  In this paper, we report on the development progress of correlation-based
optical time domain reflectometry (OTDR). Substituting the direct detection
receiver with a coherent receiver enables to extract the phase and polarization
information of the reflected signal. Furthermore, due to the mixing of a weak
probe signal with a strong local oscillator the sensitivity of the receiver
improved. This improvement was demonstrated by analyzing the reflection from an
angled physical contact (APC) connector. To further quantify the improvements,
we compare the direct detection correlation OTDR (C-OTDR) with the coherent
detection correlation OTDR (CC-OTDR) with respect to the spatial and amplitude
resolution.
