Cascaded photonic crystals in Ti-diffused LiNbO₃ strip waveguide

By using focused ion beam (FIB) technique, we have fabricated two air-hole-type triangular-lattice photonic crystals (PhCs) cascaded in a Ti-diffused LiNbO₃ strip waveguide. To ease the problem that the FIB-milled air-holes are not sufficiently deep, which results in that the holes cannot overlap most of optical mode and hence no evident stop-band feature is observed, two measures have been taken: a 0.75-μm-deep shallow trench was milled on the waveguide surface prior to the air-holes, and a scheme of two PhCs cascaded in the same waveguide was adopted. The cascaded two PhCs have the same air-hole array of 15 × 15 with a lattice constant 376.57 nm and an air-hole radius 150 nm. Transmission spectrum of the cascaded PhCs was measured, and a stop-band with a contrast ∼10 dB and a 3 dB bandwidth of ∼200 nm is observed in the spectral region 1.0–1.2 μm, in good agreement with the simulated result. Present work opens up the possibility of implementing desired PhC functions for various photonic applications by cascading multi-PhCs with different structure parameters.