Signal processing circuit and signal processing method

Abstract

This invention provides a signal processing circuit that can improve the accuracy of light intensity measurement. [Solution] The signal processing circuit 3 includes an A / D converter 31 and a noise reduction circuit 32. The A / D converter 31 converts the electrical signal Va output from the photodetector 2 in response to the input of a photon into time-series data Da including a plurality of digital signal values. The noise reduction circuit 32 determines whether the difference between each of the plurality of digital signal values ​​and the previous digital signal value satisfies a threshold condition indicating the presence of a component based on the input of a photon. In a first period that does not contain a component based on the input of a photon, the noise reduction circuit 32 replaces each of the plurality of digital signal values ​​with a predetermined value. The noise reduction circuit 32 determines the first period based on the result of the determination.

Description

Technical Field

[0001] The present disclosure relates to a signal processing circuit and a signal processing method.

Background Art

[0002] Patent Document 1 discloses a light quantity detection device. This light quantity detection device performs A / D conversion on a detection signal of a photon counting type photodetector. When the A / D converted detection signal is greater than or equal to a threshold value, the detection signal is sent as it is to a photon number calculation circuit in the subsequent stage. When the A / D converted detection signal is less than the threshold value, a preset reference value is sent to the photon number calculation circuit in the subsequent stage. The photon number calculation circuit obtains the number of photons or the light quantity incident on the photon counting type photodetector from the area of the detection signal waveform acquired until the light quantity measurement is completed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a light detection device, like the device described in Patent Document 1, in order to reduce the noise component included in the detection signal and improve the detection accuracy, there is a method of replacing the detection signal with a predetermined value when the detection signal is smaller than a threshold value. However, in this method, for example, when the base level of the detection signal gradually changes due to changes over time in an optical sensor or changes in the external environment around the optical sensor, the level of the noise component included in the detection signal also changes together with the base level and exceeds the threshold value, and the noise component is sent as it is to the photon number calculation circuit. This becomes a factor for reducing the light quantity measurement accuracy.

[0005] The purpose of this disclosure is to provide a signal processing circuit and a signal processing method that can be incorporated into a light detection device and a light detection method, respectively, to improve the accuracy of light intensity measurement. [Means for solving the problem]

[0006] [1] A signal processing circuit relating to one aspect of the present disclosure comprises an input terminal, an analog-to-digital converter, and a noise reduction unit. The input terminal receives an electrical signal output from a photodetector in response to a photon input. The analog-to-digital converter converts the electrical signal into time-series data including a plurality of digital signal values ​​having time intervals. The noise reduction unit reduces noise components in the time-series data that are different from the components based on the photon input. The noise reduction unit includes a determination unit and a replacement unit. The determination unit determines whether, for each of the plurality of digital signal values, the difference between the digital signal value and the previous digital signal value satisfies a threshold condition indicating the presence of a component based on the photon input. The replacement unit replaces each of the plurality of digital signal values ​​with a predetermined value in a first period that does not include a component based on the photon input. The replacement unit determines the first period based on the determination result in the determination unit.

[0007] In the signal processing circuit described in [1] above, the determination unit determines whether the difference between the digital signal value and the previous digital signal value satisfies a threshold condition indicating the presence of a component based on photon input. When a photon is input to the photodetector, the time-series data changes in a pulsed manner, and in many cases, the digital signal value changes significantly during the rising edge of the pulsed change. Therefore, based on the fact that the difference between the digital signal value and the previous digital signal value satisfies the threshold condition, even if the base level of the electrical signal gradually changes due to, for example, changes in the optical sensor over time or changes in the external environment (e.g., temperature) around the optical sensor, the presence of a component based on photon input can be accurately detected, and noise components different from the component based on photon input can be replaced with a predetermined value. Furthermore, unlike the method described in Patent Document 1, where the magnitude of the digital signal value and the threshold are compared, the presence of a component based on each photon can be accurately detected even when multiple photons are input at short time intervals. Therefore, according to the signal processing circuit described in [1] above, the accuracy of light intensity measurement can be improved by having the replacement unit determine the first period based on the determination result in the determination unit.

[0008] [2] In the signal processing circuit described in [1] above, the threshold condition may be that the difference value exceeds a predetermined threshold. Based on the fact that the difference value between a digital signal value and the digital signal value before that digital signal value exceeds a predetermined threshold, the presence of a component based on the photon input can be detected with high accuracy.

[0009] [3] In the signal processing circuit of [1] or [2] above, the difference value may be the difference between the digital signal value and the digital signal value immediately preceding the current digital signal value. In this case, the presence of a component based on the input of a photon can be detected with greater accuracy. Also, when multiple photons are input at short time intervals, the presence of a component based on each photon can be detected more accurately.

[0010] [4] In any of the signal processing circuits described in [1] to [3] above, the predetermined value may be a constant value. In this case, noise components can be effectively reduced.

[0011] [5] A signal processing method relating to one aspect of the present disclosure comprises an input step, an analog-to-digital conversion step, and a noise reduction step. In the input step, an electrical signal output from a photodetector in response to a photon input is input. In the analog-to-digital conversion step, the electrical signal is converted into time-series data including a plurality of digital signal values ​​having a time interval. In the noise reduction step, noise components included in the time-series data that are different from the component based on the photon input are reduced. The noise reduction step includes a determination step and a replacement step. In the determination step, for each of the plurality of digital signal values, it is determined whether the difference between the digital signal value and the digital signal value before the digital signal value satisfies a threshold condition indicating the presence of a component based on the photon input. In the replacement step, each of the plurality of digital signal values ​​is replaced with a predetermined value in a first period that does not include a component based on the photon input. In the replacement step, the first period is determined based on the determination result in the determination step.

[0012] The signal processing method described in [5] above yields the same effect as the signal processing circuit described in [1] above. That is, even when the base level of the electrical signal gradually changes, the presence of a component based on the photon input can be accurately detected, and noise components different from the component based on the photon input can be replaced with a predetermined value. Furthermore, even when multiple photons are input at short time intervals, the presence of a component based on each photon can be accurately detected. Therefore, the signal processing method described in [5] above can improve the accuracy of light intensity measurement by determining the first period of the substitution step based on the determination result of the determination step. [Effects of the Invention]

[0013] According to this disclosure, a signal processing circuit and a signal processing method can be provided that can be incorporated into a light detection device and a light detection method, respectively, to improve the accuracy of light intensity measurement. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic diagram showing the configuration of a photodetector according to one embodiment of the present disclosure. [Figure 2] Figure 2 is a graph showing an example of an electrical signal and the corresponding time-series data. [Figure 3] Figure 3 is a block diagram showing the functional configuration of the noise reduction circuit. [Figure 4] Figure 4 is a graph showing the difference value and a predetermined threshold. [Figure 5] Figure 5 is a graph showing the time-series data after the replacement. [Figure 6] Figure 6 is a flowchart that specifically shows the processing steps for each sampling point of the noise reduction circuit. [Figure 7] Figure 7 is a flowchart of a light detection method, including a signal processing method according to one embodiment. [Figure 8] Figure 8 is a graph showing an example of time-series data when 10 photons are incident on a light sensor within a certain time range. [Figure 9] Figure 9 is a graph showing an example of time-series data when 10 photons are incident on a photosensor within a certain time range, and the base level of the electrical signal changes over time. [Figure 10] Figure 10 shows an intensity mapping image generated by a photodetector. [Figure 11] Figure 11 is a graph showing time-series data for the first and second rows of the detected region. [Figure 12] Figure 12 is a graph showing time-series data for the third and fourth rows of the detected region. [Figure 13] Figure 13 is a graph showing the correlation between the integral value (output) of a pulsed time waveform and its frequency of occurrence. [Modes for carrying out the invention]

[0015] Specific examples of the present disclosure will be described below with reference to the drawings. However, the present invention is not limited to these examples, and is intended to include all modifications within the meaning and scope of the claims, as defined by the claims. In the following description, identical elements in the drawings are denoted by the same reference numerals, and redundant descriptions are omitted.

[0016] Figure 1 is a schematic diagram showing the configuration of a photodetector 1 according to one embodiment of the present disclosure. As shown in Figure 1, the photodetector 1 of this embodiment comprises a photodetector 2, a signal processing circuit 3, a signal quantity calculation circuit 4, and a computing device 5. Light P is input to the photodetector 2. The photodetector 2 outputs an electrical signal Va corresponding to each of the multiple photons that constitute the light P. The electrical signal Va is, for example, an analog voltage signal. The signal processing circuit 3 converts the electrical signal Va into digital time-series data Da, reduces the noise component contained in the time-series data Da, and generates time-series data Db. The signal quantity calculation circuit 4 calculates the signal quantity by integrating the time-series data Db over time and outputs numerical data Dc indicating the signal quantity. The computing device 5 is composed of a computing device such as a personal computer, and counts the numerical data Dc to determine the amount of incident light.

[0017] A specific example of the photodetector 2 will be described. The photodetector 2 has a photosensor 21 and an I / V conversion circuit 22. The photosensor 21 is preferably a photosensor capable of single-photon measurement, and preferably an electron tube with a photocathode, such as a photomultiplier tube or an HPD (hybrid photodetector) equipped with semiconductor elements as an electron multiplier unit and a detection unit. Light P is input to the photosensor 21. The photosensor 21 generates a photocurrent corresponding to each of the multiple photons constituting the light P and outputs a current signal Ja based on the photocurrent. The I / V conversion circuit 22 is electrically connected to the photosensor 21 and converts the current signal Ja into a voltage signal. The I / V conversion circuit 22 includes, for example, a charge amplifier or a transimpedance amplifier. The photodetector 2 outputs the voltage signal generated in the I / V conversion circuit 22 as an electrical signal Va to the signal processing circuit 3. Note that the I / V conversion circuit 22 may be included in the signal processing circuit 3 instead of the photodetector 2. Similarly, a part of the signal processing circuit 3 may be included in the photodetector 2.

[0018] The signal processing circuit 3 is a circuit that converts an electrical signal Va into time-series digital data Db. The signal processing circuit 3 has an input terminal 3a, an output terminal 3b, an analog-to-digital converter (A / D converter) 31, and a noise reduction circuit 32. The input terminal 3a is electrically connected to the signal output terminal of the photodetector 2 and receives the electrical signal Va output from the photodetector 2. The A / D converter 31 converts the electrical signal Va into time-series data Da, which includes multiple digital signal values ​​with time intervals. Part (a) of Figure 2 is a graph showing an example of the electrical signal Va. Part (b) of Figure 2 is a graph showing the time-series data Da corresponding to the electrical signal Va shown in part (a). In Figure 2, the horizontal axis represents time (ns), and the vertical axis represents voltage (V), which indicates the magnitude of the signal. As shown in part (a) of Figure 2, in this example, a pulsed time waveform Pa caused by one photon of light P is included in the time variation of the electrical signal Va. In addition, the time variation of the electrical signal Va includes noise components not caused by light P. Noise components are superimposed on the original signal components, for example, in the wiring from the signal output terminal of the photodetector 2 to the A / D converter 31, and inside the A / D converter 31. Part (b) of Figure 2 shows the time evolution of the electrical signal Va containing such noise components, converted directly into time-series data Da of digital signal values. Part (b) of Figure 2 shows N digital signal values ​​I(t1) to I(t) that constitute the time-series data Da (N is an integer of 2 or more; in the illustrated example, N=17). N ) is shown. The sampling rate of the A / D converter 31 is, for example, 10 MSa / s to 60 MSa / s. Note that 1 Sa / s means that sampling is performed once per second. When the sampling rate is 40 MSa / s, the time interval of the digital signal value (i.e., the period of A / D conversion) is 25 ns. Referring to part (b) of Figure 2, it can be seen that the time series data Da also includes a pulsed time waveform Pa, and that the digital signal value is also varied by noise components in sections other than the time waveform Pa.

[0019] The noise removal circuit 32 is a digital circuit and is the noise reduction part in the present embodiment. The noise removal circuit 32 is electrically connected to the A / D converter 31 and inputs the time-series data Da from the A / D converter 31. The noise removal circuit 32 generates time-series data Db from the time-series data Da by reducing noise components different from the components based on the photons of the light P included in the time-series data Da. The noise removal circuit may be constituted by a large-scale logic operation circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array), or a microcomputer.

[0020] FIG. 3 is a block diagram showing the functional configuration of the noise removal circuit There are a determination unit 321 and a replacement unit 322. The determination unit 321 determines whether each of a plurality of digital signal values I(t1) to I(t N ) included in the time-series data Da satisfies the threshold condition indicating the presence of the component based on the input of the photons of the light P. In the present embodiment, for a certain sampling point t n (n is an integer of 2 or more and N or less), the digital signal value I(t n ) satisfying the threshold condition means that the difference value S(t n ) = I(t n ) - I(t n-1 ) between the digital signal value I(t n ) and the digital signal value I(t n ) before (typically immediately before) the digital signal value I(t n-1 ) exceeds a predetermined threshold value. The predetermined threshold value is set in advance according to the magnitude of the noise component. In one example, the predetermined threshold value is set to be equal to three times the differential standard deviation σ' of the output intensity in a state where the light P is not input (dark state). The differential standard deviation σ' of the output intensity is calculated as σ' = (σ 2 + σ 2 ) 1 / 2 based on the standard deviation σ of the output intensity.

[0021] The determination unit 321 first calculates the difference value S(t2) ~ S(t N ) is calculated. Part (a) of Figure 4 shows the difference values ​​S(t2)~S(t) applied to the time series data Da shown in Part (b) of Figure 2. N This is a graph that superimposes the values ​​S(t). In the figure, multiple black dots represent the difference value S(t). n The determination unit 321 then determines the difference value S(t n It determines whether the difference value S(t) exceeds a predetermined threshold. Part (b) of Figure 4 shows the difference value S(t n This is a graph showing the difference between ) and predetermined thresholds TH, -TH. In this case, the difference value S(t n ) exceeds a predetermined threshold TH, which means the difference value S(t n This means that the value is either above the threshold TH or below the threshold -TH. In the example shown in part (b) of Figure 4, only the difference value S(t5) is above the threshold TH.

[0022] The substitution unit 322 sets the first period Ta and the second period Tb shown in part (b) of Figure 4. The first period Ta is the period in which the time-series data Da does not include a component based on the input of photons from optical P. The second period Tb is the period in which the time-series data Da includes a component based on the input of photons from optical P. In the illustrated example, the first period Ta and the second period Tb are consecutive, but a gap may be provided between the first period Ta and the second period Tb. The substitution unit 322 determines the first period Ta and the second period Tb based on the determination result by the determination unit 321. That is, the substitution unit 322 defines the second period Tb as the period in which the determination unit 321 determines that a digital signal value among a plurality of digital signal values ​​satisfies the threshold condition, and the digital signal values ​​counting from that digital signal value to at least one before and one after. The substitution unit 322 then sets the period excluding the second period Tb as the first period Ta. In the examples shown in parts (a) and (b) of Figure 4, the second period Tb includes a digital signal value I(t5) in which the difference value S(t5) exceeds the threshold TH, and digital signal values ​​I(t4) and S(t6) counting from the digital signal value I(t5) for at least one period before and after it.

[0023] The time width of the second period Tb may be set to be approximately equal to the total width of the pulsed time waveform Pa, or close to the total width of the pulsed time waveform Pa. For example, the time width of the second period Tb may be greater than 1.5 times the full width at half maximum of the time waveform Pa included in the time series data Da. The full width at half maximum of the time waveform Pa is the cutoff frequency f of the I / V conversion circuit 22. c Based on 1 / 2f c It is calculated as follows. Note that the digital signal values ​​I(t1)~I(t N In this case, the number of samples in this full width at half maximum interval is the sampling frequency S of the digital A / D converter 31. r Based on, Sr / 2f c It is calculated as follows.

[0024] In the examples shown in parts (a) and (b) of Figure 4, the time width of the second period Tb is set to include the digital signal value I(t4) to the digital signal value I(t8). In a system in which the photodetector 1 is used, if the total width of the pulsed time waveform Pa is known, the time width of the second period Tb should be set to be approximately equal to the total width of the pulsed time waveform Pa. The total width of the time waveform Pa can be calculated, for example, from the sampling frequency of the A / D converter 31 and the cutoff frequency of the I / V conversion circuit 22.

[0025] The substitution unit 322 replaces multiple digital signal values ​​included in the first period Ta (in the illustrated example, digital signal values ​​I(t1) to I(t3) and I(t9) to I(t9) 17 Each of the following is replaced with a predetermined value to generate time-series data Db. The predetermined value is, for example, a constant value, and in one embodiment it is zero. Line L in Figure 5 shows the time-series data Db after the replacement.

[0026] Figure 6 shows the sampling points t2~t of the noise reduction circuit 32 described above. N This flowchart specifically shows the processing steps for the given value. Note that for the first sampling point t1, since there is no difference value S(t1), the noise reduction circuit 32 does not perform the following processing.

[0027] First, the difference value S(tn Determine whether the difference value S(t) is greater than three times the difference standard deviation σ' (Step ST11). n If ) is greater than three times the difference standard deviation σ' (Step ST11: YES), it is determined that the value is due to the photon input, and the previous sampling point t n-1 Output value O(t) n-1 ) as the digital signal value I(t n-1 ) is set and the pass-through flag is turned ON (step ST12). The pass-through flag is a flag used to determine whether to output the digital signal value as is (without replacing it with a predetermined value). Also, there is a variable C that indicates the number of times the digital signal value has been output as is since the pass-through flag was turned ON. flag Initialize to 1. Then, the next sampling point t n+1 The process is repeated from step ST11 (step ST18).

[0028] Difference value S(t) n If the value is less than or equal to three times the difference standard deviation σ' (Step ST11: NO), determine whether the pass-through flag is on (Step ST13). If the pass-through flag is on (Step ST13: YES), determine that the value is still attributable to the photon input and proceed to the previous sampling point t n-1 Output value O(t) n-1 ) as the digital signal value I(t n-1 ) and variable C flag Add 1 to (step ST14). Also, if the through flag is off (step ST13: NO), it is determined that the value is not due to the photon input, and the previous sampling point t n-1 Output value O(t) n-1 Set a predetermined value as (step ST15). Then, variable C flag This is the number of samples included in the time width of the second period Tb (for example, the number of samples S included in the full width at half maximum of the component based on the photon input). r / 2f c 1.5 times, in one embodiment S r / f cWhen it reaches (Step ST16: YES), the pass flag is turned off (Step ST17). After Step ST17, or variable C flag If the number of samples included in the time width of the second period Tb has not been reached (step ST16: NO), then the next sampling point t n+1 The process is repeated from step ST11 (step ST18). Finally, the obtained output value O(t1) ~ O(t N This is output as time-series data Db.

[0029] Note that each sampling point t2~t N In this case, it should be noted that the determination in step ST11 is also performed when the pass-through flag is in the ON state. When the pass-through flag is in the ON state, the difference value S(t n If ) is greater than three times the difference standard deviation σ', it is thought that a new photon has been input, and the variable C flag This is because it needs to be initialized to 1.

[0030] Also, the final sampling point t N For determining whether replacement is possible in the following sampling point t, for example, N+1 The digital signal value I(t N+1 There are several methods, such as virtually setting the value, performing only steps ST13 to ST15 (determined solely by the pass flag), or not performing any processing.

[0031] Figure 7 is a flowchart of the photodetection method, including the signal processing method according to this embodiment. First, in the photodetection step ST21, a photon is input to the photodetector 2, and the photodetector 2 outputs an electrical signal Va corresponding to the input of the photon. Next, in the input step ST22, the electrical signal Va is input to the signal processing circuit 3. Subsequently, in the A / D conversion step ST23, the electrical signal Va is converted into a plurality of digital signal values ​​I(t1)~I(t) having a time interval. NThe data is converted into time-series data Da, which includes the input of photons. Subsequently, in the noise reduction step ST24, noise components contained in the time-series data Da are reduced to generate time-series data Db. In the noise reduction step ST24, noise components contained in the time-series data Da that are different from the components based on the photon input are reduced. The noise reduction step ST24 has a determination step ST241 and a replacement step ST242. In the determination step ST241, a threshold condition indicating the presence of components based on the input of photons is set for a plurality of digital signal values ​​I(t1)~I(t N It is determined whether each of the following conditions is satisfied. Details of the threshold conditions are as described above. In substitution step ST242, in the first period Ta which does not contain components based on the photon input, multiple digital signal values ​​I(t1)~I(t N Each of the following is replaced with a predetermined value. In replacement step ST242, multiple digital signal values ​​I(t1) to I(t N The second period Tb, which includes a digital signal value that satisfies the threshold condition and at least one digital signal value before and after that digital signal value, is excluded from the first period Ta. Details of the first period Ta and the second period Tb are as described above. Next, in the signal amount calculation step ST25, the signal amount is calculated by integrating the time series data Db over time and numerical data Dc representing the signal amount is output.

[0032] The effects obtained by the signal processing circuit 3 and signal processing method according to the present embodiment described above will be explained along with the problems of the conventional method. Part 8(a) of Figure 8 is a graph showing an example of time-series data Da when 10 photons are incident on the photosensor 21 within a certain time range. Part 8(b) of Figure 8 is a graph showing a further expanded time range of the graph shown in Part 8(a). If the base level of the electrical signal Va does not change over a long time range, as shown in these graphs, the noise component, which is different from the component based on the photon input, will fall within the range between the threshold THb and the threshold -THb. Therefore, even with the method described in Patent Document 1, for example, that is, the method in which the digital signal value is replaced with a predetermined value when the digital signal value is smaller than the threshold, the noise component can be reduced over a long period of time.

[0033] However, if the base level of the electrical signal Va changes over time due to, for example, changes in the optical sensor or changes in the external environment around the optical sensor (e.g., temperature), the level of the noise component included in the time-series data Da also changes along with the base level, as shown in part 9(a), and falls outside the range between threshold THb and threshold-THb. In part 9(a), the straight line La represents the base level of the electrical signal Va. In this case, the signal quantity is calculated with the noise component superimposed on the component caused by photons, which reduces the accuracy of light quantity measurement. It is also conceivable to widen the range between threshold THb and threshold-THb to account for long-term noise fluctuations in such an electrical signal Va. However, with such a method, if the component based on photon input is weak, the component based on photon input will also be replaced with a predetermined value. This also reduces the accuracy of light quantity measurement.

[0034] To address the above issues, the determination unit 321 determines the digital signal value I(t n ) and its digital signal value I(t n The digital signal value I(t) before ) n-1 The difference value S(t) n ) determines whether the threshold condition indicating the presence of a component based on the photon input is met. When a photon is input to the photodetector 2, the time-series data Da changes in a pulsed manner, but in most cases, at the rising edge of the pulsed change, the digital signal value I(t n ) changes significantly. Therefore, the digital signal value I(t n ) and its digital signal value I(t n The digital signal value I(t) before ) n-1 The difference value S(t) n Based on the fact that the threshold condition is met, even if the base level of the electrical signal Va gradually changes, the presence of the component based on the photon input can be accurately detected, and noise components different from the component based on the photon input can be replaced with a predetermined value. That is, as shown in part (b) of Figure 9, even if the base level of the electrical signal Va changes, the difference value S(t n) (shown as gray dots in the figure) is hardly affected, so the difference value S(t n Based on whether the digital signal value I(t) exceeds the threshold TH, the presence of a component based on the photon input can be detected with high accuracy. Furthermore, as in the method described in Patent Document 1, the digital signal value I(t) n Unlike the case where the magnitude of the photon is compared to a threshold, even when multiple photons are input at short time intervals, the presence of a component based on each photon can be accurately detected. Therefore, according to the signal processing circuit 3 of this embodiment, the light intensity measurement accuracy (S / N ratio) can be improved by having the substitution unit 322 determine the first period Ta based on the determination result in the determination unit 321.

[0035] As in this embodiment, the threshold condition is the digital signal value I(t n ) and its digital signal value I(t n The digital signal value I(t) before ) n-1 The difference value S(t) n ) may exceed a predetermined threshold TH. The difference value S(t n Based on the fact that ) exceeds a predetermined threshold, the presence of a component based on photon input can be detected with high accuracy.

[0036] As in this embodiment, the difference value S(t n ) is the digital signal value I(t n ) and the digital signal value I(t n The digital signal value I(t) immediately before ) n-1 The difference value from ) may also be used. In this case, the presence of a component based on the photon input can be detected with even greater accuracy. Furthermore, when multiple photons are input at short time intervals, the presence of a component based on each photon can be detected more accurately.

[0037] As in this embodiment, the predetermined value that replaces the digital signal value may be a constant value. In this case, noise components can be effectively reduced.

[0038] (Examples) Figure 10 shows an intensity mapping image generated by the photodetector 1. In this embodiment, for a sample defined as a detection area consisting of a total of 16 unit areas E arranged in a 4x4 grid, the light intensity emitted from each unit area E was detected by the photodetector 21 while relatively scanning either the measurement light or the photodetector 21 in two dimensions along the column direction D1 and the row direction D2. Within each unit area E, a number indicating the number of photons input to each unit area E is shown. Part (a) of Figure 11 is a graph showing time-series data Db for the first row of the detection area detected along arrow B1. Part (b) of Figure 11 is a graph showing time-series data Db for the second row of the detection area detected along arrow B2. Part (a) of Figure 12 is a graph showing time-series data Db for the third row of the detection area detected along arrow B3. Part (b) of Figure 12 is a graph showing time-series data Db for the fourth row of the detection area detected along arrow B4. In Figures 11 and 12, the solid line represents the time-series data Db, and the dashed line represents the electrical signal before D / A conversion. Referring to Figure 11, it can be seen that the noise component contained in the electrical signal is effectively reduced by the photodetector 1. Referring to Figure 12, it can be seen that the amount of incident light can be detected accurately even when photons are input at short time intervals.

[0039] Figure 13 is a graph showing the correlation between the integral value (output) of a pulsed time waveform and its frequency of occurrence. Part (a) of Figure 13 shows the case where the noise reduction circuit 32 is provided, and part (b) of Figure 13 shows the case where the noise reduction circuit 32 is not provided. The numbers indicated in part (a) of Figure 13 represent the number of photons included in the time waveform. As shown in part (b) of Figure 13, when the noise reduction circuit 32 is not provided, the light intensity measurement accuracy is low, and the difference in integral values ​​due to the number of photons is ambiguous. In contrast, as shown in part (a) of Figure 13, when the noise reduction circuit 32 is provided, the light intensity measurement accuracy is high, and the difference in integral values ​​due to the number of photons is clear. Thus, according to the signal processing circuit 3 of this embodiment, the light intensity measurement accuracy can be improved and the number of photons can be counted more accurately.

[0040] The signal processing circuit and signal processing method according to the present disclosure are not limited to the above-described embodiments, and various other modifications are possible. For example, in the above embodiment, as the difference value S(t n ), the difference between the digital signal value I(t n ) and the digital signal value I(t n-1 ) immediately before it is illustrated. The difference value S(t n ) is not limited to this, and it may be the difference between the digital signal value I(t n ) and the digital signal value I(t n-m )(m is an integer of 2 or more) two or more before it. Also, in the above embodiment, the replacement unit 322 performs signal processing by setting the first period Ta and the second period Tb. The replacement unit 322 is not limited to this, and signal processing may be performed without setting the second period Tb and the through flag.

Explanation of Signs

[0041] 1... Photo-detection device, 2... Photo-detector, 3... Signal processing circuit, 3a... Input terminal, 3b... Output terminal, 4... Signal amount calculation circuit, 5... Arithmetic unit, 21... Optical sensor, 22... I / V conversion circuit, 31... A / D converter, 32... Noise removal circuit, 321... Determination unit, 322... Replacement unit, D1... Column direction, D2... Row direction, Da, Db... Time-series data, Dc... Numerical data, E... Unit area, I(t n ), I(t1)~I(t N )... Digital signal values, Ja... Current signal, O(t1)~O(t N )... Output values, P... Light, Pa... Time waveform, S(t n ), S(t1)~S(t N )... Difference values, Ta... First period, Tb... Second period, TH, THb... Threshold values, Va... Electric signal.

Claims

1. An input terminal that receives an electrical signal output from a photodetector in response to a photon input, An analog-to-digital converter that converts the aforementioned electrical signal into time-series data including a plurality of digital signal values ​​having time intervals, A noise reduction unit that reduces noise components in the time-series data that are different from the components based on the photon input, Equipped with, The noise reduction unit is A determination unit determines whether the difference between each of the plurality of digital signal values ​​and the digital signal value preceding the digital signal value satisfies a threshold condition indicating the presence of a component based on the photon input. In a first period that does not include components based on the input of the photons, a substitution unit replaces each of the plurality of digital signal values ​​with a predetermined value, It has, The substitution unit is a signal processing circuit that determines the first period based on the determination result in the determination unit.

2. The signal processing circuit according to claim 1, wherein the threshold condition is that the difference value exceeds a predetermined threshold.

3. The signal processing circuit according to claim 1 or 2, wherein the difference value is the difference between the digital signal value and the digital signal value immediately preceding the digital signal value.

4. The signal processing circuit according to claim 1 or 2, wherein the predetermined value is a constant value.

5. An input step that inputs an electrical signal output from a photodetector in response to a photon input, The analog-to-digital conversion step converts the electrical signal into time-series data including a plurality of digital signal values ​​having time intervals, A noise reduction step that reduces noise components in the time-series data that are different from the components based on the photon input, Equipped with, The noise reduction step is, A determination step in which, for each of the plurality of digital signal values, a determination is made to determine whether the difference between the digital signal value and the digital signal value preceding the digital signal value satisfies a threshold condition indicating the presence of a component based on the photon input. A substitution step in which each of the plurality of digital signal values ​​is replaced with a predetermined value during a first period that does not include a component based on the input of the photon, It has, The substitution step is a signal processing method that determines the first period based on the determination result in the determination step.

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