1 Introduction

Pulsed solid-state lasers operating near 2 μm are useful for a variety of applications such as remote-sensing and laser medicine [1–3]. They can also be used as pump sources for optical parametric oscillator (OPO) which can efficiently convert radiation to the mid-infrared spectral range [4]. Generally, Q-switching of lasers is a simple and effective way of producing laser pulses at 2 μm. During Q-switched operation, resonator losses can be switched by using active or passive modulation schemes. Active Q-switching is obtained using electro-optic or acousto-optic devices to provide the required optical shutters. In active Q-switched lasers, laser architectures containing such elements suffer from higher cost, lack of compactness, and increased complexity. By contrast, passive Q-switching has advantages over active Q-switching in terms of inherent compactness, simplicity, and low cost of the cavity design. Up to now, 2 μm passively Q-switched (PQS) lasers have been widely demonstrated for Tm, Ho co-doped lasers [5, 6]. However, in order to achieve high power and high brightness laser output, those lasers need to be operated under liquid nitrogen temperature.

Direct resonant pumping of singly Ho-doped lasers to obtain 2 μm laser radiation allows for the advantages of high conversion efficiency, minimal thermal loading due to the low quantum defect between the pump and laser, and room-temperature operation availability. Therefore, singly Ho-doped laser is very suitable for PQS laser operations at room temperature. In the past few years, PQS Ho:YAG lasers have been demonstrated with different saturable absorbers (SA) based on semiconductor or nanometer materials. In 2001, Tsai et al. [7] inserted a Cr2+:ZnSe SA into a flash-lamp-pumped Ho:YAG laser resonator, and acquired 1.3 mJ pulse energy and 90 ns pulse duration. In the same year, Malyarevich et al. [8] proved the feasibility of using PbSe-doped phosphate glass as SA in Ho:YAG laser, and the pulse width of 85 ns was achieved. In 2010, by using a Cr2+:ZnSe SA, Terekhov et al. [9] demonstrated a compact and efficient PQS Ho:YAG laser with a pulse energy of 3 mJ and a pulse duration of 7 ns. In 2013, Chen et al. [10] reported a PQS Ho:YAG laser with a Cr2+:ZnS SA, and the single pulse energy was as high as 2.47 mJ with the pulse duration of 35 ns. In 2014, by using a graphene SA, a PQS Ho:YAG laser has been demonstrated with the pulse width of 632 ns and pulse energy of 13.3 μJ [11]. In 2015, Yao et al. [12] used Cr2+:ZnS to obtain a PQS Ho:YAG ceramic laser with the maximum energy of 0.6 mJ per pulse and the maximum pulse repetition frequency of 24.4 kHz.

Among various laser host materials, gadolinium vanadate (GdVO4) crystal doped with various ions represents a promising host material. The GdVO4 crystal has a large thermal conductivity (11.7 W/mK) which makes it favorable to be efficiently cooled down [13]. As the laser media in 2 μm spectral region, laser performances of Tm:GdVO4 and Tm, Ho:GdVO4 crystals were widely investigated [14, 15]. Nonetheless, there were few reports on Ho:GdVO4 lasers. Up to now, only continuous-wave (CW) and actively Q-switched Ho:GdVO4 lasers have been investigated [16]. However, PQS laser performances of Ho:GdVO4 crystal have never been reported. In this paper, to the best of our knowledge, we present the passively Q-switched Ho:GdVO4 laser resonantly pumped by a 1.94 μm Tm-fiber laser for the first time.

2 Experimental setup

The experimental setup is shown in Fig. 1. A 50 W FBG-locked Tm-fiber laser was used as a pump source of the Ho:GdVO4 laser. The pump wavelength was selected at 1942 nm. The fully unpolarized pump light was collimated and sent through a telescope consisting of two focusing lenses. The beam quality factor M 2 of the pump laser was about 1.8. The pump beam radius was measured to be 256 μm, resulting in a Rayleigh length (zr = πω 2 n/λM 2) of about 64 mm with refraction index of n = 1.96 inside the crystal. The Ho:GdVO4 crystal with doping concentration of 1.0 at% has a cross section of 4 × 4 mm2 and a length of 20 mm. Both end faces of the laser crystal were anti-reflection (AR) coated at 1.94 and 2.05 μm. The crystal was sandwiched between two water-cooled copper heat sinks using 0.1-mm-thick indium foil. The temperature of the cooling water was controlled at 290 K. The flat mirror M1 and the flat 45° dichroic mirror M2 had high reflectivity at 2.05 μm and high transmission at 1.94 μm. The output coupler M3 was plano-concave with the transmission of 40 % and the radius curvature of 200 mm. An AR-coated Cr2+:ZnS polycrystalline SA produced by a diffusion doped method was cut into 3 mm thickness with a small-signal transmission of about 83 %. The SA was placed in the resonator between M2 and M3, and it was also mounted in a copper heat sink which was cooled by water at 290 K. All distances between cavity elements are shown in Fig. 1. The total physical cavity length of the Ho:GdVO4 laser was 120 mm. The radius of the TEM00 mode on the SA was calculated to be about 270 μm.

Fig. 1
figure 1

Experimental setup of the passively Q-switched Ho:GdVO4 laser

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3 Experimental results and discussion

The power meter Coherent PM30 was used in this experiment. Without the SA, the CW output power of the Ho:GdVO4 laser is shown in Fig. 2. An output power of 9.3 W was obtained at the absorbed pump power of 32.6 W, corresponding to a slope efficiency of 34.1 % with respect to the absorbed pump power. The output power of the Ho:GdVO4 laser with the SA was investigated, as shown in Fig. 2. We obtained the maximum average output power of 8.4 W at the absorbed pump power of 32.6 W. The slope efficiency with respect to the absorbed pump power in PQS operation was found to be 31.1 %.

Fig. 2
figure 2

The output power of the Ho:GdVO4 laser

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The Q-switched laser pulse was detected by an InGaAs photodiode and recorded by a 350 MHz digital oscilloscope (Tektronix DPO5034B). Figure 3 showed the pulse repetition frequency (PRF) and pulse width under different absorbed pump power. As showed in Fig. 3a, the PRF increased monotonically with the increase in the absorbed pump power. The maximum PRF was 126.7 kHz at absorbed pump power of 32.6 W. As indicated in Fig. 3b, the pulse width decreased with the increase in the absorbed pump power. The minimum pulse width was 43.2 ns at absorbed pump power of 32.6 W. Figure 4a and b showed the oscilloscope trace of the minimum single pulse and the typical output pulse train, respectively. Figure 5 depicted the dependence of pulse energy on the absorbed pump power. The maximum pulse energy of 67.7 μJ was achieved at the absorbed pump power of 28.2 W, the corresponding pulse width was 69.8 ns and the PRF was 100.1 kHz. The pulse-to-pulse stability of the PQS Ho:GdVO4 laser was measured over a period of 10 min. The peak-to-peak intensity fluctuation was about 9.3 %.

Fig. 3
figure 3

a The repetition frequency and b pulse width versus the absorbed pump power

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Fig. 4
figure 4

a The typical expanded shape of the minimum single pulse; b the train of output pulses

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Fig. 5
figure 5

The pulse energy pulse versus the absorbed pump power

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To determine the beam quality factor M 2, we aligned the Ho laser radiation through a lens with 150 mm focal length. At the maximum average output power, the output beam radius was measured by a 90/10 knife-edge technique at several positions as shown in Fig. 6a. By fitting Gaussian beam standard expression to these data, the M 2 factors were determined to be 1.13 and 1.10 in x and y directions, respectively. Figure 6b showed the measured beam intensity distribution by the pyroelectric array camera (Spiricon’s Pyrocam III).

Fig. 6
figure 6

a The beam radius of the Ho:GdVO4 laser; b the typical 2D beam profiles

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The output spectrum of the CW and PQS Ho:GdVO4 laser were measured by a spectrum analyzer (Bristol Instruments 721). The spectra are shown in Fig. 7. With CW operation, the central wavelength was 2047.9 nm at the output power of 1.7 W, while the central wavelength of the Q-switched laser generated a slight redshift to 2049.1 nm at the average output power of 1.2 W.

Fig. 7
figure 7

The output spectrum of the Ho:GdVO4 laser with a CW, and b passively Q-switched operation

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4 Conclusion

In conclusion, we have demonstrated, for the first time to the best of our knowledge, a Cr2+:ZnS saturable absorber passively Q-switched Ho:GdVO4 laser operated at room temperature and resonantly pumped by a 1.94 μm Tm-fiber laser. The maximum average output power of 8.4 W was achieved with the absorbed pump power of 32.6 W, and the corresponding minimum pulse width was 43.2 ns and the maximum PRF was 126.7 kHz. The maximum pulse energy of 67.7 μJ was achieved at the absorbed pump power of 28.2 W, the corresponding pulse width was 69.8 ns, and the PRF was 100.1 kHz. The central wavelength of the Q-switched laser generated a slight redshift compared with that in CW operation, from 2047.9 to 2049.1 nm. In addition, the output beam had beam quality factors M 2 x  ~ 1.13 and M 2 y  ~ 1.10 at the maximum output level.