Abstract:
A fiber-based optical amplifier is assembled in a compact configuration by utilizing a flexible substrate to support the amplifying fiber as flat coils that are “spun” onto the substrate. The supporting structure for the amplifying fiber is configured to define the minimal acceptable bend radius for the fiber, as well as the maximum diameter that fits within the overall dimensions of the amplifier package. A pressure-sensitive adhesive coating is applied to the flexible substrate to hold the fiber in place. By using a flexible material with an acceptable insulative quality (such as a polyimide), further compactness in the final assembly is achieved by locating the electronics in a space underneath the fiber enclosure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 62/135,641, filed Mar. 19, 2015 and herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fiber-based optical amplifier and, more particularly, to a compact configuration for the amplifying fiber portion of the optical amplifier. 
     BACKGROUND 
     Various types of fiber-based optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs) and distributed Raman amplifiers (DRAs), are ubiquitous components of optical communication systems, eliminating the need to perform optical-electrical-optical signal transformations when regeneration of a fading optical signal is required. 
     In the case of EDFAs, an optical pump laser (typically operating at 980 nm) is coupled into a section of Er-doped optical fiber, and the incoming optical signal is propagated through the doped fiber with the pump light. The presence of the pump light with the erbium dopant generates amplification of the propagating optical signal by the transitions of the optically-excited erbium ions. Distributed Raman amplifiers (DRAs) operate by injecting short, high-power pulses along a section of transmission fiber that is supporting the propagation of an optical signal. The presence of these pulses (either co-propagating or counter-propagating with respect to the optical signal) excites the photons to higher energy levels, where the photons create stimulated emission as they return to their ground state. 
     The various components forming an optical amplifier module are typically made as fiber-coupled elements, and in some cases integrated (or hybridized) to form, for example, a combined isolator and WDM filter, or a combined isolator and GFF filter, or the like. Of course, lower cost and smaller-sized modules lower the overall system costs. Thus, the trend to smaller components, more hybridization and smaller modules has been taking place for some time. Indeed, the pressure for smaller form factors and lower costs continues to be exerted on the industry. 
     One path to assuage these demands is to continually reduce the size of the various components and, perhaps, increase their degree of integration. However, this is not easily accomplished in an environment where the cost of the amplifier module is also a concern. Indeed, the size of these components has decreased to the point where they cannot be readily assembled by low-cost labor (i.e., the size of some of these components can be on the order of 1 mm×1 mm×1 mm). 
     Furthermore, even with reduction in size of an optical amplifier module, such as from increasing the level of integration within the hybrid components, the different hybrids must be coupled to each other via fiber splicing and routing. As a consequence of the minimum bend radius of the optical fiber as well as the relatively large number of fiber splices and splice protectors mandating the same, the ability to further hybridize current configurations is quickly reaching its technical limits, size limits and economical possibilities of implementation. The “bend radius” is a determinative factor associated with defining an acceptable amount of signal loss. In particular, the loss exhibited by an optical signal increases with a smaller bend radius of the fiber in which the signal is propagating. At exceptionally small bend radius values, there may also be a physical failure of the fiber itself. 
     Thus, for a fiber-based optical amplifier to continue to meet the expectations of cost and size reduction, while maintaining performance requirements, a different approach to incorporating the amplifying fiber within the optical amplifier module appears to be required. 
     SUMMARY OF THE INVENTION 
     The needs remaining in the prior art are addressed by the present invention, which relates to a fiber-based optical amplifier and, more particularly, to a compact configuration for the amplifying fiber portion of the amplifier. 
     In accordance with one embodiment of the present invention, an exemplary optical amplifier is configured as comprising an optics module and a fiber module. The optics module is used to house the various optical devices utilized to process the amplified signal into an acceptable output form, and the fiber module is used to house the actual fiber within which the amplification is created. The inventive fiber module consists of a flexible substrate of insulative material (for example, a polyimide) with a pressure-sensitive adhesive top coating. The fiber itself is wound in a coil configuration on the insulative material and held in place by the adhesive coating. A support tray of a stiffer material is used to impart mechanical strength to the flexible material, and may be formed to include guides to ensure that the radius of the coil does not go below a defined minimum fiber bend radius. 
     A particular embodiment of the present invention may be configured as a rare earth-doped optical fiber amplifier, providing pump light of a specific wavelength to propagate along a coil of rare-earth (e.g., erbium) doped optical fiber at the same time as the input optical signal. Another embodiment of the present invention takes the form of a distributed Raman amplifier (DRA), where high-power laser pulses are injected into a signal path along which the input optical signal is propagating. 
     An exemplary embodiment of the present invention takes the form of an optical fiber amplifier comprising an optics module housing optical elements utilized in creating an output optical signal from an amplified version and a fiber module optically coupled to the optics module. The fiber module is used to house a section of amplifying fiber for creating gain in a propagating optical signal in the presence of pump light, where the fiber module specifically includes a flexible substrate for supporting the section of amplifying fiber in a configuration of flat coils and a support structure disposed underneath the flexible substrate, the support structure including an end termination for mechanically attaching the fiber module to the optics module. 
     Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings, where like numerals represent like parts in several views: 
         FIG. 1  is an isometric diagram of an exemplary optical fiber amplifier, formed in accordance with the present invention; 
         FIG. 2  is an exploded view of the optical fiber amplifier of  FIG. 1 ; 
         FIG. 3  depicts a section of amplifying fiber, presented as a flat coil for use within the fiber module of the amplifier; 
         FIG. 4  illustrates the coil of amplifying fiber of  FIG. 3  as disposed on an exemplary fiber module; 
         FIG. 5  is a diagram of the support structure portion of the fiber module, illustrating the parameters that control the bend radius experienced by the amplifying fiber as it is coiled; 
         FIG. 6  illustrates an alternative configuration for attaching the fiber module to the optics module of the inventive optical amplifier; and 
         FIG. 7  shows a compact configuration where the electrical isolation provided by the material of the fiber module allows for an electronic circuit board to be positioned in close proximity to the optical amplifier. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an isometric diagram of an exemplary compact optical amplifier  10 , formed in accordance with the present invention. Optical amplifier  10  comprises both an optics module  12  and an amplifying fiber module  14 . Optics module  12  is of conventional form perhaps, and includes various optical elements, hybrids, etc. utilized to create an amplified output signal that meets defined system requirements (in terms of, for example, noise floor, isolation, gain profile, insertion loss, etc.). Fiber module  14  houses a (relatively long) section of amplifying fiber that is used to perform the actual amplification function on the incoming optical signal. There exist many different configurations that may be used to provide the optical functions as required within optics module  12 , where co-pending US application Ser. No. 15/072,520, filed Mar. 17, 2016 and entitled “Optical Amplifier Module”, describes some preferred embodiments and is herein incorporated by reference. 
     For the sake of clarity, fiber module  14  is shown in  FIG. 1  without having the amplifying fiber in place. It is to be understood that when designing a rare-earth doped fiber amplifier, a section of doped fiber (e.g., erbium-doped fiber) would be placed within fiber module  14 . For the case of a distributed Raman amplifier, a section of conventional single-mode fiber is typically disposed within fiber module  14 . An exemplary process of loading the amplifying fiber into module  14  will be described below in association with  FIGS. 3-5 . 
       FIG. 2  is another view of exemplary compact optical amplifier  10 , in this case an exploded view with optics module  12  shown as separated from fiber module  14 , and a set of exemplary components forming fiber module  14  also shown. Referring to both  FIGS. 1 and 2 , an incoming optical signal is introduced in optics module  12  via a fiber pigtail connection  16 . The incoming optical signal and pump light (which may be supplied from a separate external source or co-packaged within optics module  12 ) are combined within optics module  12  and coupled into fiber module  14  via a pigtail fiber connection  17 . The optical signal and pump light then propagate through the length of amplifying fiber  20  housed with fiber module  14 , with an amplified version of the optical signal ultimately exiting fiber module  14 . The amplified signal is then coupled back into optics module  12  at an optical fiber pigtail  18 . Various post-amplification optical functions (gain flattening, power adjustments, etc., as dictated by specific system requirements) are performed on the signal, which then exits optics module  12  from optical fiber pigtail  19  as the amplified output signal from optical amplifier  10 . 
     In the specific embodiment of  FIG. 2 , fiber module  14  is shown as comprising, a stiff support member  22  configured to have a relatively circular distal portion  24  and an opposing pair of end terminations  26  and  28  that are used to attach fiber module  14  to optics module  12 . A flexible substrate  30  of a suitable insulative material (e.g., a polyimide-based material) is disposed within support member  22 , and a coating  31  of a pressure-sensitive adhesive is applied to flexible substrate  30 . The adhesive coating is used to ensure the integrity of the attachment of amplifying fiber  20  to flexible substrate  30 . Also shown in  FIG. 2  is a top cover plate  32  and a bottom cover plate  34 , used to encase amplifying fiber  20  within fiber module  14  (as shown in final form in  FIG. 1 ). 
     Variations of this specific configuration are contemplated, including the use of fewer or more end terminations to mechanically attach fiber module  14  to optics module  12 . Additionally, while it may be preferred to include an adhesive coating  31  on flexible material  30 , there may be some types of flexible material that exhibit an adhesive nature without the need for the additional coating. 
     One aspect of the present invention is the particular configuration of the amplifying fiber as a flat coil (as compared with bundling the fiber as is common in the prior art). In the particular illustration of  FIG. 3 , amplifying fiber  20  is shown as being coiled in a manner that creates a “stack” of two flat coils shown as  20 - 1  and  20 - 1 .  FIG. 4  illustrates a coil of amplifying fiber  20  as in place within circular portion  24  of support structure  22 . In the process of fabricating this portion of the compact optical amplifier, the amplifying fiber is literally spun onto flexible material  30 , using a pressure sufficient to allow for adhesive coating  31  to hold the coiled fiber in place. 
     In the implementation of an EDFA, amplifying fiber  20  comprises a section of rare-earth doped fiber about one meter in length (at times, more than a meter may be required). DRAs may utilize conventional signal mode optical fibers of lengths of several meters in the formation of amplifying fiber  20 . In attempting to incorporate these relatively long lengths of amplifying fiber within packages of relatively small dimensions, it is necessary to understand the impact of bend loss on the propagating signal. That is, if a fiber is bent into a curve (in this case, when forming a coil) with a very tight bend radius, a large fraction of the propagating signal will be scattered out of the core region; the loss increases as the fiber radius decreases. A very small fiber bend radius may also cause breakage of the fiber itself. On the other hand, if the bend radius is maintained at a large value (i.e., such that only a relatively “gentle” bend is imparted on the fiber), the size of a package required to accommodate a meter or two of amplifying fiber would be too large for many of the CFP requirements. 
       FIG. 5  is a diagram of an exemplary support structure  22  formed in accordance with these aspects of the present invention to control the curvature introduced into the coil configuration of amplifying fiber  20 . In particular, circular portion  24  of support structure  22  is shown as including a central opening  36 , where the diameter d of opening  36  is chosen to prevent amplifying fiber  20  from being coiled with a very tight bend radius. The outer diameter D of circular portion  24  is chosen to ensure that the final dimensions of optical amplifier  10  are well within the limitations imposed by the particular package design. Opening  36  may include a rim  38  to prevent the amplifying fiber from entering this central area. Similarly, the outer periphery of support structure  22  may include a rim  40  to maintain the fiber confined within its designated boundary. 
     In one exemplary assembly process, amplifying fiber  20  is spun onto the surface flexible substrate  30  of support structure  22 , using opening  36  (with rim  38 ) and outer rim  40  as guides for the process. That is, amplifying fiber  20  is spooled in a flat configuration, rather than bundled as in the prior art, with miniature splice protectors inserted in place for connection to fiber pigtails  17  and  18 . As such, this portion of the optical amplifier may be assembled with automated methods, making a highly repeatable process with high yield and low cost, as well as a small footprint. 
     It is to be understood that there are a variety of different configurations that may be used to mechanically attach fiber module  14  to optics module  12  (the optical connection provided via fiber pigtails  17  and  18 , as explained above). The exemplary configuration as shown in  FIGS. 1 and 2  utilizes a straightforward snap-fit friction fitting, between end terminations  26 ,  28  of fiber module  14  and associated attachment elements  42 ,  44  formed within the housing  46  of optics module  12  (best shown in  FIG. 2 ). 
       FIG. 6  is an isometric view of an alternative attachment configuration, in this case using a set of screws  48  to attach end terminations  26 ,  28  of fiber module  14  to housing  46  of optics module  12 . The specifics of rims  38  and  40  are also specifically shown in this view of the inventive compact optical amplifier module. from underneath optical amplifier  10 , showing the attachment of fiber module  14  to optics module  12 . In this view, both end terminations  26  and  28  are visible, with stiffening elements  42  and  42 ′ disposed as shown. Attachment screws  50  and  52  are shown as attaching end termination  26  to optics module  12 , with similar attachment screws  50 ′ and  52 ′ used to attach end termination  28 . 
     As mentioned above, another advantage of the compact amplifier module configuration of the present invention is that support structure  22  of fiber module  14  is formed of an insulative material. As such, it is possible to locate associated electric circuits in close proximity to fiber module  14  without affecting its performance.  FIG. 7  is an exemplary illustration of the inventive compact optical amplifier that illustrates this aspect. For the sake of clarity, the covering layer enclosing amplifying fiber  20  is not shown. In this case, an electronics circuit board  100  is illustrated as being disposed directly underneath compact optical amplifier  10 , extending beneath both optics module  12  and, fiber module  14 . 
     It is to be understood that the inventive fiber module for use with a fiber based optical amplifier may be used in the formation of a doped fiber amplifier (such as an EDFA) or a distributed Raman amplifier, with the only change in assembly being the type of fiber that is spooled onto the flexible substrate of the module. With that said, the particular dimensions of support structure  22  (particularly central opening  32 ) may differ as a function of a minimum, bend radius associated with a particular amplifier&#39;s design criteria. 
     It will be appreciated by those skilled in the art that changes may be made to the embodiment described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but it is intended to cover modifications within the spirit and scope of the inventive concept as defined by the appended claims.