Fiber optic interferometric electric field and voltage sensor utilizing an electrostrictive transducer

#1. Field of the Invention

#The present invention is directed to a sensor including an electrostrictive transducer and, more particularly, to a fiber optic interferometer utilizing an electrostrictive transducer for sensing low frequency electric fields, resolving extremely small voltages at frequencies between dc and 100 Hz (Hertz) when used in conjunction with a high frequency carrier, and for detecting high frequency signals in the range from 100 Hz to 100 kHz with an applied dc bias.

#2. Description of the Related Art

#FIG. 1 is a general schematic diagram of a fiber optic interferometer employing an electrostrictive transducer according to a first embodiment of the present invention;

#FIG. 2 is a graph of a power spectrum of the output of the interferometer shown in FIG. 1 when used as a low frequency electric field sensor or voltage sensor;

#FIG. 3 is a graph of the strength of the sidebands of the output of the interferometer shown in FIG. 1, dependent on the amplitude of the high frequency carrier signal applied to the electrostrictive material;

#Fiber optic interferometers are used in many applications for measuring different types of signals. Basically, a fiber optic interferometer includes a laser coupled to at least two fiber optic cables through a first coupler. A light beam from the laser is split between the fiber optic cables and rejoined at a second coupler. One of the two or both of the cables includes a sensor or transducer sensing a value of interest. Detectors detect the output from each cable and send the outputs to a differential amplifier. A difference signal can be sent to a spectrum analyzer. This type of interferometer is called a Mach-Zehnder interferometer.

#Fiber optic interferometers employing piezoelectric transducers cannot measure low frequency signals on the order of dc to 100 Hz with high resolution because of the presence of noise attributable to environment, the system, etc., and the low frequency signals are at approximately the same level and in the same low frequency range as the noise. The present invention has solved this problem by providing a fiber optic interferometer with an electrostrictive transducer which is non-linear in nature.

#A fiber optic interferometer employing an electrostrictive transducer according to a first embodiment of the present invention is shown in FIG. 1. FIG. 1 essentially shows a Mach-Zehnder type fiber optic interferometer for measuring fiber optic dc and low frequency electric fields. As discussed above, the device in FIG. 1 includes a conventional laser 20 and a conventional first coupler 22 for receiving laser light from the laser 20 and dividing the laser light into at least two equal inputs. The laser light is then input into at least two conventional single mode optical fibers (arms) 24a and 24b. One optical fiber (arm) 24b has an electrostrictive transducer 26 bonded thereto using, in the same manner as with piezoelectric transducers, epoxy, coupling fluid or any material that will maintain contact between the optical fiber and the electrostrictive material. The optical fiber 24b is bonded to the electrostrictive transducer 26 in a direction in which the largest resonance occurs to obtain the largest M coefficient. For example, if the electrostrictive transducer 26 is a tube, the optical fiber is wound circumferentially; if it is a strip of ceramic, it is placed longitudinally on the electrostrictive material. It can also be placed transverse, depending on the shape of the electrostrictive material. The electrostrictive material expands and contracts in all directions when an oscillating electric field from, for example, an oscillator (not shown, the oscillator being inside the high-frequency carrier signal circuit 25a) is applied. The oscillating electric field is applied to the electrostrictive transducer 26 by way of, for example, field plates or contacts 27A and 27B attached to the electrostrictive transducer 26 in the same manner as with piezoelectric transducers.