Chemical sensing with wireless optical remote sensor networks

Optical remote chemical sensing is one of the most efficient noncontact sensing technologies. Many chemicals show molecular rotational-vibrational absorption in the mid-wave and long-wave IR (MWIR/LWIR, 3–12μm), and multispectral sensing in these regions can enable chemical measurement and identification in real time and with high reliability. This approach avoids equipment contamination, and allows continuous chemical monitoring without deabsorption or replacement of sensing materials. It also enables effective chemical detection over a broad area, in contrast with most current chemical ‘ sniffers,’ where sensing is limited by the physical location of the sensors. Furthermore, the photodetectors used for multispectral chemical sensing have a fast response (in the order of microseconds), which enables real-time measurements.

Optical remote sensing networks with multiple distributed MWIR/LWIR remote chemical sensors connected in a wireless network can significantly improve the effectiveness of chemical sensing and monitoring. The multiple sensor cross-check and verification can also ensure system reliability should a sensor fail in the wireless network. In addition, the MWIR/LWIR optical remote sensor network can provide additional information about the chemicals, including their exact location, distribution, dispersion area, and spreading speed. Furthermore, the optical remote sensor network can share information with other wireless sensor networks and form a smart network system that can connect with personal mobile devices.

Existing optical remote chemical sensing technologies include Fourier transform IR spectroscopy,¹ tunable laser diode absorption spectroscopy,² hyperspectral imaging,³ and light detection and ranging.⁴ These systems show high spectral resolution and sensitivity. However, they are bulky, heavy, expensive, and not suitable for portable and wireless optical remote chemical sensor networks.

We have developed a miniature wireless optical remote sensor network that includes a multispectral MWIR/LWIR quantum-dot IR photodetector (QDIP) with on-chip integrated plasmonic filters and wireless sensor network connections (see Figure 1).⁵ Our system consists of MWIR/LWIR optical remote sensors (a sensor box is shown in the picture), a central wireless receiver, and a computer for sensor signal display and chemical identification and analysis. The sensors communicate with the central receiver through wireless connection.

Figure 1. The wireless mid-wave/long-wave IR optical remote sensor system, comprising wireless optical remote sensors, a central receiver, and a central computer for sensor signal display and chemical identification and analysis. The sensors communicate with the central receiver through a wireless network. ZnSe: Zinc selenide.

The multispectral sensor chip consists of nine QDIPs. One is a reference photodetector, and the other eight have integrated plasmonic filters with different passbands for spectral filtering. The reference photodetector can detect the variation of the background radiation and provide adjustment for the measurement of the other photodetectors. All the plasmonic filters can be fabricated on the QDIPs in a single fabrication step, which significantly reduces the fabrication cost of plasmonic filters. We can incorporate more QDIPs with plasmonic filters in the sensor chip to increase the spectral resolution of the optical remote sensor. The multispectral QDIPs are low-cost MWIR/LWIR detectors, with a high operating temperature (HOT).⁶ The HOT QDIP enables thermal-electric (TE) cooled chemical sensing and thus can avoid bulky, heavy, and power-hungry cryogenic cooling systems. Such TE cooled optical remote sensors can significantly reduce size, weight, and power consumption (SWaP).

By measuring the signals of the QDIPs, we can determine the absorption at these wavelengths, and by comparing these results with the signature absorptions of chemicals, we can identify the chemicals and determine their concentration.

In summary, the integrated plasmonic filters and the HOT QDIPs enable compact optical remote chemical sensors with low cost and significantly reduced SWaP. The wireless sensor network approach can significantly improve the effectiveness of optical remote chemical sensing and monitoring, including coverage area, accuracy, and reliability. In future, we will focus on applying the sensors to take measurements of carbon-based trace gases (carbon dioxide, methane, and carbon monoxide) in the atmosphere for air pollution monitoring, as well as for chemical leak detection, and for explosive detection at security checkpoints.