FIELD OF THE INVENTION
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This invention relates to optical spectrometers utilizing circular birefringence to rotate the linear polarization of light, and more particularly to deducing the photon wavelength based on an analysis of light polarization after propagating light through the circularly birefringent medium.
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OF THE INVENTION
High resolution measurement of light frequency from incoherent sources typically makes use of cavity interference such as Fabry Perot interferometers and gratings, or absorption lines from some medium. Interferometers such as a Fabry Perot or gratings are expensive and have low acceptance angles, meaning the deviation from the desired angle at which the light enters the interferometer has very little tolerance. Moreover, for such interferometers increasing the spectral resolution lowers the transmission of the signal (reduces the number of photons included in the signal). Absorption line mediums (e.g., iodine, potassium and sodium) require some atomic or molecular transition in the medium, and they only occur at discrete and fixed frequency locations. Additionally, since absorption lines absorb light, they deplete the strength of the signal being measured.
Magneto-optic spectrophotometers can be used to measure frequency, but they only distinguish light near a particular absorption line from light that is not near a particular absorption line, which provides very low frequency resolution in comparison to the current invention.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIG. 1 is a diagram of rubidium spectra, in accordance with one embodiment of the present invention.
FIG. 2 shows the basic configuration of a two-photon-absorption magneto-optic dispersion spectrometer in accordance with one embodiment of the present invention.
FIG. 3 shows the real (X′) and imaginary (X″) portions of the electric susceptibility near a split absorption line caused by a magnetic field. The figure also shows the difference in susceptibility between each circularly polarized component of a test beam for test beam paths propagating with and against a magnetic field shifted susceptibility in accordance with one embodiment of the present invention.
FIG. 4 is a plot of transmission spectra into two separate channels, a first output and second output, in accordance with one embodiment of the present invention.
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OF THE INVENTION
Several drawings illustrate physical the attributes of a magneto optic dispersion spectrometer, and quantities that may be manifested with its construction, in accordance with embodiments of the present invention. Examples are described that have particular gaseous mediums, transitions, wavelengths of complimentary light pairs, etc. for purposes of illustration. However, it should be noted that the choices of particular gaseous medium and particular transitions are abundant. Also, while concomitant to the chosen transitions, the wavelengths of the light pairs, test beam and reference beam, have wide latitude of choice upon a continuum. Thus it is recognized that the apparatus and means described herein may vary without departing from the basic underlying concepts of the invention.
The current invention is an optical spectrometer based on dispersion from two-photon-absorption. An optical spectrometer measures some property of light, typically intensity as a function of wavelength. A dispersion spectrometer utilizes a rapidly changing electric susceptibility to demarcate intensity at a particular wavelength. Embodiments of the current invention are based the creation of a medium where in selected frequency regions the dispersion changes rapidly but absorption is mostly absent. The frequency region between two absorption lines has these properties and is exploited herein. One way to create two absorption lines is to apply a magnetic field to an atomic vapor and split a single absorption line into two absorption lines.
Light that propagates through a gaseous medium is preferentially absorbed when its energy corresponds to a particular atomic transition. This preferential absorption (otherwise known as resonance absorption) also affects light phase, or dispersion. The electric susceptibility is used to describe both the absorption and dispersion effects. Whenever the real portion of the electric susceptibility, for each circular polarization state of light are different, then the medium becomes circularly birefringent. A linear polarized beam will undergo polarization rotation to another linear polarized state while traveling through a circular birefringent medium. It will be shown that the electric susceptibilities for the test beam that manifest from two-photon-absorption in an gaseous medium can be manipulated to bring about circular birefringence that changes rapidly enough to make an ultra high resolution spectrometer.
A circular birefringent medium in the present invention accomplishes circular birefringence based on a physical phenomena called two-photon-absorption. Consider an atomic transition from a ground state (lowest allowed energy state of an atom) to an intermediate excited state, which can occur with the absorption of a single photon. A single photon resonance is a photon frequency bandwidth where the energy of the photon matches an allowed atomic transition. Furthermore, consider another transition from the intermediate excited state to another still higher energy state, a final excited state that can occur with the absorption of a single photon. Two-photon-absorption is the direct transition from the ground state to the final excited state, avoiding the intermediate state, by the simultaneous absorption of two photons. A two-photon-transition identifies the states of the substance involved in two-photon-absorption. A two-photon-absorption line is a frequency bandwidth of light that can be absorbed by the process of two-photon-absorption. FIG. 1 is a diagram illustrating the process of two-photon-absorption, in accordance with one embodiment of the present invention.
In the case of two-photon-absorption, the only restriction upon the energy of the photons is that the sum of their energies match the total energy of the atomic transition: