- Email: [email protected]
Optical power supply for fiber-optic hybrid sensors
Sensors and Actuators A, 25-27 (1991) U-480
475
Optical Power Supply for Fiber-optic Hybrid Sensors WALTER
GROSS
Siemens AC ZFE Fl TPH41, Corporate Research Center, D-8520 Erlangen (F.R.G.)
Abstract
1. Introduction
Systems for optical power supply are of central importance for the concept of fiberoptic hybrid sensors. We present our systems for optical power supply which are capable of giving up to 150 mW of electrical power to the sensor head. Such a system for optical power supply consists of a combination of optical and electronic modules, namely a suitable optical power source, an optical fiber as power link and an optical energy converter with voltage control unit. This concept of a fiber-optic power supply in combination with a fiber-optic hybrid sensor covers many of the advantages given by a pure optical sensor. The galvanic isolation between the measuring circuit and the electronic evaluation unit is notable and implies use at higher voltages with high immunity to interference (EMI) of the data transmission line. A key element for our fiber-optic power supply is a specially developed photo-element array (PEA) for optical-electrical power conversion. Conversion efficiencies of lo15% with silicon material and up to 30% with GaAs material have been achieved. The available voltage with our single photoelement array (SPEA) can be up to 8 V. With a multiple photoelement array (MPEA) the voltage is switchable between 8 and 32 V. An application example for a complete fiber-optic hybrid sensor is given. With the fast rate of development of electronic and optoelectronic components, further improvements and a broad range of practical applications can be expected. An outlook to possible future improvements is given.
We present an optical power supply for fiber-optical hybrid sensors. In general, such a hybrid sensor is a combination of two technologies, fiber-optical and conventional electronic technology. This concept of a fiberoptic power supply in combination with a fiber-optic hybrid sensor has many of the advantages of pure optical sensors. Important properties include the galvanic isolation between the measuring circuit and the electronic evaluation unit. This implies use at higher voltages with high immunity to interference (EMI) of the data transmitter line.
0924-4247/91/$3.50
2. Conventional Sensor The use of conventional measuring transducers (e.g., for temperature, pressure, strain, voltage, current, etc.) requires that these units are supplied with electrical power. Usually, this power is supplied from a normal a.c. mains voltage, e.g., with a simple isolating transformer. Such a set-up is practical for simple applications with no or only few requirements in terms of electrical isolation. If we replace the metal transmission line by an optical fiber link, we obtain an electrically powered fiber-optical hybrid sensor with galvanic separation between the sensor head and receiver unit (Fig. 1) and no interference or earth problems in the data link. In a further step we may also replace the electrical power line by an optical power supply via an optical fiber. The result is a pure fiber-optical hybrid sensor consisting of an optical power transfer 6 Elsevier Sequoia/Printed in The Netherlands
476
3. Hybrid Sensor
Sensor head
Electrical
Electrical
SUPPlY
SUPPlY
Transmission line Receiver module
Fig. 1. Block diagram of a conventional electrical powered hybrid sensor with optical data transmitter (elec. = electrical, phys. = physical, opt. = optical).
Power transmitter
Datareceivel
Sensor head
Transmission lines Transmitter and receiver module
Fig. 2. Block diagram of an optically powered hybridsensor head with optical data transmitter, (elec. = electrical, phys. = physical, opt. = optical).
system, a conventional electrically operated measuring transducer for data conversion, a light-emitting diode as data transmitter, an optical fiber for data transfer and a photodiode as data receiver (Fig. 2).
Such a pure fiber-optical hybrid sensor is of special interest for applications which have extremely high demands concerning galvanic isolation between the measuring circuit and evaluation unit [l-lo]. The term ‘high demands’ means compliance with use at high voltage levels, pertinent explosion-protection regulation (for low-power systems) and the suppression of electrical interference in a transmission line. The principle of the pure fiber-optical hybrid sensor can be further expanded to a fiber-optical system consisting of several independent sensor heads (Fig. 3). The different sensor heads are supplied, e.g., from an optical power distributor, with a data-trigger signal which is used to trigger selectively the data transfer from a specific sensor head to a data receiver. Ideally, this requirement (galvanic separation between sensor head and receiver unit) is met by sensor operation at the measuring point without the requirement for an electrical data line or electrical power, as in fully-optical sensor heads (e.g., principle - _ of interferometric measurement). But the pure fiber-optical hybrid sensor has very
optical-power and trigger fiber Power transmitter
optical-data Sensor heads
fiber
Transmission lines
Data receiver
Transmitter
and
receiver module Fig. 3. Block diagram of n optically powered hybrid-sensor heads with optical power line and optical data transmitter line.
477
similar properties and is of interest in application areas in which it is not yet possible to use ‘fully’ optical sensor heads. To supply the sensor heads, electical power levels of a least a few milliwatts to some tens of milliwatts must be transmitted. Alternatively, supplying a sensor head with power from a battery is worthwhile only if the battery has sufficient power stored to guarantee longterm operation, or if it is possible to replace the battery. For local power at the sensors, the following solutions are feasible: (i) Wind, to drive a wind-turbine which generates the power for a sensor head. (ii) Sunlight, which is converted from solar cells to electrical power. (iii) A varying electromagnetic field, as the source for voltage or current transformers. With all these solutions it is not always possible in practice to guarantee the supply of power at the measuring point. For example, if the power supply for the measuring transducers is taken from the ambient power surrounding them, such systems are dependent on a power source that is not intended for the operation of these sensor units. The use of such measuring transducers is therefore restricted to cases in which the presence of ambient power is guaranteed during the whole period of measurement. A hybrid-sensor head supplied with light from an optical fiber does not need to take its power from the environment. It can therefore be positioned freely, and there is no battery to be replaced. In the sensor head, an optical energy converter converts the light, supplied via an optical fiber, into electrical power. An optically powered hybrid-sensor head provided with such an optical power supply behaves almost like a fully-optical sensor head. The following Sections discuss in detail the optical power supply in the pure fiber-optic hybrid sensor of Fig. 2.
4. Light Sources The energy transfer to the sensor head by an optical fiber can be achieved with different
light sources. Using matched peripheral components, an incandescent lamp can drive a hybrid-sensor head at up to a few mW electrical power. However, its lifetime is only at most 250 h, which is still insufficient for most of our applications. Light-emitting diodes (LEDs) as power sources are good for sensor heads with small power consumption. At the moment the maximum electrical power that can be supplied from a LED is about 300 pW. Laser diodes are currently available with radiation power (continuous ratings (cw)) & from smaller than 1 mW up to 1 W. We have done experiments with a laser diode that has a radiation power & = 40 mW. With such elements, pure hybrid-sensor heads can be supplied with electrical power approximately up to P,, = 8 mW after a 10 m fiber. Presentday high-power semiconductor strip laser arrays (LAS) emit a radiation power 300 mW < &, < 10 W. The average lifetime of a 300 mW LA is normally 10 000 h. The aperture dimension of a 10 W LA (10 mm width and 300 strips) is in most cases too big for glass-fiber optical applications. Therefore we have used LAS with 40 strips 400 pm wide and pigtail output. The conversion efficiency from electrical to optical power for a 40 strip highpower LA is nearly 25%. We obtain an output power at the pigtail end of 40,, = 1000 mW. The light from the pigtail is coupled into a 10 m fiber with a launch efficiency of nearly 75%. Therefore an optical power 4 0u,max= 750 mW is achieved at the end of the 10 m fiber. The losses due to the absorption of the transmitter fiber (attenuation of 6 dB/km) are negligible for a 10 m length.
5. Energy Converter For the operation of an optically powered hybrid-sensor head, the light supplied through an optical fiber has to be converted into electrical power. This is done by an optical energy converter. At present, electrically operated sensor heads work at voltages of 3 V up to 15 V. A gallium arsenide photoelement supplies only
478
a no-load voltage between 1 and 1.1 V (0.6 V and 0.7 V in comparison with silicon). How can this value be transformed? There exist various possibilities of voltage multiplication, some of which are: (a) voltage transformation, pulse-controlled operation at the light source and use of a single photelement with subsequent transformer; (b) the use of monolithic voltage doublers; (c) light distribution by means of a star coupler to several series-connected photoelements or photoelement arrays (PEAS); (d) light distribution to multiple photoelements or PEAS, without an intermediate component. In addition to a photoelement, the first three methods require components whose own power loss cannot be neglected. This results in a deterioration in conversion efficiency. The method of voltage multiplication with multiple photoelements requires no additional components. We have therefore concentrated on the design of special PEAS made of Si and GaAs with a cascade arrangement of single photoelements. Photoelements in GaAs have a higher no-load voltage and up to five times the conversion efficiency of Si photoelements. Therefore a new design of GaAs PEAS has been developed in a square 1 x 1 mm* pattern (Fig. 4(a), a single photoelement array, SPEA). Figure 4(b) shows an arrangement of four SPEAs (called a multiple photoelement array, MPEA) with four electrically isolated output voltages.
0.
0
I
0.5
1
I
1,5
Laser current
1
I
2 I
3 A 3,5
---+
Fig. 5. Optical to electrical conversion efficiency and electrical power vs. forward laser current for a SPEA.
To test these PEA designs, the maximum optical power c#J,,,= 690 mW was measured at the end face of a 10m long fiber. The wavelength of this LA optical source is 1 = 838 nm. The conversion efficiency of a SPEA as a function of radiation or forward laser current is demonstrated in Fig. 5. The x-coordinate at the top is the optical power and the y-coordinate shows the conversion efficiency and the converted electrical power; for a single GaAs PEA of eight segments an electrical power of 93 mW was produced (this is near to the saturation density). This corre-
laser uavekngih
+%d-k-H--13 Fig. 4. Arrangement of GaAs photoelements in different PEAS: (a) SPEA (single PEA); (b) MPEA (multiple PEA) with four electrical isolated output voltages. PES = active photoelement segment.
0
I
2,5
load resistor
R
a3anm3 kO 3.5
--)
Fig. 6. Electrical power and output voltages vs. load resistor of a GaAs SPEA.
479
We believe that we can reduce the series resistance and achieve a still better conversion efficiency. With a multiple GaAs PEA an electrical power of 118 mW was converted (Fig. 7). This corresponds to a conversion efficiency of vpEA=118mW/690mW=
17.1%
3 irradiance on a MPEA, E, = 220 kW/m2
01
/
t
I
0.5
0
I
1
1.5
I I
2
2,5
I
3
A 3,5
0
Figure 8 again shows the corresponding output voltage as a function of a load resistor value, the SPEAs being connected in series.
Losercurrentf --,
Fig. 7. Optical to electrical conversion efficiency and electrical power vs. forward laser current for a MPEA.
sponds
to a conversion efficiency of
qpEA= 93 mW/690 mW = 13.5% => irradiance on a SPEA, E, = 879 kW/m* Figure 6 compares the maximum electrical power with the corresponding output voltage of a SPEA as a function of load resistor value. The constant optical power for these measurements is 4 = 690 mW. An output voltage of up to U, x 8 V has been achieved (e.g., 5 V at maximum power PC,= 93 mW). 35
I
6. Conclusions The development and production of a PEA gives us a simple means of supplying the head of a hybrid sensor with electrical power. Light sources of suitable radiant power are already available. It has been shown that it is possible to supply a fiber-optic hybrid-sensor head with more than 100 mW of electrical power via a fiber optical power line. This power level is sufficient for running a wide range of conventional electrical sensor heads. In the near future optical power levels of 10 W by means of high-power output laser diodes or laser arrays will be available for fiber optical applications with matched optical fibers and could deliver an electrical power P,, x 1 W. The principle of such pure fiber-optical hybrid sensors has most of the advantages of fully optical sensors and gives great flexibility in choosing the appropriate sensor head.
Acknowledgements 8 0
I 5
I
I
I
I
10
15
20
25
loadresistor
I
I
50kn35
R -+
Fig. 8. Converted electrical power and output voltage vs. load resistor for a MPEA with series connection.
I would like to take this opportunity of expressing my sincere thanks to all those who made it possible to build the PEA/MPEAs, particularly to Mrs Bittel and Mr Bogner for their active support.
480
References
6 P. Schweizer, L. Neveux
and D. B. Ostrowsky, Optical fibre powered pressure sensor, Fourth Int.
1 F. Caspers and E.-G. Neumann, Optical power supply for measuring or communication devices at high voltage levels, IEEE Trans. Instrum. Meas., IM-29 (1980) 73-74. 2 H. S. Rauschenbach, Solar Cell Array Design Hundbook, Van Nostrand, London, 1980, p. 156. 3 H. Schneider and G. Zeidler, Herstellverfahren und Ausftihrungsformen von Lichtwellenleitern, Siemens Telecom Report 6 (April 1983), Supplement Nachrichtentibertragung mit Licht, pp. 29-35. 4 A. Ohte, K. Akiyama and I. Ohno, Optical-powered transducer with optical-fiber data link, Proc. SPIE, Vol. 418, Fiber Optics and Laser Sensors II, May 1984, pp. 33-38. 5 J. Niewisch, Design and application of a fiber-optic liquid level sensor, Siemens Forsch. Entwicklungsber., I5(1986)
115-119.
Symp. Optics and Opto-electronics, Applied Science and Engineering, The Hague, The Netherlands, Mar. 30-Apr. 3, 1987, p. 23. 7 R. C. Spooncer, B. E. Jones and R. Ohba, Pulse
modulated optical fibre quartz temperature sensor, Fourth Int. Symp. Optics and Opto-electronics, Applied Science and Engineering, The Hague, The Netherlands, Mar. 30-Apr. 3, 1987, p. 23. 8 W. Gross, Fiber-optic hybrid sensors with optical power supply, Siemens Forsch. Enrwicklungsber., I7 (1988) 13-17. 9 J. Lenz and P. Bjork, Optical powered sensors: a system approach to fiber optic sensors, Proc. SPIE, Vol. 961, Industrial Optical Sensing, 1988, pp. 8-25.
0 W. Kuntz and W. Mores, Energie-und Dateniibertragung iiber Lichtwellenleiter bei intelligenten Sensoren, Tech. Mess., 56 ( 1989) 166- 170.
475
Optical Power Supply for Fiber-optic Hybrid Sensors WALTER
GROSS
Siemens AC ZFE Fl TPH41, Corporate Research Center, D-8520 Erlangen (F.R.G.)
Abstract
1. Introduction
Systems for optical power supply are of central importance for the concept of fiberoptic hybrid sensors. We present our systems for optical power supply which are capable of giving up to 150 mW of electrical power to the sensor head. Such a system for optical power supply consists of a combination of optical and electronic modules, namely a suitable optical power source, an optical fiber as power link and an optical energy converter with voltage control unit. This concept of a fiber-optic power supply in combination with a fiber-optic hybrid sensor covers many of the advantages given by a pure optical sensor. The galvanic isolation between the measuring circuit and the electronic evaluation unit is notable and implies use at higher voltages with high immunity to interference (EMI) of the data transmission line. A key element for our fiber-optic power supply is a specially developed photo-element array (PEA) for optical-electrical power conversion. Conversion efficiencies of lo15% with silicon material and up to 30% with GaAs material have been achieved. The available voltage with our single photoelement array (SPEA) can be up to 8 V. With a multiple photoelement array (MPEA) the voltage is switchable between 8 and 32 V. An application example for a complete fiber-optic hybrid sensor is given. With the fast rate of development of electronic and optoelectronic components, further improvements and a broad range of practical applications can be expected. An outlook to possible future improvements is given.
We present an optical power supply for fiber-optical hybrid sensors. In general, such a hybrid sensor is a combination of two technologies, fiber-optical and conventional electronic technology. This concept of a fiberoptic power supply in combination with a fiber-optic hybrid sensor has many of the advantages of pure optical sensors. Important properties include the galvanic isolation between the measuring circuit and the electronic evaluation unit. This implies use at higher voltages with high immunity to interference (EMI) of the data transmitter line.
0924-4247/91/$3.50
2. Conventional Sensor The use of conventional measuring transducers (e.g., for temperature, pressure, strain, voltage, current, etc.) requires that these units are supplied with electrical power. Usually, this power is supplied from a normal a.c. mains voltage, e.g., with a simple isolating transformer. Such a set-up is practical for simple applications with no or only few requirements in terms of electrical isolation. If we replace the metal transmission line by an optical fiber link, we obtain an electrically powered fiber-optical hybrid sensor with galvanic separation between the sensor head and receiver unit (Fig. 1) and no interference or earth problems in the data link. In a further step we may also replace the electrical power line by an optical power supply via an optical fiber. The result is a pure fiber-optical hybrid sensor consisting of an optical power transfer 6 Elsevier Sequoia/Printed in The Netherlands
476
3. Hybrid Sensor
Sensor head
Electrical
Electrical
SUPPlY
SUPPlY
Transmission line Receiver module
Fig. 1. Block diagram of a conventional electrical powered hybrid sensor with optical data transmitter (elec. = electrical, phys. = physical, opt. = optical).
Power transmitter
Datareceivel
Sensor head
Transmission lines Transmitter and receiver module
Fig. 2. Block diagram of an optically powered hybridsensor head with optical data transmitter, (elec. = electrical, phys. = physical, opt. = optical).
system, a conventional electrically operated measuring transducer for data conversion, a light-emitting diode as data transmitter, an optical fiber for data transfer and a photodiode as data receiver (Fig. 2).
Such a pure fiber-optical hybrid sensor is of special interest for applications which have extremely high demands concerning galvanic isolation between the measuring circuit and evaluation unit [l-lo]. The term ‘high demands’ means compliance with use at high voltage levels, pertinent explosion-protection regulation (for low-power systems) and the suppression of electrical interference in a transmission line. The principle of the pure fiber-optical hybrid sensor can be further expanded to a fiber-optical system consisting of several independent sensor heads (Fig. 3). The different sensor heads are supplied, e.g., from an optical power distributor, with a data-trigger signal which is used to trigger selectively the data transfer from a specific sensor head to a data receiver. Ideally, this requirement (galvanic separation between sensor head and receiver unit) is met by sensor operation at the measuring point without the requirement for an electrical data line or electrical power, as in fully-optical sensor heads (e.g., principle - _ of interferometric measurement). But the pure fiber-optical hybrid sensor has very
optical-power and trigger fiber Power transmitter
optical-data Sensor heads
fiber
Transmission lines
Data receiver
Transmitter
and
receiver module Fig. 3. Block diagram of n optically powered hybrid-sensor heads with optical power line and optical data transmitter line.
477
similar properties and is of interest in application areas in which it is not yet possible to use ‘fully’ optical sensor heads. To supply the sensor heads, electical power levels of a least a few milliwatts to some tens of milliwatts must be transmitted. Alternatively, supplying a sensor head with power from a battery is worthwhile only if the battery has sufficient power stored to guarantee longterm operation, or if it is possible to replace the battery. For local power at the sensors, the following solutions are feasible: (i) Wind, to drive a wind-turbine which generates the power for a sensor head. (ii) Sunlight, which is converted from solar cells to electrical power. (iii) A varying electromagnetic field, as the source for voltage or current transformers. With all these solutions it is not always possible in practice to guarantee the supply of power at the measuring point. For example, if the power supply for the measuring transducers is taken from the ambient power surrounding them, such systems are dependent on a power source that is not intended for the operation of these sensor units. The use of such measuring transducers is therefore restricted to cases in which the presence of ambient power is guaranteed during the whole period of measurement. A hybrid-sensor head supplied with light from an optical fiber does not need to take its power from the environment. It can therefore be positioned freely, and there is no battery to be replaced. In the sensor head, an optical energy converter converts the light, supplied via an optical fiber, into electrical power. An optically powered hybrid-sensor head provided with such an optical power supply behaves almost like a fully-optical sensor head. The following Sections discuss in detail the optical power supply in the pure fiber-optic hybrid sensor of Fig. 2.
4. Light Sources The energy transfer to the sensor head by an optical fiber can be achieved with different
light sources. Using matched peripheral components, an incandescent lamp can drive a hybrid-sensor head at up to a few mW electrical power. However, its lifetime is only at most 250 h, which is still insufficient for most of our applications. Light-emitting diodes (LEDs) as power sources are good for sensor heads with small power consumption. At the moment the maximum electrical power that can be supplied from a LED is about 300 pW. Laser diodes are currently available with radiation power (continuous ratings (cw)) & from smaller than 1 mW up to 1 W. We have done experiments with a laser diode that has a radiation power & = 40 mW. With such elements, pure hybrid-sensor heads can be supplied with electrical power approximately up to P,, = 8 mW after a 10 m fiber. Presentday high-power semiconductor strip laser arrays (LAS) emit a radiation power 300 mW < &, < 10 W. The average lifetime of a 300 mW LA is normally 10 000 h. The aperture dimension of a 10 W LA (10 mm width and 300 strips) is in most cases too big for glass-fiber optical applications. Therefore we have used LAS with 40 strips 400 pm wide and pigtail output. The conversion efficiency from electrical to optical power for a 40 strip highpower LA is nearly 25%. We obtain an output power at the pigtail end of 40,, = 1000 mW. The light from the pigtail is coupled into a 10 m fiber with a launch efficiency of nearly 75%. Therefore an optical power 4 0u,max= 750 mW is achieved at the end of the 10 m fiber. The losses due to the absorption of the transmitter fiber (attenuation of 6 dB/km) are negligible for a 10 m length.
5. Energy Converter For the operation of an optically powered hybrid-sensor head, the light supplied through an optical fiber has to be converted into electrical power. This is done by an optical energy converter. At present, electrically operated sensor heads work at voltages of 3 V up to 15 V. A gallium arsenide photoelement supplies only
478
a no-load voltage between 1 and 1.1 V (0.6 V and 0.7 V in comparison with silicon). How can this value be transformed? There exist various possibilities of voltage multiplication, some of which are: (a) voltage transformation, pulse-controlled operation at the light source and use of a single photelement with subsequent transformer; (b) the use of monolithic voltage doublers; (c) light distribution by means of a star coupler to several series-connected photoelements or photoelement arrays (PEAS); (d) light distribution to multiple photoelements or PEAS, without an intermediate component. In addition to a photoelement, the first three methods require components whose own power loss cannot be neglected. This results in a deterioration in conversion efficiency. The method of voltage multiplication with multiple photoelements requires no additional components. We have therefore concentrated on the design of special PEAS made of Si and GaAs with a cascade arrangement of single photoelements. Photoelements in GaAs have a higher no-load voltage and up to five times the conversion efficiency of Si photoelements. Therefore a new design of GaAs PEAS has been developed in a square 1 x 1 mm* pattern (Fig. 4(a), a single photoelement array, SPEA). Figure 4(b) shows an arrangement of four SPEAs (called a multiple photoelement array, MPEA) with four electrically isolated output voltages.
0.
0
I
0.5
1
I
1,5
Laser current
1
I
2 I
3 A 3,5
---+
Fig. 5. Optical to electrical conversion efficiency and electrical power vs. forward laser current for a SPEA.
To test these PEA designs, the maximum optical power c#J,,,= 690 mW was measured at the end face of a 10m long fiber. The wavelength of this LA optical source is 1 = 838 nm. The conversion efficiency of a SPEA as a function of radiation or forward laser current is demonstrated in Fig. 5. The x-coordinate at the top is the optical power and the y-coordinate shows the conversion efficiency and the converted electrical power; for a single GaAs PEA of eight segments an electrical power of 93 mW was produced (this is near to the saturation density). This corre-
laser uavekngih
+%d-k-H--13 Fig. 4. Arrangement of GaAs photoelements in different PEAS: (a) SPEA (single PEA); (b) MPEA (multiple PEA) with four electrical isolated output voltages. PES = active photoelement segment.
0
I
2,5
load resistor
R
a3anm3 kO 3.5
--)
Fig. 6. Electrical power and output voltages vs. load resistor of a GaAs SPEA.
479
We believe that we can reduce the series resistance and achieve a still better conversion efficiency. With a multiple GaAs PEA an electrical power of 118 mW was converted (Fig. 7). This corresponds to a conversion efficiency of vpEA=118mW/690mW=
17.1%
3 irradiance on a MPEA, E, = 220 kW/m2
01
/
t
I
0.5
0
I
1
1.5
I I
2
2,5
I
3
A 3,5
0
Figure 8 again shows the corresponding output voltage as a function of a load resistor value, the SPEAs being connected in series.
Losercurrentf --,
Fig. 7. Optical to electrical conversion efficiency and electrical power vs. forward laser current for a MPEA.
sponds
to a conversion efficiency of
qpEA= 93 mW/690 mW = 13.5% => irradiance on a SPEA, E, = 879 kW/m* Figure 6 compares the maximum electrical power with the corresponding output voltage of a SPEA as a function of load resistor value. The constant optical power for these measurements is 4 = 690 mW. An output voltage of up to U, x 8 V has been achieved (e.g., 5 V at maximum power PC,= 93 mW). 35
I
6. Conclusions The development and production of a PEA gives us a simple means of supplying the head of a hybrid sensor with electrical power. Light sources of suitable radiant power are already available. It has been shown that it is possible to supply a fiber-optic hybrid-sensor head with more than 100 mW of electrical power via a fiber optical power line. This power level is sufficient for running a wide range of conventional electrical sensor heads. In the near future optical power levels of 10 W by means of high-power output laser diodes or laser arrays will be available for fiber optical applications with matched optical fibers and could deliver an electrical power P,, x 1 W. The principle of such pure fiber-optical hybrid sensors has most of the advantages of fully optical sensors and gives great flexibility in choosing the appropriate sensor head.
Acknowledgements 8 0
I 5
I
I
I
I
10
15
20
25
loadresistor
I
I
50kn35
R -+
Fig. 8. Converted electrical power and output voltage vs. load resistor for a MPEA with series connection.
I would like to take this opportunity of expressing my sincere thanks to all those who made it possible to build the PEA/MPEAs, particularly to Mrs Bittel and Mr Bogner for their active support.
480
References
6 P. Schweizer, L. Neveux
and D. B. Ostrowsky, Optical fibre powered pressure sensor, Fourth Int.
1 F. Caspers and E.-G. Neumann, Optical power supply for measuring or communication devices at high voltage levels, IEEE Trans. Instrum. Meas., IM-29 (1980) 73-74. 2 H. S. Rauschenbach, Solar Cell Array Design Hundbook, Van Nostrand, London, 1980, p. 156. 3 H. Schneider and G. Zeidler, Herstellverfahren und Ausftihrungsformen von Lichtwellenleitern, Siemens Telecom Report 6 (April 1983), Supplement Nachrichtentibertragung mit Licht, pp. 29-35. 4 A. Ohte, K. Akiyama and I. Ohno, Optical-powered transducer with optical-fiber data link, Proc. SPIE, Vol. 418, Fiber Optics and Laser Sensors II, May 1984, pp. 33-38. 5 J. Niewisch, Design and application of a fiber-optic liquid level sensor, Siemens Forsch. Entwicklungsber., I5(1986)
115-119.
Symp. Optics and Opto-electronics, Applied Science and Engineering, The Hague, The Netherlands, Mar. 30-Apr. 3, 1987, p. 23. 7 R. C. Spooncer, B. E. Jones and R. Ohba, Pulse
modulated optical fibre quartz temperature sensor, Fourth Int. Symp. Optics and Opto-electronics, Applied Science and Engineering, The Hague, The Netherlands, Mar. 30-Apr. 3, 1987, p. 23. 8 W. Gross, Fiber-optic hybrid sensors with optical power supply, Siemens Forsch. Enrwicklungsber., I7 (1988) 13-17. 9 J. Lenz and P. Bjork, Optical powered sensors: a system approach to fiber optic sensors, Proc. SPIE, Vol. 961, Industrial Optical Sensing, 1988, pp. 8-25.
0 W. Kuntz and W. Mores, Energie-und Dateniibertragung iiber Lichtwellenleiter bei intelligenten Sensoren, Tech. Mess., 56 ( 1989) 166- 170.