Fiber Optic Circulators

Fiber optic 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).  Fiber optic circulators are used in applications such as dense wavelength division multiplexing (DWDM) networks, polarization mode dispersion, optical add-drop modules (OADM), optical amplifiers, and fiber optic sensors.

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 compensating effect while traveling through the system. Fiber optical circulators are used in DWDM networks and fiber amplifiers.  Typically, they are used to separate forward and backward propagating signals with 50dB 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.

Some fiber optic circulators function use 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 add-drop modules (OADM), optical amplifiers and fiber optic sensors are all types of fiber optic circulators. Optical add-drop modules act as a primary filter. They select two channels that can be dropped by an add-drop switch. The channels that are added are combined with the remaining channels in the add filter to created a new wavelength division multiplexing (WDM) output signal. Optical amplifiers create a direct amplification of a signal without needing to convert it into an electrical signal. This type of fiber optic circulator is thought of as being a laser with suppressed feedback from the cavity. Fiber optic sensors are fiber optic circulators used to measure parameters such as strain, temperature, and pressure. 

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