Optical fiber and method for making such fiber

0001Parts of this invention were made with Government support under Agreement No. MDA972-02-3-004 awarded by DARPA. The Government may have certain rights in some of the claims of the invention.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates generally to double clad rare earth doped optical fibers, and particularly to all glass rare earth doped optical fibers suitable for use with high power light sources or in optical fiber lasers and optical amplifiers.

00042. Technical Background

0005Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Single clad rare earth doped optical fiber has been widely used in the field of optical amplifiers and fiber lasers. This type of fiber has low capability of handling high power multimode optical sources due to the difficulty of efficiently coupling multimode light from a high power optical (light) source (also referred to herein as optical pump or pump) into the rare-earth doped fiber core.

0006To solve this problem and to increase the output power of fiber lasers, those of skill in the art utilize optical fiber with a double clad structure (referred herein as double clad optical fiber). Double clad rare-earth doped optical fiber is a fiber that has a core, an inner cladding layer surrounding the core and an outer cladding layer surrounding the inner cladding layer. Optical fibers with Yb doped cores and two cladding layers surrounding the core are disclosed, for example, in U.S. Pat. Nos. 6,477,307; 6,483,973; 5,966,491 and 5,949,941.

0007Double clad optical fiber has been used in applications requiring utilization of optical sources providing between 10 to 100 Watts of optical power, because double clad optical fiber is more efficient in retaining/utilizing optical power provided by the pump than single clad optical fiber. This higher efficiency is due to fiber's utilization of clad-to-core coupling of optical pump power. More specifically, rare-earth doped double clad optical fibers accept light from the optical pump into the inner cladding and then transfer light to the rare-earth doped core through the core-to-inner cladding interface, along the length of the optical fiber. Thus, the optical fiber converts a significant part of the multi-mode light propagated through the inner cladding into a single-mode output at a longer wavelength, by coupling this pump light into the rare-earth doped core.

0008The inner cladding of the double clad optical fiber has a higher index of refraction than the outer cladding, thus the pump energy is confined inside the inner cladding and is re-directed into the core. The optical fiber is optically active due to the presence of rare-earth dopant in the core, which can be excited to higher electronic energy levels when the optical fiber is pumped by a strong optical pump. Cladding pumping can be utilized in fiber amplifiers, or employed to build high-power single mode fiber pump lasers.

0009The single-stripe broad-area diode laser remains the most efficient and least expensive pump source. Recent progress in semiconductor laser technology has led to creation of a single-stripe multi mode broad-area laser diodes with output powers of more than 10 Watts.

0010Recent progress in semiconductor laser technology has led to the creation of light sources utilizing either single stripe broad-area laser diodes or laser diode bars, directly coupled to the intermediate fiber incorporated within the light source. The maximum output power of these light sources is more than 150 Watt at a wavelength of 976 nm at the output end of the intermediate fiber. The intermediate fiber diameter and numerical aperture NA of the light source is 200 μm and 0.22, respectively.

0011In a double-clad laser, an outer cladding of the optical fiber confines the pump light provided by an optical pump in the optical fiber's multi-mode inner cladding. The much smaller cross-sectional area of the optical fiber's core is typically doped with at least one rare-earth element, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium- or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030 nm-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multi-mode first cladding for accepting and transferring pump energy to a core along the length of the device. Double-clad laser output can also be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium.

0012How much pump light can be coupled into a double-clad fiber's inner cladding depends on the cladding size and numerical aperture NA. As is known, the “etendue” (numerical aperture multiplied by the aperture dimension or spot size) of the inner cladding should be equal to or greater than the etendue of the optical pump for efficient coupling. If the numerical aperture and spot size of the optical source (optical pump are) be different in both axes, in order to have better coupling efficiency, the etendue of the inner cladding should be maintained or exceed that of the pump in both the x and y directions.

0013Typically, a high numerical aperture NA of the inner cladding, which is related to the difference in refractive index between the inner and outer cladding, is desired. In the well-known design, the first clad layer (inner cladding) is made of glass and the second layer (outer cladding) is made of plastic (for example, fluorinated polymer) with relatively low refractive index in order to increase the numerical aperture NA of the inner cladding. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage. In addition, this type of double clad optical fiber may be suitable only for sustained use with relatively low power (lower than 20 Watts) optical sources. When high power sources (more than 100 Watts) are utilized, this type of optical fiber heats and the polymer material of the outer cladding layer carbonizes or burns, resulting in device failure, especially when the fiber is bent. At medium powers (20 Watts to below 100 Watts), the polymer outer cladding ages relatively quickly, losing its mechanical and optical characteristics and becoming brittle, thus shortening the device life.

0014All-glass, Yb doped optical fibers are also known. An example of such fiber is disclosed in U.S. Pat. No. 6,411,762. The disclosed fiber, however, is not suitable for high power applications because it has a relatively low outer cladding diameter and NA, and therefore, low coupling efficiency due to light leakage outside of the optical fiber. That is, a relatively large portion of the light does not enter the optical fiber and is lost. Although this may not be an issue in applications when only a small amount of optical power needs to be coupled into the fiber, such fiber is not efficient for high power applications when the light source power is 100 Watts or more.

SUMMARY OF THE INVENTION

0015The scope of the present invention is determined by the appended claims.

0016According to one example of the invention an optical fiber comprises: (i) silica based, rare earth doped core having a first index of refraction n₁; (ii) at least one silica based cladding surrounding the core and having a second index of refraction n₂, such that n₁>n₂; wherein at least one of the core or cladding is doped with Al₂O₃, such that the ratio of max wt % to min wt % of Al₂O₃concentration is less than 2:1.

0017According to one embodiment of the present invention, the method of making an optical fiber comprises the steps of: (i) delivering desired vapor ingredients to a flame provided by a burner, wherein at least one of the vapor ingredient includes Al and the ingredient is delivered to the flame by heated helium gas; and (ii) making a glass preform by depositing the products resulting from the vapor ingredients reacting with oxygen in a flame, to form the soot-particles, wherein at least one of the products is Al₂O₃.

0018According to one embodiment of the present invention the optical fiber comprises:

0019(i) a silica based, rare earth doped core having a first index of refraction n₁;

0020(ii) at least one silica based layer surrounding the core and having a second index of refraction n₂, such that n₁>n₂; wherein the silica based layer includes in weight percent: F, 0.5 wt % to 5 wt %; and B, 0.5 wt % to 20 wt %.

0021According to one embodiment of the present invention the optical fiber comprises: (i) a silica based, rare earth doped core having a first index of refraction n₁; (ii) at least one silica based layer surrounding the core and having a second index of refraction n₂, such that n₁>n₂; wherein the silica based layer includes in weight percent: Boron—at least 8 wt %, and Fluorine—at least 2 wt %.

0022Using the fiber design and the techniques described herein the optical fiber had been made which exhibits less than 8 dB/km core background loss at a wavelength of 1280 nm.

0023According to one embodiment of the present invention a method of making an optical fiber comprises the steps of: (i) delivering desired vapor ingredients to a flame provided by a burner; (ii) making a glass preform by depositing products resulting from the vapor ingredients reacting with oxygen in a flame, to form the soot-particles; (iii) consolidating the preform in a furnace at consolidation temperatures between 1500° C. to 1600° C.; and (iv) during the consolidation step moving the preform with respect to the furnace at the rate of at least 7 mm/min.

0024Some of the advantages the optical fibers disclosed herein are: high coupling efficiency, suitability for high optical power applications (100 Watts or higher), and suitability for use as polarization maintaining fiber and long deployment life.

0025Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

0026It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

0027<figref idref="DRAWINGS">FIG. 1</figref> is a schematic cross-sectional view of one embodiment of the present invention;

0028<figref idref="DRAWINGS">FIGS. 2A-2L</figref> are a schematic cross-sectional view of other embodiments of the present invention;

0029<figref idref="DRAWINGS">FIG. 3</figref> is a schematic cross-sectional view of yet another embodiment of the present invention;

0030<figref idref="DRAWINGS">FIG. 4</figref> is a refractive index profile of a first example of optical fiber in accordance with the present invention;

0031<figref idref="DRAWINGS">FIG. 5</figref> is a schematic illustration of AlCl₃delivery mechanism;

0032<figref idref="DRAWINGS">FIG. 6</figref> illustrates Al₂O₃concentration in a preform which resulted from Argon gas delivery (bottom curve) and heated Helium gas delivery (top curve);

0033<figref idref="DRAWINGS">FIG. 7</figref> is a graph illustrating Yb₂O₃and Al₂O₃concentration within a core optical fiber preform;

0034<figref idref="DRAWINGS">FIG. 8</figref> is a schematic illustration of the formation of a core soot preform;

0035<figref idref="DRAWINGS">FIG. 9</figref> illustrates consolidation of a soot preform into a glass preform;

0036<figref idref="DRAWINGS">FIG. 10</figref> illustrates inner cladding background loss of an exemplary fiber;

0037<figref idref="DRAWINGS">FIG. 11</figref> is a graph illustrating passive core loss vs. wavelength of the optical fiber of <figref idref="DRAWINGS">FIG. 4</figref>;

0038<figref idref="DRAWINGS">FIG. 12</figref> is a graph of output power vs. launched power for the optical fiber of <figref idref="DRAWINGS">FIG. 4</figref>;

0039<figref idref="DRAWINGS">FIG. 13</figref> is a refractive index profile of an alternative example of the optical fiber of the present invention;

0040<figref idref="DRAWINGS">FIG. 14</figref> is a graph of output power vs. launched power for the optical fiber of <figref idref="DRAWINGS">FIG. 13</figref>;

0041<figref idref="DRAWINGS">FIG. 15</figref> is a graph illustrating passive core loss vs. wavelength of the optical fiber of <figref idref="DRAWINGS">FIG. 13</figref>;

0042<figref idref="DRAWINGS">FIG. 16</figref> is a refractive index profile of another alternative example of the optical fiber in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0043Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of double clad optical fiber in accordance with the present invention is shown schematically in <figref idref="DRAWINGS">FIG. 1</figref>, and is designated generally throughout by the reference numeral <b>10</b>. The optical fiber <b>10</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref> includes: a silica based, rare earth doped core <b>12</b> having a first index of refraction n₁; a first silica based cladding <b>14</b> surrounding the core and having a second index of refraction n₂, such that n₁>n₂; a silica based outer cladding <b>16</b> surrounding the first cladding and having a third index of refraction n₃. The core <b>12</b>, inner cladding <b>14</b> and the outer cladding <b>16</b> are made of glass. A protective coating <b>18</b> surrounds the outer cladding <b>16</b>. The outer coating <b>18</b> may be, for example, an organic coating which typically includes a softer primary coating and a harder secondary coating applied over the primary coating.

0044In this embodiment the silica based core <b>12</b> is doped with Yb, but other rare earth materials, such as Er may also be utilized. The core <b>12</b> may also include at least one index raising dopant. The outer cladding further <b>16</b> preferably includes an index lowering dopant, such that n₂>n₃The inner cladding diameter D_INis at least 125 μm and preferably at least 200 μm. It is even more preferable that inner cladding diameter DIN is at least 225 μm and most preferable at least 250 μm. Applicants discovered that the thick inner cladding <b>14</b> and all-glass construction of the optical fiber work in synergy to allow the optical fiber to be coupled to high energy source, and to couple the high power into the core without damaging the optical fiber. Thus, such fiber is especially suitable for high power applications.

0045It is preferable that the outer cladding <b>16</b> be relatively thin, with wall thickness less than 80 μm and preferably between about 5 μm and 35 μm. It is most preferable that the wall thickness of the outer cladding <b>16</b> be between about 10 μm to 25 μm. It is preferable that the diameter D_Cof the fiber core <b>12</b> be about 5 μm to 20 μm, the inner cladding diameter D_INbe about 125 μm to 2000 μm and more preferably about 125 μm to 1500 μm. It is even more preferable that D_INbe about 125 μm to 350 μm. It is preferable that the diameter of the outer cladding diameter (D_OUT) be about 145 to 2100 μm, more preferably between about 145 μm to 1600 μm and even more preferable that D_OUTbe about 145 μm to 500 μm. If the inner cladding <b>14</b> does not have a circular cross section, Din is defined as the smallest distance from one side of the inner cladding's crossection to the oppositely situated side of the crossection. It is also noted that the outer cladding <b>16</b> may not be circular. If the outer cladding <b>16</b> is not circular, D_OUTis defined as the smallest distance from one side of the outer cladding's crossection to the oppositely situated side of the outer cladding's crossection.

0046It is preferable that the inner cladding's <b>14</b> cross-sectional area be at least 200 times larger than the cross sectional area of the core <b>12</b>. It is even more preferable that the cross sectional area of the inner cladding <b>14</b> be between 300 and 3000 times larger than the cross sectional area of the core <b>12</b>. For example, the cross sectional area of the inner cladding <b>16</b> may be 500, 700, 1000, 1200, 1500, 1600, 2000 or 2500 times larger than the cross sectional area of the core <b>12</b>.

0047According to this embodiment, the fiber core <b>12</b> includes, in weight percent:

0048<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="42pt" align="left" /><colspec colname="1" colwidth="35pt" align="center" /><colspec colname="2" colwidth="77pt" align="right" /><colspec colname="3" colwidth="63pt" align="left" /><thead><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>Rare earth</entry><entry>0.1 to 2.5 </entry><entry>wt %;</entry></row><row><entry /><entry>P</entry><entry>0 to 5 </entry><entry>wt %;</entry></row><row><entry /><entry>Al</entry><entry>0.5 to 15 </entry><entry>wt %;</entry></row><row><entry /><entry>Ge</entry><entry>0 to 15 </entry><entry>wt %;</entry></row><row><entry /><entry>F</entry><entry>0 to 1 </entry><entry>wt %.</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0049The rare earth dopants in the fiber core <b>12</b> provide active ions to enable either a gain or a lasing action. Exemplary rare earth dopants are Yb, Er, Nd, Tm, Sm and Tb. It is preferable that the amount of rare earth dopant in the core <b>12</b> be 0.5 wt % to 1.5 wt %. Phosphorus may be added to the core materials in order to lower the softening temperature of the core glass, which may be advantageous if the core is produced by the inside vapor deposition process (described below). Phosphorus may also be utilized as a refractive index raising agent. However too much phosphorus (10% or more) provides nonlinearity through Stimulated Raman Scattering which may inhibit the lasing action. Aluminum may be added to the core as a de-clustering agent (for example, to de-cluster Yb, preferably at the ratio of Al to Yb of 5:1 to 10:1). The core <b>12</b> may also include Germanium which is an index raising dopant, and/or fluorine which is an index lowering dopant as well as a de-clustering agent.

0050The preferred ranges of the core <b>12</b> composition in weight percent are:

0051<tables id="TABLE-US-00002" num="00002"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="42pt" align="left" /><colspec colname="1" colwidth="35pt" align="center" /><colspec colname="2" colwidth="70pt" align="right" /><colspec colname="3" colwidth="70pt" align="left" /><thead><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>Rare earth</entry><entry>0.3 to 1 </entry><entry>wt %;</entry></row><row><entry /><entry>P</entry><entry>0 to 2 </entry><entry>wt %;</entry></row><row><entry /><entry>Al</entry><entry>2 to 8 </entry><entry>wt %;</entry></row><row><entry /><entry>Ge</entry><entry>3 to 15 </entry><entry>wt %; and</entry></row><row><entry /><entry>F</entry><entry>0.1 to 0.5 </entry><entry>wt %.</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0052It is preferable that the inner cladding <b>14</b> contain 5 wt % to 30 wt % Ge to in order to provide high NA. It is even more preferable that the inner cladding comprise 5 wt % to 20 wt % Ge. It is noted that 5 wt % to 10 wt % Ge works well for many applications.

0053It is preferable that the index lowering dopant of the outer cladding <b>16</b> comprises Fluorine and/or Boron in weight percent:

0054<tables id="TABLE-US-00003" num="00003"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="56pt" align="left" /><colspec colname="1" colwidth="14pt" align="center" /><colspec colname="2" colwidth="77pt" align="right" /><colspec colname="3" colwidth="70pt" align="left" /><thead><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>F</entry><entry>0 to 5 </entry><entry>wt %;</entry></row><row><entry /><entry>B</entry><entry>0 to 20 </entry><entry>wt %.</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables><br /> The amount of dopant(s) for the outer cladding <b>16</b> is chosen to preferably result in inner cladding NA of between 0.15 to 0.5. However, it is preferable that the outer cladding <b>16</b> contain at least one of B or/and F. It is preferable that the minimum amount of B and/or F is at least 0.5 wt %. It is preferable that the amount of B is at least 3 wt %. It is preferable to have more than 1 wt % and more preferably more than 2 wt % of F along with more than 8 wt % of B in the outer cladding <b>16</b>. It is preferable that the outer cladding <b>16</b> has less than 5 wt % of F, and less than 15 wt % of B. It is even more preferable that the amount of B and F be: 2 to 4 wt % of B, 3 to 10 wt % of F. <br /> Other embodiments of the double clad optical fiber of the present invention are shown schematically in <figref idref="DRAWINGS">FIGS. 2A-2L</figref> and are generally described and depicted herein with reference to several exemplary or representative embodiments with the same numbers referenced to the same or functionally similar parts. The optical fiber cross sections depicted in <figref idref="DRAWINGS">FIGS. 2A-2L</figref> are similar to the optical fiber cross section depicted in <figref idref="DRAWINGS">FIG. 1</figref>, but have non-circular inner claddings <b>14</b>. The advantage of non-circular inner cladding <b>14</b> is that non-circular shape improves the absorption of optical pump power into the core <b>12</b>. The core <b>12</b> may be located either at the geometric center of the inner cladding, or may be displaced from the geometric center of the inner cladding. The inner cladding shape may be convex as shown in <figref idref="DRAWINGS">FIGS. 2G and 2I</figref> or concave, as shown in <figref idref="DRAWINGS">FIGS. 2K and 2L</figref>.

0055The optical fiber core <b>12</b> is either circular, or elliptical, as shown in <figref idref="DRAWINGS">FIG. 3</figref>. An optical fiber with a circular core <b>12</b> is illustrated in <figref idref="DRAWINGS">FIG. 1</figref>. An elliptical core <b>12</b> may be preferred because it renders polarization maintaining properties to the optical fiber. It is preferred that the aspect ratio of the elliptical core <b>12</b> be at least 1.5:1 and more preferably be between 2:1 and 5:1, because these aspect ratios improve birefringence of the core <b>12</b>. An optical fiber with an elliptical core having a 2:1 ratio (ratio of major to minor axis) is depicted in <figref idref="DRAWINGS">FIG. 3</figref>. Thus, the optical fiber is polarization maintaining (PM) fiber if the rare earth doped core has an elliptical shape. A Yb-doped core will laze at 1.03-1.11 micron range.

0056In order to have polarization maintaining single mode fiber, the core <b>12</b> may be elliptical and should have an aspect ratio of at least 1.5 to 1. The numerical aperture NA of the core <b>12</b> is between 0.05 (for high power laser application) and 0.25 (for lower power application). The numerical aperture NA of the core <b>12</b> is defined as (N1²−N2²)^1/2. If the core <b>12</b> is not circular, it is preferable that the aspect ratio of the core be between 3:1 and 10:1.

0057The silica based inner cladding <b>14</b> may have a circular outer perimeter, as shown in <figref idref="DRAWINGS">FIG. 1</figref> (preferably with an off-center situated core), or a non-circular outer perimeter as shown in <figref idref="DRAWINGS">FIG. 2</figref>. The numerical aperture NA of the pumped inner cladding is defined as (N2²−N3²)^1/2. The inner cladding preferably has numerical aperture NA between 0.15 and 0.45 and more preferably between 0.3 and 0.4.

0058In general, a double-clad structure that could be used in a fiber laser or in an amplifier includes two claddings. A first (inner) multi-mode cladding acts as a multi-mode pumping core. The inner cladding is adjacent to the core and a second (outer) cladding surrounds the first cladding. The core <b>12</b> may be either single mode or multi mode at the core lasing wavelength. The inner cladding <b>14</b> serves as a waveguide with a high numerical aperture NA for the input (pumping) light. The larger the inner cladding diameter, the more pump light is coupled into the inner cladding from the optical source. The cross-section of the first multi-mode inner cladding (D_INis the shorter dimension of the inner cladding as seen in <figref idref="DRAWINGS">FIGS. 2A-2L</figref> may be designed to have a desired shape, e.g., matched to the near field shape of the pump source or have any other which increases coupling efficiency of the (pump) light from the light source to the inner cladding. The numerical aperture of the inner cladding must be high enough to capture the output of the light source, such as the laser diode.

0059Recent progress in semiconductor laser technology has led to the creation of light sources utilizing discrete or arrayed broad-area laser diodes coupled to the intermediate fiber incorporated within the light source. The output power of this light source is more than 150 Watt at 976 nm at the output end of the intermediate fiber. The diameter of the intermediate fiber and NA of light source is 200 μm and 0.22 NA, respectively.

0060The light from this light source is then coupled to a double clad optical fiber via high NA and large aperture lenses. With this approach one can obtain 85-90% of coupling efficiency.

EXAMPLES

0061The invention will be further clarified by the following examples.

Example 1

0062<figref idref="DRAWINGS">FIG. 4</figref> illustrates a refractive index profile of a first exemplary optical fiber of the present invention. This optical fiber has the cross-section illustrated in <figref idref="DRAWINGS">FIG. 2I</figref>. The distance D_INbetween two opposing flat sides of this inner cladding crossection is 260 μm. <figref idref="DRAWINGS">FIG. 4</figref> depicts this optical fiber's refractive index percent delta (relative to that of the pure silica) vs. the distance measured from the core center. The refractive index percent delta is defined herein as (n₁²−n_s²)/2n_i², where i=1, 2 or 3 and ns is the refractive index of pure silica. This optical fiber has a Yb doped core <b>12</b>, a Ge-silica inner cladding (% delta ≈0.46) and an outer cladding <b>16</b> which doped with Fluorine and Boron. <figref idref="DRAWINGS">FIG. 4</figref> shows that the relative refractive index difference (percent delta) of the core <b>12</b> is about 0.56, that the fluorine/boron doped outer cladding <b>16</b> has the refractive index percent delta of about −1.4. The Yb-doped fiber core is single-mode for the wavelengths above 1 μm. If the core <b>12</b> is doped with Erbium, the optical fiber will be single-mode at lasing wavelength of 1.55 μm. The optical fiber <b>10</b> has a relatively low NA (about 0.065) for the core <b>12</b>, and high NA (0.30) for the inner cladding <b>14</b>. (The NA is defined by (n_i²−n_i+1²)^1/2.) This inner cladding NA is preferably higher than that of the pump-source, allowing high coupling efficiency for the pump light of 90% or better. The small core NA (0.065) enables single mode operation with a large core size (10.5 microns diameter). If the core NA is higher (0.13, for example), the core diameter would have to be smaller (about 5 microns, for example) in order to be single mode. The bigger core diameter and lower core NA allows the core <b>12</b> to stay single-mode, while allowing the core to take more pump-power from the inner cladding, and also increases fiber power handling capability. The specific composition for this exemplary optical fiber is: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0063">Core <b>12</b>: 0.6 wt % Yb₂O₃; 4.5 wt % AL₂O₃; 3.0 wt % GeO₂; 0.2 wt % F;</li><li id="ul0002-0002" num="0064">Inner cladding <b>14</b>: 8.5 wt % GeO₂;</li><li id="ul0002-0003" num="0065">Outer cladding <b>16</b>: 9 wt % B and 2.7 wt % F.</li></ul></li></ul>

0066The fiber of <figref idref="DRAWINGS">FIG. 4</figref> was produced by the outside-vapor-deposition process (OVD). The OVD process is a way of making optical fiber by depositing from the desired vapor ingredients (including silica and the desired dopants) reacting with oxygen in a flame to form the soot-particles on a bait rod, for making fiber soot-preform. The soot-preform is then consolidated into solid transparent glass in a high temperature furnace, after the bait rod is removed. The core/inner cladding/outer cladding compositions are achieved by utilizing different vapor-ingredients for each of the layers in the soot preform forming process. The core/inner cladding performs is generated first, then consolidated, followed by an outer cladding outside vapor deposition process and then consolidated again. The final preform is then drawn into double-clad optical fiber <b>10</b> by known fiber-drawing methods.

0067More specifically, the exemplary vapor-precursor-materials used to make the fiber of <figref idref="DRAWINGS">FIG. 4</figref> are Yb(fod)₃, AlCl₃, SiF₄, SiCl₄, GeCl₄and tri-ethyl borate. During the core deposition process we achieved a uniform AlCl₃gas-phase delivery. This was accomplished by utilizing heated inert Helium as carrier gas <b>30</b> (instead of Argon gas) for AlCl₃delivery. As solid AlCl₃changes into vapor (gas) phase, it consumes a large amount of heat. Helium gas has high thermal conductivity; effectively transfers heat to AlCl₃, and maintains constant vapor pressure of AlCl₃. It is preferable that Helium gas be provided at a temperature within 150° C. to 180° C. range. As illustrated in <figref idref="DRAWINGS">FIG. 5</figref>, the heated Helium gas is provided by the He gas heater <b>32</b> to the oven <b>50</b> containing AlCl₃vessel <b>52</b>. The relatively high Helium gas temperature helps to maintain the AlCl₃containing vessel <b>52</b> at a constant temperature of about 140° C.-160° C. In order to make the optical fiber of this example, Helium gas was heated via heater <b>32</b> to 168° C. and the vessel <b>52</b> temperature was held constant at 145° C. Higher vessel temperature results higher concentration of Al in the preform. In addition, the Helium gas flow rate was also adjusted for the most uniform delivery throughout the core doping process. In this example, a 10% flow-rate slope (liter/min) is used for the delivery. (The increase in flow rate with subsequent passes was utilized for all other dopants of the core and claddings.) Heated Helium gas carries AlCl₃vapor via a heated gas line <b>54</b> to the flame burner <b>56</b>. To produce the optical fiber of this example, a 100 passes of core deposition process is started with 1.2 liter/min (pass #1) and ended (after pass # 100) with 1.65 liter/min, resulting in soot perform core thickness of about 2 mm to 3 mm. Heated Helium based AlCl₃delivery may be utilized not only to form a fiber core, but to also provide Al doping to other fiber layers (e.g. cladding), if uniform Al doping of such layers is desired. Furthermore, heated Helium assisted delivery may be also utilized for materials other than AlCl₃, which are also endothermic (i.e. heat-absorbing). An Argon gas delivery of AlCl₃results in Al₂O₃profile in a soot preform which is illustrated in <figref idref="DRAWINGS">FIG. 6</figref> (bottom curve). A Helium gas delivery of AlCl₃would result in Al₂O₃profile in a soot preform which is illustrated in <figref idref="DRAWINGS">FIG. 6</figref> (top curve). As one can see, the uniformity of Al₂O₃concentration is much better with heated Helium delivery. It is preferable that Al₂O₃is evenly distributed throughout the core layer because its presence assists in de-clustering of rare earth dopant(s) within the core. This results-in high laser/amplifier efficiency through reduced quenching. This delivery process can also be utilized in Al doped (for example, in order to replace Ge) transmission fiber (i.e. fiber without rare-earth dopants in the core) when a fiber layer with relatively high index of refraction (i.e. higher than silica) is needed.

0068As shown in <figref idref="DRAWINGS">FIG. 7</figref>, the heated Helium delivery of AlCl₃resulted in a very uniform distribution of Yb and Al throughout the preform core, which results in uniform concentration of Yb and Al within the fiber core <b>12</b>. More specifically, the resultant variability of Al₂O₃concentration in the core of is less than 2 wt % and preferably less than 0.5 wt % and more preferably less than 0.25 wt %, especially for maximum Al₂O₃concentration of over 3 wt %. It is also preferable that the ratio of max wt % to min wt % of Al₂O₃concentration in any given fiber layer (e.g. core, cladding, etc.) be less than 2:1, preferably less than 1.5:1, more preferably less than 1.2:1, and even more preferably less than 1.1:1, especially for maximum Al₂O₃concentration of over 3 wt %.

0069The Yb vapor delivery is carried by Argon gas and is accomplished by heating organometallic Yb(fod)₃in the temperature range of 150° C.-180° C., which results in a soot preform core with Yb₂O₃concentration from about 0.2 wt % to 3 wt %. In order to make the optical fiber <b>10</b> of this example, the Yb(fod)₃containing vessel temperature of 163° C. was used to achieve the Yb₂O₃concentration of about 0.6 wt %. The delivery of other materials is carried out by conventional oxygen delivery at temperatures below 100° C.

0070More specifically, according to one embodiment of the present invention, the Yb(fod)₃, AlCl₃, SiF₄, SiCl₄and GeCl₄are delivered to a gas burner <b>56</b>. (See <figref idref="DRAWINGS">FIG. 8</figref>.) The gas burner <b>56</b> operates at a temperature of about 2000° C. The pre-determined amounts of various vapor-phase materials delivered for each core or clad stage are carried by oxygen provided to the burner <b>56</b>, and react in the burner flame <b>58</b> where the desired glass-soot particles formed. The soot particles are then deposited onto a rotating bait-rod <b>59</b> or core cane <b>60</b> through the thermopheretic mechanism to result in the designed soot-preform having the Yb-doped single-mode core and Germanium up-doped inner cladding with a high NA. After the inner cladding soot preform layer is layered down and the soot preform <b>62</b> is cooled to room temperature, the bait rod <b>59</b> is removed.

0071Applicants discovered that a proper choice of high temperatures and fast down-feed rates during consolidation results in low crystallization formation in the resulting glass preform, which results in an optical fiber having very low passive (background) loss, and also eliminates the conventional double-redraw process associated with Al doped blanks.

0072More specifically, preform <b>62</b> is down fed relative to the furnace at the rate and temperature sufficient to minimize crystallization such that the background loss of the resultant fiber core is less than 8 dB/km at a wavelength of 1280 nm. As illustrated in FIG. <b>9</b>, the ‘core-inner cladding’ soot preform <b>62</b> is consolidated into solid glass-preform <b>63</b> in a high temperature (1400° C. to 1600° C.) furnace <b>64</b>. It is preferred that the furnace temperature during consolidation be 1500° C. to 1600° C., and more preferably 1530° C. to 1580° C. In order to produce the optical fiber <b>10</b> of this example we utilized the furnace temperature of 1550° C. Applicants found that for temperatures of below 1500° C. the preform glass forms crystals and the amount of crystallization is significantly reduced with furnace temperatures of above 1530° C. While in the furnace, the soot preform <b>62</b> is moved relative to the furnace <b>64</b> (e.g., down-fed) at a rate of 7 mm/min or faster. It is preferred that this rate be 8 mm/min to 12 mm/min. The optical fiber of this example made by was down-feeding the soot preform <b>62</b> at the rate of 9 mm/min. It is noted that instead of down-feeding the soot preform, the soot preform may be held in a constant position and the furnace may be moved instead. Thus, by specifying that the soot preform is moved relative to the furnace, applicants intend to cover any relative movement between the soot preform and the furnace. Generally, it is recommended that the higher the furnace temperature, the faster the rate of relative motion between the furnace and the soot preform.

0073With the above described high consolidation temperatures and fast down-feed rate, the resultant optical fiber <b>10</b> has the core background loss of less than 8 dB/km. More preferably, the core background loss of less than 5 dB/km. In this example the background loss of the core is less than 3 dB/km. The core background loss was measured by making (single mode) optical fiber without the outer cladding and measuring the background loss of this fiber. The low inner cladding loss of this fiber (measured by optical-time-domain-reflectometer OTDR) is shown in <figref idref="DRAWINGS">FIG. 10</figref>. (The two spikes in the OTDR plot represent the reflections from the end faces of the measured fiber.)

0074If a non-circular inner cladding shape is desired, the glass preform may be machined or ground to provide desired outer perimeter shape. For example, to achieve a fiber of <figref idref="DRAWINGS">FIG. 2I</figref> the glass preform was ground to that shape.

0075SiO₂with index lowering dopants was then deposited on the ground glass preform to form the outer cladding portion of the preform. In this example, the index lowering dopants are B and F. The soot particles of B and F provide low refractive index (less than that of pure silica).

0076More specifically, B₂O₃and SiO₂were vapor deposited on the ground glass preform to form a B₂O₃and SiO₂soot layer by using tri-ethyl borate and SiCl₄delivered to the burner. The blank (i.e. machined or ground glass preform) covered with the B₂O₃-doped silica soot layer was then Fluorine doped during the consolidation step by using SiF₄gas provided to the consolidation furnace. During this second consolidation step, the consolidation furnace is operated at the temperature range of 1300° C.-1400° C. At these consolidation temperatures Fluorine diffuses into the boron/silica soot layer, but does not penetrate into the underlying glass layer. For the optical fiber of this example was produced by utilizing consolidation temperature of 1350° C., so as to facilitate adequate Fluorine doping through diffusion.

0077In this example, the third layer of the preform (outer cladding) has a shape similar to that of the second layer (inner cladding).

0078The fiber drawing was conventional. The resulting all-glass double-clad optical fiber has the following core, inner-cladding, outer-cladding compositional format: <ul id="ul0003" list-style="none"><li id="ul0003-0001" num="0000"><ul id="ul0004" list-style="none"><li id="ul0004-0001" num="0079">Core: Yb₂O₃:Al₂O₃:SiO₂:GeO₂:F;</li><li id="ul0004-0002" num="0080">Inner cladding: SiO₂:GeO₂;</li><li id="ul0004-0003" num="0081">Outer cladding: B₂O₃:F:SiO₂. <br /> The amount of each dopant is optimized to ensure the high laser efficiency. The preferred inner cladding shape is not circularly symmetric, thus maximizing the pump absorption. </li></ul></li></ul>

0082The double clad fiber produced by the OVD process is especially suitable for use in a higher power fiber laser device. <figref idref="DRAWINGS">FIGS. 11 and 12</figref> correspond to the optical fiber of <figref idref="DRAWINGS">FIG. 4</figref>. More specifically, <figref idref="DRAWINGS">FIG. 5</figref> illustrates the low passive loss, for example 3 dB/km at 1280 nm, achieved in the Yb-doped core of the fiber of <figref idref="DRAWINGS">FIG. 4</figref>. The passive loss of the core (also referred to as a background loss) is the inherent loss from the core materials without the absorption-effect from the active dopants such as Yb or Er etc. <figref idref="DRAWINGS">FIG. 12</figref> illustrates the single mode fiber-laser efficiency of this fiber. More specifically, <figref idref="DRAWINGS">FIG. 12</figref> is a graph of output power (Watts) versus input power (Watts). The optical pump wavelength is 976 nm. The optical pump is fiber coupled semiconductor laser diode bars (Ga—As/InGaAs). The output from this optical pump was launched into the inner core of the double clad optical fiber of <figref idref="DRAWINGS">FIG. 4</figref>. The fiber laser shows low threshold and high lasing efficiency of 78% (which is defined by the graph's slope). The fiber has good power-handling capability and operates well with optical sources that provide optical (pump) power of over 110 Watts. The optical fiber <b>10</b> of this example has absorption per unit length (when launching pump power in the inner cladding) in the range of 0.1-2 dB/m.

Example 2

0083<figref idref="DRAWINGS">FIG. 13</figref> illustrates a refractive index profile of a second exemplary optical fiber of the present invention. More specifically, <figref idref="DRAWINGS">FIG. 13</figref> depicts refractive index delta as vs. the radius for the second exemplary optical fiber. This optical fiber has a Yb doped, silica based core <b>12</b> which is multi mode at the lasing wavelength of 1100 μm, a silica based inner cladding <b>14</b> having two sections of almost the same index of refraction (delta %≈0) and an outer cladding <b>16</b> which is doped with fluorine. The NA of the inner cladding is 0.16. <figref idref="DRAWINGS">FIG. 13</figref> illustrates that the refractive index difference (delta %) of the core <b>12</b> is about 0.7, that the fluorine doped outer cladding <b>16</b> has the refractive index delta of about −0.7. The core <b>12</b> and the first section of the inner cladding <b>14</b> are produced by an inside-vapor-deposition (IVD) process. The core Ge—Si soot is deposited inside the glass tube (first section of the inner cladding) and followed by solution Yb-doping of the core soot. The structure is then sintered into a solid preform. The preform is then used as a bait-rod for the OVD process for the additional inner cladding and outer cladding deposition.

0084The double clad optical fiber illustrated in <figref idref="DRAWINGS">FIG. 13</figref> is also suitable for use in a fiber laser device. <figref idref="DRAWINGS">FIGS. 14 and 15</figref> correspond to the optical fiber of <figref idref="DRAWINGS">FIG. 13</figref>. More specifically, <figref idref="DRAWINGS">FIG. 14</figref> illustrates fiber-laser efficiency of this optical fiber. The fiber laser shows low threshold (about 1.5 Watts) and high lasing efficiency of 78% (as defined by the graph's slope). The optical fiber has good power-handling capability with power of over 10 Watts. <figref idref="DRAWINGS">FIG. 15</figref> illustrates the low passive loss, for example less than 2 dB/km at 1280 nm, achieved in the Yb-doped core of the fiber of <figref idref="DRAWINGS">FIG. 13</figref>.

0085The specific composition for the optical fiber of the second example is: <ul id="ul0005" list-style="none"><li id="ul0005-0001" num="0000"><ul id="ul0006" list-style="none"><li id="ul0006-0001" num="0086">Core <b>12</b>: 0.8 wt % Yb₂O₃; 9.5 cwt % P₂O₃; 5.4 wt % GeO₂;</li><li id="ul0006-0002" num="0087">Inner cladding <b>14</b>: Pure Silica;</li><li id="ul0006-0003" num="0088">Outer cladding <b>16</b>: 2.3 wt % F.</li></ul></li></ul>

Example 3

0089<figref idref="DRAWINGS">FIG. 16</figref> illustrates a refractive index profile of a third exemplary optical fiber of the present invention. This optical fiber has a Yb doped core <b>12</b>, a silica based, Ge doped inner cladding <b>14</b> with the relative refractive index (% delta %) of 0.3 and an outer cladding <b>16</b> which is doped with Fluorine. <figref idref="DRAWINGS">FIG. 16</figref> shows that the refractive index difference (% delta) of the core <b>12</b> is about 0.7 and that the fluorine doped outer cladding <b>16</b> has the refractive index % delta of about −0.7. The specific composition for this optical fiber example is: <ul id="ul0007" list-style="none"><li id="ul0007-0001" num="0000"><ul id="ul0008" list-style="none"><li id="ul0008-0001" num="0090">Core <b>12</b>: 0.8 wt % Yb₂O₃; 9.5 wt % P₂O₃; 5.4 wt % GeO₂;</li><li id="ul0008-0002" num="0091">Inner cladding <b>14</b>: 6 wt % GeO₂;</li><li id="ul0008-0003" num="0092">Outer cladding <b>16</b>: 2.3 wt % F</li></ul></li></ul>

0093It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.