Fig. 1. Underwater time-frequency networks

Fig. 2. Configuration of a underwater frequency transfer system

Fig. 3. Simulation results of timing jitter PSD^[39]. Curves (i)–(iii) are calculated from the Kolmogorov model; Curves (iv)–(vi) are calculated from the von Kármán model

Fig. 4. Timing fluctuation curves^[49]. (a) Timing fluctuation of the underwater transmission link of 3 m; (b) Timing fluctuation of the underwater transmission link of 6 m; (c) Timing fluctuation of the underwater transmission link of 9 m; (d) Measurement floor for a short link

Fig. 5. Laser-based underwater frequency transfer based on electronic phase compensation technique^[50]. PS: phase shifter; CW: continuous wave; PD: photodetector; PI: proportion–integration controller; HM: half reflected mirror

Fig. 6. Experimental results^[50]. (a) Timing fluctuation curves: (i)-without compensation; (ii)-with compensation; (iii)-measurement floor; (b) Allan deviation curves: (i)-without compensation; (ii)-with compensation; (iii)-measurement floor

Fig. 7. Laser-based underwater frequency transfer based on optical phase compensation technique^[51]. CW: continuous wave; λ/2: half-wave plate; PBS: polarization beam splitter; λ/4: quarter-wave plate; PD: photodetector; PI: proportion–integration controller; HM: half reflected mirror

Fig. 8. Experimental results ^[51]. (a) Timing fluctuation curves: (i)-without compensation; (ii)-with compensation; (iii)-measurement floor; (b) Allan deviation curves: (i)-without compensation; (ii)-with compensation; (iii)-measurement floor

Fig. 9. Laser-based multiple-access underwater frequency transfer based on terminal phase compensation technique^[56]. TX: transmitting site; RX: receiving site; VCO: voltage-controlled oscillator; M1: mirror 1; M2: mirror 2; Mn: mirror n; λ/2: half-wave plate; PBS: polarization beam splitter; λ/4: quarter-wave plate; PD: photodiode; PI: proportion–integration controller

Fig. 10. Experimental results^[56]. (a) Timing fluctuation curves: (i)-without compensation; (ii)-with compensation; (b) Allan deviation curves: (i)-free running VCO B; (ii)-without compensation; (iii)-with compensation; (iv)-commercial Cs clock Microsemi-5071A