Dynamic photoacoustic spectroscopy

A novel laser-based technique remotely detects extremely low concentrations of gases and aerosols 

The ability to remotely detect trace gases and aerosols is relevant to many applications: monitoring environmental pollutants, sensing chemical warfare agents prior to human exposure, and surveilling sites of illegal drug manufacture. However, remote detection requires a highly sensitive chemical detector capable of quickly scanning large volumes of air to discern very low concentrations of materials that could be meters or even kilometers away from a contaminated area. While several chemical-sensing technologies exist, only one to date provides this combination of high sensitivity, high area-coverage rates, and long-range detection: dynamic photoacoustic spectroscopy (DPAS), a laser-based technique developed by scientists at Lincoln Laboratory.  

DPAS system from its top viewA DPAS system is composed of three major components: a tunable laser, a rotating beam director (e.g., rotating mirror), and an acoustic receiver. The current DPAS system is shown from its top view (above). All components of the optical breadboard are labeled in the corresponding block diagram (below). Underneath the breadboard is an electronics cart (not shown) with a computer, a microphone preamplifier, a mirror control system, optical power meters, and a laser chiller.

“Chemical sensing typically involves a tradeoff between sensitivity and detection range. Point sensors are ultrasensitive but must operate within close proximity to the affected site and thus cannot provide early warning of atmospheric contaminants or a terrorist attack. On the other hand, standoff sensors can detect materials remotely but are not as sensitive,” explains Charles Wynn, who developed the DPAS technique along with Michelle Clark and Ryan Sullenberger, all technical staff members in the Laboratory’s Chemical, Microsystem, and Nanoscale Technologies Group. “Our technique can remotely detect both types of trace materials (vapors and aerosols) at distances beyond 100 meters while maintaining a sensitivity competitive with many point-sensing techniques,” Wynn continues.

Because DPAS optically scans large volumes of air, its sampling rate (many 10s of m3/s) is much higher than that of technologies requiring air sample ingestion and air flow control. With its high sampling rate and sensitivity, DPAS can detect agents in well under one minute. By comparison, most techniques with similar sensitivity require more than an hour to detect agents because of the time involved in sample preparation and post-processing analysis. Unlike many other chemical agent detection methods, DPAS does not use consumables—sensor paper, columns through which air samples flow, and other materials that need to be replaced after every tested sample—which are a conduit of potential air sample contamination and thus can degrade system sensitivity.

DPAS system block diagram

A Modern Twist on an Old Technique
“Dynamic photoacoustic spectroscopy is an incredibly elegant, straightforward variant of photoacoustic spectroscopy,” says Clark. Photoacoustic spectroscopy is a well-known technique for converting optical energy into acoustic energy that is then detected by an acoustic receiver, such as a microphone or laser vibrometer. Optical energy is only converted into acoustic energy if a material that absorbs light at the wavelength of the light source (e.g., laser) is present. This wavelength can be varied to create an optical absorption spectrum for the material.

Photoacoustic spectroscopy detects trace gases and aerosols by generating an acoustic signal via a laser tuned to the specific absorption features, or chemical composition, of the chemical species of interest. The laser heats the gas or aerosol, which expands and causes a pressure front and ultimately an acoustic wave. However, the produced acoustic signal is very weak. As such, the signal is measured in a closed resonant chamber (inherently not a standoff configuration), inducing the coherent addition of acoustic waves (i.e., alignment of the wave crests and troughs). “Consider how a boat’s speed is dramatically faster when its rowers coordinate their oars to pull through the water at the same time (coherent) rather than at different times (incoherent). The acoustic waves similarly combine in phase, amplifying the signal,” says Wynn.

This coherent addition can be achieved in an open environment by sweeping the laser beam—the “dynamic” element of DPAS—through a gas or aerosol plume at the speed of sound, generating a shock-wave-like response. “In a manner similar to how shock waves are generated by supersonic jets, our technique produces a shock wave with significantly enhanced amplitude as compared to the otherwise very weak photoacoustic signal,” explains Wynn. Laboratory researchers have used the DPAS technique to generate and detect signals as high as 83 dB, a sound intensity that is easily audible to the unaided human ear. Sullenberger describes the sound as follows: “The sound produced by the DPAS system is incredible. It is sometimes louder than the sound I can make snapping my fingers.”

DPAS wavefrontIn this spatial representation of a pressure wavefront, the laser beam is being swept along the x-axis (time). Because the speed of sound is known, time can be directly related to the distance traveled by the pressure wavefront (upper axis) through the following equation: distance = speed × time. The z-axis is the direction perpendicular to the direction of the laser sweep. Acoustic energy, measured in mV, exists both above and below the laser beam location, extending roughly 20 cm along the z-axis.

From Concept to Demonstration
A successful proof-of-concept demonstration occurred in 2012. The prototype DPAS system consisted of a tunable CO2 laser, two microphones, and a rotating mirror to direct the laser beam; the technique was applied to the detection of sulfur hexaflouride (SF6), a greenhouse gas. A year later, Lincoln Laboratory provided the U.S. Army Edgewood Chemical Biological Center (ECBC), who regularly partners with the Laboratory on chemical detection–related programs, with a prototype DPAS system. The Laboratory and ECBC conducted experiments with different vapors, including a solid source of ammonia vapor, under laboratory and outdoor conditions. Using the synthetic amorphous silica Syloid 244, the team also demonstrated DPAS’ utility as a trace aerosol detection technique. The system has been shown to be minimally susceptible to interferents (i.e., background vapors or other materials that trigger a false alarm), effectively discriminating various aerosols and vapors over a wide range of concentrations.

DPAS graphIn an ambient laboratory environment at ranges exceeding 5 m, the DPAS technique was tested with a parabolic microphone and a laser vibrometer. The responses of these acoustic receivers to a burst of SF6 gas (with a concentration estimated to be in the single-digit parts per million) are shown above. Black curves represent how the devices respond in the absence of SF6. Although the microphone is more sensitive than the vibrometer is to the acoustic signal of the gas, the microphone does not allow for monostatic detection; only the vibrometer can sit adjacent to the laser source and still be able to detect the signal.

An Ongoing Collaboration  
The Laboratory continues to work with ECBC on developing DPAS. “Currently, we are determining how the DPAS system could be best deployed within the broad Department of Defense mission space,” says Wynn. “Then, we can adapt the system to meet specific size, weight, and power requirements.” Eventually, the external source that supplies power to the DPAS system will be replaced by a battery pack to facilitate field portability. In developing the future system, the team will leverage technology advances in quantum cascade lasers (QCL), which will replace the CO2 lasers that early DPAS system prototypes have relied upon. Lincoln Laboratory, other research and development laboratories, and industry are already well underway in efforts to increase the power output of QCLs and to create a compact unit that encompasses many lasers of different wavelengths. These capabilities could help extend the standoff range of the DPAS system to distances exceeding 1 km and enhance the system’s ability to distinguish between chemical agents and interferents and to detect a broader set of threats. “Advances in QCLs will allow us to dramatically reduce the size, weight, and power requirements of the DPAS sensor and are key enablers in the development of a robust sensor,” says Clark. “Compared to CO2 lasers, which only emit light at a set number of frequencies, QCLs are continuously tunable,” she adds.    

In the meantime, ECBC is leading the development of a chemical-signature library and a signal phenomenology model for the DPAS system. Staff from the U.S. Army Research Laboratory are aiding the ECBC effort, investigating optical architectures for steering and focusing the source beam and exploring new mechanisms for recreating the original signal of the vapor or aerosol via monostatic detection (i.e., colocating the laser and acoustic receiver). While it will be several years before the system is operationally implemented, circumstances could accelerate that timeline, according to Wynn: “If there was a pressing need for the system, it could be brought online in less than a year.”  

Posted February 2016

top of page