Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1998-02-12
2000-12-19
Pascal, Leslie
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359158, 359161, 359183, H04B 1004
Patent
active
061633941
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to optical data transmission and more particularly to non return to zero (NRZ) transmission of data.
When traffic is applied to an optical transmission line in NRZ format with a high launch power (typically >+18 dBm), non linear distortion is introduced by self phase modulation (SPM) causing transmission errors in the subsequent receiver at the remote end of the TX line. The distortion manifests itself by converting transitions in the data into peaks and suppressing the level of the intermediate data. Thus the receiver is more likely to detect these peaks and ignore the intermediate data. The present invention seeks to overcome or at least enable substantial reduction of errors in detection of NRZ signals.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided an optical signal transmitter comprising a coherent light generator for generating a carrier wavelength and a modulating means coupled with the output of the light generator the modulating means having a series arrangement of an optical intensity modulator and an optical phase modulator coupled between the input and an output of the modulating means, the intensity modulator having an input for a non return to zero (NRZ) digital signal of a predetermined data rate as defined by a clock generator and the phase modulator having an input coupled with the clock generator which provides pulses to the phase modulator at the data rate, or at an even multiple of the data rate, bit phase shifted relative to the data signal such that one end of each digital data period is red shifted and the other end is blue shifted to facilitate detection.
The improvement in detection results because symmetrical distortion occurs as a result of transmission and all data is effectively distorted equally as a result of transmission along the fibre.
The pulses applied to the phase modulator may be at the data rate and at a phase shift relative to the data signal such that one half of each digital data period is red shifted and the other half is blue shifted.
The transmitter may comprise a phase shifter connected between the clock generator and the input to the phase modulator to effect the relative phase displacement between the data pulse signals and the pulses applied to the phase modulator. The phase shifter may be adjustable to permit optimum adjustment of relative phase between the data pulse signal and the pulses applied to the phase modulator.
The clock generator may typically operate at 2.5 Gbit/sec.
The transmitter may include an optical signal amplifier wherein the output of the modulating means is coupled to the input of the optical signal amplifier the output of which amplifier provides an amplified signal for connection to an optical fibre transmission line.
According to another aspect of the invention there is provided an optical signal transmission system comprising a transmitter, as hereinbefore described, coupled via an optical fibre transmission line to a receiver and detector. The detector may comprise a filter and decision circuit.
According to another aspect of the invention there is provided a method of transmitting optical signals in which a coherent carrier wavelength is intensity modulated with a non return to zero digital data signal, wherein the signal data is phase modulated such that one end of each of the digital data pulse periods is red shifted and the other end of each of the data pulse periods is blue shifted thereby to facilitate detection at a receiver.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings in which:
FIG. 1 is a schematic block diagram illustrating a transmission system not employing the present invention,
FIG. 2 is a schematic block diagram illustrating a transmitter and transmission system constructed in accordance with the invention and operating in accorda
REFERENCES:
patent: 5473458 (1995-12-01), Mamyshev et al.
patent: 5515196 (1996-05-01), Kitajima et al.
patent: 5717510 (1998-02-01), Ishikawa et al.
patent: 5912755 (1999-06-01), Bergano
Patent Abstracts of Japan, vol. 095, No. 004, May 31, 1995 (JP-07-007475 A).
Patent Abstracts of Japan, vol. 095, No. 001, Feb. 28, 1995 (JP-06-303205 A).
Alcatel Alsthom Compagnie Generale d'Electriicite
Pascal Leslie
Phan Hanh
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