Frequency modulation coherent optical communications system with continuous polarization scrambling

• US 5,329,394 A
• Filed: 05/15/1992
• Issued: 07/12/1994
• Est. Priority Date: 07/04/1991
• Status: Expired due to Fees

• Current Assignee: Cselt - Centro Studi E Laboratori Telecommunicazioni SPA
• Original Assignee: Cselt - Centro Studi E Laboratori Telecommunicazioni SPA
• Inventors: Marone, Giuseppe, Caponi, Renato, Calvani, Riccardo
• Primary Examiner(s): Chilcot, Jr., Richard E.
• Assistant Examiner(s): BACARES, RAFAEL D
• Application Number:
  US07/883,836
• Time in Patent Office: 788 Days
• Field of Search: 359/156, 359/181, 359/183, 359/188, 359/189, 359/190, 359/191, 359/192, 359/182, 385/24
• US Class Current: 398/152
• CPC Class Codes: H04B 10/532 Polarisation modulation

Abstract

A coherent optical communications system with continuous polarization scrambling, wherein such scrambling is obtained by combining orthogonally polarized radiations at different frequencies emitted by two independent sources. The two sources may be employed to build the transmitted or the local oscillator of the system.

Claims

1. A method of polarization scrambling in an optical signal transmission system in which optical signals are modulated by a binary digital information signal and undergo heterodyne coherent demodulation, said method comprising the steps of:

(a) frequency modulating of linearly polarized radiation by a binary digital information signal, so that the two sources emit radiations at a first and a second frequency and at a third and a fourth frequency, respectively, in correspondence with a first and a second logic level of the information signal, the third and fourth frequencies differing from the first and the second frequencies by an amount at least equal to an inverse of a bit period in the binary signal;

(b) rendering states of polarization of the radiations emitted by the two sources orthogonal in correspondence with each bit of the information signal; and

(c) combining the two orthogonally polarized radiations to send on a transmission line in the form of an optical fiber, for each of said bits, a signal comprising two orthogonally-polarized components at the first and the third frequency and at the second and the fourth frequency, respectively, whereby a linear polarization is obtained with a polarization plane rotating by at least 360°

in a bit period.

2. The method according to claim 1, wherein the frequency difference is equal to the inverse of the bit period.

3. A coherent optical fiber transmission system using polarization scrambling comprising:

a transmitter with means for modulating an optical signal with a binary information signal and means for continuously scrambling, in each bit period of the binary information signal period, a state of polarization of the modulated optional signals; and

a receiver comprising a local oscillator generating a signal which is combined with a line signal, said transmitter comprising;

a first source of light radiations, which emits radiations linearly polarized in a first plane and is biased by the information signal so as to emit radiations at a first or a second frequency according to whether said information signal presents a first or a second of two logic levels;

a second source of light radiations, which emits radiations linearly polarized in a second plane orthogonal to the first plane and biased by the information signal so as to emit radiations at a third or a fourth frequency, according to whether said information signal presents the first or the second of the two logic levels, the third and fourth frequencies being very close to the first and second frequencies but differing therefrom by an amount at least equal to an inverse of the bit period; and

means for combining the light radiations emitted by the two sources into a single radiation which is sent onto an optical fiber transmission line and comprises, for each of the two logic levels of the information signal, two orthogonally-polarized components having the first and third frequencies and the second and fourth frequencies, respectively, so that, for both logic levels, a resulting linear polarization is obtained with a polarization plane rotating by at least 360°

in the bit period.