Radio frequency modulation of semiconductor laser as an improvement method of noise performance of scanning probe microscopy position sensitive detectors

An experimental investigation was made on use of radio frequency (RF) modulation of a semiconductor laser in order to improve the noise performance of the scanning probe micros-copy cantilever position sensitive detector. High resolution measurements of the cantilever displacement are limited by optical power intensity noise of the laser diode. In general, the intensity noise is formed by mode hopping or mode partition phenomena in the laser multimode and single mode regimes. Furthermore, the measurements are disturbed by an optical feedback, which is caused when light is scattered back into the laser cavity. In order to reduce the laser intensity noise and the optical feedback, we developed a precise laser automatic power control (APC) driver with RF modulation of the DC diode bias current. Our experiments showed that the spectral noise density of the developed scanning probe microscopy detection system was 3 times smaller than the noise density of the system without RF operating current modulation. In a low power operation, near the diode threshold current, the laser mode partition noise is dominating and it can be reduced by adjusting the modulation current frequency to 300 MHz. In this paper, the architecture of the designed system will be presented. We will also discuss the results of noise performance investigations of the scanning probe microscopy position sensitive detectors applied in precise surface measurements.