GB2247127A - Optical backplane - Google Patents

Abstract

An optical arrangement for allowing communication between a plurality of circuit boards 1 and comprising a backplane 6 having a diffusely reflecting surface 7. Each board 1 is provided with a laser diode 8a-8e which transmits at a unique frequency, thus identifying the board 1. Light reflected from the backplane 6 is received and decoded by a photodetector 9a-9e associated with each board. <IMAGE>

Description

OPTICAL BACKPLANE This invention relates to optical back-planes for rack-mounted electronic circuit boards. Optical backplanes allow optical rather than electrical coupling between circuit boards.

A known arrangement for enabling optical coupling between circuit boards is disclosed in EP 196,933. This arrangement comprises a strip of transparent material, having parallel faces and placed facing a light source mounted on a circuit board. On one face is a diffractive element opposite the light source capturing the beam from the light source and reflecting it as a collimated beam having a direction of propagation forming a known angle with the faces of the strip. Reflectors are placed alternately on the two faces of the strip, capturing and reflecting the collimated light. A second optic element with a diffracting structure is placed on the first face of the strip to capture the reflected light and direct it as a convergent beam onto a light detector mounted on a second circuit board.

The above arrangement has the following disadvantages.

Firstly, the optical components need to be aligned accurately so that the light beam is directed to its intended destination. Any subsequent relative movement of the two circuit boards, for example, would result in a misalignment thus rendering the optical coupling inoperative. Secondly, communication is possible only between one pair of boards.

The present invention provides an arrangement for optical coupling between circuit boards, the arrangement having none of the disadvantages mentioned above.

The invention comprises optical interconnecting apparatus for circuit boards for electronic components, the apparatus enabling optical connections between the circuit boards and comprising a light source associated with each circuit board, each light source radiating at a unique frequency; and a photodetector associated with each circuit board, and a diffusely reflecting surface for scattering light emitted by the light sources for subsequent detection by the photodetectors.

By virtue of the invention, communication between any number of boards is possible and not confined to pairs of boards.

Since light from each detector is scattered off the reflecting surface, it can be received by all of the photodectectors and without the need for accurate alignment of any optical components.

The light sources may be laser diodes, for example, operating in the visible or IR regions of the spectrum.

Because each light source operates at an unique frequency, identification of each circuit board by the photodetectors is possible.

The photodetectors may operate over a wide or narrow bandwidth, depending on which boards they are to receive signals from.

Preferably the light sources and photodetectors are fabricated using opto-electronic integrated circuit techniques.

An embodiment of the invention will now be described by way of example with reference to the drawing which shows a plan view of apparatus according to the invention.

The drawing shows an array of printed circuit boards 1 carrying electronic components 2. The boards are mounted inside a box 3 and secured to an inner wall 4. The inner wall 4 and side walls 5 are painted matt black. An outer wall 6 of the box 3 has a diffusely reflecting inner surface 7 of polished aluminium. Mounted on each circuit board 1 and with their active surfaces facing the reflecting surface 7, are laser diodes 8a, 8b, 8c, 8d, 8e and photodetectors 9a, 9b, 9c, 9d, 9e.

The laser diodes 8a - 8e operate in the IR region and each one is tuned to operate at a different frequency.

Turnable laser diodes suitable for this purpose are known and comprise, for example, a semi-conductor laser chip, and a lens, grating and back reflector which form an external resonant cavity. Under steady state conditions, the laser operates at the wavelength having the highest round trip gain. Thus the operating frequency is determined by the position of the back reflector. Each diode and its associated optical components are formed on a single substrate using silicon technology techniques.

A divergent beam of light emitted by the laser diode 8a is scattered off the reflecting surface 7 (see arrows on the drawing) and received by the photodetectors 9a-9e.

Radiation from the diodes 8a-8e is encoded as a series of pulses of equal intensity but variable width by electronic circuitry on each board 1. The board which is transmitting a signal at any moment is identifiable by the (carrier) frequency of the signal in question.

The detectors 9a-9e can be chosen to be of a type receptive to all emissions from the laser diodes 8a-8e.

Alternatively each one may be chosen to operate over a narrow band, thus being receptive to signals from one laser diode only. The photodetectors 9a - 9e can be made to have narrow band properties by the use of either interference filters or diffraction gratings placed over their active surfaces.

Known photodetectors suitable for this purpose comprise an array of light sensitive elements. Frequency discrimination is achieved by a diffraction grating and lens. Incoming radiation, scattered off the reflecting surface 7 is collected by the lens and directed onto the grating. The grating directs light of each unique frequency onto an associated element. The elements, lens and grating are formed on a single substrate using silicon technology techniques and GaInAs for the light sensitive elements. The array is provided with electronic circuitry for decoding the receiving signals.

The use of laser diodes operating at different frequencies enables several messages to be transmitted between boards simultaneously.

Secondary reflections of light are prevented by the light absorbent matt black inner and side walls 4, 5.

In an alternative embodiment the light source and photodetectors are formed on the one substrate and share a common input/output lens.

Claims (5)

1. Optical interconnecting apparatus for circuit boards for electronics components, the apparatus enabling optical connections between the circuit boards and comprising a light source associated with each circuit board, each light source radiating at a unique frequency; and a photodetector associated with each circuit board; and a diffusely reflecting surface for scattering light emitted by the light sources for subsequent detection by the photodetectors.

2. Apparatus as claimed in claim 1 in which each light source comprises laser diode.

3. Apparatus as claimed in either preceding claim in which the photodetector associated with each board comprises an array of light sensitive elements and a diffraction grating for selectively diffracting radiation according to its frequency onto said elements.

4. Apparatus as claimed in any preceding claim in which the light source and photodetector associated with each circuit board are formed on a common substrate.

5. Optical interconnecting apparatus for circuit boards substantially as hereinbefore described with reference to the drawing.