JPH06160013A - Interference length measuring device - Google Patents

Abstract

PURPOSE:To provide an interference length measuring device capable of accurately measuring the distance up to an object without receiving the effect of the intensity modulation component of a light source. CONSTITUTION:The modulation current to a laser beam source is cyclically changed to sheep an output wavelength. A beam splitter 2 splits the laser beam from the laser beam source 1 into reference beam and object beam. The reference beam is reflected by a reference mirror 3 and the object beam is reflected by an object 4 and the reflected beams mutually interfere on the beam splitter 2. The intensity of the beam beat due to this interference is detected by a photodetector 5. The reference mirror 3 can be displaced by prescribed quantity in synchronous relation to the sweep cycle of the oscillation wavelength of the laser beam source 1 by a piezoelectric element 6. The output of the photodetector 5 obtained when the reference mirror 3 is displaced over a plurality of stages is used to remove an intensity modulation component.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for measuring a distance to a target object by utilizing light interference.

[0002]

2. Description of the Related Art Heretofore, in order to accurately measure the distance to a target object in a factory line or the like, an interference length measuring device has been used which obtains the distance by utilizing light interference. For example, the interferometer length measuring device shown in FIG.
In -8), a light beam emitted from a laser light source 1 made of a semiconductor laser is made incident on a beam splitter 2 made of a semi-transparent mirror through an isolator 7 and split into reference light and object light in directions orthogonal to each other. The reference mirror 3 that is a reference object is irradiated, and the object light is irradiated to the target object 4 that is a target whose distance is to be measured. The reflected lights of the lights respectively radiated on the reference mirror 3 and the target object 4 return to the beam splitter 2 and interfere with each other to form an interference pattern on the beam splitter 2. The intensity of the interference pattern formed in this way is detected by the light receiving element 5 such as a photodiode, and the distance to the target object 4 is obtained by the electric signal corresponding to the intensity of the interference pattern output from the light receiving element 5. Is configured.
That is, the output signal of the light receiving element 5 is amplified by the amplifier 12, then input to the analog-digital converter 13 and converted into a digital signal, and the digital signal is processed by the arithmetic processing unit 14 as an arithmetic means including a microcomputer. The distance to the target object 4 is obtained by inputting into.

By the way, in order to accurately and easily calculate the distance to the target object 4 on the basis of the interference pattern, a method called heterodyne interferometry is known. here,
Since it is known that the laser light source 1 changes the oscillation wavelength when the injection current is changed, in the device shown in FIG.
By modulating the injection current to the laser light source 1 with the oscillator 11 into a triangular wave shape as shown in FIG. 4, the output wavelength of the laser light source 1 is swept, and the distance to the target object 4 is determined by applying the heterodyne interferometry. I am asking for it. That is, when a triangular wave-shaped modulation current having a period T _m as shown in FIG. 4 is applied to the laser light source 1, the oscillation wavelength changes as shown in FIG. 5, and at this time, the emission intensity also changes as shown in FIG.

The oscillation wavelength of the laser light source 1 is λ ₀ , and time t ₁
Light changes in wavelength and [Delta] [lambda], for receiving the optical path difference between the reference beam and the object beam when is L, with respect to optical beat caused by interference between the reference beam and the object beam by the light receiving element 5 at the time t ₂ and The intensity I of can be expressed as in equation (1). I = A + B · cos 2π (ΔλL / λ ₀ ² + L / λ ₀ ) ... (1) When the wavelength of the laser light source 1 changes by Δλ, 2πLΔλ / λ ₀ ² which is the phase term of the intensity I changes. .
If 2πLΔλ / λ ₀ ² = Δφ ₀ , the optical path difference L is
It is expressed by equation (2). L = (λ ₀ ² / Δλ) × (Δφ ₀ / 2π) (2) When the oscillation wavelength λ ₀ and the wavelength change Δλ are known Δφ
_{If 0} is known, the optical path difference L can be obtained and the distance to the target object 4 can be known.

[0005]

When the distance to the target object 4 is obtained by the above method, the measurement accuracy of the phase Δφ ₀ affects the measurement accuracy of the distance to the target object 4. As a method of obtaining the phase Δφ ₀ , a method has been proposed in which the intensity modulation component of the laser light source 1 included in the output of the light receiving element 5 is removed by a bandpass filter (Optical vol.
17, No. 6). However, as described above, when the oscillation intensity of the laser light source 1 changes in a triangular wave shape, various frequency components are included. Therefore, even if a bandpass filter is used, the intensity modulation component cannot be completely removed. However, there is a problem that the accuracy of distance measurement cannot be sufficiently improved.

On the other hand, a method has been proposed in which the intensity modulation component is removed from the output signal of the light receiving element 5 by digital processing without using a filter (Journal of Precision Engineering Vol. 58, No. 6). However, even if digital processing is performed, there is a problem that the intensity modulation component cannot be completely removed as in the case of using a filter.

An object of the present invention is to solve the above-mentioned problems, and an interferometer with which the distance to a target object can be accurately measured without being affected by the intensity modulation component of the light source. Is to provide.

[0008]

According to the present invention, in order to achieve the above object, a laser light source whose output wavelength is repeatedly swept and a light beam emitted from the laser light source are divided into two directions to irradiate a target object. A beam splitter that splits the object light and the reference light that illuminates the reference object, a light receiving unit that detects the intensity of an optical beat caused by the interference of reflected light from the target object and the reference object, and a target based on the output of the light receiving element The calculation means for measuring the distance to the object, the optical path difference setting means for changing the optical path difference between the target object and the reference object and the beam splitter by a specified amount, and the same for a few sub sweeps of the output wavelength of the laser light source The control means controls the optical path difference setting means so that different optical path differences are set at the wavelengths.

[0009]

According to the above construction, the optical path difference setting means for changing the optical path difference between the reference light and the object light by a specified amount and the output wavelength of the laser light source at the same wavelength for a few sub sweeps. By providing the control means for controlling the optical path difference setting means so that different optical path differences are set, the sweep of each time is included when the intensity modulation component that changes with the sweep of the output wavelength of the laser light source is included. Each time the intensity becomes the same, if only the optical path difference is changed and the measurement is performed multiple times, the intensity modulation component can be removed and the phase can be obtained. The distance can be measured with high accuracy.

[0010]

EXAMPLE In this example, as shown in FIG.
Is configured so that it can be displaced by using a piezoelectric element 6 such as PZT as an optical path difference setting means, and the optical path difference between the reference mirror 3 and the target object 4 and the beam splitter 2 is variable. It's different. The piezoelectric element 6 is controlled by a control means (not shown) and displaces the reference mirror 3 in synchronization with the output cycle of the oscillator 11 as described later. Further, in this embodiment, the lens 8 is interposed between the laser light source 1 and the isolator 7. The other configuration is the same as the conventional configuration.

First, the principle of this embodiment will be described. When the optical path difference is L and the optical path difference is displaced by ΔL when the optical path difference is L, the intensity I of the optical beat caused by the interference is expressed by equation (3). I = A + B · cos 2π {(L + ΔL) Δλ / λ ₀ ² + (L + ΔL) / λ ₀ } (3) Among the phase terms of the equation (3), ΔL (Δλ / λ ₀ ² ) is more than other phase terms. Is sufficiently small, this term can be ignored, and the following equation (4) can be used instead of equation (3). I = A + B · cos 2π {ΔλL / λ ₀ ² + (L + ΔL) / λ ₀ } (4) Here, the piezoelectric element 6 is controlled by the control means to set the position of the reference mirror 3 to 0, λ ₀ / 8, when is displaced so as to be λ ₀ / 4,3λ _0/8, the change ΔL of the optical path difference is, 0, lambda ₀
/ 4, and λ ₀ / 2,3λ _0/4. At this time, the phase change Δφ becomes 0, π / 2, π, 3π / 2. That is, in the equation (4), 2π {ΔλL / λ ₀ ² + (L + ΔL) /
If λ ₀ } = φ, the change ΔL in optical path difference is 0, λ ₀ /
4, with respect to _{λ 0 / 2,3λ 0/4,} (4) expression is expressed as follows. I ₁ = A + B · cos φ (5) I ₂ = A + B · cos (φ + π / 2) (6) I ₃ = A + B · cos (φ + π) (7) I ₄ = A + B · cos (φ + 3π / 2) (8) When A and B are deleted from I ₁ , I ₂ , I ₃ , and I _{4 above} to obtain φ, φ = tan ⁻¹ (I ₄ −I ₂ ) / (I ₁ −I ₃ ). (9) is obtained. As is clear from the equation (9), since the phase φ does not include the intensity modulation component of the laser light source 1, the phase φ can be determined with high accuracy. Note that, generally, if ΔL _j = jλ / N (j = 1, 2, ..., N), the phase φ can be expressed as in Equation 1.

[0012]

[Equation 1]

As described above, if the optical path difference is set in four stages, the phase φ can be obtained without being affected by the intensity modulation component. Therefore, the laser light source 1 is modulated into a triangular wave shape having a cycle T _m as shown in FIG. 4, and the piezoelectric element 6 is controlled by the control means at each cycle T _m to set the reference mirror 3 to 0, λ ₀ /
8, is displaced with λ ₀ / 4,3λ _0/8, the change of the optical path difference Δ
L is _0, λ 0/4, is set to be λ ₀ / 2,3λ _0/4.

Here, in the half cycle when the change ΔL in the optical path difference is 0, as shown in FIG. 2A, the measurement points t ₁₁ and t
₂₁ is taken, and the output signals of the light receiving element 5 at the respective measurement points t ₁₁ and t ₂₁ are I ₁₁ and I ₂₁ . Further, as shown in FIG. 2B, the output signals I ₁₂ , I _{12 of} the light receiving element 5 are also at the measurement points t ₁₂ , t ₂₂ which are delayed by the period T _{m from the} measurement points t ₁₁ , t ₂₁ .
Find I ₂₂ . The output signals I ₁₂ and I ₂₂ are ΔL = λ ₀
The measured value is / 8. Thereafter, similarly, the output signal of the light receiving element 5 is obtained at each cycle T _m from each measurement point (see FIG. 2).
(See (c) and (d)). In this way, output signals I ₁₁ , I ₂₁ , I ₁₂ , I ₂₂ , I ₁₃ , I ₂₃ , I ₁₄ ,
If I ₂₄ is calculated and applied to the equation (9), the measurement points t ₁₁ and t ₁₂
The phases φ ₁ and φ ₂ with respect to can be obtained.

The phase change Δφ ₀ from the measurement point t ₁₁ to the measurement point t ₂₁ can be expressed by the equation (10). Δφ ₀ = (2π−φ ₁ ) + φ ₂ + 2πN (10) where N is an integer. Also, the phase change Δφ ₀ is
When expressed using the integer part N and the decimal part ε (0 ≦ ε <1),
It can also be expressed as in equation (11). Δφ ₀ = 2π (N + ε) (11) Comparing the equations (10) and (11), the fractional part ε (fraction) is expressed by the equation (12) using the phases φ ₁ and φ _2. be able to. [epsilon] = {(2 [pi]-[phi] ₁ ) + [phi] ₂ } / _{2 [} pi] (12) Therefore, the phase change [Delta] [phi] ₀ can be calculated from the equation (11) by appropriately calculating the number N of optical beat waves. Then, if the phase change Δφ ₀ is obtained, the optical path difference L can be obtained by applying the equation (2). If the optical path difference L is obtained in this way, the distance to the target object 4 can be easily obtained.

[0016]

As described above, according to the present invention, a laser light source whose output wavelength is repeatedly swept, and a light beam emitted from the laser light source are divided into two directions and are applied to an object light and a reference object. A beam splitter for splitting into a reference light, a light receiving means for detecting the intensity of an optical beat caused by interference of reflected light from the target object and the reference object, and a calculation for measuring the distance to the target object based on the output of the light receiving element Means, an optical path difference setting means for changing the optical path difference between the target object and the reference object, and the beam splitter by a specified amount, and different optical paths at the time when the output wavelength of the laser light source has the same wavelength for a few sub sweeps. A control means for controlling the optical path difference setting means so that the difference is set, and an optical path difference setting means for changing the optical path difference between the reference light and the object light by a specified amount. Since the output means of the laser light source is provided with the control means for controlling the optical path difference setting means so that different optical path differences are set at the times when the output wavelength of the laser light source is swept several times Even if the intensity modulation component that changes with the wavelength sweep is included, the intensity modulation component can be removed by changing only the optical path difference and making multiple measurements when the intensity becomes the same at each sweep. Since the phase can be obtained by using the above method, there is an advantage that the distance can be measured with high accuracy without considering the intensity modulation component.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment.

FIG. 2 is an operation explanatory diagram of the embodiment.

FIG. 3 is a schematic configuration diagram showing a conventional example.

FIG. 4 is an explanatory diagram showing a modulation current of a semiconductor laser in a conventional example.

FIG. 5 is an explanatory diagram showing an oscillation wavelength of a semiconductor laser in a conventional example.

FIG. 6 is an explanatory diagram showing an output intensity of a semiconductor laser in a conventional example.

[Explanation of symbols]

 1 Laser Light Source 2 Beam Splitter 3 Reference Mirror 4 Target Object 5 Photosensitive Element 6 Piezoelectric Element

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[Procedure amendment]

[Submission date] July 5, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

[0008]

According to the present invention, in order to achieve the above object, a laser light source whose output wavelength is repeatedly swept and a light beam emitted from the laser light source are divided into two directions to irradiate a target object. A beam splitter that splits the object light and the reference light that illuminates the reference object, a light receiving unit that detects the intensity of an optical beat caused by the interference of reflected light from the target object and the reference object, and a target based on the output of the light receiving element a calculating means for measuring a distance to an object, the optical path difference setting means for changing by a predetermined amount the optical path difference between the object and the reference object and the beam splitter, the output wavelength of the laser light source double
The control means controls the optical path difference setting means so that different optical path differences are set at the same wavelength for several sweeps.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

[0009]

According to the above construction, the optical path difference setting means for changing the optical path difference between the reference light and the object light by a specified amount, and the output wavelength of the laser light source at the same wavelength for a plurality of sweeps, respectively. By providing the control means for controlling the optical path difference setting means so that different optical path differences are set, when the intensity modulation component that changes with the sweep of the output wavelength of the laser light source is included, the sweep By changing the optical path difference only and making multiple measurements at each time when the intensities become the same, the intensity modulation component can be removed and the phase can be obtained. The distance can be measured accurately.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

[0012]

[Equation 1]

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

Here, in the half cycle when the change ΔL in the optical path difference is 0, as shown in FIG. 2A, the measurement points t ₁₁ and t
₂₁ is taken, and the output signals of the light receiving element 5 at the respective measurement points t ₁₁ and t ₂₁ are I ₁₁ and I ₂₁ . Further, as shown in FIG. 2B, the output signals I ₁₂ , I _{12 of} the light receiving element 5 are also at the measurement points t ₁₂ , t ₂₂ which are delayed by the period T _{m from the} measurement points t ₁₁ , t ₂₁ .
Find I ₂₂ . The output signals I ₁₂ and I ₂₂ are ΔL = λ ₀
The measured value is / 4 . Thereafter, similarly, the output signal of the light receiving element 5 is obtained at each cycle T _m from each measurement point (see FIG. 2).
(See (c) and (d)). In this way, output signals I ₁₁ , I ₂₁ , I ₁₂ , I ₂₂ , I ₁₃ , I ₂₃ , I ₁₄ ,
If I ₂₄ is calculated and applied to the equation (9), the measurement points t ₁₁ and t ₁₂
The phases φ ₁ and φ ₂ with respect to can be obtained.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

[0016]

As described above, according to the present invention, a laser light source whose output wavelength is repeatedly swept, and a light beam emitted from the laser light source are divided into two directions and are applied to an object light and a reference object. A beam splitter for splitting into a reference light, a light receiving means for detecting the intensity of an optical beat caused by interference of reflected light from the target object and the reference object, and a calculation for measuring the distance to the target object based on the output of the light receiving element Means, an optical path difference setting means for changing the optical path difference between the target object and the reference object and the beam splitter by a specified amount, and different optical path differences at the time when the output wavelength of the laser light source becomes the same wavelength for a plurality of sweeps. And an optical path difference setting means for changing the optical path difference between the reference light and the object light by a specified amount. Since there is provided a control means for controlling the optical path difference setting means so that each different optical path difference at multiple time points as the same wavelength with respect to the sweep of the output wavelength of the laser light source is set, the output wavelength of the laser light source Even if an intensity modulation component that changes with the sweep of is included, the intensity modulation component can be removed by changing only the optical path difference and performing multiple measurements at the time when the intensity becomes the same for each sweep. Since the phase can be obtained, there is an advantage that the distance can be measured with high accuracy without considering the intensity modulation component.

Claims (1)

[Claims] 1. A laser light source whose output wavelength is repeatedly swept, and a beam splitter which divides a light beam emitted from the laser light source into two directions and divides it into object light for irradiating a target object and reference light for irradiating a reference object. And a light receiving unit that detects the intensity of an optical beat generated by interference of reflected light from the target object and the reference object, a calculation unit that measures the distance to the target object based on the output of the light receiving element, and the target object and the reference object And an optical path difference setting means for changing the optical path difference between the beam splitter and the beam splitter by a specified amount, and an optical path difference so that different optical path differences are set at the time when the output wavelength of the laser light source is swept several times for the same wavelength. An interferometer, comprising a control means for controlling the difference setting means.