Fig. 1.

Experimental setup and picture of the needle endoscope. On the lower part, the experimental setup is presented. Light focused on the fiber tip is dispersed over the different propagation modes supported by the fiber (inset). The speckled output is combined with the reference and the resulting interference pattern is digitally recorded onto the CMOS sensor. The reconstructed phase of the hologram is assigned on the Spatial Light Modulator, which then modulates the reference beam. The phase conjugate beam that is generated propagates backwards, retracing its way through the multimode fiber and coming into focus at the original position. The calibration beam can be scanned in both lateral dimensions (moving the calibration objective using motorized stages) generating a regular grid of focused spots and the corresponding phase lookup table. Upper part. Placing the fiber inside a needle tip we can make a rigid ultrathin endoscope that will allow the fiber to remain intact and at the same time allow for minimally invasive endoscopic imaging. The outer diameter of the endoscope is limited only by the needle and can be as small as 460μm for the 250μm cladding fiber used in the experiments.