Interferometer apparatus

Abstract

The present invention relates to an interferometer (2) arrangement comprising: a beamsplitter (6), a mirror (14) and a movable mirror arrangement (16); where the beamsplitter splits radiation to define a first optical pathway (10) to the mirror (14) and the second optical pathway (12) to the movable mirror arrangement (16), where an optical path difference between the first optical pathway (10) and second optical pathway (12) provides a resultant radiation (18); and where the movable mirror arrangement (16) comprises a polygonal mirror (24), the polygonal mirror (24) rotatable to vary the effective length of the second optical pathway (12).

Description

[0001] Interferometer apparatus

[0002] The present disclosure relates to an improved interferometer arrangement, in particular but not limited to, an interferometer for plastics identification.

[0003] Background of the Invention

[0004] Infrared (IR) spectroscopy may be used to identify different plastics, for example, in plastics sorting centre. A characteristic IR spectrum is shown for a number of plastics in figure 1. In prior art systems, IR radiation with low bandwidths and a wavelengths up to 2500nm are used. However, as shown in figure 1 , plastics have reflectance peaks in wavelengths exceeding 2500nm. Additionally, plastics containing carbon black pigment - i.e. , black plastics - cannot be detected using wavelengths below 2500 nm, as this pigment absorbs all light at those wavelengths. Thus, such prior art systems are not suitable for detecting such plastics.

[0005] So called Fourier Transform infrared (FTIR) spectrometers may be used to extend the detectable wavelengths to allow identification of, for example, black plastics. The inventor has found that such systems are not capable of fast operation, and therefore cannot detect fast moving objections such as plastics in a recycling centre. For example, conventional FTIR spectrometers have a scan rate of less than 1 Hz. Such a low rate means that use of FTIR spectrometers is not economical in such applications. In addition to the slow acquisition rate of existing FTIR spectrometers, conventional devices are based on halogen lamps for illumination. The limited brightness of such lamps decreases the sensitivity of measurements at long wavelengths due to environmental thermal radiation.

[0006] The present invention aims to overcome and / or ameliorate one or more of the above problems, for example, by introducing a fast-scanning FTIR combined with a bright supercontinuum illumination source. Statement of Invention

[0007] According to a first aspect, there is provided: an interferometer arrangement comprising: a beamsplitter, a mirror and a movable mirror arrangement; where the beamsplitter splits radiation to define a first optical pathway to the mirror and the second optical pathway to the movable mirror arrangement, where an optical path difference between the first optical pathway and second optical pathway provides a resultant radiation; and where the movable mirror arrangement comprises a polygonal mirror, the polygonal mirror rotatable to vary the effective length of the second optical pathway.

[0008] The movable mirror arrangement comprises a focusing mirror. The focusing may be configured to focus radiation reflected by the polygonal mirror. The focusing mirror may reflect radiation back to the polygonal mirror. Between the polygonal mirror and the focusing mirror, a second mirror may be placed. The second mirror may comprise a vertical displacement with respect to the polygon and focusing mirrors. The second mirror may be flat. A third mirror may be placed at 90 degrees from the second mirror, aligned in a way such that the beam returns to the polygon mirror and traces back the propagation path towards the beam splitter. The third mirror may be flat. The focusing mirror may comprise a spherical, parabolic and / or cylindrical mirror.

[0009] The movable mirror arrangement may comprise a first mirror. The first flat mirror may be provided at halfway to the focal point between the polygon mirror and the focusing mirror. The movable mirror arrangement may comprise a second mirror. The second mirror may be configured to receive radiation from focusing mirror reflected by the first mirror. The fixed mirror and / or second mirror may be fixed in position. The fixed mirror and / or second mirror may be flat. The second optical pathway may be defined between the beam splitter, the polygonal mirror, the focusing mirror, the first mirror and the second mirror. The polygonal mirror may be interposed the beamsplitter and the focusing mirror (i.e. such that the radiation from the beamsplitter is reflected from the polygonal mirror and onto the focusing mirror).

[0010] The focusing mirror and the second mirror may be positioned and / or sized such that the radiation returning to the beam splitter after being reflected by the polygon mirror twice co-propagates with the beam coming from the fixed arm of the interferometer.

[0011] The polygonal mirror may be rotationally symmetric and / or regular. The polygonal mirror may comprise a plurality of flat faces or facets. The polygonal mirror may comprise between 3 and 20 faces; preferably, between 5 and 15 faces. The polygonal mirror may comprise an irregular or sawtooth shape.

[0012] The polygonal mirror may be continuously movable to continuously vary the effective length of the second optical pathway.

[0013] A scan rate may be defined as the revolutions per second of the polygonal mirror multiplied by the numbers of faces of the polygonal mirror. The scan rate may be greater than or equal to 10 Hz; preferably, greater than or equal to 100 Hz. In each scan, all the wavelengths / frequencies of the light source may be recorded simultaneously in the so-called interferogram, which is a time domain signal. The Fourier Transform of the interferogram may yield the spectrum comprising the full spectral range of the light source, i.e., all of the wavelengths / frequencies. Examples of an interferogram and its corresponding spectrum are provided in Figure 1 .

[0014] According to a further aspect, there is provided: a spectrometer comprising the interferometer according to the first aspect. The spectrometer may comprise a radiation source. The radiation source may comprise an infrared radiation source. The radiation source may comprise a supercontinuum infrared radiation source. The radiation source may provide infrared radiation with wavelengths between 1.5 pm and 4.5 pm (e.g. continuously between 1.5 pm and 4.5 pm). The radiation (light) source may provide short-wave, mid-infrared and / or long- wavelength infrared radiation. The radiation source may provide infrared radiation continuously between wavelengths of 8 pm and 12 pm, preferably 9 pm and 11 pm. The radiation source may provide infrared radiation between wavelengths of 1 .5 pm and 11 pm. The radiation source may comprise a broadband quantum cascade laser operating in the infrared fingerprint region. This spectral region corresponds to wavelengths between 6.667 and 25.00 pm (conversely, 1500 and 400 cm-1)

[0015] The spectrometer may comprise a detector. The detector may receive transmitted / absorbed / reflected from a test sample. The spectrometer may comprise a Fourier transform infrared (FTIR) spectrometer.

[0016] According to a further aspect, there is provided: a sorting apparatus comprising the spectrometer of the previous aspect. The sorting apparatus may comprise a plastics sorting apparatus.

[0017] According to a further aspect, there is provided: a method of operating an interferometer arrangement comprising: providing a beamsplitter, a mirror and a movable mirror arrangement; where the beamsplitter splits radiation to define a first optical pathway to the mirror and the second optical pathway to the movable mirror arrangement, where an optical path difference between the first optical pathway and second optical pathway provides a resultant radiation; and where the movable mirror arrangement comprises a polygonal mirror; and rotating the polygonal mirror to vary the effective length of the second optical pathway.

[0018] Any aspect of the invention may be combined with any other aspect of the invention where practicable.

[0019] Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings:

[0020] Figure 1 shows spectra of various plastics;

[0021] Figure 2 shows a schematic view of an interferometer;

[0022] Figure 3 shows a schematic view of a mirror arrangement of the interferometer;

[0023] Figure 4 shows a schematic second view of the mirror arrangement of the interferometer;

[0024] Figure 5 shows a schematic view of a plastics sorting apparatus.

[0025] An interferometer arrangement 2 is shown in figure 2. The arrangement comprises a radiation source 4. Typically, the radiation source comprises a laser or the like. The radiation sources comprises a supercontinuum laser or some other broadband (i.e. the spectral coverage of which spans at least 100 nm) laser. The radiation source 4 comprises a supercontinuum infrared radiation source (i.e. a source which continuously emits throughout a specified range). For example, the radiation source provides infrared radiation with wavelengths between 1.5 pm and 4.5 pm. The radiation is thus broadband radiation covering both short-wave and mid-infrared spectral ranges. The radiation source may comprise a fluoride fiber supercontinuum laser or a Fabry-Perot quantum cascade laser. In some embodiments, the source 4 may additionally or alternatively provide radiation with wavelengths between 9 pm and 11 pm.

[0026] A beamsplitter 6 is provided. The beamsplitter 6 receives radiation 8 from the radiation source 4. Various optics (e.g. lenses, collimators etc.) may be operatively provided between the radiation source 4 and beamsplitter 6 to guide and adjust the size of the radiation beam. The beamsplitter 6 splits the radiation 8 into two different pathways 10,12. A first pathway 10 travels to a fixed / stationary mirror 14 and is reflected back to the beamsplitter 6. A second pathway 12 travels to a movable mirror arrangement 16. Likewise, the second pathway 12 is reflected back the beamsplitter 6. A difference in the length of the optical pathways 10,12 generates a periodic signal with intensity fluctuations due to different degrees of constructive and destructive interference between the radiation from optical pathways 10 and 12 in a resultant beam 18 formed at the beamsplitter 6. The resultant beam 18 is then directed toward a target 20 by a second polygonal mirror 19. The second polygonal mirror 19 rotates to scan the position of the radiation incident on the target 20, such that the radiation scans a line along the width of the conveyor belt 37 (figure 5). The resulting transmitted / absorbed / reflected light from target 20 is collected by another face of the second polygonal mirror 19 and guided to the collection optics 21 . The collection optics 21 may comprise one or more collimating / focusing lens such that the light collected from the target 20 is maximized and imaged on detector 22.

[0027] Hardware and / or software is used to determine a spectrum from the detected radiation. For example, the time-domain signal (i.e. interferogram) is then recorded by a field programmable gate array (FPGA). A software converts the interferogram to frequency domain signal (i.e. infrared spectrum) by applying Fourier Transform. The spectrum may then be used to identify one or more materials in the target. For example, the material may be identified by one or more characteristic of the infrared spectrum. Identification may be provided by the use of known spectra and / or using machine learning etc.

[0028] The moveable mirror arrangement 16 is shown in greater detail in figure 3. The mirror arrangement 16 comprises a polygonal mirror 24. The polygonal mirror comprises a plurality of faces / facets 26. The faces 26 are flat. In the present embodiment, the polygonal mirror comprises eight faces 26. The polygonal mirror 24 is configured to intercept the second radiation pathway 12. The polygonal mirror 24 is rotatably mounted to the device (e.g. about a centre- point thereof). A motor or other actuator may be provided to provide rotation of the polygonal mirror 24.

[0029] The moveable mirror arrangement 16 comprises a concave / focussing mirror 28. The concave mirror 28 is configured to receive the second pathway radiation 12 reflected from the polygonal mirror 24. The concave mirror 28 is parabolic / spherical / cylindrical. The concave mirror 28 therefore focuses radiation to a single focal point independent of the angle in which radiation intercepts the concave mirror 28. The polygonal mirror 24 is interposed the beamsplitter 6 and the focusing mirror 28 such that the radiation from the beamsplitter 6 is reflected from the polygonal mirror 24 and onto the focusing mirror 28 (i.e. the concave mirror 28 is downstream the polygonal mirror 24).

[0030] A first flat mirror 32 is provided approximately halfway the focal distance of the concave mirror 28. A second flat mirror 34 is then provided to intercept light reflected by the first flat mirror 32. The second flat mirror 34 is provided at the focal point of the concave mirror 28 (when reflected by the first flat mirror 32). The concave mirror 28, first mirror 32 and second mirror 34 are provided in a fixed position. The arrangement of mirrors 28, 32 and 34 is such that the beam returns to the same pathway 12 as the polygon mirror 28 rotates. The radiation pathway between the polygonal mirror 24, the concave mirror 28, the first flat mirror 32 and the second flat mirror 34 defines the effective path length of the second pathway 12. The difference between the effective path length of the pathway 12 and the first fixed pathway 10 is defined as the optical path difference (OPD). The OPD defines the spectral resolution of the interferometer: the larger the OPD, higher the spectral resolution.

[0031] It can be understood that the mirror arrangement in figures 3 and 4 are merely exemplary, and there may be many orientations and / or configurations that allow variation of the effective path length using the rotating polygonal mirror. Referring to figure 4, operation of mirror arrangement is described. Radiation (e.g. infrared radiation) is emitted from the light source 4. The radiation is intercepted by the beam splitter 6 and splits into the first and second pathways 10,12. The second pathway intercepts the polygonal mirror 24. The polygonal mirror 24 is shown in a first position in solid lines. The radiation is reflected from the polygonal mirror 24 and onto the concave mirror 28. The concave mirror 28 then guides the radiation onto the first mirror 32, placed approximately at half the focal distance of the concave mirror 28, which then reflects the radiation onto the second mirror 34.

[0032] The polygonal mirror 24 is shown in a second position in dashed lines. The polygonal mirror 24 has been rotated. It can be appreciated that the radiation (shown in dashed lines) follows a similar path to radiation in the first position, however, the length of the path is different in the second position than first position. The second flat mirror 34 is sufficiently sized such that light intercepting either end of the concave mirror 28 can be intercepted thereby (see solid and dashed lines). Rotation of the polygonal mirror 24 may therefore vary the optical path length of the second radiation pathway 12 (i.e. to achieve the same effect as a linearly movable mirror used in conventional FTIR interferometers). It can be appreciated that as the polygonal continuously rotates, the path length of the radiation continuously varies accordingly. The path length continuously varies as the radiation pathway 12 sweeps across a given face 26, varying the OPD accordingly. As the OPD varies, the beams from pathways 10 and 12 experience different degrees of constructive and destructive interference, generating a signal called interferogram. Once the polygonal mirror 24 rotates such that the incoming radiation 12 crosses onto a new face 26 (i.e. crosses the corner / apex therebetween) the optical length is restored back to original length, and the process is repeated. This is then repeated as the polygonal mirror 24 continues to rotate. Thus, each face 26 provides a discrete scan. The resultant radiation 18 then illuminates the sample 20. The transmitted / reflected / absorbed radiation is then detected by the detector 22.

[0033] The polygonal mirror 24 allows interferograms to be acquired in rapid succession. For example, the interferograms may be acquired much faster than conventional linearly movable mirrors (e.g. by between 10 and 100 times faster). This allows rapid successive analysis of different samples 20. This may be beneficial where many samples must be performed rapidly, for example, in manufacturing or processing environments. In the present embodiment, a scan rate of between 600 and 700 Hz may be provided. As the polygonal mirror 24 comprises eight faces, then the polygonal mirror 24 may rotate at between 75 and 88 revolutions per second. It can be appreciated that the above scan rates are merely exemplary, and the system can scan at a rate according to specific needs. The arrangement may comprise a scan rate greater than or equal to 10 Hz; preferably, greater than or equal to 50 Hz; preferably, greater than or equal to 100 Hz; preferably, greater than or equal to 250 Hz. It can be seen that such scan rates are significantly higher than prior art scan rates. The polygonal mirror allows continuously cycling of the output interferogram without the need for complex mechanical arrangements or excessive forces that may be generated using a linearly reciprocating mirror. The scans using the rotating polygonal mirror are also more stable than conventional linearly moving mirror. This means that the calibration of the timescale of the interferogram is only necessary once, and not every single scan as in conventional systems based on linearly moving mirrors.

[0034] The number of faces and / or the speed of rotation of the polygonal mirror 24 may determine the scan rate. It can be understood that increasing the number of faces and / or the speed of rotation of the polygonal mirror 24 can increase the scan / analysis rate accordingly. Where less scan speed is required, the polygonal mirror 24 may comprise between three and six faces 26. Where an intermediate speed is required, the polygonal mirror 24 may comprise between six and ten faces 26. Where a high speed is required, greater than ten faces 26 may be provided. In general terms, the polygonal mirror 24 may comprise between three and twenty faces 26; preferably, between five and fifteen faces 26.

[0035] The polygonal mirror 24 is rotationally symmetric. Typically, the polygonal mirror 24 is rotationally symmetric with the same number of rotational folds as the number of faces. The polygonal mirror 24 comprises a regular shape (i.e. all size and internal angles are the same). In some embodiments, one or more face 26 may comprise a different shape or angle. For example, such a face 26 may change the scan characteristics of the mirror 24. In some embodiments, the polygonal mirror 24 may comprise a sawtooth / saw blade tape configuration (e.g. the centre point of the face 26 is angled / non-perpendicular to the radial direction). In some embodiments, one or more face may be non-reflective (e.g. to space individual scans or to reduce scan rate).

[0036] A plastics sorting arrangement 36 is shown in figure 5. A conveyor (e.g. a conveyor belt 37) is configured to convey plastics 38. An analysis system 40, comprising the aforementioned interferometer arrangement 2, is configured to analyse and / or identify the plastics 38, for example, to determine the type thereof. A given piece of the plastics 38 is analysed and then sorted according to its type or other characteristic. Sorting arrangement 36 may comprise a camera system or other tracking system to track movement of the identified plastics material. An actuator 42 is provided to effect movement of a given plastic 38. For example, the actuator 42 comprise a pneumatic actuator configured to emit a jet of compressed air to physically blow the plastic 38 toward a first deposition area 44. Other plastics material may simply fall from the conveyor belt to a second deposition area 46. It can be appreciated that there are many ways in which analysed / identified plastics 38 may be actuated or otherwise effected. For example, pickers, robotic actuators, trap doors, gratings etc. The present application allows detection of a wide range of materials by use of a supercontinuum radiation source. In particular, the present arrangement may be used to detect black plastics. Supercontinuum light also imparts less heat to the sample, has greater brightness compared to thermal light sources, and allows measurement at longer distances. The polygonal mirror allows a fast scan rate, thus allowing detection of a wide range of materials in a fast-moving environment. The polygonal mirror provides a simple mechanical means of adjusting an optical path length at a high speed, without the need for complex mechanical actuators that would be required in a linearly movable system.

[0037] It can be appreciated that the interferometer arrangement 2 may be used in a wide range of technical applications. In particular, the interferometer arrangement 2 may be used where wide spectrum radiation is used to detect a wide range of materials, and where said materials need to be analysed / identified rapidly. For example, the arrangement 2 may be used in any of: pharmaceutical manufacturing / process; food production and / or other FMCG areas; or materials sorting (e.g. in waste / recycling environments). The arrangement 2 may be used to sort construction waste.

[0038] Whilst the present invention has been described for use with infrared, it can be appreciated that the device may use other portions of the electromagnetic spectrum, for example, UV, visible light, or visible light and infrared light.

Claims

Claims:1 . An interferometer (2) arrangement comprising: a beamsplitter (6), a mirror (14) and a movable mirror arrangement (16); where the beamsplitter splits radiation to define a first optical pathway (10) to the mirror (14) and a second optical pathway (12) to the movable mirror arrangement (16), where an optical path difference between the first optical pathway (10) and second optical pathway (12) provides a resultant radiation (18); and where the movable mirror arrangement (16) comprises a polygonal mirror (24), the polygonal mirror (24) rotatable to vary the effective length of the second optical pathway (12).

2. An interferometer arrangement according to claim 1 , where the movable mirror arrangement (16) comprises a focusing mirror (28) configured to focus radiation reflected by the polygonal mirror (24).

3. An interferometer arrangement according to claim 2, where the movable mirror arrangement (16) comprises a first flat mirror (32) provided at a position halfway to a focal point of the focusing mirror (28) between the polygon mirror (24) and the focusing mirror (28).

4. An interferometer arrangement according to claim 2, where the movable mirror arrangement (16) comprises a second mirror (34) configured to receive radiation from focusing mirror (28) reflected by the first mirror (32).

5. An interferometer arrangement according to any of claims 2-4, where focusing mirror comprises a spherical, parabolic and / or cylindrical mirror.

6. An interferometer arrangement according to any of claims 2-5, where the focussing mirror (28) and a second mirror (32) are positioned and / orsized such that radiation reflected from the polygonal mirror (24) always intercepts the focusing mirror (28) and the second mirror (32).

7. An interferometer arrangement according to any preceding claim, where the polygonal mirror (24) is interposed the beamsplitter (6) and the focusing mirror (28) such that the radiation from the beamsplitter (6) is reflected from the polygonal mirror (24) and onto the focusing mirror (28).

8. An interferometer arrangement according to any preceding claim, where the polygonal mirror (24) is continuously movable to continuously vary the effective length of the second optical pathway (12).

9. An interferometer arrangement according to any preceding claim, where the polygonal mirror (24) comprises a plurality of flat faces (26) or facets.

10. An interferometer arrangement according to any preceding claim, where the polygonal mirror comprises between 3 and 20 faces (26); preferably, between 5 and 15 faces (26).

11. An interferometer arrangement according to any preceding claim, where a scan rate is defined as the revolutions per second of the polygonal mirror (24) multiplied by the numbers of faces (26) of the polygonal mirror (24), and the scan rate is greater than greater than or equal to 10 Hz; preferably, greater than or equal to 100 Hz.

12. A spectrometer comprising: the interferometer (2) of any preceding claim; and a radiation source (4), the radiation source (4) comprising a supercontinuum infrared radiation source.

13. A spectrometer according to claim 11 , where the radiation source (4) provides infrared radiation continuously between wavelengths of 1 .5 pm and 4.5 pm and / or 9.0 pm and 11 .0 pm.

14. A sorting apparatus (36) comprising the spectrometer of any of claims 12 or 13.

15. A method of operating an interferometer arrangement (2) comprising: providing a beamsplitter (6), a mirror (14) and a movable mirror arrangement (16); where the beamsplitter (6) splits radiation to define a first optical pathway (10) to the mirror (14) and a second optical pathway (12) to the movable mirror arrangement (16), where an optical path difference between the first optical pathway (10) and second optical pathway (12) provides a resultant radiation (18); and where the movable mirror arrangement (16) comprises a polygonal mirror (24); and rotating the polygonal mirror (24) to vary the effective length of the second optical pathway (12).

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