Optical system employing near-incoherent processing for distortion correction

• US 5,515,199 A
• Filed: 01/31/1995
• Issued: 05/07/1996
• Est. Priority Date: 01/31/1995
• Status: Expired due to Term

• Current Assignee: Tyco Electronics Services GmbH (TE Connectivity Limited)
• Original Assignee: Photonic Applications, Inc.
• Inventors: Farina, James D.
• Primary Examiner(s): Lee, John D.
• Application Number:
  US08/381,228
• Time in Patent Office: 462 Days
• Field of Search: 359/326-328, 359/237-239, 359/ 245, 385/1-3, 385/5, 385/8-10, 385/122
• US Class Current: 359/326
• CPC Class Codes: G02F 2203/19 linearised modulation; reduction of harmonic distortions
• CPC Class Codes: H04B 10/2507 for the reduction or elimination of distortion or dispersion

Abstract

A system for correcting nonlinear distortion in fiber optic systems including communication systems by means of light modulation provided by interferometric modulators such as the Mach-Zehnder. The present invention involves the addition of a correction signal which contains error canceling information. The additive correction signal optical beam is frequency shifted or phase scrambled through the use of a phase or frequency modulator. This insures the corrective light signal adds incoherently or quasi-incoherently within the signal bandwidth of interest. Furthermore, the present system requires only a single light source and can be made independent of wavelength and is easily optimized with active servo devices. A system provided by the present invention also provides improvement over the state of the art because of its high degree of correction and independence from link length using ordinary single mode fibers.

Claims

1. A system for providing compensation for optical nonlinearities in an optical system that transmits data signals in a data transfer frequency band, said system comprising:

an optical source for providing a coherent main optical beam;

a beam splitter means for receiving said optical beam and providing first and second split optical beams;

a frequency generator for generating a radio frequency (RF) modulation signal whose frequency spectrum includes said data transfer frequency signal band;

a RF signal tap means for extracting a reference portion of said RF modulation signal;

a main modulator means for receiving said RF modulation signal and modulating said first split optical beam to output a modulated main optical beam having modulated and distorted optical components;

a quasi-incoherent compensation apparatus including;

a means for generating a synthesized optical beam incoherent with said main optical beam in said data frequency signal band such that no optical interference products between said main and synthesized optical beams occur within said data transfer frequency band when said main and synthesized optical beams are combined;

a means for generating an error signal indicative of the difference between said RF signal and said distortion portion of said main optical beam;

a compensation modulator means for receiving said error signal and modulating said second split optical beam to output a modulated compensation optical beam; and

an optical combiner means for receiving said main and compensation optical beams and providing therefrom a compensated output beam.

2. The system of claim 1 wherein said quasi-incoherent compensation apparatus further comprises:

a detector means for receiving a first portion of said modulated main optical beam for providing electrical signal equivalents thereof;

a signal combiner means for receiving said detector means signals and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals;

a phase modulation frequency generator for generating a phase modulation signal at a frequency outside of said data frequency signal band;

a phase modulation means for receiving said second split optical beam and phase modulation signal for providing a phase modulated second optical beam; and

a correction modulator means for receiving said difference signal and said phase modulated second optical beam to output, as part of said synthesized beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude to but opposite in phase to said modulated main optical beam distortion component.

3. The system according to claim 2 further comprising

a means for selecting the phase of said RF signal;

a means for generating signals indicative of detected optical power in said compensated output beam; and

a gain adjustment means for receiving said compensated output beam power signals and for adjusting said relative RF signal gain between said main and said synthesized optical beams;

said RF signal phase and said relative RF signal gain selected to maximize cancellation of said distorted optical components.

4. The system of claim 3 wherein said gain adjustment means further comprises a means for adjusting the magnitude of the optical power removed from said main optical beam.

5. The system of claim 3 wherein said gain adjustment means further comprises a means for adjusting the magnitude of the modulation by said correction modulator means.

6. The system of claim 1 wherein said compensated output optical beam is described by

7. The system of claim 6 wherein .o slashed.(t) corresponds to a sine/cosine wave with amplitude 2.405 radians or an amplitude corresponding to a zero in a Bessel function, J0.

8. The system of claim 6 wherein .o slashed.(t) corresponds to a frequency modulated signal with amplitude 2.405 radians or an amplitude corresponding to a zero in a Bessel function, J0.

9. The system of claim 6 wherein .o slashed.(t) corresponds to a combination of harmonic signals.

10. The system of claim 6 wherein .o slashed.(t) has a limited bandwidth random noise selected so as to deplete a carrier frequency.

11. The system of claim 1 wherein said quasi-incoherent compensation apparatus further comprises:

a detector means for receiving a first portion of said modulated main optical beam for providing electrical signal equivalents thereof;

a signal combiner means for receiving said detector means signals and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals;

a time delay means, receiving said second split optical beam, for providing a temporal delay whose magnitude is selected to be greater than a coherence time for said main optical beam; and

a correction modulator means for receiving said difference signal and said time delayed second optical beam to output, as part of said synthesized beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude to but opposite in phase to said modulated main optical beam distortion component.

12. The system of claim 1 wherein said coherent main optical beam has an initial polarization state and wherein said quasi-incoherent compensation apparatus further comprises:

a detector means for receiving a first portion of said modulated main optical beam for providing electrical signal equivalents thereof;

a signal combiner means for receiving said detector means signals and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals;

a correction modulator means for receiving said difference signal and said second optical beam to output, as part of said synthesized beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude to but opposite in phase to said modulated main optical beam distortion component;

a polarization rotator means for receiving said modulated synthesized optical beam and generating a 90 degree phase rotated synthesized optical beam; and

wherein said optical combiner means further comprises a means for maintaining said polarization states of said phase rotated, modulated synthesized optical beam and said modulated main optical beam.

13. The system of claim 1 wherein said a quasi-incoherent compensation apparatus further comprises:

a detector means for receiving a first portion of said modulated main optical beam for providing electrical signal equivalents thereof;

a signal combiner means for receiving said detector means signals and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals;

a correction modulator means for receiving said difference signal and said second optical beam to output, as part of said synthesized beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude to but opposite in phase to said modulated main optical beam distortion component;

wherein said system further comprises

a pilot tone signal generator for providing a signal a pilot frequency for presentation to said main modulator in combination with said RF modulation signal subsequent to said RF signal tap;

an output signal detector means for generating electrical signal equivalents of said output optical beam; and

a dither servo control circuit means receiving said output beam detector signals having a pilot tone signal portion, said dither servo control circuit means including automatic gain control means for varying the magnitude of gain control signals presented to said combiner means about a value thereof corresponding to a minimum of said output beam detector signal pilot tone signal portion.

14. The system of claim 13 wherein said pilot tone signal generator further comprises means for providing a signal a plurality of pilot frequencies for presentation to said main modulator in combination with said RF modulation signal subsequent to said RF signal tap;

and

wherein said dither servo control circuit means automatic gain control means further includes means for varying the magnitude of gain control signals presented to said combiner means about values corresponding to a one of corresponding minimums of said output beam detector signal pilot tone signal portions.

15. The system of claim 1 wherein said a quasi-incoherent compensation apparatus further comprises:

a detector means for receiving a first portion of said modulated main optical beam for providing electrical signal equivalents thereof;

a signal combiner means for receiving said detector means signals and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals;

a correction modulator means for receiving said difference signal and said second optical beam to output, as part of said synthesized beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude to but opposite in phase to said modulated main optical beam distortion component;

wherein said main modulator means further comprises

a means for generating two main output beams and said correction modulator means further comprises means for generating two modulated correction optical beams; and

a second combiner means for receiving said second main and modulated correction beams and generating therefrom a second compensated output beam.