Abstract: A system for optical coherence tomography (OCT) is disclosed. The system comprises an optical interferometer apparatus configured to split an optical beam into a reference beam directed to a reference reflector and a sample beam directed to a sample, and to combine a reflected beam from the reference reflector with a returning beam from the sample to form a combined optical signal. The system further comprises a two photon detector configured to detect the combined optical signal by two photon absorption and to provide a corresponding electrical signal, a frequency separation system configured to separate a low frequency component from the electrical signal, and a data processor configured for providing a topographic reconstruction of the sample based, at least in part, on the low frequency component.