We present the spectroscopic characterization and continuous-wave laser operation of a new Yb doped Y3Ga2A13O12 (Yb:YGAG) ceramic material at cryogenic temperatures. The peak absorption is centered at 942 nm, zero phonon line at 970 nm and the peak emission at 1028 nm. The emission bandwidth is as much as three times larger than Yb:YAG at cryogenic temperatures which makes this material very promising for sub-picosecond pulse generation. At cryogenic continuous-wave laser operation, a maximum output of 2.92W with a slope efficiency of 20.3% at 60K is achieved.
© 2015 Optical Society of America
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 . 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  and spectroscopic properties , 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 , Nd:Y3Sc2A13O12 (Nd:YSAG) ceramic , Nd:Y3GaxAl(5-x)O12 crystal , Nd:Y3Ga2Al3O12 (Nd:YGAG) ceramic , Yb:Y3ScA14O12 ceramic , Yb:Gd3AlxGa5-xO12 (Yb:GAGG) crystal . 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.
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  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.
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 (σa), emission (σe) and gain (σg) cross-sections we used the theoretical approach such as Beer’s Lambert law , McCumber or reciprocity method , Fuchtbauer–Ladenburg (FL)  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 (Toc) of 2%, 3%, 5%, 10% and 20% in the spectral range from 1020 nm to 1070 nm.
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 (2F5/2 level – cm−1) – 10841, 10611 and 10312 and the lower level splitting (2F7/2 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.
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%.
From Fig. 4(a) it is inferred that Toc = 20% gives maximum output power and slope efficiency when compared to others and we decided to use 20% Toc 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.
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%Toc 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%.
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.
This work is co-financed by the European Regional Development Fund, the European Social Fund and the state budget of the Czech Republic (project HiLASE: CZ.1.05/2.1.00/01.0027, project DPSSLasers: CZ.1.07/2.3.00/20.0143). This work is also supported by the Czech Science Foundation (GACR) under project GA14-01660S and by the grant RVO 68407700.
References and links
1. S. Banerjee, K. Ertel, P. D. Mason, P. J. Phillips, M. Siebold, M. Loeser, C. Hernandez-Gomez, and J. L. Collier, “High-efficiency 10 J diode pumped cryogenic gas cooled Yb:YAG multislab amplifier,” Opt. Lett. 37(12), 2175–2177 (2012). [CrossRef] [PubMed]
2. M. Hornung, S. Keppler, R. Bödefeld, A. Kessler, H. Liebetrau, J. Körner, M. Hellwing, F. Schorcht, O. Jäckel, A. Sävert, J. Polz, A. K. Arunachalam, J. Hein, and M. C. Kaluza, “High-intensity, high-contrast laser pulses generated from the fully diode-pumped Yb:glass laser system POLARIS,” Opt. Lett. 38(5), 718–720 (2013). [CrossRef] [PubMed]
3. M. Siebold, J. Hein, M. Hornung, S. Podleska, M. C. Kaluza, S. Bock, and R. Sauerbrey, “Diode-pumped lasers for ultra-high peak power,” Appl. Phys B 90(3-4), 431–437 (2008). [CrossRef]
4. T. Y. Fan, D. J. Ripin, R. L. Aggarwal, J. R. Ochoa, B. Chann, M. Tilleman, and J. Spitzberg, “Cryogenic Yb3+-doped solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 448–459 (2007). [CrossRef]
5. D. C. Brown, R. L. Cone, Y. C. Sun, and R. W. Equall, “Yb:YAG absorption at ambient and cryogenic temperatures,” IEEE J. Sel. Top. Quantum Electron. 11(3), 604–612 (2005). [CrossRef]
6. V. Jambunathan, J. Koerner, P. Sikocinski, M. Divoky, M. Sawicka, A. Lucianetti, J. Hein, and T. Mocek, “Spectroscopic characterization of various Yb3+ doped laser materials at cryogenic temperatures for the development of high energy class diode pumped solid state lasers,” High-Power, High-Energy, and High-Intensity Laser Technology; and Research Using Extreme Light: Entering New Frontiers with Petawatt-Class Lasers 8780 (2013). [CrossRef]
7. J. Korner, V. Jambunathan, J. Hein, R. Seifert, M. Loeser, M. Siebold, U. Schramm, P. Sikocinski, A. Lucianetti, T. Mocek, and M. C. Kaluza, “Spectroscopic characterization of Yb3+-doped laser materials at cryogenic temperatures,” Appl. Phys. B 116(1), 75–81 (2014). [CrossRef]
8. Y. Sato, J. Saikawa, T. Taira, and A. Ikesue, “Characteristics of Nd3+-doped Y3ScAl4O12 ceramic laser,” Opt. Mater. 29(10), 1277–1282 (2007). [CrossRef]
9. Y. Sato, J. Saikawa, I. Shoji, T. Taira, and A. Ikesue, “Spectroscopic Properties and Laser Operation of Nd:Y3ScAl4O12 Polycrystalline Gain Media, Solid-Solution of Nd: Y3Al5O12 and Nd: Y3Sc2Al3O12 Ceramics,” J. Ceram. Soc. Jpn. 112(Supplement), S313–S316 (2004).
10. B. M. Walsh, N. P. Barnes, R. L. Hutcheson, R. W. Equall, and B. Di Bartolo, “Spectroscopy and lasing characteristics of Nd-doped Y3GaxAl(5-x)O12 materials: application toward a compositionally tuned 0.94-µm laser,” J. Opt. Soc. Am. B 15(11), 2794–2801 (1998). [CrossRef]
11. Y. Oishi, K. Okamura, K. Miyazaki, N. Saito, M. Iwasaki, and S. Wada, “Amplifying high energy pulses at 1062.78 nm with diode pumped Nd:YGAG ceramic,” in Advanced Solid-State Lasers Congress, G. Huber and P. Moulton, eds., OSA Technical Digest (Optical Society of America, 2013), paper ATu3A.40. [CrossRef]
12. J. Saikawa, Y. Sato, T. Taira, and A. Ikesue, “Absorption, emission spectrum properties, and efficient laser performances of Yb:Y3ScAl4O12 ceramics,” Appl. Phys. Lett. 85(11), 1898–1900 (2004). [CrossRef]
13. B.-T. Zhang, J.-L. He, Z.-T. Jia, Y.-B. Li, S.-D. Liu, Z.-W. Wang, R.-H. Wang, X.-M. Liu, and X.-T. Tao, “Spectroscopy and laser properties of Yb-doped Gd3AlxGa5-xO12 crystal,” Appl. Phys. Express 6(8), 082702 (2013). [CrossRef]
14. J. Koerner, C. Vorholt, H. Liebetrau, M. Kahle, D. Kloepfel, R. Seifert, J. Hein, and M. C. Kaluza, “Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF2 between 20 degrees C and 200 degrees C and predictions on their influence on laser performance,” J. Opt. Soc. Am. B 29(9), 2493–2502 (2012). [CrossRef]
15. W. Koechner, “Solid-state laser engineering,” (Springer, 2006).
16. D. E. McCumber, “Einstein relations connecting broadband emission and absorption spectra,” Phys. Rev. A Gen. Phys. 136(4A), A954–A957 (1964). [CrossRef]
17. S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Infrared cross-section measurements for crystals doped with Er3+, Tm3+, Ho3+,” IEEE J. Quantum Electron. 28(11), 2619–2630 (1992). [CrossRef]