JPH0778536B2 - Ranging device - Google Patents

Description

The present invention relates to a distance measuring device, and more particularly to measuring a distance to an object to be measured by using a phase difference of light between a reference optical path and an optical path to the object to be measured. The present invention relates to a distance measuring device using an optical homodyne measurement method.

[Conventional technology]

Conventionally, there are a method of using a focus position detection mechanism of an optical disc as a means for measuring a distance to an object using light, and an optical homodyne measurement method of detecting an optical path difference from a reference optical path as a phase difference of light. There are several methods for detecting the focal position of an optical disc. For example, a detection mechanism using the astigmatism method collects light on the object plane by moving the objective lens,
A cylindrical lens is arranged in the optical path for converging the light reflected and returned. When the distance between the objective lens and the object is the reference length, the beam shape changes due to the change in the distance to place the 4-division photodetector at the position where the beam shape becomes a perfect circle. The relative distance can be measured. For details, refer to the magazine “IEE
IEEE Transactions on Consumer Electronic
s), Vol. 22, 1976, pp. 304-308, "Optical Readout of Videodis
c) ”. However, this method is not versatile because the measurement range is limited. On the other hand, the optical homodyne measurement method irradiates an object with light and causes the reflected light returned from the object to interfere with the light in the reference optical path. At this time, a beat signal is generated in the interference light by modulating the phase of the reference light or the wavelength of the light, and the distance is measured from the frequency. For details on how to modulate the wavelength of light, see, for example, the magazine "Applied Optics, Vol. 20, 1981, 400.
~ Paper on page "Absolute Distance Measurements by Variable W
avelength Interferometry) ”. This method is a highly accurate and versatile measuring method capable of measuring a distance of several tens of meters or more with an accuracy of several μm or less.

[Problems to be solved by the invention]

In the above-mentioned optical homodyne measurement method, it is necessary to make the intensities of the object light and the reference light equal on the interference surface, but in the conventional method, the light emitted from the monochromatic light source is divided by the half mirror, and It is common to put an ND filter in the reference optical path in order to make the intensity of the reference light uniform, and the output of the light was significantly lost.

[Means for solving problems]

The distance measuring device of the present invention includes a semiconductor laser, a signal generating unit that changes the wavelength of the laser, an optical system that shapes the beam emitted from the laser, and a linearly polarized component of the beam emitted from the optical system. To be a first beam and to reflect a polarization component orthogonal to the linear polarization component to a second beam.
Beam splitter for converting the first beam separated by the beam splitter, and a first phase shifter for changing the polarization state of the first beam from linearly polarized light to circularly polarized light. Child and the second
Second retarder for changing the polarization state of the beam of the beam from linearly polarized light to circularly polarized light, and the scattered light scattered by irradiating the measured object with the second beam and the first beam are caused to interfere with each other. It is characterized by comprising a photodetector for detecting an interference pattern and an arithmetic unit for processing a signal from the photodetector.

[Action / Principle]

FIG. 2 is a conceptual diagram showing the principle of the present invention, and FIG. 3 is a graph showing temporal changes in the oscillation frequency of laser light.

In the interferometer as shown in FIG. 2, the frequency-modulated light of the semiconductor laser 101 is split into two by the polarization beam splitter 102, and the reflected light from the object 106 and the reflection from the reference optical path reflected by the mirror 104. By measuring the beat frequency of interference with light,
The optical path difference between the object 106 and the reference optical path is obtained. The frequency-modulated laser light is expressed by the following equation.

E (t) = A ₀ exp [i {2π (V ₀ + ΔV (t)}] ...
(1) Here, A ₀ is the amplitude, V ₀ is the center frequency, and ΔV (t) is the frequency shift amount. The instantaneous frequency at this time is V (t) = V ₀ + ΔV (t) (2) Here, when the frequency of the laser light is modulated into a triangular wave shape as shown in FIG. 3, the amount of frequency change at a half cycle time T = 1 / 2f is constant. Let this amount be δ. On the other hand, the time difference between the signal lights is r = 2 (l ₂ -l ₁ ) / C (3), where l ₂ is the optical path length to the object surface and l ₁ is the reference optical path length. However, C is the speed of light. Photodetector at this time
The phase difference between the two optical paths on the plane 107 is Is. Therefore, the amount of phase change in the half cycle time of the interference intensity of the two light beams is Becomes Therefore, r ₂ is obtained by measuring the phase shift ΔΦ of the beat component, and l ₂ is obtained.

At this time, in order to increase the contrast of the interference fringes and increase the amplitude of the beat signal, it is necessary to equalize the intensities of the reflected light from the object and the reflected light from the reference optical path on the interference surface. However, in general, the reflected light from the reference optical path is significantly larger than the reflected light from the object, and conventionally, an ND filter was placed in the reference optical path to reduce the intensity of the reflected light. Therefore, in order to use the light effectively,
Reference light and object light are separated by 102. At this time, the polarization plane of the laser is slightly tilted, and the intensity of the scattered light from the object and the reference light are adjusted to be equal on the interference surface. For example, when the polarization plane of the light emitted from the laser 101 is almost parallel to the paper surface, most of the light is polarized beam splitter 10.
A small component that passes through 2 and is perpendicular to the paper surface is reflected.
The reflected light becomes circularly polarized light by the 1/4 wavelength plate 103, is reflected by the mirror 104, and then passes through the 1/4 wavelength plate 103 again to become polarized light parallel to the paper surface and transmitted through the polarization beam splitter 102. . On the other hand, the laser emission light transmitted through the polarization beam splitter 102 is circularly polarized by the 1/4 wavelength plate 105,
Irradiate 106. The scattered light from the object 106 is converted into polarized light perpendicular to the paper surface by the quarter-wave plate 105, reflected by the polarization beam splitter 102, condensed on the photodetector 107, and interferes with the reference light.

Since the reflected light from the object is 1/100 or less of the reference light, 99% or more of the light amount of the laser is wasted by the conventional method. However, the loss of light in the method of the present invention is a few% or less, and a light amount of 90% or more can be effectively used.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the distance measuring device of the present invention.
The light emitted from the laser 1 is collimated by the lens 2 and enters the polarization beam splitter 3. The reflected light split by the polarization beam splitter 3 becomes reference light by the mirror 5. At this time, the direction of linearly polarized light is rotated by 90 ° by transmitting the light through the ¼ wavelength plate 4 twice, and the reference light is transmitted through the polarization beam splitter 3. The other light transmitted through the polarization beam splitter 3 is focused on the object 8 by the lens 7. The scattered light from the object 8 is transmitted through the quarter-wave plate 6 twice so that the direction of the linearly polarized light is rotated by 90 °, is reflected by the polarization beam splitter 3, is condensed by the lens 9, and is then detected by the photodetector 10. It interferes with the reference light from the mirror 5.

The laser 1 is frequency-modulated as shown in FIG. 3 by a laser driving device 13 such as a function generator having a GP-IB interface, which is controlled by an arithmetic unit 12 such as a personal computer having a GP-IB interface. It The interference pattern of the reference light from the mirror 5 and the reflected light from the object 8 causes a waviness in proportion to the optical path difference. The frequency of this swell is measured by a frequency counter with GP-IB, for example.
It is possible to measure with 11 and to obtain the optical path difference with the arithmetic unit 12.

〔The invention's effect〕

As described above, the present invention provides a polarization beam splitter and
By using a quarter-wave plate to equalize the intensities of the reference light and the reflected light from the object, the light power of the light source can be effectively used.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the distance measuring device of the present invention, FIG. 2 is a conceptual diagram showing the principle of the present invention, and FIG. 3 is a graph showing temporal changes in the oscillation frequency of laser light. 1,101… Laser, 2,7,9… Lens, 3,102… Polarizing beam splitter, 4,6,103,105… 1/4 wavelength plate, 5… Mirror, 8,10
6 ... Object, 10, 107 ... Photodetector, 11 ... Frequency counter, 12
... arithmetic unit, 13 ... laser drive unit.

Claims (1)

[Claims] 1. A semiconductor laser, signal generating means for changing the wavelength of the laser, an optical system for shaping a beam emitted from the laser, and a polarization beam splitter for separating the beam emitted from the optical system. Changing the polarization state of the first beam from linear polarization to circular polarization when the beam reflected by the polarization beam splitter is the first beam and the transmitted beam is the second beam.
Phase shifter, a beam reflecting means for reflecting the first beam transmitted through the first phase shifter, and a second shifter for changing the polarization state of the second beam from linearly polarized light to circularly polarized light. Interference caused by interference between the phase beam, the first beam reflected by the reflecting means and transmitted through the first phase shifter, and the scattered light that is scattered by irradiating the object to be measured with the second beam. A distance measuring device comprising: an optical system for guiding a pattern to a photodetector; and a computing device for processing a signal from the photodetector and controlling the signal generating means, wherein the semiconductor laser is light reflected from an object. A distance measuring device characterized in that the polarization plane of the laser can be changed so that the intensity of the first beam having passed through the first retarder becomes equal on the interference plane.