WO1993016405A1

WO1993016405A1 - Optical interconnection device

Info

Publication number: WO1993016405A1
Application number: PCT/CA1993/000031
Authority: WO
Inventors: Mohsen Kavehrad; Gang Yun
Original assignee: University Of Ottawa
Priority date: 1992-02-07
Filing date: 1993-02-05
Publication date: 1993-08-19
Also published as: AU3445993A; US5469277A; CA2129627A1

Abstract

In N x N optical couplers, such as are used in local area networks and backplanes of telecommunications and computer equipment, a BRAGG volume diffraction device (101; 801; 1101; 1601) couple light beams between the inputs and outputs. The number of gratings in the diffraction device is reduced by means of so-called 'grating degeneration', whereby one grating can be shared by at least two wave pairs at the same time. In order to achieve this reductin, the directions of light waves in the dielectric slab (or the lateral positions of fibre ends) are suitably arranged. In other embodiments, a reduced number of gratings is achieved by using a 'sandwiched' structure which uses multi-layered dielectric slabs (801A, 801B; 1101A, 1101B). Both embodiments may be combined in the one coupler.

Description

OPTICAL INTERCONNECTION DEVICE

DESCRIPTION TECHNICAL FIELD:

The invention relates to optical interconnection devices and is especially, but not exclusively, applicable to N × N interconnectors or couplers such as are used in local area networks and backplanes of telecommunications and computer equipment.

BACKGROUND ART:

An optical interconnection device disclosed in a paper by M. Tabiani and M. Kavehrad entitled "Theory of an efficient N × N passive star coupler", Journal of Lightwave Technology, Vol. LT-9, No.4, pp. 448-455, April 1991, and in International patent application No. PCT/CA 91/00113, both of which are incorporated herein by reference, and US patent application No. 594,137 which is incorporated by reference and appended hereto, comprises a stratified volume BRAGG diffraction means with a spatially varying refractive index. The volume diffraction means comprises gratings which are three-dimensional periodic structures which couple planar light waves from N input fibers to N' output fibers, the prime signifying that the number of output fibers could be different from the number of input fibers. The gratings may be provided using holographic techniques.

In the coupler disclosed in PCT/CA 91/00113, the refractive index of the diffraction means varies spatially according to the expression:

where x and z are two of the three spatial ordinates of the grating medium;

is the spatial frequency vector;

m is an input position or mode, corresponding to one optical axis; m' is an output position or mode, corresponding to 7one optical axis;

m and m' take integer values that determine the number of input/output modes;

is the coefficient of coupling between m and m';

is the space vector (x,y,z). (Neglecting y gives a two-dimensional arrangement.)

Diffraction theory shows that, when N' ≥ N, such configurations can potentially achieve substantially 100 per cent efficiency. In such a coupler, when N = N', the number of different gratings in the diffraction means is ½(N² - N).

This number increases approximately with the square of the number of inputs N. Thus, while a 3 × 3 coupler requires 3 gratings, a 10 × 10 coupler requires 45 gratings. The dynamic range of the refractive index of the slab, however, can be shown to increase with N^3/2. For a practical holographic recording material, for example Dichromated Gelatin (DCG), which has a dynamic range less than 0.1, the dynamic range of the recording material will set a limit on the number of users or inputs N. The problem is compounded by the fact that high dynamic range (high modulation index) and low noise are usually conflicting requirements.

An object of the present invention is to mitigate this problem and provide an improved optical interconnection device.

DISCLOSURE OF INVENTION :

The invention seeks to reduce the number of gratings by arranging for one grating to couple at least two sets of sources with two sets of receivers, the sets being displaced relative to each other, and/or by providing two diffraction means in tandem i.e. sandwiched, so that, between them, they provide the required number of couplings or interconnections.

According to one aspect of the present invention, an optical interconnection device comprises diffraction means comprising at least one volume diffraction grating having a spatially varying refractive index, input means for directing input light beams onto said diffraction means, and output means for receiving output light beams leaving said diffraction means. The input means comprises a first set of at least two sources spaced apart in a first direction to direct a first set of light beams, preferably in a first input plane, and a second set of at least two sources spaced apart in a direction parallel to said first direction to direct a second set of light beams, preferably in a second input plane. The second set of sources preferably are displaced from the first set of sources laterally of said first direction so that the first and second sets of light beams converge towards the diffraction means. The output means comprises correspondingly arranged first and second sets of receivers to receive respective output light beams, the spatially varying refractive index and the respective positions of the sources and receivers being such that the same grating couples both sets of sources with their corresponding receivers.

With such an arrangement, each of said receivers can receive parts of input light beams from all of a corresponding set of sources.

In this specification, "sources" embraces ports, ends of optical fibers or other waveguides, laser diodes and like means for emitting light. "Receivers" embraces ports, ends of optical fibers or other waveguides, photodiodes and like means for receiving light.

The diffraction means may be arranged so that one of said parts of the input light beam is not diffracted, i.e. corresponds to the zero order. This may be achieved by varying the thickness and/or modulation depth of the grating.

Such a coupler is predicated upon the realisation that one grating can be used to couple two sets of light beams impinging upon it from different directions to emerge at correspondingly different output directions, providing that the corresponding input means and output means are suitably positioned and oriented according to Bragg conditions. Advantageously, the same grating may be used for coupling additional sets of light beams, providing that each additional set are in a different plane and also satisfy the Bragg condition.

With such an arrangement, a 4 × 4 star coupler can be made with four different gratings compared to the six different gratings required by the coupler disclosed in PCT/CA 91/00113.

According to a second aspect of the invention, an optical interconnection device, for coupling a plurality of sources with a plurality of receivers, comprises at least first and second volume diffraction means, each comprising at least one volume diffraction grating, the first volume diffraction grating means being arranged to provide a predetermined diffraction of input light beams and the second volume diffraction means being arranged to provide a predetermined diffraction of the diffracted input light beams, the arrangement being such that coupling of all of said sources with all of said receivers is provided for light beams passing through both of said two volume diffraction means.

The first and second aspects of the invention may be embodied in a single optical interconnector. Such an interconnector would then comprise at least first and second tandem diffraction means, a plurality of sources for directing input light beams onto the first diffraction means, and a plurality of receivers for receiving output light beams leaving said second diffraction means. The first and second diffraction means would each comprise at least one volume diffraction grating, the first diffraction means being arranged to provide a predetermined diffraction of input light beams, and the second diffraction means being arranged to provide a predetermined diffraction of the diffracted light beams, such that coupling of all of said sources means with all of said output means is provided for light beams passing through both of said two volume diffraction means. The plurality of sources comprises a first set spaced apart in a first direction and a second set of sources spaced apart in the same direction but laterally offset relative to the first set. The plurality of receivers comprises first and second sets of receivers correspondingly arrayed for receiving output light beams. The first volume diffraction means and said second volume diffraction means each have a refractive index varying spatially such that each receiver can receive light beams from each of a corresponding set of sources.

According to a third aspect of the invention, a diffraction means, for use with an optical interconnection device according to either the first aspect or the second aspect, comprises at least one volume diffraction grating having its refractive index n varying spatially according to the expression:

where n_o is the average refractive index;

and

Thus, if only one grating is provided, its refractive index will vary according to the expression:

If two gratings are provided, the refractive index of the combination will vary according to the expression:

According to another aspect of the invention, an optical interconnection device for interconnecting point sources, for example laser diodes, ends of optical fibres or other optical waveguides, and the like, with point receivers, for example photodiodes, ends of optical fibers or other optical waveguides and the like, comprises diffraction means comprising at least one volume diffraction grating having a spatially varying refractive index, input means comprising first lens means for collimating spherical light beams from said point sources before incidence upon the diffraction means and the output means comprising second lens means for converting said planar output light beams leaving said diffraction means to spherical light beams for reception by said point receivers.

According to a further aspect of the invention, apparatus for making a volume diffraction means comprises means for supporting a body of photorefractive material relative to a plurality of light sources and repeatedly exposing the material to light beams from said sources, and means for recording a resulting interference pattern, the sources being arranged and operable to emit said light beams from different positions in two dimensions transversely to the direction of propagation of light therefrom and an axis extending through said body.

The light sources may comprise two sources which are movable relative to each other between exposures.

Alternatively, the light sources may comprise a fixed array of sources selectively operable, in pairs, to irradiate the material, different pairs being operable to give the required interference patterns.

The sources may emit spherical light waves and the apparatus further comprise lens means disposed between the sources and the body for converting the light waves to planar waves .

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, in conjunction with the accompanying drawings, of preferred embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS:

Figure 1 is a schematic view of an optical interconnector or coupler embodying a first aspect of the invention;

Figure 1A is a detail showing a volume diffraction means for a 4 × 4 coupler;

Figure 2 illustrates lens action to convert spherical waves to plane waves;

Figure 3 is a vector diagram representing the six possible coupling combinations of a 4 × 4 star coupler; Figure 4 is a vector diagram corresponding to that of Figure 3, but with the vectors rearranged;

Figure 5 is a vector diagram illustrating coupling of two or three pairs of light beams by one grating;

Figure 6 is a vector diagram illustrating how various other coupling arrangements may be determined for a single grating;

Figure 7 is a vector tip representation corresponding to Figure 5;

Figure 8 is a schematic diagram of an interconnection device embodying a second aspect of the invention in which two diffraction means are used in tandem;

Figure 9 is a vector diagram for the interconnection device of Figure 8;

Figure 10 illustrates the logical connection of the interconnection device of Figure 8;

Figure 11 is an exploded perspective view of an interconnection device combining both aspects of the invention and employing two "degenerated" volume diffraction gratings in tandem;

Figures 12A, 12B and 12C are tip vector diagrams for, respectively, 4 × 4, 8 × 8 and 16 × 16 star couplers embodying the invention;

Figure 13 is a schematic diagram of apparatus for constructing a volume diffraction means for use in embodiments of the invention;

Figure 14 is a vector representation of the relationship between light waves used to "write" the volume diffraction means and light waves coupled by it when in use;

Figure 15 illustrates alternative input means and output means;

Figure 16 is an exploded perspective view of another embodiment of the invention comprising rod lenses;

Figure 17 is a longitudinal cross-section through the device of Figure 16; and

Figures 18A, 18B and 18C are cross-sections through the device. MODE(S) FOR CARRYING OUT THE INVENTION:

Referring to Figure 1, an optical interconnection device comprises a diffraction means 101, comprising a volume diffraction grating, disposed between a pair of convex lenses 102 and 103, respectively, with a common optical axis 104. The volume diffraction means 101 is formed from a dielectric material permitting holographic recording, such as Dichromated Gelatin (DCG). Formation of the volume diffraction grating will be described later. A rectangular array of optical fibers 105A, 105B, 105C and 105D are mounted with their respective ends in a plane 106 extending substantially perpendicular to the common optical axis 104. The optical fibres serve as point sources to direct light beams onto diffraction means 101 by way of lens 102. A similar array of optical fibers 107A, 107B, 107C and 107D are mounted with their respective ends in a plane 108 and serve as receivers to receive output light beams from lens 103.

The lenses 102 and 103 allow planar gratings to be used and recorded at a wavelength (for example 0.5145 μm from a Ar⁺ laser) different from the working wavelength of the coupler, typically 1.3 μm. Hence, as illustrated in Figure 2, each of the light beams from fibers 105A to 105D will be collimated by the lens 102 to produce a plane wave incident upon the volume grating 101. On leaving the volume grating 101, the plane output light beams are focused by the lens 103 to spherical light beams directed to the optical fibers 107A to 107D. Input optical fibres 105A to 105D comprise first and second sets. Light beams from the first set, comprising optical fibres 105A and 105B will transmit in a first plane 501 (see Figure 5) which is shown vertical. Light beams from the second set, comprising optical fibres 105C and 105D, will transmit in a second plane 502 (see Figure 5), also shown vertical. Following diffraction, light beams from optical fibres 105A and 105B will be received by optical fibres 107A and 107B, which comprise a first output set.

Each of the fibers 107A to 107D will receive output light beams comprising parts of light beams emanating from two of the input fibers. Thus the volume grating 101 couples the light beams "vertically" in sets. In particular, the arrangement is such that each output fiber receives the zero order of one of the pair of input fibers and a higher order, typically the first, from the other. This may be achieved by varying the thickness and/or modulation depth of the grating.

Such positioning of the input and output fibers means that a single grating can be used for more than one set of input/output lightbeams, so fewer individual gratings are required to give all coupling combinations. This reduction in the number of gratings will be referred to as "grating degeneration".

It should be noted that the interconnection device of Figure 1 is not a 4 × 4 coupler but rather two 2 × 2 couplers side-by-side. Figure 1A illustrates a diffraction means 101A which, substituted for the diffraction means 101 of Figure 1 , will convert the interconnection device into a 4 × 4 coupler. The diffraction means 101A comprises four different gratings, K₁₂, K₁₃, K₁₄ and K₂₃, superimposed. Gratings K₁₂ and K₁₃ are "degenerated" in that they couple two sets of sources with their respective sets of receivers, i.e. they provide the coupling which would have required gratings K₂₃ and K₃₄, which are omitted.

In order to explain the nature of a "degenerated" volume diffraction means, the theory governing operation of a 2 × 2 coupler will first be developed. In this case, the complex envelope of the light field in a volume diffraction means with a single wavelength can be represented as

where

C₁ and C₂ are constants representing initial phase and amplitude; and vector

is the wave vector of one of the two plane waves. This two-wave assumption is true if the thickness of the grating is so large that all the unwanted high order diffractions are sufficiently suppressed. From the paper and patent application by Tabiani and Kavehrad, supra , it is clear that a 2 × 2 coupler needs only one sinusoidal grating structure , the spatially varying refractive index of which can be represented as

where n_o is the average refractive index;

is the grating vector with

λ is the grating period, and φ is the initial phase of the grating. It should be noted that the same grating will result if

is changed to

so in some of the figures in this specification vectors which have arrows at both ends will be used to represent a grating vector.

From coupled wave theory, it can be shown that, in the grating depicted by Equation 2, the two plane waves will be able to exchange power, i.e., be coupled by the grating.

The strongest coupling happens when the Bragg condition is satisfied, i.e.,

where n is the average refractive index of the diffraction means, and λ is the wavelength in free space.

Thus, such a volume grating is similar to a 2 × 2 coupler, coupling a pair of lightwaves, and the N x N coupler by Kavehrad and Tabiani, supra , can actually be considered as a series of 2 × 2 couplers coupling all the possible combinations of wave pairs from the N lightwave sources. It should be noted that the number of different combinations is just equal to the number of required gratings in the slab so, for a 4 × 4 star coupler using a volume diffraction means disclosed by Kavehrad and Tabiani, six different gratings would be needed. These gratings are:

where n is the average refractive index of the

diffraction means, and λ is the wavelength in free space. It should be appreciated that either the plus sign or minus sign can be taken in the above equations without changing the final result. These vectors are illustrated in Figure 3, with the starting points of the grating vectors so located as to emphasize the relationship between wave vectors k₁, k₂, k₃ and k_4, and grating vectors K₁₂, K₁₃, K₁₄, K₂₃, K₂₄, and K₃₄. Hereafter, it is assumed that each grating has a sinusoidal waveform and a zero initial phase, so each grating vector will completely characterize A, the periodic structure of a grating.

When the directions of the vectors are re-arranged as shown in Figure 4, it can be seen clearly that

so there is redundancy. In embodiments of the present invention which use the diffraction means 101A illustrated in Figure 1A, the redundant gratings are eliminated and the sources positioned so that the wave vectors of the waves are not confined to the same plane and the angular distributions of the wave vectors are not uniform. Figure 5 illustrates how a degenerated grating represented by vector K, (which could be grating 101 of Figure 1 grating 101A in Figure 1A) couples two sets, each of two wave pairs

the first pair k₁, k₂ propagating in a first plane 501, and the second pair k₃, k₄ propagating in a second plane 502 which extends at an angle θ to plane 501. Both pairs satisfy the Bragg condition and hence can be coupled by the same grating characterized by (with no coupling between different pairs). It should be noted that the angle θ , between the two planes 501 and 502 defined by the wave vector pairs, can be arbitrary so, for a single-grating system, a number of additional sets of wave pairs can satisfy the Bragg condition and thus be coupled by the same grating. Thus, as illustrated in broken lines in Figures 1 and 5, additional input means 105E and 105F could be added emitting an additional set of two light beams

extending in a third plane 503.

More sets can be added providing that they transmit in planes which extend in the direction of the grating vector where they intersect the diffraction grating.

Also, each set may comprise more than two sources or receivers, providing that they are aligned in the corresponding plane.

General expressions governing the arrangement of the wave vectors (hence the directions of the light beams) to achieve the sharing of gratings can be derived from Equation 3 as follows:

where i, j = 1, 2, ..., N.

The geometry of these equations is shown in Figure 6. The first two equations 10(a) and 10(b) represent a spherical surface and a plane. The intersection of these surfaces makes a circle which is the trajectory of one of the wave vector tips. The other wave vector is determined by the third line (c) in Equation 10. Its trajectory makes the second circle. Without loss of generality, it can be assumed that where

is the unit vector in the x direction. This can be done by simply rotating the coordinate system. Thus,

where i, j = 1, 2,..., N. Any wave vector pairs which

satisfy Equation 10 or Equation 11 will be able to share the same grating

.

Another way of depicting the vector arrangement of

Figure 5 is a two dimensional tip-pattern diagram as shown in Figure 7. Though only the projections of the wave vector tips and grating vectors on the plane z = o can be seen in Figure 7, the vectors, and hence the directions of waves and gratings, are still uniquely determined by the tip-pattern diagram. The reason is that out of the three components of the wave vectors only two are independent. The third component and the grating vectors can be found by the following formulae:

The tip-pattern diagram is important because of its two dimensional nature. All the designs given later in this description will be presented as wave vector tip-pattern diagrams. It can be shown that, under Fresnel approximation, a tip-pattern is actually the pattern of fiber end arrangements on each focal plane when it is scaled by a factor where the plus sign is for the receiving side and the

minus sign for the transmitting side of the coupler.

When a tip-pattern is used, the tip trajectories become two dimensional and can be obtained by cancelling the z components of the wave vectors in Equation 10. These are generally two second order curves where one is the shifted version of the other. In a special case, when K_z = O, the trajectories become two parallel straight lines:

The foregoing discussion of the mathematical basis for the operation of the coupler can, of course, be applied to the vectors of Figure 1A.

Referring now to Figure 8, in a second embodiment of the invention the number of gratings required for coupling a desired number of input/output pairs is reduced by using a sandwich of at least two diffraction means, each comprising at least one volume diffraction gratings. Thus, in Figure 8, a sandwiched diffraction means 801 comprises a first diffraction means 801A, a second diffraction means and 801B and two lenses 802 and 803 with a common optical axis 804. Lens 802 is interposed between the first diffraction means 801A and an array of sources in the form of optical fibers 805A to 805D for directing light beams represented by wave vectors K₁, K₂, K₃ and K₄, respectively onto the first diffraction means 801A. A second lens 803 is interposed between the diffraction means 801B and an array of receivers in the form of optical fibers 807A to 807D. Unlike the embodiment of Figure 1, the fibers 805A to 805D and 807A to 807D are in linear, arcuate arrays.

As shown in Figure 9, the vector diagram for the sandwiched diffraction means 801, gratings are

omitted. In the first volume diffraction means 801A, two gratings are recorded, namely The second volume

diffraction means 801B contains the gratings

Referring to Figure 9, after the four input light waves k, to k₄ emerge from the first diffraction means 801A, the wave fields corresponding to fibers 105A and 105B are coupled by the grating represented by grating vector

and the wave fields corresponding to fibers 105C and 105D are coupled by the grating represented by grating vector Generally: I₁ ⁽¹⁾ = (1-η₁₂) I₁ ⁽⁰⁾ η₁₂ I₂ ⁽⁰⁾ (14)

I₁ ⁽¹⁾ = η₁₂ I₁ ⁽⁰⁾ + ( 1 - η₁₂) I₂ ⁽ ₀₎ (15)

I₃ ⁽¹⁾ = (1-η₃₄) I₃ ⁽⁰⁾ + η₃₄ I₄ ⁽⁰⁾ (16)

I₄ ⁽¹⁾ = η₃₄ I₃ ⁽⁰⁾ + ( 1 - η₁₂) I₄ ⁽ ₀₎ (17) where I₁ ^(j) ( i = 1,2,3,4, j = 0,1,2) stands for the intensity of the i-th wave at the output of the j-th diffraction means. Variable η_kl(k,l=1,2,3,4,) is the diffraction efficiency of the grating characterized by the vector .

After the waves pass through the second diffraction means 801B, the four outputs can be expressed as

It can be seen that each of the output fibers 807A to 807D can receive signals from all the input fibers 805A to 805D. In a special case, when η_ij = 50%, each input wave will have its power evenly distributed among all four outputs. Accordingly, even though gratings

and have been omitted, the interconnection device of Figure 8 is still a 4 x 4 coupler, with each slab 801A, 801B containing only two gratings.

From a logic connection point of view, the sandwiched diffraction means 801 is similar to a star coupler made from a number of 2 × 2 fiber couplers, as shown in Figure 10. The number of separate diffraction means in the sandwiched diffraction means 801 corresponds exactly to the number of stages in the coupler in Figure 10.

In a practical coupler, it is possible, and preferable, to combine both aspects of the invention, namely tandem degenerated volume diffraction means, in a single interconnection device as illustrated in Figure 11. The combined interconnection device of Figure 11 is a 4 × 4 coupler similar to that described with reference to Figures 1 and 1A. It differs, however, in that its diffraction means

1101 comprises two diffraction means 1101A and 1101B disposed between a pair of convex lenses 1102 and 1103, respectively, with a common optical axis 1104. The two diffraction means 1101A and 1101B are each degenerated to one grating and the directions of the input light beams adjusted accordingly, as in the embodiment of Figure 1. Thus, an array of input optical fibers 1105A, 1105B, 1105C and 1105D are mounted to a spherical surface 1106, with their ends in a rectangular array and normal to the spherical surface 1106 so that light beams from the fibers will be directed towards the centre of the lens 1102. A similar array of optical fibers 1107A, 1107B, 1107C and 1107D are mounted normal to the spherical surface 1108.so as to receive light beams from lens 1103. The radius of each of the spherical surfaces 1106 and 1108 is equal to the focal length of the adjacent one of the lenses

1102 and 1103.

Figure 12A shows the wave vector tip-patterns for the 4 ×4 star coupler of Figure 11. First volume diffraction means 1101A is represented by vector K₁₂ and second volume diffraction means 1101B is represented by vector K₁₃. It can be verified that these designs satisfy Equation 13.

As in the other embodiments, light beams from fibers 1105A to 1105D will be collimated by the lens 1102 to produce four plane waves incident upon the first degenerated volume grating 1101A. The degenerated first volume grating 1101A comprising grating K₁₃ couples the light beams "vertically" in pairs. The second volume grating 1101B couples the waves "horizontally" in pairs. It should be noted that the vectors

are shown vertical and horizontal merely for convenience of illustration. Other orientations, not necessarily perpendicular to each other, may be used. As described earlier, the two volume gratings 1101A and 1101B will couple the light beams differently and in such a way that their combination gives the total desired coupling of each of inputs 1105A to 1105D with all of the output fibres 1107A to 1107D. On leaving the volume grating 1101B, the light beams are focused by the lens 1103 to the corresponding optical fibers 1107A to 1107D, respectively.

Using complex amplitude, the light field at the input spherical surface 1106 can be expressed as:

where Ψ (x, y) stands for the dominant mode of a fiber, (x_i, y_i) stands for the lateral position of the i-th fiber end, and A_i represents the amplitude of the field from the i-th fiber. After propagation through free-space and passage through lens 1102, the light field at the input to volume diffraction means 1101A, becomes:

Where FF{• } stands for the two dimensional Fourier transformation, ξ and η are components of the spatial frequencies corresponding to the directions of x and y, f is the focal length of the lenses 1102 and 1103, Ψ(ξ,η) = FF{Ψ(x,y)} and p(x,y) represents the aperture area of the lens 1102. The aperture area of the lens 1102 can be considered as a spatial low-pass filter and has a Fourier transform P (ξ,η). At the output side of the second volume diffraction means 1101B, the field can be expressed as:

where

and c_ij is the coupling coefficient between the i-th and j-th waves. Equation 25 is a mathematical expression that depicts the function of the sandwich comprising volume diffraction means 1101A and 1101B.

Similarly, at the output spherical surface 1108, the field can be written as:

where it is assumed the dominant mode has the symmetry:

When the numerical aperture of the lenses 1102 and 1103 is much larger than that of the fibers:

and

which holds the same mode pattern as that of E(x,y).

Since each of the fibers 1107A to 1107D need only receive light incident normally on its end, and the numerical aperture of the lens 1103 is much larger than that of the fibers, mode matching does not present a problem because the field pattern on the receiving side of the coupler is the image of the field on the transmitting side.

The reduction in the number of gratings is quite significant. In practice, constructing six different gratings in a single slab probably requires high standard facilities and good experimental skills, while recording one volume grating in a slab with a diffraction efficiency of 50% is such a simple task that it can be done with basic holographic recording equipment.

General optimum design criteria for an arbitrary N could be deduced given the number of diffraction means, number of gratings in each and the geometry. When N is small (e.g., N ≤ 16), the arrangement is quite obvious. Besides, when N is small, the number of grating layers is also small and excess loss due to cascading need not be of concern. From experience with the material DCG, it is quite conservative to expect the excess loss of each layer to be below 0.1 dB. Therefore, the excess loss of, for example, 10 gratings can be kept below 1dB.

Figures 12B and 12C illustrate wave vector tip patterns for 8 × 8 and 16 × 16 couplers, respectively. These designs also satisfy Equation 3 or 13. As illustrated in Figure 12B, an 8 × 8 coupler comprises three slabs, the first comprising one grating K₁₅, the second slab comprising two gratings and the third slab comprising the two

gratings The eight sources are arranged in two

sets of four, each set aligned vertically in the drawings.

A 16 × 16 coupler as illustrated in Figure 12C comprises four diffraction means in tandem and the sources are arranged in four sets of four forming a rectangular array.

The first diffraction means or slab comprises two gratings the second comprises two

gratings the third comprises four gratings

and the fourth comprises four gratings Thus, for the 16 × 16

coupler, coupling of each of the sources with all of the receivers can be achieved with no more than 4 different gratings in each slab whereas, with a one-slab-no-degeneration configuration as disclosed by Tabiani and Kavehrad, 120 different gratings would be needed in one slab. It is envisaged that the diffraction gratings will be .formed using holographic techniques similar to those disclosed in International patent application number PCT/CA 91/00113, which is incorporated herein by reference, in which two lightwaves interfere with each other and the resulting interference pattern is recorded on a body of light sensitive material whose refractive index, after processing, will be directly proportional to the intensity of the interference field. Figure 13 shows a schematic diagram of an apparatus for constructing the grating. The optics are generally equivalent to the input half of the coupler in Figure 11. Two single mode, polarization-preserving fibers 1301 and 1302 are each connected at one end to a respective output of a beam splitter 1303 which is supplied by way of free space-to-fiber coupler 1304 from a laser light source 1305. The other ends of the fibers 1301 and 1302 are mounted by two optical fiber positioning means 1306 and 1307 for accurate movement in two directions defining a spherical surface 1308. The radius of curvature of spherical surface 1308 is equal to the focal length f_rec of a lens 1309. The fiber positioning means 1306 and 1307 are normal to the spherical surface 1308 and direct light to the centre of lens 1309. The positioning means 1306 and 1307 are also movable radially for focusing. After passing through the lens 1309, the light beams pass through an iris 1310 to the recording dielectric slab 1311.

In order to get a uniform illumination during the exposure, the focal length f_rec is several times larger than the focal length f of the lenses 1102 and 1103 of the coupler, so that only a small portion of the wave front from the fibers will be used.

Operation of the apparatus is similar to that in PCT/CA 90/00113 to make a diffraction means 801A or 801B for the coupler of Figure 8. Whereas Tabiani and Kavehrad would move or scan sources in one plane, embodiments of the invention require the sources to be moved, or scanned, in two transverse directions. A 16 × 16 coupler, for example, will require four diffraction means or slabs, each comprising two gratings or four gratings, as described previously. Each slab will be exposed once for each grating with the light sources in positions corresponding to those of the coupler in which the diffraction means are to be used.

Thus, in making a diffraction means 101 (Figure 1), or 1101A or 1101B (Figure 11), the fiber light sources 1306 and 1307 are moved to respective positions corresponding to a pair of inputs of the coupler and the dielectric slab 1311 exposed for each position. These positions can be directly determined from the tip-pattern design by the following equations:

Since plane wave systems are used, a different wavelength can be used for grating construction. The most important relation for the grating recording is

where

are wave vectors of the two waves used in constructing grating Equation 28 is very similar to the

second line of Equation 3 in appearance. Combining these two equations yields

The geometric interpretation is shown in Figure 14. It should be noted that where is the wavelength used

in construction, which is probably 0.514 μm when DCG is used.

It will be appreciated that, instead of two movable light sources which are moved to the desired positions between exposures, an array of light sources could be provided and different pairs energized, in sequence, to the same effect.

Because the grating patterns written into the diffraction means are planar, a single large body of photographic material could be exposed to produce the required interference patterns and diced to produce a plurality of diffraction means with the same pattern. This would reduce production costs. In the specific embodiments described herein, the light beams propagate in free space between each set of optical fibers and the respective lens, with the spherical surface being either a physical surface to which the ends of the optical fibers are mounted or simply a notional surface defined by the ends of the optical fibers and their orientations. It is envisaged, however, that other arrangements might be used. For example, as shown in Figure 15, the spherical surfaces might comprise surfaces of solid hemispherical glass blocks 1501 and 1502, their planar surfaces in contact with the volume gratings 1501 and index matched to the them. The input array of optical waveguides then comprise tapered ends of optical fibers 1504A to 1504E and the output array comprise tapered ends of optical fibers 1505A to 1505E.

Alternatively, each set of fibers might be attached to a rod lens having a cross-section matching that of the volume diffraction means and index-matched. The rod lenses would then abut respective sides of the volume diffraction means. Thus, as illustrated in Figures 16 to 18, the diffraction means 1601 comprises two sandwiched volume diffraction grating slabs 1601A and 1601B. As illustrated by the cross-sectional views of Figures 18A and 18B, the grating patterns of these diffraction means 1601A and 1601B will be the same as those of the interconnection device described with reference to Figure 11. The input optical fibers 1602A, 1602B, 1602C and 1602D are mounted in a cylindrical boss 1603, shown in cross-section in Figure 18C, which abuts the end of a cylindrical rod lens 1604. The other end of the rod lens 1604 abuts the input face of diffraction grating slab 1601A. The output means is similar in that the four output optical fibers 1605A, 1605B, 1605C and 1605D are clustered in a cylindrical boss 1606 which abuts on end of an output cylindrical rod lens 1607. The other end of rod lens 1607 abuts the output face of diffraction grating slab 1601B.

Both the input optical fibers 1602A - 1602D and the output optical fibers 1602A - 1602D are parallel to the optical axis 1608 of the device. The input rod lens 1604 comprises a one quarter pitch lens and its refractive index varies radially so that the input light beams are collimated when they arrive at the diffraction grating slab 1601A, in pairs in the required intersecting planes as previously described. The refractive index of output rod lens 1607, which also is a one quarter pitch lens, also varies radially so as to focus the light beams emerging from the diffraction grating slab 1601B so that they converge to the appropriate output optical fiber or port.

It will be appreciated that this configuration of input means and output means, employing rod lenses, is not limited to the sandwiched volume diffraction means but could be used with any of the volume diffraction means described herein.

INDUSTRIAL APPLICABILITY

Embodiments of the invention may be used not only in local area networks and backplanes of telecommunications and computer equipment, but also to interconnect components in integrated circuits, to interconnect integration circuits on a circuit board and in analogous situations in the field of optical communications, especially where single mode optical fibers are to be interconnected.

Although embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the appended claims.

Claims

CLAIMS :

1. An optical interconnection device comprising diffraction means (101; 801; 1101; 1501; 1601) comprising at least one volume diffraction grating having a spatially varying refractive index, input means (105A - D) for directing a plurality of input light beams onto said diffraction means, and output means (107A - D) for receiving a plurality of output light beams leaving said diffraction means, characterized in that said input means comprises a first set of at least two sources (105A, 105B) spaced apart in a first direction to direct a first set of light beams, and a second set of at least two sources (105C, 105D) spaced apart in a direction parallel to said first direction to direct a second set of light beams, the first set of sources and the second set of sources being displaced laterally relative to each other, said output means comprises correspondingly arranged first and second sets of receivers to receive a first set of output light beams (107A, 107B) and a second set of said output light beams (107C, 107D), the first set of receivers and the second set of receivers being displaced laterally relative to each other, and the spatially varying refractive index and the arrangement of the sources and receivers is such that said volume diffraction means couples both the first set of sources with the first set of receivers and the second set of sources with the second set of receivers.

2. An interconnection device as claimed in claim 1, characterized in that said volume diffraction means (801; 1101) comprises at least a second volume grating (801B; 1101B) superimposed with the first volume diffraction grating (801A: 1101A), the second volume diffraction grating having a spatially varying refractive index varying in a direction transverse to the direction of variation of that of the first volume diffraction grating, the arrangement being such that the second volume diffraction grating couples the first set of sources with the second set of receivers, and couples the second set of sources with the first set of receivers.

3. An interconnection device as claimed in claim 2 characterized in that said volume diffraction means comprises a plurality of said volume gratings (K₁₃, K₂₄, K₁₂, K₃₄) superimposed, the input means comprises first and second sources (105A, 105B) for directing respective ones of a first pair of said input light beams, and third and fourth sources (105C, 105D) for directing respective ones of a said second pair of said input light beams, the output means comprises first and second receivers (107A, 107B) for receiving respective ones of a first pair of output beams and third and fourth receivers (107C, 107D) for receiving corresponding ones of a second pair of output light beams, and said diffraction means comprises a first grating (K₁₃, K₂₄) for coupling said first and second sources with said first and second receivers, and said third and fourth sources with said third and fourth receivers, and a second grating (K₁₂, K₃₄) for coupling said first and second sources with said first and second receivers, and said third and fourth sources with said third and fourth receivers.

4. An interconnection device as claimed in claim 3, further characterized by a third grating (K_14,) for coupling said first source and said fourth source with said first receiver and said fourth receiver, and a fourth grating (K₂₃) for coupling said second source and said third source with said second receiver and said third receiver.

5. An interconnection device as claimed in claim 1, characterized in that the diffraction means is so arranged that each of said output light beams comprises the zero order of one of said input light beams.

6. An interconnection device as claimed in claim 1, characterized in that the input means comprises one or more additional sets of sources (105E, 105F) spaced apart in a direction parallel to said first direction for directing one or more additional sets of input light beams, and said output means comprises one or more additional sets of receivers (107E, 107F) for receiving corresponding additional sets of output light beams, such that said grating also couples each of said additional sets of sources with a respective one of said additional sets of receivers.

7. An interconnection device as claimed in claim 1, characterized by at least two said volume diffraction means (801A, 801B; 1101A, 1101B) in tandem, each comprising at least one volume diffraction grating, the first volume diffraction means (801A, 1101A) being arranged to provide a predetermined diffraction of said input light beams and the second diffraction means (801B, 1101B) being arranged to provide a predetermined diffraction of the diffracted input light beams, such that coupling of all of said sources with all of said receivers is achieved for light beams passing through both of said two volume diffraction means.

8. An interconnection device as claimed in claim 1, characterized in that the sources are configured for emitting spherical said input light beams and the receivers are configured for receiving spherical said output light beams, said input means comprising input lens means (102; 802; 1102; 1604) for collimating spherical light beams before incidence upon the diffraction means, and said output means comprising output lens means (103; 803; 1103; 1607) for converting planar output light beams leaving said diffraction means into spherical output light beams.

9. An interconnection device as claimed in claim 8, characterized in that the sources (1105A - D) are arranged in a spheric array having a radius of curvature equal to the focal length of the lens means (1102) and concentric with the lens means, said sources being aligned for emitting said input light beams radially of said spheric array.

10. An interconnection device as claimed in claim 8, characterized in that each said lens means (1604, 1607) comprises a one-quarter pitch rod lens, the input rod lens (1604) having one end juxtaposed to the diffraction means (1601) and said sources (1602A - D) arrayed at its other end and the output rod lens (1607) having one end juxtaposed to the diffraction means (1601) and the receivers (1605A - D) arrayed at its other end.

11. An interconnection device as claimed in claim 10, characterized in that said sources and said receivers are arranged to emit and receive, respectively, light beams parallel to the optical axis of the lens means, and the refractive index of the input lens means varies such that the light beams are incident upon the diffraction means from different directions, and the refractive index of the output lens means varies such that light beams leaving said diffraction means in a said output plane are parallel to said optical axis when incident upon the receivers.

12. An interconnection device as claimed in claim 1, characterized in that each of said input means (1502) and said output means (1503) comprises a hemispherical body index matched to said diffraction means (1501), the diffraction means being sandwiched between respective planar surfaces of the hemispherical bodies, the sources and receivers being disposed about the respective hemispherical surfaces.

13. An interconnection device as claimed in claim 1, characterized in that the volume diffraction grating has its refractive index varying spatially according to the expression:

where n_o is the average refractive index;

and

14. An interconnection device as claimed in claim 1, characterized in that the diffraction means comprises one grating with its refractive index varying according to the expression:

where n_o is the average refractive index;

and

15. An interconnection device as claimed in claim 1, characterized in that the diffraction means comprises two gratings their combined refractive index varying according to the expression:

where n_o is the average refractive index;

and

16. An optical interconnection device, for coupling a plurality of sources (805A - D; 1105A - D; 1602A - D) with a plurality of receivers (807A - D), characterized by at least first and second volume diffraction means (801A, 801B; 1101A, 1101B; 1601A,1601B), each comprising at least one volume diffraction grating having a spatially varying refractive index, the first volume diffraction means being arranged to provide a predetermined diffraction of input light beams from said sources and the second volume diffraction means being arranged to provide a predetermined diffraction of the diffracted input light beams, the arrangement being such that coupling of all of said sources with all of said receivers is provided for light beams passing through both of said two volume diffraction means.

17. An interconnection device as claimed in claim 16, characterized in that said sources are configured for emitting spherical said input light beams and the receivers are configured for receiving spherical said output light beams, the device being further characterized by input lens means (802, 1102, 1604) for collimating input spherical light beams before incidence upon the diffraction means, and output lens means (803, 1103,1607) for converting planar output light beams leaving said diffraction means into spherical output light beams.

18. An interconnection device as claimed in claim 17, characterized in that the sources (1105A - D) are arranged in a spheric array having a radius of curvature equal to the focal length of the input lens means (1102) and concentric with the input lens means, said sources being arranged for emitting said input light beams radially of said array, and said receivers (1107A - D) are arranged in a second spheric array having a radius of curvature equal to the focal length of the output lens means and concentric therewith, said receivers being arranged to receive output light beams radially of said second spheric array.

19. An interconnection device as claimed in claim 17, characterized in that said input lens means comprises a one quarter pitch rod lens (1604) having one end juxtaposed to the diffraction means (1601) and said sources (1604A - D) arrayed at its other end, the refractive index of the first rod lens varying radially to collimate each of said spherical light beams, and the output lens means comprises a second one quarter pitch rod lens (1607) having one end juxtaposed to the diffraction means and the receivers (1605A - D) arrayed at its other end, the refractive index of the second rod lens varying radially to focus each of said light beams leaving the diffraction means to a spherical light beam.

20. An interconnection device as claimed in claim 17, characterized in that each said lens means (1502, 1503) comprises a hemispherical body index matched to said diffraction means (1501), the diffraction means being sandwiched between the planar surfaces of the hemispherical bodies, the arrays of sources and receivers being disposed about the respective hemispherical surfaces.

21. An interconnection device as claimed in claim 16, characterized in that said volume diffraction grating has its refractive index varying spatially according to the expression:

where n is the refractive index;

22. An interconnection device as claimed in claim 16, characterized in that the diffraction means comprises one grating with its refractive index varying according to the expression:

where n₀ is the average refractive index; and

23. An interconnection device as claimed in claim 16, characterized in that the diffraction means comprises two gratings their combined refractive index varying according to the expression:

where n_o is the average refractive index;

and

24. An optical interconnection device characterized by diffraction means (101, 801, 1101; 1601) comprising at least one volume diffraction grating having a spatially varying refractive index, input means (105A - D; 805A - D; 1105A - D; 1605A - D) for directing input light beams onto said diffraction means, and output means (107A - D; 807A - D; 1107A - D.; 1605A - D)for receiving output light beams leaving said diffraction means, the input means comprising first lens means (102; 802; 1102; 1604) for collimating spherical light beams before incidence upon the diffraction means and the output means comprising second lens means (103; 803; 1103; 1607) for converting planar output light beams to spherical light beams.

25. An interconnection device as claimed in claim 24, characterized in that the input means comprises a plurality of sources (1105A - D) in a spheric array having a radius of curvature equal to the focal length of the lens means and concentric with the lens means, said sources being arranged for emitting said input light beams radially of said array.

26. An interconnection device as claimed in claim 24, characterized in that said input lens means comprises a rod lens (1604) having one end juxtaposed to the diffraction means (1601) and said sources (1602A - D) arrayed at its other end, the refractive index of the first rod lens varying radially along its length to collimate spherical light beams, and the output lens means comprises a second rod lens (1607) having one end juxtaposed to the diffraction means and said receivers (1605A - D) arrayed at its other end, the refractive index of the second rod lens varying radially along its length to convert planar light beams leaving the diffraction means to spherical light beams.

27. An interconnection device as claimed in claim 24, characterized in that each said lens means (1502, 1503) comprises a hemispherical body index matched to said diffraction means (1501), the diffraction means being sandwiched between the planar surfaces of the hemispherical bodies, the sources and receivers being arrayed about their respective hemispherical surfaces.

28. Apparatus for making a volume diffraction means by exposing a body (1311) of photorefractive material to radiation from a plurality of mutually coherent light sources to record resulting interference patterns, the apparatus being characterized by a plurality of said sources (1301, 1302), means (1306, 1307) for controlling said sources to emit radiation simultaneously and for a predetermined length of time from a first set of predetermined positions to record a first interference pattern and to emit radiation from a second set of predetermined positions to record a second interference pattern, the respective positions of the sources in the first and second set of predetermined positions being mutually laterally displaced in two dimensions relative to the direction of propagation of the radiation.

29. Apparatus as claimed in claim 28, characterized in that the sources comprise two sources (1301, 1302) movable between the first set of predetermined positions and the second set of predetermined positions,

30. Apparatus as claimed in claim 28, characterized in that the sources emit spherical light waves and the apparatus further comprises lens means (1309) disposed between the first and second sources and the body (1311) for collimating the light waves to planar waves.

31. Apparatus as claimed in claim 28, characterized in that said sources are arranged in an array extending in two dimensions transversely to the direction of propagation, said apparatus further comprising means for operating a first pair of sources at said first set of predetermined positions to record the first interference pattern, and a second pair of said sources at said second set of predetermined positions to record the second interference pattern.

32. Apparatus as claimed in claim 30, characterized in that the sources are arranged for emitting spherical light waves and the apparatus further comprises lens means (1309) disposed between the sources and the body for collimating the light waves.

33. A method of making a diffraction means for an optical interconnection device characterized by the steps of:-

(i) exposing a body (1311) of photorefractive material for a predetermined length of time to radiation from a pair of mutually coherent light sources (1301, 1302), each disposed at a first predetermined position and orientation, to record a first interference pattern; and

(ii) exposing the body for a second predetermined length of time to radiation from a pair of said light sources at different positions and orientations to record a second interference pattern;

the respective positions of said sources in the first and second predetermined positions being laterally displaced in two dimensions relative to the direction of propagation of the radiation.

34. A method as claimed in claim 33, characterized in that the body is exposed to record additional interference patterns, each with the sources at different positions and orientations.

35. A method as claimed in claim 33, further characterized by the step of dicing the body to provide a plurality of similar diffraction means each with the same interference pattern or patterns.