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All-optical signal regeneration with wavelength multicasting has been demonstrated using cross-absorption modulation in a single electroabsorption modulator for the first time. We show that the input signal wavelength can be simultaneously converted to 6 different wavelengths at 10 Gb/s with signal regeneration. The output extinction ratio, the linewidth, and the pulse shape show a significant improvement. A negative power penalty of 2 dB is obtained at 10-9 bit-error-rate level.
©2004 Optical Society of America
1. Introduction
All-optical regeneration of degraded signals is desirable for future large-scale optical networks. Cross-absorption modulation (XAM) in an eletroabsorption modulator (EAM) [1] is a promising technique for all-optical signal regeneration and wavelength conversion owing to its high-speed performance, low-chirp characteristics, and capability to maintain the same logical polarity. Much research has been focused on the investigation of the EAM-based wavelength converter, time-division demultiplexer, and its application for all-optical signal regeneration [2–6]. Recently, wavelength multicasting has been successfully demonstrated based on XAM in an EAM [7, 8]. Many advanced WDM networks require the support of multicasting, where a number of applications such as video distribution and teleconferencing require a multicast connection to be established [9–11]. In this paper, we further combine the capability of signal regeneration with wavelength multicasting of the setup. With the XAM effect of the EAM, a degraded input signal is successfully multicasted to 6 different wavelength channels with significant improvement in the extinction ratio (ER), the linewidth and the pulse shape. A complete 10 Gb/s bit-error-rate (BER) test is also performed and a negative power penalty of 2 dB is obtained at 10-9 BER level for all output channels.
2. Experiment
The experimental setup on wavelength multicasting with signal regeneration is shown in figure 1. The input signal (S) is a 231-1 bits pseudorandom return-to-zero (RZ) signal at 10 Gb/s, which is prepared by gain-switching a distributed-feedback (DFB) laser diode at 10 GHz followed by external modulation. In order to demonstrate the signal regeneration properties of our setup, an input signal with an ER of around 5 dB is prepared. The signal then propagates in 3-km standard single-mode fiber (SMF) so that the pulse width is broadened. Next, the signal is amplified by an erbium-doped fiber amplifier (EDFA) and is launched into a commercial 1.55-µm EAM through an optical circulator. The EAM module consists of an InGaAsP waveguide buried with Fe-doped InP. At the opposite port, 6 cw inputs generated from a WDM laser source are combined by a WDM multiplexer (MUX) and are launched into the EAM. The separation of each channel is 200 GHz. In our setup, the EAM is reverse-biased at -3.0 V. Nonlinear optical transmission in the EAM is realized by applying the input optical signal to produce a large number of photo-generated charge carriers in the highly absorptive waveguide. During the high-level of the NRZ signal, due to drift and diffusion the photo-generated holes and electrons are separated from each other. Therefore, the charge neutrality is lifted and gives rise to a screening of the applied electric field. The reduction of the electric field and bandfilling by the photo-generated carriers significantly decrease the absorption and create a transmission window [1]. This XAM introduced by the input signal will thus modulate the counter-propagating cw light beams [3, 7]. Due to the nonlinear characteristics of the transmission window, output signal regeneration is possible [5]. The converted signal at each wavelength is obtained through a WDM demultiplexer (DEMUX) connected to the optical circulator.
Fig. 1. Experimental setup on signal regeneration with wavelength multicasting using cross-absorption modulation in an electroabsoption modulator. EA MOD: electroabsorption modulator; PS: polarization scrambler; EDFA: erbium-doped fiber amplifier; MUX: multiplexer; DEMUX: demultiplexer; BER test set: bit-error-rate test set.
3. Results and discussion
Figure 2 shows the spectrum obtained from port B of the optical circulator without the DEMUX. The input signal is at λs=1547.7 nm, and the 6 output channels vary from λ1=1550.5 nm to λ6=1558.5 nm with a 200 GHz spacing. The input power of λs is 13 dBm and the cw input power of each channel is 7 dBm. Figure 3(a) shows the spectrum of the input signal with linear vertical scale measured by an optical spectrum analyzer with 0.01 nm resolution. Since the input RZ signal is prepared by gain-switching a DFB laser diode followed by external modulation, it is observed that the 3-dB linewidth is broadened to 60 GHz. Figure 3(b) shows the output spectrum of channel 1 with linear vertical scale and it is found that the 3-dB linewidth is greatly reduced to 2 GHz.
The power penalty of the wavelength converter has been studied by performing BER measurements at 10 Gb/s. In the experiment, the cw inputs of all the 6 channels are always “ON” and BER measurements on individual channels are performed. Figure 4 plots the output BER against the received optical power of each channel. The insets show the eye-diagrams of the input signal (upper) at port A and output signals (lower) at port C. Since the input signal has propagated in the 3-km SMF, the pulse width is broadened to 60 ps. The lower trace shows the output eye-diagram of channel 1 at -15 dBm received power. We can find that the pulse width is reduced to 25 ps. The improvement is due to the nonlinear optical transmission characteristic of the EAM under reverse bias, which allows reshaping of the degraded signal [5]. The ER of the output signal also improves from 5 dB to 12.6 dB. The low optical transmission at small pump power allows noise suppression on zeros [5] so that ER improvement is obtained. Note that similar results are also obtained for the other 5 channels. By comparing the BER performance of the input and output signals, a negative power penalty of 2 dB at 10-9 BER level is obtained for all the channels.
Since the EAM is inherently polarization independent, it is worth noting that the setup can accept arbitrary input signal polarization without affecting the output signal quality. In order to study the polarization characteristics of the converted signal, a polarization analyzer is used to monitor the output state of polarization (SOP). The SOP of the input signal is fully randomized by a polarization scrambler and the degree of polarization (DOP) is less than 5%. Figure 5(a) depicts the Poincare sphere of the input signal. It shows that the SOP is randomized over the whole sphere. Figure 5(b) depicts the corresponding Poincare sphere of the channel 1 output accumulating for 30 minutes, showing that the SOP is confined to a small spot on the sphere even though the input polarization keeps changing randomly. The DOP of the converted signal is measured to be over 98%. Since many devices used in communications, such as modulators and amplifiers, are polarization sensitive, the stable polarization output of the EAM-based wavelength converter is very desirable for optical network applications.
Fig. 4. Plot of the bit-error-rate against the received power in a 10 Gb/s BER measurement. The inset shows the eye diagrams of the input signal (upper) and the channel 1 output (lower).
Fig. 5. Poincare spheres of (a) the input signal with random polarization and (b) the output signal with a stabilized state of polarization.
4. Conclusion
In conclusion, all-optical signal regeneration with wavelength multicasting up to 6×10 Gb/s has been demonstrated using cross-absorption modulation in an electroabsorption modulator. The results show a significant improvement in the output signal quality and a negative power penalty of 2 dB is obtained.
Acknowledgments
The work described in this paper was supported by the Research Grants Council of the HKSAR, China (Project No. CUHK 4196/03E).
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