JPH06129811A - Displacement correcting method for laser length measuring instrument - Google Patents

Abstract

PURPOSE:To provide a displacement correcting method, for a laser length measuring instrument, capable of reducing any error caused when measuring a light path difference. CONSTITUTION:In a laser length measuring instrument utilizing a Michelson type interferometer for which a wavelength variable light source or a frequency variable light source is used, the wavelength or frequency of a light source is modified by producing a difference between the time required for continuously and monotonously increasing or decreasing the wavelength or frequency of the light source and the time thereafter required for continuously and monotonously increasing or decreasing the same, and then the differential quantities of the interference degree being generated within respective periods of time are measured. By utilizing such an event that under the measured errors generated condition of a constant dispacement rate withing both measurement periods of time are proportional to respective variation periods of time, the dispacement quantity is found out from the difference between both interference degrees and the time differences. Subsequently, the light path length difference between both reflection mirrors in Michelson type interometer can be corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a length measuring system (FM length measuring system) for obtaining a difference in optical path length between two reflecting mirrors in a Michelson interferometer by modulating a wavelength or a frequency of a light source. The present invention relates to a method for reducing an error caused by movement of measurement symmetry that occurs during one cycle of.

[0002]

2. Description of the Related Art Conventionally, when a wavelength (or frequency) of a light source is modulated to obtain a difference in optical path length between both reflecting mirrors in a Michelson interferometer, a frequency increasing process or a frequency decreasing process within one period of modulation is performed. On the other hand, or, the optical path length difference is obtained from the frequency change amount in each process and the change amount of the interference order caused by the frequency change. This can be expressed by the following equation. L = ΔM · (C / Δυ) ─────── (1) where, L: Optical path length difference Δυ: Light source frequency change amount C: Light speed ΔM: Interference order change amount.

However, while changing the frequency, the optical path length difference changes (displacement occurs in the measuring object).
Then, a measurement error occurs by υ / Δυ times the optical path length difference change amount (υ is the frequency at the start of change). This is because the optical path length difference at the start of frequency change is L, the frequency is υ, the interference order is M,
If the optical path length difference at the end of frequency change is L + ΔL, the frequency is υ + Δυ, and the interference order is M + ΔM, then L = M · C / υ ─ (2) L + ΔL = (M + ΔM) ・ C / (υ + Δυ) ─ (3) Therefore, ΔM = (L + ΔL) · (Δυ / C) + ΔL · (υ / C) (4) Therefore, L + ΔL = ΔM · C / Δυ−ΔL · υ / Δυ ─ (5). That is, the second item on the right side of the above equation (5) is an error. Currently, when using a semiconductor laser, Δυ / υ is 1 /
Since it is 100 to 1/1000, the length measurement error becomes 100 to 1000 times, and it is difficult to obtain submicron accuracy.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art. In the frequency modulation of the light source, the frequency change speeds of adjacent periods are alternately changed to perform modulation. By generating different errors between alternate cycles by the distance moved within both time differences, and obtaining the change speed of the optical path length difference from the difference in time required for change,
An object of the present invention is to provide a method for correcting a displacement amount of a laser length measuring device, which can reduce an error caused in measuring an optical path length difference.

[0005]

DISCLOSURE OF THE INVENTION The present invention for solving the above-mentioned problems provides a laser length measuring device using a Michelson type interferometer using a wavelength tunable light source or a frequency tunable light source. The frequency is continuously and monotonically increased or decreased, and the subsequent time is continuously and monotonically increased or decreased.The frequency is modulated with a difference, and the amount of change in the interference order occurring in each time is measured. By utilizing the fact that the measurement error that occurs when the displacement velocity is constant within the measurement time is proportional to each change time, the displacement amount is obtained from the difference between the change amounts of both interference orders and the time difference. It is characterized in that the optical path length difference between both reflecting mirrors of the Son-type interferometer is corrected.

[0006]

According to the present invention, in the frequency modulation of the light source,
By changing the frequency change speed of adjacent cycles alternately and modulating, the error which is different between the cycles by the distance moved within both time differences is generated, and the change speed of the optical path length difference is calculated from the difference in the time required for the change. Since it can be obtained, the error that occurs in the optical path length difference measurement can be reduced.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a device configuration diagram of a laser length measuring device using a wavelength tunable laser diode (hereinafter, simply referred to as an LD) for carrying out the displacement amount correcting method of the present invention. In FIG. 1, reference numeral 1 denotes a tunable LD, in which a constant current from a drive current circuit 2 is used as an active region of the tunable LD 1 and a sawtooth-shaped tuning current from a tuning current circuit 3 is used as a diffraction grating portion of the tunable LD 1. The wavelengths that continuously change can be repetitively generated by each of these. A Peltier element 4 can heat or cool the variable wavelength LD 1. A temperature detector 5 measures the temperature of the variable wavelength LD 1 and feeds it back to the Peltier element 4 via the temperature controller 6 to keep the temperature of the variable wavelength LD 1 constant. Reference numeral 7 denotes a collimator lens, which collimates the light emitted from the wavelength tunable LD 1 to form parallel light. 8 is an isolation
And the reflected light from the interferometer described later is tunable LD1.
To prevent the oscillation from becoming unstable.

The light that has passed through the isolator 8 is split into two by a half mirror 9. The reflected light is guided to the etalon 10. When the wavelength of the tunable LD 1 changes, light is emitted by the etalon 1 for each specific wavelength determined by the optical path length of the etalon 10.
Since 0 is transmitted, the transmitted light is condensed by the condenser lens 11, detected by the photodetector 12, and converted into a voltage signal by the amplifier 13. A peak detector 14 detects the peak of the etalon transmitted light signal of the amplifier 13 and triggers the timer 15. The output of the peak detector 14 is integrated by the peak counter 16 to obtain the frequency change amount of the variable wavelength LD 1.

On the other hand, the transmitted light of the harmylar-9 is further branched into two by the harmylar-17. The reflected light is reflected by the rectangular prism 18 whose position is fixed in the optical system, and becomes the reference light of the interferometer. The transmitted light from the half mirror 17 is reflected by the rectangular prism 19 whose position is to be measured and becomes the test light for the interferometer. The reference light and the test light from the right-angle prisms 18 and 19 are combined by the half mirror 17 to form an interference signal, which is condensed by the condenser lens 20, detected by the photodetector 21, and detected by the amplifier 22 as a voltage signal. Is converted to. The phase of the voltage signal is detected by the phase detector 23, and the phase integrator 24 determines the phase change amount. The outputs of the timer 15, the peak counter 16 and the phase integrator 24 are converted into distance and velocity in a signal processing device (not shown).

Here, FIG. 2 shows the optical path length difference, each electric signal and the frequency of light obtained when the rectangular prism 19 is displaced at a constant speed in the apparatus of FIG.
(A) The figure shows the optical path length difference of the interferometer corresponding to the position of the rectangular prism 19, and it is assumed that the optical path length difference changes at a constant speed. (B) The figure shows the tuning current circuit 3 to the tunable LD1.
It is a voltage signal that changes in the same way as the tuning current that flows in, and has a sawtooth waveform. The figure (c) shows how the wavelength of the emitted light of the variable wavelength LD 1 changes. (D) The figure shows an etalon transmitted light signal obtained by the amplifier 13. Etalon is a fixed FSR (Free Spectral Range) determined by its optical path length
The transmittance becomes higher with respect to the wavelength (or frequency) of each light, and the interval between the peaks extending downward in the figure indicates the wavelength (or frequency). However, in the figure, the negative side represents the presence of transmitted light due to the characteristics of the circuit of the device.
(E) The figure shows the output signal of the interferometer obtained by the amplifier 22. However, in the figure, due to the characteristics of the circuit of the device, the minus side indicates that an interference signal is output. As shown in FIG. 3, the tuning power of the tuning current circuit 3 changes the output power of the wavelength tunable LD 1, so that the amplitude of the interference signal is modulated.

In such a configuration, the method of correcting the displacement amount of the laser length measuring machine according to the present invention will be described below with reference to the signal processing flowchart shown in FIG. First, Figure 2 (b)
At point A, the tuning current is made zero. Then, the peak counter 16 (1) is reset. Next, the tuning current becomes maximum when the amount of change per unit time dI / dt = a (b).
Increase to point B in the figure.

Here, until the tuning current reaches the maximum value,
Find the peak of the transmitted light amount of the etalon. If the detected peak is the first one, timer 15 and phase integrator 24
To reset. If it is the second and subsequent peaks, the value of the timer 15 is set to t ₁ each time, and the phase integrator 24
_Is rewritten as the phase integrated value _Mab in the section AB. Further, the value of the peak counter 16 (1) is incremented by 1.

When the tuning current reaches the maximum value, the time t ₁ , the number of peaks, and the phase integrated value _Mab at the time when the peak is finally detected are recorded. The number of peaks is multiplied by the FSR of the etalon to obtain the frequency change amount Δυ.

Next, the tuning current is returned to zero. (B) C in the figure
When the point is reached, the peak counter 16 (2) is reset. Next, change amount of tuning current per time dI / dt =
At c, the maximum value is increased to point D in (b).

Here, until the tuning current reaches the maximum value,
Find the peak of the transmitted light amount of the etalon. If the detected pea
If it is the first one, reset the phase integrator 24
The value of timer 15 to t₂And Second and subsequent peaks
If so, the value of the timer 15 is changed to t each time.₃As
The value of the phase integrator 24 is set to the phase integrated value M in the section CD.
_cdRewrite as In addition, the peak counter 16 (2)
Increase the value by 1.

When the tuning current reaches the maximum value, the time t ₃ , the number of peaks, and the integrated phase value M _cd at the time when the peak is finally detected are recorded. The number of peaks is multiplied by the FSR of the etalon to obtain the frequency change amount Δυ. In addition, here, in order to simplify the description, it is assumed that the frequency change amount Δυ is the same in the section AB and the section CD. Distance and velocity are calculated from the values obtained in the above process, and the tuning current is returned to zero.

Next, the formulas for calculating the distance and velocity will be described below. First, in both sections, the times t ₀ (= 0), t ₁ , t ₂ , and t ₃ at which the first and last peaks were detected.
Therefore, the time between the detection of the first and last peaks is T ₁ = t ₁ −t ₀ ─ (6) T ₂ = t ₂ −t ₁ ─ (7) T ₃ = t ₃ −t ₂ ─ (8 ).

Here, assuming that the distance at time t ₀ is L ₀ , the interference order is M ₀ , and the speed of light is C, it can be expressed as L ₀ = M ₀ .C / υ ─ (9).

Next, assuming that the distance at time t ₁ is L ₁ and the interference order is M ₁ , it can be expressed as L ₁ = M ₁ · C / (υ + Δυ)-(10). Since the phase integrated value obtained in the section AB is M _ab = M ₁ −M ₀ ─ (11), from the above formulas (9) and (10), M _ab = L ₁ · (υ + Δυ) / C -L ₀ · υ / C = L ₁ · Δυ / C + (L ₁ −L ₀ ) · υ / C = L ₁ · Δυ / C + V · T ₁ · υ / C ─ (12) where V is the optical path length difference Change speed of.

Similarly, the distance at time t ₂ is L
₂ and the interference order is M ₂ , it can be expressed as L ₂ = M ₂ · C / υ ─ (13).

Next, assuming that the distance at time t ₃ is L ₃ and the interference order is M ₃ , it can be expressed as L ₃ = M ₃ · C / (υ + Δυ)-(14). Since the phase integrated value obtained in the section CD is M _cd = M ₃ −M ₂ ─ (15), from the above equations (13) and (14), M _cd = L ₃ · (υ + Δυ) / C -L ₂ · υ / C = L ₃ · Δυ / C + (L ₃ −L ₂ ) · υ / C = L ₃ · Δυ / C + V · T ₃ · υ / C ─ (16)

From the above equation (16)-(12), M _cd -M _ab = V · (T ₂ + T ₃ ) · Δυ / C −V · (T ₃ −T ₁ ) · υ / C (17) Therefore, the velocity is calculated as V = (M _cd -M _ab ) · C / υ / {(T ₃ −T ₁ ) + (T ₂ + T ₃ ) · Δυ / υ} ─ (18).

By substituting this value into the above equation (16), the distance at time t ₃ can be corrected. That is, L ₃ = M _cd · C / Δυ−V · T ₃ · υ / Δυ (19).

The above is the method of correcting the displacement amount of the laser length-measuring device of the present invention. Here, more detailed description will be given by substituting specific values. Optical path length difference is 100 mm, optical path length difference change speed is 1 mm / s, T ₁ is 0.3 ms, T ₂ is 0.1 m
s, T ₃ of 0.6 ms, wavelength of 850 nm to 848 n
It is assumed that the phase change amount is measured while changing to m.
That is, when the speed of light is 3 * 10 ⁸ m / s, υ = 3.529 * 10 ¹⁴ Hz υ + Δυ = 3.538 * 10 ¹⁴ Hz Δυ = 9 * 10 ¹¹ Hz. When the interference order is calculated from the above equations (9), (10), (13), and (14), M ₀ = 117633.3333 M ₁ = 1173933.6871 M ₂ = 117633.8039 M ₃ = 117934.5127 Therefore, Since the amount of phase change is M _ab = 300.3411 M _cd = 300.7088, it becomes M _cd −M _ab = 0.3547 (corresponding to 0.3547 * 2π [rad]).

From the above values, the speed is 1 from the above equation (18).
It is required to be mm / s. The largest error factor is the phase measurement accuracy, but ε (M _cd -M _ab )
Is 1/50, the velocity measurement error is ε (M _cd −M _ab ) · C / υ / (T ₃ −T ₁ ) = 0.0563 mm / s. The displacement amount correction error due to this speed error is V · T ₃ · υ / Δυ = 0.0132 mm. When the displacement amount correction by this method is not used,
Since V = 1 mm / s, an error of 0.235 mm is added. That is, the error can be reduced to 0.0132 / 0.235≈1 / 18.

In the above embodiment, instead of counting the peak number of the transmitted light amount of the etalon to obtain the frequency change amount of the light source, the phase change amount of the interferometer with a stable optical path length difference is measured to obtain the distance. The same correction can be performed by using the ratio with the phase change amount obtained from the interferometer unit that performs the measurement.

[0027]

As described above in detail with reference to the embodiments, according to the present invention, in the frequency modulation of the light source, the frequency change speeds of the adjacent periods are alternately changed so that the time difference between the two periods is reduced. It is possible to reduce the error that occurs in the measurement of the optical path length difference because it is possible to generate the error that is different between the alternating distances by the distance moved by the distance and obtain the changing speed of the optical path length difference from the difference of the time required for the change. It is possible to realize a method for correcting the displacement amount of the length measuring device.

[Brief description of drawings]

FIG. 1 is an apparatus configuration diagram of a laser length measuring device using a wavelength tunable laser diode for carrying out a displacement amount correcting method of the present invention.

FIG. 2 is a diagram showing an optical path length difference, electric signals, and light frequencies obtained when the rectangular prism 19 is displaced at a constant speed in the apparatus shown in FIG.

FIG. 3 is a diagram showing a relationship between a tuning current and a wavelength and an emission power in the wavelength tunable LD.

FIG. 4 is a signal processing flowchart showing a displacement amount correction method for a laser length measuring device according to the present invention.

[Explanation of symbols]

 1 Wavelength tunable laser diode 2 Driving current circuit 3 Tuning current circuit 4 Peltier element 5 Temperature detector 6 Temperature controller 7 Collimator lens 8 Isolator 9,17 Half mirror 10 Etalon 11, 20 Focusing lens 12, 21 Photodetector 13, 22 Amplifier 14 Peak detector 15 Timer 16 Peak counter 18, 19 Right angle prism 23 Phase detector 24 Phase accumulator

Claims (1)

[Claims] 1. A laser length measuring device using a Michelson-type interferometer using a variable wavelength light source or a variable frequency light source, wherein a time for continuously or monotonously increasing or decreasing the wavelength or frequency of the light source, Modulation is performed with a difference between the time to continuously and monotonously increase or decrease, the change amount of the interference order that occurs within each time is measured, and the length measurement occurs when the displacement velocity is constant within both measurement times. Utilizing the fact that the error is proportional to each change time, the displacement amount is obtained from the difference between the change amounts of both interference orders and the time difference, and the optical path length difference between both reflecting mirrors of the Michelson interferometer is obtained. A method for correcting a displacement amount of a laser length measuring device, which is characterized in that correction is applied.