Abstract:
A fiber optic device and a method of assembling the fiber optic device to provide at least four degrees of adjustment for an optical beam transported between an optical fiber and bulk optics in an optical device package. At an opening in the wall of the optical processor package, a washer-like structure allows initially adjustable contact at a flat surface thereof and at a spherically curved surface thereof. One of the surfaces bears against the wall about the opening and the other surface bears against a contact feature of a connecting assembly, typically a pre-assembled collimator assembly, that holds a termination portion of the optical fiber. Translational adjustment is made at the flat surface and tilting adjustment is made at the curved surface before final attachment at both surfaces. The fiber optic device is thus aligned with the optical device package and, through that package, can be aligned with another connecting assembly that is initially fixed in place at another opening of the optical device package. Circumferential tapers are provided at appropriate ones of the surfaces, the wall, and the contact feature to facilitate adhesive flow for final attachment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates broadly to devices and methods for combining bulk optical devices with optical fiber transmission, and more specifically relates to an optical fiber device having an attachment to an optical device package with improved modes of adjustment, and relates to methods for its assembly. 
     2. Discussion of the Related Art 
     The use of bulk optical devices in the midst of optical fiber transmission requires the interconversion of approximately collimated optical beams with fiber-optic transport. This interconversion employs a connecting assembly usually involving the use of collimators that have to be accurately aligned to the collimated beam path and then fixed into permanent position as part of the device packaging process. Typically, the connecting assembly is a pre-assembled assembly including a fiber termination, with one or more fiber ends, and a lens that is positioned and fixed relative to the fiber termination prior to attachment to the device package, so that an optical beam of approximately constant cross-section, a collimated beam, can be optically coupled to or from the fiber. 
     The tutorial FIG. 1 shows variations in how a collimated beam can emerge from an optical device package through a hole  12  in the package wall  11 . The beam may pass through at different places and at different angles. Specifically, beam # 3  passes through the hole highest, and beam # 2  lowest. Beam # 1  is nearly orthogonal to the wall  11 , but beam # 3  is tilted downward the most. Such variations can easily occur due to variation in optical parts and variation in placements of the internal optics and/or the other pre-assembled collimator assemblies, located at other openings (not shown) of the optical device package. 
     In placing a pre-assembled collimator assembly (hereinafter, PCA) near to the wall to couple well to each of these three illustrated beams, we need to place the PCA at the right position (height, and depth into the paper) and with a suitable tilt to match that of the actual beam. For a coordinate system, let the z-axis be orthogonal to the package wall, approximately parallel to the beam direction. Let the x-axis be perpendicular to the plane of the paper, and the y-axis in the plane of the paper, pointing up. Small angular tilts can be described as small rotations about the x-, y-, and z-axes respectively. 
     Although the complete specification of the orientation of a solid in space takes six parameters (for six degrees of freedom), the placement of a collimator to couple well to the beam is not very sensitive to displacement along the z-axis, nor to rotation about the z-axis in the absence of polarization sensitivity. Thus, the main concern is for the other four degrees of freedom and the corresponding four parameters to control. 
     Tutorial FIG. 2 shows the relative insensitivity of loss to axial displacement, that is, translational displacement along the z-axis. Moving up to three centimeters in z results in a loss of 0.06 dB, less than two percent loss. The specific curve  21  in FIG. 2 is for a beam at 980 nanometers (nm), which has been expanded conventionally in cross-section to an approximate diameter of 0.4 mm to reduce the effects of diffraction within the interior optics of an optical device package, of which the package wall is wall  11  in FIG.  1 . 
     Tutorial FIG. 3 shows in curve  31  that for a translation of the collimator in x or y, lateral motion, movement of 0.1 mm, 100 micrometers, from optimum causes a loss of about one dB, or about 20 percent loss. The specific curve  31  is also for a conventionally expanded beam of approximately 0.4 mm diameter at 980 nm. 
     In curve  41  tutorial FIG. 4 shows for lateral beam tilts a sensitivity that is considered relatively great. Lateral beam tilts are angular tilts of the beam about the x-direction or the y-direction. A tilt of less than three minutes of arc from optimum will result in a loss of about one dB. The specific curve  41  is also for a conventionally expanded beam of approximately 0.4 mm diameter at 980 nm. 
     In summary, axial displacement, displacement in z, needs control only by accurate manufacture of the package to its optical design, but for the four sensitive parameters, greater precision is needed. 
     SUMMARY OF THE INVENTION 
     According to the invention, a connecting assembly has a contact feature that contacts a first surface of a washer-like structure, and a second surface of the washer-like structure contacts the wall around the opening in the wall of the optical device package. The first and second surfaces are shaped differently and, preferably, each provides at least two directions of adjustment by relative motion of the contacting surfaces, prior to attachment. 
     According to the method of the invention, the washer-like structure is moved on its area of contact with the wall, and the connecting assembly contact feature and the washer-like structure are moved relatively on their area of contact until both the washer-like structure and the connecting assembly are centered on the beam path. 
     According to a feature of the invention, attachment is then made at both surfaces of contact. 
    
    
     Various features and advantages of the invention may be appreciated from its several implementations and as practiced according to the teaching of the following detailed description, taken together with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an optical processing package wall with possible collimated beams passing through an opening therein; 
     FIG. 2 is a theoretical plot of coupling loss versus z-displacement; 
     FIG. 3 is a theoretical plot of coupling loss versus lateral displacement; 
     FIG. 4 is a theoretical plot of coupling loss versus lateral tilt ; 
     FIG. 5 is a cross-sectional view of a first embodiment of the invention; 
     FIG. 6 is a cross-sectional view of a second embodiment of the invention; 
     FIG. 7 is a cross-sectional view of a third embodiment of the invention; and 
     FIG. 8 is a flow diagram of the basic method of the invention. 
    
    
     DETAILED DESCRIPTION 
     The following background information will enable a more precise understanding of the invention. 
     An optical device package connected to one or more fibers of a fiber optic system may typically involve propagation of a free space beam. The package may or may not involve conversion to a baseband signal and reconversion to an optical beam. The interior optical piece parts may include, but are not limited to, coated slabs such as filters, prisms, isolator sub-assemblies, and laser and photodiode optical subassemblies. 
     The optical device package will have package walls that contain the optical devices used to manipulate the beam. The walls of interest for the present invention will have one or more holes through which light can pass. Preferred wall geometries are cylindrical and box-like. 
     Immediately after leaving the PCA (the pre -assembled collimator assembly) the beam is a free-space beam with approximately constant cross-section. If it is described as a bundle of ray paths, the ray paths are drawn mutually parallel. As light is a wave phenomenon, diffraction effects play a role and prevent an actual optical beam from maintaining exactly a constant cross-section. A practical result of diffraction is that the optical coupling between two collimators does depend somewhat on their separation. The larger the initial cross-section of a collimated beam, the slighter the effects of diffraction. 
     In general, the present invention makes use of four degrees of freedom, by which is meant the ability to change the orientation of a PCA so that it can be translated lateral to the beam direction, that is, in the x, y plane, and so that it can be tilted about the x and y directions. An optional fifth degree of freedom may be useful to rotate the PCA relative to an optical polarization by rotation about the optical axis, the z-axis. 
     The present invention also makes use of bearing surfaces, which are pairs of surfaces that provide a means of sliding constraint between two parts. In the preferred embodiments, the sliding constraints used are planar and spherical. Planar constraint provides two degrees of freedom of adjustment by lateral translation, and spherical constraint provides two degrees of freedom of adjustment by tilting. Either provides a third degree of freedom corresponding to rotation about the direction of the optical beam. Following adjustment, attachment can take place between the bearing surfaces. 
     Attachment is the process of fixing the orientation of two parts together in a more or less permanent manner. Common and preferred methods are the use of adhesive, with light (e.g., ultra-violet (uv)) curing, heat curing and/or chemical reaction, or the use of solder, or welding, either resistance welding or laser-induced welding. An important aspect of attachment is the lack of uncontrolled movement, which would spoil alignment. Another important aspect is the lack of movement in response to environmental changes. 
     The preferred embodiments use a highly-filled adhesive material. Highly filled adhesives reduce adhesive swelling, most cure shifts due to cure shrinkage, and thermal displacements due to temperature changes or cycling. Specific examples are Optocast® 3410, Optocast® 3415 and Optocast® 3417, available from Electronic Materials Inc. in Breckinridge, Colo. These materials can use brief ultraviolet exposure for initial tacking, followed by longer thermal curing once sufficient material has been flowed into one or more adhesive channels that are provided. 
     For applications where thixotropic adhesives are not practical, use of cationic, unfilled non-polar adhesives (e.g., cycloaliphatic epoxy functionalized siloxanes) can significantly reduce moisture-induced swelling and improve damp heat performance of the attachment. Further, for glass or metal surfaces, proper cleaning and adhesion promotion are desirable. Silane adhesion promoters are effective. For plastic injection-molded surfaces, oxygen plasma cleaning or uv ozone cleaning are effective. 
     It has further been found to be important to provide damp heat durability of the adhesion by pre-treating the surfaces to be adhered with an adhesion promoter such as gamma glycidoxypropyltrimethoxysilane, or any one of many other adhesion promoters. 
     A particular feature of the present invention involves the use of a washer-like structure, which is a mechanical element that has a hole through which passes any or all of the following: (1) the free space beam, (2) the PCA, and (3) the fiber pigtail. The washer-like structure has two bearing surfaces that respectively mate with the package wall and with a contact feature of the PCA. These two bearing surfaces are preferably respectively flat and spherically curved, with the center of curvature of the latter approximately centered upon some portion of the free space beam in the body of the optical device package. 
     Cooperative with the washer-like structure is the PCA contact feature. Somewhere on the PCA out of the path of the beam, there is a bearing surface that will be in contact with one of the two bearing surfaces of the washer-like structure. This is the PCA contact feature. It may be part of the surface of one or more optical parts or fiber termination portion, or it may be a piece part that is added to the PCA. 
     In FIG. 5, a first embodiment of the invention is implemented in an optical fiber device including an optical device package, illustratively between two sections of optical fiber. 
     More specifically, a fiber optic device is provided with four or more degrees of freedom of adjustment according to the invention. The device includes a cylindrical optical device package  51 , including a package wall  52 , end openings  54  and  56 , and interior optics  58 . Inserted into opening  54  is a PCA  60  of known type with its pigtail  62 , a termination portion of an optical fiber. At the other end, on the portion of package wall  52  surrounding opening  56  is disposed washer-like structure  64  with its flat bearing surface # 2  in sliding contact with the correspondingly flat end wall. Its spherically-curved bearing surface # 1  is in sliding contact with the correspondingly spherically-curved bearing surface of the PCA contact feature  66  of PCA  68 . 
     The spherically curved bearing surface of the PCA contact feature  66  is provided with an initial circumferential taper  67  to facilitate adhesive flow between the contacting bearing surfaces. The centers of curvature of these spherically curved surfaces lie on the intended beam path within optical device package  51 . The initial taper  67  does not provide a significant deviation from spherical curvature for the contacting surface of PCA feature  66 . PCA  68  can be essentially identical to PCA  60 . Likewise, its pigtail  70  can be like pigtail  62 . 
     The flat bearing surface end wall of package wall  52  is provided with circumferential taper  57  to facilitate adhesive flow. Alternatively, it could be provided in flat bearing surface # 2  of washer  64 . Alternatively, one or more radially-extending grooves (not shown), instead of the tapers shown, could be used in one or more of the contacting bearing surfaces # 1  or # 2 . 
     The fiber optic device of FIG. 5 is tested by powering one of the pigtails and measuring the power transferred to the other pigtail. Optimizing the transverse alignment is achieved by sliding washer-like structure  64  at bearing surface # 2 ; and optimizing the angular alignment is achieved by sliding PCA contact feature  66  at bearing surface # 1 . Then the parts are squeezed together, the alignment re-checked, and only thereafter is attachment completed, as described in more detail in the initial portion of this detailed description, by flowing in the adhesive, uv-tacking the parts and then heating curing the adhesive. The alignment process needs to be active as described because of minor variations in the fabrication of the parts and because of variations in the assembly process. 
     Like that of FIG. 5, the fiber optic device of FIG. 6 is provided with at least four degrees of freedom of adjustment according to the invention. The device includes a cylindrical optical processing package  81 , including a package wall  82 , end openings  84  and  86 , and interior optics  88 . Inserted into opening  84  is a PCA  90  of known type. PCA  90  includes pigtail  92 , which is a termination of the optical fiber. The PCA is assembled with the lens portion in fixed relation to the pigtail  92 . The body of the PCA is formed with the lens, the pigtail, and a sleeve or sheath around everything optical in the PCA. At the other end of the device, on the portion of package wall  82  surrounding opening  86  is disposed washer-like structure  94  with its flat bearing surface # 2  in sliding contact with the correspondingly flat end wall. Taper  87  is illustratively provided in bearing surface # 2  of package wall  82 , although a taper could be provided, and a taper or grooves could be provided in the contacting flat surface of washer-like structure  94 . 
     The spherically-curved bearing surface # 1  of washer-like structure is in sliding contact with the correspondingly spherically-curved bearing surface of the PCA contact feature  96  of PCA  98 . The centers of curvature of these spherically-curved surfaces lie on the intended beam path within optical processing package  81 . Because PCA contact feature  96  is a separate piece part from PCA  98 , the latter can be essentially identical to PCA  90 . Likewise, its pigtail  100  can be like pigtail  92 . Spherically-curved bearing surface # 1  of PCA contact feature  96  has initial circumferential taper  97  to facilitate adhesive flow, although a taper or grooves could also be provided in the spherically-curved surface of washer-like structure  94 . 
     The fiber optic device of FIG. 6 is tested by optically powering one of the pigtails  92 ,  100  and measuring the optical power transferred to the other pigtail. Optimizing the transverse alignment is achieved by sliding washer-like structure  94  at bearing surface # 2 ; and optimizing the angular alignment is achieved by sliding PCA contact feature  96  at bearing surface # 1 . Then the contacting surfaces are squeezed together as described above for the implementation of FIG. 5, the alignment is re-checked, and only thereafter is attachment completed. Access for depositing the adhesive is facilitated by making package wall  82  in two or more parts that join along lines parallel to the optical axis. 
     Unlike FIGS. 5 and 6, FIG. 7 shows two adjustable ports. Like those of FIGS. 5 and 6, the fiber optic device of FIG. 7 is provided at each of its two adjustable ports at openings  116  and  117  with at least four degrees of freedom of adjustment according to the invention. The device includes a box-like optical device package  111 , including a package wall  112 , end openings  114 ,  116 , and  117 , and interior optics  118 . Inserted into opening  114  is a PCA  120  of known type with its pigtail  122 , a termination portion of an optical fiber. These parts may all be the same or similar to the parts of FIG. 6 numbered 30 digits lower. 
     At the other end of the device, on the interior portions of package wall  112  surrounding openings  116  and  117 , are disposed washer-like structures  124  and  125  with their flat bearing surfaces, in this case designated with # 1 , in sliding contact with the correspondingly flat end wall. Their spherically curved bearing surfaces # 2  are in sliding contact with the correspondingly spherically curved bearing surfaces of the PCA contact features  126  and  127  on PCA  128  and PCA  129  respectively. The centers of curvature of these spherically curved surfaces lie on the respective intended beam paths within optical processing package  111 . 
     At both of ports  116  and  117  the flat bearing surfaces # 2  of washer-like structures  124  and  125  face right against interior portions of package wall  112  and the spherically-curved bearing surfaces # 1  of washer-like structures  124  and  125  face left, and the contacting spherically-curved bearing surfaces of PCA contact features  126  and  127  face right. In this implementation, package wall  112  is preferably bathtub-shaped, that is, an open box. One can reach into the interior to place interior optics  118 , washer-like structures  124  and  125 , PCA contact features  126  and  127 , PCA  128  and PCA  129 , and any other desired components. Lateral and angular alignment at both adjustable ports is performed and attachment is then achieved. Compression is brought to bear on bearing surfaces # 1  and # 2  at each port by moving PCA  128  and PCA  129  to the right against respective washer-like structures  124  and  125 . Alignment is checked and then adhesive is placed at both sets of initial circumferential tapers  137  and  147  at the respective bearing surfaces and subsequently uv-tacked. Heat curing is subsequently performed. Optionally, a lid is placed on package wall  112  to close it. 
     Alignment is checked as in the other implementations. The fiber optic device of FIG. 7 is tested by powering, for example, pigtail  122  and measuring the power transferred to each of the other pigtails  130  and  131 . Optimizing the transverse alignments is achieved by sliding washer-like Structures  124  and  125  at bearing surfaces # 1 ; and optimizing the angular alignments is achieved by sliding PCA contact features  126  and  127  at bearing surfaces # 2 . Then the parts are squeezed together, the alignment is re-checked, and only thereafter is attachment completed. 
     In FIG. 8, a flow chart of the basic method of the invention is set out. The following starting materials are provided in step S 11 : an optical fiber; an optical device package having a wall with at least one opening; a pre-assembled collimator assembly including a termination portion (pigtail) of the optical fiber and having a contact feature; and a washer-like structure having first and second bearing surfaces, at least one of which is curved. 
     Step S 13  provides that one aligns the washer-like structure on the opening to have one of the first and second surfaces bearing on the wall and centered on the beam path. 
     Step S 15  provides that one aligns the pre-assembled collimator assembly to have its contact feature bearing on the other of the first and second surfaces and centered on the beam path. 
     Steps S 13  and S 15  can also be done in the reverse order. 
     S 17  is the finishing step. After squeezing the bearing surfaces together and re-checking the alignment, one provides attachment at both of the first and second surfaces. 
     It should be understood that the techniques and arrangements of the present invention could be varied significantly without departing from the principles of the invention as explained above and claimed hereinafter. For example, while it is preferred that the optical processing package wall be cylindrical or box-like, it is feasible to provide a spherically-curved bearing surface around an opening in the package wall to bear on a spherically-curved bearing surface of the washer-like structure. While it is preferred that the other bearing surface of the washer-like structure be flat, both of its bearing surfaces can be curved, typically spherically curved, if the washer-like structure is sufficiently elongated along the optical axis. That is, if the two bearing surfaces of the washer-like structure are sufficiently separated along the optical axis, a tilt at one of the surfaces effectively provides translation at the other of the surfaces. 
     Further, while each of the preferred implementations of the invention has been described as employing a pre-assembled collimator assembly, it should be understood that other connecting assemblies could be employed. Also, rotation of the connecting assembly around the z-axis is generally available, providing three degrees of adjustment for each pair of adjustable contacting surfaces.