JPS63298106A - Sharing interference-fringe analyzing method - Google Patents

Abstract

PURPOSE:To make it possible to measure the shape of a wave front, whose phase change is large in a space, by scanning interference fringes with a photoelectric element in a two-dimensional pattern, and freely increasing the number of samples in the space. CONSTITUTION:Laser light 1a, which is emitted from a laser light source 11, is reflected with a material to be measured 15 through a beam expander 12, a half mirror 13a and a reference lens 14. The light undergoes phase modulation. The path of the light is changed at a half mirror 13a. The light is inputted into a sharing interferometer. Luminous flux 1b is split with a half mirror 13b and relatively deviated with corner cubes 16a and 16b. The luminous flux 1b passes the half mirror 13b again and undergoes sharing interference. Then a wave front 10c is obtained. The intensity of interference fringes is converted into an electric signal through a photoelectric element 19. The interference fringes 10c are scanned with the photoelectric element 19 in the directions shown by X and Y in a two-dimensional pattern. Thus the phase of the entire body of the interference fringes 10c is obtained. In this way, the shape of the wave front of the luminous flux 1b is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a measuring method for precisely measuring the shape of a flat 1 spherical or aspherical surface of a lens, mirror, etc., and more specifically to a shearing interference fringe analysis method. /゛'s.

BACKGROUND OF THE INVENTION Conventionally, in fringe scanning type shearing interferometers, the amount of shearing depends on the spatial variation of the phase of the measured wavefront, and is also limited by the number of spatial samples of interference fringe intensity.

The prior art will be explained below using FIG. 6. Laser light 6a emitted from laser light source 61 is transmitted to beam expander 62. Half mirror 632L.

The light is reflected by the measurement object 65 through the reference lens 64, spatially phase modulated, changes its optical path by the half mirror 63a, and enters the shearing interferometer.

The light beam 6b is a half mirror ESB and is a light beam 6C.

It is divided into 6d, is relatively shifted laterally by the corner cubes eea and esb, and is again divided into no-fumin-63b.
Interference fringes 6e are generated through the. Here, when the wavelength of the laser is λ and n is an arbitrary natural number, the corner cube 6
62L is moved in the direction of arrow λ using the electrostrictive element 67 to change the phase of the wavefront 6d, and at each phase, the intensity I(x,
y) is sampled by the imaging device 69 and inputted into the one-node frame memory 610. Here, the amount of lateral shift in the X direction is ΔX, and the phase of the wavefront of the light beam 6b is h(x, y).
Then, the difference Δh(x, y) = h(x+Δx, y
)−h(x,y) is given by the following equation.

Therefore, based on equation (1), the phase h(x, y) of the wavefront of the light beam 6b is determined by determining the difference in the X direction and the X direction, integrating in the X direction and the X direction, respectively, and combining. Here, the lateral shift amount ΔX needs to satisfy the following equation.

I h(x, y)-h(X+Δx, y)l ≦π
=no=・ (2) Furthermore, the number m of samples in the X direction by the imaging device 69 needs to satisfy the following equation, where D is the diameter of the light beam 6b.

1≦m (3) Problems to be solved by the invention In the above conventional method, if the phase of the wavefront of the light beam 6b changes spatially, (2) According to equation (3), it is necessary to make ΔX smaller, and therefore pm becomes larger than in equation (3), resulting in disadvantages such as an increase in the capacity of the frame memory 610 and a slow calculation time. Further, the number of samples m is limited by the performance of the imaging device 69, and cannot be increased without limit. For these reasons, aspherical lenses with a large amount of aspherical surface could not be measured.

Means for Solving the Problems In order to solve the above problems, the shearing interference fringe analysis method of the present invention modulates the phase of the laterally shifted wavefront and the phase of the other wavefront relatively sinusoidally. , the intensity of interference fringes generated by shearing interference at a certain point is detected by a photoelectric element, and the output signal of the photoelectric element is subjected to analog processing over at least half a period to determine the phase of the interference wavefront at that point. This operation is performed two-dimensionally over the entire interference fringe to obtain the phase distribution of the entire interference wavefront, and the original wavefront is reproduced based on this information.

Effect: According to the above method, the phase changes significantly spatially from 5 to 1/2.
Even in the case of shearing interference fringes on a wavefront that is distorted, by increasing the number of spatial samples of interference fringe intensity, the amount of shearing can be reduced and interference fringe analysis can be performed, making it possible to reconstruct the wavefront. .

Embodiments Hereinafter, embodiments of the present invention will be explained based on FIGS. 1 to 6. In FIG. 1, a laser beam 1a emitted from a laser light source 11 is transmitted to a beam expander 12. half mirror
13a, it is reflected by the measurement object 15 through the reference lens 14, is phase modulated, changes its optical path by the half mirror 13a, and enters the shearing interferometer. The light beam 1b is a half mirror 1
3b, is relatively laterally shifted by the corner cubes 16a and 16b, passes through the half mirror 13b again, becomes a shearing interference wavefront 10C, and the photoelectric element 19 converts the interference fringe intensity into an electrical signal.

Furthermore, the electrostrictive element 17 is used to apply phase modulation to the optical path length. At this time, as shown in FIG. 2, an amplifier 212 amplifies the output of the photoelectric element 1e corresponding to the coordinates (x, y).
The output signal v(x, y) of the light beam e/' is the phase of the wavefront of the beam 1b, h(x, y), and the phase of the wavefront of the beam 10a.

Assuming that the amount of lateral shift of 10b is ΔX, the phase difference due to the difference in optical path length is φ, and the phase difference provided by the electrostrictive element 17 is l, it is given by the following equation.

v(x, y) = Vj+v2cos (h(x,
y)-h(x+Δx+y)-φ-1) Next, the clock pulse generator 23 generates the waveform 4 of the angular frequency ω shown in FIG.
1, and a sine wave with an angular frequency ω synchronized with this is generated by the sine wave generator 22, and the electrostrictive element drive circuit 211
supply to At this time, the waveform of the output signal of the amplifier 212 becomes the waveform 31 in FIG. 3. The waveform 31 and the waveform 41 are multiplied by the multiplier 25& to obtain the waveform 42 shown in FIG. Next, the sine and cosine of the signal generated by the sine wave generator 22 are calculated by the sine calculator 26. The cosine calculator 27 is used to perform calculations, the waveform 42 is multiplied, and the integrator 28a,
28b. Next, the output value of the integrator 28& is divided by the output value of the integrator 28b using the divider 29, and the arctangent is obtained by the arctangent calculator 20. As a result, the phase h(X
, 7)-h(x+Δx,y)-φ is obtained.

The phase of the entire interference fringe 100 is determined by scanning the photoelectric element 19 two-dimensionally over the interference fringe 10C as shown by arrows X and Y in FIG. 1 through the above-described operation.

That is, integration is performed in the X direction based on the sharing amount ΔX. The shape of the wavefront of the light beam 1b can be determined by performing the same process in the y direction and combining the results with the results in the x direction.

Effects of the Invention As stated above, according to the present invention, since the photoelectric element scans the interference fringes two-dimensionally, the number of spatial samples can be freely increased, and the amount of shear can thereby be reduced. This makes it possible to measure the shape of a wavefront with a large phase change. Furthermore, since the phase at each point of the interference fringes can be calculated in an analog manner, measurement can be performed at high speed and with high precision.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, FIG. 2 is a block diagram thereof, and FIGS. 3, 4, and 6 show signal waveforms of each block in FIGS. 1 and 2. 6 are schematic diagrams showing a conventional example. 10C...Interference fringes, 11...Laser light source, 16a. 16b... Corner cube, 17... Electrostrictive element, 19... Photoelectric element, 2o... Arctangent operator, 22... Sine wave Generator, 23...
...Clock pulse generator, 2a &, 28b...
...integrator, 29...divider.

Claims (1)

[Claims] In a fringe scanning type shearing interferometer, the phase of a laterally shifted wavefront and the phase of the other wavefront are relatively sinusoidally modulated, and the intensity of the interference fringe at a certain point of the interference fringes generated by shearing interference is measured using a photoelectric element. The phase of the interference wavefront at that point is determined by analog processing the output signal of the photoelectric element over at least half a cycle, and this operation is performed two-dimensionally over the entire interference fringe to determine the phase distribution of the entire interference wavefront. A shearing interference fringe analysis method that is characterized by determining the information and reproducing the original wavefront based on that information.