Phase fluctuation compensation for long-term transfer of stable radio frequency over fiber link

Phase conjugation frequency dissemination based on harmonics of optical comb at 10⁻¹⁷ instability level

We demonstrate a phase conjugation frequency dissemination scheme using an optical frequency comb as a coherent source. After the comb's radio frequency is detected by a photodiode, its repetition frequency's first harmonic is used to modulate a laser, which is injected into a 10 km fiber link. The round-trip signal is mixed with the triple harmonic thereby obtaining a signal that is immune to the fluctuation at the remote end. This method results in frequency instability of 1.1×10(-17) at 10(4) s. Our scheme provides a potential approach to deliver coherent frequencies to many places with high stability.

Phase fluctuation cancellation of anonymous microwave signal transmission in passive systems

A phase fluctuation cancellation approach for anonymous microwave signal transmission over fiber link is proposed and demonstrated. Unlike most previous schemes that used for active systems, our proposal is suitable for passive systems by utilizing the optical signal feedback and electrical signal phase-locking. Experimental results show that phase drifts of 7.7-ps, 54-ps and 96-ps (RMS value) for 2.45-GHz signals could be reduced to 3.1-ps, 3.8-ps and 8.5-ps after 1-km, 10-km and 25-km SMF transmission over an eight-hour period, respectively. Overall system performance is limited by the coherent Rayleigh noise and could be further optimized.

High-precision distribution of highly stable optical pulse trains with 8.8 × 10⁻¹⁹ instability

The high-precision distribution of optical pulse trains via fibre links has had a considerable impact in many fields. In most published work, the accuracy is still fundamentally limited by unavoidable noise sources, such as thermal and shot noise from conventional photodiodes and thermal noise from mixers. Here, we demonstrate a new high-precision timing distribution system that uses a highly precise phase detector to obviously reduce the effect of these limitations. Instead of using photodiodes and microwave mixers, we use several fibre Sagnac-loop-based optical-microwave phase detectors (OM-PDs) to achieve optical-electrical conversion and phase measurements, thereby suppressing the sources of noise and achieving ultra-high accuracy. The results of a distribution experiment using a 10-km fibre link indicate that our system exhibits a residual instability of 2.0 × 10⁻¹⁵ at1 s and8.8 × 10⁻¹⁹ at 40,000 s and an integrated timing jitter as low as 3.8 fs in a bandwidth of 1 Hz to 100 kHz. This low instability and timing jitter make it possible for our system to be used in the distribution of optical-clock signals or in applications that require extremely accurate frequency/time synchronisation.

Frequency comb-based microwave transfer over fiber with 7×10(-19) instability using fiber-loop optical-microwave phase detectors

We demonstrate a remote microwave/radio frequency (RF) transfer technique based on the stabilization of a fiber link using a fiber-loop optical-microwave phase detector (FLOM-PD). This method compensates for the excess phase fluctuations introduced in fiber transfer by direct phase comparison between the optical pulse train reflected from the remote site and the local microwave/RF signal using the FLOM-PD. This enables sub-fs resolution and long-term stable link stabilization while having a wide timing detection range and less of a demand in fiber dispersion compensation. The demonstrated fractional frequency instability between 2.856 GHz RF oscillators separated by a 2.3 km fiber link is 7.6×10(-18) and 6.5×10(-19) at 1000 and 82,500 s averaging times, respectively.

Stable radio-frequency transfer over optical fiber by phase-conjugate frequency mixing

We demonstrate long-distance (≥100-km) synchronization of the phase of a radio-frequency reference over an optical-fiber network without needing to actively stabilize the optical path length. Frequency mixing is used to achieve passive phase-conjugate cancellation of fiber-length fluctuations, ensuring that the phase difference between the reference and synchronized oscillators is independent of the link length. The fractional radio-frequency-transfer stability through a 100-km "real-world" urban optical-fiber network is 6 × 10(-17) with an averaging time of 10(4) s. Our compensation technique is robust, providing long-term stability superior to that of a hydrogen maser. By combining our technique with the short-term stability provided by a remote, high-quality quartz oscillator, this system is potentially applicable to transcontinental optical-fiber time and frequency dissemination where the optical round-trip propagation time is significant.