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We present a comparative study on a continuous-wave laser operation of Yb-doped Y3Ga2A13O12 (Yb:YGAG) ceramic at cryogenic temperatures with conventional pumping (940 nm) and zero phonon line pumping (969 nm) under identical experimental conditions. In CW laser operation, at 80 K with ZPL pumping, a maximum output power of 6.53 W with a slope efficiency of 52.0% is achieved with respect to incident power. When compared between two pump sources at cryogenic temperatures, ZPL pumping performs better due to the difference of quantum defect between the two pump sources that results in different heat load in the sample. In passive Q-switching experiment, at 100 K, with 85% initial transmission of Cr:YAG, an average output power of 3.37 W with a repetition rate of 17.6 kHz was achieved. The pulse energy, pulse width and peak power obtained in this case were 0.19 mJ, 164 ns and 1.16 W respectively.
© 2017 Optical Society of America
1. Introduction
Development of diode-pumped high energy / high average power short pulsed lasers especially in the pico and sub-picosecond regime has become most challenging and very attractive to the research community due to their vast applications in the field of science and technology. To develop such systems, Yb3+-doped laser materials are chosen as first choice rather than the Nd3+-doped counterpart because the former has simple energy level structure and the absence of higher energy level’s eliminates parasitic losses such as up-conversion, excited state absorption, cross-relaxation etc [1]. and as well it experiences reduction in quantum defect [2]. Many research groups that are working in this area consider Yb:YAG as active laser material due to the ease of availability either as crystal or ceramic in the market. Nevertheless, at room temperature achieving higher energy / average output power seems to be difficult because of thermal problems when pumped at high powers. This thermal issue can be solved by cooling the active medium to cryogenic temperatures where thermo-optical and spectroscopic properties are enhanced. The advantages of cryogenic cooling of solid state laser materials are reported elsewhere [3–7]. However, when Yb:YAG is cooled from room to cryogenic temperature, the absorption and emission bandwidth [8–10] reduces considerably. This reduction in emission bandwidth hinders the generation of short pulses.
To overcome the issue of generation of shorter pulses and to have similar material properties as YAG, mixed garnet Y3Ga2Al3O12 (YGAG) seems to be a valid alternative. The spectroscopic properties of Yb:YGAG were studied at cryogenic temperatures in our previous work [11], where a very wide emission band (emission bandwidth at 100 K is 5 times wider than that of Yb:YAG) was observed that clearly shows the potential of this material to generate shorter pulses. Other advantage of this material is that one can pump at zero-phonon-line (ZPL) since the ZPL bandwidth at 100 K is almost 8 times broader than that of Yb:YAG. Recently, a mode-locked Yb:YGAG generating 2.4 ps pulse using Semiconductor Saturable Absorber Mirror (SESAM) was demonstrated at liquid nitrogen temperature pumped at 940 nm [12].
Other important aspect to study is how the pump wavelength and bandwidth affects the laser characteristics of this material at cryogenic temperature, meaning how the conventional pumping (940 nm) and ZPL pumping (970 nm) affects the laser properties of Yb:YGAG at cryogenic temperature. In the case of Yb:YAG, ZPL pumping seems to be efficient rather than when pumped with conventional 940 nm pumping due to the reduction of quantum defect [13].
To understand the effect of pumping wavelength on this Yb:YGAG laser material, this work was devoted to study the cryogenic continuous-wave lasing characteristics of Yb:YGAG pumped by two different pump sources, one source emits at 940 nm and the other at ZPL under identical experimental condition. In addition, ZPL pumped passive Q-switching using a Cr:YAG saturable absorber are also demonstrated.
2. Experimental setup
The setup used for the cryogenic laser experiment is shown in Fig. 1. It is formed by “L” shaped cavity comprising of a concave mirror (M1), high reflection (HR) coated for 1030 nm, M2 – dichroic mirror HR coated for 1020 – 1200 nm range in the front side and anti-reflection (AR) coated for 900 – 980 nm in the back side, L3 – plano convex lens AR coated for 1030 nm and M3 – plane output couplers with transmissions Toc = 2, 3, 5, 10 and 20% in the 1020 – 1070 nm spectral region. To image the pump light to the sample two AR coated achromatic lenses L1 and L2 were used.
Fig. 1 Cryogenic laser setup: L1, L2 achromatic lens for imaging (150 mm and 300 mm focal length and 2 inch diameter), M1 – concave mirror (- 300 mm radius of curvature), M2 –dichroic mirror (50 mm diameter), L3 – plano convex lens (150 mm focal length), M3 – plane output coupler mirrors (Toc = 2%, 3%, 5%, 10% and 20%) and coated 10at.% Yb:YGAG ceramic.
Two fiber-coupled laser diodes of different wavelengths (940 nm and 969 nm) were used as pump sources. They both delivered a maximum output power of 25 W. The spectral bandwidth and fiber core diameter of 940 nm was 3.5 nm and 105 µm and for VBG stabilized 969 nm was 0.35 nm and 200 µm, respectively. The numerical aperture for both fibers was 0.22. The output was imaged using two achromatic lenses L1 and L2, with the magnification ratio of 1:2 for 940 nm and of 1:1 for 969 nm pumping, to the coated 10at.%Yb:YGAG ceramic from Konoshima chemical Co. Ltd, 1.5 mm thick and 4*4 mm2 aperture, having the pump beam diameter of around 200 µm. The laser mode-size in the Yb:YGAG ceramic sample was estimated to be around 220 µm using ABCD-matrix formalism from the measured distance of the laser cavity components. For passive Q-switching experiment, Cr:YAG with 85% initial transmission which has 5 mm diameter and 1.6 mm thickness was used.
Cooling of the Yb:YGAG sample was realized by mounting it in a brass holder and was placed in a cryogenic chamber that contains a two-stage closed cycle helium cryostat from Cryodyne (model no: 22C), which has heat load of 8 W at 100 K. To monitor and stablize the temperature precisely, a temperature controller from Lake Shore (model no: 335), that includes a 50 ohm resistor for heating, and two silicon diode temperature sensors (model no: DT-670), one near the cold finger and other near the sample were used.
3. Results and discussion
3.1 Cryogenic continuous-wave laser
Continuous-wave (CW) laser operation was realized at cryogenic temperature with two laser diodes one pumping at 940 nm and other pumping at 969 nm under identical experimental conditions. Although both the diode has the capability of delivering 25 W, we were able to launch a maximum of 67% of pump power for 940 nm and 55% of pump power for 969 nm with respect to seed current respectively. This was mainly due to large beam divergence of the fiber (NA = 0.22) and the limitation of the size of the imaging optics (L1 and L2 - 50.8 mm) used. Note that the launched pump power is measured after dichroic mirror (M2). To study the effect of laser performance with respect to output coupling, we fixed the temperature of the sample to 100 K and varied the transmittance of output coupler (Toc = 2%, 3%, 5%, 10% and 20%). Figure 2(a) and 2(b) shows the CW output characteristics of Yb:YGAG ceramic with the various output couplers pumped at 940 nm and 969 nm respectively. A linear dependence of output power with respect to launched pump power is observed in both pumping wavelengths. We obtained the highest slope efficiencies in both pumping wavelengths in case of the output coupler of 20%. Owing to high gain of Yb:YGAG, we expect to improve the output power and slope efficiency by higher transmittance of output coupler.
Fig. 2 CW output power characteristics of Yb:YGAG with various output couplers for (a) 940 nm pumping and (b) 969 nm pumping.
In order to study the effect of laser performance with respect to temperature, the output coupling was fixed to Toc = 20%, and the temperature was varied from 80 K to 280 K with the step size of 20 K in both pumping wavelengths. For sake of clarity, data series in step size of 40 K were plotted and the input-output characteristics of the Yb:YGAG laser at various temperatures are shown in Fig. 3(a) and 3(b) for both pump wavelengths respectively. The output powers are linearly increased with respect to the launched pump power in all cases. The slope efficiencies are also increased with respect to the sample temperature. It is worth to note that substantial improvement of output and slope efficiency is achieved when compared with uncoated sample of our previous work [11].
Fig. 3 CW output power characteristics of Yb:YGAG at various temperatures with Toc = 20% for (a) 940 nm pumping and (b) 969 nm pumping.
Figure 4(a) shows the evolution of laser threshold and slope efficiency of Yb:YGAG with respect to the launched pump power at various temperatures with Toc = 20%. Figure 4(b) shows the temperature dependence of CW output power pumped at both 940 nm and 969nm with same power level. Due to the suppression of reabsorption at the lasing wavelength, the lasing threshold decreases as the temperature decreases. This reabsorption suppression and the increase of absorption at pumping wavelength at the cryogenic temperature contribute to the increase of output power. In addition, thanks to the small quantum defect at the ZPL, we obtained further reduction in the lasing threshold and increase of the output power.
Fig. 4 (a) Evolution of laser threshold and slope efficiency of Yb:YGAG with respect to launched pump power at various temperatures for two different pump sources and Toc = 20%. (b) Evolution of maximum output power characteristics of Yb:YGAG at various temperatures for two different pump wavelengths with Toc = 20%.
The temperature dependence of lasing wavelength and the measured CW beam profile at 100 K is shown in Fig. 5(a). The lasing wavelength was shifted from 1028.5 nm to 1025 nm when the Yb:YGAG ceramics was cooled down from 260 K to 80 K. The shift of lasing wavelength is in line with the gain cross-section curves for different temperature as reported previously [11].
Fig. 5 (a) Observed laser emission wavelength at various temperatures with Toc = 20% and CW beam profile at 100 K (b) Estimated slope efficiency for Yb:YGAG with respect to absorbed power at 100 K with both pump wavelengths (940 nm and 969 nm).
We also measured the absorption of pump light in non-lasing condition for the both pump wavelengths at several cryogenic temperatures. The absorption of sample pumped at 940 nm was 72%, 70%, and 67% for 100 K, 140 K and 180 K respectively and the absorption of sample pumped at 969 nm was 93%, 86% and 80% for 100 K, 140 K and 180 K respectively. We estimated the slope efficiency with respect to the absorbed pump power for 100 K for both the pump wavelengths and are shown in Fig. 5(b). Note that the 969 nm pump shows better output and slope efficiency than that of 940 nm. This is mainly due to the difference in quantum defect between the two pump wavelengths that results in different heat load in the sample.
3.2 Passive Q-switching using Cr:YAG as saturable absorber
Passive Q-switching using Cr:YAG as a saturable absorber was realized by inserting it near the output coupler with Toc = 20%. The initial (unsaturated) transmission of Cr:YAG absorber was 85%.
Figure 6(a) and 6(b) shows the average output power, the pulse width and the repetition rate of passively Q-switched Yb:YGAG laser at cryogenic temperatures with respect to the launched pump power. The pulse width is almost conserved and the minimum pulse width obtained was 155 ns. The repetition rate increases with increase of launched pump power and increases with decrease in temperature. These changes in repetition rate are typical because the losses in the cavity are different with different temperatures which lead to different gain in the active medium that leads to different stored energy.
Fig. 6 (a) Average output power of cryogenic Yb:YGAG at different temperature with 85% initial transmission of Cr:YAG as a function of launched pump power. (b) Measured repetition rate and pulse width as a function of launched pump power for various temperatures of Yb:YGAG.
Changes in pulse energy and peak power calculated from Fig. 6(a) and 6(b) are depicted in Fig. 7(a). Typical pulse shape and beam profile at the temperature of 100 K with the launched pump power of 11.6 W are shown in Fig. 7(b). Detailed pulsed laser characteristics are summarized in Table 1 and the laser emission wavelength is 1025 nm in the case of passive Q-switching laser experiments.
Fig. 7 (a) Estimated pulse energy and peak power as a function of launched pump power of cryogenic Yb:YGAG for various temperatures with 85% initial transmission of Cr:YAG. (b) Measured laser pulse width of 164 ns, corresponding pulse repetition rate of 17.6 kHz for 85% initial transmission of Cr:YAG for a launched pump power of 11.6 W and measured beam profile at 100 K during passive Q-switching experiment.
Table 1. Output characteristics at maximum incident power of passively Q-switched cryogenic Yb:YGAG laser using 85% initial transmission Cr:YAG as saturable absorber
To achieve higher energy and to further shorten the pulses, one has to use much lower initial transmission of Cr:YAG and the cavity has to be optimized for temperature and output coupling.
4. Conclusion
In conclusion, a comparative study on continuous-wave laser operation of Yb:YGAG ceramic at cryogenic temperatures by pumping at 940 nm and ZPL (969 nm) under identical experimental conditions is presented. At 80 K with ZPL pumping, a maximum output power of 6.53 W with a slope efficiency of 52.0% is achieved. When compared between two pump wavelengths, ZPL pumping seems to be better due to lower quantum defect that results in reduced heat load in the sample. In passive Q–switching experiment, at 100 K, with 85% initial transmission of Cr:YAG, a maximum average output power of 3.37 W with a repetition rate of 17.6 kHz was achieved. The pulse energy, pulse width and peak power obtained in this case were 0.19 mJ, 164 ns and 1.16 W, respectively. Further shortening of pulses is anticipated if use much lower initial transmission of Cr:YAG and optimize the cavity for temperature and output coupling.
Funding
This work was partially supported by the state budget of the Czech Republic (projects LO1602 and LM2015086). This work is also supported by the Czech Science Foundation (GACR) under project GA14-01660S.
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