Spectral slicing filters in titanium diffused lithium niobate (ti:linbo3)



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A tunable guided-wave optical filter that performs spectral slicing at the 1530nm wavelength regime in Ti:LiNbO3 was proposed and fabricated. It is aimed at minimizing crosstalk between channels in dense wavelength division multiplexing (DWDM) optical network applications. The design utilizes a sparse grating allowing the selection of equally spaced channels in the frequency domain. Between selected channels, equally spaced nulls are also produced. The sparse grating is formed by using N coupling regions with different lengths along the direction of propagation of light in the waveguide, generating N-1 equally spaced nulls between adjacent selected channels. The distance between the centers of adjacent coupling regions is kept constant. The filtering is based on codirectional polarization coupling between transverse electric (TE) and transverse magnetic (TM) orthogonal modes in a waveguide through an overlay of strain-induced index grating, via the strain-optic effect. Two types of devices were fabricated. In the first type, the sparse gratings were produced on straight channel waveguides. Selected channels emerge from the device in a polarization state orthogonal to the input and a polarizer is needed to observe the filtered light. For the second type, an asymmetric Mach-Zehnder interferometer configuration was used to eliminate the need of the polarizer at the output, and yields an output response that is polarization independent. Both types of devices were fabricated on x-cut y-propagating LiNbO3 substrates, with N = 6 strain-induced coupling regions. The single mode channel waveguides were formed by Ti diffusion. Electrode patterns centered about the optical waveguide were defined by liftoff. In the straight channel devices, insertion loss was less than 2.5 dB on a 43 mm sample. The 3-dB channel bandwidth of the selected channels is approximately 1.0 nm. Devices were tuned thermally as well as by voltage application to surface electrodes resulting in tuning rates of 1.0 nm/oC and 0.04148 nm/V, respectively. In the polarization independent device the insertion loss for the phase-matched wavelength was 5.3 dB on a 53 mm long chip. The 3-dB bandwidth was also ~1.0 nm and the thermal tuning rate 1.0 nm/oC. The experimental results are in good agreement with design theory.