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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, JANUARY 2003

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Interplay of Fiber Nonlinearity and Optical Filtering in Ultradense WDM Ilya Lyubomirsky, Member, IEEE, Tiequn Qui, Jose Roman, Member, IEEE, Mahir Nayfeh, Member, IEEE, Michael Y. Frankel, Member, IEEE, and Michael G. Taylor, Member, IEEE

Abstract—This letter reports an experimental study on the interplay between fiber nonlinearity and optical filtering in ultradense wavelength-division-multiplexing (UDWDM) transmission systems. It is found that fiber nonlinearity-induced spectral distortion of 10-Gb/s signals has a profound effect on the performance of optical demultiplexing filters, becoming especially pronounced as channel spacing approaches UDWDM dimensions of 25 GHz. The characteristics of the interaction between fiber nonlinearity and optical filtering are found to be different for self-phase modulation compared to cross-phase modulation limited systems. Index Terms—Cross-phase modulation (XPM), dense wavelength-division multiplexing (DWDM), optical filters, self-phase modulation (SPM), ultradense wavelength-division multiplexing (UDWDM).

for narrow UDWDM optical filters. On the contrary, our experiments show that a narrow 25-GHz-spacing UDWDM optical filter with 16 GHz bandwidth can outperform a 50-GHz channel-spacing filter with 28-GHz bandwidth, despite strong SPM effects. While SPM effects are important in UDWDM as well as conventional WDM systems, XPM nonlinearity has been identified recently as the major impairment for UDWDM systems [9]. XPM induces signal phase noise, which may be converted to amplitude noise through dispersion. The dispersion can come from the fiber plant, or any other dispersive optical elements such as filters. We show that optical filter third-order dispersion can be especially problematic in XPM limited UDWDM systems.

I. INTRODUCTION

II. EXPERIMENT DESCRIPTION

T

HE OPTICAL demultiplexing filter bandwidth in wavelength-division multiplexing (WDM) scales roughly in proportion to channel spacing. Thus, as channel spacing approaches ultradense WDM (UDWDM) dimensions of 25 GHz, the optical filter bandwidth becomes comparable to a 10-Gb/s nonreturn-to-zero (NRZ) signal-spectral bandwidth. The scaling of optical filter bandwidth with channel spacing has several important implications. A narrow UDWDM optical filter rejects more amplified spontaneous emission (ASE) noise, a major impairment in optically amplified systems, compared to a 50-GHz channel-spacing filter. Indeed, a UDWDM filter can be designed with enhanced ASE-noise-limited performance, provided the receiver bandwidth is appropriately scaled to prevent over filtering of the signal [1]–[3]. A narrow UDWDM filter may also suffer from greater dispersion [4], [5]. Thus, recent research has focused on designing low-dispersion optical filters [6], [7]. However, one important aspect of UDWDM filter design has received little attention, i.e., the effect of signal spectral broadening and distortion due to fiber nonlinearity. In this letter, we investigate the effects of self-phase modulation (SPM) and cross-phase modulation (XPM) on the performance of 25- and 50-GHz channel-spacing optical filters in 10-Gb/s per channel transmission. SPM nonlinearity broadens the signal spectrum, and produces a nonlinear chirp [8]. Thus, both optical filter bandwidth and dispersion are expected to influence the performance of transmission systems dominated by SPM effects. For example, one might expect that SPM generated signal spectral broadening would cause a greater penalty Manuscript received July 25, 2002; revised September 18, 2002. The authors are with CIENA Corporation, Linthicum, MD 21090 USA. Digital Object Identifier 10.1109/LPT.2002.805864

The SPM experiment consists of single-channel 10-Gb/s NRZ transmission over 5 spans of non-zero dispersion-shifted fiber (TrueWave-RS). Each span is 75 km long, followed by an erbium-doped fiber amplifier (EDFA) line amplifier with mid-stage dispersion compensating fiber (DCF). The DCF nearly fully compensates the dispersion in each span. At the output of the amplifier chain, the signal is passed through a terminal DCF module, which sets the residual dispersion level. The launch power into each span is set for 8 dBm to ensure a strong SPM effect. The optical signal-to-noise ratio (OSNR) is adjusted to 17 dB, as measured on an optical spectrum analyzer (OSA) with 0.1 nm resolution, using an additional ASE source. Such a low OSNR is typical for long haul transmission systems utilizing forward error correction. The signal passes through an optical bandpass filter before being received in a commercial 10-Gb/s p-i-n receiver with 8 GHz bandwidth. Fig. 1 shows the measured amplitude and group delay response of the two optical bandpass filters used in the experiment. A JDSU TB9 filter with 28-GHz bandwidth, representing the performance of a typical 50-GHz channel-spacing WDM filter, is compared to a fiber Bragg grating (FBG) filter with 16-GHz bandwidth. The FBG filter is designed for 25-GHz channel-spacing UDWDM by optimizing the tradeoff between crosstalk and dispersion. Two modulated pump channels, spaced 50 GHz from the signal, are added for the XPM measurements. The pump and signal channel launch powers are set for 6 and 0 dBm, respectively. To reduce SPM effects, the signal launch power is set to a relatively low level compared to the pumps, and signal launch power into DCF is kept below 8 dBm. Furthermore, viewing the OSA trace at the output of the last amplifier we observed

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, JANUARY 2003

Fig. 1. Measured amplitude and group delay for the TB9 and FBG optical bandpass filters.

Fig. 2.

Fig. 3.

Q versus detuning measurements for SPM-limited transmission.

Q versus detuning measurements B2B. Fig. 4. Effect of residual dispersion on SPM interaction with FBG filter, as observed in the versus detuning curves.

negligible four-wave mixing (FWM), indicating that XPM is the dominant nonlinear effect in this experiment. III. RESULTS Fig. 2 shows the back-to-back (B2B) measurement of versus channel frequency detuning from filter center. The B2B measurements are dominated by ASE noise, a regime where optical filter performance depends strongly on receiver frequency response [1]–[3]. For the receiver used in these experiments, the TB9 shows a slightly higher B2B compared versus detuning curves are symmetric to FBG. The B2B about zero detuning, as also shown in previous work on FBG filters [4]. This situation changes dramatically when SPM nonlinearity is introduced during transmission. Fig. 3 shows the effects of SPM on filter performance. After versus detuning curves acquire a strong transmission, the asymmetry, leaning to negative detuning. This asymmetry is more pronounced for the UDWDM FBG filter, and can be attributed to an interaction of SPM induces chirp with the optical filter amplitude and phase response. Interestingly, SPM-induced signal spectral broadening does not result in a penalty for the narrower FBG filter. On the contrary, the FBG filter shows a

Q

higher maximum compared to the TB9 at the optimum detuning point. Note that ’s after transmission are higher compared to B2B due to SPM-induced pulse peaking, a "Soliton-like" effect which tends to open the eye. The eye opening is optimized with a proper choice of residual dispersion [10]. The optimum residual dispersion is found to be 500 ps/nm for both TB9 and FBG. Fig. 4 shows the effect of varying residual dispersion versus detuning curves for the FBG filter. on the shape of is shifted toward zero detuning, and the curves The peak become more symmetric or B2B-like as residual dispersion is increased. This effect can be attributed to positive residual dispersion canceling some of the SPM-induced chirp, thus reducing the interaction with the optical filter. versus detuning measurements for the Fig. 5 shows the versus detuning curves reXPM-limited transmission. The veal a stark difference in the interaction of optical filtering with at negative XPM compared to SPM. The sharp peaking of detuning observed in the SPM-dominated system is absent in the XPM case. Moreover, while the FBG faired well compared to TB9 in the SPM-dominated system, it suffers a penalty due

LYUBOMIRSKY et al.: INTERPLAY OF FIBER NONLINEARITY AND OPTICAL FILTERING

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ceiver. The penalty increases for the wider bandwidth receiver due to the high-pass frequency response of XPM noise [11] IV. CONCLUSION

Fig. 5.

Q versus detuning measurements for XPM-limited transmission.

In conclusion, we have investigated the interaction between SPM- and XPM-induced signal spectral distortion and optical filtering in UDWDM transmission. SPM effects versus detuning curves, which are produce asymmetric especially pronounced in UDWDM optical filters. SPM-induced high frequency components generated in different spans can interfere at the receiver, resulting in amplitude noise. The narrower UDWDM optical filter may be beneficial in reducing SPM-generated deleterious frequency components at the receiver. However, the UDWDM filter is shown to suffer a penalty in XPM-dominated transmission due to third-order dispersion effects. ACKNOWLEDGMENT The authors acknowledge useful discussion with J. Livas and J.-L. Archambault. L. Jin fabricated the FBG. REFERENCES

Fig. 6. XPM detuning penalty for FBG compared to B2B.

to XPM. As shown in Fig. 6, the XPM penalty is most severe when the signal is detuned from filter center. The FBG suffers a 0.5-dB penalty B2B when the signal is detuned by 3 GHz. The detuning penalty increases to 2 dB after transmission due to XPM. The XPM penalty is most likely due to FBG third-order dispersion. This can be understood by noting that optimum residual dispersion for XPM is measured in our experiment to be close to zero. However, when the signal is detuned from the filter center, FBG third-order dispersion generates a nonzero residual dispersion, which degrades performance due to XPM phase to amplitude noise conversion. Fig. 6 also shows the detuning penalty due to XPM for a wider electrical bandwidth 10 GHz p-i-n re-

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