We present a novel optical sensor able to measure the distance between the tip of an endoscopic probe and the anatomical object under examination. In medical endoscopy, knowledge of the real distance from the endoscope to the anatomical wall provides the actual dimensions and areas of the anatomical objects. Currently, endoscopic examination is limited to a direct and qualitative observation of anatomical cavities. The major obstacle to quantitative imaging is the inability to calibrate the acquired images because of the magnification system. However, the possibility of monitoring the actual size of anatomical objects is a powerful tool both in research and in clinical investigation. To solve this problem in a satisfactory way we study and realize an absolute distance sensor based on fiber optic low-coherence interferometry (FOLCI). Until now the sensor has been tested on pig trachea, simulating the real humidity and temperature (37°C) conditions. It showed high sensitivity, providing correct and repeatable distance measurements on biological samples even in case of very low reflected power (down to 2 to 3 nW), with an error lower than 0.1 mm.
The aim of this work is the utilization of individual multimode fibers for the purposes of microendoscopy. In the present
contribution we discuss the question of image aberration induced by transmission along the fiber proposing a restoration
algorithm. Under LP conditions, and for not too large core diameters, the main cause of aberration is intermodal
dispersion, but in this work we demonstrate that it may be computed and corrected. We implemented a restoration
algorithm based on the separation and equalization of the contribution of each mode, for both step-index and gradedindex
fibers. Simulations show that fibers with a diameter of few tens of microns can transmit even quite detailed
images, and the proposed algorithm is effective for both the above types of fibers, for different fiber lengths and for a
variety of images. Experimental tests were performed by transmitting a Gaussian beam through a graded index silica
fiber (diameter 62.5 μm, NA=0.275). After applying the proposed post-processing to the aberrated image exiting from
the fiber, we obtained an error of 0.25 &mgr;m on the FWHM of the original Gaussian beam. In conclusion, it appears
possible to "capture" an external image and transmit the same through the fiber towards an observer at the other fiber
end, after appropriate phase correction.
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