JP5590523B2 - Optical coherence tomography device - Google Patents

Description

  The present invention relates to an optical coherence tomography apparatus.

  An optical coherence tomography (OCT) apparatus is an apparatus that captures a tomographic image of a human eye or the like using a light interference phenomenon.

  The OCT apparatus includes a light source, an interferometer connected to the light source, and a tomographic image deriving unit that derives a tomographic image by measuring the output light intensity of the interferometer. The procedure for taking a tomographic image by the OCT apparatus is as follows. First, a measurement target is placed on one arm (hereinafter referred to as a measurement arm) of an interferometer. Next, an optical delay unit provided on the other arm of the interferometer (hereinafter referred to as a reference arm) is adjusted so that the optical path lengths of the reference arm and the measurement arm become substantially equal. Thereafter, the output light intensity of the interferometer is measured while changing the wavelength of the light source, and a tomographic image is derived based on the measurement result.

JP 2006-201087 A

  By the way, the optical delay device provided in the reference arm is a device that finely adjusts the optical path length by moving the reflecting mirror back and forth in the light traveling direction. Since the optical delayer adjusts the optical path length by the mechanical movement of the reflecting mirror in this way, its operability is not necessarily high. For this reason, the optical delay device is one factor that hinders improvement in operability of the OCT apparatus. In addition, the optical delay device is expensive because it is composed of precision optical parts and mechanical parts, which contributes to the cost increase of the OCT system.

  Therefore, an object of the present invention is to provide an OCT apparatus that does not require fine adjustment of the optical path length by an optical delay device.

  In order to achieve the above object, according to a first aspect of the present invention, a variable wavelength light generating unit that generates variable wavelength light whose wavelength changes with time, and the variable wavelength light as reference light and measurement light. Branching, irradiating the measurement light onto the measurement object to generate backscattered light, combining the backscattered light and the reference light to generate interference light, and measuring the intensity of the interference light In the optical coherence tomography apparatus having a tomogram deriving unit for deriving the tomogram of the measurement object based on the relationship between the measured intensity and the wavelength, the interferometer unit has a wavelength of the variable wavelength light. An optical phase modulation unit that modulates the phase of the reference light based on the intensity of the interference light to change the intensity of the interference light when the optical path length of the reference light is changed. Cal coherence tomography apparatus is provided.

  According to the present invention, it is possible to provide an OCT apparatus that does not require fine adjustment of the optical path length by an optical delay device.

It is the schematic explaining the structure of the OCT apparatus of embodiment. It is a figure explaining the time change of the wave number of the variable wavelength light which a variable wavelength light generation unit produces | generates. It is a figure explaining the relationship between the phase difference (phi) of the reference light before modulation | alteration, and the change of the wave number k of variable wavelength light.

  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. It should be noted that even if the drawings are different, corresponding parts are denoted by the same reference numerals, and description thereof is omitted.

  FIG. 1 is a schematic diagram illustrating the configuration of the OCT apparatus 2 according to the present embodiment. The solid line shown in FIG. 1 is an optical fiber that connects the following optical members. The present embodiment relates to an OCT apparatus that captures a one-dimensional tomographic image, but it is easy to extend the present embodiment to an OCT apparatus that captures a two-dimensional tomographic image or a three-dimensional image. For this purpose, for example, an optical scanning device such as a galvanometer mirror may be provided in the light irradiation / supplementing unit 26 described later.

  As shown in FIG. 1, the OCT apparatus 2 according to the present embodiment includes a variable wavelength light generating unit 4 that generates variable wavelength light 6 whose wavelength changes with time. Further, the OCT apparatus 2 splits the variable wavelength light 6 into the reference light 8 and the measurement light 10, irradiates the measurement light 18 to the measurement object 18 to generate the backscattered light 12, and the backscattered light 12 and the reference light 8. Are combined to generate interference light 14a and 14b. The OCT apparatus 2 also includes a tomographic image deriving unit 20 that measures the intensity of the interference lights 14 a and 14 b and derives a tomographic image of the measurement target 18 based on the relationship between the measured intensity and the wavelength of the variable wavelength light 6. ing.

  The interferometer unit 16 modulates the phase of the reference light 8 based on the wavelength of the variable wavelength light 6, thereby interfering with the intensity of the interference light 14 a and 14 b and changing the optical path length of the reference light 6. There is an optical phase modulation unit 22 which is substantially equal to the intensity of the light 14a, 14b.

  FIG. 2 is a diagram for explaining the change over time of the wave number (= 2π / wavelength) of the variable wavelength light 6 generated by the variable wavelength light generation unit 4. The horizontal axis in FIG. 2 is time. The vertical axis in FIG. 2 is the wave number.

  As shown in FIG. 2, the wave number of the variable wavelength light 6 changes continuously with time. As the variable wavelength light generating unit 4 that generates such variable wavelength light, for example, there is a wavelength sweep light source having a fiber ring resonator whose optical length changes with time and an optical gain medium.

  As shown in FIG. 1, the interferometer unit 16 includes an optical branching device 24 (for example, a directional coupler) that branches the variable wavelength light 6 into the measuring light 6 and the reference light 8. The interferometer unit 16 has a light irradiation / supplementing unit 26 that irradiates the measurement light 10 onto the measurement object 18 and supplements the backscattered light 12 generated by the measurement light 10 being backscattered by the measurement object 18. Yes. The interferometer unit 6 combines the reference light 8 and the backscattered light 12 and emits the first interference light 14a and the second interference light 14b (for example, the branching ratio is 1: 1). A directional coupler). Here, the intensities of the first interference light 14a and the second interference light 14b periodically change with respect to the wave number of the variable wavelength light 6, and their phases are shifted by π.

  As shown in FIG. 1, the light irradiation / supplementing unit 26 has an optical circulator 46 that emits the measurement light 10 branched by the optical branching device 24 and incident on the light incident port a from the light incident / exit port b. Yes. Further, the light irradiation / supplementing unit 26 has a collimator lens 60 that converts the measurement light emitted from the light incident / exit port b of the optical circulator 46 into parallel rays. Further, the light irradiation / supplementation unit 26 has a focusing lens 62 that collects the parallel rays and irradiates the measurement object 18.

  The measurement light 10 applied to the measurement object 18 is backscattered by the measurement object 18 to become backscattered light 12. The backscattered light 12 travels backward along the optical path along which the measurement light has traveled, enters the light incident / exit port b of the optical circulator 46, and exits from the light exit port c of the optical circulator 46.

  Further, as shown in FIG. 1, the tomogram deriving unit 20 receives the first interference light 14a and outputs a signal (for example, photocurrent) corresponding to the intensity of the first photodetector 30 ( For example, a pin photodiode) and a second photodetector 32 (for example, a pin photodiode) that receives the second interference light 14b and outputs a signal (for example, photocurrent) corresponding to the light intensity. Have. The tomographic image deriving unit 20 has a differential amplifier 34 that amplifies the difference between the output of the first photodetector 30 and the output of the second photodetector 32. The tomogram deriving unit 20 includes an AD converter 36 that performs analog-digital conversion on the output of the differential amplifier 34. With the above configuration, the tomogram deriving unit 20 measures the difference in intensity between the first interference light 14a and the second interference light 14b.

Further, the tomogram deriving unit 20 obtains the output of the AD converter 36 and determines the difference between the intensity I ₊ of the first interference light 14 a and the intensity I ₋ of the second interference light 14 b with respect to the wave number of the variable wavelength light 6. An arithmetic control unit 38 that performs Fourier transform and calculates the absolute value (or the square thereof) is included. The absolute value is a function of position coordinates along the traveling direction of the measurement light, and corresponds to the distribution in the depth direction of the backscattered light intensity of the measurement target, that is, a one-dimensional tomographic image.

  The arithmetic and control unit 38 is, for example, a computer loaded with a program for controlling the operation of the OCT apparatus 2 and deriving a tomogram, and includes a CPU (Central Processing Unit) 40 and a main memory 42 (for example, Random Access Memory). An auxiliary memory 44 (for example, a magnetic disk) is included.

―Optical phase modulation unit―
As shown in FIG. 1, the optical phase modulation unit 22 includes an optical phase modulator 48 and its control unit 50. The optical phase modulator 48 modulates the phase of incident light according to the output signal 52 of the control unit 50 and emits it. The optical phase modulator 48 has, for example, an optical waveguide made of an optical crystal such as ferroelectric crystal lithium niobate (LiNbO ₃ : LN). The optical phase modulator 48 changes the refractive index by applying a voltage to such an optical waveguide, and modulates the phase of the emitted light.

  The control unit 50 supplies the output signal 52 to the optical phase modulator 48 in accordance with instructions from the arithmetic and control unit 38.

  The optical phase modulation unit 22 is disposed in the optical path (reference light arm 56) of the reference light 8, and modulates the phase of the reference light 8 based on the change in the wavelength of the variable wavelength light 6 as described above, thereby interfering light. The intensity of 14a, 14b is set to the intensity when the optical path length of the reference light 8 is changed.

FIG. 3 shows the relationship between the phase difference φ (that is, the phase modulation amount) of the reference light 8 before and after modulation by the optical phase modulation unit 22 and the change in the wave number k (= 2π / wavelength) of the variable wavelength light 6 FIG. Here, the phase before modulation is the phase φ _a of the electric field intensity of the reference light 8 before applying a voltage to the optical phase modulator 48 at a certain position (for example, the light exit port of the optical modulator 48). . Further, the phase after modulation is the phase φ _b of the electric field intensity of the reference light 8 after applying a voltage to the optical phase modulator 48 at the same position. The phase difference φ of the reference light 8 before and after modulation is a difference φ _b −φ _a between φ _a and φ _b .

  The vertical axis in FIG. 3 is the phase difference φ. The horizontal axis represents the wave number k of the variable wavelength light 6. 3 is the phase difference φ corresponding to the wave number in the wavelength range that is not generated by the variable wavelength light generating unit 4.

  As shown in FIG. 3, the optical phase modulation unit 22 increases the phase difference φ (of the electric field strength) of the reference light 8 before and after modulation to 2π (rad) in proportion to the wave number k and reaches 0 (rad). The reference beam 8 is modulated so that the value returns to the above. Alternatively, the optical phase modulation unit 22 has a value that returns to 0 (rad) when the phase difference φ reaches -2π (rad) in proportion to the wave number k (= 2π / λ) of the wavelength λ of the variable wavelength light 6. Thus, the reference beam 8 is modulated. Therefore, the phase difference φ can be expressed by the following equation.

  Here, n is an integer.

By the way, as described above, the tomographic image to be measured is obtained by calculating the difference ΔI (= I ₊ −I ₋ ) between the intensity I ₊ of the first interference light 14 a and the intensity I ₋ of the second interference light 14 b with respect to the wave number. This is the absolute value (or its square) of the function that can be converted. This interference light intensity difference ΔI is known to be expressed by the following equation (for example, Patent Document 1). However, it is assumed that the measuring object 18 has only one reflecting surface.

Here, _{I r} and I _s, respectively, the intensity of the backscatter 12 and the reference beam 8. _Lr is the optical length of the optical path (measurement arm 54) along which the measurement light 10 and the backscattered light 12 travel. L _s is the optical length of the optical path (reference arm 56) along which the reference light 8 travels.

  An actual measurement object has a large number of reflection surfaces, and each reflection surface backscatters measurement light. The distribution derived by the tomogram deriving unit 20 is a superposition of the absolute values (or the squares thereof) of the Fourier transform of the backscattered light from the respective reflecting surfaces. Therefore, the tomographic image of the present OCT apparatus 2 can be explained by examining the equation (2).

Here, a coordinate axis along the path of the measurement light 10 in the measurement object 18 is considered. And the position coordinate of the reflective surface of the measuring object 18 is set to x. Further, L _r corresponding to the origin of the coordinate axis is L ₀ . Then, L _r corresponding to the position coordinate x of the reflection surface is L ₀ + 2x. Therefore, Formula (2) becomes as follows.

The conventional OCT apparatus 2 does not have the optical phase modulation unit 22. Therefore, φ becomes 0 (rad). In the conventional OCT apparatus, the optical length L _s of the reference arm 56 is adjusted by an optical delay so that L ₀ -L _s becomes substantially zero with respect to such ΔI. Thus, if L ₀ -L _s is not made substantially zero, the coefficient of the wave number k becomes too large, and ΔI vibrates in a short period with respect to the wave number k. As a result, the accuracy of Fourier transform is reduced.

In the present embodiment, instead of adjusting the optical length L _s of the reference arm 56, the phase of the reference light 8 is changed according to the equation (1). In this case, Equation (3) is as follows.

Therefore, L ₀ −L _s + m can be set to a desired value by adjusting the slope m of the phase modulation amount, and the coefficient of the wave number k can be adjusted without changing L _s . For example, when the slope m of the phase difference change amount with respect to the wave number increases the phase difference φ by 2π when the wavelength increases by 0.1 nm, the same effect as when the optical length L _s of the reference arm 56 is increased by 24 mm can be obtained. Can do. However, the wavelength of the variable wavelength light 6 is 1.55 μm. The wave number increment Δk can be represented by −2πΔλ / λ ² where the wavelength λ increment is Δλ.

  Here, since m is the gradient of the phase modulation amount φ, it can be easily adjusted electrically by the control unit 50 of the phase modulator 48. Therefore, the operability of the present OCT apparatus 2 is far superior to the optical phase modulation unit 22 using an optical delay device.

  As is well known, the absolute value (or its square) of the Fourier transform of the cosine function (cos (kx)) does not change even if a constant is added to the variable kx of the cosine function. That is, the absolute value (or its square) of the Fourier transform of cos (kx) and cos (kx + c) is the same value (c is a constant). Accordingly, the phase modulation amount φ may be as follows.

  However, c is a constant.

That is, in the optical phase modulation unit, the phase difference (of electric field strength) between the reference light 8 before and after modulation changes as a linear function of the wave number (= 2π / λ) of the wavelength λ of the variable wavelength light 6, and 2π The reference light 8 may be modulated so that the value returns to 0 (rad) when reaching (rad) or -2π (rad). Accordingly, the phase modulation amount φ ₀ corresponding to the initial value k ₀ of the wave number sweep can be freely set. For example, the phase modulation amount phi ₀ may be 0 (rad).

  As described above, according to the present embodiment, since the optical length of the reference arm 56 can be artificially adjusted by the optical phase modulation unit 22, fine adjustment of the optical path length by the optical delay device is not required. An OCT apparatus can be provided.

(2) Operation Next, the operation of the OCT apparatus 2 will be described.

First, the variable wavelength light generation unit 4 follows the command of the tomogram deriving unit 20 according to the command.
A variable wavelength light 6 whose wavelength continuously changes with time is generated. The initial value of the wavelength is 1530 nm, for example. On the other hand, the final value of the wavelength is 1570 nm, for example.

  Next, the optical branching device 24 branches the variable wavelength light 6 into the reference light 8 a and the measuring light 10.

  Next, the light irradiation / supplementing unit 26 irradiates the measuring object 18 with the measuring light 10 to generate the backscattered light 12. On the other hand, as described above, the optical phase modulation unit 22 modulates the phase of the reference light 8a based on the wavelength of the variable wavelength light 6 according to the command of the tomographic image deriving unit 20.

  Next, the optical coupler 28 combines the modulated reference light 8b and the backscattered light 12 to generate the first interference light 14a and the second interference light 14b.

  Next, the tomogram deriving unit 20 measures the intensity of the first interference light 14a and the intensity of the second interference light 14b, and the difference in intensity between the first interference light 14a and the second interference light 14b is Fourier-transformed. The tomographic image of the measurement object 18 is derived by converting and obtaining the absolute value (or the square of the absolute value).

  Here, the optical length of the reference arm 56 can be finely adjusted by adjusting the rate of change m of the phase modulation amount of the reference light 8a. That is, the origin of the tomographic image can be moved.

  In the present embodiment, as described above, the variable wavelength light generation unit 4 generates variable wavelength light whose wavelength continuously changes with time. However, the variable wavelength light generating unit 4 may generate variable wavelength light whose wavelength changes discretely with time (for example, light whose wavelength changes stepwise with time). In this case, the optical phase modulation unit 22 also discretely modulates the phase of the reference light 8a in response to this wavelength change. Furthermore, the variable wavelength light generation unit 4 may generate variable wavelength light that oscillates with a small wavelength around each wavelength that changes discretely. Also in this case, the optical phase modulation unit 22 oscillates the phase of the reference light 8a to be small with the phase changing discretely as the center corresponding to the wavelength change.

  In the above example, the intensity difference of the interference light is Fourier transformed, but the intensity of one interference light may be directly Fourier transformed.

2 ... OCT device 4 ... variable wavelength light generation unit 16 ... interferometer unit 20 ... tomographic image deriving unit 22 ... optical phase modulation unit

Claims (3)

A variable wavelength light generating unit for generating variable wavelength light whose wavelength changes with time;
An interferometer that splits the variable wavelength light into reference light and measurement light, irradiates the measurement light onto a measurement object to generate backscattered light, and combines the backscattered light and the reference light to generate interference light. Unit,
In an optical coherence tomography apparatus having a tomogram deriving unit that measures the intensity of the interference light and derives a tomogram of the measurement object based on the relationship between the measured intensity and the wavelength,
In the interferometer unit , the phase difference φ between the reference light before modulation and after modulation is expressed by the following equation:
φ = m · k-2π · n
(Wherein the unit of the phase difference φ is rad, m is an adjustable constant other than 0, k is the wave number, and n is an integer), and has an optical phase modulation unit that modulates the reference light. Optical coherence tomography device. The optical coherence tomography device according to claim 1,
The optical phase modulation unit, the phase difference between the reference light φ is urchin by Ru returns to 2 [pi (rad) or -2.pi. (rad) when reaches 0 (rad), characterized by modulating the reference beam Optical coherence tomography device. The optical coherence tomography device according to claim 1 or 2,
The interferometer unit is generated by splitting the variable wavelength light into the measurement light and the reference light, irradiating the measurement light onto the measurement object, and the measurement light being backscattered by the measurement object has a light irradiation / acquisition units you capture backscattered light, and an optical coupler for generating the bonds to the interference light the backscattered light and the reference light,
The optical phase modulation unit is disposed between the optical splitter and the optical coupling light,
Features optical coherence tomography equipment.

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