CA1277164C - Optical conductors - Google Patents

Abstract

OPTICAL CONDUCTORS

Abstract of the Disclosure An optical busbar, for optically interconnecting electronic and/or photonic equipment, comprises a rod of light-transmissive material, for example polycarbonate, having a longitudinal planar surface. Preferably the rod is of polygonal cross-sectional shape. Distributed along its length, opposite the planar surface, the rod has a plurality of inclined reflector surfaces for diverting light travelling along the rod so that it emerges laterally. When the rod is installed on a backplane of the equipment, the inclined reflector surfaces register with a corresponding plurality of circuit cards projecting from the backplane. The optical busbar will, of course, function with the light travelling in the opposite direction, i.e. transmitted from the circuit card and reflected to travel along the length of the busbar.

Description

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OPT I CAL ~ONDUCT~RS
This application relates to Canadian patent application serial number 503,970 by J.M. Hvezda et al. ~iled concurrently herewith.
The invention relates to optical busbars such as are used for making connections within electronic and/or photonic equipment. Photonic equipment uses light instead of electric current, e.g. uses optical communication links.
The increaslng transmission rates in present-day lU computer and telecommunications equipment have led to the use of optical busbars, often called waveguides, for the main traffic highways, which may have to operate at rates of 1 gigabit and more.
In telecommunications equipment, they have been used to interconnect circuit cards which are mounted to extend perpendicular to a backplane. (See, for example, copending patent application serial number 450,219 by A. 6raves, and assigned to the same assignee as this invention.) In such applications, the optical waveguide/busbar comprises an elongate moulding of optically transmissive plastics material.
It is desirable for such optical busbars to be manufactured cheaply in large quantities and readily mountable on backplanes and the like. To this end, one aspect of the invention provides an optical busbar comprising means for defining an elongate waveguide for conveying light in a predetermined direction along its length, the waveguide having a planar surface extending along its length; and a plurality of diverter means spaced from and aligned with each other along the length of the waveguide, each diverter means extending into the waveguide to obscure only part of its cross-' ,.
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sectiona7 area and being arranged to divert light conveyed along the waveguide in said predetermined direction and incident upon thP
diverter means transversely to the length of the waveguide to emerge from the waveguide through the plane of said planar surface.
Such a polygonal rod is relat;vely easy to manufacture, especially when made of plastics material - of which polycarbonate is preferred. Polycarbonate is preferred not only because it allows easy manufacture and has a high melt;ng point, but also because it has a relatively hiyh refractive index, making it easier to Find a coating or cladding material with a lower refractive index. Suitable coating materials include ceramics, for example silicon monoxide and silicon dioxide, and a typical thickness for the coating is about 1 micrometer. Such coating arrangements for waveguides or optical busbars are the subject of copending patent application serial number 522,264, in the name of W. Trumble, assigned to the same assignee as this invention. The reflectors may be metallized inclined surfaces.
Typically, the inclination will be 45 degrees to the longitudinal axis of the rod.
The polygonal form, with the reflected light emerging through a facet, is preferred to the cylindrical because it does not produce cylindrical lens effects. The latter would cause the light beam to spread by different amounts in different mutually perpendicular planes before arriving at the associated detector.
For ease of manufacture, and mounting upon the associated circuit board or backplane, the polygonal form is preferably regular. A square cross-section is especially advantageous since, provided with suitably disposed additional inclined surfaces, it allows light beams to emerge or enter in four mutually B

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perpendicular directions. However, other shapes could be used7 for example, triangular, with the light reflectors being formed by notching one apex so as to redirect light to emerge from the opposing facet.
In preferred embodiments, the reflectors comprise inclined surfaces each formed as an oblique truncation of a cavity of elliptical, especially circular, cross-section, the ellipkical axis preferably extending perpendicular to said facet. Small-diameter circular cavities can be made accurately more easily than other shapes because they can be formed during moulding by means of a mitred circular rod. Precision, small-diameter circular rods are usually available more readily than precision rods of other shapes.
It may be desirable to vary the areas of the reflectors in dependence upon their spacing along the rod. This may be achieved by increasing the diameter of the cavity and/or the depth to which the cavity penetrates the rod so as to alter the area of the inclined face. The last inclined reflector surface may extend completely across the end of the rod, i.e. as by mitring. The reflecting surfaces should be as close to totally reflecting as practicable. To this end, they may be coated with metal, for example, gold or aluminum.
In preferred embodiments of this aspect of the invention, the diverter means, for example reflectors are disposed rectilinearly.
The means for deflning said elongate waveguide may compr;se a rod of optically transmissive material. The rod may be coated with a material having a lower refractive index than that of ~ 3 B

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the rod. For example, the rod may be of plastics material and the coating may be glass.
~ he reflectors may have the various features mentioned earlier in relation to the first aspect of the invention. Moreover, they may be provided in the same side of the opk;cal waveguide or on different sides. Alternatively, refraction could be used instead of reflection.
Support means for supporting said optical conductor upon a backplane or the like may comprise a seating member having a seating to cooperate with said planar surface, an anchorage for securing said seating member to said backplane, and lens means adjacent said seating.
An advantage of this support arrangement is that it facilitates alignment of the individual reflector means with the associated lens(es) and the associated optical element, for example a receiver/transmitter, on the circuit card which is located by the usual pins, which are at a predetermined location relative to the anchorage.
According to another aspect of the invention, there is provided apparatus comprising a backplane, a plurality of circuit cards each associated with an optoelectric device, and an optical busbar, said circuit cards being electrically coupled to said backplane substantially parallel with each other and substantially perpendicular to the backplane, said backplane having seating means for said optical busbar, said optical busbar comprising a rod of optically transmissive material having a planar surface along one side thereof, said optical busbar being mounted to said backplane with said planar surface positively located by said seating means, said rod of .~ ~
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optica11y transmissive material having a row of diverter means along its length, the divert~r means being spaced apart so as to correspond to the aforesaid optoelectrical devices associated with said circuit cards, said diverter means being so disposed relative to said planar surface as to divert light travelling along said rod to emerge laterally through said planar surface at intervals corresponding to the spacing of said diverter means and impinge upon said optoelectrical devices.
An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:-: Figure 1 is a cross-sectional view of apparatus comprising a backplane and a plurality of circuit cards, the latter interconnected optically by way of an optical conductor embodying one aspect of the invention;
Figure 2 is a cross-sectional fragment view on the line AA of Figure l;
: Figure 3 is a perspective schematic view of a light ~-. conductor associated with a set of lenses and transmitters/receivers;
Figures 4, 5 and 6 are plan, side elevation and sectional views, respectively, of the light conductor;
; Figure 7 is a sectional side view of an alternative embodiment in which the reflector associated with the transmitter and the reflectors associated with the receivers are on opposite sides of the optical conductor; and : Figure 8 is a schematic diagram of a set of four optical conductors used to interconnect components on a backplane.

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Referring to Figure 1, a backplane 10, which may be a printed circuit board or other planar member, has a plurality of circuit cards 12, 14, 16 and 18 mounted on its one face so as to extend perpendicular to the backplane 10. The circuit cards 12, 14, 16 and 18 are coupled to the backplane by electrical connectors 20, 22, 24 and 26, respectively, adjacent holes 28, 30, 32 and 34, respectively, which extend through the backplane 10. An optical conductor 36 is mounted on the opposite face of the backplane 10 by a set of seating members 38, 40, 42 and 44 spaced apart along its length, one over each of holes 28, 30, 32 and 34, respectively.
As shown also in Figure 2, each seating member 38, 40, 42 or 44 comprises a block of aluminum having a seatin0 ;n the form of a square aperture 46 to receive and positiYely locate the optical conductor or waveguide 36. The optical conductor 36 has a polygonal, - 15 specifically square, cross-sectional shape and is a close fit in the ~ aperture 46. The facets comprise planar surfaces, at least the : lowermost one of which accurately locates the conductor 36 with its bottom facet parallel to the backplane 10. The base of each seating member 38, 40, 42 or 44 has an anchorage in the form of a set of spigots 48 (see Figure 2) which project beyond the end oP the seating : member to engage in corresponding holes 50 ln the backplane 10. The spigot holes 50 surround the corresponding one of holes 28, 30, 32 and 34 so that each seating member is located over the corresponding one of holes 28, 30, 32 and 34.
A hole or cavity 51 extends between the square aperture 46 and the anchorage end of the seatlng member 42. A lens 52 is supported to extend across the hole 51 between the base of square aperture 46 and the anchorage 48. Each lens 52 is arranged with its ~" .

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optical axis perpendicular to the longitudinal axis of the optical waveguide 36 and aligned, through the hole 28, 30, 32 or 34, with an optical element in the form of a receiver 54, 56 or 58 or an optical transmitter 60 mounted on the corresponding one oF the circuit cards 12, 14, 16 and 18. The optical transmitters may be light-emitting d;odes and the receivers photodiodes. Each LED or photodiode is fitted with a lens 53 corresponding to lens 52 (see Figure 1).
Alternatively, ~nd perhaps preferably, lasers could be used. The LED, photodiode or laser, need not be located immediately adjacent the backplane but could be positioned some distance away, poss;bly not even on the circuit card, and connected by means of another optical conductor or optical fiber, which then constitutes the transmitter or receiver.
As shown in more detail in Figures 3, 4, 5 and 6, the optical conductor 36 has a series of reflector means or taps formed by inclined planar surfaces 62, 64, 66 and 68 aligned with the lenses 52 in seating members 38, 4~, 42 and 44, respectively. The inclined surfaces 62, 64 and 66 are formed as mitred ends of a series of ; circular cavities 70, 72 and 74, respectively (see Figure 5). The 20 final inclined surface 62 is formed by mitring the ends of the rod 36.
Each cavity 70, 72 or 74 is conveniently formed during moulding of the optical waveguide 36 by means of a mould insert in the form of an obliquely truncated round rod which may readily be obtained with the requlred precision. The inclined reflector surfaces 62, 64, 66 and 68 may be coated with metal, for example gold or aluminum, to maximize their reflectance. The inclined surface 68 associated with the transmitter 60 is inclined oppositely to the other inclined surfaces so that light from the transmitter 60 is reflected through ninety . . .
, , ' ~L~7 7~L~4 degrees to travel along the optical waveguide 36 parallel to its longitudinal axis. At each of the "receiver" inclined surfaces 62, 64 and 66 a portion of the light is reflected, again through ninety degrees, to pass through the associated lens 52, the backplane 10, and the receiver's lens 53, to impinge upon the receiver 54, 56 or 58.
The amount of light reflected will depend upon the area of the inclined surFace relative to the cross-sectional area of the rod.
Typically this will be 2-4%.
The inclined surfaces may be made to have a larger area the further they are away from the transmitter 60 in order to maximize the number of taps permitted. Masking or shadowing of one inclined surface by the preceding one has not been found to be a sign;ficant problem. The combinat;on of small tap area, large inter-tap spacing, and multimode transmission serves to ensure that light by-passing one inclined re~lector surface reaches the next.
It may be convenient for the optical conductor to receive a light signal from, say, an optical fiber which is behind the backplane 10. The embodiment of Figure 7 shows a convenient way of coupling such an optical fiber 80 to the optical conductor 36. The latter is similar to the optical conductor shown in Figure 3-6, in that it has a series of reflector surfaces 64, 66 etc. but differs in that the reflector surface 82 arranged to receive light from the optical fiber 80 is on the opposite side of the optical conductor 36, i.e. adiacent the backplane 10. The associated support member 84 has spigots 86 securing it to the backplane 10, and a lens 88 mounted in a hole 90 in the part of the support member 84, that is, on the side away from the backplane 10.

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The optical fiber 80 is terminated in a connector 92 which houses a second lens 94. The connector 92 fits over the end of the support member 84 so that the axes of the lenses 88 and 94 are substantially aligned.
Thus, the light signal can be brought into the cabinet from the rear, i.e. behind the backplane, as is usual. It is Fed into the optlcal conductor 36 via the connector 92, lenses 88, 94 and directed along the optical conductor 36 by the reflector surface 82.
The other reflector surfaces 64, 66 etc. distribute the signal to the circuit cards as described with respect to Figure 1.
In the practical embodiment illustrated in Figure 8, four optical conductors 100, 102, 104 and 106 extend parallel to each other on a backplane-mounted support (not shown). One transmitter and two receivers are mounted on each of four circuit cards 108, 110, 112 and 114, respectively. The transmitters and receivers are connected to optical conductors 102 and 104, respectively. Optical conductor 104 is shown coupled at one end (light can, of course, be launched into these optical conductors through the end~ to a transmitter 116 and is coupled via its reflectors to first ones of the receiver ports of circuit cards 108, 110, 112 and 114. The other receiver ports are coupled laterally to the optical conductor 106, which is coupled at its end to a control/supervisory transmitter 118.
It is preferred for the data signals in the optical conductors 102, 104 and 106 coupled to the circuit card to travel in the same dlrection. This simplifies synchronization. Accordingly, optical conductor 102 ls coupled by a U-bend (actually two 45 deyree ; bends such as disclosed in our copending patent application number 517,834 by D.A. Kahn) to the fourth optical conductor 100, which ' ': ' ' . .
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In either the embodiment of Figure 1 or the embodiment of F;gure 7, it may be preferable for the inclined surface (68 or 82) - 5 which receives light from the transmitter to be larger than usual, for example the whole of the oblique cross-sectional area of the conductor.
Various modifications of the specific embodiments are possible without departing from the scope of the invention. For example, the rod may be of other polygonal shapes, such as hexagonal, triangular or octagonal, and the inclined reflector surfaces may be provided in the same surface as that from which the light emerges.
In preferred embodiments of the invention, the light source employs multimodal excitation, specifically with a range of angles of internal light rays of about 10 degrees. With such multimodal excitation, the spacing between aligned reflectors need not be particularly large to avoid shadowing of one reflector by the preceding reflector. In the exemplary embodiment, the spacing between the adjacent reflectors was about 50 mm, giving a ratio of reflector spacing to reflector diameter of about 150:1.
It w-ill be appreciated that although the reflector surfaces in the specific embodiment will reflect only about 2-4% of the light travelling along the conductor, -if light is being transmitted into the waveguide via such surfaces, they will reflect substantially all of the light. This is mainly because the lens system enables one to image the source onto the reflector so that substantially all of the light gets transmitted along the conductor.
The difference is that the transmitted light is still concentrated E~ .
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Moreover, although the reflector means in the specific embodiment comprise planar surfaces, other types of reflective surfaces might be employed, for example the prismatic reflector surface disclosed and claimed in our copending application number 517,834 by D.A. Kahn, or other means employing total internal reflection.
An advantage of those embodiments of the invention which involve a ro~ of reflectors in a straight line in the same surface of the waveguide, is that masking or "shadowing" of one reflector is not a s;gn;f;cant problem because of the multimode transmission in the waveguide.
The specific embodiment comprises a so-called directional coupler inasmuch as the inclined surface at each tap point is inclined in one direction only. It is envisaged that a bidirect;onal coupler could be provided by forming two oppositely-~ inclined surfaces at each tapping point. Then one would reflect light - to travel, or travelling in, one direction along the waveguide and the -~ 20 other would reflect light to travel, or travelling in, the opposite direction.
Of course, the oppos;tely-incl;ned surfaces might be spaced apart, perhaps to serve different circuit cards or d;fferent parts of the same card.
Although c;rcular cavit;es are preferred for ease of mould manufacture, other shapes are comprehended by the invention; in particular, square or otherwise rectangular cross-section m;ght be ~ B

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preferred because such a tap has maximum efficiency due to minimum loss of light.
It should be appreciated that the inclined surfaces may be provided in any combination of orientations to give 1:n distribution, n:1 concentration or multiplexing, or even n:m, i.e.
plural transmitters to plural receivers.

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Claims (21)

WHAT IS CLAIMED IS:

1. An optical busbar comprising:
means for defining an elongate waveguide for conveying light in a predetermined direction along its length, the waveguide having a planar surface extending along its length; and a plurality of reflectors of similar size spaced from and aligned with each other along the length of the waveguide and disposed along the side of the waveguide opposed to said planar surface, each reflector extending into the waveguide to obscure only a small part of its cross-sectional area and being arranged to divert light conveyed along the waveguide in said predetermined direction and incident upon the diverter means transversely to the length of the waveguide to emerge from the waveguide through the plane of said planar surface.

2. An optical busbar as defined in claim 1 wherein said means for defining an elongate waveguide comprises a rod of optically transmissive material, said planar surface comprising a longitudinal flat along one side of said rod.

3. An optical busbar as defined in claim 2, wherein each reflector has an effective reflecting area equal to about 2-4 per cent of the cross-sectional area of said waveguide.

4. An optical busbar as defined in claim 2, wherein said rod has a polygonal cross-sectional shape and said planar surface comprises one facet of the polyhedron.

5. An optical busbar as defined in claim 2 wherein each of said reflectors comprises an inclined surface formed as an oblique truncation of a respective cavity of elliptical cross-sectional shape.

6. An optical busbar as defined in claim 2, further comprising at least one reflector means disposed in the side of the rod opposite to said planar surface, the arrangement being such that light impinging upon said at least one reflector means from a direction lateral to the rod, will be reflected to impinge upon said plurality of diverter means.

7. An optical busbar as defined in claim 1, wherein said means for defining an elongate waveguide comprises a rod of optically transmissive material.

8. An optical busbar as defined in claim 2, wherein each said reflector comprises a reflective surface of a cavity in said elongate waveguide.

9. An optical busbar as defined in claim 8, wherein each said reflective surface comprises an oblique truncation of said cavity.

10. Apparatus comprising a backplane, a plurality of circuit cards each associated with an optoelectric device, and an optical busbar, said circuit cards being electrically coupled to said backplane substantially parallel with each other and substantially perpendicular to the backplane, said backplane having seating means for said optical busbar, said optical busbar comprising a rod of optically transmissive material having a planar surface along one side thereof, said optical busbar being mounted to said backplane with said planar surface positively located by said seating means, said rod of optically transmissive material having a row of diverter means along its length, the diverter means being spaced apart so as to correspond to the aforesaid optoelectrical devices associated with said circuit cards, said diverter means being so disposed relative to said planar surface as to divert light travelling along said rod to emerge laterally through said planar surface at intervals corresponding to the spacing of said diverter means and impinge upon said optoelectrical devices.

11. Apparatus as defined in claim 10, further comprising a plurality of lens means corresponding to said plurality of circuit cards, each lens means serving to couple light between said diverter means and said optoelectrical device associated with the corresponding one of said plurality of circuit cards.

12. Apparatus as defined in claim 11, wherein said lens means each comprises a first lens mounted upon said backplane and a second lens mounted upon said corresponding one of said plurality of circuit cards.

13. Apparatus as defined in claim 12, wherein each said first lens is positioned with said corresponding diverter means substantially coincident with the principal focus of said first lens.

14. Apparatus comprising:
a backplane; and an optical busbar mounted upon the backplane, the optical busbar comprising:
means for defining an elongate waveguide for conveying light in a predetermined direction along its length, the waveguide having a planar surface extending along its length, said planar surface being parallel to and spaced from the backplane, and a plurality of diverter means spaced from and aligned with each other along the length of the waveguide, each diverter means extending into the waveguide to obscure only part of its cross-sectional area and being arranged to divert light conveyed along the wave guide in said predetermined direction and incident upon the diverter means transversely to the length of the waveguide to emerge from the waveguide through the plane of said planar surface.

15. Apparatus as defined in claim 14, wherein said means for defining an elongate waveguide comprises a rod of optically transmissive material, said planar surface comprising a longitudinal flat along one side of said rod, said diverter means being disposed along the side of said rod opposed to said flat and arranged so as to divert light to pass across the rod and emerge from said flat.

16. Apparatus as defined in claim 15, wherein the busbar further comprises at least one reflector means disposed in the side of the waveguide opposite to said planar surface, the arrangement being such that light impinging upon said at least one reflector means from a direction lateral to the waveguide, will be reflected to impinge upon said plurality of diverter means.

17. Apparatus comprising:
a backplane; and an optical busbar mounted upon the backplane, the optical busbar comprising:
means for defining an elongate waveguide for conveying light in a predetermined direction along its length, the waveguide having a planar surface extending along its length, said planar surface being parallel to and spaced from the backplane; and a plurality of reflectors of similar size spaced from and aligned with each other along the length of the waveguide and disposed along the side of said waveguide opposed to said planar surface, each reflector extending into the waveguide to obscure only a small part of its cross-sectional area and being arranged to divert light conveyed along the waveguide in said predetermined direction and incident upon the reflectors transversely to the length of the waveguide to emerge from the waveguide through the plane of said planar surface.

18. Apparatus as defined in claim 17, wherein said means for defining an elongate waveguide comprises a rod of optically transmissive material, said planar surface comprising a longitudinal flat along one side of said rod.

19. Apparatus as defined in claim 18, wherein each reflector has an effective reflecting area equal to about 2-4 per cent of the cross-sectional area of said waveguide.

20. Apparatus as defined in claim 19, wherein each said reflector comprises a reflective surface of a cavity in said elongate waveguide, wherein each said reflective surface comprises an oblique truncation of said cavity.

21. Apparatus as defined in claim 20, wherein the busbar further comprises at least one reflector means disposed in the side of the waveguide opposite to said planar surface, the arrangement being such that light impinging upon said at least one reflector means from a direction lateral to the waveguide, will be reflected to impinge upon said plurality of reflectors.