Optical fibers are an example of non-planar optical waveguides.
All optical waveguides are characterized by a parameter called the V-number.
Optical fibers support the single propagation mode, LP01, when the V-number is less than 2.405.
Telecommunication applications predominantly use optical waveguides to transmit large amounts of data from one point to another. An optical waveguide’s mode structure plays a significant role in determining the wave propagation in optical applications. Depending on the mode’s structure, optical waveguides transmit a single signal (single-mode) or a multitude (multimode) of signals. Since single-mode waveguide conditions are different from multimode conditions, optical waveguides need to be specifically designed for each type of mode.
Optical fibers used in telecommunication transmission systems are the derived versions of optical waveguides. In this article, we will focus our discussion on single-mode optical fibers, but first, let’s briefly discuss optical waveguides in general.
Optical waveguides are the basic element in any optical circuit. They are often applied to integrated optical circuits, telecommunication systems, and biomedical imaging. Optical waveguides are classified into various types:
Depending on the Structure
- Fiber waveguide
Depending on the Material
Depending on the Refractive Index Distribution
- Graded index
Depending on the Mode Structure
Let’s take a closer look at single-mode optical waveguides.
Single-Mode Optical Waveguides
In fiber optics, optical waveguides are commonly used to guide the propagation of electromagnetic signals and they only support a single transmission mode.
Single-Mode Optical Fibers
An optical fiber is an example of a non-planar optical waveguide. There are also two categories of optical fibers: single-mode optical fibers and multimode optical fibers. Single-mode optical fibers consist of a “core” placed at the center, through which electromagnetic signals are guided down to a target location. The core is enclosed inside “cladding”. Cladding utilizes total internal reflection to prevent signal loss or spreading from the core. In commercial optical fibers, two protective layers cover the core and cladding unit.
Single-Mode Waveguide Conditions in Optical Fibers
Modal dispersion in optical waveguides is detrimental to their functioning. Single-mode optical waveguides are a solution to reduce modal dispersion. Single-mode optical waveguides are designed so that they prevent the existence of higher-order waveguide modes. Properly selecting waveguide parameters is critical to cutting off the higher-order waveguide modes in optical waveguides.
All optical waveguides are characterized by a parameter called the V-number, otherwise known as normalized frequency. The V-number is important in optical fibers to achieve single-mode waveguide conditions. The V-number is a unitless quantity and is given by equation (1), where λ is the wavelength of the electromagnetic signal, a is the core radius, and n1 and n2 are the refractive indices of core and cladding, respectively:
In optical fibers, the relationship to be satisfied for single-mode waveguide conditions is:
Optical fibers support the single propagation mode, LP01, when the V-number is less than 2.405. When the V-number is greater than 2.405, optical fibers support the next linearly polarized mode.
If the refractive index values are closer to each other, then thin optical fibers can establish single-mode wave propagation through them. The alignment of the core size is very important to achieve single-mode waveguide conditions in optical fibers. Cadence’s software can help you design optical waveguides that support only single-mode propagation.