Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Patent
1998-07-09
2000-09-26
Lee, John D.
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
385142, 65385, G02B 618
Patent
active
061252255
DESCRIPTION:
BRIEF SUMMARY
1. FIELD OF THE INVENTION
The present invention relates to fiber optics.
2. BACKGROUND OF THE INVENTION
A conventional mode field diameter conversion fiber comprising a length of an optical waveguide including a quartz glass cladding and a germanium-doped quartz glass core, wherein the core diameter varies along the optical waveguide, increasing towards its end, is disclosed, e.g., by K. Shiraishi, Y. Aizava, S. Kawakami in the reference titled as "Beam Expanding Fiber Using Thermal Diffusion of Dopant" in IEEE Journal of Lightwave Technology, 1990, vol.8, No.8, p.1151-1161. In the mode field diameter conversion fiber of the above reference, the longitudinal variation in a core diameter is provided owing to redistribution of the radial doping profile, which forms the refractive index structure of the optical waveguide, in the process of thermal diffusion of germanium.
The problem with the mode field diameter conversion fiber is a complicated technology caused by a small coefficient of germanium diffusion to quartz glass, resulting in a prolonged heat treatment of the optical waveguide, required to produce a mode field diameter conversion fiber. Apart from that, the diffusion occurs efficiently only at the temperature of 1600.degree. C. to 1800.degree. C. that is close to the melting point and, therefore, causes deformations in optical waveguides.
Closely approaching the claimed invention is a mode field diameter conversion fiber comprising a length of an optical waveguide including a quartz glass cladding and a doped quartz glass core, wherein the core diameter varies along the optical waveguide, increasing towards its end (see, e.g., U.S. Pat. No. 5,381,503, Int.Cl. G 02 B 6/10). In the prior art fiber, the core is initially doped with germanium and fluorine. Unlike germanium, fluorine reduces the quartz glass refractive index and additionally exhibits a greater thermal diffusion coefficient at the temperature of 1600.degree. C. to 1800.degree. C. As the result, when the optical waveguide with the double-doped core is heated, fluorine faster penetrates into the cladding, providing an efficient increase in the glass core refractive index and, therefore, a decreased mode field diameter.
A disadvantage of the above prior art is a complicated fabrication of the mode field diameter conversion fiber and a narrow range of mode field diameter variation, which is caused by a limited concentration of fluorine that can be introduced into the core along with germanium. Furthermore, the fluorine diffusion will unavoidably lead to the appearance, in such an optical waveguide, of regions with a decreased refractive index, which hampers coupling to conventional optical waveguides.
Described by O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki in the reference titled as "Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication". Appl. Phys. Lett. 1978, Vol.32, No.10, p.647-649, is a method for locally changing the refractive index of an optical waveguide, involving subjecting the optical waveguide to external exposure. Change in the refractive index occurs due to the photorefractive effect and is caused by the presence of defects that give rise to the appearance of specific bands in the absorption spectrum of the optical waveguide core. In this case, a two-photon interaction takes place. In the optical waveguide, an incoming beam and a beam reflected from the end face interfere, causing a periodic change in the refractive index.
Disadvantages of the above prior art method include a sophisticated process, a small change (.DELTA..about.10.sup.-6) in the refractive index, impossibility to vary a period of the grating produced, high cost and complexity of operating the apparatus implementing the method.
Another method for locally changing the refractive index of an optical waveguide, involving subjecting the optical waveguide to external exposure, was disclosed by G. Meltz, W. W. Morey, W. H. Glen in the reference titled as "Formation of Bragg Gratings in Optical Fibers by a Transverse
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Dianov Evgeny Mikhailovich
Golant Konstantin Mikhailovich
Grekov Mikhail Vladimirovich
Karpov Vladimir Ivanovich
Khrapko Rostislav Radievich
Lee John D.
Nauchny Tsenir Volokonnoi Optiki Pri Institute Obschei Fiziki Ro
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