By combining inexpensive and powerful laser diodes (emitter) with efficient photodiodes (detector), enough power (about 4 mA at 12 volt) is produced to operate a parked HART Field Instrument. The result is a point-to-point HART network with only optical fibers connecting the Field Instrument.
The scheme is illustrated in figure
Everything except the Optical Fiber and Field Instrument are located in the control area. A special interface built into the Field Instrument converts a conventional 2-wire HART Field Instrument to an optical Field Instrument. This equipment and the services to retrofit Field Instruments are available from NT International