Variable Laser Attenuators

To adjust the laser power (or intensity) down to the level required for a specific application, neutral density filters or other devices are often used to reduce the pump power. These techniques typically do not allow for continuous control of the laser power. On the other hand, variable laser attenuators consist of essentially two optical components, a half waveplate, and a polarizer with a good extinction ratio, and allow for continuous power control for linearly polarized lasers. The principle of operation of a laser attenuator is based on Malus's law. In this case, the intensity I of the polarized light that passes through the polarizer is given by:

I = I₀(cosθ)²

where I₀ is the input intensity, and θ is the angle between the beam's initial direction of polarization and the axis of the polarizer. Rotation of a half waveplate allows for continuous rotation of the beam's polarization direction. When following a half waveplate with a polarizer, the total system provides variable attenuation according to the above equation. Figure 1 shows the operation of a laser attenuator as well as its associated optical layout. Such an optical system would be ideal for a fully-enclosed beam routing system since the optical components are in fixed locations and the rejected light from the polarizer would be contained.