A distance-measuring interferometer developed by a group at Tohoku University (Sendai, Japan) may be the simplest ever invented. In its basic form, it consists of a laser, a detector, a mirror, and nothing else (although a lambda/4 plate and polarizing beamsplitter can be added for isolation). The trick is in the detector itself: made of silicon, it is constructed to be so thin that it is almost perfectly transparent to light. When inserted into a laser beam that is reflected back on itself to create a standing-wave pattern, the 40-nm-thick photodiode absorbs 0.6% of the light and, when either the mirror or detector is moved, maps the periodic intensity profile of the standing waves.
Appearing yellowish to the eye, the detector has interdigitated p+ and n+ regions, is 1 mm square, and has a sensitivity of 0.01 mA/W at 633 nm. Fresnel surface reflections cause the transmission to be 70% and the standing waves to appear nonsinusoidal; antireflection coatings should eliminate these effects. The researchers have mapped standing waves for a mirror movement of 20 mm. Acceptance angle of the device is ±0.3 mrad about the normal , at which point the detector begins to intersect more than one standing wave at a time. Adding a second detector with a pi/2 phase-shift plate will allow direction-sensitive distance measurement.
