Optical interference measurement device

Abstract

According to the present invention, an optical interference measurement device that measures an object W using interference light from measurement light and reference light that are emitted from a wavelength scanning light source (202), continuously irradiated at the object by arc-scanning, and reflected from the object comprises: an interference light measurement unit (201) that has an interference light detection means (204) that receives the interference light and generates an interference light signal that indicates the intensity of the interference light; and a signal processing unit (216) that samples the interference light signal to generate a plurality of signals that include a signal within the Nyquist frequency and an alias, restores the alias to the frequency domain of measured values, and acquires a surface shape profile for the object on the basis of the signal within the Nyquist frequency and the restored signal.

Description

Optical Interference Measurement Device

[0001] The present disclosure relates to an optical interference measurement apparatus that measures an object to be measured using interference light formed by measurement light and reference light.

[0002] Optical coherence tomography (OCT) is a method for capturing tomographic images of structures such as paint films or living organisms using the phenomenon of optical interference. OCT has already been put to practical use in the field of ophthalmology, where it has been used as a tomographic measurement method with a high resolution of several tens of micrometers to capture tomographic images of minute regions inside the eye, such as the retina.

[0003] There are two types of OCT: time-domain OCT (TD-OCT), which requires scanning of a reference plane, and frequency-domain OCT (FD-OCT), which does not. FD-OCT also includes two types: spectrometer-type (SD-OCT) and wavelength-scanning light source-type (SS-OCT). Both types split the light emitted from a light source into measurement light and reference light, then combine the measurement light and reference light reflected from the object to be measured, and acquire an optical tomographic image based on the beat frequency of the interference light between the measurement light and reference light.

[0004] Fig. 8 is a diagram showing a conventional SS-OCT device described in Patent Document 1. In the low-coherence interferometer in the device shown in Fig. 8, light emitted from a wavelength-scanning light source 10 is split into reference light and measurement light by a beam splitter 13, and the reference light passes through a lens 11 and a mirror 12 and enters a photodetector 17, while the measurement light passes through a lens 14 and a galvanometer mirror 15, is reflected from a measurement object 16, and then similarly enters the photodetector 17.

[0005] The signal received by the photodetector 17 is amplified by an amplifier 18 and high-frequency components are cut through an LPF (Low-pass filter) 21. There is also a scan trigger generator 19 that generates a trigger signal for each wavelength scan of the wavelength scanning light source 10, and this trigger signal is used as a trigger by a k trigger generator 20 that generates k trigger signals at equal frequency intervals of the light of the wavelength scanning light source 10 within one scan period, thereby generating k trigger signals. An optical tomographic image can be obtained by Fourier transforming the optical beat signal of the interference light based on this k trigger signal in a Fourier transform circuit 22.

[0006] Japanese Patent Application Laid-Open No. 2007-24677

[0007] In SS-OCT devices, the width in the scanning direction is limited by the presence of a lens after the galvanometer mirror, which is the scanning mechanism, and the measurable range in the depth direction, i.e., half of the maximum value of the optical path length difference between the reference light and the measurement light at which spectral interference fringes can be correctly obtained, is limited by the frequency band design of the LPF and photodetector.

[0008] Therefore, in the conventional configuration, even if there is no lens and the width of the galvanometer mirror in the scanning direction is not limited, the change in distance to the object to be measured due to scanning cannot be made larger than the measurable range in the depth direction, and there is a problem that there is a limit to the size of the object to be measured in the scanning direction that can be measured.

[0009] An object of the present disclosure is to provide an optical interference measurement device that can measure even when the change in distance to the measurement target range due to scanning is greater than the measurement range in the depth direction, i.e., that can expand the measurement range.

[0010] An optical interferometry device according to one aspect of the present disclosure is an optical interferometry device that measures an object using interference light produced by measurement light and reference light that is emitted from a wavelength scanning light source, is continuously irradiated onto the object in an arc scan, and is reflected from the object, and includes: an optical interferometry unit having interference light detection means that receives the interference light and generates an interference light signal indicating the intensity of the interference light; and a signal processing unit that samples the interference light signal to generate a plurality of signals including signals within the Nyquist frequency and aliases, restores the aliases to a frequency domain of actual measured values, and acquires a surface shape profile of the object based on the signals within the Nyquist frequency and the restored signals.

[0011] As described above, the optical interferometry device according to the above aspect of the present disclosure incorporates multiple signals from the object, including signals within the Nyquist frequency (i.e., below the Nyquist frequency) and aliases, to avoid attenuating signals above the Nyquist frequency, which is attenuated by an LPF in conventional SS-OCT devices. The aliases are then restored to the frequency domain of the actual measured values. As a result, measurements are possible even when the change in distance to the measurement target range due to scanning is greater than the measurement range in the depth direction, i.e., the measurement range can be expanded. Therefore, the surface shape of the object can be measured even if the distance to the object is greater than the measurable range. Here, multiple signals from the object are separated into signals within the Nyquist frequency and aliases by arc scanning. The term "alias" refers to signals that are aliased around the Nyquist frequency and extend into the measurable range from the zero point because signals above the Nyquist frequency are outside the maximum measurable range. Furthermore, since it is theoretically impossible to distinguish between positive and negative frequencies, negative frequency signals are aliased around the zero point into positive frequency signals.

[0012] FIG. 1 shows the overall configuration of SS-OCT in an embodiment. FIG. 2 shows an alias in an embodiment. FIG. 3 shows an excessive interference region of SS-OCT in an embodiment. FIG. 4 shows an alias waveform in an embodiment. FIG. 5 shows a flowchart of signal processing in an embodiment. FIG. 6 shows a restoration process of a downwardly convex alias waveform in an embodiment. FIG. 7 shows a restoration process of an upwardly convex alias waveform in an embodiment. FIG. 8 shows a design of a zero point position in an embodiment. FIG. 9 shows a control of a dead zone position in an embodiment. FIG. 10 shows a conventional SS-OCT device described in Patent Document 1.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0014] 1 is a diagram showing the overall configuration of an optical interference measurement apparatus according to an embodiment. The optical interference measurement apparatus includes an SS-OCT apparatus 201 as an example of an optical interference measurement unit, a measurement head 209, and a signal processing unit 216.

[0015] The SS-OCT device 201 includes a wavelength scanning light source 202, an optical fiber interferometer 203, and a photodetector 204 as an example of an interference light detecting means.

[0016] The wavelength-scanning light source 202 includes a wavelength-scanned light generating unit 205 with a long coherence length and good reproducibility, a trigger generating unit 206 that outputs a trigger signal for each wavelength scan, and a k-clock generating unit 207 that generates a k-clock signal that samples the optical frequency of the wavelength-scanned light at equal intervals based on the trigger signal. The trigger generating unit 206 for the trigger signal and the k-clock generating unit 207 for the k-clock signal are connected to a signal processing unit 216 and are used when performing Fourier transform at the timing of data acquisition. The signal processing unit 216 can obtain an optical tomographic image by Fourier transforming the interference light based on the k-clock signal.

[0017] The light generated from the wavelength-scanning light source 202 is incident on the optical fiber interferometer 203 .

[0018] In the optical fiber interferometer 203, the light output port of the wavelength tunable light source 202 is connected to a first light receiving port of two light receiving ports of a coupler 208. One light output port of the coupler 208 is connected to a measurement head 209 outside the optical fiber interferometer, and the other light output port is connected to a collimating lens 212 that transmits reference light to a reference surface 211. The coupler 208 is an example of a light splitting means, and splits the light output from the wavelength tunable light source 202 into measurement light and reference light.

[0019] The measurement light passes through a variable-focus collimator 213 and a scanning mechanism 214 in the measurement head 209 and is irradiated onto the measurement object W, where it is reflected or scattered by the measurement object W. The reflected or scattered measurement light enters the coupler 208 from the measurement head 209 and is split into two by a 50:50 coupler 215, and then each enters the photodetector 204. The photodetector 204 detects interference light resulting from the combination of the measurement light and reference light from the measurement object W.

[0020] The reference light is reflected by a reference surface 211 that can be driven forward and backward by a stepping motor 210 under the control of a control unit 290, which is an example of a reference surface moving device. The reflected light of the reference light passes through a collimating lens 212 and enters a coupler 208, similar to the measurement light, and is split into two by a 50:50 coupler 215, after which each light enters a photodetector 204. The reference surface moving device, for example, rotates a screw shaft forward and backward using a stepping motor 210 under the control of a control unit 290 connected to a signal processing unit 216, thereby driving the reference surface 211 engaged with the screw shaft forward and backward. By moving the position of the reference surface 211, the reference optical path length is changed, canceling out changes in the optical path length difference with the signal optical path length, and the zero point position, which will be described later, can be always kept constant.

[0021] In the optical fiber interferometer 203, the point where the signal optical path length of the measurement light and the reference optical path length of the reference light match is defined as the zero point, and this position is shown in Fig. 1. The position of the zero point in the embodiment can be freely changed by, for example, changing the distance between the collimating lens 212 and the reference surface 211 or the distance from the coupler 208 to the collimating lens 212.

[0022] In the photodetector 204, the interference light of the measurement light and the reference light is split into two by a 50:50 coupler 215 and then input to two input sections. The photodetector 204 extracts and amplifies the differential signal of the interference signal of the two interference lights, and then inputs it to a signal processing section 216.

[0023] The signal processing unit 216 detects these interference signals at different optical frequency sampling intervals corresponding to the sweep speed of the wavelength-scanning light source 202 by referring to the k-clock signal that samples the optical frequency of the wavelength-scanning light source 202 at equal intervals, and performs a Fourier transform on the interference signals.

[0024] In this way, by performing appropriate signal processing including Fourier transform on these interference signals in the signal processing unit 216, it becomes possible to obtain the differential of the one-dimensional refractive index distribution, i.e., the reflectance distribution, in the signal optical path of the measurement light of the measurement object W. Note that here, it is defined that the positive or negative sign of the optical path length difference is determined by the positive or negative sign of the calculation result of the signal optical path length - the reference optical path length.

[0025] In this case, as described above, if the finite measurable range in the depth direction that can be measured by the SS-OCT device 201 is LD, the maximum measurable range is the range of ±LD with the zero point as the center.

[0026] The scanning mechanism 214 is an element capable of changing the reflection direction of the measurement light, such as a galvanometer scanner, a polygon scanner, or a resonance scanner, and is capable of scanning the measurement light in the θ direction. The surface shape of the measurement object W in the X direction can be measured by continuously scanning the measurement light in the θ direction using the scanning mechanism 214, which is driven under the control of a scan control unit 291 connected to the signal processing unit 216.

[0027] <Regarding Band Design> A method for detecting interference signals outside the range of ±LD centered around the zero point, which is the maximum measurable range in the SS-OCT device 201, will be described with reference to FIGS. 2A to 2C.

[0028] The coherence region of the SS-OCT device 201 according to the embodiment is shown in FIG. 2B. Here, the range of optical path length difference in which an interference signal can be obtained is referred to as the coherence region. The coherence region is a Lorentzian function centered on the zero point when the vertical axis represents the intensity of the interference signal and the horizontal axis represents the depth z (= optical path length difference / 2) in the optical axis direction. However, for simplicity, the intensity of the interference signal is constant and it is represented as a rectangle with a width of ±LD. Near the zero point where the optical path length difference between the signal optical path and the reference optical path is 0, an interference signal is detected in the range of optical path length difference 0 ±LD. This is referred to as the zero-order coherence region.

[0029] Furthermore, when the optical path length difference between the signal optical path and the reference optical path is further increased by +4LD, that is, when the depth z is increased by +2LD from the zero point, an interference signal can also be obtained in the region from depth LD to 3LD. This is called the +1st-order coherence region.

[0030] Here, as shown in Figure 2A, +LD corresponds to the Nyquist frequency, which is half the sampling frequency when sampling the signal. Interference fringe signals at frequencies deeper than this are attenuated by the LPF in the conventional SS-OCT device shown in Figure 8.

[0031] The SS-OCT device 201 of this embodiment is designed with a band that does not attenuate signals 301 above the Nyquist frequency. However, because signals 301 above the Nyquist frequency are outside +LD, which is the maximum measurable range, they are aliased within the range from the zero point to +LD, i.e., the measurable range, with the Nyquist frequency at the center. This aliased signal is called an alias 302. Furthermore, because the SS-OCT device 201 cannot, in principle, distinguish between positive and negative frequencies, negative frequency signals are aliased into positive frequency signals with the zero point at the center.

[0032] Using these two folding patterns, the signal that can actually be acquired in the case of a circular scan is shown in Figure 2C. The original signal in the circular scan, i.e., the actual measured signal 303, falls within the measurable range of +LD by repeating the folding of the two patterns multiple times. If region A is the negative side of the zeroth-order coherence length, region B is the positive side of the zeroth-order coherence length, region C is the negative side of the +1st-order coherence length, and region D is the positive side of the +1st-order coherence length. As with the conventional SS-OCT device shown in Figure 8, the positive side of the zeroth-order coherence length is usually referred to as the measurement range. This setting of the coherence length order is just one example; for example, region A may be the negative side of the +1st-order coherence length or the negative side of the -1st-order coherence length.

[0033] An attempt is made to expand the measurable range by processing the signal that has folded back within the measurement range and returning it to the original signal 303 from the arc scan by the signal processing unit 216. Fig. 2C shows one example of the signal, and it may be, for example, an upwardly convex arc, an arc with zero folding back, or an arc with multiple folding backs.

[0034] In other words, in signal acquisition, the measurement light from the wavelength-tunable light source 202 is arc-scanned and continuously scanned across the object W. The photodetector 204 receives the interference light and generates an interference light signal indicating the intensity of the interference light. The photodetector 204 sends the interference light signal to the signal processing unit 216.

[0035] The signal processing unit 216 samples the interference light signal to generate a plurality of signals including a signal within the Nyquist frequency and an alias. The signal processing unit 216 restores the alias 302 to the frequency domain of the actual measurement value. The signal processing unit 216 further acquires a surface shape profile of the object W based on the signal within the Nyquist frequency and the restored signal.

[0036] <Signal Processing> Figure 3 is a flowchart of signal processing by the signal processing unit 216, and the signal processing will be described based on the flowchart. After acquiring a signal from a flat workpiece HW with no irregularities to obtain the reference shape, there is a folding process S1, a process S2 to remove dead zone positions, and a fitting process S3. After acquiring a signal from the measurement target W to obtain the measured shape, there is a folding process S4, a process S5 to remove dead zone positions, and an interpolation process S6. After the signal processing S3 and S6, there is a subtraction process S7.

[0037] Therefore, after signal acquisition and signal processing S1 to S3 are performed on a flat workpiece HW without irregularities to obtain the reference shape, signal acquisition and signal processing S4 to S6 are performed on the measurement target W to obtain the measured shape, and finally, subtraction processing is performed in S7 to obtain the difference between the reference shape and the measured shape, thereby obtaining the surface shape profile of the target W. However, it is desirable that the measurement target W has a thickness within +LD. Furthermore, the width dimension of the flat workpiece HW without irregularities, which is perpendicular to the transport direction when it is being transported for measurement, should be equal to or greater than the width dimension of the target W.

[0038] In the folding processes S1 and S4 from signal acquisition, and the processes S2 and S5 for removing dead zone positions, the flow is the same for a flat workpiece HW without any irregularities and the object to be measured W, so the explanation of these flows will be limited to the object to be measured W.

[0039] When scanning the scanning mechanism 214, if the incident angle of the measurement light emitted from the scanning mechanism 214 to the measurement object W is θ using the shortest distance L to the measurement object W, the distance between the measurement object W and the scanning mechanism 214 is L / cos θ. In this case, the difference with the distance L to the measurement object W is L(1-1 / cos θ). If the displacement with respect to the distance L from the center of rotation of the scanning mechanism 214 to the measurement object W with respect to the incident angle θ is W(θ), then W(θ) = L(1 / cos θ-1) (1) The displacement W(θ) indicates the surface shape profile of the measurement object W based on the distance L to the measurement object W. The waveform z(θ) obtained by observing the displacement W(θ) with this SS-OCT device 201 is shown in FIG. 2C.

[0040] 4(a) and 5(a) show the relationship between the scanning angle θ and the displacement W(θ) when the measurement object W is scanned by the scanning mechanism 214. As the scanning angle θ increases, the distance between the measurement object W and the scanning mechanism 214 increases, and the displacement W(θ) increases. In reality, the displacement W(θ) with respect to the scanning angle θ is nonlinear as shown in equation (1), but because it monotonically increases, it is treated as linear for simplicity of explanation.

[0041] Corresponding to FIG. 2C , region E shown in FIGS. 4A and 5A is the negative side of the zeroth-order coherence length, region F is the positive side of the zeroth-order coherence length, region G is the negative side of the +1st-order coherence length, and region H is the positive side of the +1st-order coherence length.

[0042] First, the folding process S4 will be described.

[0043] In the case of Fig. 4, where region E is a signal of a downwardly convex arc, the signal obtained by folding back as shown in Fig. 4(a) is returned to the signal shown in Fig. 4(f) by folding back the signal to the displacement W(θ). This process is outlined in Fig. 4(b), (c), (d), (e), and (f). For simplicity, the explanation will be limited to the region to the left of the center of the signal.

[0044] First, in Figure 4(b), areas F, G, and H are moved symmetrically around the zero point, and the resulting waveform is z1(θ), so: z1(θ) = -z(θ) (2) Next, in Figure 4(c), areas G and H are moved symmetrically around -LD, and the resulting waveform is z2(θ), so: z2(θ) = -2LD - z1(θ) (3) Next, in Figure 4(d), area H is moved symmetrically around -2LD, and the resulting waveform is z3(θ), so: z3(θ) = -4LD - z2(θ) (4) Next, in Figure 4(e), the same signal processing is performed on the right side from the center as on the left side, and the resulting waveform is z4(θ).

[0045] Finally, in FIG. 4(f), the following calculation is performed for the entire region to restore the displacement W(θ).

[0046] W(θ)=−z4(θ)+LD (5) Depending on the number of folds, it may not be necessary to use equation (5), and the displacement W(θ) can be restored using only equations (2) and (3), for example.

[0047] On the other hand, in Fig. 5, when the region E is a signal of an upwardly convex arc, the signal obtained by folding back (a) in Fig. 5 is returned to the signal (f) in Fig. 5 by folding back the signal to a displacement W(θ). This process is outlined in Fig. 5(b), (c), (d), (e), and (f). As with Fig. 4, for simplicity, the explanation will be limited to the region to the left of the center of the signal.

[0048] First, in Figure 5(b), if areas F, G, and H are moved symmetrically by LD and the resulting waveform is z1(θ), then z1(θ) = 2LD - z(θ) (6) Next, in Figure 5(c), if areas G and H are moved symmetrically by 2LD and the resulting waveform is z2(θ), then z2(θ) = 4LD - z1(θ) (7) Next, in Figure 5(d), if area H is moved symmetrically by 3LD and the resulting waveform is z3(θ), then z3(θ) = 6LD - z2(θ) (8) Next, in Figure 5(e), if the same signal processing is performed on the right side from the center as on the left side, then the displacement can be restored to W(θ).

[0049] This method is not preferable when the scanning mechanism 214 discretely changes the measurement position, because it becomes impossible to distinguish based on the inclination relative to the scanning angle θ.

[0050] Furthermore, even when the scanning mechanism 214 scans continuously, non-flat objects such as those with a stepped shape, in which the displacement W(θ) shows discrete changes, are also not desirable, and it is necessary to continuously scan the scanning angle θ for the flat measurement object W.

[0051] Next, the process S5 for removing the dead zone position will be described.

[0052] 7, the point of return becomes a dead zone 801, and the signal becomes unstable. Therefore, the signal processing unit 216 removes the signal at the dead zone position from the signal processed in step S4.

[0053] Next, the interpolation process S6 will be described.

[0054] In the signal processing unit 216, for the measurement object W, the removed signal related to the signal removal in process S5 is linearly interpolated by Fourier transform from the signal values ​​before and after the removed signal, and the linearly interpolated signal is designated as W1(θ), where W1(θ) represents the measurement shape.

[0055] Next, the fitting process S3 will be described.

[0056] In the signal processing unit 216, for a flat workpiece HW with no irregularities, if the signal after processing S1 and S2 is designated as W2(θ), then the signal W2(θ) is fitted with an n-th order function by Fourier transform, and the signal obtained by the fitting is designated as F(θ). F(θ) represents the reference shape and is stored in the signal processing unit 216. In this case, n may be any natural number such that W2(θ) - F(θ) is a linear signal that is infinitely close to 0. Also, instead of an n-th order function, fitting with a circular function, for example, x^2 + y^2 = 1, may be used.

[0057] Finally, the subtraction process S7 will be described.

[0058] The signal processing unit 216 calculates F(θ)-W1(θ), thereby completing the acquisition of the surface shape profile.

[0059] In other words, the shape obtained in advance by Fourier transforming the interference light of a flat workpiece HW with no irregularities in the signal processing unit 216 is stored as a reference shape in the signal processing unit 216. Meanwhile, the interference light obtained from an object W with irregularities as the target object W is Fourier transformed in the signal processing unit 216 to obtain a measured shape. Next, the signal processing unit 216 performs subtraction processing S7 to find the difference between the measured shape and the reference shape, thereby correcting for shape distortion due to the arc scan and obtaining a surface shape profile.

[0060] <Regarding Zero Point Position> FIG. 6 shows a design for the zero point position in the SS-OCT device 201 for obtaining stable signal intensity even when the absolute value of the scanning angle θ increases when the scanning mechanism 214 scans.

[0061] 8, the conventional SS-OCT device does not perform a wide arc scan but performs a linear scan in a localized range, so the zero point position is set between the surface of the measurement object 16 and +LD, thereby using the positive side of the zero-order coherence length where the signal intensity is the highest.

[0062] However, in the SS-OCT device 201 of this embodiment, an arc scan is performed, and the accompanying change in distance to the measurement target object W when the scanning mechanism 214 scans is required up to a coherence length region greater than the measurable range in the depth direction. The zero-point position is set between the surface of the flat workpiece HW without irregularities and +LD. This is designated as the 0th-order arc scan 701. The 0th-order arc scan 701 uses a 0th-order region with high signal strength near the front of the measurement head 209, whereas a +1st-order region with low signal strength is used where the absolute value of the scanning angle θ is large, i.e., where the measurement head 209 is far from the intersection of a line perpendicular to the surface of the flat workpiece HW without irregularities and the flat workpiece HW. In this case, where the absolute value of the scanning angle θ is large, the signal strength decreases due to the larger angle from the measurement head 209 and the greater distance compared to near the front.

[0063] To suppress signal strength reduction at locations where the absolute value of the scanning angle θ is large, the zero-point position, where the signal optical path length of the signal light (measurement light) and the optical path length of the reference light reflected by the reference surface 211 coincide, is set between 2LD and 3LD as a position advanced (or, in other words, moved away) from the surface of the flat, planar workpiece HW (or object W) in the vertical direction opposite the measurement head 209 relative to the surface of the flat, planar workpiece HW (or object W). This is shown in FIG. 6 as a -1st-order arc scan 702. Near the front, the angle is small, so reflected or scattered light from the flat, planar workpiece HW (or object W) is likely to be returned, and the distance is also close, so signal strength reduction is small. In contrast, where the absolute value of the scanning angle θ is large, signal strength reduction is large. Therefore, by positioning the zero-point position at the position of the -1st-order arc scan 702, it is possible to use the 0th-order region at that location, thereby suppressing signal strength reduction.

[0064] In this embodiment, the zero point position is located between 2LD and 3LD on the opposite side of the measurement head 209 from the surface of the flat, planar workpiece HW (or object W). However, this is just one example, and it is important to use the zeroth-order region near both ends of the width of the planar, planar workpiece HW (or object W) to be measured along the scanning direction. Therefore, for example, the zero point position may be set between -3LD and 0 in FIG. 6 . Furthermore, if orders beyond -2nd order are also required, the zero point may be set to a position more negative than -3LD, and both ends may be set in the zeroth-order region. Basically, by moving the position of the reference surface, it is desirable to set the regions near both ends of the scanning direction of the measurement workpiece HW (or object W) within the Nyquist frequency, i.e., within the measurement range, i.e., the zeroth-order coherence region. This setting can suppress a decrease in the signal intensity of the measurement light from near both ends of the object W.

[0065] <Regarding Control of Dead Zone Position> When expanding the measurable range in this embodiment, the arc scan is folded back within the measurement range. At this time, the folding back point becomes a dead zone 801 as shown in FIG. 7, making measurement impossible. Since the signal of the arc scan changes from the solid line to the dotted line in FIG. 7 by moving the zero point position, the zero point position can be moved by moving the reference surface 211 with the stepping motor 210 under the control of the control unit 290, thereby enabling control of the dead zone 801. In this case, control is performed so that the waveform in region E in FIGS. 4 and 5, where the signal can be acquired most stably, always remains at the same position.

[0066] This allows the dead zone position to be shifted each time a signal is acquired continuously, making it possible to perform measurements without the dead zone at a fixed position.

[0067] On the other hand, one factor that can be cited is the expansion and contraction of the fiber due to temperature changes, and this can cause the zero point position to change due to changes in the optical path length, resulting in an unintentional shift in the dead band position. Even if you want to fix the dead band position, this can be done by controlling the dead band position with the stepping motor 210.

[0068] According to the embodiment, in order to avoid attenuating signals above the Nyquist frequency, which are attenuated by an LPF in conventional SS-OCT devices, multiple signals from the object W, including signals within the Nyquist frequency and aliases 302, are taken in, and the aliases 302 are restored to the frequency domain of actual measurements. As a result, measurements are possible even if the change in distance to the measurement target range due to scanning is greater than the measurement range in the depth direction, i.e., the measurement range can be expanded. Therefore, the surface shape of the object W can be measured even if the distance to the object W is greater than the measurable range. Note that, by using arc scanning, multiple signals from the flat workpiece of the object W are separated into signals within the Nyquist frequency and aliases. The term "alias" refers to signals that are aliased around the Nyquist frequency into the measurable range from the zero point because signals above the Nyquist frequency are outside the maximum measurable range. Furthermore, since it is theoretically impossible to distinguish between positive and negative frequencies, negative frequency signals are aliased around the zero point into positive frequency signals.

[0069] It should be noted that any of the various embodiments or modifications described above can be combined appropriately to achieve the effects of each. In addition, combinations of embodiments, combinations of examples, or combinations of embodiments and examples are possible, and combinations of features of different embodiments or examples are also possible.

[0070] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0071] (Technology 1) An optical interference measurement device that measures an object using interference light produced by measurement light and reference light that is emitted from a wavelength scanning light source, is continuously irradiated onto the object in an arc scan, and is reflected from the object, the optical interference measurement device comprising: an optical interference measurement unit having interference light detection means that receives the interference light and generates an interference light signal that indicates the intensity of the interference light; and a signal processing unit that samples the interference light signal to generate a plurality of signals including signals within the Nyquist frequency and aliases, restores the aliases to a frequency domain of actual measured values, and acquires a surface shape profile of the object based on the signals within the Nyquist frequency and the restored signals.

[0072] (Technology 2) The optical interference measurement device according to Technology 1, wherein in the optical interference measurement unit, a zero point position where a signal optical path length of signal light which is the measurement light and an optical path length of reference light reflected at a reference surface coincide with each other is taken at a position on the opposite side of the surface of the object from a measurement head which irradiates the object with the measurement light, and advanced from the surface of the object in a direction perpendicular to the surface of the object, and by moving the position of the reference surface in the arc scan, regions near both ends of the object in the scanning direction are set within the Nyquist frequency.

[0073] (Technology 3) The optical interference measuring device according to Technology 1 or Technology 2, wherein the signal processing unit calculates a reference shape in advance by Fourier transforming interference light of a flat workpiece without irregularities, stores the reference shape, calculates a measured shape by Fourier transforming interference light obtained from an object having irregularities as the target, and calculates a difference between the measured shape and the reference shape, thereby correcting distortion of the shape due to the arc scan and acquiring the surface shape profile.

[0074] (Technology 4) The optical interference measurement device according to any one of Technologies 1 to 3, further comprising a reference surface moving device that changes the optical path length of the reference light by moving the position of a reference surface on which the reference light is reflected, cancels the change in the optical path length difference with the signal optical path length, and keeps the zero point position always constant.

[0075] These configurations allow the SS-OCT device to incorporate multiple signals from the object, including signals within the Nyquist frequency (i.e., below the Nyquist frequency) and aliases, without attenuating signals above the Nyquist frequency, which are attenuated by LPFs in conventional SS-OCT devices, and to restore the aliases to the frequency domain of actual measured values. As a result, measurements can be made even if the change in distance to the measurement object range due to scanning is greater than the measurement range in the depth direction, that is, the measurement range can be expanded. Therefore, the surface shape of the object can be measured even if the distance to the object is greater than the measurable range.

[0076] The optical interference measurement device according to the above aspect of the present disclosure has the feature of being able to measure surface shapes over a wide range over a long distance, and can be applied to applications such as precision measurements in the industrial field.

[0077] REFERENCE SIGNS LIST 10 wavelength scanning light source 11 lens 12 mirror 13 beam splitter 14 lens 15 galvanometer mirror 16 measurement object 17 photodetector 18 amplifier 19 trigger generating unit 20 k trigger generating unit 21 LPF 22 Fourier transform circuit 201 SS-OCT device 202 wavelength scanning light source 203 optical fiber interferometer 204 photodetector 205 wavelength scanning light generating unit 206 trigger generating unit 207 k clock generating unit 208 coupler 209 measuring head 210 stepping motor 211 reference surface 212 collimating lens 213 focus variable collimator 214 scanning mechanism 215 50:50 coupler 216 signal processing unit 290 control unit 291 scanning control unit 301 Signal above Nyquist frequency 302 Alias ​​303 Original signal in arc scan 701 0th order arc scan 702 -1st order arc scan 801 Dead zone L Distance to measurement object W LD Measurable range W Measurement object HW Flat workpiece with no irregularities θ Scanning angle

Claims

1. An optical interference measurement apparatus that measures an object by using interference light formed by measurement light emitted from a wavelength scanning light source and continuously irradiated onto the object by circular arc scanning and reference light reflected from the object, the apparatus comprising: an optical interference measurement unit having interference light detection means that receives the interference light and generates an interference light signal indicating the intensity of the interference light; and a signal processing unit that samples the interference light signal to generate a plurality of signals including a signal within the Nyquist frequency and an alias, restores the alias to the frequency domain of the measured value, and acquires a surface shape profile of the object based on the signal within the Nyquist frequency and the restored signal.

2. In the optical interference measurement unit, a zero point position where the signal optical path length of the signal light, which is the measurement light, matches the optical path length of the reference light reflected from the reference surface is set at a position that advances from the surface of the object in a direction perpendicular to the surface of the object, on the side opposite to the measurement head that irradiates the object with the measurement light with respect to the surface of the object. In the circular arc scan, by moving the position of the reference surface, regions near both ends in the scanning direction of the object are set within the Nyquist frequency. The optical interference measurement apparatus according to claim 1.

3. The signal processing unit: previously calculates a reference shape by Fourier-transforming the interference light of a flat workpiece having no unevenness; stores the reference shape; calculates a measurement shape by Fourier-transforming the interference light obtained from an object having unevenness as the object; and acquires the surface shape profile by correcting the shape distortion caused by the circular arc scan by calculating the difference between the measurement shape and the reference shape. The optical interference measurement apparatus according to claim 1 or 2.

4. The optical interference measurement apparatus according to claim 2, further comprising a reference surface moving device that changes the optical path length of the reference light by moving the position of the reference surface, cancels the change in the optical path length difference from the signal optical path length, and always keeps the zero point position constant.

Images