Nd:YVO$_{4}$/LuVO$_{4} $ Raman laser with a linear cavity is schematically shown in Fig. 1. An a-cut pure LuVO$_{4}$ crystal with 3$\times3\times$15 mm$^{3}$ in size was used as Raman crystal and derived by an acousto-optic Q-switched Nd:YVO$_{4}$ crystal laser. A fiber-coupled laser diode with a fiber core diameter of 200 µm and a numerical aperture of 0.22 was employed to pump an a-cut Nd:YVO$_{4}$ crystal, and the pump beam was re-imaged into a spot size of 320 µm in diameter by a pair of lenses with focal lengths of 50 mm and 80 mm. The Nd:YVO$_{4}$ crystal with 0.3 at.% Nd$^{3+}$ doped and 3$\times3\times$10 mm$^{3}$ in size was wrapped with indium foil and mounted tightly in a 18$^\circ\!$C water-cooled copper heat sink. Both end-faces of Nd:YVO$_{4}$ crystal and LuVO$_{4}$ crystal were anti-reflection (AR) coated for the pump wavelength at 808 nm, fundamental wavelength at 1.06 µm and Stokes wavelength at 1.18 µm. A 30-mm-long acousto-optic Q-switcher (AO, Gooch & Housego Co.) driven at a 40 MHz central frequency with 20 W of radio frequency power was seated after Nd:YVO$_{4}$ crystal for Q-switching operation.
cpl-36-1-014205-fig1.png
Fig. 1. Experimental arrangement of the diode end-pumped Q-switched Nd:YVO$_{4}$/LuVO$_{4}$ Raman laser. IM: pump input mirror; MM: additional middle mirror; OC: laser output coupler; AO: acousto-optic Q-switcher; LD: laser diode.
In our experiment, fundamental laser and Raman laser shared cavity (without additional middle mirror MM in Fig. 1), and coupled cavity (with MM included in Fig. 1) were designed for comparison. The pump input mirror IM was high-transmission (HT, $T>95{\%}$) coated at 808 nm and high-reflection (HR, $R>99.9{\%}$) coated from 1.06 to 1.18 µm. The laser output coupler OC was HR ($R>99.9{\%}$) coated at 1.06 µm and partial-reflection (PR, $R=97.5{\%}$) at 1.18 µm, and both plane and concave (radius of curvature 500 mm) mirrors were used for comparison. The fundamental and Raman oscillation could share the same cavity, consisting of an plane input mirror IM and an output coupler OC with a total cavity length of 75 mm for the shared cavity. For the coupled cavity, we only inserted an additional mirror MM (HT, $T>98{\%}$ at 1.06 µm and HR, $R>99.9{\%}$ at 1.18 µm) between the Q-switcher and the LuVO$_{4}$ crystal with the distance between MM and OC of about 30 mm. Therefore, the fundamental cavity formed by IM and OC, while the Raman cavity formed by MM and OC. The cavity length was kept the same as that for the shared cavity. First, the laser output characteristics with the shared cavity were studied. Both plane and concave (radius of curvature 500 mm) mirrors were used as output coupler OC for comparison. Plane IM and plane OC formed a plane-plane cavity, while plane IM and concave OC formed a plane-concave cavity. The pulse frequency repetition of acousto-optic Q-switcher was optimized at 60 kHz. Figure 2 shows the average output power versus the incident pump power for both the plane-concave cavity and the plane-plane cavity. The Raman laser thresholds were around 6 W and 7 W for the plane-concave cavity and the plane-plane cavity, respectively. The maximum average output power up to 1.03 W for the plane-concave cavity was achieved under an incident pump power of 10 W, with the corresponding conversion efficiency of 10.3%. On the contrast, only 0.41 W average output power was obtained for plane-plane cavity at the same incident pump power. The output spectra were measured by a grating monochromater (model Omni-$\lambda$ 500 with the resolution of 0.05 nm, ZOLIX). Dual-wavelength outputs with the central wavelengths 1175.8 nm and 1177.1 nm and line widths of about 0.35 nm were detected for both the cavities, as displayed in the inset of Fig. 2.
cpl-36-1-014205-fig2.png
Fig. 2. Average output power versus incident pump power for both the plane-concave cavity and the plane-plane cavity with the shared cavity. The inset shows the output laser spectra.
cpl-36-1-014205-fig3.png
Fig. 3. Measured laser output spectra for the plane-concave cavity.
Figure 3 shows the measured laser output spectra scanned from 1040 to 1200 nm for the plane-concave cavity. This indicates that Raman conversions in both the YVO$_{4}$ crystal and the LuVO$_{4}$ crystal with Raman shifts of 890 and 900 cm$^{-1}$ were achieved at the same time, respectively. The relative intensity of 1175.8 nm line was higher than that of 1177.1 nm wavelength in the plane-concave cavity. However, the relative intensity of 1175.8 nm wavelength was lower than that of 1177.1 nm wavelength in the plane-plane cavity.
cpl-36-1-014205-fig4.png
Fig. 4. Simulated cavity mode in the cavity between IM and OC with the thermal lens focal length 30 cm.
The relative intensity was mainly caused by the Raman gain of both the Raman crystals. The different relative intensities between two cavities may be caused by fundamental laser power density difference on the Nd:YVO$_{4}$ crystal and the LuVO$_{4}$ crystal for both the plane-concave cavity and the plane-plane cavity. According to the experimental setup, we simulated the fundamental cavity mode in the cavity between IM and OC when the thermal lens focal length was 30 cm, and the results are shown in Fig. 4. The plane-plane cavity owns larger cavity mode radius compared to the plane-concave cavity. The mode radius on the LuVO$_{4}$ crystal is smaller than that on the Nd:YVO$_{4}$ crystal, and the difference is much larger in the plane-plane cavity which resulted in the first-Stokes at 1177.1 nm converted by the LuVO$_{4}$ crystal with more intensity than 1175.8 nm line converted by the Nd:YVO$_{4}$ crystal. Therefore, the relative intensity between both Stokes lines depends not only on their Raman gain coefficient but also on the cavity mode on both crystals which can vary with the radius of curvature of cavity mirrors. Then, the plane-concave cavity with the coupled cavity was also studied. A plane mirror MM with HT coated at 1.06 µm and HR coated at 1.18 µm was inserted between the Q-switcher and the LuVO$_{4}$ crystal. In this experiment, we also found the Raman competition between the YVO$_{4}$ crystal and the LuVO$_{4}$ crystal. MM and OC constructed a plane-concave Raman cavity for LuVO$_{4}$ Raman operation, however, IM and MM could also construct a plane-plane Raman cavity for Nd:YVO$_{4}$ self-Raman operation. The plane-concave cavity was much easier to realize the Raman operation as its lower sensitivity to misalignment compared to the plane-plane cavity. The first-Stokes wavelength at 1177.1 nm converted in the LuVO$_{4}$ crystal with its plane-concave cavity was achieved firstly when the additional middle mirror MM was slightly tilted relative to the input mirror IM. The LuVO$_{4}$ Raman operation in the cavity formed by MM and OC could transform into the Nd:YVO$_{4}$ self-Raman operation in the cavity formed by IM and MM while the additional middle mirror MM was well aligned and resulted in lower first Stokes loss of Nd:YVO$_{4}$ self-Raman. Because both Nd:YVO$_{4}$ self-Raman cavity mirrors IM and MM were HR coated at the first Stokes wavelength, the output power of the first-Stokes at 1177.1 nm has a sudden change from the maximum value into zero when the first Stokes wavelength at 1175.8 nm was generated in the Nd:YVO$_{4}$ crystal.
cpl-36-1-014205-fig5.png
Fig. 5. Total output power versus incident pump power for coupled cavity configuration. The inset shows the pulse width and laser spectra.
Figure 5 shows its output power characteristics and the output laser spectrum. The output power was optimized under the incident pump power of 10 W. Maximum output power of 0.85 W and conversion efficiency of about 8.5% were achieved. The misalignment of laser cavity for LuVO$_{4}$ Raman operation also results in much higher threshold in Fig. 5 compared to that using the shared cavity in Fig. 2. Therefore, the coupled cavity was not optimized for lacking of suitable input mirror. If the Nd:YVO$_{4}$ self-Raman can be inhibited using an input mirror IM with high transmission at 1175.8 nm, the output power of LuVO$_{4}$ Raman laser could be further improved. The temporal pulse profiles for the first-Stokes output with both shared cavity and coupled cavity were measured. The temporal pulse profile was recorded by an InGaAs free-space photo detector (model DET08C, Thorlabs), and displayed on a 500 MHz oscilloscope. Both temporal pulse profiles measured at an incident pump power of 10 W are shown in Fig. 6. Though the laser system with coupled cavity was not well optimized for LuVO$_{4}$ Raman laser output, its pulse width was still much narrower than that using the shared cavity. Pulse widths of about 19 and 11 ns were measured for the shared cavity and the coupled cavity, respectively. Therefore, the short Raman cavity with high first-Stokes density for the coupled cavity could result in narrow pulse. We also investigated the polarization of the output laser using the Glan–Faucault prism. The polarization of the first Stokes converted in both the Nd:YVO$_{4}$ and LuVO$_{4}$ crystals was the same as the fundamental laser parallel to the $c$-axis of the Nd:YVO$_{4}$ crystal.
cpl-36-1-014205-fig6.png
Fig. 6. Temporal pulse profile for laser output with shared and coupled cavities at an incident pump power of 10 W.
In conclusion, we have investigated the output characteristics of the diode end-pumped acousto-optic Q-switched Nd:YVO$_{4}$/LuVO$_{4}$ Raman laser. Both YVO$_{4}$ and LuVO$_{4}$ have a great Raman gain and the slightly different active vibration modes of both crystals can result in different first-Stokes wavelengths. The Raman competition between the YVO$_{4}$ and LuVO$_{4}$ crystals for both the shared cavity and the coupled cavity is experimentally investigated. For the shared cavity, simultaneous Raman conversions in both the YVO$_{4}$ and LuVO$_{4}$ crystals can be obtained with dual-wavelength output at 1175.8 nm and 1177.1 nm. The relative intensity between both Stokes lines depends not only on their Raman gain coefficient but also on the cavity mode on both the crystals, which can vary with the radius of curvature of cavity mirrors. In our experiment, the maximum output power up to 1.03 W for the plane-concave cavity is achieved with the corresponding conversion efficiency of 10.3%. For the coupled cavity, 0.85 W single first Stokes wavelength at 1177.1 nm converted in the LuVO$_{4}$ crystal is obtained. The further improvement of output power is hindered because of the Nd:YVO$_{4}$ self-Raman operation. The coupled cavity with a short Raman cavity can obtain narrower pulse width compared to the shared cavity. The separated laser crystal and Raman gain media with different vanadates in a shared cavity has advantages in achieving dual-wavelength lasers with small frequency intervals. References Crystalline Raman LasersQ-Switched Raman Fiber Laser with Molybdenum Disulfide-Based Passive Saturable AbsorberAn efficient continuous-wave YVO 4 /Nd:YVO 4 /YVO 4 self-Raman laser pumped by a wavelength-locked 878.9 nm laser diodeModelling of passively Q-switched lasers with intracavity Raman conversionConditions for the occurrence of k gaps for surface polaritons on gratingsRbTiOPO4 cascaded Raman operation with multiple Raman frequency shifts derived by Q-switched Nd:YAlO3 laserHighly efficient picosecond all-solid-state Raman laser at 1179 and 1227 nm on single and combined Raman lines in a BaWO 4 crystalEfficient Diode-End-Pumped Actively Q-Switched Nd:YLF/SrWO 4 Raman LaserTetragonal vanadates YVO4 and GdVO4 – new efficient χ(3)-materials for Raman lasersHigh-power diode-pumped actively Q-switched Nd:YVO_4 self-Raman laser: influence of dopant concentrationCascaded Self-Raman Laser Emitting Around 1.2–1.3 μm Based on a c-cut Nd:YVO 4 CrystalGeneration of Higher Order Vortex Beams From a YVO4/Nd:YVO4 Self-Raman Laser via Off-Axis Pumping With Mode ConverterNear-infrared and orange–red emission from a continuous-wave, second-Stokes self-Raman Nd:GdVO_4 laserEfficient 17 μm light source based on KTA-OPO derived by Nd:YVO_4 self-Raman laserResearch and design of continuous-wave Nd:YVO4 self-Raman laser in-band pumped by a wavelength-locked laser diodeNew χ (3)-nonlinear-laser manifestations in tetragonal LuVO4 crystal: more than sesqui-octave Raman-induced Stokes and anti-Stokes comb generation and cascaded self-frequency “tripling”Nanosecond Nd 3+ :LuVO 4 self-Raman laserHigh pressure Raman scattering study on the phase stability of LuVO4Investigation on Raman spectra and thermal properties of LuVO4 and Nd:LuVO4 crystalsNew manifestations of χ (3) -nonlinear laser interactions in tetragonal LuVO 4 and YbVO 4 crystals attractive for SRS-converters and self-Raman lasersGrowth and characterization of LuVO 4 single crystalsAll-solid-state cw sodium D_2 resonance radiation based on intracavity frequency-doubled self-Raman laser operation in double-end diffusion-bonded Nd^3+:LuVO_4 crystalAn efficient cw laser at 560 nm by intracavity sum-frequency mixing in a self-Raman Nd:LuVO 4 laserCompact passively Q-switched Raman laser at 1176 nm and yellow laser at 588 nm using Nd 3+ :YAG/Cr 4+ :YAG composite crystalComparison of 115 µm Nd:YAG/KTA Raman lasers with 234 and 671 cm^−1 shiftsDiode-Pumped c -Cut Nd:Lu 0.99 La 0.01 VO 4 Self-Stimulated Raman Laser at 1181 nmEfficient Diode-Pumped Actively Q-Switched Ceramic Nd:YAG/YVO4 Raman Laser Operating at 1657 nmPower scale-up of the diode-pumped actively Q-switched Nd:YVO4 Raman laser with an undoped YVO4 crystal as a Raman shifterVelocity Measurement Based on Laser Doppler EffectSelf-mixing birefringent dual-frequency laser Doppler velocimeterInvestigation of terahertz generation from passively Q-switched dual-frequency laser pulses
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