Controlling of the bidirectional amplifier chain for optical frequency distribution based on a two-dimensional noise detector.

In the paper we present a solution for controlling the chain of bidirectional optical amplifiers, intended for long-haul fiber links used to distribute signals produced by optical atomic clocks. The solution is based on a dedicated two-channel noise detector, which allows independent measurement of the noise contributions related to interferometric signal fading and additive wideband noise. New signal quality metrics, based on two-dimensional noise detector, allows to distribute properly the needed gain among the chained amplifiers. Experimental results performed both in laboratory conditions and on a real 600 km-long link, confirming proper operation of proposed solutions, are presented.

Electrical Regeneration for Long-Haul Fiber-Optic Time and Frequency Distribution Systems.

In recent years fiber-optic-based long-haul installations for time and frequency (T&F) distribution have become operational at various sites. The common practice to cope with large attenuation of long fiber path is to use bidirectional optical amplifiers. This, however, becomes insufficient in case of very long links and/or long spans between amplifiers, because of unavoidable deterioration of the signal-to-noise ratio (SNR). In this article, we present a solution where the optical signal amplification is combined with optical-electrical-optical (OEO) regeneration, performed in a few points along the link. We analyze the impact of replacing some optical amplifiers with OEOs and demonstrate the resulting improvement in terms of SNR of the received optical signal and the phase noise at the output of the T&F distribution system. Laboratory experiments performed with both spooled and metropolitan-area fibers (total length up to 900 km) confirmed the theoretical predictions and showed that placing the OEO regenerators in appropriate points along the link allows reaching the required SNR.

Optical Multiplexing of Metrological Time and Frequency Signals in a Single 100-GHz-Grid Optical Channel.

In this article, the concept of co-locating all metrological time and frequency signals in a single optical channel of a standard, 100-GHz-spaced optical grid is presented and evaluated. The solution is intended for situations where only a narrow optical bandwidth is available in a fiber heavily loaded with standard data traffic. We localized the optical reference signals in the middle of the channel, with signals related to RF reference and time tags shifted ±12.5 GHz apart. In the experimental evaluation with a 260-km-long fiber, we demonstrate that the stability of frequency signals and the calibration of time tags remained at the very same level of stability and accuracy as for systems utilizing separate channels: the fractional long-term instability for the optical frequency reference was below 5 ×10-20 , that for the RF reference at the level of 10-17, and the mismatch of the time tag calibration was not more than 10 ps. We also identify possible issues, mainly related to a risk of unwanted Brillouin amplification and scattering.

Stability Limitations of Optical Frequency Transfer in Telecommunication DWDM Networks.

This article investigates the fundamental limitations of optical frequency transfer stability related to cost-effective implementation of signal transmission in duplex, unidirectional optical paths offered by a standard dense wavelength division multiplexing (DWDM) network. We pointed out the effect of a significant mismatch of phase fluctuations observed in pairs of fibers even when located in a common cable. We also measured the thermal sensitivities of individual DWDM optical modules in the context of the effectiveness of the signal stabilization system. Finally, we present the real implementation of the coherent optical carrier transfer in the operational DWDM network, showing the overall impact of all individual effects. We demonstrated that it is possible to obtain the long-term stability (one day averaging) within the range from 2 ×10-16 to 4 ×10-16 and typical frequency offset at the level of few times 10-16 in a 1500-km-long line by using the soil-deployed cables.

Modeling and Optimization of Bidirectional Fiber-Optic Links for Time and Frequency Transfer.

In this paper, a bidirectional fiber-optic link is considered, composed of two end terminals, connected by a number of fiber spans and bidirectional optical amplifiers. The end terminals exchange time and frequency information by sending and receiving intensity modulated optical signals in both directions, which is required to compensate the fluctuation of the propagation delay of the transmission medium. In such a link for its optimal performance, the gains of the bidirectional optical amplifiers need to be adjusted to minimize the noise resulting from Rayleigh backscattering and amplified spontaneous emission. A model of the link is proposed using a transmission matrixes approach, which allows estimating the signal-to-noise ratio (SNR) at the ends of the main link (i.e., connecting the end terminals) and at the extraction (tapping) nodes located along the main link. The transmission matrixes of a fiber span and Er-doped fiber amplifier are presented and required formulas are derived. In addition, wavelength selective isolators are considered, which allow intentional breaking of the propagation of backscattered signals and are effective in improving the SNR when long fiber spans are involved. The model is experimentally verified in a laboratory link composed of four bidirectional amplifiers and five fiber spans of total length up to 420 km, showing the agreement between the measured and calculated SNRs not worse than 2 dB.

Compensation of the Fluctuations of Differential Delay for Frequency Transfer in DWDM Networks.

This paper investigates the possibility of improving the stability of radio-frequency transfer in telecommunication dense wavelength division multiplexing fiber-optic networks. As it has been identified, the dispersion compensation fibers (DCFs), frequently used in these networks, cause substantial differential delay, whose temperature-induced fluctuations have the most significant impact on the deterioration of the stability of the frequency transfer. The authors present a method that allows achieving significant improvement in the long-term stability of the frequency transfer. The developed method is based on modeling the impact of DCFs with the help of remotely accessible temperature sensors factory installed by the manufactures in DCF modules. The effectiveness of the proposed solution has been tested on three different long-haul routes (up to 1550 km), set up in the operational Polish National Research and Education Network.

Fiber-Optic UTC(k) Timescale Distribution With Automated Link Delay Cancelation.

In this paper, we describe the idea and practical realization of an automated calibrator of fiber-optic UTC(k) distribution system, along with a high-resolution shifter of 1 PPS signal, which allows us to cancel the propagation delay and, thus, to produce the 1 PPS at the remote system output with practically zero offset. The solution was experimentally verified with 10 different optical paths, up to 300 km long. The rms offset of the output timescale in our experiments was 7.1 ps, and maximum absolute value did not exceed 15 ps.

Long Haul Time and Frequency Distribution in Different DWDM Systems.

In this paper, we have presented the possibility of time and frequency (T&F) distribution in two generations of dense wavelength-division-multiplexing (DWDM) networks: the older one, equipped with dispersion compensation fiber (DCF) modules, and the newest, without in-line chromatic dispersion compensation (dedicated for coherent signals). The experiments were performed in a 1500-km loop arranged in the PIONIER production network, with T&F signals regarded as so-called "alien wavelength" network service. In the newest DWDM version, we observed very good stability of delivered signals: modified Allan deviation approach 10-16 for averaging longer than 104 s (for 10-MHz frequency signal), and time deviation below 15 ps for averaging up to 105 s for 1 PPS time signal. These results show that the DWDM alien wavelength service can be used for high-demanding applications like cesium fountains comparisons. Results achieved for the former version of DWDM were about one magnitude worse for a long-term comparison, but it can still be useful for less demanding applications. We found that the main reason for relatively poor results observed in the older generation of DWDM is the impact of the DCFs used in this DWDM approach.

A Hybrid Solution for Simultaneous Transfer of Ultrastable Optical Frequency, RF Frequency, and UTC Time-Tags Over Optical Fiber.

We describe a fiber-optic solution for simultaneous distribution of all signals generated at today's most advanced time and frequency laboratories, i.e., an ultrastable optical reference frequency derived from an optical atomic clock, a radio frequency precisely linked to a realization of the SI-Second, and a realization of an atomic timescale, being the local representation of the virtual, global UTC timescale. In our solution both the phase of the optical carrier and the delay of electrical signals (10-MHz frequency reference and one-pulse-per-second time tags) are stabilized against environmental perturbations influencing the fiber link instability and accuracy. We experimentally demonstrate optical transfer stabilities of and for 100 s averaging period, for optical carrier and 10-MHz signals, respectively.

ELSTAB-Fiber-Optic Time and Frequency Distribution Technology: A General Characterization and Fundamental Limits.

In this paper, we present an overview of the electronically stabilized (thus named ELSTAB) fiber-optic time and frequency (T&F) distribution system based on our idea of using variable electronic delay lines as compensating elements. Various extensions of the basic system, allowing building a long-haul, multiuser network are described. The fundamental limitations of the method arising from fiber chromatic dispersion and system dynamics are discussed. We briefly characterize the main hardware challenge of the system, which is the design of a pair of low-noise, precisely matched delay lines. Finally, we present experimental results with T&F distribution over up to 615 km of fiber, where we demonstrate frequency stability in the range of 1-7 ×10(-17) for 10(5) s averaging and time calibration with accuracy well below 50 ps. Also, practical implementation of the ELSTAB in the Polish T&F distribution network is shown.

Absolute measurement of the 1S0 - 3P0 clock transition in neutral 88Sr over the 330 km-long stabilized fibre optic link.

We report a stability below 7 × 10(-17) of two independent optical lattice clocks operating with bosonic (88)Sr isotope. The value (429 228 066 418 008.3(1.9)(syst) (0.9)(stat) Hz) of the absolute frequency of the (1)S(0) - (3)P(0) transition was measured with an optical frequency comb referenced to the local representation of the UTC by the 330 km-long stabilized fibre optical link. The result was verified by series of measurements on two independent optical lattice clocks and agrees with recommendation of Bureau International des Poids et Mesures.

Multipoint dissemination of RF frequency in fiber optic link with stabilized propagation delay.

In this paper, we present the concept of accessing the signal at some midpoint of a frequency dissemination system with stabilized propagation delay, which allows building the point-to-multipoint frequency dissemination network. In the first experiments with a 160 km-long fiber link composed of a field-deployed optical cable and fibers spooled in the lab, exposed to both diurnal and seasonal temperature variations, in the access node, we obtained the Allan deviation of a 10- MHz frequency signal of about 3 × 10(-17) and the time deviation not greater than 2 ps for 10(5) s averaging.