Abstract:
A single-mode fiber includes a grating located near a fiber tip to shift transmitted light from a fundamental core mode to one or more higher cladding modes. Light exiting the fiber from the cladding mode occupies more area but is more collimated. Translational alignment tolerances are relaxed by the improvement in collimation, allowing couplings to be made directly with the single-mode fiber or through the intermediacy of a conventional lens, which can itself be aligned more readily and be less fast.

Description:
TECHNICAL FIELD 
     Light coupled into and out of single-mode fibers generally requires some form of conversion to minimize insertion losses. One such conversion involves collimating light at fiber ends to expand mismatch tolerances, particularly longitudinal mismatch. 
     BACKGROUND 
     Telecommunication and sensor systems rely on single-mode optical fibers as favored medium for conveying optical information, particularly over long distances. Coupling light into a single-mode fiber (e.g., connecting the fiber to a source) and coupling light out of a single-mode fiber (e.g., connecting the fiber to a detector) involve well-established technologies. However, such couplings remain expensive and complex because of extremely high tolerances for alignment, cleanliness, and dimensioning of the coupling system. 
     Mid-link couplings, which interrupt single-mode fibers along their length, support functions such as switching, routing, and signal modification or restoration. More such couplings are needed to accommodate a rising demand on fiber optic systems to perform increasingly complex tasks. The additional couplings add considerable cost and complexity to the fiber systems. 
     A typical single-mode fiber coupler includes a bulk optical lens, such as a gradient-index (GRIN) lens, fused to one end of a single-mode fiber section. The lens converts light diverging from a core of the fiber end into a more collimated form for further propagation through free space or another optical component. Another lens can be coupled to an end of a second single-mode fiber section for collecting the collimated light and for converging the light onto a core of the second fiber section. Although such lenses contribute to expanding longitudinal and transverse assembly tolerances for connecting single-mode fibers to or from free space and any intervening optics, the lenses themselves must also be aligned, which involves similarly tight tolerances. 
     Tapered couplers, which can be formed near the ends of single-mode fibers, shift light traveling in a single-mode core (i.e., a core mode of transmission) into the lowest order mode of a multi-mode, expanded-core region so that light exits the fibers substantially more collimated. The larger diameter modes of the expanded-core region are inherently more collimated than the smaller diameter mode of the single-mode core region. The mode shifts are accomplished by adiabatically tapering the core to enable light traveling along the core to remain in the lowest order mode as the core size increases and hence supports multiple modes. Similar couplings can be used to collect collimated light and shift the collimated light from a large-diameter mode in a multiple-mode region to the small-diameter mode of the single-mode core region for further propagation along a single-mode fiber. 
     Such tapered couplings efficiently convert light between the core mode and the lowest order mode of a multi-mode section, but manufacturing these tapered couplings to required specifications can be difficult. Fiber ends are carefully drawn down at elevated temperatures and cleaved to achieve the required form. The cost of such operations is high, particularly as an incremental cost repeated over many such couplings. 
     SUMMARY OF INVENTION 
     Lower cost couplings for single-mode optical fibers with additional performance options can be realized by using grating structures to convert light between core and cladding modes near fiber ends. Here, cladding modes refer to the modes that are guided within the cladding structure that surrounds the core of a standard single-mode fiber. The use of gratings for mode conversion improves coupling possibilities for light entering and exiting the ends of single-mode fibers. 
     A single-mode optical fiber incorporating an exemplary mode-converting coupler includes a core surrounded by a cladding and an end adapted for coupling the fiber to a continuing optical pathway. A grating formed near the end of the fiber shifts transmissions of light between the core and the cladding so that the light passing through the fiber end is substantially more collimated, thereby expanding both transverse and longitudinal alignment tolerances for coupling the fiber to the continuing optical pathway. 
     The grating can be formed by a pair of reflective short-period gratings, such as a Bragg gratings, or by a transmissive long-period grating. One of the reflective gratings is written into the core of the single-mode fiber for performing the mode conversion, and the other reflective grating is written into the cladding of the single-mode fiber for redirecting the light toward the fiber end. The transmissive grating performs the same mode conversion without requiring a change in direction. Reflective gratings have the advantage of requiring a minimum of space for reflecting narrow spectral bands, but reverse direction. Transmissive gratings require more space to achieve a similarly specific spectral response but do not reverse direction. 
     Both types of gratings can be formed directly in the single-mode fibers and subsequent assembly is not required. In addition, manufacture of the gratings can be accomplished without any drawing or furnace operations. Reflective gratings can be written by exposing a photosensitive core or cladding to actinic UV radiation in the form of an interference pattern. Transmissive gratings can be written in a similar fashion, or by using a uniform beam and a periodic shadow (or amplitude) mask, or by successively exposing relatively translated lengths of the fiber. Preferably, both types of gratings are formed near the fiber ends, and perturbations (i.e., grating lines) are written into the fiber at orientations that are substantially normal to the direction of light propagation for minimizing losses of light through sides of the fiber. 
     The mode-converting couplings can be made to free space; to other fibers or waveguides; or to optical devices such as lasers, detectors, planar or bulk optics, and micro-electro-mechanical systems (MEMS). Grating structure can be used to convert light between the fundamental core mode and one or more higher cladding modes. The mode-converting couplings can also be used as pre-collimators in combination with bulk optical lenses to meet more stringent collimating requirements. Simpler (e.g., less fast) bulk lenses can be used because of the pre-collimating function of the mode-converting couplings. 
    
    
     DRAWINGS 
     FIG. 1 is a schematic depiction of two single-mode fibers cross-connected through a bulk optical filter. 
     FIG. 2A is a schematic depiction of core-mode divergence from a single-mode fiber. 
     FIG. 2B is a schematic depiction of cladding-mode divergence from a single-mode fiber. 
     FIG. 3 is a schematic depiction of two single-mode fibers cross-connected through a bulk optical filter straddled by two lenses. 
     FIG. 4 is a schematic depiction of a single-mode fiber to planar waveguide coupling. 
     FIG. 5 is a schematic depiction of a single-mode fiber connection to a mid-link optical device using a pair of reflective gratings. 
    
    
     DETAILED DESCRIPTION 
     Exemplary couplings between two single-mode fibers  10  and  12  separated by an expanse of free space containing a bulk filter  14  are depicted in FIG.  1 . The fibers  10  and  12  are structured with the conventional features of cores  16  and  18  surrounded by claddings  20  and  22 . Within the cores  16  and  18 , however, transmissive long-period gratings  24  and  26  are written for shifting light propagating along longitudinal axes  28  and  29  of the fibers  10  and  12  between the cores  16  and  18  and the claddings  20  and  22 . 
     The transmissive grating  24  shifts light traveling primarily in the core  16  as exhibited by a core-mode intensity profile  30  to light traveling primarily in the cladding  20  as exhibited by a larger cladding-mode intensity profile  32 . Light emanates from a fiber tip  34  significantly more collimated because of the core  16  to cladding  20  shift. The cladding-mode intensity profile  30  changes little across the expanse of free space containing the bulk filter  14  between the fiber tip  34  of the fiber  10  and a fiber tip  36  of the fiber  12 . The two fiber tips  34  and  36  are preferably cleaved, cleaned, and polished to optimize the transmission of light while minimizing back reflections. 
     As a further explanation of the collimating benefits of such a core-to-cladding conversion, FIGS. 2A and 2B contrast core-mode divergence with cladding-mode divergence. In FIG. 2A, light having an intensity profile  40  propagates along a core  42  of a single-mode fiber  44  and emanates from a fiber tip  46  at an angle of divergence θ A . The core-mode intensity profile  40  rapidly expands at the divergence angle θ A  in a transverse plane as a function of longitudinal displacement. 
     In FIG. 2B, light having a similar initial intensity profile  50  propagating along a core  52  of a single-mode fiber  54  encounters a transmissive long-period grating  56  that shifts most of the light out of the core  52  into a surrounding cladding  58 . Following the Brightness Theorem, which postulates that area can be traded for divergence, a larger cladding-mode intensity profile  60  emanates through a fiber tip  62  at a substantially smaller divergence angle θ B  (i.e., is substantially more collimated than the core-mode intensity profile  40  emanating from the fiber tip  46 ). Thus, the cladding-mode intensity profile  60  expands at a much slower rate than the core-mode intensity profile  40  and can be more readily collected for further transmission. 
     Referring back to the example depicted in FIG. 1, the cladding-mode intensity profile  32  similarly holds form through the free space region that includes the bulk filter  14  and is collected as a nearly identical cladding-mode intensity profile  66  within the cladding  22  of the single-mode fiber  12 . The transmissive long-period grating  26  converts the cladding-mode intensity profile  66  to a core-mode intensity profile  68  for further transmission along the single-mode fiber  12 . 
     The cladding-mode transmissions  32  and  66  can be guided by an air-to-cladding interface, but the extent of the cladding-mode transmissions  32  and  66  should be limited to avoid losses of light due to imperfections in the air-to-cladding interface. For this reason, the transmissive long-period gratings  24  and  26  are located adjacent to the fiber tips  34  and  36 . Other cladding interfaces, particularly polymers, could be used to guide light over short distances or to influence the intensity profile or spectral content of the light over longer distances. 
     In addition, the mode conversions preferably take place symmetrically about the longitudinal axes  28  and  29 . Care is taken during the manufacture of the transmissive gratings  24  and  26  to orient perturbations in refractive index (i.e., grating lines formed at interfaces between longitudinal regions of different refractive index) substantially normal to the longitudinal axes  28  and  29 . 
     The core-to-cladding directional conversions enlarge transverse areas and correspondingly reduce divergence of beams emanating from single-mode fibers. The cladding-to-core directional conversions collect light beams over enlarged transverse areas of single-mode fiber claddings and converge the collected light within the cores of the same fibers for further transmission. Together, such single-mode fiber-to-fiber couplings relax transverse alignment tolerances approximately in proportion to a ratio of the cladding-mode to core-mode diameters. Longitudinal tolerances are relaxed approximately in proportion to a square of the cladding-mode to core-mode diameters. The angular tolerances are tightened in proportion to the same ratio of diameters, but the loosening of the two translational (i.e., transverse and longitudinal) tolerances are regarded as more significant for purposes of assembly and use. 
     Although the core-to-cladding mode conversions significantly improve the collimation of light emanating from the tips of single-mode fibers, some applications require even more collimated light. The embodiment of FIG. 3 exemplifies this situation. Interposed between tips  72  and  74  of two single-mode fibers  76  and  78  is a bulk filter  80  straddled by two bulk lenses (e.g., gradient index or Fresnel lenses)  82  and  84 . 
     A grating  86  shifts light from a core-mode intensity profile  88  to a cladding-mode intensity profile  90  for pre-collimating light exiting the fiber tip  72 . The lens  82 , which is preferably fused to the fiber tip  72 , completes a remainder of the desired collimation. The pre-collimation provided by the grating  86  allows both the power of the lens  82  to be reduced and the translational alignment tolerances of the lens  82  to be loosened. Fresnel lenses, in particular, can be made simpler if less speed is involved. 
     The filtered light is collected by the bulk lens  84 , which converges the collected light into a cladding-mode intensity profile  92  at the tip  74  of the single-mode fiber  78 . A grating  94  converts the cladding-mode intensity profile  92  into a core-mode intensity profile  96  for guiding the light more efficiently over larger distances. Both the power of the lens  84  can be reduced and the translational alignment tolerance of the lens  84  can be loosened with respect to conventional lens-to-fiber couplings. 
     FIG. 4 illustrates an improved coupling between a single-mode fiber  100  and a planar waveguide  102 . A grating  104 , formed in the single-mode fiber  100  adjacent to a fiber tip  106 , shifts light between a core-mode intensity pattern  110  and a cladding-mode intensity pattern  112 . The grating  104  is formed in a core  114  of the single-mode fiber  100  along a longitudinal axis  116  that extends normal to a direction of light propagation  118  within the waveguide  102 . 
     A blazed grating  120  within the planar waveguide  102  redirects light emanating from the fiber tip  106  of the single-mode fiber  100  through a right angle into the waveguide  102 . Conversely, the grating  120  also redirects light propagating along the waveguide  102  through a right angle into alignment with the longitudinal axis  116  of the single-mode fiber  100 . The light emanating from the planar waveguide  102  is collected within a cladding  122  of the single-mode fiber  100 . The grating  104  converts the collected light from the cladding-mode intensity pattern  112  to the core-mode intensity pattern  110  for further transmission. 
     An exemplary mid-link coupling of a single-mode fiber  126  using a pair of reflective short-period gratings  128  and  130  is shown in FIG.  5 . The reflective grating  128  is written into a core  132  of the single-mode fiber  126  for reflecting light from the core  132  into a cladding  134  of the fiber  126 . The reflective grating  130  is written into the cladding  134  to reflect light in the cladding back toward a fiber tip  136 . 
     Light traveling primarily in the core  132  and having a core-mode intensity profile  140  is largely unaffected by the reflective grating  130  as it passes through the reflective grating  130  en route to the reflective grating  128 . However, light converted and reflected by the reflective grating  128  and having a cladding-mode intensity profile  142  is substantially reflected by the reflective grating  130 . The reflected light having a cladding-mode intensity profile  144  passes through the reflective grating  128  largely unaffected en route to the fiber tip  136 . 
     The combined affect of the two reflective gratings  128  and  130  is similar to the effect of a single transmissive grating as described in the earlier embodiments. Light passing through the fiber tip  136  is substantially more collimated, which expands translational tolerances for coupling the single-mode fiber  126  to a mid-link device  146 , such as a micro-electro-mechanical device as well as to free space or another fiber. A similar combination of reflective gratings could be used in another fiber to reflect light collected in a cladding mode and to both convert and reflect the light again into a core mode for further transmission along the fiber. 
     The two reflective gratings  128  and  130  can be designed together to exhibit a more specific or complex spectral response. For example, the two gratings can be spectrally offset so that only a portion of the converted light is reflected onward in the cladding mode. The remaining portion of the converted light scatters from the less efficient cladding en route to its source. Couplings between single-mode fibers also produce opportunities for both reflective and transmissive gratings to further influence spectral response. Once a spectrum of light is shifted into the cladding by a first single-mode fiber, a selected portion of the spectrum can be shifted back into the core for further transmission by the second single-mode fiber. Light remaining in the cladding of the second single-mode fiber decays rapidly owing to scattering losses and bends. In addition, the gratings in this or any other of the embodiments can be chirped, concatenated, or otherwise designed according to known practices to exhibit a more complex spectral response for accomplishing filtering functions in addition to coupling.