You’re probably aware that lasers can be used to measure distances, and laser rangefinders are fairly common. Their operation is straightforward: the laser beam is turned on momentarily, and the time it takes to bounce off a surface and return is measured. But, what you may not know is that the optical hardware you need to take sub-micron measurements is built into many laser diodes, and Ben Krasnow of Applied Science explains how to utilize that capability in his newest video.
This project relies on the fact that many laser diode units, like the kind you’d find in a laser pointer, have a monitor photo diode mounted in the package right next to the laser diode itself. The purpose of the monitor photo diode is to create a closed loop system that can be used to regulate the actual light output of the laser diode, instead of just regulating the current. Not all laser diodes have the monitor photo diode these days, but they can still be found if you look.
Because that monitor photo diode responds to the intensity of the reflected light, there is a detectable change based on how close a surface is to the laser. To make that more useful, a special amplification circuit can be used to convert the current readings to voltage readings. This essentially turns the laser diode into a self-mixing interferometer. Altering the current slightly changes the color of the light, and can ultimately be used to measure tiny distance changes. Krasnow goes into a lot of detail about how exactly this works, and how to use it, so be sure to watch the whole video.