by Steve Bush
Friday 11 May 2007
Getting power to a sensor is easy. Just use a 20mA current loop, or a battery at the far end, or a solar cell. The options are almost endless.
But what if the sensor is 20,000V away from the central unit, or the other side of an MRI scanner’s 2T alternating magnetic field?
There is a fairly straight forward answer - send the power down an optical fibre.
“It is relatively easy if you make it a large multi-mode fibre, with a core something like 60µm in diameter,” Dr Neil Broderick of the University of Southampton’s http://www.orc.soton.ac.uk/, tells EW.
Energy is injected from a laser. “Ideally you want a nice beam shape, with a Gaussian energy distribution,” says Broderick. “Efficiency does depend on the input quality.”
The laser energy can simply be focused into the end of the fibre with a lens. However, coupling in this way is prone to energy loss from the various interfaces and some energy will miss the core.
Well behaved circular beams, like those from a fibre laser, can be focused into the core fairly effectively, says Broderick, whereas the odd-shaped beams produced by semiconductor lasers are more prone to waste energy.
Single-mode fibres of the type used in long-haul telecoms can also carry power, but the 10µm core means it is significantly more difficult coupling in the light energy.
So how much energy can a fibre carry? “From very low power up to kilowatts,” says Broderick. But only if the source has a fairly wide bandwidth, more than a couple of gigahertz, he adds.
With high optical quality low bandwidth lasers “you get scattering. Anything above a couple of milliwatts will be reflected back,” says Broderick.
At anything over a few milliwatts, there are also increasing safety issues if the fibre can be cut, broken, or disconnected.
Multi-watt lasers and optical fibres “are the sort of thing used in laser surgery”, says Broderick.
Even with milliwatts, eye safety must be considered - particularly as the energy is most likely to be infra-red and therefore invisible. Some form of monitored feedback signal along the fibre can be incorporated to shut the laser down if the optical path could be unintentionally breached.
As semiconductor lasers with built-in fibre pig-tails can be bought over the counter, the engineering challenge for an OEM wishing to produce a power-over-fibre system is concentrated at the receiving end.
California-based JDS Uniphase (http://www.jdsu.com/) produces power-over-fibre systems. Last year it announced optical-to-electrical conversion efficiency greater than 50 per cent for a GaAs remote-end solar cell, in which it specialises.
“An efficiency of 50 per cent pushes the boundaries of the maximum theoretical limit for photovoltaic power conversion. This improvement enables more power- hungry electronics such as transducers, transceivers and sensors to be powered over fibre,” says JDSU. “The higher power efficiency also permits remote electronics to be powered by fibre over longer distances, such as tower-mounted installations for cellular and digital TV relay stations.”
The firm’s standard offering produces a few hundred milliwatts at the sensor end from a 1W laser power source and systems are available that include two channels of 16-bit data on the same fibre over 500m.
A higher output version uses a 5W source to deliver 1W, and can be paralleled to generate 5W at the sensor.
Wavelengths are 800 or 900nm in 62.5 or 105µm multi-mode fibres and data rates up to 50MHz are possible, claims the firm.
Although it can, as an option, the firm does not include regulation of the solar cell output as standard.
Solar cells are notoriously tricky to get the best out of as they have different I-V curves for each illumination level.
Loads that draw significantly less than the maximum output power can be fed by simple linear series or shunt regulators. Those which demand close to the maximum output power will require techniques such as peak-power-tracking .
Although fibre-over-power is touted as a useful way to cross voltage and magnetic barriers, power can actually be sent over quite long distances which opens up other applications.
“It depends on what wavelength of light is used,” says the firm. “For 780-850nm [near IR], a fibre length of 1km is probably the limit since a percentage of the light will be lost due to fibre attenuation. For 1310-1550nm, fibre lengths above 10km can be used.”
JDSU sees the future of power-over-fibre in sensing. “Photonic power replaces copper and batteries for remote sensors, coaxial cable in wireless applications, and oil or gas-filled measurement transformers in high voltage applications,” says the firm. “It can be leveraged across multiple markets, including medical, wireless communications, electrical power, industrial sensor, and aerospace applications.”
California-based RLH Industries, a firm new to power-over-fibre with a system due out early this year, is equally enthusiastic about the technology: “Fibre optics is the most reliable and safest method of transmitting data or voice services verses all other known copper or radio transmission methods.”
Peak power tracking
Solar cells have I-V characteristics that foil conventional feedback loops if maximum power is to be extracted under varying illumination and load impedance.
A technique which works is peak-power tracking where the operating point of a DC-DC converter is artificially dithered by an oscillator at a few Hertz.
By synchronously detecting the resultant output power ripple, a signal can be developed which is then used to push the operating point of the converter in the appropriate direction.
Loads in power-over-fibre systems have to be designed within the limitations of the power source. Start up surges, for example, must be limited.