JP2671667B2 - Laser doppler velocimeter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser Doppler velocimeter for measuring the moving speed of a moving object in a non-contact manner by utilizing the Doppler effect of light.

[0002]

2. Description of the Related Art Generally, a velocity meter as shown in FIG. 4 has been used to measure the velocity of a moving object by utilizing the Doppler effect of light. FIG. 4 shows, for example, JP-A-54-8387.
Is a diagram showing a conventional laser Doppler velocimeter shown in 2 discloses, in FIG, 1 is a laser oscillator, 2 denotes a beam collimator for obtaining a required laser beam diameter, 3 a beam splitter, 4 _a, 4 _b and 4 _c are total reflection mirrors, 5
Is an object to be measured, 6 is a light receiving lens, 7 is a photodetector, and 8 is a signal processing unit.

As shown in FIG. 4, a laser beam from a laser oscillator 1 is collimated by a beam collimator 2 to have a required beam diameter, and then divided into two by a beam splitter 3 and directed in opposite directions by total reflection mirrors 4 _a and 4 _b . When the light is crossed onto the object to be measured 5 from the object, the wavelength of the scattered light from the object to be measured 5 corresponding to each irradiation laser beam is
A so-called positive and negative Doppler shift is caused in accordance with the speed v of. The scattered light that has undergone the two positive and negative Doppler shifts is received via the light receiving lens 6 and the total reflection mirror 4 _c , and converted into an electric signal by the photodetector 7, and a DC signal proportional to the received light and An AC signal having a Doppler frequency f _d (hereinafter referred to as “Doppler signal”) is obtained.

[0004]

(Equation 1)

Here, v: velocity of the object to be measured λ: wavelength of laser light Ψ: crossing angle of two irradiation laser beams

The speed v of the object 5 to be measured is calculated by inputting the Doppler signal converted into an electric signal by the photodetector 7 to the signal processing unit 8 to obtain the Doppler frequency f _d and calculating the speed by the "equation 1". You can ask.

[0007]

The spatial measurement area of the laser Doppler velocimeter shown in FIG. 4 is determined by the area where two irradiation laser beams intersect, and L _m is the area where two irradiation laser beams intersect. , D _b is the diameter of the irradiation laser beam, Ψ is the crossing angle of the two irradiation laser beams, w _m is the beam width when the two irradiation laser beams intersect concentrically with the DUT 5, and h _m is This shows the beam height at this time, and L _m is given by “Equation 2”.

[0008]

(Equation 2)

On the other hand, when this laser Doppler velocimeter is applied to various manufacturing lines, the line fluctuation (positional fluctuation of the object to be measured) is generally large at ± 10 mm to ± 100 mm. It is necessary to increase the diameter D _{b of the} laser beam or decrease the crossing angle Ψ of the two irradiation laser beams. When the crossing angle Ψ is decreased, the Doppler frequency f _d per unit speed decreases from “Equation 1”. There is a limit because it lowers the accuracy, and the laser beam diameter D _b
Has been taken to increase. However, when the diameter D _{b of the} laser beam is increased, the area S ₁ irradiated on the object to be measured becomes large as shown in "Equation 3", so that the irradiation laser power per unit area decreases and the Doppler shift occurs. In addition, there is a problem that the power of the signal light whose phase is aligned in the scattered light is reduced and the S / N of the Doppler signal is reduced.

[0010]

(Equation 3)

The present invention has been made to solve the above problems, and an object thereof is to provide a highly accurate laser Doppler velocimeter having a high Doppler signal S / N and a wide spatial measurement region. I am trying.

[0012]

SUMMARY OF THE INVENTION A laser Doppler velocimeter according to the present invention enables a beam waist at an intersecting position only for a laser beam component perpendicular to a plane formed by two intersecting irradiation laser beams. A cylindrical lens is provided which narrows down and irradiates the object to be measured as an elliptical beam.

Further, a cylindrical lens is arranged in front of the optical system for blocking the outside air, and a receiving lens system for scattered light is provided with a cylindrical lens and a condenser lens which are orthogonal to the cylindrical lens.

Furthermore, an optical system is provided for expanding only the laser beam component of the irradiation laser beam, which is the same as the plane formed by the two intersecting irradiation laser beams, by a required magnification.

[0015]

SUMMARY OF cylindrical lens in the present invention, for irradiating the object to be measured irradiated laser beam as an oval shape beam, the beam width in the direction that determines the L _m remains beam height is secured is compressed, L _m Is maintained, the irradiation laser power per unit area is increased and the signal S / N of the Doppler signal is improved.

Further, by arranging a cylindrical lens on the front surface of the optical system for shielding the outside air, even in an optical system in which the optical path length from the split by the beam splitter to the intersection of the laser beams is different, two irradiation lasers are used. It is possible to function with one cylindrical lens for the beam, the configuration is simplified, and the time required for optical axis adjustment can be shortened.

Furthermore, in an optical system in which the frequency of the irradiation laser beam is shifted by a required frequency by using the acousto-optic modulator, the incident laser beam diameter of the acousto-optic modulator is limited, so that after passing through the acousto-optic modulator. Laser beam,
The range of L _m can be expanded by arranging an optical system that expands only the beam width in the direction that determines L _m by a required magnification.

[0018]

【Example】

Embodiment 1 FIG. FIG. 1 shows a schematic configuration diagram of an embodiment of the present invention, in which 1 to 8 and D _b , Ψ, w _m are exactly the same as those of the conventional device. Reference numeral 9 denotes a cylindrical lens whose focal length f matches the optical path length L ₁ + L ₂ from the insertion position to the intersection of the two laser beams, and h
Is the beam waist height of the irradiation laser beam focused by the cylindrical lens.

In the laser Doppler velocimeter configured as described above, the laser beam from the laser oscillator 1 is collimated by the beam collimator 2 to a required beam diameter, and the cylindrical lens 9 narrows it down only in the height direction. When the beam is divided into two by 3, and the total reflection mirrors 4 _a and 4 _b cross and irradiate the DUT 5 from opposite directions, an elliptical beam is formed on the DUT 5, and each irradiation laser is irradiated. The wavelength of the scattered light from the DUT 5 corresponding to the beam causes a positive and negative Doppler shift depending on the velocity v of the DUT 5. The scattered light that has undergone the two positive and negative Doppler shifts is received via the light receiving lens 6 and the total reflection mirror _4c, and the signal converted into an electric signal by the photodetector 7 has the Doppler value shown in "Equation 1". Since the frequency f _d is included, as in the conventional laser Doppler velocimeter, the Doppler signal converted into an electric signal by the photodetector 7 is input to the signal processing unit 8, and the Doppler frequency f _d is calculated to calculate the velocity. The velocity v of the DUT 5 can be obtained.

The beam waist height h of the irradiation laser beam at this time is expressed by the following equation.

[0021]

(Equation 4)

Where f is the focal length of the cylindrical lens, k is the ratio of h that can be achieved by the actual lens to h that is determined by the diffraction limit.

Therefore, the elliptical irradiation beam area S ₂ on the surface of the object to be measured is given by "Equation 5".

[0024]

(Equation 5)

Here, the ratio of the irradiation beam area S ₁ of the conventional system and the irradiation beam area S ₂ of the present invention is calculated to be "Equation 6", for example, f = 500 mm, λ = 632.8 nm,
If D _b = 5 mm and K = 10, then S ₂ / S ₁ = 0.08, and the irradiation laser power per unit area is about 12.5.
And the Doppler signal S / N is significantly improved.

[0026]

(Equation 6)

Embodiment 2 FIG. FIG. 2 shows a cylindrical lens 9.
Is arranged on the front surface of the optical system for blocking the outside air, and the scattered light from the DUT 5 is converted into parallel light by the cylindrical lens 9 and the cylindrical lens 10 orthogonal to this, and then the condenser lens 11 is used. To the photodetector. According to this FIG. 2, even in an optical system in which the optical path length from the beam splitter being divided into two to the laser beam crossing is different, it is possible to function with two irradiation laser beams with one cylindrical lens. ,
The configuration is simplified, and the time required for adjusting the optical axis can be shortened.

Embodiment 3 FIG. Figure 3 is arranged acousto-optic modulator 12a, and 12b in the irradiated laser beam path, the frequency of each of the two irradiation laser beam is shifted by the required frequency, in an optical system provide a constant offset frequency in Doppler frequency, the In this case, since the incident laser beam diameters of the acousto-optic modulators 12a and 12b are limited, only the beam width of the laser beam after passing through the acousto-optic modulators in the direction that determines L _m by the cylindrical lenses 13a, 13b, 14a and 14b. After the required magnification n is enlarged, the cylindrical lenses 9a and 9b are narrowed down so that a beam waist is formed in the height direction at the intersecting position of the two irradiation laser beams, and the optical component has a limited laser beam diameter. Even if is used, L _m can be expanded.

Although a combination of cylindrical lenses has been described as the beam width expanding optical system, since the beam waist is narrowed down so as to be formed after the beam width is expanded, the beam diameter is circularly formed by a general beam expander. It goes without saying that the same effect can be obtained even if expanded.

[0030]

Since the present invention is constructed as described above, it has the following effects.

By irradiating the object to be measured with an irradiation laser beam as an elliptical beam using a cylindrical lens, the irradiation laser power per unit area is increased while keeping the spatial measurement region wide, and the Doppler signal The signal S / N can be improved.

Further, to the construction of arranging the front of the optical system the cylindrical lens as ambient air shutoff
Therefore , the structure is simplified, and the time required for adjusting the optical axis can be shortened.

[0033]

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a laser Doppler velocimeter showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a laser Doppler velocimeter showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram of a laser Doppler velocimeter showing a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a conventional laser Doppler velocimeter.

[Explanation of symbols]

1 Laser Oscillator 2 Beam Collimator 3 Beam Splitter 4 _a Total Reflection Mirror 4 _b Total Reflection Mirror 4 _c Total Reflection Mirror 5 DUT 6 Light Receiving Lens 7 Photo Detector 8 Signal Processor 9 Cylindrical Lens 10 Cylindrical Lens 11 Condensing Lens 12 Acousto-optic modulator 13 Cylindrical lens 14 Cylindrical lens

Claims (1)

(57) [Claims] 1. A laser that outputs a specific wavelength, and
Laser output laser beam is divided into two,
The direction of the laser beam can cross and illuminate the moving object.
For irradiating each of the two irradiation laser beams
About Doppler shift according to the moving speed of the moving object
Light-receiving lens system that receives the scattered light generated with the required aperture diameter
And a diffuser containing the Doppler signal received by the above-mentioned receiving lens system.
Laser Doppler equipped with photodetector for photoelectrically converting irregular light
In the speedometer, two intersecting irradiation laser beams are produced.
Laser beam components perpendicular to the
Cylindrica to narrow down so that mu west can be made
The external lens is placed in front of the light-receiving lens system to block outside air.
It has a function and the light receiving lens system is cylindrical.
A laser characterized by comprising a lens and a condenser lens
Doppler speedometer .