Nd:YAG waveguide laser Q-switched by evanescent-field interaction with graphene Yang Tan,1,3 Chen Cheng,1 Shavkat Akhmadaliev,2 Shengqiang Zhou,2 and Feng Chen1,* 1 2
School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and Particle Irradiation (Ministry of Education), Shandong University, Jinan 250100, China Helmholtz-Zentrum Dresden- Rossendorf, Institute of Ion Beam and Materials Research, Dresden 01314, Germany 3 [email protected] * [email protected]
Abstract: We report on the Q-switched laser operation by the evanescentfield interaction with the graphene layers deposited on a Nd:YAG surface planar waveguide, which was fabricated by the 15 MeV carbon ion irradiation. Based on the simple and compact design of the cavity with saturable-absorber features, the Q-switched pulsed waveguide laser operation was achieved at the wavelength of 1064 nm through the interaction between the graphene layer and the evanescent-field of the waveguide mode. The maximum output pulse repetition rate was ~29 kHz with the pulse duration of ~9.8 µs. ©2014 Optical Society of America OCIS codes: (230.7390) Waveguides, planar; (140.3540) Lasers, Q-switched; (160.5690) Rareearth-doped materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). F. Chen and J. R. Vázquez de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). F. Chen, “Micro-and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications,” Laser Photonics Rev. 6(5), 622–640 (2012). H. Jeong, S. Y. Choi, E. Jeong, S. J. Cha, F. Rotermund, and D. Yeom, “Ultrafast Mode-Locked Fiber Laser Using a Waveguide-Type Saturable Absorber Based on Single-Walled Carbon Nanotubes,” Appl. Phys. Express 6(5), 052705 (2013). G. A. Torchia, P. F. Meilán, A. Rodenas, D. Jaque, C. Mendez, and L. Roso, “Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics,” Opt. Express 15(20), 13266–13271 (2007). Y. Ren, Y. Jia, N. Dong, L. Pang, Z. Wang, Q. Lu, and F. Chen, “Guided-wave second harmonics in Nd:YCOB optical waveguides for integrated green lasers,” Opt. Lett. 37(2), 244–246 (2012). Y. Jia, N. Dong, F. Chen, J. R. Vázquez de Aldana, S. Akhmadaliev, and S. Zhou, “Continuous wave ridge waveguide lasers in femtosecond laser micromachined ion irradiated Nd:YAG single crystals,” Opt. Mater. Express 2(5), 657–662 (2012). Y. Tan, S. Akhmadaliev, S. Zhou, S. Sun, and F. Chen, “Guided continuous-wave and graphene-based Qswitched lasers in carbon ion irradiated Nd:YAG ceramic channel waveguide,” Opt. Express 22(3), 3572–3577 (2014). Y. Yao, Y. Jia, F. Chen, Sh. Akhmadaliev, and Sh. Zhou, “Channel waveguide lasers at 1064 nm in Nd:YAG crystal produced by C⁵⁺ ion irradiation with shadow masking,” Appl. Opt. 53(2), 195–199 (2014). N. Pavel, G. Salamu, F. Voicu, F. Jipa, M. Zamfirescu, and T. Dascalu, “Efficient laser emission in diodepumped Nd:YAG buried waveguides realized by direct femtosecond-laser writing,” Laser Phys. Lett. 10(9), 095802 (2013). M. Iwai, T. Yoshino, S. Yamaguchi, M. Imaeda, N. Pavel, I. Shoji, and T. Taira, “High power blue generation from a periodically poled MgO:LiNbO3 ridge-type waveguide by frequency-doubling of a diode end-pumped Nd:Y3Al5O12 laser,” Appl. Phys. Lett. 83(18), 3659–3661 (2003). A. G. Okhrimchuk, V. K. Mezentsev, V. V. Dvoyrin, A. S. Kurkov, E. M. Sholokhov, S. K. Turitsyn, A. V. Shestakov, and I. Bennion, “Waveguide-saturable absorber fabricated by femtosecond pulses in YAG:Cr4+ crystal for Q-switched operation of Yb-fiber laser,” Opt. Lett. 34(24), 3881–3883 (2009). D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). Y. Tan, F. Chen, J. R. Vázquez de Aldana, H. Yu, H. Zhang, and H. Zhang, “Tri-wavelength laser generation based on neodymium doped disordered crystal waveguide,” Opt. Express 21(19), 22263–22268 (2013).
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9101
15. H. H. Yu, V. Petrov, U. Griebner, D. Parisi, S. Veronesi, and M. Tonelli, “Compact passively Q-switched diodepumped Tm:LiLuF4 laser with 1.26 mJ output energy,” Opt. Lett. 37(13), 2544–2546 (2012). 16. O. Sandu, G. Salamu, N. Pavel, T. Dascalu, D. Chuchumishev, A. Gaydardzhiev, and I. Buchvarov, “High-peak power, passively Q-switched, composite, all-poly-crystalline ceramics Nd:YAG/Cr4+:YAG lasers,” Quantum Electron. 42(3), 211–215 (2012). 17. H. H. Yu, H. Zhang, Z. Wang, J. Wang, Y. Yu, M. Jiang, D. Tang, G. Xie, and H. Luo, “Passively mode-locked Nd:LuVO4 laser with a GaAs wafer,” Opt. Lett. 33(3), 225–227 (2008). 18. H. H. Yu, H. Zhang, Y. Wang, C. Zhao, B. Wang, S. Wen, H. Zhang, and J. Wang, “Topological insulator as an optical modulator for pulsed solid-state lasers,” Laser Photonics Rev. 7(6), L77–L83 (2013). 19. P. Tang, X. Zhang, C. Zhao, Y. Wang, H. Zhang, D. Shen, S. Wen, D. Tang, and D. Fan, “Topological insulator: Bi2Te3 saturable absorber for the passive Q switching operation of an in-band pumped 1645-nm Er:YAG Ceramic laser,” IEEE Photon. J. 5(2), 1500707 (2013). 20. G. Q. Xie, J. Ma, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene saturable absorber for Q-switching and mode locking at 2 μm wavelength,” Opt. Mater. Express 2(6), 878–883 (2012). 21. R. Mary, G. Brown, S. J. Beecher, F. Torrisi, S. Milana, D. Popa, T. Hasan, Z. Sun, E. Lidorikis, S. Ohara, A. C. Ferrari, and A. K. Kar, “1.5 GHz picosecond pulse generation from a monolithic waveguide laser with a graphene-film saturable output coupler,” Opt. Express 21(7), 7943–7950 (2013). 22. J. W. Kim, S. Y. Choi, D. I. Yeom, Sh. Aravazhi, M. Pollnau, U. Griebner, V. Petrov, and F. Rotermund, “Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes,” Opt. Lett. 38(23), 5090–5093 (2013). 23. B. Charlet, L. Bastard, and J. E. Broquin, “1 kW peak power passively Q-switched Nd3+-doped glass integrated waveguide laser,” Opt. Lett. 36(11), 1987–1989 (2011). 24. R. Salas-Montiel, L. Bastard, G. Grosa, and J.-E. Broquin, “Hybrid Nd-doped passively Q-switched waveguide laser,” Mater. Sci. Eng. B 149(2), 181–184 (2008). 25. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18(24), 24994–24999 (2010). 26. Y. O. Barmenkov, S. A. Kolpakov, A. V. Kir’yanov, L. Escalante-Zarate, J. L. Cruz, and M. V. Andrés, “Inuence of Cavity Loss Upon Performance of Q-Switched Erbium-Doped Fiber Laser,” IEEE Photon. Technol. Lett. 25(10), 977–980 (2013). 27. X. Zhang, Sh. Zhao, Q. Wang, B. Ozygus, and H. Weber, “Modeling of passively Q-switched lasers,” J. Opt. Soc. Am. B 17(7), 1166–1175 (2000). 28. Y. Tan, F. Chen, J. R. Vázquez de Aldana, G. A. Torchia, A. Benayas, and D. Jaque, “Continuous wave laser generation at 1064 nm in femtosecond laser inscribed Nd:YVO4 channel waveguides,” Appl. Phys. Lett. 97(3), 031119 (2010).
1. Introduction As the basic active devices for integrated photonic applications, the waveguide laser has attracted much attention since its first proposal in 1961 from the laboratory curiosity to the application area [1–5]. In the waveguide platform with gains, the high intra-cavity intensity of the pumping light could be achieved even at the low pumping powers. Consequently, superior laser performances, such as lower thresholds, enhanced slope efficiency and efficient heat dissipation, to bulk lasers could be obtained in compact waveguide structures [1]. The realization of the waveguide laser systems depends on the technologies of the waveguide fabrication in various gain media. Ion irradiation/implantation has been proved to be an efficient technique to produce the waveguide structure in a number of gain media with the high qualities, such as low propagation losses and well-preserved fluorescence features [6, 7]. Particularly, the neodymium ion doped yttrium aluminum garnet (Nd:YAG) crystals are good examples for ion irradiated waveguides. Both surface and buried guiding structures have been fabricated by diverse ion beam techniques. As of yet, the waveguide lasers with high efficiencies have been achieved in these Nd:YAG waveguides produced by the ion irradiation [8, 9]. Up to now, the majority of the research on waveguide lasers is related to the continuous wave (cw) regime, whilst limited work was performed on pulsed waveguide lasers [10–14]. Nevertheless, significant progress on pulsed waveguide lasers has shown the intriguing potential for applications in a few topics, e.g., in nonlinear microscopy, frequency comb generation and spectroscopy. The Q-switching process is one of the major techniques to obtain pulsed lasers. Passive Q-switching lasing depends on the modulation by a saturable absorber (SA), which has the variable transmission along with the intensity of light exceeding the threshold. One advantage of the passive Q-switching over the active configuration is that
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9102
the additional switching electronics are not necessary for the passive systems, which reduces the production cost for high energy pulses. A number of materials have been utilized for the passive Q-switching, including semiconductor saturable absorber mirrors (SESAMs), transition metal-doped bulk crystals (e.g., Cr4+:YAG) and single-walled carbon nanotubes (SWCNT) [15–17]. Recently, graphene, as a single carbon atomic layer, has been proved to be a promising material as an efficient SA. As graphene has a linear dispersion of Dirac electrons, the absorption of graphene is only determined by the optical conductivity constant. As a result, graphene has a wide-range absorption from visible to THz wavelengths, which could be considered to be ultra-broadband SAs. In addition, it was found that graphene has an ultrafast recovery time and the moderate modulation depth [18–20]. All these features make graphene a good candidate of low-cost, highly efficient SAs for the pulsed lasing. In most cases, the typical design to achieve the passive Q-switched waveguide laser is to set the SA between the active waveguide and the resonator mirrors. Recently, a novel design with better performance was proposed, in which the Q-switching was realized by evanescentfield interaction with the carbon nanotube SA that was deposited on the surface of plane waveguides [21–24]. In this work, we report, for the first time, on the realization and characteristics of the pulsed waveguide laser at 1064 nm on the Nd:YAG planar waveguide Q-switched by evanescent-field interaction with a surface-coated 16-layer graphene. 2. Experiments
Fig. 1. The experimental scheme for the indirect interaction graphene Q-switched waveguide laser generation.
The Nd:YAG planar waveguide was fabricated by the carbon ion irradiation. In this work, a Nd:YAG crystal (doped by 1 at. % Nd3+ ions) was cut into pieces with dimensions of 10 × 10 × 2 mm3 and optically polished. Using a 3 MV tandem accelerator, the C5+ ions were irradiated onto one 10 × 10 mm2 face at the energy of 15 MeV and at the fluence of 2 × 1014 ions/cm2. Through the ion irradiation process, the planar waveguide with thickness of ~9 μm was produced at the near-surface region of the Nd:YAG crystal. The graphene with ~16 layers manufactured by the chemical vapor deposition (CVD) on the copper and nickel disks was transferred to the surface of the planar waveguide. The absorption of graphene depends on the overlap between the evanescent-field from the waveguide and the graphene layers. To enhance the evanescent-field near the surface, the refractive index matching liquid was dripped onto the surface of the graphene, which was with refractive index of ~1.52 and thickness of ~200 µm. Figure 1 shows the schematic for the generation of the indirect interaction Q-switched waveguide lasers. For the laser oscillation, the Fabry-Perot oscillator was constructed by two special designed mirrors that were adhered to the end-facets of the planar waveguide. The input mirror (M1) was with high reflectivity (HR) at 1064 nm (reflectivity, R> 99.9%) and with high transmission (HT) at 810 nm (transmission, T> 99.9%), whilst the out-coupling #207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9103
mirror M2 had T~5% at 1064 nm. Through the end-coupling arrangement, the pumping laser at 810 nm from a Ti:Sapphire cw laser (Coherent MBR 110) was coupled into the waveguide. The output light was collected by a long work-distance microscope objective and imaged by an infrared CCD camera. 3. Results and discussion Figure 2 shows the Raman spectrum of the graphene on the surface of the waveguide. In the Raman spectrum, the G peak and the 2D peak were observed, respectively. The G peak was located at ~1580 cm−1 with a FWHM (full width at half maximum) of ~42 cm−1, meanwhile the 2D peak was at 2700 cm−1 with a FWHM 40 cm−1, which indicates the high quality of the graphene film. The intensity ratio of G peak to 2D peak is around 2.4. Hence the thickness of the graphene film on the surface of the planar waveguide was assumed to be 12-16 layers, which is similar with our experiments.
Fig. 2. The G (a) and the 2D (b) bands of the Raman spectrum of the graphene on the surface of Nd:YAG waveguide.
Fig. 3. (a) Cross-sectional microscope image of the planar waveguide in Nd:YAG crystal; (b) the reconstructed refractive index distribution of the planar waveguide; the (c) calculated and (d) measured modal profiles of the waveguide laser at the wavelength of 1064 nm. The dashed lines indicate the position of the graphene layer.
Figure 3(a) shows the cross section of the waveguide structure obtained from one polished end facet. After irradiation, the waveguide structure with thickness of ~9 μm was observed near the surface of the Nd:YAG crystal. According to the method described in Ref [25], we reconstructed the refractive index distribution of the planar waveguide. The maximum refractive index was ~1.8356 at the depth of ~2 μm beneath the surface of the sample. In order
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9104
to increase the evanescent-field near the surface, we annealed this planar waveguide at 180 °C for 30 min. After the annealing, the maximum Δn was decreased to 1.8345 and the refractive index distribution of the waveguide was depicted in Fig. 3(b). Based on the reconstructed refractive index profile, we calculated the modal intensity distributions of the waveguide at the wavelength of 1064 nm by the beam propagation method (Rsoft© BeamProp 8.0), as shown in Fig. 3(c). By comparing Fig. 3(c) with the measured propagation mode (Fig. 3(d)), we found that it was a reasonable similarity. The position of graphene layers (surface of the waveguide) was also indicated in Figs. 3(a)-3(d) (the dashed lines).
Fig. 4. The laser oscillation spectra of the graphene Q-switched waveguide laser (a) and the modal profile of the output laser (b).
Fig. 5. (a) The power of the output laser as a function of the incident pumping power with (blue doted line) and without (red doted line) Graphene; (b) the variation ratio of the output laser power as a function of time; (c) the variation of the pulse duration and the repetition rate of the graphene Q-switched pulse waveguide laser as a function of the input pumping power.
Figure 4 depicts the measured emission spectrum of the output beam with the pumping power (240 mW) above the lasing threshold. The polarization of the pumping laser was perpendicular to the graphene layers (i.e., with TM polarization) and the wavelength was ~810 nm. In Fig. 4(a), a peak at 1064 nm with the FWHM ~2 nm (less than the measurement error of the spectrograph) was observed, denoting the laser oscillations in the waveguide structure. Figure 4(b) shows the obtained modal profile of the output waveguide laser. The output power (Pout) of the 1064 nm waveguide laser as a function of the pumping power (Pin) was shown in Fig. 5(a). Without the graphene layers, the lasing threshold was determined to be 72 mW corresponding to the slope efficiency of ~7.4%. The maximum output power was ~14 mW at the incident pumping power of ~262 mW. Compared with the variation of the output power with Graphene modulation, the threshold was increased to 99
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9105
mW and the slope efficiency was decreased to 6%. The variation of the laser emission could be explained by Eqs. (1) and (2) in Ref [26]. As one can see, the laser threshold and the slope efficiency were proportional and reciprocal to the round-trip cavity loss (δ), respectively. In this work, the graphene layers added onto the surface of the waveguide will induce the extra loss through the absorption of Graphene. As a result, we believe the variations described in Fig. 5(a) were induced by the increasing of δ in the waveguide. The intensity of the output laser was measured along with time. As depicted in Fig. 5(b), the typical Q-switched pulse trains were found and the pulse duration of the Q-switched pulses was ~11 μs with the repetition rate of ~24 kHz. Figure 5(c) depicts the repetition rate and the pulse duration with the variation of the power of the pumping laser. Along with the pumping power variation, a near linear variation of repetition rate was observed. At the pumping power ~262 mW, the maximum repetition rate was ~29 kHz and the pulse energy was calculated to be around 0.37 μJ. The stored energy in the waveguide could be estimated to be ~66 μJ following Equ. (1). According to Ref [26], a total loss of per pulse was calculated to be ~18 dB. E = hν N 0 SL (1) where hv is the laser photon energy (h is Plank constant and v is the optical frequency), S is the area of the waveguide cross-section, L is the length of the waveguide, N0 is the concentration of Nd ions in the material. Figure 5(c) shows the variation of the pulse duration. It was slightly decreased from 12 μs to 9 μs, which was typical for a passively Q-switched laser [27]. For passively Q-switched laser, the pulse duration could be theoretically modeled as the equation below. ΔR ≈
3.52TR
τp
(2)
where ΔR is the modulation depth of the saturable absorber; τp is the pulse duration; TR is the cavity round-trip time. Considering about the waveguide length of ~1 cm, TR was ~0.122 ns and the maximum modulation depth was calculated to be less than 1% in this work. We would like to compare it with another design of Graphene-based Q-switched waveguide laser in Ref [28]. As reported in Ref [28], the pulse duration was around 60 ns indicating the modulation depth of 7%, which has a similar pumping condition as this work. It seems Graphene with evanescent-field interaction design has a larger satruation intensity, which indicates the difficulty of this design gets fully saturate and the potential to generating largeenergy pulses. 4. Conclusions We demonstrate the Q-switched pulse laser operation the Nd:YAG planar waveguide by the interaction between the evanescent field and the graphene layer that was coated on the surface of the Nd:YAG waveguide. Owing to the coupling of the evanescent-field, the passive Q switching was realized with the 10-mm long waveguide laser cavity. The maximum output power of 10 mW was obtained with the pulse duration ~9.8 µs, and the maximum repetition rate of 29 kHz. Acknowledgments This work is carried out under the financial support by the National Natural Science Foundation of China (No. U1332121) and the 973 Project (No. 2010CB832906) of China. Y. T. acknowledges the support by the National Natural Science Foundation of China (No. 11305094) and the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (No. BS2010CL035). S. Z. acknowledges the funding by the Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713).
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9106
School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and Particle Irradiation (Ministry of Education), Shandong University, Jinan 250100, China Helmholtz-Zentrum Dresden- Rossendorf, Institute of Ion Beam and Materials Research, Dresden 01314, Germany 3 [email protected] * [email protected]
Abstract: We report on the Q-switched laser operation by the evanescentfield interaction with the graphene layers deposited on a Nd:YAG surface planar waveguide, which was fabricated by the 15 MeV carbon ion irradiation. Based on the simple and compact design of the cavity with saturable-absorber features, the Q-switched pulsed waveguide laser operation was achieved at the wavelength of 1064 nm through the interaction between the graphene layer and the evanescent-field of the waveguide mode. The maximum output pulse repetition rate was ~29 kHz with the pulse duration of ~9.8 µs. ©2014 Optical Society of America OCIS codes: (230.7390) Waveguides, planar; (140.3540) Lasers, Q-switched; (160.5690) Rareearth-doped materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). F. Chen and J. R. Vázquez de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). F. Chen, “Micro-and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications,” Laser Photonics Rev. 6(5), 622–640 (2012). H. Jeong, S. Y. Choi, E. Jeong, S. J. Cha, F. Rotermund, and D. Yeom, “Ultrafast Mode-Locked Fiber Laser Using a Waveguide-Type Saturable Absorber Based on Single-Walled Carbon Nanotubes,” Appl. Phys. Express 6(5), 052705 (2013). G. A. Torchia, P. F. Meilán, A. Rodenas, D. Jaque, C. Mendez, and L. Roso, “Femtosecond laser written surface waveguides fabricated in Nd:YAG ceramics,” Opt. Express 15(20), 13266–13271 (2007). Y. Ren, Y. Jia, N. Dong, L. Pang, Z. Wang, Q. Lu, and F. Chen, “Guided-wave second harmonics in Nd:YCOB optical waveguides for integrated green lasers,” Opt. Lett. 37(2), 244–246 (2012). Y. Jia, N. Dong, F. Chen, J. R. Vázquez de Aldana, S. Akhmadaliev, and S. Zhou, “Continuous wave ridge waveguide lasers in femtosecond laser micromachined ion irradiated Nd:YAG single crystals,” Opt. Mater. Express 2(5), 657–662 (2012). Y. Tan, S. Akhmadaliev, S. Zhou, S. Sun, and F. Chen, “Guided continuous-wave and graphene-based Qswitched lasers in carbon ion irradiated Nd:YAG ceramic channel waveguide,” Opt. Express 22(3), 3572–3577 (2014). Y. Yao, Y. Jia, F. Chen, Sh. Akhmadaliev, and Sh. Zhou, “Channel waveguide lasers at 1064 nm in Nd:YAG crystal produced by C⁵⁺ ion irradiation with shadow masking,” Appl. Opt. 53(2), 195–199 (2014). N. Pavel, G. Salamu, F. Voicu, F. Jipa, M. Zamfirescu, and T. Dascalu, “Efficient laser emission in diodepumped Nd:YAG buried waveguides realized by direct femtosecond-laser writing,” Laser Phys. Lett. 10(9), 095802 (2013). M. Iwai, T. Yoshino, S. Yamaguchi, M. Imaeda, N. Pavel, I. Shoji, and T. Taira, “High power blue generation from a periodically poled MgO:LiNbO3 ridge-type waveguide by frequency-doubling of a diode end-pumped Nd:Y3Al5O12 laser,” Appl. Phys. Lett. 83(18), 3659–3661 (2003). A. G. Okhrimchuk, V. K. Mezentsev, V. V. Dvoyrin, A. S. Kurkov, E. M. Sholokhov, S. K. Turitsyn, A. V. Shestakov, and I. Bennion, “Waveguide-saturable absorber fabricated by femtosecond pulses in YAG:Cr4+ crystal for Q-switched operation of Yb-fiber laser,” Opt. Lett. 34(24), 3881–3883 (2009). D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). Y. Tan, F. Chen, J. R. Vázquez de Aldana, H. Yu, H. Zhang, and H. Zhang, “Tri-wavelength laser generation based on neodymium doped disordered crystal waveguide,” Opt. Express 21(19), 22263–22268 (2013).
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9101
15. H. H. Yu, V. Petrov, U. Griebner, D. Parisi, S. Veronesi, and M. Tonelli, “Compact passively Q-switched diodepumped Tm:LiLuF4 laser with 1.26 mJ output energy,” Opt. Lett. 37(13), 2544–2546 (2012). 16. O. Sandu, G. Salamu, N. Pavel, T. Dascalu, D. Chuchumishev, A. Gaydardzhiev, and I. Buchvarov, “High-peak power, passively Q-switched, composite, all-poly-crystalline ceramics Nd:YAG/Cr4+:YAG lasers,” Quantum Electron. 42(3), 211–215 (2012). 17. H. H. Yu, H. Zhang, Z. Wang, J. Wang, Y. Yu, M. Jiang, D. Tang, G. Xie, and H. Luo, “Passively mode-locked Nd:LuVO4 laser with a GaAs wafer,” Opt. Lett. 33(3), 225–227 (2008). 18. H. H. Yu, H. Zhang, Y. Wang, C. Zhao, B. Wang, S. Wen, H. Zhang, and J. Wang, “Topological insulator as an optical modulator for pulsed solid-state lasers,” Laser Photonics Rev. 7(6), L77–L83 (2013). 19. P. Tang, X. Zhang, C. Zhao, Y. Wang, H. Zhang, D. Shen, S. Wen, D. Tang, and D. Fan, “Topological insulator: Bi2Te3 saturable absorber for the passive Q switching operation of an in-band pumped 1645-nm Er:YAG Ceramic laser,” IEEE Photon. J. 5(2), 1500707 (2013). 20. G. Q. Xie, J. Ma, P. Lv, W. L. Gao, P. Yuan, L. J. Qian, H. H. Yu, H. J. Zhang, J. Y. Wang, and D. Y. Tang, “Graphene saturable absorber for Q-switching and mode locking at 2 μm wavelength,” Opt. Mater. Express 2(6), 878–883 (2012). 21. R. Mary, G. Brown, S. J. Beecher, F. Torrisi, S. Milana, D. Popa, T. Hasan, Z. Sun, E. Lidorikis, S. Ohara, A. C. Ferrari, and A. K. Kar, “1.5 GHz picosecond pulse generation from a monolithic waveguide laser with a graphene-film saturable output coupler,” Opt. Express 21(7), 7943–7950 (2013). 22. J. W. Kim, S. Y. Choi, D. I. Yeom, Sh. Aravazhi, M. Pollnau, U. Griebner, V. Petrov, and F. Rotermund, “Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes,” Opt. Lett. 38(23), 5090–5093 (2013). 23. B. Charlet, L. Bastard, and J. E. Broquin, “1 kW peak power passively Q-switched Nd3+-doped glass integrated waveguide laser,” Opt. Lett. 36(11), 1987–1989 (2011). 24. R. Salas-Montiel, L. Bastard, G. Grosa, and J.-E. Broquin, “Hybrid Nd-doped passively Q-switched waveguide laser,” Mater. Sci. Eng. B 149(2), 181–184 (2008). 25. Y. Tan, A. Rodenas, F. Chen, R. R. Thomson, A. K. Kar, D. Jaque, and Q. Lu, “70% slope efficiency from an ultrafast laser-written Nd:GdVO4 channel waveguide laser,” Opt. Express 18(24), 24994–24999 (2010). 26. Y. O. Barmenkov, S. A. Kolpakov, A. V. Kir’yanov, L. Escalante-Zarate, J. L. Cruz, and M. V. Andrés, “Inuence of Cavity Loss Upon Performance of Q-Switched Erbium-Doped Fiber Laser,” IEEE Photon. Technol. Lett. 25(10), 977–980 (2013). 27. X. Zhang, Sh. Zhao, Q. Wang, B. Ozygus, and H. Weber, “Modeling of passively Q-switched lasers,” J. Opt. Soc. Am. B 17(7), 1166–1175 (2000). 28. Y. Tan, F. Chen, J. R. Vázquez de Aldana, G. A. Torchia, A. Benayas, and D. Jaque, “Continuous wave laser generation at 1064 nm in femtosecond laser inscribed Nd:YVO4 channel waveguides,” Appl. Phys. Lett. 97(3), 031119 (2010).
1. Introduction As the basic active devices for integrated photonic applications, the waveguide laser has attracted much attention since its first proposal in 1961 from the laboratory curiosity to the application area [1–5]. In the waveguide platform with gains, the high intra-cavity intensity of the pumping light could be achieved even at the low pumping powers. Consequently, superior laser performances, such as lower thresholds, enhanced slope efficiency and efficient heat dissipation, to bulk lasers could be obtained in compact waveguide structures [1]. The realization of the waveguide laser systems depends on the technologies of the waveguide fabrication in various gain media. Ion irradiation/implantation has been proved to be an efficient technique to produce the waveguide structure in a number of gain media with the high qualities, such as low propagation losses and well-preserved fluorescence features [6, 7]. Particularly, the neodymium ion doped yttrium aluminum garnet (Nd:YAG) crystals are good examples for ion irradiated waveguides. Both surface and buried guiding structures have been fabricated by diverse ion beam techniques. As of yet, the waveguide lasers with high efficiencies have been achieved in these Nd:YAG waveguides produced by the ion irradiation [8, 9]. Up to now, the majority of the research on waveguide lasers is related to the continuous wave (cw) regime, whilst limited work was performed on pulsed waveguide lasers [10–14]. Nevertheless, significant progress on pulsed waveguide lasers has shown the intriguing potential for applications in a few topics, e.g., in nonlinear microscopy, frequency comb generation and spectroscopy. The Q-switching process is one of the major techniques to obtain pulsed lasers. Passive Q-switching lasing depends on the modulation by a saturable absorber (SA), which has the variable transmission along with the intensity of light exceeding the threshold. One advantage of the passive Q-switching over the active configuration is that
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9102
the additional switching electronics are not necessary for the passive systems, which reduces the production cost for high energy pulses. A number of materials have been utilized for the passive Q-switching, including semiconductor saturable absorber mirrors (SESAMs), transition metal-doped bulk crystals (e.g., Cr4+:YAG) and single-walled carbon nanotubes (SWCNT) [15–17]. Recently, graphene, as a single carbon atomic layer, has been proved to be a promising material as an efficient SA. As graphene has a linear dispersion of Dirac electrons, the absorption of graphene is only determined by the optical conductivity constant. As a result, graphene has a wide-range absorption from visible to THz wavelengths, which could be considered to be ultra-broadband SAs. In addition, it was found that graphene has an ultrafast recovery time and the moderate modulation depth [18–20]. All these features make graphene a good candidate of low-cost, highly efficient SAs for the pulsed lasing. In most cases, the typical design to achieve the passive Q-switched waveguide laser is to set the SA between the active waveguide and the resonator mirrors. Recently, a novel design with better performance was proposed, in which the Q-switching was realized by evanescentfield interaction with the carbon nanotube SA that was deposited on the surface of plane waveguides [21–24]. In this work, we report, for the first time, on the realization and characteristics of the pulsed waveguide laser at 1064 nm on the Nd:YAG planar waveguide Q-switched by evanescent-field interaction with a surface-coated 16-layer graphene. 2. Experiments
Fig. 1. The experimental scheme for the indirect interaction graphene Q-switched waveguide laser generation.
The Nd:YAG planar waveguide was fabricated by the carbon ion irradiation. In this work, a Nd:YAG crystal (doped by 1 at. % Nd3+ ions) was cut into pieces with dimensions of 10 × 10 × 2 mm3 and optically polished. Using a 3 MV tandem accelerator, the C5+ ions were irradiated onto one 10 × 10 mm2 face at the energy of 15 MeV and at the fluence of 2 × 1014 ions/cm2. Through the ion irradiation process, the planar waveguide with thickness of ~9 μm was produced at the near-surface region of the Nd:YAG crystal. The graphene with ~16 layers manufactured by the chemical vapor deposition (CVD) on the copper and nickel disks was transferred to the surface of the planar waveguide. The absorption of graphene depends on the overlap between the evanescent-field from the waveguide and the graphene layers. To enhance the evanescent-field near the surface, the refractive index matching liquid was dripped onto the surface of the graphene, which was with refractive index of ~1.52 and thickness of ~200 µm. Figure 1 shows the schematic for the generation of the indirect interaction Q-switched waveguide lasers. For the laser oscillation, the Fabry-Perot oscillator was constructed by two special designed mirrors that were adhered to the end-facets of the planar waveguide. The input mirror (M1) was with high reflectivity (HR) at 1064 nm (reflectivity, R> 99.9%) and with high transmission (HT) at 810 nm (transmission, T> 99.9%), whilst the out-coupling #207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9103
mirror M2 had T~5% at 1064 nm. Through the end-coupling arrangement, the pumping laser at 810 nm from a Ti:Sapphire cw laser (Coherent MBR 110) was coupled into the waveguide. The output light was collected by a long work-distance microscope objective and imaged by an infrared CCD camera. 3. Results and discussion Figure 2 shows the Raman spectrum of the graphene on the surface of the waveguide. In the Raman spectrum, the G peak and the 2D peak were observed, respectively. The G peak was located at ~1580 cm−1 with a FWHM (full width at half maximum) of ~42 cm−1, meanwhile the 2D peak was at 2700 cm−1 with a FWHM 40 cm−1, which indicates the high quality of the graphene film. The intensity ratio of G peak to 2D peak is around 2.4. Hence the thickness of the graphene film on the surface of the planar waveguide was assumed to be 12-16 layers, which is similar with our experiments.
Fig. 2. The G (a) and the 2D (b) bands of the Raman spectrum of the graphene on the surface of Nd:YAG waveguide.
Fig. 3. (a) Cross-sectional microscope image of the planar waveguide in Nd:YAG crystal; (b) the reconstructed refractive index distribution of the planar waveguide; the (c) calculated and (d) measured modal profiles of the waveguide laser at the wavelength of 1064 nm. The dashed lines indicate the position of the graphene layer.
Figure 3(a) shows the cross section of the waveguide structure obtained from one polished end facet. After irradiation, the waveguide structure with thickness of ~9 μm was observed near the surface of the Nd:YAG crystal. According to the method described in Ref [25], we reconstructed the refractive index distribution of the planar waveguide. The maximum refractive index was ~1.8356 at the depth of ~2 μm beneath the surface of the sample. In order
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9104
to increase the evanescent-field near the surface, we annealed this planar waveguide at 180 °C for 30 min. After the annealing, the maximum Δn was decreased to 1.8345 and the refractive index distribution of the waveguide was depicted in Fig. 3(b). Based on the reconstructed refractive index profile, we calculated the modal intensity distributions of the waveguide at the wavelength of 1064 nm by the beam propagation method (Rsoft© BeamProp 8.0), as shown in Fig. 3(c). By comparing Fig. 3(c) with the measured propagation mode (Fig. 3(d)), we found that it was a reasonable similarity. The position of graphene layers (surface of the waveguide) was also indicated in Figs. 3(a)-3(d) (the dashed lines).
Fig. 4. The laser oscillation spectra of the graphene Q-switched waveguide laser (a) and the modal profile of the output laser (b).
Fig. 5. (a) The power of the output laser as a function of the incident pumping power with (blue doted line) and without (red doted line) Graphene; (b) the variation ratio of the output laser power as a function of time; (c) the variation of the pulse duration and the repetition rate of the graphene Q-switched pulse waveguide laser as a function of the input pumping power.
Figure 4 depicts the measured emission spectrum of the output beam with the pumping power (240 mW) above the lasing threshold. The polarization of the pumping laser was perpendicular to the graphene layers (i.e., with TM polarization) and the wavelength was ~810 nm. In Fig. 4(a), a peak at 1064 nm with the FWHM ~2 nm (less than the measurement error of the spectrograph) was observed, denoting the laser oscillations in the waveguide structure. Figure 4(b) shows the obtained modal profile of the output waveguide laser. The output power (Pout) of the 1064 nm waveguide laser as a function of the pumping power (Pin) was shown in Fig. 5(a). Without the graphene layers, the lasing threshold was determined to be 72 mW corresponding to the slope efficiency of ~7.4%. The maximum output power was ~14 mW at the incident pumping power of ~262 mW. Compared with the variation of the output power with Graphene modulation, the threshold was increased to 99
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9105
mW and the slope efficiency was decreased to 6%. The variation of the laser emission could be explained by Eqs. (1) and (2) in Ref [26]. As one can see, the laser threshold and the slope efficiency were proportional and reciprocal to the round-trip cavity loss (δ), respectively. In this work, the graphene layers added onto the surface of the waveguide will induce the extra loss through the absorption of Graphene. As a result, we believe the variations described in Fig. 5(a) were induced by the increasing of δ in the waveguide. The intensity of the output laser was measured along with time. As depicted in Fig. 5(b), the typical Q-switched pulse trains were found and the pulse duration of the Q-switched pulses was ~11 μs with the repetition rate of ~24 kHz. Figure 5(c) depicts the repetition rate and the pulse duration with the variation of the power of the pumping laser. Along with the pumping power variation, a near linear variation of repetition rate was observed. At the pumping power ~262 mW, the maximum repetition rate was ~29 kHz and the pulse energy was calculated to be around 0.37 μJ. The stored energy in the waveguide could be estimated to be ~66 μJ following Equ. (1). According to Ref [26], a total loss of per pulse was calculated to be ~18 dB. E = hν N 0 SL (1) where hv is the laser photon energy (h is Plank constant and v is the optical frequency), S is the area of the waveguide cross-section, L is the length of the waveguide, N0 is the concentration of Nd ions in the material. Figure 5(c) shows the variation of the pulse duration. It was slightly decreased from 12 μs to 9 μs, which was typical for a passively Q-switched laser [27]. For passively Q-switched laser, the pulse duration could be theoretically modeled as the equation below. ΔR ≈
3.52TR
τp
(2)
where ΔR is the modulation depth of the saturable absorber; τp is the pulse duration; TR is the cavity round-trip time. Considering about the waveguide length of ~1 cm, TR was ~0.122 ns and the maximum modulation depth was calculated to be less than 1% in this work. We would like to compare it with another design of Graphene-based Q-switched waveguide laser in Ref [28]. As reported in Ref [28], the pulse duration was around 60 ns indicating the modulation depth of 7%, which has a similar pumping condition as this work. It seems Graphene with evanescent-field interaction design has a larger satruation intensity, which indicates the difficulty of this design gets fully saturate and the potential to generating largeenergy pulses. 4. Conclusions We demonstrate the Q-switched pulse laser operation the Nd:YAG planar waveguide by the interaction between the evanescent field and the graphene layer that was coated on the surface of the Nd:YAG waveguide. Owing to the coupling of the evanescent-field, the passive Q switching was realized with the 10-mm long waveguide laser cavity. The maximum output power of 10 mW was obtained with the pulse duration ~9.8 µs, and the maximum repetition rate of 29 kHz. Acknowledgments This work is carried out under the financial support by the National Natural Science Foundation of China (No. U1332121) and the 973 Project (No. 2010CB832906) of China. Y. T. acknowledges the support by the National Natural Science Foundation of China (No. 11305094) and the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (No. BS2010CL035). S. Z. acknowledges the funding by the Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713).
#207313 - $15.00 USD (C) 2014 OSA
Received 27 Feb 2014; revised 1 Apr 2014; accepted 1 Apr 2014; published 7 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009101 | OPTICS EXPRESS 9106
