Polarization-Multiplexed Optical Transmission System, Polarization-Multiplexed Optical Transmitter, and Polarization-Multiplexed Optical Receiver

• US 20120134676A1
• Filed: 11/22/2011
• Published: 05/31/2012
• Est. Priority Date: 11/29/2010
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: KIKUCHI, Nobuhiko
• Granted Patent: US 8,805,188 B2
• Time in Patent Office: Days
• US Class Current: 398/65
• CPC Class Codes: H04B 10/532 Polarisation modulation
• CPC Class Codes: H04B 10/614 comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers H04J14/06 takes precedence
• CPC Class Codes: H04J 14/06 Polarisation multiplex systems

Abstract

There is a need to prevent two receivers from converging on a state of receiving the same polarization state, fast start receivers, and ensure highly reliable operations. A polarization-multiplexed transmitter previously applies frequency shifts of frequencies +Δf and −Δf to X-polarization and Y-polarization digital information signals to be transmitted. Optical field modulators modulate and polarization-multiplex the signals. As a result, a frequency difference of 2Δf is supplied to X-polarization and Y-polarization components. A polarization diversity coherent optical receiver 215 receives the signal. A frequency estimation portion in a digital signal processing circuit detects a frequency difference signal in both polarization components. This signal is used to a polarization splitting circuit in the digital signal processing circuit.

Claims

1. A polarization-multiplexed optical transmission system comprising:

a polarization-multiplexed optical transmitter that includes two optical modulators and a polarization beam combiner, modulates input light input to the two optical modulators using mutually different multilevel information signals being one of one-dimensional and two-dimensional to generate an optical multilevel signal, and allows the polarization beam combiner to multiplex the optical multilevel signal into mutually orthogonal polarizations for transmission, wherein the polarization-multiplexed optical transmitter supplies a specified frequency difference to a center frequency of an optical multilevel signal for the mutually orthogonal polarization components; and

a polarization-multiplexed optical receiver including;

an adaptive polarization splitting circuit for adaptively separating two orthogonal polarization components from a polarization-multiplexed signal; and

two sets of demodulation parts for demodulating two separated polarization components,

wherein the polarization-multiplexed optical receiver detects a frequency difference component in two received polarization components and identifies or separates a multiplexed original orthogonal polarization component based on a detected frequency component.

2. The polarization-multiplexed optical transmission system according to claim 1,

wherein the polarization-multiplexed optical receiver resets an operation of the adaptive polarization splitting circuit in any of cases, in which intensity for a frequency difference component is detected from two polarization components and is smaller than or equal to a predetermined value;

a frequency difference between two polarization components is smaller than or equal to a predetermined value; and

a difference between the center optical frequency of an optical signal demodulated at one of the demodulation parts and that of an optical signal demodulated at the other thereof is smaller than a predetermined value.

3. The polarization-multiplexed optical transmission system according to claim 1,

wherein the polarization-multiplexed optical receiver extracts one of a frequency difference component detected from the two polarization components and frequency information corresponding to a center frequency of an optical signal received by each of the two sets of demodulation parts;

and

wherein the polarization-multiplexed optical receiver adaptively adjusts the adaptive polarization splitting circuit in consideration of any of;

one of maximizing and minimizing a difference between the center frequencies;

maximizing intensity of a frequency difference component; and

making one of pieces of frequency information corresponding to the center frequencies greater than the other.

4. The polarization-multiplexed optical transmission system according to claim 1,

wherein the polarization-multiplexed optical transmitter has an input terminal for externally specifying the specified frequency difference between two orthogonal polarization components and has a function of specifying the specified frequency difference;

and

wherein the polarization-multiplexed optical receiver has an input terminal for externally specifying a specified frequency difference to be detected and has a function of specifying the specified frequency difference.

5. The polarization-multiplexed optical transmission system according to claim 1,

wherein the polarization-multiplexed optical receiver has a function of outward outputting any of:

difference information between center frequencies of signals received at the two sets of demodulation parts;

frequency information corresponding to each of center frequencies of signals received at the two sets of demodulation parts; and

intensity of a component of the specified frequency difference.

6. The polarization-multiplexed optical transmission system according to claim 1,

wherein the polarization-multiplexed optical receiver is equivalent to a digital coherent polarization-multiplexed optical receiver that receives a signal based on polarization diversity coherent detection and provides the adaptive polarization splitting circuit based on digital signal processing.

7. The polarization-multiplexed optical transmission system according to claim 1,

wherein the adaptive polarization splitting circuit includes an automatic polarization controller and a polarization-dependent transparent device.

8. A polarization-multiplexed optical transmitter comprising:

two optical modulators and a polarization beam combiner,

wherein the polarization-multiplexed optical transmitter modulates input light input to the two optical modulators using mutually different multilevel information signals being one of one-dimensional and two-dimensional to generate an optical multilevel signal;

wherein the polarization-multiplexed optical transmitter allows the polarization beam combiner to multiplex the optical multilevel signal into mutually orthogonal polarizations for transmission; and

wherein the polarization-multiplexed optical transmitter supplies a specified frequency difference to a center frequency of an optical multilevel signal for the mutually orthogonal polarizations.

9. The polarization-multiplexed optical transmitter according to claim 8,

wherein the specified frequency difference is 20% or less of an entire bandwidth occupied by a transmitted optical signal.

10. The polarization-multiplexed optical transmitter according to claim 8,

wherein the polarization-multiplexed optical transmitter includes one transmission light source, splits output light from the light source into two each to be input to optical modulators, and provides an optical frequency shift function portion for, before, or after the optical modulator;

and

wherein the optical frequency shift function portion shifts center frequencies of the two output light beams so as to differ from each other by an amount equivalent to the specified frequency difference.

11. The polarization-multiplexed optical transmitter according to claim 8,

wherein input light to the two optical modulators is used as one of :

output light from two light sources having different oscillation frequencies differing from each other by a specified frequency; and

a branched version of output light from a multi-wavelength light source that outputs a plurality of light beams differing from each other by a specified frequency.

12. The polarization-multiplexed optical transmitter according to claim 8,

wherein the optical modulator is equivalent to an optical field modulator that outputs any two-dimensional optical field corresponding to a two-dimensional voltage signal applied to a plurality of modulation electrodes;

and

wherein the two optical modulators generate a two-dimensional rotational optical multilevel modulation signal by applying center frequency shifts, differing from each other by the specified frequency, to two-dimensional multilevel modulation signals.

13. The polarization-multiplexed optical transmitter according to claim 12,

wherein the polarization-multiplexed optical transmitter includes a DA converter connected to each modulation electrode of the optical modulator, generates a two-dimensional rotational multilevel modulation voltage signal corresponding to the two-dimensional rotational optical multilevel modulation signal, and applies the voltage to a modulation electrode of the optical modulator.

14. The polarization-multiplexed optical transmitter according to claim 12, further comprising:

a low-frequency analog oscillator; and

one of analog multiplier and an analog adder connected to each modulation electrode of the optical modulator,

wherein the polarization-multiplexed optical transmitter generates a two-dimensional rotational optical multilevel modulation by performing one of complex multiplication and addition on the multilevel information signal and a sine wave signal output from the low-frequency analog oscillator.

15. A polarization-multiplexed optical receiver in a polarization-multiplexed optical transmission system that also includes a polarization-multiplexed optical transmitter, the receiver comprising:

an adaptive polarization splitting circuit for adaptively separating two orthogonal polarization components from a polarization-multiplexed signal; and

two sets of demodulation parts for demodulating two separated polarization components,

wherein the polarization-multiplexed optical receiver detects a frequency difference component in two received polarization components and identifies or separates a multiplexed original orthogonal polarization component based on a detected frequency component; and

wherein the polarization-multiplexed optical transmitter includes two optical modulators and a polarization beam combiner, modulates input light input to the two optical modulators using mutually different multilevel information signals being one of one-dimensional and two-dimensional to generate an optical multilevel signal, allows the polarization beam combiner to multiplex the optical multilevel signal into mutually orthogonal polarizations for transmission, and supplies a specified frequency difference to a center frequency of an optical multilevel signal for the mutually orthogonal polarizations.

16. The polarization-multiplexed optical receiver according to claim 15,

wherein the polarization-multiplexed optical receiver resets an operation of the adaptive polarization splitting circuit in any of cases, in which intensity for a frequency difference component is detected from two polarization components and is smaller than or equal to a predetermined value;

a frequency difference between two polarization components is smaller than or equal to a predetermined value; and

a difference between the center optical frequency of an optical signal received at one of the demodulation parts and that of an optical signal demodulated at the other thereof is smaller than a predetermined value.

17. The polarization-multiplexed optical receiver according to claim 15,

wherein the polarization-multiplexed optical receiver extracts one of a frequency difference component detected from the two polarization components and frequency information corresponding to a center frequency of an optical signal demodulated by each of the two sets of demodulation parts;

and

wherein the polarization-multiplexed optical receiver adaptively adjusts the adaptive polarization splitting circuit in consideration of any of;

one of maximizing and minimizing a difference between the center frequencies;

maximizing intensity of a frequency difference component; and

making one of pieces of frequency information corresponding to the center frequencies greater than the other.

18. The polarization-multiplexed optical receiver according to claim 15, comprising:

an input terminal for externally specifying a specified frequency difference to be detected; and

a function of specifying the specified frequency difference.

19. The polarization-multiplexed optical receiver according to claim 15, comprising:

a function of outward outputting any of;

difference information between center frequencies of signals received at the two sets of demodulation parts;

frequency information corresponding to each of center frequencies of signals received at the two sets of demodulation parts; and

intensity of a component of the specified frequency difference.

20. The polarization-multiplexed optical receiver according to claim 15,

wherein the polarization-multiplexed optical receiver is equivalent to a digital coherent polarization-multiplexed optical receiver that receives a signal based on polarization diversity coherent and provides the adaptive polarization splitting circuit based on digital signal processing.