Quasi-needle-like focus synthesized by optical coherence tomography.

It is known that lateral resolution and depth of focus (DOF) in an optical imaging system are coupled, and a compromise between them has to be made. In this Letter, we propose to resolve the trade-off between lateral resolution and the DOF by a synthetic effective point spread function in optical path length (OPL) domain. A quasi-needle-like focus is synthesized by optical coherence tomography. We demonstrate that the synthesized quasi-needle-like focus provides a four-fold extension of a conventional DOF, while maintaining a high lateral resolution of 2.5 µm over a depth range of approximately 240 µm. The focal range can be further extended with more optical path length coded beams for synthesis involved.