JP2013160699A - Optical tomographic image measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical tomographic image measuring device by which it becomes possible to acquire interference reflected light intensity distribution of a measuring object at high speed, etc.SOLUTION: An optical tomographic image measuring device includes a unit which is constituted of: first optical division means for dividing light emitted from a light source into reference light and signal light; irradiation means for irradiating a sample with the signal light; second optical division means for dividing the reference light into a plurality of reference light groups; third optical division means for dividing the signal light reflected or diffused from the sample into a plurality of signal light groups; and a plurality of detectors which detect an interference signal between one piece of reference light of the reference light groups and one piece of signal light of the signal light groups, in which the reference light and the signal light form the interference signal to be detected by the same detector, and difference between optical path length through which the reference signal passes from the first division means to the detector and optical path length through which the signal light passes from the first division means to the detector is different in each of the plurality of detectors.

Description

  The present invention relates to an optical tomographic image measurement apparatus, and in particular, measures light by dividing light from a light source into signal light and reference light, and optically interfering the signal light and reference light that have passed through the inspection object for detection processing. The present invention relates to an optical tomographic image measurement apparatus that acquires structural information in the depth direction from the surface of a target object.

  In recent years, Optical Coherence Tomography (OCT: Optical Coherence Tomography) is known as a means that can obtain the internal structure of a measurement sample in a non-destructive manner with high resolution. The OCT apparatus divides the light emitted from the light source into signal light and reference light, optically interferes with the signal light and the reference light that have passed through the object to be detected, and detects interference light. This is an optical tomographic image measurement apparatus that acquires structural information in the depth direction from the surface of a measurement target object.

  The above-described OCT measurement methods are roughly classified into two types: a time domain OCT (TD-OCT) measurement method and a Fourier Domain OCT (FD-OCT) measurement method.

  The TD-OCT measurement method is a method for acquiring an interference reflected light intensity distribution corresponding to a position in the depth direction of a measurement object by measuring the interference light intensity while changing the optical path length of the reference light. A typical patent of TD-OCT is shown in Patent Document 1.

  In the FD-OCT measurement, the interference signal between the signal light and the reference light that has passed through the measurement object is measured by decomposing the signal into a wavelength spectrum, and the measurement signal is Fourier-transformed to obtain a position in the depth direction of the measurement object. This is a method of acquiring a corresponding interference reflected light intensity distribution.

  There are two methods for FD-OCT measurement. One is the Spectrum Domain-OCT (SD-OCT) method. The interference light between the reflected or scattered light from the measurement sample and the reference light is dispersed with a spectroscope, and the interference signal is generated by using an array detector or the like. This is a method of detecting a spectrum and acquiring an interference reflected light intensity distribution.

  The other is a method called Swept Source-OCT (SS-OCT), which uses a light source that instantaneously oscillates a single wavelength and scans the oscillation wavelength temporally. In this method, the interference reflected light intensity distribution is acquired by Fourier-transforming the spectrum of the interference signal obtained by scanning the oscillation wavelength from the light source.

  Patent Document 2 is cited as an apparatus invention for performing surface profile measurement and optical cross-sectional image capturing by the SS-OCT method.

  Patent Document 3 is an invention of an optical measurement apparatus that measures the optical characteristics of a measurement sample using TD-OCT measurement. The present invention discloses an optical measurement apparatus that shortens the measurement time in TD-OCT measurement. The realization method includes photoelectric conversion by providing an optical multiplexer / demultiplexer that divides the reference light in an optical measurement device using light of a short coherent length and a reference light modulation mechanism that performs different modulation on each divided reference light. Information on multiple measurement points with different depths is included in the combined light of the reference light and measurement light incident on the detector, and the optical characteristics of these multiple measurement points are output from the output of the photoelectric converter by the computer. The optical measurement device is configured so that data is calculated.

JP-A-4-174345 JP 2007-24677 A Japanese Patent Laid-Open No. 10-267830

In OCT, it is required to measure a wide range in a short time. The measurement range in the optical axis direction of SS-OCT (the distance in the depth direction that can be measured by one wavelength scan) is proportional to the coherent length of the light source. The coherent length of a wavelength scanning laser depends on the wavelength shift that occurs during the data sampling time. The wavelength shift Δλ [nm] is determined by the data sampling time Δt [s] of the interference signal and the wavelength sweep speed V [nm / s], and is expressed by equation (1).
[Equation 1]
Δλ = Δt · V (1)

  The wavelength shift is reduced by reducing the data sampling time. As a result, the coherent length is increased and the measurement range can be widened. However, the data sampling time is determined by the time required for sampling in the analog / digital (AD) conversion circuit when converting the analog electric signal of the interference signal detected by the light receiver into a digital signal. Therefore, the data sampling time is finite, and there is a limit to expanding the measurement range in the optical axis direction determined from the data sampling time.

  By reducing the wavelength sweep speed, the wavelength shift can be reduced and the measurement range can be widened. However, if the wavelength sweep speed is decreased, the measurement time becomes longer. The measurement range in the optical axis direction and the wavelength sweep speed are in a trade-off relationship, and there is a problem in simultaneously reducing the measurement time and expanding the measurement range.

  Further, in order to further expand the measurement range that can be measured by one wavelength scan, it is necessary to move the position of the mirror of the reference light as in TD-OCT and perform measurement again. For this reason, since it is necessary to prepare a precise movement mechanism of the submicron order of the mirror for the measurement, a long time is required for the measurement due to the movement of the mirror.

  In the long-time measurement, the signal-to-noise ratio is deteriorated because the variation of the measurement target position during the measurement becomes large.

  In view of the above-described problems, the present invention does not require a reference light mirror moving mechanism, enables both shortening of the measurement time and expansion of the depth direction measurement range, and provides a highly sensitive optical tomography measurement apparatus. It is to provide.

  The optical tomography measuring apparatus according to the present invention has the following configuration.

  A first light splitting means for splitting the light emitted from the light source into reference light and signal light; an irradiating means for irradiating the sample with the signal light; and a second light splitting the reference light into a plurality of reference light groups. A light splitting means, a third light splitting means for splitting the signal light reflected or scattered from the sample into a plurality of signal light groups, one reference light of the reference light group and one signal of the signal light group A plurality of detectors for detecting an interference signal with light, and a reference light and a signal light forming an interference signal detected by the same detector, wherein the detection from the first dividing means A unit in which a difference between an optical path length through which the reference light has passed to the detector and an optical path length through which the signal light has passed from the first dividing means to the detector is different for each of a plurality of detectors. Optical tomographic image measuring device.

  According to the present invention, the measurement range in the depth direction by a single wavelength scan is expanded, so that the drive system of the reference mirror is not required and measurement can be performed in a short time. Also, the cost of the equipment will be low. In addition, since measurement can be performed for a short time, noise caused by movement of the measurement sample during measurement can be reduced, and optical tomographic measurement can be performed with high sensitivity and high accuracy.

1 shows an embodiment of an optical tomographic imaging apparatus according to the present invention. 1 shows an embodiment of an optical tomographic imaging apparatus according to the present invention (simultaneous measurement of different measurement positions). 1 is an example of an optical tomographic imaging apparatus according to the present invention (preventing light interference between units);

  The principle of the present invention will be described.

  The light emitted from the light generating means characterized by instantaneously oscillating a single wavelength and scanning the oscillation wavelength temporally is divided into signal light and reference light. Further, the reference light is divided into a plurality of reference lights (reference light group).

  The signal light is applied to the sample to be measured, and the signal light scattered or reflected from the sample is divided into a plurality of signal lights (signal light group). The reference light group and the signal light group are optical path bodies that give different optical path lengths for each reference light of the reference light group, or optical paths that give different optical path lengths for each signal light of the signal light group The signal light group that passes through the body and the signal light group that has passed through the sample is multiplexed by a multiplexing means. The interference light from the combining means is detected by the detecting means. The reflected light intensity distribution corresponding to the depth position of the sample is acquired from the Fourier transform of the wavelength spectrum obtained by detecting the interference signal.

  Interfering the reference light and the signal light that have passed through the optical path lengths having different lengths means that the presence or absence of cross-sections at different material positions in the depth direction of the sample is measured by the TD-OCT measurement technique. Scanning the transmission wavelength and detecting the light intensity of the interference light continuously measures the presence or absence of cross sections around the different sample positions in the different depth directions using the SS-OCT measurement technique. It corresponds to.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an optical tomographic image measurement apparatus according to an embodiment of the present invention.

  There is a light source 1 characterized by instantaneously oscillating a single wavelength and temporally scanning the oscillation wavelength. The light emitted from the light source 1 is divided into signal light LS1 and reference light LR1. It has a light splitting means 2.

  As the light source 1, for example, a wavelength scanning laser constituted by a semiconductor amplifier SOA (semiconductor optical amplifier), a resonator, and a wavelength selection filter can be used. As the wavelength selection filter, for example, a combination of a diffraction grating and a polygon mirror, a combination of a diffraction grating and a polygon mirror, a one using a Fabry-Perot filter, or the like is used. Further, as the light splitting means 2, for example, a fiber coupler may be used. More specifically, it is composed of a 1 × 2 optical fiber coupler, and the light from the light source can be divided into two.

  There is a light splitting means 5R for further splitting the reference light LR1 dispersed by the light splitting means 2 into a reference light group LR2 which is a plurality of reference lights. On the other hand, the signal light LS1 dispersed by the light splitting means 2 passes through the probe 6 for guiding the signal light LS1 to the sample 11, and further passes through the collimator lens 12 depending on the sample 11, and is irradiated on the sample 11. The

  The reflected or scattered light LS3 from the measurement sample due to the irradiation of the signal light LS2 from the probe 6 passes through the optical circulator 3 for guiding the reflected or scattered light LS3 to the light detection unit, and further passes through the optical circulator 3. The signal light LS4 passes through the light dividing means 5S that divides the signal light LS4 into a plurality of lights, and becomes the signal light group LS5.

  A fiber coupler can be used for the light dividing means 5R and 5S. More specifically, the light dividing means 5R is composed of, for example, a 1 × 3 optical fiber coupler, and divides one reference light into two or more, for example, three reference light groups. Similarly, the light dividing means 5R is composed of, for example, a 1 × 3 optical fiber coupler, and divides one reference light into two or more, for example, three reference light groups.

  Each reference light of the reference light group LR2 passes through the optical path bodies 17 having different optical path lengths, and is combined with each signal light of the signal light group LS5 passing through the sample 11 by the optical multiplexing means 7. A fiber coupler can be used as the optical multiplexing means. More specifically, for example, a 2 × 1 optical fiber coupler can be used as the optical multiplexing means 7 that combines the reference light and the signal light.

  The combined interference light group LC1 is detected by the light detection means 8D, and the A / D conversion circuit 9 generates a digital wavelength spectrum based on the analog signal obtained from the light detection means 8D.

  The wavelength spectrum is Fourier transformed by the computer 10 to calculate a reflected light intensity distribution corresponding to the depth position of the sample. The cross section included in the sample 11 can be measured by the reflected light intensity distribution.

  In the present embodiment, a combination of an SOA, a resonator, and a wavelength selection filter is used as the light source 1, but a combination of a light source having a broad spectrum and a wavelength variable filter may be used. For example, a supercontinuum light source can be used as the light source having a broad spectrum, and an FFP (fiber Fabry-Perot) filter can be used as the wavelength tunable filter.

  Furthermore, in this embodiment, the optical path body 17 that gives different optical path lengths to the optical path lengths of the respective reference lights of the reference light group LR2 gives different optical path lengths to the respective reference lights with respect to the optical paths of the reference light group LR2. The optical path lengths of 17 are different (for example, optical fibers having different lengths are used), and the signal light group LS5 passing through the sample 11 has the same optical path length. Here, the configuration is such that different optical path lengths are given to the optical path lengths of the respective reference lights in the reference light group LR2, but a different optical path length may be given to the optical path lengths of the respective signal lights in the signal light group LS5. In this case, for example, the optical path lengths of the reference light group LR2 may be the same, and the optical fibers 4 used for the signal light group LS5 may have different lengths in order to provide different optical path lengths for the signal light group LS5. .

  Essentially, it is not important that the length of the optical path of the reference light group LR2 is the same or the length of the signal light group LS5 is the same, but the optical path lengths of the reference light and the signal light that interfere with each other are equal. Or approximately equal. Then, by varying the optical path difference between the reference light and the signal light that interfere with each other, different depth regions of the sample 11 can be observed simultaneously.

  According to the present invention, the interference between the reference light and the signal light that have passed through the optical path lengths having different lengths is performed by measuring the presence / absence of a cross section at a different sample position in the depth direction of the sample, scanning the transmission wavelength, and performing interference. By detecting the light intensity of light, it is possible to continuously detect the presence or absence of cross sections around different sample positions in the different depth directions. Thereby, the measurement of the cross-sectional direction of the sample 11 can be investigated over a wide range simultaneously in a short time.

  FIG. 2 is a schematic configuration diagram of an optical tomographic image measuring apparatus configured to include a unit that divides the light from the light source 1 into a plurality of units, and is a form using a plurality of units 18 of the first embodiment.

  The light source 1 of Example 1 and the light splitting means 19 that splits the light emitted from the light source 1 into a plurality of lights. The light split by the light splitting means 19 corresponds to the light emitted from the light source 1 of the first embodiment, and is split by the light splitting means 2 into reference light LR1 and signal light LS1.

  If the signal light LS2 is irradiated to different measurement positions (two-dimensional positions other than the depth direction) of the sample, an optical tomographic image can be acquired at different measurement positions of the sample.

  For example, a 1 × 3 optical fiber coupler can be used as the light dividing means 19 for dividing the light from the light source 1 into a plurality of parts. There are two or more light splitting means.

  In FIG. 2, for convenience, there are a plurality of samples, but different positions of the same sample may be measured simultaneously, or different samples may be measured simultaneously.

  FIG. 3 is another schematic configuration diagram of the optical tomographic image measurement apparatus. In the third embodiment, a light splitting unit 19 that splits light from the light source 1 into a plurality of light paths, and optical path bodies 20 that give different optical path lengths between the light splitting unit 19 and each of the light splitting units 2 are provided. An optical tomographic image measurement apparatus system comprising: The configuration of each unit is the same as in the second embodiment.

  In the third embodiment, the optical path length between the light splitting means 19 that splits the light from the light source 1 into a plurality and the light splitting means that splits the split light into two of signal light and reference light is different. As the optical path body 20 that gives the optical path length, for example, fibers having different lengths are used. This makes it possible to distinguish the sample reflected light of a plurality of optical axes even when the measurement samples are close to each other or overlapped.

DESCRIPTION OF SYMBOLS 1 Light source 2, 5R, 5S, 19 Light splitting means 3 Optical circulator 4 Optical fiber 6 Probe 7 Optical multiplexing means 8D Optical detection means 9 A / D conversion circuit 10 Computer 11 Sample 12 Collimator lens 17, 20 Optical path body 18 Unit LC1 Interference Light group LR1 Reference light LR2 Reference light group LN Split light LS1, LS2, LS4 Signal light LS3 Reflected or scattered light LS5 Signal light group

Claims (9)

First light splitting means for splitting light emitted from the light source into reference light and signal light;
Irradiating means for irradiating the sample with the signal light;
Second light splitting means for splitting the reference light into a plurality of reference light groups;
Third light splitting means for splitting the signal light reflected or scattered from the sample into a plurality of signal light groups;
A plurality of detectors for detecting an interference signal between one reference light of the reference light group and one signal light of the signal light group;
Reference light and signal light forming an interference signal detected by the same detector, the optical path length through which the reference light has passed from the first splitting means to the detector, and the first splitting means An optical tomographic image measurement apparatus comprising a unit, wherein a difference from an optical path length through which the signal light has passed to the detector differs for each of a plurality of detectors. The optical tomographic image measurement apparatus according to claim 1.
An optical tomographic image measurement apparatus comprising a plurality of detectors for each pair of reference light of a reference light group and signal light of a signal light group. The optical tomographic image measurement apparatus according to claim 1.
An optical tomographic image measurement apparatus comprising: a light source that changes a wavelength of the emitted light with time. The optical tomographic image measurement apparatus according to claim 1.
An optical tomographic image measuring apparatus comprising optical combining means for combining reference light and signal light forming an interference signal detected by the same detector. The optical tomographic image measurement apparatus according to claim 1.
Reference light and signal light that form an interference signal detected by the same detector, the optical path length through which the reference light has passed from the second splitting means to the detector, and the third splitting means An optical tomographic image measurement apparatus, wherein a difference from an optical path length through which the signal light has passed to the detector differs for each of a plurality of detectors. A plurality of units of claim 1;
An optical tomographic image measurement apparatus comprising: a fourth light splitting unit that splits light emitted from a light source for each unit. The optical tomographic image measurement apparatus according to claim 6.
An optical tomographic image measurement apparatus comprising: a light source that changes a wavelength of the emitted light with time. The optical tomographic image measurement apparatus according to claim 6.
The optical tomographic image measuring apparatus according to claim 1, wherein the samples measured in each unit are different sample positions of the same sample. A plurality of units of claim 1;
Having a fourth light splitting means for splitting the light emitted from the light source for each unit;
An optical tomographic image measurement apparatus, wherein optical path lengths from the fourth light dividing means to the first light dividing means of each unit are different from each other.