William N. Gibler

In-line fiber Fabry-Perot interferometer with high-reflectance internal mirrors

A finesse of 21 has been measured for a fiber Fabry-Perot interferometer (FFPI) with multilayer TiO/sub 2//SiO/sub 2/ internal mirrors. Experimental reflectance and transmittance data for the thermally tuned interferometer were fit with calculated curves using values of 86% for the reflectance and 7.2% (0.33 dB) for the excess optical loss of each mirror. Applications in sensing and communications are discussed. >

FIBER-OPTIC PRESSURE SENSORS FOR INTERNAL COMBUSTION ENGINES

Two designs incorporating embedded fiber Fabry-Perot interferometers as strain gauges were used for monitoring gas pressure in internal combustion engines. Measurements on a Diesel engine, a gasoline-fueled engine, and a natural-gas engine are reported.

Fiber optic application for thermal switching in vanadium dioxide films

Large thermally induced changes in the end-interface reflectance and transmittance of silica fibers coated with vanadium dioxide films have been observed.

Interferometric fiber-optic sensor embedded in a spark plug for in-cylinder pressure measurement in engines.

Pressure sensing in an internal combustion engine with an intrinsic fiber Fabry-Perot interferometer (FFPI) integrated with a spark plug is demonstrated for the first time. The spark plug was used for the ignition of the cylinder in which it was mounted. The FFPI element, protected with a copper/gold coating, was embedded in a groove in the spark-plug housing. Gas pressure in the engine induced longitudinal strain in this housing, which was also experienced by the fiber-optic sensing element. The sensor was monitored with a signal conditioning unit containing a chirped distributed-feedback laser. Pressure sensitivities as high as 0.00339 radians round-trip phase shift per pounds per square inch of pressure were observed. Measured pressure versus time traces showed good agreement with those from a piezoelectric reference sensor mounted in the same engine cylinder.

Metal-embedded fiber-optic Fabry–Perot sensors

The sensing of temperature and of ultrasonic pressure with fiber-optic Fabry-Perot interferometers embedded in aluminum is demonstrated. The metal parts are cast in air by using graphite molds. Breakage of the fibers at the air-metal interface during the casting process is avoided through the use of stainless-steel stress-relief tubes. The optical phase in an embedded interferometer is found to be 2.9 times more sensitive to temperature change than for the same interferometer in air, in good agreement with model calculations. An embedded interferometer has also been used to detect ultrasonic waves over the frequency range of 0.1-8 MHz.

Optical fiber Fabry-Perot sensors for smart structures

Fiber Fabry-Perot interferometers (FFPIs) utilizing internal mirrors have been developed to sense temperature, strain, acoustic waves and other physical perturbations in structural materials, and have been successfully embedded in composites and in metals. The construction, performance and application of the FFPI sensors to smart structures are described.

Fiber optic sensor for substrate temperature monitoring

The use of an optical fiber Fabry–Perot interferometric temperature sensor for monitoring substrate temperature in a rf sputtering system is demonstrated. The sensor head consists of a continuous length of single mode silica fiber which contains two internal mirrors a distance of 1.08 cm apart to form the interferometer cavity. The laser light source for the sensor is located outside the vacuum system and connected to the sensor head via a fiber optic feedthrough. The accuracy of the sensor was 0.05 °C in this experiment, but considerable improvement is possible with better signal averaging techniques. The fiber optic sensor provided substrate temperature information during rf plasma excitation, when a nearby thermocouple ceased to function due to electromagnetic interference.

Method for embedding optical fibers and optical fiber sensors in metal parts and structures

A technique for embedding one or more optical fibers in a cast metal part or structure while maintaining optical transmission through the fiber is presented. This technique provides nondestructive monitor of internal perturbations of the structure. Application of the method to embedded fiber optic sensors in metallic structures and to fiber-embedded metal feedthrough are reported and the performances of temperature and ultrasound fiber sensor embedded in a cast aluminum block are demonstrated.

An interferometric fiber optic sensor embedded in a spark plug for in-cylinder pressure measurement in engines

The feasibility of measuring in-cylinder engine pressure with a novel spark plug embedded fiber optic sensor is demonstrated. This result, combined with the proven ability of FFPI (fiber Fabry-Perot interferometer) sensors to operate reliably at high temperatures in engines, suggests that fiber optic sensors merit consideration for use with engine control systems for reducing emissions and improving fuel economy in automobiles of the future.

Fiber Optic In-Cylinder Combustion Pressure Sensor System

The need for improved in-cylinder pressure sensors for reciprocating engines is widely recognized in both user and R&D communities. The ideal sensor would give accurate readings of combustion chamber gas pressure with high temporal resolution within each engine cycle. It would also operate reliably for several years - for at least one year without calibration - and be affordable (i. e., cost a few hundred dollars per cylinder). Sensors meeting these criteria are the key to engine control systems for reducing harmful emissions levels and improving fuel economy, and to parametric monitoring for predicting emission levels. They will also find immediate application in instrumentation for engine research and development.