Power Over Fiber Shines at Voltage Isolation

Jul 25, 2007 11:46 AM
By Mark Valentine, Technical Editor, Power Electronics Technology

The basic PoF system consists of the Tx unit (see Figure 3), located at the substation, and the Rx unit (see Figure 4), located at the remote fiber-copper interface. The two units are linked by three optical fibers. Two fibers transfer the power, with each fiber forming a single power channel for each of the two continuous-wave 830-nm laser diodes in the Tx unit. The third fiber transfers a power-integrity feedback signal from a conventional LED in the Rx unit to a conventional optical receiver in the Tx unit.

According to Furey, the PoF’s energy-transfer efficiency is low because the technology is new, and the main purpose is safety, not performance. The lasers in the Tx unit consume about 48 W in order to deliver about 720 mW at the Rx unit. While power losses increase with fiber length, useful power is still transmitted at the maximum recommended fiber length of 1500 feet.

Furey also states that the lasers have a narrow operating temperature range that requires both active heating and cooling systems onboard the Tx unit. Therefore, the ambient system operating temperature for the Tx unit is from 0°C to 35°C. A more robust version with an ambient operating temperature range of -20°C to +55°C is under development.

Furey states that each of the two energy-carrying optical fibers feeds a respective photovoltaic power converter (PPC) in the Rx unit that produces an output of 12 V. The two receivers are tied in series to generate 24 V at the output. The outputs of several Rx units can also be tied in series or parallel to produce even greater output voltages or currents, respectively.

RLH recently filed the patent for PoF technology, and four evaluation units have been successfully deployed and in service for over a month. According to Furey, the primary application is for powering RLH’s fiber-optic telephone cards that require low power in remote locations without access to grid or portable power. It is certainly interesting to note a certain parallel in history. Specifically, after Bell Labs invented the transistor, it envisioned the sole application of this device as a replacement for the mechanical relays in automated switchboards, to be used exclusively in their own telephone network.

For more information on PoF technology, as well as utility-scale high-voltage isolation, please visit the RLH Industries web site.

Figure 3. The Tx unit of the PoF system is rack-mountable, and provides status indicators, including when the laser temperature control systems are engaged.

Figure 4. The Rx unit is compact and may be installed in any RLH fiber-optic-link-card housing.

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© 2007 Penton Media, Inc.