In addition to CD and DVD, the blu ray disc BD has a single 3 wavelength playback / recording optical head, capable of recording and playing back 3 formats. There is need for BD blu-ray disc recorder to record and playback DVD-R and CD-R. Therefore, a 3 wavelength recording / playback optical head is required to make it possible. A single unit 3 wavelength laser and also an objective lens corresponding to the 3 wavelengths have been developed in order to develop a 3 wavelength recording and playback optical head with most simple structure.
A prototype optical head was made to realize a common optical path for the 3 wavelengths, by utilizing the laser and the objective lens. A variable magnification function is the key to realize the 3 wavelength optical head in the 3 wavelength objective and laser lens. The function was realized by introducing expander lens assemblies whose magnifications were different for corresponding objective lens apertures for BD, CD and DVD. The efficiency from each laser to BD, DVD and CD can be maximized with the help of this function. This function enables high speed recording for each format. As for the object lens, introduction of aspherical glass and hologram lens to achieve spherical aberration correction corresponding to each wavelength enables one optical system to deal with 3 wavelengths.
With advancements in technology, the number of parts and size of the prototype optical head can be reduced by combining, for example, optoelectronic integrated circuit (OEIC) and so on. From the above discussion, we come to the conclusion that the single 3 wavelength playback / recording optical head is key to the expansion of BD bluray disc markets.
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