Measuring wavelength change

An optical wavelength analyser including: an entrance slit (4) for receiving a light beam (3) including signals with various wavelengths and passing the beam at least partly; a diffractor (6, 7, 9) for receiving the passed beam and diffracting the signals dependent on their wavelength; a detector (8) including adjacent detector elements (32, 33, 35, 36, 38, 39) for receiving the diffracted signals and generating their output signals; a processor (21) for determining the wavelengths from the output signals, in which the received light beam has a spatially uniform intensity; the diffractor diffracts each signal on a different detector element subset, consisting of at least a first element (32, 33, 35, 36, 38, 39.) for receiving at least a first signal with a first signal level; the processor determines each signal's wavelength dependent on the first signal level and a calibration value.

FIELD OF THE INVENTION

The present invention relates to an optical wavelength analysis arrangement comprising:entrance selection means for receiving a light beam comprising one or more signals each with its own wavelength and passing at least part of the beam;diffractive means arranged to receive the at least part of the beam and to diffract each of the signals at an angle dependent on the wavelength;detector means comprising a plurality of detector elements arranged to receive the diffracted signals and to generate one or more detector output signals in dependence on the diffracted signals; andprocessing means connected to the detector means for receiving the detector output signals and determining the wavelength of each of the signals.

PRIOR ART

An arrangement as defined in the outset, is known from WO 99/09370, in which such an arrangement is described for usage in Fiber Bragg Grating (FBG) based, structure monitoring applications. In such applications, physical parameters like strain, temperature, pressure and others, are measured by a fiber network, containing a plurality of optical FBG sensors distributed over the structure. FBG sensors are capable of precise and absolute measurement of physical parameters as mentioned above. A FBG sensor installed in a fiber-optic network, reflects light signals that travel through the optical fiber, with a wavelength λ that relates to the FBG periodicity Λ as given by equation [1]:
λ=2nΛ[1],

where λ is the wavelength of the light reflected by the FBG sensor,n is the effective index of refraction of the optical fiber,

and Λ is the periodicity of the FBG sensor, respectively.

Physical parameters that can be measured with FBGs, are related to the reflected wavelength due to the coupling of the physical parameters to the refractive index or the periodicity of the grating.

In structure monitoring applications, measurement of one or more specific physical parameters derived from a signal of an optical sensor in the fiber, is performed at a plurality of locations in the structure (e.g., a fuselage of an aircraft). To identify the origin of signals, each optical sensor generates a signal with a wavelength, specific for that sensor in that location. Thus, each wavelength corresponds to a location in the network. The signal wavelengths are well separated by intervals. The intervals are large enough to prevent overlap of sensor signals, when the response of a sensor changes due to change in a physical parameter, measured at the location of the sensor.

The light beam reflected by the sensors on the fiber network thus comprises a plurality of signals with different wavelengths has to be analysed by e.g., spectrometric means.

In many optical applications like FBG sensor networks, the wavelength of incident light is measured by a spectrometric arrangement with the purpose to determine a physical parameter related to the wavelength.

The method of spectrometry to determine the wavelength of light originated by the optical sensor, is well known. Light, gathered from a source (e.g., an optical sensor), is projected on a grating. Due to the wave characteristics of the light and the periodicity of the grating, the light is diffracted by the grating in one or more orders with their own direction as related to the wavelength(s) of the light, the angle of incidence, and the periodicity of the grating. By measurement of the angle of the diffraction direction(s) in the spectrometer, the wavelength of the light is determined. In spectrometers, as known in the art, the diffracted light is projected on a detector array (e.g., a linear or two-dimensional CCD system). In such an arrangement the position of the projected light on the detector is proportional to the wavelength of the light. The position of the projection is determined by fitting a mathematical model to the intensity data as measured by the detector's elements. The fitting procedure is needed here, since the spatial intensity distribution of the incident light beam that enters the spectrometer's slit is not uniform, because typically, the beam is focussed on the slit to collect as much optical power as possible. The spatial intensity profile of the projected light beam is usually described by a peak-shaped curve. To determine the centroid of the signal, a model describing the shape of the peak is fitted to the measured signal. Due to the non-linearity of the spatial intensity distribution, a useful fit can be accomplished only if a plurality of data points within the profile are measured. To obtain a reasonable accuracy with a resolution higher than the size of a detector element (a “pixel”), the spot projected on the detector array must cover a sufficiently large number of elements in the array, all of which must be sampled in the fitting procedure. Typically, a resolution of approximately 1/10 pixel is possible on a range of 10 pixels.

For a measuring range, which is, for example, 50 times larger, the range on which the light beam is projected must be extended to 50 detector elements. Taking into account a cross-talk separation between signals of about 10 detector elements, in that case a range of more than 60 detector elements on the detector is needed for one signal. Usually, in a spectrometric application, many wavelengths are to be measured simultaneously, which requires that the spectrometer provides a sufficient large detector array. When, for example, 32 signals must be measured simultaneously, the detector array needs approximately 2000 elements.

In WO99/09370 a number of fiber channels each comprising a plurality of signals with different wavelengths are monitored by spectrometric means using a two-dimensional detector array on which the spectra of each fiber channel are projected on elongated regions of the array.

As known to those skilled in the art, in a such FBG sensor network, measurement of wavelengths of optical signals must be carried out at a rate, sufficiently efficient with respect to the number of FBG sensors in the network and the requirements for the type of application e.g., monitoring a structure by the FBG sensor network. By consequence of the method of fitting the peak shapes, a disadvantage of the arrangement of WO 99/09370 to obtain sub-element accuracy is, the rate at which data can be collected and calculated, especially, when high sample rates are required. From the paper of S. Chen et al., “Multiplexing of large-scale FBG arrays using a two-dimensional spectrometer”, SPIE vol. 3330 (1999), p. 245–252, it can be found that with a projection of a signal from a light beam on a field of 7×7 detector elements a resolution of approximately 1/56 of an element can be accomplished.

Due to the large amount of detector elements in an array, in such systems the overall sampling rate is in the order of only 25–100 Hz.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an arrangement and a method to improve the measurement of a signal and the processing of the signal by processing means, in order to determine the wavelength of the signal in a simple, efficient, and fast manner, with high accuracy.

This object is obtained in an optical wavelength analysis arrangement as defined in the outset characterised in that:

the entrance selection means are arranged for outputting a beam with a spatial uniform intensity distribution;the diffractive means are arranged to diffract each of the signals such that each diffracted signal impinges on a different subset of detector elements, each subset comprising at least a first detector element for receiving at least a first signal portion with a first signal portion level;
the processing means are arranged to determine, for each subset the wavelength of the diffracted signal received in dependence on the first signal portion level and a calibration value.

With such an arrangement, the spatial intensity profile of a signal is already well known before detection. This implies that to determine the wavelength of the signal as measured by the detection means, the detection means needs a subset of at least one, at most two of detector elements to measure the signal. In case of a uniform distribution and a known constant optical power level of the light beam, only a single detector element, comprising part of the image of a signal, is sufficient. If however, the optical power level is unknown due to e.g. fluctuations, a subset of two adjacent detector elements, that comprise the complete image of a signal, will suffice to determine the wavelength. Due to the small number of detector elements to be sampled, the calculation of the wavelength of a signal in the arrangement of the present invention is strongly simplified. Accordingly, the computation time involved to measure the wavelength of a signal is strongly reduced.

Consequently, the measurement of wavelengths of a plurality of signals in the arrangement of the present invention requires less time than is known from the prior art. Also, since less detection elements are necessary to measure a signal, the detection means in this arrangement can have a smaller number of detection elements than is known from the prior art. Therefore, the total readout time for the detector array is also reduced.

The present invention also relates to a method of optical wavelength analysing comprising the steps of:receiving a light beam comprising one or more signals each with its own wavelength;diffracting each of the signals at an angle dependent on the wavelength;receiving the diffracted signals and generating one or more detector output signals in dependence on the diffracted signals by a plurality of detector elements;determining the wavelength of each of the signals from the detector output signals characterised by:the beam having a spatial uniform intensity distribution;diffracting each of the signals such that each diffracted signal impinges on a different subset of detector elements, each subset comprising at least a first detector element for receiving at least a first signal portion with a first signal portion level;determining, for each one of the subsets the wavelength of the signal received in dependence on the first signal portion level and a calibration value.

The present invention also relates to a computer arrangement comprising processor means and arranged to receive detector output signals from detector means comprising one or more subsets of detector elements, each subset having at least a first detector element for receiving a first signal portion with a first signal portion level of a signal derived from a beam with a spatial uniform intensity distribution, the arrangement being programmed to determine, for each one of the subsets the wavelength of the signal received in dependence on the first signal portion level and a calibration value.

Moreover, the invention relates to a computer program product to be loaded by a computer arrangement comprising processor means and arranged to receive detector output signals from detector means comprising one or more subsets of detector elements, each subset having at least a first detector element for receiving a first signal portion with a first signal portion level of a signal derived from a beam with a spatial uniform intensity distribution, the computer program product, after being loaded by the computer arrangement, providing the computer arrangement with the capability to determine, for each one of the subsets the wavelength of the signal received in dependence on the first signal portion level and a calibration value.

Furthermore, the present invention relates to an optical wavelength analyser comprising:an entrance selector with a slit for receiving a light beam comprising one or more signals each with its own wavelength and passing at least part of the beam;a diffractor to receive the at least part of the beam and to diffract each of the signals at an angle dependent on the wavelength;a detector comprising one or more pairs of adjacent detector elements arranged to receive the diffracted signals and to generate one or more detector output signals in dependence on the diffracted signals;a processor connected to the detector for receiving the detector output signals and determining the wavelength of each of the signals
characterised in that:the entrance selector is arranged for receiving a beam with a spatial uniform intensity distribution;the diffractor is arranged to diffract each of the signals such that each diffracted signal impinges on a different subset of detector elements, each subset comprising at least a first detector element for receiving at least a first signal portion with a first signal portion level;
the processor is arranged to determine, for each subset the wavelength of the diffracted signal received in dependence on the first signal portion level and a calibration value.

Finally, the invention relates to a data carrier provided with a computer program product as defined above.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to an arrangement and method of simple and fast measurement of a wavelength of a signal from an optical sensor, which may be an FBG sensor, or any other type of optical sensor as known in the art.

In the present invention the signal to be measured is adapted in such a way that the signal has a spatial uniform intensity distribution. The intensity uniformity of the signal simplifies measurement of the wavelength of such a signal. Also, computations relating to the wavelength of the measured signal are simplified by the spatial uniformity of the signal.

InFIG. 1a schematic overview of an arrangement in accordance with the present invention is shown. In the arrangement1, a spectrometer2is used to measure the wavelength of light signals from one or more optical sensors. Light comprising signals from one or more sensors is projected by, for example, a beam expander element, as a beam3on an entrance slit4of the spectrometer2. From the light beam3, a selection is made by the slit4to obtain a smaller beam5with a spatial uniform intensity. By means of a lens6at focal point distance from the slit4, the beam is projected on a dispersive element like a grating7. The grating7diffracts the beam at a diffraction angle relative to the incoming beam. The diffraction angle is dependent on the wavelength, the periodicity of the grating7, and the incident angle of the beam on the grating7. Thus, the beam is dispersed into its spectral components. The spectrum is projected on a detector8, by a lens9at focal point distance from the detector. The detector8, comprising sensor10and control electronics11, is capable of measuring the optical power of the projected spectrum as a function of the position on the detector8, in which the position is correlated to the wavelength of the light. The detector8may be a CCD sensor, a CMOS imager, or any other capable type of sensor as known in the art. Using this optical set-up, the projection of a monochromatic beam on the detector8is an image of the slit opening4. In case the focal length of lens6is equal to that of lens9, the image is a one to one image of the slit opening.

By means of the control electronics, the detector8is connected to a computer20, which records the spectrum measured by the detector, and calculates the wavelengths of signals in the spectrum.

FIG. 2shows a schematic overview of a computer arrangement20comprising processor means21with peripherals. The processor means21are connected to memory units18,22,23,24which store instructions and data, an I/O connection25which connects the processor means21to the control electronics11of detector8, one or more reading units26(to read, e.g., floppy disks19, CD ROM's20, DVD's, etc.), a keyboard27and a mouse28as input devices, and as output devices, a monitor29and a printer30.

The memory units shown comprise RAM22, (E)EPROM23, ROM24and hard disk18. However, it should be understood that there may be provided more and/or other memory units known to persons skilled in the art. Moreover, one or more of them may be physically located remote from the processor means21, if required. The processor means21are shown as one box, however, they may comprise several processing units functioning in parallel or controlled by one main processor, that may be located remote from one another, as is known to persons skilled in the art. Moreover, other input/output devices than those shown (i.e.,27,28,29,30) may be provided.

FIG. 3shows a schematic drawing of a monochromatic signal projected on detector8in accordance with a first preferred embodiment of the present invention. The detector elements32,33,35,36,38,39,41,42of the detector8are arranged in a row. The position of the center of the image of the slit for specific wavelengths λ0, λ1, λ2on the interfaces between detector elements32,33, and35,36and38,39respectively is indicated by dashed vertical lines. On the detector8a signal's image31of the slit opening4is projected by the optics of the spectrometer2as shown inFIG. 1.

The shape of the image31is conformal with the rectangular slit opening4. By design of the projection system, the width B of the image31is smaller than the width W of one of the detector elements32,33. Thus, the image only partially covers both detector elements in the one (e.g., horizontal) direction as indicated by arrow X. The covered length on the elements32,33is denoted as x32, x33respectively. In the other perpendicular vertical direction as indicated by arrow Y, the height H of the image is also smaller than the detector element height h. However, the height H of the image may be larger than the detector element height h, thus covering a detector element completely in this direction.

By a calibration procedure as known in the art, the position of the detector elements is translated to a wavelength scale. By means of this calibration procedure the wavelength of a signal can be determined from the position of the signal's image on the detector elements32,33. InFIG. 3a dashed line denotes the position of the center of the image of the slit for wavelength λ1the interface between elements32and33. Other detector element pairs35,36and38,39with their respective wavelengths λ0and λ2are shown. In between the detector element pairs a spacing, for example by means of one or more unused detector elements41,42, is included, in order to prevent cross-talk of signals. It is to be noted that in stead of unused detector elements41,42an empty gap may be implemented in between the detector element pairs (32,33), (35,36) and (38,39).

FIGS. 4aand4bshow an illustration of the method to calculate the position and wavelength of a signal, projected on detector8, and their respective change, in an arrangement of the present invention.

InFIG. 4aas an example, the centreline of the image31is projected on the detector elements32,33and coincides with the interface of the two detector elements.

If a spatial uniform intensity of the image is assumed, the optical power43,44measured on a detector element32,33will be proportional to the area H*x32, H*x33covered on the element, which is directly proportional to the coverage in the direction X, since the coverage in the direction Y is constant.

The difference between the optical power43,44is determined. In order to correct for fluctuations of the optical power of the beam, the difference is normalised by dividing by the sum of optical power43and44.

In this case, the optical power43,44measured is equal on each element, since x32equals x33. The difference in optical power is zero, which indicates that the image is projected symmetrically on the detector elements, with the centre of the image at the interface between the two elements. Thus, the wavelength of the signal equals λ1.

FIG. 4bshows the projection of image31on detector8for a signal with a wavelength that differs from λ1by an amount δλ.

Because the position of the image31on the detector8is directly proportional (as calibrated) to the wavelength of the projected image, a wavelength difference δλ is directly proportional to a shift δx of the image's centreline. The centreline of the image is projected shifted over a distance δx with respect to the interface between the two elements32,33. Since the illuminated width x′32is not equal to x′33, the normalised difference of optical power45and46measured on the respective detector elements32,33and divided by the sum of the optical powers45and46, is unequal to zero.

In this way, the wavelength of a signal can be measured advantageously, by determining the normalised difference of the optical power received by two adjacent detector elements. The measurement requires a limited number of detector elements and the calculation requires few and simple computations by processor means21with relatively short computation times.

InFIG. 5a block diagram is shown of a method related to the present invention, to be carried out by the processor means21, to determine the wavelength of a signal projected on a pair of detector elements, according to the measurement principle as shown inFIGS. 4aand4b.

In step51, the processor means21enter the procedure by a request to select two adjacent detector elements Ej, Ej+1for measurement.

In steps52and53, the computer addresses the control electronics11to read optical power I(Ej), I(Ej+1) of elements Ej, Ej+1, and to transfer the data in a readable format to the computer.

In step54, it is checked if a signal is present on the selected elements.

If light is measured, then results are calculated. Otherwise, the following steps55–58are skipped.

Step55calculates the optical power difference between the elements Ej, Ej+1, normalised by the sum of the optical power I(Ej) and I(Ej+1)

Step56calculates the corresponding shift δx on the detector8.

In step57the wavelength shift δλ with respect to the wavelength λ1corresponding to the centreline between detector elements Ej, Ej+1is calculated from shift δx by using wavelength calibration data for the detector.

Step58calculates the measured wavelength by adding the wavelength shift δλ to the wavelength λ1, corresponding to the position of the detector elements centreline. This wavelength λ1is derived from the wavelength calibration data for the detector8.

In step59the procedure ends. The processor means21return to the procedure where the request for step51originated from, with the value of the measured wavelength, or if no optical power was measured on the detector elements, with a predetermined value e.g., zero to signal this state.

Change of wavelength as a function of time can be measured by repeating the procedure as shown inFIG. 5, at given time intervals. In each measurement the wavelength of the signal is determined by the procedure ofFIG. 5. The difference δλ relative to the first measured value, as a function of time, can be calculated, stored, and processed further.

FIG. 6shows the results of measuring the wavelength of a signal carried out in an arrangement of the present invention.

A monochromatic light source of which the wavelength changes directly proportional with time, is projected as a parallel beam on the slit opening4of the spectrometer. The intensity distribution of the light beam at the slit opening4has a spatial uniform intensity distribution. InFIG. 6the normalised measured wavelength of the signal from the light source is shown as a function of time.

The sub-detector element accuracy in such arrangement depends on the signal to noise ratio of the detector elements. In this arrangement, using commercially available detectors an accuracy of 1/500 can be obtained, at an overall sampling rate in the order of a several kHz.

It should be understood that measuring wavelength change using a light beam with a spatial uniform intensity distribution can even be done by measurement of the diffracted signal on only a single detector element, without the use (or need) of a second detector element. As illustrated byFIG. 4a, a diffracted signal covers each of the detector elements in a pair just partially. Thus, when the diffracted signal shifts, due to a change of wavelength, the signal's coverage31on the detector element32will change from x32to say x′32, and cause a change in the measured optical power on the detector element from measured optical power level43to measured level45.

If the light beam3has an optical power level which is time invariant, i.e. the intensity of the beam does not change over time, no normalisation step is necessary and it will be sufficient to measure the optical power level of a diffracted signal impinging on a single detector element32. In that case, the shift of the wavelength can be calculated from the difference of the measured optical power level on the single detector element32and the optical power level of a reference diffracted signal, which has been calibrated with respect to its location on the detector element32. Such calibration procedures to obtain the latter optical power level are known to those versed in the art. In fact, in an embodiment for this type of measurement with a single detector element, the second element33in the detector element pair32,33may even be omitted.

Therefore, depending on the application, it may be possible to have only one single detector element, or a few single detector elements set-up separately at various locations in the arrangement to measure wavelength(s).

Also, if the optical power level of the beam is monitored constantly, for example, by a separate detector, measurement on only a single detector element is sufficient to determine the wavelength of a diffracted signal. Here, the shift of the wavelength can be calculated from the quotient of the measured optical power level on the single detector element32and the optical power level of a reference signal, used for calibration. In such an embodiment, the optical power level of the reference signal is to be corrected by dividing by the actual optical power level of the diffracted signal.

FIG. 7shows a schematic drawing of a signal projected on detection means in accordance with a second preferred embodiment of the present invention.

InFIG. 7, entities with the same reference numbers as used in preceding figures, refer to the same entities as shown in those figures. In this second preferred embodiment, the spatially uniform signal is projected as an image31on more than two detector elements of the detector8. Still, in this embodiment the advantage of a significant simple and fast calculation scheme as presented above, can to a large extent be achieved here also.

The detector elements33d,34,35,35a,35b,35c,35d,36,37,37a, of the detector8are arranged in a row. On the detector8a signal's image31of the slit opening4is projected by the optics of the spectrometer2as shown inFIG. 1.

The shape of the image31is conformal with the rectangular slit opening4. By design of the projection system, the height H of the image31is smaller than the height h of the detector elements33d,34,35,35a,35b,35c,35d,36,37,37a. Thus, in the one (e.g., horizontal) direction as indicated by arrow X, the image31covers a plurality of N detector elements35,35a,35b,35c,35d,36of which the two exterior detector elements35,36are only partially covered.

However, the height H of the image may be larger than the detector element height h, thus covering a detector element completely in this direction.

In this second preferred embodiment the center of the image31is determined by weighing of the signals received by the individual detector elements. By weighing, the “center of gravity” of the image intensity distribution can be calculated. The position of the “center of gravity” will depend on the actual intensity distribution. In case of a spatially uniform intensity distribution the result of weighing will be exactly the center position of the image31.

Assuming the image31covers N detector elements Ej. . . Ek(35,35a,35b,35c,35d,36) on the detector, each element Eiwithin that range Ej. . . Ekmeasuring an intensity I(Ei), the (horizontal) center position C of image31can be calculated by:

In case of a uniform intensity distribution of the light beam3, the intensity I measured on the detector elements35a,35b,35c,35d(i.e., N−2 elements Ej+1. . . Ek−1) will be identical for all detector elements Ej+1. . . Ek−1. Only the intensity on the exterior two elements Ej, Ek(35,36) will depend on the actual coverage of respective element Ej, and Ekby the image31. The center position C can then be calculated by:

If the light beam3has an optical power level which is time invariant, the signals of the detector elements35a,35b,35c,35dwill be identical for all these elements35a,35b,35c,35dand also constant over time. In that case, the term (N−2)*I is equal to a constant Q. The center position C can then be calculated by:

Using a calculation scheme according to one of the latter two equations, the center position C of the signal on the detector8can be obtained relatively simply.

FIG. 8shows a schematic drawing of a signal projected on detection means in accordance with a third preferred embodiment of the present invention.

InFIG. 8, entities with the same reference numbers as used in preceding figures, refer to the same entities as shown in those figures. In this third embodiment, the detector elements33d,34,35,35a,35b,35c,35d,36,37,37aare grouped in two imaginary detector elements indicated as A and B.

The spatially uniform signal is projected as an image31on a plurality of N detector elements35,35a,35b,35c,35d,36of the detector8. The two exterior detector elements35,36are only partially covered. The signal of the imaginary detector elements A and B is the sum of the intensities measured on the respective detector elements belonging to imaginary detector element A, and imaginary detector element B, respectively.

The calculation scheme as explained inFIGS. 4aand4bfor two detector elements, can be used in a similar way for determining the position and wavelength of the signal of image31by means of two imaginary detector elements A, B. Although, more detector elements need to be sampled in such a calculation, in this embodiment the advantage of a significant simple and relatively fast calculation scheme as presented above, can to a large extent be achieved here also.

FIG. 9shows an arrangement of the present invention in which multiple sets of signals are measured simultaneously. This arrangement illustrates the possibility of the present invention to provide means of de-multiplexing, an aspect essential to detection systems used in applications where a signal from one of a plurality of optical sensors needs to measured. InFIG. 9, entities with the same reference numbers as used in preceding figures, refer to the same entities as shown in those figures.

The arrangement as shown inFIG. 1can be modified to extend the number of light sources that can be measured by replacing a single slit opening by two or more slit openings.

InFIG. 9, as an example, an arrangement is shown in which the slit opening4is replaced by two slit openings74,75on the spectrometer. On each slit opening74,75a broadband light source comprising multiple signals of different wavelengths (e.g., from a FBG array network) is projected. The light beam generated in each slit opening74,75has a spatial uniform distribution. In a similar way to the embodiment shown inFIG. 1, each signal with its particular wavelength is projected on a pair of detector elements of a detector array8. It is to be noted that due to the displacement of slit opening75with respect to slit opening74, the optical path of light from slit opening74differs from that from slit opening75. As illustrated inFIG. 9this results in an additional separation of the signals originating from the respective slit openings74,75: on the detector8the spectrum from the light source at slit opening74is projected next to the spectrum from the light source at slit opening75.

In the arrangement as shown inFIG. 9, de-multiplexing of a plurality of broadband light sources comprising multiple signals of different wavelengths can be performed on a one-dimensional detector array8according to one of the preferred embodiments as described above.

A plurality of spectra is projected adjacent to each other on the array. When properly calibrated, all spectra can be analysed simultaneously using the corresponding procedure for that particular embodiment.

Finally,FIGS. 10a–10dshow arrangements of the present invention in which alternative optical means are utilised. InFIG. 10a–10d, entities with the same reference numbers as used in preceding figures, refer to the same entities as shown in those figures.

FIG. 10ashows an arrangement of the present invention in which the lenses6,9of the diffractive means are combined in a single lens81.

As is known to those skilled in the art, the lens81may also be combined with dispersive means7into a single optical element as a concave grating82with the same functionality as defined by the diffractive means comprising separate projection means6,9and dispersive means7. The latter arrangement is shown inFIG. 10b.

FIG. 10cshows an arrangement of the present invention, in which prismatic means is applied. Instead of a grating7as dispersive element, a prism83is used to redirect each signal in a direction depending on the wavelength λ1, λ2, λ3of the respective signal.

An unproved sensitivity can be achieved by replacing the grating7by a combination of two or more dispersive elements. An example of such an arrangement is shown inFIG. 10d.

FIG. 10dshows an arrangement of the present invention, in which a combination of dispersive means are utilised. A light beam encompassing a plurality of signals with wavelength λ1, λ2, λ3, respectively, entering through the entrance slit4is projected by lens6on the grating7. Here the light beam is diffracted in directions depending on the wavelengthλ1, λ2, λ3of the respective signals. The diffracted beams are projected on a second dispersive element83, e.g., a prism. The prism redirects each diffracted beam in a direction depending on its wavelength. By dispersing the incoming light beam by more than one dispersive element, a larger separation between individual signals can be achieved. Typically, by this arrangement a higher sensitivity of the instrument can be obtained.

It will be appreciated that other combinations of dispersive elements are possible with similar improved sensitivity.

Moreover, as known to persons skilled in the art, lenses6,9may also be replaced by reflective optical elements, like concave mirrors (not shown), without changing the functionality of the optical wavelength analysis arrangement. It is also noted that in the present invention a holographic element may provide the same functionality as a prism or grating.