P. Krehlik

Optical fibers in time and frequency transfer

In the paper we analyze the fundamental accuracy limits of the time/frequency transfer in fiber-optic transmission systems based on intensity modulation and direct detection (IM-DD) of the light signal. The unidirectional and bidirectional time/frequency transfer schemes are considered, and their main limitations are pointed out. In particular, the impact of the fiber backscattering and the temperature dependence of the chromatic dispersion are examined in the context of bidirectional transfer. Finally, experimental results are presented and related to the preceding considerations. The experiments performed with a 60 km long fiber demonstrate single-picosecond accuracy of the time transfer. Our measurements suggest that it should be possible to obtain better accuracy of time/frequency transfer than that reported in the literature for systems based on the IM-DD principle.

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 105 s averaging.

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 88Sr isotope. The value (429 228 066 418 008.3(1.9)syst (0.9)stat Hz) of the absolute frequency of the 1S0 – 3P0 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.

Ultrastable long-distance fibre-optic time transfer: active compensation over a wide range of delays

In this paper we describe a new solution of active delay stabilization for fibre-optic distribution of time and RF-frequency signals, which allows one to obtain both high precision and a potentially unlimited range of compensation of the fibre delay fluctuations. The solution is based on a hybrid system exploiting a pair of continuously tuned electronic variable delay lines, and a set of switched optical delays. We present a fully operational prototype of the time and frequency distribution setup based on this idea, which is capable of compensating more than 1 µs of the fiber delay fluctuations, and thus may be used in very long-haul links up to about 1000 km, without the need for any seasonal maintenance. We also report measurements of the time and frequency distribution stability, and the verification of the time transfer calibration.

Characterization of semiconductor laser frequency chirp based on signal distortion in dispersive optical fiber

In the paper, the simple method of laser chirp parameters estimation is presented. It is based on measuring time-domain distortions of chirped signal transmitted through dispersive fiber and finding laser chirp parameters by matching measured distortions to calculated ones. Experiments undertaken with 1.55 μm telecommunication grade distributed feedback (DFB) lasers and standard single-mode fiber are described, together with some practical remarks on measurement setup and main conclusions.

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 105 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.

Directly modulated lasers in negative dispersion fiber links

In the paper the impact of frequency chirp of directly modulated lasers is considered in context of high-speed data transmission over negative dispersion fiber. Chirp/dispersion induced signal distortions are described, and detailed investigations of their influence on 10 Gb/s transmission system performance are presented. The desired laser chirp characteristics and optimal driving conditions are determined. It is also demonstrated that directly modulated laser with low chirp offers similar or even better dispersion tolerance than unchirped (externally modulated) source. Experimental verification of described investigations is presented at the end.

Multipoint joint time and frequency dissemination in delay-stabilized fiber optic links

This paper presents the system for dissemination of both the RF frequency (e.g., 5, 10, or 100 MHz) and time (pulse per second) signals using an actively tapped fiber-optic link with electronic stabilization of the propagation delay. In principle several nodes for accessing the time/frequency signals may be added without the degradation of the dissemination in the main link. We are discussing the algorithm of determining the propagation delay from the local end of the link to the access node that is required for calibration of the time dissemination. Performed analysis shows that the uncertainty of the time calibration at the access node may in practice be dominated by the dependence of the propagation delay of the receivers on impinging optical powers and is only weakly affected by the distance between the local and access modules. The uncertainty is, however, still low, being only about two times higher compared with the calibration uncertainty of the main link. Experimental results performed on several spooled fibers show that the accuracy of described calibration procedures, expressed as a difference from the results of direct measurement, is not worse than 35 ps.

Measurement of acoustic noise in field-deployed fiber optic cables

In the paper we are presenting the results of the measurements of the phase noise occurring in the optical fiber because of mechanical (acoustic) vibrations. The system used for these measurements, based on a dual-mixer time-difference method, is also presented. We tested two different fiber spans, one running along the motorway and the second one running along the railroad tracks. After analyzing more than 100 h of recorded acoustic noise signals we found substantial differences depending not only on the route of the fiber but even on the time of the day.

Multipoint dissemination of RF frequency in delay-stabilized fiber optic link in a side-branch configuration

In this paper we are presenting the concept of the multipoint dissemination of RF frequency signal via a fiber optic network with actively stabilized propagation delays. The idea allows to add tapping nodes and side branches to the main point-to-point frequency transfer link, constituting a tree-like dissemination network. We are demonstrating the experimental results with the 100 km-long side branch starting at the 110th km of the 170 km-long main link. At the end of the side branch we obtained the Allan deviation of a 10-MHz frequency signal of about 4×10-17, and the time deviation not greater than 1.2 ps for 105 s averaging.