Laser sensors

Researchers at Rice University have successfully used laser-based sensors to detect the presence of specific gases in concentrations in the range of parts per billion to parts per trillion.

Researchers at RiceUniversity, Houston, TX have successfully used laser-based sensors to detect the presence of specific gases in concentrations in the range of parts per billion to (ppb) parts per trillion (ppt).

The Rice researchers’ method is based on Tunable Diode Laser Absorption Spectroscopy (TDLAS), which uses a semiconductor laser as the excitation light source for absorption spectroscopic measurement. Different types of molecule absorb light at different wavelengths, and the amount of light absorbed depends on the number of molecules in the light’s path. The device measures the change in the light intensity as it resonates with molecules’ absorption wavelengths.

Inside the laser, a Texas Instruments 32-bit TMS320F2812 DSP controller performs calibration, filtering and numeric processing to precisely detect these intensity changes in real time. A technician can then monitor the concentration of molecules encountered in a person’s breath or in the air to detect the presence of a particular gas.

The conventional method of determining the composition of a gas is to run it through a laboratory instrument using a method like gas chromatography/mass spectroscopy (GC/MS). GC/MS separates the gas into its component parts and determines the composition of each component by splitting it into ions and measuring their various weights. The GC/MS approach and comparable technologies are accurate and sensitive, but are used mainly in a scientific laboratory because they require costly, bulky instruments and highly skilled technicians.

Also, GC/MS is very time intensive, both in terms of preparing the sample for analysis and providing results.

In contrast, the TDLAS method can maintain a high level of precision for gas detection outside of a laboratory setting with a fast detection time of less than one second.

After receiving data from the detector, the F2812 controller in the laser uses an on-chip analog-to-digital converter (ADC) to convert the detector signal and digitally average a large number of samples in order to increase the signal-to-noise ratio.

Then, the system processes the data and performs corrections before performing linear fits with data from a known sample of the target gas to determine the concentration. The F2812 offers 256 kilobytes of flash memory for storing the laser sensor program and booting the device.

The RiceUniversity team used TI’s Code Composer Studio software to program the F2812 controller, which made it possible to move through all phases of the development process within a single integrated development environment (IDE).

Dr. Frank Tittel, the J.S. Abercrombie Professor in Electrical and Computer Engineering at RiceUniversity, and his research team have worked with doctors at the Baylor College of Medicine in Houston to detect elevated levels of carbonyl sulphide that indicate problems with rejection of a lung transplant. The team is also working with NASA to measure the air quality in the space shuttle during missions. Another potential application under investigation is the use of the new generation of instruments to provide early warning of chemical or biological attack.

The estimated commercial cost, according to Dr. Tittel, of producing the current design in volume is about $4,000, but it is expected that further improvements will reduce this figure by an order of magnitude.