Fiber optic circulators, commonly referred to as optical circulators, are nonreciprocal devices that direct an optical signal (light) from one port to the next, in only one direction at a time. While the direction of the light may be redirected as needed, the light must pass through ports sequentially (from port 1, to port 2, before traveling to port 3).
How Optical Circulator Works
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Fiber optic circulators are used to reduce the overall dispersion of light within a fiber optic system. In conjunction with a dispersion compensating module (DCM), fiber optic circulators are able 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.
Optical circulators support bi-directional ports and allow a single fiber to be used for both transmission and reception of an optical signal. Fiber optic circulators are used in applications such as dense wavelength division multiplexing (DWDM) networks, polarization mode dispersion, chromatic dispersion compensation, optical add-drop modules (OADM), optical amplifiers, and fiber optic sensors.
Division Multiplexing (DWDM) Networks
Fiber optical circulators used in DWDM networks are used to separate forward and backward propagating signals with 50 dB of isolation (protection of the input fiber from returning power while employing the rejected light). Fiber optic circulators also provide a level of cross-talk (the ratio of the output power produced by the desired input to the output power produced by undesired inputs) of more than 60 dB. This allows a single fiber to effectively transmit a bi-directional signal.
Optical Add-drop Multiplexing (OADM)
Optical add-drop modules act as a primary filter. Optical couplers can achieve this behavior by the use of a Fiber Bragg Grating (FBG). The full bandwidth signal enters the coupler and is directed toward the next port where the FBG is placed. The FBG reflects the desired signal back to the coupler while the dropped channels exit through the port where the FBG was placed.
Polarization Mode Dispersion (PMD)
Some fiber optic systems experience polarization mode dispersion (PMD), an inherent property of all optical media. PMD is caused by the difference in the propagation velocities of light in the orthogonal principal polarization states of the transmission medium. If the optical pulse contains both polarization components, then the different polarization components in fiber optic circulators will travel at different speeds and arrive at different times, smearing the received optical signal. Optical couplers can correct PMD by rotating the electric and magnetic field of the optical signal.
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Chromatic Dispersion Compensation
Optical circulators compensate for chromatic dispersion by using a chipped Fiber Bragg Grating (FBG). FBG's are wavelengths dependent reflectors. A section of optical fiber is treated or doped with a material that changes the refractive index of the fiber, causing wavelength dependent reflections. Chipped Fiber Bragg Gratings have several gratings displaced throughout a fiber and allow for compensation of chromatic dispersion.
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Optical amplifiers create a direct amplification of a signal without needing to convert it into an electrical signal. Optical circulators feed the input signal into the amplifier, receive the amplified signal, and reroute the signal to an output port. In this application the fiber optic circulator supplies suppressed feedback from the cavity.
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Fiber Optic Sensors
Fiber optic sensors are used to measure parameters such as strain, temperature, and pressure. They use fiber optic circulators to reroute signals. The high isolation between the input and reflected optical power coupled with a low insertion loss make optical couplers the preferred component for interfacing with optical sensors.