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A subwavelength-diameter optical fibre (SDF or SDOF) is an optical fibre whose diameter is less than the wavelength of the light being propagated through it. An SDF usually consists of long thick parts (same as conventional optical fibres) at both ends, transition regions (tapers) where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part. Due to such a strong geometrical confinement, the guided electromagnetic field in an SDF is restricted to a single mode called fundamental.
There is no general agreement on how these optical elements are to be named; different groups prefer to emphasize different properties of such fibres, sometimes even using different terms. The names in use include subwavelength waveguide, [1] subwavelength optical wire, [2] subwavelength-diameter silica wire, [3] subwavelength diameter fibre taper, [4] [5] (photonic) wire waveguide, [6] [7] photonic wire, [8] [9] [10] photonic nanowire, [11] [12] [13] optical nanowires, [14] optical fibre nanowires, [15] tapered (optical) fibre, [16] [17] [18] [19] fibre taper, [20] submicron-diameter silica fibre, [21] [22] ultrathin optical fibres, [23] optical nanofibre, [24] [25] optical microfibres, [26] submicron fibre waveguides, [27] micro/nano optical wires (MNOW).
The term waveguide can be applied not only to fibres, but also to other waveguiding structures such as silicon photonic subwavelength waveguides. [28] The term submicron is often synonymous to subwavelength, as the majority of experiments are carried out using light with a wavelength between 0.5 and 1.6 µm. [11] All the names with the prefix nano- are somewhat misleading, since it is usually applied to objects with dimensions on the scale of nanometers (e.g., nanoparticle, nanotechnology). The characteristic behaviour of the SDF appears when the fibre diameter is about half of the wavelength of light. That is why the term subwavelength is the most appropriate for these objects.[ original research? ]
An SDF is usually created by tapering a commercial, usually step-index, optical fibre. Special pulling machines accomplish the process.
An optical fibre usually consists of a core, a cladding, and a protective coating. Before pulling a fibre, its coating is removed (i.e., the fibre is stripped). The ends of the bare fibre are fixed onto movable "translation" stages on the machine. The middle of the fibre (between the stages) is then heated with a flame (such as of burning oxyhydrogen) or a laser beam; at the same time, the translation stages move in opposite directions. The glass melts and the fibre is elongated, while its diameter decreases. [29]
Using the described method, waists between 1 and 10 mm in length and diameters down to 100 nm are obtained. In order to minimize the losses of light to unbound modes, one must control the pulling process so that the tapering angles satisfy the adiabatic condition [30] by not exceeding a certain value, usually in the order of a few milliradian. For this purpose, a laser beam is coupled to the fibre being pulled and the output light is monitored by an optical power meter throughout the whole process. A good-quality SDF would transmit over 95% of the coupled light, [29] most losses being due to scattering on the surface imperfections or impurities at the waist region.
If the fibre being tapered is uniformly pulled over a stationary heating source, the resulting SDF has an exponential radius profile. [31] In many cases it is convenient to have a cylindrical waist region, that is the waist of a constant thickness. Fabrication of such a fibre requires continuous adjustments of the hotzone by moving the heating source, [29] and the fabrication process becomes significantly longer.
Being extremely thin, an SDF is also extremely fragile. Therefore, an SDF is usually mounted onto a special frame immediately after pulling and is never detached from this frame. The common way of securing a fibre to the mount is by a polymer glue such as an epoxy resin or an optical adhesive.
Dust, however, may attach to the surface of an SDF. If significant laser power is coupled into the fibre, the dust particles will scatter light in the evanescent field, heat up, and may thermally destroy the waist. In order to prevent this, SDFs are pulled and used in dust-free environments such as flowboxes or vacuum chambers. For some applications, it is useful to immerse the freshly tapered SDF into purified water and thus protect the waist from contamination.
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Applications include sensors, [32] nonlinear optics, fibre couplers, atom trapping and guiding, [25] [33] [34] [35] quantum interface for quantum information processing, [36] [37] all-optical switches, [38] optical manipulation of dielectric particles. [39] [40]
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.
Photonic-crystal fiber (PCF) is a class of optical fiber based on the properties of photonic crystals. It was first explored in 1996 at University of Bath, UK. Because of its ability to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber, PCF is now finding applications in fiber-optic communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive gas sensors, and other areas. More specific categories of PCF include photonic-bandgap fiber, holey fiber, hole-assisted fiber, and Bragg fiber. Photonic crystal fibers may be considered a subgroup of a more general class of microstructured optical fibers, where light is guided by structural modifications, and not only by refractive index differences.
A superprism is a photonic crystal in which an entering beam of light will lead to an extremely large angular dispersion. The ability of the photonic crystal to send optical beams with different wavelengths to considerably different angles in space in superprisms has been used to demonstrate wavelength demultiplexing in these structures. The first superprism also modified group velocity rather than phase velocity in order to achieve the "superprism phenomena". This effect was interpreted as anisotropic dispersion in contrast to an isotropic dispersion. Furthermore, the two beams of light appear to show negative bending within the crystal.
Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices.
A fiber laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.
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A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.
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The Mamyshev 2R regenerator is an all-optical regenerator used in optical communications. In 1998, Pavel V. Mamyshev of Bell Labs proposed and patented the use of the self-phase modulation (SPM) for single channel optical pulse reshaping and re-amplification. More recent applications target the field of ultrashort high peak-power pulse generation.
In optics, a nematicon is a spatial soliton in nematic liquid crystals (NLC). The name was invented in 2003 by G. Assanto. and used thereafter Nematicons are generated by a special type of optical nonlinearity present in NLC: the light induced reorientation of the molecular director. This nonlinearity arises from the fact that the molecular director tends to align along the electric field of light. Nematicons are easy to generate because the NLC dielectric medium exhibits the following properties:
Optofluidics is a research and technology area that combines the advantages of fluidics and optics. Applications of the technology include displays, biosensors, lab-on-chip devices, lenses, and molecular imaging tools and energy.
The superconducting nanowire single-photon detector is a type of optical and near-infrared single-photon detector based on a current-biased superconducting nanowire. It was first developed by scientists at Moscow State Pedagogical University and at the University of Rochester in 2001. The first fully operational prototype was demonstrated in 2005 by the National Institute of Standards and Technology (Boulder), and BBN Technologies as part of the DARPA Quantum Network.
Orbital angular momentum (OAM) multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum of the electromagnetic waves to distinguish between the different orthogonal signals.
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A hybrid plasmonic waveguide is an optical waveguide that achieves strong light confinement by coupling the light guided by a dielectric waveguide and a plasmonic waveguide. It is formed by separating a medium of high refractive index from a metal surface by a small gap.
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.
Jonathan C. Knight, is a British physicist. He is the Pro Vice-Chancellor (Research) for the University of Bath where he has been Professor in the Department of Physics since 2000, and served as head of department. From 2005 to 2008, he was founding Director of the university's Centre for Photonics and Photonic Materials.
A tapered double-clad fiber (T-DCF) is a double-clad optical fiber which is formed using a specialised fiber drawing process, in which temperature and pulling forces are controlled to form a taper along the length of the fiber. By using pre-clad fiber preforms both the fiber core and the inner and outer cladding layers vary in diameter and thickness along the full length of the fiber. This tapering of the fiber enables the combination of the characteristics of conventional 8–10 µm diameter double-clad single-mode fibers to propagate light in fundamental mode with those of larger diameter (50–100 µm) double-clad multi-mode fibers used for optical amplification and lasing. The result is improved maintenance of pulse fidelity compared to conventional consistent diameter fiber amplifiers. By virtue of the large cladding diameter T-DCF can be pumped by optical sources with very poor brightness factor such as laser diode bars or even VECSELs matrices, significantly reducing the cost of fiber lasers/amplifiers.
Luc Thévenaz is a Swiss physicist who specializes in fibre optics. He is a professor of physics at EPFL and the head of the Group for Fibre Optics School of Engineering.