Active Propagation Delay Stabilization for Fiber-Optic Frequency Distribution Using Controlled...

1480 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 60, NO. 4, APRIL 2011

Active Propagation Delay Stabilization forFiber-Optic Frequency Distribution

Using Controlled Electronic Delay LinesŁukasz Sliwczynski, Przemysław Krehlik, Łukasz Buczek, and Marcin Lipinski

Abstract—In this paper, we present the novel method of stabiliz-ing the propagation delay in the fiber-optic link using two matchedelectronic delay lines. For this purpose, we developed a dedicatedintegrated circuit using Austria Mikro Systeme 0.35-μm CMOSprocess. Two transmission links were tested at two independentlaboratories, operating at different distances (20 and 60 km, re-spectively) and using slightly different setups. Both links showthe Allan deviation of about 2 × 10−17 after 1-d averaging andresidual fluctuations of the propagation delay of around 12 ps peakto peak. The obtained results show that such systems are suitablefor distributing the signals from hydrogen masers or cesium clocksat the distances spanning tens of kilometers.

Index Terms—Atomic clock, fiber network, frequency control,frequency distribution, optical fibers, time transfer.

I. INTRODUCTION

SYNCHRONIZATION with some source of the referencefrequency is often necessary in many areas of human ac-

tivity. Because the most precise references are usually operatedand maintained by specialized laboratories, the problem ofpassing the signal from such reference to the remote location(also called transfer, dissemination, or distribution) is quitecommon. It is the basis of many commercial and scientificapplications required, e.g., in telecommunication, navigation,development of the atomic timescales, or performance of geo-dynamical, astronomical, and physical experiments. It alsoappears in metrological applications, where comparing theclock under test with the reference is necessary to performthe calibration. Throughout the years, varieties of techniqueswere developed for the distribution of reference timing signals,depending on the accuracy required in a particular application[1]–[28]. Often, satellite techniques are employed, either basedon the omnipresent global positioning system (GPS) [1], [2]or the so-called two-way satellite time and frequency transfer(TWSTFT) [3]–[5]. The advantage of satellite systems is thepossibility to operate even on distances spanning thousands ofkilometers. Because of easy access to the GPS, this is often the

Manuscript received May 25, 2010; revised October 7, 2010; acceptedOctober 8, 2010. Date of publication December 6, 2010; date of current versionMarch 8, 2011. The Associate Editor coordinating the review process for thispaper was Dr. George Xiao.

The authors are with the AGH University of Science and Technology,30-059 Krakow, Poland (e-mail: [email protected]; [email protected];[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2010.2090696

method of choice for performing time/frequency comparisons,even for relatively short distances of a single kilometer.

Satellite methods, however, have moderate accuracy [1]–[5] and, thus, are often unsuitable for performing scientificexperiments or comparing modern high-performance atomicsources. For such applications, the distribution of the timingsignals using optical fibers is appropriate. This area has beenextensively explored in the last years, and a number of ref-erences are available on the subject [6]–[28], together withsome review papers [29], [30]. Recently, with the advent ofmicrowave fountain clocks offering stability on the order of10−16 in 1 d [31] and optical clocks featuring even betteraccuracy [31], [32], the frequency transfer by optical fibers hasbecome a priority for modern frequency metrology with theresearch projects running in the U.S., Europe, and Japan.

Because the propagation delay introduced by the fiber is in-fluenced by the temperature [33], [34] and mechanical stressesdistributed along the link, the accuracy available in the sim-plest unidirectional applications is also rather low [7], [12],[14], [34]. Thus, some measures must be taken to upgradethe accuracy of fiber-optic links. Two approaches are com-monly applied to mitigate this problem, both based on bidi-rectional transmission of the light in the link. One methodrequires two clocks at both ends of the link (so is unsuitablefor distribution of the frequency signals). It is analogous tothe aforementioned TWSTFT and may be implemented eitherby exploiting existing telecommunication synchronous-optical-network/synchronous-digital-hierarchy infrastructure [6], [8]–[11] or by using the so-called “black fiber” or single densewavelength division multiplex (DWDM) channel [12], [19],[34]. The last two approaches allow obtaining much betteraccuracy because of greater symmetry between both directionsof transmission.

The second group of methods exploits operation in thefeedback loop to stabilize the propagation delay of the link.Such methods are well suited for both distribution of referencesignals and comparisons of independent clocks. The transmittedsignal may either be in the electrical form [13]–[19] or bebrought from some optical reference [25]–[28]. In this paper,we describe the closed-loop transmission system with stabi-lization of the propagation delay developed to distribute the5- or 10-MHz reference signal generated, e.g., by hydrogenmasers or cesium clocks. Until now, two main approaches tobuilding such systems exist. The first one is based on controlledoptical delays [13]–[17] used to compensate variations of the

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SLIWCZYNSKI et al.: PROPAGATION DELAY STABILIZATION FOR FIBER-OPTIC FREQUENCY DISTRIBUTION 1481

Fig. 1. General concept of active stabilization of propagation delay in the fiber-optic link.

Fig. 2. Timing model of the loop for active stabilization of the delay.

propagation delay of the fiber. The second one exploits theso-called phase conjugators [14], [15], [18]. Such a system,exploiting a set of mixers and frequency multipliers/dividers,induces the phase modulation to the signal driving the laser thatis exactly opposite to the fluctuations occurring in the fiber.

The proposed new scheme avoids cumbersome optical delaysand exploits a simple structure based on matched electricalvariable delay lines, being easy to implement and fabricateusing a standard CMOS process. This allows reducing the sizeof the equipment, energy consumption, and associated costscompared to previously reported solutions.

II. CONCEPT OF CLOSED-LOOP DELAY STABILIZATION

The variations of the propagation delay originating in theoptical fiber may be substantially reduced using some sort ofdelay-locked-loop architecture. The general concept of such aloop is shown in Fig. 1. At the local end, the phase of the signalfrom the reference is compared with its delayed version aftertraveling around the loop, and any delay variations are detectedand cancelled by controlling the variable delay element placedin either an optical or an electrical path. In each case, because ofthe optical circulators, the fiber link operates in both directions,and for the best performance, the highest possible symmetry(concerning delays introduced by components constituting thelink) is required.

Using the timing model displayed in Fig. 2, it may be shownthat the propagation delay τL→R between the local and remoteends is

τL→R =τRT

2+

τxF − τxB

2+

τFF − τFB

2(1)

where τxF and τxB denote the delays introduced in the forwardand backward directions by the controlled delay element (elec-trical τEF and τEB or optical τOF and τOB), respectively, τFF

and τFB are the forward and backward delays of the opticalfiber, respectively, and τRT is the round-trip delay which isstabilized by the loop at some constant value. It may be noticedfrom (1) that, if τFF = τFB and τxF = τxB, then τL→R isconstant and, thus, unaffected by the influence of temperatureor mechanical stress on the optical fiber. (It was assumed thatthe propagation delays associated with the transmit and receiveelectronics do not contribute substantially to the stability of thedelay.)

Fulfilling the aforementioned conditions forces using thesame fiber to transmit signals in both directions and operatingthe lasers in the local and remote ends with exactly the samewavelengths. Some small laser detuning is, however, requiredin practice, particularly at longer distances [34], because ithelps to reduce the noise resulting from beating of the receivedsignal with the Rayleigh backscattering generated from thelocal transmitter [35], [36]. In addition, detuning allows usingoptical filters to further reduce any noise components arisingfrom backscattering and also from reflections occurring at theoptical connectors [34].

Detuning of the lasers introduces some difference of thepropagation delay for signals traveling through the fiber inopposite directions. Together with a nonzero thermal coefficientof fiber chromatic dispersion D, this results in some depen-dence of τL→R on temperature. This, however, is only slightas, for most fibers nowadays, the value of ∂D/∂T is around8 × 10−4 ps/(nm · km · K) [37], [38]. Thus, for detuning ofaround 0.4 nm (which allows operating the system in anystandard DWDM fiber-optic systems) and for a 100-km linkundergoing seasonal temperature variations of 25 K, the prop-agation delay mismatch not greater than about 8 ps may beexpected. If necessary, this value may be even further reducedusing smaller nonstandard laser detuning [34].

Some difference between the propagation delay in the for-ward and backward directions may also be caused by the polar-ization mode dispersion (PMD) of the fiber [39]. Fortunately,for fibers nowadays, the PMD coefficient is rather low, being onthe order of 0.05–0.1 ps/

√km [40], [41], so the expected delay

mismatch should not exceed a few picoseconds even for linksspanning hundreds of kilometers. However, the phenomenonof PMD is inherently statistical, and there is no upper limitfor it in the strict sense. Compensation of the PMD, althoughpossible, is rather difficult [42]. A much simpler solution thatmay be applied in the considered case is to use the polarizationscrambler for changing the state of polarization at the output


of the laser transmitter with some kilohertz frequency [34].This way, the long-term fluctuations of the propagation delaymismatch are converted to some short-term jitter, which maybe quite easily filtered out.

Thus, by exploiting the idea of bidirectional transmissionin the fiber-optic link, it is possible to reduce the instabilityof the propagation delay from some nanoseconds/kilometerto less than 0.1 ps/km, i.e., about four orders of magnitude.The optical delay lines used to realize the stabilized links arebased either on the mechanical change of the optical length[13], [29] or the dependence of the fiber propagation delay ontemperature or mechanical stress [14]–[17]. Such delays areconsiderably large and bulky, requiring a substantial amount ofpower for control. To obtain a wide range of delay variations,usually, both methods must be exploited, which complicatesthe circuitry needed for controlling the delay lines. Moreover,applying thermal or mechanical stress on the fiber (particularlywhen tightly wound on the spool) affects its birefringence,increasing the fluctuations caused by the PMD [14], [16].

Changing bulky optical delays into compact electronic onesis desirable because, this way, the size of the entire transceiverlocated at the local end may be substantially reduced, makingthe solution cost effective and easy to operate and maintain.This, however, requires developing dedicated delay lines withclosely matched tuning characteristics, which is the subject ofthe next section.

III. ELECTRONIC CONTROLLED DELAY LINE

There are a few requirements that must be met when de-signing the electronic controlled delay line for application inthe fiber-optic system with stabilized propagation delay. Therequired range of tuning results from the temperature-inducedvariations of the propagation delay in the length of the fiberconnecting the local and remote ends of the system. Consid-ering the usual thermal coefficient of the propagation delaybeing below 40 ps/km · K [33], [34] and assuming seasonaltemperature variations of around 25 K, it stems that about1 ns of delay tuning is required per kilometer of the link.The mismatch between the forward and backward delay linesshould be possibly low, preferably not exceeding a dozen ofpicoseconds, and the control of the delay should be monotonic.The delay line should accept the digital square-wave low-voltage CMOS signals and operate with a speed in the range of5–10 MHz. Because there are no readily available componentswith desired properties, we decided to design an applicationspecific integrated circuit (ASIC) for this purpose.

There are a few possibilities to realize such delay lines[43]–[46]. Basing on the Monte Carlo simulations, we chosethe design based on a CMOS current-starved inverter loadedwith additional capacitance. In such a structure, uncertaintiesunavoidable in any IC manufacturing process appeared to de-grade the matching between both delay lines less than in othersolutions.

The diagram showing the single section of our delay line isshown in Fig. 3. The delay of this block is determined by thespeed of charging the capacitor C by the inner inverter M2–M3.The current available for this is controlled by transistors M1

Fig. 3. Single cell of the controlled delay line along with the microphotographshowing details of fabricated chip.

and M4 with gate–source voltages generated in the circuitcomposed of transistors MU and ML. Additional small-areatransistors M1A and M4A, being always on, improve the char-acteristics of the circuit at the extremes of the tuning voltageVCTRL where the main transistors M1 and M4 are turned off.Due to the relatively high gain of the inverter, such a delay celldoes not require additional buffer to form the output signal; theonly buffer that is necessary should be located at the output ofthe entire delay line.

The delay range available from the single section is on theorder of 3 ns for the VCTRL changing from 0 to 3.3 V. Thecomplete delay line for one channel is composed of 40 sections,resulting in a theoretical delay range exceeding 120 ns. Bothdelay lines were implemented on the same chip because of therequirement of close matching of the delays, with transistorsMU and ML common for the entire design.

The circuit was fabricated on the standard Austria MikroSysteme (AMS) 0.35-μm process (see the microphotograph ofthe chip shown in Fig. 3) [47]. Together with the input andoutput buffers (not shown in the microphotograph), the chipconsumes 1.6 mA at 3.3 V with 5-MHz input signals (bothdelay lines are active). The measured rise and fall times at theoutputs of the delay lines are on the order of 600 ps, whereasthe intrinsic short-term jitter is below 7 ps RMS for short delays(∼45 ns) and increases gradually to about 10 ps for longerdelays (∼160 ns).

In Fig. 4(a), the delay versus the control voltage measuredfor the fabricated chip is shown, whereas, in Fig. 4(b), themismatch between the two coupled delay lines is shown. Thesemeasurements were performed on a stand-alone chip, feedingboth delay lines with the same 5-MHz square-wave signal.The outputs of the delay lines were observed alternately usingthe single channel of the high-speed oscilloscope (HP83480mainframe with HP83485B plug-in), triggered directly from theinput signal. Reasonable values of the mismatch are observed


Fig. 4. (a) Variation of the propagation delay versus control voltage and (b) mismatch between delay lines in fabricated circuit.

Fig. 5. Configuration of the fiber-optic link used in the experiment.

for the tuning voltage ranging from 0.75 to 2 V, which givesthe delay range of around 90 ns. This is enough for stabilizingthe delay in the link several dozens of kilometers long. Thedifferential thermal coefficient of the delay was estimated tobe below 0.5 ps/K which is low enough to allow operating thechip even in a non-air-conditioned laboratory.

IV. CONFIGURATION OF THE SYSTEM

The block diagram showing the configuration of the com-plete frequency distribution system is shown in Fig. 5. Theinput to the system was a 5-MHz sinusoidal signal takenfrom the reference source. At first, the signal is convertedto the square wave by the former to match it to the delayline. Because this process introduces some jitter, it is removednext by the jitter cleaner based on a phase-locked-loop filterfeaturing the bandwidth of around 10 kHz. To perform thistask, the LMK0300 precision clock conditioner from NationalSemiconductor was used. A similar cleaner is used at the outputof the system to reduce the short-term jitter originating inthe fiber-optic receiver. The sinusoidal signal at the output ofthe system is obtained by low-pass filtering the square wavefrom the second jitter cleaner using the filter with a 5-MHzcutoff frequency. As the transmitters for both forward and back-ward directions, telecommunication-grade distributed feedback(DFB) laser diodes are used equipped with power and wave-length stabilization loops.

An important part of the entire system is the phase detectorresponsible for developing the signal controlling the delay lines.The circuit based on the charge-pump architecture featuring nodead zone was particularly designed for this purpose. The phase

detector is implemented, together with the delay lines, insidethe fabricated ASIC.

The round-trip delay introduced by the fiber substantiallyaffects the transfer function of the closed loop system (seealso [15] and [26]), turning the feedback into positive forfluctuations of the propagation delay with frequencies of around0.5 · τ−1

RT and higher. By setting appropriate parameters of theloop filter following the phase comparator, we limited thebandwidth of the system to about 0.25 · τ−1

RT to avoid possibleamplification of some noise components.

The feedback loop of the type presented in Fig. 5 shows a fewstable points where it may lock, which are determined by thecoincidence of the rising slopes of the outgoing and returningsignals at the inputs of the charge-pump phase detector. For ourdelay lines with the tuning range of around 100 ns and for a5-MHz signal, there will be one or two stable points, dependingon the length of the fiber. Without some additional circuitry, thelocking point would be chosen on random during the systemstart-up. This is undesirable because, this way, there is no anycontrol on the value of the VCTRL voltage that would developat the output of the loop filter. For proper operation of thesystem, this value must be located somewhere in the middleof the tuning characteristics (cf. Fig. 4) to ensure sufficientsafety margins for variations of environmental conditions. Tosolve this problem in the system shown in Fig. 5, the digitallycontrolled delay is included (the stepper). This device exploitsthe 50-MHz signal available at one of the outputs of the jittercleaner and divides it by 10 to get back the 5-MHz signal that isa synchronous replica of the 5-MHz reference from the maser.Halting the counter of the divider for a number of periodsis equivalent to delaying the 5-MHz signal by a multiple of


Fig. 6. Variations of the air temperature in Warsaw during the measurementperiod (May 1–18, 2010).

20 ns. Denoting the shift introduced by the stepper by τstep,the locking condition of the loop may be stated as (cf. Fig. 2)

2(τEF + τFF) + τstep = N · T (2)

where T = 200 ns is the period of the 5-MHz reference and Nis some integer number. Thus, varying the τstep at the systemstart-up, appropriate operating conditions for the delay linesmay be set independently on the length of the link.

V. EVALUATION OF THE SYSTEM PARAMETERS

For performing the tests, we developed two complete fre-quency distribution systems. One of them was installed in theLaboratory of Time and Frequency, Polish Central Office ofMeasures (GUM), Warsaw, Poland, and the second one wasevaluated at our laboratory (AGH).

In the first case, the 5-MHz sinusoidal input to the systemwas brought from the VCH-1005 active hydrogen maser, andthe instability of the output frequency relative to the inputsignal was measured using a Quartzlock A7-MX frequencycomparator with a 10-Hz measurement bandwidth. The signalwas transmitted through a 20-km-long spooled piece of anonzero dispersion shifted large effective area fiber displayingthe attenuation of 7 dB, including 4 subscription channel/angledphysical contact (SC/APC) connectors. The spool was placedoutside the building where it was directly affected by thesunlight. Through the measurement period (May 1–18, 2010),the variations of the air temperature greater than 15 K wereobserved (see Fig. 6). Basing on the usual value of the thermalcoefficient, the variations of the propagation delay of the fiberover 12 ns might be expected. Both local and remote endsof the system, along with the frequency comparator, werelocated in the air-conditioned room with limited access, wherethe temperature was constant within ±0.5 K. The p-i-n-basedreceivers featuring a 2-GHz bandwidth and directly modulatedDFB lasers (operating near 1550 nm with 0.4-nm detuning)were used.

The instability of the propagation delay measured betweenthe local and remote ends of the link is shown in Fig. 7. Thepeak–peak jitter is about 12 ps, whereas most of the plot showseven better stability, with fluctuations on the order of 5 ps(the RMS jitter is lower than 2 ps). The reason for the peak

Fig. 7. Instability of the propagation delay for the link evaluated at GUM.

Fig. 8. Plot of the overlapping ADEV for 20-km-long link evaluated at GUM.

visible in Fig. 7 is unclear; however, some explanation maybe that, at the beginning of the experiment, we occasionallyentered the room to check the operation of the equipment. Then,nobody interrupted the experiment, so the thermal conditionsstabilized, giving better stability of the result. No direct rela-tionship between the fiber temperature and the instability of thepropagation delay plots is visible (cf. Figs. 6 and 7).

The graph of the overlapping Allan deviation (ADEV) isshown in Fig. 8, together with the noise floor of the measure-ment system (this is just for the A7-MX comparator alone withbooth inputs fed with the same 5-MHz signal from the maservia the power splitter). The curve starting at τ = 50 s, computedfrom the data shown in Fig. 7 and registered between May 1 andMay 18, decreases monotonically with almost constant slopeτ−1 and no bump at τ = 43 200 s, despite the aforementionedlarge changes of the fiber temperature. After 1-d averaging, thecurve reaches the level of about 2 × 10−17. The bump in thegraph near τ = 500 s is probably caused by some interferencewith the air-conditioning system installed in the laboratory andworking with some 15-min cycle. A similar bump occurringat the curve showing the measurement noise floor seems toconfirm this conjecture.

The second curve showing the ADEV for shorter averagingtimes was measured 1 d after finishing the main experiment (onMay 19) and shows σy(τ = 1 s) ≈ 3 × 10−13. There is somesmall discrepancy between both curves, but the measurementperiods do not overlap and have substantially different duration(1 600 000 s compared to just 32 000 s). We had to divide theentire measurement into parts to be able to show the plots of


Fig. 9. Fluctuations of the propagation delay of 60-km-long link without thestabilization of the propagation delay.

propagation delay instability because the A7-MX frequencycomparator allows registering only 32 000 samples of phasedata.

In the case of the second setup that was evaluated at AGH,the 60-km-long link was used, exploiting a standard single-mode fiber (equivalent to an SMF-28 type) granted for testingby Polish Telecom operator (Telekomunikacja Polska S.A.)and as part of their standard telecommunication infrastructure.The link was composed of nine fiber spans, connected usingSC/APC connectors, with relatively large total attenuation of23 dB. The cable was placed mainly under the ground (exceptsmall parts at the crossing points), partially in the ducts andpartially buried. Both ends of the fiber were available at our lab-oratory. During the experiments, we had the access to two pairsof the fibers running in the same cable. One of them was usedfor building the stabilized link, whereas the second one enabledus to trace simultaneously the changes of the propagation delayof the fiber, shown in Fig. 9. The measurement period lastedfor 25 d, between April 23 and May 5, 2010. Even though thediurnal variations of the temperature of the fiber buried underthe ground are rather small, a strong seasonal trend in the graphis visible anyway, resulting in a change of the propagation delayof about 8 ns.

The frequency distribution system used in this part of theexperiment was slightly modified compared to the systemevaluated at GUM, mainly because of longer transmissiondistance. The receivers used were based on avalanche photo-diodes to increase the sensitivity, with the bandwidth of around2 GHz. The transmitters, operating near 1537 nm with 0.4-nmdetuning, were externally modulated using DFB laser moduleswith integrated electroabsorption modulators. This way, thedistortions of transmitted pulses resulting from the chromaticdispersion of the fiber were substantially limited.

A highly stable Rohde–Schwartz SMB100A signal genera-tor equipped with an SMB-B1 OCXO reference was used asthe source of transmitted frequency. To measure the stabilityof the propagation delay, we used the high-speed samplingoscilloscope (HP83480 mainframe with HP83485B plug-in).The trigger and input to the oscilloscope were taken from theSMB100A generator and from the remote end of the transmis-sion system, respectively. All the equipment was placed in anon-air-conditioned room, where the temperature variations ofabout 4–5 K were observed.

Fig. 10. Plot of the overlapping ADEV for the link evaluated at AGHoperating with and without the stabilization of the propagation delay.

Fig. 11. Residual fluctuations of the propagation delay of 60-km-long linkwith the stabilization.

The gathered data were used to calculate the overlappingADEV, shown in Fig. 10. In the same figure, the ADEV fora nonstabilized link is also shown, along with the noise floorof the measurement system (determined for the oscilloscopealone). The results for the stabilized link are very similar tothat obtained at GUM. The graph is monotonic with mostlyτ−1 slope. After 1-d averaging, it is possible to obtain theADEV of around 2 × 10−17. A slight ADEV increase aroundthe averaging time equal to half of the delay (τ = 43 200 s)may be explained by observing the curve representing themeasurement noise floor. It is visible that, for large values ofτ , the obtained results are close to the limits resulting fromthe equipment that we were using. This is probably caused bythe influence of the temperature variations on the measurementequipment in our laboratory as no such effect was observedin the system evaluated at GUM (see Figs. 7 and 8), wherethe laboratory was air conditioned. Moreover, the plot of thedata used for calculating the ADEV (see Fig. 11) displayssome diurnal fluctuations. Nevertheless, after 1-d averaging, theADEV for the stabilized link is over two orders of magnitudebetter compared to the nonstabilized link.

The performed experimental investigations confirmed thatthe stabilized system we built using matched variable electronicdelay lines allows reducing the fluctuations of the propagationdelay of the fiber link from some dozens of nanoseconds to therange of some picoseconds, resulting in the reduction factorof around three orders of magnitude. The obtained values ofthe ADEV are in the range of 2 × 10−17 for 1-d averaging,


i.e., at least an order of magnitude better than the ADEV ofmost commonly used frequency standards, based on cesiumsources or hydrogen masers. Thus, such links are suitable fordistributing precise frequency signals from such atomic sourcesat the distances spanning tens of kilometers.

VI. CONCLUSION

The accuracy of the frequency reference signal distributedusing the fiber-optic link has been determined by a variety offactors. The one that is dominating on the hour and day scale isthe thermal dependence of the propagation delay of the opticalfiber, limiting the fractional frequency instability to not betterthan 10−15 for 1-d averaging time for links some dozen ofkilometers long. This drawback may be substantially reducedby transmitting the signal in both directions through the fiberand building the feedback loop to stabilize the propagationdelay between the local and remote ends of the link. Thesystems designed for stabilization of the propagation delay thatare known from the literature are based on optical delay linesor phase conjugators.

In this paper, we have described a different approach basedon using matched variable electronic delay lines. Such a so-lution is very compact, consuming little power, and allowingreducing substantially the size and cost of the frequency dis-tribution system. To realize such a system, we designed andfabricated a dedicated ASIC using the AMS 0.35-μm CMOSprocess, composed of two closely matched delay lines alongwith a charge-pump-based phase comparator featuring no deadzone.

Exploiting the aforementioned architecture, we built twocomplete systems with stabilized propagation delay. One ofthese systems, operating on the 20-km-long link and influencedby large temperature variations, was brought for testing tothe Laboratory of Time and Frequency at GUM. The secondone, using a 60-km-long telecommunication fiber, was testedin our laboratory (AGH). Despite substantial differences inthe environmental conditions affecting the fibers, using a bitdifferent components at the receivers and transmitters, and ap-plying different measurement procedures, we get a comparableperformance in both cases.

The performed experiments have proven that the methodexploiting matched variable electronic delay lines is suitablefor active stabilization of the propagation delay in fiber-opticlinks. The obtained results (ADEV ≈ 2 × 10−17 at τ = 105 s)are comparable with that reported in the literature for muchmore complicated systems, exploiting either optical delay linesor phase conjugators (ADEV ≈ 1 × 10−17 at τ = 105 s) [13]–[15], [18]. Slightly better results were reported for the sta-bilized system exploiting polarization scrambling [16] or us-ing much higher frequency for the transfer (ADEV ≈ 2−3 ×10−18 at τ = 105 s) [17]. In the setup reported herein, we havenot implemented the polarization scrambling yet; however, animprovement resulting from using this technique in two-waytime/frequency transfer was also pointed out in our recentpaper [34]. Significantly better results are reported only forthe systems using an unmodulated optical carrier [20]–[27]or based on mode-locked laser sources [28]. Such solutions,

however, are unsuitable for transmitting frequency signals thatare already in the electrical form, e.g., generated by hydrogenmasers.

Increasing the reach of the system would be possible intwo ways. The most straightforward approach is to use Nsections to connect the local and remote ends. Each sectionmay use exactly the same equipment as described herein; inparticular, the matched delay lines need not to be redesigned.An additional benefit resulting from such an approach is thateach intermediate point in the chain may serve as the remoteend as well, offering the access to the transferred reference.Basing on the results presented in [18], it might be expectedthat the stability of the concatenated system would degrade as√

N . Thus, the system composed of N = 10 sections shouldhave ADEV ≈ 6−7 × 10−17 at τ = 105 s, allowing increasingthe transmission distance to some 1000 km. The other method isto use bidirectional erbium-doped fiber amplifiers (EDFAs) toregenerate optical signals attenuated in the fiber. This approach,however, is more challenging because the matched delay linesmust be redesigned and their controlling scheme optimized toaccommodate N times larger fluctuations of the delay. Addi-tionally, the round-trip delay of the system with bidirectionalEDFAs would increase, resulting in the necessity of reducingthe control bandwidth of the loop.

The parameters of the frequency distribution system that wecurrently developed are sufficient to use them for distribut-ing the frequency signals generated by most commonly usedatomic sources, like cesium clocks and hydrogen masers, atthe distances on the order of tens of kilometers. To upgradethe performance of our system, we are working on adding thepolarization scrambling in the transmitters and on increasingthe system reach using bidirectional EDFAs. We would alsolike to extend the capabilities of the system to enable the timetransfer, i.e., transmission of 1 pulse/s being the standard outputfrom atomic clocks. This way, such a system might be useful fordeveloping the atomic timescales or to perform as a “virtual”clock, delivering atomic-accuracy reference to some remotelocation.

ACKNOWLEDGMENT

The authors would like to thank Dr. A. Czubla from the Lab-oratory of Time and Frequency, Polish Central Office of Mea-sures, Warsaw, Poland, for arranging the measurements (seeFigs. 7 and 8) and for the many helpful discussions. The authorswould also like to thank T. Pawszak and P. Zetkowski fromthe Telekomunikacja Polska S.A. for their help in performingthe field trial over the 60-km-long fiber span and J. Godek,J. Kolodziej, and W. Machowski from the Application SpecificIntegrated Circuit Design Team, AGH University of Scienceand Technology, for their help in developing the prototype ofthe chip with the delay lines.

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Łukasz Sliwczynski was born in Rajcza, Poland, in 1969. He received theM.Sc. and Ph.D. degrees from the AGH University of Science and Technology,Kraków, Poland, in 1993 and 2001, respectively.

He is currently with the AGH University of Science and Technology. Hewas working on high-speed transmitters and receivers for digital fiber-optictransmission systems, particularly operating with unbalanced data streams. Healso investigated 10-Gb/s transmitters with directly modulated lasers usingoptical filtering to mitigate distortions caused by chromatic dispersion ofthe fiber. His current research interests include developing precise time andfrequency transfer systems based on optical fibers.

Przemysław Krehlik received the M.Sc. and Ph.D. degrees in electronics fromthe AGH University of Science and Technology, Kraków, Poland, in 1988 and1998, respectively.

Since 1988, he has been with the Fiber Optic Transmission Group, Instituteof Electronics, AGH University of Science and Technology. His R&D activity isfocused on high-speed electronic circuits, direct modulation of semiconductorlasers, application specific fiber-optic systems, and optoelectronic measurementtechniques.

Łukasz Buczek was born in Kraków, Poland, in 1984. He received the M.Sc.degree from the AGH University of Science and Technology, Kraków, in 2009.Since 2009, he has been working toward the Ph.D. degree in the Fiber OpticTransmission Group, AGH University of Science and Technology.

His main research interests include precise stabilization of wavelength ofsemiconductor laser sources.

Marcin Lipinski was born in Kraków, Poland, in 1947. He received the M.S.degree in electronics from Wrocław Technical University, Wrocław, Poland,in 1971 and the Ph.D. degree from the AGH University of Science andTechnology, Kraków, 1984.

He was with the R&D Department, POLON Nuclear Equipment Plant, wherehe worked as a Project Manager on many computer automated measurementand control system products. Since 1977, he has been with the Institute ofElectronics, AGH University of Science and Technology, where is currentlymanaging the Fiber Optic Transmission Group. He has been engaged in severalprojects concerned with fiber-optic transmission applied in nontelecommuni-cation fields, such as military or contribution television. His current researchinterests include precise fiber-optic reference time and frequency transfer.

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