Objective Laser are used in many research fields such as quantum communication, atom cooling, atom clock, and materials processing. The power stability of laser is very important, especially in the field of quantum precision measurements where it directly affects the experimental measurement accuracy. For the atom clock, the power stability of the laser affects its stability and accuracy. Therefore, it is necessary to make the active laser power stabilization system. As a general control method, the fuzzy proportional-integral-differential (PID) control has been widely used in the closed-loop control systems, such as temperature control, path planning, flight attitude adjustment, etc. A recent study investigates the laser power stabilization with the analog circuit PID, but in which the values of PID parameters need to be readjusted if it is used in different environments and the stabilized value of laser power cannot be changed during the experiments. In order to solve these problems, the fuzzy PID control scheme is proposed. We hope that our solution can reduce the stable time of the feedback loop, improve the relative intensity noise, and achieve long-term stabilization of laser power.
Methods There are two types of feedback loop for the laser power stabilization, one is feedback to the laser current (internal loop), and the other is feedback to an acoustic optical modulator (AOM) (external loop). Generally, the external lock loop is used because the internal loop will disturb the laser current and thus the frequency. In this paper, an embedded system of laser power stabilization based on fuzzy control is investigated. The lock loop is realized by feedback to an AOM. After passing through AOM, the laser generates diffractive light. By adjusting the diaphragm only +1 order diffraction light is allowed to pass through. After passing through the beam splitter, it is divided into two beams. One beam is detected by the photodetector, the other beam is used for experiments. The digital control circuit consists of an analog-to-digital (AD) converter, a digital-to-analog (DA) converter, and a digital signal processing chip. First of all, the laser power is detected by photodetector. Then the voltage signal is converted to a digital signal by an AD conversion. The error signal is obtained by comparing with the standard set voltage. After that the error signal and its rate of change as well as the three parameters of PID are fuzzy, and the fuzzy algorithm controller performs the calculations. The results of the parameters of PID are clarified. Finally, the amplitude modulation voltage of AOM is output through DA after the PID operation. The key to the performance of the laser power stabilization is setting the fuzzy rules. Table 1 shows the fuzzy rules adjusted according to the actual situation.
Results and Discussions The set voltage of laser power is 3.5V. It is defined that the loop stable time is the one required for the photodetector voltage that increases from 0 V to 3.5V. The stable time of laser power after fuzzy control can be obtained by monitoring the voltage of the photodetector in the feedback loop. Compared to traditional PID, the stable time is reduced from 4.7ms to 1.8ms due to the absence of overshoot (Fig. 5). The relative intensity noise of the laser power can be measured by placing the photodetector outside the loop (the beam for physical experiments). The results show that the power spectral density of relative intensity noise of the laser is depressed from -88dBc/Hz to -110dBc/Hz at 1 Hz and from -93dBc/Hz to -110dBc/Hz at 10Hz, and is lower than -110dBc/Hz over a wide frequency range, meanwhile the relative intensity noise of DA output voltage is lower than that of the laser (Fig. 6), meeting the experimental requirements. In addition, the relative fluctuation of the laser power is measured over three hours and improved from 0.29% to 0.035% after power stabilization (Fig. 7). Here, the relative fluctuation of the laser power is the ratio of the laser power fluctuation to the average.
Conclusions In this paper, a fuzzy control is applied to laser power stabilization using an embedded technique. The amplitude modulation voltage of the AOM is used to change the diffraction efficiency of the laser and thus achieve the laser power stabilization. Compared with traditional PID, after adding the fuzzy control, the feedback loop will not oscillate due to overshoot, and the stable time of the feedback loop is reduced from 4.7ms to 1.8ms. After power stabilization, the power spectral density of laser relative intensity noise is greatly improved in the low-frequency part, which is suppressed by 22 dB at 1Hz, and is lower than -110dBc/Hz over a wide frequency range. The time domain test results show that the relative fluctuation of the laser power improves from 0.29% to 0.035% within 3h. In the field of quantum precision measurement, the power stabilization technique is important for improving the measurement accuracy, such as improving the stability of atom clock and the accuracy of interferometer measurements, and because the power stabilization technique can change the stabilized laser power in real time, it is suitable for some experimental procedures that need to change the laser power in specific situations.
