Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2001-12-12
2003-07-15
Juba, John (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S001000, C359S566000, C369S112100, C369S112150, C369S110020, C369S110030, C369S110040
Reexamination Certificate
active
06594042
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup used to record or reproduce signals on or from optical disks, and an optical element, a light irradiating method, and a polarizing hologram which are used to construct the optical pickup.
2. Description of the Related Art
The prior art will be described with reference to
FIGS. 4 and 5
.
FIG. 4B
shows a sectional configuration of a conventional optical element and part of an optical system of an optical pickup using this optical element. Further,
FIG. 4A
is a plan view showing the shape of a pattern for aperture restricting means for use in this optical element. Two lights of different wavelengths, for example, a light of 650 nm wavelength used for recording or reproducing signals on or from DVDs or the like and a light of 800 nm wavelength used for recording or reproducing signals on or from CDs or the like, impinge on and pass through this element.
In these figures, the optical element is formed of a glass substrate
61
, another glass substrate
63
, a polarizing hologram
62
composed of an uneven grating made of a refractive-index-anisotropic material, and a wavelength plate
66
, both the polarizing plate and the wavelength plate being sandwiched between the glass substrates
61
and
63
. The wavelength plate
66
has an optical thickness as applies a phase difference to the wavelength of 650 nm which difference equals five-fourths of the wavelength. This corresponds to the application of a phase difference to the wavelength of 800 nm which difference equals about one wavelength.
The polarizing hologram is formed of an uneven grating composed of a refractive-index-anisotropic material (for example, refractive indices n1 and n2) and having a depth d
1
and an isotropic material having the same refractive index as either of the two refractive indices of the anisotropic material in recesses in the grating (for example, the refractive index n1) In general, when light passing through the grooves is assumed to have a phase difference &phgr;, a transmittance T is expressed as follows:
[Equation 1]
T
=cos
2
(&phgr;/2) (Equation 1)
When the refractive indices of the grating for lights having orthogonal polarizing directions are defined as n1 and n2, a polarized light that is parallel with the direction in which the grating exhibits the refractive index n1 has the following phase difference:
[Equation 2]
&phgr;=0 (Equation 2)
Accordingly, the transmittance is 1.
On the other hand, a polarized light of which polarization is orthogonal to the direction in which the grating exhibits the refractive index n1 has the following phase difference:
[Equation 3]
&phgr;=2&pgr;(
n
1
−n
2)
d
1/&lgr; (Equation 3)
Thus, if a depth d1 is set so that:
[Equation 4]
&phgr;=&pgr; (Equation 4)
then, the transmittance will be zero and the light will be totally diffracted. In the above equations, the 650-nm light shown in
FIG. 4
corresponds to a wavelength &lgr;1, and the 800-nm light corresponds to a wavelength &lgr;2.
That is, when a linearly polarized light is incident on this grating, all the incident light is transmitted therethrough because the anisotropic material does not substantially function as a diffraction grating for the polarizing direction in which the anisotropic material exhibits the refractive index n1. On the other hand, the material functions as a diffraction grating for a polarizing direction orthogonal to the above polarizing direction and diffracts the incident light.
In
FIG. 4B
, of the lights emitted from two light sources having different wavelengths, a light of a wavelength &lgr;1 is linearly polarized and has a polarizing direction parallel with the sheet of the drawing, and a light of a wavelength &lgr;2 has a polarizing direction perpendicular to the sheet of the drawing. The light of the wavelength &lgr;1 is reflected by a wavelength separating filter
70
and is incident on an optical element
72
. On the other hand, the light of the wavelength &lgr;2 has its polarization plane rotated through 90° by a half wavelength plate
71
to become a linearly polarized light having a polarizing direction parallel with the sheet of the drawing. This linearly polarized light passes through the wavelength separating filter
70
and enters the optical element
72
.
Accordingly, the respective polarizing directions of light, upon impinging on the element, corresponds to the direction in which the grating of the polarizing hologram exhibits a refractive index of n1. Thus, all light entering the optical element passes through the polarizing hologram
62
regardless of the wavelength.
A light of 650 nm wavelength passes through the polarizing hologram
62
and is then converted into a circularly polarized light by a wavelength plate
66
that applies a phase difference to this wavelength which difference equals five-fourths of the wavelength. Then, the light exits the element and is then reflected by the reflector
67
to follow a backward path. On this path, the light is converted by the wavelength plate
66
into a linearly polarized light of which a direction of polarization is orthogonal to that of the forward path. Therefore, the light is totally diffracted by the polarizing hologram
62
.
On the other hand, after passing through the polarizing hologram
62
, a light of 800 nm wavelength is not converted into any polarized light because it passes through the wavelength plate
66
having an optical thickness substantially equal to one wavelength. The light is then reflected by the reflector
67
to travel along the backward path, and also on this path, the light is not converted into any polarized light. Consequently, as in the case with the forward path, the light is not diffracted by the polarizing hologram
62
. More generally speaking, when the wavelengths of the two light sources are defined as &lgr;1 and &lgr;2, the wavelength plate
66
is designed under the following conditions:
[Equation 5]
(
N
1+1/4)&lgr;1
≈N
2×&lgr;2(
N
1 and
N
2 are integers) (Equation 5)
In this case, the light of 650 nm wavelength corresponds to the wavelength &lgr;1, and the light of 800 nm wavelength corresponds to the wavelength &lgr;2.
Further, one
63
of the glass substrates has a wavelength-selective optical thin film formed thereon and which varies the transmittance depending on the wavelength of light passing through the glass substrate, that is, an aperture restricting film
68
. In this case, since the light of 650 nm wavelength is totally transmitted through the aperture restricting film
68
(which is formed like a ring in the area A in FIGS.
4
A and
4
B), a phase compensating film
69
is formed in an area B so as not to create a phase difference between a light passing through the area A and a light passing through the area B, which is located in the vicinity of the center of the element.
On the other hand, the light of 800 nm wavelength has such film design conditions that it exhibits a high transmittance only in the area B in the figures and does not substantially pass through the area A. By blocking the light in the area A, the aperture is restricted.
As described above, when two lights of different wavelengths pass through the optical element
72
forward and backward, one of the lights is substantially totally transmitted through the element on the forward path and is totally diffracted on the backward path by the polarizing hologram
62
. Further, this light is subjected to no aperture restrictions by aperture restricting film
68
. The other light undergoes no diffraction by the polarizing hologram
62
on the forward or backward path, but undergoes aperture restrictions by the aperture restricting film
68
.
An optical element having such functions can be used for an optical pickup having a plurality of light sources with different wavelengths, for example, as an element used for an optical pickup or the like as
Asada Jun-ichi
Nishiwaki Seiji
Saitoh Youichi
Boutsikaris Leo
Juba John
RatnerPrestia
LandOfFree
Optical element, optical pickup, information recording and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical element, optical pickup, information recording and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical element, optical pickup, information recording and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3045057