Power Over Fiber
Definition: delivery of power for electronic devices via light in an optical fiber which is converted to electricity
German: Energieübertragung durch Glasfasern
Category: fiber optics and waveguides
Author: Dr. Rüdiger Paschotta
How to cite the article; suggest additional literature
Summary: This article explains
- what is the basic concept of “power over fiber”,
- what efficiency can be achieved, and on what factors it depends,
- what are typical applications, and the advantages over other methods of supplying power, and
- what laser safety issues are involved.
Optical fibers or fiber cables can be used for transmitting optical power from a source to some application. The term power over fiber or photonic power implies that optical power is converted to electrical power for some electronic device. That conversion can be done with a photovoltaic cell, i.e., a semiconductor device based on a material such as gallium arsenide, indium phosphide, or indium gallium arsenide. A typical system contains
- a laser diode emitting a few watts of optical power, being the original optical power source,
- a multimode fiber with a length between a couple of meters and a few hundred meters, and
- a photovoltaic cell with an active area of several square millimeters.
For short-range transmission, laser diodes emitting around 750–850 nm are typically used in combination with GaAs-based or silicon-based photovoltaic cells. Long transmission distances (possibly several kilometers) can be realized with systems operating at longer optical wavelengths because this drastically reduces Rayleigh scattering.
Typical transmitted powers are some hundreds of milliwatts or a few watts, but there is no principal reason why one should not be able to transmit much more, such as dozens or even hundreds of watts, given that multimode fibers with a sufficiently large core can transmit many kilowatts. It is only that the required photovoltaic cells would lead to a very large receiver.
A possible alternative is the light transmission through free space, but that approach is normally less practical, since it involves alignment and a higher risk of interruptions of the beam, and for large transmission distances also limitations due to beam divergence. In addition, there may be problems with laser safety.
Power Efficiency of Power over Fiber
The following sources of power losses need to be considered:
- Efficient laser diodes typically have efficiencies around 50% to 60% including fiber coupling.
- Propagation losses in the fiber (through scattering and absorption) are usually negligible, unless the transmission distance is quite long.
- While photovoltaic cells typically feature efficiencies of only ≈25% when used as solar cells, they can be far more efficient when operated with quasi-monochromatic light: efficiencies well about 50% are easily reachable, even nearly 70% have been demonstrated already, and even somewhat more should be possible. For that, the photon energy of the laser must be somewhat above (but not too far above) the band gap energy.
- Some additional losses may occur in electronics needed to transform the generated voltage to the required level and to stabilize it for the application.
Overall, a power conversion efficiency (electrical-to-electrical) around 20% to 30% should typically be feasible. For low required power levels, such losses should normally be well acceptable, while for higher power levels one will normally try to further optimize the efficiency to 40% or higher.
Advantages of Power over Fiber
Although an insulated copper wire is a simpler technology for transferring electric power, power over fiber offers advantages in specific situations:
- Non-conducting fiber cables (based on glass fibers or plastics) can be installed where high electric voltages occur. For example, a fiber can transmit power for a current transducer in a high-voltage transmission line. (Note that there are also fiber-optic sensors where no electrical power is needed locally.) Such current sensors with an optical power isolator can replace bulky transformer systems.
- The insulating property is also useful when a device (e.g. some radio signal receiver) is connected to an antenna, which could be hit by lightnings. There is then no risk that lightning strokes are transmitted via the cable.
- Optical delivery of power avoids any sensitivity to strong magnetic fields (e.g. in magnetic resonance imaging) and to electromagnetic interference. Conversely, no electromagnetic radiation, which might disturb other devices, can be emitted, and also no DC magnetic fields are generated.
- There is no risk that explosive materials (e.g. in a fuel tank of an airplane) can be ignited, as could occur e.g. via an electric spark.
- In a system for optical fiber communications, there may be spare fibers which can be used for transmitting power when an electrical connection does not exist. Other possibilities are to use some separate fiber core(s) of a multi-core fiber, or even to use one fiber core for both power delivery and data transmission.
- A fiber can have a far lower weight than an electrical cable, and may also be thinner.
- The same fiber may be used to send back data e.g. from a sensor, using some other wavelength channel.
Therefore, a number of applications can be envisaged in areas such as industrial sensors, aerospace, and optical communications.
Obvious disadvantages are the cost of optical components and the limited potential in terms of available power and conversion efficiency. There may also be a laser safety issue associated with several watts of optical power, which can leave the fiber when it is broken.
During normal operation, the laser light is fully confined in the fiber, and there is no risk e.g. for nearby persons. However, a substantial laser safety issue may arise when the fiber is broken, so that the laser light can exit. Despite the substantial divergence, a few watts of near-infrared light need to be considered as fairly dangerous for human eyes. Therefore, one may need to use a well protected fiber cable, or possibly include additional features for automatically switching off the laser when a fault is recognized.
The RP Photonics Buyer's Guide contains three suppliers for power over fiber systems. Among them:
See also: fibers, fiber cables, laser diodes, fiber optics
Questions and Comments from Users
What kind of optical fiber is best suited for PoF – single-mode or multimode?
How can you transmit data and power using the same cable?
What software can simulate PoF?
The author's answer:
Generally, it will depend on the used laser source which kind of fiber is most suitable. It is a (spatially) single-mode laser, you may well work with single-mode fiber, otherwise with multimode fiber.
Yes, you could at the same time transmit data, e.g. on a separate optical wavelength. But that is probably unusual.
Concerning simulations, I am not sure what aspect you would like to simulate. If it is just light propagation in the fiber, various software tools are available, including our RP Fiber Power.
What wavelengths can be transmitted over single mode fiber? I am specifically interested in the wavelengths found in sunlight that reaches the Earth’s surface. I am only concerned about transmission lengths less than 50 meters.
The author's answer:
This won't work, since you cannot efficiently couple sunlight into a single-mode fiber. This is basically because sunlight lacks spatial coherence.
Does an optical fiber have a lower power loss compared to a copper cable?
The author's answer:
That depends on the circumstances – for example, on the thickness of wires, the applied electric voltage etc.
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Can a single-mode fiber withstand 1 W from a semiconductor laser, or are there limitations on the fiber? For example, would it have to be made out of quartz instead of silica?
The author's answer:
Yes, a single-mode fiber can carry that power in the infrared region which is usually used for such purposes. However, one would usually use a multimode fiber.
Note that quartz is a crystalline material, from which one can hardly make flexible fibers. But maybe you mean “fused quartz”, a term sometimes used for amorphous material. That would be the same as fused silica.