Kenji Nagashima

A new technique for speckle noise reduction of laser projection displays using waveplates

In laser projection displays, countermeasure technique against glares on the screen called speckle noise is one of the focuses. In the experiments, a quarter-waveplate with slow axis rotated by 45 degrees against the polarization axis of the incident green laser source was inserted into the optical path of our laser projection display system. Only in such a setup, an enough coherence alleviation was attained. We have also carried out quantitative evaluations of the speckle noise by overlaying the projected images using a CCD camera on the histogram-based estimations of speckle noise intensities. Noise reduction of about 20% was accomplished.

Effective speckle noise reduction of laser projection displays by high frequency driving current superposition for blue, red, anddirect green emission laser diode

In this work, we have achieved for the first time an effective speckle noise reduction simply by means of just high frequency LD driving current superposition for an ultla compact micro-electro-mechanical-systems (MEMS) based high resolution scanning beam RGB-laser projectors including the direct green emission LD. In the experiments, we have carried out quantitative evaluations of the speckle noise by overlaying the projected images using a CCD camera with histogram based estimation of speckle noise intensities. Moreover, we have examined the reduction of granular images seen by humane eyes in higher ratios in color stereo image pairs using double-stimulus continuous-quality scale (DSCQS) method. As a consequence, the noise reduction rate of 53.5% for the red laser diode, 80.1% for the green laser diode, and 50.3% for the blue laser diode was attained with histogram based estimation, and the image quality was also enhanced by speckle noise reduction with all RGB LDs using DSCQS method. Moreover, we have also investigated the noise reduction mechanisms in these RGB LDs in relation with the relaxation oscillation frequencies from viewpoint of chaotic multi-mode oscillations with existence of optical feedback.

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.