EP0646251A1 - Coupleur en etoile de guide d'ondes utilisant une interference multimode - Google Patents

Coupleur en etoile de guide d'ondes utilisant une interference multimode

Info

Publication number
EP0646251A1
EP0646251A1 EP93910213A EP93910213A EP0646251A1 EP 0646251 A1 EP0646251 A1 EP 0646251A1 EP 93910213 A EP93910213 A EP 93910213A EP 93910213 A EP93910213 A EP 93910213A EP 0646251 A1 EP0646251 A1 EP 0646251A1
Authority
EP
European Patent Office
Prior art keywords
waveguide
waveguides
subsidiary
multimode
radiation
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.)
Withdrawn
Application number
EP93910213A
Other languages
German (de)
English (en)
Inventor
Richard Michael Dra Malvern Jenkins
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.)
UK Secretary of State for Defence
Original Assignee
UK Secretary of State for Defence
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 UK Secretary of State for Defence filed Critical UK Secretary of State for Defence
Publication of EP0646251A1 publication Critical patent/EP0646251A1/fr
Withdrawn legal-status Critical Current

Links

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
    • G02B6/2804—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
    • G02B6/2817—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 reflective elements 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
    • G02B6/2804—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
    • G02B6/2808—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 a mixing element which evenly distributes an input signal over a number of outputs
    • G02B6/2813—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 a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging

Definitions

  • This invention relates to a radiation coupling device for coupling a single input to a plurality of outputs.
  • the present invention provides a radiation coupling device incorporating a lrtulti ode waveguide, and wherein:
  • a plurality of subsidiary waveguides arranged for fundamental mode operation are connected to the multimode waveguide, (2) reflecting means are arranged to return to the subsidiary waveguides radiation received by the multimode waveguide therefrom, and
  • the relative dimensions and positioning of the subsidiary and multimode waveguides and the reflecting means are arranged to provide for input radiation propagating as a fundamental mode of any one of the subsidiary waveguides to undergo modal dispersion within the multimode waveguide and thereafter to excite the fundamental mode of each of the subsidiary waveguides after return from the reflecting means.
  • the invention provides the advantage that it lends itself to construction in compact form with relatively efficient radiation intensity coupling. It may be implemented in differing waveguide media, such as hollow waveguides suitable for coupling to fibre optics and solid waveguides suitable for optical micro-circuitry.
  • the subsidiary waveguides are of square cross-section
  • the multimode waveguide is of rectangular cross- section
  • the reflecting means is retroreflecting
  • the subsidiary waveguides are ported centrally to respective like subdivisions of the multimode waveguide's transverse cross-section
  • the path length of radiation within the multimode waveguide is 8b ⁇ / ⁇ K, where K is the number of subsidiary waveguides, b is half the transverse width of the multimode waveguide and ⁇ is the radiation wavelength within the multimode waveguide.
  • the subsidiary waveguides may be arranged for connection to respective fibre optic waveguides.
  • the subsidiary and multimode waveguides may be hollow cored with alumina ceramic waveguide walls.
  • the subsidiary and multimode waveguides may alternatively be ridge waveguides of semiconductor material.
  • Figure 1 is a schematic sectional plan view of a coupling device of the invention
  • Figures 2 and 3 are sectional side views on lines II-II and III-III respectively in Figure 1 ;
  • Figure 4 illustrates transverse electric field intensity distributions at a number of longitudinal positions in a multimode rectangular waveguide
  • Figure 5 illustrates the variation of radiation power coupling to waveguide modes as a function of the aspect ratio of a multimode rectangular waveguide
  • Figure 6 provides perspective views of waveguide modes
  • Figure 7 illustrates the variation in modal amplitudes in a multimode rectangular waveguide as a function of input waveguide displacement from a coaxial location
  • Figures 8, 9 and 10 are schematic sectional views of a further device of the invention.
  • FIG 1 there is. shown in a plan a central horizontal section of a radiation coupling device of the invention indicated generally by 10. Vertical sections on lines II-II and III-III in Figure 1 are shown in Figures 2 and 3 respectively.
  • the coupling device 10 is formed from three parallel-surfaced sheets of alumina ceramic material, these being a base sheet 12, a central sheet 14 and a cover sheet 16 indicated between chain lines in Figures 2 and 3.
  • the central sheet 14 is slotted by milling through its thickness, which defines four input waveguides 18a to 18d and a beamsplitter waveguide 20.
  • the input waveguides will be referred to collectively as 18.
  • the waveguides 18 and 20 have side walls (not shown) defined by flat surfaces formed in the milling of the central sheet 1 . They have upper and lower walls (not shown) provided respectively by a lower surface 16' of the cover sheet 16 and an upper surface 12' of the base sheet 12.
  • the beamsplitter waveguide 20 is of rectangular cross-section, being of length , width 2b and height 2a as indicated by scales 30.
  • L, a and b are parameters which may vary between different embodiments of the invention.
  • b 8a.
  • the input waveguides 18 are of square section with side 2a.
  • the length L of the beamsplitter waveguide 20 is given by:-
  • the input waveguides 18a to 18d have inserted within them respective fibre optic waveguides 22a to 22d referred to collectively as 22.
  • the beamspli ⁇ r._r waveguide 20 is connected to a retroreflecting mirror 24.
  • the beamsplitter waveguide 20 has a central longitudinal axis indicated by a dotted line 26.
  • the input waveguides 18a to 18d have respective, central longitudinal axes 28a to 28d parallel to and coplanar with the beamsplitter waveguide axis 26.
  • the axes 28a to 28d are referred to collectively as 28. They are located centrally of respective quarters of the transverse cross-section of the beamsplitter waveguide 20, as indicated by an uppermost scale 30 in Figure 1.
  • the uppermost scale 30 is calibrated for the width (2b) of the beamsplitter waveguide 20. It has a zero position on the beamsplitter waveguide axis 26.
  • Scale positions -3b/4, -b/4, +b/4 and +3b/4 locate the input waveguide axes 28a to 28d respectively. These positions are located centrally of beamsplitter waveguide quarters; these quarters are defined by scale intervals -b to -b/2, -b/2 to 0, 0 to +b/2 and +b/2 to +b respectively of which -b/2 and +b/2 are not shown.
  • the axes 28 are therefore located periodically (in the spatial sense) across the transverse cross-section of the beamsplitter waveguide 20.
  • the input waveguides 18a to 18d have respective entrance and exit apertures (not referenced) located in planes orthogonal to the axis 26.
  • y and z Cartesian co-ordinate axes are shown at 32.
  • the z axis is the device's longitudinal axis 26.
  • the x and y axes are transverse vertical and transverse horizontal respectively; of these, the x axis is not shown at 32 as it is perpendicular to the plane of the drawing.
  • Figure 4 provides graphs of transverse electric field intensity distributions calculated for a reference waveguide (not shown) .
  • This waveguide is 8L in length, eight times that of the beamsplitter waveguide 20, but it has the same transverse cross-section as the latter.
  • an intensity distribution curve 74 indicates initial conditions at one end of the reference waveguide.
  • the maximum 74a is equivalent to radiation propagating as a fundamental mode (a half-cycle of a sine wave) of one of the input waveguides 18, and is located at a position corresponding to that of the input waveguide 18a.
  • the curve 74 is zero.
  • the maximum 74a is of constant optical phase. It is treated as an input excitation of the reference waveguide.
  • the former produces multimode excitation of the latter.
  • the reference waveguide modes which are excited have different propagation constants in the longitudinal z direction. In consequence, their phase relationships with respect to one another vary with z.
  • the phases of maxima 78a to 78d are ⁇ n /A , re, 0, " ⁇ /4 respectively.
  • the transverse intensity distribution is shown by a curve 80 having two maxima 80a and 80b centered at y value -3b/4 and +3b/4.
  • the maxima 80a and 80b are not of like phase.
  • the distribution is shown by a curve 82 having four maxima 82a to 82d. Between adjacent maxima on the curve 82, the intensity is zero.
  • the maxima 82a to 82d are located in the y dimension exactly as maxima 78a to 78d respectively.
  • the radiation phase variations along the curves 78 and 82 differ however.
  • radiation is input from a coherent source (not shown) along one of the fibre optic waveguides 22a to 22d, from which the radiation passes to a respective one of the input waveguides 18a to 18d associated therewith.
  • the radiation is arranged to excite the fundamental mode of the chosen input waveguide.
  • the input waveguides 18 may be designed to support only the fundamental mode of radiation propagation. Alternatively, the input waveguides 18 may be capable of supporting higher order modes of radiation propagation, in which case the input radiation is arranged to excite only the fundamental mode. Radiation propagates along the chosen input waveguide (eg 18a) until it reaches the beamsplitter waveguide 20.
  • the apertures referred to are in the plane 34 where the input and beamsplitter waveguides 18 and 20 merge together, ie where the input waveguides 18 are connected or ported to the beamsplitter waveguide 20. All four input waveguides 18 therefore receive retroreflected radiation, and couple it to respective fibre optic waveguides 22. In consequence, each of the fibre optic waveguides 22 receives input of radiation corresponding to a respective one of the maxima 78a to 78d. This shows that input radiation to any one of the fibre optic waveguides 22 is returned to all four of these waveguides.
  • the device 10 therefore acts as a star coupler.
  • the device 10 is suitable for use with C0 2 laser radiation for which ⁇ Q is 10.59 microns.
  • the length of the beamsplitter waveguide 20 is therefore 425 mm.
  • the lengths of the input and fibre optic waveguides 18 and 22 do not affect the operation.of the device 10 to any appreciable extent (ignoring imperfections).
  • the theoretical propagation characteristics of a rectangular waveguide (such as the beamsplitter waveguide 20) will now be analysed. It is assumed that this waveguide has height 2a, width 2b and is bounded by walls of a homogeneous dielectric material with complex dielectric constant ⁇ . It is also assumed that these walls are highly reflecting, and do not attenuate propagating waveguide modes significantly.
  • the waveguide has height, width and length dimensions which are parallel to the x, y and z axes respectively. It has normalized linearly polarised modes of the kind EH ⁇ .
  • m is the mode number relating to the field dependency along the x axis
  • n is the mode number relating to the field dependency along the y axis
  • phase coefficient ⁇ mn is given by
  • Equation (6.1) Equation (6.1)
  • ⁇ is the wavelength of the radiation propagating in the waveguide.
  • EH11 ⁇ A,mn .EHmn (7)
  • the A mn amplitude coupling coefficients are the coefficients of a Fourier series which represents the electric field at an input aperture where the relevant input waveguide 18 merges into the beamsplitter waveguide 20.
  • the EHmT , modes are mutually orthogonal, and in consequence the coefficients A mn can be calculated from overlap integrals of the form:
  • Equations (5) to (8) it is possible to calculate how the amplitude + coefficients of the excited rectangular waveguide modes vary as a function of b/a.
  • the ratio b/a is that of the widths of the central and input waveguides.
  • Figure 5 illustrates the variation of lAmnl 2 with b/a. This shows the effect on power coupling of varying the beamsplitter waveguide width to height ratio.
  • Figure 5 illustrates modal power coupling to the beamsplitter waveguide 20 which would occur from an input waveguide located coaxially about the device axis 26.
  • A, C and E are symmetric modes and B, D and F are antisymmetric modes.
  • E(y) and E(-y) be the equivalents for the y axis.
  • an input waveguide 18 connected at a periodic position with respect to the beamsplitter waveguide's transverse (y) dimension produces a number of periodically located maxima after some propagation distance.
  • the waveguides 18 have axes 28 located centrally of respective quarters of the beamsplitter waveguide 20.
  • Each input waveguide is capable of producing four maxima 78a to 78d.
  • an input waveguide coaxial with the centre of such a subdivision and offset from the axis 38 would produce N periodically spaced maxima distant 8L/N along a reference waveguide as described in relation to Figure 3.
  • the device 10 may be adapted for any number of inputs. Where they merge into the beamsplitter waveguide, these inputs must have waveguide axes or centres at spatially periodic locations offset from the beamsplitter waveguide axis.
  • the location periodicity or corresponding notional number of subdivisions N sets the required length of the beamsplitter waveguide, which is 4L/N.
  • the invention offers the advantage that for coupling devices requiring a relatively low degree of splitting, ie a low value of N, the number of modes that the beamsplitter waveguide 20 must support is small.
  • the modes required are in the region of the 2N lowest order modes.
  • solid semiconductor material waveguides may also be employed.
  • Nd-YAG laser radiation is suitable for use with ridge waveguides of the ternary semiconductor material system Al ⁇ Ga ⁇ _ ⁇ As.
  • Metal microwave waveguides may also be used.
  • Figure 9 is a horizontal section of the device 100
  • Figures 8 and 10 are vertical sections on lines VIII-VIII and X-X respectively.
  • the device 100 is equivalent to that described earlier, except that it is implemented as a ridge waveguide structure suitable for construction by semiconductor lithography techniques. Description of the device 100 will be restricted to aspects where it differs from the device 10.
  • the device 100 comprises a GaAs substrate 112 surmounted by a multilayer waveguide structure 114 shown as a single layer for convenience.
  • the structure 114 incorporates a plurality of semiconductor layers (not shown) of the Al ⁇ Ga ⁇ _ ⁇ As system. It is configured to form four input waveguides 118a to 118d and a beamsplitter waveguide 120.
  • the input waveguides are of square cross-section with side 2a, and the beamsplitter waveguide is of length L, width 2b and height 2a.
  • the length L obeys Equation (1), in which the refractive index n is equal to the value in an Al ⁇ Ga ⁇ _ ⁇ As waveguide core layer of appropriate value of x.
  • the free space wavelength ⁇ Q of operation may be that of an Nd-YAG laser, ie 1.06 ⁇ m.
  • the device 100 incorporates a retroreflecting end mirror 124 connected to the beamsplitter waveguide 120.
  • the device 100 operates as described earlier, except that the input waveguides 118a to 118d do not communicate with fibre optic waveguides. Instead it is envisaged that they would extend directly to other parts of an integrated optic circuit (not shown) .
  • ⁇ is the radiation wavelength within the reference waveguide.
  • the waveguide cross-section dimensions b/a will be given by equating L j and L ⁇ and taking the square root as follows:
  • the required length of beamsplitter waveguide is one half of the reference wavelength length of L ⁇ or j as defined above.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

Un dispositif (10) de couplage de rayonnement comprend un guide d'ondes diviseur de faisceau, multimode et rectangulaire (20), connecté, au niveau d'une extrémité, à un ensemble de guides d'ondes d'entrée (18). Des guides d'ondes respectifs (22) en fibre optique sont insérés dans les guides d'ondes d'entrée (18). Ces derniers sont connectés selon les positions périodiques à travers la section transversale du guide d'ondes diviseur de faisceau. Ce dernier (20) est également connecté, au niveau de son autre extrémité, à un miroir rétroréfléchissant (24). Un rayonnement se propageant en mode fondamental dans n'importe lequel des guides d'ondes d'entrée (18) passe le long du guide d'onde diviseur de faisceau (20), et est rétroréfléchi par le miroir (24). En retournant, il est divisé entre les guides d'ondes d'entrée (18) en raison de la dispersion modale dans le guide d'ondes diviseur de faisceau (20). Le dispositif (10) agit ainsi comme un coupleur en étoile.
EP93910213A 1992-06-16 1993-05-17 Coupleur en etoile de guide d'ondes utilisant une interference multimode Withdrawn EP0646251A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GB929212727A GB9212727D0 (en) 1992-06-16 1992-06-16 Radiation coupling device
GB9212727 1992-06-16
PCT/GB1993/001005 WO1993025923A1 (fr) 1992-06-16 1993-05-17 Coupleur en etoile de guide d'ondes utilisant une interference multimode

Publications (1)

Publication Number Publication Date
EP0646251A1 true EP0646251A1 (fr) 1995-04-05

Family

ID=10717158

Family Applications (1)

Application Number Title Priority Date Filing Date
EP93910213A Withdrawn EP0646251A1 (fr) 1992-06-16 1993-05-17 Coupleur en etoile de guide d'ondes utilisant une interference multimode

Country Status (3)

Country Link
EP (1) EP0646251A1 (fr)
GB (1) GB9212727D0 (fr)
WO (1) WO1993025923A1 (fr)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3425150B2 (ja) * 1994-02-11 2003-07-07 コーニンクレッカ、フィリップス、エレクトロニクス、エヌ．ヴィ． 位相調整アレイを有する光学装置
GB2344692A (en) 1998-12-11 2000-06-14 Bookham Technology Ltd Optical amplifier
SE523638C2 (sv) * 2001-09-28 2004-05-04 Ericsson Telefon Ab L M Omkopplare baserad på flermodsinterferensvågledare
GB0201950D0 (en) * 2002-01-29 2002-03-13 Qinetiq Ltd Multimode interference optical waveguide device
GB0201969D0 (en) 2002-01-29 2002-03-13 Qinetiq Ltd Integrated optics devices
ITMI20020655A1 (it) * 2002-03-28 2003-09-29 Castelli Clino Trini Pannello luminoso a doppia faccia avente uniformita' di illuminazione

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2285623A1 (fr) * 1974-09-20 1976-04-16 Max Planck Gesellschaft Dispositif auto-formateur d'images, comportant un guide d'ondes
GB2220764B (en) * 1988-07-15 1992-02-19 Stc Plc Single mode couplers

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9325923A1 *

Also Published As

Publication number Publication date
WO1993025923A1 (fr) 1993-12-23
GB9212727D0 (en) 1992-07-29

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