JP3213250B2 - Optical measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical measurement apparatus for irradiating a subject with coherent light and measuring the light of the subject using light transmitted or reflected through the subject.

[0002]

2. Description of the Related Art Light, such as the human body and other living tissues, is exposed to light.
The biggest difficulty in optical measurement with a remarkable light scatterer as the subject
The points indicate transmitted light or reflected light emitted from the subject in all directions.
How the light along the traceable light path
To extract. How to make this possible
For example, an extremely short laser pulse (picosecond; 10
^-12 laser pulse for about sec)
The time profile of the emitted light
Light component that has passed through the shortest distance,
There is a known method for extracting an apparent transmitted straight light component.
(Eg, S. Anderson-Engele)
s, R.S. Berg, S.M. Svanberg, O .;
Jarman "Optics Letters"
vol. 15, 1179 (1990). ) Meanwhile, scattered
Focusing on the loss of directionality of scattered light, it is known as antenna characteristics.
Optical heterodyne detection method with excellent direction selectivity
The transmitted straight light component and the paraxial front
A method of detecting only the scattered light component has also been proposed (eg,
For example, M. Toida, M .; Kondo, T .; Ic
himura, H .; Inaba "Electron
ic Letters "vol. 26, 700 (19
90)).

[0003] Optical measurement based on the optical heterodyne detection method,
It has also been pointed out that, in principle, it has the characteristic of moving forward while being scattered and selectively detecting an optical signal emitted from the scatterer while partially retaining the time coherence of the incident light ( For example, KP Chan, M. Ya
mada, H .; Inaba "Applied Ph
ysics "B, vol. 63, 249 (199)
6)).

Further, in order to achieve high-speed and high-resolution optical measurement based on the optical heterodyne detection method, the introduction of a two-dimensional optical heterodyne detector array for two-dimensionally detecting transmitted light has been proposed (K. P. Chan, M. Yama
da, H .; Inaba "Electronics"
Letters, vol. 30, 1753 (199
4)).

[0005]

However, the optical heterodyne detection method requires coherence in the signal light,
Following each optical path, the paraxial forward scattered lights having different optical path lengths each have an indefinite (random) combined phase on the light receiver, and the intensity of the optical interference signal is also indefinite. This is well known as the laser speckle phenomenon (for example,
J. C. Dainty Edition "Laser Speckle"
And Related Phenomena "S
publisher-Verlag Publishing (New Yor
k, 1975)).

FIG. 5 to FIG. 7 are diagrams showing the time change of the signal intensity due to the laser speckle phenomenon. Figure 5, Figure 6,
FIG. 7 shows that a laser beam (wavelength: 1.06 μm) having a diameter of about 400 μm was incident on potato, lean beef, and chicken breast each having a thickness of 1 cm, and the light transmitted through each sample was subjected to a heterodyne method. The time course of the heterodyne signal strength at the time of detection is shown.

As can be seen from these results, the temporal change of speckles, that is, the period in which the heterodyne signal intensity changes, varies greatly from sample to sample. It is well known that the intensity distribution of an optical heterodyne signal due to the effect of speckle statistically follows a Rayleigh distribution.Speckle averaging, that is, time averaging of signal intensity, can effectively eliminate the effect of speckle. It has been known. However, for a sample such as the example in FIG. 7 where the speckles change to a statistically random situation for about one minute, averaging multiple independent speckles can be quite significant. It takes a long time.

An object of the present invention is to provide an optical measurement device capable of effectively eliminating the effect of speckle in a short time regardless of a sample.

[0009]

According to the present invention, there is provided an optical measuring apparatus which emits coherent light,
The object placement section where the object is placed, the coherent light emitted from the light source, the signal light passing through the object placement section, and the reference light passing through an optical path different from the optical path passing through the subject placement section. And the signal light after passing through the subject placement section and the reference light via different optical paths are superimposed on each other to generate interference light that interferes with the signal light and the reference light. When an optical system, a light receiving device for receiving the interference light generated by the interference optical system, and a signal light before reaching the object placement portion are arranged on the optical path of the signal light, and the inside of the signal light beam is divided into a plurality of regions. And a phase distribution modulator that sequentially changes the phase patterns of the plurality of divided regions with time, while making the phases of the divided regions different from each other.

Here, in the optical measuring apparatus of the present invention, the interference optical system typically expands the beam diameter of the signal light and guides the signal light to the phase distribution modulator. Further, the optical measurement device of the present invention has a plurality of light receiving elements arranged so as to independently receive each of the divided regions when the light receiver divides the inside of the beam of the interference light into a plurality of regions. It may be.

Further, the optical measuring device of the present invention is provided with an averaging unit for averaging the output of the photodetector while the phase distribution distribution modulator sequentially changes the phase pattern of the signal light. Or connected. The optical measurement device of the present invention includes the above phase distribution modulator and sequentially changes the phase pattern of the incident signal light. A random speckle pattern is obtained every time the phase pattern of the incident signal light is changed. Therefore, the speckle pattern can be changed every time the phase pattern of the incident light is changed, even in the case of the sample itself in which the change speed of the speckle is extremely slow, and the averaging can be performed at a high speed. Light measurement becomes possible.

[0012]

Embodiments of the present invention will be described below. FIG. 1 is a configuration diagram of a first embodiment of the optical measurement device of the present invention. The laser light 11a emitted from the laser light source 11 is split into two parts by a beam splitter 12 into a signal light 11b and a reference light 11c. The signal light 11b is applied to a beam expander 13 including two lenses 13a and 13b.
As a result, the beam diameter is expanded, and the beam enters the phase distribution modulator 14 driven by the drive circuit 141. As will be described in detail later, the phase distribution modulator 14 makes the phases of the respective divided regions different from each other when the beam of the signal light whose beam diameter is expanded by the beam expander 13 is divided into a plurality of regions. The phase patterns of the plurality of divided regions are sequentially changed in time. The signal light having passed through the phase distribution modulator 14 passes through the light scattering object 10 arranged in the object arrangement section 15 and enters the beam splitter 16.

On the other hand, the reference light 11c, which has been split into the signal light 11b by the beam splitter 12, is reflected by the mirror 18, frequency-shifted by the acousto-optic modulator (AOM) 19, and transmitted from the two lenses 20a, 20b. The beam diameter is expanded by the beam expander 20, and the light is further reflected by the mirror 21 and enters the beam splitter 16.

As described above, both the signal light and the reference light enter the beam splitter 16 and are superimposed on each other. I do. The light receiver 17 is provided with a plurality of light receiving elements 17a, 17b,..., 17n arranged two-dimensionally, and the plurality of light receiving elements 17a, 17b,.
, 17n, a plurality of beat signals representing the transmittance distribution of the subject 10 are generated. Each light receiving element 17a, 1
The beat signals obtained at 7b,..., 17n are signals having a frequency corresponding to the frequency shift amount by the AOM 19. A plurality of light receiving elements 17a, 17 constituting the light receiver 17
The plurality of beat signals obtained in b,..., 17n are input to the signal processing unit 22, and the signal processing unit 22 obtains the transmittance distribution of the subject 10 represented by the signal intensities of the plurality of beat signals. . In determining the transmittance distribution, a beat signal is fetched every time the phase pattern of the signal light is changed by the phase distribution modulator 14, and based on the beat signals of a plurality of phase patterns sequentially input in time, the beat signal is obtained. , A time-average transmittance distribution is obtained, whereby the effect of speckle is removed with high accuracy and at high speed. The display 23 includes the signal processing unit 2
An image representing the transmittance distribution obtained in step 2 is displayed.

FIG. 2 is a diagram showing a configuration example of the phase distribution modulator 14 shown by the block in FIG. Each is
For example, a plurality of mirrors 142 having an area of 1 mm × 1 mm
are arranged two-dimensionally, and on the back surface of each of the mirrors 142a, 142b, 142c,..., piezoelectric elements 143a, 143b, for driving each of the mirrors 142a, 142b, 142c,. 143c, ...
Has been fixed. The drive circuit 141 is connected to each piezoelectric element 143
When a voltage is applied to a, 143b, 143c,..., the thickness of the piezoelectric element to which the voltage is applied changes by an amount corresponding to the applied voltage, and the mirror to which the piezoelectric element is fixed moves minutely in the x direction. These mirrors 142a, 142b,
As shown in FIG. 1, signal lights having the same phase are incident on the mirrors 142c,... When the driving circuit 141 applies different voltages to the piezoelectric elements 143a, 143b, 143c,. 142a, 142b, 1
The signal light reflected at 42c,... Becomes a signal light having a phase distribution in the signal light beam, as schematically shown in FIG. The phase pattern changes each time the drive circuit 141 changes the voltage pattern applied to the piezoelectric elements 143a, 143b, 143c,.

As shown in experimental data, the signal light emitted from the light scattering object 10 shown in FIG. 1 is displaced by several mm with respect to the originally incident optical axis while maintaining a certain degree of coherence. There is paraxial forward scattered light that exits almost parallel to the optical axis, and such paraxial forward scattered light enters each other. Therefore, the phase of the incident signal light is changed to, for example, about 1 mm × 1 mm in the beam. In this case, a signal light with a completely different speckle pattern can be obtained.

Therefore, while the phase distribution modulator 14 changes the phase pattern of the signal light at high speed and variously, the signal processing unit 22 obtains the average transmittance distribution between them, so that the subject 10 itself can, for example, Even if only a slow change is shown as shown in FIG. 7, the influence of the speckle pattern that appears randomly can be effectively and rapidly removed.

The phase distribution modulator 14 shown in FIG. 2 is of a reflection type that reflects signal light. For example, an element whose refractive index changes according to an applied voltage is arranged, and the phase of the signal light transmitted therethrough is changed. A distribution may be given, and the phase distribution modulator according to the present invention is not limited to a specific configuration.
FIG. 3 is a diagram showing experimental results showing the effectiveness of providing a phase distribution modulator.

FIG. 3A shows an Nd: YAG laser (beam diameter: about 400 μm) having a wavelength of 1.06 μm incident on a 1 mm thick fillet, and transmitting the transmitted light to an 8 × 8 two-dimensional heterodyne detector. Array (the size of each light receiving element is 3
00 μm × 300 μm, the entire array size is 3 mm ×
3 mm). FIG. 3 (B)
It is the result of having carried out the same experiment with 7 mm thick chicken meat.

From these results, even in a medium where remarkable light scattering occurs, the paraxial forward scattered light propagated while being scattered in the vicinity of the incident optical axis has coherence. As a result, the incident light measured by the optical heterodyne detection method The beam image is an image wider than the incident beam diameter. Based on the above experimental results, we adopted a method of expanding the incident beam and dividing it into plane waves with many phase differences.
It is clear that the transmitted light components of each light wave interfere with each other on the observation plane if the width of the divided area of the plane wave is appropriately set. If the phase difference between the divided regions is randomly changed at high speed between 0 and 2π, it is possible to perform speckle averaging based on the speckle change following the phase difference.

As a prior example using a two-dimensional heterodyne array, for example, “KP Chan, DK Ki
llinger "Optics Letters"
vol. 16, 1219 (1991) "is also referred to as the first example of such a kind, but the experiment there is performed by combining the outputs of the respective array elements to generate a signal-to-noise ratio of an optical signal that causes a large spatial coherence loss on the propagating optical path. This is essentially different from the present invention.

FIG. 4 is a block diagram of a second embodiment of the optical measuring device of the present invention. The same components as those of the embodiment shown in FIG. 1 are denoted by the same reference numerals as those in FIG. Laser light 11a emitted from laser light source 11
After being converted into linearly polarized light by the polarizer 100, the polarized light is split by the polarizing beam splitter 12 into signal light 11b and reference light 11c, which are polarized in directions orthogonal to each other. Signal light 11
b denotes the beam expander 13 and the phase distribution modulator 1
After passing through 4, the light enters the polarizing prism 101. The polarizing prism 101 is arranged in a direction to transmit the signal light incident from the phase distribution modulator 14 side. The signal light that has passed through the polarizing prism 101 further passes through the λ / 4 plate 102, passes through the subject 10 disposed in the subject placement unit 15, is reflected by the mirror 103, and passes through the subject 10 again. Further, by passing through the λ / 4 plate 102 once again, the light enters the polarizing prism 101 again with its polarization direction rotated by 90 degrees as compared with the signal light on the outward path. The signal light that reenters the polarizing prism 101 is compared with the signal light on the outward path,
Since the polarization directions are different from each other by 90 degrees, the reflected light is reflected by the polarization prism 101 and is superposed by the beam splitter 104 in intensity on the reference light 11c having the same polarization direction.

On the other hand, the reference light 11c, which has been split into the signal light 11b by the polarizing beam splitter 12, is reflected by the mirror 18, frequency-shifted by the AOM 19, and transmitted by the beam expander 20 comprising two lenses 20a and 20b. The beam diameter is enlarged, and the beam is split by the beam splitter 104 in intensity with the signal light. As described above, the signal light is λ /
By transmitting through the four plates 102 twice, the polarization direction becomes 9
The light is rotated by 0 degrees, and becomes light having the same polarization direction as the reference light. Therefore, the interference light in which the signal light and the reference light interfere with each other is incident on the light receiver 17.

In the present embodiment, the phase distribution modulator 14 driven by the driving circuit 141 generates signal light whose phase space distribution is changed at various high speeds, and the time during which the phase distribution is changed variously. An average transmittance distribution is determined, and the effect of speckle is removed. In the present embodiment, since the signal light has transmitted through the subject 10 twice, the subject 1
It is possible to obtain an image in which the transmittance distribution of 0 appears as a more clear signal intensity distribution.

[0025]

As described above, according to the present invention,
Statistically random speckle patterns can be forcibly changed at high speed, noise caused by speckle patterns can be effectively and quickly removed, and high-speed optical measurement can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of an optical measurement device of the present invention.

FIG. 2 is a diagram showing an example of a configuration of a phase distribution modulator shown by a block in FIG.

FIG. 3 is a diagram showing experimental results showing the effectiveness of providing a phase distribution modulator.

FIG. 4 is a configuration diagram of a second embodiment of the optical measurement device of the present invention.

FIG. 5 is a diagram showing a time change of a signal intensity due to a laser speckle phenomenon.

FIG. 6 is a diagram showing a time change of signal intensity due to a laser speckle phenomenon.

FIG. 7 is a diagram showing a time change of signal intensity due to a laser speckle phenomenon.

[Explanation of symbols]

 Reference Signs List 10 light scattering object 11 laser light source 11a laser light 11b signal light 11c reference light 12 beam splitter 13 beam expander 14 phase distribution modulator 15 object placement unit 16 beam splitter 17 light receiver 17a, 17b,..., 17n light receiving element 18 Mirror 19 AOM 20 beam expander 22 signal processing unit 23 display 100 polarizer 101 polarizing prism 102 λ / 4 plate 103 mirror 141 drive circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01J 9/00-9/04 G01N 21/00-21/61 G01B 9/00-9/10

Claims (1)

(57) [Claims] 1. A light source that emits coherent light, an object placement section on which an object is placed, and coherent light emitted from the light source are split into two to increase a beam diameter passing through the object placement section. The signal light that has passed through an optical path different from the optical path that passes through the subject placement section , the beam diameter has been enlarged, and the signal light is relatively
A reference light having a different frequency is generated , and the signal light after passing through the object placement section and the reference light having passed through different optical paths are superimposed on each other, whereby the signal light and the reference light interfere with each other. An interference optical system that generates the interfering light , and a plurality of beams within the beam of the interference light generated by the interference optical system .
Independently receive each divided area when divided into areas
To, a photodetector to have a plurality of light receiving elements arranged, the is disposed on the optical path of the signal light before reaching the subject placement unit, when dividing the inside of the beam of the signal light into a plurality of regions
Of the phase distribution modulator to temporally sequentially changing the phase pattern of these plurality of divided regions, the phase pattern of the signal light by the phase distribution modulator sequentially
Averaging to average the output of the receiver while changing
Optical measuring device is characterized in that a part.