Fig. 1. (a) Setup of the ASOPS system. CIR+WDM, 980/1550 nm wavelength-division multiplexer fiber circulator; EDF, Er-doped fiber; SESAM, semiconductor saturable absorption mirror; PZT, piezoelectric transducer; FDL, electrically controlled fiber delay line; IWDM, wavelength-division multiplexer with isolator; PD, fiber-coupled photodiode; Raman, polarization-maintaining Raman fiber; PS, power splitter; PLO, phase-locked oscillator; DDS, direct-digital synthesis component; BP, electronic band-pass filter; AMP, amplifier; M1 and M2, electronic mixers; PLL1 and PPL2, phase-locked loops; f_M, master repetition rate; f_S, slave repetition rate; Δf_set, desired offset frequency. The formulas in blue give the corresponding frequencies inside the error-signal unit branches. The straight and dashed lines correspond to the optical beams and electronic connections, respectively. (b) Internal structure diagram of the ASOPS system. The master and slave lasers were enclosed in two closed copper boxes, respectively. (c) Integrated ASOPS system prototype.

Fig. 2. Spectra of the (a) master and (b) slave lasers centered at 1565 nm. Autocorrelation traces of the (c) master and (d) slave lasers. The dashed lines represent the Gaussian curves fitted to the autocorrelation traces.

Fig. 3. Recorded repetition rates of (a) master laser f_M, (d) slave laser f_S and (g) Δf. Counts of (b) f_M, (e) f_S and (h) Δf. Overlapping Allan deviations of the recorded (c) f_M, (f) f_S and (i) Δf. (j) Phase noise measurements of the locking master and slave lasers.

Fig. 4. Time-domain interference signals of the ASOPS system with Δf of (a) 100 Hz, (b) 200 Hz and (c) 300 Hz. (d) Repetition rate of the slave laser locked at different Δf from 1 Hz to 1 MHz. (e) Tunable repetition rate of the slave laser with FDL modulations.

Fig. 5. (a) Experimental setup of the TDS spectrometer. (b) Temporal waveform and (c) power spectrum of the pulsed THz radiation obtained at a 50-sweep measurement.