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Register a new userComplete Polarization Control in Multimode Fibers with Polarization and Mode Coupling
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Multimode optical fibers have seen increasing applications in communication, imaging, high-power lasers and amplifiers. However, inherent imperfections and environmental perturbations cause random polarization and mode mixing, making the output polarization states very different from the input one. This poses a serious issue for employing polarization sensitive techniques to control light-matter interactions or nonlinear optical processes at the distal end of a fiber probe. Here we demonstrate a complete control of polarization states for all output channels by only manipulating the spatial wavefront of a laser beam into the fiber. Arbitrary polarization states for individual output channels are generated by wavefront shaping without constraint on input polarizations. The strong coupling between spatial and polarization degrees of freedom in a multimode fiber enables full polarization control with spatial degrees of freedom alone, transforming a multimode fiber to a highly-efficient reconfigurable matrix of waveplates.
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A scanning white light interferometer is developed to measure the distributed polarization coupling (DPC) in high birefringence polarization maintaining fibers (PMFs). Traditionally, this technique requests only one polarization mode to be excited or both polarization modes to be excited with equal intensity in the PMF. Thus, an accurate alignment of the polarization direction with the principal axis in PMF is strictly required, which is not facilely realized in practical measurement. This paper develops a method to measure the spatial distribution of polarization mode coupling with random modes excited using a white light Michelson interferometer. The influence of incident polarization extinction ratio (PER) on polarization coupling detection is evaluated theoretically and experimentally. It is also analyzed and validated in corresponding measurement that the sensitivity of the polarization coupling detection system can be improved more than 100 times with the rotation of the analyzer.
Hyperbolic materials offer a much wider freedom in designing optical properties of nanostructures than ones with isotropic and elliptical dispersion, both metallic or dielectric. Here, we present a detailed theoretical and numerical study of the unique optical properties of spherical nanoantennas composed of such materials. Hyperbolic nanospheres exhibit a rich modal structure that, depending on the polarization and direction of incident light, can exhibit either a full plasmonic-like response with multiple electric resonances, a single, dominant electric dipole or one with mixed magnetic and electric modes with an atypical reversed modal order. We derive resonance conditions for observing these resonances in the dipolar approximation and offer insight into how the modal response evolves with the size, material composition, and illumination. Specifically, the origin of the magnetic dipole mode lies in the hyperbolic dispersion and its existence is determined by two diagonal permittivity components of different sign. Our analysis shows that the origin of this unusual behavior stems from complex coupling between electric and magnetic multipoles, which leads to very strongly scattering or absorbing modes. These observations assert that hyperbolic nanoantennas offer a promising route towards novel light-matter interaction regimes.
We analyze coherent wave transport in a new physical setting associated with multimode wave systems where reflection is completely suppressed and mode-dependent losses together with mode-mixing are dictating the wave propagation. An additional physical constraint is the fact that in realistic circumstances the access to the scattering (or transmission) matrix is incomplete. We have addressed all these challenges by providing a statistical description of wave transport which fuses together a free probability theory approach with a Filtered Random Matrix ensemble. Our theoretical predictions have been tested successfully against experimental data of light transport in multimode fibers.
We investigate the coupling efficiency of parametric downconversion light (PDC) into single and multi-mode optical fibers as a function of the pump beam diameter, crystal length and walk-off. We outline two different theoretical models for the preparation and collection of either single-mode or multi-mode PDC light (defined by, for instance, multi-mode fibers or apertures, corresponding to bucket detection). Moreover, we define the mode-matching collection efficiency, important for realizing a single-photon source based on PDC output into a well-defined single spatial mode. We also define a multimode collection efficiency that is useful for single-photon detector calibration applications.
We provide a new analysis technique to measure the effect of the isotropic polarization rotation, induced by e.g. the isotropic cosmic birefringence from axion-like particles and a miscalibration of CMB polarization angle, via mode coupling in the cosmic microwave background (CMB). Several secondary effects such as gravitational lensing and CMB optical-depth anisotropies lead to mode coupling in observed CMB anisotropies, i.e., non-zero off-diagonal elements in the observed CMB covariance. To derive the mode coupling, however, we usually assume no parity violation in the observed CMB anisotropies. We first derive a new contribution to the CMB mode coupling arising from parity violation in observed CMB. Since the isotropic polarization rotation leads to parity violation in the observed CMB anisotropies, we then discuss the use of the new mode coupling for constraining the isotropic polarization angle. We find that constraints on the isotropic polarization angle by measuring the new mode-coupling contribution are comparable to that using the $EB$ cross-power spectrum in future high-sensitivity polarization experiments such as CMB-S4 and LiteBIRD. Thus, this technique can be used to cross-check results obtained by the use of the $EB$ cross-power spectrum.
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