JPS5933209B2 - Laser beam divergence angle measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for measuring a beam divergence angle, which is a fundamental parameter of a laser beam.

The output beam from a laser oscillator has different properties depending on whether it has a single transverse mode or not, but since a single mode beam is better in terms of application, most practical laser oscillators are currently using single mode beams. It is designed to output a so-called Gaussian beam. Therefore, the laser beam to which the present invention is applied is also a Gaussian beam. Among the parameters that indicate beam characteristics, the divergence angle is one of the most important parameters, and its size is always indicated in the product catalog of a laser oscillator. However, no effective method for measuring the beam divergence angle, which is an important parameter, has been known so far, and in many cases only theoretical values are displayed. The conventional method for measuring the beam divergence angle has been to measure the beam diameter at several locations along the beam axis and find it from the slope of a straight line connecting the diameters. By the way, the intensity distribution of the beam cross section in a Gaussian beam has a Gaussian shape,
Its radius is defined as the distance from the axis to the point where the intensity is 1/e2 of that on the axis. It is known that the curve drawn by the beam diameter along the axis becomes a hyperbola as shown in FIG. In other words, there is a region near the minimum diameter of the beam (hereinafter referred to as beam waist) lSfj in which the diameter does not change much, and the beam exhibits a constant inclination spread for the first time sufficiently far away from this region. The beam divergence angle is defined by the angle of this region of constant inclination. Therefore, when measuring the divergence angle, it is clear that if the beam diameter loss I stagnation range happens to be near the beam waist, measuring using the method described above will yield results that are different from the actual results. The beam radius w(2) at the axial distance z from the beam waist is W at the beam waist. , is known to be theoretically expressed by the following equation, where the wavelength is λ.

(1) w(2) ■woto+(force)2 From this, the slope θ(2) of the tangent to the locus of the beam radius W at the position of distance z is derived and expressed as the following equation.

However, θ<<1. dwA2) θ(2)■ − Tilt at z=1, that is, beam divergence angle θ(d)
(obtained by setting z=1 in equation (2)) becomes vν▼▼υ.

Here, if we define the distance when
) to (4).

Zd is the slope of the tangent to the trajectory of the beam radius w is 0.99θ (
d) represents the distance from the beam waist to the point where

In other words, in order to accurately measure the divergence angle of the beam using the method described above, it is necessary to stagnate at a distance of approximately Zd from the beam waist. Figure 2 shows a typical laser (wavelength: 0.63μMO) He-Ne laser and wavelength: 10.6μ.
Taking the case of an m(7) CO2 laser as an example, an example of calculation results of the magnitude of Zd with respect to the magnitude of the radius WO of the rebeam waist will be shown. In a He-Ne laser, the beam waist is at the output end of the oscillator and is W. Many of them are made with a size of about 0.5. In this case, Zd is approximately 1 from Figure 2.
It turns out that it is 0m. Therefore, it can be said that it is necessary to measure the spread angle at a distance of about 10 m from the oscillator. Since the CO2 laser has a long wavelength, Zd is much shorter than that of the He-Ne laser for the same value of WO. However, in CO2 laser, He
-W of the output from the oscillator compared to the Ne laser. is generally large, and in applications such as when transmitting beams for laser radars where the divergence angle is an important parameter, W. In many cases, Zd is converted to a number of about 10 and then transmitted, so Zd becomes as long as an elbow. For example, if WO is 20w and dew, Zd will be 800m. Furthermore, in the case of beams that generally require measurement, the position of the beam waist is often unknown, so in any case, there are various problems with measuring the divergence angle using the method described above. As described above, it is an object of the present invention to provide a measuring device that can easily and accurately measure the divergence angle of a laser beam using a new method, which has many problems with conventional methods.

The principle of the present invention will be explained below with reference to FIGS. 3 to 6.

In Fig. 3, when a laser beam 31 is passed through a lens 32 with a focal length of f, the image of the beam at a cross section perpendicular to the beam axis at the focal length position 33 of the lens is an image of the incident beam at infinity. It is nothing but a reflection.

Therefore, if the radius of the beam at the focal plane is Wf, and the divergence angle θ(d) of the beam incident on the lens is at infinity, the following relationship holds. Based on this relationship, the present invention can measure Wf if f is known. This method takes advantage of the fact that θ (to) can be found from . Diameter measurement techniques are required.

The knife edge method can be applied as a beam diameter measuring method for this purpose. The intensity 1(X,y) in a cross section perpendicular to the axis of the Gaussian beam is expressed by the following equation. P here.

is the total output of the laser, t is the beam radius, and X and y represent the position on the coordinates whose origin is the axis assumed in the cross section perpendicular to the beam axis. When the knife edge is used to scan the beam in a direction perpendicular to the axis, the output change is expressed by the following equation. The diameter w can be found from the characteristic curve of P(XVPO. Now, if the amount of movement of the knife edge until P(XVPO changes from 0.9 to 0.1 is X2 - X1, then w can be expressed by the following formula. A razor blade or a prism edge is used for the knife edge.

Figure 5 shows the results of this method using He with a wavelength of 0.63 μm.
An example of measuring the diameter of a -Ne laser beam along the optical axis is shown. The condensed line is a theoretical curve when the diameter of the beam waist is 0.167 mn, and the beam divergence angle θ(8) obtained from this is 1.21 mrad. Incidentally, if you know the approximate position of the beam waist, you can carefully measure the beam diameter along the optical axis using the knife edge method as shown in Figure 5, and find a theoretical curve that best matches all the measured values. There is also a method to find the divergence angle from . However, this method is not necessarily a good method from a practical standpoint because it requires the work of finding a theoretical curve that matches the large number of measurement points. Now, using the beam shown in FIG. 5 as a test beam, a sixth example of verification of the beam divergence angle measurement method of the present invention using the knife edge method explained above will be described.
As shown in the figure. The lens used was a plano-convex lens with a focal length f of 65.0, and this was placed at five different positions on the axis including the beam waist of the beam in Figure 5, and the beam diameter Wf at each focal position was measured using the knife edge method. θ was determined as a measured value at each position using the relationship shown in equation (6). In the figure, the horizontal axis indicates the position of the lens on the beam axis, and the vertical axis indicates the size of the divergence angle θ. The measured values were in good agreement with the theoretical value of 1.21 mrad at all positions, and the error was within ±2%. As is clear from these results, according to the method of the present invention, the divergence angle can be easily determined by one measurement of the beam diameter at the focal length of the lens. Moreover, the lens can be placed at any position in the beam, which means that with the conventional method of determining the beam diameter by connecting the beam diameter trajectories, accuracy cannot be achieved unless the beam diameter is suppressed at a sufficient distance from the beam waist. It turns out that this is a big advantage compared to the previous version. 7th
The figure shows, as a specific embodiment of the present invention, an example of application to an apparatus for measuring and calibrating the divergence angle of a transmitted beam of a laser radar. Laser radars have various uses, and the device parameters differ depending on the purpose, but the divergence angle of the transmitted laser beam is an important parameter for any form of laser radar, and its measurement and calibration are important. is greatly needed. In FIG. 7, 701 is the output laser beam from the oscillator, 702 is a beam expander for controlling the divergence angle of the laser beam (generally used in four laser radars), and 703 is the expander 70.
The transmitting laser beam is sent out from 2, 704 is a mirror and the sliding table 7
05 at an angle of 45 to the axis of the laser beam. 706 is a lens with a known focal length, and mirror 70
4 is inserted into the transmission beam 703, and when the transmission beam is reflected, the center thereof is aligned with the optical axis of the reflected beam 707.

Reference numeral 708 denotes a knife edge, which can be moved in and out perpendicularly to the beam axis at the focal point of the lens 706, and is attached to a sliding table 710 that is fed by a micrometer head 709 so that the amount of movement thereof can be read.

A detector 711 detects the output of the beam 712 partially cut off by the knife edge 708 .

713 is an output meter for reading the output.

With the device configuration as shown in the figure, the divergence angle of the output beam from the EXPANTER can be easily measured and calibrated. That is, during measurement, a mirror 704 is placed in the transmitted beam, and the beam is reflected into a lens 706 to be focused, which is then sharpened by a knife edge 708. At this time, the amount of movement of the knife edge is outputted by a micrometer head 709. A total of 713 is read, and the output P is when the output is not shielding the beam with a knife edge. , find the amount of movement of the knife edge (X2-X1) until it changes from 0.9P0 to 0.1P0, calculate Wf from the relationship of equation (9), and use equation (6) from this and the focal length f of the lens. θ- can be calculated based on the relationship. According to the measuring device of the present invention, the laser beam divergence angle, which has been extremely difficult to measure due to the lack of an effective method known in the United States, can be easily and accurately measured. Possible uses include measuring the output beam in the inspection process at a laser oscillator manufacturing site, or measuring and calibrating the spread angle of the transmitted beam of a laser radar.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the trajectory drawn by the laser beam radius along the beam axis and the divergence angle. Figure 2 is a diagram explaining the distance from the beam waist where the slope of the tangent to the trajectory traced by the beam radius can be considered as the beam divergence angle. Figure 3 is a diagram explaining the beam diameter at the focal point when the beam passes through a lens, Figure 4 is a diagram showing the output change when cutting the beam with a knife edge, and Figure 5 is a diagram explaining the beam diameter at the focal point when the beam is passed through a lens. FIG. 6 is a diagram showing an example of verifying the beam divergence angle by the method of the present invention, and FIG. 7 is a diagram illustrating a specific embodiment of the present invention. Explanation of symbols, 706...Lens, 708...
...Knife Edge, 713...Output meter.

Claims (1)

[Claims] 1 Consists of a lens with a known focal length, a knife edge, and a power meter, the laser beam to be measured is passed through the lens, and the laser beam after passing is directly aligned perpendicular to the beam axis at the focal length of the lens. The knife edge is placed so that it can be scanned, and when the edge cuts the beam, an output meter is placed so that the output change can be read. From the moving length of the edge and the output change of the laser beam, the beam diameter at the focal length of the lens is determined. The measurement position of the laser beam divergence angle is characterized by determining the divergence angle of the original beam from it and the focal length of the lens.