초록 ▼

A method and system of coherent detection of optical signals. The system utilizes a digital signal processor to recover an incoming optical signal. The system employs a local oscillator, which does not need to be phase locked to the signal. The signal may be consistently recovered, even when the pol

A method and system of coherent detection of optical signals. The system utilizes a digital signal processor to recover an incoming optical signal. The system employs a local oscillator, which does not need to be phase locked to the signal. The signal may be consistently recovered, even when the polarization state varies over time. Additionally, the signal may be recovered when it comprises two channels of the same wavelength that are polarization multiplexed together. In addition, any impairment to the signal may be reversed or eliminated.

대표청구항 ▼

What is claimed is: 1. A coherent optical detection system receiving an incoming optical signal in an optical communications network, said system comprising: a local oscillator emitting light; a phase diverse hybrid for generating two replicas of the incoming signal and two replicas of the local os

What is claimed is: 1. A coherent optical detection system receiving an incoming optical signal in an optical communications network, said system comprising: a local oscillator emitting light; a phase diverse hybrid for generating two replicas of the incoming signal and two replicas of the local oscillator light, said phase diverse hybrid combining the first replica of the incoming optical signal and the first replica of the local oscillator light into a first output and combining the second replica of the incoming optical signal and the second replica of the local oscillator light into a second output and wherein said local oscillator does not have to be phase locked to the incoming optical signal; wherein the phase relationship between the optical signal and the local oscillator light in the first output is different from 0 degrees and different from 180 degrees compared to the phase relationship between the local oscillator light and the optical signal in the second output and the state of polarization of the optical signal relative to the local oscillator light in the first output is not orthogonal to the state of polarization of the optical signal relative to the local oscillator light in the second output; and two photodetectors communicating with the phase diverse hybrid, wherein said two photodetectors receive optical signals from the two outputs and convert them to electrical signals; the electrical signals received by the two photodetectors are digitized by two A/D converters to create digital values; and a digital signal processor multiplies the digital values from the two A/D converters by two coefficients and sums them to form a complex number, then multiplies the complex number by a rotating phase factor to provide a complex representation of the envelope of the electric field of the incoming optical signal or a component of the complex representation of the envelope of the electric field of the incoming optical signal, the rotating phase factor having a frequency and a phase equal to estimates of the frequency and the phase of the incoming optical signal with respect to the local oscillator light. 2. The coherent optical detection system of claim 1 wherein the digital signal processor at least partially reverses the effect of propagation of the incoming optical signal through an optical fiber transmission system. 3. The coherent optical detection system of claim 2 wherein the digital signal processor compensates for the chromatic dispersion of the optical fiber transmission system. 4. The coherent optical detection system of claim 1 wherein the digital signal processor performs an optical filtering function on the electric field envelope. 5. The coherent optical detection system of claim 1 wherein the digital signal processor improves the quality of the incoming optical signal, the digital signal processor applying an algorithm which utilizes parameters that are adjusted to give different signal processing functions, and the values of those parameters are chosen for improving the quality of the recovered signal. 6. A method of receiving an incoming optical signal in a coherent optical detection system, said method comprising the steps of: emitting light from a local oscillator, said local oscillator not requiring a phase lock with the incoming optical signal; generating two replicas of the incoming signal and two replicas of the local oscillator light by a phase diverse hybrid; combining, by the phase diverse hybrid, the first replica of the incoming optical signal and the first replica of the local oscillator light into a first output; combining the second replica of the incoming optical signal and the second replica of the local oscillator light into a second output, wherein the phase relationship between the optical signal and the local oscillator light in the first output is different from 0 degrees and different from 180 degrees compared to the phase relationship between the local oscillator light and the optical signal in the second output and the state of polarization of the optical signal relative to the local oscillator light in the first output is not orthogonal to the state of polarization of the optical signal relative to the local oscillator light in the second output; receiving optical signals from the two outputs by two photodetectors in communication with the phase diverse hybrid; converting the optical signals from the two outputs into electrical signals; digitizing the electrical signals by two A/D converters to create digital values; multiplying digital values from the two A/D converters by coefficients and summing them to form a sequence of complex numbers; and multiplying the complex numbers by a rotating phase factor whose frequency is equal to an estimate of the frequency of the signal with respect to the local oscillator light and whose phase is equal to an estimate of the phase of the incoming optical signal with respect to the local oscillator light, to provide a complex representation or component thereof of the electric field envelope of the incoming optical signal. 7. The method of receiving an incoming optical signal of claim 6 further comprising the step of producing an output from the digital signal processor which is the result of a signal processing operation on a plurality of samples over time of the complex envelope of the electric field of the incoming optical signal, wherein the output has the information contained in the incoming optical signal. 8. The method of receiving an incoming optical signal of claim 6 wherein the incoming optical signal contains a plurality of WDM channels, further comprising the steps of: compensating for crosstalk imposed on a first of the plurality of WDM channels by at least one of the remainder of the plurality of WDM channels; and estimating the information carried on the first of the plurality of WDM channels. 9. The method of receiving an incoming optical signal of claim 8 wherein the crosstalk is cross phase modulation imposed on the first WDM channel by a second WDM channel during passage through an optical fiber transmission system. 10. The method of receiving an incoming optical signal of claim 8 wherein the crosstalk is caused by four wave mixing occurring when the plurality of WDM channels generates a four wave mixing product which at least partially overlaps the optical spectrum of the WDM channel that experiences the crosstalk. 11. A coherent optical detection system receiving an incoming optical signal in an optical communications network, said system comprising: a local oscillator emitting light; a phase diverse hybrid for generating two replicas of the incoming signal and two replicas of the local oscillator light, said phase diverse hybrid combining the first replica of the incoming optical signal and the first replica of the local oscillator light into a first output and combining the second replica of the incoming optical signal and the second replica of the local oscillator light into a second output and wherein said local oscillator does not have to be phase locked to the incoming optical signal; wherein the phase relationship between the optical signal and the local oscillator light in the first output is different from 0 degrees and different from 180 degrees compared to the phase relationship between the local oscillator light and the optical signal in the second output and the state of polarization of the optical signal relative to the local oscillator light in the first output is not orthogonal to the state of polarization of the optical signal relative to the local oscillator light in the second output; and two photodetectors communicating with the phase diverse hybrid, wherein said two photodetectors receive optical signals from the two outputs and convert them to electrical signals; wherein the electrical signals received by the two photodetectors are digitized by two A/D converters; and a digital signal processor performing a computation on digital values from the A/D converters to provide a complex representation of the envelope of the electric field of the incoming optical signal or a component of the complex representation of the envelope of the electric field of the incoming optical signal. 12. The coherent optical detection system of claim 11 wherein the digital signal processor produces an output which is the result of a signal processing operation on a plurality of samples over time of the complex envelope of the electric field of the incoming optical signal. 13. The coherent optical detection system of claim 12 wherein the digital signal processor at least partially reverses the effect of propagation of the incoming optical signal through an optical fiber transmission system. 14. The coherent optical detection system of claim 13 wherein the digital signal processor compensates for the chromatic dispersion of the optical fiber transmission system. 15. The coherent optical detection system of claim 14 wherein the signal processing operation performed by the digital signal processor compensates for the chromatic dispersion experienced by the optical signal by applying to the complex envelope of the incoming optical signal a convolution with a specified mathematical function, the mathematical function being close to the impulse response of the transfer function corresponding to a chromatic dispersion equal in magnitude and opposite to the chromatic dispersion of the optical fiber transmission system. 16. The coherent optical detection system of claim 14 wherein the signal processing operation performed by the digital signal processor to compensate for the chromatic dispersion experienced by the optical signal includes the calculation of the Fourier transform of an interval of time of the electric field of the optical signal. 17. The coherent optical detection system of claim 13 wherein the signal processing operation performed by the digital signal processor at least partially reverses the effect of self phase modulation imposed on the incoming optical signal. 18. The coherent optical detection system of original claim 11 wherein the signal processing operation performed by the digital signal processor at least partially reverses the effect of multipath interference imposed on the incoming optical signal. 19. The coherent optical detection system of claim 11 wherein the signal processing operation performed by the digital signal processor includes performing an optical filtering function on the complex envelope of the electric field. 20. The coherent optical detection system of claim 11 wherein the signal processing operation performed by the digital signal processor improves the quality of the incoming optical signal, the digital signal processor applying an algorithm which utilizes parameters that are adjusted to give different signal processing functions, and the values of those parameters are chosen for improving the quality of the recovered signal. 21. The coherent optical detection system of claim 20 wherein the signal processing operation that improves the quality of the recovered signal is a feedforward equalization-decision feedback equalization function. 22. The coherent detection system of claim 20 wherein the signal processing operation that improves the quality of the recovered signal is a maximum likelihood sequence estimation function. 23. A method of receiving an incoming optical signal in a coherent optical detection system, said method comprising the steps of: emitting light from a local oscillator, said local oscillator not requiring a phase lock with the incoming optical signal; generating two replicas of the incoming signal and two replicas of the local oscillator light by a phase diverse hybrid; combining, by the phase diverse hybrid, the first replica of the incoming optical signal and the first replica of the local oscillator light into a first output; combining the second replica of the incoming optical signal and the second replica of the local oscillator light into a second output, wherein the phase relationship between the optical signal and the local oscillator light in the first output is different from 0 degrees and different from 180 degrees compared to the phase relationship between the local oscillator light and the optical signal in the second output and the state of polarization of the optical signal relative to the local oscillator light in the first output is not orthogonal to the state of polarization of the optical signal relative to the local oscillator light in the second output; receiving optical signals from the two outputs by two photodetectors in communication with the phase diverse hybrid; and converting the optical signals from the two outputs into electrical signals; digitizing the electrical signals by two A/D converters; and performing a computation on digital values from the A/D converters by a digital signal processor to provide a complex representation of the envelope of the electric field of the incoming optical signal or a component of the complex representation of the envelope of the electric field of the incoming optical signal. 24. The method of receiving an optical signal of claim 23 wherein: the phase relationship between the optical signal and the local oscillator light in the first output of the phase diverse hybrid is a certain phase angle compared to the phase relationship between the local oscillator light and the optical signal in the second output of the phase diverse hybrid, said phase angle deviating from 90 degrees; and the step of performing a computation on digital values from the A/D converters by the digital signal processor includes compensating for the deviation of the phase angle from 90 degrees so as to produce a complex representation of the envelope of the electric field of the incoming optical signal or a component thereof that is substantially the same as if the phase angle were 90 degrees. 25. The method of receiving an incoming optical signal of claim 23 further comprising the step of producing an output from the digital signal processor which is the result of a signal processing operation on a plurality of samples over time of the complex envelope of the electric field of the incoming optical signal, wherein the output has the information contained in the incoming optical signal. 26. The method of receiving an incoming optical signal of claim 25 wherein the step of performing computations by a digital signal processor includes reversing at least partially the effect of propagation of the signal through an optical fiber transmission system. 27. The method of receiving an incoming optical signal of claim 26 wherein the step of performing computations by a digital signal processor includes reversing at least partially the effect of the chromatic dispersion of the optical fiber transmission system on the optical signal. 28. The method of receiving an incoming optical signal of claim 27 wherein the step of performing computations by a digital signal processor includes calculating the Fourier transform of an interval of time of the electric field of the optical signal, multiplying the Fourier transform by a set of coefficients equal to the inverse of the transfer function of the chromatic dispersion experienced by the incoming optical signal to produce a compensated Fourier transform, and calculating the inverse Fourier transform of the compensated Fourier transform. 29. The method of receiving an incoming optical signal of claim 25 wherein the step of performing computations by a digital signal processor includes reversing at least partially the effect of multipath interference imposed on the incoming optical signal. 30. The method of receiving an incoming optical signal of claim 25 wherein the step of performing computations by a digital signal processor includes performing an optical filtering function on the complex envelope of the electric field. 31. The method of receiving an incoming optical signal of claim 23 wherein the incoming optical signal contains a plurality of WDM channels, further comprising the steps of: compensating for crosstalk imposed on a first of the plurality of WDM channels by at least one of the remainder of the plurality of WDM channels; and estimating the information carried on the first of the plurality of WDM channels. 32. The method of receiving an incoming optical signal of claim 31 wherein the crosstalk is cross phase modulation imposed on the first WDM channel by a second WDM channel during passage through an optical fiber transmission system. 33. The method of receiving an incoming optical signal of claim 31 wherein the crosstalk is caused by four wave mixing occurring when the plurality of WDM channels generates a four wave mixing product which at least partially overlaps the optical spectrum of the WDM channel that experiences the crosstalk. 34. The method of receiving an optical signal of claim 23 wherein: the two digitized signals from the two A/D converters arriving at the digital signal processor at the same time correspond to samples taken at different times with respect to the envelope of the electric of the incoming optical signal; and the step of performing a computation on digital values from the A/D converters by the digital signal processor includes compensating for the different path delays experienced by two signal paths.