JPH032583A - Method and instrument for optical length measurement - Google Patents

Abstract

PURPOSE:To measure the length of an optical path by measuring the required for the propagation of light with different wavelength and performing arithmetic operation according to a specific expression based upon meteorological observation data. CONSTITUTION:A YAG laser 5 emits basic light and a mode lock device 6 converts the output light of the laser 5 into a pulse train with a synchronizing signal generated by an original oscillation part 3 ad supplies the pulse train to an electrooptic switch 7 to generate a light pulse burst. Further, a light higher harmonic generation part 8 outputs the light pulse burst of the 2nd higher harmonic and an optical transmitting telescope 9 mixes and sends the two light pulse bursts to a reflection part 4 at a target point. Then a photodetecting arithmetic part 2 receives reflected light by a receiving telescope 10, and detects and modulates the pulse bursts of the basic light and higher harmonic light, and a phase comparing device 13 compares their phase differences from a synchronizing signal. An arithmetic unit 15 calculates the times t1 and t2 for the propagation of the basic light and higher harmonic light from a reference point to the target point, prescribes the refracting powers N1 and N2 of the wavelength light beams by equations I according to dry air density rhos and steam density rhow measured by a meteorological observation device 14 to calculate the length D of the optical path from an equation II.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical length measurement method and apparatus for determining the length of an optical path from a base point to a target point using light waves.

[Conventional technology]

Conventionally, methods and devices for determining the length of an optical path by measuring the propagation time of a light wave have been widely used.

[Problem to be solved by the invention]

The conventionally used 41 length method and device are 1
Most of them used light waves of different wavelengths. That is,
The propagation time t of the light wave, the temperature T1 of the optical path, the atmospheric pressure P, the humidity RH, etc. are measured, and the refractive power N (T) of the optical path is measured.

P, RHL...), and the length of the optical path D is calculated as D= i
t/(N+1)] ・C. However, C is the speed of light in vacuum. Therefore, in order to accurately measure the dIII length, it is essential to accurately know the weather of the optical path, ie, the temperature, pressure, humidity, etc. Accurate weather measurements can be made in a laboratory or underground tunnel where the environmental conditions are sufficiently controlled, or over short distances, so conventional methods can provide sufficient length measurement accuracy and pose no particular problem.

However, when measuring several kilometers to several tens of kilometers in the horizontal direction in the atmosphere, as is often required in surveying, a major problem arises because the average refractive power of the optical path cannot be easily estimated. Therefore, it is naturally difficult to obtain all lengths with high accuracy. Some attempts have been made to collect measurement data by placing many meteorological observation instruments in the optical path and estimate a more accurate average refractive power of the optical path, thereby performing highly accurate length measurements under such conditions. Not only would the equipment be large-scale, but new challenges would arise, such as how to calibrate each meteorological observation instrument, so high accuracy could not be expected.

On the other hand, the refractive power N of an optical path filled with n types of nonpolar substances
If the density of the first 116 substance is ρ1 and the wavelength-dependent coefficient is R3, then by using the following relationship, and measuring the propagation time of the optical path using light waves of multiple wavelengths, Weather perspective 1fl
There are also attempts to make ll itself unnecessary.

In the method using two light waves with different wavelengths, the time required for the optical path of wavelength 2 to propagate through the measurement optical path is 1. 1
, and when the dry air density of the optical path is ρ, and the refractive powers of wavelength 1 and wavelength 2 are N and N2, we define the relationship N1 = α1 °ρ3 N − α2 °ρ8, and the length D of the optical path is D= [t →-a (tl-tl)/(α
−α 2) Calculated from C・C■. Since this method does not take into account the water vapor density, or humidity, in the optical path, it is extremely accurate.
I couldn't have expected it to be that long.

In the method using three light waves with different wavelengths, the times t, t2, and t3 used for the light waves of wavelengths 1, 2, and 3 to propagate through the measurement optical path are measured, and the dry air density in the optical path is expressed as ρ, The water vapor density is ρ, the wavelength S
W1, wavelength 2
, when the refractive powers at wavelength 3 are N, N, and N, N −α ・ρ + β ・R7 11S I N −α ・ρ 10β ・β1 2 2 s 2 N −α ・ρ + β ・ρ 3 3 S 3 , and the length D of the optical path is defined as D = [t + ((α ・ β − α
・β )(t −t )l/+(α2−α
1) (β - β ) - (α3 - α1) (β - β )l + + (α ・β1 - α1 ・β
) (t3-tl))/((α2α) (β
-β)-(α3-αl)(R2-β1))] - Determined from C. Although this method is excellent in theory, it requires the development of new technologies such as light sources, light transmission control devices, light reception detection devices, etc., and it is currently difficult to realize effective devices. be.

The present invention aims to solve the above problems.

[Means and effects for solving the problem] Therefore, in the present invention, the dry air density in the optical path is ρ, the water vapor density is ρ,
When the refractive power at wavelength] and wavelength 2 are N1 and N2, we define the following relationship: N = α ・ρ + β1 ・ρ1 1 1 s α 〉〉β
(ρ /ρ ) 1 1 w sα 〉〉
Focusing on the relationship β ・ (ρ / ρ ) 2 2 w s , D−[t + ((α + β ・ ρ
/ρ)1], 1.
w s(t 2 t l ) l / ((α
1+β1・ρ/ρ )−(α 1β・ρ/v
s 2 2 vρ ))] ・C A formula for calculating the length of the optical path was derived, making it possible to easily perform more accurate length measurements with a lower meteorological measurement accuracy than conventional methods and devices. It is characterized by

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram of an embodiment in which a sub-length device is constructed using the optical length measurement method according to the present invention.

The length measuring device shown in Fig. 1 includes a light transmitting section 1 that transmits light waves toward a reflecting section 4 at a target point, and a light receiving section that receives reflected light from the reflecting section 4 and performs length measurement calculation, processing, and display. Arithmetic unit 2;
It includes an original oscillation section 3 that supplies a synchronizing signal to the light transmitting section 1 and the light receiving computing section 2, and a reflecting section 4 that sends back the light waves sent from the light transmitting section 1 to the light receiving computing section 2. It is assumed that the light transmitting section 1, the light receiving section 2, and the original oscillating section 3 are installed at a reference point, and the reflecting section 4 is installed at a target point.

The light transmitting unit 1 includes a YAG laser 5 that generates fundamental light, and a Y, AG laser 5 based on a synchronization signal output from the original oscillation unit 3.
a mode-locking device 6 that converts the output light of into a pulse train;
An electro-optical switch 7 that outputs an optical pulse burst from the output pulse train of the mode-locking device 6, and an optical harmonic generator that manually outputs the optical pulse burst of the electro-optical switch to output its second harmonic, that is, an optical pulse burst of wavelength 5B2nII1. The output light pulse burst of the electro-optical switch 7 and the optical harmonic generator 8 is mixed with the reflector 4 at the target point.
It consists of a transmitting telescope 9 that transmits light to.

The light receiving calculation section 2 includes a light receiving telescope 10 that receives the optical pulse burst reflected by the reflecting section 4 at the target point, and a light receiving telescope 10.
A main optical detection section 11, an optical harmonic detection section 12, which detect the fundamental light pulse burst and the optical harmonic pulse burst received at the Main optical detection section 11, optical height 1 solenoid detection section 12
a phase comparator 13 that compares the phase difference of each demodulated electrical signal;
Based on the comparison results between the meteorological observation device 14 that measures the weather on the optical path and the phase comparator 13, the time required for the fundamental light and optical harmonics to propagate from the reference point to the target point is calculated, and the meteorological observation device 14, and a display device 16 for displaying the calculation results.

The original oscillation unit 3 includes a crystal oscillator 17, a light transmitting unit 1, and a light receiving calculation unit 2.
It consists of a synchronization signal generation section 18 that generates a synchronization signal to be supplied to the.

The reflecting section 4 is composed of a plurality of corner cubes 19.

Note that the transmitting telescope and the receiving telescope 10 are coaxial or have the same beam.

Using this example, that is, wavelength 1 = 1064r + n + wavelength 2 = 532 nm, the optical path length of 1.00 km is set to ±0.5 × 10
In order to measure with an accuracy of 6, the measurement accuracy of optical path temperature measurement ± 7°C, atmospheric pressure measurement ± 400 mb, humidity measurement ± 22%, and propagation time i + t constant ± 8 ps is required.

The measurement accuracy required when performing similar measurements using the conventional method, i.e., the length measurement method using one wavelength, is as follows: optical path temperature measurement ± 0.5°C atmospheric pressure 711 ± 1.8 mb humidity measurement ± 55% When compared with the propagation time measurement of ±1.66 ps, it can be seen that the temperature and pressure measurement accuracy has become a more achievable value. Relaxing the required accuracy of temperature measurements is particularly significant because temperature measurement errors are always the largest source of error. On the other hand, although the measurement accuracy of humidity and propagation time has become a severe value, it is still a value that can be adequately handled with the current technology.

Furthermore, the present invention is not limited to the method and apparatus using the fundamental wave of the YAG laser and its harmonics as shown in the above embodiments. For example, even when using a HeNe laser and a Ho-Cd laser, that is, wavelength 1 = 633 nm and wavelength 2 = 442 nm, it can be expected that the measurement accuracy of temperature and atmospheric pressure will be similarly reduced.

Incidentally, the measurement accuracy required to perform the same measurement as described above using these two wavelengths is as follows: optical path temperature measurement ±7°C atmospheric pressure measurement ±427 mb humidity i+t constant ±22% propagation time 4I constant ±8 ps.

The above evaluation was made with reference to the following paper.

Paper [JalIles C, Owens, “0ptica
l rel'active 1ndexof air
:Dependence on pressure, t
cvperature.

and composition, Applied
0ptjcs, vol, 6. No, l.

51-59. (1967) J [Effects of the Invention] As explained above, according to the present invention, an optical system that can easily perform more accurate length measurement with a lower meteorological measurement accuracy than conventional methods and devices. A length measuring method and device can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention. 1...Light transmitting section, 2...Light receiving calculation section, 3...Original oscillation section, 4...Reflecting section.

Claims (1)

[Claims] 1. Measure the time t_1 and t_2 required for light of different wavelengths, wavelength 1 and wavelength 2, to propagate from a reference point to a target point, and determine the length D of the optical path from the reference point to the target point. When calculating, when the speed of light in vacuum is C, the dry air density in the optical path is ρ_S, the water vapor density is ρ_W, and the refractive powers of wavelength 1 and wavelength 2 are N_1 and N_2, N_1=α_1・ρ_S+β_1・ρ_W N_2 =α_2・ρ_S+β_2・ρ_W, D=[t_1+{(α_1+β_1・ρ_W/ρ_S)
・(t_2-t_1)}/{(α_1+β_1・ρ_W
/ρ_S)−(α_2+β_2・ρ_W/ρ_S)}]
-C Or an optical length measurement method characterized by using this approximate expansion formula. 2. When measuring the time t_1 and t_2 required for light of different wavelengths (wavelength 1 and wavelength 2) to propagate from the reference point to the target point and finding the length D of the optical path from the reference point to the target point, When the speed of light in the optical path is C, the dry air density in the optical path is ρ_S, the water vapor density is ρ_W, and the refractive powers at wavelength 1 and wavelength 2 are N_1 and N_2, N_1=α_1・ρ_S+β_1・ρ_W N_2=α_2・ρ_S+β_2・ρ_W Define the relationship D=[t_1+{(α_1+β_1・ρ_W/ρ_S)
・(t_2-t_1)}/{(α_1+β_1・ρ_W
/ρ_S)−(α_2+β_2・ρ_W/ρ_S)}]
-C Or an optical length measuring device characterized by using this approximate expansion formula.