Old Lyme, Conn.-based Sennheiser is the latest vendor–and one of the few–to offer a microphone that uses optics rather than electrical signals to capture and transmit sound. The underlying concept is relatively straightforward. A light source–usually a light-emitting diode (LED)–shines against the diaphragm, and the reflections are picked up by a photodetector that's on the same side of the diaphragm. When the diaphragm moves, so do the reflections, creating changes in light intensity. The photodetector notes these changes, beginning the process of capturing them as sound waves. The light waves travel along a fiber optic cable to a unit that, besides providing power, includes a photodiode that converts the light into electrical signals.
Sennheiser's new mics, along with those from rivals such as Israel-based Optoacoustics, are aimed primarily at specific vertical markets...
...optical mics are a potential fit for high-security environments, such as government and defense contractor offices, where eavesdropping is a concern. That's because the alternative–mics with copper cables–even when they're shielded can double as antennas, radiating whatever content is traversing them.
How far those "broadcasts" travel depends on factors such as whether there are multiple walls in the area to attenuate the signal. But if the copper cables are in, say, an executive conference room that has lots of windows, there's a better chance that the signals can be picked up by someone in the parking lot below.
That scenario is one of the reasons why many government guidelines, such as the National Security Agency's TEMPEST, require fiber for secure applications. Often, the concerns such guidelines address often apply to the general enterprise market, too. (more)
In 1994, while optical microphones were still esoteric spy tools, I created a fiber optic microphone teacup for my clients. (front view) (rear view) (bottom view). Only 323 were made. If you still have yours, hold on to it. It's rare. ~Kevin
