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Optical Circulator Configurations and Advantages

An optical circulator is a generalized isolator having three or more ports. While an isolator causes loss in the isolation direction, a circulator collects the light and directs it to a non-reciprocal output port.

circulator connections

The figure drawn above shows the severa l possible circulator configurations.

Figure (a) illustrates the port mapping for a four-port circulator. The ports cyclically map 1 -> 2 -> 3 -> 4 -> 1. This is called a strict-sense circulator because every input port has a specific non- reciprocal output port. Construction of a strict-sense circulator with more ports becomes inelegant but ones with three ports becomes simple.

Figure (b) illustrates a non-strict-sense circulator having any number of ports greater than two. In this case each input port has a specific non-reciprocal output port except for the last port; the light input to the last port is lost. The ladder diagram reflects the optical path within the component and indicates the disconnect between the first and the last ports.

Figure (c) illustrates the three-port non-strict-sense circulator. This circulator has significance in telecommunication applications because return of light from port 3 to port 1 is not often required. For instance, the reflected light from a fibre Bragg grating need only be separated from the input light without loss, but as optical links are not typically operated in reverse there is no need for strict-sense behaviour.

An optical circulator allows the routing of light from one fibre to another based upon the direction of the light propagation. Nonreciprocal optical devices, such as optical isolators and optical circulators, are essential components of optical communication systems. Optical isolators pass light propagating in a forward direction while inhibiting the propagation of light in a backward direction. Since the optical circulator is an inherently non-reciprocal device, the light never goes to other ports. Optical circulators have wide applications. They are used to convert an existing unidirectional fibre optic communication link to a full duplex communication link by installing an optical circulator at each end of the link. Optical circulators are also used in fibre amplification systems, wavelength division multiplex (WDM) networks, optical time-domain reflectometers (OTDRs) and for test instruments.

Optical Circulator

Advantages

Although the optical sophistication of circulators makes them comparatively expensive, designers will appreciate the reliability and elegance imparted by fewer components. The resultant performance improvement may eliminate the need for more powerful transmitters, more sensitive receivers, and intermediate amplifiers, thus making optical circulators an economically interesting proposition. And, as with all technology, we can anticipate sharply lower prices as the optical circulator gains wider acceptance.

As one examines optical circulator applications, the notion of integrating the circulator, transmitter, and receiver into a single housing becomes very attractive. All elements would be aligned on a common platform, thus avoiding a multitude of external connectors and splices.

Where to Buy

Optical circulators is widely used in WDM networks, polarization mode dispersion, chromatic dispersion compensation, optical add-drop modules (OADM), optical amplifiers, OTDR and fibre sensing applications. Fiberstore offer 3/4 Ports Polarization-Insensitive optical circulator and 1310/1550/1064 Polarization-Maintaining (PM) optic circulators. Our fibre optical circulators can provide high isolation, very low insertion loss, low polarization dependent loss (PDL), low polarization mode dispersion (PMD), and excellent environmental stability.  Any other wavelengths, without or with any connector can customised according to your requirement. Fiberstore also provide Cisco Optical Transceivers, such as Cisco GLC-SX-MMD, Cisco GLC-LH-SMD and Cisco GLC-T 1000base-T, etc.

The Working Principle of the Optical Circulator

In fibre optical networks passive components such as optical isolators are essential for delivering of signals with minimum loss. Another type of passive element that is commonly used in fibre optic systems is the optical circulator. These devices that are 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. Optical circulators have continued to increase their presence in a broad array of applications, including optical amplifiers, optical add and drop systems, dense wavelength-division multiplexing (DWDM Mux) networks and, optical time domain reflectometers (OTDRs).

In a 3-port circulator a signal is transmitted from port 1 to port 2, another signal is transmitted from port 2 to port 3 and, finally, a third signal can be transmitted from port 3 to port 1. This behavior is represented by the following.

Optical circulators

Figure 1 Conventional figure to represent the behavior of an optical circulator.

The name derives from the fact that a signal is transmitted from Port 1 to Port 2, another signal can be transmitted from Port 2 to Port 3 and, finally, a signal can be transmitted from Port 3 to Port 1. In practice, one or two ports are used as inputs and the third port is used as the output.

Two simple examples of optical circulators can be considered. The first is an EDFA amplifier (erbium doped fibre amplifier) application to amplify a signal. The configuration to do this with a three-port optical circulator is shown in the following figure.

a pump laser

Figure 2. Amplification of an input optical signal by a pump laser.

In the figure, a weak optical signal at 1550nm is input to Port 1 and is directed to Port 2. The weak signal at Port 2 is pumped by a 980nm pump lasers and the amplified signal is then transmitted from port 2 to the output Port 3.

In the second example, we consider the application of a Fibre-Bragg grating compensator to correct a distorted signal. This can also be done using a three-port optical circulator as shown in the following figure.

Fiber dispersion compersation

Figure 3. Fibre dispersion compensation of a distorted signal.

In the figure the distorted signal at Port 1 is conditioned by transmitting the delay distorted input signal to the input of Port 2. At Port 2 dispersion compensation is applied and the compensated (corrected) signal is transmitted to Port 3.

We now analyze the operation of the three-port optical circulator. We begin by considering the figure shown below.

Configuration of a three-port optical circulator

Figure 4. Configuration of a three-port optical circulator.

The components are a beam splitting polarizer (1), a reflection prism (2), two birefringent crystals (3,6), a Faraday rotator (4), and a half-waveplate (5). The upper figure (a) describes the propagation from Port 1 to Port 2 and lower figure (b) describes the propagation from Port 2 to Port 3.

Optical circulators can be used to achieve bi-directional optical signal transmission over a single fibre. Optical circulators is commonly used in WDM networks, polarization mode dispersion, chromatic dispersion compensation, optical add-drop modules (DWDM OADM), optical amplifiers, OTDR and fibre sensing applications. Fiberstore offer 3/4 ports polarization-insensitive optical circulator and 1310/1550/1064 polarization-maintaining (PM) optic circulators. Our fibre optical circulators can provide high isolation, very low insertion loss, low polarization dependent loss (PDL), low polarization mode dispersion (PMD), and excellent environmental stability. Any other wavelengths, without or with any connector can customised according to your requirement.

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