We report on the Q-switched laser operation by the evanescent-field 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
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 . 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 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 evanescent-field 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.
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 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.
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 , 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 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).
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 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 . 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 , a total loss of per pulse was calculated to be ~18 dB.
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 . For passively Q-switched laser, the pulse duration could be theoretically modeled as the equation below.
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 . As reported in Ref , 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 large-energy pulses.
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.
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).
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