Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
1998-10-16
2001-06-05
Chang, Audrey (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S580000, C359S588000, C359S589000, C359S359000
Reexamination Certificate
active
06243203
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical system for guiding a beam of electromagnetic radiation, which system has at least one transition from a first medium to a second medium.
The invention also relates to an optical device in which such an optical system is used, such as an optical scanning device or a lithographic projection apparatus.
BACKGROUND OF THE INVENTION
The optical system may be a lens system comprising one, a plurality, or a large number of lens elements in which each lens surface constitutes said transition, namely the transition between the lens material, for example glass or a synthetic material, and the medium surrounding the lens, for example air. The optical system may alternatively comprise components other than lens elements, such as a beam splitter, a birefringent plate and the like.
It is common practice to provide an optical coating, or cladding, such as an anti-reflection coating on the transitions so as to prevent unwanted reflections from occurring at said transitions. Not only do such reflections reduce the transmission through these transitions, but also the reflected radiation may reach unwanted positions in the relevant optical apparatus where it may cause troublesome effects.
An optical apparatus which has become very popular and is currently manufactured in large numbers is the playback apparatus for an optical record carrier in which an audio program is stored, which apparatus is known as CD player. This apparatus comprises, inter alia, an objective lens, preferably in the form of a single lens element having one or two aspherical surfaces with which a scanning beam from a diode laser is focused to form a scanning spot with a diameter of the order of 1 &mgr;m on the information plane of the record. For novel uses of optical record carriers, for example as a storage medium for a digital audio program and for a video program or film, either or not in a digital form, the information contents of such a record carrier must be increased considerably, so that, with the same dimension of the record carrier, the information density must be increased considerably. This means that the information elements, for example in the form of pits in the information layer in which the information is stored in an encoded form, must be reduced considerably. To be able to read the smaller information elements separately, the scanning spot must also be reduced. The size of the scanning spot is proportional to &lgr;/NA, in which &lgr; is the wavelength of the scanning beam and NA is the numerical aperture of the objective system. It has therefore been proposed to use a scanning beam having a smaller wavelength, for example 650 nm instead of the customary 860 or 780 nm and to increase the numerical aperture of the objective system.
The article “High-numerical-aperture lens systems for optical recording” in Optics Letters, vol. 18, no. 4, Feb. 15, 1993, pp. 305-307 describes how the numerical aperture can be increased considerably in a scanning device for optical record carriers, namely by providing a plano-convex lens between the customary objective lens and the record carrier, with the flat side of this lens facing the record carrier. This plano-convex lens is also referred to as SIL (solid immersion lens) in analogy with the immersion lens used in microscopes. An NA of 0.85 can be realized with the combination of the objective lens and the plano-convex lens.
SUMMARY OF THE INVENTION
However, it has been found that, when using such an optical system with a high numerical aperture, the scanning beam no longer has a uniform intensity distribution after it has passed the lens combination, so that the scanning spot formed by this beam no longer has the desired quality. Consequently, the read signal will have a considerably reduced quality.
It is an object of the present invention to provide an optical system with a high numerical aperture for, inter alia, a scanning device for optical record carriers which does not affect the intensity distribution of a beam passing therethrough. This optical system is characterized in that an anti-reflection coating provided at the area of said transition comprises at least two sub-coatings which are maximally anti-reflective for different angles of incidence.
The invention is based on the recognition that, at larger numerical apertures, the border rays of the beam are incident on the lens surface and on the anti-reflection coating provided on this surface at such a large angle, for example, larger than 40°, to the normal on the lens surface that a standard coating loses its effect for these rays. Consequently, the transmission of the relevant lens element will be dependent on the distance to the optical axis; the transmission will be smaller as the distance to the optical axis increases. According to the invention, this new problem is solved by using a combination of a number of sub-coatings each of which is optimized for a different angle of incidence, instead of a standard coating which is optimized for an angle of incidence of 0°. Such a composite coating maintains the desired effect for a great spread of angles of incidence such as occurs at a high numerical aperture.
In principle, a sub-coating in the optical system may consist of a single layer whose product of the thickness and the refractive index has a given value. The optical system is preferably further characterized in that each sub-coating consists of at least two layers of a different material.
Similarly as a sub-coating consisting of a single layer, a sub-coating consisting of two layers has a reflection of 0% for the angle for which it is designed, hereinafter referred to as design angle, but it has also a small reflection, for example of less than 1% for a small range of angles of incidence around the design angle. Instead of subcoatings consisting of two layers, sub-coatings consisting of three or more layers may be used. A coating consisting of three layers has a small reflection through a wider range of angles of incidence, while the reflection for the design angle is slightly larger than zero percent.
A practical embodiment of the optical system suitable for a numerical aperture of the order of 0.85 is further characterized in that the coating comprises a stack of seven two-layer sub-coatings each consisting of two layers of a different material, which subcoatings are maximally anti-reflective for angles of incidence of 0°, 15°, 30°, 40°, 45°, 50° and 55°, respectively.
With this number of sub-coatings and the limited total number of layers, and the choice of the angles of incidence for which the sub-coatings are optimized, an optical system having said numerical aperture can be realized which has a substantially uniform transmission.
Instead of the coating comprising only sub-coatings with an equal number of layers, the optical system may be characterized in that the coating comprises a combination of two-layer sub-coatings and three-layer sub-coatings each consisting of three layers of a different material.
Such a coating combines the advantages of two-layer sub-coatings and three-layer sub-coatings.
The two-layer and three-layer sub-coatings may be arranged in different ways. A preferred embodiment of the optical system with such a coating is further characterized in that the coating comprises a first stack of two-layer sub-coatings and a second stack of three-layer sub-coatings.
An attractive embodiment of the optical system with this coating is characterized in that the first and the second stack each comprise eight sub-coatings each, which are maximally anti-reflective for angles of incidence of 0°, 10°, 20°, 30°, 40°, 50°, 55° and 60°, respectively.
The invention may be used in different types of optical systems. A first use is an optical system in the form of a lens system comprising at least one lens element, the two lens surfaces of which each constitute said transition. Such a lens system is then characterized in that the lens surfaces are provided with said coating.
The lens system may alternatively comprise a number of lens elements
Leibbrandt Godert W. R.
Schleipen Johannes J. H. B.
Belk Michael E.
Chang Audrey
U.S. Philips Corporation
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