JPH0213878A - Optical distance measuring device - Google Patents

Abstract

PURPOSE:To minimize the disturbance of a demodulated wave by reducing the effect of the scattering light from a midway route to at least 1/10 that of a conventional light modulation phase comparing method by bringing a transmitting wave to a light burst state. CONSTITUTION:A transmitted light wave is modulated into a pulse state by a mode lock apparatus 2 and divided at every definite time by an electro-optical switch and converted to higher harmonics by a light higher harmonics generating part 4 to be sent out from a transmission telescope. One unit of division at this time forms a light pulse burst state and this light pulse burst is reflected by a scheduled target reflecting mirror to be received by a receiving telescope 7. This operation can be performed by electrically measuring timing to open a gate. By this method, the jamming component scattered on the way of route and returned is blocked and only the reflected time from a target or in the vicinity thereof can be taken out.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light wave distance measuring device that measures the distance to a target using light waves.

[Conventional technology]

There are two conventional methods for measuring distance using light: In other words, there are two methods: one is to emit a light pulse, reflect it from a target object, directly measure the round trip time, and convert the round trip time into distance, and the other is to send out a continuous wave modulated by a sine wave and detect the reflected received wave. After that, there is a method of comparing the phase with the modulated wave (optical modulation phase comparison method). The former method has an average distance measurement accuracy, but the equipment is large and expensive, so it is not commonly used. On the other hand, the latter method usually has a distance measurement accuracy of about 10-6, which is sufficient for short distances but insufficient for long distances, and the measurement distance range is limited. Not suitable for distance. However, it is generally widely used because the device can be made small and relatively inexpensive.

Therefore, the optical modulation phase comparison method will be further explained. In the optical modulation phase comparison method, the phase of the modulated fundamental wave during transmission and the detected phase of the reflected received wave are compared, and the phase difference is calculated as φ.
Then, when the distance to be measured is less than half a wavelength (λ/2), the relationship is D−(λ/2)·(φ/2π) (1). Note that when the non-measurement distance is longer than half a wavelength, it is necessary to add only a certain integer multiple.

There are various ways to determine this integer, but the integer is usually determined by changing the modulation frequency, making similar phase measurements for two or more frequencies, and solving simultaneous equations from these results.

[Problem to be solved by the invention]

By the way, many errors occur in actual field measurements. In particular, in addition to the reflected waves from the target object, scattered waves from aerosols, fog, dust, etc. along the route, and scattered waves from objects on the ground mix in, and overlap with the original reflected waves, significantly distorting the waveform. , causing a large error in determining the phase difference. In fact, outdoors at a distance of 1k11 or more, unless the weather is very clear, the reflected waves from the target are significantly weakened by scattering and attenuation in the atmosphere, and even if there is a small amount of scattering in the atmosphere, the amplitude will be smaller than that. Furthermore, since the influence of dust is large at construction sites, etc., measurement accuracy can only be maintained over extremely short distances.

An object of the present invention is to solve these problems.

[Means to solve the problem]

In order to solve the above problems, the light wave ranging device of the present invention has the following features:
The light transmitted is a repeating pulse or sinusoidal burst.

[Effect]

By operating on the receiving side only at the pre-scheduled arrival time of the burst, it is possible to block interference components that are scattered along the route and come back.

〔Example〕

The figure is a block diagram showing one embodiment of the present invention.

First, the transmitting side will be explained. YAG laser oscillator 1
A mode locking device 2 is added to the .

The mode-locking device 2 is a so-called active mode-locking device in which a reflecting mirror is placed so as to have a required resonator length with respect to the YAG laser 1, and an electro-optical switch is also provided to forcibly apply phase synchronization. It is a means of realizing the law. By applying mode lock in this manner, the output laser light of the YAG laser 1 can be made into a pulse train, that is, can be modulated. The original modulation wave is preferably UH
A high frequency such as the F band is preferable, and this can be easily achieved using the mode-locking method. In the experiment, high distance resolution could be obtained by mode-locking the YAG laser 1 at 300 MHz.

Note that the electro-optical switch used in the mode-lock device 2 can maintain an accurate modulation frequency by multiplying the output signal of the high-precision crystal oscillator 12, which is the main oscillator.

The electro-optic switch 3 is a switch for creating an optical pulse burst from the mode-locked laser output. The burst time length and interval are appropriately selected depending on the distance a'I11 is to be determined and the like.

That is, the burst interval is generally chosen to be longer than the time it takes the light to travel to and from the target reflector, e.g.
It is preferable to set the time to about 100 μS for a measurement distance of 10 ka. However, the burst interval is not limited to this, and a shorter burst interval may be used if a timing gate (described later) on the receiving side is used. The burst time length is selected depending on timing reproduction accuracy, and is preferably about 10 μS, for example. This electro-optic switch 3 can also maintain accurate burst time length and interval by multiplying the output signal of the crystal oscillator 12, similar to the electro-optic switch in the mode-lock device rIt2. The optical harmonic generator 4 inputs the optical pulse burst obtained from the electro-optic switch 3 and outputs the optical pulse burst having a wavelength of 5320 m, which is a harmonic thereof, for example, a second harmonic. The optical pulse burst that has passed through the optical harmonic generator 4 is emitted by a transmitting telescope 5 to an object to be measured (target object), which is not shown.

Next, the receiving side will be explained. The receiving telescope 7 is coaxial or parallel to the transmitting telescope 5, and receives the optical pulse burst reflected by the reflecting mirror of the object to be measured.

The block 8 is composed of a photodetector and a narrowband amplifier, and in this block 8, the received optical pulse burst is photodetected, converted into a demodulated electric wave, and further narrowband amplified.

The injection locking device 9 connects the oscillator causing self-oscillation to
This is a type of amplifier that utilizes the phenomenon that when a frequency signal close to the oscillation frequency is input as an injection wave, the oscillation frequency is phase-locked to the injection wave. In this embodiment, a demodulated electric wave obtained from a received optical pulse burst is input as an injection wave, and a reference wave having the same phase as the demodulated electric wave is reproduced. By creating the demodulated reference wave in this way, it becomes possible to constantly compare the phase with the fundamental modulated wave in the phase comparator 10 described below.

The phase comparator 10 is a device that performs phase comparison of a demodulated reference wave with a modulated reference waveform in the LF band by beatdown. For phase comparison, higher accuracy can be obtained by beating down the demodulated reference wave to a lower frequency than by comparing the demodulated reference wave as it is, so this method is adopted in this embodiment. ing. According to experiments, 20KHz
By converting to the LF band, it was possible to perform ultra-high-precision phase comparison, and it was confirmed that the phase of the destination pulse burst was accurately preserved even in the LF band. The frequency required for beatdown is local oscillator 1.
It is supplied from 3.

The calculation means 11 is means for calculating the distance to the target object based on the phase difference obtained by the phase comparator 10 based on the above-mentioned equation (1), and is actually composed of a microcomputer or the like. The calculation results are displayed on the display means 14.

Next, a series of operations of this embodiment configured as described above will be explained. The transmitted light wave is modulated into pulses by the mode-locking device 2, divided into fixed time periods by the electro-optic switch 3, converted into harmonics by the optical harmonic generator 4, and transmitted from the transmitting telescope 5. One unit of the division at this time is in the form of a light pulse burst, and this light pulse burst is reflected by a scheduled target reflecting mirror and received by the receiving telescope 7. Since the time at which the optical pulse burst reaches the receiver can be scheduled in advance, the receiver is operated only at that time. This can be done by electrically timing and opening the gate. With this method, it is possible to block the disturbing components that are scattered on the path and return, and extract only the reflected pulse bursts from the target at or near a specific time. The thus obtained received optical pulse burst containing no interfering components is optically detected and narrowband amplified, and then regenerated as a received reference wave by the injection locking device 9.
0, the phase is constantly compared with the fundamental modulated wave.

The phase difference detected by the phase comparison device 10 is calculated by the calculation means 11
is converted into a distance and displayed on the display means 14.

In this embodiment, a transmission wave having one type of wavelength is used, but this does not preclude the use of two or more types of wavelengths to correct atmospheric temperature, pressure, etc.

Further, in this embodiment, the light is modulated in a pulse shape, but the light is not limited to this, and may be modulated in a sine wave shape, for example. Furthermore, in this example, an active mode-locking method is used to perform modulation at a high frequency.
The modulation method is not limited to this, and a passive mode-locking method or other modulation methods may be used.

〔Effect of the invention〕

As explained above, according to the light wave ranging device of the present invention,
Since the transmitted wave is in the form of an optical burst, the influence of scattered light from the intermediate path can be reduced by at least one method using the conventional optical modulation phase comparison method.
/10 or less, and disturbances in the demodulated waveform can be minimized.

Therefore, its measurement distance range can be significantly increased.

[Brief explanation of the drawing]

The figure is a block diagram showing one embodiment of the present invention. 1...YAG laser, 2...Mode-locking device, 3
... Electro-optical switch, 4... Optical harmonic generation section, 5
... Transmitting telescope, 7... Receiving telescope, 8... Optical detection and narrowband amplifier, 9... Injection locking device, 10.
... Phase comparator, 11... Calculating means, 12... Crystal oscillator, 13... Local oscillator, 14... Display means.

Claims (1)

[Scope of Claims] 1. In a light wave distance measuring device that uses light as a carrier wave and determines distance based on the phase difference between modulated signals of transmitted and received light, the transmitted light is a repetition of pulses or sinusoidal bursts. A light wave ranging device featuring: 2. The light wave ranging device according to claim 1, wherein the receiving section operates only at a timing when a burst is received at or near a specific time. 3. The optical distance measuring device according to claim 1, further comprising means for determining the phase of the basic reception period by reproducing the basic pulse or sine wave period from the reception burst signal. 4. The optical distance measuring device according to claim 1, wherein the optical burst is generated by mode-locking a laser oscillator and further intermittent operation using an optical switch. 5. Claim 1, further comprising means for calculating the distance to the object by comparing the phase difference between the phase reproduced from the received burst signal and the phase of the fundamental transmission period.
The light wave ranging device described. 6. The optical distance measuring device according to claim 5, wherein the phase comparison is performed after beatdown of the timing regenerated wave regenerated from the received burst signal. 7. The light wave distance measuring device according to claim 2, further comprising means for calculating the approximate distance to the object by shifting the reception gate timing.