PM Optical Circulators: Technology used and Categorization

Since 1990, polarization maintaining optical circulator has become one of the essential components in advanced optical communication systems. Nowadays, its applications have expanded widely not only in telecommunication industry but also in medical and imaging fields. Here, we will discuss this indispensable component in detail. So, let’s start with the basics.

What is a PM Optical Circulator?
PM optical circulator is a three or four port non reciprocal passive component which functions similar to an isolator. It transmits the light wave from one port to next port with maximum intensity while maintaining polarization and blocking any light transmission from one port to the previous port. Thus, it is also featured as a unidirectional circulator.

Technology Used in PM Optical Circulators
Polarization Maintaining Optical Circulators are designed on the basis of nonreciprocal polarization rotation of the Faraday effect. As the working of optical circulators is based on several components such as Faraday rotator, birefringent crystal, waveplate and beam displacer, let’s take a quick look at each of them.

Faraday Effect:
It is a magneto-optical effect which explains the phenomenon in which polarization plane of electromagnetic wave (or light wave) is rotated inside a material under magnetic field applied in parallel to the direction of wave propagation. The unique aspect of this effect is that the direction of rotation is independent of the propagation direction of light wave, which implies that rotation is non-reciprocal.

Light Propagation in Birefringent Crystal:
Birefringent crystal is a common material used in the designing of optical circulators. The crystals used in optical circulators are typically anisotropic uniaxial which means they have two refractive indices with one optical axis. The function of this birefringent crystal depends on the propagation direction of light and its optic axis orientation (crystal cutting). The crystals which are generally used include quartz, rutile, YVO4, etc.

Waveplate:
Also called retardation plate, a waveplate is one of the applications of birefringent crystal. It is made by cutting a crystal into a particular orientation. Due to small birefringence, crystal quartz is broadly used for making waveplates.

Beam Displacer:
A birefringent crystal based beam displacer is used to split an incoming light beam into two beams with orthogonal polarization states.

Categorization of Optical Circulators
Optical circulators are mainly divided into two categories:
Polarization-dependent optical circulator
Polarization-independent optical circulator

The former type is only functional for a light wave with a particular polarization state and is only used in a few applications such as free space communications between satellites and crystal sensing.

On the other hand, the latter type is functionally independent of the polarization state of light. While in ordinary circulators, the polarization is not maintained but there are polarization maintaining optical circulators available in the market also. They are used in a large variety of applications.

According to their functionality, optical circulators can also be divided into two groups.
Full Circulator - light passes through all ports in a full circle
Quasi Circulator - light passes through all ports but the light from last port is lost

Fortunately, due to the advancement in technology, you can avail highly reliable and efficient Polarization Maintaining Optical circulators not only in standard specifications but in customized specifications too.

Want to Know What a PM Tap Coupler is? Read this

Instead of diving straight into complexities of polarization maintaining tap coupler, let’s first discuss what an optical coupler simply is. An optical coupler is a device which can join one or more optic fiber ends for transmitting light waves in multiple paths. It is capable of combining two or more inputs into a single output as well as dividing a single input into two or more outputs. If you compare it with a splice or a connector, you will find that with an optical coupler, the signals can be more attenuated as the input signal can be distributed among the output ports.

In general, fiber optic couplers can be divided into two categories: active and passive.

For active coupler, an external energy source is required. On the other hand, there is no requirement of power to operate passive couplers.

In the market, you will find different types of fiber optical couplers such as X coupler, combiner, splitter, stars and trees. This categorization is carried out on the basis of the number of input and output ports.

As the name implies, a combiner does the work of combining two signals and providing a single output. Similarly, a splitter does the functioning of dividing a single optical signal to facilitate two output signals. It can further be divided into Y couplers and T couplers. If we talk about star couplers, they distribute power from inputs to outputs. While some tree couplers take multiple input signals and provide one output signal, others take a single input and provide multiple output signals.

Fiber optic couplers are manufactured using a variety of technologies such as planar waveguide, fused-fiber and micro optics.

Now, let’s move to polarization maintaining optical couplers. They are the micro optic filter type devices which facilitate optical signal splitting while maintaining polarization with high extinction ratio. Being a coupler in the essence, they are capable of either splitting the light from an input polarization maintaining fiber between two output PM fibers or can join signals from two PM input fibers into a single PM output fiber.

By now, it is clear that what fiber optical couplers and PM optical couplers are. So, what exactly is a polarization maintaining tap coupler? It is a type of PM coupler which can split light waves with accurate tap ratio.

The polarization state can be aligned either with slow axis or fast axis of the polarization maintaining fibers. It is made from separate light crystals and the output port is also different from PM filter coupler. The benefit of using a PM tap coupler is that it can handle higher power and also has more precise coupling ratio than PM filter coupler. So, PM tap coupler is a better device with high optical performance and stability to be used for splitting high power linearly polarized light wave to provide multiple outputs while still preserving the polarization state.

This tap coupler can also be used as a power tap in a PM fiber system to monitor signal power without disturbing polarization state of the light traveling in PM fiber.

Applications of PM Tap Coupler

• Power sharing in polarization sensitive systems
• PM fiber interferometers
• Signal monitoring in PM fiber systems
• Coherent detection
• Fiber optic instruments
• Fiber sensors

Features of PM Tap Coupler

• Compact-in-line package
• Low insertion loss
• High extinction ratio
• Excellent reliability
• Superior stability
• Optimum optical performance
• Accurate tap ratio
• Can handle higher power
• Precise coupling ratio

If you have requirement of polarization maintaining tap couplers or another kind of PM coupler with custom specifications, contact one of the esteemed PM coupler manufacturers so that you can be sure of that you are investing your time and money at the right place.