Application Cases

Experiment Name: Simulated Turbulence Tilt Aberration Compensation Experiment

Research Direction:
This section completes the experimental study of using Adaptive Fiber Coupler (AFC) to correct simulated turbulence tilt aberration under small receiving aperture conditions, improving single-mode fiber coupling efficiency and stability. Spectral analysis was performed on the experimental results, and the correction bandwidth of AFC for simulated turbulence was measured.

Test Equipment:
High-voltage amplifier, photodetector, adaptive fiber coupler, laser, turbulence simulator, etc.

Experimental Process:

Figure 1: Single-mode Fiber Coupling Experimental Setup Diagram

Figure 1 shows the schematic diagram of the AFC correction simulated turbulence experiment. This setup replaces the fast steering mirror in the previous scheme with a turbulence simulator, where the simulated turbulence consists of airflow with specific wind speed and temperature. The AFC receiving lens has an aperture of D=18mm and focal length f=120mm. A planar beam with wavelength 1064nm experiences wavefront distortion after passing through the simulated turbulence. The distorted beam is focused by the coupling lens onto the single-mode fiber core to achieve single-mode fiber coupling. The single-mode fiber mode field radius is w₀=5μm. A photodetector (PD) is placed at the end of the single-mode fiber to detect the optical energy coupled into the fiber and transmit the detected energy to the controller. The controller generates control signals that are amplified 100 times by a high-voltage amplifier (HVA) and applied to the AFC to achieve closed-loop tilt correction control. The control algorithm uses SPGD algorithm, with the performance metric being the optical energy J detected by the photodetector, and a closed-loop iteration rate of 625Hz.

Experimental Results:

Figure 2: Single-mode Fiber Coupling Efficiency Iteration Curve with Simulated Turbulence

Figure 2 shows the variation curve of single-mode fiber coupling efficiency over time obtained through 10,000 SPGD iterations each in open-loop and closed-loop conditions with simulated turbulence. When AFC does not correct the simulated turbulence tilt aberration (open-loop), the average single-mode fiber coupling efficiency over 10,000 iterations is <η>=30.07%, with a coupling efficiency MSE of 7.28%, indicating that simulated turbulence significantly degrades both coupling efficiency and stability. When AFC corrects the simulated turbulence tilt aberration (closed-loop), the average single-mode fiber coupling efficiency improves to 61.72%, very close to the experimentally obtained maximum coupling efficiency of 67%, while the coupling efficiency MSE reduces to 2.16%, enhancing the stability of single-mode fiber coupling efficiency. Therefore, under small receiving aperture conditions, correcting only the turbulence tilt aberration can achieve good single-mode fiber coupling efficiency and stability, with experimental results consistent with simulation results.

Figure 3: Spectral Analysis Curve of Coupling Efficiency Iteration

Figure 3 shows the spectral analysis curve of the single-mode fiber coupling efficiency iteration curve with simulated turbulence. The solid line in the figure represents the spectral analysis curve of the open-loop iteration curve from Figure 2, showing that in open-loop condition, the disturbance energy of simulated turbulence first decreases slowly and then rapidly with increasing frequency, with disturbances mainly distributed in low-frequency components (within 20Hz), containing an extreme value with relatively large disturbance energy between 80-90Hz. The dashed line represents the spectral analysis curve of the closed-loop iteration curve from Figure 3. Comparison with the open-loop frequency analysis curve shows that AFC correction of tilt aberration significantly suppresses turbulence disturbance energy below 20Hz while enhancing high-frequency disturbance energy, though the enhanced portion affects single-mode fiber coupling efficiency and stability. Therefore, the correction bandwidth of AFC for simulated turbulence is approximately 20Hz.

The experiment demonstrates that the correction bandwidth of AFC for simulated turbulence is about 20Hz. Under small receiving aperture conditions, correcting only atmospheric turbulence tilt aberration can achieve good single-mode fiber coupling efficiency and stability.

Recommended High-Voltage Amplifier: ATA-7020

Figure: ATA-7020 High-Voltage Amplifier Specifications

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