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We report the first experimental demonstration of pulsed pump wavelength exchange in a highly nonlinear dispersion-shifted fiber for alloptical time de-multiplexing of 80 Gb/s return-to-zero (RZ) signals. Orthogonal pulsed pump and continuous wave (cw) pump are used in the fiber-based wavelength exchange for ultra-fast power switching. Error-free operation was achieved for the proposed all-optical 1:8 de-multiplexer over all time de-multiplexed 10 Gb/s RZ signals with ≤ 2.1 dB power penalty at 10-9 bit-error rate. The experimental results show that a wide-band phasematching is feasible for fiber-based nonlinear parametric processes.
©2008 Optical Society of America
1. Introduction
With ultra-fast nonlinear response, fiber-based all-optical signal processing techniques are promising core solutions for the future high speed optical communication networks. Various all-optical signal processing techniques utilizing cross-gain modulation (XGM), cross-phase modulation (XPM) and four-wave mixing (FWM) effects in fibers have been explored to enable applications in optical communication networks including wavelength conversion, packet routing and switching [1, 2], and optical add-drop. Among these fiber nonlinear effects, all-optical wavelength exchange based on parametric effects in optical fibers is of particular interest owing to its unique features of simultaneous power exchange between different wavelengths [3, 4], low-noise operation [5, 6] and non-interferometric configuration. Exchange of two adjacent wavelengths is achieved with two phase-matched pumps under FWM process in which the signals undergo a periodic exchange in their power. This power exchange operation between signals is particular attractive for manipulating high speed optical time-division multiplexing (OTDM) data stream at its raw bit-rate compared with the approach of using electric bandwidth dependent electro-absorption modulators for demultiplexing of OTDM signals.
All-optical wavelength exchange has been demonstrated using two parallel or orthogonal continuous wave (cw) pumps [4, 7]. Yet, the gating pumps remain in cw or quasi-cw for applications like wavelength routing and on-off gating of signals mainly at low bit-rate (≤ 40 Gb/s) operations with non-return-to-zero (NRZ) modulation format due to the tight phase-matching condition. This tight phase-matching requirement for broad spectral width pumps and signals over a wide wavelength range makes a fully exchange between two short pulse channels difficult and can only be achieved for fibers with small dispersion effects.
In this paper, instead of using cw or quasi-cw pumps as in the previous demonstrations [3–7], a synchronized pulsed pump and a cw pump were used in a wavelength exchange-based OTDM de-multiplexer. With a synchronized pulsed pump in the time domain, wavelength exchange can be applied in de-multiplexing or simultaneous add-drop an OTDM signal. The advantage of using wavelength exchange as the add-drop element for OTDM system over the conventional wavelength conversion schemes is the complete exchange nature between signals. In other words, a specific channel at certain time instance can be dropped from the OTDM data stream, whereas another synchronized channel can be added to the OTDM data stream simultaneously. The potential polarization insensitive operation of wavelength exchange using polarization diversity scheme [8 , 9] further strengthens its attractiveness over the interferometric approach using nonlinear optical loop mirror (NOLM) for add-drop of OTDM channel [10].
Here, we demonstrate an experimental validation of periodic wavelength exchange using a cw pump, a 10 GHz pulsed pump and an 80 Gb/s OTDM return-to-zero (RZ) signal for OTDM signal de-multiplexing applications.
2. Operating principle
Fig. 1. Wavelength allocation and SOP of the pumps and the signals in an orthogonal-polarized pulsed pump wavelength exchange.
A complete wavelength exchange operation consists of four waves arranged symmetrically with respect to the zero-dispersion wavelength λ0 of the fiber, so that their angular frequencies (ω = 2π c / λ) have a relation of
where λpi and λsi (i = 1, 2) are respectively the wavelength of the interplaying pumps and signals. Perfect phase-matching condition is generally not possible due to the change of dispersion profile along the propagation direction. Consequently, a propagation constant mismatch
among these four waves is introduced for propagation constants (βpi and βsi) in the fiber. Effects from this propagation constant mismatch are particularly severe for a long nonlinear interaction in the fiber, and are magnified with spectral width of the interplaying waves.
A pulsed pump wavelength exchange is demonstrated by de-multiplexing an OTDM RZ signal with a synchronized pulsed pump and a cw pump. The wavelength allocation is shown in figure 1. An orthogonal-polarized wavelength exchange operation is proposed by setting the state-of-polarization (SOP) of the pumps in orthogonal state [4]. This orthogonal-polarized operation between two intense neighboring pumps minimizes the undesirable nonlinear interaction between them and suppresses the spurious FWM components.
Fig. 2. Schematic illustration of (upper) de-multiplexing, and (lower) simultaneous add-drop of OTDM signal in a pulsed pump wavelength exchange. Inset shows the wavelength allocation and the fiber dispersion curve.
In this proof-of-principle demonstration of a pulsed pump wavelength exchange, only two pumps and a signal are input to the nonlinear fiber leading to a power transfer of the signal from λs1 to λs2 with the presence of the pulsed pump at λp2. For applications of de-multiplexing an OTDM signal, the OTDM channel aligned (in the time domain) with the pulsed pump will be switched to a new wavelength at λs2 and extracted by spectral filtering. Based on the unique characteristics of wavelength exchange, however, the same configuration can be applied to realize a simultaneous add-drop of an OTDM channel by switching a time-aligned RZ signal at λs2 as a possible extension of the proposed scheme. These two operations are schematically illustrated in figure 2. The peak power max {Pp2} of the pulsed pump is adjusted to follow [4]
for a fiber length L, so that maximum wavelength exchange occurs at the center of an OTDM channel. As the pump power is time-dependent, a near complete signal transfer occurs at the peak of the OTDM signal, whereas this power transfer is incomplete at other times towards the edges of the pulsed pump leaving an M-shaped residual at the original wavelength. A consequence of this incomplete power transfer within the signal pulse is a distorted pulse shape of the de-multiplexed signal. The wavelength exchanged, de-multiplexed signal will therefore exhibit a narrower pulse width. A potential problem of this incomplete power transfer is the interference between the residual power and the original signals in the simultaneous add-drop operation. The challenge of incomplete power transfer over a bit slot could be alleviated by using a pump pulse with a flat top as demonstrated in [11].
3. Experiment
Fig. 3. Experimental setup of the pulsed pump wavelength exchange for de-multiplexing of OTDM signals.
Figure 3 shows the experimental setup of the pulsed pump wavelength exchange for de-multiplexing of OTDM signals. The input 80 Gb/s OTDM RZ signal at 1562 nm (λs1) was generated by time-multiplexing an intensity-modulated 2 ps 10 GHz pulse train from a mode-locked fiber laser (MLFL) with a Mach-Zehnder modulator. An erbium-doped fiber amplifier (EDFA) was used before the bit-rate multiplier to compensate its loss (≈ 16 dB). The pseudo-random binary sequence (PRBS) length of the resultant bit-rate multiplied 80 Gb/s OTDM signal was 27-1 bits. This 80 Gb/s signal was then amplified to 13 dBm average power with another stage of EDFA.
On the other hand, a pulsed pump at 1548 nm was prepared through wavelength conversion of the tapped MLFL output. This tapped output was amplified to 16 dBm average power and combined with an amplified cw probe light at 1548 nm. The combined signals were then launched into a 100 m dispersion-flattened photonic crystal fiber (PCF) followed by a fiber optic polarizer (POL) for wavelength conversion of the pulse train to the cw wavelength at 1548 nm (λp2) through cross-polarization modulation (XPolM) [12]. The nominal dispersion, dispersion slope, and nonlinear coefficient of the PCF used are respectively ≈ -0.23 ps/nm.km, -0.0034 ps/nm2.km, and 11 W-1km-1. The measured pulse width of the wavelength-converted pulsed pump was ≤ 9 ps, and was limited by the operating bandwidth of the photo-detector. The wavelength-converted pulse train at 1548 nm was then amplified to 19 dBm average power and combined with a cw pump light at 1540 nm (λp1) and 27 dBm average power. The cw pump light was phase-dithered with a phase modulator (PM) driven by a 10 Gb/s NRZ signal with 27-1 bits PRBS length for stimulated Brillouin scattering (SBS) suppression. The pumps (cw and pulsed) and the 80 Gb/s OTDM signal were then combined with polarization controller (PCs) at each corresponding arm and launched into a 400 m highly-nonlinear fiber (HNLF) for wavelength exchange-based signal de-multiplexing. A low optical power was set for the OTDM signal to avoid the undesirable nonlinear effects. This HNLF having a nonlinear coefficient γ = 14 W-1km-1, a zero-dispersion wavelength λ 0 = 1554 nm and a dispersion slope dD/dλ = 0.03 ps/nm2km, served as the nonlinear medium for wavelength exchange process.
Channel selection was carried out by aligning the pulsed pump with the bit slot of the target channel using an optical delay line (ODL). After wavelength exchange, the surviving 70 Gb/s signal and the de-multiplexed 10 Gb/s signal were separated using tunable optical bandpass filters (TBPF).
4. Results and discussion
Fig. 4. Measured eye diagrams of (a) the original 10 Gb/s RZ signal and (b) the bit-rate multiplied 80 Gb/s signals. (c) Waveform of the wavelength converted pulsed pump at 1548 nm. Time base: (a) and (b) 20 ps/div; (c) 10 ps/div.
Figure 4(a)-(c) show respectively the generated 10 Gb/s RZ signal, the time-multiplexed 80 Gb/s OTDM signal, and the wavelength-converted pulse train. As indicated in Fig. 4(c), a pulse pedestal was observed in the PCF-based wavelength converter due to the walk-off effect in the nonlinear interaction under a finite fiber birefringence. This walk-off effect is the major limitation for higher speed operations in the current setup and can be overcome by using a separate pulse source to serve as the pulsed pump.
Fig. 5. Measured eye diagrams of (a) the 10 Gb/s de-multiplexed signals and (b) the 70 Gb/s surviving signals at different time delays of the pulsed pump. Time base: 20 ps/div.
Figure 5 lists the measured eye diagrams of the wavelength exchange de-multiplexed 10 Gb/s signals and their corresponding 70 Gb/s surviving signals, with the measured optical spectra shown in Fig. 6. Clear and widely open eye diagrams were observed simultaneously for the de-multiplexed and the surviving signals. The pulse ringing observed in the de-multiplexed signal was due to the limited operating bandwidth of the photo-detector and the sampling system. On the other side, the corresponding OTDM channel was dropped with a slight power drop for the neighbor channel of the 70 Gb/s surviving signal due to the distorted pulse shape of the wavelength converted pulsed pump as shown in Fig. 4(c). The pulse pedestal in the wavelength converted pulsed pump depleted the power of the signal pulse after to the target channel slot. Consequently, the neighbor channel observed a finite pump power at λp2 completing the wavelength exchange conditions. Such power drop can be avoided by using another synchronized pulse source as the pulsed pump in a full scale implementation to further improve the performance of the proposed wavelength exchange-based de-multiplexer.
Fig. 6. Optical spectrum of the 80 Gb/s OTDM signal, the wavelength-converted pulsed pump, the output of the HNLF, and the filtered and amplified de-multiplexed 10 Gb/s signal.
To quantify the performance of the de-multiplexer, bit-error rate (BER) of the de-multiplexed signals were measured and compared against the back-to-back (btb) 10 Gb/s RZ signal as shown in Fig. 7. Error-free operations were attained for all wavelength exchange-based de-multiplexed channels. The power penalty, compared with the btb RZ signal of similar pulse width, incurred in the de-multiplexing ranged from 0.6 to 2.1 dB at 10-9 BER. This power penalty was mainly caused by the amplitude fluctuation of the bit-rate multiplied OTDM signal; whereas its variation among different channels was believed to be caused by the phase noise under the phase-dithering of the cw pump light. As the rapidly changing phase of the dithered cw pump light was transferred to the generated idler, i.e. the de-multiplexed signal, the channels aligned to the rising or falling edges of the dithering signal suffer from a greater frequency chirp, which was transformed into amplitude fluctuation after spectral filtering. Such degradation can be avoided by using two pulsed pumps. Nevertheless, the results suggest a wide-band phase-matched wavelength exchange is possible for OTDM applications with pulse width down to 2 ps.
5. Conclusions
Pulsed pump fiber-based wavelength exchange has been experimentally demonstrated for alloptical time de-multiplexing. An 80 Gb/s OTDM signal has been successfully de-multiplexed into 8 individual 10 Gb/s signals. Clear and widely open eye diagrams are obtained for all 8 de-multiplexed channels, with a less than or equal to 2.1 dB power penalty at 10-9 BER. The experimental results show that a wide-band phase-matching is feasible for wavelength exchange of 2 ps pulses. The proposed wavelength exchange-based de-multiplexer is promising for high performance simultaneous add-drop of OTDM signals. Further performance enhancement is anticipated with improved pulsed pump sources for higher bitrate operations.
Acknowledgment
The work described in this paper was partially supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKU 7179/06E and HKU 7172/07E). The authors would also like to acknowledge Sumitomo Electric Industries for providing the HNLF.
References and links
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