BROADBAND OPTICAL DETECTOR WITH ADAPTED CHANNELS FOR SPECIFIC SPECTRAL DETECTION

FIELD OF THE INVENTION

The present invention generally relates to specialized spectroscopic detection techniques, and more particularly to a broadband optical detector with adapted channels for specific spectral detection.

BACKGROUND OF THE INVENTION

Such specialized detection techniques as, for instance, for non-invasive glucose detection, are often based on optical spectroscopy. This spectroscopic technology requires an optical detector for multiple wavelengths.

In many applications, indeed, a spectroscopic analysis is performed to identify certain substances or to analyse their concentration. For this technology, an absorption spectrum is recorded and analysed. Usually, some parts of the spectrum are of higher interest and some parts contain less information. Typically, a high resolution will usually be required at those parts of the optical spectrum where high variations can be observed. In the prior art, usually detector arrays are used that have channels with equal size and sensitivity. A high resolution for the whole spectral range requires expensive optical detectors that are usually difficult to manufacture.

In US 5,857,462 A, there is described a method and an apparatus for systematic wavelength selection for improved multivariate spectral analysis. The method includes selecting multiple wavelength subsets, from the electromagnetic spectral region appropriate for determining the known characteristic of the analyte (such as glucose), for use by an algorithm wherein the selection of wavelength subsets improves the model fitness of the determination for the unknown values of the known characteristic. However, in such applications as glucose detection, for instance, only small parts of the spectrum contain the most important information but also the larger part of the remaining spectrum needs to be measured for accurate analysis. The solution disclosed in US 5,857,462 A is therefore not suitable.

SUMMARY OF THE INVENTION

An aim of the present invention is to overcome the above mentioned disadvantages of the prior art.

To achieve this aim, the suggested solution is based on a resolution of the optical detection system that is not equally distributed over the whole spectral range. The distribution of the different resolutions will be adapted to the spectrum of the desired analyte.

According to a first aspect of the present invention, there is thus proposed an optical detector for spectroscopic analysis of a substance within a given spectrum, which comprises a detection arrangement having at least a first resolution at a first part of the spectrum and a second resolution, different from said first resolution, at a second part of the spectrum, different from said first part of the spectrum.

The invention makes use of high resolution only at the important parts of the spectrum, and thus results in less overall resolution. Furthermore, less pixel (as suggested in one embodiment of this invention) require less readout time and thus allow for higher processing speed.

Another advantage of one or more embodiments is a faster data analysis, as less data points are to be processed.

A second aspect of the present invention relates to a non-invasive glucose detection system based on optical spectroscopy which incorporates a system according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements and in which:

- Figure 1 is a chart showing an example of spectrum that would require higher resolution at certain parts;

- Figure 2 is a diagram illustrating a modified photo-detector used in a first embodiment of the optical detector; - Figure 3 is a diagram illustrating the first embodiment of the optical detector;

- Figure 4 provides front and top views of a specialized grating used in a second embodiment of the optical detector;

- Figure 5 is diagram showing the second embodiment of the optical detector; - Figure 6 is a diagram illustrating a third embodiment of the optical detector;

- Figure 7 is a diagram illustrating a first variant of a fourth embodiment of the optical detector;

- Figure 8 gives curves illustrating the operation of the optical detector according to Figure 7; - Figure 9 is a diagram illustrating a second variant of the fourth embodiment of the optical detector;

- Figure 10 gives curves illustrating the operation of the optical detector according to Figure 9; and,

- Figure 11 is a diagram illustrating a fifth embodiment of the optical detector.

DESCRIPTION OF PREFERRED EMBODIMENTS

The description that follows is intended at defining the main principles of the proposed detection system for spectroscopy with non-equally distributed spectral resolution.

By way of example, it will be considered the case of a non- invasive glucose detection system. A spectroscopic analysis is performed on an absorption spectrum to analyse glucose concentration. Glucose concentration monitoring requires an optical detector for multiple wavelengths, within the spectral range [1 μm ; 2,5 μm].

With reference to the chart in Figure 1, which gives the absorptivity (A, in AU/mm) as a function of the wavelength (λ, in μm) within the prescribed spectral range, usually, some parts of the spectrum are of higher interest because they exhibit high variation, and some parts contain less information.

Typically, a higher resolution will then be required at those parts of the optical spectrum, like here the parts 101 and 102 around wavelength values λ=l,50 μm and λ=2,00 μm, where high variation can be observed. Amongst several ways to build the described invention, five examplary embodiments will be described in detail below. All are aiming for an optical detecting

system that has different resolution at respective parts of the recorded spectrum. The approaches to this solution are different in the sense that either the detector array itself is different from currently available ones or some specialized optics/mechanics are used in combination with a readily available detector array.

First Exemplary Embodiment: Large Standard Detector with Binned Pixels

The more pixels a photo detector has the more expensive it becomes, because more pixel errors might occur. As stated above, however, not all pixels are needed for the optical detection.

Thus, in this first embodiment, a standard multi-pixel detector is used, where some pixels are binned together according to the requirements of the desired spectrum.

For instance the system may comprises a detector array having a first group of adjacent pixels which are binned together (defining a region of the detector surface having lower resolution), and a second group of adjacent pixels, different from the pixels of said first group of pixels, which are not binned together (defining a region of the detector surface having higher resolution, corresponding to the resolution of the standard, i.e., with un-binned pixels, detector array).

Alternately or additionally, the detector array may have a first group of a number Nl of adjacent pixels which are binned together, and a second group of a number N2 of adjacent pixels, different from the pixels of said first group of pixels, which are also binned together, where the N2 is however different from Nl. If N1>N2, the first group is defining a region of the detector surface with lower resolution than the second group, and vice versa.

With reference to Figure 2, an example modified detector array that is based on a linear 16-pixel detector array (with pixels bO to bl5), has at least one group of pixels which are binned together, in a region of the detector surface that corresponds to a part of the spectrum of less interest. In the example as shown, there are three such groups of pixels, corresponding to pixels bθ-b2, b6-b7 and bl2-bl5, respectively. The detector is thus so arranged and operated as if it had only 10 effective pixels, namely pixels bO' to b9'. This way, the detector array exhibits the highest resolution (which is the resolution of the standard 16-pixel detector array) in the other regions where pixels are

not binned together, corresponding to the parts of the spectrum of higher interest, namely parts 101 and 102. These parts of the spectrum are sensed by effective pixels b2'-b3' and b5'-b8', respectively. The other parts of the spectrum are sensed, with a lower resolution, by the remaining effective pixels. Stated otherwise, all parts of the desired spectrum are covered, but the optical detecting system has a non-equally distributed spectral resolution over said spectrum.

Figure 3 shows an example of use of a modified detector array in accordance with this first embodiment. The incoming light, with optical wavelengths in the spectrum of interest, is input via an entrance slit Sl. The incoming light is then mirrored to an optical unit for the dispersion thereof, like for instance a grating Gl, through a first concave mirror Ml. The grating Gl is a standard grating, namely it has a constant refractive index. The dispersed light reflected by the grating is then mirrored to a detector array Dl through another concave mirror M2.

The detector array Dl is designed as a modified detector array in the sense indicated above.

Second Embodiment: Specialized Grating

The diffraction of incoming light onto the different photo-detector pixels may be achieved by means of a grating, like indicated above with reference to the first embodiment. Usually, gratings with constant dispersion are used. This results in an equal distribution of the spectrum over the photo-detector surface.

According to a second embodiment, however, a specialized grating with non- equal refractive index can be used to achieve non-equal distribution of the dispersed light.

An example is depicted in Figure 4, on which the bottom part shows a top view of the specialized grating G2 and the upper part shows a front view of the specialized grating G2. According to this example, the specialized grating has a first grating portion 41 with a lower pitch value and a second grating portion 42 with a higher pitch value. Portions 41 and 42 extend in respective directions which form a slight angle one with the other, to compensate for the difference of inclination of the outgoing rays of the dispersed light.

An example of use of the adapted grating G2 is given in Figure 5. This figure is the same as Figure 4, in which the standard grating Gl is replaced by the adapted grating G2, and in which the modified detector array Dl is replaced by a standard (i.e., un-modified) linear detector array D2. While a standard grating would show equally distributed wavelengths, the specialized grating G2 exhibits more dispersion for some wavelengths and less dispersion for other wavelengths. When combined with the standard detector array D2, the adapted grating G2 form a detection system for spectroscopy having different resolutions for different parts of the spectrum that is covered. More than one specialized grating like grating G2 can be arranged in the optically path, whereby the each of them contributes to the technical effect.

Third Embodiment: Curved Mirror

According to a third embodiment, the different resolutions may also be obtained by changing the focussing of the dispersed light in such a way that some parts of the spectrum are imaged to a greater part of the photo-detector surface while other parts (of equivalent spectral width) are imaged to a smaller part of the photo-detector surface.

With reference to Figure 6, this embodiment comprises an optical unit for the dispersion of the incoming light, like a prism or a grating, which bear reference Gl in the figure. This diffracted light is then imaged to the detector array D2 by means of an optical device CMl having different focal areas, like an adaptive optic or a curved mirror, through concave mirrors M3 and M4 interposed on the optical path.

The detector array may be a conventional (linear) photo-detector array D2, like the one used in the second embodiment (Figure 5). Such combination of the optical device CMl and of the standard detector array

D2 forms a detection system for spectroscopy having different resolutions for different parts of the spectrum that is covered.

More than one optical device like device CMl may be inserted in the optical path between the tool for dispersion and the linear detector array.

Fourth Embodiment: Movable Grating or Movable Detector

Instead of having a detector array, a fourth embodiment uses an optical unit for dispersion (grating or prism) Gl optically coupled with a single spot photo detector D3, as depicted in Figure 7. Control means are provided which are so arranged and operated that different parts of the spectrum are recorded by moving the detector along the dispersed spectrum such that different parts of the spectrum are focussed onto the photo detector.

Controlling the detector in such a way that it is put in movement at a non- constant velocity will do the adaptation of the recorded spectrum to the spectrum of the desired analyte. This will result in higher resolution (higher integration time per spatial unit) for parts with slow movement and lower resolution (less integration time per spatial unit) for regimes with faster movement.

The curves in Figures 8(a), 8(b) and 8(c) give the shape of the intensity I of the light along the recorded spectrum, the displacement x(t) of the movable single spot photo-detector D3 as function of time, and the recorded signal S(t) as function of time, respectively.

In a variant illustrated by Figure 9, the control means are operated such that different parts of the spectrum are recorded by moving the unit for dispersion Gl

(instead of the photo-detector D3), at respective velocities. For instance, the unit for dispersion Gl may be a rotating grating which is rotated at non-constant velocity around an axis during the recording.

The curves in Figures 10(a), 10(b) and 10(c) give the shape of the intensity I of the light along the recorded spectrum, the angle φ(t) of the movable unit for dispersion Gl as function of time, and the recorded signal S(t) as function of time, respectively. In other variants, both the unit for dispersion Gl and the single spot photo- detector D3 are moved, simultaneously or not, during the recording. Putting one of them in movement at some moments and both of them at other moment may provide the same result as changing the velocity of the movement of a single one of the unit Gl and detector D3.

Fifth Embodiment: Multiple Detectors

In a fifth embodiment, the incoming light is split up by means of optical filters into parts for high resolution recording and parts for low resolution recording. The different parts of the spectrum are then guided to different dispersion units (prism or grating) according to the spectral requirements of that specific spectral region. Each spectral region is recorded using a respective photo-detector array. On average, the means for photo-detection have a lower resolution than the maximum resolution that is obtained over the spectrum.

With reference to Figure 11 , a multi-detector arrangement for adapted spectral detection according to this fifth embodiment may comprise two filters Fl and F2 arranged in cascade to split the incoming light in three parts.

A first part of the incoming light is processed by a first grating G4 coupled with a first photo-detector D4. A second part of the incoming light is processed by another grating G5 coupled with another photo -detector D5. Finally, a third part of the incoming light is processed by still another grating G6 coupled with still another photo- detector D6. Stated otherwise, each filter is followed by a detector arrangement comprising an optical unit for dispersion of a respective part of the incoming light, optically coupled with a photo-detector.

The gratings G4, G5 and G6 are designed so as to select a respective part of the spectrum each. The detectors D4, D5 and D6 are photo-detector arrays having respective resolutions. They may be linear photo-detector arrays.

The invention according to this fifth embodiment is not limited by the number of filters, which will depend on the number of spectral ranges that must be recorded with a specific resolution. A minimum of one filter is required, to split the light in two parts. For an incoming light of a given intensity, the maximum number of filters is limited only by the transmission factor and losses of the used filters, and by the sensitivity of the used photo-detectors.

Conclusion

The invention could be applied in any specialized spectrometer for the detection of analytes. A special use case is the non-invasive detection of glucose by means of spectroscopic detection of glucose in a body fluid.

While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, one embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.