Novel transmitter of dark RZ signal based on external modulator

Abstract

A novel transmitter to generate a dark RZ signal with tunable duty cycle and extinction ratio is proposed, by modifying the process of precoding, modulating and coding. A dark RZ signal is generated simply by using one dual-arm Mach-Zehnder LiNbO 3 modulator. We demonstrate experimentally that this optical dark RZ signal can be directly measured by a conventional binary intensity modulation direct detection (IM-DD) receiver. When different values of duty cycles at 2.5 Gbit/s are adjusted, the experimental results show different BER curves and eye diagrams of the optical dark RZ signal. CLC number: TN832 Document code: A Article ID: 1673-1905(2007)02-0109-03 DOIIt is well known that a few advanced optical modulation formats have been proposed recently [1][2][3][4][5][6] . In order to reduce complication of modulating or coding in the transmitter and decoding in the receiver, Jianjun Yu [7] demonstrated a simple method to generate the MD-RZ by using only one dual-arm Mach-Zehnder LiNbO 3 modulator (LN-MOD). Moreover, half-bit delayed dark RZ payload with orthogonal modulation DPSK label has been proposed [8] , where the dark-RZ pulse was generated by using a nonlinear optical loop mirror (NOLM). The dark RZ signal generated by this method can be used in orthogonal labeling optical networks. But using an NOLM to generate dark RZ pulse needs high power consumption. In this paper, we report a simple and novel dark RZ signal transmitter implementation, which only requires one dual-arm Mach-Zehnder LiNbO 3 modulator. The principle for dark RZ signal generation is similar to a modified duo-binary signal generation in Ref. [7]. Through analyzing the process of modulating and coding theoretically, we propose and experimentally demonstrate a novel method to generate a dark RZ signal with tunable duty cycle and extinction ratio. This dark RZ signal is easily generated only by using a dual-arm LiNbO 3 M-Z modulator. We demonstrate experimentally that this optical dark RZ signal can be directly measured by a conventional binary intensity modulation direct detection (IM-DD) receiver. The experimental results show the BER performances and eye diagrams of the optical dark RZ signal with different duty cycles at 2.5 Gbit/s.Precoding is unavoidable to generate a dark RZ signal in the optical transmitter, because it can avoid error propagation for transmission [9] .Assuming that the NRZ pulse has a waveform shape of q(t), the transmitted signal of the NRZ pulse can be written aswhere T is the bit time period, and d k is the binary data "1" or "0". Here k is the word-length of the pseudorandom sequence bits. The signal after precoding can be expressed as 1 ( ) ( ), 0,1 k k