JP2007144024A - Three-dimensional measurement endoscope using self-mixing laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the acquisition of a distance by a conventional endoscope by triangulation by irradiating an object with spotlight to measure the distance to the object poses a precision problem due to a short baseline length and that a self-mixing semiconductor laser can precisely measure even a short distance but has a difficulty in measuring at many points. <P>SOLUTION: The accuracy of the three-dimensional measurement of an object can be improved by solving the fewness of measurement points of the self-mixing semiconductor laser by performing distance measurement for the central portion of the range of observation by the self-mixing semiconductor laser and by performing the distance measurement for the peripheral portion by a pattern projection method. By reflecting the result of measurement by the self-mixing semiconductor laser in the central portion on the result of measurement by the pattern projection method, the result of measurement by the pattern projection method can be also corrected. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for measuring a distance to an object in an endoscope.

Conventionally, in order to measure the distance to an object in an endoscope, the object is irradiated with spot light, and the image is captured by the camera, based on the principle of triangulation from the position of the camera, the irradiator and the spot. The distance was obtained (see Patent Documents 1 and 2).
However, in this method, the base axis length between the camera and the irradiator cannot exceed the size of the distal end portion of the endoscope, and there is a problem in accuracy.
Further, a distance measurement technique that does not rely on triangulation has been considered (see Patent Documents 3 and 4 and Non-Patent Document 1). This is a technique for obtaining the distance from the change in oscillation frequency by self-interference (self-mixing) of the laser light irradiated to the object and the reflected light from the object, but requires a two-dimensional scanning means. The device was large.
Japanese Patent No. 2875832 JP-A-5-52531 JP 10-246782 A Japanese Patent Laid-Open No. 2-112784 S.Shinohara et al. "Compact and High-precision range finder with wide dynamic range and its application" IEEE Trans.Instrumrntation and Measurement.Vol.41, No.1, pp.40-44 (1992)

  As long as the triangulation method is used, it is difficult to overcome the short base length of the endoscope. Further, the distance measurement by the self-mixing laser has been performed with two-dimensional scanning by mechanical means, and therefore has a small number of measurement points and has low utility value.

The present invention increases practical value by combining the triangulation method and the self-mixing laser distance measurement method.
As a non-contact type distance measuring method, the triangulation method, the light section method, and the moire topography method have been used so far. On the other hand, the distance measurement method using a self-mixing laser is a technology for measuring the distance by frequency-modulating laser light with a continuous triangular wave, and the triangulation method or the like is a technology that sets a line.
In the present invention, a distance meter using a self-mixing laser diode (Self-Mixing Laser Diode: hereinafter referred to as “SM-LD”) that modulates the frequency of laser light with a continuous triangular wave is used in combination with the triangulation method.

The principle of the distance meter using SM-LD is already known, but the outline will be described below.
FIG. 1 shows the basic configuration of the SM-LD range velocimeter. Since the triangular wave voltage generated by the oscillator (1) is applied to the laser diode current source (2), the current applied to the laser diode (3) is modulated by the triangular wave. When the injection current to the diode increases, the refractive index in the resonator decreases and the temperature rises, and the frequency of the laser light decreases. The laser light intensity is also modulated. Light emitted from the frequency-modulated laser diode passes through the lens and is irradiated onto the target surface. A part of the light scattered on the target surface reversely follows the same path as the emitted light and returns to the laser diode resonator.
Then, the return light causes self-interference (self-mixing) with the light in the resonator, and the output waveform of the photodiode (4) incorporated in the laser diode package changes to the modulation triangular wave and the resonance state of the external resonator. The resulting step-like changes (mode hops) appear to overlap.

That is, as shown in FIG. 2, the internal resonator having a length L1 constituted by the internal reflection surfaces (11, 12) of the laser diode (3) and the reflection surface of the surface of the external object (6) ( 13) and an external resonator having a length L2 constituted by the reflection surface (12) of the laser surface are combined, and interference occurs between the modulation frequency and the resonance frequency of the composite resonator. Affects the oscillation intensity.
This step-like change (mode hop) is periodic at the rising and falling portions of the modulated triangular wave. The frequency (period) of the mode hop signal depends on the distance L between the laser diode and the target and the target velocity v in the optical axis direction of the laser diode. Therefore, if the mode hop frequency (period) is measured, the distance from the laser diode to the target and the speed of the target are determined. As described above, the SM-LD distance-velocity measuring optical system can be configured to be very compact by using only a laser diode package and a condenser lens with a built-in photodiode.

The self-mixing semiconductor laser irradiates an object with light emitted from the semiconductor laser that periodically changes the oscillation frequency and returns a part of the reflected light from the object to the semiconductor laser. The optical system to be coupled, a photoelectric conversion element that measures the output of the semiconductor laser, and an arithmetic circuit that obtains the distance from the change in output of the photoelectric conversion element to the object can be miniaturized.
Although the SM-LD distance velocity measurement optical system is small, only the distance to one irradiation point can be measured with the basic structure. Therefore, the distance of a plurality of points has been measured so far by scanning with a rotating mirror or a vibrating mirror. However, it is difficult to use such a configuration for a small device such as an endoscope.

FIG. 3 shows an example in which a CCD (22) and an SM-LD (8) as an image pickup device are provided at the distal end portion of the endoscope (20). The CCD (22) is provided with an imaging lens (21), and the SM-LD (8) is provided with a projection lens (5). Since a single SM-LD irradiates light only at one point, distance measurement is performed only at one point. Therefore, it is desirable here to use a plurality of two-dimensionally arranged SM-LD elements.
If a plurality of SM-LD elements are arranged, signals from each are processed to obtain distance information up to a plurality of points. There are a form provided at the distal end of the endoscope and a form provided at the intermediate part or the operation side and transmitted by an optical fiber. The arrangement of the SM-LD elements is also preferably a two-dimensional planar arrangement, but may be a one-dimensional array arrangement due to storage space constraints.

The image of the object (6) picked up by the CCD (22) is processed by the signal processing circuit (32) in the control device (30), and the display device (with the distance information obtained from the SM-LD (8)). 31). If the distance information is only one point, it is displayed as numerical information in a part of the display area. However, if the distance information is obtained at multiple points, three-dimensional display is possible.
In obtaining distance information of only one point, laser light is irradiated to a target point for which distance display is desired using one of a plurality of arranged SM-LD elements. When scanning by an actuator, which will be described later, displacement by the actuator is controlled and an arbitrary target point is irradiated with laser light. These can be considered as temporary suspension of the scanning operation.
On the other hand, for three-dimensional display, it is necessary to create a right-eye image and a left-eye image and use a stereoscopic display device as the display device (31). If simplified, contour lines are displayed, the contour lines at the reference position are displayed in white or yellow, contour lines far from the reference position are gradually blue, and contour lines near the reference position are gradually displayed in red. A display can be used.
Further, even if a single SM-LD element is small, if it is two-dimensionally arranged for three-dimensional display, it will become large, and it will be difficult to store it at the distal end of the endoscope (20). In this case, as shown in FIG. 4, it can be solved by optical transmission using an optical fiber (23).
The laser light from the SM-LD (8) is irradiated to the end face of the optical fiber (23) by the imaging lens (10). This light passes through the optical fiber (23) and is emitted from the other end face, but is adjusted by the projection lens (5) to focus on the surface of the object (6). Similarly, the reflected light from the object follows the reverse path and is fed back to the SM-LD (8).

If a two-dimensional arrangement is made using a plurality of SM-LD elements, the size increases and the cost is high.
As a solution, the irradiation point is displaced by a small actuator such as a piezo element to obtain distance information up to a plurality of points. For this purpose, there are a form in which the SM-LD element itself is displaced and a form in which the end face of the optical fiber that has transmitted light from the SM-LD element is displaced. An example of this is shown in FIG.
In FIG. 5A, the actuator (24) is coupled to the SM-LD (8) and displaced in the direction perpendicular to the optical axis. Thereby, the displacement of the irradiation point can be expected. FIG. 5B is an example when the optical fiber (23) is used for optical relay. Similar to FIG. 5A, the SM-LD (8) is displaced. FIG. 5 (c) is an example using the optical fiber (23) as in FIG. 5 (b), but here the end surface of the optical fiber (23) is not the SM-LD (8) but the optical axis by the actuator (24). It is moved in the orthogonal direction. It will be obvious to those skilled in the art that the end face of the optical fiber (23) on the object (6) side may be displaced as a modification of FIG. 5 (c).
Further, the same effect can be obtained by displacing the lenses (5, 10) in the direction orthogonal to the optical axis instead of the displacement of the end face of the SM-LD (8) or the optical fiber (23). FIG. 5D shows an example in which a plurality of SM-LD elements described above are arranged and light is emitted sequentially in a time-division manner to irradiate different positions of the object (6). Instead of time-division driving, an equivalent sequential light emission using an electronic shutter such as a liquid crystal shutter is also possible.

A dot pattern is projected over the entire observation range of the object. After capturing this image with an image sensor and performing pattern recognition processing, a distance is obtained for each corresponding point using a triangulation method. In parallel with this, a narrow range centering on the central portion (attention point) of the range that becomes the field of view is focused by the beam light from the SM-LD, and the distance is obtained.
In the current state of the art, the distance measurement by SM-LD is more accurate than that by triangulation because of the short baseline length in the endoscope.
Therefore, according to the present invention, the vicinity of the central portion can be measured with high accuracy, and the measurement point density can also be measured with high accuracy by limiting the scanning range. The distance measurement pattern is not limited to a dot pattern, and may be various patterns such as a cross pattern and a stripe pattern.

FIG. 6 shows the relationship between the observation range and the distance measurement range.
The range photographed by the CCD (22) that is the image sensor is shown as an observation range (26), which is the widest range. The range in which the dot pattern is projected by the pattern projection unit (25) is indicated by the pattern projection range (27), and is set slightly inside the observation range. The range in which the distance is measured by the SM-LD (8) is set at the center as the SM-LD measurement range (28). The measurement range is appropriately increased or decreased depending on the desired arrangement density of measurement points.

  If the information measured by the triangulation method is corrected by the distance information measured by the SM-LD, the measurement error by the triangulation method can be corrected. In this correction, if a measurement value at the same point is obtained by both, the difference is regarded as an error, and if there is no measurement value at the same point, it is calculated by interpolation, or an average value within a certain area is regarded as an error. It is also possible to place a flat plate at a known distance and calculate and store a correction value in advance for each distance and / or position in the screen. These corrections are basically made by the signal processing circuit (32) in the control device (30).

  In the present invention, by using an SM-LD (self-mixing semiconductor laser), the distance to the object can be measured without being affected by the baseline length as compared with the conventional measurement based on triangulation. In addition, surface scanning is possible by displacing a single SM-LD element (or an optical fiber end face or lens) in a direction perpendicular to the optical axis, thereby enabling three-dimensional image display (stereoscopic view).

Diagram showing the basic configuration of a self-mixing semiconductor laser (SM-LD) The figure which shows the internal resonance circuit and external resonance circuit in SM-LD The figure which shows the structure which provided SM-LD in the front-end | tip part of an endoscope The figure which shows the structure which provided SM-LD in the operation part of the endoscope The figure which shows the various structural examples which displace an irradiation point. (a) is an example in which the SM-LD is displaced by an actuator, (b) is an example in which an optical fiber is provided in the middle, (c) is an example in which the end face of the optical fiber is displaced by an actuator, and (d) is an array of SM-LD elements. Each of the examples is arranged in the shape and sequentially irradiated. Diagram showing measurement range

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Oscillator 2 Driver 3 Photodiode 4 Laser diode 5, 10, 21 Lens 6 Object 7 Measuring circuit 8 SM-LD (self-mixing semiconductor laser)
20 Endoscope 22 CCD
23 Optical fiber

Claims (3)

In the endoscope, a three-dimensional measurement endoscope provided with a distance measuring means by a pattern projection method and a distance measuring means by a self-mixing laser method. The three-dimensional measurement endoscope according to claim 1, wherein the distance measuring means by the self-mixing laser method measures intensively a central portion of an observation range. The three-dimensional measuring endoscope according to claim 1, further comprising a control device that corrects the distance information obtained by the pattern projection method based on the distance information obtained by the self-mixing laser method.

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