Scaling information pathways in optical fibers by topological confinement.

Spatial mode-count scalability in optical fibers is of paramount importance for addressing the upcoming information-capacity crunch, reducing energy consumption per bit, and for enabling advanced quantum computing networks, but this scalability is severely limited by perturbative mode mixing. We show an alternative means of light guidance, in which light's orbital angular momentum creates a centrifugal barrier for itself, thereby enabling low-loss transmission of light in a conventionally forbidden regime wherein the mode mixing can be naturally curtailed. This enables kilometer-length-scale transmission of a record ~50 low-loss modes with cross-talk as low as -45 decibels/kilometer and mode areas of ~800 square micrometers over a 130-nanometer telecommunications spectral window. This distinctive light-guidance regime promises to substantially increase the information content per photon for quantum or classical networks.

12 mode, WDM, MIMO-free orbital angular momentum transmission.

Simultaneous MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber is demonstrated. WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals. System performance is evaluated by measuring bit error rates, which are found to be below the soft FEC limit, and limited by inter-modal crosstalk. The crosstalk in the system is analyzed, and it is concluded that it can be significantly reduced with an improved multiplexer and de-multiplexer.

13.4km OAM state propagation by recirculating fiber loop.

Enabled by an enhanced effective index separation (Δneff = 1.7 × 10-4) and low transmission loss (0.8dB/km), OAM states are propagated over 13.4km in an air core fiber using a recirculating fiber loop. We observe that intermodal crosstalk decreases rapidly with increasing effective index separation, Δneff, and an order of magnitude lower crosstalk may be achieved just by doubling Δneff. We find that, in agreement with coupled power theory, our fiber has mode coupling properties analogous to elliptical core PM fibers, which yield ~10 × or more lower crosstalk than for conventional LP fiber mode orders with the same Δneff. This confirms that, for OAM modes, birefringent perturbations rather than shape perturbations matter most. In the process of performing the loop experiment, we demonstrate that OAM states in these fibers can be preserved with low loss (≤ 0.2dB) and low crosstalk (-15dB) while splicing distinct segments of the air-core fiber. For well-designed fibers, we demonstrate that OAM modes can travel distances relevant for large-scale data centers.

Determination of femtosecond-laser-induced refractive-index changes in an optical fiber from far-field measurements.

A new method for direct writing of localized, circularly symmetric refractive-index changes in optical fibers with a femtosecond laser is demonstrated. The refractive-index changes are characterized using a novel approach employing comparison of numerical simulations to the measured far-field profiles of unmodified and modified fibers. From the analysis, a negative refractive-index change of -0.015±0.005 within a radius of (0.6±0.1) µm is determined.

Terabit-scale orbital angular momentum mode division multiplexing in fibers.

Internet data traffic capacity is rapidly reaching limits imposed by optical fiber nonlinear effects. Having almost exhausted available degrees of freedom to orthogonally multiplex data, the possibility is now being explored of using spatial modes of fibers to enhance data capacity. We demonstrate the viability of using the orbital angular momentum (OAM) of light to create orthogonal, spatially distinct streams of data-transmitting channels that are multiplexed in a single fiber. Over 1.1 kilometers of a specially designed optical fiber that minimizes mode coupling, we achieved 400-gigabits-per-second data transmission using four angular momentum modes at a single wavelength, and 1.6 terabits per second using two OAM modes over 10 wavelengths. These demonstrations suggest that OAM could provide an additional degree of freedom for data multiplexing in future fiber networks.

Complex mode amplitude measurement for a six-mode optical fiber.

We propose a measurement protocol and parameter estimation algorithm to recover the powers and relative phases of each of the vector modes present at the output of an optical fiber that supports the HE11, TE01, HE21, and TM01 modes. The measurements consist of polarization filtered near-field intensity images that are easily implemented with standard off-shelf components. We demonstrate the accuracy of the method on both simulated and measured data from a recently demonstrated fiber that supports stable orbital angular momentum states.

Experimental demonstration of intermodal nonlinear effects between full vectorial modes in a few moded fiber.

We experimentally investigate intermodal nonlinear interactions, such as Raman scattering and four wave mixing. The fiber used is a specially designed few moded fiber, which splits the degeneracy of the first mode group, leading to stable propagation of the two full vectorial modes, TM01 and TE01. For the Raman experiments pumping occur in either the fundamental mode or the two full vectorial modes, whereas the signal is in the fundamental mode. In all three experiments approximately 40 dB of gain is achieved using 307 W of pump peak power. When pumping in either of the full vectorial modes four wave mixing is observed.

Measurement of effective refractive-index differences in a few-mode fiber by axial fiber stretching.

A method for measuring the effective refractive-index differences in a few-mode fiber by applying axial fiber stretching is described. This method represents a straightforward technique for characterization of few-mode fibers. Interference between LP01 and LP11 and in some cases also between LP11 and LP21 are observed in a fiber designed for support of LP01 and LP11. The relative strength of the coupled modes depends on specific splicing characteristics, and in some cases only two modes are seen. The results agree well with theoretical predictions for the fiber under investigation.

Control of orbital angular momentum of light with optical fibers.

We present a fiber-based method for generating vortex beams with a tunable value of orbital angular momentum from -1ℏ to +1ℏ per photon. We propose a new (to our knowledge) method to determine the modal content of the fiber and demonstrate high purity of the desired vortex state (97% after 20 m, even after bends and twists). This method has immediate utility for the multitude of applications in science and technology that exploit vortex light states.

Nonlinear generation of broadband polarisation vortices.

Polarisation-vortex beams over a broad wavelength region are generated by nonlinear transformation of a radially-polarized mode in a specially-designed optical fiber. The beams are produced by stimulated Raman scattering from 20-ns 1064-nm laser pulses, and up to the 4th order Stokes shift is observed. Measurements of polarization-selected intensity profiles of individual Stokes components show that the generated light maintains the desired spatial intensity distribution and radial polarization of the pump mode. At the highest pump power, 300 W, the process creates a coherent vortex beam from 1064 nm to 1310 nm, which is a span of nearly 250 nm.

Generation and propagation of radially polarized beams in optical fibers.

Beams with polarization singularities have attracted immense recent attention in a wide array of scientific and technological disciplines. We demonstrate a class of optical fibers in which these beams can be generated and propagated over long lengths with unprecedented stability, even in the presence of strong bend perturbations. This opens the door to exploiting nonlinear fiber optics to manipulate such beams. This fiber also possesses the intriguingly counterintuitive property of being polarization maintaining despite being strictly cylindrically symmetric, a prospect hitherto considered infeasible with optical fibers.

Adrm1, a putative cell adhesion regulating protein, is a novel proteasome-associated factor.

We have identified Adrm1 as a novel component of the regulatory ATPase complex of the 26 S proteasome: Adrm1 was precipitated with an antibody to proteasomes and vice versa. Adrm1 co-migrated with proteasomes on gel-filtration chromatography and non-denaturing polyacrylamide gel electrophoresis. Adrm1 has been described as an interferon-gamma-inducible, heavily glycosylated membrane protein of 110 kDa. However, we found Adrm1 in mouse tissues only as a 42 kDa peptide, corresponding to the mass of the non-glycosylated peptide chain, and it could not be induced in HeLa cells with interferon. Adrm1 was present almost exclusively in soluble 26 S proteasomes, albeit a small fraction was membrane-associated, like proteasomes. Adrm1 was found in cells in amounts equimolar with S6a, a 26 S proteasome subunit. HeLa cells contain no pool of free Adrm1 but recombinant Adrm1 could bind to pre-existing 26 S proteasomes in cell extracts. Adrm1 may be distantly related to the yeast proteasome subunit Rpn13, mutants of which are reported to display no obvious phenotype. Accordingly, knock-down of Adrm1 in HeLa cells had no effect on the amount of proteasomes, or on degradation of bulk cell protein, or accumulation of polyubiquitinylated proteins. This indicates that Adrm1 has a specialised role in proteasome function.

Mitochondria-rich cells as experimental model in studies of epithelial chloride channels.

The mitochondria-rich (mr) cell of amphibian skin epithelium is differentiated as a highly specialised pathway for passive transepithelial transport of chloride. The apical membrane of mr cells expresses several types of Cl(-) channels, of which the function of only two types has been studied in detail. (i) One type of channel is gated by voltage and external chloride concentration. This intriguing type of regulation leads to opening of channels only if [Cl(-)](o) is in the millimolar range and if the electrical potential is of a polarity that secures an inwardly directed net flux of this ion. Reversible voltage activations of the conductance proceed with long time constants, which depend on V in such a way that the rate of conductance activation increases when V is clamped at more negative values (serosal bath grounded). The gating seems to involve processes that are dependent on F-actin localised in the submembrane domain in the neck region of the flask-shaped mr cell. (ii) The other identified Cl(-) pathway of mr cells is mediated by small-conductance apical CFTR chloride channels as concluded from its activation via beta-adrenergic receptors, ion selectivity, genistein stimulation and inhibition by glibenclamide. bbCFTR has been cloned, and immunostaining has shown that the gene product is selectively expressed in mr cells. There is cross-talk between the two pathways in the sense that activation of the conductance of the mr cell by voltage clamping excludes activation via receptor occupation, and vice versa. The mechanism of this cross-talk is unknown.