GB2589622A

GB2589622A - Sensor

Info

Publication number: GB2589622A
Application number: GB1917813.6A
Authority: GB
Inventors: Naydenova Izabela; Amarandei George; Grogan Catherine
Original assignee: Univ Dublin Technological; Technological University Dublin
Current assignee: Univ Dublin Technological; Technological University Dublin
Priority date: 2019-12-05
Filing date: 2019-12-05
Publication date: 2021-06-09
Anticipated expiration: 2039-12-05
Also published as: GB201917813D0; GB2589622B

Abstract

A sensor element 10 is provided comprising; a first layer 11 comprising a fixed part and a moveable part; a second layer 12 comprising a holographic diffraction grating 121 fixed to the moveable part of the first layer; wherein the first and second layers have differential expansion in response to an analyte, such that the sensor element is configured to respond to an analyte by deflecting and changing the angle of the second layer relative to the fixed part of the first layer. The first layer may be a cantilever, bridge or membrane. The first layer may comprise a holographic grating with different spatial frequency to the second layer grating 121.

Description

SENSOR

TECHNICAL FIELD

The present invention relates to a sensor, and a method for sensing.

BACKGROUND

Cantilever sensors utilise a cantilever beam which is configured to deflect in response to a change in an environmental condition, such as temperature and relative humidity or a chemical analyte. A typical cantilever sensor comprises a silicon micro-cantilever. Such sensors have gained extensive interest due to their small size, mass production and high sensitivity. Micro-and nano-cantilever based sensors have been demonstrated as sensors to detect a variety of biological, physical and chemical analytes. They have potential applications in biosensors, chemical sensors, portable devices, medical devices and in security control.

Cantilever based sensors can be divided into two groups: the static and the dynamic.

The static mode operates by measuring a change in deflection of the cantilever beam due to a change in surface stress of the cantilever beam on one side compared to the other, thus inducing a deflection, in response to a change in an environmental condition, similar to the bimetallic effect. This is achieved by providing a 'sensing layer' on one side of the beam, which is subject to the change in surface stress compared to the other side of the beam. The sensing layer interacts with the target analyte, which changes the surface stress of the sensing layer, thus producing a cantilever deflection. The dynamic mode operates by measuring the resonant frequency of a vibrating cantilever. Dynamic sensors may detect a change in resonant frequency due to mass loading by an analyte that is adsorbed on the beam.

To improve their sensitivity, cantilever sensors may require a large number of units and processing steps controlled through powered interface circuits, with the readout performed by expensive equipment. Alternatives to silicon cantilever sensors are needed. Such previously proposed alternatives include paper based cantilever sensors to detect the presence of volatile organic solvents and a piezoresistive layer integrated into a PDMS substrate to measure changes in the mechanical activity of heart cells, but enhancement of the sensitivity is still a challenge.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a sensor element comprising a first layer and a second layer. The first layer comprises a fixed part and a moveable part. The second layer comprises a holographic diffraction grating fixed to the moveable part of the first layer. The first and second layers have differential expansion in response to an analyte, such that the sensor element is configured to respond to an analyte by deflecting and changing the angle of the second layer relative to the fixed part of the first layer.

The sensor element advantageously provides two means of measuring an analyte; the first being a measurable mechanical deflection in the sensor element, and the second being a measurable change in a characteristic of a light beam diffracted by the diffraction grating of the sensor clement. The sensor clement therefore provides both a low sensitivity measure of an analyte, which can be quickly and simply obtained visually, and a high sensitivity measure of an analyte As such, different experimental needs can be met using the sensor element.

The first layer may also comprise a holographic diffraction grating. The holographic diffraction grating of the first layer and the holographic diffraction grating of the second layer may have different spatial frequencies. The first layer may be sensitive to the analyte and the second layer may be sensitive to a different analyte. Alternatively, the first and second layers each have a different sensitivity to the analyte.

Providing first and second layers which are each sensitive to a different analyte, and providing a holographic diffraction grating with a different spatial frequency on each layer enables the sensor element to be used to identify the presence of different analytes.

Providing first and second layers which each have a different sensitivity to the analyte, enables the sensor element to be used to measure the analyte over a wider range compared to first and second layers both having the same sensitivity to the analyte.

The second layer may comprise a plurality of holographic diffraction gratings, each with a different sensitivity. This enables a broad total range of analyte concentration to be detected.

The or each holographic diffraction grating may comprise a transmission holographic diffraction grating. A transmission holographic diffraction grating is easier to mass produce than, for example, a reflection grating, since there are strict requirements for vibration free environment during recording of reflection gratings.

Alternatively, the or each holographic diffraction grating may comprise a reflection holographic diffraction grating. A reflection grating may be advantageous in reducing the volume of the system. Another advantage is that also in some embodiments, for example in the case of a membrane or a bridge, the colour distribution of the reflected light can be observed by a naked eye, thus it is possible to have a visual detection.

The holographic diffraction grating may comprise a photopolymer with a periodic variation in refractive index. This may provide a diffraction efficiency of up to 100%, which is advantageous over, for example, a surface sinusoidal grating which may have a maximum diffraction efficiency of only 33%.

The holographic diffraction grating may comprise a surface holographic grating. For example, the holographic diffraction grating may comprise a surface sinusoidal grating or a blazed surface grating. The diffraction efficiency of a blazed surface grating may be up to 100%. A surface holographic grating may be used instead of a holographic grating comprising a photopolymer with a periodic variation in refractive index when a thinner holographic grating is required, or when faster response times are required (faster response times are provided by shorter diffusion lengths).

The sensor element may comprise a further layer configured to change strain, relative to the first layer, in response to an analyte. The further layer may enhance the deflection of the sensor element, making the deflection easier to measure.

The analyte may be, for example, water vapour, so the sensor will be responsive to humidity. Embodiments of the invention provide a humidity sensor that has a lower cost of production and is easier to operate compared to known humidity sensors.

The second layer may have a different coefficient of thermal expansion to the first layer, so as to cause deflection of the second layer in response to temperature. This enables the sensor element to detect changes in temperature with a high resolution; for example higher than that of a conventional thermometer.

The first laver may be arranged as a cantilever. Alternatively or additionally, the sensor element may be arranged as a bridge or membrane. Embodiments of the invention therefore provide a sensor element that can be used in a variety of configurations according to experimental needs.

According to another aspect of the invention, there is provided a sensor comprising a sensor clement according to any of the above described embodiments of the invention. The sensor comprises a light source configured to direct an incident light beam on to the second layer of the sensor element, and a detector arranged to measure an intensity of a diffracted beam produced by diffraction of the incident light by the second layer. Alternatively the detector can measure the intensity of the transmitted beam.

By measuring the intensity of the diffracted beam, small changes in the analyte can be readily detected, since this will result in a relatively large change in the intensity of the diffracted light. This provides for a high sensitivity sensor.

According to another aspect of the invention, there is provided a method of detecting a analyte using the sensor element or sensor according to any of the above described embodiments of the invention. The method comprises illuminating the diffractive element with an incident light beam, and measuring an intensity of a diffracted beam produced by diffraction of the incident light beam by the diffractive element and/or measuring the spectral characteristics of the diffracted beam by using a spectrometer.

A change in the spectral characteristics of the diffracted beam, for example a spectral shift in the diffracted beam spectral profile, could be caused by shrinkage/swelling of the second layer. If the spectral shift is just due to a change of the effective refractive index of the second layer, the sensitivity will be relatively small (1). Cody, 1. Naydenoya, Theoretical modeling and design of photonic structures in zeolite nanocomposites for gas sensing. Part IL volume gratings, JOSA A. Vol. 35, Issue I, (2018). pp. 12-19.) The method may further comprise adjusting the sensitivity of the sensor by adjusting the spatial frequency of the holographic diffraction grating.

BRIEF DESCRIPTION OF THE DRAWINGS Figure I shows a sensor element; Figure 2 shows a sensor comprising the sensor element of Figure 1; Figure 3 shows a graph showing the relationship between relative humidity and an angle of deflection of the sensor element of the sensor of Figure 2; and Figure 4 shows a graph showing the relationship between relative humidity and the intensity of a laser beam of the sensor of Figure 2 as diffracted by a diffractive element of the sensor element.

Figures 5a, 5b and 5c each shows a sensor element comprising two layers each containing a diffraction grating with different period; Figures 6a, 6b and 6c each show a sensor element comprising two layers -one layer which is not sensitive to the analyte (inactive layer) and a second layer in which two gratings with different periods are recorded simultaneously (multiplexed)

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration of a portion of a sensor element 10. The sensor element 10 comprises a first layer 11 and a second layer 12. The second layer 12 comprises a diffractive optical element. The first layer 11 is preferably shaped into a structure comprising a fixed part and a moveable part such as a cantilever (with a fixed part at one end), bridge (with fixed parts at either end) or membrane (fixed around the edge). The second layer 12 is adhered to the first layer 11, and may cover only a portion of the first layer 11. In some embodiments the first laver and the second layer may have the same layout (e.g. be both patterned with a diffractive pattern).

The first layer 11 may comprise any suitable mechanical material, and may be crystalline (e.g. silicon), polycrystalline (e.g. polysilicon), amorphous (e.g. silicon oxide, glass etc), metallic (e.g. aluminium, titanium, platinum, gold etc) or polymeric (e.g. polyimide, polydimethylsiloxane/PDMS, poly methylmethylacrylate/PMMA etc.).

The second layer 12 may comprise a holographic diffraction grating 121, such as a transmission volume phase holographic coating, comprising a material with a periodic (e.g. sinusoidal) variation in refractive index. The second layer 12 may comprise a photopolymer layer that has been exposed to an interference pattern in order to generate the periodic variation in refractive index. In other embodiments the second layer 12 may comprise a reflective holographic diffraction grating.

The first and second layers 11, 12 have differential expansion in response to an analyte. For example, the first and second layer 11, 12 may swell by different amounts in response to humidity and/or may have different thermal coefficients of expansion. In some embodiments, one of the first and second layer 11, 12 may be functionalised with a furthcr layer that changes strain in response to an analyte, thereby causing the cantilever to curve by an amount dependent on the concentration of the analyte. The change in strain will induce a change in stress, due to the first and second layers being fixed together.

In use, a portion of an incident light beam 13 is diffracted by the second layer 12 at an angle to the incident light beam 13 as diffracted light beam 15. At least some of the incident light beam 13 may be transmitted as the transmitted light beam 14. The diffraction efficiency of the second layer 12 is the ratio of the power of the diffracted light beam 15 to the power of the incident light beam 13. The diffraction efficiency of a holographic diffraction grating is strongly dependent on the angle of incidence of the incident light beam 13 with the grating (as illustrated in Figure 4). A change in the angle of the incident light beam 13 relative to the second layer 12 of the cantilever may be caused by deflection of the sensor element 10. This sensing mechanism enables small changes in the analyte to be readily detected, since this will result in a relatively large change in the intensity of the diffracted light 15.

Figure 2 shows a sensor 20 comprising a cantilever sensor element 10. The sensor element 10 comprises a bi-layer cantilever in which the first layer 11 is a PDMS layer, and the second layer 12 is a photopolymer such as acrylamide based photopolymer comprised of a polyacrylamide, polyvinyl alcohol. Triethanolamine, erythrosine B dye.

The sensor 20 also comprises a light source 22 (e.g. a laser) and a detector 21. The light source 22 is configured to direct an incident light beam 13 on to the second laver 12 of the sensor element 10. The detector 21 measures an intensity of the diffracted light beam 15 (as diffracted by the diffractive element 121). The detector 21 may :" comprise a spectrometer, a photodiode or the like. A protractor 23 is also shown for measurement of the deflection of the sensor element 10, for comparison with a measurement according to an embodiment.

In the embodiment of Figures 1 and 2, the sensor element is arranged as a transmission grating; the incident light beam 13 passes through the sensor element 10 and the diffracted light beam 15 is picked up by the detector 21. In an alternative embodiment, the sensor element 10 is arranged as a reflection grating and the diffracted light beam 15 is reflected by the sensor element 10 towards a detector located on the same side of the sensor element 10 as the light source 22.

In an example embodiment, when exposed to a change in relative humidity (RH), the sensor element 10 deflects as a result of a change in strain of the first layer 11 relative to the second layer 12. This relative change in strain is caused by the difference in absorption of moisture between the first and second layers I I. 12 in the event of an increase in RH, and the relative desorption of moisture between the first and second layers 11, 12 in the event of a decrease in RH. The sensor element 10 deflects from a neutral position (indicated by the dotted line 'n' in Figure 2), and the direction of deflection relative to the neutral position depends on if the change in relative humidity is an increase or a decrease (relative to a reference humidity at which the cantilever element 10 is straight). In the example of Figure 2, the cantilever sensor element 10 deflects to the left of the neutral position (as viewed in Figure 2) with a decrease in RH and to the right of the neutral position with an increase in RH; however it will be appreciated that the sensor 20 can be reconfigured such that the sensor element 10 will deflect in the opposite direction in response to a respective decrease or increase in RH, for example by rearranging the sensor element 10 such that the first laver 11 faces the opposite direction to the that shown in Figure 2, and by rearranging the laser source 22 and the intensity sensor 21 accordingly.

The angle of deflection of the sensor element 10 from the neutral position may be measured by eye using the protractor 23 and, based on a known relationship between the angle of deflection and RH. RH can be determined. This relationship is demonstrated by the trend line 30 shown in Figure 3 which is derived from a plurality of empirical points 31. The empirical points 31 are obtained by observing the deflection of the sensor element 10 for a plurality of values of RH measured by a different, pre-calibrated sensor to that of the sensor 20. This simple readout has a relatively low resolution, since small changes in humidity cannot be detected accurately due to the limited accuracy of the protractor.

A higher sensitivity measurement (e.g. of RH) can be obtained using the sensor 20 and the detector 21. The detector 21 will measure a change in the intensity of the diffracted beam 15 as the sensor element 10 deflects in response to a change in RH. The incident angle of the incident beam 13 on the second layer 12 when the sensor clement 10 is in the neutral position 'n' may be equal to the Bragg angle of the grating of the second layer 12, and the detector 21 may be arranged to receive the diffracted beam 15 corresponding with this position of the sensor element 10 From the measured value of the intensity of the diffracted beam 15, the diffraction efficiency of the grating of the second layer 12 of the sensor element 10 can be derived, and from this a Bragg selectivity curve 40, shown in Figure 4, is obtained.

The intensity of the diffracted beam 13 for incidence angles either side of the Bragg angle, where A is the grating period, A is the wavelength of the incident light, and 0 the Bragg angle (measured between the incident beam 15 and the plane of the second layer 12) has a high sensitivity to a changes in RH. The Bragg angle may be defined as the angle for which maximum diffraction efficiency is detected and which meets the Bragg condition of -2: = 2 -2: sin(0).

As the sensor element 10 deflects from the neutral position, the incidence angle of the incident beam 13 on the second laver 12 deviates from the Bragg angle, and the diffraction efficiency may change from 0 to [00% with a sensor element 10 deflection in the order of 1-2 degrees It will be appreciated that although RH is the analyte being measured in the above described example, the sensor 20 can be used to measure any suitable analyte. For example the sensor clement 10 could be doped with zeolite nanoparticics that have shown selective response to different volatile organic components such as methanol, ethanol, isopropanol, toluene and metal di-cations in water solutions etc. The sensor 20 combines a cantilever sensor platform with a photopolymer grating to produce a simple and low-cost sensor as well as a highly sensitive sensor with sensitivity that can be tuned by the spatial frequency of the diffraction grating.

The mechanical response of the sensor element 10 (i.e. the change in curvature for an incremental change in analyte concentration) depends on the thickness of the layers 11, 12, and may be modelled using the Smits equation for a cantilever bimorph (with strain resulting from the analyte substituted for piezoelectric strain), or more generally based on the approach of DeVae et at (DeVoe, Don L., and Albert P. Pisano. "Modeling and optimal design of piezoelectric cantilever microactuators." Journal of Microelectromechanical systems 6.3 (1997): 266-270).

In general, thinner cantilevers (in the direction of deflection 'd' in Figure 2) may be more responsive, assuming that the stress resulting from the analyte is confined to a relatively thin layer at/near the surface. There may be a linear relationship between sensitivity and, therefore, the detection limit, with thickness of the sensor element 10, particularly when observing the angle of deflection of the sensor element 10 using the protractor 23.

The detection limit of RH determined from the intensity of the diffracted laser beam 211 depends on both the thickness of the sensor element 10 in the direction of deflection and the spatial frequency of the grating of the second layer 12, i.e. the space between the individual fringes of the grating For a given sensor element thickness, the FWHM (full width half maximum) of the Bragg selectivity curve obtained from measuring the intensity of the diffracted beam 15 decreases with increasing spatial frequency of the grating of the second layer 12.

The FWHM is also determined by the thickness of the second layer 12; the thicker the layer the smaller the FWHM. A smaller FWHM indicates a greater sensitivity to changes in RH.

The detection limit of the sensor 20 using the sensitivity of the linear region of the Bragg selectivity curve can be calculated. The value for resolution 'R.' in the RH measurement of the example embodiment is for example, 0.01, which corresponds to a 1 % change in diffraction efficiency. A lower detection limit of close to 0.01% RH can be achieved by detecting the change in intensity of the diffracted beam 15 at the detector 21. Such measurements, determined from the intensity of the diffracted beam 15, which represents a 100X increase in sensitivity compared to determining RH from the deflection of the sensor element 10 alone.

In some embodiments, the sensor 20 provides both a low sensitivity measure of RH provided by the deflection of the sensor element 10 which can be quickly and simply obtained visually (e.g. using the protractor 23), and a high sensitivity measure of RH provided by the intensity of the diffracted laser beam 15. As such different experimental needs can be met using the sensor 20.

The sensitivity of the sensor 20 is dependent on: the spatial frequency of the diffraction grating; the orientation of the fringes in the diffraction grating; the thickness of the layer containing the diffraction grating; the surface profile of the diffraction grating (for example symmetric versus asymmetric); the dimensions of the sensor element 10; and the material chemistry and the mechanical (for example elasticity) and thermal properties of the first and second lavers 11, 12.

The response time of the sensor 20 is dependent on: the mechanical properties (for example elastic modulus) of the first and second layers II, 12; the porosity of the second layer 12; and the material chemistry of the second layer 12, for example the make-up of photopolymer receptors.

The selectivity of the sensor 20, i.e. the particular analyte or analvtes that the sensor can detect a change in, is dependent on: the chemical affinity of either the first or second layer 11, 12; and the material chemistry of the second laver 12, for example the make-up and concentration of photopolymer receptors; The sensor 20 has applications in a number of fields, including point-of-care health monitoring, environmental testing monitoring of industrial processes and security control.

Figures 5a-c show an alternative embodiment of the sensor element. The sensor element 50 comprises a first layer 51 having a diffraction grating having a first spatial frequency, and a second layer 52 having a diffraction grating having second spatial frequency. The first layer 51 is responsive to a first analyte and the second layer 52 is responsive to a second analyte, or the same analyte. The sensor element 50 can be employed in a sensor comprising a light source (not shown) configured to deliver an incident beam 13 to the first layer 51 of the sensor element 50. Detectors (not shown) are located at '1' and '2' as labelled in Figures 5a-c to measure the intensity of a diffracted beam 15. The sensor element 50 is fixed at one end to provide a cantilever sensor.

Figure 5b shows readout from the second layer 52, with a diffracted beam 15 from the first layer 51 detected at the second detector 2. Figure Sc shows readout from the second layer 52, with a diffracted beam 15 detected at the first detector I. in embodiments where the first and second layer 51, 52 are responsive to different analytes, the sensor element is responsive to either analyte (on its own).

In an alternative embodiment, the first and second layers each have a different sensitivity to the same analyte. In this case, detection of the diffracted beam 15 at the first detector may be used to measure analyte concentration within a first range, and detection of the diffracted beam 15 at the second detector may be used to measure analyte concentration within a second range Figures 6a-c show an alternative embodiment of the sensor element. The sensor element 60 comprises a first layer 61 which does not comprise a diffraction grating, and a second layer 62 which comprises two diffraction gratings. The sensor element 60 can be employed in a sensor comprising a light source (not shown) configured to deliver an incident beam 13 to the second layer 62 of the sensor element 60. Detectors (not shown) are located at '1' and '2' as labelled in Figures 6a-c to measure the intensity of a diffracted beam 15. The sensor dement 60 is fixed at one end to provide a cantilever sensor.

Each of the diffraction gratings of the second layer 62 has a different sensitivity.

When a diffracted beam 15 is detected at the first detector, as shown in Figure 6b, the incident beam 13 has been diffracted by the first diffraction grating. When the diffracted beam 15 is detected at the second detector, as shown in Figure 6c, the incident beam 13 has been diffracted by the second diffraction grating. The sensitivities of the first and second diffraction gratings are such that the analyte is measured in a first range when the incident beam is diffracted by the first diffraction grating, and measured in a following, second range when the incident beam is diffracted by the second diffraction grating. As such, a larger range of the analvte can be measured by providing two diffraction gratings of different sensitivities on the second layer 62 compared to providing a single diffraction grating.

In alternative embodiments, more than two diffraction gratings, each having a different sensitivity, may be provided on the second layer 62 to further increase the range over which the analyte can be measure In one embodiment, five diffraction gratings are provided on the second layer 62.

There could be a number of gratings, for example 5, recorded in the same layer (this is called multiplexing). Each grating will diffract when the incident angle is at the correct angle for this grating (and only for this grating). Thus a larger range of analyte concentrations can be covered (otherwise a single grating will produce a signal with extremely high sensitivity, but only for a very small range of incident angles -equivalent to a very small range of deviations from the starting position of the cantilever, or very small range of concentrations).

The present invention provides a highly sensitive sensor platform based on an opto-mechanical hybrid structure utilising a diffractive optical element, and a method for sensing based on the strong dependence of the diffraction efficiency of the diffractive optical element on the angle of incidence of the probe light beam. Due to the hybrid nature of the mechanical structure and the differential mechanical response of the sensor components to the present target analyte the diffractive optical element changes its position with respect to the probe beam and this leads to a change in the diffracted light intensity, which is used for quantitative measure of the amount of the analyte present.

Claims

CLAIMSA sensor element comprising: a first layer comprising a fixed part and a moveable part; and a second layer comprising a holographic diffraction grating fixed to the moveable part of the first layer; wherein the first and second layers have differential expansion in response to an analyte, such that the sensor dement is configured to respond to an analyte by deflecting and changing the angle of the second layer relative to the fixed part of the first laver.
2. The sensor element according to claim 1, wherein the first layer comprises a holographic diffraction grating.
IS 3. The sensor dement according to claim 2, wherein the holographic diffraction grating of the first layer and the holographic diffraction grating of the second layer have different spatial frequencies.
4. The sensor dement according to claim 3, wherein the first layer is sensitive to the analyte and the second layer is sensitive to a different analyte.
5. The sensor element according to claim 3, wherein the first and second layers each have a different sensitivity to the analyte.
6. The sensor element according to claim 1, wherein the second layer comprises a plurality of holographic diffraction gratings, and wherein each holographic diffraction grating comprises a different sensitivity.
7. The sensor element according to any preceding claim, wherein the or each holographic diffraction grating comprises a transmission holographic diffraction grating.
8. The sensor element according to any of claims 1 to 6, wherein the or each holographic diffraction grating comprises a reflection holographic diffraction grating.
9. The sensor clement according to any preceding claim, wherein the or each holographic diffraction grating comprises a photopolymer with a periodic variation in refractive index.
10. The sensor clement according to any preceding claim, wherein the or each holographic diffraction grating comprises a surface holographic grating.
The sensor element according to any preceding claim, comprising a further layer configured to change strain, relative to the first layer, in response to an analyte.
12. The sensor element of any preceding claim, wherein the analyte is water vapour, so the sensor is responsive to humidity.
13. The sensor element according to any preceding claim, wherein the second laver IS has a different coefficient of thermal expansion to the first laver, so as to cause deflection of the second layer in response to temperature.
14. The sensor element according to any preceding claim, wherein the first layer is arranged as a cantilever.
15. The sensor element according to any preceding claim, wherein the sensor element is arranged as a bridge or membrane.
16. A sensor, comprising the sensor element of any preceding claim, the sensor comprising: a light source configured to direct an incident light beam on to the second layer; a detector arranged to measure an intensity of a diffracted beam produced by diffraction of the incident light by the second laver.
17. A method ofdetecting an analyte using the sensor element of any of claims 1 to or the sensor according to claim 16, the method comprising; illuminating the diffractive element with an incident light beam; and measuring an intensity of a diffracted beam produced by diffraction of the incident light beam by the diffractive element, and/or measuring the spectral characteristics of the diffracted beam by using a spectrometer
18. A method of detecting an analyte using the sensor element of any of claims 1 to 15 or the sensor according to claim 16, the method comprising; illuminating the diffractive element with an incident light beam; and measuring an intensity of the transmitted straight through beam affected by diffraction of the incident light beam by the diffractive element, and/or measuring the spectral characteristics of the transmitted straight through beam by using a spectrometer.
19. The method of claim 17, comprising adjusting the sensitivity of the sensor by adjusting the spatial frequency of the holographic diffraction grating.