JPH09280946A - Laser-beam measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a laser-beam measuring apparatus in which stray light is shielded and in which an influence on a laser beam to be measured is reduced to a minimum by using a spatial filter in which the Fourier transform of an optical amplitude transmittance distribution does not take a negative value. SOLUTION: A spatial filter 6 has a Gaussian distribution-shaped optical transmittance distribution in which a Fourier transform does not take a negative value, and its transmittance peak position is made to agree with the intensity peak position of a laser spot 7. In addition, the transmittance of the filter 6 is set in such a way that the transmittance becomes sufficiently low in a position in which a ghost spot 8 is incident. Consequently, stray light due to the rear reflection of a semitransparent mirror 2 can be suppressed sufficiently by the filter 6. The spot 7 which is transmitted through the filter 6 is changed into a parallel beam by a lens 9 so as to be incident on a CCD camera 10. The cross-sectional intensity distribution (the diffraction pattern) of a laser beam which is taken into by the camera 10 is displayed on a monitor 11. Since the filter 6 has the Gaussian distribution-shaped optical transmittance distribution, an undulation is not generated in the diffraction pattern, and high-reliability data is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a measuring device for measuring a cross-sectional intensity distribution of laser light.

[0002]

2. Description of the Related Art Laser light is used in various measuring devices, but the object to be finally measured is often the cross-sectional intensity distribution of the laser light. When measuring the cross-sectional intensity distribution of laser light, the influence of stray light in the optical system, such as the back reflection of a half mirror, may be a problem. In order to remove this stray light, conventionally, a laser beam was focused on a laser spot, a minute pinhole was arranged at the focusing position, and only the light that passed through this pinhole was measured. If the stray light is condensed at a position slightly displaced from the pinhole, the stray light is blocked by the pinhole.

[0003]

However, when the object to be finally measured is the intensity distribution of the laser beam cross section, there is a big problem in inserting the pinhole. That is, the diffraction pattern due to the pinhole is superimposed on the laser beam cross-sectional intensity distribution after passing through the pinhole.
Since the amplitude distribution of the diffraction pattern of the pinhole has positive and negative values, superposition thereof causes a fine periodic undulation in the laser beam cross-sectional intensity distribution, which is a significant obstacle to measurement. Therefore, it is an object of the present invention to provide a laser beam measuring device capable of sufficiently blocking stray light and minimizing the influence on the laser beam to be measured.

[0004]

In order to solve the above problems, the present invention uses a spatial filter in which the Fourier transform of the optical amplitude transmittance distribution does not take a negative value in place of the pinhole.
A Gaussian distribution can be given as an example of a distribution in which the Fourier transform does not take a negative value.

Next, the principle of the present invention will be described in terms of wave optics with reference to FIG. Let u (x, y) be the amplitude distribution of the laser spot immediately before entering the spatial filter, and p (x, y) be the amplitude transmittance distribution of the spatial filter. Amplitude distribution v (x, y) of the laser spot immediately after passing through the spatial filter
Is given by v (x, y) = u (x, y) .p (x, y) ... (1).

The far-field diffraction pattern of this spot is f
If (ξ, η), f (ξ, η) is given by the Fourier transform of v (x, y). Therefore, when the Fourier transform is expressed by FT, f (ξ, η) = FT [v (x, y)] = FT [u (x, y) · p (x, y)] = FT [u (x, y) )] * FT [p (x, y)] (2) Here, the symbol * means a junction product. Also, coordinates
(ξ, η) is a pupil coordinate and represents the position on the diffraction pattern by a component orthogonal to the optical axis of the direction cosine of the straight line connecting the focal point and each point on the diffraction pattern.

From the equation (2), it can be seen that the diffraction pattern after passing through the spatial filter has a distribution corresponding to the Fourier transform of the spatial filter superimposed on the diffraction pattern before passing in the form of a junction product. This causes the diffraction pattern to be deformed.

When the spatial filter is a pinhole having a radius a as in the conventional example, p (x, y) is a function such that it is 1 within the radius and 0 outside the radius. The Fourier transform of such a function is generally D · J ₁ [C · (ξ ² + η ² ) ^1/2 ] / [C · (ξ ^{2 where} C and D are constants and J ₁ is a first-order Bessel function. + Η ² ) ^1/2 ] ... (3), which is a function that takes positive and negative values almost periodically. As is clear from the equation (2), in such a case, the periodic undulation determined by the equation (3) is superimposed on the diffraction pattern, which is very inconvenient in measurement.

Here, the spatial filter is changed to one having a Gaussian distribution-like amplitude transmittance. The Fourier transform of the Gaussian distribution is also Gaussian, and the Gaussian distribution does not have a negative value. Therefore, as is clear from the equation (2),
No fine periodic waviness occurs in the far-field diffraction pattern f (ξ, η). Of course, even in this case, the obtained diffraction pattern is not exactly equal to the pattern without the spatial filter. However, since the fine undulations are eliminated, the variation in the data in actual measurement is drastically reduced, and highly reliable data can be obtained.

The half-value width w of the Gaussian distribution of the spatial filter
Is less likely to deform the original diffraction pattern if it is made as wide as possible to prevent stray light. Although the spatial filter having the Gaussian distribution-like amplitude transmittance is described here, the spatial filter having the Gaussian distribution-like light intensity transmittance distribution has the same value. Although the spatial filter having the Gaussian distribution-like amplitude transmittance is described as an example here, in order to prevent a fine periodic undulation in the far-field diffraction pattern f (ξ, η), the spatial filter is not necessarily limited to the Gaussian distribution. First, the Fourier transform of the light amplitude transmittance distribution does not have to take a negative value.

[0011]

Embodiments of the present invention will be described.
FIG. 2 shows an embodiment in which the present invention is applied to an apparatus for observing a far field diffraction pattern of a laser spot reflected on a sample surface. The laser beam emitted from the laser light source 1 passes through the half mirror 2 and enters the objective lens 3. The laser beam focused by the objective lens 3 forms a diffraction-limited spot on the sample surface 4. Sample surface 4
The laser beam diffracted and reflected by the laser beam passes through the objective lens 3 again, is reflected by the surface of the half mirror 2, and then forms a laser spot 7 on the spatial filter 6 by the condenser lens 5. At this time, the half mirror 2
The unnecessary laser beam reflected on the back surface of the laser also enters the condenser lens 5 and produces a ghost spot 8. However, if the wedge angle θ is provided on the front surface and the back surface of the half mirror 2, the unnecessary ghost spot 8 is formed on the spatial filter 6 at a position apart from the laser spot 7.

The spatial filter 6 has a light transmittance distribution in the form of a Gaussian distribution, and its transmittance peak position coincides with the intensity peak position of the laser spot 7. Further, the transmittance of the spatial filter 6 is set to be sufficiently low at the position where the ghost spot 8 is incident. Therefore, the spatial filter 6 can sufficiently suppress the stray light due to the back surface reflection of the half mirror. The spot 7 passing through the spatial filter 6 becomes a parallel beam by the lens 9 and is incident on the CCD camera 10. CC
The cross-sectional intensity distribution (diffraction pattern) of the laser beam captured by the D camera 10 is displayed on the monitor 11. Here, since the spatial filter 6 has a Gaussian distribution-like light transmittance distribution, no waviness occurs in the diffraction pattern observed as described above, and highly reliable data can be obtained.

Next, the spread of the light transmittance distribution of the spatial filter 6 will be described. As described above, the smaller the Gaussian distribution spread of the spatial filter 6, the more the diffraction pattern is deformed. If the light intensity transmittance half-width w of the spatial filter 6 becomes smaller than the intensity distribution half-width s of the incident laser spot, this deformation becomes extremely large, which hinders observation and measurement. Therefore, it is preferable to form so that s <w.

On the other hand, the half width w of the light intensity transmittance of the spatial filter 6 is the half width s of the intensity distribution of the incident laser spot.
If it is much larger than this, the function of preventing stray light will deteriorate. Considering again the case of using the pinhole, the number of undulations generated in the diffraction pattern is approximately 2a obtained by dividing the pinhole diameter 2a by the laser spot diameter s.
/ S. Therefore, considering the number of pixels of a normal two-dimensional CCD camera, if this value 2a / s exceeds 500,
It is considered that there will be no effect due to the relationship of resolution. In other words, in such a case, observation /
Inconvenience due to undulation does not occur in measurement. Therefore, it can be said that it is necessary to use the Gaussian distribution type spatial filter 6 when w / s <500.

Therefore, the Gaussian distribution spatial filter 6 according to the present invention is effective in that the light intensity transmittance half width w is 1 to 500 of the intensity distribution half width s of the incident laser spot 7.
It can be concluded that there is a double range, ie, s <w <500s. Of course, it is preferable to find an appropriate size in this range depending on the position of the ghost spot 8.

[0016]

As described above, according to the present invention, it is possible to obtain a laser beam measuring device capable of sufficiently shielding stray light and minimizing the influence on the laser beam to be measured.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the principle of the present invention.

FIG. 2 is a configuration diagram showing one embodiment of the present invention.

[Explanation of symbols]

1 ... Laser light source 2 ... Half mirror 3 ... Objective lens 4 ... Sample surface 5 ... Condensing lens 6 ... Spatial filter 7 ... Laser spot 8 ... Ghost spot 9 ... Lens 10 ... CCD camera 11 ... Monitor

Claims (3)

[Claims] 1. A laser beam measuring apparatus, wherein a spatial filter is arranged at a position where a laser beam is focused, and a cross-sectional intensity distribution of a focused beam, a divergent beam or a parallel beam after passing through the spatial filter is measured. As a laser beam measuring device, a spatial filter that does not take a negative value in the Fourier transform of the optical amplitude transmittance distribution is used. 2. The laser light measuring device according to claim 1, wherein the spatial filter has a light amplitude transmittance distributed in a Gaussian distribution. 3. When the full width at half maximum of the intensity distribution of the laser spot at the focus position is s and the full width at half maximum of the optical amplitude transmittance of the spatial filter is w, it is formed so that s <w <500s. The laser light measuring device according to claim 1 or 2.