Crosstalk Coherence in Optical Switching Chips

With the rapid development of information and communication technology, optical switching is more desirable for communication networks with low latency and high data rates than traditional Opto-Electro-Opto(O-E-O)operating. The latter easily suffers from the so-called "electronic bottleneck", and then high latency and power consumption. All-optical switching can steer data signals to the destination ports in the optical domain, and possesses the transparency of signal format, bit rate, and protocol. Silicon-based optical switching chips have been employed to realize Wavelength-Division Multiplexing (WDM) network nodes, with small size, low power consumption, and fast switching speed, which are also expected to be suitable for future Space-Division Multiplexing (SDM) all-optical switching nodes. In these chips, the connection paths are usually shorter than the coherent length of light, and the coherent effect between two beams from the same light source will take place, resulting in the power fluctuation of optical signals or crosstalk increase. In the paper, we focus on the 4x4 Benes optical switching chip, composed of 2x2 Mach-Zehnder Interferometer (MZI) optical switches, in which the optical couplers are realized by Multimode Interference (MMI) devices. The transmission matrix method is used to deduce the transfer function of input to output optical fields for the optical switches and the whole chip. Here, we take into account the crosstalk coherence caused by the imperfect 3 dB MMIs in the MZI optical switches. Based on coherence theory, the assumption is made that the optical fields of only coherent light beams can be added. In other words, the transmission matrix model is suitable for single-port input cases in calculations. For multi-port inputs, optical intensities are calculated separately for each input port to avoid superimposing the optical fields of different sources during calculations. By analyzing the expression of the transmission matrix, it can be observed that if the states of middle switches are the same, the coherence effects have a significant impact on the crosstalk performance of the switching array. This switch state can be referred to as a "strongly coherent" switch state. On the other hand, if the states of middle switches are different, this switch state can be referred to as a "weakly coherent" switch state. Furthermore, for a 4x4 non-blocking optical switch array, there are 24 different switching requirements. By examining the corresponding switch states for each switching requirement, it is found that 8 of the switching requirements consistently correspond to strongly coherent switch states, while the remaining 16 switching requirements consistently correspond to "weakly coherent" switch states. In other words, if the switching requirement is determined, it is not possible to avoid the occurrence of strong coherent phenomena by selecting an appropriate switch state. For the given parameters of the MZI optical switch element, and the influence of crosstalk coherence on the insertion loss and crosstalk performance for the optical switching chip are simulated. It is shown that, 1) the change of the insertion loss is negligible for each channel; 2) under strongly coherent switch states, channel crosstalk can increase by a maximum of 2.22 dB due to coherence effects, and it can be reduced by up to 4.78 dB, resulting in fluctuations of up to 7 dB, to the disadvantage of data transmission. Furthermore, the impact of crosstalk on Bit Error Rate (BER) was assessed through the experiment. Assuming that the initial phase shift introduced by the connected waveguides is random, according to the phase dependency of coherence, we propose a scheme to effectively eliminate crosstalk coherence in the optical switching chip by using three phase shifters nested on the connection waveguides. The analysis process or method presented here is also applied to large-scale optical switching chips or network nodes in the presence of crosstalk.