Multiple Access Techniques for Bipolar Optical Code Division in Wireless Optical Communications

In this paper, multiple access techniques for bipolar optical code division are proposed for application in wireless optical communications. First, several fiber Bragg gratings (FBGs) and optical circulators are used to encode optical signals. Then, the horizontal or vertical polarization state can be controlled using bipolar data. Experiments were conducted using two devices to determine the feasibility of using multiple access techniques for bipolar optical code division in free-space optical communication. The first experiment tested the first architecture, in which each user operates a mechanical optical switch by switching data bits to generate optical signals with specific polarization states through a collimator in a free-space channel. As the decoding components, several FBGs and optical circulators are used as primary devices. Decoded signals are sent to a balanced photodetector, which can reconstruct original data from the encoder. The second experiment involved a second architecture, in which an encoder with an erbium-doped fiber amplifier and a decoder with an attenuator are used to improve system stability. The third experiment was designed to test whether multiple access interference can be alleviated with the proposed architectures. In the final experiment, an ultrafast optical switch was used instead of the original optical switch for selecting optical signals with a specific polarization state to improve the overall transmission rate. The experimental results indicate that the proposed scheme could be successfully implemented in free-space optical communication systems. The architectures of the proposed scheme mitigate multiple access interference with a simple and cost-effective design.