The capacity of CD is only about 700MB, while that of Blu-ray disc can reach 25GB. If you use a Blu-ray disc, you can put 9G files.
The capacity difference between them is closely related to the wavelength of the related laser beam.
Generally speaking, the recording density of an optical disc is limited by the size of the readout spot, that is, the optical diffraction limit, which includes the laser wavelength λ and the numerical aperture NA of the objective lens.
Therefore, in order to improve the recording density, the traditional optical disc technology can generally use short-wavelength laser or increase the numerical aperture of the objective lens to reduce the light spot, such as upgrading CD(780nm, Na: 0.45) to DVD(650nm, Na: 0.6) and then to Blu-ray disc (405nm, Na: 0.85).
For CD, the laser wavelength is 780nm, the numerical aperture NA of the objective lens is 0.45, and the distance that the laser beam will converge to a point needs 1.2 mm, which determines that the thickness of the CD substrate is1.2 mm. No matter whether the substrate of CD is too thick or too thin, the laser beam cannot converge to a point, which seriously affects the recording and reading of data.
Extended data
The development trend of optical discs is to develop into large-capacity storage (such as DVD+R DL products produced in 20 10), and the technical research and development of the industry is also guided by this.
Single-sided and double-layer DVD discs have appeared. Single-sided double-layer disc (DVD+R double-layer) is to make two recording layers on the same surface by using different laser beam focusing positions, and the laser power of single-sided double-layer disc in the first layer and the second layer is the same (laser power is
References:
Baidu encyclopedia-CD