Optics: measuring and testing – By dispersed light spectroscopy – With monochromator structure
Reexamination Certificate
2000-11-17
2003-07-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With monochromator structure
C356S328000
Reexamination Certificate
active
06597452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention relates to diffraction-grating spectrometers and monochromators and, more particularly, to a Littrow-type diffraction grating spectrometer.
2. Background Information
It is known in the art to use various optical modules that are assembled into a desired configuration to perform a specified optical function. Such configuration may take the form of a scientific instrument, or may find employment in a spectroscopy application. Modules that in turn comprise a number of devices for performing optical functions are also known. It is usually advantageous to make each module as compact as possible.
A spectroscope, an instrument which produces a spectrum, is one particularly useful example of such an optical instrument. Another is a spectrograph which is a spectroscope provided with a recording device, or other light-capture means, or the like to receive and record or otherwise process the spectrum generated. A spectrometer is a spectrograph enhanced with means to quantify the output, for example a scale to measure particular wavelengths, or a detector to determine intensity, at one or more wavelengths. The present invention relates particularly to spectrometers, and that term will be used hereinafter. However it will be understood that the novel optics described herein can be employed in spectroscopes or spectrographs for applications where recording, quantification or similar capabilities are not required and the invention extends to such novel spectroscopic and spectrographic applications. To the extent that the invention may be applied to output a single spectral band or to provide a scanned output comprising a series of individual spectral bands, the term “spectrometer” should also be understood to include monochromators.
Diffraction grating spectrographs use one or more diffraction gratings to diffract input light into a spectrum of specific wavelengths or spectral bands. In a typical configuration, spectrographs are designed to select a single wavelength, or a narrow spectral band from the input light, for examination or recordal.
In one known embodiment of spectrometer employing a planar diffraction grating, a concave mirror is illuminated by a point source whose spectrographic composition is to be analyzed. The light from the point source is collimated by the concave mirror to form a parallel bundle of rays, which are caused to fall upon the surface of a planar diffraction grating. This concave mirror is known as a collimator in a typical spectrometer instrument. Because the planar diffraction grating has a number of grooves etched in its surface, light falling on the surface of the diffraction grating is diffracted, that is, reflected an angle which is a function of the wavelength of the light. If the input light source comprises a number of wavelengths, the result is that light of different wavelengths will be diffracted, or reflected, at an angle which is a function of wavelength.
The diffracted light may then be received by a second concave mirror which focuses the diffracted light to form an image of the point source under analysis. However, because light of different wavelengths has been diffracted at different angles, the point source is imaged by the second concave mirror, also known as a focusing mirror, at different points for different wavelengths. Accordingly, it is possible to select out individual wavelengths, or more precisely a narrow region of the spectrum, or spectral band, consisting essentially of a single wavelength, to measure the intensity of the same and to utilize this information, for example for elemental analysis of an emissive source material.
Spectrometric elemental analysis of samples has many industrial uses. For example, in the case of the analysis of industrial slag, such as might be obtained from crucible of molten metal in a steel furnace, the slag may be put into a plasma, excited and the emission spectrum analyzed and measured with a spectrometer. The wavelengths appearing in the plasma emission band indicate the nature and quantity of the impurities in the slag, enabling plant operators to adjust production parameters to achieve a desired product.
While the above discussion has centered on spectrometer devices using mirrors, and such devices are usually preferred because of the quality of imaging using mirrors, it is possible to construct devices using focusing lenses, such as convex lenses or compound multielement lenses having an overall convex optical characteristic. In principle, it is also possible to combine lenses and mirrors in an instrument.
It is also noted that diffraction gratings in spectrometers may be either classical mechanically ruled diffraction gratings of the type invented and made by applicant's assignee at the beginning of the 1800's, or holographic diffraction gratings of the type manufactured by applicant's assignee since the 1960's.
It is also known that spectrometers may be constructed using concave diffraction gratings, such as concave holographic diffraction gratings of the type invented by Flamand in the late 1960's working at the applicant company as illustrated by his U.S. Pat. No. 3,628,849.
A Littrow-mounted system is a relatively common method of utilizing large plane reflection gratings, providing simplicity and good optical quality arising from the use of a single nmirror to perform both collimating and focusing functions. Moreover, in this configuration, the collimating and focusing functions are both performed in the same geometric space, resulting in efficient use of that space. In a typical Littrow setup, a mirror delivers parallel incident light from an input point source to the grating, and focuses diffracted light received from the grating to an output point often proximate the input point source. In such devices, a single mirror acts as both collimator and focusing element at once, minimizing the number of optical elements required.
In addition to its simplicity, employing the Littrow configuration is particularly desirable for its high quality output. Because the input and output light beams traverse the same optical path, in opposite directions, optical aberrations in the collimating and focusing components are auto-corrected, or self compensating, so that image quality is diffraction limited, i.e. limited by the physical properties of the optical system not by the deficiencies of the optics.
Referring now to
FIG. 1
, one embodiment of a prior art, Littrow-mounted, plane grating spectrometer
1
is shown schematically. Spectrometer
1
employs a rotatably mounted diffraction grating
7
having an inlet aperture, or slit
2
, providing a point or line light source and admits light from a source
3
in the direction of arrows
4
toward a concave focusing mirror
5
. Light reflected from mirror
5
is focused to travel in a collimated beam in the direction of arrows
6
which then strikes diffraction grating
7
.
When the collimated light
6
strikes grating
7
it is diffracted at an angle which varies as a function of wavelength. Accordingly, grating
7
is rotated to variably select one wavelength from a number of wavelengths, as desired, or to scan through the spectrum of available wavelengths. Thus, a selected wavelength of light
8
is reflected back, at the Littrow angle, to travel in a parallel beam in the direction of arrows
9
, oppositely to the direction of arrows
6
. The returned selected wavelength of light traveling in directions
9
strikes concave mirror
5
and is focused to aperture
2
.
An image can then be formed on a detector placed at aperture
2
, if desired, or otherwise recorded or processed and quantified, if desired. From a practical standpoint, placement of the detector at a point on a slit may be undesirable. Therefore, the detector may be slightly offset, and the system tuned to select the desired wavelength, or other desired wavelengths, by rotation of grating
7
to an angular position that results in the imaging of that wavelength on the detector. As described
Jiang Wu
Lange Kevin
Slutter Warren Stephen
Handal & Morofsky
Jobin Yvon Inc.
Lauchman Layla
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