We describe a technique that allows the improvement of the resolution of optical microscopes for nanofiber measurements beyond the diffraction limit. It can be readily implemented on any microscope. We demonstrated it by measuring tapered fibers radii from 0.4 to 4 µm with a resolution below the diffraction limit, from a few nanometers up to 50 nm in the worst case, depending on the radii. This technique is a non-contact measurement with the microscope objective placed a few centimeters from the nanofiber. We acquire the experimental diffraction pattern by scanning the object plane of the microscope system, upstream and downstream the nanofiber. We compare this experimental diffraction pattern to a bank of all the simulated patterns for all the radii. The radius of the simulated diffraction pattern that best matches to the experimental one is the sought radius.
In this paper, we present a design of an all-fiber source of correlated photon pairs based on standard telecommunications tapered fibers. We examine the generation of correlated photon pairs using parametric process (") in silica tapered optical fibers. This nonlinear process is ensured thanks to surface dipole and bulk multipole nonlinearities. The process of photons creation is modeled by taking into account the vector aspect of the propagation of the optical field in a silica nanofiber. The phase matching is provided by propagating the pump field in one spatial mode, while generating a photon pair in another spatial mode. The generation efficiency of photon pairs depends on diameter uniformity of the nanofiber after the manufacturing process. We size this nanofiber for a good optimization of photon pair generation efficiency, we report that the tolerance in diameter uniformity is = 2 nm for a generation rate of photon pairs estimated to $% ≈ 22 000 pairs/s, for 1 W power pump and a nanofiber length of 1.1 mm. Deposits on the nanofiber can be used in order to relax the manufacturing constraints on diameter to maximize the generation rate of photon pairs. As an example, the use of Polytetrafluoroethylene (PTFE) on the nanofiber applied as a cladding whose thickness is infinite makes it possible to relax the constraints on the nanofiber diameter. For the same = 2 nm, a generation rate of photon pairs estimated to $% ≈ 78 000 pairs/s for 1 W power pump and a nanofiber length of 2.4 mm is predicted.
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