1 Introduction

Recently, all-solid-state lasers operating around 1.6 µm have attracted great attention due to their wide applications in the areas of laser medicine, difference absorption lidar, remote sensing, range finding, spectroscopy and wavelength conversion. In addition, such lasers operating around 1.6 µm have been widely desired in a variety of surgical and dermatological applications due to their eye-safe characteristics [1–7]. Traditionally, efforts are mostly focused on the development of Er-doped lasers for generating 1.6-µm operations [1–5]. Additionally, optical parametric oscillators (OPOs) driven by Nd-doped lasers also have been demonstrated for obtaining 1.6-µm eye-safe laser operations, and OPOs have the advantages of high output power and optical conversion efficiency; however, OPO is also easily affected by the influence of phase matching; additionally, in practice, OPO is easily affected by external factors such as temperature [6, 7]. Stimulated Raman scattering (SRS) in crystalline materials, another efficient method for achieving new laser lines based on a third-order nonlinear optical process, is rarely reported for achieving 1.6-µm lasers. To our knowledge, CVD-diamond external cavity Raman lasers operating at 1.63 µm pumped by 1.34-µm flashlamp Nd:YAP lasers have been demonstrated by Jelínková et al. [8] and Jelínek et al. [9], respectively. Raman laser output energies of 18 µJ [8] and 47 µJ [9] were obtained, and the corresponding peak powers were 3 kW [8] and 7.8 kW [9]. Besides CVD-diamond, no other Raman active media were employed for generating Raman generations around 1.6 µm, among the well-used Raman active media including YVO4 [10, 11], PbWO4 [12], KGd(WO4)2 [13], BaWO4 [14–16] and SrWO4 [17–19]. SrWO4 has been regarded as an excellent Raman active crystal for its advantages of high Raman gain coefficient, good mechanical and optical properties [17–19]. Since SrWO4 exhibits a Raman frequency shift of 921 cm−1 [20], the first-Stokes generation around 1.6 µm will be obtained if a SrWO4 crystal pumped by a fundamental laser operating around 1.4 µm.

In the past, for obtaining 1.4-µm laser operations, Nd-doped crystals including Nd:YAG, Nd:YAP, ceramic Nd:YAG and Nd:GGG have been employed as laser gain media. To our knowledge, the first diode-pumped 1.44-µm Nd:YAG laser with 69 mW output power was reported in Kubecek et al. [21]; since then, 1.44-µm Nd:YAG lasers with CW output powers of 1, 2 and 2.5 W with optical-to-optical efficiencies of 10, 13.3 and 15.6 % were demonstrated by Agnesi et al. [22], Šulc et al. [23] and Huang et al. [24], respectively; in our previous work, we have demonstrated a LD-pumped Nd:YAG ceramic laser operating at 1442.8 nm with a output power of up to 3.96 W and an optical-to-optical efficiency of up to 19.1 %; in that work, the laser performance of a Nd:YAG single crystal also has been studied as a comparison, and the results of our work prove that ceramic Nd:YAG is more efficient for achieving 1.4 μm laser operations [25].

In this work, we successfully demonstrated a SrWO4 Raman laser operating at 1664 nm by employing a 1.44-µm Nd:YAG ceramic laser as pump source. At a pump power of 27.7 W and a pulse repetition rate of 10 kHz, an average output power of 1.16 W at 1664 nm was obtained. The maximum pulse energy of 266 µJ was obtained at the pulse repetition frequency (PRF) of 1 kHz. Traditionally, higher pulse energy can be obtained from flashlamp-pumped laser system. Due to the reasons of low PRF and high pump power in our work, 266 µJ pulse energy obtained in our work is the highest of a 1.6-µm Raman laser which is approximately five times bigger than the result of 47 µJ obtained by Jelínek et al. [9].

2 Experiment setup

Figure 1 shows the experimental setup of the diode-pumped actively Q-switched ceramic Nd:YAG/SrWO4 intracavity Raman laser. The pump source was a 50-W fiber-coupled 808-nm laser diode with the core diameter of 400 µm and the numerical aperture of 0.22. A focusing lens system with a focal length of 50 mm and a coupling efficiency of 95 % was used to focus the pump beam into the laser crystal, and the focus spot diameter in the laser crystal was 400 µm. M1 was a concave mirror with a curvature radius of 1000 mm. As shown in Fig. 2a, it was coated for high reflection (HR) at 1444 and 1664 nm (R > 99.8 %) and high transmission (HT) at 808 nm (T > 95 %). The output coupler M2 was a plane mirror. The transmission spectrum of M2 is shown in Fig. 2b, and it was coated for HR at 1444 nm (R > 99.8 %) and partial reflection (PR) at the first-Stokes wavelength 1664 nm (R = 85 %). For suppressing the compete emissions around 1.06, M1 was also HT coated for 1064 nm. Additionally, for suppressing the 1.3-μm laser emission, M1 and M2 were all HR (R > 99.9) coated for the 1319-nm wavelength operation. Also, for the inhibition of the first-Stokes generation at 1.5 μm corresponding to the 1.3-μm fundamental wavelength, both mirrors were HR (R > 99.9) coated for the 1.5-μm wavelength region. The laser medium was a 1.0 at.% Nd:YAG ceramic with a length of 5 mm and a diameter of 3 mm. The Raman active medium was a SrWO4 crystal with a length of 35 mm. The crystals used in our experiment were both supplied by State Key Laboratory of Crystal Materials of Shandong University. Both sides of the Nd:YAG ceramic and SrWO4 crystals were antireflection (AR) coated at 1444 and 1664 nm (R < 0.2 %). The entrance face of the Nd:YAG ceramic was also HT coated at 808 nm. Both crystals were wrapped with indium foil and mounted in water-cooled Cu blocks. The water temperature was maintained at 18 °C. A 35-mm-long acousto-optic (AO) Q-switch (Gooch & Housego Company) driven at 41 MHz center frequency with 15 W of rf power was placed between the ceramic Nd:YAG and the SrWO4 crystal, and both surfaces of the AO were coated for HT at 1444 nm. The overall laser cavity length was approximately 12 cm. A dichroic mirror was used to block the 1444-nm fundamental laser. The average output power was measured by a power meter (Molectron PM10) connected to Molectron EPM2000.

Fig. 1
figure 1

Experimental arrangement of the diode-pumped, actively Q-switched intracavity ceramic Nd:YAG/SrWO4 Raman lasers

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Fig. 2
figure 2

a Transmission spectrum of M1 and b transmission spectrum of M2

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3 Results and discussion

Firstly, the characteristic of the diode-pumped actively Q-switched ceramic Nd:YAG laser at 1444 nm was studied, and three output couplers with different transmissions of 0.5, 1 and 3 % at 1444 nm were employed instead of the Raman output coupler mentioned above. Firstly, the spectrum of the laser is measured, and the spectral information of the laser recorded by the optical spectrum analyzer (AQ-6315A.350–1750 nm) with a resolution of 1 nm is shown in Fig. 3. It can be seen that the laser emission is at 1444 nm, and no other emissions appeared. Without the AO, the relationships between output powers and pump powers with different output couplers under the PRF of 10 kHz are shown in Fig. 4a; as is shown, the optimum transmission of the output coupler for the 1444-nm operation is found to be 0.5 %. The dependence of the output power on the pump power with the T = 0.5 % output coupler at pulse repetition frequencies of 1, 2, 5 and 10 kHz is shown in Fig. 4b. Under a pump power of 20.5 W, the average output power at 1444 nm is 2.01, 2.31, 2.96 and 3 W at PRFs of 1, 2, 5 and 10 kHz with the T = 0.5 % output coupler, respectively.

Fig. 3
figure 3

Optical spectrum for the actively Q-switched ceramic Nd:YAG laser operating at 1444 nm

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Fig. 4
figure 4

a Average output powers of the 1444-nm laser with respect to the pump power at PRF of 10 kHz with different OC. b Average output powers of the 1444-nm laser with respect to the pump power at PRFs of 1, 2, 5 and 10 kHz with T = 0.5 % OC

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When the Raman output coupler M2 was used, the eye-safe Raman laser operation at 1664 nm was generated. Firstly, the spectrum of the Raman laser was measured. The spectral information of the Raman laser which was recorded by a wide range optical spectrum analyzer (AQ-6315A.350–1750 nm) with a resolution of 1 nm is shown in Fig. 5. It can be seen that the fundamental laser emission is at 1444 nm and the first Stokes is at 1664 nm. The frequency shift between the Raman laser and the fundamental laser is 921 cm−1, which is in agreement with the Raman shift of the SrWO4 crystal [20].

Fig. 5
figure 5

Optical spectrum for the actively Q-switched ceramic Nd:YAG/SrWO4 Raman laser operating at 1664 nm

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Figure 6 shows the dependence of output powers of the first-Stokes Raman generation on pump powers at PRFs of 1, 2, 5 and 10 kHz. In the experiment, when the pump power is higher than 21 W at PRFs of 1 and 2 kHz, the laser crystal has been destroyed, so the highest pump power at PRFs of 1 and 2 kHz was limited to 21 W. At a pump power of 27.7 W and a PRF of 10 kHz, the maximum output power is as high as 1.16 W, corresponding to optical conversion efficiency from diode laser to the first Stokes of 4.2 %. It is obvious that the output power is strongly depended on the PRF. Also the average output power goes saturated at higher pump power under PRFs of 1, 2 and 5 kHz. However, no saturation is found under the PRF of 10 kHz. The main reason for this problem may be self-focusing, which was discussed by Chen et al. [16].

Fig. 6
figure 6

Average output powers of the 1664-nm laser with respect to the pump power at PRFs of 1, 2, 5 and 10 kHz

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Figure 7 gives the relationship between the pulse energies of the Raman laser and the pump powers. The maximum pulse energies of as high as 266, 150.5, 189 and 116 µJ were achieved at PRFs of 1, 2, 5 and 10 kHz. To our knowledge, 266 µJ is the highest pulse energy of a 1.6-µm Raman laser.

Fig. 7
figure 7

Pulse energies of the 1664-nm laser with respect to the pump power at PRFs of 1, 2, 5 and 10 kHz

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The characteristic of the pulse width was studied for investigating the phenomenon of pulse shortening in the SRS process. Figure 8 shows the typical single-pulse shapes of the fundamental and Raman pulses monitored by a Tektronix digital phosphor oscilloscope (TDS 5052B, 5 G Samples/s 500-MHz bandwidth) at a pump power of 27.7 W and a PRF of 5 kHz. The pulse duration of the depleted fundamental and the first-Stokes line were measured to be 93 and 5 ns, respectively. The corresponding peak power is 38 kW which is much higher than the results reported by Jelínek et al. [9] and Jelínková et al. [8]. The results shown in Fig. 8 indicate that SRS frequency conversion leads to pulse shortening of the Stokes component. Also in comparison with the previous results obtained in Raman laser demonstrations which are pump by Nd-doped lasers operating at 1.06 and 1.3 µm, the phenomenon of pulse shortening in our experiment is much significant [10–19].

Fig. 8
figure 8

Typical pulse shapes of the fundamental and Raman fields at the pump power of 27.7 W and a PRR of 5 kHz

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4 Summary

In summary, we demonstrated a high-energy diode-pumped actively Q-switched intracavity Raman laser with SrWO4 as the Raman active medium, and the first-Stokes Raman generation at 1664 nm was achieved. A maximum output power of 1.16 W was obtained at a PRF of 10 kHz, corresponding to an optical-to-optical conversion efficiency of 4.2 %. The maximum pulse energy of as high as 266 µJ was achieved at the PRF of 1 kHz.