JP2006300801A - Optical coherent tomography device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical coherent tomography apparatus in which a cause of deterioration of a dynamic range is removed and a measurement range is widened.
In the optical coherent tomography apparatus, interference preventing means (fibers 43, 44) for preventing interference between the measurement light leaked directly from the light receiving port of the optical circulator 35 to the light transmitting port and the reference light is provided. , Provided in the optical path of the measurement light and the reference light, and further, isolators 45 and 46 for preventing reflection from the differential amplifier 37, and a variable attenuator 47 for reducing the difference between the DC components of the two output lights from the coupler 36, Provided between the coupler 36 and the differential amplifier 37.
[Selection] Figure 6

Description

  The present invention relates to an optical coherent tomography apparatus, for example, to measure various structures such as a coating film and a tomographic image of a living body using a light interference phenomenon.

(1) Features of OFDR-OCT method The optical coherent tomography method (OCT method) is a method for photographing a structure such as a coating film or a tomographic image of a living body using a light interference phenomenon (Non-Patent Document 1). .
The OCT method has already been put into practical use in the medical field, and is used for tomographic imaging of a fine tissue such as the retina by taking advantage of a high resolution of ten and several μm. This is not only for the positive reason that the resolution is high, but also because it has a mechanical drive part in the measurement system, so it is not suitable for high-speed measurement. There is also a negative reason that the possible range is limited to a narrow region of 1 to 2 mm at most in the depth direction.

The present inventors have developed a new OCT method to solve this problem (Non-Patent Document 2) and succeeded in measuring a wide range of the anterior segment (Non-Patent Document 5). This method is a completely new method using a variable wavelength light source as a light source, and can measure very fast because there is no mechanical drive part. The present inventors call this method the OFDR-OCT method (0ptical-frequency-domain-reflectmetory-OCT).
This method will be described below. The conventional OCT method is referred to as an OCDR-OCT method (0ptical-coherence-domain-reflectometory-OCT).

(2) Configuration of OFDR-OCT Apparatus FIG. 13 is a tomographic imaging apparatus for the anterior segment by the OFDR-OCT method developed by the present inventors.
A light emitting port of a variable wavelength light generating device 71 that is a variable wavelength light generating means capable of emitting light while changing the wavelength, such as a super-periodic structure diffraction grating distributed reflection semiconductor laser light generating device (Non-patent Document 3), The light is optically connected to the light receiving port of the first coupler 72 including a directional coupler that divides the light into two (for example, 90:10).

  The light transmission port on one side (division ratio 90% side) of the first coupler 72 composed of a directional coupler or the like is a dividing means composed of a directional coupler or the like that divides light into two (for example, 70:30). Optically connected to the light receiving port of the second coupler 73.

  The light transmission port on one side (the division ratio 70% side) of the second coupler 73 is optically connected to the light receiving port of the traveling direction control means composed of an optical circulator 75 (hereinafter abbreviated as circulator). . The light transmission port on the other side of the second coupler 73 (the division ratio 30% side) is a third coupler 76 that is a multiplexing means including a directional coupler that divides light into two (for example, 50:50). Optically connected to the light receiving port. The light transmission port of the circulator 75 is optically connected to the light reception port of the third coupler 76. The light sending / receiving port of the circulator 75 is connected to a measuring head 90 (measuring light irradiating means) as shown in FIG. The measuring head 90 also functions as means for capturing signal light reflected or back-scattered by the eye 96 that is a measurement target (signal light capturing means). That is, the measurement head 90 serves as measurement light irradiation / signal light capturing means.

  As shown in FIG. 14, the measurement head 90 includes a collimating lens 92 that shapes the measurement light that has passed through the optical fiber into a parallel beam, a focusing lens 94 that condenses the parallel beam on the anterior eye portion, and the progress of the measurement light. It comprises a galvano mirror 93 that scans the direction.

  The measuring head 90 is attached to the vacant space by removing the slit light (slit light) irradiation system from the slit lamp microscope 95 supported by the support 85. By utilizing the alignment function of the slit lamp microscope 95, the measurement light can be guided near a desired position of the eye 96 of the subject.

  As shown in FIG. 13, the light transmission ports on one side and the other side of the third coupler 76 are optically connected to the light reception port of the first differential amplifier 77 having a light detection function. The Log output section of the first differential amplifier 77 is electrically connected to one input section of the second differential amplifier 78 that corrects and calculates fluctuations in the input signal intensity.

  On the other hand, the light transmission port on the other side (the division ratio 10% side) of the first coupler 72 is optically connected to the light reception port of the photodetector 79. The output part of the photodetector 79 is electrically connected to the input part of the Log amplifier 80. The Log output unit of the Log amplifier 80 is electrically connected to the other input unit of the second differential amplifier 78.

  The output section of the second differential amplifier 78 is electrically connected to the input section of the arithmetic control device 81 for synthesizing the coherence interference waveform, that is, the reflection or backscattering intensity distribution, via an analog / digital converter (not shown). Yes. The output unit of the calculation control device 81 is electrically connected to the input unit of the display device 82 such as a monitor or a printer that displays the calculation result. The arithmetic and control unit 81 can control the variable wavelength light generator 71 and the galvanometer mirror 93 based on the input information.

(3) Measurement principle of OFDR-OCT method The signal light generated by the measurement object (for example, the laser beam divided into 70% by the second coupler 73) reflected or back-scattered by the anterior segment is as follows. The third coupler 76 combines and interferes with reference light (variable wavelength light divided by 30% by the second coupler 73).

The combined light is the sum of the DC component and the interference component, but the first differential amplifier 77 extracts only this interference component. Equation (1) below indicates the magnitude of the interference component I _d (k _i ) detected by the first differential amplifier 77 when the object to be measured has only one reflecting surface 100 as shown in FIG. It represents.

2L is the optical path length traveled by the first split light (division ratio 70%) before being split by the second coupler 73 and multiplexed by the third coupler 76 (the travel distance of light multiplied by the refractive index). The same shall apply hereinafter) and the second split light (split ratio 30%), that is, the difference between the optical path length traveled by the reference light, and k _i is the wave number of the light emitted from the variable wavelength light generator 71 in the i-th position. (= 2π / λ, λ is the wavelength), I _s and I _r is the intensity of the intensity and the reference light reflected or backscattered light (signal light) by the respective measured. The first differential amplifier 77 generates an output proportional to the above I _d (k _i ) (more precisely, its logarithm), and the second differential amplifier 78 corrects fluctuations in the output of the variable wavelength light generator 71. .

  FIG. 15 shows a case where the reflecting surface 100 exists at a position away from the position where 2L = 0 by the distance D. Since the distance traveled until the light reflected by the reflecting surface 100 returns to the position of 2L = 0 is 2D, 2L = 2D at the position of the reflecting surface. Therefore, the value of L corresponding to the position of the reflecting surface is D.

The tomographic image is synthesized by subjecting I _d (k _i ) to Fourier transform by the arithmetic and control unit 81. Hereinafter, a process of constructing a tomographic image will be described.

First, Fourier cosine transformation and Fourier sine transformation are performed on I _d (k _i ). That is, the following formulas (2) and (3) are calculated.

Here, z is a position coordinate. N is the total number of wave numbers emitted from the variable wavelength light source 71. If the wave number interval is Δk and the starting point of wave number scanning is k ₀ + Δk, k _i is expressed by the following equation (4). Note that i = 1, 2,..., N.

Next, the following Y _t (z) is obtained from the calculated Y _c (z) and Y _s (z).

Y _t ² (z) of the above formula (5) or the square root Y _t (z) represents the distribution of the reflection intensity (or back scattering intensity) of the reflection surface (or scattering surface) with respect to the depth direction of the measurement target. To express. In the case of this example having one reflecting surface, the reflection distribution intensity represented by the following formula (6) is obtained.

  Here, B (z) is expressed by the following equation (7) and forms a part of the noise floor.

If x = (z−2L) / 2 × Δk in the first term of Equation (6), the first term becomes (sin (N · x) / sinx) ² .

This equation has a large value N ² when x = 0, that is, z = 2L, and approaches zero rapidly when moving away from z = 2L. Similarly, in the second term, a large value N ² is obtained when z = −2L, and it approaches zero rapidly when it is away from z = −2L. That is, this term generates a folded image.

Accordingly, by plotting x = z / 2 on the horizontal axis and Y _t ² (2x) on the vertical axis y, y = N ² · I _r · I _{s is} obtained when x = ± L, and at other positions. It becomes substantially zero.

Normally, the optical path length is adjusted so that there is no measurement target for x <0, and Y _t ² (2x) is plotted only for x ≧ 0. Therefore, even if Y _t ² (2x) is plotted against x, a folded image does not appear, and the distribution of reflection (or backscattering) intensity in the depth direction can be obtained by the plot.

Chen Kenmei 0PTRONICS (2002), N07, 179 T. Amano, H. Hiro-0ka, D. Choi, H. Furukawa, F. Kano, M. Takeda, M. Nakanishi, K. Shimizu, K. Obayashi, Proceeding of SPIE, Vol. 5531, p.375, 2004. Yoshikuni, Yuzo, Applied Physics, Vol. 71, No. 11 (2002), p1362-1366. Handbook of Optical Coherence Tomography, edited by Brett E. Bouma and Guillermo J. Tearney, p.364-p.367. 40th Annual Meeting of the Japanese Ophthalmological Society 19th Annual Meeting of the Ophthalmology ME Society General Meeting Program and Abstracts 2004 p.61.

(1) Relation between dynamic range and measurement limit in depth direction Dynamic range is one of the important factors that determine the performance of the OCT method.
The dynamic range means the ratio of noise to signal intensity, and the theoretical limit is the maximum value N ² of the signal intensity (equation (6)) and the noise floor (z >> 0) in equation (6). Value) intensity ratio.

FIG. 16 shows changes in signal intensity (formula (6)) with respect to the position coordinate x in the depth direction. The horizontal axis represents the position coordinate x with respect to the depth direction of the measurement object, and the vertical axis represents the logarithm of the signal intensity Y _t ² (2x) expressed by the equation (6) (Y _t ² (0) value). It has been standardized.) This figure shows an example where L = 0, that is, x = 0 and Y _t ² (2x) has the maximum value, plotted against x ≧ 0 (that is, the signal intensity peak 101). Only one half is shown.)

  In most cases, the OFDR-OCT signal from a living tissue or the like is much stronger in reflected light intensity from the surface than the backscattered light intensity from the inside. For this reason, the noise floor of the surface reflection peak hinders measurement of backscattered light from the inside of the measurement target. This situation will be specifically described with reference to FIG. It is assumed that there is a surface at a position of L = 0, and a peak 101 represents an OFDR-OCT signal due to surface reflection. Suppose that the backscattering rate (the ratio of the intensity of the backscattered light to the intensity of the measurement light incident on the scatterer) inside the measurement object is equal to the reflectance on the surface of the measurement object. The measurement light is scattered and exponentially decreases as it goes into the measurement object. Therefore, the peak intensity of the OFDR-OCT signal from the scatterer inside the measurement object decreases linearly with respect to the depth direction as indicated by the attenuation line 103 in FIG.

Since the noise floor 102 due to one surface reflection decreases only gradually with respect to the position coordinate x, the two will eventually intersect. At a position deeper than the intersection 104, the noise floor 102 is an OFDR-OCT signal 103 from the inside of the measurement object. Become stronger.
Therefore, it is impossible to take a tomographic image at a position deeper than the intersection 104, that is, the larger the ratio of the peak 101 to the noise floor 102, that is, the larger the dynamic range, the deeper the tomographic image can be taken.

(2) determined noise floor in the real dynamic range equation (6), when calculating the equations (2) and (3) by Y _c (z) and Y _s (z), the measured value I _d (k _{If i} ) is multiplied by a window function (for example, a Gaussian function), it can be drastically reduced (however, the resolution is degraded. Japanese Patent Application No. 2004-202957).
However, when the OFDR-OCT signal of the biological tissue is measured with the apparatus shown in FIG. 13, when evaluating the noise floor with the signal intensity of the tissue surface where the reflected light intensity is usually strongest being 0 dB, an appropriate window function is used. Even if the theoretical noise floor value is -70 dB, the measured value is -45 dB or more, which is much larger than the expected value -70 dB. For this reason, the conventional OFDR-OCT apparatus has a problem that a sufficient measurement range cannot be taken.

(3) Problem to be Solved by the Present Invention The present invention has been made in view of the above problems, and an object thereof is to provide an optical coherent tomography apparatus in which the cause of dynamic range degradation is eliminated and the measurement range is widened. And

An optical coherent tomography apparatus according to a first invention for solving the above-described problems is provided.
Variable wavelength light generating means;
Splitting means for splitting the output light from the variable wavelength light generating means into measurement light and reference light;
Measurement light irradiation / signal light supplementing means for irradiating the measurement light to the measurement object and capturing the signal light reflected or backscattered by the measurement object.
A bidirectional optical path connected to the measurement light irradiation / signal light supplement means, and the measurement light and the signal light travel in opposite directions;
A light receiving port for inputting the measurement light divided by the dividing means, and a light sending / light for outputting the inputted measurement light to the bidirectional optical path and inputting the signal light from the bidirectional optical path A traveling direction control means having a receiving port and a light transmission port for outputting the inputted signal light;
A multiplexing means for multiplexing the signal light and the reference light;
Measuring means for measuring the intensity of the output light from the multiplexing means;
From the intensity of the output light from the multiplexing means measured by the measuring means, the position where the measurement light is reflected or backscattered by the measurement object, and the reflection intensity or backscattering intensity at the position, In an optical coherent tomography apparatus having a specifying means for specifying the depth direction of a measurement object,
The present invention is characterized in that there is provided interference preventing means for preventing interference between the leaked light in which the measurement light directly leaks from the light receiving port to the light sending / receiving port of the traveling direction control means.

An optical coherent tomography device according to a second invention for solving the above-mentioned problems is provided:
In the optical coherent tomography device according to the first invention,
The interference preventing means is
From the sum of the optical path length of the measurement light from the splitting means to the traveling direction control means and the optical path length of the signal light from the traveling direction control means to the multiplexing means, the splitting means to the multiplexing means The optical path length of the reaching reference light is an optical path set so as to be longer than the maximum value of the coherence distance of each output light of the variable wavelength light generating means.

An optical coherent tomography apparatus according to a third invention for solving the above-described problem is
In the optical coherent tomography device according to the second invention,
From the splitting means to the traveling direction control means, the optical path length of the measuring light from the measuring object to the measuring object via the bidirectional optical path, and from the measuring object to the bidirectional optical path, the traveling direction control means The optical path length of the bidirectional optical path is set to be approximately equal to the sum of the optical path length of the signal light reaching the multiplexing means and the optical path length of the reference light from the dividing means to the multiplexing means. It is characterized by that.

  For example, the sum of the optical path length of the measurement light from the dividing means to the traveling direction control means and the optical path length of the signal light from the traveling direction control means to the multiplexing means, and the optical path length of the reference light from the dividing means to the multiplexing means If the optical path length of the bidirectional optical path is set to half of the optical path length of the reference light from the dividing means to the multiplexing means, the dividing means passes through the traveling direction control means and the bidirectional optical path. The optical path length of the measurement light reaching the measurement light irradiation / signal light supplementing means and the optical path length of the signal light from the measurement light irradiation / signal light supplementing means to the multiplexing means via the bidirectional optical path and the traveling direction control means And the optical path length of the reference light from the dividing means to the multiplexing means.

An optical coherent tomography device according to a fourth invention for solving the above-described problems is provided as follows:
In the optical coherent tomography device according to any one of the first to third inventions,
The interference preventing means is
The traveling direction control means attenuates the leakage light with respect to the measurement light incident on the light receiving port by 60 dB or more.

An optical coherent tomography device according to a fifth invention for solving the above-described problems is provided.
In the optical coherent tomography device according to any one of the first, second, and fourth inventions,
The sum of the optical path length of the measurement light from the dividing means to the traveling direction control means and the optical path length of the signal light from the traveling direction control means to the multiplexing means, and from the dividing means to the multiplexing means When the optical path length of the reference light is different,
The interference preventing means is
It is an intermittent light extinguishing means for intermittently extinguishing the output light from the variable wavelength light generating means so that the leakage light and the reference light do not enter the multiplexing means at the same time.

An optical coherent tomography apparatus according to a sixth invention for solving the above-described problem is provided.
In the optical coherent tomography device according to any one of the first to fifth inventions,
The multiplexing means is
When the intensity of the signal light and the reference light is constant regardless of the wave number of the variable wavelength light generating means, the first component having a constant light intensity with respect to the wave number and the second light intensity oscillating with respect to the wave number. A first output port that outputs interference light composed of components;
When the intensity of the signal light and the reference light is constant regardless of the wave number, the light intensity oscillates with respect to the third component having a constant light intensity with respect to the wave number and the wave component, and has a phase opposite to the second component. And a second output port that outputs interference light composed of the fourth component of
The measuring means includes
The first input port is optically coupled to the first input port, and the second output port is optically coupled to the second input port. The difference between the intensity and the intensity of light incident on the second input port is measured.

An optical coherent tomography apparatus according to a seventh invention for solving the above-described problems is provided.
In the optical coherent tomography device according to the sixth invention,
An antireflection means for preventing light reflected by the first input port from returning to the first output port is provided between the first output port and the first input port,
In addition, another antireflection means for preventing light reflected by the second input port from returning to the second output port is provided between the second output port and the second input port. And

An optical coherent tomography apparatus according to an eighth invention for solving the above-described problems is provided.
In the optical coherent tomography device according to the sixth or seventh invention,
An adjusting means for reducing the difference between the first component and the third component measured by the measuring means is provided.

An optical coherent tomography device according to a ninth invention for solving the above-described problems is provided.
In the optical coherent tomography device according to the eighth invention,
While the adjusting means is a variable optical attenuator,
The variable attenuator is disposed between at least one of the first output port and the first input port or between the second output port and the second input port.

An optical coherent tomography device according to a tenth aspect of the present invention for solving the above-described problem,
In the optical coherent tomography device according to the eighth invention,
The adjusting means includes
Weighting one or both of the intensity of light incident on the first input port and the intensity of light incident on the second input port to reduce the difference between the first component and the third component It is characterized by being.

An optical coherent tomography device according to an eleventh invention for solving the above-mentioned problems is provided as follows:
In the optical coherent tomography device according to any one of the first to tenth inventions,
The variable wavelength light generating means comprises a variable wavelength laser.

An optical coherent tomography device according to a twelfth invention for solving the above-described problem is
In the optical coherent tomography device according to any one of the first to eleventh inventions,
The measuring means includes
Means for measuring the intensity of output light from the multiplexing means for each wave number of the variable wavelength light generating means;
The specifying means is:
A position where the measurement light is reflected or back-scattered by the measurement object from the set of intensity of the output light from the multiplexing means measured for each wave number by the measurement means, and the reflection intensity or backward at the position. The scattering intensity is specified with respect to the depth direction of the measurement object.

An optical coherent tomography device according to a thirteenth invention for solving the above-mentioned problems is
In the optical coherent tomography device according to the twelfth invention,
The specifying means is:
Reflection intensity or backscattering intensity with respect to the depth direction of the measurement object by Fourier-transforming the combination of the intensity of the output light from the combining means and the real number consisting of the wave numbers measured for each wave number by the measuring means It is characterized by specifying.

An optical coherent tomography device according to a fourteenth aspect of the present invention for solving the above-described problem is provided.
In the optical coherent tomography device according to the twelfth invention,
The measuring means includes
A first output light intensity at which the intensity of the output light from the multiplexing means changes as a cosine function with respect to the wave number; and an intensity of the output light from the multiplexing means with respect to the wave number as a sine function or its reverse sign. If it changes as a function, both the second output light intensity can be measured,
The specifying means is:
From the set of the first output light intensity and the second output light intensity, the position at which the measurement light is reflected or backscattered by the measurement object, and the reflection or backscattering intensity at the position are determined by the measurement object. It is characterized in that it is specified without folding back in the depth direction.

An optical coherent tomography device according to a fifteenth aspect of the present invention for solving the above problems is provided.
In the optical coherent tomography device according to the fourteenth aspect,
The specifying means is:
When the measurement object is composed of only one reflecting surface,
z is a variable indicating a position map, and 2L is the sum of the optical path length of the measurement light from the dividing means to the measurement object and the optical path length of the signal light from the measurement object to the multiplexing means, When a value obtained by subtracting the optical path length of the reference light from the dividing means to the multiplexing means,
From either the first output light intensity or the second output light intensity, only one of k × (z−2L) or k × (z + 2L) is obtained for each wave number k of the output light of the variable wavelength light generating means. For a function that is proportional to either or both of the cosine or sine function,
By calculating the sum of the functions calculated for each wave number k, the reflection intensity or the backscattering intensity with respect to the depth direction of the measurement object is specified without being folded back.

  According to the present invention, since the interference preventing means is used, it is possible to prevent the deterioration of the dynamic range and to expand the measurement range (measurement depth) in the OFDR-OCT method. Further, by applying the interference preventing means of the present invention to other OCT methods using a variable wavelength laser light source (for example, chirp OCT method (Non-patent Document 4)), the dynamic range of the other OCT methods is also deteriorated. And the measurement range can be expanded.

(I) Cause of dynamic range deterioration (A) Possible cause of noise floor generation In actual measurement, not only the noise floor based on the above-described measurement principle but also various noises shown below can generate a noise floor. .
(1) Thermal noise Normally, thermal noise of amplifiers becomes a problem.
(2) Shot noise Noise caused by the current being quantized by the elementary charge of electrons (3) A / D board quantization noise, etc. (4) RIN (re1ative intensity noise)
Noise caused by fluctuations in laser light intensity due to wave number switching, fluctuations from the set value of wave number, mechanical and thermal fluctuations of the interferometer, etc. (5) Interference noise Not intended Self-interference of the reference light itself caused by reflection of the reference light, interference with the reference light caused by unintentional reflection of the measurement light, etc.

(B) Elucidation of noise floor generation source As a result of detailed studies, the present inventors have described (1) to (4) above of the noise floor (hereinafter referred to as “excessive noise floor”). I found out that it was not the main cause. The remaining cause may be interference noise (5).
The reflection of the reference light and the measurement light can occur at all the connection points of the optical components constituting the apparatus shown in FIG. 13, and the identification thereof is not easy.
However, as a result of intensive studies as described below, the present inventors have succeeded in identifying the generation source.

(1) Step 1: Identification of reflected light generation point First, an isolator was appropriately inserted before and after each optical component constituting the measurement system, and an attempt was made to specify the reflected light generation point.
FIG. 1 shows an outline of the experimental apparatus, and its configuration will be briefly described.
In the experimental apparatus, a coupler 2 that splits light into two is optically connected to the light exit of the variable wavelength light generator 1, and the split optical path 3 is connected to the coupler 6 via the circulator 5. Optically connected, and the other divided optical path 4 is optically connected directly to the coupler 6. The light supplied from the circulator 5 irradiates the sample 10 through the collimator lens 7, the galvano mirror 8, and the focusing lens 9, and the light reflected (backscattered) from the sample 10 again becomes the focusing lens 9 and the galvano mirror. 8, and input to the coupler 6 through the collimator lens 7. Then, the two optical paths from the coupler 6 that multiplexes the light are optically connected to the photo receiver 11, the output is converted by the A / D converter 12, and input to the computer 13. Thus, the output of the variable wavelength light generator 1 is controlled.

  First, isolators were sequentially inserted at positions a to g shown in FIG. 1, and the change in the noise floor was observed. No change in a, no change even if paired into b and c, no change even if paired in d and e, and a few dB reduction in noise floor due to paired insertion in f and g It was. This is considered to be a result of the isolator blocking the reflected light from the detector in the photo receiver 11 (corresponding to the first differential amplifier 77 in FIG. 13).

(2) Step 2: Partial removal of RIN In FIG. 13, the method of using the first differential amplifier 77 with the third coupler 76 set to a division ratio of 50:50 is known as a balance detection method. It is known as an effective method for removing and extracting only the interference signal of equation (1). However, the division ratio of the third coupler 76 cannot be strictly set to 50:50, and a slight shift increases the noise floor. Therefore, when the variable attenuator (attenuator 14) is inserted at the position f or g on the side where the DC component of the output at positions f and g in FIG. 1 is slightly larger (see FIG. 2), the attenuation is adjusted. An improvement of several dB was observed.

  However, the noise floor is still large, suggesting that some noise sources exist in addition to interference noise caused by reflection of reference light and measurement light.

(3) Step 3: Influence of Crosstalk Light After extensive investigations, the crosstalk finally remaining between the light inlet and the light outlet of the circulator 5 is the main source of the excessive noise floor. I found out.

The elucidation process is shown below.
As shown in FIG. 3, in the optical path from the circulator 5 to the measurement target (sample 10), the previous part was removed from the collimator 7 (the optical connectors (15a, 15b) were removed), and the change in the noise floor was observed. In this state, the optical path of the sample light S1 (the optical path of the measurement light and the signal light) is blocked, and the signal light does not reach the multiplexing means composed of the coupler 6, so that no noise floor should be generated. Surprisingly, however, this did not reduce the noise floor. In order to explain this phenomenon, a part of the measurement light incident from the light receiving port h of the circulator 5 leaks to the light transmission port j to become leakage light (crosstalk light), and this leakage light and the reference light S2 interfere with each other. I hypothesized that a noise floor might be formed.

  Therefore, in order to reliably block the optical path of the sample light S1, the optical connector 16 disposed after the circulator 5 was removed, and changes in the noise floor were observed. As a result, the noise floor was reduced by more than a dozen dB, supporting the above hypothesis.

  In the OCT apparatus, there has never been a case where such a phenomenon (a phenomenon in which a noise floor occurs due to leakage of measurement light into the optical path of signal light) has been reported. Therefore, it is extremely difficult for those skilled in the art to foresee the crosstalk light of the circulator 5 as a cause of occurrence of an excessive noise floor, and it is believed that this phenomenon is first elucidated by the insight of the present inventors.

(II) Elimination of interference (1) Reduction of crosstalk of circulator itself In order to eliminate the influence of crosstalk light, the performance of the circulator 5 can be increased to eliminate crosstalk. The circulator used was often used when assembling the fiber optical system, and the crosstalk was 50 to 60 dB. When the circulator was replaced with a crosstalk of 60 dB or more, the noise floor decreased. Therefore, it is considered as one solution to use a circulator having a crosstalk of 60 dB or more, preferably 70 dB or more, and more preferably 80 dB or more.

(2) Device configuration considering coherence distance However, it is not easy to reduce the crosstalk of the circulator 5. Therefore, the present inventors have sought an apparatus that does not contribute to the formation of a noise floor even if crosstalk light is generated in the circulator 5.

  To that end, it was considered most effective to construct means for preventing crosstalk light and reference light from interfering with each other, and several methods were tried. Among them, the difference between the optical path length on the crosstalk light side (sample light S1 side) and the optical path length on the reference light S2 is determined based on the coherence distance of the variable wavelength light generator 1 (for example, a semiconductor laser such as SSG-DBR laser light). The present inventors have found that it is most convenient and effective to make the length longer.

  Specifically, as shown in FIG. 4, an optical fiber 18 having a length of 7 m (optical path length (optical length) 10 m) comparable to the coherent distance of the semiconductor laser was inserted between the optical paths mn of the reference light S2. Further, in order to adjust the optical path length of the optical path of the sample light S1 and the optical path of the reference light S2, between the light receiving port / light transmitting port i of the circulator 5 and the previous kl, half of the optical fiber 18 inserted between mn. An optical fiber 17 having a length (3.5 m) was inserted. As a result, the noise floor was reduced by 15 dB. As described above, the noise floor is greatly reduced, and the fluctuation of the laser used for the measurement, that is, the RIN can be observed. This indicates that interference noise is no longer the main cause of noise floor generation. Such a significant noise floor reduction greatly expands the measurable range.

The preferred length of the optical fiber to be inserted is determined by the coherence distance of a variable wavelength semiconductor laser used as a light source. The coherence distance of the laser beam of the semiconductor laser can be measured using an interferometer, for example, a Michelson interferometer as shown in FIG. Assuming that the electric field of light incident on the Michelson interferometer is E (t) and the delay time is τ, the output i _{d of the} photodetector 21 is expressed by the following equation (8).

Here, the bar above the character expression means the time average. The delay time τ can be obtained from the distance L ₁ between the half mirror 22 and the mirror 23 and the distance L ₂ between the half mirror 22 and the movable mirror 24 by the following equation (9).

ここで、ｃは光の速度を表す。

Here, c represents the speed of light.

  Expression (8) is composed of a component that does not depend on τ and a component that depends on it. It is known that the component that depends on τ is represented by the following equation (10) when C (τ).

ここで、ω₀は光の角周波数、パラメータτ_cはレーザ電磁界のコヒーレント時間と呼ばれている。

Here, ω ₀ is called the angular frequency of light, and the parameter τ _c is called the coherent time of the laser electromagnetic field.

C (τ) is a term representing an interference component. From the equations (9) and (10), the envelope of C (τ) is the difference “2 · | L ₁ −L ₂ |” between the sample optical path and the reference optical path. It can be seen that there is an exponential decrease with respect to "." Therefore, in the present invention, the optical path length difference “2 · | L ₁ −L ₂ |” when C (τ) (interference signal component depending on the delay time τ) is half of C (0) is coherent. It is defined as distance.

  As defined above, the value of the optical length of the optical fiber 18 inserted between the mn is preferably a coherent distance (the maximum of the coherent distances of all scanning wave numbers, the same applies hereinafter). Preferably, it is 2 times the coherence distance, further 4 times, further 8 times, and further 16 times. The preferable value is a value when there is no difference in optical path length between the optical path of the sample light S1 and the optical path of the reference light S2 in a state where no fiber is inserted between mn.

  As a result of confirming that the noise floor can be effectively reduced with the SSG-DBR laser, the optical path length of the optical fiber to be inserted (a value obtained by multiplying the length along the optical path by the refractive index. Is different from the length of each part multiplied by the refractive index of that part.) The preferred value of mn is 5 m or more (between kl: 2.5 m or more), more preferably mn 10 m or more (between kl: 5 m or more), more preferably 20 m or more between mn (between kl: 10 m or more), and most preferably 40 m or more between mn (between kl: 20 m or more). The coherence distance of the SSG-DBR laser is typical as a semiconductor, and a preferable value is approximately the same value even in a variable wavelength laser made of another semiconductor laser.

(3) Device configuration in which signal light and reference light do not reach at the same time As means for preventing crosstalk light from interfering with reference light (interference prevention means), in addition to the means for adjusting the optical path length, variable wavelength light is used as an interferometer. The reference light is extinguished when the crosstalk light reaches the coupler 6 and the reference light is turned on when the signal light arrives. Intermittent light-off means). In order to make the variable wavelength light travel intermittently, an optical modulator, for example, a Mach-Zehnder modulator (preferably not generating a wavelength chirp) may be disposed between the SSG-DBR laser 1 and the coupler 2.

  FIG. 6 is an OFDR-OCT tomographic imaging apparatus with a reduced noise floor developed by the present inventors. The object to be measured is the human anterior segment.

  A light emitting port of a variable wavelength light generating device 31 that is a variable wavelength light generating means capable of emitting light while changing the wavelength, such as a superperiodic structure diffraction grating distributed reflection semiconductor laser light generating device (Non-patent Document 3), The light is optically connected to the light receiving port of the first coupler 32 including a directional coupler that divides the light into two (for example, 90:10).

  The light transmission port on one side (division ratio 90% side) of the first coupler 32 composed of a directional coupler or the like is a dividing means composed of a directional coupler or the like that divides light into two (for example, 70:30). The second coupler 33 is optically connected to the light receiving port. That is, the output light from the variable wavelength light generator 31 is divided into the measurement light side (division ratio 70% side) and the reference light side (division ratio 30% side).

  The light transmission port on one side (the division ratio 70% side) of the second coupler 33 is optically connected to the light receiving port of the traveling direction control means composed of the circulator 35 (crosstalk 50 to 60 dB). The light transmission port on the other side of the second coupler 33 (dividing ratio 30% side) is a third coupler 36, which is a multiplexing means including a directional coupler that divides light into two (for example, 50:50). Optically connected to the light receiving port.

  The light sending / receiving port of the circulator 35 is connected to a measuring head 50 as shown in FIG. 7 via an optical fiber 43 serving as a bidirectional optical path that allows measurement light and signal light to travel in opposite directions. The light transmission port of the circulator 35 is optically connected to the light reception port of the third coupler 36. That is, in the circulator 35, the measurement light divided by the second coupler 33 is input to the light receiving port, and the input measurement light is output from the light sending / light receiving port to the optical fiber 43 and the optical fiber 43. Is input to the optical transmission / reception port, and the input signal light is output to the third coupler 36 from the optical transmission port.

  The measurement head 50 is means for irradiating the measurement light to the measurement object (measurement light irradiation means) and means for capturing the signal light reflected or backscattered by the eye 66 that is the measurement object ( It also functions as a signal light capturing means) (measurement light irradiation / signal light capturing means).

  Specifically, as shown in FIG. 7, the measurement head 50 is provided on a movable stage 61 supported by a support 60, is supported by the movable stage 61, and enters and exits a part of the peripheral wall on the distal end side. A main body cylinder 51 in which a window 51a is formed, and a collimator arranged on the proximal end side inside the main body cylinder 51 and optically connected to the circulator 35 to shape the measurement light passing through the optical fiber 43 into a parallel beam. A lens 52, a galvanometer mirror 53 that is disposed on the front end side inside the main body cylinder 51, changes its orientation direction, and scans the measurement light in the horizontal direction, and a collimating lens 52 inside the main body cylinder 51, A focusing lens 54 is provided between the galvano mirror 53 and condenses a parallel beam on the anterior segment.

  The support 60 is provided with support arms 62 and 63 for fixing and supporting the subject's face in a sitting position with the subject's eye 66 oriented in the horizontal direction, and for silent confirmation as irradiation position confirmation means. The microscope 65 is attached. In other words, the measuring head 50 is usually attached to a vacant space by removing the slit light (slit light) irradiation system from the slit lamp microscope used for ophthalmic diagnosis. Then, by utilizing the alignment function of the slit lamp microscope, the measurement light can be guided near a desired position of the eye 66 of the subject.

  That is, the measurement light input to the light receiving port of the circulator 35 enters the collimating lens 52 inside the main body cylinder 51 of the measuring head 50 from the light sending / receiving port of the circulator 35, and is shaped into a parallel beam to be a focusing lens. After being condensed at 54, the light is emitted from the entrance / exit light window 51 a of the main body cylinder 51 through the galvanometer mirror 53 and is irradiated to the eye 66. The measurement light applied to the eye 66 is reflected (or back-scattered) by the eye 66 to become signal light, and the reflected (or back-scattered) signal light enters the inside through the input / output light window 51 a of the main body cylinder 51. Then, the light is reflected by the galvanometer mirror 53, and enters the light sending / receiving port of the circulator 35 from the base end side of the main body cylinder 51 through the focusing lens 54 and the collimating lens 52. The incident signal light is output from the light transmission port of the circulator 35 and input to the third coupler 36. The signal light and the reference light are combined in the third coupler 36 and divided into two (for example, 50:50) and output.

  In this embodiment, the optical path length of the reference optical path (division ratio 30%) between the second coupler 33 and the third coupler 36 is the optical path length between the second coupler 33 and the circulator 35 and the circulator 35 and the third coupler. The length of the optical fiber 44 constituting the optical path of the reference light is adjusted so that the maximum coherence distance 10 m of the variable wavelength light source 31 is longer than the sum of the optical path lengths 36. In other words, by appropriately adjusting the length of the optical fiber 44, interference between the leaked light of the measurement light leaked directly from the light receiving port of the circulator 35 to the light transmitting port and the reference light is prevented (interference prevention). means).

  Further, the length of the optical fiber 43 between the circulator 35 and the measurement target is set so that the optical path length between the circulator 35 and the measurement target becomes equal to half 5 m of the maximum coherence distance of the variable wavelength light generator 31. adjust. This is between the optical path length between the second coupler 33 and the circulator 35 and the optical path length between the circulator 35 and the third coupler 36, and between the second coupler 33 and the third coupler 36 excluding the optical fiber 44. If the optical path length of the optical fiber 43 is set to half of the optical path length of the optical fiber 44 when the optical path length of the reference optical path (division ratio 30%) is equal, the circulator 35 and the optical fiber 43 are connected from the second coupler 33. The sum of the optical path length of the measurement light that reaches the measurement target (eye 66) and the optical path length of the signal light from the measurement target (eye 66) via the optical fiber 43 and the circulator 35 to the third coupler 36; This means that the optical path length of the reference light between the second coupler 33 and the third coupler 36 is substantially equal.

  In addition, as shown in FIG. 6, the light transmission outlet (first output opening) on one side of the third coupler 36 passes through an isolator 45 serving as an antireflection means and a variable attenuator 47 serving as an adjustment means. Is optically connected to a light receiving port (first input port) of a first differential amplifier 37 (measuring means) having a light detecting function for detecting light. The other optical transmission port (second output port) of the third coupler 36 is connected to another light receiving port (second input port) of the first differential amplifier 37 via an isolator 46 serving as an antireflection means. Is optically connected. That is, by inserting the isolators 45 and 46 between the first differential amplifier 37 and the third coupler 36, the light reflected from the first differential amplifier 37 is prevented from returning to the light transmission port of the third coupler 36. is doing. The Log output section of the first differential amplifier 37 is electrically connected to the input section of the second differential amplifier 38 that corrects the fluctuation of the input signal strength.

  On the other hand, the light transmission port on the other side (the division ratio 10% side) of the first coupler 32 is optically connected to the light reception port of the photodetector 39. The output part of the photodetector 39 is electrically connected to the input part of the Log amplifier 40. The Log output part of the Log amplifier 40 is electrically connected to the input part of the second differential amplifier 38.

  The output section of the second differential amplifier 38 is electrically connected to the input section of the arithmetic control device 41 (specifying means) via an analog / digital converter (not shown). The arithmetic and control unit 41 obtains the position where the measurement light is reflected or backscattered from the measured light intensity, the reflection intensity or the backscattering intensity at the position, and the depth direction (depth direction) of the measurement target. A backscattering intensity distribution, that is, a coherence interference waveform is synthesized. The output unit of the calculation control device 41 is electrically connected to the input unit of the display device 42 such as a monitor or a printer that displays the calculation result. The arithmetic and control unit 41 can control the variable wavelength light generator 31 and the galvanometer mirror 53 based on the input information.

  In FIG. 6, a variable attenuator 47 (variable attenuator) is inserted into one of the two optical output ports of the third coupler 36, specifically, the isolator 45 side, but the variable attenuator 47 is inserted. The optical path is determined as follows.

  The split ratio of a 3 dB coupler that can actually be used is not completely 50:50. The sensitivity of the photodetector in the differential amplifier is also slightly different between the two inputs. Therefore, when the optical path of the signal light is cut at the positions 48a and 48b and only the reference light is input to the third coupler 36 as shown in FIG. The output of the instrument (Auto-balanced photoreceiver) should be zero, but not completely zero. Therefore, the interference optical path on the side where the optical signal is strongly detected is identified from the positive / negative of the output of the photodetector, and the variable attenuator 47 is inserted into the identified optical path.

  Then, the attenuation rate of the variable attenuator 47 is adjusted so that the output of the photodetector becomes smaller than that before the insertion of the variable attenuator 47 in a state where the optical path of the signal light is cut off at 48a and 48b. Alternatively, the attenuation factor of the variable attenuator 47 may be adjusted so that the noise floor is minimized while actually observing the A-line (scanning in the depth direction).

  Further, for example, when the intensity of the signal light and the reference light is constant regardless of the wave number of the variable wavelength light generator 31, the light intensity vibrates with respect to the first component and the wave number whose light intensity is constant with respect to the wave number. The interference light composed of the second component is output from one light transmission port (first output port) of the third coupler 36, and the light intensity is constant with respect to the third component and the wave number. Measured by the first differential amplifier 37 in such a manner that interference light consisting of a fourth component that oscillates and has a phase opposite to the second component is output from the other light transmission port (second output port). The attenuation factor of the variable attenuator 47 may be adjusted so that the difference in light intensity between the first component and the third component becomes small.

(Method of operation)
First, the measuring light is guided near the desired position of the eye 66 of the subject by utilizing the alignment function of the slit lamp microscope.

  Next, a command is issued from the arithmetic and control unit 41, and light is emitted from the variable wavelength light generator 31 while switching the wave number in a stepwise manner with respect to time (see FIG. 9), and measurement is performed for each wave number.

Therefore, the second differential amplifier 38 outputs a signal proportional to the following equation (11) for each wave number k _i .

This output is converted into a digital signal by an analog / digital converter and read by the arithmetic and control unit 41. The arithmetic and control unit 41 stores this value in association with k _i , thereby collecting a set (data) of measurement results for each wave number.

  Next, a command is issued from the arithmetic and control unit 41 to the galvanometer mirror 53, and the irradiation position of the variable wavelength light on the surface of the measurement object is slightly moved on a straight line in the horizontal direction. The same measurement as described above is performed for the new irradiation position. By repeating the above operation, data necessary for constructing a tomographic image is collected. The number of scanning points in the horizontal direction is, for example, 100 points.

After the measurement is completed, the arithmetic and control unit 41 uses the collected data to calculate the reflection or backscattering intensity distribution Y _t ² (z in the depth direction for each measurement point based on the equations (2) to (5). ) To construct a tomographic image.

  In the constructed tomogram, the noise floor is improved by 5 dB by inserting the isolators 45 and 46, improved by 5 dB by adjusting the attenuation amount of the variable attenuator 47, and improved by 15 dB by adjusting the optical path length. The total was reduced by 25 dB.

  Note that the variable attenuator 47 may be disposed behind both the isolators 45 and 46. In this case, it is not necessary to examine in advance the output port of the third coupler 36 that outputs larger interference light.

  In this embodiment, the wave number is scanned so as to increase stepwise. However, the wave number scan is not necessarily performed in this manner, and all the necessary wave numbers can be scanned within a predetermined time. Any scanning method can be used. For example, the wave number may not gradually increase stepwise but gradually decrease, or all wave numbers necessary for constructing a tomographic image may be scanned randomly.

  In this embodiment, the variable attenuator 47 is used to correct the output imbalance of the third coupler 36. For example, as shown below, the input light intensity is weighted to obtain a difference. A differential amplifier may be used.

Typically, the differential amplifier is designed to output an output V ₀ = β (V ₂ −V ₁ ) that is proportional to the difference between the _two inputs V ₁ and V ₂ . A3 four circuits of the combination of the resistance of the resistance value R _c of FIG. 10, it becomes beta = 1 the differential amplifier, a _{V 0 = V 2 '-V 1} '.

  As a method of making the output of the differential amplifier a weighted subtracting circuit instead of an equivalent subtraction of two input voltages, for example, as shown in FIG. A1 and A2) are included, and the amplification levels are adjusted so that these amplification levels have a desired weight.

In FIG. 10, V ₁ ′, V ₂ ′ and the output voltage V ₀ are expressed by the following equation (12).

Ｒ_f1とＲ_f2を可変抵抗にすれば、電圧Ｖ₁とＶ₂の重みを変えることができる。上記構成により、第３カプラ３６の一方の光送出口（第１出力口）から出力され、波数に対して光強度が一定の第１成分と、第３カプラ３６の他方の光送出口（第２出力口）から出力され、波数に対して光強度が一定の第３成分に重みを付けて、これらの差を小さくするように補正することが可能となる（調整手段）。

If R _f1 and R _f2 are variable resistors, the weights of the voltages V ₁ and V ₂ can be changed. With the above configuration, the first component output from one light transmission port (first output port) of the third coupler 36 and having a constant light intensity with respect to the wave number, and the other light transmission port (the first light output port of the third coupler 36). It is possible to weight the third component that is output from the two output ports) and has a constant light intensity with respect to the wave number, and make corrections so as to reduce these differences (adjustment means).

In this embodiment, a logarithmic output is required. This can be easily realized by a circuit for further converting V ₀ into a logarithm.

  In the differential amplifier, the gain for each input is adjusted so that the output of the differential amplifier becomes smaller with the optical path of the signal light cut at 48a and 48b (see FIG. 8). Alternatively, the gain for each input may be adjusted so that the noise floor is minimized while actually observing the A-line (scanning in the depth direction).

  The present embodiment is an example in which the present invention is applied to an OFDR-OCT apparatus that has recently been newly developed by the present inventors and does not generate a folded image (Japanese Patent Application No. 2005-14650).

(Device configuration)
FIG. 11 is an example of an OFDR-OCT apparatus using the present invention. The object to be measured is the anterior segment of the person, similar to the OFDR-OCT apparatus described in the prior art.

  A light emitting port of a variable wavelength light generating device 31 that is a variable wavelength light generating means capable of emitting light while changing the wavelength, such as a superperiodic structure diffraction grating distributed reflection semiconductor laser light generating device (Non-patent Document 3), The light is optically connected to the light receiving port of the first coupler 32 including a directional coupler that divides the light into two (for example, 90:10).

  The light transmission port on one side (division ratio 90% side) of the first coupler 32 composed of a directional coupler or the like is a dividing means composed of a directional coupler or the like that divides light into two (for example, 70:30). The second coupler 33 is optically connected to the light receiving port.

  The light transmission port on one side (the division ratio 70% side) of the second coupler 33 is optically connected to the light reception port of the circulator 35 (crosstalk 50 to 60 dB). The light transmission port on the other side of the second coupler 33 (the division ratio 30% side) is connected to the input of the optical phase modulator 34, and the output of the optical phase modulator 34 divides the light into two (for example, 50:50). It is optically connected to one light receiving port of the third coupler 36 which is a multiplexing means comprising a directional coupler or the like. As the optical phase modulator, for example, an optical phase modulator comprising an LN modulator and its control device can be used.

  The light transmission port of the circulator 35 is optically connected to the light reception port of the third coupler 36, and the light transmission / light reception port is connected to a measurement head 50 as shown in FIG. The measurement head 50 is a means for irradiating the measurement target with the measurement light, and also functions as a means for capturing the signal light reflected or backscattered by the eye as the measurement target (measurement light irradiation / signal light). Supplementary means).

  Since FIG. 7 has been described in the first embodiment, a detailed description thereof is omitted, but as shown in FIG. 7, the measurement head 50 collimates the measurement light that has passed through the optical fiber 43 into a parallel beam. The lens 52, a focusing lens 54 for condensing the parallel beam on the anterior eye part, and a galvano mirror 53 for scanning the measurement light in the horizontal direction, are slit light (slit light) from the slit lamp microscope. It is installed in an empty space with the irradiation system removed. By utilizing the alignment function of the slit lamp microscope, the measurement light can be guided near a desired position of the eye 66 of the subject.

  Also in the present embodiment, the optical path length of the reference optical path (division ratio 30%) between the second coupler 33 and the third coupler 36 is the optical path length between the second coupler 33 and the circulator 35 and the circulator 35 and the third coupler. The length of the optical fiber 44 constituting the optical path of the reference light is adjusted so that the maximum coherence distance 10 m of the variable wavelength light source 31 is longer than the sum of the optical path lengths 36. The length of the optical fiber 43 between the circulator 35 and the measurement target is set so that the optical path length between the circulator 35 and the measurement target is equal to half 5 m of the maximum coherence distance of the variable wavelength light generator 31. Is adjusted. The sum of the optical path length between the second coupler 33 and the circulator 35, the optical path length between the circulator 35 and the third coupler 36, and the reference light between the second coupler 33 and the third coupler 36 excluding the optical fiber 44. The optical path length of the optical path (division ratio 30%) is assumed to be equal.

  Also, isolators 45 and 46 serving as antireflection means are inserted between the first differential amplifier 37 having the light detection function and the third coupler 36. Furthermore, a variable attenuator 47 serving as an adjusting unit is inserted after one of the isolators connected to the two output ports of the third coupler 36, for example, the isolator 45. The optical path into which the variable attenuator 47 is inserted may be determined by a method similar to the method described in the first embodiment.

  As described above, the first optical amplifier 37 having the light detection function of detecting the intensity of light via the isolator 45 and the variable attenuator 47 is provided at one side of the third coupler 36. It is optically connected to the light receiving port (first input port) of (measuring means). Further, the other optical transmission port (second output port) of the third coupler 36 is optically connected to the other light receiving port (second input port) of the first differential amplifier 37 via the isolator 46. is doing. The Log output section of the first differential amplifier 37 is electrically connected to one input section of the second differential amplifier 38 that corrects the fluctuation of the input signal strength.

  On the other hand, the light transmission port on the other side (the division ratio 10% side) of the first coupler 32 is optically connected to the light reception port of the photodetector 39. The output part of the photodetector 39 is electrically connected to the input part of the Log amplifier 40. The Log output part of the Log amplifier 40 is electrically connected to the input part of the second differential amplifier 38.

  The output section of the second differential amplifier 38 is electrically connected to the input section of the arithmetic control device 41 (specifying means) via an analog / digital converter (not shown). The arithmetic and control unit 41 obtains the position where the measurement light is reflected or backscattered from the measured light intensity, the reflection intensity or the backscattering intensity at the position, and the depth direction (depth direction) of the measurement target. A backscattering intensity distribution, that is, a coherence interference waveform is synthesized. The output unit of the calculation control device 41 is electrically connected to the input unit of the display device 42 such as a monitor or a printer that displays the calculation result. The arithmetic and control unit 41 can control the variable wavelength light generator 31, the optical phase modulator 34, and the galvanometer mirror 53 based on the input information.

The output of the first differential amplifier 37 takes a log of I (k _i , φ) = 2 (I _r I _s ) ^1/2 cos (2L × k _i + φ). Φ is the phase modulation amount of the optical phase modulator 34. On the other hand, since the output of the Log amplifier 40 has a value proportional to logI _r , the output of the second differential amplifier 38 is expressed by the following equation (13) (the constant term is omitted).

  Note that the log in Expression (13) is an expression when there is one reflecting surface, but for the sake of simplicity, the case where there is one reflecting surface will be considered.

(Method of operation)
A command is issued from the arithmetic and control unit 41, and light is emitted from the variable wavelength light generator 31 while switching the wave number in a stepped manner with respect to time (lower part of FIG. 12).

  The arithmetic and control unit 41 issues a command to the optical phase modulator 34 simultaneously with the wave number scanning command. Based on this command, the optical phase modulator 34 synchronizes with the wave number switching of the variable wavelength light generator 31 to change the phase of the reference light between 0 [rad] and −π / 2 [rad] as shown in the upper part of FIG. Alternately modulate between. That is, the reference light is phase-modulated by 0 [rad] in the first half of the wave number holding period and by −π / 2 [rad] in the second half.

In the second differential amplifier 38, the first half of the holding time of each wave number k _i outputs a signal proportional to the following equation (14).

In the second half of the holding time of each wave number k _i , a signal proportional to the following equation (15) is output.

  When log is removed from the above equation, the following equations (16) and (17) are obtained.

That, I (k _i, 0) becomes a cosine function with wavenumber (first output light _{intensity), I (k i, -π} / 2) is a sinusoidal function (second output light intensity). Incidentally, the output light intensity when the reflecting surface is only one becomes a cosine function with wavenumber as above I (k _i, 0), "output light that varies as a cosine function with wavenumber" I will call it. _{Further, I (k i, -π /} 2) output light intensity becomes a sine function if the reflecting surface as there is only one, referred to as "output light varies as a sine function with wavenumber" I will do it.

This output is converted into a digital signal by an analog / digital converter and read by the arithmetic and control unit 41. The arithmetic and control unit 41 stores this value in association with k _i and φ = 0, −π / 2.

  Note that φ = π / 2 may be used, and in this case, the signal processing may be performed after the output sign is reversed, and there is no essential difference from the case where the sign is not reversed.

  Next, a command is issued from the arithmetic and control unit 41 to the galvanometer mirror 53, and the irradiation position of the variable wavelength light on the surface of the measurement object is slightly moved on a straight line in the horizontal direction. The same measurement as described above is performed for the new irradiation position. By repeating the above operation, data necessary for constructing a tomographic image is collected. The number of scanning points in the horizontal direction is, for example, 100 points.

After the measurement is completed, the arithmetic and control unit 41 uses the collected data to calculate the reflection or backscattering intensity distribution Y _t ^"2 (z) in the depth direction for each measurement point according to the following equations (18) to (19). Calculate and construct a tomographic image based on this distribution.

  Note that the first term of the equation (18) is a Fourier cosine transform of the intensity of the output light that changes as a cosine function with respect to the wave number. Similarly, the second term of the equation (18) is a Fourier sine transform of the intensity of the output light that changes as a sine function with respect to the wave number. Further, the first term of the equation (19) is for performing a Fourier sine transformation on the intensity of the output light that changes as a cosine function with respect to the wave number, and the second term of the equation (19) is a sine function with respect to the wave number. The intensity of the output light that changes as follows is subjected to Fourier cosine transform.

  When there is one reflecting surface or backscattering body, the following equation (21) is obtained.

  The function represented by the equation (21) takes a large value at z = 2L, and rapidly decreases as the distance from z = 2L increases. There is no term that generates a folded tomographic image such as the second term on the right side of Equation (6). That is, based on Expression (21), a tomographic image without a folded image can be constructed. Note that z is a variable indicating a position table, and 2L is an optical path length of measurement light from the second coupler 33 to the measurement target (eye 66) and a signal from the measurement target (eye 66) to the third coupler 36. This is a value obtained by subtracting the optical path length of the reference light from the second coupler 33 to the third coupler 36 from the sum of the optical path lengths of the light.

As described in Japanese Patent Application No. 2005-14650, the above calculation is based on the first output light intensity and the second output light intensity when the measurement target is composed of only one reflecting surface (or scatterer). For each wave number k _i of the output light of the variable wavelength light generator 31, a function proportional to the cosine function and sine function is calculated for k _i × (z−2L), and these calculated for each wave number k _i are calculated. By calculating the sum of the functions, the reflection intensity or the backscattering intensity in the depth direction of the measurement target is specified without being folded back. _{Incidentally, k i × (z-2L} ) , rather than with respect to _{k i × (z + 2L)} , calculates a function proportional cosine function, a sine function may be the total sum. However, in this case, the obtained image is a mirror image with respect to the origin. Also, a function that is proportional to either the cosine function or the sine function may be calculated for k _i × (z−2L) or k _i × (z + 2L) to obtain the sum.

When there are a plurality of reflecting surfaces or scatterers, the sum of a term consisting of the following equation (22) corresponding to signals from the plurality of reflecting surfaces (or scatterers) and a term that is negligibly small (2L _i represents an optical path length difference with respect to the i-th reflecting surface, and M represents the number of reflecting surfaces.) Therefore, even if there are a plurality of reflecting surfaces (or scatterers), a tomographic image without folding is obtained.

  In the constructed tomographic image, the noise floor is improved by 5 dB by inserting the isolators 45 and 46, improved by 5 dB by inserting the variable attenuator 47 and adjusting the attenuation, and 15 dB by adjusting the optical path length. Improved and reduced by 25 dB in total. Note that the variable attenuator 47 may be disposed behind both the isolators 45 and 46. In this case, it is not necessary to examine in advance the output port of the third coupler 36 that outputs a larger amount of interference light.

  In this embodiment, the wave number is scanned so as to increase stepwise, but the wave number scan does not necessarily have to be performed in this way, and all the necessary wave numbers can be scanned within a predetermined time. Any scanning method may be used. For example, the wave number may not gradually increase stepwise but gradually decrease, or all wave numbers necessary for constructing a tomographic image may be scanned randomly.

  In this embodiment, an optical phase modulator that dynamically changes the phase of the reference light is used as means for modulating (shifting) the phase of the reference light. However, the optical path of the reference light is divided into two parts. A means for statically shifting the phase in the optical path (for example, an optical phase modulator having a fixed phase) may be arranged. However, since the reference light divided into two is combined with the signal light, the signal light also needs to be divided into two, and the optical paths of the divided reference light and signal light need to be combined one-on-one. In this way, it is possible to simultaneously obtain an interference signal that changes as a cosine function and an interference signal that changes as a sine function with respect to the wave number. The phase to be shifted is, for example, π / 2. At this time, it is necessary to adjust the optical path lengths of the divided reference lights so that both are equal. The same applies to the optical path length of the divided signal light. When a directional coupler is used as a means for splitting light, a phase difference π / 2 is generated between the light immediately after splitting into two, and it is necessary to perform signal processing in consideration of this influence. . However, one of the two interference lights changes as a cosine function and the other changes as a sine function regardless of the way of multiplexing (including the case where the sign is reversed).

  In addition, an optical component that produces a difference of π / 2 in the phase of the divided light, such as a directional coupler, can be used. By appropriately arranging these in the optical path of the plurality of signal lights and the optical path of the reference light, the phase difference after multiplexing can be set to, for example, π / 2, and the interference changes as a cosine function and a sine function with respect to the wave number You can also get light.

  Further, instead of the variable attenuator (variable attenuator), the differential amplifier with gain adjustment function of each input shown in the first embodiment (FIG. 10) may be used.

  In the first and second embodiments, the interference noise due to the crosstalk light is eliminated by adjusting the optical path length. However, the noise floor is reduced by reducing the crosstalk of the circulator 35 without doing so. be able to.

  Specifically, in the first and second embodiments, the optical fibers 43 and 44 for adjusting the optical path length are not inserted into the reference optical path or the like, and the circulator 35 has a crosstalk of 50 to 60 dB to 60 to 70 dB. Changed to. That is, since the circulator 35 attenuates the leakage light with respect to the measurement light incident on the light receiving port by 60 dB or more, the circulator 35 serves as an interference prevention means for preventing the leakage light and the reference light from interfering with each other.

  In the tomogram constructed using the above configuration, the noise floor was improved by 5 dB by inserting the isolators 45 and 46, improved by 10 dB by changing the circulator 35, and reduced by 15 dB in total.

  In Examples 1 to 3, a Mach-Zender interferometer was used as an interferometer. However, usable interferometers are not limited to this type, and other interferometers such as a Michelson interferometer can be used. is there. When the Michelson interferometer is used, the means for dividing the variable wavelength light and the means for multiplexing the signal light and the reference light are the same.

  In the first to third embodiments, the Fourier transform is used for the analysis of the measurement signal. However, the Fourier transform is not necessarily required, and an analysis method that can extract a large number of frequency components from the signal. Other methods are acceptable if there are. Specifically, when the reflected light (or backscattered light) from the measurement object interferes with the reference light and the light intensity is measured while changing the wave number, the frequency corresponding to the position of the reflected light (or backscatterer) is obtained. It becomes a function of many cosine functions that oscillate. Therefore, if a function having a frequency component corresponding to each position can be extracted from this signal, a tomographic image can be constructed. For example, the Fourier transform is included in a more general wavelet transform, and the present invention can be applied to a case where the wavelet transform is used for analysis of a measurement signal.

  Furthermore, in Examples 1 to 3, an optical circulator is used as the traveling direction control means. However, other optical elements such as a 3 dB coupler including a directional coupler can also be used.

  In addition, in the first to third embodiments, the wave number of the variable wavelength generator is changed discontinuously (discretely) with respect to time, the wave number is maintained for a certain time, and the intensity of the interference light is measured during that time. ing. However, the present invention is naturally applicable to OCT that measures interference light while continuously changing the wave number, for example, chirp CT (Non-Patent Document 4). In the measurement methods described in the first to third embodiments, the interference light intensity is measured while continuously changing the wave number, and the output of the photodetector is sampled when a predetermined wave number is reached. Good. At this time, if the interference light intensity is averaged over a certain range of wave numbers around a predetermined wave number, the S / N ratio is improved.

  By using this optical coherent tomography apparatus according to the present invention, it can be used not only in living bodies but also in manufacturing industries such as precision instruments.

It is a figure explaining the procedure which pinpoints a reflected light generation point in the optical component which comprises an optical coherent tomography apparatus. It is a figure which shows the state which inserted the attenuator in the optical component which comprises an optical coherent tomography apparatus. In the circulator of an optical coherent tomography apparatus, it is a figure which shows the state which removed the connector in order to pinpoint a reflected light generation | occurrence | production point. It is a figure which shows the state which lengthened the optical path length in the optical coherent tomography apparatus. It is a figure explaining the Michelson interferometer which measures a coherence distance. It is a schematic block diagram which shows an example of embodiment of the optical coherent tomography apparatus which concerns on this invention. It is a schematic block diagram of the measurement head of the optical coherent tomography apparatus of FIG. FIG. 5 is a diagram illustrating adjustment of an attenuator in the optical coherent tomography apparatus of FIG. 4. It is a time chart of the wave number of the emitted light from a variable wavelength generator. It is a figure which shows a weighted differential amplifier. It is a schematic block diagram which shows another example of embodiment of the optical coherent tomography apparatus which concerns on this invention. It is a time chart of the wave number of the emitted light from the variable wavelength generator and the phase modulation of the reference light. It is a schematic block diagram which shows the conventional optical coherent tomography apparatus. It is a schematic block diagram of the measurement head of the optical coherent tomography apparatus of FIG. It is a figure explaining the measurement principle of the conventional optical coherent tomography apparatus. It is a graph showing the change of the signal intensity with respect to the position coordinate of a depth direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 31 Variable wavelength light generator 32 1st coupler 33 2nd coupler 34 Optical phase modulator 35 Optical circulator 36 3rd coupler 37 1st differential amplifier 38 2nd differential amplifier 39 Photo detector 40 Log amplifier 41 Operation control apparatus 42 Display devices 43 and 44 Optical fibers 45 and 46 Variable isolators 47 Variable attenuators 50 Measuring heads 52 Collimating lenses 53 Galvano mirrors 54 Focusing lenses 60 Supports 61 Movable stages 65 Microscopes

Claims (15)

Variable wavelength light generating means;
Splitting means for splitting the output light from the variable wavelength light generating means into measurement light and reference light;
Measurement light irradiation / signal light supplementing means for irradiating the measurement light to the measurement object and capturing the signal light reflected or backscattered by the measurement object.
A bidirectional optical path connected to the measurement light irradiation / signal light supplement means, and the measurement light and the signal light travel in opposite directions;
A light receiving port for inputting the measurement light divided by the dividing means, and a light sending / light for outputting the inputted measurement light to the bidirectional optical path and inputting the signal light from the bidirectional optical path A traveling direction control means having a receiving port and a light transmission port for outputting the inputted signal light;
A multiplexing means for multiplexing the signal light and the reference light;
Measuring means for measuring the intensity of the output light from the multiplexing means;
From the intensity of the output light from the multiplexing means measured by the measuring means, the position where the measurement light is reflected or backscattered by the measurement object, and the reflection intensity or backscattering intensity at the position, In an optical coherent tomography apparatus having a specifying means for specifying the depth direction of a measurement object,
An optical anti-reflection device is provided that prevents interference between the leaked light in which the measurement light leaks directly from the light receiving port of the traveling direction control unit to the light transmitting port and the reference light. Coherent tomography device. The optical coherent tomography device according to claim 1,
The interference preventing means is
From the sum of the optical path length of the measurement light from the splitting means to the traveling direction control means and the optical path length of the signal light from the traveling direction control means to the multiplexing means, the splitting means to the multiplexing means An optical coherent tomography apparatus characterized in that the optical path length of the reference light reaching the optical path is set to be longer than the maximum value of the coherence distance of each output light of the variable wavelength light generating means. The optical coherent tomography device according to claim 2,
From the splitting means to the traveling direction control means, the optical path length of the measuring light from the measuring object to the measuring object via the bidirectional optical path, and from the measuring object to the bidirectional optical path, the traveling direction control means The optical path length of the bidirectional optical path is set to be approximately equal to the sum of the optical path length of the signal light reaching the multiplexing means and the optical path length of the reference light from the dividing means to the multiplexing means. An optical coherent tomography device characterized by that. The optical coherent tomography apparatus according to any one of claims 1 to 3,
The interference preventing means is
The optical coherent tomography apparatus, wherein the traveling direction control means attenuates the leakage light with respect to the measurement light incident on the light receiving port by 60 dB or more. In the optical coherent tomography device according to any one of claims 1, 2 and 4,
The sum of the optical path length of the measurement light from the dividing means to the traveling direction control means and the optical path length of the signal light from the traveling direction control means to the multiplexing means, and from the dividing means to the multiplexing means When the optical path length of the reference light is different,
The interference preventing means is
Optical coherent, characterized in that it is an intermittent light extinguishing means for intermittently extinguishing the output light from the variable wavelength light generating means so that the leakage light and the reference light do not enter the multiplexing means simultaneously. Tomography device. The optical coherent tomography apparatus according to any one of claims 1 to 5,
The multiplexing means is
When the intensity of the signal light and the reference light is constant regardless of the wave number of the variable wavelength light generating means, the first component having a constant light intensity with respect to the wave number and the second light intensity oscillating with respect to the wave number. A first output port that outputs interference light composed of components;
When the intensity of the signal light and the reference light is constant regardless of the wave number, the light intensity oscillates with respect to the third component having a constant light intensity with respect to the wave number and the wave component, and has a phase opposite to the second component. And a second output port that outputs interference light composed of the fourth component of
The measuring means includes
An intensity of light incident on the first input port, the first input port having an optically coupled first output port and a second input port having an optically coupled second output port. And an optical coherent tomography apparatus for measuring the difference between the intensity of light incident on the second input port. The optical coherent tomography device according to claim 6,
An antireflection means for preventing light reflected by the first input port from returning to the first output port is provided between the first output port and the first input port,
In addition, another antireflection means for preventing light reflected by the second input port from returning to the second output port is provided between the second output port and the second input port. Optical coherent tomography device. The optical coherent tomography device according to claim 6 or 7,
An optical coherent tomography apparatus comprising adjusting means for reducing a difference between the first component and the third component measured by the measuring means. The optical coherent tomography device according to claim 8,
While the adjusting means is a variable optical attenuator,
An optical coherent optical system characterized in that the variable attenuator is arranged between the first output port and the first input port, or at least one of the second output port and the second input port. Tomography device. The optical coherent tomography device according to claim 8,
The adjusting means includes
Weighting one or both of the intensity of light incident on the first input port and the intensity of light incident on the second input port to reduce the difference between the first component and the third component An optical coherent tomography device. The optical coherent tomography device according to any one of claims 1 to 10,
An optical coherent tomography apparatus characterized in that the variable wavelength light generating means comprises a variable wavelength laser. The optical coherent tomography device according to any one of claims 1 to 11,
The measuring means includes
Means for measuring the intensity of output light from the multiplexing means for each wave number of the variable wavelength light generating means;
The specifying means is:
A position where the measurement light is reflected or back-scattered by the measurement object from the set of intensity of the output light from the multiplexing means measured for each wave number by the measurement means, and the reflection intensity or backward at the position. An optical coherent tomography apparatus characterized by specifying a scattering intensity with respect to a depth direction of the measurement object. The optical coherent tomography device according to claim 12,
The specifying means is:
Reflection intensity or backscattering intensity with respect to the depth direction of the measurement object by Fourier-transforming the combination of the intensity of the output light from the combining means and the real number consisting of the wave numbers measured for each wave number by the measuring means Optical coherent tomography apparatus characterized by specifying The optical coherent tomography device according to claim 12,
The measuring means includes
A first output light intensity at which the intensity of the output light from the multiplexing means changes as a cosine function with respect to the wave number; and an intensity of the output light from the multiplexing means with respect to the wave number as a sine function or its reverse sign. If it changes as a function, both the second output light intensity can be measured,
The specifying means is:
From the set of the first output light intensity and the second output light intensity, the position at which the measurement light is reflected or backscattered by the measurement object, and the reflection or backscattering intensity at the position are determined by the measurement object. An optical coherent tomography apparatus characterized by being specified without being folded back in the depth direction. The optical coherent tomography device according to claim 14,
The specifying means is:
When the measurement object is composed of only one reflecting surface,
z is a variable indicating a position map, and 2L is the sum of the optical path length of the measurement light from the dividing means to the measurement object and the optical path length of the signal light from the measurement object to the multiplexing means, When a value obtained by subtracting the optical path length of the reference light from the dividing means to the multiplexing means,
From either the first output light intensity or the second output light intensity, only one of k × (z−2L) or k × (z + 2L) is obtained for each wave number k of the output light of the variable wavelength light generating means. For a function that is proportional to either or both of the cosine or sine function,
An optical coherent tomography apparatus characterized in that, by obtaining the sum of the functions calculated for each wave number k, the reflection intensity or the backscattering intensity in the depth direction of the measurement object is specified without being folded back.

Images