How it works – OFDR vs OTDR

OFDR (optical frequency domain reflectometry) serves a similar purpose as OTDR (Optical Time Domain Reflectometry)  and yet these two technologies function very differently.  An interrogator using OTDR transmits a lightpulse of known width and measures the reflected energy and it’s time of flight to determine the magnitude and location of an event along the length of a fiber network.  A known disadvantage of OTDR’s is the presence of a deadzone in which the interrogator is momentarily blind to measuring reflected energy.  This deadzone manifests itself in a relatively high spatial resolution.  Spatial resolution is the ability to detect closely spaced events along the length of a fiber network.  Deadzones are typically in the order of meters and this makes OTDR unsuitable for applications where low spatial resolution or high accuracy is required. 

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Simplified Diagram of OFDR

OFDR – No deadzone

By contrast, OFDR scans a fiber network using light from a variable frequency swept wavelength coherent laser coupled into an interferometer.  One leg of the interferometer is a reference path of fixed length while the other leg represents the optical network under test.  Backscattered light from fiber under test is combined with light from the reference arm and the intersection of these creates an interference signal.  This interference signal contains information pertaining to the precise location and magnitude of reflective events along the length of the network under test.  In order to extract this information a series of Fourier transforms are performed on the interference signal, the output of which is a display indicating the precise location and magnitude of reflective events along the length of fiber network.  OFDR is able to measure the precise location of reflective events with no deadzone.  Using OFDR, cable lengths and fault locations can be measured precisely and optical networks and components can be characterized.  

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OBR 4600 Trace of a fiber optic connector demonstrating sub millimeter spatial resolution
OBR 4600 Trace of a fiber optic connector demonstrating sub millimeter spatial resolution