JPH0455702A - Interference-signal processing method for absolute-length measuring machine - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an improvement in a method for counting the number of interference fringes when performing absolute length measurement using a variable wavelength (frequency) light source.

<Conventional technology> Conventionally, when performing absolute length measurement using a variable wavelength (frequency) light source, a Fabry-Bello type laser diode is used as the light source, and the difference between mode hops when the drive current is varied is measured. Continuous wavelength variations have been used. In this method, the change in wavelength with respect to the change in drive current is linear, and the absolute distance can be determined from the amount of phase change of the interference signal by continuously and monotonically changing the light source wavelength (frequency). In contrast, wavelength tunable laser diodes (hereinafter referred to as
Using a variable wavelength LD (simply referred to as a wavelength tunable LD) allows the amount of change in frequency (wavelength) to be increased by 10 to 100 times, which has the advantage of improving length measurement accuracy.

<Problem to be solved by the invention> However, when performing absolute length measurement using the wavelength tunable LD shown in the above-mentioned prior art, the current for wavelength change (
Since the change in the wavelength with respect to the change in the tuning current (hereinafter simply referred to as the tuning current) is not linear as shown in ■ in Figure 8, the change in the phase of the interference signal is also not linear, and the output power (■ in Figure 8), the amplitude of the interference signal is also not constant. Therefore, when counting the number of interference fringes, the periodic interval of the interference fringes changes depending on the frequency, so the fractional part on the time axis When attempting to measure the fraction of the phase based on the time, an error occurs and the absolute length (optical path length difference) cannot be determined with high precision.

The present invention has been made based on the above-mentioned problems of the prior art.
The purpose of the present invention is to provide an interference signal processing method for an absolute length measuring device that can obtain a highly accurate absolute length using a frequency-variable light source.

Means for Solving the Problems〉 The interference signal processing method of the present invention for solving the above tasks uses an absolute length measuring device using a Michelson type interferometer using a variable wavelength light source or a variable frequency light source. , find the upper and lower envelopes of the interference signal generated when the frequency or wavelength of the light source is continuously and monotonically changed, find the zero level of the interference signal from the median of these upper and lower envelopes, and calculate the zero level of the interference signal. Find the zero-crossing point of the interference signal from the intersection of By finding a correlation curve between the phase integrated value and the phase integrated value at this time, and finding the phase integrated value between constant wavelength changes during the wavelength change from the correlation curve, This feature is characterized in that the optical path length difference between the two is determined.

According to the present invention, the phase integrated value during a certain wavelength change is determined from the correlation curve between the time of the zero crossing point of the interference signal and the phase integrated value at this time. (Frequency) Even if the frequency of the variable light source does not change linearly with respect to time, the phase integrated value of the interference signal can be determined with high accuracy, so highly accurate absolute length measurement can be performed.

<Example> Hereinafter, the present invention will be explained based on the drawings.

FIG. 1 is a diagram showing the configuration of an absolute length measuring device using a wavelength tunable LD for implementing the interference signal processing method for an absolute length measuring device of the present invention.

In Fig. 1, reference numeral 1 denotes a wavelength tunable LD, which supplies a constant current from a drive current circuit 2 to the active region of the wavelength tunable LDI, and transmits a sawtooth wave tuning from a tuning current circuit 3 to the diffraction of the wavelength tunable LDI. By flowing the light through each of the grating sections, continuously changing wavelengths can be repeatedly generated. 4 is a Vertier element, which can heat or cool the wavelength tunable LDI. 5 is a temperature detection end, which is a wavelength variable LDI
The temperature of the Vertier element 4 is measured via the temperature controller 6.
The temperature of the wavelength tunable LDI is kept constant by feedback. Reference numeral 7 denotes a collimator lens, which collimates the light emitted from the variable wavelength LDI and converts it into parallel light. Reference numeral 8 denotes an isolator, which prevents reflected light from an interferometer (described later) from returning to the wavelength tunable LDl, thereby preventing oscillation from becoming unstable.

The light passing through the isolator 8 is split into two by a half mirror 9, and the zero reflected light is guided to an etalon 10. When the wavelength of the wavelength tunable LDI changes, light passes through the etalon IO for each specific wavelength determined by the optical path length of the etalon 10, so the transmitted light is collected by the condenser lens 11 and sent to the photodetector 12. The signal is detected by the amplifier 13 and converted into an electrical signal by the amplifier 13. On the other hand, the transmitted light of the half mirror 9 is further split into two by the half mirror 14, and the 0 reflected light is reflected by a right angle prism 15 whose position is fixed within the optical system and becomes a reference light of the interferometer. C. The light transmitted through the mirror 14 is reflected by the right angle prism 16 whose position is to be measured, and becomes test light for the interferometer. The reference light and test light from the right angle prisms 15 and 16 are combined by a half mirror 14 to become an interference signal, focused by a condenser lens 17, detected by a photodetector 18, and converted into an electrical signal by an amplifier 19. be done.

Here, FIG. 2 shows an electrical signal obtained from the device shown in FIG. (a) The figure shows the wavelength variable LD from the tuning current circuit 3.
This is a voltage signal that changes in the same way as the tuning current flowing through I, and has a sawtooth wave shape.

(b) The figure shows an etalon transmitted optical signal obtained by the amplifier 13. The etalon has a constant FSR determined by its own optical path length.
The transmittance increases for each wavelength (or frequency) of light,
In the figure, the interval between peaks extending downward indicates the wavelength (or frequency). However, in the figure, due to the characteristics of the circuit of the device, the negative side represents the presence of transmitted light. (c) The figure shows the interferometer output signal (g(t)) obtained by amplifier I9.
However, due to the characteristics of the device's circuit, the diagram shows that the negative side is where the interference signal is coming from. The amplitude of the interference signal is determined by the change in the output power that is changed by the tuning current of the tuning circuit 3 and the change in the spectral line width of the wavelength tunable LDI, and the frequency of the interference signal is determined by the change in the wavelength of the wavelength tunable LDI. are not linear, so they are modulated. Also, in Figures (B) and (C), the part where the waves are dense is because the sawtooth wave shown in Figure (B) returns to the zero level in a short time, and this part is not used for signal processing.

In such a configuration, the interference signal processing method of the absolute length measuring device of the present invention will be sequentially explained below using the signal processing flowchart shown in FIG.

(1) In the voltage signal that changes in the same way as the tIt flow shown in Figure 2 (a), use the point at which the tuning current of the tuning current circuit 3 reaches its peak (point T in the diagram) as the trigger for starting data reading. is used to capture N etalon transmission optical signals (Fig. 2 (b)) and interference signals (Fig. 2 (c)) from amplifiers 13 and 19 at regular intervals, A/D convert them, and convert them into numerical data. Hold.

(2) In the etalon-transmitted optical signal shown in FIG. 2 (b), the number (1) is calculated by adding 1 to the quotient k obtained by dividing the frequency variable range, which is known in advance from the frequency characteristics of the wavelength-tunable LDI with respect to the tuning current, by the FSR of the etalon. In this working example, 0,1
．． 2.・・・・・・, k) minimum point (Mo , M+ ,
M2 , ......, Mt > time (what number among N data, t (No), t (H+ 1°t[H,), ......, t( Find Hb)).

(3) In the interference signal shown in FIG. 2(c), a maximum point P between the times of the minimum points MO to Mk of the etalon transmitted optical signal (FIG. 2(b)). ~Pl time and minimum point VO~Vt
Find the time of.

(4) Using the time of the maximum points P0 to P1 and the value of the interference signal for that time, a linear approximation equation connecting the maximum points is determined by the least squares method to obtain an envelope. Note that the order of the linear approximation equation is determined by the number of local maximum points. However, wavelength variable L
Find out the upper limit based on the frequency characteristics of DI.

The value of the linear approximation equation at time t is expressed as y = P (t)...■ (see Figure 4) (5) Minimum point ■ as in item (4) above. At the time of ~VIN, a linear approximation equation connecting the minimum points is found to obtain the envelope line.

The value of the linear approximation formula at time t is expressed as y = V (t)...■ (see Figure 4) (6) The interference signal is calculated from the median value of the upper and lower envelopes obtained from the above equations 0 and 0. virtual zero level y=Z(t) −
Find (P(t) +V[t) ) /2=-・■0
Next, time Z of the zero cross point where the interference signal (g(t)) and the zero level (Z(t)) intersect. Find -, -29. (See Figure 4) (7) Time z of the zero cross point obtained in the above (6),
The phase difference between Z, ... +2g is π (180°)
Therefore, the phase of the zero cross point at time Z0 is set to 0.
The phase at time Z of the zero cross point is π,...
, the phase at time zg of the zero cross point is given as qπ and the phase integrated values are sequentially given, and a linear approximation equation is determined by the least squares method using the phase integrated values for the times of the zero cross points.

The value of the linear approximation equation at time t is expressed as y=θ(1)...■ (see Figure 5) (8) Minimum points M0 and Mk of the etalon transmitted light signal obtained from the above (2) The phase values at times t(No) and t(Hk) of are obtained from the above equation 0, and the phase difference Δθ
seek.

〇 to phase difference is Δθ=θ(t(Hk))−〇(i(Ha))
...Represented by ■. (Refer to Figure 6) (9) The absolute length of the interferometer is determined from the difference Δθ in the phase values at times t(Ha) and t(Hk) between the minimum points M0 and Mk of the etalon-transmitted light signal obtained in item (8) above. (Optical path length difference) ΔL is determined. The absolute length ΔL is determined by the following formula.

ΔL=cXΔθ/(4πxΔνxkxNair)...
・■ However, C: Speed of light Δν in vacuum: FSR (frequency) of etalon Nair: Refractive index of air.

The above is the interference signal processing method of the absolute length measuring device according to the present invention. Here, a specific example of the length measurement result according to the present invention is shown in FIG.

The FSR (Δν) of the etalon is 5Gl (Z, the quotient (k) of dividing the frequency variable range by the FSR (Δν) of the etalon is 8
Then, one period of the interference fringe is calculated from the above equation 0 as follows: ΔL=3X10"X2π/(4πx 5 x 10' x8x1.o003)=
3.75m. Also, from Figure 7, the linearity error (reference length 23 and 8
The error in the measurement value from a straight line connecting two 3m points is 46μ
m, the counting accuracy of the fractional part of the interference fringe is 0.046/3.75 = 0.0123 - approximately 1/80. Therefore, one period of the interference fringe, 3.75B, can be read with an accuracy of approximately 1/80. There will be.

Note that the device configuration of an absolute length measuring device using a variable frequency (wavelength) light source is not limited to the device configuration shown in Figure 1, but may also include a configuration in which the emitted light from the variable wavelength (frequency) light source is guided to an etalon and an interferometer. Further, the device for generating signals at fixed frequency or wavelength intervals is not limited to an etalon, and a gas absorption cell or the like may be used.

In addition, by differentiating the etalon transmitted light signal on an electric circuit and detecting the zero cross point, it is possible to obtain the times t(No) to t (Hk) of the minimum points M0 to Mk of the etalon transmitted light signal, and to detect the interference signal. By directly determining the times of zero cross points Zo to zq by AC coupling, the load on the arithmetic unit can be reduced, and the processing speed can be improved.

<Effects of the Invention> As specifically explained above in conjunction with the embodiments, according to the present invention, the difference in the phase integrated value between a constant wavelength change during a wavelength change is calculated from the time of the zero cross point of the interference signal and the time of the zero cross point of the interference signal. It is calculated from the correlation curve with the phase integrated value at time, and it is possible to accurately calculate the phase integrated value of the interference signal even if the wavelength of the wavelength (frequency) variable light source does not change linearly with time. Therefore, highly accurate absolute length measurement can be performed.

Also, since the correlation curve between the time of the zero crossing point and the phase integrated value at that time is obtained from the zero crossing point of the interference signal,
It is possible to realize an interference signal processing method for an absolute length measuring device that has effects such as having little effect even if the output power of the light source changes with the wavelength.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an embodiment of an absolute length measuring device using a wavelength tunable LD, Fig. 2 is a diagram showing an electric signal obtained from the device shown in Fig. 1, and Fig. 3 is an absolute length measuring device of the present invention. Fig. 4 is a signal processing flowchart showing the interference signal processing method of the device; Fig. 4 is a diagram showing the linear approximation formula (envelope), zero level, and zero cross point obtained using the interference signal value obtained from the device in Fig. 1; , Figures 5 and 6 are zero cross point 2°~
A diagram showing the correlation curve of the time-phase integrated value at the minimum points M0 and Mk of the Zq + etalon transmitted light signal, FIG. 7 is a diagram showing a specific example of the length measurement results according to the present invention, and FIG. 8 is a diagram showing the tuning in the wavelength tunable LD. FIG. 3 is a diagram showing the relationship between current, wavelength, and output power. DESCRIPTION OF SYMBOLS 1... Tunable wavelength laser diode, 2... Driving flow path, 3... Tuning current circuit, 4... Vertier element,
5... Temperature detection end, 6... Temperature controller, 7... Collimator lens, 8... Isolator, 9.14...
・Half mirror 10... etalon, 11.17...
Condensing lens, 12.18... Photodetector, 13.19.
...Amplifier, 15.16...Right angle prism. Figure 2 Interference signal value Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Tuning current A

Claims (1)

[Claims] In an absolute length measurement device that uses a Michelson type interferometer using a variable wavelength light source or a variable frequency light source, the upper and lower envelopes of the interference signal generated when the frequency or wavelength of the light source is continuously and monotonically changed are measured. Find the zero level of the interference signal from the median value of the upper and lower envelopes, find the zero cross point of the interference signal from the intersection of this zero level and the interference signal, and calculate the adjacent zero cross points of this interference signal. Using the fact that the phase difference at a point is 180°, a correlation curve between the time of the zero-crossing point of the interference signal and the phase integrated value at this time is obtained, and the correlation curve between a constant wavelength change during a wavelength change is calculated. An interference signal processing method for an absolute length measuring device, characterized in that an optical path length difference between both reflecting mirrors of the Michelson type interferometer is determined by determining a phase integrated value from the correlation curve.