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Description:
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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 . |