⇦Chinese Physics Letters, 2020, Vol. 37, No. 4, Article code 044202 Coaxial Multi-Wavelength Generation in YVO$_{4}$ Crystal with Stimulated Raman Scattering Excited by a Picosecond-Pulsed 1064 Laser * Jing-Jie Hao (郝婧婕)1,2,3, Wei Tu (涂玮)1,2**, Nan Zong (宗楠)1,2, Yu Shen (申玉)1,2**, Shen-Jin Zhang (张申金)1,2, Yong Bo (薄勇)1,2, Qin-Jun Peng (彭钦军)1,2, Zu-Yan Xu (许祖彦)1,2 Affiliations 1Key Lab of Solid State Laser, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 2Key Lab of Function Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 3University of Chinese Academy of Sciences, Beijing 100190 Received 19 December 2019, online 24 March 2020 *Supported by the Key Laboratory Foundation from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.
**Corresponding authors. Email: ywtu@mail.ipc.ac.cn; shenyu@mail.ipc.ac.cn Citation Text: Hao J J, Tu W, Zong N, Shen Y and Zhang S J et al 2020 Chin. Phys. Lett. 37 044202    Abstract The multiwavelength characteristics of stimulated Raman scattering (SRS) in YVO$_{4}$ crystal excited by a picosecond laser at 1064 nm are investigated theoretically and experimentally. Laser output with seven wavelengths is achieved coaxially and synchronously at 894, 972, 1175, 1312, 1486, 1713 and 2022 nm in a YVO$_{4}$ crystal. The maximum total Raman output energy is as high as 2.77 mJ under the pump energy of 7.75 mJ. A maximum total Raman conversion efficiency of 47.8% is obtained when the pump energy is 6.54 mJ. This is the highest order of Stokes components and the highest output energy generated by YVO$_{4}$ reported up to date. This work expands the Raman spectrum of YVO$_{4}$ crystal to the near-IR regime, with seven wavelengths covered at the same time, paving the way for new wavelength generation in the near-IR regime and its multiwavelength application. DOI:10.1088/0256-307X/37/4/044202 PACS:42.55.Ye, 42.65.Dr, 42.65.Ky © 2020 Chinese Physics Society Article Text Stimulated Raman scattering (SRS) is a practical method for achieving simultaneous output of multiple wavelength, which will play an important role in optical imaging systems, multiwavelength communication, and biochemical detection, etc. In the last few decades, stimulated Raman scattering has been widely utilized to extend the spectral coverage of existing laser technologies and many Raman crystals have been developed and studied including nitrates, tungstates, and vanadates.[1–3] As a promising Raman medium, YVO$_{4}$ crystal has advantages of high Raman gain, good mechanical and optical properties, especially a mature growth technology for large physical size with high optical quality.[4] Kaminskii et al. demonstrated its attractive $\chi ^{(3)}$ nonlinear properties in 2001.[3] Their discoveries have made vanadate materials to be widely used in solid-state Raman lasers. As for the visible regime, Raman scattering of YVO$_{4}$ crystal pumped by picosecond 532 nm laser with an external single-pass configuration in 2006.[5] Zong et al. have reported a transient state SRS with a picosecond pulse in YVO$_{4}$ crystal.[6] In 2013, Xu et al. obtained three Stokes lines and three anti-Stokes lines by picosecond 526.5 nm laser with a Raman amplification.[7] At present, the application of YVO$_{4}$ at near-infrared region mainly focused on its combination with Nd:YVO$_{4}$ in self-Raman research, 1176 nm and 1313 nm have been reported.[8,9] However, few attention has been paid on SRS. In 2009, Hu et al. reported three Stokes lines from external resonator by using a YVO$_{4}$ crystal pumped by 1064 nm picosecond laser.[10] With the excitation of single-pass SRS, the emission of four Stokes components was achieved by Kaminskii et al.,[11] which is the highest Stokes lines reported up to date. Because the Raman gain coefficient is inversely proportional to the pump wavelength, expanding the output spectrum to longer wavelength is still a problem remains to be solved.[12] In this Letter, the multi-wavelength generation in a $c$-cut YVO$_{4}$ crystal based on picosecond 1064 nm pumping laser is investigated. Up to five coaxial Stokes components are achieved, which is the highest order of Stokes in YVO$_{4}$ in near-infrared region. A total Raman output energy of 2.77 mJ is obtained under the pump energy of 7.75 mJ, which corresponds to a Raman conversion efficiency of 44.6%. When the pump energy is 6.54 mJ, the maximum total Raman conversion efficiency is obtained to be 47.8%. To achieve the multiwavelength output by YVO$_{4}$, a preliminary theoretical research is conducted. It takes a finite time for a material to respond to the pump field, which results in two distinct temporal regimes for SRS: one is steady-state regime, for which the pump pulse duration is much longer than the response time of the medium; and the other is a transient regime when the pump pulse duration becomes comparable to or even shorter than the dephasing time of the vibrational excitation. For the vibronic Raman mode, the dephasing time $T_{\rm R}~(\approx \,$3.5 ps)[3] is in the same order of magnitude as the pulse duration of pump source, $\tau_{\rm P} \approx 30$ ps, hence the SRS experiment excitation provides a transient state regime for YVO$_{4}$ crystal. In transient Raman regime, the Stokes signal intensity is related to the pump energy $\tau_{\rm P}I_{\rm P}$,[13] thus an optimum coverage volume of the pump beam in YVO$_{4}$ crystal is necessary so that higher energy could be input without damaging the crystal. The generation position of Stokes and the corresponding divergence angle are the main influences taken into consideration. The relative intensity of each order of Stokes versus transmitting distance are shown in Fig. 1. The following Raman coupled equations are applied for simulating the generation of Stokes,[14] $$ \begin{array}{l} \frac{dI_{\rm P} }{dz}=-g_{\rm R} I_{\rm P} I_{\rm S,1} -\alpha_{0} I_{\rm P}, \\ \frac{dI_{\rm S,1} }{dz}=g_{{\rm R},1} I_{\rm S,1} (I_{\rm P} -I_{\rm S,2})-\alpha_{0} I_{\rm S,1}, \\ \frac{dI_{{\rm S},n} }{dz}=g_{{\rm R},n} I_{{\rm S},n} (I_{{\rm S},n-1} -I_{{\rm S},n+1})-\alpha_{0} I_{{\rm S},n}, \\ \end{array}~~ \tag {1} $$ where $I_{{\rm S},n}$ ($n = 2$, 3, 4, 5) and $I_{\rm P}$ are the Stokes and pump intensity; $z$ is the transmitting distance in Raman crystal; $g_{{\rm R},i}$ derived from $g_{{\rm R},i}=g_{\rm R}\omega_{{\rm S},i}/\omega_{\rm P}$ is the Raman gain coefficient for each order of Stokes component, and $\alpha_{0}$ is the extinction coefficient.
cpl-37-4-044202-fig1.png
Fig. 1. The relative intensity of each order of Stokes components with respect to the transmitting distance in crystal under different pump intensities.
To calculate the divergence angle of Raman laser, the phase matching condition[15] and the Sellmeier equation[3] are used. The phase matching condition is $\boldsymbol{k}_{0} +\boldsymbol{k}_{-1} =\boldsymbol{k}_{n-1} +\boldsymbol{k}_{n}$, where $\boldsymbol{k}_{0}$, $\boldsymbol{k}_{-1}$, $\boldsymbol{k}_{n-1}$ and $\boldsymbol{k}_{-n}$ are the pump laser, the first-order Stokes, the $(n-1)$th order anti-Stokes and the $n$th order Stokes. The refractive index of YVO$_{4}$ crystal is obtained by the Sellmeier equation $$ n_{\rm o}^{2} =1+\frac{2.7665\lambda^{2}}{\lambda^{2}-0.026884},~~n_{\rm e}^{2} =1+\frac{3.5930\lambda^{2}}{\lambda^{2}-0.032103}.~~ \tag {2} $$ Then, the divergence angles of five Stokes components are obtained to be 1.568$^{\circ}$, 3.513$^{\circ}$, 6.082$^{\circ}$, 9.45$^{\circ}$, and 14.057$^{\circ}$, respectively. These results suggest that the position of the occurrence of higher-order Stokes is gradually moved forward and the divergence angle is significantly increased. The Raman gain is generally proportional to the length of Raman medium.[16] A larger transverse size will be required if a longer YVO$_{4}$ crystal is used so that high-order Stokes components could not be blocked by the sidewall of the crystal. Figure 2 schematically shows the experimental setup. The pump source is a diode-pumped passively mode-locked Nd:YVO$_{4}$/YAG 1064 nm laser (PL2251 A-10-SH). The pump pulse duration and frequency repetition are 30 ps and 10 Hz, respectively. The diameter of the pump beam is 8 mm and the divergence is less than 0.5 mrad. The maximum output energy of each pulse in our experiment is 7.75 mJ. The size of the $c$-cut YVO$_{4}$ crystal is 5 mm $\times 5$ mm $\times 40$ mm. M is 45$^{\circ}$ high-reflection mirror for the pump laser. The pump beam diameter is decreased to smaller than 3 mm by a telescope system (comprised of L$_1$ and L$_2$, with $f_1$=300 mm and $f_2$=100 mm, respectively) to effectively increase the pump energy and to match the dimensions of the Raman crystal. Two spectrometers are used to detect the Raman spectrum, and an energy meter is set up to measure the output energy.
cpl-37-4-044202-fig2.png
Fig. 2. Experimental setup for SRS in the $c$-cut YVO$_{4}$ crystal.
Table 1. Spectral compositions of YVO$_{4}$ crystal under the pump of picosecond 1064 nm laser.
$\lambda_{\rm P}$ (nm) Stokes and anti-Stokes generation
Wavelength (nm) $\chi^{(3)}$ lasing component SRS and RFWM-Line attribution
1064 894 AS$_{\rm t2}$ $\omega_{\rm P}+2\omega_{\rm SRS}$
972 AS$_{\rm t1}$ $\omega_{\rm P}+\omega_{\rm SRS}$
1064 $\lambda_{\rm P}$ $\omega_{\rm P}$
1175 $S_{\rm t1}$ $\omega_{\rm P}-\omega_{\rm SRS}$
1312 $S_{\rm t2}$ $\omega_{\rm P}-2\omega_{\rm SRS}$
1486 $S_{\rm t3}$ $\omega_{\rm P}-3\omega_{\rm SRS}$
1713 $S_{\rm t4}$ $\omega_{\rm P}-4\omega_{\rm SRS}$
2022 $S_{\rm t5}$ $\omega_{\rm P}-5\omega_{\rm SRS}$
Using the experimental setup, the SRS is generated by firing the fundamental laser beam into the YVO$_{4}$ crystal. The first-Stokes radiation wavelength is measured to be 1175 nm when reaching the SRS threshold. Increasing pump energy, higher-order Stokes and anti-Stokes lines are obtained subsequently. Spectrum characteristics of the laser output are monitored by commercial grating monochromators (Ocean optical NIR Quest-256) for the wavelength range of 1000–2500 nm in the optical resolution of 9.5 nm and by a spectrometer (AvaSpec-2048FT-SPU) for the wavelength range of 200–1100 nm in the optical resolution of 0.4 nm. The output spectrum of the Raman laser under the maximum pump energy of 7.75 mJ is summarized in Table 1 and shown in Fig. 3. As shown in Fig. 3(a), when the pump intensity is 4.6 GW/cm$^{2}$, except for the fundamental wavelength at 1064 nm, there are five emission peaks corresponding to the 891 cm$^{-1}$ Raman frequency shift. The spectra of Stokes components match with the above simulation results. The first and second anti-Stokes components are beyond the spectral range of the spectrometer (Ocean optical NIR Quest-256) and they are displayed in Fig. 3(b). Totally eight coaxial components are observed within the spectral range from 894 nm to 2022 nm.
cpl-37-4-044202-fig3.png
Fig. 3. SRS spectrum of $c$-cut YVO$_{4}$ crystal pumped by 1064 nm picosecond laser. (a) Stokes spectrum is measured by an Ocean optical NIR Quest-256 spectrometer. (b) Anti-Stokes spectrum is observed by an AvaSpec-2048FT-SPU spectrometer.
A band-pass filter is employed to separate the Stokes light from the pump. The output energy of the Stokes is measured by an energy meter. Figure 4 shows the total Raman output energy and the conversion efficiency versus incident pump energy. It could be seen that total Raman energy increases with the increasing pump energy. The threshold for the first Stokes laser is 2.18 mJ, and the maximum total Raman energy is 2.77 mJ under the pump energy of 7.75 mJ. As the pump energy increases, the conversion efficiency reaches a maximum value and then decreases slightly due to the inherent thermal effect. The highest total Raman conversion efficiency is measured to be 47.8% at the incident pump energy of 6.54 mJ. From the energy dependence measured, it is possible to estimate the Raman gain coefficient with the simple expression $g_{\rm R}I_{\rm th}L=25$,[2] where $I_{\rm th}$ is the pump intensity corresponding to 1% conversion efficiency, and $L$ is the length of the crystal. From Fig. 4, the threshold pump intensity is estimated to be $I_{\rm th}=1.301$ GW/cm$^{2}$, $g_{\rm R}$ for the $c$-cut YVO$_{4}$ crystal pumped by 1064 nm laser is calculated to be 4.8 cm/GW, which is almost equal to the previously reported result.[10] Due to this high level of gain, YVO$_{4}$ is evidently confirmed to be an excellent Raman crystal.
cpl-37-4-044202-fig4.png
Fig. 4. The total combined output energy of the Stokes and anti-Stokes components and the corresponding conversion efficiency with respect to the incident pump energy.
In summary, we have demonstrated a Raman generator based on $c$-cut YVO$_{4}$ crystal pumped by a 1064 nm picosecond laser. Without any tuning technology, up to two anti-Stokes and five Stokes components are generated, which is the highest order of Stokes reported under the same conditions. The maximum Raman output energy is as high as 2.77 mJ under the pump energy of 7.75 mJ. A maximum SRS conversion efficiency of 47.8% into all Stokes and anti-Stokes components in the near-IR range is achieved under the pump energy of 6.54 mJ. This research has obtained coaxial multi-wavelength output, as well as high energy in YVO$_{4}$ crystal, which will expand the utilization of this satisfying crystal. References Stimulated Raman scattering of picosecond pulses in barium nitrate crystalsEfficient Raman shifting of picosecond pulses using BaWO4 crystalTetragonal vanadates YVO4 and GdVO4 – new efficient χ(3)-materials for Raman lasersGrowth of Large Yttrium Vanadate Single Crystals for Optical Maser StudiesStimulated Raman scattering of picosecond pulses in a YVO4 crystalRaman Frequency Conversion of Picosecond Pulses in the YVO 4 CrystalEfficient continuous-wave YVO4/Nd:YVO4 Raman laser at 1176 nmSecond-Stokes YVO_4/Nd:YVO_4/YVO_4self-frequency Raman laserNew manifestations of nonlinear χ (3) -laser properties in tetragonal YVO 4 crystal: many-phonon SRS, cascaded self-frequency “tripling”, and self-sum-frequency generation in blue spectral range with the involving of Stokes components under one-micron picosecondpumpingSteady-State Raman Gain in Diamond as a Function of Pump WavelengthRaman spectroscopy of crystals for stimulated Raman scatteringNear-quantum-limit efficiency of picosecond stimulated Raman scattering in BaWO_4 crystalAngular Dependence of Maser-Stimulated Raman Radiation in CalciteDiode-pumped mid-infrared YVO4 Raman laser at 2418 nm
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