Piezoelectric bimorph optical-fiber sensor

We propose and demonstrate a novel high-voltage optical-fiber sensor. This sensor consists of an emitting fiber, a receiving fiber, and a piezoelectric bimorph transducer. The emitting fiber is fixed in a base, whereas the receiving fiber is mounted on the free end of the piezoelectric bimorph transducer. When a voltage is applied to the piezoelectric bimorph transducer, its free end is displaced over a distance delta. The displacement induces a loss in the optical coupling between the emitting and the receiving fiber. The voltage can be measured by monitoring the coupling loss.     

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.     

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.     

Development of a differential optical-fiber displacement sensor

A novel optical-fiber displacement sensor is proposed and demonstrated. It consists of a laser diode light source, an optical-fiber probe, and two photodetectors. The bundling of the probe is sectioned into three parts: a centrally positioned fiber in the bundle for illumination, the first-neighbor fibers for receiving (part I), and the remaining fibers for receiving (part II). The ratio of the difference to the sum of the output signals from the part I and the part II receiving fibers can eliminate the variation in the sensitivity of the sensor to reflectivity of the target. The performance of the sensor is geometrically analyzed. The working distance is determined by the distance from the centered illuminating fiber to the boundary between the part I and the part II receiving fibers. The experimental measurements made with three different reflectivity targets confirm that the sensor performance is independent of the three reflectivities, as predicted by the analysis.     

High-resolution method of monitoring optical-fiber geometrical parameters

Translated from Izmeritel'naya Tekhnika, No. 12, pp. 35–37, December, 1991.     

Field trial of an electrically passive optical-fiber magnetometer

A field trial of an electrically passive optical-fiber magnetometer incorporated into the transient electromagnetic method of geomagnetic surveying is described. The transducer of the magnetometer consisted of metallic glass wire cocoated with a length of optical fiber. The magnetometer was capable of measuring the decay rate of the surface magnetic field produced by the surveying technique. The sensor's minimum detectable field was 2.5 × 10(-8) G/√Hz at 2 kHz.     

Intrinsic optical-fiber sensor for nerve agent sensing

A novel chemical-sensing technique to detect the nerve agent sarin stimulant dimethylmethylphosphonate (DMMP) is presented. This technique uses a combination of doped polypyrrole as an active chemical material coated on an optical fiber to form an intrinsic fiber-optic sensor. Sensitivity of up to 26 ppm of DMMP with response time of a few seconds is demonstrated. Influence of three different dopants, i.e., 1,5 naphthalene disulphonic acid, anthraquinone 2 sulphonic acid, and hydrochloric acid is investigated for sensor response and sensitivity. Two polymer processing techniques, i.e., in situ deposition and monomer vapor phase deposition is investigated for optimal polypyrrole morphology for DMMP sensitivity. The influence of substrate nature, i.e., hydrophilic and hydrophobic, on sensor sensitivity is studied. Organophosphate specific binding sites have been created in polypyrrole structure using Cu/sup 2+/ ions to enhance DMMP response. The selectivity issue is addressed by testing the sensor in the presence of other gases like ammonia, water vapor, and acetone which influence the electronic properties of polypyrrole.     

Reflectometric measurements of polarization properties in optical-fiber links

Polarization sensitive reflectometric measurements are effective tools for the characterization of polarization properties of optical-fiber links. These techniques show two advantages compared with the standard ones: they can perform measurements using only one fiber end (both for transmission and for reception of the probe signal) and, most important, they can characterize the local evolution of the polarization properties of the fiber link. Reflectometric measurements of differential group delay and fiber birefringence have been already successfully performed. More recently, the possibility of measuring, also, polarization dependent loss has been theoretically explored. In this paper the theory and main applications of polarization sensitive reflectometric techniques are reviewed.     

Three-dimensional phase imaging with a scanning optical-fiber interferometer

We describe a quantitative method for measuring the phase of a propagating wave field in three dimensions by use of a scanning optical-fiber interferometer. Because phase modulation in the reference arm is exploited, this technique is insensitive to large variations in the intensity of the field being studied and is therefore highly suitable for measurement of phase within spatially confined optical beams. It uses only a single detector and is not reliant on lock-in electronics. The technique is applied to the measurement of the near field of a cleaved optical fiber and is shown to produce results in good agreement with theory.     

Interferometric optical time-domain reflectometry for distributed optical-fiber sensing

The technique of optical time-domain reflectometry is analyzed to determine the effect of an optical phase modulation on light backscattered in an optical fiber. It is shown that the spatial distribution along the fiber of an external phase modulation can be measured with a spatial resolution close to that of optical time-domain reflectometry. A distributed interferometric sensor arrangement that employs this technique is investigated experimentally, and a satisfactory interrogation of more than 1000 resolution intervals is demonstrated.     

White-light reconstruction setup for shearograms using optical-fiber waveguides

A white-light reconstruction setup for shearograms using optical-fiber waveguides is presented. The setup is compact, is not susceptible to dust and scratches, and enables the two-dimensional fringe pattern on shearograms to be viewed directly under uniform and enhanced illumination with minimal image distortion.     

Conical and biconical ultra-high-Q optical-fiber nanowire microcoil resonator

New 3D geometries of the optical nanowire microcoil resonator are suggested and investigated theoretically. The dependence of the Q factor on coupling parameters is calculated and compared for three different profiles. Results suggest that ultra-high-Q resonators can be fabricated more easily when the nanowire microcoil resonator has a biconical profile.     

Frequency-derived distributed optical-fiber sensing technique: theory and characterization

Frequency-derived distributed optical-fiber sensing is a method for remote measurement of the spatial distribution of linear birefringence in an optical fiber, allowing a corresponding measurement of those external measurands that influence this birefringence. The method employs a pump-probe scheme, which, by use of the optical Kerr effect, generates an optical modulation of the probe beam, with a modulation frequency whose temporal variation maps the spatial distribution of birefringence. We provide a complete theoretical analysis of this method by using Jones calculus and graphic representation on the Poincaré sphere. The relevant characterization of the technique and some experimental results are also presented; these show good agreement with the theory.     

Characterization of an optical-fiber reflectometer for in situ measurement applications

We present studies of apparatus based on optical-fiber probes for measuring the reflectivity of opaque materials. The probes are designed for in situ applications at high temperatures and in situations for which access is difficult. First, we present a method in which the hemispherical reflectivity and the angular distribution of the reflected flux are obtained from bidirectional measurements. Second, we describe a method to measure the reflectivity in only the normal direction; this method is available only for diffuse materials. Both methods of use of the device were validated through the use of commercial diffuse standards and specific materials with particular surface microstructures.     

Characterization of time-resolved fluorescence response measurements for distributed optical-fiber sensing

A distributed optical-fiber sensing system based on pulsed excitation and time-gated photon counting has been used to locate a fluorescent region along the fiber. The complex Alq3 and the infrared dye IR-125 were examined with 405 and 780 nm excitation, respectively. A model to characterize the response of the distributed fluorescence sensor to a Gaussian input pulse was developed and tested. Analysis of the Alq3 fluorescent response confirmed the validity of the model and enabled the fluorescence lifetime to be determined. The intrinsic lifetime obtained (18.2±0.9 ns) is in good agreement with published data. The decay rate was found to be proportional to concentration, which is indicative of collisional deactivation. The model allows the spatial resolution of a distributed sensing system to be improved for fluorophores with lifetimes that are longer than the resolution of the sensing system.     

Circular-array optical-fiber probe for backscattering photon correlation spectroscopy measurements

A miniaturized probe comprising a circular array of optical fibers coupled to a graded-index microlens has been designed for analyzing colloidal solutions by photon correlation spectroscopy (PCS). The system's suitability for PCS measurements was validated by experimental testing performed in bulk solutions and small drops of monodisperse and bimodal test colloidal solutions.