Russell G. May

Self-calibrated interferometric-intensity-based optical fiber sensors

This paper presents self-calibrated interferometric-intensity-based optical fiber sensors, which combine for the first time fiber interferometry and intensity-based devices into a single sensor system. The sensor involves an extrinsic Fabry-Perot (FP) interferometric cavity. The broadband light returned from the FP cavity is split into two channels in such a way that one channel has a coherence length much longer than the doubled air-gap separation in the sensor so the FP generates effective interference, while the coherence length in the other channel is so short that no effective interference takes place. As a result, the optical signal in the channel with a long coherence length yields information about the FP cavity length while the signal in the other channel is proportional only to the source power, fiber attenuation, and other optical loss factors in the optical path. To eliminate fringe direction ambiguity and relative measurement limitations associated with interferometric sensors, the sensor is designed such that it is operated over the linear range between a valley and a peak of one interference fringe in the first channel. Moreover, the ratiometric signal-processing method is applied for the signals in the two channels to obtain self-calibrating measurement to compensate for all unwanted factors, including source power variations and fiber bending losses. Various pressure and temperature sensors based on the self-calibrated interferometric/intensity-based scheme are designed, fabricated, and tested. Experimental results show that a resolution as high as 0.02% of full scale can be obtained for both the pressure and temperature measurements.

Novel data processing techniques for dispersive white light interferometer

White light interferometry has been used in the sensing area for many years. A novel data processing method for demodulating the information from the interference spectrum of a white light system is presented. Compared with traditional algorithms, both high-resolution and large dynamic range have been achieved with a relatively low-cost system. Details of this arithmetic are discussed. A compact white light interferometric system employing this algorithm has been developed, combined with fiber Fabry-Perot sensors. A60.5-nm stability over 48 hours with a dynamic range on the order of tens of microns has been achieved with this system. The temperature dependence of this system has been analyzed, and a self-compensating data processing approach is adopted. Experimental results demonstrated a 61.5-nm shift in the temperature range of 10 to 45°C.

Novel techniques for the fabrication of holey optical fibers

Recently developed optical fibers rely on an array of air holes in the cladding to confine light to the fiber core as opposed to conventional telecommunications fibers that require a refractive index difference produced by different composition glasses in the core and cladding regions. Holey fibers have been fabricated by drawing an array of tubes stacked around a solid central core. In this paper, we describe a new technique to produce the holes (or pores) in the cladding region. These new fibers have been made by drawing a preform, consisting of a porous outer cladding region surrounding a solid central core region, into a fiber. During the fiber drawing process, the pores initially present in the preform cladding region are drawn into small, long, thin tubular pores. Controlling the dimensions and distribution of the pores in the preform can control the physical dimensions and distribution of the pores in the fiber. In some of the preforms, the porous cladding region in the preform was prepared by sol gel techniques. The preform fabrication process and fiber drawing process used to produce these new holey fibers as well as the results of the morphological study elucidating the size, shape and distribution of the porous phase are presented.

Single-crystal sapphire fiber-based strain sensor for high-temperature applications

Single-crystal sapphire fibers have a very high melting point (up to 2050/spl deg/C), which renders them a very good candidate for sensing applications at a very high temperature. We present in this paper the recent work of developing single-crystal sapphire fiber extrinsic Fabry-Perot interferometric strain sensors based on the white-light interferometric spectrum demodulation technique. Prototype sapphire strain sensors were fabricated and tested at high temperatures up to 1004/spl deg/C. The preliminary experimental results indicate that the sensors are promising to be used under high-temperature environments for making strain measurements with strain measurement resolution of 0.2-/spl mu/ strain.

Sapphire-fiber-based intrinsic Fabry-Perot interferometer

A sapphire optical fiber intrinsic Fabry–Perot interferometric sensor is demonstrated. A length of multimode sapphire fiber that functions as a Fabry–Perot cavity is spliced to a silica single-mode fiber. The interferometric signals of this sensor are produced by the interference between the reflection from the silica–sapphire fiber splice and the reflection from the free end face of the sapphire fiber. This sensor has been demonstrated for temperature measurement. A resolution of 0.2°C has been obtained over a measurement range of 310°C to 976°C.

Fiber optic pressure sensor with self-compensation capability for harsh environment applications

A novel fiber optic pressure sensor system with self-compensation capability for harsh environment applications is reported. The system compensates for the fluctuation of source power and the variation of fiber losses by self-referencing the two channel outputs of a fiber optic extrinsic Fabry-Perot interfrometric (EFPI) sensor probe. A novel sensor fabrication system based on the controlled thermal bonding method is also described. For the first time, high-performance fiber optic EFPI sensor probes can be fabricated in a controlled fashion with excellent mechanical strength and temperature stability to survive and operate in the high-pressure and high-temperature coexisting harsh environment. Using a single-mode fiber sensor probe and the prototype signal-processing unit, we demonstrate pressure measurement up to 8400 psi and achieved resolution of 0.005% (2σ=0.4 psi) at atmospheric pressure, repeatability of ±0.15% (±13 psi), and 25-h stability of 0.09% (7 psi). The system also shows excellent remote operation capability when tested by separating the sensor probe from its signal-processing unit at a distance of 6.4 km.

Advances in sapphire-fiber-based intrinsic interferometric sensors

We present recent advances in the development of a sapphire-fiber-based interferometric sensor. The dependence of the fringe contrast of the sensor output on the quality of the silica-to-sapphire fiber splice is investigated. Sensor performance has been improved by optimizing both the sensor geometry and its method of fabrication. This sensor was demonstrated for the measurement temperature above 1510°C, and a resolution of 0.1°C has been obtained.

All-Dielectric Frequency Selective Surface for High Power Microwaves

In this work, an all-dielectric frequency selective surface was developed for high power microwaves. By avoiding the use of metals, arcing at field concentration points and heating in the conductors was avoided. To do this in a compact form factor while still producing a strong frequency response, we based our design on guided-mode resonance (GMR). To make this approach viable for radio and microwave frequencies, we overcame three major challenges. First, conventional GMR devices have less than 1% fractional bandwidth and we extended this to 16%. Second, conventional GMR devices have a field-of-view less than 1° and we extended this to over 40°. Third, conventional GMR devices must be composed of hundreds of periods to operate, but our device operated very well with only eight. In this paper, we present our design and experimental results at 1.7 GW/m2.

Location of impacts on composite panels by embedded fiber optic sensors and neural network processing

Location of impacts on an anisotropic polymer matrix composite panel was demonstrated by using a neural network to process the outputs of embedded fiber optic strain sensors. Three extrinsic Fabry-Perot interferometer sensors were embedded in a graphite/bismaleimide composite with a unidirectional lay-up. The location of an impact can be calculated directly by triangulation from the difference in arrival times of the impact-generated stress waves at the embedded sensors. A data set of 132 experimental results was generated by impacting the panel at evenly spaced locations, and measuring the time differences. A back-propagation neural network was simulated using commercial software, and the data set was used to train the network. Following training, the network was capable of determining the location of randomly located impacts with an accuracy of a few centimeters. These results were comparable to the accuracy achieved for impact location on an isotropic aluminum plate, indicating that the neural network performance is independent of material anisotropy.

Miniaturized 3-D surface profilometer using a fiber optic coupler

A 3-D surface profilometer is described which uses a simple fiber optic coupler to form a Youngs double pinhole interferometer. The Youngs fringes are projected onto a surface, captured by a camera and analyzed using the Fourier transform method. The phase of the fringe pattern on the object is used to reconstruct the surface profile. System analysis, results from a simulation, and preliminary experimental results are provided which indicate a system resolution on the order of a tenth of a millimeter.