Results and Discussions First, we evaluate the locking performance for the control of the phase difference between the two optical paths in the DD-MZM. Fig. 5(a) shows the DC output signal of photodetector 2 (PD 2) after measurement for half an hour. The figure shows that, without locking, the DC signal changes with a relative proportion of 8%, and this drift increases further as the observation time increases. However, with locking, the drift of the DC output signal is significantly suppressed, and the value is stabilized at the setpoint, which is 8.1 V in the current system. To further evaluate the locking bandwidth, we perform noise power spectral density analysis on the acquired data, and the results are shown in Fig. 5(b). It is clear that the locking bandwidth can reach 120 kHz, which is much larger than the linewidth of the fiber laser (1 kHz). Consequently, the relative phase noise of the two lasers can be greatly suppressed, and the coherence of the two lasers is improved.To evaluate the effect of phase locking on the ranging results, the distance of the target mirror is fixed. The distance results are continuously monitored for half an hour and are shown in Fig. 6. It is clear that when the phase difference of the two optical paths in the DD-MZM is unlocked, the obtained distance value drifts from 144.04 mm to 144.33 mm over a range of 0.27 mm. With locking, the drift of the phase measurement results is greatly suppressed from 0.004π to 0.0003π, and the suppression ratio reaches 11 dB. The remaining noise exhibits a white noise behavior. The retrieved distance with locking remains at approximately 144.32 mm.To analyze the stability of the ranging system and to assess its sensitivity, we perform Allan variance analysis based on the results shown in Fig. 6, and the obtained results are shown in Fig. 7. It can be seen that when the phase is unlocked, the uncertainty of ranging is 2.56 μm in the short term, and due to the influence of long-term drift, the Allan variance rapidly increases when the integration time exceeds 5 s. However, with locking, both the white noise and long-term drift of the system are suppressed, and the uncertainty of the distance measurement reaches 2.26 μm in the short term. When the integration time increases, the white noise of the system is further restrained. Further, the system achieves an optimal uncertainty of 0.4 μm when the integration time is 100 s. When repeating the experiments with different distances, the phase detection accuracy is found to be better than 0.00032π, and the ranging stability can be less than 0.42 μm when the measurement distance is 200-1800 mm.

Ranging technology has important applications in many fields. Lasers have excellent coherence and high brightness, and therefore, their employment in ranging has rapidly promoted the development of various absolute or relative distance measurement technologies. Among them, dual-frequency laser ranging technology, which combines multi-wavelength interferometry and the phase method, has been demonstrated to be a powerful ranging tool. The distance is deduced by the relative phase difference between two beat frequencies generated in the echo light and reference light, each of which is composed of two lasers with different frequencies. However, the application and generalization of the traditional dual-frequency laser ranging technology are impeded by its complexity, limited measuring range, low phase detection stability, large phase noise, and low sensitivity.

We propose a novel dual-frequency laser ranging technology based on a dual-drive Mach-Zehnder modulator (DD-MZM). Only the light in one of the two phase modulators inside the DD-MZM is modulated by a radio frequency and then is combined with that within the other phase modulator. Consequently, a light source analogous to a dual-frequency laser is generated in the output of the DD-MZM. The noise intensity induced by the phase difference is significantly suppressed due to the tiny difference in the phase shifts between the two light paths inside the DD-MZM. In addition, to further restrict the phase difference noise, a real-time phase shift control is implemented in one path of the DD-MZM using a feedback loop. The error signal of the feedback is generated from the direct current (DC) output of one detector monitoring the beat note relative to the phase noise. In this paper, the theoretical model for laser ranging is first presented using the electromagnetic transfer function. Subsequently, the principle of feedback control is analyzed. Based on the model, the experimental setup is established. The signal is acquired using frequency down-conversion and the distance information is retrieved using the phase method.

In this paper, we propose a novel laser ranging technology based on a DD-MZM. A dual-frequency laser for ranging is generated by modulating one light path inside the DD-MZM. To facilitate the detection system, the beat note is analyzed by the down-frequency method, and the ranging result is deduced using the phase method. Compared with the traditional method of generating dual-frequency lasers, which is based on the combination of fiber beam splitters, fiber beam combiners, and acousto-optic modulators or electro-optic modulators, the system has a more compact structure and a much smaller path difference between the two light paths. Therefore, its relative phase noise is smaller, and the ranging stability is better. In addition, to further suppress the relative phase noise of the two light paths, we use the DC output signal to generate a correction signal to perform real-time dynamic control of the DC input of one path of the DD-MZM . The results demonstrate that the long-term drift of the ranging results with locking is greatly suppressed with a suppression ratio of 11 dB, and the distance measurement stability can reach 0.4 μm. This novel scheme is conducive to realizing a long-term stable, miniaturized, and highly integrated laser ranging system.