High performance optical link

• US 6,634,811 B1
• Filed: 11/30/1999
• Issued: 10/21/2003
• Est. Priority Date: 11/30/1999
• Status: Expired due to Term

• Current Assignee: Emcore Corporation
• Original Assignee: JDS UK Limited
• Inventors: Colyer, Mark, Gertel, Eitan
• Primary Examiner(s): Chan, Jason
• Assistant Examiner(s): Leung, Christina Y
• Application Number:
  US09/451,417
• Time in Patent Office: 1,421 Days
• Field of Search: 359/182, 359/188, 359/124, 359/189, 398/187, 398/150, 398/159, 398/141, 398/200
• US Class Current: 398/159
• CPC Class Codes: H04B 10/2507 for the reduction or elimination of distortion or dispersion
• CPC Class Codes: H04B 10/2575 Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier

Abstract

An optical link is described for transmission of an RF signal using an optical fiber having opposing ends and configured such that unwanted distortion signals are produced when optical signals pass through the optical fiber. At least first and second optical signals are transmitted through the optical fiber, where the optical signals have been modulated with an RF signal in such a way that a predetermined relationship is produced between the RF signals modulated on the first and second optical signals. At the opposing end of the optical fiber, a receiving arrangement receives the modulated first and second optical signals including the unwanted distortion signals produced during the transmission through the optical fiber. The receiving arrangement then regenerates the RF signal from the modulated first and second optical signals while causing the unwanted distortion signals to be canceled based on the predetermined relationship between the RF signals modulated on the first and second optical signals.

Claims

1. An optical link for transmission of an RF signal using an optical fiber having opposing ends such that unwanted distortion signals are produced between said opposing ends when optical signals pass through the optical fiber, said optical link comprising:

at least two light sources which emit first and second optical carriers at first and second optical wavelengths, respectively;

a combiner for combining the first and second optical carriers into a combined optical signal;

a modulator for modulating the combined optical signal using the RF signal, and an for producing first and second optical signals, such that each of the first and second optical signals includes a portion of the first and second optical carriers and said RF signal in a way which produces a predetermined relationship between the first and second optical signals;

transmission means for selectively multiplexing certain portions of the first and second optical signals to produce a multiplexed signal and for introducing the multiplexed signal into one end of said optical fiber; and

receiving means, at the opposing end of said optical fiber, for receiving the multiplexed signal including said unwanted distortion signals produced during transmission through said optical fiber and for recovering from the received multiplexed signal said first optical carrier affected by said unwanted distortion signals and having said RF signal modulated thereon and said second optical carrier affected by said unwanted distortion signals and having said RF signal modulated thereon and, thereafter, for using the modulated RF signals including said unwanted distortion signals obtained from the modulated first and second optical carriers in a way which re-generates the RF signal based on said predetermined relationship while causing said distortion signals to be canceled.

2. The optical link of claim 1, wherein said combiner includes a polarization-maintaining combiner which combines the first and second optical carriers for use in producing the first and second optical signals.

3. The optical link of claim 2, wherein said modulator includes a Mach Zehnder modulator for modulating the RF signal onto the combined first and second optical carriers and for producing the first and second optical signals in a way which produces said predetermined relationship between the first and second optical signals.

4. The optical link of claim 1, wherein said predetermined relationship between the first and second optical signals is a phase relationship, such that the modulated RF signal on the first optical carrier is 180°

out of phase with respect to the modulated RF signal on the second optical carrier; and

wherein the modulated RF signals obtained from the first and second optical signals are combined by said using an additional 180°

phase shift such that the modulated RF signal from the first optical carrier adds with the modulated RF signal from the second optical carrier while the additional 180°

phase shift causes the distortion signals from the first and second optical carriers to cancel.

5. The optical link of claim 4, wherein said transmission means includes a wavelength-division multiplexer having first and second input channels and an output channel,

wherein said wavelength division multiplexer extracts the first optical carrier as the certain portion from the first optical signal at the first input channel, and extracts the second optical carrier as the certain portion from said second optical signal at the second input channel, such that said multiplexed signal at the output channel contains only the first and second optical carriers having said predetermined phase relationship.

6. The optical link of claim 1, wherein said predetermined relationship between the first and second optical signals is a phase relationship such that the modulated RF signal on the first optical carrier is 180°

out of phase with respect to the modulated RF signal on the second optical carrier; and

wherein said receiving means includes a demultiplexer for separating the first and second optical carriers from said multiplexed signal into first and second optical output signals such that the first optical output signal includes said first optical carrier and the second optical output signal includes said second optical carrier, while maintaining the phase relationship between the modulated RF signals forming part of each of said first and second optical carriers within the first and second optical output signals.

7. The optical link of claim 6 wherein said receiving means includes means for converting the modulated RF signals forming part of said first and second optical output signals into first and second RF output signals, while preserving the phase relationship between the modulated RF signals, and wherein the modulated RF signals are combined using combining means for inverting the phase of one of the modulated RF signals and adding the inverted modulated RF signal to the other one of the modulated RF signals.

8. The optical link of claim 7 wherein said receiving means includes a phase compensator configured for introducing an additional 180°

phase shift to the modulated RF signal on one of said optical output signals such that the modulated RF output from one of the optical output signals is brought into phase with the other one of said optical output signals while said unwanted distortion signals on one of the optical output signals is forced substantially out of phase with respect to said unwanted distortion signal on the other one of said optical output signals.

9. A method of transmitting an RF signal using an optical fiber having opposing ends such that unwanted distortion signals are at least produced between said opposing ends when optical signals pass through the fiber, said method comprising the steps of:

a) emitting first and second optical carriers at different optical wavelengths;

b) combining the first and second optical carriers into a combined optical single;

c) modulating the combined optical signal using the RF signal forming a modulated combined optical signal;

d) producing first and second optical signals from the modulated combined optical signal in a way which provides a predetermined relationship between the first and second optical signals, so that each of the first and second optical signals includes at least a portion of the first and second optical carriers and said RF signal modulated thereon;

e) selectively multiplexing certain portions of the first and second optical signals to produce a multiplexed signal, and then introducing the multiplexed signal into one end of said optical fiber;

f) receiving the multiplexed signal at an opposing end of said optical fiber, including said unwanted distortion signals produced during transmission through said optical fiber; and

g) at the opposing end of said fiber, recovering from the multiplexed signal said first optical carrier having said RF signal modulated thereon as affected by said unwanted distortion signals and said second optical carrier having said RF signal modulated thereon as affected by said unwanted distortion signals, and, thereafter, using the modulated RF signals obtained from the first and second optical carriers in a way which re-generates the RF signal based on said predetermined relationship while causing said distortion signals to be canceled.

10. The method of claim 9, wherein said first and second optical carriers include first and second polarization states, respectively, and wherein step b) includes combining the first and second optical carriers in a way which maintains said first and second polarization states for use in producing the first and second optical signals.

11. The method of claim 9, wherein said certain portions of said first and second optical signals include a phase relationship such that the modulated RF signal on the first optical carrier is 180°

out of phase with respect to the modulated RF signal on the second optical carrier, and

wherein step (g) includes recovering the modulated RF signals from the first and second optical carriers, introducing an additional 180°

phase shift to the recovered RF signal obtained from the first or second optical carrier to produce an inverted recovered RF signal, and adding the inverted recovered RF signal to the other recovered RF signal such that the additional 180°

phase shift causes the unwanted distortion signals to cancel.

12. The method of claim 9, wherein certain portions of the first and second optical signals are multiplexed by:

i) extracting the first optical carrier from the first optical signal, including said RF signal modulated thereon, to reject the second optical carrier, ii) extracting the second optical carrier from the second optical signal, including said RF signal modulated thereon, to reject the first optical carrier, and, iii) combining the extracted first and second optical carriers to produce said multiplexed signal while preserving said predetermined relationship.

13. The method of claim 12, wherein step (d) includes inducing a phase difference of 180°

between the RF signal modulated on the first optical carrier and the RF signal modulated on the second optical carrier as the predetermined relationship between the first and second optical signals.

14. The method of claim 13, wherein step (f) includes separating the first and second optical carriers from said multiplexed signal into first and second optical output signals, such that the first optical output signal includes said first optical carrier as affected by said unwanted distortion signals and the second optical output signal includes said second optical carrier as affected by said unwanted distortion signals, while maintaining the phase relationship between the modulated RF signals on said first and second optical carriers in the first and second optical output signals.

15. The method of claim 14, wherein step (g) includes converting said first and second optical output signals into first and second RF output signals;

and

wherein said predetermined phase relationship is preserved between the first and second RF output signals, at least in part, using a phase compensation step which brings the first and second RF outputs substantially into phase while said unwanted distortion signals on said first and second optical output signals, produced during transmission through said optical fiber, are forced substantially out of phase.

16. A method of transmitting an RF signal using a wavelength division multiplexer including first and second input ports matched to first and second optical wavelengths, respectively, and an output port, and further using an optical fiber having opposing ends such that unwanted distortion signals are at least produced between said opposing ends when optical signals pass through the fiber, said method comprising the steps of:

a) emitting first and second optical carriers having said first and second optical wavelengths, respectively;

b) combining the first and second optical carriers into a combined optical signal;

c) modulating the combined optical signal using the RF signal forming a modulated combined optical signal;

d) producing first and second optical signals from the modulated combined optical signal in a way which provides a predetermined relationship between the first and second optical signals and so that each of the first and second optical signals includes at least a portion of the first and second optical carriers having said RF signal modulated thereon;

e) selectively multiplexing certain portions of the first and second optical signals using said wavelength division multiplexer to produce a multiplexed signal and then introducing the multiplexed signal from the wavelength division multiplexer into one end of said optical fiber;

f) at the opposing end of said fiber, receiving and recovering said first optical carrier having said RF signal modulated thereon as affected by the unwanted distortion signals and said second optical carrier having said RF signal modulated thereon as affected by the unwanted distortion signals;

g) removing the modulated RF signals from the modulated first and second optical carriers while preserving said predetermined relationship; and

h) combining the modulated RF signals in a way which regenerates said RF signal based on said predetermined relationship while causing said distortion signals to be canceled.

17. The method of claim 16, wherein said step (e) includes using said wavelength division multiplexer to extract the first optical wavelength from the first optical signal at the first input port of said wavelength-division multiplexer so as to reject the second optical signal, and to extract the second optical wavelength from the second optical signal at the second input port of said wavelength-division multiplexer so as to reject the first optical signal, such that said multiplexed signal at the output port of said wavelength division multiplexer includes only the first optical wavelength from the first optical signal as affected by said unwanted distortion signals and the second optical wavelength from the second optical signal as affected by said unwanted distortion signals, each of which is modulated by said RF signal while preserving said predetermined relationship.