JP2017096885A - Image capturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce cost of an image capturing device with a wavelength-swept light source by doubling the frequency of a k-clock signal using an inexpensively configured frequency doubling circuit.SOLUTION: An image capturing device includes; a light source unit configured to emit light while sweeping the frequency of the light; an interference unit configured to split the light from the light source into reference light and irradiation light for irradiating a subject, and generate interference light from the reference light and light reflected from the subject; a clock signal generator unit configured to generate a clock signal on the basis of the light from the light source; a detection unit configured to detect the interference light and generate an interference signal in synchronization with the clock signal; and an information acquisition unit configured to acquire information on the subject on the basis of the interference signal. The clock signal generator unit includes a frequency doubling circuit configured to double the frequency of the clock signal by obtaining an exclusive OR of a signal corresponding to portions of the clock signal at a voltage level higher than a positive first threshold and a signal corresponding to portions of the clock signal at a voltage level lower than a negative second threshold.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an imaging apparatus.

  Currently, an imaging apparatus (hereinafter referred to as an OCT apparatus) using an optical coherence tomography (hereinafter referred to as OCT) has been developed. The OCT apparatus irradiates an object with light and causes reflected light returning from different depths of the object to interfere with reference light according to the wavelength of the irradiated light. The OCT apparatus can obtain information relating to a tomography of an object, specifically a tomographic image, by analyzing a frequency component included in a time waveform of interference light intensity (hereinafter referred to as an interference spectrum). The OCT apparatus is used, for example, for fundus examination as an ophthalmic imaging apparatus.

  In the examination using the OCT apparatus, the behavior of the subject is limited during the examination in order to obtain an accurate image. Therefore, when the measurement speed of the OCT apparatus is slow, the period during which the subject's behavior is restricted becomes long, and the physical burden on the subject increases. For this reason, in the OCT apparatus, it is desired to improve the measurement speed and reduce the physical burden on the subject.

  Therefore, as an OCT apparatus with improved measurement speed, an OCT apparatus using a wavelength swept light source (Swept Source OCT apparatus, hereinafter referred to as an SS-OCT apparatus) has been actively developed. Patent Document 1 discloses an example of an SS-OCT apparatus, and exemplifies a light source using a fiber ring resonator and a wavelength selection filter as a wavelength tunable light source.

  In the SS-OCT apparatus, in order to further improve the measurement speed, it is preferable to acquire a wider range of tomographic information with one imaging. For this reason, a technique has been studied in which the imaging range in the depth direction of a living tissue that is an examination subject using an SS-OCT apparatus is made wider. In addition, the depth direction of the living tissue as the subject is generally called the A scan direction.

  Here, in the SS-OCT apparatus, the imaging range in the A-scan direction can be made wider by increasing the number of times of sampling the interference light between the reflected light from the subject and the reference light, that is, the sampling number of the interference light. Are known. For this reason, in order to increase the sampling number of the interference light and widen the imaging range in the A-scan direction, the frequency of the clock signal indicating the sampling timing (hereinafter referred to as k clock signal) is increased. .

  Generally, in order to generate a high-frequency k clock signal using light from a light source, it is necessary to use a light source having a long coherent length. However, since a light source having a long coherent length is expensive, the cost of the SS-OCT apparatus increases when such a light source is used. Therefore, a method for increasing the k clock frequency without using a light source having a long coherent length has been proposed.

  Patent Document 2 discloses a method of doubling the frequency of the sample clock of the SS-OCT apparatus using a multiplier.

  Non-Patent Document 1 discloses a method of doubling the frequency by taking the exclusive OR of the original waveform and the waveform obtained by shifting the phase of the original waveform by 90 ° using a 90 ° phase shift circuit. ing.

JP 2012-115578 A US Pat. No. 7,916,387

J. et al. Xi, L. Huo, J. et al. Li, X. Li, “Generic real-time uniform K-space Sampling method for high-speed swept-Source optical coherence tomography”, OPTICS EXPRESS, e. 18, no. 9, p. 9511-9517

  However, the multiplier described in Patent Document 2 and the 90 ° phase shift circuit described in Non-Patent Document 1 are also expensive, which leads to an increase in the cost of the SS-OCT apparatus.

  Therefore, in the present invention, the frequency of the k clock signal is doubled using a frequency doubling circuit having an inexpensive configuration, thereby reducing the cost of the imaging apparatus using the wavelength swept light source.

  According to one embodiment of the present invention, while sweeping the frequency of light, the light source unit that emits the light, the light from the light source unit is branched into reference light and irradiation light that irradiates the subject, An interference unit that generates interference light by the reference light and reflected light of the irradiation light reflected from the subject, a clock signal generation unit that generates a clock signal based on light from the light source unit, and the light source A light branching unit that splits the light from the light into the light incident on the clock signal generation unit and the light incident on the interference unit, and a detection unit that detects the interference light and generates an interference signal in synchronization with the clock signal And an information acquisition unit that acquires information on the subject based on the interference signal, wherein the clock signal generation unit doubles the frequency of the clock signal based on the light from the light source unit. Circuit including the frequency The logic circuit takes the logical sum of a signal composed of a portion having a potential equal to or higher than the first threshold of the clock signal and a signal composed of a portion having a potential equal to or lower than the second threshold of the clock signal. An imaging device is provided in which the frequency of the signal is doubled and the first threshold has a positive polarity and the second threshold has a negative polarity.

  Since the imaging apparatus according to the present invention has the above-described configuration, the frequency of the k clock can be doubled at low cost, and the cost of the imaging apparatus using the wavelength swept light source can be reduced.

1 schematically shows a configuration example of an OCT apparatus according to a first embodiment of the present invention. 1 schematically illustrates an example of the configuration of a k clock generation unit in an OCT apparatus according to a first embodiment of the present invention. 1 schematically shows a configuration example of a frequency doubling circuit in an OCT apparatus according to Embodiment 1 of the present invention. 4 shows waveforms of signals that propagate in a frequency doubling circuit in the OCT apparatus according to the first embodiment of the present invention. 1 schematically shows an example of the configuration of an OCT apparatus according to a modification of Embodiment 1 of the present invention. An example of the structure relevant to the frequency doubling circuit of the OCT apparatus by Example 2 of this invention is shown roughly. 3 schematically shows a configuration example of a frequency doubling circuit in an OCT apparatus according to a second embodiment of the present invention. 5 shows waveforms of signals that propagate through a frequency doubling circuit in an OCT apparatus according to Embodiment 2 of the present invention. An example of the point spread function of the OCT interference signal in the OCT apparatus of Example 2 of the present invention is schematically shown. 7 is a flowchart illustrating an operation for adjusting a reference potential of a comparator in the OCT apparatus according to the second embodiment of the present invention.

  Hereinafter, exemplary embodiments for carrying out the present invention will be described in detail with reference to the drawings. However, dimensions, materials, shapes, relative positions of components, and the like described in the following embodiments are arbitrary and can be changed according to the configuration of the apparatus to which the present invention is applied or various conditions. Also, in the drawings, the same reference numerals are used between the drawings to indicate the same or functionally similar elements.

  Hereinafter, an imaging apparatus (OCT apparatus) using the optical coherence tomography according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 1 schematically shows a configuration example of an OCT apparatus according to a first embodiment of the present invention. Hereinafter, the OCT apparatus according to the present invention will be described as an OCT apparatus used for examining the fundus of a subject. However, the OCT apparatus according to the present invention can be used for purposes other than fundus examination, and may be used for examination of any object such as an organ. The OCT apparatus according to the present invention is an SS-OCT apparatus using a wavelength swept light source. However, for the sake of simplicity, the OCT apparatus is hereinafter simply referred to as an OCT apparatus.

  As shown in FIG. 1, the OCT apparatus 1 includes a light source unit 10 that emits light, an interference unit 20 that generates interference light, a detection unit 30 that detects interference light, and a fundus of an eye that is a subject 100. An information acquisition unit 40 that acquires the information and a display unit 70 that displays the acquired information are provided. Furthermore, the OCT apparatus 1 irradiates the subject 100 with irradiation light, and emits the reflected light from the subject 100 to the interference unit 20, and the reference for causing interference with the reflected light emitted from the measurement arm 50. A reference arm 60 that emits light to the interference unit 20 is provided. Further, in the OCT apparatus 1, a k clock generation unit (clock signal generation unit) 80 that generates a k clock signal, and light from the light source unit 10 is incident on the light incident on the k clock generation unit 80 and the interference unit 20. An optical fiber coupler (branching unit) 90 that branches into light is provided.

  The light source unit 10 includes a wavelength swept light source 11 that sweeps the wavelength of emitted light and sweeps the optical frequency. As the wavelength swept light source 11, any light source can be used as long as it can sweep the wavelength of emitted light and sweep the optical frequency. Therefore, the wavelength swept light source 11 may be, for example, a light source using a fiber ring resonator and a wavelength selection filter described in Patent Document 1, or other commercially available wavelength swept laser. The light source unit 10 is connected to an optical fiber coupler 90 via an optical fiber.

  The optical fiber coupler 90 is connected to the light source unit 10, the interference unit 20, and the k clock generation unit 80 via an optical fiber. The optical fiber coupler 90 branches the light from the light source unit 10 into light that enters the k clock generation unit 80 and light that enters the interference unit 20. A beam splitter or the like may be used instead of the optical fiber coupler 90.

  The k clock generation unit 80 generates a k clock signal based on the light emitted from the light source unit 10 via the optical fiber coupler 90. Further, the k clock generation unit 80 sends the generated k clock signal to the detection unit 30, and the detection unit 30 detects interference light and is based on the interference light in synchronization with the k clock signal received from the k clock generation unit 80. Generate an interference signal.

  Optical fiber couplers 21 and 22 are provided in the interference unit 20. The optical fiber coupler 21 is connected to the optical fiber couplers 90 and 22, the measurement arm 50, and the reference arm 60 through an optical fiber. The optical fiber coupler 21 branches the light emitted from the light source unit 10 via the optical fiber coupler 90 into irradiation light irradiated onto the fundus via the measurement arm 50 and reference light via the reference arm 60. .

  Irradiation light is irradiated to the subject 100 via the measurement arm 50, and enters the optical fiber coupler 22 via the measurement arm 50 and the optical fiber coupler 21 as reflected light reflected by the subject 100. On the other hand, the reference light enters the optical fiber coupler 22 via the reference arm 60. The reflected light and the reference light interfere with each other at the optical fiber coupler 22 and are emitted from the optical fiber coupler 22 as interference light.

  The optical fiber coupler 22 is connected to the detection unit 30 via two optical fibers having different optical path lengths. The interference unit 20 may be configured using a beam splitter or the like instead of the optical fiber coupler.

  The measurement arm 50 is provided with a polarization controller 51, a collimator 52, an X-axis scanner 53, a Y-axis scanner 54, and a focus lens 55. The polarization controller 51 is provided in an optical fiber connected from the optical fiber coupler 21 to the measurement arm 50 and adjusts the polarization state of the irradiation light and the reflected light passing through the measurement arm 50. The collimator 52 is connected to the optical fiber coupler 21 via an optical fiber, and irradiates the irradiation light whose polarization state is adjusted by the polarization controller 51 as spatial light. The irradiation light irradiated as the spatial light is irradiated to the fundus of the subject 100 via the X-axis scanner 53, the Y-axis scanner 54, and the focus lens 55.

  The X-axis scanner 53 and the Y-axis scanner 54 are configured by deflection mirrors arranged so that their rotation axes are orthogonal to each other. The X-axis scanner 53 and the Y-axis scanner 54 constitute a scanning unit having a function of scanning the fundus with irradiation light, and can change the irradiation position of the irradiation light on the fundus. Here, the X-axis scanner 53 performs scanning in the X-axis direction, and the Y-axis scanner 54 performs scanning in the Y-axis direction. Note that the X-axis direction and the Y-axis direction are directions orthogonal to the eye axis direction of the eye that is the subject 100, and are also orthogonal to each other.

  Irradiation light applied to the fundus is reflected as backscattered light (reflected light) on the fundus. The reflected light from the fundus is emitted from the measurement arm 50 via the focus lens 55, the Y-axis scanner 54, the X-axis scanner 53, the collimator 52, and the polarization controller 51 again. Then, it propagates through the optical fiber and enters the optical fiber coupler 22 via the optical fiber coupler 21.

  On the other hand, the reference arm 60 is provided with a polarization controller 61, a collimator 62, a dispersion compensation glass 63, an optical path length adjustment optical system 64, a dispersion adjustment prism pair 65, and a collimator 66. The polarization controller 61 is provided in an optical fiber connected to the reference arm 60 from the optical fiber coupler 21 and adjusts the polarization state of the reference light passing through the reference arm 60. The collimator 62 is connected to the optical fiber coupler 21 via an optical fiber, and emits the reference light whose polarization state is adjusted by the polarization controller 61 as spatial light. The reference light emitted as the spatial light enters the collimator 66 via the dispersion compensation glass 63, the optical path length adjustment optical system 64, and the dispersion adjustment prism pair 65.

  The dispersion compensation glass 63 and the dispersion adjustment prism pair 65 can adjust the dispersion of the reference light. Therefore, by using the dispersion compensation glass 63 and the dispersion adjustment prism pair 65, the dispersion of the reference light can be adjusted so as to correspond to the dispersion of the reflected light passing through the measurement arm 50. Further, the optical path length adjusting optical system 64 can move in a direction approaching or moving away from the collimators 62 and 66 as indicated by an arrow A1 in FIG. 1, and can adjust the optical path length of the reference arm 60. Therefore, the optical path length of the reference arm 60 can be adjusted by the optical path length adjusting optical system 64 according to the optical path length to the fundus through which the irradiation light passes.

  The reference light incident on the collimator 66 propagates through the optical fiber connecting the collimator 66 and the optical fiber coupler 22 and enters the optical fiber coupler 22.

  As described above, the reflected light and the reference light that have entered the optical fiber coupler 22 interfere with each other, and are output as branched interference light from the optical fiber coupler 22 into two optical fibers having different optical path lengths. Is incident on.

  The detector 30 is provided with a detector 31 and an A / D converter 32. The detector 31 detects each interference light incident from the optical fiber coupler 22 via two optical fibers. Here, since the two optical fibers from the optical fiber coupler 22 to the detector 31 have different optical path lengths, the interference light passing through the two optical fibers has different phases according to the optical path lengths of the respective optical fibers. Have.

  The detector 31 sends an interference signal based on the detected interference light to the A / D converter 32, and the A / D converter 32 converts the received interference signal into a digital signal. The A / D converter 32 is connected to the k clock generation unit 80, and the A / D converter 32 samples the interference signal in synchronization with the k clock signal sent from the k clock generation unit 80. , Convert to digital signal. The A / D converter 32 sends the interference signal converted into a digital signal to the information acquisition unit 40. Therefore, the detection unit 30 can detect the interference light, generate a digitized interference signal in synchronization with the k clock signal, and send the interference signal to the information acquisition unit 40.

  The information acquisition unit 40 performs frequency analysis such as Fourier transform on the digital signal received from the detection unit 30 to obtain fundus information. The information acquisition unit 40 detects the differential of the interference signal by taking the difference between the interference signals detected by the detector 31 and having different phases, and reduces noise based on the non-interference component of the interference signal. can do. Therefore, the information acquisition unit 40 can improve the signal-to-noise ratio (S / N ratio) of the fundus information based on the interference signal by performing the differential detection. The information acquisition unit 40 sends the obtained fundus information to the display unit 70, and the display unit 70 displays the received information as a tomographic image.

  Note that the information acquisition unit 40 may be configured in the OCT apparatus 1 as an arbitrary information processing unit including a CPU, an MPU, or the like, or may be configured using a general-purpose computer. The display unit 70 may be a monitor provided in the OCT apparatus 1 or an individual monitor connected to the OCT apparatus 1.

  Through the series of operations described above, the OCT apparatus 1 can acquire information related to a tomogram at a certain point of the subject 100. Acquiring information related to the tomography in the depth direction of the subject 100 in this way is called an A-scan. In the OCT apparatus 1, the scanning unit configured by the X-axis scanner 53 and the Y-axis scanner 54 scans the subject 100 to obtain information on a two-dimensional tomographic image and a three-dimensional tomographic image of the subject. Can be acquired.

  Here, scanning the subject 100 in a direction in which information related to the tomogram of the subject 100 in a direction orthogonal to the A-scan, that is, information on a two-dimensional tomographic image, is referred to as B-scan. Furthermore, scanning the subject 100 in a direction orthogonal to both the A-scan and B-scan scan directions is called a C-scan. In particular, when two-dimensional raster scanning is performed within the fundus of the subject 100 when acquiring three-dimensional tomographic information, the direction in which scanning is performed at high speed is referred to as the B-scan direction, and is orthogonal to the B-scan direction. A direction in which scanning is performed at a low speed is referred to as a C-scan direction.

  The OCT apparatus 1 can obtain a two-dimensional tomographic image of the subject 100 by performing A-scan and B-scan, and can perform the subject 100 by performing A-scan, B-scan, and C-scan. 3D tomographic images can be obtained. The B-scan and the C-scan are performed by a scanning unit configured by the X-axis scanner 53 and the Y-axis scanner 54 described above. Note that the line scanning direction such as the B-scanning direction and the C-scanning direction may not coincide with the X-axis direction or the Y-axis direction. For this reason, the B-scan and C-scan line scanning directions can be appropriately determined according to a two-dimensional tomographic image or a three-dimensional tomographic image to be imaged.

  In the OCT apparatus 1, the sampling of the interference signal by the A / D converter 32 of the detection unit 30 is performed at the same optical frequency (with respect to the light from the light source unit 10 based on the k clock signal generated by the k clock generation unit 80. (Equal wave number) interval.

  Here, since the light from the light source unit 10 of the OCT apparatus 1 is swept in wavelength, the optical frequency changes with time. On the other hand, since the k clock generation unit 80 generates a k clock signal based on the light from the light source unit 10, the k clock signal has an equal wave number with respect to the interference signal based on the interference light detected by the detection unit 30. Sampling timing at intervals can be shown.

  Next, the k clock generation unit 80 will be described in more detail with reference to FIG. FIG. 2 shows a configuration example of the k clock generation unit 80.

  As shown in FIG. 2, the k clock generation unit 80 includes a k clock interference unit (clock interference unit) 82, an optical sensor (clock detection unit) 83, and a frequency doubling circuit 84.

  The k clock interference unit 82 branches the light from the light source unit 10 into light incident on two optical paths having different optical path lengths, and generates clock interference light by the light emitted from the two optical paths. The k clock interference unit 82 includes optical fiber couplers 821 and 828, connectors 822, 824, 825, and 827, and optical fibers 823 and 826.

  Light 811 from the light source unit 10 via the optical fiber coupler 90 propagates through the optical fiber connected to the optical fiber couplers 90 and 821 and enters the optical fiber coupler 821. The optical fiber coupler 821 transmits light from the light source unit 10 to a first optical path configured by the connector 822, the optical fiber 823, and the connector 824, and a second optical path configured by the connector 825, the optical fiber 826, and the connector 827. 811 is branched and incident. Here, since the optical fiber 823 and the optical fiber 826 have predetermined different optical path lengths, the first optical path and the second optical path have different optical path lengths. The light passing through the first optical path enters the optical fiber coupler 828 from the connector 824 via the optical fiber, and the light passing through the second optical path enters the optical fiber coupler 828 from the connector 827 via the optical fiber. To do.

  The light passing through the first optical path and the light passing through the second optical path interfere with each other in the optical fiber coupler 828 and enter the optical sensor 83 via the optical fiber as interference light.

  The optical sensor 83 detects the incident interference light and generates an interference signal (k clock signal) 812. The optical sensor 83 sends the generated interference signal 812 to the frequency doubling circuit 84.

  The frequency doubling circuit 84 generates a k clock signal 813 obtained by doubling the frequency of the interference signal 812 based on the received interference signal 812, and k clock obtained by doubling the frequency in the A / D converter 32 of the OCT apparatus 1. Send a signal 813.

  Thereby, the A / D converter 32 can sample the interference signal based on the reflected light and the reference light from the subject 100 in synchronization with the k clock signal 813 whose frequency is doubled.

  Next, the frequency doubling circuit 84 in the OCT apparatus 1 according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 3 schematically shows a configuration example of the frequency doubling circuit 84 that doubles the frequency of the interference signal. FIG. 4 shows the waveform of each signal propagating in the frequency doubling circuit 84.

  As shown in FIG. 3, the frequency doubling circuit 84 includes a comparator 841 (first comparator), a comparator 842 (second comparator), and an OR circuit 843.

  The comparators 841 and 842 compare the input signals and switch the output according to the comparison result. The minus side input terminal (reverse phase input terminal) of the comparator 841 is connected to GND, and the potential of the minus side input terminal of the comparator 841 is a predetermined reference potential A having a positive polarity. Further, the + side input terminal (positive phase input terminal) of the comparator 842 is connected to GND, and the potential of the + side input terminal of the comparator 842 is a predetermined reference potential B having a negative polarity. An interference signal 812 (signal 401) is input to the + side input terminal of the comparator 841 and the − side input terminal of the comparator 842. In this embodiment, the absolute values of the reference potentials A and B are the same value.

  The input terminal of the logical sum circuit 843 is connected to the output terminals of the comparators 841 and 842, and the logical sum circuit 843 outputs the logical sum of the output signal 402 of the comparator 841 and the output signal 403 of the comparator 842.

  Next, a signal flow in the frequency doubling circuit 84 will be described.

  The interference signal 812 output from the optical sensor 83 is input to the frequency doubling circuit 84 and is input as a signal 401 to the + side input terminal of the comparator 841 and the − side input terminal of the comparator 842.

  The comparator 841 compares the input signal 401 with the reference potential A, and indicates High when the signal 401 has a potential higher than the reference potential A, as shown in FIG. The signal 402 is output.

  Further, the comparator 842 compares the input signal 401 with the reference potential B, and outputs a signal 403 indicating High when the signal 401 has a potential lower than the reference potential B, and indicating Low otherwise. To do.

  A logical sum circuit 843 calculates the logical sum of the input signals 402 and 403 and outputs a signal 404. As shown in FIG. 4, the signal 404 corresponds to a pulse indicating High corresponding to the portion of the interference signal 401 having a potential higher than the reference potential A and a portion of the interference signal 401 having a potential smaller than the reference potential B. Correspondingly, a pulse indicating High is included. Since the interference signal 401 is an AC signal, the number of pulses indicating High in the signal 404 is doubled in one cycle of the interference signal 401 compared to the pulse signal including the portion having the positive potential of the interference signal 401. . Therefore, the frequency of the signal 403 is doubled compared to the frequency of the pulse signal composed of a portion having a positive potential of the interference signal 401 (interference signal 812). The signal 404 output from the OR circuit 843 is sent to the A / D converter 32 as a k clock signal 813 whose frequency is doubled. Therefore, the A / D converter 32 can sample the interference signal based on the reflected light and the reference light from the subject 100 in synchronization with the k clock signal 813 whose frequency is doubled.

  Therefore, the frequency doubling circuit 84 according to the present embodiment can generate the k clock signal 813 whose frequency is doubled by using an inexpensive comparator without using an expensive multiplier or a 90 ° phase shift circuit. it can.

  As described above, the OCT apparatus 1 according to the present embodiment includes the light source unit 10 that sweeps the optical frequency of light and emits the light. Furthermore, the OCT apparatus 1 branches the light from the light source unit 10 into reference light and irradiation light that irradiates the subject 100, and interference between the reference light and reflected light of the irradiation light reflected from the subject 100. An interference unit 20 that generates light is provided. The OCT apparatus 1 also includes a k clock generation unit 80 that generates a k clock signal based on light from the light source unit 10, and light that enters the k clock generation unit 80 from the light from the light source unit 10 and the interference unit 20. And an optical fiber coupler 90 that branches into incident light. Further, the OCT apparatus 1 detects interference light generated by the interference unit 20, and acquires information on the subject 100 based on the detection unit 30 that generates an interference signal in synchronization with the k clock signal and the interference signal. And an information acquisition unit 40. The k clock generation unit 80 includes a frequency doubling circuit 84 that doubles the frequency of the k clock signal based on the light from the light source unit 10. The frequency doubling circuit 84 calculates the logic between the signal 402 corresponding to the portion having a potential higher than the first threshold of the k clock signal and the signal 403 corresponding to the portion having a potential lower than the second threshold of the k clock signal. The frequency of the k clock signal is doubled by taking the sum. Here, the first threshold value has a positive polarity, and the second threshold value has a negative polarity. As a result, the OCT apparatus 1 according to the present embodiment can generate a k clock signal whose frequency is doubled using an inexpensive comparator without using an expensive multiplier or a 90 ° phase shift circuit. The cost increase of the OCT apparatus 1 can be suppressed.

  In addition, when generating interference light, it is known that the frequency of interference light increases as the optical path difference between the two optical paths is increased. In this regard, in the OCT apparatus 1 according to the present embodiment, the frequency doubling circuit 84 can double the frequency of the interference signal 812. Therefore, in the OCT apparatus 1, since it is not necessary to interfere with light passing through an optical path having a longer optical path difference in generating a k clock signal having a high frequency, a stable k clock using a light source having a short coherent length is eliminated. A signal can be generated.

  As for the sampling of the interference signal by the A / D converter 32, it is known that sampling is performed at a frequency that is at least twice the frequency of the interference signal based on the sampling theorem. Further, in the OCT apparatus, light used for measuring the subject reciprocates between the measurement arm and the fundus of the subject. Therefore, it is known that in an ordinary OCT apparatus, the optical path difference between the first optical path and the second optical path of the k clock generation unit is required to be at least four times the distance in the depth direction of the acquired image. Yes.

  On the other hand, as described above, in the OCT apparatus 1 according to the present embodiment, the frequency doubling circuit 84 can double the frequency of the interference signal 812. Therefore, in the OCT apparatus 1 according to the present embodiment, the optical path difference between the first optical path and the second optical path of the k clock generation unit 80 may be about twice the distance in the depth direction of the image to be acquired. As described above, in the OCT apparatus 1 according to the present embodiment, the k clock generation unit 80 uses a light source having a short coherent length in which the optical path difference between the first optical path and the second optical path is shorter than that of a normal OCT apparatus. Thus, the same performance as that of a normal OCT apparatus can be exhibited. In this case, since a light source having a short coherent length can be used, the cost can be reduced as compared with a normal OCT apparatus.

  In the present embodiment, the absolute values of the reference potentials A and B are the same, but the values of the reference potentials A and B are not limited to this. The reference potentials A and B may be set so that the phase difference between the signals 402 and 403 output from the comparators 841 and 842 is approximately 180 °, and the absolute values of the reference potentials A and B are different. Also good.

  Here, the frequency doubling circuit 84 according to the present embodiment includes a comparator 841 (first comparator), a comparator 842 (second comparator), and an OR circuit 843. The comparator 841 compares the interference signal (k clock signal) 812 (signal 401) input to the positive phase input terminal with a predetermined voltage (reference potential A) having a positive polarity applied to the negative phase input terminal, A signal 402 as a comparison result is output. The comparator 842 compares the interference signal (k clock signal) 812 (signal 401) input to the negative phase input terminal with a predetermined voltage (reference potential B) having a negative polarity applied to the positive phase input terminal, A signal 403 as a comparison result is output. The logical sum circuit 843 calculates the logical sum of the signals 402 and 403 from the comparators 841 and 842, and outputs a k clock signal 813 (signal 404) whose frequency is doubled.

  In the OCT apparatus 1 according to the present embodiment, the k clock generation unit 80 and the optical fiber coupler 90 are provided outside the light source unit 10. However, as illustrated in FIG. 5, the k clock generation unit 80 and the optical fiber coupler 90 may be provided inside the light source unit 10.

  The interference signal 401 is ideally a sine wave. However, when the amplification in the positive region and the negative region in an amplifier (not shown) that amplifies the interference signal 401 is asymmetric, the amplitudes of the positive region and the negative region of the interference signal 401 are asymmetric. Therefore, when the absolute values of the reference potentials of the comparator 841 and the comparator 842 are equal, the phase difference between the output signal 402 of the comparator 841 and the output signal 403 of the comparator 842 may not be 180 °. In this case, the equifrequency characteristic of the signal 404 obtained by taking the logical sum of the output signals 402 and 403, that is, the k clock signal 813 whose frequency is doubled, is lost.

  When the iso-frequency characteristic of the k clock signal 813 is lost, the sampling interval of the interference signal of the subject 100 is disturbed. As a result, the interference signal of the subject 100 cannot be sampled at regular wave intervals, and the frequency component of the interference spectrum of the subject 100 grasped from the sampled data may not be appropriately analyzed. For this reason, noise may occur in the tomographic image of the subject 100 acquired by the OCT apparatus.

  Therefore, in the OCT apparatus according to the second embodiment, the frequency is doubled by adjusting the reference potential of the comparator in the frequency doubling circuit and adjusting the phase difference of the output signal of each comparator in the frequency doubling circuit. This prevents the iso-frequency characteristics of the k clock signal from being destroyed.

  Hereinafter, the OCT apparatus according to the second embodiment will be described with reference to FIGS. In the OCT apparatus according to the present embodiment, the constituent elements other than the information acquisition unit 41, the frequency doubling circuit 85, and the D / A converter 42 are the same as the respective constituent elements in the OCT apparatus 1 according to the first embodiment. The same reference numerals are assigned and description thereof is omitted. Hereinafter, the OCT apparatus according to the second embodiment will be described focusing on differences from the OCT apparatus 1 according to the first embodiment.

  FIG. 6 shows the A / D converter 32, the information acquisition unit 41, the D / A converter 42, the display unit 70, and the frequency doubling circuit 85 in the OCT apparatus.

  The A / D converter 32 is the same as the A / D converter 32 in the first embodiment, and is connected to the detector 31 and the information acquisition unit 41. The A / D converter 32 samples and binarizes the interference signal (OCT interference signal) 301 output from the detector 31 in synchronization with the k clock signal 815 sent from the frequency doubling circuit 85. Convert to OCT interference signal 302.

  The information acquisition unit 41 is connected to the A / D converter 32 and the display unit 70. Similar to the information acquisition unit 40 in the first embodiment, the information acquisition unit 41 performs frequency analysis such as Fourier transform on the OCT interference signal 302 received from the A / D converter 32 to obtain fundus information. Thereafter, the information acquisition unit 41 sends a signal 411 including the obtained fundus information to the display unit 70. The information acquisition unit 41 is connected to the D / A converter 42 and controls the D / A converter 42 using a control signal 412 including a setting value of the D / A converter 42 described later.

  The D / A converter 42 is connected to the information acquisition unit 41 and the frequency doubling circuit 85. The D / A converter 42 can change the signal 421 output to the frequency doubling circuit 85 according to the control signal 412 from the information acquisition unit 41. Here, the signal 421 corresponds to the reference potential Ref 1 of the comparator 852 in the frequency doubling circuit 85 corresponding to the comparator 842 in the frequency doubling circuit 84. That is, the D / A converter 42 can adjust the reference potential Ref1 of the comparator 852. Therefore, the information acquisition unit 41 can control the reference potential Ref1 of the comparator 852 by controlling the D / A converter 42.

  The frequency doubling circuit 85 is connected to the A / D converter 32 and the D / A converter 42. In addition, the frequency doubling circuit 85 is connected to the optical sensor 83 in the k clock generation unit 80, receives an interference signal (k clock signal) 814 sent from the optical sensor 83, and has a frequency doubled k clock signal. 815 is sent to the A / D converter 32. The interference signal 814 is the same as the interference signal 812 output from the optical sensor 83 in the first embodiment.

  Further, the frequency doubling circuit 85 is configured so that the reference potential Ref1 of the comparator 852 in the frequency doubling circuit 85 can be set from the information acquisition unit 41 via the D / A converter 42.

  Hereinafter, the frequency doubling circuit 85 will be described with reference to FIG. FIG. 7 schematically shows a configuration example of the frequency doubling circuit 85 according to the present embodiment. Similar to the frequency doubling circuit 84 in the first embodiment, the frequency doubling circuit 85 is provided with comparators 851 and 852 and an OR circuit 853.

  The comparators 851 and 852 compare the input signals as in the comparators 841 and 842, and switch the output according to the comparison result. The negative input terminal of the comparator 851 is connected to GND, and the negative input terminal of the comparator 851 has a predetermined reference potential A having a positive polarity. On the other hand, the + side input terminal of the comparator 852 is connected to the D / A converter 42, and the potential of the + side input terminal of the comparator 852 has a negative polarity based on the signal 421 output from the D / A converter. The reference potential Ref1 has. The interference signal 814 (signal 801) generated by the optical sensor 83 in the k clock generation unit 80 is input to the + side input terminal of the comparator 851 and the − side input terminal of the comparator 852.

  The input terminal of the OR circuit 853 is connected to the output terminals of the comparators 851 and 852. The OR circuit 853 calculates the logical sum of the output signal 802 of the comparator 851 and the output signal 803 of the comparator 852, and The frequency-doubled k clock signal 804 is output.

  The signal flow in the frequency doubling circuit 85 is the same as that in the frequency doubling circuit 84 except that the signal 801 input to the comparator 852 is compared with the reference potential Ref1 input from the D / A converter 42. Since this is the same as the signal flow, the description is omitted. Note that the comparator 852 outputs a signal 803 indicating High when the signal 801 has a potential lower than the reference potential Ref1, and otherwise indicating Low.

  Next, referring to FIGS. 8 to 10, the reference potential Ref1 of the comparator 852 is adjusted in the OCT apparatus according to the present embodiment, and the output signals 802 and 803 of the comparators 851 and 852 of the frequency doubling circuit 85 are adjusted. An operation for adjusting the phase difference will be described. FIG. 8 shows the waveform of each signal propagating through the frequency doubling circuit 85.

  As described above, the interference signal 801 (signal 814) input to the frequency doubling circuit 85 is ideally a sine wave. However, the interference signal 801 generated by the optical sensor 83 is amplified by an amplifier (not shown), and the amplitude is asymmetric between the positive region and the negative region due to amplification asymmetry in the positive region and the negative region of the amplifier. There is a case where the waveform becomes as follows.

  Hereinafter, the case where the interference signal 801 has a waveform in which the amplitude is asymmetric between the positive region and the negative region will be described. FIG. 8 shows an interference signal 801 having a waveform in which the amplitude is asymmetric between the positive region and the negative region. The interference signal 801 shown in FIG. 8 has a waveform in which the amplitude of the negative region is smaller than the amplitude of the positive region.

  The interference signal 801 having such a waveform is converted into reference potentials (threshold potentials) A and B having positive or negative polarities having the same absolute value in the comparators 841 and 842 as in the frequency doubling circuit 84 in the first embodiment. Assuming that In such a case, the phase difference between the signals 402 and 403 output from the comparators 841 and 842 is not 180 ° based on the amplitude asymmetry in the positive region and the negative region of the interference signal 801. For this reason, in the interference signal 404 obtained by ORing the signals 402 and 403, the rising interval of the wave corresponding to the signal 402 and the subsequent signal 403, the wave corresponding to the signal 403, and the subsequent signal 402 Corresponding wave rise intervals are different. For this reason, the interference signal 404, that is, the k clock signal 813 whose frequency has been doubled, does not have equal wave number characteristics. Therefore, as described above, noise may occur in the tomographic image of the subject 100 acquired by the OCT apparatus.

  Therefore, in this embodiment, the reference potential Ref1 of the comparator 852 is adjusted based on the OCT interference signal, and the phase difference between the signals 802 and 803 output from the comparators 851 and 852 is adjusted. Thereby, the OCT apparatus according to the present embodiment prevents the equal wave number property of the k clock signal 815 whose wave number is doubled from being broken.

  In this operation, a mirror is placed at the position of the retina of the subject 100 to totally reflect the irradiation light, and a point spread function (Point as shown in FIG. 9) is obtained from the OCT interference signal based on the reflected light from the mirror and the reference light. A waveform called “Spread Function” (hereinafter referred to as “PSF”) is obtained. FIG. 9 shows the PSF of the OCT interference signal when the mirror is placed at a position where the optical path difference between the reference arm and the measurement arm of the OCT apparatus is about 830 μm. In FIG. 9, the horizontal axis indicates the optical path difference between the reference arm and the measurement arm, and the vertical axis indicates the signal intensity. This PSF has an effect on the waveform when the frequency analysis on the OCT interference signal is not properly performed. The PSF has a low peak height especially when the iso-frequency characteristic of the k clock signal 815 whose frequency is doubled is broken, and a waveform appears in other parts of the peak. Therefore, the output value of the D / A converter 42 is changed so that the peak height of the PSF is maximized, and the phase difference between the signals 802 and 803 output from the comparators 851 and 852 is adjusted. , K-wave signal 815 can be adjusted in the equifrequency characteristics.

  FIG. 10 shows a flowchart of an operation for adjusting the reference potential Ref1 of the comparator 852 based on the PSF of the OCT interference signal. Hereinafter, an operation of adjusting the reference potential Ref1 of the comparator 852 based on the PSF of the OCT interference signal will be described with reference to FIG.

  When the operation starts in step S1001, the information acquisition unit 41 sets the setting value of the D / A converter 42 to 0 and initializes it in step S1002. The set value of the D / A converter 42 corresponds to the output value (signal 421) of the D / A converter 42. Here, every time the set value of the D / A converter 42 is increased by one. Further, the output value of the D / A converter 42 is lowered by a predetermined amount. Further, when the set value of the D / A converter 42 is 0 (initial value), the output value of the D / A converter 42 may be 0V or a predetermined value.

  Next, in step S1003, the information acquisition unit 41 obtains the PSF from the OCT interference signal 301 based on the reflected light from the mirror placed at the position where the fundus of the subject 100 is placed and the reference light, and the peak of the PSF is obtained. The value of (signal strength) is calculated. After calculating the peak value, the information acquisition unit 41 associates the peak value with the setting value of the D / A converter 42 and stores it in a storage device (not shown).

  When the information acquisition unit 41 stores the peak value, whether the output value of the D / A converter 42 is equal to or smaller than the amplitude of the negative region of the interference signal 814 input to the frequency doubling circuit 85 in step S1004. Judge whether or not. If the output value of the D / A converter 42 is equal to or smaller than the amplitude of the negative region of the interference signal 814, the operation proceeds to step S1006, and if not, the operation proceeds to step S1005. Note that the information acquisition unit 41 may have any configuration for obtaining information about the amplitude of the negative region of the interference signal 814. For example, the information acquisition unit 41 may be configured to receive the interference signal 814 from the k clock generation unit 80.

  If the output value of the D / A converter 42 is not less than or equal to the amplitude of the negative region of the interference signal 814, the information acquisition unit 41 increases the set value of the D / A converter 42 by 1 in step S1005. When the setting value of the D / A converter 42 is increased by 1, the information acquisition unit 41 returns to step S1003, obtains the OCT interference signal again, calculates the PSF peak value of the OCT interference signal, and the D / A converter 42. It is stored in association with the set value.

  In step S1004, when the output value of the D / A converter 42 is equal to or smaller than the amplitude of the negative region of the interference signal 814, the comparator 852 determines that the amplitude of the interference signal 814 is based on the output value of the D / A converter 42. The potential (threshold potential) Ref1 is not exceeded. Therefore, a signal indicating High is not output from the comparator 852, and the frequency doubling circuit 85 cannot double the frequency of the interference signal 814.

  Therefore, in such a case, the process for increasing the set value of the D / A converter 42 is not performed, and the loop for determining the PSF peak value consisting of steps S1003, S1004, and S1005 is terminated, and the operation proceeds to step S1006. Proceed to In step S1006, the information acquisition unit 41 determines the maximum peak value from the PSF peak values that have already been stored, and the D / A converter 42 associated with the maximum peak value. Only the set value is stored in a storage device (not shown). The information acquisition unit 41 sets the output value of the D / A converter 42, that is, the reference potential Ref1 of the comparator 852, using the stored setting value. Thereby, in the OCT apparatus, it is possible to obtain an appropriate reference potential (threshold potential) Ref1 of the frequency doubling circuit 85 that maximizes the PSF peak value.

  By the above operation, the reference potential Ref1 of the comparator 852 is adjusted so that the PSF peak value of the OCT interference signal is maximized, and the phase difference between the signals 802 and 803 output from the comparators 851 and 852 is adjusted. be able to. Thereby, as shown in FIG. 8, the phase difference between the signals 802 and 803 output from the comparators 851 and 852 is adjusted to be approximately 180 °. Therefore, the signal 804 obtained from the logical sum of the signals 802 and 803 corresponds to the rising interval of the wave corresponding to the signal 802 and the subsequent signal 803, the wave corresponding to the signal 803, and the subsequent signal 802. The rising intervals of the waves to be made are almost equal. Therefore, it is possible to prevent the equifrequency characteristics of the k clock signal 815 (signal 804) whose frequency has been doubled from being destroyed due to the asymmetry of amplification in the positive region and the negative region of the amplifier.

  As described above, in the OCT apparatus according to the present embodiment, the information acquisition unit 41 sets the reference potential Ref1 (second threshold) of the comparator 852 based on the point spread function of the OCT interference signal. More specifically, the information acquisition unit 41 sets the reference potential Ref1 (second threshold value) of the comparator 852 so that the peak of the point spread function of the OCT interference signal is maximized. Therefore, the frequency doubling circuit 85 configured to be able to adjust the reference potential Ref1 of the comparator 852 by the information acquisition unit 41 causes the waveform of the signal 804 corresponding to the waveform in the negative region of the interference signal 814. The phase can be adjusted. Accordingly, it is possible to prevent the iso-wave characteristics of the k clock signal 815 whose frequency has been doubled from being lost due to amplification asymmetry in the positive region and the negative region of the amplifier that amplifies the interference signal 814.

  In the above description, the operation of the OCT apparatus according to the present embodiment has been described using the interference signal 801 having a waveform in which the amplitude of the negative region is smaller than the amplitude of the positive region. However, even when the interference signal 801 has a waveform in which the amplitude of the negative region is larger than the amplitude of the positive region, the OCT apparatus according to the present embodiment can operate similarly.

  In the present embodiment, the reference potential Ref1 of the comparator 852 is adjusted. Instead, the reference potential A (first threshold) of the comparator 851 is adjusted according to the PSF of the OCT interference signal. It is good also as composition to do. In this case, every time the set value of the D / A converter 42 is increased by 1, the output value of the D / A converter 42 is increased by a predetermined amount. In step S1004, the information acquisition unit 41 determines whether the output value of the D / A converter 42 is equal to or greater than the amplitude in the positive region of the interference signal 814. Thereafter, when the output value of the D / A converter 42 is equal to or larger than the amplitude in the positive region of the interference signal 814, the operation proceeds to step S1006, and otherwise, the operation proceeds to step S1005.

  Even in such a case, the reference potential of the comparator 851 is adjusted so that the PSF peak value of the OCT interference signal is maximized, and the phase difference between the signals 802 and 803 output from the comparators 851 and 852 is adjusted. can do. For this reason, it is possible to prevent the equi-frequency property of the k clock signal 815 whose frequency is doubled from being lost due to the asymmetry of amplification in the positive region and the negative region of the amplifier.

  Further, the reference potentials of the comparators 851 and 852 may be adjusted together. The operation in this case can be performed by a combination of the operation for adjusting the reference potential of the comparator 851 and the operation for adjusting the reference potential of the comparator 852. Even in this case, the same effects as described above can be obtained.

  Note that the PSF waveform of the OCT interference signal is more susceptible to the disruption of the iso-frequency characteristics of the k clock signal when the frequency of the OCT interference signal is high. Therefore, in this embodiment, in the OCT apparatus, the PSF of the OCT interference signal is measured by placing a mirror at a position where the optical path difference between the reference arm and the measurement arm is approximately 830 μm. However, the position where the mirror is placed is not limited to this. The mirror may be placed at an arbitrary position corresponding to the position of the retina portion of the subject 100 to be examined for the purpose of measuring the PSF in a state close to the actual examination conditions using the OCT apparatus. Therefore, the mirror can be placed at any position such as a position where the optical path difference between the reference arm and the measurement arm is about 170 μm, about 400 μm, and about 700 μm.

  Regarding the operation flow shown in FIG. 10, in step S1003, the PSF peak previously obtained and stored in the loop for obtaining the PSF peak is compared with the currently obtained PSF peak value, and only the larger one is stored. Also good. In this case, when it is determined in S1004 that the loop is to be ended, the output value of the D / A converter 42 is determined using the set value of the D / A converter 42 associated with the peak stored in S1006. Even if it does in this way, there can exist an effect | action similar to the operation | movement demonstrated in the present Example.

  Step S1005 may be performed before step S1004. In this case, when the output value of the D / A converter is not less than or equal to the amplitude of the interference signal in step S1004, the process proceeds to step S1003. Even if it does in this way, there can exist an effect | action similar to the operation | movement demonstrated in the present Example.

  In the above embodiment, the configuration of the OCT apparatus other than the k clock generation unit 80 has been described based on the configuration example of the OCT apparatus illustrated in FIG. 1, but the configuration of the OCT apparatus other than the k clock generation unit 80 is not limited thereto. I can't. For example, instead of the optical fiber connecting the optical fiber coupler, a predetermined space may be provided to transmit light as spatial light, or a configuration that does not perform differential detection of interference light may be employed.

  While the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Inventions modified within the scope not departing from the spirit of the present invention and inventions equivalent to the present invention are also included in the present invention. Moreover, each above-mentioned Example and modification can be combined suitably in the range which is not contrary to the meaning of this invention.

  10: light source unit, 20: interference unit, 30: detection unit, 40: information acquisition unit, 80: k clock generation unit (clock signal generation unit), 90: coupler (branch unit), 100: fundus (subject), 84: Frequency doubling circuit, 843: OR circuit, A: Reference potential (first threshold), B, Ref1: Reference potential (second threshold)

Claims (8)

A light source unit that sweeps the frequency of light and emits the light;
An interference unit for branching light from the light source unit into reference light and irradiation light for irradiating the subject, and generating interference light by the reference light and reflected light of the irradiation light reflected from the subject; ,
A clock signal generation unit that generates a clock signal based on light from the light source unit;
A branching unit that branches light from the light source unit into light incident on the clock signal generation unit and light incident on the interference unit;
A detection unit that detects the interference light and generates an interference signal in synchronization with the clock signal;
An information acquisition unit for acquiring information of the subject based on the interference signal;
With
The clock signal generation unit includes a frequency doubling circuit that doubles the frequency of the clock signal based on the light from the light source unit,
The frequency doubling circuit performs a logical sum of a signal corresponding to a portion having a potential higher than the first threshold of the clock signal and a signal corresponding to a portion having a potential lower than the second threshold of the clock signal. By doubling the frequency of the clock signal,
The first threshold has a positive polarity;
The imaging apparatus, wherein the second threshold has a negative polarity.   The imaging apparatus according to claim 1, wherein the information acquisition unit sets at least one of the first threshold and the second threshold based on the interference signal.   The imaging apparatus according to claim 2, wherein the information acquisition unit sets at least one of the first threshold and the second threshold based on a point spread function of the interference signal.   The imaging apparatus according to claim 3, wherein the information acquisition unit sets the at least one of the first threshold and the second threshold so that a peak of a point spread function of the interference signal is maximized. . The frequency doubling circuit includes:
A first comparator for comparing the clock signal input to the positive phase input terminal with a predetermined voltage having a positive polarity applied to the negative phase input terminal;
A second comparator for comparing the clock signal input to the negative phase input terminal with a predetermined voltage having a negative polarity applied to the positive phase input terminal;
A logical sum circuit for computing a logical sum of signals from the first and second comparators;
The imaging device according to any one of claims 1 to 4, further comprising: The clock signal generator is
A clock interference unit for branching light from the light source unit into light incident on two optical paths having different optical path lengths and generating clock interference light by light emitted from the two optical paths;
A clock detector for detecting the clock interference light and generating the clock signal;
The imaging device according to any one of claims 1 to 5, further comprising:   The imaging device according to claim 1, wherein the clock signal generation unit is provided outside the light source unit.   The imaging device according to claim 1, wherein the clock signal generation unit is provided inside the light source unit.

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