OTDR Index of Refraction

The basic optical property of a material is the index of refraction. 
It is also one of the key parameter that needs to be key-in into OTDR as to get accurate result on actual fiber optics distance measurement.
The index of refraction (n) measures the speed of light in an optical medium. 

The speed of light (c) in free space or vacumm is 

Therefore index of refraction of a material is equivalent to :-

When light enters  a different material ( from glass into air or from air into glass)is   the speed changes. 
This causes the light to bend or refract.
Light will always travel slower in the fiber material than in air. 

The index of refraction for glass  is approximately  1.50 or lower.
The index of refraction for air  is exactly 1.00. 

In order to get the actual fiber optics length measurement using OTDR, one  is required to enter fiber optics index of refraction into one of OTDR key parameters. The fiber optics index of refraction could be obtained from the fiber optics cable manufacturer.

OTDR Setup - Index of Refraction & Fiber Distance

All OTDRs regardless of brand have four basic setup requirement i.e. the OTDR user is required to key in these four basic data parameters into OTDR in order to get good and accurate fiber trace analysis.

The required data parameters are :-

1. Testing Range - Refer to Article HERE
2. Pulse Width - Refer to Article HERE
3. Index of Refraction 
4. Averaging Time

Index of Refraction

The relationship between distance and velocity or speed is as shown below -

You can review previous article on Index of Refraction HERE or HERE

As previously mentioned index of refraction measures the speed of light in optical medium. It is a function of velocity of light in vacuum and velocity of light in the opticel fiber medium or simply stated as below -

Velocity of light in fiber optics can be further expressed in terms of velocity of light in vacuum and index of refraction. This is done by rearranging the index of refraction equation as stated above.

Knowing that C or velocity of light in vacuum is a constant value

Then the distance of fiber optics as measured by OTDR can be simplied as shown below

The index of refraction, n,  is a known value which can be obtained from the fiber optics manufacturer. While t , OTDR pulse width  and its backscatter light travelling time , can be measured by OTDR automatically ie. the initial time laser pulse width launched into fiber optics and time taken by backscattered light returned back into OTDR.

Therefore, the measured of fiber optics distance is a function of index of refraction.

As value of IOR increases the measured fiber optic distance or length is getting shorter.

As value of IOR decreases the measured fiber optic distance or length is getting longer.

If the OTDR index of refraction does not match to that of fiber optics under testing IOR then the OTDR distance traces might show the incorrect results.

If the fiber optics under testing IOR is not known, then the end-user can use the OTDR default  IOR setting.

Typical fiber optics IOR value is between 1.4000 to 1.6000

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Rule of Thumb Always obtain the fiber optics Index of Refraction values from corresponding fiber optics manufacturer.