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DIY Optical Isolation

Lasers are used to send data down optical fibers, to sense trace amounts of chemicals in air or water and to observe single molecules in a living cell. The key to all these applications, and many more, is the stability of the laser. A laser beam is consistently bright, it's narrow and straight, and it consists of a single color. Those properties are consequences of the laser cavity design -- the space between the two mirrors of a laser. But if laser light from outside the cavity makes it back into the cavity, the laser can be fooled -- it acts as if there is more than one optical cavity, and the stability of the laser is shot. That's why you need to build an optical isolator to keep reflections from making their way back into the laser.

Things You'll Need

Linear polarizer
Quarter-waveplate at your laser wavelength
Flat mirror
Laser

Instructions

- 1
  Place the linear polarizer in the output beam of your laser. If your laser is linearly polarized, as many are, line the polarizer up with the output polarization of the laser. You can do this by monitoring the beam energy or intensity "downstream" of the linear polarizer as you rotate it. When the energy of the beam is maximum, you've matched the laser polarization.
- 2
  Place the quarter-waveplate in the beam after the linear polarizer. The quarter waveplate has a fast axis and a slow axis. The waveplate slows down light polarized along its slow axis -- delaying or retarding it by a quarter wavelength relative to the fast axis.
- 3
  Put a flat mirror in the beam after the quarter-waveplate. Adjust the mirror so you can see the reflected beam on the side of the linear polarizer closest to the laser. You can shine the reflected beam on the laser housing itself, or hold a piece of paper by the side of the output beam.
- 4
  Rotate the quarter-waveplate while looking at the reflected beam. The beam will disappear when the waveplate is oriented to shift the polarization so the reflected beam can't make it back through the linear polarizer.

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