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 fiber optic system testing and
fiber-optic LAN system and CATV fiber optic 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.