Patent Publication Number: US-7215422-B2

Title: Assembly and method for wavelength calibration in an echelle spectrometer

Description:
This application claims the benefit of German Application No. 102 05 142.9 filed Feb. 7, 2002 and PCT/EP03/00832 filed Jan. 28, 2003. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to a spectrometer. Furthermore, the invention relates to a method for the wavelength calibration of echelle spectrometers. 
     Echelle spectrometers are spectrometers operating wi have a high angular dispersion, i.e. the ability to angularly separate closly proximate wavelengths. This has the advantage of a high resolution and yet small dimensions of the assembly. Therefore, echelle gratings are particularity suitable for high resolution spectroscopy, such as atomic absorption spectroscopy with continuous light sources. A spectrometer with an echelle grating normally operates in very high diffraction orders. Typical values are 20 th  to 150 th  order. The free spectral range in each order is comparatively small. 
     In order to avoid spectral overlapping of different diffraction orders in the exit plane of the spectrometer, echelle spectrometers are used in combination with an internal separation of the orders which is perpendicular to the direction of the echelle dispersion and leading to two-dimensional spectra. The use of echelle gratings in combination with a pre-monochramator for the separation of the orders is also known as a so called double echelle spectrometer assembly. The radiation is specrally pre-selected for example by means of a prism. Only the radiation from a limited spectral range essentially corresponding to one order enteres the echelle spectrometer. The generated echelle spectrum has a linear spectral form. Diffraction gratings or prisms are used as dispersing optical elements for the selection of the order with a pre-monochromator. The directions of the dispersion of the pre-monochromator and the echelle grating are parallel to each other. 
     For most applications it is necessary to calibrate the spectrometer. A wavelength is allocated to each geometric position in the exit plane of the spectrometer. The calibration can vary due to temperature changes, vibrations or other mechanical changes. In this case it may be necessary to re-calibrate the device. 
     2. Prior Art 
     From the DE 41 18 760 A1 a double spectrometer assembly is known having a fluid prism with variable prism angle which generates a spectrum with a low, adjustable dispersion. The spectrum is imaged on an intermediate slit simultaneously forming the entrance slit, i.e. the field stop for the following echelle spectrometer. The intermediate slit cuts out a partial spectrum from the entire spectrum of the light source to be measured, the spectral band width of such partial spectrum being at least smaller than the band width of the corresponding diffraction order of the echelle grating. Such an assembly operates with a small intermediate slit with constant width. The width of the intermediate slit is selected similar to the width of a picture element of the detector (pixel). The width of the entrance slit is comparatively large. The selection of the spectral band width of the light entering through the intermediate slit is effected by varying the linear dispersion in such a way that the prism angle is adjusted accordingly. The position of the wavelength of the portion of the spectrum is adjusted by rotating the prism. The position of the portion of the spectrum on the detector of the echelle spectrometer is adjusted by rotating the echelle grating. The precision of the adjustment of the wavelength position is determined by the precision of the mechanical adjustment of the angle of the echelle grating and the prism of the pre-monochromator, respectively. 
     From DE 195 45 178 A1, a spectrometer assembly is known consisting of an echelle spectrometer and a preceding prism spectrometer for the separation of the orders, the assembly using the Neon spectrum of a low pressure discharge lamp as a line source for the wavelength calibration of the echelle spectrometer. The light from the line source enters the echelle spectrometer bypassing the prism spectrometer through an auxiliary slit in the plane of the intermediate slit and is detected with additional detector elements at the light detector. In such an assembly the widths of the auxiliary slit and the intermediate slit are constant, have the same width and they are smaller than the entrance slit of the pre-monochromator. The intermediate slit forms the field stop for the double spectrometer assembly in a known way. The width of the entrance slit of the described assembly can be changed in steps. Using a fixed prism angle the entrance slit serves as the spectral limitation of the light beam entering the echelle spectrometer. The light entering through the auxiliary slit without pre-dispersion for the wavelength calibration generates a characteristic pattern of spectral lines on the detector. Not all the lines belong to one diffraction order of the Echelle grating, but represent the superposition of the different diffraction orders of the grating. Each line exactly represents one pair of values for the incident and diffraction angle at the grating. With a sufficient line density at least on line is imaged on the reference detector for each position of the grating. By mechanically coupling of the detectors on a common silicon chip a wavelength calibration of the measuring detector can be performed for each measuring wavelength using the position of the reference line with the second, parallelly arranged reference detector. The accuracy of the adjustment of the wavelength position is only determined by the measuring accuracy of the measurement of the reference spectrum, apart from the various imaging errors of the measurement and reference spectra. Thereby, it is now independent from the accuracy of the mechanical adjustment of the Echelle grating. 
     It is a disadvantage of the known assemblies, that each detector element of the light receiving detector is illuminated by the light from a different position of the entrance slit which is significantly wider than the intermediate slit. Thereby a measuring error can be generated, when using the double spectrometer assembly especially for the investigation of light sources with an inhomogeneous light density distribution. 
     Furthermore, the accuracy of the adjustment of the wavelength position of the wavelength range selected by means of the pre-monochromator is completely dominated by the accuracy of the mechanical adjustment of the dispersing element used for the pre-monochromator. Furthermore, the line density of the calibrating light source often is not sufficient with very high linear dispersion of the echelle spectrometer to image at least one calibration line on the detector for each grating position. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the invention to provide a double spectrometer assembly of the above mentioned kind, where a complete wavelength calibration for the entire assembly is possible by controlled rotation of the echelle grating and the dispersing element of the pre-monochromator. Furthermore, it is an object of the invention to provide a spectrometer, which can be calibrated, wherein the relative distribution of the spectral intensity values within the selected wavelength range is insensitive to intensity changes in the light source with inhomogeneous light density distribution. 
     With the use of the narrow entrance slit and a wide intermediate slit the entrance slit forms the field stop for the entire optical assembly. For different wavelengths, the same position of the light source is imaged on each detector pixel at all times. The image of the entrance slit can move due to, for example, thermal and mechanical influences. Due to the possibiltiy of imaging this image as a wavelength range of a continuous spectrum on the detector and adjusting the pre-monochromator always in the same position on the detector this changing movement can be compensated quasi-online. A highly precisely adjustable assembly is generated which provides correct spectral intensity values which are independent of geometric changes in the light source and which are, to a large extent, independent of thermal and mechanical influences. The continuous spectrum ensures that a positionable intensity peak or a positionable intensity profile is present in each order and for each grating position. No ideal continuum is, however, required, in which exactly the same intensity is present at all wavelengths. It is sufficient if the light is emitted at all relevant wavelengths. Such light sources are, for example, noble gas high pressure short arc lamps. 
     The device is particularly suitable for applications using a light source with a continuous spectrum anyway. These are, amongst others, atomic absorption spectrometers, wherein the correction of background interference is effected with continuous light sources or atomic absorption spectrometers with a continuous light source as a measuring light source. However, the continuous light source can also be entered into the optical path exclusively for the calibration of the assembly. 
     The width of the entrance slit is preferably selected such, that the width of its image on the detector is equal to the width of a detector element. Thereby a good compromise between maximum resolution and maximum light transmission is achieved. A reduction of the entrance slit width does not lead to an increase of the resolution. Enlargement would lead to higher light transmission, but, at the same time, would also lead to decreasing resolution. 
     Preferably the width of the intermediate slit is adjustable. Then the intensity profile on the detector can be adjusted for each wavelength such that only the required spectral bandwidth is used for spectroscopic measurements and all other wavelengths are blocked. This has, amongst others, the advantage, that the stray light is reduced. For the calibration of the pre-monochromator by means of a continuous spectrum the measurement of the flancs of the intensity profile with minimum slit width can be used for the positioning. 
     In an embodiment of the invention the pre-monochromator comprises a prism. Quartz prisms in particular are very suitable for applications in the UV/VIS-spectral range due to their high transmissivity. In a further embodiment of the invention the spatially resolving detector is a CCD- or a PDA-detector. 
     In a particularity preferred embodiment of the invention the pre-monochromator is arranged in a Littrow arrangement. Thereby a compact arrangement with small imaging errors and high resolution can be achieved with only few components. Spacial requirements and costs are thereby reduced. The same applies to the arrangement of the echelle spectrometer. 
     Preferably the wavelength adjustment is effected by rotation of the respective dispersive elements. However, it is also possible to adjust the other optical components, such as mirrors or the detector. 
     In a particularity preferred embodiment of the invention the means for the wavelength calibration of the echelle spectrometer comprise a light source with a line spectrum, emitting light which can be imaged on the intermediate slit and means are provided for adjusting a line detected with the detector in a reference position. This kind of calibration enables a wavelength adjustment in such a way that the remaining error is determined by the measuring error of the detector only and not by the error of the mechanical adjustment of the rotation of the grating. The line spectrum of the calibration light source can occur in a wavelength range which is at a large distance from the wavelength to be measured, i.e. in a diffraction order, which is substantially different from the order of the measuring wavelength, as long as the diffraction angles at the echelle grating are sufficiently close to each other. By computing the diffraction angle of the measuring wavelength and the reference wavelength in the different diffraction orders the corresponding angular position of the grating can be adjusted in a very simple way. 
     In case that all overlapping lines of the calibration light source are too distant from each other for all diffraction orders of the echelle grating, in a further embodiment of the invention one or more additional calibrating slits can be provided next to the intermediate slit in the direction of dispersion of the echelle grating and one ore more light sources with line spectra for illuminating such calibrating slits can be provided. In this case the line spectrum shifted in the direction of dispersion occurs several times at the detector. Lines of the same wavelength are shifted by the geometric distance of their respective slit relative to the intermediate slit with respect to such line, which is generated by the intermediate slit itself. The line density generated thereby on the detector enables the reduction of the measuring error with the wavelength calibration. The real geometric distances between the slit openings can be measured exactly by means of the detector for slit images of the same wavelength. 
     Preferably the prism and the echelle grating are arranged in such a way that drifts of the image of the entrance slit in the intermediate slit and of the image of the entrance slit on the detector in the common dispersion direction of the prism pre-monochromator and echelle spectrometer caused by changes of the prism- and grating dispersion due to temperature changes occur in opposite directions. With increasing environmental temperature and the thermal extension of the grating carrier resulting therefrom the grating constant increases. Thereby the diffraction angle for a monochromatic wavelength decreases for a constant incident angle at the echelle grating. The corresponding spectral line is shifted towards smaller wavelengths on the detector. 
     The environmental temperature also influences the diffraction constant of the prism material. Thereby the monochromatic image of the entrance slit in the intermediate slit is shifted. The echelle grating is illuminated with the incident angle of the wavelenght shifted in such a way. A larger incident angle at the echelle grating results in a smaller diffraction angle. With a suitable positioning of the grating, prism and—if present—optical components changing the dispersion direction (mirrors) both thermal effects can be made to operate in opposite directions and, as a result, only cause a minimal drift of the spectrum on the detector. Thereby higher adjustment accuracy of the pre-positioning of the wavelength positions can be achieved. Furthermore the required repetition rate of wavelength calibrations can be reduced. 
     The invention is described below in greater detail with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an echelle spectrometer with pre-monochromator. 
         FIG. 2  shows an intermediate slit assembly in detail. 
         FIG. 3  shows the intermediate slit assembly of  FIG. 2  with additional calibration slits. 
         FIG. 4   a  shows a portion with the intermediate slit and the calibrated position of a monochromatic image of the entrance slit. 
         FIG. 4   b  shows the intensity distribution of a light source with continuous spectrum and a light source with a line spectrum in a reference position at the detector. 
         FIG. 5   a  is a representation of  FIG. 4   a  in a non-calibrated position 
         FIG. 5   b  is a representation of  FIG. 5   b  in a non-calibrated position 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     In  FIG. 1  numeral  10  denotes a spectrometer assembly. The assembly  10  comprises a pre-monochromator  2  and an echelle spectrometer  4 . Numeral  21  denotes the entrance slit of the pre-monochromator  2 . It is illuminated by a light source  11  with a continuous wavelength spectrum for calibration. Such a light source is for example a Xenon high pressure short arc lamp. For this purpose a rotatable mirror  13  and a lens  12  are arranged in the measuring optical path  14  as an imaging element. The entrance slit  21  has a fixed slit width of 25 microns, corresponding to the width of a detector element  51  of a CCD-linear array  5  used as a spatially resolving light detector. The entrance slit  21  forms the field stop for the double spectrometer assembly and defines the width of a monochromatic beam at the location of the detector  5 . 
     The incident divergent light beam is deflected and collimated by a paraboloidal mirror  22 . The parallel light passes through a prism  23 . Thereby the light is dispersed for a first time. After the reflection at a plane mirror  24  which is arranged behind the prism  23 , the radiation passes through the prism  23  for a second time. Thereby almost a doubling of the dispersion is achieved. The light dispersed by the prism to a degree depending on the wavelength is then focused on the intermediate slit  3  by the mirror  22 . Due to the double passage through the prism, maximum angular dispersion can be generated. This is particularly important in the spectral range of long wavelengths over 600 nm to cut out sufficiently small spectral intervals from the continuous spectrum of the light source  11  at the intermediate slit  3 . The positioning of the wavelength of the spectral range at the position of the intermediate slit  3  can be achieved by electromotoric rotation of the prism  23  and the plane mirror  24  about a common axis  25 . This is indicated by an arrow  26 . 
     A portion of the light with a well-defined reduced spectral bandwidth enters the echelle spectrometer  4  through the intermediate slit  3 . Therein, the divergent beam hits the paraboloidal mirror  41  and is collimated. The paraboloidal mirror  41  reflects the parallel light beam in the direction of an echelle grating  42 . After diffraction at the echelle grating  42  the light is again reflected by the paraboloidal mirror  41  and focused on the CCD-linear array  5 . The latter converts the spectral intensity distribution to electric signals which afterwards are digitised and transferred for further data processing. For the selection of the wavelength, the echelle grating  42  can be rotated about a rotational axis  43  extending parallel to the grooves of the grating. This is indicated by a double arrow  44 . 
     The grating and the prism are mounted, in this assembly, in such a way, that the thermally caused wavelength drifts in the pre-monochromator and the echelle spectrometer occur in opposite directions. 
     The intermediate slit  3  has two moveable slit jaws for adjusting the width of the intermediate slit. With the minimum adjustable width, the slit jaws touch a pin having a well defined diameter. Thereby, the slit jaws cannot approach each other any further and the slit width is adjusted to a reproducible value at a well-defined position. The minimum width is larger than the width of the detector elements and the entrance slit. 
       FIG. 2  shows an embodiment of the mechanical assembly  50  of the intermediate slit with adjustable slit width in detail. The intermediate slit is formed by two slit edges  52 . The slit edges  52  are mounted on a flat slit edge carrier  57 , respectively, which is connected to an essentially rectangular base body  58 . The base body  58  is fixed to a base plate (not shown) of the spectrometer assembly. The connecting portion between the carrier  57  and the base body  58  is tapered and forms the spring joint  51  of a bending spring. 
     The carriers  57  are cut out at the lower end in  FIG. 2 . An eccentric disc  54  which is controlled by a stepper motor is arranged in the space formed thereby. If the eccentric disc  54  is rotated about an axis of rotation  55 , both carriers  57  are pushed apart against the force of the bending spring  51  or approach each other again. Therefore, the slit edges are not exactly parallel to each other, but perform a scissors-like movement. However, the influence on the limitation of the bandwidth generated thereby is negligible especially for small slit heights of, for example, 1 mm. 
     In order to provide sufficient space for movement of the carriers  57  within the base body  58  spaces  59  are provided therebetween. Furthermore a pin  53  is provided defining the minimum slit width. 
     A further embodiment of the intermediate slit assembly  50  of the same kind is shown in  FIG. 3 . There, on each side of the base body  58 , fixed auxiliary slits  56  are additionally provided. The auxiliary slits  56  serve as additional calibration slits for increasing the line density at the detector when the echelle spectrometer is calibrated. 
     Overall, the real image of the light source  11  in the entrance slit is, at first, exactly imaged on the intermediate slit  3  by the optical system of the pre-monochromator  2  and, subsequently, it is imaged on the detector by the optical system of the echelle spectrometer ( FIG. 1 ). 
     By rotating a rotating mirror  33  into the measuring optical path the light of a neon lamp can be focused by means of an imaging element  32  in the plane of the intermediate slit  3 , dispersed in the echelle spectrometer, without preceding separation of the orders, and detected as a spectrum on the detector  5 . 
     The described assembly operates as follows for the wavelength calibration: 
     At first, the intermediate slit  3  is adjusted to a width which is slightly wider than the width of the entrance slit, for example to 30 microns. Then the mirror  33  is rotated about the axis  34  into the light path. Thereby the light from the light source  11  with the continuous spectrum is blocked and the light from the light source  31  with a line spectrum is passed, through the intermediate slit, into the echelle spectrometer. 
     The echelle grating  42  is roughly positioned by rotating about the axis  43 . This means, that a reference line selected for the calibration of a desired measuring wavelength can be unambiguously identified on the linear detector array. This reference line is selected depending on the measuring wavelength from a wavelength catalogue of known reference lines of the line source. The reached position of the reference line on the linear detector array is determined. Then this position is compared with a previously calculated desired position. The desired position is calculated from the difference between the calculated diffraction angle of the measuring wavelength and the reference line. 
     A deviation of the position of the reference line from its desired position is corrected by a fine correction of the angular position of the echelle grating and, thereby, also the measuring wavelength is adjusted to its desired position. This means that, by rotating the grating, the line is shifted to its position. The echelle spectrometer is completely calibrated after this step. A wavelength can be unambiguously and very precisely allocated to each detector element, when light of a known order enters the spectrometer. 
     For the calibration of the prism arranged in the pre-monochromator the entrance slit is again illuminated by the continuum light source. The mirror  33  is again rotated out of the light path and the line spectrum is blocked. Here also the prism is first only roughly positioned. The positioning is effected in a way to ensure that the deviation from the desired measuring wavelength is smaller than the section, detected by the linear detector array, of the free spectral range of the order in the echelle spectrum in which the measuring wavelength is measured with the maximum blaze efficiency. In other words: The “correct” order is coupled into the echelle spectrometer. In this case the spectral portion can unambiguously be identified on the linear detector array. 
     As the intermediate slit is adjusted to a small width, the spectral interval appears as a peak-shaped profile enabling the easy determination of a maximum, a half-width value or the like. It is also possible to calibrate with a wider intermediate slit. In this case the spectral interval appears with a trapezoidal intensity profile, allowing the definition of the position for example as the middle of the half-width value. The spectral interval is selected by the intermediate slit and dispersed at the echelle grating. 
       FIG. 4   a  shows, at a heavily enlarged scale, the situation at the intermediate slit  3  of the pre-monochromator  2  in  FIG. 1 . The case of the ideal adjustment of the pre-monochromator is shown, after the position of the echelle grating  42  ( FIG. 1 ) has been exactly adjusted by means of the internal line source  31 . 
     The measuring wavelength represented by the emission line  82  is positioned exactly in the middle of the intermediate slit with the slit edges  80 . The width of the intermediate slit determined by the distance between the slit edges  80  is selected such that, for the spectral bandwidth of the selected spectral interval its geometric width after the dispersion and imaging on the linear detector array  5  in the echelle spectrometer  4  is smaller than the width of the detector. In the present case the width of the intermediate slit is between 0,05 and 0,1 mm. The width of the linear detector array is about 10 mm. 
       FIG. 4   b  shows the resulting intensity distributions on the linear detector array  5  for the cases of the measurement of the emission line  82  and a continuum  81  according to  FIG. 4   a . The intensity distribution  83  of the emission line  82  is symmetrical to its desired position  86  after the calibration of the echelle spectrometer has been effected. The centre of the half-width value  85  of the essentially trapezoidal intensity distribution  84  for a spectral portion selected from a continuum is exactly equal to the desired position  86  of the measuring wavelength after calibration of the pre-monochromator has been performed. 
     Due to temperature variations or other interference this situation can change. Such a disturbed situation is shown in  FIG. 5 .  FIG. 5   a  shows, at a heavily enlarged scale, the intermediate slit in the case, where the spectrum of the pre-monochromator is shifted with respect to the intermediate slit. The measuring wavelength, represented by the shifted emission line  92 , is no longer located in the middle between the slit edges  80 , i.e. also the position of the continuum  91  is shifted by the same amount. 
       FIG. 5   b  shows the associated intensity values on the linear detector array for the cases of measuring the emission line  92  and the continuum  91 . The centre position of the intensity distribution  93  of the emission line  92  is shifted on the linear detector array by about the same amount  98  from the desired position  86  as the emission line  92  is shifted from the centre between the slit edges  80  of the intermediate slit. However, the shift  99  of the centre of the half-width value  95  of the trapezoidal intensity distribution  94  of the spectral interval selected from a continuum is considerably larger, because the spectral interval selected by the intermediate slit, due to the echelle dispersion, appears to be heavily stretched on the linear detector array. This is the case because the echelle dispersion is considerably higher than the dispersion of the pre-monochromator. The image of a monochromatic emission line, however, is not broadened very much by the echelle dispersion. 
     A shift of such an intermediate image generates completely different results with respect to the intensity distribution on the linear detector array, if the measuring spectrum is either a line spectrum or a continuum. The much larger shift of the half-width value centre of the trapezoidal intensity profile of the continuum as compared to the line shift, which can be explained by the relation between the linear dispersions of the echelle spectrometer and the pre-monochromator, enables the highly accurate positioning of the measuring wavelength in the intermediate slit using the continuum measurement. 
     The assembly is particularity suitable for measuring methods which make use of a light source emitting a continuous spectrum anyway, for example atomic absorption spectroscopy with continuum sources (CSAAS) or with atomic absorption spectroscopy with a background compensation by means of a deuterium lamp.