POLARIZATION MULTIPLEXED SIGNALING USING TIME SHIFTING IN RETURN-TO-ZERO FORMAT
Polarization multiplexing by encoding data using a return-to-zero format, and by interleaving the constituent orthogonal polarization components such that the data-carrying portion of the bit window from one orthogonal polarization component occupies the zero portion of the bit window for the other orthogonal polarization component.
1 . An optical transmitter comprising:
a light source that generates and outputs a pulsed or continuous wave optical beam;
a polarization beam splitter that splits the optical beam into first and second orthogonal polarization components;
a first optical data modulator that receives the first orthogonal polarization component and modulates data onto the first orthogonal polarization component using a return-to-zero format;
a second optical data modulator that receives the second orthogonal polarization component and modulates data onto the second orthogonal polarization component using a return-to-zero format;
a delay component that delays the modulated second orthogonal polarization component; and
an optical multiplexer that combines the modulated first orthogonal polarization component and the delayed and modulated second orthogonal polarization component, wherein the delay introduced by the delay component is sufficient that the orthogonal polarization components are interleaved when combined by the optical multiplexer.
2 . The optical transmitter in accordance with claim 1 , wherein the return-to-zero format of the first orthogonal polarization component, and the return-to-zero format of the second orthogonal polarization component is the same return-to-zero format.
3 . The optical transmitter in accordance with claim 1 , wherein the return-to-zero format of the first orthogonal polarization component is return-to-zero on-off keying (RZ-OOK).
4 . The optical transmitter in accordance with claim 1 , wherein the return-to-zero format of the first orthogonal polarization component is return-to-zero differential phase shift keying (RZ-DPSK).
5 . The optical transmitter in accordance with claim 1 , wherein the delay component introduces a one-half bit of delay.
6 . The optical transmitter in accordance with claim 1 , wherein the delay component introduces an N+½ bit relative delay, where N is a whole number.
7 . An optical receiver, comprising:
a polarization controller that adjusts a polarization state of an externally received polarization multiplexed optical signal based on receipt of a control signal, said polarization multiplexed optical signal having a clock signal modulated thereon;
a polarization splitter that splits the polarization multiplexed optical signal received from the polarization controller into first and second orthogonal polarization components;
a first optical detector for converting the first orthogonal polarization components into a first electrical signal;
a second optical detector for converting the second orthogonal polarization component into a second electrical signal; and
a feedback circuit for generating the control signal based on a characteristic of the clock signal extracted from the first or second electrical signals.
8 . The optical receiver in accordance with claim 7 , wherein the control signal causes the polarization controller to adjust the polarization state of the externally received polarization multiplexed optical signal so that an amplitude of the clock signal is maximized.
9 . The optical receiver in accordance with claim 7 , wherein the feedback circuit includes a filter tuned to a frequency of the clock signal and coupled to an output of the first optical detector.
10 . The optical receiver in accordance with claim 9 , wherein the feedback circuit further includes:
a peak detector arrangement for receiving the clock signal from the filter and generating an output signal representative of the clock signal amplitude; and
a control circuit for generating the control signal in response to receipt of the output signal from the peak detector arrangement.
11 . The optical receiver in accordance with claim 7 , wherein the polarization multiplexed optical signal is an optical signal modulated in accordance with an RZ format having RZ pulses based on the clock signal.
12 . The optical receiver in accordance with claim 11 , wherein the optical signals is an RZ-DPSK signal.
13 . The optical receiver in accordance with claim 7 , wherein the feedback circuit is configured to generate the control signal based on a characteristic of the clock signal that causes a reduction in cross-talk between the first and second orthogonal polarization components.
14 . The optical receiver in accordance with claim 7 , wherein the control signal causes the polarization controller to adjust the polarization state of the externally received polarization multiplexed optical signal so that an amplitude of the clock signal is minimized.
15 . A method for demultiplexing an optical signal, comprising:
receiving a polarization multiplexed optical signal having a clock signal modulated thereon;
splitting the polarization multiplexed optical signal received from the polarization controller into first and second orthogonal polarization components; and
adjusting a polarization state of the polarization multiplexed optical signal based on a characteristic of the clock signal derived from at least one of the first or second orthogonal polarization components.
16 . The method in accordance with claim 15 , further comprising adjusting the polarization state of the polarization multiplexed optical signal to align the polarization state of the polarization multiplexed optical signal with a polarization axis of a polarization splitter used to split the polarization multiplexed optical signal into the first and second orthogonal polarization components.
17 . The method in accordance with claim 15 wherein the characteristic of the clocks signal is its amplitude.
18 . The method in accordance with claim 17 , further comprising adjusting the polarization state of the polarization multiplexed optical signal to maximize the amplitude of the clock signal.
19 . The method in accordance with claim 17 , further comprising adjusting the polarization state of the polarization multiplexed optical signal to minimize the amplitude of the clock signal.
20 . The method in accordance with claim 19 , wherein the clock signal is twice the bit rate of each of the first and second orthogonal polarization components.