Fabrication and Characterization of Femtosecond Laser Written Low Loss Depressed Cladding and Half-Ring Waveguides in Diamond

We report on the fabrication and characterization of femtosecond (fs) laser written depressed cladding and half-ring waveguides in diamond. The propagation losses of 2.05 dB/cm for the depressed claddings and 1.2 dB/cm for the half rings are the lowest propagation losses reported to date for fs laser written waveguides in diamond. Such low loss values reveal the potential of diamond waveguides in efficient quantum sensing applications.<br/>A polished single crystal CVD diamond was used in the experiments. The sample was positioned on a 3D translational stage and an amplified Ti³⁺:Sapphire laser generating 120 fs pulses at 800 nm and at 1 kHz repetition rate was used during waveguide fabrication. Laser pulses were focused into the sample with a 40X objective. With 100 nJ of incident pulse energy and 0.4 mm/s scan speed, 6 waveguides were fabricated across the full length of the sample. Three of the waveguides had depressed circular cladding, whereas the rest had semi-circular, half-ring configuration. Fabrication depth of depressed cladding waveguides was determined as 100 μm. Core diameter (D) of the depressed claddings was kept constant as 60 μm and the number of written tracks (N) was set as N=36, 48 and 60. Half rings were fabricated directly below the crystal surface to create an interface between air and the waveguide core. Design parameters (D, N) of the three half-ring waveguides were set as (60 μm, 18), (90 μm, 28), and (120 μm, 36).<br/>A He-Ne laser operating at 633 nm was utilized to characterize the waveguides. The laser beam was coupled to the waveguides with a converging lens and the guided beam exiting the waveguide was collimated with an aspheric lens (focal length f=6 mm). In order to reduce the mode mismatch between the focused beam and the waveguide core, converging lenses with focal lengths of f=4, 5 and 7.5 cm were used for half ring waveguides with D=60, 90 and 120 μm, respectively. For the depressed claddings, the optimum focal length for the converging lens was determined as f=7.5 cm.<br/>By comparing the power transmission of the waveguide with the bulk transmission of the crystal, propagation loss was measured as 2.05, 3.04 and 3.99 dB/cm for the depressed cladding waveguides with N=36, 48 and 60, respectively. For the half rings, losses were found as 1.2, 1.8 and 6.1 dB/cm for the waveguides with D=60, 90 and 120 μm, respectively. The trend from the depressed claddings shows that an increase in N increases the propagation loss as well. This trend is expected since the fs laser written tracks introduce graphitic content to the track region, resulting in an increase in the loss of the waveguide due to the optical properties of graphite. We further examined the structure of the written tracks by using Raman microscopy and observed the G-band for graphite at 1575 cm^-1, which supports the mentioned trend. For the half rings, results indicate that as the core diameter increases, the propagation loss decreases. This trend is expected also, since increasing the core diameter also increases the size of the unmodified core region in relation to the modified graphitic region.<br/>To summarize, the propagation loss of 2.05 dB/cm is the lowest loss reported to date for fs laser written depressed cladding waveguides in diamond. Results from the depressed cladding waveguide measurements showed that decreasing the number of written tracks reduces the loss. However, since reducing the number of tracks also decreases the refractive index contrast (Δn) and the ability of the waveguides to confine light, an optimum number of written tracks is needed for low-loss guiding. So, use of half-ring waveguides in diamond enables the confinement of light with reduced number of tracks, by taking advantage of the high Δn occuring at the air-waveguide interface. Switching to the half-ring waveguides resulted in propagation losses as low as 1.20 dB/cm, which is the lowest propagation loss reported to date for fs laser written waveguides in diamond.