Patent Publication Number: US-2023152154-A1

Title: Fabry-perot interferometer having supporting elements

Description:
FIELD 
     The aspects of the disclosed embodiments relate to optical Fabry-Perot interferometers. 
     BACKGROUND 
     A Fabry-Perot interferometer comprises a pair of semi-transparent mirrors, which are arranged to operate as an optical cavity. 
     Referring to  FIGS.  1   a    and  1   b,  a Fabry-Perot interferometer FPI of prior art may comprise a first mirror plate  100  and a second mirror plate  200 . The first mirror plate  100  has a first semi-transparent mirror M 1 , and the second mirror plate  200  has a second semi-transparent mirror M 2 . The interferometer may comprise one or more actuators ACU 1 , ACU 2  for changing the distance d F  between the mirrors M 1 , M 2 . 
     The first mirror plate  100  may be bonded to a base plate BASE 1  by one or more joints J 0 . The second mirror plate  200  may be bonded to the actuators ACU 1 , ACU 2  by one or more joints J 0 . The actuators may be bonded to the base plate BASE 1  by joints J 3 . Thermal expansion and/or shrinking of the joints J 0 , J 3  may cause deforming forces FX 1 , FY 1 , FY 2 , which may cause significant geometric deformation (DEFORM 1 ) of the mirrors. The geometric deformation of the shape of the mirrors may disturb or prevent operation of the Fabry Perot interferometer. 
     SUMMARY 
     The aspects of the disclosed embodiments are directed to provide a Fabry-Perot interferometer. An aspect of the disclosed embodiments is to provide a method for producing a Fabry-Perot interferometer. An aspect of the disclosed embodiments is to provide a spectrometer, which comprises a Fabry-Perot interferometer. 
     According to an aspect, there is provided a Fabry-Perot interferometer ( 300 ), comprising:
         a first mirror plate ( 100 ) comprising a first semi-transparent mirror (M 1 ),   a second semi-transparent mirror (M 2 ) to define an optical cavity together with the first mirror (M 1 ), and   one or more first supporting elements (S 1 ) to support the first mirror plate ( 100 ),       

     wherein the first mirror plate has a first substantially planar surface (SRF 11 ) and a second substantially planar surface (SRF 12 ) defining the maximum thickness (h 100 ) of the first mirror plate ( 100 ), 
     wherein the first mirror plate ( 100 ) is bonded to the one or more first supporting elements (S 1 ) by three or more joints (J 1 ), 
     wherein each joint (J 1 ) is bonded to the first mirror plate ( 100 ) at a bonding region (REG 1 ), 
     wherein the distance (d 1 ) between each bonding region (REG 1 ) and the first substantially planar surface (SRF 11 ) is greater than 30% of the thickness (h 100 ) of the mirror plate ( 100 ), and the distance (d 2 ) between each bonding region (REG 1 ) and the second substantially planar surface (SRF 12 ) is greater than 30% of the thickness (h 100 ) of the mirror plate ( 100 ). 
     The interferometer may comprise a first semi-transparent mirror implemented on a first mirror plate, and a second semi-transparent mirror implemented on a second mirror plate. The interferometer may comprise one or more actuators to change the distance between the first mirror and the second mirror. 
     The width or diameter of the mirror plates of the interferometer may be e.g. in the range of 5 mm to 50 mm. The semi-transparent mirrors of the interferometer may be produced with a high degree of accuracy. The deviations of the semi-transparent mirror from the perfect planar shape may initially be e.g. smaller than λ/200, before the mirror plate is attached to the interferometer. 
     The parts of the interferometer may be connected to each other by joints, e.g. by adhesive joints. Thermal expansion of the joints, humidity-induced expansion of the joints, and/or shrinking of the joints may cause that deforming forces are transferred to the mirror plates of the interferometer. Adhesive joint may absorb moisture e.g. from ambient air, depending on the relative humidity of the ambient air. 
     The second mirror may be substantially parallel with the first mirror during operation. The parallelism of the second mirror with respect to the first mirror may be adjusted by using the actuators. However, geometric deformation of the shape of the mirror cannot be typically corrected by using the actuators. 
     The mirror plate may be supported by supporting elements, wherein the mirror plate may be bonded to supporting elements by joints. The positions of the bonding regions of the mirror plate may be selected to overlap with a middle plane of the mirror plate, so as to minimize deviations of the shape of the semi-transparent mirror from the initial shape. In particular, coupling the supporting and deforming forces to the mirror plate via bonding regions located at the middle plane may minimize bending of the mirror of the mirror plate. Coupling the supporting and deforming forces to the mirror plate via bonding regions located at the middle plane may ensure that deformations of the upper surface of the mirror plate may substantially compensate deformations of the lower surface of said mirror plate. 
     Local deviations of the shape of the semi-transparent mirror from the initial planar shape may preferably be smaller than e.g. λ/500. Local deviations of the shape of the semi-transparent mirror from the initial planar shape may preferably be smaller than e.g. 10 nm. Positioning the joints to overlap with the middle plane of the mirror plate may allow keeping the stress-induced deviations smaller than a predetermined limit, e.g. smaller than 10 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following examples, several variations will be described in more detail with reference to the appended drawings, in which 
         FIG.  1   a    shows, by way of example, in a cross-sectional side view, a Fabry-Perot interferometer of Prior Art, 
         FIG.  1   b    shows, by way of example, in a cross-sectional side view, geometric deformation of the mirrors of the interferometer of  FIG.  1     a,    
         FIG.  2    shows, by way of example, in a cross-sectional side view, a spectrometer, which comprises a Fabry-Perot interferometer, 
         FIG.  3    shows, by way of example, spectral transmittance peaks of a 
       Fabry-Perot interferometer. 
         FIG.  4   a    shows, by way of example, in a cross-sectional side view, a Fabry-Perot interferometer, 
         FIG.  4   b    shows, by way of example, in a cross-sectional side view, a Fabry-Perot interferometer, 
         FIG.  5   a    shows, by way of example, in a side view, a joint of the mirror plate, 
         FIG.  5   b    shows, by way of example, in an end view, the joint of  FIG.  5   a   , 
         FIG.  5   c    shows, by way of example, in a side view, a joint of the second mirror plate, 
         FIG.  5   d    shows, by way of example, in a side view, a support element for a mirror plate, 
         FIG.  5   e    shows, by way of example, in a side view, a support element for a mirror plate, 
         FIG.  6   a    shows, by way of example, an exploded three-dimensional view of the Fabry-Perot interferometer, 
         FIG.  6   b    shows, by way of example, in a top view, positions of support elements with respect to the second mirror plate, 
         FIG.  6   c    shows, by way of example, in a top view, positions of support elements with respect to the second mirror plate, 
         FIG.  6   d    shows, by way of example, in a top view, positions of support elements with respect to the second mirror plate, 
         FIG.  7   a    shows, by way of example, in a side view, a capacitive sensor for monitoring the distance between the mirrors of the interferometer, 
         FIG.  7   b    shows, by way of example, determining a distance between the mirrors by monitoring a capacitor signal, and 
         FIG.  8    shows, by way of example, measuring spectral intensity by using the interferometer. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  2   , a spectrometer  700  may comprise a Fabry-Perot interferometer  300 . An object OBJ 1  may reflect, emit and/or transmit light LB 1 , which may be transmitted through the interferometer  300  in order to monitor the spectrum of the light LB 1 . The interferometer  300  may be used e.g. for measuring reflection, transmission (absorption) and/or emission of the light LB 1  of the object OBJ 1 . 
     The Fabry-Perot interferometer  300  comprises a first mirror plate  100  and a second mirror plate  200 . The first mirror plate  100  comprises a first semi-transparent mirror M 1 , and the second mirror plate  200  comprises a second semi-transparent mirror M 2 . The mirrors M 1 , M 2  may have e.g. a substantially circular form or a substantially rectangular form. 
     The distance d F  between the mirrors M 1 , M 2  may be adjusted to provide constructive interference for transmitted light at one or more given wavelengths so that the interferometer  300  may transmit light. The distance d F  may also be adjusted to provide destructive interference for transmitted light at the given wavelength so that the interferometer  300  may reflect light. 
     The mirror distance d F  may be adjusted by one or more actuators ACU 1 , ACU 2 . One or more actuators may be arranged to move the second mirror plate  200  with respect to the first mirror plate  100 . The actuator ACU 1 , ACU 2  may be e.g. a piezoelectric actuator, an electrostatic actuator, an electrostrictive actuator, or a flexoelectric actuator. 
     The spectrometer  700  may comprise a control unit CNT 1 . The control unit CNT 1  may be arranged to send a control signal SET D  to the interferometer  300  in order to adjust the mirror gap d F . The interferometer  300  may comprise a driver unit  420 . The driver unit  420  may e.g. convert a digital control signal SET D  into an analog signal suitable for driving one or more actuators. The driver unit  420  may provide a signal HV 1  for driving an actuator. The driver unit  420  may provide e.g. a high voltage signal HV 1  for driving a piezoelectric actuator. 
     The mirrors M 1 , M 2  may be substantially flat and substantially parallel to each other. The flatness of the mirror M 1 , M 2  may be e.g. better than λ N /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). λ N  denotes a predetermined operating wavelength. The predetermined operating wavelength λ N  may be e.g. in the range of 500 nm to 4000 nm. The predetermined operating wavelength λ N  may be e.g. 2000 nm or 4000 nm. 
     The interferometer  300  may optionally comprise means for monitoring the distance d F  between the mirrors and/or the mirror plates. The interferometer  300  may comprise e.g. capacitive means for monitoring the distance. The interferometer  300  may comprise e.g. inductive means for monitoring the distance. The interferometer  300  may comprise e.g. interferometric means for monitoring the distance. 
     The interferometer  300  may optionally comprise capacitive sensor electrodes for capacitively monitoring mirror distance d F . Sensor electrodes G 1   a,  G 2   a  may together form a sensor capacitor C 1 , wherein the capacitance value of the sensor capacitor C 1  may depend on the mirror distance d F . Consequently, the mirror distance d F  may be monitored by monitoring the capacitance value of the sensor capacitor C 1 . The sensor capacitor C 1  may be connected to a capacitance monitoring unit  410  e.g. by conductors CONa, CONb. The capacitance monitoring unit  410  may provide a sensor signal S d  indicative of the mirror distance d F . The sensor capacitor C 1  may also be implemented e.g. as shown in  FIG.  7   a   . 
     The capacitance monitoring unit  410  may provide a sensor signal S d . The sensor signal may be used for monitoring the mirror gap d F . The spectral response of the spectrometer  700  may be calibrated e.g. as a function of the mirror gap d F . The spectrometer  700  may comprise a memory MEM 2  for storing spectral calibration parameters DPAR 2 . The mirror gap d F  and/or a spectral position  2 ,, may be determined from the sensor signal S d  e.g. by using the spectral calibration parameters DPAR 2 . 
     The interferometer  300  may also comprise two or more sensor capacitors for monitoring the parallelism of the mirrors M 1 , M 2 , in addition to monitoring the mirror distance d F  between the mirrors M 1 , M 2 . The mirror M 2  may be adjusted to be parallel with the mirror M 1  e.g. by controlling one or more of the actuators ACU 1 , ACU 2  of the interferometer  300 . However, geometric deformation of the shape of the mirror M 1  and/or M 2  cannot be typically corrected by using the actuators ACU 1 , ACU 2   
     The Fabry-Perot interferometer  300  may form transmitted light LB 2  by filtering the light LB 1  obtained from the object OBJ 1 . The spectrometer  700  may comprise an optical detector DET 1 . The interferometer  300  may be optically coupled to the detector DET 1 . The transmitted light LB 2  may impinge on the detector DET 1 . The detector DET 1  may be e.g. an image sensor or a non-imaging detector. The detector DET 1  may provide one or more intensity signals S DET1  indicative of the intensity of the transmitted light LB 2 . 
     The spectrometer  700  may optionally comprise imaging, focusing, or collimating optics  500 . The optics  500  may be arranged to focus light LB 2  to the detector DET 1 . The optics  500  may also be positioned between the interferometer  300  and the sensor DET 1 . 
     The interferometer  300  may also be positioned e.g. within multi-element optics  500 . The interferometer  300  may be positioned between elements of multi-element optics  500 . 
     The spectrometer  700  may optionally comprise a memory MEM 1  for storing intensity calibration parameters CALPAR 1 . The spectrometer  700  may be arranged to obtain detector signal values S DET1  from the detector DET 1 , and to determine intensity values X(λ) from the detector signal values S DET1  by using one or more intensity calibration parameters CALPAR 1 . At each mirror distance value d F , an intensity value X(λ) of the light LB 1  may be determined from a detector signal S DET1  by using the one or more intensity calibration parameters CALPAR 1 . 
     The spectrometer  700  may optionally comprise a memory MEM 3  for storing output OUT 1 . The output OUT 1  may comprise e.g. detector signals S DET1  and/or intensity values determined from the detector signals. The output OUT 1  may e.g. comprise one or more digital images of the object OBJ 1 , captured by using the interferometer  300  as a spectrally selective filter. 
     The spectrometer  700  may comprise a memory MEM 4  for storing a computer program PROG 1 . The computer program PROG 1  may be configured, when executed by one or more data processors (e.g. CNT 1 ), cause the apparatus  300 ,  700  perform one or more of the following:
         measure a distance d F  between the mirrors M 1 , M 2 ,   set a transmittance peak of the interferometer  300  to a selected position,   cause spectral scanning of the interferometer  300 ,   measure a spectral intensity value,   measure a spectrum,   adjust parallelism (tilt angle) of the mirrors M 1 , M 2 .       

     The spectrometer  700  may optionally comprise a user interface USR 1  e.g. for displaying information to a user and/or for receiving commands from the user. The user interface USR 1  may comprise e.g. a display, a keypad and/or a touch screen. 
     The spectrometer  700  may optionally comprise a communication unit RXTX 1 . The communication unit RXTX 1  may transmit and/or receive a signal 
     COM 1  e.g. in order to receive commands, to receive calibration data, and/or to send output data OUT 1 . The communication unit RXTX 1  may have e.g. wired and/or wireless communication capabilities. The communication unit RXTX 1  may 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 spectrometer  700  may optionally comprise one or more optical cut-off filters  510  to limit the spectral response of the detector DET 1 . 
     SX, SY and SZ denote orthogonal directions. The light LB 2  may propagate substantially in the direction −SZ. The optical axis AX 1  of the interferometer  300  may be parallel with the direction SZ. The light LB 2  may propagate substantially in the direction of the optical axis AX 1 . The light LB 2  may propagate substantially in the direction −SZ. 
       FIG.  3    shows, by way of example, a spectral transmittance of a Fabry-Perot interferometer  300 , and the pass band of an optional filter  510 . The uppermost curve of  FIG.  3    shows the spectral transmittance T F (λ) of the Fabry-Perot interferometer  300 . The spectral transmittance T F (λ) may have one or more adjacent transmittance peaks PEAK 1 , PEAK 2 , PEAK 3  of the Fabry-Perot interferometer  300 . For example, a first transmittance peak PEAK 1  may be at a wavelength λ 0 , a second transmittance peak PEAK 2  may be at a wavelength λ 1 , and a third transmittance peak PEAK 3  may be at a wavelength  22 . The spectral positions λ 0 , λ 1 , λ 2  of the transmission peaks PEAK 1 , PEAK 2 , PEAK 3  may depend on the mirror distance d F  according to the Fabry-Perot transmission function. The spectral positions of the transmission peaks may be changed by changing the mirror gap d F . The spectral positions of the transmission peaks may be changed by tuning the mirror gap d F . The transmission peaks PEAK 1 , PEAK 2 , PEAK 3  may also be called passbands of the Fabry-Perot interferometer. 
     The spectrometer  700  may optionally comprise one or more optical cut-off filters  510  to limit the spectral response of the spectrometer  700 . The one or more filters  510  may together provide a spectral transmittance T S (λ). The one or more filters  510  may provide an optical band pass filter defined by cut-off wavelengths λ min  and λ max . 
     Referring to  FIGS.  4   a  and  4   b   , the Fabry-Perot interferometer  300  may comprise a first mirror plate  100 , a second mirror plate  200 , and one or more actuators ACU 1 , ACU 2  to change the distance d F  between the mirrors M 1 , M 2  of the interferometer  300 . The first mirror plate  100  may have a first semi-transmissive mirror M 1 , and the second mirror plate  200  may have a second semi-transmissive mirror M 2 . The M 1 , M 2  may be planar with an accuracy, which is e.g. better than λ/200. 
     The width w 100  of the first mirror plate  100  may be e.g. in the range of 5 mm to 50 mm. 
     The distance d F  between the semi-transparent mirrors M 1 , M 2  may be e.g. in the range of 0.5 μm to 10 μm. The distance d F  between the semi-transparent mirrors M 1 , M 2  may be e.g. in the range of 0.2 μm to 10 μm. 
     The first mirror plate  100  may be stationary, and the second mirror plate  200  may be moved by the actuators ACU 1 , ACU 2 , with respect to the first mirror plate  100 . 
     The Fabry-Perot interferometer  300  may comprise two or more actuators ACU 1 , ACU 2  to change the distance d F  between the mirrors M 1 , M 2  of the interferometer  300  and/or to adjust the tilt angle between the mirrors M 1 , M 2 . 
     The first mirror plate  100  and/or the second mirror plate  200  may be supported by supporting elements S 1 . The supporting elements S 1  may be e.g. bent pieces of metal sheet or blocks. The mirror plates  100 ,  200  may be bonded to the supporting elements S 1  by joints J 1 . The joints J 1  may be positioned close to the middle plane of the mirror plates  100 ,  200 , so as to reduce deformation of the shape of the mirrors M 1 , M 2 . 
     The semi-transparent mirrors M 1 , M 2  may be e.g. dielectric multilayer coatings deposited on a transparent substrate. The substrate material of the mirror plates  100 ,  200  may be transparent in the operating wavelength range of the interferometer  300 . The material of the mirror plates  100 ,  200  may be e.g. glass, silica, silicon or sapphire. 
     The supporting elements S 1  may e.g. comprise or consist of a metal, glass, silicon (Si), silica (SiO 2 ), sapphire (Al 2 O 3 ) or ceramic material. Supporting block elements S 1  ( FIG.  4   b   ) may provide stable support for the mirror plate  100 ,  200 . 
     Block elements S 1  of  FIG.  4   b    may sometimes transfer horizontal forces (e.g. in direction SX) to the mirror plate  100 ,  200 . Using the slightly flexible supporting elements of  FIG.  4   a    may reduce the magnitude of forces transferred to the mirror plate  100 ,  200 , when compared with the supporting blocks of  FIG.  4   b   . Flexible supporting elements S 1  may e.g. comprise spring steel or Invar alloy (FeNi36). 
     The material of the supporting elements S 1  for the stationary mirror plate (e.g. M 1 ) may be selected such that the thermal expansion of the supporting elements S 1  may at least partly compensate thermal expansion of the joints J 2 . The material of the supporting elements S 1  for the movable mirror plate (e.g. M 2 ) may be selected such that the thermal expansion of the supporting elements S 1  may at least partly compensate thermal expansion of the joints J 2 , J 3  and thermal expansion of the one or more actuators ACU 1 . 
     The slightly flexible supporting elements of  FIG.  4   a    may provide a less stable support for the mirror plates, when compared with the supporting blocks of  FIG.  4   b   . However, the reduced stability of the flexible elements S 1  may be at least partly compensated by monitoring and correcting the position of the mirror plate  200  by using the actuators. The position of the mirror plate  200  may be monitored e.g. by using the capacitive sensor electrodes G 1   a,  G 2   a,  G 1   b,  G 2   b.  The position of the mirror plate  200  may be corrected by using the actuators ACU 1 , ACU 2 . 
     The interferometer  300  may comprise a base plate BASE 1 . The first mirror plate  100  and the second mirror plate  200  may be attached to the base plate BASE 1  via the supporting elements S 1 . 
     The first mirror plate  100  may be bonded to the supporting elements S 1  by joints J 1 . The support elements S 1  of the first mirror plate  100  may be attached to the base plate BASE 1  by joints J 2 . The support elements S 1  of the first mirror plate  100  may be attached e.g. to a mounting surface SRF 3  of the base plate BASE 1  by the joints J 2 . The surface SRF 3  may be e.g. substantially perpendicular to the optical axis AX 1  of the interferometer  300 . 
     The second mirror plate  200  may be bonded to the supporting elements S 1  by joints J 1 . The support elements S 1  of the second mirror plate  200  may be attached to the actuators ACU 1 , ACU 2  by joints J 2 . The actuators ACU 1 , ACU 2  may be attached to the base plate BASE 1  by joints J 2 . The support elements S 1  of the second mirror plate  200  may be attached e.g. to the mounting surface SRF 3  by the joints J 2 . 
     The joints J 1 , J 2 , J 3  may be e.g. adhesive joints. The joints J 1 , J 2 , J 3  may comprise e.g. an adhesive (ADH 1 ). In an embodiment, the joints J 1 , J 2 , J 3  may also be formed e.g. by welding or soldering. 
     The interferometer  300  may be optionally assembled such that a gap GAP 3  remains between the first mirror plate  100  and the base plate BASE 1 . The gap GAP 3  may reduce the probability of causing deformation of the mirror plate  100 . 
     The interferometer  300  may optionally comprise capacitive sensor electrodes (G 1   a,  G 2   a,  G 1   b,  G 2   b ) for monitoring the distance d F  between the mirrors M 1 , M 2  and/or for monitoring a tilt angle of the mirror M 2  with respect to the mirror M 1 . 
     The first mirror plate  100  may comprise sensor electrodes G 1   a,  G 1   b.  The second mirror plate  200  may comprise sensor electrodes G 2   a,  G 2   b.  The electrodes G 1   a  and G 2   a  may together form a first sensor capacitor C 1 , which has a capacitance C 1 . The electrodes G 1   b  and G 2   b  may together form a second sensor capacitor C 2 , which has a capacitance C 2 . The sensor capacitors C 1 , C 2  may also be implemented e.g. as shown in  FIG.  7   a   . 
     The distance between the electrodes G 1   a,  G 2   a  may depend on the mirror distance d F , and the capacitance C 1  of the first sensor capacitor C 1  may depend on the distance between the electrodes G 1   a,  G 2   a  such that the mirror distance d F  near the electrodes G 1   a,  G 2   a  may be monitored by monitoring the capacitance C 1  of the first sensor capacitor C 1 . The mirror distance d F  close to the electrodes G 2   a,  G 2   b  may be monitored by monitoring the capacitance C 2  of the second sensor capacitor C 2 , respectively. 
     The mirror distance d F  and/or tilt angle may also be monitored e.g. by analyzing one or more optical signals transmitted through the interferometer  300 . The sensor electrodes may also be omitted. 
     The base plate BASE 1  may optionally comprise an opening OPE 1  for light LB 1 , LB 2 , which is transmitted through the interferometer  300 . 
     Referring to  FIGS.  5   a  and  5   b   , each supporting element S 1  of the first mirror plate  100  may be bonded to the first mirror plate  100  by a joint J 1 . The mirror plate  100  may be bonded to one or more supporting elements S 1  by three or more joints J 1 . In particular, the number of the joints J 1  of the first mirror plate  100  may be equal to three in order to minimize internal stress of the mirror plate  100 . The joints J 1  may be e.g. adhesive joints. Each joint J 1  may be bonded to the first mirror plate  100  at a bonding region REG 1 . 
     The mirror plate  100  may have a first surface SRF 11  and a second surface SRF 12 . The surface SRF 11  may be at least partly defined by the outer surface of the first mirror M 1 . The surface SRF 11  may be parallel with the first mirror M 1 . The first surface SRF 11  and a second surface SRF 12  may define the thickness h 100  of the mirror plate  100 . The second surface SRF 12  may be e.g. substantially parallel with the first surface SRF 11 . The second surface SRF 12  may also be slightly tilted with respect to the first surface SRF 11  in order to reduce an effect of unwanted reflections. The first surface SRF 11  and the second surface SRF 12  may define a non-zero wedge angle. 
     The joints J 1  of the first mirror plate  100  may be positioned close to the middle plane PLN 1  of the first mirror plate  100 . Positioning the joints J 1  close to the middle plane may reduce or minimize geometric deformation of the mirrors M 1 , M 2  away from the planar shape. A joint J 1  may transfer a horizontal deforming force (e.g. in the direction SX) to the mirror plate. 
     The distance d 1  between each bonding region REG 1  and the first substantially planar surface SRF 11  may be e.g. greater than 30% of the thickness h 100  of the mirror plate  100 , and the distance d 2  between each bonding region REG 1  and the second substantially planar surface SRF 12  may be e.g. greater than 30% of the thickness h 100  of the mirror plate  100 . 
     The distance d 1  between each bonding region REG 1  and the first substantially planar surface SRF 11  may be e.g. greater than 30% of the thickness h 100  of the mirror plate  100 , and the distance d 3  between the lowermost edge of each bonding region REG 1  and the first substantially planar surface SRF 11  may be e.g. smaller than 70% of the thickness h 100  of the mirror plate  100 . 
     In particular, each bonding region REG 1  may overlap the central plane PLN 1  of the first mirror plate  100 . The central plane PLN 1  may meet each bonding region REG 1  of the first mirror plate  100 . 
     In case of a horizontal deforming force acting on the middle plane PLN 1 , resulting deformations of the upper surface SRF 11  and the lower surface SRF 12  may be substantially symmetric with respect to the middle plane PLN 1 . For example, a horizontal deforming force may cause a tendency of the upper surface SRF 11  to deflect upwards, but this tendency may be effectively compensated by a corresponding tendency of the lower surface SRF 12  to deflect downwards. Consequently, a deformation of the lower surface SRF 12  may substantially compensate a deformation of the upper surface SRF 11 . 
     Causing the deforming force to act on the middle plane may minimize geometric deformation of the mirrors M 1 , M 2 . In particular, causing the deforming force to act on the middle plane may minimize bending of the mirrors M 1 , M 2 . 
     Each bonding region REG 1  may be substantially perpendicular to the central plane PLN 1  of the first mirror plate  100 , e.g. in order to further ensure that horizontal forces are transferred symmetrically from the supporting elements S 1  to the mirror plate. 
     Each bonding region REG 1  may be located on a side surface (SRF 10 ) of the mirror plate, the side surface (SRF 10 ) being substantially perpendicular to the central plane PLN 1  of the first mirror plate  100 . A mirror plate  100  with perpendicular side surfaces (SRF 10 ) may be substantially symmetrical with respect to the central plane PLN 1 , e.g. in order to minimize deformations and/or in order to simplify production of the mirror plate  100 . 
     A joint J 1  may also transfer a deforming torque to the mirror plate. The lo maximum dimension d MAX  of the bonding region REG 1  may be smaller than a predetermined limit in order to minimize or avoid transferring a deforming torque from the supporting element S 1  to the mirror plate  100 . For example, the maximum dimension d MAX  of each bonding region REG 1  of the first mirror plate  100  may be e.g. smaller than 30% of the thickness h 100  of the mirror plate  100 . 
     The bonding region REG 1  may have a first dimension w REG1  in a first direction along the central plane PLN 1 , and the bonding region REG 1  may have a second dimension h REG1  in a second direction, which is perpendicular to the first direction. The first dimension w REG1  may be e.g. the width of the bonding region REG 1 , and the second dimension h REG1  may be e.g. the height of the bonding region REG 1 . 
     For example, the height h REG1  of each bonding region REG 1  of the first mirror plate  100  may be e.g. smaller than 30% of the thickness h 100  of the mirror plate  100 . For example, the width w REG1  of each bonding region REG 1  of the first mirror plate  100  may also be e.g. smaller than 30% of the thickness h 100  of the mirror plate  100 . 
     SX, SY, and SZ may denote orthogonal directions. The mirror M 1  of the first mirror plate  100  may 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 SRF 11  may be above the “lower” surface SRF 12  in the direction SZ. The “lower” surface SRF 12  may be below the “upper” surface SRF 11 . Incoming light LB 1  may propagate in the direction -SZ from the surface SRF 21  to the surface SRF 22 . The mirror M 1  may be below the mirror M 2 . Transmitted light LB 2  may propagate in the direction -SZ from the surface SRF 11  to the surface SRF 12 . 
     The distances d 1 , d 2 , d 3  may be defined in the direction of the optical axis (SZ). 
     The middle plane PLN 1  of the first mirror plate  100  is at the halfway between the upper surface SRF 11  and the lower surface SRF 12 . The distance dpi between the middle plane PLN 1  and the upper surface SRF 11  is equal to the distance d P2  between the middle plane PLN 1  and the lower surface SRF 12 . 
     The first surface SRF 11  may be substantially parallel with the second surface SRF 12 . The middle plane PLN 1  may be substantially parallel with the first surface SRF 11 . 
     The first surface SRF 11  and the second surface SRF 12  may also define a non-zero wedge angle e.g. in order to reduce unwanted reflections. 
     The supporting element S 1  may also be e.g. an annular ring, which may be attached to the first mirror plate  100  by three joints J 1 . 
     Referring to  FIG.  5   c   , each bonding region REG 1  of the second mirror plate  200  may overlap the central plane PLN 2  of the second mirror plate  200 . The central plane PLN 2  may meet each bonding region REG 1  of the second mirror plate  200 . 
     The second mirror plate  200  may have a first substantially planar surface SRF 21  and a second substantially planar surface SRF 22 . The surface SRF 22  may be at least partly defined by the outer surface of the second mirror M 2 . The surface SRF 22  may be parallel with the mirror M 2 . The first substantially planar surface SRF 21  and the second substantially planar surface SRF 22  may define the maximum thickness h 200  of the second mirror plate  200 . The middle plane PLN 2  of the second mirror plate  200  is at the halfway between the upper surface SRF 21  and the lower surface SRF 22 . 
     The first surface SRF 21  may be substantially parallel with the second surface SRF 22 . The middle plane PLN 2  may be substantially parallel with the surface SRF 22 . The first surface SRF 21  and the second surface SRF 22  of the plate  200  may also define a non-zero wedge angle e.g. in order to reduce an effect of unwanted reflections. 
     The second mirror plate  200  may be bonded to one or more first supporting elements S 1  by three or more joints J 1 . Each joint J 1  may be bonded to the second mirror plate  200  at a bonding region REG 1 . The distance d 21  between each bonding region REG 1  and the first substantially planar surface SRF 21  may be greater than 30% of the thickness h 200  of the second mirror plate, and the distance d 22  between each bonding region REG 1  and the second substantially planar surface SRF 22  may be greater than 30% of the thickness h 200  of the second mirror plate  200 . 
     The distance d 22  between each bonding region REG 1  and the substantially planar surface SRF 22  may be greater than 30% of the thickness h 200  of the second mirror plate  200 , and the distance d 23  between the uppermost edge of each bonding region REG 1  and the substantially planar surface SRF 22  may be smaller than 70% of the thickness h 200  of the second mirror plate  200 . 
     The distances d 21 , d 22 , d 23  may be defined in the direction of the optical axis (SZ). 
     Each bonding region REG 1  of the second mirror plate  200  may be substantially perpendicular to the first substantially planar surface SRF 21  of the second mirror plate  200 . 
     The maximum dimension d MAX  of each bonding region REG 1  of the second mirror plate  200  may be smaller than 30% of the thickness h 200  of the second mirror plate  200 . 
     Referring to  FIG.  5   d    and  FIG.  5   e   , the supporting elements S 1  may have a first bonding region REG 11  and a second bonding region REG 12 . The first bonding region REG 11  may be attached to the bonding region REG 1  of the mirror plate ( 100 ,  200 ) by a joint J 1 . The second bonding region REG 12  may be attached the surface SRF 3  of the base plate BASE 1  or a bonding surface of an actuator (ACU 1 ) by a joint J 2 . After attaching, the first bonding region REG 11  may be substantially perpendicular to the mirror M 1 , M 2  of the mirror plate. After attaching, the second bonding region REG 12  may be substantially parallel with the mirror M 1 , M 2  of the mirror plate. An angle al between the regions REG 11 , REG 12  may be substantially equal to 90 degrees. The flexible support element S 1  of  FIG.  5   d    may reduce the magnitude of forces transferred to the mirror plate. The support element of  FIG.  5   e    may provide more stable support, when compared with the support element of  FIG.  5   d   . 
     The mirrors M 1 , M 2  may be rather close to each other. The interferometer  300  may be produced such that a target value of the mirror distance d F  is 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 M 1  with the mirror M 2  may cause permanent damage to the interferometer  300 . For example, the mirrors, the actuators, and/or other structures of the interferometer  300  may be damaged in case of the contact. For example, in case of the comparative interferometer shown in  FIG.  1    a, shrinking of the joints J 0  of the second plate  200  may pull the second plate  200  towards the first plate  100  so that mirror M 1  may come into contact with the mirror M 2 . 
     Each bonding region REG 1  may be substantially perpendicular to the central plane of the mirror plate. The perpendicular orientation of the bonding region REG 1  may reduce the risk of damaging the interferometer  300  e.g. in case of shrinking and/or expansion of the joints. In an embodiment, the interferometer  300  may be produced e.g. such that the joints J 1  are formed on the bonding regions REG 1  after the joints J 2 , J 3  have been formed, wherein the bonding regions REG 1  may have said perpendicular orientation. For example, the joints J 1  may be adhesive joints, which may be cured after the joints (J 2 , J 3 ) have been formed. 
     In an embodiment, the interferometer  300  may be produced such that the joints J 2  for the first mirror plate  100  and the joints J 2  for the second mirror plate  200  are cured substantially simultaneously at a substantially similar rate, so as to reduce or minimize relative movement of the second mirror plate  200  with respect to the first mirror plate during said curing. Consequently, the risk of damaging the mirrors M 1 , M 2  due to mutual contact may be reduced. 
       FIG.  6   a    shows, by way of example, in a three-dimensional exploded view of a Fabry-Perot interferometer  300 . The interferometer  300  may comprise a first mirror plate  100 , a second mirror plate  200 , and three actuators ACU 1 , ACU 2 , ACU 3 . The first mirror plate  100  may be supported by three joints J 1 , and the second mirror plate  200  may be supported by three joints J 1 . The first mirror plate  100  may be attached to the base plate BASE 1  via the joints J 1  and via supporting elements S 1 . The second mirror plate  200  may be attached to the actuators ACU 1 , ACU 2 , ACU 3  via the joints J 1  and via supporting elements S 1 . 
     Using the three actuators may allow adjusting an average value of the mirror distance d F  and adjusting the tilt of the mirror M 2  about a first axis (e.g. direction SX) and about a second axis (e.g. direction SY). 
     The interferometer  300  may comprise sensor electrodes G 1   a,  G 2   a,  G 1   b,  G 2   b,  G 3   a,  G 3   c  to implement three or more sensor capacitors. Using the three or more sensor capacitors may allow monitoring the average value of the mirror distance d F  and monitoring the tilt of the mirror M 2  about a first axis and about a second axis. The monitoring unit  410  may comprise multiple inputs for the sensor capacitors. The sensor electrodes may be arranged to monitor the alignment of the second mirror plate  200  with respect to the first mirror plate  100 . For example, a non-zero difference between the capacitance of a first sensor capacitor (formed by electrodes G 1   a,  G 2   a ) and the capacitance of a third sensor capacitor (formed by electrodes G 1   c,  G 2   c ) may indicate that the second plate  200  is tilted about the axis SX. 
       FIGS.  6   b ,  6   c , and  6   d    show, by way of example, in a top view, different positions of support elements S 1  for supporting the mirror plate  200  of the interferometer  300 .  FIGS.  6   b  and  6   c    show supporting a substantially rectangular mirror plate  200 .  FIG.  6   d    shows supporting a substantially circular mirror plate  200 . 
     Referring to  FIG.  7   a   , the interferometer  300  may comprise one or more capacitive sensors C 1  for monitoring the mirror distance d F . The sensor C 1  may comprise a first sensor electrode G 1   a,  a second sensor electrode G 2   a,  and an intermediate sensor electrode G 0   a.  The electrodes G 1   a,  G 2   a  may be attached to the first stationary mirror plate  100 . The intermediate electrode G 0   a  may be attached to the movable second mirror plate  200 . When using the intermediate electrode G 0   a,  it is not necessary to connect conductors CONa, CONb to the movable mirror plate  200 . 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 C 11 . The intermediate electrode and the second electrode may form a second capacitor C 12 . The capacitive sensor C 1  may comprise the capacitors C 11  and C 12 , which may be connected in series via the intermediate electrode G 0   a.  The electrodes G 1   a,  G 2   a  of the sensor capacitor C 1  may be connected to a capacitance monitoring unit  410  e.g. by conductors CONa, CONb. The capacitance monitoring unit  410  may provide a sensor signal S d  indicative of the mirror distance d F . 
       FIG.  7   b    shows, by way of example, a relation between the mirror distance d F  and the capacitance value C d  of a sensor capacitor. The curve CCRV 1  of  FIG.  8    shows the sensor capacitance C d  as the function of the mirror gap d F . To the first approximation, the value of the sensor capacitance C d  may be inversely proportional to the value of the electrode distance. C d,1  denotes the sensor capacitance at a first mirror distance value d F,1 . C d,2  denotes the sensor capacitance at a second mirror distance value d F,2 . 
     The control unit CNT 1  may be arranged to determine the value of the mirror distance d F  from the measured value of the sensor capacitance C d . The capacitance monitoring unit  410  may provide a sensor signal value S d,1  when the sensor capacitance has a value C d,1 . The capacitance monitoring unit  410  may provide a sensor signal value S d,2  when the sensor capacitance has a value C d,2 . 
       FIG.  8    shows, by way of example, the spectral intensity I(λ) of light LB 1  received from an object OBJ 1 . The spectral intensity I(λ) may have a value X(λ 0 ) at a wavelength λ 0 . The value X(λ 0 ) may be determined from the detector signal S DET1  obtained from the optical detector DET 1 . The wavelength λ 0  may be selected by adjusting the mirror gap d F  before the detector signal S DET1  is obtained from the optical detector DET 1 . The mirror gap d F  may be scanned during a measurement in order to measure spectral range of the spectrum OSPEC 1  of the object OBJ 1 . 
     The object OBJ 1  may be e.g. a real object or a virtual object. A real object OBJ 1  may be e.g. in solid, liquid, or gaseous form. The real object OBJ 1  may be a cuvette filled with a gas. The real object OBJ 1  may be e.g. a plant (e.g. tree or a flower), a combustion flame, or an oil spill floating on water. The real object OBJ 1  may 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 OBJ 1  may be e.g. an optical image formed by another optical device. 
     The interferometer  300  may be suitable for filtering and/or analyzing infrared light. The materials and the dimensions of the mirror plate  100  may be selected such that a Fabry Perot interferometer  300  comprising the mirror plate  100  may 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 plate  100  may be e.g. rectangular or circular, when viewed in the direction of the optical axis AX 1  (direction -SZ). The second mirror plate  200  may be e.g. rectangular or circular, when viewed in the direction of the optical axis AX 1 . The first mirror plate  100  may be smaller than the second mirror plate  200 , the first mirror plate  100  may be larger than the second mirror plate  200 , or the mirror plates  100 ,  200  may be of equal size. The width or diameter moo of the first mirror plate  100  may also be greater than or equal to the width or diameter wax) of the second mirror plate  200 . 
     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 S 1  may provide more stable support for the mirror plate M 1  and/or M 2 , when compared with the slightly flexible supporting elements. In an embodiment, the first supporting elements S 1  may be rigid. In particular, the supporting blocks S 1  may be implemented as rigid non-flexible elements S 1 . 
     In an embodiment, the interferometer  300  may be produced such that the joints J 2  are formed after the joints J 1  and J 3  have been formed. For example, the joints J 2  may be adhesive joints, which may be cured after the joints J 1  and J 3  have been formed. 
     The width moo of the first mirror plate  100  may be e.g. in the range of 5 mm to 100 mm. In particular, the width w 100  of the first mirror plate  100  may be e.g. in the range of 5 mm to 50 mm. 
     The materials and the dimensions of the mirror plate  100  may also be selected such that a Fabry-Perot interferometer  300  comprising the mirror plate  100  may 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 plate  100  may also be selected such that a Fabry-Perot interferometer  300  comprising the mirror plate  100  may 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 λ N  of the Fabry-Perot interferometer  300  may 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 (M 1 ),   a second semi-transparent mirror (M 2 ) to define an optical cavity together with the first mirror (M 1 ), and   one or more first supporting elements (S 1 ) to support the first mirror plate ( 100 ),       

     wherein the first mirror plate has a first substantially planar surface (SRF 11 ) and a second substantially planar surface (SRF 12 ) defining the maximum thickness (h 100 ) of the first mirror plate ( 100 ), 
     wherein the first mirror plate ( 100 ) is bonded to the one or more first supporting elements (S 1 ) by three or more joints (J 1 ), 
     wherein each joint (J 1 ) is bonded to the first mirror plate ( 100 ) at a bonding region (REG 1 ), 
     wherein the distance (d 1 ) between each bonding region (REG 1 ) and the first substantially planar surface (SRF 11 ) is greater than 30% of the thickness (h 100 ) of the mirror plate ( 100 ), and the distance (d 2 ) between each bonding region (REG 1 ) and the second substantially planar surface (SRF 12 ) is greater than 30% of the thickness (h 100 ) of the mirror plate ( 100 ). 
     Example 2. The interferometer ( 300 ) of example 1, wherein each bonding region (REG 1 ) of the first mirror plate ( 100 ) overlaps the central plane (PLN 1 ) of the first mirror plate ( 100 ). 
     Example 3. The interferometer ( 300 ) of example 1 or 2, wherein a maximum dimension (d MAX ) of each bonding region (REG 1 ) 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 (REG 1 ) of the first mirror plate ( 100 ) is substantially perpendicular to the first substantially planar surface (SRF 11 ) of the first mirror plate ( 100 ). 
     Example 5. The interferometer ( 300 ) according to any of examples 1 to 4, wherein the joints (J 1 ) are adhesive joints (ADH 1 ). 
     Example 6. The interferometer ( 300 ) according to any of examples 1 to 5, wherein the first supporting elements (S 1 ) are flexible. 
     Example 7. The interferometer ( 300 ) according to any of examples 1 to 6, comprising three actuators (ACU 1 , ACU 2 , ACU 3 ) to change a distance (d F ) between the first mirror (M 1 ) and the second mirror (M 2 ). 
     Example 8. The interferometer ( 300 ) according to any of examples 1 to 7, comprising a second mirror plate ( 200 ), second supporting elements (S 1 ), a base plate (BASE 1 ), and three actuators (ACU 1 , ACU 2 , ACU 3 ) wherein the second mirror plate ( 200 ) comprises the second semi-transparent mirror (M 2 ), wherein the first mirror plate ( 100 ) is attached to the base plate (BASE 1 ) by three supporting elements (S 1 ), wherein the actuators (ACU 1 , ACU 2 , ACU 3 ) are bonded to the base plate (BASE 1 ), wherein the second mirror plate ( 200 ) is attached to the actuators (ACU 1 , ACU 2 , ACU 3 ) by the second supporting elements (S 1 ). 
     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 (M 2 ), and
         one or more second supporting elements (S 1 ) to support the first mirror plate ( 100 ),       

     wherein the second mirror plate ( 200 ) has a first substantially planar surface (SRF 21 ) and a second substantially planar surface (SRF 22 ) defining the maximum thickness (h 200 ) of the second mirror plate ( 200 ), 
     wherein the second mirror plate ( 200 ) is bonded to the one or more second supporting elements (S 1 ) by three or more joints (J 1 ), 
     wherein each joint (J 1 ) is bonded to the second mirror plate ( 200 ) at a bonding region (REG 1 ), 
     wherein the distance (d 21 ) between each bonding region (REG 1 ) and the second substantially planar surface (SRF 21 ) is greater than 30% of the thickness (h 200 ) of the mirror plate ( 200 ), and the distance (d 22 ) between each bonding region (REG 1 ) and the second substantially planar surface (SRF 22 ) is greater than 30% of the thickness (h 200 ) 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.