Middle-infrared (mid-IR) laser sources have played a crucial role in a variety of applications, such as molecular spectroscopy, free-space communication, laser surgery, eye-safe laser radars, and many others^[1–4]. At present, the main methods of generating mid-IR wavelengths include nonlinear optical frequency conversion technology^[5], quantum cascade semiconductor lasers^[6], rare-earth ions-doped crystal or fiber lasers^[7,8], and transition-metal (TM)-doped II-VI chalcogenide lasers^[9,10]. Chromium-doped II-VI chalcogenides, like Cr:ZnSe/ZnS, have favorable physical and spectroscopic characteristics, such as a 100% quantum efficiency of fluorescence at room temperature, a high gain cross section, and an extremely broad emission band, and thus have been widely used as the gain media to build tunable or ultrafast laser sources around 2–3 µm^[1,11].

In particular, the wide absorption spectrum of Cr:ZnSe/ZnS, ranging from 1.3 µm to 2.1 µm^[9], makes them less restrictive to the choice of pump sources. Multiple pump sources have been used to pump Cr:ZnSe/ZnS lasers^[12–23]. Both 1.56 µm Er3+ fiber lasers and 1.93 µm Tm3+ fiber lasers have been used to pump Cr:ZnSe or Cr:ZnS lasers^[12,13]. Currently, most of the optical pump sources used for Cr:ZnSe/ZnS lasers are fiber lasers^[12–19]. The use of bulk lasers to pump Cr:ZnSe/ZnS has also been demonstrated in a few studies^[20–22]. A continuous-wave (CW) Tm:YLF laser at 1918 nm and a CW Tm:YAP laser at 2000 nm have been employed to pump Cr:ZnSe single crystal lasers, respectively^[20,21]. In 2019, a CW Tm:YLF laser at 1908 nm pumped Cr:ZnSe crystal laser was demonstrated, and a maximum output power of 1.8 W was obtained^[22]. The λabs of the absorption peak of Cr:ZnSe^[9] is located at 1.77 µm. Theoretically, a pump with a wavelength closer to the peak is more favorable, which could lower the requirement for the doping concentration of Cr2+ ions. Compared to the absorption coefficient of the absorption peak of 1.77 µm, a decline of less than 20% can be realized if the pump wavelength is between 1.7 µm and 1.85 µm, but the absorption coefficient would decline to ∼2/3 at 1908 nm or to ∼1/3 at 1560 nm, respectively^[23]. The uniform Cr doping of the ZnSe media is critical for obtaining multi-watt output powers. Moreover, insufficient Cr concentration and low pump absorption make them non-suitable for high-power lasers, but it is not easy to fabricate high quality Cr:ZnSe crystals with sufficient concentration. Therefore, if a high-quality Cr:ZnSe crystal with low concentration is used, a pump with a wavelength closer to the absorption peak is in need. Recently, an ∼1.7μm laser diode (LD) pumped CW/pulsed Cr:ZnSe single crystal lasers have also been reported^[23]. It is well known that LDs have bad beam quality, which usually needs a complex pump system to reshape the pump beam. A bulk crystal pump source with a good beam quality and pump wavelength closer to the absorption peak of the Cr:ZnSe/ZnS crystals is quite desired.

In this Letter, a CW and broadly tunable Cr:ZnSe laser pumped by a Tm:YLF bulk laser with 1845 nm and 1887 nm wavelengths is demonstrated for the first time and to the best of our knowledge. For the pump wavelength of 1887 nm, the maximum output power is 1.79 W, and the corresponding slope efficiency is 28.8%. For the pump wavelength of 1845 nm, the maximum output power is 1.39 W, and the corresponding slope efficiency is 28.3%. Furthermore, the wavelength-tuning properties of this Cr:ZnSe laser at both pump wavelengths using a reflective diffraction grating are also investigated. The widest tuning range achieved at the pump wavelength of 1887 nm is 582 nm from 2066 nm to 2648 nm. For the pump wavelength of 1845 nm, a tuning range of ∼700nm (696 nm) from 2040 nm to 2736 nm is achieved. This is the widest tuning range currently achieved in this configuration, to the best of our knowledge.

The experimental setups for the CW and wavelength tuning operation of the Cr:ZnSe bulk crystal laser are illustrated in Figs. 1(a) and 1(b), respectively. The configurations for the Tm:YLF bulk laser are the same as in Ref. [24]. For the first output coupler (OC1), two flat-flat mirrors (OC1-a, OC1-b) with different transmittances at 1.8–2.0 µm are used, so laser outputs of 1845 nm and 1887 nm are obtained to pump the Cr:ZnSe crystal, respectively. The specific transmittance of OC1-a and OC1-b and the oscillating wavelengths are summarized in Table 1. The beam quality of the 1845 nm pump light is measured to be 1.59 and 1.91 in the horizontal and vertical directions, respectively, and the beam quality of the 1887 nm pump light is measured to be 1.42 and 1.92 in the horizontal and vertical directions, respectively. Dichroic mirrors DM1 and DM2 with the same parameters are used to filter out the 793 nm pump light. An optical isolator (Thorlabs, IO-5-1950-HP) is placed after DM2 to prevent the back-feedback light from affecting the laser output intensity stability of the Tm:YLF pump laser. A plano-convex lens L3 with a focal length of 75 mm and high transmittance (HT) coating for 1.65–3 µm is used to focus the pump beam into the Cr:ZnSe crystal, and the focused spot diameter of the pump beam inside the Cr:ZnSe bulk crystal is calculated to be ∼150μm using the software WinABCD.

In Fig. 1(a), a V-shaped folded cavity is built for the CW operation of the Cr:ZnSe laser. The flat mirror M2 with HT coating for 1.6–1.9 µm and high reflectivity (HR) coating for 2.1–2.7 µm is used as the input mirror for the folded cavity. A Cr:ZnSe bulk crystal with dimensions of 2mm×2mm×5mm and a nominal doping concentration of 3×1018cm−3 is used as the gain medium. Both faces of the crystal are HT coated for the laser wavelengths in the range of 2.2–2.6 µm and the pump wavelengths in the range of 1.52–1.95 µm. The folded mirror M3 is a plano-concave mirror with a radius of curvature (ROC) of 100 mm, with HT coating for 1.6–1.9 µm and HR coating for 2.1–2.7 µm. For OC2, two OCs with 5% and 8% transmittance, respectively, in the range of 2.1–2.7 µm are used. In the V-shaped folded cavity, the waist diameter (DL) of the oscillating laser in the cavity is designed to be 232 µm, 203 µm, and 187 µm by adjusting the distance between M3 and OC2, respectively. To remove the heat generated in the crystals, the Tm:YLF crystal and the Cr:ZnSe crystal are both wrapped in indium foils and mounted in copper heat sinks whose temperatures are maintained at 18°C by water chillers.

In Fig. 1(b), an X-shaped folded cavity is built for the wavelength tuning operation of the Cr:ZnSe laser. The same Tm:YLF pump system and Cr:ZnSe crystal as in Fig. 1(a) are still used. The input mirror M4 and the folded mirror M5 have the same parameters as M3, and a 450 g/mm reflective mid-IR diffraction grating (Thorlabs, GR1325-45031) is used as the tuning element. In this configuration, the spot diameter of the 1.8 µm pump beam focused in the crystal is 193 µm, and the waist spot diameter of the laser beam is 185 µm. The diffraction grating is placed at an initial angle of incidence of 58°, and the wavelength-tuning is obtained by adjusting the angle of the diffraction grating.

The spectra of the 1845 nm and 1887 nm pump light from the Tm:YLF laser measured by a spectrometer (Yokogawa, AQ6375B) are shown in Fig. 2(a). The linewidth of the Tm:YLF laser at 1845 nm is 1.8 nm, and at 1887 nm is 1.3 nm. When the 793 nm LD works at the maximum output power of 30 W, for the 1887 nm pump light, the maximum incident pump power into the Cr:ZnSe crystal is 6.31 W. For the 1845 nm pump light, the maximum incident pump power is 5.23 W due to the larger insertion loss of the commercial optical isolator, which is optimized for 1950 nm wavelength. The absorption efficiencies of the Cr:ZnSe crystal for the two pump lights are shown in Fig. 2(b), and it can be found that the absorption efficiency decreases gradually with the increase of the incident pump power.

For the CW operation of the Cr:ZnSe laser, different configurations are used. Figure 3 shows the output power curves under 1845 nm pumping. Figure 3(a) shows that when TOC2 of 5% is used, the maximum output power is 1.14 W with a slope efficiency of 23.1% corresponding to a laser waist diameter of 203 µm. Figure 3(b) shows that when TOC2 of 8% is used, the maximum output power is 1.39 W with a slope efficiency of 28.3%, corresponding to a laser waist spot diameter of 187 µm.

Figure 4 shows the output power curves under 1887 nm pumping. Figure 4(a) shows the results when TOC2 of 5% is used; the maximum output power is 1.28 W with a slope efficiency of 20.9%, corresponding to a laser waist spot diameter of 187 µm. It can be seen that the output power tends to be saturated when the incident pump power exceeds 5 W. During the experiment, we found that if the incident pump power exceeded 5 W, the laser output would become unstable. Figure 4(b) shows that when TOC2 of 8% is used, the maximum output power is 1.79 W with a slope efficiency of 28.8%, corresponding to a laser waist spot diameter of 203 µm. This is the maximum output power and the highest slope efficiency obtained in the experiment. We believe the efficiency of only 28.8% should be attributed to the following reasons. First, the pump absorption is not sufficient in our experiment. We used a Cr:ZnSe crystal with the size of 2mm×2mm×5mm and doping concentration 3×1018cm−3, which has an absorption coefficient of ∼2.4cm−1 (1845 nm small signal incidence). Second, the TOC in our experiments is only 8%, it can be found that the efficiency will be higher if the transmittance of the OC is higher^[16,25]. For a DL of 187 µm and 232 µm, the maximum output power is 1.63 W and 1.33 W, respectively, and a slight saturation can be observed as well.

Comparing Fig. 3 with Fig. 4, one can see that the laser waist diameter affects the laser output power when a 1887 nm pump is used, but this phenomenon is not obvious when the 1845 nm pump is used. This difference should be attributed to the better mode-matching effect of the 1845 nm pump created by the stronger absorption of the 1845 nm light. Also, it can be seen that when an output coupler with a transmittance of 8% is used, the slope efficiency and the obtained maximum output power are both higher than the case where the transmittance of OCs is 5%.

We measure the power stability of the laser at the maximum output power for the Cr:ZnSe laser pumped by 1845 nm and 1887 nm, the RMS values for its power stability are measured over 60 minutes to be 0.84% and 0.75%, respectively and are shown in Fig. 5. Figures 6(a)–6(d) show the spectra of the CW operation of the Cr:ZnSe laser, which are measured with a spectrometer (Thorlabs, OSA 207C) and are essentially in the range of 2420–2550 nm.

The M²-factors of the CW Cr:ZnSe laser are measured with a BeamGage professional camera (Spiricon, PY-IV-C-A Pro). As shown in Fig. 7(a), for the 1845 nm pump, the M2 factors are measured to be 1.65 and 1.51 in the horizontal and vertical directions, respectively. The OC used in the measurement has a transmittance of 8%, and the laser wavelength used in the calculations is 2520 nm. It should be noted that the measurement is taken at the incident pump power of 2.89 W with a corresponding output power of 730 mW. If the incident pump power continues to rise, then the laser beam will no longer be a Gaussian TEM00 but a higher-order mode, as shown in Fig. 8. As shown in Fig. 7(b), for the 1887 nm pump, the M2 factors are measured to be 1.56 and 1.82 in the horizontal and vertical directions, respectively. The OC used in the measurement also has a transmittance of 8%, and the laser wavelength used in the calculations is 2450 nm. This corresponds to an incident pump power of 4.6 W and an output power of 1.2 W. Similarly, if the incident pump power continues to be elevated, then higher-order modes will appear. Although the temperature coefficient of the refractive index of the Cr:ZnSe crystal is as high as 2.6×10−5K−1^[26], we believe that the reason for the onset of the higher-order mode at such a low-level pump may be mainly the quality of the used crystal.

The tuning properties of the CW Cr:ZnSe crystal laser are also tested under the 1845 nm and the 1887 nm pump, respectively. The spectrometer is still the OSA 207C. Figure 9(a) shows the tuning curves of the output power for the OCs with two different transmittances at 6.31 W of the 1887 nm pump. By adjusting the angle of the diffraction grating, a tuning range of 582 nm from 2066 nm to 2648 nm is achieved, and the spectra of the tuned output are shown in Fig. 9(b). For the 1845 nm pump, Fig. 10(a) shows the tuning curves of the output power for the OCs with two different transmittances at 5.3 W of the 1845 nm pump. As shown in Fig. 10(b), a tuning range of 696 nm from 2040 nm to 2736 nm is realized, which is the widest tuning range realized with a diffraction grating as the tuning element of a Cr:ZnSe bulk crystal laser. The linewidths of the Cr:ZnSe-tuned spectra are basically in the range of 0.3–0.6 nm. We compare our results in Table 2 with five other publications that also reported on mid-IR tunable lasers based on the Cr:ZnSe bulk crystal. The gain bandwidth is directly connected with the total absorption in the crystal. Generally, a pump with a wavelength closer to the absorption peak will bring more intense absorption at the incidence facet of the crystal, which creates a higher population inversion ratio and is more beneficial for increasing the tuning range. For the maximum and minimum values of the tuning range, we believe they are mainly limited by the coatings of our dichroic mirrors. Theoretically, the increase of the transmission of the OC mirror will result in a reduction of the tuning range at the same pump power level. The spectral tuning bandwidth behavior shown in Fig. 10 (1845 nm pump) agrees well with the theoretical expectation, but the spectral tuning bandwidth behavior shown in Fig. 9 (1887 nm pump) looks contradictory to the theoretical expectation. Analyzing Figs. 3 and 4, one can find that for 1887 nm pump with a 5% OC, the laser performance is the worse, and the mode-matching exerts an evident influence on the laser output. Therefore, the abnormal spectral tuning bandwidth behavior by 1887 nm pump should be due to the greater internal loss created by the 5% OC. In Figs. 9(a) and 10(a), one can see a well-marked peak between 2600 nm and 2650 nm on the tuned curves. The gray dashed lines show the water-vapor absorption. We believe the appearance of the power surge is caused by the valley in the water-vapor absorption band.

In conclusion, a CW and broadly tunable Cr:ZnSe laser pumped by a Tm-doped bulk laser with a wavelength below 1900 nm is demonstrated for the first time and to the best of our knowledge. When the laser operates in the CW regime, a maximum output power of 1.79 W with corresponding slope efficiency of 28.8% is obtained with a 1887 nm Tm:YLF pump laser and a maximum output power of 1.39 W with a corresponding slope efficiency of 28.3% is obtained with a 1845 nm Tm:YLF pump laser. In terms of the tuning property of the CW Cr:ZnSe crystal laser, a wide tuning range of 696 nm from 2040 to 2736 nm by using a reflective mid-IR diffraction grating and a 1845 nm Tm:YLF pump laser is realized. In addition, the laser beam quality has been measured as well, the fundamental mode operation is limited to a pump power level of 4.6 W at most, and further improvement of the crystal fabrication should be a key point in a future power scale. The experimental results show that a compact and cost-effective Tm-doped bulk laser emitting below 1900 nm could be considered to be a quite suitable source for the pumping of Cr2+ doped II-VI chalcogenides lasers.