WO1994018589A1

WO1994018589A1 - Optical backplanes

Info

Publication number: WO1994018589A1
Application number: PCT/GB1994/000222
Authority: WO
Inventors: Leslie Charles Laycock; Stuart Charles Robertson; Paul Stephen Cawte; Michael Bains
Original assignee: Gec-Marconi Limited
Priority date: 1993-02-05
Filing date: 1994-02-04
Publication date: 1994-08-18
Also published as: WO1994018588A1; WO1994018587A1; GB9302324D0

Abstract

An optical backplane uses deflection holograms to route a signal from a transmitter to a receiver. The use of a holographic deflector/lens to produce a slightly defocussed image at the receiver reduces the sensitivity to angular misalignment of the transmitter. The board is then mounted so that the position of the board locking/hinging mechanism coincides with the apparent position of the source.

Description

OPTICAL BACKPLANES

The present invention relates to the implementation of optical backplanes using mass produced optical components and subsystems such as holograms and CD-type laser diodes.

According to the present invention there is provided a telecommunications optical backplane connecting system comprising a holographic deflector/iens mounted

on a plane surface forming part of the backplane and an optical transmitter and receiver mounted on respective component boards, said boards being perpendicular to the backplane, wherein the holographic deflector/lens is arranged to provide a defocussed image of the transmitter at the receiver.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 shows a schematic representation of a holographic backplane;

Figure 2 shows the geometry of the backplane of Figure 1 ;

Figure 3 shows graphically the relationship between lateral displacement and hologram deflection angle;

Figure 4 shows diagrammatically a self-aligning optical module;

Figure 5 illustrates a multi-bounce backplane configuration;

Figure 6 illustrates a typical layout of a rack of transmitter/receiver boards and auxiliary reflectors/holograms;

Figures 7(a) to 7 (f) show examples of multiple-bounce interconnection paths;

Figure 8 illustrates the use of a combined holographic deflector/lens;

SUBSTITUTE SHEET Figure 9 illustrates the effect of a collimated transmitter in the embodiment of

Figure 8; and

Figure 10 illustrates the effect of a defocussed transmitter in the embodiment

of Figure 8.

A schematic of a basic system is shown in Figure 1. A pair of deflection holograms route each signal to the desired location using a single reflection from a mirrored backplane. These holograms are simple linear gratings and their translational alignment with respect to the transmitters and receivers are not critical: any lateral movement in the transmitter will be mapped one-to-one at the receiver, thus the holograms need only be aligned to within ~1mm in order to achieve satisfactory performance (compare this with a guided interconnect system in which μm size tolerances are necessary). However one parameter which does need to be considered carefully is the angular misalignment of the transmitter. It will be shown below that this factor places a physical restraint on the dimensions and geometry of the backplane itself. Solutions are proposed which allow the full length of a 480mm (19") shelf to be interconnected.

With reference to Figure 2, consider the transmitter being angularly displaced by Δθ' to the normal of the deflection hologram. From Bragg's law, the change in angle, Δθ, from the desired angle θ, is given by,

Δθ⁷

Δθ = (1 ) cosθ

SUBSTITUTE SHEET The dimensions of the backplane, viz. depth, t, and length, L, are related to θ by,

tanθ = — 2t

Differentiating with respect to θ gives

dL 2t

(2) dθ cos²θ

Hence, from equations (1 ) and (2), an angular displacement of the transmitter of Δθ'

results in a lateral beam displacement, ΔL, at the receiver of

AL - -i ^ - <L^, * 4ty_jAe, cos³θ 4t²

This very strong dependence of lateral displacement on θ is illustrated in Figure 3 where ΔL is plotted against θ for a value of angular displacement of 0.1° and a backplane depth of 50mm. It can be seen that above θ ^« 50°, the value of ΔL begins to increase rapidly: if the latter is limited to 1mm then the maximum value θ can take is 56°, and the maximum interconnect length L is 150mm. In order to extend this interconnection length up to the width of a 480mm (19") shelf, it is proposed that one or more of the following three opto-mechanical solutions can be adopted: 1 ) the use of self-aligning optical modules, 2) the use of multiple reflections in dead space and

3) the use of deflector holograms incorporating lens-elements and a front hinging board mechanism.

This technique will limit the value which Δθ' can take and consequently permit greater interconnect lengths for a given backplane depth and lateral displacement.

In the optical backplane system, the holograms are fixed to a glass plate for

SUBSTITUTE SHEET mechanical support, alignment and protection. The float glass plate used possesses

very flat surfaces and is thus ideal as a reference for other optical components in the backplane. It is proposed that the transmit and receive optoelectronics are housed in light-weight, compact modules which are connected to the printed circuit boards (PCBs) by means of flexible mechanical mounts and electrical connectors, see Figure 4. They would be positioned along the back edge of the boards such that when the latter are inserted into the shelf the front faces of the modules come in contact with the glass plate and are automatically aligned parallel to the holograms irrespective of the angle at which the PCBs are positioned.

It was shown earlier that a single-reflection backplane allowed interconnection up to a length of 150mm. A multiple reflection technique, as illustrated in Figure 5, would allow one end of a 480mm (19") shelf to be interconnected to the other. However in order to maintain the superior flexibility of free space optical interconnects, it is important that these auxiliary reflections do not occur at the locations of intermediate receiver holograms, resulting in unacceptable crosstalk. It is therefore proposed to allocate the 'real-estate' between receiver holograms to particular transmitters and to use this dead-space to locate either simple plane reflectors or auxiliary holograms depending upon the relay function required. With reference to Figure 6, the backplane would be 'divided' into three sections. Communications within each individual section would be achieved using the standard one-bounce technique, with deflection angles always <56°. For links between adjacent sections, an auxiliary hologram would be used when L > 140mm. This device would redirect the light to the required receiver in the second section. For communications between the two end

SUBSTITUTE SHEET sections an additional reflection would be employed before the redirecting hologram is addressed if L > 280mm. In the particular design illustrated, where adjacent boards are separated by 20mm, the first reflector/hologram is located 7¹/₃ board pitches away

from the transmitter and the second device 14% pitches away.

A selection of various interconnect paths are shown in figures 7(a) to 7(f).

Figure 7(a) shows a single reflection interconnection within a single one of the sections shown in Figure 6.

Figures 7(b) and 7(c) show two examples of interconnection between adjacent sections using an auxiliary hologram.

Figure 7(d) shows interconnection between the end sections using an auxiliary hologram.

Figure 7(e) shows interconnection between the end sections using an auxiliary reflector and a second hologram.

Figure 7(f) shows an interconnection network exploiting the fan-out capabilities of auxiliary holograms.

The final example shown in Figure 7(f) illustrates how each auxiliary hologram can provide a fan-out capability thus enhancing the flexibility of the system without putting undue strain on any individual holographic element.

One of the prime benefits of employing holograms as the deflecting elements is their capability to incorporate other optical functions, such as lensing. A holographic optical element (HOE) could thus be used not only to deflect the transmitter beam but in addition to image the source onto the receiver, thus minimising the extent of lateral misplacement of the beam at the receiver due to shifts in the position of the

SUBSTITUTE SHEET transmitter, see Figure 8. In order to optimise the demagnification factor, D, of any

transmitter misalignment, the transmitter/lens distance, U, must be maximised with respect to the lens/receiver, distance, V, since

D = (4)

V

If the transmitter produces a collimated beam, then the use of a holographic lens whose focal length equals V, results in a value of D = ∞ ie. the system is totally immune to lateral displacements of the transmitter, see Figure 9. It would however now be particularly sensitive to any residual angular misalignment of the transmitter. A more beneficial situation would be to slightly defocus the collimation of the transmitter such that the source appeared to emanate from a position at the front edge of the board, and to locate the board locking (and consequent hinging) mechanism at this same location, so that any angular misalignment of the board was centred on the apparent source position, see Figure 10.

The optimal solution will depend upon the relative magnitudes of the boards potential angular and lateral misalignment. In cases where the former is greater than the latter, the defocussed condition described above should be chosen; in cases where the situation is reversed, the transmitter should be more collimated.

SUBSTITUTE SHEET

Claims

1. A telecommunications optical backplane connecting system comprising a holographic deflector/lens mounted on a plane surface forming part of the backplane and an optical transmitter and receiver mounted on respective component boards, said boards being perpendicular to the backplane, wherein the holographic deflector/lens is arranged to provide a defocussed image of the transmitter at the receiver.

2. A telecommunications optical backplane as claimed in claim 1 , wherein the transmitter component board has a locking/hinging point at the apparent transmitter position.

3. A telecommunications optical backplane as claimed in claim 1 or 2, wherein the transmitter and/or the receiver are flexibly mounted on the respective component boards and have a plane front surface perpendicular to their respective optical axes, so as to bring the transmitter and/or receiver front surface into mating contact with the plane surface of the backplane or a further surface parallel thereto by flexing of the flexible mounting, so as to bring the transmitter and/or receiver optical axis normal to the backplane.

SUBSTITUTE SHEET