GB2238879A - Modulation device - Google Patents

Modulation device Download PDF

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Publication number
GB2238879A
GB2238879A GB8924834A GB8924834A GB2238879A GB 2238879 A GB2238879 A GB 2238879A GB 8924834 A GB8924834 A GB 8924834A GB 8924834 A GB8924834 A GB 8924834A GB 2238879 A GB2238879 A GB 2238879A
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GB
United Kingdom
Prior art keywords
liquid crystal
electro
optical fibre
optic
crystal material
Prior art date
Legal status (The legal status 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 status listed.)
Granted
Application number
GB8924834A
Other versions
GB2238879B (en
GB8924834D0 (en
Inventor
Carolyn Bowry
Zacharias K Ioannidis
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
General Electric Co PLC
Original Assignee
General Electric Co PLC
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 General Electric Co PLC filed Critical General Electric Co PLC
Priority to GB8924834A priority Critical patent/GB2238879B/en
Publication of GB8924834D0 publication Critical patent/GB8924834D0/en
Publication of GB2238879A publication Critical patent/GB2238879A/en
Application granted granted Critical
Publication of GB2238879B publication Critical patent/GB2238879B/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

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Classifications

    • 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 
    • G02F1/13—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 liquid crystals, e.g. single liquid crystal display cells
    • G02F1/1326—Liquid crystal optical waveguides or liquid crystal cells specially adapted for gating or modulating between optical waveguides
    • 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 
    • G02F1/011—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  in optical waveguides, not otherwise provided for in this subclass
    • 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 
    • G02F1/011—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  in optical waveguides, not otherwise provided for in this subclass
    • G02F1/0115—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  in optical waveguides, not otherwise provided for in this subclass in optical fibres
    • G02F1/0118—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  in optical waveguides, not otherwise provided for in this subclass in optical fibres by controlling the evanescent coupling of light from a fibre into an active, e.g. electro-optic, overlay

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Liquid Crystal (AREA)

Abstract

A device for modulating the intensity of light transmitted along an optical fibre (13) comprises an electro-optic (for example liquid crystal) cell (17) which is switch able to couple material of one or other of two different refractive indices to the optical fibre. The cell comprises a body (24) of electro-optic material disposed between two electrically-conductive layers (16, 19). One conductive layer (16) is disposed between the body of electro-optic material and the surface of the optical fibre. <IMAGE>

Description

Nodulation Device This invention relates to a modulation device for modulating the intensity of light in a fibre optic system.
The device is of the so-called "all-fibre" type, i.e. the type which does not require the cutting of the optical fibre to allow insertion of the device. Such devices are of considerable importance in fibre optic systems, because they are rugged devices which operate with low loss and low back-reflection.
All-fibre modulation devices have previously been proposed in which a portion of the cladding is removed from the fibre, and access to the evanescent field of the light-guided mode is achieved by either polishing or etching the fibre. The modulation is effected by virtue of the electro-optic properties of a nematic liquid crystal which is coupled to the fibre at the polished or etched region.
Nematic liquid crystals exhibit a high refractive index ne for light polarised along the director and a low refractive index nO for the orthogonal polarisation direction. Under the influence of an electric field applied to the liquid crystal the director aligns either in the direction of the field (positive dielectric anisotropy) or perpendicular to it (negative dielectric anisotropy), which results in modulation of the refractive index experienced by the guided mode.
Ideally, ne and nO should span the effective refractive index of the fibre mode to provide low-loss, high-contrast modulation.
Figure 1 of the accompanying drawing is a schematic cross-sectional view of a known liquid crystal all-fibre modulation device. An optical fibre 1 comprises a core 2 encircled by cladding 3 and a protective outer sheath (not shown). After removal of part of the sheath, the fibre 1 is embedded in a block of glass 4. The glass is ground and polished to remove a portion of the cladding from the core. A layer 5 of MgF2 is deposited on the surface of the block, to act as an aligning layer, and two gold electrodes 6 and 7 of, say, 2ym thickness are deposited on the layer 5, spaced apart by, say, looum. A glass cover sheet 8 is located over the electrodes 6 and 7, and the space defined between the cover sheet, the layer 5 and the electrodes is filled with a nematic liquid crystal material 9.
Such devices suffer from the disadvantage that, in order to avoid interaction between the electrodes 6 and 7 and the guided mode, the electrodes must be positioned far from the interaction zone, and are therefore spaced far apart, for example, lOOym as mentioned above.
The result is that a large operating voltage, up to hundreds of volts, is required to effect switching of the device.
It is an object of the invention to provide an improved liquid crystal light-modulation device.
According to the invention there is provided a modulation device for modulating the intensity of light transmitted along an optical fibre, the device comprising an electro-optic cell which is switchable to couple material of either of two different refractive indices to the optical fibre, wherein the cell comprises a body of electro-optic material disposed between two electrically-conductive layers, one of the conductive layers lying between the body of electro-optic material and the surface of the optical fibre.
Preferably the electro-optic material comprises a liquid crystal material.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 shows a schematic cross section of a prior liquid crystal modulation device, as aforesaid, Figure 2 shows a schematic cross section of a liquid crystal modulation device in accordance with the invention, and Figure 3 illustrates transmission curves for non-coated and ITO-coated optical fibres.
Referring to Figure 2, a modulation device 10 comprises a block of glass 11 having a curved slot 12 of, say, 25cm radius cut therein. A strand 13 of single mode optical fibre is stripped of part of its sheath and is glued into the slot, for example using an epoxy resin. The surface 14 of the glass block 11 nearest to the fibre 13 is ground and polished until a portion of the cladding 15 of the fibre is removed, so that access to the evanescent field of the guided mode in the fibre core can be obtained. A layer 16 of indium tin oxide (ITO) of thickness, say, 15nm is sputtered on to the polished surface 14 to form one electrode of a liquid crystal cell 17.
A glass cover sheet 18 having an ITO layer 19 similar to the layer 16 deposited on its underside is separated by dielectric spacers 20,21 from the layer 16, so that the gap between the ITO layers is only a few pm. Before assembly, the layers 16 and 19 are rubbed so that they also act as aligning layers for the liquid crystal material which is to be confined therebetween. Alternatively, aligning layers 22,23 formed of, for example, rubbed polymer or evaporated silicon monoxide can be deposited over the ITO layers 19,16. The glass block 11, the spacers 20 and 21 and the cover sheet 18 are assembled, and the space defined between them is filled, by capillary action, with a nematic liquid crystal material 24, such as a material designated 14616 obtainable from BDH Limited. This material can be aligned homogeneously.
An electric field can be applied to the liquid crystal material by applying a voltage to the ITO layers via thin wires 25 and 26, respectively, which are attached to the layers before assembly of the device. The field is therefore induced in a direction perpendicular to the surface 14 of the block 11.
It will be apparent that, due to the very small separation which exists between the ITO layers 16 and 19, the voltage required to achieve the field (e.g. 1-2 volts/pm) required to switch the nematic liquid crystal will be quite small, as compared with the large voltage required for the prior devices, in which the gold electrodes are spaced apart by, say, fifty times the distance.
The use of indium tin oxide has other advantages, in that it is easy to deposit on the glass and it is mechanically strong and not easily scratched. However, it affects the characteristic transmission curve when the refractive index of the overlayer is changed. ITO has a positive optical dielectric constant and surface plasmon effects therefore do not occur. However, resonance occurs, giving effects similar to plasmon effects. ITO has a high refractive index (around 1.7) compared with the low refractive index of the media, i.e. the glass block and the nematic liquid crystal material, on either side of it.Normally the differences between the refractive indices would be too large for waveguiding to occur within the 110 but, due to the very small thickness of the 110 layer 14, the fundamental TEo and TMo modes alone are supported. This produces resonance dips in the TE and TM modes of the transmission versus refractive index curves for the device. The positions and magnitudes of these dips depend on the thickness of the ITO layer and the distance of the 110 layer from the core. Curve (a) in Figure 3 is a typical transmission curve for an optical fibre without an ITO layer for both the TE and TM modes. When an 110 layer is placed on top of the fibre, the TE and TM modes split.For this particular fibre the TM mode remains as in curve (a) but the TE mode now shows a resonance dip as in curve (b). By increasing the 110 thickness, it is also possible to alter the TM mode. An 110 layer can therefore be used an a excellent polariser if a material of suitable refractive index is used as an overlayer.
For the homogeneous alignment of the liquid crystal material along the fibre axis, both modes encounter the ordinary refractive index no of the material, nO being 1.452 at 25 C. For a material having a positive dielectric anisotropy, under the influence of the applied electric field the nematic liquid crystal molecules tend to align with the field, and the TM mode encounters the extraordinary refractive index ne = 1.5. A high modulation (for example 45dB) therefore results. The TE mode encounters a refractive index nO of 1.452 at 250C and remains in the fibre. There is no modulation because the TE mode also encounters nO when the liquid crystal material is switched.
If the liquid crystal material is initially homogeneously aligned perpendicular to the axis of the optical fibre, the TE mode will be modulated and the TM mode will not be modulated.
If a liquid crystal material having a negative dielectric anisotropy is used, an initial homeotropic alignment is required.
When the material is switched, a random homogeneous alignment is obtained, so that both the TE and TM modes are modulated but out of phase with each other.
Initial alignment of the liquid crystal material between these two orthogonal directions is possible, and can be chosen depending upon the modulation effect required.
In one application of the device described above, a second block, similar to the block 11, containing a second optical fibre in a curved slot, is inverted over the liquid crystal cell in place of the cover sheet 18. The ITO electrode 19 is deposited on the surface of this second block. Switching of the liquid crystal material causes coupling together of the fibres, so that light is transmitted from one fibre to the other.
Although the embodiment described above uses a nematic liquid crystal material, a ferroelectric liquid crystal material might alternatively be used, provided that a sufficiently low refractive index can be achieved. Sudn materials exhibit much shorter switching times than the nematic materials. Alternate polarity switching pulses may be required for driving devices using ferroelectric materials. Electro-optic polymers might alternatively be used instead of such nematic or ferroelectric materials, thereby enabling very fast switching of the device, for example in the GHz range.

Claims (13)

1. A modulation device for modulating the intensity of light transmitted along an optical fibre, the device comprising an electro-optic cell which is switchable to couple material of either of two different refractive indices to the optical fibre, wherein the cell comprises a body of electro-optic material disposed between two electrically-conductive layers, one of the conductive layers lying between the body of electro-optic material and the surface of the optical fibre.
2. A device as claimed in Claim 1, wherein the electro-optic material comprises a liquid crystal material.
3. A device as claimed in Claim 2, wherein the liquid crystal material is a nematic material.
4. A device as claimed in Claim 2, wherein the liquid crystal material is a ferroelectric liquid crystal material.
5. A device as claimed in Claim 1, wherein the electro-optic material comprises a non-linear optical polymer.
6. A device as claimed in Claim 1, wherein each of the electrically-conductive layers is formed of indium tin oxide.
7. A device as claimed in any preceding claim, comprising a transparent block in which a portion of the optical fibre is mounted so that said portion is at least partly close to a surface of the block, the electro-optic cell being disposed in contact with said surface.
8. A device as claimed in Claim 7, wherein the other of said conductive layers is deposited on a transparent cover sheet which is separated from the transparent block by dielectric spacers.
9. A device as claimed in Claim 7, wherein the other of said conductive layers is deposited on a second transparent block containing a second optical fibre, whereby switching of the liquid crystal material by application of a voltage between the electrically-conductive layers enables mutual coupling of the optical fibres so that light is transmitted from one fibre to the other.
10. A device as claimed in any one of Claims 2-4, including means to cause initial alignment of the molecules of the liquid crystal material.
11. A device as claimed in Claim 1, wherein said one conductive layer is formed of indium tin oxide, which layer acts additionally as a light-polarising layer.
12. A modulation device substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawing.
13. A light-polarising device comprising a layer of indium tin oxide in contact with an optical fibre.
GB8924834A 1989-11-03 1989-11-03 Modulation device Expired - Fee Related GB2238879B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB8924834A GB2238879B (en) 1989-11-03 1989-11-03 Modulation device

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB8924834A GB2238879B (en) 1989-11-03 1989-11-03 Modulation device

Publications (3)

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GB8924834D0 GB8924834D0 (en) 1989-12-20
GB2238879A true GB2238879A (en) 1991-06-12
GB2238879B GB2238879B (en) 1994-01-19

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0926536A1 (en) * 1997-12-23 1999-06-30 BRITISH TELECOMMUNICATIONS public limited company Tunable optical device
FR2776784A1 (en) * 1998-03-30 1999-10-01 France Telecom Electro-optical modulator, forming a Pockels cell
WO2001067166A1 (en) * 2000-03-03 2001-09-13 Lightwave Microsystems Corporation Variable optical attenuators
EP1243962A2 (en) * 2001-03-19 2002-09-25 BTI Photonics Inc. Wavelength tunable reflector and methods

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2176626A (en) * 1985-05-29 1986-12-31 Gen Electric Plc Fibre optic coupler
GB2184859A (en) * 1985-12-04 1987-07-01 Gen Electric Plc Fibre optic devices

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2176626A (en) * 1985-05-29 1986-12-31 Gen Electric Plc Fibre optic coupler
GB2184859A (en) * 1985-12-04 1987-07-01 Gen Electric Plc Fibre optic devices

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0926536A1 (en) * 1997-12-23 1999-06-30 BRITISH TELECOMMUNICATIONS public limited company Tunable optical device
WO1999034249A1 (en) * 1997-12-23 1999-07-08 British Telecommunications Public Limited Company Tunable optical device
FR2776784A1 (en) * 1998-03-30 1999-10-01 France Telecom Electro-optical modulator, forming a Pockels cell
EP0961154A1 (en) * 1998-03-30 1999-12-01 France Telecom Hybrid integrated electro-optic modulator of the Pockels-effect type
US6185033B1 (en) 1998-03-30 2001-02-06 France Telecom Hybrid integrated electro-optical modulator of the pockels effect type
WO2001067166A1 (en) * 2000-03-03 2001-09-13 Lightwave Microsystems Corporation Variable optical attenuators
EP1243962A2 (en) * 2001-03-19 2002-09-25 BTI Photonics Inc. Wavelength tunable reflector and methods
EP1243962A3 (en) * 2001-03-19 2003-11-19 BTI Photonics Inc. Wavelength tunable reflector and methods

Also Published As

Publication number Publication date
GB2238879B (en) 1994-01-19
GB8924834D0 (en) 1989-12-20

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PCNP Patent ceased through non-payment of renewal fee

Effective date: 19941103