JPH0221203A - Phase difference detection device - Google Patents

Abstract

PURPOSE:To obtain extremely stable beat frequency by interposing an acousto- optical modulation element to one of two optical paths and forming frequency difference between the laser beams of two optical paths. CONSTITUTION:The linear polarized laser beam of single frequency from a laser beam source is split into an S-wave and a P-wave, both of which have planes of polarization crossing each other at a right angle, by a polarized beam splitter 12. For example, the plane of polarization of laser beam is rotated by 45 deg. with respect to the incident surface of the splitter 12. The P-wave transmitted through the splitter 12 is condensed to an object 16 to be measured by the first lens 14 and the transmitted P-wave is brought to parallel beam by the second lens 18 and reflected by a mirror 24 to reach a polarized beam splitter 28 through the first acousto-optical modulation element 26. This modulation element 26 is equipped with a light pervious substrate 30 and the ultrasonic oscillator 32 provided to the end part thereof and transmitted beam is deflected by diffraction action according to the cyclical refractive index change corresponding to the surface elastic wave generated from the oscillator 32 to shift frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a phase difference detection device that can be used in digital phase contrast microscopes and the like for detecting the thickness, shape, etc. of minute transparent objects.

Conventional technology Transparent micro objects, such as transparent electrodes on a glass plate, and living cells and tissues, which are colorless and transparent but have many parts that have only a different refractive index or thickness than their surroundings, have extremely weak contrast. Phase contrast microscopes have been developed to observe the thickness, dimensions, shape, etc. of measurement objects. One type of phase contrast microscope is a Zeeman laser that outputs laser light, and a polarization beam splitter that splits the laser light into S waves and P waves, passes one through the object to be measured, and then splits the laser light into S waves and P waves. An optical system that combines the waves using a polarizing beam splitter, a photosensor that outputs an interference beat signal by detecting this combined light, and a phase difference between the interference beat signal and the reference beat signal that is detected and shown. It is equipped with a phase meter that outputs an analog signal, and displays the thickness and shape of the object to be measured based on the sequentially obtained phase differences while moving the object to be measured relative to the direction perpendicular to the laser beam. be.

Problems to be Solved by the Invention In such a digital phase contrast microscope, a laser light source with high stability performance, such as a highly stabilized Zeeman laser, is used, but even with such a highly stabilized Zeeman laser, -a Although a high frequency stability of about 10-9 can be obtained for the laser beam outputted from it, a low frequency stability of about 10-3 can be obtained for the beat frequency of the orthogonal two-frequency laser beam outputted from it. do not have. For this reason, the phase difference caused by the passage of the object to be measured cannot be measured with high accuracy, and the thickness, shape, etc. of the object to be measured cannot be detected with sufficient accuracy in some cases.

The present invention has been made against the background of the above circumstances,
The purpose of this is to enable, for example, a phase contrast microscope to detect the phase difference caused by the passage of an object to be measured, and the thickness and shape of the object to be measured based on this phase difference with sufficient accuracy. In order to achieve this, it is an object of the present invention to provide a phase difference detection device capable of detecting a phase difference caused by passage of an object to be measured with high accuracy.

Means for Solving the Problems In order to achieve the above object, the gist of the present invention is to provide a phase difference detection device that detects with high precision the phase difference received by light transmitted through an object to be measured. a) a laser light source that outputs a laser beam; (b) the laser beam is separated into two optical paths, the laser beam of one optical path is passed through the object to be measured, and then the laser beam of the two optical paths is an optical device for multiplexing; (C1 an acousto-optic modulator that is provided in at least one of the two optical paths to create a frequency difference between the laser beams in the two optical paths and changes the frequency of the laser beam that passes therethrough; d) Receives the laser beam multiplexed by the optical device, has basically the same frequency as the frequency difference formed between the laser beams of the two optical paths, and includes phase difference information due to the object to be measured. (e) a photo sensor that outputs a phase difference beat signal; and (e) a phase difference given by the object to be measured based on the reference beat signal having a frequency difference formed by the acousto-optic modulator and the phase difference beat signal. and a digital phase difference detection circuit for counting.

Operation and Effect of the Invention In this way, the acousto-optic modulation element interposed in at least one of the two optical paths creates a frequency difference between the laser beams in the two optical paths, so that an extremely stable beat frequency can be achieved. is obtained.

In other words, the acousto-optic modulator is provided with a substrate on which surface acoustic waves from an ultrasonic oscillator such as a crystal are propagated, and the ultrasonic oscillator receives signals from an oscillation circuit equipped with a crystal oscillator. Since the laser beam is excited by a drive signal with a stable frequency, an extremely stable frequency shift is imparted to the laser beam that has passed through the acousto-optic modulation element configured in this manner. Therefore, compared to the conventional case using a laser light source with low beat frequency stability, the phase difference can be measured with high accuracy, and the thickness and shape of the object to be measured can be detected with sufficient accuracy. It can be done.

EXAMPLE Hereinafter, an example of the present invention will be described in detail based on the drawings. In the following description, each optical element is fixed to a machine frame (not shown) so that its relative position does not change.

In FIG. 1, a laser light source 10 outputs laser light that is linearly polarized light with a single frequency, and a polarization beam splitter 12 generates S waves (S polarized light components) whose polarization planes are orthogonal to each other. and P waves (P polarized light components). The relative rotational position of the deflection plane of the laser beam with respect to the polarizing beam splitter 12 is set in this manner. For example, the plane of deflection of the laser beam is rotated by 45 degrees with respect to the plane of incidence of the polarizing beam splitter 12. The P waves transmitted through the polarizing beam splitter 12 are focused on the object to be measured 16 by the second lens 18, and the P waves transmitted through the object to be measured 16 are converted into parallel light beams by the second lens 18. Ru. The first lens 14 and the second lens 18 are each positioned apart from the object to be measured 16 by the focal length. The object to be measured 16 is placed on an X-Y table 22 that is positioned by an X-Y drive device 20, so that the point at which the P wave passes through the object to be measured 16 is scanned. .

The P wave transmitted through the object to be measured 16 as described above is reflected by the mirror 24 and then transmitted to the first acousto-optic modulation element 26.
and reaches the polarizing beam splitter 28. The first acousto-optic modulator 26 is known as a so-called AOM, and includes a transparent substrate 30 having a high acousto-optic effect, and an ultrasonic oscillator 32 provided at the end of this substrate 30. , the transmitted light is deflected and its frequency is shifted by a diffraction effect according to a periodic change in refractive index corresponding to a surface acoustic wave generated from the ultrasonic oscillator 32. The ultrasonic oscillator 32 is supplied with an output signal from a well-known oscillation drive circuit 36 including a crystal oscillator 34 having a unique oscillation frequency f1.

The S wave reflected by the polarization beam splinter 12 is reflected by the mirror 38 and then transmitted through the second acousto-optic modulation element 40 to reach the polarization beam splitter 28 . The second acousto-optic modulator 40 also includes a transparent substrate 42 having a high acousto-optic effect and an ultrasonic oscillator 44 provided at the end of this substrate 42. Due to the diffraction effect that follows periodic changes in the refractive index corresponding to the surface acoustic waves generated, the transmitted light is deflected and its frequency is shifted. The ultrasonic oscillator 44
is supplied with the output signal of a well-known oscillation drive circuit 48 comprising a crystal oscillator 46 having a natural oscillation frequency f2.

In the first acousto-optic modulator 26 and the second acousto-optic modulator 40, as is well known, when the angular frequency of the surface acoustic wave is Ω and the order of the diffracted light is m,
Only the frequency mΩ is shifted. For example, in the case of first-order diffracted light, the frequency is shifted by Ω. As a result, P that has passed through the first acousto-optic modulator 26
wave, and the S wave that has passed through the second acousto-optic modulation element 40, has an extremely high stability based on the operation of the crystal oscillators 34 and 46, for example, an accurate frequency f with a frequency stability of about 10-8, and A shift of f2 is applied to each. Note that in the first acousto-optic modulation element 26 and the second acousto-optic modulation element 40 in FIG. The first-order diffracted light that has passed through the second acousto-optic modulator 40 reaches the deflection beam splinter 28.

The polarizing beam splitter 28 combines the P wave and the S wave. The thus combined phase difference beat light passes through an analyzer 50 tilted at 45 degrees with respect to the polarization planes of the P wave and S wave, respectively, and passes through a photo sensor 5.
2, and a phase difference beat signal f is output. The polarizing beam splitter 12.28 and the mirror 24.38 constitute a so-called Mannhat-Zender interferometer.
An optical device 56 is configured that separates the laser beam into two optical paths, passes the laser beam on the optical path in the τ direction through the object to be measured, and then combines the laser beams on the two optical paths. In addition,
The analyzer 50 is for adjusting the ratio of the P wave component and the S wave component detected by the photosensor 52. The intensity i of the phase difference beat light can be expressed as in the following equation (1), and has a fundamental beat frequency (rt fz) and also includes a phase difference effect imparted from the object to be measured 16. . Similarly, the phase difference beat signal fD can also be expressed by equation (1). For example, the frequencies f1 and f2
8QM)Iz and 79MHz are selected respectively, and in such a case, the above fundamental beat frequency (f+
rt) is IMHz.

1,, = 'l + 2cos (2π(rt-rg)
+Δl・2π/λ+Φ, ty) However, Δl is the optical path difference (constant) between the optical path of the P wave and the optical path of the S wave, λ is the wavelength of the laser beam, and Φ8. is the ( of the object to be measured 16)
x, y) is the phase difference at the coordinate point.

Furthermore, the output signal of the oscillation drive circuit 36 having the frequency f, and the frequency f! The output signal of the oscillation drive circuit 48 is
A reference beat signal f having a beat frequency (ft f2) is output from the signal synthesis circuit 54. This reference beat signal r can be expressed by equation (2).

f5=co! (2π(ft fz) )
...(21) As is clear from equations (1) and (2) above, the optical path difference Δl between the optical path of the P wave and the optical path of the S wave is constant, so basically, from equation (11) When formula (2) is subtracted, that is, when the reference/standard beat signal f is subtracted from the phase difference beat signal fD, the (x, y
) The phase difference ΦX at the coordinate point is calculated. Note that the phase difference Φ8. can be expressed by the following equation (3), where n is the density and h is the thickness of the object to be measured 16.

Φ□=n −h・2π/λ ・ ・ ・ (3) Figure 2 shows the phase difference Φ8. This shows a digital circuit for calculating . In the figure, the phase difference beat signal fD and the reference beat signal f8 are amplified and shaped into rectangular waves by amplifiers and waveform shapers 60 and 62, and then supplied to a first counter 64 and a second counter 66. These counters 64 and 66 are configured, for example, as ring counters to count the phase difference beat signal fD and the reference beat signal f, which are made into rectangular waves, respectively, and count according to a timing control signal (not shown). The contents are output to the first latch 68 and the second latch 70. The latches 68 and 70 newly store the count contents supplied from the counters 64 and 66 in response to a signal synchronized with the timing control signal, and the subtracter 7
The memory contents are supplied to 2. Subtractor 72 calculates the difference between the counts supplied from latches 68 and 70 and supplies it to third latch 74. The third latch 74 newly stores the difference between the count contents supplied from the subtracter 72 in accordance with a signal synchronized with the timing control signal, and outputs the stored contents to the data bus line 76. This third latch 74
, a phase difference beat signal f, and a reference beat signal fll are obtained.
A signal representing the phase difference with respect to the signal as a multiple (integer) of 2π is output. That is, the phase difference between the phase difference beat signal rD and the reference beat signal r is φ8. is expressed by the following equation (4), a signal representing Nxy is output from the third latch 74.

Φ,,=<2π・NXy+φmV) ・ ・ ・(
4) Furthermore, the phase difference beat signal f and the reference beat signal f are amplified and shaped into rectangular waves by other amplification/waveform shapers 78 and 80, and then supplied to the AND gate 82. As a result, the AND gate 82 is opened for a time corresponding to a phase difference smaller than 2π among the phase differences between the phase difference beat signal fD and the reference beat signal f.

The clock signal generator 84 generates a clock signal SK of a predetermined accurate frequency based on the natural vibration of the crystal oscillator 86 and supplies it to the third counter 88 . Therefore, the third counter 88 counts the number of clock signals SK corresponding to the phase difference smaller than 2π. The third counter 88 outputs the count to the fourth latch 90 in accordance with the timing control signal, and at the same time is cleared in preparation for the next count. A fourth latch 90 responds to a signal synchronized with the timing control signal to newly store the input signal and supply the stored contents to the data bus line 76. The phase difference between the beat signal f and the reference beat signal f is Φ8. The part smaller than 2π, that is, the formula (
4) φ8. The third latch 74 outputs a signal representing . The above-mentioned circuit, which outputs a signal representing the phase difference as a multiple of 2π NXy and a signal representing the phase difference φU, constitutes the digital phase difference detection circuit 91.

The CPU 92 has memory (ROM, RAM, etc.)
While processing the input signal according to a program stored in
direction at a predetermined speed. As a result, CPU9
2 calculates and stores the phase difference Φxy for each unit time, and displays it on the display 96. FIG. 3 shows an example of a display when the object to be measured 16 is a transparent electrode on a glass plate. The phase difference on the vertical axis in Figure 3 corresponds to the thickness dimension, and the time on the horizontal axis corresponds to the feeding distance (scanning distance). , cross-sectional profile, inter-electrode distance d, etc. can be measured.

As described above, according to this embodiment, the frequency shift by the first acousto-optic modulator 26 and the second acousto-optic modulator 40 is based on the high frequency stability of about 10-' by the crystal oscillators 34 and 46. Oscillation drive circuit 36.48
The beat frequency (r+- rz) and the fundamental beat frequency (fl-f2) of the phase difference beat signal, the phase difference can be measured with extremely high accuracy.

Therefore, the dimensions and shape of the object to be measured 160 can be measured with sufficient accuracy.

Next, another embodiment of the present invention will be described. In the following description, parts common to those in the above-described embodiments are designated by the same reference numerals, and the description thereof will be omitted.

In FIG. 4, a first acousto-optic modulator 26 and a second acousto-optic modulator 40 are inserted in the optical path of the S wave. When the frequency of the laser beam (S wave) is ro, the frequencies of the S wave that have passed through the first acousto-optic modulator 26 and the second acousto-optic modulator 40 are fo-11 and -f2. FIG. 5 is a diagram showing this effect in detail. In the figure, the O-order diffracted light of the light transmitted through the first acoustic modulation element 26 is blocked by the light shielding plate 100, and only the first-order diffraction light reaches the second acoustic modulation element 40. This first-order diffracted light is f
After receiving a shift of l, its frequency becomes f0+r. Subsequently, the O-order diffracted light of the light transmitted through the second acoustic modulation element 40 is blocked by the light shielding plate 102, and only the -1st-order diffracted light reaches the polarizing beam splitter 28. This -1st-order diffracted light is further shifted by -f2, and its frequency eventually becomes f0+f, -f2. On the other hand, the frequency of P waves is [. Therefore, the fundamental frequency of the phase difference beat signal f output from the photosensor 52 is (fl fz) as in the above-described embodiment. Therefore, the same effects as in the previous embodiment can be obtained.

FIG. 6 shows another embodiment corresponding to FIG. 1, and FIG. 7 shows another embodiment corresponding to FIG. 4. 6 and 7, a non-polarizing beam splitter 104 is provided in place of the polarizing beam splitter 12, and a λ/2 plate 106 is provided in one of the two optical paths split by the non-polarizing beam splitter 104. is installed.

In such an embodiment, the linearly polarized light, e.g. The polarization plane of the other S wave is rotated by the λ/2 plate 106, so that it can pass through the polarizing beam splitter 28, and it heads toward the photosensor 52. As a result, even in the embodiments shown in FIGS. 6 and 7,
The same effects as in the previous embodiment can be obtained.

In the embodiment of FIG. 8, the laser light (linearly polarized light) output from the laser light source 10 is transmitted to the polarizing beam splitter 1
10 into S-polarized light and P-polarized light. One of the P-polarized lights is given a frequency shift of f by the first acousto-optic modulator 26, and then reflected by the mirror 112 and reaches the non-polarized beam splinter 114. the other S
After the polarized light is given a frequency shift of f2 by the second acousto-optic modulator 40, it is reflected by the mirror 116 and reaches the non-polarizing beam splitter 114.

The S-polarized light and the P-polarized light that have reached the non-polarized beam splinter 114 in this way are combined with each other, for example f0
The combined light is made up of two waves having two frequencies, +f, and f, +f, and whose planes of polarization are orthogonal to each other. A part of this combined light is detected by a photosensor 120 through an analyzer 118, and a reference beat signal fll is output. The other part of the combined light is separated into S-polarized light and P-polarized light by the polarization beam splitter 12 in the optical path configured similarly to the embodiment shown in FIG. 1 or 4, and then
After the P-polarized light passes through the object to be measured 16, the S-polarized light and the P-polarized light are combined again by the polarization beam splinter 28, and detected by the photosensor 52, and a phase difference beat signal f.

is output. In this embodiment, the reference beat signal f
There is a feature that Il is detected by a photosensor 120 through an analyzer 118, but other aspects are the same as in the previous embodiment.

Although one embodiment of the present invention has been described above based on the drawings,
The invention also applies in other aspects.

For example, in the embodiment described above, the phase difference is detected while moving the X-Y table 22 in one dimension, and the object to be measured 1
Although the thickness change in the one-dimensional direction in 6 was measured, the phase difference may be detected with the X-Y table 22 fixed, and the displacement of the surface of the object to be measured 16 may be measured.
It is also possible to sequentially store phase differences while moving the X-Y table 22 in two-dimensional directions, and display a two-dimensional image based on the phase differences.

Furthermore, although the oscillation drive circuits 36 and 48 of the above embodiments were configured to output drive signals with high frequency stability by using the crystal oscillators 34 and 46, other crystals may be used. , other types of high stability oscillation circuits may be used.

The above-mentioned embodiment is merely one embodiment of the present invention, and various modifications may be made to the present invention without departing from its spirit.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing the circuit of the embodiment of FIG. 1. FIG. 3 is a diagram showing an example of the output from the circuit of FIG. 2. Figures 4, 6, 7, and 8 are
FIG. 2 is a diagram corresponding to FIG. 1 showing the configuration of another embodiment of the present invention. FIG. 5 is an enlarged view of the first and second acousto-optic modulation elements in FIG. 4 and their effects in detail. lO: Laser light source 16: Measured object 26: First acousto-optic modulator 40: Second acousto-optic modulator 52: Photosensor 56: Optical device 91: Digital phase difference detection circuit

Claims (1)

[Scope of Claims] A phase difference detection device for detecting with high precision the phase difference experienced by light transmitted through an object to be measured, comprising: a laser light source that outputs laser light; and a laser light source that directs the laser light into two optical paths. an optical device that separates the laser beams of the two optical paths and combines the laser beams of the two optical paths after passing the laser beams of one optical path to the object to be measured; and for forming a frequency difference between the laser beams of the two optical paths. an acousto-optic modulator that is provided on at least one of the optical paths and changes the frequency of the laser beam that passes therethrough; a reference beat signal having a frequency difference formed by the acousto-optic modulation element; a digital phase difference detection circuit that counts the phase difference given by the object to be measured based on the phase difference beat signal.