JPH04248490A - Laser doppler speedometer - Google Patents

Abstract

PURPOSE:To obtain the maximum S/N and thereby to intend to raise accuracy by providing a controlling measures of an acousto-optic modulator so as to make a light value ratio of object light and reference light to be a desired light value ratio, and a monitoring measures of light value of the object light. CONSTITUTION:Object light comes into a photodetector 23 in order to be monitored its light value. The object light is inputted to a controller 25 which stores a correspondence chart of the detected object light and driving power of an acousto-optic optical modulator 3, and signal showing the driving power of the acousto-optic optical modulator 3 is outputted to a driver 20, corresponding to the input signal. The driver 20, receiving the signal, drives the acousto- optic optical modulator 3 so that light value ratio of the object light and the reference light may be optimized. Into a photodetector 21, the object light and the reference light of which light value ratio is already optimized, are projected, and therewith the signal having the best S/N corresponding to reflectivity and the like, of an object to be measured, can be obtained, and accuracy of a speedometer can be highly improved.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention aims to irradiate a moving object to be measured with laser light (irradiation light) emitted from a laser light source, and collect the reflected light (object light) reflected from the object to be measured and the The present invention relates to a laser Doppler velocimeter that obtains a signal corresponding to the movement of the object to be measured by interfering with a reference light extracted from a part of the laser light.

0002

[Prior Art] A beat is generated by causing a laser beam (object beam) reflected from a moving object to be measured to undergo frequency modulation due to the movement to interfere with a reference beam of a constant frequency that interferes with the object beam. A laser Doppler velocimeter is known that obtains a signal corresponding to the movement of the object to be measured by detecting this beating with a photodetector. Among these laser Doppler velocimeters, there is a type called an optical fiber laser Doppler velocimeter, which is configured to transmit laser light through an optical fiber in order to provide flexibility in the positional relationship between the speedometer body and the object to be measured. There are some things.

In the above-mentioned laser Doppler velocimeter, in order to detect both the movement of the object to be measured toward the speedometer and the movement away from the speedometer by distinguishing them from each other, interference occurs when the object to be measured is stopped. The frequency of light is set to a frequency other than zero frequency. This is achieved by configuring a so-called heterodyne interferometer that shifts the frequency of either the irradiation light or the reference light using, for example, an acousto-optic light modulator.

When the frequency of light is shifted using the acousto-optic light modulator, the acousto-optic light modulator produces not only the first-order light whose frequency has been shifted but also the zero-order light whose frequency has not been shifted. are simultaneously emitted, and 100% of the light incident on this acousto-optic light modulator cannot be used as frequency-shifted light. For this reason, the laser light emitted from the laser light source is first made incident on the acousto-optic light modulator without dividing it into the irradiation light and the reference light, and the zero-order light emitted from the acousto-optic light modulator and the A speedometer configured to use one of the secondary lights as irradiation light and the other as reflected light is also conceivable.

FIG. 2 shows a laser Doppler velocimeter configured to use one of the zero-order light and the first-order light emitted from the acousto-optic light modulator as irradiation light and the other as reflected light as described above. FIG. Laser light 2 emitted from laser light source 1 enters an acousto-optic light modulator 3 driven by a driver 20, and from this acousto-optic light modulator 3, zero-order light 4 and no frequency shift are generated. Primary light 5 that has undergone a frequency shift is emitted. This zero-order light 4 is light used as irradiation light to irradiate the object to be measured 30, and is a polarizing beam splitter (hereinafter abbreviated as "PBS") 6.
incident on . This PBS 6 is originally arranged to transmit polarized laser light 2 (0th order light (irradiation light) 4), and the irradiation light 4 that has passed through this PBS 6 is λ/4
The light is converted into circularly polarized light by passing through the plate 7, and after passing through the objective lens 8, the object to be measured 30 is irradiated. Here, it is assumed that the object to be measured 30 is repeatedly vibrating in the direction A-B, which is inclined by an angle θ with respect to the optical path of the irradiated light.

The irradiated light is reflected by the object to be measured 30, and this reflected light (this reflected light is referred to as "object light") passes through the objective lens 8 and the λ/4 plate 7 again and enters the PBS 6. Here, the light that irradiates the object to be measured 30 is the light that has been converted into circularly polarized light by passing through the λ/4 plate 7 once,
Since the object light reflected from the object to be measured 30 passes through the λ/4 plate 7 again, this object light becomes linearly polarized light whose polarization direction is 90 degrees different from that of the light emitted from the PBS 6. Therefore, the object light incident on this PBS6 is
6, and further reflected by mirror 9, and enters beam splitter 10. This beam splitter 10 is
Unlike PBS, it splits the laser beam 2 into two regardless of the polarization direction of the laser beam 2, and out of the object light incident on this beam splitter 10, this beam splitter 1
The component reflected by 0 enters the photodetector 21.

The primary light 5 emitted from the acousto-optic light modulator 3 is used as a reference light to interfere with the object light, and after being reflected by a mirror 11, it is transmitted to a λ/2 plate 12.
By passing through the object light, its polarization direction is matched with that of the object light, and it enters the beam splitter 10, and the component transmitted through the beam splitter 10 enters the photodetector 21 together with the object light. The object light and reference light that entered the photodetector 21 in this way interfere with each other on the photodetector 21, and this interfered light is detected by the photodetector 21, thereby obtaining a beat signal of frequency fb. It will be done. After this beat signal is amplified by the preamplifier 22, it is input to a signal processing circuit (not shown), and the vibration frequency of the object 30 to be measured is determined.

Furthermore, in FIG. 2, for example, an optical fiber laser Doppler velocimeter can be constructed by inserting an optical fiber such as a polarization maintaining optical fiber at the location indicated by symbol F. Here, what is polarization maintaining optical fiber?
A single mode optical fiber that transmits light polarized in two predetermined directions that are orthogonal to each other.

In the laser Doppler velocimeter configured as described above, the laser beam 2 emitted from the laser light source 1
Let the wavelength and frequency of the measured object 3 be λ and fo, respectively.
When the vibration velocity of 0 is V (a function of time t), the frequency shift amount fd of the object light reflected from the object to be measured 30 from the laser light 2 emitted from the laser light source 1 is fd
=(2・V/λ)・cos θ... (1
) Therefore, the frequency fs of the object light is
fs = fo + fd = fo + (2・
V/λ)・cos θ... (2) Further, the reference light is transmitted to the acousto-optic light modulator 3 as described above.
Since the primary light 5 which has undergone a frequency shift is used, if the modulation frequency of this acousto-optic light modulator 3 is fm, the frequency fr of the reference light is fr = fo + fm... (3)
becomes. Since the object light and the reference light interfere on the photodetector 21, a sum frequency and a difference frequency appear in the above equations (2) and (3). The frequency is detected, and the frequency fb of this difference is calculated from the above equations (2) and (3) as follows: fb = |f
r − fs | = | fm −
(2・V/λ)・cos θ｜ ... (
4) It becomes. The frequency fb of this difference is detected as a beat signal, and thereby the velocity Vcos θ of the object to be measured 30 in the optical axis direction of the irradiated light is detected.

0010

[Problems to be Solved by the Invention] Although a laser Doppler velocimeter is realized with the above-described configuration, the amount of object light reflected from the object to be measured 30 largely depends on the reflectance of the object to be measured 30. If the reflectance or the like is low and therefore the amount of object light is small, there is a risk that the signal obtained by the photodetector 21 will be a signal with a poor S/N ratio. Also, the object to be measured 3
0, the ratio of the object light and the reference light changes greatly due to the difference in reflectance, etc., and the light that does not contribute to the interference between the object light and the reference light remains as a DC component, and this DC component also affects the S/N ratio. This is one of the causes of the decline.

In view of the above circumstances, the present invention provides a laser that can always obtain a signal with the highest S/N ratio depending on the object to be measured, even if the reflectance etc. of the object to be measured differs depending on the object to be measured. The purpose is to provide a Doppler velocimeter.

0012

[Means for Solving the Problems] A laser Doppler velocimeter of the present invention for achieving the above object converts laser light emitted from a laser light source into zero-order light and first-order light using an acousto-optic light modulator. The object to be measured is irradiated with either the zero-order light or the first-order light, and the other of the zero-order light or the first-order light is used as a reference light. A laser that interferes with object light obtained by reflecting one of the lights from the object to be measured, and receives the light obtained by the interference to obtain a signal representing the state of movement of the object to be measured. In the Doppler velocimeter, a monitor means for monitoring the light quantity of the object light, and a light quantity ratio of the object light and the reference light to a desired light quantity ratio based on the light quantity of the object light monitored by the monitor means. The present invention is characterized by comprising a control means for controlling the acousto-optic light modulator so that the following is achieved.

[0013]

[Operation] The laser Doppler velocimeter of the present invention monitors the light amount of the object light and uses an acousto-optic method based on the monitored light amount so that the light amount ratio of the object light and the reference light becomes a desired light amount ratio. Since the target light modulator is controlled, the light intensity ratio between the object light and the reference light is appropriately determined according to the reflectance of the object to be measured, and the highest S/
This means that a signal with a high N ratio can be obtained.

[0014]

[Examples] Examples of the present invention will be described below. Figure 1
1 is a schematic configuration diagram of a laser Doppler velocimeter according to an embodiment of the present invention. In this figure, components corresponding to those of the laser Doppler velocimeter shown in FIG. 2 are given the same reference numerals as those used in FIG. 2, and their explanations will be omitted.

In the laser Doppler velocimeter shown in FIG. 1, a mirror 13 that transmits a part of the object light is used in place of the mirror 9 used in the laser Doppler velocimeter shown in FIG. The transmitted object light enters a photodetector 23, and the amount of light is monitored by the photodetector 23. The signal representing the light intensity of the object light obtained by the photodetector 23 is amplified by the amplifier 24, and then amplified by the light intensity of the detected object light and the light intensity of the detected object light in order to make the light intensity ratio between the object light and the reference light an optimum value. A correspondence table with the driving power of the acousto-optic light modulator 3 is input to the controller 25 in which it is stored. The controller 25 refers to the above correspondence table and outputs a signal representing the drive power of the acousto-optic light modulator 3 to the driver 20 in accordance with the input signal. The driver 20 receives this signal and drives the acousto-optic light modulator 3 so that the light intensity ratio between the object light and the reference light is optimized.

In this way, since the light intensity of the object light is monitored and the driving power of the acousto-optic light modulator 3 is adjusted based on the light intensity, the photodetector 21 receives an object whose light intensity ratio is optimized. The light and the reference light are incident on the photodetector 2.
1, the best S/
A signal with an N ratio will be obtained.

Although the above embodiment is an example of a laser Doppler velocimeter that does not use an optical fiber, the present invention includes an optical fiber laser Doppler velocimeter that uses an optical fiber, The present invention can be widely applied to laser Doppler velocimeters that use one of the zero-order light and first-order light as an object light and the other as a reference light.

[0018]

As explained in detail above, the laser Doppler velocimeter of the present invention uses one of the zero-order light and the first-order light emitted from the acousto-optic light modulator as the irradiation light and the other as the reference light. Then, the light intensity of the object light obtained by irradiating and reflecting the irradiation light on the object to be measured is monitored, and the acousto-optic control is performed so that the light intensity ratio of the object light and reference light becomes the desired light intensity ratio. Since the driving power of the optical modulator is controlled, it is possible to obtain a signal with the best S/N ratio for each object to be measured depending on the reflectance of the object to be measured, making it possible to use a highly accurate laser Doppler velocimeter. Realized.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a laser Doppler velocimeter according to an embodiment of the present invention.

[Figure 2] 0th-order light and 1st-order light emitted from an acousto-optic light modulator
Schematic configuration diagram of a laser Doppler velocimeter configured to use one side of the secondary light as irradiation light and the other as reflected light

[Explanation of symbols]

1 Laser light source 2 Laser light 3. Acousto-optic light modulator 4 0th order light (irradiation light) 5 Primary light (reference light) 20 Driver 21 Photodetector 22 Preamplifier 23 Photodetector 24 Amplifier 25 Controller 30 Object to be measured

Claims (1)

[Claims] 1. Laser light emitted from a laser light source is divided into zero-order light and first-order light by an acousto-optic light modulator, and the
The object to be measured is irradiated with either the secondary light or the first-order light, and the reference light and the one light are combined using the other of the zero-order light or the first-order light as a reference light. In a laser Doppler velocimeter that obtains a signal representing a state of movement of the object to be measured by interfering with object light obtained by being reflected from the object to be measured and receiving the light obtained by the interference. , a monitor means for monitoring the light amount of the object light, and a light amount ratio of the object light and the reference light is set to a desired light amount ratio based on the light amount of the object light monitored by the monitor means. A laser Doppler velocimeter comprising: control means for controlling the acousto-optic light modulator.