While a variety of techniques exist to monitor trace gases, methods relying on absorption of laser light are the most commonly used in terrestrial applications. Cavity-enhanced absorption techniques typically use high-reflectivity mirrors to form a resonant cavity, inside of which a sample gas can be analyzed. The effective absorption length is augmented by the cavity’s high quality factor, or Q, because the light reflects many times between the mirrors. The sensitivity of such mirrorbased sensors scales with size, generally making them somewhat bulky in volume. Also, specialized coatings for the high-reflectivity mirrors have limited bandwidth (typically just a few nanometers), and the delicate mirror surfaces can easily be degraded by dust or chemical films.

The Trace Gas Detection System features a concentrated sample gas flowing to the free-space gap in the microfabricated disc, packaged inside a compact sealed enclosure with external laser and detection electronics. Also shown is a SEM image of a high-Q calcium fluoride disc that has been augmented with a notch using FIB.
As a highly sensitive and compact alternative, JPL is developing a novel trace gas sensor based on a monolithic optical resonator structure that has been modified such that a gas sample can be directly injected into the cavity. This device concept combines ultrahigh Q optical whispering gallery mode resonators (WGMR) with microfabrication technology used in the semiconductor industry. For direct access to the optical mode inside a resonator, material can be precisely milled from its perimeter, creating an open gap within the WGMR. Within this open notch, the full optical mode of the resonator can be accessed. While this modification may limit the obtainable Q, calculations show that the reduction is not significant enough to outweigh its utility for trace gas detection. The notch can be milled from the high-Q crystalline WGMR with a focused ion beam (FIB) instrument with resolution much finer than an optical wavelength, thereby minimizing scattering losses and preserving the optical quality. Initial experimental demonstrations have shown that these opened cavities still support high-Q whispering gallery modes.

This technology could provide ultrasensitive detection of a variety of molecular species in an extremely compact and robust package. With this type of modified WGMR, one can inject a gas sample into the open gap, allowing highly sensitive trace molecule detection within a roughly 1-cm volume. Other critical components of the instrument, such as the detector and a semiconductor laser, could be directly packaged with the resonator so as to not significantly increase the size of the device.

Besides its low mass, volume, and power consumption, the monolithic design makes these resonators intrinsically robust devices, capable of handling significant temperature excursions, without moving parts to wear out or delicate coatings that can be easily damaged. A sensor could integrate with microfluidics technology for a chip-scale device. It could be mounted to the end of a deployable arm, or inserted into a borehole. Also, a network of individual sensors could be dispersed to monitor conditions over a wide region.

This work was done by David C. Aveline, Nan Yu, Robert J. Thompson, and Dmitry V. Strekalov of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47173

Topics:
Chemicals Coatings, colorants and finishes Coatings, colorants, and finishes Energy consumption Fluoride Gases Lasers Optics Physical Sciences Scale models Semiconductors Sensors and actuators Simulation and modeling
This Brief includes a Technical Support Package (TSP).
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Miniature Trace Gas Detector Based on Microfabricated Optical Resonators

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NASA Tech Briefs Magazine

This article first appeared in the October, 2013 issue of NASA Tech Briefs Magazine (Vol. 37 No. 10).

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Overview

The document presents a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing advancements in miniature trace gas detection using microfabricated optical resonators, specifically modified Whispering Gallery Mode resonators (WGMR). The primary objective of the research was to develop WGMRs with an internal opening, allowing access to the optical fields within the resonator, which enhances their functionality and application potential.

Traditional WGMRs are limited in their accessibility due to the confinement of their mode volume, which restricts interaction to the perimeter of the disk where the evanescent field leaks out. The innovative design of "unlocked" WGMRs, featuring a precise notch cut into the perimeter, overcomes this limitation. This modification enables the integration of internal structures within the optical cavity, significantly broadening the scope of applications.

The document outlines several promising applications for these modified WGMRs, including trace gas detection and spectroscopy, opto-mechanical sensors, precision displacement and force sensors, molecular mass detection, and optical modulators. These devices are particularly suited for various technologies utilized by JPL and NASA, such as miniature accelerometers and gyroscopes for spacecraft navigation, as well as applications in the Department of Defense (DoD) and civil sectors.

Key results from the research include the successful development of methods for precision milling using Focused Ion Beam (FIB) technology on materials like CaF2 and fused silica. The team fabricated resonator discs with diameters of approximately 2.5 mm and thicknesses of 50-75 micrometers, incorporating notches of 10-20 micrometers wide. Optical analysis systems were built to measure the quality factor (Q) of the resonators, achieving a loaded Q of approximately 4x10^5 and a finesse of 56.

The significance of these advancements lies in the WGMRs' combination of extremely high optical quality, small volume, robust packaging, low power requirements, and high optical intensity without the need for special coatings. This research not only contributes to the field of optical resonators but also holds the potential for significant technological advancements in various applications, particularly in aerospace and environmental monitoring. The document serves as a comprehensive overview of the project, its objectives, results, and implications for future technology development.