Reflecting mirror and optical pickup
A reflecting mirror with a reflective multi-layer coating capable reflecting light beams of discrete wavelengths, e.g., laser beams in the wave ranges of CD, DVD and mass storage optical discs, free of the problem of light absorption.
1. An optical pickup, comprising:
a light source for emitting at least a light beam at wavelength of 405 nm, a light beam at wavelength 640 nm, and a light beam at wavelength 780 nm;
a polarized beam splitter;
a λ/4 wave plate and photodetector; and
a reflecting mirror, in which a light path is formed to and from said light source, said polarized beam splitter, said λ/4 wave plate and photodetector and said reflecting mirror;
said reflecting mirror comprising:
a first reflective multi-layer coating formed on a substrate plate by alternately depositing a low refractivity layer and a first high refractivity layer absorptive of a wavelength of 405 nm having a refractivity higher than said low refractivity layer of the first reflective multi-layer coating; and
a second reflective multi-layer coating formed at a light incident side of said first reflective multi-layer coating, by alternately depositing a low refractivity layer and a non-absorptive second high refractivity layer having a refractivity higher than said low refractivity layer of the second reflective multi-layer coating but lower than said first high refractivity layer in said first reflective multi-layer coating;
wherein an incident light beam at a wavelength of 405 nm is reflected off by said second reflective multi-layer coating while incident light beams at wavelengths of 640 nm and 780 nm are reflected off by cooperative actions of said first and second reflective multi-layer coatings; and
wherein said second reflective multi-layer coating reflects off part of incident light beams at wavelengths of 640 nm and 780 nm.
2. The optical pickup as defined in claim 1 , wherein said low refractivity layers of the first and second reflective multilayer coatings are formed by deposition of an SiO 2 -base material, said first high refractivity layer is formed by deposition of TiO 2 , and said second high refractivity layer is formed by deposition of at least one of Ta 2 O 5 , Nb 2 O 5 , ZrO 2 , CeO 2 , a lanthanum-titanium mixture oxide and a zirconium-titanium mixture oxide.
3. The optical pickup as defined in claim 1 , wherein said low refractivity layer is formed by deposition of an SiO 2 -base material, said first high refractivity layer is formed by deposition of TiO 2 , and said second high refractivity layer is formed by deposition of Nb 2 O 5 ;
said second reflective multi-layer coating being formed by alternately depositing Nb 2 O 5 and SiO 2 in more than 8 layers in total, and said first reflective multi-layer coating being formed by alternately depositing TiO 2 and SiO 2 in more than 24 layers in total.
4. The optical pickup as defined in claim 1 , wherein said low refractivity layers of the first and second reflective multilayer coatings, first high refractivity layer and second high refractivity layer are deposited by an ion plating, vacuum deposition, ion assist or sputtering process.