JP4620577B2 - Optical coherence tomographic image measuring method and apparatus - Google Patents

Description

  The present invention relates to an optical coherence tomographic image measurement method and apparatus for visualizing, for example, the internal structure of an object using light pulses.

  In order to elucidate the functions of biological organs such as the eyeball and internal organs, a system that can measure the tissue structure in vivo is required. For these purposes, optical coherence tomography (OCT) devices are now widely used. An optical coherence tomographic image (OCT) measuring apparatus is a Michelson interferometer type apparatus using a light source that is broadband and has low temporal coherence. In an optical coherence tomographic image (OCT) measurement apparatus using a Michelson interferometer, light from a sample to be measured (object) is object light, and light in the other optical path is reference light. The temporal coherence information of the object light is read out by causing two lights to interfere, and the structure in the depth direction of the sample to be measured (object) is detected from this coherence information. As a conventional technique, an optical coherence tomographic image measurement apparatus of this type is disclosed (for example, refer to Patent Document 1 and Patent Document 2).

  According to such an optical coherence tomographic image measuring apparatus, it is possible to obtain a high spatial resolution performance of 10 μm or less in the depth direction by using a low coherence light source such as a titanium sapphire laser. Currently, an optical coherence tomographic image measurement apparatus can image a tomographic structure of a retina of a living eye noninvasively with an accuracy close to that of an optical tissue section. However, since the low-coherence light source is used as described above, the performance of the two-dimensional spatial resolution in the horizontal direction of the sample (object) to be measured is about several tens of μm. Therefore, the optical coherence tomographic image measurement apparatus cannot be used for image diagnosis that requires a two-dimensional spatial resolution of 1 μm or less, such as photocytodiagnosis.

  As a technique for improving the spatial resolution in the horizontal direction, a microscope technique has been reported that improves the spatial resolution beyond the diffraction limit due to nonlinear characteristics of a material using a picosecond laser (for example, see Non-Patent Document 1). In this technique, a two-wavelength laser (excitation and erasing laser) is used to realize a microscope with higher resolution than a conventional fluorescence microscope. The method for improving the spatial resolution is as follows. First, the sample is irradiated with an excitation laser to be excited to the upper level. When the transition to non-radiation occurs and falls to the lower level, a doughnut-shaped optical electric field distribution is generated at the position of the beam waist, so that stimulated emission occurs at the beam periphery and fluorescence emission is observed at the beam center. As a result, fluorescence below the diffraction limit can be observed.

  The vertical resolution L in the optical coherence tomographic image can be expressed as L = λ / Δλ (λ is the center wavelength, and Δλ is the laser spectrum width). Therefore, in order to acquire a high-resolution tomographic image in the vertical direction, a laser pulse (for example, a femtosecond titanium sapphire laser) having a wide spectrum width must be used. At present, a high-resolution optical tomographic image is obtained in the vertical direction using a titanium sapphire laser having a spectral band with a full width at half maximum of about 200 nm (see, for example, Non-Patent Document 2). In the conventional technique, a λ / 2 delay plate, an optical spatial modulator, or the like is used, and a delay time of λ / 2 is given to the laser pulse envelope as a spatial length. The laser pulse is a laser pulse in which the optical carrier electric field phase is pseudo-inverted. By superimposing these laser pulses, the optical electric field at the center of the beam becomes zero, and a ring-shaped beam profile is formed.

With the ultrashort pulse laser technology in the visible / near infrared wavelength region in recent years, it is possible to easily generate an ultrashort optical pulse whose full width at half maximum (pulse width) is very close to the oscillation time of light. The poles of the electric field oscillating in the optical short pulse phase is called the optical carrier absolute phase is defined by [Delta] [phi _CE. FIG. 6 shows an electric field (optical carrier electric field) E of an optical pulse and a pulse envelope ENV of electric field oscillation that takes a peak value at time zero. Time difference between the time corresponding to the maximum value of the time and the electric field vibration corresponding to the peak value of the intensity waveform of the optical pulse as shown in FIG. 6 is a [Delta] [phi _CE. With an ultrashort laser pulse, the optical electric field at the center of the beam cannot be made completely zero even if the optical carrier electric field phase is artificially inverted using a λ / 2 delay plate or an optical spatial modulator. . In the example of FIG. 7, since the laser pulse with the delay time 0 and the laser pulse with the delay time π / 2 are not line-symmetric with respect to the horizontal axis of the electric field 0, even if these laser pulses are overlapped, You can see that the world does not become zero.

  Therefore, in order to use an extremely short laser pulse and completely reduce the optical electric field at the center of the beam to zero, a technique for making the optical carrier absolute phase shift between the optical pulses constant is required. In a normal optical pulse, the shift amount of the optical carrier absolute phase between the pulses is not constant. In recent years, a control technique has been established that makes the shift amount of the optical carrier absolute phase between each pulse constant (for example, see Non-Patent Document 3). By using this control technique, it is possible to produce a laser pulse whose phase is completely reversed. By utilizing this control technique, the optical electric field at the center of the beam of the ultrashort pulse laser can be made completely zero by superimposing laser pulses whose optical carrier electric field phase is completely inverted as shown in FIG. .

Japanese Patent No. 3539726 Japanese Patent No. 3626110 M.Dyba et al., “Focal Spots of Size λ / 23 Open Up Far-Field Florescence Microscopy at 33 nm Axial Resolution”, PHYSICAL REVIEW LETTERS, 2002, Vol.88, No.16 A.D.Aguirre et al., “High-resolution optical coherence microscopy for high-speed, in vivo cellular imaging”, OPTICS LETTERS, 2003, Vol.28, No.21, p.2064-2066 D.J. Jones et al., “Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis”, SCIENCE, 2000, Vol.288, p.635-639

  As described above, in the conventional optical coherence tomographic image measurement method using a low-coherence light source such as a titanium sapphire laser, the two-dimensional spatial resolution in the horizontal direction is about several tens of μm. It was necessary to further improve the two-dimensional spatial resolution of directions. However, a method for improving the horizontal two-dimensional spatial resolution in the optical coherence tomographic image measurement method has not been established at present.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to improve horizontal two-dimensional spatial resolution in an optical coherence tomographic image measurement method and apparatus.

The optical coherence tomographic image measurement method according to the present invention includes a separation step in which the light pulse emitted from the light pulse output means is divided into two, one is irradiated to the sample to be measured, and the other is the reference light. A generation step for sequentially generating light pulses of the first and second types of beam profiles from light, and reflection from the sample to be measured by the light pulses of the two types of beam profiles sequentially irradiated on the sample to be measured. An interference intensity measuring step for measuring the interference intensity between the light and the reference light for each beam profile; and the interference intensity obtained by the optical pulse of the first beam profile and the optical pulse of the second beam profile. An interference intensity difference calculating step for calculating a difference value from the interference intensity obtained, a storing step for storing the difference value of the interference intensity, and the reference And an optical path length control step of the optical path length is a predetermined change in the amount of light, repeated while changing the optical path length of the reference light, and outputs a difference value of a series of interference intensity stored in said storing step as an optical tomographic image light In the coherence tomographic image measurement method, the optical pulses having the two types of beam profiles are an optical pulse having an optical carrier absolute phase difference of 0 at a central portion and a peripheral portion of the beam and an optical pulse having an optical carrier absolute phase difference of π. It is characterized by being.
In one configuration example of the optical coherence tomographic image measurement method of the present invention, the optical pulse emitted from the optical pulse output means has a fixed shift amount of the optical carrier absolute phase between the optical pulses. There is .

Further, the optical coherence tomographic image measuring apparatus of the present invention divides the light pulse output means and the light pulse emitted from the light pulse output means into two parts, one of which is used as the irradiation light to the sample to be measured, and the other reference A beam splitter for generating light, a generating means for sequentially generating light pulses of first and second types of beam profiles from the irradiated light, and light of the two types of beam profiles sequentially irradiated on the sample to be measured A light detecting means for measuring the interference intensity between the reflected light from the sample to be measured and the reference light by a pulse for each beam profile; the interference intensity obtained by the light pulse of the first beam profile; An interference intensity difference calculating means for calculating a difference value from the interference intensity obtained by the optical pulse of the beam profile of, and a memory for storing the difference value of the interference intensity And optical path length control means for changing the optical path length of the reference light by a predetermined amount, repeatedly calculating the difference value of the interference intensity while changing the optical path length of the reference light, and stored in the storage means An optical coherence tomographic image measurement apparatus that outputs a series of interference intensity difference values as an optical tomographic image , wherein the optical pulse of the two types of beam profiles has an optical carrier absolute phase difference of 0 between the central part and the peripheral part of the beam. The optical pulse is an optical pulse whose optical carrier absolute phase difference is π .

  According to the present invention, optical pulses having two types of beam profiles are generated, and the interference intensity obtained by the optical pulse having the first beam profile and the interference intensity obtained by the optical pulse having the second beam profile are calculated. By calculating the difference value, an optical tomographic image having a horizontal resolution equal to or lower than the diffraction limit of the optical pulse can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an optical coherence tomographic image measurement apparatus according to an embodiment of the present invention. This optical coherence tomographic image measuring device is an ultrashort pulse laser device 1 which is an optical pulse output means, beam splitters 3 and 4, a polarizing beam splitter 5, and a generating means for generating optical pulses of two types of beam profiles. An irradiation light beam profile generation device 6, a condensing optical element 12, a first optical delay mechanism 14 that is an optical path length control means for changing the optical path length of the reference light by a predetermined amount, a photodetector 15, and a computer 16 and a polarization rotation element 19. The beam splitter 4, the condensing optical element 12, the polarizing beam splitter 5, the photodetector 15, and the polarization rotating element 19 are light detection means for measuring the interference intensity between the reflected light from the sample 13 to be measured and the reference light. The computer 16 comprises an interference intensity difference calculating means for calculating the difference value of the interference intensity and a storage means for storing the difference value of the interference intensity.

The ultrashort pulse laser device 1 emits linearly polarized ultrashort pulse laser light 2 in which the shift amount of the optical carrier absolute phase between optical pulses is fixed to π. As a technique for fixing the shift amount of the optical carrier absolute phase to π, for example, the technique disclosed in Non-Patent Document 3 can be used.
The beam splitter 3 separates the ultrashort pulse laser light 2 emitted from the ultrashort pulse laser device 1 into irradiation light for the sample 13 to be measured and reference light toward the optical delay mechanism 14.

  The irradiation light beam profile generation device 6 includes a Michelson interferometer type optical system in a vacuum vessel in order to generate two types of beam profiles. The ultrashort pulse laser beam 2 that has entered the vacuum container of the irradiation light beam profile generator 6 is first divided into reflected light and transmitted light by the beam splitter 7. The beam splitter 7 has a structure in which a branch plane 72 is sandwiched between substrates 70 and 71 having the same material and thickness as shown in FIG. This structure takes into account that the dispersion values of reflected light and transmitted light are equal.

  Reflected light from the beam splitter 7 enters a second optical delay mechanism composed of mirrors 8 and 9, and transmitted light from the beam splitter 7 enters a third optical delay mechanism composed of mirrors 10 and 11. The reflected light incident on the second optical delay mechanism repeats reflection between the mirrors 8 and 9 and then returns to the beam splitter 7. Similarly, the transmitted light incident on the third optical delay mechanism returns to the beam splitter 7 after being repeatedly reflected between the mirrors 10 and 11. In order to make the dispersion value of the laser pulse equal between the reflected light and the transmitted light, the number of reflections between the mirrors 8 and 9 and the number of reflections between the mirrors 10 and 11 are set to be equal.

  On the optical path between the mirrors 8 and 9, a dot 17 that blocks the beam central portion is provided. On the other hand, on the optical path between the mirrors 10 and 11, an aperture 18 that blocks the beam peripheral portion. Is installed. The pulse laser light returned from the second optical delay mechanism and the pulse laser light returned from the third optical delay mechanism are combined by the beam splitter 7 and emitted from the irradiation light beam profile generation device 6.

  Here, the two types of beam profiles are generated by the optical path length difference between the light passing through the second optical delay mechanism and the light passing through the third optical delay mechanism. The pulse laser beam having the first beam profile is generated with an optical path length difference of 2 (N−1) (N is a natural number) times the laser repetition frequency, and the pulse laser beam having the second beam profile has the laser repetition frequency. It is generated with an optical path length difference of 2N-1 times. When an optical path length difference of 2 (N-1) times the laser repetition frequency is given, the difference in optical carrier absolute phase between the center portion and the peripheral portion of the beam is zero. On the other hand, when an optical path length difference of 2N-1 times the laser repetition frequency is given, the difference in the optical carrier absolute phase between the center portion and the peripheral portion of the beam is π.

  The positions of the mirrors 8 and 9 or the positions of the mirrors 10 and 11 in the vacuum vessel can be adjusted according to an instruction from the computer 16 described later. As a result, the irradiation light beam profile generation device 6 sets the optical path length difference between the light passing through the second optical delay mechanism and the light passing through the third optical delay mechanism to 2 (N−1) times the laser repetition frequency or 2N−. It can be set to either one (ie, the beam profile is set to either the first beam profile or the second beam profile).

The pulsed laser light generated by the irradiation light beam profile generation device 6 passes through the beam splitter 4.
The condensing optical element 12 condenses the pulse laser beam having the first beam profile or the pulse laser beam having the second beam profile that has passed through the beam splitter 4 and irradiates the sample 13 to be measured.

  When the pulsed laser beam having the first beam profile in which the phase difference between the beam center portion and the peripheral portion is zero is irradiated onto the sample 13 to be measured, the shape of the focused spot on the sample 13 to be measured is shown in FIG. So that it becomes circular. On the other hand, when the measurement sample 13 is irradiated with pulsed laser light having a second beam profile in which the phase difference between the central portion of the beam and the peripheral portion is π, a laser pulse whose optical carrier absolute phase is inverted at the central portion during condensing. Since they overlap, the focused spot shape on the sample 13 to be measured has an annular shape as shown in FIG.

  Depending on the variation of the optical system, the condensing spot shape may not be circular or annular. In such a case, means for adjusting the optical path length of the pulsed laser light is provided in the condensing optical element 12. You may do it. Thereby, it can adjust so that a beam waist may be located in the surface of the to-be-measured sample 13, and a condensing spot shape can be made circular or an annular shape.

Next, the reflected light from the sample 13 to be measured passes through the condensing optical element 12 again, is reflected by the beam splitter 4, and enters the polarizing beam splitter 5.
On the other hand, the first optical delay mechanism 14 has a function capable of adjusting the optical path length of the reference light reflected by the beam splitter 3 in accordance with an instruction from the computer 16 described later.

The polarization rotation element 19 rotates the polarization plane of the reference light that has passed through the first optical delay mechanism 14. At this time, the rotation angle of the polarization rotation element 19 is adjusted in advance so that the interference intensity between the reflected light from the sample 13 to be measured and the reference light that has passed through the first optical delay mechanism 14 is maximized.
The polarization beam splitter 5 combines the reflected light from the sample 13 to be measured and the reference light that has passed through the polarization rotation element 19.

The photodetector 15 measures the interference intensity between the reflected light and the reference light synthesized by the polarization beam splitter 5. The photodetector 15 is composed of, for example, a photomultiplier tube or a semiconductor photodetector.
FIG. 4 shows an example of the configuration of the computer 16. The computer 16 includes a beam profile selection unit 21, an optical delay control unit 22, an interference intensity storage unit 23, an interference intensity difference calculation unit 24, and an optical tomographic image output unit 25.

The beam profile selection unit 21 selects one of the first and second beam profiles, and instructs the irradiation light beam profile generation device 6 to output pulse laser light of the selected beam profile. .
The optical delay control unit 22 instructs the first optical delay mechanism 14 to change the optical path length of the reference light by a predetermined amount.

The interference intensity storage unit 23 stores the interference intensity value measured by the photodetector 15 and also stores the interference intensity difference value calculated by the interference intensity difference calculation unit 24.
The interference intensity difference calculation unit 24 interferes when the measurement sample 13 is irradiated with the pulse intensity of the first beam profile and the sample 13 with the pulse intensity of the second beam profile. The difference value from the intensity is calculated.

The optical tomographic image output unit 25 outputs a series of interference intensity difference values calculated while changing the optical path length of the reference light as an optical tomographic image.
The computer 16 as described above can be realized by hardware including a CPU, a storage device, and an interface, and a program that controls these hardware resources. The CPU executes processing according to a program stored in the storage device.

  Next, the operation of the optical coherence tomographic image measurement apparatus according to the present embodiment will be described with reference to FIG. First, the beam profile selection unit 21 of the computer 16 selects one of the first and second beam profiles, and generates an irradiation light beam profile so as to output pulse laser light of the selected beam profile. The device 6 is instructed (step S1 in FIG. 5).

The ultrashort pulse laser device 1 emits an ultrashort pulse laser beam 2. The irradiation light beam profile generation device 6 generates pulse laser light having the beam profile selected in step S <b> 1 from the ultrashort pulse laser light 2 incident through the beam splitter 3. The pulsed laser light generated by the irradiation light beam profile generation device 6 is irradiated to the sample 13 to be measured via the beam splitter 4 and the condensing optical element 12 (step S2).
The reflected light of the laser pulse irradiated on the sample 13 to be measured interferes with the reference light by the polarization beam splitter 5. The interference intensity storage unit 23 stores the value of the interference intensity measured by the photodetector 15 (step S3).

Next, the beam profile selection unit 21 determines whether or not the interference intensity has been measured for all the beam profiles (step S4). For example, when the second beam profile is not selected, the beam profile selection unit 21 returns to step S1, selects the second beam profile, and outputs the irradiation light beam so as to output the pulse laser light of the second beam profile. Instructs the profile generation device 6.
Similarly to the above, the interference intensity value is stored in the interference intensity storage unit 23 by the processing of steps S2 and S3. Thus, the processes in steps S1 to S4 are repeated until the first and second beam profiles are sequentially selected and the interference intensities of the two types of beam profiles are measured.

  After acquiring the interference intensity data of the two types of beam profiles (YES in step S4), the interference intensity difference calculation unit 24 causes the interference intensity when the pulsed laser beam of the first beam profile is irradiated to the sample 13 to be measured. And a difference value between the interference intensity when the measured sample 13 is irradiated with the pulse laser beam having the second beam profile (step S5). This difference value of the interference intensity corresponds to an interference signal from a space below the diffraction limit of the laser pulse. The interference intensity storage unit 23 stores the difference value of the interference intensity (step S6).

Subsequently, the optical delay control unit 22 instructs the first optical delay mechanism 14 to change the optical path length of the reference light by a predetermined amount (step S7).
Next, the beam profile selection unit 21 determines whether or not the measurement has been completed a predetermined number of times (step S8). This predetermined number of times is a value obtained by dividing the predetermined measurement range by the amount of change in optical path length per one time of the optical delay mechanism 14. In this way, the processes of steps S1 to S8 are repeated until the measurement is completed a predetermined number of times, and the difference value of the interference intensity is calculated each time the optical path length of the reference light changes while increasing or decreasing the optical path length of the reference light. Remember.

  When the measurement is completed a predetermined number of times (YES in step S8), the optical tomographic image output unit 25 outputs a series of interference intensity difference values stored in the interference intensity storage unit 23 as an optical tomographic image (step S9). This completes the measurement.

  In the present embodiment, when the optical path length of the reference light is changed corresponding to the deep position of the sample 13 to be measured, and the optical path length to the reflection point of the deep layer of the sample 13 to be measured matches. Only by using that the maximum interference signal can be obtained, it can be obtained as an optical tomographic image. In this embodiment, an ultrashort pulse laser beam in which the shift amount of the optical carrier absolute phase between the optical pulses is fixed to a constant value is used, and a pulse laser having two types of beam profiles is obtained from the ultrashort pulse laser beam. By generating light and calculating a difference value between the interference intensity obtained by the optical pulse of the first beam profile and the interference intensity obtained by the optical pulse of the second beam profile, the diffraction limit of the optical pulse is calculated. It is possible to obtain an optical tomographic image having the following horizontal resolution.

  The present invention can be applied to an optical coherence tomographic image measurement apparatus.

It is a block diagram which shows the structure of the optical coherence tomographic image measuring apparatus which concerns on embodiment of this invention. It is sectional drawing which shows the structure of the beam splitter used with the irradiation light beam profile production | generation apparatus in the optical coherence tomographic image measuring apparatus of FIG. It is a figure which shows the condensing spot shape of the laser pulse irradiated to the to-be-measured sample in embodiment of this invention. It is a block diagram which shows one structural example of the computer in the optical coherence tomographic image measuring apparatus of FIG. It is a flowchart which shows operation | movement of the optical coherence tomographic image measuring apparatus of FIG. It is a figure for demonstrating the optical carrier absolute phase of the ultrashort optical pulse in a time domain. It is a figure which shows the optical electric field of the ultrashort optical pulse at the time of giving a delay time and inverting the optical carrier electric field phase. It is a figure which shows the optical electric field of the ultrashort optical pulse at the time of reversing the optical carrier absolute phase using the control technique which makes the shift amount of the optical carrier absolute phase between optical pulses constant.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Ultra short pulse laser apparatus, 2 ... Ultra short pulse laser beam, 3, 4, 7 ... Beam splitter, 5 ... Polarizing beam splitter, 6 ... Irradiation light beam profile production | generation apparatus, 8, 9, 10, 11 ... Mirror, DESCRIPTION OF SYMBOLS 12 ... Optical element for condensing, 13 ... Sample to be measured, 14 ... Optical delay mechanism, 15 ... Photo detector, 16 ... Computer, 17 ... Dot, 18 ... Aperture, 19 ... Polarization rotating element, 21 ... Beam profile selection part , 22: an optical delay control unit, 23: an interference intensity storage unit, 24: an interference intensity difference calculation unit, 25: an optical tomographic image output unit.

Claims (4)

A separation step in which the light pulse emitted from the light pulse output means is divided into two and one is used as irradiation light to the sample to be measured and the other is used as reference light;
A generation step of sequentially generating optical pulses of the first and second types of beam profiles from the irradiation light;
An interference intensity measuring step for measuring, for each beam profile, an interference intensity between the reflected light from the measurement sample and the reference light by the light pulses of the two types of beam profiles sequentially irradiated on the measurement sample;
An interference intensity difference calculating step for calculating a difference value between the interference intensity obtained by the optical pulse of the first beam profile and the interference intensity obtained by the optical pulse of the second beam profile;
A storage step of storing a difference value of the interference intensity;
An optical path length control step of changing the optical path length of the reference light by a predetermined amount;
An optical coherence tomographic image measurement method that repeatedly performs while changing the optical path length of the reference light, and outputs a series of interference intensity difference values stored in the storage step as an optical tomographic image,
The optical pulses of the two types of beam profiles are an optical pulse having an optical carrier absolute phase difference of 0 between a central portion and a peripheral portion of the beam and an optical pulse having an optical carrier absolute phase difference of π. Tomographic image measurement method. The optical coherence tomographic image measurement method according to claim 1,
The optical coherence tomographic image measuring method, wherein the optical pulse emitted from the optical pulse output means has a fixed shift amount of the optical carrier absolute phase between the optical pulses. Optical pulse output means;
A beam splitter that divides the light pulse emitted from the light pulse output means into two parts, one of which is irradiated to the sample to be measured and the other as reference light;
Generating means for sequentially generating light pulses of first and second types of beam profiles from the irradiation light;
A light detection means for measuring, for each beam profile, an interference intensity between the reflected light from the measurement sample and the reference light by the light pulses of the two types of beam profiles sequentially irradiated on the measurement sample;
Interference intensity difference calculating means for calculating a difference value between the interference intensity obtained by the optical pulse of the first beam profile and the interference intensity obtained by the optical pulse of the second beam profile;
Storage means for storing a difference value of the interference intensity;
Optical path length control means for changing the optical path length of the reference light by a predetermined amount,
An optical coherence tomographic image measurement apparatus that repeatedly calculates a difference value of the interference intensity while changing an optical path length of the reference light, and outputs a series of interference intensity difference values stored in the storage unit as an optical tomographic image. And
The optical pulses of the two types of beam profiles are an optical pulse having an optical carrier absolute phase difference of 0 at the central part and a peripheral part of the beam and an optical pulse having an optical carrier absolute phase difference of π. Tomographic image measuring device. The optical coherence tomographic image measurement apparatus according to claim 3,
The optical pulse output means emits an optical pulse in which the shift amount of the optical carrier absolute phase between the optical pulses is fixed to a constant value .

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