The 3 lasers have played an import role in many fields, such as scientific research, industrial production and medical care.^[1,2] In the last two decades, as a medium could generate lasers over this spectral range directly, the Fe:ZnSe crystal has attracted much attention. The first Fe:ZnSe laser pumped by a 2.698- Er:YAG laser was reported by Adams in 1999.^[3] From then on, the Er:YAG, Er:YSGG, Cr:ZnSe, HF, and other lasers which had the wavelength around were employed as the pump.^[4] Up to now, the HF lasers and xenon flash lamp pumped Er:YAG lasers have been the primary pumps for the high energy Fe:ZnSe lasers while the other pumps generated the Fe:ZnSe lasers with the output energy of several millijoules or less.^[4] The HF lasers are usually employed as the pumps for short-pulse-duration Fe:ZnSe lasers at room temperature,^[5–12] and the Fe:ZnSe lasers and the pump both have the pulse durations of 100 ns–200 ns. For high energy Fe:ZnSe lasers with long pulse duration, the free-running 2.94- Er:YAG lasers at low temperature are the efficient pumps which have the pulse durations of several hundreds of microseconds.^[13,14]

Although Er:YAG is widely used as the gain medium to generate the 2.94- lasers, the self-terminal state, in which the lifetime of the upper level (0.1 ms) is far less than that of the lower (6 ms), goes against the population inversion.^[15] Doping the sensitizer Cr³⁺ into the Er:YAG is benefit to the laser operation because Cr³⁺ has two broad absorption bands in the red and blue–green wavelength regions corresponding to the transitions from to and energy levels, which overlap well with the emission of the xenon flash lamp.^[16] The transfer of the excitation from Cr³⁺ to the level and the cross-relaxation together populate the level, which weakens the self-terminal state,^[17,18] as shown in Fig. 1.

Fig. 1

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	Fig. 1 Energy diagram of Cr3+ and Er3+.

In this work, we demonstrated an Fe:ZnSe laser pumped by a Cr, Er:YAG laser in single-shot free-running mode for the first time. For the xenon flash lamp pumped Cr, Er:YAG laser, a maximum single pulse energy of 1.414 J was obtained. The threshold was reduced by 29.3% and the slop efficiency was increased by 52.2%, compared with the Er:YAG laser under the same condition. For the Fe:ZnSe laser, at liquid nitrogen and room temperature, the maximum output energies were 197.6 mJ and 3.5 mJ, the central wavelengths were 4037.4 nm and 4509.6 nm, corresponding to the slop efficiency of 13.4% and 0.27%, respectively.

The optical schematic diagram of the Cr, Er:YAG, and Er:YAG lasers is shown in Fig. 2. The Cr, Er:YAG crystal doped with 0.8 at.% Cr³⁺ and 50-at.% Er³⁺, the Er:YAG crystal doped with 50-at.% Er³⁺ both had a diameter of 5 mm and a length of 150 mm. The circular facets of the two crystals were anti-reflection coated at 2700 nm–3100 nm. Two xenon flash lamps with the electric arc of 140 mm were placed into the double-ellipse condenser cavity. The minimum pulse width of the lamps was with the maximum voltage of 2000 V in our power supply system which ran in single-shot mode. The crystals and lamps were cooled in the deionized water at a temperature of 20 °C. The 324-mm-long straight cavity was composed of a flat rear mirror M1 (high-reflection coated at 2700 nm–3100 nm) and an output coupler M2 (curvature radius equals −1500 mm, and transmittance equals 28.5% at 2700 nm–3100 nm).

Fig. 2

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	Fig. 2 Schematic diagram of the Cr, Er:YAG/Er:YAG laser.

Figure 3 shows the output pulse energy versus the incident pump energy for the Er:YAG and Cr, Er:YAG lasers. The output pulse energy was measured with the Joule meter Coherent J-50MB-YAG-IR (energy range: 1 mJ–3 J) and the energy of the flash lamp was calculated with the formula , where C and U presents the capacitance and voltage of the capacitor, respectively.

Fig. 3

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	Fig. 3 Output pulse energy versus incident energy of the Cr, Er:YAG/Er:YAG laser.

For the Er:YAG laser, the maximum pulse energy was 0.941 J with the pump energy of 400 J. The calculated threshold and slope efficiency were 200.45 J and 0.46%, respectively. While for the Cr, Er:YAG laser, the maximum pulse energy was 1.414 J with the pump energy of 342.25 J. We did not inject the whole pump energy of 400 J to avoid the coating films of the crystal or mirrors being damaged. The calculated threshold and slope efficiency were 141.70 J and 0.70%, which were respectively reduced by 29.3% and increased by 52.2% compared with the Er:YAG laser.

The temporal characteristics of the Cr, Er:YAG laser and flash lamp were detected simultaneously by a PbSe detector (spectral range: 1000 nm–4700 nm, response time: ) and a silicon pin photodiode (spectral range: 200 nm–1100 nm, response time: 50 ns) respectively, and recorded by a Tektronix MSO 3034 digital oscilloscope (sampling rate: 2.5 GS/s, bandwidth: 300 MHz). A typical temporal profile is shown in Fig. 4, and the pulse duration of the Cr, Er:YAG laser was about .

Fig. 4

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	Fig. 4 Temporal profile of the Cr, Er:YAG laser (red) and flash lamp (black).

The wavelength of the Cr, Er:YAG laser was recorded with the help of a WDG-30 monochromator (resolution: 0.3 nm). At the maximum pulse energy of 1.414 J, the wavelength covered the range of 2923 nm–2940 nm, centered at 2931.2 nm with a full width at half maximum (FWHM) of 10 nm according to the Gauss fitting, as shown in Fig. 5.

Fig. 5

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	Fig. 5 Output spectrum of the Cr, Er:YAG laser.

As described above, we obtained a Cr, Er:YAG laser in single-shot free-running mode. As the central wavelength of the Cr, Er:YAG laser was near the absorption peak of the Fe:ZnSe crystal,^[19] we took advantage of this laser as a pump for the Fe:ZnSe laser.

The optical schematic diagram of the Fe:ZnSe laser is shown in Fig. 6.

Fig. 6

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	Fig. 6 Schematic diagram of the Fe:ZnSe laser.

The 50-mm-long straight cavity was composed of a flat input mirror M3 (high-transmission coated at 2700 nm–3100 nm and high-reflection coated at 3800 nm–4700 nm) and an output coupler M4 (coated with different transmittances at 3800 nm–4700 nm). A 45° dichroic mirror M5 was placed behind M4 to filtrate the residual pump.

The Fe:ZnSe crystal, the dimension of which was 4 mm in thickness and 10 mm×10 mm in cross-section, was grown by high-pressure vertical zone melting with the Fe²⁺ doping concentration of 5×10¹⁸/cm³. The two working facets were uncoated, high-quality polished, and parallel to each other within . The crystal was wrapped by indium foil and mounted in a copper heat sink. The crystal temperature was controlled by a liquid nitrogen cooled cryogenic vacuum Dewars, which had two parallel plane uncoated CaF₂ windows aligned perpendicular to the optical axis of the cavity, and detected by a thermocouple of PT100. The Cr, Er:YAG laser was focused via F1 and injected into the center of the Fe:ZnSe crystal along the optical axis.

We have a study on the Fe:ZnSe laser at liquid nitrogen temperature of 77 K and room temperature of 298 K. The measuring instruments were the same as that used in the Cr, Er:YAG laser.

Figure 7 shows the output pulse energy versus incident energy of the Fe:ZnSe laser at 77 K. The maximum pulse energy of 197.6 mJ was obtained under the incident pump energy of 1.38 J when the flat output coupler with a transmittance equal to 50% was used, corresponding to a slope efficiency of 13.4%.

Fig. 7

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	Fig. 7 Output pulse energy versus incident energy of the Fe:ZnSe laser at 77 K (R and T represent the curvature radius and transmittance of M4, respectively).

Figure 8 shows a typical temporal profile of the Fe:ZnSe laser at 77 K, which has the similar shape with the Cr, Er:YAG laser.

Fig. 8

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	Fig. 8 Temporal profile of the Fe:ZnSe laser at 77 K.

The central wavelength of the Fe:ZnSe laser at 77 K was 4037.4 nm with an FWHM linewidth of 122.0 nm according to the Gauss fitting, which is shown in Fig. 9.

Fig. 9

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	Fig. 9 Output spectrum of the Fe:ZnSe laser at 77 K.

Figure 10 shows the output pulse energy versus incident energy of the Fe:ZnSe laser at 298 K. Although the lifetime of upper energy level was as short as 370 ns at room temperature, we still acquired the lasing output on account of the spiked nature of the pump.^[13] The maximum pulse energy of 3.5 mJ was obtained under the incident pump energy of 1.38 J when the flat output coupler with a transmittance equal to 30% was used, corresponding to a slope efficiency of 0.27%.

Fig. 10

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	Fig. 10 Output pulse energy versus incident energy of the Fe:ZnSe laser at 298 K.

Figure 11 shows a temporal profile of the Fe:ZnSe laser at 298 K, the pulse duration of which was sharper and shorter compared with that at 77 K.

Fig. 11

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	Fig. 11 Temporal profile of the Fe:ZnSe laser at 298 K.

The central wavelength of the Fe:ZnSe laser at 298 K was 4509.6 nm with an FWHM linewidth of 171.5 nm according to the Gauss fitting, which is shown in Fig. 12. It had a redshift of about 470 nm compared with that at 77 K.

Fig. 12

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	Fig. 12 Output spectrum of the Fe:ZnSe laser at 298 K.

In conclusion, we demonstrated an Fe:ZnSe laser in single-shot free-running operation pumped by a Cr, Er:YAG laser for the first time. The maximum single pulse energy of the Cr, Er:YAG laser was 1.414 J, the threshold and slope efficiency were reduced by 29.3% and increased by 52.2% compared with the Er:YAG laser due to the doping iron of Cr³⁺, which weakened the self-terminal state. The maximum single pulse energy and central wavelength of the Fe:ZnSe laser were 197.6 mJ and 4037.4 nm at 77 K, respectively, in contrast to 3.5 mJ and 4509.6 nm at room temperature. A higher single pulse energy could be obtained by increasing the pump energy because the gain was not saturated. This work demonstrated that Cr, Er:YAG is a more effective crystal to generate the 2.93- laser compared with Er:YAG, and the Cr, Er:YAG laser is a feasible pump for the Fe:ZnSe crystal to generate the lasers.