Abstract:
An optical fiber coupling reduces power loss by creating breaks in one or both of the optical fibers being joined so that destructive interference between modes of the signal escaping at the coupling and the breaks reduces total power loss. The breaks can be in the receiving and/or transmitting optical fiber, and each break generally correspond to a small region of changed refractive index in the cladding and/or core of an optical fiber. The breaks can be created using a laser or by removal and replacement of small regions of cladding material. An active alignment process can position the optical fibers for physical attachment.

Description:
BACKGROUND  
         [0001]    Optical fibers can carry light signals for great distances without significant loss. Light losses, however, can be significant at a fiber-to-fiber coupling, where two optical fibers are joined. A variety of optical fiber-to-fiber coupling systems have been developed. One class of optical coupling systems uses optical elements such as lens pairs between the ends of the optical fibers. The optical elements collect the light emitted from the source optical fiber and focus the light on the core of the receiving optical fiber. For a high efficiency at coupling optical power into the receiving optical fiber, the focused light needs to be at the proper angles for transmission along the core of the receiving optical fiber. The intervening optical elements can achieve a high coupling efficiency but increase the cost and alignment difficulties involved in creating a fiber-to-fiber coupling.  
           [0002]    Butt coupling provides a simple fiber-to-fiber coupling in which the ends of two optical fibers are adjacent to or in contact with each other. This type of fiber-to-fiber coupling is typically made with a tool that adjoins the ends, aligns the optical fibers, and then attaches a physical coupling device and/or adhesive meant to hold the two optical fibers in the established alignment. Several types of errors or differences in the fiber-to-fiber couplings can cause power loss.  
           [0003]    One source of power loss is defects in the end faces of the optical fibers. For optimal coupling efficiency the end faces of each optical fiber must be perpendicular to the length of the fiber and free of defects such as chips or roughness.  
           [0004]    Another source of power loss in fiber-to-fiber couplings is misalignment of the optical fibers. FIG. 1A, for example, illustrates an example where the centers of the cores  110 A and  110 B of optical fibers  100 A and  100 B are offset from each other. FIG. 1B illustrates optical fibers  100 A and  100 B having an angular misalignment, and FIG. 1C illustrates optical fibers separated by too large of a gap  120 . Such alignment errors are inherent in mechanical alignment techniques and result in signal loss. This signal loss can be reduced or minimized using an active alignment procedure that measures the optical power coupled into the receiving optical fiber and adjusts the position and orientation of the optical fibers to maximize power coupling efficiency.  
           [0005]    Intrinsic differences in properties of the two optical fibers being coupled are yet another source of power loss in a fiber-to-fiber coupling. FIG. 1D illustrates optical fibers  100 A and  100 B that have different numerical apertures so that a portion of the light signal from transmitting fiber  100 A is at angles that cannot be coupled into receiving fiber  100 B. FIG. 1E illustrates a situation where optical fibers  100 A and  100 B have cores  110 A and  110 B with different diameters so that less than the full cross-section of the optical signal from the transmitting fiber  100 A can be coupled into the receiving optical fiber. Differences in the cladding diameter (as illustrated in FIG. 1F) or differences in the diffractive index profiles of the optical fibers  100 A and  100 B (as illustrated in FIG. 1G) can result in similar power loss at the optical coupling. This type of power loss is inherent to the optical fibers&#39; properties and variations and is present even if the optical fibers can be ideally aligned.  
           [0006]    In view of the current state of the art, optical coupling structures and methods are sought that can reduce optical power loss at the couplings of optical fibers. Such structures and methods would ideally reduce power losses regardless of whether misalignment or differences in the properties of the optical fibers caused the power losses.  
         SUMMARY  
         [0007]    In accordance with an aspect of the invention, a portion of a receiving optical fiber and/or a transmitting optical fiber is patterned to include discontinuities or breaks. Resulting signal leakages at the end face of the transmitting fiber and from the breaks destructively interfere to reduce total signal loss and increase the power efficiency of the fiber-to-fiber coupling. The patterning of the optical fibers can include discontinuities or breaks in the cladding and/or the cores of one or both of the optical fibers.  
           [0008]    One specific embodiment of the invention is an optical device such as an optical coupling for a pair of optical fibers. The ends of the optical fibers are positioned so that one optical fiber transmits an optical signal into the other optical fiber. One or both of the optical fibers includes one or more breaks. Each break in an optical fiber is a change in the refractive index of the core or the cladding of the optical fiber, and the one or more breaks are positioned so that the optical signal escaping at the breaks interferes with the optical signal escaping between the ends of the optical fibers. The interference reduces the total power loss in the optical device.  
           [0009]    Another embodiment of the invention is a process for coupling optical fibers. The process begins by treating a first optical fiber to create one or more breaks near an end of the first optical fiber. Optionally, a second optical fiber is similarly treated to create breaks in the second optical fiber. The patterns on the optical fibers generally depend on the wavelength of an optical signal to be transmitted through the optical coupling and on the properties of the optical fibers. The first and second optical fibers are then aligned to permit transmission of the optical signal between the ends of the optical fibers. The alignment of the first and second optical fibers is such that the optical signal escaping at the breaks in one or both of the optical fibers interferes with the optical signal escaping between the ends of the optical fibers. An active alignment process can measure the power coupled into the receiving optical fiber and position the optical fibers to minimize power loss.  
           [0010]    Several methods for treating the optical fibers are available. The treatment generally changes a refractive index of the fiber core or cladding in regions corresponding to the breaks. One such treatment technique irradiates an optical fiber with a laser to change a refractive index of the optical fiber in the breaks. Another treatment technique removes cladding from one or more portions of an optical fiber and fills the resulting space with a material having a refractive index that differs from a refractive index of the cladding (e.g., material having the same refractive index as the core of the optical fiber.) 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIGS. 1A, 1B,  1 C,  1 D,  1 E,  1 F, and  1 G illustrate causes of optical power loss at a fiber-to-fiber coupling.  
         [0012]    [0012]FIG. 2 illustrates an embodiment of the invention having breaks in a receiving fiber to create destructive interference that results in the recapture of power lost at a fiber-to-fiber junction.  
         [0013]    [0013]FIG. 3 illustrates an embodiment of the invention having breaks in just the cladding of a receiving fiber to create destructive interference that results in the recapture of power lost at a fiber-to-fiber junction.  
         [0014]    [0014]FIG. 4 illustrates an embodiment of the invention having breaks in transmitting and receiving optical fibers to create interference that reduces power lost at a fiber-to-fiber coupling.  
         [0015]    [0015]FIG. 5 illustrates an embodiment of the invention having breaks in the cladding of transmitting and receiving optical fibers to create interference that reduces the power loss at a fiber-to-fiber coupling.  
         [0016]    [0016]FIG. 6 is a flow chart of a fiber-to-fiber coupling process in accordance with an embodiment of the invention. 
     
    
       [0017]    Use of the same reference symbols in different figures indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0018]    In accordance with an aspect of the invention, forming a pattern of breaks, cuts, or marks in a receiving optical fiber and/or a transmitting optical fiber can reduce optical power loss at a fiber-to-fiber junction. Generally, the discontinuity between the end of one optical fiber and the start of the next fiber activates secondary transmission modes. The secondary transmission modes are not viable for long distances but may travel a short distance in the cladding of the receiving optical fiber. The pattern of breaks provides other sources of signal leakage and results in interference between different modes of a light signal propagating along the optical fibers, and the interference can channel the optical power that would otherwise escape at the junction back into the core of the receiving fiber.  
         [0019]    [0019]FIG. 2 illustrates an embodiment of an optical coupling  200  in accordance with an embodiment of the invention. Optical coupling  200  joins optical fibers  100 A and  100 B, which include respective cores  110 A and  110 B and surrounding cladding  120 A and  120 B. Optical fibers  100 A and  100 B can be of any desired type capable of carrying an optical signal  130 . A physical coupling device such as a sleeve  250  holds optical fibers  100 A and  100 B in the proper alignment for use in a system.  
         [0020]    In optical fiber  100 A, optical signal  130  has one or more propagation modes along core  110 A, where the propagation mode depends on the design of optical fiber  100 A, e.g., according to whether optical fiber  100 A is a single mode fiber for the wavelength of light used in optical signal  130 . At the end  140 A of optical fiber  110 A, optical signal  130  exits optical fiber  100 A and enters optical fiber  100 B, but the differences between optical fibers  100 A and  110 B and other sources of power loss described above with reference to FIGS. 1A to  1 G couple some of the power from the optical signal into the secondary propagation modes of optical fiber  100 B. The secondary modes generally include propagation modes of light in cladding  120 B of optical fiber  100 B, and for a conventional optical fiber coupling, the power coupled into such secondary modes is lost.  
         [0021]    Optical coupling  200  includes breaks  210  in optical fiber  100 B to recapture (or avoid the loss of) optical power normally coupled into the secondary propagation modes at end  140 A of optical fiber  100 A. Breaks  210  can be actual breaks in optical fiber  100 B or merely regions of changed refractive index in cladding  120 B and/or core  110 B. In particular, the refractive index of core  110 B can be changed in breaks  210  to match the refractive index of cladding  120 B, or the refractive index of cladding  120 B can be changed in breaks  210  to match the refractive index of core  110 B.  
         [0022]    Breaks  210  couple power from optical signal  130  into the secondary propagation modes of optical fiber  100 B. However, in accordance with an aspect of the invention, the locations of breaks  210  are chosen so that the light coupled into the secondary modes from breaks  210  in optical fiber  100 B constructively (or destructively) interferes with the light coupled into the secondary modes at end  140 A. As a result of the interference, less signal power is lost through the secondary modes, thereby reducing the total power loss in coupling  200 .  
         [0023]    The size and locations of breaks  210  in optical fiber  110  generally depend on the carrier wavelength of light in optical signal  130 , the optical and geometric characteristics of optical fibers  100 A and  100 B, and the alignment and separation of optical fibers  100 A and  100 B. The spacing and number of periodic breaks can be determined using a beam propagation method and/or a coupled mode analysis. Such analysis methods are known in the art and described in “Quantum Electronics” Yariv (1989), U.S. patent application Ser. No. 10/027,498, entitled “A Method and Apparatus for Reducing Optical Insertion Loss in Planar Lightwave Circuits Through Dielectric Perturbation Optimization” and Venkatesh et al., “Insertion Loss Reduction by Optimization of Waveguide Perturbations”, OFC 2002 Postdeadline Papers, FA4-1-FA4-3 (2002) further describe use of such analysis for reducing insertion loss in planar lightwave circuits (PLCs). Since these analysis methods are generally approximations, some experimental trial and error will normally be required to determine the ideal spacing for a particular configuration.  
         [0024]    The interference that reduces the power loss of a fiber coupling can be achieved in a variety of ways. FIG. 3, for example, illustrates an embodiment of the invention having breaks  310  that are confined to the cladding  120 B of optical fiber  100 B. FIGS. 4 and 5 illustrate that breaks  410  or  510  can also be introduced into cladding  120 A and/or core  110 A of the transmitting optical fiber  100 A in addition to or in place of breaks  210  or  310  in the receiving optical fiber  100 B.  
         [0025]    [0025]FIG. 6 is a flow diagram of a coupling process  600  for optical fibers. For coupling process  600 , an initial step  610  selects a pattern for breaks in the optical fibers being joined. Selection of whether the break pattern changes the core and/or cladding of the receiving optical fiber and/or the transmitting optical fiber may depend on the technique used to create the breaks. As noted above, the pattern of breaks will generally depend on the wavelength of light in the optical signal and the optical and geometric characteristics of optical fibers being joined and can be optimized for a particular alignment and separation of the optical fibers.  
         [0026]    Step  620  marks, cuts, or otherwise treats one or both of the optical fibers to create the selected break pattern. The end of one or both of the optical fibers thus includes one or more breaks carefully placed as required to cause the constructive (or destructive) interference that will reduce power loss. One technique for forming the breaks uses an ultraviolet laser similar to those used to write optical fiber gratings. Such lasers can create discontinuities at precise locations in a fiber. Ideally, the discontinuities have the same refractive index for the cladding and the core. Alternatively, the refractive index of the cladding in breaks may be changed, for example, by removing a section of cladding without disturbing the fiber core, and then filling the removed section of cladding with a material matching the refractive index of the core. An advantage of having breaks that only affect the cladding is the lower level of defects in the core. Such defects could scatter light at the break points, causing power loss.  
         [0027]    Once the optical fibers are treated to create the desired break pattern, the two optical fibers are aligned in step  630 . Preferably, an active alignment process adjusts the alignment and separation of optical fibers while measuring the amount of optical power coupled into the receiving optical. Commercially available systems such as a 17 FCM 301 3-Axis Fiber-to-Fiber Coupling System from Melles Griot can be used to scan the relative position and orientation of the optical fibers while observing a power meter to detect maxima in the power coupling efficiency. An active alignment thus finds the orientation and separation of the optical fibers that minimizes power loss.  
         [0028]    After the optimal orientation and separation are found, step  640  attaches a physical coupling device that attaches the two optical fibers and holds the optical fibers in the optimal alignment. Any type of conventional coupling device that can hold the optical fibers in alignment for use can be employed.  
         [0029]    Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. In particular, although embodiments have been described primarily for butt-coupling of optical fibers, other embodiments of the invention can include optical fibers with breaks to utilizes interference-induced reduction in power loss in fiber-to-fiber couplings having intervening optical elements between the fibers being coupled. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.