1 Introduction

Solid-state lasers in the eye, safe range of 2 μm [1], are important owing to the potential applications in atmospheric sensing [2], wind lidar [3], medicine [4], and so on. Owing to the strong absorption by water, the 2 μm laser is an ideal light source for biomedical applications. The 2 μm laser can also be used for range finding, coherent laser lidar and atmospheric sensing since it is in the eye-safe spectral range. In addition, high-power 2 μm lasers are preferable as pump sources for optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs) in the mid-infrared region than 1 μm lasers since they provide higher quantum efficiency. Based on these considerations, solid-state lasers operating around the 2 μm waveband have been intensively investigated in recent years [5–7]. Thulium trivalent ions have long fluorescence lifetimes, high quantum efficiency and the absorption of thulium trivalent ions is near 790 nm which matches well with the commercially laser diodes [8, 9]. Various 2 μm solid-state lasers based on Tm-doping crystal, such as Tm:YAG, Tm:YLF and Tm:KLu (WO4)2 have been studied and reported [10–16].

The Tm:YAlO3 (Tm:YAP) single crystal is attractive as active material for 2 μm lasers mainly due to its natural birefringence combined with good thermal and mechanical properties similar to those of the YAG crystal [17]. In addition, the emission cross section of thulium in the YAP crystal (5.5 × 10−21 cm2) is twice as high as that in the YAG crystal (2.2 × 10−21 cm2) [18]. In [19], they demonstrate the first use of InGaAs/GaAs as a saturable absorber in the Q-switching of a diode-pumped Tm:YAP laser. With a pump power of 35 W, the maximum pulse energy of 28.1 μJ with a pulse width of 447 ns was obtained.

Single-walled carbon nanotube (SWCNT) absorber is a promising material for passively Q-switched and mode-locked lasers. It exhibits fast recovery times, chemical stability, and broad spectral range, roughly between 1 and 2 μm. SWCNT-based saturable absorbers can be fabricated in a much simpler and cost efficient way with well-known techniques such as spray [20], spin coating [21] or evaporation methods [22–25]. Just a few months ago, vertical evaporation method was used to fabricate single walled carbon nanotubes absorber for 1 μm mode locking solid-state lasers and the output power of the pulse laser was as high as 3.6 W [25, 26]. But so far, researches on the Q-switched and mode-locking 2 μm laser with SWCNT absorber have been hardly reported.

In this paper, using SWCNT as the saturable absorber, a diode-pumped passively Q-switched and QML Tm:YAP laser at 2.01 and 1.97 μm are realized for the first time. For the passively Q-switched, the minimum pulse width of 255.1 ns and the maximum average output power of 295 mW have been obtained at the pump power of 8.64 W. In this pump power, the pulse energy of 13.6 μJ and the pulse repetition frequency of 21.76 kHz were obtained. In order to compare different cavity and investigate the performance of SWCNT, QML Tm:YAP laser has been also studied and the related laser parameters have been given. The QML laser produced a maximum average output power of 780 mW and the maximum pulse energy of 32.1 μJ. The repetition rate of the mode-locked pulse inside the Q-switched envelope was 244.1 MHz and the pulse width is estimated to be about 1 ns. Several aspects of the passively Q-switched and Q-switch mode-locking Tm:YAP lasers with SWCNT saturable absorber have all been investigated experimentally.

2 Fabrication of SWCNT absorber

The SWCNTs used in this experiment were grown by electric arc discharge technique. The mean diameter of the SWCNTs is about 1.5 nm. At the first step of preparing SWCNT-SA, several mg of SWCNTs powder was poured into 10-ml 0.1 % sodium dodecyl sulfate (SDS) aqueous solution. SDS was used as a surfactant to disperse SWCNTs in aqueous solution. The SWCNT aqueous solution was then ultrasonically agitated for 12 h. After the ultrasonic process, the dispersed solution of SWCNTs was centrifuged to remove sedimentation of larger SWCNTs bundles. The upper portion of the centrifuged solution was decanted to a bottle, diluted and poured into a polystyrene cell. Then, we inserted vertically a hydrophilic quartz substrate into the polystyrene cell and put on steady for being gradually evaporated at atmosphere. It tooks about 2 weeks for the complete evaporation. Finally, a 100 nm thick ZnO film was coated on both side of the quartz to isolate the SWCNT from the air by atom layer deposition. Then the substrate coated with SWCNTs and ZnO is ready for using as a saturable absorber. An UV-visible-NIR spectrophotometer was employed to measure the linear optical transmission of the SWCNT absorbers of different concentrations, as shown in Fig. 1.

Fig. 1
figure 1

Absorption spectrum of SWCNT absorber

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3 Diode-pumped passively Q-switched Tm:YAP lasers

The setup of the passively SWCNT Q-switched laser is the flat-concave cavity. A fiber-coupled diode 795 nm laser with a core-diameter of 400 μm and N. A. of 0.22 was used as the pump source. The pump light was focused into the Tm:YAP crystal with a radius of 100 μm by a 1:0.5 focus lens. The b-cut Tm:YAP crystal (5 at. % Tm-doped) has the dimensions of 3 × 3 × 5 mm3. To efficiently remove the generated heat under diode-pumping, the crystal was wrapped with indium foil and mounted in a water-cooled copper block, and the temperature of the water was maintained at 14 °C. The input mirror is a concave mirror with radius of 100 mm, which is anti-reflection (AR) coated at 780–810 nm on its outside surface and high-reflection (HR) coated at 1,900–2,000 nm on its inside surface. The output mirror is a plane mirror with a transmission of 5 %. The transmission-type SWCNT absorber used in the experiment was put next to the output mirror. In addition, optical absorption of pump power could lead to thermal gradients inherently inducing stress and expansion in the laser crystal. So, we designed the cavity by ABCD matrix theory. According to the ABCD matrix theory, the whole cavity length was at 30 mm to ensure the cavity stability through the available pump power and intense focus laser mode with a spot size of 68 μm in radius on the SWCNT.

The dependence of output power of the passively Q-switched and continuous wave (cw) output powers on the incident pump power are shown in Fig. 2. Under the pump power of 8.64 W, the highest average output power of the passively Q-switched laser and cw output powers were 295, and 582 mW, respectively. The average output power of the passively Q-switched laser was smaller than cw output powers for insertion loss. The laser output power did not exhibit a clear saturation tendency as the pump power increased. In order to protect the Tm:YAP from damage, we did not increase the pump power any more.

Fig. 2
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Variation of the output power with the incident pump power for the Q-switched

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Figure 3 shows the pulse width and the repetition rate versus the pump power. We can see that the pulse width decreased with the increasing of the incident pump power, while the repetition rate increased with increasing pump power. The shortest pulse width was 255.1 ns corresponding to the repetition rate of 21.76 kHz when the incident pump power was 8.64 W. From Fig. 4, we can see that the peak power increased with the increasing of the incident pump power. The maximum peak power of 53.1 W was obtained at the incident pump power of 8.64 W. The largest pulse energy was 13.6 μJ. Under the pump power of 8.64 W, the pulse profile was shown in Fig. 5, which was detected by a fast photodiode detector (ET-3500) with a rise time of 250 ps and recorded with a 1 GHz digital storage oscilloscope (Tektronix TDS5104). The inset corresponds to a train of the Q-switched pulses at the pump power of 8.64 W. The pulse-to-pulse intensity fluctuation was <20 %. This fluctuation could be induced by the intrinsic nonlinear dynamics of the system such as the deterministic chaos. Another possible reason is related to the structure of the gain media. Local heating of the active channel could also induce visible instability of the Q-switching pulses. With an optical spectrum analyzer (AvaSpec-NIR256-2.5), the spectrum of the Q-switched laser is centered at 2.01 μm with a FWHM of ~20 nm (see Fig. 9).

Fig. 3
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The Q-switched of pulse width and repetition rate versus incident pump power

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Fig. 4
figure 4

The Q-switched of pulse peak power versus incident pump power

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Fig. 5
figure 5

A temporal profile of Q-switched with 1 μs at the incident pump power 8.64 W. The pulse width was 255.1 ns

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4 Diode-pumped passively Q-switched mode-locking Tm:YAP lasers

For enhancing the intensity in the saturable absorber, the mode size in it should be small enough. In order to get smaller radius on the SWCNT, we designed the cavity by ABCD matrix theory. According to the ABCD matrix theory, we applied a V-type resonator. There was a spot size of 53 μm in radius on the SWCNT. We obtained QML, not cw mode locking. Nearly 100 % modulation depth for the mode-locked pulses inside the Q-switched envelope was obtained for the first time (as far as we know) with a V-type cavity. The schematic diagram for the passively Q-switched mode-locked Tm:YAP laser is shown in Fig. 6. The total length of the folded cavity was 0.62 m. The input cavity mirror M1 was a flat mirror, which was antireflection coated at 780–810 nm and high reflection at 1,900–2,000 nm. The cavity mirror M2 (R2 = 300 mm), was coated for high reflection at 1,900–2,000 nm. The output mirror M3 is a plane mirror with a transmission of 5 %. The transmission-type SWCNT absorber used in the experiment was put next to M3.

Fig. 6
figure 6

Cavity configuration of the passively Q-switched mode-locking Tm:YAP laser

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Figure 7 showed the average output power of the passively Q-switched mode-locking and cw output powers as a function of the incident pump power. At an incident pump power of 8.64 W, the passively QML and cw output powers of 780 mW and 1.13 W was obtained, respectively. Similarly, in order to protect the Tm:YAP from damage, we did not increase the pump power any more. The repetition rates of the Q-switched envelop increased from 11.38 to 24.27 kHz when the incident pump power increased from 4.51 to 8.64 W. The maximum energy and peak power of single Q-switched pulse was 32.1 μJ and 32.2 W, respectively. Figure 8 displayed the temporal traces of the QML laser pulses obtained under the incident pump power of 8.64 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of ~244.1 MHz. Theoretically, the mode-locked repetition rate is given by f = c/2l (c is the speed of light, l is the optical length of the resonator). The inset presents the expanded oscilloscope traces of a train of mode-locked pulses obtained at the incident pump power of 8.64 W, the mode-locked pulses within the Q-switch envelope were separated by 4.1 ns. Apparently, the experimental result is in good agreement with the theoretical calculation.

Fig. 7
figure 7

Variation of the output power with the incident pump power for the QML

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Fig. 8
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The temporal trace of the QML laser pulses at the incident pump power 8.64 W. The inset corresponds to expanded oscilloscope traces of a train of the mode-locked pulses inside the Q-switched envelope at the pump power of 8.64 W

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With a close observation, the average rise time of the mode-locked pulse is found to be about 1 ns. The formula \( t_{\hbox{real}} = \sqrt {(t_{\hbox{measure}}^{2} - t_{\hbox{probe}}^{2} - t_{\hbox{oscilloscope}}^{2} )} \) describes the relationships among the real rise time \( t_{\hbox{real}} \) of the pulse, the measured rise time \( t_{\hbox{measure}} \) of the pulse, the rise time \( t_{\hbox{probe}} \) of the probe and the rise time \( t_{\hbox{oscilloscope}} \) of the oscilloscope employed. The rise time of oscilloscope \( t_{\hbox{oscilloscope}} \) is determined by \( t_{\hbox{oscilloscope}} \) *BW = 0.35–0.4, where BW is the bandwidth of the oscilloscope. The employed oscilloscope in our experiment is with a bandwidth of 1 GHz and the rise time of employed probe is about 0.25 ns. Using the above formula, the real rise time of the mode-locked pulse is about 800 ps. According to the definition of the rise time and considering the symmetric shape of the mode-locked pulse, we can assume that the width of the pulse is approximately 1.25 times more than the rise time of the pulse. Then the duration of the mode-locked pulse is estimated to be about 1 ns. The optical spectrum of the QML laser is centered at 1.97 μm with a FWHM of ~18 nm (see Fig. 9).

Fig. 9
figure 9

Optical spectrum of Q-switched and Q-switched mode locked laser

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5 Conclusions

In summary, the Q-switched and QML of a diode-pumped Tm:YAP operating at 2 μm with the SWCNTs are demonstrated for the first time. The maximum peak power of 53.1 W has been obtained at the incident pump power 8.64 W for Q-switched laser. The shortest pulse width was 255.1 ns corresponding to the repetition rate of 21.76 kHz when the incident pump power was 8.64 W. The QML laser produced a maximum average output power of 780 mW. The repetition rate of the mode-locked pulse inside the Q-switched envelope was 244.1 MHz and the pulse width is estimated to be about 1 ns. Our experimental results have shown that the SWCNT is promising for the potential use in the diode-pumped Tm:YAP near 2 μm laser.