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Ultrasensitive sensor causes stir at show

Photonics Spectra
Mar 2010
Melinda Rose,

A prototype laser spectroscopy platform developed by electrical engineers at Princeton University attracted a lot of attention when it was demonstrated during SPIE Photonics West 2010 in January. The sensor, a new development in wireless sensor networks for monitoring trace gases and chemicals, uses an infrared laser operating at 2 µm and can detect atmospheric carbon dioxide with a sensitivity of 113 ppb in an average time of 1 s.

“Several people stopping by the booth have expressed interest in commercializing the technology, which gives us a good impression that our efforts are going in the right direction, because we want to develop technology that is truly field-deployable and useful for many different applications,” said Gerard Wysocki, an assistant professor of electrical engineering whose Princeton group developed the sensor in collaboration with MIRTHE (Mid-InfraRed Technologies for Health and the Environment), a National Science Foundation Engineering Research Center based at Princeton.

“Of course, there is large interest among scientists who consider this kind of development very useful because they could start new applications that are not possible with today’s large and bulky instrumentation that is commercially available,” Wysocki said.

His group, which includes postdoctoral scientist Stephen So and graduate student Clinton Smith, designed the tunable diode laser absorption spectroscopic (TDLAS) CO2 sensor to be lightweight, highly sensitive and extremely energy efficient, with the potential to operate for years on batteries or solar power.

“The main focus of this work was on developing ultralow-power operation of those sensors, and this particular sensor is based on a VCSEL [vertical-cavity surface-emitting] laser working at two micrometers, and the total power dissipation of the sensor is about 0.3 watts,” Wysocki said.

A low-power, portable wireless laser spectroscopic sensor for atmospheric CO2 monitoring attracted a lot of attention when it was demonstrated during SPIE Photonics West 2010 in January. The sensor uses an infrared laser operating at 2 µm and can detect atmospheric carbon dioxide with a sensitivity of 113 ppb (1ss) in an average time of 1 s.

The sensor can run several days on a single 10.5-Ah lithium-polymer battery (with continuous wireless transmission of data). It also has the capability for low operational duty cycles – 100 nA sleep current with short wake time – and it can be deployed with a solar panel or other energy-harvesting unit for continuous monitoring.

“Depending on the radio, we can get about 100 meters between sensors,” So said. Using the cellular infrastructure means that the sensors could be placed miles apart. A critical requirement of the system, he said, is that it must use as little energy as possible so that it can be powered by batteries or solar panels, providing data continuously for years at a time.

“There’s no technology currently available that can do that.”

The main applications of the technology are focused on atmospheric science, for carbon monitoring, as well as on carbon sequestration science needed for monitoring carbon dioxide leaks over large areas.

“Basically, we’ve redeveloped all of the laboratory equipment and tried to jam it all into a small box, to make it portable, and so that we can actually go out into the field with these things and place them next to a tree or on top of a rock and have them wirelessly communicate their data over radio and send it to your computer in your office,” said So, the lead researcher on the project. “And if you have a lot of these nodes, you can do some mapping and correlations to determine carbon flux and greenhouse gas flux.”

Clinton Smith (center) and Stephen So, electrical engineers at Princeton University, and Lijun Xia (far right) from Johns Hopkins University test a prototype wireless CO2 sensor developed in collaboration with MIRTHE (Mid-InfraRed Technologies for Health and the Environment), a National Science Foundation Engineering Research Center based at Princeton. In the field tests, the tunable diode laser absorption spectroscopic (TDLAS) sensor measured CO2 respiration on a forest floor at the Smithsonian Environmental Research Center and produced results well correlated with a commercially available Vaisala sensor. Photos courtesy of MIRTHE.

The technology could be used in the future by governments needing to implement precise cap-and-trade systems for CO2 monitoring, to help appropriately assign industrial carbon credits or for emission controls for industry to minimize atmospheric pollution, Wysocki said.

With their new sensor, “lasers can target particular absorption lines, and we can perform very precise and selective measurements of different molecules,” Wysocki said. “The technology’s also very universal. By changing the laser source and detector, we could target different molecules, and since the sensor is small, we can have several molecules in a reasonably small package at the same time.

“The main advantages of spectroscopic sensing is that we have both sensitivity and selectivity,” Wysocki said. “So we can’t run into the same problems standard sensors do when two molecules that cause similar response in the sensor can confuse the sensor, and one molecule can be misinterpreted as another one.”

The TDLAS sensor was field-tested for atmospheric CO2 and soil respiration monitoring – which also produces high levels of CO2 – by Princeton’s MIRTHE partners from Johns Hopkins University at the Smithsonian Environmental Research Center. Tested against a commercially available sensor, the TDLAS produced well-correlated measurements.

“We are currently developing new sensing methods that can be used with the same electronics that we have developed for spectroscopic sensing in general, with flexible functionality,” Wysocki said. “The same electronics can be used for oxygen sensing using a quite complex sensing method called Faraday rotation spectroscopy. This kind of method can provide much higher sensitivities and would allow for further miniaturization of the sensor – with the same capabilities or even better,” he said.

laser spectroscopy
That part of the science involved in the study of the theory and interpretation of spectra that uses the unique characteristics of the laser as an integral part in the development of information for analysis. Raman spectroscopy and emission spectroscopy are two areas where lasers are used.
1. A generic term for detector. 2. A complete optical/mechanical/electronic system that contains some form of radiation detector.
atmosphericatmospheric sciencecap-and-tradecarboncarbon dioxidecarbon sequestrationchemicalsClinton SmithCO2diode lasersemission controlenergyFaraday rotation spectroscopyfield testGerard Wysockigreenhouse gasimagingindustrialinfraredJohns Hopkinslaser spectroscopyMelinda RoseMid-Infrared Technologies for Heatlh and the EnrironmentMIRTHEmonitoringNational Science Foundationparts per billionpollutionportablePrincetonResearch & TechnologysensorSensors & DetectorsSmithsonian Environmental Research Centersoil respirationsolarStephen SoTDLASTech PulseTest & MeasurementtunableVCSELwirelesslasers

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