JP2621792B2 - Method and apparatus for measuring spatial coherence - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring the spatial coherence of electromagnetic waves such as light and X-rays.

[0002]

2. Description of the Related Art Measurement of the spatial coherence of electromagnetic waves is an important technical subject in many fields such as laser microscopes, laser lithography, second harmonic generation technology using nonlinear optical crystals, optical information processing and laser processing. In a laser microscope, a stepper using a laser as a light source, and the like, speckle patterns and interference fringes caused by lenses cause a reduction in imaging performance, and a laser processing machine also suffers from a problem of a reduction in processing accuracy due to the interference pattern. The conversion efficiency of the second harmonic generation by the nonlinear optical crystal is related to the spatial coherence. It is extremely important to grasp the spatial coherence of the light source used in the design or technical development of these devices and reflect it in the design. On the other hand, also in the field of optical information processing that actively utilizes light interference, it is important to measure the spatial coherence of the light source used. As described above, the measurement of the spatial coherence of a light source is an extremely important subject in various fields, and the following two methods have been conventionally known as the measuring method. That is, the first method is a method called Young's interference method or the double pinhole method, and the second method is a shearing interference method.

FIG. 6 shows the principle of the first method, Young's interferometry. Double pinhole plate 61 with two holes
Is irradiated with a light beam 60 and the light from the double pinhole interferes with the focal plane 63 of the lens 62 installed behind the double pinhole plate 61. The Young's interferometer is simple and compact, but the distance between the pinholes is fixed. However, there are drawbacks in that the measurement accuracy is poor because of this, and it takes time to replace the pinhole. FIG. 7 is a diagram illustrating an example of the shearing interferometry that is the second method. The light beam 70 is split by a beam splitter 71, and each light beam is reflected by a reflecting mirror 72, 73, 74 or a roof prism 75 as shown in the figure.
Are again synthesized by the beam splitter 76 and interfere with each other. At this time, the split light beams are symmetrically combined with each other and interfere with each other. Although this method has improved the disadvantages of the first conventional example, it requires many high-precision optical components, and therefore requires high alignment accuracy, and in particular, high accuracy is required for adjusting the distance between two divided optical paths. Cost. Japanese Patent Application Laid-Open No. 63-1940 discloses a method of measuring the coherence length by AD-converting an interference fringe signal obtained by using these methods, but performs a special operation such as Hilbert transform. Operation device is required.

[0004]

Although the first method, the Young's interferometry, is compact, the pinhole must be replaced in order to perform a highly accurate measurement, and the adjustment for that requires time. There is such a disadvantage. The second shearing interferometry has a disadvantage that a high-precision optical system is required and many high-precision optical components are required. The present invention provides a measuring apparatus and a method for easily measuring the spatial coherence of light with high accuracy with a simple configuration in view of these conventional methods.

[0005]

Means for Solving the Problems 1 consisting of two transmission holes
A first reflecting mirror having a plurality of pairs of holes in the vertical direction of the pair of holes and a second reflecting mirror installed in parallel with the first reflecting mirror;
And a lens array corresponding to each pair of transmission holes, wherein the distance between a pair of holes in the first reflection mirror is an integral multiple of the distance between the first holes, It is a spatial coherence measuring device composed of sensors for measuring an interference pattern from each pair of holes.

[0006] Further, the first device of the first reflector having a plurality of pairs of two transmission holes is provided by the above apparatus.
The reflected light beam of the parallel light beams incident on the vicinity including the pair of holes sequentially irradiates the transmission hole while repeating reflection between the second plane mirror and the first reflection mirror, thereby forming the hole of the first reflection mirror. This is a spatial coherence measurement method in which an interference pattern is formed by a pair of holes on the focal plane of a lens array installed corresponding to the above, and the spatial coherence of incident light is determined from the sharpness of the interference pattern.

[0007]

According to the present invention, the interval provided in the first reflecting mirror is one.
A plurality of pairs of transmission holes, which is an integral multiple of the second pair of transmission holes, are sequentially illuminated by multiple reflection with the second plane mirror, and a number of interference fringes are formed by light transmitted from the pair of holes,
Further, the spatial coherence length of the irradiation light is measured by obtaining the definition of the interference fringes.

[0008]

FIG. 1 is a schematic view of an apparatus according to an embodiment of the present invention. The first reflecting mirror 11 is provided with a pair of transmission holes 12a and 13a at a minute interval h, and a second transmission hole 12a, 13a is located at a distance L in the vertical direction of a line connecting the two holes. The minute transmission holes 12b and 13b with a spacing of 2h are opened,
Similarly, at the position of a distance L from the second pair of holes, third minute transmission holes 12c and 13c are provided at an interval of 3h. Although FIG. 1 illustrates an example of four pairs of holes, an n-th minute transmission hole (not shown) having an n-th interval (not shown) is formed at a position of a distance nL where n is an integer. A second plane mirror 14 is provided in parallel with and facing the first reflecting mirror 11. A lens array 15 is provided corresponding to each pair of transmission holes formed in the first reflecting mirror 11.

The transmission holes 12a, 13a of the first reflecting mirror 11
A part of the collimated light flux 10 incident at an incident angle θ passes through a pair of transmission holes 12a and 13a, and forms an interference pattern at a position of the focal length f of the lens array 15. Most of the light flux 10 that has entered the transmission holes 12a and 13a of the first reflecting mirror is reflected, then reflected by the second reflecting mirror, and
The area near the second transmission holes 12b and 13b located at a distance L from the first transmission hole of the reflecting mirror 11 is irradiated. The light passing through the second transmission holes 12b and 13b is transmitted to the first transmission hole 12b.
A interference fringe is formed at the position of the focal length f of the lens array 15 in the same manner as the light transmitted through a and 13a. Second transmission hole 1
Most of the light beams irradiated to 2b and 13b are reflected, further reflected by the second reflecting mirror 14, and irradiated to the third transmission holes 12c and 13c located at a distance L from the second transmission hole. Similarly, the light beam passing through the transmission hole forms an interference fringe at the position of the focal length f of the lens array 15. At a position 16 of the lens array 15 at the focal length f, a two-dimensional imaging device is installed as a means for measuring interference fringes.

FIG. 2 is a diagram for further explaining the principle of the device shown in FIG. 1, in which the light flux of wavelength λ illuminating the first reflecting mirror 11 is spaced at an integer multiple of the spacing h. This figure shows a state in which the light passes through two holes and forms an interference fringe on the focal plane 16 of the lens array 15. Light irradiating the reflecting mirror 11 having a large number of pairs of holes passes through the holes and forms interference fringes on the focal plane of the lens array via the lens array in accordance with the distance between the holes. FIG. 3 is a diagram showing an example of the interference pattern obtained at that time. In FIG. 3, the numbers 1, 2, 3... In the Y direction correspond to n which is an integral multiple of L of the distance between the pair of holes in FIG.
Is the distance in the direction connecting the pair of holes on the focal plane 16.
The vertical axis indicates the light intensity obtained at that time. FIG. 3 shows that the pattern of the interference fringes changes and the sharpness also changes depending on the distance between the pair of holes. FIG. 5 is a diagram showing one of the interference fringe patterns shown in FIG. At this time, it is well known that when the _maximum value of the light intensity is I _max and the adjacent minimum value is I _min , the sharpness V is defined by the following equation 1.

[0011]

(Equation 1)

FIG. 4 shows the calculated sharpness V from the light intensity distribution of the interference pattern shown in FIG. In FIG. 4, the interval between the holes where the definition V becomes 0 is the spatial coherence length. That is, the spatial coherence length of the light source used in the examples of FIGS. 3 and 4 is 6 h.
In addition, 1 / e of sharpness which standardized spatial coherence length
² may be defined as the spatial coherence length. 1 /
Since e ² = 0.135, the coherence length at this time is 5.2h in FIG. Thus, the spatial coherence length of light can be easily and accurately measured.

In the embodiment shown in FIG. 1, an image sensor is provided on the focal plane 16 of the lens array 15, and the light intensity distribution shown in FIG. 3 is measured. An imaging device such as a CCD camera or a vidicon can be used as the imaging device. It is also possible to record with a photosensitive material such as a film and read the light intensity of interference fringes. The intensity of light that is multiple-reflected between the first reflector 11 and the second reflector 14 attenuates in accordance with the number of reflections, but does not affect interference fringes and sharpness by changing the sensitivity of the image sensor. I can do it. Further, by sequentially changing the reflectance of the first reflecting mirror, the second reflecting mirror, or both reflecting mirrors, the light intensity of the interference fringes on the focal plane 16 of the lens array on which the imaging device is installed is made substantially equal. This can be easily realized. Although an example using light has been described above, it is clear that the present invention can be applied to all electromagnetic waves including X-rays, and the target light source can be continuous wave light or pulsed light.

[0014]

As described above, according to the present invention, the spatial coherence length of light can be easily and accurately measured with a simple apparatus including a reflector having a plurality of pairs of holes, a plane reflector and a lens array. .

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating the principle of the present invention.

FIG. 3 is a diagram showing a measurement example of interference fringes obtained by the apparatus of the present invention.

FIG. 4 is a diagram showing sharpness obtained from the interference fringes of FIG. 3;

FIG. 5 is a diagram for explaining sharpness.

FIG. 6 is a diagram showing the principle of a conventional method of Young's interferometry.

FIG. 7 is a diagram showing an example of a conventional method of the shearing interferometry.

[Explanation of symbols]

 Reference Signs List 10 collimated light source 11 first reflecting mirror 12a, 13a, 12b, 13b having a pair of transmitting holes 14 transmitting hole 14 second reflecting mirror 15 lens array 16 focal plane 60 light flux 61 double pinhole plate 62 lens 63 focal plane 70 Light beam 71, 76 Beam splitter 72, 73, 74 Reflector 75 Roof prism

Claims (2)

(57) [Claims] 1. A first reflecting mirror having a plurality of pairs of two transmission holes with a plurality of transmission holes at different intervals is reflected out of a parallel light beam incident near the first reflection mirror including the first pair of holes. By sequentially irradiating the transmission hole of the first reflecting mirror while the light beam repeatedly reflects between the second plane mirror and the first reflecting mirror,
A spatial coherence measurement method for forming a plurality of interference patterns on a focal plane of a lens array provided corresponding to a hole of a first reflecting mirror and obtaining a spatial coherence of incident light from the sharpness of the interference pattern . 2. A first reflecting mirror having a plurality of pairs of holes formed of two transmitting holes, a second plane reflecting mirror installed opposite to the first reflecting mirror, and a pair of respective ones. It has a lens array corresponding to the transmission holes, and the interval between a pair of holes in the first reflecting mirror is an integral multiple of the interval between the first holes, and the interference formed by each pair of holes. A spatial coherence measurement device composed of sensors that measure patterns.