EP0634025A1

EP0634025A1 - Improvements to optical phase shifting

Info

Publication number: EP0634025A1
Application number: EP93907651A
Authority: EP
Inventors: Steven James Friskin; Ralph Alexander Betts
Original assignee: Australian and Overseas Telecommunications Corp Ltd
Current assignee: Australian and Overseas Telecommunications Corp Ltd
Priority date: 1992-04-03
Filing date: 1993-04-02
Publication date: 1995-01-18
Also published as: CA2133556A1; JPH07508355A; WO1993020475A1; EP0634025A4

Abstract

A non-reciprocal optical phase shifter is disclosed, together with various applications. The phase shifter includes direction dependent and independent polarisation rotating devices (52, 53) in parallel optical paths which are recombined, so that the relative phase shift of the output signal depends on the direction of propagation. The device can be employed to form simple isolators, bidirectional wavelength dependent isolators, circulators, and enables bidirectional communications down a single fibre at different wavelengths without polarisation selection. Bulk optics and fibre implementations are disclosed.

Description

IMPROVEMENTS TO OPTICAL PHASE SHIFTING Technical Field

The present invention relates to optical systems, including phase shifters, isolators, circulators and bi-directional communication systems, particularly but not exclusively for optical fibre communication systems. Background Art

An optical isolator is an important component in many optical systems, including communications applications and bulk lasers. The role of the isolator is to allow transmission of light in only one direction. Isolators are used in communications systems to prevent feedback resulting from reflections returning to a laser diode, and are used in conjunction with optical amplifiers to ensure there is no lasing or noise degradations due to feedback. A typical amplifier may have a conventional isolator at its input, its output or both input and output. It is not practically possible to operate a very high gain amplifier without isolation because of residual reflections and scattering.

The most common type of prior art isolator relies on the Faraday effect to achieve non reciprocal rotation of the polarisation state of light. In the presence of an applied magnetic field the polarisation of light is rotated in a direction independent of the propagation direction, and proportional to the Verde constant of the material in which the light propagates. By using a crystal with a high Verde constant, a rotation of 45 degrees can be effected in a short distance. A single polarisation isolator can be constructed as shown in Figure 1. Light travelling from left to right is polarised in the vertical direction, then rotated by 45 degrees by the Faraday rotator. A second polariser is placed at this angle, allowing the light to pass undisturbed. Light travelling from right to left is polarised in the 45 degree direction, then rotated by the Faraday element a further 45 degrees, so that it is now in the horizontal plane. This light will be completely blocked by the vertical polarisation so there will be no transmission in the right to left direction.

One technique for achieving polarisation independent isolation is to split incident light into two polarisation components, isolating each of the components, and then recombining the two polarisation components. The polarisation splitters themselves may act as the polarisers for the isolators, so a polarisation independent isolator can be constructed as shown in Figure 2.

Whilst such isolators are essential in many amplification applications, they negate the possibility of two way communication down a single fibre, and of bi-directional amplification. It would be advantageous to be able to provide single fibre two way communication in systems which use optical amplifiers.

It is one object of the present invention to provide an improved isolator which at least ameliorates the disadvantages of the prior art.

According to one aspect the present invention provides a non- reciprocal phase shifter, comprising at least one first polarisation rotating means, the direction of polarisation rotation being dependant on the direction of propagation of transmitted light, and at least one second means for altering polarisation, the direction of polarisation alteration being independent of the direction of propagation of transmitted light, characterised in that substantially half of any light propagating from either end of the phase shifter having an arbitrary polarisation travels through each of the first polarisation rotating means and second means for altering polarisation.

According to another aspect the invention comprises a non- reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said paths including respectively first and second means for altering the polarisation of incident light having arbitrary polarisation, at least one of said means for altering polarisation having a first rotation in one direction of propagation and another rotation in a second direction of propagation, and the other or one of said means for altering having a polarisation change independent of the direction of propagation, the arrangement being such that a first relative phase shift between the paths occurs for light propagating in one direction, and a second relative phase shift between the paths occurs for light propagating in a second direction.

According to a further aspect the invention comprises a non- reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said first path including in a first propagation direction successively first polarisation rotating means having a rotation dependent on propagation direction and second polarisation altering means having a change independent of the direction of propagation, said second path including in said first propagation direction successively third polarisation altering means having a change independent of the direction of propagation and fourth polarisation rotating means having a rotation dependent on propagation direction, the arrangement being such that substantially half of the incident light travels through each of said first and second paths, and the relative phase shift of output light is dependent on the direction of propagation of the incident light.

A further aspect of the present invention provides a non- reciprocal phase shifter, comprising means for transmitting substantially half of incident light into two optical paths each having arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that in a first direction of propagation the paths have outputs which are have a first relative phase shift, and in the reverse direction the paths have outputs which have a different relative phase shift.

Another aspect of the present invention provides an optical isolator, comprising a non-reciprocal phase shifter characterised in that for a selected wavelength, in a first propagation direction the output light is substantially transmitted, and in the reverse direction the output light is substantially attenuated. A further aspect of the present invention provides bidirectional optical isolator, comprising means for transmitting substantially half of incident light through each of first and second optical paths, at least said first path including polarisation rotating means having a rotation dependent on propagation direction and at least said second path including second polarisation altering means having a rotation independent of the direction of propagation, said first and second paths having a path length difference, and means for recombining said first and second paths, the arrangement being such that for light having a first wavelength propagating in a first direction total attenuation occurs, while for light having said first wavelength propagating in a second reverse direction substantial transmission occurs; and for light having a second wavelength propagating in said first direction substantial transmission occurs, while for light having said second wavelength propagating in said second reverse direction substantial attenuation occurs.

A further aspect of the present invention provides an optical circulator, comprising at least one input to an non-reciprocal isolator , and means for coupling the isolator to two outputs, the arrangement being such that light having one wavelength is output through one output, and light having a second wavelength is output through the second output.

A further aspect of the present invention provides bidirectional optical fibre communications system, in which signals travelling in a first direction have a first wavelength, and signals travelling in the other direction have a second wavelength, both signals travelling in the same optical fibre, characterised in that the system includes one or more wavelength selective bidirectional isolators.

A further aspect of the present invention provides a bidirectional optical amplifier for allowing amplification of signals at a first wavelength in a first direction, and at a second wavelength in the second, reverse direction, comprising means for inducing gain at said first and second wavelengths, and bidirectional wavelength dependent isolation means arranged such that signals at said first wavelength travelling in said first direction, and signals at said second wavelength travelling in said second direction, are transmitted, and signals at said first wavelength travelling in said second direction, and signals at said second wavelength travelling in said first direction, are attenuated, and such that undesired feedback at said first and second wavelengths is substantially suppressed. A further aspect of the present invention provides an optical isolator, comprising means for transmitting substantially half of incident light into two optical paths each halving arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that in a first direction of propagation the paths have outputs which are in phase, and so transmit the incident light, and in the reverse direction the paths have outputs which are 180° out of phase, and so do not transmit the incident light.

Brief Description of Drawings

Several embodiments of the invention will be described with reference to the drawings, in which:

Figure 1 is a schematic view of a prior art isolator;

Figure 2 is a schematic view of another prior art isolator; Figure 3 is a schematic view of an optical circulator;

Figure 4 illustrates a first technique for bi-directional isolation in a wavelength diversity transmission system;

Figure 5 illustrates a second technique for bi-directional isolation in a wavelength diversity transmission system; Figure 6 illustrates conceptually a technique for bi-directional isolation in a wavelength diversity system;

Figure 7A and 7B illustrate interference within the system of figure 10;

Figure 8 illustrates one implementation of the system of figure 10;

Figure 9 illustrates another implementation of the system of figure 10;

Figures 10A and 10B illustrate a preferred implementation of a non-reciprocal phase shifter;

Figure 11 illustrates a polarisation dispersion free isolator;

Figure 12 illustrates a balanced polarisation dispersion free design;

Figure 13 illustrates an integrated implementation of a circulator/isolator according to the present invention;

Figure 14 is a graph showing wavelength dependence in an experimental device; Figure 15 illustrates a preferred implementation of the system of figure 10;

Figure 16 illustrates schematically a bidirectional amplifier.

Figure 17 illustrates schematically a fibre embedded circulator according to the present invention;

Figure 18 illustrates an implementation of the device of Figure 17;

Figure 19 illustrates a mode-converting circulator;

Figure 20 illustrates a fused Mach-Zender implementation of an isolator/circulator according to the present invention;

Figure 21 illustrates a filtered isolator according to the present invention;

Figure 22 illustrates a high isolation device;

Figure 23 illustrates a network utilising a bi-directional amplifier;and

Figure 24 illustrates schematically a low polarisation dispersion amplifier. ^• Detailed description

This aspect of the present invention is particularly adapted to be implemented in bi-directional networks. In the transmission system envisaged using this form of isolation, transmission in one direction takes place at one wavelength, and in the opposite direction at a second wavelength, as is illustrated in principle in Figure 23. A particular difficulty in this arrangement is in constructing a simple isolator which achieves wavelength selective isolation. If such an isolator could be constructed, it is clear that the amplifier would be adequately isolated from reflections, as such reflections would in general maintain the wavelength of the light and so be passed out of the system upon meeting an isolator.

There are several approaches that can be taken to achieve the desired end. A first embodiment of this type of isolator depends upon splitting the light into the two wavelengths using a grating or wavelength division multiplexer 34, isolating each wavelength separately in each direction using isolators 32, 33, then recombining the light afterwards 31, as shown in Figure 4. A slightly more refined approach can be constructed with an optical circulator 42 and a single wavelength multiplexing element 41 as shown in Figure 5. This aspect of the present invention is based on a new approach to isolation which is inter. erometric instead of being polarisation dependent. One immediate advantage that arises from this is that all of the embodiments described are in general polarisation independent, without the need to split and recombine polarisations. Some of the implementations illustrated are for single moded waveguiding applications only, but the general concept is equally applicable to bulk isolators.

A general form of this aspect of the invention is shown in Figure 6. A single beam of light entering at point 1 is split by beamsplitter 51 into two paths formed using mirrors 54, 55. In one path a 90 degree Faraday rotator 53 is placed, and in the second path a pair of half-wave retardation plates 52 at a relative angle of 45 degrees is placed. The pair of retarders 52 rotates the polarisation of any arbitrary polarisation of incident light by 90 degrees.

This can be demonstrated by the appropriate Jones' Matrix multiplications. The two polarisations are now in the same direction, and the optical path length can now be adjusted to be equal in both arms ensuring constructive interference on the through path (1) to (4). In the reverse path with light incident at port 4, the polarisation of the two beams will be opposite because the Faraday rotation is independent of the propagation direction, whereas the birefringent rotation is reciprocal. This corresponds to a 180 degrees phase change in the electric field vector between the two beams and destructive interference will occur at port 1. The light will instead exit through port 2 forming an isolator.

The device created is related to a Mach-Zender interferometer. It will be apparent that wavelength dependence in the isolation can be achieved. If an optical path length difference is introduced between the two arms of the Mach-Zender, then there will be only certain wavelengths for which the interference will be constructive in light travelling from port 1 to port 4. This will give a sin2 dependence to the intensity of light emerging in port 4 as a function of λ, with a periodicity proportional to

1/Δ/, the difference in optical path lengths, with the remaining light being redirected to port 3. Because of the 180 degree phase shift between beams travelling right to left, the transmission curve at port 1, Figure 7A, will be complimentary to that obtained at port 4, Figure 7B. For wavelengths having destructive interference at port 4 for forward propagation, constructive interference will be obtained at port 1 for reverse propagation.

This is precisely the characteristics required for a wavelength dependent isolator. For instance, by using an optical path difference of 0.12 mm a device can be constructed to be forward propagating at 1535 nm (corresponding to the first gain peak of the erbium-doped fibre amplifier) and reverse propagating at 1555 nm (the second gain peak of the erbium doped fibre amplifier). Isolation is provided for both wavelengths. The main obstacle to be overcome is the stability of the device to external perturbations. As an interferometric device it is potentially much more sensitive to any thermally or mechanically induced variations in the relative path lengths. Although thermal and mechanical stabilisation is possible, it is unlikely that any bulk device built according to Figure 6 would have the required stability for a field device.

One solution is to build .the entire device from solid components, with a balance in the construction of the arms such that any thermal expansion affects each arm equally (except for the very small path length difference 1/Δ/. This is shown in Figure 8. Incident light 62 is incident on silvered face 63, and passes either through rotator 67 and half wave plate 68, or rotator 64 and half wave plate 65, to eventually exit 66. It will be apparent that an isolator similar to Figure 6 is created. It is noted that instead of a 90 degree Faraday rotation in one arm we now have a 45 degree rotation in one arm and minus 45 degrees in the second arm, and one of the half-wave plates is in one arm and the second is in the second arm. It can again be shown by Jones matrices that this is equivalent to the previous arrangement. It is, however, very difficult to produce a 50% beam-splitting cube which is polarisation insensitive at many of the wavelengths of particular interest to optical communications.

A second solution is to physically split the beam into the top half and bottom half and to use the same rotators and birefringent plates as before ,as shown in Figure 9. Lens 73, 71 collimate/focus the incident light to and from beam 75 and project the light onto mirror set 70. It is now much less obvious that this should work as an isolator, or even that there should be any interference at all. In fact this will no longer act as a bulk isolator, and it is only when the input 74 and output 72 are both single mode fibres that the interference or isolation will take place. This is because of the quantum nature of the modes of a fibre.

A qualitative theoretical understanding can be obtained by considering the device as a black box, which provides two paths of equal probability and the same phase for photons travelling in the forward direction. The expectation value of a photon arriving at a point in the second fibre is the sum of {the different possible wave functions times the probability of that wave function}. In the forward case the wave functions will be in phase for a given wavelength photon and the normalised expectation value will be 1 (100% probability of the photon arriving). In the reverse direction for that same wavelength the wave functions will be opposite sign and so the probability of the photon arriving will be 0.

The above argument is easily verified by using the formalism of Fourier optics. An assumed Gaussian mode is transformed by the lens into a Gaussian beam in Fourier space. In the absence of any phase delay, the second lens transforms the beam back to the original Gaussian, which excites fully the fundamental mode of the second fibre. In quantum mechanical terms, the overlap integral of the mode and the exciting light is 1. We have 100% transmission. If a phase delay of π is incurred in the top half of the beam, then this is equivalent to multiplying by -1 the top half of the Fourier transform. Upon performing the Fourier transform corresponding to the second lens, an odd function is obtained. The overlap integral of an odd function with an even function is necessarily zero. It is now clear what is happening to the photons which are lost to the system. They are simply trying to excite higher order modes which are cut off and so cannot propagate along the fibre. These photons are lost very quickly into the cladding of the fibre.

This principle has been demonstrated in the laboratory by constructing a very simple interferometer, which has proved surprisingly stable. An optical flat was inserted so as to occupy half of the beam in a beam expander between two single mode fibres. There is a clear modulation of the received light as a function of wavelength, corresponding to the two paths being in phase and out of phase. This is shown in Figure 14.

This leads us to another device for bi-directional isolation. In this the expanded beam is never physically separated. A compound element consisting of a 90 degree Faraday 76 rotator and a pair of crossed half wave plates 74 is made up, by polishing and joining on one side. This compound element is now located in the beam so as to occupy exactly one half of the beam as shown in Figure 15. The path difference can be achieved by having a slightly different optical path between the Faraday rotator and the birefringent element. This is conceptually identical to the device outlined in Figure 9. The exact wavelength of transmission can be tuned by varying slightly the angle at which the element is inserted into the beam. The thickness of this compound element need not be much larger than 500 μm with state-of- the-art Faraday rotators. Insertion loss should be negligible (<0.5 dB) with good beam expansion optics, and stability is maximised because there is only a small length of crystal (~1 mm) generating the path difference. One disadvantage of the embodiments described is that the construction of the non-reciprocal phase shifter used different materials to act upon each half of the beam which is split. The device is therefore susceptible to temperature dependence as the thermal expansion coefficients and thermal refractive index coefficients for each half will be different, thus shifting the wavelength of maximum isolation.

Figures 10A, 10B illustrate a preferred stable design for the non- reciprocal phase shifting element.

This implementation uses a 45 degree Faraday rotator 101 in both halves and a half-wave plate 100, 102 in both halves. In one half the half waveplate 100 is to the left of the Faraday rotator 101, and in the second half, the half waveplate 102 is to the right of the Faraday rotator 101. The second waveplate 102 is orientated to have its optic axis at 45 degree relative to the first waveplate 100.

To understand the operation of the device consider the path of light travelling left to right in the vertical polarisation 104 for a wavelength independent device. In the top half of the beam, the light travels through the fast axis of the half-wave plate 100 then through the Faraday rotator 101 to rotate the polarisation 45 degree clockwise. The light in the bottom half of the beam passes first through the Faraday rotator 101, to be rotated clockwise into line with the fast axis of the half wave plate 102. The two beams upon recombining will be in phase. Similarly, for light in the horizontal polarisation 105, light travelling- through both the top and bottom halves will travel through the slow axis of the half wave plates 100, 102 and so will be in phase. Light travelling from right to left, incident at 45 degree clockwise to the vertical in the top half will travel through the Faraday rotator 101 first to be rotated to the horizontal axis and then pass through the slow axis of the half wave plate 100. Light travelling from right to left, incident at 45 degree clockwise to the vertical in the bottom half will pass first through the fast axis of the half wave plate 102 then through the Faraday rotator 101 to be in the horizontal axis. As the top beam has travelled through the fast axis and the bottom beam has travelled through the slow axis there is a 180 degree relative phase shift between both halves of the beam. In the forward direction, recombination of the light leads to constructive interference and hence transmission, and in the reverse direction recombination of the light leads to destructive interference and hence attenuation. This is the basis for non-reciprocal transmission, or isolation. This may be readily extended to a circulator application, as will be apparent to the reader.

The isolator described with reference to Figure 10 has a very small (yet finite) polarisation dispersion of one wavelength difference between the two polarisation states in the forward direction. This is because one polarisation travels in the fast axis (top and bottom halves) and the other polarisation travels in the slow axis of the half wave plates. The resultant polarisation dispersion is for 1.5 μm light equal to 5 fs.

Although this value is much lower than even the best commercially available isolators, it is possible to design the present isolator to have intrinsically zero polarisation dispersion. This is done by making the forward transmissive path equivalent to a fast axis and a slow axis transit. Figure 11 shows how this can be achieved. The top half consists of for the vertical polarisation: fast axis: Faraday rotator: slow axis for the horizontal polarisation slow axis: Faraday rotator: fast axis

The bottom half consists for both polarisations of an optical path length equivalent to fast axis + Faraday rotator + slow axis. There is no intrinsic polarisation dispersion. Figure 12 is equivalent to Figure 11 except that the design is entirely balanced again.

A simple optical circulator using the beam splitting isolator as the basis may be readily implemented as shown in Figure 19. In the isolator the light which is rejected in the non-transmission direction is antisymmetric about the axis of the interface and so cannot excite the fundamental mode of a single mode fibre. It can however excite the first higher order mode of a multimode fibre 110 which can have the same symmetry. By using a fibre which supports this higher order mode and then producing a coupler 115 which couples the higher order mode to a second fibre we can perform the function of a three or four port circulator. Single mode fibres 116 are coupled 114, 115 to multimode fibre 118, 110. Lens 111, 113 and phase shifter 112 form a beam-expanded isolator as described above. Such a coupler can couple light from the correct symmetry higher order mode to the fundamental mode of a single mode fibre, where the propagation coefficients are matched in the coupling region. Either fused or polished coupling techniques can be used. Note that with the crystal interface in the parallel expanded beam, non-optimal excitation of the higher order mode is achieved with the remaining light lost to the system. This can be improved by having the interface where the beam of light is in transition from the near-field to far- field image. This is achieved by focussing the light between the lenses and placing the non reciprocal phase shifting crystals at an appropriate position. Referring to Figure 13, in this implementation a Mach-Zender wave guide is made in integrated optics. These are commercially available. A slot is cut through both arms of the Mach-Zender 113 and the two halves of the non-reciprocal phase shifter 112 are inserted into the slot so that light from one arm passes through one half and light from the second arm passes through the second half. Beam expansion techniques can be used if necessary to reduce the loss due to the non- waveguiding propagation through this region. As usual a magnetic field must be supplied around the Faraday rotator. This device could be combined with other integrated optic devices in a useful fashion, such as combining it with an integrated splitter.

Figure 20 illustrates a further Mach-Zender implementation involving two fibres using fused coupler technology. The implementation illustrated insert two fibres 115 into a glass tube 114 of lower refractive index than the fibre cladding index, and taper the tube down in two closely spaced regions 113 to form two 50:50 couplers. With the fibres 115 held firmly in place by the surrounding collapsed glass tubing 114, the device can be cut and polished, before the non-reciprocal phase shifting elements 112 are placed between the fibre waveguides. Alternatively, a slot 116 can be used as for the integrated optic implementation shown in Figure 13. This implementation has the potential to provide low loss coupling of the light through the non- reciprocal phase shifter. By expanding the fundamental mode, the diffraction effects are reduced and propagation through non-waveguiding regions can be achieved with low loss. Beam expansion is achieved through using a tapered region of the fibre, or by core diffusion techniques.

A single polarisation fibre embedded isolator has been described in the scientific literature. The application of single polarisation devices is, however, extremely limited. The device shown in Figure 15 provides a polarisation insensitive isolator. Light from a single mode fibre 74 is expanded via lens 73 into an expanded beam. Non-reciprocal phase shifter 112 is formed from a Faraday rotator 76 in one half of beam 75, and a pair of half wave plates 77 in the other half. Lens 71 conveys the light into fibre 72. It will be appreciated that the principle of operation is analogous to the device of Figure 6. These could be potentially made into isolating connectors. Such devices can be either wavelength independent or wavelength selective.

Another feature which can be incorporated into a isolator of the split beam type (or a standard isolator for that matter) is filtering using the split beam technique. By splitting the beam with a non-birefringent reciprocal element for instance a non-birefringent wave plate 117, with different optical path lengths on both sides a sinusoidal filtering can be applied. This can be seen in Figure 21. This is particularly useful in amplifier applications to equalise the gain over a certain band width. To combine this with an isolator of the type discussed above the interface of the reciprocal filtering element should be orthogonal to the interface of the non-reciprocal phase shifter 112. By splitting the beam using ratios other than 50:50, the required degree of extinction can be obtained.

The ability to cascade these devices to form higher isolation or temperature independent features is remarkably simple. Two non- reciprocal phase shifters, 112, 122 can be cascaded in the same beam expander by ensuring that the interfaces of the devices are perpendicular to each other. This is shown in Figure 22. Some tuning of the characteristics of the device can also be achieved with small variations from 90 degree relative orientation. Two identical devices can be cascaded to increase the peak isolation and isolation bandwidth of the device, or two devices with slightly different central wavelengths can be cascaded to achieve a broader isolation bandwidth. Peak isolation wavelength tuning

It is useful in manufacture to be able to tune the wavelength of peak isolation to be different to the wavelengths of 45 degree Faraday rotation. This can be achieved according to the present invention by choosing the relative angle between the optic axis of the half wave plates to be equal to the Faraday rotation angle at the wavelengths of desired peak isolation. ^• This feature can be combined with the previous feature for broadband isolation, or to achieve high isolation in opposite directions for wavelengths which are well separated. Tuning of device

One important practical issue, distinct from the wavelength of peak isolation, is the tuning of the device to ensure that the maximum extinction occurs at the wavelength(s) which are required. This is achieved by ensuring that the light from each half of the non-reciprocal phase element is exactly in phase at the wavelength(s) of interest for the transmission direction. The relative phase between each half is most easily tuned by an angular variation of the crystals. Where the optical path length is different or there is a difference in the angular orientation of both halves, this can be achieved by rotating the whole device. This should be rotated in a plane such that the interface of the crystals remains perpendicular to the beam, for the split beam implementation.

A separate tuning mechanism is possible. A small phase shift in half of the Fourier plane (i.e. in the expanded beam) is equivalent to a lateral displacement in the direction perpendicular to the interface in the image plane. So an alternative is to laterally displace the fibre from the true image position to tune the phase. This tuning mechanism can incur some small losses, but is useful for fine tuning.

Temperature independence of phase change through material balancing It should be noted that although the balance design has exactly the same material on both halves of the non-reciprocal phase shifter for the broad band case, the bi-directional design requires an additional optical path length, and so there will be potentially some temperature dependence to the phase, due to the thermal expansion, and thermal refractive index dependence in this unbalanced portion. At least two approaches could be taken. One is to use a material with a very small temperature dependent phase shifter. Another is to use two different materials in each half of the non-reciprocal phase shifter, to produce the optical path difference. These materials are chosen to have slightly different thermal coefficients such that the smaller optical length has the larger thermal coefficients. Using this method the relative phase can be made to remain essentially constant over a wide temperature range. Fibre embedded optical circulator

Figure 17 illustrates a circulator using the principle of the devices described earlier, for instance Figure 6, except that the beam splitting will be done in fibre using an optical coupler. A 90 degree Faraday rotator 88 is embedded in one arm of the Mach-Zender and a 90 degree birefringent rotator 88 in the other arm (or other combinations as described earlier), to form a circulator.

One way to manufacture this and keep the arm lengths down to an absolute minimum is described with reference to Figure 18. A fused silica V-grove 90, 92 is used to align each of the fibres, and a 90 degree Faraday rotator 93 is embedded into one fibre 130 half way along the fused silica and a 90 degree polarisation rotating birefringent plate 94 is embedded into the other fibre 131. The silica V-grooves are then used to make a polished coupler, by continually polishing until 100 percent coupling is achieved in the forward direction. This should correspond to 50% coupling before and after the rotating devices. The device is now identical in operation to that shown in Figure 17, but is extremely compact and resistant to the environment. The device will behave as a circulator and is capable of mass production. This is of course an alternative implementation of the isolator, as all circulators are also isolators. It may be necessary to bury slightly the fibre in the silica at the point where the crystal embedding takes place, but this will have no real effect on the operation of the device.

Figure 24 illustrates an amplifier arrangement using an isolator according to the present invention . This arrangement is low polarisation dispersive. Pump source 131 and input signal 142 enter wavelength division multiplexer 135, pass through low birefringence erbium doped fibre 132, and enter polarisation dispersion isolator 138 (as described with reference to figures 11 and 12 for example). The signal then passes through low birefringence erbium doped fibre 134, wavelength division multiplexer 136, and is output 143. Pump source 137 drives the erbium fibre amplifier.

Figure 25 illustrates a bidirectional amplifier, of the type which enables bidirectional communications down a single fibre, utilising the same amplifiers for both wavelengths. The arrangement is similar to figure 24, but incorporates a bidirectional wavelength dependant isolator 133 (as described, for example, with reference to figure 6). The isolator 133 is positioned between two erbium doped fibre amplifiers 132, 134. At 140, signals at λ_r are input, and at λ₂ are output. At 141, signals at λ₂ are

input, and at λ- are output. Signals counterpropagating to the allowed

directions at the different frequencies, for instance at λ₂ input at .141, are suppressed. Thus, a true bidirectional system is possible, as is a true bidirectional amplifier, without undesired feedback to the amplifiers becoming a problem.

It will be appreciated that variations and additions are possible within the spirit and scope of the invention.

Claims

1 . A non-reciprocal optical phase shifter, comprising at least one first polarisation rotating means, the direction of polarisation rotation being dependant on the direction of propagation of transmitted light, and at least one second means for altering polarisation, the polarisation change being independent of the direction of propagation of transmitted light, characterised in that substantially half of any light propagating from either end of the phase shifter having an arbitrary polarisation travels through each of the first polarisation rotating means and second means for altering polarisation.

2. A non-reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said paths each including respectively first and second means for altering the polarisation of incident light having arbitrary polarisation, at least one of said means for altering having a first rotation in one direction of propagation and another rotation in a second direction of propagation, and the other or one of said means for altering having a change independent of the direction of propagation, the arrangement being such that a first relative phase shift between the paths occurs for light propagating in one direction, and a second relative phase shift between the paths occurs for light propagating in a second direction.

3. A non-reciprocal optical phase shifter, comprising means for transmitting incident light through first and second optical paths and recombining said paths at an output, said first path including in a first propagation direction successively first polarisation rotating means having a rotation dependent on propagation direction and second means for altering polarisation having a rotation independent of the direction of propagation, said second path including in said first propagation direction successively third means for altering polarisation having a change independent of the direction of propagation and fourth polarisation rotating means having a rotation dependent on propagation direction, the arrangement being such that substantially half of the incident light travels through each of said first and second paths, and the relative phase shift of output light is dependent on the direction of propagation of the incident light.

4. A non-reciprocal phase shifter according to claim 3, wherein the first and third polarisation rotating means are provided by a single faraday rotator extending across both optical paths.

5. A non-reciprocal phase shifter, comprising means for transmitting substantially half of incident light into two optical paths each having arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that in a first direction of propagation the paths have outputs which are have a first relative phase shift, and in the reverse direction the paths have outputs which, have a different relative phase shift.

6. A bidirectional optical isolator, comprising means for transmitting substantially half of incident light through each of first and second optical paths, at least said first path including polarisation rotating means having a rotation dependent on propagation direction and. at least said second path including second means for altering polarisation having a change independent of the direction of propagation, said first and second paths having a path length difference, and means for recombining said first and second paths, the arrangement being such that for light having a first wavelength propagating in a first direction total attenuation occurs, while for light having said first wavelength propagating in a second reverse direction substantial transmission occurs; and for light having a second wavelength propagating in said first direction substantial transmission occurs, while for light having said second wavelength propagating in said second reverse direction substantial attenuation occurs.

7. An optical circulator, comprising at least one input to an isolator according to claim 13 , and means for coupling the isolator to two ports, the arrangement being such that light travelling in one direction travels through the isolator and one port, and light travelling in the other direction travels through the other port and the isolator.

8. An optical circulator, comprising at least one input to an isolator according to claim 13, and means for coupling the isolator to two ports, the arrangement being such that light having one mode is output through one ports, and light having a second mode is output through the second ports.

9. A bidirectional optical fibre communications system, in which signals travelling in a first direction have a first wavelength, and signals travelling in the other direction have a second wavelength, both signals travelling in the same optical fibre, characterised in that the system includes one or more wavelength selective bidirectional isolators .

10. A bidirectional optical fibre communications system according to claim 9, wherein the isolators are in accordance with claim 12.

1 1. A bidirectional optical amplifier for allowing amplification of signals at a first wavelength in a first direction, and at a second wavelength in the second, reverse direction, comprising means for inducing gain at said first and second wavelengths, and bidirectional wavelength dependent isolation means arranged such that signals at said first wavelength travelling in said first direction, and signals at said second wavelength travelling in said second direction, are transmitted, and signals at said first wavelength travelling in said second direction, and signals at said second wavelength travelling in said first direction, are attenuated, and such that undesired feedback at said first and second wavelengths is substantially suppressed.

12. An optical isolator, comprising means for transmitting substantially half of incident light into two optical paths each halving arbitrary polarisation, and recombining the paths to produce an output, each of said paths comprising means for altering the polarisation of the transmitted light, characterised in that in a first direction of propagation the paths have outputs which are in phase, and so transmit the incident light, and in the reverse direction the paths have outputs which are 180° out of phase, and so do not transmit the incident light.

13. An optical isolator, comprising a non-reciprocal phase shifter characterised in that for a selected wavelength, in a first propagation direction the output light is substantially transmitted, and in the reverse direction the output light is substantially attenuated.

14. A device for creating an inter. erometric effect, comprising means for collimating light into a beam, a plurality of optical elements disposed intermediate the beam such that part of the light travels through each element, and means for converging the light.