Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2002-08-14
2004-11-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S477000, C356S479000, C356S484000, C356S499000
Reexamination Certificate
active
06813030
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a technique for detecting back-scattering light from a scattering potential which has a scattering center at a micro object or the like located within, for example, a living body, which is a medium that strongly scatters light; obtaining information regarding a scattering position and information regarding reflection amplitude by use of interference measurement means which utilizes a phenomenon that coherence is present even in reflection light from an object that strongly scatters light and which utilizes the shortness of coherence length of low-coherence light; and obtaining single-dimensional or two-dimensional image data, or multidimensional image data such as three-dimensional image data, while scanning the interior of the object. More particularly, the present invention relates to an optical interference tomographic image observing apparatus which enables easy observation of a tomographic image of a light-scattering medium such as a living body by use of a remote device.
BACKGROUND ART
An attempt for obtaining a reflection tomographic image of a living body, which is a medium that strongly scatters light, starts from construction of an interferometer by use of low-coherence light (see Naohiro Tanno, “
Kogaku
” Vol. 28, No. 3, pp. 116-125 (1999)). A conventional technique will be described with reference to
FIGS. 1 and 2
.
FIG. 1
is a diagram showing the structure of a conventional light-wave reflection image measurement apparatus proposed by the present inventors.
In this light-wave reflection image measurement apparatus, a light beam from a low-coherence (also referred to as partial-coherence) light source
71
is introduced directly to a Michelson's interferometer in order to split the beam into two beams by means of a beam splitter
73
. One of the split beams, which is to be used as reference light, undergoes frequency shift. The frequency-shifted light beam is reflected by a movable reflection mirror
72
, which also serves to change a depth position within an object, and is caused to enter a photo detector
75
. The other light beam or transmission light is supplied to an object
74
to be measured as object irradiation light. The light is scatter-reflected by a layer of scattering objects located at a deep portion of the object
74
and having a different refraction index. The reflection light, serving as object reflection light, is mixed with the reference light by means of the beam splitter
73
so as to cause interference. As a result, a beat signal is detected by means of the photo detector
75
. While the positional relation between the illumination light and the object is changed in order to effect scanning, the detected electric signals are fed to a computer via a filter and an amplification/signal processing section, whereby the detected electric signals are stored in the computer. The thus-stored electric signals are converted to image data in order to obtain a reflection tomographic image.
FIG. 2
is a diagram showing the structure of a conventional tomographic-image observation apparatus which employs a structure on the basis of the above-described principle and in which optical fibers are disposed to form optical paths in order to cope with vibration and facilitate handling (see, for example, Japanese
Kohyo
(PCT) Patent Publication No. 6-511312).
As shown in
FIG. 2
, a light beam from a light source
81
propagates through a fiber
82
and enters a splitter/mixer circuit
86
. One light beam emitted from the splitter/mixer circuit
86
propagates through a fiber. Light coming out from the outgoing end of the fiber is converged by means of a convex lens
83
. As a result, the light is reflected by an object
84
, and the reflection light serves as object reflection light. After impartment of a frequency shift by means of a piezo-oscillation phase shifter
85
, the other light beam emitted from the splitter/mixer circuit
86
is reflected by a movable reflection mirror
80
, and the reflection light serves as reference light, which is mixed and is caused to interfere with the object reflection light by means of the splitter/mixer circuit
86
. The mixed light enters a photo detector
87
, whereby a reflection tomographic image can be observed in the same manner as described above.
Conventional interference measurement methods all utilize a movable reflection mirror for changing light reflection position (reference reflection position). In general, the movable reflection mirror is a reflection mirror attached to a linear actuator or a galvano-motor. Since the linear actuator moves an object back and forth via gears, the moving speed is as low as several mm/sec. In another method, a long fiber is wound around an electrostrictive element such as an element made of PZT, and the length of a reflection light path is changed through extension and contraction of the fiber.
DISCLOSURE OF THE INVENTION
Among the above-described conventional methods, the method employing a reflection mirror attached to a linear actuator or the like involves problems in that high-speed sweeping is difficult, and that when the mirror is moved back and forth periodically, linearity is deteriorated due to backlash and other causes.
Meanwhile, the method in which a long fiber is wound around an electrostrictive element such as an element made of PZT and the length of a reflection light path is changed through extension and contraction of the fiber involves a problem in that since a path for reference light becomes excessively long, the temperature varies, and the length of a path for object light must be increased.
Furthermore, the apparatus utilizing a linear actuator and the apparatus utilizing an electrostrictive element are both large in size, and fabrication of a compact, transportable apparatus including an interferometer is difficult.
Moreover, when sweeping speed is low, a very long time is needed to complete tomographic image measurement, which makes applying the tomographic image measurement to examination of a living body or a moving object difficult. In addition, when the path for reference light and the path for reflection light are made excessively long, optical signals attenuate excessively. In this case, the SN ratio of an obtained image decreases, which makes observation of a deep portion of an object difficult.
In order to solve the above-described problems, the present invention proposes a method and a specific apparatus which utilizes a rotating Littrow reflector prism and which can reflect a light beam in order to produce a delay reflection light beam or a progressive reflection light beam which travels toward the incoming direction of the light beam, even when the reflection point moves along a circumference of a rotary body upon rotation thereof or a surface of the prism facing the vertex thereof inclines. The method and the apparatus utilize the features of the prism such that the prism reflects a light beam toward the direction from which the light beam comes, and even when the incoming light beam inclines with respect to the surface facing the 90-degree vertex, the prism accurately reflects a light beam toward the incoming direction. Further, the present invention realizes a reliable, high-speed-scanning reflection mirror by attaching prisms on a small, high-speed motor, and opens to the road to a compact, simplified, transportable apparatus which can be used practically for optical interference tomographic image observation, which is an object of the present invention. Moreover, another object of the present invention is to provide an optical interference tomographic image observing apparatus which extracts reflection signals of wide dynamic range and high SN ratio through high-speed scanning in order to detect a static or dynamic structure of a deep portion of, for example, a living body and to produce a multidimensional image for observation.
In order to achieve the above objects. the present invention provides the following.
[1] An optical interference tomographic image observing apparatu
Brown Khaled
Font Frank G.
Japan Science and Technology Corporation
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