Fig. 1. Configuration of all-PM NALM mode-locked fiber laser. WDM, 980/1550 nm wavelength division multiplexer; PBC, polarization beam coupler; Col., collimator; FR, Faraday rotator; QWP, quarter waveplate. The FR and QWP form the nonreciprocal phase shifter. Optical spectra of two mode-locked lasers. Blue line, laser 1; red line, laser 2.

Fig. 2. Working principle of ASOPS. Two laser sources that have slightly different repetition frequency rates (fr and fr+Δfr) are used. Every period Tupdate, the LO will take a complete sampling signal of the SL pulse via sum-frequency generation (SFG).

Fig. 3. Experimental setup of the robust LIDAR system. Two free-running all-PM lasers with slightly different repetition rates (∼80.267 MHz, ∼82.269 MHz) are used as the laser sources. An FC/PC connector having 4% Fresnel reflection at the surface and a hollow retroreflector are used as the reference and target mirror, respectively. The absolute distance between the reference and target mirrors is obtained through optical intensity cross-correlation between the reflected SL and LO pulses. Note that the telescope is focal length adjustable. SL, single laser; LO, local oscillator; ISO, isolator; HWP, half-waveplate; Col., collimator; Ref., reference mirror; Tar., target mirror; PBS, polarization beam splitter; PPKTP, periodically poled KTiOPO4; APD, avalanche photo-detector; LPF, low pass filter.

Fig. 4. (a) Absolute distance measurement results of the femtosecond absolute ranger compared with the standard interferometers. (b) Residuals of measured distance versus truth data from standard interferometers at the NIM. The averaging period for the TOF measurement is 500 ms.

Fig. 5. Allan deviations at absolute distances of 30 and 60 m. The measurement accuracy increases with the increase of averaging time and reaches less than 1 μm at 1 s averaging time.

Fig. 6. Residuals of measured distance after the Kalman filter compared with the standard interferometers. The residuals and deviations are less than 3 μm, while the update rate is 200 Hz.

Table 1. Output Parameters of Two Lasers