Robert A. Atkins

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. >

Performance of a fiber-optic temperature sensor from −200 to 1050°C

A reflectively monitored Fabry-Perot temperature sensor is characterized over a 1250 degrees C temperature range. Dielectric mirrors for the interferometer are incorporated into a continuous length of single-mode fiber by a fusionsplicing technique. The change in optical phase per unit temperature is nearly constant above room temperature but decreases at lower temperatures. Phase-shift data for the germania-doped silica fiber used in these experiments are compared with calculations based on previously reported information on the temperature dependence of refractive index and length in bulk fused silica.

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.

Railroad bridge instrumentation with fiber-optic sensors

Fiber-optic sensors were installed on fatigue-critical components in the superstructure of a railroad bridge to monitor dynamic strains induced by trains crossing the bridge as well as to detect the onset of cracks. Each fiber Fabry–Perot interferometer (FFPI) strain gage was adhesively bonded to a stainless-steel strip to facilitate all-weather installation on the steel bridge members by spot welding. FFPI strain sensors were also installed on a rail at an approach to the bridge. Electrical resistive strain gages were colocated with the fiber-optic sensors on the bridge for the purpose of performance verification. In addition to the strain gages, fiber-optic continuity sensors for crack detection were bonded to the structure at critical locations. A telemetry system for transmitting the data over telephone lines was also installed at the bridge site. Dynamic response of the fiber-optic strain sensors is comparable with that of the electrical gages, and their performance has not degraded in the year since the initial installation.

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.

Optical characterization of a-As2S3 thin films prepared by magnetron sputtering

It is well known that thermally evaporated a-As2S3 thin films are prone to oxidation when exposed to ambient environment. These As2O3 crystals can have a devastating effect on propagating light by introducing a major source of scattering loss in submicron optically integrated circuits. Magnetron sputtering a-As2S3 not only produces films that have optical properties closer to their equilibrium state like their bulk glass counter parts, the as-deposited films also show no detectable signs of As2O3 crystals in them when they are exposed to the ambient environment. These attributes are unique to a magnetron sputtered a-As2S3 film and are probably caused by the “photoannealing” effect from the visible light emitted by the argon plasma during the sputtering process. The optical properties of a magnetron sputtered a-As2S3 film and its propagation loss on a Ti diffused LiNbO3 waveguide are reported here. The thin film results agree closely with published data on As2S3 bulk glass, and the optical properties of th...