Optics 101

Photonic Crystal Fibers - Paving the Way for Better Optical Fibers

Introduction
The Properties of Photonic Crystals
What is Photonic Crystal Fibers
Manufacture of Photonic Crystal Fibers
Classification of Photonic Crystal Fibers
High Index Guiding Fibers
Low Index Guiding Fibers
Applications of Photonic Crystal Fibers

Introduction

Photonic crystals are materials which consist of a periodic structure in the nanometer scale, which is in the order of wavelengths of light. Photonic crystal uses their periodic reflections, refractions, and interference to trap and guide light in an innovative manner.

The Properties of Photonic Crystals

The properties of photonic crystals are governed by a photonic band gap. Photonic band gap is a term which is used to describe the frequency range to inhibit the propagation of light through the photonic crystal structure.

The photonic band gap can also be considered as an anti-resonance of the photonic crystal. The destructive interference stops the light from propagating at a particular frequency.

A large amount of interest has been generated by the development and the numerous applications of photonic crystal fibers. The photonic crystal field can be thought of as the most currently active areas of optical research.

What is Photonic Crystal Fibers

Photonic crystal fibers or PCFs is an optical fiber which is constructed using a two-layered microstructured arrangement. The top layer consists of a material with a low refractive index, while the background layer is made from a material with high refractive index. The material that makes up the background layer is usually undoped silica.

Air voids that runs along the length of the fiber is typically used for regions with low refractive index. Such air voids can be achieved by employing a preform with holes made by, for example, stacking capillary tubes. Plastics and soft glasses also permit the manufacturing of preform for photonic crystal fibers. The numerous configurations of air void arrangements result in the photonic crystal fibers to have different properties.

Manufacture of Photonic Crystal Fibers

Hair-thin photonic crystal fibers are readily manufactured once the desired preform has been made and drawn to a fiber at high temperatures. Processing precautions needs to be taken to preserve the air void arrangements during the drawing process.

Typically, photonic crystal fibers are coated with a protective coating to improve handling properties of the fibers.

The most frequently used and simplest photonic crystal fiber contains air holes in a triangular pattern with a hole missing or an array of air holes surrounding a solid core. The areas with the missing hole can be considered as having a higher refractive index. This setup is similar to the core in a conventional optical fiber.

Classification of Photonic Crystal Fibers

Photonic crystal fibers can be divided into two modes of operations based on the refractive index. They are sometimes referred to as high index guiding fibers and low index guiding fibers.

High Index Guiding Fibers

High index guiding fibers guides light in a solid core by a principle called Modified Total Internal Reflection, which is similar to conventional optical fibers. Total internal reflection is the result of the lower effective index in the microstructured air-filled region.

The refractive index of the microstructured cladding within the photonic crystal fibers demonstrates a dependency of wavelength that is very different to the core material. In other words, a higher-effective-index contrasts between the core and cladding. This strong wavelength dependence of the refractive index permits the design of, for example, single-mode fibers.

Low Index Guiding Fibers

Sometimes referred to as photonic bandgap fibers. It operates under a totally different guiding principal that is based on a photonic bandgap located in the cladding region.

A defect such as an extra air hole is used to create a core in the photonic bandgap fiber structure. This results in the creation of an area where light can propagate. The light is trapped in the low index core as the photonic bandgap effect causes the propagation in the microstructured cladding area impossible. As the light can only propagate at the defect areas, a low index guiding core is formed.

Photonic bandgap fiber therefore opens new possibility to guide light in vacuum, in air or any gas compatible with the fiber material.

Applications of Photonic Crystal Fibers

Here are some of the application examples of photonic crystal fibers:

  • Fiber amplifiers and lasers
  • Nonlinear devices
  • Telecom components
  • Fiber optic sensors
  • Quantum optics

Source: AZoOptics

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