1 Introduction

Pulsed lasers operating in 2-μm spectral regions with durations from nanosecond to microsecond are of particular interest for wind velocity measurements, material processing, laser ranging, remote sensing, and optical frequency conversion [1–4]. Such kinds of laser can be obtained using active or passive Q-switching techniques. The latter is usually preferred due to the advantages of simplicity, compactness, and reliability [5, 6]. To date, Graphene [7, 8], Cr:ZnS, Cr:ZnSe [9, 10], InGaAs/GaAs quantum [11], and PbS-doped glass [12] have been reported as saturable absorbers for passively Q-switched 2-μm lasers. Compared with the aforementioned saturable absorbers, GaSb- and InGaAs-based semiconductor saturable absorber mirrors (SESAMs) are known well as passive mode locker for ultrafast 2-μm lasers with free design parameters such as nonsaturable losses, absorption band, modulation depth, and saturation fluence [13, 14]. Generally, Q-switching is an unwanted state for mode-locked 2-μm lasers; however, on the other hand, it provides a simple way to generate 2-μm laser pulses with durations in an order of microsecond. Recently, Kivistö et al. [15] reported a passively Q-switched Tm,Ho co-doped fiber laser operating at 2 μm based on the antimonide semiconductor, 385 ns pulse with repetition rate of 160 kHz was achieved. After that, Yang et al. [16] demonstrated a passively Q-switched Tm-doped fiber laser with SESAM, pulse duration range from 1 μs to 362 ns under repetition rate from 20 to 80 kHz was obtained. However, there is no report on passively Q-switched bulk lasers with SESAM in 2-μm region.

Tm3+-doped crystals have gained precedence for the generation of 2-μm lasers, since Tm3+ ions have absorption bands around 800 nm where commercial GaAs/AlGaAs laser diodes are available. By now, continuous wave (CW) or pulsed operations of Tm3+-doped tungstate [17], garnet [18], sesquioxide [19], and silicate [20, 21] lasers have been reported, which showed excellent properties of high efficiency and broad wavelength tunability, etc. Monoclinic silicate crystal Sc2SiO5 (short for SSO) belonging to biaxial SC2O3–SiO2 system [22] has exhibited favorable physical, chemical, and thermal properties and has been doped with several rare earth dopants including Er3+ [23], Yb3+ [24], Tm3+ [25], and Nd3+ [26] for generation of lasers from 1 to 2 μm. Especially, the large ground-state splitting of about 691 cm−1 in Tm:SSO crystal is helpful to suppress the reabsorption process, thus can reduce the reabsorption loss and enhance the laser efficiency [27]. Although the ground-state splitting is smaller than those of Tm:LSO (1,094 cm−1) and Tm:YSO (1,021 cm−1), a larger thermal conductivity of about 7.5 Wm−1 K−1 is attractive (5.3 Wm−1 K−1 for un-doped LSO host and 4.4 Wm−1 K−1 for un-doped YSO host) [28]. Moreover, the long upper-level lifetime of about 1.14 ms indicates its excellent energy storage capacity [29]. However, the current research on Tm:SSO lasers was mainly focused on CW and wavelength-tunable operations [27, 29]. As far as we known, there is no any report on Q-switched Tm:SSO lasers so far.

Here, we report, for the first time, to the best of our knowledge, a diode-pumped CW wavelength-tunable and passively Q-switched Tm:SSO laser using an InGaAs-based SESAM as saturable absorber. Under CW operation, a maximum output power of 340 mW at 1,980.7 nm was obtained, corresponding to a slope efficiency of 11 %. With a 2-mm quartz plate as the wavelength selector, wavelength-tunable CW operation was achieved from 1,922 to 2,020 nm, exhibiting a broad tunability range over 100 nm. In passive Q-switching regime, by changing the positions of the SESAM, the pulse width and the pulse repetition rate could be varied from 7.6 to 11.4 μs and from 800 Hz to 8.8 kHz, respectively. A maximum output pulse energy of 14.7 μJ with a repetition rate of 800 Hz and a minimum pulse width of 7.6 μs corresponding to a repetition rate of 8.8 kHz around 1,974.4 nm were obtained from the passively Q-switched Tm:SSO lasers.

2 Experimental setup

The schematic diagram of diode-pumped Tm:SSO laser is shown in the Fig. 1. A fiber-coupled diode laser was used as pump resource with a fiber core size of 100 μm in diameter and numerical aperture of 0.22. Its emission wavelength was 787 nm at 15 °C with a maximum output power of 50 W. A 1:1 imaging module was employed to focus the pump light into Tm:SSO crystal with a pump spot diameter of 100 μm inside it. The 5 × 5×3 mm3 Tm:SSO single crystal was grown by the Czochralski method and doped with 4 at % Tm3+. Both surfaces of the Tm:SSO crystal were antireflection coated from 750 to 850 nm (reflectivity <2 %) and from 1,930 to 2,230 nm (reflectivity <0.8 %). The laser crystal was wrapped in indium foil and mounted in a copper block cooled by a watercooler to 12 °C. Mirrors M1 was a concave mirror (R = 75 mm) with antireflection coated from 750 to 850 nm (reflectivity <2 %) and high reflectivity coated (reflectivity >99.9 %) from 1,820 to 2,150 nm. Flat mirror M3 and concave mirrors M2 (R = 75 mm), M4 (R = 30 mm), M5 (R = 50 mm), and M6 (R = 100 mm) were all high reflectivity coated from 1,820 to 2,150 nm (reflectivity >99.9 %). The laser pulse trains were recorded by a digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7102, USA) and a fast InGaAs photodetector with rise time of 35 ps, (EOT, ET-5000, USA). A laser power meter (MAX 500AD, Coherent, USA) was used to measure the average output power. The laser spectrometer used for measuring the output spectra with a resolution bandwidth of 0.4 nm (APE WaveScan, APE Inc.)

Fig. 1
figure 1

CW and passively Q-switched Tm:SSO laser setup

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3 Experimental results and discussions

3.1 CW operation

The CW operation of diode-pumped Tm:SSO laser was investigated first using a X-type cavity with mirrors of M1, M2, M3, and OC2. Three OCs with different transmissions of T = 1, 2, and 3 % were employed in the experiment for comparisons. The threshold pump powers were 0.23, 0.29, and 0.59 W for OC of T = 1, 2, and 3 %, respectively. The increase in threshold pump power was caused by the additional loss introduced by a higher transmission of OCs. The measured output powers and spectra are shown in Figs. 2, 3, respectively. In Fig. 2, the incident pump powers were measured behind mirror M1. With OC of T = 2 % employed, a maximum output power of 340 mW was achieved at 1,980.7 nm, corresponding to a slope efficiency of 11 %. The maximum output powers of 265 mW at 1,983.1 nm and 205 mW at 1,973 nm were obtained using OCs of T = 1 and 3 %, respectively. Although the power saturation was not observed in our experiment, we did not further increase the incident pump power to prevent the laser crystal from fragmentation, which may be induced by a higher incident pump power.

Fig. 2
figure 2

Output power characteristics of CW Tm:SSO laser under different OCs

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Fig. 3
figure 3

Output spectra from Tm:SSO lasers in CW regime with different OCs. The curves are vertically offset for clarity

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The wavelength tunability of CW Tm:SSO laser was studied using a 2-mm-thick quartz plate and OC of T = 2 % at the incident pump power of 3.4 W. The birefringent quartz plate had an optical axis in the plane of input face and was inserted into the cavity at Brewster’s angle. By rotating the quartz plate and aligning the OC carefully, the output wavelengths were tuned continuously from 1,923.9 nm to 2,023.8 nm with a total tunable range over 100 nm. The output powers versus output wavelengths are shown in Fig. 4, from which two power peaks were observed around 1,980 and 1,992 nm. A maximum output power of 160 mW was achieved around 1,980 nm. Although the wavelength-tunable range of 100 nm in our experiment was somehow smaller than that of 130 nm obtained in [27], the wavelength could be tuned with a resolution of about 1 nm here.

Fig. 4
figure 4

Wavelength Tunability of CW Tm:SSO laser at incident pump power of 3.4 W with OC of T = 2 %

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3.2 Q-switched operation

The SESAM (Batop Inc.) employed here was based on an anti-resonant design with a 50-nm-thick Ga0.33In0.67As/GaAs absorber layer, corresponding to a saturation fluence of 70 μJ/cm2 and a modulation depth of about 1 % at 1,980 nm, which is the same one that has been successfully used for mode locking of Tm,Ho:YAG [13] and Tm:LYSO [28] lasers. In the experiment, we also tried mode-locking operation of Tm:SSO laser in a 127-cm-long cavity consisting of mirrors M1, M2, M4, M5, M6, OC1, and SESAM as shown in the Fig. 2; however, no any mode-locking phenomena were observed within our pump range. To find out the reason, we consider the condition for stable passive mode locking without Q-switching instabilities as shown in Ref. [30]:

$$E_{\text{p}}^{2} \ge E_{\text{p,c}}^{2} = E_{\text{sat,L}} E_{\text{sat,A}} \varDelta R$$

Here, E p is the intracavity pulse energy, E p,c is the minimum pulse energy required to realize stable CW mode locking, and E sat,L is the saturation energy of the gain medium, which is given by the product of saturation fluence F sat,L = hv/mσ L and the effective laser mode area inside the gain medium \(A_{\text{eff,L}} = \pi \omega_{\text{L}}^{2}\), where m is the number of passes through the gain medium per cavity round trip, σ L is the emission cross section of the laser crystal, and ω L is the beam radius inside the gain medium. E sat,A = F sat,A A eff,A, F sat,A is the saturation fluence of SESAM; \(A_{\text{eff,A}} = \pi \omega_{\text{A}}^{2}\) is the effective laser mode area on SESAM, and ΔR is the modulation depth of SESAM, respectively. Based on the result of E p,c, the required threshold output power for CW mode-locking operation can be given byP thr ≥ E pc × REF × T OC, where REFis the repetition rate of the mode-locking pulse, and T OC is the transmission of OC. Considering the parameters in our experiment, ω L = 40 μm, ω A = 110 μm, m = 2, ΔR = 1 %, T oc = 1 %, REF = 118 MHz, F sat,A = 70 μJ/cm2, σ L = 9.58 × 10−21 cm2, we obtained P thr = 309 mW, which is much higher than 20 mW achieved in our experiment. The corresponding intracavity power intensity on SESAM was too low to introduce efficient intensity fluctuation required by mode locking, to which we attributed the absence of mode-locking phenomena.

Although the laser could not run in mode-locking operation, stable passive Q-switching operation was easily achieved. The characteristics of the passively Q-switched Tm:SSO laser were studied at a fixed pump power of 3.4 W with OC of T = 1 %. By aligning the cavity carefully, stable Q-switching operation was achieved when the distance between M6 and SESAM (hereafter D) was in the range of 590–598 mm within the cavity stability region. The average output power and pulse duration as well as pulse repetition rate were observed to be varied with D changed (see Fig. 4). As can be seen from Fig. 5, the pulse repetition rate decreased and the pulse duration increased with the augment of D.

Fig. 5
figure 5

Characteristics of the passively Q-switched Tm:SSO laser at the incident pump power of 3.4 W with OC of T = 1 %. (D is the distance between SESAM and mirror M6)

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When D was 590 mm, the Tm:SSO laser produced a pulse of 7.4 μs with a repetition rate of 8.8 kHz. The lowest repetition rate of 856 Hz with pulse duration of 11.4 μs was obtained at D = 598 mm, corresponding to a maximum pulse energy of 14.7 μJ. The temporal profiles of pulse trains are shown in Fig. 6, showing an excellent Q-switching stability. The pulse to pulse fluctuation was estimated to be <5 %. Under the passive Q-switching operation, the output wavelength was located at 1,974.4 nm without change when D was varied from 590 to 598 mm, which spectrum is shown in Fig. 7. Compared with the output wavelength in CW running regime, it was much narrowed and slightly blue shifted in the Q-switched regime. We attribute the spectral blue shifting to the increased inversion rate introduced by the insertion losses of the SESAM in a typical three-level laser system [31], and the introduced loss also narrowed the spectrum.

Fig. 6
figure 6

The temporal profiles of the pulse trains from passively Q-switched Tm:SSO laser with OC of T = 1 % at D = 598 mm (upper) and 590 mm (lower). (Insets: temporal profiles of Tm:SSO laser pulses)

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Fig. 7
figure 7

Spectral characteristics for passively Q-switched Tm:SSO laser

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To understand the dependences of the pulse repetition rate and durations on the variations in D, we calculated the beam radii and power intensity on SESAM for different values of D using ABCD propagation matrix theory, which are shown in Fig. 8. It can be concluded from Fig. 5 and 8 that large power intensity on SESAM leads to a high pulse repetition rate and short pulse duration. We attribute this to the time needed to bleach the saturable absorber and establish that the pulse is short when the power intensity on SESAM is large.

Fig. 8
figure 8

The beam radii and power intensity on SESAM with change of D

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4 Conclusion

In this paper, we have presented the characteristics of diode-pumped CW wavelength-tunable and passively Q-switched Tm:SSO lasers. Under CW regime, the Tm:SSO laser produced a maximum output power of 340 mW at 1,980.7 nm, corresponding to a slope efficiency of 11 %. With a 2-mm-long quartz plate inserted into the cavity at Brewster angle, the CW Tm:SSO laser wavelengths could be continuously tuned from 1,922 to 2,020 nm within a resolution of 1 nm. With an GaInAs-based SESAM as saturable absorber, passively Q-switched Tm:SSO laser was realized. A maximum pulse energy of 14.7 μJ at a repetition rate of 856 Hz and a minimum pulse width of 7.6 μs with a repetition rate of 8.8 kHz both at 1,974.4 nm were obtained by changing the positions of SESAM. As far as we know, this is the first report on diode-pumped wavelength-tunable and passively Q-switched Tm:SSO lasers.