Inscription of type I and depressed cladding waveguides in lithium niobate using a femtosecond laser

Abstract

We describe two types of waveguides (type I and depressed cladding) inscribed in lithium niobate using a variable repetition rate (200kHz-25MHz), 270fs duration fiber laser. The type I modification-based waveguides have propagation losses in the range from 1.2 to 10dB/cm at 1550nm, depending on experimental parameters. These waveguides are not permanent; they deteriorate over time. Such deterioration of waveguides can be slowed down from 30 days to 100 days by pre-annealing the samples and by writing at a 720kHz laser repetition rate. The propagation losses measured at 1550nm show significant improvement for pre-annealed samples. The depressed cladding-inscribed waveguides are permanent, but the propagation loss depends on the number of damage tracks. A track separation of ∼1 μm between adjacent damage tracks yields the lowest propagation loss of 0.5dB/cm at 1550nm for a 40μm diameter waveguide. We observe multimode guidance for sizes in the range of 20-80μm in these waveguide structures at 1550nm. Their crystalline nature is found to remain intact, as inferred from second-harmonic generation within the waveguide region.