refractive index from joint loss

two compatible MM SI fibers joined with small air gap.
If joint loss is 0.47dB , how can I find refractive index of perfectly aligned fiber ?

I think that you need more then one result, try 2 different air gaps and their respective losses. If the light was travelling along the axis of the fibre then all light leaving the first would land onto the face of the other (loss = 0dB). If the light was travelling more or less across the fibre, just zig-zaging its way down it. As it leaves the end of one fibre a lot of the light exiting would strike the connector walls and miss the face of the second fibre. In this case the losses would be high (> 20 dB?). So now you know the air gap, the diam of the cable and from the losses, you can work out the "spray " angle of the exiting beam and look up the refractive index.
Frank