Abstract:
In a method of manufacturing a polarization maintaining optical coupler, protective jackets of the optical fibers are tapered adjacent the fused portions. In one embodiment of the method a fusing heat source travels repeatedly over a fixed predetermined distance. The fused portion is surrounded by air.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of and claims the benefit of prior U.S. patent application Ser. No. (not yet assigned) filed on Jun. 19, 2001 and assigned to a common assignee. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention pertains to optical fiber couplers, in general, and to polarization maintaining optical fiber couplers, in particular.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical fiber couplers are used for splitting optical power and for wavelength division multiplexing. Polarization maintaining (PM) couplers also maintain the polarization of light launched into the coupler input. PM couplers are utilized in fiber optical communication products and in fiber optic sensor products such as fiber optic gyroscopes.  
           [0004]    Single mode optical fiber carries two orthogonal polarized modes with almost identical velocities. As a result, cross coupling of the two polarization modes occurs whenever temperature changes or mechanical vibrations take place. The polarization cross coupling causes polarization mode dispersion (PMD). PMD, in turn, leads to broadening of optical pulses.  
           [0005]    Polarization maintaining fiber is fabricated by introducing stress applying members during manufacture of the fiber. The stress applying parts create a birefringence as a result of the refractive index difference between the two polarizations. This birefringence makes the two polarization modes propagate at different speeds, slow and fast, which is the reason that there are two orthogonal principal axes corresponding to the two speeds. There are three commonly used types of PM fiber based on the geometries of the stress applying members: PANDA bow tie and elliptically stressed. PANDA is the most commonly used telecom PM fiber. PANDA fiber includes a protective jacket over the fiber and the fiber comprises two stress rods disposed on either side of the optical core.  
           [0006]    Optical fiber PM couplers are either of a mechanical lapped and polished type or a fused taper type. In the fabrication of both types, alignment of the polarization principal axis is essential. The mechanical lapped and polished type coupler is fabricated by embedding an unjacketed fiber in a grooved quartz block and mechanically lapping and polishing the block until the fiber core is reached. Two such blocks are bonded together to form a coupler. Couplers of this type demonstrate low loss and high polarization extinction ratio performance. However, the performance is achieved only over a limited temperature range. Additionally, the fabrication of mechanically lapped and polished type couplers is a labor intensive and time consuming process with the result that production costs are expensive.  
           [0007]    The fabrication of fused taper type couplers involves aligning the fibers; and fusing and tapering the fibers until a desired coupling of optical power is realized. The fusion and tapering process produces a single piece of glass in the coupling region resulting in a more stable type of coupler than the mechanically lapped and polished type.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the principles of the invention, a method of manufacturing an optical coupler includes the step of orienting first and second polarization maintaining optical fibers to a first predetermined orientation. A first portion of the first and second optical fibers are placed in a side-by-side relationship. In a fusing step the first portion of the first and second optical fibers are heated with heat from a heat source to produce a fused portion. The fused portion is subjected to a tapering to produce a predetermined taper over the fused portion. During the fusing and tapering steps the heat source is moved repeatedly over a predetermined fixed distance.  
           [0009]    In a preferred embodiment of the invention the first and second polarization maintaining fibers are PANDA fiber. The first and second optical fibers each have first and second polarization modes corresponding to first and second orthogonal principal axes and the first predetermined orientation comprises one of the first or second polarization modes.  
           [0010]    Further in accordance with one aspect of the invention, each of orienting step includes illuminating a respective one fiber with a laser source while rotating the fiber around its respective longitudinal axis. The interference pattern produced in the fiber is monitored. Rotation is ceased when the interference pattern corresponds to a predetermined pattern.  
           [0011]    Yet further in accordance with the invention the first and second optical fibers are supported on a substrate. A dielectric gel is disposed on the first and second optical fibers and the substrate proximate each end of the fused portion.  
           [0012]    In one method in accordance with the invention each of optical fiber comprises a jacket; and the method includes removing the jacket from the first and second optical fibers in a region corresponding to the first portion. The first optical fiber jacket is bonded to the second optical fiber jacket adjacent each end of said first portion. The method includes tapering the first optical fiber jacket adjacent each end of the first portion to produce first and second tapered portions; and tapering the second optical fiber jacket adjacent each end of the first portion to produce first and second tapered portions. In accordance with certain aspects of the invention steps are included of bonding the first optical fiber first tapered portion to the second optical fiber first portion; and bonding the first optical fiber second tapered portion to the second optical fiber second tapered portion.  
           [0013]    In accordance with another aspect of the invention steps are included of mounting and bonding the first and second optical fibers to a substrate; providing dielectric gel on the substrate and on each of said first and second optical fibers in regions proximate the ends of the first portion; assembling the substrate and with the first and second optical fibers in a housing; and providing air around the first portion in the coupler. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    The invention will be better understood from a reading of the following detailed description in conjunction with the drawing figures in which like reference numerals are used to designate like elements, and in which:  
         [0015]    [0015]FIG. 1 shows an optical coupler in accordance with the principles of the invention;  
         [0016]    [0016]FIG. 2 shows the optical coupler of FIG. 1 in longitudinal cross section;  
         [0017]    [0017]FIG. 3 is a cross-section of the optical coupler of FIG. 2 taken at lines  3 - 3 ;  
         [0018]    [0018]FIG. 4 is a cross-section of the optical coupler of FIG. 2 taken at lines  4 - 4 ;  
         [0019]    [0019]FIG. 5 is a cross-section of the optical coupler of FIG. 2 taken at lines  5 - 5 ;  
         [0020]    [0020]FIG. 6 is a cross-section of the optical coupler of FIG. 2 taken at lines  6 - 6 ;  
         [0021]    [0021]FIG. 7 illustrates a fiber alignment station arrangement for manufacturing the coupler of FIG. 1;  
         [0022]    [0022]FIG. 8 illustrates a coupler draw station utilized to form the coupler of FIG. 1;  
         [0023]    [0023]FIG. 9 illustrates two optical fibers utilized to construct the coupler of FIG. 1;  
         [0024]    [0024]FIG. 10 illustrates the two fibers of FIG. 9 fused together in accordance with the invention; and  
         [0025]    [0025]FIGS. 11, 12, and  13  are cross-section drawings taken at lines A-A of FIG. 10. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Turning now to FIG. 1, a small sized polarization maintaining optical fiber coupler  100  is shown. Coupler  100  utilizes standard  125  micron cladding diameter polarization maintaining optical fiber  101 ,  103 . The specific fiber utilized in the illustrative embodiment of the invention is PANDA fiber. Coupler  100  includes a stainless steel tubular package  105  that is sealed at each end  107 ,  109  with epoxy. As more clearly seen in the longitudinal cross-section of FIG. 2, optical fibers  101 ,  103  are disposed in a channel  113  formed in a substrate  111 . In the illustrative embodiment of the invention, substrate  111  is a fused silica substrate.  
         [0027]    As will be explained in greater detail hereinafter, the two optical fibers  101 ,  103  each have the plastic jacket cladding removed over a length that in the illustrative embodiment ranges from 20 to 24 mm. Fiber  101  is aligned vertically to either the slow or fast principle polarization axis. Fiber  103  is aligned identically to fiber  101 . The cladding of each fiber  101 ,  103  that contacts the other fiber  103 ,  101  is shaved to produce a taper to bring the fibers  101 ,  103  closer to each other and the fibers  101 ,  103  are epoxy bonded to each other UV curable epoxy. The aligned fibers are fused and tapered to produce resulting coupler section  115 . The resulting coupler is disposed into channel  113  of substrate  111 . Fibers  101 ,  103  and substrate  111  are encapsulated into stainless steel tube  105  with epoxy end capping  107 ,  109  as shown in FIG. 3. As shown in the cross-section of FIG. 4, fibers  101 ,  103  are bonded into channel  113  with heat curable epoxy  117  in regions  119 ,  121  proximate end caps  107 ,  109 . To minimize vibration effects, a dielectric gel  123  is disposed in regions  125 ,  127  of channel  113  as shown in the cross-section of FIG. 5. Regions  125 ,  127  are proximate the ends of the coupling region  115  of optical fibers  101 ,  103 . In coupling region  115 , the bare, fused fiber is surrounded by air  133  as shown in FIG. 6.  
         [0028]    The length of the packaged coupler is 32 to 34 mm with a diameter of 3 mm. Typical losses are less than 0.5 dB and polarization extinction rations at the two output fibers is better than 20 dB.  
         [0029]    Optical fiber coupler  100  is manufactured utilizing an alignment station  700  shown in FIG. 7 and a draw station  800  shown in FIG. 8.  
         [0030]    Fiber alignment station  700  is utilized to vertically align optical fibers so as to identically align fibers according to a selected polarization axis. As more clearly shown in FIG. 9, each PANDA fiber  101 ,  103  includes a protective plastic jacket  901  surrounding its fiber  902 . Prior to subjecting each fiber to alignment, the protective plastic jacket  901  is removed over a region  131  in which the optical fiber  101  will be fused to a second optical fiber  103 . After removal of protective plastic jacket or cladding  901  in region  131 , the optical fiber  902  is exposed.  
         [0031]    Turning back to FIG. 7, the optical fiber  101  is then fed into the alignment station  700 . At the alignment station, the optical fiber  101  is positioned on an x-y-z stage  701 . Optical fiber  101  is captured by fiber clamps  703 ,  705  each respectively coupled to single axis stages  707 ,  709  having stepper motors  711 ,  713 , respectively. The optical fiber is supported with a predetermined tension, as monitored by tension gauge  735 , between the two stepper motors  711 ,  713  and supported on x-y-z stage  701 . A computer  715  is coupled to stepper motor controllers  717 ,  719  and is used to axially rotate optical fiber  101 ,  103  to a predetermined position. The predetermined position is determined by utilizing a helium neon laser to illuminate the fiber  101 ,  103 . The laser light passes through a reflector  723  having an aperture  725  formed therein for passage of the laser beam  729 . Reflector  723  is disposed at a 45° angle to laser beam  729  and disposed to reflect the image from optical fiber  101 ,  103  to a CCD camera  727 . CCD camera  727  is coupled to a monitor  731 .  
         [0032]    Computer  715  is utilized to cause both stepper motors  711 ,  713  to rotate optical fiber  101 ,  103  while the fiber is illuminated by laser beam  729 . The illumination of fiber  101 ,  103  by laser beam  729  causes a visible interference or “dot” pattern to occur in the illuminated fiber  101 ,  103 . The fiber is rotated until the predetermined dot pattern  733  appears on monitor  731 . At that time the optical fiber  101 ,  103  is retained in position. As shown if FIG. 9, the plastic jacket or cladding  901  immediately adjacent the bare fiber portion  131  of the optical fiber  101 ,  103  held in position is shaved to produce a flat surface  903 ,  905  tapered at a predetermined angle a to the longitudinal axis of the optical fiber. By providing tapered surfaces  903 ,  905  at an angle “a”, the bare portions  131  of optical fibers  101 ,  103  may be placed in side-by-side relationship without producing significant stress on the optical fibers  101 ,  103 . An ultraviolet curable epoxy is disposed on the shaved surfaces  903 ,  905  and the optical fibers  101 ,  103  are placed side by side with surfaces  903 ,  905  on each of the two fibers  101 ,  103  mating against each other and the bare optical fiber cores of fibers  101 ,  103  being in contact with each other.  
         [0033]    The fiber assembly of optical fibers  101 ,  103  is then placed in draw station  800  shown in FIG. 8. Draw station  800  is used to fuse and taper optical fibers  101 ,  103  using predetermined fabrication parameters in menu-driven computer  809  that controls operation of the draw station. Draw station  800  includes stepper motors  801 ,  803  that have clamps  805 ,  807  that capture and support optical fibers  101 ,  103 . Computer  809  via stepper motor interfaces  811 ,  813  controls each stepper motor. Draw station  800  also includes an H 2 /O 2  micro gas torch  815  that is positionally controlled by computer  809  via interface  817 . Details of the gas generator that supplies the gases to torch  815  are not shown in the drawing figures for clarity. A tunable laser  819  is coupled to one end of fiber  103 . Optical power and polarization measurement apparatus  821  is coupled to fiber  101  and  103 . With this arrangement, the amount of coupling between fiber  101  and fiber  103  is precisely determined during the manufacture of the coupler. The amount of coupling between fibers  101 ,  103  is determined by the amount of taper of the fused fibers  101 ,  103 .  
         [0034]    [0034]FIGS. 11, 12,  13  illustrate three different types of fusion of fibers  101 ,  103  in cross-sectional FIG. 11 illustrates the case where there is light fusion of the two fibers  101 ,  103 . FIG. 12 illustrates the instance with medium fusion and FIG. 13 illustrates strong fusion. As shown in FIG. 11, PANDA fibers  101 ,  103  each include stress rods  151  and an optical core  153 .  
         [0035]    Draw station  800  operates by having torch  815  travel at a constant velocity back and forth over the entirety of the coupling region of fibers  101 ,  103  while stepper motors  801 ,  803  draw the heated fibers such that fusion occurs along the entirely of the travel range of torch  815 . Tunable laser  821  couples light into fiber  103  and apparatus  821  monitors the light output from fibers  101 ,  103  until the desired couple power between fibers  101 ,  103  is obtained. Torch  821  is then turned of. The resulting fused optical fibers  101 ,  103  are then placed in a channel  113  of a fused silicon substrate  111  as shown in FIG. 2. Fibers  101 ,  103  are attached to substrate  111  with heat curable epoxy  119 ,  121  while maintaining the fibers  101 ,  103  straight. The epoxy is cured at 120° Centigrade for 10 minutes. The bare fibers are covered with a dielectric gel  125 ,  127  while leaving the coupling region of the fibers  101 ,  103  exposed to air with no surrounding material immediately proximate the fibers. The substrate assembly is cured in an oven at 50° Centigrade for a predetermined time. The substrate  111  is then inserted into a stainless steel tube  105  with three dots of epoxy at the bottom of the substrate  111 . The ends of the assembly are sealed with epoxy  107 .  
         [0036]    As will be appreciated by those skilled in the art, various modifications can be made to the embodiments shown in the various drawing figures and described above without departing from the spirit or scope of the invention. In addition, reference is made to various directions in the above description. It will be understood that the directional orientations are with reference to the particular drawing layout and are not intended to be limiting or restrictive. It is not intended that the invention be limited to the illustrative embodiments shown and described. It is intended that the invention be limited in scope only by the claims appended hereto.