Havstad, S. A.

Lee, S.

Pan, Z.

Petersen, Martin Nordal

Willner, A. E.

In this letter, we demonstrate a simple technique for dispersion monitoring by adding a single inband subcarrier tone to the transmitted data signal. A measurable dispersion of up to 1200 ps/nm is demonstrated in a 10-Gb/s channel using a 7-9 GHz subcarrier and the addition of the subcarrier induced a power penalty o

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Online chromatic dispersion monitoring and compensation using a single inbandsubcarrier tonePetersen, Martin Nordal; Pan, Z.; Lee, S.; Havstad, S. A.; Willner, A. E.Published in:I E E E Photonics Technology LettersLink to article, DOI:10.1109/68.992614Publication date:2002Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Petersen, M. N., Pan, Z., Lee, S., Havstad, S. A., & Willner, A. E. (2002). Online chromatic dispersion monitoringand compensation using a single inband subcarrier tone. I E E E Photonics Technology Letters, 14(4), 570-572.DOI: 10.1109/68.992614570 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002Online Chromatic Dispersion Monitoringand Compensation Using a SingleInband Subcarrier ToneM. N. Petersen, Z. Pan, S. Lee, S. A. Havstad, Member, IEEE, and A. E. Willner, Senior Member, IEEEAbstract—In this letter, we demonstrate a simple technique fordispersion monitoring by adding a single inband subcarrier tone tothe transmitted data signal. A measurable dispersion of up to 1200ps/nm is demonstrated in a 10-Gb/s channel using a 7–9 GHz sub-carrier and the addition of the subcarrier induced a power penaltyof 0.5 dB. Dynamic ranges exceeding 20 dB and resolution sensi-tivities better than 10 (ps/nm)/dB are shown. With an 8-GHz toneand a 15% modulation depth for 10-Gb/s signals, we show a mea-surement range of 975 ps/nm with a 22-dB dynamic range. We usedthe monitor output signal to achieve accurate tunable dispersioncompensation.Index Terms—Chromatic dispersion, dispersion compensation,dispersion monitoring.I. INTRODUCTIONF IBER CHROMATIC DISPERSION has long been consid-ered a main limitation in high bit rate and long-distanceoptical transmission systems. With the introduction of recon-figurable optical networking, any change in path lengths dueto switching will produce a different accumulation of disper-sion, thereby minimizing the effectiveness of fixed chromaticdispersion compensation solutions. Furthermore, future mainte-nance or scalability of an optical system may change the accu-mulated dispersion. Moreover, chromatic dispersion varies withtemperature [1], becoming a significant problem for long-dis-tance transmission of 40-Gb/s signals [2], [3]. All of these sce-narios would necessitate tunable dispersion compensation and,therefore, the ability to monitor the actual data signal while thesystem is still online. For management and monitoring purposesin a robust system, a flexible and cost-effective dispersion mon-itoring technique could prove quite beneficial.Of course, some level of dispersion monitoring can beachieved by actual electronic measurement of the eye-openingpenalty, -factor, or bit-error rate (BER). However, thesemethods require electronics that are fairly difficult to im-plement for higher data rates. Previous reported work onoptical techniques for dispersion monitoring has included: 1)measuring the phase delay between two out-of-band subcarrierManuscript received August 29, 2001; revised December 17, 2001.M. N. Petersen is with the Research Center COM, Technical University ofDenmark, 2800 Lyngby, Denmark (e-mail: mnp@com.dtu.dk).Z. Pan and A. E. Willner are with the Department of Electrical Engi-neering—Systems, University of Southern California, Los Angeles, CA90089-2565 USA.S. Lee and S. A. Havstad are with Phaethon Communications, Fremont, CA94538 USA.Publisher Item Identifier S 1041-1135(02)01668-3.tones [4]; 2) adding a phase modulation at the transmitter andmeasuring the phase-to-amplitude modulation conversion at thereceiver [5]; and 3) modulating the frequency of the transmitteddata signal and then monitoring the clock phase deviation atthe receiver [6].We demonstrate a simple technique for dispersion monitoringby adding a single inband subcarrier tone to the transmitteddata signal. A measurable dispersion of up to 1200 ps/nm isdemonstrated in a 10-Gb/s channel using a 7–9 GHz subcarrierand the addition of the subcarrier induced a power penalty of0.5 dB. Dynamic ranges exceeding 20 dB and resolution sen-sitivities better than 10 (ps/nm)/dB are shown. 10 (ps/nm)/dBmeans that a 10 ps/nm dispersion change will give rise to 1-dBpower change in the electrical monitor signal for this particularsystem. With an 8-GHz tone and a 15% modulation depthfor 10-Gb/s signals, we show a measurement range of 975ps/nm with a 22-dB dynamic range. We use the monitor outputsignal to achieve accurate tunable dispersion compensation anddemonstrate a reduction in power penalty from 2 dB to lessthan 0.5 dB. In general, this technique has a reconfigurablemeasurement range.II. OPERATION PRINCIPLEIn general, chromatic dispersion causes periodic powerfading of an RF subcarrier tone after detection by a squarelaw detector [7]. The reason is that the frequency-dependentdispersion produces a deleterious time delay between the trans-mitted sidebands, thereby causing serious RF power fading.This effect is problematic for subcarrier multiplexed systems.However, RF fading can be exploited for dispersion monitoringif a sinusoidal subcarrier signal is added at the transmitter asa tone in digital systems.Fig. 1 shows the basic principle of RF fading of a subcar-rier tone within the data band. The tone carries no data and hasa narrow spectral linewidth that is less than a few megahertz.Since the tone is within the frequency band of the data, it ex-periences the same dispersion as the data and can be used as adispersion sensor. The upper and lower side bands of the subcar-rier will initially be inphase with each other at the transmitter.However, a phase difference will accumulate during transmis-sion between the two sidebands as a result of the frequency-de-pendent fiber chromatic dispersion. The degree of phase mis-match is related to the amount of accumulated dispersion whenthe subcarrier is optoelectronically detected and the subcarrierpower fades according to the phase mismatch in a predictable1041-1135/02$17.00 © 2002 IEEEAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 10,2010 at 07:41:02 EST from IEEE Xplore.  Restrictions apply. PETERSEN et al.: ONLINE CHROMATIC DISPERSION MONITORING AND COMPENSATION 571Fig.1. Principle of RF fading used for dispersion monitoring. RF tone withindata band fades due to dispersion.Fig. 2. Setup used for dispersion monitoring and compensation using inbandtone as dispersion sensor.manner. The detected photocurrent at the tone frequency willfade according to (1)(1)where is the peak photocurrent, is the modulation depth,is the subcarrier tone frequency, is carrier wavelength,is the total accumulated dispersion, and is the speed oflight in vacuum.The dispersion monitor resolution and measurement rangeare dependent on the specific frequency of the subcarrier tone.The tone frequency determines the point of total fading, forwhich unambiguous measurement requires the range of disper-sion to be within one period of fading. Therefore, the point oftotal fading is referred to as the measurement the range. Conse-quently, by changing the tone frequency it is possible to selectthe measurable dispersion range and the sensitivity accordingly.III. EXPERIMENTAL SETUPFig. 2 shows the experimental setup used for dispersion mon-itoring. The optical source is modulated by a combined signalconsisting of 10-Gb/s data and a single frequency tone in therange of 7–9 GHz. The modulation depth of the subcarrier tonerelative to the data is 15%. The {data subcarrier} modulatedsignal is transmitted through conventional singlemode fiber ofdifferent lengths to simulate different amounts of accumulateddispersion. Some optical power is tapped off before the data re-ceiver in order to detect the subcarrier monitoring tone. Thistone is extracted after detection by using a tunable electricalbandpass filter. Note that this technique uses a very simple setupand all equipment used for the dispersion monitoring system iscommercially available without requiring any fast electronics.(a)(b)Fig. 3. (a) RF spectra of an 8-GHz tone in a 10-Gb/s transmission system atdifferent amounts of accumulated dispersion. (b) RF fading due to dispersionfor a 7- and 9-GHz tone. The solid lines represent the theoretical results and thepoints are experimental results.A nonlinearly chirped FBG, which has the periodicity thatvaries nonlinearly along the length of the fiber, is used as atunable dispersion compensator [8]. This FBG produces atime delay that varies nonlinearly with wavelength. When it isstretched uniformly, the dispersion at a specific wavelength ischanged. The FBG is controlled by the detected RF power tocompensate for the accumulated dispersion.IV. RESULTS AND DISCUSSIONIn our experiment, we transmitted the optical signal throughdifferent lengths of fiber and measured the electrical subcarriertone power for each transmission length; note that we maintaina constant optical power before the receiver. Fig. 3(a) shows theobservedelectricalpowerspectrumfordifferentaccumulateddis-persions. The total fading of the 8-GHz tone occurs at 975 ps/nm.Fig. 3(b) shows the measured and theoretical fading curves fortonefrequenciesof7and9GHz,inwhichthefadingfollowsafunction with increasing dispersion. With a 7- and 9-GHz subcar-rier, a range of 675 and 1200 ps/nm, respectively, is possible withsensitivitiesup to10(ps/nm)inaccordingto1dBofpowerfading.Based on Fig. 3(b), an important characteristic of this techniqueis that the sensitivity increases for higher dispersion values sincehigher values reside on a steeper portion of the curve.The subcarrier fading is also sensitive to differential groupdelay (DGD). However, it was experimentally verified that DGDvalues up to 10–15 ps do not influence the dispersion measure-ment. At larger values additional fading will be observed.range results in a higher sensitivity.It is possible to recon-figure the measurement range and sensitivity of the monitorby simply tuning the tone frequency. Fig. 4 shows the relation-ship between the tone frequency and the measurable dispersionrange, in which a smaller Using the high subcarrier frequencyAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 10,2010 at 07:41:02 EST from IEEE Xplore.  Restrictions apply. 572 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002Fig. 4. Theoretical curve for measurable range of chromatic dispersion.Fig. 5. Measured BERs. The inband tone is at 8.0 GHz and at 15% modulationdepth. The 3-dB bandwidth of the tone is 5 MHz.will provide higher sensitivity. However in order not to occupymore bandwidth, the frequency cannot be out of the data band.On the other hand, using the low subcarrier frequency will pro-vide a wider measurement range, but the sensitivity will not beenough for dispersion compensation. A 3-GHz subcarrier willfor example enable a measurement range of about 7000 ps/nm,but a 40-km transmission in single-mode fiber will cause lessthan 0.5-dB drop in the 3-GHz subcarrier power.We evaluated the system impairment caused by the additionof the subcarrier tone to the data at the transmitter. The higherthe modulation index of the subcarrier is, the higher the systemimpairment will be. Fig. 5 shows the BER measurementsfor the back-to-back and the 30-km transmission cases, withand without the 15% modulation depth 8-GHz subcarrier. A0.5-dB power penalty is observed due to the inclusion of themonitor tone and complete tone fading occurs at 975 ps/nm.Measurements for several tone frequencies in the range of 7 to9 GHz found no measurable difference in power penalty.By using the measured tone power as a control signal, we op-timize the system by stretching the nonlinearly chirped FBG thathas a dispersion range of 350 to 1000 ps/nm. We transmitthe signal through 60 km of fiber and accumulate 830 ps/nm ofdispersion. We use an 8.3-GHz tone corresponding to a mea-surement range of 910 ps/nm. Fig. 6(a) shows the observed eyediagrams and electrically filtered tone before and after compen-sation. In this experiment, a 22-dB rise in tone power is achieveddue to dispersion compensation. The BER measurement results(a)(b)Fig. 6. (a) Eye of data and filtered tone before and after dispersioncompensation. A 22-dB rise in tone power is observed. (b) BER curvesshowing reduction of power penalty due to dispersion compensation.are shown in Fig. 6(b). The power penalty is reduced from 2 dBto less than 0.5 dB due to the dispersion compensation.In summary, we demonstrate a simple technique for disper-sion monitoring by adding a single inband tone to the trans-mitted data. We also show tunable dispersion compensation byusing the measured tone power as the control signal to vary anonlinearly chirped FBG.REFERENCES[1] W. H. Hatton and M. Nishimura, “Temperature dependence of chromaticdispersion in single mode fibers,” J. Lightwave Technol., vol. LT-4, pp.1552–1555, 1986.[2] S. Kuwahara, A. Sano, K. Yonenaga, Y. Miyamoto, and Y. Yam-abayashi, “Adaptive dispersion equalization by detecting dispersionfluctuations using PM-AM conversion,” Electron. Lett., vol. 34, no. 20,pp. 1956–1958, 1998.[3] A. Sano, T. Kataoka, M. Tomizawa, K. Hagimoto, K. Sato, K. Wakita,and K. Kato, “Automatic dispersion equalization by monitoringextracted-clock power in a 40-Gbit/s, 200-km transmission line,” inECOC, 1996, TuD.3.5, pp. 2207–2210.[4] T. E. Dimmick, G. Rossi, and D. J. Blumenthal, “Optical dispersionmonitoring technique using double sideband subcarriers,” IEEE Photon.Technol. Lett., vol. 12, pp. 900–902, July 2000.[5] M. Tomizawa, Y. Yamabayashi, Y. Sato, and T. Kataoka, “Nonlinear in-fluence on PM-AM conversion measurement of group velocity disper-sion in optical fibers,” Electron. Lett., vol. 30, no. 1434, 1994.[6] Y. Takushima and K. Kikuchi, “In-service monitor for group-velocitydispersion of optical fiber transmission systems,” Electron. Lett., vol.37, pp. 743–745, 2001.[7] F. Devaux, Y. Sorel, and F. Kerdiles, “Simple measurement of fiber dis-persion and of chirp parameter of intensity modulated light emitter,” J.Lightwave Technol., vol. 11, pp. 1937–1940, Dec. 1993.[8] K.-M. Feng, J.-X. Cai, V. Grubsky, D. S. Starodubov, M. I. Hayee, S.Lee, X. Jiang, A. E. Willner, and J. Feinberg, “Dynamic dispersion com-pensation in a 10-Gb/s optical system using novel voltage tuned nonlin-early-chirped fiber bragg grating,” IEEE Photon. Technol. Lett., vol. 11,pp. 373–375, Dec. 1999.Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 10,2010 at 07:41:02 EST from IEEE Xplore.  Restrictions apply. 

Online chromatic dispersion monitoring and compensation using a single inband subcarrier tone

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Online chromatic dispersion monitoring and compensation using a single inband subcarrier tone

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