Ultra-thin endoscopes are highly desirable for many applications involving remote imaging. Current ultra-thin endoscopes are primarily video-endoscopes and have a shaft diameter of 6 mm or less. Fiberscopes, on the other hand, can reach a micro-meter diameter, thus allowing examination of small, difficult-to-reach, spaces for medical and other applications. Multimode fibers are being explored as ultra-thin lensless replacements for the commonly used endoscopes. The difficulty with imaging or focusing light through a multimode fiber is phase randomization of light propagating through the fiber, which results in a complex speckle pattern at the fiber output. To overcome this obstacle, an access to both fiber ends is required for pre-calibration.
A novel endoscopic method that was developed by Prof. Silberberg at the Weizmann Institute of Science allows light focusing through a multimode fiber by approaching solely the proximal end and retrieving information about the distal end using non-linear optical feedback.
· Clinical imaging of narrow cavities (blood vessels, respiratory system, joints, etc.)
· Selective targeting and burning of fluorescent targets (imaging and treatment)
- Ultra-thin (micro-meter scale) and flexible
- Lensless endoscopy
- High resolution and accuracy
We consider a two-photon lensless multimode fiber-based endoscope, where an ultrashort pulse is delivered to a fluorescently tagged sample through the fiber. The pulses excite two photon fluorescence (2PF) from a 2PF screen placed against the fiber distal end. The back-propagated 2PF that is collected by the same fiber is separated from the excitation light at the proximal end by a dichroic mirror (DM), and the Fourier-transformed image of the fiber facet is recorded by an EMCCD camera. It is then used as feedback for a wavefront-shaping optimization algorithm, controlling a spatial light modulator (SLM) at the proximal fiber end. The nature of the light propagation in the fiber allows for scanning and controlling the focus position at the fiber distal end.