August 15, 2012 / Vol. 37, No. 16 / OPTICS LETTERS
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Mid-infrared waveguide lasers in rare-earth-doped YAG Yingying Ren,1,2 Graeme Brown,2 Airán Ródenas,2 Stephen Beecher,2 Feng Chen,1,3 and Ajoy K. Kar2,* 1 2
School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK 3 e-mail:
[email protected] *Corresponding author:
[email protected]
Received June 19, 2012; accepted July 10, 2012; posted July 12, 2012 (Doc. ID 170967); published August 6, 2012 We report near-infrared (IR) to mid-IR (up to 3.4 μm wavelength) multimode waveguiding in deep buried channel waveguides fabricated inside rare-earth ion-doped ceramic YAG for the first time to our knowledge. Waveguide laser operation at around 2 μm wavelength with multi- or single-transverse modes is also preliminarily demonstrated from these waveguides. © 2012 Optical Society of America OCIS codes: 230.7380, 320.2250.
In recent years, ultrafast laser inscription (ULI) has been shown to be a powerful technique for the fabrication of three-dimensional waveguides inside numerous transparent materials, among which rare-earth-doped transparent yttrium aluminum garnet (YAG) ceramic has started to draw increasing attention [1–3], mainly because of its significant advantages over single crystals such as good optical and thermal properties, the possibility of larger size, higher doping concentration, and lower fabrication costs [4–6]. Tm3 is one of the most effective rare-earth laser ions for achieving 2 μm wavelength eye-safe laser operation, which offers many potential applications in atmospheric testing, space communication, or in medicine [7–10]. Highly efficient, 2 μm laser operation from Tm:YAG in both bulk and planar waveguide configuration has been reported [11–14]. The formation of buried waveguides by the ULI technique relies on the permanent increase or reduction of the substrate refractive index at the nonlinear focal volume of the laser pulses [3]. For the latter case, it is possible to inscribe a waveguide directly inside the sample, which consists of a low-index tubular cladding surrounding an undamaged crystal core, which therefore allows for highly efficient laser action. Such a waveguide structure can feature low propagation loss and geometric flexibility for arbitrarily shape and size [15–17]. In previous work, depressed cladding waveguides have been fabricated in Cr:YAG, Nd:YAG, and Tm:ZBLAN by ULI [15–19], but no mid-infrared (IR) waveguiding has yet been demonstrated in YAG. In this letter, we demonstrate multimode (MM) waveguiding up to 3.4 μm wavelength in YAG, which therefore proves the possibility of guiding in the whole transparency range of YAG. MM and single-mode (SM) waveguide lasers with wavelengths of ∼2 μm are further reported for the first time to the best of our knowledge. The Tm:YAG ceramic (doped by Tm3 ions of 1 at.%) used in our work was polished to an optical quality. Waveguides were fabricated by ULI with an ultrafast Yb-doped fiber master-oscillator power amplifier laser (IMRA FCPA μ-Jewel D400), operating at 500 kHz, with a pulse duration of 460 fs and a center wavelength of 1047 nm, as previously reported [1,20]. The laser beam, with a pulse energy of 220 nJ, was circularly polarized and focused by a 0.4 NA aspheric lens into the substrate. 0146-9592/12/163339-03$15.00/0
The substrate was translated perpendicular to the laser beam axis at a speed of 3 mm ∕ s. Under these conditions, an array of parallel tracks was inscribed 300 μm below the polished top surface to form a symmetric cladding. The tubular cladding, of 100 μm diameter, was inscribed using 80 equally spaced tracks around the circumference. Figure 1(a) shows the cross-sectional optical microscope image of the waveguide, from which visible light transmission can be seen clearly. Further investigation was performed with μ-Raman confocal mapping analysis to assess the quality of the 100 μm cladding structure. Figure 1(b) shows the μ-Raman intensity mapping result of the waveguide cross-section, showing that the cladding distribution is composed of damaged crystalline material where the intensity of the Raman signal has decreased by about 50% at the track centers. It has been proven that, for the case of YAG, the local lattice damage can lead to a local reduction in the refractive index, which can therefore act as a low-index cladding [3,21]. As the guided modes are highly confined well inside the cladding, the core guiding region is
Fig. 1. (Color online) (a) Microscope visible light transmission of 100 μm diameter cladding waveguide; (b) Tubular Cladding Index Morphology distribution analyzed by μ-Raman mapping. Near-field images of propagation modes from (c) near-IR (1.95 μm) to (d) mid-IR (3.39 μm). © 2012 Optical Society of America
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composed by high quality unirradiated Tm3 :YAG, which preserves the sample design properties for laser applications. Nonetheless, μ-Raman results also showed the signature of stress fields surrounding the cladding structure. These results are out of the scope of the present letter and will therefore be published elsewhere. To investigate the guiding behavior of the waveguide, a 1.94 μm diode laser was coupled by a SMF-28 fiber to the facet at different input positions within the waveguide core region, and out-coupling was performed using a 0.25 NA lens. For longer mid-IR wavelengths, a He–Ne laser with a wavelength of 3.39 μm was used, and ZnSe focusing lenses were applied for in-coupling and outcoupling the waveguide. Some near-field images of highorder transverse modes of the waveguide are shown in Figs. 1(c) and 1(d). As expected, the waveguide was highly MM even at a wavelength of 3.39 μm, which demonstrates the potential for waveguiding in the whole mid-IR transmission range of YAG up to 6 μm wavelength. For laser characterization experiments, a CW Ti:sapphire (Spectra-Physics, 3900S) was used as a pump source. The best laser oscillation was found for a pumping wavelength of ∼800 nm. Two dielectric mirrors were adhered to the facets of the 10.5 mm long sample with index matching fluid to form a laser cavity. The polarized laser was coupled into the waveguide using a planoconvex lens with a focal length of 100 mm, resulting in a calculated diffraction limited spot size of 107 μm. The generated laser beam from the waveguide’s output facet was collected with a 10× microscope objective. After being separated from the residual pump with a silicon filter, the laser emission from the waveguide was detected. The laser output power versus input power was measured for a range of different output couplers and is presented in Fig. 2. The best laser performance was achieved by using a 20% output coupler, giving a 27% slope efficiency, 312 mW threshold, and 93.2 mW of output corresponding to an input power of 660 mW. The inset of Fig. 2 shows the laser emission spectrum from this waveguide. The center wavelength of the laser spectrum
is 1985 nm, corresponding to the emission line correlated to the 4 F3 → 3 H6 transition of Tm3 ions. As the 100 μm cladding waveguide is highly MM even for mid-IR light, as can be seen in Figs. 1(c) and 1(d), two smaller cladding structures with a 36 μm diameter were fabricated in the same sample with the aim of achieving SM waveguides for laser operation at 2 μm. The central depth of the waveguides were 250 μm under the polished surface, and the number of written tracks that the cladding structures were composed of were 28 (W 1 ) and 16 (W 2 ), respectively. By changing the number of inscription tracks of which the cladding structure was composed, we aimed to tailor some of the waveguide properties including the mode field diameter (MFD) and propagation loss to increase the laser efficiency. Figures 3(a) and 3(b) show the optical micrograph of these two structures. The same laser characterization setup as for the MM waveguide was used except for the input plano-convex lens, which was replaced by one with a focal length of 32 μm to reduce coupling losses. The measured near-field images and intensity profiles of the output lasers are shown in Figs. 3(c) and 3(d) confirming SM operation at 1985 nm. For waveguide W 2 , the MFD is measured to be 32.7 μm and 36.9 μm in horizontal and vertical direction, respectively, while for waveguide W 1 , the MFD is reduced to 32.3 μm (horizontal) and 28.7 μm (vertical), which could be the result of a tighter cladding confinement due to the increased number of laser written tracks. The laser performance of W1, with a 20% output coupler is shown in Fig. 4. The maximum laser power achieved from W 1 is 56.2 mW for 515 mW of input pump power. In addition, as one can see, compared with the MM waveguide, the thresholds of laser operation for the SM waveguide lasers are much lower, around 100 mW, which is probably due to the improved coupling efficiency between pump laser and the waveguide mode as well as to the reduced MFD of the waveguide. In conclusion, we have demonstrated multi- and singlemode channel waveguiding from the near-IR to mid-IR in YAG by ultrafast laser writing depressed cladding
Fig. 2. (Color online) Output laser power as a function of input pump power obtained from the MM cladding waveguide measured with different output couplers. The inset shows the laser emission spectrum at 1985 nm for the 20% output coupler.
Fig. 3. (Color online) The end micrograph view of 36 μm cladding structures with 28 (a) and 16 (b) written tracks. Near-field images of SM output lasers from cladding waveguides with 28 (c) and 16 (d) inscription tracks.
August 15, 2012 / Vol. 37, No. 16 / OPTICS LETTERS
Fig. 4. (Color online) Output laser power versus input power measured from SM cladding waveguides.
structures. Channel waveguide lasers were also realized in Tm:YAG at a wavelength of 2 μm, with the maximum slope efficiency of 27% from multimode laser action and a minimum threshold of 100 mW input power from the single-mode one. This work was supported by the National Natural Science Foundation of China (11111130200) and Royal Society international joint projects NSFC 2010 (JP 100985). We gratefully acknowledge financial support from the UK EPSRC through grant EP/GO30227/1. A. Rodenas acknowledges financial support from the Spanish Ministerio de Educación under the Programa de Movilidad de Recursos Humanos del Plan Nacional de I+D+I 2008/2011 for abroad postdoctoral researchers. The authors would also like to thank Renishaw for the long-term loan of an inVia Reflex Raman microscope, as part of the Renishaw Heriot-Watt University Strategic Alliance. References 1. A. Ródenas, A. Benayas, J. R. Macdonald, J. Zhang, D. Y. Tang, D. Jaque, and A. K. Kar, Opt. Lett. 36, 3395 (2011).
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