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Multipoint Dissemination of RF Frequency in Fiber Optic Link With Stabilized Propagation Delay Przemysław Krehlik, Łukasz Śliwczyński, Łukasz Buczek, and Marcin Lipiński Abstract—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 10MHz frequency signal of about 3 × 10−17 and the time deviation not greater than 2 ps for 105 s averaging.

I. Introduction

A

frequency and time transfer over optical fibers becomes an attractive alternative to the classical methods based on the satellite links, as the TWSTF or various variants of the GPS-based schemes. Many ideas for frequency (and sometimes also time) transfer in optical networks have been presented, based on transmission of either a highly coherent optical carrier [1]–[3], an optical comb [4], an optical signal from a mode-locked laser phase-locked to an optical reference [5], or an intensitymodulated light [6]–[11]. The main problem, which should be overcome using any such approach, is a fluctuation of the propagation delay of the fiber, caused mainly by varying temperature, and on much smaller scale by some mechanical stresses and random fiber birefringence (known also as polarization-mode dispersion). The solutions based on the intensity modulation, although offering slightly worse stability of frequency transfer than possible while transmitting the coherent optical carrier, are more economical and robust, and are compatible with current commercial clocks (e.g., cesium clocks and fountains or hydrogen masers) which generate time and frequency signals in the electrical domain. Thus, these systems seem to be a realistic solution that may be attractive for relatively wide range of the most demanding users requiring access to high-quality frequency and time signals. However, a serious problem is that most of the solutions presented thus far are just point-to-point systems, without the possibility of point-to-multipoint dis-

Manuscript received December 7, 2012; accepted May 18, 2013. The work presented in the paper was funded by the Polish National Science Centre under the decision DEC-2011/03/B/ST7/01833. The authors are with the AGH University of Science and Technology, Krakow, Poland (e-mail: [email protected]). DOI http://dx.doi.org/10.1109/TUFFC.2013.2766

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semination. The idea of multiple access was proposed for the first time in [12] for the dissemination of the optical carrier, and a similar idea was adopted in the multipleaccess system for dissemination of a microwave frequency described in [13]. In recent years, our group presented a new idea of frequency (10 MHz) and time [1 pulse/s (PPS)] dissemination based on an active electronic stabilization of the propagation delay of a fiber optic link [9] and dedicated bidirectional optical amplifiers for its long-haul extension [14], [15]. Reported experiments that we performed, both in the laboratory and in the field, showed the Allan deviation (ADEV) between 1 × 10−17 and 5 × 10−17 and the time deviation (TDEV) below 1 ps for one-day averaging. Since January 2012, one of our systems has been continuously operated between two Polish UTC laboratories: the Time and Frequency Laboratory of the Central Office of Measures (GUM, located in Warsaw), and the Astrogeodynamic Observatory of the Space Research Center of the Polish Academy of Science (AOS, located in Borowiec, near Poznan), over a distance of 420 km [16]. In this paper, we report on the multipoint frequency dissemination in our transfer system with the stabilization of the propagation delay. The frequency access node we developed and tested may be configured as a simple tap, making the frequency signal available at some point along the main fiber (hereafter referred to as a trunk fiber) which connects the local and remote ends of the link. It is also possible to configure it as a side branch that allows dissemination of the frequency signal in a tree-like architecture. II. Accessing the Frequency Signal in an Actively Stabilized Transfer Link The general concept of our system is depicted in Fig. 1(a). It comprises the local and remote modules, responsible for stabilization of the propagation delay between the input and the remote output of the fiber link. The frequency access node, located at any point along the trunk fiber, taps some portions of the optical signals propagating in both the forward and backward directions and is responsible for stabilizing the propagation delay from the input of the system to the access node. The details of the access node are shown in Fig. 1(b); its operation, as well as possible configurations of the frequency dissemination network will be discussed further in the text.

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Fig. 1. (a) Block diagram of the actively stabilized frequency transfer system, and (b) details of the tapping module. The nodes marked in red (A, B, and N) correspond to the nodes of the timing model shown in Fig. 2.

A. Stabilization of the Propagation Delay in the Trunk Path Ignoring the access node for a moment, we will recall the core idea of stabilizing the propagation delay of the main (trunk) link, illustrated in Fig. 1(a). In the local module, the incoming frequency signal goes through the electronic variable delay line, and then reaches the electro/optic (E/O) converter (the intensity-modulated laser). Next, the signal passes the optical circulator and propagates in the forward direction down the trunk optical fiber to the remote location. There, it is back-converted to the electrical form and feed to the output, but also turned back to the trunk fiber in the backward direction, using a laser with a slightly different wavelength. (The circulators act as diplexers, allowing bidirectional transmission over the same fiber.) The signal coming back to the local module is converted to the electrical form, enters the second variable delay line, and reaches the phase detector, whose second input is connected to the input port of the module. The phase detector senses the phase difference between the input and feedback signals, and cancels it by varying the delays of both the forward and backward delay lines

Fig. 2. Timing model of the system. 

by the same amount. Thus, the round-trip delay in the system is kept constant, unaffected by the fluctuations of the fiber delay. [Such a closed-loop system with variable delays and a phase detector is often called a delay-locked loop (DLL.)] Using the simplified timing model (i.e., ignoring all constant delays introduced by the components of the system that do not affect the further reasoning) presented in Fig. 2, one may write the expression reflecting the stabilization of the round-trip delay: τ DF + τ L → R + τ R → L + τ DB = const., (1a)

and thus

∆τ DF + ∆τ L → R + ∆τ R → L + ∆τ DB = 0, (1b)

where ΔτXX denotes the changes of the corresponding propagation delays, provoked by variations of the temperature or other environmental factors influencing the fiber. Provided the full symmetry of the forward and backward directions, i.e.,

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∆τ DF = ∆τ DB and ∆τ L → R = ∆τ R → L, (2)

one can obtain

∆τ DF + ∆τ L → R = 0. (3)

This means that also the input-output delay is kept constant, and thus the stability of the frequency transfer is not affected by the variations of the fiber propagation delay. The detailed discussion of potential impairments of such a system, caused by hardware factors and phenomena occurring in the fiber, is presented in [9] and [17]. Here, we recall that many experiments have shown that our system can compensate the fluctuations of the fiber delay within the range of about 100 ns, with the residual instability of 5 to 20 ps (depending on the particular fiber link). B. Frequency Access Module Now let us focus our attention on the access module shown in Fig. 1(b). The tap is realized by inserting the 2 × 2 optical directional coupler into the trunk fiber, which extracts some part of the forward-traveling optical signal into the point A, and similarly a fraction of the backwardpropagating signal into the point B. Analyzing the delay fluctuations present at points A and B of the access node, one may write

∆τ A = ∆τ DF + ∆τ L → A, (4)

and

∆τ B = ∆τ DF + ∆τ L → R + ∆τ R → B. (5)

Using (3), and noting that ΔτL→A + ΔτR→B = ΔτL→R, we may finally arrive at

∆τ A = −∆τ B, (6)

which means that the fluctuations of the signal delay (phase) at points A and B are exactly opposite. This observation suggests that a circuit producing a signal with the phase being a mean value of the phases present at points A and B gives, in fact, a signal with a constant, stable phase. For this purpose, we proposed the solution depicted in Fig. 1(b). The cascade of two simultaneously controlled delay lines is driven from the point A. The total delay of both lines is adjusted by the phase detector in such a way that the phase of the delayed signal is matched to that occurring at the point B, which may be reflected by the following equation for the delay differences:

∆τ A + ∆τ D1 + ∆τ D2 = ∆τ B. (7)

Provided that both delay lines used in the access module are matched, i.e., ΔτD1 = ΔτD2, and exploiting (6), one can obtain the delay variations at the point N:



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∆τ N = ∆τ A + ∆τ D1 = 0. (8)

This proves that the delay between the system input and the access point N is also kept constant, which constitutes the stabilized tap at some point of the trunk line. C. Side Branch Configuration This concept may be extended in the way presented in Fig. 3, where the tapping of the trunk link forms a side branch. To prove the stabilization feature in such a configuration, one may modify (7) by accounting for the variations of the propagation delay of the branch fiber in the forward (ΔτBF) and backward (ΔτBB) directions:

∆τ A + ∆τ D1 + ∆τ BF + ∆τ BB + ∆τ D2 = ∆τ B. (9)

Exploiting the symmetry conditions in the form ΔτD1 = ΔτD2 and ΔτBF = ΔτBB, and using (6), it may be calculated that the delay from the system input to the branch output SB (see Fig. 3) is kept constant; i.e.,

∆τ SB = ∆τ A + ∆τ D1 + ∆τ BF = 0. (10)

III. Point-to-Multipoint Frequency Dissemination Architecture Exploiting the basic idea of the delay stabilization, and implementing the concepts presented herein, a flexible architecture of the frequency distribution network may be arranged. In the case of a link with a high power budget, tapping of the signal from the trunk fiber may be realized simply by means of the 2 × 2 fiber directional coupler [see Fig. 4(a)]. The coupling ratio r determines the amount of the optical power split from the trunk fiber and should be chosen in such a way as to assure a sufficient optical power in the O/E converters in both the main system and the access module. In a network displaying high optical attenuation, it may be impossible to use such simple configuration, and thus the tapping should be combined with the bidirectional optical amplification. One possible arrangement is presented in Fig. 4(b), in which two directional couplers are placed symmetrically at either side of the bidirectional optical amplifier to tap a strong, amplified signal. The overall architecture of a point-to-multipoint frequency dissemination network exploiting our concepts is shown in Fig. 5. The main trunk link may be accessed and (if necessary) amplified several times. The access node in the main trunk may locally produce the electrical output [basic access module, as shown in Fig. 1(b)], or may form the optical side-branch and be terminated in a remote location (as shown in Fig. 3). The extension of the network may be done by adding new access nodes in the main trunk, or by using any of the outputs to feed the “slave” segment of the network, as illustrated in Fig. 5.

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Fig. 4. (a) Simple tapping by use of a directional coupler and (b) tapping combined with amplification. 

Fig. 3. Tapping with a side-branch fiber. 

Finally, we should comment on the expected quality of the frequency distribution in the point-to-multipoint configuration, relative to the basic, point-to-point configuration. The impact of the tapping nodes on the trunk link is unnoticeable as long as the optical power entering both O/E converters of the main link is held at a sufficient level. This is because the tapping does not affect the fundamental symmetry of the forward and backward light propagation in the main fiber link, and does not introduce any active devices into the main signal path. Accessing the trunk fiber also does not influence the propagation of Rayleigh backscattering and amplified spontaneous emission (ASE). The frequency signal outputting the access node is, in general, affected by the same degradation factors as are present in the trunk path and, in addition, by its own electronic and opto-electronic modules. Taking a reason-

Fig. 5. Example of point-to-multipoint dissemination architecture.

able assumption that our electronic modules have a dominant impact on an imperfection of the frequency transfer, and that the access node uses an additional set of the electronic modules similar to that used in the basic link, the stability degradation factor of about 2 may be roughly expected at the access node output (with respect to the trunk-link output). In a cascaded architecture with slave segments, the expected stability degradation factor of the frequency transfer is about n, where n is the effective number of the modules with the electronic delay lines necessary to stabilize a particular output (focusing, e.g., on the “slave” segment of the network shown in Fig. 5, n = 3 for its remote module and n = 4 for its branch output module). However, some deterministic factors, as e.g., the mismatch between the delay lines in the local or access modules, may be correlated, so may either increase or decrease the phase fluctuations at some particular output. IV. Measurement Results An experimental setup we used to check the performance of the proposed solutions is shown in Fig. 6. The optical path consisted of Kraków–Skawina–Kraków field-

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Fig. 6. (a) The experimental setup and (b) route of field-deployed looped telecommunication cable used in the experiment. 

deployed fiber loop being a part of real telecom network (62 km, attenuation 19 dB), the single-path bidirectional optical amplifier (SPBA) [14], [15] (14 dB gain), and two spans (49 km, 9.5 dB each) of the fibers spooled in the thermal chamber, with the second SPBA (10 dB gain) inserted between them. The access node was placed after the first bidirectional amplifier. Our intention when planning the experiment was to avoid a possible advantage of any kind of the symmetry in the fiber path on either side of the access node. Thus, we chose fibers with substantially different lengths and attenuations, undergoing different environmental influence. The underground cable from the real network was subjected to slow temperature variations, whereas the spools kept in the thermal chamber were exposed to the diurnal cycles with ΔT ≈ 2.5°C. The stability of the frequency transfer was determined by observing simultaneously the phases of both the remote output and the access node output, referenced to the 10 MHz signal from an oven-controlled crystal oscillator (OCXO) feeding the local module. The measurement was performed using a high-speed, real-time digital oscilloscope (DSO81004A, Agilent Technologies Inc., Santa Clara, CA) with 10 GHz

bandwidth, 40 Gsample/s sampling rate, and 0.8 ps jitter floor. The signals from the OCXO and both the remote and access modules were connected to the oscilloscope using short electrical cables. We gathered all this equipment, together with the local module, in the same limited-access laboratory with the temperature stable within ±0.5°C. The SPBAs were placed in other locations where the temperature was not controlled. The data were collected for each 100 s and the positions of the slopes were determined based on the mean value of registered histograms. The experiment lasted from October 20th to November 1st, 2012. The variations of the delay of the fiber link, resolved from the registered fluctuations of the voltage controlling the delay lines in the local module, are shown in Fig. 7. Visible diurnal delay fluctuations of about 5 ns may be ascribed to the spooled fibers located in the thermal chamber, whereas the slow trend started around the middle of the measurement period was caused by a rapid drop of the outdoor temperature and the first snowfall after October 27th, affecting the field-deployed Kraków–Skawina– Kraków fiber.

Fig. 7. Fiber delay fluctuations during the experiment period. 

Fig. 8. Delay fluctuations at the remote output and the access node (plots are separated vertically by 10 ps for better readability). 

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Fig. 9. (a) Allan deviation and (b) time deviation for remote and access node outputs. In both graphs, a dashed line shows the noise floor resulting from the oscilloscope noise. 

The fluctuations of the phase of the 10 MHz frequency signal observed at the remote output and at the access node are presented in Fig. 8. The peak-to-peak fluctuations at the remote output were about 8 ps, whereas at the access node, the value of about 13 ps was observed. Comparing these plots to the delay fluctuations of the fiber (Fig. 7), one may conclude that because of the active stabilization of the propagation delay, the stability of the frequency transfer was improved by more than three orders of magnitude at both outputs compared with an unstabilized link. Some correlation visible in Fig. 8 may be caused by the mismatch between the forward and backward delay lines that is responsible for some residual transfer of the fluctuations of the propagation delay of the fiber (shown in Fig. 7) to both the remote and access module outputs. The plots of the overlapping Allan deviation (ADEV) are shown in Fig. 9(a). The curves for both the remote and access node outputs decrease monotonically with a slope close to τ−1, dropping to about 2 × 10−17 at the remote output and to about 3 × 10−17 at the access node for the averaging time of 105 s. For the same averaging time, the time deviation (TDEV) shown in Fig. 9(b) is less than 0.5 ps for the remote output and less than 2 ps for the access node. This huge improvement compared with the ADEV and TDEV calculated for an open-loop link confirms a proper operation of both the trunk stabilization system and the access node. Analyzing the experimental results presented here, we may observe that the frequency transfer quality measured at the main output is very similar to the results reported for various experiments in our previous papers [9], [11], [14], so the tapping indeed does not deteriorate the stability of the frequency transfer in the trunk dissemination system. As expected, the tapped output shows only slightly lower transfer quality compared to the main output. Determined value of the ADEV proves that this system is capable of disseminating the frequency signals generated by current state-of-the-art cesium clocks, hydrogen masers, or even cesium fountains.

V. Conclusions In the paper, we presented the concept of tapping the signal from a frequency dissemination system with active stabilization of the propagation delay. This gives the possibility of accessing the stable frequency signal at practically an unlimited number of midpoints located along the trunk fiber link. Additionally, the tapping allows the building of side branches conveying the frequency signal to locations lying outside the trunk fiber path. In this way, the point-to-multipoint dissemination network may be built with an economical usage of the optical fibers and with the possibility to successively extend the reach of the network. Because the access nodes use the same crucial electronic blocks as the basic link, a modular and reconfigurable organization of the electronic equipment is possible. In our future work, we are going to incorporate time transfer to the multipoint distribution concept presented herein, similarly to the way it was done in our basic, pointto-point system [11]. Because we transmit the time signal in the form of some modulation of the frequency signal, we expect stability of the delay of the time signal very similar to that obtained for the frequency signal. However, the challenging issue will be to develop an accurate and convenient method for an absolute calibration of the delay of the time signal to the access node. References [1] O. Terra, G. Grosche, K. Predehl, R. Holzwarth, T. Legero, U. Sterr, B. Lipphardt, and H. Schnatz, “Phase-coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link,” Appl. Phys. B, vol. 97, no. 3, pp. 541–551, 2009. [2] K. Predehl, G. Grosche, S. Raupach, S. Droste, O. Terra, J. Alnis, T. Legero, T. Hänsch, T. Udem, R. Holzwarth, and H. Schnatz, “A 920-kilometer optical fiber link for frequency metrology at the 19th decimal place,” Science, vol. 336, no. 6080, pp. 441–444, 2012. [3] O. Lopez, A. Kanj, P.E. Pottie, D. Rovera, J. Achkar, Ch. Chardonnet, A. Amy-Klein, and G. Santarelli, “Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network,” Appl. Phys. B, vol. 110, no. 1, pp. 3–6, 2009.

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[4] G. Marra, H. S. Margolis, and D. J. Richardson, “Dissemination of an optical frequency comb over fiber with 3×10−18 fractional accuracy,” Opt. Express, vol. 20, no. 2, pp. 1775–1782, 2012. [5] K. Holman, D. Jones, D. Hudson, and J. Ye, “Precise frequency transfer through a fiber network by use of 1.5-μm mode-locked source,” Opt. Lett., vol. 29, no. 13, pp. 1554–1556, 2004. [6] M. Fujieda, M. Kumagi, and S. Nagano, “Coherent microwave transfer over 204-km telecom fiber link by a cascaded system,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 57, no. 1, pp. 168– 174, 2010. [7] O. Lopez, A. Amy-Klein, M. Lours, Ch. Chardonnet, and G. Santarelli, “High-resolution microwave frequency dissemination on an 86-km urban optical link,” Appl. Phys. B, vol. 98, no. 4, pp. 723–727, 2010. [8] M. Amemiya, M. Imae, Y. Fujii, T. Suzuyama, F. Hong, and M. Takamoto, “Precise frequency comparison system using bidirectional optical amplifiers,” IEEE Trans. Instrum. Meas., vol. 59, no. 3, pp. 632–640, Mar. 2010. [9] Ł. Śliwczyński, P. Krehlik, Ł. Buczek, and M. Lipiński, “Active propagation delay stabilization for fiber optic frequency distribution using controlled electronic delay lines,” IEEE Trans. Instrum. Meas., vol. 60, no. 4, pp. 1480–1488, 2011. [10] M. T. L. Hsu, Y. He, D. A. Shaddock, R. B. Warrington, and M. B. Gray, “All-digital radio-frequency signal distribution via optical fibers,” IEEE Photon. Technol. Lett., vol. 24, no. 12, pp. 1015–1017, 2012. [11] P. Krehlik, Ł. Śliwczyński, Ł. Buczek, and M. Lipiński, “Fiber-optic joint time and frequency transfer with active stabilization of the propagation delay,” IEEE Trans. Instrum. Meas., vol. 61, no. 10, pp. 2844–2851, 2012. [12] G. Grosche, “Verfahren zum Bereitstellen einer Referenz-Frequenz”, DPMA Patent application DE 10 2008 062 139 A1, Jun. 24, 2010. [13] C. Gao, B. Wang, W. L. Chen, Y. Bai, J. Miao, X. Zhu, T. C. Li, and L. J. Wang, “Fiber-based multiple-access ultrastable frequency dissemination,” Opt. Lett., vol. 37, no. 22, pp. 4690–4692, 2012. [14] Ł. Śliwczyński, P. Krehlik, Ł. Buczek, and M. Lipiński, “Frequency transfer in electronically stabilized fiber optic link exploiting bidirectional optical amplifiers,” IEEE Trans. Instrum. Meas., vol. 61, no. 9, pp. 2573–2580, 2012. [15] Ł. Śliwczyński and J. Kołodziej, “Bidirectional optical amplification in long-distance two-way fiber optic time and frequency transfer systems,” IEEE Trans. Instrum. Meas., vol. 62, no. 1, pp. 253–262, 2013. [16] Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via stabilized fiber optic link at the distance of 420 km”, Metrologia, vol. 50, no. 2, pp. 133–145, 2013. [17] Ł. Śliwczyński, P. Krehlik, and M. Lipiński, “Optical fibers in time and frequency transfer,” Meas. Sci. Technol., vol. 21, no. 7, art. no. 075302, 2010.

Przemysław Krehlik received his M.Sc. and Ph.D. degrees in electronics from the AGH University of Science and Technology in Kraków, Poland, in 1988 and 1998, respectively. Since 1988, he has worked in the Fiber Optic Transmission Group in the Institute of Electronics, AGH. His R&D activity is focused on high-speed electronic circuits, direct modulation of semiconductor lasers, application specific fiber-optic systems, and optoelectronic measurement techniques.

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Łukasz Śliwczyński was born in Rajcza, Poland, in 1969. He received M.Sc. and Ph.D. degrees from the AGH University of Science and Technology in Kraków, Poland, in 1993 and 2001, respectively. He worked on high-speed transmitters and receivers for digital fiber optic transmission systems, especially operating with unbalanced data streams. He also investigated 10 GB/s transmitters with directly modulated lasers using optical filtering to mitigate distortions caused by chromatic dispersion of the fiber. His interest now includes developing precise time and frequency transfer systems based on optical fibers.

Łukasz Buczek was born in Kraków, Poland, in 1984. He received the M.Sc. degree from the AGH University of Science and Technology in Kraków, Poland, in 2009. In 2009, he joined the Fiber Optic Transmission Group at the AGH University of Science and Technology, where he is currently working toward a Ph.D. degree. His main interest is in precise stabilization of the wavelength of semiconductor laser sources.

Marcin Lipiński was born in Kraków, Poland, in 1947. He received his M.S. degree in electronics in 1971 from Wrocław Technical University and joined the R&D Department of the POLON Nuclear Equipment Plant, where he worked as project manager on many CAMAC system products. In 1977, he moved to the Institute of Electronics at the AGH University of Science and Technology, where he received his Ph.D. degree in 1984. He has been engaged in several projects concerned with fiber-optic transmission applied in non-telecommunication fields, such as military or contribution TV. He is currently managing the Fiber Optic Transmission Group, and his present interests include precise fiber-optic reference time and frequency transfer.

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, ...
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