Optical: systems and elements – Optical aperture or tube – or transparent closure
Reexamination Certificate
2000-02-25
2003-04-22
Chang, Audrey (Department: 2872)
Optical: systems and elements
Optical aperture or tube, or transparent closure
C359S558000, C359S562000, C250S493100, C250S503100, C369S112010, C369S112230
Reexamination Certificate
active
06552864
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of beam generation techniques, particularly useful in near-field applications such as, for example, high resolution scanning for optical data storage, inspection, recording, microscopy, etc.
BACKGROUND OF THE INVENTION
There is a great variety of light scanning systems, typically comprising a light source for generating a light beam of a certain wavelength and light directing means for directing the light beam onto the object. A common goal of such systems consists of increasing the system's resolution. It is known that a resolution depends on the diameter of a light beam striking the object, namely the less the diameter of the beam, the higher the resolution of the system. Laws of electromagnetism, governing the propagation of light, state that a propagating light wave cannot be focused to a spot of a size significantly smaller than the light wavelength.
One approach to overcome this impediment is based on broader subject known as “near field optics”. According to this approach, a point-like light source, having dimensions smaller than the light wavelength, is typically generated by means of either defining small apertures on opaque screens, or passing the light through point-like tips of sub-wavelength dimensions. However, these means have an inherent property consisting of that the spot-size provided by light emerging from a point-like source expands rapidly away from the source. As a result, high optical resolution can be achieved solely at very close proximity of the source. This is a serious impediment common to all known methods in near field optics.
Systems for generating propagating optical beams that do not expand the size of a central lobe in the transverse profile of the beam while propagating have been developed and disclosed, for example, in U.S. Pat. Nos. 4,852,973 and 4,887,885. Such beams are identified as “non-diffracting beams” or Bessel beams. The technique disclosed in these patents provides for generating a traveling wave beam substantially unaffected by diffractive spreading, namely a beam having a transverse Bessel function profile, such that its effective spatial width is not smaller than several wavelengths. This condition is inherent to the propagating character of the disclosed solutions of the optical fields and methods of generating them.
As illustrated in
FIG. 1
, a system of the kind, generally designated
1
, comprises a light source
2
for emitting a light beam
4
and a collimating and focusing arrangement, generally at
6
, which typically includes a lens
8
or plurality of such lenses (not shown). A circular annular source
10
of the beam
4
, defining the radius R of a circular slit in screen, is located in the back focal plane of the lens
8
. As shown, the passage of the beam
4
through such a circular annular source
10
forms a narrow beam
4
′ whose profile across the circular annular source
10
is in the form of a Bessel function. The beam
4
′ propagates along an axis A
p
and impinges onto an object
12
(constituting a target plane), while substantially retaining its form at
4
″. A sharp central spot size s is related to the radius R of the circular slit in the screen, a focal length f of the lens
8
and the wavelength
&lgr;
of the light beam as follows:
s
=
3
4
·
λ
f
R
Thus, the system
1
represents a “diffraction free arrangement” which enables to generate an axially symmetric, non-diffraction, non-evanescent field in the form of a known zero-order Bessel function of the first kind.
Turning now to
FIGS. 2
a
and
2
b
, there are illustrated the intensity distributions of the zero-order Bessel beam J
0
(solid line
13
) in comparison to a Gaussian beam (dotted line
14
) at two different distances z
1
and z
2
of propagation, respectively.
FIG. 2
a
shows the position of z
1
=0, that is an initial plane where the beams are formed, while
FIG. 2
b
shows the position after propagating a distance z
2
=50 cm. It is evident that the Bessel beam, while propagating along the axis A
p
, substantially retains its transverse shape at a central part along the axis of propagation A
p
. It should be specifically noted that this method, as many other conventional methods, relates to propagating beams and distances much larger than the size of the aperture.
Another solution for producing a Bessel beam is disclosed in U.S. Pat. No. 5,349,592. According to this technique, a three-portion apodizer is used aimed at reducing the sidelobe intensity of a light beam and obtaining a relatively high center peak intensity ratio. This can facilitate data reading in a high recording density data carrier. The apodizer changes characteristics of the wavefront of part of the light beam, so as to split the wavefront and to change the beam spot size on a target plane (image bearing member). This is implemented by deviating the phases of light beam components.
U.S. Pat. No. 5,497,359 discloses a system aimed at reducing the diameter of a scanning (reading) beam. The system relates to an optical disk data storage, typically comprising a light source for emitting a light beam and light directing means. The operation of the system is based on the transition of tightly focused beams between two dielectric interfaces. To this end, the light directing means comprises a super-hemispherical solid immersion lens (SIL) which is in the form of an air-bearing slider (ABS) having a lens section located on its back side opposite the side with the ABS. The slider and the lens section are made of the same transparent material having the same refraction index n. According to a so-called “evanescent field coupling” phenomenon, the appearance of an evanescent field associated with light internally reflected within the SIL is provided. An evanescent mode is a wave-guide propagation mode which is known per se and therefore need not be more specifically described, except to note that in this mode the amplitude of a wave diminishes rapidly along the direction of its propagation, but the phase does not change.
Generally speaking, the technique disclosed in the above patent utilizes the effect of coupling evanescent fields of high angle light beams to a recording medium (optical disc), and is aimed at reducing the spot size on a target plane (optical disc). Actually, this technique improves the known SIL-based recording technique, by coupling those rays, which are internally reflected at the base of the SIL, to the optical disc via their evanescent field. This technique deals with the coupling of both propagating evanescent and non-evanescent parts of incoming beams resulting in the undesirable spreading of the spot size with the beam propagation. Here, however, beams having planar wavefront are used, and the target plane is placed less than a wavelength distance from the base of the SIL. The need for such a small distance between the SIL and the target plane is associated with unavoidable beam spreading with the increase of this distance, due to the fact that both evanescent and non-evanescent components are coupled out of the arrangement.
Thus, a common unavoidable condition of the above configuration is again a very small distance (less than 0.25 wavelengths) between the object and the light directing means, i.e. the aperture and slider, respectively. This is owing to the following undesirable effects:
(1) a decaying character of the evanescent components of a field generated by the aperture; and
(2) a rapidly expanding property of a remaining field, which causes the spot size of a transmitted field to increase many times within a distance equivalent of a few aperture sizes.
If the fast signal decay problem may be eliminated by employing either a stronger light source or more sensitive detection means, neither of these means will help to overcome the rapid expansion related problem.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel method and system for generating a beam of radiation, particularly such
Boutsikaris Leo
Chang Audrey
Friedman Mark M.
Ramot at Tel Aviv University Ltd.
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