Grantee Research Project Results
1999 Progress Report: An Integrated Near Infrared Spectroscopy Sensor for In-Situ Environmental Monitoring
EPA Grant Number: R826190Title: An Integrated Near Infrared Spectroscopy Sensor for In-Situ Environmental Monitoring
Investigators: Levy, Roland A.
Institution: New Jersey Institute of Technology
EPA Project Officer: Aja, Hayley
Project Period: February 20, 1998 through February 19, 2001 (Extended to February 19, 2003)
Project Period Covered by this Report: February 20, 1999 through February 19, 2000
Project Amount: $322,230
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals
Objective:
The integrated optical sensor under consideration is a device that uses the principles of interferometry in order to monitor and determine in-situ the concentration of numerous organic analyte species simultaneously. Operation of the sensor is based on the detection of refractive index changes on waveguide surfaces resulting from the presence of volatile organic compounds (VOCs) in the environment. The sensor consists of a symmetric, single mode Mach-Zehnder interferometer with one arm (sampling) that is exposed directly to the analyte. A glass buffer layer protects the second arm (reference) from the influence of different gases. Light is coupled into the waveguide and split between the sampling and reference arms using a Y-splitter configuration. Since the phase difference is directly proportional to the effective index difference between the waveguide arms, the interferometer's output intensity changes with the concentration of gas present in the air.
Such a sensor offers numerous advantages over conventional analytical techniques such as gas chromatography and mass spectrometry including small physical size, geometric flexibility, environmental versatility, immunity to the electromagnetic interference, low loss for long length sensing and high sensitivity. This device has been designed so as to be mass-produced using standard silicon based processes resulting in low cost. The envisioned instrumentation that incorporates this sensor is expected to be portable, rugged and suitable for real time monitoring of organics. The feedback from such a device combined with current optical fiber technology allows for the device to be used as a remote, in-situ sensor.
Progress Summary:
The waveguides used to fabricate the sensor consisted of silica-based films on silicon wafers. A 15 mm thick SiO2 film was first synthesized by low pressure chemical vapor deposition (LPCVD) to act as cladding material for the waveguide and prevent light from coupling with the underlying silicon. A 7 mm thick phosphorus-doped (8 weight percent P) LPCVD SiO2 film was then synthesized to act as core material for the waveguide. This layer underwent patterning using standard lithographic exposure and plasma etching techniques and subjected to a 1050?C anneal to cause viscous flow and rounding of the edges. This rounding-off procedure was found to be necessary to minimize coupling losses between fiber and waveguide. The refractive index of the doped glass was measured to be near 1.466, thus, producing with the underlying SiO2 (n=1.458) cladding a single mode waveguide device. Deposition of a thick (~1mm) LPCVD SiO2 buffer layer over the entire wafer and a subsequent lithographic step resulted in selective removal of that layer over the sampling arm of the interferometer. This configuration allowed for exposure of the sampling arm to various gases in the air environment in order to cause a change in the effective refractive index of that arm. The arm coated with the SiO2 buffer layer saw a constant refractive index of n = 1.458. Three, four, five, and six microns-wide waveguides formed the two interferometer paths using a splitting angle of 2?. The sampling and reference arms had a fixed separation of 50 mm and variable lengths (2, 4, 6, 8, and 10 mm). A comparison between calculated and measured dependence of interferometer output on arm length was used for sensor calibration purposes. The processing sequence used in the fabrication of the sensor is shown below:
Future Activities:
Our effort has focused on developing the technology required to design the waveguide structures, the processes needed to produce integrated optical sensors and the methodology necessary to model the device performance. With the existence of such a technology platform, the stage is set to develop our prototype sensors and evaluate the performance in terms of their capabilities for monitoring and quantifying multicomponent mixtures of contaminants in air and water during the remaining period of this grant.Journal Articles:
No journal articles submitted with this report: View all 4 publications for this projectSupplemental Keywords:
air, water, VOC, environmental chemistry, monitoring, analytical., Scientific Discipline, Air, Environmental Chemistry, Chemistry, Engineering, Chemistry, & Physics, Electron Microscopy, environmental monitoring, organic analyte species, remote sensing, optical sensor, infrared spectroscopy sensor, Mach-Zender interferometer, waveguide surfaces, real time monitoring, organic contaminantsRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.