Computer Controlled Variable Attenuator for Tunable Lasers

This application note describes a closed loop, computer-controlled, variable attenuator (Figure 1) for the control of output power from a tunable laser. It is based on the combination of a 1/2 wave-plate and a polarizer. The algorithm used to control power operates in a closed loop with Proportional Integral Derivative (PID) feedback, compensating for slow changes in output power.

For the manual variable attenuator, the power is measured directly at the output of the attenuator for each wavelength and set accordingly by rotating the 1/2 wave-plate. In contrast, the computer-controlled closed loop attenuator sets and maintains the power by positioning the 1/2 wave-plate based on a feedback signal from a photo-detector. The photodetector can exhibit a strong dependence on wavelength (www.newport.com/818Series). This chromaticity becomes significant when the tuning range is large. Consequently, a spectrometer is incorporated into the setup, which provides real-time information about the laser's wavelength, thus taking advantage of the photo-detector's precise wavelength calibration.