Patent Application
20230358527
The present invention relates to an optical interference tomographic imaging device.
An example of the technique for performing tomographic imaging in the vicinity of the surface of the object to be measured includes an optical coherence tomography (OCT) technique. In the OCT technique, tomographic imaging in the vicinity of the surface of the object to be measured is performed using interference between scattered light (hereinafter, also referred to as “backscattered light”) from the inside of the object to be measured when the object to be measured is irradiated with the light beam and reference light beam. In recent years, the application of the OCT technique to medical diagnosis and industrial product inspection has been expanded.
In the OCT technique, a position in an optical axis direction, that is, a depth direction of a portion (light scattering point) where the object light beam is scattered in the object to be measured is identified by using interference between the object light beam irradiated to and scattered by the object to be measured and the reference light beam, thereby obtaining structure data spatially resolved in the depth direction inside the object to be measured. Examples of the OCT technique include a time domain (TD-OCT) system and a Fourier domain (FD-OCT) system, and the FD-OCT system is more promising in terms of high speed and high sensitivity. In the FD-OCT method, when object light beam and reference light beam are caused to interfere with each other, an interference light spectrum in a wide wavelength band is measured, and Fourier transform is performed on the interference light spectrum to obtain structure data in the depth direction. Examples of the method for obtaining an interference light spectrum include a spectral domain (SD-OCT) method using a spectrometer and a swept source (SS-OCT) method using a light source that sweeps a wavelength.
Furthermore, by scanning the object light beam radiation position of the object to be measured in an in-plane direction perpendicular to the depth direction of the object to be measured, it is possible to obtain tomographic structure data spatially resolved in the in-plane direction and spatially resolved in the depth direction, that is, three-dimensional tomographic structure data of the object to be measured.
The OCT technique has been put into practical use as a tomographic imaging device for the fundus in ophthalmic diagnosis, and has been studied to be applied as a non-invasive tomographic imaging device for various parts of a living body.
By measuring the change in the intensity ratio between the interference light beam R151 and the interference light beam R161 accompanying the wavelength change of the light emitted from the wavelength swept laser light source 501, the interference light spectrum data is obtained. The wavelength dependency of the photoelectric conversion output of the balance type light receiver 507 represents the interference light spectrum. By measuring the interference light spectrum and performing Fourier transform on it, data indicating the intensity of the backscattered light (object light beam) at different positions in the depth direction (Z direction) can be obtained (hereinafter, the operation of obtaining data indicating the intensity of the backscattered light (object light beam) in the depth direction (Z direction) at a certain position of the object to be measured 520 is referred to as an “A scan”).
Interference between object light beam having a wavelength λ and a wave number k (=2π/λ) and reference light beam is considered. In a case where the optical path length until the reference light beam is reflected by the reference light beam mirror 506 and returns to the optical splitting/merging unit 503 after the reference light beam is split at the optical splitting/merging unit 503 is PR, and the optical path length until the object light beam is backscattered at one light scattering point of the object to be measured 520 and returns to the optical splitting/merging unit 503 after the object light beam is split at the optical splitting/merging unit 503 is PS=PR+z0, the object light beam R131 and the reference light beam R141 interfering at the optical splitting/merging unit 503 interfere with each other at a phase difference kz0+φ. Here, φ is a constant that does not depend on k or z0. The amplitude of the object light beam R131 is denoted by ES and the amplitude of the reference light beam R141 is denoted by ER, where they interfere with each other at the optical splitting/merging unit 503.
I(k)∝ES·ERcos(kz0+φ) [Math. 1]
The intensity difference between the interference light beam R151 and the interference light beam R161 represented by above expression is photoelectrically converted by the balance type light receiver 507. A light spectrum data generation unit 508 generates interference light spectrum data based on the information on the wavelength change of the emission light from the wavelength swept laser light source 501 and the information on the intensity difference between the interference light beam R151 and the interference light beam R161 from the balance type light receiver 507. The modulation with the period 2π/z0 appears in the interference light spectrum data I(k) obtained by measuring from the wave number k0−Δk/2 to k0+Δk/2. The obtained interference light spectrum data is transmitted from the light spectrum data generation unit 508 to an A scan waveform generation unit 509.
The A scan waveform generation unit 509 performs Fourier transform on the interference light spectrum data. The amplitude J(z) of the Fourier transform of I(k) is expressed as follows.
J(z)=|∫I(k)eizkdk|∝δ(z−z0)+δ(z+z0) [Math. 2]
A peak of a δ function is shown at z=z0 (and z=−z0) reflecting a light scattering point position z0.
When the object to be measured is a mirror, the light scattering point is located at one position. However, in general, the object light beam irradiated to the object to be measured is sequentially backscattered while being attenuated and propagating into the inside to some extent, and the light scattering points of the object light beam are distributed in a range from the surface to a certain depth. In the case where the light scattering points are distributed from z0−Δz to z0+Δz in the depth direction, the modulation from the period 2π/(z0−Δz) to 2π/(z0+Δz) appears in an overlapping manner in the interference light spectrum.
Further, the radiation position of the object light beam R111 is scanned on the object to be measured 520 by the irradiation optical system 505. By repeatedly performing the A scan operation while moving the radiation position of the object light beam R111 in the scanning line direction (X direction) by the irradiation optical system 505 and connecting the measurement results, a map of two-dimensional intensity of backscattered light (object light beam) in the scanning line direction and the depth direction is obtained as tomographic structure data (hereinafter, the operation of repeatedly performing the A scan operation in the scanning line direction (X direction) and connecting the measurement results is referred to as a “B scan”).
Further, by repeatedly performing the B scan operation while moving the radiation position of the object light beam R111 not only in the scanning line direction but also in the direction (Y direction) perpendicular to the scanning line by the irradiation optical system 505, and connecting the measurement results, three-dimensional tomographic structure data is obtained (hereinafter, the operation of repeatedly performing the B scan operation in the direction perpendicular to the scanning line (Y direction) and connecting the measurement results is referred to as a “C scan”).
In a case where the living body is an object to be measured, it is usually difficult to perform measurement while completely fixing the living body, and thus, it is necessary to perform measurement at high speed. In a case where a wide range of measurement is required, it is difficult to measure a wide range at a high speed only by perform scanning with one object light beam at a high speed. Therefore, a configuration in which a plurality of object light beams is irradiated has been proposed (PTL 1). A plurality of object light beams simultaneously scans a plurality of different regions of the object to be measured to perform measurement, thereby enabling high-speed measurement in a wide range.
PTL 2 relates to an optical measurement method, and offers an OCT device including an optical probe. PTL 3 relates to a system for generating data using endoscopic microscopy, and offers an OCT device including a multimode fiber.
The inventors have found that it is possible to achieve an irradiation optical system used for irradiation with a plurality of object light beams without changing an irradiation optical system used for irradiation with a single object light beam by connecting a multi-core optical fiber to a fiber collimator placed before the irradiation optical system. The inventors have found the problem that the difference in the wavelength dispersion between the optical path of the object light beam using the multi-core optical fiber (MCF) and the optical path of the reference light beam using the standard single mode optical fiber (SMF) adversely affects the spatial resolution in the depth direction. In PTLs 1 to 3, no description about attention to this problem and means for solving the problem is not found.
An object of the present invention is to provide a configuration for suppressing degradation in spatial resolution in a case where wavelength dispersion of an optical path of an object light beam and wavelength dispersion of an optical path of a reference light beam are different in an optical interference tomographic imaging device.
In order to achieve the above object, an optical interference tomographic imaging device according to the present invention includes a wavelength swept laser light source, a splitting means configured to split light emitted from the wavelength swept laser light source into an object light beam and a reference light beam, an irradiation means configured to irradiate an object to be measured with the object light beam output from the splitting means, and scan a predetermined range, a light spectrum data generation means configured to generate information on wavelength dependency of an intensity ratio of an interference light beam between an object light beam, irradiated to and scattered by the object to be measured, and the reference light beam, a wavelength dispersion compensation processing means configured to perform compensation for information on the wavelength dependency of the intensity ratio of the interference light beam generated by the light spectrum data generation means, the compensation being carried out by using a multiplication process based on a difference in wavelength dispersion of an optical path of the object light beam and an optical path of the reference light beam, and a tomographic structure information generation means configured to generate tomographic structure information on the object to be measured, based on a result of the wavelength dispersion compensation process.
The optical interference tomographic imaging device according to the present invention suppresses degradation in spatial resolution even in a case where wavelength dispersion of an optical path of an object light beam is different from wavelength dispersion of an optical path of a reference light beam, such as when a multi-core optical fiber is used to irradiate an object to be measured with a plurality of object light beams.
Hereinafter, example embodiments of the present invention will be described with reference to the drawings. Before describing specific example embodiments, an example embodiment of a superordinate concept of the present invention will be described.
In the optical interference tomographic imaging device of
The light spectrum data generation means 64 of the optical interference tomographic imaging device of
The control means 66 controls the irradiation optical system 59 in such a way as to move the object light beams R11 and R12 in the scanning line direction and the direction perpendicular to the scanning line on one plane of the object to be measured. Preferably, the control means 66 controls a period and a speed at which the irradiation optical system 59 scans the object to be measured.
The wavelength dispersion compensation processing means 65 compensates for a difference between the wavelength dispersion of the optical path of the object light beam and the wavelength dispersion of the optical path of the reference light beam due to the use of the MCF 57 when irradiating the object to be measured with the plurality of object light beams R11 and R12.
According to the optical interference tomographic imaging device of
The wavelength swept laser light source 101 generates a wavelength-swept light pulse. Specifically, the wavelength swept laser light source 101 generates light pulses whose wavelength increases from 1250 nm to 1350 nm for a duration of 10 μs. The wavelength swept laser light source 101 generates the light pulses repeatedly at 50 kHz every 20 μs.
The light emitted from the wavelength swept laser light source 101 is split into a plurality of light beams R01 and R02 by the optical splitter 102, and then is split into object light beams R11 and R12 and reference light beams R21 and R22 by the plurality of optical splitting/merging units 105 via the plurality of optical delayers 103 and the plurality of circulators 104.
The plurality of object light beams R11 and R12 output from the optical splitting/merging units 105 is irradiated to an object to be measured 120 via the optical connection unit 106, the MCF 107, the fiber collimator 108, and the irradiation optical system 109, and scan is performed. More specifically, the irradiation optical system 109 includes a scanning mirror and a lens, and irradiates different positions on the X-Y plane of the object to be measured 120 with the plurality of object light beams 110a and 110b to scan a certain range.
The object light beams 110a and 110b with which the object to be measured 120 is irradiated are scattered backward (in a direction opposite to the radiation direction of the object light beam) from the object to be measured 120. Then, the object light beams (backscattered light) R31 and R32 scattered from the object to be measured 120 return to the optical splitting/merging unit 105 via the irradiation optical system 109 and the MCF 107.
The plurality of reference light beams R41 and R42 output from the optical splitting/merging unit 105 is reflected by the reference light beam mirror 112 and return to the optical splitting/merging unit 105.
Therefore, in the optical splitting/merging unit 105, the object light beam R31 scattered from the object to be measured 120 and the reference light beam R41 reflected from the reference light beam mirror 112 interfere with each other, and the interference light beam R51 and the interference light beam R61 are obtained. Similarly, in the optical splitting/merging unit 105, the object light beam R32 scattered from the object to be measured 120 and the reference light beam R42 reflected from the reference light beam mirror 112 interfere with each other, and the interference light beam R52 and the interference light beam R62 are obtained. Therefore, the intensity ratio between the interference light beam R51 and the interference light beam R61 is determined by the phase difference between the object light beam R31 and the reference light beam R41, and the intensity ratio between the interference light beam R52 and the interference light beam R62 is determined by the phase difference between the object light beam R32 and the reference light beam R42.
The interference light beams R51 and R52 pass through the circulator 104 and is input to, and the interference light beams R61 and R62 are directly input to the related balance type light receiver 113. Then, the information on the change in the intensity ratio between the interference light beam R51 and the interference light beam R61 and the information on the change in the intensity ratio between the interference light beam R52 and the interference light beam R62 are input from the balance type light receiver 113 to the light spectrum data generation unit 114.
The balance type light receiver 113 is a light receiver in which two photodiodes are connected in series and the connection is an output (differential output). The band of the balance type light receiver 113 is 1 GHz or less.
The light spectrum data generation unit 114 generates interference light spectrum data based on the information on the wavelength change of the emission light from the wavelength swept laser light source 101 and the information on the change in the intensity ratio between the interference light beams R51 and R61. Similarly, the light spectrum data generation unit 114 generates an interference light spectrum based on the information on the wavelength change of the emission light from the wavelength swept laser light source 101 and the information on the change in the intensity ratio between the interference light beams R52 and R62. The interference light spectrum data generated by the light spectrum data generation unit 114 is input to the A scan waveform generation unit 116 via the wavelength dispersion compensation processing unit 115.
In order to describe the effect of the wavelength dispersion compensation processing unit 115, first, regarding the A scan waveform generated without passing through the wavelength dispersion compensation processing unit 115, a waveform obtained in the case of one light scattering point is illustrated in
A cause of degradation of the position resolution in the A scan waveform generated without passing through the wavelength dispersion compensation processing unit 115 will be described below. The SMF is used for an optical path until the reference light beam is reflected by the reference light beam mirror 112 and returns to the optical splitting/merging unit 105 to interfere with the object light beam after the reference light beam is split by the optical splitting/merging unit 105, and the optical path length is PR. On the other hand, the SMF having the length L1 and the optical path length P1, and the MCF having the length L2 and the optical path length P2 are used for the optical path until the object light beam is backscattered at one light scattering point of the object to be measured 120 and returns to the optical splitting/merging unit 105 to interfere with the reference light beam after the object light beam is split by the optical splitting/merging unit 105, and the optical path length is PS=P1+P2+z0. Here, P2=PR−P1 is satisfied at a certain wavelength λ0 and a certain wave number k0, but P2=PR−P1 is not necessarily satisfied at any wavelength λ and wave number k in the wavelength sweep range.
This is because the wavelength dispersion of the MCF forming the optical path length P2 and the wavelength dispersion of the SMF forming the optical path length PR−P1 are different. The phase difference between the object light beam and the reference light beam interfering at the optical splitting/merging unit 105 is k0z0+φ at the wavelength λ0 and the wave number k0, while it is kz0+k(P1+P2−PR)+φ at any wavelength λ and wave number k. Here, φ is a constant that does not depend on k or z0. Using the equivalent refractive index nM of the MCF, the equivalent refractive index nS of the SMF, and the difference Δn thereof, the following is expressed.
P
1
+P
2
−P
R
=n
M
L
1
−n
S(L2−LR)=ΔnL
It is conceivable that Δn has k dependency. In the wavelength range of 1250 nm to 1350 nm, An increases as the wavelength decreases, and can be approximately expressed as Δn˜αk (α>0) as k dependency. As described above, when the amplitude of the object light beam interfering at the optical splitting/merging unit 105 is denoted by ES and the amplitude of the reference light beam is denoted by ER, the generated interference light spectrum is expressed as follows.
I(k)∝ES·ER·cos(kz0+kΔnL+φ) [Math. 3]
kΔnL appears in the phase term, which is not proportional to k. In the A scan waveform obtained by Fourier-transforming this, degradation in position resolution occurs as illustrated in (a) to (e) of
Therefore, in the example embodiment of the present invention, the A scan waveform is generated via the wavelength dispersion compensation processing unit 115. The wavelength dispersion compensation processing unit 115 performs the following multiplication process using k dependency of Δn grasped in advance.
I(k)·exp(−ikΔnL)∝ES·ER·cos(kz0+kΔnL+ϕ)·exp(−ikΔnL) [Math. 4]
By the Fourier transform performed by the A scan waveform generation unit 116 in the subsequent stage, the following is expressed.
J(z)=|∫I(k)e−iΔnLkeizkdk)∝δ(z−z0)+f(z) [Math. 5]
As illustrated in (f) to (j) of
In general, the object light beam irradiated to the object to be measured is sequentially backscattered while being attenuated and propagating into the inside to some extent, and the light scattering points of the object light beam are distributed in a range from the surface to a certain depth. In a case where the light scattering points are distributed from z0−Δx to z0+Δz in the depth direction, modulation from the period 2π/(z0−Δz) to 2π/(z0+Δz) appears in an overlapping manner in the interference light spectrum, and this forms the A scan waveform.
The A scan waveform generation unit 116 generates an A scan waveform. The A scan waveform generation is repeatedly performed while the radiation positions of the object light beams R11 and R12 are moved in the scanning line direction (X direction) by the irradiation optical system 109 based on the control by the object light beam radiation position setting unit 118, and by connecting the measurement results, a map of the two-dimensional intensity of the backscattered light (object light beam) in the scanning line direction and the depth direction is obtained as the B scan tomographic structure data.
Furthermore, the tomographic image generation unit 117 generates three-dimensional tomographic structure data in the X, Y, and Z directions (C scan) by connecting measurement results obtained by repeatedly performing the B scan operation while moving the radiation positions of the object light beams R11 and R12 in the scanning line direction and the direction perpendicular to the scanning line based on the control by the object light beam radiation position setting unit 118.
In the optical interference tomographic imaging device 100 of
Since the wavelength dispersion compensation processing unit 115 compensates for the difference between the wavelength dispersion of the optical path of the object light beam and the wavelength dispersion of the optical path of the reference light beam due to the use of the MCF 107 when irradiating the object to be measured 120 with the plurality of object light beams R11 and R12, it is possible to suppress degradation in position resolution due to the difference in the wavelength dispersion. Even in a case where the wavelength dispersion of the optical path of the object light beam and the wavelength dispersion of the optical path of the reference light beam are different, for example, when the MCF 107 is used for irradiating the object to be measured 120 with the plurality of object light beams R11 and R12, degradation in the spatial resolution of the scanning waveform can be suppressed by compensating for the difference in the wavelength dispersion.
An optical interference tomographic imaging device 300 according to the second example embodiment of the present invention will be described.
As illustrated in
The wavelength swept laser light source 301 generates a wavelength-swept light pulse. Specifically, the wavelength swept laser light source 301 generates light pulses whose wavelength increases from 1250 nm to 1350 nm for a duration of 10 μs. The wavelength swept laser light source 301 generates the light pulses repeatedly at 50 kHz every 20 μs.
The light emitted from the wavelength swept laser light source 301 is split into a plurality of light beams R01 and R02 by the first optical splitter 302, and then split into object light beams R11 and R12 and reference light beams R21 and R22 by the plurality of second optical splitters 305 via the plurality of optical delayers 303.
The plurality of object light beams R11 and R12 output from the second optical splitter 305 is irradiated to an object to be measured 320 via the plurality of circulators 304, the optical connection unit 306, the MCF 307, the fiber collimator 308, and the irradiation optical system 309, and scan is performed. More specifically, the irradiation optical system 309 irradiates different positions on the X-Y plane of the object to be measured 320 with the plurality of object light beams 310a and 310b, and scans a certain range.
The object light beams 310a and 310b with which the object to be measured 320 is irradiated are scattered backward (in a direction opposite to the radiation direction of the object light beam) from the object to be measured 320. Then, the object light beams (backscattered light) R31 and R32 scattered from the object to be measured 320 are input to the coherent light receiver 311 via the irradiation optical system 309, the MCF 307, and the plurality of circulators 304.
The plurality of reference light beams R21 and R22 output from the second optical splitter 305 is input to the coherent light receiver 311.
An internal configuration example of the coherent light receiver 311 that causes the object light beam and the reference light beam to interfere with each other is illustrated in
The light spectrum data generation unit 312 generates interference light spectrum data based on the information on the wavelength change of the emission light from the wavelength swept laser light source 301 and the information on the change in the interference light intensity ratio between the object light beam R31 and the reference light beam R21 from the coherent light receiver. Similarly, the light spectrum data generation unit 312 generates the interference light spectrum data based on the information on the wavelength change of the emission light from the wavelength swept laser light source 301 and the information on the change in the interference light intensity ratio between the object light beam R32 and the reference light beam R22 from the coherent light receiver.
The interference light spectrum data generated by the light spectrum data generation unit 312 reflects a difference between an optical path length until the reference light beam reaches the optical merging unit inside the coherent light receiver 311 after the reference light beam is split by the second optical splitter 305 and an optical path length until the object light beam is irradiated to the object to be measured 320, backscattered, and reaches the optical merging unit inside the coherent light receiver 311 after the object light beam is split by the second optical splitter 305. The SMF is used for an optical path until the reference light beam reaches the optical merging unit inside the coherent light receiver 311 after the reference light beam is split by the second optical splitter 305, and the optical path length is PR. On the other hand, the MCF having the length L1 and the optical path length P1, and the SMF having the length L2, and the optical path length P2 is used for the optical path until the object light beam is backscattered at one light scattering point of the object to be measured 320 and reaches the optical merging unit inside the coherent light receiver 311 after the object light beam is split by the second optical splitter 305, and the optical path length is PS=P1+P2+z0. Using the equivalent refractive index nM of the MCF, the equivalent refractive index nS of the SMF, and the difference Δn thereof, the following is expressed.
P
1
+P
2
−P
R
=n
M
L
1
−n
S(L2−LR)=ΔnL
I(k)∝ES·ER·exp[i(kz0+kΔnL+ϕ)] [Math. 6]
The interference light spectrum data represented by the above expression is generated by the light spectrum data generation unit 312. By using the coherent light receiver, it is possible to detect a state in which interference between the object light beam and the reference light beam differs by the phase difference π (quadrature phase). A term represented by kΔnL appears in the phase term of the interference light spectrum data, and is not proportional to k. The following multiplication process is performed via the wavelength dispersion compensation processing unit 313.
I(k)·exp(−ikΔnL)∝ESER·exp[i(kz0+kΔnL+ϕ]·exp(−ikΔnL) [Math. 7]
Therefore, by the Fourier transform performed by the A scan waveform generation unit 314 the following is expressed.
□(□)=|∫□(□)□−□Δ□□□□□□□∝□(□−□0) [Math. 9]
A peak of a δ function is shown at z=z0, and an A scan waveform for one light scattering point position is obtained without degradation in position resolution.
The A scan waveform generation is repeatedly performed while the radiation positions of the object light beams R11 and R12 are moved in the scanning line direction (X direction) by the irradiation optical system 309 based on the control by the object light beam radiation position setting unit 316, and by connecting the measurement results, a map of the two-dimensional intensity of the backscattered light (object light beam) in the scanning line direction and the depth direction is obtained as the B-scan tomographic structure data.
Further, the three-dimensional tomographic structure data in the X, Y, and Z directions is generated by connecting the measurement results obtained by repeatedly performing the B scan operation while moving the radiation positions of the object light beams R11 and R12 in the scanning line direction and the direction perpendicular to the scanning line based on the control by the object light beam radiation position setting unit 316 (C scan).
As in the above-described first example embodiment, in the optical interference tomographic imaging device 300 of
As in the first example embodiment described above, even in a case where the wavelength dispersion of the optical path of the object light beam is different from the wavelength dispersion of the optical path of the reference light beam, it is possible to suppress degradation in spatial resolution of the scanning waveform by compensating for the difference in wavelength dispersion.
While the invention has been particularly shown and described with reference to example embodiments thereof, the invention is not limited to these example embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/032367 | 8/27/2020 | WO |
