JOURNAL OF TELECOMMUNICATIONS, VOLUME 16, ISSUE 1, SEPTEMBER 2012 1

WDM Transmission after All-Optical NRZ to CSRZ and RZ to CSRZ Format Conversion

Using an SOA-NOLM Mousaab M. Nahas

Abstract— This paper presents experimental results for the performance of a CSRZ format after all-optical conversion from NRZ and RZ formats using a semiconductor laser amplifier based nonlinear optical loop mirror (SOA-NOLM). The paper starts with showing successful conversion results for 4 × 10 Gbit/s WDM signals with 100 GHz spacing. The optical conversion bandwidth and limitations for the all-optical converter device is also presented. The paper then demonstrates transmission results over a 195 km fiber span for the all-optical converted CSRZ signal and its original NRZ/RZ signals. The receiver sensitivity for the converted four wavelengths is compared with the sensitivity for the original NRZ and RZ counterparts. The paper proves that the power required at the receiver for 10-9 BER is less for the converted CSRZ format in all signals. We believe that such an investigation is useful since the all-optical devices are now considered to be of key importance in current and future all-optical networks.

Index Terms— All-optical processing, nonlinear optical loop mirror, modulation format conversion, carrier-suppressed RZ modulation format, WDM transmission.

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1 INTRODUCTION

t has already been proven that the performance of the optical fiber communication system largely depends on the modulation format [1]-[6]. In fact, the best modula-

tion format is dictated by many system parameters such as system length, fiber type, dispersion management and the optical bandwidth [7]. In general, the carrier-suppressed return-to-zero (CSRZ) modulation format can deliver excellent transmission performance due to its high tolerance to nonlinear effects and chromatic dispersion compared to that of the conventional non-return-to-zero (NRZ) and return-to-zero (RZ) formats [8]. The CSRZ reduces the main carrier component of the signal, which carries most of the power in the NRZ and RZ formats but contains no useful information. This results in that the input power into the fiber is reduced, allowing the use of optical amplifiers in transmission with lower nonlinear effects. Furthermore, the CSRZ also narrows the optical spectrum, thus the signal becomes more tolerant to chro-matic dispersion with higher spectral efficiency [9]. For WDM applications, CSRZ offers better performance be-cause it is less sensitive to four-wave-mixing (FWM) caused by the WDM channels interaction [10]. However, CSRZ signal generation is more complex than the conven-tional NRZ and RZ since at least one additional modula-tor is required [11]. This modulator can be a LiNbO3 MZ modulator which is an electro-optic device. Recently, elec-tro-optic devices have been thought to be replaced by all-

optical devices which have been considered to be of key importance in all-optical networks where the signal re-mains in the optical domain all over the network without being converted to electronics [12]. This indeed can in-crease the cost effectiveness of the WDM network. As a result of this, all-optical signal processing including switching, demultiplexing, signal regeneration and for-mat conversion have received much interest in a recent research [13]-[15], where this paper will consider the all-optical format conversion from NRZ and RZ to CSRZ due to the advantages mentioned above. In particular, the paper aims to examine the transmission performance of 4 × 10 Gbit/s WDM CSRZ signals after all-optical format conversion, where single-channel results for the same device have already been presented in [16]. The CSRZ signal performance will be compared with that of the original NRZ and RZ signals, so any improvement being noticed on the converted signal behavior would encour-age the operators to accommodate all-optical modulators in place of the conventional electro-optic devices that ex-ist in the current WDM networks. 1.1 NRZ/RZ to CSRZ Format Conversion The conversion from NRZ and RZ to CSRZ is conven-tionally implemented as shown in Fig. 1 [17], where the electro-optic MZ modulator is biased at the minimum power transmission point. An electrical clock with half the data frequency is inserted into the modulator to modulate the input NRZ or RZ optical data. As a result, the optical field changes the polarity for every other pulse, i.e. the phase alternates systematically between 0

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———————————————— • M.M. Nahas is with the Electrical and Computer Engineering, Faculty of

Engineering – North Jeddah, King Abdulaziz University, KSA.

© 2012 JOT

www.journaloftelecommunications.co.uk

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and π from pulse to pulse. Therefore, the time-averaged optical field is zero, and the spectrum shows no carrier where the two characteristic frequency components will be at ±ν; since ν is half the frequency of the data [4], [17].

Fig. 1. NRZ/RZ to CSRZ conversion principle.

1.2 All-Optical Format Conversion

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(b) Fig. 2. (a) SOA-NOLM converter. (b) All-optical format conversion.

The all-optical conversion from NRZ and RZ to CSRZ was first shown in [18] using a semiconductor optical amplifier based nonlinear optical loop mirror (SOA-NOLM), which is sometimes referred to as SLALOM (semiconductor laser amplifier in a loop mirror) [16], [19]. The all-optical conversion is implemented as shown in Fig. 2, which applies to 10 Gbit/s data. The SOA-NOLM is biased at around 2π, i.e. the phase difference between

the clockwise and counter-clockwise signals is 2π. A con-trol pulse, which is a sinusoidal optical clock at half the data channel clock frequency i.e. 5 GHz, is injected to saturate the SOA hence modulates the refractive index of the SOA’s active region periodically. At the output of the loop mirror, when the clockwise and the counter clock-wise signals are recombined, their phase modulation is converted into amplitude modulation. Thus, the output data is alternating between 0 and π at adjacent bits; hence the input NRZ or RZ data signal is converted into CSRZ format. Paper [17] has also shown the conversion of a CSRZ back to the original NRZ and RZ by following the same principle. However, if the control pulse is absent, the clockwise and counter-clockwise signals will destruc-tively interfere at the output of the loop mirror. If the phase difference is adjusted from 2π to π, the interference will be constructive, thus the input NRZ or RZ signal is switched out of the SOA-NOLM without changing its modulation format [20].

2 WDM CONVERSION EXPERIMENT In this part, we investigate the feasibility of the SOA-NOLM converter in converting 4 × 10 Gbit/s WDM sig-nals from NRZ/RZ to CSRZ formats, and then explore the available optical conversion bandwidth for this device and its limitations. 2.1 Conversion Experimental Setup

Fig. 3. Experimental setup for WDM conversion.

The experimental setup for all-optical conversion is shown in Fig. 3. To generate the 4 × 10 Gbit/s WDM data signals, four CW laser sources, starting at 1554.7 nm with an even separation of 100 GHz, are applied with a 231–1 PRBS via one LiNbO3 modulator for the NRZ format, and two LiNbO3 modulators for the RZ format. The pattern generator is driven by an external clock reference of 10.7 GHz where no forward error correction is applied. The resultant 4 × 10 Gbit/s WDM data stream is amplified and launched into the converter. The SOA-NOLM com-prises of a 50/50 coupler with polarization controllers (PCs) on each arm, a 50/50 coupler on the clockwise path and an SOA being offset from the centre by a 140 ps on counter-clockwise direction using variable optical delay line on the clockwise arm. The SOA provides a peak fiber-to-fiber gain of 20 dB, and a saturation output power of 3.85 dBm when biased at 200 mA. For the control pulse, a CW DFB laser with a wavelength of 1542.3 nm is modu-lated at 5.35 GHz, delayed and coupled via a 50/50 cou-pler into the SOA-NOLM at a power of 3.2 dBm. A vari-

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able optical delay line, to adjust the control pulse arrival time relative to the data, controls the phase seen by the clockwise and counter clockwise data at the SOA. On the output of the SOA-NOLM, a tunable grating band-pass filter with a 0.24 nm 3 dB bandwidth and a 7 dB insertion loss is used to pass the data traffic signal while the 1542.3 nm control wavelength is filtered out. Two circulators are used within the system; one before the SOA-NOLM to prevent its reflected signal from interfering with the in-coming data, and one after the SOA-NOLM to prevent any reflected light from re-entering the loop mirror that can change the whole characteristics of the device. On the output of the SOA-NOLM, a WDM coupler, whose char-acteristics are shown in Fig. 4, is used to extract the 1542.3 nm control wavelength whilst allowing the WDM data to propagate along the fiber. It is seen from Fig. 4 that the control signal can be suppressed by approximately 37 dB through the WDM coupler while the whole WDM data pass without considerable attenuation.

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2.2 Conversion Results and Analysis Successful conversion results have been obtained from running the above experiment as shown in Fig. 5, which presents the spectra and corresponding eye diagrams for all WDM signals before and after conversion in both NRZ and RZ cases. It can be clearly seen that the main compo-nent of the NRZ and RZ signals is suppressed after con-version as in (b) and (d), where the difference between the main two characteristic components in the converted CSRZ is 0.08 nm (10 GHz) in all signals. The converted signals are slightly broadened compared to the original signals and that can be due to the ASE noise and some other undesired nonlinear effects such as cross-gain modulation and FWM exist in the SOA.

Having achieved a conversion for the WDM signals, it is necessary to explore the maximum bandwidth of such a converter. This enables determining the maximum num-ber of WDM channels that can be converted via this de-vice without considerable penalty. For this test, the WDM coupler is replaced by a fiber Bragg grating (FBG) with 3 dB bandwidth of ~1.1 nm and isolation of ~19.8 dB as shown in Fig. 6 to reflect the control wavelength at 1542.3 nm.

The reason for using the FBG is that the transmission bandwidth of the WDM coupler used before is a small fraction of that for the whole C-band wavelength region.

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Fig. 5. OSA Spectra (power in dBm vs wavelength in nm using 0.06nm OSA resolution bandwidth) and corresponding eye diagrams forWDM signals. (a) NRZ; (b) CSRZ converted from NRZ; (c) RZ; (d)CSRZ converted from RZ.

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Fig. 4. WDM coupler spectrum, depicting the transmitted and ex-tracted wavelengths.

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This basically allows some wavelengths to pass through which in turn disables finding the entire conversion bandwidth. Again, the reflected signal from the FBG is prevented from re-entering the SOA-NOLM by the sec-ond circulator.

Fig. 7 depicts the spectral outputs taken after the SOA-NOLM for four arbitrary wavelengths spanning the available conversion bandwidth of the converter for RZ to CSRZ conversion. The lowest and the highest wave-lengths at which the signal is fully converted to CSRZ with BER ≤10-9 are 1531.4 nm and 1562.2 nm, respec-tively. This means that the signal will be either uncon-verted fully or converted with considerable error-rate (>10-9) outside this region. In fact, this bandwidth is lim-ited by the active region of the SOA as well as the wave-length of the control pulse used to saturate the SOA in the SOA-NOLM. So even with the 19.8 dB isolation provided by the fiber Bragg grating, a peak 2 dB above the noise floor at 1542.3 nm can be clearly seen in the spectrum. This partial suppression of the switching signal reduces the conversion region by approximately 1 nm either side of the control signal resulting in a total available band-width of 28.8 nm (depicted as a lined block in Fig. 7). However, the WDM coupler which was used in place of the fiber FBG earlier can increase the suppression of the control pulse by an additional 17.2 dB albeit at the ex-pense of available bandwidth, as the 3 dB cut-off for the transmission port on the WDM is at 1551 nm. Ideally a

device with the suppression performance of the WDM coupler but with the bandwidth of the fiber Bragg grating could be used to increase the bandwidth of the SOA-NOLM converter by at least 2 nm.

3 WDM TRANSMISSION EXPERIMENT In this part, we examine the transmission performance of the all-optical converted CSRZ signal against its original NRZ and RZ counterparts over a 195 km fiber span.

Fig. 6. Spectrum of the fiber Bragg grating.

Fig. 8. Experimental setup for WDM transmission, depicting theSOA-NOLM converter and the 195 km fiber link.

Fig. 7. The spectral outputs taken after the SOA-NOLM for four arbi-trary wavelengths spanning the available conversion bandwidth of the converter for RZ to CSRZ conversion.

3.1 Transmission Experimental Setup The transmission experiment setup is shown in Fig. 8. After conversion to CSRZ, the WDM data signals are post-amplified to 4.6 dBm per channel (i.e. ~10.6 dBm in total) before being launched into 195 km fiber span. This power is relatively high and has been chosen to ensure nonlinear effect during transmission. Using this, it is pos-sible to compare the tolerance of the converted and un-converted signals towards nonlinearities. The fiber span consists of two dispersion-managed sections of SMF-DCF-SMF. The first consists of 40.7 km of SMF, 16.5 km of DCF with -1383 ps/nm and 41.8 km of SMF. The second section has 42.9 km SMF, 15.2 km DCF with -1387 ps/nm and 38 km SMF. The SMFs have 0.2 dB/km attenuation coefficient and dispersion of 17 ps/nm/km at 1550 nm. The losses are compensated using two C-band erbium-doped fiber amplifier (EDFA) repeaters in each section. The EDFA denoted A in the figure has a 30 dB maximum small signal gain and the EDFA denoted B has a 40 dB maximum small signal gain. The noise Fig. is ~5 dB for each EDFA. An ASE filter is used after the last SMF span to remove the accumulated ASE noise outside the signals band including the gain peak at 1530 nm. At the receiver, the WDM signals are pr-amplified and then demulti-plexed by a tunable band-pass grating filter with a 0.24 nm 3 dB bandwidth and a 7 dB insertion loss. The indi-vidual channels are then isolated by a 10 GHz clock re-covery unit and detected by the BERT. An optical attenu-ator is used after the band-pass filter for receiver sensitiv-ity measurements.

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3.2 Transmission Results and Analysis The main results of the transmission experiment are pre-sented in Fig. 9. It shows the BER performance versus received power after transmission over 195 km span for the four WDM channels in both NRZ and its converted CSRZ (top graph), and RZ and its converted CSRZ (bot-tom graph). It is essential to mention that, for consistency, the measurements of Fig. 9 were all performed with the loop mirror, i.e. all signals are passed through the SOA-NOLM thus they have the same environmental condition where any impairment or instability being introduced by the loop mirror is common to all measurements. There-fore, in the case of unconverted NRZ/RZ signal, the sig-nal is passed through the SOA-NOLM with absence of the control pulse and bias at π before being transmitted over the fibre span. The SOA-NOLM would switch the input NRZ or RZ signal out without changing its modulation format as explained in section 1.2.

As a result, a received power of -17.9 dBm per channel is required to achieve a BER of 10-9 for the NRZ format, whereas the converted CSRZ format required a lower power of -18.8 dBm to attain the same error-rate. For the RZ case, the received power required for 10-9 BER is -22 dBm, while for the converted CSRZ format it is -22.5 dBm. The slope of the BER-power curve for both con-

verted carrier-suppressed formats decays slower than that of their original counterparts. Therefore, at a BER of 10-5, the improvement in the required received power can increase from 0.9 dB to 1.2 dB for NRZ case and from 0.5 dB to 0.9 dB for RZ. This would imply that the receiver sensitivity improvement at 10-9 BER could be further in-creased if forward error correction (FEC) is used to cor-rect errors at 10-5 error-rate. However, the larger im-provement in the NRZ case was due to the entire change of data format, i.e. NRZ to RZ with the carrier being sup-pressed. In RZ to CSRZ case, the signal is still return-to-zero after conversion but its carrier was suppressed. This implies that in NRZ case, part of the improvement was attained before transmission just because the NRZ has become CSRZ. In fact, the receiver sensitivities at the out-put of the SOA-NOLM were measured without transmis-sion (i.e. back-to-back) for all formats using single-channel at 1555.2 nm. It was found that in the case of NRZ to CSRZ, the received power required for the con-verted CSRZ for 10-9 BER was less than that for the un-converted NRZ by 0.5 dB. In the case of RZ to CSRZ, the converted and unconverted formats had almost the same required power for 10-9 BER. This implies that the trans-mission improved the sensitivity of the CSRZ format by 0.4 dB in the case of NRZ, and 0.5 dB in the case of RZ format. However, this result would indicate that the CSRZ signal can propagate over longer fiber distance with acceptable error-rate in both NRZ and RZ cases if more fiber spans are added to the system or a recirculat-ing loop is used on the testbed [16].

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Although the impairments of the SOA-NOLM con-verter limits the performance of all formats, the results can still be useful as relative comparisons for the con-verted and unconverted signals in transmission. Future work may involve optimizing the performance of the loop mirror by using better SOA characteristics. On the other hand, as this particular system configuration pre-sented 195 km strongly dispersion-managed fiber link with ~40 km maximum accumulated dispersion and 100 GHz channel spacing; it did not present huge transmis-sion penalties for the formats studied. This can be under-stood as the majority of the nonlinear contribution comes from self-phase modulation (SPM) of the individual sig-nals. More considerable improvement would be expected

the transmission configuration is modified to demon-strate significant four-wave mixing (FWM) by using lower local dispersion, e.g. DSF, or higher inter-channel crosstalk by using lower channel spacing as in DWDM systems, which was not considered as a part of this study.

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Nevertheless, it is still necessary to ensure that the im-provement obtained above is only caused due to the sig-nal being converted to CSRZ. The other possible reason for such an improvement may be that either the con-verted or unconverted signal is being chirped somewhere within the loop mirror, while the other signal is not being affected. It also could be that the signals are chirped dif-ferently thus they have different performance after transmission. To examine this, the SOA-NOLM output signals were passed through three lengths of SMF, where the broadening of the signal was compared as shown in

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Fig. 9. BER versus received power after 195 km transmission for: (a)NRZ (Solid) and its converted CSRZ (Dashed); (b) RZ (Solid) and itsconverted CSRZ (Dashed).

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Fig. 10. The figure shows the results for RZ and its con-verted CSRZ for one selected wavelength (λ2=1555.5 nm). In fact, if one signal is positively chirped, it will broaden more rapidly than the other during propagation thus its performance is worse. In contrast, if it is negatively chirped, it will see negative dispersion at the beginning of transmission until its pre-chirp and the dispersion-induced chirp along the fiber cancel each other. This would result in that the signal starts broadening later than the un-chirped signal hence better performance is expected. Fig. 10 shows similar broadening evolution for the converted CSRZ and unconverted RZ pulse, giving that there is no improvement caused by chirp.

4 CONCLUSION This paper presented experimental results for the per-formance of a CSRZ format after all-optical conversion from NRZ and RZ formats using a semiconductor laser amplifier based nonlinear optical loop mirror (SOA-NOLM). The paper showed successful conversion results for 4 × 10 Gbit/s WDM signals with 100 GHz spacing. The optical conversion bandwidth and limitations for the all-optical converter device was explored as well. The paper then demonstrated transmission results over a 195 km fiber span for the all-optical converted CSRZ signal and its original NRZ/RZ signals. The receiver sensitivity for the converted four WDM signals was compared with the sensitivity for the original NRZ and RZ counterparts. The paper proved that the receiver requires less power in the converted CSRZ signals to satisfy 10-9 BER. More sig-nificant improvement would be expected if the system uses fibers with low local dispersion, or less channel spac-ing so more inter-channel interaction is induced. In gen-eral, the SOA-NOLM converter limits the performance of all formats in our experiments, but the results can still be used to show relative performance for the converted and unconverted signals in transmission.

ACKNOWLEDGMENT The experimental work of this paper was carried out at Aston University (UK) in collaboration with Mohammad H. Wahid and under the supervision of Prof. Keith J. Blow whom the author is thankful. The author is also

grateful to Robin Ibbotson who has made valuable con-tribution to the work of this paper.

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Mousaab M. Nahas received a BSc degree from the University of Jordan in 1999 and an MSc degree from Aston University in 2002. He then joined the Photonics Research Group of Aston University and received a PhD degree in optical fiber communications in 2007. He worked in telecommunications industry between 2007-2009. In 2009, he joined the Electrical Engineering Department in the Faculty of Engineering at Rabigh branch of King Abdulaziz University in KSA, and worked as Assistant Professor until 2011. In 2011, he moved to North Jeddah branch of King Abdulaziz University and is now Assistant Professor in Electrical and Computer Engineering. His main research interest is upgrading legacy WDM communication systems using different techniques including data patterning and modulation formats. He is also interested in line monitoring tech-niques for legacy optically amplified long-haul undersea systems.