1 Introduction

Pulsed solid-state lasers operating around 2 μm are useful for a variety of applications such as in remote sensing and medical field [1, 2]. They can also be used as pump sources for optical parametric oscillator (OPO) which can efficiently convert radiation to the 3- to 12-μm mid-infrared spectral range [3]. Passively Q-switched (PQS) lasers with saturable absorbers (SAs) were usually accompanied with significant advantages such as inherent compactness, simplicity and low-cavity design cost. Until now, 2-μm PQS lasers with different SAs have been presented using Tm-doped [4, 5] and Tm,Ho co-doped crystals [6, 7]. Compared with the single Tm-doped and Tm,Ho co-doped laser crystals, single Ho-doped laser crystals have larger emission cross section and the lower quantum defect between pump and laser. In 2001, Tsai et al. [8] 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. [9] proved the probability of using PbSe-doped phosphate glass as SA in Ho:YAG laser, and the pulse width of 85 ns was achieved. In 2010, using a Cr2+:ZnSe SA with the initial transmittance 70 %, Terekhov et al. [10] demonstrated a compact and efficient PQS Tm-fiber-Ho:YAG laser with pulse energy of 3 mJ and duration of 7 ns. In 2013, Chen et al. [11] reported a PQS Tm:YLF pumping Ho:YAG laser with Cr2+:ZnS SA, and the single pulse energy was as high as 2.47 mJ with the pulse duration of 35 ns. In 2014, the PQS Ho:YAG laser in-band pumped by a diode-pumped Tm:YLF has been demonstrated with the shortest pulse width of 632 ns and pulse energy of 13.3 μJ [12].

Ho:YAG and Ho:LuAG crystals are both oxide crystals, so their characteristics are similar. However, compared with Ho:YAG crystal, Ho:LuAG has large manifold splitting and low thermal occupation for the lower laser level owing to its higher crystal field. Ho:LuAG has the highest branching ratio for a level in the 5 I 7 manifold at room temperature among all the garnet laser materials considered [13]. And the slight molar mass difference of 5.8 % between Ho and Lu (46.7 % between Y and Ho) makes a weak decrease in thermal conductivity of LuAG with Ho3+ ion [14]. These advantages make Ho:LuAG crystal be suitable to generate 2-μm laser at room temperature. To the authors’ best knowledge, PQS laser performances of Ho:LuAG crystal have not been reported as widely as Ho:LuAG laser in the literature [15, 16]. In this paper, we report the PQS performance of Ho:LuAG laser using Cr2+:ZnS as SA for the first time. Compared with other SAs, the Cr2+:ZnS material naturally possesses higher optical damage threshold (1.5 J/cm2) [17] and larger thermal conductivity (0.27 W cm−1 K−1) [18], which lead to weaker thermal lens effect. Particularly, Cr2+:ZnS SAs are very promising for passively Q-switching of the rare-earth lasers owing to about two orders of magnitude greater absorption and emission cross sections than that of the rare-earth ions [19].

In CW operation, we obtained the maximum output power of 2.35 W at the center wavelength of 2,100.22 nm, corresponding to a slope efficiency of 37.2 %. Meanwhile, under PQS operation, we obtained a stable PQS Ho:LuAG pulse radiation at the maximum pump power of 16 W with the center wavelength of 2,076.56 nm at the output coupler of T = 30 %. The 36-ns short pulse duration was obtained as well as the 1.54 mJ maximum pulse energy. The output beam is close to fundamental transverse electromagnetic mode (TEM00).

2 Experimental setup

The experimental setup is shown in Fig. 1. The pump source is a diode-pumped Tm:YLF laser at 1,907.6 nm with maximum output power of 16.0 W. The pump beam radius in the center of the Ho crystal was about 0.4 mm. The total transmittance efficiency of the beam shaping system was more than 92 % at 1,907.6 nm. The size of Ho:LuAG crystal is Φ5 × 30 mm3, corresponding to Ho-doped concentration of 0.5 %at., and the end surfaces of crystal were coated with anti-reflection film on 1.91 μm (R < 0.5 %) and 2 μm (R < 0.3 %). The Ho:LuAG crystal wrapped with indium foil was installed on a copper heat sink which was cooled by water. Temperature of the Ho:LuAG crystal was held at 17 °C. A simple L-shaped Plano–Plano cavity was adopted for the Ho:LuAG resonant cavity. Plane mirror M1 and 45° dichroic mirror M2 were coated with 1.91 μm anti-reflection coatings and 2 μm high-reflection coatings. The plane output coupler M3 has, respectively, transmittance of 30 and 25 % at ~2 μm. The distances between M1 and M2, and M3 and M2 are 45 and 70 mm, respectively. The physical length of resonant cavity is 115 mm. A 2-mm thickness Cr2+: ZnS SA was cut into 9 × 9 mm2 cross section with small signal transmittance of ~82 % at ~2 μm, and the end surfaces of the crystal were coated with anti-reflection film on 2 μm (R < 0.3 %). It was mounted in a copper heat sink which was cooled by water. The SA is placed in the resonator 10 mm away from the output coupler M3. The beam radius of the Ho:LuAG laser inside the resonator was calculated by using the software MATLAB. In the middle of the Ho:LuAG crystal, the radius of the oscillating mode was about 402.2 μm, and the radius of the oscillating mode on the Cr2+:ZnS SA was about 437.5 μm.

Fig. 1
figure 1

Experiment setup of the PQS Ho:LuAG laser

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3 Results and discussion

The CW output power of Ho:LuAG laser as a function of the total incident pump power is shown in Fig. 2a with the output coupler transmittance of T = 30 % and T = 25 %. The maximum output power of 2.35 W is obtained for T = 30 %, corresponding to slope efficiency of 37.2 %. When output coupler transmittance was changed to T = 25 %, the maximum output power was 2.19 W, corresponding to a slope efficiency of 34.2 %. The unstable Plano–Plano resonator leads to the high laser threshold.

Fig. 2
figure 2

Output power versus total incident pump power, a CW and b PQS operations

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We investigated the output characteristics of the PQS Ho:LuAG laser after inserting the SA Cr2+:ZnS into the resonant cavity. Figure 2b shows the average output power for the output coupler transmittance T = 30 % and T = 25 % with the total incident pump power. From Fig. 2b, it is noted that the average output power increases linearly with the total incident pump power. The maximum average output power of 1.14 W is obtained for the output coupler transmittance T = 30 %, corresponding to a slope efficiency of 24 %. Meanwhile, for T = 25 %, the maximum average output power is 0.69 W, corresponding to a slope efficiency of 18.4 %. The laser thresholds of 12.3 and 13.2 W are obtained for the output coupler transmittance of T = 30 % and T = 25 %, respectively. Here, it is worth to mention that the single-pass absorption efficiency of Ho crystal was 66 % in lasing operation.

The CW laser spectrum recorded with Spectrum Analyzer (Bristol Instruments 721) at the pump power of 10 W is shown in Fig. 3a. The CW lasers with two different output couplers T = 30 % and T = 25 % have same central wavelength of 2,100.22 nm. For the PQS mode, the wavelength was measured by a WDM1–3 grating monochromator and a LeCroy WaveJet 332 digital oscilloscope (350 MHz bandwidth) with a fast response InGaAs detector. Compared with CW operation, PQS lasers generate obviously red shift of the central wavelengths and the emission central wavelength of 2,076.56 and 2,097.74 nm for the two different output couplers T = 30 % and T = 25 % were obtained as shown in Fig. 3b. In the PQS mode, the losses in the resonator increases drastically due to the inserting of the Cr2+:ZnS SA, leading to a higher population inversion density while the laser pulse is being constructed; thus, the gain coefficient changes related to emission cross section and emitting wavelength [20].

Fig. 3
figure 3

Laser spectra in a CW and b PQS operations

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Figure 4a shows the pulse width versus the total incident pump power with two different output coupler transmittances. For the output coupler transmittance of T = 30 % and T = 25 %, the pulse width maintains at 39 and 36 ns, respectively, and keeps almost constant with the total incident pump power. The output coupler transmittance has obvious effect on the pulse width. The lower output coupler transmittance leads to the narrower pulse width. Owing to the decrease in the output coupler transmittance, the lower cavity loss decreases the turn threshold number density. Thus, narrow pulse width was achieved due to more inversion population during the formation of Q pulse under the same effective pump power [21]. Figure 4b shows the pulse repetition rate versus the total incident pump power for two different output coupler transmittances. For the output coupler of T = 30 % and T = 25 %, the pulse repetition rate increases from 167 to 790 Hz and from 156 to 554 Hz, respectively. Under the condition of the same output coupler transmittance, the pulse repetition rate increases with the total incident pump power and it decreases with the transmittance of the output coupler under the same total incident pump power.

Fig. 4
figure 4

a Pulse width and b pulse repetition rate versus total incident pump power

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For the Ho:LuAG laser with different output couplers of T = 30 % and T = 25 %, the output pulse trains and single pulse at maximum total incident pump power are shown in Fig. 5a and b, respectively. The pulse amplitude variation is <5 % among pulse to pulse. Moreover, the output pulses show fine stability with the change of the total incident pump power.

Fig. 5
figure 5

Single-pulse profile and pulse trains for output coupler transmittance of a T = 30 % and b T = 25 %

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Figure 6 shows the pulse energy versus total incident pump power. For the Ho:LuAG laser with different output couplers of T = 30 % and T = 25 %, the pulse energy increase from 1.36 to 1.54 mJ and from 1.28 to 1.45 mJ with the increase in the total incident pump power, respectively. Using the same pump power, the pulse energy increases with the increase in output coupler transmittance. For the output coupler of T = 30 %, the transverse beam quality was obtained by 90/10 knife-edge method at the maximum pump power of 16 W, as shown in Fig. 7. The beam quality factor is up to M 2 = 1.05, which indicated the output beam is close to fundamental TEM00. A low spatial resolution laser beam profile (insert in Fig. 7) at the highest pump power was observed by a pyroelectric camera. The damage threshold of the SA was approximately 7.21 W/mm2. Moreover, a Glan prism was used to detect output laser polarization state, and then, vertically polarized output light was found.

Fig. 6
figure 6

Pulse energy versus total incident pump power

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Fig. 7
figure 7

Beam radius versus the distance from a lens at 10.4 W pump power. Insert typical 2D beam profiles

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4 Conclusion

We reported resonant pumped CW and PQS Ho:LuAG lasers with two different output couplers. For the output coupler of T = 30 %, the maximum continuous wave output power of 2.35 W at 2,100.22 nm was obtained, corresponding to a slope efficiency of 37.2 %. Then, inserting a Cr2+:ZnS crystal as SA, for the output coupler of T = 30 % and T = 25 %, the center wavelength decreased from 2,100.22 to 2,076.56 nm and from 2,100.22 to 2,097.74 nm, respectively. The minimum pulse width of 36 ns was obtained for T = 25 %, and highest pulse energy of 1.54 mJ was achieved for T = 30 %, corresponding to the pulse repetition rate of 554 and 790 Hz, respectively. The beam quality factor of M 2 was 1.05, and the output beam is close to fundamental TEM00. For higher output power of Q-switch pulse, the pump power was increased on the situation of the improvement of the output coupler transmittance and the resonator spot size at the position of Cr2+:ZnS crystal.