JPS6367673B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a velocity meter and a current meter using a laser.

A conventional device of this type is shown in FIG. In the figure, 1 is a laser, 2 is a beam splitter, 3 is a velocity probe consisting of an optical fiber or an optical fiber and a lens system, 4 is an optical fiber, 5 is an optical receiver, and 6 is a spectrum analyzer as a measuring device. , 7 is the object to be measured.

FIG. 2 shows an example of observation using a spectrum analyzer using a conventional device.

Next, the operation will be explained. The emitted light from the laser 1 passes through a beam splitter 2 and is guided to a velocity probe 3 using an optical fiber. This light is emitted toward the object to be measured 7 at an appropriate angle.
The light subjected to the Doppler effect is again received by the velocity probe 3, passes through the beam splitter 2 and the optical fiber 4, and is guided to the optical receiver 5. The scattered light subjected to the Doppler effect is optically heterodyne detected with the light reflected at the tip of the velocity probe (this reflected light is used as reference light) in the optical receiver 5. The frequency fs of this detection output is fs=2‖V|/λcosθ...(1) In equation (1), ‖V| is the velocity of the object to be measured,
λ is the wavelength of the laser, and the angle between the light emitted from the θ velocity probe and the velocity of the object to be measured |V|. From (1), by measuring f, the speed |V| can be determined.

However, even if the laser itself oscillates in multiple modes, or even if a single mode oscillation laser is used, by using an optical fiber, etc., the reflected light returns to the laser resonator and oscillates in multiple modes. There are many. At this time, a heterodyne detection output between laser modes is generated in addition to the measurement signal. That is,
If the oscillation frequency of each mode is represented by f _n , f _o , etc., and the output signal of the optical receiver is Vout, then Vout = A _s exp (j2π f _s t) + 〓 ^m≠n A _i exp (j2π (f _n −f _o )t) + 〓 ^m≠n B _i exp(j2π(f _n −f _o +fs)t). In equation (2), the first term on the right side is a signal term containing velocity information, the second term is a heterodyne detection output term between laser modes, and the third term is the scattered light that has undergone the Drupler effect for each mode, and the optical This is the heterodyne output of each mode of the reference light reflected from the fiber end face. However, since scattered light is generally weak and interference between different modes is weaker than interference between the same modes, the output of the third term can be ignored.

FIG. 2 is an example of observation using a spectrum analyzer. Noise is included in the low frequency region.

Conventional laser velocimeters are configured as described above, so when the laser itself oscillates in multiple modes, or when the laser oscillates in multiple modes by using an optical fiber, the difference between the laser modes is The heterodyne detection power and the heterodyne detection output of the laser fundamental mode and the signal subjected to the Doppler effect appear as outputs at the same time, which has disadvantages such as lowering speed measurement accuracy and narrowing the measurement dynamic range.

This invention was made in order to eliminate the drawbacks of the conventional ones as described above. After the laser emitted light is split into two by a beam splitter, one of the split lights is subjected to heterodyne detection using a separately prepared optical receiver. By electrically calculating the difference between this detection output (reference signal) and the detection output including the speed signal,
It aims to provide high performance laser speed.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 3, 1 is a laser, 2 is a beam splitter, 3 is a speed probe composed of an optical fiber or an optical fiber and a lens system, 4 is an optical fiber, 5 is an optical receiver, 7 is an object to be measured, 8 is a differential amplifier, and 6 is a measurement device such as a spectrum analyzer.

Next, the operating principle of the present invention will be explained. Third
In the figure, the electrical output signal Va from the optical receiver 5
is the same as the conventional example, that is, equation (2), and can be expressed as Va=A _s exp(j2π f _s t) + 〓 ^m≠n A _o exp(j2π(f _{n −fo} ) t).

On the other hand, one of the laser beams split into two by the two beam splitters is input to the optical receiver 5' either directly or through a variable optical attenuator or the like. In principle, this light does not include light subjected to the Doppler effect, so the output voltage Vb of the optical receiver 5' is V _b = 〓 ^m≠n B _o exp (j2π (f _n - f _o )t) becomes. Since this output voltage (reference signal) is obtained by optical heterodyne detection between laser modes, compared with the second term of equation (2), An=αBn...(4) f _n - f _o ＝f′ _n −f′ _o ...(5).

Therefore, by adjusting the gains of the amplifiers included in the optical receivers 5 and 5' and inputting them into the differential amplifier 8, the output signal of the differential amplifier is Vout=βA _s exp(j2π f _s t) ...(6) is obtained, and the effect of interference between laser modes can be removed. β is a constant determined by the gain of the differential amplifier 8, which is an amplifier in the optical receiver 5.

FIG. 4 shows an example of the signal spectrum and analyzer output obtained using this device. Noise components occurring in the low range can be removed.

In the above embodiments, an example was described in which the speed probe is composed of one optical fiber or one optical fiber and a lens system. The output speed probe is configured to be separated into an output speed probe that receives scattered light from the measured object and emits it to the measured object, and a light receiving speed probe that receives scattered light from the measured object and introduces this scattered light to an optical receiver. Both the speed probe and the speed probe for light reception may be constructed of optical fibers.

In addition, there are reference beam method Doppler velocimeters that do not use optical fibers, and two-beam mode method (Differential
It can also be applied to the 1-beam mode method, etc.

Furthermore, in the above embodiment, in order to eliminate noise due to heterodyne detection output between laser modes other than the measurement signal, two optical receivers are independently installed as an optical receiving system so that the DC components of the two output signals are equal. Although the method of adjusting, operating, and differentially amplifying their output signals has been described, it is also possible to remove noise by configuring as shown in FIG.

That is, in FIG. 5, 9 is a light receiving element, 1
0 is a preamplifier, and this light receiving element 9 and preamplifier 10 constitute the above-mentioned optical receiver. In addition, 11 is a differential amplifier for AGC amplifier, 1
2 is an AGC amplifier, and 13 is a main amplifier. In the system configured in this way, the magnitude of the DC component of the output of one of the receivers consisting of the light receiving element 9 and the preamplifier 10 is detected by the differential amplifier 11 for AGC amplification, and this is used to detect the magnitude of the DC component of the output of one of the receivers consisting of the light receiving element 9 and the preamplifier 10. AGC amplifier 12 of the other receiver so that the amplitude of the output noise voltage is equal.
The gain of the two receivers can be automatically adjusted and the two receiver outputs can be fed into the differential amplifier 8 to eliminate noise due to interference between laser modes.

In addition, as shown in Figure 6, after dividing each AC output signal of the two receivers by their respective DC components and equalizing the amplitude of the noise voltage due to interference between laser modes of the output of each receiver, Differential amplification also makes it possible to remove noise due to interference between laser modes. In this FIG.
is configured to divide the AC output signal from 3.

As described above, according to the present invention, two types of optical receivers and a differential amplifier are used to remove noise caused by interference between laser modes, and the measurement accuracy is high.
Also, there is an effect that a laser Doppler velocimeter or current meter with a wide dynamic range can be obtained.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of a conventional laser Doppler speedometer, Fig. 2 is a waveform diagram showing an example of observation by a spectrum analyzer when using the conventional device, and Fig. 3 shows a laser Doppler speedometer according to an embodiment of the present invention. FIG. 4 is a waveform diagram showing an observation example using the laser velocimeter of the present invention, and FIGS. 5 and 6 are block diagrams showing other embodiments of the signal processing system. In the figure, 1 is a laser, 2 is a beam splitter, 3 is a velocity probe consisting of an optical fiber or an optical fiber and a lens system, 4 is an optical fiber, 5 is an optical receiver, 6 is a spectrum analyzer, and 7 is a receiver. Measuring object, 8 is a differential amplifier, 9 is a light receiving element, 10 is a preamplifier, 11 is a differential amplifier for AGC amplifier, 12 is an AGC amplifier, 13 is a main amplifier,
15 is a divider. 14 is a reference voltage terminal. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. Receives the emitted light from the laser via a beam splitter, emits the received emitted light from the laser toward the object to be measured, and scatters the Doppler effect from the object to be measured. a speed probe that receives light; a reference light based on the scattered light received by the speed probe and the light emitted from the laser;
a first optical receiver that performs optical heterodyne detection;
A second optical receiver receives the emitted light from the laser split by the beam splitter and performs optical heterodyne detection, and differentially detects the detected electrical outputs from the first and second optical receivers. Laser Doppler velocimeter with differential amplifier to amplify. 2. The speed probe is composed of an optical fiber or an optical fiber and a lens system, and the optical fiber receives light emitted from a laser via a beam splitter and emits it to the object to be measured, and also receives light from the object to be measured. The laser doppler according to claim 1, wherein the reference light received by the first optical receiver is light reflected at the tip of the optical fiber. speedometer. 3. The speed probe has an output speed probe that receives the emitted light from the laser via a beam splitter and emits it to the measured object, and a light receiving speed probe that receives the scattered light from the measured object. ,
2. The laser Doppler velocimeter according to claim 1, wherein both probes are configured separately. 4 The first and second optical receivers, as a signal processing system, automatically adjust the amplifier gain of the other optical receiver depending on the magnitude of the DC component of the output of one optical receiver, and 2. The laser Doppler velocimeter according to claim 1, wherein the AC output of the optical receiver is input into a differential amplifier to remove noise. 5. Noise is removed by dividing each AC output signal of the first and second optical receivers by their respective DC components and inputting them into a differential amplifier. The laser Doppler velocimeter according to item 1.