# Patent

## Title
Optical fiber apparatus with suppression of higher order modes

## Abstract

Optical fiber apparatus having a wavelength of operation, that comprises an optical fiber including a core comprising an active material for providing light having the operating wavelength responsive to the optical apparatus receiving pump optical energy having a pump wavelength; a cladding disposed about the core; at least one region spaced from the core; and wherein the optical fiber is configured and arranged such that at the wavelength of operation the optical fiber can propagate a plurality of modes and wherein the optical fiber comprises a fundamental mode that is primarily a mode of the core and at least one higher order mode (HOM) that is a mixed mode of a selected mode of the core and of a selected mode of the at least one region.

## Background
FIELD OF THE INVENTION
The present invention relates to optical fibers and optical fiber apparatus, such as, for example, optical fiber amplifiers, lasers and amplified spontaneous emission (ASE) sources.
BACKGROUND OF THE DISCLOSURE
Lasers, amplifiers and other optical apparatus based on optical fiber can provide flexible, rugged and relatively simple sources of optical energy. Accordingly, in many applications such optical fiber apparatus can often have one or more advantages as compared to counterparts based on a gas medium (e.g., CO2) or on a bulk solid-state medium (e.g., a Nd:YAG rod). For example, optical fiber lasers can have a smaller footprint, or can be more efficient, or can require less sophisticated cooling arrangements as compared to using a gas or bulk rod solid-state laser in a similar application. Often, however, it can be desirable to increase the output power of optical fiber apparatus, as certain gas and bulk solid-state lasers can readily produce high CW output powers or pulses of optical energy having high energy and/or high peak power.
Unfortunately, because of the high power density inherent in confining optical energy to the relatively small cross sectional area of an optical fiber, non-linear phenomena, such as Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS), can severely limit scaling the output power of a fiber laser or amplifier to higher powers. Though these non-linear processes are complex, each can be addressed, at least in part, by reducing the power density in the core of the fiber. One way to reduce power density is to increase the diameter of the core of the fiber and/or reduce the numerical aperture (NA) of the core, such that the fiber has a larger mode field diameter (MFD). Reducing the power density in this manner can increase the power threshold for the onset of the undesirable non-linear phenomena.
This approach, however, is not without drawbacks. Fibers having larger core diameters can typically support higher order transverse modes (e.g., LP11, LP21, LP02 etc.) in addition to the fundamental mode (e.g., LP01). Such higher orders modes (HOMs) tend to degrade the quality of output optical energy provided by the fiber apparatus and hence raise the M2 parameter (lower M2 means better beam quality). In many applications a low M2 is desired. Forestalling the onset of non-linear effects while also maintaining good beam quality can present a challenge to the designer of optical fiber apparatus.
Some approaches to this challenge are known in the art. For example, U.S. Pat. No. 6,496,301, issued on Dec. 17, 2002 to Koplow, Kliner and Goldberg, teaches bending a multimode fiber having a larger core to substantially attenuate, via increased bend loss, higher order modes such that a fiber amplifier provides gain in substantially a single mode. See also U.S. Pat. No. 7,424,193, issued on Sep. 9, 2008 to Alamantas Galvanauskas, which teaches a composite waveguide having a central core and at least one side core helically wound about the central core and in optical proximity to the central core. According to the '193 patent, higher order modes of the central core selectively couple to the helical side core and experience high loss such that the central core is effectively single-mode.
Existing techniques, however, are not necessarily entirely satisfactory in all circumstances. Accordingly, it is an object of the present invention to address one or more of the deficiencies or drawbacks of the prior art.
SUMMARY OF THE DISCLOSURE
In one aspect of the disclosure there is provided an optical fiber apparatus having a wavelength of operation and comprising an optical fiber, wherein the optical fiber comprises a core including an active material for providing light having the operating wavelength responsive to the optical apparatus receiving pump optical energy having a pump wavelength; a cladding disposed about the core; at least one region spaced from the core; and wherein the optical fiber is configured and arranged such that at the wavelength of operation the optical fiber can propagate a plurality of modes, including a fundamental mode that is primarily a mode of the core and at least one higher order mode (HOM) that is a mixed mode of a selected mode of the core and of a selected mode of the at least one region.
In another aspect of the disclosure, an optical fiber apparatus having a wavelength of operation comprises an optical fiber, where the optical fiber can include a core; a cladding disposed about the core; and at least one region spaced from the core. The optical fiber can be configured and arranged such that the optical fiber comprises a first mode that is primarily a mode of one of the core and the least one region and a second mode that is a mixed mode of a selected mode of the core and of a selected mode of the at least one region; and wherein the selected mode of the core and the selected mode of the at least one region are of the same azimuthal order. The same order can comprises the zero order or a non-zero order. The first mode can be primarily a mode of the core, and can comprise the fundamental mode. The second mode can comprise a higher order mode.
In an additional aspect of the disclosure, an optical fiber apparatus having a wavelength of operation at which the optical apparatus is configured to propagate optical energy comprises an optical fiber including a core; a cladding disposed about the core; and at least one region spaced from the core. The optical fiber can be configured and arranged such that at the wavelength of operation the fiber supports a plurality of modes wherein the fundamental mode is primarily a mode of the core, at least one higher order mode (HOM) is a mixed mode of a selected mode of the core and a selected mode of the at least one region, and wherein at least another HOM having a lower mode order than the first HOM is not a mixed mode and is primarily a mode of the core or of the at least one region. “Lower mode order” means that the at least another mode is nearer in terms of effective index to the fundamental mode than the at least one HOM. The at least another HOM can comprises a mode of non-zero azimuthal order, such as, for example, the LP11 mode.
In further aspects of the disclosure, the active material can comprise a rare earth material, which can comprise, for example, one or more of erbium, ytterbium, neodymium or thulium. The at least one region can comprise an absorbing material for absorbing optical energy having the wavelength of operation and which, if absorptive of optical energy having the pump wavelength, can have a higher absorption for optical energy having the wavelength of operation than for optical energy having the pump wavelength. The absorbing material can comprise, for example, one or more of samarium, praseodymium or terbium. The at least one HOM can have a substantially higher propagation loss than the fundamental mode at the wavelength of operation. The operating wavelength can be, for example, about 1060 nanometers, about 1550 nanometers or about 2000 nanometers (e.g., the wavelength range at which thulium ions lase). The at least one HOM can comprise an HOM of zero azimuthal order. The selected mode of the core can comprise the LP02 core mode. The at least one HOM can comprise an HOM of non-zero azimuthal order. The selected mode of the core can comprise the LP11 mode. The at least one region can comprise a ring-shaped region. The at least one region can comprise a conventional ring core. The at least one region can comprise a plurality of satellite regions, which can be arranged in a ring. The at least one HOM can have a substantially higher propagation loss than the fundamental mode at the wavelength of operation. The propagation loss of the at least one HOM can be at least 5 times, in terms of dB per unit distance, higher than the propagation loss of the fundamental mode at the wavelength of operation.
In more aspects of the disclosure, the optical fiber comprises at least one longitudinally extending stress inducing region having a thermal coefficient of expansion that is different from material of the optical fiber disposed about the stress inducing region. The stress inducing region can increase the birefringence of the optical fiber. The birefringence can be increased such that optical fiber comprises a polarization maintaining (“PM”) optical fiber. The optical fiber can comprise a rare earth material for providing optical energy having the operating wavelength response to the optical apparatus receiving optical energy having a pump wavelength. The core of the optical fiber can comprise a diameter of at least 15 microns, a selected numerical aperture, and a V-number at the operating wavelength of greater than 3. The V-number can be greater than 5. The selected numerical aperture can be no greater than 0.10, or, alternatively, no less than 0.13 or no less than 0.15.
In even more aspects of the disclosure, the optical fiber apparatus can be configured as a laser. The laser can comprise a source of pump optical energy, which can include one or more pump diodes The laser can include a laser cavity defined by at least one optical fiber Bragg grating. The optical fiber apparatus can be configured as a master oscillator-power amplifier (MOPA) arrangement, wherein a seed oscillator feeds a power amplifier, which can comprises an optical fiber amplifier. The master oscillator need not comprise a fiber-based device, and can, for example, comprise a laser diode, and in this case the optical fiber apparatus may not, in some cases, include a laser cavity defined by at least one optical fiber Bragg Grating. The optical fiber apparatus can be constructed and adapted such the optical fiber is “end-pumped” or “side-pumped.” Also, the optical apparatus can include a second fiber disposed alongside the optical fiber, as is described in more detail elsewhere herein, for providing pumping optical energy to the optical fiber.
According to yet a further aspect of the disclosure there is also taught a method of designing and/or fabricating an optical fiber having a mixed mode, which method can comprise the steps of: selecting a first mode of the optical fiber, such as a mode of a region of the optical fiber, that is to be mixed with another mode to form a mixed mode; determining the azimuthal order and effective refractive index of the selected first mode; selecting a mode of at least one other region of the fiber to have substantially the same effective refractive index and same azimuthal order as the first mode; and constructing and arranging the design of the fiber such the selected modes mix to form a mixed mode. “Substantially the same” in this context means close enough so that the selected modes will mix to form the mixed mode.
In one general aspect, the present disclosure includes the realization that (HOMs, while typically understood to be generally undesirable for applications requiring good beam quality, may not be equally problematic in a practical application. Typically, HOMs that include substantial overlap in intensity distribution with the fundamental mode are more likely to be excited and to cause beam quality degradation; modes with less overlap may be less problematic. In some circumstances HOMs of zero azimuthal order (e.g., LP02) or modes that do not have a substantially central minima or null in the intensity distribution map will have higher overlap, and a well aligned splice, such as between a single mode fiber and the multimode optical fiber of an amplifier, can produce less excitation of modes of non-zero azimuthal order (e.g., LP11) or of modes that do have a substantially central minima or null in the intensity distribution as these modes have less overlap. In other circumstance, such modes may be of more concern, however.
For example, if the LP02 mode of the core is likely to be excited, that mode can be mixed with a mode of the at least one region and the mixed mode can be substantially attenuated relative to the fundamental mode by, for example, the mixed mode being “leaky” or, alternatively or additionally, by introducing an absorbing material to the at least one region. Any additional loss introduced to the fundamental mode, which is typically not converted to a mixed mode and hence much less affected by the at least one region, is lessened and is usually not unduly detrimental. Not all HOMs (e.g., one or more of the less problematic ones, such as, in some circumstances, the LP11 core mode) need be converted to mixed modes and can remain primarily modes of the core or of the at least one region spaced therefrom. Thus, in one sense, the disclosure teaches a more “surgical,” and hence simpler, approach that focuses more on the “problem” HOMs.
Certain terms used herein are now generally discussed. Others are discussed in the Specific Description and elsewhere below.
“Primarily a mode of the core” or “primarily a core mode” means that the mode (e.g., the fundamental mode) is not a mixed mode of the core and the ring core, where at least one HOM is a mixed mode of the core and the ring core spaced therefrom. In other words, the properties of the mode that is primarily a mode of the core are substantially determined by the core properties and the properties of the cladding, with the presence of the spaced ring core of which the at least one HOM is a mixed mode having little effect on the properties of the mode.
“Substantially higher propagation loss,” as that term is used herein, means that the loss, as measured in dB per unit distance (e.g., per meter) is at least five (5) times higher at the wavelength of operation (e.g., at least 1.0 dB/meter if the baseline for comparison is 0.2 dB/meter). Such propagation loss can be determined on the basis of a test fiber that does not include a rare earth material, as such material may also absorb optical energy at the operating wavelength and may make comparisons difficult (e.g., the problem of measuring a relatively small difference between relatively large numbers). Stating that one mode is suppressed relative to another mode means that it has substantially higher propagation loss than the other mode. It is noted that the terms “index of refraction” and “refractive index” are at times used interchangeably herein. “Multimode” means not single mode, and includes what is sometimes referred to in the art as “few-moded.” Typically a multimode fiber has a V-number of greater than 2.405 at its operating wavelength. “Material” includes material in the forms of ions (e.g., “comprising a concentration of erbium” includes comprising a concentration of Er3+ ions).
Further advantages, novel features, and objects of the invention will become apparent from the following detailed description of non-limiting embodiments of the invention when considered in conjunction with the accompanying FIGURES, which are schematic and which are not necessarily drawn to scale. For purposes of clarity, not every component is labeled in every one of the following FIGURES, nor is every component of each embodiment of the invention shown where illustration is not considered necessary to allow those of ordinary skill in the art to understand the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates a cross section of an optical fiber, taken perpendicular to the longitudinal axis along which the fiber extends;
FIG. 2 is an idealized plot of a possible refractive index profile for the core and cladding of the fiber of FIG. 1 as well as corresponding normalized intensity profiles for the fundamental and selected higher order modes (HOMs);
FIG. 3 schematically illustrates a cross section of an optical fiber according to the disclosure having a core and at least one region spaced from the core;
FIG. 4 is an idealized plot of a possible refractive index profile for the fiber of FIG. 3 as well as corresponding normalized intensity profiles for the fundamental and selected HOMs;
FIG. 5 shows intensity distribution maps for the fundamental and higher order modes of another optical fiber, where only the core and cladding are considered in the analysis;
FIG. 6 shows intensity distribution maps for the fundamental and higher order modes corresponding to the optical fiber of FIG. 5, with the proviso that only a ring core and cladding are considered in the analysis;
FIG. 7A shows intensity distribution maps for the fundamental and higher order modes of the optical fiber considered in FIGS. 5 and 6, where all of the core, ring core and cladding are considered in the analysis;
FIG. 7B shows intensity distribution maps for additional higher order modes of the optical fiber considered in FIG. 7A;
FIG. 8 shows intensity distribution maps for fundamental and higher order modes of a additional optical fiber, with the proviso that only the core and cladding are considered in the analysis;
FIG. 9A shows intensity distribution maps for the fundamental and higher order modes corresponding to the optical fiber of FIG. 8, with the proviso that only a ring core and cladding are considered in the analysis;
FIG. 9B shows intensity distribution maps for additional higher order modes of the optical fiber of FIG. 9A;
FIG. 10 shows intensity distribution maps for the fundamental and higher order modes of the optical fiber considered in FIGS. 8 and 9, where all of the core, ring core and cladding are considered in the analysis;
FIG. 11 schematically illustrates a cross section of an optical fiber according to the disclosure including a pair of longitudinally extending stress inducing regions; and
FIG. 12 is a perspective view schematically illustrating one example of an optical apparatus according to the disclosure including a second optical fiber located alongside a first optical fiber and a common cladding disposed about the first and second optical fibers.



SPECIFIC DESCRIPTION
FIG. 1 schematically illustrates a cross section of an optical fiber 12, taken perpendicular to the longitudinal axis along which the fiber 12 extends. The optical fiber 12 of FIG. 1 can include a core 14 and a cladding 16 disposed about the core 14. Typically the cladding 16 contactingly surrounds the core 12 and tends to confine optical energy propagated by the optical fiber 12 to the core 14 for guidance thereby, such as by via the phenomenon of total internal reflection. The optical fiber 12 can include at least one additional region 18 disposed about the cladding 16. The region 18 shown in FIG. 1 can represent a second cladding disposed about the cladding 16 for tending to confine optical energy, such as pump optical energy, to the cladding 16 for guidance thereby. Such a fiber is referred to in the art as a “cladding pumped” or “double-clad” optical fiber, and the cladding 16 can be referred to as a “pump cladding.” The second cladding can comprise, for example, a glass or a fluorinated low index polymer coating applied and cured during draw of the optical fiber 12. Alternatively, the region 18 can represent a high index polymer protective region, typically comprising an outer higher modulus layer disposed over an inner lower modulus layer. One or both of the layers can be applied as coatings to the optical fiber as part of the draw process. Variations of the foregoing are of course possible. For example, the region 18 can comprise a glass second cladding for tending to confine pump light to the cladding 16, and the region 18 can in turn have disposed thereabout a polymer protective region having one or more layers.
The core 14 can comprise an active material for providing optical energy (e.g., via the process of stimulated emission) responsive to the optical fiber 12 receiving pump optical energy having a pump wavelength. The active material can comprise a rare earth material, such as, for example, one or more of erbium, ytterbium, neodymium or thulium (e.g., a concentration of Er, Yb, Nd or Th ions).
The core 14 is typically multimode, and has a diameter D that is larger than a standard single mode core (e.g., a 5 micron diameter core) to provide a fundamental mode having an increased mode field diameter (MFD). The increased MFD can reduce power density and hence increase the power threshold for the onset of non-linear phenomena. The exact MFD can depend on other factors, such as refractive index profile, of course, but for many standard designs a larger diameter core will typically having a fundamental mode have a larger MFD.
FIG. 2 includes a plot or one possible refractive index profile 30 for the core 14 and cladding 16 of the fiber 12 of FIG. 1, and also includes plots of normalized intensity profiles for the fundamental and selected HOMs. (Intensities are normalized for each mode by setting the integral over r·dr·dφ to 1.) As shown in FIG. 2, the optical fiber 12 can support, in addition to the fundamental mode LP01 indicated by reference numeral 36 with a central maxima 48, multiple HOMs, such as, among others, the LP11 mode indicated by reference numeral 38 and the LP02 mode, indicated by reference numeral 42. Note that whereas certain of the HOMs, such as the LP11 mode 38 and, has a central minima or null, the LP02 mode comprises an intensity profile that does not include such a minima or null, and rather is substantially non-zero at its center, as indicated in FIG. 2 by reference numeral 46. A substantially central maxima (which is typically absolute, not just local) is characteristic of modes of zero azimuthal order, at least for lower order modes. Azimuthal order refers to the value of the first subscript in the mode designation.
The HOMs, if excited, can degrade beam quality, as noted above. In many instances the LP02 mode is the most problematic, because it shares with the fundamental mode a intensity profile that has a substantially central maxima such that excitation of the fundamental mode, such as by a simple splice to single mode fiber, could be very likely to excite the LP02 mode. In other instances, the LP11 mode can be problematic.
Consider now the optical fiber 412 of FIG. 3, which can be substantially similar to optical fiber 12 of FIG. 1, and hence can include some or all of the structural features thereof noted above. The optical fiber 412 includes a core 414, a cladding 416 disposed about the core 414, and, optionally, a second region 418 disposed about the cladding 416. However, in addition to the features of the optical fiber 12 of FIG. 1, the optical fiber 412 can further include at least one region 423 spaced from the core 414. In the embodiment shown in FIG. 4 the at least one region 423 comprises a ring core 425. The at least one region 423 can include an absorbing material for absorbing optical energy having a selected wavelength, such as, for example, the wavelength of operation of the optical fiber 412. The absorbing material is preferably of low absorption at the pump wavelength.
FIG. 4 plots one possible refractive index profile 430 for the optical fiber 412 of FIG. 3 and normalized intensity profiles for the fundamental and selected HOMs. The refractive index profile 430 includes an inner section 435 corresponding to the core 414 and outer sections 431 corresponding to the ring core 425. Sections of the refractive index profile between the section 435 (corresponding to the core) and the sections 431 (representative of the ring core 425) correspond to the cladding 416, as do sections outward of the sections 431. Cladding sections are not indicated by reference numerals in FIG. 4. Note that refractive profile section 435 and sections 431 can have substantially the same index difference with respect to the cladding.
Reference numerals 436 and 438 indicate respective fundamental mode LP01 and HOM LP11, all of which are at least relatively similar to, respectively, the LP01 and LP11 modes of FIG. 4 (indicated in FIG. 2 by reference numerals 36 and 38, respectively). Note that FIGS. 2 and 4 differ considerably in that the intensity profile of the LP02 mode of FIG. 2, which had a substantially non-zero central portion 46 of FIG. 2, is largely absent from FIG. 4. Present are the intensity profile 448 of the fundamental mode LP01 that is non-zero at substantially the center of the fiber, in addition to the substantially centrally non-zero intensity profile 446 of the HOM LP02, but the peak of the intensity profile 446 is reduced considerably in comparison with the LP02 intensity profile 46. The LP02 mode of FIG. 2 has been “converted” to a mixed mode 440 of the core 414 and the at least one region 423 (or in any event the LP02 seems to have disappeared and the mixed mode has appeared). Because the mixed mode has appropriate intensity in the ring core 425, it is substantially attenuated, including the intensity portion of the mixed mode present in the core 414, by the absorbing material comprised by the ring core 425. The fundamental mode LP01, however, which is not a mixed mode, is largely unaffected, or at least any increase in attenuation thereof is easily accommodated and/or not overly detrimental in many practical applications.
If the optical fiber apparatus comprises a rare earth material that is to be pumped, it can be desirable to select an absorbing material that tends not to absorb the pump optical energy. Absorbing materials useful with typical rare earths include samarium, praseodymium or terbium. Sm3+ and Pr3+ ions, which have strong absorption around 1064 nm and 1030 nm, respectively, can be useful when the optical fiber apparatus include a rare earth material, such as ytterbium, providing light at around 1060 nm. Ytterbium can be pumped, for example, at 915, 940 or 975 nm, and, as one example, samarium and praseodymium have low absorption at 975 nm. Samarium can also be particularly useful when the rare earth material comprises neodymium or ytterbium and erbium. Terbium can be particularly useful when, for example, pumping the rare earth material thulium at 1576 nm and samarium when pumping at 790 nm. Thulium can provide optical energy at about 2000 nm, as is known in the art. The absorbing material can have concentration of, for example, about 500 ppm, about 1000 ppm, about 1500 ppm about 2000 ppm or greater than about 2000 ppm.
The ring core can comprise a silica-based glass. The silica-based glass can comprise, for example, one or more of a concentration of aluminum, phosphorus, germanium or fluorine. In one example, the ring core comprises a concentration of phosphorus and fluorine, such as in a silica based glass; in another example, the ring core can comprise a concentration of aluminum and can include, for example, a concentration of germanium. In one example, a silica based glass can comprise the aluminum and germanium concentrations, and the absorbing material can comprise samarium. The concentration ranges specified above for the absorbing material can be useful for the Al, Ge, P and F materials noted above.
Modeling indicates the attenuation of the mixed mode can be at least between one and two orders of magnitude greater that than of the fundamental mode, where the attenuation is specified in terms of dB/meter (e.g., tenths of a dB/meter for LP01 compared to tens of dB/meter for the mixed mode to which the LP02 mode is converted). Sec Table I below.







TABLE I







Modeled Losses of Fundamental and


HOMs - Optical Fiber of FIGS. 3 & 4









Modeled losses at 1060



nm (dB/m), ring core


Mode
doped with 1000 ppm Sm











LP01 (fundamental)
0.1586


LP11
1.3708


LP21 (converted to mixed mode)
—


LP(3) = (mixed core/ring core mode 446)
74.8062


LP(4) = (mixed core/ring core mode 440)
38.0906





(Core diameter = 19 μm, Δn = 0.006, ring = 12.5/16.5 μm, Δn = 0.006)






Thus the optical fiber of FIGS. 3 and 4 can provide fiber having the benefit of fundamental mode that has an increased MFD, which reduces the core power density and allows higher power transmission before the onset of non linear phenomena. The optical fiber 412 can be configured and arranged, such as by, for example, selecting one or more of the composition, geometrical shape, location and refractive index profiles of the core 414 and/or ring core 425, such that at the desired wavelength of operation the optical fiber 412 supports a fundamental mode and at least one HOM that is a mixed mode of the core and ring core. The at least one HOM can be attenuated, while the fundamental mode can remain primarily a mode of the core 414.
Often obtaining a higher power laser or optical amplifier means providing a higher power of pump light to the optical fiber 412, and the optical fiber 412 should be able to handle the high power without the fiber degrading, such as by photo darkening. As noted above in the discussion regarding FIG. 1, the cladding 416 can comprise glass, such as, for example, a silica-based glass. It is often desirable that the cladding 416 consist of or consist essentially of glass, such as a silica based glass or pure silica glass, to help ensure that the optical fiber 412 can handle high power levels of light having the pump wavelength, especially when the optical fiber 412 comprises a cladding pumped fiber. The cladding 416 can, in certain aspects of the disclosure, be substantially homogenous and (except for the presence of the at least one region that effects mixed modes and, if desired, stress inducing regions) and hence not be considered as including a “microstructure” for guiding optical energy.
With reference to FIGS. 1-4 above, the modeled fiber has a core 14 having a diameter D of 19 microns having a step index profile 30, with the raised section corresponding to the refractive index of the core 14 and having an index difference Δn of about 0.006 with respect to the cladding 16. Calculations were performed for an operating wavelength of about 1060 nm. The ring core 425 comprised a thickness T of about 4 microns and is spaced from the core 414 by a spacing S of about 3 microns (inner radius of ring core=12.5 microns, outer radius of ring core=16.5 microns). The ring core 425 was considered as doped with about 1000 ppm of an absorbing material (samarium). The index difference or Δn between the outer sections 431 corresponding to the ring core 425 (or more generally corresponding to the at least one region) and the cladding was also considered as about 0.006, but more generally need not be equal to the Δ between the inner section 435 (corresponding to the core 425) and the cladding. The Δn between the inner section 435 and cladding can be less than, substantially equal to, or greater than the Δn between the outer sections 431 and the cladding.
To further facilitate understanding of the disclosure, additional details regarding optical fibers having one or more mixed modes as well one or more modes that are primarily of the core or that at the least one region are provided below. To illustrate the versatility of the disclosure, different fiber designs are considered.
Consider a fiber generally as shown in FIGS. 3 and 4 and having a central core having a diameter of about 21 μm and a core having an inner radius of about 16 μm and an outer radius of about 21 μm. The fiber has a step refractive profile generally as shown in FIG. 4 with a Δn for the central core of about 0.0025. Note, however, the Δn for the ring core, unlike the fiber shown in FIG. 4, is different than that of the core. For the example presently under consideration, the Δn of the ring core is less than the Δn of the central core, and is about 0.00154. The index of refraction of the cladding is about 1.44968 at a wavelength of about 1060 nm. The core, considered individually, has a numerical aperture (NA) of about 0.085 and V-number at 1060 nm of about 5.30.
The fiber can be analyzed as if the ring core is absent, that is, replaced by the cladding material. This is referred to herein as an “individual core” analysis and the modes as “core modes.” However, although the ring core is not present, the overlap integral between the normalized mode intensity and the portion of the fiber the ring core would occupy if present can be calculated. See Table II below.







TABLE II







Waveguide modes in fiber with core diameter


of 21.0 μm & Δn = 0.0025.













Mode Effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral with
integral


mode
Type
index
core
with ring





1
LP01
1.45182
0.9733
0.00040


2
LP11o
1.45129
—
—


3
LP11e
1.45129
0.9267
0.00179


4
LP21o
1.45061
0.8529
0.00693


5
LP21e
1.45061
—
—


6
LP02
1.45040
0.8041
0.01586


7
LP31o
1.44982
0.7165
0.03404


8
LP31e
1.44982
—
—





nclad = 1.44968 at 1060 nm. NA = 0.085. V = 5.30.






Modes are ordered by their effective index value, with higher order modes having lower effective indices. Modes having an effective index greater than that of the cladding are considered guided. Odd and even degenerate modes are indicated by “o” and “e” subscripts, respectively. Only one calculation is made where values are expected to be the same for the odd and even modes. FIG. 5 shows two dimensional intensity distribution maps for the modes of Table II.
The optical fiber can also be analyzed as if the ring core is present and the core is absent. Table III presents the results of such an analysis, and FIG. 6 shows intensity maps for the modes of Table III. This analysis is referred to herein as “individual ring core” analysis and the modes as “ring core” or “ring” modes.







TABLE III







Waveguide modes in fiber with ring r1/r2 = 16.0/21.0


μm & Δn = 0.00154.













Mode effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral mode
integral


mode
type
index
with core
with ring














1
R01
1.45041
0.00137
0.67097


2
R11o
1.45037
—
—


3
R11e
1.45037
0.00065
0.67395


4
R21o
1.45029
0.00023
0.67764


5
R21e
1.45028
—
—


6
R31o
1.45013
0.00062
0.67857


7
R31e
1.45013
—
—


8
R41o
1.44994
0.00002
0.67354


9
R41e
1.44993
—
—


10
R51o
1.44969
0.00000
0.65995


11
R51e
1.44968
—
—





nclad = 1.44968 at 1060 nm.






Table IV below tabulates modeling analysis of the actual optical fiber, that is, where the central and the ring core are both present. FIG. 7 shows two dimensional intensity distribution maps for the modes of the actual fiber analysis of Table IV.







TABLE IV







Waveguide modes in fiber with core diameter of 21.0 μm & Δn = 0.0025


and ring r1/r2 = 16.0/21.0 μm & Δn = 0.00154.













Mode Effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral with
integral


mode
Type
index
core
with ring














1
LP01
1.45182
0.97237
0.00045


2
LP11o
1.45129
—
—


3
LP11e
1.45129
0.92049
0.00343


4
LP21o
1.45063
0.77994
0.04831


5
LP21e
1.45063
—
—


6
(LP02-R01)m1
1.45051
0.38302
0.33125


7
R11o
1.45037
—
—


8
R11e
1.45037
0.01737
0.66734


9
(LP02-R01)m2
1.45026
0.47590
0.34334


10
R21o
1.45025
0.00664
0.64030


11
R21e
1.45024
—
—


12
R31o
1.45016
0.06734
0.60407


13
R31e
1.45016
—
—


14
R41o
1.44995
0.00741
0.66361


15
R41e
1.44993
—
—


16
LP31o
1.44976
0.68473
0.08991


17
R51e
1.44968
0.00234
0.65680





nclad = 1.44968 at 1060 nm.






This analysis is referred to as an “actual fiber” analysis, and the modes as “actual fiber modes” or “actual modes.”
The data presented herein in the various FIGURES and Tables is now discussed to demonstrate the approach used to classify modes as mixed or primarily of a region and to determine how to identify the modes that have mixed to form a particular mixed mode. To better demonstrate the modal intensity distribution maps, the analysis used to generate the maps of FIGS. 5-10 does not include the effect of the absorbing material, such as the absorbing material comprised by the ring 425 of FIG. 3.
The following criteria represents one way to establish that a mode identified in an actual fiber analysis is a mixed mode wherein particular modes identified in individual analyses have mixed: (1) the intensity map for the actual fiber mixed mode appears to be a combination of the individual intensity maps of the individual modes that are mixing to form the mixed mode; (2) the modes that are mixing are of the same azimuthal order; and (3) whereas the modes considered individually might not have intensities in the core and ring core that are significantly of the same order of magnitude (as can be indicated by the overlap integrations being generally the same), for the mixed mode intensities are significantly of the same order. “Significantly of the same order,” for the purposes of (3), means that the larger quantity is no greater than about ten (10) times the smaller quantity. Finally, to facilitate mixing, (4) the individual modes of the core and of the at least one region should have effective indices that are not too disparate. An exact match of effective indices is not understood to be required, however.
With reference to Tables II and III, note that the effective refractive index of the LP02 core mode (1.45040) is substantially the same as that of the effective refractive index of the R01 ring core mode (1.54041). (Note that the difference, 1×10−5, is certainly considered narrower than the full ambit of “substantially the same.”) Furthermore, both the modes are of the same (zero in this case) azimuthal order. In addition, whereas FIGS. 7A and 7B include intensity maps that bear a strong resemblance to each intensity maps for individual core modes or intensity maps for individual ring modes, FIGS. 7A and 7B do not include an analog to the LP02 core mode. For example, the intensity maps for modes LP(1)-LP(5) of the actual fiber analysis of FIG. 7A each bear a strong resemblance to the intensity maps for the modes LP(1)-LP(5) of the core analysis of FIG. 5. The LP(6) mode of the actual analysis of FIG. 7A, however, does not resemble any one core mode intensity map of FIG. 5 or ring core mode intensity map of FIG. 6, but rather appears to be a combination of the LP02 core mode (LP(6) of the individual core analysis of FIG. 5) and the R01 (LP(1) of the individual ring core analysis of FIG. 6).
The overlap integrals of Tables II-IV confirm that the LP(6) actual fiber mode is a mixed mode of the LP02 core mode and the R01 ring core mode. Whereas the overlap for the R01 ring core mode with the ring core is approximately 500 times the overlap with the core, and the overlap of LP02 core with the core is approximately 50 times the overlap with the ring core, the ratio of the larger overlap value to the smaller overlap value for the LP(6) actual mode is now reduced to 1.1563, indicating a close to even distribution between the core and ring core.
Based on a similar analysis for the LP(9) actual mode, it is therefore concluded that the LP02 core mode and R01 ring mode mix to form two mixed modes—the LP(6) and LP(9) modes of Table IV and FIG. 7A. Inclusion of absorbing material in the ring core should suppress these mixed modes, yet leave the LP(1), or LP01, fundamental actual fiber mode far less attenuated, as it primarily a mode of the core (as are the core modes LP(2)-LP(5), which modes include modes corresponding to the LP11 and LP21 odd and even modes of the core).
Note that certain actual HOMs can have intensity profiles that appear to be a mixture core and cladding modes, yet that are not considered to be mixed modes. For example, LP(10) of FIG. 7B appears to be a combination of the LP(4), or R21o, ring core mode of FIG. 6 and the LP(4), or LP21o, core mode of FIG. 5. However, the effective refractive indices of the R21o mode (1.45029) is not that close to that of the LP21o mode (1.45069), and most importantly the values for the overlap integrals for the LP(10) actual fiber HOM differ by a factor of about 100 (see Table IV), with the vast majority of the intensity overlapping with the ring core. Accordingly, although the azimuthal order of the R21o and LP21o modes is the same, and both are of the odd orientation, they are not considered to be mixing to form a mixed mode, despite the LP(10) actual fiber intensity distribution appearing to be a direct combination of R21o ring core mode and the LP21o cladding mode. The LP(10) actual fiber mode appears to be primarily a mode of the ring core, despite the added features of intensity map. A mode such at the LP(10) is referred to herein as a “composite mode,” because of the appearance of the intensity map, but not a mixed mode.
Accordingly, one approach to mixing a selected mode of region, for example, a selected mode of the core (perhaps so that it can be suppressed) is to design the ring core such that a ring mode of the same azimuthal order as the targeted core mode has a similar effective index to that of the targeted core mode. Analysis of the actual fiber data can confirm the existence of the mixed mode. Iterations can be performed as necessary, varying one or more of the geometry of the core and ring cores, spacing therebetween, refractive index profiles, etc. to arrive at the design where the desired modes mix to form a desired mixed mode. The effect of an absorbing material comprised in one of the regions (e.g., the core or ring core) can be ascertained to establish suppression of a mode or to further confirm that a mode is a mixed mode or primary mode. For example, absorbing material in the ring core should affect mixed modes and modes that are primarily of the ring core, but typically do not substantially affect modes that are primarily modes of the core.
In the above examples, the LP02 core mode is mixed with ring core modes to form an actual mode, but no attempt was made to mix the LP11 core modes. Data demonstrating an optical fiber wherein the LP11 core modes mix with ring core modes to form mixed modes are presented in Tables V-VII and FIGS. 8-10. The procedure follows that described above in conjunction with Tables II-IV and FIGS. 5-7B. In this example the optical fiber has a core having a diameter of about 14.0 μm and Δn of about 0.003 with respect to the cladding, which, as in the previous example, has an index of refraction at 1060 nm of 1.44968. The core, considered alone, has a numerical aperture (NA) of about 0.093 and a V-number of about 3.87. The optical fiber includes a ring core having an inner radius of about 14 μm and an outer radius of about 20 μm, and a Δn with respect to the cladding of about 0.00215. Again, note that the Δn of the core and ring core differ.
Table V presents the individual core analysis, and FIG. 8 shows the corresponding intensity maps for the modes presented in Table V; Table VI presents the ring core considered individually, and FIG. 9 shows the corresponding intensity maps for the modes of table VI; and Table VII presents actual fiber mode data, with FIG. 10 presenting the intensity distribution maps for actual fiber modes of the Table VII.







TABLE V







Waveguide modes in fiber with core diameter


of 14.0 μm & Δn = 0.003.













Mode Effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral with
integral


mode
Type
index
core
with ring





1
LP01
1.45198
0.94157
0.00007


2
LP11o
1.45096
—
—


3
LP11e
1.45096
0.83074
0.00098


4
LP21o
1.44978
0.58417
0.03368


5
LP21e
1.44977
—
—





nclad = 1.44968 at 1060 nm. NA = 0.093. V = 3.87.













TABLE VI







Waveguide modes in fiber with ring r1/r2 = 14.0/20.0


μm & Δn = 0.00215.













Mode Effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral with
integral


mode
Type
index
core
with ring














1
R01
1.45100
0.00070
0.80418


2
R11o
1.45096
—
—


3
R11e
1.45096
0.00041
0.80945


4
R21o
1.45086
0.00019
0.81169


5
R21e
1.45085
—
—


6
R31o
1.45068
0.00007
0.81247


7
R31e
1.45068
—
—


8
R41o
1.45044
0.00002
0.80934


9
R41e
1.45043
—
—


10
R51o
1.45014
0.00001
0.79959


11
R51e
1.45014
—
—


12
R61o
1.44978
0.00000
0.78037


13
R61e
1.44978
—
—





nclad = 1.44968 at 1060 nm.













TABLE VII







Waveguide modes in fiber with core diameter of


14.0 μm & Δn = 0.003 ring r1/r2 =


14.0/20.0 μm & Δn = 0.00215.













Effective
Mode overlap
Mode overlap


LP
Mode
refractive
integral with
integral


mode
Type
index
core
with ring














1
LP01
1.45198
0.94077
0.00038


2
(LP11-R11)m1o
1.45101
—
—


3
(LP11-R11)m1e
1.45101
0.39740
0.39548


4
R01
1.45100
0.00258
0.80793


5
(LP11-R11)m2o
1.45091
—
—


6
(LP11-R11)m2e
1.45091
0.42913
0.42612


7
R21o
1.45086
0.00146
0.80846


8
R21e
1.45085
—
—


9
R31o
1.45068
0.00027
0.81170


10
R31e
1.45068
—
—


11
R41o
1.45044
0.00005
0.80919


12
R41e
1.45043
—
—


13
R51o
1.45014
0.00001
0.79953


14
R51e
1.45014
—
—


15
R61o
1.44978
0.00073
0.77957


16
R61e
1.44978
—
—


17
LP21o
1.44976
0.60800
0.01824





nclad = 1.44968 at 1060 nm.






From Tables V and VI, note the LP11o and LP11e core modes have substantially the same effective indices as the R11e and R11o ring core modes. The modes, of course, are of the same azimuthal order (azimuthal order is 1 in this case). The odd modes and even modes each mix to form two mixed actual fiber modes, resulting in a total of four mixed modes. That is, LP11o mixes with R11o to form the mixed modes LP(2) and LP(5) of Table VII and FIG. 10 (labeled (LP11-R11)m1o and (LP11-R11)m2o, respectively). Similarly, the LP11e core mode mixes with the R11e ring core mode to form the LP(3) and LP(6) mixed actual fiber modes indicated in Table VII and FIG. 10 (labeled LP11-R11)m1e and (LP11-R11)m2e, respectively). The mode field distribution maps of the actual fiber LP(2) and LP(5) modes appear to be logical combinations of the LP11o and R11o modes, and the mode field distribution maps for the LP(3) and LP(6) modes appear as one would expect for combinations of the LP11e and R11e modes. Consideration of the overlap integrals also supports the formation of the identified mixed modes. The overlap integrals for the LP(2), LP(3), LP(5) and LP(6) actual fiber modes have ratios of the higher to lower values on the order of 1, indicating nearly equal distribution in the core and ring core.
Note that the LP21 core modes (i.e., the LP21o and LP21e core modes) of Table V have effective refractive indices (1.44978, 1.44979) that are nearly identical those of the R61 ring core modes (1.44978) of Table VI. However, full consideration of all data presented in Tables V-VII and FIGS. 8-10 indicates that the LP21 and R61 modes, though “index matched,” do not mix to form a mixed mode, and remain primarily modes of the core and ring core, respectively, in the analysis of the actual fiber. Consider also that the effective indices of the Applicants consider that this is because the LP21 core modes are of significantly different azimuthal order (order=2) than the 6th order R61 ring core modes. This insight—that matching azimuthal order can greatly facilitate selecting modes for mode mixing and is, at least in some circumstances, more important than strict effective index matching and can be a condition for modes to mix—does not appear to be appreciated by the prior art.
Although examples provided herein have focused on preserving the integrity of the fundamental mode and selective suppression of certain HOMs, the teachings herein could be applied, in certain circumstances, to favoring a selected HOM over another HOM at the expense, perhaps, of the fundamental mode. Such an approach is within the scope of the disclosure. It is also considered within the scope of the present disclosure to have both the core LP11 and LP02 modes mix with ring core modes to form mixed modes. The design may include two ring cores, one surrounding another, where the core LP11 mode mixes with a mode of one ring core and the core LP02 mode mixes with a mode of the other ring core. In another approach, the core LP02 can mix with the ring R02 ring core mode and the core LP11 mix with a mode of the ring core having a lower order than the R02 mode.
Thus, according to one aspect of the disclosure, Applicant has realized that it may not be necessary to address all HOMs according to the same proscription. Certain HOMs, in many applications, are much more likely to be problematic than others, and, accordingly it may not be as important to address those that are less important in the same manner as those that are more problematic. A splice from an SM fiber to a MM fiber is much more likely to excite a HOM having an intensity distribution map that is also substantially central and azimuthally symmetric than other HOMs that are not substantially central and azimuthally symmetric. For example, such a splice is considered more likely to excite the LP02 mode shown in FIG. 3 than the LP11 mode.
V-number and NA are parameters that are often specified for an optical fiber. Unless otherwise specified, V-number and NA of a core refer to the V-number and the NA of the core considered individually, that is, without consideration of the at least one region that does contribute to the formation of mixed modes. It is noted that the a fiber can be “microstructured,” that is, can include features, such as an array of longitudinally extending index modified regions (e.g., an array of voids having an index of refraction different than that of the material defining the voids) that provide a photonic bandgap effect or that macroscopically change the average index of the cladding via a weighted average analysis of the indices of refraction of the silica regions and index modified regions. In the latter instance guidance by the core is still considered to be by total internal reflection (TIR). Microstructured fibers are considered to be within the scope of the present disclosure. For example, in one microstructured design, the “ring” can be formed by leaving out the voids in an annular region disposed about the core. In this instance, analysis of the core individually would include the cladding including the voids (and with the ring including the otherwise missing voids), and a mode considered to be guided “primarily” by the core would of course be affected by the voids.
In another example, an optical fiber according to the present disclosure can comprise a core, a cladding disposed about the core, and optionally a region disposed about the cladding. The optical fiber can include at least one region spaced from the core, where the at least one region can comprise a plurality of satellite regions, which can be individual longitudinally extending voids or index modified regions arranged in a ring or other configuration.
In certain aspects, an optical fiber according to the present disclosure can have a core having a V-number at the wavelength of operation of the fiber of no less than 4.0; no less than 5.0; no less than 6.0; no less than 7.0; or no less than 7.5. In certain aspects of the disclosure the V-number can be from 3.0 to 5.0; from 5.0 to 7.0; or from 7.0 to 10.0. In other aspects of the disclosure, the V-number is not greater than 3, not greater than 3.5, not greater than 4, not greater than 4.5, not greater than 5, or not greater than 5.5.
In other aspects of the disclosure, the core of a fiber can have an NA of no less than 0.12, no less than 0.15, no less than 0.16, or no less than 0.17 at the wavelength of operation of the optical fiber. The NA of the core can be about 0.17.
In additional aspects of the disclosure, the core of a fiber can have a diameter of at least 15 microns; at least 20 microns; at least 25 microns; at least 30 microns; at least 35 microns; at least 40 microns; or at least 50 microns.
Combinations of the foregoing aspects are within the scope of the disclosure, as is appreciated by one of ordinary skill apprised of the disclosure herein. Additional embodiments of optical fiber are now described.
FIG. 11 schematically illustrates a cross section of an optical fiber 1012 according to the disclosure that includes, in addition to a core 1014 and at least one region 1023 spaced from the core 1014, a pair of longitudinally, or axially, extending stress inducing regions, indicated by reference numerals 1033A and 1033B. The stress inducing regions 1033A and 1033B can help induce selected birefringence in the optical fiber, such as, for example, via the stress-optic effect. The stress inducing regions can have a thermal coefficient of expansion selected to be different than that of the material of the fiber disposed about the stress inducing regions such that when fiber cools after being drawn stresses are permanently induced. Birefringence refers to at least a region of the fiber, such as, for example, the core 1014, having a substantially different refractive index for one polarization of light than for the orthogonal polarization of light. The fiber 1012 can be a polarization maintaining fiber or a polarizing fiber, depending, at least in part, on the choice of one or more of composition, shape and location of the stress inducing regions. The index of refraction of the stress inducing regions 1033 can be adjusted, via the use of various dopants, including, for example, those noted above, to be lower than that of the cladding 1016, substantially matched to that of the cladding 1016, or even to be higher than that of the cladding 1016.
FIG. 12 shows a perspective view schematically illustrating one example of an optical fiber apparatus according to the disclosure. The optical apparatus 1110 can include a first optical fiber 1112 that can include, as described above, a core 1114, a cladding 1116 disposed about the core 1114, and at least one region 1123 spaced from the core 1114 for supporting selected mixed modes with the core 1114. The optical fiber 1112 can have a wavelength of operation and can include a rare earth material for providing optical energy having the wavelength of operation responsive to the optical fiber 1112 receiving pump optical energy having a pump wavelength. The optical fiber apparatus 1110 can include a second optical fiber 1135 located alongside the first optical fiber, and the second optical fiber can include at least a core 1137. The optical fiber apparatus 1110 can include a common cladding 1145 disposed about the first and second optical fibers. The common cladding 1145 can be constructed and arranged so as to tend to confine optical energy to the core 1137 of the second optical fiber 1135 for guidance by the core 1137. The second optical fiber 1135 can propagate pump optical energy and the first and second optical fiber located alongside one another, as shown in FIG. 10, such that the pump optical energy couples to the first optical fiber 1112 for optically pumping the rare earth material comprised by, for example, the core 1114 of the first optical fiber 1112. The first and second optical fibers, 1112 and 1135, respectively, can be drawn together within the common cladding 1145. The optical apparatus 1110 can be constructed and arranged such that the first and second optical fibers can be accessed individually at the ends of a length of the optical fiber apparatus 1110 so as to, for example, couple pump optical energy to the second optical fiber for subsequent coupling to the first optical fiber and to deliver a signal to and/or extract a signal from the core 1114 of the first optical fiber 1112.
An optical fiber apparatus can be configured, according to one aspect of the disclosure, as a laser. Such as laser can comprise at least one reflector, which can comprise a grating, such as, for example, a Bragg grating formed via the selective application of actinic radiation to, for example, a photosensitive section of optical fiber. The laser can comprise a second reflector. One of the reflectors is usually less reflective than the other of the reflectors, as is known in the art. Two spaced reflectors can form a laser cavity therebetween. The laser can also be configured as a distributed feedback (DFB) laser, and can use a distributed reflector, typically in the form of one grating having a phase change therein, and can provide narrow linewidth light. A laser can also be configured in a master oscillator-power amplifier (MOPA) arrangement, where a master oscillator, such as a diode or fiber laser, seeds an optical fiber amplifier. Optical fiber apparatus according to the disclosure can include a fiber optical coupler for coupling pump light to the optical fiber apparatus, as well as a source of pump optical energy, which can comprise one or more pump diodes.
As noted above, an optical fiber can comprise a rare earth material for providing light of a first wavelength responsive to the fiber receiving (e.g., being “pumped by”) light of a second wavelength (e.g., “pump light”). “Rare earth material,” as used herein, means one or more rare earths, typically included in the fiber in the form of rare earth ions. The rare earths can be selected by those of ordinary skill in the art of the field of pumped fibers, for example from the Lanthanide group of elements in the periodic table (materials having the atomic numbers 57-71). The optical fiber can be pumped as shown in FIG. 8 and discussed above. Also, the optical fiber can be “end-pumped” as is known in the art, and can include a second, or “pump” cladding for propagating the pump light delivered to the optical fiber via the end pumping.
The refractive index profiles shown in the foregoing FIGURES are idealized. Actual refractive index profiles measured on a preform or from an actual optical fiber drawn from the preform can include other features, as is well known in the art, such as rounded edges between sections and the signature “dip” in the index of refraction of the core due to the burnoff of dopants in the core during the collapse stage of the Modified Chemical Vapor Deposition (MCVD) process (assuming that the MCVD process is used to fabricate the optical fiber preform). Also, some of the sections of the refractive index profile corresponding to a particular region of the fiber are drawn to portray the index of refraction as substantially constant for the region. This need not be true in all practices of the disclosure. As is well known in the art, the index of refraction of a region of a fiber, such as the core of a fiber, need not be constant, and can be varied according to a predetermined function to provide a particular result. For example, it is known in the art to provide a core comprising a graded refractive index profile, where the profile corresponds to a parabola or other suitable function.
Several embodiments of the invention have been described and illustrated herein. Those of ordinary skill in the art will readily envision a variety of other means and structures for performing the functions and/or obtain the results or advantages described herein and, each of such variations or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art would readily appreciate that all parameters, dimensions, materials and configurations described herein are meant to be exemplary and that actual parameters, dimensions, materials and configurations will depend on specific applications for which the teaching of the present disclosure is used.
Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments of the invention described herein. It is therefore to be understood that the foregoing embodiments are presented by way of example only and that within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. The present invention is directed to each individual feature, system, material and/or method described herein. In addition, any combination of two or more such features, systems, materials and/or methods, if such features, systems, materials and/or methods are not mutually inconsistent, is included within the scope of the present invention.
In the claims as well as in the specification above all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving” and the like are understood to be open-ended. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the U.S. Patent Office Manual of Patent Examining Procedure §2111.03, 7th Edition, Revision.
The phrase “A or B” as in “one of A or B” is generally meant to express the inclusive “or” function, meaning that all three of the possibilities of A, B or both A and B are included, unless the context clearly indicates that the exclusive “or” is appropriate (i.e., A and B are mutually exclusive and cannot be present at the same time).
It is generally well accepted in patent law that “a” means “at least one” or “one or more.” Nevertheless, there are occasionally holdings to the contrary. For clarity, as used herein “a” and the like mean “at least one” or “one or more.” The phrase “at least one” may at times be explicitly used to emphasize this point. Use of the phrase “at least one” in one claim recitation is not to be taken to mean that the absence of such a term in another recitation (e.g., simply using “a”) is somehow more limiting. Furthermore, later reference to the term “at least one” as in “said at least one” should not be taken to introduce additional limitations absent express recitation of such limitations. For example, recitation that an apparatus includes “at least one widget” and subsequent recitation that “said at least one widget is colored red” does not mean that the claim requires all widgets of an apparatus that has more than one widget to be red. The claim shall read on an apparatus having one or more widgets provided simply that at least one of the widgets is colored red.

## Claims

1. Optical fiber apparatus having a wavelength of operation, comprising:
an optical fiber, said optical fiber including
a core comprising an active material for providing light having the operating wavelength responsive to said optical apparatus receiving pump optical energy having a pump wavelength;
a cladding disposed about said core;
at least one region spaced from said core; and
wherein said optical fiber is configured and arranged such that at said wavelength of operation said optical fiber can propagate a plurality of modes and wherein said optical fiber comprises a fundamental mode that is primarily a mode of said core and at least one higher order mode (HOM) that is a mixed mode of a selected mode of said core and of a selected mode of said at least one region.

2. The optical fiber apparatus of claim 1 wherein said active material comprises a rare earth material.

3. The optical fiber apparatus of claim 2 wherein said at least one region comprises an absorbing material for absorbing optical energy having the wavelength of operation and, if absorptive of optical energy having the pump wavelength, has a higher absorption for optical energy having the wavelength of operation than for optical energy having the pump wavelength.

4. The optical fiber apparatus of claim 1 wherein said at least one HOM comprises an HOM of zero azimuthal order.

5. The optical fiber apparatus of claim 1 wherein said selected mode of said core comprises the LP02 core mode.

6. The optical fiber apparatus of claim 1 wherein said at least one HOM comprises an HOM of non-zero azimuthal order.

7. The optical apparatus of claim 1 wherein said selected mode of said core comprises the LP11 mode.

8. The optical fiber apparatus of claim 1 wherein said optical fiber comprises at least one longitudinally extending stress inducing region having a thermal coefficient of expansion that is different from material of said optical fiber disposed about said stress inducing region, said stress inducing region for increasing the birefringence of said optical fiber.

9. The optical fiber apparatus of claim 1 wherein said at least one region comprises a ring-shaped region.

10. The optical fiber apparatus of claim 1 wherein said apparatus is configured as a fiber laser.

11. Optical fiber apparatus having a wavelength of operation, comprising:
an optical fiber, said optical fiber including
a core;
a cladding disposed about said core;
at least one region spaced from said core;
said optical fiber being configured and arranged such that at the wavelength of operation said optical fiber comprises a first mode that is primarily a mode of one of said core and said at least one region and a second mode that is a mixed mode of a selected mode of said core and of a selected mode of said at least one region; and
wherein said selected mode of said core and said selected mode of said at least one region are of the same azimuthal order.

12. The optical fiber apparatus of claim 11 wherein same order comprises the zero order.

13. The optical fiber apparatus of claim 11 wherein said first mode is primarily a mode of said core and comprises the fundamental mode and said second mode comprises a higher order mode (HOM).

14. Optical fiber apparatus having a wavelength of operation at which said optical apparatus is configured to propagate optical energy, comprising:
an optical fiber comprising
a core;
a cladding disposed about said core;
at least one region spaced from said core;
said optical fiber being configured and arranged such that at said wavelength of operation said fiber supports a plurality of modes wherein the fundamental mode is primarily a mode of said core, at least one higher order mode (HOM) is a mixed mode of a selected mode of said core and a selected mode of said at least one region, and wherein at least another HOM having a lower mode order than said at least one HOM is not a mixed mode and is primarily a mode of said or said at least one region; and

wherein “lower mode order” means that the at least another mode is nearer in terms of effective index to the fundamental mode than the at least a first HOM.

15. The optical fiber apparatus of claim 14 wherein said at least a first HOM comprises an HOM of zero azimuthal order.

16. The optical fiber apparatus of claim 15 wherein said selected mode of said core comprises the LP02 core mode.

17. The optical fiber apparatus of claim 15 wherein said selected mode of said at least one region comprises a mode of zero azimuthal order.

18. The optical fiber apparatus of claim 13 wherein said at least another HOM comprises an HOM of non-zero azimuthal order.

19. The optical fiber apparatus of claim 18 said at least another HOM comprises the LP11 core mode.

20. The optical fiber apparatus of claim 13 wherein said optical fiber comprises a rare earth material for providing optical energy having the operating wavelength responsive to said optical apparatus receiving optical energy having a pump wavelength.

21. The optical fiber apparatus of claim 14 wherein said core comprises a diameter of at least 15 microns, a numerical aperture of no greater than 0.10 and a V-number at said operating wavelength of greater than 3.

22. The optical fiber apparatus of claim 14 wherein said core comprises a diameter of at least 15 microns, a numerical aperture of no less than 0.13 and a V-number at said operating wavelength of greater than 3.