GB2119536A - Fibre optic Faraday rotation device and method - Google Patents

Abstract

A fibre optic Faraday rotation device comprises an optical fibre wound in a helical coil. Optical radiation of a frequency such that its beat length is equal to or is a sub- multiple of the length of one turn of the coil is passed through the coil. A magnetic field is applied orthogonally to the axis of the coil.

Description

SPECIFICATION Fibre optic Faraday rotation device and method This invention relates to fibre optic devices which utilise the Faraday effect. The devices may be optical isolators or filters or circulators or modulators, ("optical" referring to visible or infra red or ultra violet radiation), or the device may be a magnetic field sensor.

In fibre optic systems, especially communications systems, it is often a requirement to have an isolator, a filter etc., and it is convenient if the device is itself of fibre optic form to minimise the coupling problems and retain the advantages of the small size of optical fibre devices. Several fibre optic devices are already known which use the effect of Faraday rotation to provide an isolator, filter, etc.

The Faraday effect arises when light propagates through a medium in a direction parallel to a magnetic field, and the effect causes rotation of the plane of polarisation of linearly polarised light through an angle H where: 0=VBz (1) V is the Verdet constant, B the magnetic field strength and z the length. The direction of the rotation 0 is determined by the direction of the field, not the direction of propagation, so that a second passage of the light through the medium in the reverse direction causes an additional rotation 0 and not a compensating effect.

Many optical fibres consist of fused silica, and the Verdet constant V of silica is not large so that long path lengths are needed to achieve a rotation 0 of, say, 450. A difficulty arises because when an optical fibre is bent to give a compact device, the bend-induced birefringence tends to quench the Faraday rotation.

In the prior art, optical fibre devices utilising the Faraday effect have been designed to minimise the need to bend the optical fibres, and therefore to minimise the introduction of stress birefringence.

For example in Optics Letters Vol. 6, No. 7, July 1981 pages 322 to 323, Turner and Stolen illustrate a device in which an optical fibre is folded to pass nine times through a linear array of fourteen magnets, the magnetic field being applied to only the straight lengths of the fibre and not to the loops of the connecting parts of the fibre. In the example given, the magnetic field extends over many centimetres, while the end loops add to the length of the device.

In a different arrangement described in Applied Optics, 20, No. 23, st December 1981, pages 3989 to 3990, Findakly describes an optical fibre wound circumferentially on an 8 centimeter diameter wooden ring, with a copper wire wound toroidally to apply a magnetic field, giving a device about 10 centimetres in diameter.

It is clear that these prior art devices are much larger than most components in optical fibre systems. Further, one device requires the provision of an electric current to one or more electromagnets to give a high magnetic field.

It is an object of the invention to provide a fibre optic device using the Faraday effect which is more compact than known devices, which in some embodiments can use a permanent magnet to induce the Faraday effect, and which in one embodiment can act as a detector of a magnetic field.

According to the invention, there is provided a fibre optic Faraday rotation device for use with radiation of predetermined frequency comprising an optical fibre wound in a helical coil the diameter of which is such that the phase shift of one component of polarisation of radiation of said predetermined frequency differs from the phase shift of an orthogonal component of polarisation of said radiation by a multiple of 2n per turn of the coil.

Usually there will be further provided an associated polarisation means.

In many embodiments of the device there is further provided means for applying a magnetic field to the spiral coil in a direction perpendicular to the spiral axis of the coil.

Optionally the device may further comprise variable support means arranged to vary the birefringence of the fibre by applying tension to the fibre.

When the device includes means for providing a magnetic field, the type of fibre and the dimensions of the spiral coil may be chosen so that the device applies a known angular rotation to radiation passing through the device; the device may act as an isolator, or a filter or a circulator.

In a variation, there may be provided means for applying a variable magnetic field to the coil, when the device will be an optical modulator.

A device in accordance with the invention may, in association with an optical source and an optical detector, act as a magnetic field sensor.

Also according to the invention, a method of sensing a magnetic field comprises placing at a test position a fibre optic Faraday rotation device as hereinbefore defined; supplying to the device optical radiation at a known direction of polarisation; and sensing where any angular rotation of the polarised radiation due to the Faraday effect occurs as it passes through the Faraday rotation device.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a free-standing Faraday rotation coil; Figure 2 shows a variable-diameter Faraday rotation coil; Figure 3 illustrates schematically a Faraday rotation device; Figure 4 is a plot of the power variations of radiation at two orthogonal directions propagating through a Faraday rotation device; Figure 5 illustrates schematically a Faraday effect isolator; Figure 6 shows the variation of the transmitted radiation power with coil birefringence change or with wavelength change; and Figure 7 illustrates schematically a Faraday effect magnetic field sensor.

In Figure 1, a single optical fibre is wound into a spiral coil 1 OA of precisely controlled diameter 2R, the coils of the spiral being contiguous. Light may be transmitted through the fibre ends 1 2, 14.

The optical properties of such a free-standing coil change only slightly with temperature. In Figure 2, a single optical fibre is wound into a spiral 1 OB around a variable diameter cylindrical former 18 which consists of two half-cylinders 20, 22, held together by screws 24 which allow the separation between the half cylinders to be varied slightly, thus creating a stress by tensioning the fibre and altering the birefringence but not substantially affecting the shape of the fibre coil. A magnetic field 13 is applied in a direction with respect to the coil 10 indicated by the arrows.

Figure 3 shows schematically a device 34 in which a coil 10, which may take either of the forms shown in Figures 1, and 2, is placed between the poles of a permanent magnet 26, the coil and magnet being attached to a plane support 28 and the fibre ends 12, 14 being attached to fibre connectors 30, 32.

When radiation passes along a birefringent optical fibre, the component of polarisation aligned with the fast axis of the fibre is retarded less than the component aligned with the slow axis of the fibre, so a phase difference is introduced between the two components. After a certain length, which depends on the fibre properties, the slow-axis-aligned component will have retarded by 3600 more than the fast-axis-aligned component, so that the components are again in phase. The length of fibre which causes such a 3600 difference in the phase shifts is known as the beat length of the fibre. When a fibre is bent, birefringence is induced by the stress. The beat length is then dependent on both the bend radius and the radius of the fibre itself.

In the spiral coil 10 of the present invention, the coil radius R is chosen so that the coil circumference substantially equals the beat length of the stressed fibre, or is an odd multiple of that length. The result is that the bend-induced birefringence does not quench the much smaller effect of Faraday rotation. The Faraday rotation is then cumulative, each coil of the spiral in the magnetic field contributing an equal angular rotation.

If both the Faraday rotation and the linear birefringence are uniform throughout a straight optical fibre, the input and output polarization states E of radiation passing through The fibre may be related by the use of Jones calculus and are given byL

where x and y are the principal axes of the fibre and z is measured along the fibre axis: #ss=2# (nx-ny) # is the fibre birefringence; F is the Faraday rotation per unit length of the fibre; and

If the input radiation is linearly polarised along the x direction the value of I Ey(z) I / i Ex(o) oscillates along the fibre and reaches a maximum of only 2F". However, with the fibre arranged so that F alternates in sign in alternate half beat increments, as in the coiled fibre arrangements shown in Figures 1 to 4, 1 Ey I grows monotonically and I Ex I decreases to zero. Thus the effect of Faraday rotation can be cumulative.

In the coiled fibre arrangement according to the invention, if magnetic field B is uniform, then: z F=VB cos (-) (4) R The condition for efficient interaction is that 4)='/R or, if F Ap, (which is usually the case), then: 1 #ss=- (5) R If K is a constant relating the stress-induced birefringence AjB to the fibre radius rand coil radius R, i.e.

Kr #ss=- (6) R then the required radius R of the fibre spiral 10 can be determined for a fibre of known properties.

Taking typical values for a high silica fibre, with V1 .7 degrees centimetre -1 T1 at a wavelength of A=633 nanometres, and K < 7.5 x107 degrees per metre, then for r=75 micrometres and B=0.29 tesla, calculation gives R=7.38 millimetres. This gives | Ex | = | Ey | at A,B z < 36 times 2, i.e. at about 36 turns, (equivalent to 167 centimetres of fibre).

Figure 4 shows the theoretical variation with distance along the fibre of power of radiation in the two polarisations states, using the numerical example given above and substituting equations (4) and (5) in (2) for the case when EX(O)=1 and Ey(O)=O. The inset is a magnification of |Ey| between N=25 and N=26, showing the changes in one turn of the spiral coil as the direction of the magnetic field with respect to the fibre axis changes through alignment to transverse to alignment at 1 800, then through a transverse relative direction back to alignment at 3600.

When Ap z equals a whole number multiple of 7r, as in a fibre coil having a circumference equal to the birefringent beat length, the instantaneous state of polarisation of the light in the fibre is linear, and makes an angle of # tan-1 [| Ey(z) |/| Ex(z) |] with the x axis, one sign applying to even and the other to odd multiples, depending on the sense of the magnetic field.

The cumulative effect of Faraday rotation illustrated in Figure 4 shows that, by controlling the radius R and the number of turns of a spiral coil of optical fibre of known properties, a known angle of rotation can be applied to polarised light travelling along the fibre. For example, a device giving a rotation of 450 can be used as a fibre optic isolator which will transmit light in one direction with little loss but which will prevent transmission in the reverse direction so that, for example, laser light is prevented from returning to the laser source. Other angles of rotation may be selected depending on the intended use of the fibre optic device.

An example of the mode of use of a fibre optic isolator according to the invention is given in Figure 5. A laser source 36 is coupled to a fibre optic input fibre 38 through a coupling device 40. The fibre 38 passes through a fibre optic polariser 42 to the input connector of a Faraday device 34 according to the invention, arranged to apply a Faraday rotation of 450. The output connector of the device 34 is connected through a fibre optic analyser 44 to a fibre optic system indicated schematically at 46.

The polariser 42 sets the polarisation direction of the input light, and the Faraday device rotates it through 450 to match the direction of the analyser. Light from the laser 36 is transmitted into the system 46 with minimum loss, but if any light is reflected back through the polariser 44, it is rotated a further 450 by the Faraday device 34 and is therefore crossed with respect to the polariser 42, which does not allow passage of the light into the laser 36.

For a Faraday device according to the invention, the birefringence of the fibre is wavelength dependent, i.e. there is a wavelength at which equation (5) is satisfied exactly, while at other wavelengths the polarisation beat-length will not be matched exactly to the coil circumference. The effect of a slight mismatch X, given by the equation (1 + #) #ss=-- (7) R is illustrated in Figure 6 in which for a 40-turn coil |Ey| is plotted against A tOta=8N27r. The curve falls sharply on either side of a maximum to zero at +2.Figure 6 can also be regarded as illustrating the effect of a change of wavelength AR on a fixed-radius coil, the abscissa then corresponding to A This effect allows a Faraday device according to the invention to act as a filter, a selected narrow band of wavelengths passing through the device.

In addition to the birefringence induced by bending the fibre a further birefringence can be provided by applying tension. In a spirally-coiled Faraday device according to the invention, this tension can be applied by use of the arrangement illustrated in Figure 2; the screws 24 are adjusted to tension the fibre slightly. In this way the birefringence can be altered so that equation (5) is satisfied for a chosen wavelength. The device may be varied in this way over several multiplies of 7c, giving a tunable filter.

The bandwidth of a Faraday device filter is inversely proportional to the number of turns N in the spiral coil, so that the filter properties are easily adjusted.

In another embodiment (not illustrated), the magnetic field B applied to the spiral coil is caused to vary, for example by the use of an electromagnet in place of the permanent magnet shown in Figure 3.

The device can then act as an optical modulator.

In addition to a device in which a spiral coil fixed in a magnetic field is used to apply a known angular rotation, a Faraday device according to the invention can also be used as a magnetic field detector. A coil, such as illustrated in Figures 1, and 2 but without the provision of a magnet as in Figure 3, can be used as such a detector in an arrangement shown schematically in Figure 7. A laser 48 is coupled through a coupler 50 and a fibre optic polariser 52 to a Faraday device, comprising a coil 10 of optical fibre, and through a fibre optic analyser 54 to a photodetector 56. The directions of the polariser 52 and analyser 54 are crossed and parallel respectively to the fast axes of the fibre, so that equation (7) is the transfer function.In the absence of a magnetic field in the vicinity of the coil 10, when there is no Faraday rotation, the photodetector receives no light from the laser 48. If the device 10 is placed in a magnetic field with at least a component perpendicular to the spiral axis of the spiral coil, then a Faraday rotation will be applied to the laser light and the photodetector will receive a signal.

The signal strength can be related to the strength of the magnetic field or its component in the plane of each coil in the spiral. Thus the device can act as a detector of the strength or the direction of a magnetic field.

Considering the efficiency of the device mathematically and restricting attention to values of Ap z=N2 where N=O, 1, 2 etc, it is possible to define T perpendicular as:

It can be seen that the magnitude of the effect is half that which would be observed in a nonbirefringent fibre in a uniform magnetic field.

If the extinction ratio of the polariser 52 and analyser 54 is about 10-4, the minimum detectable field is about 10-3 tesla for a coil of the dimensions and properties given above. Alternatively, if a configuration is chosen so that Ex(o)=Ey(o), then equation (7) is in effect shifted by 7t/4 and the sensitivity of the device becomes dTpe,JdB=Vz/2, i.e. the device operates on a linear part of the curve. If the detector is shot noise limited and if the maximum photodetector current is 1 milliamp, then a field of the order 1 0-8 tesla may be detected. The use of interferometric methods may further reduce the detectable field strength.

It is an advantage of a Faraday device magnetic field sensor that it may be entirely constructed of non-conducting materials.

In any of the Faraday devices according to the invention, it is preferable to use a spun fibre which has an inherently low birefringence; this minimises the effect of any variation in ambient temperature.

In all embodiments so far considered, the coil circumference has been made equal to the beat length of the fibre. It is also possible to use a coil circumference equal to an integral odd number of beat lengths, if a large coil radius is essential and if a lower efficiency is tolerable. A coil circumference equal to an integral even number of beat lengths would give cancellation of the Faraday effect.

Claims (12)

Claims

1. A fibre optic Faraday rotation device for use with radiation of predetermined frequency comprising an optical fibre wound in a helical coil the diameter of which is such that the phase shift of one component of polarisation of radiation of said predetermined frequency differs from the phase shift of an orthogonal component of polarisation of said radiation by a multiplq of 27r per turn of the coil.

2. A fibre optic Faraday rotation device as claimed in Claim 1 further including stress application means to vary the relative phase shift of the two components.

3. A fibre optic Faraday rotation device as claimed in Claim 2 wherein said stress application means comprises a substantially cylindrical former for said helical coil, said former having adjusting means to alter its diameter.

4. A fibre optic Faraday rotation device as claimed in any one of the preceding claims incorporating magnetic field generating means to create a magnetic field in a direction orthogonal to the axis of the helical coil.

5. A fibre optic Faraday rotation device as claimed in Claim 4 wherein said magnetic field generating means is a permanent magnet.

6. A fibre optic Faraday rotation device as claimed in Claim 4 wherein said magnetic field generating means is an electromagnet.

7. An optical modulation incorporating a fibre optic Faraday rotation device as claimed in Claim 6 together with means for varying a current applied to said electromagnet.

8. A magnetic field sensor including a fibre optic Faraday rotation device as claimed in any one of Claims 1 to 3 together with a source of optical radiation and a detection of optical radiation.

9. A magnetic field sensor as claimed in Claim 8 comprising a source of optical radiation, first polarising means to polarise said optical radiation, a fibre optic Faraday rotation device positioned to receive polarised radiation from said source, second polarising means to analyse polarised radiation from said rotation device and detector means to detect radiation received from said second polarising means.

10. An optical filter including a fibre optic Faraday rotation device as claimed in any one of Claims 1 to3.

11. A method of sensing a magnetic field comprising placing at a test position a fibre optic Faraday rotation device comprising an optical fibre wound in a helical coil, supplying to the device optical radiation polarised in a known direction, and measuring the direction of polarisation of radiation emergent from said device to determine the rotation thereof.

12. A fibre optic F.araday rotation device substantially as herein described with reference to the accompanying drawings.