Rapid thermal annealing in high repetition rate ultrafast laser waveguide writing in lithium niobate

For the first time to our knowledge, bulk modification of lithium niobate using high repetition rate ultrashort laser pulses has been studied. A fiber based ultrafast laser has been applied in a range of 0.1 to 1.5 MHz repetition rate to directly inscribe optical waveguides in z-cut lithium niobate. Circularly polarized light with stretched 600 fs pulses produced waveguides with nearly circular mode profiles that guided in the telecom band of 1300 nm. Higher laser repetition rate of 700 kHz was found to offer smooth waveguides with low propagation loss of 0.6 dB/cm, matching the best reported value so far, with the advantage of 50 fold faster writing speed. At repetition rates of 250 kHz and higher, the tracks exhibited a cladding-like modification zone that extended outside the main laser interaction volume, yielding smoother structures, despite higher net fluence delivery, providing concrete evidence of heat accumulation and thermal annealing effects. We also present the first observation of periodic micro-structures in the bulk laser interaction volume of a non-glass material.