Stephanus J. Spammer

Interferometric distributed optical-fiber sensor

An interferometric technique is described for detecting and locating perturbations along an optical fiber. This distributed sensor, based on a modified fiber-ring interferometer, has a position-dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift that is detected and converted to spatial information. The sensor consists of two parts, namely, a reflecting-fiber-ring interferometer and a differentiating-ring interferometer. The reflecting ring consists of a fiber ring with one port of the coupler connected to a reflector. Consequently the output port of the reflecting-ring interferometer is the same as the input port. Because it is an inherent zero-path-imbalanced system, a short-coherence-length source such as a light-emitting diode can be used. Any time-varying perturbation on the fiber in the ring results in a detector signal proportional to the product of the rate-of-phase change caused by the perturbation and the distance of the perturbation relative to the center of the fiber ring. The second part of the system, a differentiating-ring interferometer, consists of the same fiber-ring interferometer modified only slightly. The output of this part of the sensor is proportional only to the rate of phase change as a result of the unknown perturbation and contains no distance information. By dividing the output of the reflecting-ring interferometer by the output of the differentiating-ring interferometer, we determine disturbance location. Results obtained with a 155-m distributed fiber sensor are discussed.

Temperature insensitive fiber optic accelerometer using a chirped Bragg grating

We report the development and demonstration of a fiber optic accelerometer. The principle of operation is based in the use of chirped Bragg gratings where the chirp width varies as the grating is physically deformed. The theory of operation is presented and experimentally compared with a conventional accelerometers measurements for a vibrating beam.

Mach–Zehnder and modified Sagnac-distributed fiber-optic impact sensor

An interferometric technique is described to detect and locate perturbations along an optical fiber. This distributed sensor has a position-dependent response to time-varying disturbances such as strain or temperature. A modified Sagnac interferometer configuration that incorporates an additional coupler and a mirror allows separation of the Sagnac and the Mach-Zehnder signals. Operation of the new configuration was verified experimentally with a 100-m-long sensing fiber. The discrepancy between actual and measured locations of disturbances applied to the fiber did not exceed 0.6 m.

Differentiating optical-fiber Mach–Zehnder interferometer

We introduce a new type of optical-fiber Mach-Zehnder interferometer whose output depends on phase differentials or the time rate of change of the unknown phase-modulating signal. Whereas the actual phase excursion introduced by the signal could cause interference over several fringes in a conventional Mach-Zehnder interferometer, the differential phase shifts may be restricted to the linear range of the phase detector. Being of simple construction, the interferometer can be operated without active biasing, additional phase modulation, or complex signal-processing techniques. We analyze a prototype architecture to explain the principle of operation of the system and to derive design formulas. This is followed by experimental evaluation of a more practical configuration.

A quadrature phase tracker for open-loop fiber-optic gyroscopes

A phase detection system for open-loop fiber-optic gyroscopes which is able to determine the Sagnac phase over several interference fringes is presented. It employs two quadrature signals of the Sagnac phase, as well as an electronic feedback signal that is a first-order estimate of the phase. Linearity is achieved by means of a trigonometric transformation, several of which have been identified, and a first-order control system that generates the feedback signal. The dynamic properties such as ramp and step response of the phase tracker have been studied. It is shown that the introduction of a simple feedforward loop completely eliminates the velocity error which is inherent in the first-order feedback system. For the particular loop gain used in the experimental system, a ramp of 280*10/sup 3/ rad/s could be tracked without detectable velocity error. The introduction of feedforward improved the risetime for a step input of 0.86 rad from 2.4 mu s to 550 ns. >

A novel signal demodulation technique for chirped Bragg grating strain sensors

While Bragg grating strain sensors have been used in many applications, the need to monitor the variation in the gratings peak wavelength as well as the bandwidth has frequently required the use of expensive and bulky spectral decomposition systems. The increased use of chirped Bragg grating sensors, where the bandwidth varies as the grating is subjected to external perturbations, also requires the use of these wavelength demodulation systems for proper use in many applications. In this paper we report the development and demonstration of a novel demodulation technique which may be readily used for chirped Bragg grating strain sensors.

Fiber optic sensors in the Waterbury Bridge

The use of fiber optic sensors for the internal state measurements of large civil structures has been increasing in recent years. In many instances, sensors are embedded into the reinforced concrete structure is an attempt to measure a single parameter of interest. Installation and preliminary measurements obtained from a suite of fiber optic sensors which were embedded into a 67 m steel truss bridge spanning the Winooski River in Vermont (USA) are presented.

Output power characteristics of a Fabry-Pe´rot Er3+-doped fiber laser

An analysis is performed to obtain explicit closed-form ex- pressions for the output power, threshold pump power, and the optimum length of an erbium-doped Fabry-Perot cavity fiber laser. To verify the validity of the model, we used an electron cyclotron resonance plasma- enhanced chemical vapor deposition apparatus to deposit dielectric thin films on one fiber end to change the reflectance of a cavity mirror, while monitoring the reflectance in situ. We also varied the active fiber length and compared the measured values of output power and threshold pump power with the theoretically predicted values. Good agreement between theory and experiment was obtained.

Modified Sagnac/Mach-Zehnder interferometer for distributed disturbance sensing

We propose an alternative configuration for an interferometric distributed fiber optic sensor. The sensor uses a combination of Mach-Zehnder and Sagnac interferometers that share the same fiber. The output of the Sagnac interferometer is proportional to the product of the position where the disturbance was applied and the magnitude of the disturbance. The output of the Mach-Zehnder interferometer is a function of the disturbance magnitude only. The position and magnitude of a disturbance applied to the sensing fiber can thus be obtained from these two signals. Results obtained with a 200 m distributed fiber sensor are discussed.

Polymer dispersed liquid crystal fiber optic electric field probe

Polymer dispersed liquid crystal inserted between two multimode optical fiber end faces forms the basis of an electric field probe. This probe has an active volume of about 0.001 mm/sup 3/ and approximates a point measurement. The linear relation between detector output and electric field in the 600 V/cm to 800 V/cm range is adequate for most electric power distribution systems. As the contrast ratio of this transducer is large, it can also be used as a safety on-off detector for high-voltage equipment.