What would diameter differences between spliced fibers look like on an OTDR?

When observing diameter differences between spliced fibers on an Optical Time-Domain Reflectometer (OTDR), the resulting behavior is indicative of how light travels through the differing diameters at the splice point. If there is a disparity in fiber diameters, this will create a mismatch in the mode field diameters, leading to a significant phenomenon where light is not efficiently transmitted through the splice.

The splice will likely allow for light to pass in one direction with some degree of gain, due to certain optical characteristics favoring transmission in that direction. However, the same fiber mismatch can cause a much higher loss when attempting to transmit light back in the opposite direction because of the increased scattering and reflection that occurs at the splice.

Thus, the situation results in the observed characteristic: gain when light travels in one direction and significant loss in the return path. This behavior is a crucial aspect to understand when diagnosing fiber optic networks and assessing the quality of splices, particularly in terms of maintaining effective communication pathways.

In contrast, the other options describe scenarios that are not effectively representative of the distinct transmission effects caused by diameter mismatches in spliced fibers.