Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
1999-12-09
2001-06-19
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S451000, C356S453000
Reexamination Certificate
active
06249350
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an interferometer and a method for compensating for the dispersion and a method for increasing the spectral resolution of an interferometer.
Related Technology
The measuring principle behind an interferometer for electromagnetic radiation is based on the interference between two coherent electromagnetic partial waves &psgr;
1
and &psgr;
2
, which have a defined phase relationship to each other, i.e., the same wavelength &lgr;, and on which a fixed phase difference &Dgr;&phgr;, which is constant over time, is superimposed. The intensity I
det
of the superimposed wave is detected. I
det
is proportionate to a+cos(&Dgr;&phgr;), where a is a constant. Only identical polarization components, such as electrical field vectors along the x axis, always interfere with one another.
In a two-beam interferometer, the radiation from a radiation source is split into two beam components with a relative phase difference of zero and each beam component supplied to one interferometer arm. The interferometer arms can have different optical path lengths, superimposing a phase difference not equal to zero upon the partial waves after they pass through the interferometer. In a two-beam interferometer with an optical path length difference of &Dgr;ln between the two interferometer arms, the phase difference between the partial waves is
Δϕ
=
2
π
Δ
(
ln
)
λ
.
l is the actual path length and n the refractive index of the medium. This interferometer phase &Dgr;&phgr; is dependent on wavelength &lgr; and on &Dgr;ln, and can therefore be used, if one quantity is known, to precisely measure the other quantity.
Interferometers are used, for example, to measure the length of such objects as gauge blocks, to measure the refractive index, or in spectroscopy.
Length measurements, i.e., measurements of the variation in optical path length difference &Dgr;ln, become corrupted by wavelength fluctuations, since variations in the wavelength produce a variation in the interferometer output signal I
det
, even though &Dgr;ln remains the same. To remedy this disadvantage, Y. Troitski, Applied Optics 34 44717 (1995) proposes a dispersion-free interferometer which nearly compensates for the wavelength dependence of the output signal by applying vapor to the interferometer mirror. The disadvantage of method is that the application of vapor to the mirror makes it expensive and inflexible. The working area of the interferometer, i.e., the range around a basic wavelength &lgr;
0
, within which the output signal is nearly independent of wavelength &lgr; of the radiation source, is set to basic wavelength &lgr;
0
by adjustment after application of the vapor and can no longer be adjusted to actual conditions, e.g., after changing radiation sources. A further disadvantage is that the working area is narrow, which means that dispersion compensation cannot be used for a broader band &Dgr;&lgr; around the basic wavelength &lgr;
0
.
One disadvantage of interferometers used for spectroscopic studies is the great sensitivity of the output signal not only to the wavelength but also to variations in the path length difference of the interferometer arms, caused, for example, by vibrations in the measurement apparatus. This leads to an error-prone spectroscopic measurement result.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an interferometer for electromagnetic radiation which compensates for the wavelength dependence of the output signal in a broad wavelength range &Dgr;&lgr; around an adjustable basic wavelength &lgr;
0
, at the same time maintaining high sensitivity to variations in the interferometer arm length. The object is also to provide an interferometer for electromagnetic radiation in which the spectral resolution is higher than that of conventional two-beam interferometers with comparable arm lengths, thereby stabilizing the output signal toward variations in the interferometer arm length.
The present invention provides an interferometer for electromagnetic radiation, including at least two interferometer arms and one beam splitter, with the light from an electromagnetic radiation source being split by the beam splitter into two beam components and supplied to each of the two interferometer arms; with the same or a further beam splitter again superimposing, on the beam components after they pass through the interferometer arms, a phase difference
Δϕ
=
2
π
Δ
(
ln
)
λ
,
dependent on optical path length difference &Dgr;ln within the interferometer arms and on wavelength &lgr;; and with the intensity of the superimposed wave being measured by a detector that is characterized by the following features:
a) At least one polarizer is provided in the optical path, which establishes a defined polarization state P
½
0
of the electromagnetic partial waves entering the interferometer arms, where the polarization state is independent of the wavelength and can be different for the two beam components.
b) At least one optical element is provided in at least one interferometer arm, with the optical element varying the polarization state P
½
0
of the electromagnetic partial wave as a function of wavelength &lgr;, i.e., each spectral component &lgr;
i
is encoded with a polarization P
½
(&lgr;
i
), so that the electromagnetic partial waves take on polarization states P
1
(&lgr;) or P
2
(&lgr;). The regions P
1
(&lgr;) and P
2
(&lgr;), shown on the Poincaré sphere, are at least partially different from each other.
c) An analyzer, located at the output if the interferometer, transmits an adjustable polarization state P
det
, thereby forming projections P
det
(P
½
(&lgr;)) for each spectral component &lgr;
i
and producing an additional wavelength-dependent phase difference &ggr;(&lgr;) between the partial waves of the spectral components, where &ggr; is a function of P
½
(&lgr;) and P
det
.
The present invention also provides a method using an interferometer of this type, characterized in that the dispersion or wavelength dependence of the output intensity in the range around a basic wavelength &lgr;
0
, is compensated for by the following steps (to achieve stabilization of the interferometer output signal during spectral wavelength fluctuations around &lgr;
0
and thus increased sensitivity of the interferometer to variations in arm length difference &Dgr;l).
a) Production of a defined wavelength-independent output polarization P
½
0
of the electromagnetic partial waves entering the interferometer arms, where P
1
0
≠P
2
0
is possible.
b) Encoding of the individual spectral components of the electromagnetic partial waves with a wavelength-dependent polarization P
1
(&lgr;) or P
2
(&lgr;).
c) Detection of a defined polarization P
det
.
d) Adjustment of the polarization quantities P
½
0
, P
½
(&lgr;), and P
det
to each other so that the following applies to wavelength &lgr; in the area of &lgr;
0
:
Υ
(
λ
)
=
-
½
Ω
(
P
det
,
P
1
(
λ
)
,
P
2
(
λ
)
)
≈
d
+
2
π
Δ
ln
λ
o
2
(
λ
-
λ
0
)
or
ⅆ
Ω
ⅆ
λ
(
λ
0
)
≈
-
4
π
Δ
ln
λ
0
2
where d is a constant and &OHgr;(P
det
, P
1
(&lgr;), P
2
(&lgr;)) is the area of the spherical triangle defined for a fixed &lgr; by points P
det
, P
1
(&lgr;), and P
2
(&lgr;) when representing the polarization quantities on the Poincaré unity sphere.
The present invention also provides a method using an interferometer of the type described above, characterized in that the spectral resolution of the interferometer is increased in the range around a basic wavelength &lgr;
0
by the following steps:
a) Production of a defined wavelength-independent output polarization P
½
0
of the electromagnetic partial waves entering the interferometer arms,
Dultz Wolfgang
Frins Erna
Hils Bernd
Schmitzer Heidrun
Deutsche Telekom AG
Kenyon & Kenyon
Kim Robert
LandOfFree
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