# Patent

## Title
Rare earth doped single polarization double clad optical fiber with plurality of air holes

## Abstract

An optical fiber including: (i) a silica based, rare earth doped core having a first index of refraction n1; (ii) a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber; (iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3, wherein said optical fiber supports a single polarization mode within the operating wavelength range.

## Background
Parts 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.


FIELD OF THE INVENTION
The present invention relates generally to optical waveguide fibers, and more particularly to a rare earth doped optical fiber exhibiting single polarization properties.
FIELD OF THE INVENTION
The 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.
TECHNICAL BACKGROUND
Optical 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.
To 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.
Double 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.
The 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.
The 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.
Recent 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.
In 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 inert 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.
How 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.
Typically, 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.
All-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.
Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with, and connection to, optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). The polarization characteristic (single polarization) propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss.
Polarization retaining fibers (sometimes referred to as a polarization maintaining fibers) can maintain the input polarizations on two generally-orthogonal axes. These fibers are not single polarization fibers. A common polarization maintaining fiber includes stress birefringence members and includes, as shown in FIG. 1A, a circular core 12′ surrounded by an cladding region 14′. Core 12′ and the cladding region 14′ are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material is greater than that of the cladding material.
In FIG. 1A, diametrically opposed relative to core 12′, are two stress-inducing regions 13′ formed of a glass material having a Thermal Coefficient of Expansion (TCE) different from that of cladding material 14′. When such a fiber is drawn, the longitudinally-extending stress-inducing regions 13′ and the cladding region will shrink different amounts, whereby regions 13′ will be put into a state of tension or compression strain. Strain induced birefringence (otherwise referred to a stress-induced birefringence) is imparted in the fiber and thereby reduces coupling between the two orthogonally polarized fundamental modes. It should be recognized that such fibers including these stress-inducing regions 13′ do not provide single polarization properties.
Slight improvement in the polarization performance of single mode optical fibers has been achieved by elongating or distorting the fiber core geometry, as a means of decoupling the differently polarized light components. Examples of such optical fiber waveguides with elongated cores are disclosed in U.S. Pat. Nos. 4,184,859, 4,274,854 and 4,307,938. However, the noncircular geometry of the core alone is, generally, not sufficient to provide the desired single polarization properties. It is also noted that this type of optical fiber has relatively low birefringence (i.e., 10−5 or less). Furthermore, these fibers are not optically active fibers and, therefore are not suitable for use as a laser or an amplifier fiber.
It has, therefore, been an area of ongoing development to obtain an optical fiber that will single polarization performance while being suitable for use as optical amplification medium, and which is also easily manufacturable.
SUMMARY OF THE INVENTION
Definitions:
The following definitions and terminology are commonly used in the art.
Refractive index profile—the refractive index profile is the relationship between the refractive index (Δ %) and the optical fiber radius (as measured from the centerline of the optical fiber) over a selected portion of the fiber.
Birefringence—birefringence is the difference between the effective refractive indices of the two polarization modes.
Radii—the radii of the segments of the fiber are generally defined in terms of points where the index of refraction of the material used takes on a different composition. For example, the central core has an inner radius of zero because the first point of the segment is on the centerline. The outer radius of the central core segment is the radius drawn from the waveguide centerline to the last point of the refractive index of the central core having a positive delta. For a segment having a first point away from the centerline, the radius of the waveguide centerline to the location of its first refractive index point is the inner radius of that segment. Likewise, the radius from the waveguide to centerline to the location of the last refractive index point of the segment is the outer radius of that segment. For example, an down-doped annular segment surrounding the central core would have an outer radii located at the interface between the annular segment and the cladding.
Relative refractive index percent Δ % —the term Δ % represents a relative measure of refractive index defined by the equation:

Δ %=100×(ni2−nc2)/2ni2

where Δ % is the maximum refractive index of the index profile segment denoted as i, and nc, the reference refractive index, is taken to be the refractive index of the cladding layer. Every point in the segment has an associated relative index measured relative to the cladding.

In accordance with some embodiments of the present invention, an optical fiber is provided which exhibits polarization maintaining (retaining) properties while being suitable for use as an optical amplification media. In accordance with some of the embodiments of the present invention, a rare earth doped optical fiber is provided which exhibits single polarization properties within a Single Polarization Band (SPB). The fibers parameters are preferably selected such that the SPB coincides with an operating wavelength band.
According to the present invention the optical fiber includes:

    
    
        (i) a silica based, rare earth doped core having a first index of refraction n1;
        (ii) a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, the inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
        (iii) a silica based outer cladding surrounding the inner cladding and having a third index of refraction n3, such that n2>n3; 
        wherein the optical fiber supports a single polarization mode within the operating wavelength range.
    
    


One advantage of the optical fiber of the present invention is its capability to produce gain, thus being capable for use in a laser or an optical amplifier while (i) performing as single polarization fiber and exhibiting a single polarization band SPB width of greater than 10 nm and even more preferably greater than 15 nm, and (ii) being capable of handling relatively large amounts of optical power. Another advantage of the optical fiber of the present invention is that because it performs both as a gain fiber and the SP fiber, it eliminates the need to for splicing together gain fiber and the single polarization fiber, thereby reducing the splicing loss, the overall fiber length, while eliminating work and cost associated with splicing the two fibers together.
More particularly it is believed that in these embodiments the effective refractive index of one of the polarizations is such that this polarization cannot propagate within the SPB, while the other orthogonal polarization associated with different effective refractive index is such that this polarization may still propagate in the SPB. Accordingly, single polarization propagation within the SPB is provided by the rare earth doped fiber with a relative simple structure. In some of the embodiments of the optical fibers according to the present invention the SPB width is 20 to 40 nm.
Additional features and advantages of the invention will be set forth in the detail 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.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic cross-sectional view of a prior art optical fiber;
FIG. 1B is a schematic cross-sectional view of one embodiment of the present invention;
FIGS. 2A–2C are schematic cross-sectional views of other embodiments of the present invention;
FIGS. 3A and 3B illustrate schematically relative a refractive index profiles of an exemplary optical fiber of the present invention, across two different cross-sections of the fiber;
FIG. 3C is a measured refractive index profile of a first example of optical fiber in accordance with the present invention;
FIG. 4 is a graph illustrating passive core loss vs. wavelength of the optical fiber of FIG. 2B;
FIG. 5A is a graph of output power vs. launched power for the optical fiber of FIG. 2B;
FIG. 5B is a graph illustrating single polarization property of the optical fiber of FIG. 2B;
FIG. 6 is a refractive index profile of an alternative example of the optical fiber of the present invention;
FIG. 7 is a graph illustrating passive core loss vs. wavelength of the optical fiber of FIG. 6;
FIG. 8 is a schematic illustration of AlCl3 delivery mechanism;
FIG. 9 illustrates Al2O3 concentration in a preform which resulted from Argon gas delivery (bottom curve) and heated Helium gas delivery (top curve);
FIG. 10 is a graph illustrating Yb2O3 and Al2O3 concentration within a core optical fiber preform;
FIG. 11 is a schematic illustration of the formation of a core soot preform;
FIG. 12 illustrates consolidation of a soot preform into a glass preform;
FIG. 13 illustrates inner cladding background loss of an exemplary fiber;
FIG. 14 illustrates schematically a core cane utilized to manufacture the fiber of FIGS. 1B and 2A–2C;
FIG. 15 illustrates schematically a core-clad cane utilized to manufacture the fiber of FIGS. 1B and 2A–2C;
FIG. 16 illustrates schematically a grooved cane utilized to manufacture the fiber of FIGS. 1B and 2A–2C;
FIG. 17 illustrates schematically a glass tube with the inserted grooved cane of FIG. 16;
FIG. 18 illustrates schematically an exemplary consolidation process utilized to manufacture the fiber of FIGS. 1 and 2A–2C;
FIG. 19 illustrates schematically a redraw tower utilized to manufacture the fiber of FIGS. 1B and 2A–2C;
FIG. 20 illustrates a preform subassembly that includes a silica tube overclad with silica soot;
FIG. 21 illustrates a machined core/inner clad blank;
FIG. 22 illustrates a consolidated blank after the core/inner cladding blank it has been machined as shown in FIG. 21 and overclad with the silica based outer cladding material;
FIG. 23 illustrates schematically a process for drawing fiber utilized to manufacture the fiber of FIGS. 1B and 2A–2C.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE SINGLE POLARIZATION DOUBLE CLAD OPTICAL FIBER WITH AIR HOLES
Reference 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 single polarization optical fiber in accordance with the present invention is shown schematically in FIG. 1B, and is designated generally throughout by the reference numeral 10. The optical fiber 10 illustrated in FIG. 1B includes: an elongated, silica based, rare earth doped core 12 having a first index of refraction n1; a first silica based cladding 14 (inner cladding) surrounding the core 12 and having a second index of refraction n2, such that n1>n2; and a silica based outer cladding 16 surrounding the first cladding 14 and having a third index of refraction n3. The inner cladding 14 contains at least two air holes 24, 26, preferably situated on diametrically opposite sides of the core 12 and extending along the core 12, through the length of the fiber 10. The air holes 24, 26 lower the effective refractive index of the inner cladding 14 along the line A—A illustrated in FIG. 1B and enhance or enable single polarization property of this fiber. The core 12, inner cladding 14 and the outer cladding 16 are made of glass. A protective coating 18 surrounds the outer cladding 16. The outer coating 18 may be, for example, an organic coating which typically includes a softer primary coating and a harder secondary coating applied over the primary coating.
In this embodiment the silica based core 12 is doped with Yb, but other rare earth materials, such as Er may also be utilized. The core 12 may also include at least one index raising dopant. The outer cladding further 16 preferably includes an index lowering dopant, such that n2>n3 The inner cladding diameter DIN is preferably at least 125 μm and more 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 14 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, while two air holes make this fiber a single polarization fiber. The size of the air holes may vary, preferably from 7 to 20 μm in diameter, depending on the desired size (minor axis) of the fiber core.
It is preferable that the outer cladding 16 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 16 be between about 10 μm to 25 μm. It is preferable that the diameter DC of the fiber core 12 be about 5 μm to 20 μm, the inner cladding diameter DIN be about 125 μm to 2000 μm and more preferably about 125 μm to 1500 μm. It is even more preferable that DIN be about 125 μm to 350 μm. It is preferable that the diameter of the outer cladding diameter (DOUT) be about 145 to 2100 μm, more preferably between about 145 μm to 1600 μm and even more preferable that DOUT be about 145 μm to 500 μm. If the inner cladding 14 does not have a circular cross section, Din is defined as the smallest distance from one side of the inner cladding's cross section to the oppositely situated side of the cross section. It is also noted that the outer cladding 16 may not be circular. If the outer cladding 16 is not circular, DOUT is defined as the smallest distance from one side of the outer cladding's cross section to the oppositely situated side of the outer cladding's cross section. It is preferable that the inner cladding's 14 cross-sectional area be at least 200 times larger than the cross sectional area of the core 12. It is even more preferable that the cross sectional area of the inner cladding 14 be between 300 and 3000 times larger than the cross sectional area of the core 12. For example, the cross sectional area of the inner cladding 16 may be 500, 700, 1000, 1200, 1500, 1600, 2000 or 2500 times larger than the cross sectional area of the core 12.
According to this embodiment, the fiber core 12 includes, in weight percent:

















Rare earth
0.1 to 2.5
wt %;



P
0 to 5
wt %;



Al
0.5 to 15
wt %;



Ge
0.1 to 15
wt %;



F
0 to 1
wt %.










The rare earth dopants in the fiber core 12 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 12 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. 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 12 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.
The preferred ranges of the core 12 composition in weight percent are:

















Rare earth
0.3 to 1
wt %;



P
0 to 2
wt %;



Al
2 to 8
wt %;



Ge
3 to 15
wt %; and



F
0.1 to 0.5
wt %.










The Yb-doped core 12 will laze at 1.03–1.11 micron range.

It is preferable that the inner cladding 14 contain 5 wt % to 30 wt % Ge 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.
It is preferable that the index lowering dopant of the outer cladding 16 comprises Fluorine and/or Boron in weight percent:
















F
 0.5 to 5 wt %;



B
0.5 to 20 wt %.










The amount of dopant(s) for the outer cladding 16 is chosen to preferably result in inner cladding NA of between 0.15 to 0.5. However, it is preferable that the outer cladding 16 contain at least one of of B or/and F. 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 16. It is preferable that the outer cladding 16 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 F and 3 to 15 wt % of B.
Other embodiments of the double clad optical fiber of the present invention are shown schematically in FIGS. 2A–2C 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 inner cladding 14 of the optical fiber of FIGS. 2A–2C is non-circular. The advantage of non-circular inner cladding 14 is that non-circular shape improves the absorption of optical pump power into the core 12. The elongated core 12 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 optical fiber core 12 is preferably elliptical, as shown in FIGS. 1B and 2A–2C, but may have other elongated shapes. Adjacent to the core and situated at least partially within the inner cladding 14 are at least two air holes 24, 26. The elongated (elliptical) core 12, in conjunctions with the air holes 24, 26 renders this optical fiber a single polarization (SP) fiber. It is preferred that the aspect ratio (ratio of major to minor axis) of the elliptical core 12 be at least 1.5:1 and more preferably be between 2:1 and 10:1, because these aspect ratios improve birefringence of the core 12.
The core delta is less than 1% Δ and preferably less than 0.5% Δ. The numerical aperture NA of the core 12 is between 0.05 (for high power laser application) and 0.25 (for lower power application). The numerical aperture NA of the core 12 is defined as (n12−n22)1/2, where n1 is the index of refraction of the core 12 and n2 is the index of refraction of the inner cladding 14.
The silica based inner cladding 14 may have a circular outer perimeter, as shown in FIG. 1B (preferably with an off-center situated core), or a non-circular outer perimeter as shown in FIG. 2A–C. The numerical aperture NA of the inner cladding 14 is defined as (n22−n32)1/2, where n3 is the index of refraction of the outer cladding layer 16. The inner cladding preferably has numerical aperture NA between 0.15 and 0.45 and more preferably between 0.3 and 0.4.
In 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 14 is adjacent to the core 12 and a second (outer) cladding 16 surrounds the first or the inner cladding 14. The core 12 may be either single mode or multi-mode at the core lasing wavelength. The inner cladding 14 serves as a waveguide with a high numerical aperture NA for the input (pumping) light. That is, the inner cladding serves as a pump cavity. 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 (DIN is the shorter dimension of the inner cladding as seen in FIGS. 2A–2C) 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. Recent 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.
The 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
The invention will be further clarified by the following examples.
Example 1
FIGS. 3A and 3B illustrate schematically a relative refractive index profile of a first exemplary optical fiber of the present invention. More specifically, FIGS. 3A and 3B depicts optical fiber's refractive index percent delta (relative to that of the pure silica) vs. the distance measured from the core center. FIG. 3A illustrates schematically a refractive index profile taken across the region that does not contain the air holes, for example, along the line Y—Y of the fiber depicted in FIG. 1B. FIG. 3B illustrates schematically a refractive index profile of the same fiber, but taken across the region that contains the air hole 24 (for example, along the line A—A of the fiber depicted in FIG. 1B).
FIG. 3C illustrates measured refractive index profile (percent delta, relative to that of the pure silica) of a first exemplary optical fiber of the present invention along the Y—Y axis, measured from the core center. This optical fiber has the cross-section illustrated in FIG. 2B. The distance DIN between two opposing flat sides of this inner cladding cross section is 260 μm. The refractive index percent delta is defined herein as (ni2−nS2)/2ni2, where i=1, 2 or 3 and nS is the refractive index of pure silica. This optical fiber has a Yb doped core 12, a Ge-silica inner cladding (% delta≈0.46) and an outer cladding 16 which doped with Fluorine and Boron.
FIG. 3C shows that the relative refractive index difference (percent delta) of the core 12 is about 0.56, that the fluorine/boron doped outer cladding 16 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 12 is doped with Erbium, the optical fiber will be single-mode at lasing wavelength of 1.55 μm. The optical fiber 10 has a relatively low NA (about 0.065) for the core 12, and high NA (0.30) for the inner cladding 14. (The NA is defined by (ni2−ni+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 12 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:

    
    
        Core 12: 0.6 wt % Yb2O3; 4.5 wt % Al2O3; 3.0 wt % GeO2; 0.2 wt % F;
        Inner cladding 14: 8.5 wt % GeO2;
        Outer cladding 16: 9 wt % B and 2.7 wt % F.
    
    


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.
The double clad fiber produced by the OVD process is especially suitable for use in a higher power fiber laser device. FIGS. 4 and 5 correspond to the optical fiber of FIG. 2B. More specifically, FIG. 4 illustrates the low passive loss, for example 3 dB/km at 1280 nm, achieved in the Yb-doped core of the fiber of FIG. 2B. 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. FIG. 5 illustrates the single mode fiber-laser efficiency of this fiber. More specifically, FIG. 5 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 cladding 14 of the double clad optical fiber of FIG. 2B. The fiber laser shows low threshold and lasing efficiency of 52% (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 100 Watts. The optical fiber 10 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.
FIG. 5B illustrates measured transmission spectrum of double clad single polarization fiber shown in FIG. 2B. The single polarization bandwidth is around 20 nm, centered at 1080 nm in which lasing taking place. In this exemplary fiber the first cutoff wavelength λ1 is about 1070 nm and the second cutoff wavelength λ2 is about 1090 nm.
Example 2
FIG. 6 illustrates a refractive index profile of a second exemplary optical fiber of the present invention, across the region that does not intersect the airholes 24, 26 (along the axis Y—Y, for example). More specifically, FIG. 6 depicts refractive index delta as vs. the radius for the second exemplary optical fiber. This optical fiber has a Yb doped, silica based core 12 which is multi mode at the lasing wavelength of 1100 μm, a silica based inner cladding 14 having two sections of almost the same index of refraction (delta % ≈0) and an outer cladding 16 which is doped with fluorine. The NA of the inner cladding is 0.16. FIG. 6 illustrates that the refractive index difference (delta %) of the core 12 is about 0.7, that the fluorine doped outer cladding 16 has the refractive index delta of about −0.7.
The double clad optical fiber illustrated in FIG. 6 is also suitable for use in a fiber laser device. FIG. 7 corresponds to the optical fiber of FIG. 6. More specifically, FIG. 7 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 FIG. 6. The optical fiber has good power-handling capability with power of over 10 Watts.
The specific composition for the optical fiber of the second example is:

    
    
        Core 12: 0.8 wt % Yb2O3; 9.5 wt % P2O3; 5.4 wt % GeO2;
        Inner cladding 14: Pure Silica;
        Outer cladding 16: 2.3 wt % F.

The optical fiber of example 2 would have a single polarization range of 20 nm to 40 nm, depending on the size (along the minor axis) of the core 12.

    
    


The Process for Making Fiber
The fiber of FIGS. 1B and 2A–2C is 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 soot-preform. The soot-preform is then consolidated into solid 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 preform is generated first, then consolidated, followed by core/inner cladding preform generation and consolidation, which in turn, is followed by the outer cladding outside vapor deposition process and another consolidation step. The final preform is then drawn into double-clad single polarization optical fiber 10 by known fiber-drawing methods.
More specifically, the following steps are utilized to make the rare earth doped double clad single polarization fiber of FIGS. 1B and 2A–2C.
1. Core cane formation. The core cane is formed first. The core is manufactured, for example, by a standard OVD process. The core materials are deposited onto the bait rod during the laydown step. The exemplary vapor-precursor-materials used to make the fiber core cane are Yb(fod)3, AlCl3, SiF4, SiCl4, GeCl4 and tri-ethyl borate. Other rare-earth materials may be utilized either in addition to Yb, or instead of Yb. During the core deposition process we achieved a uniform AlCl3 gas-phase delivery. This was accomplished by utilizing heated inert Helium as carrier gas 30 (instead of Argon gas) for AlCl3 delivery illustrated schematically in FIG. 8. As solid AlCl3 changes 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 is provided at a temperature within 150° C. to 180° C. range. As illustrated in FIG. 8, the heated Helium gas is provided by the He gas heater 32 to the oven 50 containing AlCl3 vessel 52. The relatively high Helium gas temperature helps to maintain the AlCl3 containing vessel 52 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 32 to 168° C. and the vessel 52 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 AlCl3 vapor via a heated gas line 54 to the flame burner (gas burner) 56. To produce the core preform 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 preform core thickness of about 2 mm to 3 mm. Heated Helium based AlCl3 delivery 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 instead of the Helium gas delivery of AlCl3 may be utilized, but a Helium gas delivery of AlCl3 results better uniformity of Al2O3 concentration. (See FIG. 9). It is preferable that Al2O3 is 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.
As shown in FIG. 10, the heated Helium delivery of AlCl3 resulted 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 12. More specifically, the resultant variability of Al2O3 concentration in the core is less than 2 wt % and preferably less than 0.5 wt % and more preferably less than 0.25 wt %, especially for maximum Al2O3 concentration of over 3 wt %. It is also preferable that the ratio of max wt % to min wt % of Al2O3 concentration 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 Al2O3 concentration of over 3 wt %.
The Yb vapor delivery is carried by Argon gas and is accomplished by heating organometallic Yb(fod)3 in the temperature range of 150° C.–180° C., which results in a soot preform core with Yb2O3 concentration from about 0.2 wt % to 3 wt %. In order to make the optical fiber 10 of this example, the Yb(fod)3 containing vessel temperature of 163° C. was used to achieve the Yb2O3 concentration of about 0.6 wt %. The delivery of other materials is carried out by conventional oxygen delivery at temperatures below 100° C.
More specifically, according to one embodiment of the present invention, the Yb(fod)3, AlCl3, SiF4, SiCl4 and GeCl4 are delivered to a gas burner 56. (See FIG. 11.) The gas burner 56 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 56, and react in the burner flame 58 where the desired glass-soot particles formed. The soot particles are then deposited onto a rotating bait-rod 59 or core cane 60 through the thermopheretic mechanism to result in the designed soot-preform 62 which will be used to manufacture single polarization fiber with the Yb-doped single-mode core.
After the core soot preform layer is layered down and the soot preform 62 is cooled to room temperature, the bait rod 59 is removed from the center of core soot preform 62. The core soot preform 62 is then consolidated (densified into the solid glass) to become a solid glass-preform 62A which is drawn into core cane 62B. (See FIGS. 12 and 14.)
Applicants discovered that a proper choice of high temperature and fast down-feed rate during consolidation results in low crystallization formation in the resulting solid 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. More specifically, soot preform 62 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, and preferably 3 dB or less, at a wavelength of 1280 nm. As illustrated in FIG. 12, the ‘core’ soot preform 62 is consolidated into solid glass-preform 62A in a high temperature (1400° C. to 1600° C.) furnace 64. 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 10 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 62 is moved relative to the furnace 64 (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 down-feeding the soot preform 62 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.
With the above described high consolidation temperatures and fast down-feed rate, the resultant optical fiber 10 has the core background loss of less than 8 dB/km. More preferably, the optical fiber exhibits 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 core soot preform 62 has sufficient amount of Ge to produce, after the cladding process is completed, a fiber with core delta of 0.06 to 0.1%. After the core preform 62 has been consolidated, as described above, it is drawn into the core cane 62B. The core cane 62B is preferably 1 meter long and about 8 mm in diameter. The core cane 62B is illustrated schematically in FIG. 14.
2. First clad blank formation. The core cane 62B is overclad with silica soot to form a core/clad (soot) blank (referred herein as the first clad bank 63). The first clad blank is then consolidated to form cane 63A. The first clad blank 63 has a core to the first clad diameter ratio of 0.4 to 0.6. The cane 63A is about 42 mm in diameter. Cane 63A is illustrated schematically in FIG. 15.
Alternatively a sleeving process may be utilized to form cane 63A, by placing a silica sleeve around the core cane 62A.
3. Grooved cane formation. The cane 63A includes sections 112, 114 which correspond to the core 12 and the first cladding layer 14 of the optical fiber 10. Cane 63A is preferably about 1 meter long and about 8 mm in diameter. Grooves 54 are then ground into the diametrically opposite longitudinal sides of the cane 63A to a width of about 6.4 mm and to a depth of about 8 to 10 mm, thereby forming grooved cane 63B. (See FIG. 13.) The groove depth depends on the thickness of the first clad layer, but should be such that its bottom substantially abuts the section 112 (corresponding to the fiber core 12), as illustrated in FIG. 18. The grooved cane 63B is HF etched for about 30 minutes to clean any grinding residue and then redrawn to an appropriate size cane (OD of about 8 mm).
The grooved and redrawn cane 63B is then inserted into a 1 meter long silica tube or sleeve 65 overclad with silica soot 67 (for example, about 800–1000 gms.), as shown in FIG. 19, to form a preform subassembly 70. Silica overcladding 67 method on the sleeve 65 is preferably produced by an Outside Vapor Deposition (OVD). The exemplary silica tube 65 may have an inner diameter of about 8.8 mm and an outer diameter of about 11.8 mm which supports a layer of silica soot 67. The silica tube baring soot is cleaned, both inside and outside, with a chemical solvent or alcohol (IPA for example), prior to the insertion of the etched and re-drawn cane 63B into the tube 65. If needed, the two holes 24, 26 in the preform subassembly 70 may be further etched via HF to enlarge the holes.
The preform subassembly 70 of FIG. 17 is then consolidated in accordance with a conventional consolidation process as shown in FIG. 18 by first drying in a consolidation furnace 64 in an atmosphere of Cl2, and then consolidating in the furnace in a He-containing atmosphere to produce a consolidated preform 70A. The consolidated preform 70A is then inserted into a redraw tower 74 as shown in FIG. 19. The preferred down feeding rate is about 7 mm/min. Heat is applied to preform 70A by heating element 75 and it is drawn down by tension applying wheels 76 into an approximately 7–8 mm diameter cane 78. While the redraw process (drawing to a smaller diameter core cane from the preform) is occurring, a positive pressure (about 1 psi) is applied to the holes 24, 26 sufficient to keep them from closing. The pressure may be sufficient to cause the central core to elongate slightly. The pressure used is a function of the draw temperature, glass viscosity, and draw speed among other factors.
This cane 78, now having an elliptically shaped central core and air holes, is again inserted into a 1 meter long silica tube 65A which is overclad with about 1000 grams of silica soot 67A, as shown in FIG. 20 to form preform subassembly 70B. This preform subassembly 70B is consolidated in the same manner as heretofore to form consolidated blanks 70C. The consolidated blanks 70C will form the basis for the core and the inner clad portion of the optical fiber 10.
The consolidated blanks 70C are then machined, if needed, to desired shape. Breaking circular symmetry in the inner clad layer enhances pump light absorption efficiency. A machined core/inner cladding blank 70D is illustrated schematically in FIG. 21. The machined blank 70D is overclad again, for example by SiO2 with index lowering dopants and then consolidated to a consolidated blank 71. The down-doped silica layer of the consolidated blank 71 will form the second, or outer cladding 16 of the optical fiber 10. FIG. 22 illustrates schematically an exemplary consolidated blank 71. If boron is used in overcladding, it is preferred that consolidation is performed in Fluorine environment. In this example, the index lowering dopants are B and F.
More specifically, B2O3 and SiO2 were vapor deposited on the ground glass preform to form a B2O3 and SiO2 soot layer by using tri-ethyl borate and SiCl4 delivered 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 SiF4 gas 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. The optical fiber of this example was produced by utilizing consolidation temperature of 1350° C., so as to facilitate adequate Fluorine doping through diffusion. In this example, the third layer of the preform (outer cladding) has a shape similar to that of the second layer (inner cladding).
The consolidated blank 71 is then suspended from a handle 81 in a draw furnace 80 as shown in FIG. 23 and a fiber 82 is drawn therefrom. During draw, a small positive pressure (about 1 psi or less) is applied to the holes to keep them from closing. This causes the core to become (more) elliptically shaped. In the exemplary fiber depicted in FIGS. 1B and 2A–2C we utilized positive pressure of less than 0.1 psi. The draw speed was about 1 m/sec. The resulting fiber has an elliptically shaped core and dual air holes.
As should be recognized, the elongation of the core may occur in the redraw step, the draw step, or combinations thereof to achieve the desired aspect ratio of the central core. In either case, a positive pressure is applied to the holes in the preform (and fiber) to cause the elongation to occur.
It will be apparent to those skilled in the art that variations and modifications can be made to the present invention without departing from the scope of the invention. For example, although step index structures are show, other graded index structures may be employed. Moreover a ring structure may be added to the fiber profile as well and would still function acceptably. 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.

## Claims

1. An optical fiber comprising:
(i) a silica based, rare earth doped core having a first index of refraction n1;
(ii) a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
(iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3;
wherein said optical fiber supports only a single polarization mode within the operating wavelength range, such that said fiber has a single polarization band SPB of at least 10 nm.

2. The optical fiber according to claim 1 wherein said rare earth core is an elongated core.

3. The optical fiber according to claim 1 wherein said core is elliptical and the ratio of its maximum dimension to its minimum dimension is at least 1:1.5.

4. The optical fiber according to claim 1, wherein said core is doped with Al2O3, such that the ratio of max wt % to min wt % of Al2O3 concentration in said core is less than 2:1.

5. The optical fiber according to claim 1 wherein said rare earth core includes Yb.

6. An optical fiber comprising:
(i) a silica based, rare earth doped core having a first index of refraction n1;
(ii) a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
(iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3;
wherein said optical fiber supports only a single polarization mode within the operating wavelength range, wherein said air holes are circular with a diameter of 7 to 20 μm.

7. The optical fiber according to claim 1 wherein said inner cladding contains an even number of air holes.

8. The optical fiber according to claim 1 wherein the shortest dimension of said inner cladding is at least 145 μm.

9. The optical fiber according to claim 1 wherein said silica based outer cladding contains at least F or Boron.

10. The optical fiber according to claim 1 wherein the core delta is 0.5% Δ or less.

11. The optical fiber according to claim 2 wherein the core has a maximum dimension of 1.5 μm to 15 μm and a minimum dimension of 1 μm to 5 μm.

12. The optical fiber according to claim 1 wherein said operating wavelength range is 1000 to 1120 nm.

13. An optical fiber comprising:
(i) a 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, said cladding having a plurality of air holes extending longitudinally through the length of said optical fiber, wherein said optical fiber supports only a single polarization mode within the operating wavelength range, and has a single polarization band SPB of at least 10 nm.

14. The optical fiber according to claim 13 wherein said rare earth core is an elongated core.

15. The optical fiber according to claim 13 wherein said core is elliptical and the ratio of its maximum dimension to its minimum dimension is at least 1:1.5.

16. The optical fiber according to claim 13, wherein said rare earth core includes Yb.

17. An optical fiber comprising:
(i) a 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 said cladding having a plurality of air holes extending longitudinally through the length of said optical fiber, wherein said optical fiber supports only a single polarization mode within the operating wavelength range, wherein said air holes are circular with a diameter of 7 to 20 μm.

18. The optical fiber according to claim 1, wherein said air holes are in contact with said core.