Joint time and frequency transfer in optical fiber has been widely discussed and there are several kinds of schemes. However, they all have such limitations as employing additional wavelength channels in metropolitan network and introducing electronics noise in the process of sine frequency signal converting into square wave.
In a recent study, a team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made new progress on time and frequency signal joint transfer via the same wavelength. Related work was published on Optics Letters.
In their experiment, researchers proposed a simple and efficient scheme to overcome the above-mentioned problems. The superiority of the novel scheme is to reduce the channel number by half in fiber network. Based on the proposed scheme, a precise and continuous time and frequency transfer system was demonstrated over a 110km fiber link, which showed the effectiveness of the scheme.
At the local site, time signal was phase modulated and frequency signal was intensity modulated on the same carrier. At the remote site, the transferred optical carrier was split into two beams, and time and frequency signals were recovered by coherent and incoherent demodulation, respectively.
They found that the noise caused by fiber link was compensated by the optical delay line. Relative stabilities of 1.7E-14/1s and 5.9E-17/104s for frequency and time interval for 1PPS with sub-ps stability after 1000s were obtained.
This scheme can be used for ultra-stable frequency dissemination and time synchronization system based on metropolitan network.
The research was supported by Strategic Priority Research Program of the Chinese Academy of Sciences.
Fig 1. Measured time deviation (TDEV) and Allan deviation (ADEV)
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS