Silicon-based electro-optic phase modulator with reduced residual amplitude modulation
An arrangement for removing unwanted amplitude modulation from the output of an electro-optic phase modulator (formed within a silicon-on-insulator (SOI) system) includes resonant filters that are biased on the positive and negative slopes of the response signal. Therefore, as the amplitude response of one filter decreases, the amplitude response of the other filter increases, resulting in balancing the output and essentially eliminating amplitude modulation from the phase-modulated output signal. In one embodiment, ring resonators (formed in the SOI layer) are used to provide the filtering, where as the number of resonators is increased, the performance of the filtering arrangement is improved accordingly.
1. A filtering arrangement for reducing amplitude modulation in a silicon-based electro-optic phase modulator output, the filtering arrangement formed in a silicon-on-insulator (SOI) structure including a sub-micron thick silicon surface layer (SOI layer) and comprising:
an optical waveguiding structure formed within the SOI layer for supporting the propagation of an optical output signal from an associated electro-optic phase modulator;
a first optical filter, coupled to the optical waveguiding structure, and formed to out-couple a predetermined first wavelength along a rising edge of the filter characteristic curve; and
a second optical filter, coupled to the optical waveguiding structure, and formed to out-couple a predetermined second wavelength along a falling edge of the filter characteristic curve so as to reduce amplitude modulated components from the propagating optical output signal.
2. A filtering arrangement as defined in claim 1 wherein the optical waveguiding structure comprises a single optical waveguide formed within the SOI layer of the SOI structure.
3. A filtering arrangement as defined in claim 1 wherein the optical waveguiding structure comprises an optical splitter/combiner geometry including a first waveguiding arm and a second waveguiding arm disposed in parallel between an input waveguiding section and an output waveguiding section, where the first optical filter is disposed alongside the first waveguiding arm and the second optical filter is disposed alongside the second waveguiding arm.
4. A filtering arrangement as defined in claim 3 wherein the optical splitter/combiner further comprises an optical phase adjustment element disposed along either one or both of the first waveguiding arm and the second waveguiding arm, the optical phase adjustment element for reducing phase differences between the signals propagating therealong.
5. A filtering arrangement as defined in claim 1 wherein the first and second optical filters comprise optical ring resonators, formed as ring waveguides within the SOI layer of the SOI structure.
6. A filtering arrangement as defined in claim 5 wherein the optical ring resonators include a highly doped region to provide for tuning of the filtered wavelength value.
7. A filtering arrangement as defined in claim 1 wherein the first optical filter comprises a first plurality of separate optical filtering elements and the second optical filter comprises a second plurality of separate optical filtering elements where each filtering element is configured to out-couple a difference wavelength and the resonance of the filtering arrangement response increases with an increasing number of separate filtering elements.