Development of periodically poled lithium niobate zinc-indiffused ridge waveguides at blue wavelengths

Tailored laser sources for UV/blue wavelengths are in ever-increasing demand due to the advancing fields of quantum ion-trapping, metrology and computation. In particular, compact sources are required for the field deployment of quantum technologies. In recent work, we demonstrated the ability to fabricate ridge waveguides in Periodically Poled Lithium Niobate (PPLN) for Rb atom trapping based on 1560 nm Second Harmonic Generation (SHG) and mode matched to standard telecommunications optical fibre [1]. Here, we present our advancement of this work toward UV generation at 369 nm for Yb+ Doppler cooling, with an emphasis on manufacturing scalability.Our first example is a zinc-indiffused PPLN ridge waveguide for conversion of ~780 nm to 390 nm light. Electric field poling was used to achieve periodic gratings in 5% MgO-doped lithium niobate with a width of 200 μm and a phase-matching period of 2.2 μm through the entire 500 μm thickness of the crystal. Metallic zinc was deposited and indiffused to create a planar waveguide in the MgO:PPLN crystal, followed by ridge and facet definition via ultra-precision ductile dicing.Fig. 1. (a) SHG spectra generated from a 5.6 μm wide PPLN ridge waveguide with an overlaid model of a theoretical sinc2 with the corresponding Fabry-Perot interference fringes. (b) Pump mode profile, demonstrating single-mode operation. (c) Photograph of the device and the input/output coupling optics for characterisation. (d) Colour photograph of prism separated fundamental and second harmonic output.Fig. 1(a) shows a SHG spectra from a 5.6 μm wide diced ridge waveguide. Detailed spectra in low power operation is taken using lock-in amplification. As the sample is non-AR coated, the associated Fabry-Perot interference fringes for a 14 mm cavity can modelled to explain the SHG spectrum, also shown in Fig. 1(a).Fig. 1(b), the corresponding pump 2nd moment MFD is calculated as 4.7 x 6.1 μm in the x and y axes, respectively.Fig. 1(c) shows the device and the characterisation optics for simple input coupling using a 15 mm focal length aspheric lens from a collimated, fibre coupled source; the 390nm signal can be seen in Fig. 1(d). A conversion efficiency of ~10 %/W/cm2 has been attained with a 14 mm-long waveguide. This device offers a higher efficiency than typical UV-transmissive, nonlinear crystals in low-power operation, such as LBO, which typically require cavity enhancement for application [2]. This approach provides a simpler fabrication process to other ridge-style PPLN waveguides achieved through thinning, resin bonding and dicing to 1.5 μm depth ridge waveguides [3].We will present the optimisation of parameters for both periodic poling and waveguide fabrication. Current results on device efficiency, spectra and advancements to longer devices will also be presented. The effects of ridge width variation to understand distortion from the expected sinc2 SHG spectra will also be discussed to further understand the fabrication limitations in these indiffused, ridge waveguides.References:1. L. G. , S. A. Berry, R. H. Bannerman, A. C. Gray, J. W. Field, C. Holmes, J. C. Gates, P. G. R. Smith and C. B. E. Gawith, “Developing PPLN Waveguides for Quantum Rubidium Atom Traps in Space” Nonlinear Photonics Optical Society of America, (pp. NpTh1C-6). (2018).2. M. Pizzocaro, D. Calonico, P. C. Pastor, J. Catani, G. A. Costanzo, F. Levi, and L. Lorini. "Efficient frequency doubling at 399nm" Applied optics, 53, 3388-3392 (2014).3. K. Mizuuchi, T. Sugita, K. Yamamoto, T. Kawaguchi, T. Yoshino, and M. Imaeda "Efficient 340-nm light generation by a ridge-type waveguide in a first-order periodically poled MgO:LiNbO3" Optics letters 28, 1344-1346 (2003).