Figure 1. System performances of 40Gbit/s x 9 WDM and single-channel transmissions with and without phase alternation.
Carrier-suppressed dispersion managed soliton transmission using Novel OTDM technique
Oki Electric Industry Co,. Ltd., 550-5, Higashiasakawa, Hachioji, Tokyo 193-8550,
Japan
TEL: +81-426-6762, Fax: +81-426-6581, E-mail: murai572@oki.co.jp
Abstract
DWDM transmission characteristics of DM solitons with duty-cycle of more than 50% and initial phase alternation are investigated, both numerically and experimentally.
Introduction
The dispersion-managed soliton enables the ultra-high bit-rate transmission over the distances of several thousands km without regeneration. In the WDM regime with a fixed channel spacing, however, inter-channel degradation mechanisms such as the cross phase modulation (XPM) begin to dominate, limiting the bit-rate increase per channel. The use of broader pulses alleviates the inter-channel degradation because of its less non-linearity, if the intra-channel pulse-to-pulse interaction is controlled. An initial phase alternation technique, where the optical phase of the pulses toggles between 0 and p at the bit-rate [1][2], could be the key to overcome the problem. With the initial phase alternation, the neighboring pulses have negative optical phase correlation, leading to less pulse-to-pulse interaction and carrier-suppressed spectrum [3]. For its simpler implementation, the optical time division multiplexing (OTDM) can be utilized. In this work, the effect of initial phase alternation with OTDM technique is evaluated with the 40Gbit/s DM soliton in both single-channel and DWDM transmissions, both numerically and experimentally.
Numerical evaluation on the effect of phase alternation
For the sake of simpler comparison between alternating-phase and uniform-phase cases, the system performances of 40Gbit/s single-channel and nine channels WDM DM-soliton transmissions were numerically evaluated with Q value-estimates using 27-1 pseudo-random-bit-sequence (PRBS). For optical signals, Gaussian pulses with FWHM of 13ps (duty cycle; 52%) were assumed, and were launched into the transmission line with an appropriate pre-chirp. An amplifier spacing was 60km, consisting of 30km-long positive dispersion fiber (D= 3.14ps/nm/km, dD/dl= 0.07ps/nm2/km, g =1.01/W/km) and negative dispersion fiber (D= -3.02ps/nm/km, dD/dl= -0.07ps/nm2/km, g = 5.06/W/km). The condition on the dispersions was optimized to obtain the maximum reduction of pulse-to-pulse interaction by the phase alternation technique [4][5]. Span loss, path-average dispersion, and noise figure (NF) of optical amplifier were 12dB, 0.06ps/nm/km, and 5dB, respectively. In the WDM transmission, a channel spacing was set at 100GHz, and we assumed neighboring wavelength-channels were multiplexed wit h orthogonal polarization state to reduce the effect of XPM [6]. In this calculation, the effect of polarization mode dispersion was excluded, so that the orthogonal polarization state between the neighboring wavelength-channels is maintained over the transmission. The wavelength-channels were de-multiplexed by a 3rd-order Butterworth filter with the bandwidth of 75GHz at the receiver. Figure 1 illustrates the system performances of WDM (the worst channels, thick lines) and single-channel (thin lines) transmission with (solid lines) and without (dashed lines) phase alternation. The average launched power and the pre-chirp value for each wavelength-channel was 1.73dBm and Ð47.95ps/nm. In the single-channel case, the transmission distance is improved up to more than 5,000km by employing phase alternation, while the pulse-to-pulse interaction limits the transmission to less than 2,000km in the uniform-phase case. The transmission of alternating-phase case is to be limited by the non-linear interaction between signal and ASE noise. On the other hand, in the WDM regime, the transmission of the uniform-phase soliton is still limited by the pulse-to-pulse interaction, based on the comparison with the results of single channel case. In such a situation, the phase alternation also improves the transmission performance as well as the single-channel case. In this example, the WDM transmission is extended up to the distance of more than 3,000km by employing the phase alternation technique. The WDM transmission of the alternating-phase case is mainly deteriorated by XPM coming from 200GHz-apart channels.
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Figure 1. System performances of 40Gbit/s x 9 WDM and single-channel transmissions with and without phase alternation. |
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Figure 2. Experimental setup and optical spectrum of 40Gbit/s x 8 WDM signal. |
Transmission experiments
To examine this concept, the transmission experiment was conducted. Figure 2 describes the setup of re-circulating loop experiment for 40Gbit/s WDM-DM solitons. The transmission line consisted of two amplifier spans. An amplifier spacing was 45km comprising two sets of 10km of standard fiber (SMF, 16.5ps/nm/km @1550nm) and 2.5km of dispersion compensating fiber (DCF, -66ps/nm/km @1550nm), followed by 20km of SMF and 5km of DCF. This configuration is known as the dense DM line [6], which is a suitable way to stabilize DM soliton transmission using the large dispersion fibers such as standard fibers. The span loss was 14.5dB. The noise figures of EDFAs were about 6dB. The path-average dispersion was set 0.05ps/nm/km at wavelength of 1555 nm and the average dispersion slope was 0.008 ps/nm2/km. At the transmitter, nearly Fourier transform-limited Gaussian pulses with FWHM of 14ps (corresponding to duty cycle of 56%) were generated by 20GHz-sinusoidally driven electro-absorption modulator (EAM), and were encoded at 40Gbit/s using OTDM module [7]. 40Gbit/s data was pre-chirped, and launched into the re-circulating loop. Since the OTDM uses the optical delay of 25ps-delay to combine two 20Gbit/s pulse streams, the optical phase difference between the interleaved 20Gbit/s-pulse streams can be adjusted by slightly changing the wavelength of the optical pulses. In the experiment, tweaking the wavelength by ~0.16nm turned out to provide p-phase difference between the adjacent pulses. After transmission, a 40 Gbit/s signal was selected by bandpass filter with bandwidth of about 90GHz, and then down-converted to four-10 Gbit/s signals by using two EAMs.
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Figure 3. Measured BER and eye-diagrams, taken at 1800km, for single-channel transmissions with (filled circle, (a)) and without (open circle, (b)) phase alternation. |
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Figure 4. Achieved transmission distances (BER<10-9) for WDM and single channel transmissions. |
Figure 3 shows the measured bit error rate (BER) for 40Gbit/s single channel transmission with and without phase alternation using this technique. Insets are measured eye-diagrams taken at the distance of 1800km for (a) alternating-phase and (b) uniform-phase cases. The achieved transmission distance in the case of alternating-phase soliton was 2,070km at average launched power of 4.5dBm and the average dispersion of 0.045ps/nm/km, whereas that of uniform-phase case was only 1,260km. The eye-diagram (a) does not reveal any waveform distortion due to pulse-to-pulse interaction and the transmission distance was only limited by OSNR degradation due to ASE accumulation. In eight (four) channels WDM experiment, all wavelength-channels were multiplexed with single polarization state. The channel spacing was set to be about 120GHz (240GHz), which meets conditions that p-phase difference between adjacent pulses for all wavelength-channels were realized as shown in an inset of figure 2. Figure 4 shows the achieved transmission distances (BER<10-9) of 8 WDM (squares) and 4 WDM (triangles) transmissions, with an average channel power of 4.5dBm. For the comparison, the results of single channel transmissions with (circles) and without (crosses) phase alternation are also plotted in figure 4. The transmission performances of 8 WDM case were deteriorated due to XPM, compared with the single-channel cases with phase alternation. However, the transmission distances of more than 1,260km were obtained for all channels, while single channel cases without phase alternation were limited to the distance of less than 1,260km. On the other hand, in 4 WDM case, the transmission performance for each channel shows no clear evidence for the degradation due to XPM. The experimental results are consistent with the predictions of our numerical calculation, and show the possibility of long haul DWDM transmission based on DM soliton.
Conclusion
The transmission improvement of DM soliton with initial phase alternation was confirmed both numerically and experimentally. The simulation revealed that initial phase alternation was an effective method to reduce the pulse-to-pulse interaction induced degradation for very wide pulse with duty cycle of more than 50 %. By use of OTDM technique, it was verified experimentally that the phase alternation enables 40 Gbit/s DM soliton transmissions beyond 2,000 km, and the eight channels WDM transmission over 1,260 km with a 120GHz channel spacing. While the WDM transmission was limited by XPM, the achieved distance was better than that of single-channel transmission with uniform-phase soliton. The phase-alternating soliton transmission over properly designed DM fiber line will realize the ultra-high bit-rate long haul DWDM systems at higher spectral efficiency.
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