Polarizing diffraction grating and magneto-optical head made...

Details Polarizing diffraction grating and magneto-optical head made... Polarizing diffraction grating and magneto-optical head made...

1999-01-19

2001-08-21

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Diffraction

From zone plate

56, 56, 56, 56, C369S044120, C369S112040

Reexamination Certificate

active

06278548

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarized beam splitter using a diffraction grating and a magneto-optical head using the polarized beam splitter.
2. Description of the Related Art
In general, a polarized beam splitter used for a magneto-optical head is provided by producing dielectric multi-layers of an isotropic medium having a constant refractive index regardless of an incident polarized light beam direction and putting the dielectric multi-layers together onto the slope of a triangle prism such as a polarized light prism
11
in FIG.
1
. For a boundary plane of the dielectric multi-layers, a P polarized light beam (a component of linearly polarized light beam vibrating in the plane perpendicular to the boundary plane) can be transmitted 100% and a S polarized light beam (a component of linearly polarized light beam vibrating in the boundary plane) can be reflected 100%. For detecting a polarized light beam rotation in the magneto-optical recording medium by using these polarized light beams, a signal generation is performed by sloping the direction of a linearly polarized light beam of the reflection light beam by 45° for a splitting direction so that a transmission light beam has the same optical amount as reflection light beam and detecting the change of a balance between the transmission light beam and the reflection light beam along the polarized light beam rotation from a difference of a detected amount of light beam.
As another prior method described in a patent application No. 4-19522, there is provided a magneto-optical head using a specific Wollaston prism
21
and configured in a Japanese patent application No. 6-77351. This Wollaston prism
21
is provided by putting an anisotropic medium together in a specified direction of a principal axis and is configured as shown in FIG.
2
. In other words, there is a light beam having the same status of polarized light beam as an incident light beam in the center and two divided polarized light beam components crossing orthogonal to both sides. A magneto-optical head can be miniaturized by packaging into a unit because three optical detectors
22
,
23
and
24
can be configured in a plane by using this dividing method. It is possible to detect a magneto-optical signal from light beams on both sides and a servo signal such as a focusing signal or tracking signal from a light beam in the center.
Contrary to the polarized beam splitter of a magneto-optical head, a polarized beam splitter using a hologram is described in pages 622 through 628 of National Technical Report, Vol. 41, No. 6, 1995, and shown in
FIG. 3. A
light beam incident on hologram device
31
and being polarized in a specific direction transmits through without an occurrence of a diffracted light beam, becomes a rotated polarized light beam through a &lgr;/4 board
32
, reflects at a reflection substance
33
, transmits through the &lgr;/4 board
32
again, becomes a linearly polarized light beam in the direction crossing orthogonal to what it was at the time of incidence and is incident on the polarizing diffraction hologram device
31
. An anisotropic optical crystal such as a lithium niobate is used for a medium to configure the polarizing diffraction hologram device
31
and a configuration that differentiates a phase difference acted by a grating to two linearly polarized light beam crossing each other orthogonally. The polarizing diffraction hologram device
31
can be utilized as a polarized beam splitter
31
that splits a linearly polarized light beam in a polarized light beam direction at the time of incidence as a 0
th
order light beam and a linearly polarized light beam in the direction crossing orthogonal to the 0
th
order light beam as a ±1
st
order diffracted light beam.
Although an optical crystal is necessary, since a diffraction hologram is provided, the polarized beam splitter can be produced through an exposure development process using a photo-mask and have an advantage of mass production. Further, it is easy to miniaturize the polarized beam splitter and there is a feature in the beam splitter that its splitting ratio and an angle of splitting can be designed freely.
SUMMARY OF THE INVENTION
As a polarizing beam splitter for a magneto-optical signal detection, the conventional wollaston prism is not suitable for miniaturization and thinning. In normal polarizing diffraction, since a polarized light status of ±1
st
order diffracted light is identical and a 0
th
order light must be used for a polarized light detection, a symmetric characteristic goes worse and a noise increases.
Generally, a polarized beam splitter in
FIG. 1
, used in a magneto-optical head is not suitable for miniaturizing the magneto-optical head because two planes
12
and
13
are necessary for the beam splitter.
A prior art Wollaston prism
21
as shown in FIG.
2
and as described in a Japanese patent application No. 4-19522, has a problem presently concerning its own thickness for an introduction of a further miniaturization using an exposure development process by a photo-mask generally used in a semiconductor integrated circuit, because the Wollaston prism has a cubic structure. For a further miniaturization, there is another problem that if a common path optical system making common an optical path of a light beam incident on a disk and another optical path directed to an optical detector through reflection is configured by putting a semiconductor laser and the optical detector into a package, an unnecessary polarized light beam split on a projected path mixes into a signal light beam on a returning path through a reflection at a disk.
A polarized beam splitter using a grating and described in pages 622 through 628 of National Technical Report, Vol. 41, No. 6, 1995, causes a deterioration of a common mode rejection ratio and a signal to noise ratio because if the beam splitter is used for detecting a magneto-optical signal, one of two components of polarized light beam crossing each other orthogonally becomes a sum of each output of two optical detectors, another one of two components of polarized light beam becomes a signal from one optical detector, a shot noise caused by an optical detector is generated and a symmetry of a rear stage amplifier is deteriorated by a thermal noise.
Additionally, it is difficult to unify an optical detector useful for detecting a servo signal and there is a problem that a common path optical system distribution placing an optical source of a semiconductor laser on a bisector of an optical axis of a diffracted light beam is impossible to realize.
With respect to the consideration of the above subject, an object of the present invention is to provide a polarizing diffraction grating performing a polarized light beam splitting different from the conventional polarizing diffraction grating.
An object of the present invention is to differentiate a polarized light beam status of ±nth order diffracted light beam in the diffraction grating.
An object of the present invention is to acquire a magneto-optical signal from a differential output of the respective amount of an optical detection.
An object of the present invention is to use a 0
th
order light beam for a servo signal detection and to place an optical source of a semiconductor laser on a central optical axis of a ±nth order diffracted light beam.
An object of the present invention is to miniaturize a magneto-optical head.
An object of the present invention is to provide a magneto-optical head having a mass production capability.
The present invention has a feature that for a natural number n, there is provided a diffraction grating wherein a polarized light beam status of +nth order diffracted light beam is different from a polarized light beam status of −nth order diffracted light beam. A polarized light beam status means a size of a particular polarized light beam. A phase difference between two components of a particular polarized light beam means the two components of a l

Affiliated with

Also associated with

Rating

Polarizing diffraction grating and magneto-optical head made... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polarizing diffraction grating and magneto-optical head made..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarizing diffraction grating and magneto-optical head made... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2505975