JP2540430B2 - Laser beam condensing characteristics measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-power laser, in particular, a pulse laser in the infrared region, for measuring a light-collecting characteristic such as a focal position, a light-collecting diameter, a beam divergence angle and a beam spot size of a laser beam. The present invention relates to a laser beam focusing characteristic measuring device. In particular, the present invention relates to a laser beam focusing characteristic measuring device which is required to measure these values at high speed and with high accuracy.

[0002]

2. Description of the Related Art Conventionally, in the measurement of the spatial intensity distribution of a laser beam, in the case of a low-output laser, a device in which elements are arranged one-dimensionally or two-dimensionally, such as a CCD or a pyroelectric element, is used. Have been.

[0003] However, in order to measure the spatial intensity distribution of a high-power laser, particularly a pulse laser in the infrared region, a highly reliable pinhole scanning method capable of directly inputting a high intensity is often used. In this pinhole scanning method, the energy of the laser beam reaching the energy detector is limited by placing a pinhole plate with a small pinhole in front of the energy detector. Integrate with the plate and move vertically to the laser beam,
The energy transmitted through the pinhole is measured for each position.

[0004]

However, in the above-described pinhole scanning method, since a small pinhole is moved little by little with respect to the optical axis, a high-precision measurement is performed by increasing the number of data samplings. In order to
There was a problem that it took a lot of time.

Although the pinhole scanning is performed by using a stepping motor and further controlled by a computer in synchronization with the laser pulse, the measurement time can be reduced to some extent. There are inherent limitations in speeding up the measurement.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a laser beam focusing characteristic measurement method capable of measuring the laser beam focusing characteristics in a shorter time and with higher accuracy than in the past. It is intended to provide a device.

[0007]

According to the first aspect of the present invention, there is provided a laser beam focusing characteristic measuring apparatus for measuring the intensity of a laser beam to be measured, and the laser beam intensity measuring device. A Hadamard-array-type slit in which slits are arranged in accordance with matrix elements of a Hadamard matrix, and driving means for moving the Hadamard-array slit so as to scan in the beam radial direction of the laser beam to be measured. And characterized in that:

According to a second aspect of the present invention, there is provided a laser beam focusing characteristic measuring apparatus for introducing a laser beam to be measured, and a laser beam to be measured introduced by the laser beam introducing means. Focusing means for focusing, and at least two laser beam intensity measuring means arranged at intervals on the optical path of the laser beam to be measured between the focusing means and a focal point by the focusing means; A Hadamard array-type slit in which slits are arranged in accordance with matrix elements of a Hadamard matrix, respectively, arranged in front of the laser beam intensity measuring means, and scan the Hadamard-array slit in the beam radial direction of the laser beam to be measured. Driving means for moving the laser beam to be measured, and measuring the intensity of the laser beam to be measured. A monitoring laser beam intensity measuring means for monitoring fluctuations in the intensity of the beam, a beam splitter for dividing the laser beam to be measured, and entering the laser beam intensity measuring means and the monitoring laser beam intensity measuring means; A calculating means for calculating a focal position and a condensing diameter of the laser beam to be measured from measurement results of the laser beam intensity measuring means and the monitoring laser beam intensity measuring means.

That is, in the present invention, data acquisition is multiplexed by slits reflecting a Hadamard matrix, instead of measuring data sequentially for each point. This can reduce the measurement time by simultaneously capturing several data. After the measurement is completed, the data is subjected to Hadamard transform, so that the same result as the data obtained by pinhole for each point can be obtained.

[0010]

[Effect] Conventionally, in a case where a signal (light intensity) is very weak or a detector sensitivity is very weak, in a spectroscopic measurement,
A method of improving the S / N ratio by using a Hadamard array type slit in which slits are arranged according to matrix elements of a Hadamard matrix is known (see “R. Horiba, Y. Okazaki and
K. Nagasaka: Jpn. J. Appl. Phys. 31 (1992) 3469. ").

The laser beam focusing characteristic measuring apparatus of the present invention measures the laser beam focusing characteristics in a short time with high accuracy by using this Hadamard array type slit. Hereinafter, the measurement principle will be described.

The intensity distribution of light is represented by n elements x in the spatial direction.
_It is decomposed into _i , the corresponding n observations are denoted by y _i , and the measurement error of each time is denoted by e _i . When these values are represented by vertical vectors X, Y, and E of n rows, there is a relationship of Y = AX + E (1) therebetween. Here, A is called an n × n mask matrix. When this is a unit matrix I, it corresponds to simple pinhole scanning, and x _i and y _i correspond one-to-one.

On the other hand, instead of multiplexing the measurements and measuring all of x _i to x _n independently,
Performing the measurement in an appropriate combination reduces errors, shortens the measurement time, and narrows the dynamic range of the measuring instrument. An optimized version of this mask matrix is called an S matrix obtained from a Hadamard matrix. In a matrix S ′, S ′ = (１ ／) (B−H) (2) obtained from the following formula from a (n + 1) × (n + 1) Hadamard matrix H and a matrix B in which all elements are 1: An n × n matrix with one row and the first column removed is the S matrix, and all elements are 0 or 1.

A position corresponding to 0 is masked to prevent light from reaching the measuring instrument, and a position corresponding to 1 is a Hadamard array type slit in which a slit (pinhole) is arranged so that light reaches. For example, the observation value y _i is detected as a sum of some of x _i to x _n reflecting the S matrix.

The actual laser light intensity distribution vector is
It is obtained by multiplying the observation value vector Y by the inverse matrix S ⁻¹ of the S matrix.

As described above, according to the present invention, when measuring the intensity distribution at n places, the mean square error is reduced to 4n / (n + 1) ² as compared with simple pinhole scanning. In this case, if the measurement is to be performed to obtain the same error in the pinhole scanning, the number m of data acquisition must be increased to (n + 1) ² / 4n times. Therefore, according to the present invention, the measurement time is reduced to 4n / (n + 1) ² as compared with the case of the pinhole scanning when a result of the same accuracy is obtained.

For example, when measuring 60 measurement points, assuming that the laser pulse is operating at 1 Hz,
If the Hadamard array type slit is used, the measurement is completed in 60 seconds = 1 minute. In order to obtain the same precision as this by pinhole scanning, the number of times of capturing data at one location must be 16 (60 + 1) ² / (4 × 60) = 16 times. Therefore, the measurement time is 60 × 16 = 960
Seconds = 16 minutes. That is, in this case, the measurement can be performed in 1/16 of the time using the Hadamard array type slit as compared with the case using the pinhole scanning.

[0018]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration of a spatial intensity distribution measuring unit in the laser beam focusing characteristic measuring apparatus of the present embodiment. FIG. 2 schematically shows a configuration of a Hadamard array type slit used in this apparatus. It is shown.

As shown in FIG. 2, the position corresponding to 0 is masked in the Hadamard array type slit 10, reflecting the matrix element of the 7 × 7 S matrix shown in the upper part of FIG. In the position corresponding to, the slit (pinhole) 1
1 is arranged.

That is, the Hadamard array type slit 10 has slits 11 and 11 corresponding to the matrix elements of 1 to 7 rows (or columns) of the upper S matrix in the range shown by arrows 1 to 7 at the bottom in the figure. A mask portion (a portion where the slit 11 is not formed) is formed.

As shown in FIG. 1, the Hadamard array type slit 10 constructed as described above is provided with an energy meter 1 in front of an energy meter 12 as a laser beam intensity measuring means for measuring the intensity of the laser beam L.
2 is arranged so as to limit the incidence of the laser beam L on the laser beam 2. Further, a drive mechanism 13 is connected to the Hadamard-array slit 10 and, based on a command signal from a computer 14, a predetermined distance indicated by an arrow in the drawing in steps corresponding to one matrix element of the S matrix. It is configured to be moved in the direction. Note that in FIG.
Reference numeral 5 denotes a mask for limiting the incident area of the laser beam L to a predetermined area of the Hadamard array type slit 10.

As the energy meter 12, for example, a calorimeter manufactured by Sensor Physics Co., Ltd. can be used, and a signal measured by the energy meter 12 is input to a computer 14 via an amplifier, an A / D converter and the like (not shown). And configured to process and store data. Computer 14
Is configured to receive a control signal from a laser generator (not shown), and perform movement control and data collection of the Hadamard array type slit 10 in accordance with the emission timing of the laser beam L emitted in a pulse shape. .

In the apparatus of this embodiment having the above configuration, the energy meter 12 measures the sum of the intensities of the laser beams L at positions corresponding to the seven matrix elements of the S matrix at a time. By performing such a measurement at least seven times while moving the Hadamard array type slit 10 in a step-like manner, an observation vector Y as shown in FIG. 3 is obtained. ^-1
, A distribution vector X (showing the spatial intensity distribution of the laser beam L) as shown in FIG. 4 can be obtained.

Therefore, in this embodiment, for example, four times for one position of the Hadamard array type slit 10 in comparison with the case where the measurement is performed 70 times in total, 10 times per one place by the pinhole scanning. The measurement with the same accuracy can be performed only by performing a total of 28 measurements.

In order to know the focusing characteristics such as the focal position of the laser beam, the focusing diameter, the beam divergence angle, and the beam spot size, the spatial intensity distribution of the laser beam L must be at least two different distances from the light source. It is necessary to measure at several places. FIG. 5 shows a configuration of an embodiment of the apparatus for obtaining light condensing characteristics by simultaneously performing measurements at such a plurality of points.

As shown in FIG. 1, the apparatus of this embodiment is provided with a laser beam introducing mirror 21 for introducing a laser beam L into the apparatus. The laser beam introducing mirror 21 is formed of a metal mirror or a beam splitter, and is held by a support rod 22.

The support rod 22 is made of Mo (molybdenum) from which the laser beam L introduced into the apparatus is first reflected.
The support rod 22 is connected to a mirror 23, and is provided with an index indicating the distance between the laser beam introducing mirror 21 and the Mo mirror 23. Further, the laser beam introducing mirror 21 is configured to freely swing an angle with respect to the Mo mirror 23, so that the positional relationship between the laser beam L and the device can be set flexibly.

The laser beam L introduced into the apparatus by the laser beam introducing mirror 21 is reflected by the Mo mirror 23 and the Mo mirror 24, and is reflected by the aperture 2.
5 and is led to a KCl beam splitter 26. The Mo mirror 23 and the Mo mirror 2
Reference numeral 2 is for performing beam alignment.
Here, a part of the laser beam L guided to the KCl beam splitter 26 is taken out and introduced into an energy meter 27 for energy monitoring. The energy monitor energy meter 27 monitors the energy of the pulse of the incident laser beam L and calibrates the energy variation.

On the other hand, the laser beam L that has passed through the KCl beam splitter 26 is incident on a concave mirror 28 having a radius of curvature of 4 m and is collected. On the optical path of the laser beam L condensed by the concave mirror 28, a beam splitter 29 made of a ZnSe plate, a Mo mirror 30, an aperture 31, and a Mo mirror 32 are arranged in this order.

The beam splitter 29 is set so that the reflection and transmission are 50% -50%.
% Of the laser beam L is reflected downward by 90 degrees. Below the beam splitter 29, a Hadamard array type slit 10, an energy meter 12, a driving mechanism 13, a mask 15, and the like, which are configured in the same manner as in the above-described embodiment, are arranged. It is configured to measure the spatial intensity distribution of the reflected laser beam L.

The Mo mirror 32 is also set so as to reflect the laser beam L downward by 90 degrees.
Below that, a Hadamard array type slit 10, an energy meter 12, and a
The drive mechanism 13, the mask 15, and the like are arranged, and are configured to measure the spatial intensity distribution of the laser beam L reflected downward by the beam splitter 29.

That is, immediately after being reflected by the concave mirror 28, the spatial intensity distribution of the laser beam L is measured below the beam splitter 29, and at the same time, the laser beam L is measured below the Mo mirror 32 immediately before the focal point. It is configured to measure the spatial intensity distribution of L.

These measurement signals are supplied to an amplifier (not shown)
The light is input to the computer 14 shown in FIG. 1 via an A / D converter and the like, and the light-collecting characteristics are calculated as follows.

That is, in the computer 14, first,
Based on the measurement signals at the above two points and the measurement signal from the energy meter 27 for energy monitoring, a Gaussian curve is obtained by the least square method, that is, I = I ₀ exp [− ｛(rr− ₀ ) / ω｝ ² ]. And the spot size ω at the two measurement points is calculated.

Where I is the intensity of the radial coordinate r in the spatial distribution, r ₀ is the coordinate of the center of the laser beam L,
I ₀ represents the intensity at the center of the laser beam L.

Next, the focal position and the condensing diameter of the laser beam L are obtained from the change in the beam diameter ω measured at two places as described above. That is, the beam diameter ω at two different points (position z) in the optical path of the laser beam (wavelength λ)
By calculating the equation, ω ² = ω ₀ ² [1+ {λ (z−z ₀ ) / (πω ₀ ² )}
² ] is used to obtain the focused diameter ω ₀ of the laser beam and its focused position z ₀ .

Next, an equation representing the curvature of the wavefront of the laser beam is as follows: R = (z−z ₀ ) [1+ ｛πω ₀ ² / (λ (z−
z ₀ ))｝ ² ], the divergence angle of the laser beam is calculated from the curvature R of the wavefront, and the radius of curvature of the laser beam L changes before and after the concave mirror 28. the radius of curvature R of the beam, 1 / R = (1 / R m) - with the relationship (1 / f), re-converted from the radius of curvature R _m in the device. Note that f represents the focal length of the concave mirror 28.

The apparatus according to this embodiment having the above-described structure can obtain the same effects as those of the above-described embodiment, and simultaneously scans the laser beam L at two places while scanning in synchronization with the laser pulse. Since the measurement of the spatial intensity distribution is performed, it is possible to measure the focusing characteristics of the laser beam at high speed. Further, the laser beam L using the concave mirror 28 is used.
Is condensed and the spatial intensity distribution (beam profile) is measured at two points where the beam diameter changes greatly, so that the apparatus can be downsized and highly accurate measurement can be performed.

Further, by optimizing a wideband energy meter and an optical material, it is possible to measure the focusing characteristics of a laser beam in a wide wavelength range from ultraviolet to infrared, and to measure a laser beam having a wide range of energy levels. Can be measured directly, and is suitable for measurement of a CO ₂ laser or the like which is a high-intensity laser of far infrared rays.

[0041]

As described above, according to the laser beam condensing characteristic measuring apparatus of the present invention, the laser beam condensing characteristics can be measured in a shorter time and with higher accuracy than in the prior art.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a spatial intensity distribution measuring unit in a laser beam focusing characteristic measuring apparatus according to one embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a Hadamard array type slit used in the laser beam focusing characteristic measuring apparatus of FIG.

FIG. 3 is a diagram showing an example of a laser beam intensity observed by the laser beam focusing characteristic measuring apparatus of FIG. 2;

FIG. 4 is a diagram showing an example of a spatial intensity distribution of a laser beam.

FIG. 5 is a diagram showing an overall configuration of a laser beam focusing characteristic measuring apparatus according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10 Hadamard array type slit 11 Slit 12 Energy meter 13 Drive mechanism 14 Computer 15 Mask L Laser beam

Claims (2)

(57) [Claims] A laser beam intensity measuring means for measuring the intensity of a laser beam to be measured; and a Hadamard array type slit disposed in front of the laser beam intensity measuring means and having slits arranged according to matrix elements of a Hadamard matrix. And a driving means for moving the Hadamard-array-type slit so as to scan in the beam diameter direction of the laser beam to be measured. 2. A laser beam introducing means for introducing a laser beam to be measured, a condensing means for condensing the laser beam to be measured introduced by the laser beam introducing means, a condensing means, and the condensing means. At least two laser beam intensity measuring means arranged at intervals on an optical path of the laser beam to be measured between a focal point and a laser beam intensity measuring means; A Hadamard array-type slit in which slits are arranged according to matrix elements; a driving unit for moving the Hadamard-array slit so as to scan in the beam radial direction of the laser beam to be measured; and measuring the intensity of the laser beam to be measured. And monitor laser beam intensity to monitor the intensity variation of the measured laser beam. Measuring means, a beam splitter for dividing the laser beam to be measured and entering the laser beam intensity measuring means and the monitoring laser beam intensity measuring means, the laser beam intensity measuring means and the monitoring laser beam intensity measuring means Calculating means for calculating a focal position and a converging diameter of the laser beam to be measured from the measurement result of (1).