JP2005114473A - Light detection method and biological light-measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a method for detecting a faint light signal, with high accuracy. <P>SOLUTION: In this light detection method, high wave surface selectivity of a heterodyne system is merged with a photon measurement method for measuring light on a photon level, by noticing that the behavior of light wave as photons is dominant in the range of faint light signals due to the particulate properties of light wave. The number of photons or the physical quantity correlating with the number of photons are measured of a heterodyne beat signal, obtained by causing signal light to interfere with reference light (1, 3). The intensity of the signal light is found by a calculation, based on its measurement value (4). The change with time of the faint light is measured with sufficient accuracy, without being limited by the Schottky noise of a photodetector or without lowering the time response of a measurement system. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a light detection method and a biological light measurement apparatus using the method, and in particular, irradiates a subject with light and detects the transmitted or scattered light to measure and image the optical characteristics of the living body. The present invention relates to a biological light measurement device.

  As an image diagnostic apparatus capable of observing the inside of a subject such as a human body in a non-destructive manner, various types such as X-ray CT, MRI, ultrasonic diagnosis, and the like that mainly image morphological information of living tissue have been put into practical use . In addition, various proposals have been made to measure and image biological function information. For example, functional information such as biological metabolites and blood flow is measured by visible to near-infrared light, and brain activity and various diseases An optical imaging apparatus has been proposed that simply measures the above (Patent Documents 1 and 2). However, since the devices described in Patent Documents 1 and 2 use scattered light that spreads spatially, the spatial resolution is low, and it is difficult to image the details of tissues and structures in cells.

  In view of this, development of an optical coherence tomography (OCT) apparatus that selectively detects non-scattered light from the transmitted light of an object and forms an image is in progress. For example, an OCT apparatus has been proposed that irradiates a living body with coherent light, and detects and images only the straight light of the transmitted light of the subject using an optical heterodyne method (Patent Document 3). In addition, an OCT apparatus that detects reflected light from a living body by an optical heterodyne method and selects only a signal having a specific depth for imaging has been proposed (Patent Document 4).

  However, since the ratio of non-scattered light that can be measured is very small and the intensity of incident light is limited from the viewpoint of safety, the intensity of the heterodyne signal generated by the non-scattered light is very small. For this reason, in the OCT apparatus described in Patent Documents 3 and 4, a highly sensitive photomultiplier tube or avalanche photodiode is used for light detection. In the optical heterodyne method, a part of the irradiation light is branched into reference light, and the intensity of the signal light is measured using an interference phenomenon between the reference light and the signal light transmitted or reflected by the subject. In general, signal light and wavelength-shifted (frequency-modulated) reference light are superimposed on a detector, and a beat signal generated by interference between the two is measured. Since the beat signal intensity is a product of the intensity of the signal light and the reference light, the beat signal is amplified by increasing the reference light intensity to improve the SN.

JP 57-115232 A JP-A 63-275323 JP-A-2-110345 JP-A-8-86745

However, when the transmitted light of the subject is measured by the OCT apparatus, the intensity of the detection signal rapidly decreases as the thickness of the subject increases. For example, when an adult head of about 15 cm is used as a subject and measurement is performed using near-infrared light having a wavelength of about 800 nm, the ratio of transmitted light that passes through the subject and reaches the opposing surface is the detector area. Is 1 cm ² , it attenuates to about 10 ⁻¹⁵ . That is, when the incident light intensity is 1 mW, the detected light intensity is 10 ⁻¹⁸ W. When this is converted into the number of photons, it becomes about 60 counts / second, the particle nature of the photons becomes dominant, and the measurement signal noise becomes statistical fluctuation dominant. In this case, in order to suppress the influence of the shot noise of the detector, it is necessary to increase the reference light level. However, even if the reference light level is increased and the heterodyne beat signal is increased, the SN fluctuation is caused by the influence of the statistical fluctuation of the signal light. The ratio is not improved beyond a certain level. In particular, in order to obtain an image effective for diagnosis, a certain SN or more is required. Therefore, it is necessary to increase the irradiation light intensity to the subject or to reduce the fluctuation by narrowing the time bandwidth.

On the other hand, in medical applications, the irradiation light intensity is limited to a certain level or less for safety, so it is difficult to increase the detected light amount. Therefore, in the conventional heterodyne system, it is necessary to slow down the time response and narrow the signal bandwidth. For example, when a photomultiplier tube (image magnification 10 ⁷ ) is used for light detection at a reference frequency modulation of 50 Hz, a stationary current can be electrically measured at 80 pA in the photon region where the number of incident photons is 50 counts / second. It is. However, in order to obtain a 50 Hz beat signal, the time constant of the circuit needs to be about 10 ms or less. In this case, the input photon count rate becomes comparable to the response time of the circuit, and the accuracy of the beat signal cannot be obtained due to the time fluctuation of the incident photons. Therefore, in such a low signal region, it is necessary to lower the modulation frequency of the heterodyne so that the number of incident photons is sufficiently within the period, but if the modulation frequency is lowered, the measurement response time will be delayed accordingly. In clinical application, there is a problem that it cannot sufficiently follow the time change of the living body.

  As described above, the conventional heterodyne method has a problem in that an effective diagnostic image cannot be obtained because a sufficient amount of signal light cannot be obtained if the subject size is large. In addition, in order to obtain an effective diagnostic image, it is necessary to appropriately change the modulation frequency and the circuit time constant according to the size of the subject, and there is a problem that the configuration of the apparatus is complicated and the operation becomes complicated.

  Further, if the modulation frequency of the reference light is lowered to a certain degree or more, it becomes difficult to remove optical and electrical disturbance noise, and it becomes difficult to ensure accuracy. In the case of a reflection type OCT apparatus, if the measurement depth is set deep, the attenuation of the reflected light increases, so that the amount of incident light on the photodetector is significantly reduced, and signal accuracy cannot be obtained sufficiently as in the transmission method. There is a problem.

  An object of the present invention is to realize a method for detecting a weak optical signal with high accuracy, and to provide a biological optical measurement device with an increased measurement depth using the detection method.

  The principle of the light detection method of the present invention that solves the above-mentioned problem is that the light wave in the weak light signal region has a dominant behavior as a photon due to the particle nature of the light wave. It is characterized by a combination of high wavefront selectivity and a photon measurement method that measures light at the photon level. In other words, by measuring the amplitude of the heterodyne beat signal of the signal light and the reference light using the photon measurement method, the weak or extremely weak optical signal is measured with high sensitivity, and the weak optical signal is detected with high accuracy. Made possible.

  Specifically, the number of photons of a heterodyne beat signal in which signal light and reference light interfere with each other or a physical quantity correlated with the number of photons is measured, and the intensity of the signal light is calculated based on the measured value. And As a result, it is possible to measure the temporal change of the weak light with sufficient accuracy without being limited by the Schottky noise of the photodetector and without reducing the time response of the measurement system.

  Further, the transmitted light or reflected light of the irradiation light irradiated to the measurement object is used as signal light, a heterodyne beat signal is formed between the signal light and the reference light whose frequency is modulated, and the number of photons of the heterodyne beat signal Alternatively, a physical quantity correlated with the number of photons can be measured, and the intensity of the signal light can be obtained by calculation based on the measured value.

  In these cases, the number of photons may be counted continuously, but even if counted continuously, the measurement accuracy does not necessarily improve. Therefore, it is preferable to perform the measurement for each of a plurality of measurement gates having a fixed time width set discretely in the time axis direction. This measurement gate may divide a plurality of signal periods of the heterodyne beat signal into sections and set a sampling time of a certain time for each section. For example, the measurement gate may be an integer of 3 or more of the signal period of the heterodyne beat signal.

  The biological light measurement device of the present invention using the above-described light detection method includes a light source that generates light to be irradiated to a measurement target, a modulation unit that generates a reference light that is frequency-modulated by branching the light emitted from the light source, Optical means for forming a heterodyne beat signal obtained by interfering the signal light of the transmitted light or reflected light to be measured with the reference light, and photoelectric conversion means for entering the heterodyne beat signal and converting photons into electric pulses Counting means for counting the electric pulses converted by the photoelectric conversion means for each set time width, calculating means for obtaining the intensity of the signal light based on the number of electric pulses counted by the counting means, and the calculation An image generating means for generating an image based on the intensity of the signal light obtained by the means can be provided.

  According to this, it is possible to measure faint light that has passed through a thick subject to be measured, or faint scattered light or non-scattered light with high accuracy, so that an optical measurement image with improved resolution can be generated. Can contribute to diagnosis.

  In this case, it is possible to extract the beat signal intensity only by computer processing without setting a measurement gate in advance by providing storage means for storing the electrical pulse output from the photoelectric conversion means together with the detection time.

  Note that if the light detection method of the present invention is applied to an OCT apparatus that can obtain a conventional level optical signal, noise can be reduced by selecting the straight component of the photon of the signal light. Can be further improved. In addition, when a light signal with sufficient intensity can be obtained, a conventional light detection method is applied, and when a weak light signal is measured, the light detection method of the present invention is switched and applied, so that it corresponds to the measurement object. A diagnostic image with an appropriate resolution can be obtained.

  According to the present invention, it is possible to realize a method for detecting a weak optical signal with high accuracy. In addition, by using the light detection method of the present invention, it is possible to realize a biological light measurement device with an increased measurement depth.

  Hereinafter, the present invention will be described in detail based on embodiments.

Embodiment 1

  FIG. 1 shows a block configuration diagram of an embodiment of a photon counting detection apparatus to which the light detection method of the present invention is applied. As shown in the figure, the photon counting detector includes a photodetector 1, and a heterodyne beat signal obtained by interfering the signal light to be measured with the reference light is incident on the photodetector 1. The photodetector 1 is a photoelectric conversion means that detects photons of an incident heterodyne beat signal and converts them into electric pulses (hereinafter simply referred to as pulses). For example, the photomultiplier tube has a high multiplication factor and a high-speed response. Apply. The pulse train output from the photodetector 1 is the number of photons incident on the photodetector 1 or a physical quantity that correlates with the number of photons. The pulse train is input to the wave height discriminator 2 to remove non-photon signals such as Schottky noise. As a result, the pulse height discriminator 2 outputs a fixed pulse train indicating the input time of each photon. This regular pulse train is a pulse train having a time density proportional to the intensity of light incident on the photodetector 1. The fixed pulse train output from the pulse height discriminator 2 is input to the counter 3, where the number of pulses is counted for each sampling gate having a fixed time width set here, and converted into a histogram for each set time width. The histogram of the number of pulses output from the counter 3 is input to the arithmetic processing unit 4 constituted by a computer. The arithmetic processing unit is configured using a computer such as a personal computer, and includes a display monitor that displays an image of the calculation result. The arithmetic processing unit 4 calculates the intensity of the heterodyne beat signal based on the histogram of the number of pulses, obtains the intensity of the signal light based on this, and outputs and displays the result on the display monitor.

  That is, the photon counting detection apparatus of FIG. 1 pays attention to the fact that the light wave of a weak optical signal becomes dominant as a photon, and photon measurement that measures light at the photon level with high heterodyne wavefront selectivity. It is characterized by the fusion of law. In other words, by measuring the time distribution of the number of photons of the heterodyne beat signal obtained by interfering the signal light with the frequency-modulated reference light and the signal light, and measuring the amplitude of the heterodyne beat signal, the weak or extremely weak optical signal Was measured with high sensitivity.

  The case where the intensity of the signal light is detected in real time based on the heterodyne beat signal incident on the photodetector 1 has been described above, but the intensity of the signal light can also be detected off-line. In this case, as shown in FIG. 1, the pulse train output from the photodetector 1 is input to the time discriminator 5 to discriminate the detection time of the pulse train, and the pulse train is stored in the memory 6 together with the detection time. To do. According to this, the arithmetic processing means can read out the pulse train from the memory 6, obtain a histogram for each arbitrarily set time width, and extract the intensity of the telodyne beat signal.

  As described above, according to the embodiment of FIG. 1, it is not limited to the Schottky noise of the photodetector 1 and the time response of the measurement system is not deteriorated. Time changes can be measured with sufficient accuracy.

Embodiment 2

  FIG. 2 shows a conceptual configuration diagram of an embodiment of a biological light measurement device to which the photon counting detection device of FIG. 1 is applied. Irradiation light having a frequency fo emitted from the light source 7 is incident on the beam splitter 8, divided at a set ratio, and irradiated onto the subject 9. If the ratio of this branching is set to a light amount that does not exceed the allowable irradiation amount to the living body that is the subject 9 in consideration of attenuation due to passing through the subject 9, for example, a good SN can be obtained. The signal light that has passed through the subject 9 passes through the half mirror 10 and enters the photodetector 1 of the photon counting detector 11. On the other hand, the reference light branched by the beam splitter 8 enters the optical modulator 12 and undergoes modulation (energy shift) by the modulation frequency fs. Thereby, the irradiation light and the reference light have a frequency difference of fs while being output from the same light source. This frequency shift, for example, can be realized by using an AOM element using ultrasonic waves. The frequency-modulated reference light is guided to the mirror 14 via the movable mirror 13, further guided to the half mirror 10 via the mirror 15, and superimposed on the signal light to be incident on the photodetector 1. ing. At this time, the positions of the movable mirror 13, the mirrors 14, 15 and the half mirror 10 are adjusted so that interference occurs between the signal light and the reference light.

  The light source 7 can be composed of a semiconductor laser that emits light (for example, around 830 nm) in the visible to infrared wavelength region. In addition, the wavelength of irradiation light selects the light wavelength suitable for the optical characteristic of a measurement substance. For example, although 830 nm can be selected as a wavelength having a large absorption change by blood hemoglobin for blood flow measurement, the wavelength is not limited to this, and two or more wavelengths may be selected.

  Next, the operation of this embodiment configured as described above will be described. If the intensity of the signal light that has passed through the subject 9 is Is (t) and the intensity of the reference light is Ir (t), both can be expressed by the following equations (1) and (2). Here, Is and Ir are peak values, and δ is a phase difference between them.

(Equation 1)
Is (t) = Is · cos {2πfo · t + δ}

(Equation 2)
Ir (t) = Ir · cos {2π (fo + fs) t}
The light intensity Ihb (t) of the heterodyne beat signal incident on the photodetector 1 is expressed by the following equation (3) and has the waveform shown in FIG.

(Equation 3)
Ihb (t) = Ir + Is + 2√ (Ir · Is) · cos2π (fs · t + δ)
Here, if the detection efficiency of the photodetector 1 is α, the number of photon pulses Nhb (t) detected by the photodetector 1 can be expressed by the following equation 4, and the pulse train shown in FIG. Become.

(Equation 4)
Nhb (t) = α (Ir + Is) + 2α√ (Ir · Is) · cos2π (fs · t + δ)
If the time change of the heterodyne beat signal is detected based on the pulse train thus measured, the intensity of the signal light can be easily measured. Therefore, as shown in FIG. 3 (c), a plurality of measurement gates τi (i = 0, 1, 2) are set in synchronization with the phase of the modulation frequency fs so as to be discrete in the time axis direction. The number of pulses is counted by the counter 3. This measurement gate τi has the same time width, and for example, three time widths τ0, τ1, and τ2 are set as shown in FIG. 3C based on the period of the modulation wave with reference to the phase of the modulation frequency fs. . As shown in the figure, the interval between the phase delays φi (i = 0, 1, 2) of each measurement gate is 2π / 3 · fs, and φ1 = 0, φ2 = 2π / 3 · fs, and φ3 = 4π /, respectively. 3 · fs. When the phase delay φi is represented by the time axis ti, ti = φi / 2πfs, and t1 = 0, t2 = 1 / 3fs, and t3 = 2 / 3fs.

  By counting the number of pulses in the pulse train by the counter 3 in accordance with the measurement gate τi set in this way, a histogram representing a change in the time density of the number of pulses is obtained. The count value Si (i = 0, 1, 2) at each measurement gate τi is expressed by the following equation (5).

この計数値Ｓｉに基づいてヘテロダインビートの各周期におけるビート振幅を、ビート各周期でのビート振幅を容易に求めることができる。そして、求めたビート強度に基づいて、次式の数６により信号光の強度Ｉｓを求める。 (Equation 5)

Based on this count value Si, the beat amplitude in each cycle of the heterodyne beat and the beat amplitude in each beat cycle can be easily obtained. Then, based on the obtained beat intensity, the intensity Is of the signal light is obtained by the following equation (6).

演算処理装置４は、計数器３から出力される計数値Ｓｉに基づいてオンライン処理により信号光の強度Ｉｓを求めてもよいが、演算処理装置４内のメモリにパルス数のヒストグラムを格納しておき、オフラインで信号光の強度Ｉｓを求めてもよい。特に、オンライン処理する場合は、専用のソフトを組み込んだＤＳＰ（ディジタル・シグナル・プロセッサ）を用いることで、高速処理を並行して行えることから、安価なＰＣ(パーソナル・コンピュータ)を用いて小型安価な装置を構成することができる。 (Equation 6)

The arithmetic processing unit 4 may obtain the signal light intensity Is by online processing based on the count value Si output from the counter 3, but stores a histogram of the number of pulses in a memory in the arithmetic processing unit 4. Alternatively, the signal light intensity Is may be obtained offline. Especially for online processing, high-speed processing can be performed in parallel by using a DSP (digital signal processor) incorporating dedicated software, so it is small and inexpensive using an inexpensive PC (personal computer). Can be configured.

  In this way, the two-dimensional distribution of the signal light intensity passing through the subject 9 is obtained and imaged, or the three-dimensional distribution of the signal light intensity is obtained and imaged, and the image is displayed on the display monitor. Thereby, the density | concentration of the specific substance in the subject 9 can be measured and the image of density | concentration distribution can be obtained.

  The number of pulses need not be measured in every cycle of the modulation frequency fs, and may be measured, for example, every N periods (N = 2, 3,...) According to the required response time. Further, the number i of measurement gates τi in each cycle can be set to be larger than three in FIG. If the set number of measurement gates in each cycle is increased, the accuracy of beat amplitude measurement can be improved. However, as the number of measurement gates set increases, the accuracy does not improve. Efficient measurement can be realized by considering the optimum number from the relationship with the time density of the number of photon measurements of the measurement signal.

  In this embodiment, pulse counting is performed in real time (real time), but it is not always necessary to perform in real time. If a sufficiently fast time measurement circuit is used, the input time for each photon pulse is measured. By creating a list storing the count values, it is possible to set an arbitrary measurement gate and perform measurement offline. Thereby, the optimal measurement gate width and phase according to the time density of the input photons can be arbitrarily set. If the data obtained by the list method is used, the amplitude of the beat signal can be obtained from the optimum fitting with the model function of an appropriate beat waveform without using the count value histogram for each measurement gate.

  Here, when the time density of photons incident on the photodetector 1 increases, for example, electric pulses overlap at the anode output of the photomultiplier tube, and so-called counting down may occur in the counter 3. In general, the counting efficiency α tends to decrease as the intensity of the measurement signal increases. In this regard, when photons are input at a substantially uniform time density as in the present embodiment, the photon input time interval has a statistical distribution, and the count can be estimated from the pulse waveform. Therefore, by adding a calculation process for correcting the count rate in accordance with the pulse measurement density, it is possible to perform accurate measurement over a wide count range.

  Further, when the amount of light further increases and the counting efficiency decreases, it can be dealt with by reducing the intensity of the light source 7 or inserting an optical filter having an appropriate attenuation factor into the input part of the photodetector 1. In addition, although the device configuration is large, when the light intensity is high, measurement is performed using a conventional analog heterodyne method, and the photon counting detection device according to the present embodiment can be switched when weak light suitable for photon measurement is obtained. It may be configured. As a result, a heterodyne optical measurement device with a large dynamic range can be realized, and therefore it can be easily applied to a tomographic image device (OCT device) described below.

Embodiment 3

  FIG. 4 shows an embodiment of a biological light measurement device to which the photon counting detection device of the present invention and a conventional analog heterodyne detection device are applied. The present embodiment is an OCT apparatus that measures an optical characteristic distribution in an arbitrary slice in a scatterer and obtains a three-dimensional image based on the measured distribution, and can be applied to, for example, tomographic imaging of the skin or eye retina. In the figure, components having the same reference numerals as those in the embodiment of FIGS. 1 and 2 have the same functional configuration.

  As the light source 7, an SLD (Super Luminescence Diode) having a very short coherent time and a uniform wavefront is used. Irradiation light emitted from the light source 7 is guided to a Michelson type optical interferometer (OC: Optical Coupler) 22 through an optical fiber 21. In this optical interferometer 22, the irradiation light from the light source 1 is branched into irradiation light for the subject 9 and reference light. Irradiation light to the subject 9 is guided to the collimator 24 via the optical fiber 23 and is irradiated to the subject 9 via the lens 25. A part of the light irradiated to the subject 9 undergoes coherent reflection according to the depth position of the subject 9, and the reflected light is incident on the optical interferometer 22 again via the fiber 23.

  On the other hand, the reference light branched by the optical interferometer 22 is guided to the optical modulator 12 via the optical fiber 26 and subjected to frequency modulation, and then irradiated to the movable mirror 28 via the lens 27. ing. The movable mirror 28 is for returning the reference light to the optical interferometer 22 with an appropriate time delay, and the distance from the lens 27 can be adjusted. The reference light reflected by the movable mirror 28 enters the optical interferometer 22 again through the lens 27, the optical modulator 12, and the optical fiber 26.

  Here, assuming that the depth of the measurement position of the subject 9 is d and the speed of light in the subject is c, the signal light having a time delay of 2 dc with respect to the irradiation light to the subject 9 is the optical interferometer 22. Return to. The signal light and the reference light are overlapped by the optical interferometer 22 and enter the photodetector 1 through the optical fiber 29. At this time, since the coherent time of the light source 7 is very short, interference occurs in a range where the phase of the signal light and the reference light coincide with each other, and a beat occurs at the modulation frequency fs of the reference light. Further, by scanning the position of the movable mirror 28, backscattered light from layers having different depths corresponding to the mirror position interferes to generate a beat signal.

  The beat signal output from the photodetector 1 is guided to the switch 31 via the signal line 30. The optical switch 31 switches whether the output of the photodetector 1 is guided to the wave height discriminator 2 via the signal line 32 or to the demodulator 34 via the signal line 33. The beat signal output from the wave height discriminator 2 is input to the counter 3 to count the number of pulses, and the count value is input to the arithmetic processing unit 4 via the switch 35. On the other hand, the beat signal input to the demodulator 34 is selected by the demodulator 34 and the low-pass filter (LPF) 36 as in the conventional analog heterodyne system, and the beat signal is analog-digital (A / D) by the lock-in system. The beat signal output to the converter 37 and converted into a digital signal is input to the arithmetic processing unit 4 via the switch 35.

  Further, the modulation frequency fs is variably input from the transmitter 38 to the optical modulator 12. The modulation frequency fs is input to the counter 3 and the demodulator 34, and is used as a timing signal for the pulse number measurement gate or a synchronization signal for the demodulator 34, respectively.

  As described above, the present embodiment is a combination of the two systems of the photon counting detector according to the present invention and the conventional analog heterodyne photodetector, and switches the measurement mode according to the signal light intensity. It has become. That is, when the amount of signal light incident on the photodetector 1 is sufficiently large, the switches 31 and 35 are connected to the arithmetic processing unit 4 via the system of the conventional demodulator 34, LPF 36, and A / D converter 37. Entered. On the other hand, when the intensity of the signal light is below a predetermined level, the switches 31 and 35 are input to the arithmetic processing unit 4 through the system of the wave height discriminator 2 and the counter 3. In the arithmetic processing unit 4, the intensity of the signal light is obtained based on the intensity of the beat signal input from each system, and the function information in the measurement target part of the subject 9 is imaged as in the embodiment of FIG. To do.

  In addition, according to the present embodiment, the three-dimensional image inside the subject 9 is formed by two-dimensionally scanning the irradiation light with which the subject 9 is irradiated and moving the movable mirror 28 to scan the measurement depth. it can.

  According to the above-described embodiment, the amount of coherent light reaching the surface from the tomographic plane greatly decreases as the depth increases, so that even if an SLD is used, a conventional analog heterodyne photodetection device Then, since a sufficient S / N ratio cannot be obtained due to Schottky noise of the photodetector, only a depth of about 100 μm can be measured. On the other hand, according to the present embodiment, since the photon counting detection device according to the present invention is provided in combination, it is possible to measure weak light. The application range can be expanded.

Embodiment 4

  FIG. 5 shows a configuration diagram of another embodiment of an OCT apparatus to which the photon counting detection apparatus of the present invention is applied. In the figure, components having the same reference numerals as those in the embodiment of FIGS. 1 and 2 have the same functional configuration. As shown in the figure, the irradiation light emitted from the light source 7 is expanded into parallel light by the lens 41 and is irradiated onto the subject 9 through the half mirror 42. The signal light reflected from the subject 9 is reflected by the half mirror 42 and is incident on the photodetector 43. On the other hand, the reference light reflected and branched by the half mirror 42 is reflected in the incident direction by the movable mirror, passes through the half mirror 42, is superimposed on the signal light, and enters the photodetector 43. Yes. The movable mirror 44 is constantly moved and scanned in the direction of the optical axis of the reference light at a constant speed V, so that the reference light is subjected to frequency modulation at a constant frequency.

  Here, a two-dimensional image intensifier capable of simultaneously measuring signal light in two dimensions is applied to the photodetector 43 of the present embodiment. This image intensifier is a two-dimensional image sensor such as a photodiode array having a two-dimensional function for detecting a two-dimensional light input or a CCD camera such as a linear image sensor (see paragraph number 0012 of Patent Document 4). A two-dimensional light input can be detected and multiplied.

  In the photodetector 43, light having the same phase difference between the reference light and the signal light causes interference, as in the above-described embodiment. Here, since the movable mirror 44 is always scanned at a constant speed, a beat having a frequency corresponding to the scanning speed is generated. Then, the scanning position of the movable mirror 44 determines the measurement depth of the subject 9. The two-dimensional heterodyne beat signal light output from the photodetector 43 is projected onto the CCD camera 46 via the lens 45. The CCD camera 46 is controlled by the gate controller 47 to capture the signal light. As described with reference to FIG. 3C, the gate control unit 47 is a signal input to the time width of the measurement gate τi every 1 / M (M = 2, 3,...) Of the modulation frequency period. Count the photon distribution of light. The time width of the measurement gate τi is set so that the number of input photons to each pixel of the CCD camera 46 does not exceed 1. That is, when tw = 1 / (M · fs) and the number of photon inputs per unit time of the CCD camera 46 is Np, tw × Np≈1 is set. As a result, since it can be assumed that each pixel has no input of one photon or more in each image frame, the number of photon inputs to the image can be counted using a predetermined dark value. It is also possible to cope with higher-speed measurement by providing a plurality of dark values and identifying two or more photon inputs. Here, if a sufficient number of frames can be set within the cycle of the modulation frequency period fs, a two-dimensional image of the beat signal intensity can be measured based on this.

  According to the present embodiment, in addition to the effects of the above-described embodiments, two-dimensional measurement can be performed simultaneously, so that optical topography can be performed at high speed. Also in the present embodiment, similarly to the embodiment of FIG. 4, it is possible to switch and measure by using a conventional analog heterodyne type photodetector in accordance with the amount of signal light.

Embodiment 5

  FIG. 6 shows a configuration diagram of another embodiment of an OCT apparatus to which the photon counting detection apparatus of the present invention is applied. In the figure, components having the same reference numerals as those in the embodiment of FIGS. 1 and 2 have the same functional configuration. As shown in the figure, the irradiation light emitted from the light source 7 (for example, a near-infrared laser having a wavelength of 800 nm) enters the optical modulator 12 and is modulated by the modulation frequency fs. The output light of the light modulator 12 is converted into parallel light by the lens 48, and modulated light and non-modulated light are generated as two adjacent light beams. The object 9 is irradiated with this light beam, and the transmitted light is narrowed down by collimators 49 and 50 and input to the photodetector 1. Here, the diameters dl and d2 of the collimators 49 and 50 satisfy the condition of R> dl · d2 / λ with respect to the pinhole interval R and the measurement wavelength λ as conditions for selectively transmitting non-scattered light. Set to. Thereby, only light with good straightness reaches the photodetector 1. As a result, the optical absorption on the specific line of the subject 9 can be measured. Therefore, if a scan in the t direction and the θ direction for obtaining projection ellipses from multiple directions is added to this, a tomographic image can be constructed by the back projection method used in X-ray CT or the like.

  The pulse of the beat signal detected by the photodetector 1 is imaged by the processing of the wave height discriminator 2, the counter 3, and the arithmetic processing unit 4 as in the embodiment of FIG. 1 or FIG.

  According to the present embodiment, it is possible to measure a beat of light having a frequency difference of the frequency fs incident in the vicinity with high accuracy. In addition to scanning the subject 9 as a scanning method for imaging, the optical system may be configured to move and rotate around the subject 9. Furthermore, high-speed measurement is possible by using a fan beam method widely used for X-ray CT as an imaging method. In this embodiment, since the beat is detected by making two optical beams having a frequency difference close to each other on the same axis, strong reference is made as in the embodiment shown in FIGS. Since signal multiplication by light cannot be used, the effect of the light detection method of the present invention is great. Also in the present embodiment, similarly to the embodiment of FIG. 4, it is possible to switch and measure by using a conventional analog heterodyne type photodetector in accordance with the amount of signal light.

It is a block diagram of the photon counting detection apparatus of one Embodiment to which the photon detection method of this invention is applied. It is a conceptual block diagram of one Embodiment of the biological light measuring device to which the photon counting detection apparatus of FIG. 1 is applied. It is a wave form diagram of each part explaining operation | movement of embodiment of FIG. It is a conceptual block diagram of other embodiment of the biological light measuring device to which the photon counting detection apparatus of this invention is applied. It is a conceptual lineblock diagram of an embodiment of a two-dimensional type biological light measuring device to which a photon counting detection device of the present invention is applied. It is a conceptual block diagram of other embodiment of the biological light measuring device to which the photon counting detection apparatus of this invention is applied.

Explanation of symbols

1 Photodetector 2 Wave height discriminator 3 Counter 4 Arithmetic processing device 5 Time discriminator 6 Memory 7 Light source 8 Beam splitter 9 Subject 10 Half mirror 11 Photon counting detector 12 Optical modulator 13, 14, 15 Mirror

Claims (5)

  A photodetection method in which the number of photons of a heterodyne beat signal in which signal light and reference light interfere with each other or a physical quantity correlated with the number of photons is measured, and the intensity of the signal light is calculated based on the measured value.   The transmitted light or reflected light of the irradiation light irradiated to the measurement object is used as signal light, the signal light and reference light whose frequency of the irradiation light is modulated are formed to form a heterodyne beat signal, and the photon of the heterodyne beat signal The light detection method which calculates | requires the physical quantity correlated with the number or the number of photons and calculates | requires the intensity | strength of signal light based on this measured value.   3. The photodetection according to claim 1, wherein the measurement of the number of photons or the physical quantity correlated with the number of photons is performed for each of a plurality of measurement gates having a fixed time width discretely set in a time axis direction. Method.   A light source that generates light to irradiate the measurement target, a modulation unit that generates a reference light that is frequency-modulated by branching the light emitted from the light source, a signal light that is transmitted or reflected from the measurement target, and the reference light. Optical means for forming a heterodyne beat signal by interfering with each other, photoelectric conversion means for entering the heterodyne beat signal to convert photons into electric pulses, and electric pulses converted by the photoelectric conversion means for each set time width Counting means for counting, computing means for obtaining the intensity of the signal light based on the number of electric pulses counted by the counting means, and image generation for generating an image based on the intensity of the signal light obtained by the computing means A biological light measuring device having means.   The living body light measurement apparatus according to claim 4, further comprising a storage unit that stores the electric pulse output from the photoelectric conversion unit together with a detection time.

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