Abstract: We demonstrate a unique solution to use a one-pulse control to achieve simultaneous two-channel all-optical demultiplexing that usually requires a two-pulse control or a two-step operation based on the conventional approaches. By applying a dispersion asymmetric nonlinear optical loop mirror (DA-NOLM) to introduce cross phase modulation (XPM) in both the clockwise (CW) and the counter clockwise (CCW) propagating branches, we have demonstrated reconfigurable, error-free 40-to-10 Gb/s two-channel demultiplexing (DEMUX) for OTDM OOK signals. Switchable operation between two-channel and single-channel DEMUX is also realized based on the proposed all-optical DEMUX configuration.
©2010 Optical Society of America
Optical time division multiplexing (OTDM) is an important technology for generating high speed optical signals while occupying only one single wavelength channel. In OTDM systems, demultiplexing (DEMUX) can be implemented either by electrical or optical means. Along with the rapid increase of bit rates up to tens and hundreds of gigabit/s, it is more favorable to perform all-optical DEMUX, avoiding OEO conversion and offering ultra-fast response of the processing [1,2]. All-optical DEMUX has been previously demonstrated based on various nonlinear effects in optical fibers and semiconductor optical amplifiers (SOA), such as cross phase modulation (XPM) in a nonlinear optical loop mirror (NOLM) and nonlinear polarization rotation (NPR) [3–5].
In addition, in a reconfigurable optical network, it is necessary to perform dynamic allocation of the bandwidth . An example is in allocating different tributaries from the OTDM signal to one local distribution point, followed by subdividing the tributaries to sub-local distribution points or local users. Such a scenario requires reconfigurable DEMUX which can be realized with cascaded stages of DEMUX and multiple control pulses with reconfigurable operation. In order to achieve two-channel or multi-channel DEMUX with the above all-optical DEMUX techniques, multi-step operation or multiple pulse control will be required.
In this work, we demonstrate a solution to use only a single control pulse for simultaneous (one-step) two-channel DEMUX based on a NOLM with asymmetric dispersion inside the loop. Besides, no extra fiber coupler is needed inside the 3-dB fiber loop mirror [7,8]. Channel selection can be easily realized by wavelength tuning owing to asymmetric dispersion in the fiber loop. All-optical two-channel DEMUX has been achieved for 40-Gb/s OTDM OOK signals. Switchable operation between two-channel and single-channel DEMUX has also been demonstrated, offering the flexibility for reconfigurable operation and for dynamic bandwidth allocation.
DA-NOLM: Principle for Two-Channel DEMUX
The structure of the dispersion asymmetric NOLM (DA-NOLM) is schematically shown in Fig. 1(a) . The loop mirror contains a nonlinear medium such as a highly nonlinear fiber (HNLF) next to a group velocity dispersion (GVD) element. The GVD element can simply be a standard single mode fiber (SMF) that introduces a relative delay between light branches at different wavelengths (the delay between the control pulse and the OTDM signal in this case). In the counter clockwise (CCW) operation, the control pulse at λ1 and signal at λ2 propagate together in the nonlinear medium. XPM will result in a phase change of the signal at λ2. As for the clockwise (CW) operation, the control and the signal will first arrive at the GVD element where a relative delay will be introduced. Subsequently, the signal will undergo XPM initiated by the relatively delayed control pulse in the nonlinear medium. However, the phase change now occurs at a different OTDM channel compared to that of the CCW operation. Hence, two different baseband channels can be switched out.
In Eq. (2), γ and L stand for the nonlinear coefficient and the length of the HNLF, and P1/2cw/ccw is the peak control power for the XPM process over C-1/C-2 in the CW/CCW operation. Assume that a control power of P0 is required for a π phase change and that P1cw = 0, P1ccw = P0, P2cw = P0, P2ccw = 0 in Fig. 1(b), a π phase change will be introduced for both C-1 and C-2, giving rise to simultaneous two-channel DEMUX according to Eq. (1).
The relative delay (Δt) is adjustable by wavelength tuning of the control pulse due to the presence of GVD. This feature offers reconfigurability of the demultiplexer. For example, with ~25 ps relative delay, C-1 and C-2 can be simultaneously demultiplexed from the 40-Gb/s OTDM signal, while C-1 and C-3 will be demultiplexed when the relative delay is ~50 ps. The relation between the wavelength spacing and the relative delay is shown in Table 1 for different DEMUX channels obtained with a GVD of 10 ps/nm.
Figure 2 shows the experimental setup for two-channel DEMUX of a 40-Gb/s OTDM OOK signal. In Fig. 2(a), the upper branch of the 10-GHz pulse train at λ2 is amplified to ~26 dBm (average power) for spectral broadening by self-phase modulation (SPM) in HNLF-1, which has a nonlinear coefficient of 11-W−1km−1 at ~1550 nm. The input 10-GHz pulse train at λ2 has a pulse width of 10.6 ps. The SPM process in HNLF-1 significantly broadens the spectrum from 2 nm to 14 nm (20-dB bandwidth) as shown in Fig. 3 . By spectral slicing with a tunable bandpass filter (2-nm Gaussian filter, 3-dB bandwidth) after HNLF-1, we obtain a wavelength tunable 10-GHz control pulsed source with a typical width of 11 ps.
In Fig. 2(a), the lower branch of the 10-GHz pulse train at λ2 is used in 40-Gb/s OTDM OOK signal generation through electro-optic modulation at 10-Gb/s with pseudo random binary sequence (PRBS, 231-1) followed by 1 × 4 optical multiplexing. In this work, we set λ2 at 1547-nm and tune λ1 over the range of 1542 to 1556-nm.
The 40-Gb/s OTDM OOK signal is then combined with the 10-GHz amplified control pulse in the optical coupler, as illustrated in Fig. 2(b). The signal and the control pulse are together launched into the DA-NOLM through an isolator. Inside the loop, a 638-m SMF is used to introduce the GVD and another 1-km HNLF (HNLF-2) is used as the nonlinear medium. HNLF-2 has a nonlinear coefficient of 11.7 W−1km−1 and a dispersion coefficient of 0.02 ps∙nm−1km−1 around 1550 nm. The GVD of the SMF is 10 ps/nm, resulting in a relative delay of 50 ps when Δλ = | λ1 - λ2 | = 5 nm. With the 10-GHz control pulse overlapped temporally with C-1 but spaced spectrally at 2.5, 5.0, and 7.5 nm apart from the signal as shown in Fig. 4(a) , we achieve two-channel DEMUX for C-1/C-2, C-1/C-3 and C-1/C-4, respectively. The corresponding eye diagrams are shown in Fig. 4(b).
Meanwhile, according to Fig. 1, C-1 is synchronized with the control pulse before it enters the DA-NOLM. Hence, C-1 rather than the other channels experience XPM in the CCW operation. By synchronizing C-2 instead of C-1 with the control pulse, one can also achieve C-2/C-3 and C-2/C-4 DEMUX with 2.5 and 5.0-nm wavelength detunings, respectively. The two-channel DEMUX results, with widely opened eye-diagrams shown in Fig. 4(b), are in good agreement with the analysis in Tab. 1.
The proposed DA-NOLM is also capable of single-channel DEMUX. Switchable operation between two-channel and single-channel DEMUX is shown in Fig. 5 . Here, λ1 is set at 1554.5 nm and C-1 overlaps temporally with the control pulse. The XPM process for C-1 occurs in the CCW operation while that of C-4 occurs in the CW operation after the GVD medium. Since the control and signal pulses are broadened and distorted before demultiplexing takes place through optical gating with XPM, the performance is poorer in the CW operation. In this work, we use a polarization controller (PC) in the fiber loop to balance the DEMUX performances between the two channels. A small difference can still be observed as depicted in Fig. 4(b) and Fig. 5. The CCW operation DEMUX (C-1) performs slightly better than that of the CW operation (C-2, C-3, C-4).
As shown in Fig. 5, the eye openings of the selectable single-channel DEMUX obtained by polarization readjustment are comparable to those of two-channel DEMUX. Based on single-channel DEMUX with control wavelength detunings of 2.5, 5.0 and 7.5 nm, we have performed BER measurements and the results are shown in Fig. 6 . Error free DEMUX has been obtained for all channels. C-1 DEMUX is obtained by XPM in the CCW operation while C-2, C-3 and C-4 DEMUX are obtained by XPM in the CW operation. The results indicate error free two-channel DEMUX with ~2-dB difference in receiver sensitivity between the two output channels. Compared to the back-to-back 10-Gb/s BER measurement, our demultiplexer introduces ~2-dB and ~4-dB power penalty for CCW and CW operations, respectively.
It is worth mentioning that to perform a BER measurement for simultaneous two-channel DEMUX, one will need to carry out an extra demultiplexing step to retrieve a single 10-Gb/s tributary, thus resulting in additional power penalty. Hence, the result will not be a fair comparison with that obtained from conventional single-channel DEMUX.
In this work, we use a relatively large GVD (10 ps/nm) to introduce the relative delay. It appears that the GVD can lead to pulse broadening and will be a concern in high-bit-rate DEMUX. To minimize the broadening effect while keeping a large relative delay, one can use a smaller GVD with a larger control-signal wavelength detuning, or a specially designed GVD mapping to compensate the broadening. Considering the operation in C-band for 10-Gb/s baseband OTDM signals, we believe reconfigurable two-channel DEMUX for 160 Gb/s is achievable.
It is foreseeable that the proposed DA-NOLM can also be used for other formats like phase shift keying (PSK). In particular, with the use of four-wave mixing rather than XPM , improved applications such as simultaneous demodulation and demultiplexing of DPSK-OTDM signals can be achieved and the work is under investigation.
We have demonstrated reconfigurable two-channel DEMUX based on a DA-NOLM using a single baseband control pulse. Error free 40-to-10 Gb/s DEMUX has been achieved for an OTDM OOK signal using a 10-GHz control pulse. The new scheme of demultiplexing also provides dynamic channel selection by tuning of the control wavelength. DEMUX operations with both two-channel and single-channel outputs have been demonstrated.
This work is supported by the Research Grants Council of Hong Kong (Projects CUHK 415907, 416808, 416509).
References and links
1. S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber Commun. Rep. 3(1), 1–24 (2006). [CrossRef]
2. S. Boscolo, S. K. Turitsyn, and K. J. Blow, “Nonlinear loop mirror-based all-optical signal processing in fiber-optic communications,” Opt. Fiber Technol. 14(4), 299–316 (2008). [CrossRef]
3. T. Yamamoto, E. Yoshida, and M. Nakazawa, “Ultrafast nonlinear optical loop mirror for demultiplexing 640 Gbit/s TDM signals,” Electron. Lett. 34(10), 1013–1014 (1998). [CrossRef]
4. I. D. Phillips, A. Gloag, P. N. Kean, N. J. Doran, I. Bennion, and A. D. Ellis, “Simultaneous demultiplexing, data regeneration, and clock recovery with a single semiconductor optical amplifier based nonlinear-optical loop mirror,” Opt. Lett. 22(17), 1326–1328 (1997). [CrossRef]
5. J. H. Lee, T. Tanemura, K. Kikuchi, T. Nagashima, T. Hasegawa, S. Ohara, and N. Sugimoto, “Use of 1-m Bi2O3 nonlinear fiber for 160-Gbit/s optical time-division demultiplexing based on polarization rotation and a wavelength shift induced by cross-phase modulation,” Opt. Lett. 30(11), 1267–1269 (2005). [CrossRef] [PubMed]
6. M. Roelens, J. A. Bolger, D. Williams, S. Frisken, G. W. Baxter, A. M. Clarke, and B. J. Eggleton, “Flexible and Reconfigurable Time-Domain De-Multiplexing of Optical Signals at 160 Gbit/s,” IEEE Photon. Technol. Lett. 21(10), 618–620 (2009). [CrossRef]
7. N. Chi, L. Xu, K. S. Berg, T. Tokle, and P. Jeppesen, “All-optical wavelength conversion and multichannel 2R regeneration based on highly nonlinear dispersion-imbalanced loop mirror,” IEEE Photon. Technol. Lett. 14, 469–471 (2002).