A Brief Introduction to Polarization Maintaining Isolators

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Polarization maintaining isolator which ‘at
times’ is also called fiber optic isolator and polarization maintaining optical
isolator. It allows and keeps light to travel in one direction only. Its prime
job is to prevent back reflection and backscattering in the reverse direction,
for all states of polarization. In technical terms, the device is a two-port
micro-optic isolator built with PM panda fiber. The isolator is commonly used
in lasers, fiber optic systems, and amplifier systems. It actually prevents
feedback which is not at all required in an optical oscillator.

 

Some devices in which this isolator is used

 

PM
isolator is utilized all over the world majorly
in communication systems, instrumentation applications, and polarization
maintaining fiber-optic amplifiers. The isolator is also used in fiberoptic
system testing and fiber-optic LAN system and CATV fiberoptic links.

 

Some of the many great features of these
isolators

 

High isolation capacity

High Extinction Ratio

High Return Loss

Low Insertion Loss

Every fiber optic isolator has an optical
fiber inside of it which is the most important component. Let’s now discuss how
it works.

 

Optical fiber inside such isolators is a
thin strand made of pure glass. It acts as a guide for the light wave over long
distances by following the principle of ‘total internal reflection’. These are
very effective when the light waves try to pass between two varying media.

 

The fiber inside these devices including
polarization maintaining optical isolator is composed of two layers of glass –
the core and the cladding. The core typically carries the actual signal of
light and the glass layer surrounding the core is called cladding. In
comparison to the core, the cladding has a lower refractive index. All of this
causes total internal reflection successfully within the core.

 

What is transmitted over fiber?

 

Most fibers work in pairs where digital
signals are encoded in light’s analog pulses preferably via the NRZ modulation
– Non-Return to Zero. Since they operate in pairs, one is used to transmit
while the other to receive, however, both signals can also be sent over a
single stand.

 

Basic yet most used fiber types

 

SMF – Single Mode Fiber

MMF – Multi-Mode Fiber

 

The actual difference basically lies in the
size of the core. SMF has an in-depth narrow core not more than 9µm which
allows the propagation to just a single mode of light, whereas, MMF has a
greatly wider core somewhere around 50µm and 62.5µm is also available on the
market. MMF allows multiple modes of light to propagate. They both have their
different characteristics along with their own pros & cons.