Acoustic modes in optical fiberlike waveguides.

A perturbation analysis of guided and leaky modes in fiber acoustic waveguides with core and cladding parameters that are slightly different is presented. The perturbing parameter is the shear-velocity difference between the core and cladding material epsilon(s). Acoustic fields and eigenvalues are expanded in power series of epsilon (s)(1/2) for radial and flexural modes and in powers of epsilon(3) for torsional modes. Expansion of leaky longitudinal modes is also in terms of epsilon(s), but the nature of perturbation analysis for these modes is somewhat different from that of guided modes. Zero-order solutions for all types of modes are obtained, and some important higher-order effects are discussed. Common features of optical and acoustic modes in weakly guiding fibers are addressed. It is shown that with respect to zero-order solutions of guided modes, optical and acoustic fibers have identical propagation characteristics. Exact and zero-order propagation characteristics for several lower-order shear-type modes are calculated and compared.

Robot gripper control system using PVDF piezoelectric sensors.

A novel robot gripper control system is presented that uses PVDF (polyvinylidene fluoride) piezoelectric sensors to damp exerted force actively. By monitoring the current developed by the PVDF sensor, an output proportional to the rate of change of the force exerted by the gripper is obtained. The signals from the PVDF sensor and strain-gauge force sensor are arranged in a proportional and derivative control system for the control of force. The control system was tested on an instrumented Rhino XR-1 manipulator hand. The capabilities of the control system are analyzed and are verified experimentally. The results for this particular gripper indicate that the additional sensory feedback can decrease the force step response rise time by 88% while maintaining a monotonic zero-overshoot response. The inclusion of the rate feedback increases the damping ratio of the dominant poles while maintaining the step response rise time.

Endovascular histologic effects of ultrathin gold- or vitronectin-coated platinum aneurysm coils in a rodent arterial occlusion model: a preliminary investigation.

BACKGROUND AND PURPOSE: Novel stratagems to improve the efficacy of platinum coils in occluding cerebral aneurysms have primarily involved coating coils with materials thought likely to provoke more desirable histologic reactions. No investigations to date, however, have evaluated the utility of gold or vitronectin coatings, despite known endovascular histologic effects of these agents, which may be favorable for treating cerebral aneurysms. This study was conducted to evaluate the degree of endovascular histologic change associated with ultrathin gold- or vitronectin-coated platinum coils. It was hypothesized that such coatings would increase intra-aneurysmal intimal hyperplasia and the degree of luminal occlusion compared with standard platinum coils. MATERIALS AND METHODS: The ligated carotid artery rat model was used to study 4 different aneurysm coil conditions: no coil (sham-surgery controls), uncoated platinum coil, and gold- or vitronectin-coated platinum coil. Two weeks postimplantation, the aneurysms were harvested and stained with hematoxylin-eosin. Slides were evaluated for the degree of neointimal response by a pathologist blinded to treatment. Additional quantitative evaluation was performed blindly by using the ratio of intimal-to-luminal cross-sectional area. RESULTS: A gold- or vitronectin-coated platinum aneurysm coil produced a statistically significant increase in neointimal response compared with a sham (no coil). Arterial segments treated with gold-coated platinum coils also demonstrated a statistically significant 100% increase in neointimal response compared with those treated with bare platinum coils. CONCLUSIONS: In concordance with our hypothesis, ultrathin coatings of gold provoked a neointimal response and degree of luminal occlusion greater than that of plain platinum aneurysm coils in a rat arterial occlusion model.

Polarization-independent all-fiber wavelength-division multiplexer based on a Sagnac interferometer.

An all-fiber wavelength-division multiplexer (WDM) based on the nonreciprocity of the birefringence to the polarization states is proposed. The transfer function of a Sagnac interferometer is wavelength dependent if the loop birefringence of the interferometer consists of both circular and linear parts. Theoretical analysis shows that the output characteristics of this WDM are similar to those of a fiber taper-based device. Both the bandwidth and the peak wavelength of the new WDM can be tuned by changing the loop birefringence. Experimental prototypes exhibit a channel isolation greater than 25 dB with peak passband insertion loss of less than 1 dB.

Split-cavity cross-coupled extrinsic fiber-optic interferometric sensor.

A new configuration of the fiber-optic extrinsic Fabry-Perot interferometer is demonstrated. This configuration utilizes two sensor heads on a single directional coupler in a split-cavity cross-coupled extrinsic fiber interferometric (SCEFI) arrangement to provide a four-beam interference. The need for quadrature phase biasing is eliminated, with a new spectrum analysis detection scheme devised for the SCEFI in a no-feedback condition. Good agreement between the model for interference and the experimental results is demonstrated. Wide applications for fiber sensor multiplexing and split-cavity étalons for filters in wavelength-division-multiplexing systems are indicated.

Interferometric model for phase analysis in fiber couplers.

An interferometric model is proposed to estimate the phase differences in lossless, strongly coupled biconical fiber couplers. This approximate method is simpler than the traditional s-parameter network theory-based analysis technique and minimizes the number of unknowns. The phase difference between the transmitted and coupled light fields is directly related to the field interaction and can be estimated by employing the energy conservation and mode orthogonality principles. The maximum coupling coefficient and dependence of phase difference on coupling conditions can be analyzed for multiport single-mode fiber couplers.

Ultrasonic wave measurement by differential interferometry.

The theory and performance of a two beam differential interferometer for measurement of both surface and bulk waves are described. The system is insensitive to small errors (<1 mm) in focus or in specimen flatness. Both amplitude and phase measurements are demonstrated. The system has been absolutely calibrated and can detect 6 x 10(-4)-A surface wave displacements on glass.

Direct numerical analysis of dual-mode elliptical-core optical fibers.

A direct solution of the scalar wave equation (SWE) for an elliptical-core weakly guiding fiber is considered. We decompose the SWE into two ordinary differential equations by using the method of separation of variables, demand the periodicity of the solution in terms of the angular coordinate, and, by matching the boundary conditions numerically, obtain the characteristic propagation curves for the two modes. A simple computer program permits the acquisition of two-mode fiber device parameters as functions of the core ellipticity, the core dimensions, and the refractive-index difference.

Effect of external index of refraction on multimode fiber couplers.

The dependence of the performance of fused-taper multimode fiber couplers on the refractive index of the material surrounding the taper region has been investigated both theoretically and experimentally. It has been identified that for a 2 × 2 multimode fiber coupler there is a range of output-power-coupling ratios for which the effect of the external refractive index is negligible. When the coupler is tapered beyond this region, the performance becomes dependent on the external index of refraction and lossy. To analyze the multimode coupler-loss mechanism, we develop a two-dimensional ray-optics model that incorporates trapped cladding-mode loss and core-mode loss through frustrated total internal reflection.

Computer-simulation results support the experimental observations. Related issues such as coupler fabrication and packaging are also discussed.

Simultaneous strain and temperature measurement with long-period gratings.

The differential modulation of the attenuation bands in a long-period grating is used for simultaneous sensing of axial strain and temperature. A grating fabricated in a conventional optical fiber is demonstrated for concurrent measurements of strain over a range of 2100 micro? and temperature over a range of 125 degrees C, with maximum errors of 58 micro? and 1 degrees C, respectively.

Modeling of fiber-optic sensors based on micromechanical vibrations in liquid.

Fiber-optic chemical sensors based on optical power absorption or wavelength changes are well known. A new type of sensing element is considered. A micromechanical vibrated fiber-optic tip changes its resonance frequency during its operation. Sensors of this type are simple and convenient and do not require adjustment while in use. They are useful in industry and in medical applications. The action of this sensitive element in a liquid is considered.

Optical scanning extrinsic Fabry-Perot interferometer for absolute microdisplacement measurement.

We report an optical-scanning, dual-fiber, extrinsic Fabry-Perot interferometer system for absolute measurement of microdisplacement. The system involves two air-gapped Fabry-Perot cavities, formed by fiber end faces, functioning as sensing and reference elements. Taking the scanning wavelength as an interconverter to compare the gap length of the sensing head with the reference-cavity length yields the absolute measurement of the sensing-cavity length. The measurement is independent of the wavelength-scanning accuracy, and the reference-cavity length can be self-calibrated simply by one's changing the sensing-head length by an accurate value.

Photoinduced birefringence rotation in germanosilicate glass optical fiber.

Experimental results are reported for the permanent photoinduced rotation of the principal axes of the birefringence in a circular-core single-mode optical fiber on exposure of the fiber to powerful argon-laser radiation at 488 nm. The linearly polarized laser beam was launched into the fiber along one of the principal axes of the initial birefringence in the fiber. After several exposures totaling 4 h, a rotation of as much as 57 degrees was observed in the orientations of the principal axes in addition to the photoinduced birefringence.

Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors.

We demonstrate the operation of a quadrature phase-shifted extrinsic Fabry-Perot fiber-optic sensor for the detection of the amplitude and the relative polarity of dynamically varying strain. Two laterally displaced single-mode fibers inserted within a hollow silica tube form the 90 degrees phase-shifted sensing system. A multimode fiber, placed in the tube facing the two fibers, acts as a reflector, thereby creating an air gap that acts as a Fabry-Perot cavity. A theoretical description of the sensor is given, and its operation as a dynamically varying strain sensor is described. Strain sensitivities of 5.54 degrees phase shift/microstrain cm(-1) are obtained.

Polarimetric analysis of two-mode, elliptical-core optical fibers.

We show that linearly polarized light launched at different angles with respect to the major axis of a two-mode, elliptical-core fiber results in an amplitude-modulated beat-length pattern in the far field. The variation of the output pattern is explained theoretically and confirmed experimentally. A polarizer placed at the output of the fiber is shown to eliminate the amplitude modulation in the output signal. A simple theoretical model, based on the weakly guiding assumption, is shown to be helpful in determining few-mode fiber sensor parameters in practical systems.

Attometer resolution of wavelength shifts by use of fiber modal interferometers.

A method for on-line, real-time detection of wavelength drifts in laser diodes is proposed. This technique uses the wavelength dependence of the differential propagation constant between the LP(01)/LP(even)(11) or LP(01)/LP(02) modes in two-mode fibers to measure wavelength changes that manifest themselves as intensity modulation in the output far field. Theoretical calculations show that wavelength drifts of the order of 10(-18) m can be resolved by this technique.

Optical fiber polarizer based on highly birefringent single-mode fiber.

An in-line fiber polarizer is proposed and demonstrated. The attenuation of a single-mode fiber experiencing an axial periodic perturbation shows peaks at certain wavelengths, resulting from the power coupling between the guided mode and the cladding modes. When the fiber has significant linear birefringence, each of these peaks is split into two peaks. The fiber thus exhibits polarization-selective attenuation characteristics. An example of such a polarizer is demonstrated based on a highly birefringent fiber with a beat length of 1.3 mm. A 25-dB polarization extinction ratio is obtained, with an attenuation of 1.3 dB at an operating wavelength of 1177 nm.

Fiber-optic temperature sensors based on differential spectral transmittance/reflectivity and multiplexed sensing systems.

A concept for optical temperature sensing based on the differential spectral reflectivity/transmittance from a multilayer dielectric edge filter is described and demonstrated. Two wavelengths, λ(1) and λ(2), from the spectrum of a broadband light source are selected so that they are located on the sloped and flat regions of the reflection or transmission spectrum of the filter, respectively. As temperature variations shift the reflection or transmission spectrum of the filter, they change the output power of the light at λ(1), but the output power of the light at λ(2) is insensitive to the shift and therefore to the temperature variation. The temperature information can be extracted from the ratio of the light powers at λ(1) to the light at λ(2). This ratio is immune to changes in the output power of the light source, fiber losses induced by microbending, and hence modal-power distribution fluctuations. The best resolution of 0.2 °C has been obtained over a range of 30-120 °C. Based on such a basic temperature-sensing concept, a wavelength-division-multiplexed, temperature-sensing system is constructed by cascading three sensing-edge filters that have different cutoff wavelengths along a multimode fiber. The signals from the three sensors are resolved by detecting the correspondent outputs at different wavelengths.

Sapphire fibers: optical attenuation and splicing techniques.

The optical attenuation in sapphire fibers was examined. Attenuation was found to depend heavily on injection conditions. A number of techniques for making sapphire-silica fiber splices were attempted, with an effort toward optimizing injection conditions in the sapphire fiber. The most successful of these techniques, interior capillary-tube splicing, produced robust splices with an attenuation of less than 1 dB.

Optical fiber nanometer-scale Fabry-Perot interferometer formed by the ionic self-assembly monolayer process.

The ionic self-assembly monolayer process is a novel technique that has already been used to deposit ultrathin films on glass, polymer, and silicon substrates of different sizes and shapes. This technique is presented as a new tool with which to apply coatings on optical fibers. A nanometer-scale interferometric cavity was built up at the end of an optical fiber with discrete thickness increments of 4.75 nm for a total thickness of 1 mum . Theoretical and experimental aspects of the nanometer-scale Fabry-Perot cavity are described, and both theoretical and experimental results show good agreement.