Reshaping of picosecond optical pulses with a simple interferometer

Abstract

In optical fiber communication systems, reshaping is crucial for long-haul transmission and networks due to the accumulated noise and dispersion, which will gradually degrade the signals. In this article, a SOA (Semiconductor Optical Amplifier)-based interferometer Figure 6Comparisons of return losses between measurements and simulations for two bonding wires INTRODUCTIONIn future optical networks, transmission of high bit rate optical signals will likely involve regenerators to suppress signal degradation induced by accumulation of noise, jitter, and dispersion, which otherwise would severely limit the network size [1]. The chromatic dispersion and polarization mode dispersion of the transmission fibers may be the major contributors to the degradations in the time domain [2]. Now many kinds of schemes have been investigated to overcome these problems, including the different kinds of all-optical regenerators [3, 4]. Still, in most methods reported so far, the regeneration is performed at the expense of an increased complexity, which is not suitable for future highspeed networks. In this article, the SOA-based interferometer is studied both in theory and experiments, which is similar to the ultrafast nonlinear interferometer (UNI) [5] except that no pump pulse is injected. For the ultrafast response of UNI, it is used in clock and data recovery [6], optical sampling [7], and as a demultiplexer [8, 9] in highspeed OTDM systems. Here, the reshaping capability of this simple interferometer without control pulse injecting is demonstrated. The picosecond optical pulses with repetition rate of 10 GHz distorted by dispersion shift fiber (DSF) and dispersion compensation fiber (DCF), respectively are reshaped successfully in experiments. THEORYThe configuration used to demonstrate the reshaping capability of UNI without control pulse injecting is shown in Figure 1. The folded configuration shown in the figure was adopted for better stability of the interferometer. The pulse was aligned with the polarization maintaining (PM) ports of the polarization beam splitter (PBS) using PC1. The common port of the PBS was spliced to the polarization maintaining fiber (PMF) with 45°offset. Then the input signal pulse is split into two orthogonally polarized pulses that are separated by T interval after traversing the PMF. The delay time T is determined by the length of PMF and refractive index difference between fast axis and slow axis in PMF. The amplitude of the orthogonal polarization components is E l and E twhere E in is the amplitude of input optical pulse. Then the two orthogonal polarization components enter the SOA with the time interval of T. Because of the refractive index and gain nonlinearities of SOA, the trailing component will experience different gain and phase shift, compared with the leading one. Let G l ͑t͒ and l ͑t͒ be the gain and phase shift of the leading component after passing the SOA. G t ͑t͒ and t ͑t͒ are for the trailing one. Then the amplitudes of the two orthogonal polarization components ...