Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1996-04-17
1998-07-14
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356351, G01B 902
Patent
active
057812934
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns spectrometers, particularly Fourier-transform spectrometers.
BACKGROUND OF THE INVENTION
Conventional Fourier-transform spectrometers are based on Michelson interferometer arrangements.
In a Fourier-transform spectrometer input light is divided into two arms by a beam splitter. If the output of the interferometer is recorded as a function of the path difference between the two arms, an interferogram is obtained. The power spectrum of the Fourier-transform of the interferogram corresponds to the spectral distribution of the input light.
The resolution of a Fourier-transform spectrometer depends on the maximum path difference between the two arms. ##EQU1##
The shortest wavelength that can be unambiguously measured depends both on the maximum path difference and the number of data points recorded across the interferogram. ##EQU2##
Most Fourier-transform spectrometers are designed for operation in the infra-red (1-10 um) and have resolutions of 10's-100's GHz.
Conventional Fourier-transform interferometer using a Michelson interferometer have a moving mirror in one arm to introduce the path difference and an internal HeNe laser to measure it. As such the mechanical quality of the movement is critical to the performance of the instrument. In addition, a conventional instrument requires a finite time over which to scan the mirror and thereby record a spectrum.
OBJECT OF THE INVENTION
It is an object of the invention to provide an improved Fourier-transform spectrometer.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, the Michelson interferometer in a Fourier-transform spectrometer is replaced by a birefringent optical component.
Birefringent materials exhibit a refractive index that depends on the polarisation of the incident light. Therefore, by using a birefringent component in the path of a polarised beam of light it is possible to introduce a path difference between two light beams following the same path.
A basic implementation of a birefringent component within a Fourier-transform spectrometer is later described with reference to FIG. 3 of the accompanying drawings.
Preferably the incident light is polarised at 45.degree. to the fast and slow axes of the birefringent component. This divides the light equally between the "fast" and "slow" polarisation components. The path difference introduced between the two polarisations depends in part on the lateral position of the birefringent component. One suitable birefringement component is a Wollaston prism, as later described with reference to FIG. 4 of the accompanying drawings.
The path difference between the two polarisation components is given by: indices of the birefringement materials,
By translating the Wollaston prism the path difference between the two polarisations can be varied. To obtain an interferogram between the two polarisations a second polariser, also at 45.degree., may be used to extract a common polarisation component. As before, a Fourier-transform of the interferogram gives the spectral content of the input light.
According to a second aspect of the invention, the requirement for any moving parts within a Fourier-transform spectrometer is removed by using an extended light source so that all areas of the birefringent component are illuminated simultaneously. Different positions on the birefringent component thus correspond to different path differences between the two polarisations.
By providing an array of detectors the entire interference pattern corresponding to all the different path differences can be recorded simultaneously. Again a Fourier-transform of the interferogram will give the spectral distribution of the input light. In this way it is possible to produce a simultaneous, Fourier-transform spectrometer which has no moving parts.
A Wollaston prism, as previously mentioned in connection with the first aspect of the invention, is preferably used in the design of a Fourier-transform spectrometer having no moving parts. The birefringence in the Wolla
REFERENCES:
patent: 4320973 (1982-03-01), Fortunato et al.
patent: 5179424 (1993-01-01), Lequime et al.
Duncan Alan James
Harvey Andrew Robert
Padgett John Miles
Sibbett Wilson
Siemens Plc.
Turner Samuel A.
Weinstein Louis
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