Vincent A. Handerek

A fully distributed simultaneous strain and temperature sensor using spontaneous Brillouin backscatter

We report here the first simultaneous measurement of strain and temperature using Brillouin backscatter in an optical fiber. A new sensor arrangement is presented which allows the distributed measurement of Brillouin spectra. Simultaneous measurement of spontaneous Brillouin power and Brillouin shift distributions are made from these spectra, and from this information, we obtain fully distributed measurements of strain and temperature. Our sensor achieves a 100-/spl mu//spl epsiv/ strain and 4/spl deg/C temperature resolution, with 40-m spatial resolution, over a sensing length of 1200 m.

Simultaneous strain and temperature sensing with photogenerated in-fiber gratings

A new technique for simultaneous strain and temperature sensing is demonstrated. The approach employs two different types of photogenerated fiber grating, namely, a fiber Bragg grating and a fiber polarization-rocking filter. The method relies on the different dependencies of the fiber refractive index and birefringence on strain and temperature. Both of these measurands can be determined from the effect that they have on the resonant wavelength of each grating. The information is provided in the frequency domain, avoiding the problem of limited unambiguous signal range associated with the use of competing optical fiber interferometers.

High resolution instrumentation system for fibre-Bragg grating aerospace sensors

Abstract A passive technique for high-resolution detection of the wavelength of peak reflection from fibre Bragg grating (FBG) sensors is presented. The approach, based on a de-convolution of the FBG spectrum and the spectrometer resolution, essentially overcomes the low spectral resolution (≈0.1 nm) associated with commercial spectrometers employing CCD detection. The performance of two algorithms, the Centroid Detection algorithm (CDA) and the least squares method (LSQ), are compared. The LSQ algorithm offers resolution enhancement of at least two orders of magnitude and is shown to be independent of the FBG intensity and bandwidth over a broad range.

A high spatial resolution distributed optical fiber sensor for high-temperature measurements

One of the advantages of the use of optical fibers for sensing is their ability to perform distributed measurements. Several distributed sensors that measure temperature along optical fibers are already commercially available. All of them are based on Raman thermometry. However, the spatial resolution that they achieve is limited to 1 m, making them unsuitable for applications where higher spatial resolution is needed. Also, their temperature range is very dependent on the behavior of fibers at high temperature, a subject which still needs investigation. In this article, we present a distributed optical fiber sensor that addresses these two issues. On one hand, the combination of Raman thermometry with the time-correlated single-photon counting technique permits the achievement of high spatial resolutions (0.1 m). On the other hand, the use of specially coated fibers allows measurement of high temperatures. We have investigated the system temperature sensitivity and have evaluated the measurement errors i...

Compact Mach-Zehnder fiber interferometer incorporating photoinduced gratings in elliptical-core fibers

A new type of Mach-Zehnder interferometer has been fabricated in an elliptical-core high-birefringence fiber. The directional couplers of a conventional fiber interferometer are replaced by two photoinduced polarization rocking filters written on the same fiber. The resonant wavelength of the interferometer was 787 nm, with an operational bandwidth of 18 nm. The device can be used in two different modes for temperature sensing.

Frequency-derived distributed optical-fiber sensing: Rayleigh backscatter analysis

Frequency-derived distributed optical-fiber sensing is a powerful and convenient method for measuring the spatial distribution of birefringence in a high-birefringence fiber. The method relies on the special statistical characteristics of Rayleigh backscatter for its action, and these are analyzed in the context of the sensing arrangement, with an emphasis on the physical mechanisms. Implications for system design are also discussed.

Photorefractive polarization couplers in elliptical core fibres

Photorefractive polarization couplers written internally in germanium-doped elliptical core fibers at 488, 514, and 532 nm are reported. Complete power transfer between the orthogonal polarization modes of the fiber was achieved for couplers written at 514 and 488 nm, respectively. It is shown that the couplers are nonuniform in length because of the high photoinduced attenuation and also due to two-photon absorption. Polarization coupling of higher order modes is also demonstrated at shorter wavelengths where their polarization beat lengths match the polarization beat length of the fundamental mode at which the coupler was written.<<ETX>>

Distributed optical-fiber sensor for spatial location of mode coupling by using the optical Kerr effect

We describe a method for determining the locations of discrete mode-coupling points along a polarization-maintaining fiber using a pump-probe architecture based on the optical Kerr effect.

Frequency-derived remote measurement of birefringence in polarization-maintaining fiber by using the optical Kerr effect

A technique to measure remotely the birefringence of polarization-maintaining fiber is described. The method uses a pump-and-probe scheme that employs the optical Kerr effect together with a small wavelength difference between the sources to allow a measurable derived frequency to be generated. The frequency depends on the birefringence of the fiber and the source wavelength offset. This latter feature provides a tunable frequency and hence an extra degree of freedom in the system design.

Simultaneous observation of photobleaching and photorefractive effects in germanium-doped optical fibers

Photobleaching and photorefractive effects caused by intense pulsed laser irradiation at 532 nm have been observed simultaneously in a germanium-doped monomode fiber. A decrease of up to 4.1*10/sup -4/ in the refractive index of the core at 633 nm was inferred from the change in the radiation pattern of a probe beam and also from the change in the cutoff wavelength of the LP/sub 11/ mode. The effect is attributed to the bleaching of preexisting Ge(1) color centers in the fiber.<<ETX>>