Microstructured optical fiber

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Microstructured optical fibers (MOF) are optical fiber waveguides where guiding is obtained through manipulation of waveguide structure rather than its index of refraction.

In conventional optical fibers, light is guided through the effect of total internal reflection. The guiding occurs within a core of refractive index higher than refractive index of the surrounding material (cladding). The index change is obtained through different doping of the core and the cladding or through the use of different materials. In microstructured fibers, a completely different approach is applied. Fiber is built of one material (usually silica) and light guiding is obtained through the presence of air holes in the area surrounding the solid core. The holes are often arranged in the regular pattern in two dimensional arrays, however other patterns of holes exist, including non-periodic ones. While periodic arrangement of the holes would justify the use of term "photonic crystal fiber", the term is reserved for those fibers where propagation occurs within a photonic defect or due to photonic bandgap effect. As such, photonic crystal fibers may be considered a subgroup of microstructured optical fibers.

There are two main classes of MOF

  1. Index guided fibers, where guiding is obtained through effect of total internal reflection
  2. Photonic bandgap fibers, where guiding is obtained through constructive interference of scattered light (including photonic bandgap effect.)

Structured optical fibers, those based on channels running along their entire length go back to Kaiser and Co in 1974. These include air-clad optical fibers, microstructured optical fibers sometimes called photonic crystal fiber when the arrays of holes are periodic and look like a crystal, and many other subclasses. Martelli and Canning realized that the crystal structures that have identical interstitial regions are actually not the most ideal structure for practical applications and pointed out aperiodic structured fibers, such as Fractal fibers, are a better option for low bend losses. [1] Aperiodic fibers are a subclass of Fresnel fibers which describe optical propagation in analogous terms to diffraction free beams. [2] These too can be made by using air channels appropriately positioned on the virtual zones of the optical fiber. [3]

Photonic crystal fibers are a variant of the microstructured fibers reported by Kaiser et al. They are an attempt to incorporate the bandgap ideas of Yeh et al. in a simple way by stacking periodically a regular array of channels and drawing into fiber form. The first such fibers did not propagate by such a bandgap but rather by an effective step index – however, the name has, for historical reasons, remained unchanged although some researchers prefer to call these fibers "holey" fibers or "microstructured" optical fibers in reference to the pre-existing work from Bell Labs. The shift into the nanoscale [4] was pre-empted by the more recent label "structured" fibers. An extremely important variant was the air-clad fiber invented by DiGiovanni at Bell Labs in 1986/87 based on work by Marcatili et al. in 1984. [5] This is perhaps the single most successful fiber design to date based on structuring the fiber design using air holes and has important applications regarding high numerical aperture and light collection especially when implemented in laser form, but with great promise in areas as diverse as biophotonics and astrophotonics. [6]

Periodic structure may not be the best solution for many applications. Fibers that go well beyond shaping the near field now can be deliberately designed to shape the far-field for the first time, including focusing light beyond the end of the fiber. [7] These Fresnel fibers use well known Fresnel optics which has long been applied to lens design, including more advanced forms used in aperiodic, fractal, and irregular adaptive optics, or Fresnel/fractal zones. Many other practical design benefits include broader photonic bandgaps in diffraction based propagating waveguides and reduced bend losses, important for achieving structured optical fibers with propagation losses below that of step-index fibers.

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Refractive index contrast, in an optical waveguide, such as an optical fiber, is a measure of the relative difference in refractive index of the core and cladding. The refractive index contrast, Δ, is often given by , where n1 is the maximum refractive index in the core and n2 is the refractive index of the cladding. The criterion n2 < n1 must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include and . Normal optical fibers, constructed of different glasses, have very low refractive index contrast (Δ<<1) and hence are weakly-guiding. The weak guiding will cause a greater portion of the cross-sectional Electric field profile to reside within the cladding as compared to strongly-guided waveguides. Integrated optics can make use of higher core index to obtain Δ>1 allowing light to be efficiently guided around corners on the micro-scale, where popular high-Δ material platform is silicon-on-insulator. High-Δ allows sub-wavelength core dimensions and so greater control over the size of the evanescent tails. The most efficient low-loss optical fibers require low Δ to minimise losses to light scattered outwards.

Laser diode Semiconductor laser

A laser diode is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

Photonic crystal Periodic optical nanostructure that affects the motion of photons

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 Class of optical fiber based on the properties of photonic crystals

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.

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Fiber Bragg grating Type of distributed Bragg reflector constructed in a short segment of optical fiber

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Distributed Bragg reflector Structure used in waveguides

A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.

Optical fiber Light-conducting fiber

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and transparent dielectric waveguides made of plastic and glass.

Double-clad 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.

Extraordinary optical transmission

Extraordinary optical transmission (EOT) is the phenomenon of greatly enhanced transmission of light through a subwavelength aperture in an otherwise opaque metallic film which has been patterned with a regularly repeating periodic structure. Generally when light of a certain wavelength falls on a subwavelength aperture, it is diffracted isotropically in all directions evenly, with minimal far-field transmission. This is the understanding from classical aperture theory as described by Bethe. In EOT however, the regularly repeating structure enables much higher transmission efficiency to occur, up to several orders of magnitude greater than that predicted by classical aperture theory. It was first described in 1998.

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Digital planar holography

Digital planar holography (DPH) is a method for designing and fabricating miniature components for integrated optics. It was invented by Vladimir Yankov and first published in 2003. The essence of the DPH technology is embedding computer designed digital holograms inside a planar waveguide. Light propagates through the plane of the hologram instead of perpendicularly, allowing for a long interaction path. Benefits of a long interaction path have long been used by volume or thick holograms. Planar configuration of the hologram provider for easier access to the embedded diagram aiding in its manufacture.

Slot-waveguide

A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.

Subwavelength-diameter optical fibre

A subwavelength-diameter optical fibre 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 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.

Hybrid plasmonic waveguide

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.

Ravindra Kumar Sinha (physicist) Indian physicist and administrator

Prof. Ravindra Kumar Sinha is the Vice Chancellor of Gautam Buddha University, Greater Noida, Gautam Budh Nagar Under UP Government. 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.

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References

  1. ↑ Martelli, C; Canning, J; Gibson, B; Huntington, S (2007). "Bend loss in structured optical fibres". Optics Express. 15 (26): 17639–44. Bibcode:2007OExpr..1517639M. doi: 10.1364/OE.15.017639 . PMID   19551059.
  2. ↑ Canning, J (2002). "Diffraction-free mode generation and propagation in optical waveguides" (PDF). Optics Communications. 207: 35–39. Bibcode:2002OptCo.207...35C. doi:10.1016/S0030-4018(02)01418-9.[ permanent dead link ]
  3. ↑ Canning, J; Buckley, E; Lyytikainen, K (2003). "Propagation in air by field superposition of scattered light within a Fresnel fiber". Optics Letters. 28 (4): 230–2. Bibcode:2003OptL...28..230C. doi:10.1364/OL.28.000230. PMID   12661527.
  4. ↑ Huntington, S; Katsifolis, J; Gibson, B; Canning, J; Lyytikainen, K; Zagari, J; Cahill, L; Love, J (2003). "Retaining and characterising nano-structure within tapered air-silica structured optical fibers" (PDF). Optics Express. 11 (2): 98–104. Bibcode:2003OExpr..11...98H. doi: 10.1364/OE.11.000098 . PMID   19461711. Archived from the original (PDF) on 2020-02-10.
  5. ↑ .J. DiGiovanni, R.S. Windeler, "Article comprising an air-clad optical fiber", U.S. Patent 5,907,652 ; G02B 006/20 (1998 filed 1997); based on previous patent: E.A.J. Marcatili, "Air clad optical fiber waveguide," U.S. Patent 3,712,705 (1973)
  6. ↑ Åslund, Mattias L.; Canning, John (2009). "Air-clad fibres for astronomical instrumentation: focal-ratio degradation". Experimental Astronomy. 24: 1–7. Bibcode:2009ExA....24....1A. doi:10.1007/s10686-008-9132-7.
  7. ↑ J. Canning, Fresnel Optics Inside Optical Fibres, in Photonics Research Developments, Chapter 5, Nova Science Publishers, United States, (2008) and refs therein
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