Nicholas Lagakos

Temperature-induced optical phase shifts in fibers.

The static thermal sensitivity of the optical phase in bare and jacketed fibers has been studied both analytically and experimentally. Taking into account the exact fiber composition and geometry, the strains have been determined from the thermally induced stresses using the appropriate boundary conditions, and the resulting phase shift has been calculated. The results of this analysis are found to be in agreement with experimental results obtained from measurements employing a Mach-Zehnder fiber interferometer.

Frequency and temperature dependence of elastic moduli of polymers

The frequency and temperature dependence of the elastic moduli of a number of commercially available polymers has been studied in the temperature range of 0–35 °C and for frequencies 102–106 Hz. Away from transitions a significant new relationship has been obtained, i.e., the Young’s modulus of these polymers is proportional to log of frequency. Using this relationship, together with the low and high frequency data, transitions in some of the polymers were identified.

Acoustic sensitivity predictions of single-mode optical fibers using Brillouin scattering

Elastic and elastooptic coefficients used to predict acoustic response sensitivity for two single-mode optical fibers have been determined from Brillouin scattering measurements. These measurements were made on two ITT single-mode fiber preforms currently of interest in the fabrication of fiber-optic acoustic sensors. Previous predictions of acoustic sensitivity assumed the optical fiber waveguides as homogeneous fused silica cylinders. It was found that this assumption introduces no more than a 5% error in the pressure sensitivity for a low numerical aperture (N.A.) fiber and a 30% error for a high N.A. fiber.

Planar fiber-optic interferometric acoustic sensor

A sensing portion for an interferometric acousto-optic sensor and a sensor using that sensing fiber are disclosed. The sensor fiber has an outer coating of a material having a low bulk modulus. This coating greatly enhances the sensitivity of the sensor fiber. In a preferred embodiment, the material of low bulk modulus embeds the sensor fiber and any included reference fiber. Most preferably, the material having a low bulk modulus is a polymer such as polyurethane. The sensing portions made according to the present invention will usually be incorporated into sensors used for underwater exploration.

Miniature, high performance, low-cost fiber optic microphone

A small, high performance fiber optic microphone has been designed, fabricated, and tested. The device builds on a previous design utilizing a thin, seven-fiber optical probe, but now adds a micromachined 1.5μm thick silicon diaphragm active element. The resulting sensor head is thin (several millimeters) and light, and the overall microphone system is less expensive than conventional microphones with comparable performance. Measurements in the laboratory using a standard free-field technique at high frequencies, an enclosed calibrator at lower frequencies, and pseudostatic pressure changes demonstrate uniform broadband response from near dc (0.01 Hz) up to near 20 kHz. The measured microphone internal noise is nearly flat over this band and does not exhibit noticeable levels of 1∕f noise. Over the audible portion of this band, the minimum detectable pressure is determined to be 680μPa per root Hz with further reductions possible using lower noise∕higher power light sources and∕or improvements in the diap...

Planar and linear fiber optic acoustic sensors embedded in an elastomer material

A interferometric planar and linear fiber optic sensor system comprised of a sensor element and a reference element. In the planar fiber optic sensor system the sensor and reference fibers are shaped in loops circularly and uniformly, heat treated or bonded together and embedded in a spiral pattern within a low Bulk Modulus and Youngs Modulus elastomer encapsulant. The invention results in high and frequency independent sensitivity which minimizes acceleration effects. For the linear sensor, the sensing fiber is shaped in loops forming a linear chain which is embedded in an appropriate low bulk modulus elastomeric encapsulant. The reference fiber is shaped in loops around a cylindrical aluminum mandrel within which the input and output fiber cables and couplers are encapsulated in a high bulk modulus material epoxy resin. In both the planar and linear forms, the sensing and reference fiber may be of equal length, however, the reference fiber may be of a shorter length when a coherent light source is utilized.

Microbend fiber-optic sensor as extended hydrophone

A novel microbend fiber-optic acoustic sensor has been studied, both analytically and experimentally. The sensor is simple mechanically, insensitive to acceleration, and achieves shape flexibility by utilizing fairly long fiber lengths for the sensing element. The acoustic sensitivity and minimum detectable pressure of the sensor were found to be significantly improved over previously reported microbend sensors. Further optimization of the sensor appears possible.

Pressure desensitization of optical fibers

The pressure sensitivity of the phase in optical fibers has been studied analytically by taking into account the exact composition and geometry of multilayer fibers. This analysis shows that there are combinations of glass and coating materials and corresponding thicknesses which make fibers pressure insensitive.

Optimizing fiber coatings for interferometric acoustic sensors

The pressure sensitivity of the phase of light propagating in an optical fiber is studied both analytically and experimentally. The analysis, which takes into account the exact composition and geometry of multilayer fibers, is utilized to identify coating properties which optimize the fiber acoustic sensitivity. In order to predict the fiber acoustic sensitivity, the elastic parameters of commonly used coating materials, thermoplastics, and UV curable elastomers have been studied in bulk samples as a function of frequency ( 10^{2}-10^{4} Hz) and temperature ( 0-35\deg C). The analytically predicted frequency dependence of the acoustic sensitivity is found to be in agreement with that obtained experimentally from fibers with coatings of various materials.

Dynamic thermal response of single-mode optical fiber for interferometric sensors.

The dynamic temperature phase sensitivity of a three-layer optical fiber is calculated for unjacketed as well as Al- and Hytrel-coated fibers. The calculations include both the variation of the refractive index with temperature and the thermally induced axial and radial strains. The calculated phase sensitivity indicates that it is currently possible to measure a 1-microdegree C temperature change at frequencies exceeding 50 kHz with 1 cm of a metal coated optical fiber.