Optics: measuring and testing – By polarized light examination
Reexamination Certificate
1999-04-28
2002-04-16
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
C356S368000
Reexamination Certificate
active
06373569
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method and device for the spectral analysis of electromagnetic radiation.
BACKGROUND ART
Currently there exist two basic physical approaches to the spectral analysis of light utilizing either the phenomenon of angular separation or interference of light.
In the method based on the angular separation, the analyzed light beam interacts with an appropriate optical element and changes the direction of its propagation depending on the wavelength. This way spectral components are spatially separated and can be independently analyzed. The spatial separation is based either on the dispersion of the refractive index in optical prisms or on the properties of the optical grating where the angle of reflection of the incident radiation depends on the wavelength. Commercially available instruments based on these principles are called monochromators which have been widely used in ultraviolet (UV), visible (VIS) and infrared (IR) spectral regions.
Instruments directly utilizing the interference of light are generally based on the Michelson interferometer. The analyzed light is split into the two interfering light beams and so called “interferogram” is measured as a function of the variable optical path in one arm of the interferometer. The spectrum of the analyzed radiation is then extracted from the interferogram by methods of Fourier-transform analysis. The interferometric techniques are preferentially utilized in the IR spectral region because for longer wavelength it is easier to reach the required accuracy of the position of the movable mirror in the interferometer.
DISCLOSURE OF INVENTION
The nature of the invention lies in the utilization of a new physical method for the spectral analysis of electromagnetic radiation, especially light, and in the technical design and construction of devices which utilize this new principle.
The new method according to the present invention utilizes the physical phenomenon known as dispersion of optical rotation, wherein the polarization plane of linearly polarized electromagnetic radiation is rotated during propagation through an active environment of a proper optical element (rotator) and the rotation angle depends on the wavelength of the radiation. If the total intensity of polarized polychromatic radiation is I=∫I(&lgr;) d&lgr;, then after passage through the rotator and through an analyzing polarizer (analyzer) the “rotogram” R(p) can be measured as a function of the parameter p, which characterizes physical and geometrical properties of the rotator:
R
(
p
)
=
∫
λ
I
(
λ
)
cos
2
[
ϕ
0
+
ϕ
(
λ
,
p
)
]
ⅆ
λ
(
1
)
wherein I(&lgr;) is the spectrum of input radiation, &phgr;
0
denotes an angle between a direction of maximum transmittance of the analyzer and a polarization plane of input radiation, and &phgr;(&lgr;,p) stands for a rotation angle of the polarization plane of radiation with the wavelength &lgr; after passage through the rotator. The equation (1) is a Fredholm's integral equation of the first type (Press W. H., Teukolski S. A., Wetterling W. T. and Flannery B. P.:
Integral Equations and Inverse Theory in Numerical Recipies in Fortran
, Cambridge University Press, 1992, p.779) with the kernel K(&lgr;,p)=cos
2
[&phgr;
0
+&phgr;(&lgr;,p)]. If R(p) is measured, I(&lgr;) can be unambiguously calculated from equation (1) using modern advanced methods of numerical analysis, especially the maximum entropy method (MEM) (Skilling J. and Bryan R. K.: Maximum Entropy Image Reconstruction: General Algorithm,
Mon. Not. R. astr. Soc
., 211 (1984) 111-124). In the special case of a rotator made from an optically active material with a specific optical rotation D(&lgr;), the parameter p can represent its adjustable thickness in the direction of the light beam propagation, which gives &phgr;(&lgr;,p)=pD(&lgr;).
The invention determines a spectrum of electromagnetic radiation, particularly of light characterized by a measurement of a rotogram R(p) of the radiation defined by equation (1), wherein a dispersion element made from an optically active medium exhibiting dispersion of optical rotation, placed between two polarizers with arbitrarily oriented polarization planes, preferably parallel or perpendicular, and subsequent mathematical analysis of the rotogram, preferably by the maximum entropy method, is used to determine the spectrum of the electromagnetic radiation.
In the device based on this principle, the linearly polarized beam of radiation first propagates through an optical rotator where the polarization planes of the individual spectral components are rotated depending on their wavelength &lgr;, then passes through an analyzer and strikes a detector that measures R(p) as a function of the parameter p. Finally the spectrum I(&lgr;) is calculated from equation (1).
In an apparatus in accordance with the invention there may be utilized one of the many possible configurations of a rotator, which can be manufactured from optically active left-hand and right-hand-rotating forms of quartz crystals (FIG.
1
A). The rotator may comprise two left-hand-rotating (−) prisms
1
and
2
and one right-hand-rotating (+) compensation plate
3
. The function of the rotator remains the same when the right-hand-rotating prisms and the left-hand-rotating compensation plate are used. The light beam propagates along a direction of parallel aligned optical axes of all three optical elements. A shift of the larger prism
1
by a distance x along its common plane with the smaller prism
2
from the position when the path d of the beam in the right-hand and left-hand-rotating materials is the same, d
(+)
=d
(−)
, causes a change of effective thickness of the active environment equal to p=d
(+)
−d
(−)
=x·:tan&agr;. The direction of the movement and the angle &agr; are depicted in FIG.
1
A. For a set of different shifts x the rotogram R(p) can be measured. Then the spectrum I(&lgr;) can be calculated from the measured R(p).
In some special applications the change of the thickness of the rotator can be a disadvantage. The rotator depicted in
FIG. 1B
, (Hariharan P., Meas. Sci. Technol. 4 (1993) 136-137), does not suffer from this drawback. The rotator consists of four geometrically identical quartz prisms
13
,
14
,
15
,
16
, from which two prisms
13
,
16
are made from right-hand-rotating quartz and the other two prisms
14
,
15
from left-hand-rotating quartz. The optical axes of all four prisms are again oriented parallel with the optical axis of the rotator and the input light propagates through the rotator in the direction of the axis. A shift of the mutually fixed pair of prisms
15
,
16
relative to the other pair of mutually fixed prisms
13
,
14
, in the direction of the x-axis, perpendicular to the direction of the light propagation, causes a change of the parameter p=d
(+)
−d
(−)
=2x·tan&agr;. The change of p is the same over the whole cross section of the beam. Any shift produces a uniform rotation of polarization planes of rays of the same wavelength.
The invention is further a method for determining the spectrum of a point-size source of light in a single-channel setup with a passage of polarized electromagnetic radiation through an optical element which exhibits uniform dispersion of optical rotation the whole cross-section of the beam of radiation. After passage through the analyzing polarizer, the intensity of the radiation is sequentially measured by a single-channel detector as a function of the parameter p. In this case p represents the thickness of the optically active medium in the direction of the beam propagation. Multiple measurements of the output intensity for different values of p creates the rotogram R(p) from which the desired spectrum is calculated.
The invention is further a method for determining a spectrum of electromagnetic radiation of a two-dimensional source of radiation.
He{haeck over (r)}man Petr
Ve{haeck over (c)}e{haeck over (r)} Jaroslav
Antonelli Terry Stout & Kraus LLP
Font Frank G.
Natividad Phil S.
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