An interferometer is used to _____.

An interferometer is a sophisticated optical instrument that employs the principles of interference to achieve very high precision measurements of various physical quantities. The correct application of an interferometer, specifically in the context given, is to examine the ferrule end-face of a fiber optic connection.

When looking at fiber optic connectors, ensuring proper centering and curvature of the ferrule end-face is crucial for optimal light transmission, minimizing signal loss, and reducing back reflections. An interferometer can create an interference pattern by combining light from a reference beam and the reflected light from the end face, allowing for detailed assessments of its surface characteristics, such as flatness and alignment. This capability is integral in maintaining high-quality connections in fiber optic systems.

In terms of the other options, while measuring light intensity, joining fibers, and determining fiber length are important aspects of fiber optics, they do not specifically apply to the unique capability of an interferometer. For instance, intensity measurements typically rely on photodetectors, and fusion splicing is a method used to join fibers, while fiber length can be determined using various techniques involving time-domain reflectometry or optical time domain methods, which do not involve the same principles as interference.