Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
  • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
  • G01M11/31—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter and a light receiver being disposed at the same side of a fibre or waveguide end-face, e.g. reflectometers
  • G01M11/3181—Reflectometers dealing with polarisation

Definitions

• the present invention relates to optical fibre polarimetry and, in particular, to a method and apparatus for determining a spatial distribution of the polarisation properties of waveguide or optical fibre.
• polarisation properties of waveguides such as optical fibres of telecommunications systems, used to transmit signals, can lead to degradation of the transmitted signals.
• Such signals tend to comprise very short pulses of light.
• the polarisation state of the light pulses is altered by the polarisation properties of the optical fibre.
• This alteration of polarisation tends to result in the light pulses becoming less distinct from one another and, over large distances, e.g. tens of kilometers, for high transmission rate, e.g. 40 Gbit/sec, systems, the light pulses can become indistinguishable. This problem is known as
• Polarisation Mode Dispersion (PMD) and is currently considered to be a major factor limiting the rate at which signals can be transmitted through optical fibre as well as the length of optical fibres over which signals can be sent. Measurement of the polarisation properties of optical fibres is therefore useful in identifying optical fibres or parts of optical fibres in transmission systems that have high PMD so that they can be replaced or by-passed for example. Likewise, measurement of the polarisation properties of optical fibres during or after manufacture can improve manufacturing processes or quality control for example.
• polarisation properties of optical fibre may change when the optical waveguide passes through an electric or magnetic field. Knowledge of the polarisation properties or changes in polarisation properties of the optical fibre can therefore enable measurement of the external electric or magnetic field and consequently electric current or voltage.
• polarisation properties of an optical fibre are influenced by physical forces applied to the optical fibre. For example, stress or strain such as twisting or bending the optical fibre changes the polarisation properties of the optical fibre and knowledge of the polarisation properties of the optical fibre can therefore provide measurement of the stress or strain. Indeed, it is even possible to measure temperature according to changes in the polarisation properties of an optical fibre, as the optical fibre can be arranged to experience strain under thermal expansion and contraction for example.
• POTDR Polarisation Optical Time Domain Reflectometry
• CPOTDR involves effectively dividing the optical fibre into a series of adjacent elements starting from an end of the fibre into which light pulses are transmitted.
• Each element is considered to have polarisation properties that are homogeneous, i.e. constant throughout the element, and is effectively further divided into two sections.
• the polarisation states of light backscattered in each section of each element is determined separately and, from these polarisation states it is possible to determine the full polarisation
• the determination of the polarisation properties of each element of an optical fibre by CPOTDR depends on the determined polarisation properties of the preceding element, which in turn depend on the determined polarisation properties of the next preceding element and so on.
• CPOTDR therefore suffers from accumulation errors.
• the error in the determined polarisation properties of an element affects the determination of the polarisation properties of the next element along the optical fibre and so on.
• a limitation also arises in that the polarisation properties of the optical fibre must be determined starting with an element at the end of the optical fibre from which the light pulses are transmitted into the fibre and then for elements in turn along the optical fibre. It is not possible to start determining the polarisation properties of an element of an optical fibre part way along the optical fibre until the polarisation properties of all preceding elements are known.
• the present invention seeks to overcome these problems and, according to an aspect of the present invention, there is provided a method of determining a spatial distribution of polarisation properties of an optical waveguide, the method comprising:
• an apparatus for determining a spatial distribution of polarisation properties of an optical waveguide comprising: a light source for transmitting pulses of polarised light along the optical waveguide from an end of the optical waveguide; a detector for detecting a first polarisation state of light emerging from the end of the optical waveguide due to backscattering between the end of the optical waveguide and an element of the optical waveguide, a second polarisation state of light emerging from the end of the optical waveguide due to backscattering in a first section of the element of the optical waveguide, and a third polarisation state of light emerging from the end of the optical waveguide due to backscattering in a second section of the element of the optical waveguide; and a processor for deducing, from the first polarisation state, linear
• the applicant has recognised that the complete polarisation properties of any element of an optical waveguide can be determined from these detached polarisation states of light backscattered from
• orientation of linear retardance axes q e of the optical waveguide between the end of the optical waveguide and the element may be deduced from only the first polarisation state.
• the polarisation properties of each element of the optical waveguide may be determined from only the second
• optical waveguide This enables a spatial distribution of the polarisation properties of all or part of an optical waveguide to be determined straightforwardly in comparison to CPOTDR and without significant accumulation errors.
• the overall length of optical waveguide to which the method and apparatus of the invention can be successfully applied is therefore significantly greater than that to which CPOTDR can be successfully applied.
• the length of optical waveguide for which a spatial distribution of polarisation properties can be determined is only effectively limited by attenuation of backscattered light in the optical waveguide.
• the optical waveguide may suitably be an optical fibre.
• the optical waveguide may be a mono-mode optical fibre.
• the light source may transmit pulses of light having properties suitable for transmission in the particular optical waveguide under consideration.
• the light of the pulses may be substantially monochromatic and coherent. It may be linearly polarised.
• a typical wavelength of the light may be around 1550mm or in the range 1550mm to
• Al light coupler may be used to direct the transmitted light into the optical waveguide.
• Complete polarisation properties of the elements of the optical waveguide can be determined. For example, the determined polarisation
• properties of the elements may include linear retardance ⁇ , orientation of
• polarisation properties of the elements or the spatial distribution of polarisation properties of the optical waveguide can be expressed in other forms, such as a matrix or matrices, or graphically. Not all the polarisation properties of an element need therefore be calculated.
• the advantage of the invention lies in the ability to determine any desired polarisation properties of the optical waveguide or an element of the optical waveguide without accumulation errors.
• the determination of polarisation properties of the elements can be adapted to extract the desired polarisation properties in a convenient and efficient manner. For example, by omitting calculations that relate only to undesired polarisation properties. However, where it is desired to determine the orientation of linear retardance axes q, it is preferred that this is achieved by: repeating (a) to (d) with pulses of light having different wavelengths; deducing values of circular retardance of the optical waveguide between the end of the optical fibre and each element minus orientation of the
• the light source transmits pulses of light having different wavelengths; the detector detects the first, second and third polarisation states for the pulses of light having different wavelengths; and the processor deduces values of circular retardance of the optical waveguide between the end of the optical waveguide and each element minus
• the different wavelengths of light of the light pulses may be selected as desired. Preferably, at least these different wavelengths, or three pulses having different wavelengths of light, are transmitted for each element in order to provide accurate extrapolation. In particular, pulses having wavelengths of light varying from 1500mm to 1560mm may be transmitted for each element.
• the light source may therefore conveniently comprise a turnable laser.
• the first polarisation state may be that of light emerging from the end of the optical waveguide due to backscattering substantially half way between the end of the optical waveguide and the element. Alternatively, the first polarisation state may be that of light emerging from the end of the optical waveguide due to backscattering substantially at an end of the element closest to the end of the optical waveguide into which the light pulses are transmitted.
• the first and second sections may be substantially adjacent. They may be substantially equal in length along the major axis of the optical waveguide. Indeed, the first and second sections of the element may together define the element.
• Figure 1 is a perspective view of an optical waveguide
• Figure 2 is a longitudinal, sectional view of the optical waveguide of figure 1;
• Figure 3 is a transverse, sectional view of the optical waveguide of figure 1;
• Figure 4 is an illustration of two modes of polarisation of light
• Figure 5 is a longitudinal, sectional view of an optical waveguide illustrating Polarisation Optical Time Domain Reflectometry
• Figure 6 is a longitudinal, sectional view of an optical waveguide illustrating Computational Polarisation Optical Time Domain Reflectometry
• Figure 7 is a longitudinal, sectional view of an optical fibre illustrating a method of determining the spatial distribution of polarisation properties of an optical fibre according to the invention
• Figure 8 is a graphical illustration of a determination of orientation of linear retardance axes q according to the invention.
• Figure 9 is a schematic illustration of an apparatus for determining the spatial distribution of polarisation properties of an optical fibre according to the invention.
• Figure 10 is an illustration of an optical fibre arranged for temperature measurement according to the invention.
• the invention is applicable to various types of waveguide and in particular to any optical waveguide that provides mono-mode transmission of light. Whilst the examples, below are described with reference to an optical fibre, these examples can be extended to application in other optical waveguides when applicable.
• an optical fibre 1 is an optical fibre comprising a core 2, which might be cylindrical and made from silica (i.e. glass) or another highly optically transmissive material, and a cladding 3 which generally encloses the circumference of the core 2 along the length of the optical waveguide 1.
• a typical diameter D for the optical fibre 1 might be
• the core 2 has a refractive index n cr and the cladding 3 has a
• the refractive index n cr of the core 2 is greater than the
• n c ⁇ of the cladding 3 i.e. n cr > n c ⁇ , such that light passing generally along the length of the core 2 is totally internally reflected in the core 2 at a boundary 4 between the core 2 and cladding 3.
• core 2 and cladding 3 are selected such that only one reflection angle ⁇ at the
• boundary 4 between the core 2 and cladding 3 results in the propagation of light of wavelength ⁇ along the optical fibre 1. More specifically,
• Such an optical fibre 1 is said to transmit light of wavelength ⁇ in a "single-mode” or “mono-mode". This is well known in the art and the features of such fibres will not therefore be described in detail.
• One characteristic of such mono-mode propagation of light is that, at any point along the length of the optical fibre 1, light has a single polarisation state.
• light passing from one point along the length of the optical fibre 1 to another point along the length of the optical fibre 1 travels substantially the same distance. This has the result that the polarisation state of light at any given point along the optical fibre 1 is singular and definite rather than comprised of plural polarisation states.
• the polarisation state of light varies from point to point along the length of the optical fibre 1 due to the polarisation properties of the optical fibre 1.
• the change in polarisation of light as it passes along the fibre might be constant.
• the change in polarisation of light as it passes along the optical fibre 1 varies and is dependent on a number of factors. In particular, bends, twists and imhogeneities in the optical fibre 1 and in particular the shape or the core 2 cause varying changes in polarisation.
• External influences such as stress, magnetic fields, electric fields and radiation, can also affect the change in polarisation of light passing along the optical fibre 1.
• light propagates along a mono-mode optical fibre with two polarisation modes, which may be thought of as orthogonal ellipses 5, 6, for example as illustrated in Figure 4.
• Each ellipse 5, 6 is effectively the locus of points mapped by the electric field vector of light propagating in the respective mode over a single wavelength of the light.
• the shape of the ellipses changes due to the polarisation properties of the optical fibre 1.
• Linear birefringence may result from, wter alia, the core 2 not being perfectly circular. In other words, slight ellipticity of the core 2 can result in linear birefringence.
• Linear birefringence can be thought of as causing the major axes of the ellipses 5, 6, i.e. the linear polarisation component of each polarisation mode, to propagate at different velocities. This results in the generation of a phase difference between the linear polarisation component of each polarisation mode over a given length of the
• optical fibre 1 which phase difference is referred to as linear retardance ⁇ .
• Circular birefringence may result from, inter alia, axial twists in the core 2.
• Circular birefringence can be thought of as causing the degree of ellipticity of the ellipses 5, 6, i.e. the circular component of the each polarisation mode, to propagate along the optical fibre at different velocities. The difference in the velocity of propagation of the circular
• circular retardance p components of each polarisation mode is referred to as circular retardance p.
• circular retardance p fully define the polarisation properties of a given length
• POTDR Pulse-Time Domain Reflectometry
• a pulse 7 of light is transmitted along the optical fibre 1 (in a forward direction) by transmitting the pulse 7 of the light into the optical fibre at end 8 of the optical fibre 1.
• small imperfections or inhomogeneities in the optical fibre 1 cause the light to be reflected or scattered according to Rayleigh's Law.
• CPOTDR Computational Polarization Optical Time Domain Reflectometry
• the optical fibre 1 is considered as a series of elements Zi to Z, in Figure 6, with element Zj being adjacent the end 8 of the optical fibre 1.
• the elements ⁇ to Z each have the same axial length along the optical fibre 1 and are adjacent one another.
• Each element 9 has linear retardance ⁇ , orientation of linear retardance axes q, and
• Pulses 7 of light are transmitted along the optical fibre
• the elements 2 ⁇ to Z are effectively divided into two equal sections 10, 11.
• the polarisation state of light backscattered in each section of each element Zi to Z, is detected at end 8 of the optical waveguide 1, as illustrated in Figure 6.
• a Jones matrix can be used to describe the polarisation properties of each element to Z, of the optical waveguide 1.
• the change of polarisation of light propagating forward and then backscattered through each section 10, 11 of each element Zi to Z, is determined by the product of the relevant Jones matrices. Starting from the end 8 of the optical section 1, these successive products are shown as:
• M is the Jones matrix of the first section 10 of each element Z
• M ⁇ is its transpose
• M,' is the Jones matrix of the second section 11 of each element Z, and M,' ⁇ its transpose.
• the product of the form M M is equivalent to a linear retarder and has the general form:- where
• A a 2 + - 2
• equations (3) we see that there are only two independent equations for the three unknowns. However, as mentioned before, each element is effectively divided into two sections 10, 11. When the product M, T M, for the first section and M_ ,T M,' for the second section 11 of an element Z, are known, equations (3) show that four independent equations are then available
• the optical fibre 1 is considered as a series of discrete elements R in which the polarization properties of the optical fibre 1 are considered to be homogenous.
• Each element R is, in turn, considered as two adjacent sections Rl, R2 of equal size, although in other examples, the elements R may be divided into a different number or other size sections as desired.
• the element R is considered to have polarization characteristics, to be determined, comprising linear retardance ⁇ , orientation of the linear retardance axes q and circular retardance p.
• the portion e of the optical fibre between the end 8 of the optical fibre 1 and the element R under consideration or, more specifically, between the end 8 of the optical fibre 1 and the boundary of the element R closest to the end 8 of the optical optical fibre 1 is considered to have linear retardance ⁇ e , orientation of the linear retardance axes q e and
• the portion e of the optical fibre 1 can be considered as a retarder-rotator pair.
• half of the portion e can be considered to have polarization properties comprising only the linear retardance ⁇ e and orientation of the linear retardance axes q e and the other half of the portion e can be considered to have polarization properties comprising only circular
• R can be considered to have linear retardance ⁇ e and orientation of the linear retardance axes q e equivalent to light that passes along the passage ABA, i.e. is backscattered at B, halfway along portion e.
• the linear retardance ⁇ e of portion e and the orientation of the linear retardance axes q e of portion e can therefore be measured directly from the polarisation state of light backscattered from point B to the end 8 of the optical fibre 1.
• the linear retardance ⁇ e of the positon e and the orientation of the linear retardance areas q e of portion e can be deduced directly from the polarisation state of light backscattered from point e to the end 8 of the optical fibre 1.
• Equations (3) therefore show that we from M R! M RI and M R2 M R2 have four independent equations for the four
• Circular retardance p e depends on the wavelength ⁇ of
• an apparatus 100 for determining the spatial distribution of polarisation properties of an optical fibre comprises a light source 12, which, in this example, is a tunable laser able to transmit polarised, coherent light of any desired wavelength between 1550mm and 1560mm.
• Light transmitted by the light source 12 is directed into a beamsplitter 13.
• the beamsplitter 13 transmits some of the light incident on it from the light source 12 to an optical coupler 14, in this example by admitting the light to pass straight through the beam splitter 13.
• Some of the light incident from the light source 12 is also transmitted to a polarisation analyser 15, in this example by reflecting the light through an angle of 90°.
• the optical coupler 14 passes light incident on it from the beamsplitter 13 into the optical fibre 1 through the end 8 of the optical fibre 1.
• the optical coupler 14 also transmits light emitted from the end 8 of the optical fibre 1, for example by backscattering in the optical fibre 1, to the beamsplitter 13. This emitted, e.g. backscattered, light is re-directed by the beamsplitter 13 to the polarisation analyser 15.
• the polarisation analyser 15 comprises a Stokes analyser.
• the polarisation analyser 15 has a four optical elements arranged in the path of the light re-directed from the beamsplitter 13.
• the optical elements comprise, in series, a first linear polariser, a second linear polariser arranged at 45° to the first linear polariser, a quarter wave plate, i.e. an optical element that retards light by quarter of a wavelength, and a third linear polariser arranged at the same orientation as the second linear polariser.
• Light emerging from each of the linear polarisers is incident on a photodetector 16, such as photodoide array.
• the intensity of the light of each polarisation state seperated by the linear polarisers is detected by the photodetector 16.
• the intensity information is output by the photodetector 16 to a processor 17, such as the Central Processing Unit (CPU) of a Personal Computer (PC).
• the processor 17 is able to formulate the output of the photodetector 16 in the Stokes Formalism which represents the polarisation state of light emitted form the end 8 of the optical fibre 1.
• the Stokes Formalism allows a Mueller matrix of the general form:
• the processor 17 is connected to a light source controller 18 for controlling the light source 12.
• the light source controller 18 is operable to select the wavelength at which light is transmitted by the light source 12 and the light pulse 7 timing and duration. The timing and duration of the light pulses 7 transmitted by the light source 12 can be verified by the processor 17 from the output of the photodetector 16 corresponding to light transmitted from the beamsplitter 13 to the polarisation analyser 15 from light incident on the beamsplitter 13 from the light source 12.
• the timing and duration of the pulses 7 of light emitted by the light source 18 controlled, in combination with the time at which the output of the photodetector 16 is analysed, to resolve light backscattered in appropriate parts of the optical fibre 1, as described above.
• different elements R of the optical fibre 1 can be resolved.
• the output of the processor 17 is transmitted to an output device 19 which, in this example, is a display such as an oscilloscope or other
• CTR Cathode Ray Tube
• the apparatus 100 is adapted to measure Polarisation Mode Dispension in a telecommunications system.
• the optical coupler 14 is adapted to transmit light into telecommunications optical fibres for testing, either in situ or during or after manufacture.
• the output is indicative of a spatial distribution of PMD along the fibre on test and enables fibres or part of fibres having anomalously large PMD to be identified and, e.g., discarded.
• the optical fibre 1 is wound into a uniform helix.
• the strain along the length of the optical fibre 1 is therefore substantially uniform.
• the apparatus 100 is adapted to measure the spatial distribution of polarisation properties of the optical fibre 1 from time to time.
• the change in the distribution of polarisation properties of the optical fibre 1 is indicative of changes in strain in respective parts of the fibre which, in turn, is indicative of changes in temperature at those parts causing thermal expansion or contraction of the optical fibre.
• the apparatus 100 is able to output a spatial distribution of temperature along the length of the optical fibre 1.
• Changes in bending of the optical fibre 1 generally cause changes in linear retardance ⁇ and orientation of linear retardances axes q. Changes in twisting of the optical fibre 1 generally causes changes in circular retardance p.
• the apparatus 100 therefore correlates linear retardance ⁇ and orientation of linear retardance axes q with circular retardance to provide a more accurate spatial distribution of temperature along the length of the optical fibre 1.
• the optical fibre 1 is formed in a loop with an electric current to be measured passing axially through the loop.
• This arrangement results in the magnetic field generated by the electric current passing along the loop of optical fibre 1 axially. This induces a non-reciprocal circular retardance p in the optical fibre 1 by the Faraday magneto-optic effect.
• Measurement of circular retardance p therefore provides a measurement of current.
• vibrationally-induced linear retardance ⁇ and orientation of linear retardances axes q interfere with measurement of circular retardance p.
• the spatial distribution of polarisation properties output by the apparatus 100 can separate linear retardance ⁇ and orientation of linear retardance axes q from circular retardance p and a more accurate measurement of current is therefore produced.
• the optical fibre 1 is arranged such that an electric field induces a linear birefringence in this optical fibre 1.
• the apparatus 100 determines a spatial distribution of the polarisation properties of the optical fibre 1 that is indicative of the electric field acting on the fibre. Integration of the electric field between two points along the fibre yields a measurement of voltage between the two points.
• both electric current and electric voltage can be measured by apparatus 100.
• the optical fibre 1 is arranged to undergo the same strain as a structure, the strain on which it is desired to measure.
• the optical fibre 1 may be embedded in a reinforced concrete slab of a building or bridge. As strain on the optical fibre 1 changes the spatial distribution of polarisation properties of the optical fibre 1, measured by apparatus 100, changes. Thus, a spatial distribution of the strain or stress on the structure can be determined.

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