We report high efficiency continuous wave laser oscillations at 1063.6 nm from an ultrafast laser written Nd3+:GdVO4 channel waveguide under the 808 nm optical excitation. A record 17 mm·s−1 writing speed was used while the low propagation loss of the waveguide (~0.5 dB·cm−1) enabled laser performance with a threshold pump power as low as 52 mW and a near to quantum defect limited laser slope efficiency of 70%.
© 2010 OSA
Neodymium doped gadolinium vanadate (Nd:GdVO4) is a well known crystal that is of special relevance for the development of compact near-infrared solid state lasers. This is owing to its excellent spectroscopic properties (such as large emission cross section and broad absorption bands), high thermal conductivity, high damage threshold, high Raman gain [1–4], and also to its high laser slope efficiencies comparable or even larger than those obtained from Nd:YAG or Nd:YVO4 . In integrated photonics, channel waveguide structures are used to counteract the effect of diffraction and confine light over an indefinite length. For lasers applications, channel waveguides can be used to tightly confine the pump and laser modes and achieve a high spatial overlap. If appropriately controlled, waveguide confinement can facilitate lower lasing thresholds and higher pumping efficiencies than are obtainable in bulk laser counterparts . Furthermore, low loss butt-coupling of light between single mode optical fibers and optical waveguides can also be achieved if the channel waveguide refractive index profile is suitably controlled. Due to their robust monolithic nature, waveguide lasers are becoming potential candidates as optically efficient laser sources for the future integrated optics [6–8]. Nevertheless, all this potential ultimately relies on the development of microfabrication processes capable of providing low loss waveguides while offering design flexibility and single step fast processing times.
In previous works, both ion implantation  and pulse laser deposition  techniques have been used for Nd:GdVO4 crystal waveguide microfabrication. However, as of yet there have been no reports of the laser operations of the fabricated waveguides. In addition, both of these techniques are limited to the fabrication of two-dimensional (2D) surface waveguide structures. Recently, ultrashort pulse direct laser writing (DLW) has emerged as one of the most efficient techniques for the three-dimensional (3D) microfabrication of channel waveguides in a plethora of solids [11,12]. This technique has been successfully applied to fabricate channel waveguide devices in many transparent optical materials including crystals, ceramics, glasses and polymers [13–20]. Recently, the DLW technique has been applied by different groups to fabricate channel waveguide lasers in Nd:YAG ceramics which exhibited a laser slope efficiency of 60% , in single Nd:YAG crystals with output power in excess of 1 W , and in Yb:YAG crystals with up to 75% slope efficiencies. Apart from YAG crystals, similar waveguide lasers have also been performed in nonlinear crystals such as Nd:YVO4, yielding slope efficiencies close to 40% , but to the best of our knowledge there has not been yet any attempt to develop efficient channel waveguide lasers in Nd:GdVO4 crystals using DLW.
In this work, we report on the ultrafast DLW of low-loss channel waveguides and lasers in a Nd:GdVO4 crystal. Ultrafast DLW was achieved by using very high writing speeds in excess of 17 mm·s−1. The fabricated waveguides exhibited low propagation losses of 0.5 dB·cm−1 which enabled laser action around 1064 nm with low laser thresholds and high laser slope efficiencies close to the theoretical quantum defect limit.
2. Experimental details
The Nd:GdVO4 crystal (doped with 1 at.% Nd3+ ions) used in the present work was cut into dimensions of 8 × 5 × 1.5 mm3 along a, b and c axes, respectively. Waveguides were fabricated by focusing through the a-b face, and the waveguides were written along the b axis of the sample. For DLW an IMRA μ-Jewel D400 ultrafast fiber laser system, which emitted a 200 kHz train of ≈350 fs (FWHM) pulses at a central wavelength of 1047 nm, was used. The pulses were focused ~100 µm below the sample surface using a 0.6 numerical aperture (NA) lens. The laser beam power on the sample was set to 140 mW (700 nJ per pulse), and the laser polarization was set parallel to the a axis of the Nd:GdVO4 crystal. A high precision, air bearing three-axis Aerotech translation stage was used for fast computer controlled movement of the sample through the laser focus. Following latest works on DLW of channel waveguides in laser crystals [13–20], the double-line stress-guiding design was chosen for our device. However, a record writing speed of 17 mm·s−1 was used, this being a 2 to 3 orders of magnitude higher fabrication speed than that previously demonstrated in highly efficient crystalline waveguide lasers (10-50 μm·s−1) [15,17]. The written tracks were parallel to each other and were fabricated scanning the sample in the same direction. The separation between the tracks was set to ~30 µm along the a-axis. After the DLW process, confocal µ-Raman surface mapping characterization of the waveguide facet was performed using a Renishaw inVia Reflex microscope attached to a 514 nm Argon laser. After μ-Raman analysis the linear guiding properties of the channel waveguide were studied by using an endface coupling optical system at 780 nm.
Figure 1 shows a schematic diagram of the experimental setup used for laser experiments. A continuous wave (cw) Ti:Sapphire laser (Coherent MBR 110) tuned to 808 nm was used as the pumping source. The power and polarization state of the 808 nm incident beam were adjusted by a pair of waveplates (WP1 and WP2) and a Glan Taylor prism (GTP). A convex lens with a focal length of f = 25 mm was used to focus the pump light into the input face of the waveguide. The 1064 nm laser radiation was collected from the opposite face of the waveguide with a 20 × microscope objective lens (NA = 0.4). A dichroic beamsplitter was used to separate the residual non-absorbed 808 nm pump radiation. The 808 nm in-coupling efficiency was estimated to be 80 ± 10%, this being the input coupling efficiency measured for a non absorbed 780 nm beam. In this work, 1064 nm laser oscillation was achieved both without and with laser mirrors. In the former case, the two polished end facets formed the Fabry-Perot cavity for laser oscillations, which results in an effective transmittance of 90% determined from the refractive index of Nd:GdVO4 (n ≈2). In the latter case, an input mirror (>99% reflectivity at 1064 nm and 98% transmission at 808 nm) and an output coupler (OC) (30% transmission at 1064 nm) were mechanically attached to the input and output waveguide’s faces.
3. Results and discussion
Figure 2(a) shows the three most intense stimulated Raman scattering (SRS) active modes of the Nd:GdVO4 sample taken both at the center of the waveguide core and in a non irradiated point of the crystal. It is observed that the phonon modes at the waveguide core are almost identical in intensity and shape to the original non irradiated crystal, therefore suggesting that the waveguide should keep all the crystal optical properties. Figures 2(b) and (c) show the waveguide cross section optical microscope transmission image and the corresponding μ-Raman analysis of the waveguide horizontal cross section, respectively. The waveguide core position, where the refractive index has been increased, is labeled W in Figs. 2(b) and (c). For the sake of brevity only the energy shifts of the most SRS-active A1g(ν1) phonon at 881.29 cm−1 energy are presented here. This phonon has been previously assigned to totally symmetric stretching optical vibration modes of the tetragonal VO4 3- ionic groups of the vanadate tetragonal crystal , and therefore its energy blue-/red-shift can be associated to compression/dilatation of the crystal lattice. Confirming previous detailed studies on this type of crystalline waveguides , the laser written darker tracks which can be seen in Fig. 2(b) consist of slightly damaged but crystalline volumes where the phonon mode width has only slightly increased + 0.04 cm−1, and its intensity dropped a 50%, therefore having a lowered index of refraction with respect to the bulk. These tracks are surrounded at about 10 μm by compressed defect free regions of increased refractive index which provide the channel waveguiding at W point. Differently compressed regions along the horizontal cross section are identified by the blue-shift of the A1g phonon mode presented in Fig. 2(c). At the waveguide core between tracks, the 881.3 cm−1 mode blue-shifts + 0.07 cm−1, clearly indicating compressive stress as the main cause for the refractive index increment responsible for channel waveguiding. Also no increase in the phonon width or decrease in its intensity is observed in this waveguiding region, therefore demonstrating the waveguide keeps all the Nd:GdVO4 crystal properties.
Figure 3(a) depicts the measured modal profiles of the 780 nm guided light with the polarization along c axis (TM mode). According to the µ-Raman results of stress and intensity distribution, we have reconstructed the 2D refractive index distribution of the waveguide cross section following the same arguments as previously done in Ref . Figure 3(b) shows the reconstructed 2D extraordinary refractive index (n e) profile of the waveguide. As previously observed in Nd:YAG waveguides , the two laser tracks consist of reduced refractive index volumes (Δn ~-1.2 × 10−3) with a region between them characterized by a stress-induced refractive index increment (Δn ~ + 0.3 × 10−3). Based on this refractive index distribution we have applied the finite-difference beam propagation method (FD-BPM)  to simulate the propagation of the light in the waveguide. Figure 3(c) depicts the calculated modal profile of the TM mode, in good agreement with the experimental modes, which is shown in Figure. 3(b). The propagation loss of the Nd:GdVO4 channel waveguides were determined to be as low as 0.5 dB·cm−1 for TM polarized light at 780 nm by following the method in Ref , which is similar to that previously obtained for Nd:YAG (0.6 dB·cm−1) and Nd:YVO4 (0.8 dB/cm) and even lower than in Yb:YAG crystals (1.3 dB·cm−1), with the important advantage of using a 2 to 3 orders of magnitude faster fabrication process.
Figure 4 shows the measured emission spectra around 1064 nm from the Nd:GdVO4 channel waveguide as obtained when pumping well above threshold. Single line oscillation at 1063.6 nm is observed, this corresponding to the wavelength at which the 4F3/2→4I11/2 emission cross section peaks. Inset in Fig. 4 shows the measured near-field distribution of the output laser beam at 1063.6 nm, which exhibits a single mode configuration. The 1063.6 nm laser radiation shows excellent long term time stability in both the level power and polarization state is found to be parallel to the optical axis.
Figure 5 shows the output power at 1063.6 nm as a function of the absorbed pump power at 808 nm with no laser mirrors as well as with an OC of 30% transmittance. In the former case, the reflectivity of the two end faces were 10% due to Fresnel reflection. The maximum output power of the waveguide laser was 256 mW at a pump power of 569 mW, leading to an optical conversion efficiency of 45% and a laser slope efficiency of 70%. This is very close to the quantum defect limit between pump and laser photons of 76%. The use of the mirror-less cavity leads to a very high laser slope efficiency but at the expense of large pump threshold. When the laser experiments were performed with the OC of 30%, the threshold pump power was reduced to 52 mW, the optical conversion efficiency was also reduced to 21%, corresponding to a laser slope efficiency of 23%.
In summary, we have reported the ultrafast laser fabrication of highly efficient cw waveguide laser radiation at ~1064 nm wavelength in Nd:GdVO4 low-loss channel waveguides with slope efficiencies as high as ~70%, which is close to the quantum defect limit. Well preserved Raman properties in conjunction with the excellent guiding and laser operation suggest the fs-laser written Nd:GdVO4 waveguides would serve as new efficient integrated laser devices for diverse photonic applications such as self-SRS integrated waveguide lasers.
The work is supported by the National Natural Science Foundation of China (No. 10925524). D. Jaque thanks the Universidad Autónoma de Madrid, Comunidad Autónoma de Madrid and Ministerio de Ciencia e Innovación for financial support under projects MAT2007-64686, CCG08-UAM/MAT-4434 and Phama S2009/MAT-1756). We also acknowledge the support from UK Engineering and Physical Sciences (EPSRC), grant numbers EP/G030227/1 and EP/D047269/1. We would like to thank Renishaw for the long-term loan of an inVia Reflex Raman microscope, as part of the Renishaw Heriot Watt Strategic Alliance.
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