1 Introduction

Solid-state Raman lasers based on stimulated Raman scattering (SRS), as a practical and efficient nonlinear frequency conversion method, can generate laser output at new wavelength. The spectra obtained with SRS in crystalline media cover wide wavelength range from ultraviolet to infrared [1,2,3,4], depending on the fundamental wavelength and Raman medium used. Compared with liquid and gas Raman gain media, crystalline Raman media have many advantages, such as large Raman gain coefficient, high thermal conductivity, good mechanical properties, high optical damage threshold, and chemical stability [4]. Therefore, with emergence of excellent crystalline Raman media, solid-state Raman lasers based on crystalline Raman media have attracted considerable attentions, and have a practical potential in laser spectroscopy, atmospheric monitoring, and remote sensing.

In the visible and near-infrared spectral region, the majority of solid-state Raman laser with high conversion efficiency have been demonstrated. For instance, Raman conversion efficiencies in Ba(NO3)2 [5], BaWO4 [6, 7], diamond [8, 9], are approaching quantum limit. Raman gain coefficient in crystalline Raman media is a key parameter, which is relevant to the Raman threshold and Raman conversion efficiency. In theory, Raman gain coefficient is closely related to the wavelength of the pump laser. Experimentally, many measurements of Raman gain coefficient in crystalline media have been performed in visible to near-infrared spectral region [10,11,12,13,14]. These measurements indicated that Raman gain coefficient declines towards longer pump wavelengths. This means that realization of Raman laser in the mid-infrared and longer wavelength range is difficult, due to smaller Raman gain coefficient.

Even so, realization of crystalline Raman lasers in mid-infrared spectral region has been carried out, owing to their potential application. Batay et al. realized self-Raman conversion in Tm:KY(WO4)2 crystal [15]. The Raman conversion from the fundamental component at 1950 nm to the first Stokes wavelength at 2365 nm, in the passively Q-switched Tm:KY(WO4)2 laser, was observed. Sabella et al. reported a pulsed mid-infrared diamond Raman laser with tunable output wavelength range from 3.38 to 3.80 μm, and 80 μJ pulse energy was generated [16]. Kuzucu presented a high average-power external-cavity BaWO4 Raman laser at 2602 nm [17]. In our previous works, with a BaWO4 crystal as Raman gain medium in intracavity configurations, the first Stokes outputs at 2533 and 2360 nm were obtained, respectively [18, 19]. Among the commonly crystalline media, yttrium vanadate crystals (YVO4) is a promising Raman gain medium in near-infrared region. Especially, Nd:YVO4, as an excellent self-Raman host material and laser gain medium, has been widely used in diode-pumped CW, actively and passively Q-switched self-Raman lasers [20,21,22,23,24,25,26]. The goal of the work described here is to realize Raman conversion in YVO4 crystal in mid-infrared spectral region.

In this paper, we demonstrate a mid-infrared 2.4 μm YVO4 Raman laser intracavity-pumped by a diode-pumped actively Q-switched Tm:YAP laser at 1990 nm. To the best of our knowledge, this is the first time that YVO4 Raman conversion in a 1990 nm Tm:YAP laser has been presented. With an incident diode pump power of 10.9 W, an average output powers of 272 mW for the first Stokes component at 2.4 μm are obtained at a PRR of 1 kHz. The corresponding pulse width and single pulse energy of the first Stokes component at 2.4 μm are approximately 15 ns and 0.27 mJ, respectively. Broadening of the first Stoke spectrum is observed for the Raman laser. The spectrum bandwidth is about 10 nm.

2 Experimental setup

In this experiment, an intracavity configuration is adopted. The experimental setup of the mid-infrared YVO4 Raman laser intracavity-pumped by a diode-pumped Tm:YAP laser is shown in Fig. 1. To avoid confusion, 1990 nm “pump” laser for the intracavity YVO4 Raman laser is called the “fundamental” in this experiment. Compared with external cavity implementation, the intracavity configuration provides higher laser energy intensity in the Raman resonant cavity so that it is relatively easy to exceed the Raman threshold intensity. The pump source is a fiber-coupled continuous-wave diode laser operating at 794 nm, with a fiber core diameter of 400 μm and a numerical aperture (NA) of 0.22. The pump beam from the diode laser is unpolarized. An optical focusing system, composed of two identical plane-convex lenses with a focal length of 25 mm, is utilized to reimage the diode pump beam into the Tm:YAP crystal. The pump beam waist radius in the Tm:YAP crystal is approximately 200 μm. The coupling efficiency of the optical focusing system is approximately 95%.

Fig. 1
figure 1

Configuration of the mid-infrared YVO4 Raman laser intracavity-pumped by a diode-pumped Tm:YAP laser

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An a-cut Tm:YAP crystal, as laser gain medium, has dimensions of 3 mm × 3 mm × 10 mm. The crystal is wrapped in indium foil, and held in a water-cooled copper heat sink with a thermoelectric cooler to maintain a temperature in 16.0 ± 0.1 °C. The Tm:YAP crystal is doped with 3.0 at% Tm3+. Both end faces of the Tm:YAP crystal are polished and antireflection coated for the fundamental wavelength at 1990 nm, and for the diode pump wavelength at 794 nm. An a-cut YVO4 crystal with dimensions of 4 mm × 4 mm × 30 mm is used as Raman gain medium. The YVO4 crystal is provided by Castech Inc. Both end faces of the YVO4 crystal are antireflection coated in the range of 1800–2550 nm (R < 3%). It is wrapped with indium foil, and mounted on a water-cooled copper heat sink, whose temperature is maintained in a temperature of 18 ± 0.1 °C by a thermoelectric cooler. A 30-mm-long fused silica acousto-optic Q-switch is antireflection coated at 2 μm, and driven at 27.12 MHz center frequency by 50 W of RF power.

The resonant cavity is devised to obtain the first Stokes emission. The flat input mirror M1 has high transmission at diode pump wavelength 794 nm (T > 99.7%), and high reflectivity at the fundamental wavelength 1990 nm (R > 99.7%). The mirror M3, used as Raman output coupler for the first Stokes component, is coated for high reflectivity in the range of 1900–2015 nm (R > 99.5%), and partial transmission at 2418 nm (T = 8%), with a curvature radius of 300 mm. The flat intracavity mirror M2 has high transmission at 1990 nm (T > 98%), and high reflectivity in the range of 2350–2600 nm (R > 99.8%). The input mirror M1 and mirror M3 compose the fundamental laser cavity. The intracavity mirrors M2 and mirror M3 compose the Raman resonant cavity. All of the cavity mirrors are made of CaF2. The physical length of the fundamental laser cavity is about 145 mm, and the physical length of the Raman laser cavity is about 40 mm. A 45° flat dichroic mirror M4 is utilized to separate the residual fundamental and the first Stokes components.

3 Experimental results and discussion

The spectral information of the Raman laser in Q-switched operation is monitored by a commercial grating monochromator (Omni-λ 300, resolution of the spectrum is 0.1 nm) with an InAs detector. The average output power is measured by a power meter (Coherent, PM10). The pulse temporal behaviors of the fundamental and the first Stokes are recorded by a 300 MHz bandwidth digital oscilloscope (Tektronix, TDS3032B) with a > 100 MHz bandwidth IR detector (Vigo, PVM-10.6).

The diode-pumped actively Q-switched Tm:YAP laser at 1990 nm is investigated first. An output coupler with T = 5% at 1990 nm is used instead of the Raman cavity output coupler in Fig. 1. Figure 2 shows the average power and pulsed width (FWHM) at 1990 nm versus the incident diode pump power at 794 nm for a PRR of 1 kHz. With an incident pump power of 13.1 W, an average output power of 1.27 W and a pulse width of 196 ns are obtained. The absorbed power is 72% of the incident diode pump power.

Fig. 2
figure 2

Average output power and pulse width at 1990 nm versus incident diode pump power for a PRR of 1 kHz

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The operation of the Raman laser is investigated at a PRR of 1 kHz. The fundamental polarization is linear, along the c-axis of the YVO4. The output spectrum of the YVO4 Raman laser is shown in Fig. 3. It is recorded at an incident pump power of 9.7 W. The central wavelengths of the fundamental and first Stokes are 1990 and 2418 nm, respectively. The corresponding Raman frequency shift is ~ 890 cm−1, which is in good agreement with the frequency shift of a-cut YVO4 crystal reported in [20, 27]. Since the coatings of all cavity mirrors are optimized for the first Stokes component, and the Raman gain coefficient is smaller for the second Stokes, the second Stokes emission is not detected in this experiment. The output spectrum of the first Stokes laser is shown alone in Fig. 4. Compared with the fundamental laser spectrum, the spectrum of the first Stokes laser is broader. The linewidth (FWHM) of the first Stokes emission is approximately 10 nm. Similar experimental results have been described and discussed in [28, 29].

Fig. 3
figure 3

Optical spectrum of the YVO4 Raman laser at an incident diode pump power of 9.7 W for a PRR of 1 kHz

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Fig. 4
figure 4

Output optical spectrum of the first Stokes for the mid-infrared YVO4 Raman laser

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Figure 5 shows the average output powers and corresponding pulse width at the first Stokes wavelength of 2418 nm as function of the incident diode pump power. An average output power of 272 mW at 2418 nm is obtained at an incident diode pump power of 10.9 W. The optical efficiency with respect to incident diode pump is about 2.5%. In this experiment, to prevent damage to the coating of the intracavity mirror M2, the incident diode pump power does not continue to increase. If quality of the mirror coating can be improved, the average output power of the Raman laser will continue to grow. The mode matching between the fundamental and the first Stokes is not easily achieved in the intracavity configuration, which influences the Raman conversion efficiency. The Raman gain is generally proportional to the length of Raman medium. If a longer YVO4 crystal is used, the conversion efficiency of the Raman laser will be improved.

Fig. 5
figure 5

Average output power and pulse width at the first Stokes wavelength of 2148 nm versus incident diode pump power for a PRR of 1 kHz

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Pulse shapes for the fundamental and the first Stokes components are measured at an incident diode pump power of 10.9 W (Fig. 6). Once the first Stokes laser pulse arises, the envelope of the fundamental laser pulse will change. The depletion of the fundamental laser pulse results in building up of the first Stokes pulse, so the time trace of the fundamental pulse is not symmetric. The spectral FWHM of the first Stokes laser is approximately 15 ns. The corresponding single pulse energy and peak power are calculated to be 0.27 mJ and 18 kW, respectively. Figure 7 shows the output beam profiles of the first Stokes laser at a maximum output power of 272 mW, which are recorded by a commercial beam analyzer (Electrophysics, Micron Viewer 7290A). The far-field intensity distribution of the first Stokes beam is near-Gaussian and close to fundamental transverse electromagnetic mode (TEM00) (Fig. 7a), owing to the intracavity Raman beam-cleanup effect [30].

Fig. 6
figure 6

Typical oscilloscope traces for the fundamental and first Stokes pulses at an incident pump power of 10.9 W

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Fig. 7
figure 7

Far-field intensity distributions of the first Stokes beam at an average output power of 272 mW: a two-dimensional distributions; b three-dimensional distributions

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4 Conclusions

In conclusion, laser performance of the mid-infrared YVO4 Raman laser, intracavity-pumped by a diode-pumped actively Q-switched Tm:YAP laser at 1990 nm, has been demonstrated. An a-cut 30-mm-long YVO4 is used as Raman gain medium in the intracavity configuration. With an incident diode pump power of 10.9 W, an average output power of 272 mW at the first Stokes wavelength of 2418 nm is obtained for a PRR of 1 kHz. The optical conversion efficiency from incident diode pump to the first Stokes is about 2.5%. The pulse width and single pulse energy of the first Stokes are approximately 15 ns and 0.27 mJ, respectively. And the broadening of output spectrum for the first Stokes component is observed in this experiment. The optical conversion can be improved with an external cavity configuration because the mode matching between the fundamental and the Raman beam can be realized easily. It is potential that the average output power of the Raman laser is increased by optimizing cavity design and employing high-quality mirrors.