EFFECTS OF PMD WITH RANDOM MODE COUPLING ON 2.48 Gb/s IM-DD LIGHTWAVE SYSTEMS'

C. H. Prola Jr, J. A. Pereira da Silva, A. 0. Dal Fonno, R. Passy, J. P. von der Weid

Center for Telecommunications Studies, Pontificia Universidade Catblica do Rio de Janeiro Rua MarquEs de SHo Vicente 225, Rio de Janeiro, 22453-9100, Brazil

E-Mail: prola@cetuc.puc-no.br TeWax: +55 2 1 2397425-

Abstract

Effects of Polarization Mode Dispersion (PMD) on 2.48 Gb/s IM-DD lightwave systems are experimentally observed by using PMD emulators made with segments of highly birehgent fiber with random mode coupling splices. Random birefringence between mode couplings was also included in the emulator. The measurements indicate that the eye diagram is corrupted by PMD values above 10 ps. BER measurements show more than 1.5 dB power penalty for PMD values of 30 ps.

1. INTRODUCTION

Polarization mode dispersion (PMD) is the residual dispersion of a single mode liber due to the degeneracy of the fundamental spatial mode, which supports two orthogonal pola~ization states. Residual stress and external effects over the fiber give rise to a differential polarization mode delay which affects the ultimate transmission properties of single mode fibers [ 11. In the last few years much effort have been made in PMD measurements [2,3] as well as in the description of the signal distortion and PMD penalties in lighlmave systems [44 5). More recently, it was shown that PMD penalties in digital "mission systems can be severely affected by the combined effect of the spectral width of the optical source and the wavelength dependence of PMD (2"d0rder PMD) [6]. Because of the small values of PMD.delays it is hard to separate PMD effects from other distortion sources, like chromatic dispersion, amplifier noise, etc. Hence the approach to understand PMD effects in optical lightwave systems has been to use simulated links or virtual optical links in which all other distortion sources are negligiile, except PMD.

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High Birefringence Fibers or polarization split variable delay lines have been usecl to simulate the PMD value of an optical link, and this is true whenever the source is monochromatic or has its cohtmce time much smaller than the PMD which is being simulated. However mode coupling, which is normally found in single mode optical cables, gives rise to a wavelength dependent polarization mode &layy so that laser chirp or mode coupling fluctuations wil l strongly affect the system performauce. Hence, the simulation of PMD in optical lransmission systems must include random mode coupling, otherwise the conclusions obtained with mode coupling-free experiments may be far from the reality.

Random mode coupling in PMD effects were indeed evaluated by splicing equid lengths of HiBi fiber segments with random angles between the polarizations axes, m a first attempt to simulate a real single mode optical link with random mode coupling [7'J Howevery merent values of the total PMD were- simulated by conci&nating increasingly d e r segments, keeping constant the total length of the samplle. More recently, random mode coupling was investigated in different concatenations by varying both the number of splices and the mean coupling length [81-

Because of the statistical nature of PMD; its effects in an optical link are constandly varying, giving rise to fading in lightwave systems. Two approaches may be used to " i z e these effects: keep the PMD of the link as small as possible or design adaptive equakers to compensate for PMD effects. The first one is a matter of fiberkable fabrication. As a second approach, an adaptive equalizer was indeed proposed for PMD compensation [9], but the experimental test included only three random angle splices in the simulation of die optical link, a number certainly too small to descnie a real long span optical link.

In order to understand and control PMD effects in lightwave systems PMD emula.tors are very useful, but care must be taken in order to properly simulate long optical fiber spans because Merent total PMD values can be obtained .. .._-

WO& developed under the CpSD Telebms research contract #JPQD-513193. 1

0-7803-4165-1/97/$10.00 0 1997 IEEE. 687 SBMODEEE MTT-S IMOC'97 Proceedings

according to the number of splices or the length of the individual HBi fiber segments. In this letter we present PMD emulators for optical llnks which includes random mode coupling and PMD values ranging from 1 to 100 ps. Because of the statistical nature of the PMD, a given emulator will present different polarization mode delays, depending of the input polarkation state of the light signal. The effect of PMD in 2.48 Gbls IM-DD lightwave systems is evaluated here by varying the polarization state at the input port of the PMD emulator.

2. PMD EMULATOR

When a fiber is excited with a light source whose coherence is much larger than its PMD, the fiber behave like a 2-mode fiber. The Differential Group Delay (DGD) between these two modes 6.r is -the most relevant parameter determining the polarization properties of the transmission through this fiber. However, the DGD is wavelength dependent, unstable and fluctuates in time. Hence, the PMD AT is defined as a statistical quantity. We shall use the RMS definition: AT = ,/m . This definition of PMD is directly related to the distribution I(t) of time of flight of optical pulses much shorter than the PMD [IO]. The general form of I(t) can be computed introducing a discrete mode coupling model, which considers the single-mode fiber as a concatenation of many polarization maintaining fiber segments with random coupling angles [3]. IQ this model, dz can be expressed in terms of the fiber length 1, the mean coupling length h and the average modal birehgence dn.

We simulate random mode coupling by splicing merent lengths of high birefringence fibers with their axes randomly oriented. In this way, the mode dispersion of the short HiBi fiber segment emulates the PMD of long standard fiber pieces, whereas the random axes splicing emulates the random mode coupling. In long span optical links, a great number of splices guarantees the condition I/12 >> 1 even for very low birefiingence fibers and ideal cabling process. Hence, our approach to build a PMD emulator was to keep a great number of mode couplings in order to insure a random mode coupling and a continuous distriiution of polarization time-of-flights. This distriiution was calculated by considering a short pulse with random polarization launched into the first HiBi fiber segment and coupled in both the fast and slow fiber axes. The signal propagates along the segment with different group velocities, arriving in the output with different time of flights. T h a the signal intensity is coupled in the polarization axes of the next segment, which has a random angle with respect to the previous one. The reason to couple the signal intensity and not the field amplitude is that the coherence time of the LED source used in the PMD interferometric measurement is much smaller than the polarization mode delay of each HiBi fiber segment. It should be noted that although DFB lasers are normally used in optical systems, the equivalence of the coherent and incoherent approaches to the PMD measurement is well established Ell]. Repeating this coupling procedure for all segments, the output of the segment n is given by:

where I‘,fand I: are the signals intensities in the fast and slow axes at the output of the nrh segment, D,, is a matrix that represents the time delay between the signals in its fast and slow axes, C, is the matrix which determines the signal intensity that is coupled in each polarization axis and 4,, is the angle between the axes of neighboring segments. The time of flight distribution can now be computed by applying expression (1) successively until the last segment (n=IV) of the PMD emulator is included, thus generating ZN different total delay times. Of come, if all 6z, are equal, which is the case for equal lengths HiBi segments, only N different equally spaced delay times will be generated. In order to avoid this degeneracy, we used a different length for each fiber segment.

In our calculation the splicing angles were randomly generated following a uniform distriiution with 0 I 4 s 2n. The length of each segment was generated with a Gaussian distriiution probability around a mean value h, which deternnines the PMD of the emulator. Hence, the randomness of mode coupling and birefiingence of each fiber section were both included. PMD emulators with dr of I, 3,10,30 and 100 ps were made using the results of these calculations. These values cover the range of interest in almost all practical cases. Each emulator has 20 segments of a HiBi fiber, generating 2M different delay times, a number which was considered great enough to provide for a continuum time-of-flights distn’bu.tion in practical cases. The PlMD value of the HiBi fiber was 1.78 pdm, and the width of the coupling length distnl>ution was 20% of the mean coupling length. The computer generated lengths and

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coupling angles were reproduced in the construction of the PMD emulators. ]Mechanical Splices and a Splice Organizer Tray were used. Measurements of dz were done using the interferometric technique [3] for many states of polarization of the input signal.

PMD = ‘I 0.76 ps

c, .- - .- fi 2

t

U)

P) m 5

PMD = 10.83 ps .-

-20 0 20 Relative delay [ps]

Figure 1 - Simulation (a) and interferometric measurement @) of the :LO ps PMD emulator.

Figure 1 shows the interferometric measurement of the PMD ( l ~ 1 . 3 p) of a 10 1)s emulator and the corresponding theoretical simulation. In this case, the mean coupling length was 1.36 m so that between the fastest ancl slowest time-of-flight in the emulator the time interval scale is -0.7 fs, much smaller than the time resolution of any lh4-DD actual transmission system. The agreement between the calculated and measured values is remarkable, and it was better than 5%. in all cases. The total attenuation of the PMD emulators varied between 7 and 11 dl3, which corresponds to a mean 0.47 dE3 loss per mechanical splice. This bad performance is mainly due to our cleaving tool and the fact that the angle between the fiber axes were preset by the calculation.

3. EFFECTS OF PMD ON A 2.48 Gb/S IM-DD LIGHTWAVE SYSTEM

In order to evaluate the effects of PMD on digital systems, a simulated optical li~& was mounted as in figure 2. A (291) pseudo-random sequence generator modulates the injection current of a 1.3 p DFB laser diode. The optical signal passes through a polarization controller that varies the polarization at the input of the PMD emulator. An APD photodetector and SDH iilter were used to detect the signal. A clock rewveqy circuit triggers a sampling scope which presents the detected signal. An optical attenuator was used to compensltte for the different valules of the attenuation of each PMD emulator, keepmg constant the optical power at the photodetector input.

PRBS 2” - 1 Generator

APD PMD +

Recover optical

Attenuator s p l i i

Figure 2 - Experimental set-up for eye diagram measurements

Another optical link was mounted for bit error rate @ER) measurements, with a SDWSONET Analyser (transmiter and receiver), as shown in figure 3. A (291) pseudo-random sequence modulates the injection current of a 1.3 pm DFB laser diode. The signal passes through a EDFA optical ampUier and a variaible optical attenuator, wch adjust the received optical power.

689

w Emulator

Figure 3 - Experimental set-up for BER measurements at 2.48 Gb/s.

Eye diagrams were obtained for each PMD emulator. The state of polarization was adjusted fiom maximum to minimum eye degradation, depending on maximum or minimum mismatch between the input polarization state and the principal states of the PMD emulator. We also observed that the eye diagram varies with time due to the evolution of the input polarization state produced by thermal variations in the fiber environment. Figure 3 shows typical eye diagrams for maximum signal distortion obtained with the 10, 30 and 100 ps PMD emulators, together with the reference eye. without PMD. We can see that the 10 ps PMD emulator already presents a slight eye degradation. For a 30 ps PMD a severe degradation is observed, whereas the eye is practically closed for 100 ps PMD. No degradation could be observed for PMD smaller than 10 ps.

Figure 4 .. Eye diagrams for PMD values: (a) reference eye, (b) 10 ps, (c) 30 ps and (d) 100 ps

2 8

m

e E j

QI w

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Fi,pre 5 - Dependence of the receiver sensitivity on the PhaD value

BER measurements at different received optical powers were obtained with 10 and 30 ps PMD emulators. Strong variations with time were observed in the BER values at the Same received optical power due to the evolution of the input polarization state. With 30 ps PMD emulator BER values change from 10” up to lo-’’- Figure 5 shows the worst BER values with 10 and 30 ps emulators. These measurements show a power penalty @ER = lo’) of more than 0.5 dB and 1.5 dB with 10 ps and 30 ps, respectively.

These effects are somewhat greater than those reported by Suetsup et a1 [7] and sugest power penalties even larger than those measured at 5 Gbls systems [SI. Theoretically predicted mean penalties for 30 ps PMD at 2.48 Gb/s are smaller than 0.1 dE3 [l], but this calculation considers only intersymbol interference due to pulse broadening, and neglects chirp or wavelen,d dependence of the PMD. In our case the laser frequency modulation was close to its resonance frequency, enhancing the laser chirp amplitude. We believe that 2“d order PMD combined with the chirp of our laser source is the responsible for these high distortion levels.

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4. CONCLUSION

PMD emulators with PMD values ranging from 1 to 100 ps were made by concatenating 20 segments of HiBi fibers with variable coupling lengths. A Gaussian probability distribution was used for the coupling lengths. The width of the distributions were 20% of the mean value in all cases. Random mode coupling was also imposed at each splice. The PMD effects on 2.48 Gb/s IM-DD lightwave systems were evaluated with such devices, and the results show that eye degradation i s observed for PMD values above 10 ps, which are believed to be enhanced by laser chirping. BER measurements confirm the observations of the eye diagrams.

5. ACKNOWLEDGMENTS

The authors acknowledge partial support fkom the Brazilian National Research Caiuncil CNPq and CAPES.

6. REFERENCES

[l] C. D. Poole, R W. "kach, A. R. Chraplyvy e D. A. Fishman, "Fading in lightwave systems due to

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[9] T. Takahashi, T. Imai and M. Aiki, "Automatic compensation technique for timewise fluctuating po:larization mode dispersion in in-line amplifier systems" Electronics Lett., vol. 30,4, pp 348-349,1994.

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