JP2016148539A - OCT device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical coherence tomography (OCT) device with which it is possible to perform measurement without using a reference interferometer in which the effect of the nonlinearity of the optical frequency sweep speed of a sweep light source is eliminated.SOLUTION: An OCT device comprises: a sweep light source 11 for outputting a light that is wavelength-swept at a prescribed optical frequency sweep speed; an optical coupler 12 for dividing an output light from the sweep light source 11 into a measurement light and a reference light; an optical circulator 13 for making the measurement light enter a measurement object 100 and a reflection mirror 15, and also combining a measurement light reflected by the measurement object 100 and a measurement light reflected by the reflection mirror 15; an optical coupler 16 for combining the measurement light combined by the optical circulator 13 and the reference light, and thereby outputting an interference light; and a reflectance distribution calculation unit 19 for extracting, from the spectrum of the interference light, a signal corresponding to the measurement light reflected by the reflection mirror 15 as a reference signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an OCT apparatus, and more particularly to an optical frequency sweep type OCT apparatus.

  2. Description of the Related Art Conventionally, an apparatus that captures a tomographic image of a retina or skin using optical coherence tomography (OCT) is known.

  FIG. 10 shows a basic configuration of a conventional OCT apparatus. The sweep light source 41 sweeps the wavelength of the output light so that the frequency of the light changes linearly with time. Part of the light from the sweep light source 41 is branched by the optical coupler 42 and enters the measurement interferometer 44. One of the lights branched by the optical coupler 43 enters the delay fiber 45, and the other enters the measurement object 100 via the lens 46 and is reflected.

  The light reflected by the measurement object 100 is incident on the measurement interferometer 44 again. In the optical coupler 47, interference light between the light incident on the measurement object 100 and reflected and the delayed light propagated through the delay fiber 45 is generated.

  The interference light from the measurement interferometer 44 is heterodyne detected by the light receiver 48a and sampled by the A / D converter 49a. When the optical frequency is swept linearly in the sweep light source 41, the frequency of the beat signal obtained by heterodyne detection is the light incident on the measurement object 100 and reflected, and the delayed light propagated through the delay fiber 45. Is proportional to the optical path difference.

  If the lens 46 is disposed at a position where the optical path difference becomes zero, the frequency of the beat signal is proportional to the distance from the lens 46 to the measurement object 100. Therefore, if the beat signal is Fourier-transformed and its spectrum is calculated, the reflectance distribution inside the measuring object 100 can be measured.

  However, the frequency of the light output from the actual sweep light source 41 is swept non-linearly or superimposed with modulation. As a result, the spectrum of the heterodyne signal output from the measurement interferometer 44 is blurred or has sidebands, which hinders highly accurate measurement.

  Therefore, a configuration has been proposed in which light from the sweep light source 41 is branched by the optical coupler 42 and input not only to the measurement interferometer 44 but also to the reference interferometer 51 (see, for example, Patent Document 1). The interference light from the reference interferometer 51 is heterodyne detected by the light receiver 48b and sampled by the A / D converter 49b. The influence of the non-linearity of the sweep by the sweep light source 41 appears not only in the measurement interferometer 44 but also in the reference interferometer 51. Therefore, if the controller 50 in the subsequent stage of the A / D converters 49a and 49b resamples the beat signal from the measurement interferometer 44 at the timing when the signal intensity from the reference interferometer 51 becomes 0, the sweep is performed. It is possible to obtain a signal from which fluctuations in the above are removed. This is called linearization.

JP 2012-154790 A

  However, in the conventional method of performing linearization using a reference interferometer, in addition to the photoreceiver and A / D converter for measurement interferometer, the photoreceiver and A / D converter for reference interferometer There is a problem that the apparatus becomes expensive.

  The present invention has been made in order to solve the above-described conventional problems, and performs measurement without the influence of nonlinearity of the optical frequency sweep speed of the sweep light source without using a reference interferometer. An object of the present invention is to provide an OCT apparatus capable of performing the above.

  In order to solve the above-mentioned problems, an OCT apparatus according to claim 1 of the present invention includes a light source that outputs light that has been wavelength-swept at a predetermined optical frequency sweep speed, output light from the light source, measurement light, and reference light. A light branching portion that branches into two, the measurement light incident on the measurement object and the reference reflection surface, the measurement light reflected by the measurement object, and the measurement light reflected by the reference reflection surface; A second optical multiplexing unit that outputs interference light by multiplexing the measurement light and the reference light multiplexed by the first optical multiplexing unit, and the interference A light receiving unit that receives light to generate a beat signal, an A / D conversion unit that converts the beat signal into a digital signal, and a reflection in the propagation direction of the measurement light on the measurement object based on the digital signal Reflectance distribution calculation to calculate the rate distribution The reflectance distribution calculation unit includes a first Fourier transform unit that Fourier-transforms the digital signal to calculate a spectrum of the interference light, and the spectrum reflected from the spectrum by the reference reflecting surface. A signal extraction unit that extracts a signal corresponding to the measurement light as a reference signal, an inverse Fourier transform unit that calculates a time waveform signal by performing an inverse Fourier transform on the reference signal, and the time waveform signal of the reference signal as a reference, A resampling unit for resampling the digital signal; and a second Fourier transform unit for calculating the reflectance distribution by Fourier transforming the digital signal resampled by the resampling unit. To do.

  In the OCT apparatus according to claim 2 of the present invention, the optical path length of the reference light output from the optical branching unit to the second optical multiplexing unit, and the measurement object output from the optical branching unit The optical path difference between the optical path length of the measurement light reflected by the optical path and reaching the second optical multiplexing section is output from the optical branching section and the optical path length of the reference light reaching the second optical multiplexing section, and the light The reference reflecting surface is disposed at a position that is shorter than an optical path difference from the optical path length of the measurement light that is output from the branching portion and reflected by the reference reflecting surface to reach the second optical multiplexing portion. To do.

  In the OCT apparatus according to claim 3 of the present invention, the optical path length of the reference light output from the optical branching unit to the second optical multiplexing unit, and the measurement object output from the optical branching unit The optical path difference between the optical path length of the measurement light reflected by the optical path and reaching the second optical multiplexing section is output from the optical branching section and the optical path length of the reference light reaching the second optical multiplexing section, and the light The reference reflecting surface is arranged at a position that is longer than an optical path difference from an optical path length of the measurement light that is output from the branching portion and reflected by the reference reflecting surface to reach the second optical multiplexing portion. To do.

  The present invention provides an OCT apparatus capable of performing measurement without the influence of nonlinearity of the optical frequency sweep speed of a swept light source without using a reference interferometer.

It is a figure which shows the structure of the OCT apparatus as the 1st Embodiment of this invention. It is a figure which shows the structure of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is FIG. (1) explaining the process of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is FIG. (2) explaining the process of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is a figure explaining the process which the signal extraction part in the OCT apparatus as the 1st Embodiment of this invention extracts the signal corresponding to the measurement light reflected by the measuring object. It is a flowchart explaining the process of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is a figure which shows the other structure of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is a flowchart explaining the other process of the reflectance distribution calculation part in the OCT apparatus as the 1st Embodiment of this invention. It is a figure which shows the structure of the OCT apparatus as the 2nd Embodiment of this invention. It is a figure which shows the structure of the conventional OCT apparatus.

  Hereinafter, embodiments of an OCT apparatus according to the present invention will be described with reference to the drawings.

(First embodiment)
As shown in FIG. 1, an OCT apparatus 1 according to a first embodiment of the present invention is an optical frequency sweep type OCT (Swept-Source OCT: SS-OCT), and includes a sweep light source 11 and an optical branching unit. An optical coupler 12, an optical circulator 13 as a first optical multiplexing unit, a measurement optical system 14, an optical coupler 16 as a second optical multiplexing unit, a balanced receiver 17 as a light receiving unit, an A / D converter 18, and a reflectance A distribution calculation unit 19 is provided.

  The sweep light source 11 is configured to output laser light that has been wavelength-swept in a prescribed sweep wavelength range and optical frequency sweep speed (or wavelength sweep speed). The oscillation wavelength is swept by, for example, changing the resonance wavelength by changing the angle of the diffraction grating or mirror in an external resonator laser using a diffraction grating.

  The optical coupler 12 branches the output light of the sweep light source 11 into two. One of the lights branched by the optical coupler 12 is input to the delay fiber 20 as reference light. The other of the lights branched by the optical coupler 12 is input as measurement light to the first terminal 13a of the optical circulator 13 and input to the measurement fiber 21 via the second terminal 13b.

  The measurement optical system 14 includes at least one lens 14a. The measurement optical system 14 condenses the measurement light output from the second terminal 13 b of the optical circulator 13 and emitted from the tip of the measurement fiber 21 on the measurement object 100, and the measurement light reflected by the measurement object 100. Is input to the second terminal 13 b of the optical circulator 13 through the measurement fiber 21.

  On the surface of the lens 14a, a reflection film 15 is formed as a reference reflection surface that reflects light in a wavelength band including at least the wavelength of the output light of the sweep light source 11. The measurement light reflected by the reflective film 15 is input to the second terminal 13 b of the optical circulator 13.

  The optical circulator 13 causes measurement light to enter the measurement object 100 and the reflection film 15 from the second terminal 13b, and combines the measurement light reflected by the measurement object 100 and the measurement light reflected by the reflection film 15. The wave is output from the third terminal 13c.

  Hereinafter, the optical path length of the reference light output from the optical coupler 12 and reaching the optical coupler 16 is also referred to as “reference optical path length”. In addition, the optical path length of the measurement light output from the optical coupler 12 and reflected by the measurement object 100 to reach the optical coupler 16 is also referred to as “measurement optical path length”. Further, the optical path length of the measurement light output from the optical coupler 12 and reflected by the reflecting film 15 to reach the optical coupler 16 is also referred to as “reference optical path length”.

  The reflective film 15 is disposed at a position where the optical path difference between the reference light optical path length and the measurement light optical path length is shorter than the optical path difference between the reference light optical path length and the reference light optical path length. FIG. 1 shows an example in which the position where the optical path difference from the reference light optical path length is zero is set on the back side from the surface of the measurement object 100 along the propagation direction of the measurement light.

  Alternatively, the reflective film 15 may be disposed at a position where the optical path difference between the reference light optical path length and the measurement light optical path length is longer than the optical path difference between the reference light optical path length and the reference light optical path length.

  The optical coupler 16 combines the reference light from the delay fiber 20 and the combined light of the measurement light output from the third terminal 13c of the optical circulator 13 (reflected light from the measurement object 100 and the reflective film 15). It is like that. Thereby, from the optical coupler 16, the object 100 is measured from the distance from the position where the optical path difference from the reference light optical path length becomes zero to the reflection film 15, and from the position where the optical path difference from the reference light optical path length becomes zero. Interference light having a frequency component corresponding to the distance up to is output.

  The delay fiber 20 and the measurement fiber 21 are made of, for example, a single mode fiber. In order to avoid a situation in which the reference light from the delay fiber 20 input to the optical coupler 16 and the measurement light from the measurement fiber 21 are orthogonal to each other and the interference signal cannot be obtained, the delay fiber 20 side or the measurement fiber 21 is used. A polarization controller may be inserted into at least one of the sides to adjust the polarization. Alternatively, polarization diversity reception that detects two orthogonal polarization components of reflected light from the measurement fiber 21 may be used. Alternatively, all or some of the fibers may be polarization maintaining fibers.

  Further, an optical isolator is inserted between the sweep light source 11 and the optical coupler 12 as necessary in order to prevent adverse effects caused by Rayleigh scattered light generated in the delay fiber 20 returning to the sweep light source 11 via the optical coupler 12. Also good.

  The phases of the interference light respectively output from the two output ports of the optical coupler 16 are opposite to each other. These two combined lights are input to the balanced receiver 17. The balanced receiver 17 includes, for example, two photodiodes (PD) and a subtracter. The PD outputs an electrical signal proportional to the intensity of input light.

  The balanced receiver 17 receives the two interference lights from the optical coupler 16 by two PDs, converts them into electric signals, subtracts the electric signals by a subtractor, and outputs the measurement light from the measurement fiber 21. A beat signal due to interference of the reference light from the delay fiber 20 is generated.

  By such balance reception, the amplitude of the beat signal due to interference is doubled, the in-phase noise included in the two combined lights is canceled, and the random noise is doubled in amplitude, so the signal-to-noise ratio Can be improved.

  The A / D converter 18 converts an analog electric signal (beat signal) output from the balanced receiver 17 into a digital signal.

  The reflectance distribution calculation unit 19 is composed of, for example, a microcomputer including a CPU, a ROM, a RAM, and the like, and calculates the reflectance distribution of the measurement object 100 in the propagation direction of the measurement light based on the digital signal of the beat signal. It has become. As shown in FIG. 2, the reflectance distribution calculation unit 19 executes a predetermined program, thereby causing a first Fourier transform unit 31, a signal extraction unit 32, an inverse Fourier transform unit 33, a resampling unit 34, and a second sampling unit. The Fourier transform unit 35 is configured by software.

  The first Fourier transform unit 31 performs discrete Fourier transform on the digital signal of the beat signal and calculates the spectrum of the interference light.

  The signal extraction unit 32 is a digital filter that functions as a low-pass filter, a high-pass filter, or a band-pass filter, and a signal corresponding to the measurement light reflected by the reflective film 15 from the spectrum calculated by the first Fourier transform unit 31. Is extracted as a reference signal.

  The inverse Fourier transform unit 33 calculates a time waveform signal by performing inverse discrete Fourier transform on the reference signal extracted by the signal extraction unit 32. This time waveform signal is equivalent to the signal of the reference interferometer in the conventional OCT.

The resampling unit 34 resamples the digital signal of the beat signal with reference to the time waveform signal of the reference signal. This resampling is performed, for example, at a time when the signal intensity of the time waveform signal of the reference signal becomes zero. Note that the resampling timing is not limited to this, and for example, a time point when the signal intensity of the time waveform signal of the reference signal takes a peak value may be used as a trigger. Further, it is possible to signal strength larger measurement bandwidth if trigger when the next signal strength and the time T _k to be zero is equally divided between the time T _{k + 1} becomes zero. FIG. 4 (c) shows an example divided into four equal parts. The timing of resampling by the resampling unit 34 and the value to be resampled are determined by a known interpolation method or the like.

  The second Fourier transform unit 35 performs discrete Fourier transform on the digital signal of the beat signal resampled by the resampling unit 34 to calculate a reflectance distribution in the measurement light propagation direction on the measurement object 100. Yes.

  Hereinafter, the operation of the reflectance distribution calculation unit 19 in the OCT apparatus 1 of the present embodiment will be described with reference to the flowcharts of FIGS. 3 to 5 and FIG. 6.

  First, the first Fourier transform unit 31 performs discrete Fourier transform on the digital signal of the beat signal output from the A / D converter 18 and reflects the reflected light from the measurement object 100 as shown in FIG. Then, the spectrum of the reflected light from the reflective film 15 is calculated (step S1).

  In OCT, a sweep in which the optical frequency changes linearly with time is ideal. However, the frequency of the laser light actually output from the sweep light source 11 may be swept nonlinearly or modulated. Due to this, the spectrum of the reflected light from the reflective film 15 is not a line spectrum but a broad shape and has sidebands.

  Next, the signal extraction unit 32 extracts a reference signal corresponding to the measurement light reflected by the reflection film 15 from the spectrum of the interference light in FIG. At this time, sidebands are also extracted as shown in FIG. 3B (step S2).

  Next, the inverse Fourier transform unit 33 performs inverse discrete Fourier transform on the extracted reference signal to calculate a time waveform signal of the reference signal as shown in FIG. 3C (step S3).

  Next, the resampling unit 34 resamples the beat signal output from the A / D converter 18 with reference to the time waveform signal of the reference signal as shown in FIG. 4A (step S4). FIG. 4A shows an example in which the beat signal is resampled at a frequency twice that of the reference signal, triggered by the time when the signal intensity of the reference signal becomes zero.

  Then, the second Fourier transform unit 35 performs discrete Fourier transform on the resampled beat signal, and removes the influence of the nonlinearity of the optical frequency sweep speed of the sweep light source 11 as shown in FIG. 4B. Is calculated (step S5). From this spectrum, the reflectance distribution in the propagation direction of the measurement light in the measurement object 100 and the reflective film 15 is obtained.

  In the process shown in the flowchart of FIG. 6, the beat signal output from the A / D converter 18 in step S4 is resampled. The present invention is not limited to this, and a signal corresponding to the measurement light reflected by the measurement object 100 as well as the reference signal is extracted from the spectrum of FIG. 3A as the measurement object signal shown in FIG. The target signal may be resampled. The configuration and flowchart of the reflectance distribution calculation unit 19 in that case are shown in FIGS.

  The processes in steps S11, S12, and S14 in the flowchart of FIG. 8 are the same as steps S1, S2, and S3 in the flowchart of FIG.

  The signal extraction unit 32 extracts a measurement target signal corresponding to the measurement light reflected by the measurement target 100 from the interference light spectrum of FIG. 3A as shown in FIG. 5 (step S13).

  The inverse Fourier transform unit 33 performs inverse discrete Fourier transform on the extracted measurement target signal to calculate a time waveform signal of the measurement target signal (step S15).

  Next, the resampling unit 34 resamples the time waveform signal of the measurement target signal output from the inverse Fourier transform unit 33 using the time waveform signal of the reference signal as a reference (step S16).

  Then, the second Fourier transform unit 35 performs discrete Fourier transform on the resampled measurement target signal, and calculates a spectrum of only the reflected light from the measurement target 100 (step S17).

  As a result, it is possible to further eliminate the influence of nonlinearity of the optical frequency sweep speed of the sweep light source 11 as compared with the case where the beat signal output from the A / D converter 18 is resampled.

  As described above, the OCT apparatus 1 according to the present embodiment uses the single light receiver to reflect light from the reflective film 15 for linearization (reference signal) in OCT (SS-OCT) using the sweep light source 11. Then, the reflected light (measurement target signal) from the measurement target 100 is received. The frequency of the reference signal and the signal to be measured is separated by a digital filter. The OCT apparatus 1 resamples and linearizes the beat signal or the measurement target signal based on the reference signal. Further, the OCT apparatus 1 performs discrete Fourier transform on the linearized beat signal or measurement target signal to obtain the reflectance distribution of the measurement target 100 from which the influence of the nonlinearity of the optical frequency sweep speed of the sweep light source 11 is removed. .

  As a result, since the OCT apparatus can be configured with one light receiver and one A / D converter without using a reference interferometer in the conventional OCT, simplification and cost reduction of the OCT apparatus can be realized.

  In the OCT apparatus 1 according to the present embodiment, the optical path difference between the reference optical path length and the measurement optical optical path length is shorter (or longer) than the optical path difference between the reference optical path length and the reference optical optical path length. A reflective film 15 is disposed. Thereby, the spectrum of the reflected light from the reflective film 15 and the spectrum of the reflected light from the measurement object 100 can be separated on the frequency axis, and the reference signal and the measurement target signal can be extracted separately.

(Second Embodiment)
Next, an OCT apparatus 2 as a second embodiment of the present invention will be described with reference to the drawings. Note that the description of the same configuration and operation as in the first embodiment will be omitted as appropriate.

  As shown in FIG. 9, the OCT apparatus 2 of the present embodiment branches measurement light output from the second terminal 13b of the optical circulator 13 through the measurement fiber 21a into two, and the measurement fiber 21b and An optical coupler 22 for inputting to the measurement fiber 21c is provided. The measurement fibers 21b and 21c can be detached from the optical coupler 22.

  Further, the optical coupler 22 includes measurement light emitted from the tip of the measurement fiber 21b and reflected by the measurement object 100, and measurement light reflected by the reflective film 15 formed at the tip of the measurement fiber 21c. The light is inputted and outputted to the second terminal 13 b of the optical circulator 13.

  In the OCT apparatus 2 of the present embodiment configured as described above, the measurement fibers 21b and 21c can be appropriately replaced with ones having appropriate lengths. For this reason, the optical path difference between the reference light optical path length and the measurement light optical path length can be easily adjusted. Similarly, the optical path difference between the reference light optical path length and the reference light optical path length can be easily adjusted. Thereby, the spectrum of the reflected light from the reflective film 15 and the spectrum of the reflected light from the measurement object 100 can be separated into arbitrary positions on the frequency axis.

1, 2 OCT device 11 Sweep light source 12 Optical coupler (optical branching unit)
13 Optical circulator (first optical multiplexing unit)
14 Measurement optical system 14a Lens 15 Reflective film (reference reflective surface)
16 Optical coupler (second optical multiplexer)
17 Balanced receiver (receiver)
18 A / D converter (A / D converter)
DESCRIPTION OF SYMBOLS 19 Reflectance distribution calculation part 20 Delay fiber 21,21a-21c Measurement fiber 22 Optical coupler 31 1st Fourier transform part 32 Signal extraction part 33 Inverse Fourier transform part 34 Resampling part 35 2nd Fourier transform part 100 Measurement object

Claims (3)

A light source (11) that outputs light swept in wavelength at a predetermined optical frequency sweep speed;
A light branching section (12) for branching the output light from the light source into measurement light and reference light;
The measurement light is incident on the measurement object (100) and the reference reflection surface (15), and the measurement light reflected by the measurement object and the measurement light reflected by the reference reflection surface are combined. A first optical multiplexing unit (13) to perform,
A second optical multiplexing unit (16) that outputs interference light by multiplexing the measurement light and the reference light combined in the first optical multiplexing unit;
A light receiving unit (17) that receives the interference light and generates a beat signal;
An A / D converter (18) for converting the beat signal into a digital signal;
A reflectance distribution calculation unit (19) for calculating a reflectance distribution in the propagation direction of the measurement light in the measurement object based on the digital signal,
The reflectance distribution calculation unit
A first Fourier transform unit (31) for Fourier transforming the digital signal to calculate a spectrum of the interference light;
A signal extraction unit (32) for extracting, as a reference signal, a signal corresponding to the measurement light reflected by the reference reflecting surface from the spectrum;
An inverse Fourier transform unit (33) for calculating a time waveform signal by performing an inverse Fourier transform on the reference signal;
A resampling unit (34) for resampling the digital signal with reference to the time waveform signal of the reference signal;
An OCT apparatus comprising: a second Fourier transform unit (35) that performs Fourier transform on the digital signal resampled by the resampling unit to calculate the reflectance distribution.   The optical path length of the reference light that is output from the optical branching unit and reaches the second optical multiplexing unit, and the measurement that is output from the optical branching unit and reflected by the measurement object to reach the second optical multiplexing unit An optical path difference from the optical path length of the light is output from the optical branching unit and reaches the second optical multiplexing unit, and is output from the optical branching unit and reflected by the reference reflecting surface. 2. The OCT apparatus according to claim 1, wherein the reference reflection surface is disposed at a position that is shorter than an optical path difference from an optical path length of the measurement light reaching the second optical multiplexing unit.   The optical path length of the reference light that is output from the optical branching unit and reaches the second optical multiplexing unit, and the measurement that is output from the optical branching unit and reflected by the measurement object to reach the second optical multiplexing unit An optical path difference from the optical path length of the light is output from the optical branching unit and reaches the second optical multiplexing unit, and is output from the optical branching unit and reflected by the reference reflecting surface. 2. The OCT apparatus according to claim 1, wherein the reference reflection surface is disposed at a position that is longer than an optical path difference with an optical path length of the measurement light reaching the second optical multiplexing unit.

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