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
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/105—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type having optical polarisation effects

Definitions

• This invention concerns optical fibres. More particularly, it concerns optical fibres comprising an inner glass core having a first refractive index and a glass cladding surrounding the core, the cladding having a second refractive index, with the core and the cladding having dimensions which permit the transmission through the fibre of a single mode of propagation of electromagnetic radiation in the form of light.
• glass is used in a sense to encompass a wide range of materials that are usually termed “glass” in this field, including silica and doped silicas, and "light” includes radiation in the ultraviolet and near infrared frequency bands.
• Optical fibres per se are well known. They are currently used to transmit pulses of light to convey information in communications systems, and they are also used in a wide variety of instruments. They may be constructed in one of several ways. One construction has the optical fibre as a very small bore hollow glass tube filled with a very pure liquid. In another construction, the optical fibre is constructed entirely of glass with the refractive index varying progressively from the centre of the fibre to its outer surface. The most common type of optical fibre currently in use, however, consists of a core of glass having a constant, high refractive index surrounded by a sheath or cladding of glass having a constant, lower refractive index. This is called a "step profile fibre". For convenience, in this specification we will illustrate the invention for step profile fibres but it applies to other types of fibres as well.
• Their fibre production technique uses a complex procedure which involves assymetrical doping of a silica tube which is to be the preform of the fibre, followed by selective hightemperature etching with a fluorine-liberated gas, then collapsing the tube to form the solid optical fibre preform.
• the main objective of the present invention is to produce a single-mode optical fibre that will propagate only one state of polarised light. This objective is achieved by constructing the fibre so that the unwanted linearly polarised mode of propagation is "leaked" from the fibre. If the leakage of the unwanted mode is very rapid, the fibre effectively will not support this mode at all. If the leakage is less rapid, the fibre becomes an attenuator of the unwanted mode of propagation, and in practice (for example, when a relatively long length of fibre is used in, for instance, communication systems) the unwanted mode is eliminated from the fibre.
• the present inventors have established that such fibres can be realised by fibres which have a glass core surrounded by a glass cladding, with anisotropy introduced into the fibres.
• the anisotropy may be induced, for example, by the application of transverse stress to the fibre preform. If the anisotropy of the fibre is appropriately chosen, one of the two orthogonally polarised fundamental modes will effectively not propagate.
• the present invention may be realised by either of two alternative approaches. Let x and y denote the two orthogonal directions of polarisation of light in the fibre. For convenience, we assume, for the purposes of this specification, that the fibre is to propagate the fundamental x-polarised mode and eliminate the fundamental y-polarised mode.
• the first approach to produce the present invention requires the anisotropy induced in the fibre to be such that the refractive index profile for xpolarised light is guiding and the refractive index for y-polarised light is effectively constant throughout the fibre cross-section. (That is, in the case of a step-profile fibre, the refractive indices for ypolarised light in the core and cladding are substantially the same.) In this case the fibre cannot support the y-polarised fundamental mode at all.
• the second approach which realises the present invention requires the induced anisotropy in the fibre to be such that the refractive index profile for xpolarised light is guiding and the propagation constant for the fundamental y-polarised mode, ⁇ y , satisfies the relationship ⁇ y ⁇ k.n x2 .
• the wavenumber k is 2 ⁇ / ⁇ and A is the wavelength of light transmitted in the fibre.
• an optical fibre comprises a single-mode optical fibre with anisotropy induced therein, characterised in that the anisotropy is such that one mode of linearly polarised light effectively leaks from the fibre.
• the present invention can be defined as an optical fibre comprising: a) a core of a first glass having a first refractive index, n 1 , when in an unstressed state; and b) a cladding of a second glass having a second refractive index, n 2 , when in an unstressed state; characterised in that said core and cladding are stressed so that the first refractive index, n 1 , becomes n x1 for x-polarised light and n y1 for ypolarised light, and the second refractive index, n 2 , becomes n x2 for x-polarised light and n y2 for ypolarised light; and one of the following sets of relationships is satisfied:
• V y [2 ⁇ / ⁇ ].p.n y1 .[2 ⁇ y ] 1 ⁇ 2 , and ⁇ x and V x are defined correspondingly.
• W is the wavelength of light being transmitted and p is, the diameter of the fibre core.
• the fabrication technique for such fibres will involve the application of transverse stress to the preform of the fibre. This stress is preserved in the fibre as it is drawn from the preform.
• the y-polarised light either is not propagated at all in the fibre or otherwise leaks from the stressed fibre, thus effectively eliminating the y-polarised fundamental mode in a sufficiently long length of fibre.
• Figure 1 is a schematic representation of the end of a step profile type of optical fibre
• Figure 2 is a graph showing the variation of the refractive index, n, with the radial distance from the centre of the fibre of Figure 1, after a transverse stress has been applied to the fibre.
• the refractive index now depends on the state of polarisation of the light;
• Figure 3 is a graph showing the variation of the refractive index, n, for a step-profile fibre constructed in accordance with the first approach noted above ;
• Figure 4 is a graph showing the variation of the refractive index, n, for a step-profile fibre constructed in accordance with the second approach noted above.
• the present invention will be illustrated with reference to its realisation in a step-profile optical fibre of circular symmetry.
• the present inventors emphasise that this invention does not require a particular fibre geometry or refractive index profile.
• the invention allows for great flexibility in this area and the inventive concept may be embodied in a fibre constructed as simply as a circularly symmetric step-profile fibre (which has been chosen, for simplicity, to illustrate the invention in this specification) or as complex as a "bow-tie fibre".
• the important feature of the invention is that sufficient anisotropy is introduced into the optical fibre to cause one mode of polarised light to leak from the fibre. There is no limitation on the particular way by which the anisotropy is introduced into the optical fibre.
• a circularly symmetric step-profile optical fibre comprises a glass core 10 of circular cross-section surrounded by a cladding 11 of a different kind of glass.
• the cladding has an annular cross-section.
• the actual fibre geometry is not essential to the present invention.
• the refractive index of the core is higher than the refractive index of the cladding material, and the boundary between the core 10 and cladding 11 is continuous.
• the diameter of the core of the fibre, p ranges from 3 to 20 micrometres.
• the stress applied to the optical fibre has resulted in the refractive index for y-polarised light in the core being the same as the refractive index for y-polarised light in the cladding.
• the fibre cannot support any y-polarised bound modes, and the ypolarised light will rapidly leak from the fibre, even when a short length of fibre is used to transmit light. Very rapid leakage of the y-polarised light also occurs when n y2 n y1 and
• Figure 4 illustrates the refractive index profile of a circularly symmetric anisotropic step-profile fibre.
• the refractive index profile depends on the state of polarisation of the light.
• the modes of the fibre are not completely linearly polarised.
• the "x-polarised mode” has also a small y-polarised component and thus "sees” something of the refractive index profile for the y-polarised light and similarly the "y-polarised mode” "sees” something of the refractive index profile for x-polarised light.
• fibres were constructed and transversely stressed using the "bowtie" stressing technique described in the aforementioned paper by R D Birch, D N Payne and M P Varnham.
• the fibres were constructed to have a core of germano-silicate glass surrounded by a cladding of fluoro-phosphorous silicate glass.
• the cladding was integrally bound to an outer region of the same glass as the cladding, but containing zones of borosilicate glass.
• test fibres exhibited both low loss and an extinction ratio of 50 dB/km.
• the present invention provides an optical fibre in which the y-polarised mode of light transmission is effectively eliminated from the fibre, thus permitting true single-mode, single-polarisation light transmission by the fibre.
• Optical fibres which transmit only one mode of linearly polarised light are used in a wide variety of optical fibre applications. They are especially useful in coherent detection systems and in optical fibre interferometers.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
• Optical Couplings Of Light Guides (AREA)

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• 1984
  • 1984-02-24 WO PCT/AU1984/000024 patent/WO1984003362A1/en not_active Application Discontinuation
  • 1984-02-24 EP EP19840900837 patent/EP0136306A4/de not_active Withdrawn
  • 1984-02-24 JP JP50095884A patent/JPS60501183A/ja active Pending

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