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Structural optimization of femtosecond-laser-written lithium niobate waveguide by Raman spectroscopy

A new optimization method for femtosecond-laser-written lithium niobate (LN) waveguides is proposed and experimentally demonstrated to achieve low propagation loss. In this method, we use Raman spectroscopy to find detailed structural changes in LN crystal. We then use the results to optimize the laser irradiation conditions, including pulse laser energy, and laser irradiation design. In order to confirm the validity of this method, depressed cladding type LN waveguides were formed and their structural and optical characteristics were investigated in detail. Using proposed method, we succeeded in fabricating depressed cladding type LN waveguides with low propagation loss (∼0.64 dB cm−1) in the communications wavelength band. Furthermore, by analyzing the crystal structure of the depressed cladding waveguide cross-section using a Raman spectroscopy mapping system, the asymmetry of the waveguide cladding was newly clarified, providing a new insight for further improvement of the optical properties.