1. Introduction

Laser systems with an active medium doped by erbium ions and generating eye-safe radiation around 1.6 $\mu$m wavelength are very interesting for applications such as satellite communication, range finding, lidar, spectroscopy, medicine, and atmospheric sensing [1]. A number of laser hosts is still expanding, the famous are YLF, YAP, or Sc$_2$O$_3$. To group of a well established Er-doped laser material belongs a crystal of Er:YAG which exhibits excellent thermal properties caused by its strictly ordered structure. Less ordered or disordered systems, like mixed fluoride crystals and ceramics, have much broader and smoother spectra in comparison with YAG. Other promising alternative to these materials could be special garnets, as Gd$_3$Ga$_5$O$_{12}$ (GGG) [2,3]. The GGG crystal possesses a relatively low-phonon energy (600 cm$^{-1}$) [4,5] given by heavier elements such as Ga and Gd. A similar phenomenon was observed, e.g., in heavy metal glasses [6]. Multi-site structure of these crystals results in mentioned broadband absorption and emission, preserving good thermal conductivity. In this study we have investigated erbium-doped mixed gadolinium-gallium-aluminium garnet Er:Gd$_3$Ga$_3$Al$_2$O$_{12}$ (Er:GGAG). It could be expected that the Er:GGAG crystal will have a comparable value of the phonon energy as GGG crystal doped by Er ions. Moreover, the Er:GGAG has a lower melting point (2100 K) than YAG, YAP, YSGG, or sesquioxides [7] and it can be easily grown by the Czochralski method.

Laser operations in the Er:GGAG laser host were already observed at a wavelength of 2.9 $\mu$m [8]. The transition $^4$I$_{13/2}$ $\rightarrow \, ^4$I$_{15/2}$ enables to generate eye-safe laser radiation around $\sim$ 1.6 $\mu$m. An eye-safe region includes radiation with a relatively low probability of retinal damage connected with a loss of vision. Radiation at longer wavelength than 1.4 $\mu$m is strongly absorbed by an eye front part (cornea, aqueous humor, and lens) and therefore cannot reach a significantly more sensitive retina [6,9].

As a pumping system for Er:GGAG crystal, a diode laser emitting radiation at 1.46 $\mu$m could be used. While using a pumping system emitting within this spectral region (around 1.5 $\mu$m), a laser radiation generation profits from resonant pumping, i.e., pumping directly into lasing bands [10]. In comparison with non-resonant pumping (e.g. at 970 nm), this pumping type provides small thermal losses due to a low quantum defect and lower thermal stress of laser rod [11–13]. Nevertheless, residual heat deposition inside the pumped medium is always present. The spectroscopic properties (absorption and emission cross-sections) of solid-state active laser media can also be very often improved at lower temperatures. Laser generation at low temperature permits more efficient pumping of an active medium, resulting for example in higher output power [14–16]. For in-band pumping of cryo-cooled Er:YAG laser (pumping radiation wavelength 1532 nm, incident power $\sim$ 53 W), a quasi CW output power of 25 W was achieved [17]. Moreover, highly efficient diode resonant pumping of Er-doped vanadates (Er:YVO$_4$ and Er:GdVO$_4$) was demonstrated [18]. The corresponding slope efficiencies for Er:GdVO$_4$ and Er:YVO$_4$ laser systems up to 83 and 85 %, respectively, were reached using an Er-fiber laser pumping at 1538.8 nm. The submitted study is a continuation of our cryogenic laser research with Er:YAP and Er:YAG active media [19,20]. The main goal was to investigate the temperature influence on spectroscopic and lasing properties of the Er:GGAG crystal.

2. Experimental setup

The gadolinium-gallium-aluminium garnet laser rod doped by erbium ions with 0.53 at.% (Er/Gd) concentration had a form of a cylinder (9.5 mm in length, 15 mm in diameter) with plane-parallel polished faces without anti-reflection coatings. The Er:GGAG crystal was grown by the Czochralski method in a slightly oxidative atmosphere using an iridium crucible. The tested Er:GGAG crystal had a composition Gd$_{2.985}$Er$_{0.017}$Ga$_{2.641}$Al$_{2.357}$O$_{12}$. From the X-ray measurements, it follows that the GGAG crystal has a cubic structure (lattice constant a = 12.231 A) with space group O$^{10}_h$ (Ia3d). From the ion radius and oxidation states of the Er$^{3+}$, one could predict that Er$^{3+}$ substitutes Gd$^{3+}$ in the GGAG crystal lattice. Due to partial substitution of Ga$^{3+}$ by Al$^{3+}$ ions, the crystal field is distorted, and the spectral line width becomes larger because of the inhomogeneous broadening [21].

The tested sample was placed in a temperature-stabilized copper holder inside the vacuum chamber of a liquid nitrogen-cooled cryostat (Janis Research, VPF-100). The temperature was controlled within the 80 - 340 K temperature range by a controller Lake Shore (model 325). On both side of vacuum chamber there were uncoated CaF$_2$ windows for spectroscopy and also for laser experiments. These windows could not be aligned independently of the laser crystal (due to the cryostat construction), so it could cause some power loss due to Fresnel reflection (about 3 % at the border). The spectrophotometer (Shimadzu UV - 3600) with spectral resolution 0.8 nm in mid-infrared region was used for measuring the transmission spectra and calculation of the absorption coefficient. The fluorescence spectra and laser emission were measured by the fiber-coupled spectrometer (Ocean Optics NIR 512) sensitive in the 0.9 - 1.7 $\mu$m spectral range.

For longitudinal end-pumping, a fiber (core diameter 400 $\mu$m, NA = 0.22) coupled laser diode system (M1F4S22-1470.5-30C-IS30.1, DILAS) was used. The laser diode was operating in the pulsed regime (10 ms pulse length, 10 Hz repetition rate, maximum mean power 24 W). It was possible to temperature-tune the laser diode emission wavelength down to local absorption maximum of the Er:GGAG at wavelength 1461 nm (8 nm linewidth, FWHM). Pumping radiation was collimated and focused into the sample by two achromatic lens doublets with the same focal lengths f = 75 mm. The beam waist diameter was investigated to be 310 $\mu$m. The 142 mm long hemispherical laser resonator consisted of a flat pumping mirror with high transmission for pumping radiation and high reflectivity for laser emission and curved (r = 150 mm) output coupler with reflectivity of 98 % @ 1645 nm. The pumping mirror and the active medium were placed inside the vacuum chamber of the liquid nitrogen cryostat, while the output coupler remained outside it. The laser layout is shown in Fig. 1.

 

Fig. 1. (a) Scheme of resonantly pumped Er:GGAG laser - hemispherical resonator formed by the spherical pumping mirror (PM) placed in vacuum chamber of liquid nitrogen cooled cryostat and output coupler (OC) in the air; (b) Photo of Er:GGAG crystal with Er$^{3+}$ doping concentrations 0.53 % (length 9.5 mm).

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The output mean power of the laser was measured using a broadband thermopile probes (PM3 and PM10, Coherent) in connection with energy/power meter (Molectron EMP2000, Coherent). The temporal structure of the generated radiation was observed by PbSe detector (PDA20H-EC, sensitivity 1.5 - 4.8 $\mu$m, Thorlabs) connected to oscilloscope (TDS 3052B, 500 MHz, 5 GS/s, Tektronix). The beam spatial structure was analyzed by the infra-red sensitive camera (Pyrocam III, Spiricon).

3. Results and discussion

The spectroscopic properties and laser output characteristics in dependence on Er:GGAG crystal temperature within 80 K and 340 K were investigated. The measured absorption coefficient in the spectral range 1430 - 1660 nm for selected temperatures of 80 K, 180 K, 300 K, and 340 K is shown in Fig. 2(a). At the pumping wavelength 1461 nm, an increase in this coefficient with decreasing Er:GGAG crystal temperature could be observed. The absorption decrease with decreasing temperature in lasing part of the spectra (above 1.6 $\mu$m) is also remarkable. These two dependencies greatly improve the efficiency of laser radiation generation at lower temperatures.

 

Fig. 2. (a) Absorption spectra of the Er:GGAG crystal for selected temperatures: 80 K, 180 K, 300 K, and 340 K, inset graph - spectral overlap of the absorption peak (sample temperature 80 K) and 1461 nm emission line of the pump diode; (b) Fluorescence spectra of the Er:GGAG laser crystal for selected temperatures of 80 K, 200 K, 300 K, and 350 K, inset graph - dependence of the fluorescence decay time on temperature.

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To investigate the fluorescence spectra in the temperature range from 80 K up to 350 K, the Er:GGAG sample was excited by a laser diode emitting radiation at 971 nm. The spectra for selected temperatures 80 K, 200 K, 300 K, and 350 K can be seen in Fig. 2(b). From this figure, it can be seen that with decreasing crystal temperature, the peaks became narrower, mainly from 1500 to 1550 nm. To evaluate the decay time of the upper laser level $^4$I$_{13/2}$ the exponential fit was used. From inset in Fig. 2(b), one can see that the fluorescence decay time was decreasing from 6.4 ms to 5.5 ms with rising temperature from 100 K to 340 K, respectively.

The dependence of output peak power on absorbed pumping peak power with respect to the Er:GGAG active medium temperature was also investigated (Fig. 3(a)). The highest corresponding slope efficiency was 54 % at 80 K. The maximal output peak power was 2.8 W (pulse duration 9.6 ms). Fig. 3(b) shows the dependencies of the slope efficiency and laser threshold on crystal temperature. The slope efficiency was decreasing from 54 % to 29 % with increasing temperature from 80 K to 340 K, respectively. The laser threshold had an inverse trend - it increased from 0.5 W to 1.7 W for temperature increase from 80 K to 340 K, respectively.

 

Fig. 3. (a) Output peak power dependence of the Er:GGAG laser pumped by 1461 nm fibre-coupled diode on absorbed pumping peak power for various crystal temperatures; (b) Dependence of slope efficiency and laser threshold on crystal temperature.

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The Er:GGAG laser emitted radiation at $\sim$ 1650 nm (4 nm FWHM) in whole temperature range. For temperatures from 80 to 140 K, the second minority emission peak was observed at 1639 nm (5 nm FWHM). The spectrum of emitted wavelength can be seen in Fig. 4(a). The beam spatial structure was not entirely symmetrical, nevertheless it was close to a fundamental Gaussian mode (Fig. 4(b)). This could be due to the local crystal in-homogeneity or heat distribution in the crystal.

 

Fig. 4. (a) Spectrum of emitted wavelength by the Er:GGAG laser; (b) Beam laser spatial structure for 80 K crystal temperature and 2.8 W output peak power.

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4. Conclusion

The Er:GGAG crystal temperature has a significant influence on its spectroscopic and lasing properties in the temperature range from 80 to 340 K. In the case of the active material temperature decrease and the pumping wavelength 1461 nm, the increase of the Er:GGAG crystal absorption coefficient was observed. Moreover, with decreasing active crystal temperature, the fluorescence spectrum peaks became narrower, mainly from 1500 to 1550 nm. The fluorescence decay time at the upper laser level ($^4$I$_{13/2}$) was decreasing from 6.4 ms to 5.5 ms with increasing temperature from 100 to 340 K, respectively. The temperature decrease to 80 K improved the absorption coefficient at wavelength 1461 nm and therefore the corresponding output peak power value was measured higher (2.4 times) with respect to room temperature. The highest corresponding slope efficiency was 54 % at 80 K. The maximal output peak power was 2.8 W (pulse duration 9.6 ms). To the best of our knowledge, these results with Er:GGAG crystal as a new active laser medium enabling to generate radiation in the eye-safe region have not been presented so far.