JPS63179224A - Optical phase difference measuring instrument - Google Patents

Abstract

PURPOSE:To measure the surface shape of a body to be measured, the uniformity of a medium, etc., with high accuracy by utilizing an optical heterodyne interference system, and moving the body to be measured and a light wave to be detected relatively and obtaining a beat signal to be detected. CONSTITUTION:Luminous flux emitted by a light source 2 is passed through a lambda/2 plate 3 and split by a polarization beam splitter (PBS) 4 into transmitted light and reflected light. Here, a 1st light wave which is diffracted by an acoustooptic (AO) element 6 and a 2nd light wave which is diffracted by an AO element 5 are superimposed by a PBS 15, passed through an image formation optical system 16 and a lambda/4 plate 17, and polarized reversely circularly to form interference fringes. The 1st and 2nd light waves are split by a PBS 18 into two, one light beam is image-formed on a detector 19, and the other is image-formed on a detector 20; and the respective detectors detect beat signals, which become a reference signal and a signal to be detected. Here, the reference signal of one measurement point obtained by the detector 19 is recorded 21 and the phase difference from the signal to be detected which is obtained by the detector 20 when a stage 24 is moved according to the reference signal is found.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical phase difference measuring device, and in particular, uses a light wave interference method using optical heterodyne technology to measure changes in refractive index distribution and surface shape of a measured object. The present invention relates to an optical phase difference measuring device suitable for measuring an optical phase change of an incident light wave caused by a change in the optical phase of an incident light wave.

(Prior Art) Various interferometers, such as the Matsuhatsu-Ender interferometer and the Twyman-Green interferometer, have been used to measure the surface shape of the object to be measured and the homogeneity of the medium of the optical material. There is.

In this type of interferometer, for example, when determining the surface shape of an object to be measured, interference fringes are formed by the phase change preserved in the surface shape of the incident light wave, and when determining the uniformity of the medium of an optical material, the incident light wave Interference fringes are formed from phase changes that depend on the non-uniformity of the medium, and the interference state of these interference fringes is measured and determined.

Generally, at this time, a plane wave of a predetermined aperture is made incident on the object to be measured, and the phase change of the reflected or transmitted light wave is converted into an intensity distribution of interference fringes for measurement.

For example, when determining the uniformity of the medium of an optical material, that is, the refractive index difference with other regions, measure the refractive index difference with the uniform medium region by Δn, and the thickness of that region in the direction of light wave propagation with d. Let λ be the wavelength of the light, then the phase change ΔΦ of the incident light wave with respect to other regions is as follows.

In order to measure the uniformity of the medium of the optical material with high precision at this time, it is necessary to obtain the phase change ΔΦ, that is, the interference state of the interference fringes with high precision.

However, in conventional interferometers, a beam diameter corresponding to the size of the measured object is incident on the measured object, so as the measured object becomes larger, the intensity of the light wave incident on the measured object decreases.
The contrast of interference fringes decreases.

In order to measure the interference state of interference fringes with high precision, it is possible to use a highly sensitive photodetector or a high-output laser light source, but these elements generally have limitations, and in many cases it is difficult to measure the interference state highly. There were problems such as it became difficult to measure accurately and the overall size of the device increased.

(Problems to be Solved by the Invention) The present invention uses an optical heterodyne interference method to measure the surface shape and material uniformity of an optical member with high precision by detecting the phase change of an incident light wave. The purpose of the present invention is to provide an optical phase difference measuring device that can perform the following steps.

(Means for Solving the Problem) A reference beat signal is obtained by generating two light waves, a first light wave and a second light wave, having different frequencies, and combining the first light wave and the second light wave. After one of the first light wave and the second light wave is incident on the object to be measured and is transmitted or reflected, when the test light wave and the other light wave are combined, the object to be measured and the object to be measured are combined. A beat signal to be measured is obtained by moving the light wave relatively, and the optical phase change state of the object to be measured relative to the incident light wave is measured using these two beat signals.

(Embodiment) FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention. In the figure, 1 is an object to be measured, which is a transparent sample with a refractive index distribution inside, 2 is a coherent light source such as a laser, and 3 is a λ/2 plate, which determines the polarization plane of the light beam emitted from the coherent light source 2. It is tilted 45' with respect to the normal to the paper surface. Reference numeral 4 denotes a polarizing beam splitter (hereinafter referred to as PBS), which divides the light beam that has passed through the λ/2 plate 3 into P direction and S direction of the polarization plane, and generates two light waves with approximately equal intensity and whose polarization planes are orthogonal to each other. is being generated. 5 and 6 are acousto-optic modulation elements (hereinafter referred to as A10 elements) that diffract the two light waves generated by the PBS 4, respectively, and frequency-modulate the light waves to, for example, 80 MH2 and 81 MH2.

7 and 8 are plane reflecting mirrors that reflect the light waves diffracted by the A10 element and change the optical path; 9 is a PBS for directing the two light waves in the same direction; and 10 is a predetermined mirror from the diffracted light generated by the A10 element 5.6. 12 is a PBS that separates the incident light wave; 13 and 14 are plane reflectors that reflect the two light waves separated by PBS 12 and change the optical path; 15 is a plane reflector 14 that reflects the light waves. 16 is an imaging optical system that guides the light waves obtained through the PBS 15 to detectors 19 and 20, which will be described later. The system is configured to image the object 1 to be measured on the sensor surface of the detector 19.20. 17 is the combined P
A λ/4 plate that converts the light waves in the azimuth and S azimuth into circularly polarized light,
18 is a circularly polarized light wave that has passed through the λ/4 plate 15 and is split and guided to a detector 19.21 <PBS, 19.20 is a detector whose position is fixed by a minute aperture, and 21 is a detector 19
A recording circuit 22 is an arithmetic circuit which calculates the phase difference between the signal from the recording circuit 21 and the signal from the detector 20. 23 is a display means for displaying or outputting the calculation results obtained by the calculation circuit 22.

Reference numeral 24 denotes a stage which is movable in the x and y directions and on which the object to be measured 1 is placed.

Next, the operation in this embodiment will be explained.

A light beam emitted from the coherent light source 2, such as a laser beam, passes through a λ/2 plate 3 and is split into transmitted light and reflected light by the PBS 4. Here, the reflected light is a light wave having a polarization plane in the S direction (hereinafter referred to as S-polarized light), and is linearly polarized in a direction perpendicular to the plane of the paper. This light wave is diffracted by the A10 element 6. Here, if the frequency of the ultrasonic wave generated by the A/O element 6 is 81 MH2 and the +1st-order diffracted light is used for measurement, the light wave that is the +1st-order diffracted light (hereinafter referred to as the first light wave) is +81 MH2. M) After receiving a frequency shift of 12, P
Directed to BS15.

On the other hand, the transmitted light that has passed through the PBS 4 is a light wave having a polarization plane in the P direction (hereinafter referred to as P polarization), and is linearly polarized in a direction parallel to the paper surface, and this light wave is diffracted by the A10 element 5. receive. Here, if the frequency of the ultrasonic wave generated by the A/O element 5 is 80 MHz and +1st order diffracted light is used for measurement, the light wave (+1st order diffracted light)
Hereinafter, this will be referred to as a second light wave. ) is subjected to a frequency shift of +80MH2, and the plane reflector 7, PBS 9, aperture 1
0, PBS 12, plane reflection @ 13, and object to be measured 1 are directed to PBS 15 through the optical path in this order. Therefore, the phase of the second light wave undergoes a phase change according to the refractive index distribution of the object to be measured 1, and the second light wave and the first light wave are connected to the PBS 15.
The light is multiplexed by the imaging optical system 16, passes through the λ/4 plate 1 and the metal plate 7, and becomes circularly polarized light in opposite directions, thereby forming interference fringes.

Now, the light intensity distribution i (x, y, t) when the second light wave that has passed through the object to be measured 1 and the first light wave, which is a plane wave whose frequency differs by IMH2, is combined is position x, y
If ΔΦ(x, y) is the phase change of the second light wave at
os (ΔΦ(x,y) to 2πx 10't+)
−・・・・・・・−(1) can be expressed. In addition, here K is the wave number and = 2π/λ (λ is the wavelength)
It is indicated by.

Therefore, as shown in (1) above, the phase change ΔΦ(x,y) of the second light wave is determined as the phase change of the beat signal IMHz"2π×106.

As mentioned above, the first light wave and the second light wave, which are circularly polarized in opposite directions from each other due to the λ/4 plate 1 and the metal plate 7, are split by the PB 318, and one of them is sent to the sensor of the phase-locked micro-aperture detector 19. The other side is imaged onto a detector 20, and an I MHz beat signal is detected by each detector. here,
The light waves that are imaged on the detectors 19 and 20 are composed of the second light wave that has passed through an arbitrary position (x(1, yo)) of the object 1 to be measured, and the first light wave. IM due to interference
The H2 beat signal is detected by detectors 19 and 20, and serves as a reference signal and a test signal for measurement.

In this embodiment, a reference signal obtained by the detector 19 at one measurement point is recorded by the recording circuit 21, and when the stage 24 is moved using this reference signal as a reference, the detector 2
The phase difference with the test signal obtained from 0 is calculated.

That is, in the detector 20, when the stage 24 is moved and the object to be measured 1 is scanned, beat signals obtained at each position are obtained as the test signals.

Then, the phase difference between the reference signal and the test signal is determined by the arithmetic circuit 22, and the result is displayed by the display means 23.

FIG. 2 shows the beat signal R, which is the reference signal output from the detector 19, and the phase information (ΔΦ
) is shown on the time axis. By scanning the stage 24 one-dimensionally or two-dimensionally, a signal as shown in FIG. 2 is obtained, and the phase difference ΔΦ(x, y) of the signal S from the reference signal R at each position (x, y) is calculated. By electrically detecting the arithmetic circuit 22, the relative phase difference ΔΦ(x, y
), that is, the object to be measured! The refractive index distribution of

FIG. 3 shows a schematic block diagram of calculations in the calculation circuit 22 from the beat signals R and S to obtaining the refractive index distribution, where 30 is a phase detector, 31 is a microprocessor, and 32 is an output of a CRT, printer, etc. Show the device. Peat R and S obtained from the detectors 19 and 20 are processed by the phase detector 30 to calculate the phase difference ΔΦ(x,y). At this time, the microprocessor 31 calculates the scanning position (x, y) by the stage 24. , y), and the beat signal S at that scanning position (x, y) is sequentially fetched into the phase detector 30 and processed. Note that the beat signal R is substantially constant.

The phase difference ΔΦ(x.

The signal of y) is A/D converted and sent to the microprocessor 31.
is input. The microprocessor 31 calculates the refractive index distribution n from the phase difference ΔΦ(x, y) according to a predetermined conversion procedure.
(x, y) and outputs a signal regarding n (x, y) to the output device 32.

Then, the output device 32 displays information regarding the refractive index distribution of the object to be measured on the display means 23 in the form of numerical values or images.

As described above, in this embodiment, the refractive index distribution of the object to be measured 1 is determined.

In this embodiment, in addition to measuring the refractive index distribution of the object to be measured, the surface shape of a lens, a reflecting mirror, etc. can be similarly determined by detecting the phase difference of the incident light wave.

In this embodiment, instead of moving the object to be measured, the light wave to be measured that is incident on the object to be measured is scanned by an optical scanning means or the like, and the object to be measured and the light wave to be measured are moved relative to each other. It's okay.

(-Effects of the Invention) According to the present invention, by using an optical heterodyne interference method and obtaining a beat signal by relatively moving the object to be measured and the light wave to be measured, the object to be measured is large. It is also possible to achieve an optical phase difference measuring device that can measure the surface shape of an object to be measured, the homogeneity of a medium, etc. with high precision without increasing the size of the entire device.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the relationship between a reference beat signal and a test beat signal according to the present invention, and FIG. 3 is a refraction diagram according to the present invention. 1 is a block diagram of signal processing when measuring rate distribution. In the figure, 1 is the object to be measured, 2 is the light source, 3 is the λ/2 plate, 4.9,
12, 15.18 are polarizing beam splitters, 15, 6 are acousto-optic elements, 10 are apertures, 7, 8, 13.1
4 is a plane mirror, 17 is a λ/4 plate, 19.20 is an optical sensor.
, 24 is a stage, 21 is a recording circuit, 22 is an arithmetic circuit,
23 is a display means.

Claims (1)

[Claims] A reference beat signal is obtained by generating two light waves, a first light wave and a second light wave, having different frequencies, and combining the first light wave and the second light wave, and generating one of the first light wave and the second light wave. After the light wave to be measured is made incident on the object to be measured and transmitted or reflected, when the light wave to be measured and the other light wave are combined, by moving the object to be measured and the light wave to be measured relatively. An optical phase difference measuring device characterized in that a beat signal to be measured is obtained, and an optical phase change state of the object to be measured with respect to an incident light wave is measured using these two beat signals.