IEEE Photonics Technology Letters - Semantic Scholar

2 downloads 0 Views 331KB Size Report
Lab, Holmdel, NJ 07733-0400 USA (e-mail: [email protected]). H. Haunstein is with ..... A. Shakouri is with the Jack Baskin School of Engineering, University of ... J. E. Bowers is with the Department of Electrical and Computer Engineering,.
IEEE Photonics Technology Letters:

http://www.i-leos.org/archives/

April 2002 VOLUME 14 NUMBER 4 IPTLEL (ISSN 1041-1135) PAPER

Copyright © 2002 IEEE.

Reprinted from IEEE Photonics Technology Letters, Vol. 14, No. 4 , Apr. 2002, pp. 558 -560 & IEEE Photonics Technology Letters , Vol. 14 , No. 7 , July 2002, pp. 1019 Higher Order PMD Distortion Mitigation based on Optical Narrow Bandwidth Signal Filtering L. Moller, J.H. Sinsky, H. Haunstein, S. Chandrasekhar, C. Doerr, J. Leuthold, C.A. Burrus, L.L. Buhl

This material is posted here with permission of the IEEE. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by sending a blank email message to [email protected]. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.

558

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002

Higher Order PMD Distortion Mitigation Based on Optical Narrow Bandwidth Signal Filtering L. Möller, J. H. Sinsky, Member, IEEE, H. Haunstein, S. Chandrasekhar, Fellow, IEEE, C. R. Doerr, Member, IEEE, J. Leuthold, Member, IEEE, C. A. Burrus, Life Fellow, IEEE, and L. L. Buhl

Abstract—We show a novel higher order polarization-mode dispersion (PMD) mitigation technique for a preamplified system based on narrow optical bandwidth filtering of the signal. Reducing the signal bandwidth results in increased signal immunity against higher order PMD distortions. The method is theoretically explained and experimentally verified. Index Terms—Communication systems, dispersive channels, optical fiber polarization, optical filters, polarization.

I. INTRODUCTION

P

OLARIZATION-MODE dispersion (PMD) is considered to be the main limitation for high-speed transmission system in terms of maximum single channel bit rate and transmission length [1]. Several different electrical and optical methods for mitigation of PMD-induced intersymbol interference (ISI) have been demonstrated [2]. Roughly speaking, electrical equalizers which are only available up to 10 Gb/s, offer less system performance improvement than optical PMD compensators (PMDC), which can partially eliminate the signal distortion in the optical domain. Various optical PMDC concepts were demonstrated based on different operation mode and implementation forms like principal state of polarization (PSP) launch or PMDCs that generate the inverse birefringence of the transmission link. The majority of PMDCs are built by incorporating a series of concatenated stages, where each stage contains a polarization controller (PC) followed by a birefrigent element (BE). The simplest architecture in terms of complexity, consists of only one stage, while more complex versions were demonstrated with 73 cells [3], resulting in better equalization performance, but requiring a more complex control algorithm. Clearly, there is a tradeoff to be made between PMD compensation capability, numbers of stages, control algorithm, feedback generation, insertion loss, tuning speed, size, and cost. However, all of these optical approaches have in common that they can compensate for link PMD up to a certain amount and order. For example, the simplest PMDC (one stage) can compensate completely for an arbitrary amount of first-order PMD, and thus, higher order PMD becomes the significant limitation for equalization, as explained in the theoretical section to follow.

Manuscript received September 26, 2001; revised December 13, 2001. L. Möller, J. H. Sinsky, S. Chandrasekhar, C. R. Doerr, J. Leuthold, C. A. Burrus, and L. L. Buhl are with Bell Labs, Lucent Technologies, Crawford Hill Lab, Holmdel, NJ 07733-0400 USA (e-mail: [email protected]). H. Haunstein is with Lucent Technologies, 90411 Nuremberg, Germany. Publisher Item Identifier S 1041-1135(02)01856-6.

Fig. 1. Alignment of PMD vectors and output SOP to obtain first-order PMD Mueller matrix of PMDC) [5]. undistorted signals (R

It is known that return-to-zero (RZ) signals suffer a lot of degradation from higher order PMD. Here, we show that an optical bandwidth reduction on the receiver side can mitigate signal distortion from higher order PMD. Due to nonlinear propagation effects, lower bandwidth data formats, like nonreturn-to-zero (NRZ), are not suitable for on–off keying in long-haul transmission, making the use of RZ signals necessary. However, a narrow bandwidth optical filter on the receiver can provide both advantages: RZ pulse transmission and less higher order PMD sensitive narrow bandwidth signals in front of the detector. II. THEORY Fig. 1 illustrates the main idea behind first-order PMD compensation using a one stage PMDC in front of the receiver. Let us assume that a transmission link and PMDC are adequately and in described by their first-order PMD vectors Stokes space. At the output of the PMDC, the polarization state (at the carrier frequency) is (anti) par(SOP) of the signal allel to the overall PMD vector of fiber and PMDC. The freis given by quency dependence of

(1) stands for a small variation of angular frequency. where is It can be seen that to a first-order approximation, frequency independent (vector product is zero). This condition indicates a PSP launch to the combined devices of fiber and PMDC, in which first-order PMD distortions are eliminated in the signal [4]. The PMD compensation limitation arises now from higher order PMD distortion. The second-order PMD is , which in this case is proportional to the first-order given by PMD vectors’ product [5]. Notice the SOP change goes like . Since the output SOP varies with , a bandwidth limitation on the signal reduces the relative SOP change, and thus, PMD distortion effects resulting from PSP changes are reduced.

1041-1135/02$17.00 © 2002 IEEE

MÖLLER et al.: HIGHER ORDER PMD DISTORTION MITIGATION

559

Fig. 2. Pulse deformation due to perturbation proportional to the second derivative of the original electrical fields for pulses with different edge steepness (dotted line original pulse field, dashed line perturbation, solid line distorted pulse field).

This is the key idea of the method. Stokes space comfortably illustrates the relationship between SOPs and PMD vectors. However, when pulse distortions are computed, the Jones space has been proven to be more suitable. After PMD compensation, the output electrical field in the frequency domain is given by

Fig. 3. Experimental setup for higher order PMD generation with overall first-order PMD equal zero. A narrow bandwidth filter is used for mitigation of higher order PMD pulse distortion.

depends on several parameters (PMD amount, launched SOP, filter shape, residual first-order PMD, electrical bandwidth). However, under reasonable assumptions, the optimal (3 dB) filter bandwidth in case of no PMD is around twice the bit rate, whereas in case of higher order PMD, the optimal filter bandwidth is close to the bit rate. Asymmetrical filter alignment with respect to the carrier frequency, as used by vestigial side band filtering, could give additional advantages. It should be mentioned that besides filter bandwidth, the filter shape has an impact on the compensation efficiency.

(2) III. EXPERIMENTAL PROOF OF PRINCIPLE (3) , , stand for the Jones vectors of the electrical where field in front of the photo receiver, at the output of the transmitter, and the overall Jones matrix describing the transmission fiber and the PMDC, respectively. Dots indicate the derivative with respect to the angular frequency. In the case of first-order becomes zero. The Fourier transPMD compensation, . form of (2) shows the impact of the perturbation term in Depending on the launched SOP ( ) and , the perturbation term on the right-hand side of (3) can cause a significant pulse distortion. These distortions become stronger the steeper the pulse flanks (edges) are. A narrow optical filtering of such a pulse would reduce the steepness of the pulse flanks, which is correlated to the optical bandwidth of the signal, and therefore, lower the magnitude of the perturbation. Qualitatively, this is shown in Fig. 2, where two Gaussian pulses with same peak power, but different flank steepnesses are shown. When the perturbation becomes strong enough, optical power is shifted to the edges of the pulses leaving a dip in the centers. To evaluate the potential of the novel method, an advanced link and receiver model is necessary. As a second-order PMD representation in Jones space, we use the Francia–Bruyere model [6]. When the electrical and optical filter impulse responses and the electrical fields of the pulses are assumed to be Gaussian functions, a lengthy but closed form solution can be found for the -factor by extending the formalism from [7] to a dual polarization signal representation that includes the Jones matrix. We assume an OSNR limited receiver [signal amplified spontaneous emission (ASE) beat noise is dominant], which is the relevant case in long-haul transmission. The result of this analysis shows that for the case of higher order PMD, the optimal optical filter bandwidth should be chosen to be smaller than in the case without PMD. The optimum

As explained by theory, we expect an improvement when the filter bandwidth is optimized. These conditions cannot be obtained with arbitrary accuracy in the laboratory due to a limited selection of available optical filters. Instead, for proof of principle, we used an experimental setup that was not optimized in terms of bandwidth and filter shape fitting, but showed qualitatively the impact of narrow-band filtering on PMD distorted eye diagrams. The experimental setup is shown in Fig. 3. A pseudorandom binary sequence (PRBS) RZ signals with a 33% duty cycle is launched into a series of three polarization-maintaining fibers (PMF), which are connected by PCs. Each section has purely first-order PMD described by the DGD – . Using a polarization analyzer and sweeping the frequency of the unmodulated carrier, it is possible to align the PMD such that first-order PMD is completely canceled. The PMD distorted signal is amplified and passes through a wide bandpass filter for ASE reduction without any further signal degradation, and is split into to two paths. One of these paths contains the filter (NBF) for spectral bandwidth reduction. Thus, by switching between both paths, the filter impact can be studied. The signals are detected for eye diagram or bit-error rate (BER) measurement. First, we choose setup parameters that make the eye closure and reopening clearly visible. A 5-Gb/s RZ signal is launched with a SOP into the PMDE such that maximum eye closure is obtained. The PMDE was built out of PMFs described by DGD 130.8 ps, DGD 140 ps. The receiver is represented by – a fourth order Bessel–Thompson filter with a 3750-MHz bandwidth (75% of the bit rate). For a narrow bandwidth optical filter (NBF), we used an integrated Mach–Zehnder (MZ) filter (designed in silicon optical bench technology) with a 5-GHz 3-dB bandwidth. We show in Fig. 4(a) the eye of the heavily PMD distorted signal. As explained in Fig. 2, higher order PMD causes the clearly visible

560

Fig. 4. Eye diagrams of the unfiltered PMD distorted 5-Gb/s 33% RZ signal (a) after MZ filtering with 5-GHz bandwidth (b), (c) of the undistorted signal with and without MZ filtering (d), (e) and the corresponding signal spectra (f).

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002

and filtered signal are shown when the PMD was eliminated. Due to thermal drifts in the PMF’s BER curves cannot be taken over a longer time interval. Even with thermal isolation of the PMF, the SOP remains constant only over a minute or so. Therefore, we measured the receiver sensitivity at BER 1 10 (required measurement time less than a second). With PMD and without MZ filtering the required OSNR was 20.8 dB. When the same PMD distorted signal was MZ filtered a 18.4 dB was measured. The receiver sensitivity of OSNR 13.7 dB, sensitivity in the case without PMD was OSNR 14.6 dB. Filtering gives an and with MZ filtering, OSNR improvement of 2.4 dB in the case of higher order PMD, and in the case of no PMD, a penalty of 0.9 dB. Notice, since PMD is fluctuating over time, the filter should be designed such that distortions, especially those resulting from high amounts of PMD, are mitigated. Even if this would cause smaller penalties for low PMD, the outage probability of the system, determined by high penalty amounts, would decrease. The PMDE generates a kind of second-order PMD that is referred to as depolarization, which is the dominant effect in terms of signal degradation. IV. CONCLUSION

Fig. 5. Eye diagrams of PMD distorted 10 Gb/s 33% RZ signal with (b) and without MZ filtering (a) compared to the eye diagrams of the undistorted signals (c), (d) (MZ filtered). The corresponding spectra are shown in (e).

dip in the pulse center. MZ filtering of the signal can reopen the eye as shown in Fig. 4(b), where the carrier frequency of the signal resides in the transmission maximum of the filter. Better results can be achieved under certain condition when the center frequency of the filter is detuned from the carrier frequency [see Fig. 4(c)]. This effect is known from vestigial side-band filtering [8]. The three corresponding signal spectra were detected with a 2-GHz resolution [see Fig. 4(f)]. The MZ filtering results in a quasi RZ-NRZ format conversion of the signal. The eye of the original signal (no PMD distortion) before and after MZ filtering is shown in Fig. 4(d) and (e). All eyes are taken when same amounts of power are launched to the detector. Next, we used 10 Gb/s 33% RZ signals, a PMDE with 50 ps, DGD3 66 ps, a MZ filter with 15-GHz DGD – 3-dB bandwidth and a detector with an electrical 8-GHz Gaussian lowpass filter characteristic. The launched SOP was again set to obtain maximum eye closure. We show the eye diagrams of the PMD distorted signal before and after optical filtering [see Fig. 5(a) and (b)]. The wider bandwidth of the filter causes less ISI (compared to the 5-Gb/s experiment) as visible in Fig. 5(c) and (d), where the eyes of the unfiltered

We have demonstrated that optical narrow bandwidth filtering of RZ signals can reduce pulse distortions that result from higher order PMD. The required filter bandwidth is significantly smaller than conventional filter rules predicted for optimal detection in the case of no PMD. Advantageously, this higher order PMD mitigation method requires no active channel adaptation, only a passive filter. REFERENCES [1] P. A. Andrekson, “High speed soliton transmission on installed fibers,” in OFC 2000, pp. 229–231. [2] D. Pennincks and S. Lanne, “Reducing PMD impairments,” in OFC 2001, Paper TuP1. [3] Noe et al., “Polarization mode dipersion compensation at 10, 20 and 40 Gb/s with various optical equalizers,” J. Lightwave Technol., vol. 17, pp. 1602–1616, Sept. 1999. [4] F. Roy, “A simple dynamic polarization mode dispersion compensator,” in OFC’ 99, 1999, Paper TuS4. [5] J. P. Gordon et al., “PMD fundamentals: Polarization mode dispersion in optical fibers,” PNAS, pp. 4541–4550, 2000. [6] C. Francia, F. Bruyere, D. Pennincks, and M. Chbat, “PMD secondorder effects on pulse propagation in single-mode optical fibers,” IEEE Photon. Technol. Lett., pp. 1739–1741, Dec. 1998. [7] S. Saito, T. Matsuda, and A. Naka, “An analytical signal and noise expression for optical preamplifier receivers and its application,” in Optical Amplifiers and Their Applications’ 97, Victoria, B.C., 1997, Paper TuD11. [8] S. Bigo et al., “10.2 Tb/s (256 42.7 Gb/s PDM/WDM) transmission over 100 km TeraLight fiber with 1.28 b/s/Hz spectral efficiency,” in OFC’01, 2001, Postdeadline Paper PD 25.

2

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 7, JULY 2002

1019

Comments and Corrections__________________________________________________ Correction to “Wide Tunable Double Ring Resonator Coupled Lasers”

Correction to “Higher Order PMD Distortion Mitigation Based in Optical Narrow Bandwidth Signal Filtering”

Bin Liu, Ali Shakouri, and John E. Bowers

L. Möller, J. H. Sinsky, H. Haunstein, S. Chandrasekhar, C. R. Doerr, J. Leuthold, C. A. Burrus, and L. L. Buhl

In the above letter,1 there was a misprint of Fig. 3. The correct figure is reproduced below:

In the above letter,1 there was a misprint of Fig. 1. The correct figure is reproduced below:

Fig. 1. Alignment of PMD vectors and output SOP to obtain first-order PMD Mueller matrix of PMDC). undistorted signals (R

Fig. 3. The effects of loss on SMSR of the adjacent resonant mode at different tuning enhancements and coupling coefficients without considering the material gain difference. ( M ,l m, n : ,n ,   = l to get the maximum transmission, and the output power is 1 mW).

1 = 30 = 500 = 1 0 (1 0 ) exp(0 )

= 33

=3

Manuscript received November 5, 2001; revised January 27, 2002. B. Liu is with Calient Networks, Goleta, CA 93117 USA (e-mail: [email protected]). A. Shakouri is with the Jack Baskin School of Engineering, University of California, Santa Cruz, CA 95064 USA. J. E. Bowers is with the Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93206 USA. Publisher Item Identifier S 1041-1135(02)03235-4. 1IEEE

Photon. Technol. Lett, vol. 14, pp. 600–602, May 2002

Manuscript received September 26, 2001; revised December 13, 2001. L. Möller, J. H. Sinsky, S. Chandrasekhar, C. R. Doerr, J. Leuthold, C. A. Burrus, and L. L. Buhl are with Bell Labs, Lucent Technologies, Crawford Hill Laboratory, Holmdel, NJ 07733-0400 USA (e-mail: [email protected]). H. Haunstein is with Lucent Technologies, 90411 Nuremberg, Germany. Publisher Item Identifier S 1041-1135(02)01856-6. 1IEEE

Photon. Technol. Lett, vol. 14, pp. 558–560, Apr. 2002

1041-1135/02$17.00 © 2002 IEEE