JP2008145376A - Optical tomographic imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve an S/N ratio of an interference signal, in an optical tomographic imaging system using optical tomographic measurement. <P>SOLUTION: In the optical tomographic measurement using light wherein a wavelength is swept at a fixed period, interference light L4 between reflected light L3 and reference light L2 is branched into the first interference light L4a and the second interference light L4b by a light branching means 5. The first interference light L4a and the second interference light L4b are attenuated with each different attenuation factor in each wavelength band respectively so that each light quantity becomes approximately uniform by variable light attenuators 60A, 60B, and then balance-detected by an interference light detection means 70. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a tomographic image processing method, apparatus and program for generating an optical tomographic image by OCT (Optical Coherence Tomography) measurement, and a tomographic imaging system using the same.

  Conventionally, when an optical tomographic image of a living tissue is acquired, an optical tomographic image acquisition device using OCT measurement is sometimes used. It is applied to various parts such as observation of the fundus, anterior eye, skin, arterial blood vessel wall using a fiber probe, and observation of a digestive tract in which a fiber probe is inserted from a forceps channel of an endoscope. In this optical tomographic image acquisition apparatus, after the low-coherent light emitted from the light source is divided into measurement light and reference light, reflected light from the measurement object when the measurement light is irradiated to the measurement object, or backscattering The light and the reference light are combined, and an optical tomographic image is acquired based on the intensity of the interference light between the reflected light and the reference light.

  The OCT measurement is roughly divided into two types: TD-OCT (Time domain OCT) measurement and FD (Fourier Domain) -OCT measurement. The TD-OCT (Time domain OCT) measurement shown in Patent Document 1 measures the interference light intensity while changing the optical path length of the reference light, thereby measuring the position in the depth direction of the measurement object (hereinafter referred to as the depth position). Is a method for obtaining a reflected light intensity distribution corresponding to the above.

  On the other hand, the FD (Fourier Domain) -OCT measurement measures the interference light intensity for each spectral component of the light without changing the optical path lengths of the reference light and the signal light, and uses the obtained spectral interference intensity signal as a computer. In this method, the reflected light intensity distribution corresponding to the depth position is obtained by performing frequency analysis represented by Fourier transform. In recent years, it has attracted attention as a technique that enables high-speed measurement by eliminating the need for mechanical scanning existing in TD-OCT.

  Typical examples of the apparatus configuration for performing FD (Fourier Domain) -OCT measurement include an SD-OCT (Spectral Domain OCT) apparatus and an SS-OCT (Swept source OCT). Among these, the SS-OCT apparatus emits laser light whose wavelength is temporally swept from the light source unit, causes reflected light and reference light to interfere at each wavelength, and a signal time corresponding to a temporal change in optical frequency. An optical tomographic image is constructed by measuring a waveform and Fourier-transforming a spectrum interference intensity signal obtained thereby by a computer (see Patent Document 2).

In the SS-OCT measurement described above, in order to improve the S / N ratio, as shown in Patent Document 1, an optical fiber coupler or the like is used to branch the interference light into two so that the amount of light is substantially uniform. It is disclosed that each of the branched interference lights is detected by a detector, and the difference between the detected signals is detected as an interference signal (balance detection). As a result, the interference signal is amplified by a factor of 2 and the other in-phase optical noise is canceled, and the S / N ratio can be improved by removing the non-interference component.
JP 2001-264246 A JP 2006-132996 A

  The S / N ratio is improved by performing so-called balance detection of the interference light in the TD-OCT measurement of Patent Document 1 described above. This balance detection can also be applied to the SS-OCT measurement shown in Patent Document 2. it can. In order to improve the S / N ratio by this balance detection, when the interference light is split into two, it is necessary to branch the interference light so that the light quantity becomes substantially equal. However, since the optical fiber coupler used for the actual branching of the interference light has a wavelength dependency, even if the interference light can be branched at a ratio of 50:50 in a certain wavelength band, In the wavelength band, there is a problem that the S / N ratio is not reduced to 50:50.

  That is, in SS-OCT measurement, light is emitted while sweeping between predetermined wavelength bands. In order to improve the S / N ratio by balance detection, the branching ratio is set over the entire wavelength band to be swept. It must be set to 50:50. However, due to the wavelength dependence of the optical fiber coupler that is actually used for branching interference light, it is difficult to maintain a 50:50 branching ratio over the entire wavelength band, and the light quantities of the two interference lights differ. As a result, there is a problem that the S / N ratio is lowered.

  Therefore, an object of the present invention is to provide an optical tomographic imaging system capable of improving the S / N ratio.

  An optical tomographic imaging system of the present invention includes a light source unit that emits light while sweeping a wavelength at a constant period, a light splitting unit that splits light emitted from the light source unit into measurement light and reference light, A combining unit that combines the reflected light and the reference light when the measurement light divided by the dividing unit is reflected on the measurement target, and an interference light between the reflected light combined by the combining unit and the reference light An optical branching means for branching into one interference light and a second interference light, a variable optical attenuator for attenuating the first interference light and the second interference light branched by the optical branching means with different attenuation rates for each wavelength band; Interference light detecting means for detecting a difference between the first interference light and the second interference light attenuated by the variable optical attenuator as an interference signal, and a tomographic image processing means for generating a tomographic image from the interference signal sampled by the sampling means It is characterized in that it comprises and.

  Here, the variable optical attenuator may have any configuration as long as it attenuates the first interference light and the second interference light at different attenuation rates for each wavelength band. For example, the variable optical attenuator includes a disk-shaped neutral density filter having different light attenuation factors along the circumferential direction, into which the first interference light and / or the second interference light branched by the light branching unit is incident. It may be provided with a rotation driving means for rotating the neutral density filter.

  At this time, the variable optical attenuator may be provided separately for each of the first interference light and the second interference light, or only one may be provided. When only one is provided, for example, the neutral density filter has a first attenuation region on the outer peripheral side in which the attenuation factor for each wavelength band for the first interference light is set, and an attenuation factor for each wavelength band for the first interference light is set. A second attenuation region on the circumferential side, and the first interference light may be incident on the first attenuation region of the neutral density filter, and the second interference light may be incident on the second attenuation region. .

  In addition, the variable optical attenuator is a multilayer filter in which the wavelength transmission characteristic changes depending on the incident angle of the first interference light or the second interference light on which the first interference light or the second interference light branched by the light branching unit is incident And a filter tilting means for tilting the multilayer filter with respect to the optical axis of the first interference light or the second interference light.

  Further, the variable optical attenuator collects the first interference light or the second interference light branched by the light branching means and enters the optical fiber optically connected to the interference light detection means, and the optical fiber. There may be provided a fiber end face moving means for adjusting the incident light quantity of the first interference light or the second interference light by moving the incident end face of the optical fiber.

  Alternatively, the variable optical attenuator may use a Faraday rotator.

  According to the optical tomographic imaging system of the present invention, the light source unit that emits light while sweeping the wavelength at a constant period, and the light dividing means that divides the light emitted from the light source unit into measurement light and reference light A combining means for combining the reflected light and the reference light when the measurement light divided by the light dividing means is reflected by the measurement object, and an interference light between the reflected light combined by the combining means and the reference light Branching into a first interference light and a second interference light, and a variable optical attenuator for attenuating the first interference light and the second interference light branched by the light branching means at different attenuation rates for each wavelength band An interference light detecting means for detecting a difference between the first interference light and the second interference light attenuated by the variable optical attenuator as an interference signal; and obtaining tomographic information from the interference signal sampled by the sampling means to obtain a tomographic image; And the tomographic image processing means for forming the optical branching means when generating an interference signal using the difference between the branched first interference light and the second interference light due to the wavelength-dependent characteristics of the light branching means. Even when the interference light cannot be evenly branched in the entire wavelength band, the variable light attenuator can attenuate the light amounts of the first interference light and the second interference light so as to be substantially equal in each wavelength band. The S / N ratio can be improved.

  Hereinafter, embodiments of the optical tomographic imaging system of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing a preferred embodiment of the optical tomographic imaging system of the present invention. The optical tomographic imaging system 1 obtains a tomographic image of a measurement target S such as a living tissue or a cell in a body cavity by SS-OCT (Swept source OCT) measurement by inserting an optical probe 10 into the body cavity. is there. The optical tomographic imaging system 1 includes an optical probe 10, an interferometer 20, a light source unit 30, a periodic clock generation unit 80, an A / D conversion unit 90, a tomographic image processing unit 100, a display device 110, and the like.

  FIG. 2 is a schematic diagram showing an example of a tip portion of the optical probe 10 of FIG. 2 is inserted into a body cavity through, for example, a forceps opening, and includes a probe outer tube (sheath) 11, an optical fiber 12, an optical lens 15, and the like. The probe outer cylinder 11 is made of a flexible cylindrical member, and is made of a material through which the measurement light L1 and the reflected light L3 are transmitted. The probe outer cylinder 11 has a structure in which the tip is closed by a cap 11a.

  The optical fiber 12 guides the measurement light L1 emitted from the interferometer 20 to the measurement target S, and also reflects light (backscattered light) from the measurement target S when the measurement light L1 is irradiated onto the measurement target S. L3 is guided to the interferometer 20 and is accommodated in the probe outer cylinder 11. A spring 13 is fixed to the outer peripheral side of the optical fiber 12, and the optical fiber 12 and the spring 13 are mechanically connected to the rotary drive unit 10A. The optical fiber 12 and the spring 13 are rotated in the arrow R1 direction with respect to the probe outer cylinder 11 by the rotation drive unit 10A. The rotation drive unit 10A includes a rotation encoder (not shown), and the rotation control unit 10B recognizes the irradiation position of the measurement light L1 based on a signal from the rotation encoder.

  The optical lens 15 has a substantially spherical shape for condensing the measuring light L1 emitted from the optical fiber 12 on the measuring object S, and condenses the reflected light L3 from the measuring object S to collect the optical fiber 12. Is incident on. Here, the focal length of the optical lens 15 is formed, for example, at a distance D = 3 mm from the optical axis LP of the optical fiber 12 in the radial direction of the probe outer cylinder. The optical lens 15 is fixed to the light emitting end of the optical fiber 12 using a fixing member 14, and when the optical fiber 12 rotates in the direction of arrow R1, the optical lens 15 also rotates integrally in the direction of arrow R1. Therefore, the optical probe 10 irradiates the measuring object S with the measuring light L1 emitted from the optical lens 15 while scanning in the arrow R1 direction (circumferential direction of the probe outer cylinder 11).

The operation of the rotation drive unit 10A for rotating the optical fiber 12 and the optical lens 15 in FIG. 1 is controlled by the rotation control means 10B. The rotation control means 10B rotates in the direction of the arrow R1 with respect to the probe outer cylinder 11 at about 20 Hz, for example. Control to do. The rotation control means 10B outputs the rotation clock signal R _CLK to the tomographic image processing means 100 when determining that the optical fiber 12 has made one rotation based on the signal from the rotation encoder of the rotation drive unit 10A. .

FIG. 3 is a schematic diagram illustrating an example of the light source unit 30. The light source unit 30 is adapted to emit laser light L while sweeping the wavelength at a constant period T _0. Specifically, the light source unit 30 includes a semiconductor optical amplifier (semiconductor gain medium) 311 and an optical fiber FB30, and the optical fiber FB30 is connected to both ends of the semiconductor optical amplifier 311. . The semiconductor optical amplifier 311 has a function of emitting weak emission light to one end side of the optical fiber FB30 by injecting drive current and amplifying light incident from the other end side of the optical fiber FB30. When a drive current is supplied to the semiconductor optical amplifier 311, the laser light L is emitted to the optical fiber FB 30 by an optical resonator formed by the semiconductor optical amplifier 311 and the optical fiber FB 30.

  Further, an optical branching device 312 is coupled to the optical fiber FB30, and a part of the light guided in the optical fiber FB30 is emitted from the optical branching device 312 to the optical fiber FB31 side. Light emitted from the optical fiber FB31 is reflected by a rotating polygon mirror (polygon mirror) 316 via a collimator lens 313, a diffraction grating element 314, and an optical system 315. The reflected light enters the optical fiber FB31 again via the optical system 315, the diffraction grating element 314, and the collimator lens 313.

  Here, the rotary polygon mirror 316 is rotated in the direction of the arrow R30, and the angle of each reflecting surface is changed with respect to the optical axis of the optical system 315. Thereby, only the light of a specific wavelength band among the lights dispersed in the diffraction grating element 314 returns to the optical fiber FB31 again. The wavelength of the light returning to the optical fiber FB31 is determined by the angle between the optical axis of the optical system 315 and the reflecting surface. The light having a specific wavelength incident on the optical fiber FB31 is incident on the optical fiber FB30 from the optical splitter 312 and the laser light L having a specific wavelength is emitted toward the optical fiber FB1a.

Therefore, when the rotary polygon mirror 316 rotates at a constant speed in the direction of the arrow R30, the wavelength λ of the light incident on the optical fiber FB1a again changes with a constant period as time passes. Specifically, as shown in FIG. 4, the light source unit 30 emits light L having a wavelength swept from the minimum sweep wavelength λmin to the maximum sweep wavelength λmax at a constant period T ₀ (for example, about 50 μsec). The light L emitted from the light source unit 30 is branched into the optical fibers FB1b and FB1c by the optical branching unit 2 made of an optical fiber coupler or the like, and is incident on the interferometer 20 and the periodic clock generation unit 80, respectively.

  Although the case where the wavelength is swept by rotating the polygon mirror as the light source unit 30 is illustrated, the light source unit 30 emits light while sweeping the wavelength at a constant period by a known technique such as an ASE light source unit. May be.

  FIG. 5 is a schematic diagram showing an example of the interferometer 20 in the optical tomographic imaging system 1 of FIG. The interferometer 20 is a Mach-Zehnder type interferometer, and is configured by housing various optical components in a housing 20A. The interferometer 20 divides the light L emitted from the light source unit 30 into measurement light L1 and reference light L2, and the measurement light L1 divided by the light division means 3 is irradiated onto the measurement object S. And combining means 4 for combining the reflected light L3 from the measurement object S and the reference light L2, and detecting the interference light L4 between the reflected light L3 combined by the combining means 4 and the reference light L2. Interference light detecting means 70. The interferometer 20 and the light source unit 30 are connected using an APC (Angled physical contact) connector. By using the APC connector, the reflected return light from the connection end face of the optical connector (optical fiber) can be reduced to the limit, and deterioration of the tomographic image P can be prevented.

  The light splitting means 3 is composed of, for example, a 2 × 2 optical fiber coupler, and splits the light L guided from the light source unit 30 through the optical fiber FB1c into the measurement light L1 and the reference light L2. At this time, the light dividing means 3 divides the light at a ratio of, for example, measurement light L1: reference light L2 = 99: 1. The light splitting means 3 is optically connected to each of the two optical fibers FB2 and FB3. The split measurement light L1 is incident on the optical fiber FB2 side, and the reference light L2 is incident on the optical fiber FB3 side. It is like that.

  An optical circulator 21 is connected to the optical fiber FB2, and optical fibers FB4 and FB5 are connected to the optical circulator 21, respectively. An optical probe 10 that guides the measurement light L1 to the measurement target S is connected to the optical fiber FB4. The measurement light L1 emitted from the optical probe 10 is guided from the optical fiber FB2 to the optical probe 10, and the measurement target S Is irradiated. The reflected light L3 reflected from the measuring object S is incident on the optical circulator 21 through the optical fiber FB4 and is emitted from the optical circulator 21 to the optical fiber FB5 side. The optical fiber FB4 and the optical probe 10 are connected using an APC (Angled physical contact) connector, and the reflected return light from the connection end face of the optical connector (optical fiber) is reduced to the limit. Image quality degradation is prevented.

  On the other hand, an optical circulator 22 is connected to the optical fiber FB3, and optical fibers FB6 and FB7 are connected to the optical circulator 22, respectively. The optical fiber FB6 is connected with an optical path length adjusting means 40 for changing the optical path length of the reference light L2 in order to adjust the tomographic image acquisition region. The optical path length adjusting means 40 includes an optical path length rough adjusting optical fiber 40A for coarsely adjusting the optical path length, and an optical path length fine adjusting means 40B for finely adjusting the optical path length.

  One end side of the optical path length coarse adjustment optical fiber 40A is detachably connected to the optical fiber FB2, and the other end side is detachably connected to the optical path length fine adjustment means 40B. A plurality of optical path length rough adjustment optical fibers 40A having different lengths are prepared in advance, and an optical path length rough adjustment optical fiber 40A having an appropriate length is appropriately attached as necessary. The optical path length coarse adjustment optical fiber 40A is connected to the optical fiber FB6 and the optical path length fine adjustment means 40B using an APC (Angled physical contact) connector, and is connected to the end face of the optical connector (optical fiber). The reflected return light is reduced to the limit, and the image quality deterioration of the tomographic image P is prevented.

  The optical path length fine adjustment means 40B includes a reflection mirror 43, an optical terminator 44, and the like. The reflection mirror 43 reflects the reference light L2 emitted from the optical path length coarse adjustment optical fiber 40A to the optical terminator 44 side, and again reflects the reference light L2 reflected from the optical terminator 44 to the optical path length coarse adjustment optical fiber 40A side. Is reflected. The reflection mirror 43 is fixed on a movable stage (not shown), and is moved in the optical axis direction (arrow A direction) of the reference light L2 by the mirror moving means. The length changes. This movable stage is configured to move the reflecting mirror 43 in the direction of arrow A when the optical path length adjusting operation unit 46 is operated by a doctor or the like.

  Further, a polarization controller 50 is optically connected to the optical fiber FB7. The polarization controller 50 has a function of rotating the polarization direction of the reference light L2. As the polarization controller 50, for example, a known technique such as JP-A-2001-264246 can be used. The polarization controller 50 adjusts the polarization direction by operating the polarization adjustment operation unit 51 by a doctor or the like. For example, the reflected light L3 and the reference light L2 are combined by the combining means 4. By operating the polarization adjustment operation unit 51 so that the respective polarization directions coincide with each other, the tomographic image can be adjusted to be clear.

  The multiplexing means 4 is composed of a 2 × 2 optical fiber coupler, and combines the reflected light L3 guided through the optical fiber FB5 and the reference light L2 guided through the optical fiber FB7. Specifically, the multiplexing unit 4 branches the reflected light L3 guided through the optical fiber FB5 into two optical fibers FB8 and FB9, and the reference light L2 guided through the optical fiber FB7 into two optical fibers FB8, Branch to FB9. Accordingly, the reflected light L3 and the reference light L2 are combined in each of the optical fibers FB8 and FB9, the first interference light L4a is guided in the optical fiber FB8, and the second interference light L4b is guided in the optical fiber FB9. Will wave. That is, the multiplexing unit 4 also functions as an optical branching unit 5 that branches the interference light L4 between the reflected light L3 and the reference light L2 into two interference lights L4a and L4b.

  The interference light detection means 70 includes a first light detection unit 71 that detects the first interference light L4a, a second light detection unit 72 that detects the second interference light L4b, and a first light detection unit 71 detected by the first light detection unit 71. And a differential amplifier 73 that outputs the difference between the first interference light L4a and the second interference light L4b detected by the second light detection unit 72 as the interference signal IS. Each of the light detection units 71 and 72 includes, for example, a photodiode or the like, and photoelectrically converts each of the interference lights L4a and L4b incident via the variable light attenuators 60A and 60B and inputs them to the differential amplifier 73. The difference amplifier 73 amplifies the difference between the interference lights L4a and L4b and outputs it as an interference signal IS. In this way, by performing balance detection on the interference lights L4a and L4b by the differential amplifier 73, in-phase optical noise other than the interference signal IS can be removed while amplifying and outputting the interference signal IS. The image quality can be improved.

  Variable optical attenuators 60A and 60B are provided between the optical branching unit 5 (the multiplexing unit 4) and the interference light detecting unit 70. These variable optical attenuators 60A and 60B attenuate the respective light amounts of the first interference light L4a and the second interference light L4b at different attenuation rates for each wavelength band, and emit them to the interference light detection means 70 side. Note that variable optical attenuators 60A and 60B are provided for the first interference light L4a and the second interference light L4b, respectively.

  FIG. 6 is a schematic diagram showing an example of the variable optical attenuator 60A. The variable optical attenuator 60 </ b> A includes a disk-shaped neutral density filter (ND filter) 62 and a rotation drive unit (rotational motor) 64 that rotates the neutral density filter 62. As shown in FIG. 7, the neutral density filter 62 is formed, for example, in such a way that the degree of darkness of black varies along the circumferential direction (arrow R10 direction), and the attenuation factor (transmittance) of light differs. ing. Then, the first interference light L4a is incident on the spot position 62a of the neutral density filter 62, and the first interference light L4a is attenuated according to the attenuation factor (transmittance) of the spot position 62a and is incident on the optical fiber FB10. Therefore, when the neutral density filter 62 is rotated in the direction of the arrow R10 by the driving means 64, the attenuation rate at the position where the interference light L4a is transmitted changes over time. The variable optical attenuator 60B also has the same configuration as that in FIG. The attenuation rates by the variable optical attenuators 60A and 60B are set so that the light amounts of the interference light beams L4a and L4b are substantially equal in each wavelength band.

  Therefore, when the interference light beams L4a and L4b having different wavelengths are incident on the variable optical attenuators 60A and 60B as time changes, the variable optical attenuators 60A and 60B change the attenuation rates of the interference light beams L4a and L4b according to the wavelength changes. The interference lights L4a and L4b are attenuated. As a result, the light intensity detection signal levels of the interference lights L4a and L4b detected by the light detection units 71 and 72 are substantially equal in the entire wavelength band, and the S / N when the interference light detection means 70 performs balance detection. The ratio can be improved.

  That is, as described above, the first interference light L4a and the second interference light L4b are branched using the optical branching means 5 such as an optical fiber coupler or a beam splitter. This optical fiber coupler or the like has wavelength-dependent characteristics, and does not have a 50:50 branching ratio over the entire wavelength range of the interference light L4 (the sweep wavelength band of the laser light L). That is, depending on the wavelength bands of the interference lights L4a and L4b, they may be branched at different branching ratios.

  For example, FIG. 8 is a graph showing branching characteristics depending on the wavelength of an optical fiber coupler. When the reflected light L3 is branched into the optical fiber FB8 and the optical fiber FB9 using the optical branching means (optical fiber coupler) 5 having the branching characteristics as shown in FIG. 8, the reflected light L3 is split into the optical fiber FB8 and the optical fiber FB9, respectively. The amount of the reflected light L3 is different, and is particularly remarkable on the long wavelength side. Similarly, when the reference light L2 is divided into the optical fiber FB8 and the optical fiber FB9 by the optical branching means (optical fiber coupler) 5, the amount of the reflected light L3 that is divided into the optical fiber FB8 and the optical fiber FB9, respectively. In particular, it becomes particularly prominent on the long wavelength side. When the light amounts of the reflected light L3 and the reference light L2 incident on the optical fibers FB8 and FB9 are different from each other, the light amounts of the first interference light L4a and the second interference light L4b are also different from each other.

  On the other hand, in order to improve the S / N ratio by balance detection of the interference light detection means 70, the light amounts of the first interference light L4a and the second interference light L4b need to be substantially uniform. On the other hand, when the light amounts of the first interference light L4a and the second interference light L4b are different, the white noise component increases and the S / N ratio decreases, and the purpose of balance detection is sufficiently achieved. Can not do it. Therefore, variable optical attenuators 60A and 60B whose attenuation factors change depending on the wavelength of the interference light L4 are inserted between the optical branching means 5 and the interference light detection means 70.

  Specifically, when the optical branching unit 5 uses an optical fiber coupler having a wavelength dependency characteristic as shown in FIG. 9A, the first interference light L4a has a light quantity as it goes from the minimum sweep wavelength λmin to the maximum sweep wavelength λmax. The second interference light L4b increases in light quantity from the minimum sweep wavelength λmin to the maximum sweep wavelength λmax. Therefore, as shown in FIG. 9B, a variable optical attenuator 60A is inserted which increases the transmittance (decreases the attenuation factor) as the first interference light L4a is swept from the minimum sweep wavelength λmin to the maximum sweep wavelength λmax. Is done. Similarly, a variable optical attenuator 60B is inserted that decreases the transmittance (increases the attenuation factor) as the second interference light L4a is swept from the minimum sweep wavelength λmin to the maximum sweep wavelength λmax.

That is, the amount of light for each wavelength band of the first interference light L4a when branched by the light branching means 5 is Pa (λ), the amount of light for each wavelength band of L4b of the second interference light is Pb (λ), When the transmittance of each wavelength band of the first interference light L4a in the variable optical attenuator 60A is Ta (λ), and the transmittance of each wavelength band of the second interference light L4b in the variable optical attenuator 60B is Tb (λ). ,
Pa (λ) · Ta (λ) = Pb (λ) · Tb (λ)
The attenuation rate (transmittance) of each variable optical attenuator 60A, 60B is set so that

  As a result, the light amounts of the branched interference lights L4a and L4b are substantially equal in the entire wavelength band, and the white noise component is removed by balance detection in the interference light detection means 70 to improve the S / N ratio. it can.

  The interference signal IS output from the interference light detection means 70 is amplified by the amplifier 74 and then output to the A / D conversion unit 90 via the signal band filter 75. By providing this signal band filter 75, it is possible to remove noise from the interference signal IS and improve the S / N ratio.

  FIG. 10 is a block diagram showing an example of the A / D conversion unit 90 shown in FIG. The A / D conversion unit 90 converts the interference signal IS detected by the interference light detection means 70 into a digital signal and outputs the digital signal. The A / D converter 91, the sampling clock generation circuit 92, the control controller 93, Interference signal storage means 94 is provided. The A / D converter 91 converts the interference signal IS output as an analog signal from the interferometer 20 into a digital signal. The A / D converter 91 performs A / D conversion of the interference signal IS based on the sampling clock output from the sampling clock generation circuit 92. The interference signal storage means 94 is composed of, for example, a RAM (Random Access Memory) or the like, and stores the interference signal IS converted into a digital signal. The operations of the A / D converter 91, sampling clock generation circuit 92, and interference signal storage means 94 are controlled by a controller 93.

Here, the interference signal IS that has been stored by the interference signal storage means 94, when the periodic clock signal T _CLK is output, the interference signal acquisition timing of the periodic clock signal T _CLK is outputted by one cycle as a reference It is acquired by means 101. Specifically, for example, as shown in FIG. 12B, the interference signal acquisition unit 101 acquires the interference signal IS in the wavelength band DT before and after the output timing of the periodic clock signal _TCLK . Note that the output timing of the periodic clock signal T _CLK is not limited to the case of FIG. 12B as long as it is within the wavelength band to be swept, and the interference signal IS for one period is set to the wavelength immediately after the start of the wavelength sweeping. Alternatively, it may be acquired, or may be set immediately before the end of the wavelength sweep to acquire the interference signal IS for one period.

FIG. 11 is a schematic diagram showing an example of the periodic clock generating means 80 for generating the above-described periodic clock signal _TCLK . The periodic clock generation means 80 outputs one periodic clock signal T _CLK each time the wavelength of the light L emitted from the light source unit 30 is swept by one period. The photodetection unit 84 is provided. Then, the light L emitted from the optical fiber FB 1 c enters the optical filter 82 via the optical lens 81. The light L transmitted through the optical filter 82 is detected by the light detection unit 84 via the optical lens 83, and the periodic clock signal T _CLK is output to the A / D conversion unit 90.

The optical filter 82 is made of, for example, an etalon or the like, and has a function of transmitting only light of the set wavelength λref and shielding light of other wavelength bands. As shown in FIG. 12A, it has a plurality of transmission wavelengths. The optical filter 82 has a light transmission period FSR (free spectrum range) in which one transmission wavelength is set in the wavelength band λmin to λmax among the plurality of transmission wavelengths. Therefore, only the light of the set wavelength λref set in the wavelength band λmin to λmax in which the wavelength of the light emitted from the light source unit 30 is swept is transmitted, and the light of the other wavelength bands is shielded. Therefore, as shown in FIG. 12B, when the light L whose wavelength is periodically swept is emitted from the light source unit 30 and the wavelength of the light L becomes the set wavelength λref, the periodic clock signal T _CLK is output. Will be. As shown in FIG. 12B, depending on the characteristics of the optical filter (etalon) 82, the transmission band width (FWHM: Full Width at Half Maximum) becomes wide, and the generation timing of the periodic clock signal _TCLK is increased. There is a case where it is shifted within a range within the transmission band width. In this case, it is preferable that the interference signal acquisition unit 101 described later is accurate and preferable if, for example, the middle of the transmission bandwidth is set as the generation timing of the periodic clock signal _TCLK .

Thus, by generating and outputting the periodic clock signal T _CLK using the light L actually emitted from the light source unit 30, the light L emitted from the light source unit 30 has a predetermined light intensity from the start of wavelength sweeping. Even when the time until becomes different for each period, the interference signal IS in the wavelength band of the predetermined period T (see FIG. 4) can be acquired from the set wavelength λref. Therefore, the periodic clock signal _TCLK can be output at the timing of acquiring the interference signal IS in the assumed wavelength band in the tomographic image processing means 100, and degradation in resolution can be suppressed.

  FIG. 13 is a block diagram showing an example of the tomographic image processing means 100. The configuration of the tomographic image processing means 100 as shown in FIG. 3 is realized by executing a tomographic image processing program read into the auxiliary storage device on a computer (for example, a personal computer). The tomographic image processing unit 100 includes an interference signal acquisition unit 101, an interference signal conversion unit 102, an interference signal analysis unit 103, a tomographic image generation unit 105, and the like.

The interference signal acquisition unit 101 acquires, from the interference signal storage unit 94, one period of the interference signal IS detected by the interference light detection unit 70 based on the periodic clock signal _TCLK output from the periodic clock generation unit 80. Is. Specifically, for example, the interference signal acquisition unit 101 acquires the interference signal IS in the wavelength band DT before and after the output timing of the periodic clock signal _TCLK as shown in FIG. Note that the interference signal obtaining means 101 as long as it acquires the interference signal IS for one period of the output timing of the periodic clock signal T _CLK as a reference, the output timing of the periodic clock signal T _CLK is within the wavelength band to be swept If so, the present invention is not limited to the case of FIG. 10B, and may be set to a wavelength immediately after the start of the wavelength sweep, or may be set immediately before the end of the wavelength sweep.

  The interference signal converting means 102 makes the interference signal IS acquired over time in the A / D conversion unit 90 as shown in FIG. 14 at equal intervals on the wavenumber k (= 2π / λ) axis as shown in FIG. It has the function to rearrange as follows. Specifically, the interference signal conversion means 102 has a time-wavelength sweep characteristic data table or function of the light source unit 30 in advance, and uses this time-wavelength sweep characteristic data table and the like at equal intervals on the wavenumber k axis. The interference signal IS is rearranged so that As a result, when calculating tomographic information from the interference signal IS, highly accurate tomographic information is obtained by a spectrum analysis method that assumes that the frequency space is equal in frequency space such as Fourier transform processing and processing by the maximum entropy method. Can do. The details of this signal conversion method are disclosed in US Pat. No. 5,956,355.

  Here, the interference signal conversion means 102 acquires the output from the optical filter 182 as shown in FIG. 12A, and based on the time-wavelength sweep characteristic data table or function and the output from the optical filter 182, the signal It is also possible to perform conversion. However, since the output from the optical filter 182 is processed, the signal conversion process takes time. On the other hand, the interference signal acquisition unit 101 acquires an interference signal IS in a preset wavelength band, and the interference signal conversion unit 102 performs conversion processing using a time-wavelength sweep characteristic data table or the like, thereby performing signal conversion processing. Can be made more efficient.

  The interference signal analysis means 103 analyzes the interference signal IS signal-converted by the interference signal conversion means 102 using, for example, a known spectrum analysis technique such as Fourier transform processing, maximum entropy method (MEM), Yule-Walker method, The tomographic information r (z) is acquired.

The tomographic image generation unit 105 acquires the tomographic information r (z) for one period (one line) acquired by the interference signal analysis unit 103 in the radial direction (arrow R1 direction) of the optical probe 10, and FIG. One tomographic image P as shown is generated. Here, the tomographic image generation unit 105 stores the tomographic information r (z) for one line sequentially acquired in the tomographic information storage unit 105a, and the rotation clock signal R _CLK is received from the rotation control unit 10B of FIG. When output, the tomographic image P is generated using the stored tomographic information r (z) for n lines as shown in FIG. For example, when the periodic clock T _CLK from the light source unit 30 is 20 kHz and the optical probe 10 scans the measuring light L1 in the direction of the arrow R1 at 20 Hz, the tomographic image generation means 105 has n = 1024 lines. One tomographic image P is generated using the tomographic information r (z).

  In order to improve the image quality, a method of acquiring, acquiring and averaging a plurality of tomographic images may be used. That is, when the optical probe 10 irradiates the same part of the measuring object S while scanning the measurement light L1 a plurality of times, the tomographic image generation means 105 acquires a plurality of tomographic images from the same part. Then, the tomographic image generation unit 105 calculates the average value of the tomographic information r (x, z) at each depth position z at the position x with respect to the length direction of the optical probe 10 using the plurality of tomographic images. Thereby, the noise component contained in each tomographic image is canceled, and a tomographic image with good image quality can be acquired.

  Further, when the tomographic image generation unit 105 generates the tomographic image using the tomographic information r (z) for a plurality of lines in the scanning direction (arrow R1 direction), the tomographic information of a plurality of adjacent lines is averaged. You may make it produce | generate a tomographic image using a thing. For example, the tomographic image generation unit 105 uses the average value of the tomographic information for three adjacent lines as the tomographic information used for generating the tomographic image. Thereby, the noise component contained in the tomographic information of each line is canceled, and a tomographic image with good image quality can be generated.

  The image quality correction unit 106 corrects the image quality by performing sharpening processing, smoothing processing, and the like on the tomographic image P generated by the tomographic image generation unit 105. Then, the tomographic image P subjected to the image quality correction is displayed on the display device 110 in FIG.

  An operation example of the optical tomographic imaging system will be described with reference to FIGS. First, a light beam swept at a constant period within a predetermined wavelength band is emitted from the light source unit 30. The light L is divided into two at the optical branching means 2 and is incident on the interferometer 20 and the periodic clock generating means 80, respectively. In the light splitting means 3 of the interferometer 20, the light L is split into measurement light L1 and reference light L2, the measurement light L1 is emitted toward the optical fiber FB2, and the reference light L2 is emitted toward the optical fiber FB3.

  The measurement light L1 is guided through the optical circulator 21, the optical fiber FB4, and the optical probe 10, and is irradiated onto the measurement object S. Then, the reflected light L3 reflected at each depth position z of the measuring object S and the backscattered light are incident on the optical probe 10 again. The reflected light L3 is incident on the multiplexing means 4 via the optical probe 10, the optical circulator 21, and the optical fiber FB5.

  On the other hand, the reference light L2 is incident on the optical path length adjusting means 40 via the optical fiber FB3, the optical circulator 22, and the optical fiber FB6. Then, the reference light L2 whose optical path length is adjusted by the optical path length adjusting means 40 is again guided through the optical fiber FB6, the optical circulator 22, the polarization controller 50, and the optical fiber FB7 and is incident on the multiplexing means 4.

  In the multiplexing unit 4, the reflected light L3 and the reference light L2 are combined, and the interference light L4 when combined is branched in the multiplexing unit 4 (optical branching unit 5), and the two interference lights L4a. 4b are emitted to the optical fibers FB8 and FB9, respectively. Then, the interference lights L4a and L4b guided through the optical fibers FB8 and FB9 are attenuated by the variable optical attenuators 60A and 60B, and balance detection is performed by the interference light detection means 70.

  Here, before the balance detection in the interference light detection means 70, the wavelength band is set so that the branched first interference light L4a and second interference light L4b are substantially equal in the entire wavelength bands of the interference light L4a and L4b. By providing the variable optical attenuators 60A and 60B that are attenuated at different attenuation rates every time, the non-interference component can be reliably removed by balance detection in the interference light detection means 70, and the S / N ratio can be improved.

  The interference light L4 detected by the interference detection by the interference light detection means 70 is output as an interference signal IS, and is output to the A / D conversion unit 90 through the amplifier 74 and the signal band filter 75. Thereafter, the interference signal IS is A / D converted by the A / D conversion unit 90, and data for one cycle (one line) of the light source unit 30 is stored in the interference signal storage means 94.

When the wavelength of the light L incident on the periodic clock generation means 80 from the light source unit 30 via the light branching means 2 is the set wavelength λref, the light L that has passed through the optical filter 82 passes through the optical lens 83 and is a light detection unit. 84. Then, the periodic clock signal T _CLK is output from the periodic clock generation means 80 to the interference signal acquisition means 101, and the interference signal IS for one period is acquired from the interference signals IS stored in the interference signal storage means 94. .

In the tomographic image processing means 100, the interference signal conversion means 102 performs signal conversion processing on the interference signal IS for one line so that the wave number k is equally spaced. Thereafter, the interference signal IS is subjected to spectrum analysis by the interference signal analysis means 103, whereby tomographic information (reflectance) is acquired from the interference signal IS as tomographic information r (z). In the tomographic image generation means 105, the acquired tomographic information r (z) is accumulated for n lines in the scanning direction (arrow R1 direction) of the measuring light L1. When the rotation clock signal R _CLK is detected, one tomographic image P is generated using the accumulated plurality of tomographic information r (z). Thereafter, the image quality correction means 106 performs image quality correction on the generated tomographic image P, and the image quality corrected tomographic image P is displayed on the display device 110 of FIG.

  FIG. 17 is a schematic view showing a second embodiment of the variable optical attenuator in the optical tomographic imaging system of the present invention. The variable optical attenuator will be described with reference to FIG. In the variable optical attenuator shown in FIG. 17, parts having the same configurations as those of the variable optical attenuators 60A and 60B shown in FIG. The variable optical attenuator 160 in FIG. 17 is different from the variable optical attenuator 60A in FIG. 6 in that the first interference light L4a and the second interference light L4b are attenuated by one neutral density filter 162.

  That is, as shown in FIG. 17B, the neutral density filter 162 includes a first attenuation region 162X in which an attenuation factor is set on the outer peripheral side for each wavelength band of the first interference light L4a along the circumferential direction. And a second attenuation region 162Y in which the attenuation factor is set on the inner peripheral side for each wavelength band of the second interference light L4b along the circumferential direction. When the first interference light L4a is incident on the spot position 162a on the outer peripheral side of the neutral density filter 162 and the neutral density filter 162 rotates in the direction of arrow R10, the first interference light L4a is attenuated at the spot position 162a (transmittance). ) And is incident on the optical fiber FB10. Similarly, when the second interference light L4b is incident on the spot position 162b of the neutral density filter 162 and the neutral density filter 162 rotates in the direction of the arrow R10, the second interference light L4b is attenuated at the spot position 162b (transmittance). ) And is incident on the optical fiber FB11.

  Even in this case, it is possible to improve the S / N ratio by balance detection by making the light amounts of the first interference light L4a and the second interference light L4b substantially equal, and using one variable light attenuator 160 for each. Since the interference lights L4a and L4b can be attenuated, the apparatus can be downsized.

  In addition, like the variable light attenuator 160 in FIG. 18, the first interference light L4a and the second interference light L4b are incident on the spot positions 162a and 162b that are symmetric with respect to the rotation axis with respect to the neutral density filter 162. It may be.

  FIG. 19 is a schematic diagram showing a third embodiment of the variable optical attenuator in the optical tomographic imaging system of the present invention. The variable optical attenuator 260A will be described with reference to FIG. In the variable optical attenuator of FIG. 19, parts having the same configuration as the variable optical attenuator 60A of FIG. Note that the variable optical attenuator 260B for the second interference light L4b has the same configuration.

  A variable optical attenuator 260A in FIG. 19 includes a multilayer filter 262 that receives the first interference light L4a branched by the optical branching unit 5 and has a wavelength transmission characteristic that changes according to the incident angle of the first interference light L4a. Swing means 264 for tilting the filter 262 with respect to the optical axis of the first interference light L4a is provided.

  The first interference light L4a emitted from the optical fiber FB8 is converted into parallel light by the optical lens 61 and is incident on the multilayer filter 262. The multilayer filter 262 can be tilted in the direction of the arrow R20 by the swinging means 264. The swinging means 264 and the multilayer filter 262 and the first interference light are matched to the wavelength of the emitted first interference light L4a. The attenuation rate (transmittance) of the first interference light L4a is adjusted by changing the angle of the L4a with the optical axis. The first interference light L4a that has passed through the multilayer filter 262 enters the optical fiber FB10 via the optical lens 63. Even in this case, the light amounts of the first interference light L4a and the second interference light L4b can be made substantially equal, and the S / N ratio can be improved by balance detection.

  FIG. 20 is a schematic diagram showing a fourth embodiment of the variable optical attenuator in the optical tomographic imaging system of the present invention. The variable optical attenuator 360A will be described with reference to FIG. In the variable optical attenuator 360A of FIG. 20, the same reference numerals are given to the portions having the same configuration as the variable optical attenuator 60A of FIG. 6, and the description thereof is omitted. Note that the variable optical attenuator 360B for the second interference light L4b has the same configuration.

  The variable optical attenuator 360A in FIG. 20 collects the first interference light L4a branched by the light branching means 5 and enters the optical fiber FB10 optically connected to the interference light detection means 70, and a light Fiber end face moving means 364 for adjusting the amount of incident light of the first interference light L4a to the fiber FB10 by moving the incident end face of the optical fiber FB10 is provided.

  Then, the first interference light L4a emitted from the optical fiber FB8 is condensed on the incident end face of the optical fiber FB10 by the optical lens 361 and is incident on the optical fiber FB10. The fiber end face moving means 364 includes, for example, a piezo element and moves the incident end face in a direction orthogonal to the optical axis according to the wavelength of the first interference light L4a emitted from the optical fiber FB10. Thereby, the light quantity of the 1st interference light L4a which injects into optical fiber FB10 will be attenuated. Note that the variable optical attenuator 360B for the second interference light L4b has the same configuration. Even in this case, the light amounts of the first interference light L4a and the second interference light L4b can be made substantially equal, and the S / N ratio can be improved by balance detection.

  FIG. 21 is a schematic diagram showing a fifth embodiment of the variable optical attenuator in the optical tomographic imaging system of the present invention. The variable optical attenuator 460A will be described with reference to FIG. In the variable optical attenuator 460A shown in FIG. 21, the same reference numerals are given to portions having the same configuration as the variable optical attenuator 60A shown in FIG. Note that the variable optical attenuator 460B for the second interference light L4b has the same configuration.

  A variable optical attenuator 460A shown in FIG. 21 is a variable optical attenuator using a Faraday rotator, and includes a birefringent crystal plate 461, an optical lens 462, a permanent magnet 463, an electromagnet (coil) 464, a Faraday rotator 465, and a permanent magnet 466. , A reflection plate 467, variable excitation means (current supply source) 468, and the like. A static magnetic field having a constant strength is applied to the Faraday rotator 465 in the optical axis direction by permanent magnets 463 and 466. The electromagnet 464 applies a magnetic field in the optical axis direction corresponding to the application of current from the variable excitation means 468 to the Faraday rotator 465 to change the polarization angle of the Faraday rotator 465.

  Then, the first interference light L4a input to the variable light attenuator 460A is incident on the birefringent crystal plate 461 and is separated into an ordinary ray and an extraordinary ray in the birefringent crystal plate 461. Thereafter, the first interference light L4a separated into two light rays passes through a static magnetic field having a constant intensity formed in the optical axis direction by the optical lens 462 and the permanent magnet 463, and is incident on the Faraday rotator 465.

  Thereafter, the polarization angle of the first interference light L4a is rotated by the Faraday rotator 465, passes through the static magnetic field having a constant intensity formed in the optical axis direction by the permanent magnet 466, and is reflected again to the permanent magnet 466 side by the reflecting plate 467. Then, the first interference light L4a is emitted following the same optical path. Here, when the first interference light L4a reflected by the reflecting plate 467 is incident on the birefringent crystal plate 461, the ordinary ray travels straight on the optical axis, and the extraordinary ray deviates from the optical axis. By controlling the current flowing through the variable excitation means 468 and changing the polarization angle of the Faraday rotator 465, the light intensity of ordinary light can be changed. Therefore, the attenuation factor of the light quantity of the first interference light L4a can be changed for each wavelength band. Even in this case, the light amounts of the first interference light L4a and the second interference light L4b can be made substantially equal, and the S / N ratio can be improved by balance detection.

  According to each of the above embodiments, the variable optical attenuators 60A and 60B for attenuating the first interference light L4a and the second interference light L4b branched by the light branching means 5 with different attenuation rates for the respective wavelength bands, and the variable light Interference light detection means 70 for detecting the difference between the first interference light L4a and the second interference light L4b attenuated by the attenuators 60A, 60B (160, 260A, 260B, 360, 460) as the interference signal IS, and interference light detection means The tomographic image processing means 100 that obtains tomographic information at each depth position from the interference signal IS detected by 70 and generates a tomographic image is provided, so that the branched first interference light L4a and second interference light L4b When the interference signal IS is generated by using the difference between the optical branching means 5 and the optical branching means 5 in the entire wavelength band of the interference light due to the wavelength dependence characteristics of the optical branching means 5. Even when it is not possible to branch, the variable optical attenuator can attenuate the light amounts of the first interference light L4a and the second interference light L4b so as to be substantially equal in each wavelength band, so that the S / N ratio is improved. Can be achieved.

  The embodiment of the present invention is not limited to the above embodiment. For example, although various embodiments of the variable optical attenuator have been described with reference to FIGS. 6 and 17 to 21, a known variable optical attenuator such as a variable optical attenuator using a Faraday rotator or other known variable optical attenuator is used. Can do.

  2 illustrates the case where the measurement light L1 is irradiated while scanning in the circumferential direction (arrow R1 direction), but the measurement light L1 is scanned in the longitudinal direction of the optical probe 10. The person who irradiates may be sufficient. At this time, the optical fiber 12 and the optical lens 15 have a structure that can move in the longitudinal direction of the optical probe 10 with respect to the probe outer cylinder 11.

Schematic configuration diagram showing a preferred embodiment of the optical tomographic imaging system of the present invention Schematic diagram showing an example of an optical probe used in the optical tomographic imaging system of FIG. The schematic diagram which shows an example of the light source unit in the optical tomographic imaging system of FIG. The graph which shows a mode that the wavelength of the light inject | emitted from the light source unit of FIG. 3 is swept Schematic diagram showing an example of an interferometer in the optical tomographic imaging system of FIG. Schematic diagram showing an example of a variable optical attenuator in the interferometer of FIG. The schematic diagram which shows an example of the neutral density filter in the variable optical attenuator of FIG. FIG. 5 is a graph showing an example of wavelength-branching ratio characteristics of the light branching means in FIG. 6 is a graph showing an example of wavelength-attenuation characteristics of the variable optical attenuator in FIG. 1 is a block diagram illustrating an example of an A / D conversion unit in the optical tomographic imaging system of FIG. Schematic diagram showing an example of periodic clock signal generation means in the optical tomographic imaging system of FIG. The graph which shows an example of the periodic clock signal produced | generated by the periodic clock signal production | generation means of FIG. Block diagram showing an example of the tomographic image processing means of FIG. The graph which shows an example of the interference signal input into the resampling means of FIG. The graph which shows an example of the interference signal resampled by the resampling means of FIG. Schematic diagram showing an example of a tomographic image generated by the tomographic image generating means of FIG. Schematic diagram showing a second embodiment of a variable optical attenuator in the optical tomographic imaging system of FIG. FIG. 17 is a schematic diagram showing a modification of the variable optical attenuator in FIG. Schematic diagram showing a third embodiment of a variable optical attenuator in the optical tomographic imaging system of FIG. Schematic diagram showing a fourth embodiment of a variable optical attenuator in the optical tomographic imaging system of FIG. Schematic diagram showing a fifth embodiment of a variable optical attenuator in the optical tomographic imaging system of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical tomographic imaging system 3 Optical dividing means 4 Combined means 5 Optical branching means 10 Optical probe 20 Interferometer 30 Light source unit 60A, 60B, 160, 260A, 260B, 360A, 360B Variable optical attenuator 62, 162 Neutral filter 64 Rotation drive means 70 Interference light detection means 75 Signal band filter 80 Periodic clock generation means 90 A / D conversion unit 100 Tomographic image processing means 262 Multilayer filter 264 Oscillating means 361 Optical lens 364 Fiber end face moving means IS Interference signal L Light L1 Measurement light L2 Reference light L3 Reflected light L4 Interference light L4a First interference light L4b Second interference light P Tomographic image r (z) Tomographic information S Measurement object

Claims (6)

A light source unit that emits light while sweeping a wavelength at a constant period;
A light splitting means for splitting the light emitted from the light source unit into measurement light and reference light;
Multiplexing means for multiplexing the reflected light and the reference light when the measurement light divided by the light dividing means is reflected by the measurement object;
Light branching means for branching the interference light between the reflected light and the reference light combined by the multiplexing means into a first interference light and a second interference light;
The first interference light and the second interference light branched by the light branching unit have different attenuation rates for each wavelength band, and the light amounts of the first interference light and the second interference light are approximately equal. A variable optical attenuator that attenuates to
Interference light detecting means for detecting a difference between the first interference light and the second interference light attenuated by the variable optical attenuator as an interference signal;
An optical tomographic imaging system comprising: a tomographic image processing means for generating a tomographic image from the interference signal detected by the interference light detecting means.   The variable optical attenuator is a disc-shaped neutral density filter with different light attenuation factors along the circumferential direction on which the first interference light and / or the second interference light branched by the light branching unit is incident. The optical tomographic imaging system according to claim 1, further comprising: a rotation driving unit that rotates the neutral density filter.   The neutral density filter includes an outer peripheral first attenuation region in which an attenuation factor for each wavelength band with respect to the first interference light is set along the circumferential direction, and the second interference light along the circumferential direction. The optical tomographic imaging system according to claim 2, further comprising: an inner peripheral second attenuation region in which an attenuation factor for each wavelength band is set.   The variable optical attenuator changes the wavelength transmission characteristic according to the incident angle of the first interference light or the second interference light on which the first interference light or the second interference light branched by the light branching unit is incident. 2. The optical tom according to claim 1, further comprising: a multilayer filter; and a filter tilting unit that tilts the multilayer filter with respect to an optical axis of the first interference light or the second interference light. Imaging system.   An optical lens that the variable light attenuator collects the first interference light or the second interference light branched by the light branching means and enters an optical fiber optically connected to the interference light detection means; And a fiber end face moving means for adjusting the amount of incident light of the first interference light or the second interference light to the optical fiber by moving the incident end face of the optical fiber. The optical tomographic imaging system according to claim 1.   The optical tomographic imaging system according to claim 1, wherein the variable optical attenuator uses a Faraday rotator.