Stimulated Raman scattering (SRS) has been shown to be an effective method for wavelength conversion of laser radiation in recent years. Due to various advantages of compactness, high stability, small scale, low loss, and high efficiency, Raman crystals have been widely employed in many fields such as information, communication, medical treatment, and so on. Nd:YVO₄ crystals have been considered as an efficient Raman medium in diode-pumped solid-state self-Raman lasers because of its high Raman gain (4.5 cm/GW)^[1] and large emission cross section.

In 2001, Kaminskii et al.^[1] predicted that Nd:YVO4 would be promising self-Raman laser media. In 2004, Chen et al.^[2] first reported a self-Raman laser with Nd:YVO4 crystal. For the following few years, an increasing number of works have focused major attention on the first-Stokes Raman laser operating at the wavelength of λSt1=1176nm and λSt1=1525nm^[3–7]. Since 2012, Chen et al.^[8] and our group^[9–11] have both reported a second-Stokes self-Raman laser at a wavelength of λSt2=1313nm and λSt1=1764nm, respectively. However, there has rarely been a report about the third-Stokes self-Raman laser at λSt3=1487nm.

For a self-Raman laser, thermal effects, caused by the absorption of pump light and the SRS cascading frequency conversion process in the Nd:YVO4 crystal, have been the most important factors affecting the overall performance. Consequently, a composite Nd:YVO4 crystal has been applied as the laser gain medium to reduce the influence of the thermal effects^[11]. It can not only mitigate the thermal effects but also increase the Raman interaction length for the SRS frequency conversion.

In this work, we demonstrated a diode-end-pumped actively Q-switched YVO4/Nd:YVO4/YVO4 self-Raman laser at a third-Stokes wavelength of 1487 nm. The maximum average output power at 1487 nm was up to 506 mW at an incident pump power of 34 W and a pulse repetition frequency (PRF) of 30 kHz, with a corresponding optical conversion efficiency of 1.49%. The highest pulse energy and peak power were obtained to be 24.5 μJ and 20.6 kW, respectively.

Our experimental setup of a Q-switched third-Stokes self-Raman laser is shown in Fig. 1. The cavity configuration was a plano-concave resonator. The pump source was a high-power fiber-coupled diode-laser-array at 808 nm, which was commercially available. The core diameter and numerical aperture (NA) of the fiber were 0.4 mm and 0.22, respectively. By employing an optical imaging system with an imaging ratio of 1:0.5, a fiber output beam at a wavelength of 808 nm was focused into the laser crystal with a spot size of 0.2 mm in diameter. The pump mirror M1 was a flat mirror, high-transmittance (HT) coated at 808 nm, and high-reflection (HR) coated at 1064, 1178, and 1313 nm. The output mirror M2 was a concave mirror, with a radius-of-curvature of 250 mm, HR coated at 1064, 1178, and 1313 nm. The transmittances of M1 and M2 at 1487 nm were measured to be 50%. The length of the resonator cavity from M1 to M2 was 80 mm. An a-cut YVO4/Nd:YVO4/YVO4 composite crystal was adopted as the laser material, with dimensions of 3×3×10(mm). The 10-mm-long 0.3 at.% Nd3+ doped crystal was bounded by a 2-mm-long pure YVO4 at the pumped end and an 18-mm-long pure YVO4 at another end. It was anti-reflection (AR) coated at 808, 1064, 1178, 1313, and 1487 nm on both of its faces. To remove the heat generated in the crystal, the laser crystal was wrapped with indium foil and mounted in a water-cooled copper block heat sink. The water temperature was maintained at about 20°C. An acousto-optic Q-switch (AOS) with a length of 35 mm, AR-coated at 1064 nm on both of its faces, was placed close to the laser crystal. The repetition rate of AOS could be tuned from 1 to 100 kHz continuously. An optical spectrum analyzer (YOKOGAWA AQ6370B, with a resolution of 0.02 nm) was used to measure the spectral information of the laser. A digital phosphor oscilloscope (Tektronix DPO 7104C, with 1 GHz electrical bandwidth) and a fast photodiode detector (EOT ET-3500) were used to record the temporal profiles of the output pulses. A diffraction grating (THORLABS GR25-1210) was used to disperse the mixed wavelengths.

Using the aforementioned experimental configuration described, the average output powers of third-Stokes Raman lasers at 1487 nm versus the incident pump power were measured at different PRFs of 15, 30, and 50 kHz, as shown in Fig. 2. The threshold of the third-Stokes emission for the PRFs of 15, 30, and 50 kHz were measured to be 6, 6, and 8 W, respectively. From Fig. 2, we can see that the third-Stokes output powers gradually tend to saturate, but the output power would descended with the pump power increasing higher. The strong thermal lens effect resulting from the SRS process can induce the instability of the resonator^[12], and the end-face of the crystal would be damaged by the excessively high pump power. Therefore, high pump power becomes a limiting factor in the development of our self-Raman laser. The maximum average output power was up to 506 mW at an incident pump power of 34 W and a PRF of 30 kHz, with a corresponding optical conversion efficiency of 1.49%. The maximum output powers at the PRFs of 15 and 50 kHz were measured to be 368 and 430 mW, with a corresponding optical conversion efficiency of 1.23% and 1.26%, respectively. There were several reasons for the low optical conversion efficiency. First, the transmittance of the pump mirror at 1487 nm was measured to be 50%. Observed from the optical spectrum analyzer, much third-Stokes Raman emission leaked from the pump mirror. Higher conversion efficiency and output power will be expected with optimum mirrors. Second, the laser crystal and the Q-switch, which were not AR-coated at 1487 nm, would lead to more loss in the cavity. Third, the SRS was an inelastic process, thus the thermal loading of the third-Stokes Raman emission was more serious than the second-Stokes Raman emission. Moreover, the duty cycle was not adequately adjusted in the work reported in this Letter. Therefore, a higher conversion efficiency is expected with optimization of the experimental facilities.

A typical optical spectrum of output radiation at an incident pump power of 34 W and a PRF of 30 kHz is shown in Fig. 3. The highest single pulse energy was measured to be 24.5 μJ at an incident pump power of 30 W and a PRF of 15 kHz. The highest peak power was measured to be 20.6 kW at an incident pump power of 34 W and a PRF of 30 kHz. The central wavelengths of the fundamental, first-Stokes, second-Stokes, and third-Stokes radiations were determined to be 1064.4, 1176, 1313.4, and 1487.2 nm. Accordingly, the frequency shift of each adjacent radiation was calculated to be about 890cm−1, which corresponds to the main optical vibration mode of the tetrahedral VO43− ionic groups.

The line widths at the wavelengths of 1064.4, 1313.4, and 1487.2 nm were measured to be 0.18, 0.32, and 0.175 nm, respectively. The spectrum of the third-Stokes radiation is shown in Fig. 4 (with a line width of 0.175 nm.)

A diffraction grating was used to disperse the fundamental, second-, and third-Stokes output radiation. The energy of the first-Stokes radiation was mostly converted to higher-order Stokes. The temporal profiles of the output pulses were recorded by a digital phosphor oscilloscope with a fast photodiode detector at a pump power of 34 W and a PRF of 30 kHz, as shown in Fig. 5. The shortest pulse width was measured to be 810.6 ps at the third-Stokes radiation, with the corresponding fundamental and the second-Stokes pulse widths of 5.1 and 1.601 ns, respectively. The pulse will be compressed in the cascading nonlinear frequency conversion process of SRS^[12,13]; therefore, it can be found that the pulse width of higher-order Stokes radiation is much shorter.

Finally, the beam quality factor (M2) of the third-Stokes was measured to be 1.53×2.74 in the horizontal and vertical directions at a pump power of 34 W and a PRF of 30 kHz.

In conclusion, a diode-end-pumped actively Q-switched third-Stokes YVO4/Nd:YVO4/YVO4 self-Raman laser is investigated at a wavelength of 1487 nm. The maximum average output power at 1487 nm is up to 506 mW at an incident pump power of 34 W and a PRF of 30 kHz, with a corresponding optical conversion efficiency of 1.49%. The highest pulse energy and peak power are obtained to be 24.5 μJ and 20.6 kW, respectively. It is expected that a higher output power of third-Stokes radiation can be achieved with optimization of the output coupling and a pump mirror that is HR-coated at 1487 nm.