The cascadable recirculating buffer based on nonlinear polarization rotation in a semiconductor optical amplifier

ABSTRACT Optical buffer is an important component in optical switching and optical signal processing. In this paper, we present a novel all-optical buffer based on nonlinear polarization rotation induced by injection current of semiconductor optical amplifier. With Mueller matrix, we theoretically investigate the change of polarization state induced by nonlinear polarization rotation in semiconductor optical amplifier. The operation of the proposed optical buffer at 2.5Gb/s is experimentally setup. The loading and reading operations with 1024bit with this optical buffer are demonstrated. Polarization maintenance is taken into account to give the optimum operation condition Keywords: Optical buffer, nonlinear polarization rotation, principal states of polarizations, semiconductor optical amplifier 1. INTRODUCTION Optical buffers (OBs) have attracted widely attentions in optical router, optical switching, all-optical computer, optically phased array, and so on. In past years, several fiber-loop buffers have been reported. K. L. Hall et al. [1, 2] demonstrated an all-optical pulse storage ring storing a 1.25 kb packet at 10Gb/s. As [3, 4] showed, the optical buffer based on a wavelength converter was reported. R.Langenhorst et al. [5] extended the theory and described a model of the fiber loop buffer, incorporating semiconductor optical amplifier (SOA) as switching gates. Agarwal et al. [6, 7] presented the esable all-optical buffer based two NOLM. As reported [8, 9], Aiming Liu et al. presented the DLOB all-optical buffer. As the SOA can provide a high gain and a strong refractive-index change and allow photonic integration, it is widely used in the reported fiber-loop buffers [3-9]. However, in fiber loop buffers which configurations based on F-P cavity [6, 7] and DLOB [8, 9], whether the data is buffered or passed depends on the appearance of the interference between the lights that propagate in the two opposite directions in the fiber loop. Which result that the data must propagate in the fiber loop at least one circle whether the signal need buffer or not. Therefore, these buffers are difficult to cascade. Moreover, in these types of buffers [6-9], the SOA provides the nonlinear phase shift induced by the control light between the two opposite light beams, and thus, the SOA must deviate from the center of the fiber loop to prevent lights in the two opposite directions encountering in the SOA. According to the above reasons, the buffer depth (the ratio between the length of the data and the leng th of the fiber-loop) does not exceed 50%. In this paper, we present a new fiber loop buffer which is ba sed on the nonlinear polarization rotation (NPR) in SOA. In our buffer, we utilize two orthogonal polarization states to realize the data buffering and passing. A SOA provides the nonlinear polarization rotation induced by the injected curren t. The passing data can go through the SOA directly without propagating one circle in the fiber loop , so the buffer is easy to cascade. In the buffer, SOA does not place in the fiber loop; the buffer depth can exceed 50%. Moreover, because the po larization rotation switch can be operated at the time of 120fs [10, 11], the all-optical buffer based on polarization rotation can be operated ultra fast. In our experiments, we use the concept of Principal States of Polarizations (PSP) to find two orthogonal states of polarizations (SOPs) in the SOA [12, 13]. Although the polarization rotation in SOA attracts more and more attentio n, to our knowledge, an all-optical buffer based on the polarization rotation in SOA has not been reported.