Joint time and frequency transfer through one International Telecommunication Union 100 GHz wavelength division multiplexing channel with commercial devices.

We proposed a joint time and frequency transfer scheme over a single International Telecommunication Union 100 GHz wavelength division multiplexing (WDM) channel using a normal commercial WDM device and commercial offset WDM device. A standard 100 GHz WDM channel is divided into three sub-channels with a frequency interval of more than 20 GHz for a time and frequency transfer, which could help to avoid the interference among time, frequency, and data signals in other WDM channels. A joint high-precision time and frequency transfer is, therefore, able to be performed with data transmission over WDM optical communication links without extra requirements on devices. A joint time and frequency transfer in a single 100 GHz WDM channel is experimentally demonstrated over a 60 km fiber link with the communication data transmission in the adjacent channels. The stability of the time transfer can be better than 15 ps at 1 s, and the stability of the frequency transfer can be better than 2.7×10-14 at 1 s, while the bit error rates of the adjacent channels are at the same level as the separate transmission.

Fiber-optic joint time and frequency transmission with enhanced time precision.

We propose a high-precision joint time and frequency transmission scheme based on bidirectional wavelength division multiplexing transmission over an optical fiber link. The time signal is generated based on the phase-stable frequency signal with employment of a dedicated designed low-jitter event generator. Time synchronization is realized by eliminating the time difference between the time signals of the master and slave stations, which is determined by accurate two-way time comparison. In this way, thanks to the high stability of the frequency transmission, low jitter of the dedicated designed event generator, and the high accuracy of the two-way time comparison, a high precision time signal with enhanced time stability and accuracy can be obtained at the slave station, which is synchronized to the master station. Experimentally, a joint time and frequency transmission system is demonstrated over a 62-km urban fiber link. The results show a time stability in terms of time deviation (TDEV) of 3.5 ps/s and 430 fs/10,000 s, and an accuracy of better than 20 ps can be realized.

Multi-access fiber-optic time dissemination with bidirectional optical-electrical-optical nodes.

We first report multi-access time dissemination with bidirectional optical-electrical-optical (BOEO) nodes based on bidirectional time division multiplexing transmission over a single fiber with the same wavelength. Each BOEO node can be simultaneously served as a bidirectional amplifier, an isolator, and a multi-access time dissemination node to boost the faded bidirectional optical signals, to effectively suppress the accumulated noise along the main fiber link, and to extract synchronized time signals, respectively. We experimentally demonstrate multi-access time dissemination with two BOEO nodes in our time dissemination apparatus with the main fiber link up to 350 km. The results illustrate that stabilities in terms of time deviation of 23.2 ps at the integration time of 1 s and 3.8 ps at the integration time of 105 s, 24.3 ps at the integration time of 1 s and 1.3 ps at the integration time of 105 s, and 23.9 ps at the integration time of 1 s and 2.9 ps at the integration time of 105 s are obtained for the main link and access nodes 1 (AN1) and 2 (AN2), respectively. The proposed scheme demonstrated together with preliminary measurement results provides a guidance for future development ultra-long haul fiber-optic time dissemination with the BOEO nodes.

Time transfer through the optical supervisory channel in wavelength division multiplexing systems.

We present an optical fiber time transfer scheme through the optical supervisory channel (OSC) in wavelength division multiplexing (WDM) systems. A sub-band of the standard OSC band is used to transmit time signals by only adding sub-OSC filters in commercial WDM systems. The transmission of the existing communication data service is not affected. The wavelengths of bidirectional time transmission in the same sub-band can be very close, or even the same, to maximize the bidirectional propagation symmetry and improve the precision of time transfer. Combined time, optical supervisory signal, and communication data transmission is experimentally demonstrated along a single 100 km optical fiber in a WDM system in laboratory.