Polarization Dependent Isolator VS. Polarization Independent Isolator

Connectors and other types of optical devices on the output of the transmitter may cause reflection, absorption, or scattering of the optical signal. These effects on the light beam may cause light energy to be reflected back at the source and interfere with source operation. In order to reduce the effects of the interference, an optical isolator is usually used. Optical isolator allows a beam of light to stream through a single one way direction. At the same time, it prevents the light from going back in the opposite direction. According to the polarization characteristics, optical isolators can be divided into two types, including polarization dependent isolator and polarization independent isolator. The polarizer-based module makes a polarization dependent isolator, and the birefringent crystal-based structure makes a polarization independent isolator. You may be very confused about them as you find that there is only a little difference via their names. So, what are they and what are the differences between them? This paper will give you the answer.

Polarization Dependent Isolator

The polarization dependent isolator consists of three parts, an input polarizer , a Faraday rotator, and an output polarizer. Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45°. The analyser then enables the light to be transmitted through the isolator. Light traveling in the backward direction becomes polarized at 45° by the analyser. The Faraday rotator will again rotate the polarization by 45°. This means the light is polarized horizontally. Since the polarizer is vertically aligned, the light will be extinguished.

principle of polarization dependent isolator

The picture shows us a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser, is set to 45°. The Faraday rotator is chosen to give a 45° rotation. Because the polarization of the source is typically maintained by the system, polarization dependent isolator is widely used in free space optical systems.

Polarization Independent Isolator

The polarization independent isolator also consists of three parts, an input birefringent wedge, a Faraday rotator, and an output birefringent wedge. Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0°) and horizontal (90°) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45°. This means the o-ray is now at 45°, and the e-ray is at −45°. The output birefringent wedge then recombines the two components.

principle of polarization independent isolator

Light traveling in the backward direction is separated into the o-ray at 45, and the e-ray at −45° by the birefringent wedge. The Faraday Rotator again rotates both the rays by 45°. Now the o-ray is at 90°, and the e-ray is at 0°. Instead of being focused by the second birefringent wedge, the rays diverge. The picture shows the propagation of light through a polarization independent isolator. While polarization dependent isolator allows only the light polarized in a specific direction, polarization independent isolator transmit all polarized light. So it is usually widely used in optical fiber amplifier.

Comparison of Polarization Dependent Isolator and Polarization Independent Isolator

In fact, you have already understood these two types of isolators according to the contents above. We can see their similarities and differences through the comparison of their definition, working principle and applications. Both of them consist of three parts and have a same principle based on Faraday effect. However, to overcome the limitation of polarization dependent isolator, polarization independent isolator has been developed. Regardless of the polarization state of the input beam, the beam will propagate through the isolator to the output fiber and the reflected beam will be isolated from the optical source. If the extinction ratio is important, a polarization dependent isolator should be used with either polarization maintaining fibers or even regular single-mode fibers. If the system has no polarization dependence, a polarization independent isolator will be the obvious choice.

How Much Do You Know About Optical Circulator?

What is Optical Circulator?

Optical circulator is a non reciprocal device allowing for the routing of light from one fiber to another based upon the direction of the light propagation. It is a special fiber optical component that could be used to separate optical signals in an optical fiber. It usually has at least three ports designed such that light entering any port exits from the next. With its features of high isolation of the input and reflected optical powers, as well as its low insertion loss, optical circulator is widely used in advanced communication systems and fiber optical sensing applications. 3-port optical circulator is the most commonly used optical circulator. In a 3-port optical circulator, there are generally two ports used as input ports and only one port used as output port. Here are some 3-port optical circulator samples shown in the following picture.

optical circulator

Principle of Optical Circulator

Optical circulator, as a passive element is widely used in fiber optical system. It is usually used to direct the optical signal from one port to another port and in one direction only. This action prevents the signal from propagating in an unintended direction. In a 3-port optical  circulator, a signal is transmitted from port 1 to port 2, another signal is transmitted from port 2 to port 3, finally a third signal can be transmitted from port 3 to port 1. The following picture shows us the working principle of the optical circulator with 3 ports.

principle of optical circulator

Main Features of Optical Circulator
  • High isolation
  • Low insertion loss
  • Low polarization dependent loss
  • Low polarization mode dispersion
  • Excellent environmental stability
Applications of Optical Circulator

Optical circulator can be utilized to achieve bi-directional optical signal transmission over a single fiber. It is a very important optical component which is commonly used in passive optical network, (wavelength division multiplexing) WDM network, polarization mode dispersion, chromatic dispersion compensation, optical add-drop modules (OADM), optical amplifiers, optical time domain reflectometry (OTDR) and fiber sensing applications. Fiberstore offers 3/4 ports polarization-insensitive optical circulator and 1310/1550/1064 polarization-maintaining (PM) optical circulators to satisfy your different applications.

In this blog, we have learnt some basic knowledge of optical circulator. We know that it is usually used to reduce the overall dispersion of light within a fiber-optic system. Especially in passive optical network, optical circulator is in a position to transmit light through the system and use half as much of the fiber to achieve the desired compensating effect while traveling through the system. In a word, optical circulator is an efficient means for conveying a light signal and makes communication systems more convenient and economical.

For more detailed information about optical circulator, welcome to visit our website or contact us over sales@fiberstore.com.