WEDELSBÄCK, Haakan (Kamomillgatan 23, Ängelholm, S-262 52, SE)
FOSS ANALYTICAL AB (Pål Anders väg, Höganäs, S-263 21, SE)
JACOB RIIS, Folkenberg (Ved Skovgaerdet 33, Hilleroed, DK-3400, DK)
WEDELSBÄCK, Haakan (Kamomillgatan 23, Ängelholm, S-262 52, SE)
| Claims 1. A spectrometric instrument (2;22;32;46) comprising a radiation source (4;24;34) for generating radiation from within the ultraviolet to infra-red spectral region with which to illuminate a sample (8;42); a complementary detector arrangement (6;38;50) for detecting intensity variations in the radiation from the source (4;24;34) after its interaction with the sample (8;42) and a measurement system (10;40;54) for receiving radiation and generating sample dependent wavelength resolvable intensities at the detector arrangement (6;38;52) chartacterised in that the instrument (2;22;32;46) further includes a fiber Bragg grating (20;44;48;48') constructed to provide a known spectral signature (I1..13) and disposed in the instrument (2;22;32;46) so as to generate at the detector arrangement (6;38;52) wavelength dependent intensity variations characteristic of the spectral signature (I1..13). 2. A spectrometric instrument as claimed in claim 1 characterised in that the instrument (2;22;32;46) further comprises a computer part (18; 18') adapted to receive an output from the detector arrangement (6;38;50) representative of the detected sample dependent intensity variations and to index it against a wavelength value; and to receive an output from the detector arrangement (6;38;52) representative of the intensity variations characteristic of the spectral signature and generate a correction for the wavelength value in dependence thereof. 3. A spectrometric instrument as claimed in claim 1 or claim 2 characterised in that the detector arrangement (6;38;50,52) is cooperable with the measurement system (10;40;54) and the fiber Bragg grating (20;44;48,48') to simultaneously detect the sample dependent wavelength resolvable intensities and the wavelength dependent intensity variations characteristic of the spectral signature. 4. A spectrometric instrument as claimed in claim 3 characterised in that the detector arrangement comprises a single detector (6;38). 5. A spectrometric instrument as claimed in any preceding claim characterised in that the detector arrangement comprises a first detector (50) for detecting the intensity variations in the radiation from the source after its interaction with the sample (42) and a second detector (52) for detecting the wavelength dependent intensity variations characteristic of the spectral signature of the fiber Bragg grating (48). 6. A spectrometric instrument as claimed in claim 5 characterised in that the fiber Bragg grating (48) is constructed to provide the known spectral signature comprising a continuous wavelength dependent variation (I -i 3). 7. A spectrometric instrument as claimed in claim 6 characterised in that the fiber Bragg grating (48) is constructed to provide the known spectral signature as a continuous sinusoidal variation (113). 8. A spectrometric instrument as claimed in claim 6 or claim 7 characterised in that the fiber Bragg grating (48,48') is constructed to provide the spectral signature which additionally comprises one or more distinct spectral features (I 11..12) at one or more known wavelengths superimposed on the continuous, known wavelength dependent variation (113). 9. A spectrometric instrument as claimed in any of the claims 1 to 5 characterised in that the fiber Bragg grating is constructed to provide the spectral signature which consists of one or more distinct spectral features at one or more known wavelengths (1 .. 2)- 10. A spectrometric instrument as claimed in any preceding claim characterised in that the fibre Bragg grating comprises a plurality of individual fiber Bragg gratings (48,48') each constructed to generate a different portion of the desired spectral signature and which are arranged in the instrument to generate these portions sequentially or simultaneously at the detector (52) to together generate a composite spectral signature as the characteristic spectral signature. 1 1. A spectrometric instrument as claimed in any preceding claim characterised in that the radiation source (24) comprises a first radiation supply (26) for illuminating the sample (8) and a second radiation supply (28) for illuminating the fiber Bragg grating (20). 12. A spectrometric instrument as claimed in claim 1 1 characterised in that each supply (26;28) is independently energisable. 13. A method of providing a wavelength drift correction of a spectrometric instrument as claimed in any of the preceding claims characterised by the steps of: a) generating at the detector arrangement wavelength dependent intensity variations characteristic of the spectral signature of the fiber Bragg grating; b) outputting from the detector arrangement a signal representative of the spectral signature of the fiber Bragg grating for receipt by a computer; c) generating in the computer a correction dependent on a comparison of wavelength positions of features of the characteristic spectral signature represented by the received output and known reference wavelength positions of the corresponding features; and d) applying in the computer the correction to spectral data generated from an output from the detector arrangement which is representative of sample dependent intensity variations to thereby compensate for wavelength drift of the instrument. 14. A method as claimed in Claim 13 characterised by an addition step of generating at the detector arrangement wavelength dependent intensity variations characteristic of a sample simultaneously with those generated at step a). 15. A method as claimed in Claim 13 characterised by generating the wavelength dependent intensity variations characteristic of the sample and of the fiber Bragg grating at separate detectors of the detector arrangement. |
Spectronnetric Instrument including a Wavelength Reference
[0001] The present invention relates to a spectronnetric instrument including a wavelength reference.
[0002] Spectronnetric instruments typically comprise a radiation source for
generating radiation from within the ultraviolet to infra-red spectral region with which to illuminate a sample, a complementary detector arrangement for detecting intensity variations in the radiation from the source after its interaction with the sample and a measurement system disposed in a light path between source and detector for receiving radiation from the source and generating sample dependent signals at the detector arrangement from which wavelength resolvable intensity variations may be derived. Spectrometric instruments may also be categorised as pre-dispersive or post-dispersive instruments depending on whether the measurement system is arranged to receive the radiation respectively before or after its interaction with the sample.
[0003] In one known type of spectrometric instrument the measurement system comprises a fixed dispersion element and the detector system comprises a conventional solid state device having an array of individual pixel photodetectors in which pixel location represents wavelength of the dispersed radiation. Such a detector typically is a photodiode array, a charge coupled device or a charge injection device. In another known type of spectrometric instrument the measurement system comprises a scanning dispersion element which cooperates with the detector system to scan individual wavelengths over a single photodetector. In a further type of spectrometric instrument the measurement system comprises an interferometer which generates an interferogram at the detector. A Fourier transform of an interferogram provides the spectral information for subsequent analysis. Modern instruments also include a computer that is receptive of spectral data from the detector and operates to analyze and manipulate spectra. However, irrespective of the type of spectrometric instrument the basic operating principle is the same, namely wavelength dependent intensity variations are generated for subsequent analysis. [0004] Spectrometric instruments typically also comprise means for confining a sample within a light path between source and detector, such as a sample cuvette or flow cell, the material of which additionally interacts with the light as well as, mirrors, prisms, gratings, lenses and other optical elements that may be introduced in the radiation path in order to deflect the radiation in a predetermined manner.
[0005] With improvements in optical elements, detectors, sources and
computerization, there has evolved an ability to perform very high precision measurements which are then employed in the determination of one or both physical and chemical properties of the sample under investigation. Differences in these properties are associated with subtle differences in the absorption spectra that are measured by the
spectrometric instrument. The very small changes in spectral
characteristics cannot effectively be detected directly by personnel, and computerized analysis has become a necessity.
[0006] Unfortunately, the state of one or more of the different components may vary (or drift) over time and/or with the conditions of the surroundings. For example, the relative angular position of a moving diffraction grating may change with time so that the start and stop positions of a wavelength scan may vary or the absolute positions of the wavelengths on the detector elements of a diode array may change with temperature. Such drifts will tend to more or less influence the spectra generated by the spectrometric instrument and hence the accuracy of any sample property determination.
[0007] Typically, the drift of the spectrometric instrument may be described as a wavelength drift as a cause of which the same wavelength may not be represented identically by two otherwise similar spectrometers or in two spectra generated by the same instrument at different times, and an intensity drift in which different intensities are measured at the same wavelengths for the same sample in two otherwise similar instruments or in two spectra recoded at different times by the same instrument.
Therefore, spectrometric instruments generally need wavelength dependent calibration at regular intervals in order to produce precise spectra. [0008] In a typical calibration procedure a reference sample is used which has a characteristic spectral signature comprising distinct spectral features (peaks or troughs in spectral intensity) at a number of known wavelengths. This reference sample is introduced into the beam from the radiation source and the wavelength positions of the spectral features that make up the spectral signature are recorded by the spectrometric instrument and compared with the known (or true) wavelength positions in order to provide a correction function for the wavelength scale of the instrument. This correction function is then applied to correct spectra that are subsequently obtained using the instrument.
[0009] The reference sample is often separate from the spectrometric instrument and is typically shipped to the site of the individual instrument, usually together with a spectrum of the sample or some other indication of the true wavelength positions of the features that constitute the spectral signature of the reference sample. Other solutions involve the use of an internal reference sample which is swapped with the measurement sample when recording a reference spectrum for calibration purposes or the use of an internal or external emission source providing characteristic emission wavelengths.
[0010] It is a clear disadvantage, in means of working time and precision of the existing methods for calibration, that they require the regular introduction of a reference sample into the spectrometric instrument.
[001 1] In an attempt to reduce this problem US 6,420,695 discloses a method for wavelength calibration of a spectrometric instrument, here a tuneable Fabry-Perot interferometer, using the absorbing lines of methane or CO2 as an internal wavelength references. Since these gases are naturally found in ambient air their spectra may be recorded from the air in the light path between source and detector and may be recorded during the acquisition of a sample spectrum without the need either to introduce a reference sample or to acquire a separate reference spectrum. However, this is only useful if the absorbance lines of the naturally occurring gas lie within a spectral region of interest and if the gas is present in amounts sufficient to produce an absorption line which is not masked by the sample spectrum. This typically not the case when the spectral instrument is intended for use in the near infra red spectral region.
[0012] According to one aspect of the present invention there is provided a
spectrometric instrument comprising a radiation source for generating radiation from within some or all of the ultraviolet to infra-red spectral region with which to illuminate a static or moving sample which may or may not be confined by a sample chamber, a complementary detector arrangement for detecting intensity variations in the radiation from the source after its interaction with the sample and a measurement system for receiving radiation from the source and generating sample dependent wavelength resolvable intensities at the detector arrangement. The instrument further comprising a fiber Bragg grating constructed to provide a known spectral signature and disposed in the instrument so as to generate at the detector arrangement wavelength dependent intensity variations characteristic of the spectral signature.
[0013] A computer part may also be included as a component of the instrument which is adapted to receive an output from the detector arrangement representative of the detected sample dependent intensity variations and to index it against a wavelength scale, for example based on pixel location in a detector diode array (DDA) based instrument or angle of rotation in a moving diffraction element based instrument. The computer part being further adapted to receive an output from the detector arrangement representative of the intensity variations characteristic of the spectral signature and generate a correction for the wavelength scale in
dependence thereof. In this manner spectral information generated by the instrument may be standardised.
[0014] By configuring a spectrometric instrument according to the present
invention in a manner by which the detector arrangement cooperates with the measurement system and the fiber Bragg grating to simultaneously detect the sample dependent wavelength resolvable intensities and the wavelength dependent intensity variations characteristic of the spectral signature then a wavelength calibration may be obtained concomitantly with the acquisition of a sample spectrum. [0015] By employing a radiation source which comprises a first radiation supply for illuminating the sample and a second radiation supply for illuminating the fiber Bragg grating then the illumination of the sample and of the fiber Bragg grating may be selected, temporally or spectrally, independent of one another. For example each supply may be independently and selectably energisable so that the characteristic spectral signature of the fiber Bragg grating is collected less frequently than sample spectral features, such as only in conjunction with the collection of spectral features from a reference sample. This would enable both the photometric response of the instrument from the reference sample and a wavelength calibration from the fiber Bragg grating to be obtained at the same time.
[0016] By employing a detector arrangement which comprises a first detector for detecting the intensity variations in the radiation from the source after its interaction with the sample and a second detector for detecting the wavelength dependent intensity variations characteristic of the spectral signature then the two sets of variations are not superimposed on the same detector so that the wavelength positions of the fiber Bragg grating spectral signature may be chosen independently of the sample spectral features. Usefully, the fiber Bragg grating may be designed to provide an output at the second detector that varies continuously and in a known manner, for example sinusoidally, across substantially the entire wavelength region of interest in the analysis of the sample. In this manner the angular position of a moving grating may be monitored continuously. Additionally, such a fiber Bragg grating may also be constructed to generate one or more sharp peaks superimposed on the continuous variation in order to mark precise and known wavelength positions, such as to mark start and end wavelengths of a desired spectral range. Such a fibre Bragg grating may conveniently comprise a plurality of fibres where each is constructed to generate a different portion of the desired spectral signature and which may be arranged to generate these portions sequentially or simultaneously at the detector to together generate a composite spectral signature. Indeed the use of such an arrangement need not be confined to the generation of the above described superimposition of peaks but may also be employed for the generation of any desired characteristic spectral signature. Since each fibre may be designed independently of one another in order to generate a desired spectral signature then the signature may be made more complex without necessarily increasing the complexity of the design of the individual fibre.
[0017] According to a second aspect of the present invention there is provided a method of providing a wavelength drift correction of a spectrometric instrument according to the first aspect of the present invention and comprising the steps of: a) generating at the detector arrangement wavelength dependent intensity variations characteristic of the spectral signature of the fiber Bragg grating; b) outputting from the detector arrangement a signal representative of the spectral signature of the fiber Bragg grating for receipt by a computer; c) generating in the computer a correction dependent on a comparison of wavelength positions of features of the characteristic spectral signature represented by the received output and known reference wavelength positions of the corresponding features; and d) applying in the computer the correction to an output from the detector arrangement dependent on sample dependent intensity variations to thereby compensate for wavelength drift of the instrument.
[0018] The method may also comprise an addition step of generating at the
detector arrangement wavelength dependent intensity variations characteristic of a sample simultaneously with those generated at step a).
[0019] Furthermore, the wavelength dependent intensity variations characteristic of the sample and of the fiber Bragg grating may be generated at separate detectors of the detector arrangement. This may permit the selection of the characteristic signature of the fiber Bragg grating to be made substantially independently of a consideration of the expected variations characteristic of the sample. Additionally this may permit the selected detection of the intensity variation that is characteristic of the Fiber Bragg grating.
[0020] Exemplary embodiments of the present invention will now be described with reference to the drawings in the accompanying figures, of which: Fig.1 shows a first example of a spectrometric instrument according to the present invention in a post-dispersive measurement configuration; Fig.2 shows a second example of a spectrometric instrument according to the present invention in a post-dispersive measurement configuration; Fig.3 shows a first example of a spectrometric instrument according to the present invention in a pre-dispersive measurement configuration; Fig.4 shows a second example of a spectrometric instrument according to the present invention in a pre-dispersive measurement configuration; Fig.5 shows a representation of a plot of a wavelength dependent intensity variation recordable by a spectrographic instrument according to any of the spectrographic instruments illustrated in Figs. 1 and 3; Figs. 6 show at Fig. 6a a representation of a plot of a sample generated wavelength dependent intensity variation and at Fig.6b a representation of a plot of a wavelength dependent intensity variation using a first example of fiber Bragg grating recordable using the spectrographic instrument illustrated in Figs. 2 and 4; and Figs. 7 show at Fig. 7a a representation of a plot of a sample generated wavelength dependent intensity variation and at Fig.7b a representation of a plot of a wavelength dependent intensity variation using a second example of fiber Bragg grating recordable using the spectrographic instrument illustrated in Fig. 4.
[0021] Considering now the spectrometric instrument 2 which, in this
embodiment, comprises a radiation source 4 for generating a beam of radiation 12 and a complementary detector arrangement 6. The source 4 is selected to generate radiation from the whole or part of the ultra-violet to infra-red regions of the electromagnetic spectrum dependent on the nature of analysis of the sample to be performed. According to the present exemplary spectrometric instrument 2 a sample 8 is, in use, disposed between the source 4 and a measurement system 10. The source 4 and detector arrangement 6 may be mutually arranged to facilitate any one of known spectral measurement methodologies, such as transmission, reflection, transflection or lateral transmission methodologies.
[0022] In this post-dispersive measurement configuration the measurement
system 10 is disposed to receive the radiation beam 12 from the source after it has interacted with a sample material 8. The measurement system 10 may be any measurement system which is adapted to receive the incident beam 12 and provide an output beam 1 12 to the detector arrangement 6 to generate a signal that can be resolved to provide an indication of wavelength dependent intensity variations within the beam 12 that is received by the measurement system 10. Such a system 10 may, for example, comprise a fixed or movable diffraction grating spectrometer or a Fourier Transform interferometer as are known in the art.
[0023] In the present example the signal from the detector arrangement 6 is
passed to a computer for resolution. This computer may either be a unit 18 that is integral with and forms a part of the spectrometric instrument 2 or, as is also illustrated in Fig. 1 , be a unit 18' that is external to, even geographically remote from, and is connectable with the detector arrangement 6, for example via the internet or via Local or Wide Area Networks.
[0024] The spectrometric instrument 2 according to the present invention also comprises a fiber Bragg grating 20 that is constructed to provide a characteristic spectral signature of known form. The grating 20 is disposed to receive a portion of the radiation beam 12' generated by the radiation source 4 and to transmit it to the measurement system 10 simultaneously with the beam 12 that has interacted with a sample 8. In the present embodiment and by way of example only the fiber Bragg grating 20 is disposed in the spectrometric instrument 2 so that the transmitted beam portion 12' does not pass through the sample 8. In other embodiments the fiber Bragg grating 20 may be disposed in the spectrometric instrument 2 so that the transmitted beam portion 12' will pass through the sample 8. However, the amount of energy that may be transmitted by a single-mode (or few-mode) optical fiber of the type from which the fiber Bragg grating 20 is constructed is typically orders of magnitude less than the energy that can be transmitted in multi-mode fibers or "free-space" beams which comprise the radiation beam 12 illuminating the sample 8. Thus if the fiber Bragg grating 20 is positioned directly in the beam path, before or after the sample 8, the spectral modulation may not be resolvable in the measured spectrum at the detector arrangement 6 since it is too small compared with the S/N ratio and this arrangement is, for many measurement situations, preferably avoided.
[0025] It will be appreciated by those skilled in the art that the spectrographic instrument 2 is comprised of several individual components, which in the present exemplary embodiment can be considered as the source 4, the detector arrangement 6, the measurement system 10, the computer unit 18 or 18' and the fiber Bragg grating 20. Each of these may be considered as performing individual functions that collectively provide the functionality of the instrument 2 and some or all of which may be included in a single instrument housing or as separate, interconnectable units without departing from the invention as claimed.
[0026] The measurement system 10 of the present embodiment operates to
provide an output 1 12' which is associated with the fiber Bragg grating 20 to the detector arrangement 6 simultaneously with and superimposed on the output 1 12 from the incident beam 12 that is received after its interaction with a sample 8. As with the output 1 12, the detector arrangement 6 is configured to receive the output 1 12' associated with the fiber Bragg grating and to generate a signal which is resolvable by the computer 18, 18' to generate an indication of a wavelength dependent intensity variation which is characteristic of the spectral signature of the fiber Bragg grating 20.
[0027] An example of the superimposed outputs 1 12 and 1 12' incident on the detector arrangement 6 is provided graphically in Fig. 5. A plot of intensity variation (I) versus wavelength (λ) positions of the spectral signatures of each of the sample (solid line) and the fiber Bragg grating 20 (broken line) is illustrated in Fig. 5. By way of example only the fiber Bragg grating 20 of the present embodiment is shown as being designed to provide three sharp peaks l-i , I2, I3, distributed across the wavelength range of interest for the spectral signature of the sample. Two peaks 11 , 13 are selected to mark the beginning (l-i) and the end (I3) respectively of the wavelength range of interest and, in this example, the third peak (I2) is selected to lie in a known region of the spectral signature of the sample that provides substantially no spectral information of interest for the analysis of the sample. [0028] It will be appreciated by those skilled in the art that the outputs monitored by the detector 6 are due to changes in the absorption properties of the sample/grating through which the associated beam passes and that although the variations are here illustrated as increases of intensity they may also be represented as decreases in intensity. This largely depends on the Bragg grating design employed. For sake of clarity all references within this description refer to increases in intensity but apply equally to decreases in intensity.
[0029] The computer 18,18' is programmed to provide a correction for the
wavelength drift associated with the instrument 2 which correction is dependent on the resolved spectral signature of the fiber Bragg grating 20. This correction is usable within the computer 18, 18' to correct the wavelength positions of the wavelength dependent intensity variations within the beam 12 that is received by the measurement system 10 after its interaction with a sample 8. This correction may be performed in a number of ways known in the art in relation to the application of other, known wavelength standards to spectral correction. For example a calibration equation may be generated, such as by linear or non-linear regression, based on an association of peaks detected from the fiber Bragg grating 20 with the known reference positions of those peaks. This equation may then be applied to a spectrum measured from a sample material in order to provide a wavelength standardised spectrum.
[0030] A second exemplary embodiment of post-dispersive measurement
configuration of a spectrometric instrument 22 according to the present invention is illustrated in Fig. 2, in which elements in common with the embodiment of Fig. 1 are given the same reference numeral.
[0031] The spectrometric instrument 22 here is shown to comprise a source of radiation 24 which consists of a first radiation supply 26 and a second radiation supply 28 and a complementary detector arrangement 6. The first radiation supply 26 is provided to generate a beam of radiation 12 with which to illuminate a sample 8. The second radiation supply is to generate a beam of radiation 30 which is received by a fiber Bragg grating 20. The two supplies 26, 28 may be configured in one embodiment to be operated independently of one another such that the spectral signature of the Bragg grating 20 is superimposed less frequently on the spectra from a sample in the sample chamber 8. In this manner the spectral signature of the fiber Bragg grating 20 may be superimposed only on a portion of the spectrum collected from the sample 8 or may be superimposed only on spectra from reference samples. In the latter case this facilitates the simultaneous collection of data for both corrections of the photometric response, using the reference sample, and of the wavelength drift, using the fiber Bragg grating 20, of the instrument 22.
[0032] It will be appreciated this control of the generation of data representative of the spectral signature characteristic of the fiber Bragg grating 20 may be achieved in other manners in other embodiments, for example by using a source having a single supply (as element 4 in Fig. 1 say) and a
mechanical chopper disposed in the portion of the radiation beam (12' say of Fig. 1 ) from the source intended for the Bragg grating 20.
[0033] Similar to the configuration of the spectrometric instrument 2 of Fig. 1 the fiber Bragg grating 20 is disposed within the instrument 22 to transmit radiation 30 from the second radiation supply 28 of the radiation source 24 to the measurement system 10 without it passing through the sample 8.
[0034] The measurement system 10 is configured to provide an output 130 which is associated with the fiber Bragg grating 20 to the detector arrangement 6 simultaneously with or separately from that output 1 12 from the incident beam 12 received after its interaction with the sample 8.
[0035] As with the output 1 12, the detector arrangement 6 is configured to receive the output 130 that is associated with the fiber Bragg grating and to generate a signal which is resolvable by the computer 18, 18' to derive an indication of a wavelength dependent intensity variation which is
characteristic of the spectral signature of the fiber Bragg grating 20. This indication is then used within the computer 18 to correct any wavelength drift of the instrument 22, such as in a manner described above with respect to the instrument 2 of Fig. 1.
[0036] Considering now an embodiment of a pre-dispersive spectrometric
instrument 32 which is illustrated in Fig.3. The instrument 32 comprises a radiation source 34 for generating a beam of radiation 36 and a
complementary detector arrangement 38. The instrument also comprises a measurement system in the form of a monochromator 40. In use a sample 42 is disposed relative to the source 34 and detector arrangement 38 so that the beam 36 is received by the monochromator 40 before its interaction with a sample in the sample chamber 42. In this conventional pre-dispersive measurement arrangement the monochromator 40 acts to separate the incident beam 36 into discrete wavelength regions and to provide in turn each of these discrete wavelength regions as an output beam 136 for interaction with a sample in the sample chamber 42 before being detected by the detector arrangement 38.
[0037] A computer 18 may be provided as a part of the spectrometric instrument 32 to receive an output signal from the detector arrangement 38 or may be provided as a separate unit 18' that is connectable to the detector arrangement 38 to receive such an output.
[0038] According to the present invention a portion 136' of this output beam 136 is provided simultaneously directly to a fiber Bragg grating 44. After passing through the fiber Bragg grating 44 the beam 136' is transmitted to the detector arrangement 38 where it is detected simultaneously with the beam 136 of the same wavelength region that has passed through the sample 42. The detector arrangement 38 is configured to generate a signal which is resolvable by the computer 18, 18' to generate an indication of a wavelength dependent intensity variation which is characteristic of the spectral signature of the fiber Bragg grating 44 simultaneously with an indication of a wavelength dependent intensity variation which is characteristic of the sample 42.
[0039] As discussed above with respect to the other embodiments the computer 18, 18' is configured to employ the so resolved spectral signature characteristic of the fiber Bragg grating 44 in generating a correction which is usable in correcting the wavelength positions of resolved spectral signatures characteristic of the sample 42.
[0040] A second embodiment of a pre-dispersive measurement configuration is illustrated in Fig. 4 in which components similar to those of the embodiment illustrated in Fig. 3 are given the same reference numerals.
[0041] In the present example a spectrometric instrument 46 comprises a source of radiation 34, a measurement system comprising a monochromator 40 and a fiber Bragg grating 48. A detector arrangement is also provide as a part of the instrument 46 and comprises a first detector 50 and a second detector 52. The first detector 50 is disposed to receive a radiation beam 136 output by the monochromator 40 after passing through a sample 42. The second detector is adapted to receive a portion 136' of the radiation beam that has been output directly from the monochromator 40 via the fiber Bragg grating 48. As illustrated in the present embodiment it may be desirable to house the monochromator 40, the fiber Bragg grating 48 and the second detector 52 in a same measurement instrument system housing 54.
[0042] As with the previous embodiments a computer 18 may be provided as a part of the spectrometric instrument 46 to receive an output signal from the detector arrangement 50, 52 or may be provided as a separate unit 18' that is connectable to the detector arrangement 50, 52 to receive such an output.
[0043] In the present embodiment the first detector 50 is configured to generate an output signal which is resolvable by the computer 18, 18' to generate an indication of a wavelength dependent intensity variation which is characteristic of the sample 42. The second detector 52 is configured to generate an output signal which is resolvable by the computer 18,18' to generate an indication of a wavelength dependent intensity variation which is characteristic of the spectral signature of the fiber Bragg grating 48.
[0044] With this detector arrangement 50, 52 the signals representing the incident radiation beams 136, 136' respectively are not superimposed at the same detector. This has an advantage that the characteristic spectral signature of the fiber Bragg grating 48 may be selected independently of the features in the spectral signature of the sample. The fiber Bragg grating 48 may, for example, be designed to provide a plurality of sharp peaks equally spaced throughout the wavelength region of interest for the analysis of a sample. An example of signals representing the incident radiation beams at the first detector 50 and the second detector 52 is represented graphically in Figs. 6 as plots of intensity variation (I) versus wavelength (λ) positions of the spectral signatures of each of the sample 42 (Fig. 6a) and the fiber Bragg grating 20 (Fig 6.b). This will provide a number of reference wavelength positions (peaks I4..10 in Fig 6b) with which a wavelength calibration may be determined having an improved accuracy over those employing only one or a few peaks (peaks 11. 3 say of Fig. 5).
[0045] Alternatively, or additionally the fiber Bragg grating 48 may, for example, be designed to provide a continuously, such as sinusoidally, varying intensity variation as its characteristic spectral signature. Such a signal may be employed to continuously monitor the angular position of a movable dispersion element, for example a grating, which may be employed in some examples of the monochromator 40. An example of signals representing the incident radiation beams at the first detector 50 and the second detector 52 is represented graphically in Figs. 7 as plots of intensity variation (I) versus wavelength (λ) positions of the spectral signatures of each of the sample (Fig. 7a) and the fiber Bragg grating 20 (Fig 7.b). As can be seen, this example of the fiber Bragg grating 48 is designed to provide a characteristic spectral signature consisting of a number of relatively sharp peaks In , 112 superimposed on a periodic, sinusoidal intensity variation 113.
[0046] In one exemplary embodiment the fiber Bragg grating may comprise a plurality of individual fibres, here a parallel arrangement of two fibres 48, 48', designed to together generate the characteristic spectral signature. In the present example the first fiber, 48 say, is designed to provide the continuously varying signal (113) and the second fiber, 48' say, is designed to provide the number of relatively sharp peaks (l n J i2)- In the present example the fibre Bragg grating 48,48' are illuminated simultaneously by the monochromator 40 and their individual characteristic spectral signatures are superimposed on one another at the detector 52 to generate a composite spectral signature.
[0047] In a further configuration of the present embodiment the two detectors 50, 52 may be independently controllable to generate signals representative of the characteristic spectral signature of the fiber Bragg grating (48) at selected intervals, such as only in conjunction with measurements on a reference sample.
[0048] The fiber Bragg gratings 20, 44, 48 according to the present invention may all be constructed to provide a desired characteristic signature using techniques well known in the art as outlined below.
[0049] The optical fiber employed in the fiber Bragg grating is designed such that the refractive index of its core changes with intensity and duration of its exposure to a particular wavelength of intense radiation, typically ultraviolet (UV) radiation. Generally, fiber Bragg gratings are created by inscribing a systematic periodic or aperiodic variation of refractive index into the core of this type of optical fiber, such as a germanium-doped silica fiber, using an intense source of optical radiation.
[0050] Three main processes are used to manufacture the grating, namely
interference; masking and point-writing. Uniform gratings may be constructed using a two-beam interference process. In this process the source, typically a laser source, is split into two beams which interfere with each other to create a periodic intensity distribution. The refractive index of the photosensitive fiber changes according to this periodic intensity distribution. A photomask having the intended grating features may also be employed to construct the fiber Bragg grating. The photomask is placed between the UV light source and the photosensitive fiber. The variation in the opacity of the photomask then determines the grating structure.
Photomasks are particularly useful in the manufacture of fiber Bragg gratings having additional spectral features beyond those which can be produced from a grating manufactured using an interference pattern.
[0051] A single UV laser beam may also be used to 'write' the grating into the fiber point-by-point. Here, the laser has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period fiber gratings. Point-by-point is also used in the fabrication of tilted gratings.
[0052] When designing the fiber Bragg grating wavelength reference standard according to the present invention some parameters of the spectrum are particularly important to consider. In order to get sufficient power transmitted, it is advantageous if the band width of the fiber Bragg grating peaks are as large as possible and the spectral shape of the fiber Bragg grating peaks are smooth and single-peaked (e.g. a Gauss or Lorentz shape), such that the convolution of the fiber Bragg grating peaks and the instrument line shape also has a well-defined peak. Another desired feature is a large ratio between the peak transmission, and the residual transmission between the peaks. Any residual transmission in the fiber Bragg grating tends to act like stray light when superimposed on the sample spectra. This contribution may be measured separately and subtracted from the spectra, but optimally the residual transmission is below the noise level such that it need not be considered.
[0053] It is also known that strain on the optical fiber and changes in temperature can influence the wavelength position(s) of the reference peak(s). It is therefore advantageous that the fiber Bragg grating is mounted in the instrument in an unstrained condition. The temperature of the fiber may be controlled to maintain it at a known, fixed value or a calibration of reference wavelength shift with temperature may be provided for use within the computer as a correction factor to be applied to the measured wavelength position.
[0054] It will be understood that the sample 8,42 may be confined in to flow
through a flow cell element of a sample flow system, such as flow injection system or through a section of a process pipeline in an industrial process environment or may be held stationary in a sample cuvette during measurement without departing from the present invention. Alternatively the sample may be a free-standing sample on which measurements may be made.