Chemical sensors based on optical ring resonators are undergoing development. A ring resonator according to this concept is a closed-circuit dielectric optical waveguide. The outermost layer of this waveguide, analogous to the optical cladding layer on an optical fiber, is a made of a polymer that (1) has an index of refraction lower than that of the waveguide core and (2) absorbs chemicals from the surrounding air. The index of refraction of the polymer changes with the concentration of absorbed chemical(s). The resonator is designed to operate with relatively strong evanescent-wave coupling between the outer polymer layer and the electromagnetic field propagating along the waveguide core. By virtue of this coupling, the chemically induced change in index of refraction of the polymer causes a measurable shift in the resonance peaks of the ring.

Figure 1. A Polymer-Clad Optical Ring Resonator acts as a chemical sensor in that the resonance spectrum becomes shifted in wavelength when the polymer absorbs chemicals from the air.

In a prototype that has been used to demonstrate the feasibility of this sensor concept, the ring resonator is a dielectric optical waveguide laid out along a closed path resembling a racetrack (see Figure 1). The prototype was fabricated on a silicon substrate by use of standard techniques of thermal oxidation, chemical vapor deposition, photolithography, etching, and spin coating. The prototype resonator waveguide features an inner cladding of SiO2, a core of SixNy, and a chemical-sensing outer cladding of ethyl cellulose. In addition to the ring resonator, there are input and output waveguides separated from the straight segments of the ring resonator by an evanescent- wave-coupling gap of 2 mm.

Figure 2. Shifts in the Wavelength of a peak in the resonance spectrum of the device of Figure 1

Figure 2 presents results of a test of the prototype in an open room. During the test, the temperature of the sensor was stabilized to ±0.1 K. The sensor was left undisturbed by chemicals, except during a short interval when a cotton swab wetted with isopropyl was placed 4 in. (~10 cm) away from the sensor and another short interval when a cotton swab wetted with acetone was similarly placed near the sensor. The chemical exposures resulted in easily detectable signals that exceeded background variations by at least an order of magnitude. The jagged nature of the portions of the plot corresponding to the chemical exposures has been attributed to “mode hops,” in which the specific ring-resonator mode that was being followed moved out of the tuning range of a laser used as the input light source, causing the laser to lock onto a new mode.

The results have been interpreted as demonstrating the feasibility of optical polymer-based sensors. Inasmuch as the index of refraction of ethyl cellulose is known to respond to wide variety of volatiles, sensors like this one could be useful as non-specific indicators of spills of volatile compounds.

This work was done by Margie Homer, Allison Manfreda, Kamjou Mansour, Ying Lin, and Alexander Ksendzov of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
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Refer to NPO-40601, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Chemicals Coatings, colorants and finishes Coatings, colorants, and finishes Corrosion Electronic equipment Fiber optics Lasers Optics Physical Sciences Polymers Sensors and actuators Waveguides
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Chemical Sensors Based on Optical Ring Resonators

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    This article first appeared in the October, 2005 issue of NASA Tech Briefs Magazine (Vol. 29 No. 10).

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    Overview

    The document titled "Chemical Sensors Based on Optical Ring Resonators" presents research conducted by a team including A. Ksendzov, M.L. Homer, A. M. Manfreda, K. Mansour, and Y. Lin, focusing on the development of an optical ring resonator readout system for polymer-based chemical sensors. The primary aim is to enhance the detection of volatile compounds in gaseous environments using a novel sensing technique.

    The researchers utilized ethyl cellulose as the sensing layer due to its well-characterized properties and responsiveness to a wide range of volatiles, including acetone and isopropyl alcohol. The optical readout method is based on evanescent wave sensing, which allows for increased sensitivity and circumvents challenges associated with traditional conductometric sensors that rely on conductive particle loading in polymers.

    The ring resonator, designed with a racetrack shape, features a bend radius of 1.5 mm and incorporates a waveguide structure that ensures significant overlap between the propagating field and the polymer cladding. The fabrication process involved multiple layers, including thermal oxide and PECVD SiₓNy, followed by patterning and annealing to optimize optical performance. The resulting sensor demonstrated a loss of 1.5 dB per round trip and a crossover fraction close to theoretical predictions.

    Preliminary tests conducted in an open room environment showed the sensor's capability to detect chemical exposure effectively. The data collection involved monitoring the position of resonator peaks while exposing the sensor to isopropyl alcohol and acetone. The results indicated that chemical exposure produced detectable signals that significantly surpassed background variations, confirming the sensor's sensitivity.

    The document emphasizes the potential applications of this technology, including the development of non-specific indicators for volatile compounds and the possibility of array sensing for more complex chemical identification. The ongoing research aims to calibrate the sensors in controlled atmospheres to further validate their performance.

    Overall, this work represents a significant advancement in chemical sensing technology, leveraging optical ring resonators to improve detection capabilities and broaden the scope of applications in environmental monitoring and safety. The findings contribute to the broader field of sensor technology, with implications for aerospace and other industries.