What type of OTDR measurements require the correct fiber index of refraction value?

What type of OTDR measurements require the correct fiber index of refraction value?

• A. Length
• B. Attenuation
• C. Backscatter
• D. Pulse width

Answer: (A)

The measurement that depends on knowing the fiber’s refractive index is length. An OTDR sends a light pulse into the fiber and records how long it takes for light to travel to a point (and back). To turn that travel time into a physical distance, you need the light’s propagation speed in the fiber, which is roughly c divided by the effective refractive index (v ≈ c/n). Since the pulse makes a round trip, distance is proportional to t/(2n). If the index isn’t known or is used incorrectly, the distance scale on the OTDR trace becomes off, skewing length measurements along the fiber. Attenuation, on the other hand, is about how the signal’s power decreases with distance and is not converted from time to distance in the same way. Backscatter reflects the amount of light scattered along the fiber and is a property of the material and imperfections, not a direct conversion from time to length. Pulse width is a parameter of the emitted pulse that affects resolution, not a calculation that requires n to determine distance.

The measurement that depends on knowing the fiber’s refractive index is length. An OTDR sends a light pulse into the fiber and records how long it takes for light to travel to a point (and back). To turn that travel time into a physical distance, you need the light’s propagation speed in the fiber, which is roughly c divided by the effective refractive index (v ≈ c/n). Since the pulse makes a round trip, distance is proportional to t/(2n). If the index isn’t known or is used incorrectly, the distance scale on the OTDR trace becomes off, skewing length measurements along the fiber.

Attenuation, on the other hand, is about how the signal’s power decreases with distance and is not converted from time to distance in the same way. Backscatter reflects the amount of light scattered along the fiber and is a property of the material and imperfections, not a direct conversion from time to length. Pulse width is a parameter of the emitted pulse that affects resolution, not a calculation that requires n to determine distance.