Claims:CLAIMS
We Claim:

1. An optical fiber (100, 200, 300, 400) comprising:
one or more cores (102); and
a clad enveloping the one or more cores (102), wherein the clad comprising at least two clad layers (104, 106), wherein a first clad layer (104) of the at least two clad layers (104, 106) is made of silica with less than 0.1% metallic impurity, and a second clad layer (106) of the at least two clad layers (104, 106) is made of silica with greater than 0.1% metallic impurity.

2. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the core (102) is either based of pure silica or of germanium doped silica and having OH content less than 1 ppm.
3. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the first clad layer (104) and second clad (106) has OH content of less than 5 ppm.
4. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the first clad layer has a radius in a range of 10-20 microns.
5. The optical fiber (100, 200, 300, 400) as claimed in claim 1 further comprising a buffer clad layer (202, 302, 402) between the first clad layer (104) and the second clad layer (106).
6. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the first clad layer (104) has less than 800 ppm OH content, less than 10 ppm aluminium and less than 2 ppm sodium.
7. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the second clad layer (106) has less than 50 ppm OH content, more than 10 ppm aluminium and more than 2 ppm sodium.
8. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the at least two clad layers comprise the first clad layer (104) surrounding the one or more cores (102) and the second clad layer (106) surrounding the first clad layer (104), wherein the first clad layer (104) and the second clad layer (106) are defined by a first purity level of silica and a second purity level of silica, wherein the first purity level is higher than the second purity level.
9. The optical fiber (100, 200, 300, 400) as claimed in claim 1, wherein the first clad layer (104) is made of type 3 silica and the second clad layer (106) is made of type 1 silica or type 2 silica.
, Description:TECHNICAL FIELD
[0001] The present disclosure relates to the field of optical fibers, and more particularly, relates to multi-clad optical fibers.

BACKGROUND
[0002] Optical fiber cables have secured an important position in building optical network of modern communication systems across the globe. An optical fiber cable consists of one or more optical fibers. The one or more optical fibers use light to transfer information from one end of the one or more optical fibers to other. The one or more optical fibers are made by optical fiber preforms which are manufactured by depositing layers of specially formulated silicon dioxide using chemical vapor deposition (CVD) processes, such as outside vapor deposition (OVD) and vapor axial deposition (VAD) processes. Such processes often utilize only a portion of a starting raw material due to limitations in deposition efficiency of the deposition processes. A lower deposition efficiency for the deposition processes leads to wastage of generated silica soot. Thus, resulting in significant increase in raw material cost. Some of the prior art references are given below:
[0003] US5076825A teaches about preform having three layer construction, wherein a support layer made of a silica glass having a drawable temperature of at least 1800° C. In order to eliminate the problem that the cores of some adjacent optical fibers come into direct contact with each other i.e. to avoid sticking of cores of optical fibre.
[0004] US5090979A discloses a manufacturing process for an optical fibre preform whose cladding layer is doped in order to prevent loss of useful light from the optical fibre without reducing range of light wavelength over which the fibre provides effective transmission.
[0005] US20030026584A1 discloses an optical fibre having cladding made of nano particles in order to bear mechanical stresses and is hydrophobic in nature.
[0006] While the prior arts cover various solutions for the aforesaid drawbacks related to material wastage and high production cost, however, there still remains a need of material that can be used for producing the optical fiber preforms and incurs low cost without reducing function, performance and quality of the optical fiber preforms. In light of the above-stated discussion, there is a need to overcome the above stated disadvantages.

OBJECT OF THE DISCLOSURE
[0007] A primary object of the present disclosure is to provide a multi-clad optical fiber to address the aforesaid drawbacks.
[0008] Another object of the present disclosure is to manufacture optical fiber preforms using a material that incurs low cost without reducing function, performance and quality of the optical fiber preforms, wherein the material is a combination of type 1, type 2, type 3 and type 4 silica forming optical fiber preform claddings.

SUMMARY
[0009] Accordingly, the present disclosure provides an optical fiber. The optical fiber comprises one or more cores and a clad enveloping the one or more cores. The clad comprises at least two clad layers, wherein a first clad layer of the at least two clad layers is made of silica with less than 0.1% metallic impurity, and a second clad layer of the at least two clad layers is made of silica with greater than 0.1% metallic impurity. The first clad layer and the second clad layer are defined by a first purity level of silica and a second purity level of silica, wherein the first purity level is higher than the second purity level. The first clad layer surrounds the one or more cores and the second clad layer surrounds the first clad layer. The first clad layer is made of type 3 silica and the second clad layer is made of type 1 silica or type 2 silica. The first clad layer has less than 800 ppm OH content, less than 10 ppm aluminium and less than 2 ppm sodium and the second clad layer has less than 50 ppm OH content, more than 10 ppm aluminium and more than 2 ppm sodium.
[0010] The optical fiber further comprises a buffer clad layer between the first clad layer and the second clad layer.
[0011] These and other aspects herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawing. It should be understood, however, that the following descriptions are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention herein without departing from the spirit thereof.

BRIEF DESCRIPTION OF FIGURES
[0012] The invention is illustrated in the accompanying drawing, throughout which like reference letters indicate corresponding parts in the figure. The invention herein will be better understood from the following description with reference to the drawing, in which:
[0013] FIG. 1 to FIG. 4 illustrate various multi-clad optical fibers.

DETAILED DESCRIPTION
[0014] In the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be obvious to a person skilled in the art that the invention may be practiced with or without these specific details. In other instances, well known methods, procedures and components have not been described in details so as not to unnecessarily obscure aspects of the invention.
[0015] Furthermore, it will be clear that the invention is not limited to these alternatives only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without parting from the scope of the invention.
[0016] The accompanying drawing is used to help easily understand various technical features and it should be understood that the alternatives presented herein are not limited by the accompanying drawing. As such, the present disclosure should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawing. Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another.
[0017] The drawbacks mentioned earlier (in the background section) are overcome by the proposed disclosure that provides multi-clad optical fibers manufactured by optical fiber preforms. The preforms are formed using a material that incurs low cost without reducing function, performance and quality of the optical fiber preforms, wherein the material is a combination of type 1, type 2, type 3 and type 4 silica forming optical fiber preform claddings.
[0018] Referring now to the drawings, and more particularly to FIG. 1 through FIG. 4 that illustrate various multi-clad optical fibers (interchangeably “optical fibers”) 100, 200, 300, 400.
[0019] Generally, an optical fiber refers to a medium associated with transmission of information over long distances in the form of light pulses. The optical fiber uses light to transmit voice and data communications over long distances when encapsulated in a jacket/sheath. The optical fiber may be of ITU.T G.657.A2 category. Alternatively, the optical fiber may be of ITU.T G.657.A1 or G.657.B3 or G.652.D or a multi-core or other suitable category. The ITU.T, stands for International Telecommunication Union-Telecommunication Standardization Sector, is one of the three sectors of the ITU. The ITU is the United Nations specialized agency in the field of telecommunications and is responsible for studying technical, operating and tariff questions and issuing recommendations on them with a view to standardizing telecommunications on a worldwide basis. The optical fiber may be a bend insensitive fiber that has less degradation in optical properties or less increment in optical attenuation during multiple winding/unwinding operations of an optical fiber cable.
[0020] Referring back to FIG. 1 through FIG. 4, the optical fiber 100, 200, 300, 400 may comprise one or more cores 102 and a clad enveloping the one or more cores 102.
[0021] The one or more cores 102 are a light-carrying portion of the optical fiber 100, 200, 300, 400 using total internal reflection. In Fig. 1, Fig. 2 ad Fig. 3 there is a single core 102 having plurality of clad represented by 104, 106, 108 and so on surrounding the core 102. In Fig. 4 there are plurality of cores represented by 102 having plurality of clad 104 and 106 surrounding the core 102.The clad may comprise at least two clad layers 104, 106, 108 i.e., a first clad layer 104, a second clad layer 106 and a third clad layer 108. The first clad layer 104 may surround the one or more cores 102, the second clad layer 106 may surround the first clad layer 104 and the third clad layer 108 may surround the second clad layer 106. It may be noted that the optical fiber 100, 200, 300, 400 may comprise more than three clad layers or less than three clad layers.
[0022] The first clad layer 104 of the at least two clad layers may be made of silica with less than 0.1% metallic impurity and the second clad layer 106 of the at least two clad layers may be made of silica with greater than 0.1% metallic impurity. Particularly, the first clad layer 104 may be made of type 3 or type 4 silica, the second clad layer 106 may be made of type 1 silica or type 2 silica, the third clad layer 108 may be composed of type 1 silica or type 2 silica or type 3 silica or any combination of type 1 silica, type 2 silica, type 3 and type 4 silica. The type 1 silica or silica glasses may be produced from natural quartz by electric fusion under vacuum or under an inert gas atmosphere. The type 2 silica or silica glasses may be produced from quartz crystal powder by flame fusion (Verneuil process). The type 3 silica or silica glasses may be synthetic vitreous silicas produced by hydrolyzation of SiCl4 (Silicon tetrachloride) when spraying into an oxygen–hydrogen flame. The type 3 silica or silica glass may be synthetic clear silica glass made by pyrolysis of SiCl4 vapours blown into a plasma burner flame in the presence of oxygen. The core is either based of pure silica or of germanium doped silica wherein the content of OH has to be maintained at less than 1 ppm. The first clad layer which surrounds the core and the second clad layer which surrounds the first clad layer has OH content below 5ppm. The first clad layer is of 10-20 microns in radius.
[0023] Advantageously, due to combination of silica clad layers and a reduced coating layer in the optical fiber 100, 200, 300, 400, wastage of soot is significantly reduced, thereby significant reduction in production cost of the optical fiber 100, 200, 300 400.
[0024] The first clad layer 104 and the second clad layer 106 may be defined by a first purity level of silica and a second purity level of silica respectively, wherein the first purity level is higher than the second purity level. The first clad layer 104 may have less than 800 ppm OH content, less than 10 ppm aluminium and less than 5 ppm sodium. The second clad layer 106 may have less than 50 ppm OH content, more than 1ppm aluminium and more than 2 ppm sodium. Alternatively, the amount of OH content, aluminium and sodium in the first clad layer 104 and the second clad layer 106 may vary. Alternatively, other metallic impurities except aluminium and sodium may be used.
[0025] Due to the presence of OH content or group, the optical fiber 100, 200, 300, 400 remains dry so as to prevent signal deterioration as presence of moisture may cause attenuation or other adverse phenomena related to signal loss.
[0026] Electron probe micro-analyser (EPMA) and laser ablated ICP and Atomic absorption spectroscopy and Fourier transform infrared spectroscopy (FTIR) are techniques for the elemental analysis and quantification of the impurities present in the glass used for making optical fibre and thereby differentiating the different types of silica. This further helps in determining type of silica to be used in an optical fibre as either a combination or individual for inner and outer clad.
[0027] The optical fiber may further comprise a buffer clad layer 202, 302, 402 between the first clad layer 104 and the second clad layer 106 as shown in FIG. 2, FIG. 3 and FIG. 4. The buffer clad layer 202, 302, 402 may be composed of pure silica.
[0028] Conditional language used herein, such as, among others, "can," "may," "might," "may," “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain alternatives include, while other alternatives do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more alternatives or that one or more alternatives necessarily include logic for deciding, with or without other input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular alternative. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list.
[0029] Disjunctive language such as the phrase “at least one of X, Y, Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain alternatives require at least one of X, at least one of Y, or at least one of Z to each be present.
[0030] While the detailed description has shown, described, and pointed out novel features as applied to various alternatives, it can be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the scope of the disclosure. As can be recognized, certain alternatives described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others.