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A fully integrated all-fiber implementation of common-path interferometer-based optical coherence tomography (OCT) has been successfully demonstrated. Unlike conventional Michelson-based OCT systems, there is no separate reference arm as the reference signal is derived from the probe-end back reflection. This leads to "downlead insensitivity", which makes the device more compatible with existing endoscopic medical instruments. We also theoretically analyze the optimization of system sensitivity in balanced and unbalanced detection. We have conducted experiments to confirm some of our signal-to-noise ratio (SNR) analysis, which are in excellent agreement with the theory. Interestingly, our analysis predicts existence of much higher beat noise in common-path OCT as compared with the conventional OCT. We predict a maximum upper limit of only 6-dB improvement in the SNR when the balanced detection is used, as compared with the unbalanced detection.