Types of Optical Isolator and their Working

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An optical isolator, especially a Faraday
isolator, is a device which transmits light in a certain direction while
eliminating the back reflection and backscattering at any polarized state. It
is generally categorized into two categories – Polarization Sensitive Optical
Isolators and Polarization-Insensitive Optical Isolators. As I have already
mentioned them as Faraday Isolators, it is obvious that they use the Faraday
Effect of the Magneto-optical crystal. To know more about these isolators,
let’s discuss the two types of optical isolators and how they work.

 

Polarization Sensitive Optical Isolators

 

These are the simplest Faraday Isolators
which work only when the input beam has a guided linear polarization.

 

Working:

 

Their working is simple in which a polarized
beam is passed through the first polarizer with minimum loss, then pass through
45 degree Faraday rotator and finally passed through the second polarizer with
its transmitting axis being rotated by 45 degree in order to ensure that
transmission losses are as low as possible.

 

When this light is reflected back to the
output port with unmodified polarization state, it will fully pass through the
output polarizer, but due to 45 degree rotated direction of polarization, the
light will be blocked at the input polarizer or can be sent to separate output
port. In case if the rotator’s rotation angle deviates from 45 degree due to
any reason such as fabrication errors, the degree of isolation would be
reduced. The problem is that we always need an isolator with high isolation
which may be reduced in these kinds of isolators due to several reasons.

 

Polarization Insensitive Optical Isolators

 

A Polarization Insensitive Optical
Isolator is the device which functions for arbitrary polarization of
the input beam. As many fibers don’t maintain the polarization, such devices
are often suitable and required in the context of fiber optics. Moreover,
optical fiber communication systems are operated with arbitrary polarization
state; so you need to use the Faraday Isolators and other components which can
cope with undefined polarization state.

 

Principle:

 

The basic principle of PI optical isolator
is to spatially separate the orthogonal polarization components of i/p beam
with the help of a polarizer. Then, send them through Faraday rotator and
combine the components again in the second polarizer.

 

The thing to note here is that polarization
insensitive optical isolator doesn’t preserve the polarization state as there
is an undefined relative phase change between the two components of
polarization. This phase change is dependent on temperature and wavelength.

 

These isolators are widely used in
telecommunication industry and various other applications in laser technology.
They are characterized by high isolation, low insertion loss and excellent
temperature stability. In the market, these isolators are available in various
wavelengths and bandwidths. So, before making a purchase, clarify which
specific isolator would be suitable for your use.