Abstract

High-precision distribution of frequency and time standards through a kilometer-scale network is demanded for several precision applications, such as atomic clock comparison and synchronization and long-baseline array synchronization of phase-coherent radio telescopes [1]. Optical fiber is the best long-distance medium for transferring microwave frequency standards. To achieve the required stability through the transmission link, the transmitted and the round-trip signal are compared and the phase error extracted is used to actively cancel out the phase jitter. For long distance frequency dissemination, regenerator nodes are required to provide acceptable SNR and high loop bandwidth, especially for cascaded optical links via the telecommunication network [2]. In this paper, we propose novel intermediate node and receiver architecture for the dual wavelength approach, based on the Two Mode Injection Locking – TMIL technique. The incoming optical wavelength λ1 is intensity modulated with the sinusoidal frequency standard (fig. 1a). With the use of a fiber coupler, it is combined with the CW wavelength λ2 and both are injected to the FP laser through circulator 2, at two different lasing modes. Wavelength conversion and reproduction to λ2 and optical regeneration of the original sinusoidal microwave frequency standard carried in λ1 takes place in the FP laser [3]. The proposed regenerator system uses only two lasers, and low cost off-the-shelf passive optical components eliminating the need for optical amplifiers, photodiodes and modulators. The solution is also transparent to the value of the modulation frequency and it can enable complex 1 to M dissemination scenarios by using multiple CW lasers as probes that will reproduce the incoming microwave reference and distribute it to different destinations. The experimentally realized transmission test-bed for the proposed scheme evaluation is shown in fig. 1b. The frequency is set at 5 GHz and the RF generator is controlled by the 10 MHz reference signal from Spectracom Pendulum CNT-91R rubidium standard frequency counter. Measurements carried out with the CNT-91R and Spectratime pico-time frequency counter provided Allan deviation (ADEV) values of 2.10−14 at one second for the electrical sub-system operating at 5 GHz. The optical carrier, at λ1 = 1552.75 nm, is generated and intensity modulated by a DFB laser diode followed by an electro-absorption modulator (EAM). The modulation depth was set to 73%. The optical power launched into the 50 km of single-mode fiber (SMF) is +9 dBm. The received power at the input of the regenerator is −1.7 dBm without and −7.6 dBm with the use of the dispersion compensating fiber (DCF) respectively. The CW laser’s output power was set to −4.3 dBm at λ2=1550.15 nm. The ADEV performance is depicted in fig. 1c. To compare the stability performance of the regenerator to that of the typical IM system, the same phase correction scheme and settings were used. The repeater closed loop performance is slightly worse than that of the original system, especially at short averaging times due to mechanical instabilities, whilst at the long time scale its performance is identical to that of the noise floor. The ADEV measurements prove its regenerative nature recording noise floor limited stability after 50 km of transmission without the need for optical amplification at the receiver.

© 2015 IEEE

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