Method and apparatus for compensation of spectral dispersion in an interferometer

Abstract

The invention relates to a method (500) of acquiring an interference signal, comprising: - splitting an incident light wave into a first light wave and a second light wave, the first and second light waves propagating in a first and second optical path of an interferometer, - adjusting an optical path difference between the first and second light waves, and - generating the interference signal by combining the first and second light waves on a detector, the method being characterized in that it further comprises: - measuring a spectral dispersion using the interference signal, and - compensating (505) the spectral dispersion by positioning a dispersion compensation plate made of a dispersive material at a compensation angular position with respect to an optical axis of the first or second light wave at an angle to satisfy a spectral dispersion criterion. The invention also relates to an interferometer device implementing such a method.

Description

Technical Field

[0001] This invention relates to a method for compensating spectral dispersion in an interferometer apparatus. The invention also relates to an interferometer apparatus for implementing this method.

[0002] The field of this invention is interferometry, interferometric imaging, and Fourier transform spectroscopy. Background Technology

[0003] Fourier transform spectrometers (FT spectrometers) are well-known instruments used for spectral analysis of incident light. They can be implemented in a variety of ways. One of the most common implementations is based on the Michelson interferometer scheme.

[0004] The incident light is split by a central beam splitter into a reference wave propagating in a reference arm with a static reflector and a measurement wave propagating in a measurement arm with a movable reflector. The light reflected by the two reflectors combines and interferes at the detector.

[0005] The FT spectrometer works by acquiring the intensity of interference light received by a movable mirror at different positions on the detector, which correspond to different optical path differences or delays between the measured wave and the reference wave.

[0006] When a complete interferogram of the measurement light is obtained, its Fourier transform provides the power spectral density of the measurement light.

[0007] To obtain accurate spectral information, it is necessary to acquire interference signals that are unaffected by any spectral dispersion introduced by the interferometer. The primary cause of spectral dispersion is the refractive index of materials through which light waves pass, excluding air or a vacuum, which varies significantly with wavelength. This variation in refractive index results in a change in the optical path length of light within the material with wavelength.

[0008] If dispersion cannot be fully compensated between the optical paths of the interferometer, it is clearly impossible to achieve the "contact" position, which corresponds to zero optical path difference between waves propagating in the corresponding arms of the interferometer and applies simultaneously to all target wavelengths, thus making it impossible for all these wavelengths to simultaneously satisfy the constructive interference condition. In this case, the Fourier Transform (FT) of the interferogram cannot fully represent the power spectral density of the light.

[0009] Therefore, in Michelson interferometer configurations, it is known to use a beam splitter plate (e.g., instead of a beam splitter cube) to minimize the thickness of the glass material through which light waves pass, and to add a compensation plate in the arm of the interferometer where the light passes through the beam splitter plate for a shorter distance.

[0010] Ideally, if the compensation plate has the same material as the beam splitter or the same refractive index, thickness, and angular orientation as the beam splitter, the light will travel the same path through the glass material in both arms of the interferometer, resulting in perfectly equal dispersion along these paths, thus achieving compensation.

[0011] However, for a beam splitter or compensation plate with a thickness of 3 mm, the thickness tolerance of the component is approximately + / - 0.1 mm.

[0012] For example, for a glass plate made of fused silica with a thickness uncertainty of + / -0.1 mm, considering the refractive index of fused silica (n=1.458 when λ=600nm), the single-pass optical path difference in the glass material between the two arms of the interferometer can reach more than 50µm, or more than 100µm round trip, which is unacceptable for most applications.

[0013] Furthermore, since the refractive index varies with wavelength, the optical path difference in glass materials also varies significantly with wavelength. For example, taking fused silica as an example again, due to the thickness uncertainty between the beam splitter and the compensation plate being + / -0.1 mm, the optical path difference in the glass material varies by more than 2 µm in the wavelength range of 400 nm to 800 nm. Summary of the Invention

[0014] One object of the present invention is to provide an interferometer device that overcomes at least one drawback of the known art.

[0015] Another object of the present invention is to provide an interferometer device, which implements an apparatus and method for compensating spectral dispersion.

[0016] Another object of the present invention is to provide an interferometer having dispersion compensation devices and methods, which can be implemented using off-the-shelf components with conventional or standard tolerances.

[0017] Another object of the present invention is to provide an interferometer having a dispersion compensation device and method, which allows spectral dispersion to be compensated with an accuracy down to a fraction of the wavelength over a wide spectral range.

[0018] Another object of the present invention is to provide an interferometer having a dispersion compensation device and method, which allows for the monitoring and compensation of dispersion during operation.

[0019] Another object of the present invention is to provide a Michelson interferometer with complete spectral dispersion compensation.

[0020] Another object of the present invention is to provide an interferometer for implementing a Fourier transform spectrometer, which has thorough spectral dispersion compensation to obtain spectral information of the measurement light.

[0021] Another object of the present invention is to provide an interferometer having thorough spectral dispersion compensation for obtaining spectral information from measurement light generated by photoluminescence or fluorescence.

[0022] At least one of the above objectives is achieved by a method for acquiring an interference signal, the method comprising:

[0023] - The incident light wave is split into a first light wave and a second light wave, which then propagate along the first and second optical paths of the interferometer, respectively.

[0024] - Adjust the optical path difference between the first and second light waves, and

[0025] - An interference signal is generated by combining the first and second light waves on a detector.

[0026] The method is characterized by the following:

[0027] - Measuring spectral dispersion using interferometric signals, and

[0028] - Spectral dispersion is compensated by positioning a dispersion compensation plate made of dispersive material at an angle relative to the optical axis of the first or second light wave at a compensation angle position to satisfy the spectral dispersion criterion.

[0029] The method according to the invention allows for the acquisition of interference signals for which spectral dispersion is measured and minimized. Spectral dispersion compensation is achieved by positioning a dispersion compensation plate at a specific angular position relative to the optical axis of one of the light waves. The specific angular position is determined by measuring the spectral dispersion and changing the angular position until a spectral dispersion criterion is satisfied.

[0030] Spectral dispersion criteria may include one of the following: the minimum spectral dispersion in a specific spectral range, or spectral dispersion below a predetermined threshold in the spectral range.

[0031] The implementation of the method according to the invention does not require prior measurement of the actual thickness of the components used.

[0032] Using the method of this invention, the optical path difference between the first and second light waves is substantially the same for all target wavelengths of the incident light wave. The optical path difference can then be equalized with an accuracy as low as a fraction of the wavelength or tens of nanometers.

[0033] Dispersion compensation plates are made of dispersive materials and are not in a vacuum or air. For example, they can be made of glass.

[0034] According to an embodiment, measuring spectral dispersion may include determining the contact position of the interferometer at a control wavelength, the contact position corresponding to the zero optical path difference between a first light wave and a second light wave at the control wavelength.

[0035] Determining the contact position can include obtaining the interference signal by changing the optical path difference between the first and second optical paths. It can also include identifying interference conditions, such as bright fringes, the highest central fringe, or the highest constructive interference.

[0036] According to an embodiment, measuring spectral dispersion may include:

[0037] - Set the dispersion compensation plate at the corner position.

[0038] - Determine the first contact position for the first control wavelength and the second contact position for the second control wavelength, and

[0039] - Spectral dispersion is calculated by calculating the difference between the first contact position and the second contact position.

[0040] According to an embodiment, determining the compensation angle position may include at least one of the following steps:

[0041] - The compensated angular position satisfying the spectral dispersion criterion is calculated by applying an analytical and / or numerical model of spectral dispersion in the interferometer based on the angular position.

[0042] - Spectral dispersion was measured at multiple angular positions.

[0043] - Interpolate or extrapolate the spectral dispersion measurements obtained at multiple angular positions to find the compensation angular positions that satisfy the spectral dispersion criteria.

[0044] - Iteratively measure spectral dispersion for angular positions to converge to a compensated angular position that satisfies the spectral dispersion criterion.

[0045] According to an embodiment, the method of the present invention may further include the following steps:

[0046] - Obtain the interference signal of the incident light wave for multiple optical path differences.

[0047] - Use interference signals to calculate the spectral information of the measured light wave.

[0048] Measuring a light wave involves seeking its spectral information. Spectral information can include, for example, the spectral power density of the entire spectrum obtained by applying a Fourier transform to the interferogram. Alternatively, it can include spectral information of a specific wavelength obtained by applying a specific sampling scheme to the interferogram.

[0049] According to another aspect of the present invention, an interferometer device is provided, comprising:

[0050] - A beam splitter configured to split an incident light wave into a first light wave and a second light wave, the first light wave and the second light wave propagating along a first optical path and a second optical path, respectively.

[0051] - Adjustment device, used to adjust the optical path difference between the first and second light waves.

[0052] - A detector used to generate an interference signal by combining a first light wave and a second light wave.

[0053] The interferometer device is characterized by further comprising:

[0054] - A rotating device configured to position a dispersion compensation plate made of a dispersive material at an angle relative to the optical axis of a first light wave or a second light wave, and

[0055] - A processing device configured as follows:

[0056] ○Measuring spectral dispersion using interferometric signals, and

[0057] ○ Determine the compensation angle position used to position the dispersion compensation plate to meet the spectral dispersion criterion.

[0058] Adjustment devices for adjusting optical path difference may include, but are not limited to, mechanical displacement devices, such as translation stages or piezoelectric actuators for shifting mirrors. Rotation devices may include, for example, rotary tables or tilt adjustment frames. Processing devices may include, but are not limited to, computers, microprocessors, control boards, and interface hardware with analog-to-digital converters and I / O boards, to acquire and process interference signals and control the orientation of the dispersion compensation plate.

[0059] The interferometer apparatus may also include a control light source configured to illuminate the interferometer with a controlled incident light wave to measure spectral dispersion.

[0060] The controlled incident light wave is a known light wave configured to measure spectral dispersion. In this case, the interferometer can be irradiated with an incident light wave in the form of a controlled incident light wave to measure spectral dispersion, and simultaneously or sequentially irradiated with an incident light wave in the form of a measurement incident light wave for which an interference signal needs to be measured.

[0061] The interferometer device may also include a control detector configured to generate a control interference signal for measuring spectral dispersion by collecting light waves emitted by a control light source and passing through the interferometer.

[0062] The interferometer may then include a detector in the form of a control detector for measuring spectral dispersion, and a detector in the form of a measurement detector for measuring the interference signal from the incident light wave.

[0063] The control light source may include at least one light source element with low coherence and narrow spectral range.

[0064] In this case, the center fringe corresponding to the contact position of the interferometer device can be easily identified in the detected interferogram.

[0065] Alternatively, the interferometer device may include a broadband control light source, a control detector, and a bandpass filter located between the control light source and the control detector.

[0066] According to some embodiments, the interferometer device may include a monochromatic control light source, such as a laser light source.

[0067] This monochromatic control light source can be used to monitor changes in the optical path difference between the first and second optical paths.

[0068] According to an embodiment, the interferometer device may further include an imaging lens configured to image incident measurement light waves emitted from the field of view onto a linear or area array detector.

[0069] The imaging lens may include, for example, a first lens used as an optical collection device, arranged to collect incident measurement light, such as light from an object plane or a sample, for which information is to be acquired. The imaging lens may also include a second lens arranged such that, when combined with the first lens, the incident measurement light from the object plane is imaged onto a detector.

[0070] The detector may be or include a CCD or CMOS linear or area array detector, or a TDI camera.

[0071] According to an embodiment, the interferometer device may include an optical collecting device for collecting measurement incident light, a measurement detector, and at least one set of control light sources and control detectors, wherein the control detectors are illuminated by the control light sources.

[0072] Optical collection devices may include any means for introducing light into the interferometer. They may include, for example, lenses as described above. They may also include light guides or light inlets, windows, fiber optic connectors (if the measurement light is brought in by an optical fiber), or, for example, apertures or aperture stops if the measurement incident light is propagating in free space or is a collimated beam.

[0073] In this configuration, the interferometer is illuminated by measurement incident light through an optical collection device, and simultaneously or sequentially by control incident light emitted from a control light source. The interferometer also includes a detector in the form of a measurement detector for generating interference signals using the measurement incident waves, and a detector in the form of a control detector for generating interference signals using the incident control light. Therefore, two incident light waves illuminate the same interferometer through the same beam splitter, the same first and second optical paths, and the same dispersion compensation plate, but are projected onto different detectors.

[0074] According to an embodiment, the interferometer device may include multiple sets of control light sources and control detectors, which are located around the optical collection device and the measurement detector, respectively.

[0075] As before, the same interferometer is used to measure the incident light wave and control the incident light wave to illuminate it, but their respective light sources and detectors are spatially offset. For example, this can allow for monitoring and correcting tilt or angular uncertainties of elements that alter the optical path length between the first and second optical paths when correcting spectral dispersion.

[0076] The interferometer device according to the present invention can be of the Michelson type, which includes a reflector and a central beam splitter in the first optical path and the second optical path, respectively.

[0077] At least one reflector may be mounted on the displacement device to change the optical path length of the first optical path and / or the second optical path.

[0078] Interferometer equipment may include a beam splitter in the form of a beam splitter plate, and a dispersion compensation plate made of the same material as the beam splitter plate.

[0079] Alternatively, the interferometer device can be of the Mach-Zehnder type.

[0080] According to an embodiment, the interferometer apparatus may further include an excitation source for illuminating the sample and exciting photoluminescence emission, and an optical collection device configured to guide the photoluminescence as an incident light wave to the interferometer.

[0081] Therefore, the interferometer device according to this embodiment can be used in photoluminescence imaging applications, particularly as a Fourier transform imaging spectrometer. This system allows, for example, imaging and inspection of semiconductor samples (e.g., silicon carbide (SiC) substrates) to identify and classify crystal defects.

[0082] Of course, in the interferometer device according to the invention, any other type of fluorescence signal can be used as the incident light. Attached Figure Description

[0083] Other advantages and features will become apparent from a review of the detailed description of the non-limiting embodiments and the accompanying drawings, wherein:

[0084] - Figure 1 The interferogram and corresponding power spectral density obtained using a Fourier transform spectrometer are shown.

[0085] - Figure 2 A non-limiting example of an interferometer device according to the present invention is illustrated schematically.

[0086] - Figure 3This is a schematic diagram illustrating the spectral dispersion compensation principle in the interferometer device according to the present invention.

[0087] - Figure 4 An example of spectral dispersion compensation obtained using the method according to the invention is shown, and

[0088] - Figure 5 An embodiment of a method for compensating spectral dispersion in an interferometer apparatus according to the present invention is shown.

[0089] It should be understood that the embodiments described below are not limiting. In particular, variations of the invention are conceivable to include only a portion of the features described below, rather than independently of other features, if the selection of such feature is sufficient to provide a technical advantage or to distinguish the invention from the prior art. Such a selection includes at least one preferred functional feature without structural details, or only a portion of the structural details if the portion alone is sufficient to provide a technical advantage or to distinguish the invention from the prior art.

[0090] In the accompanying drawings, elements common to multiple drawings may retain the same reference numerals.

[0091] The embodiments described below relate to a Fourier transform (FT) spectrometer arrangement in which an interferometer is used to obtain spectral information from an incident light source. In such a spectrometer, control of spectral dispersion in the measuring interferometer is particularly important because dispersion over the entire effective spectral range must be limited to a fraction of the wavelength to allow for accurate measurement of spectral information.

[0092] Of course, the described interferometer scheme can be used for any other type of application.

[0093] As mentioned earlier, in a Fourier transform spectrometer, the incident light is split into a first wave and a second wave. These waves then combine and interfere at the detector.

[0094] A variable optical path difference or delay is introduced between the first and second waves, for example, by moving the interferometer's mirrors. The FT spectrometer works by acquiring the intensity of the received interference light at different optical path differences on the detector.

[0095] Broadband light typically produces, for example Figure 1 The interference signal shown in (a). Figure 1 (a) shows the interferogram A(d) obtained using an FT spectrometer as a function of optical path difference or displacement of a movable mirror.

[0096] Signal A(d) is also known as the autocorrelation function of the light being analyzed:

[0097]

[0098] Where 𝐸(𝑡) is the electric field of the light being analyzed, d is the optical position of the movable mirror, and 𝑐 is the speed of light.

[0099] All the spectral information of the analyzed light is contained in the autocorrelation signal A(d). This spectral information can be recovered by applying a simple Fourier transform (FT) to the autocorrelation signal.

[0100] Spectral power density = φ(A(d)).

[0101] As an example, Figure 1 (b) shows the relationship with Figure 1 (a) Power spectral density corresponding to the signal.

[0102] Figure 2 This is a schematic diagram of a non-limiting example of an interferometer device according to the present invention.

[0103] Figure 2 The interferometer device 100 shown is a Michelson type. The interferometer device 100 includes a beam-splitting element, indicated by a beam splitter plate 112, for splitting the incident light wave 113 into a first light wave 114 and a second light wave 115. Light waves 114 and 115 propagate back and forth along the first and second optical paths, respectively. The first and second light waves are reflected by a static mirror 116 and a movable mirror 117, respectively.

[0104] The static reflector 116 can also be called a reference reflector, and the movable reflector 117 can also be called a measuring reflector.

[0105] The first light wave 114 can also be called the reference wave, and the second light wave 115 can also be called the measurement light wave.

[0106] The incident light wave can correspond to the light wave emitted from the sample or object to be measured or imaged, such as reflected light and / or photoluminescence.

[0107] The interferometer device 100 also includes a detector 102. The first and second light waves reflected by the mirrors 116 and 117 are recombined on the detector 102.

[0108] like Figure 2 As shown in the example, a dispersion compensation plate 105 is arranged in the second optical path, located between the movable reflector 117 and the beam splitter plate 112. The dispersion compensation plate 105 has the same material as the beam splitter plate 112 or a material with the same refractive index as the beam splitter material. The dispersion compensation plate 105 is positioned in the optical path through which the light wave passes through the material of the beam splitter plate 112 with minimal obstruction. In this example, the first light wave passes through the beam splitter plate 112 three times, while the second light wave passes through the beam splitter plate 112 only once. The function of the dispersion compensation plate 105 is to compensate for the difference in optical path length between the first and second light waves in the material of the beam splitter plate 112.

[0109] The interference signal acquired by detector 102 is a function of the optical path difference between the first and second light waves. The optical path difference δ can be controlled by moving the movable mirror 117 a distance d, such that the optical path difference between the light waves is δ = 2d. The detected signal can then be processed by processing device 130, specifically to obtain spectral information of the measured light radiation.

[0110] To move the movable reflector 117, the reflector 117 may be equipped with a translation actuator 118, such as a piezoelectric actuator. Alternatively, any other displacement device suitable for moving the movable reflector 117 may be provided.

[0111] Of course, the reference mirror 116 can also be a movable mirror.

[0112] according to Figure 2 The interferometer apparatus of the embodiment is arranged in an imaging configuration. Light 113 emitted from sample 104 (e.g., photoluminescence generated by excitation from an excitation source (not shown)) is collimated by an optical collecting device formed by a first lens 123 before being split by beam splitter plate 112. Reference wave 114 and measurement wave 115 are recombined on detector 102 via second lens 124.

[0113] exist Figure 2 In the illustrated embodiment, detector 102 is a camera positioned in an imaging plane that is optically conjugate to an object plane via first and second lenses 123, 124, the object plane being positioned on the surface of sample 104. Therefore, light emitted from the surface of sample 104 in the field of view is imaged onto detector 102 by an interferometer.

[0114] It should be noted that the first and second lenses 123 and 124 are positioned in the optical path shared by the measurement wave 115 and the reference wave 114. In this way, lenses 123 and 124 do not contribute to the spectral dispersion between the measurement wave and the reference wave.

[0115] In the illustrated embodiment, the camera includes a CCD or CMOS detector with linear or area array pixels to image the linear field of view or two-dimensional field of view of the surface of sample 104, respectively.

[0116] According to other embodiments, the device of the present invention may have a single-point measurement configuration or a single-beam measurement configuration, wherein the incident light or measurement light 113 is in the form of a single beam and the detector 102 is a point detector, such as a photodiode.

[0117] In one embodiment, incident light 113 from a point light source can be collimated by a first lens 113 and focused onto detector 102 by a second lens 124.

[0118] In another embodiment, the incident light 113 may be a collimated beam, and the first lens 123 and the optional second lens 124 may be omitted. In this case, the optical collecting device for collecting the incident light 113 may include an optical window or aperture.

[0119] according to Figure 2 The interferometer apparatus 100 of the illustrated embodiment includes a rotating device 106 configured to position a dispersion compensation plate 105 at an angle relative to a second light wave 115. The rotating device may be, for example, a rotary table 106 on which the dispersion compensation plate 105 is mounted. The rotary table 106 is arranged to rotate the compensation plate 105 about a rotation axis perpendicular to the optical axis of the second light wave 115, thereby changing the angle of incidence of the second light wave 115 on the dispersion compensation plate 105. Therefore, the angle of incidence of the second light wave 115 on the dispersion compensation plate 105 depends on the rotational orientation of the plate 105.

[0120] The interferometer device 100 is also provided with a processing unit 130, which is configured to acquire and process the signal detected by the detector 102.

[0121] The processing device 130 is specifically configured to acquire spectral information of the measurement light wave 113 based on the interference signal detected by the detector 102.

[0122] The processing device 130 is also configured to determine the spectral dispersion of the interferometer (particularly for the measurement of the interference signal detected by the detector 102) and to correct the spectral dispersion by adjusting the angular position of the dispersion compensation plate 105 using the rotation device 106 to satisfy the spectral dispersion criterion. Therefore, the processing device is also configured to implement and execute the method for spectral dispersion compensation according to the present invention described below.

[0123] The processing device may include, but is not limited to, a computer, a microprocessor, a controller board, and all the hardware required to perform the method steps, such as volatile and non-volatile memory, analog-to-digital converters, and I / O boards.

[0124] according to Figure 2 The interferometer device of the illustrated embodiment includes a control interferometer. The control interferometer has the same beam splitter plate 112 and reflectors 116 and 117 as the interferometer described above.

[0125] Controlling the interferometer allows for monitoring and correcting optical dispersion within the device, and can optionally be used to measure the optical path difference between the two arms of the interferometer, as described below with respect to the method according to the invention.

[0126] like Figure 2As shown, the control interferometer includes a control light source 120. The control light source 120 includes at least one light source element. The control light source 120 is positioned to emit at least one control light wave 121, which is different from the incident light wave or measurement light wave 113 described above. The control light wave 121 is transmitted through the interferometer in the same manner as the measurement light wave 113 and through the same components. Therefore, the control light wave 121 is subject to the same optical delay and the same spectral dispersion as the measurement light wave.

[0127] The control light source 120 may also include multiple light source elements that are combined to emit control light along the same optical axis.

[0128] The control light source 120 may include one or more low-coherence light sources that selectively emit light within a narrow spectral range near a calibration wavelength or control wavelength distributed within the target spectral range of the measurement light.

[0129] For example, to analyze the target spectrum in the visible light range (400-800nm), the control light source may include light-emitting diodes (LEDs) that emit light in the red (R), green (G), and blue (B) portions of the spectrum and are optically combined, or switchable RGB LEDs.

[0130] The spectral ranges of these light sources are chosen such that their coherence lengths correspond to a small number of wavelengths of the respective light source. Therefore, the central constructive interference (corresponding to zero optical path difference) can be well identified by its amplitude compared to adjacent interferences, and at least several interference fringes can be observed.

[0131] The control light source can also be a broadband control light source combined with a bandpass filter.

[0132] The control interferometer also includes a control detector 122, which differs from the detector 102 described above. The control light wave 121 is split into a first control light wave 125 and a second control light wave 126 by a beam splitter 112. The first and second control light waves 125 and 126 propagate back and forth along the first and second optical paths, respectively, and are reflected by a static mirror 116 and a movable mirror 117, respectively. The first and second control light waves reflected by mirrors 116 and 117 are recombined on the control detector 122.

[0133] When using a broadband controlled light source, the detector may include a bandpass filter.

[0134] Controlling the interference signal generated within the interferometer allows for monitoring the optical dispersion of the interferometer device. The control detector 122 selectively detects signals at multiple control wavelengths. These control wavelengths can be used sequentially with a single detector 122, or simultaneously with detectors, for example, including dichroic filters that direct the respective calibration wavelengths to different detector elements.

[0135] In addition to dispersion monitoring, the control interferometer can be arranged to allow optical measurement of the optical path difference between the two arms of the interferometer; that is, to allow displacement measurement. This can be done in place of or added to an internal metrology system (e.g., using a capacitive sensor) that monitors and measures the translation of the reflector 117.

[0136] In this configuration, the control light source 120 comprises a laser source (e.g., a laser diode) that emits light with a stable and known optical wavelength and a coherence length much greater than the effective displacement range of the measuring mirror. The light waves emitted from the laser source (referred to as metrological light) are split into metrological measurement waves and metrological reference waves, which interfere at the control detector 122. The phase of the interference signal can then be obtained using known methods to measure the relative displacement of the interferometer mirrors.

[0137] In order to measure displacement while measuring dispersion, the control detector 122 may include an optical splitter (dichroic splitter or polarization splitter) and separate detector elements to allow simultaneous detection of the metrological laser and the dispersion monitoring light.

[0138] The apparatus according to the invention may include more than one control interferometer, such as two, three, or four control interferometers. Multiple control interferometers may be positioned such that their respective control beams pass through beam splitter 112 at locations laterally offset relative to the position where the measurement beam 113 passes through the beam splitter. These positions may also be positioned symmetrically relative to the measurement beam 113. Therefore, spectral dispersion can be monitored at multiple locations relative to the beam splitter and the mirror. This provides more accurate measurements and allows for correction of parallelism mismatches.

[0139] According to embodiments, these multiple control interferometers may each include all control light sources for dispersion monitoring (and displacement measurement as appropriate), or only a portion thereof, with different control light sources (e.g., control light sources of different wavelengths) set in different control interferometers.

[0140] In particular, the control interferometer used for dispersion monitoring can be different from the control interferometer used for displacement measurement.

[0141] like Figure 2 The interferometer device shown can implement the method for compensating spectral dispersion according to the present invention.

[0142] refer to Figure 3 and Figure 4 The principle of dispersion compensation according to the present invention will be explained in detail.

[0143] refer to Figure 3 For the reference arm of the interferometer, the following variables are defined:

[0144] - D bsThe geometric thickness of beam splitter 112

[0145] - D bm The geometric distance between the center of beam splitter 112 and reference mirror 116 along the optical axis of reference wave 114.

[0146] - θ ibs The angle of incidence of the light wave on the beam splitter 112 relative to the surface normal.

[0147] - n bs : The refractive index of beam splitter 112.

[0148] For the measuring arm of the interferometer, the following variables are defined:

[0149] - D c The geometric thickness of the dispersion compensation plate 105.

[0150] - D bc The geometric distance along the optical axis of the reflected wave between the point of contact of the light wave on the beam splitter 112 and the center of the compensation plate 105, assuming the compensation plate rotates around this center.

[0151] - D cm The geometric distance between the center of the dispersion compensation plate 105 and the measuring mirror 117.

[0152] - θ ic : Measure the angle of incidence of light wave 115 on the dispersion compensation plate 105 relative to the surface normal.

[0153] - n c : The refractive index of dispersion compensation plate 105.

[0154] In the measuring arm, the optical path length (unidirectional) of the measuring wave from the contact point (or center) of the beam splitter 112 to the measuring mirror 117 is:

[0155] (Equation 1)

[0156] The optical path length between the beam splitter and the dispersion compensation plate is:

[0157] (Equation 2)

[0158] The optical path length is defined as the geometric path length multiplied by the refractive index of the material.

[0159] The optical path length within the compensation plate 105 is:

[0160] (Equation 3)

[0161] And the optical path length between the compensation plate 105 and the measuring mirror 117 is:

[0162] (Equation 4)

[0163] The lateral displacement of the light wave is

[0164] (Equation 5)

[0165] In the reference arm, the optical path length (unidirectional) of the reference wave from the contact point on the beam splitter 112 to the reference mirror 116 is:

[0166] (Equation 6)

[0167] The optical path length within beam splitter 112 is:

[0168] (Equation 7)

[0169] And the optical path length between beam splitter 112 and reference mirror 116 is:

[0170] (Equation 8)

[0171] The lateral displacement of the beam is:

[0172] (Equation 9)

[0173] A balanced interferometer refers to adjusting the optical path lengths of the measuring arm and / or the reference arm until their lengths match.

[0174] (Equation 10)

[0175] For the application under consideration, it is also crucial to satisfy this matching condition as much as possible across the entire target spectral range. Therefore, the matching condition can be restated as follows:

[0176] (Equation 11)

[0177] If the thickness, refractive index, or material of the beam splitter and dispersion compensation plate are perfectly matched, the beams will pass through the same angle of incidence. (For example, 45 degrees) can make the corresponding optical path lengths in the measuring arm and the reference arm match all wavelengths.

[0178] However, as mentioned earlier, there may actually be a thickness mismatch between the beam splitter and the compensation plate. ,therefore:

[0179] (Equation 12)

[0180] Because of the optical dispersion inherent in this thickness mismatch, simply changing the optical path length (e.g. in air) is insufficient to compensate for the dispersion across the entire target wavelength range.

[0181] According to the present invention, by rotating the dispersion compensation plate by an additional compensation angle θ icomp This can compensate for the difference in optical path length between the two arms of the interferometer. Then, the dispersion is minimized to a level suitable for a variety of applications.

[0182] The dispersion compensation according to the present invention can be achieved, for example, by calculating the compensation angle θ. icomp To achieve this, making

[0183] (Equation 13)

[0184] Satisfying the matching condition (Equation 11) means that the optical path dispersion within the target spectral range is minimized.

[0185] Figure 4 An example of this compensation is shown. Figure 4 The example calculations are for a beam splitter and a dispersion compensation plate made of fused silica. The beam splitter is 3 mm thick. The dispersion compensation plate is 3.1 mm thick, corresponding to a nominal thickness of 3 mm and a thickness tolerance of +0.1 mm, which is common in catalog products. The incident angle θ on the beam splitter... ibs It is 45 degrees.

[0186] exist Figure 4 In (a), the optical path length difference (L) between the two arms of the interferometer was calculated for wavelengths of 400 nm, 600 nm, and 800 nm. M -L R ), which is the incident angle θ of the dispersion compensation plate. ic The optical path difference in the diagram is zero (corresponding to angular position). This corresponds to a balanced interferometer, where the thickness of the dispersion compensation plate matches the thickness of the beam splitter (3 mm), and the incident angles of the beam splitter and the dispersion compensation plate are the same, i.e., θ. ic =θ ibs =45 degrees. The diagram illustrates how the optical path length difference at different wavelengths changes with the compensation rotation of the compensation plate.

[0187] exist Figure 4 (b) shows the optical path length difference (“OPD”) of wavelengths of 400 nm and 800 nm relative to wavelength of 600 nm as a function of compensation angle θ. icomp The graph shows the variation. This close-up view illustrates the residual dispersion after applying the compensation angle. In this case, the residual optical path difference between the 400 nm and 800 nm wavelengths is less than 10 nm, which is satisfactory for many applications, especially spectral analysis.

[0188] Figure 4 (a) and 4(b) show the compensation angle θ icomp = -4.5 degrees, or the tilt angle θ of the compensation plate ic At 40.5 degrees, both optical path difference and dispersion can be minimized.

[0189] It should be noted that under these conditions, both positive and negative thickness errors (whether the compensation plate is thicker or thinner than the beam splitter) can be compensated.

[0190] Figure 5 A method for compensating spectral dispersion in an interferometer apparatus according to an embodiment of the present invention is shown.

[0191] Figure 5 The method 500 shown includes determining spectral dispersion and compensating for spectral dispersion. In the compensation phase, a dispersion compensation plate is rotated according to a compensation angle to minimize the dispersion criterion described below.

[0192] In step 501, the compensation angle θ icomp It is set to the first value (e.g., zero) or initialized.

[0193] In step 502, an interference signal from a first control wavelength of a control interferometer having a first control light source is detected. For this purpose, the position of the measuring mirror is scanned to obtain an autocorrelation interference signal corresponding to the control light source, for example... Figure 1 As shown in (a).

[0194] By locating the vertices of the central bright fringe or constructive interference and retrieving the position provided by the internal metrology of the interferometer (using a controlled interferometer with a laser light source as described above, or a metrology system inside an actuator with one or more mirrors), the contact position with zero optical path difference is located on the interferogram.

[0195] In step 503, similar to step 502, the second contact position is measured for at least a second wavelength (i.e., using a second control light source different from the first control light source).

[0196] Additional contact locations can be measured for different control wavelengths. Preferably, the control wavelength is selected to cover the target spectral range.

[0197] In step 504, the position difference (or mirror position) of each contact position is calculated for different wavelengths, thereby providing a measurement of spectral dispersion.

[0198] Then, the determination phase includes step 505: calculating the new compensation angle θ. icomp To compensate for spectral dispersion, the angular position of the compensation plate is adjusted to that value.

[0199] Then steps 501 to 505 can be iteratively repeated until the spectral dispersion meets the criterion, for example:

[0200] - Spectral dispersion is less than the threshold (e.g., a fraction of the wavelength, 1 / 10 of the center wavelength, etc.).

[0201] - The change in spectral dispersion between the two iterations of steps 501-505 is below the threshold (convergence criterion).

[0202] Steps 501 to 505 correspond to the dispersion adjustment phase for determining and compensating for spectral dispersion in the interferometer. After completing these steps, the device can then perform measurements of the measurement light in the measurement phase shown in step 510. Such measurements may include measuring the spectral information of the measurement light as described above. Of course, steps 501 to 505 of the dispersion adjustment phase can be repeated periodically, alternating with or in parallel with measurement phase 510, to check whether the spectral dispersion in the device remains within the expected limits.

[0203] like Figure 4 As shown, the dispersion at the optical contact position at different wavelengths varies monotonically and nearly linearly with the compensation angle, at least within a small range. Therefore, by measuring the contact position for at least two wavelengths and at least two compensation angles in steps 501-505, the data can be extrapolated linearly for each wavelength and their intersection points can be found, thereby providing a new compensation angle θ. icomp If necessary, the process can be iteratively repeated by measuring a new set of optical contact positions for different wavelengths. This allows spectral dispersion to be minimized through an iterative process without any prior knowledge of component uncertainties.

[0204] Of course, if sufficient prior information is available, such as the actual thickness of the beam splitter and dispersion compensation plate, the compensation angle θ can be calculated directly using (Equation 1) and (Equation 6) in step 501. icomp The verification can be performed in steps 502 and 503, and if necessary, steps 501-505 can be iteratively repeated as described above.

[0205] Therefore, prior information can be used to directly calculate the compensation angle θ. icomp The set value, or as a means of initializing the iteration method, can be used to improve convergence efficiency and speed.

[0206] Of course, the present invention is not limited to the examples described in the above description.

Claims

1. A method (500) for acquiring an interference signal, comprising: The incident light wave is split into a first light wave and a second light wave, which then propagate along the first and second optical paths of the interferometer, respectively. - Adjust the optical path difference between the first light wave and the second light wave, and - An interference signal is generated by combining the first and second light waves on a detector. The method is characterized in that it further includes: - Spectral dispersion is measured using the aforementioned interference signal, and - The spectral dispersion (505) is compensated by positioning a dispersion compensation plate made of a dispersive material at a compensation angle relative to the optical axis of the first or second light wave at a compensation angle position to satisfy the spectral dispersion criterion.

2. The method (500) according to claim 1, wherein, Measuring spectral dispersion includes determining the contact position of the interferometer at a controlled wavelength, the contact position corresponding to the zero optical path difference between the first and second light waves at the controlled wavelength.

3. The method (500) according to claim 2, wherein, Measuring spectral dispersion includes: - The dispersion compensation plate is positioned (501) at an angle. - Determine (502, 503) the first contact position for the first control wavelength and the second contact position for the second control wavelength, and - The spectral dispersion (504) is calculated by calculating the difference between the first contact position and the second contact position.

4. The method (500) according to any one of the preceding claims, wherein, Determining the position of the compensation angle includes at least one of the following steps: - The compensated angular position satisfying the spectral dispersion criterion is calculated by applying an analytical and / or numerical model of spectral dispersion in the interferometer based on the angular position. - Spectral dispersion is measured at multiple angular positions (501-504). - Interpolate or extrapolate the spectral dispersion measurements obtained at multiple angular positions to find the compensation angular positions that satisfy the spectral dispersion criteria. - Iteratively measure the spectral dispersion (501-504) for the angular position to converge to a compensated angular position that satisfies the spectral dispersion criterion.

5. The method (500) according to any one of the preceding claims, wherein, The spectral dispersion criterion includes one of the following: the spectral dispersion is below a predetermined threshold in the spectral range, or the minimum value of dispersion in the spectral range.

6. The method (500) according to any one of the preceding claims further includes the following step: - To acquire the interference signal of the incident light wave for multiple optical path differences (510), - Use the interference signal to calculate the spectral information of the measured light wave.

7. An interferometer device (100), comprising: - A beam splitter (112) is configured to split an incident light wave (113) into a first light wave (114) and a second light wave (115), the first light wave (114) and the second light wave (115) propagating along a first optical path and a second optical path, respectively. - Adjustment device (118) for adjusting the optical path difference between the first light wave (114) and the second light wave (115), - Detector (102) for generating an interference signal by combining the first light wave (114) and the second light wave (115), The interferometer device (100) is characterized in that it further includes: - A rotating device (106) configured to position a dispersion compensation plate (105) made of a dispersive material at an angle relative to the optical axis of the first light wave (114) or the second light wave (115), and - Processing device (130), which is configured as follows: ■ The spectral dispersion is measured using the aforementioned interference signal, and ■ Determine the compensation angle position for positioning the dispersion compensation plate (105) to satisfy the spectral dispersion criterion.

8. The interferometer apparatus (100) according to the preceding claim further includes a control light source (120) configured to illuminate the interferometer with a control incident light wave (121) to measure spectral dispersion.

9. The interferometer device (100) according to the preceding claim further includes a control detector (122) configured to generate a control interference signal for measuring the spectral dispersion by collecting light waves (125, 126) emitted from the control light source (120) and passing through the interferometer.

10. The interferometer apparatus (100) according to claim 8 or 9, wherein, The control light source (120) includes at least one low-coherence narrow-spectral-range light source element.

11. The interferometer device (100) according to claim 9, comprising a broadband control light source (120), a control detector (122), and a bandpass filter disposed between the control light source (120) and the control detector (122).

12. The interferometer apparatus (100) according to any one of claims 7 to 11, further comprising an imaging lens (123, 124) configured to image incident measurement light waves emitted from the field of view onto a linear or area array detector (102).

13. The interferometer apparatus (100) according to any one of claims 7 to 12, comprising an optical collecting device for collecting measurement incident light, a measurement detector, and at least one set of control light sources and control detectors, wherein the control detectors are illuminated by the control light sources.

14. The interferometer device (100) according to the preceding claim, comprising a plurality of control light sources and control detectors, the plurality of control light sources and control detectors being located around the optical collection device and the measurement detector, respectively.

15. The interferometer apparatus (102) according to any one of claims 7 to 14, further comprising an excitation source for illuminating the sample and exciting photoluminescence emission, and an optical collection device configured to guide the photoluminescence as an incident light wave to the interferometer.

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