1 Introduction

Due to the advantages of eye-safe characteristic and being in atmospheric weak absorption band, lasers in the range of 2 μm have important applications in atmospheric sounding, medicals, and material processing [1–3]. Especially, 2 µm pulse lasers are efficient pump sources for optical parametric oscillators (OPOs) and optical parametric amplifications (OPAs), which can generate infrared laser output [4, 5]. Compared with passively mode-locked laser [6], actively mode-locked (AML) laser can only compress laser pulse width to picosecond magnitude, but it does not exist damage to saturable absorption mirror in AML laser, which makes it easier to obtain output power of watt magnitude [7]. In recent years, researches of 2 μm AML laser mainly devoted to Tm3+/Ho3+ laser materials. In 2007, Gatti et al. [8] reported AML Tm–Ho:LiYF4 and Tm–Ho:BaY2F8 lasers with central wavelength tunable around 2.06 μm. In 2013, Dergachev [9] reported an AML Ho:YLF laser that could generate 3.5 W laser output in 2.05 μm, corresponding to the pulse width of 250–300 ps. In 2015, Muzik and Jelinek [7] reported a 1.91-μm Tm:YLF AML laser with maximum output power of 2.6 W, corresponding to the shortest pulse width of 170 ps at a repetition rate of 149.3 MHz.

LuAG material belongs to cubic crystal system that has similar characters as YAG crystal. However, compared with YAG material, LuAG material has large manifold splitting and low thermal occupation for the lower level owing to its higher crystal field. Compared with YLF crystal, the LuAG material has better mechanical properties and thermal performance, and its growth process was more mature. All the advantages above make LuAG material a better choice as the main host for the Yb-doped [10], Nd-doped [11], Er-doped [12], and Tm-doped lasers [13]. In recent years, Ho-doped LuAG crystal has been proved to be splendid laser material for 2 μm laser output. Yao et al. of our group successively reported the output performances of actively Q-switched Ho:LuAG laser and passively Q-switched Ho:LuAG laser [14, 15], but performance of mode-locked Ho:LuAG laser is still unreported.

In this paper, we reported the properties of resonantly pumped AML Ho:LuAG laser with an acousto-optic modulator (AOM). In continuous-wave (CW) operation, the maximum output power of 2.9 W with slope efficiency of 31.1 % was obtained under the incident pump power of 11.4 W. In continuous-wave mode-locked (CWML) operation, with the same incident pump power, maximum single-pulse energy of 32.7 nJ and a mode-locked pulse width of 333.4 ps were achieved. Besides, the central wavelength of 2100.4 nm was obtained in both CW and CWML operations. As far as we know, this is the first time that AOM was used in a Ho:LuAG laser to produce a 2-μm AML output.

2 Experimental setup

Figure 1 shows the schematic of the experimental setup. We employed a Tm3+-doped fiber laser as the pump source, with a maximum output power of 11.4 W. The Tm3+-doped fiber laser with a beam quality factor M 2 of 1.2 had two central wavelengths of 1907.1 and 1908.8 nm. The pump light was focused into the crystal center by a beam coupling system (f = 7 mm and f = 150 mm), and the beam radius was 0.19 mm in the crystal center. The size of Ho:LuAG crystal was Φ5 × 30 mm3, corresponding to Ho3+-doped concentration of 0.5 at. %. The end surfaces of the crystal were coated with anti-reflection film at 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 circulating water with the temperature of the Ho:LuAG crystal held at 17 °C. We employed a fold cavity resonator with eight mirrors in the experiment. M1 was a plane mirror, which was high reflectivity (HR)-coated at 2.1 μm. M2 and M3 were 10° concave mirrors with the curvature of 600 mm and HR-coated at 2.1 μm. M4 and M5 were 45° dichroic mirrors, which were high transmission (HT)-coated at 1.91 μm and HR-coated at 2.1 μm. M6 was 10° concave mirror with a curvature of 300 mm, HT-coated at 1.91 μm, and HR-coated at 2.1 μm. M7 was a 10° concave mirror with a curvature of 400 mm and HR-coated at 2.1 μm. M8 was a 0° output coupler with a curvature of 150 mm and had a transmittance of 10 % at 2.1 μm. M9, M10, and M11 were 45° dichroic mirrors, which were HT-coated at 792 nm and HR-coated at 1.91 μm, for the purpose of evacuating the remaining pump light. M1 was mounted on an adjustable submillimeter translation stage for fine-tuning the resonator length. The physical length of the resonant cavity was 1767 mm. The distances between M1 and M2, M2 and M3, M3 and M4, M4 and M5, M5 and M6, M6 and M7, and M7 and M8 were 294, 343, 176, 75, 210, 354, and 315 mm, respectively. The calculated diameter of the TEM00 mode was about 376 µm based on cold resonator. The AML employed an AOM, which consisted of a 30-mm-long Brewster-cut fused quartz. The AOM was HT at 2.1 μm with an optical aperture of 2 mm and the radio-frequency power of 20 W. The radio-frequency power and the peak transmission modulation of the mode locker were measured to be approximately 20 W and 27.5 % at 2.1 μm, respectively.

Fig. 1
figure 1

Schematic diagram of the experimental setup for active mode locking of the Ho:LuAG laser

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3 Experimental results and discussion

The output power of Ho:LuAG laser as a function of the incident pump power is shown in Fig. 2 at CW and CWML operations. The output power of Ho:LuAG laser in the experiment was measured by a Coherent PM30 power meter. When the AOM was switched off at the incident pump power of 11.4 W, the maximum output power of 2.9 W and a slope efficiency of 31.1 % were obtained for the CW Ho:LuAG laser, which indicated an optical–optical conversion efficiency of 25.4 %. In CWML regime, the laser achieved a 2.7-W output power under the incident pump power of 11.4 W, corresponding to a slope efficiency of 29.3 % and an optical–optical conversion efficiency of 23.7 %.

Fig. 2
figure 2

Output power of Ho:LuAG laser versus incident pump power at CW and CWML operations

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The CW and CWML laser spectrum recorded with Spectrum Analyzer (BRISTOL INSTRUMENTS 721) at the incident pump power of 11.4 W is shown in Fig. 3. As can be seen from Fig. 3, the CW laser had central wavelength of 2100.4 nm. Compared with CW operation, the CWML laser also had the same central wavelength of 2100.4 nm. In addition, it was found that the output wavelength of the laser was not sensitive to temperature changes.

Fig. 3
figure 3

Laser spectra in CW and CWML operations

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The pulses were recorded with a high-speed detector (EOT, ET-5000F) combined to a 12.5-GHz-bandwidth oscilloscope (Tektronix, DPO7000C). Figure 4 shows a typical mode-locked pulse train of Ho:LuAG laser. It can be found from Fig. 4 that the output pulses always show good stability. As shown in Fig. 4, the pulse repetition rate was 82.48 MHz. The mode-locked pulse period was about 12 ns, and in a cavity cycle, only a mode-locked pulse was observed. Furthermore, we measured the temporal trace of the mode-locking pulse under the incident pump power of 11.4 W, as shown in Fig. 5. The full width at half maximum (FWHM) of a mode-locked pulse was 333.4 ps, corresponding to the maximum single-pulse energy of 32.7 nJ.

Fig. 4
figure 4

Typical mode-locked pulse trains

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Fig. 5
figure 5

Temporal trace of a mode-locking pulse with a FWHM of 333.4 ps

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The radio-frequency (RF) spectrum of the mode locking is recorded by a RF spectrum analyzer with a bandwidth of 3 GHz (N9320A, Agilent). The RF spectrum obtained in Fig. 6 shows a clean peak at the fundamental repetition rate of 82.48 MHz under the incident pump power of 11.4 W, corresponding to the laser optical path length of 1818.6 mm. The signal-to-noise ratio of the RF spectrum reached 50 dB. Moreover, there was no obvious side peaks near the center peak, which proved that the laser operated in CWML mode rather than the Q-switched mode-locking mode. The signal-to-noise ratio we obtained was lower than most of the passively mode-locked results that reported before [16]. The reason is that both the matching degree of AOM modulation frequency and the resonator optical length are hard to control, but the passively mode-locked laser is free from this problem.

Fig. 6
figure 6

RF spectrum of the mode-locked pulses

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Figure 7 shows the measured beam radius under maximum CW output power of 2.9 W at various distances from the lens with f = 93 mm. The transverse output beam profile was measured by using a 90∕10 knife-edge technique. The M 2 factor was calculated to be 1.1, which indicated that the output beam was close to fundamental TEM00. The inset in Fig. 7 is the transverse output beam profile obtained in the near field at the highest pump power, which was measured by a Spiricon Pyrocam I pyroelectric camera.

Fig. 7
figure 7

The beam radius of the Ho:LuAG laser. Inset, typical 2D beam profiles

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4 Conclusion

In conclusion, to our knowledge, we first demonstrated an AML Ho:LuAG laser with a diode-pumped Tm3+-doped fiber laser. For the CW operation, at a maximum incident pump power of 11.4 W, we obtained the maximum output power of 2.9 W at the central wavelength of 2100.4 nm, corresponding to a slope efficiency of 31.1 %. The beam quality factor of M 2 was 1.1, and the output beam was close to fundamental TEM00. For the CWML operation, the CWML laser output had the same central wavelength of 2100.4 nm just as the CW operation. The repetition frequency of mode-locked pulse was 82.48 MHz. At the incident pump power of 11.4 W, we obtained the maximum average output power of 2.7 W with a slope efficiency of 29.3 %, corresponding to the single-pulse energy of 32.7 nJ and pulse width of 333.4 ps.