The computer controlled closed loop attenuator sets and maintains the power by positioning the ½ wave-plate based on a feedback signal from a photo detector which can exhibit a strong dependence on wavelength. 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 wavelength of the laser, thus taking advantage of the precise wavelength calibration of the photo-detector. This is especially important when the laser power needs to be held constant while scanning wavelength. The algorithm used to control power operates in a closed loop with PID (proportional, integral, derivative) feedback, compensating for slow changes in output power within the bandwidth of the device greater than 500 ms.