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Navy

Optical time domain reflectometer (OTDR) calibration standard and instrument

Precise calibration of OTDR’s in the field

Photonics

Fiber optic cables must be tested for quality and inspected for potential defects which can be caused by spooling of the cable or other events. This is done with ODTRs. The instrument connects to one end of a fiber optic cable. It sends a pulse of light into the fiber, which then travels down its length. Some light energy in the pulse is scattered and reflected back through the fiber optic cable and to the OTDR. The OTDR has an internal timer, which measures the time elapsed between the sending of the pulse, and any energy that is reflected and returned to the OTDR.  Calibration relies on use of a sample spool with an often imprecise length and index of refraction.  Further complicating the calibration regime, the OTDR calculates a time measurement whereas a distance measurement is more meaningful to end users.  To date, there has not been a standard or method available that allows for a direct length comparison between the OTDR and a fiber optic distance standard.

Navy researchers have developed a fiber optic cable calibration standard in combination with a device for calibrating distance and attenuation parameters of an OTDR. The attenuation and distance fiber optic cable calibration standard (ADFOCS) utilizes spooled fiber optic cable with a well-known length and well-known index of refraction assembled into a single, rack-mountable enclosure which also includes a visual inspection scope for checking fiber optic cable connectors for cleanliness. By use of the invention, well-known lengths of fiber optic cables are measured allowing for a direct comparison in units of length, rather than time.

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