**4.1 Yes... twisted light through the light pipe...**

Today's optical-fiber communication systems use wavelength division multiplexing, as discussed in Section 2, to squeeze multiple channels of data through the same fiber simultaneously, offering much speedier data transfer rates.

But there is another breakthrough research in squeezing photons into this light pipe! It is the "twisted light" or the photons with an orbital angular momentum (OAM) that can also be utilized to encode data channels. It is like another degree of freedom or another dimension on which data can be transferred. The angular momentum has an infinite number of states. Each wavelength can carry different values of this angular momentum. Thus, OAM appears to be one more parameter of light that was not explored till date for communications (**Figure 11**).

Researcher Miles Padgett and his coworkers in the University of Glasgow discovered in 2004 that OAM modes can be sent through air. They used a holographic

**Figure 11.** *Twisted light through an optical fiber (https://goo.gl/images/vAZD3C).*

**Figure 13.**

*Wireless communication through twisted light (https://www.rdmag.com/news/2017/10/twisted-light-couldilluminate-new-path-wireless-communications).*

pattern to split a twisted laser light into nine separate helical beams and sent them 15 m through the air to a telescope. At the receiving end, this telescope was able to distinguish and read out all the beams simultaneously. The bandwidth of the experiment was not quite high though. A multiplexer and demultiplexer for such twisted or helical beams were presented in a research paper at the Optical Fiber Communication Conference and Exposition, in Los Angeles, in March. The multiplexing device presented in the conference was one with multiple waveguides that were carved onto a single chip. Later, Willner et al., researchers from the University of Southern California, reported a research work related to transfer of data using OAM modes of light in Nature Photonics in 2012. They had used twisted light to transfer data at approx. 2.5 terabits per second over a distance of about 1 m. But twisted beams would need to travel lot farther in order to be used for optical communications. Later, a team in Vienna, in 2014, set a record by sending pixelated images of few famous Austrians by using twisted light. The images were sent to another site in Vienna that was 3 km apart. The researchers used helical beams with four helices or twists, such that a data transfer rate of 4 pixels per second could be achieved (**Figures 12** and **13**).

The improvements should be welcome news to companies such as Intel and Luxtera, which have been racing to find ways to replace the expensive exotic semiconductors and separate components in most optical communications systems with cheap integrated chips made of silicon. Twisted light arrays could allow communication channels between chips in a computer.
