Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2001-08-08
2003-11-25
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S364000
Reexamination Certificate
active
06654121
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus and a method for detecting the state of polarization (SOP) of light.
BACKGROUND ART
There are various ways of describing polarization of light, among which the Stokes parameter method is representative. The Stokes parameters are comprised of four elements, i.e., S
0
, S
1
, S
2
and S
3
. S
0
is the total intensity of light, S
1
is the intensity of 0-degree linear polarization minus the intensity of 90-degree linear polarization, S
2
is the intensity of 45-degree linear polarization minus the intensity of −45-degree linear polarization, S
3
is the intensity of right-hand circular polarization minus the intensity of left-hand circular polarization. When the incident light E(t) is expressed as in Equation 1 below, the Stokes parameters can be obtained by Equation 2 below:
E
(
t
)=
{circumflex over (x)}E
x
(
t
)
e
i&phgr;
x
(t)
+ŷE
y
(
t
)
e
−i&phgr;
y
(t)
Equation 1,
where both E
x
and E
y
are real numbers; and
S
0
=<E
x
2
>+<E
y
2
>
S
1
=<E
x
2
>−<E
y
2
>
S
3
=<2
E
x
E
y
cos &eegr;>
S
4
=<2
E
x
E
y
sin &eegr;> Equation 2,
where &eegr;=&phgr;
x
−&phgr;
y
and <> is time average.
The method of measuring the Stokes parameters can. be divided into two types, i.e., the space division measurement method and the time division measurement method.
FIG. 1
is a diagram illustrating the space division measurement method.
Referring to
FIG. 1
, a first photo detector
30
a
measures the total intensity lo of the light coming out of a 4-way beam splitter
10
without a polarizer. In this manner, a linear polarizer
20
a
is aligned at a 0-degree direction in front of a second photo detector
30
b
, and a linear polarizer
20
b
is aligned at a 45-degree direction in front of a third photo detector
30
c
. In this manner, the second and third photo detectors
30
b
and
30
c
measure the intensities of the respective linear polarization components I
1
and I
2
of each direction. A fourth photo detector
30
d
measures the intensity I
3
of the light that has propagated through the 1/4 wavelength phase retarder
15
aligned at a 45-degree direction and a linear polarizer
20
c
aligned at a 0-degree direction. The intensity I
3
corresponds to the intensity of the right-hand circular polarization. The measured intensities I
0
, I
1
, I
2
,and I
3
have a relationship with the Stokes parameter as follows:
S
0
=I
0
S
1
=2
I
1
−I
0
S
2
=2
I
2
−I
0
S
2
=2
I
2
−I
0
Equation3.
According to the method illustrated in
FIG. 1
, the measurement can be done rapidly because light is equally divided into four components and measured at the same time by use of four photo detectors. However, the apparatus is complicated and the sensitivity can vary with photo detectors, because four photo detectors are used.
FIG. 2
is a diagram illustrating the time division measurement method. Referring to
FIG. 2
, a phase retarder
200
and a polarizer
210
which are capable of rotation to a certain angle are located sequentially in front of-a photo detector
220
. By adjusting the angle, it is possible to measure the light intensities I
0
, I
1
, I
2
, and I
3
that have a relation with the Stokes parameters as illustrated in Equation 3. However, this method has a disadvantage that the relative angle between the phase retarder
200
and the polarizer
210
must be very precisely controlled.
The above-illustrated space division or time division measurement methods are capable of modification. However, the disadvantage of these methods is that they use a 1/4 wavelength retarder as the phase retarder in which error easily occurs, because the amount of retardation is a function of wavelength.
DISCLOSURE OF THE INVENTION
Therefore, it is an object of the present invention is to provide an apparatus for detecting polarization with a simplified structure.
It is another object of the present invention to provide a method for detecting polarization with an enhanced speed.
It is still another object of the present invention to provide a method of detecting the state of polarization of light in a wide range of wavelength by reducing errors in detecting the state of polarization. In particular, it is the object of the present invention to reduce the measurement error arising from the change in the amount of phase retardation with respect to the wavelength in the phase retardation plate.
To this end, an apparatus for detecting polarization is provided, the apparatus comprising a phase retarder rotating at a first constant speed for causing phase retardation between polarization components of an incident light with respect to a fast axis and a slow axis of the phase retarder, a polarizer rotating at a second constant speed for changing a state of polarization of light that propagated through the phase retarder, a detector for detecting an intensity of light that propagated through the polarizer, and a signal processing unit for deriving frequency components of electrical signals of light that was detected at the detector to analyze the state of polarization of the incident light.
Preferably, the phase retarder is a 1/4 wavelength plate.
The method of detecting a state of polarization of an incident light according to the present invention comprises the steps of periodically changing the state of polarization of polarization components of the incident light by propagating the incident light through a phase retarder that has a fast axis and a slow axis and rotates at a first constant speed, changing an intensity of the incident light by propagating the incident light of which the state of polarization was changed through a polarizer rotating at a second constant speed, detecting the changed intensity of the incident light, and deriving frequency components from the detected intensity of the incident light to obtain Stokes parameters.
Preferably, the first constant speed and the second constant speed are different from each other. Alternatively, the first constant speed and the second constant speed may have same magnitudes but opposite directions.
Still alternatively, the first constant speed and the second constant speed may have same directions and the ratio of a magnitude of the first constant speed to a magnitude of the second constant speed may range from 1/3 to 3/1.
REFERENCES:
patent: 4960319 (1990-10-01), Dankowych
patent: 5005977 (1991-04-01), Tomoff
patent: 5247176 (1993-09-01), Goldstein
patent: 5396329 (1995-03-01), Kalawsky
patent: 5519493 (1996-05-01), Reiley
patent: 5659412 (1997-08-01), Hakki
patent: 6396575 (2002-05-01), Holland
patent: 63-055519 (1988-03-01), None
patent: 2-250026 (1990-10-01), None
English-language abstract of JP2-250026 A.
English-language abstract of JP63-055519.
Jeong Ho Jin
Lee Bong Wan
Donam Systems Inc.
Marger & Johnson & McCollom, P.C.
Stafira Michael P.
LandOfFree
Apparatus and method for detecting polarization does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Apparatus and method for detecting polarization, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Apparatus and method for detecting polarization will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3156391