## 3 Experimental Steps and Results
The positive and negative values represent the direction and the value represents the steps of the motor adjustment. The ordinate represents the depth of a certain motor state. Scanning was conducted to identify the area with the minimum FM-AM depth quickly and to improve feedback efficiency. The figure is concave toward the center along the circumference; this concave point indicates the minimum FM-AM depth. The time-domain pulse diagram obtained before and after compensation is shown in Figure 5. The relationship between intensity and depth can be expressed as Equation (6). The pulse featured a significantly large depth before the compensation for dispersion, reaching 96%. By contrast, the depth was reduced to 3.8% when employing the active closed-loop feedback GVD compensation device. We think that a depth of less than 5% is a relatively good state, which is also the standard and aim of our experiment and engineering. We can adjust other parameters to further reduce the depth, such as adjusting the grating spacing, but it is difficult to control it manually to meet the adjustment accuracy. Thus, FM-AM can be well suppressed because the compensation system compensates for the GVD of the long transmission optical fiber, and also because of the GVD introduced by the optical fiber in the seed source. Figure 4: Diagram of motor scan. Figure 3: Diagram of compensation device (**a**) photograph; (**b**) schematic. Figure 6 shows the results of the depth test over 12 h. As indicated by the graph, the depth varied between 2.28% and 5.22%. This also demonstrates the good long-term reliability of the feedback device. However, certain slight drifts of approximately 0.1-0.3% can also be detected in the graph; these can possibly be attributed to the influence of the variations in the laboratory temperature, because temperature is closely related to the dispersion of the fiber. Therefore, we recorded the laboratory temperature under the same environmental and time conditions for verification. As shown in Figure 7, the first 6 h are the night when researchers work in the laboratory. The temperature of the equipment and of the people will cause temperature changes. The next six hours are the night break. The result of the test is unstable, with fluctuations of approximately 0.5-1.5 \({}^{\circ}\)C; this rightfully has a direct impact on the dispersion compensation. The corresponding depth change was approximately 0.3%. Clearly, the change in temperature is a long-term, slow drift, and we think the effect of temperature on the depth is a slow cumulative process, rather than a violent sudden change; additional means are required to overcome this. This will be a focus of our future research. Figure 5: Time–domain pulse diagram before and after compensation. Figure 6: Depth test over 12 h.  
When adjusting the grating spacing, the theoretical calculation can initially only provide a rough estimate of the grating pair position. Thus, it is necessary to fine-tune the spacing in the experiment via manual adjustments to accurately compensate for the second-order dispersion of the system. If only the distance between the grating pairs is adjusted manually, it is difficult to control the adjustment accuracy; it also difficult to ensure that the angle of the grating is appropriate, which makes it difficult to adjust the optical path. Notably, the scheme of controlling the grating-pair spacing by using an electric guide rail suffers from certain limitations, such as poor robustness, difficultly in adjustments, and high economic costs. Hence, using an electric, adjustable mirror mount and a fixed parallel grating pair is still the better choice for realizing small amounts of automatic dispersion compensation.