Future all-optical communication systems may consist of local networks interconnected through nodes. In local network applications, the non-return-to-zero (NRZ) signal format is preferable to the return-to-zero (RZ) format because it requires less bandwidth for a given data transfer rate. However, bit-rate enhancement between network nodes can be achieved by time division multiplexing of optical signals in the RZ format with the use of an optical multiplexer. Thus, format conversion between NRZ and RZ is a key technology for future all-optical networks.
Several format conversion schemes have been demonstrated using injection-locked laser diodes1 or semiconductor optical loop mirrors.2 However, they are limited by potential bottlenecks. Furthermore, few schemes have been reported that operate at bit rates of 40 Gbit/s or above. It is obvious that future developments in high-speed optical communications will require better performance than is currently available.
We have demonstrated a symmetric Sagnac interferometric device capable of ultrafast operation at 40 Gbit/s or above. The advantage of this scheme compared with the terahertz optical asymmetric demultiplexer (TOAD) structure2 is that the influence of the carrier recovery time of the semiconductor optical amplifier (SOA) on the operation rate is further weakened because of the symmetric structure used. In addition, because the SOA is used as a nonlinear switching medium, it can be integrated with other semiconductor components.
Figure 1. Proposed implementation of an all-optical NRZ-to-RZ format converter. SOA: Semiconductor optical amplifier. CW: Clockwise. CCW: Counterclockwise. NRZ: Non-return to zero. RZ: Return to zero.
The novel symmetric Sagnac interferometric structure, similar in principle to that of the TOAD, is illustrated in Figure 1. The main components in the loop, two identical SOAs, are placed symmetrically around the upper coupler. The pump signal, which is a clock signal at a different wavelength from the probe signal data streams, is injected into the loop from port B via the upper coupler. The probe signal is coupled into the loop mirror via the lower 50/50 coupler, and is injected into NRZ signal data streams from port A, where it is split into two beams counterpropagating around the loop and recombining at the lower coupler. In operation, the clock signal, acting as pump beams, modifies the optical characteristics of the right SOA, and thereby controls the phase shift of the clockwise (CW) and counterclockwise (CCW) probe signal with sufficiently small intensity. As a result, the CW and CCW pulses interfere at port C either constructively or destructively, depending on the phase shift between them, and the NRZ data signal is converted to an RZ signal at output port C. The following equation describes the transfer function at the Sagnac-switching window:
Here,t is the pulse round-trip time, and i(i=1, 2) represents the SOAs. Gi(t)=Pi_out(t)/Pi_in(t), i=1, 2; Pi_out and Pi_in are the output and input signal power of the CW or CCW probe signals, respectively. Pin is the input signal power of the probe pulse, and PC is the output power of the converted RZ data signal at port C.
Figure 2 presents the results of a numerical simulation of an all-optical NRZ-to-RZ format converter based on the novel scheme3 using 40Gbit/s transmission of 128bit pseudo-random binary sequences. Figure 2(a) and (b) shows the input pulses (probe and pump, respectively), while Figure 2(b) shows the output pulses. The simulation is consistent with an all-optical NRZ-to-RZ format converter, with an extinction ratio for the output RZ data signals of 19.1dB. The scheme performed well and shows good potential for application in ultrafast all-optical NRZ-to-RZ format conversion.
Figure 2. Simulation results of a 40Gbit/s all-optical NRZ-to-RZ format converter using 128bit NRZ pseudo-random binary sequences: (a) the input NRZ data signal serving as the probe; (b) the output result of the converted RZ data signal; (c) the input clock signal functioning as the pump.
We believe that the symmetric Sagnac interferometer structure can overcome a growing electronic bottleneck and enhance network capacity in future communication systems. This structure has proven to be a serious candidate and has been extensively studied in various all-optical signal processing applications. In the future, we plan to focus on novel devices in all-optical signal processing at high bit rates, which is still a subject of intensive research.
School of Information
Central University of Finance and Economics
Zhixin Chen received his MS from South China Normal University in 2004 and a PhD from Beijing University of Posts and Telecommunications in 2007. His interests include high-speed optical communication systems and networks, and optical switching.