Modulated (as opposed to steady) light can be used in the optical transmission of power to remotely located sensor circuitry in a system like that described in the preceding article, "General-Purpose Optically Powered Sensor and Control System" (NPO-19604). This is analogous to the use of alternating (as opposed to direct) current on electrical power lines, and the benefit is similar; namely, that the voltage can be stepped up to a desired level by use of a transformer.
The figure illustrates two alternative techniques as well as the present technique for fiber-optic transmission of power from a base station to a remote station (sensor node) and conversion of optical to electrical power at the remote station. In the first technique, steady light is sent to an array of photodiodes, which are connected in series to build up the voltage to the required level. The disadvantage of this technique is that an array of photodiodes can be expensive, and there can be significant losses in the coupling of light from the optical fiber to the array, depending on the shape of the array.
In the second technique, steady light is sent to a single photodiode, and the output steady voltage of the photodiode is increased to the required higher level by a chopper, a step-up transformer, and a rectifier. The disadvantage of this technique is that the chopper consumes a substantial part of the power, thereby reducing the overall power-conversion efficiency of the remote station.
In the present technique, the optical power is modulated at the source in the base station, where efficiency is somewhat less of a consideration because power is more abundant there. Preferably, the light is either modulated with a sine wave or is chopped into square pulses ("on" during the first half cycle, "off" during the second half cycle). The modulated light illuminates a single photodiode, the output of which is now modulated. The modulated output of the photodiode is fed directly to a step-up transformer; because of the modulation, there is no need to process it through a chopper. Therefore, the chopper is eliminated and the power-conversion efficiency correspondingly increased. In practice, the conversion efficiency will not be high unless care is taken in the design of the step-up transformer and the rectifier circuit. Techniques to recover the magnetic energy in the transformer that are similar to those used in chopper circuits should be used.
This work was done by Shannon P. Jackson, Harold Kirkham, and Colonel McLyman of Caltech for NASA's Jet Propulsion Laboratory.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category, or circle no. 173 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).