Application Cases

Experiment Title: Closed-Loop Experiment of Adaptive Optics System

Testing Equipment:High-voltage amplifier, wavefront controller, wavefront sensor, deformable mirror, computer, etc.

Experimental Process:

Figure 1: Physical Diagram of Adaptive Optics System

The experiment to test the effectiveness of the normalized cross-correlation slope algorithm was conducted on the adaptive optics system platform shown in Figure 1, which displays all components of the adaptive optics system except for the high-voltage amplifier and computer.

Figure 2: Schematic Diagram of Adaptive Optics System

The optical layout of the adaptive optics system is shown in Figure 2. A laser beam with a central wavelength λ of 650 nm is emitted from a 70-μm-diameter fiber and collimated into parallel light by a doublet lens L1 with an aperture of 55 mm and a focal length of 300 mm. The parallel light is reflected at a small angle by a deformable mirror with an aperture of 20 mm and then enters the beam reduction system composed of L2 and L3. The reduced beam passes through a beam splitter P, with one part entering the sensor for aberration detection and the other part passing through a lens L4 with a focal length of 300 mm to form an image on the CCD to observe the far field of the wavefront at P. Both parts transmit the captured images to the computer for image display and real-time data processing.

After receiving the image captured by the wavefront sensor, the computer uses the optimized normalized cross-correlation slope program to extract the slope and generate a slope vector. The slope vector is processed through wavefront reconstruction and control calculations to obtain a control signal. The control signal is then converted from digital to analog (D/A) and output from the computer to the high-voltage amplifier for voltage amplification, which in turn controls the deformable mirror for wavefront correction. The corrected image is fed back to the computer by the sensor and CCD imaging system, achieving system closed-loop operation.

Experimental Results:

Figure 3: Spot Image Before Correction

Figure 4: Spot Image After Correction

Figure 5: (a) Wavefront Map Before Closed Loop; (b) Wavefront Map After Closed Loop (Data in Figures Are in Units of Wavelength λ)

Figure 3 shows the far-field spot image detected by the CCD before system closed-loop correction. Figure (a) is a two-dimensional image of the spot, with the horizontal and vertical coordinates representing the position of pixels in the image. Figure (b) is a three-dimensional plot of the spot intensity distribution, with the horizontal and vertical coordinates representing the position of pixels in the image and the vertical coordinate representing the brightness value of the pixels. From Figure 3, it can be seen that due to the influence of aberrations, the spot shape is diffuse and the central energy is not concentrated before correction. Figure 4 shows the laser spot image after system correction. Figure (a) is the spot image captured on the target surface, and Figure (b) is the corresponding three-dimensional intensity distribution plot, with the same coordinate meanings as in Figure 3. From Figure 4, it can be seen that the corrected spot image clearly shows the central spot and the concentric circular rings produced by diffraction, and compared with the spot before correction, the energy of the corrected spot is more concentrated.

Figure 5 shows the changes in the wavefront phase and specific parameters before and after correction. As can be seen from the figure, after system correction, the peak-to-valley value PV of the wavefront phase decreased from 1.958λ to 0.350λ, and the root mean square value RMS decreased from 0.496λ to 0.056λ (λ is the central wavelength of the above laser beam, i.e., 650 nm). The corrected wavefront has smaller fluctuations and is closer to ideal parallel light.

By comparing Figures 3 and 4 and analyzing Figure 5, it can be seen that the optimized normalized cross-correlation slope detection program can indeed effectively detect wavefront slopes, and this detection method can work well in the presence of pseudo-spot interference, with good anti-interference performance. In the case of pseudo-spots, when using the centroid algorithm for calculation, it is necessary to reduce the threshold to eliminate the pseudo-spots in order to effectively detect the target. However, when the pseudo-spots are reduced, the signal of the real spot is also reduced, especially when the intensity of the pseudo-spots is not much different from or even exceeds that of the real spot, the threshold-reduced centroid algorithm will not work properly.

Recommended High-Voltage Amplifier: ATA-7030

Figure: ATA-7030 High-Voltage Amplifier Specifications

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