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(12) (10) Patent N0.: US 6,947,650 B1 Homa (45) Date of Patent: Sep. 20, 2005 (54) LONG WAVELENGTH, PURE SILICA CORE 6,280,850 B1 8/2001 Oh et al. SINGLE MODE FIBER AND METHOD OF 6,307,994 B1 10/2001 Pack et al. FQRMING THE SAME 6,247,174 B1 * 2/2002 Onishi et iii. ............. .. 385/122 6, 29,666 B1 3/2003 Du tz et a . (75) Inventor: Daniel Scott Homa, Bloomsbury, NJ 200 ES(:1jg1t‘;It11al°~t ‘*1’ """"" " 385/123 (US) 2003/0110811 A1 6/2003 Nunome et al. . 2003/0145629 A1 8/2003 And t l. (73) Asslgneei Luna Energy LLC> B1a°kSb“rg> VA 2003/0147619 A1 8/2003 8663231 :1 :1. (U5) 2005/0063663 A1* 3/2005 Anderson et 61. ......... .. 385/142 ( * ) Notice: Subject to any disclaimer, the term of this OTHER PUBLICATIONS guserg iiszyéggngedo (ga agjusted under 35 Hecht, J . (2002) Understanding Fiber Optics, 4th Edition, p. ' ' ' y y ' 135-137, NeW Jersey, Ohio, Prentice Hall.* (21) Appl. NO.Z 10/840,130 * erred by eXarr1rr1er (22) Filed; May 6, 2004 Primary Examiner—Brian Healy Assistant Examiner—Charlie Peng (51) Int. Cl.7 ......................... .. G02B 6/16; G02B 6/02; G92]; 6/()9 (57) ABSTRACT (52) U.S. Cl. ..................... .. 385/123; 385/126; 385/144 (58) FIEId Of SEHPCII ............................... .. 385/126-128 Arr Qptreal fiber suitable [Q suppgrt single rr1()de Qptreal transmission at longer Wavelengths (e.g., 1550 nm) is (56) References Cited formed to comprise a pure silica core region and a “doWn doped” cladding layer. The core region is defined as having U.S. PATENT DOCUMENTS a diameter d and the cladding layer is defined has having an 4,439,007 A 3/1984 I-111413’ ct a1~ outer diameter D. In accordance With the present invention, 2 2/ Ilfzay ct al" single mode propagation Will be supported When D/d>8.5, 4i691:99O A 41987 Carlin ct a1‘ and is preferably in the range of 9-10. 5,596,668 A 1/1997 DiGiovanni et al. 5,942,296 A 8/1999 Oh et al. 9 Claims, 2 Drawing Sheets 16 10
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U.S. Patent Sep. 20, 2005 Shfifit 2 61 2 US 6,947,650 B1
LONG WAVELENGTH, PURE SILICA CORE SINGLE MODE FIBER AND METHOD OF FORMING THE SAME
The present invention relates to a single mode fiber for long Wavelength (e.g., 7»=1550 nm) applications and, more particularly, to a single mode fiber comprising a pure silica core and a relatively thick cladding such that the ratio of the diameter of the cladding (defined as “D”) to the diameter of the core (defined is “d”) is greater than 8.5.
BACKGROUND OF THE INVENTION
In single mode optical fibers, a significant fraction of the total guided energy is not confined to the core, and the “tail” of the poWer distribution extends a considerable distance into the cladding material. Since the substrate (or sleeve) tube material outer cladding is typically less pure (and therefore much more lossy) than the cladding material, it is necessary to ensure that no significant fraction of the total poWer propagates in the substrate- or tube-derived material.
In one type of prior art single mode fiber, referred to as a “depressed cladding” fiber, the effective refractive index of the cladding material is chosen to be substantially less than the refractive index of the core. In most of these depressed cladding prior art designs, the core region is “up doped” and the cladding region is “doWn doped” so as to obtain the largest difference in refractive index With the smallest overall fiber diameter. The ratio of the cladding diameter D to the core diameter d, is used in determining various performance parameters of optical fiber made from the perform. For example, to obtain optical fiber having desired transmission characteristics, the D/d ratio should be Within an acceptable, but relatively narroW, range of values. The single mode cut-off Wavelength must also be taken into account in the determination of the appropriate D/d value. The cut-off Wavelength is the Wavelength beloW Which the optical fiber behaves as a step-index multimode fiber and above Which behaves as a single mode fiber. Also, the D/d ratio affects the mode field diameter (MFD) Which is a measure of the Width of the light intensity in a single mode fiber—also referred to as the “spot size”. In most cases, it is desired to maintain the ratio D/d less than 2.5 While this value is acceptable for most short Wavelength arrangements, long Wavelengths (e.g., 1550 nm) cannot be supported in such an arrangement.
In the case Where a depressed clad/pure silica core fiber is used (i.e., cladding is doped to exhibit a refractive index less than silica, a non-negligible fraction of the total poWer Will to leak to the outer cladding. The fiber Would thus have relatively high loss, even if the outer cladding has a loW absorption loss, comparable to that of the deposited cladding material. This type of loss is referred to as a “leaky mode” loss, since the radiation propagating in the outer cladding is unguided and Will “leak” aWay. Leaky mode loss can be avoided by depositing a significantly thick cladding layer.
Therefore, for a pure silica core fiber (such as fabricated by MCVD), the depressed cladding Which provides the index difference necessary for a Waveguide must be large enough to contain the single mode, While not alloWing the energy to leak from the fiber and drastically increase attenuation at the specified Wavelength. Furthermore, the perform must be designed to have a cutoff Wavelength that is relatively close to the operating Wavelength to adequately contain the mode. Further, the depressed cladding material
should have a thickness sufficient to contain the operating Wavelength mode Without suffering from huge bending loss.
The present invention addressed the need remaining in the prior art and relates to a single mode fiber for long Wavelength (e.g., 7»=1550 nm) applications and, more particularly, to a single mode fiber comprising a pure silica sore and a relatively thick cladding such that the ratio of the diameter of the cladding (defined is as “D”) to the diameter of the core (defined as “d”) greater than is approximately 8.5. By “approximately”, it is to be understood that the value may be someWhat less than 8.5 (for example, approaching 8) or someWhat greater than 8 (for example, 9 or 10). An upper bound is not critical as long as the desired single mode propagation at relatively long Wavelengths is achieved.
In accordance With the present invention, the core, is formed from pure silica, With a relatively thick cladding comprising fluorine-doped silica. The addition of the fluorine species serves to reduce the effective refractive index of the cladding material With respect to the pure silica core material. Using conventional MCVD processes, approximately 15-90 layers of fluorine-doped silica are deposited Within a glass perform tube, With the core material thereafter deposited over the deposited layers of fluorine-doped silica.
Advantageously, by forming a pure silica core fiber With such a large D/d ratio, the fiber Will be radiation resistant—a necessary feature for some applications, at standard telecommunication operating Wavelengths (usually less than 1700 nm). The fiber has also been shoWn to be hydrogen resistant (i.e., performs Well in a hydrogen environment), and therefore, exhibits improved resistance to the hydrogeninduced loss typically seen in harsh environments (“doWnhole” fibers, for example).
Other and further advantages and features of the present invention Will become apparent during the course of the folloWing discussion and by reference to the accompanying draWings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring noW to the draWings,
FIG. 1 illustrates a cross-sectional vieW and associated refractive index profile for a single mode, long Wavelength fiber formed in accordance With the present invention; and
FIGS. 2-5 illustrate an exemplary process for forming the single mode, long Wavelength fiber of the present invention.
FIG. 1 contains a cross-sectional vieW (FIG. 1(a)) and associated refractive index profile (FIG. 1(b)) of a long Wavelength, single mode fiber 10 formed in accordance With the present invention. The fiber comprises a relatively small diameter pure silica core region 12, Where the diameter of core region 12 is referred to as “d” in the illustrations. A relatively thick cladding layer 14 surrounds core region 12, Where the diameter of cladding layer 14 is defined as “D” in the illustrations. In accordance With the present invention, cladding layer 14 is doped With fluorine, Which functions to loWer the effective refractive index of the material, ensuring that most of the propagating signal Will remain in the core region. A“tube” layer 16 is shoWn as surrounding cladding layer 14, Where tube layer 16 may also comprise pure silica. FIG. 1(b) illustrates the refractive index profile for fiber 10, Where the difference betWeen the refractive index of the core