We propose a modified nonlinear amplifying loop mirror (NALM) for phase-preserving 2R regeneration of wavelength division multiplexed (WDM) return-to-zero differential phase-shift-keyed signals. As proof of principle the regeneration capability of this NALM setup has been investigated experimentally for two 10 Gbit/s wavelength channels. A significant eye-opening improvement and a negative power penalty of 1.2 dB have been observed in both channels.
©2008 Optical Society of America
The continuing interest in phase-encoded transmission formats such as return-to-zero differential phase-shift keying (RZ-DPSK)  has driven the search for suitable reamplifying and reshaping regenerator schemes (2R regenerators). They counteract the growth of amplitude noise in optical transmission lines. The main origins of such amplitude noise are amplified spontaneous emission (ASE) at each in-line amplifier and nonlinear intra- and inter-channel effects. Moreover, for phase-encoded formats it is of importance to prevent the generation of nonlinear phase noise by nonlinear effects such as the Gordon-Mollenauer effect  since both amplitude and phase noise decrease the signal quality. Therefore, the accumulation of amplitude noise should be tackled first of all. However, a 2R regenerator suitable for DPSK signals must be able to suppress amplitude noise while preserving the phase relations between pulses in which the data are encoded.
Several 2R regenerator schemes for DPSK signals have already been proposed. Most of them use nonlinear effects in a single fiber such as self-phase modulation (SPM) [3,4] or four-wave-mixing (FWM)  in order to regenerate the signal. These regeneration methods work only for signals comprised of a single wavelength. However, in order to use the bandwidth of a transmission fiber efficiently, in current systems wavelength division multiplexed (WDM) signals are used. Trying to regenerate a WDM signal with 2R regenerators that exploit the ultra fast Kerr effect in a single fiber, common for all WDM channels, would result in amplitude and phase distortions due to cross-phase modulation (XPM) and FWM between different wavelength channels. The reason for this is that in order to regenerate a certain wavelength channel all XPM and FWM contributions from other channels must also be taken into account. Such a regenerator scheme would demand fixed relationships between all WDM channels in bit sequence, channel power and time. However, in operational systems this is generally not the case. Therefore, it is not possible to regenerate all channels at the same time by this means.
For WDM On-Off-keyed (OOK) signals, regenerator schemes which can cope with this problem have already been developed. There are basically two approaches: In the first one the WDM signal is demultiplexed first so that each channel can be regenerated in a separate regenerator. After the regeneration the channels are recombined by a wavelength multiplexer into a WDM signal again [6,7]. This scheme is quite straightforward but has the disadvantage that the setup can become rather complex and expensive if a large number of channels has to be regenerated. In the second scheme the pulses of the particular channels are separated temporally by e.g. a dispersive walk-off in a dispersion-managed fiber  or by converting the WDM signal into an optical time-domain multiplexed signal (OTDM) . In this scheme the nonlinear interactions between the channels can be kept low. This concept has the disadvantage that signals with a high duty cycle cannot be regenerated and, therefore, the bandwidth of the transmission line can not be used efficiently.
Our goal was to develop an amplitude regenerator for phase-encoded RZ-DPSK WDM signals with high bit rates independent of the duty cycle using a minimum number of components. We propose an adapted demultiplexing-multiplexing approach integrated within a nonlinear amplifying loop mirror (NALM) with an asymmetrical splitting ratio. As proof of principle we have investigated experimentally its suitability for simultaneous amplitude regeneration by evaluating eye-diagrams and bit error ratio (BER) measurements for two 10 Gbit/s RZ-DPSK wavelength channels.
2. Principle of operation
A NALM is a nonlinear fiber Sagnac interferometer based on SPM with an optical bidirectional amplifier in the loop. In order to prevent XPM and FWM effects from disturbing the SPM in the loop we propose for WDM signal regeneration a modified NALM setup. This setup (Fig. 1) uses wavelength multiplexer and demultiplexer inside the interferometer loop to split and recombine the WDM signals. Between the multiplexer and demultiplexer separate nonlinear fibers are provided for each channel of the WDM signal. This setup has the advantage that only separate nonlinear fibers are used in contrast to the common approach where for each channel a separate complete regenerator is necessary. This concept significantly decreases the complexity of the setup because now only one bidirectional EDFA and fiber coupler are needed.
The operation principle of the modified NALM is as follows: The incoming WDM signal is split asymmetrically at the fiber coupler into two counter-propagating partial signals. The weaker signal propagates through the EDFA first where it is strongly amplified. Then it passes the demultiplexer “B” after which different wavelength channels propagate in separate nonlinear fibers. Because the signals in this propagation direction have been strongly amplified, they acquire significant phase shifts due to SPM in the nonlinear fibers. Afterwards the different channels are again multiplexed by the second multiplexer “A”.
The counter-propagating stronger signal is first split at the multiplexer “A” so that again the different channels propagate separately through the nonlinear fibers. However, the power of the signals in this direction is much lower compared to the amplified, originally weak signals. Therefore, the acquired nonlinear phase shifts in this direction are almost negligible. After the propagation through the nonlinear fibers the wavelength channels are recombined by the multiplexer “B” and the resulting WDM signal is amplified in the EDFA with the same gain as the weak partial signal. At the output port of the fiber coupler the weak partial signal with a large nonlinear phase shift interferes with the strong partial signal which has an almost unchanged phase. The latter, being much stronger, mainly determines the phase of the output signal and thus ensures negligible phase distortions. At the same time a nonlinear power characteristic with a plateau region is obtained for all wavelength channels, which is necessary for amplitude noise reduction.
3. Experimental results and discussion
To prove the suitability of this 2R regenerator for WDM phase-encoded transmission an RZ-DPSK signal consisting of two wavelength channels at 1544 nm and 1553 nm was used in our experiment (Fig. 2). Consequently, the modified NALM consisted also of two nonlinear fibers. The fiber used at 1544 nm had a length of 2 km, a nonlinearity of 3.5W-1km-1, a dispersion of -1.2 ps·nm-1km-1 and losses of 1.45 dB/km. The fiber used for the 1553-nm channel had a length of 3.1 km, a nonlinearity of 8.5W-1km-1, a dispersion of +0.54 ps·nm-1km-1 and losses of 0.64 dB/km with splice losses of 1 dB at each end. The fiber coupler in the NALM had a splitting ratio of 20:80.
The spectral widths of the 2-ps pulses from the two 10-GHz tunable mode-locked lasers (TMLLs) were about 2 nm at FWHM. The multiplexer and demultiplexer inside the NALM had a 200-GHz channel spacing and about 3 dB insertion loss for a CW-signal. Both signal bandwidths were adapted to the WDM multiplexer and demultiplexer channel spacing with narrow optical bandpass filters (75 GHz and 120 GHz) which had different transmission characteristics thus resulting in 4-ps long pulses measured in a FROG-setup. Thus the signals without the NALM and signals passing the NALM had comparable bandwidths also at the receiver. However, because of the insufficient edge steepness of these filters there would have been rather strong crosstalk of about -10 dB between neighboring channels in the multiplexers because of the overlap of the spectral wings. Therefore, a spectral separation of 1100 GHz between the channels was chosen. 4-ps pulse duration at high duty cycles corresponds to a data rate of about 100 Gb/s/channel. In this experiment only 10 Gb/s/channel were used because only then the pulse peak power from the saturated output of the bidirectional EDFA is high enough to achieve the necessary nonlinear phase shift in the available nonlinear fibers.
As optical amplifier a bidirectional EDFA operated in constant pump current regime was used. It consisted of a single erbium-doped fiber in order to have the same path length for both propagation directions inside the loop. The small-signal gain of the EDFA was about 23 dB, decreasing down to 16 dB in saturation for an input power of 5 mW. The gain was the same for both propagation directions. Polarization controllers were placed in front and inside the NALM to optimize the interference between the counter-propagating partial signals. Additionally, by using the polarization controllers the phase bias of the Sagnac interferometer could be set independently for each wavelength channel [10,11].
In order to characterize the regeneration capability of the proposed NALM setup, its influence on eye diagrams of a strongly amplitude-impaired RZ-DPSK signal was studied for both wavelength channels. First of all, a wavelength-multiplexed pulse train was produced by synchronously combining two 10-GHz pulse trains coming from the two TMLLs operating at different wavelengths in a 50:50 fiber coupler (see Fig. 2). This WDM pulse train was then modulated by a 27-1 pseudo-random bit sequence in a dual-drive Mach-Zehnder-Modulator (MZM). If there would be any nonlinear crosstalk between the channels inside the regenerator, regeneration of such a synchronized WDM pulse train would result in maximal nonlinear cross talk, amounting to a worst case scenario for the nonlinear phase accumulation inside the NALM. Since the intention was to show that the modified NALM has no difficulties operating under worst-case conditions decorrelation of the data patterns was not performed.
In order to achieve strong and fast amplitude fluctuations, a MZM driving voltage 40% smaller than Vπ was used and the bias voltage of the MZM was detuned from the optimal value. This approach makes it possible to attribute a broadening of the upper level in the eye diagram to an increase of phase distortions and a broadening of the lower level to an increase of amplitude distortions as is described in . The optically pre-amplified RZ-DPSK receiver comprised a tunable 1-nm band-pass filter for wavelength channel selection, a fiber based delay-line interferometer (DLI) for demodulation of the DPSK signal and a balanced photodiode (BPD).
To achieve optimal settings, the eye opening after the NALM was maximized for both wavelength channels by adjusting the input power into the NALM and also by optimizing the polarization controllers in front and inside the NALM. Best performance was found at an average input power of 12 mW for the channel at 1544 nm and 3.7 mW for the channel at 1553 nm. The resulting eye diagrams are shown in Fig. 3. The amplitude-distorted 1544 nm and 1553 nm channels are depicted in (a) and (c) whereas the regenerated signals are shown in (b) and (d), respectively.
The eye diagrams consist of 4·106 samples each. The eye opening, calculated for a unit interval of 0.1, is 0.94 dB larger for the regenerated 1544-nm channel than for the corresponding amplitude-distorted case. For the 1553-nm channel the eye opening is 1.2 dB larger when using the NALM. Clearly, the lower rails of the eye diagrams are significantly narrower for the regenerated channels indicating an efficient suppression of amplitude distortions. At the same time there is practically no broadening of the upper rail and thus there are no additional phase distortions in the regenerated 1544-nm channel. The slightly broadened upper rail of the regenerated 1553-nm channel indicates that small intensity-dependent phase distortions due to the NALM are present, but, since the eye-opening improvement is even bigger for this channel, these phase distortions have no impact on the signal quality. Also, the successful regeneration of the 1544-nm channel where the upper rail stays the same suggests that it is in principle possible to achieve negligible phase distortions. The reason for the non-ideal eye openings is most probably a polarization drift induced by a change of fiber-birefringence due to temperature fluctuations in the laboratory.
The bit error ratio (BER) was measured vs. the input power Prec into the DLI (Fig. 2) with the same amplitude-impaired signal as was used for the eye-diagram analysis. The power into the DLI was adjusted by the variable optical attenuator (VOA) in front of the optical preamplifier. The NALM settings were kept unchanged. The results for both channels are shown in Fig. 4 together with the data for the optimal DPSK signal with no intentional distortions. To characterize the amount of a possible crosstalk between the channels, single-channel BER-measurements were performed separately for both wavelength channels. In this case a CW-laser at 1536 nm was used to saturate the bidirectional EDFA in order to keep the NALM input power as well as the gain of the bidirectional EDFA the same as for the dual channel operation. The results of these measurements are also shown in Fig. 4. By using the amplitude-distorted DPSK signal, the receiver sensitivity was decreased by 1.8 dB in the case of the 1544-nm channel and by 1.6 dB for the 1553-nm channel for a BER of 10-9. Using the NALM to regenerate both signals led to an improvement of 1.2 dB in receiver sensitivity for both channels. The regeneration performance measured in single-channel operation is in good agreement with the performance of the dual-channel operation as the BER measurements differ less than 0.2 dB in the receiver input power. This suggests that there is no substantial crosstalk between the two channels inside the NALM.
To investigate the pulse shape of the regenerated pulses additional amplitude and phase measurements using the frequency-resolved optical gating (FROG) technique were performed in front and after the NALM. For both channels it was found that there are no significant pulse shape distortions in the regenerated signal besides a slight pulse broadening to about 6.5 ps at FWHM due to a slight mismatch in the filter functions of initial filters and multiplexer. Also the nonlinear phase shift imposed on the pulses due to the NALM is indeed small since the measured chirp was even smaller than for the not regenerated pulses which had already an initial chirp.
The results demonstrate that the modified NALM can be successfully used as a phase preserving amplitude regenerator for WDM DPSK signals. The proposed setup of the NALM is easily scalable for larger channel numbers given that the bidirectional EDFA provides sufficient power or that the length of the nonlinear fibers is adapted accordingly. Also, it is possible to dispense with the polarization controllers inside the NALM by using polarization maintaining components.
A new scheme for a phase-preserving NALM-based 2R regenerator has been proposed that uses a multiplexer and demultiplexer inside the NALM so that each wavelength channel of a WDM signal propagates in a separate nonlinear fiber. This reduces significantly the complexity of the setup since only one bidirectional EDFA and a fiber coupler are needed. In eye-diagram and BER measurements, the capability of this modified NALM has been demonstrated for WDM operation of two wavelength-multiplexed RZ-DPSK signals. Eye-diagram measurements have shown that the phase information is kept intact during the amplitude regeneration. Moreover, the nonlinear fibers used in the NALM setup had different lengths and even different signs of the group velocity dispersion thus demonstrating that the modified NALM can operate easily under a large range of conditions. Considering recent advances in highly nonlinear materials  also a full integration of all components of this NALM such as multiplexers, nonlinear waveguides and amplifying waveguides seems to be possible.
The authors thank Alcatel-Lucent, Nuremberg for the loan of the nonlinear fibers that were used in the NALM setup. This work was partly supported by the Federal Ministry of Education and Research under Grant 01BP564.
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