Abstract:
An optical assembly includes a fiber optic pigtail, a sleeve in which the pigtail is housed, a collimating lens, a rod upon which is mounted an optical device and a lens sleeve in which the lens and rod are housed. Bonding lines bond the rod and lens to the lens sleeve, the pigtail to the pigtail sleeve, and the pigtail sleeve to the lens sleeve. The optical assembly and method of fabrication thereof support multiple optical device geometries, longitudinal adjustment of the position of the optical device within the optical assembly, an extended bonding line between the optical device and the environment outside the optical assembly, full x-y-z positioning of the pigtail relative to the lens and reduced reliance on bonding line thickness irregularities.

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
   This application claims the benefit of U.S. provisional application No. 60/437,195, filed on Dec. 31, 2002, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   Component alignment is an important part of the fabrication process for many types of optical assemblies. One reason is that light emitted from an optical fiber is highly divergent. Due to this high degree of divergence, unacceptable insertion loss, insertion loss ripple and wavelength dependent loss will often occur within an optical assembly unless light is directed between the components thereof with precision. Such precision is typically ensured only through careful alignment of the components during fabrication. Another reason component alignment is important is the desire for adaptable optical assemblies that support varied performance requirements. Fine-tuning the alignment of components of an optical assembly during fabrication is one way to meet varied performance requirements. 
   Unfortunately, the geometries of known optical assemblies have been ill-suited to either achieving acceptable insertion loss and insertion loss ripple, or accommodating varied performance requirements, or both, while still maintaining acceptable system robustness and cost characteristics. There is accordingly a need for an improved optical assembly geometry, and a fabrication process therefor, that are better suited to achieve these stated objectives. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention provides an optical assembly of a type that couples light from one or more optical fibers to an optical device that induces an action on the light and reflects the light back to one or more optical fibers. In a preferred embodiment, the optical assembly includes a fiber optic pigtail, a sleeve in which the pigtail is housed, a collimating lens, a rod upon which is mounted an optical device, and a lens sleeve in which the lens and rod are housed. Bonding lines bond the rod and lens to the lens sleeve, the pigtail to the pigtail sleeve, and the pigtail sleeve to the lens sleeve. 
   The mounting of the optical device on the rod before insertion into a lens sleeve provides several advantages relative to direct insertion of the optical device into the lens sleeve. Among them, it allows the optical assembly to support multiple optical device geometries; it allows longitudinal adjustment of the position of the optical device within the optical assembly to meet performance objectives; it allows an extended bonding line between the optical device and the environment outside the optical assembly; and it reduces reliance on bonding line thickness irregularities in the optical assembly to meet performance objectives. 
   The use of a separate pigtail sleeve and lens sleeve provides several advantages relative to the use of an integrated sleeve. Among them, it allows for full x-y-z positioning of the pigtail relative to the lens to meet performance objectives; and it reduces reliance on bonding line thickness irregularities in the optical assembly to meet performance objectives. 
   In another aspect, the present invention provides a method for fabrication of the optical assembly. In a preferred embodiment, the fabrication method includes the steps of bonding the lens to the lens sleeve; bonding the optical device to the rod; adjustably combining the pigtail and the pigtail sleeve; adjustably combining the lens sleeve, rod and pigtail sleeve; monitoring performance of the assembly and adjusting the relative positions of the rod, lens sleeve, pigtail and pigtail sleeve until initial performance objectives are met; bonding the rod to the lens sleeve; monitoring performance of the assembly and adjusting the relative positions of the lens sleeve, pigtail and pigtail sleeve until final performance objectives are met; and bonding the pigtail to the pigtail sleeve and the pigtail sleeve to the lens sleeve. 
   The first monitoring and adjusting step advantageously allows the z position and θ roll angle of the rod, and the x-y-z positions and θ roll angle of the pigtail, to be adjusted to meet initial performance objectives. The second monitoring and adjusting step advantageously allows the pigtail and pigtail sleeve to be adjusted in x-y-z-θ relative to one another and the other components of the optical assembly to meet final performance objectives. 
   These and other aspects of the present invention may be better understood by reference to the following detailed description, taken in conjunction with the accompanying drawings that are briefly described below. Of course, the actual scope of the invention is defined by the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an optical assembly in a preferred embodiment of the invention. 
       FIG. 2  is a cross-sectional view of an optical assembly in a preferred embodiment of the invention. 
       FIG. 3  is a flow diagram describing a method for fabricating an optical assembly in a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1 , a perspective view of an optical assembly  100  is shown in a preferred embodiment. Optical assembly  100  includes optical fibers  110  housed within a pigtail  120 , which is in turn housed within a pigtail sleeve  130 . Although a dual fiber arrangement is shown, an optical assembly operative within the scope of the invention may have one or more fibers. Pigtail sleeve  130  is coupled to a lens sleeve  150  in which is housed a lens  140  and a rod  160 . Mounted on rod  160  is an optical device  170 , such as a Gires-Tournois etalon (GTE), a thin-film filter, a Faraday garnet, a waveplate, or a more complex optical structure. Optical device  170  may have any of numerous geometries, such as rectangular, square or round. Components  110  through  160  are preferably made of glass, although other material compositions are possible. 
   In operation, inbound light enters optical assembly  100  on one of fibers  110 , travels through pigtail  120  on the one of fibers  110  and is emitted from the one of fibers  110  into free space between pigtail  120  and lens  140 . The light reaches lens  140  where it is subjected to angular and focal adjustments prior to being emitted from lens  140  into free space between lens  140  and optical device  170 . The light reaches optical device  170  where a desired action is induced on the light prior to reflecting the light back through lens  140  and into the other one of fibers  110 . 
   For example, where optical device  170  is a GTE, optical device  170  has a first mirror that is partially reflective, a second mirror that is fully reflective and a glass cavity in between. The spacing between the mirrors (i.e. the thickness of the glass cavity) is generally a function of the channel spacing of a Dense Wave Division Multiplexing (DWDM) system in which optical assembly  100  is operative. Light arriving from lens  140  enters and exits optical device  170  through the partially reflective mirror. Optical device  170  subjects different wavelength components of the light to variable delay in accordance with its resonant properties. That is, the partial reflectivity of the first mirror causes certain wavelength components to be restrained in the glass cavity between the first mirror and the second mirror longer than others. Optical device  170  thereby imposes a group delay on the wavelength components of the light which can correct group velocity dispersion previously induced on the light&#39;s pulses by a high speed, long haul, DWDM transmission system. The light may make multiple passes through optical assembly  100  in order to amplify the actions induced by optical device  170 . 
   Of course, optical device  170  need not be a GTE and optical assembly  100  may be operative in systems other than DWDM systems. Moreover, it will be appreciated that the light may enter and exit optical assembly  110  on the same one of fibers  110 . 
   Turning to  FIG. 2 , a cross sectional view of an optical assembly  200  is shown in a preferred embodiment. Optical assembly  200  includes components  210 ,  220 ,  230 ,  240 ,  250 ,  260 ,  270  that have counterparts in components  110 ,  120 ,  130 ,  140 ,  150 ,  160 ,  170  discussed above in connection with FIG.  1 . Additionally,  FIG. 2  shows bonding lines  225 ,  235 ,  245 ,  255 ,  265  that bond components together. Bonding line  225  bonds pigtail  220  with pigtail sleeve  230 , bonding line  235  bonds pigtail sleeve  230  with lens sleeve  250 , bonding line  245  bonds lens  240  with lens sleeve  250 , bonding line  255  bonds rod  260  with lens sleeve  250  and bonding line  265  bonds optical device  270  with rod  260 . Bonding lines  225 ,  235 ,  245 ,  255 ,  265  are preferably epoxy bonding lines created by allowing epoxy to wick into the interfaces between the respective components at elevated temperatures and holding the respective components securely until the epoxy cures. Each one of bonding lines  225 ,  235 ,  245 ,  255 ,  265  is preferably thin and of substantially uniform thickness, improving the imperviousness and insensitivity of optical assembly  200  to external environmental fluctuations after assembly, such as wide thermal excursions, high humidity and harsh mechanical shock. The coupling of optical device  270  to lens sleeve  250  via rod  260  advantageously enables bonding line  255  to be longer than a bonding line that would result from direct coupling of optical device  270  to lens sleeve  250 , further reducing the susceptibility of optical assembly  200  to external environmental influences that could adversely impact on performance. 
   Turning now to  FIG. 3 , in conjunction with  FIG. 2 , a flow diagram illustrates a method for fabricating optical assembly  200  in a preferred embodiment. 
   In Step  310 , lens is bonded to lens sleeve  250 . Lens  240  is inserted into lens sleeve  250  a pre-defined but tolerant depth. Lens  240  and lens sleeve  250  are heated and epoxy is allowed to wick between lens  240  and lens sleeve  250 . The components are held securely until the epoxy cures. When cured, the epoxy forms bonding line  245 . 
   In Step  315 , optical device  270  is bonded to rod  260 . Optical device  270  is placed adjacent to one end of rod  260 . Optical device  270  and rod  260  are heated and epoxy is allowed to wick between optical device  270  and rod  260 . The components are held securely until the epoxy cures. When cured, the epoxy forms bonding line  265 . Alternatively, optical device  270  may be optically contacted to one end of rod  260 . 
   In Step  320 , pigtail  220  and pigtail sleeve  230  are adjustably combined. Pigtail  220  is inserted into pigtail sleeve  230  a pre-defined but tolerant depth. 
   In Step  325 , lens sleeve  250 , rod  260  and pigtail sleeve  230  are adjustably combined to form an adjustably combined optical assembly. Lens sleeve  250 , rod  260  and pigtail sleeve  230  are held relative to one another at a position that approximates their position in the finally assembled state, and the components are heated. 
   In Steps  330 ,  335  and  340 , performance of the adjustably combined optical assembly is monitored and the relative position of the components is adjusted in response to performance feedback until initial performance objectives are met. For example, insertion loss as a function of frequency is monitored using either a light source and an optical spectrum analyzer, or a wavelength domain component analyzer. Adjustments in relative position are made manually to reduce overall insertion loss while at the same time obtaining a desired insertion loss ripple pattern. Alternatively, adjustments to relative position may be automated using equipment known in the art. Adjustments may be made, for example, to the z position and θ roll angle of rod  260  and the x-y-z positions and θ roll angle of pigtail  220 . 
   In Step  345 , rod  260  is bonded to lens sleeve  250  once initial performance objectives are met. When, for example, insertion loss, insertion loss ripple and wavelength dependent loss measurements are in sufficient conformance with initial performance objectives, epoxy is allowed to wick between rod  260  and lens sleeve  250 . The components are held securely until the epoxy cures. When cured, the epoxy forms bonding line  255 . 
   In Steps  350 ,  355  and  360 , performance of the adjustably combined optical assembly is monitored and the relative position of the components is adjusted in response to performance feedback until final performance objectives are met. These steps allows for fine adjustments in system performance that may be necessary or desirable after initial performance objectives have been met. Pigtail  220  and pigtail sleeve  230  may be adjusted in x-y-z-θ relative to one another and lens sleeve  235  until final performance objectives are met. 
   In Step  365 , pigtail  220  is bonded to pigtail sleeve  230  and pigtail sleeve  230  is bonded to lens sleeve  250  once final performance objectives are met. When, for example, insertion loss, insertion loss ripple and wavelength dependent loss measurements are in sufficient conformance with final performance objectives, epoxy is allowed to wick between pigtail  220  and pigtail sleeve  230 , and between pigtail sleeve  230  and lens sleeve  250 . The components are held securely until the epoxy cures. When cured, the epoxy forms bonding lines  225  and  235 , respectively. 
   It will be appreciated by those of ordinary skill in the art that the invention may be embodied in other specific forms with out departing from the spirit or essential character hereof. The present description is therefore considered in all respects illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.