An optical isolator is a passive magneto-optic device that allows light transmission in one direction only. This article describes optical isolation, its definition, components, types, working principles, advantages, and applications.
What is an Optical Isolator?
An optical isolator is a device that allows the light signals to travel only in one direction and absolutely blocks the signals in the other direction. It is also known as an optical diode, photocoupler, or optocoupler. Its function is similar to a semiconductor diode, which allows electric currents to flow only in one direction. Hence, it is known as an optical diode.
When a light signal propagates through the optical fiber, some portion of this light may be reflected. Therefore, in order to avoid the reflection of light signals in the backward direction, optical isolation is used.
Optical isolation is done through an optical device that allows unidirectional propagation of light signals through an optical fiber. Its primary function is to stop the undesirable backward reflection of light signals. Hence, it reduces signal degradation and improves the stability of the optical communication system.
Components of Optical Isolator
An optical isolation system typically consists of three main components, namely, input polarizer, Faraday rotator, and output polarizer (also called analyzer).
The incoming optical signal passes through the input polarizer. The input polarizer blocks the light signals of orthogonal polarization while allowing light signals of only a specific polarization through it. The light signals then enter the Faraday rotator, which polarizes the light signals to rotate with the help of a magnetic field. Finally, the light signals enter the output polarizer, which allows them to pass through it.
All three components of the isolator together allow the light signal to propagate in the forward direction and block in the opposite direction.
Types of Optical Isolators
They are mainly classified into the following three types.
Polarization Dependent Isolator
The polarization-dependent isolator allows the light signal to propagate in the forward direction and blocks the light signal to propagate in the backward direction by utilizing the polarization axis.
Polarization Independent Isolator
A polarization-independent isolator consists of components like wavelength division multiplexer (WDM), pumping diode laser, erbium-doped fiber, etc., instead of polarizers and Faraday rotator. This type of optical isolator is mainly used in EDFA optical amplifiers.
Magnetic Optical Isolator
The magnetic optical isolator is a type of polarization-dependent optical isolator. In this type of isolator, the polarization rotation within the Faraday rotator is done by controlling the strength of the magnetic field.
Working of Optical Isolator
The working of optical isolation is based on the Faraday effect, which is a magneto-optic effect. According to this effect, the polarization of a light signal passing through a material is rotated in the existence of a magnetic field.
The working of an isolator is explained in the following steps:
Step 1 – The light signal passes through the input polarizer that polarizes it vertically.
Step 2 – The vertically polarized light signal then enters the Faraday rotator that rotates the polarization of the light signal by utilizing the magneto-optic effect (called the Faraday effect). It typically rotates the polarization of light signal by an angle of 45°. The most important characteristic of the Faraday polarizer is that it rotates the polarization only in a single direction.
Step 3 – Finally, the rotated light signal leaves the optical isolator through the output polarizer.
Now, let us understand how the optical isolator blocks the backward reflection of the light signal.
The light signal is rotated by an angle of 45° (say in the clockwise direction as shown in the above figure). This rotation is non-reciprocal. The light signal gets horizontally polarized after the rotation of polarization. When this horizontally polarized light signal is reflected in the backward direction, the input polarizer, being a vertical polarizer, rejects it. This way, no light signal will get reflected through the optical isolator.
Advantages of Optical Isolator
The following are the major advantages of the use of optical isolation in the optical fiber system:
- Reduces the signal degradation due to backward reflection
- Enhances the transmission quality, efficiency, and performance of a fiber optic system.
- It helps in maintaining a high signal-to-noise ratio in the system.
- Eliminates undesirable interference by providing unidirectional transmission of light signals.
- It helps in achieving isolation between the forward and reverse directions.
Applications of Optical Isolator
Optical isolation is used in a variety of applications. Some key applications of optical isolation are listed below:
- Optical isolation is widely used in optical communication systems to reduce signal degradation due to backward reflection.
- Used in fiber optic-based telecommunication systems to protect sensitive devices like laser diodes, photodetectors, etc., from reflected signals.
- Used in optical amplifiers to ensure stable and efficient optical signal generation and amplification.
- Optical isolations are also used in optical measurement systems to prevent undesirable reflection and minimize errors in measurement.