WO1989003055A1 - Metal clad fibre optic polarizer - Google Patents

Metal clad fibre optic polarizer Download PDF

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Publication number
WO1989003055A1
WO1989003055A1 PCT/GB1988/000771 GB8800771W WO8903055A1 WO 1989003055 A1 WO1989003055 A1 WO 1989003055A1 GB 8800771 W GB8800771 W GB 8800771W WO 8903055 A1 WO8903055 A1 WO 8903055A1
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WO
WIPO (PCT)
Prior art keywords
optical
waveguide device
metal film
refractive index
thickness
Prior art date
Application number
PCT/GB1988/000771
Other languages
French (fr)
Inventor
George Stewart
Walter Johnstone
Brian Culshaw
Original Assignee
University Of Strathclyde
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 University Of Strathclyde filed Critical University Of Strathclyde
Publication of WO1989003055A1 publication Critical patent/WO1989003055A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2726—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide
    • G02B6/274—Optical coupling means with polarisation selective and adjusting means in or on light guides, e.g. polarisation means assembled in a light guide based on light guide birefringence, e.g. due to coupling between light guides
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G01N21/552—Attenuated total reflection
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
    • G02B6/276—Removing selected polarisation component of light, i.e. polarizers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
    • G02B6/2821—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29331—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by evanescent wave coupling
    • G02B6/29332—Wavelength selective couplers, i.e. based on evanescent coupling between light guides, e.g. fused fibre couplers with transverse coupling between fibres having different propagation constant wavelength dependency
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G01N21/552—Attenuated total reflection
    • G01N21/553—Attenuated total reflection and using surface plasmons
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/12—Circuits of general importance; Signal processing
    • G01N2201/121—Correction signals

Definitions

  • an optical device comprising a first waveguide device having a predetermined effective refractive index and arranged, in use, to carry an optical wave having an accessible evanescent field in a predeteremined region of the first waveguide device, a second waveguide device connected to the first waveguide device in said predeter ⁇ mined region and arranged to interact with said evanescent field, wherein said second waveguide device comprises a thin metal film in surface contact with said predetermined region of the first waveguide device and of suitable thickness to support an antisymmetric plasma wave, and a body of optical material in surface contact with said metal film, said body having sufficient thickness substantially to contain the evanescent field of the plasm wave and having a refractive index which is selected relative to the effective refractive index of said first waveguide device in combination with the thickness of said thin metal film to permit polarisation-selective coupling of said optical and plasma waves.
  • the antisymmetric plasma wave referred to is that which in the bound mode case is carried within the thickness of the thin metal film and has an antisymmetric charge distribution with symmetric field distribution within themetal and an evanescent ⁇ field extending from both surfaces of themetal as distinct from a surface plasm wave which may be present at a single surface of the metal and has an evanescent field extending in both directions from that surface.
  • the TM polarised antisymmetric plasma wave propagates in the thin metal fil at a velocity controlled by selection of the refractive index of the overlay body of optical material in combination with the metal thickness to match the propagation velocity of the optical field (strictly speaking the TM polarisation mode of the optical field) of the first waveguide device.
  • Matching of the propagation velocities of the waves is the condition for coupling and the TM polarised nature of the plasma wave to which the optical field couples gives the device its polarisation selectivity.
  • Particular forms of such devices are polarisers and polarising couplers.
  • first waveguide device having a known effective refractive index
  • coupling of the optical and plasma waves is achieved over a first range of metal film thicknesses when the refractive index of the optical material is at least as great as the known effective refractive index and the resultant plasma wave is leaky, ie. radiative into the overlay body of optical material.
  • Coupling is also achieved over a second range of metal film thicknesses when therefractive inclex of the optical material is less than the known effective refractive index and the resultant plasma wave is bound ie., no ⁇ - radiative.
  • the first metal film thickness range comprises thickness values which are less than those contained in the second metal film thickness range.
  • the first waveguide device may comprise an optical fibre (which may be a standard fibre or a highly birefringen fibre) having a core surrounded by cladding.
  • an optical fibre which may be a standard fibre or a highly birefringen fibre
  • access to the evanescent field of the optical wave may be provided by localised reduction in thickness . of the cladding in the predetermined region.
  • This thickness reduction may be effected in full or in part during manufacturing of the optical fibre or thereafter by etching, grinding and/or polishing, and requires removal of substantially all (e.g. 99%) of the cladding from one side of the fibre in order to form the predetermined region, as is already known.
  • access to the evanescent field of the optical wave may be provided by heating and drawing a portion of the fibre until the core is of.a reduced section which is insufficiently great to contain the optical wave so that the optical wave at least partly travels along the cladding with an evanescent field external to the cladding.
  • the first waveguide device may comprise an integrated optical device incorporating channel or planar light guides which can be on the surface of a substrate or buried therein to some extent.
  • the evanescent field of the optical wave may be accessed directly, or possibly after some material removal by abutment of the thin metal film with the light guide.
  • the thin metal film may be, for example, aluminium or chromium or silver but many other metals are expected to give acceptable results.
  • the thickness required to support the antisymmetric plasma wave is from a few Angstroms up to about 500 Angstroms.
  • a thickness within the range 70-100 Angstroms demonstrated leaky plasma waves coupled efficiently whilst the range 170-270 Angstroms demonstrated bound plasma waves coupled efficiently.
  • the body of optical material may be inactive (passive) and formed for example of a glass.
  • the body of optical material may be active for example, electro-optic whereby its refractive index is electrically controllable via an appropriate electrode structure.
  • the body of optical material may be such that its refractive index is dependent upon temperature, pressure, chemical absorption or other influence in which case the device according to the present invention is operable as a sensor for such influence.
  • the body of optical material may also be in contact with a further optic waveguide device.
  • the device according to the present invention can be arranged to function as a polariser so that light of any state of polarisation, eg.
  • the thin metal film is aluminium having a thickness within either of the above specified ranges, the extinction ratio and loss factors (as explained hereafter) were demonstrated to be >40 dB and ⁇ 1.00 dB respectively.
  • the device according to the present invention can be arranged to function as a polarisation sensitive intensity modulator.
  • the device according to the present invention can function as a polarisation-selective coupler so that light of a particular state of polarisation delivered at one end (ie. input port) of the first waveguide device is coupled into the further optic waveguide device and emanates therefrom (at the coupled output port) whilst light of the orthogonal state of polarisation delivered at the one end (ie. input port) of the first waveguide device continues to be delivered at the other end thereof (ie. at the throughput port) .
  • a polarisation selective coupler Such a device constitutes apolarisation selective coupler.
  • Fig. 1 illustrates a device according to the present invention (not to scale)
  • Fig. 2 illustrates typical dispersion curves for the operationally relevant surface plasmons supported by the thin metal fibre of the Fig. 1 device;
  • Fig. 3 illustrates particular extinction ratio curves for different constructions of the Fig. 1 device
  • Fig. 4 illustrates particular loss figures for a typical construction of the Fig. 1 device
  • Fig. 5 illustrates a further form of device according to the present invention.
  • Fig. 6 is a cross sectional view taken on the line S-S of Fig. 6.
  • the device 10 shown in the drawing is a polariser comprising a first waveguide device 15 comprising fibre core 11 with cladding 12 only a short axial length of which is illustrated in the interests of clarity.
  • a first waveguide device 15 comprising fibre core 11 with cladding 12 only a short axial length of which is illustrated in the interests of clarity.
  • the cladding 12 is locally removed by thickness reduction in the region 12A in order to render accessible the evanescent field of the optical wave delivered to the first waveguide device 15, and to this region is connected a second waveguide device 20 in the form of a thin metal film 16 in surface contact with the cladding region 12A and an overlay 17 of optical material.
  • the film 16 is of suitable thickness to support an antisymmetric plasma wave as illustrated at 18 which in the bound mode case has an antisymmetric charge distribution with symmetric field distribution within the thickness of film 16 and an evanescent field extending from both surfaces of the metal film.
  • the overlay 17 is sufficiently thick to contain the evanescent field of the plasma wave 18 and has a refractive index which is selected relative to the effective refractive index of the device 15 and in combination with the thickness of film 16, as will be explained.
  • an optical wave delivered to device 15 and having TM and TE modes is delivered from the device 15 with only TE mode due to the TM mode having been coupled into device 20.
  • the efficiency with which this is performed is denoted by the 'TM to TE extinction ratio' as measured at the output of the device 15 and by the 'loss' (attenuation) figure of the TE mode in propagating through the device 15.
  • the first waveguide device 15 is formed by a 1.3 ⁇ m standard Telecommunications fibre having a silica cladding with refractive index of 1.447 (approximately) and having a core with refractive index of 1.451 (approximately).
  • the metal .film is aluminium in the range 70-100 Angstroms for efficient coupling to the leaky modes or 170-270 r Angstroms for efficient coupling to the bound modes and the overlay has a refractive index selected for optimum extinc ⁇ tion ratio from the ranges 1.451 to.1.470 (approximately) or 1.436 to 1.444 (approximately respectively.
  • the overlay 17 forms an optical matching layer to a further waveguide device which like device 15 has its cladding locally reduced for evanescent field exposure.
  • the TM mode coupled from device 15 via the metal film 16 is delivered into the further waveguide device and becomes available at the output thereof.
  • Fig. 2 illustrates the theoretical dispersion relationships with metal thickness (T, in Angstroms of Aluminium) of the bound (solid lines) and leaky
  • the effective refractive index of the optical fibre (first waveguide device) is approximately 1.450 and coupling occurs from the first waveguide device to a given plasma wave when the effective refractive index of the plasma wave equals 1.450. From this illustration it is evident that when the effective refractive index of the first waveguide device (n ) is less than n_, coupling of optical and plasma waves occurs in leaky mode (dotted lines) at relatively thin metal thicknesses whereas when n 3 is less than n coupling occurs in bound mode at relatively thick metal thicknesses.
  • Fig. 3 illustrates curves of the observed TM to TE extinction ratio (in dB) against the refractive index (n-) of the overlay 17 for various metal film thicknesses when the index n_S is approximately 1.450. It will be seen that when n., is greater than n thin metal thicknesses provide good extinction ratios whereas when n- is less than n thick metal thicknesses provide good extinction ratios.
  • Fig. 4 illustrates for the specific device above the attenuation (in dB) of the TE mode in propagating through device 15 as a function of Aluminium film thickness (in Angstroms) for an overlay index, n-, of 1.456. The loss is high at low thickness values and decreases with increasing thickness. A 1 dB loss is achieved at 70
  • optimisation of the extinction ratio and insertion loss factors is simply a matter of selecting the appropriate metal thickness and overlay index.
  • the metal film is aluminium having a thickness of 90 Angstroms and the overlay index is 1.456 the extinction ratio is > 40 dB (which is excellent) and the insertion loss factor is about 0.5 dB (which also is excellent).
  • the extinction ratio is greater than 40 dB and the insertion loss factor is less than 0.2 dB.
  • Figs. 5 and 6 schematically illustrate a device 30 in accordance with the present invention in which access to the evanescent field is provided by local tapering of the fibre 31.
  • this is achieved by locally heating and pulling a fibre of standard diameter (say 125 microns) in a controlled manner until the guided optical wave has an evanescent field which propagates external to the fibre cladding (until the core diameter is reduced below the level at which it can contain the optical field) .
  • the diameter of the tapered fibre portion is of the order of 25 microns and permits circularly symmetric access to the evanescent field.
  • the metal film 32 is applied asymmetrically to the fibre 31, for example by deposition, and is overcoated with a body 33 of optical material as previously.

Abstract

An optical device comprising a first waveguide device (15) having a predetermined effective refractive index is arranged, in use, to carry an optical wave having an accessible evanescent field in a predetermined region (12A) of the first waveguide device (15). A second waveguide device (20) is connected to the first waveguide device (15) in the predetermined region (12A) and is arranged to interact with said evanescent field. The second waveguide device (20) comprises a thin metal film (16) in surface contact with the predetermined region (12A) of the first waveguide device (15) and of suitable thickness to support an antisymmetric plasma wave, and a body (17) of optical material in surface contact with the metal film (16). Body (17) has sufficient thickness substantially to contain the evanescent field of the plasma wave and has a refractive index which is selected relative to the effective refractive index of said first waveguide device (15) in combination with the thickness of the thin metal film (16) to permit polarisation-selective coupling of the optical and plasma waves.

Description

METAL CLAD FIBRE OPTTC POLARIZER
This invention relates to optical devices. According to the present invention there is provided an optical device comprising a first waveguide device having a predetermined effective refractive index and arranged, in use, to carry an optical wave having an accessible evanescent field in a predeteremined region of the first waveguide device, a second waveguide device connected to the first waveguide device in said predeter¬ mined region and arranged to interact with said evanescent field, wherein said second waveguide device comprises a thin metal film in surface contact with said predetermined region of the first waveguide device and of suitable thickness to support an antisymmetric plasma wave, and a body of optical material in surface contact with said metal film, said body having sufficient thickness substantially to contain the evanescent field of the plasm wave and having a refractive index which is selected relative to the effective refractive index of said first waveguide device in combination with the thickness of said thin metal film to permit polarisation-selective coupling of said optical and plasma waves.
The antisymmetric plasma wave referred to is that which in the bound mode case is carried within the thickness of the thin metal film and has an antisymmetric charge distribution with symmetric field distribution within themetal and an evanescent■ field extending from both surfaces of themetal as distinct from a surface plasm wave which may be present at a single surface of the metal and has an evanescent field extending in both directions from that surface.
By virtue of the present invention the TM polarised antisymmetric plasma wave propagates in the thin metal fil at a velocity controlled by selection of the refractive index of the overlay body of optical material in combination with the metal thickness to match the propagation velocity of the optical field (strictly speaking the TM polarisation mode of the optical field) of the first waveguide device. Matching of the propagation velocities of the waves is the condition for coupling and the TM polarised nature of the plasma wave to which the optical field couples gives the device its polarisation selectivity. Particular forms of such devices are polarisers and polarising couplers.
For a particular first waveguide device having a known effective refractive index, coupling of the optical and plasma waves is achieved over a first range of metal film thicknesses when the refractive index of the optical material is at least as great as the known effective refractive index and the resultant plasma wave is leaky, ie. radiative into the overlay body of optical material. Coupling is also achieved over a second range of metal film thicknesses when therefractive inclex of the optical material is less than the known effective refractive index and the resultant plasma wave is bound ie., noή- radiative. The first metal film thickness range comprises thickness values which are less than those contained in the second metal film thickness range.
The first waveguide device may comprise an optical fibre (which may be a standard fibre or a highly birefringen fibre) having a core surrounded by cladding. In this case access to the evanescent field of the optical wave may be provided by localised reduction in thickness . of the cladding in the predetermined region. This thickness reduction may be effected in full or in part during manufacturing of the optical fibre or thereafter by etching, grinding and/or polishing, and requires removal of substantially all (e.g. 99%) of the cladding from one side of the fibre in order to form the predetermined region, as is already known. Alternatively, access to the evanescent field of the optical wave may be provided by heating and drawing a portion of the fibre until the core is of.a reduced section which is insufficiently great to contain the optical wave so that the optical wave at least partly travels along the cladding with an evanescent field external to the cladding.
In an alternative form the first waveguide device may comprise an integrated optical device incorporating channel or planar light guides which can be on the surface of a substrate or buried therein to some extent. In this case the evanescent field of the optical wave may be accessed directly, or possibly after some material removal by abutment of the thin metal film with the light guide. The thin metal film may be, for example, aluminium or chromium or silver but many other metals are expected to give acceptable results. In the case of aluminium the thickness required to support the antisymmetric plasma wave is from a few Angstroms up to about 500 Angstroms. In a particular embodiment of the device (as described hereafter) a thickness within the range 70-100 Angstroms demonstrated leaky plasma waves coupled efficiently whilst the range 170-270 Angstroms demonstrated bound plasma waves coupled efficiently.
The body of optical material may be inactive (passive) and formed for example of a glass. Alternatively the body of optical material may be active for example, electro-optic whereby its refractive index is electrically controllable via an appropriate electrode structure. The body of optical material may be such that its refractive index is dependent upon temperature, pressure, chemical absorption or other influence in which case the device according to the present invention is operable as a sensor for such influence. The body of optical material may also be in contact with a further optic waveguide device. In its simplest form, when the body of optical material is inactive, the device according to the present invention can be arranged to function as a polariser so that light of any state of polarisation, eg. circular, elliptical or random polarisation, received at one end (ie. input port) of the first waveguide device is delivered at the other end (ie. the output port) thereof with a predetermined state of polarisation. In the aforementioned embodiment, when the thin metal film is aluminium having a thickness within either of the above specified ranges, the extinction ratio and loss factors (as explained hereafter) were demonstrated to be >40 dB and <1.00 dB respectively.
In the case where the body of optical material is active and is an electrically controlled body of electro-optic material the device according to the present invention can be arranged to function as a polarisation sensitive intensity modulator.
In the case where the body of optical material, whether active or inactive, is in contact with a further optic waveguide device, the device according to the present invention can function as a polarisation-selective coupler so that light of a particular state of polarisation delivered at one end (ie. input port) of the first waveguide device is coupled into the further optic waveguide device and emanates therefrom (at the coupled output port) whilst light of the orthogonal state of polarisation delivered at the one end (ie. input port) of the first waveguide device continues to be delivered at the other end thereof (ie. at the throughput port) . Such a device constitutes apolarisation selective coupler. Embodiments ofthe present invention will now be described by way of example with reference to the accompanying drawings in whichs-
Fig. 1 illustrates a device according to the present invention (not to scale) ; Fig. 2 illustrates typical dispersion curves for the operationally relevant surface plasmons supported by the thin metal fibre of the Fig. 1 device;
Fig. 3 illustrates particular extinction ratio curves for different constructions of the Fig. 1 device; Fig. 4 illustrates particular loss figures for a typical construction of the Fig. 1 device;
Fig. 5 illustrates a further form of device according to the present invention; and
Fig. 6 is a cross sectional view taken on the line S-S of Fig. 6.
The device 10 shown in the drawing is a polariser comprising a first waveguide device 15 comprising fibre core 11 with cladding 12 only a short axial length of which is illustrated in the interests of clarity. Within the illustrated length of the first waveguide device 15 the cladding 12 is locally removed by thickness reduction in the region 12A in order to render accessible the evanescent field of the optical wave delivered to the first waveguide device 15, and to this region is connected a second waveguide device 20 in the form of a thin metal film 16 in surface contact with the cladding region 12A and an overlay 17 of optical material. The film 16 is of suitable thickness to support an antisymmetric plasma wave as illustrated at 18 which in the bound mode case has an antisymmetric charge distribution with symmetric field distribution within the thickness of film 16 and an evanescent field extending from both surfaces of the metal film. The overlay 17 is sufficiently thick to contain the evanescent field of the plasma wave 18 and has a refractive index which is selected relative to the effective refractive index of the device 15 and in combination with the thickness of film 16, as will be explained.
In operation, an optical wave delivered to device 15 and having TM and TE modes (ie. states of polarisation) is delivered from the device 15 with only TE mode due to the TM mode having been coupled into device 20. The efficiency with which this is performed is denoted by the 'TM to TE extinction ratio' as measured at the output of the device 15 and by the 'loss' (attenuation) figure of the TE mode in propagating through the device 15.
In a specific device 10, operating at 1.3μm, the first waveguide device 15 is formed by a 1.3μm standard Telecommunications fibre having a silica cladding with refractive index of 1.447 (approximately) and having a core with refractive index of 1.451 (approximately). The metal .film is aluminium in the range 70-100 Angstroms for efficient coupling to the leaky modes or 170-270 r Angstroms for efficient coupling to the bound modes and the overlay has a refractive index selected for optimum extinc¬ tion ratio from the ranges 1.451 to.1.470 (approximately) or 1.436 to 1.444 (approximately respectively. In each case the extinction ratio is > 40 dB and the insertion loss is < 1 dB as is clear from Figs. 3 and 4. In a modified form of the illustrated device which functions as a polarising coupler the overlay 17 forms an optical matching layer to a further waveguide device which like device 15 has its cladding locally reduced for evanescent field exposure. In this arrangement the TM mode coupled from device 15 via the metal film 16 is delivered into the further waveguide device and becomes available at the output thereof.
Fig. 2 illustrates the theoretical dispersion relationships with metal thickness (T, in Angstroms of Aluminium) of the bound (solid lines) and leaky
(dotted and chain link lines) antisymmetric plasma waves for several values of the index (n_) of the overlay 17 for the specific device described above. Leaky waves denoted by chain link lines are not sustainable in practise and consequently cannot be coupled to. The effective refractive index of the optical fibre (first waveguide device) is approximately 1.450 and coupling occurs from the first waveguide device to a given plasma wave when the effective refractive index of the plasma wave equals 1.450. From this illustration it is evident that when the effective refractive index of the first waveguide device (n ) is less than n_, coupling of optical and plasma waves occurs in leaky mode (dotted lines) at relatively thin metal thicknesses whereas when n3 is less than n coupling occurs in bound mode at relatively thick metal thicknesses.
Fig. 3 illustrates curves of the observed TM to TE extinction ratio (in dB) against the refractive index (n-) of the overlay 17 for various metal film thicknesses when the index n_S is approximately 1.450. It will be seen that when n., is greater than n thin metal thicknesses provide good extinction ratios whereas when n- is less than n thick metal thicknesses provide good extinction ratios. Fig. 4 illustrates for the specific device above the attenuation (in dB) of the TE mode in propagating through device 15 as a function of Aluminium film thickness (in Angstroms) for an overlay index, n-, of 1.456. The loss is high at low thickness values and decreases with increasing thickness. A 1 dB loss is achieved at 70
Angstroms falling to 0.5 dB at 100 Angstroms. The loss is independent of overlay index over the index range which results in high TM/TE extinction ratio. At the higher metal thickness associated with coupling to bound plasmohs the losses for the TE mode are less than 0.2 dB for the overlay indices which lead to high extinction ratios.
It will therefore be appreciated from Figs. 3 and 4 that for any given waveguide device 15 having a particular value of n , optimisation of the extinction ratio and insertion loss factors is simply a matter of selecting the appropriate metal thickness and overlay index. For example, for the specific device described above, when the metal film is aluminium having a thickness of 90 Angstroms and the overlay index is 1.456 the extinction ratio is > 40 dB (which is excellent) and the insertion loss factor is about 0.5 dB (which also is excellent). Alternatively for an aluminium film thickness of 170 Angstroms and an overlay index of 1.440 the extinction ratio is greater than 40 dB and the insertion loss factor is less than 0.2 dB.
In practice from a manufacturing viewpoint, utilisation of the leaky modes is preferred due to the relatively relaxed constraints on the overlay index.
Figs. 5 and 6 schematically illustrate a device 30 in accordance with the present invention in which access to the evanescent field is provided by local tapering of the fibre 31. As previously explained this is achieved by locally heating and pulling a fibre of standard diameter (say 125 microns) in a controlled manner until the guided optical wave has an evanescent field which propagates external to the fibre cladding (until the core diameter is reduced below the level at which it can contain the optical field) . Typically the diameter of the tapered fibre portion is of the order of 25 microns and permits circularly symmetric access to the evanescent field. The metal film 32 is applied asymmetrically to the fibre 31, for example by deposition, and is overcoated with a body 33 of optical material as previously.

Claims

, 1. An optical device comprising a first waveguide device (15) having a predetermined effective refractive index and arranged, in use, to carry an optical wave having an accessible evanescent field in a predetermined region (12A) of the first waveguide device (15) , a second waveguide device (20) connected to the first waveguide device (15) in said predetermined region (12A) and arranged to interact with said evanescent field, characterised in that said second waveguide device (20) comprises a thin metal film (16) in surface contact with said predetermined region (12A) of the first waveguide device (15) and of suitable thickness to support an antisymmetric plasma wave, 'and a body (17) of optical material in surface contact with said metal film (16) , said body (17) having sufficient thickness substantially to contain the evanescent field of the plasma wave and having a refractive index which is selected relative to the effective refractive index of said first waveguide device (15) in combination with the thickness of said thin metal film (16) to permit polarisation-selective coupling of said optical and plasma waves.
2. An optical device as claimed in claim 1, characterised in that the body (17) of optical material in surface contact with said metal film (16) is inactive.
3. An optical device as claimed in claim 1, characterised in that the body (17) of optical material in surface contact with said metal film (16) is active.
4. An optical device as claimed in any preceding claim, characterised in that the body (17) of optical material in surface contact with said metal film (16) has a refractive index which is influenced by an external parameter whereby the device (10) is operable as a sensor for such parameter.
5. An optical device as claimed in any preceding claim characterised in that said thin metal film (16) is aluminium with a thickness not greater than 500 Angstroms.
6. An optical device as claimed in any preceding claim, characterised in that the first waveguide device (15) comprises an optical fibre having a core (11) surrounded by cladding (12) and access to the evanescent field of the optical wave is provided by localised reduction of cladding thickness at one side of the fibre in order to form the predetermined region (12A) .
7. An optical device as claimed in any one of claims
1 to 5, characterised in that the first waveguide device (15) comprises an optical fibre (31) having a core surrounded by cladding and access to the evanescent field of the optical wave is provided by localised tapering of the fibre (31) , the thin metal film (32) being asymmetrically applied to the tapered portion of the fibre at the predetermined region (12A) .
8. An optical device as claimed in any one of claims 1 to 5 , characterised in that the first waveguide device (15) comprises an integrated optical device incorporating a channel or planar light guide.
9. An optical device as claimed in any preceding claim, characterised in that the body (17) of optical material is in contact with a further optic waveguide device so as to form a light coupler.
PCT/GB1988/000771 1987-09-22 1988-09-08 Metal clad fibre optic polarizer WO1989003055A1 (en)

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GB878722338A GB8722338D0 (en) 1987-09-22 1987-09-22 Optical devices
GB8722338 1987-09-22

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Cited By (4)

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Publication number Priority date Publication date Assignee Title
WO1991001489A1 (en) * 1989-07-21 1991-02-07 National Research Development Corporation Optical devices
GB2239715A (en) * 1990-01-06 1991-07-10 Plessey Co Plc Integrated optical device
EP0606377A1 (en) * 1991-10-03 1994-07-20 Foster-Miller, Inc. Optical fiber for spectroscopic monitoring
CN114114546A (en) * 2021-11-22 2022-03-01 曲阜师范大学 All-fiber polarizer based on mixed plasmon waveguide structure

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DE3011663A1 (en) * 1980-03-26 1981-10-01 Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt Optical fibre polarisation device - has crystal block with edge face in contact with polished surface of optical conductor
EP0212773A2 (en) * 1985-08-20 1987-03-04 Litton Systems, Inc. Cutoff polarizer and method
DE3534737A1 (en) * 1985-09-28 1987-04-09 Licentia Gmbh Method for producing a fibre-optic polariser

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DE3011663A1 (en) * 1980-03-26 1981-10-01 Licentia Patent-Verwaltungs-Gmbh, 6000 Frankfurt Optical fibre polarisation device - has crystal block with edge face in contact with polished surface of optical conductor
EP0212773A2 (en) * 1985-08-20 1987-03-04 Litton Systems, Inc. Cutoff polarizer and method
DE3534737A1 (en) * 1985-09-28 1987-04-09 Licentia Gmbh Method for producing a fibre-optic polariser

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7th European Conference on Optical Communication, 8-11 September 1981 Copenhagen (DK) O. Parriaux et al.: "Fiber optic polarizer using plasmon - guided wave resonance" pages P6-1 - P6-3 *
Optics Communications, vol. 16, no. 3, March 1976, North-Holland Publishing Co. Amsterdam (NL) H.F. Mahlein: "Integrated optical polarizer", pages 420-424, *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1991001489A1 (en) * 1989-07-21 1991-02-07 National Research Development Corporation Optical devices
US5598267A (en) * 1989-07-21 1997-01-28 British Technology Group Limited Optical sensor for orthogonal radiation modes
GB2239715A (en) * 1990-01-06 1991-07-10 Plessey Co Plc Integrated optical device
US5082341A (en) * 1990-01-06 1992-01-21 Gec-Marconi Limited Integrated optical device with zero-gap and well-spaced regions
GB2239715B (en) * 1990-01-06 1994-04-27 Plessey Co Plc Integrated optical device
EP0606377A1 (en) * 1991-10-03 1994-07-20 Foster-Miller, Inc. Optical fiber for spectroscopic monitoring
EP0606377A4 (en) * 1991-10-03 1994-08-10 Foster Miller Inc Optical fiber for spectroscopic monitoring.
CN114114546A (en) * 2021-11-22 2022-03-01 曲阜师范大学 All-fiber polarizer based on mixed plasmon waveguide structure
CN114114546B (en) * 2021-11-22 2023-12-19 曲阜师范大学 All-fiber polarizer based on hybrid plasmon waveguide structure

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AU2521088A (en) 1989-04-18
GB8722338D0 (en) 1987-10-28

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