JPH11351814A - Stabilized interference measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interference fringe analyzing device which is simple and stable. SOLUTION: This device has 2-frequency light 4, a 2-beam interferometer 11, a phase meter 18, and a servo signal 22. A specimen 26 is installed at a measurement part 27. The optical path difference of an interferometer is made not to be so large between a control part 10 and a measurement part 27. A two-frequency beam is split by a beam splitter 5b, and only one frequency component of the two-frequency beam is extracted by a 0 deg. or 90 deg. -azimuth polarizer 28, reflected by a mirror 29 and then expanded in beam width by a beam expander 30, and made incident on the measurement part 27 of the interferometer. In this way, interference fringes 31 of a Fizeau interferometer by normal single frequency light are obtained representing the unevenness distribution of the measured body 26. The interference fringes are detected by a CCD camera 33 through a beam splitter 32, processed by using a PC(personal computer) 25 with an image memory, and displayed as interference fringes 35 on a TV 34.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for non-contact and accurate measurement of the surface shape, thickness, refractive index distribution, etc. of an object using an interferometer. is there. 2. Description of the Related Art Conventionally, an interferometer has been used as a means for precisely measuring the two-dimensional distribution of the surface shape, thickness or refractive index of an object. Normally, in an interferometer, a light beam is irradiated on an object to be measured, and the reflected light or transmitted light interferes with a reference light, and the generated interference fringes include phase information of the object. It is numerically obtained from the situation by visual observation or signal processing by a CCD camera and a personal computer.
For automatic fringe analysis by a personal computer, a phase shift interferometry in which an optical path difference is shifted stepwise by π / 2 and a corresponding changed interference fringe is used is often used. [0003] However, in the conventional interference measurement method, even if the interferometer is mounted on a high-performance anti-vibration table, if disturbance such as air turbulence or vibration is present, the measurement will not be performed during the measurement time. In this case, the interference fringes fluctuate, which makes it impossible to analyze the fringes or significantly lowers the measurement accuracy. In addition, in order to perform accurate measurement, the frequency of the light source must be stable, and there is a disadvantage that an expensive frequency stabilized light source is required. [0004] The present invention relates to a new type of interferometer which eliminates the instability due to disturbance, which is a drawback of the conventional interferometer, and does not require an expensive frequency stabilized light source as a light source. Conventionally, as a method of stabilizing interference fringes,
A method is known in which the interference fringe intensity is detected and a servo is applied so that the interference fringe intensity becomes a constant value. However, in this method, there is a null point in the fringe intensity versus phase slope sensitivity (sine function). Therefore, there is a problem that the phase shift method cannot be applied and the light source is affected by a change in the intensity of the light source. SUMMARY OF THE INVENTION The present invention provides an interferometer using a servo system which detects a phase fluctuation of an interferometer caused by a disturbance or a frequency fluctuation of a light source by a heterodyne method and uses it as an error signal. Is compensated by the frequency of the light beam or the mirror position of the interferometer, thereby enabling the interferometer to be stabilized at an arbitrary phase. According to this method, the phase shift can be performed by changing the servo setting phase signal stepwise by π / 2, so that remarkable disturbance stability can be obtained compared to the conventional interference measurement, and the light source Performance required for After passing a laser beam through an isolator, it passes through an acousto-optic frequency shifter to generate orthogonally (0 ° azimuth, 90 ° azimuth) linearly polarized two frequencies f ₁ and f ₂ . As an example of the interference measurement according to the present invention, a case will be considered in which a Fizeau interferometer including two mirrors (a front mirror and a rear mirror) arranged opposite to each other is used to measure the unevenness distribution on the surface of a mirror-like object. . The sample object is fixed on a rear mirror. In the Fizeau interferometer, when high coherence light is incident on the interferometer, interference occurs between the partially reflected light from the front mirror and the reflected light on the sample surface, and as a result, interference fringes are generated according to the phase distribution due to the surface unevenness of the sample. A part of the Fizeau interferometer (for example, one end where no sample is placed) is used as a control unit, and a quarter-wave plate is inserted into the optical path. An orthogonal (0
Azimuth, 90 ° azimuth) linearly polarized two frequencies f ₁ , f ₂
Is incident, and the reflected light passes through a polarizer and a pinhole having an azimuth of 0 ° or 90 °, a beat signal having a difference frequency Δf = f _{1 to} f ₂ and a phase δ = 4πLf ₀ / C is generated. here,
L is the optical path length difference between the two mirrors reflecting light, f ₀ is the center frequency of the laser beam, C is the speed of light in vacuum. When the interferometer receives a disturbance such as vibration or air fluctuation or a change in the frequency of the light source, the interference fringes change. The phase δ of the beat signal fluctuates according to the fluctuation of the interference fringes. Therefore, if the phase fluctuation of the beat signal is suppressed, the interference fringes are stabilized, and the stationary interference fringes can be observed even when there is disturbance or frequency fluctuation of the light source. Since the phase δ of the beat signal depends on f ₀ and L, δ can be controlled by adjusting the laser frequency or the mirror interval. Therefore, detected using a phase meter [delta], phase meter output (voltage) as compared to the reference voltage V _R in a differential amplifier, and amplifies the astigmatic difference, variable [delta] using the servo system it as an error signal By feeding back to the mechanism, the fluctuation of δ is suppressed, and the interferometer is stabilized.
Further, if the reference voltage V _R only stepwise shift the combined amount of the phase / voltage conversion ratio of the phase meter, the phase shift interference fringe is obtained needed for fringe analysis. In the interferometer system, a tunable laser can be used to change δ. For example, an oscillation frequency shift due to the current of a semiconductor laser can be used. The oscillation frequency of the semiconductor laser changes linearly with the drive current. In the interferometer, when a semiconductor laser is used as a light source and the output of the differential amplifier is fed back to the drive current of the semiconductor laser, a phase change due to disturbance or a change in laser frequency is compensated for by a change in the drive current of the semiconductor laser. Be stabilized. In this case, when the control unit and the measurement unit in the interferometer have substantially the same optical path length difference, the stability achieved is increased. In the interferometer system, a mirror movement by an actuator (PZT, giant magnetostrictive element, etc.) can be used to change δ. In this case, any light source may be used as long as it is a monochromatic light source, and a general laser light source (for example, a helium neon laser) may be used.
By feeding back the output of the differential amplifier to a voltage for driving the PZT or a current for driving the giant magnetostrictive element, interference fringes can be stabilized. In the stabilized interferometer, when the reference voltage of the differential amplifier is shifted stepwise by an amount corresponding to the phase / voltage conversion ratio of the phase meter, the bias phase of the interferometer is maintained while the interferometer is stabilized. Are obtained, four interference fringes having a stepwise change at π / 2 pitch are obtained. By reading this with a CCD and following the algorithm of phase shift interference fringe analysis, the two-dimensional unevenness distribution of the test object can be measured. In the stabilized interferometer, the two-dimensional unevenness distribution of the object can be obtained by applying the tilt to the stationary interference fringes according to the algorithm of the Fourier transform method using a spatial carrier fringe generated by the Fizeau interferometer by giving a tilt. Can be measured. When it is desired to measure the two-dimensional distribution of the thickness and the refractive index of the transparent object as the object to be measured by the interferometer, the two are inserted into the measuring section. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an overall configuration diagram showing an embodiment of the present invention. The parallel light from the semiconductor laser light source 1 is
After passing through the isolator 2, the acousto-optic frequency shifter 3
, Two orthogonally polarized (0 ° azimuth, 90 ° azimuth) linearly polarized light having two frequencies f ₁ and f ₂ are generated. A part of the two-frequency light beam 4 is converted into a beam splitter 5a.
When it is passed through a polarizer 6 having a 45 ° azimuth and detected by a detector 7, a beat signal 8 having a frequency Δf = f _{1 to} f ₂ is generated. The two-frequency light beam 4 enters a part (one end) of a Fizeau interferometer 11 composed of mirrors 9 and 10. This is used as the control unit 12, and a quarter-wave plate 13 is inserted into a portion where the light beam passes. The light emitted from the interferometer is composed of partially reflected light from the front semi-transmissive mirror 9 and reflected light from the back mirror 10. The back reflected light has a phase of δ =
It is delayed by (4π / C) Lf ₀ , and the polarization plane is rotated by 90 ° by the action of the １ ／ wavelength plate 13. Then, after the output light of the control interferometer passes through the beam splitter 5b, the intensity of one point of the interference fringes is taken out through the pinhole 14, passed through the analyzer 15 of 0 ° or 90 ° azimuth, and detected by the detector 16. A beat signal 17 is generated, and its phase includes the phase δ of the control interferometer. When the two beat signals 8 and 17 are input to the phase meter 18, a voltage signal V19 proportional to the phase difference δ = (4π / C) Lf ₀ is output. This voltage signal 19 is led to a differential amplifier 20 to amplify a difference voltage 22 from a reference voltage V _R 21 and to feed back to a drive current 24 of the semiconductor laser 1 via a drawbar 23. This servo mechanism makes δ = constant. As a result, the stabilization of the control interferometer 12, and hence the stability of the entire Fizeau interferometer 11, which is mechanically integrated with the control interferometer, against disturbances and fluctuations in the frequency of the light source is achieved. Since the interference fringes stabilized in this way are stabilized in the phase region, they have the advantage that they are not affected by fluctuations in light intensity and can be further stabilized at an arbitrary phase value. The test object 26 is set on the measuring section 27. The optical path difference between the interferometer between the control unit 10 and the measurement unit 27 should not be too large. Beam splitter 5b
, And only one frequency component of the two frequencies is taken out by a polarizer 28 having a azimuth of 0 ° or 90 °, reflected by a mirror 29, and then expanded in a beam width by a beam espander 30. 27. As a result, an interference fringe 31 of the Fizeau interferometer using ordinary one-frequency light is obtained, and the fringe represents the unevenness distribution of the measured object 26. The interference fringes are detected by a CCD camera 33 through a beam splitter 32, processed using a PC (personal computer) 25 with an image memory, and displayed on a TV 34 as interference fringes 35. FIG. 2 is a diagram for stabilizing the fringe. When vibration is applied to the entire interferometer, the interference fringes do not become a stationary pattern when the hood back mechanism is turned off, and disappear after a long period of averaging (FIG. 2A). However, when the hood-back mechanism is turned on, the interference fringes are stabilized and stand still (FIG. 2b). The stabilized interference fringes stabilized in this way can be analyzed by various methods. To use the Fourier transform method, the mirror of the interferometer is slightly tilted to generate a carrier fringe, which is processed by a personal computer. When the phase shift method is used, the reference voltage V _R 21 of the differential amplifier 20 is applied to the V
_R = Vo + (π / 2) iα (where i = 0, 1, 2,
3, alpha phase-voltage conversion coefficient of the differential amplifier, the V ₀ changes the V _R stepwise so that the relationship between the initial voltage),
In the Fizeau interferometer, a phase shift of π / 2 pitch can be performed. FIG. 3 is an experimental example of a stabilized phase shift. FIG. 4 is a diagram illustrating characteristics when a disturbance (mechanical vibration) exists. When the feedback control is not performed (off), there is a sharp phase fluctuation. However, when the feedback circuit is turned off, the phase is stabilized, and a shift of π / 2 = 90 ° is given stepwise. I understand. FIG. 4 is a diagram for checking the effect of the phase shift. The interference fringes are shifted at a phase pitch of π / 2. Four different interference fringes I ₁ , I ₂ , I ₃ , I ₄ are converted to CC
Reads in D, captures it in the frame, and two-dimensionally measures the surface shape (in the case of a reflective object) 37 and thickness distribution (in the case of a transmissive object) of the object to be inspected by PC processing using a normal phase shift interferometry algorithm. I do. FIG. 5 is an overall configuration diagram showing another embodiment of the present invention. A general laser is used as a light source, and an actuator is used for phase control.
The differential amplifier output 22 is fed back to the drive electric signal of the actuator 36 on which the mirror 10 of the interferometer is mounted. As the actuator, a giant magnetostrictive element (current drive) or an electrostrictive element (voltage drive) is used. The other device parts are the same as in the case of the semiconductor laser current feedback of FIG. It is obvious that the interferometer according to the present invention may be any two-beam interferometer, such as a Twyman-Green interferometer or a Mach-Zehnder interferometer. . As means for controlling the frequency of the light beam in the interferometer according to the present invention, in addition to the current modulation of the semiconductor laser,
Various types of phase modulation, such as phase modulation by an electro-optic crystal inside a resonator, movement of a resonator mirror by an actuator, tuning of a tunable laser by controlling a wavelength dispersion element inside a resonator, and frequency modulation by an electro-optic element outside a resonator. Obviously, frequency modulation means can be used. According to the present invention, with a simple configuration,
Automatic analysis of interference fringes can be performed with high accuracy and high stability even in an environment with a lot of air fluctuations and mechanical vibrations, and even when the frequency of the light source fluctuates. This is not realized by the conventional interference measurement technology, and has a wide application range as a new measurement technology. [0032]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall view for explaining an embodiment of an interferometer according to the present invention. FIG. 2 is a diagram showing an embodiment of phase shift and stabilization according to the present invention. FIG. 3 is a diagram showing an embodiment of phase shift and stabilization according to the present invention. FIG. 4 is a diagram for explaining an embodiment of a phase shift wave of an interference fringe according to the present invention. FIG. 5 is an overall view for explaining an embodiment of an interferometer according to the present invention. [Description of Signs] 1 light source 19 output voltage 2 isolator 20 differential amplifier 3 frequency shifter 21 reference voltage 4 orthogonal linearly polarized light 2 frequency light 22 output 5a beam splitter 23 drava 5b beam splitter 24 feedback signal 6 polarizer 25 personal computer 7 light detection Instrument 26 Test object 8 Beat signal 27 Interferometer measuring unit 9 Translucent mirror 28 Polarizer 10 Mirror 29 Mirror 11 Interferometer 30 Beam expander 12 Interferometer controller 31 Interference fringe 13 Quarter wave plate 32 Beam splitter 14 pin Hall 33 CC
D camera 15 Polarizer 34 TV 16 Photodetector 35 Interference fringe 17 Beat signal 36 Actuator 18 Phase meter 37 Surface shape

Claims (1)

Claims: 1. An optical path difference of an interferometer is detected by two-frequency optical heterodyne interference, and the phase of an interference fringe is stabilized at an arbitrary variable phase value by phase control using a servo mechanism. An apparatus characterized by performing fringe analysis. 2. The interferometer according to claim 1, wherein a semiconductor laser is used as a light source, and a two-beam interferometer comprising a control unit and a measurement unit mechanically fixed to each other is configured. A 1/4 wavelength plate is inserted in the optical path, two-frequency light of orthogonal linearly polarized light generated by an acousto-optic element or the like is incident, and an optical heterodyne including a phase corresponding to the optical path difference of the control unit interferometer using a polarizer. A signal is generated, the phase is detected by a phase meter, the output of the phase meter is compared and amplified with a reference voltage by a differential amplifier, fed back to the drive current of the semiconductor laser, and the interferometer is controlled by controlling the laser frequency. In the interferometer, the measurement unit of the interferometer generates interference fringes of the object to be measured by one of the two-frequency light, and stabilizes the interference fringes against disturbance and frequency fluctuation of the light source by the operation of the control unit. Interferometer with features. 3. The interferometer according to claim 1, wherein a general laser is used as a light source, and a two-beam interferometer comprising a control unit and a measurement unit mechanically fixed to each other is formed, and the control unit of the interferometer is used. In this method, a quarter-wave plate is inserted in the optical path, two-frequency light of orthogonal linearly polarized light generated by an acousto-optic element or the like is incident, and light including a phase corresponding to the optical path difference of the control unit interferometer using a polarizer. Generates a heterodyne signal, detects the phase with a phase meter, compares and amplifies the output of the phase meter with a reference voltage with a differential amplifier, feeds back to the drive signal of the actuator mounted with the mirror that constitutes the interferometer, and The interferometer is stabilized by controlling the position.
An interferometer characterized in that an interference fringe of an object to be measured is generated by one light of a frequency light, and the interference fringe is stabilized against disturbance or frequency fluctuation of a light source by an operation of a control unit. In the interferometer according to claims 2 and 3, the reference voltage of the differential amplifier is shifted stepwise so that the bias phase becomes π.
To generate interference fringes shifted stepwise at a pitch of
A phase shift interferometer that performs fringe analysis using the interference fringes.