JP2006250848A - High speed light delay producing method in light coherence tomography and phase modulated light coherence tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high speed light delay producing method in light coherence tomography for obtaining a tomographic image in a high S/N ratio by detecting light using an independently stable narrow band beat frequency without being restricted by an angle-of-rotation speed while putting the high speed scanning due to a rotary body to practical use and without relying on unstable Doppler frequency modulation, and also to provide a phase modulated light coherence tomography apparatus. <P>SOLUTION: In the high speed light delay producing method by a rotary reflecting body in the light coherence tomography, the light from a wide band wavelength light source 1 is split into the matter light and reference light to an inspection target 10 by a two-splitting half mirror 2 and a phase modulating element 6 is provided in this light path to allow the reference light to reflect and regress from the light delaying and reflecting mechanism 4 by a rotary reflecting body to combine the same with the matter light, which is regressed from the inspection target 10, by the two-splitting half mirror 2 while the heterodyne interference signal based on the phase modulating element is detected. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-speed optical delay generation method in optical coherence tomography and a phase modulation optical coherence tomography apparatus in optical coherence tomography.

  An optical coherence tomography apparatus using a light source having a broad wavelength range has been developed, is currently put into practical use, is used in the medical field, and is widely used (for example, see Patent Document 1 and Non-Patent Document 1 below). As a method of generating the optical heterodyne interference beat signal in the high-speed optical delayed reflection mechanism by the conventional rotating reflector, the reflected light from the high-speed rotating reflector is subjected to Doppler shift frequency modulation. A method for detecting the beat frequency signal is known (see, for example, Patent Document 2 below).

  FIG. 4 is a schematic diagram of a conventional optical coherence tomography apparatus.

In this figure, 101 is a low coherence light source (for example, an SLD (super luminescent diode) light source), 102 is a half mirror (half mirror for splitting), 103 is a rotating body, and 104 is a rotating reflector (for example, a corner cube). , 105 a fixed mirror, 105 a a position adjuster for the fixed mirror 105, 107 an X axis scanning mirror, 108 a Y axis scanning mirror, 109 an objective lens, 110 an object to be inspected, 111 an optical detector, 112 Is a signal processor, and 113 is a computer display.
Japanese Patent No. 20110042 Japanese Patent No. 3553451 Optics, Vol. 28, No. 3, 1999 pp. 116-125

However, in the method using Doppler frequency modulation in the above-described conventional optical coherence tomography, the rotational angular velocity of the rotating reflector (corner cube mirror) 104 is fast, and the resulting Doppler beat signal frequency is as high as several tens of MHz, and the rotation As a result, the center frequency shifted by several MHz, and narrow band detection was impossible. Furthermore, there is a problem that the Doppler frequency from a plurality of reflectors shifts depending on the rotation unevenness of the rotator and the mounting angle of the reflectors.

  Therefore, there is a drawback that the beat frequency cannot be narrowed or stabilized in light detection, and only a tomographic image with a poor signal-to-noise ratio (SN ratio) can be obtained.

  In view of the above situation, the present invention provides an independent stable and narrow-band beat without relying on unstable Doppler frequency modulation, and without being constrained by the rotational angular velocity while utilizing high-speed scanning by a rotating body. It is an object of the present invention to provide a high-speed optical delay generation method and a phase modulation optical coherence tomography apparatus in optical coherence tomography that can detect light at a frequency and construct a tomographic image with a high S / N ratio.

In order to achieve the above object, the present invention provides
[1] In a method for generating a high-speed optical delay in optical coherence tomography, light from a low-coherence light source is divided into an object beam and a reference beam to be inspected by a half mirror for splitting into two , and a phase modulation element in this optical path And the reference light is reflected and returned from the optical delayed reflection mechanism by the rotating reflector, and is combined with the object light returning from the inspection object by the half mirror for division into two, and based on the phase modulation element. A heterodyne interference beat signal is detected.

  [2] In a phase-modulated light coherence tomography apparatus, a low-coherence light source, a half-mirror for splitting the light from the low-coherence light source into object light and reference light to be inspected into two parts, A provided phase modulation element that operates at a predetermined frequency, a light delay reflection mechanism that delays and reflects the reference light by a rotating reflector, an object light that returns from the inspection object, and a reference light that returns from the light delay reflection mechanism And a photodetector for detecting a heterodyne interference beat signal based on the phase modulation element.

  [3] In a phase-modulated optical coherence tomography apparatus, a low-coherence light source, a half-mirror for splitting the light from the low-coherence light source into object light and reference light to be inspected into two parts, and in the middle of the optical path In-plane scanning using the X-axis scanning mirror and the Y-axis scanning mirror with the provided phase modulation element that operates at a predetermined frequency, the optical delay reflection mechanism that delays and reflects the reference light by a rotating reflector, and the object light A surface scanning mechanism, an objective lens, the half-dividing half mirror that combines the object light returning from the inspection object and the reference light returning from the optical delay reflection mechanism, and a heterodyne based on the phase modulation element And a photodetector for detecting an interference beat signal.

  [4] In the phase modulation optical coherence tomography device according to [2] or [3], the phase modulation element is a fixed mirror having an electrostrictive element in close contact, and the fixed mirror is provided in a reference optical path or an object optical path. It is characterized by that.

  [5] In the phase modulation optical coherence tomography device according to [2] or [3], the phase modulation element is an electrostrictive element made of a light transmitting member, and the electrostrictive element is placed in a reference optical path or an object optical path. It is characterized by comprising.

  [6] The phase modulation optical coherence tomography device according to [2] or [3], further including an electrical filter that extracts only a constant frequency component based on the phase modulation element from the output from the photodetector. Features.

  As described above in detail, according to the present invention, the rotational angular velocity is constrained without relying on the unstable Doppler frequency modulation by the conventional rotating reflector and taking advantage of the high-speed scanning by the rotating member. In addition, light detection can be performed independently and with a stable and narrow-band beat frequency, and a tomographic image can be obtained with a high S / N ratio.

The method for generating a high-speed optical delay in optical coherence tomography according to the present invention splits light from a low-coherence light source into object light and reference light to be inspected by a half mirror for splitting into two , and a phase modulation element in this optical path And the reference light is reflected and returned from the optical delayed reflection mechanism by the rotating reflector, and is combined with the object light returning from the inspection object by the half mirror for division into two, and based on the phase modulation element. Heterodyne interference beat signal is detected. Therefore, by providing a phase modulation element independent from the rotating reflector, the bandwidth is narrowed, phase modulation is performed at a low frequency that can be detected with a high S / N ratio, and the resolution determined by the coherence length of the light source can be kept to a minimum. A beat signal can be detected at several KHz according to the sampling theorem.

Hereinafter, embodiments of the present invention will be described in detail.
[Example 1]
FIG. 1 is a schematic diagram of an optical coherence tomography apparatus showing a first embodiment of the present invention. In this figure, 1 is a broadband wavelength light source [for example, SLD (super luminescent diode) light source], 2 is a half mirror (half mirror for splitting), 3 is a rotating body, 4 is a rotating reflector (for example, a corner cube) Mirror), 5 is a fixed mirror, 5a is a position adjuster of the fixed mirror 5, 6 is an electrostrictive element (transducer) (phase modulation element), and 6a is a modulation frequency power source of the electrostrictive element. The modulation frequency power source 6a constitutes a phase modulation element that vibrates the fixed mirror 5 with an electrostrictive element (for example, a piezoelectric element) 6 and applies phase modulation to reflected light. 7 is an X-axis scanning mirror, 8 is a Y-axis scanning mirror, 9 is an objective lens, 10 is an object to be inspected, 11 is a photodetector, 12 is an electric filter and signal processor, and 13 is a computer display.

  A light wave from an SLD light source 1 having a broad wavelength range, which is a low-coherence light source, is split by a half mirror 2, one into object light to the object to be inspected 10 and the other into reference light. After passing through the rotary reflector 4, it is reflected by the phase modulation by the fixed mirror 5, returns on the same optical path, and returns to the half mirror 2. At this time, the reference light that returns to the reflection is also subjected to Doppler frequency modulation by the rotating reflector 4 at the same time. In this embodiment, the phase modulation element is a reflection type, and the electrostrictive element 6 is in close contact with the fixed mirror 5 in the reference optical path.

  On the other hand, the object light divided into two is incident on the inspection object 10 through the surface scanning mechanism (the X-axis scanning mirror 7 and the Y-axis scanning mirror 8) and the objective lens 9, and the object reflected light is reflected and returned along the same optical path. Return to half mirror 2.

  The optical delay reflection mechanism is composed of a rotating body 3, a rotating reflector 4, and a fixed mirror 5. The rotating reflector 4 is arranged so as to reflect light radially on the rotating body 3 and in the tangential direction of the rotating body 3. A plurality of them are arranged (only one is shown in the figure). Since the rotating reflector 4 rotates at high speed in the forward or backward direction with respect to the optical axis, a Doppler frequency shift toward purple or red is given to the reflected reference light.

  In this embodiment, an example in which an SLD is used for the broadband wavelength light source 1 has been described. However, it is obvious that a laser having a wide wavelength width and a short coherence length may be used as the light source.

  With the above configuration, the reference light is subjected to Doppler frequency shift simultaneously with the phase modulation. Since the phase modulation is a temporal shift with respect to the phase of the light wave, a well-known Bessel function expansion at the modulation frequency causes a frequency shift including the fundamental frequency and the frequency components of the harmonics. As a result, a plurality of beat signal frequencies including these frequencies are output from the photodetector 11 from the interference between the reference light and the object light combined by the half mirror 2.

  The Doppler beat signal frequency by the conventional rotating reflector is as high as several tens of MHz, has a bandwidth due to rotation, and has a wide band characteristic of several MHz due to uneven rotation and differences in the mounting angle of the reflector. Yes. For this reason, the S / N ratio inversely proportional to the bandwidth of the detection signal frequency is small, and the quality of the constituent image in the subsequent stage is degraded.

  In the present invention, as shown in this embodiment, a phase modulation element (consisting of a fixed mirror 5 and an electrostrictive element 6) independent from the rotating reflector 4 is provided to solve these problems. That is, it is possible to detect a beat signal at a few KHz according to a sampling theorem that can narrow the bandwidth and perform phase modulation at a low frequency that can be detected with a high signal-to-noise ratio and maintain the resolution determined by the coherence length of the light source to the minimum. It is a thing. From the output of a plurality of beat signal frequencies, only the fundamental frequency component of the phase modulation is extracted and amplified by the electric filter and the signal processor 12. As a result, it is possible to increase the signal-to-noise ratio by 100 to 1000 times for detecting Doppler beat signals of several tens of MHz.

Since it is only necessary to detect a fixed beat signal by driving a piezo vibrator or the like at a predetermined fixed frequency in this phase modulation element, an electric signal close to the audio frequency is not required, which does not require a wideband amplifier that reduces the S / N ratio. for narrow band amplification process, is characterized to be achieved also reduces the thermal noise in an electronic circuit or a transmission medium. Also, since the device is not high frequency, there is an advantage that it can be manufactured at low cost.

  Furthermore, even if the scanning speed in the optical axis (Z-axis) direction is variable depending on the rotational speed of the rotating body 3, the beat signal frequency to be detected is constant and unchanged, and the signal processing circuit is switched unlike the case of the Doppler frequency. There is a feature that does not need.

  With the above features, it is possible to easily obtain a clear image with a high S / N ratio, that is, with little background noise from the output as an image signal, and to visualize a tomographic image of the inspection object 10 with the computer display 13 or the like. it can.

A two-dimensional or three-dimensional tomographic image signal is a surface scanning mechanism in which a mirror is attached to a rocking part of a device that performs rocking motion at high speed and high accuracy, generally called a galvanometer, for X-axis and Y-axis, respectively. Used as (X-axis scanning mirror 7 and Y-axis scanning mirror 8), the object light is scanned on the XY plane, and the envelope distribution in the Z-axis direction of the beat signal is detected and acquired each time.
[Example 2]
FIG. 2 is a schematic diagram of an optical coherence tomography apparatus showing a second embodiment of the present invention. In this embodiment, a transparent piezo vibrator is disposed in the object optical path.

  In this figure, reference numeral 14 denotes a piezo vibrator (piezo piezoelectric element) made of a transparent crystal, and 6b denotes a drive electrode, which constitutes a phase modulator. The other components are the same as in FIG.

  Since the transparent piezoelectric piezo vibrator 14 is slightly changed in thickness by the application of an electric field, the transmitted light is subjected to phase modulation of the frequency of the AC driving electric field.

  In this embodiment, the object light is phase-modulated by passing through the piezo vibrator (transparent crystal body) 14 and is combined with the reference light. As in the embodiment of FIG. 11, a beat signal having a phase modulation frequency is obtained.

In the present embodiment, an example in which the transparent piezo vibrator 14 is provided in the object optical path is shown, but the same effect can be obtained even if the transparent piezo vibrator is provided in the reference optical path. Alternatively, a plurality of transparent piezo vibrators with different modulation frequencies may be provided in the reference optical path and the object optical path, respectively, and a beat signal having a frequency difference between the modulation frequencies may be detected. In general, the drive frequency of a transparent piezo vibrator may be a high frequency. When a low-frequency beat signal is detected to obtain a high S / N ratio and to perform signal processing, a plurality of such transparent piezo vibrators are used. One method is also used.
Example 3
FIG. 3 is a schematic diagram of an optical coherence tomography apparatus showing a third embodiment of the present invention. In this embodiment, a reflection type phase modulation element is arranged in the object optical path.

  In this figure, a piezo vibrator (piezo piezoelectric element) 6 is provided on the reflection surface of the delay fixed prism 15 in the object optical path, and phase modulation is performed at a predetermined frequency by a modulation frequency power source (drive power source) 6a of the electrostrictive element. Is.

  The object light subjected to the phase modulation is combined with the reference light, and the photodetector 11 can detect the heterodyne interference beat signal having the phase modulation frequency in the same manner as in the first and second embodiments. In FIG. 3, reference numerals 16 and 17 denote object light reflecting mirrors. The other components are the same as those in FIG.

  In addition, this invention is not limited to the said Example, A various deformation | transformation and combination are possible based on the meaning of this invention, These are not excluded from the scope of the invention.

  The high-speed optical delay generation method and the phase modulation optical coherence tomography apparatus in optical coherence tomography according to the present invention can be widely used in the medical field.

1 is a schematic diagram of a phase modulation optical coherence tomography apparatus showing a first embodiment of the present invention. FIG. 6 is a schematic diagram of a phase modulation optical coherence tomography apparatus in which a transparent piezo vibrator is disposed in an object optical path according to a second embodiment of the present invention. It is a schematic diagram of a phase modulation optical coherence tomography apparatus in which a reflection type phase modulation element is arranged in an object optical path showing a third embodiment of the present invention. It is a schematic diagram of the conventional optical coherence tomography apparatus by Doppler frequency shift.

Explanation of symbols

1 Broadband wavelength light source (SLD light source)
2 Half mirror 3 Rotating body 4 Rotating reflector 5 Fixed mirror 5a Fixed mirror position adjuster 6 Electrostrictive element (transducer) (phase modulation element)
6a Modulation frequency power source of electrostrictive element 6b Drive electrode 7 X-axis scanning mirror 8 Y-axis scanning mirror 9 Objective lens 10 Inspected object 11 Photo detector 12 Electric filter and signal processor 13 Computer display 14 Piezo consisting of transparent crystal Vibrator 15 Fixed prism for delay in object optical path 16, 17 Reflection mirror for object light

Claims (6)

  The light from the low-coherence light source is divided into the object light to be inspected and the reference light by a half-dividing half mirror into two parts, and a phase modulation element is provided in the optical path. Optical coherence characterized by detecting reflection of a heterodyne interference beat signal based on the phase modulation element by combining the object light returning from the object to be inspected and combining the object light returning from the inspected object by the half-dividing half mirror. Fast optical delay generation method in tomography. (A) a low coherence light source;
(B) a half mirror for splitting that splits light from the low-coherence light source into object light and reference light for the object to be inspected;
(C) a phase modulation element operating at a predetermined frequency provided in the middle of the optical path;
(D) an optical delay reflection mechanism for delay-reflecting the reference light by a rotating reflector;
(E) the half mirror for two divisions that combines the object light returning from the inspection object and the reference light returning from the optical delayed reflection mechanism;
(F) A phase modulation optical coherence tomography apparatus comprising: a photodetector that detects a heterodyne interference beat signal based on the phase modulation element. (A) a low coherence light source;
(B) a half mirror for splitting that splits light from the low-coherence light source into object light and reference light for the object to be inspected;
(C) a phase modulation element operating at a predetermined frequency provided in the middle of the optical path;
(D) an optical delay reflection mechanism for delay-reflecting the reference light by a rotating reflector;
(E) a surface scanning mechanism that scans an in-plane with the object light using an X-axis scanning mirror and a Y-axis scanning mirror;
(F) an objective lens;
(G) the half mirror for bisection for combining the object light returning from the inspection object and the reference light returning from the delayed optical reflection mechanism;
(H) A phase-modulated optical coherence tomography apparatus comprising: a photodetector that detects a heterodyne interference beat signal based on the phase modulation element.   4. The phase modulation optical coherence tomography apparatus according to claim 2, wherein the phase modulation element is a fixed mirror having an electrostrictive element in close contact, and the fixed mirror is provided in a reference optical path or an object optical path. Modulated optical coherence tomography device.   4. The phase modulation optical coherence tomography apparatus according to claim 2, wherein the phase modulation element is an electrostrictive element made of a light transmitting member, and the electrostrictive element is provided in a reference optical path or an object optical path. A phase modulation optical coherence tomography device.   4. The phase-modulated light coherence tomography apparatus according to claim 2, further comprising an electric filter for extracting only a constant frequency component based on the phase modulation element from the output from the photodetector. Coherence tomography device.

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