Transmission of a 20 Gb / s NRZ OOK Signal Throughout a 390 km Fiber Link and a Cascade of 11 x 50 GHz Filters and 9 x EDFAs

This paper presents a qualitative analysis of the impact caused by fiber propagation effects and by a cascade of optical filters, designed for 10 Gb/s NRZ OOK transmission, and optical amplifiers on a 20 Gb/s NRZ OOK optical signal traveling a 390 km fiber link. The choice for such bit rate and modulation format has been driven by the 50 GHz bandwidth of the filters (in the ITU DWDM grid) and by simplicity and low cost, respectively. The system, investigated experimentally and numerically, comprises 4 spans of standard fiber and a concatenation of 7 dispersion compensating modules, 4 wavelength selective switch reconfigurable optical add and drop multiplexers, 9 erbium-doped fiber amplifiers and 1 WDMDEMUX (> 75 GHz bandwidth). We demonstrate the feasibility of such bit rate upgrade in a cost-effective network planning strategy, which may be more adequate for regions where the demand for bandwidth do not require a jump to higher capacities (i.e. 40 and 100 Gb/s), thus allowing for reusing the 10 Gb/s installed infrastructure.

detection, which present higher spectral efficiency and may be less affected by linear regime physical impairments [2], [3]. However, phase modulation and coherent detection not necessarily lead to higher spectral efficiency. This happens only if such modulation techniques are associated with multilevel signaling. Such higher bit rates imply that the emerging technologies must support a wide bandwidth signal [2]. Under such spectral constrain, the low cost requirement is not well accomplished since part of the network legacy may not be used, added to the fact that those solutions are expensive and technically complex [4]. Phase Shift Keying (DP QPSK) [5]. Furthermore, despite the advantages provided by the new formats and technologies, traffic demands across the network may not require 40 and 100 Gb/s capacity everywhere. So, a cost-effective network design needs to exploit the scenario of operation with Mixed Line Rates (MLRs) [1], [6].
In that approach, we have investigated the possibility of implementing an intermediate upgrade in an existing 10 Gb/s infrastructure without much expenses, by using NRZ OOK, direct detection and low loss optical dispersion compensator, i.e. 10 Gb/s-designed DCM based on chirped gratings, thus allowing for propagation in linear regime. We then analyze the effect of a large concatenation of filter and ASE across a nearly 400 km long optical path traversed by a 20 Gb/s NRZ OOK signal. That bit rate has been chosen due to the 50 GHz bandwidth of each filter, which does not allow the transmission of a 40 Gb/s NRZ OOK signal. We have carried out experimental and numerical measurements for evaluating the transmission performance.
The paper is organized as follows. Section II presents the implications of optical filters concatenation, Section III describes the experimental setup and the numerical simulation parameters.
Section IV presents the results and discussion and, in Section V, the conclusions are described.

II. CASCADE OF OPTICAL FILTERS
In a WDM optical network, a signal can be filtered several times from its origin to its destination.
That filtering comes from the signal passage, for example, through ROADMs, DCMs and Optical Cross-Connects (OXCs), which may be installed along the optical path. Those filters exhibit nonideal transfer functions for optical amplitude and phase, which causes passband curvature in their frequency response, tilt and ripple. Moreover, the phase transfer function can not vary linearly with frequency.
Those effects are magnified by increasing the concatenation of devices, thus causing spectral clipping of the signal spectrum and/or dispersion problems due to nonlinearities of the phase response, which can limit the maximum link length [7], [8].
The spectral clipping and dispersion of the filter can lead to the signal distortion in the time domain, causing a Q-factor penalty. The effective spectral transfer function of a cascade of filters results from the multiplication of all filter responses and, generally, is much narrower than that due to a single filter. Figure 1 shows the effective transfer functions of a single optical filter with 36 GHz bandwidth and 12 cascaded and aligned filters, obtained with the Optiwave Systems Inc. OptiSystem 9.0 software tool. As expected, it can be noticed that the effective filter function after 12 filters is much narrower than after a single filter. The optical impairment penalty associated with filter concatenation effects is define as an eyeclosure penalty (ECP). For a thermal noise-limited system, the eye-closure penalty may be further translated into a Q-factor penalty. The ECP calculation considering only contributions to the eye closure from signal distortion can be obtained as: where the numerator represents the eye opening in the back-to-back condition and the denominator represents the eye opening under the fiber transmission conditions [9]. Note that the ECP in decibel is defined in the 20 log ( ) format, here, but may be estimated in the 10 log ( ) format in case the intensity I has been given in power unit. In equation (1), I 1,min,BB and I 1,min , correspond to the minimum value of level '1', in the back-to-back and fiber transmission configurations, respectively. Similarly, I 0,max,BB and I 0,max , to the maximum value of level '0', in the back-to-back and fiber transmission configurations, respectively and, finally, I ave,BB and I ave correspond to the eye-diagram average intensity in the back-to-back and fiber transmission configurations, respectively.

III. SETUP
A. Experiment

B. Simulation
We used the same experimental diagram in the OptiSystem 9.0 simulator, that solves the nonlinear Schrödinger equation, configured with the parameters indicated in Table 1. Some parameters, such as responsivity, thermal noise and dark current are not available, and have been set to default values.

IV. RESULTS AND DISCUSSION
In order to evaluate the physical impairments on a 40 Gb/s NRZ OOK signal, we simulated its transmission on the setup seen in Figure 2. The eye diagrams, presented in Figure 3, confirm the strong limitation imposed by the impairments on that bit rate and modulation format propagation. The eye diagram degradation is a consequence of many effects in mutual combination, but by observing the spectral narrowing due to the cascade of filters (WSSs, DCMs and DEMUX) 3 along the optical path, as indicated in Figure 4, it is possible to infer how strongly the narrow spectral window might penalize the system performance. This fact, alone, implies that for such system configuration, the transmission of 40 Gb/s NRZ OOK is not feasible, despite of other problems associated to other physical impairments. From those indications, and considering the aim of reusing a 10 Gb/s infrastructure, we have chosen 20 Gb/s NRZ OOK as the basis for our investigations.
Therefore, from then on, the results are referred to it. The 20 Gb/s eye-diagrams, for the simulation and experiment, are shown in Figures 5 and 6, respectively. As the first dispersion compensation stage, DCM1, does not match the required dispersion value, the performance degradation due to residual dispersion is notable. However, we observe an improvement in performance after DCM2, that also compensated the residual dispersion, and a more natural degradation from then on, due to the other impairments. Three of them, ASE accumulation, electrical noise (the front end bandwidth, 40 GHz, is higher than necessary) and PMD were expected. One must notice that the deleterious effects associated to PMD have not been combated in any way.  For evaluating the filter concatenation influence, we have carried out spectral simulations and measurements, which are summarized in Figures 7 and 8, respectively. From Figure 7 we notice a loss of side lobe with spectral narrowing after propagation throughout the filters, which lead to the conclusion that even at 20 Gb/s the cascade of seven 50 GHz filters represent a limitation for the system performance.  We also notice, in the experimental data, that the spectral narrowing may be slightly observed with an alignment, in amplitude, of the curves, thus highlighting the similarity between the spectra that cross the filters and the one that does not cross them (the back to back transmission). Furthermore, for the ASE accumulation effect, the modulated signal spectra after the 390 km path does not show a significant Optical Signal-to-Noise Ratio (OSNR) penalty (OSNR ≈15 dB). One should note that the excess of filtering along the optical path, if by one side limits the overall bandwidth, by the other side acts to reduce the impact caused by ASE concatenation due a cascade of optical amplifiers, which has been confirmed by our results.
Finally, we should remember the extra electrical bandwidth of the optical receiver, designed for 40 Gb/s measurements, which is likely to add extra electrical noise to the measured eye diagrams seen in Figure 7. Considering that, it is reasonable to expect that a receiver designed for 20 Gb/s would allow the reception of cleaner eye diagrams. Other effects, besides ASE accumulation, electrical noise and PMD, such as nonlinear effects, could be further investigated in configurations with higher optical power levels. For those used in the setup, we have not observed any strong impact caused by fiber nonlinearities.
By using Equation (1)  large amount of filters (4 x WSS ROADMs, 7 x DCMs) and EDFAs (9 units). By contrary, considering the possibility of including some forward error corrector technique, which could improve the system performance, it is reasonable to affirm that the use of 20 Gb/s NRZ OOK may be a cost effective strategy for network upgrades where the increase in capacity does not require a jump to higher bit rates, which would imply in more complex and expensive technologies, such as coherent detection with phase modulation formats. This way, the legacy of 10 Gb/s systems could be preserved and reused, in a good choice for the network planning made by carriers that aim to keep in balance the Capex.
Finally, one should notice that bit rates of 10, 40 and 100 Gb/s are standardized in Telecommunications industry and the analysis presented in the paper is performed in a system with 20 Gb/s which is not a standard bit rate. However, the use of 20 Gb/s has increased in recent years, mainly due to applications where modulation formats such DQPSK (and OFDM (Orthogonal Frequency Division Multiplexing) are employed {14], [15]. ACKNOWLEDGMENT This work has been supported by Funttel (Finep) and Fapesp (grant 06-04546-4).