Time-of-flight distance measurement based on a dual-comb approach is widely applied in the fields of laser radar, topography scanning, and vibration measurement by using two femtosecond lasers with a small repetition frequency difference for asynchronous optical sampling (ASOPS). In this manner, the high temporal resolution, comb-shaped spectrum, and ultra-low noise performance of femtosecond lasers can be fully utilized. However, the update rate of a dual-comb ranging system based on ASOPS is limited to a few kilohertz (determined by the repetition frequency difference) so as to avoid insufficient optical sampling. Given the extremely small duty cycle determined by the ratio of the femtosecond pulse width to the millisecond sampling period, most of the sampling time during a full sampling cycle is wasted in the process of pulse walk-off. To solve this problem, an electro-optical modulator (EOM) is added to the ASOPS system to modulate the repetition frequency periodically in this work. The so-called electronically controlled optical sampling (ECOPS) approach breaks the update rate limitation in the ASOPS system and can increase the update rate to hundreds of kilohertz, further enriching the application fields of dual-comb distance measurement technology.
ECOPS uses two lasers with tightly phase-locked repetition frequency (fr) as the light source. One is called a local laser with an EOM inserted in the cavity, and the other is called a signal laser. The EOM in the cavity is used to modulate the repetition frequency of the local laser. As a square wave is imposed on the EOM, the repetition frequency is switched between fr-Δfr and fr+Δfr, and the modulation period is determined by the square wave modulation frequency fm. Therefore, the repetition frequency difference between the two lasers switches between -Δfr and Δfr at the modulation frequency fm. Different from ASOPS with only a fixed Δfr, the rapid switching of ±Δfr effectively drives the output pulse of the local laser to scan back and forth on both sides of the output pulse of the signal laser, resulting in a controlled, bounded optical sampling and avoiding the unwanted pulse walk-off. The update rate is determined by the modulation frequency fm, which breaks the limitation of the ASOPS-based measurement system where the update rate is determined by the repetition frequency difference Δfr. In the experiment, a pair of nonlinear polarization rotation (NPR) mode-locked fiber lasers with a repetition frequency of ~158 MHz are selected as the signal laser and the local laser. In the local laser, the pulse duration is 140 fs, the spectral width is 21 nm, the central wavelength is 1569 nm and the average power is 20 mW. As for the signal laser, the pulse duration is 92 fs, the spectral width is 51 nm, the central wavelength is 1557 nm and the average power is 50 mW. Part of the output of the two lasers is combined and directed to a balanced optical cross-correlator (BOC), which detects the relative timing error between the two lasers with sub-femtosecond resolution. The error signal is fed back to the end mirror mounted on a fast piezo-actuator such that the repetition frequencies of the two fiber lasers are tightly phase-locked, and the residual timing jitter is lower than a few femtoseconds. After the phase locking of repetition frequencies is established, the main parts of the laser output are directed to the distance measurement module. A mechanical delay line is used to adjust the optical path from the output of one laser so as to make sure that the pulses from the two lasers overlap in time. As square wave modulation is applied to the EOM, the signal pulses naturally scan back and forth on both sides of the local pulses, which enables ECOPS-based distance measurement.
This paper proposes a dual-comb absolute distance measurement system based on ECOPS and selects two NPR mode-locked lasers with a repetition frequency of ~158 MHz. In the experiment, their repetition frequencies are locked by the synchronization module. After the two femtosecond pulse trains are aligned through the spatial delay line, the sampling pulse is moved back and forth on both sides of the signal pulse by adding a square wave to the EOM in the local laser, which overcomes the problem in the ASOPS approach where the update rate is limited by the repetition frequency difference. The update rate of the experimental setup can be up to 200 kHz, and the measurement accuracy can reach 16.7 nm with an average time of 20.5 ms in the measurement of an absolute distance of 41 μm. In theory, when the update rate is reduced to 40 kHz, the detectable range can reach 1.75 mm, which meets the detection requirements of trenches in most micromechanical structures, semiconductor devices, and other micro-nano devices. The experiments show that the system has good repeatability, reproducibility, stability, and accuracy. The trench depth in a micromechanical structure is measured with the designed system. The dual-comb system based on ECOPS can be widely used in micro-distance measurement, and also has potential application prospects in the fields of 3D topography scanning, such as surface profilometry and flatness analysis.