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Opto-Electronics and Communications Conference (OECC)

High-precision Underwater Frequency Transmission based on Green Laser

Fei Yang, Lili Han, Kang Cao, Dijun Chen, Huaguo Zang, Weibiao Chen

Published Jun 30, 2024• 0 citations
Publisher

Abstract

Underwater frequency transmission is the key technology for time-space reference in the marine environment. A high-precision underwater frequency transmission method based on green laser and digital phase noise compensation is demonstrated. The transmission stability of 400 MHz and 100 MHz frequency signal after 10 m underwater transmission is 3.9×10-14@1 s and 6.4×10-14@1 s. It is the first time for that the stability at 1 s of underwater frequency transmission has entered the order of 10-14. It is proven that the stability of underwater frequency transmission could be closed to the level of optical fiber microwave frequency transmission on land.

DOI
10.1109/oecc54135.2024.10975361

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Underwater frequency transmission is the key technology for time-space reference in the marine environment. A high-precision underwater frequency transmission method based on green laser and digital phase noise compensation is demonstrated. The transmission stability of 400 MHz and 100 MHz frequency signal after 10 m underwater transmission is 3.9×10-14@1 s and 6.4×10-14@1 s. It is the first time for that the stability at 1 s of underwater frequency transmission has entered the order of 10-14. It is proven that the stability of underwater frequency transmission could be closed to the level of optical fiber microwave frequency transmission on land.
We demonstrated an underwater frequency transfer technique with a green diode laser. The characteristic of the timing fluctuation and instability for the transfer technique was analyzed and simulated. With this technique, we had transferred a highly stable 100-MHz frequency signal over an underwater link with distances of 3 m, 6 m, and 9 m for 5000 s, respectively. The experimental results involving the underwater transfer of the 100-MHz radio-frequency signal shows that the rms timing fluctuations are 5.9 ps (3-m link), 6.4 ps (6-m link), and 8.4 ps (9-m link). The calculations also show that the relative Allan deviations for the 3-m, 6-m, and 9-m transmission links are 5.6 × 10−13 at 1 s and 5.3 × 10−15 at 1000 s, 5.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000, and 6.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000 s, respectively. The measured instabilities were lower than Rb and Cs atomic clocks, implying that the proposed frequency transfer scheme can potentially be used to transfer the signals of these atomic clocks over underwater links.
Transmitting an ultra-high precision frequency of an optical clock via wavelength division multiplexing (WDM) is a key technology for economically realizing innovative applications enabled by significantly accurate and stable time and frequency. In this study, we investigated an optical frequency transmission method using a commercial WDM system, employing our highly stable optical frequency repeater systems with a fiber noise compensation (FNC) function. For the 20-km WDM transmission, we achieved the frequency instability of 4 × 10–15at an averaging time of 1 second (s) and reaches ~ 1 × 10–18at 34,000 s. This frequency stability enables optical clock reference frequencies to be transmitted over long distances, demonstrating the potential of our WDM link to enable the widespread deployment of the optical clock frequency at the frontier of precision.
We demonstrate a photonic radio-frequency transmission system via optical fiber. Optical radio-frequency signal is generated utilizing a Mach-Zehnder modulator based on double-side-band with carrier suppression modulation scheme. The phase error induced by optical fiber transmission is transferred to an intermediate frequency signal by the dual-heterodyne phase error transfer scheme, and then canceled by a phase locked loop. With precise phase compensation, a radio frequency with high-phase stability can be obtained at the remote end. We performed 20.07-GHz radio-frequency transfer over 100-km optical fiber, and achieved residual phase noise of -65 dBc/Hz at 1-Hz offset frequency, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 110 fs. The long-term frequency stability also achieves 8×10(-17) at 10,000 s averaging time.
A multiple-access underwater frequency transfer scheme using terminal phase compensation is demonstrated. With this scheme, a highly stable 100 MHz frequency signal was disseminated over a 3 m underwater link for 5000 s. The timing fluctuation and fractional frequency instability were both measured and analyzed. The experimental results show that with the phase compensation technique, the total root-mean-square (RMS) timing fluctuation is about 3 ps, and the fractional frequency instabilities are on the order of 5.9 × 1 0 − 13 at 1 s and 5.3 × 1 0 − 15 at 1000 s. The experiment results indicate that the proposed frequency transfer technique has a potential application of disseminating an atomic clock to multiple terminals.
We proposed and experimentally demonstrated a high-precision frequency transmission based on IQ modulation on silicon. The silicon photonic chip (SIPC) includes a photonic IQ modulator and some couplers. According to the calculated Allan deviation of discriminated phase, the stability of the 500MHz microwave frequency signal after EO-OE based on SIPC is3.6×10−143.6\times 10^{-14}3.6×10−14@1s and5.4×10−175.4\times 10^{-17}5.4×10−17@10000s. After 82km fiber link transmission, the transmission stability of 500MHz microwave frequency signal is7.4×10−147.4\times 10^{-14}7.4×10−14@1s and7.0×10−157.0\times 10^{-15}7.0×10−15@10000s separately. With the noise compensation of the fiber transmission, the stability of 500MHz microwave frequency signal is improved to5.6×10−145.6\times 10^{-14}5.6×10−14@1s and9.9×10−169.9\times 10^{-16}9.9×10−16@10000s respectively.
We have demonstrated accurate frequency transfer via an ultra long span optical-fiber link by passive phase conjugation, which is based on frequency mixing. Combing with the phase-lock-loop, our compensation technique can eliminate the effect of phase noise that introduced during the optical-fiber transmission, so the phase of remote signal is synchronized to the phase of local reference signal. The transfer stability of our experiment with 2.4 GHz frequency signal through a 1007 km fiber link is 6.9_10-14/s and 4.1_10-16/104s.