A chemical sensor prototype was developed that can detect single-fingerprint quantities of substances from a distance of more than 100 feet away, and could be shrunk to the size of a shoebox. The portable infrared chemical sensor could be mounted on a drone or carried by users such as doctors, police, border officials, and soldiers.
The sensor is made possible by a new optical-fiber-based laser that combines high power with a beam that covers a broad band of infrared frequencies — from 1.6 to 12 microns — over the mid-wave and long-wave infrared. Most chemicals have fingerprint signatures between 2 and 11 microns; this wavelength range is called the spectral fingerprint region. The new device enables identification of solid, liquid, and gas targets based on their chemical signature.
The device was built using off-the-shelf fiber optics and telecommunications components, except for one custom-made optical fiber. This approach ensures that the laser will be reliable and practical to manufacture at a reasonable cost.
Broadband infrared lasers are typically built up from a laser that produces very short pulses of light, and then a series of amplifiers ramps up the power, but this approach is limited to laboratories. In addition to their high costs, these components can't yet shrink small enough to fit into a handheld device. The use of lenses and mirrors would make the device sensitive to jostling and changes in temperature.
To craft the new laser, a standard laser diode similar to those in laser pointers and barcode scanners was used. This pulse was then boosted in power with telecom amplifiers similar to those used in the field to periodically ramp voice signals back up as they diminish over long travels through the fiber optic lines. Then the broadband signal was run through a 2-meter coil of optical fiber. One-nanosecond pulses were introduced at high power; they break up into very narrow series of small, short pulses, typically less than a picosecond in width.
In a process known as “supercontinuum generation,” the wavelengths covered by that light were expanded by sending it through specialized softer glass fibers. Most lasers emit light of just one wavelength, or color, but supercontinuum lasers give off a focused beam packed with light from a much broader range of wavelengths. Visible-wavelength supercontinuum lasers, for example, discharge tight columns that appear white because they contain light from across the visible spectrum. The new broadband infrared supercontinuum laser does the equivalent, but in longer infrared wavelengths.
To use the device, one shines the laser on an object and analyzes the reflected light to identify what wavelengths did not bounce back. Chemicals are identified by the unique pattern of infrared wavelengths that they absorb.
In addition to applications in policing and defense, tissue samples that currently are chemically analyzed in a laboratory — a process that takes time and materials — could be assessed on the spot using the new system. It may even be possible to run the beam through a scope and analyze tissue in the body.