Method for stabilizing a spectrometer using single spectral notch

A method for determining spectral calibration data (λcal(Sd), Sd,cal(λ)) of a Fabry-Perot interferometer (100) comprises:

FIELD

Some variations relate to spectral analysis of light.

BACKGROUND

The spectrum of an object may be measured by using spectrometer, which comprises a Fabry-Perot interferometer. The spectral scale of the spectrometer may be determined by calibration measurements, e.g. by using the excitation spectrum of a gas discharge lamp. The gas discharge lamp may typically contain e.g. argon, neon, xenon, krypton, hydrogen, or mercury.

SUMMARY

Some variations may relate to providing a spectrometer. Some variations may relate to providing a method for measuring a spectrum. Some variations may relate to providing a computer program for measuring a spectrum. Some variations may relate to providing a computer program product, which comprises computer program code for measuring a spectrum.

According to a first aspect, there is provided a method according to claim1.

According to a second aspect, there is provided an apparatus according to claim13.

Further aspects are defined in the other claims.

A method for determining spectral calibration data (λcal(Sd), Sd,cal(λ)) of a Fabry-Perot interferometer (100) may comprise:forming a spectral notch (NC2) by filtering input light (LB1) with a notch filter (60) such that the spectral notch (NC2) corresponds to a transmittance notch (NC1) of the notch filter (60),measuring a spectral intensity distribution (M(Sd)) of the spectral notch (NC2) by varying the mirror gap (dFP) of the Fabry-Perot interferometer (100), and by providing a control signal (Sd) indicative of the mirror gap (dFP), anddetermining the spectral calibration data (λcal(Sd), Sd,cal(λ)) by matching the measured spectral intensity distribution (M(Sd)) with the spectral transmittance (TN(λ)) of the notch filter (60).

A method for verifying spectral calibration data (λcal(Sd), Sd,cal(λ)) of a Fabry-Perot interferometer (100) may comprise:forming a spectral notch (NC2) by filtering input light (LB1) with a notch filter (60) such that the spectral notch (NC2) corresponds to a transmittance notch (NC1) of the notch filter (60),measuring a spectral intensity distribution (M(Sd)) of the spectral notch (NC2) by varying the mirror gap (dFP) of the Fabry-Perot interferometer (100), and by providing a control signal (Sd) indicative of the mirror gap (dFP), andverifying the spectral calibration data (λcal(Sd), Sd,cal(λ)) by checking whether the measured spectral intensity distribution (M(Sd)) matches with the spectral transmittance (TN(λ)) of the notch filter (60).

The spectrometer may comprise a Fabry-Perot interferometer and a detector for monitoring intensity of light transmitted through the Fabry-Perot interferometer. The spectral position of the Fabry-Perot interferometer may be scanned by varying the mirror gap of the Fabry-Perot interferometer. The spectrometer may provide a control signal indicative of the mirror gap. The control signal may be provided e.g. by a control unit, and the mirror gap may be controlled according to the control signal. Alternatively, the control signal may be provided by monitoring the mirror gap, e.g. by using a capacitive sensor. The control signal may be e.g. a digital control signal or an analog control signal. Each spectral position may be associated with a control signal value such that the relationship between the spectral positions and the control signal values may be expressed by calibration data.

The spectral scale of the interferometer may be calibrated in order to perform accurate spectral analysis. The spectral scale may be defined according to the calibration data. The calibration data may define the relationship between each spectral position of the transmission peak and the control signal value corresponding to said spectral position.

When monitoring an unknown spectrum, the spectrometer may be arranged to obtain intensity values from the detector as a function of the control signal. Measured intensity values may be associated with calibrated spectral positions based on the calibration data. The calibration data may comprise e.g. parameters of a regression function, which defines the relationship between each spectral position and the control signal value corresponding to said spectral position. The calibration data may be stored e.g. in a memory of the spectrometer, and/or in a database server.

The Fabry-Perot interferometer comprises a first semi-transparent mirror and a second semi-transparent mirror, which are arranged to form an optical cavity of the interferometer. The Fabry-Perot interferometer may provide a narrow transmission peak, which has adjustable spectral position, and which can be used for spectral analysis. The spectral position of the transmission peak may be changed by changing the distance between the mirrors. The distance between mirrors may be called e.g. as the mirror gap or as the mirror spacing. The Fabry-Perot interferometer may have an adjustable mirror gap.

The spectral position of the transmittance peak may be changed according to a control signal. The control signal may be e.g. a voltage signal, which is applied to electrodes of an electrostatic actuator in order to change the mirror gap of the Fabry-Perot interferometer. The control signal may be e.g. a voltage signal, which is applied to a piezoelectric actuator of the Fabry-Perot interferometer in order to change the mirror gap of the Fabry-Perot interferometer. Yet, the control signal may be provided by using a capacitive sensor, which is arranged to monitor the mirror gap of the Fabry-Perot interferometer.

The relationship between each spectral position of the transmission peak and the control signal value corresponding to said spectral position may depend e.g. on the operating temperature of the Fabry-Perot interferometer. Said relationship may depend on the operating life (i.e. age) of the interferometer. Said relationship may be substantially changed e.g. if the interferometer experiences an impact (i.e. an acceleration shock). Said relationship may be substantially changed also due to chemical corrosion.

The spectral scale of the interferometer may be stabilized by using an optical notch filter. The relationship between each spectral position and the control signal value corresponding to said spectral position may be determined and/or verified based on a spectral notch formed by using the optical notch filter. The notch filter may be placed in the optical path of the spectrometer. In particular, the notch filter may be integrated with a bandwidth-limiting filter of the spectrometer so that using the notch filter does not significantly increase the mechanical complexity of the spectrometer. The notch filter and the bandwidth-limiting filter may be implemented e.g. by using dielectric multilayer coatings deposited on a substrate. The combination of the notch filter and the bandwidth-limiting filter may be implemented as a substantially monolithic structure, which may be mechanically stable. In an embodiment, the notch filter may also be used as a mechanically protective input window for the spectrometer. The notch filter may provide a simple and highly stabile spectral reference for stabilizing the spectral scale of the spectrometer. The spectral scale of the spectrometer may be stabilized by using the notch filter.

The notch filter may be arranged to form a spectral notch. The Fabry-Perot interferometer may be used for measuring the spectral intensity distribution of the spectral notch. The spectral calibration data of the interferometer may be determined by matching the measured distribution with the transmittance notch of the notch filter. The matching may be performed e.g. by using cross correlation, and/or by associating a control signal value with predetermined a predetermined spectral position. The measured distribution may be matched with the spectral transmittance e.g. by using cross-correlation. The spectral calibration data may be checked by using cross-correlation analysis. The spectral calibration data of the interferometer may be checked by comparing the measured distribution with the spectral transmittance of the notch filter.

The spectral calibration data may be determined by matching spectral features of the measured spectral distribution with spectral features of the spectral transmittance of the notch filter.

The spectral calibration data may be determined by matching the spectral notch of the measured spectral distribution with the notch of the spectral transmittance of the notch filter.

The spectral calibration data may be determined such that the measured spectral distribution matches with the spectral transmittance of the notch filter, when the relation between the control signal and the spectral position is determined by using said spectral calibration data.

The spectral calibration data may be determined such that spectral features of the measured spectral distribution substantially coincide with spectral features of the spectral transmittance of the notch filter, when the relation between the control signal and the spectral position is determined using said spectral calibration data.

The spectral calibration data may be determined such that the spectral position of a first spectral feature of the measured spectral distribution substantially coincides with the spectral position of a first spectral feature of the spectral transmittance of the notch filter, when the relation between the control signal and the spectral position is determined using said spectral calibration data.

Input light may be filtered by the notch filter in order to provide a filtered spectrum, which has a spectral notch. The spectral position of the spectral notch may be highly stable. The spectral position of the spectral notch may be substantially independent of air pressure, variations of humidity, ageing, and/or corrosion. The spectral position of the spectral notch may remain unaltered even after the spectrometer has experienced a mechanical impact.

The notch filter may be implemented as a highly stable monolithic structure. In particular, the notch filter and the bandwidth-limiting filter may be implemented by using a structure, which is mechanically and thermally stable.

The spectrometer may permanently comprise the notch filter in order to enable on-line stabilization and/or verification of the spectral scale. In an embodiment, the spectral scale of the spectrometer may be determined and/or verified even when measuring an unknown spectrum of an object.

Spectral stability may be a key parameter when analyzing spectra by using the spectrometer. By using the notch filter, the spectral scale may be stabilized even when the spectrometer is used in a harsh environment. A highly stable spectrometer may be provided by combining the scanning Fabry-Perot interferometer with the notch filter. The notch filter may be easily integrated in an on-line measurement system.

The operation of the notch filter as such does not require operating power. However, optional monitoring the temperature of the notch filter may sometimes require a very low power. The notch filter may have a highly reproducible thermal drift. In an embodiment, the spectral calibration data may comprise information about the effect of the operating temperature of the notch filter on the spectral scale. The operating temperature of the notch filter may be monitored, and the spectral scale of the interferometer may be determined according to the operating temperature of the notch filter.

The calibration of the spectrometer may optionally comprise performing intensity calibration in addition to performing the spectral calibration. The intensity values of the spectrometer may be calibrated e.g. by measuring spectral intensity values of light obtained from a blackbody radiator or a tungsten ribbon lamp, and by comparing the measured spectral intensity values with intensity calibration data associated with said radiator or lamp.

The spectrometer may be used for analyzing spectra of samples e.g. in the pharmaceutical industry, in the beverage industry, in the food industry, or in petrochemical industry. The sample may comprise e.g. food, beverage, medicament, or a substance for producing a medicament.

DETAILED DESCRIPTION

Referring toFIG. 1, a spectrometer500may comprise a Fabry-Perot interferometer100, a detector DET1, and a notch filter60.

An object OBJ1may reflect, emit and/or transmit light LB1. The light LB1may be coupled into the spectrometer500in order to monitor the spectrum of the light LB1. In particular, the notch filter60may be used as an input window, and the light LB1may be coupled into the spectrometer500through the notch filter60. The spectrometer500may optionally comprise a housing400. The spectrometer500may comprise a protective housing400. The housing400may be arranged to mechanically protect the interferometer100.

In an embodiment, the housing400may be hermetic. The housing400may be filled with a gas, which has a predetermined composition. For example, the housing400may be filled with nitrogen or argon.

The spectrometer500may be used e.g. for monitoring spectral properties of light LB1received from the object OBJ1. The light LB1may be e.g. reflected by the object, transmitted through the object, scattered by the object and/or emitted from the object.

The Fabry-Perot interferometer100comprises a first semi-transparent mirror110and a second semi-transparent mirror120. The distance between the first mirror110and the second mirror120is equal to a mirror gap dFP.

The mirror gap dFPis adjustable. The first mirror110may have a solid-gas interface111, and the second mirror121may have a solid-gas interface121. The mirror gap dFPmay denote the distance between the interfaces111and121. The Fabry-Perot interferometer100may provide a transmission peak PFP,k(FIG. 2a), wherein the spectral position of the transmission peak PFP,kmay depend on the mirror gap dFP. The spectral position of the transmission peak PFP,kmay be changed by changing the mirror spacing dFP. The transmission peak PFP,kmay also be called as the passband of the Fabry-Perot interferometer100.

The filter60may provide filtered light LB2by filtering the light LB1received from the object OBJ1.

The Fabry-Perot interferometer100may form transmitted light LB3by transmitting a portion of the filtered light LB2to the detector DET1. The interferometer100may be optically coupled to the detector DET1. The transmitted light LB3may at least partly impinge on the detector DET1.

An actuator140may be arranged to move the first mirror110with respect to the second mirror120. The actuator140may be e.g. electrostatic actuator (FIG. 6). The actuator140may be e.g. a piezoelectric actuator. The mirrors110,120may be substantially planar and substantially parallel to each other. The mirrors110,120may be flat. The semi-transparent mirrors110,120may comprise e.g. a metallic reflective layer and/or a reflective dielectric multilayer. One of the mirrors110,120may be attached to a frame, and the other mirror may be moved by one or more actuators140.

In an embodiment, the object OBJ1may be a real or virtual object. For example, the object OBJ1may be a tangible piece of material. The object OBJ1may be a real object. The object OBJ1may be e.g. in solid, liquid, or gaseous form. The object OBJ1may comprise a sample. The object OBJ1may a combination of a cuvette and a chemical substance contained in the cuvette. The object OBJ1may be e.g. a plant (e.g. tree or a flower), a combustion flame, or an oil spill floating on water. The object may be e.g. the sun or a star observed through a layer of absorbing gas. The object OBJ1may be a display screen, which emits or reflects light of an image. The object OBJ1may be an optical image formed by another optical device. The object OBJ1may also be called as a target. The light LB1may be provided e.g. directly from a light source, by reflecting light obtained from a light source, by transmitting light obtained from a light source. The light source may comprise e.g. an incandescent lamp, a blackbody radiator, an infrared light emitting glow-bar, a tungsten halogen lamp, a fluorescent lamp, or a light emitting diode.

The spectrometer500may comprise a control unit CNT1. The control unit may comprise one or more data processors. The spectrometer may provide a control signal Sdindicative of the mirror gap dFP. In an embodiment, the control unit CNT1may provide the control signal Sd, and the mirror gap may be controlled according to the control signal Sd. For example, the spectrometer500may comprise a driving unit142, which may be arranged to convert a digital control signal Sdinto a voltage signal Vab. The voltage signal Vabmay be coupled e.g. to an electrostatic actuator or to a piezoelectric actuator in order to adjust the mirror gap dFP. The driver unit142may convert a digital signal Sdinto an analog signal Vabsuitable for driving the actuator140.

In an embodiment, the control signal Sdmay be provided by using a sensor. The interferometer may comprise e.g. a capacitive sensor150for monitoring the mirror gap dFP(FIG. 7). The capacitive sensor150may be arranged provide a control signal Sdby monitoring the mirror gap dFP. The control signal Sdmay be used e.g. as a feedback signal indicative of the mirror spacing dFP.

In an embodiment, the control signal Sdmay be provided by monitoring a varying voltage signal Vabcoupled to the interferometer100. For example, the control signal Sdmay be proportional to the voltage signal Vabcoupled to the actuator140.

The spectrometer500may optionally comprise light concentrating optics300e.g. for concentrating light into the detector DET1. The optics300may comprise e.g. one or more refractive lenses and/or one or more reflective surfaces (e.g. a paraboloid reflector). The optics300may be positioned e.g. between the interferometer100and the detector DET1. One or more components of the optics300may also be positioned before the interferometer100. In an embodiment, the notch filter60may also be positioned between the interferometer100and the detector DET1.

The detector DET1may be arranged to provide a detector signal SDET1. The detector signal SDET1may be indicative of the intensity I3of light LB3impinging on the detector DET1. The detector DET1may convert the intensity I3of light LB3impinging on the detector DET1into detector signal values SDET1.

The detector DET1may be sensitive e.g. in the ultraviolet, visible and/or infrared region. The spectrometer500may be arranged to measure spectral intensities e.g. in the ultraviolet, visible and/or infrared region. The detector DET1may be an imaging detector or a non-imaging detector. The detector DET1may be selected according to the detection range of the spectrometer500. For example, the detector may comprise e.g. a silicon photodiode. The detector may comprise a P-N junction. The detector may be a pyroelectric detector. The detector may be a bolometer. The detector may comprise a thermocouple. The detector may comprise a thermopile. The detector may be an Indium gallium arsenide (InGaAs) photodiode. The detector may be a germanium photodiode. The detector may be a photoconductive lead selenide (PbSe) detector. The detector may be a photoconductive Indium antimonide (InSb) detector. The detector may be a photovoltaic Indium arsenide (InAs) detector. The detector may be a photovoltaic Platinum silicide (PtSi) detector. The detector may be an Indium antimonide (InSb) photodiode. The detector may be a photoconductive Mercury cadmium telluride (MCT, HgCdTe) detector. The detector may be a photoconductive Mercury zinc telluride (MZT, HgZnTe) detector. The detector may be a pyroelectric Lithium tantalate (LiTaO3) detector. The detector may be a pyroelectric Triglycine sulfate (TGS and DTGS) detector. The detector may comprise one or more pixels of a CMOS detector array. The detector may comprise one or more pixels of a CCD detector array.

The spectrometer500may comprise a memory MEM4for storing intensity calibration data CPAR1. One or more intensity values I1of the light LB1may be determined from the detector signals SDET1by using the intensity calibration data CPAR1. The intensity calibration data CPAR1may comprise e.g. one or more parameters of a regression function, which allows determining intensity values I1of the light LB1from the detector signal values SDET1.

Each determined intensity value I1may be associated with a value of the control signal Sd, and the determined intensity value I1may be associated with a spectral position λ based on the control signal value Sdand spectral calibration data λcal(Sd).

Changes of operating temperature of the Fabry-Perot interferometer, operating life (i.e. age) of the interferometer, acceleration shocks and/or chemical corrosion may have an adverse effect on the spectral accuracy of the spectrometer, such that a spectral position determined by using the calibration data λcal(Sd) may substantially deviate from the true spectral position. The stability of the spectral scale of the interferometer may be improved by using the notch filter60.

The spectral calibration data λcal(Sd) may be determined and/or verified by a method, which comprises:forming a spectral notch (NC2) by filtering input light (LB1) with a notch filter (60) such that the spectral notch (NC2) corresponds to a transmittance notch (NC1) of the notch filter (60),varying the mirror gap (dFP) of a Fabry-Perot interferometer (100), and providing a control signal (Sd) indicative of the mirror gap (dFP),analyzing the intensity of light (LB3) transmitted through the notch filter (60) and the Fabry-Perot interferometer (100) in order to determine a first control signal value (SN1) associated with a first mirror gap value (dFP) when the transmission peak (PFP,k) of the interferometer (100) substantially coincides with the spectral notch (NC2),forming a first association (λN1,SN1) between the first control signal value (SN1) and a first spectral position (λN1) of the transmittance notch (NC1), anddetermining and/or verifying spectral calibration data (λcal(Sd)) of the interferometer (100) based on the first association (λN1,SN1).

A spectral distribution M(Sd) may be measured by varying the mirror gap dFP, and by recording the detector signal values SDET1as the function of the control signal Sd. The measured spectral distribution M(Sd) may give detector signal values SDET1as the function of the control signal Sd. The intensity of the transmitted light LB3may be analyzed by analyzing the distribution M(Sd).

The first association (λN1,SN1) may also be called e.g. as the relationship between the first control signal value SN1and the first spectral position (λN1) of the transmittance notch (NC1). The first association (λN1,SN1) may be specified e.g. by a data pair (λN1,SN1). The spectrometer500may comprise a memory MEM2for storing reference data RDATA1, which may define one or more associations (λN1,SN1). The data RDATA1may comprise information about the spectral transmittance function TN(λ) of the notch filter.

Each measured detector signal value SDET1may be associated with a value of the control signal Sd, and the detector signal value SDET1may be associated with a spectral position λ based on the control signal value Sdand spectral calibration data λcal(Sd).

The spectrometer500may comprise a memory MEM3for storing spectral calibration data λcal(Sd). The spectral calibration data λcal(Sd) may comprise e.g. one or more parameters of a regression function, which allows determining the relationship between spectral positions λ and the detector signal values SDET1.

The spectrometer500may be arranged to determine spectral positions λ from control signal values Sdby using the spectral calibration data λcal(Sd). The spectrometer500may comprise a memory MEM5for storing a computer program PROG1. The computer program PROG1may be configured, when executed by one or more data processors (e.g. CNT1), to determine spectral positions λ from control signal values Sdby using the spectral calibration data λcal(Sd).

The spectrometer500may be arranged to obtain detector signal values SDET1from the detector DET1, and to determine intensity values I1from the detector signal values SDET1by using the intensity calibration data CPAR1. The computer program PROG1may be configured, when executed by one or more data processors (e.g. CNT1), to obtain detector signal values SDET1from the detector DET1, and to determine intensity values I1from the detector signal values SDET1by using the intensity calibration data CPAR1.

The computer program PROG1may be configured, when executed by one or more data processors (e.g. CNT1), to determine calibration data λcal(Sd) byvarying the mirror gap (dFP) of a Fabry-Perot interferometer (100),providing a control signal (Sd) indicative of the mirror gap (dFP), analyzing the intensity of light (LB3) transmitted through the notch filter (60) and the Fabry-Perot interferometer (100) in order to determine a first control signal value (SN1) associated with a first mirror gap (dFP) when the transmission peak (PFP,k) of the interferometer (100) substantially coincides with the spectral notch (NC2),forming a first association (λN1,SN1) between the first control signal value (SN1) and a first spectral position (λN1) of the transmittance notch (NC1), anddetermining spectral calibration data (λcal(Sd)) of the interferometer (100) based on the first association (λN1,SN1).

The spectrometer500may optionally comprise a memory MEM1for storing spectral data XS(λ). The spectral data XS(λ) may comprise e.g. intensity values I1determined as a function I1(λ) of the spectral position λ. The spectral data XS(λ) may comprise e.g. detector signal values SDET1determined as a function SDET1(λ) of the spectral position λ. The spectral data XS(λ) may be calibrated measured spectral intensity distribution.

The spectrometer500may optionally comprise a user interface USR1e.g. for displaying information and/or for receiving commands. The user interface USR1may comprise e.g. a display, a keypad and/or a touch screen.

The spectrometer500may 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 λcal(Sd), and/or to send spectral data XS(λ). The communication unit RXTX1may be capable of wired and/or wireless communication. For example, the communication unit RXTX1may be capable of communicating with a local wireless network (WLAN), with the Internet and/or with a mobile telephone network.

The spectrometer500may be implemented as a single physical unit or as a combination of separate units. In an embodiment, the interferometer100, and the units CNT1, MEM1, MEM3, MEM4, MEM5, USR1, RXTX1may be implemented in the same housing. In an embodiment, the spectrometer500may be arranged to communicate detector signals SDET1and control signals Sdwith a remote data processing unit, e.g. with a remote server. Spectral positions λ may be determined from the control signals Sdby the remote data processing unit.

The spectrometer500may comprise one or more bandwidth-limiting filters to define a detection band ΔλBof the spectrometer500. The bandwidth-limiting filters may limit the spectral response of the detector DET1. The filters may be positioned before and/or after the interferometer100. The bandwidth-limiting filters may be combined with the notch filter60. In particular, the notch filter60may also be arranged to provide the bandwidth-limiting functionality.

The spectrometer500may optionally comprise e.g. a lens and/or an aperture230, which is arranged to limit the divergence of the light LB3transmitted through the interferometer100to the detector DET1, in order to provide a narrow bandwidth ΔλFPof the transmission peak PFP,k. For example, the divergence of the light LB3may be limited to be e.g. smaller than or equal to 10 degrees. When using light concentrating optics300, the divergence of light LB3contributing to the spectral measurement may also be limited by the dimensions of the detector DET1.

The spectrometer500may optionally comprise a temperature sensor61for monitoring the operating temperature Topof the notch filter60. Information about the operating temperature Topmay be communicated to the control unit CNT1e.g. by using a temperature signal STemp.

SX, SY and SZ denote orthogonal directions. The light LB2may propagate substantially in the direction SZ. The mirrors110,120of the interferometer may be parallel to a plane defined by the directions SX and SY. Only the directions SZ and SY are shown inFIG. 1.

FIG. 2ashows, by way of example, the spectral transmittance TFP(λ) of a Fabry-Perot interferometer100, and the spectrum B(λ) of light LB1received from an object OBJ1. The spectral transmittance TFP(λ) of the interferometer100may have a plurality of transmission peaks PFP,k−1, PFP,k, PFP,k+1, . . . at respective spectral positions λFP,k−1, λFP,k, λFP,k+1. The spectrometer500may be arranged to detect light LB3transmitted by a predetermined peak PFP,k. The spectral position λFP,kof the transmission peak PFP,kmay be adjusted by changing the mirror gap dFP.

The detection band ΔλPBof the spectrometer500may be defined e.g. by the filter60. The spectrometer500may be arranged to operate such that the spectrometer500is substantially insensitive to spectral components, whose wavelengths are outside a detection range ΔλPB. The filter60may be arranged to reject spectral components at wavelengths which are shorter than a first cut off value λCUT1, and the filter60may be arranged to reject spectral components at wavelengths which are longer than a second cut off value λCUT2. The filter60may be implemented e.g. by using dielectric multilayer coatings. The filter60may block spectral components at wavelengths outside the detection band ΔλPBfrom reaching the detector DET1. The cut-off limits λCUT1, λCUT2may be selected such that only spectral components within the detection range ΔλPBmay propagate to the detector DET1, depending on the spectral position λFP,kof the transmission peak PFP,kof the interferometer100. The cut-off limits λCUT1, λCUT2may be selected such that spectral components overlapping the other transmission peaks λFP,k−1, λFP,k+1do not propagate to the detector DET1. Adjacent peaks PFP,k, PFP,k+1of the interferometer100are separated by the free spectral range ΔλFSR,FP. The cut-off limits λCUT1, λCUT2may be selected such that the detection range ΔλPBof the spectrometer500is narrower than the free spectral range λΔFSR,FP. Spectral components at wavelengths outside the detection range ΔλPBmay also be rejected by utilizing spectral selectivity of the detector DET1and/or another optical component of the spectrometer.

For example, the cut-off wavelengths λCUT1, λCUT2of the detection range of the spectrometer500may be e.g. in the range of 780 nm to 15000 nm. In particular, the detection range of the spectrometer500may be in the near infrared range (780 nm to 3000 nm).

I2(λ) denotes spectral intensity of light LB2impinging on the interferometer100, and I3(λ) denotes spectral intensity of light LB3transmitted through the interferometer100. The transmittance TFP(λ) means the ratio I3(λ)/I2(λ).

The lowermost curve ofFIG. 2ashows an input spectrum B(λ) of the light LB1received from an object OBJ1. The input spectrum B(λ) may also be called as the spectral intensity distribution I1(λ) of the input light LB1. The spectrum B(λ) may have a maximum value BMAX. The spectral transmittance of the peak PFP,kmay have a maximum value TFP,MAX. The maximum value BMAXof the input spectrum B(λ) may be attained e.g. at a spectral position λX1.

The calibration data λcal(Sd) may be determined and/or verified by using the notch filter60.

FIG. 2bshows, by way of example, the spectral transmittance TN(λ) of the notch filter60. The spectral transmittance TN(λ) may have a transmittance notch NC1. The transmittance notch NC1may have a first edge EDGE1at a spectral position λN1. The transmittance notch NC1may have a second edge EDGE2at a spectral position λN2. The minimum spectral transmittance TMINbetween the positions λN1and λN2may be e.g. smaller than 50% of the maximum spectral transmittance TMAXof the notch filter60. The minimum spectral transmittance TMINbetween the positions λN1and λN2may be e.g. smaller than 10% of the maximum spectral transmittance TMAXof the notch filter60.

The transmittance notch NC1may have a spectral width ΔλFWHM,NC1. The spectral width ΔλFWHM,NC1may be defined as the spectral separation between the positions λN1and λN2. In this case, the acronym FWHM means full width at half minimum. The spectral positions λN1and λN2may indicate positions where the spectral transmittance TNC1(λ) reaches a value, which is equal to the value 0.5·(TMAX+TMIN). The spectral width ΔλFWHM,NC1may denote the full spectral width defined by the spectral points where the spectral transmittance TNC1(λ) reaches a value, which is equal to the value 0.5·(TMAX+TMIN).

The spectral width ΔλFWHM,NC1of the notch NC1may be e.g. smaller than 10% of the spectral width of the detection range ΔλPBof the spectrometer500. The spectral width ΔλFWHM,NC1of the notch NC1may be e.g. smaller than 20 nm.

If the minimum spectral transmittance TMINis very low, this may cause loss of spectral information. In that case, spectral information between the wavelengths λN1, λN2may be lost.

FIG. 2cshows, by way of example, the spectral transmittance TN(λ) of a notch filter60where the minimum spectral transmittance TMINis greater than or equal to 10% of the maximum spectral transmittance TMAX. For example, the minimum spectral transmittance TMINmay be e.g. in the range of 10% to 80% of the maximum spectral transmittance TMAX. Consequently, loss of spectral information may be avoided between the wavelengths λN1and λN2.

FIG. 2dshows, by way of example, a filtered spectrum C(λ), which may be provided by filtering the input light LB1ofFIG. 2awith the notch filter60ofFIG. 2b. The spectrum C(λ) may have a maximum value CMAX.

FIG. 2eillustrates the effect of the notch filter60on the spectrum of light LB2transmitted through the notch filter60. Input light LB1impinging on the notch filter60may have an input spectrum B(λ), and filtered light LB2transmitted through the notch filter60may have a filtered spectrum C(λ). The notch filter60may provide the filtered light LB2by filtering the input light LB1. The filtered spectrum C(λ) may be obtained by multiplying the input spectrum B(λ) with the transmittance TN(λ) of the notch filter60:
C(λ)=TN(κ)·B(λ)  (1)

The uppermost curve ofFIG. 2eshows the spectral transmittance TN(λ) of the notch filter60. The transmittance TN(λ) has a transmittance notch NC1, which may have a first spectral position λN1and a second spectral position λN2. The positions λN1and λN2may be known at high accuracy.

The spectral position of the transmittance notch NC1may also be specified e.g. by a single wavelength, which indicates the position of the center of the notch NC1.

The second curve from the top ofFIG. 2eshows an input spectrum B(λ) of input light LB1received from an object OBJ1. The light LB1may be e.g. reflected from the object OBJ1, emitted by the object OBJ1, and/or transmitted through the object OBJ1. The third curve from the top ofFIG. 2eshows a filtered spectrum C(λ), which is formed by filtering the input spectrum B(λ) with the notch filter60. The filtered spectrum may be expressed as a function C(λ) of spectral position λ. The filtered spectrum C(λ) may have a spectral notch NC2at the positions λN1and λN2. The spectral notch NC2may be formed by multiplying the input spectrum B(λ) with the transmittance TN(λ) in the vicinity of the transmittance notch NC1.

The lowermost curve ofFIG. 2eshows a measured spectral intensity distribution M(Sd). The distribution M(Sd) may be measured by scanning over the filtered spectrum C(λ). The distribution M(Sd) may be measured by varying the mirror gap dFP, and by recording the detector signal values SDET1as the function of the control signal Sd. The spectral notch NC2of the filtered spectrum C(λ) may be represented by the notch NC2′ of the measured distribution M(Sd). The measured spectral intensity distribution M(Sd) of the filtered notch NC2may have a notch NC2′, which corresponds to the spectral notch NC2of the filtered spectrum C(λ).

The distribution M(Sd) is measured by scanning the interferometer100. Different parts of the spectrum may be scanned at different times. The distribution M(Sd) may represent a time-averaged spectrum of the filtered light. The distribution M(Sd) does not need to represent an instantaneous spectrum of light transmitted through the notch filter.

When checking/determining spectral calibration data, it is not necessary to scan over the whole spectral range from λCUT1to λCUT2. When checking/determining spectral calibration data, it may be sufficient to scan over a spectral range, which is slightly wider than the notch NC2. When checking/determining spectral calibration data, the method may comprise measuring a spectral intensity distribution M(Sd) of the spectral notch NC2.

The spectral position of the transmission peak PFP,kof the Fabry-Perot interferometer100may be scanned by varying the mirror gap dFP. The control signal Sdmay be provided such that the control signal Sdis indicative of the mirror gap dFP. The spectral position of the transmission peak PFP,kof the Fabry-Perot interferometer100may be scanned by varying the value of the control signal Sd. The detector signal SDET1may be monitored as the function of the control signal Sdin order to detect the edges of the notch NC2, NC2′.

The spectral position of the transmission peak PFP,kmay coincide with a first edge of the spectral notch NC2when the control signal Sdof the Fabry-Perot interferometer100is equal to a first marker value SN1. The spectral position of the transmission peak PFP,kmay coincide with a second edge of the spectral notch NC2when the control signal Sdof the Fabry-Perot interferometer100is equal to a second marker value SN2. The marker values SN1, SN2may be determined by scanning the Fabry-Perot interferometer100and by analyzing the detector signal SDET1. The marker values SN1, SN2may be determined by varying the control signal Sd, measuring intensity values (I3or SDET1) of the distribution M(Sd) as a function of the control signal Sd, and by determining a control signal value SN1associated with the notch NC2′ of the distribution M(Sd).

In an embodiment, the marker values SN1, SN2may be qualified only if the difference between said marker values SN1, SN2substantially corresponds to the spectral width ΔλFWHM,NC1of the notch NC1.

The method may optionally comprise:checking whether the first marker value SN1is a first predetermined range,checking whether the second marker value SN2is a second predetermined range, andchecking whether the difference between the second marker value SN2and the first marker value SN1is in a third predetermined range.

A calibrated spectral intensity distribution M(Sd,cal(λ)) may be subsequently determined from the measured distribution M(Sd) by using a calibration function Scal,d(λ) or λcal(Sd). The calibration function Scal,d(λ) and/or λcal(Sd) may be determined and/or verified by matching the measured distribution M(Sd) with the transmittance function TN(λ). The calibration function Scal,d(λ) and/or λcal(Sd) may be determined and/or verified by using the marker values SN1, SN2of the measured distribution M(Sd) and the accurately known wavelengths λN1and λN2of the transmittance notch NC1of the notch filter60. The marker values SN1, SN2may be used for determining the calibration function Scal,d(λ) and/or λcal(Sd).

The limits λCUT1, λCUT2of the detection band ΔλPB may be optionally used to provide additional marker values SC1, SC2.

Data pairs (λN1, SN1), (λN2, SN2), (λCUT1, SC1), (λCUT2, SC2) for determining the calibration function λcal(Sd) may be obtained by:determining one or more marker values SN1, SN2, . . . from a measured distribution M(Sd), andforming the data pairs (λN1, SN1), (λN2, SN2), (λCUT1, SC1), (λCUT2, SC2) by associating each marker value SN1, SN2. . . with a corresponding spectral position λN1, λN2, λCUT1, λCUT2defined by a spectral feature of the spectral transmittance TN(λ) of the filter60.

The accuracy of a calibration function λcal(Sd) may be improved and/or checked by using one or more of the data pairs (λN1, SN1), (λN2, SN2), (λCUT1, SC1), (λCUT2, SC2). A corrected calibration function λcal(Sd) may be determined by using one or more of the data pairs (λN1, SN1), (λN2, SN2), (λCUT1, SC1), (λCUT2, SC2). The calibration function λcal(Sd) may be stored e.g. in a memory MEM3of the spectrometer.

The filtered spectrum C(λ) ofFIG. 2emay represent the spectrum of light LB2transmitted through the notch filter60. In an embodiment, a first part of the input light LB1may be coupled to the interferometer via the notch filter60, and a second part of the input light may be simultaneously coupled to the interferometer100without passing through the notch filter60. For example, the filter60may cover less than 100% of the cross-section of the aperture of the interferometer100. For example, the input light LB1may be divided into a first part and a second part by using a beam splitter, wherein the first part may be coupled to the interferometer100through the filter60, and the second part may be coupled to the interferometer100without passing through the filter60. The spectrometer500may comprise e.g. optical fibers, prisms and/or mirrors for guiding the first part and/or the second part. Consequently, the spectrum C(λ) may have a well-defined filtered notch NC2without causing significant loss of intensity data between the wavelengths λN1, λN2. The filtering effect of the notch filter may be compensated e.g. by dividing the calibrated measured spectral intensity distribution M(Sd,cal(λ)) with the spectral transmittance TN(λ) of the notch filter50.

Referring toFIG. 2f, the intensity of the input light LB1may sometimes be too low for accurate determination of the marker values and/or the spectral features of the spectrum of the input light LB1may disturb accurate determination of the marker values. The validity of a marker value SN1, SN2may be checked before said marker value SN1, SN2is used for determining the spectral scale. The method may comprise checking the validity of a marker value SN1, SN2. The method may comprise providing an indication when one or both marker values SN1, SN2are invalid. In case of an invalid marker value SN1, SN2the spectrometer may e.g. display a warning message, which indicates that the spectral scale could not be properly determined and/or verified.

The spectral intensity I2of the filtered light LB2may be determined from the detector signal SDET1. For example, the spectral intensity I2of the filtered light LB2may be substantially proportional to the detector signal SDET1. The spectral intensity I2may be determined from the detector signal SDET1e.g. by using intensity calibration data.

M(SN1) may denote the magnitude of the detector signal SDET1when the control signal Sdis equal to the first marker value SN1. M(SN2) may denote the magnitude of the detector signal SDET1when the control signal Sdis equal to the second marker value SN2. M(SN1L) may denote the magnitude of the detector signal SDET1when the control signal Sdis equal to a first auxiliary value SN1L. M(SN2R) may denote the magnitude of the detector signal SDET1when the control signal Sdis equal to a second auxiliary value SN2R. MMAXmay denote the maximum value of the detector signal SDET1.

The auxiliary value SN1Lmay be selected such that the value M(SN1L) represents the filtered spectrum I2immediately outside the filtered notch NC2, on a first side of the notch NC2′. For example the difference SN1−SN1Lmay be equal to a predetermined value ΔS. The predetermined value ΔS may be e.g. in the range of 1% to 5% of the difference SC2−SC1.

The auxiliary value SN2Rmay be selected such that the value M(SN2R) represents the filtered spectrum I2immediately outside the filtered notch NC2, on a second side of the notch NC2′. For example the difference SN2R−SN2may be equal to a predetermined value ΔS. The predetermined value ΔS may be e.g. in the range of 1% to 5% of the difference SC2−SC1.

The method may comprise performing one or more validity tests for determining the validity of a marker value SN1. In an embodiment, a marker value SN1may be determined to be invalid if the marker value SN1is classified to be invalid according to at least one performed validity test. The marker value SN1may be determined to be valid if the marker value SN1is classified to be valid according to all performed validity tests. The marker value SN1may be classified to be valid according to a validity test when the marker value SN1is not classified to be invalid according to said test.

For example, the method may comprise performing a validity test where the first marker value SN1is classified to be invalid if the value M(SN1L) is smaller than a predetermined value. The first marker value SN1may be classified to be invalid if the value M(SN1L) is smaller than e.g. 5% of the maximum value MMAX.

For example, the method may comprise performing a validity test where the second marker value SN2is classified to be invalid if the value M(SN2R) is smaller than a predetermined value. The second marker value SN2may be classified to be invalid if the value M(SN2R) is smaller than e.g. 5% of the maximum value MMAX.

For example, the method may comprise performing a validity test where the first marker value SN1is classified to be invalid if the absolute value of the difference M(SN2R)−M(SN1L) is greater than a predetermined value. The first marker value SN1may be classified to be invalid if the absolute value of the difference M(SN2R)−M(SN1L) is greater than e.g. 10% of the maximum value MMAX.

For example, the method may comprise performing a validity test where a marker value SN1is classified to be invalid if the difference SN2−SN1is not within a predetermined range.

FIG. 2gshows determining a calibrated measured spectrum XS(λ) from the measured spectral intensity distribution M(Sd) ofFIG. 2e. The uppermost curve ofFIG. 2gshows the measured spectral intensity distribution M(Sd), which may be obtained by varying the mirror gap dFP, and by recording the detector signal values SDET1as the function of the control signal Sd. The second curve from the top ofFIG. 2gshows a calibrated spectral intensity distribution M(Sd,cal(λ)) determined from the distribution M(Sd) by using the calibration function λcal(Sd) and/or Sd,cal(λ). The spectrum XS(λ) may be determined from the measured distribution M(Sd,cal(λ)) by using the intensity calibration data CPAR1. Calibrated intensity values may be determined from the detector signal values SDET1by using the intensity calibration data CPAR1. The calibrated measured spectrum XS(λ) may be obtained by using the intensity calibration data CPAR1to convert detector signal values into calibrated intensity values. The calibrated measured spectrum XS(λ) may represent the spectrum B(λ) of the input light LB1. The calibrated measured spectrum XS(λ) of the input light LB1may represent the spectrum of the object OBJ1.

Referring toFIG. 3a, the spectral scale of the interferometer100may be determined by scanning the transmission peak PFP,kof the interferometer100over the spectrum C(λ) of the filtered light LB2. The spectral scale of the interferometer100may be determined by scanning the transmission peak PFP,kof the interferometer100over the spectral notch NC2of the spectrum C(λ). The spectral scale may be determined and/or verified by scanning the transmission peak PFP,kof the interferometer100over the spectrum C(λ) of the filtered light LB2. The filtered light LB2may be provided e.g. by filtering broadband light LB1with the notch filter60.

During the scanning, the mirror gap dFPof the interferometer100may be varied such that the control signal Sdis indicative of the mirror gap dFP. In particular, the mirror gap dFPmay be varied according to the control signal Sd. For example, the mirror gap dFPmay be adjusted by converting the control signal Sdinto driving voltage, which is coupled to the actuator140of the interferometer100. Alternatively, the mirror gap dFPmay be monitored e.g. by a capacitive sensor150, which may be utilized to provide the control signal Sd(FIG. 7).

The combined transmittance of notch filter60and the Fabry-Perot interferometer100may be proportional to the intensity I3of light LB3impinging on the detector DET1. The combined transmittance may be analyzed by monitoring the intensity I3of light LB3impinging on the detector DET1. The detector signal SDET1of the detector DET1may be indicative of the intensity I3of light LB3impinging on the detector DET1.

The mirror gap dFPmay be varied and the detector signal SDET1may be analyzed in order to determine a first control signal value SN1associated with a first mirror gap dFPwhen the transmission peak PFP,kof the interferometer100substantially coincides with a first edge of the filtered notch NC2.

The mirror gap dFPmay be varied and the detector signal SDET1may be analyzed in order to determine a second control signal value SN2associated with a second mirror gap dFPwhen the transmission peak PFP,kof the interferometer100substantially coincides with a second edge of the filtered notch NC2.

The spectral position λN1and the first control signal value SN1may be associated to form a first pair (λN1,SN1). The spectral position λN2and the second control signal value SN2may be associated to form a second pair (λN2,SN2). Spectral calibration data Scal,d(λ) of the interferometer (100) may be determined and/or verified by using the first pair (λN1,SN1) and the second pair (λN2,SN2). Further pairs (λCUT1,SC1), (λCUT2,SC2), . . . may be formed based on the spectral positions of the edges of the detection band ΔλPB. The spectral calibration data Scal,d(λ) of the interferometer (100) may be determined and/or verified also by using the further pairs (λCUT1,SC1), (λCUT2,SC2).

In an embodiment, the calibration function may depend on the operating temperature of the interferometer100. The spectral calibration data Scal,d(λ) may be determined for different operating temperatures of the interferometer100. First spectral calibration data Scal,d,T1(λ) may be determined for use at a first operating temperature T1of the interferometer100. Second spectral calibration data Scal,d,T2(λ) may be determined for use at a second operating temperature T2of the interferometer100.

In an embodiment, the spectrometer500may be arranged to operate such that the operating temperature Topof the notch filter is close to the operating temperature of the interferometer100. The difference between the operating temperature Topof the notch filter60and the operating temperature of the interferometer100may be kept e.g. smaller than 3° C. during operation of the spectrometer500. The spectrometer500may optionally comprise a temperature sensor for monitoring the operating temperature of the interferometer100. The spectrometer500may optionally comprise a temperature sensor61for monitoring the operating temperature Topof the notch filter60.

In an embodiment, the operating temperature Topof the notch filter60may substantially deviate from the operating temperature of the interferometer100. In that case, the spectral positions may be determined from the control signal values Sdby taking into account the operating temperature Topof the notch filter60. The calibration function λcal(Sd) may depend on the operating temperature Topof the notch filter60. The operating temperature Topof the notch filter60may be an input variable of the calibration function λcal(Sd).

The calibration function λcal(Sd) may be determined by monitoring the operating temperature Topof the notch filter60, and by calculating the temperature-induced spectral shift of the spectral positions λN1, λN2from the operating temperature Topof the notch filter60. Consequently, a first calibration function λcal(Sd) determined for a first operating temperature Topof the notch filter may be different from a second calibration function λcal(Sd) determined for a second operating temperature Topof the notch filter.

The method may comprise determining first spectral calibration data λcal(Sd) at a first operating temperature Topof the notch filter (60), and determining second spectral calibration data λcal(Sd) at a second operating temperature Topof the notch filter (60), wherein the second spectral calibration data may be different from the first spectral calibration data.

Spectral deviations caused by a change of temperature may be larger at the shorter wavelengths of the detection band ΔλPBthan at the longer wavelengths of the detection band ΔλPB. The transmittance notch NC1may be positioned e.g. substantially in the middle of the detection band ΔλPBor at the shorter wavelengths of the detection band ΔλPB. The difference between the wavelengths λN1and λCUT1may be e.g. in the range of 5% to 50% of the spectral width of the detection band ΔλPB.

The relationship between the spectral positions λ and the corresponding control signal values Sd(λ) may be expressed e.g. by a calibration function λcal(Sd) and/or by a calibration function Scal,d(λ). The calibration function Scal,d(λ) may be the inverse function of the calibration λcal(Sd). The function Scal,d(λ) may also be called e.g. as the marker signal function. Examples of the function Scal,d(λ) are shown inFIG. 3a, and examples of the function λcal(Sd) are shown inFIG. 3b.

The calibration function λcal(Sd) and/or Scal,d(λ) may be determined e.g. by fitting a regression function to a plurality of data points. The calibration function λcal(Sd) and/or Scal,d(λ) may be e.g. a polynomial function. The calibration function λcal(Sd) may be e.g. a third order polynomial function. The calibration function λcal(Sd) and/or Scal,d(λ) may be verified e.g. by fitting a regression function to the data pair (λN1,SN1). The calibration function λcal(Sd) and/or Scal,d(λ) may be determined and/or verified e.g. by fitting a regression function to the data pairs (λN1,SN1), (λN2,SN2), (λCUT1,SC1), (λCUT2,SC2).

In an embodiment, the notch filter60may enable on-line stabilization of the Fabry-Perot interferometer even when utilizing only one edge of the spectral notch NC2. In that case, the calibration function λcal(Sd) and/or Scal,d(λ) may be determined e.g. by fitting a regression function to a single data point (λN1,SN1) or (λN2,SN2).

The spectral position λX1corresponding to a value SX1of the control signal Sdmay be determined by using the calibration function λcal(Sd). For example, the maximum value MMAXof the measured distribution M(Sd) may be attained when the control signal Sdis equal to the value SX1. The accurate spectral position λX1of the peak of the input spectrum B(λ) may be determined from the control signal value SX1by using the calibration function λcal(Sd), which has been determined and/or verified by using the spectral notch NC2.

Referring back toFIG. 2e, an arbitrary control signal value SX2may be associated with a detector signal value M(SX2). The spectral position λX2corresponding to a value SX2of the control signal Sdmay be determined by using the calibration function λcal(Sd). Consequently, a detector signal value M(SX2) of a measured distribution M(Sd) may be associated with the corresponding spectral position λX2by using the calibration function λcal(Sd) where Sd=SX2. I1(λX2) denotes the intensity of input light LB1at the spectral position λX2. I2(λX2) denotes the intensity of filtered light LB2at the spectral position λX2.

The control signal values SX1, SX2may be different from the marker values SC1, SN1, SN2, and SC2.

Calibration data defining the calibration function λcal(Sd) may be stored in a memory MEM3of the spectrometer500and/or in a memory of a database server. The calibration data may comprise e.g. a look-up-table, which corresponds to the calibration function λcal(Sd). The calibration data may comprise e.g. parameters, which define a polynomial calibration function λcal(Sd).

When needed, the calibration data may be retrieved from the memory. The calibration data may be used for determining the spectral scale for a measured spectrum. The calibration data may be determined and/or verified. The determined calibration data may be optionally stored in a memory MEM3of the spectrometer500and/or in a memory of a database server. The calibration data may be determined by modifying previous calibration data. Modified calibration data may be optionally stored in a memory MEM3of the spectrometer500and/or in a memory of a database server, again.

A first calibration function λcal,T1(Sd) may be determined for use at a first operating temperature T1of the interferometer100. A second calibration function λcal,T2(Sd) may be determined for use at a second operating temperature T2of the interferometer100.

Using the first calibration function λcal,T1(Sd) for control signal values Sdmeasured at the second operating temperature T2may cause a spectral error Δλ.

The measured distribution M(Sd) may be compared with the spectral transmittance TN(λ) of the notch filter60by using cross-correlation analysis. Matching the distribution M(Sd) with the transmittance TN(λ) may be performed by using cross-correlation. The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by correlation analysis. The distribution M(Sd) may indicate intensity values as the function of control signal Sd. A calibrated distribution M(Sd,cal(λ)) may be determined from the measured distribution M(Sd) by using the calibration function Sd,cal(λ). The calibrated distribution M(Sd,cal(λ)) may provide intensity values as the function of spectral position λ.

The calibration function Sd,cal(λ) may be a regression function, which has one or more adjustable parameters. For example, the calibration function Sd,cal(λ) may be a polynomial function, and the adjustable parameters may be the coefficients of the terms of the polynomial function.

The cross-correlation of the calibrated distribution M(Sd,cal(λ)) with the transmittance function TN(λ) may provide a value, which indicates the degree of similarity between the calibrated distribution M(Sd,cal(λ)) and the transmittance function TN(λ). The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by adjusting one or more parameters of the regression function Sd,cal(λ), and calculating the cross-correlation of the calibrated distribution M(Sd,cal(λ)) with the transmittance function TN(λ). One or more parameters of the regression function may be adjusted until the cross-correlation of the calibrated distribution M(Sd,cal(λ)) with the transmittance function TN(λ) reaches a maximum value. The cross-correlation may reach a maximum value when the spectral position of the notch NC2′ of the calibrated distribution M(Sd,cal(λ)) substantially coincides with the notch NC1of the transmittance function TN(λ).

An auxiliary transmittance function TN(λcal(Sd)) of the notch filter60may give the transmittance of the notch filter60as the function of control signal Sd. The calibration function λcal(Sd) may be expressed as a regression function, which has one or more adjustable parameters. One or more parameters of the regression function may be adjusted until the cross-correlation of the measured distribution M(Sd) with the auxiliary transmittance function TN(λcal(Sd)) reaches a maximum value.

The method may comprise:providing a regression function Sd,cal(λ) or (λcal(Sd),determining a calibrated spectral intensity distribution (M(Sd,cal(λ)) from the measured spectral intensity distribution (M(Sd)) by using the regression function, anddetermining one or more parameters of the regression function (Sd,cal(λ)) such that the cross-correlation of the calibrated spectral intensity distribution (M(Sd,cal(λ)) with the spectral transmittance (TE,MAX) of the notch filter60reaches a maximum value.

The method may comprise:providing a regression function Sd,cal(λ) or (λcal(Sd),determining an auxiliary transmittance (TN(λcal(Sd)) from the spectral transmittance (TN(λ)) of the notch filter60by using the regression function, anddetermining one or more parameters of the regression function (λcal(Sd)) such that the cross-correlation of the distribution (M(Sd)) with the auxiliary transmittance (TN(λcal(Sd)) reaches a maximum value.

In an embodiment, the accuracy of the calibration function λcal(Sd) and/or Sd,cal(λ) may be verified by checking whether the maximum value of the cross-correlation is higher than or equal to a predetermined limit. If the maximum value of the cross-correlation is lower than the predetermined limit, this may be an indication that the calibration function is not valid.

FIG. 4ashows an apparatus700suitable for absorption or reflection measurements. The apparatus700may comprise a spectrometer500and a light source unit210. The light source unit210may provide illuminating light LB0. The apparatus700may be arranged to analyze an object OBJ1. The object OBJ1may be e.g. an amount of chemical substance contained in a cuvette. The object OBJ1may be a sample. The object OBJ1may be e.g. a piece of material. The light source unit210may be arranged to illuminate the object OBJ1. The spectrometer500may be arranged to receive light LB1transmitted through the object OBJ1and/or to receive light LB1reflected from the object OBJ1. The apparatus700may comprise a notch filter60. The apparatus may further comprise the units60,100,300, DET1, CNT1, MEM1, MEM2, MEM3, MEM4, MEM5, RXTX1, USR1. The units60,100,300, DET1, CNT1, MEM1, MEM2, MEM3, MEM4, MEM5, RXTX1, USR1may operate in a similar manner as described in the other parts of this description.

Referring toFIG. 4b, the light source unit210may comprise a light source221, and optionally a light-directing element222. The light source221may comprise e.g. an incandescent lamp, a blackbody radiator, an infrared light emitting glow-bar, a tungsten halogen lamp, a fluorescent lamp, or a light emitting diode. The light-directing element222may comprise e.g. a lens or a paraboloid reflector. The light source unit210may be arranged provide illuminating light LB0. The illuminating light LB0may have a broad spectrum. In an embodiment, the illuminating light LB0may have a broadband spectrum which is substantially continuous at least in the spectral range from λCUT1to λCUT2.

FIG. 5aillustrates filtering of light by the notch filter60in case of an absorption (or reflectance) measurement. Input light LB1impinging on the filter60may have an input spectrum B(λ), and filtered light LB2transmitted through the filter60may have a filtered spectrum C(λ). The filter60may provide the filtered light LB2by filtering the input light LB1. The filtered spectrum C(λ) may be obtained by multiplying the input spectrum B(λ) with the transmittance TN(λ) of the notch filter60(see equation 1).

The uppermost curve ofFIG. 5ashows the spectral transmittance TN(λ) of the filter50. The second curve from the top ofFIG. 5ashows an input spectrum B(λ). The input spectrum B(λ) may be e.g. an absorption spectrum or a reflectance spectrum. The third curve from the top ofFIG. 5ashows a filtered spectrum C(λ), which is formed by filtering the input spectrum B(λ) with the notch filter60.

The lowermost curve ofFIG. 5bshows a measured spectral intensity distribution M(Sd). The distribution M(Sd) may be measured by scanning over the filtered spectrum C(λ). The spectral position of the transmission peak PFP,kof the Fabry-Perot interferometer100may be scanned by varying the mirror gap dFP. The control signal Sdmay be provided such that the control signal Sdis indicative of the mirror gap dFP. The spectral position of the transmission peak PFP,kof the Fabry-Perot interferometer100may be scanned by varying the value of the control signal Sd. The detector signal SDET1may be monitored as the function of the control signal Sdin order to detect the edges of the notch NC2, NC2′.

The measured distribution may be expressed as a function M(Sd) of the control signal Sd. The calibration function λcal(Sd) may be determined and/or verified by comparing the measured distribution M(Sd) with the transmittance function TN(λ). The calibration function λcal(Sd) may be determined and/or checked by using marker values SN1, SN2, SC1, . . . of the measured distribution M(Sd) and by using the accurately known wavelengths λN1, λN2, λCUT1, λCUT2of the transmittance function TN(λ).

FIG. 5bshows forming a measured absorption spectrum I1(λ)/I0(λ) from the measured spectral intensity distribution M(Sd) ofFIG. 5a. The uppermost curve ofFIG. 5bshows the measured spectral intensity distribution M(Sd), which may be obtained by varying the mirror gap dFP, and by recording the detector signal values SDET1as the function of the control signal Sd. The second curve from the top ofFIG. 5bshows a calibrated spectral intensity distribution M(Sd,cal(λ)) determined from the measured spectral intensity distribution M(Sd) by using the calibration function λcal(Sd) and/or Sd,cal(λ).

If desired, a calibrated measured spectrum XS(λ) may be optionally obtained by using the intensity calibration data CPAR1to convert detector signal values to calibrated intensity values. The calibrated measured spectrum XS(λ) may represent the spectrum of light transmitted through the object OBJ1or the spectrum of light reflected by the object OBJ1.

A reference distribution MREF(Sd,cal(λ)) may be obtained by measuring the spectral intensity distribution without the absorbing sample OBJ1. The reference distribution MREF(Sd,cal(λ)) may represent the spectrum of the illuminating light LB0. The reference distribution MREF(Sd,cal(λ)) may be stored e.g. in the memory MEM4of the apparatus700. The measured absorption spectrum I1(λ)/I0(λ) may be determined from the calibrated spectral intensity distribution M(Sd,cal(λ)) by using the reference distribution MREF(Sd,cal(λ)). The measured absorption spectrum I1(λ)/I0(λ) may be determined by dividing the reference distribution MREF(Sd,cal(λ)) with the spectral intensity distribution M(Sd,cal(λ)).

FIG. 6shows, by way of example, an interferometer100where the optical cavity has been formed by etching. The spectrometer500may comprise the interferometer shown inFIG. 6. The spectrometer500may comprise an interferometer100where the optical cavity has been formed by etching. The spectrometer500may comprise an interferometer100where an empty space ESPACE1between the mirrors110,120has been formed by etching, after the material layers of the mirrors110,120have been formed.

The spacer115may be deposited on top of the mirror120, two or more material layers of the mirror110may be deposited top of the spacer115, and the empty space (ESPACE1) between the mirrors110,120may be formed by etching material away from between the mirrors110,120after the two or more material layers of the mirror110have been deposited.

The first mirror110may have a movable portion MPOR1, and the first mirror110may be called e.g. as the movable mirror. The movable portion MPOR1of the movable mirror110may be moved with respect to the stationary mirror120in order to adjust the mirror gap dFP. The second mirror120may be called e.g. as the stationary mirror.

The stationary mirror120may comprise a plurality of material layers supported by substrate130. The movable mirror110may be supported by a spacer layer115. The spacer layer115may be formed on top of the stationary mirror120, and the movable mirror110may be supported by the spacer layer115. The movable mirror110may comprise e.g. material layers110a,110b,110c,110d, and/or110e. The stationary mirror120may comprise e.g. material layers120a,120b,120c,120d, and/or120e. The mirrors110,120may comprise reflective multilayer coatings. The mirrors110,120may be implemented by using reflective multilayer coatings.

The material layers of the stationary mirror120may be formed e.g. by depositing material on top of a substrate130and/or by locally converting material of the substrate130. The spacer layer115may be deposited on top of the stationary mirror120after the material layers of the stationary mirror120have been formed. The material layers of the movable portion MPOR1may be formed after the spacer layer115has been deposited, by depositing material layers of the movable mirror110on top of the spacer layer115. The material layers of the mirrors110,120may be e.g. e.g. silicon-rich silicon nitride, polycrystalline silicon, doped polycrystalline silicon, silicon oxide and/or aluminum oxide. The layers may be deposited e.g. by using a LPCVD process. LPCVD means low pressure chemical vapor deposition. The substrate130may be e.g. monocrystalline silicon or fused silica. The spacer layer115may comprise e.g. silicon dioxide. The spacer layer115may consist essentially of silicon dioxide.

The empty space ESPACE1between the mirrors110,120of the interferometer100may be formed by etching. The material of the spacer layer115may etched away e.g. by using hydrofluoric acid (HF). The mirror110may comprise a plurality of miniature holes H1for guiding hydrofluoric acid (HF) into the space between the mirrors110,120and for removing the material of the spacer layer115. The width of the holes H1may be so small that they do not significantly degrade the optical properties of the interferometer100.

The movable portion MPOR1may be moved e.g. by an electrostatic actuator140. The electrostatic actuator140may comprise two or more electrodes Ga, Gb. A first electrode Ga may have a voltage Va, and a second electrode Gb may have a voltage Vb. The electrodes Ga, Gb may generate an attractive electrostatic force F1when a voltage difference Va−Vb is applied between the electrodes Ga, Gb. The electrostatic force F1may pull the movable portion MPOR1towards the stationary mirror120.

The electrostatic actuator140may be implemented as a rugged, shock-proof, miniature, stable and/or low-cost structure. Thanks to using the notch filter for spectral stabilization, an interferometer100having the electrostatic actuator140may also be implemented without using a capacitive sensor for monitoring the mirror gap.

The voltage Va may applied to the electrode Ga by using a conductor CON1and a terminal N1. The voltage Vb may applied to the electrode Gb by using a conductor CON2and a terminal N2. The voltages Va, Vb may be provided by a voltage supply, which may be controlled by the control unit CNT1. The voltages Va, Vb may be provided according to the control signal Sd. The terminals N1, N2may be e.g. metallic, and the conductors CON1, CON2may be e.g. bonded to the terminals N1, N2.

The aperture portion AP1of the movable portion MPOR1may have a width w1. The aperture portion AP1of the movable mirror110may be highly planar in order to provide sufficient spectral resolution. The magnitude of electrostatic forces directly acting on the aperture portion AP1may be kept low in order to preserve the planar shape of the aperture portion AP1. The attractive force F1may be generated by a substantially annular electrode Gb, which surrounds the aperture portion AP2of the stationary mirror120. The mirror120may optionally comprise a neutralizing electrode Gc, which may be arranged to reduce deformation of the aperture portion AP1of the movable mirror110. The neutralizing electrode Gc may be substantially opposite the aperture portion AP1of the movable mirror110. The voltage of the neutralizing electrode Gc may be kept substantially equal to the voltage Va of the electrode Ga, in order to reduce deformation of the aperture portion AP1of the movable mirror110. The voltage difference between the electrodes Ga and Gc may be kept smaller than a predetermined limit in order to reduce deformation of the aperture portion AP1of the movable mirror110. In an embodiment, the neutralizing electrode Gc may be galvanically connected to the electrode Ga e.g. by using a connecting portion N1b. The annular electrode Gb may be positioned around the neutralizing electrode Gc. The electrodes Ga and Gc may be substantially transparent at the operating spectral region of the interferometer100. For example, the electrodes Ga, Gb and Gc may comprise doped polycrystalline silicon, which may be substantially transparent for infrared light LB3.

The electrodes Ga, Gb may generate an attractive electrostatic force F1when a driving voltage Vabis coupled to the electrodes Ga, Gb. The driving voltage Vabmay be equal to the voltage difference Va−Vb. The voltage may be coupled to the electrodes Ga, Gb e.g. via the conductors CON1, CON2and the terminals N1, N2. The mirror110may be flexible and/or the spacer115may be mechanically compressible such that the mirror gap dFPmay be changed by changing the magnitude of the electrostatic force F1. The magnitude of the electrostatic force F1may be changed by changing the driving voltage Va−Vb(=Vab).

Coupling a first driving voltage Vabto the electrodes Ga, Gb may cause adjusting the transmission peak PFP,kof the interferometer100to a first spectral position (e.g. to the position λP1), and coupling a second different driving voltage Vabto the electrodes Ga, Gb may cause adjusting the transmission peak PFP,kof the interferometer100to a second different spectral position (e.g. to the position λP2).

During normal operation, the space ESPACE1between the mirrors110,120may be filled with a gas. However, the interferometer100may also be operated in vacuum, and the pressure in the space ESPACE1may be low.

The interferometer100produced by depositing and etching may be considered to have a substantially monolithic structure. Said interferometer100may be e.g. shock resistant and small. The mass of the moving portion MPOR1may be small, and the interferometer100may have a high scanning speed.

Referring toFIG. 7, the spectrometer500may comprise an interferometer100, which has a distance sensor150for monitoring the mirror gap dFP. The distance sensor150may be e.g. a capacitive sensor, which comprises two or more capacitor plates G1, G2. A first capacitor plate G1may be attached to the first mirror110, and a second capacitor plate G2may be attached to the second mirror120so that the distance between the plates G1, G2depends on the mirror gap dFP. The capacitor plates G1, G2may together form a capacitor, which has a capacitance Cxsuch that the capacitance Cxmay depend on the mirror gap dFP. The capacitance value Cxmay be indicative of the mirror gap dFP. The capacitor plates G1, G2may be connected to a capacitance monitoring unit152e.g. by conductors CONa, CONb. The capacitance monitoring unit152may provide a signal Sdindicative of the capacitance Cxof the sensor150. The capacitance monitoring unit152may provide a signal Sdindicative of the mirror gap dFP.

The capacitance monitoring unit152may be arranged to measure the capacitance Cxe.g. by charging the capacitive sensor150with a predetermined current, and measuring the time needed to charge the sensor150to a predetermined voltage. The capacitance monitoring unit152may be arranged to measure the capacitance Cxe.g. by coupling the capacitive sensor150as a part of a resonance circuit, and measuring the resonance frequency of the resonance circuit. The capacitance monitoring unit152may be arranged to measure the capacitance Cxe.g. by using the capacitive sensor150to repetitively transfer charge to a tank capacitor, and counting the number of charge transfer cycles needed to reach a predetermined tank capacitor voltage.

The interferometer100may comprise a driver unit142. The driver unit142may e.g. convert a digital driving signal S140into an analog signal suitable for driving the actuator140. The driver unit142may provide e.g. a voltage signal Vabfor driving an electrostatic actuator140, or for driving a piezoelectric actuator140.

In an embodiment, the control unit CNT1may be configured to provide a digital driving signal S140for changing the mirror gap dFP, and the control unit CNT1may be arranged to receive the control signal Sdindicative of the mirror gap dFP.

The spectrometer500may be implemented e.g. in a first mobile unit. Determining spectral positions λ from the control signal values Sdmay be carried out in the first mobile unit. The spectral positions may be expressed e.g. in terms of wavelengths or wavenumbers. Determining spectral positions λ from the control signal values Sdmay be carried out in a second mobile or in stationary unit, which is separate from the first unit. The stationary unit may be implemented e.g. in a server, which may be accessed via the Internet.

The spectrometer500may be used e.g. for remote sensing applications. The spectrometer500may be used e.g. for measuring the color of an object OBJ1. The spectrometer500may be used e.g. for an absorption measurement, where the transmission peak of the interferometer100may be adjusted to a first spectral position to match with an absorption band of an object OBJ1, and the transmission peak of the interferometer100may be adjusted to a second spectral position to match with a reference band. The spectrometer500may be used e.g. for a fluorescence measurement, where the first spectral position of the transmission peak of the interferometer is matched with fluorescent light emitted from an object OBJ1, and the second spectral position is matched with the illuminating light, which induces the fluorescence.

When measuring the reflectance spectrum of an object OBJ1, the object OBJ1may be illuminated with illuminating light. The illuminating light may have a broad spectrum. In an embodiment, the bandwidth of the illuminating light may be greater than or equal to the detection range of the spectrometer500.

When measuring broadband light, the spectrometer may provide a filtered spectrum, which has the spectral notch NC2at a stable spectral position. The spectral scale of the interferometer100may be determined and/or verified based on the spectral notch NC2. Therefore, the accuracy requirements of the scanning interferometer may become highly relaxed as the spectral positions of the edges of the notch NC2may be detected at high accuracy.

The spectral scale of the spectrometer500may be calibrated by using notch filter60, which provides only one transmittance notch NC1. However, the spectrometer500may comprise a filter unit60, which provides two or more transmittance notches. Referring toFIG. 8a, the filter unit60of the spectrometer500may comprise e.g. two or more notch filters60a,60b. referring toFIG. 8b, the filter unit60may provide a first transmittance notch NC1and a second transmittance notch NC1B such that the second transmittance notch NC1B is spectrally separate from the first transmittance notch NC1. The first transmittance notch NC1may be used for providing a first data pair (λN1,SN1) as described above. The second transmittance notch NC1B may be used for providing a first auxiliary data pair (λN1B,Sd(λN1B)) and/or a second auxiliary data pair (λN2B,Sd(λN2B)). The first auxiliary data pair (λN1B,Sd(λN1B)) and/or a second auxiliary data pair (λN2B,Sd(λN2B)) may be used to further improve the accuracy of determining the spectral calibration data λcal(Sd).

The term “light” may refer to electromagnetic radiation in the ultraviolet, visible and/or infrared regime.

In an embodiment, light may be coupled into a spectrometer by using one or more optical fibers. For example, light may be guided to the spectrometer from an optical probe by using one or more optical fibers.

A spectral position may be defined e.g. by providing a wavelength value and/or by providing a wavenumber value. The spectral scale may be defined e.g. by using wavelength values and/or by using wavenumber values.

For the person skilled in the art, it will be clear that modifications and variations of the devices and the methods according to the present invention 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 invention, which is defined by the appended claims.