Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-06-06
2003-09-23
Kim, Robert H. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S126000, C385S144000, C359S341100
Reexamination Certificate
active
06625363
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical fibers and, more particularly, to an optical fiber having an inner cladding for receiving pump radiation that is to be absorbed by active material in the core of the optical fiber.
BACKGROUND
Optical fiber lasers and amplifiers are known in the art. In such lasers and amplifiers, rare earth materials disposed in the core of the optical fiber laser or amplifier absorb pump radiation of a predetermined wavelength and, responsive thereto, provide or amplify light of a different wavelength for propagation in the core. For example, the well-known erbium doped fiber amplifier (EDFA) receives pump radiation having a wavelength of 980 or 1480 nanometers (nm) and amplifies an optical signal having a wavelength in the 1550 nm region and propagating in the core.
In such optical fiber lasers and amplifiers, the pump radiation can be introduced directly to the core, which can be difficult due to the small size of the core, or can be introduced to the cladding layer surrounding the core and absorbed by the core as the rays propagating in the cladding layer intersect the core. Lasers and amplifiers wherein the pump radiation is to be introduced to the cladding layer are known as “cladding-pumped” optical devices, and facilitate the scale-up of lasers and amplifiers to higher power systems.
FIG. 1
illustrates an optical fiber having a core
20
, an inner, or pump, multimode cladding layer
22
, and an outer cladding layer
24
. The inner cladding layer
22
confines light rays
26
, which represent the light generated or amplified in the core
20
, to the core
20
. Similarly, the outer cladding
24
confines light rays
28
, which represent pump radiation propagating in the inner cladding
22
, to the inner cladding
22
. Note that the rays
28
periodically intersect the core
20
for generating or amplifying the light in the core
20
, represented by the rays
26
. Because the inner cladding
22
is multimode, many rays other than those shown by reference numeral
28
can propagate in the inner cladding
22
.
Absorption per unit length is a useful figure of merit for evaluating a cladding-pumped optical fiber laser or amplifier. It is typically desirable that the amplifier or laser has a high absorption per unit length, indicating that the pump radiation frequently intersects the core
20
. It has been determined by various researchers over the years that a standard circular fiber geometry, such as is desirable when fabricating an optical fiber for transmission over substantial distances, does not optimally promote absorption by the core
20
of the radiation pumped into the cladding layer
24
. Unfortunately, some rays (referred to in the art as skew rays) of the pump radiation
28
can essentially propagate down the optical fiber while spiraling around the core without substantially intersecting the core
20
. See
FIG. 1B
, where pump radiation rays
28
A do not intersect the core
20
. This leads to a low absorption per unit length of the optical fiber device, and hence detracts from the performance of the optical fiber laser or amplifier.
The prior art teaches two approaches for enhancing the intersection of the pump radiation with the core and hence raising the absorption per unit length of the optical fiber amplifier or laser. In the first approach, the core is relocated to intersect more of the rays of the pump radiation. For example, as shown in FIG.
2
A and disclosed in U.S. Pat. No. 4,815,079, issued Mar. 21, 1989 to Snitzer et al., the core can be offset from the center of the optical fiber so as to enhance the intersection of pump light with the core.
In the second approach, the shape of the outer circumference of the inner, or pump, cladding layer is modified to scatter more rays towards the core so as to intersect with the core. For example, as shown in FIG.
2
B and also disclosed in the '079 patent to Snitzer, the inner cladding can have a rectangular outer circumference. See also
FIG. 2C
, where the inner cladding has a “D”-shaped outer circumference that includes a flat section, as disclosed in U.S. Pat. No. 5,864,645, issued Jan. 26, 1999 to Zellmer et al. In yet another example of this approach, the outer circumference of the cladding is shaped as a polygon, such as a hexagon, as disclosed in U.S. Pat. No. 5,533,163, issued Jul. 2, 1996 to Muendel and shown in FIG.
2
D. In yet further examples, the outer circumference of the inner cladding has a “star” shape, as disclosed in U.S. Pat. No. 5,949,941, issued Sep. 7, 1999 to DiGiovanni and illustrated in FIG.
2
E. See also WO 99/30391, published Jun. 17, 1999, disclosing an optical fiber having a core, inner and outer claddings, and a series of circularly shaped perturbations or irregularities formed in the otherwise circular outer boundary of the inner cladding, as shown in FIG.
2
F. The optical fiber is drawn from a preform having rods inserted into holes drilled into the preform.
The prior art approaches discussed above can have disadvantages. For example, the resultant fibers can be difficult to splice to a fiber having a standard, circular geometry in a manner that provides for an acceptably low loss of light, as is often required in a practical application. The offset core fiber of
FIG. 2A
can be particularly difficult to splice. Furthermore, designs shown in
FIGS. 2B-2F
, wherein the outer circumference of the inner cladding is shaped, can require shaping of the preform from which the fiber is drawn. Shapes that include flat areas, such as the polygon design discussed above, can be difficult and/or time consuming, and hence more expensive, to fabricate. The flat areas are typically first machined into the preform from which the optical fiber is drawn. In addition, shaped areas of the preform tend to deform and change shape when the fiber is drawn at the most desirable temperatures. Accordingly, often the draw temperature is reduced to preserve the desired shape of the outer circumference of the cladding. A reduced draw temperature typically produces optical fibers having higher attenuation and lower mechanical strength.
Accordingly, although the approaches described above may represent an improvement in the art, a cladding-pumped fiber that addresses one or more of the foregoing disadvantages and drawbacks of the prior art approaches would be a welcome advance in the art.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an optical apparatus that includes a cladding-pumped optical fiber. The cladding-pumped optical fiber includes the following: a core including a material having a first index of refraction and an active material; a multimode inner cladding layer for receiving pump radiation, where the inner cladding layer is disposed about the core and includes material having a second index of refraction that is less than the first index of refraction; a second cladding layer disposed about the inner cladding layer, where the second cladding layer includes material having a third index of refraction that is less than the second index of refraction; and a layer disposed about the inner cladding layer, where the layer includes granular matter for applying stress to the fiber for enhancing the absorption of pump radiation by the core. The second cladding layer can include granular matter. The cladding-pumped optical fiber can include a third layer disposed about the second cladding layer, where the third layer includes granular matter. The cladding-pumped optical fiber can include at least one bend.
In another aspect, the present invention provides an optical apparatus comprising a cladding-pumped optical fiber. The cladding-pumped optical fiber includes the following: a core including a material having a first index of refraction and an active material; a multimode inner cladding layer for receiving pump radiation, where the inner cladding layer is disposed about the core and includes material having a second index of refraction that is less than the first index of refraction; a second cladding layer disposed about the inner
Carter Adrian
Jacobson Nils J.
Tankala Kanishka
Kim Robert H.
Nufern
Rainville Peter J
Suchecki Krystyna
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