Coherent optical frequency transfer via 972-km fiber link

We demonstrate coherent optical frequency dissemination over a distance of 972 km by cascading two spans where the phase noise is passively compensated for.Instead of employing a phase discriminator and a phase locking loop in the conventional active phase control scheme,the passive phase noise cancellation is realized by feeding double-trip beat-note frequency to the driver of the acoustic optical modulator at the local site.This passive scheme exhibits fine robustness and reliability,making it suitable for long-distance and noisy fiber links.An optical regeneration station is used in the link for signal amplification and cascaded transmission.The phase noise cancellation and transfer instability of the 972-km link is investigated,and transfer instability of 1.1×10^(-19)at 10^(4)s is achieved.This work provides a promising method for realizing optical frequency distribution over thousands of kilometers by using fiber links.

Phase noise approximation analysis for the OFDM system

The influence of phase noise（PN） present at the access point（AP） and mobile terminator（MT） may degrade the performance in orthogonal frequency division multiplexing （OFDM） system, because the OFDM system is sensitive to the PN. The method of PN approximation based on the PN power spectrum was proposed to combat the influence of the PN present. The idea of the PN cancellation is the PN is approximated for the sum of the different frequency between the AP and the MT as well as Doppler shift supposing that the fixed different frequency during several OFDM symbols are identical, so the variable phase may be considered as the combination of the fixed partition and the random partition. The approximation method is utilized to remove the fixed and the random phase partition so that the signal-to-noise ratio （SNR） may be improved. The analysis and the simtilation proposed show that the method may effectively improve the performance of the OFDM system.

Experimental Study on the Lattice-End-Mirror Vibration Noise Cancellation of Ytterbium Clock Laser

In this paper,we describe an optical path stabilization method in order to keep the phase of the clock laser fixed during the interrogation.The phase noise along the optical path of the clock laser is extracted from the heterodyne beat signals.Then an acousto-optic modulator(AOM)driven by direct digital synthesizer(DDS)receives a feedback signal to cancel the phase noise.The experimental results show that our system can effectively suppress additional phase noise.The fractional frequency instability is dropped to 1.76×10^-17/τ at 1 s,which is improved by two orders of magnitude than before.In the case of Rabi excitation with a pulse duration T=50 ms,frequency shift is 7.9 mHz due to the feedback system relock process.For phase noise from different frequency,the system has a good suppression effect on low frequency phase noise.