An optical head has a first light source for emitting a first light beam having a first wavelength, a second light source for emitting a second light beam having a second wavelength larger than the first wavelength, and an optical system for guiding the first and second light beams to an optical disc...http://www.google.ca/patents/US5513164?utm_source=gb-gplus-sharePatent US5513164 - Optical recording and reproducing apparatus
