Femtosecond laser writing of low-loss waveguides with different geometries in diamond

We report on the fabrication and optical characterization of femtosecond (fs) laser written waveguides with varying geometries and parameters in a single crystal CVD grown diamond. Depressed circular cladding, half-ring, and double-line waveguides were fabricated. Design parameters such as core size and number of written tracks were varied to minimize propagation losses. Characterization of the waveguides was performed at 633 nm, which is close to the peak fluorescence wavelength of the nitrogen vacancy (NV) centers in diamond. Important experimental results revealing the dependence of the propagation loss and refractive index contrast on the design parameters were obtained. The maximum refractive index contrast was estimated as 22.7 × 10⁻⁵ for the fabricated waveguides. The measured propagation loss values of 2.05 dB/cm and 1.20 dB/cm, obtained with circular depressed cladding and half-ring waveguides, respectively, are, to the best of our knowledge, the lowest propagation loss values reported so far among fs laser written diamond waveguides.