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.
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 xe2x80x9ccladding-pumpedxe2x80x9d 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 28A 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. 2A 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. 2B 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 xe2x80x9cDxe2x80x9d-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. 2D. In yet further examples, the outer circumference of the inner cladding has a xe2x80x9cstarxe2x80x9d shape, as disclosed in U.S. Pat. No. 5,949,941, issued Sep. 7, 1999 to DiGiovanni and illustrated in FIG. 2E. 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. 2F. 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.
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 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 asymmetrically about the inner cladding layer for applying stress to the fiber for enhancing the absorption of pump radiation by the core. The asymmetrically disposed layer can include a hard polymer. The second cladding layer can be disposed asymmetrically about the inner cladding. The cladding-pumped optical fiber can include at least one bend.
In yet a further 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 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 compressive layer that contracts after being applied to the fiber and that is disposed about the first cladding layer for applying stress to the fiber. The compressive layer can be disposed asymmetrically about the inner cladding and/or can be disposed about both the first and second cladding layers. The cladding-pumped optical fiber can include at least one bend.
In each of the foregoing practices of the invention, the optical apparatus can include an external means for applying stress to the cladding-pumped optical fiber. The external means can include a mechanical structure about which the cladding-pumped optical fiber is bent.
Thus while the prior art typically teaches away from certain features of the invention as claimed above, such as the inclusion of granular matter, normally considered a contaminant, in a layer of an optical fiber and the application of stress to the fiber, it is understood, according to the invention, that including such features can provide an improved optical apparatus including a cladding-pumped optical fiber.