Bit-rate-variable and order-switchable opticalmultiplexing of high-speed

pseudorandom bit sequence using optical delaysXiaoxia Wu,1,* Jian Wang,1 Omer F. Yilmaz,1 Scott R. Nuccio,1 Antonella Bogoni,1,2 and Alan E. Willner11Department of Electrical Engineering—Systems, University of Southern California, Los Angeles, California 90089, USA

2Consorzio Nazionale Interuniversitario per le Telecomunicazioni (CNIT), Pisa, Italy*Corresponding author: xiaoxia@usc.edu

Received April 23, 2010; revised August 9, 2010; accepted August 12, 2010;posted August 20, 2010 (Doc. ID 127455); published September 3, 2010

We experimentally demonstrate high-speed optical pseudorandom bit sequence (PRBS) multiplexing with coarseand fine bit-rate tuning capability and a switchable order using optical delays. Data multiplexing of 80 Gbit=s and160 Gbit=s is shown, each with a tunable rate using a conversion/dispersion-based continuously tunable opticaldelay and tunable PRBS order with large switchable fiber delays. A 7% bit-rate tunability, i.e., 80–85:6 Gbit=sand 160–171:2 Gbit=s, is shown for both 27 − 1 and 215 − 1 PRBS. The rf spectra before and after multiplexing aremeasured in each case and show a suppression ratio of >30 dB, exhibiting the expected PRBS spectral character-istics. © 2010 Optical Society of AmericaOCIS codes: 060.2330, 060.7140.

High-data-rate communication channels are gainingmuch interest, as evidenced by the planned issuing ofa 100 Gbit=s Ethernet IEEE standard. A key instrumentto probe the performance of such communication sys-tems is a pseudorandom bit sequence (PRBS) generator[1]. Desirable features of such a PRBS generator includethe ability to (i) change the order of the PRBS, (ii) varythe data rate between different base data rates (i.e.,40, 80, and 160 Gbit=s), and (iii) tune the bit ratearound each data rate to accommodate forward-error-correction (FEC) overhead up to several percent (i.e.,40 to 42:7 Gbit=s, 80 to 85:6 Gbit=s, and 160 to171:2 Gbit=s) [2].In general, true PRBS data can be generated by time

multiplexing several lower-rate PRBS channels into a sin-gle higher-rate channel. To ensure a correct pseudoran-dom sequence, the lower-rate channels must beaccurately delayed by required fractions of the PRBSword length. Such accurate delays and multiplexing ofparallel streams represent a significant part of thecomplexity of high-speed PRBS data generation in theelectronic domain, thus making >100 Gbit=s PRBS gen-eration difficult to achieve. Previous research on opticalPRBS generation has included the use of optical shift reg-isters to generate a PRBS at 1 Gbit=s [3]. Another ap-proach reported was to use an optical OR gate that iscomposed of two nonlinear-fiber-based Sagnac inter-ferometers to obtain quadrupling of 12:5 Gbit=s to a50 Gbit=s PRBS [4].Traditionally, optical PRBS multiplexing is realized

by the split-delay-and-combine approach, in whichthe optical delay is either fixed or motorized. Recently,conversion/dispersion-based tunable optical delays haveattracted much attention because of their large achiev-able delay tuning range, continuous tunability, fine-tuning resolution, modulation format transparency, andthe potential for integration [5–8]. Although such delayshave been achieved, only a few demonstrations havebeen reported on using widely tunable delays to achievehigh-rate PRBS multiplexing of varying rate or PRBS

order. In [9], we proposed and demonstrated bit-rate-tunable, all-optical 27 − 1 PRBS multiplexing from 20 to40 Gbit=s, using a conversion/dispersion-based tunableoptical delay element. The capability of going to a higherbit rate was limited by the achievable tunable delay formultiple signals.

In this Letter, we experimentally demonstrate high-speed optical multiplexing with a coarse and fine bit-ratetuning capability and switchable order up to 171:2 Gbit=sfor a 215 − 1 PRBS [10]. By using large switchable fiberdelays and a conversion/dispersion-based continuouslytunable optical delay, we switch the order between 7and 15, change the PRBS generation between two mainbit rates (80 and 160 Gbit=s) and tune around a given bitrate to accommodate up to a 7% FEC overhead, i.e., ratefine-tuning from 80 to 85:6 Gbit=s and from 160to 171:2 Gbit=s.

Shown in Fig. 1 is a conceptual diagram of the PRBSmultiplexing scheme using large switchable fiber delaysfor order switching and a small conversion/dispersion-based continuously tunable optical delay for rate fine-tuning. A short-pulse return-to-zero optical PRBS signalwith tunable rate, e.g., 40 to 42:8 Gbit=s, is split into two

Fig. 1. (Color online) Concept of using a large switchableoptical delay for PRBS order tuning and a conversion/dispersion-based continuously tunable optical delay for PRBSrate fine-tuning.

3042 OPTICS LETTERS / Vol. 35, No. 18 / September 15, 2010

0146-9592/10/183042-03$15.00/0 © 2010 Optical Society of America

copies by a passive splitter. One path has a switchablefiber delay for order switching, which consists of aswitchable bank of large fixed half-word delays, e.g.,half-word of 27 − 1 and 215 − 1 PRBS at 40 Gbit=s. Theother path has a conversion/dispersion-based tunable op-tical delay for rate fine-tuning of the multiplexed high-rate PRBS, e.g., from 80 to 85:6 Gbit=s. In this manner,the appropriate order can be selected and the bit ratecan be fine-tuned. Consequently, a true high-rate PRBSdata stream with a tunable order and rate is obtained,given the input PRBS rate is tuned accordingly.The experimental setup is shown in Fig. 2. At the trans-

mitter, a mode-locked laser at 1550 nm with a repetitionrate of 40–42:8 GHz and a pulse width of ∼1:5 ps is used.Data modulation is applied using a Mach–Zehnder mod-ulator (MZM) driven by a 215 − 1 or 27 − 1 PRBS at theappropriate rate. For the 80 Gbit=s PRBS multiplexingcase, the 40 Gbit=s signal is split into two paths, oneof which passes through the conversion/dispersion-based tunable delay and the other of which passesthrough a switchable fiber delay for order switching.For 160 Gbit=s PRBS multiplexing, the 40 Gbit=s signalfirst passes through a self-built tunable optical multiplex-er before splitting, as shown in Fig. 2. In the path with theconversion/dispersion-based tunable delay, the signal isconverted via four-wave mixing (FWM) in a 100 m pieceof highly nonlinear fiber (HNLF). The HNLF used has anonlinear coefficient (γ) of ∼25 W−1 km−1, a zero-disper-sion wavelength (ZDW) of ∼1558:2 nm, and a dispersionslope (S) of ∼0:026 ps=nm2=km. After being filtered out,the converted signal is then sent through ∼13:6 km ofdispersion compensation fiber [(DCF) −4000 ps=nm] toimpose a wavelength-dependent delay for 80 Gbit=sPRBS multiplexing (∼6:8 km for 160 Gbit=s). The de-layed signal is converted back to the original wavelengthusing FWM in a 330 m HNLF, with γ ∼ 25 W−1 km−1,ZDW ∼1562:2 nm, and S ∼ 0:026 ps=nm2=km. 1 nmbandpass filters (BPFs) are used for 80 Gbit=s PRBSmul-tiplexing and are replaced by 2 nm BPFs for the160 Gbit=s multiplexing. A tunable dispersion compen-sating module (TDCM) with a �30 ps=nm tuning rangeand 1 ps=nm tuning resolution is used to minimize theresidual dispersion. The output of the TDCM is combinedwith the other path to generate an 80 or 160 Gbit=s PRBS,which is then demultiplexed to 40 Gbit=s for BERmeasurements.Shown in Fig. 3(a) is the relative delay for 80 Gbit=s

PRBS multiplexing as a function of the converted wave-

length. We first consider the conversion to 1558:5 nm asa reference and adjust the path with the switchable fiberdelay to give a PRBS at 80 Gbit=s, e.g., 215 − 1 PRBS. Therequired tunable delays to add 3% (i.e., 82:4 Gbit=s) and7% FEC (i.e., 85:6 Gbit=s) are ∼12 and ∼26:8 ns, respec-tively. This corresponds to ∼3 and ∼6:7 nm shifts from1558:5 nm in the conversion/dispersion-based delay.Shown in Fig. 3(b) are the optical spectra after boththe first and second wavelength conversion stages for80 Gbit=s 215 − 1 PRBS multiplexing. The signal isconverted to ∼1558:5 nm by placing the pump at∼1554:25 nm in the first stage. After passing throughthe DCF and by sharing the same pump, the signal isphase conjugated and converted back to the originalwavelength in the second stage. Similarly, shown in Figs.3(c) and 3(d) are the spectra for 82.4 and 85:6 Gbit=s215 − 1 PRBS multiplexing, respectively. By switchingthe fiber delay and tuning the pump wavelength, 27 − 1PRBS multiplexing is obtained accordingly.

The 80 Gbit=s PRBS is demultiplexed to 40 Gbit=s forperformance evaluation. Shown in Fig. 4 are the BERcurves of the demultiplexed 80 Gbit=s 215 − 1 PRBSand the corresponding eye diagrams. For the tributary

Fig. 2. Experimental setup for PRBS multiplexing. For 80 Gbit=s PRBS multiplexing, the switchable optical multiplexer (shown inthe dashed box) is not used: TDL, tunable delay line.

Fig. 3. (Color online) Experimental results for 80 Gbit=s 215 −1 PRBS multiplexing. (a) Relative delay versus convertedwavelength (λC) and (b)–(d) optical spectra at different con-verted wavelengths for rate tuning: (b) λC ¼ 1558:5 nm, (c)λC ¼ 1561:5 nm, and (d) λC ¼ 1565:2 nm, with λS fixed at1550 nm.

September 15, 2010 / Vol. 35, No. 18 / OPTICS LETTERS 3043

that does not pass through the conversion/dispersion de-lay element, an ∼2 dB penalty compared to 40 Gbit=sback to back is observed, mainly from the demultiplexingand the beating effect with the noise from the tributarythat passes through the tunable delay line. An∼5 dB pen-alty is observed for the tributary that passes through thetunable delay line. We attribute the extra ∼3 dB penaltyto the residual dispersion/dispersion slope caused bynonideal dispersion compensation, and the opticalsignal-to-noise-ratio (OSNR) degradations caused by cas-cading erbium-doped fiber amplifiers (EDFAs) and lim-ited wavelength conversion efficiency.Figure 5 shows the rf spectra before and after

the PRBS multiplexing for different cases of 80 Gbit=sPRBS multiplexing. Prior to multiplexing, the rf spec-trum consists of equally spaced tones separated by∼ðsymbol rateÞ=ðword lengthÞ, i.e., ð40 Gbit=sÞ=ð215 −1Þ ¼ 1:22 MHz for a 215 − 1 PRBS. After multiplexing,the tone spacing is increased by a factor of 2, i.e.,ð80 Gbit=sÞ=ð215 − 1Þ ¼ 2:44 MHz, showing that propermultiplexing is achieved. Similarly, the tone spacing isincreased to 2.52 and 2:61 MHz when multiplexing to82.4 and 85:6 Gbit=s 215 − 1 PRBS, respectively. For mul-tiplexing to 27 − 1 PRBS, the tone spacing at 80, 82.4, and85:6 Gbit=s is 0.63, 0.66, and 0:68 GHz, respectively. Notethat for proper multiplexing, the unwanted tones at oddinteger multiples of the tone spacing are suppressed by

>30 dB, but for inaccurate bit shifts, the tones increaserapidly, as shown in Fig. 5.

Similarly, Fig. 6 shows the BER curves of the160 Gbit=s 215 − 1 PRBS and corresponding eye dia-grams. For the tributaries that do not pass through theconversion/dispersion-based delay line, an ∼5 dB pen-alty is observed, while an ∼9 dB penalty is observedfor the tributaries that pass through the tunable delayline. The slightly higher penalty at 160 Gbit=s is likelydue to the fact that the tolerance to dispersion, disper-sion slope, and OSNR degradations are reduced withthe increasing of the bit rate.

This material is based on research sponsored by theUnited States Air Force Research Laboratory (USAFRL)and the Defense Advanced Research Agency (DARPA)under agreements FA8650-08-1-7820 and N00014-05-1-0053. We also thank support from the National ScienceFoundation (NSF)-funded Center for the IntegratedAccess Networks (CIAN).

References

1. S. W. Golomb, Shift Register Sequences (AegeanPark, 1982).

2. H. D. Kidorf, A. Lucero, J.-X. Cai, G. Lenner, F. W. Kerfoot,C. R. Davidson, Y. Cai, J. Li, J. Chen, M. Nissov, and A.Pilipetskii, in The 14th Annual Meeting of the IEEE Lasers

and Electro-Optics Society (LEOS) (IEEE, 2001), Vol. 2, pp.477–478.

3. A. J. Poustie, K. J. Blow, R. J. Manning, and A. E. Kelly, Opt.Commun. 159, 208 (1999).

4. C. Kouloumentas, C. Stamatiadis, P. Zakynthinos, and H.Avramopoulos, IEEE Photon. Technol. Lett. 21, 456 (2009).

5. Y. Dai, X. Chen, Y. Okawachi, A. C. Turner-Foster, M. A.Foster, M. Lipson, A. L. Gaeta, and C. Xu, Opt. Express17, 7004 (2009).

6. E. Myslivets, N. Alic, S. Moro, B. P. P. Kuo, R. M. Jopson,C. J. McKinstrie, M. Karlsson, and S. Radic, Opt. Express17, 11958 (2009).

7. X. Wu, S. Nuccio, O. F. Yilmaz, J. Wang, A. Bogoni, and A. E.Willner, Opt. Lett. 34, 3926 (2009).

8. Y. Dai, Y. Okawachi, A. C. Turner-Foster, M. Lipson, A. L.Gaeta, and C. Xu, Opt. Express 18, 333 (2010).

9. X. Wu, L. Christen, O. F. Yilmaz, S. R. Nuccio, and A. E.Willner, Opt. Lett. 33, 1518 (2008).

10. X. Wu, J. Wang, O. F. Yilmaz, S. R. Nuccio, A. Bogoni, andA. E. Willner, in European Conference on Optical Commu-

nications (ECOC) (IEEE, 2009), paper 5.3.5.Fig. 5. Radio-frequency spectra showing the successful multi-plexing to 80–85:6 Gbit=s PRBS with the orders of 15 and 7.

Fig. 6. (Color online) BER performance of the 40 Gbit=stributaries of the multiplexed 160 Gbit=s 215 − 1 PRBS.

Fig. 4. (Color online) BER performance of the 40 Gbit=stributaries of the multiplexed 80 Gbit=s 215 − 1 PRBS.

3044 OPTICS LETTERS / Vol. 35, No. 18 / September 15, 2010