Fourier transform spectrometer and method of fourier transform spectroscopy

Abstract

To provide a miniaturized Fourier transform spectrometer.SOLUTION: The invention relates to a Fourier transform spectrometer comprising an interferometer 10 comprising a fixed mirror 11, a movable mirror 12 and a detector. The fixed and movable mirrors are positioned parallel to each other and form an optical cavity 13, and the movable mirror is operable to move relative to the fixed mirror to change the size of the optical cavity. A fixed mirror is positioned such that light 14 received from a sample enters the optical cavity through a back of the fixed mirror, the detector is positioned and operable to detect light which has exited the optical cavity through a back of the movable mirror and output an interferogram, and the fixed and movable mirrors are distanced from each other. The movable mirror is at or can move to a position where at least two paths 16, 31 to the detector for received light reflected within the interferometer are equal in length.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] FIELD OF THE INVENTION The present invention relates to Fourier transform spectrometers and methods of Fourier transform spectroscopy, particularly, but not exclusively, the present invention relates to infrared Fourier transform spectrometers and spectroscopy. [Background technology]

[0002] Every molecule and / or substance has a distinct absorption spectrum, absorbing different wavelengths of light to different extents. Figure 1 shows an example absorption spectrum, in this case, that of dichloromethane. Thus, molecules and / or substances and their composition can be determined using spectroscopy by obtaining their absorption spectrum.

[0003] Given the ability to determine molecules and / or substances and their composition, spectroscopy has the following advantages: monitoring food quality (e.g., food contamination or dating); environmental monitoring (e.g., air pollution or water quality); medical uses (e.g., blood analysis and drug testing); and Material control, It has a wide range of applications including

[0004] However, many currently available spectrometers are relatively large, cumbersome, and / or expensive, limiting their use outside of a laboratory setting and therefore their usefulness. For use outside of the laboratory, there is a need to miniaturize spectrometers while also reducing costs. An ideal spectrometer would be scalable down to a size of a few millimeters square and sufficiently inexpensive so that the spectrometer can be incorporated into mobile electronic devices.

[0005] There are three main configurations of spectrometers that can be miniaturized: gratings, Fabry-Perot interferometers, and Michelson interferometers.

[0006] A grating is shown in Figure 2. With respect to the grating, light 1 incident on a sample is directed at a grating 2 at an angle of incidence α. Light passing through or reflected from different slits in the grating interferes, resulting in an interference pattern 3 that depends on the output angle β, which can be detected by a detector. If the wavelength defines the location of the maximum and minimum in the pattern for each wavelength of light, the detector can detect multiple wavelengths, and thus construct a spectrum.

[0007] A wide array of detectors is required to adequately detect pattern 3 and obtain all the necessary information to form the entire absorption spectrum. Furthermore, pattern 3 requires sufficient space to spread out to achieve sufficient resolution. Finally, due to higher order reflections, gratings have a limited free spectral range. These challenges mean that there are limits to how miniaturized a spectrometer using a grating can be, and the resulting spectrometer has a limited optical frequency range.

[0008] A Fabry-Perot interferometer is shown in Figure 3. Light 1 incident on a sample is transmitted through a first mirror 4 into a cavity formed between two parallel mirrors 4 and 5. Light 1 is reflected between mirrors 4 and 5, such that only the wavelength of light 1 that is resonant with the cavity size is transmitted through a second mirror 5, and light 6 then travels onto a detector. The distance between mirrors 4 and 5 can be changed, allowing the user to scan through different wavelengths of light. In this way, the spectrometer can move through various wavelengths and build up an absorption spectrum.

[0009] There are several challenges associated with using a Fabry-Perot interferometer. Fabry-Perot interferometers require uniform mirror spacing at the wavelength of light, which is difficult to manufacture. A further challenge is that high-order reflections mean that the interferometer has a limited free spectral range. Furthermore, Fabry-Perot interferometers require high-quality optical mirror surfaces, usually layered Bragg mirrors. This means that spectrometers using Fabry-Perot interferometers are both relatively expensive to manufacture and have a limited optical frequency range.

[0010] In addition to the above, alternating layers of different materials are required to construct the Bragg mirror. Standard microelectromechanical systems (MEMS) processing techniques, such as gas-phase hydrofluoric acid, are incompatible with many materials, thus limiting material choices when scaling down spectrometers that use Fabry-Perot interferometers.

[0011] A Michelson interferometer is shown in Figure 4. Light 1 is incident on beam splitter 7, at which point part of the beam is reflected onto top mirror 8 and another part is transmitted through the beam splitter to side mirror 9. These parts are reflected back to beam splitter 7 where they recombine. The position of side mirror 9 is adjustable, which allows the optical path length for one part of the light to be changed, thus introducing an optical path difference.

[0012] The Michelson interferometer is used as part of a Fourier transform infrared spectrometer. The side mirror 9 is moved rapidly, and a detector records as the side mirror 9 moves. The side mirror 9 must be moved from zero optical path difference to the desired maximum resolution, but it is usually easier to move the mirror through the position where the zero optical path difference falls. The resulting recorded output can be converted to an absorption spectrum via Fourier transform.

[0013] Fourier transform infrared spectrometers have a wide wavelength range and high spectral resolution with a high signal-to-noise ratio (i.e., Fellgett's gain) compared to gratings and Fabry-Perots. However, challenges remain in miniaturizing Fourier transform infrared spectrometers. The 90-degree rotated beam path is difficult to manufacture when scaled down, and is especially difficult for mass production. This means that it is difficult to provide a miniaturized spectrometer at a sufficiently reduced cost.

[0014] Furthermore, there are difficulties involved in providing two optical path lengths that can be made equal when designing MEMS due to issues such as pulling phenomena and tuning range. Furthermore, light must be coupled and decoupled into such systems via optical fibers, further increasing the difficulty (and associated cost) in manufacturing the system.

[0015] In addition to the above, in many miniaturized spectrometers, comb drives are used to move the movable mirror. The need for comb drives increases the size and associated cost of the physical device because they are relatively complex to manufacture.

[0016] It is an object of the present invention to overcome some or all of the problems associated with spectrometers and methods of spectroscopy described above. Summary of the Invention

[0017] According to a first aspect of the present invention, there is provided a Fourier transform spectrometer comprising an interferometer comprising a fixed mirror, a movable mirror, circuitry, electrodes, and a detector, wherein the mirrors are arranged parallel to each other to form an optical cavity, the movable mirror is operable to move relative to the fixed mirror to vary the length of the optical cavity, a first of the mirrors is arranged such that received light enters the optical cavity through a rear part of the first mirror, the detector is arranged and operable to detect light that exits the optical cavity through a rear part of a second of the mirrors and output an interferogram, the mirrors are spaced apart from each other so that the movable mirror is at or movable to a position where at least two optical paths to the detector for the received light reflected within the interferometer are equal in length, the movable mirror and the electrodes are arranged at a distance from each other, a circuit is connected to the movable mirror and the electrodes and is operable to apply an electric field between the movable mirror and the electrodes to move the movable mirror relative to the fixed mirror, and the electrodes are arranged such that the movable mirror is between the fixed mirror and the electrodes.

[0018] The Fourier transform spectrometer of the present invention can apply a Fourier transform to certain high-order combinations of these interferences of multiple reflections. These interferences occur when at least two paths of reflected light within the interferometer are equal in length. This contrasts with Fourier transform spectrometers using a Michelson interferometer, in which only one reflection occurs at each mirror surface and the Fourier transform is applied only to the interference of the two zero-order reflected beams. It is not possible to measure the zero-order interference of two beams reflected from the front side of a mirror in the required "in-line" mirror-detector configuration. This is because this interference occurs when the path difference between the reflected light and the light transmitted straight through the optical cavity is equal, and therefore occurs when the distance between the mirrors is zero.

[0019] While still a Fourier transform spectrometer, the spectrometer combines the advantages of placing the mirror and detector "in-line" with each other and with the light from the sample (the spectrometer is easier to miniaturize and therefore cheaper) with the associated advantages of a wide wavelength range, high spectral resolution, and high signal-to-noise ratio.

[0020] The mirrors may be spaced apart from one another such that the movable mirror can move through positions where at least two paths for reflected light within the interferometer are of equal length.

[0021] The interferometer may include three or more reflective surfaces arranged so that light received on its path to the detector is reflected from two or more reflective surfaces.

[0022] One or more mirrors may have one or more reflective surfaces. One or more mirrors may have two or more reflective surfaces. The or each front portion of one or more mirrors may be a reflective surface. The or each rear portion of one or more mirrors may be a reflective surface. Each reflective surface may be operable to reflect received light from its front and / or rear portion. The reflective surfaces may be arranged such that received light on two or more optical paths may be reflected from one or more of the same reflective surfaces.

[0023] The mirrors may be positioned such that received light on one or more optical paths reflects within the one or more mirrors. The mirrors may be positioned such that received light on one or more optical paths reflects within the optical cavity. The mirrors may be positioned such that received light on one or more optical paths circulates within the one or more mirrors. The mirrors may be positioned such that received light on one or more optical paths circulates within the optical cavity.

[0024] One or more mirrors may have a coating on their front and / or rear. The coating may be an anti-reflective coating. The one or more coatings may each comprise one or more reflective surfaces. The one or more coatings may comprise an electrical insulator. The fixed mirror may have a coating on its front. The fixed mirror may have a coating on its rear. The movable mirror may have an uncoated front. The movable mirror may have an uncoated rear.

[0025] The Fourier transform spectrometer may include a light source positioned to direct light into the optical cavity through the back of the first mirror.

[0026] The Fourier transform spectrometer may include a sample holder operable to receive and hold the sample in a fixed position such that light from the light source passes through or is reflected from the sample before entering the optical cavity.

[0027] Each mirror may include silicon. Each mirror may be comprised of silicon.

[0028] Forming the mirrors entirely from silicon can simplify the process of forming a Fourier transform spectrometer because, for example, there is no need to fabricate Bragg mirrors.

[0029] Each mirror may be flat. Each mirror may be rectangular. The first mirror may be a fixed mirror. The second mirror may be a movable mirror.

[0030] The interferometer may include a housing. The mirror may be connected to the housing. The fixed mirror may be connected to the housing such that its position is fixed relative to the housing. The fixed mirror may be connected to the housing by essentially forming it with the housing. The movable mirror may be connected to the housing such that the movable mirror is movable relative to the housing. The movement of the movable mirror may be elastic. The interferometer may include one or more elastic elements connecting the movable mirror to the housing, the movable mirror being movable on the or each elastic element. The or each elastic element may be a flexure. The or each flexure may be a helical flexure. The second mirror may be circular. The or each helical flexure may partially or completely surround the circular mirror. There may be four flexures. If there are multiple flexures, the locations where each flexure connects to the mirror and / or housing may be spaced apart from each other at regular intervals. The or each flexure may surround the mirror 360 degrees before connecting to the housing. The or each flexure may comprise silicone. The or each elastic element may be constructed from silicone.

[0031] The Fourier transform spectrometer may include an analysis unit operable to receive the interferogram and perform a Fourier transform on the interferogram to obtain a spectrum for the sample. The analysis unit may be operable to perform a Fourier transform on a specific higher order combination of reflections other than the zeroth order. The analysis unit may be operable to perform a Fourier transform on first order interference. The spectrum may cover a wavelength range of 1 μm to 2.6 μm. The spectrum may cover one or more, or all, of visible, near-infrared, short-wavelength infrared, mid-wavelength infrared, and / or long-wavelength infrared.

[0032] The Fourier transform spectrometer comprises a circuit and electrodes, the circuit connected to the movable mirror and the electrodes and operable to apply an electric field between the movable mirror and the electrodes to move the movable mirror relative to the fixed mirror. The circuit may be operable to apply a voltage difference between the movable mirror and the electrodes to apply the electric field. The electrodes are positioned such that the movable mirror is between the fixed mirror and the electrodes. The circuit may be operable to apply a voltage difference between the movable mirror and the electrodes to apply the electric field.

[0033] By positioning the electrodes so that the movable mirror is between them, the means for actuating the movable mirror (application of an electric field between the movable mirror and the electrode) is decoupled from the optical cavity. If an electric field were applied across the optical cavity (i.e., between the fixed and movable mirror), the range of movement of the movable mirror and the minimum size of the optical cavity would be limited to approximately one-third of the original distance between the mirrors. If the movable mirror were moved closer, the size of the electric field required would lead to pulling.

[0034] By arranging the electrodes as described, the pulling does not limit the minimum size of the optical cavity or the range over which the movable mirror can move, because when the electric field is zero, the movable mirror is as close as desired to the fixed mirror, and as the electric field is increased, the movable mirror moves away from the fixed mirror.

[0035] The electrode may be positioned between the movable mirror and the detector. The electrode may have an aperture therethrough. The detector may be positioned below the aperture. The detector may be positioned within the aperture. The electrode may be positioned alongside the detector. The electrode and detector may be located together on a base.

[0036] The electrode may comprise a 2D material. The 2D material is less than 10 nanometers thick. The 2D material may be conductive. The 2D material may be transparent. The electrode may comprise graphene. The graphene may be conductive. The graphene may be transparent. The electrode may comprise graphene on quartz. The electrode may comprise graphene on quartz. The graphene may have a thickness of less than 10 nanometers.

[0037] By forming the electrodes from graphene, they are transparent and can ultimately be made thin so that they do not affect the optical properties of the interferometer, even when placed between the movable mirror and the detector.

[0038] The electrodes may be parallel to each mirror. The electrodes may form a base of the housing. Portions of the walls may extend across each other from the movable mirror to the electrodes.

[0039] The mirrors may be spaced apart from one another such that the movable mirror is at or can be moved to a position where the length of the optical cavity is between 20 μm and 45 μm. The mirrors may be spaced apart from one another such that the movable mirror is at or can be moved to a position where the length of the optical cavity is between 25 μm and 40 μm. The mirrors may be spaced apart from one another such that the movable mirror is at or can be moved to a position where the length of the optical cavity is between 30 μm and 35 μm.

[0040] The movable mirror may be movable along a line. The line may be straight. The movable mirror may only be movable along a line. The line may extend perpendicularly from the movable mirror. The line may extend perpendicularly from the rear of the movable mirror. The line may start at the rear of the movable mirror.

[0041] The Fourier transform spectrometer may be an infrared Fourier transform spectrometer.

[0042] According to a second aspect of the present invention, there is provided a mobile electronic device comprising the Fourier transform spectrometer of the first aspect.

[0043] Providing a Fourier transform spectrometer in a mobile electronic device would bring spectroscopy into everyday use for the general public, greatly expanding the applications and usefulness of spectroscopy.

[0044] The mobile electronic device may be a mobile phone, a tablet, or a laptop.

[0045] According to a third aspect of the present invention there is provided an interferometer for use in a Fourier transform spectrometer comprising: a fixed mirror and a movable mirror, the mirrors forming an optical cavity, the movable mirror operable to move relative to the fixed mirror so as to vary the size of the optical cavity; and circuitry and electrodes connected to the movable mirror and the electrodes and operable to form an electric field between the two to cause the movable mirror to move relative to the fixed mirror, the movable mirror being disposed between the fixed mirror and the electrodes.

[0046] By positioning the electrodes so that the movable mirror is between them and the fixed mirror, the means for actuating the movable mirror (application of an electric field between the movable mirror and the electrodes) is decoupled from the optical cavity. Thus, retraction does not limit the minimum size of the optical cavity or the range over which the movable mirror can move. This is because when the electric field is zero, the movable mirror is as close as desired to the fixed mirror, and as the electric field is increased, the movable mirror moves away from the fixed mirror. The electrodes may be positioned between the movable mirror and the detector.

[0047] The mirrors may be parallel to each other.

[0048] The third aspect of the invention may include any of the optional features of the first aspect as desired and / or appropriate.

[0049] According to a fourth aspect of the present invention, there is provided a Fourier transform spectrometer comprising the interferometer of the third aspect.

[0050] According to a fifth aspect of the present invention there is provided an interferometer for use in a Fourier transform spectrometer comprising a housing, a movable mirror, one or more flexures, circuitry and electrodes, the movable mirror being connected to the housing via the or each flexure and movable on the or each flexure relative to the housing, and the circuitry being connected to the movable mirror and the electrodes and operable to apply an electric field between the movable mirror and the electrodes to cause movement of the movable mirror.

[0051] The required configuration is much simpler to build than a comb drive, and therefore the interferometer is easier and cheaper to manufacture.

[0052] The interferometer may comprise a fixed mirror, the fixed mirror and a movable mirror arranged to form an optical cavity, the movable mirror being movable relative to the fixed mirror. The mirrors may be parallel.

[0053] The fifth aspect of the invention may include any of the optional features of the first aspect as desired and / or appropriate.

[0054] According to a sixth aspect of the present invention, there is provided a Fourier transform spectrometer comprising the interferometer of the fifth aspect.

[0055] According to a seventh aspect of the present invention, there is provided a method of Fourier transform spectroscopy comprising the steps of: moving a moveable mirror of an interferometer from or through a position where at least two paths for light reflected in the interferometer are of equal length, and detecting the light with a detector of the interferometer to obtain an interferogram for the sample; and applying a Fourier transform to the interference of a dimension of the interferogram other than the zeroth order interference to obtain a spectrum.

[0056] By applying a Fourier transform to interferences of other orders than the zeroth order, types of interferometers that cannot provide interferograms for zeroth order interferences but can provide interferograms for higher order interferences can be used in this method. Such interferometers do not need to have beams rotated by 90 degrees, making it easier and cheaper to build miniaturized spectrometers.

[0057] Fourier transform spectroscopy may involve applying a Fourier transform to the first order interference.

[0058] The Fourier transform spectroscopy method may use the Fourier transform spectrometer of the first aspect to perform each step.

[0059] The Fourier transform spectroscopy may be infrared Fourier transform spectroscopy. The spectrum may cover a wavelength range from 1 μm to 2.6 μm.

[0060] In order that the invention may be more clearly understood, one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0061] [Figure 1] FIG. 1 is a diagram of the absorption spectrum of dichloromethane. [Figure 2] FIG. 1 is a diagram of a lattice configuration. [Figure 3]FIG. 1 is a diagram of a Fabry-Perot interferometer. [Figure 4] FIG. 1 is a diagram of a Michelson interferometer. [Figure 5] FIG. 1 is a diagram of an interferometer of an infrared Fourier transform spectrometer. [Figure 6] FIG. 1 is a diagram of a simulated interferogram from an infrared Fourier transform spectrometer. [Figure 7] FIG. 1 shows an interferogram of a first-order sideburst. [Figure 8] 1 is a diagram of the spectrum of white light obtained by an infrared Fourier transform spectrometer. [Figure 9] FIG. 6 is a diagram of a second, movable mirror of the interferometer of FIG. 5. [Figure 10] FIG. 1 is a diagram of an interferometer of an infrared Fourier transform spectrometer, including the electrodes and voltage sources required to move the second mirror. [Figure 11] 1 illustrates a step-by-step process in forming the first mirror of an infrared Fourier transform spectrometer. [Figure 12] 1 illustrates a step-by-step process in forming the second mirror of an infrared Fourier transform spectrometer. [Figure 13] 1 shows an infrared Fourier transform spectrometer. [Figure 14] 1 shows spectra obtained for acrylic by an infrared Fourier transform spectrometer and a prior art spectrometer. [Figure 15] FIG. 1 shows spectra obtained for milk of different fat contents. DETAILED DESCRIPTION OF THE INVENTION

[0062] As shown in FIG. 5, an interferometer 10 of an infrared Fourier transform spectrometer includes two mirrors 11 and 12. Mirrors 11 and 12 are parallel to each other and face each other to form a cavity 13. A first mirror 11 (a fixed mirror) is disposed above a second mirror 12 (a movable mirror). The second mirror 12 is movable relative to the first mirror 11 toward and away from the first mirror 11 to change the size and distance of the cavity between the mirrors 11 and 12. The mirrors 11 and 12 are held in a housing formed by two walls 30, where the first mirror 11 is essentially formed with the wall 30 and the second mirror 12 is connected to the wall 30 so that the second mirror 12 is movable relative to the walls.

[0063] Light 14 incident on the sample of interest is transmitted through the rear of the first mirror 11. Thus, there are multiple paths through mirrors 11, 12, and optical cavity 13. A first path 15 of light is transmitted straight out of cavity 13 and through second mirror 12, with the majority of light 14 taking this first path 15. However, some of the light 14 takes other paths within and through first and second mirrors 11, 12, and through cavity 13. For example, as shown in FIG. 5, a second path 16 of light is transmitted out the rear of second mirror 12, where it is reflected back and forth between the front faces of mirrors 11 and 12 that form the cavity before exiting cavity 13. A further path 31 of light 14 is reflected off the rear and front faces within second mirror 12 before exiting the rear of second mirror 12. Another example is a path in which light enters the cavity 13 and is reflected at the front and back surfaces of the first mirror 11 before being transmitted straight through the second mirror 12 .

[0064] The position of the second mirror 12 can be adjusted so that path 31 (and other paths along which light is reflected within the interferometer) can have the same length as the second path 16 and / or other paths along which light is reflected within the interferometer, leading to interference. This interference is a higher-order interference than zero-order interference and is known as a "sideburst" on the interferogram. As shown in FIG. 6, the sidebursts 17, 18 are not as strong a signal as the zero-order interference 19 between the light on the first path 15 and the light on the second path 16. However, the first-order sideburst 17 is a signal large enough to be converted into a spectrum, and this occurs when the mirrors are at a certain distance from each other, making it possible to obtain a complete measurement of the sideburst needed to obtain a spectrum. In contrast, zero-order interference occurs when the gap distance is zero, which is not measurable.

[0065] In use, the position of the second mirror 12 will be moved through the gap distance where the second path 16 and the third path 31 are of equal length (or the gap distance where two other paths other than the first path 15 are of equal length), which for the configuration shown in this description is approximately 35 μm. Detector 101 (shown in FIG. 13) measures as the second mirror 12 moves to acquire first-order sidebursts 17, as shown in FIG. 7.

[0066] The displacement of the second mirror 12 is measured using a non-contact distance sensor such as a laser interferometer, a capacitance sensor, or the like.

[0067] The analysis unit 102 of the Fourier transform infrared spectrometer 100, also shown in Figure 13, can then perform a Fourier transform on the acquired interferogram, resulting in an absorption spectrum. Figure 8 shows the resulting constructed spectrum for white light in the absence of a sample.

[0068] The first mirror 11 and the second mirror 12 are constructed of silicon. Figure 9 shows the second mirror 12 used in this configuration. The mirror 12 is circular and is partially cut out of a sheet 20 of silicon. The mirror 12 remains connected to the remainder of the sheet 20 by a strip of silicon 21 that spirals from the mirror 12 to the remainder of the sheet 20, which thus forms part of the wall 30 of the interferometer 10. There are four flexures 21, each spiraling around the entire circumference of the mirror 12, so that the location where the flexure 21 connects to the mirror 12 around the circumference is the same relative to the circumference where the flexure 21 connects to the remainder of the sheet 20. There are four flexures 21 evenly spaced around the circumference.

[0069] Flexure 21 allows second mirror 12 to move up and down relative to the rest of sheet 20 (i.e., move perpendicular to the plane of sheet 20), thereby allowing this mirror to move relative to first mirror 11 and therefore change the distance and size of the gap in cavity 13.

[0070] As shown in FIG. 10 , an electrode is positioned away from the rear surface of the second mirror 12, such that the first mirror 11, the second mirror 12, and the electrode 22 lie in three parallel planes, with the second mirror 12 between the first mirror 11 and the electrode 22. The second mirror 12 and the electrode 22 are connected so that a voltage can be applied between the second mirror 12 and the electrode 22, thereby creating an electric field. The second mirror 12 is a circular mirror held in place by a helical curvature 21, meaning that the presence of an electric field displaces the second mirror 12 away from the first mirror 11 toward the electrode 22, the distance of displacement depending on the strength of the electric field. The second mirror 12 can be displaced by tens of microns. In use, an electric field is applied via the electrode, and its strength is controlled and varied to move the second mirror 12 through the primary sideburst.

[0071] The electrode 22 is graphene on quartz, which makes it transparent and therefore has minimal effect on the optical cavity 13 formed between the first mirror 11 and the second mirror 12.

[0072] As shown in Figure 11, the first mirror 11 is formed from a block 23 that includes two layers of silicon 24, 25. The first layer 24 is approximately 500 μm thick, and the second layer 25 is approximately 30 μm to approximately 50 μm thick. Sandwiched between the first layer 24 and the second layer 25 is a thin insulating layer 26. Sandwiching the first layer 24, the second layer 25, and the insulating layer 26 are two thin layers 27 of silicon nitride.

[0073] To form the first mirror 11, as shown in Figure 11, part of the second layer of silicon 25 and the layer of silicon nitride 27 covering part of the second layer of silicon 25 are removed using photolithography (reactive ion etching for the silicon nitride layer 27 and potassium hydroxide for etching the silicon 25) to expose the insulating layer 26. The second layer 26 and the covering layer of silicon nitride 27 are etched so that the cut-out section has a square cross-section and slopes down to the silicon 24.

[0074] The resulting cap 28 forms the first mirror 11, the cavity 13, and part of the wall 30, with the middle portion of the first layer of silicon 24 being the first mirror 11, the cut-out section forming the cavity 13, and the remaining portions of the block 23 forming parts of the wall 30.

[0075] As shown in Figure 12, the second mirror 12 is formed from a block 32 that includes two layers of silicon 33, 34. The first layer 33 is approximately 10 μm thick, and the second layer 34 is several hundred micrometers thick, e.g., approximately 500 μm. Sandwiched between the first layer 33 and the second layer 34 is a thin insulating layer 35. Sandwiching the first layer 33, the second layer 34, and the insulating layer 35 are two thin layers 36 of silicon nitride.

[0076] For the second mirror 12, a cap 28 is formed like the cap for the first mirror 11, as shown in Figure 12. Once this is done, lithography, specifically deep reactive ion etching, is used to cut a spiral bend (not shown in Figure 12) into the first layer 33. In doing so, the layer of silicon nitride 36 on top of the first layer 33 is also removed from the top of the spiral bend 21.

[0077] The next step is to remove the insulator 35 on the mirror 12 and spiral flexure 21 using vapor phase hydrofluoric acid. To complete the process, a microelectromechanical system (MEMS) 29 is formed with the remaining layer of silicon nitride 36 from on top of the first layer 33. The sections of the MEMS 29 other than the second mirror 12 and spiral flexure 21, along with sections of cap 28, form the interferometer walls 30.

[0078] To form the interferometer 10, the MEMS 29 is placed on top of the electrode 22, with the cutout section of the MEMS 29 between the second mirror 12 and the electrode 22. The cap 28 is then placed on top of the MEMS 29, such that the MEMS 29 is between the cap 28 and the electrode 22, and the cutout section of the MEMS 29 is between the second mirror 12 and the electrode 22. All three are then fixed in place, with the detector positioned behind the electrode 22 and the second mirror 12. A light source can be integrated into the system behind the front mirror 11. Alternatively, the system can be configured to use an external light source.

[0079] The infrared Fourier transform spectrometer 10 has a broad spectral range from about 1 μm to about 2.6 μm, with a spectral resolution down to about 10 nm, while the interferometer is only a few tens of millimeters cubed in size (having dimensions of roughly 4×4×1.5 mm, so 24 mm 3 (where .gtoreq.1.0, .gtoreq.1.0). Thus, the infrared Fourier transform spectrometer 10 can be integrated into mobile electronic devices such as mobile phones and tablets, providing the mobile electronic devices with spectrometer functionality.

[0080] As shown in FIG. 14, by comparing a spectrum for acrylic obtained by infrared Fourier transform spectrometer 100 side-by-side with a spectrum for the same material obtained by a prior art spectrometer, the obtained spectrum can be compared to conventional spectrometers and used to identify materials with known spectra.

[0081] As shown by FIG. 15, the infrared Fourier transform spectrometer 100 is sufficiently accurate to distinguish between substances with similar spectra, in the illustrated case milks with different fat contents.

[0082] One or more embodiments have been described above by way of example only, and many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

1. A Fourier transform spectrometer comprising an interferometer comprising a fixed mirror, a movable mirror, circuitry, electrodes, and a detector, the mirrors being arranged parallel to one another to form an optical cavity, the movable mirror being operable to move relative to the fixed mirror to vary the length of the optical cavity, a first one of the mirrors being arranged such that received light enters the optical cavity through a rear portion of the first mirror, and the detector being arranged and operable to detect light exiting the optical cavity through a rear portion of a second one of the mirrors and outputting an interferogram. the mirrors are spaced apart from one another so that the movable mirror is at or movable to a position where at least two optical paths to the detector for received light reflected within the interferometer are equal in length; the movable mirror and electrodes are positioned at a distance from one another; the circuit is connected to the movable mirror and the electrodes and is operable to apply an electric field between the movable mirror and the electrodes to move the movable mirror relative to the fixed mirror; and the electrodes are positioned so that the movable mirror is between the fixed mirror and the electrodes.

2. The Fourier transform spectrometer of claim 1 , wherein the mirror comprises silicon.

3. 3. A Fourier transform spectrometer according to claim 1 or claim 2, wherein the interferometer comprises a housing.

4. 4. A Fourier transform spectrometer according to claim 3, wherein the interferometer comprises one or more elastic elements connecting the movable mirror to the housing, the movable mirror being movable on the or each elastic element.

5. 5. A Fourier transform spectrometer as claimed in claim 4, wherein the or each elastic element is a flexure.

6. 6. The Fourier transform spectrometer of claim 1, wherein the electrodes comprise graphene.

7. The Fourier transform spectrometer of claim 6 , wherein the electrodes comprise graphene on quartz.

8. 8. A Fourier transform spectrometer according to any one of claims 1 to 7, wherein the mirrors are spaced apart from each other such that the movable mirror is at or can be moved to a position where the optical cavity is between 20 μm and 45 μm apart.

9. 9. The Fourier transform spectrometer of claim 8, wherein the mirrors are spaced apart such that the movable mirror is at or can be moved to a position where the optical cavity is between 25 μm and 40 μm apart.

10. 10. The Fourier transform spectrometer of claim 9, wherein the mirrors are spaced apart from one another such that the movable mirror is at or can be moved to a position where the optical cavity is between 30 μm and 35 μm apart.

11. 11. A Fourier transform spectrometer according to any one of claims 1 to 10, comprising an analysis unit operable to receive the interferogram and perform a Fourier transform on the interferogram to obtain an optical spectrum for a sample.

12. 12. The Fourier transform spectrometer of claim 11, wherein the analysis unit is operable to perform the Fourier transform on an order of interference other than zero-order interference.

13. 13. The Fourier transform spectrometer of claim 12, wherein the analysis unit is operable to perform the Fourier transform on a first order interference.

14. 14. A method of Fourier transform spectroscopy, comprising the steps of providing a Fourier transform spectrometer according to any one of claims 1 to 13, moving the movable mirror of the interferometer from or through a position where at least two paths for light reflected within the interferometer are equal in length and detecting light with the detector of the interferometer to obtain an interferogram for a sample, and applying a Fourier transform to interference of a dimension of the interferogram other than the zeroth order interference to obtain a spectrum.

15. 15. The method of Fourier transform spectroscopy of claim 14, comprising applying the Fourier transform to first order interference.

Images