1. Field of the Disclosure
The disclosure relates to an optical fiber configured with a plurality of optical components which are selectively coupled together to define the fiber's composite mosaic core with a controllable refractive index. More specifically, the disclosure relates to optical fibers configured with the desired refractive index and dopant profiles.
2. Discussion of Prior Art
There exist several methods of manufacturing optical fibers. However, the procedure always includes first preparing a preform, i.e. a rod of vitreous silica of very high purity, and then drawing the fiber from the preform. A variety of light non-amplifying dopants are added to modify the refractive index of the base silica. The selection of non-amplifying dopants depends on the desired index difference. For example, boron and fluorine can be used to reduce the refractive index of silica, while phosphorous and germanium can be used to increase it.
Active optical fibers are obtained by doping the silica with ions of rare earth elements, e.g. erbium (Er), ytterbium (Yb), neodymium (Nd), holmium (Ho), samarium (Sm), thulium (Tm) and others. These dopants are generally used together with another dopant, such as phosphate or alumina, for the purpose of minimizing the phenomenon of rare earth ions “clustering” well known to one of ordinary skill.
The efficiency of active optical fibers depends on interaction between light and matter. The efficiency can be increased in two ways. Firstly, by increasing energy density in the core of the optical fiber by reducing its diameter. This makes it necessary to have a large index difference and a high concentration of dopants other than rare earth ions. Alternatively, interaction between light and matter can be improved by confining the rare earth ions in the small central region of the core, i.e. designing the desired dopant profile.
A problem then arises of the dopant diffusion which makes it difficult to obtain the above-mentioned objectives. In a widely used technique of internal modified chemical vapor deposition (MCVD), the diffusion problem manifests itself practically throughout the process including fiber drawing stages. While MCVD has many advantages over other methods, one of ordinary skill in fiber fabrication is well aware that, using this method, it is difficult to provide the desired refractive index profile in all types of fibers and, particularly, in active fibers and with a low refractive index difference. For example, the step index profile is often characterized by a uniform refractive index within the core. The reality is that, typically, the core refractive index varies and varies significantly, as diagrammatically shown in
A need therefore exists for a technology that can provide an easy and effective control of the refractive index profile in fiber manufacturing.
The single preform has the length of just a few tens of centimeters and diameter not exceeding a few centimeters. The drawing of fiber from such a preform is very time consuming, and the process output is limited by a relatively short fiber length not exceeding several meters.
Accordingly, another need exists for a technology that allows the fabrication of individual fibers in a time-efficient manner.
Along with the refractive index profile, a doping spatial profile is very important to all fibers and particularly, active multimode (“MM”) fibers. Depending on the doping profile, which can be characterized, among others, by a doping radius and location of dopants within the fiber core, the fiber may be specifically tailored to amplify, for example, a fundamental mode or other selective modes, while suppressing or at least not amplifying other modes. An exemplary structure illustrating the latter may be configured with a central region of MM core heavily doped with ions that partake in amplifying predominantly a fundamental mode at a given wavelength, while doping the core periphery with another type of ions that typically suppress higher order modes at the same wavelength. However, as known to one of ordinary skill in the fiber laser art, very few different types of ions can productively coexist in the same core and, as discussed above, control of the ion deposition is far from being simple and particularly effective.
Hence another need exists for a simple, effective technique that allows controllably forming the desired dopant profile in optical fibers.
Referring specifically to MM active fibers configured to emit output in a fundamental mode, it is known that the scalability of such fibers is limited by the onset of nonlinear effects (“NLE”). One well-known way to reduce the consequences of NLE is to increase a mode area while lowering the core numerical aperture (NA) and increasing the core diameter. In general, this type of fibers is known in the art as large mode area (“LMA”) fibers. Achieving single mode operation then requires modal discrimination techniques, such as bend loss. But for large core and low NA, bending deforms the mode field distribution and reduces the mode area.
Thus, a further need exists for the fibers with a large mode area that are less sensitive to bending stresses prevalent with LMA fibers.
The above and other needs are satisfied by the disclosed here structure. The disclosed fiber, like any typical optical fiber, is configured with a core and at least one cladding surrounding the core and having a refractive index lower than that of the core.
The inventive concept relates to the core produced by controllably arranging and coupling together individual components into the desired pattern. Consequently, the thus produced structure is further referred to as a mosaic core.
The components each are configured with a silica-glass medium that has an inner region. The coupling of components to one another produces a matrix, i.e., the cumulative mass of silica-glass medium with embedded therein spaced apart elements which correspond to respective central regions of individual components. The individual components may be selected to have different or uniform physical properties and geometries which together provide the mosaic core with the desired cumulative refractive index.
In accordance with one aspect of the inventive concept, the elements of the mosaic core each are dimensioned to not waveguide light at a given wavelength. Accordingly, as light is coupled into these none-waveguiding elements, it cannot propagate therealong and bleeds into and further guided through the silica-glass medium of the mosaic core. The refractive index of silica-glass is a well-defined step with the top that is substantially more flat, which is desired because of the repeatable uniformity of the index differential, than that of a standard core doped, for example, with light amplifying ions or emitters. The desired refractive index profile, i.e., the difference between refractive indices of respective composite silica-glass-based core and silica-glass-based outer cladding can be easily realized by providing, for example, the cladding with the desired concentration of non-amplifying dopants.
In accordance with another aspect of the inventive concept, individual components may be doped with light amplifying ions of rare earth elements or light non-amplifying ions or with a combination of these. Controllably arranging the components in the predetermined pattern can provide the mosaic core with the desired dopant profile. Thus, the components may be arranged so that elements doped with ions of rare earth elements are concentrated in a specific area of the mosaic core. With the predetermined topography of the dopant profile, the mosaic core is configured to provide the desired gain for selective transverse modes, if the core supports propagation of multiple transverse modes while other high order modes are suppressed.
The above and other aspects and features of the invention will become more readily apparent from the following specific description accompanied by the drawings, in which:
The inventive concept underlying the disclosed fiber allows one of ordinary skill in the laser arts to configure an optical fiber with practically any desired refractive index profile. In addition to the desired refractive index profile, based on the inventive concept, an optical fiber may be provided with any desired dopant profile. The optical fiber configured in accordance with the inventive concept may support propagation of single mode (“SM”) or multiple modes (“MM”) and be passive or active.
Referring to
While reducing the initial stack of rods 20 to the desire diameter of mosaic core 25, individual rods each are reduced to the diameter that considerably inhibits and preferably completely prevents waveguiding at a five wavelength. In other words, a substantial portion of light coupled into the thus reduced individual rod expands beyond it and starts coupling with modes decoupled from neighboring rods 20. But this light is wave-guided by the silica-based medium of core 25 which is the result of fused peripheries of respective rods 20. Based on the foregoing, thus, light mostly propagates through silica which has a refractive index without the undesirable fluctuations so characteristic to prior art fibers, particularly active fibers. Because the cladding 24 is also silica-based the refractive index profile of fiber 10, if it is a step index, is characterized by a uniform index differential Δn between the core's and cladding's respective indices, as shown in
Returning to
The original rods 20 may be configured to support propagation of a single mode or multiple transverse modes. The rods may be made from silica-glass doped with phosphate or alumina. No light amplifying ions may be doped in silica-based medium 24, i.e., fiber 25 can be passive. For many applications, it is desirable that fiber 25 is configured as a polarization maintaining fiber.
Alternatively, silica-based medium 24 may be doped with light emitters, i.e. ions of any suitable rare earth element individually or a combination of the latter. For example, the ions may include Ytterbium (“Yb”), Erbium (“Er”); Neodymium (“Nd”), Thulium (“Tm”), Samarium (“Sm”), Europium (“Eu”), Holium (“Ho”) and others. According to the inventive concept, rods 20 may be doped with the same type of light amplifying ions. Alternatively, using different preforms and, therefore rods, different types of rare earth ions can easily coexist in one core. For example, some of the rods can be doped with Er and others with Yb. Turning briefly to
Referring to
The individual rods 20 may be doped with a high concentration of light amplifying ions of up to 5000 ppm and even higher. As one of ordinary skill in the laser arts knows, high concentrations of light amplifying ions cause lower thresholds for formation of colored centers, which are highly dangerous to fibers and their operation. In the inventive structure, however, the ion concentration is low (when compared to standard fiber which often have a dopant concentration may vary within 2000-5000 ppm range) since the area of core 25 is significantly greater than that of individual rods 20.
In accordance with a further concept of the present invention, rods 20 doped with light amplifying ions can be controllably arranged within any desired core region. For example, as shown in
The inventive fiber 10 having different topographies. The neighboring rods 20 may be spaced from one another, for example, at a distance varying from about 7 to 17 microns. The rod diameter, for example, may vary from 2 to 3 microns. The core diameter is not limited by any particular consideration and can be constructed based on the desired requirements. The cumulative area occupied by rods 20 may vary between 5 and about 50% of the entire core area, but preferably is about 10%.
The foregoing clearly illustrates the flexibility offered by the inventive fibers. With such a variety of easily controllable physical and geometrical parameters, core 25 can be easily constructed in accordance with any given specification, polarization etc., and thus can be properly described as mosaic.
The input and output ends of respective rods 20 are dimensioned to not waveguide light which is coupled into the mosaic core and thus is guided by silica-based medium. In the input tapered region, the rods each begin to gradually widen expanding to the individual core size that is capable of supporting a single mode, i.e., each rod 20 supports propagating of a single mode along the central large-diameter core region. As the rods each narrow along the output tapered region, individual cores do not support propagation of light. The single modes expand beyond respective cores interacting with one another so as to form a mega mode that is further guided along the silica-based medium of the core. The fiber of
A variety of changes of the disclosed structure may be made without departing from the spirit and essential characteristics thereof. Thus, it is intended that all matter contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the disclosure being defined by the appended claims
Number | Date | Country | |
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61909060 | Nov 2013 | US |
Number | Date | Country | |
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Parent | PCT/US2014/063397 | Oct 2014 | US |
Child | 15165297 | US |