Described are a sensor and a method for measuring a vibration of a surface
obscured from view. The sensor includes a narrowband source of a
terahertz beam, a beamsplitter, a beam combiner and a terahertz detector.
The beamsplitter splits the terahertz beam into a sample beam for
irradiating the surface and a reference beam. The beam combiner combines
the sample beam scattered from the surface and the reference beam. The
terahertz detector generates an electrical signal based on a modulation
of the power of the combined beams due to the vibrating surface. The
electrical signal indicates a characteristic of the surface vibration.
Homodyne or heterodyne detection can be utilized. Advantageously, the
sensor can see surfaces that are covered, concealed or otherwise obscured
behind optically opaque materials, including plastic, cloth, foam, paper
and other materials. Thus the sensor has a wide variety of applications
where conventional vibrometers are not practical.