WO1988006745A1 - Optical device with a non reciprocal polarization rotator - Google Patents

Optical device with a non reciprocal polarization rotator Download PDF

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Publication number
WO1988006745A1
WO1988006745A1 PCT/GB1988/000133 GB8800133W WO8806745A1 WO 1988006745 A1 WO1988006745 A1 WO 1988006745A1 GB 8800133 W GB8800133 W GB 8800133W WO 8806745 A1 WO8806745 A1 WO 8806745A1
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WO
WIPO (PCT)
Prior art keywords
rotator
rotator element
polarised light
optical device
light signals
Prior art date
Application number
PCT/GB1988/000133
Other languages
French (fr)
Inventor
William James Stewart
Colin Edge
Original Assignee
Plessey Overseas Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Plessey Overseas Limited filed Critical Plessey Overseas Limited
Publication of WO1988006745A1 publication Critical patent/WO1988006745A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/032—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect
    • G01R33/0322—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect using the Faraday or Voigt effect
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on magneto-optical elements, e.g. exhibiting Faraday effect
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3515—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F3/00—Optical logic elements; Optical bistable devices
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2589—Bidirectional transmission
    • H04B10/25891—Transmission components
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/213—Fabry-Perot type

Definitions

  • the present invention relates to an optical device and more particularly but not exclusively to a device to be used in a bi ⁇ directional fibre communication system.
  • a reciprical rotator or retarders such as a -/4 plate.
  • a A/4 plate acts to retard light rays in respective ordinary and extra-ordinary planes, the degree of retardation in respective ordinary and extra-ordinary planes differing by a quarter period.
  • a reciprical rotator provides a positive rotation for one traversal direction and a reciprical negative rotation for the reverse traversal with a consequent nett nil effect. This presents problems when rotation is required in single fibre communication systems ,as usually ""double pass routing" of signals thorough the reciprical rotator is necessary.
  • the A/4 plate must be set at 45° to an incident light beam.
  • An objective of the present invention is to provide a device which can substantially achieve an orthogonal polarisation transition in an incident light beam without "double pass routing".
  • an optical device for polarised light beams comprising a non-reciprical rotator element and a reflective surface, the reflective surface being arranged relative to the rotator element for reflecting light beams incident from the rotator element back to the rotator element whereby * in operation, light beams incident upon the rotator element traverse through the rotator element at least twice with consequential axial rotation of the polarised light beam plane on each traversal 1 being accumulative and additive to resultant chirality inversion produced by the mirror.
  • the rotator element is a Faraday Rotator which may include a YIG crystal and inductive means.
  • the rotator element may have a rotation of 45°.
  • the device may be adapted to form a Fabry-Perot resonant cavity.
  • a bidirectional fibre communication system comprising a light source of polarised light signals, a detector, a non- reciprical rotator element and a reflective surface, light signals from the light source being, in operation incident upon both the detector and the rotator element, light sign?ls incident upon the rotator element being rotated in the rotator element in the plane of the beam both on traversal to the reflective surface and reflection back through the rotator element to become incident upon the detector whereby light signals received at the detector directly from the light source can be distinguished from those light signals received via the rotator element by their respective polarisation states.
  • a method of rotation of polarised light signals comprising: - (i) directing the polarised light signals to a non-reciprical rotator element; (ii) causing the polarised light signals to traverse the rotator element to become incident upon a reflective surface; (iii) the reflective surface reflecting the polarised light signals back through the rotator element whereby the polarised light signals are rotated in the plane of the signals by the rotator element said rotations being accumulative.
  • Figure 1 is a schematic diagram of an embodiment of the present invention.
  • Figure 2 illustrates orthogonal polarisation of linear, circular and elliptical light beams at positions A. B. C and D in figure 1.
  • a conventional mirror reflects light incident upon it in such a way that superposes the onward light field that would have existed without the mirror backwards towards the source.
  • a more complex device known as a 'phase conjugate reflector' can be constructed that returns a light field such that each source point has its light returned to it. This function requires a non-linear transform (i.e. active adaptive component) and may be initialised to remove 'turbulence' in a light beam. Most of these devices work for single input/output polarisations but general devices have been reported.
  • a typical device would be a 2.65 mm length of YIG crystal 7 as the Faraday rotator 5 and an aluminised mirror 3.
  • Incident linearly polarised light at the YIG crystals 7 design wavelength of 1.55 was found to be returned in the orthogonal state with an extinction of better than 20-dB (measurement limited).
  • a variety of reciprocal birefringent media and a 2 metre length of single mode fibre have been inserted between the incident polarised light 1 and the rotator 7, and the orthogonality of the returned light is still maintained to better than 20-dB.
  • the rotation produced by the YIG crystal 7 had a wavelength variation of 5.14°/nm therefore an isolation of 20-dB should be possible over a wavelength range of lOOnm.
  • Other Faraday rotators 5 are, however, available which have a smaller wavelength dependence and could be used to produce broadband orthoconjugate reflectors 5.
  • a potential application of such a device would be in fibre bidirectional systems and measurements.
  • the device can be used to separate the upstream and downstream signals into orthogonal states, but by varying the Faraday rotator polarisation modulation of the returned signal can be achieved.
  • a broadband orthoconjugate reflector would cancel, on the returned signal, any spectral variations in polarisation caused by the fibre and hence would effectively restore any depolarisation effects.
  • Localised non-reciprocities such as might be produced by external magnetic fields, would • disrupt the orthogonality of the returned state and hence a transducer or detector instrument may be constructed.
  • a Fabry-Perot cavity can also be formed by use of one or two such devices as reflectors having simple, regularly spaced modes even with random birefringment elements in the cavity which would otherwise disrupt them (because light would return with altered polarisation from a round trip).
  • This has fibre resonator applications including in fibre lasers.
  • An alternative use may be in a non linear fibre logic system where a double pass system gives, at low power, a simple output because of reciprocity but more complex behaviour at higher powers.
  • the Faraday rotator and the mirror have both been made from fibre components, so on all-fibre orthoconjugate reflector is possible.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Optics & Photonics (AREA)
  • Power Engineering (AREA)
  • Electromagnetism (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Optical Communication System (AREA)

Abstract

An optical device wherein rotation of a polarised beam of light can be achieved with passive optical elements. The device comprises a non-reciprocal rotator element before a reflective surface. The light passes in to the rotator and is reflected back again by the reflective surface the beam thus being rotated on each passage to give an accumulative rotation. An assembly including a further partially reflective surface can be constructed which approximates a Fabry-Perot cavity.

Description

Optical device with a non reciprocal polarization rotator
The present invention relates to an optical device and more particularly but not exclusively to a device to be used in a bi¬ directional fibre communication system.
In bi-directional fibre communication systems a convenient way of separating incident and returned signals is to ensure the signals are in respective orthogonal polarisation states. An orthoganal polarisation relationship ensures minimum interaction between signals.
Previously, provision of signals in an orthoganal polarisation relationship has involved reciprical rotators or retarders such as a -/4 plate. A A/4 plate acts to retard light rays in respective ordinary and extra-ordinary planes, the degree of retardation in respective ordinary and extra-ordinary planes differing by a quarter period. A reciprical rotator provides a positive rotation for one traversal direction and a reciprical negative rotation for the reverse traversal with a consequent nett nil effect. This presents problems when rotation is required in single fibre communication systems ,as usually ""double pass routing" of signals thorough the reciprical rotator is necessary. Furthermore, to achieve circularly polarised light the A/4 plate must be set at 45° to an incident light beam.
An objective of the present invention is to provide a device which can substantially achieve an orthogonal polarisation transition in an incident light beam without " double pass routing".
According to the present invention there is provided an optical device for polarised light beams comprising a non-reciprical rotator element and a reflective surface, the reflective surface being arranged relative to the rotator element for reflecting light beams incident from the rotator element back to the rotator element whereby* in operation, light beams incident upon the rotator element traverse through the rotator element at least twice with consequential axial rotation of the polarised light beam plane on each traversal 1 being accumulative and additive to resultant chirality inversion produced by the mirror.
Preferably, the rotator element is a Faraday Rotator which may include a YIG crystal and inductive means.
The rotator element may have a rotation of 45°.
The device may be adapted to form a Fabry-Perot resonant cavity.
Also disclosed is a bidirectional fibre communication system comprising a light source of polarised light signals, a detector, a non- reciprical rotator element and a reflective surface, light signals from the light source being, in operation incident upon both the detector and the rotator element, light sign?ls incident upon the rotator element being rotated in the rotator element in the plane of the beam both on traversal to the reflective surface and reflection back through the rotator element to become incident upon the detector whereby light signals received at the detector directly from the light source can be distinguished from those light signals received via the rotator element by their respective polarisation states.
A method of rotation of polarised light signals comprising: - (i) directing the polarised light signals to a non-reciprical rotator element; (ii) causing the polarised light signals to traverse the rotator element to become incident upon a reflective surface; (iii) the reflective surface reflecting the polarised light signals back through the rotator element whereby the polarised light signals are rotated in the plane of the signals by the rotator element said rotations being accumulative.
An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:-
Figure 1 is a schematic diagram of an embodiment of the present invention; and,
Figure 2 illustrates orthogonal polarisation of linear, circular and elliptical light beams at positions A. B. C and D in figure 1.
A conventional mirror reflects light incident upon it in such a way that superposes the onward light field that would have existed without the mirror backwards towards the source. A more complex device known as a 'phase conjugate reflector' can be constructed that returns a light field such that each source point has its light returned to it. This function requires a non-linear transform (i.e. active adaptive component) and may be initialised to remove 'turbulence' in a light beam. Most of these devices work for single input/output polarisations but general devices have been reported.
With single mode fibre spatial reconstruction is not required as only a single source/receiver point exists. It can be shown that the conjugate reflector function (i.e. returning always a polarisation identical to the input) is still non-linear and thus active. However, the 'orthoconjugate' function that returns always the orthogonal polarisation can be made from passive parts.
To achieve the orthogonal state of a light beam requires its polarisation to be rotated by 90° (or π/2) and its chirality to be reversed (i.e. left to right and vice versa).
Consider Figure 1, chirality reversal occurs when light 1 is reflected from a plane mirror 3, converting circularly polarised light into its orthogonal state but leaving linear states unchanged. As stated previously to produce an orthogonal linear state a 90° rotation is required, in this embodiment a 45° Faraday rotator 5 before mirror 3 is used. The rotator 5 effects a 45° rotation on both forward and return passses across the YIG crystal 7 consequently as such rotations are non-reciprocal and additive, the rotator 5 gives substantially the 90° rotation required. An arrangement such as that shown in Figure 1 will therefore retro-reflect the orthogonal polarisation state of any incident polarised light as shown in Figure 2 for linear, circular and elliptical polarisation states.
A typical device would be a 2.65 mm length of YIG crystal 7 as the Faraday rotator 5 and an aluminised mirror 3. Incident linearly polarised light at the YIG crystals 7 design wavelength of 1.55
Figure imgf000006_0001
was found to be returned in the orthogonal state with an extinction of better than 20-dB (measurement limited). A variety of reciprocal birefringent media and a 2 metre length of single mode fibre have been inserted between the incident polarised light 1 and the rotator 7, and the orthogonality of the returned light is still maintained to better than 20-dB. The rotation produced by the YIG crystal 7 had a wavelength variation of 5.14°/nm therefore an isolation of 20-dB should be possible over a wavelength range of lOOnm. Other Faraday rotators 5 are, however, available which have a smaller wavelength dependence and could be used to produce broadband orthoconjugate reflectors 5.
A potential application of such a device would be in fibre bidirectional systems and measurements. The device can be used to separate the upstream and downstream signals into orthogonal states, but by varying the Faraday rotator polarisation modulation of the returned signal can be achieved. A broadband orthoconjugate reflector would cancel, on the returned signal, any spectral variations in polarisation caused by the fibre and hence would effectively restore any depolarisation effects. Localised non-reciprocities such as might be produced by external magnetic fields, would • disrupt the orthogonality of the returned state and hence a transducer or detector instrument may be constructed.
A Fabry-Perot cavity can also be formed by use of one or two such devices as reflectors having simple, regularly spaced modes even with random birefringment elements in the cavity which would otherwise disrupt them (because light would return with altered polarisation from a round trip). This has fibre resonator applications including in fibre lasers.
An alternative use may be in a non linear fibre logic system where a double pass system gives, at low power, a simple output because of reciprocity but more complex behaviour at higher powers. The Faraday rotator and the mirror have both been made from fibre components, so on all-fibre orthoconjugate reflector is possible.

Claims

1. An optical device for polarised light beams comprising a non- reciprical rotator element and a reflective surface, the reflective surface being arranged relative to the rotator element for reflecting light beams incident from the rotator element back to the rotator element whereby, in operation, light beams incident upon the non- reciprical rotator element traverse through the rotator element at
-least twice with consequential axial rotation of the polarised light beam plane on each traversal being accumulative and additive to resultant chirality inversion produced by the mirror.
2. An optical device as claimed in claim 1 wherein a partially reflecting surface is located at the opposite end of the rotator element to the reflective surface.
3. An optical device as claimed in claim 1 wherein the rotator element is a Faraday Rotator.
4. An optical device as claimed in any preceding claim wherein an inductive element is perpherial with the Faraday Rotator.
5. An optical device as claimed in claim 3 wherein the Faraday Rotator includes a YΪG crystal.
6. An optical device as claimed in claim 5 wherein the YIG crystal in the Faraday Rotator has a length substantially of 2.65 mm.
7. An optical device as claimed in claim 4 wherein the inductive element provides a constant magnetic field to the rotator element whereby variation in rotation of the plane of the polarised light beam are thus indicative of variation in external magnetic fields.
8. An optical device as claimed in any preceding claim wherein the rotator element has a rotation of 45°.
9. An optical device substantially as hereinbefore described with reference to figure 1.
10. An optical device comprising a rotator element for rotation of polarised light about its axis with first and second ends, said first end being reflective and the second end being partially reflective with the polarised light passing through the partially reflective end before becoming incident upon the reflective end such that the device constitutes substantially a Fabry-Perot cavity.
1 1. An optical device for detection of external magnetic field variations wherein operation a rotator element under the influence of an imposed constant magnetic field rotates a polarised light beam in its plane, the polarised light beam being further altered by an external magnetic field on passage through an optical fibre whereby said further alteration of the polarised light can be determined to indicate external magnetic field variation.
12. A bidirectional fibre communication system comprising a light source of polarised light signals, a detector, a non-reciprical rotator element and a reflective surface, light signals from the light source in operation being incident upon both the detector and the rotator element, light signals incident upon the rotator element being rotated in the rotator element in the plane of the beam both on traversal to the reflective surface and reflection back through the rotator element to become incident upon the detector whereby light signals received at the detector directly from the light source can be distinguished from those light signals received via the rotator element by their respective polarisation states.
13. A bidirectional fibre communications system as claimed in claim 12 wherein modulation of the light signals is "achieved by modulating the magnetic field applied to the Faraday rotator.
14. A bidirectional fibre communications system as claimed in claim 12 wherein modulation of the light signals is achieved by an electro-optic modulator between the Faraday rotator and the reflector.
15. A method of rotation of polarised light signals comprising:- (i) directing the polarised light signals to a non-reciprical rotator element;
(ii) causing the polarised light signals to traverse the rotator element to become incident upon a reflective surface;
(iii) the reflective surface reflecting the polarised light signals back through the rotator element whereby the polarised light signals are rotated in the plane of t1 e signals by the rotator element said rotations being accumulative.
16. A method of rotation of polarised light signals as claimed in claim 14 wherein the non-reciprical rotator element is a Faraday Rotator.
17. A method of rotation of polarised light signals as claimed in claim 14 wherein the Faraday Rotator includes a YIG crystal.
18. A method of rotation of polarised light signals substantially as hereinbefore described.
19. An optical logic device using a non-linear birefringent element, such as a fibre, followed by a rotator and reflector such as claimed in claim 13 wherein operation light is returned in its orthogonal state at low magnetic field strengths but at a different state at higher magnetic field strengths.
PCT/GB1988/000133 1987-02-26 1988-02-26 Optical device with a non reciprocal polarization rotator WO1988006745A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8704528 1987-02-26
GB08704528A GB2203259A (en) 1987-02-26 1987-02-26 Polarisation rotator associated with reflective surface

Publications (1)

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WO1988006745A1 true WO1988006745A1 (en) 1988-09-07

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JP (1) JPH01502780A (en)
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WO (1) WO1988006745A1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0500157A1 (en) * 1991-02-19 1992-08-26 Koninklijke KPN N.V. Optical transmitting and receiving system incorporating optical circulator

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GB1163431A (en) * 1966-04-22 1969-09-04 Mullard Ltd Light-Switching Arrangement
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EP0078731A1 (en) * 1981-11-03 1983-05-11 Thomson-Csf Interferometer for measuring a magnetic field, and electric current measuring device comprising such an interferometer
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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0500157A1 (en) * 1991-02-19 1992-08-26 Koninklijke KPN N.V. Optical transmitting and receiving system incorporating optical circulator
US5327278A (en) * 1991-02-19 1994-07-05 Koninklijke Ptt Nederland N.V. Optical transmitting and receiving system having polarization diversity in each hybrid circuit

Also Published As

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GB2203259A (en) 1988-10-12
GB8704528D0 (en) 1987-04-01
JPH01502780A (en) 1989-09-21
EP0305430A1 (en) 1989-03-08

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