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
  • G02B6/28—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
  • G02B6/29304—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 diffraction, e.g. grating
  • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
  • G02B6/29317—Light guides of the optical fibre type
• • 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
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
  • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
  • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
  • G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
• • 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
  • G02B6/29304—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 diffraction, e.g. grating
  • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
  • G02B6/29323—Coupling to or out of the diffractive element through the lateral surface of the light guide
• • 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
  • G02B6/29335—Evanescent coupling to a resonator cavity, i.e. between a waveguide mode and a resonant mode of the cavity
  • G02B6/29337—Cavities of the linear kind, e.g. formed by reflectors at ends of a light guide
• • 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
  • G02B6/42—Coupling light guides with opto-electronic elements
  • G02B6/4201—Packages, e.g. shape, construction, internal or external details
  • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
  • G02B6/4215—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers

Definitions

• the present invention relates to a method and a device for coupling light to or from an optical waveguide .
• Light guiding in optical waveguides, and light guiding in optical fibres in particular, is a well-known technology for transporting energy and information in the form of light.
• one-dimensional optical waveguides are based on light guiding in a medium of cylindrical symmetry.
• the light guiding takes place in a core, which is surrounded by a medium having a lower refractive index, the so-call- ed cladding, light guiding according to a simple model being obtained by means of repeated total internal reflections between the core and the cladding.
• the light can only propagate in certain predetermined directions, so-called modes, which are defined by certain phase conditions which must be met in connection with the propagation of the light.
• these modes consist of eigensolutions to Maxwell's equations applying existing cylindrical boundary conditions . If the cross-sectional dimension of the core is sufficiently small, the light can only propagate in a single such mode.
• An optical waveguide with this characteristic is called an optical monomode waveguide.
• Monomode waveguides have certain important advantages over a waveguide permitting several modes (multimode waveguide) . For example, the information transfer capacity of an optical monomode fibre, often called an optical single-mode fibre, is much greater than that of a multimode fibre when light is guided through a long fibre. Another important advantage of a monomode waveguide such as a single-mode fibre is its lack of ambiguity.
• the characteristics of the light will be well-defined along the entire waveguide.
• the intensity distribution of the light will be well-defined along the entire waveguide. This is extremely important in order to provide predictable operation of waveguide-based components.
• a detailed description of the characteristics of the single-mode fibre is provided in, for example, L.B. Jeun Subscribe "Single-mode fiber optics: Principles and applications", Marcel Dekker, New York (1990) .
• WDM wavelength- multiplexed transfer
• optical phase grating is a structure of essentially periodically varying refractive index in an optically transparent medium.
• a review of the technology is provided in, for example, M. C. Hutley "Diffraction gratings", Academic Press, London (1982) .
• a small part of the incident light is reflected by each grating element (period) .
• a plurality of grating elements are arranged in succession (i.e. arranged in a phase grating) the total amount of reflected light will be the sum of all of these separate reflections.
• each grating element depends on the depth (amplitude) of the refractive index modulation of the phase grating, i.e. on the refractive index difference of the grating elements. The greater the modulation the greater the part of the inci- dent light that is reflected by each phase element. If the propagation direction of the light which is incident upon a phase grating is essentially perpendicular to the grating, i.e. to the normal of the grating elements, the grating is said to be operating in the Bragg domain and is called a Bragg grating. As a result of the perpendicular incidence the light will be reflected essentially parallel to the direction of incidence (i.e.
• each grating element will thus overlap the light reflected by all the other grating elements, thus giving rise to interference.
• all reflections within a certain angle cone will couple to the only mode (propagation direction) permitted by the waveguide.
• constructive interference arises, and despite the fact that each grating element only provides a low intensity reflection, substantial reflection will be obtained for this wavelength from the grating as a whole.
• This wavelength, at which a substan- tial reflection is obtained from the grating as a whole, is called the Bragg wavelength ⁇ bra gg and is given (in connection with perpendicular incidence) by
• n is the average value of the refractive index and ⁇ is the period of the phase grating.
• the reflectance for the Bragg wavelength is given by
• Rbragg tanh 2 KL where L is the length of the Bragg grating in the propagation direction of the light and K is defined as
• ⁇ n is the amplitude of the refractive index modulation. Since the refractive index modulation ⁇ n typically is small (10 ⁇ 5 - 10 ⁇ 3 ) , the above expression of the reflectance can be expanded into a power series, whereby it can be seen that the reflectance is approximately proportional to the square of ⁇ n.
• a method for providing a phase grating in an optical waveguide is known from, for example, US-4,725,110 (Glenn et al) . According to this method a waveguide is illuminated by ultraviolet light through an interferometer, resulting in periodic exposure of the waveguide, which gives rise to a periodic alteration of the refractive index in the waveguide.
• This refractive index alteration remains in the waveguide subsequent to the exposure.
• the period can be chosen so that the desired Bragg wavelength is obtained.
• the angles of incidence of the interfering, ultraviolet light rays are usually chosen to be symmetrically arranged relative to the axis of propagation of the waveguide in order to provide grating elements whose planes are oriented essen- tially at right angles to the propagation axis of the waveguide, the grating thus operating in the Bragg domain.
• This technology has been found to be most effective for waveguides in which the waveguiding structure is composed of germanium silicate, i.e. where the waveguiding structure is composed of quartz to which a certain amount of germanium has been added.
• the angle at which the light will be coupled from the waveguide is determined by the angle of_inclination of the grating elements in relation to the propagation axis of the waveguide (the transverse phase matching condition) as well as by the wavelength (the longitudinal phase matching condition) . See, for example, R. Kashyap. "Fiber Bragg Gratings",
• the tilted grating elements function as small, almost completely transparent, mirrors.
• the diameter of the mirrors (grating elements) is essentially equal to the diameter of the waveguiding structure.
• the waveguiding structure is composed of the core of the fibre, which usually has a diameter of about 10 micrometers. Since this diameter is not much greater than the wavelength of the light, the mirrors (grating elements) will cause diffraction of the reflected light. Consequently, the reflected light will spread out in a cone around the angle defined by the angle of inclination of the grating elements.
• the transverse phase matching condition gives that this angle is about twice as large as the angle of inclination.
• the grating elements reflect light which is partially overlapping, a certain wavelength will only give rise to constructive interference if the light from each consecutive grating element is in phase with the light from the preceding grating element. This occurs at a certain predetermined angle, which is given by the longitudinal phase matching condition
• N e f f and n olad are the refractive indices of the waveguiding structure (core) and the substrate (cladding) respectively, the substrate being assumed, in the above expression, to have an infinite extension, ⁇ - ⁇ being the output-coupling angle in the cladding, and ⁇ g being the angle of inclination.
• ⁇ - ⁇ being the output-coupling angle in the cladding
• ⁇ g being the angle of inclination.
• chirp frequency sweep
• a prism having the same refractive index as the cladding of a fibre can be utilised, the prism being brought into optical contact with the fibre with the aid of a contacting liquid.
• This technology permits angles of inclination of less than 15° and thereby avoids the above drawbacks to some extent.
• the prism is also used for spatial separation of output-coupled wavelengths with the aid of the dispersion of the prism.
• this output -coupling has some remaining drawbacks. Firstly, the resolution of wavelengths is limited by the fact that the chirp function only serves its intended purpose for a certain wavelength. Secondly, the limited length of the chirped grating causes significant diffraction in connection with small angles of inclination. Thirdly, the coupling efficiency will be different for different wavelengths.
• the main object of the present invention is to improve the possibilities of coupling light to or from optical waveguides. This object is achieved by the use of a device and a method for light coupling of the kind stated in the appended claims.
• a specific object of the present invention is to provide a device for wavelength-selective light coupling to or from an optical waveguide, which has a spectral resolution that is substantially higher than that enabled by the prior art .
• Another object of the invention is to provide a device for light coupling to or from an optical waveguide, enabling, in relation to the prior art, weaker and more precise coupling mechanisms while maintaining coupling efficiency, so that, for example, a signal having a plurality of wavelength components which propagate in an optical waveguide can be analysed without affecting the signal as a whole to any significant extent.
• a further object of the invention is to provide a device for light coupling to or from an optical waveguide which is easy to produce and which is mechanically sturdy.
• the invention is based on the insight that a resonance in an optical waveguide provides improved possi- bilities of coupling light in connection with the waveguide. Since a specific wavelength component is resonant in a specific portion of the waveguide not only does one obtain more efficient coupling of the resonant wavelength component, but it is also separated spatially from the other wavelength components by means of a concentration (local power density increase) in the resonant portion. An alternative way of describing this is that the coupling strength of a certain wavelength component, when this wavelength component is coupled to or from the waveguide, increases significantly at the portion of the waveguide which is resonant to this wavelength component.
• Wavelength components of light propagating in a waveguide can be separated spatially by the provision of a number of resonant portions in the waveguide, each portion being resonant to a specific wavelength component of the light.
• the resonance increases the coupling strength of a specific wavelength component in the corresponding resonance portion.
• Wavelength selectivity is thus achieved by the spatial separation of said resonance portions, as well as by the increased coupling efficiency of the respective wavelength components at the corresponding resonance portion. Coupling of specific wavelength components to or from the waveguide can thereby take place very advantageously at said resonant portions.
• the present invention enables output -coupling of a specific wavelength compo- nent from an optical waveguide, in which a plurality of wavelength components are propagating, without any significant impact on the wavelength components which are not being coupled.
• the wavelength-specific, local resonances in the waveguide will result in a local power density increase of the associated wavelength components, thereby permitting the utilisation of a coupling which is so weak that the impact on wavelength components having the original power density is negligible in most applications.
• the invention enables coupling of light to or from an optical waveguide, where different wavelength components are coupled to or from the waveguide at spatially separate portions.
• the in- vention also enables very smooth coupling of the respective waveguide components to an associated connecting waveguide at the corresponding resonant portion.
• different waveguide components can be coupled to or from an optical waveguide, such as an optical fibre, at different positions along the waveguide.
• a device thus comprises at least one optical waveguide and means for coupling light to or from the optical waveguide and is provided with means for providing a portion in the optical waveguide which is locally resonant to a specific waveguide component.
• said means for light coupling of said wavelength component to or from the waveguiding structure are adapted to couple light at the resonance portion corresponding to said wavelength component.
• the waveguiding structure is a fibre core in an optical fibre, preferably an optical single-mode fibre, in which the local resonance portions are provided by a phase grating which is arranged in the fibre core.
• a phase grating which is arranged in the fibre core.
• the phase grating is preferably a Bragg grating with a monotonically increasing or decreasing period; a so-called chirped Bragg grating. The Bragg wavelength is thus different in different parts of the grating, and, consequently, different wavelength components correspond with the Bragg wavelength at different portions of the grating.
• the wavelength which corresponds with the local Bragg wavelength will exhibit resonance, and thus increased power density, locally by virtue of the fact that the light is at least partially reflected back and forth by the grating in this portion.
• a plurality of spatially separate portions, in which light propagating in the fibre core exhibits resonance to a certain wavelength component forming part of the light, are thereby obtained along the extent of the chirped grating. The deeper the index modulation of the chirped grating, the more the respective wavelength component is concentrated to the corresponding resonant portion.
• FIG. 1 shows output-coupling from an optical fibre with the aid of a tilted phase grating according to the prior art
• Fig. 2 shows output -coupling from an optical fibre with the aid of a chirped, tilted phase grating according to the prior art, for obtaining a focal line of the output -coupled light;
• Fig. 3 shows output-coupling from an optical fibre with the aid of a chirped, tilted phase grating, in which a prism is used for enabling smaller angles of inclination according to the prior art
• Fig. 4 is an outline diagram showing how resonance portions are created for three arbitrarily chosen wavelength components at different portions of an optical waveguide with the aid of a chirped Bragg grating;
• Fig. 5 is an outline diagram showing wavelength selective output -coupling of light from a waveguide according to a first preferred embodiment of the present invention
• Fig. 6 is an outline diagram showing wavelength selective output -coupling of light from a waveguide according to a second preferred embodiment of the present invention
• Fig. 7 is an outline diagram showing wavelength selective output-coupling of light from a waveguide according to a third preferred embodiment of the present invention.
• Fig. 8 is an outline diagram showing wavelength selective output-coupling of light from a waveguide according to a preferred embodiment in which a secondary waveguide is utilised as an intermediary step in connection with the output-coupling;
• Fig. 9 is an outline diagram showing input -coupling of light into a waveguide according to the present inven- tion, in which the grating structure of the waveguide forms part of a light -generating means, for example a laser.
• a light -generating means for example a laser.
• an optical waveguide for example an optical single-mode fibre 1
• a chirped Bragg grating 2 has been manufactured according to the prior art .
• the grating is formed with a monotonically increasing or decreasing period, i.e. it is chirped, dif- ferent Bragg wavelengths are obtained at different portions along the grating. More specifically, the Bragg wavelength increases or decreases monotonically, in accordance with the period of the grating, as a function of the longitudinal position along the grating.
• the light propagating in the waveguide has arbitrarily been assumed to be composed of three wavelength components ⁇ i , ⁇ 2 and ⁇ 3 , which are indicated by reference numerals 11, 12, and 13 in the Figures.
• the grating 1 is chirped different wavelength components 11, 12, 13 will correspond with the Bragg wavelength of the grating at different portions 21, 22, 23 along the grating.
• wavelength component 11 for example, will be reflected by the chirped grating in the area designated by reference numeral 21.
• the reflection is equally efficient in the case of light incident from the opposite direction and, consequently, the reflected light will be reflected again by the grating in said area 21.
• a resonance effect occurs which causes the power density to increase locally for the wavelength component which corresponds with the local Bragg wavelength in said area 21.
• resonances are obtained for the other wavelength components 12, 13 at the resonance portions 22, 23 of the grating corresponding thereto.
• the purpose of providing these resonance portions with increased power density is that light can be coupled from the waveguide in a wavelength-selective manner by means of output-coupling means which are arranged adjacent to (or operatively connected to) the waveguide at the respective portions.
• a major advantage of an optical coupling according to the invention is that the coupling factor can be made so weak that wavelengths which are not resonant (do not have increased power density) are essentially unaffected.
• Fig. 5 shows a first preferred embodiment of the invention according to which said means for coupling light to or from the optical waveguide is composed of a phase grating 3 having grating elements whose planes intersect the propagation axis of the waveguiding structure under an angle which is different from 90 degrees, i.e. a tilted grating.
• This tilted grating is formed in such a way that the output-coupling is negli- gible at the positions and wavelengths where the chirped grating is not resonant. However, in the areas with resonance (with increased power density) 21, 22, 23 efficient coupling is obtained. Since each wavelength component circulates in the respective portion, light will be output -coupled in two directions 31ab, 32ab,
• Output-coupling using titled gratings is polarisation-dependent and, consequently, in this case, output- coupling mainly takes place for one of the two polarisation directions of the light.
• Two tilted gratings can advantageously be arranged in the waveguiding structure, one grating being turned 90 degrees about the propagation axis of the waveguiding structure, output-coupled light being obtained in four lobes located opposite each other in pairs (not shown) . Each opposite pair of lobes thus contains light with the same polarisation.
• Fig. 6 shows a second preferred embodiment of the invention.
• the coupling means comprises a Bragg grating 4 having an index modulation which decreases transversally across the grating.
• the amplitude (modulation depth) is thus lower at one edge 41 of the grating (radially) than at the opposite edge 42.
• this type of grating is referred to as a transversally asymmetrical phase grating. If the transversal modulation depth variation is sufficiently large it will be possible to couple light to and from the waveguide with the aid of the transversally asymmetrical phase grating.
• the chirped Bragg grating 2 i.e. the means for providing local resonances (locally increased power densities) and the transversally asymmetrical phase grating 4 are the same grating, as illustrated in the Figure.
• the above-mentioned tilted grating can also be a transversally asymmetrical phase grating, a less pronounced tilt being required for obtaining light coupling to or from the optical fibre. This reduces the polarisation-dependence of the coupling, which is an advantage in some applications.
• Fig. 7 shows a third preferred embodiment of the present invention.
• the local, wavelength-specific resonances 21, 22, 23 are created by a chirped phase grating 2 arranged in a waveguide 1.
• the optical waveguide is preferably an
• Suitable means 61, 62, 63 for output- or input -coupling of light are suitably arranged adjacent to (or operatively connected to) said secondary waveguiding structure. In the Figure these are shown as means for evanescent coupling, but they can, of course, comprise an arbitrary, suitable means, the above embodiments being examples thereof.
• An important advantage of coupling light to or from a waveguide 1 with the aid of the secondary waveguiding structure 5a as described above is that the coupling has no significant impact on light which is propagating in the main waveguide, except for those wavelength components which are being coupled.
• the evanescent coupling can be made sufficiently weak to ensure that coupling of unintended wavelength components is essentially negligible.
• Another advantage of this embodiment is that the requirement of deep index modulation of said phase grating is not as strict, which, in some cases, is an advantage from a manufacturing point of view.
• FIG. 9 A preferred embodiment for input-coupling of light into an optical waveguide according to the present invention is illustrated in Fig. 9.
• said means for creating local, wavelength-specific resonance portions are represented by a chirped phase grating 2.
• said means for coupling light to or from the optical waveguide are represented by a tilted grating 3.
• the Figure shows three separate light sources 71, 72, 73, lasers for example, which emit three different wavelength components 11, 12, 13 of light. The emitted light is coupled into the waveguide 1 at the resonance portion 21, 22, 23 corresponding to the respective wavelength components.
• some type of focusing optics 81, 82, 83 are used for this input -coupling .
• the respective resonant portions 21, 22, 23 in the waveguide perform the function of one of the cavi- ty mirrors in a laser.
• the above-mentioned light sources 71, 72, 73 comprise a light -generating medium and one of the mirrors of the laser cavity, feedback, and thus laser action, being provided with the aid of the resonance in the waveguide, which resonance serves as a feedback cavity mirror in the laser.
• a major advantage of this embodiment is that the emission wavelength of the laser will be locked to the wavelength to which the corresponding portion of the waveguide is resonant, since sufficient feedback will occur only at this wave- length.
• a separate, external laser can be used, the emitted wavelength of the laser being coupled into the waveguide at a portion which is resonant to this wavelength.
• An alternative way of providing output-coupling of light is to bend said waveguide, whereby a controlled leakage of light from the waveguiding structure is obtained.
• the waveguide component which is coupled from the waveguide at a certain position along the waveguide can then be controlled by varying the bending.
• An optical fibre can, for example, be wound onto a cylinder body, said control being effected by expanding or contracting the cylinder body.
• input -coupling of light by bending the optical fibre is also possible, although this is somewhat more difficult from a technical point of view.
• wavelength components stated above can, but need not, in themselves comprise several discrete wavelengths.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Couplings Of Light Guides (AREA)
• Optical Integrated Circuits (AREA)
• Diffracting Gratings Or Hologram Optical Elements (AREA)

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• 2000
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  • 2000-06-28 WO PCT/SE2000/001373 patent/WO2001002885A1/en active Search and Examination
  • 2000-06-28 CA CA002377493A patent/CA2377493A1/en not_active Abandoned
  • 2000-06-28 EP EP00946665A patent/EP1192493A1/de not_active Withdrawn
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