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16 Jun 2006
Photonic Power provides an all-fibre solution that enhances power and data transmission of remote sensing devices. Mort Cohen explains why power-over-fibre is an attractive alternative to batteries and bulky copper cabling when operating in harsh environments.
Many applications use sensors, transducers and other communication devices in remote situations that subject the electronics to high radio frequency (RF) noise, electromagnetic interference (EMI), magnetic fields or very high voltage levels.
Duplex communication with these devices often uses standard optical fibre. Photonic Power's technology generates electricity by launching laser-light down a long optical fibre onto a photovoltaic, which in turn powers any remote electronics and associated signal conversion circuitry.
Delivering power-over-fibre in this fashion, and in lieu of copper wiring, has several significant benefits. Firstly, the system is immune to surrounding RF and magnetic fields, lightning and high voltage effects. It also offers spark-free operation that is impervious to RF heating and will not be affected by radiation from the power source. Optical fibre is less bulky than copper cabling and is capable of operating over longer distances.
Principles of power-over-fibre
Figure 1 shows a photonic power module (PPM); using this, up to 5 W of optical power from a laser diode is launched into a multimode or singlemode fibre. The operating wavelength ranges between 780 and 980 nm depending on distance and power considerations. This optical power illuminates a photovoltaic power converter (PPC) that is co-located with the remote electronics up to several kilometres from the laser source.
The PPC converts the delivered optical power to electrical power with greater than 50% efficiency to drive the remote electronics. For example, approximately 1 W of optical power illuminating the PPC would result in 0.5 W of delivered electrical power.
The PPC is typically a 2 × 2 mm gallium arsenide or indium phosphide chip composed of several p-n junctions that are connected in series such that the voltages from the junctions are additive. The amount of current delivered is proportional to the level of light illuminating the chip.
Optically powered data links
The PPM is the core technology in an optically powered data link (OPDL) system. OPDLs, as shown in figure 2, are noise-immune turnkey solutions for powering and transmitting data over standard optical fibre.
The schematic in figure 3 shows how an OPDL system integrates with a remote sensor. Optical power from a laser diode and driver assembly is launched into the fibre. The optical power is converted into electrical power using a PPC. This converted power drives the sensor and associated analogue-to-digital conversion electronics at the remote end. The analogue sensor data is then converted into a 16-bit digital format and is transmitted through a second fibre using an LED in most cases.
The data are transmitted to a central processing area where they are either reconverted to analogue format or digitally analysed. Feedback and self-check features monitor all vital functions, including laser output level, data-link integrity, recovery and, if necessary, synchronization. This all-fibre approach provides a unique, electrically isolated, lightning-proof power delivery and data transmission system.
Enhancing sensor performance
Electronic sensors are used in many industrial monitoring and test and measurement applications to record operating parameters or environmental conditions, such as temperature, pressure, humidity, voltage and current. Many sensors are installed in environments that are hazardous, electrically noisy, inaccessible, or exposed to extreme weather. Other sensors use batteries as their power source even though they require frequent replacement or recharging.
Using OPDL technology to power the sensor and transmit the sensor data ensures continuous, fully isolated power, which is free from the effects of RF, EMI or magnetic fields. It also ensures spark-free operation in environments where safety is paramount.
OPDL technology can be used across a diverse array of industries for a wide range of applications, including test and measurement; sensing in industrial, medical, aerospace and homeland security applications; and electric power current monitoring.
Photonic Power believes that a number of electronic measurement devices can benefit from its all-fibre solution. These include field probes for electromagnetic compatibility measurements; fuel gauges and sensors on aircraft; underground exploration, mapping, and seismic monitors; location tracking equipment in underground mining operations; video surveillance cameras; medical monitoring devices; and current transducers in electric power transmission grids.
The key advantage in most of the applications listed above is immunity to the surrounding environment, which permits operation in high radiation fields, potentially explosive environments, and hazardous environmental conditions such as high temperatures or pressures.
In addition, the use of optical fibre fully isolates the electronics from lightning and high voltage effects and eliminates the possibility of RF-induced heating. All of these factors translate into increased personnel safety, improved equipment reliability and reduced maintenance costs.
Electromagnetic interference testing
The test and measurement industry is increasingly using OPDL technology to monitor electromagnetic emissions from a variety of commercial products including mobile phones and other wireless devices; computers; televisions; and automotive equipment.
Electrical circuits produce electric and magnetic fields that radiate RF energy, increasing the possibility of the electronics exceeding regulated EMI limits. Electric field probes are used in EMI testing to quantify emission levels and help identify EMI sources. Traditional EMI chamber testing can be long and arduous. Batteries typically provide the power for the probes and the testing must be halted when recharging or replacement is required. This downtime limits the number of tests that can be performed in a 24 h period.
As shown in figure 4, OPDL technology can continuously power the probe while generating sampling data at high rates, providing performance measurement. The OPDL serves as an isolated power supply to drive the EMI probes, offering a pure power source that contributes no extraneous interference and eliminates the need for batteries and battery exchange or recharge.
EMI testing can be completed more efficiently and more cost-effectively. The small form factor of the PPC also permits the use of compact probes in confined spaces with reduced electric field perturbation.
Photonic power harnesses laser light to offer a totally new method of driving electronic circuitry. The use of an OPDL system eliminates large, costly instrument transformers, bulky coaxial cable, batteries and lightning suppressors for many industry applications while offering fully isolated electric power and data transmission at a competitive price.
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