Data signals can be transmitted via the same optical fibers that are used to transmit power from base stations to remote stations containing sensors and associated circuitry. [A prototype system based on fiber-optic transmission of data and power is described in "General-Purpose Optically Powered Sensor and Control System" (NPO-19604), which is the first of two articles preceding this one.] The implementation of the present data-transmission/power-transmission concept involves the choice of one of several possible modulation schemes (see figure); the choice of a scheme for a specific system depends, in part, on whether the power-supply optical signal is steady or is modulated as described in the immediately preceding article, "Transmitting Power to Sensor Circuitry via Modulated Light" (NPO-19603).
In the case of a nominally steady power-supply optical signal, the data modulation can consist of brief interruptions of this signal; the modulation can be picked off by a simple amplifier circuit added to the power-conversion circuit in the remote station. In an experiment, data were transmitted at rates as high as 9.6 kb/s. The power-conversion circuit can be designed with sufficient reserve capacity and with a capacitor, or other energy-storage device to supply power to the other circuits in the remote station during the interruptions.
If the power-supply optical signal is nominally a steady pulse train in which the light is on half the time and off half the time, then the pulse train can be modified for transmission of data by the differential Manchester code. In the absence of data, the pulse train continues undisturbed; when data are present, some of the "on" pulses are changed to "off" pulses, and an equal number of "off" pulses are changed to "on" pulses. Because the total numbers of "on" and "off" pulses remain the same, the time-averaged transmitted power does not change.
In principle, it should also be possible to transmit data from the remote station back to the base station along the power-supply optical fiber. A microprocessor-controlled data-transmission optoelectronic circuit in the remote station would be synchronized with the Manchester-code pulses; during the "off" periods of the Manchester code, this circuit would transmit trains of relatively high-frequency data pulses.