Patent Application
20170138790
The present invention relates to a method for determining spectral calibration data of a Fabry-Perot interferometer. Some variations relate to spectral analysis of light. Further, the present invention relates to an apparatus.
The wavelength scale of a Fabry-Perot interferometer can be calibrated e.g. by measuring the excitation spectrum of a gas discharge lamp. The gas discharge lamp may typically contain e.g. argon, neon, xenon, krypton, hydrogen, or mercury. The spectrum of the gas discharge lamp comprises a high number of atomic emission lines, which are characteristic to the gas contained in the lamp. However, gas discharge lamps are not available for all wavelength regions of interest. The spectral separation between atomic lines may sometimes be too narrow for accurate calibration. The spectral separation between atomic lines may sometimes be too large for accurate calibration. The calibration lamps consume electrical power. The calibration lamps may be fragile.
Document US 2004/070768 A1 discloses, for example, a wavelength reference apparatus for use in calibrating a tunable Fabry-Perot filter or a tunable VCSEL, whereby the device may be tuned to a precise, known wavelength, the wavelength reference apparatus comprising an LED, where the LED is chosen so as to have an emission profile which varies with wavelength. Further, the reference apparatus comprises an etalon, where the etalon is chosen so as to have a transmission profile which comprises a comb of transmission peaks, with each transmission peak occurring at a precise, known wavelength. Furthermore, the reference apparatus comprises a detector for detecting the light emitted by the LED and passing through the etalon. When a tunable Fabry-Perot filter or tunable VCSEL is positioned between the etalon and the detector, and the device is swept through its tuning range by varying the tuning voltage applied to the device, the known transmission wavelengths established by the LED and the etalon can be correlated to counterpart tuning voltages of the device, whereby to calibrate the device. The specific wavelengths of transmission peaks are a function of the etalon's substrate thickness and refractive index. The thickness of the etalon and the refractive index are configured such that very narrow transmission peaks are obtained which are located relatively close to each other.
Such a configuration is disadvantageous when calibrating mid-resolution devices such as MEM5 Fabry-Perot interferometer based devices. Two adjacent transmission peaks may be located too close to each other in order to unambiguously distinguish the peaks. For example, in case that the spectral resolution of a device is 10 nm and the specific wavelengths of transmission peaks of the etalon are spectrally 1 nm wide, about 90% of the signal power is lost compared to a case in which the specific wavelengths of transmission peaks of the etalon have spectral width of 10 nm. Thus, very narrow transmission peaks which are located relatively close to each other can increase the calibration time per device by the multiplier 100. Additionally, the signal power transmitted through the etalon is relatively small.
Therefore, it would be beneficial to provide a method for determining spectral calibration data of a Fabry-Perot interferometer and an apparatus, wherein calibration time can be reduced and the signal power transmitted through the etalon can be increased. Additionally, it would be beneficial to provide a method and apparatus by means of which variation of performance characteristics due to temperature changes can be considered, thus improving precision of calibration.
Some variations may relate to a method for calibrating a spectrometer. Some variations may relate to a method for measuring a spectrum. Some variations may relate to a spectrometer. Some variations may relate to a calibration device for calibrating a spectrometer. Some variations may relate to a computer program for calibrating a spectrometer. Some variations may relate to a computer program for measuring a spectrum. Some variations may relate to a computer program product, which comprises computer program code for calibrating a spectrometer. Some variations may relate to 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 claim 1.
According to a second aspect, there is provided an apparatus according to claim 11.
Further aspects are described in the dependent claims.
A method for determining spectral calibration data (λcal(Sd), Sd,cal(λ)) of a Fabry-Perot interferometer (100) may comprise:
A method for verifying spectral calibration data (λcal(Sd), Sd,cal(λ)) of a Fabry-Perot interferometer (100) may comprise:
A spectrometer may comprise a Fabry-Perot interferometer and a detector for monitoring intensity of light transmitted through the Fabry-Perot interferometer. The Fabry-Perot interferometer may be used for measuring an intensity distribution by scanning the interferometer. The interferometer may be scanned by varying the mirror gap of the 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. Spectral calibration data of the interferometer may determine a relation for obtaining spectral positions from values of the control signal. The spectral calibration data may define the spectral scale of the interferometer. Each spectral position may be associated with a control signal value by using the spectral calibration data.
When monitoring an unknown spectrum, the spectrometer may be arranged to obtain intensity values from the detector as a function of the control signal. The measured intensity values may be associated with calibrated spectral positions by using the spectral calibration data. The spectral 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 spectral 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 adjustable mirror gap.
The spectral position of the transmittance peak may be changed according to the control signal. 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. 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.
In an embodiment, the control signal may also be provided by a sensor. The control signal may indicate e.g. capacitor value of 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 e.g. due to chemical corrosion.
A Fabry-Perot etalon may be arranged to form a plurality of filtered spectral peaks. Fabry-Perot interferometer may be used for measuring the spectral intensity distribution of the filtered spectral peaks. The spectral calibration data of the interferometer may be determined by matching the peaks of the measured distribution with the peaks of the spectral transmittance of the etalon. The spectral calibration data of the interferometer may be checked by comparing the measured distribution with the spectral transmittance of the etalon. 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 may be determined by matching spectral features of the measured spectral distribution with spectral features of the spectral transmittance of the etalon.
The spectral calibration data may be determined by matching spectral peaks of the measured spectral distribution with spectral peaks of the spectral transmittance of the etalon.
The spectral calibration data may be determined such that the measured spectral distribution matches with the spectral transmittance of the etalon, 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 etalon, 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 etalon, when the relation between the control signal and the spectral position is determined using said spectral calibration data, and such that the spectral position of a second spectral feature of the measured spectral distribution substantially coincides with the spectral position of a second spectral feature of the spectral transmittance of the etalon, when the relation between the control signal and the spectral position is determined using said spectral calibration data,
The etalon may be placed in the optical path of the spectrometer. The etalon may provide a simple and highly stabile spectral reference for calibration and/or for measurement purposes. The spectral scale of the spectrometer may be stabilized by using a Fabry-Perot etalon, which has fixed mirror spacing.
Input light may be filtered by using a Fabry-Perot etalon in order to provide a plurality of spectral peaks. Said spectral peaks may be called e.g. as reference peaks or as filtered peaks. The etalon may comprise a substrate, which has a first planar surface and a second planar surface. The first planar surface and the second planar surface may be flat. The second planar surface is parallel to the first planar surface. The distance between the planar surfaces may be called as the mirror spacing of the etalon. The planar surfaces may form an optical cavity, which causes constructive and destructive interference such that broadband input light transmitted through the planar surfaces may have a plurality of the spectral reference peaks. The spectral position of transmittance peaks of the Fabry-Perot etalon may be highly stable.
The spectral position of transmittance peaks of the Fabry-Perot etalon may mainly depend on the mirror spacing of the etalon. The mirror spacing of the etalon may be substantially constant. The mirror spacing of the etalon may be substantially independent of air pressure, variations of humidity, ageing, and/or corrosion. The mirror spacing of the etalon may remain constant even after a mechanical impact. The mirror spacing of the etalon may have highly reproducible thermal expansion.
The Fabry-Perot etalon may have highly stable monolithic structure. The monolithic structure may be mechanically and thermally stabile. The monolithic etalon may be more stable than an etalon, where the reflectors are separated by an air gap. The mirror spacing of the etalon may be defined by the thickness of a substrate of the etalon. In that case, the stability of the etalon may mainly depend on the thermal stability of the substrate. In that case, the mirror spacing of the etalon may mainly depend on the magnitude of temperature variation and on the coefficient of thermal expansion (CTE) of the substrate. For example, the substrate may be silicon. For example, when the substrate is silicon and when the temperature of the substrate is monitored with an accuracy, which is better than 2° C., the wavelength stability may be better than 0.01 nm at the wavelength of 2 μm. In practice, the stability of the spectral scale may be e.g. better than 1 ppm. The coefficient of thermal expansion of silicon is approximately 2.6·10−6/° C. The stability of the spectral scale may be e.g. better than 1 ppm (= 1/106). The deviation λ(Sd1)−λP1 between a determined wavelength λ(Sd1) and the true wavelength λP1 may be smaller than 10−6·λP1.
In an embodiment, the spectral positions of the reference peaks may depend on the temperature of the substrate of the etalon, but the spectral positions of the reference peaks may be accurately determined based on temperature of the substrate. The temperature of the substrate may be monitored by a temperature sensor. The spectral positions of the reference peaks may be accurately known as a function of the temperature of the substrate. The temperature of the substrate of the etalon may be optionally monitored by a temperature sensor. The sensor may be implemented e.g. by a thermocouple, Pt100 sensor, or by a P-N junction.
In an embodiment, a Fabry-Perot spectrometer may be calibrated by using a light source unit, which comprises the etalon. In an embodiment, a spectroscopic apparatus may comprise a light source unit, the etalon, and the Fabry-Perot interferometer.
In an embodiment, the spectral scale of a spectrometer may be determined and/or verified when measuring an unknown spectrum of an object. The spectral calibration data may be determined and/or verified by using light received from said object. The spectral calibration may be performed on-line, when measuring the unknown spectrum of the object. The etalon may be e.g. temporarily positioned between the object and the spectrometer, or the spectrometer may permanently comprise the etalon. Spectral stability may be a key parameter when analyzing spectra by the spectrometer. By using the etalon, 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 etalon. The etalon may be easily integrated in an on-line measurement system. In an embodiment, a Fabry-Perot spectrometer may comprise a permanently attached etalon for providing reference peaks.
The operation of the etalon as such does not require operating power. However, optional monitoring the temperature of the etalon may sometimes require a very low power.
Calibration by using an etalon may be used at various different wavelength regions, by selecting the material of the substrate and the optional coatings of the planar reflective surfaces of the etalon. By using the etalon, several reference peaks may be provided to cover a wide portion of the detection range of the spectrometer. In an embodiment, several reference peaks may be provided to cover substantially the whole detection range of the spectrometer.
In an embodiment, light may be coupled into a spectrometer by using one or more optical fibers.
In an embodiment, a light source unit may comprise the etalon, and calibration light provided by a light source unit may be coupled into a spectrometer for determining and/or checking the spectral scale of a spectrometer. In an embodiment, calibration light from a single light source unit may be distributed to several spectrometers in order to calibrate the spectral scales of said spectrometers substantially simultaneously. The calibration may be performed e.g. during production of the spectrometers. In an embodiment, even thousands of interferometers may be calibrated at a factory rapidly and/or with relatively low costs.
In an embodiment, the calibration light provided by a light source unit may be simultaneously distributed to a plurality of spectrometers by using optical fibers.
The calibration of the spectrometer may optionally comprise intensity calibration in addition to 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.
Certain embodiments provide a method for determining spectral calibration data of a Fabry-Perot interferometer and an apparatus, wherein calibration time can be reduced and the signal power transmitted through the etalon can be increased. Additionally, certain embodiments provide a method and apparatus by means of which variation of performance characteristics due to temperature changes can be considered, thus improving precision of calibration.
In the following examples, several variations will be described in more detail with reference to the appended drawings, in which
Referring to
The Fabry-Perot interferometer 100 comprises a first semi-transparent mirror 110 and a second semi-transparent mirror 120. The distance between the first mirror 110 and the second mirror 120 is equal to a mirror gap dFP. The mirror gap dFP may be adjustable. The first mirror 110 may have a solid-gas interface 111, and the second mirror 121 may have a solid-gas interface 121. The mirror gap dFP may denote the distance between the interfaces 111 and 121. The Fabry-Perot interferometer 100 may provide a transmission peak PFP,k (
The spectrometer 500 may comprise one or more filters 60 to define a detection band ΔλPB of the spectrometer 500. The filter 60 may provide filtered light LB2 by filtering the light LB1 received from the object OBJ1.
The Fabry-Perot interferometer 100 may form transmitted light LB3 by transmitting a portion of the filtered light LB2 to the detector DET1. Transmitted light LB3 obtained from interferometer 100 may be coupled to the detector DET1. The transmitted light LB3 may at least partly impinge on the detector DET1.
An actuator 140 may be arranged to move the first mirror 110 with respect to the second mirror 120. The actuator 140 may be e.g. an electrostatic actuator (
The light LB1 may be obtained from an object OBJ1. For example, the light LB1 may be emitted from the object, the light LB1 may be reflected from the object, and/or the light LB1 may be transmitted through the object. The spectrum of the light LB1 may be measured e.g. in order to determine emission spectrum, reflectance spectrum, and/or absorption spectrum of the object OBJ1.
The object OBJ1 may be e.g. a real or virtual object. For example, the object OBJ1 may be a tangible piece of material. The object OBJ1 may be a real object. The object OBJ1 may be e.g. in solid, liquid, or gaseous form. The object OBJ1 may comprise a sample. The object OBJ1 may a combination of a cuvette and a chemical substance contained in the cuvette. The object OBJ1 may 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 OBJ1 may be a display screen, which emits or reflects light of an image. The object OBJ1 may be an optical image formed by another optical device. The object OBJ1 may also be called as a target.
The light LB1 may also 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 mirror gap dFP of the interferometer 100 may be varied according to the control signal Sd. For example, the mirror gap dFP may be adjusted by converting the control signal Sd into driving voltage, which is applied to the actuator 140 of the interferometer 100. Alternatively, the mirror gap dFP may be monitored e.g. by a capacitive sensor, which may provide the control signal Sd.
The spectrometer 500 may comprise a control unit CNT1. The control unit may comprise one or more data processors. The control unit CNT1 may be arranged to provide a control signal Sd for controlling the mirror spacing dFP of the interferometer 100. For example, the spectrometer 500 may comprise a driving unit, which may be arranged to convert a digital control signal Sd into a voltage signal Vab. The voltage signal Vab may be coupled to a piezoelectric actuator or to en electrostatic actuator in order to adjust the mirror gap dFP (
The control signal Sd may also be a sensor signal. The interferometer may comprise e.g. a capacitive sensor for monitoring the mirror gap dFP (
The spectrometer 500 may optionally comprise light concentrating optics 300 for concentrating light into the detector DET1. The optics 300 may comprise e.g. one or more lenses and/or one or more reflective surfaces (e.g. a paraboloid reflector). The optics 300 may be positioned before the interferometer 100. The optics 300 may be positioned after the interferometer 100 (i.e. between the interferometer 100 and the detector DET1). One or more components of the optics 300 may be positioned before the interferometer 300, and one or more components of the optics 300 may be positioned after the interferometer.
The detector DET1 may be arranged to provide a detector signal SDE1. The detector signal SDET1 may be indicative of the intensity I3 of light LB3 impinging on the detector DET1. The detector DET1 may convert the intensity I3 of light LB3 impinging on the detector DET1 into a detector signal value SDET1.
The detector DET1 may be sensitive e.g. in the ultraviolet, visible and/or infrared region. The spectrometer 500 may be arranged to measure spectral intensities e.g. in the ultraviolet, visible and/or infrared region. The detector DET1 may be selected according to the detection range of the spectrometer 500. 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 DET1 may be an imaging detector or a non-imaging detector. The detector may comprise one or more pixels of a CMOS detector. The detector may comprise one or more pixels of a CCD detector.
The spectrometer 500 may comprise a memory MEM4 for storing intensity calibration data CPAR1. One or more intensity values I1 of the light LB1 may be determined from the detector signals SDET1 by using the intensity calibration data CPAR1. The intensity calibration data CPAR1 may comprise e.g. one or more parameters of a regression function, which allows determining intensity values I1 of the light LB1 from the detector signal values SDET1.
Spectral calibration data may determine a relation between values of the control signal Sd and spectral positions λ. A calibration function λcal(Sd) may determine a relation for obtaining spectral positions λ from values of the control signal Sd. Spectral calibration data may comprise parameters of a function λcal(Sd), which gives spectral position λ as the function of the control signal Sd.
Spectral calibration data Sd,cal(λ) may determine a relation for obtaining values of the control signal Sd from spectral positions λ. Spectral calibration data may comprise parameters of a function Sd,cal(λ) which gives control signal Sd as the function of the spectral position λ.
Each determined intensity value 11 may be associated with a value of the control signal Sd, and the determined intensity value 11 may be associated with a spectral position λbased on said control signal value Sd and spectral calibration data.
Each measured detector signal value SDET1 may be associated with a value of the control signal Sd, and the detector signal value SDET1 may be associated with a spectral position λ based on the control signal value Sd and spectral calibration data.
The spectrometer 500 may comprise a memory MEM3 for storing spectral calibration data. The spectral calibration data λcal(Sd) may comprise e.g. one or more parameters of a regression function, which allows determining the relationship between control signal values Sd and spectral positions λ. The spectrometer 500 may be arranged to determine spectral positions λ from control signal values Sd by using the spectral calibration data. The spectrometer 500 may comprise a memory MEM5 for storing a computer program PROG1. The computer program PROG1 may be configured, when executed by one or more data processors (e.g. CNT1), to determine spectral positions λ from control signal values Sd by using the spectral calibration data. The spectrometer 500 may be arranged to obtain detector signal values SDET1 from the detector DET1, and to determine intensity values I1 from the detector signal values SDET1 by using the intensity calibration data CPAR1. The computer program PROG1 may be configured, when executed by one or more data processors (e.g. CNT1), to obtain detector signal values SDET1 from the detector DET1, and to determine intensity values I1 from the detector signal values SDET1 by using the intensity calibration data CPAR1.
The spectrometer 500 may optionally comprise a memory MEM1 for storing spectral data λS(λ).The spectral data λS(λ) may comprise e.g. intensity values I1 determined as a function I1(λ) of the spectral position λ. The spectral data λS(λ) may comprise a calibrated measured spectrum I1(λ). The spectral data λS(λ) may comprise e.g. detector signal values SDET1 determined as a function SDET1(λ) of the spectral position λ.
The spectrometer 500 may optionally comprise a user interface USR1 e.g. for displaying information and/or for receiving commands. The user interface USR1 may comprise e.g. a display, a keypad and/or a touch screen.
The spectrometer 500 may optionally comprise a communication unit RXTX1. The communication unit RXTX1 may transmit and/or receive a signal COM1 e.g. in order to receive commands, to receive calibration data, and/or to send spectral data. The communication unit RXTX1 may be capable of wired and/or wireless communication. For example, the communication unit RXTX1 may be capable of communicating with a local wireless network (WLAN), with the Internet and/or with a mobile telephone network.
The spectrometer 500 may be implemented as a single physical unit or as a combination of separate units. In an embodiment, the interferometer 100, and the units CNT1, MEM1, MEM3, MEM4, MEM5, USR1, RXTX1 may be implemented in the same housing. In an embodiment, the spectrometer 500 may be arranged to communicate detector signals SDET1 and control signals Sd with a remote data processing unit, e.g. with a remote server. Spectral positions λ may be determined from the control signals Sd by the remote data processing unit.
The spectrometer 500 may optionally comprise one or more optical cut-off filters 60 to limit the spectral response of the detector DET1. The filters 60 may define the detection band of the spectrometer 500. The filters 60 may be positioned before and/or after the interferometer 100.
The spectrometer 500 may optionally comprise e.g. a lens and/or an aperture 230, which is arranged to limit the divergence of the light LB3 transmitted through the interferometer 100 to the detector DET1, in order to provide a narrow bandwidth ΔλFP of the transmission peak PFP,k. For example, the divergence of the light LB3 may be limited to be e.g. smaller than or equal to 10 degrees. When using light concentrating optics 300, the divergence of light LB3 contributing to the spectral measurement may also be limited by the dimensions of the detector DET1.
SX, SY and SZ denote orthogonal directions. The light LB2 may propagate substantially in the direction SZ. The mirrors 110, 120 of the interferometer may be substantially perpendicular to the direction SZ. The directions SZ and SY are shown in
The spectrometer of
The Fabry-Perot etalon 50 may be, for example, formed using silicon on insulator (SOI) technology. The Fabry-Perot etalon 50 and the Fabry-Perot interferometer 100 may be, for example, formed using micro-electro-mechanical system (MEM5) technology. Typically, the Fabry-Perot etalon 50 is configured such that the specific wavelengths of transmission peaks of the etalon 50 and the spectral resolution of the system are synchronized. Thus, adjacent transmission peaks can be located in order to unambiguously distinguish the peaks. According to certain embodiments, the transmitted signal power in the blocking bands may be, for example, in the range between 1% and 30% of the original signal power.
The spectrometer 500 may comprise one or more cut-off filters 60 to define a detection band ΔλPB of the spectrometer 500. The spectrometer 500 may be arranged to operate such that the spectrometer 500 is substantially insensitive to spectral components, whose wavelengths are outside a detection range ΔλPB. The detection range ΔλPB may be defined e.g. by a bandpass or cut-off filter 60, which rejects wavelengths which are shorter than a first cut off value λCUT1 and longer than a second cut off value λCUT2. The filter unit 60 may be implemented by using one or more optical filters. For example, the filter unit 60 may be implemented by stacking two or more cut-off filters. The filters 60 may block wavelengths outside the detection band ΔλPB from reaching the detector DET1. A cut-off filter 60 may prevent spectral components at wavelengths λ shorter than a first cut-off limit λCUT1 from impinging on the detector DET1. A cut-off filter 60 may prevent spectral components at wavelengths λ longer than a second cut-off limit λCUT1, λCUT2 from impinging on the detector DET1. The cut-off limits λCUT1, λCUT2 may be selected such that only spectral components within the detection range ΔλPB propagate to the detector DET1, depending on the spectral position λFP,k of the transmission peak PFP,k of the interferometer 100. The cut-off limits λCUT1, λCUT2 may be selected such that spectral components overlapping the other transmission peaks λFP,k−1, λFP,k+1 do not propagate to the detector DET1. Adjacent peaks PFP,k, PFP,k+1 of the interferometer 100 are separated by the free spectral range ΔλFSR,FP. The cut-off limits λCUT1, λCUT2 may be selected such that the detection range ΔλPB of the spectrometer 500 is narrower than the free spectral range ΔλFSR,FP. Wavelengths outside the detection range ΔλPB may also be rejected by utilizing spectral selectivity of the detector DET1 and/or another optical component of the spectrometer. The filters 60 may be omitted e.g. when the detector DET1 is not sensitive to light outside the range refined by the cut-off limits λCUT1, λCUT2. The filters 60 may be omitted when the input light LB1 does not contain spectral components at wavelengths outside the range defined by the cut-off limits λCUT1, λCUT2.
I2(λ) may denote spectral intensity of light LB2 impinging on the interferometer 100, and I3(λ) may denote spectral intensity of light LB3 transmitted through the interferometer 100. The spectral transmittance TFP(λ) means the ratio I3(λ)/I2(λ).
The lowermost curve of
Referring to
The light source unit 210 may comprise a light source 221, and optionally a light-directing element 222. The light source 221 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 light-directing element 222 may comprise e.g. a lens or a paraboloid reflector. The light source unit 210 may be arranged provide illuminating light LB0. The Fabry Perot etalon 50 may be arranged to provide filtered light LB00 by filtering the illuminating light LB0. The illuminating light LB0 may have a broad spectrum, and the filtered light LB00 may have a comb-like spectral intensity distribution I00(λ), corresponding to the spectral transmittance TE(λ) of the etalon 50.
The uppermost curve of
The spectral transmittance TE(λ) is equal to the relative fraction of incident light at a specified wavelength that passes through the etalon 50. When generating calibration light LB00 by filtering with the etalon 50, the spectral transmittance TE(λ) may be equal to the ratio I00(λ)/I0(λ), where I0(λ) denotes the spectral intensity of the illuminating light LB0, and I00(λ) denotes the spectral intensity of the filtered light LB00.
The spectral transmittance TE(λ) of the etalon 50 comprises a plurality of transmittance peaks P1, P2, P3, . . . Each transmittance peak P1, P2, P3, . . . may have a peak wavelength λP1, λP2, λp3, . . . The etalon 50 may have a plurality of adjacent transmission peaks P1, P2, P3, . . . at the wavelengths λP1, λP2, λP3, . . . The detection range LλPB of the spectrometer 500 may comprise e.g. three or more transmission peaks P1, P2, P3, . . .
The transmission peaks P1, P2, P3 may have a spectral width ΔλFWHM,E. The acronym FWHM means full width at half maximum.
Referring to the second curve from the top, the etalon 50 may form a plurality of filtered spectral peaks P′1, P′2, P′3, P′4, P′5, . . . by filtering illuminating light LB0 such that a first filtered peak P′1 corresponds to a first transmittance peak P1 of the etalon 50, and a second filtered peak P′2 corresponds to a second transmittance peak P1 of the etalon 50. The spectrum I00(λ) of the calibration light LB00 may have a plurality of filtered spectral peaks P′1, P′2, P′3, P′4, P′5, . . . The spectral positions of the filtered spectral peaks P′1, P′2, P′3, P′4, P′5, . . . may substantially coincide with the spectral positions λp1, λP2, λp3, . . . of transmittance peaks of the etalon 50.
Referring to the lowermost curve of
The spectral intensity distribution M(Sd) may be measured by scanning the interferometer 100 over the filtered spectral peaks P′1, P′2, P′3, P′4, P′5, . . . The spectral intensity distribution M(Sd) may be measured by varying the mirror gap dFP and by recording the detector signal SDET1 as the function of the control signal Sd. The spectral intensity distribution M(Sd) may be measured by varying the mirror gap dFP of the Fabry-Perot interferometer 100 according to a control signal Sd, or by varying the mirror gap dFP of the Fabry-Perot interferometer 100 and providing the control signal Sd by monitoring the mirror gap.
Spectral calibration data for the interferometer 100 may be determined by matching the measured spectral intensity distribution M(Sd) with the spectral transmittance TE(λ) of the etalon (50).
A first peak P1 of the spectral transmittance TE(λ) may have a spectral position λp1, and a second peak P2 of the spectral transmittance TE(λ) may have a spectral position λP2. A first filtered peak P′1 of the distribution M(Sd) corresponds to the first transmittance peak P1 of the etalon 50, and a second filtered peak P′2 of the distribution M(Sd) corresponds to the second transmittance peak P2 of the etalon 50. The first filtered peak P′1 of the measured distribution M(Sd) may coincide with a first control signal value Sd1, and the second filtered peak P′2 of the measured distribution M(Sd) may coincide with a second control signal value Sd2. Consequently, the first control signal value Sd1 may be associated with the spectral position λp1, and the second control signal value Sd2 may be associated with the second control signal value Sd2.
Referring to
The spectral intensity distribution M(Sd) may be measured by scanning the interferometer 100. During the scanning, the mirror gap dFP may be varied according to the control signal Sd, or the spectrometer may provide a control signal Sd by monitoring the mirror gap dFP. The spectral intensity distribution M(Sd) may be measured by varying the mirror gap dFP and by recording the detector signal SDET1 as the function of the control signal Sd.
The spectrometer 500 may comprise a memory MEM2 for storing information about the spectral transmittance TE(λ) of the etalon 50. For example, the memory MEM2 may comprise data, which numerically defines the spectral transmittance function TE(λ). For example, the memory MEM2 may comprise data, which specifies the spectral positions λP1, λP2, λp3, . . . of the transmittance peaks of the etalon 50.
The combined transmittance of the Fabry-Perot etalon 50 and the Fabry-Perot interferometer 100 may be proportional to the intensity I3 of light LB3 impinging on the detector DET1. The combined transmittance may be monitored by monitoring the intensity I3 of light LB3 impinging on the detector DET1. The detector signal SDET1 of the detector DET1 may be indicative of the intensity I3 of light LB3 impinging on the detector DET1. The mirror gap dFP may be varied and the combined transmittance may be monitored in order to determine a first control signal value Sd1 associated with a first mirror gap dFP when the transmission peak PFP,k of the interferometer 100 substantially coincides with a first filtered spectral peak P′1 of the calibration light LB00. The mirror gap dFP may be varied and the combined transmittance may be monitored in order to determine a second control signal value Sd2 associated with a second mirror gap dFP when the transmission peak PFP,k of the interferometer 100 substantially coincides with a second filtered spectral peak P2 of the calibration light LB00.
The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by matching the measured distribution M(Sd) with the transmittance function TE(λ), wherein the matching may comprise associating control signal values with spectral positions. The first filtered spectral peak P′1 has a spectral position λP1, and the second filtered spectral peak P′2 has a spectral position λP2. The spectral position λP1 and the first control signal value Sd1 may be associated to form a first pair (λP1, Sd1). The spectral position λP2 and the second control signal value Sd2 may be associated to form a second pair (λP2, Sd2). Spectral calibration data λcal(Sd) and/or Sd,cal(λ) may be determined by using the first pair (λP1, Sd1) and the second pair (λP2, Sd2). Additional pairs (λP3, Sd3), (λP4, Sd4), . . . may be formed based on the spectral positions of the other filtered spectral peaks. The spectral calibration data λcal(Sd) of the interferometer (100) may be determined also by using the additional pairs (λp3, Sd3), (44, Sd4),
A first control signal value Sd1 may be associated with a first mirror gap dFP in a situation where the spectral position of the transmission peak PFP,k of the interferometer 100 coincides with the spectral position λP1 of a first transmittance peak P1 of the etalon 50. A second control signal value Sd2 may be associated with a second mirror gap dFP in a situation where the spectral position of the transmission peak PFP,k of the interferometer 100 coincides with the spectral position λP2 of a second transmittance peak P2 of the etalon 50. A third control signal value Sd3 may be associated with a third mirror gap dFP in a situation where the spectral position of the transmission peak PFP,k of the interferometer 100 coincides with the spectral position λp3 of a third transmittance peak P3 of the etalon 50. Values (Sd4, λP4), (Sd5, λP5), (Sd6, λP6), (Sd7, λP7), (Sd8, λP8), (Sd9, λP9) may be paired, respectively.
The relationship between the spectral positions λ and the corresponding control signal values Sd(λ) may be expressed e.g. by a calibration function Scal,d(λ). The relationship between the control signal values Sd(λ) and the corresponding spectral positions λmay be expressed e.g. by a calibration function λcal(Sd).
The calibration function λ,(Sd) and/or Sd,cal(λ) may be determined e.g. by fitting a regression function to a plurality of data points (λi, Sd j) wherein the spectral positions of said data points (λi, Sd j) may substantially coincide with the spectral positions λP1, λP2, λp3, . . . of the transmission peaks P1, P2, P3, . . . of the etalon 50. Spectral calibration data λcal(Sd) and/or Sd,cal(λ) may comprise e.g. a regression function, which may be fitted to the data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3), (λP4, Sd4),.. The calibration function Sd,cal(λ) may be e.g. a polynomial function. The calibration function Sd,cal(λ) may be e.g. a third order polynomial function.
The calibration function λcal(Sd) and/or Sd,cal(λ) or a look-up-table corresponding to the calibration function λcal(Sd) and/or Sd,cal(λ) may be stored in a memory MEM3 of the spectrometer 500 and/or in a memory of a database server. 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 verified and/or modified. Modified calibration data may be optionally stored in a memory MEM3 of the spectrometer 500 and/or in a memory of a database server, again.
The transmittance TE(λ) of the etalon 50 may, in turn, be calibrated e.g. by using a monochromator, by using a Fourier transform infrared spectrometer and/or by comparing the transmittance TE(λ) with the spectral positions of emission lines of a calibration lamp (based on atomic line emission).
The spectral width ΔλFWHM,E of the transmittance peaks P1, P2, P3 may depend on the reflectivity of the planar surfaces 51, 52, and on the mirror spacing dE of the etalon 50 (
The planar surfaces 51, 52 of the etalon 50 may be optionally coated with semi-reflective coatings. The planar surfaces of the etalon 50 may be implemented by using semi-reflective coatings. The etalon 50 may comprise semi-reflective coatings. The reflectivity of the coatings may be selected to provide a suitable spectral width ΔλFWHM,E. However, the etalon 50 may also be implemented without reflective coatings. The surfaces 51, 52 may operate as semi-reflective mirrors based on the difference between the refractive index of the substrate 53, and the refractive index of the surrounding gas.
Referring to
C(λ)=TE(λ)·B(λ) (1)
The uppermost curve of
The peaks P1, P2, P3, . . . may have, for example a maximum transmittance TEMAX. Between adjacent peaks P1, P2, P3, . . . a minimum transmittance peak with an intensity ratio TEMIN is located at accurately known wavelengths. According to certain embodiments, the minimum transmittance peaks and the maximum transmittance peaks of the etalon 50 can be used for determining spectral calibration data λcal(Sd), Sd,cal(λ) of a Fabry-Perot interferometer 100.
The second curve from the top of
reflected from the object OBJ1, emitted by the object OBJ1, and/or transmitted through the object OBJ1. The input spectrum B(λ) may have one or more moderately sloped portions POR1. The moderately sloped portion POR1 means a portion where the absolute value of the derivative ∂B(λ)/∂λ is smaller than or equal to a predetermined limit at each spectral position of said portion, and where the spectrum B(λ) is greater than zero. The input spectrum B(λ) may also have one or more steeply sloped portions POR2. The steeply sloped portion POR2 means a portion where the absolute value of the derivative ∂B(λ)/∂λ is higher than said predetermined limit. The moderately sloped portion POR1 may also be called e.g. as a substantially flat portion. The steeply sloped portion POR2 may also be called e.g. as a steep portion.
The third curve from the top of
The maximum value of an individual filtered peak P′1 may be attained when the control signal Sd of the Fabry-Perot interferometer 100 is equal to a marker value Sd1. The marker value Sd1 of the filtered peak P′1 may be determined by scanning the Fabry-Perot interferometer 100 and by analyzing when the detector signal SDET1 attains a local maximum. The marker value Sd1 of the filtered peak P′1 may be determined by varying the control signal Sd, measuring the distribution M(Sd) as a function of the control signal Sd, and by determining a control signal value Sd1 where the distribution M(Sd) attains a local maximum.
Referring to the lowermost curve of
The filtered spectrum C(λ) may attain a local maximum value at a peak wavelength λP1. The filtered spectrum C(λ) may attain local maximum values at peak wavelengths λP1, λP2, λP3, . . . The measured spectral intensity distribution M(Sd) may attain a local maximum value when the control signal is equal to a marker value Sd1. The distribution M(Sd) may attain local maximum values at marker values Sd1, Sd2, Sd3, . . .
The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by matching the distribution M(Sd) with the transmittance function TE(λ). The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by using matching marker values Sd1, Sd2, Sd3, . . . determined from the measured distribution M(Sd), and by using the accurately known wavelengths λP1, λP2, λp3, . . . of the transmittance peaks P1, P2, P3.
The spectral position λP1 of the transmission peak P1 is accurately known, and the spectral position PP′1 of a filtered peak P′1 of the filtered spectrum C(λ) may substantially coincide with the spectral position λP1 of a transmission peak P1 of the etalon 50 when the transmission peak P1 is within a moderately sloped portion POR1 of the spectrum B(λ). The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined and/or checked based on the position of the filtered peak P′1. In particular, the calibration function λcal(Sd) and/or Sd,cal(λ) may be determined and/or checked based on the marker signal value Sd1 associated with the filtered peak P′1.
On the other hand, the spectral position of a filtered peak P′4 of the filtered spectrum C(λ) may substantially deviate from the spectral position λP4 of a transmission peak P4 of the etalon 50 when the transmission peak P4 is within a steeply sloped portion POR2. One or more marker values Sd4, Sd5 may be associated with filtered peaks P′4, P′5, which deviate from the spectral positions λ4, λ5 of the corresponding transmission peaks P4, P5, because the spectral positions λ4, λ5 are within a steeply sloped portion POR2 of the input spectrum B(λ). One or more non-matching marker values Sd4, Sd5 may be omitted when determining and/or checking the calibration function λcal(Sd) and/or Sd,cal(λ).
The distribution M(Sd) may comprise a plurality of filtered peaks P′1, P′2, P′3, . . . . The shape of each filtered peak P′1, P′2, P′3, . . . may be compared with the shape of a transmittance peak P1, P2, P3 of the transmittance TE(λ) in order to determine whether the marker value Sd1 of the filtered peak P′1 can be used for checking the spectral calibration. The shape of a filtered peak may correspond to the shape of a transmittance peak if the input spectrum is a slowly varying function of the wavelength.
The method may comprise determining whether a marker value Sd1 corresponds to a peak wavelength λp1, which is within a moderately sloped portion POR1 of the input spectrum B(λ).Said determining may be performed e.g. by determining an estimate BM(λ) of the input spectrum B(λ), and checking whether the absolute value of the derivative ∂BM(λ)/∂λis smaller than or equal to a predetermined limit in the vicinity of a spectral position λ1. A peak wavelength λP1 may be within a moderately sloped portion POR1 of the input spectrum B(λ) if the absolute value of the derivative ∂BM(λ)/∂λis smaller than or equal to a predetermined limit b1 in the vicinity of the peak wavelength λP1.
Said estimate BM(λ) of the input spectrum B(λ) may be calculated e.g. by providing a calibrated distribution M(Scal,d(λ)) from the measured distribution M(Sd), and by dividing the calibrated distribution M(Scal,d(λ)) with the transmittance TE(λ) of the etalon 50:
kint denotes an intensity calibration coefficient. kint may also depend on the wavelength. The intensity calibration data CPAR1 may comprise the calibration coefficient kint.
A control signal value Sd(λP1) corresponding to the peak wavelength λP1 may be estimated by using the calibration function Scal,d(λ), or by using a preliminary calibration function Scal,d(λ).
Sd(λP1)≈Scal,d(λP1) (3)
The estimated control signal value Scal,d(λP1) may be compared with the marker values Sd1, Sd2, Sd3, . . . in order to determine whether the estimated control signal value Scal,d(λP1) substantially coincides with any of the marker values Sd1, Sd2, Sd3, . . . The estimated control signal value Scal,d(λP1) may substantially coincide with a marker value Sd1 e.g. when the difference between the marker value Sd1 and the estimated control signal value Scal,d(λP1) is smaller than or equal to a predetermined limit value ΔSLIM.
|Sd1−Scal,d(λd1)|ΔSLIM (4)
Each marker value Sd1, Sd2, Sd3, Sd4, . . . may be classified to be a matching value or a non-matching value. A marker value Sd1 may be classified to be a matching value if the marker value Sd1 corresponds to a peak wavelength λP1, which is within a moderately sloped portion POR1 of the input spectrum B(λ), and if the marker value Sd1 substantially coincides with an estimated control signal value Scal,d(λP1). A marker value Sd4 may be classified to be a non-matching value if the marker value Sd4 corresponds to a peak wavelength λP4, which is within a steeply sloped portion POR2 of the input spectrum B(λ), and/or if the marker value Sd4 substantially deviates from all estimated control signal values Scal,d(λP1), Scal,d(λP2), Scal,d(λP3), Scal,d(λP4), . . . The matching marker values Sd1, Sd2, Sd3, . . . may be used for checking and/or improving the accuracy of the calibration function Scal,d(λ).
Data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3), . . . for determining and/or checking the calibration function λcal(Sd) and/or Sd,cal(λ) may be obtained by:
The accuracy of a calibration function λcal(Sd) and/or Sd,cal(λ) may be improved and/or checked by using the data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3). A modified calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by using the data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3). The modified calibration function λcal(Sd) and/or Sd,cal(λ) may be slightly different from the preliminary calibration function. The modified calibration function λcal(Sd) and/or Sd,cal(λ) may be stored e.g. in a memory MEM3.
An improved estimate BM(λ) for input spectrum B(λ) may subsequently be determined by providing a calibrated distribution M(Scal,d(λ)) from the measured distribution M(Sd) by using the modified calibration function Scal,d(λ), and by dividing the calibrated distribution M(Scal,d(λ)) with the transmittance TE(λ) of the etalon 50:
The filtered spectrum C(λ) of
The distribution M(Sd) is measured by scanning the interferometer 100. Different peaks of the spectrum may be scanned at different times. The distribution M(Sd) may represent a time-averaged spectrum of the filtered peaks provided by the etalon 50. The distribution M(Sd) does not need to represent an instantaneous spectrum of light transmitted through the etalon 50. Increasing the finesse of the etalon 50 may reduce the minimum spectral transmittance TE,MIN of the etalon 50. If the minimum spectral transmittance TE,MIN is very low, this may reduce accuracy of the intensity values and/or may cause loss of spectral data. The reflectance of the coatings of the etalon 50 may be selected such that the minimum spectral transmittance TE,MIN of the etalon 50 e.g. lower than or equal to 90% of the maximum spectral transmittance TE,MAX of the etalon 50. The reflectance of the coatings of the etalon 50 may be selected such that the minimum spectral transmittance TE,MIN of the etalon 50 e.g. is in the range of 10% to 90% of the maximum spectral transmittance TE,MAX of the etalon 50.
In an embodiment, a first part of the input light LB1 may be coupled to the interferometer via the etalon 50, and a second part of the input light may be simultaneously coupled to the interferometer 100 without passing through the etalon. For example, the etalon 50 may cover less than 100% of the cross-section of the aperture of the interferometer 100. For example, the input light LB1 may be divided into a first part and a second part by using a beam splitter, wherein the first part may be coupled to the interferometer 100 through the etalon 50, and the second part may be coupled to the interferometer 100 without passing through the etalon 50. The spectrometer 500 may 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 narrow well-defined filtered peaks P′1, P′2, P′3, . . . without causing significant loss of data between the peaks of the spectrum C(λ). Consequently, the spectrum C(λ) may have narrow well-defined filtered peaks P′1, P′2, P′3, . . . without causing significant reduction of accuracy of the intensity values between the peaks of the spectrum C(λ).
In an embodiment, the minimum spectral transmittance TE,MIN of the etalon 50 may be very low when spectral information is not needed from the spectral regions between the adjacent transmittance peaks P1, P2, P3, . . . The minimum spectral transmittance TE,MIN may be e.g. lower than 10% of the maximum spectral transmittance TE,MAX. The minimum spectral transmittance TE,MIN may be e.g. lower than 1% of the maximum spectral transmittance TE,MAX.
The calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by matching the measured distribution M(Sd) with the transmittance function TE(λ), wherein the matching may comprise using cross-correlation. The measured distribution M(Sd) may be compared with the spectral transmittance (TE(λ)) of the etalon (50) by using cross-correlation analysis. 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 TE(λ) may provide a value, which indicates the degree of similarity between the calibrated distribution M(Sd,cal(λ))and the transmittance function TE(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 TE(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 TE(λ) reaches a maximum value. The cross-correlation may reach a maximum value when the spectral positions of the peaks of the calibrated distribution M(Sd,cal(λ)) substantially coincide with the spectral positions of the peaks of the transmittance function TE(λ).
An auxiliary transmittance function TE(λcal(Sd)) of the etalon 50 may give the transmittance of the etalon 50 as 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 TE(λcal(Sd)) reaches a maximum value.
The method may comprise:
The method may comprise:
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.
The spectrometer 500 may comprise a temperature sensor 58 for monitoring operating temperature of the etalon 50 (see e.g.
According to certain embodiments, variation of performance characteristics due to temperature changes can be considered by means of the temperature signal STEMP indicative of the operating temperature of the substrate of the etalon 50, thus improving precision of calibration. The spectral positions of the transmittance peaks of the etalon 50 as a function of the operating temperature may be, for example, calculated by the control unit CNT1.
As changes of temperatures of the environment typically also affect the operating temperature of the Fabry-Perot interferometer 100, temperature drift will occur in the wavelength response of the interferometer 100. Suprisingly, the temperature signal STEMP indicative of the operating temperature of the substrate of the etalon 50 can also be used for calculation of temperature related performance characteristics of the Fabry-Perot interferometer 100 according to certain embodiments. Further, the temperature signal STEMP indicative of the operating temperature of the substrate of the etalon 50 can be used for calculation of temperature related performance characteristics of any given unit of the system according to certain embodiments.
In step 805, a plurality of filtered peaks P′1, P′2 may be provided by filtering input light LB1 with the etalon 50.
In step 810, the distribution M(Sd) may be measured by scanning the interferometer 100 over the filtered peaks P′1, P′2.
In step 815, a first marker signal value Sd1 may be determined by analyzing a first peak P′1 of the distribution M(Sd). The first marker signal value Sd1 may be associated with the spectral position λP1 of the first peak P1 of the spectral transmittance TE(λ) of the etalon 50.
In step 820, a second marker signal value Sd2 may be determined by analyzing a second peak P′2 of the distribution M(Sd). The second marker signal value Sd2 may be associated with the spectral position λP2 of the second peak P2 of the spectral transmittance TE(λ) of the etalon 50.
In step 830, the calibration data λcal(Sd) and/or Sd,cal(λ) may be determined from the associated pairs (λP1, Sd1), (λP2, Sd2).
The apparatus 700 may comprise an etalon 50. The etalon may be arranged to filter light transmitted via the optical path of the apparatus 700. For example, the etalon 50 may be positioned between the object OBJ1 and the spectrometer 500. For example, the light source unit 210 may comprise the etalon 50. For example, the spectrometer 500 may comprise the etalon 50. For example, the etalon may be positioned between the light source unit 210 and the object OBJ1 (see
The uppermost curve of
The spectral intensity distribution M(Sd) of the filtered light may be measured by scanning the Fabry-Perot interferometer 100. The measured distribution M(Sd) may be converted into a calibrated distribution by using the calibration function λcal(Sd) and/or Sd,cal(λ). The calibration function λcal(Sd) and/or Sd,cal(λ) may checked by comparing the measured distribution M(Sd) with the spectral transmittance TE(λ) of the etalon 50
The spectral position λP1 of the transmission peak P1 is accurately known, and the spectral position of a filtered peak P′1 of the filtered spectrum C(λ) may substantially coincide with the spectral position λP1 of a transmission peak P1 of the etalon 50 when the transmission peak P1 is within a moderately sloped portion POR1 of the spectrum B(λ). Each marker value Sd1, Sd2, Sd3, Sd4, . . . may be classified to be a matching value or a non-matching value. A marker value Sd1 may be classified to be a matching value if the marker value Sd1 corresponds to a peak wavelength λP1, which is within a moderately sloped portion POR1 of the input spectrum B(λ), and if the marker value Sd1 substantially coincides with an estimated control signal value Scal,d(λP1). One or more spectral positions, e.g. the position λ4 may be within a steeply sloped portion POR2 of the input spectrum B(λ). One or more marker values Sd4 may be omitted when determining and/or checking the calibration function λcal(Sd) and/or Sd,cal(λ).
Data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3), . . . for determining and/or checking the calibration function λcal(Sd) and/or Sd,cal(λ) may be obtained by:
The accuracy of a calibration function λcal(Sd) and/or Sd,cal(λ) may be improved and/or checked by using the data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3). A modified calibration function λcal(Sd) and/or Sd,cal(λ) may be determined by using the data pairs (λP1, Sd1), (λP2, Sd2), (λP3, Sd3). The modified calibration function λcal(Sd) and/or Sd,cal(λ) may be slightly different from the preliminary calibration function. The modified calibration function λcal(Sd) and/or Sd,cal(λ) may be stored e.g. in a memory MEM3.
An improved estimate BM(λ) for input spectrum B(λ) may subsequently be determined by providing a calibrated distribution M(Scal,d(λ)) from the measured distribution M(Sd) by using the modified calibration function Scal,d(λ), and by dividing the calibrated distribution M(Scal,d(λ)) with the transmittance TE(λ) of the etalon 50 (see equation 5).
If desired, a calibrated measured spectrum XS(λ) may be obtained by multiplying the calibrated spectral intensity distribution M(Sd,cal(λ)) with the function 1/TE(λ), and by using the intensity calibration data CPAR1 to convert detector signal values to calibrated intensity values. The calibrated measured spectrum XS(λ) may represent the spectrum of light transmitted through the object OBJ1 or the spectrum of light reflected by the object OBJ1.
The absorption spectrum I1(λ)/I0(λ) may also be determined by using a reference distribution. In an embodiment, the absorption spectrum I1(λ)/I0(λ) may be determined without determining calibrated intensity values. The 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 e.g. the spectrum of the illuminating light LB0. The reference distribution MREF(Sd,cal(λ)) may be stored e.g. in the memory MEM4 of the apparatus 700. The measured absorption spectrum I1(λ)/I0(λ) may be determined from the calibrated spectral intensity distribution M(Sd,cal(λ)) by using the spectral transmittance TE(λ) of the etalon, and by using the reference distribution MREF(Sd,cal(λ)). A compensated spectral intensity distribution may be determined from the calibrated spectral intensity distribution M(Sd,cal(λ)) by dividing with the spectral transmittance 1/TE(λ), and the measured absorption spectrum I1(λ)/I0(λ) may be determined by dividing the reference distribution MREF(Sd,cal(λ)) with the compensated spectral intensity distribution M(Sd,cal(λ))/TE(λ).
Referring to
When measuring the reflectance spectrum of an object OBJ1, the object OBJ1 may 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 spectrometer 500.
The mirror 110 may be supported by a spacer 115. The spacer 115 may be deposited on top of the mirror 120. The interferometer 100 may be produced by a method, which comprises depositing two or more material layers of the mirror 110 on top of the spacer 115. The empty space ESPACE1 between the mirrors 110, 120 may be formed by etching material away from between the mirrors 110, 120 after the two or more material layers of the mirror 110 have been deposited.
The first mirror 110 may have a movable portion MPOR1, and the first mirror 110 may be called e.g. as the movable mirror. The movable portion MPOR1 of the movable mirror 110 may be moved with respect to the stationary mirror 120 in order to adjust the mirror gap dFP. The second mirror 120 may be called e.g. as the stationary mirror.
The stationary mirror 120 may comprise a plurality of material layers supported by substrate 130. The movable mirror 110 may be supported by the spacer layer 115. The spacer layer 115 may be formed on top of the stationary mirror 120, and the movable mirror 110 may be supported by the spacer layer 115. The movable mirror 110 may comprise e.g. material layers 110a, 110b, 110c, 110d, and/or 110e. The stationary mirror 120 may comprise e.g. material layers 120a, 120b, 120c, 120d, and/or 120e. The mirrors 110, 120 may be implemented by using reflective multilayer coatings. The mirrors 110, 120 may comprise reflective multilayer coatings. The material layers of the stationary mirror 120 may be formed e.g. by depositing material on top of a substrate 130 and/or by locally converting material of the substrate 130. The spacer layer 115 may be deposited on top of the stationary mirror 120 after the material layers of the stationary mirror 120 have been formed. The material layers of the movable portion MPOR1 may be formed after the spacer layer 115 has been deposited, by depositing material layers of the movable mirror 110 on top of the spacer layer 115. The material layers of the mirrors 110, 120 may be 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 substrate 130 may be e.g. monocrystalline silicon or fused silica. The spacer layer 115 may comprise e.g. silicon dioxide. The spacer layer 115 may consist essentially of silicon dioxide. The spacer layer 115 may consist of silicon dioxide. The empty space ESPACE1 between the mirrors 110, 120 of the interferometer 100 may be formed by etching. The material of the spacer layer 115 may etched away e.g. by using hydrofluoric acid (HF). The mirror 110 may comprise a plurality of miniature holes H1 for guiding hydrofluoric acid (HF) into the space between the mirrors 110, 120 and for removing the material of the spacer layer 115. The width of the holes H1 may be so small that they do not significantly degrade the optical properties of the interferometer 100.
The movable portion MPOR1 may be moved e.g. by an electrostatic actuator 140. The electrostatic actuator 140 may comprise two or more electrodes Ga, Gb. Va denotes the voltage of the first electrode Ga, and Vb denotes the voltage of the second electrode Gb. The electrodes Ga, Gb may generate an attractive electrostatic force F1 when a voltage difference Va-Vb (=Vab) is applied between the electrodes Ga, Gb. The electrostatic force F1 may pull the movable portion MPOR1 towards the stationary mirror 120.
The electrostatic actuator 140 may be rugged, and/or mechanically stable and/or shock-proof. The electrostatic actuator 140 may have small size. The interferometer 100 may be produced at low costs when the interferometer 100 comprises the electrostatic actuator 140.
The interferometer 100 may optionally comprise a capacitive sensor for monitoring the mirror gap. The interferometer 100 may comprise an electrostatic actuator 140, and a capacitive sensor for monitoring the mirror gap. However, the use of the capacitive sensor is not necessary. Thanks to using the etalon for spectral stabilization, an interferometer 100 having an electrostatic actuator 140 may be implemented without a capacitive sensor for monitoring the mirror gap.
The voltage Va may applied to the electrode Ga by using a conductor CON1 and a terminal N1. The voltage Vb may applied to the electrode Gb by using a conductor CON2 and 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, N2 may be e.g. metallic, and the conductors CON1, CON2 may be e.g. bonded to the terminals N1, N2.
The aperture portion AP1 of the movable portion MPOR1 may have a width w1. The aperture portion AP1 of the movable mirror 110 may be substantially flat in order to provide sufficient spectral resolution. The magnitude of electrostatic forces directly exerted on the aperture portion AP1 may be kept low in order to preserve the flatness of the aperture portion AP1. An electrostatic force F1 for moving the movable portion MPOR1 may be generated by using a substantially annular electrode Gb, which surrounds the aperture portion AP2 of the stationary mirror 120. The mirror 120 may optionally comprise a neutralizing electrode Gc, which may be arranged to keep the aperture portion AP1 flat during the scanning, by reducing forces exerted on the aperture portion AP1. The neutralizing electrode Gc may be substantially opposite the aperture portion AP1 of the movable mirror 110. 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 AP1 of the movable mirror 110. 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 AP1 of the movable mirror 110. Consequently, the movable portion MPOR1 may be moved by the electrostatic force F1 such that said force F1 pulls an annular region surrounding the aperture portion AP1, wherein the aperture portion AP1 may remain as a substantially force-free region.
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 interferometer 100. The electrodes Ga, Gb and Gc may comprise e.g. doped polycrystalline silicon, which may be substantially transparent for infrared light LB3.
The electrodes Ga, Gb may generate the electrostatic force F1 when a driving voltage Vab is applied to the electrodes Ga, Gb. The driving voltage Vab may be equal to the voltage difference Va-Vb. The voltage may be applied to the electrodes Ga, Gb e.g. via the conductors CON1, CON2 and the terminals N1, N2. The electrode Gc may be galvanically connected to the electrode Ga. The mirror 110 may be flexible and/or the spacer 115 may be mechanically compressible such that the mirror gap dFP may be changed by changing the magnitude of the electrostatic force F1. The magnitude of the electrostatic force F1 may be changed by changing the driving voltage Va-Vb (=Vab). The spectrometer 500 may comprise a driving voltage unit 142, which may be arranged to generate the driving voltage Vab according to the control signal Sd.
A portion of the mirror 110 between the aperture portion AP1 and spacer 115 may be flexible so as to allow varying the mirror gap dFP. The thickness of the mirror 110 may be selected so as to allow repeated local bending.
Applying a first driving voltage Vab to the electrodes Ga, Gb may cause adjusting the transmission peak PFP,k of the interferometer 100 to a first spectral position (e.g. to the position λP1), and applying a second different driving voltage Vab to the electrodes Ga, Gb may cause adjusting the transmission peak PFP,k of the interferometer 100 to a second different spectral position (e.g. to the position λP2).
During normal operation, the space ESPACE1 between the mirrors 110, 120 may be filled with a gas. However, the interferometer 100 may also be operated in vacuum so that the gas pressure in the space ESPACE1 is substantially equal to zero.
The interferometer 100 produced by depositing and etching may be considered to have a substantially monolithic structure. Said interferometer 100 may be e.g. shock resistant and small. The mass of the movable portion MPOR1 may be small, and the interferometer 100 may have a high scanning speed. The movable portion MPOR1 may be rapidly accelerated to the full scanning speed.
Referring to
The capacitance monitoring unit 152 may be arranged to measure the capacitance C, e.g. by charging the capacitive sensor 150 with a predetermined current, and by measuring a time needed to charge the sensor 150 to a predetermined voltage. The capacitance monitoring unit 152 may be arranged to measure the capacitance Cx e.g. by coupling the capacitive sensor 150 as a part of a resonance circuit, and measuring the resonance frequency of the resonance circuit. The capacitance monitoring unit 152 may be arranged to measure the capacitance Cx e.g. by using the capacitive sensor 150 to repetitively transfer charge to a tank capacitor, and counting the number of charge transfer cycles needed to reach a predetermined tank capacitor voltage.
The interferometer 100 may comprise a driving unit 142. The driving unit 142 may e.g. convert a digital driving signal S140 into an analog signal suitable for driving the actuator 140. The driving unit 142 may provide e.g. a voltage signal Vab for driving an electrostatic actuator 140, or for driving a piezoelectric actuator 140.
In an embodiment, the control unit CNT1 may be configured to provide a digital driving signal S140 for changing the mirror gap dFP, and the control unit CNT1 may be arranged to receive the control signal Sd.
The spectral scale of the spectrometer may be stabilized by using a Fabry-Perot etalon, which has fixed mirror spacing. The mirror spacing of the etalon may remain constant during scanning of the interferometer.
The optical cavity of the etalon 50 may consist of one or more solid materials, preferably silicon Si and/or silicon dioxide SiO2.
The spectrometer 500 may be implemented e.g. in a first mobile unit. Determining spectral positions λ from the control signal values Sd may be carried out in the first mobile unit. Determining spectral positions λ from the control signal values Sd may be carried out in a second unit, which is separate from the first unit. The second unit may be stationary or mobile. The stationary unit may be implemented e.g. in a server, which may be accessed e.g. via the Internet.
The spectrometer 500 may be used e.g. for remote sensing applications. The spectrometer 500 may be used e.g. for measuring the color of an object OBJ1. The spectrometer 500 may be used e.g. for an absorption measurement, where the transmission peak of the interferometer 100 may be adjusted to a first spectral position to match with an absorption band of an object OBJ1, and the transmission peak of the interferometer 100 may be adjusted to a second spectral position to match with a reference band. The spectrometer 500 may be used e.g. for a fluorescence measurement, where the first spectral position of the transmission peak of the interferometer may be matched with fluorescent light emitted from an object OBJ1, and the second spectral position may be matched with the illuminating light, which induces the fluorescence.
When measuring the reflectance spectrum of an object OBJ1, the object OBJ1 may 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 spectrometer 500.
When broadband light is coupled into the spectrometer, the etalon may provide a filtered spectrum, which has a plurality of filtered peaks at stable spectral positions. The spectral scale of the interferometer 100 may be determined and/or verified by using the filtered peaks. The filtered spectrum may be measured by using the Fabry-Perot interferometer, in order to provide a measured filtered spectrum. The measured spectral intensity distribution M(Sd) may substantially reproduce the transmittance peaks of the etalon, and the spectral scale may be determined and/or verified by using the peaks of the distribution M(Sd). In particular, the spectral scale of the spectrometer may be stabilized in a situation where the relationship between the mirror gap and the control signal changes e.g. due to variations of operating temperature, mechanical shocks, and/or ageing.
The accuracy of the spectral scale of the interferometer 100 may be determined by the accuracy at which the spectral transmittance TE(λ) of the etalon is known. ΔλDEV1 may denote a difference between an actual spectral position of a transmittance peak and a nominal spectral position of said transmittance peak. ΔλDEV2 may denote a difference between an actual spectral position of the interferometer and a calibrated spectral position corresponding to said actual spectral position of the interferometer. The deviation ΔλDEV2 may be e.g. smaller than 2·ΔλPEV1. The error (ΔλDEV2) of detecting the spectral positions of the peaks of the filtered spectrum may be e.g. smaller than two times the error (ΔλDEV1) of the known spectral positions of the peaks of the transmittance of the etalon.
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.
The term “light” may refer to electromagnetic radiation in the ultraviolet, visible and/or infrared regime.
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. The spectral scale may be called e.g. as the wavelength scale. Spectral calibration may also be called e.g. as wavelength calibration. Spectral calibration data may be called e.g. as wavelength calibration data.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20145626 | Jun 2014 | FI | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2015/050476 | 6/29/2015 | WO | 00 |
