This application claims the benefit of Korean Patent Application No. 2007-80843, filed on Aug. 10, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
Aspects of the present invention relate to a solid immersion lens (SIL) near-field system, and more particularly, to an SIL near-field system having a long working distance so as to sufficiently control a gap to prevent the SIL and a disc surface from colliding with each other in the near-field system.
2. Description of the Related Art
In order to increase a storage capacity of a recording medium, research to develop multilayer recording media using a laser beam of a short wavelength and an objective lens having a high numerical aperture (NA) is being performed. As a result of such research, Blu-ray discs, having a storage capacity of 25 GB for each layer in its multilayer structure, have been developed using a blue-violet laser diode and an objective lens having an NA of 0.85. A Blu-ray disc can be used to record two hours of high-definition television or thirteen hours of standard-definition television. However, such a conventional optical storage method cannot satisfy future storage capacity requirements. Therefore, a new kind of storage method is required.
As such, a conventional near-field storage using a solid immersion lens (SIL) has been developed to increase the storage capacity using characteristics of a near-field optical system.
Referring to
In general, the SIL can be classified into two types, that is, the hemispherical type and the super-hemispherical type. A thickness of the super-hemispherical SIL is (1+1/nSIL)r (where, r is a radius of the sphere, and nSIL is a refractive index of the material forming the SIL). In a system using the hemispherical SIL, an effective NA (NAeff) can be calculated as defined by the following Equation 1:
NAeff=NAobj×nSIL (Equation 1)
In a system using the super-hemispherical SIL, an effective NA (NAeff) can be calculated as defined by the following Equation 2:
NAeff=NAobj×n2SIL (Equation 2)
In Equations 1 and 2, NAobj is the NA of the objective lens, and nSIL and n2SIL are the refractive indexes of the materials forming the respective SILs.
In the conventional near-field optical storage system including the SIL, the air gap between the disc and the SIL is very small, that is, about 20 to 30 nm, so as to prevent the small focus spot from being spread.
Aspects of the present invention provide an SIL near-field system having a longer working distance by using an incident light that is radially polarized in order to solve a problem caused by the small working distance in the SIL near-field system.
According to an aspect of the present invention, there is provided a solid immersion lens (SIL) near-field system including: a radially polarized beam generator to generate a radially polarized beam; an SIL; an objective lens to focus the radially polarized beam on a bottom surface of the SIL; and a mask to shield a center portion of the radially polarized beam, the center portion being about an optical axis of the radially polarized beam.
According to an aspect of the present invention, the radially polarized beam generator may include: a light source to emit a linearly polarized beam of a predetermined wavelength; and a radial polarization converter to convert the linear polarization of the incident beam into a radial polarization.
According to an aspect of the present invention, the radial polarization converter may be a diffractive optical element or a liquid crystal element to convert the polarization status of the incident beam from the linear polarization to a radial polarization.
According to an aspect of the present invention, the radially polarized beam generator may further include a collimating lens to collimate the beam emitted from the light source.
According to an aspect of the present invention, the system may further include: a hollow beam generator to generate a hollow incident beam in order to reduce a light loss caused by the shielding operation of the mask.
According to an aspect of the present invention, the hollow beam generator may include: a first conical lens disposed so that the radially polarized beam emitted from the radially polarized beam generator can be incident upon a flat incident surface of the first conical lens; and a second conical lens disposed so that the radially polarized beam incident from the first conical lens can exit through a flat exit surface of the second conical lens.
According to an aspect of the present invention, a minimum diameter of the mask (Dmask) may be calculated as Dmask=2×EFLobj×sin(1/nSIL), where a focal length of the objective lens is EFLobj and a refractive index the SIL is nSIL.
According to an aspect of the present invention, the system may further include: a magnifying lens to adjust the focal point of the near-field system.
According to an aspect of the present invention, the SIL may be formed as a hemisphere, a super-hemisphere, a truncated hemisphere, an oval, or an aspherical shape.
According to an aspect of the present invention, the system may further include: a metal film formed on the bottom surface of the SIL to have a sub-micron opening in a center portion of the metal film to restrain side lobes in an intensity profile of the focus spot.
According to an aspect of the present invention, the near-field system may be used for optical storage, optical lithography, and optical trapping of a particle.
According to an aspect of the present invention, the near-field system may irradiate the beam focused by the objective lens and the SIL onto a disc, and the near-field system used for optical recording/reproducing may further include: a first photodetector to receive the beam reflected by the disc to detect an information signal or an error signal; and a first optical path changer to change an optical path of the radially polarized beam that is incident thereupon.
According to an aspect of the present invention, the system may further include: a second photodetector to detect signals to control a gap servo; and a second optical path changer disposed between the radially polarized beam generator and the first optical path changer or between the first optical path changer and the objective lens to change an optical path of the radially polarized beam that is incident thereon so that a portion of the beam reflected by the disc can proceed toward the second photodetector.
According to an aspect of the present invention, the system may further include: a magnifying lens to adjust the focus of the radially polarized beam with respect to the disc, the magnifying lens being disposed between the radially polarized beam generator and the objective lens.
Aspects of the present invention provide an SIL near-field system having a long working distance by using a radially polarized incident beam. In the SIL near-field system according to aspects of the present invention, the working distance can be increased to 100 nm or longer. By comparing the SIL near-field system according to aspects of the present invention with the conventional SIL near-field system, a gap servo and a tilt margin according to aspects of the present invention can be relaxed, and a scratch and a collision between an SIL and a disc can be prevented, and thus, the disc and the SIL can be protected.
Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.
A near-field recording using an SIL can realize a high recording density using a lens having a high effective numerical aperture (NA). However, an air gap existing between the SIL and a recording medium must be maintained within a range of 20 to 30 nm due to a rapid increase in a spot size and a decay of an evanescent wave, which results in the need for a strict gap servo and a tight tilt margin.
In a general near-field system having the SIL, a linearly polarized beam or a circularly polarized beam is used as an incident beam.
A radial polarization is different from the above-described types of polarization, that is, an electric field vector at each point of the incident beam is in a radial direction as shown in
When the radially polarized beam is focused onto a lens having a high NA, a sharp focus spot can be generated. The size of the focus spot generated from the radially polarized beam is relatively smaller than that formed by the linearly polarized beam or the circularly polarized beam. Moreover, a longitudinal component of a focus field with respect to the radially polarized beam has a non-diffraction property that allows a constant spot size to be maintained along a propagation direction at a certain distance. Therefore, the size of the focus spot can be constantly maintained within a certain distance range, and thus, the radially polarized beam can increase a working distance of the SIL in near-field recording.
Here, the non-diffraction property of the longitudinal component can be further strengthened by shielding the central portion of the beam incident on the collimating lens with the low NA from the portion of the beam incident on the collimating lens with the high NA. Therefore, in the near-field system including the SIL according to aspects of the present invention, a mask may be used in order to shield the incident beam having the low NA so that the longitudinal component is strengthened. In addition, a pair of conical lenses disposed to make a hollow beam may be used so as to reduce a light loss caused by the shielding of the mask.
Referring to
The light source 21 that emits the laser beam can emit a linearly polarized beam. Therefore, a diffractive optical element or a liquid crystal element to convert the polarization of the incident light into the radial polarization can be used as the radial polarization converter 25. A radial polarization converter 25 formed of a diffractive optical element is disclosed in Radially and Azimuthally Polarized Beams Generated by Space-Variant Dielectric Sub-Wavelength Gratings, Ze'ev Bomzon, et al., OPTICS LETTERS Vol. 27, No. 5, published on Mar. 1, 2002. A radial polarization converter 25 formed of a liquid crystal element is disclosed in Linearly Polarized Light with Axial Symmetry Generated by Liquid-Crystal Polarization Converters, M. Stalder, et al., OPTICS LETTERS Vol. 21, No. 23, published on Dec. 1, 1996.
Referring back to
A minimum diameter (Dmask) of the mask 40 can be set as Dmask=2×EFLobj×sin(1/nSIL), when it is assumed that a focal length of the objective lens 45 is EFLobj and a refractive index of a material forming the SIL 50 is nSIL. A maximum diameter of the mask 40 should be less than an entrance pupil diameter (EPD) of the objective lens 45.
The hollow beam generator 30 prevents light loss that is caused by the shielding of the incident beam near the optical axis by the mask 40 and may be disposed between the radially polarized beam generator 20 and the mask 40. The hollow beam generator 30 includes a first conical lens 31 and a second conical lens 35 that are disposed to generate hollow incident beams. The first conical lens 31 is disposed so that a flat surface facing the radially polarized beam generator 20 is an incident surface 31 a into which the RPB emitted from the radially polarized beam generator 20 is incident as a parallel beam. The second conical lens 35 is disposed so that a flat surface becomes an exit surface 35a from which the beam that is hollowed by passing through the first conical lens 31 as a parallel beam exits.
Therefore, when the parallel beam is incident to the first conical lens 31, the beam passing through the first and second conical lenses 31 and 35 becomes a hollow parallel beam. A diameter of hollow circle at a center of the hollow beam formed by the first and second conical lenses 31 and 35 may be the same as or similar to that of the mask 40 in order to minimize the light loss.
The objective lens 45 focuses the incident RPB onto the bottom surface 50a of the SIL 50. The SIL 50 can be a hemispherical type, a super-hemispherical type, a truncated hemispherical type, an oval type, or an aspherical type.
The SIL near-field system 10 having the above structure according to the present embodiment can have a working distance that is longer than that of the conventional near-field system by using the incident RPB. The SIL near-field system 10 having the long working distance that is longer than that of the conventional near-field system can be applied to various optical systems requiring a small light spot and a long working distance. For example, the SIL near-field system 10 can be used as an optical storage system for Blu-ray discs (BDs) or high definition digital versatile discs (HDDVDs), in optical lithography, and in optical trapping of a particle.
As shown in
The intensity distributions of the simulation model of
The spot profiles when the air gap is 30 nm and 100 nm are similar to each other as shown in
As shown in
In the focus spot intensity profile, there are relatively large side lobes that can affect signal qualities in terms of recording and reproducing data, as shown in
Referring to
As described with reference to
A power of the light source 21 can be monitored by a monitor photodetector 135. The beam emitted from the light source 21 passes through the collimating lens 23 that changes a diverging beam into a parallel beam. The parallel beam then passes through the radial polarization converter 25, the first and second conical lenses 31 and 35, the mask 40, the second and first optical path changers 110 and 115, and the magnifying lens 120, and then, is incident to the objective lens 45.
The first and second conical lenses 31 and 35 change the beam proceeding from the radial polarization converter 25 into the hollow beam in order to avoid the light loss caused by the shielding operation of the mask 40.
The first optical path changer 115 changes the optical path of the incident radially polarized beam such that the radially polarized beam incident from the radial polarization beam generator 20 proceeds toward the objective lens 45, and a portion of the radially polarized beam reflected by the disc 101 passes through the SIL 50 and the objective lens 45 to proceed toward the first photodetector 118.
The second optical path changer 110 changes the optical path of the radially polarized beam so that the radially polarized beam incident from the radial polarization beam generator 20 proceeds toward the objective lens 45, and a portion of the radially polarized beam reflected by the disc 101 passes through the SIL 50 and the objective lens 45 to proceed toward the second photodetector 113.
A beam splitter can be used as the first or second optical path changers 115 or 110. In
Sensor lenses 116 and 111 can be further disposed on the optical path between the first optical path changer 115 and the first photodetector 118 and between the second optical path changer 110 and the second photodetector 113, respectively.
Also, as illustrated in
The magnifying lens 120 is used to adjust the focal point of the SIL near-field system 100 for optical recording/reproducing of the present embodiment, and the magnifying lens 120 can be adjusted so that the beam is accurately focused on the bottom surface 50a of the SIL 50.
The objective lens 45 focuses the beam on the bottom surface 50a of the SIL 50. Data can be recorded and/or reproduced onto/from a recording layer of the disc 101 by a near-field coupling of the objective lens 45 and the SIL 50. The objective lens 45 can have a high NA, for example, about 0.77, and can obtain an effective NA of about 1.84 when the refractive index of the SIL 50 material is about 2.38. Due to the non-diffractive property of the longitudinal component in the radial polarization, the profile of the spot formed on the bottom surface 50a of the SIL 50 can be maintained, and the size of the spot can be constant up to an air gap of 100 nm. Thus, the working distance of the SIL 50 can be increased.
In order to restrain the side lobes from being generated in the intensity profile of the focused spot, the metal film 51 having the opening 55 on the center portion of the metal film 51 can be coated on the bottom surface 50a of the SIL 50 as shown in
On the other hand, in
According to the SIL near-field system 100 for optical recording/reproducing having the above structure, the radially polarized beam incident into the disc 101 is reflected by the disc 101 and is condensed by the SIL 50 and the objective lens 45. After that, the beam passes through the magnifying lens 120, and is partially reflected by the first and second optical path changers 115 and 110. Here, the first photodetector 118 detects an information signal, that is, an RF signal, and the second photodetector 113 detects a gap servo signal which is a signal to maintain the air gap between the SIL 50 and the disc 101 constant.
In the above description, the SIL near-field system 100 using the radially polarized beam according to aspects of the present invention is used to perform the optical recording/reproducing in optical data storage. Further, the SIL near-field system 100 according to aspects of the present invention can be used in optical trapping and optical lithography, etc.
Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
Number | Date | Country | Kind |
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2007-80843 | Aug 2007 | KR | national |