1 Introduction

Stimulated Raman scattering (SRS) in crystalline materials is one of the most efficient methods for extending the spectral range of existing lasers based on a third-order nonlinear optical process [1–6]. With the development of high-quality Raman crystals in recent years, there has been a rapid increase of interest in solid-state Raman lasers [7–25]. Actively Q-switched Raman lasers, with the advantages of high conversion efficiency and output power, have been widely investigated [16–25]. Compared with the actively Q-switched Raman lasers, the passively Q-switched Raman lasers are compact, simple, inexpensive, and easy to operate. In 2004, Chen reported the generation of efficient subnanosecond self-stimulated Raman laser pulses by a diode-pumped passively Q-switched Nd:GdVO4/Cr4+:YAG laser. At an incident pump power of 2.0 W, the self-Raman laser produced stable 750-ps pulses at 1175.6 nm with the pulse energy of 6.3 μJ at a 22-kHz repetition rate [26]. By using Nd:YVO4 as the self-Raman crystal, they obtained stable 710-ps pulses at 1178.6 nm with 7.2 μJ of pulse energy at a 17-kHz repetition rate with an incident pump power of 2.0 W [27]. In 2005, Yb:YVO4/Cr4+:YAG self-Raman laser was realized [28]. The 1119.5-nm laser pulse energy was 3.6 μJ and the pulse duration was 6 ns at a repetition rate of 25 kHz. The passively Q-switched Yb:KLu(WO4)2 self-Raman laser with 0.4 W output power at 1137.0 nm was reported in 2006 [29]. In 2009, Basiev et al. realized the passively Q-switched self-Raman laser oscillation in the SrMoO4:Nd3+ crystal pumped by LD at 804 nm, and a pulse energy of 21 μJ was obtained at 1163 nm [30]. In the same year, passively Q-switched self-Raman output from a composite Nd:YVO4/YVO4 laser using the Cr:YAG saturable absorber was reported. When the initial transmission of the Cr:YAG was 95 %, the Raman output of 570 mW was observed at 1176 nm [31]. In 2010, Basiev et al. reported a diode-pumped \(\mbox{LiF:F}_{2}^{-}\) passively Q-switched Nd3+:SrMoO4 Raman laser. When the pulse energy was as high as 60 μJ, the repetition rate was less than 5 kHz. When the repetition rate was more than 30 kHz, the pulse energy was less than 1 μJ. And the maximum average power was less than 300 mW [32].

As a new promising Raman-active crystal, SrWO4 has attracted much attention for its good mechanical and optical properties. The Raman mode at 921 cm−1 has the strongest intensity, and its linewidth is 2.7 cm−1 at room temperature [33]. Its steady-state Raman gain coefficient is 5.0 cm/GW at 1064 nm. The spontaneous Raman spectroscopy of SrWO4 crystal was investigated by Basiev et al. in 2000 [34]. Shortly thereafter, the intracavity Raman generation employing SrWO4 as the Raman-active material was studied using an alexandrite free-running multimode laser as the pump source [32, 35, 36]. In 2006, Ding et al. developed an external cavity Raman converter with SrWO4 Raman crystal [37]. In 2008, a highly efficient diode-pumped actively Q-switched intracavity Raman laser with SrWO4 as the Raman active medium was presented by Chen et al. [24]. As high as 23.8 % diode-to-Stokes optical conversion efficiency was obtained with an incident pump power of 7.17 W and a pulse repetition rate of 15 kHz. Then Chen et al. demonstrated a diode-side-pumped actively Q-switched Nd:YAG/SrWO4 intracavity Raman laser, and an average output power of 10.5 W at 1180 nm was obtained [25]. However, to our knowledge, no research on the LD end-pumped passively Q-switched intracavity Raman laser based on the SrWO4 crystal has been carried out.

In this paper, we present the output properties of intracavity Raman generation in a diode-pumped passively Q-switched Nd:YAG/SrWO4 laser with Cr4+:YAG as the saturable absorber. At an incident pump power of 14.9 W, the intracavity Raman laser system delivers 1.78 W average output at 1180 nm with a pulse repetition frequency (PRF) of 21.2 kHz. The highest pulse energy of 88.1 μJ is achieved at a pump power of 13.7 W. The average output power and pulse energy are much higher than the previously obtained values in diode-pumped passively Q-switched intracavity Raman lasers [26–32].

2 Experimental arrangement

Figure 1 shows the experimental arrangement of the diode-pumped passively Q-switched Nd:YAG/SrWO4 Raman laser. The pump source is a 25 W fiber-coupled 808 nm laser diode with a core diameter of 600 μm and a numerical aperture of 0.22. The resonator has a concave-plane configuration. The rear mirror M1 is a concave mirror with a curvature radius of 300 mm. It is coated for high reflection (HR) at 1064 and 1180 nm (R>99.8 %) and high transmission (HT) at 808 nm (T>96 %). Three output couplers (OC1, OC2 and OC3) with different reflectivities at 1180 nm (R=90 %, 84 %, and 73 %) are employed. All the output couplers are plane mirrors and coated for HR at 1064 nm (R>99.9 %). The laser gain medium is an Nd:YAG with 1.0 at.% Nd doping and dimensions of ∅4 mm×8 mm. The Raman active medium is a SrWO4 crystal with a length of 34 mm. Both sides of the Nd:YAG and SrWO4 crystals are coated for antireflection (AR) at 1064 and 1180 nm (R<0.2 %). The entrance face of the Nd:YAG is also coated for HT at 808 nm. Both crystals are wrapped with indium foil and mounted in water-cooled copper heat sinks. The water temperature is maintained at 20 ∘C. The three Cr4+:YAG crystals with different initial transmissions of 94 %, 87 %, and 81 % have antireflection coatings at 1000∼1350 nm on both faces. The overall laser cavity length is approximately 9 cm. A dichroic mirror is used to block the residual fundamental laser at 1064 nm. The average output power is measured by a power meter (Molectron PM3) connected to Molectron EPM2000. The spectral information is monitored by a wide range optical spectrum analyzer (Yokogawa AQ6315A, 350–1750 nm). The Raman pulse’s temporal behavior is recorded by a Tektronix digital phosphor oscilloscope (TDS 5052B, 5 G Samples/s, 500 MHz bandwidth) with a fast p-i-n photodiode.

Fig. 1
figure 1

Experimental arrangement of the diode-pumped passively Q-switched Nd:YAG/SrWO4 Raman laser

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3 Experimental results

Figure 2 depicts the optical spectra of the passively Q-switched Nd:YAG/SrWO4 Raman laser output, which is recorded at an incident pump power of 14.9 W for OC3 at T 0=81 % without the dichroic mirror. Corresponding to the fundamental laser at 1064 nm, the first-Stokes laser is at 1180 nm. For all pumping powers, no second Stokes is observed. It can be seen that the frequency shift between the first-Stokes and the fundamental laser is 921 cm−1, which is in agreement with the optical vibration modes of the tetrahedral \(\mathrm{WO}_{4}^{2-}\) ionic groups.

Fig. 2
figure 2

Optical spectra for the LD pumped passively Q-switched Nd:YAG/SrWO4 intracavity Raman laser

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The relations between the average output power of the first Stokes and the incident pump power are shown in Figs. 3, 4, 5 while OC1, OC2, and OC3 are used, respectively. The squares, circles, and triangles represent the results for T 0=94 %, 87 %, and 81 %, respectively. As seen from Figs. 3–5, the first-Stokes power increases with the increasing of the pumping power and the threshold decreases with the increasing of the Cr4+:YAG initial transmission. The highest average output power is 1.78 W with a PRF of 21.2 kHz for OC3 at T 0=81 %, which is obtained at a pump power of 14.9 W. The corresponding conversion efficiency from the diode pump power to Raman output power is 12.0 %. The highest pulse energy of 88.1 μJ is obtained while the output coupler OC3 and the Cr4+:YAG crystal of T 0=81 % are used at a pump power of 13.7 W, corresponding to a diode-to-Stokes conversion efficiency of 12.2 %. The obtained average output power and pulse energy are much higher than the previously reported values in LD-pumped passively Q-switched intracavity Raman lasers [26–32].

Fig. 3
figure 3

Average output power at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC1 is used

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Fig. 4
figure 4

Average output power at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC2 is used

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Fig. 5
figure 5

Average output power at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC3 is used

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There are two main reasons for the high pulse energy and the high average power. One is the good mechanical and optical properties of SrWO4 as the Raman active medium. The other is that the thermal effect of Cr4+:YAG crystals is reduced by a heat sink. At the beginning, we studied the characteristics of Raman laser without cooling the Cr4+:YAG crystals because they were too thin to be put in a heat sink like Nd:YAG and SrWO4 crystals. The laser had low efficiency, and the average output power was less than 1 W at T 0=81 %. The thermal effect in the Cr4+:YAG crystals became serious with increasing pump power, which impacted the stabilization of the resonator and impeded the increase of the Raman laser output power. So in the following experiment, the Cr4+:YAG crystal is stuck to a water-cooled copper heat sinks with a hole, and the water temperature is maintained at 20 ∘C. In this way, the Raman laser output power and conversion efficiency have significant increases.

Figures 6, 7, 8 give the pulse width with increasing incident pump power while OC1, OC2, and OC3 are used, respectively. Figures 9, 10, 11 show pulse repetition rate with increasing incident pump power while OC1, OC2, and OC3 are used, respectively. The squares, circles, and triangles represent the results for T 0=94 %, 87 %, and 81 %, respectively. As shown in Figs. 6–11, the pulse width decreases slightly and the pulse repetition rate increases with increasing pumping power. With decreasing initial transmission of the saturable absorber, the pulse width of the Raman laser becomes narrower, and the pulse repetition rate of the Raman laser becomes smaller.

Fig. 6
figure 6

Pulse width at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC1 is used

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Fig. 7
figure 7

Pulse width at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC2 is used

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Fig. 8
figure 8

Pulse width at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC3 is used

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Fig. 9
figure 9

Pulse repetition rate at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC1 is used

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Fig. 10
figure 10

Pulse repetition rate at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC2 is used

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Fig. 11
figure 11

Pulse repetition rate at 1180 nm with respect to the incident pump power for Cr4+:YAG different initial transmissions of 94 %, 87 %, and 81 % when OC3 is used

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As shown in Figs. 3–5, there are the obvious saturation effects for the output Raman laser power, and there is obvious relation between the saturation and the initial transmission T 0. The larger the initial transmission T 0 is, the easier the output power goes saturated. As seen from Figs. 9–11, at a given high pump power, the higher the initial transmission of the saturable absorber T 0 is, the higher the pulse repetition rate of the Raman laser is. And when the pulse repetition rate becomes higher, the laser goes through the Cr4+:YAG crystal more frequently, the accumulated heat in the Cr4+:YAG crystal becomes larger, the stabilization of the resonator becomes worse, and the output power at 1180 nm is easier to go saturated. So the Raman laser output power is easier to go saturated while initial transmission of Cr4+:YAG crystal gets higher, which can be seen from Figs. 3–5.

As seen from Figs. 3–11, the shortest pulse duration and simultaneously the highest pulse energy are obtained at the lowest initial transmission. We can find the reason from the basic theory of passively Q-switched lasers [38–40]. The initial population inversion density in the laser medium n i is determined by the following Eq. (1). The lower the initial transmission of the saturable absorber is, the larger the population inversion density in the laser medium n i is [38], the higher the output laser pulse energy is. Simultaneously, the larger the accumulated population inversion density is, the faster the photon population density varies when the Q-switched pulse is built up, the shorter the pulse duration is [38–40]. This is the reason for the fact that the shortest pulse duration and simultaneously the highest pulse energy are obtained at the lowest initial transmission of 81 %.

$$ n_{i} = \frac{\ln \left( \frac{1}{R_{L}} \right) + \ln \left(\frac{1}{T_{0}^{2}} \right) + L_{L}}{2\sigma l}$$
(1)

where R L is the reflectivity of the output coupler for the fundamental laser, σ is the stimulated emission cross section, l is the length of the laser medium, T 0 is the initial transmission of the saturable absorber.

The typical time shape for Raman laser pulse is shown in Fig. 12, which is recorded at a pump power of 13.7 W for OC3 at T 0=81 %. The pulse duration of the Raman lasers is approximately 5.5 ns.

Fig. 12
figure 12

Typical oscilloscope trace for the Raman pulse at 1180 nm

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4 Conclusions

In summary, the characteristics of a diode-pumped Cr4+:YAG passively Q-switched Nd:YAG/SrWO4 intracavity Raman laser has been investigated for the first time. With an incident pump power of 14.9 W, as much as 1.78 W of average output power at the Stokes wavelength of 1180 nm is generated with a PRF of 21.2 kHz. The highest pulse energy of 88.1 μJ is achieved at a pump power of 13.7 W. The obtained pulse energy and average power are much higher than those of the previously reported diode-pumped passively Q-switched intracavity Raman lasers.