Lens-less 3D raster-scanning endomicroscopy via multicore fibers (MCFs) enables minimally invasive applications for instance auto fluorescent imaging for cancer diagnostics in the brain. However, it suffers from various issues: (i) periodic core arrangements, which result in higher diffraction orders and a limited field of view, (ii) bend-sensitive transfer functions which require constant on-line calibration, and (iii) inherent (static) differential path length differences of the individual fiber cores. To overcome these limitations, we present an MCF with 1200 aperiodically arranged cores, which is twisted to decrease dynamic bending sensitivity. Furthermore, diffractive optical elements (DOEs) were directly imprinted on the fiber facet using 2-Photon-Polymerization to compensate the inter-core-dispersion.
As a first demonstration, a simple imaging system consisting only of a camera and an MCF with an integrated DOE for phase compensation and focusing is realized for direct imaging. As a result, a flexible phase preserving fiber waveguide is realized, that can easily be included in standard microscopes to extend their field of applications to deep tissue and in vivo imaging.
Coherent fiber bundles used in endoscopic imaging suffer from inter-core dispersion resulting in pseudo random phase distortions for a transmitted wavefront. This limits their application to relaying intensity patterns for pixelated, 2D near field imaging. In the last years, employing spatial light modulators for digital optical conjugation of these distortions and unpixellated 3D raster scanning has been demonstrated. Here we present using 2-Photon Polymerization for writing phase compensation holograms onto the CFB facet enabling direct far field imaging in a simplified and robust manner. Robustness and field of view were increased by aperiodic and twisted fibers.
Lens-less endoscopy based on multi-core fibers (MCFs) with aperiodic core arrangements enables 3D imaging deep inside tissue with reduced imaging artifacts such as higher-order diffraction. With a scalable iterative stack-and-draw process, we fabricated and characterized (e.g. cross-talk) two aperiodic MCFs: (i) a 250 µm fiber with 420 cores and (ii) a 333 µm fiber with 1281 cores. Since lens-less endoscopy is sensitive to dynamic bending, two different approaches to twist the fibers were evaluated: i) rotation of the fiber preform during fiber drawing and (ii) post-production twisting of the MCF within a fiber processing station.
We experimentally demonstrate the possibility of adiabatic conversion of LP11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the values of effective indices, the LP11 modes are selectively converted to right-/left-handed circularly polarized vortex modes HE21 with a total angular momentum of ±2 and to quasi-TE01/TM01 modes with a total angular momentum of 0. The proposed conversion method has a purely topological origin, therefore, it is broadband in nature, in contrast to the methods based on resonant effects, and can be applied as an all-fiber broadband source of vortex beams.
Funding: Narodowe Centrum Nauki (DEC-2016/22/A/ST7/00089, Maestro 8).
Controllable excitation of spatial and polarization modes is of high importance in numerous applications, such as nonlinear optics, mode division multiplexing, interferometric measurements or sensing. We propose an effective method for selective excitation of different combinations of modes from LP01 and LP11 groups in a birefringent fiber. In the proposed method, the mode selection is realized with only a Wollaston prism, a rotatable polarizer and a half-wave plate, which ensures the possibility of high-power operation, low wavelength dependence, and tunability. Our approach makes it possible to excite almost all possible combinations of the LP01 and LP11 polarization modes and to continuously tune the relative coupling efficiencies of different modes by transverse shifting of the Wollaston prism. We demonstrate experimentally that the suppression rate of the unwanted modes with respect to the targeted mode exceeds 20 dB, and discuss the system configurations ensuring the highest possible coupling efficiencies for specific modes combinations. As example applications we show direct soliton and supercontinuum generation in the LP11 mode, broadband conversion of a supercontinuum from the LP01 to LP11 mode, broadband generation of vortex beams, gain tunability of intermodal four-wave mixing and cross-polarization four-wave mixing.
We report a white-light interferometric method for broad-band measurements of chromatic dispersion of higher-order-modes (HOMs) selectively excited in an optical fiber using a spatial light modulator (SLM). To excite a specific mode we appropriately modulated a phase distribution across a supercontinuum input beam with the SLM used in reflective configuration. For this purpose, the SLM surface was divided into azimuthally and radially distributed sectors which introduce the phase shifts equal alternately to 0 or π radians, similarly as in the targeted mode. The voltage applied to respective sectors of the SLM was corrected versus wavelength to ensure broad-band dispersion measurements for the required mode. For a given voltage setting, the dispersion measurements were possible without any correction over 250 nm in the visible and over even greater range in the infrared. We demonstrate feasibility of the proposed approach in the measurements of chromatic dispersion for all modes supported by Corning SMF-28e, i.e., LP01, LP11, LP21, LP02, and LP31. The measurements were conducted in the spectral range from 450 nm up to the cut-off wavelengths of respective higher order modes and up to 1600 nm for the fundamental mode.
We present a novel method for determining a spectral phase derivative from a single spectral interferogram obtained by using a white-light interferometry technique, which is further used for calculation of chromatic dispersion profile. The method is based on direct calculations of a second derivative of the registered spectral intensity at extremal points. We explain theoretical background of the method, discuss its limitations and evaluate an accuracy by numerical simulations. Additionally, we validate the proposed method in dispersion measurements of the BK7 glass plate and the SMF-28 fiber.
We present an alignment procedure which allows for precise gluing of a structure with an optically pumped quantum emitter to the end face of zirconia ferrule with a specially fabricated high numerical aperture single-mode fiber. The proposed method is an important step towards building a single-photon source based on an InGaAs quantum dot emitting in 1.3 μm range and located deterministically in a microlens fabricated by in-situ electron beam lithography and plasma etching to improve the photon extraction efficiency. Since single QDs are very dim at room temperature which hinders QD-fiber adjustment by maximizing the collected photoluminescence signal, the developed method uses light back-reflected from the top surface of the sample with microlens as a feedback signal. Using this approach, we were able to position the high-NA fiber over the center of the microlens with an accuracy of about 150 nm in a lateral direction and 50 nm in a vertical direction. The alignment accuracy was confirmed by following the room temperature emission from quantum wells embedded in a reference microlens. We also present initial low temperature tests of the coupling system mounted in a compact and portable Stirling cryocooler.
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