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66 ADVANCES IN ELECTRONICS AND TELECOMMUNICATIONS, VOL. 1, NO. 2, NOVEMBER 2010 Fig. 5. Modulated laser output power (a), and fiber output power corrupted by interplay of the laser chirp and the fiber chromatic dispersion (b). distortions after the 20 km long fiber were calculated, and compared with the measurement. As it is visible in Fig. 5(b), the calculatedandmeasuredfiberoutputsignalsare practically identical, which proves the adequate chirp modeling. VI. CONCLUSIONS The influence of the carrier transport between the SCH and QW regions is analyzed in the paper in the context of chirp modeling. It was shown that even for high values of carrier capture time, when the transport effects seriously affect the laser IM and FM characteristics, the simple relations coupling intensity modulation with chirp, derived for bulk lasers, may beused.Itisofseriouspracticalimportancebecauseitallowed us to determine the chirp from IM characteristics, using the model requiring only two parameters: the line enhancement factor and the adiabatic chirp coefficient. Namely, the time domain evolution of chirp may be obtained from the measured (or somehow modeled) time domain evolution of the laser output power, by means of (5). Alternatively, the frequency domain FM transfer function may be obtained from the frequency domain IM transfer function, using (6). This way in many cases the troublesome full QW laser modeling may be omitted without sacrificing the accuracy of considerations. REFERENCES [1] R. Ribeiro, J. da Rocha, A. Cartaxo, H. da Silva, B. Franz, and B. Wedding, “FM response of quantum-well lasers taking into account carrier transport effects,” IEEE Photon. Technol. Lett., vol. 7, no. 8, 1995. [2] E. Peral, W. Marshall, and A. Yariv, “Precise measurement of semiconductor laser chirp using effect of propagation in dispersive fiber and application to simulation of transmission through fiber gratings,” J. Lightw. Technol., vol. 16, no. 10, 1998. [3] E. Peral and A. Yariv, “Measurement and characterization of laser chirp ofmultiquantum-well distributed-feedback lasers,” IEEEPhoton. Technol. Lett., vol. 11, no. 3, 1999. [4] O. Nobuyuki, K. Masahiro, I. Masato, and M. Yasushi, “1.5-µm Strained- Layer MQW-DFB Lasers with High Relaxation-Oscillation Frequency and Low-Chirp Characteris-tics,” IEEE J. Quantum Electron., vol. 32, no. 7, 1996. [5] L.Bjerkan, A.Royset, L.Hafskjaer, andD.Myhre,“Measurement oflaser parameters for simulation of high-speed fiberoptic systems,” J. Lightw. Technol., vol. 14, no. 5, 1996. [6] J. Morgado and A. Cartaxo, “Directly modulated laser parameters optimization for metropolitan area networks utilizing negative dispersion fibers,” IEEE J. Sel. Topics Quantum Electron., vol. 9, no. 5, 2003. [7] K. Czotscher, S. Weisser, A. Leven, and J. Rosenzweig, “Intensity Modulation and Chirp of 1.55-µm Multiple-Quantum-Well Laser Diodes: Modeling and Experimental Verification,” IEEE J. Sel. Topics Quantum Electron., vol. 5, no. 3, 1999. [8] P. Krehlik, “Characterization of semiconductor laser frequency chirp based on signal distortion in dispersive optical fiber,” Opto-Electron. Rev., vol. 14, no. 2, 2006. [9] H. da Silva and M. Freire, “Multi-quantum well laser parameters for simulation of optical transmission systems up to 40 gbit/s,” in IEEE Global Telecommun. Conf., 1998.
ADVANCES IN ELECTRONICS AND TELECOMMUNICATIONS, VOL. 1, NO. 2, NOVEMBER 2010 67 Precise Measurements of Highly Attenuated Optical Eye Diagrams Przemysław Krehlik, Łukasz ´Sliwczyński, and Grzegorz Sikorski Abstract—The idea and practical realization of a measurement system dedicated for highly attenuated eye diagrams diagnostics is presented in the paper. It is specially oriented on high-speed modulated optical data transmission signals which amplification is difficult and/or undesired. The presented measurements displayed the usefulness of proposed solution. Index Terms—eye diagram, optical measurements, noise reduction I. INTRODUCTION A NALYSISoftheeyediagram(calledalsotheeyepattern) is a simple but powerful method of digital transmission channel diagnostics. The eye diagram arises from overlapping many differentdata patterns time-shiftedby an integer number of unit intervals (i.e. serial clock cycles) – see Fig. 1a. Degradation of the digital signal, caused by the transmission channel, may be thus recognized and measured. Some well known cases of signal distortions are illustrated on Fig. 1b. The simplest way to obtain the eye diagram is to register the data signal with an oscilloscope having long persistence, duringthesynchronizationofthetimebasefromthedataclock signal (alternatively divided by any integer factor). In case of fast optical signals, the best choice is to use the sampling oscilloscope with the optical-to-electrical (O/E) converter integrated with the sampling unit. This solution offers the outstanding equivalent bandwidth up to 70 GHz, with flat frequency response and low group delay dispersion [1]. However, it suffers from relatively high noise, in range of 10 ... 20 µWRMS of equivalent optical power. The noise disturbs or even completely blurs the observed eye diagram when the measured signal is strongly attenuated by long fibers or other optical devices. In some cases the problem may be overcome by using an optical amplifier, or external O/E converter followed by electronic amplifier. Unfortunately,in some situationsthose solutionscouldnot be usedor are suspectedof introducing some artefacts affecting the measurement results. Therefore, some method of noise reductionin the eye diagram measurements is desired. II. THE IDEA OF EYE NOISE REDUCTION A well known method of noise reduction, used in the measurements of periodic signals on digitising oscilloscopes, is to P. Krehlik is with the Institute of Electronics, AGH University of Science and Technology, Mickiewicza 30, 30-059 Kraków, Poland; e-mail: krehlik@agh.edu.pl. Ł. ´Sliwczyński is with the Institute of Electronics, AGH University of Science and Technology, Mickiewicza 30, 30-059 Kraków, Poland; e-mail: sliwczyn@agh.edu.pl. G. Sikorski graduated from AGH University of Science and Technology in 2007. Fig. 1. The idea of the eye diagram construction (a), and common eye distortions (b). average many registrations of the same trace (so called boxcar averaging). When the noise is zero mean and uncorrelated in subsequent measurements, the root-mean-square (RMS) of the noise is reduced accordingly to the square root of the number of averaged registrations. In the ordinary eye diagram measurement, however, the overlapping of different patterns on the scope screen prohibits direct averaging. The presented idea changes the manner of collecting signal samples to allow averaging-basednoise reduction. The pattern generator, connected to the input of transmission link under test, outputs a set of different data sequences. Each sequence is repeated a number of times to allow the averaging of particular patterns measured at the tested link output. Finally, all stored averaged patterns are overlapped and shown on “virtual” oscilloscope display – see Fig. 2. It should be realized that the described method of the eye diagram construction changes in some way the information gathered in the eye diagram. By reducing the measurement noise it clarified all pattern dependent signal distortions (intersymbol interferences (ISI), nonlinear distortions, pattern dependent jitter and so on). At the other hand, however, the averaging reduces not only measurement noise but also any possiblerandomeventsinthereceivedsignal,suchastransmitter relative intensity noise (RIN), optical amplifier amplified spontaneous emission (ASE), adjacent signals crosstalks in multichannel systems etc. III. MEASUREMENT SYSTEM IMPLEMENTATION Based on the idea presented above, a measurement system dedicated for measuring highly attenuated optical eye diagrams was built. The system (see Fig. 3) is based on
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