Fibre optic cables form the backbone of our communications networks and are used everywhere, from your internet connection to medical imaging. These cables are incredibly thin strands of glass or plastic that guide light over long distances by bouncing it off the inner walls, a process called total internal reflection. It has allowed information to be carried over long distances incredibly fast and reliably. Traditional optical fibres, however, have some limitations, like loss or scrambling of the information over long distances. Scientists are constantly looking for ways to improve how light is guided and manipulated within the fibre.
In a new study, researchers from the Delhi Technological University, the University of Delhi, and Gautam Buddha University have proposed a novel fibre optic design for efficient and robust information transmission. Based on computer modelling and analysis, they have proposed a novel solid-core photonic crystal fibre (PCF) design that, theoretically, performs better than traditional fibres.
Instead of just a solid core of glass surrounded by another layer called cladding, like in a traditional fibre optic cable, PCFs have a unique structure. They are made with a pattern of tiny air holes running along the entire length of the cable, like a honeycomb structure within the glass. By carefully designing the size and arrangement of these holes, scientists can control how light travels through the fibre in ways that are impossible with traditional fibres.
There are two main ways light can be guided in a PCF. In a solid-core PCF, like the one focused on in this research, the centre of the fibre is a solid piece of glass, and the surrounding area with air holes acts like a special kind of cladding. Light is guided here through a process related to total internal reflection, but the air holes create a periodic structure that affects the effective refractive index of the cladding. The refractive index is a measure of how much light slows down when it passes through a material. The pattern of air holes lowers the effective refractive index of the cladding than the solid core, causing light to stay trapped in the core.
The second type, hollow-core PCFs, have a central air-filled core and guide light through a different mechanism called bandgap guidance. This mechanism is more complex but allows light to travel through the air, which can reduce losses even further.
For the current study, the researchers propose a solid-core PCF made from chalcogenide glass (As2Se3). This glass is particularly good at transmitting light in the mid-infrared (mid-IR) region of the electromagnetic spectrum. Their specific design involves a solid core of chalcogenide glass surrounded by five rings of tiny air holes arranged in a hexagonal pattern. The size of these air holes gradually decreases from the outer rings towards the centre core. This variation in hole size creates what’s called a graded index effect. Like an optical lens with a gradually changing refractive index helps to focus light more effectively, the graded arrangement in the fibre helps to control how light travels.
In the simulations, the structure displayed ultra-high numerical aperture (NA) which is a measure of how much light a fibre can collect or the range of angles at which light can enter the fibre and still be guided. A higher NA means the fibre can gather more light. At a specific wavelength in the mid-IR, the new PCF design shows a significant negative chromatic dispersion, a phenomenon where different colours (wavelengths) of light travel at slightly different speeds, causing the pulse to spread out over time. Negative dispersion means that some colours of light are slowed down more than others in a specific way. This can be used to compensate for the normal positive dispersion that occurs in other types of fibres, allowing for more precise signal transmission over longer distances. The new PCF design also exhibits a remarkably low confinement loss, meaning very little light escapes as it travels along the fibre.
This research presents a significant advancement in solid-core PCF technology. The unique properties of the PCF open up possibilities for various applications in the mid-infrared region, like High-sensitivity sensing in environmental monitoring or medical diagnostics, improved spectroscopy, advanced optical communication systems and specialised mid-IR lasers and light sources
The research, however, is based on computer simulations and theoretical analysis. Building such a precise fibre with tiny, graded air holes can be challenging. Any imperfections in the manufacturing process could also affect the fibre's actual numerical aperture, dispersion, and losses.
The researchers themselves point out that “very precise fabrication techniques would be needed to achieve the predicted performance”.
Nevertheless, the research demonstrates the exciting potential of carefully designed photonic crystal fibres for manipulating light. While the practical fabrication of such a fibre is still a challenge, the theoretical findings pave the way for future innovations in photonics.
This research article was written with the help of generative AI and edited by an editor at Research Matters.