Ultrafast laser waveguide writing: Lithium niobate and the role of circular polarization and picosecond pulse width

For the first time to our knowledge, ultrafast laser writing has generated room-temperature stable guided-wave optics in bulk lithium niobate for the telecommunication spectrum. Among a seven-dimensional parameter space for waveguide optimization, two frequently overlooked parameters, pulse duration and polarization, were found to be key in overcoming undesired nonlinear optical responses imposed by this material. Single-mode waveguides were best formed with circularly polarized light having a relatively long pulse duration of approximately 1.0 ps. The waveguides were highly polarization dependent and guided in both telecommunication bands near 1300 and 1550 nm, exhibiting losses as low as 0.7 dB/cm.