JPH07260418A - Method for measuring displacement - Google Patents

Abstract

PURPOSE:To measure micro displacement on the surface of an object highly accurately by causing a measuring light subjected to phase variation to interfere with a reference light to produce an interfering light and then measuring fringes using a specific comparison value. CONSTITUTION:A reference light and a measuring light are subjected to regulation of phase variation through a phase modulator 8 prior to returning back to a 3dB coupler 17. The phase variation is regulated by applying a predetermined rectangular voltage in synchronism with a transmitter 20 by means of a DA converter 18 through an analog switch 19. One or both of the reference light and the measuring light are then subjected to phase modulation in stepwise as shown by a formula for an interval of 0<=t<=n (t is a time variable) and the average intensity of interfering light during that interval is employed as a comparison value. In the formula, n is an integer of 2 or above, tau is the time interval of stepwise phase modulation, and the function S(X) is a binary step function defined such that S(X)=0 for X<0 and S(X)=1 for X>=0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is based on laser light interference.
The present invention relates to a method for measuring the displacement and movement direction of a minute portion of an object to be measured.

[0002]

2. Description of the Related Art With the development of lasers, electronics, microcomputers, sensor elements, etc., laser light having excellent coherence is used to measure the displacement and moving direction of a minute portion of the surface of an object to be measured by optical interference. The method is popular. Since this measuring method can be performed in a non-contact manner and has high accuracy, it is used for measuring the surface roughness of crystal wafers, optical components, X-ray mirrors, and the like.

As one of such displacement measuring methods by optical interference, two sets of Michelson type interference systems are combined to make the measuring light interfere with two interference lights having a phase difference of π / 2. A method of measuring the direction of displacement and the direction of movement is known. The method will be described with reference to FIG.

The laser light emitted from the laser diode 1 is guided to the optical waveguide substrate 3 via the polarization-maintaining optical fiber 2 and divided into three by the Y branch 4 of the optical waveguide and the 3 dB couplers 5 and 6. . Two of the split lights are reflected by the reflecting mirror 7 provided on the end face of the optical waveguide substrate 3 and return to the 3 dB couplers 5 and 6 as reference lights.

On the other hand, another light passing through the center of the optical waveguide substrate 3 passes through the phase modulator 8 and is emitted from the end face of the optical waveguide substrate 3. Since this emitted light spreads due to diffraction, it is converted into parallel light by the condenser lens 9. This parallel light is reflected on the surface of the DUT 10, is guided to the optical waveguide substrate 3 by following the opposite optical path, and is guided by the phase modulator 8 and the Y branch 1.
Return to 3dB couplers 5 and 6 through 1. 3 dB
The couplers 5 and 6 interfere with the reference light reflected from the reflecting mirror 7 and the measurement light reflected from the DUT 10, and the interference light passes through the multimode optical fiber 12 to the photodetection diodes 13 and 14. Be guided. The interference light guided to the photodetection diodes 13 and 14 is converted into electric signals and output as interference signals L ′ and R ′ through the comparators 15 and 16, respectively.

Since it is impossible to obtain the intensities of the two reference lights exactly the same, the comparison reference voltage V _REF1 ,
V _REF2 is applied to each of the comparators 15 and 16.

The optical path lengths of the two interference outputs L and R guided to the two optical fibers 12 are adjusted so that they have a phase difference of π / 2 with each other, and the position of the DUT 10 changes in the optical path length direction. When it is displaced to, the interference outputs L and R change as shown in the upper stage of FIG. 4, and the interference signals L ′ and R ′ also change accordingly as shown in the lower stage of FIG. From the phase relationship between the interference signals L ′ and R ′ thus obtained and the number of pulses (fringe measurement), the minute displacement and moving direction of the DUT 10 can be obtained.

[0008]

When displacement measurement by optical interference using such a Michelson interferometer is performed by fringe measurement, interference outputs L and R which are sine waves are converted into rectangular wave signals L'and R. In order to convert into a binary number as', a comparison value which is an intermediate value of the interference output is required. Conventionally, this comparison value has been obtained by averaging the interference output by an analog circuit or manually adjusting and setting it.

However, in the method of averaging the interference outputs in an analog manner, an accurate comparison value cannot be obtained unless the interference outputs are constantly changing. When setting the comparison value manually, if the object to be measured is moving or the printed material has a large contrast between white and black, the reflectance of light and the light receiving efficiency of the measuring light during measurement Changes greatly, and accurate displacement measurement is not possible.

The present invention has been made in view of the above problems, and an object of the present invention is to divide a laser beam of the same light source into a measuring beam and a reference beam and receive the measuring beam. Irradiate the measurement object, the measurement light reflected from the measurement object and having a phase change caused by the change of the optical path difference with the reference light is caused to interfere with the reference light, and the obtained interference light is used as a comparison value. It is an object of the present invention to provide a method for accurately measuring the displacement and movement direction of a minute portion by giving an accurate comparison value when measuring the fringe measurement and measuring the minute displacement and movement direction of the surface of the measured object.

[0011]

A displacement measuring method of the present invention for achieving the above object is to divide a laser beam of the same light source into a measuring beam and a reference beam and irradiate the object to be measured with the measuring beam. Then, the measurement light reflected from the object to be measured and having a phase change caused by a change in optical path difference with the reference light is caused to interfere with the reference light, and the obtained interference light is measured by fringe using a comparison value,
In the method of measuring a minute displacement or moving direction of the surface of the object to be measured, in one or both of the reference light and the measurement light, a time variable is t, and a period of 0 ≦ t ≦ nτ,

[0012]

[Formula 1] The stepwise phase modulation represented by is given, and the average value of the interference light intensity in the above period is used as the comparison value.

However, n is an integer of 2 or more, τ is a step width time in the above stepwise phase modulation, and the function S (x) is S (x) = 0 when x <0, and S when x ≧ 0. It is a binary step function defined as (x) = 1.

In the method of the present invention, when the phase modulation amount with respect to the modulation time is, for example, n = 4, the pattern becomes as shown in FIG. This is a step-like pattern in which the amount of phase modulation increases with the step width τ as the modulation time elapses.
If the total sum of the respective times to which the phase modulation amount of the discrete value is given does not change within the period of ≤t≤nτ, then, as shown in Figs.
The same effect can be obtained even with a phase modulation pattern as shown in FIG. For example, in FIG. 7, the phase modulation amounts of discrete values 0, π / 2, π, 3π / 2 are 0 ≦ t ≦
The sum of the times given in the period of nτ is τ
The same applies to FIGS. 8, 9 and 10, and the same effect is obtained.

[0015]

FIG. 1 shows an example of a displacement measuring device using a set of Michelson type interferometers for realizing the displacement measuring method of the present invention. In this example of the apparatus, a modulated signal is time-divided so that one set of interfering systems has a role of two sets of interfering systems.
It has the same function as the device having the above-mentioned configuration.

The laser light emitted from the laser diode 1 is guided to the optical waveguide substrate 3 via the polarization-maintaining optical fiber 2 and is split into two by the 3 dB coupler 17. One of the split lights is reflected by the reflecting mirror 7 formed on the end face of the optical waveguide substrate 3, and the 3 dB coupler 17 is used as the reference light.
Come back to. The other light is emitted from the optical waveguide substrate 3, collimated by the condenser lens 9, and irradiated onto the DUT 10. The light reflected by the DUT 10 follows the reverse path and returns to the 3 dB coupler 17 as the measurement light.

When returning to the 3 dB coupler 17, the phase change of the two reflected lights is adjusted by the phase modulator 8 formed on the optical waveguide substrate 3. In this adjustment, the DA converter 18 converts the predetermined rectangular wave voltage into the analog switch 19
Through the oscillator 20 in synchronism with the oscillator 20.

The interference light interfered by the 3 dB coupler 17 is emitted from the end face of the optical waveguide substrate 3, received by the photodetection diode 13 via the multimode optical fiber 12, and converted into an electric signal. This electric signal is binarized by the comparator 15 through the low-pass filter 23, and is input to the sample and hold circuits 24 and 25 in synchronization with the oscillator 20. Interference signals L ′ and R ′ are output from the sample and hold circuits 24 and 25, respectively. When the position of the DUT 10 is displaced in the direction in which the optical path length changes, the interference outputs L and R and the binarized interference signals L ′ and R ′ are as shown in FIG.

When the object to be measured 10 is stationary, the interference output is D, A, B, C, A'B 'in FIG. 5 due to the phase difference.
It takes a value such as C '. Here, the phase modulation is expressed by Equation 1 and n = 2
When the interference output is A, B, or C, 0
In the period of ≦ t <2τ, the left diagram, the center diagram, and
The interference output shown in the right figure is obtained. And the period 2τ
When the interference output in Fig. 6 is averaged, the same value is obtained in any of the left diagram, the center diagram, and the right diagram in Fig. 6, and this value is the value of the interference output A. The comparison value thus obtained is an ideal comparison value for fringe counting, regardless of n or τ.

When τ is set to infinity and n is set to infinity, the change in phase modulation becomes a ramp wave. Combining two ramp waves results in a triangular wave. Therefore, the present invention also includes the case where the change in phase modulation is a triangular wave. In the case of performing triangular wave phase modulation, assuming that the amplitude is x and the average value for ideal fringe counting is R, the relationship between them can be expressed by Equation 2.

[0021]

[Formula 2] When the amplitude x is set to an integral multiple of 2πrad, the ideal value R = 1 can be obtained regardless of where the interference output point is modulated.

When detecting the moving direction by fringe measurement, the two interference signals L'and R'are π / 2 r
It is desirable to keep the phase difference of ad, but if the phase difference deviates by π, it becomes impossible to detect the moving direction, so the value R is

[0023]

[Formula 3] 1- (0.5) ^1/2 <R <1+ (0.5) ^1/2 . FIG. 11 shows Equation 2 and Equation 3. From FIG. 11, the phase modulation amplitude x for enabling detection of the moving direction by fringe measurement is 0.886.
It can be seen that it needs to be larger than π rad.

[0024]

EXAMPLE An example of the present invention will be described with reference to FIG. x
On a −cut LiNbO ₃ substrate, a 3 dB coupler 17 by a proton exchange optical waveguide propagating in the Y direction and a phase modulator 8 were provided. 2πra for the light traveling back and forth in the phase modulator 8
Since the voltage for producing the phase difference of d was 2.0V, 1.5V was output to the reference power supply 18. The extinction ratio of light passing through the proton exchange optical waveguide is 50.
It is at least dB, and this optical waveguide will act as a polarizer as a whole.

Laser diode 1 having an emission wavelength of 830 nm
Was introduced into the polarization-maintaining optical fiber 2 and connected to the optical waveguide substrate 3. The light from the laser diode 1 is 3d
The B coupler 17 divides the beam into two parts, one toward the reflecting mirror 7 and the other toward the condenser lens 9. A non-reflection coating was applied between the optical fibers 2 and 12 and the optical waveguide substrate 3.

The light traveling to the reflecting mirror 7 is transmitted to the optical waveguide substrate 3
The light is reflected by the reflecting mirror 7 made of an aluminum vapor-deposited film on the end face, returns to the 3 dB coupler 17 again, and becomes reference light.

The light traveling to the condenser lens 9 becomes parallel light here and is irradiated onto the object to be measured 10, and the reflected light returns to the optical waveguide again. An antireflection coating was also applied to the end surface of the optical waveguide substrate 3 on the measurement light side. Reference light and measurement light are phase modulator 8
3 dB by receiving phase modulation due to electro-optic effect
The multimode optical fiber 12 interferes with the coupler 17.
The light is detected to be guided to the photodetection diode 13 via.

A rectangular wave oscillator having a frequency of 2 MHz was used as the oscillator 20. The output of the oscillator 20 is 4 by the counter 21.
It is divided and connected to the demultiplexer 22. The four outputs Y0 to Y3 of the demultiplexer 22 are 0.5 μs
It becomes active one after another every ec. Y0 and Y1 are connected to the sample and hold circuits 24 and 25, and π / 2r
The interference results of light having a phase difference of ad are held respectively. Further, Y0 to Y3 are also connected to the analog switch 19, and the phase modulator 8 causes 0 to Y3 as shown in FIG.
From rad to 3π / 2 rad, the phase modulation that changes every π / 2 rad is continuously performed. Formula 1
Of the reference voltage V _REF for comparison as a comparison value when the interference output is passed through the low-pass filter 23.
Got In addition, the phase modulator 8 has 0 rad and π / 2 r
Interference signals L'and R'when performing phase modulation of ad
Was held by the sample and hold circuits 24 and 25. In this way, n = 4 and τ = 0.5 μse in the equation 1
When the device of the example c) was constructed, signals equivalent to L ′ and R ′ obtained by the conventional method were obtained.

In the present embodiment, even if the surface of the printed material whose reflected light amount greatly changes is the object to be measured, is moved at 0.02 m / sec, or is stationary for a long time, a minute displacement is detected while detecting the moving direction. Could be measured.

[0030]

EFFECT OF THE INVENTION Laser light of the same light source is split into measurement light and reference light, the measurement light is applied to the object to be measured, and the phase is changed by the change in optical path difference from the object to be measured and reflected from the object. The measured light that has changed is caused to interfere with the reference light, and the obtained interference light is measured by fringe using a comparison value, and an accurate comparison is made when measuring the minute displacement and the moving direction of the surface of the measured object. By giving a value, it is possible to accurately measure the displacement and moving direction of a minute portion. In particular, it is suitable for measuring the displacement of an object to be measured such as a printed material with a large change in the amount of reflected light or an object to be measured that has been stationary for a long time.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a state of phase modulation in the embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a displacement measuring device.

FIG. 4 is a diagram showing a state of interference output with respect to a displacement amount of an object to be measured.

FIG. 5 is a diagram showing a value of an interference output according to a phase difference.

6 is a diagram showing a difference in interference output with respect to modulation time in A, B, and C of FIG.

FIG. 7 is a diagram showing changes in phase modulation amount with respect to modulation time.

FIG. 8 is a diagram showing changes in the amount of phase modulation with respect to the modulation time.

FIG. 9 is a diagram showing changes in the amount of phase modulation with respect to the modulation time.

FIG. 10 is a diagram showing changes in the amount of phase modulation with respect to the modulation time.

FIG. 11 is an interference output average value R with respect to the phase modulation amplitude x.
Expression 2 and Expression 3 are shown, and the diagonal line represents the range in which the movement direction can be detected by the fringe measurement.

[Explanation of symbols]

 1 Laser Diode 2 Polarization-Maintaining Optical Fiber 3 Optical Waveguide Substrate 4, 11 Y Branch 5, 6, 17 3 dB Coupler 7 Reflector 8 Phase Modulator 9 Condenser Lens 10 Object to be Measured 12 Multimode Optical Fiber 13, 14 Light Detection diode 15 and 16 Comparator 18 Reference power supply 19 Analog switch 20 Oscillator 21 Counter 22 Demultiplexer 23 Low pass filter 24 and 25 Sample and hold circuit

Claims (1)

[Claims] 1. A laser beam of the same light source is divided into a measuring beam and a reference beam, the measuring beam is irradiated to the object to be measured, and the phase is changed by a change in optical path difference from the object to be measured and reflected from the object to be measured. In the method of causing the measurement light that has changed to interfere with the reference light, measuring the fringe of the obtained interference light using a comparison value, and measuring the minute displacement or the moving direction of the surface of the object to be measured,
In either or both of the reference light and the measurement light, with a time variable being t, in a period of 0 ≦ t ≦ nτ, The displacement measuring method is characterized by applying a stepwise phase modulation represented by, and using the average value of the interference light intensity in the period as the comparison value. However, n is an integer of 2 or more,
τ is the step width time in the stepwise phase modulation, and the function S (x) is a binary value defined as S (x) = 0 when x <0 and S (x) = 1 when x ≧ 0. Is a step function of.