WO2008029506A1

WO2008029506A1 - Optical coherence tomography apparatus

Info

Publication number: WO2008029506A1
Application number: PCT/JP2007/000933
Authority: WO
Inventors: Kohji Ohbayashi; Kimiya Shimizu
Original assignee: School Juridical Person Kitasato Gakuen
Priority date: 2006-09-04
Filing date: 2007-08-30
Publication date: 2008-03-13
Also published as: JPWO2008029506A1

Abstract

An optical coherence tomography apparatus comprised of a light generating device, an interferometer and an optical detector, is further provided with an optical amplifier in a sample optical path of the interferometer to improve the measuring sensitivity.

Description

Specification

Optical Coherence Tomography equipment

Technical field

The present invention relates to an optical coherence tomography apparatus, and more particularly to a high-sensitivity optical coherence tomography apparatus using an optical amplifier.

Background art

[0002] (1) Current status of 0GT

Optical ■ Coherence ■ Tomography (Optical

Coherence Tomography (OCT) is a high-resolution optical tomography technique that uses the interference phenomenon of light. Since the optical interference phenomenon is used, high resolution (approximately 10) close to the wavelength of light can be easily achieved with 0GT. In addition, because the probe for tomography is light, X-ray GT (Computed

X-ray exposure is not a problem as in Tomography). Utilizing this high-resolution and non-invasive feature, 0GT has realized a diagnostic device for observing the fundus and anterior segment with high resolution at the microscope level.

[0003] There are three types of 0GT: TD-0GT, which has already been put into practical use, and SD-0GT and 0FDR-0GT in the research and development stage. TD—0GT is the time domain

Domain) method, which was first developed. SD-0GT is a spectral domain method that has been studied for a long time. 0FDR—0GT is optical, frequency, domain, refractometry (Optical

frequency domain ref lectometry), which was recently developed (Non-patent Document 1).

[0004] Although TD-0GT has a simple device configuration, it has limitations in increasing the measurement speed, and is not suitable for taking moving images of slice images. On the other hand, SD-0GT and 0FDR-0GT are easy to measure at high speed, and it is also possible to take tomographic videos. [0005] Most human tissues, except for some tissues such as the lens of the eye, strongly scatter or reflect light (hereinafter simply referred to as “scattering”). 0GT is a measurement technology that uses this light scattering. When the human body is irradiated with light, the light is backscattered or reflected inside the tissue (hereinafter simply referred to as “backscattering”). 0GT supplements this backscattered light to construct a tomographic image. However, the scattering of light by human tissues is intense. Therefore, the irradiation light rapidly attenuates inside the tissue and does not reach the deep part. For this reason, even the light backscattered at a position just a few countries from the human body surface is attenuated to near the 0GT measurement limit. For this reason, the range that can be measured with 0GT is limited to a depth of several countries from the surface of the human body.

[0006] The measurable depth of 0GT is determined by its sensitivity. The most sensitive method among the above three methods 0GT is 0FDR-0GT. The sensitivity of SD_0GT and TD-0GT decreases in this order. Among these, the sensitivity of 0FDR-0GT is conspicuous, reaching several hundred times that of the TD-0GT system (Non-patent Document 2).

[0007] (2) Configuration of 0FDR-0GT device

The following is a brief description of the device configuration and principle of 0FDR-0GT, which is the most excellent in terms of measurement speed and sensitivity. Figure 4 shows the equipment configuration of 0FDR-0GT.

[0008] The light exit port of the variable wavelength light generator 1 that can emit light while changing the wavelength, such as a super-periodic structure diffraction grating distributed reflection semiconductor laser light generator (Non-patent Document 3), divides the light into two It is optically connected to the light receiving port of the first force bra 2 (optical splitter) consisting of a directional coupler (eg 90:10). The optical connection is made by an optical fiber indicated by a solid line.

[0009] The light transmission port on one side (the division ratio 90% side) of the first force bra 2 is optically connected to the light receiving port of the optical circuit 3. The light exit / light entrance of the optical circulator 3 is connected to the first light irradiation / capture device 5 that irradiates the measurement light to the measurement object 4 and captures the signal light backscattered by the measurement target 4. It is connected. The optical outlet of the optical circulator 3 is connected to the light receiving port on one side of the second force bra 6 (optical coupler) consisting of a directional coupler (division ratio 50: 50). Has been.

Note that the measurement light refers to light emitted to the measurement object 4 among the emitted light of the variable wavelength light generator 1 divided by the first force bra 2. Further, the other side of the emitted light of the variable wavelength light generator 1 divided by the first force bra 2 is referred to as reference light. The measurement light is backscattered by the measurement object 4, and the light incident on the interferometer (the optical system consisting of the first and second force bras 2, 6 and the first and second circulators 3, 7) again is the signal light. Called.

[001 1] The first light irradiating / capturing device 5 includes a collimating lens 14 for shaping the measurement light emitted from the light emitting port / light receiving port of the optical circulator 3 into a parallel beam, and the parallel beam. A focusing lens 16 that focuses light on the measurement object 4 and a galvanometer mirror 15 that linearly scans the surface of the measurement object 4 by deflecting measurement light are provided.

The optical transmission port on the other side of the first force bra 2 (division ratio of 10% side) is optically connected to the optical reception port of the optical circuit 7. The optical emission port / light receiving port of the optical circulator 7 is connected to a second light irradiation / capturing device 9 that irradiates the reference light to the reference mirror 8 and captures the reference light backscattered by the reference mirror 8. Has been. The light exit port of the optical circulator 7 is optically connected to the light receiving port on the other side of the second force bra 6 composed of a directional coupler (division ratio 50:50). The reference mirror 8 is carried on a support body that can move back and forth, and its position is adjusted so that the optical path lengths of the reference optical path 17 and the sample optical path 18 are substantially equal.

[0013] The light transmission ports on one side and the other side of the second force bra 6 are optically connected to the first and second photodetectors 10, 11 having the same quantum efficiency. The outputs of the first and second photodetectors 10, 11 are electrically connected to the differential amplifier 12.

[0014] The output section of the differential amplifier 12 has a reflectance distribution (ref l ect i v i ty

That is, it is electrically connected to the input unit of the arithmetic and control unit 13 that synthesizes the reflection or backscattering intensity distribution via an analog / digital converter (not shown). The output unit of the calculation control device 13 is electrically connected to the input unit of a display device (not shown) such as a monitor or a printer that displays the calculation results. This arithmetic and control unit 1 3 controls the galvanometer mirror 15 of the variable wavelength light generating device 1 and the first light irradiation / capturing device 5 based on the input information.

The tomographic image is constructed as follows. The variable wavelength light generator 1 emits the wave number (= 2π / wavelength) of the laser light while continuously switching at an extremely narrow wave number interval. The light emitted from the variable wavelength light generator 1 is incident on an interferometer comprising the first and second force bras 2, 6 and the first and second circulators 3, 7, and is scattered by the measurement object 4. The reference light reflected by the reference mirror 8 interferes with the second force bra 6. The intensity of the interference light is detected by the first and second photodetectors 10 and 11, and the DC component (proportional to the sum of the reference light intensity and the signal light intensity) contained in the interference light is detected by the differential amplifier 12. Only the removed interference component (hereinafter referred to as signal current) is input to the arithmetic and control unit 13. The arithmetic and control unit 13 records the wave number of the laser beam emitted from the variable wavelength light generator 1 and the signal intensity for the laser beam for all the wave numbers. When the wave number scanning of the variable wavelength light generator 1 is completed, the arithmetic and control unit 13 performs a Fourier transform on the recorded signal intensity with respect to the wave number. The result of the Fourier transform is a function of the position where the measurement light is backscattered by the measurement object 4 and the intensity of the backscattered light. In other words, the depth direction distribution of the backscattering rate for the measurement object (more precisely, the distribution of the backscattering rate for the measurement object with respect to the irradiation direction of the measurement light) is obtained. The arithmetic control device 13 measures this distribution while moving the measurement light irradiation position little by little along the straight line on the surface of the measurement object 4. Finally, the measured distributions are combined to construct a tomographic image of measurement target 4. The movement of the measurement light irradiation position is performed by the first light irradiation / capturing device 5 based on a command from the arithmetic control device 13 (Non-patent Document 1).

Non-Patent Document 1: T. Amano, H. Hi ro-oka, D. Choi, H. Furukawa, F. Kano, M. Takeda, M. Nakanishi, K. Shimizu, and K. Obayashi, Proceeding of SPIE, Vol .5531, p.375, 2004.

Non-Patent Document 2: S. H. Yun, G. J. Tearnery, J. F. de Boer, N. Iftimia, and B. E. Bouma, OPTICS EXPRESS, Vol.11, p.2953, 2003.

Non-Patent Document 3: Yuzo Yoshikuni, “Development Trends of Wavelength Tunable Lasers and Their Expectations for System Applications”, Applied Physics, Japan Society of Applied Physics, 2002, Vol. 71, No. 11 , p. 1 3 6 2-1 3 6 6.

Non-Patent Document 4: Kiyoshi Nakagawa, Masataka Nakazawa, Kazuo Aida, Kazuo Enomoto, “Optical Amplifier and its Applications J, Ohmsha, 1992, ρ · 22.

Non-Patent Document 5: Kiyoshi Nakagawa, Masataka Nakazawa, Kazuo Aida, Kazuo Enomoto, “Optical Amplifiers and Their Applications J, Ohmsha, 1992, ρ. 32.

Disclosure of the invention

Problems to be solved by the invention

[0016] As described above, the sensitivity of 0FDR-0GT is improved several hundred times compared to TD-0GT.

However, even with such high sensitivity, the measurable depth is at most 2 to 3 mm from the surface of the human body.

[0017] Since the measurable range is so narrow, the application range of 0GT is limited. If the measurement range (a depth

If range is one or two countries deeper, the range of application of 0GT will be greatly expanded. For example, in tomography of the anterior eye with 0GT, it is still impossible to observe the ciliary body hidden behind the iris. It becomes possible to observe the ciliary body with only one or two countries deeper. Observation of the ciliary body is important for the diagnosis of glaucoma and is eagerly desired in ophthalmic medicine. In addition, there is a concept to diagnose cancer tissue by combining 0GT with an endoscope, but the measurement depth is insufficient in 2 to 3 countries from the tissue surface, and the measurement range in several countries is further expanded. Expected.

[0018] On the other hand, 0GT must be operated at high speed for moving image shooting, but the sensitivity of 0GT decreases as the measurement speed increases (Non-patent Document 2). Therefore, the image at a deep position becomes unclear, and the range in which movie shooting is possible is limited to a depth of 1 to 2 mm near the surface.

Therefore, an object of the present invention is to provide a 0GT device that improves the measurement sensitivity of 0GT and expands the depth at which a tomographic image can be observed.

Means for solving the problem

[0020] In order to achieve the above object, a first invention includes a light generator, an optical splitter that divides light output from the light generator into measurement light and reference light, and the measurement A light irradiation / capturing device that irradiates the measurement object with the constant light and captures the signal light that is reflected or backscattered by the measurement object; and an optical coupler that combines the signal light and the reference light And a light detection device that measures the intensity of the output light coupled by the optical coupler, and a reflection of the measurement light with respect to the irradiation direction of the measurement light in the measurement target based on the output of the light detection device Alternatively, an optical coherence tomography apparatus having a backscattering position and an arithmetic and control unit for specifying a reflection intensity or a backscattering intensity is characterized by having an optical amplifier for amplifying the signal light.

[0021] By adopting such a configuration, the first invention has the effect of improving the measurement sensitivity of 0GT and expanding the observable depth of the tomographic image.

[0022] A second invention is the first invention according to the first invention, wherein the intensity of the reference light is optimized so that the sensitivity of the optical coherence tomography device is best when the optical amplifier is not disposed. It is characterized in that it is larger than the reference light intensity and smaller than the maximum value of the input light intensity at which the light detection device operates normally.

[0023] By adopting such a configuration, the second invention improves the measurement sensitivity of 0GT and expands the observable depth of the tomographic image (observable depth from the surface of the measurement target). The effect of doing is surely produced.

[0024] A third invention is characterized in that, in the first invention, the intensity of the reference light incident on the photodetecting device is larger than 15 W and smaller than 1 O mW.

[0025] By adopting such a configuration, the second invention improves the measurement sensitivity of 0GT and expands the observable depth of the tomographic image (observable depth from the surface of the measurement target). The effect of doing is surely produced.

[0026] A fourth invention is characterized in that, in the first to third inventions, the light generating device is a variable wavelength light generating device capable of changing a wave number of emitted light.

[0027] By adopting such a configuration, the fourth invention has the effect of further improving the measurement sensitivity of 0FDR-0GT, which is the most sensitive of 0GT, and expanding the observable depth of tomographic images. Play. [0028] According to a fifth aspect of the present invention, there is provided a light generator, a light output port of the light generator is connected to a light receiving port, and the light output from the light generator is divided into measurement light and reference light An optical branching device, a first optical scanner that has an optical transmission port on one side of the optical branching device connected to the optical receiving port, and an optical transmission port of the first optical circuit / A first light irradiating / capturing device that is connected to a light receiving port, irradiates the measurement light on the measurement object, and captures signal light that is reflected or backscattered by the measurement object; A light output port of the optical circuit 1 is connected to the light receiving port, an optical amplifier that amplifies the signal light, and a second light transmitting port of the optical branching device that is connected to the light receiving port. Of the optical sensor and the second optical sensor of the second An emission port / light reception port connected to irradiate the reference mirror with the reference light and capture the reference light reflected by the reference mirror; and The light exit port is connected to the light receiving port on one side, the light exit port of the second optical circulator is connected to the light receiving port on the other side, and optical coupling that couples the signal light and the reference light And a first light detection device for measuring the intensity of the output light coupled by the optical coupler, and the other optical coupler on the other side of the optical coupler. A second light detection device for measuring the intensity of the output light coupled by the optical coupler, and a differential in which the outputs of the first and second light detection devices are electrically connected. An amplifier and the differential amplifier are electrically connected, and the first and second photodetectors are electrically connected. Based on the force, and an arithmetic control unit for identifying a reflection intensity or backscatter intensity and the reflected or backscattered position of the measuring light with respect to the irradiation direction of the measuring light in the measurement target.

By adopting such a configuration, the fifth invention has the effect of improving the 0GT measurement sensitivity and expanding the observable depth of the tomographic image.

[0030] A sixth invention is the method according to the fifth invention, wherein the intensity of the reference light is optimized so that the sensitivity of the optical coherence tomography device is best when the optical amplifier is not disposed. The reference light intensity is larger than the reference light intensity and smaller than the maximum value of the input light intensity at which the light detection device operates normally. The

[0031] By adopting such a configuration, the second invention improves the measurement sensitivity of 0GT, and expands the observable depth of the tomographic image (observable depth from the surface of the measurement target). The effect of doing is surely produced.

[0032] A seventh invention is characterized in that, in the fifth invention, the intensity of the reference light incident on the first and second photodetectors is greater than 1 and less than 1 OmW.

[0033] By adopting such a configuration, the second invention improves the measurement sensitivity of 0GT and expands the observable depth of the tomographic image (observable depth from the surface of the measurement target). The effect of doing is surely produced.

[0034] An eighth invention is characterized in that, in the fifth to seventh inventions, the light generating device is a variable wavelength light generating device capable of changing a wave number of emitted light.

By adopting such a configuration, the eighth invention has the effect of further improving the measurement sensitivity of 0FDR-0GT, which is the most sensitive of 0GT, and expanding the observable depth of tomograms. .

The invention's effect

[0035] According to the present invention, it is possible to improve the measurement sensitivity of 0GT and expand the depth at which a tomographic image can be observed.

Brief Description of Drawings

[0036] FIG. 1 is a block diagram of a 0FDR-0GT device according to the present invention.

[Figure 2] Sensitivity of the conventional 0FDR-0GT device.

FIG. 3 is an SNR of the 0FDR-0GT device according to the present invention.

FIG. 4 is a configuration diagram of a conventional 0FDR-0GT device.

Explanation of symbols

[0037] 1 Variable wavelength light generator

2 First coupler

3 First optical sensor

4 Measurement target 5 First light irradiation / capture device

6 Second coupler

7 Second optical sensor

9 Second light irradiation / capture device

1 3 Arithmetic control unit

1 9 Optical amplifier

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

[0039] (1) Device configuration

Fig. 1 shows the configuration of the 0GT device according to this embodiment. This apparatus has a configuration in which an optical amplifier 19 is arranged in the sample optical path 18 of the conventional 0FDR-0GT apparatus shown in FIG. The newly introduced optical amplifier 19 is disposed between the sampler 3 and the second force bra 6 in the sample optical path 18.

[0040] Furthermore, the first force bra 2 included in the 0FDR-0GT device shown in FIG. 1 has a split ratio different from that of the first force bra 2 included in the 0FDR-0GT device shown in FIG. The split ratio of the first force bra 2 of the conventional 0FDR-0GT device shown in Fig. 4 is 10: 90 (reference optical path 17: sample optical path 18 = 10: 90). However, the split ratio of the first force bra 2 in FIG. 1 is 50:50 (reference optical path: sample optical path = 50: 50). For this reason, the intensity of the light emitted from the first force bra 2 of FIG. 1 to the reference optical path 17 is five times larger than that of the apparatus of FIG. With such a device configuration, the sensitivity of 0FDR-0GT of this embodiment is greatly improved. Hereinafter, the configuration of the 0FDR-0GT apparatus according to the present embodiment shown in FIG. 1 will be described in detail. The parts common to those in Fig. 4 are given the same reference numerals.

[0041] The 0FDR-0GT device in FIG. 1 is a variable wavelength light generator 1 that can emit light while changing the wavelength, such as a super-periodic structure grating distributed reflection semiconductor laser light generator (Non-Patent Document 3). The light exit of the directional coupler that divides light into two (division ratio 50: 50 ) Is optically connected to the light receiving port of the first force bra 2 comprising an optical fiber. Here, the optical connection means being connected by an optical fiber. The same applies to the following description.

The output light intensity of the variable wavelength light generator 1 is 4 mW, and the intensity of the reference light reaching the photodetectors 10 and 11 is 400 w. In addition, the variable wavelength light generator 1 has a wave number interval of 2.62 x 10-? In the wavelength scanning range between 1530 nm and 1570 nm. ^-1 emits laser light while switching the wave number. Therefore, the number of wavelengths (or wave numbers) used for measurement is 400. The wave number holding time from switching the wave number to switching to the next wave number is 500ns. The light transmission port on one side (division ratio 50%) of the first force bra 2 is optically connected to the light receiving port of the first optical scanner 3. The light exit / light entrance of the first optical circulator 3 irradiates the measurement light onto the measurement object 4 and also captures the signal light backscattered by the measurement object 4. Is optically connected. The light output port of the first optical circulator 3 is optically connected to the light receiving port of an optical amplifier 19 composed of a semiconductor optical amplifier or an optical fiber amplifier. The light output port of the optical amplifier 19 is optically connected to the light receiving port on one side of the second force bra 6 composed of a directional coupler (division ratio 50:50).

[0043] The optical amplifier must be capable of amplifying light having a wavelength within the wavelength scanning range 1530 nm to 1570 nm of the variable wavelength light generator 1 (all wavelengths emitted by the variable wavelength light generator 1). Don't be. Furthermore, it is preferable that the optical gain has as little wavelength dependency as possible and the maximum gain is 20 dB or more. As an optical amplifier satisfying such conditions, there is a traveling wave semiconductor optical amplifier whose optical amplification layer is made of InGaAsP. An optical fiber amplifier that satisfies these conditions is the C-band erbium-doped optical fiber (EDFA) used in optical communications.

[0044] The configuration of the first light irradiation / capturing device 5 is the same as that of the conventional 0FDR-0GT shown in FIG.

[0045] The light transmission port on the other side (division ratio 50%) of the first force bra 2 is optically connected to the light reception port of the second optical scanner 7. Second optica The light emitting port / light receiving port of the Luzer modulator 7 is connected to a second light irradiation / capturing device 9 that irradiates the reference light to the reference mirror 8 and captures the reference light reflected by the reference mirror 8. The light output port of the second optical scanner 7 is optically connected to the light receiving port on the other side of the second force bra 6 composed of a directional coupler (division ratio 50:50). ing. The reference mirror 8 is carried on a support body that can be moved back and forth, and its position is adjusted so that the optical path lengths of the reference optical path 17 and the sample optical path 18 are substantially equal.

The light transmission ports on one side and the other side of the second force bra 6 are optically connected to the first and second photodetectors 10 and 11 having the same quantum efficiency. The outputs of the first and second photodetectors 10, 11 are electrically connected to the differential amplifier 12.

[0047] The output section of the differential amplifier 12 has a reflectance distribution (ref l ect i v i ty

prof i ele) That is, it is electrically connected to the input unit of the arithmetic and control unit 13 for synthesizing the reflection or backscattering intensity distribution via an analog / digital converter (not shown). The output unit of the arithmetic control device 13 is electrically connected to the input unit of a display device (not shown) such as a monitor or a printer that displays the calculation results. The arithmetic control device 13 controls the variable wavelength light generating device 1 and the first light irradiation / capturing device 5 based on the input information.

[0048] The tomographic image is constructed as follows. From the variable-wavelength light generating apparatus 1, in the wave number (= 2 [pi / wavelength) a very narrow wave number intervals (this embodiment of the laser beam, the 2 continuously switched while emit 62 χ 10- ⁷ η in m- The light emitted from the variable wavelength light generator 1 is incident on an interferometer comprising the first and second force bras 2, 6 and the first and second circulators 3, 7, and is scattered by the measurement object 4. The reference light reflected by the reference mirror 8 interferes with the second force bra 6. The intensity of the interference light is detected by the first and second photodetectors 10, 11 and is included in the interference light. The direct current component (proportional to the sum of the reference light intensity and the signal light intensity) is removed by the differential amplifier 12 and only the interference component (hereinafter referred to as signal current) is input to the arithmetic and control unit 13.

The arithmetic and control unit 13 records the wave number of the laser beam emitted from the variable wavelength light generator 1 and the signal intensity for the laser beam with respect to all the wave numbers. Variable wavelength light emission When the wave number scanning of the live device 1 is completed, the arithmetic and control unit 13 performs a Fourier transform on the recorded signal intensity with respect to the wave number. The result of the Fourier transform is a function of the position where the measurement light is backscattered by the measurement object 4 and its backscattering intensity. In other words, the depth distribution of the backscattering rate for the measurement object is obtained. The arithmetic and control unit 13 measures this distribution while moving the measurement light irradiation position little by little along the straight line on the surface of the measurement object 4. Finally, the tomogram of the measurement object 4 is constructed by bundling the measured distributions. The movement of the measurement light irradiation position is performed by the first light irradiation / capturing device 5 based on a command from the arithmetic control device 13 (Non-patent Document 1).

[0050] (2) Principle

Next, we will explain the reason why sensitivity is dramatically improved by using the device shown in Fig. 1.

[0051] (i) Conventional 0FDR-0GT sensitivity (shot noise limit)

In a photodetection device such as that shown in Fig. 4 using an interferometer, it is known that if the intensity of the reference light is sufficiently increased, the sensitivity increases to the shot (2) noise limit (Non-patent Document 2). First, this point will be explained.

[0052] The noise current included in the photocurrent detected by the photodetector is the thermal noise of the amplifier to which the photodetector is connected, the shot generated when the photocurrent flows to the photodetector, noise, And a noise current generated due to optical noise (so-called relative intensity noise (RIN)) included in the reference light and signal light incident on the photodetector. That is, the noise current can be expressed as follows.

[0053] [Equation 1]

Here, i ² _th is the thermal noise of the amplifier to which the photodetector is connected, 7? Is the sensitivity of the photodetector, _Pr is the intensity of the reference light, and P _s is the intensity of the signal light. Q is the charge of the electron, h is the Planck's constant, and 2 / is the optical frequency of the signal light and reference light. BW is the frequency band of the photodetector. <> Represents a time average. [0054] Equation (1) is obtained by using dual photodetectors 10 and 11 and a differential amplifier 12 as shown in FIG.

It does not represent noise for “balanced detection”, but represents noise for a light detection method using one photodetector and one input amplifier. However, there is no essential difference between the two, and the explanation is simpler if Equation (1) is used. Therefore, the sensitivity of the conventional 0FDR-0GT device will be described below based on equation (1). The accurate noise in dual balance detection is expressed by equation (8) used in the following explanation. In this equation, the quantum noise generated by the analog / digital converter to which the output of the differential amplifier 12 is connected is also taken into account.

[0055] The first term on the right side of equation (1) is thermal noise of the amplifier to which the photodetector is connected.

The second term is the photo detector shot noise. The third term is the RIN noise of the reference light and signal light.

Of the noises expressed in the respective terms on the right side of the equation (1), the shot noise caused by the reference light can be easily controlled by increasing or decreasing the reference light intensity. The third term on the right side, that is, RIN noise also depends on the reference light intensity, but usually the value of RIN is very small, so the existence of this term can be ignored.

[0057] As can be seen from Equation (1), when the intensity of the reference light is increased, the reference light is distorted. ■ Noise increases, shots with signal light. ■ Noise, thermal noise, and R IN noise can be ignored. Become. In this state, noise is proportional to the reference light intensity. On the other hand, the mean square of the signal current is expressed as shown in Equation (2) (Non-patent Document 2). According to Eq. (2), the root mean square of the signal current is also proportional to the reference light intensity.

[0058] [Equation 2]

SNR (signal to noise ratio: signal to

noise ratio) is calculated based on the signal current i _s (t). And the ratio of the output of the arithmetic and control unit 13 calculated from the noise current i _n (t). This ratio can be obtained from the signal current time average <i ² _s (t)> and the noise current time average <i ² _n (t)> using the relational expression shown in Equation (3). .

[0059] [Equation 3]

Here, Ns is the number of wave numbers emitted by the variable wavelength light generator 1 during one wavelength scan. Note that the photocurrent i (t) flowing through the photodetector is the sum of i _s (t) and i _n (t), i (t) =

i _s (t) + i _n (t) (However, to simplify the explanation, the photocurrent due to the DC component of the interference light is omitted in this equation.)

Based on the equations (1) to (3), the SNR when the reference light intensity is sufficiently strong is derived as follows.

[0061] [number 4] inheritance (P _r Univ.)

Here, f _A is the frequency at which the variable wavelength light generator 1 repeatedly scans the wave number (the reciprocal of the time required to scan all wave numbers). According to Eq. (4), in the limit where the reference light is large, the SNR depends only on the signal light intensity Ps and is independent of the reference light intensity Pr. (However, is constant.)

[0062] Shot noise is based on the natural fluctuation of the current that occurs because the movement of each charged particle is independently random, and inevitably exists in semiconductor elements where charged particles such as electrons and holes move. .

[0063] It is known that in a photodetection device as shown in Fig. 4 using an interferometer, the SNR is the strongest in a state where the shot noise is dominant (Non-patent Document 2). This condition is called the shot noise limit. Equation (4) shows the SNR of the 0FDR-0GT device at this shot noise limit. [0064] Incidentally, the sensitivity of OFDR-OGT device, that would otherwise generate noise equal signal currents, (measurement of interest) based on the backscattering factor r ² is defined (r ² = Po / Ps, Po Is the intensity of the measurement light immediately before it is irradiated on the measurement object). Because backscatter rate r ² in the biological tissue over several orders of magnitude, a material obtained by 10 times logarithm of r ^2, is defined as sensitivity of 0FDR-0GT device.

[0065] The sensitivity of 0FDR-0GT at the shot noise limit can be derived from Equation (4) as follows (Non-patent Document 2). Specifically, assuming that the right side of Equation (4) is equal to 1, substitute Ps = r ² Po into Equation (4) and calculate r ² .

[0066] [Equation 5]

ηΡ.

Sensitivity [dB] =-lOlog (5) As is clear from Equation (5), at the shot noise limit, the sensitivity of 0FDR-0GT is determined only by the intensity Po of the measurement light, regardless of the intensity Pr of the reference light. Therefore, in order to increase the sensitivity of 0 FDR-0GT, it is effective to increase the intensity Po of the measurement light.

[0067] For this reason, conventionally, in order to increase the sensitivity of 0FDR-0GT, the split ratio of the first force bra 2 has been made as large as possible on the sample optical path 18 side. In this way, as long as the 0 FDR-0GT device is at the shot noise limit (or its vicinity), the sensitivity of 0FDR-0GT can be sufficiently increased (Non-patent Document 2).

[0068] (i i) Sensitivity in the present embodiment

In the 0FDR-0GT apparatus according to the present embodiment shown in FIG. 1, an optical amplifier 19 is disposed in the sample optical path 18 to amplify the signal light. Furthermore, the sensitivity of the 0FDR-0GT device is improved by making the split ratio of the first force bra 2 equal to increase the intensity of the reference beam.

[0069] If the gain of the optical amplifier 19 is g, the intensity of the amplified signal light is g ■ Ps. Follow go-between, the formula <i _s ² (t)> (2) is proportional to, rather than P _{_s} g ■ P _s. If the noise generated by the optical amplifier 19 is sufficiently smaller than the shot noise due to the reference light, the sensitivity of the 0FDR-0GT device shown in Fig. 1 is as follows: Po in Equation (5) is replaced with g ■ Po It can be expressed by the following formula. [0070] [Equation 6] Sensitivity [dB] = (6)

Equation (6) shows that the sensitivity of the 0FDR-0GT device (inside log) in Fig. 1 is proportional to the gain g of the optical amplifier. If a semiconductor optical amplifier or an optical fiber single amplifier is used, the gain g of the optical amplifier can be easily increased to 20 dB or more. In other words, if an optical amplifier is used, the sensitivity of 0FDR-0GT can be easily improved by 20 dB or more. This improvement in sensitivity expands the measurable range of 0FDR-0GT by about two countries.

[0071] Assuming that the noise generated by the optical amplifier 19 is sufficiently smaller than the shot noise caused by the reference light, which is a precondition for the derivation of Equation (6), is 0FDR-0G as shown in Fig. 1. There is no guarantee that it will be established with the T device. Rather, the ASE generated by the optical amplifier (Amp I i f i ed

It is natural to think that the sensitivity deteriorates due to noise such as Spountaneous Em i ss i on. For this reason, as far as the inventor is aware, no attempt has been made to improve the sensitivity of 0GT using an optical amplifier.

[0072] The sensitivity of the 0FDR-0GT device shown in Fig. 1 was measured with the reference light intensity actually increased. As a result, according to the present embodiment, it was confirmed that the sensitivity of the 0FDR-0GT device was greatly improved.

[0073] In the following, the reason why it is considered that there is a high possibility that the sensitivity is deteriorated simply by placing the optical amplifier in the sample optical path 18 will be described in detail in "(i V) Sensitivity deterioration due to optical amplifier" below. To do. Finally, “(3) Operation (sensitivity improvement)” describes the results of sensitivity improvement attempted with the 0FDR-0GT device shown in Fig. 1.

[0074] (i i i) Details of sensitivity of conventional 0FDR-0GT device

First, the SNR of a conventional 0FDR-0GT device will be described.

[0075] As described above, the SNR of the conventional 0FDR-0GT device shown in Fig. 4 can be expressed by equation (3). According to Non-Patent Document 2, <)>)> It is expressed as below.

[0076] [Equation 7] (H 丑) ² :,.. ( ⁷⁾

[0077] [Equation 8]

(8) where p _r = Pr / 2 and p _s = Ps / 2.

[0078] Equations (7) and (8) will be described in a little more detail. There are some differences between the device described in Non-Patent Document 2 and the device shown in FIG. Since the difference is not essential, equations (7) and (8) apply directly to the device of Fig. 4.

[0079] _pr and _ps are the intensities of the reference light and the signal light incident on the photodetectors 10 and 11, respectively. The first term on the right-hand side of equation (8) is the DAQ board (analog / digital converter D / A converter “Pod”) (by shifting the lowest bit to 1 or 0) quantization noise, second The term is excess noise if the DAQ port produces noise other than quantization noise, and G is the amplification of the differential amplifier 12. The third term is the thermal noise of the differential amplifier 12. The fourth term is the sum of shot noise generated by the photodetectors 10 and 11. The fifth term is RIN noise, and is the common signal rejection efficiency of the differential amplifier. The RIN can be removed by the differential amplifier 12 only (the self-beat) term of the reference light or signal light (which is multiplied by and becomes small). The common signal rejection rate is not applied to the noise generated by the mutual beat of the reference light and signal light incoherently. In the case of "1, RIN noise of mutual beat, even if p _r is weaker than p _s, there may be can not be ignored in comparison with the self-beat noise.

[0080] Quantization noise and excess noise can be increased by increasing the value to G = 2x10 ⁵ , for example. Can be ignored. Fig. 2 is described in Non-Patent Document 2, and compares the theoretical curve calculated using Eqs. (7), (8), and (3) with the experimental value. The parameter values used to calculate the theoretical curve are: p _s = 3.8nW, i _th = 6pA / Hz- ² , 77 = 1, RIN = 1 x 10-"/ Hz (-110dB / Hz;), = 3.16x10-3 (-25dB), BW = 5x10 ⁶ HZ and Ns / 2 = 260.

The horizontal axis represents the intensity of the reference light detected by the photodetectors 10 and 11. The vertical axis represents the sensitivity of the 0FDR-0GT device. As is clear from Fig. 2, the sensitivity is highest in the region where the intensity of the reference light is 10-20; UW. When calculated based on the parameters described above, thermal noise is dominant in the region where the reference light intensity is lower than 10 to 20 W, and the sensitivity gradually deteriorates in the region where the reference light intensity is higher than 10 to 20 W. It can be seen that the cause is RIN by the reference beam. Based on this figure, conventionally, tomographic images were taken with the reference light intensity set at around 15 W (per detector).

[0082] (iv) Sensitivity degradation due to optical amplifier

Next, the reason why it is natural to think that the sensitivity is degraded by the optical amplifier will be explained. The optical amplifier 19 disposed in the sample optical path not only amplifies the signal light but also generates amplified spontaneous emission, that is, ASE. The intensity of this ASE is usually more than an order of magnitude higher than the optimum value of the reference light intensity described above. For example, the ASE intensity of a semiconductor optical amplifier operating at a gain of 19 dB was measured just before the photodetectors 10 and 11, and found to be 350 W.

[0083] Like the reference light, ASE is incident on photodetectors 10 and 11 to generate shot noise.

Specifically, it is added to the fourth term of Equation (8). In this situation, Eq. (6), which was based on the assumption that the sensitivity is improved in proportion to the gain of the optical amplifier, is no longer valid.

In addition, various noises such as shot noise due to fluctuations in ASE intensity itself, signal-to-AES beat signal, beat signal between AES, and the like are generated in the optical amplifier (Non-patent Document 4). Naturally, these noises are added to the right side of Eq. (8). Moreover, these noises are proportional to the first or second gain g. Therefore, the sensitivity of 0FD R-0GT deteriorates rapidly as the gain increases. [0085] If the optical amplifier is originally used by placing the optical amplifier in front of the photodetector, it is possible to improve the SNR of the photodetector by amplifying a weak optical signal more than thermal noise. . However, in photodetection devices that use interferometers such as those shown in Fig. 4 and Fig. 1, the shot noise limit is easily eliminated by increasing the intensity of the reference light without using an optical amplifier. Can be achieved. Therefore, it has been considered that the use of an optical amplifier is meaningless in a light detection system using an interferometer (Non-patent Document 5).

[0086] (3) Operation (sensitivity improvement)

However, the present inventor considers that there is no risk of such sensitivity deterioration if the intensity of the reference light is increased, and the division ratio of the first force bra 2 is set to be higher than that of the conventional 0FDR-0 GT device on the reference light path side It was expensive. In this way, the shot noise due to the reference light becomes larger than the noise such as AES using the optical amplifier 19 as the source, and the shot noise limit (or a state close to it) can be realized again.

That is, the present inventor believes that the sensitivity deterioration as described above is alleviated if the intensity of the reference light is made larger than the optimum value of the reference light intensity (in a state where the optical amplifier 19 is not disposed). It was. In this way, we thought that the ratio of shot noise caused by reference light in the total noise increased, and as a result, we approached the shot noise limit again. Therefore, the present inventors set the split ratio of the first force bra 2 higher on the reference optical path side than the conventional 0FDR-0GT device so that more light is supplied to the reference optical path side 17. .

It should be noted that “the optimum value of the reference light intensity (when the optical amplifier 19 is not disposed)” means that the sensitivity of the 0FDR-0GT device is highest when the optical amplifier 19 is not disposed (mostly This is the reference light intensity.

Specifically, it refers to the optimum value of the reference light intensity obtained by the same method as in Non-Patent Document 2. First, the SNR is obtained as a function of the reference light intensity. Measurement target 4 consists of an optical attenuator (neutradensity fil ers: ND filter) and a metal mirror. Sensitivity is calculated from the measured SNR. The reference light intensity is variable variable attenuation placed in front of the reference mirror 8: S device (var i ab le Adjust the strength with neutra l -dens i ty fil er). The position of the reference mirror 8 is adjusted so that the peak position of the 0FDR-0GT signal is at the center of the measurement range (a depth range or the ranging window). Note that since the sensitivity changes gradually near the maximum value, it is not easy to directly determine the optimum value of the reference light from the measured sensitivity value. In such a case, the median value of the reference light intensity (there are two points) whose sensitivity slightly deteriorates from the maximum value may be set as the optimum value of the reference light intensity.

[0090] Hereinafter, the results of demonstrating this idea by operating the 0FDR-0GT device shown in Fig. 1 will be described.

The light emitted from the variable wavelength light generator 1 is divided into two by the first force bra 2 at a ratio of 50:50. One of the divided lights (measurement light) is guided to the measurement object 4 by the first optical scanner 3 and the first light irradiation / capturing device 5, and from the measurement object 4. The signal light is again guided to the light receiving port of the optical amplifier 19 by the first light irradiating / capturing device 5 and the optical force ruthe-curator 3. The light amplified by the optical amplifier 19 is guided to the light receiving port on one side of the second force bra 6.

[0092] The other of the light divided by the first force bra 2 (reference light) is guided to the reference mirror 8 by the optical circulator 7 and the second light irradiation / capturing device 9, and the reference mirror 8 The reference light reflected by the second light irradiation / capturing device 9 and the second optical circulator 7 is again guided to the light receiving port on the other side of the second force bra 6.

The signal light and the reference light guided to the second force bra 6 are combined and guided to the first and second photodetectors 10 and 11 and converted into electric signals. The electrical signals detected by the first and second optical detectors 10, 11 are guided to the differential amplifier 12, and the signal current (interference component) is extracted. This signal current is converted into a digital signal by an analog / digital converter (not shown) and input to the arithmetic and control unit 13. The arithmetic and control unit 13 constructs a tomogram based on the input digital signal.

FIG. 3 shows the SNR of the 0FDR-0GT device shown in FIG. 1, measured by changing the gain of the optical amplifier 19. The horizontal axis represents the gain of the optical amplifier 19 and the vertical axis represents SNR. In the figure The parameter is the intensity of the signal light. The output of the variable wavelength light generator 1 is 4 mW, and the intensity of the reference light received by the photodetectors 10, 11 is 400 w. The intensity of the reference light is 4 times that of the conventional 0FDR-0GT device invented by the present inventor and 27 times that of Non-Patent Document 2.

[0095] As expected, the SNR is improved by amplifying the signal light by the optical amplifier 19. In addition, the SNR improves as the gain increases. In particular, in the region where the gain is 10 dB or less, the SNR increases almost in proportion to the gain. This is the ideal amplification itself shown in Equation (6).

[0096] What should be noted in FIG. 3 is the case where the intensity of the signal light is 0.006 nW and 0.02 nw. When the gain of the optical amplifier 19 was OdB, that is, when no optical amplification was performed, such a weak optical signal was buried in noise and could not be observed. However, as the gain is increased, the signal light is also amplified. Finally, the noise level is exceeded and the 0FDR-0GT signal can be observed. Figure 3 shows this well.

That is, it was confirmed that the sensitivity of the 0FDR-0GT device can be improved by optically amplifying the signal light while increasing the intensity of the reference light. The intensity of the reference light is not limited to 400 W described above. The intensity of the reference light should be larger than the intensity of the reference light optimized for the best sensitivity without an optical amplifier. In this way, the sensitivity is improved according to the extent to which the reference light is increased. However, if the reference light intensity becomes too high, the photodetector and the differential amplifier may not operate normally. Therefore, there is an upper limit to the preferred reference light intensity. Therefore, the intensity of the reference light must be lower than the input light intensity at which the light detection device consisting of the light detector and the amplifier operates normally.

[0098] Specifically, the intensity of the reference light incident on the photodetector is larger than the reference light optimized for the best sensitivity without an optical amplifier, and consists of a photodetector and an amplifier. It is preferably smaller than the maximum value of the input light intensity at which the light detection device operates normally, and more preferably, it is at least twice the intensity of the optimized reference light and the maximum value of the input light intensity at which the light detector operates normally. 0.9 times or less, more preferably 6 times or more the intensity of the optimized reference light and the normally operating input light. 0.8 times or less of the maximum value of the intensity, more preferably 20 times or more of the optimized reference light intensity and 0.7 times or less of the maximum value of the normally operating input light intensity. . More specifically, the intensity of the reference light incident on the photodetector is preferably greater than 10 mW, more preferably 30 W or more and 9 mW or less, more preferably 90; u W or more and 8 mW or less, and most preferably 300 W or more and 7 mW or less.

[0099] The position where the optical amplifier 19 is disposed is preferably between the first light irradiation / capturing device 5 and the second force bra. This is because if the first force bra and the first light irradiation / capturing device 5 are disposed, strong human measurement light may be irradiated to the human body and damage the human body. In particular, if the object to be measured is the eye, serious damage may occur. In other words, the optical amplifier is preferably arranged so as to amplify only the signal light and not the measurement light.

[0100] In the above embodiment, the 0FDR-0GT device, particularly the device in which the wave number changes stepwise has been described. The present invention is also applicable to 0FDR-0GT devices, SD-0GT devices, and TD-0GT devices in which the wave number changes continuously.

Industrial applicability

[0101] The present invention can be used in the medical device field, particularly in the manufacturing industry of ophthalmic diagnostic devices.

Claims

The scope of the claims

[1] a light generator;

An optical splitter that divides the light output from the light generator into measurement light and reference light;

A light irradiating / capturing device that irradiates the measurement light onto the measurement object, captures the signal light that is reflected or backscattered by the measurement object, and combines the signal light and the reference light. An optical coupler,

A light detection device for measuring the intensity of the output light coupled by the optical coupler; and based on the output of the light detection device, reflection or backscattering of the measurement light with respect to the measurement light irradiation direction on the measurement target In an optical coherence tomography apparatus having an arithmetic and control unit for specifying a position and reflection intensity or backscattering intensity,

An optical coherence tomography apparatus comprising an optical amplifier for amplifying the signal light.

[2] The intensity of the reference light is

Intensity of input light that is larger than the intensity of the reference light optimized so that the sensitivity of the optical coherence tomography apparatus is optimized and the optical detection apparatus operates normally without the optical amplifier being disposed. The optical coherence tomography device according to claim 1, wherein the optical coherence tomography device is smaller than the maximum value.

[3] The intensity of the reference light incident on the photodetection device is

2. The optical coherence tomography device according to claim 1, wherein the optical coherence tomography device is larger than 15 W and smaller than 1 O mW.

4. The optical coherence tomography device according to claim 1, wherein the light generation device is a variable wavelength light generation device capable of changing a wave number of emitted light.

[5] a light generator; A light output port of the light generation device connected to a light reception port, and an optical branching device for dividing the light output from the light generation device into measurement light and reference light;

A first optical circulator in which an optical outlet on one side of the optical splitter is connected to an optical inlet;

A light transmission / reception port of the first optical circuit is connected to irradiate the measurement light with the measurement light, and the measurement light is reflected or backscattered by the measurement target. A first light irradiating / capturing device for capturing; an optical amplifier for amplifying the signal light, wherein the light output port of the first optical circuit is connected to the light receiving port;

A second optical circulator in which the optical transmission port on the other side of the optical splitter is connected to the optical reception port;

A light emitting port / light receiving port of a second optical circuit is connected to irradiate the reference mirror with the reference light and to capture the reference light reflected by the reference mirror. / Capture device,

The light output port of the optical amplifier is connected to the light receiving port on one side, the light output port of the second optical circulator is connected to the light receiving port on the other side, and combines the signal light and the reference light An optical coupler;

A first optical detector connected to an optical outlet on one side of the optical coupler and measuring the intensity of the output light coupled by the optical coupler;

A second light detection device for measuring the intensity of the output light coupled to the other optical transmission port of the optical coupler and coupled by the optical coupler;

A differential amplifier in which the outputs of the first and second photodetectors are electrically connected; and the differential amplifier is electrically connected, based on the outputs of the first and second photodetectors, An optical coherence tomography device comprising a calculation control device for specifying a reflection or backscattering position and a reflection intensity or backscattering intensity of the measurement light with respect to the measurement light irradiation direction in the measurement object. The intensity of the reference light is

Without the optical amplifier, the optical coherence 6. The apparatus according to claim 5, wherein the intensity of the reference light optimized for the best sensitivity of the apparatus is smaller than the maximum value of the intensity of the input light for the light detection apparatus to operate normally. Optical coherence tomography device.

[7] The intensity of the reference light incident on the first and second photodetectors is

6. The optical coherence tomography device according to claim 5, wherein the optical coherence tomography device is larger than 1 5 W and smaller than 1 O mW.

8. The optical coherence tomography device according to claim 5, wherein the light generation device is a variable wavelength light generation device capable of changing a wave number of emitted light.