Laser-based sensors that are tiny, wireless, and open source
Accurate, precise, and continuous wide-area mapping of trace gas concentrations over long time periods is a difficult problem. Various remote sensing methods attempt to provide such capability, but the larger the area, the less temporal and spatial resolution is available because of slow raster scanning and limited sensitivity at remote locations. Thus, it would be ideal to deploy multiple distributed sensors with adequate sensitivity and time response that could be placed anywhere.
However, experimental laser-based sensors usually require a full suite of tabletop equipment to operate, which makes it difficult to deploy the sensors in real-world field applications. Most research and development in this field has been concentrated on maximizing performance of single-sensor units for field applications in specialized facilities with easily available infrastructure such as electrical power, logistics, and air-conditioned areas. In applications such as remote sensing, most systems use powerful laser sources for increased ranges, causing more complexity and deployment difficulties.
We have developed a solution that can exploit wireless networking technology and infrastructure, as well as novel space and power-saving optoelectronics in order to implement multiple sensors that can be deployed anywhere with minimum to non-existent infrastructure. Our sensing instrumentation, which we call openPHOTONS, is being developed as an open-source platform that enables wireless networking of compact, power-efficient photonic sensors.
Figure 1 depicts the structure of the open repository. Technologies available within the platform include the latest embedded electronic systems and efficient firmware to produce flexible sensor systems. Our approach focuses on gas-phase molecular sensing using laser spectroscopy, which allows for detection of trace amounts of chemicals (at parts-per-million and parts-per-billion levels) and mapping movements of chemicals in time and space. This technology is especially useful for carbon and greenhouse gas monitoring in the atmosphere (see Figure 2).
The electronic platform provides flexible configuration capabilities with digital lock-in amplification of a single photodetector at up to 20kHz and three channels for measuring independent phase and harmonics. It also has a modulated current driver with <2ppm noise at 1kHz in 1 second and a temperature controller that can supply about 6W of cooling power with <0.001K noise over at least 3 hours. The integrated preamplifier has <3pA/Hz1/2 of noise at 1kHz. In addition, there is a continuous universal serial bus or wireless transmission of data. Most photonic sensors require these functions for complete sensor operation.
The open-source nature of the project will allow researchers to collaborate and quickly adapt this technology to new and powerful applications (see Figure 1). The basic architecture is the same used in many remote sensors,1 and it could be made to provide a quickly deployable, multi-point remote-sensing wireless network.
To date, we demonstrated openPHOTONS-based sensing instrumentation for three different detection methods of laser-based gas sensing. They are tunable diode laser absorption spectroscopy (TDLAS),2 quartz-enhanced photoacoustic spectroscopy (QEPAS),3 and Faraday rotation spectroscopy (FRS).4 A TDLAS sensor for carbon dioxide (CO2) using a vertical-cavity surface-emitting laser (VCSEL) at 2μm wavelength was able to reach ~1×10−5/Hz1/2 minimum detectable absorption (MDA) while transmitting data wirelessly and consuming only 0.3W of electrical power.2 This MDA level allowed us to detect 0.1ppm of CO2 in 1 second average time in an optical path of 3.5 meters.
Additionally, the stability of the system evaluated using Allan variance analysis allowed for ~100 seconds of drift-free performance, a typical value for absorption spectrometers. For a QEPAS sensor using a telecom diode laser at 1.57μm, we were able to achieve 1.08×10−6cm−1/Hz1/2 with 26mW of optical power, or about 480ppm of CO2 in air with 1 second average.3 For Faraday rotation spectroscopy4 with static magnetic fields, we were able to achieve an equivalent MDA of ~1×10−6/Hz1/2 resulting in a minimum detection limit of 400ppmv of O2 in air in a 15cm path and 1 second lock-in time constant.
We have used these sensors for a variety of applications including a triangulating gas plume detection test, a field portable soil CO2efflux monitor, and a system to detect the respiration of insects (California isopods). In conclusion, to enable wide-area, long-term, high-spatio-temporal resolution of trace-gas mapping and other applications, we have implemented a wireless photonic sensor platform for remote and point sensors. The platform is open source, and has the capability to generate both research and field sensors. Future work will implement the platform in other types of photonic sensors as reference designs and characterize long-term performance measurements in demanding field applications.
The authors would like to acknowledge the financial support of the Mid InfraRed Technologies for Health and the Environment National Science Foundation (NSF) Engineering Research Center and an NSF major research infrastructure award #0723190 for the openPHOTONS systems.
Stephen So is a postdoctoral research fellow in Princeton's Laser Sensing Laboratory supported by the National Institutes of Health. He received his PhD degree from Rice University, supported by an Autrey Fellowship. His research interests are developing trace-gas sensor networks and wearable trace gas sensors.
Gerard Wysocki is assistant professor of electrical engineering. He received his PhD in physics in 2003 from Johannes Kepler University in Linz, Austria, and his MS in electronics in 1999 from the Wroclaw University of Technology in Poland. His Princeton University Laser Sensing (PULSE) group conducts research on the development of laser spectroscopic instrumentation for applications in chemical sensing. His current research interests include applications of molecular dispersion sensing for in-situ and remote trace gas detection, tunable mid-infrared lasers, and distributed wide-area spectroscopic sensor networks.
