Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2002-06-06
2004-06-01
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S333000, C356S334000
Reexamination Certificate
active
06744506
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a depolarizing plate for use in eliminating polarization dependency, as well as a monochromator, an optical spectrum analyzer and other optical apparatus that use the depolarizing plate.
A conventional depolarizing plate is shown in FIG.
2
. In
FIG. 2
, numerals
13
a
and
13
b
refer to wedge plates each made of a birefringent material such as quartz.
As shown in
FIG. 2
, each wedge plate has a crystallographic optical axis 45 degrees from the vertical direction (as indicated by the solid arrow for wedge plate
13
a
and by the dashed arrow for wedged plate
13
b
).
The wedge plates
13
a
and
13
b
are so cut that their thickness varies in a vertical direction and they are joined together such that their crystallographic optical axes cross each other at right angles.
Therefore, the thickness of each wedge plate varies continuously in a direction 45 degrees from a crystallographic optical axis thereof.
A birefringent material has the ability to confer a phase difference between two components of light that passes through it, one vibrating in a direction parallel to a crystallographic optical axis thereof and the other vibrating in a direction normal to the crystallographic optical axis. The conferred phase difference is proportional to the thickness of the birefringent material.
In the depolarizing plate shown in
FIG. 2
, the thickness of each wedge plate varies in the vertical direction which is 45 degrees from a crystallographic optical axis thereof; hence, the phase difference conferred differs with the position where light passes and the transmitted light is spatially a mixture of many states of polarization.
The incident light passing through the conventional depolarizing plate shown in
FIG. 2
is split into two rays at the wedge portion.
This splitting of light is shown below with reference to FIG.
5
.
A ray of light which is ordinary for the wedge plate
13
a
is extraordinary for the wedge plate
13
b
whereas an extraordinary ray for the wedge plate
13
a
is ordinary for the wedge plate
13
b
. Therefore, the materials difference in refractive index causes refraction at the wedge portion but in different directions, splitting the incident light into two rays.
The split rays satisfy the following relation:
&agr;=2(
n
e
−n
o
)tan &thgr;
0
where &agr;: the angle between the two split rays;
&thgr;
0
: the angle of the wedge
n
e
: the refractive index for the ordinary light
n
o
: the refractive index for the extraordinary light.
The conventional depolarizing plate
13
shown in
FIG. 2
may be applied to a conventional monochromator of the type shown in
FIG. 3
which has a concave mirror
3
which causes incident light
1
to emerge after it is converted to parallel light through an entrance slit
2
, a plane diffraction grating
4
which diffracts the parallel light emerging from the concave mirror, a concave mirror
5
which condenses the diffracted light from the plane diffraction grating, and an exit slit
6
for selecting only a specified wavelength component of light.
Further referring to
FIG. 3
, the incident light
1
is launched onto the depolarizing plate via the entrance slit
2
, where it is split into two rays; the split rays are incident on the plane diffraction grating in the manner described below with reference to FIG.
6
.
The two split rays of light (
14
a
,
14
b
) emerging from the depolarizing plate
13
are collimated by the first concave mirror
3
and incident on the plane diffraction grating
4
to be diffracted respectively.
Details of diffraction by the grating
4
are given below with reference to FIG.
6
.
The relationship between the angle of incidence on the plane diffraction grating
4
and the angle of diffraction is described by the following equation:
m&lgr;=d
·cos &thgr;(sin &agr;
1
+sin &agr;
2
)
where m: the order of diffraction
d: grating constant
&lgr;: wavelength
&thgr;: the angle formed between incident light and the direction of groove depth
&agr;
1
: the angle of incidence of light on the diffraction grating
&agr;
2
Z: the angle of emergence of light from the diffraction grating.
In the equation given above, the two split rays
14
a
and
14
b
have the same incident angle &agr;
1
.
However, due to the angle &agr; between the two split rays from the depolarizing plate
13
, the angle &thgr; formed between the angle of incidence on the plane diffraction grating
4
and the direction of the depth of grooves in the plane diffraction grating will take different values except in the case where the height of intercept of the concave mirror
3
by the incident light coincides with the central axis of the concave mirror.
Thus, the two split rays have different values for the angle of emergence &agr;
2
.
Hence, as shown by dots in
FIG. 4A
, the two split rays are skewed with respect to the longitudinal direction of the rectangular opening in the exit slit
6
.
As a result, one of the two split rays will not be able to pass through the exit slit.
However, the exit slit has to choose a specified wavelength component from the condensed light.
Since the components of light condensed at the two points have the same wavelength, all of the light at those points need emerge from the exit slit
6
and to this end, the following adjustment is required.
In order to ensure that the two split rays are both transmitted through the narrow exit slit
6
, the parallel light obtained by collimating the split incident light with the concave mirror
3
need be launched onto the plane diffraction grating
4
with the angle &thgr; between the incident light and the depth of grooves in the plane diffraction grating being adjusted to be the same in all situations.
In other words, the height of intercept by the incident light is brought into registry with the central axis of the concave mirror.
This puts a constraint on the parts layout of the monochromator, introducing greater difficulty into apparatus designing.
With a view to increasing the resolving power of the monochromator or expanding a dynamic range thereof toward the near end, the incident light may be diffracted by the plane diffraction grating two or more times but it is all the more difficult to design a capability for ensuring that only the light that has been diffracted a plurality of times will pass through the exit slit.
As an alternative, the offset between two condensed spots of light may be eliminated by adjusting the tilting of the exit slit. However, if the height of light intercept varies due to disturbances such as temperature changes, the angle setting for the plane diffraction grating may be offset from the wavelength of the emerging light to deteriorate a spectral characteristics thereof.
As described above, the use of the conventional depolarizing plate of
FIG. 2
in a monochromator has involved the problem that two rays of light emerging from the depolarizing plate are split obliquely to the longitudinal direction of the rectangular opening in the exit slit on account of the diffraction by the plane diffraction grating and cannot pass through the exit slit simultaneously.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel depolarizing plate which splits incident light into two rays along the length of a rectangular opening in an exit slit in such a way that both rays can pass through the exit slit.
Another object of the invention is to provide a monochromator and an optical spectrum analyzer that assure high precision using the depolarizing plate.
In order to attain these objects, the invention first provides a depolarizing plate
7
comprising a first rectangular wedge plate
7
a
that has a first crystallographic optical axis in a diagonal direction of the rectangle and which has a thickness thereof in a vertical direction vary continuously in a direction 45 degrees from the first crystallographic optical axis and a second rectangular wedge plate
7
b
that has a second crystallographic optical axis in a diagonal direction of the rectangle crossing the first c
Kaneko Tsutomu
Yamamoto Toshikazu
Ando Electric Co. Ltd.
Evans F. L.
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