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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. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically. [1]
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, [2] subwavelength optical wire, [3] subwavelength-diameter silica wire, [4] subwavelength diameter fibre taper, [5] [6] (photonic) wire waveguide, [7] [8] photonic wire, [9] [10] [11] photonic nanowire, [12] [13] [14] optical nanowires, [15] optical fibre nanowires, [16] tapered (optical) fibre, [17] [18] [19] [20] fibre taper, [21] submicron-diameter silica fibre, [22] [23] ultrathin optical fibres, [24] optical nanofibre, [25] [26] optical microfibres, [27] submicron fibre waveguides, [28] 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. [29] 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. [12] 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. [30]
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 [31] 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, [30] 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. [32] 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, [30] 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, [33] nonlinear optics, fibre couplers, atom trapping and guiding, [26] [34] [35] [36] quantum interface for quantum information processing, [37] [38] all-optical switches, [39] optical manipulation of dielectric particles. [40] [41]
Subwavelength-diameter optical fibers have various applications owing to the special conditions of confining light in nanoscale dimensions. Some of the key usages are:
The SDFs increase the sensitivities to environment factors like temperature and humidity.
They play an important role in second-order harmonic generation and in all-optical switching processes, important in photonics and quantum communication.
These fibers make the manipulation of atoms and photons possible; thus, they are very vital in quantum information processing.
SDFs are used for moving nanoparticles in optical tweezers, useful in nanotechnology.
Their broad applications make them fundamental in advanced optics and quantum technologies.
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.
An optical ring resonator is a set of waveguides in which at least one is a closed loop coupled to some sort of light input and output. The concepts behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is output to the output bus waveguide which serves as a detector waveguide. Because only a select few wavelengths will be at resonance within the loop, the optical ring resonator functions as a filter. Additionally, as implied earlier, two or more ring waveguides can be coupled to each other to form an add/drop optical filter.
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. Hollow-core fibers (HCFs) are a related type of optical fiber which bears some resemblance to holey optical fiber.
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.
Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commonly at the 1.55 micrometre wavelength used by most fiber optic telecommunication systems. The silicon typically lies on top of a layer of silica in what is known as silicon on insulator (SOI).
In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is a smooth spectral continuum. There is no consensus on how much broadening constitutes a supercontinuum; however researchers have published work claiming as little as 60 nm of broadening as a supercontinuum. There is also no agreement on the spectral flatness required to define the bandwidth of the source, with authors using anything from 5 dB to 40 dB or more. In addition the term supercontinuum itself did not gain widespread acceptance until this century, with many authors using alternative phrases to describe their continua during the 1970s, 1980s and 1990s.
A spaser or plasmonic laser is a type of laser which aims to confine light at a subwavelength scale far below Rayleigh's diffraction limit of light, by storing some of the light energy in electron oscillations called surface plasmon polaritons. The phenomenon was first described by David J. Bergman and Mark Stockman in 2003. The word spaser is an acronym for "surface plasmon amplification by stimulated emission of radiation". The first such devices were announced in 2009 by three groups: a 44-nanometer-diameter nanoparticle with a gold core surrounded by a dyed silica gain medium created by researchers from Purdue, Norfolk State and Cornell universities, a nanowire on a silver screen by a Berkeley group, and a semiconductor layer of 90 nm surrounded by silver pumped electrically by groups at the Eindhoven University of Technology and at Arizona State University. While the Purdue-Norfolk State-Cornell team demonstrated the confined plasmonic mode, the Berkeley team and the Eindhoven-Arizona State team demonstrated lasing in the so-called plasmonic gap mode. In 2018, a team from Northwestern University demonstrated a tunable nanolaser that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism.
A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.
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.
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.
Microstructured optical fibers (MOF) are optical fiber waveguides where guiding is obtained through manipulation of waveguide structure rather than its index of refraction.
In physics, a high contrast grating is a single layer near-wavelength grating physical structure where the grating material has a large contrast in index of refraction with its surroundings. The term near-wavelength refers to the grating period, which has a value between one optical wavelength in the grating material and that in its surrounding materials.
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.
Prof. R K Sinha is the Vice Chancellor of Gautam Buddha University, Greater Noida, Gautam Budh Nagar under Uttar Pradesh Government since January 2022. He was the Director of the CSIR-Central Scientific Instruments Organisation (CSIR-CSIO) Sector-30C, Chandigarh-160 030, India. He has been a Professor - Applied Physics, Dean-Academic [UG] & Chief Coordinator: TIFAC-Center of Relevance and Excellence in Fiber Optics and Optical Communication, Mission REACH Program, Technology Vision-2020, Govt. of India Delhi Technological University Bawana Road, Delhi-110042, India.
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
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.
In photonics, a meta-waveguide is a physical structures that guides electromagnetic waves with engineered functional subwavelength structures. Meta-waveguides are the result of combining the fields of metamaterials and metasurfaces into integrated optics. The design of the subwavelength architecture allows exotic waveguiding phenomena to be explored.