Photonic Power Technical Questions
1. What is a photovoltaic power converter (PPC) and how does it work?
2. How much power can be obtained?
3. What voltage and current can I expect?
4. Is this voltage regulated?
5. What happens if the illuminating light intensity gets too low (< 10 mW) ?
6. How is the conversion efficiency calculated?
7. Is the impedance of the device constant?
8. What different voltage output does JDSU offer?
9. What packages are available?
10. What is the most suitable light source to use?
11. Does JDSU offer any light sources?
12. What fiber should be used?
13. How long can the fiber be?
14. Do I need a heat sink for power converter?
15. Are there any special considerations for a circuit design engineer?
16. Can the power converter also receive data?
17. Can multiple converters be connected in parallel for higher power output?
1. What is a photovoltaic power converter (PPC) and how does it work?
The term power converter is used for a variety of power conversion devices. At JDSU, we narrowly define power converter as a photovoltaic power converter (PPC). PPC is a semiconductor device designed to convert light into electrical power at voltages which are convenient to drive electronic circuitry and sensors. It can receive power either from a light source, such as a laser diode or an LED connected to a fiber optic cable, or from direct light illumination. The way it works is very similar to how a solar cell works, except with different voltage outputs, at much higher conversion efficiency, and power delivery. When a high power laser source is connected to the PPC via fiber, it basically forms a power line, except the "power" is carried by light. This makes photonic power a viable technique for applications where elimination of electromagnetic interference (EMI) or isolation of the electronics from the surrounding environment is a requirement
2. How much power can be obtained?
Normally, hundreds of milliwatts. Since the power conversion efficiency of the PPC is in the 40% to 50% range, when illuminated with laser light, the laser power at the fiber output is required to be in the one watt range to deliver a few hundred milliwatts of electrical power from the PPC. Please note the fiber attenuation of laser power is not included in the above consideration.
3. What voltage and current can I expect?
As in an illuminated solar cell, the maximum output voltage is limited by its open-circuit voltage, and the maximum current output is limited by its short circuit current. However, the maximum power output is neither at open-circuit voltage point, nor at short-circuit current point. In fact, at both of those points, the (electrical) power output is zero. To actually obtain the (converted) electrical power output, the PPC device needs to be set at a voltage less than its open-circuit voltage, which is determined by the PPC device, and at a current level very close to its short-circuit current level, which is proportional to the illumination intensity.
Although not a standard feature, voltage regulation can be incorporated. The only restriction is that the regulated voltage has to be less than open-circuit voltage for the particular PPC device that is used.
5. What happens if the illuminating light intensity gets too low (<10 mW)?
The open-circuit voltage becomes more or less a constant from a PPC device when the illuminating light intensity is high enough, for example, more than 10 mW. When the light intensity gets too low, say 100 µW, then the open-circuit voltage drops with decreasing light intensity. This is not desired for most applications because a constant driving voltage is usually required.
6. How is the conversion efficiency calculated?
Efficiency (É) = Electrical Power Out / Optical Power In
The efficiency is a measure of the ability of the power converter to change optical power into electrical power. Optical Power In is considered the power out of the fiber. Electrical Power Out is the product of the output voltage and the output current of the PPC device, taken from the current-voltage curve (IV curve).
This conversion efficiency is in fact not a constant even under constant optical power illumination. However, there is a maximum power output point, where the voltage value is Vm, which is less the open-circuit voltage (Voc) but close to it, and the current value is Im, which is less the short-circuit current (Isc) but close to it as well. The maximum power point is represented by: (Voc) x (Isc) x (FF), where the fill factor (FF) is defined as the ratio of Vm x Im, and Voc x Isc. We normally specify this maximum power conversion efficiency point to represent the capability of a PPC.
7. Is the impedance of the device constant?
No. As can be seen from the response to the previous question, the matching impedance for the maximum power output is Vm / Im. But, because Im changes with light intensity, the matching impedance for the maximum power output changes with light intensity as well.
8. What different voltage output does JDSU offer?
Currently, JDSU offers 2, 3, 4, 5, 6, and 12 volts of open circuit voltage.
9. What packages are available?
The PPC is generally packaged in a fiber optic receptacle or with a pig-tailed fiber. The fiber optic receptacles are standard ST, FC, and SMA. The pig-tailed fiber includes a ST, FC, or SMA connector.
JDSU's Photonic Power Module (PPM) delivers a comprehensive solution that includes the laser, laser driver, heat sink package, system calibration, warranty, and support.
10. What is the most suitable light source to use?
The most suitable light source is a high power laser with fiber pig-tailed output. However, the wavelength and the fiber need to be carefully selected in order to obtain the best performance from the PPC. For assured specifications, please contact JDSU for recommendations on a complete PPM solution optimized for output power, distance, and efficiency under various environmental conditions.
11. Does JDSU offer any light sources?
JDSU offers the most suitable laser light source for optimizing the efficiency of our power converters. This laser is generally in a very compact package with adjustable current control. The laser output is typically pigtailed. All of these components are included in the PPM.
12. What fiber should be used?
The main consideration for choosing the fiber is to match output beam profile from the fiber with the PPC. Different kinds of fibers may affect the performance of PPC. JDSU will provide the optimal laser/fiber/PPC configuration if a PPM is ordered. Please contact JDSU for help with selecting the most suitable fiber for your application if you are opting to acquire the PPC and laser separately.
13. How long can the fiber be?
This depends on what wavelength of light is used. For near-IR (780-850 nm), a fiber length of 1km is probably the limit since a percentage of the light will be lost due to fiber attenuation. For longer wavelengths (1310-1550 nm), fiber lengths up to and above 10km can be used.
14. Do I need a heat sink for power converter?
Generally not. JDSU can provide additional heat sinking if desired. For certain situations like very high output power applications (>1 W), or custom packaging, special provision for heat sinking may be required.
15. Are there any special considerations for a circuit design engineer?
In any circuit application involving a PPC, the design engineer must ensure that there is adequate power for the circuitry at all times. This is particularly true at a start-up phase when the circuit is first turned on via light illumination. Since the PPC is a limited power source and only delivers electrical power directly proportional to the incoming light power, the PPC cannot withstand any current drain. So, it is well advised to include a capacitor that has to be charged before any load is applied.
16. Can the power converter also receive data?
Yes. The power converter can receive data with rates up to 50 MHz, depending on the size of the power converter.
17. Can multiple converters be connected in parallel for higher power output?
Yes, they can. Power converters can be connected in parallel and be illuminated individually. The electrical output is simply the addition of the current from each power converter, while maintaining the same output voltage. Our HiPOD product now offers up to 5 watts of electrical output power.
