Higher sensitivity in timing jitter measurement has great importance in studies related to precise measurements. Timing jitter noise floors contribute one of the main parts in existing measurements. In this article, a phase error signal is obtained by superposition of outputs of two optical heterodyne discrimination apparatus to suppress the noise floor. Excess phase noise of the electrical amplifier is avoided. We demonstrate 2.6 × 10⁻¹⁴ fs²/Hz (~160 ys/√Hz) timing jitter noise floor between two identical 99 MHz repetition-rate mode-locked Ti:sapphire lasers after their repetition rates are tightly synchronized. The performance is extensible to reach an integrated timing jitter resolution of one attosecond.

A practical frequency stabilization method for opto-electronic oscillators (OEOs) using an Adaptive Proportional-Integral-Derivative (PID) controller with Trend-based Adjustment (APTA) is proposed and experimentally demonstrated. The method dynamically adjusts the laser driver current of the distributed feedback (DFB), enhancing frequency stability without needing costly tunable lasers. The APTA approach achieves a frequency drift reduction to ±0.005 ppm/K and an Allan deviation of 8.5 × 10 ^-11 at 1000 s under thermal variations while maintaining phase noise performance.

The propagation distance confines the development of precise time-frequency transmission using optical frequency combs due to the dispersion of the link. Here we disseminate a fiber re-circulating loop to emulate 504-km comb-based transmission. An optical filter in combination with a spool of dispersion compensation fiber is utilized to restrict the dispersion effect. The residual instability reached 4.0 × 10⁻¹⁴ at 1 s and 7.32 × 10⁻¹⁸ at 10,000 s over the 504-km link. The result indicated that this approach could meet the demand for long-haul clock transmission and comparison in the giant fiber-optic gyroscope to detect the seismic and gravitational potential.

A method to improve the frequency stability of microwave frequency comb (MFC) signals generated by an actively mode-locked optoelectronic oscillator (AML-OEO) is proposed and experimentally demonstrated. In the experiment, fundamental mode locking in the constructed AML-OEO is achieved, producing MFC signals with a center frequency of 2.165 GHz and a repetition rate of 396.629 kHz. By locking the MFC comb tooth to a stable reference source using a phase-locked loop (PLL), the stability of MFC’s center comb tooth reaches 2.4 × 10⁻¹³@1000s and the frequency stability improves by more than two orders of magnitude on a second-level time scale compared to the free-running AML-OEO. The improved frequency stability of MFC signals makes AML-OEO suitable as high-performance microwave sources for various critical applications.

We apply the Empirical Mode Decomposition (EMD) algorithm and the Time Convolutional Network (TCN) structure, predicated on Convolutional Neural Networks, to successfully enable feature extraction within high-precision optical time-frequency signals, and provide effective identification and alerts for abnormal link states. Experimental validation confirms that the proposed method not only delivers an efficacy on par with traditional manual techniques, but also excels in swiftly identifying anomalies that typically elude conventional approaches. This investigation furnishes novel theoretical backing and forecasting tools for high-precision optical transmission.

Optical clocks can be used as the absolute frequency reference due to their high accuracy and stability. In a precise optical clock transmission system, the instability loss of the link is usually evaluated by beating the remote optical signal with the reference clock, which is not suitable for real frequency distribution applications. Therefore, it is necessary to assess the performance directly at the remote site for the optical frequency transfer, because the two sites of the link are usually not co-located. In this paper, we proposed a comb-based remote residual instability evaluation scheme. Two coherent optical combs with different wavelengths were extracted from a frequency stable comb and transmitted after multiplexing. The residual instability was evaluated directly at the remote site by measuring the phase fluctuation difference between the two combs. We achieved 8.61 × 10⁻¹⁹ at 40,000 s over a 10 km fiber link, reaching the instability of optical clocks. These results revealed that our scheme can evaluate optical clock frequency transmission directly at the remote site, which made the method truly practical.