Noise reduction of violet laser diodes by selection of feedback light polarization

We have investigated for the first time optical feedback noise characteristics of violet laser diode (LD) and also demonstrated its noise reduction by selecting the polarization of the feedback light in a typical optical disk pick-up system using 160mW type 405 nm GaN violet LD. The polarization axes of feedback light have been selected by means of rotatable quarter-wave plate (QWP). In this method, we have attained certain noise reduction without superposition of a high frequency (HF) component on driving current, which makes the pick-up system simple and inexpensive. As a result of our experiments, by selecting the angle of QWP relative to the original polarization axis of LD, we could suppress the optical output fluctuations caused by optical feedback from external cavity to less than 20% of peak-to-peak value in our violet laser diodes. In addition, we also measured noise reduction effect by the method of HF superposition. We achieved the noise suppression by selection of feedback light polarization as good as that of HF superposition. As a consequence, our present method could be a new approach for practical and simplified industrial fabrications of inexpensive, substantially low noise optical disk pick-up systems for high power violet laser diode.