**1. Introduction**

Light is an electromagnetic wave, composed of electric and magnetic fields. The fields oscillate in a direction perpendicular to the direction in which the wave is moving. If the electric field is always oscillating in the same plane, the light is said to be linearly polarized. Photons of such light possess a linear momentum. Such photons have the power to propel a boat if the solar sails absorb the linear momentum. If the direction of the plane in which the light's electric field is vibrating is itself rotating as the wave moves, light is said to be circularly polarized. In this case, light is said to possess a spin angular momentum. When such a light hits a floating ball, it will start to spin like a planet, rotating about its own axis. We thus had these two types, and surprisingly, it turned out that these are not the only ways light behaves. Allen et al., in 1992, showed in his seminal paper that light may possess another strikingly different characteristic behavior [1]. It happens when the wave fronts, instead of moving in a straight line or diverging/converging, tend to bend, rotate, and propagate in a helical fashion. This was something unexpected and very different from the already known phenomenon. This implied that the energy propagation direction is not a straight line but that too forms a helical trace

as the wave moves in the forward direction. Hence, the Poynting vector (S = E × H) changes its direction continuously as the wave moves. Therefore, a given phase front will rotate around the center and trace a helix as it propagates. Such a wave is said to possess a momentum, apart from the linear and spin momentum, which is termed as the orbital angular momentum or OAM. The center of the wave thus carries a singularity of phase called the optical vortex, and the intensity profile represents a doughnut [2–17]. Hitting with this type of light, a free-floating ball will revolve in a circle about a central point as a planet orbits a star. Thus, a "vortex beam" represents a column of light with a hole in the center. In 1991, physicist Robert Spreeuw, at the time a PhD student in Han Woerdman's lab at Leiden University in The Netherlands, sat down during a team coffee break and presented some ideas about how to make twisted light. "The first reactions were a bit doubtful," Spreeuw recollects. "But we kept thinking about it and, bit by bit, it started to look more realistic."

Angular momentum of photons consists of two different components. The first one is the spin angular momentum (SAM), which corresponds to the polarization of the photon. The second component is the orbital angular momentum (OAM), which relates to the spatial phase profile of the photon." Both the components have been used extensively in optical experiments in the laboratories. Moreover, polarization has been used successfully in quantum experiments in free space for about an order of 100 km [18–23]. The polarization of a photon is still more easily controllable and resistant against atmospherical influences and resides in a two-dimensional state space. This places an inherent limit on how much information one can send per photon. An alternate way to encode information is in the OAM, another degree-of-freedom of a photon that offers infinite unbounded number of discrete levels theoretically and is able to render faster effective communication over long distances [18–23].

A brief note on the light-carrying OAM would give an insight into what it looks like. This light has a "twisted" or helical wave front, with an azimuthal phase varying from 0 to 2 *πl*. The integer *l* stands for the topological charge or helicity, and *lħ* is the OAM of the photon. In 1992, Woerdman and his colleague Les Allen created twisted light in the lab and showed that even a single photon of light has OAM [1]. In the next year, they showed how to convert a normal helium-neon laser to one that carried OAM. One way to create twisted light is to send it through a phase plate with thickness varying azimuthally. This shapes the wavefonts of the field into a form resembling a helix. It is as if you took a rod and swirled it around to create a vortex in the phases of the electromagnetic waves. This is similar to a vortex in water waves. Topological charge (*l*) 1 means there is 1 helix propagating clockwise (say) in space, charge 2 means 2 interwined helices propagate, while −1, −2, etc., mean that 1 and 2 helices propagate anticlockwise in space. Higher order charges would increase the number of helices. **Figure 1** shows a plane wave (*l* = 0) and helical waves with *l* = 1 and −1, respectively.

Beams carrying OAM can also be generated in the lab using other techniques like a helical mirror or using a computer-generated hologram. Although many of these and other techniques are available, researchers are excited about the potential uses of these beams. Such beams have the potential to trap and rotate particles (optical tweezers), render fast optical communication, used for quantum computing, and use in various other areas. In 1995, an Australian team placed small particles in the dark, central cavity of an OAM laser and watched them whirl around, providing visual proof that the light was carrying OAM. The researchers could even reverse the direction of the OAM laser's twist and spin the particles the opposite way. Thus, an exciting new degree of freedom has been made available to the researchers to extract the potential of these twisted wavefronts. These can be launched into the optical fibers (OF) and photonic crystal fibers (PCF) and provide an all new technique to communicate data at speeds higher than the available ones. But before

**29**

**Figure 2.**

*Structured Light Fields in Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.85958*

how they are used in data communication.

*new-understanding-twisted-light-affect-use-quantum-computing).*

shows a typical sketch of an optical fiber.

**2. What are optical fibers?**

**Figure 1.**

moving further, let me give you a brief description of what optical fibers are and

*Phase fronts/wave fronts, phase profiles and transverse intensity profiles of a positive helical wave with m = 1, a plane wave, and a negative helical wave with m = −1. (ref: phot https://futurism.com/*

Optical fibers are dielectric wave-guides, circular in shape that can transport optical information and energy. They have a central core that is surrounded by a concentric cladding that has a slightly lower refractive index than that of the core. Optical fibers are normally made of silica doped with index-modifying dopants such as GeO2 [24–29]. A protective coating of one or two layers of a cushioning material (such as acrylate) is used to reduce cross talk between adjacent fibers and microbending, which occurs when fibers are pressed against rough surfaces. Microbending and cross talk increase the loss of optical energy as the optical beams propagate through the fiber. Fibers are typically incorporated into cables in order to provide environmental protection. Typical cables have a polyethylene sheath that encases the fiber within a strength member such as steel or Kevlar strands. **Figure 2**

Since the core has a higher index of refraction than the cladding, light will be confined to the core when the condition for total internal reflectance (TIR) is

*Cross-section view of an optical fiber (https://www.newport.com/t/fiber-optic-basics).*

### **Figure 1.**

*Fiber Optics - From Fundamentals to Industrial Applications*

look more realistic."

as the wave moves in the forward direction. Hence, the Poynting vector (S = E × H) changes its direction continuously as the wave moves. Therefore, a given phase front will rotate around the center and trace a helix as it propagates. Such a wave is said to possess a momentum, apart from the linear and spin momentum, which is termed as the orbital angular momentum or OAM. The center of the wave thus carries a singularity of phase called the optical vortex, and the intensity profile represents a doughnut [2–17]. Hitting with this type of light, a free-floating ball will revolve in a circle about a central point as a planet orbits a star. Thus, a "vortex beam" represents a column of light with a hole in the center. In 1991, physicist Robert Spreeuw, at the time a PhD student in Han Woerdman's lab at Leiden University in The Netherlands, sat down during a team coffee break and presented some ideas about how to make twisted light. "The first reactions were a bit doubtful," Spreeuw recollects. "But we kept thinking about it and, bit by bit, it started to

Angular momentum of photons consists of two different components. The first one is the spin angular momentum (SAM), which corresponds to the polarization of the photon. The second component is the orbital angular momentum (OAM), which relates to the spatial phase profile of the photon." Both the components have been used extensively in optical experiments in the laboratories. Moreover, polarization has been used successfully in quantum experiments in free space for about an order of 100 km [18–23]. The polarization of a photon is still more easily controllable and resistant against atmospherical influences and resides in a two-dimensional state space. This places an inherent limit on how much information one can send per photon. An alternate way to encode information is in the OAM, another degree-of-freedom of a photon that offers infinite unbounded number of discrete levels theoretically and is able to

A brief note on the light-carrying OAM would give an insight into what it looks like. This light has a "twisted" or helical wave front, with an azimuthal phase varying from 0 to 2 *πl*. The integer *l* stands for the topological charge or helicity, and *lħ* is the OAM of the photon. In 1992, Woerdman and his colleague Les Allen created twisted light in the lab and showed that even a single photon of light has OAM [1]. In the next year, they showed how to convert a normal helium-neon laser to one that carried OAM. One way to create twisted light is to send it through a phase plate with thickness varying azimuthally. This shapes the wavefonts of the field into a form resembling a helix. It is as if you took a rod and swirled it around to create a vortex in the phases of the electromagnetic waves. This is similar to a vortex in water waves. Topological charge (*l*) 1 means there is 1 helix propagating clockwise (say) in space, charge 2 means 2 interwined helices propagate, while −1, −2, etc., mean that 1 and 2 helices propagate anticlockwise in space. Higher order charges would increase the number of helices. **Figure 1** shows a

Beams carrying OAM can also be generated in the lab using other techniques like a helical mirror or using a computer-generated hologram. Although many of these and other techniques are available, researchers are excited about the potential uses of these beams. Such beams have the potential to trap and rotate particles (optical tweezers), render fast optical communication, used for quantum computing, and use in various other areas. In 1995, an Australian team placed small particles in the dark, central cavity of an OAM laser and watched them whirl around, providing visual proof that the light was carrying OAM. The researchers could even reverse the direction of the OAM laser's twist and spin the particles the opposite way. Thus, an exciting new degree of freedom has been made available to the researchers to extract the potential of these twisted wavefronts. These can be launched into the optical fibers (OF) and photonic crystal fibers (PCF) and provide an all new technique to communicate data at speeds higher than the available ones. But before

render faster effective communication over long distances [18–23].

plane wave (*l* = 0) and helical waves with *l* = 1 and −1, respectively.

**28**

*Phase fronts/wave fronts, phase profiles and transverse intensity profiles of a positive helical wave with m = 1, a plane wave, and a negative helical wave with m = −1. (ref: phot https://futurism.com/ new-understanding-twisted-light-affect-use-quantum-computing).*

moving further, let me give you a brief description of what optical fibers are and how they are used in data communication.
