1. Introduction

In recent years, due to the fast development of diode pumped solid state lasers, research in the area of high energy/high average power lasers with different geometry such as slab and thin disks has greatly expanded. As a consequence, several new applications in the field of science and technology have emerged such as laser shock peening, high harmonic generation, soft X-ray source via laser Compton source, laser induced damage threshold measurements, and laser driven inertial fusion energy.

To develop systems of this class researchers used Ytterbium (Yb³⁺ - here after Yb) ion, because of its simplest energy level structure and easy availability of semiconductor laser diodes in the kilowatt level to use as pump source [1, 2]. However, for generation of picosecond or sub-picosecond short pulses, laser host materials with broad emission band-widths are necessary. Several Yb doped host materials were investigated in the past and a good comparison with respect to band-width limited pulse duration was reported in [3]. In this context, Yb: Y₃Al₅O₁₂ (Yb:YAG), which belongs to the family of garnet, seems to be promising due to the excellent material properties and furthermore ease of availability in the market either as crystal or ceramic when compared to other laser hosts.

To avoid thermal issues and to enhance the efficiency, the lasers based on Yb:YAG is cooled down to cryogenic temperatures. The cryogenic cooling enhances the thermo-optic [4] and spectroscopic properties [5], but at the same time emission band-width [6, 7] of Yb:YAG decreases tremendously, impeding the generation of short pulses.

The alternative way to overcome the band-width issue is to have mixed garnets which will have similar material properties as that of YAG. However, the thermal conductivity of these mixed garnets is expected to be lower than that of YAG. These mixed garnets can be achieved by replacing a part of aluminum metal ion with other metal ions such as gallium, scandium in the lattice structure which gives rise to an inhomogeneous broadening in the host matrix. In addition, change of the material composition in the host matrix, results in variation of crystal field strength, which will allow having different emission wavelengths of a particular active dopant ion of our choice. Several mixed garnets have been reported in the past based on Nd and Yb active ions, some among them are Nd:Y₃ScA1₄O₁₂ ceramic [8], Nd:Y₃Sc₂A1₃O₁₂ (Nd:YSAG) ceramic [9], Nd:Y₃Ga_xAl_(5-x)O₁₂ crystal [10], Nd:Y₃Ga₂Al₃O₁₂ (Nd:YGAG) ceramic [11], Yb:Y₃ScA1₄O₁₂ ceramic [12], Yb:Gd₃Al_xGa_5-xO₁₂ (Yb:GAGG) crystal [13]. This gives us the motivation to explore the promise of a new mixed garnet based on Yb: Y₃Ga₂Al₃O₁₂ (Yb:YGAG) fabricated as ceramic.

Here in this work, we studied the spectroscopic characteristics of Yb:YGAG in terms of absorption and emission cross-sections at various cryogenic temperatures. In addition, cryogenic continuous-wave laser operation at several cryogenic temperatures was also demonstrated.

2. Experimental

The experimental setup used for the cryogenic spectroscopic measurement is depicted in Fig. 1(a). The setup used is similar to the setup reported in [14] to reduce re-absorption effect, and it allows carrying out the absorption and emission measurements at the same run. For absorption measurement we used a fiber coupled white light source (Bentham U.K model no: WLS100) where the intensity is flat in the 800 to 1200 nm range. For emission measurement we used a fiber coupled laser diode as pump source centered at 940 nm and it was focused on the sample surface at an angle about 40° with respect to the detection beam path.

 

Fig. 1 (a). Experimental setup used for absorption and fluorescence spectra measurements. L1, L2 – Achromatic lens. M1, M2 – Silver mirrors. SM1& SM2, Silver coated spherical mirrors. WLS – White light source, FLD – Fiber coupled laser diode centered at 940 nm, High resolution spectrophotometer. Figure 1(b). 10at.% Yb: YGAG ceramic; 2 mm thick.

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All the obtained spectra were collected using High resolution spectrophotometer (Horiba Jobin Yvon, model no: 1250M) which has a resolution down to 15 pm, and includes a diffraction grating of 600 grooves/mm for the 1 μm region. Two kinds of photomultiplier tubes were used, one covering the region of 800 – 1000 nm (Hamamatsu, model no: R2658P) and other covering the region of 950 – 1200 nm (Hamamatsu, model no: H10330A-25). For measuring the fluorescence decay curves, 940 nm pulsed pumping was employed at pulse duration of 4 ms that includes a rise time of 8µs and fall time of 4µs. The decay was detected using the H10330A-25 photomultiplier tube which has a time response of 900 ps rise time. All the decay curves were recorded and averaged with a digital oscilloscope (Lecroy, model no: 334A).

Efficient cooling of the 10at.% Yb:YGAG sample (Konoshima chemical Co. Ltd - see Fig. 1(b)) was provided by a brass holder placed in a cryogenic chamber that contains a two-stage closed-cycle helium cryostat (Cryodyne, model no: 22C) with heat load of 1W at 10K. To precisely monitor and maintain the temperature, a temperature controller (Lake Shore, model no: 335) which includes a 50 ohm resistor for heating and two silicon diode temperature sensors (model no: DT-670), one near the sample and other near the cold fringe was used. For determination of absorption (σ_a), emission (σ_e) and gain (σ_g) cross-sections we used the theoretical approach such as Beer’s Lambert law [15], McCumber or reciprocity method [16], Fuchtbauer–Ladenburg (FL) [17] method and same methodology as described in our previous works [6, 7].

Cryogenic laser experiments using uncoated sample were carried out using the “L” shaped cavity setup as shown in Fig. 2. It consists of anti-reflection (AR) coated achromatic lenses (L1 and L2), concave mirror (M1) high reflection (HR) coated for 1030 nm (radius of curvature = −300 mm), dichroic mirror (M2) HR coated for 1020 – 1200 nm in the front side and AR coated for 900 – 980 nm in the rear side, plano convex lens (f = 150mm) (L3) AR coated for 1030 nm, plane output couplers (M3) with the transmissions (T_oc) of 2%, 3%, 5%, 10% and 20% in the spectral range from 1020 nm to 1070 nm.

 

Fig. 2 Cryo laser setup: L1, L2 achromatic lens, M1 – concave mirror, M2 – dichroic mirror, L3 – plano convex lens, M3 – plane output coupler mirrors (Toc = 2%, 3%, 5%, 10% and 20%) and 10at.% Yb:YGAG ceramic.

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The pump source was a fibre coupled diode laser that delivers a maximum output of 25W at 940 nm with a numerical aperture of 0.15 and the spectral bandwidth is 3.5 nm (BWT Beijing LTD). The output was imaged using the achromatic lenses L1 and L2 in the ratio of 1:2 to the uncoated 10at.%Yb:YGAG, 2 mm thick and 18 mm diameter, ceramic with normal incidence to have the pump beam size of around 210 µm. The mode-size near the Yb:YGAG ceramic was estimated to be around 220 µm based on the ABCD-matrix formalism. The cryostat that was used during spectroscopic experiment was used to cool down the sample.

3. Results and discussion

3.1 Cryogenic spectroscopic results

Absorption and emission measurement were carried in the 20 – 340K range in step size of 20K. In addition, the stark level splitting of Yb:YGAG ceramic was determined using the low temperature absorption and emission spectra at 10 K. The energy level values are: upper level splitting (²F_5/2 level – cm⁻¹) – 10841, 10611 and 10312 and the lower level splitting (²F_7/2 level – cm ⁻¹) – 690, 563, 373, 0. For this 10at.%Yb:YGAG sample, the life time at room temperature was estimated to be 1.02 ms and the theoretical ion density was estimated to be 1.20 × 10²¹ atoms/cm³. All these data were used for the determination of absorption and emission cross-sections as mentioned in the experimental section. For the sake of clarity, only the results from some selected temperatures are presented.

Figures 3(a)-3(c) show the absorption, emission and gain cross-section of Yb:YGAG at various temperatures (40K,100K, 200K and 300K), respectively. In general, the peak cross-sections increase with decrease in temperature. From Fig. 3(a) one can see a broader absorption in the 910 to 945 nm range, peak absorption centering at 942 nm and zero phonon line at 970 nm which is close that of Yb:YAG [6, 7] and Yb:LuAG [6, 7]. This is an advantage, so one can use the same pump sources as that of Yb:YAG and LuAG. From Fig. 3(b) one can infer a broader emission band in the 1000 to 1050 nm range and peak emission centered around 1028 nm. Furthermore, for temperature less than 180K, the broad 1028 nm peak splits into two narrow peaks around 1026 nm and 1028.5 nm. In addition, one can observe that both absorption bandwidth and emission bandwidth (crucial parameter for short pulse generation) decrease with decrease in temperature. It is worth to note that the emission band-width is three times larger (see Table 1) when compared with the same family of materials Yb:YAG [6, 7] at cryogenic temperatures, which makes this material promising for the generation of short pulses. This inhomogeneous broadening in Yb:YGAG occurs due to the presence of mixed occupancy of Al and Ga on the octahedral and tetrahedrally coordinated sites resulting in the variation of crystal field at different Yb³⁺ sites. A detailed comparison of cross-sections and bandwidth’s of Yb:YAG and Yb:YGAG for various temperatures can be found in Table 1.

 

Fig. 3 (a) Absorption, (b) emission and (c) gain cross-sections (β is inversion rate) of Yb:YGAG ceramic

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Table 1. Comparison of cross-sections and bandwidth’s of Yb:YAG [6, 7] and Yb:YGAG for various temperatures

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3.2 Cryogenic continuous-wave laser performance

Cryogenic continuous-wave laser experiments using uncoated sample were carried out by pumping at 940 nm as mentioned in the experimental session. As a first step, input-output power characteristics were measured for different transmission of output couplers and the temperature of the sample was maintained at 100 K. Since we are limited to measure the real absorbed power due to the large divergence of the unabsorbed power after the sample, the slope efficiency was estimated with respect to incident power for all the measured values of output couplers and is presented in Fig. 4(a). In addition, to have an idea of how much pump light is absorbed during laser experiment at 100K, we used the absorption cross-section values at 100K and estimated the absorption in the sample to be around 85%.

 

Fig. 4 (a) Input-output power characteristics of uncoated Yb:YGAG at different transmission of output couplers, (b) laser emission wavelength and fluorescence emission spectra at 100K and (c) Input-output power characteristics of uncoated Yb:YGAG at various temperatures with 20%Toc

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From Fig. 4(a) it is inferred that T_oc = 20% gives maximum output power and slope efficiency when compared to others and we decided to use 20% T_oc to study the effect of temperature. The laser wavelength observed was 1025 nm irrespective of the transmission of output coupler used and the obtained laser wavelength is in accordance with the emission spectra. Figure 4(b) shows the obtained laser emission wavelength and fluorescence emission spectra at 100K .

Figure 4(c) shows the input-output power characteristics of Yb:YGAG laser with respect to various temperatures in step size of 20K. For the sake of clarity only few temperatures are presented. From Fig. 4(c), one can infer the increase in output power when the sample is cooled down from room temperature to cryogenic temperature. This increase in output power is related to the increase in stored energy and gain, as well as reduction of re-absorption at laser wavelength when sample is cooled down to cryogenic temperatures.

Figure 5(a) shows the evolution of slope efficiency and laser threshold with respect to incident power at various temperatures and Fig. 5(b) shows the temperature versus maximum output power characteristics for Yb:YGAG laser at a pump power of 15.52W. From Fig. 5(a) it is inferred that slope efficiency increases and the laser threshold decreases with decrease in temperature. From Fig. 5(b) we observed a maximum output of 2.92W and one could observe saturation in the maximum output power for temperature lower than 60K which could be related with the bleaching of the sample. Although the surface flatness of Yb:YGAG sample was relatively low (λ/2), the observed beam profile which is Gaussian at 100K is shown as inset in Fig. 5(b). Further improvement of beam quality is possible by applying precise polishing for laser applications.

 

Fig. 5 (a) Evolution of slope efficiency and laser threshold with respect to incident power for Yb:YGAG at various temperatures and (b) Temperature versus maximum output power for Yb:YGAG laser at a pump power of 15.52W (beam profile shown as inset).

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Since the Fresnel loss of uncoated sample is approximately 8.5% per surface which corresponds to the cavity round-trip loss of 29.8%, the net cavity round-trip loss using 20%T_oc would be 50%. From this early laser results, one can infer that even introducing higher losses in the cavity, the material show the potentiality of lasing. Further increase of output and efficiency is possible by AR coating on Yb:YGAG samples with optimum thickness and doping. We are expecting that the optimum output coupler for AR coated Yb:YGAG is higher than 50%.

4. Conclusion

In conclusion, we present detail spectroscopic characterization and demonstrate continuous-wave laser operation of a new Yb:YGAG ceramic material at cryogenic temperatures. The peak cross-sections increase with decrease of temperature, while at the same time the absorption and emission bandwidth decreases with decrease of temperature. The emission bandwidth of Yb:YGAG is as much as three times broader than that of widely used Yb:YAG at cryogenic temperatures which makes this new material promising for ultrashort pulse generation. In preliminary continuous-wave laser operation, a maximum output of 2.9W with a slope efficiency of 20.3% is achieved at a temperature of 60K and laser is emitted at 1025 nm. Future work will be based on precise value of cross-sections using low doped samples and to verify if the present results are not affected by reabsorption. The structural properties of this promising material will be studied. We are also planning to optimize the transmission of output coupling in this cryogenic laser cavity by using AR coated samples with optimum thickness to substantially increase the output and slope efficiency. In addition, tuning range at cryogenic temperatures by inserting a birefringent filter made of quartz will also be studied.