John D. Gordon and Dr. Stephen Schultz, Electrical and Computer Engineering
Volatile organic compounds (VOC’s) are valuable throughout many industrial and academic settings. These chemicals, however, often raise environmental and health related concerns. For example, chlorinated hydrocarbons (CHC’s) are often used as cleaning solvents but have significant ozone depleting potential. In addition, many CHC’s are known human carcinogens, requiring restrictions to their use.
Accurate real-time monitoring of such chemicals can be extremely important to prevent health and environmental disasters. Through monitoring the concentrations of VOC’s in either gas or liquid forms, VOC sensors can help maintain human and environmental safety. In addition, chemical concentration data acquired through the sensors can be applied to process control systems to improve product quality and yield.
Fiber-optic based chemical sensors provide in-situ remote monitoring of VOC concentrations in liquid or gaseous forms. The small size and ease of integration with electronic systems allows the fiber-optic chemical sensor to be placed in extremely small and remote areas. Fiber-optic chemical sensors are also chemically inert to a wide range of hazardous chemicals. These sensors can be used to monitor several concentrations of VOC’s throughout an array of sensing locations, providing detailed and accurate monitoring of VOC concentration levels.
However, current fiber-optic chemical sensors are in need of improvement in both size and response times. Through this project, we have developed a reusable fiber-optic chemical sensor design based on a single mode D-shaped fiber (D-fiber). The D-fiber provides a real-time VOC sensing platform with a significant decrease in size and chemical response times.
The D-fiber was chosen as a sensing platform because it offers significant improvement over standard round fiber. The D-fiber is significantly smaller, allowing the sensor to be placed in more remote locations. In addition, the fiber has a D shape, with one side of the cross section significantly closer to the fiber core. This provides an opportunity to easily interact with the evanescent field of the signal while maintaining linear geometry.
In order to make the fiber chemically sensitive, a thin polymer membrane was placed on the flat side of the D-fiber. When the polymer absorbs certain chemicals its optical properties change. This change alters the light signal’s phase as it travels though the D-fiber, allowing us to know chemical concentrations at the polymer-coated section of the fiber.
This project involved both sensor design as described above, and sensor optimization. Our goal was to make out fiber-optic chemical sensor as responsive as possible. This involved several iterations and characterization of the sensor. We found that we also had to consider mechanical constraints within our fabrication process.
Through development and optimization of the sensor, we have learned a great deal about the potential for fiber-optic chemical sensors. Our sensor can determine concentration of hazardous chemicals in both liquid and gas forms. In addition, our sensor is able to detect the scent of various chemicals including acetone, which was a threat in the recent airline liquid bomb scare.
Resulting from this project, we expect several publications. My thesis, which describes the project in detail, has been accepted by the Department of Physics and Astronomy. In addition, a journal article is currently pending peer review for publication in Optics Express.
We also expect utilization of what we have learned for application in other types of fiber-optic chemical sensors. We are currently testing a new design, which uses a microscopic grating on the D-fiber flat to increase sensitivity and accuracy of the sensor. Publication of the results is also expected within a few months.
In conclusion, this project has opened several doors to new and innovative chemical sensors. In addition, it has provided me with the valuable opportunity to work closely with my mentor. I have gained experience in optical engineering that I could have not received from any other classroom environment.