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 each of the plurality of digital signal values ​​satisfies a threshold condition indicating the presence of a component based on the input of a photon. In the first period, the noise reduction circuit 32 replaces each of the plurality of digital signal values ​​with a predetermined value. The noise reduction circuit 32 considers the period excluding the second period as the first period. The second period is a period including the digital signal value that has been determined to satisfy the threshold condition among the plurality of digital signal values, and at least one digital signal value before and after that digital signal value.

Description

[Technical Field]

[0001] This disclosure relates to a signal processing circuit and a signal processing method. [Background technology]

[0002] Patent Document 1 discloses a light intensity detection device. This light intensity detection device performs A / D conversion on the detection signal of a photon count type photodetector. If the A / D converted detection signal is above a threshold, the detection signal is sent directly to the subsequent photon count calculation circuit. If the A / D converted detection signal is below the threshold, a preset reference value is sent to the subsequent photon count calculation circuit. The photon count calculation circuit determines the number of photons or light intensity incident on the photon count type photodetector from the area of ​​the detection signal waveform acquired until the light intensity measurement is completed. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2012-37267 [Overview of the project] [Problems that the invention aims to solve]

[0004] As described in Patent Document 1, in photodetectors, there is a method to reduce noise components in the detection signal and improve detection accuracy by replacing the detection signal with a predetermined value when the detection signal is smaller than a threshold. However, in this method, even if the detection signal contains components caused by the photon input, if the magnitude of the detection signal is smaller than the threshold, the detection signal will be replaced with a predetermined value. This is a factor that reduces the accuracy of light intensity measurement.

[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 a time interval. The noise reduction unit reduces noise components in the time-series data that are different from the component based on the photon input. The noise reduction unit includes a determination unit and a replacement unit. The determination unit determines whether each of the plurality of digital signal values ​​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 the first period. The replacement unit defines the period excluding the second period as the first period. The second period is a period including the digital signal value that the determination unit has determined to satisfy the threshold condition among the plurality of digital signal values, and at least one digital signal value before and after that digital signal value.

[0007] When detecting a photon, not only the digital signal value that satisfies the threshold condition, but also the digital signal values ​​before and after that digital signal value may contain components attributable to the photon input. In the signal processing circuit described in [1] above, the second period includes not only the digital signal value that satisfies the threshold condition, but also at least one digital signal value before and after that digital signal value. The first period in which the digital signal value is replaced with a predetermined value is the period excluding the second period. This makes it possible to output digital signal values ​​that contain components attributable to the photon input, which exist before and after the digital signal value that satisfies the threshold condition, without replacing them with a predetermined value. Therefore, the signal processing circuit described in [1] above can be incorporated into a photodetector to improve the accuracy of light intensity measurement.

[0008] [2] In the signal processing circuit described in [1] above, a digital signal value satisfying the threshold condition may mean that the difference between the digital signal value and the digital signal value before it exceeds a predetermined threshold. 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 at the rising edge of the pulsed change. Therefore, based on the fact that the difference between the digital signal value and the digital signal value before it exceeds a predetermined threshold, the presence of a component based on the input of a photon can be detected with high accuracy. Furthermore, unlike the method described in Patent Document 1, which compares the magnitude of the digital signal value and the threshold, even when multiple photons are input at short time intervals, the presence of a component based on each photon can be accurately detected.

[0009] [3] In the signal processing circuit described in [2] above, the difference value may be the difference between the digital signal value and the digital signal value immediately preceding it. 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] In any of the signal processing circuits described in [1] to [4] above, the time width of the second period may be greater than 1.5 times the full width at half maximum of the component based on the photon input. In this case, the digital signal value including the component caused by the photon input can be output over a sufficient time width without being replaced with a predetermined value.

[0012] [6] 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, it is determined whether each of the plurality of digital signal values ​​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. In the replacement step, the first period is defined as the period excluding a second period which includes the digital signal value that was determined in the determination step to satisfy the threshold condition, and at least one digital signal value before and after that digital signal value.

[0013] The signal processing method described in [6] above yields the same effect as the signal processing circuit described in [1] above. That is, it is possible to output digital signal values ​​that include components caused by the photon input, which exist before and after the digital signal value that satisfies the threshold condition, without replacing them with a predetermined value. Therefore, it can be incorporated into a photodetection method to improve the accuracy of light intensity measurement. [Effects of the Invention]

[0014] 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]

[0015] [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 diagram illustrating the effect. [Figure 9] Figure 9 is a graph showing time-series data when two photons are input at short time intervals. [Figure 10] Figure 10 is a graph showing time-series data when two photons are input at short time intervals. [Figure 11] Figure 11 shows an intensity mapping image generated by a photodetector. [Figure 12] Figure 12 is a graph showing time-series data for the first and second rows of the detected region. [Figure 13] Figure 13 is a graph showing time-series data for the third and fourth rows of the detected region. [Figure 14] Figure 14 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]

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] The noise removal circuit 32 is a digital circuit and is the noise reduction unit 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 32 can 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.

[0021] FIG. 3 is a block diagram showing the functional configuration of the noise removal circuit 32. The noise removal circuit 32 of the present embodiment has 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 a threshold condition indicating the presence of a 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) that digital signal value I(t n-1 ) exceeds a predetermined threshold. The predetermined threshold is set in advance according to the magnitude of the noise component. In one example, the predetermined threshold 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.

[0022] 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 if 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 following. 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.

[0028] 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).

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The effects obtained by the signal processing circuit 3 and signal processing method according to the above embodiment will now be explained. When a photon is detected, the digital signal value I(t) that satisfies the threshold condition is n ) as well as the digital signal value I(t n Digital signal values ​​I(t) before and after ) n-1 ) and I(t n+1 ) may also contain components resulting from the photon input. In the signal processing circuit 3 of this embodiment, the digital signal value I(t) that satisfies the threshold condition is n ) as well as the digital signal value I(t n ) and counting from at least one digital signal value I(t n-1 ) and I(t n+1The digital signal value I(t) that satisfies the threshold condition is included in the second period Tb. In other words, the first period Ta, in which the digital signal value is replaced with a predetermined value, is the period excluding the second period. As a result, the digital signal value I(t) that satisfies the threshold condition is included in the second period Tb. n The digital signal value I(t) that includes components caused by the photon input, which are present before and after the input. n-1 ) and I(t n+1 ) can be output without replacing it with a predetermined value. Therefore, according to the signal processing circuit 3 and signal processing method of this embodiment, it can be incorporated into the light detection device 1 and light detection method to improve the light intensity measurement accuracy (S / N ratio).

[0034] As in this embodiment, the digital signal value I(t n ) satisfies the threshold condition if its 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. When a photon is input to the photodetector 2, the time-series data Da changes in a pulsed manner, but in many 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 input exceeds a predetermined threshold TH, the presence of a component based on the input photon can be detected with high accuracy. Furthermore, unlike the method described in Patent Document 1, which compares the magnitude of the digital signal value with the threshold, the presence of a component based on each photon can be accurately detected even when multiple photons are input at short time intervals.

[0035] 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-1The 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.

[0036] 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.

[0037] As in this embodiment, the time width of the second period Tb is 1 / 2f, which is the full width at half maximum of the component based on the photon input. c It may be greater than 1.5 times. In this case, the digital signal value, which includes components caused by the photon input, can be output over a sufficient time width without being replaced with a predetermined value.

[0038] Figure 8 is a diagram illustrating the effects obtained by this embodiment. Part 8(a) of Figure 8 is a graph showing the noise reduction effect according to the method described in Patent Document 1, showing time-series data Da and time-series data Dd obtained by replacing digital signal values ​​smaller than the threshold with zero. Part 8(b) of Figure 8 is a graph showing the noise reduction effect according to this embodiment, showing time-series data Da and Db.

[0039] In the method described in Patent Document 1, as shown in part 8(a) of Figure 8, if the digital signal value falls below a threshold, it is replaced with a predetermined value, even if it is the tail portion of the pulsed time waveform Pa caused by the photon input. Therefore, region A shown in part 8(a) of Figure 8 is not added to the time integral for calculating the signal amount and becomes a loss. As a result, the accuracy of light intensity measurement decreases. In contrast, in this embodiment, as shown in part 8(b) of Figure 8, the tail portion of the pulsed time waveform Pa caused by the photon input is output as time-series data Db without being replaced with a predetermined value. Therefore, the loss can be reduced and the accuracy of light intensity measurement can be improved.

[0040] Figures 9 and 10 are graphs showing time-series data Da and Db when two photons are input at short time intervals. Figure 9 shows, as a comparative example, the case in which the method described in Patent Document 1 is applied, that is, the method in which the digital signal value is replaced with a predetermined value when the digital signal value is smaller than a threshold. In Figure 9, part (a) shows the time-series data Da and the threshold THb, and part (b) shows the time-series data Dd in which the digital signal value smaller than the threshold THb is replaced with zero. Figure 10 shows the case in which the method of this embodiment is applied. In Figure 10, part (a) shows the time-series data Da and the difference value S(t1)~S(t N (b) shows the time series data Db, and the threshold TH is shown in (b).

[0041] In the method described in Patent Document 1, as shown in part 9(b) of Figure 9, the tail portion of the time waveform Pa at the very end is replaced with a predetermined value. Consequently, loss occurs, and the accuracy of light intensity measurement decreases. In contrast, in this embodiment, as shown in part 10(b) of Figure 10, the tail portion of the time waveform Pa at the very end is output as time-series data Db without being replaced with a predetermined value. This is because the difference value S(t) occurs at the rising edge of the time waveform Pa at the very end. 11 This is because the value exceeds the threshold TH, causing the end of the second period Tb (see Figure 5), in which no substitution to a predetermined value is performed, to be further delayed in time. Thus, according to this embodiment, even when multiple photons are input at short time intervals, losses can be reduced and the accuracy of light intensity measurement can be improved.

[0042] (Examples) Figure 11 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 12 is a graph showing time-series data Db for the first row of the detection area detected along arrow B1. Part (b) of Figure 12 is a graph showing time-series data Db for the second row of the detection area detected along arrow B2. Part (a) of Figure 13 is a graph showing time-series data Db for the third row of the detection area detected along arrow B3. Part (b) of Figure 13 is a graph showing time-series data Db for the fourth row of the detection area detected along arrow B4. In Figures 12 and 13, the solid line represents the time-series data Db, and the dashed line represents the electrical signal before D / A conversion. Referring to Figure 12, it can be seen that the noise component contained in the electrical signal is effectively reduced by the photodetector 1, and that the component caused by photons input to the photosensor 21 is accurately detected down to the base. Referring to Figure 13, it can be seen that the amount of incident light can be accurately detected even when photons are input at short time intervals.

[0043] FIG. 14 is a graph showing the correlation between the integrated value (output) of a pulsed time waveform and the occurrence frequency. The (a) part of FIG. 14 shows the case where the noise removal circuit 32 is provided, and the (b) part of FIG. 14 shows the case where the noise removal circuit 32 is not provided. The numbers shown in the (a) part of FIG. 14 represent the number of photons included in the time waveform. As shown in the (b) part of FIG. 14, when the noise removal circuit 32 is not provided, the light quantity measurement accuracy is low, and the difference in the integrated value due to the number of photons is ambiguous. On the other hand, as shown in the (a) part of FIG. 14, when the noise removal circuit 32 is provided, the light quantity measurement accuracy is high, and the difference in the integrated value due to the number of photons is clear. Thus, according to the signal processing circuit 3 of the present embodiment, the light quantity measurement accuracy can be improved and the number of photons can be counted more accurately.

[0044] 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 exemplified. The difference value S(t n ) is not limited to this, and 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. Further, the signal processing circuit and signal processing method according to the present disclosure may be implemented using the digital signal value I(t n ) without using the difference value S(t n ).

[0045] Also, in the above embodiment, as the case where the digital signal value I(t n ) satisfies the threshold condition, the case where the difference value S(t n ) exceeds a predetermined threshold TH is exemplified, but the threshold condition indicating the presence of a component based on the input of photons is not limited to this. For example, like the method described in Patent Document 1, the digital signal value I(t n) may exceed a predetermined threshold. Even in that case, the first period Ta and the second period Tb may be set to the digital signal value I(t) as in the above embodiment. n By setting the timing of ) as a reference, the same effects as in the above embodiment can be obtained. [Explanation of symbols]

[0046] 1... Light detection device, 2... Photodetector, 3... Signal processing circuit, 3a... Input terminal, 3b... Output terminal, 4... Signal quantity calculation circuit, 5... Calculation unit, 21... Light sensor, 22... I / V conversion circuit, 31... A / D converter, 32... Noise reduction circuit, 321... Judgment unit, 322... Replacement unit, A... Area, D1... Column direction, D2... Row direction, Da, Db, Dd... Time series data, Dc... Numerical data, E... Unit area, I(t n ),I(t1)~I(t N )...Digital signal value, Ja...Current signal, O(t1)~O(t N )...Output value, P...Light, Pa...Time waveform, S(t n ), S(t1)~S(t N )...Difference value, Ta...First period, Tb...Second period, TH,THb...Threshold, Va...Electrical 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 that determines whether each of the plurality of digital signal values ​​satisfies a threshold condition indicating the presence of a component based on the input of the photon, During the first period, a replacement unit replaces each of the plurality of digital signal values ​​with a predetermined value, It has, The replacement unit is a signal processing circuit that defines the first period as a period excluding a second period which includes a digital signal value that the determination unit has determined to satisfy the threshold condition among the plurality of digital signal values, and at least one digital signal value before and after that digital signal value.

2. The signal processing circuit according to claim 1, wherein the digital signal value satisfies the threshold condition if the difference between the digital signal value and the digital signal value preceding the digital signal value exceeds a predetermined threshold.

3. The signal processing circuit according to claim 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 any one of claims 1 to 3, wherein the predetermined value is a constant value.

5. The signal processing circuit according to any one of claims 1 to 3, wherein the time width of the second period is greater than 1.5 times the full width at half maximum of the component based on the photon input.

6. 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 of determining whether each of the plurality of digital signal values ​​satisfies a threshold condition indicating the presence of a component based on the input of the photon, In the first period, a substitution step is performed in which each of the plurality of digital signal values ​​is replaced with a predetermined value, It has, The substitution step is a signal processing method in which the first period is defined as a period excluding a second period that includes a digital signal value among the plurality of digital signal values ​​that is determined in the determination step to satisfy the threshold condition, and digital signal values ​​counting from the said digital signal value and one before and one after it.

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