Spectrometer with a shaped oriented slit

Abstract: An improved spectrometer comprises a slit in light blocking plate, an echelle grating, and a detector array. The slit is shaped and oriented to align an image of a light beam projected through the slit, onto the echelle grating and onto the detector in a desired orientation and shape relative to the detector array. Precise adjustment of the shape and orientation of the slit is dependent on the orientation of the detector with respect to the dispersion direction of the echelle grating. The spectrometer provides high detector resolution with reduced read-out time.

Description: The invention relates in general to the field of atomic spectroscopy. Specifically, the invention describes a spectrometer which has been modified to provide high detector resolution by using a shaped, orientated entrance slit which corrects the optical distortion caused by the grating geometry so that the image of the entrance slit will match the orientation and shape of the resolution element of the detector. In atomic spectroscopy, which includes atomic absorption, atomic emission, and atomic fluorescence, electromagnetic radiation of a wavelength characteristic of a particular element is quantitatively determined and used as a measure of the concentration of that particular element in a sample. The intensity of the radiation at the characteristic wavelength is proportional to the concentration of the element in the sample. The sensitivity of any of these atomic spectroscopic techniques depends in part on the ability to select radiation of a particular wavelength range, resolve the selected wavelength range into a pattern of spectral lines, and detect the intensity of the radiation associated with the spectral lines. Wavelength selection and resolution is typically carried out by a spectrometer. Radiation of a range of wavelength is passed through an entrance slit located at the focus of a collimating mirror (first mirror). The collimated ray is then reflected onto a grating and dispersed into a pattern of spectral lines (i.e., a dispersion pattern), with each wavelength being dispersed at a characteristic angle (i.e., a dispersion angle) from the grating. In spectrometers using conventional high ruling density gratings, measurement of radiation is typically conducted for the lower diffraction orders, most preferably the first order. This is done because of the potential overlap of wavelengths of different orders emitted at the same angle from the grating and because a much higher percentage of radiation can be obtained using the first order in these conventional gratings. However, both the dispersion (i.e., angular dispersion) and resolving power of the grating are improved when higher orders are used. Echelle gratings with blazed lines enable the user to take advantage of the greater resolution and angular dispersion obtainable at higher orders. Echelle gratings are characterized by a smaller size with fewer ruled lines, typically 95 to 100 lines/mm compared with more than 1000 lines in a conventional grating. The lines are cut into the surface of the grating at an angle or blaze to the surface. The blaze changes the diffraction properties of the grating so that for any wavelength range it is possible to channel a higher proportion of radiation into the higher orders at the desired diffractive angle. The various diffraction orders generally lie on top of one another. A cross dispersion element (e.g., a prism) can be used to further separate the overlapping orders into the orthogonal direction. Once the grating, either echelle or conventional, resolves the radiation into different diffracted angles, a camera mirror (second mirror) can then be used to create a dispersion pattern on the focal plane of the mirror. This pattern consists of images of the entrance slit for each wavelength being dispersed. The orientation and shape of the image of the entrance slit depends on the geometry of the spectrometer. There are two possible orientations of the incident radiation relative to the echelle grating rulings or grating normal. In the first orientation, the grating rulings are orthogonal to the meridional plane of the incident ray. In this configuration, the slit and its image lie in the horizontal foci of the mirrors and the grating has no effect on the vertical aberration of the system. The resulting image of the entrance slit then has the same shape as the entrance slit itself and is oriented parallel to the grating rulings. In the second orientation, the grating rulings are parallel to the meridional plane and the incident and the diffracted rays form a small angle  with the plane through the grating normal. The slit and its image lie in the vertical foci of the mirrors and the grating will then cause vertical aberration. The resulting image will then tilt at an angle X = arctan(2 tanα tan) with respect to the orientation of the grating rulings and entrance slit, where α is the grating blazed angle. This assumes that the grating is used in the Littrow position where the angle of the incident and diffracted rays with the grating normal is the same as the grating blazed angle. This second type of orientation, although having a tilted image, is generally preferred in an echelle spectrometer due to higher grating efficiency and better optical quality. Various types of detectors have been used to detect spectral images at the focal plane of the spectrometer. Linear and area arrays of photodiodes, charge-coupled devices (CCD), charge injection devices (CID), and plasma coupled devices have been used to retrieve optical signals from conventional grating spectrometers as well as echelle grating spectrometers. A CCD which could be used in an echelle spectrometer would contain an x-y grid of pixels that might be comprised of 1,000 rows and 1,000 columns of pixels, for a total of 1,000,000 pixels. Detectors of this type have the advantage of being able to detect all signals within the spectrometer, including background noise. The x-y grid of detector pixels is aligned with the grating rulings. If the pixels have different sizes in the x and y dimension, the grid with the wider dimension is usually aligned parallel to the grating rulings. To achieve the highest resolution, the image of the entrance slit should closely match the orientation and shape of the detector pixels. A general rule of practice is that the width of the slit is used as the resolution element and is matched to twice the width of a CCD pixel. This requirement of matching the slit image to the orientation and shape of the detector pixels presents no problem for the first type of grating spectrometer where the grating rulings are orthogonal to the meridional plane of the incident ray. However, the tilted images of the second type of grating spectrometer are a problem when using an x-y grid type of CCD detector. The tilted image and consequent mismatch with the shape of the detector pixels results in a loss of spectral information. The lost information means lower resolution and a diminished performance for the spectrometer. The use of an x-y grid type of CCD detector also results in a mismatch of shape between the detector and the diffraction pattern. Trade-offs exist between underfilling or overfilling of the detector. In addition, because every pixel is read in an x-y grid type of CCD detector, read-out time is long, even though many of the pixels may have no useful information. It may be possible with a tilted slit image to improve the spectral line shape reproduction by using a computer algorithm to recover and reconstruct the spectrum. However, since the signal in the direction of the slit height may fall upon more than two pixels, this would require the use of an x-y grid type of CCD detector having additional pixels to recover the entire signal. Moreover, in such a case, a multiple read-out must be performed for any single spectral line which will reduce the Signal/Noise ratio and hence diminish the performance of the spectrometer. Another type of solid-state detector has arrays of sensing elements positioned exactly along and on the locations of diffraction pattern on a focal plane of an echelle grating spectrometer. This type of detector provides continuous spectral coverage with much faster read-out time. To achieve high resolution in this type of detector, the image of the entrance slit should match closely to the orientation and shape of the pixels in the detector arrays. The invention is set out in its broadest terms in claim 1 of this specification. Examples of the prior art and of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is an illustration of the main components of an echelle spectrometer embodying the present invention. Figure 2 is an illustration of a straight edge (i.e., horizontal) prior-art slit in a light blocking plate. Figure 3 is an illustration of a slit which has been translated and stretched as described in an illustrative embodiment of the invention. Figure 4 is an illustration of an echelle grating. Figure 5(a) is an illustration of a two-dimensional diffraction pattern created in an echelle spectrometer and oriented horizontally. Figure 5(b) is an illustration of a two-dimensional diffraction in pattern of an echelle spectrometer, oriented at a skew angle. Figure 6(a) is an illustration of a plurality of pixels in a linear detector array. Figure 6(b) is an illustration of the image of a conventional straight-edged rectangular slit on a detector array, the image tilted by an angle with respect to the direction of the skewed linear array. Figure 6(c) is an illustration of the image of a shape orientated slit on a detector array, the image aligning squarely with the skewed linear array. Figure 1 illustrates the main components of an echelle spectrometer in which the slit of the present invention may be used. The spectrometer generally includes an echelle grating 105 that has a plurality of ruling lines, a cross dispersion element 145, for example, a prism, and a detector array 120. Additional optical elements, such as mirrors 118 and 132, are used to reflect and focus the incoming radiation, the diffracted radiation, and ultimately the dispersed radiation. The electromagnetic radiation to be analyzed, typically emanating from a source 137, should uniformly illuminate the entrance slit. As is well known in the art, the term "slit" means an opening 125 in a plate or other light blocking structure 127 that allows radiation to enter into the spectrometer. The radiation is then collimated by the first mirror 118 and incident on the echelle grating 105. Radiation of a range of wavelengths incident at the grating at the same incident angle are diffracted at angles which varied with wavelength. The diffracted beam 128 includes relatively high orders overlapping together. A prism 145 may be used to disperse the various orders, and a two-dimensional diffraction pattern is formed. Reflector 132 may be used to form the image of the entrance slit in a predetermined focal plane in the spectrometer. A detector 120 is positioned to intercept the spectrum at the image plane. Signals developed at detector 120 are processed in read-out electronics 117. The echelle grating 105 is preferred to be used in quasi-Littrow mode as shown in Figure 4 where the incident ray 135, and the diffracted ray 128, measured in the x-z plane, form an angle α 137 with respect to the grating normal 138. This angle has the same magnitude as the blaze angle of the grating, for example 45 degrees. The grating 105 is mounted with the ruling grooves 110 parallel to the meridional (horizontal) plane. The grating may have approximately 95 grooves per millimetre. The incident 135 and the diffracted 128 rays make an angle  140 with respect to the plane through the grating normal. Figure 5(a) illustrates a two-dimensional diffraction pattern created in the image plane of the echelle spectrometer. In a preferred detector design, the sensing elements are arranged to form a plurality of skewed linear arrays such that the angle of each array tracks the angle and curvature of the particular diffraction order to be detected by that array. Figure 5(b) illustrates a two-dimensional diffraction pattern orientated at a skew angle. Depending on the particular spectrometer design, each individual array 152 may include a plurality of optical sensing elements 151 of varying sizes, called pixels, as shown in Figure 6(a). The resolution element is generally chosen to match to twice the width of the pixel along the dispersion direction. Because the lengths of the pixel arrays may vary due to the change of free spectra range, the number of pixels in the orders will be different. A typical size of the pixels is 25 micron by 50 micron. For a conventional straight-edged rectangular slit 125 as shown in Figure 2, the image of the slit 150 on the focal plane will be tilted by an angle with respect to the direction of the skewed linear array 152 as shown in Figure 6(b). The image of the rectangular entrance slit will not match with the resolution element and will extend beyond the ideal two pixels 151. Figure 3 illustrates the geometric configuration of one embodiment of the slit in the present invention. The slit having a shape defined by translating a short side 205 of a rectangular slit in a direction parallel to said short side, while simultaneously stretching two long sides 210 of said rectangular slit, in an amount sufficient to rotate the said two long sides by an angle of 2. Computation of the amount of translation of the short sides and stretching of the long sides of the rectangular slit is a matter of basic trigonometry. The image 150 of the entrance slit of the invention on the focal plane of the echelle spectrometer will align squarely with the skewed linear array 162 as shown in Figure 6(c). The image of the entrance slit matches the shape and orientation of the resolution element of two pixels 161. A principal advantage of the echelle spectrometer described above is that it achieves higher detector resolution at comparatively low cost, without complicated signal processing of raw information from the detector array. It also facilitates achieving extended spectrum coverage by orienting the detector array to match the dispersion pattern of the echelle grating, e.g., rather than a horizontal orientation, the detector array may be oriented along skewed lines or a diagonal line extending between two opposite corners of the detector surface. See Figures 5(a) and 5(b). The illustrative embodiments described above are particularly useful for an echelle spectrometer since by its nature it is a two dimensional detection system. However, the invention could also be applied to any spectrometer where the image of the slit should align and match the detector array.

Claims: An echelle spectrometer comprising: (a) an echelle grating (105) that has a face and a plurality of ruling lines (110); (b) a detector array (120); and (c) a slit (125) shaped and oriented to align an image (150) of a light beam (135) projected through the slit (125), onto the grating (105), and onto the detector array (120), to a desired orientation relative to the detector array, an axis of the detector array (120) being parallel to the rulings of the grating (105), and the slit (125) having a non-rectangular shape defined by translating one short side (205) of an initially rectangular slit while simultaneously stretching at least one long side (210) of said initially rectangular slit, wherein: the light beam (135) falls on the echelle grating (105) at an angle  with respect to a plane perpendicular to the ruling lines (110) of the grating, and an angle α with respect to a plane parallel to the rulings, and the one short side (205) of the initial rectangle is translated in an amount sufficient to rotate the one long side (210) by an angle of arctan (2tanα tan). A spectrometer as claimed in claim 1 wherein the grating (105) is adapted to disperse radiation passing through the grating into a dispersion pattern, and the detector array (120) is oriented so that the dispersion pattern falls along a skewed line extending between two opposite sides of the detector array. A spectrometer as claimed in claim I or 2 wherein the one short side (205) of the initial rectangle is translated in a direction parallel to said one short side (205). A spectrometer as claimed in any one of claims 1 to 3 wherein a cross dispersion element (145) is placed between the echelle grating (105) and the detector (120). A spectrometer as claimed in any one of claims 1 to 4 wherein a reflector (118) is used to reflect radiation onto the grating (105). A spectrometer as claimed in any one of claims 1 to 5 wherein a reflector (132) is used to reflect the dispersion pattern of the grating onto the detector (120). A spectrometer as claimed in any one of claims 1 to 6 wherein a mask (130) is placed between the echelle grating (105) and the detector (120) for screening unwanted radiation from the detector array. A spectrometer as claimed in any one of claims 1 to 7 wherein the lines on said grating are spaced so as to have about 100 lines/mm. A spectrometer as claimed in any one of claims 1 to 8 wherein the detector array is oriented so that a dispersion pattern of the grating falls along a diagonal line extending between two opposite corners of the detector array.

Inventors: CHIEN RING-LING, CHIEN, RING-LING
Applicants: VARIAN INC, VARIAN, INC.
Application year: 1997
Main classification: G01J3
IPC codes: G21K1, G01J3, G02B5