Day: October 27, 2022

Why Do We Need Polarization Maintaining Fibers?

Polarization maintaining fibers has been around since the development of fiber optics in the mid 20th century.

In fact, these fibers are considered to be the next generation of fiber optic technology with capabilities such as high bandwidth, low loss, and high temperature stability.

In order to understand this concept, you must first have an understanding of what polarization is and how it works in fiber optics.

What is Polarization?

Polarization is a fundamental property of electromagnetic waves, which have oscillating electric and magnetic fields. This phenomenon occurs in light waves and radio waves, for example.

Light that is polarized can be described as having an electric field vibrating at right angles to its direction of travel, or an electric field vibrating parallel to its direction of travel.

Light with an uneven distribution of polarization (i.e., light that has both vertical and horizontal components) can be said to be unpolarized.

What is Polarization Maintaining Fibers?

A polarization-maintaining fiber (PM Fiber) is a specialty single-mode fiber. Normally, single-mode fibers can carry randomly polarized light. In contrast, PM fiber propagates a single polarization of light.

Polarization-maintaining fibers maintain linearly-polarized light waves during propagation and do not cross-couple optical power between polarizations.

Some fiber optic components require polarized light input, such as external modulators. This polarization-maintaining feature plays a crucial role in these components.

In order to achieve this characteristic, stresses are induced in the material during the manufacturing process. PMFs come in two types: linear polarization maintaining fibers (LPMFs) and circular polarization maintaining fibers (CPMFs)

Uses of Polarization Maintaining Fibers

A PMF fiber optic cable is used in many applications. In fact, a PMF fiber optic cable might be what you are using to read these words right now!

The most common applications of PMF optical cables include lightwave transmission, telecommunications, and medical equipment.

Many telecommunications companies use PMF optical cables to transmit information over long distances in order to support communication between different countries.

Common medical devices that utilize fiber optics are sonography devices and endoscopes.

Ways of Realizing Polarization-maintaining Fibers

First, a polarization preserving fiber (PPF) has three layers: core, clad, and coating. Inside an optical fiber is a cavity known as core that is surrounded by a cladding.

There are also two polarizations in fiber optics – linear and circular polarization. Circular polarization states that when light propagates through a medium with time-varying ellipticity, its direction rotates continuously around an axis parallel to the propagation direction.

Cross Section of Polarization Maintaining Fibers

Polarization-maintaining fibers (PMF) or polarization-maintaining optical fibers (PMOF) are optical fibers which have a property that preserves the polarization state of light as it travels through them.

This is an important feature for applications in areas such as telecommunications and fiber-optic gyroscopes, and is usually provided by intrinsic material properties (such as birefringence), by inner cladding layers, or by external polarization controllers.

Conclusion

Polarization-maintaining fibers are well known for their ability to allow different polarized components (vertical and horizontal) to be transmitted through the fiber simultaneously. Their applications range from fiber optic sensing to interferometry to slab dielectric waveguides.

A Brief Guide on Optical Isolators, Their Types, and Working

Michael Faraday established that the optical isolator operations are based on Faraday Effect in 1842. According to Faraday Effect, the polarized light plane turns when light energy propagates through the glass that can be exposed to a magnetic field. The direction of rotation of the plane is based on the magnetic field as an alternative to the light transmission direction.

Optical devices and connectors that are used in a fiber optic system also cause some effects such as the absorption of energy and reflection of the optical signal on the output of the transmitter. These effects can cause the light energy to be reproduced back at the supply, which in turn causes interruption or obstruction in the supply function.

To overcome such interference effects, we need optical diodes, also called optical isolators.

What is an optical isolator?

An optical isolator goes by multiple names such as an optical diode, photocoupler, and optocoupler. An optical isolator is a magneto-optic device designed specifically to allow the transmission of the light signal in one direction only.

Due to its functionality, it plays a key role when you need to prevent unnecessary feedback to an optical oscillator, also called a laser cavity. The working of this device mainly depends on Faraday Effect, which plays an important role in the main component, namely the Faraday rotator.

How does an optical isolator work?

An optical isolator is composed of three main components, namely, a Faraday rotator, an input polarizer, and an output polarizer. When a light signal passes through an input polarizer in the forward direction, it turns into polarized light with the vertical plane. The function mode of this optical device is classified into two types depending on different directions of light such as forward mode or backward mode.

In forward mode, the light signal enters an input polarizer and becomes linearly polarized. When this linearly polarized light signal passes through the Faraday rotator, the Faraday rotator rod will turn 45 degrees, allowing the linearly polarized signal to leave the output polarizer at 45 degrees.

In backward mode, as the light signal rotated by 45 degrees enters the output polarizer, it again turns by 45 degrees in a similar path when passing through the Faraday rotator. After that, 90 degrees polarized light turns into a vertical plane toward the input polarizer and hence, cannot leave the isolator. At this point, the light beam is either absorbed or reflected.

What are the different types of optical isolators?

Based on working, optical isolators are categorized into different types:

  • Polarized Type Optical Isolator: This type of isolator utilizes the polarization axis to transmit light in one direction. It allows light to travel in the forward direction and stops light to transmit back. In this category, you can find polarization-dependent isolators and polarization-independent isolators.
  • Composite Type Optical Isolator: It is a polarization-independent polarizer that is often used in EDFA that has different components such as WDM, erbium-doped fiber, pumping diode laser, and more.
  • Magnetic Type Optical Isolator: It is a polarized optical isolator that pressurizes the magnetic element of a Faraday rotator that works on the Faraday Effect.

Based on the amount of power they can handle, optical isolators can be categorized into two types:

  • High-power isolator: A high-power optical isolator is the optical passive component that allows the transmission of light in one direction and can handle high levels of power. One example of a high-power isolator is a 30Watt 1064nm high-power isolator.
  • Low-power isolator: A low-power optical isolator is the optical passive component that works the same as a high-power isolator but operates at low power. For example:  2Watt 980 PM optical isolator  

If you need optical isolators for your project or application, contact DK Photonics no matter your requirement is for low-power isolators or high-power isolators.