PMD Penalty Monitor for Automatic PMD Compensation ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004

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PMD Penalty Monitor for Automatic PMD Compensation of 40-Gb/s RZ Data R. Pajntar, M. Vidmar, Member, IEEE, H. Suche, A. Paoletti, and A. Schiffini

Abstract—A dual-window spectral analysis polarization-mode dispersion (PMD) penalty monitor of novel design for 40-Gb/s return-to-zero signals is presented. Its good performance was verified in a PMD compensation experiment using an electrooptical Ti : LiNbO3 distributed compensator. Index Terms—Adaptive equalizers, optical fiber communication, polarization-mode dispersion (PMD), polarization-mode dispersion compensation.

I. INTRODUCTION

S

IGNAL distortion due to polarization-mode dispersion (PMD) is considered to be a major limiting factor when transmitting high bit-rate optical signals over long distances. Since PMD varies stochastically with time, some kind of adaptive PMD compensation is often indispensable. An essential part of any automatic PMD compensator (PMDC) is a PMD penalty monitor, the output of which yields an appropriate feedback error signal to be used for proper adjustment of the PMD equalizer. The ultimate goal of the PMD compensation is to minimize the bit-error rate (and outage probability), but, unfortunately, this quantity cannot be measured with adequate accuracy in sufficiently short time. There are several other possible means of detecting PMD-induced penalties, like eye monitoring [1], measurements of the degree of polarization [2], and coarse spectral analysis of the detected signal [3]–[7]. It is well known that first-order PMD gives rise to spectral hole burning in the electrical power spectrum of the detected optical signal [3]. A feasible method for the reduction of the PMD-induced impairments is to restore the radio-frequency (RF) spectrum of the detected signal by suitably adjusting the PMDC. PMD-induced distortion can be evaluated by integrating the detected electrical power spectral density over appropriately large bandwidths, a function usually performed by band-limited power detectors. This type of PMD monitoring has been readily used for nonreturn-to-zero (NRZ) as well as return-to-zero (RZ) signals of different data rates, investigating two or more spectral windows. RZ signals are spectrally

Manuscript received August 17, 2003; revised November 23, 2003. This work was supported by the European IST Program, Project ATLAS under Contract IST-1999-10626. R. Pajntar was with the Faculty of Electrical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia. He is now with the Slovenian Institute of Quality and Metrology, 1000 Ljubljana, Slovenia (e-mail: [email protected]). M. Vidmar is with the Faculty of Electrical Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia. H. Suche is with the Applied Physics Department, University of Paderborn, 33098 Paderborn, Germany. A. Paoletti and A. Schiffini are with Pirelli Labs S.p.A., 20126 Milan, Italy. Digital Object Identifier 10.1109/LPT.2004.824663

broader than NRZ signals and have distinct overtones. Since particularly good compensation of RZ signals can only be obtained by selecting the strong clock frequency line, they are more difficult to tackle in comparison to NRZ signals of the same data rate. In this letter, we present a novel PMD penalty monitor for 40-Gb/s RZ signals, based on the principle of coarse spectral analysis, which uses frequency translating to the intermediate frequency (IF) by heterodyning. A functionally similar penalty monitor for 40-Gb/s RZ signals has already been realized [7], but this is a rather complex measuring system consisting of a general purpose spectrum analyzer tuned to the 40-GHz clock spectral line and of a band-limited power meter, which is limited in the dynamic PMD penalty tracking capability. To the best of our knowledge, there is no report of a simple, high dynamic range, and reliable PMD penalty monitor of this type so far. II. CIRCUIT DESCRIPITON A block scheme of the PMD penalty monitor is shown in Fig. 1. The 40-Gb/s RZ optical data signal is first converted to an by a fast optical receiver electrical signal with spectrum consisting of a p-i-n photodiode and a linear transimpedance amplifier (TIA). The heart of the PMD penalty monitor is the RF module. Its basic function is to downconvert the two specand , to the tral windows of interest, namely is spectrally split and appropriIF. The input signal ately filtered by a branching filter which consists of a bandpass filter BPF1 and of a combination of several filters with overall . Thus, two distinctive channels transfer characteristic are formed. Although the function of the BPF1 is primarily to select the 40-GHz spectral line, its bandwidth is not critical since the 40-GHz spectral line is very strong in comparison to the surrounding power spectral density. Furthermore, the PMD of the optical line influences the 40-GHz spectral line exactly in the same manner as the surrounding power spectral density so there is no need to extract the 40-GHz line with a very narrow bandpass filter. The whole spectrum that is passed through the BPF1 is translated downward in frequency by a harmonic ( 2) mixer with high-side subharmonic (21 GHz) local oscillator (LO) signal injection. The IF signal is then filtered by a low-pass filter LPF1, amplified and fed to a power detector, which at its output produces voltage proportional to the IF signal power. The role of the IF amplifier is to improve the noise figure of the power detector and, thus, also the sensitivity of the PMD penalty monitor. The function of detecting the IF signal power was realized using an integrated circuit (IC) logarithmic detector (Analog Devices AD8313), which is

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004

Fig. 1. PMD penalty monitor block scheme and idealized spectrum of the detected optical signal impaired by PMD.

Fig. 3. Frequency response of Channel 2 measured using frequency swept CW signal generator.

Fig. 2.

Transfer functions and power spectral densities in the Channel 2.

usable over a 2.5-GHz-wide bandwidth and has high dynamic range (70 dB) and good response time (40 ns full scale). Due to the high-side LO signal injection in harmonic Mixer 1, the is power spectral density within the frequency band not only translated in frequency but also spectrally inverted. It appears in the IF with the discrete spectral line being at 2 GHz. is very strong in comparThe 40-GHz spectral line in ison to the total power within the frequency band . Therefore, it is of utmost importance that it is prevented from , reaching harmonic Mixer 2, where it could mix with the causing a strong mixing product of frequency equal to 2 GHz at the IF output of the mixer. The combination of filters with the acts as a band selector and overall transfer function an image rejection filter at the same time. Frequency conversion in Channel 2 is also obtained in the harmonic ( 2) mixer, but this time by low-side subharmonic (10.5 GHz) LO injection (Fig. 2). Assuming that the frequency conversion efficiency is results from a product constant, the spectral window and an upconverted IF transfer of the transfer function function . The frequency translated power spectral denappears in the IF as . sity The output voltages of the power detectors are translated to the digital domain by the analog-to-digital conversion. The re-

sulting digital data are appropriately processed using the dedicated microcontroller and output through the RS232 interface. Subharmonic signals for the harmonic mixers are derived from a single LO and a frequency doubler. Since the measured is broad-band “noise” signal in the spectral window and the IF is very broad for the 40-GHz clock line to pass through, the requirements for the LO are relaxed in terms of the phase noise and frequency stability. The entire RF module was realized as a microstrip circuit on a 0.25-mm-thick woven glass reinforced PTFE microwave substrate of dimensions 80 mm 30 mm. All electronic circuits of the PMD penalty monitor were built of commercially available discrete components. Although the PMD penalty monitor was made up of several modules they could all be integrated into a single module, the size of which would greatly depend on the technology used. The use of unpackaged components and advanced microwave substrates would greatly reduce the size of the circuit. The measured sensitivity of Channel 2 for continuous wave (CW) RF signals varied somewhat within the passband, but was always better than 53 dBm. The sensitivity of Channel 1 for 40-GHz CW excitation was even better, around 58 dBm. Although the bandwidth of Channel 2 of the RF module was de], it was limsigned to be slightly greater than 5 GHz [ ited to 2.75 GHz (Fig. 3) with the introduction of an integrated power detector, since our attempt to build a high dynamic range power detector with the bandwidth of 5 GHz failed.

PAJNTAR et al.: PMD PENALTY MONITOR FOR AUTOMATIC PMD COMPENSATION OF 40-Gb/s RZ DATA

Fig. 4.

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Experimental setup of the automatic PMD compensation.

III. EXPERIMENT Good performance of the PMD penalty monitor was confirmed in an automatic PMD compensation experiment (setup shown in Fig. 4) using a 40-Gb/s RZ transmitter (TX) at 1552 nm (pseudorandom binary sequence (PRBS) of word length ), polarization controller, PMD emulator, distributed cut propagation Ti : LiNbO PMDC, PMD penalty monitor, personal computer (PC) running original software, PMDC drive electronics, and fast oscilloscope. The optical power at the input of the PMD penalty monitor varied considerably with changes in the PMD, but its average value was approximately 7.5 dBm. The adjustment of the PMDC was possible by means of 32 50 V), each driving a bipolar control voltages (limited to group of four electrooptic transverse electric–transverse magnetic mode converter electrodes. The PMDC had 24 ps of the total compensatory power of which only 18 ps were actually used due to the limited number of available high voltage am1.1 plifiers. Polarization-dependent loss of the PMDC was dB. The control software made possible three modes of operation: manual adjustment, random search for optimum, and track optimum using dithering of PMDC control voltages for slope investigation of the PMD penalty signal. In this mode of operation, the control algorithm tries to maximize the total electrical power in both spectral windows. The results of the static compensation of 17-ps differential group delay (DGD) are shown in Fig. 5. An automatic PMD compensation running in the tracking mode was also perfectly able to dynamically track the PMD changes but its speed was somewhat limited due to the slow serial communication between the PMD penalty monitor, PC, and PMDC driver. IV. CONCLUSION We have presented a simple yet efficient PMD penalty monitor. The essential advantage of the proposed and realized scheme is that it allows the detection of power of millimeter-wave signals by downconversion to much lower frequencies and, thus, enables the use of IC power detectors.

Fig. 5. Automatic PMD compensation of 17 ps of DGD introduced by a JDS model PE4 PMD emulator. (a) PMD emulator output. (b) Output of the PMDC with 0 V applied to all electrodes. (c) Output of the PMDC for the best hit found by a short automatic random search for optima. (d) Eye opening improvement after the gradient method algorithm was started.

The realized PMD penalty monitor has its own fast optical receiver, which is by far the most expensive component. In principle, the electrical signal of the detected optical data can be “borrowed” from the optical data receiver as long as it operates in linear (nonlimiting) regime, thus, sharing the cost of the photodiode. The presented PMD penalty monitor can be modified for other data rates and the NRZ modulation format, in which case additional spectral windows are needed. Although this was not tested, we suppose that the proposed scheme has the potential of redoubling the bandwidth of the simply by changing the transfer spectral window , as shown on Fig. 2. The limited function of BPF 2 to bandwidth of the IC power detector is thus effectively doubled. REFERENCES [1] F. Buchali, S. Lanne, J. P. Thiéry, W. Baumert, and H. Bülow, “Fast eye monitor for 10 Gbit/s and its application for optical PMD compensation,” in Proc. Optical Fiber Communication Conf., Anaheim, CA, 2001, Paper TuP5. [2] S. Lanne, W. Idler, J. P. Thiéry, and J. P. Hamaide, “Demonstration of adaptive PMD compensation at 40 Gb/s,” in Proc. Optical Fiber Communication Conf., Anaheim, CA, 2001, Paper TuP3. [3] F. Heismann, D. A. Fishman, and D. L. Wilson, “Automatic compensation of first order polarization mode dispersion in a 10 Gb/s transmission system,” in Proc. Eur. Conf. Optical Communications, vol. 1, Madrid, Spain, 1998, pp. 529–530. [4] R. Noé et al., “Polarization mode dispersion compensation at 10, 20, and 40 Gb/s with various optical equalizers,” J. Lightwave Technol., vol. 17, pp. 1602–1616, Sept. 1999. [5] S. Hinz et al., “Distributed fiberoptic PMD compensation of 60 ps differential group delay at 40 Gbit/s,” in Proc. Eur. Conf. Optical Communications, vol. II, Nice, France, 1999, pp. 136–137. [6] H. Ooi, Y. Akiyama, and G. Ishikawa, “Automatic polarization-mode dispersion compensation in 40 Gbit/s transmission,” in Proc. Optical Fiber Communication Conf., San Diego, CA, 1999, Paper WE5–1. [7] S. Hinz et al., “Polarization mode dispersion compensation for 6 ps, 40 Gbit/s pulses using distributed equaliser in LiNbO ,” Electron. Lett., vol. 35, no. 14, pp. 1185–1186, 1999.