JPH0816602B2 - Displacement measuring device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement measuring device, and more particularly, to heterodyne interference capable of measuring displacement of an object to be measured with high accuracy by using a semiconductor laser whose oscillation frequency can be linearly modulated by a driving current. Type frequency modulation optical fiber displacement measuring device.

[0002]

2. Description of the Related Art Conventionally, a frequency modulation type heterodyne interferometric displacement measuring device has been known, for example, Japanese Patent Laid-Open No. 63-10.
1702 gazette. That is, this measuring device periodically changes the drive current applied to the semiconductor laser in a triangular wave shape to modulate the oscillation frequency and the emission intensity of the semiconductor laser. Then, this laser light is split into a reference light and an object light (light that is irradiated on the object to be measured) by a beam splitter, and the object light is emitted from the emission surface of the beam splitter to the measurement surface,
The reflected light is made incident again from the emission surface and is superimposed on the reference light. There is a time delay corresponding to the distance from the emission surface to the measurement surface between the object light reflected by the measurement surface and returned, and the reference light, and the frequencies of the two differ. Therefore, heterodyne interference occurs between the reference light and the object light, and the beat frequency thereof corresponds to the distance from the emission surface to the measurement surface. Therefore, the displacement of the measurement object is measured by counting the beat frequency.

However, since the above-mentioned conventional heterodyne interferometric displacement measuring device does not use an optical fiber in the optical path of the laser beam, it is easily affected by the environment and the size of the device becomes large due to the beam splitter, the mirror and the like. There's a problem. Further, since the measurement object is measured in a stationary state, the displacement of the moving measurement object cannot be measured in real time.

On the other hand, the present applicant has recently constructed most of the optical paths in the Fizeau interferometer by using optical fibers and optical fiber couplers to make a compact device that is not easily affected by the measurement environment, and to measure the measurement surface. We proposed a frequency modulation optical fiber displacement measuring device that can measure the displacement of a moving object in real time by utilizing the Doppler frequency shift due to the movement of the object (Japanese Patent Application No. 4-2114).
30 specification).

The interference signal between the reference light and the object light is detected by the photodiode, and the detected signal is affected by the change in emission intensity caused by the frequency modulation of the semiconductor laser. Therefore, in the above displacement measuring apparatus, the influence of the change in the emission intensity is removed from the interference signal by dividing the interference signal by the emission intensity of the semiconductor laser with a divider.

[0006]

By the way, in the above displacement measuring apparatus, it is preferable to increase the modulation frequency of the semiconductor laser.
Higher response speed has been clarified by theoretical studies and experimental results. However, the improvement of the response speed due to the increase of the modulation frequency is limited by the frequency modulation characteristic of the semiconductor laser and the frequency band of the signal processing circuit. In particular, the divider has a high frequency band in addition to the drawback that the error increases when the magnitude of the denominator signal approaches zero. Therefore, the divider has been one of the limiting factors for improving the response speed of the displacement measuring device.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a displacement measuring device which can eliminate the divider, speed up the measurement, and reduce the number of parts.

[0008]

In order to achieve the above object, the present invention provides a semiconductor laser and a sawtooth for inputting a sawtooth wave or triangular wave modulation current having a predetermined period T _s (frequency f _s ) to the semiconductor laser. First or second wave-shaped or triangular wave generating means
And a third optical fiber, and a first optical fiber for guiding laser light oscillated from the semiconductor laser to a tip of the first optical fiber.
And a fiber optic coupler for optically coupling the rear ends of the first, second and third optical fibers with each other,
An optical fiber coupler that guides light that has passed through the first optical fiber to the second optical fiber, and guides light that returns via the second optical fiber to the third optical fiber; A second lens disposed at the tip of the optical fiber for reflecting a part of the light on the emission end face and transmitting the rest of the light for substantially parallel light or condensing and irradiating the measurement surface; and the third optical fiber. A photoelectric conversion element that is disposed at the tip and photoelectrically converts light that enters through the third optical fiber, and a frequency nf that is n times the frequency f _s of the sawtooth wave or the triangular wave from the output of the photoelectric conversion element. _a bandpass filter for extracting a frequency component having a predetermined bandwidth with _s as a center frequency; first pulse output means for outputting a pulse signal having a frequency obtained by multiplying the frequency of the output waveform of the bandpass filter by m times; Saw blade A second pulse output for outputting a pulse signal having a frequency obtained by multiplying the same frequency as the frequency nf _s by m times in synchronization with the cycle T _s of the sawtooth wave or triangular wave modulation current output from the sawtooth wave or triangle wave generating means. Means, and a counter that integrates the difference in frequency generated by the relative movement of the second lens and the measurement surface in the pulse signals output from the first and second pulse output means, respectively. It is characterized by that.

[0009]

According to the present invention, most of the optical path of the laser light is constituted by the optical fiber and the optical fiber coupler, and a compact measuring device which is not easily affected by the measuring environment is provided. Moreover, since the Doppler frequency shift due to the movement of the measurement surface is used, the displacement of the moving object can be measured in real time. Furthermore, Japanese Patent Application No. 4-21143
Compared with the device described in the specification No. 0, the divider, which is one of the limiting factors of the response speed, is omitted, so that the measurement speed can be increased.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a displacement measuring device according to the present invention will be described in detail below with reference to the accompanying drawings. 1 is a block diagram showing an embodiment of the displacement measuring device according to the present invention.
(A)-(H) is a signal waveform diagram output from each part of FIG. 1 when using a sawtooth wave modulation current.

As shown in FIG. 1, this displacement measuring device is
Mainly sawtooth wave or triangular wave generator 10, semiconductor laser 12, collimator lens 14, objective lens 16, optical fiber coupler 18, optical fibers 21 to 23, selfoc lens 26, photodiode 30, bandpass filter 34, waveform shaper 36. , 38, frequency multiplication circuits 40, 4
2, a gate circuit 43, and an up / down counter 44.

The measurement principle when a sawtooth wave modulation current is used will be described below. The sawtooth wave or triangular wave generator 10 has a period T
Sawtooth wave modulation current S of _S (frequency f _s , angular frequency ω _s )
a (FIG. 2A) is a semiconductor laser 12 and a waveform shaper 3
6 is output. The sawtooth wave or triangular wave generator 10
Outputs a sawtooth wave modulation current Sa having a modulation width smaller than that of the bias current as shown in FIG.

The oscillation frequency and the emission intensity of the semiconductor laser 12 are modulated by the input sawtooth wave modulation current Sa.
The semiconductor laser 12 has a heating and / or cooling element 13
A and a temperature sensor 13B are provided, and the temperature controller 13 controls the heating and / or cooling element 13A so that the temperature detected by the temperature sensor 13B becomes a constant value. Thereby, the wavelength change due to the temperature change of the semiconductor laser 12 is suppressed and the measurement accuracy is improved.

The modulated laser light output from the semiconductor laser 12 is focused on the tip of the optical fiber 21 via the collimator lens 14 and the objective lens 16. The rear ends of the optical fibers 21 and 23 are optically connected to the rear ends of the optical fibers 22 by the optical fiber coupler 18. Therefore, the light that has passed through the optical fiber 21 is guided to the optical fiber 22 through the optical fiber coupler 18, and a part of the light that returns through the optical fiber 22 is guided to the optical fiber 23.

At the tip of the optical fiber 22, a part of the light is reflected at the exit end face, and the rest is transmitted and the substantially parallel light or condensed light is irradiated onto the measurement surface.
Is provided. That is, the emission end face of the SELFOC lens 26 is a plane perpendicular to the optical axis, and the reflected light reflected by the emission end face is converged again by the SELFOC lens itself and returns to the optical fiber 22. Further, the reflection on the exit end face of the SELFOC lens 26 is due to the difference in the refractive index between the SELFOC lens 26 and air, and about 4% of the incident light of the SELFOC lens 26 is reflected. The remaining approximately 96% of the light amount illuminates the measurement surface. Therefore, even if the measurement surface is a rough surface, and the rough measurement surface is not exactly perpendicular to the optical axis of the SELFOC lens 26, light having a light amount of a few percent of the light irradiating the measurement surface is generated by the cell. If the light is focused on the Fock lens 26, an interference signal of sufficient intensity can be obtained and displacement measurement can be performed.

Now, the end face reflected light (reference light) of the SELFOC lens 26 and the measurement surface reflected light (object which is emitted from the SELFOC lens 26 and reflected by the measurement surface of the object 27 to be incident on the SELFOC lens 26 again). Light)
There is a time delay corresponding to the distance D between the emission end surface of the SELFOC lens 26 and the measurement surface of the measurement object 27, and the frequencies of both are different. Therefore, heterodyne interference occurs between the reference light and the object light.

This interference signal is guided through the optical fiber 22, the optical fiber coupler 18 and the optical fiber 23, and detected by the photodiode 30 installed at the tip of the optical fiber 23. The frequency and phase of the signal Se (FIG. 2C) detected by the photodiode 30 are proportional to the distance D between the emission end face of the SELFOC lens 26 and the measurement surface of the measurement object 27. The level of the signal Se is influenced by the emission intensity of the semiconductor laser 12 and the reflectance of the measurement surface.

By the way, in the device described in Japanese Patent Application No. 4-211430, in order to remove the influence of the change in the emission intensity of the semiconductor laser 12 from the signal detected by the photodiode 30, a photodiode is used by a divider. The signal detected by 30 was divided by the signal indicating the emission intensity of the semiconductor laser 12. In the present invention, the use of the divider can be omitted according to the following principle.

First, the oscillation frequency modulation and the emission intensity modulation of the semiconductor laser will be described. As a semiconductor laser,
When a sawtooth wave modulation current Sa (t) having a period T _s and a modulation width Δi as shown in (A) is input, the oscillation angular frequency ω (t) and the emission intensity I (t) of the semiconductor laser are also modulated in a sawtooth shape. (See FIGS. 3B and 3C). Here, one modulation period (-T _s
/ 2 ≦ t ≦ T _s / 2), the sawtooth wave modulation current Sa
Let (t) be the following expression: Sa (t) = i ₀ + Δi · t / T _s = i ₀ + α _i · t (1). Oscillation angular frequency ω (t) and emission intensity I (t)
Is the following equation: ω (t) = ω ₀ + Δω · t / T _s = ω ₀ + k _wi · α _i · t (2) I (t) = I ₀ + ΔI · t / T _s = I ₀ + k _Ii・ It can be written as α _i · t (3).

However, the definitions of i ₀ , ω ₀ , Δi, Δω, and ΔI are shown in FIG. Further, α _i = Δi / T _s (current modulation rate) k _wi = Δω / Δi (modulation constant of oscillation angular frequency) k _Ii = ΔI / Δi (modulation constant of emission intensity)

Next, the interference signal detected by the photodiode 30 will be described. SELFOC lens 2
The emission intensity of the semiconductor laser 12 reaching 6 is ηI (t). However, η is a constant determined by the coupling rate of the laser light to the optical fiber 21 and the distribution rate of the optical fiber coupler 18. Further, the intensity of the end surface reflected light (reference light) of the SELFOC lens 26 is I _r (t), and the reflected light (object light) of the measurement surface returning to the SELFOC lens 26 is I
Assuming that ₀ (t), I _r (t) and I ₀ (t) are expressed by the following equation: I _r (t) = β _r · ηI (t) (4) I ₀ (t) = β ₀ · ηI (t)… It becomes like (5). However, β _r and β ₀ are reflection coefficients.

The reference light and the object light undergo heterodyne interference, and the interference signal Se detected by the photodiode 30 is detected.
(t) is given by the following equation. Se (t) = K · ηI (t) × _{[β r + β 0 +2 (β} r β 0) 1/2 cos (ω b t + φ b) ] ... (6) where, K is the photoelectric conversion of the photodiode 30 , Ω _b ,
φ _b is the angular frequency and phase of the beat signal, and is expressed by the following equation, ω _b = 2Δω / T _S τ (7) φ _b = ω ₀ τ (8) In Expressions (7) and (8), τ is the time delay between the reference light and the object light, as shown in FIG.

In the processing method described in Japanese Patent Application No. 4-211430, I (t) in the equation (6) is removed by a divider, and the direct current component (β _r + β ₀ ) is further cut. , Se (t) = A · cos (ω _b t + φ _b ) ... (9) was obtained. On the other hand, according to the present invention, I (t) of the formula (6) is
Can be represented by the formula (3), the formula (3)
If (k _Ii · α _i · t) is smaller than I ₀ , the following equation: I ₀ >> | k _Ii · α _i · t _max | = k _Ii · α _i · T _s / 2 = k _{If Ii} (Δi / T _s ) (T _s / 2) = k _Ii · Δi / 2 = ΔI / 2 (10) is satisfied, (k _Ii · α _i · t) can be ignored. Then, in this case, the above equation (6) is given by the following equation: Se (t) ≈K · ηI ₀ [β _r + β ₀ +2 (β _r β ₀ ) ^1/2 cos (ω _b t + φ _b )] ... (11) is obtained, and the above equation (9) can be obtained by cutting the DC component (β _r + β ₀ ).

In order to satisfy the above equation (10),
Can be done as follows: (1) I₀To increase. (2) Reduce Δi. (3) k_IiSmaller. The above (1) and (2) are the modulation current Sa (t) shown in the equation (1).
At the bias current i₀To increase the modulation width Δi
It means to make it smaller. Also, k in (3)_IiIs a semiconductor
It is not possible to freely change this value due to the intrinsic characteristics of the laser.
I can't, but k _IiIs small and k_wiLarge semiconductor
A laser may be selected and used.

In the present embodiment, as described above, the sawtooth wave or triangular wave generator 10 has a sawtooth wave modulation current S having a small modulation width with respect to the bias current as shown in FIG.
As a result, the influence of the change in emission intensity of the semiconductor laser 12 on the interference signal Se detected by the photodiode 30 is negligibly small as shown in FIG. 2 (C).

Now, referring to FIG. 1, the photodiode 30
The interference signal Se detected by is output to the bandpass filter 34. The bandpass filter 34 has a center frequency f _s and a bandwidth Δν (for example, f _s / 10), takes out a component of f _s from the input signal Se, and waveform-shapes the taken out signal Sf (FIG. 2 (D)). To the container 38.
The waveform shaper 38 converts the input signal Sf into a rectangular wave signal Sg (FIG. 2 (E)), and the frequency multiplication circuit 42 outputs this signal Sg.
The frequency g is a preset magnification m (m = 2 in FIG. 2)
So that the signal becomes a pulse signal.
Sg ′ is output to the input G ₁ of the gate circuit 43 (see FIG. 2).
(See (G) and (H)).

On the other hand, the waveform shaper 36 converts the sawtooth wave modulation current Sa input from the sawtooth wave or triangle wave generator 10 into a rectangular wave signal Sb (FIG. 2 (B)), and the frequency multiplication circuit 40 uses this. The frequency of the signal Sb is multiplied by the frequency multiplication circuit 42.
Multiplied by the same multiplication factor, and the multiplied signal (pulse signal)
Sb ′ is output to the input G ₂ of the gate circuit 43 (see FIG. 2).
(See (F)).

The gate circuit 43, the input G ₁ 'and enter G ₂ of the pulse signal Sb' pulse signal Sg create a new pulse signal by the frequency difference, and outputs the up input U or down input D of the up-down counter 44 . Therefore, the up / down counter 44 integrates the frequency difference between the pulse signal Sb 'and the pulse signal Sg'. By the way, when the measurement object 27 is stopped, the frequency of the pulse signal Sg ′ output from the frequency multiplication circuit 42 is the reference pulse signal S output from the frequency multiplication circuit 40.
Although the count value of the up / down counter 44 does not change, the signal Sg output from the waveform shaper 38 due to the Doppler frequency shift (the output from the frequency multiplication circuit 42). The frequency of the pulse signal Sg ') is changed. That is, when the measurement object 27 moves in a direction away from the SELFOC lens 26,
The frequency of the pulse signal Sg 'increases (see FIG. 2G), and when the measurement object 27 moves toward the SELFOC lens 26, the frequency of the pulse signal Sg' decreases (see FIG. 2H). .

Therefore, the pulse signal Sb 'and the pulse signal S
The difference in the number of pulses of g ′ corresponds to the moving speed and the moving direction of the measurement object 27, and the integrated value of the difference in the number of pulses is the measurement object 27.
Corresponding to the amount of movement. Accordingly, the moving distance of the measurement object 27 can be measured based on the amount of increase or decrease in the count value of the up / down counter 44 when the measurement object 27 is moved from a certain position to another position.

The modulation current waveform is not limited to the sawtooth wave, but a triangular wave may be used. When a triangular wave is used, the doubled sensitivity can be obtained by inverting the phase of the folded portion and adding it to the first half portion. Next, the maximum velocity of the measurement surface that can measure displacement will be described. The interference signal S detected by the photodiode 30
As described above, e can be expressed by the equation (9), and the signal Sf (t) output from the bandpass filter 34 can be expressed by the following equation: Sf (t) = B · cos (ω _S t + φ _b ) ... (12)

Now, considering the case where the measurement surface moves at a speed v from the initial distance D ₀ and the distance D can be expressed as D = D ₀ + vt (13), equation (12) gives Sf (t) = B · cos {2π [(f _s + Δf _D ) t + 2D ₀ / λ ₀ ]} (14) Here, Δf _D is the Doppler frequency shift due to the movement of the measurement surface and can be expressed by the following equation.

Δf _D = 2v / λ ₀ (15) On the other hand, in order to obtain Sf (t) from the bandpass filter 34, Δf _D is the bandwidth Δν of the bandpass filter 34.
It needs to be smaller. Therefore, the maximum moving speed v _max of the measurement surface on which the displacement can be measured is as follows. _{v max = λ 0/2 ·} Δν ... as shown in (16) (16), to increase the maximum moving speed v _max, it is necessary to increase the bandwidth .DELTA..nu of the bandpass filter 34. In the embodiment shown in FIG. 1, although the center frequency of the bandpass filter 34 and the f _s, the signal Se output from the photodiode 30, because it is continuously modulated in period T _S, the period T _S Is a periodic function, f
Each frequency component of _s , 2f _s , ..., Nf _s ,. Now, the center band-pass filter of the frequency nf _s, when to extract the component of the frequency nf _s, equation (12), Sf (t) = B · cos (nω S t + φ b) = B · cos [2π (nf _s + Δf _D ) t]. Then, Δν is 1/10 of the center frequency nf _s
When set to, formula (16), and _{v max = λ 0/2 ·} Δν = λ 0/2 · nf s / 10 ... (17). Therefore, the maximum speed can be increased by increasing n and f _s .

By the way, when the oscillation frequency of the semiconductor laser 12 is linearly modulated by the drive current, the upper limit of the frequency f _s of the drive current is limited by the modulation characteristic of the semiconductor laser 12. On the other hand, the intensity of the frequency component extracted by the bandpass filter is the sine function sin [(
ω _b −n ω _S ) T _S / 2] / [(ω _b −n ω _S ) T _S /
2]. The value of the sink function is ω _b = nω _S
At time 1, the maximum value becomes 1, and it sharply decreases according to the difference between ω _b and n ω _{S. When} it exceeds a certain level, the intensity of the output signal of the bandpass filter 34 becomes small, and the waveform shaper 38 is accurate. The pulse signal is no longer output and measurement becomes impossible.

Although the value of n is limited under the above conditions, a large n can be obtained by increasing the angular frequency ω _b of the beat signal. In order to increase the angular frequency ω _b , the frequency modulation width of the semiconductor laser 12 may be increased or the distance D may be increased. FIG. 5 is a schematic view of a main part showing another embodiment of the present invention. As shown in the figure, in this embodiment, an SELFOC lens 26 and an offset setting optical fiber 25 for increasing the optical path difference from the measurement surface of the measurement object 27 are provided. By using the spiral optical fiber 25, the SELFOC lens 2
The optical path difference can be made significantly larger than the distance between 6 and the object to be measured, and the influence of the measurement environment therebetween can be reduced. In addition, quarter-pitch SELFOC lenses 25A and 25B with AR coating are connected to both ends of the optical fiber 25. As described above, by increasing the distance D by the optical fiber 25, a large n can be taken, that is, the maximum moving speed v _max can be increased.

When the center frequency of the bandpass filter 34 is set to nf _s , the frequency of the pulse signal Sb 'of the frequency multiplication circuit 40 in FIG. 1 also needs to be n times.

[0036]

As described above, according to the displacement measuring apparatus of the present invention, most of the optical path of the laser beam is composed of the optical fiber and the optical fiber coupler. The measurement can be performed with high resolution, and the device can be made compact. Moreover, since the Doppler frequency shift due to the movement of the measurement surface is used, the displacement of the moving object can be measured in real time. Further, conventionally, it is general to provide a reflecting mirror on the measurement surface, but according to the present invention, the object to be measured is not limited to the reflecting mirror, and a rough surface of various materials such as metal, plastic, acrylic, and paper, It has been confirmed by a measurement test that measurement is possible even if the rough measurement surface is inclined.

Further, as compared with the device described in Japanese Patent Application No. 4-21140, the divider, which is one of the limiting factors of the response speed, is omitted, so that the measurement speed can be increased. You can

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a displacement measuring device according to the present invention.

FIGS. 2A to 2H are signal waveform diagrams output from the respective units of FIG. 1 when a sawtooth wave modulation current is used.

3 (A) to 3 (C) are waveform diagrams showing a modulation current of a sawtooth wave input to a semiconductor laser, an oscillation angular frequency of the semiconductor laser generated by the modulation current, and a light emission intensity, respectively.

FIG. 4 is a waveform diagram showing oscillation angular frequencies and changes in emission intensity of object light and reference light when a sawtooth wave modulation current is used.

FIG. 5 is a schematic view of a main part showing another embodiment of the present invention.

[Explanation of symbols]

 10 ... Sawtooth wave or triangular wave generator 12 ... Semiconductor laser 13 ... Temperature controller 13A ... Heating and / or cooling element 13B ... Temperature sensor 14 ... Collimator lens 16 ... Objective lens 18 ... Optical fiber coupler 21, 22, 23, 25 ... Optical fibers 25A, 25B, 26 ... Selfoc lens 27 ... Measured object 30 ... Photodiode 34 ... Bandpass filter 36, 38 ... Waveform shaper 40, 42 ... Frequency multiplication circuit 43 ... Gate circuit 44 ... Up-down counter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor's chapter 耀 耀 Chinese People's Republic of China, Beijing, Haidian District (no address) Inside Tinghoa University (72) Inventor Tang Dong Lei 9 Shimorenjaku, Mitaka City, Tokyo 7-1 Tokyo Seimitsu Co., Ltd. (72) Inventor Akira Shimokawa 1-20-17 Ogawa, Machida-shi, Tokyo (72) Inventor Masahiro Akabane 9-7 Shimorenjaku, Mitaka-shi, Tokyo In-house Tokyo Seimitsu Co., Ltd. (72) Inventor Kenji Sakai 9-7 Shimorenjaku, Mitaka-shi, Tokyo Tokyo Seimitsu Co., Ltd. (56) References JP-A-3-15709 (JP, A) JP-A-4-121826 (JP, A) JP-A-4-121612 (JP, A) JP-A-4-120402 (JP, A) JP-A-1-153923 (JP, A) JP-B-6-63726 (JP, B2)

Claims (3)

[Claims] 1. A semiconductor laser, a sawtooth wave or triangle wave generating means for inputting a sawtooth wave or triangle wave modulation current having a predetermined period T _s (frequency f _s ) to the semiconductor laser, and first, second and An optical fiber No. 3; a first lens for guiding the laser light oscillated from the semiconductor laser to the tip of the first optical fiber; and an optical path between the rear ends of the first, second, and third optical fibers. An optical fiber coupler that optically couples the light that has passed through the first optical fiber to the second optical fiber and returns the light that returns via the second optical fiber to the third optical fiber. An optical fiber coupler that guides to the second optical fiber, and a second optical fiber that is disposed at the tip of the second optical fiber and reflects a part of the light at the emission end face and allows the rest to pass through and to irradiate the measurement surface with substantially parallel light or condensed light. Lens, and the third optical fiber A photoelectric conversion element that is disposed at the tip of the fiber and photoelectrically converts light that enters through the third optical fiber; and an output of the photoelectric conversion element that is n times the frequency f _s of the sawtooth wave or the triangular wave. A bandpass filter for extracting a frequency component having a predetermined bandwidth centered on the frequency nf _s ; and a first pulse output means for outputting a pulse signal having a frequency obtained by multiplying the frequency of the output waveform of the bandpass filter by m times. A second pulse signal having a frequency obtained by multiplying the same frequency as the frequency nf _s by m times in synchronization with the cycle T _s of the sawtooth wave or triangular wave modulation current output from the sawtooth wave or triangular wave generating means; Difference between the pulse output means and the frequency signals generated by the relative movement of the second lens and the measurement surface in the pulse signals respectively output from the first and second pulse output means. Displacement measuring apparatus characterized by comprising a counter for accumulating and. 2. The displacement measuring device according to claim 1, wherein the sawtooth wave or triangular wave generating means generates a sawtooth wave or triangular wave modulation current having a small modulation width with respect to a bias current. 3. An offset setting optical fiber for increasing an optical path difference between the second lens and the measurement surface is provided between the second lens and the measurement surface. 1 or 2 displacement measuring device.