Multi-channel optical transceiver with offset quadrature amplitude modulation
The present disclosure provides a multi-carrier optical transmitter, receiver, transceiver, and associated methods utilizing offset quadrature amplitude modulation thereby achieving significant increases in spectral efficiency, with negligible sensitivity penalties. In an exemplary embodiment, an optical transmitter includes circuitry configured to generate a plurality of optical subcarriers, a plurality of data signals for each of the plurality of subcarriers, and a plurality of modulator circuits for each of the plurality of subcarriers, wherein each of the plurality of modulator circuits includes circuitry configured to offset an in-phase component from a quadrature component of one of the plurality data signals by one-half baud period.
1. An optical transmitter, comprising:
circuitry configured to generate a plurality of optical subcarriers;
a plurality of data signals for each of the plurality of subcarriers;
a plurality of modulator circuits for each of the plurality of subcarriers, wherein each of the plurality of modulator circuits comprises circuitry configured to offset an in-phase component from a quadrature component of one of the plurality data signals by one-half baud period;
a splitter splitting each of the plurality of subcarriers into two components, wherein the plurality of modulator circuits comprise modulator circuits for each of the subcarrier components; and
a polarization combiner combining each of the subcarrier components to form a polarization multiplexed signal with horizontal and vertical components.
2. The optical transmitter of claim 1 , further comprising:
prefiltering circuitry for each of the plurality of modulator circuits, wherein the prefiltering circuitry is configured to provide chromatic dispersion precompensation and pulse shaping to localize signal time and frequency content of one of the plurality data signals.
3. The optical transmitter of claim 2 , wherein the prefiltering circuitry comprises one of an analog filter or a digital filter performing pulse shaping through one of Root-Raised-Cosine, Isotropic Orthogonal Transform Algorithm, or Extended Gaussian functions, and wherein the prefiltering circuitry precompensates for a predetermined amount of chromatic dispersion.
4. The optical transmitter of claim 2 , wherein each of the plurality of modulator circuits comprises a Mach-Zehnder optical modulator, and wherein the prefiltering circuitry is configured to provide a predistortion function for compensating Mach-Zehnder optical modulator transfer curve nonlinearities.
5. The optical transmitter of claim 2 , wherein the prefiltering circuitry comprises any of finite impulse response (FIR) or infinite impulse response (IIR) components, and wherein a number of taps is set responsive to an amount of chromatic dispersion.
6. The optical transmitter of claim 1 , wherein the circuitry configured to generate the plurality of subcarriers is configured to generate the plurality of subcarriers at frequency offsets locked to a data baud rate.
7. The optical transmitter of claim 6 , wherein the circuitry configured to generate a plurality of subcarriers comprises at least one of filtering of mode-locked pulse lasers and phase modulation of a continuous wave laser.
8. The optical transmitter of claim 1 , wherein the plurality data signals are encoded with amplitude and phase modulation to provide a quadrature amplitude modulation constellation.
9. An optical modulation/demodulation method, comprising:
providing a data signal;
generating a plurality of optical subcarriers;
splitting the data signal into a plurality of sub-data signals for each of the plurality of optical subcarriers;
offsetting in-phase components and quadrature components of each of the plurality of sub-data signals by one-half baud rate;
modulating each of the plurality of optical subcarriers with the offset in-phase components and quadrature components of an associated sub-data signal of the plurality of sub-data signals;
splitting each of the plurality of optical subcarriers into two components, wherein a plurality of modulator circuits comprise modulator circuits for each of the optical subcarrier components; and
combining each of the optical subcarrier components to form a polarization multiplexed signal with horizontal and vertical components.
10. The optical modulation/demodulation method of claim 9 , further comprising:
formatting the plurality of sub-data signals for optical modulation.
11. The optical modulation/demodulation method of claim 10 , further comprising:
prefiltering the offset in-phase components and quadrature components prior to modulating.
12. The optical modulation/demodulation method of claim 11 , further comprising:
transmitting a modulated optical signal over a fiber link;
receiving the modulated optical signal;
splitting the modulated optical signal into N copies with N comprising a number of the plurality of optical subcarriers;
inputting each of the N copies into a separate coherent optical hybrid along with a local oscillator at a frequency based on a frequency of an associated optical subcarrier of the plurality of optical subcarriers; and
decoding the offset in-phase components and quadrature components for each of the plurality of optical subcarriers.
13. The optical modulation/demodulation method of claim 12 , further comprising:
postfiltering an output of each of the coherent optical hybrids.