AU602079B2

AU602079B2 - Optical communication system

Info

Publication number: AU602079B2
Application number: AU15657/88A
Authority: AU
Inventors: Kurt Fussganger
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1987-05-15
Filing date: 1988-05-06
Publication date: 1990-09-27
Anticipated expiration: 2008-05-06
Also published as: AU1565788A; DE3716247C2; DE3716247A1

Description

602079 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION F'OR THE INVENTION ENTITLED "OPTICAL COMMUNICATION SYSTEM" The following statement is a full description of this invention, including the best method of performing it known to us:- This invention relates to a communication system for unidirectional or bidirectional transmission of information between at least two stations using optical signals of different wavelength.

In a prior art optical communication system, adjacent wavelengths differ by 40 nm 60 nm. This wavelength difference is determined by the selectivity of the multiplexer/demultiplexers used. Since the attenuation of the optical signals in an optical waveguide is strongly :0 wavelength-dependent, and the wavelength range with low attenuation is limited, in the prior art system only few optical signals can be transmitted over an optical waveguide.

An object of the present invention is to provide an optical communication system which has at least twice the transmission capacity of the prior art system or increased crosstalk attenuation between adjacent optical channels.

According to the invention there is provided a communication system for unidirectional or bidirectional transmission of information between at least two stations using optical signals of different wavelength, wherein optical signals with adjacent wavelengths have different directions of polarization, and each station contains a wavelengthselective and polarization-direction-selective multiplexer and/or demultiplexer.

In the novel communication system, multiplexers and demultiplexers with the same wavelength selectivity as in the prior art communication system can be employed. It is only necessary to add polarization-division multiplexers and demultiplexers, so that transmission capacity can be doubled at very low cost.

L The invention will now be described with the aid of an embodiment which is shown schematically in the single figure of the accompanying drawing. Further embodiments will be pointed out as the description proceeds.

The optical communication system comprises two stations 4 and a transmission link 2, which has one optical waveguide. One of the stations contains a multiplexer 1, and the other a demultiplexer 3. The multiplexer 1 is fed four optical signals of different wavelength, with the di- 1 rections of polarization of the first and third optical signals being perpendicular to those of the second and fourth optical signals. The optical signals are combined into a multiplex signal by the multiplexer 1 and transmitted over the optical ,aveguide to the demultiplexer 3, which separates the multiplex signal int>l the four original optical signals.

The transmission link 2 comprises one polarizationmaintaining optical waveguide or one non-polarizationmaintaining optical waveguide and a polarization controller.

It is preferable to use single-mode optical waveguides.

If discrete optical components are used for the multiplexer 1 and the demultiplexer 3, a polarizationdivision multiplexer or demultiplexer having one wavelengthdivison multiplexer or demultiplexer for each direction of polarization connected thereto will be connected to the transmission link 2 in each station 4.

Because of the reversibility of light propagation, the polarization- and wavelength-division multiplexers can be identical in construction to the demultiplexers.

L Examples of suitable discrete polarization-division multiplexers are: S beam-splitter tubes of quartz or flint glass, S Foster beam splitters of quartz or calcite, S doubly refracting quartz prisms according to Wollaston, Rochon, Senarmont or Dove, S fused quartz bodies with a transparent intermediate metal or semiconductor GaP, InP, Si or GaAs) layer according to Fresnel equations, or fused quartz plates of different indices of refraction according to Brewster's law.

Suitable discrete wavelength-division multiplexers are, for example, dichroic interference filters (cut-off filters), diffraction gratings, or wavelength-selective singlemode fibre couplers.

In other embodiments of the communication system, each station 4 contains a multiplexer and a demultiplexer or a muldexer or a bomudex (bidirectional optical multiplexer and demultiplexer), so that information can be transmitted between the stations in both directions. The transmission link 2 needs to have only one optical waveguide, but it is also possible to provide one optical waveguide for each direction of transmission.

The above-mentioned wavelength-division multiplexers with interference cut-off filters have a typical wavelength selectivity of 40 nm. Therefore, optical signals with the same directions of polarization must have a wavelength spacing of 40 nm, while optical signals with adjacent wavelengths and different directions of polarization mist have a wavelength spacing of only 20 nm, so that the wavelengths 1 for the individual optical signals are, for example: 1 1(1 )=1280nm 12() =1300nm 13( I)=1320nm 14( )=1340nm.

In preferred embodiments of the communication system, the multiplexer 1 and the demultiplexer 3 are polarizationand wavelength-selective PANDA-fibre multiplexer/demultiplexers as are described, for example, in "Electronics Letters", February 13, 1986, Vol. 22, No. 4, pp. 181 et seq., or integrated optical devices.

Instead of doubling the transmission capacity by using optical signals with a wavelength spacing of 20 nm, an improvement in crosstalk attenuation can be achieved by retaining the 40-nm wavelength spacing determined by the wavelength-division multiplexers and nevertheless transmitted optical signals with adjacent wavelengths which have different directions of polarization. In such a system, the order of polarisation- and wavelength-division multiplex can be reversed, the wavelength-division multiplex-r is connected to the transmission link 2, and the polarizationdivision multiplexers to the wavelength-division multiplexer.

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Claims

1. A communication system for unidirectional or bidirectional transmission of information between at least two stations using optical signals of different wavelength, wherein optical signals with adjacent wavelengths have dif- ferent directions of polarization, and each station contains a wavelength-selective and polarization-direction-selective multiplexer and/or demultiplexer.

2. A system as claimed in claim 1, wherein the trans- mission link comprises at least one polarization-maintaining optical waveguide.

3. A system as claimed in claim 1, wherein the trans- mission link comprises at least one optical waveguide and one polarization controller.

4. A system as claimed in claim or 3, wherein the op- tical waveguide is a single-mode optical waveguide.

A system as claimed in any one of the preceding claims, wherein the multiplexer and/or the demultiplexer are/is constituted by at least one fibre-optic coupler, par- ticularly at least one wavelength- and polarization- selective PANDA-fibre coupler.

6. A system as claimed in any one of claims 1 to 4, wherein the multiplexer and/or the demultiplexer are/is con- stituted by integrated optical devices.

7. A system as claimed in any one of claims 1 to 4, wherein the multiplexer and/or the demultiplexer are/is con- stituted by discrete optical components.

8. A communication system substantially as her'&ln de- scr'ibed with reference to the figure of the drawing. DATED TH~IS TWENTY-EIGHTH DAY OF APRIL, 1988 ALJCATEL N.V.