Measurement device

Abstract

To provide a measurement device capable of improving measurement accuracy by more accurately changing a parameter of light such as a phase in object measurement by light.SOLUTION: A measurement device includes a light source configured to emit light for irradiating an object, an optical system including a phase modulator configured to change a phase of at least a part of the light, a signal generation circuit configured to generate a digital signal, a digital-analog conversion circuit configured to convert the digital signal into an analog signal and output the analog signal, an amplitude changing circuit configured to adjust an amplitude of the analog signal by a gain to generate a drive signal, a drive circuit configured to drive the phase modulator based on the drive signal, and a signal line electrically connected to the amplitude changing circuit and configured to control the gain.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a measurement device. [Background technology]

[0002] Optical interferometry using laser light is widely used as a method for obtaining information about the distance and / or shape of an object without contact. Optical interferometry enables measurements with nanometer accuracy by precisely controlling the phenomenon of optical interference. For example, Michelson interferometry using single-wavelength laser light is one method for measuring distance differences on the nanometer scale as light intensity.

[0003] A typical example of homodyne optical interferometry, which allows for non-contact and highly accurate measurements, is the phase shift method, which performs optical measurements with nanometer accuracy, as disclosed in Non-Patent Documents 1 and 2. In the phase shift method, in an optical system that generates interference light between object light and reference light, the phase of the object light is estimated based on the intensities of multiple interference lights obtained by, for example, changing the phase of the reference light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-008475 [Non-patent literature]

[0005] [Non-Patent Document 1] P. Carre, Metrologia, vol 2 1, 1966. [Non-patent document 2] A.Dubois, J.Opt.Soc.Am.A, vol 18 8, 2002. Summary of the Invention [Problem to be solved by the invention]

[0006] The present disclosure provides a measurement device that can improve measurement accuracy in optical object measurement by more accurately changing optical parameters such as phase. [Means for solving the problem]

[0007] A measurement device according to one aspect of the present disclosure includes a light source that emits light to illuminate an object, an optical system including a phase modulator that changes the phase of at least a portion of the light, a signal generation circuit that generates a digital signal, a digital-to-analog conversion circuit that converts the digital signal into an analog signal and outputs it, an amplitude change circuit that adjusts the amplitude of the analog signal using a gain to generate a drive signal, a drive circuit that drives the phase modulator based on the drive signal, and a signal line electrically connected to the amplitude change circuit, the signal line being for controlling the gain.

[0008] A general or specific aspect of the present disclosure may be realized as a system, an apparatus, a method, an integrated circuit, a computer program, or a recording medium such as a computer-readable recording disk, or as any combination of a system, an apparatus, a method, an integrated circuit, a computer program, and a recording medium. A computer-readable recording medium may include a non-volatile recording medium such as a CD-ROM (Compact Disc-Read Only Memory). An apparatus may consist of one or more devices. When an apparatus consists of two or more devices, the two or more devices may be located in a single device or may be located separately in two or more separate devices. In this specification and claims, the term "apparatus" may refer not only to a single device but also to a system consisting of multiple devices. A "system" may include devices installed in remote locations away from other devices and connected via a communication network. [Effects of the Invention]

[0009] According to the technology of the present disclosure, it is possible to realize a measurement device that can improve measurement accuracy in optical object measurement by changing optical parameters such as phase more accurately. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram schematically illustrating the configuration of a measurement device according to an exemplary embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram showing an example of the time change of a clock signal generated in a processing circuit and the time change of a drive voltage applied to a phase modulator. [Figure 3] FIG. 3 is a graph showing an example of the relationship between the intensity of the interference light and the phase of the reference light. [Figure 4] FIG. 4 is a block diagram showing a schematic configuration of a processing circuit included in a measurement device of a comparative example. [Figure 5] FIG. 5 is a block diagram showing a schematic configuration example of a processing circuit included in the measurement device according to this embodiment. [Figure 6] FIG. 6 is a diagram illustrating a configuration example of the amplitude changing circuit. [Figure 7] FIG. 7 is a graph schematically showing the relationship between the amplitude of the drive signal and the SN ratio in this embodiment. [Figure 8] FIG. 8 is a flowchart showing an example of a measurement operation executed by the processing circuit in this embodiment. [Figure 9] FIG. 9 is a flowchart showing an example of a driving operation of a phase modulator executed by the processing circuit in this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this disclosure, all or part of a circuit, unit, device, component, or part, or all or part of a functional block in a block diagram, may be implemented by one or more electronic circuits, including, for example, a semiconductor device, a semiconductor integrated circuit (IC), or an LSI (large scale integration). An LSI or IC may be integrated on a single chip or may be configured by combining multiple chips. For example, functional blocks other than memory elements may be integrated on a single chip. While the terms LSI and IC are used here, the term may be changed depending on the degree of integration, and may be referred to as a system LSI, a VLSI (very large scale integration), or an ULSI (ultra large scale integration). A field programmable gate array (FPGA), which is programmable after LSI fabrication, or a reconfigurable logic device, which can reconfigure connections within an LSI or set up circuit partitions within an LSI, may also be used for the same purpose.

[0012] Furthermore, all or part of the functions or operations of a circuit, unit, device, component, or section can be implemented by software processing. In this case, the software is recorded on one or more non-transitory recording media such as ROMs, optical disks, hard disk drives, etc., and when the software is executed by a processor, the functions specified in the software are performed by the processor and peripheral devices. A system or device may include one or more non-transitory recording media on which software is recorded, a processor, and necessary hardware devices, such as interfaces.

[0013] In this disclosure, "light" refers to electromagnetic waves including not only visible light (wavelength of about 400 nm to about 700 nm), but also ultraviolet light (wavelength of about 10 nm to about 400 nm) and infrared light (wavelength of about 700 nm to about 1 mm). In this specification, ultraviolet light may be referred to as "ultraviolet light," and infrared light may be referred to as "infrared light."

[0014] (Embodiment) [Measuring equipment] First, an example configuration of a measurement device according to an embodiment of the present disclosure will be described with reference to Fig. 1. Fig. 1 is a block diagram that schematically illustrates the configuration of a measurement device according to an exemplary embodiment of the present disclosure. Fig. 1 also illustrates an object 10 to be measured. The measurement device 100 shown in Fig. 1 measures information related to the distance and / or shape of the object 10 using a phase shift method.

[0015] As shown in FIG. 1, the measurement device 100 includes a light source 20, an interference optical system 30, a photodetector 40, and a processing circuit 50. The interference optical system 30 includes lenses 32a, 32b, and 32c, a beam splitter 34, a mirror 36, and a phase modulator 38. In this specification, the interference optical system 30 is also referred to simply as the "optical system." The thick solid, dashed, and dotted lines with arrows in FIG. 1 represent the flow of light with a divergence. The thin lines with arrows in FIG. 1 represent the flow of signals, and the thick curves in FIG. 1 represent optical fibers. When the focal point of lens 32a is located on the light emission surface of the light source 20 and the focal point of lens 32c is located on the light detection surface of the photodetector 40, optical fibers are not necessarily required.

[0016] The light source 20 emits laser light 20L of a single wavelength λ through an optical fiber. The thick solid line with an arrow in FIG. 1 represents the laser light 20L. The light source 20 may include a light-emitting element such as a distributed feedback (DFB) laser diode, an external cavity (EC) laser diode, or a vertical cavity surface-emitting (VCSEL) laser diode. These laser diodes are inexpensive, compact, and capable of single-mode oscillation.

[0017] The interference optical system 30 converts the laser light 20L into the object light 20L as follows.О and reference light 20L R The object beam 20L is branched and returned. О and reference light 20L R Interference light 20L I The thick dashed line with an arrow in Figure 1 indicates the object beam 20L. О The thick dotted line with an arrow in Figure 1 represents the reference beam 20L. R In this specification, the reference light 20L R is also called the "first light" and the object light 20L О The laser beam 20L is also referred to as the "second light." О Therefore, it can be said that the laser light 20L is light for irradiating the object 10.

[0018] The laser light 20L emitted from the light source 20 through an optical fiber is collimated by the lens 32a and enters the beam splitter 34, which then splits the object light 20L О and reference light 20L R Object beam 20L О is focused by the lens 32b, enters the object 10, is reflected by the object 10, is collimated by the lens 32b, and returns to the beam splitter 34. R passes through the phase modulator 38, is reflected by the mirror 36, passes through the phase modulator 38 again, and returns to the beam splitter 34. О and reference light 20L R are superimposed by the beam splitter 34 to form an interference light 20L I Interference light 20L I is collected by lens 32c.

[0019] The phase modulator 38 receives the reference light 20L, which is a part of the laser light 20L. R The phase modulator 38 may include, for example, an electro-optic material whose refractive index changes with application of a drive voltage. By changing the drive voltage over time in a sawtooth, triangular, or sinusoidal waveform, the refractive index of the electro-optic material also changes over time. As a result, the reference beam 20L passing through the electro-optic material RThe phase of the interference light 20L changes over time. I The intensity also changes with time.

[0020] Alternatively, the phase modulator 38 may change the position of the mirror 36 with nanometer precision by applying a drive voltage to the mirror 36, thereby adjusting the reference beam 20L from the beam splitter 34 to the mirror 36. R The optical path length of the reference light 20L may be changed by a piezoelectric element. R The phase of also changes over time.

[0021] The photodetector 40 detects the interference light 20L via an optical fiber. I Detects the interference light 20L I The photodetector 40 outputs a signal according to the intensity of the light. The photodetector 40 includes one or more photodiodes. The photodetector 40 may include a preamplifier that amplifies the signal. When the photodetector 40 includes multiple photodiodes arranged two-dimensionally, information about the shape of the object 10 can be obtained based on the signal output from the photodetector 40.

[0022] The processing circuitry 50 controls the operation of the light source 20 and the phase modulator 38. The processing circuitry 50 also performs processing operations based on the signal output from the photodetector 40.

[0023] As will be explained in detail later, in the measurement device 100 according to this embodiment, the processing circuit 50 includes a circuit configuration that improves the measurement accuracy in the phase shift method. The measurement accuracy in the phase shift method is length measurement accuracy. Length measurement accuracy indicates the degree of accuracy when measuring distance. Length measurement accuracy is also a measure of how accurately a distance was measured, and the higher the length measurement accuracy, the more accurately the distance can be measured.

[0024] [Principle of the phase shift method] Next, the principle of the phase shift method for performing optical measurement with nanometer accuracy will be explained. О The optical path length is L О , the reference beam 20L from the beam splitter 34 to the mirror 36R The optical path length is L R Then, the object light is 20L О The phase of is φ О =(2π / λ)(2L O ) and the reference beam is 20L. R The phase of is φ R =(2π / λ)(2L R ) The interference light 20L detected by the photodetector 40 I The intensity I of the light is expressed by the following formula (1): In formula (1), A and B are coefficients.

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[0025] Reference light 20L R Phase φ of R can be changed by applying a drive voltage to the phase modulator 38. As a result, the object beam 20L О and reference light 20L R The optical path length difference between the reference light 20L and the laser light 20L varies. It is known that the optical path length difference to be varied within the interference optical system 30 may be about half a period to one period with respect to the wavelength λ of the laser light 20L, depending on the phase estimation algorithm. R Phase φ of R The driving voltage applied to the phase modulator 38 is adjusted so that the amount of change in the reference light 20L corresponds to the optical path length difference. R Phase φ of R The change in the reference light is 20L. R Phase φ of R means the amount obtained by subtracting the minimum value from the maximum value.

[0026] FIG. 2 is a diagram showing an example of the time change of the clock signal generated in the processing circuit 50 and the time change of the drive voltage applied to the phase modulator 38. In the example shown in FIG. 2, each time a clock signal is generated, the drive voltage increases from 0 V to a predetermined amplitude. Therefore, the drive voltage changes over time in a sawtooth waveform. As the drive voltage changes over time, the reference light 20L R Phase φ of RIt changes over time like a sawtooth wave, with one period of the sawtooth wave corresponding to one measurement.

[0027] Figure 3 shows the interference light 20L I Intensity I and reference light 20L R Phase φ of R 3 is a graph showing an example of the relationship between the reference light 20L and the amplitude of the driving voltage when the amplitude of the driving voltage is 2 V. R Phase φ of R Therefore, as shown in FIG. 3, when the amplitude of the driving voltage is 2 V, the interference light 20L I The intensity I of the interference light 20L changes by one period. When the amplitude of the driving voltage is 6V, I The intensity I of the wave changes over three periods.

[0028] 4-step algorithm Object light 20L О Phase φ of О An example of a method for estimating is the four-step algorithm disclosed in Non-Patent Document 1. In this algorithm, a reference beam 20L spaced at equal intervals is R The four phases φ R Four interference lights at 20L I The intensity I of the four interfering lights 20L is obtained. I Based on the intensity of the object beam 20L О Phase φ of О Specifically, the following is estimated:

[0029] Phase φ of reference light 20LR R By changing the value of 2α at regular intervals, such as 3α, α, -α, and -3α, the interference light 20L I The intensity I of the interference light 20L is obtained at four points. I Intensities I1 to I4 are obtained. In formula (2), α is equal to or smaller than π / 2.

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[0030] Interference light 20LI Based on the intensities I1 to I4, the object beam 20L О Phase φ of О are expressed by the following equations (3) and (4): u=I2-I3, and m=(I2+I3)-(I1+I4).

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[0031] Estimated object light 20L О Phase φ of О from the optical path length L O , it is possible to obtain information about the distance and / or shape of the object 10.

[0032] In addition, interference light 20L I Obtain the intensity of the point and reference light 20L R Phase φ of R The interval is not limited to the above example, and varies depending on the algorithm.

[0033] 4-bucket algorithm Object light 20L О Phase φ of О Another example of a method for estimating the reference beam 20L is the 4-bucket algorithm disclosed in Non-Patent Document 2. In this algorithm, R Phase φ of R Interfered light 20L in four consecutive equally spaced sections I The intensity I is integrated to calculate four areas, and the object light 20L is calculated based on these four areas. О Phase φ of О is estimated.

[0034] The number of areas and the integration intervals corresponding to each area are not limited to the above example, but differ depending on the algorithm.

[0035] [Configuration of the processing circuit included in the measurement device of the comparative example and its problems] Before describing the configuration of the processing circuit 50 included in the measurement device 100 according to this embodiment, the configuration of the processing circuit included in a measurement device of a comparative example will be described with reference to FIG. 4. FIG. 4 is a block diagram that schematically shows the configuration of the processing circuit included in the measurement device of the comparative example. FIG. 4 also shows the light source 20, phase modulator 38, and photodetector 40. The measurement device of the comparative example has the same components as the measurement device 100 according to this embodiment, except for the processing circuit.

[0036] 4 includes a control circuit 51, a signal generating circuit 52, a digital-to-analog (DA) conversion circuit 53, a low-pass filter 54, a driving circuit 55, an AD conversion circuit 56, and a signal processing circuit 57. The configuration including the signal generating circuit 52, the DA conversion circuit 53, the low-pass filter 54, and the driving circuit 55 is a conventional circuit configuration as disclosed in, for example, Patent Document 1.

[0037] The control circuit 51 controls the operation of the signal generation circuit 52. The signal generation circuit 52 receives a control signal from the control circuit 51 and generates a digital signal forming a waveform such as a sawtooth wave, a triangular wave, or a sine wave. The signal generation circuit 52 may be, for example, an FPGA. The DA conversion circuit 53 converts the digital signal into an analog signal and outputs it. The analog signal is a voltage signal. The low-pass filter 54 removes high-frequency components from the analog signal output from the DA conversion circuit 53 to generate a drive signal. The drive circuit 55 drives the phase modulator 38 based on the drive signal. Specifically, the drive circuit 55 applies a drive voltage to the phase modulator 38 based on the drive signal. The phase modulator 38 adjusts the reference light 20L in accordance with the drive voltage. R Phase φ of R The four bubbles in Figure 4 show examples of time-dependent changes in the digital signal, analog signal, drive signal, and drive voltage.

[0038] The control circuit 51 further controls the operations of the light source 20 and the signal processing circuit 57. The light source 20 receives a control signal from the control circuit 51 and emits a laser beam 20L. The signal processing circuit 57 receives a control signal from the control circuit 51 and converts the object beam 20L into a signal based on the signal output from the photodetector 40 and obtained via the AD conversion circuit 56. О Phase φ of О The AD conversion circuit 56 converts the analog signal output from the photodetector 40 into a digital signal and outputs it. The signal processing circuit 57 further acquires information about the distance and / or shape of the object 10 based on the estimation result. The acquired information can be displayed, for example, on a separately prepared display device.

[0039] As a result of examining the conventional circuit configuration disclosed in Patent Document 1, the inventors of the present invention found that the smaller the amplitude of the analog signal, the lower the measurement accuracy. This is because the smaller the amplitude of the analog signal, the larger the voltage fluctuation caused by noise generated by DA conversion becomes relative to the amplitude. As a result, the signal-to-noise ratio of the drive signal decreases. In the phase shift method, for example, in a four-step algorithm, the reference light 20L R Phase φ of R Since it is assumed that =3α, α, -α, and -3α are known, noise occurring in the drive signal affects the measurement accuracy.

[0040] An example of noise is quantization error. In DA conversion, a digital signal is composed of a finite number of levels and a clock, so a stepped voltage is generated as an analog signal, as shown in Figure 4. Quantization error refers to the difference between the linear voltage of an ideal analog signal and the stepped voltage of an actual analog signal.

[0041] The measurement accuracy in the phase shift method depends on the smoothness of the analog signal. R Phase φ of R Since the value of the interfering light 20L changes over time, the step-like shape of the analog signal is I This is because it affects the time change of the intensity I of the

[0042] The higher the bit depth, which is the resolution of the DA conversion circuit 53, the smaller the LSB, which is the analog voltage value per level in the digital signal, can be. Therefore, the quantization error can be reduced, and the analog signal becomes smoother.

[0043] For example, if the bit depth is 12 bits and the analog signal range is 0 to 2 V, i.e., 12 bits / 2 V, the LSB is approximately 5.0 x 10 -4 V. In the case of 10 bits / 2 V, the LSB is approximately 2.0 × 10 -3 Therefore, the quantization error can be reduced in the case of 12 bits / 2V compared to the case of 10 bits / 2V, resulting in a smoother analog signal.

[0044] Since the amplitude of the digital signal output from the signal generating circuit 52 and the bit depth of the DA conversion circuit 53 are fixed values, hardware elements capable of outputting an analog signal with the smallest quantization error are selected depending on the application.

[0045] At this time, not only the application, the phase estimation algorithm, the type of optical element, and the wavelength λ of the laser light 20L but also the reference light 20L R Phase φ of R If the amplitude and bit depth of the digital signal are changed according to the various conditions, it becomes necessary to modify the hardware elements each time.

[0046] For example, in a circuit configuration in which a 4V amplitude digital signal output from a signal generating circuit 52 is converted into an analog signal by a 14-bit DA conversion circuit 53, changing the amplitude of the digital signal from 4V to 2V results in a 13-bit / 2V signal. In this case, only a portion of the bit depth of the DA conversion circuit 53 can be used. Therefore, in the case of 13-bit / 2V, the quantization error is larger than in the case of 14-bit / 2V.

[0047] Other examples of noise include differential nonlinearity error and integral nonlinearity error. The step voltage generated by ideal DA conversion is expressed as an integer multiple of the LSB. However, slight fluctuations occur in actual step voltages. Differential nonlinearity error refers to this fluctuation. Integral nonlinearity error refers to the difference between the ideal voltage and the actual voltage in the entire signal waveform, and corresponds to the accumulation of differential nonlinearity error.

[0048] The inventors discovered a problem in that the smaller the amplitude of the analog signal, the lower the measurement accuracy due to the noise described above, and came up with the measurement device 100 according to this embodiment, which solves this problem. The measurement device 100 according to this embodiment has a circuit configuration in the processing circuit 50 that can improve measurement accuracy even when the amplitude of the analog signal is small.

[0049] [Configuration of the processing circuit 50 included in the measurement device 100 according to this embodiment] Next, with reference to FIG. 5, an example configuration of the processing circuit 50 included in the measurement device 100 according to this embodiment will be described. FIG. 5 is a block diagram schematically showing an example configuration of the processing circuit 50 included in the measurement device 100 according to this embodiment. The processing circuit 50 shown in FIG. 5 includes a control circuit 51, a signal generation circuit 52, a DA conversion circuit 53, a low-pass filter 54, a drive circuit 55, an AD conversion circuit 56, a signal processing circuit 57, an amplitude modification circuit 58, and a signal line 59. The components other than the amplitude modification circuit 58 and the signal line 59 are as described with reference to FIG. 4. In this specification, the operations performed by each circuit included in the processing circuit 50 may be described as operations performed by the processing circuit 50.

[0050] The processing circuit 50 shown in Fig. 5 differs from the processing circuit 49 shown in Fig. 4 in that an amplitude change circuit 58 is provided between the DA conversion circuit 53 and the drive circuit 55, and a signal line 59 electrically connects the control circuit 51 and the amplitude change circuit 58. In the example shown in Fig. 5, the amplitude change circuit 58 is provided between the DA conversion circuit 53 and the low-pass filter 54, but this example is not limiting. The amplitude change circuit 58 may also be provided between the low-pass filter 54 and the drive circuit 55. The low-pass filter 54 may be omitted.

[0051] In the measurement device 100 according to this embodiment, with some exceptions described below, the amplitude of the analog signal generated by the DA conversion circuit 53 is fixed so as to be close to the maximum value. In this specification, unless otherwise specified, the amplitude of the analog signal generated by the DA conversion circuit 53 is simply referred to as the "amplitude of the analog signal." The amplitude of the analog signal may be, for example, 0.8 to 1.0 times the maximum value. The closer the amplitude of the analog signal is to the maximum value, the more the aforementioned noise influence on measurement accuracy can be reduced. When the amplitude of the analog signal is maximum, the influence of noise is lowest.

[0052] In the measurement device 100 according to this embodiment, the amplitude change circuit 58 adjusts the amplitude of the analog signal by a gain. The control circuit 51 controls the gain via a signal line 59. The two balloons shown in FIG. 5 represent the time changes of the analog signal before adjustment input to the amplitude change circuit 58 and the analog signal after adjustment output from the amplitude change circuit 58. By adjusting the amplitude of the analog signal by a gain, the amplitude of the drive signal can be changed, and the reference light 20L R Phase φ of R The gain is adjustable. R Phase φ of R It depends on the amount of change in

[0053] Therefore, the S / N ratio of the drive signal can be optimized according to the situation without the need to modify hardware elements. For example, if an analog signal with an amplitude of 4 V and a bit depth of 14 bits is output from the DA conversion circuit 53, even if the amplitude is changed to 2 V or 3 V by the amplitude change circuit 58, the bit depth remains at 14 bits.

[0054] Next, a configuration example of the amplitude change circuit 58 will be described with reference to Fig. 6. Fig. 6 is a diagram schematically illustrating a configuration example of the amplitude change circuit 58. The amplitude change circuit 58 illustrated in Fig. 6 is a general amplifier circuit capable of adjusting gain. As illustrated in Fig. 6, the amplitude change circuit 58 includes an operational amplifier 58a and a digital potentiometer 58b. The operational amplifier 58a includes a positive terminal which is a grounded non-inverting input terminal, and a negative terminal which is an inverting input terminal electrically connected to the digital potentiometer 58b.

[0055] The digital potentiometer 58b determines the gain as follows: The digital potentiometer 58b digitally controls the position of a wiper electrically connected to the negative terminal of the operational amplifier 58a based on a signal from the control circuit 51 via a signal line 59. With the wiper position as a reference, the input voltage V in The value of the resistor on the side is R1, and the output voltage V out The value of the resistor on the side is R2. These resistors are variable resistors. R1 and R2 change depending on the wiper position.

[0056] The operational amplifier 58a operates so that the voltage difference between the positive and negative terminals becomes zero. in / R1+V out The relationship / R2=0 holds, and V out =-(R2 / R1)V in R2 / R1 corresponds to the gain. The gain may be greater than 1, equal to 1, or less than 1. In this specification, the gain is defined as |V out | / |V in | and has a positive value.

[0057] The method for adjusting the gain is not limited to the above example. For example, instead of using the digital potentiometer 58b, a separately prepared FPGA and the operational amplifier 58a may be electrically connected via a signal line 59, and the user may adjust the gain by controlling the operation of the FPGA from a PC.

[0058] Unlike the measurement device 100 according to this embodiment, the reference light 20L is generated by the conventional circuit configuration in the processing circuit 49 shown in FIG. R Phase φ of R When changing the reference light 20L, the amplitude of the analog signal is changed via a digital signal by the signal generating circuit 52. Specifically, by increasing the amplitude of the analog signal, R Phase φ of R In addition, by reducing the amplitude of the analog signal, the amount of change in the reference light 20L R Phase φ of R The amount of change in the analog signal is reduced. However, as mentioned above, if the amplitude of the analog signal is small, the signal-to-noise ratio of the drive signal is reduced, resulting in a decrease in measurement accuracy. In this specification, "changing the amplitude of the analog signal via a digital signal by the signal generation circuit 52" means that the signal generation circuit 52 changes the digital signal, resulting in a change in the amplitude of the analog signal generated by the DA conversion circuit 53.

[0059] In contrast, in the measurement device 100 according to this embodiment, the amplitude of the analog signal is fixed so as to be close to the maximum value, so that the voltage fluctuation caused by the noise generated by the DA conversion can be made smaller relative to the amplitude. Therefore, the signal-to-noise ratio of the drive signal obtained by combining the amplitude and gain of the analog signal is improved. As a result, the reference light 20L R Phase φ of R It is possible to more accurately change the value of the saturation voltage to improve measurement accuracy.

[0060] For example, in the above-described four-step algorithm and four-bucket algorithm in the phase shifting method, the measurement accuracy can be improved as follows.

[0061] 4-step algorithm Obtained interference light 20L I In principle, if the number of points of the intensity I is four, there is no problem even if the resolution of the DA conversion circuit 53 is 2 bits. I The signal showing the intensity I of the signal is superimposed with random noise, and (2) there is a problem that it is not easy to analyze the measurement error using only four points.

[0062] Regarding (1), the interference light 20L before and after I The influence of random noise can be suppressed by taking several points of intensity I and using a moving average. I By smoothing the overall intensity I, analysis such as coherence becomes possible. Therefore, by increasing the resolution of the analog signal and reducing the quantization error, a smooth interference light 20L I If we can obtain a signal that indicates the intensity I of the signal, we can solve problems (1) and (2).

[0063] As described above, by optimizing the smoothness of the analog signal, the moving average can be used to reduce noise and improve measurement accuracy.

[0064] 4-bucket algorithm Interference light 20L I Ideally, the intensity I of the reference beam 20L is R Phase φ of R However, in reality, the analog signal becomes stepped as shown in Figure 4, so the interference light 20L I If the resolution of the DA conversion circuit 53 is low, the intensity I of the interference light 20L I The deviation from the ideal smooth change in intensity I becomes larger.

[0065] In this algorithm, the interference light 20L ISince it is assumed that the intensity I of the analog signal changes smoothly, the accuracy of phase estimation improves as the resolution of the DA conversion circuit 53 increases. Therefore, by optimizing the smoothness of the analog signal, it becomes possible to improve the measurement accuracy.

[0066] [Relationship between drive signal amplitude and SNR] Next, the relationship between the amplitude of the drive signal and the SN ratio in the measurement device 100 according to this embodiment will be described with reference to Fig. 7. Fig. 7 is a graph that schematically shows the relationship between the amplitude of the drive signal and the SN ratio in this embodiment.

[0067] Point A in Figure 7 represents the state where the analog signal has the maximum amplitude and the minimum gain. Point B in Figure 7 represents the state where the analog signal has the maximum amplitude and the maximum gain. Point C in Figure 7 represents the state where the analog signal has the minimum amplitude and the maximum gain. Point D in Figure 7 represents the state where the analog signal has the minimum amplitude and the minimum gain. "Signal A" in Figure 7 represents the analog signal. The rectangles ABCD in Figure 7 represent the range of drive signal amplitude and S / N that can be achieved by adjusting the analog signal amplitude and gain.

[0068] As mentioned above, the application, phase estimation algorithm, type of optical element, wavelength λ of the laser light 20L, as well as the reference light 20L depending on the development stage, R Phase φ of R In the measurement apparatus 100 according to this embodiment, the amplitude of the analog signal is fixed to the maximum on the line segment AB indicated by the thick solid line, and the gain is changed by the amplitude change circuit 58 to change the reference light 20L. R Phase φ of R In other words, the desired amplitude of the drive signal corresponding to the desired amount of change in the reference light 20L is realized. R Phase φ of RWhen the amount of change in the drive signal is changed from the first amount of change to the second amount of change, the amplitude of the analog signal is the same for the first amount of change and the second amount of change, but the gain is different for the first amount of change and the second amount of change. Unlike the measurement device 100 according to this embodiment, it is also possible to realize a desired amplitude of the drive signal by changing the amplitude of the analog signal via a digital signal by the signal generation circuit 52 on the line segment BC represented by the thin solid line, with the gain fixed at the maximum.

[0069] Comparing the two, as shown in Figure 7, the S / N ratio of the drive signal when the amplitude of the drive signal is adjusted on line segment AB is higher than the S / N ratio of the drive signal when the amplitude of the drive signal is adjusted on line segment BC. In the former case, the S / N ratio of the drive signal can be maintained at a high value even if the amplitude of the drive signal is small. This is because the S / N ratio of the drive signal depends on the amplitude of the analog signal, but not on the gain. In range α shown in Figure 7, the S / N ratio of the drive signal is highest on line segment AB. Range α is the range where the amplitude of the drive signal is greater than the value obtained by multiplying the maximum amplitude of the analog signal by the minimum gain, and less than the value obtained by multiplying the maximum amplitude of the analog signal by the maximum gain.

[0070] However, if the amplitude of the desired drive signal is too small, it may not be possible to realize the desired amplitude of the drive signal on the line segment AB. In this case, the measurement device 100 according to this embodiment realizes the desired amplitude of the drive signal on the line segment AD represented by the thick dashed line by changing the amplitude of the analog signal via a digital signal using the signal generation circuit 52, with the gain fixed to a minimum. In other words, the reference light 20L R Phase φ of R When the amount of change in the gain changes from the first amount of change to the second amount of change, and the second amount of change is smaller than the amount of change when the gain is set to the minimum value, the signal generating circuit 52 changes the digital signal. Unlike the measuring device 100 according to this embodiment, it is also possible to realize a desired amplitude of the drive signal by changing the gain using the amplitude changing circuit 58 on the line segment CD represented by the thin dashed line, while keeping the amplitude of the analog signal fixed at the minimum.

[0071] Comparing the two, as shown in Figure 7, the S / N ratio of the drive signal when the amplitude of the drive signal is adjusted on line segment AD is higher than the S / N ratio of the drive signal when the amplitude of the drive signal is adjusted on line segment CD. In the former case, the S / N ratio of the drive signal increases the further away from point D. In range β shown in Figure 7, the S / N ratio of the drive signal is highest on line segment AD. Range β is the range where the amplitude of the drive signal is equal to or greater than the value obtained by multiplying the minimum amplitude value of the analog signal by the minimum gain value, and is equal to or less than the value obtained by multiplying the maximum amplitude value of the analog signal by the minimum gain value.

[0072] From the above, in the measurement device 100 according to this embodiment, the reference light 20L R Phase φ of R In order to achieve the desired amount of change in the amplitude of the drive signal on the line segments AB and AD, the amplitude of the drive signal is adjusted. As a result, the signal-to-noise ratio of the drive signal is improved and a decrease in measurement accuracy can be suppressed compared to when the amplitude of the drive signal is adjusted on the line segments BC and CD.

[0073] If both the first and second change amounts can be achieved by adjusting the amplitude of the drive signal on the line segment AB, then both can be achieved while maintaining a high S / N ratio for the drive signal. There are cases where one of the first and second change amounts can be achieved by adjusting the amplitude of the drive signal on the line segment AB, but the other change amount cannot be achieved by adjusting the amplitude of the drive signal on the line segment AB. Even in such cases, the other change amount can be achieved by adjusting the amplitude of the drive signal on the line segment AD, so that the other change amount can be achieved while maintaining a relatively high S / N ratio for the drive signal. Even when both the first and second change amounts are achieved by adjusting the amplitude of the drive signal on the line segment AD, both can be achieved while maintaining a relatively high S / N ratio for the drive signal.

[0074] When the first and second change amounts are achieved by adjusting the amplitude of the drive signal on line segment AB, the gain is different in both modes. Even if one of the first and second change amounts is achieved by adjusting the amplitude of the drive signal on line segment AB and the other change amount is achieved by adjusting the amplitude of the drive signal on line segment AD, the gain is different for the first and second change amounts unless one of the change amounts is achieved at point A. In either case, it can be said that the gain differs depending on the first and second change amounts. However, when both the first and second change amounts are achieved by adjusting the amplitude of the drive signal on line segment AD, the gain is the same regardless of the first and second change amounts.

[0075] At points A and B, the amplitude of the analog signal does not need to be maximum, and may be, for example, 0.8 to 1.0 times its maximum value. At points C and D, the amplitude of the analog signal does not need to be minimum, and may be, for example, 1.0 to 1.2 times its minimum value. At points B and C, the gain does not need to be maximum, and may be, for example, 0.8 to 1.0 times its maximum value. At points A and D, the gain does not need to be minimum, and may be, for example, 1.0 to 1.2 times its minimum value.

[0076] [Example of measurement operation] Next, an example of the measurement operation performed by the processing circuit 50 will be described with reference to Fig. 8. Fig. 8 is a flowchart showing an example of the measurement operation performed by the processing circuit 50 in this embodiment. The processing circuit 50 performs the operations of steps S101 to S104 shown in Fig. 8.

[0077] <Step S101> The processing circuit 50 causes the light source 20 to emit the laser light 20L.

[0078] <Step S102> The processing circuit 50 outputs the reference light 20L to the phase modulator 38. R Phase φ of R is varied over time, for example, in the form of a sawtooth wave, a triangular wave, or a sine wave.

[0079] <Step S103> The processing circuit 50 calculates the object light 20L based on the signal output from the photodetector 40. О Phase φ of О Specifically, the interference light 20L shown in FIG. I Intensity I and reference light 20L R Phase φ of R From the relationship between the object beam 20L and the phase, a phase estimation algorithm such as the 4-step algorithm or the 4-bucket algorithm is used. О Phase φ of О is estimated.

[0080] <Step S104> The processing circuit 50 obtains information about the distance and / or shape of the object 10 based on the estimation result.

[0081] By the above measurement operation, information about the distance and / or shape of the object 10 can be obtained more accurately.

[0082] [Example of driving operation of phase modulator 38] Next, an example of the driving operation of the phase modulator 38 executed by the processing circuit 50 in step S102 shown in Fig. 8 will be described with reference to Fig. 9. Fig. 9 is a flowchart showing an example of the driving operation of the phase modulator 38 executed by the processing circuit 50 in this embodiment. A control circuit 51 included in the processing circuit 50 executes the operations of steps S201 to S206 shown in Fig. 9.

[0083] <Step S201> The control circuit 51 controls the reference light 20L R Phase φ of R The amount of change is determined according to conditions such as the wavelength λ of the laser light 20L, the shape of the object 10, and the phase estimation algorithm. The amount of change may be, for example, a half period or one period of the wavelength λ. The amount of change may be determined by a user or by a dedicated determination algorithm.

[0084] <Step S202> The control circuit 51 stores the reference light 20L from the following memory. R Phase φ of R The measurement device 100 according to this embodiment reads out the set value of the amplitude of the drive signal according to the amount of change in the reference light 20L. R Phase φ of R The drive signal amplitude setting value corresponding to the amount of change in the drive signal is stored in a memory.

[0085] <Step S203> The control circuit 51 determines whether the set value of the amplitude of the drive signal is greater than the value obtained by multiplying the maximum value of the amplitude of the analog signal by the minimum value of the gain. This value corresponds to the value on the horizontal axis of point A in FIG. 9. If the determination is Yes, the control circuit 51 performs the operation of step S204. If the determination is No, the control circuit 51 performs the operation of step S205.

[0086] <Step S204> The control circuit 51 determines the amplitude of the analog signal to be the maximum value, and determines the gain by dividing the set value of the amplitude of the drive signal by the maximum amplitude of the analog signal. The gain and amplitude of the analog signal determined in this way can realize the amplitude of the drive signal on the line segment AB shown in Figure 7.

[0087] <Step S205> The control circuit 51 determines the gain to be the minimum value, and determines the amplitude of the analog signal as the value obtained by dividing the set value of the amplitude of the drive signal by the minimum value of the gain. The gain and amplitude of the analog signal determined in this way can realize the amplitude of the drive signal on the line segment AD shown in Figure 7.

[0088] <Step S206> The control circuit 51 causes the signal generation circuit 52 to output a digital signal corresponding to the analog signal whose amplitude has been determined as described above to the DA conversion circuit 53, as shown in Fig. 5. The "D signal" shown in Fig. 9 represents the digital signal. The control circuit 51 further outputs a signal corresponding to the gain determined as described above to the amplitude change circuit 58 via a signal line 59, as shown in Fig. 5.

[0089] The above driving operation makes it possible to drive the phase modulator 38 with a driving signal having a high S / N ratio. Therefore, the measurement device 100 according to this embodiment has improved measurement accuracy and can more accurately acquire information about the distance and / or shape of the object 10, compared to a comparative measurement device equipped with the processing circuitry 49 shown in FIG.

[0090] The above driving operation may be partially modified as follows: Before the measurement operation, the following table created based on the processing in steps S203 to S205 is stored in memory. The table includes correspondence relationships between the amplitudes of multiple driving signals, the amplitudes of multiple analog signals, and multiple gains. Each of the amplitudes of the multiple driving signals corresponds to one of the amplitudes of the multiple analog signals and one of the gains. In the driving operation shown in FIG. 9 , the control circuit 51 may read the table from memory instead of steps S203 to S205, and determine the amplitude and gain of the analog signal corresponding to the set value of the amplitude of the driving signal based on the table.

[0091] In the description of the measurement apparatus 100 according to this embodiment, an example of object measurement using the phase shift method has been given, but the present invention is not limited to this example. Depending on the application, other methods such as heterodyne optical interferometry and OCT (Optical Coherence Tomography) may also be used. In an optical system including a phase modulator, other components such as lenses, beam splitters, and mirrors may be changed as appropriate depending on the application.

[0092] Furthermore, in the description of the measurement device 100 according to this embodiment, the reference light 20L, which is a part of the laser light 20L emitted from the light source 20, R Phase φ of R However, the present invention is not limited to this example. Depending on the application, the phase of not only a part of the laser beam 20L emitted from the light source 20 but the entire laser beam 20L, i.e., the phase of at least a part of the laser beam 20L, may be changed.

[0093] Furthermore, although the measurement device 100 according to this embodiment has been described with reference to an example in which the phase is changed, the present invention is not limited to this example. Depending on the application, the parameter of light changed by the modulator may be, for example, at least one of the phase, intensity, and polarization.

[0094] [Note] The above description of the embodiments discloses the following techniques.

[0095] [Technology 1] a light source that emits light for illuminating an object; an optical system including a phase modulator that changes the phase of at least a portion of the light; a signal generating circuit that generates a digital signal; a digital-to-analog conversion circuit that converts the digital signal into an analog signal and outputs the analog signal; an amplitude change circuit that adjusts the amplitude of the analog signal by a gain to generate a drive signal; a drive circuit that drives the phase modulator based on the drive signal; a signal line electrically connected to the amplitude change circuit for controlling the gain; Equipped with Measuring equipment.

[0096] In this measurement device, in measuring an object using light, the phase can be changed more accurately, thereby improving measurement accuracy.

[0097] [Technology 2] The gain varies depending on the amount of change in the phase. The measurement device described in Technology 1.

[0098] This measurement device allows the gain to be adjusted to change the amount of phase change.

[0099] [Technology 3] the amplitude changing circuit includes a digital potentiometer that defines the gain; The measuring device according to technique 1 or 2.

[0100] This measurement device allows the gain to be adjusted using a variable resistor.

[0101] [Technology 4] When changing the amount of change in the phase, the amplitude change circuit changes the gain while keeping the amplitude of the analog signal output from the digital-to-analog conversion circuit fixed. A measuring device according to any one of techniques 1 to 3.

[0102] In this measurement device, the amplitude of the analog periodic signal is fixed, so the amount of phase change can be changed while maintaining a high signal-to-noise ratio of the drive signal.

[0103] [Technology 5] When changing the amount of change in the phase, if a desired amount of change in the phase cannot be achieved even if the amplitude change circuit changes the gain while the amplitude of the analog signal output from the digital-analog conversion circuit is fixed, the signal generation circuit changes the digital signal. A measuring device according to any one of techniques 1 to 3.

[0104] This measurement device can achieve a desired small amount of phase change while maintaining a relatively high signal-to-noise ratio of the drive signal.

[0105] [Technology 6] When the amount of change in the phase is changed from a first amount of change to a second amount of change, the amplitude of the analog signal is the same in the case of the first amount of change and in the case of the second amount of change, and the gain is different in the case of the first amount of change and in the case of the second amount of change. A measuring device according to any one of techniques 1 to 4.

[0106] In this measurement device, the amplitude of the analog periodic signal is fixed, so the amount of phase change can be changed while maintaining a high signal-to-noise ratio of the drive signal.

[0107] [Technology 7] When the amount of change in the phase is changed from a first amount of change to a second amount of change, and the second amount of change is smaller than the amount of change in the phase when the gain is set to a minimum value, the signal generating circuit changes the digital signal. A measuring device according to any one of techniques 1 to 3 or technique 5.

[0108] This measurement device can achieve a desired small amount of phase change while maintaining a relatively high signal-to-noise ratio of the drive signal.

[0109] [Technology 8] The optical system comprises: The light is split into a first light and a second light, and the phase is the phase of the first light; generating interference light between the first light, the phase of which has been changed by the phase modulator, and the second light; A measuring device according to any one of techniques 1 to 7.

[0110] This measurement device enables object measurement based on the interference light of the first light and the second light.

[0111] [Technology 9] the analog signal is time-varying; A measuring device according to any one of techniques 1 to 8.

[0112] In this measurement device, the intensity of light obtained from the optical system changes over time in accordance with the time change of the analog signal, and the measurement accuracy of the object can be improved based on the time-varying light intensity.

[0113] [Technology 10] a light source that emits light for illuminating an object; an optical system including a modulator that changes a parameter of at least a portion of the light; a signal generating circuit that generates a digital signal; a digital-to-analog conversion circuit that converts the digital signal into an analog signal and outputs the analog signal; an amplitude change circuit that adjusts the amplitude of the analog signal by a gain to generate a drive signal; a drive circuit that drives the modulator based on the drive signal; a signal line electrically connected to the amplitude change circuit for controlling the gain; Equipped with Measuring equipment.

[0114] With this measurement device, in measuring an object using light, the parameters of the light can be changed more accurately, thereby improving the measurement accuracy.

[0115] [Technology 11] the parameter of the at least some of the light is at least one of a phase, a polarization, and an intensity of the at least some of the light. The measuring device described in Technology 10.

[0116] This measurement device allows for more precise variation of at least one of the phase, polarization, and intensity. [Industrial Applicability]

[0117] The measurement device according to the present disclosure can be used, for example, to measure information relating to the distance and shape of an object. [Explanation of symbols]

[0118] 10 objects 20 light source 20L laser light 20L O object light 20L R reference light 20L I Interferometric light 30 Interference Optical System 32a~32c lenses 34 Beam Splitter 36 Mirror 38 Phase Modulator 40 Photodetector 49, 50 Processing circuit 51 Control circuit 52 Signal generation circuit 53 DA conversion circuit 54 Low-pass filter 55 Drive circuit 56 AD conversion circuit 57 Signal Processing Circuit 58 Amplitude deflection circuit 59 Signal Line 100 Measuring Equipment

Claims

1. a light source that emits light for illuminating an object; an optical system including a phase modulator that changes the phase of at least a portion of the light; a signal generating circuit that generates a digital signal; a digital-to-analog conversion circuit that converts the digital signal into an analog signal and outputs the analog signal; an amplitude change circuit that adjusts the amplitude of the analog signal by a gain to generate a drive signal; a drive circuit that drives the phase modulator based on the drive signal; a signal line electrically connected to the amplitude change circuit for controlling the gain; Equipped with Measuring equipment.

2. The gain varies depending on the amount of change in the phase. The measurement device according to claim 1 .

3. the amplitude changing circuit includes a digital potentiometer that defines the gain; The measuring device according to claim 1 or 2.

4. When changing the amount of change in the phase, the amplitude change circuit changes the gain while keeping the amplitude of the analog signal output from the digital-to-analog conversion circuit fixed. The measuring device according to claim 1 or 2.

5. When changing the amount of change in the phase, if a desired amount of change in the phase cannot be achieved even if the amplitude change circuit changes the gain while the amplitude of the analog signal output from the digital-analog conversion circuit is fixed, the signal generation circuit changes the digital signal. The measuring device according to claim 1 or 2.

6. when the amount of change in the phase is changed from a first amount of change to a second amount of change, the amplitude of the analog signal is the same in the case of the first amount of change and in the case of the second amount of change, and the gain is different in the case of the first amount of change and in the case of the second amount of change; The measuring device according to claim 1 or 2.

7. When the amount of change in the phase is changed from a first amount of change to a second amount of change, and the second amount of change is smaller than the amount of change in the phase when the gain is set to a minimum value, the signal generating circuit changes the digital signal. The measuring device according to claim 1 or 2.

8. The optical system comprises: the light is split into a first light and a second light, the phase being the phase of the first light; generating interference light between the first light, the phase of which has been changed by the phase modulator, and the second light; The measuring device according to claim 1 or 2.

9. the analog signal is time-varying; The measuring device according to claim 1 or 2.

10. a light source that emits light for illuminating an object; an optical system including a modulator that changes a parameter of at least a portion of the light; a signal generating circuit that generates a digital signal; a digital-to-analog conversion circuit that converts the digital signal into an analog signal and outputs the analog signal; an amplitude change circuit that adjusts the amplitude of the analog signal by a gain to generate a drive signal; a drive circuit that drives the modulator based on the drive signal; a signal line electrically connected to the amplitude change circuit for controlling the gain; Equipped with Measuring equipment.

11. the parameter of the at least some of the light is at least one of a phase, a polarization, and an intensity of the at least some of the light. The measurement device according to claim 10.

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