1 Introduction

Pulsed solid-state lasers based on Ho3+ ions operating slightly above 2 μm have potential applications in medicine, spectroscopy, remote sensing and as a pump source for mid-infrared optical parametric oscillators (OPO's) [1, 2]. Previously, stable pulsed holmium solid-state lasers were mostly realized using acousto-optic active Q-switches [3–6]. Additional electronics and complex mechanical components generally make the active Q-switching scheme inconvenient and costly in practical applications. Passively switched lasers possess advantages such as simple volume structure and low cost. However, only a few passive Q-switched Ho3+ solid-state lasers at 2.1 μm have been reported so far [7] because saturable absorbers in this wavelength range are rare and difficult to fabricate [8]. As for passive Q-switching, only Cr2+:ZnSe has ever been used as the saturable absorber in a Ho:YAG crystal laser. With this saturable absorber, they obtained a Q-switched Ho laser with 1.3 mJ pulse energy and ~90 ns pulse duration, but the efficiency was only 5 % [9]. With a Dirac-type electronic states distribution near the Fermi energy, graphene exhibits saturable absorption properties covering a ultra-broad wavelength range from visible to mid-IR. Moreover, graphene has outstanding linear and nonlinear optical properties, such as low threshold level of saturable absorpting and ultra-fast recovery time. Since first demonstration of mode locking with graphene near 1.5 μm in Er3+ fiber laser was reported by Bao et al. [10]. Q-switched or mode-locked operations have been widely investigated in many other systems [11–13]. However, to the best of our knowledge, there is no report of Q-switched or mode-locked Ho3+ laser using graphene as the saturable absorber yet.

In recent years, polycrystalline ceramics as laser gain host materials have attracted enormous interests in laser community due to a number of important advantages over single crystals, including rapid and large volume fabrication, extreme flexibility in doping concentration profile and sample structure [14, 15]. With the technical improvement in fabrication, high-performance laser operations at ~2.1 μm wavelength region have been demonstrated with ceramic Ho:Y2O3, Ho:Lu2O3 and Ho:YAG gain materials [16–19]. Generally, holmium-doped sesquioxide ceramic lasers needed cryogenic condition that exhibited poor operations at room temperature. On the other hand, high-efficiency and high-power laser output of Ho:YAG ceramic has been demonstrated at room temperature [18].

In this work, we report on the first atomic layer graphene passively Q-switched Ho:YAG ceramic laser. The laser operated with a slope efficiency of 33.8 % for the CW mode without graphene in the cavity while the slope efficiency decreased to 16.5 % under the modulation of the graphene. Stable Q-switched operations with 2.6–9 μs pulse durations and 28–64 kHz repetition rates were demonstrated as the pump power provided by a diode pumped Tm fiber laser was increased from 2 to 3.27 W. Both repetition rates and pulse durations show smooth and monotonous tendency with increasing pump power. Average output power up to 264 mW was generated, and the maximum pulse energy was 9.3 μJ at a repetition rate of 64 kHz.

2 Experiment and results

We employed a simple folded resonator design configuration (as shown in Fig. 1) for passively Q-switched polycrystalline ceramic Ho:YAG laser. The cavity comprised two flat mirrors, M1 (with a high reflectivity (>99.7 %) at the lasing wavelength (2,097 nm) and a high transmission (>97 %) at the pump wavelength (1,907 nm)) and M4 (with a high reflectivity (>99.5 %) at both pump and lasing wavelength), a concave mirror, M3, with a 100-mm radius of curvature and with a high reflectivity (>99.7 %) from 2,040 to 2,250 nm and a high transmission (~97 %) from 1,850 to 1,960 nm to filter out the unabsorbed pump power, and an output coupler, M2, with a transmission of 3 % at the lasing wavelength (2,000–2,250 nm) with a 300-mm radius of curvature. The angle of incidence on M2 and M3 was made as small as possible (~3°) in order to minimize astigmatism. The spacing of mirrors M1 and M2, M2 and M3 and M3 and M4 were selected 185, 400 and 50 mm to produce a TEM00 beam radius of 125 μm in the laser crystal and ~40 μm in the graphene. A polycrystalline Ho:YAG ceramic with 2.0 at.% Ho3+ doping (fabricated at Nanyang Technological University, Singapore) was cut and polished to have a cross section of 2 × 3 mm2 and 7.4 mm in length. Both end facets were antireflection (AR) coated in the 1,850–2,250 nm wavelength range in order to minimize reflection losses at the pump and lasing wavelengths. The ceramic sample was wrapped with indium foil (~0.1 mm in thickness), mounted within a water-cooled copper heat sink maintained at ~15 °C to insure efficient heat removal and positioned near M1 for a good mode matching of the pump beam and the laser mode inside the laser ceramic. A few layer of graphene on a quartz substrate formed a graphene saturable absorber that was placed in front of M4.

Fig. 1
figure 1

Schematic diagram of the experimental setup

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The pump source used in the experiment was an in-house constructed Tm-doped fiber laser comprising a ~3-m length of double-clad fiber with a 25-μm-diameter (0.17 NA) Tm-doped alumina silicate core surrounded by a 300-μm-diameter D-shaped pure silica inner cladding with a calculated NA of 0.46. The fiber laser was pumped through opposite ends by a high-power 796-nm diode laser which was split into two beams of roughly equal power. A volume Bragg grating (VBG) of 1,912 nm center wavelength and >99 % diffraction efficiency were employed for wavelength selection and spectrum narrowing (details can be found in Ref. [20]). Operating wavelength of the Tm-doped fiber laser was tuned to match the absorption peak of Ho:YAG ceramic at 1,907 nm by adjusting the working angle of the VBG. Laser output from the fiber pump source (M 2 ~ 2) was collimated by a 30-mm focal length plano-convex lens and subsequently focused to a beam of ~150 μm radius at the center of Ho:YAG using a 200-mm focal length lens, resulting a confocal parameter of ~66 mm inside the ceramic.

At first, we investigated the performance of the continuous-wave (CW) polycrystalline ceramic Ho:YAG laser. The combined power of the two output beams was measured by the “Coherent PM200F-19” power meter. Laser operation was realized at a threshold pump power of ~1.3 W and generated 711 mW output (see Fig. 2) at 2,097 nm with the maximum incident pump power of 3.27 W, corresponding to a slope efficiency of 33.8 % with respect to incident pump power. No self-Q-switching was observed during the experiment.

Fig. 2
figure 2

Laser output power as a function of incident pump power for CW and Q-switched mode

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Graphene coated on the quartz plate (developed at Nanchang University, Jiangxi Province, P.R.China) was placed before the rear mirror M4 as saturable absorber (see the layout of the setup in Fig. 1). Pulsed laser oscillation was observed once the incident pump power exceeded the threshold of ~1.8 W. Average output power as function of incident pump power is also plotted versus the cw operation in Fig. 2. Maximum average output power of 264 mW was obtained under 3.27 W of incident pump power, corresponding to an incident slope efficiency of ~16.5 %. The increased threshold and decreased slope efficiency of this Q-switched laser operation can be mainly attributed to the intrinsic loss of graphene and the insertion loss of quartz substrate. For Q-switched lasing with a graphene saturable absorber, the modulation depth related to the number of the graphene layers plays an important role in the pulse duration. The modulation depth of graphene saturable absorber is dramatically increased by increasing the number of graphene layers, which will shorten the pulse duration in the Q-switched laser system [8, 21]. In addition, since low output coupler transmission is usually beneficial to the energy storage and to realize the pulsed laser easily, further improving the pulse energy and reducing the pulse duration of Q-switched laser should be achievable by optimizing layer number of graphene flake and transmission coefficient of the output coupler. The M 2 parameter of the output beam under the maximum average output power was measured with nano-scan to be ~1.1, which indicates that the laser output is nearly diffraction limited. It can be seen that laser average output power shows a linear dependence on the incident pump power till the maximum pump power of 3.27 W, suggesting that there is scope for further scaling in laser output by simply increasing the incident pump power.

Pulse width and repetition rate were detected using a fast InGaAs photodiode (DET10D/M) and then recorded with a 1-GHz bandwidth oscilloscope (LeCroy 104MXs-A). Pulse width (line with symbol ●) and repetition rate (line with symbol ■) related to incident pump power are presented in Fig. 3. That shows a dramatic monotonous decrease in pulse durations from 9 to 2.6 μs and increase in repetition rate from 28 to 64 kHz with respect to incident pump power from threshold to maximum power of 3.27 W. Figure 4a depicts a typical single-pulse envelope at maximum pump power of 3.27 W, corresponding to a pulse duration of 2.6 μs. The pulse train is also shown in Fig. 4b with repetition rate of 64 kHz.

Fig. 3
figure 3

Pulse width and repetition rate versus incident pump power for Q-switching operation

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Fig. 4
figure 4

a Single-pulse envelope, b Typical pulse train under the incident pump power of 3.27 W

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3 Conclusion

In conclusion, passively Q-switched operation of the Ho:YAG ceramic laser at 2,097 nm was demonstrated using a few layer of graphene thin films as saturable absorber. Stable pulses of 28–64 kHz repetition rate and 2.6–9 μs pulse widths were generated. The maximum average output power of the Q-switched Ho:YAG ceramic laser was over 264 mW and maximum single-pulse energy was 9.3 μJ at a repetition rate of 64 kHz. Both repetition rate and pulse width of the graphene Q-switched laser change monotonously with the pump power. To the best of our knowledge, this is the first report of graphene passively Q-switched Ho:YAG ceramic laser at 2.1 μm.