A voltage sensor utilizing a non-reciprocal phase shift induced by a time variation of an electric field of the voltage to be measured. A light source provides broadband light to a polarizer. The polarized light is distributed to two linear polarizations. The two polarizations of light are provided with a dynamic or static phase shift relative to each other before or after being birefrigently modulated with an electro-optic sensor in accordance with the electric field of a voltage to be measured. The modulated light goes through a delay line that is terminated with a quarter-wave plate and mirror. The quarterwave plate may be an optical fiber. The portion of light entering the quarter-wave device in a first polarization is reflected in a second polarization and the portion of light entering the quarter-wave device in the second polarization is reflected in the first polarization. The electro-optic sensor again birefringently modulates the light. The light is converted to electrical signals that are processed into a signal representing a non-reciprocal phase shift that is proportional to the sensed electric field. The latter signal is integrated into a signal proportional to the electric field of the voltage to be measured.

BACKGROUND 
The invention pertains to fiber optic sensors and, particularly, to fiber 
optic voltage sensors. 
Over the past decade, fiber optic sensors have received attention in the 
application of magnetic field sensing and current sensing. Fiber optic 
current sensors are advantageous over iron-core current transformers, 
since fiber optic sensors are non-conductive and lightweight. Furthermore, 
fiber optic sensors also do not exhibit hysteresis and provide a much 
larger dynamic range and frequency response. 
Fiber optic current sensors work on the principle of the Faraday effect. 
Current flowing in a wire induces a magnetic field, which, through the 
Faraday effect, rotates the plane of polarization of the light traveling 
in the optical fiber wound around the current carrying wire. Faraday's 
law, stated as 
EQU I=H.multidot.dL 
where I is the electrical current, H is the magnetic field and the integral 
is taken over a closed path around the current. If the sensing fiber is 
wound around the current carrying wire with an integral number of turns, 
and each point in the sensing fiber has a constant sensitivity to the 
magnetic field, then the rotation of the plane of polarization of the 
light in the fiber depends on the current being carried in the wire and is 
insensitive to all externally generated magnetic fields such as those 
caused by currents carried in nearby wires. The angle, .DELTA..phi., 
through which the plane of polarization of light rotates in the presence 
of a magnetic field is given by 
EQU .DELTA..phi.=V.intg.H.multidot.dL 
where V is the Verdet constant of the fiber glass. The sensing optical 
fiber performs the line integral of the magnetic field along its path, 
which is proportional to the current in the wire, when that path closes on 
itself. Thus, one has .DELTA..phi.=VNI where N is the number of turns of 
sensing fiber wound around the current carrying wire. The rotation of the 
state of polarization of the light due to the presence of an electrical 
current is measured by injecting light with a well defined linear 
polarization state into the sensing region, and then analyzing the 
polarization state of the light after it exits the sensing region. 
Alternatively, .DELTA..phi. represents the excess phase shift encountered 
by a circularly polarized light wave propagating the sensing fiber. 
This technology is related to the in-line optical fiber current sensor as 
disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor 
James N. Blake and entitled "Fiber Optic Interferometric Circuit and 
Magnetic Field Sensor", which is incorporated herein by reference. Optical 
fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 
1997, to inventor James N. Blake and entitled "Fiber Optics Apparatus and 
Method for Accurate Current Sensing", which is incorporated herein by 
reference. 
However, a need has arisen for a fiber optic voltage sensor. 
SUMMARY OF THE INVENTION 
The present invention utilizes a non-reciprocal phase shift induced by a 
time variation of an electric field of the voltage to be measured. A light 
source provides broadband light to a polarizer. The polarized light is 
distributed to two linear polarizations. The two polarizations of light 
are provided with a dynamic or static phase shift either before or after 
being birefrigently modulated with an electro-optic sensor in accordance 
with the electric field of a voltage to be measured. The modulated light 
goes through a delay line that is terminated with a quarter-wave plate and 
mirror. The quarter-wave plate maybe an optical fiber. The portion of 
light entering the quarter-wave device in a first polarization is 
reflected in a second polarization and the portion of light entering the 
quarter-wave device in the second polarization is reflected in the first 
polarization. The electro-optic sensor again birefringently modulates the 
light. The light is converted to electrical signals that are processed 
into a signal representing a non-reciprocal phase shift that is 
proportional to the sensed electric field. The latter signal is integrated 
into a signal proportional to the electric field of the voltage to be 
measured. The intensity of the electric field indicates the magnitude of 
the voltage.

DESCRIPTION OF THE EMBODIMENT 
FIG. 1a shows an embodiment of a voltage sensor 10 having a dynamic phase 
shift between the polarizations of light. It is a closed-loop system. A 
light source 11 emits light that goes through coupler 12 and onto 
polarizer 13. Coupler 12 and detector 21 may be excluded with the 
returning light going to and through light source 11 to a backfacet 
photodiode 47 at the back of light source 11, as shown in FIG. 1b. 
Further, polarizer 13 may have a pigtail 48 that is coupled directly to 
modulator 15 having a connection 43 with the principal birefringence axes 
of pigtail 48 and modulator 15 aligned at 45 degrees. The output fiber of 
polarizer 13 is connected to a polarization-maintaining (PM) input fiber 
14 of birefringence modulator 15. The axes of this connection 43 are 
aligned at 45 degrees to distribute the light evenly in both polarization 
axes of PM fiber 14. Light travels through modulator 15 in both 
polarization axes and down a PM fiber 16 to an electro-optic (i.e., 
electric field) sensitive element 17. Modulator 15 provides a bias 
modulation of the birefringence between the two axes of polarization of 
light medium in modulator 15. The bias modulation occurs at a frequency of 
about several hundred kilohertz (for example, 300 kHz). Electro-optic 
sensor 17 provides additional birefringence modulation at about 60 hertz, 
since the voltage being measured would be that of a 60 hertz power line. 
The electric field, say, from the voltage being monitored, modulates the 
birefringence between the two light waves in sensor 17. The light from 
sensor 17, enters a PM fiber delay line 18, which has axes aligned with 
sensor 17, and is connected to and terminated by a quarter-wave plate or 
PM fiber 19 and a mirror 20, via connection 44, such that light traveling 
down fibers 18 and 19 in the x-axis returns in the y-axis and vice versa. 
The axes of delay line 18 and termination 19 are at a 45-degree alignment 
at connection 44. The light returns from fiber 18 to sensor 17 and 
receives further birefringence modulation. The two portions of the light 
entering fiber 19 in the first (x) and second (y) polarizations, 
respectively, return from fiber 19 in the other polarizations, that is, in 
the second (y) and first (x) polarizations, respectively. The 
birefringence modulation on the two waves is not canceled for ac fields 
because of the delay time between the passage of the two waves through 
sensor 17. The light then travels from sensor 17 through fiber 16, 
modulator 15, fiber 14, polarizer 13 and coupler 12 to detector 21. 
The output of detector 15 is an electrical representation of the light 
waves entering the detector. The AC (i.e., alternating current) fields 
detected are typically of a low frequency compared to the delay time 
introduced by delay line 18 between sensing element 17 and mirror 20. In 
this case, the non-reciprocal phase shift introduced is proportional to 
the time derivative of the field to be sensed. Closed-loop signal 
processing electronics 22 may be incorporated to take in the signal from 
detector 21 and provide a feedback signal to birefringence modulator 15. 
This signal has a non-reciprocal phase-shift, as noted above, which is 
proportional to the time derivative of the field to be sensed. Thus, the 
signal may be integrated by integrator 23 to yield an output proportional 
to the field sensed by the element 17. The output signal from integrator 
23 goes to a voltage indicator 45, which provides the voltage reading of 
measuring device 10 with respect to the voltage being measured at sensor 
17. 
An analysis of device 10 of FIG. 1 is shown below. Phase difference 
modulation is Q.sub.x (t)-Q.sub.y (t) There are similarities to the 
in-line current sensor. BM(t) is birefringence modulation. BM.sub.1 
represents the modulation of birefringence modulator 15 and BM.sub.2 
represents the modulation of electro-optic sensor 17. Going from source 11 
to mirror 20, one has Q.sub.x (t) and Q.sub.y (t) of the light intensity 
for the x and y polarizations. Going from mirror 20 to detector 21, one 
has Q.sub.x (t+.tau.) and Q.sub.y (t+.tau.) of the light intensity. 
Formulas for explaining phase difference modulation are noted. 
EQU Q.sub.x (t)-Q.sub.y (t)-Q.sub.x (t+.tau.)+Q.sub.y (t+.tau.) 
EQU BM(t).tbd.Q.sub.x (t)-Q.sub.y (t) 
Phase difference modulation is BM(t)-BM(t-.tau.) 
EQU For bias modulation at the eigen frequency, BM.sub.1 (t+.tau.)=-BM.sub.1 (t 
) 
EQU Sensor modulation is BM.sub.2 (t)-BM.sub.1 (t+.tau.)=-.tau.(dBM.sub.2 
(t)/dt) 
EQU Total modulation is 2BM.sub.1 (t)-.tau.(dBM.sub.2 (t)/dt) 
where: 
-.tau.(dBM.sub.2 (t)/dt) is the quasi-static non-reciprocal phase shift. 
The loop output is --.tau.(dBM.sub.2 /dt) and the output is 
-.tau.TBM.sub.2. BM.sub.2 is proportional to the voltage being measured. 
FIG. 2 is a device 24 that is similar to device 10 of FIG. 1a, except that 
birefringence modulator 15 is replaced with a Faraday rotator 25. The 
alternate light source 11 and detector 47 arrangement of FIG. 1b may be 
used in lieu of the coupler 12 and detector 21 arrangement of system 24. 
System 24 is an open-loop device having a static phase shift. Light from 
source 11 goes through coupler 12. From coupler 12, the light is polarized 
by polarizer 13. The output light of polarizer 13 goes to Faraday rotator 
25, which passively rotates the polarization of the light about 22.5 
degrees. 
Faraday rotator 25, in lieu of birefringence modulator 15, results in a 
lower cost device although with somewhat lower performance. Faraday 
rotator 25 has a quarter-wave plate 46 at each end. Rotator 25 could have 
just one plate 46 at the end towards polarizer 13. A bulk optic rotator 25 
may have a sliver of quartz glued to the respective end as a quarter-wave 
plate 46. The output of Faraday rotator 25 goes to electro-optic sensor 17 
which provides birefringence modulation at about 60 hertz. The magnitude 
of an electric field from the voltage being measured or monitored 
modulates the birefringence between the two polarization axes of the 
medium of the two light waves that propagate respectively in sensor 17. 
The light from sensor 17 enters a PM optical fiber delay line 18 which is 
terminated by, via connection 44, a quarter-wave plate or PM fiber 19 and 
a mirror 20, such that light traveling down fibers 18 and 19 in the x-axis 
returns in the y-axis and light in the y-axis returns in the x-axis. The 
birefringence axes of PM fiber 18 are aligned with the axes of sensor 17 
and are at 45 degrees with the axes of quarter-wave plate or fiber 19. The 
light returns from fiber 18 to sensor 17 and receives further 
birefringence modulation. The light goes through rotator 25, which 
includes at least one quarter-wave plate 46, polarizer 13 and coupler 12, 
and on to detector 21. The output of detector 21 is an electrical 
representation of the light waves entering the detector. A non-reciprocal 
phase shift is introduced which is proportional to the time derivative of 
the field to be sensed. Open-loop signal processor 26 may be incorporated 
to process the electrical signal from detector 21 and provide a signal 
that is proportional to the time derivative of the field being sensed, to 
integrator 23. Integrator 23 yields a signal that is proportional to the 
electric field sensed by element 17. The signal from integrator 23 goes to 
a voltage indicator 45 that provides the voltage reading of device 24. 
FIG. 3 shows a bulk optics alternative 27 for birefringence modulator 15 or 
Faraday rotator 25 of FIGS. 1 and 2, respectively, for a third 
configuration 28. System 28 is an open-loop device having a static phase 
shift of the light polarizations. Light 29 comes from source 11 and goes 
through polarizer 13 to a passive two-polarization detection device 27. 
Light 29 is polarized into a first direction by polarizer 13. Light 30 of 
a first polarization enters device 27 and goes to a light splitter 31. 
About half of beam 30 is reflected out of and dumped from splitter 31. The 
remaining portion of beam 30, which is designated beam 32, goes to 
polarization maintaining (PM) fiber 16. Fiber 16 has polarization axes 
that are aligned at about 45 degrees relative to the axes of polarizer 13. 
In other words, a PM fiber 16 is connected to the polarized output of 
device 27 at axis angles of 45 degrees. Light 32 is now distributed in the 
two axes of linear polarizations of fiber 16. Light 32 goes from fiber 16 
to electro-optic sensor 17. Sensor 17 birefringently modulates light 32 in 
accordance with a sensed electric field of a voltage to be measured. The 
birefringence modulation of the light medium of electro-optic sensor 17 
results in a nonreciprocal phase shift of the light waves in the first (x) 
and second (y) polarizations of beam 32, that is proportional to a time 
derivative of the electric field of the voltage that is being measured. 
Light 32 from sensor 17 goes to delay line 18, quarter-wave length plate 
or fiber 19 and is reflected by mirror 20. Optical fiber delay line 18 is 
about 10 to 100 meters of polarization maintaining fiber that has 
birefringence polarization axes that are aligned with such axes of 
electro-optic sensor 17. The quarter-wave plate or PM fiber 19 has its 
birefringence axes aligned at about 45 degrees to the axes of delay line 
18. Fiber 19 is terminated with a mirror 20. 
Light 32 is reflected back from mirror 20 as a light 33. The first portion 
of light beam 32 is of the first polarization and the second portion of 
light beam 32 is of the second polarization. When the light of beam 32 is 
reflected back, the first portion of beam 32 is of the second polarization 
and the second portion is of the first polarization. This is designated as 
light 33. Light 33 goes through sensor 17 and is further modulated. Delay 
line 18 prevents the second birefringence modulation of light 33 from 
canceling the first modulation of light 32, because of the delaying of the 
light by delay line 18. Light 33 then goes through fiber 16 from sensor 
17, to device 27. Light 33 enters splitter 31 and a portion of light 33 is 
reflected to quarter-wave plate. The other portion of light 33 passes 
through splitter 31, polarizer 13 and to light source 11. This portion of 
light 32 is dumped. Light 33 reflected to a quarter wave plate 35 is 
designated has light 34 which as a first portion of light in the first 
polarization and a second portion of light in the second polarization. 
Quarter-wave plate 35 introduces a 90-degree static phase shift bias 
between the first and second portions of light 34. This 90 degree biased 
light is designated as light 36. Plate 35 is a bulk optics substitute for 
birefringence modulator 15 of configuration 10 in FIG. 1, and Faraday 
rotator 25 of configuration 24 in FIG. 2. 
From plate 35, light 36 goes to a prism or polarization sensitive splitter 
37. A first portion of light 36 is of the first polarization and is 
designated as light 38. A second portion of light 36 is of the second 
polarization and is designated as light 39. Polarization sensitive 
splitter 37 reflects one portion of light 36 as light 38 of the first 
polarization to detector 40 and another portion of light 36 as light 39 of 
the second polarization to detector 41. Detectors 40 and 41 may be 
photodiodes. Detector 40 sends out electrical signals or currents 
representative of light 38. Detector 41 sends out electrical signals or 
currents representative of light 39. Detectors 40 and 41 are connected to 
a signal-processing device 42. Processor 42 takes the signals from 
detectors 40 and 41 and processes them. With signal 38 designated as 
signal I.sub.1, and signal 39 designated as I.sub.2, processor 42 
processes the signals according to a formula (I.sub.1 -I.sub.2)/(I.sub.1 
+I.sub.2), into a signal representing the non-reciprocal phase shift that 
is proportional to a time derivative of the electric field of the voltage 
being measured. The output of processor 42 is sent to integrator 23, which 
integrates the signal from processor 42 into a signal proportional to the 
electric field. The signal from integrator 23 goes to a voltage indicator 
45, which provides the voltage reading of measuring device 28 of the 
voltage at sensor 17.