WO1998036256A1

WO1998036256A1 - Apparatus for measuring characteristics of an optical fibre

Info

Publication number: WO1998036256A1
Application number: PCT/IB1998/000225
Authority: WO
Inventors: Andrea Galtarossa
Original assignee: Andrea Galtarossa
Priority date: 1997-02-13
Filing date: 1998-02-10
Publication date: 1998-08-20
Also published as: JP3262337B2; IT1291413B1; CA2280020C; CA2280020A1; US6229599B1; JP2000510246A; ITPD970025A1; EP0972179A1; AU5875998A

Abstract

Apparatus for measuring characteristics of an optical fiber (11), at different positions along the length of the optical fiber (11), which apparatus comprises: tuneable source means (2) for providing optical pulses of light (3) which have a variable wavelength and a narrow wavelength bandwidth; polarisation selecting coupler means (5) for conveying light between three separate ports (4, 6, 15), such that a state of polarisation of optical pulses of light (3) becomes a particular launch state of polarisation of light (7), and such that one or more particular receive states of polarisation of light (14) become or more separate channels of light (16); optical connector means (8) for making an optical connection between a bi-directional port (6) of the polarisation selecting coupler means (5) and one end (10) of the optical fibre (11); photodetector means (17) for converting the intensity of one or more separate channels of light (16) into one or more separate electrical signals; launch controller means (1); receiver controller means (18); and processor means (19) for measuring and processing the electrical signals provided by the photodetector means (17) into measurements of the characteristics of the optical fibre (11).

Description

APPARATUS FOR MEASURING CHARACTERISTICS OF AN OPTICAL FIBRE

This invention relates to apparatus for measuring characteristics of an optical

fibre. More specifically, this invention relates to apparatus for measuring characteristics

of an optical fibre such as beat length, correlation length, and polarisation mode

dispersion, at different positions along the length of the optical fibre.

Within the field of optical fibre telecommunications, the current upper limit on

the bit rate of detection arises from the birefringence distributed along the length of a

single-mode optical fibre. The birefringence may be due to non-circularity of the core

of the optical fibre, and to stresses within the fibre. The birefringence may also vary

along the length of the fibre. However, the birefringence may also be caused by

external forces acting upon the fibre, and to temperature variations, and these effects

vary both along the length of the fibre and with time. The overall birefringence of a

length of fibre thus varies over time with a magnitude which is a random process.

An ideal single-mode optical fibre guides optical power in the fundamental

mode as two identical but orthogonal polarisation modes, so that the modes are

completely interchangeable. However, imperfections in the fibre and the effect of

external parameters lead to the optical power within the two polarisation modes both

differing in magnitude and travelling at slightly different speeds so that a differential

group delay exists between the modes. The birefringence determines the magnitude of

the differential group delay, and the optical power within the two polarization, and so

the width of an optical pulse travelling along the optical fibre will vary randomly by an amount determined by the random fluctuations in the birefringence. This effect is called

the polarisation mode dispersion, and it is of particular concern because it limits the

performance of optical fibre telecommunications systems in a way that cannot be

predicted accurately, and hence cannot be compensated.

Although polarisation mode dispersion is usually considered to be a

characteristic of the total length of an optical fibre, the effects which give rise to it may

act over relatively short sections of the fibre. In particular, the polarisation mode

dispersion of an optical fibre cable may increase significantly after installation, arising

from a change in the birefringence over one particular short length within the fibre.

Accordingly, it would be useful to be able to measure characteristics related to the two

polarisation modes of a single-mode optical fibre, such as polarisation mode dispersion,

at different positions along the length of the optical fibre so that any local effect can be

identified at a particular length position in the optical fibre. This would be particularly

useful if the measurement were able to be made with access to just one end of the fibre

in the same way that optical time domain reflectometers are used to measure the optical

loss of fibres. Existing commercial apparatus can measure the overall polarisation

mode dispersion of an optical fibre and requires access to both ends of the fibre. The

existing commercial apparatus does not enable the measurement of the polarisation

mode dispersion at different positions along the length of the optical fibre. The

magnitude of the birefringence as it varies along the fibre may be characterised as a beat

length, and the statistical correlation between two sections of fibre may be related by a

correlation length. Both these parameters are useful for describing the behaviour of the fibre, and the environment it is experiencing, and are inherently characteristics of length

position within the fibre.

There are a number of known methods of measuring polarisation mode

dispersion, and associated characteristics of single-mode optical fibres, which provide a

single measurement for the total length requiring access to both ends of the fibre. In

addition, optical time domain reflectometry is a well-established technique for

measuring the optical loss of an optical fibre at different positions along the length of

the fibre and requiring access to only one end of the fibre. The present invention is

based on the discovery that it is possible to apply the existing measurement techniques

of polarisation mode dispersion to modified versions of optical time domain

reflectometry apparatus, and thus to derive useful measurements.

The present invention aims to provide apparatus to measure the beat length, the

correlation length, the polarisation mode dispersion, and other characteristics of single

mode optical fibres related to the two polarisation modes of the fibre at different

positions along the length of the fibre. The present invention also aims to provide

apparatus for the measurement of characteristics of an optical fibre with access to only

one end of the fibre.

According to a non-limiting embodiment of the present invention, there is

provided apparatus for measuring characteristics of an optical fibre, at different

positions along the length of the optical fibre, which apparatus comprises:

tunable source means for providing optical pulses of light which have a variable

wavelength and a narrow wavelength bandwidth; polarisation selecting coupler means which comprises an input port, a bi¬

directional port and an output port, and which is for conveying light between the input

port, the bi-directional port and the output port, such that a state of polarisation of the

optical pulses of light input at the input port becomes a particular launch state of

polarisation of light output from the bi-directional port, and such that one or more

particular receive states of polarisation of light input at the bi-directional port become

one or more separate channels of light output from the output port;

optical connector means for making an optical connection between the bi¬

directional port of the polarisation selecting coupler means and one end of the optical

fibre, so that the light output from the bi-directional port of the polarisation selecting

coupler means is launched into the optical fibre and light backscattered within the

optical fibre is received as the light input at the bi-directional port of the polarisation

selecting coupler means;

photodetector means for converting the intensity of each of the one or more

separate channels of light output from the output port of the polarisation selecting

coupler means into one or more separate electrical signals;

launch controller means for controlling the timing, duration and wavelength of the

optical pulses provided by the tunable source means, and for specifying the particular

launch state of polarisation of the light output from the bi-directional port of the

polarisation selecting coupler means;

receive controller means for controlling the timing of measuring the electrical

signals provided by the photodetector means, and for specifying the one or more particular receive states of polarisation of light input at the bi-directional port of the

polarisation selecting coupler means; and

processor means for measuring and processing the electrical signals provided by

the photodetector means into measurements of the characteristics of the optical fibre.

The tunable source means may be a semiconductor laser diode, a solid state

laser, or a gas laser, with a wavelength bandwidth preferably less than 0.1 nanometers

and a peak power preferably greater than one milliwatt. The tunable source means may

include optical amplifiers, which may take the form of semiconductor or optical fibre

amplifiers. The tunable source means may also include means for defining a specific

state of polarisation of the light emitted. The light emitted by the tunable source means

may be any electromagnetic radiation at wavelengths appropriate for the measurements

being made and the optical fibre under test. Preferably, the wavelength of the tunable

source means is in the range of approximately 1100 nanometers to 1800 nanometers.

The wavelength of the tunable source means may be varied by any method appropriate

for the embodiment of the tunable source means, such as by varying the temperature of

a semiconductor laser diode. Preferably, the wavelength of the tunable source means is

varied by increments of wavelength which may be a small fraction of the wavelength

bandwidth, and over a range which may be substantially greater than the wavelength

bandwidth. More preferably, the wavelength of the tunable source is varied by

increments of less than 0.1 nanometers over a range of greater than 10 nanometers. The optical connector means may comprise of one of the many optical fibre

connectors commonly used to join two optical fibres. Preferably, the optical connector

means has very low polarisation dependent loss.

The photodetector means may comprise one or more PIN diodes, avalanche

photodiodes, phototransistors, or photomultipliers.

The launch controller means, receive controller means, and processor means

may comprise personal computers, microprocessors, dedicated electronic processors,

analogue to digital convertors, digital to analogue convertors, electric motors, and/or

other electronic and electromechanical components, as are well-known to someone

skilled in the art of measurement instrumentation. The processor means may include

processing algorithms which may be related to the physical theory of measurement,

such as for polarisation mode dispersion. The processor means may also include

algorithms which may be appropriate only for a specific optical fibre and specific

conditions, where the measurement problem may not be fully tractable to known

theoretical analysis, but, nevertheless, measurements obtained by the current invention

may be correlated with the optical fibre to provide useful information about the

behaviour of the specific optical fibre.

In a first embodiment of the present invention, the apparatus is one in which

measurements of the state of polarisation of the backscattered light are made

consecutively, and such apparatus is one in which there is only one separate channel of

light output from the output port of the polarisation selecting coupler means, such that

the photodetector means converts the intensity of the said one separate channel of light; and in which the polarisation selecting coupler means comprises:

polarisation independent coupler means which comprises an input port, a bi¬

port, the bi-directional port and the output port, with negligible polarisation dependent

loss of the light, such that light input at the input port becomes light output from the bi¬

directional port, light input at the bi-directional port becomes light output from the

output port, and the light is substantially unchanged as it passes in either direction

between the bi-directional port of the polarisation selecting coupler means and the bi¬

directional port of the polarisation independent coupler means;

polarisation controller means for converting the state of polarisation of light input

at the input port of the polarisation selecting coupler means into a particular launch state

of polarisation of light input at the input port of the polarisation independent coupler

means, such that the particular launch state of polarisation is selected as specified by the

launch controller means; and

polarisation analyser means for selecting a particular receive state of polarisation

of the light output from the output port of the polarisation independent coupler means to

become the one separate channel of light output from the output port of the polarisation

selecting coupler means, such that the particular receive state of polarisation is selected

as specified by the receive controller means.

The polarisation independent coupler means may include one or more bulk-optic

components, such as non-polarising beam-splitters, or may include one or more optical

fibre components, such as directional fiber couplers or optical fibre circulators. In addition, the polarisation independent coupler means may include an active element

such as an acousto-optic deflector.

The polarisation controller means may comprise one or more bulk-optic

birefiingent waveplates, such as half-wave and quarter-wave plates, and linear or

circular polarisers, that may be rotated about an optical axis in an optical path, and/or

inserted and removed from the optical path, to select a specific state of polarisation of

the light output from the polarisation controller means. The polarisation controller

means may also comprise contiguous sections of birefiingent optical fibre that may be

positioned relative to each other to alter the state of polarisation of the light output. The

polarisation controller means may also include one or more planar integrated optics

circuits in which the state of polarisation of the light output may be selected by

electrical signals supplied by the launch controller means.

The polarisation analyser means may comprise one or more bulk-optic

the light input from the polarisation independent coupler means. The polarisation

analyser means may also include one or more planar integrated optics circuits in which

the state of polarisation of the light output is selected by electrical signals supplied by

the receiver controller means.

In the first embodiment of the present invention, the apparatus may be one in

which the launch controller means specifies, and the polarisation controller means selects, only two particular launch states of polarisation at each of the wavelengths of

the tunable source means controlled by the launch controller means.

In the first embodiment of the present invention, the apparatus may alternatively

be one in which the launch controller means specifies, and the polarisation controller

means selects, only one separate particular launch state of polarisation at each of the

wavelengths of the tunable source means controlled by the launch controller means.

The polarisation controller means may comprise a single bulk-optic component, such as

a linear polariser or a circular polariser.

In the first embodiment of the present invention, the apparatus may be one in

which the receive controller means specifies, and the polarisation analyser means

selects, only two particular receive states of polarisation of the light output from the

output port of the polarisation independent coupler means for each of the particular

launch states of polarisation of the light input at the input port of the polarisation

independent coupler means at each of the wavelengths of the tunable source means; and

is one in which the processor means is such that the state of polarisation of the light

backscattered by the optical fibre is deduced from the two separate particular receive

states of polarisation.

In a second embodiment of the present invention, the apparatus is one in which

the polarisation controller means comprises an input polariser means for selecting a

single particular launch state of polarisation for the light input to the input port of the

polarisation independent coupler means, and the polarisation analyser means comprises

an output polariser means for selecting a single particular receive state of polarisation of the light output from the output port of the polarisation independent coupler means. The

input polariser means may comprise a linear polariser, and the output polariser means

may comprise a linear polariser.

In a third embodiment of the present invention, the apparatus is one in which

measurements of the state of polarisation of the backscattered light are made

the photodetector means converts the intensity of the one separate channel of light; and

in which the polarisation selecting coupler means comprises:

polarisation independent coupler means which comprises an input port, a bi¬

output port, the light is substantially unchanged as it passes from the input port of the

polarisation selecting coupler means to the input port of the polarisation independent

coupler means, and the light is substantially unchanged as it passes from the output port

of the polarisation independent coupler means to the output port of the polarisation

selecting coupler means; and

bi-directional polariser means for selecting a particular state of polarisation of the

light that passes in either direction between the bi-directional port of the polarisation independent coupler means and the bi-directional port of the polarisation selecting

coupler means.

In the third embodiment of the present invention, the bi-directional polariser

means may comprise a linear polariser.

In a fourth embodiment of the present invention, the apparatus is one in which

measurements of the state of polarisation of the backscattered light may be made in

parallel, and such apparatus is one in which there are two or more separate channels of

the photodetector means converts the intensities of the said two or more separate

channels of light, and such that the polarisation selecting coupler means comprises:

polarisation independent coupler means which comprises an input port, a bi¬

directional port of the polarisation independent coupler means;

of polarisation of light input at the input port of the polarisation independent coupler means, such that the particular launch state of polarisation is selected as specified by the

launch controller means; and

polarisation parallel analyser means for selecting two or more particular receive

states of polarisation of the light output from the output port of the polarisation

independent coupler means to become the two or more separate channels of light output

from the output port of the polarisation selecting coupler means.

In the fourth embodiment of the present invention, the polarisation controller

means may be such that only two separate particular launch states of polarisation may

be selected at each of the wavelengths of the tunable source means controlled by the

launch controller means, or, alternatively, the polarisation controller means may be such

that only one separate particular launch state of polarisation may be selected at each of

the wavelengths of the tunable source means controlled by the launch controller means.

The polarisation parallel analyser means may include bulk-optic birefiingent

waveplates, such as half-wave and quarter-wave plates, and linear or circular polarisers,

that may be placed in two or more separate optical paths, to select two or more

particular receive states of polarisation of the light input from the polarisation

independent coupler means, such that the light output comprises two or more separate

channels. The polarisation parallel analyser means may also include one or more

planar integrated optics circuits in which two or more receive states of polarisation of

the light output in two or more separate channels may be selected by electrical signals

supplied by the receiver controller means.

In the fourth embodiment of the present invention, the apparatus may be one in which the polarisation parallel analyser means comprises a beam separator means for

separating the light input into two or more separate beams with negligible polarisation

dependent loss, and comprises two or more single polariser means for selecting a

particular receive state of polarisation for each of the said two or more separate beams.

The two or more single polariser means may be such that a first single polariser

means comprises a linear polariser, such that a second single polariser means also

comprises a linear polariser but with a different orientation of its polarisation axis, and

such that a third single polariser means comprises a circular polariser.

Alternatively, in the fourth embodiment of the present invention, the apparatus

may be one in which the polarisation parallel analyser means selects only two particular

receive states of polarisation of the light output from the output port of the polarisation

independent coupler means for each of the separate particular launch states of

polarisation light input at the input port of the polarisation independent coupler means at

each of the wavelengths of the tunable source means; and is one in which the processor

means is such that the state of polarisation of the light backscattered by the optical fibre

is deduced from the two particular receive states of polarisation.

In this alternative embodiment of the present invention, the apparatus may be

one in which the polarisation parallel analyser means comprises a beam separator means

for separating the light input into only two separate beams, and in which the apparatus

includes only two single polariser means for selecting a particular receive state of

polarisation for each of the said two separate beams. The two single polariser means may be such that a first single polariser means

comprises a linear polariser, and such that a second single polariser means also

comprises a linear polariser but with a different orientation of its polarisation axis, or

such that the second single polariser means comprises a circular polariser.

Embodiments of the invention will now be described solely by way of example

and with reference to the accompanying drawings in which:

Figure 1 shows first apparatus for measuring characteristics of an optical fibre,

and illustrates the relationships between the tunable source means, the polarisation

selecting coupler means, the optical connector means, the optical fibre, the

photodetector means, the launch controller means, the receive controller means, and the

processor means;

Figure 2 shows apparatus of the present invention in which the polarisation

controller means, the polarisation independent coupler means, and the polarisation

analyser means are shown for only one separate channel of light output from the output

port of the polarisation selecting coupler means;

Figure 3 shows apparatus of the present invention in which the polarisation

selecting coupler means includes the input polariser means and the output polariser

means;

Figure 4 shows apparatus of the present invention in which the polarisation

selecting coupler means includes the bi-directional polariser means;

Figure 5 shows apparatus of the present invention in which the polarisation

controller means, the polarisation independent coupler means, and the polarisation parallel analyser means are shown for two or more separate channels of light output

from the output port of the polarisation selecting coupler means; and

Figure 6 shows apparatus of the present invention in which the polarisation

parallel analyser means includes two or more single polariser means.

Referring to Figure 1, there is shown a launch controller means 1 which selects

the wavelength of a tunable source means 2 and the timing parameters of a sequence of

launch optical pulses 3 via a communications link 20. The launch optical pulses 3 enter

a polarisation selecting coupler means 5 at an input port 4 and are output at a bi¬

directional port 6 as launch optical pulses 7 with a particular launch state of polarisation

specified by the launch controller means 1 and selected by the polarisation selecting

coupler means 5 via the communications link 20. The launch optical pulses 7 pass

through an optical connector means 8 and enter the near end 10 of an optical fibre 11 as

launch optical pulses 9. The optical connector means 8 facilitates the optical connection

between the optical fibre 11 and the polarisation selecting coupler means 5. As the

launch optical pulses 9 pass along the optical fibre 11, some light is backscattered by the

physical process of Rayleigh scattering, and returns back along the optical fibre 11 ,

arriving at the near end 10 as backscattered light 13 with a specific state of polarisation,

determined by the state of polarisation of the launch optical pulses 9 and the polarisation

characteristics of the route taken through the optical fibre 11. The backscattered light

13 passes back through optical connector means 8 to enter the bi-directional port 6 of

the polarisation selecting coupler means 5 as backscattered light 14. One or more

particular receive states of polarisation of the backscattered light 14 are selected by the polarisation selecting coupler means 5 via the communications link 20 as the

backscattered light 14 passes from bi-directional port 6 to output port 15 to become one

or more separate channels of light 16. A receive controller means 18 specifies, via the

communications link 20, and the polarisation selecting coupler means 5 selects a

particular receive state of polarisation of the backscattered light 14 to be output in each

of the separate channels of light 16. The photodetector means 17 converts the intensity

of the light in each of the separate channels of light 16 into separate electrical signals

which are controlled by the receive controller means 18 so that they are transferred to a

processor means 19 by the communications link 20 at selected intervals of time relative

to the launch optical pulses 3. These intervals of time determine the position in length

within the optical fibre 11 from which the light 16 was backscattered. In this way, the

processor means 19 collects a set of data consisting of the state of polarisation of the

backscattered light 13 as a function of the state of polarisation of the launch optical

pulses 9, a function of the position in length within the optical fibre 11 , and a function

of the wavelength of the launch optical pulses 3 generated by the tunable source means

2. The processor means 19 may then compute a range of different characteristics of the

optical fibre 11, including beat length, correlation length, and polarisation mode

dispersion. Polarisation mode dispersion may be computed by a number of well-known

methods, some of which are detailed below with reference to other embodiments of the

present invention.

Referring to Figure 2, the polarisation selecting coupler means 5 is such that

only one channel of light 41 is output from the output port 15. The launch optical pulses 3 enter input port 4 to become launch optical pulses 30 which enter polarisation

controller means 31. The launch controller means 1 specifies, and the polarisation

controller means 31 selects, a particular launch state of polarisation for the launch

optical pulses 32 which leave the polarisation controller means 31 and enter a

polarisation independent coupler means 34 through input port 33. The launch optical

pulses 32 are output from the polarisation independent coupler means 34 at a bi¬

directional port 35 as launch optical pulses 36 and then output from bi-directional port 6

as launch optical pulses 7, having incurred negligible polarisation dependent loss. The

backscattered light 14 enters through the bi-directional port 6 to become backscattered

light 21 which enters the bi-directional port 35 and is output from the output port 37 as

backscattered light 38, having incurred negligible polarisation dependent loss. A

polarisation analyser means 39 selects a particular receive state of polarisation of

backscattered light 38 to be output as backscattered light 40, where the particular

receive state of polarisation is specified by the receive controller means 18 via the

communications link 20. The backscattered light 40 is output from the output port 15 as

backscattered light 41, and the intensity of backscattered light 41 is then measured by

photodetector means 17. In a typical measurement sequence determined by the

processor means 19, one or more particular receive states of polarisation of

backscattered light 38 may be selected, and the intensity of backscattered light 41

measured, consecutively, at each of one or more particular launch states of polarisation

selected by the polarisation controller means 31 at each of the selected wavelengths of

tunable source means 2, over a range of timing intervals which represent a range of length position within the optical fibre 11. A complete state of polarisation of the

backscattered light 13 may be computed by the processor means 19 from the

measurements made by the photodetector means 17 for three or more separate particular

receive states of polarisation selected by the polarisation analyser means 39.

Polarisation mode dispersion may be computed by the well-known method of Jones

Matrix Eigenanalysis. As can be appreciated from Figure 2, at each of two separate

wavelengths of tunable source means 2, a Jones matrix of the optical fibre 11 is obtained

by measuring three complete states of polarisation of the backscattered light 13 for three

separate particular launch states of polarisation of the launch optical pulses 9 and using

a processing algorithm such as that described in, for example, R.C.Jones, "A new

calculus for the treatment of optical systems VI: Experimental determination of the

Matrix", J. Optical.Soc.Amer., Vol.37, No.2, ρpl lO-112, 1947. The two Jones matrices

at the two separate wavelengths are then processed by an algorithm such as that

described in, for example, ANSI/TIA/EIA-455-122-1996, "FOTP-122 Polarisation-

Mode Dispersion Measurement for Single-Mode Optical Fibers by Jones Matrix

Eigenanalysis", to give the polarisation mode dispersion as a single value for the

wavelength range defined by the two wavelengths at which the measurements were

made. It may be assumed that the range of wavelengths in the backscattered light 13

may be small enough for the backscattered light 13 to be considered to be fully

polarised.

In another embodiment of the present invention, and also with reference to

Figure 2, the launch controller means 1 specifies and the polarisation controller means 31 selects only two separate particular launch states of polarisation for the launch

optical pulses 9 so that only two separate complete states of polarisation of the

backscattered light 13 are measured at each of the wavelengths of the tunable source

means 2 selected by the launch controller means 1. Although two complete states of

polarisation are insufficient to define a Jones matrix of the optical fibre 11, it is possible

to compute the polarisation mode dispersion using the well-known Poincare sphere

method as described in, for example, N.S.Bergano, C.D.Poole, R.E.Wagner,

"Investigation of Polarization Dispersion in Long Lengths of Single-Mode Fiber using

Multilongitudinal Mode Lasers", IEEE Journal of Lightwave Technology, Vol.LT-5,

No.11, pp. 1618-1622, 1987. It is well-known that a complete state of polarisation of

light may be described by a Stokes vector, which may be visualised as a line connecting

a point on the surface of a sphere of unit radius to the centre of the sphere where the

sphere is called the Poincare sphere, as described in, for example, D.S.Kliger,

J.W.Lewis, C.E.Randall, "Polarised Light in Optics and Spectroscopy", Academic

Press, ISBN 0-12-414975-8, 1990. The radius of unity for the Poincare sphere follows

from the assumption that the range of wavelengths in the backscattered light 13 is small

enough for the backscattered light 13 to be considered to be fully polarised. At any one

wavelength of the tunable source means 2, and for timing of measurement by the

receive controller means 18 so that the backscattered light 13 comes from a specific

length position in the optical fibre 11, the Stokes vector of the measured state of

polarisation of backscattered light 13 describes a great circle plane of the Poincare

sphere, as the state of polarisation of the launch optical pulses 9 is changed by the polarisation controller means 31. A great circle is defined as being a circle on the

surface of a sphere whose centre coincides with the centre of the circle. Accordingly,

the great circle plane may be defined uniquely by the measurement of just two complete

states of polarisation of backscattered light 13 for two separate, preferably orthogonal,

states of polarisation of launch optical pulses 9. At two different wavelengths of the

tunable source means 2, two different great circle planes may be obtained, and these

plane intersect in a diameter of the Poincare sphere. The two points where this diameter

intersects with the surface of the sphere are called the principal states of polarisation,

and may be related to a well-known mathematical formulation as described in, for

example, C.D.Poole, R.E.Wagner, "Phenomenological approach to polarisation

dispersion in long single-mode fibres", Electronics Letters, Vol.22, No.19, ppl029-

1030, 1986. Accordingly, the great circle plane can be visualised as rotating about the

principal states of polarisation as the wavelength changes, and it is well-known that the

angle of rotation leads directly to the polarisation mode dispersion, as described in, for

example, C.D.Poole, N.S.Bergano, R.E.Wagner, H.J.Schulte, "Polarization Dispersion

and Principal States in a 147-km Undersea Lightwave Cable", IEEE Journal of

Lightwave Technology, Vol.6, No.7, ppl 185-1190, 1988.

In another embodiment of the present invention, and also with reference to

Figure 2, the launch controller means 1 specifies and the polarisation controller means

31 selects only one separate particular launch state of polarisation for the launch optical

pulses 9 so that only one complete state of polarisation of the backscattered light 13 is

measured at each of the wavelengths of the tunable source means 2 selected by the launch controller means 1. In the description of the previous embodiment of the present

invention, it was described how the polarisation mode dispersion was obtained from the

angle between two great circle planes, each measured at a separate wavelength. A

single Stokes vector in one of these planes will describe a small circle on the surface of

the Poincare sphere as the wavelength changes and the great circle plane rotates, where

the centre of the small circle lies on the diameter that is the intersection of the great

circle planes which links the principal states of polarisation. Three separate

measurements of position on the small circle, obtained from three separate

measurements of the complete state of polarisation of backscattered light 13 at three

separate wavelengths of tunable source means 2, are sufficient to define the centre of the

small circle and thus the axis of rotation of the great circle planes, as shown in, for

example, D.Andresciani, F.Curti, F.Matera, B.Daino, "Measurement of group-delay

difference between the principal states of polarization on a low-birefringence terrestrial

fiber cable", Optics Letters, Vol.12, pp.844-846, 1987. The polarisation mode

dispersion may then be computed from the angle of rotation of the great circle planes.

In another embodiment of the present invention, and also with reference to

Figure 2, the polarisation analyser means 39 selects only two separate particular receive

states of polarisation of the backscattered light 13 which are measured at each of the

wavelengths of the tunable source means 2 selected by the launch controller means 1. It

is well-known that the complete state of polarisation of light is described by three

independent parameters, representing the amplitude of the electrical vector in two

orthogonal polarisations and the phase angle between the two, which leads to three independent components for a Stokes vector for fully polarised light. Thus, three or

more separate measurements are required to obtain the three independent components.

However, with reference to the Poincare sphere, the modulus of the magnitude of one

component may be deduced from independent measurements of the other two

components by the constraint that the magnitude of the Stokes vector is unity and hence

the sum of the squares of the magnitudes of all three components must equal unity,

where the factor of unity follows from the assumption that the range of wavelengths in

the backscattered light 13 is small enough for the backscattered light 13 to be considered

to be fully polarised. With respect to visualisation on the Poincare sphere, this means

that a Stokes vector is known to the uncertainty that the third component may be at

either end of a diameter of the sphere. In general, this is not sufficient to fully

characterise the complete state of polarisation. However, for some optical fibres 11, the

rate of rotation of the great circle planes about the intersecting diameter is small enough,

as the wavelength of tunable source means 2 changes, such that the relative change in

the Stokes vector is small enough to remove any ambiguity caused by the uncertainty in

sign of the deduced component of the state of polarisation of the backscattered light 13.

Accordingly, it becomes possible to estimate the polarisation mode dispersion by the

Jones Matrix Eigenanalysis method, the Poincare sphere great circle method, or the

Poincare sphere small circle method as previously described.

For the embodiments of the present invention previously described, the

measurement of the polarisation mode dispersion depends upon two or more

measurements made at two or more separate wavelengths of the tunable source means 2. Preferably, the range of wavelengths in the launch optical pulses 3 may be less than the

wavelength interval between the measurements, so that accurate results may be obtained

from the wavelength independence of the measurements. In a preferred embodiment of

the present invention, the wavelength bandwidth of the tunable source means 2 may be

less than 0.1 nanometers and the smallest spacing of wavelength selectable by launch

controller means 1 may be less than 0.1 nanometers, which may be particularly

advantageous for investigating the change in polarisation mode dispersion over the

range of a few tens of nanometers of wavelength that are of cuπent interest for

wavelength division multiplexed optical communication systems.

For the embodiments of the present invention previously described, three

different methods may be used to obtain a measurement of the same parameter,

polarisation mode dispersion. Each method is well-known and used for the

measurement of the polarisation mode dispersion where the state of polarisation of the

light passing from one end of an optical fibre to the other is measured, and so the light

has made a single pass through the fibre. However, a particular characteristic of the

present invention is that the light measured has traversed the optical fibre 11 in one

direction, been backscattered at a particular point in the optical fibre 11, and then

returned back through the same length of optical fibre 11. Accordingly, the

measurement of the polarisation mode dispersion made by the present invention, and

described above, is that of the combined forward and backward pass of the light through

the optical fibre 11. In general, the most useful parameter may be that of the

polarisation mode dispersion between one end of a fibre and the other end, and so it may be necessary to understand the relationship between the two measurements. The use of

polarised light to study the characteristics of light backscattered from an optical fibre

has been well-known since early work in the field of polarisation optical time domain

reflectometry (POTDR) was published, such as, for example, A.J.Rogers, "Polarization

optical time domain reflectometry", Electronics Letters, Vol.16, No.13, pp.489-490,

1980. It is also well-known that circular birefringence in the fibre is not detectable in a

simple and direct way by POTDR as shown in, for example, J.N.Ross, "Birefringence

measurement in optical fibers by polarization-optical time-domain reflectometry",

Applied Optics, Vol.21. No.19, pp.3489-3495, 1982. The inventor of the present

invention has studied this problem and has concluded that the polarisation mode

dispersion of an optical fibre from one end to the other end may be equal to the

polarisation mode dispersion, when measured by light which has made a forward and a

backward pass through the optical fibre, multiplied by a factor 0.64, on average. This

numerical value has been obtained from many numerical simulations using

mathematical models of optical fibres, as described in, for example, F.Curti, B.Daino,

Q.Mao, F.Matera, C.G.Someda, "Concatenation of Polarisation Dispersion in Single-

Mode Fibres", Electronics Letters, Vol.25, No.4, pp.290-292, 1989. It is believed that

this value is different to the value of 0.707, which would be expected for the

polarisation mode dispersion of two concatenated but uncoπelated lengths of fibre,

because the light may be correlated between the forward and backward passes as each

section of the optical fibre is always traversed twice, once in each direction. The

uncertainty in the numerical factor of 0.64 is expected to be much less than the variation in the measured polarisation mode dispersion arising from a general statistical

Maxwellian distribution, which is typical of modern single-mode optical fibres, as

discussed in, for example, F.Curti, B.Daino, G.Marchis, F.Matera, "Statistical

Treatment of the Evolution of the Principal States of Polarization in Single-Mode

Fibers", IEEE Journal of Lightwave Technology, Vol.8, No.8, ppl 162-1166, 1990.

The embodiments of the present invention, as previously described, obtain one

or more states of polarisation as a function of both wavelength of the tunable source 2

and length position within the optical fibre 11. Thus, at each wavelength the complete

state of polarisation as a function of length position within the optical fibre 11 may be

derived. This function may be interpreted as resulting in an estimate of the beat length

within the optical fibre, where the birefringence of the fibre is approximately constant,

as described in, for example, A.Galtarossa, G.Gianello, C.G.Someda, M.Schiano,

"Stress Investigation in Optical Fiber Ribbon Cable by Means of Polarization Sensitive

Techniques", IEEE Photonics Technology Letters, Vol.6, No.10, pp.1232-1235, 1994.

Alternatively, where the states of polarisation are changing rapidly with length position

arising from short range fluctuations in birefringence in the optical fibre, it may be

appropriate to autocorrelate the states of polarisation to arrive at a correlation length

which estimates how rapidly the fluctuations in the birefringence takes place.

A particular advantage of the present invention is that the complete state of

polarisation of the backscattered light is measured, and this is independent of the

absolute intensity of the backscattered light, which means that the loss of optical power

which occurs as the light traverses forward and then back through the optical fibre 11 is not important to the measurement of characteristics such as polarisation mode

dispersion. However, the absolute intensity of the backscattered light may be known,

just as with a conventional optical time domain reflectometer, and so it may be possible

to measure the optical power loss of the optical fibre 11 at the same time as making the

measurements of the states of polarisation.

Referring now to Figures 2 and 3, the polarisation controller means 31 comprises

an input polariser means 42, which selects a single particular launch state of polarisation

for the light 32 input to the input port 33 of the polarisation independent coupler means

34. The input polariser means 42 may be a linear polariser. In addition, the polarisation

analyser means 39 comprises an output polariser means 43 which selects a single

particular receive state of polarisation of the light output 38 from the output port 37 of

the polarisation independent coupler means 34. The output polariser means 43 may be a

linear polariser. This embodiment of the present invention provides only one particular

launch state of polarisation of the launch optical pulses 9 and measures the intensity of

only one particular receive state of polarisation of the backscattered light 13, at each of

the wavelengths of the tunable source means 2 and the length position in the optical

fibre 11, determined by launch controller means 1 and receive controller means 20.

This measurement of only one particular receive state of polarisation of the

backscattered light 13 is insufficient for the determination of the great circle planes

within the Poincare sphere representation as described previously. However, it is well-

known that over a sufficient range of wavelengths the states of polarisation of the

backscattered light 13 will give Stokes vectors that cover most of the surface of the Poincare sphere, and it then becomes possible to use the well known fixed-polariser

wavelength-scanning method of computing the approximate value of the polarisation

mode dispersion, as described in, for example, C.D.Poole, D.L.Favin, "Polarization-

Mode Dispersion Measurements Based on Transmission Spectra Through a Polarizer",

IEEE Journal of Lightwave Technology, Vol.12, No.6, pp917-929, 1994.

Referring to Figures 1 and 4, the launch optical pulses 3 are input to the input

port 4 of the polarisation selecting coupler means 5 to become launch optical pulses 23.

The launch optical pulses 23 are input to the input port 33 of the polarisation

independent coupler means 34 and are output from the bi-directional port 35 as launch

optical pulses 36, having incurred negligible polarisation dependent loss. Bi-directional

polariser means 46 selects one particular launch state of polarisation of launch optical

pulses 36 to become launch optical pulses 45 which are output from the bi-directional

port 6 as the launch optical pulses 7. The backscattered light 14 is input to bi¬

directional port 6 to become the backscattered light 21 which returns through the bi¬

directional polariser means 46 to become backscattered light 47 with one particular

receive state of polarisation. The backscattered light 47 is input to the bi-directional

port 35 of the polarisation independent coupler means 34 and is output from the output

port 37 as backscattered light 24 which becomes the backscattered light 41 output from

output port 15, having incurred negligible polarisation dependent loss. In this way, the

bi-directional polariser means 46 selects both the particular launch state of polarisation

of the launch optical pulses 7, and the particular receive state of polarisation of the

backscattered light 41. The bi-directional polariser means 46 may be a linear polariser which may be of particular value for a portable and low cost embodiment of the present

invention.

As with measuring polarisation mode dispersion using the Jones Matrix

Eigenanalysis method and the Poincare sphere methods, described previously, the value

of polarisation mode dispersion measured using the fixed polariser wavelength scanning

method, described previously, needs to be coπected to take into account the coπelation

between the light as it passes forward through the optical fibre 11 and as it returns after

being backscattered. The inventor of the present invention has studied this problem and

has concluded that the polarisation mode dispersion of the fibre from one end to the

other end may be equal to the polarisation mode dispersion, when measured by light

which has made a forward and a backward pass through the optical fibre, multiplied by

the factor 0.60, on average. This numerical value has been obtained from many

numerical simulations using mathematical models of optical fibres, as described in, for

example, F.Curti, B.Daino, Q.Mao, F.Matera, C.G.Someda, cited previously, and over

the same range of models as used to obtain the numerical factor of 0.64 cited previously.

The uncertainty in the numerical factor of 0.60 is expected to be much less than the

variation in the measured polarisation mode dispersion because of the nature of the

statistical Maxwellian distribution as described in, for example, F.Curti, B.Daino,

G.Marchis, F.Matera, cited previously.

Referring to Figure 5, the polarisation selecting coupler means 5 is such that two

or more separate channels of light 25 are output from the output port 15. The launch

optical pulses 3 enter input port 4 and are output from bi-directional port 6 to become the launch optical pulses 7 with a particular launch state of polarisation selected by the

polarisation controller means 31, and the backscattered light 13 becomes the

backscattered light 38, as previously described. The backscattered light 38 is input to an

input port 48 of a polarisation parallel analyser means 49 and is separated into two or

more separate channels of light 22 and output from an output port 50, and which are

output from the output port 15 as the separate channels of light 25. Each of the separate

channels of light 22 results from the selection of a separate particular receive state of

polarisation of the backscattered light 38, and so photodetector means 17 may convert

the intensities of the channels of light 25 into separate electrical signals in parallel. The

capability of this embodiment of the present invention of making measurements in

parallel may be applied to all the different methods of measuring the complete state of

polarisation of the backscattered light 13 described previously, and may be particularly

advantageous because there may be no need to make measurements of the individual

components of the complete state of polarisation of the backscattered light 13

consecutively, and so the time taken to make an overall measurement may be reduced.

Referring to Figures 5 and 6, the polarisation parallel analyser means 49 includes

a beam separator means 52. Light 51 input at the input port 48 is separated by the beam

separator means 52 into two or more separate beams 53 with negligible polarisation

dependent loss. Each of the separate beams 53 passes through a separate single

polariser means 54 to become a separate output beam 55, each with a particular receive

state of polarisation, and where the output beams 55 are output from an output port 50

as the separate channels of light 22. In a further embodiment of the present invention, there are three of the single

polariser means 54, in which a first one of the single polariser means 54 comprises a

linear polariser, in which a second one of the single polariser means 54 also comprises a

linear polariser but with a different orientation of its polarisation axis, preferably

orthogonal, and in which a third one of the single polariser means 54 comprises a

circular polariser. The resulting three output beams 55 may be measured in parallel to

provide a complete state of polarisation of the backscattered light 13.

In a further embodiment of the present invention, there are two of the single

linear polariser, and in which a second one of the single polariser means 54 also

comprises a linear polariser but with a different orientation of its polarisation axis,

preferably orthogonal. Alternatively, the first one of the single polariser means 54

comprises a linear polariser, and the second one of the single polariser means 54

comprises a circular polariser. The resulting two output beams 55 may be measured in

parallel to provide an estimate of a complete state of polarisation of the backscattered

light 13.

In any embodiment of the present invention the optical components may be such

that there may be a residual change in state of polarisation which is added to that

resulting from the optical fibre 11. The residual change in state of polarisation may be

measured by a process of calibration in which measurements may be made with the

present invention for a particular known optical fibre 11, or with an artefact substituted

for optical fibre 11, whose optical transfer function is known. The calibration measurement may then be used to coπect a subsequent measurement of an unknown

optical fibre 11, and the calibration process may be carried out as often as required to

obtain a satisfactory measurement of the residual change.

It is to be appreciated that the embodiments of the invention described above

with reference to the accompanying drawings have been given by way of example only.

Thus, parts of the illustrated embodiments may be combined in any suitable appropriate

order. Also, modifications and additional components may be provided to enhance the

performance of the apparatus. Thus, for example, the apparatus may include means for

reducing chirping, that is, the variation in wavelength of the tunable source means 2

throughout the duration of each of the launch optical pulses 3. The apparatus may

include means to guide and form the light as it passes through the apparatus. Such

means may include optical components, such as lenses and minors. The apparatus may

also include means to improve the signal to noise ratio of the measurements. Such

means may include optical components to reduce stray light, or to reduce undesirable

changes in the polarisation properties of the apparatus of the present invention.

Claims

1. Apparatus for measuring characteristics of an optical fibre (11), at different positions

along the length of the optical fibre (11), which apparatus comprises:

tunable source means (2) for providing optical pulses of light (3) which have a

variable wavelength and a na╧Çow wavelength bandwidth;

polarisation selecting coupler means (5) which comprises an input port (4), a bi┬¼

directional port (6) and an output port (15), and which is for conveying light between

the input port (4), the bi-directional port (6) and the output port (15), such that a state of

polarisation of the optical pulses of light (3) input at the input port (4) becomes a

particular launch state of polarisation of light (7) output from the bi-directional port (6),

and such that one or more particular receive states of polarisation of light (14) input at

the bi-directional port (6) become one or more separate channels of light (16) output

from the output port (15);

optical connector means (8) for making an optical connection between the bi┬¼

directional port (6) of the polarisation selecting coupler means (5) and one end (10) of

the optical fibre (11), so that the light (7) output from the bi-directional port (6) of the

polarisation selecting coupler means (5) is launched into the optical fibre (11) and light

(13) backscattered within the optical fibre (11) is received as the light (14) input at the

bi-directional port (6) of the polarisation selecting coupler means (5); photodetector means (17) for converting the intensity of each of the one or more

separate channels of light (16) output from the output port (15) of the polarisation

selecting coupler means (5) into one or more separate electrical signals;

launch controller means (1) for controlling the timing, duration and wavelength of

the optical pulses (3) provided by the tunable source means (2), and for specifying the

particular launch state of polarisation of the light (7) output from the bi-directional port

(6) of the polarisation selecting coupler means (5);

receive controller means (18) for controlling the timing of measuring the electrical

signals provided by the photodetector means (17), and for specifying the one or more

particular receive states of polarisation of light (14) input at the bi-directional port (6) of

the polarisation selecting coupler means (5); and

processor means (19) for measuring and processing the electrical signals provided

by the photodetector means (17) into measurements of the characteristics of the optical

fibre (11).

2. Apparatus according to claim 1 in which there is only one separate channel of light

(41) output from the output port (15) of the polarisation selecting coupler means (5),

such that the photodetector means (17) converts the intensity of the said one separate

channel of light (41); and in which the polarisation selecting coupler means (5)

comprises:

polarisation independent coupler means (34) which comprises an input port (33), a

bi-directional port (35) and an output port (37), and which is for conveying light between the input port (33), the bi-directional port (35) and the output port (37), with

negligible polarisation dependent loss of the light, such that light (32) input at the input

port (33) becomes light (36) output from the bi-directional port (35), light (21) input at

the bi-directional port (35) becomes light (38) output from the output port (37), and the

light (36,21) is substantially unchanged as it passes in either direction between the bi┬¼

directional port (6) of the polarisation selecting coupler means (5) and the bi-directional

port (35) of the polarisation independent coupler means (34);

polarisation controller means (31) for converting the state of polarisation of light

(30) input at the input port (4) of the polarisation selecting coupler means (5) into a

particular launch state of polarisation of light (32) input at the input port (33) of the

polarisation independent coupler means (34), such that the particular launch state of

polarisation is selected as specified by the launch controller means (1); and

polarisation analyser means (39) for selecting a particular receive state of

polarisation of the light (38) output from the output port (37) of the polarisation

independent coupler means (34) to become the one separate channel of light (41) output

from the output port (15) of the polarisation selecting coupler means (5), such that the

particular receive state of polarisation is selected as specified by the receive controller

means (18).

3. Apparatus according to claim 2 in which the launch controller means (1) specifies,

and the polarisation controller means (31) selects, only two particular launch states of polarisation at each of the wavelengths of the tunable source means (2) controlled by the

launch controller means (1).

4. Apparatus according to claim 2 in which the launch controller means (1) specifies,

and the polarisation controller means (31) selects, only one particular launch state of

polarisation at each of the wavelengths of the tunable source means (2) controlled by the

launch controller means (1).

5. Apparatus according to any one of claims 2-4 in which the receive controller means

(18) specifies, and the polarisation analyser means (39) selects, only two particular

receive states of polarisation of the light (38) output from the output port (37) of the

polarisation independent coupler means (34) for each of the particular launch states of

polarisation of the light (32) input at the input port (33) of the polarisation independent

coupler means (34) at each of the wavelengths of the tunable source means (2); and in

which the processor means (19) is such that the state of polarisation of the light (13)

backscattered by the optical fibre (11) is deduced from the two particular receive states

of polarisation.

6. Apparatus according to claim 2 in which the polarisation controller means (31)

comprises an input polariser means (42) for selecting a single particular launch state of

polarisation for the light (32) input to the input port (33) of the polarisation independent

coupler means (34), and the polarisation analyser means (39) comprises an output polariser means (43) for selecting a single particular receive state of polarisation of the

light (38) output from the output port (37) of the polarisation independent coupler

means (34).

7. Apparatus according to claim 6 in which the input polariser means (42) comprises a

linear polariser, and in which the output polariser means (43) comprises a linear

polariser.

8. Apparatus according to claim 1 in which there is only one separate channel of light

such that the photodetector means (17) converts the intensity of the one separate

comprises:

bi-directional port (35) and an output port (37), and which is for conveying light

between the input port (33), the bi-directional port (35) and the output port (37), with

negligible polarisation dependent loss of the light, such that light (23) input at the input

port (33) becomes light (36) output from the bi-directional port (35), light (47) input at

the bi-directional port (35) becomes light (24) output from the output port (37), the light

(23) is substantially unchanged as it passes from the input port (4) of the polarisation

selecting coupler means (5) to the input port (33) of the polarisation independent

coupler means (34), and the light (24) is substantially unchanged as it passes from the output port (37) of the polarisation independent coupler means (34) to the output port

(15) of the polarisation selecting coupler means (5); and

bi-directional polariser means (46) for selecting a particular state of polarisation of

the light (36,21) that passes in either direction between the bi-directional port (35) of the

polarisation independent coupler means (34) and the bi-directional port (6) of the

polarisation selecting coupler means (5).

9. Apparatus according to claim 8 in which the bi-directional polariser means (46)

comprises a linear polariser.

10. Apparatus according to claim 1 in which there are two or more separate channels of

light (25) output from the output port (15) of the polarisation selecting coupler means

(5), such that the photodetector means (17) converts the intensities of the said two or

more separate channels of light (25), and such that the polarisation selecting coupler

means (5) comprises:

light (36,21) is substantially unchanged as it passes in either direction between the bi- directional port (6) of the polarisation selecting coupler means (5) and the bi-directional

port (35) of the polarisation independent coupler means (34);

polarisation is selected as specified by the launch controller means (1); and

polarisation parallel analyser means (49) for selecting two or more particular

polarisation independent coupler means (34) to become the two or more separate

channels of light (25) output from the output port (15) of the polarisation selecting

coupler means (5).

11. Apparatus according to claim 10 in which the launch controller means (1) specifies,

and the polarisation controller means (31) selects, only two particular launch states of

launch controller means (1).

12. Apparatus according to claim 10 in which the launch controller means (1) specifies,

launch controller means (1).

13. Apparatus according to any one of claims 10-12 in which the polarisation parallel

analyser means (49) comprises a beam separator means (52) for separating input light

(51) into two or more separate beams (53) with negligible polarisation dependent loss;

and in which the apparatus includes two or more single polariser means (54) for

selecting a particular receive state of polarisation for each of the said two or more

separate beams (53).

14. Apparatus according to claim 13 in which there are three of the single polariser

means (54), in which a first one of the single polariser means (54) comprises a linear

polariser, in which a second one of the single polariser means (54) also comprises a

linear polariser but with a different orientation of its polarisation axis, and in which a

third one of the single polariser means (54) comprises a circular polariser.

15. Apparatus according to any one of claims 10-12 in which the polarisation parallel

analyser means (49) selects only two particular receive states of polarisation of the light

(38) output from the output port (37) of the polarisation independent coupler means (34)

for each of the particular launch states of polarisation light (32) input at the input port

(33) of the polarisation independent coupler means (34) at each of the wavelengths of

the tunable source means (2); and in which the processor means (19) is such that the

state of polarisation of the light (13) backscattered by the optical fibre (11) is deduced

from the two particular receive states of polarisation.

16. Apparatus according to claim 15 in which the polarisation parallel analyser means

(49) comprises a beam separator means (52) for separating input light (51) into only two

separate beams (53); and in which the apparatus includes only two single polariser

means (54) for selecting a particular receive state of polarisation for each of the said

two separate beams (53).

17. Apparatus according to claim 16 in which a first one of the single polariser means

(54) comprises a linear polariser, and in which a second one of the single polariser

means (54) also comprises a linear polariser but with a different orientation of its

polarisation axis.

18. Apparatus according to claim 16 in which a first one of the single polariser means

means (54) comprises a circular polariser.