JP2014115161A - Dynamic focal shift type optical interference tomographic microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dynamic focal shift type optical interference tomographic microscope which is high-resolution, high-speed and inexpensive and uses an optical interference tomographic method.SOLUTION: A laser light whose wavelength is swept enters a single-mode fiber 101. The incident light is divided by a fiber coupler 102, and one moves toward an object to be measured 120 while the other is reflected by a mirror 108 as a reference light. The light reflected in the object to be measured 120 passes through a lens 105, a variable-focal length lens 104 and a lens 103 and is reconnected to the single-mode fiber 101. At that time, an incident position of the single-mode fiber and a focal point within the object to be measured are confocal, and only a reflected light from a focal position out of all the reflected lights from the inside of the object to be measured 120 is reconnected to the single-mode fiber 101. The focal position can be changed with the use of the variable-focal length lens 104, and reflectance distribution in a depth direction can be obtained.

Description

  The present invention relates to a dynamic focal shift optical coherence tomography microscope using optical coherence tomography (OCT).

  Conventionally, optical coherence tomography (OCT) has been used in living body tomography in medical treatment, internal stress inspection and internal structure inspection in manufacturing industry.

  OCT can be broadly classified into two types: Time Domain OCT (TD-OCT) and Fourier Domain OCT (FD-OCT). In the former TD-OCT, a light source having a short coherence distance is divided into two light beams, one of which is signal light and the other is reference light. The reference light is reflected by the reference mirror, the signal light is reflected by the measurement object, the two lights are interfered, and the reflectance of the object is obtained by the magnitude of the interference signal. Here, since the coherence distance of the used light is short, an interference signal appears only when the optical path lengths of the reference light and the signal light are equal. By moving the reference mirror back and forth in the optical axis direction, it is possible to obtain reflectances of different depths of the object.

  Reflection by the object always occurs regardless of the position of the reference mirror, but reflected light that does not correspond to the position of the reference mirror is discarded without being used. Therefore, although the light utilization efficiency of TD-OCT is not high, the resolution in the depth direction is generally better than that of the latter FD-OCT. In TD-OCT, a method called in-focus OCT using a lens is used as a method of advancing high resolution, which is a feature, improving resolution in the in-plane direction normal to the depth direction, and increasing the depth of penetration. Has also been examined (Non-patent Document 1).

  In addition, FD-OCT, which is more sensitive than TD-OCT, is further classified into Spectral Domain OCT (SD-OCT) and Swept Source OCT (SS-OCT). SD-OCT is coherent like TD-OCT. Although a light source with a short distance is used, SS-OCT uses a light source with a long coherence distance and sweeps its center wavelength. FD-OCT is characterized by high sensitivity by the effective use of light, but it does not require a reference mirror movement mechanism like TD-OCT to acquire interference signals, so it can measure faster than TD-OCT. It is.

JP 2010-286730 A

Masami Omi, Akihiro Kakimoto, Megumi Nishigaki, Masamitsu Haruna, “In-Focus OCT Using the Condensing Effect of Irradiating Lenses”, Proc. 59-64, Dec. 2003

  However, in this in-focus OCT, it is difficult to move the focusing depth by the lens at high speed, the light use efficiency is low, and the reference mirror needs to be mechanically driven, so that the speed is increased. There is a problem that is difficult.

  In SS-OCT, when the interference signal received by the photodetector (PD) is subjected to Fourier transform, the frequency corresponds to the depth. Therefore, SS-OCT is suitable for high speed because there is no mechanically movable part. On the other hand, the load of electrical processing is large. Usually, the PD signal is AD-converted, and the Fourier transform is performed by a DSP or FPGA that performs fast Fourier transform (FFT). For example, if the wavelength sweep is performed at 2.5 μs and it is performed with 4096 points of sampling, the AD converter must have a sampling rate of 1.64 GSPS, including an FFT of 4096 sample points including data transfer. Must complete in 2.5 μs. These are values that can be realized at present, but they require several million yen and are expensive.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a high-resolution, high-speed, and low-cost dynamic focal shift optical coherence tomography microscope using optical coherence tomography. It is to provide.

  In order to solve the above-described problems, the present invention is an optical coherence tomographic microscope including a lens having a focal point in a measurement object, and outputs wavelength swept light in which the center frequency of light is swept in time. A light source, a confocal optical system that irradiates the focus position of the measurement object with the wavelength-swept light and selectively extracts reflected light from the focus position, and a variable focus that can move the focus position in the optical axis direction A lens, a photodetector for receiving reflected light from the focal position, and a frequency variable bandpass filter capable of changing a filter frequency in synchronization with the focal position.

  According to a second aspect of the present invention, in the dynamic focus shift optical coherence tomographic microscope according to the first aspect, the optical multi / demultiplexer and a mirror are further included, and the optical multi / demultiplexer includes the wavelength swept light. Is split into light that irradiates the object to be measured and light that irradiates the mirror, and the reflected light from the object to be measured and the mirror is combined and emitted to the photodetector. The optical path length from the demultiplexer is arranged to be equal to the optical path length between the optical multiplexer / demultiplexer and the focal position of the measurement object.

According to a third aspect of the present invention, in the dynamic focus shift optical coherence tomographic microscope according to the first aspect, the variable focus lens includes an electro-optic crystal K _1-y Li _y Ta _1-x Nb _x O ₃ . The variable focus lens used is included.

  According to a fourth aspect of the present invention, in the dynamic focus shift optical coherence tomographic microscope according to the first aspect, the optical multiplexer / demultiplexer is one of an optical waveguide type multiplexer / demultiplexer and a beam splitter. It is characterized by.

  According to a fifth aspect of the present invention, in the dynamic focus shift optical coherence tomographic microscope according to the first aspect, the frequency variable bandpass filter has the filter frequency that matches the beat frequency of the reflected light from the focal position. It is set to do.

  The present invention can realize a dynamic focal shift optical coherence tomography microscope using optical coherence tomography with high resolution, high speed, and low cost.

It is a figure which shows the optical system of the dynamic focus movement type | mold optical coherence tomographic microscope which concerns on one Embodiment of this invention. It is a figure which shows the structure of the frequency variable band pass filter connected to the photodetector. (A) is a figure which shows the time change of the signal strength output from the photodetector, (b) is a figure which shows the time change of the signal strength output from the band pass filter. It is a figure which shows the optical system of the dynamic focus movement type | mold optical coherence tomographic microscope which concerns on one Embodiment of this invention. It is a figure which shows the optical system of the dynamic focal movement type | mold optical coherence tomography microscope which concerns on one Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

  The present invention combines the advantages of in-focus OCT used in TD-OCT and SS-OCT without a mechanical drive part when acquiring depth information to acquire a high-speed and high-definition OCT image. Features.

(Embodiment 1)
FIG. 1 shows an optical system of a dynamic focus shift optical coherence tomographic microscope according to an embodiment of the present invention. In FIG. 1, only the part related to data acquisition in the depth direction is shown, and data acquisition in the in-plane direction with the depth direction as a normal line is not shown.

Laser light having a line width δλ is incident on the single mode fiber 101. It is assumed that the wavelength of this laser light is swept at a period T, a wavelength sweep width Δλ, and a center wavelength λ ₀ . The incident light is divided by the fiber coupler 102, one of which is directed to the device under test 120, and the other is reflected by the mirror 108 as reference light. The light directed to the object to be measured 120 is collimated by the lens 103 and then condensed inside the object to be measured 120 via the variable focus lens 104 and the lens 105 having a fixed focal length.

  The light reflected in the DUT 120 is recombined with the single mode fiber 101 via the lens 105, the variable focus lens 104, and the lens 103. At this time, the incident position of the single mode fiber and the focal point in the object to be measured are in a confocal relationship, and it is the reflection from the focal position that recombines with the single mode fiber 101 in the reflected light from the object to be measured 120. Only the light is reflected, and the reflected light from other than the focal position does not recombine with the single mode fiber 101. The focal position can be changed using the variable focus lens 104, and the reflectance distribution in the depth direction can be obtained.

  In the case of the measurement object 120 with small scattering and absorption, the same resolution as a normal confocal microscope can be achieved without using interference with the reference light. However, in the case of the object 120 to be measured that is highly scattered and absorbed, the intensity of stray light is large because the reflected light from the vicinity of the surface is strong, and the reflected light from the focal position is buried, making measurement impossible.

  Therefore, in order to extract signal light from stray light, interference with reference light is used. The reflected light from the DUT 120 and the reflected light from the mirror 108 are combined through the fiber coupler 102 and received by the photodetector 109.

  Signal processing after the photodetector 109 will be described with reference to FIGS.

  FIG. 2 shows the configuration of a frequency variable bandpass filter connected to the photodetector. The frequency variable bandpass filter 110 can extract only an arbitrary frequency component from the signal output from the photodetector 109 by adjusting the filter frequency. On the surface of the object 120 to be measured, the optical path length difference from the reference light is 0, and the beat frequency f of the reflected light from the depth L from the surface and the reference light is

It is represented by Here, n is the refractive index of the DUT 120. If the filter frequency of the frequency variable bandpass filter 110 is set to f given by the equation (1) when the focal position is L by the variable focus lens 104, only the signal from the focal position is obtained regardless of the stray light from the surface. It can be extracted as a beat signal. For example, as a typical example, the depth L = 1 mm from the surface of the object 120 to be measured, the refractive index n = 1.3, the wavelength sweep width Δλ = 100 nm of laser light, the period T = 5 μs, and the center wavelength λ _0. When = 1.3 μm, the beat frequency f = 30 MHz.

  Further, in practice, the amplification frequency and the lens power of the variable focus lens 104 are modulated in synchronization with the wavelength sweep of the wavelength swept light source. FIG. 3A shows a time change of the signal intensity output from the photodetector, and FIG. 3B shows a time change of the signal intensity output from the bandpass filter. When the signal from the photodetector 109 is a composite from a plurality of reflecting surfaces including stray light as shown in FIG. 3A, the signal whose filter frequency and focus position are modulated in synchronization with the wavelength sweep is shown in FIG. As in b), the depth information is obtained by converting it into time. By this method, it is possible to measure the reflectance at different depths of the object at high speed without an expensive high-speed AD converter or high-speed FFT circuit.

  Here, when a variable focus lens using a KTN crystal is used as the variable focus lens 104, high-speed focus movement is possible (see Patent Document 1).

  1 illustrates an optical system using the single mode fiber 101 and the fiber coupler 102. What is important in the present invention is a combination of a confocal optical system, interference between reference light and signal light, and a wavelength swept light source. The present invention can also be implemented in a configuration using other spatial optical systems.

(Embodiment 2)
FIG. 4 shows an optical system of a dynamic focus shift optical coherence tomographic microscope according to an embodiment of the present invention. The wavelength swept light is split into two by the beam splitter 201, one of which is reflected by the mirror 202 as reference light and proceeds to the photodetector 203. The other light beam is condensed in the pinhole 205 by the lens 204, collimated by the lens 206, and then condensed in the measured object 220 by the variable focus lens 207 and the lens 208. The reflected light passes through the lens 208, the variable focus lens 207, and the lens 206, passes through the pinhole 205, and proceeds to the photodetector 203 through the lens 204 and the beam splitter 201. The processes after the photodetector 203 are the same as those in the first embodiment.

  FIG. 5 shows an optical system of a dynamic focus shift optical coherence tomographic microscope according to one embodiment of the present invention. The wavelength swept light is linearly polarized light having a center wavelength of 1.3 μm, a sweep width of 100 nm, a sweep frequency of 200 kHz (duty 90%), and a beam diameter of 3 mmφ.

  The beam is split into two light beams by the deflecting beam splitter 301, and light traveling upward in the drawing is referred to as reference light, and light traveling in the right direction in the drawing is referred to as signal light.

  The reference light passes through the λ / 4 wavelength plate 302, is reflected by the mirror 303, and reaches the photodetector 304 again through the λ / 4 wavelength plate 302 and the deflecting beam splitter 301.

  The signal light passes through the λ / 4 wavelength plate 302, passes through the lens 305 having a focal length of 7.5 mm, passes through the pinhole 306 having a diameter of 7 μm, and is collimated by the lens 307 having the focal length of 7.5 mm. Four KTN variable focus lenses 308 are used, and a half-wave plate 309 is sandwiched between them. The four KTN variable focus lenses 308 and the half-wave plate 309 are voltage-controlled so that the focal length can be changed between ∞ and 20 cm as a whole.

  Next, the signal light is condensed in the measured object 320 via a lens 311 having a focal length of 20 mm via a mechanism whose direction is modulated by a movable mirror (galvanometer mirror) 310 driven at 1 kHz.

  The focal position variable width obtained by combining the KTN variable focal lens 308 and the lens 311 is about 2 mm. If the refractive index of the object 320 to be measured is 1.3, the focused spot size is 13 μm, so the in-plane resolution is about 10 μm. Since the depth of focus is 130 μm, the resolution in the depth direction is significantly degraded compared to the in-plane direction unless interference with the reference light is used.

  Since the beam period T = 4.5 μs at a duty of 90% at 200 kHz, if a depth resolution ΔL = 10 μm is required, the bandwidth of the frequency variable bandpass filter is required to be 340 kHz. Note that the wavelengths of the wavelength sweep, the variable focus lens, and the frequency variable bandpass filter are synchronized as in the first and second embodiments.

101 Single mode fiber 102 Fiber coupler 103, 105, 106, 107, 204, 206, 208, 305, 307, 311 Lens 104, 207 Variable focus lens 108, 202, 303 Mirror 109, 203, 304 Photo detector 120, 220, 320 DUT 110 Frequency variable bandpass filter 201, 301 Beam splitter 205, 306 Pinhole 302 λ / 4 wavelength plate 308 KTN variable focus lens 309 Half wavelength plate 310 Movable mirror

Claims (5)

An optical coherence tomography microscope having a lens having a focal point in a measurement object,
A light source that outputs wavelength-swept light in which the center frequency of light is swept in time;
A confocal optical system that irradiates the focus position of the measurement object with the wavelength swept light and selectively extracts reflected light from the focus position;
A variable focus lens capable of moving the focal position in the optical axis direction;
A photodetector for receiving reflected light from the focal position;
A variable frequency bandpass filter capable of changing a filter frequency in synchronization with the focal position;
A dynamic focal shift optical coherence tomographic microscope characterized by comprising: Optical multiplexer / demultiplexer,
Mirror,
The optical multiplexer / demultiplexer demultiplexes the wavelength swept light into light that irradiates the object to be measured and light that irradiates the mirror, and combines the reflected light from the object to be measured and the mirror. And the mirror is arranged such that the optical path length from the optical multiplexer / demultiplexer is equal to the optical path length between the optical multiplexer / demultiplexer and the focal position of the measurement object. The dynamic focus shift optical coherence tomographic microscope according to claim 1, wherein The dynamic focus shift optical coherence tomography according to claim 1, wherein the variable focus lens includes a variable focus lens using an electro-optic crystal K _1-y Li _y Ta _1-x Nb _x O _3. microscope.   2. The dynamic focus shift optical coherence tomography microscope according to claim 1, wherein the optical multiplexer / demultiplexer is one of an optical waveguide type multiplexer / demultiplexer and a beam splitter.   2. The dynamic focus shift optical coherence tomography microscope according to claim 1, wherein the variable frequency bandpass filter is set so that the filter frequency matches a beat frequency of reflected light from the focal position.

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