Patent ID: 12209905

DETAILED DESCRIPTION

Referring toFIG.2, a spectrometer700may comprise a Fabry-Perot interferometer300. An object OBJ1may reflect, emit and/or transmit light LB1, which may be transmitted through the interferometer300in order to monitor the spectrum of the light LB1. The interferometer300may be used e.g. for measuring reflection, transmission (absorption) and/or emission of the light LB1of the object OBJ1.

The Fabry-Perot interferometer300comprises a first mirror plate100and a second mirror plate200. The first mirror plate100comprises a first semi-transparent mirror M1, and the second mirror plate200comprises a second semi-transparent mirror M2. The mirrors M1, M2may have e.g. a substantially circular form or a substantially rectangular form.

The distance dFbetween the mirrors M1, M2may be adjusted to provide constructive interference for transmitted light at one or more given wavelengths so that the interferometer300may transmit light. The distance dFmay also be adjusted to provide destructive interference for transmitted light at the given wavelength so that the interferometer300may reflect light.

The mirror distance dFmay be adjusted by one or more actuators ACU1, ACU2. One or more actuators may be arranged to move the second mirror plate200with respect to the first mirror plate100. The actuator ACU1, ACU2may be e.g. a piezoelectric actuator, an electrostatic actuator, an electrostrictive actuator, or a flexoelectric actuator.

The spectrometer700may comprise a control unit CNT1. The control unit CNT1may be arranged to send a control signal SETDto the interferometer300in order to adjust the mirror gap dF. The interferometer300may comprise a driver unit420. The driver unit420may e.g. convert a digital control signal SETDinto an analog signal suitable for driving one or more actuators. The driver unit420may provide a signal HV1for driving an actuator. The driver unit420may provide e.g. a high voltage signal HV1for driving a piezoelectric actuator.

The mirrors M1, M2may be substantially flat and substantially parallel to each other. The flatness of the mirror M1, M2may be e.g. better than λN/200, in order to provide a suitable finesse (i.e. the ratio of the free spectral range to the spectral width of a transmission peak). λNdenotes a predetermined operating wavelength. The predetermined operating wavelength λNmay be e.g. in the range of 500 nm to 4000 nm. The predetermined operating wavelength λNmay be e.g. 2000 nm or 4000 nm.

The interferometer300may optionally comprise means for monitoring the distance dFbetween the mirrors and/or the mirror plates. The interferometer300may comprise e.g. capacitive means for monitoring the distance. The interferometer300may comprise e.g. inductive means for monitoring the distance. The interferometer300may comprise e.g. interferometric means for monitoring the distance.

The interferometer300may optionally comprise capacitive sensor electrodes for capacitively monitoring mirror distance dF. Sensor electrodes G1a,G2amay together form a sensor capacitor C1, wherein the capacitance value of the sensor capacitor C1may depend on the mirror distance dF. Consequently, the mirror distance dFmay be monitored by monitoring the capacitance value of the sensor capacitor C1. The sensor capacitor C1may be connected to a capacitance monitoring unit410e.g. by conductors CONa, CONb. The capacitance monitoring unit410may provide a sensor signal Sdindicative of the mirror distance dF. The sensor capacitor C1may also be implemented e.g. as shown inFIG.7a.

The capacitance monitoring unit410may provide a sensor signal Sd. The sensor signal may be used for monitoring the mirror gap dF. The spectral response of the spectrometer700may be calibrated e.g. as a function of the mirror gap dF. The spectrometer700may comprise a memory MEM2for storing spectral calibration parameters DPAR2. The mirror gap dFand/or a spectral position λ may be determined from the sensor signal Sde.g. by using the spectral calibration parameters DPAR2.

The interferometer300may also comprise two or more sensor capacitors for monitoring the parallelism of the mirrors M1, M2, in addition to monitoring the mirror distance dFbetween the mirrors M1, M2. The mirror M2may be adjusted to be parallel with the mirror M1e.g. by controlling one or more of the actuators ACU1, ACU2of the interferometer300. However, geometric deformation of the shape of the mirror M1and/or M2cannot be typically corrected by using the actuators ACU1, ACU2

The Fabry-Perot interferometer300may form transmitted light LB2by filtering the light LB1obtained from the object OBJ1. The spectrometer700may comprise an optical detector DET1. The interferometer300may be optically coupled to the detector DET1. The transmitted light LB2may impinge on the detector DET1. The detector DET1may be e.g. an image sensor or a non-imaging detector. The detector DET1may provide one or more intensity signals SDET1indicative of the intensity of the transmitted light LB2.

The spectrometer700may optionally comprise imaging, focusing, or collimating optics500. The optics500may be arranged to focus light LB2to the detector DET1. The optics500may also be positioned between the interferometer300and the sensor DET1.

The interferometer300may also be positioned e.g. within multi-element optics500. The interferometer300may be positioned between elements of multi-element optics500.

The spectrometer700may optionally comprise a memory MEM1for storing intensity calibration parameters CALPAR1. The spectrometer700may be arranged to obtain detector signal values SDET1from the detector DET1, and to determine intensity values X(λ) from the detector signal values SDET1by using one or more intensity calibration parameters CALPAR1. At each mirror distance value dF, an intensity value X(λ) of the light LB1may be determined from a detector signal SDET1by using the one or more intensity calibration parameters CALPAR1.

The spectrometer700may optionally comprise a memory MEM3for storing output OUT1. The output OUT1may comprise e.g. detector signals SDET1and/or intensity values determined from the detector signals. The output OUT1may e.g. comprise one or more digital images of the object OBJ1, captured by using the interferometer300as a spectrally selective filter.

The spectrometer700may comprise a memory MEM4for storing a computer program PROG1. The computer program PROG1may be configured, when executed by one or more data processors (e.g. CNT1), cause the apparatus300,700perform one or more of the following:measure a distance dFbetween the mirrors M1, M2,set a transmittance peak of the interferometer300to a selected position,cause spectral scanning of the interferometer300,measure a spectral intensity value,measure a spectrum,adjust parallelism (tilt angle) of the mirrors M1, M2.

The spectrometer700may optionally comprise a user interface USR1e.g. for displaying information to a user and/or for receiving commands from the user. The user interface USR1may comprise e.g. a display, a keypad and/or a touch screen.

The spectrometer700may optionally comprise a communication unit RXTX1. The communication unit RXTX1may transmit and/or receive a signal COM1e.g. in order to receive commands, to receive calibration data, and/or to send output data OUT1. The communication unit RXTX1may have e.g. wired and/or wireless communication capabilities. The communication unit RXTX1may be arranged to communicate e.g. with a local wireless network (Bluetooth, WLAN), with the Internet and/or with a mobile communications network (4G, 5G).

The spectrometer700may optionally comprise one or more optical cut-off filters510to limit the spectral response of the detector DET1.

SX, SY and SZ denote orthogonal directions. The light LB2may propagate substantially in the direction −SZ. The optical axis AX1of the interferometer300may be parallel with the direction SZ. The light LB2may propagate substantially in the direction of the optical axis AX1. The light LB2may propagate substantially in the direction −SZ.

FIG.3shows, by way of example, a spectral transmittance of a Fabry-Perot interferometer300, and the pass band of an optional filter510. The uppermost curve ofFIG.3shows the spectral transmittance TF(λ) of the Fabry-Perot interferometer300. The spectral transmittance TF(λ) may have one or more adjacent transmittance peaks PEAK1, PEAK2, PEAK3of the Fabry-Perot interferometer300. For example, a first transmittance peak PEAK1may be at a wavelength λ0, a second transmittance peak PEAK2may be at a wavelength λ1, and a third transmittance peak PEAK3may be at a wavelength λ2. The spectral positions λ0, λ1, λ2of the transmission peaks PEAK1, PEAK2, PEAK3may depend on the mirror distance dFaccording to the Fabry-Perot transmission function. The spectral positions of the transmission peaks may be changed by changing the mirror gap dF. The spectral positions of the transmission peaks may be changed by tuning the mirror gap dF. The transmission peaks PEAK1, PEAK2, PEAK3may also be called passbands of the Fabry-Perot interferometer.

The spectrometer700may optionally comprise one or more optical cut-off filters510to limit the spectral response of the spectrometer700. The one or more filters510may together provide a spectral transmittance TS(λ). The one or more filters510may provide an optical band pass filter defined by cut-off wavelengths λminand λmax.

Referring toFIGS.4aand4b, the Fabry-Perot interferometer300may comprise a first mirror plate100, a second mirror plate200, and one or more actuators ACU1, ACU2to change the distance dFbetween the mirrors M1, M2of the interferometer300. The first mirror plate100may have a first semi-transmissive mirror M1, and the second mirror plate200may have a second semi-transmissive mirror M2. The M1, M2may be planar with an accuracy, which is e.g. better than λ/200.

The width w100of the first mirror plate100may be e.g. in the range of 5 mm to 50 mm.

The distance dFbetween the semi-transparent mirrors M1, M2may be e.g. in the range of 0.5 μm to 10 μm. The distance dFbetween the semi-transparent mirrors M1, M2may be e.g. in the range of 0.2 μm to 10 μm.

The first mirror plate100may be stationary, and the second mirror plate200may be moved by the actuators ACU1, ACU2, with respect to the first mirror plate100.

The Fabry-Perot interferometer300may comprise two or more actuators ACU1, ACU2to change the distance dFbetween the mirrors M1, M2of the interferometer300and/or to adjust the tilt angle between the mirrors M1, M2.

The first mirror plate100and/or the second mirror plate200may be supported by supporting elements S1. The supporting elements S1may be e.g. bent pieces of metal sheet or blocks. The mirror plates100,200may be bonded to the supporting elements S1by joints J1. The joints J1may be positioned close to the middle plane of the mirror plates100,200, so as to reduce deformation of the shape of the mirrors M1, M2.

The semi-transparent mirrors M1, M2may be e.g. dielectric multilayer coatings deposited on a transparent substrate. The substrate material of the mirror plates100,200may be transparent in the operating wavelength range of the interferometer300. The material of the mirror plates100,200may be e.g. glass, silica, silicon or sapphire.

The supporting elements S1may e.g. comprise or consist of a metal, glass, silicon (Si), silica (SiO2), sapphire (Al203) or ceramic material. Supporting block elements S1(FIG.4b) may provide stable support for the mirror plate100,200.

Block elements S1ofFIG.4bmay sometimes transfer horizontal forces (e.g. in direction SX) to the mirror plate100,200. Using the slightly flexible supporting elements ofFIG.4amay reduce the magnitude of forces transferred to the mirror plate100,200, when compared with the supporting blocks ofFIG.4b. Flexible supporting elements S1may e.g. comprise spring steel or Invar alloy (FeNi36).

The material of the supporting elements S1for the stationary mirror plate (e.g. M1) may be selected such that the thermal expansion of the supporting elements S1may at least partly compensate thermal expansion of the joints J2. The material of the supporting elements S1for the movable mirror plate (e.g. M2) may be selected such that the thermal expansion of the supporting elements S1may at least partly compensate thermal expansion of the joints J2, J3and thermal expansion of the one or more actuators ACU1.

The slightly flexible supporting elements ofFIG.4amay provide a less stable support for the mirror plates, when compared with the supporting blocks ofFIG.4b. However, the reduced stability of the flexible elements S1may be at least partly compensated by monitoring and correcting the position of the mirror plate200by using the actuators. The position of the mirror plate200may be monitored e.g. by using the capacitive sensor electrodes G1a,G2a,G1b,G2b.The position of the mirror plate200may be corrected by using the actuators ACU1, ACU2.

The interferometer300may comprise a base plate BASE1. The first mirror plate100and the second mirror plate200may be attached to the base plate BASE1via the supporting elements S1.

The first mirror plate100may be bonded to the supporting elements S1by joints J1. The support elements S1of the first mirror plate100may be attached to the base plate BASE1by joints J2. The support elements S1of the first mirror plate100may be attached e.g. to a mounting surface SRF3of the base plate BASE1by the joints J2. The surface SRF3may be e.g. substantially perpendicular to the optical axis AX1of the interferometer300.

The second mirror plate200may be bonded to the supporting elements S1by joints J1. The support elements S1of the second mirror plate200may be attached to the actuators ACU1, ACU2by joints J2. The actuators ACU1, ACU2may be attached to the base plate BASE1by joints J2. The support elements S1of the second mirror plate200may be attached e.g. to the mounting surface SRF3by the joints J2.

The joints J1, J2, J3may be e.g. adhesive joints. The joints J1, J2, J3may comprise e.g. an adhesive (ADH1). In an embodiment, the joints J1, J2, J3may also be formed e.g. by welding or soldering.

The interferometer300may be optionally assembled such that a gap GAP3remains between the first mirror plate100and the base plate BASE1. The gap GAP3may reduce the probability of causing deformation of the mirror plate100.

The interferometer300may optionally comprise capacitive sensor electrodes (G1a,G2a,G1b,G2b) for monitoring the distance dFbetween the mirrors M1, M2and/or for monitoring a tilt angle of the mirror M2with respect to the mirror M1.

The first mirror plate100may comprise sensor electrodes G1a,G1b.The second mirror plate200may comprise sensor electrodes G2a,G2b.The electrodes G1aand G2amay together form a first sensor capacitor C1, which has a capacitance C1. The electrodes G1band G2bmay together form a second sensor capacitor C2, which has a capacitance C2. The sensor capacitors C1, C2may also be implemented e.g. as shown inFIG.7a.

The distance between the electrodes G1a,G2amay depend on the mirror distance dF, and the capacitance C1of the first sensor capacitor C1may depend on the distance between the electrodes G1a,G2asuch that the mirror distance dFnear the electrodes G1a,G2amay be monitored by monitoring the capacitance C1of the first sensor capacitor C1. The mirror distance dFclose to the electrodes G2a,G2bmay be monitored by monitoring the capacitance C2of the second sensor capacitor C2, respectively.

The mirror distance dFand/or tilt angle may also be monitored e.g. by analyzing one or more optical signals transmitted through the interferometer300. The sensor electrodes may also be omitted.

The base plate BASE1may optionally comprise an opening OPE1for light LB1, LB2, which is transmitted through the interferometer300.

Referring toFIGS.5aand5b, each supporting element S1of the first mirror plate100may be bonded to the first mirror plate100by a joint J1. The mirror plate100may be bonded to one or more supporting elements S1by three or more joints J1. In particular, the number of the joints J1of the first mirror plate100may be equal to three in order to minimize internal stress of the mirror plate100. The joints J1may be e.g. adhesive joints. Each joint J1may be bonded to the first mirror plate100at a bonding region REG1.

The mirror plate100may have a first surface SRF11and a second surface SRF12. The surface SRF11may be at least partly defined by the outer surface of the first mirror M1. The surface SRF11may be parallel with the first mirror M1. The first surface SRF11and a second surface SRF12may define the thickness h100of the mirror plate100. The second surface SRF12may be e.g. substantially parallel with the first surface SRF11. The second surface SRF12may also be slightly tilted with respect to the first surface SRF11in order to reduce an effect of unwanted reflections. The first surface SRF11and the second surface SRF12may define a non-zero wedge angle.

The joints J1of the first mirror plate100may be positioned close to the middle plane PLN1of the first mirror plate100. Positioning the joints J1close to the middle plane may reduce or minimize geometric deformation of the mirrors M1, M2away from the planar shape. A joint J1may transfer a horizontal deforming force (e.g. in the direction SX) to the mirror plate.

The distance d1between each bonding region REG1and the first substantially planar surface SRF11may be e.g. greater than 30% of the thickness h100of the mirror plate100, and the distance d2between each bonding region REG1and the second substantially planar surface SRF12may be e.g. greater than 30% of the thickness h100of the mirror plate100.

The distance d1between each bonding region REG1and the first substantially planar surface SRF11may be e.g. greater than 30% of the thickness h100of the mirror plate100, and the distance d3between the lowermost edge of each bonding region REG1and the first substantially planar surface SRF11may be e.g. smaller than 70% of the thickness h100of the mirror plate100.

In particular, each bonding region REG1may overlap the central plane PLN1of the first mirror plate100. The central plane PLN1may meet each bonding region REG1of the first mirror plate100.

In case of a horizontal deforming force acting on the middle plane PLN1, resulting deformations of the upper surface SRF11and the lower surface SRF12may be substantially symmetric with respect to the middle plane PLN1. For example, a horizontal deforming force may cause a tendency of the upper surface SRF11to deflect upwards, but this tendency may be effectively compensated by a corresponding tendency of the lower surface SRF12to deflect downwards. Consequently, a deformation of the lower surface SRF12may substantially compensate a deformation of the upper surface SRF11.

Causing the deforming force to act on the middle plane may minimize geometric deformation of the mirrors M1, M2. In particular, causing the deforming force to act on the middle plane may minimize bending of the mirrors M1, M2.

Each bonding region REG1may be substantially perpendicular to the central plane PLN1of the first mirror plate100, e.g. in order to further ensure that horizontal forces are transferred symmetrically from the supporting elements S1to the mirror plate.

Each bonding region REG1may be located on a side surface (SRF10) of the mirror plate, the side surface (SRF10) being substantially perpendicular to the central plane PLN1of the first mirror plate100. A mirror plate100with perpendicular side surfaces (SRF10) may be substantially symmetrical with respect to the central plane PLN1, e.g. in order to minimize deformations and/or in order to simplify production of the mirror plate100.

A joint J1may also transfer a deforming torque to the mirror plate. The lo maximum dimension dMAXof the bonding region REG1may be smaller than a predetermined limit in order to minimize or avoid transferring a deforming torque from the supporting element S1to the mirror plate100. For example, the maximum dimension dMAXof each bonding region REG1of the first mirror plate100may be e.g. smaller than 30% of the thickness h100of the mirror plate100.

The bonding region REG1may have a first dimension wREG1in a first direction along the central plane PLN1, and the bonding region REG1may have a second dimension hREG1in a second direction, which is perpendicular to the first direction. The first dimension wREG1may be e.g. the width of the bonding region REG1, and the second dimension hREG1may be e.g. the height of the bonding region REG1.

For example, the height hREG1of each bonding region REG1of the first mirror plate100may be e.g. smaller than 30% of the thickness h100of the mirror plate100. For example, the width wREG1of each bonding region REG1of the first mirror plate100may also be e.g. smaller than 30% of the thickness h100of the mirror plate100.

SX, SY, and SZ may denote orthogonal directions. The mirror M1of the first mirror plate100may be in a plane defined by the directions SX and SY. SZ may denote the “vertical direction”. A mirror plate may have an “upper” surface and a “lower” surface with respect to the direction SZ. The “upper” surface SRF11may be above the “lower” surface SRF12in the direction SZ. The “lower” surface SRF12may be below the “upper” surface SRF11. Incoming light LB1may propagate in the direction -SZ from the surface SRF21to the surface SRF22. The mirror M1may be below the mirror M2. Transmitted light LB2may propagate in the direction -SZ from the surface SRF11to the surface SRF12.

The distances d1, d2, d3may be defined in the direction of the optical axis (SZ).

The middle plane PLN1of the first mirror plate100is at the halfway between the upper surface SRF11and the lower surface SRF12. The distance dpi between the middle plane PLN1and the upper surface SRF11is equal to the distance dP2between the middle plane PLN1and the lower surface SRF12.

The first surface SRF11may be substantially parallel with the second surface SRF12. The middle plane PLN1may be substantially parallel with the first surface SRF11.

The first surface SRF11and the second surface SRF12may also define a non-zero wedge angle e.g. in order to reduce unwanted reflections.

The supporting element S1may also be e.g. an annular ring, which may be attached to the first mirror plate100by three joints J1.

Referring toFIG.5c, each bonding region REG1of the second mirror plate200may overlap the central plane PLN2of the second mirror plate200. The central plane PLN2may meet each bonding region REG1of the second mirror plate200.

The second mirror plate200may have a first substantially planar surface SRF21and a second substantially planar surface SRF22. The surface SRF22may be at least partly defined by the outer surface of the second mirror M2. The surface SRF22may be parallel with the mirror M2. The first substantially planar surface SRF21and the second substantially planar surface SRF22may define the maximum thickness h200of the second mirror plate200. The middle plane PLN2of the second mirror plate200is at the halfway between the upper surface SRF21and the lower surface SRF22.

The first surface SRF21may be substantially parallel with the second surface SRF22. The middle plane PLN2may be substantially parallel with the surface SRF22. The first surface SRF21and the second surface SRF22of the plate200may also define a non-zero wedge angle e.g. in order to reduce an effect of unwanted reflections.

The second mirror plate200may be bonded to one or more first supporting elements S1by three or more joints J1. Each joint J1may be bonded to the second mirror plate200at a bonding region REG1. The distance d21between each bonding region REG1and the first substantially planar surface SRF21may be greater than 30% of the thickness h200of the second mirror plate, and the distance d22between each bonding region REG1and the second substantially planar surface SRF22may be greater than 30% of the thickness h200of the second mirror plate200.

The distance d22between each bonding region REG1and the substantially planar surface SRF22may be greater than 30% of the thickness h200of the second mirror plate200, and the distance d23between the uppermost edge of each bonding region REG1and the substantially planar surface SRF22may be smaller than 70% of the thickness h200of the second mirror plate200.

The distances d21, d22, d23may be defined in the direction of the optical axis (SZ).

Each bonding region REG1of the second mirror plate200may be substantially perpendicular to the first substantially planar surface SRF21of the second mirror plate200.

The maximum dimension dMAXof each bonding region REG1of the second mirror plate200may be smaller than 30% of the thickness h200of the second mirror plate200.

Referring toFIG.5dandFIG.5e, the supporting elements S1may have a first bonding region REG11and a second bonding region REG12. The first bonding region REG11may be attached to the bonding region REG1of the mirror plate (100,200) by a joint J1. The second bonding region REG12may be attached the surface SRF3of the base plate BASE1or a bonding surface of an actuator (ACU1) by a joint J2. After attaching, the first bonding region REG11may be substantially perpendicular to the mirror M1, M2of the mirror plate. After attaching, the second bonding region REG12may be substantially parallel with the mirror M1, M2of the mirror plate. An angle α1between the regions REG11, REG12may be substantially equal to 90 degrees. The flexible support element S1ofFIG.5dmay reduce the magnitude of forces transferred to the mirror plate. The support element ofFIG.5emay provide more stable support, when compared with the support element ofFIG.5d.

The mirrors M1, M2may be rather close to each other. The interferometer300may be produced such that a target value of the mirror distance dFis e.g. in the range of 0.2 μm to 10 μm. The risk of mutual contact between the mirrors may be high when the target value is smaller than 0.5 μm.

Accidental contact of the mirror M1with the mirror M2may cause permanent damage to the interferometer300. For example, the mirrors, the actuators, and/or other structures of the interferometer300may be damaged in case of the contact. For example, in case of the comparative interferometer shown inFIG.1a, shrinking of the joints J0of the second plate200may pull the second plate200towards the first plate100so that mirror M1may come into contact with the mirror M2.

Each bonding region REG1may be substantially perpendicular to the central plane of the mirror plate. The perpendicular orientation of the bonding region REG1may reduce the risk of damaging the interferometer300e.g. in case of shrinking and/or expansion of the joints. In an embodiment, the interferometer300may be produced e.g. such that the joints J1are formed on the bonding regions REG1after the joints J2, J3have been formed, wherein the bonding regions REG1may have said perpendicular orientation. For example, the joints J1may be adhesive joints, which may be cured after the joints (J2, J3) have been formed.

In an embodiment, the interferometer300may be produced such that the joints J2for the first mirror plate100and the joints J2for the second mirror plate200are cured substantially simultaneously at a substantially similar rate, so as to reduce or minimize relative movement of the second mirror plate200with respect to the first mirror plate during said curing. Consequently, the risk of damaging the mirrors M1, M2due to mutual contact may be reduced.

FIG.6ashows, by way of example, in a three-dimensional exploded view of a Fabry-Perot interferometer300. The interferometer300may comprise a first mirror plate100, a second mirror plate200, and three actuators ACU1, ACU2, ACU3. The first mirror plate100may be supported by three joints J1, and the second mirror plate200may be supported by three joints J1. The first mirror plate100may be attached to the base plate BASE1via the joints J1and via supporting elements S1. The second mirror plate200may be attached to the actuators ACU1, ACU2, ACU3via the joints J1and via supporting elements S1.

Using the three actuators may allow adjusting an average value of the mirror distance dFand adjusting the tilt of the mirror M2about a first axis (e.g. direction SX) and about a second axis (e.g. direction SY).

The interferometer300may comprise sensor electrodes G1a,G2a,G1b,G2b,G3a,G3cto implement three or more sensor capacitors. Using the three or more sensor capacitors may allow monitoring the average value of the mirror distance dFand monitoring the tilt of the mirror M2about a first axis and about a second axis. The monitoring unit410may comprise multiple inputs for the sensor capacitors. The sensor electrodes may be arranged to monitor the alignment of the second mirror plate200with respect to the first mirror plate100. For example, a non-zero difference between the capacitance of a first sensor capacitor (formed by electrodes G1a,G2a) and the capacitance of a third sensor capacitor (formed by electrodes G1c,G2c) may indicate that the second plate200is tilted about the axis SX.

FIGS.6b,6c, and6dshow, by way of example, in a top view, different positions of support elements S1for supporting the mirror plate200of the interferometer300.FIGS.6band6cshow supporting a substantially rectangular mirror plate200.FIG.6dshows supporting a substantially circular mirror plate200.

Referring toFIG.7a, the interferometer300may comprise one or more capacitive sensors C1for monitoring the mirror distance dF. The sensor C1may comprise a first sensor electrode G1a,a second sensor electrode G2a,and an intermediate sensor electrode G0a.The electrodes G1a,G2amay be attached to the first stationary mirror plate100. The intermediate electrode G0amay be attached to the movable second mirror plate200. When using the intermediate electrode G0a,it is not necessary to connect conductors CONa, CONb to the movable mirror plate200. Avoiding the use of movable conductors may provide e.g. simplified construction, faster response and/or more reliable operation.

The first electrode and the intermediate electrode may form a first capacitor C11. The intermediate electrode and the second electrode may form a second capacitor C12. The capacitive sensor C1may comprise the capacitors C11and C12, which may be connected in series via the intermediate electrode G0a.The electrodes G1a,G2aof the sensor capacitor C1may be connected to a capacitance monitoring unit410e.g. by conductors CONa, CONb. The capacitance monitoring unit410may provide a sensor signal Sdindicative of the mirror distance dF.

FIG.7bshows, by way of example, a relation between the mirror distance dFand the capacitance value Cdof a sensor capacitor. The curve CCRV1ofFIG.8shows the sensor capacitance Cdas the function of the mirror gap dF. To the first approximation, the value of the sensor capacitance Cdmay be inversely proportional to the value of the electrode distance. Cd,1denotes the sensor capacitance at a first mirror distance value dF,1. Cd,2denotes the sensor capacitance at a second mirror distance value dF,2.

The control unit CNT1may be arranged to determine the value of the mirror distance dFfrom the measured value of the sensor capacitance Cd. The capacitance monitoring unit410may provide a sensor signal value Sd,1when the sensor capacitance has a value Cd,1. The capacitance monitoring unit410may provide a sensor signal value Sd,2when the sensor capacitance has a value Cd,2.

FIG.8shows, by way of example, the spectral intensity I(λ) of light LB1received from an object OBJ1. The spectral intensity I(λ) may have a value X(λ0) at a wavelength λ0. The value X(λ0) may be determined from the detector signal SDET1obtained from the optical detector DET1. The wavelength λ0may be selected by adjusting the mirror gap dFbefore the detector signal SDET1is obtained from the optical detector DET1. The mirror gap dFmay be scanned during a measurement in order to measure spectral range of the spectrum OSPEC1of the object OBJ1.

The object OBJ1may be e.g. a real object or a virtual object. A real object OBJ1may be e.g. in solid, liquid, or gaseous form. The real object OBJ1may be a cuvette filled with a gas. The real object OBJ1may be e.g. a plant (e.g. tree or a flower), a combustion flame, or an oil spill floating on water. The real object OBJ1may be e.g. the sun or a star observed through a layer of absorbing gas. The real object may be e.g. an image printed on a paper. A virtual object OBJ1may be e.g. an optical image formed by another optical device.

The interferometer300may be suitable for filtering and/or analyzing infrared light. The materials and the dimensions of the mirror plate100may be selected such that a Fabry Perot interferometer300comprising the mirror plate100may be applicable for spectral analysis of infrared light.

The Fabry-Perot interferometer may be used as an optical filter, which has a variable mirror gap. An optical device may comprise one or more Fabry-Perot interferometers. The optical device may be e.g. a non-imaging spectrometer, an imaging spectrometer, a chemical analyzer, a biomedical sensor, and/or a component of a telecommunication system.

The first mirror plate100may be e.g. rectangular or circular, when viewed in the direction of the optical axis AX1(direction −SZ). The second mirror plate200may be e.g. rectangular or circular, when viewed in the direction of the optical axis AX1. The first mirror plate100may be smaller than the second mirror plate200, the first mirror plate100may be larger than the second mirror plate200, or the mirror plates100,200may be of equal size. The width or diameter w100of the first mirror plate100may also be greater than or equal to the width or diameter wax) of the second mirror plate200.

The term “plate” may refer to a body, which has one or more substantially planar portions. The plate may have a first substantially planar portion so as to minimize wavefront distortion of light transmitted and/or reflected by said planar portion. The plate may optionally have a second substantially planar portion, so as to minimize wavefront distortion of light transmitted through the first substantially planar portion and the second substantially planar portion. The first planar portion may cover the entire top surface of the plate, or the first planar portion may cover less than 100% of the top surface of the plate. The second planar portion may cover the entire bottom surface of the plate, or the second planar portion may cover less than 100% of the bottom surface of the plate. The plate may optionally have e.g. one or more protruding portions and/or recessed portions. In an embodiment, first planar portion may be substantially parallel to the second planar portion. In an embodiment, first planar portion and the second planar portion may define a non-zero wedge angle e.g. in order to reduce unwanted reflections.

The supporting blocks S1may provide more stable support for the mirror plate M1and/or M2, when compared with the slightly flexible supporting elements. In an embodiment, the first supporting elements S1may be rigid. In particular, the supporting blocks S1may be implemented as rigid non-flexible elements S1.

In an embodiment, the interferometer300may be produced such that the joints J2are formed after the joints J1and J3have been formed. For example, the joints J2may be adhesive joints, which may be cured after the joints J1and J3have been formed.

The width w100of the first mirror plate100may be e.g. in the range of 5 mm to 100 mm. In particular, the width w100of the first mirror plate100may be e.g. in the range of 5 mm to 50 mm.

The materials and the dimensions of the mirror plate100may also be selected such that a Fabry-Perot interferometer300comprising the mirror plate100may be applicable for spectral analysis of visible light. The operating wavelength range of the interferometer may include e.g. the range of 380 nm to 760 nm.

The materials and the dimensions of the mirror plate100may also be selected such that a Fabry-Perot interferometer300comprising the mirror plate100may be applicable for spectral analysis of ultraviolet light. The operating wavelength range of the interferometer may include e.g. the range of 150 nm to 380 nm.

An operating wavelength λNof the Fabry-Perot interferometer300may be e.g. in the range of 150 nm to 6000 nm.

Various embodiments are illustrated by the following examples.

Example 1. A Fabry-Perot interferometer (300), comprising:a first mirror plate (100) comprising a first semi-transparent mirror (M1),a second semi-transparent mirror (M2) to define an optical cavity together with the first mirror (M1), andone or more first supporting elements (S1) to support the first mirror plate (100),

wherein the first mirror plate has a first substantially planar surface (SRF11) and a second substantially planar surface (SRF12) defining the maximum thickness (h100) of the first mirror plate (100),

wherein the first mirror plate (100) is bonded to the one or more first supporting elements (S1) by three or more joints (J1),

wherein each joint (J1) is bonded to the first mirror plate (100) at a bonding region (REG1),

wherein the distance (d1) between each bonding region (REG1) and the first substantially planar surface (SRF11) is greater than 30% of the thickness (h100) of the mirror plate (100), and the distance (d2) between each bonding region (REG1) and the second substantially planar surface (SRF12) is greater than 30% of the thickness (h100) of the mirror plate (100).

Example 2. The interferometer (300) of example 1, wherein each bonding region (REG1) of the first mirror plate (100) overlaps the central plane (PLN1) of the first mirror plate (100).

Example 3. The interferometer (300) of example 1 or 2, wherein a maximum dimension (dMAX) of each bonding region (REG1) of the first mirror plate (100) is smaller than 30% of the thickness (100) of the mirror plate (100).

Example 4. The interferometer (300) according to any of examples 1 to 3, wherein each bonding region (REG1) of the first mirror plate (100) is substantially perpendicular to the first substantially planar surface (SRF11) of the first mirror plate (100).

Example 5. The interferometer (300) according to any of examples 1 to 4, wherein the joints (J1) are adhesive joints (ADH1).

Example 6. The interferometer (300) according to any of examples 1 to 5, wherein the first supporting elements (S1) are flexible.

Example 7. The interferometer (300) according to any of examples 1 to 6, comprising three actuators (ACU1, ACU2, ACU3) to change a distance (dF) between the first mirror (M1) and the second mirror (M2).

Example 8. The interferometer (300) according to any of examples 1 to 7, comprising a second mirror plate (200), second supporting elements (S1), a base plate (BASE1), and three actuators (ACU1, ACU2, ACU3) wherein the second mirror plate (200) comprises the second semi-transparent mirror (M2), wherein the first mirror plate (100) is attached to the base plate (BASE1) by three supporting elements (S1), wherein the actuators (ACU1, ACU2, ACU3) are bonded to the base plate (BASE1), wherein the second mirror plate (200) is attached to the actuators (ACU1, ACU2, ACU3) by the second supporting elements (S1).

Example 9. The interferometer (300) according to any of examples 1 to 8, comprising a second mirror plate (200) comprising the second semi-transparent mirror (M2), andone or more second supporting elements (S1) to support the first mirror plate (100),

wherein the second mirror plate (200) has a first substantially planar surface (SRF21) and a second substantially planar surface (SRF22) defining the maximum thickness (h200) of the second mirror plate (200),

wherein the second mirror plate (200) is bonded to the one or more second supporting elements (S1) by three or more joints (J1),

wherein each joint (J1) is bonded to the second mirror plate (200) at a bonding region (REG1),

wherein the distance (d21) between each bonding region (REG1) and the second substantially planar surface (SRF21) is greater than 30% of the thickness (h200) of the mirror plate (200), and the distance (d22) between each bonding region (REG1) and the second substantially planar surface (SRF22) is greater than 30% of the thickness (h200) of the mirror plate (200).

For the person skilled in the art, it will be clear that modifications and variations of the devices and methods according to the present disclosure are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the disclosed embodiments, which is defined by the appended claims.