Homodyne interconnections of integrated circuits

This invention relates to interconnection of electrical elements by optical means, and in particular to interconnection of elements on semiconductor wafers and interconnections between separate wafers. Within the context of this specification 'wafers' includes both comparatively small dimensioned chips and also larger slices.

It has been recognised that the performance and size of integrated circuits is limited, in some instances, by the characteristics of the electrical interconnections; for example the capacitance and inductance of the contacts and leads may introduce time delays in the signal as it is transmitted from one part of a circuit to another.

Optical interconnection has been proposed as a way of reducing delays and adverse electrical interactions, with direct detection of the incoming optical signal by photo detectors. (J. W. Goodman et al "Optical Interconnection for VLSI Systems" Proc. IEEE, Vol. 72, No. 7, July 1984).

Such an interconnection system has also been proposed in EP patent application No. 0,150,929 to provide an optical communications system between chips mounted on a substrate in which signals from each of several integrated circuits are transmitted to other integrated circuits the light signals being transmitted along light conducting members in the silicon board. This, too, employed a direct detector.

A disadvantage of proposed systems is that direct detection necessitates low noise, multi-stage amplification and therefore at the detection end of the optical link fabrication complexity is greatly increased, although it is possible to fabricate all the elements from silicon. At the transmission end, in order to modulate an optical signal in accordance with an output signal from a silicon integrated circuit with sufficient strength to enable direct detection it has been considered necessary to incorporate separate modulators or to fabricate modulators or light sources from Group III-V materials, again adding greatly to fabrication complexity.

EP patent application No. 0,168,192 discloses electromagnetic wave transmission system which includes an optical transmission medium for propagating a beam including two coherent electromagnetic wave signals. When received the two signals are separated into an information component and a reference component. The reference component drives an optical amplifier for producing an amplified reference component that is coherent with the information component. Mixing the coherent amplified reference component with the information component in a homodyne detector produces a baseband information signal.

The present invention provides an optical interconnection system system for integrated circuits including a first circuit providing an output signal and a second circuit including an optical detector characterised in that the optical detector is a homodyne detector and the system includes an optical source for supplying a carrier wave which comprises at least two parts; a modulator for modulating one part of the carrier wave in accordance with an output signal from the first circuit; coupling means for coupling the modulated part of the carrier wave and the other part of the carrier wave, unmodulated, to the homodyne detector where it is mixed with the modulated part of the carrier wave to provide an input signal to the second circuit.

In one embodiment of the invention the modulation and detection take place respectively on the same wafer as the first and second circuits and are fabricated integrally of the same material. The wafer or wafers are preferably made of silicon and the optical transmission frequency is on or close to the band edge absorption frequency.

Within the context of this specification the expression 'optical' source includes electro-optical sources. Optical is to be construed as including the visible spectrum and also the infra-red and ultra-violet parts of the electromagnetic spectrum.

The invention is now described by way of example with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic diagram of a first embodiment of the invention showing a single interconnection;</li><li>Figure 2 is a schematic diagram of a second embodiment of the invention showing multiple interconnections, and</li><li>Figure 3 is a schematic diagram of a third embodiment of the invention showing further multiple interconnections.</li></ul>

Homodyne detection of modulated optical signals is generally acknowledged to be the most sensitive technique of detection and yields high gain even from weak modulation. However, because homodyne detection requires the detector to be provided with a signal (termed the local oscillator signal) of the same frequency and phase as the incoming carrier; it is also generally the most difficult detection technique to implement, especially when the carrier frequency may be subject to drift. In the present invention the carrier beam is split prior to modulation and the unmodulated beam is utilised as the local oscillator input.

Referring now to Figure 1, a silicon chip 1 has a first circuit area A that requires connection to a second circuit area B. The connection between areas A and B is made by a waveguide or optical fibre 2 that connects a modulator 3 associated with area A to a detector 4 associated with area B. An external narrow optical line width laser 5 provides an unmodulated input optical signal to the modulator 3 where the optical signal is modulated by the electrical signal output from circuit area A. The modulated signal is transmitted along waveguide 2 to detector 4 where it is mixed with light of the same phase and frequency for homodyne detection.

The local oscillator input to the detector 4 is provided by supplying light directly from laser 5, along local oscillator path 6 by splitting the output from laser 5 upstream of the modulator. Thus by using the same coherent source the light from the local oscillator path 6 is automatically in the required phase and of the same frequency. Any tendency for frequency drift in the laser, and hence in the carrier frequency, is immediately compensated by a similar change in the unmodulated carrier wave input as the local oscillator wave.

The sensitivity of homodyne detection enables both silicon based modulators and detectors to be used in order to simplify the fabrication process for an all silicon integrated circuit. However, for some applications it is envisaged that other materials will be deposited on to the silicon to form the modulator. Optical detectors operating at wavelengths up to the region of about 1100 nm can readily be fabricated in silicon and a silicon optical modulator operating close to or on that wavelength (that is, close to or on the absorption band edge) would be usable because the signal, although weak, would be readily detectable by the homodyne method. Also the amplification requirements after homodyne detection are less stringent than required in direct detection and a single narrow band amplifier can be used, again simplifying fabrication.

It is possible for the optical link to comprise optical fibres either free or attached to the wafer, or for a waveguide to be fabricated within the wafer itself, for example an etched channel filled with suitably doped silicon dioxide or other optically transmissive medium (as Phys. Vol. 19 No. 7, July 1980, ppL372-L374). Alternatively the optical communication may be through free space (or atmosphere), but in this instance alignment and scattering can create problems. In general it is preferred for the links from laser to wafer and between wafers to be via optical fibres and the links within a wafer to be via on-wafer routes.

Figure 2 illustrates a second embodiment of the invention in which a plurality of interconnections between circuits A and B utilise the same source laser. Each circuit A has a modulator associated with each input carrier wave and each circuit B has a detector associated with each unmodulated local oscillator input. The circuits A and B may be on a single wafer or they may be on separate wafers. In Figure 2 the output beam from each end of laser 5 is used, the two output beams being identical. Each of the beams could be used for either A or B circuits or for combinations. Figure 3 illustrates a further progression with more circuits from which it will be realised that a single source laser may power a complete system of circuits. Again the interconnection may be made in free space or, preferably, in waveguides such as optical fibres. For larger systems gas lasers may be employed as each modulator detector pair will preferably operate on a wave of 10 to 100 microwatts power.

The optical interconnections may be utilised as part of an integrated optical or optoelectronic device in addition to providing the function of conducting the signal between parts of an electronic system. It is envisaged that a given detector may receive signals from a plurality of modulators. The electrical signals from several circuits may be combined and transmitted along a single optical link, the sensitivity of homodyne detection enabling easier separation than direct detection.

The delay caused by optical transmission of signals depends upon the speed of light in the optical medium concerned. In silica light propagates at about 2 x 10⁸ m/sec and therefore a 1 cm track introduces a delay of only 50 psec. Preferably the optical interconnections are less than 1 metre in length.

Within the context of this specification 'integrated circuit' includes circuits comprising electronic, optical or optoelectronic elements.