Optical multiplexing techniques and their marriage for on-chip and optical fiber communication: a review

Optical multiplexing techniques is an urgent research topic, and scholars all over the world are conducting in-depth research in this field. Over the years, many multiplex technologies such as wavelength division multiplexing (WDM), mode division multiplexing (MDM), space division multiplexing (SDM), polarization multiplexing (PDM) and orbital angular momentum multiplexing (OAMM) have been proposed. Wavelength division multiplexing (WDM) technology enables two or more optical signals of different wavelengths to be transmitted simultaneously through a single fiber, making full use of the fiber’s low loss characteristics in a large wavelength range. The theory was first proposed by Delange in 1970, and it was not until 1977 that the basic research of WDM technology began, which focused on the application of communication networks. Since then, with the continuous development of optical fiber, light source, photodetector and other fields, people’s exploration of WDM technology has also accelerated. The advantage of polarization multiplexing (PDM) is that the amount of signal transmission can be multiplied, because two independent signals can be distributed at the orthogonal polarization position of the same beam of light, and the two polarization channels are separated and independently identified at the receiving end.

As the demand for higher data rates continues to grow, the last degree of freedom of multiplexing, space, has been intensively studied over the past decade. Among them, the mode division multiplexing (MDM) is mainly generated by N transmitters, which is realized by spatial mode multiplexer. Finally, the signal supported by the spatial mode is transmitted to the low-mode fiber. During signal propagation, all modes on the same wavelength are treated as a unit of the Space Division multiplexing (SDM) super channel, i.e. they are amplified, attenuated and added simultaneously, without being able to achieve separate mode processing. In MDM, different spatial contours (that is, different shapes) of a pattern are assigned to different channels. For example, a channel is sent over a laser beam that is shaped like a triangle, square, or circle. The shapes used by MDM in real-world applications are more complex and have unique mathematical and physical characteristics. This technology is arguably the most revolutionary breakthrough in fiber optic data transmission since the 1980s. MDM technology provides a new strategy to implement more channels and increase link capacity using a single wavelength carrier. Orbital angular momentum (OAM) is a physical characteristic of electromagnetic waves in which the propagation path is determined by the helical phase wavefront. Since this feature can be used to establish multiple separate channels, wireless orbital angular momentum multiplexing (OAMM) can effectively increase the transmission rate in high-to-point transmissions (such as wireless backhaul or forward).