*Corresponding author:

Large-energy, narrow-bandwidth laser pulse at 1645 nm in a diode-pumped Er:YAG solid-state laser passively Q-switched by a monolayer graphene saturable absorber Rong Zhou, 1 Pinghua Tang, 1 Yu Chen, 1 Shuqing Chen, 1 Chujun Zhao, 1, * Han Zhang, 1,2 and Shuangchun Wen 1 1 Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China 2 e-mail: hanzhang@hnu.edu.cn *Corresponding author: cjzhao@hnu.edu.cn Received 28 October 2013; revised 3 December 2013; accepted 5 December 2013; posted 11 December 2013 (Doc. ID 200119); published 9 January 2014 Nonlinear transmission parameters of monolayer graphene at 1645 nm were obtained. Based on the monolayer graphene saturable absorber, a 1532 nm LD pumped 1645 nm passively Q-switched Er:YAG laser was demonstrated. Under the pump power of 20.8 W, a 1645 nm Q-switched pulse with FWHM of 0.13 nm (without the use of etalon) and energy of 13.5 μj per pulse can be obtained. To the best of our knowledge, this is the highest pulse energy for graphene-based passively Q-switched Er:YAG laser operating at 1645 nm, suggesting the potentials of graphene materials for high-energy solid-state laser applications. 2014 Optical Society of America OCIS codes: (190.4400) Nonlinear optics, materials; (140.3500) Lasers, erbium; (140.3540) Lasers, Q-switched. http://dx.doi.org/10.1364/ao.53.000254 1. Introduction Pulsed solid-state lasers with an eye-safe wavelength around 1.65 μm possess a number of important applications, such as remote sensing, optical communication, laser radars, and designation [1,2]. Er:YAG crystal as the solid laser gain medium can be used to generate 1645 nm high-energy pulses directly [3 5]. The manifolds of Er:YAG energy levels indicate that in-band pumping can efficiently excite the laser crystal to lase at 1645 nm owing to the optical transition between the 4 I 15 2 4 I 13 2, as shown in Fig. 1. The in-band pumping allows for transforming a significant portion of the system s thermal load 1559-128X/14/020254-05$15.00/0 2014 Optical Society of America from the gain medium to the pump diodes, which therefore suppresses the distortion of gain medium and limits the thermally induced birefringence and the optical lens effect [6]. Either active or passive Q-switching technology can lead to pulsed operation. Different from the active Q-switching regime, which is enabled by the electro-optical or acoustic-optical modulators, the passive Q-switching operation allows a more compact geometry and simpler setup. It is well known that the saturable absorber (SA) devices play a central role in the passive Q-switching operation. Conventional SAs, such as semiconductor SA mirrors, were not yet off the shelf for the 1645 nm wavelength. Some crystals, such as Cr: ZnSe and Co: MALO, have been used to produce the Q-switching pulse at 1645 nm [7,8]. 254 APPLIED OPTICS / Vol. 53, No. 2 / 10 January 2014

Fig. 1. Manifolds of Er:YAG crystal field energy levels. Graphene, a 2D atomic layer of carbon atoms, was successfully demonstrated as the next-generation broadband SA benefiting from its ultrafast recovery time, low-saturation intensity, tunable SA modulation depth, and high damage threshold [9 17]. Besides few layer graphene, monolayer graphene has been widely used in the mode-locked fiber laser [12] and bulk solid-state laser [13]. Recently, much effort using graphene as SA for 1645 nm wavelength laser also has been demonstrated by Gao et al. [14] and Zhu et al. [15], with a single pulse energy of 7.05 and 7.08 μj. All those contributions used the few-layer graphene as SA. However, there are no detailed descriptions on the nonlinear transmission properties of graphene at 1645 nm, which is vital to understanding the principle of Q-switching. Furthermore, compared to multilayer graphene, monolayer graphene has a much smaller nonsaturable loss and higher damage threshold, which means the potential to obtain high-energy and high efficient laser output. Equally important, the Er/Yb co-doped double-clad fiber laser was usually used as the pump laser to generate 1645 nm laser, which complicates the laser system and further limits the maximum incident pumping ability. Here we reported the nonlinear SA characteristics of monolayer graphene at 1645 nm, and that, in a 1532 nm diode-pumped Er:YAG laser cavity with a monolayer graphene as the SA component, the single-pulse energy can reach up to 13.5 μj, which is the highest pulse energy reported for graphenebased passively Q-switching Er: YAG laser at 1645 nm wavelength up to now. the typically employed method, such as the polymethylmethacrylate (PMMA) transfer method, the ultrasonic processing method used here can improve the hydrophobicity of the substrate, and thus the graphene with less wrinkles and defects can be transferred. Moreover, the method is considered as the green technology, thanks to its high efficiency, economic performance, and fewer requirements for the facility. Raman spectrum measurement was used to characterize the layer number of the graphene sample, as shown in Fig. 2. The G and 2D peaks are located at 1600 and 2700 cm 1, respectively. The FWHM of the 2D peak was about 38 cm 1, and the height of G peak versus its relative height (G/2D) was calculated to be about 0.48, suggesting that the graphene sample is monolayer [19,20]. Figure 3 shows the linear and nonlinear transmission spectrum of the monolayer graphene SA. It exhibits about 2.5% absorption in the 1000 2000 nm wavelength range, which deviates slightly from the theoretical value of universal absorption, as shown in Fig. 3(a). A balanced twin-detector measurement technique was applied to investigate the nonlinear SA properties of the graphene sample [21]. The laser source is an acousto-optic Q-switching Er:YAG solid-state laser (center wavelength: 1645 nm, pulse duration: 350 ns and repetition rate: 2 khz). A beam splitter was used to separate the laser source into two beams, namely, one as a reference beam monitored by detector 1 used to characterize the input power before the sample; the other was focused on the sample as the transmitted power monitored by the detector 2. By adjusting the optical attenuator, we can continuously tune the incident laser power on the sample. The nonlinear transmission curve is shown in Fig. 3(b). By fitting the curve with equation T F 1 ΔT exp F F sat α ns (T is normalized transmittance, ΔT is normalized modulation depth, F is the fluence of the incident laser, F sat is saturable fluence, and α ns is nonsaturable loss), one can obtain that the saturable energy density, normalized modulation depth and non-saturable loss of the graphene sample 2. Experimental Setup In the present paper, the CVD-grown graphene (ACS MATERIAL LLC, Grown on Cu, 5 cm 5 cm, monolayer) films were transferred to quartz plate by an ultrasonic processing method [18]. Compared with Fig. 2. Raman spectrum of the graphene SA. 10 January 2014 / Vol. 53, No. 2 / APPLIED OPTICS 255

Fig. 3. Linear and nonlinear transmission curve of monolayer graphene SA, and the experimental setup for saturable absorption measurement (inset). are about 7.8 mj cm 2, 10.7%, and 0.25%, respectively. During the measurement, no optical damage of the monolayer graphene SA was observed. To verify the effectiveness of the SA, we have increased the laser intensity from a low- to high-power regime and then decreased the laser intensity from a high- to low-power regime several times, and almost the same nonlinear transmission curve can be still observed. In each testing round, the operating characteristics of the Q-switching laser are nearly the same. A triple mirror system was specially designed for the passively Q-switching Er:YAG laser, as shown in Fig. 4. It consists of a plane pump in-coupling mirror (M1) with high reflectivity (>99.8%) at the lasing wavelength ranging from 1600 to 1700 nm and high transmittance (>94%) for the pump wavelength of 1532 nm. The concave mirror (M2) with the radius of 200 mm was coated with an antireflection coating at 1532 nm and a high reflection coating at 1645 nm. The plane output coupler (M3) had a transmittance of 10% at 1645 nm. This type of resonator can eliminate the influence of the residual pump laser upon the SA. In order to boost the performance of the passive Q-switching operation, the monolayer graphene SA deposited on a quartz substrate was placed near the output coupler. The Er:YAG crystal doped with a concentration of 0.25 at. % Er ion has a diameter of 4 mm and a length of 40 mm. Both of the end facets were antireflection coated with a range of operation band from 1400 to 1700 nm in order to reduce reflection losses. It was then wrapped by indium foil and mounted in a copper heat sink to maintain the temperature around 288 K Fig. 4. Setup of the Q-switched Er:YAG laser. by the water cooler. The total length of resonator was about 220 mm. The pump source used in the experiment was a fiber-coupled laser diode operated at 1532 nm with a core diameter of 200 μm and a numerical aperture of 0.22. Its maximum output power can reach up to 35 W. The temperature of cooling water of LD was set at 286 K in order to ensure efficient absorption of pump power at the maximum pump power. The pump beam was collimated by a plano convex lens L1 (f 1 40 mm) first, and then focused on the Er:YAG crystal with a radius of about 300 μm through another plano-convex lens L2 (f 2 125 mm). According to the ABCD matrix theory, the radii of the oscillating TEM 00 mode were calculated to be about 340 μm and 150 μm at the middle of the Er:YAG and graphene SA, respectively. 3. Experimental Results and Discussion Without introducing SA into the cavity, the laser operates in the CW operation regime and starts to lase at a threshold value of 14.3 W. The maximum output power quasi-linearly increased up to 2.1 W under an incident pump power of 20.8 W. The overall optical to optical conversion efficiency was about 10%, and the slope efficiency was about 33% with respect to the incident power. No self Q-switching was observed during the process of either increasing or decreasing the pumping power. Then the graphene sample was placed inside the resonant cavity. After fine tuning the cavity and the position of the SA, a stable Q-switching operation state was achieved. Due to the increase of optical loss from the SA, the threshold power increased up to 17.6 W (in comparison with 14.3 W, where SA is absent). And the average output power increased quasi-linearly up to 474 mw at a pump power of 20.8 W, corresponding to a slope efficiency of 10.9%. The quasi-linear feature of the two curves can be traced back to the wavelength shifting of the laser pump. In our experiment, the pump wavelength shifts from 1528 nm at 14 W toward 1529.1 nm at 21 W. Compared with that of the CW operation regime, the lower output power and smaller slope efficiency of the Q-switched mode are due to the additional loss induced by the absorption 256 APPLIED OPTICS / Vol. 53, No. 2 / 10 January 2014

Fig. 5. Relation between the output power and the incident pump power for CW and Q-switched operation. Fig. 7. Pulse train profiles of the Q-switched Er:YAG laser. of graphene and the reflection of uncoated quartz substrate. The relations between the output power and the incident pump power for CW and Q-switched operation were both plotted in Fig. 5. Figure 6 shows the output spectrum of the Q- switched Er:YAG laser, with a central wavelength of 1645.06 nm. Experimentally, the lasing wavelength was almost kept constant around 1645.06 nm without deviation. By placing a wavelength tunable filter component inside the cavity, a wavelength tunable Q-switching operation may be anticipated. And the 3 db bandwidth was about 0.13 nm. The reason why we can still obtain a narrowband lasing without the use of etalon is because our SA component, which is graphene deposited onto an optical quartz, can function as an equivalent F-P etalon. The corresponding optical trace of the Q-switched pulse at the pump power 20.8 W is shown in Fig. 7. The repetition rate and pulse width were 35.09 khz and 6.64 μs, respectively. When the laser is Q- switched, there are no obvious modulations for the pulse, and the laser output is stable. The longest operation time we have tested is about 1 h. The dependence of the pulse duration and repetition rate on the incident pump power is plotted in Fig. 8. Pulse width and repetition rate versus incident pump power for Q-switched operation. Fig. 8. The pulse width decreased from 15 to 7.78 μs while the repetition rate increased from 26 to 35.44 khz as the incident power varied from 17.5 to 20.3 W. The single pulse energy as a function of incident pump power is shown in Fig. 9. The maximum single pulse energy could reach up to 13.5 μj when the incident power was 20.8 W. This single pulse energy for Fig. 6. Output spectrum of the Q-switched Er:YAG laser. Fig. 9. Pulse energy versus incident pump power for Q-switched operation. 10 January 2014 / Vol. 53, No. 2 / APPLIED OPTICS 257

graphene-based passively Q-switched laser was nearly twice of that reported in previous work [14,15]. 4. Conclusion In conclusion, we have obtained the nonlinear SA parameters of monolayer graphene at 1645 nm in this work. By introducing the graphene with quartz substrate into the laser cavity, we demonstrated a high-energy passively Q-switched 1645 nm Er:YAG solid state laser pumped by a 1532 nm LD. The maximum single pulse energy of 13.5 μj, and wavelength bandwidth of 0.13 nm were obtained. By further optimizing the resonant cavity and employing additional treatment (such as the incorporation of an in-cavity tunable filter or F-P etalon component), a much higher single-pulse energy with shorter pulse, tunable wavelength operation, and narrower operation bandwidth could be anticipated. Rong Zhou and Pinghua Tang contributed equally to this work. 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