Gaussian laser beam divergence angle measurement device and method

Abstract

The present invention relates to the field of laser detection technology and discloses a Gaussian laser beam divergence angle measurement device and method. The method comprises: passing the laser beam to be measured through a polarization beam splitter prism, being incident normally on a volume phase grating and a quarter-wave plate, and being split into three light spots with light intensities of approximately 0.25:0.5:0.25. The light is then reflected by a reflector, passed again through the quarter-wave plate and the volume phase grating, and reflected by the polarization beam splitter prism to a CCD camera. By varying the distance from a reflector with a movable mirror frame to the volume phase grating, the proportion of the zero-order light spot intensity in the total light spot intensity at different distances x0 is obtained. The divergence angle of the laser beam to be measured is then fitted using a formula based on the relationship between the intensity proportion of the zero-order light spot and the beam divergence angle.

Description

Technical Field

[0001] The present invention relates to the technical field of laser detection, and in particular to a device and method for measuring the divergence angle of a Gaussian laser beam. Background Art

[0002] Beam divergence is a key parameter in measuring laser beam quality, reflecting the divergence characteristics of a beam as it travels over different distances. A laser beam with a smaller divergence angle has better directionality, enabling more efficient use of laser energy. A larger divergence angle rapidly expands the beam's coverage area, enabling wide-area detection. Therefore, accurately measuring the divergence angle of a laser beam is of paramount importance.

[0003] Currently, the methods for measuring the divergence angle of a laser beam include the slit scanning method [Chinese Patent No. CN91107468.6], the trepanation method [Chinese Patent No. CN201110317316.X], the CCD measurement method [Chinese Patent No. CN01108756.0], and the measurement method based on a transmission-type volume Bragg grating [Chinese Patent No. CN202011582959.2].

[0004] The slit scanning method requires a long scanning time, and its stability and accuracy are difficult to guarantee during the scanning process. The trepanning method requires manual alignment of the beam and the center of the pinhole diaphragm during measurement, which not only depends on the operator's personal experience but is also not conducive to batch measurement. The CCD measurement method is currently the more common method for detecting laser far-field divergence angles, but the measurement accuracy is limited by lens aberrations and the length of the device. The measurement method based on a transmission volume Bragg grating (VBRG) measures the divergence angle of the measured beam by measuring the relationship between the diffraction efficiency of the VBRG and the divergence angle, with a measurement accuracy of up to 100 microradians. Summary of the Invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a device for measuring the divergence angle of a Gaussian laser beam based on twice passing through a volume phase grating. The divergence angle of the laser beam to be measured is obtained by fitting the data of the light intensity ratio of the zero-order spot after the Gaussian laser beam to be measured passes through the volume phase grating twice to the equivalent grating spacing.

[0006] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0007] A Gaussian laser beam divergence angle measuring device is characterized in that it comprises: a polarization beam splitter prism, a volume phase grating, a quarter wave plate, a reflector, an achromatic lens and a CCD camera;

[0008] The Gaussian laser beam to be measured is incident on the polarization beam splitter prism, is transmitted through the polarization beam splitter prism, sequentially passes through the volume phase grating and the quarter-wave plate, and then is incident on the reflector. After being reflected by the reflector, it returns along the original path, sequentially passes through the quarter-wave plate and the volume phase grating, and then is incident on the polarization beam splitter prism. It is emitted from the side in the z direction and is incident on the CCD camera through the achromatic lens. The CCD camera is connected to a computer for monitoring the light intensity distribution. The position of the reflector is moved along the optical path, thereby changing the distance x0 between the reflector and the rear surface of the volume phase grating. The ratio P of the zero-order spot intensity to the total spot intensity at different distances x0 is obtained, and the divergence angle θ of the Gaussian laser beam to be measured is fitted and calculated. The formula is as follows:

[0009]

[0010] Where: A is the distortion factor of the wavefront caused by the reflector and volume phase grating, S is the periodic distance of the reflector movement corresponding to the periodic change of the spot intensity, θ diff is the diffraction angle between the ±1st order light and the 0th order light of the volume phase grating, L = 2(x0 + Δx) is the equivalent distance between two passes through the volume phase grating, where Δx is the correction caused by the phase difference between the 0th order light and the +1st order light at the output surface of the volume phase grating.

[0011] Preferably, the volume phase grating is an acousto-optic modulator.

[0012] Preferably, the reflector is placed on a movable bracket.

[0013] The measuring principle of the present invention is:

[0014] The Gaussian laser beam to be measured is incident on a volume phase grating. The required volume phase grating is a grating with a cosine-varying refractive index along the z-direction. The parameters are set so that the output is three plane waves, namely +1, 0, and -1, with an intensity ratio of 0.25:0.5:0.25. The propagation direction of the 0-order light is the same as that of the incident light. The specific parameters of the required volume phase grating can be calculated using coupled wave theory. Based on the transformation matrix of the light beam passing through the volume phase grating and the propagation process, and applying the function form of the Gaussian beam, the formula (1) is obtained for the change of the intensity of the 0-order light after the Gaussian beam has been spatially transmitted over a certain distance and diffracted twice by the grating as the distance x0 between the reflector and the rear surface of the volume phase grating. By measuring the ratio P of the zero-order spot intensity to the total spot intensity at different distances x0, the divergence angle θ of the Gaussian laser beam to be measured is fitted using formula (1).

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] By fitting the data of the light intensity ratio of the zero-order spot after the Gaussian laser beam to be measured passes through the volume phase grating twice to the equivalent grating spacing, the present invention can measure Gaussian laser beams with a divergence angle as low as 10 microradians. Compared with the CCD measurement method, it has higher accuracy and smaller measurement divergence angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a light path diagram of the Gaussian laser beam divergence angle measuring device of the present invention;

[0018] Figure 2 This is an equivalent optical path diagram of the Gaussian laser beam divergence angle measuring device of the present invention;

[0019] Figure 3 1 is a schematic structural diagram of an application embodiment of a Gaussian laser beam divergence angle measuring device according to the present invention;

[0020] Figure 4 It is a curve relationship diagram of the measured different x0 and the proportion of 0-level light spot.

[0021] In the figure: 1-polarization beam splitter prism, 2-body phase grating, 3-1 / 4 wave plate, 4-reflecting mirror, 5-first achromatic lens, 6-first CCD camera, 7-laser emission module, 8-second achromatic lens, 9-second CCD camera, 10-filter DETAILED DESCRIPTION

[0022] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below with reference to the accompanying drawings and examples.

[0023] See also Figure 1 , Figure 1 This is the optical path diagram of the Gaussian laser beam divergence angle measurement device of the present invention. As shown in the figure, the laser to be measured is transmitted through the polarization beam splitter prism 1 and is normally incident on the volume phase grating 2 and the quarter-wave plate 3. The volume phase grating 2 is a grating with a refractive index that varies cosine along the z direction. The parameters are set so that the output is three plane waves, namely ±1st order and 0th order light. The 0th order light has the same propagation direction as the incident light, and the diffraction angle between the ±1st order and the 0th order light is θ. diffThe intensity ratios of the -1, 0, and +1 levels are 0.25:0.5:0.25. The specific parameters of the required volume phase grating can be calculated by coupled wave theory. The references are [H. Kogelnik,"Coupled wave theory for thick hologranm gratings,"Bell Syst. Tech. J. 48, 2909-2947 (1969)], [R. Magnusson and T. K. Gaylord, "Analysis of multiwave diffracting by thick gratings," J. Opt. Soc. Am. 67, 1165–1170 (1977).], and [F. G. Kaspar, "Diffraction by thick periodically stratified gratings with complex dielectric constant," J. Opt. Soc. Am. 63, 37–45 (1973).]. The optical axis of the quarter-wave plate 3 forms an angle of 45 degrees with the polarization direction of the light. The beam is then reflected by a mirror 4 with a movable mirror frame in the x-direction, with positive reflection for the zero-order light. The beam is incident on the quarter-wave plate 3 and volume phase grating 2 for a second time. After exiting the volume phase grating 2 again, the beam enters the polarization beam splitter prism 1 and exits from its side in the z-direction. It then passes through the achromatic lens 5 and is incident on the CCD camera 6, whose array surface is located at the back focal plane of the achromatic lens 5. The light intensity distribution is acquired, and the proportion P of the zero-order spot intensity in all spot intensities is calculated.

[0024] In this embodiment, the volume phase grating is realized by the ultrasonic grating of the acousto-optic crystal in the acousto-optic modulator. The acousto-optic modulator crystal is SGT40-780-2TA-T produced by the 26th Institute of China Electronics Technology Group Corporation. The diffraction angle θ between the 0th and ±1st orders of the acousto-optic modulator is diff 7.57×10 -3 After passing through the AOM, the Gaussian laser beam is reflected by a plane mirror, returning the zero-order beam along its original path. The ±1-order and zero-order beams then pass through the AOM again, effectively passing through two identical volume phase gratings.

[0025] See also Figure 3 , Figure 3This is a schematic diagram of the structure of an application embodiment of the Gaussian laser beam divergence angle measurement device of the present invention, which completes the measurement of the laser beam divergence angle. As shown in the figure, the laser to be measured is incident on the polarization beam splitter prism 1 from the laser emitting module 7, transmitted into the acousto-optic modulator 2, and three light spots of zero order, positive and negative first order are obtained, and then transmitted through the 1 / 4 wave plate 3. At this time, the reflector 4 with a movable mirror frame is removed, and the light beam is Fourier transformed by the second achromatic lens 8 and incident on the photosensitive surface of the second CCD camera 9, which is on the back focal plane of the second achromatic lens 8. At this time, the three light spots of zero order and positive and negative first order can be seen on the photosensitive surface. The light intensity ratio of the three light spots can be observed using computer monitoring, and the light intensity ratio of the three light spots can be adjusted to approximately 0.25:0.5:0.25 by adjusting the incident angle of the laser emitting module 7.

[0026] After obtaining the above three spot intensity ratios, the reflector 4 with a movable frame is mounted on the quarter-wave plate 3. The light beam is then reflected by the reflector through the quarter-wave plate 3 and incident on the AOM 2 for a second time. After exiting, it enters the polarization beam splitter prism 1. Because the light beam has passed through the quarter-wave plate 3 twice, its polarization direction has rotated 90°. Therefore, the light beam exits from the side of the polarization beam splitter prism 1, then passes through the first achromatic lens 5 and is incident on the photosensitive surface of the first CCD camera 6. This screen is at the back focal plane of the first achromatic lens 5. The light beam intensity distribution is then monitored by a computer, and the proportion P of the zero-order spot intensity to the total spot intensity is further calculated. By moving and recording the distance x0 between the reflector 4 with the movable frame and the rear surface of the AOM 2, the zero-order spot intensity can be observed in the first CCD camera 6 as it changes periodically and decays continuously. The proportion P of the zero-order spot intensity at various distances x0 is calculated using a computer.

[0027] For the data processing part, we use formula (1) for fitting, and apply the genetic algorithm at the same time, and replace the fitness function with the orthogonal distance regression method. Among them, for the orthogonal distance method, since the horizontal coordinates of adjacent data points are the same, the faster the vertical coordinate value changes, the lower its credibility. Therefore, the data points are multiplied by the following weight factor

[0028]

[0029] Where f'(x) is the derivative of the fitting function, and σ is an adjustable scaling factor, which is set to 0.01 during the fitting process.

[0030] According to the above processing method, for different divergence angles θ, we can get Figure 4 (a)(b), the fitting results are θ a =8.17×10 -4 ±1.45×10 -5rad, △x=4.27mm, A=0.964, S=13.92mm; θ b =3.12×10 -4 ±1.23×10 -5 rad, △x=4.27mm, A=0.975, S=13.92mm. At the same time, the second achromatic lens 8 and the second CCD camera 9 are used to directly measure the divergence angles of the above two cases, and the fitting results are θ a =8.10×10 -4 ±6.84×10 -6 rad,θ b =2.97×10 -4 ±1.96×10 -5 rad, it can be seen that the two methods are consistent within the error range. Since the effective aperture of the acousto-optic modulator 2 used in this embodiment limits the maximum size of the measurable Gaussian beam. But in principle, under the existing technology, a volume phase grating with an aperture on the order of centimeters can be prepared, and the grating constant can be reduced by one order of magnitude compared with the embodiment. Therefore, a Gaussian beam with a wider beam width and a smaller divergence angle can be measured. In addition, the distance x0 from the rear surface of the acousto-optic modulator 2 to the reflector 4 can also be increased by one order of magnitude to improve the measurement accuracy. Conservatively estimated, the measurement method proposed in the present invention can measure Gaussian laser beams with a divergence angle as low as 10 microradians, which is more accurate than the CCD measurement method and can measure a smaller divergence angle.

[0031] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A Gaussian laser beam divergence angle measuring device, characterized in that: include: Polarization beam splitter, volume phase grating, quarter wave plate, mirror, achromatic lens and CCD camera; The Gaussian laser beam to be measured is vertically incident on the polarization beam splitter prism, transmitted through the polarization beam splitter prism, sequentially passes through the volume phase grating and the quarter-wave plate, and then is incident on the reflector. After being reflected by the reflector, it returns along the original path, sequentially passes through the quarter-wave plate and the volume phase grating, and then is incident on the polarization beam splitter prism. After being reflected by the polarization beam splitter prism, it is incident on the CCD camera through the achromatic lens. The CCD camera is connected to a computer and is used to monitor the light intensity distribution. The position of the reflector is moved along the optical path, thereby changing the distance x0 between the reflector and the rear surface of the volume phase grating. The ratio P of the zero-order spot intensity to the total spot intensity at different distances x0 is obtained, and the divergence angle θ of the Gaussian laser beam to be measured is calculated by fitting. The formula is as follows: Where: A is the distortion factor of the wavefront caused by the reflector and volume phase grating, S is the periodic distance of the reflector movement corresponding to the periodic change of the spot intensity, θ diff is the diffraction angle between the ±1st order light and the 0th order light of the volume phase grating, L = 2(x0 + △x) is the equivalent distance between two passes through the volume phase grating; where △x is the correction caused by the phase difference between the 0th order light and the +1st order light at the output surface of the volume phase grating.

2. The Gaussian laser beam divergence angle measuring device according to claim 1, characterized in that: It is characterized in that The volume phase grating is an acousto-optic modulator.

3. The Gaussian laser beam divergence angle measuring device according to claim 1, characterized in that: It is characterized in that The reflecting mirror is placed on a movable bracket.

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