Electronic circuit board with internal optically transmissive layer forming optical bus

A method of constructing an electronic circuit assembly comprises forming a circuit board having first and second sets of circuit layers and an internal optically transmissive layer, sandwiched between the first and second sets of circuit layers. Holes are formed through the first set of circuit layers by excimer laser drilling, the holes extending as far as the optically transmissive layer without passing through that layer. Electronic devices are mounted on the circuit board over said holes, at least some of the devices including optical receiver and/or transmitter means for receiving/transmitting optical signals from/to the optically transmissive layer. An optical fibre is coupled to the optically transmissive layer. The optically transmissive layer thus acts as a bus interconnecting the devices.