Edgar A. Mendoza

Embeddable distributed moisture and pH sensors for nondestructive inspection of aircraft lap joints

Two distributed fiber optic sensors for use in the prevention and monitoring of corrosion in aircraft are described. These sensors, based on optical fibers that are intrinsically sensitive to either water or changes in pH, will alert maintenance personnel to the presence of water in lap joints and other inaccessible critical areas. Furthermore, the sensors can also locate precisely where the moisture infiltration has occurred. In a typical application, a sensor fiber would be embedded in a lap joint along the bottom panel of an aircrafts body, or on a wing, where water is likely to collect. Changes in the optical transmission through the fiber can be monitored either periodically or continuously to determine the extent of water penetration.

Photolithography of integrated optic devices in sol-gel glasses

Advances in integrated optics devices require the development of materials and technologies that can transmit, guide, receive, multiplex, demultiplex, modulate, and demodulate optical signals. These requirements are crucial for the realization of advanced integrated optic devices that fully employ the potentially wide bandwidth (approximately THz) of optical signal processing and computing. The authors are developing an innovative new technology for the photolithographic fabrication of sol-gel derived integrated optic devices (sol-gel IODs). The fabrication procedure is based on direct photolithographic writing of IODs onto a photoactive sol-gel glass matrix. Sol-gel glasses are made by a two-step process: first a gel film is chemically formed and dried to a porous state, and second the porous film is densified into solid glass at high temperature. An organometallic photosensitizer is doped into the porous matrix after the first step. Waveguide patterns are then formed by straightforward photolithographic techniques, and the unexposed sensitizer is removed. The final densification step locks in the waveguide patterns, creating a durable, impermeable integrated optic device. Depending on the choice of sensitizer, these waveguide patterns can be passive (simply having a higher index of refraction than the surrounding host glass) or active (possessing optical properties that can be influenced by the application of electric fields). The uncomplicated nature of this process makes this a very promising approach for the fabrication of commercially viable integrated optic devices.

Photolithography of Bragg gratings in sol-gel-derived fibers

We report a novel photolithographic technique that facilitates the photogeneration of refractive index gratings in sol-gel derived silica fibers, based on the ability to produce changes in the index of refraction of a silica glass of the order of 0.1 to 0.001. A porous sol-gel fiber impregnated with a photosensitive organometallic compound exhibits optical changes when exposed to light. Changes in transmission and refractive index can be induced and are permanent after consolidation of the porous glass matrix. Gratings have been produced in doped sol-gel fibers by exposing the fiber to UV light through a contact mask. The photoinduced grating is permanent after heat treatment of the fiber to 1200 degree(s)C. The uncomplicated nature of this process makes this a very promising approach for the fabrication of commercial fiber optic refractive index gratings for fiber lasers, fiber optic sensors, filters, and multiplexing schemes.

Active intrinsic optical chemical sensor for the detection and measurement of carbon monoxide in air

In our ongoing work developing a reversible optical chemical sensor (OCS) for CO, we have produced a greatly simplified and improved sensor design. Chemically impregnated porous optical fiber has been replaced with impregnated porous Vycor glass; a battery powered blue LED has replaced a compact Hg lamp and power supply; optical fibers are no longer necessary; The Vycor acts as both chemical transducer and optical waveguide. The CO OCS now consists of five components: a 9 V battery and a blue LED; a transition metal impregnated porous Vycor matrix/waveguide; a short pass interference filter or a colloidally colored glass bandpass filter; and a large area, resin coated Si photodetector. The CO OCS responds reversibly to the presence of [CO] in both air and in N2, over the 40 to 950 ppm range at STP. The sensor has shown a lower limit of detection of approximately 40 ppm at STP. In terms of the transmitted intensity, the sensor is very slow in responding to very slow changes or buildups in the [CO]ambient. Yet, in the presence of rapid changes in the [CO]ambient, the sensor displays a 100% time constant of 60 to 70 seconds irrespective of the delta [CO]ambient. The new design simplifies sensing greatly in that optical fibers (and the various problems associated with their use), bulky light sources, and extremely fragile porous optical fibers have been supplanted by small, durable, inexpensive, commercially available components. The sensor could find application in environments in which the [CO] can or does change rapidly.

Miniaturized integrated optic chemical sensors for environmental monitoring and remediation

The use of integrated optic chemical sensors in environmental monitoring and remediation applications offers many advantages, the most prominent being that they can easily accommodate multiple sensing-elements, are miniaturized, lightweight, and immune to electromagnetic interference. We are developing versatile, multi-analyte, micro-miniaturized integrated optic chemical sensor (IOCS) technologies for use in closed-loop control and process monitoring for industrial and environmental applications. We have demonstrated a complete integrated optic sensor system and tested it by simultaneously identifying and quantifying samples containing traces of metal ions in a water stream. Conversion of the optical information into control signals is accomplished by a wavelength-division multiplexed optoelectronic unit, which can be remotely located, connected via optical fibers. The simplicity of this process makes this a very promising approach for the fabrication of commercially viable integrated optic devices.

Demonstration of Self-Referenced Fiber Optic Moisture and pH Sensors Using Optical Time Domain Reflectometry (OTDR)

The authors are developing a family of distributed fiber optic sensors (DIFOS) for use in prevention and monitoring of corrosion in advanced structural components of aging aircraft. These sensors, based on optical fibers that are intrinsically sensitive to either water or changes in pH, will alert maintenance personnel to the presence of water in aircraft lap joint structures and other inaccessible critical areas. We have demonstrated that the distributed fiber optic moisture and pH sensors can detect and localize water and/or changes in the pH environment surrounding the fiber sensor with 10-cm spatial resolution. A dual- wavelength (850 nm and 1300 nm) optical time domain reflectometer (OTDR) was used to characterize the spatial sensitivity and resolution of 20-meter lengths of both the moisture-sensitive and pH-sensitive optical fibers. The results of the characterization efforts demonstrated conclusively the ability of these distributed sensors to detect and localize their respective target measurands within 10 cm, with built-in self-referencing to distinguish between moisture (or pH changes) and spurious effect (e.g., fiber bending).

Fiber optic NO2 sensor for combustion control

We report here experimental results for a nitrogen dioxide sensor based on optical transmission through porous silica fiber. The NO2 optrode is activated by using a proprietary chemical pretreatment process on the porous silica. Light is coupled to and from the porous silica optrode via 600 micrometers diameter optical fibers. The peak response of the sensor is at approximately 420 nm and therefore can be monitored using a blue LED. The sensor demonstrated a reversible, full-range response from 0 to 10,000 ppm NO2 in a balance of nitrogen. The sensor also demonstrated a sensitivity of 10 ppm. The range and sensitivity of this sensor make it suitable for monitoring nitrogen dioxide emission from combustion stacks.

Intrinsic fiber optic sensor for distributed water detection

A distributed fiber optic moisture sensor based on intrinsic changes in the optical properties of the cladding is reported. A 10-meter-long fiber sensor was fabricated that demonstrated a response to humidity in less than 5 minutes. The humidity-sensitive cladding was fabricated on-line during fiber draw by continuously coating a multimode glass core fiber with a polyvinyl acetate cladding, in which a water-sensitive indicator had been dissolved. The indicator was a solvatochromic dye that showed a pronounced hypsochromic shift in its absorption spectrum in the presence of water. The moisture response of the sensor was monitored by measuring changes in the optical attenuation of the fiber in the region between 580 nm and 650 nm. This spectral region facilitates the use of commercially available solid state optoelectronic devices such as LEDs, laser diodes, and PIN photodiode detectors, in order to produce a low-cost, compact, lightweight humidity sensor.

Photolithography of integrated optical devices in porous glasses

We are developing a completely new technology for the low cost fabrication of integrated optic devices (IODs) in silica glasses. This technology is based on direct photolithographic writing of IODs onto photoactive sol-gel glasses. Waveguide patterns are formed by straightforward photolithographic techniques. Depending on the choice of photosensitive glass, these waveguide patterns can be passive (simply having a higher index of refraction than the surrounding `host glass) or active (with optical properties that can be influenced by the application of electric or magnetic fields). The simplicity of this process makes it very promising for the fabrication of commercially viable integrated optic devices.

Sol-gel-based fiber optic and integrated optic chemical sensors for environmental monitoring and process control

Sol-gel technology has been used to fabricate fiber optic and integrated optic chemical sensors for environmental monitoring and process control applications. These multi-sensing element sensors offer many advantages, the most prominent being that they are miniaturized, lightweight, and immune to electromagnetic interference. We are developing versatile, multi-analyte, micro-miniaturized fiber optic chemical sensor (FOCS) and integrated optic chemical sensor (IOCS) technologies for use in closed-loop control and process monitoring for industrial and environmental applications.