Lab Notes

Posted June 2008

Good Vibrations

A different take on terahertz radiation can measure moving motors or beating hearts.

Terahertz radiation—the part of the electromagnetic spectrum that lies between infrared and microwaves—has recently become a hot technology for security applications. T-rays, as they're called, can penetrate such barriers as clothing, paper, plastic, and cardboard, and can identify the chemical make-up and physical shapes of substances that they find. And they do so without the hazardous ionizing effects of X-rays. Now Lincoln Laboratory researchers Jerry Chen and Sumanth Kaushik have taken T-rays in a new direction: using this radiation to listen for vibrations. "As far as I know," Chen says, "this is the first time anybody has used T-ray technology for vibration sensing."


Terahertz (T-ray) interferometry can detect vibrations. T-rays that bounce off a vibrating object (such as a loudspeaker) are combined with a reference T-ray beam; the resulting interference between the beams yields information about the vibration.

The interferometric technique starts by splitting the T-ray into two separate beams. Next, Chen and Kaushik aim one beam at the object they want to examine. The beam hits the object and bounces back to a detector while a reference beam is routed directly to the detector. A beam reflected from a vibrating object will be out of phase with the reference beam, causing an interference pattern on the detector. Taking a Fourier transform of these time-varying patterns reveals the object's vibrational frequency.

Chen, an electrical engineer in the Laboratory's Active Optical Systems Group, tested his system by placing an ordinary stereo speaker behind a cardboard barrier. Using the T-ray interferometer, he measured the peak velocity of the speaker as it vibrated. The results told him not only that the speaker was moving, but also which pitches it was producing. To check the validity of his approach, he performed the same test without the barrier, using a helium-neon laser beam in conventional optical interferometry, and got the same results.

The advantage of T-rays over a laser-based vibrometer is their ability to penetrate many nonmetallic barriers. For example, T-rays could detect the ticking of a time bomb inside a leather briefcase. Other ways of measuring vibrations often involve physical contact with the object being measured, an intrusion that can throw off the measurement.

T-ray vibration sensors could check the efficiency of motors, whether they're inside huge aircraft or tiny microelectromechanical systems such as those used in projectors. The technology, Chen says, could thus provide a method of testing industrial machinery without having to take it apart. An automobile designer might use a T-ray system to trace the source of a particular frequency to see just which part is causing an unwanted sound.

The device could also detect a beating human heart or vibrating vocal cords. That ability might prove useful in, say, examining a battlefield to quickly separate the injured from the dead. T-rays could work well in such a setting because they can penetrate smoke and dust as well as cotton and Kevlar. T-rays' speed and accuracy in triage for both military and civilian emergencies (such as a major traffic accident or building collapse) could save lives.

Chen filed a patent application on the system early last year. He says he'd welcome the chance to refine his detector, and sees no reason it couldn't be commercialized.



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