1. Introduction

Laser sources operating at about 2 μm spectral range have become a region of increasing interest in view of a wide variety of emerging scientific and technological applications, such as remote sensing of atmospheric species, laser radar systems, laser spectroscopy, medical surgery, plastic processing and pump sources for mid-infrared (3-5 µm) OPOs. The timely progress in 2-μm laser technology is, therefore, vital for the development of modern diagnostic tools for novel environmental, chemical, biological, and medical applications. Using an AlGaAs diode laser as pump source, trivalent thulium (Tm³⁺) can provide efficient laser oscillation at this specific wavelength through the ³F₄ → ³H₆ transition channel. Based on the various applications and easily commercially available pump source, investigation on Tm³⁺ lasers is one of the most attractive research fields during the past decades. With a rising research interest on Tm³⁺ lasers, research on Tm³⁺-doped laser materials has also attracted more and more attention, which has been indeed developed greatly in the past years, such as typical crystalline oxides like Tm:YAG [1,2], Tm:GdVO₄ [3,4], Tm:YVO₄ [4] and Tm:LuVO₄ [4,5], as well as typical fluorides like Tm:YLF [6] and Tm:BaY₂F₈ [7]. More particularly, to operate (ultra-)short laser pulses with high pulse energy and peak power, Tm-doped multisite or structurally disordered materials, leading to wide emission transitions with reasonably large cross sections and presenting more favorable thermo-mechanical properties than glasses, are worth to be investigated.

CaGdAlO₄, abbreviated as CGA, is a complex disordered oxide and belongs to tetragonal system. Aluminum is octahedrally coordinated by oxygen and the obtained AlO₆ octahedra form a framework structure. Ca²⁺ and Gd³⁺ ions are randomly located between layers of AlO₆ octahedra [8]. CGA is very suitable for laser host material thanks to its high mechanical strength with Mohs hardness 6, high thermal conductivity (6.9 W m⁻¹ K⁻¹ and 6.3 W m⁻¹ K⁻¹ along a and c axes) and moderate thermal expansion coefficient (10.1 × 10⁻⁶ /K and 16.2 × 10⁻⁶ /K along a and c axes) [8]. In fact, CGA doped with Yb³⁺ has been gained great success [9,10]. Nevertheless, Yb:CGA laser can only operate at about 1-μm spectral domain. To realize 2-μm lasers, CGA with Tm³⁺ dopant is necessary. Fortunately, Tm:CGA crystal, like Yb:CGA, can be grown by the standard Czochralski method [8]. In 2015, continuous-wave and mode-locked Tm:CGA lasers at 1949 nm were reported with output power of 730 mW and slope efficiency of 27%, as well as pulse width of 27 ps [11]. In the same year, continuous-wave and mode-locked Tm:CGA laser were reported again with output power 337 mW and slope efficiency of 50.9%, as well as greatly reduced pulse width of 650 fs [12]. However, for high-power continuous-wave laser generation in Tm:CGA, a real advancement has been very recently reported by Wu et al. [13]. The authors, using a 1.7-μm Raman fiber laser as pump source, achieved a maximum output power of 7.2 W with slope efficiency of 59.8% at 1.96 μm. However, the specific pump source limits the applications of this work.

To date, on the one hand, no passively Q-switched Tm:CGA laser has been reported. In contrast to active Q-switching, passive Q-switching under the help of saturable absorbers has been a more desirable method, as it has the advantages of being compact, simple, and low cost. On the other hand, the laser performance of continuous-wave Tm:CGA laser, using a more standard 790-nm AlGaAs as pump source, has been still very limited to a maximum output power of less than watt-level. Therefore, in this work, with a 790-nm diode laser, we operated power scaled continuous-wave Tm:CGA lasers not only at the commonly reported 1.94 μm wavelength but also at far less demonstrated 1.85 μm wavelength for Tm³⁺ lasers. Furthermore, with a Cr:ZnSe saturable absorber, the two lasers can also be operated in Q-switched regimes.

2. Experimental setup

Figure 1 schematically shows the laser experimental setup configuration of the diode-pumped Tm:CGA lasers in continuous-wave and passively Q-switched regimes. The pump source was a 790-nm fiber-coupled diode laser with core diameter of 105 μm and numerical aperture of 0.22. The pump beam coupling optics consisted of two doublet lenses with focal lengths of 30 and 50 mm, respectively playing role in collimating and focusing the pump beam. Thus, the size of the pump beam was expanded by 5/3 times. The laser resonator was a simple and compact plane-concave configuration. The flat input mirror (IM) has a transmission of 84% at pumping wavelength and high reflection of 99.8% from about 1700 to 2000 nm. Two curved output couplers (OCs), both having a curvature radius of 50 mm, were used to realize the two goals of this work. OC1 has a broad and flat transmission of about 8.5% from 1800 to 2000 nm, while OC2 has monotonously increasing transmissions from about 5.0% at 1854 nm to 38% at 1938 nm.

 

Fig. 1 The schematic of diode-pumped continuous-wave and Q-switched Tm:CGA laser.

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Inside the laser resonator, the laser gain medium of a Tm:CGA crystal, with dimensions of 4 × 4 × 7 mm³ (7 mm in thickness) and dopant concentration of 2at%, was placed very close to the IM. Single pass absorption of the laser crystal was measured to be about 85% of the pump power. To improve the laser output power, it is very crucial to alleviate the thermal lensing effect by removing the generated heat inside the laser crystal. Hence, the laser crystal was wrapped with indium foil and then mounted inside a copper block, which was temperature controlled at 14°C by a water-cooled chiller. Passive Q-switching was realized by introducing an anti-reflection coated Cr:ZnSe crystal with initial transmission of about 95%. The Cr:ZnSe crystal was sandwiched between two pieces of copper plates without additional cooling.

3. Results and discussion

The un-polarized and polarized emission spectra of the a-cut Tm:CGA crystal associated with the ³F₄ to ³H₆ optical transition are shown in Fig. 2, and the spectral ranges extend from 1600 to 2100 nm. For the un-polarized emission spectrum, the FWHM of it is about 200 nm, while for the two polarized emission spectra, i.e. σ polarization ($E\perp c$) and π polarization (E//c), the FWHMs of them are similar to be about 170 nm. Moreover, the σ-polarized emission peaks at about 1826 nm and the π-polarized emission has a peak at about 1801 nm. According to Ref [8], Tm:CGA crystal has an intense ³H₆ → ³F₄ absorption transition corresponding to spectral range from about 1550 to 1900 nm, and as a consequence laser performance at relatively short wavelength must be very limited.

 

Fig. 2 Polarization-independent (black line) and polarization-dependent ($E\perp c$ in red and $E//c$ in blue) fluorescence spectra of Tm:CGA crystal.

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For continuous-wave laser operation, we obtained maximum output powers when the laser cavities were optimized to be about 45 mm in physical lengths. The experimentally optimized cavity lengths should correspond to optimized mode overlap efficiencies to realize maximum energy extractions from the Tm:CGA laser cavities. Figure 3(a) shows the output power characteristics obtained by using OC1 and OC2, respectively. Using OC1, the maximum output power reached 3.05 W at 1938 nm, with threshold absorbed power of 0.96 W, which led to a slope efficiency of about 33.7%. Using OC2, lasing at 1854 nm, the maximum output power decreased to 2.41 W with increased threshold of 1.31 W. Thus, the slope efficiency diminished to 28.5%. The lasing wavelength, at 1938 or at 1854 nm, was determined by the reabsorption and emission intensities of the laser crystal at corresponding transition lines, as well as by the transmission loss of the OC. It is clear that using the OC2 with far higher transmission at 1938 nm than at 1854 nm is vital to operate a short-wavelength 1854-nm laser. The laser spectra of the two lasers were shown in Fig. 3(b) with emission peaks at about 1938 and 1854 nm. In addition, the output powers have not shown any saturation effect and in fact as shown in Fig. 3(a) the output powers for the two wavelengths increased very linearly, which indicated a good thermal property of Tm:CGA crystal as mentioned above. By optimizing the transmission of the OC, the pump-to-mode overlap efficiency, the concentration and length of the Tm:CGA crystal for optimization of the laser performance and for higher absorption of the pump power, further power scaling (to 5 W) could be anticipated in the near future.

 

Fig. 3 (a) Output power characteristics of continuous-wave Tm:CGA lasers at 1938 and 1854 nm, and (b) corresponding laser spectra at 1938 and 1854 nm.

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Using a Glan-Taylor polarizer, the polarizations of the two output lasers were also measured and shown in Fig. 4 at low pumping level, from which one can find that the two lasers exhibited the same polarizations, since with the increase of the rotation angles of the polarizer the output powers of the two lasers evolved the same. Further, the two lasers are fully σ linearly polarized, the intensity maxima being found at angles for which $E\perp c$. The polarizations of the two lasers agree with the spectra presented in Fig. 2, which indeed shows higher intensities in σ than in π at the 1938 and 1854 nm.

 

Fig. 4 The variations of output powers with the rotation angle of Glan-Taylor polarizer.

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Passively Q-switched laser operation at the two lasing wavelengths were realized by inserting a Cr:ZnSe crystal acting as saturable absorber. Figure 5 shows the average output powers varying with the increase of absorbed powers. Because of the insertion of the saturable absorber, the laser threshold for both cases increased to 1.68 and 2.67 W. For the 1938 nm laser, the average output power increased to 0.64 W at absorbed power of 6.71 W, while the average output power of the 1854 nm laser also increased to 0.37 W at the same pumping level. Correspondingly, the slope efficiencies of the two lasers were fitted to be about 12.6% and 9.2%. Further increasing the pump powers led to degraded stabilities of the pulse trains, and we have ascribed the failures of stability maintenances to increase of the thermal effect with respect to the saturable absorber, which have not provided extra cooling. To protect the Cr:ZnSe saturable absorber from fracturing, no higher pump power has been injected.

 

Fig. 5 Average output power characteristics of Q-switched Tm:CGA lasers at 1938 and 1954 nm.

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The typical single pulse profile and pulse trains are recorded in Fig. 6. For the 1938 nm laser, as Fig. 6(a) shown, the shortest pulse width was obtained to be about 44 ns and the pulse trains obtained at the maximum output power showed a repetition rate of 13.9 kHz. Operating the Q-switching of Tm:CGA laser at 1854 nm by using OC2, the shortest pulse width a little broadened to about 47 ns and the maximum pulse repetition rate was about 9.9 kHz. During the measurements, for two cases, no secondary pulses nor parasitic emission between pulses were observed. Note that the present results also showed much narrower pulse widths than that of a 75 ns pulse obtained by active Q-switching reported in Ref [13].

 

Fig. 6 Single pulse widths and pulse trains of Tm:CGA lasers at (a) 1938 nm and (b) 1854 nm.

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Figure 7(a-b) shows the whole evolutions of pulse widths and pulse repetition rates of the two lasers. According to theoretical deduction of passively Q-switched laser [14], increased gain induced by increased pump power will lead to the narrowing of pulse duration, while increasing the pump power itself incurs a linear increase of pulse repetition rate, if without taking the saturation effect into account. For 1938 nm laser (see Fig. 7(a)), the pulse width decreased from 61 to 44 ns, while decreasing from 63 to 47 ns for 1854 nm laser (see Fig. 7(b)). For both cases, the pulse widths showed quick saturations with the increasing of the pump powers. Correspondingly, the pulse repetition rates for the lasers increased from 2.76 to 13.9 kHz and from 1.16 to 9.9 kHz, both with good linearity. Using these data, it allows estimating the pulse energy and pulsing peak power, as shown in Fig. 7(b) and 7(c). For the 1938 nm laser, the maximum pulse energy and pulse peak power were estimated to be about 46 μJ and 1.04 kW, while 37.4 μJ and 0.79 kW for 1854 nm laser.

 

Fig. 7 The evolutions of pulse width, pulse repetition rate, pulse energy and pulse peak power with the increase of absorbed power for (a) and (b) at 1938 nm, as well as (c) and (d) at 1854 nm.

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Finally, we would mention our recently published laser results to have a comparison with the present results in this work, which has concerned diode-pumped continuous-wave and passively Q-switched Tm:CYA laser using a MoS₂ saturable absorber [15]. In [15], the maximum output power was only limited to about 1 W for continuous-wave operation. One of the reason for much lower Tm:CYA laser power than the present Tm:CGA laser power is that the absorbed power of the Tm:CYA crystal was almost only half of Tm:CGA. Another reason is the utilization of flat output coupler in that work, which in general leads to worse overlap between the pump beam and cavity mode, as well as a bigger laser beam waist than the optimal inside the laser gain medium. For Q-switching, the shortest pulse width for Tm:CYA laser was about 0.48 μs, i.e. far broader than the present 44 ns achieved in this work. The conventional Cr:ZnSe saturable absorber has advantage in achieving shorter Q-switched laser pulse in comparison with now-popular (2D nanomaterials) MoS₂. Moreover, MoS₂, especially for those 2D nanomaterials fabricated by liquid phase exfoliation, in the form of thin film, has relatively lower damage threshold than Cr:ZnSe, which restricted the laser performance of Q-switching, to a great extent. For these reasons, the present Tm:CGA lasers showed about 10 times higher pulse energy and more than 100 times higher pulse peak power than that of Tm:CYA laser.

4. Conclusion

In summary, diode-pumped Tm:CGA lasers at 1938 and 1854 nm were demonstrated in continuous-wave and Q-switched regimes. The maximum continuous-wave output powers for the two lasers were 3.05 and 2.41 W, respectively. For Q-switching, using a Cr:ZnSe crystal as saturable absorber, the maximum average output powers reached 0.64 and 0.37 W for the 1938 and 1854 nm lasers. The corresponding shortest pulse widths, maximum pulse repetition rates, maximum pulse energies and maximum pulse peak powers were (44ns, 13.9 kHz, 46 μJ, 1.04 kW) and (47 ns, 9.9 kHz, 37.4 μJ, 0.79 kW), respectively.

Power scaling of the continuous-wave lasers could be realized by further optimizing the laser configuration, laser crystal and transmission of the output coupler. Based on these optimizations, with cooling of the saturable absorber, we hope that the laser results of the Q-switching could also be much improved.