Abstract:
Apparatus and method for allowing fine adjustment of a diffraction grating within an optical wavelength-division multiplexing (WDM) device and for maintaining the grating in near-littrow alignment (i.e., to within an accuracy of +/−0.003 degrees) over the device&#39;s operating temperature range. Precise alignment is maintained by use of a 0.3 PPM/° C. “super invar” material in a wedge-shaped grating mount to eliminate an angular shift of the grating over typical operating temperatures. The wedge-shaped grating mount and thereby the orientation of the surface of the grating are adjusted precisely and maintained in position over the operating temperatures via a dual flexure structure wherein four precision adjustment screws with jeweled contact tips are tightened to achieve the desired orientation and stiffen the structure and then locked via locking nuts to prevent slippage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical communications and, more specifically, to precision mechanical mounting mechanisms for optical components. 
     2. Description of the Related Art 
     Wavelength-division multiplexing (WDM) (also known as dense-wavelength-division multiplexing (DWDM)) has been shown as a promising approach for increasing the capacity of existing fiber optic networks. A communications system employing WDM uses multiple optical signal channels, each channel being assigned a particular channel wavelength. In such a WDM system, optical signal channels are typically generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a waveguide, and demultiplexed such that each channel wavelength may be individually routed or switched. 
     One demultiplexing technique involves the use of an optical diffraction grating to separate the wavelengths of an incident optical signal into its constituent channels. Such a technique is described in detail in U.S. Pat. No. 6,307,657 (herein “the &#39;657 patent”), incorporated herein by reference in its entirety. 
     As described the &#39;657 patent, relative alignment of various optical components within the device including the optical grating is important to proper operation of the device. The &#39;657 patent discusses techniques for relative adjustment of these components, maintenance of reasonable relative alignment between the components over temperature, and aspects of the design that afford tolerance to misalignment. However, as the wavelength spacing between channels in WDM systems decreases, the tolerance of the grating angle with respect to the incident light also decreases. Additionally, as optical components (e.g. micro-mirror arrays) and the devices that incorporate them are made smaller and smaller to accommodate more channels in less space, the linear distance associated with the angular tolerance of the grating location also decreases. These trends lead to the need for increasingly more accurate adjustment and temperature compensation, maintenance, and athermalization mechanisms. 
     SUMMARY OF THE INVENTION 
     Problems in the prior art are addressed in accordance with principles of the invention by an apparatus and method for enabling coarse and fine alignment adjustment of a diffraction grating within an optical wavelength-division multiplexing (WDM) device and for maintaining the diffraction grating in near-littrow alignment (e.g., to within an accuracy of +/−0.003 degrees) over the device&#39;s operating temperature (e.g., −5.0° C. to 65° C.). 
     In a preferred embodiment, precise grating alignment is maintained in a WDM device by use of a 0.3 parts-per-million per degree centigrade (PPM/° C.) Super Invar alloy (e.g., 63Fe-32Ni-5Co) in a wedge-shaped grating mount for the optical grating to eliminate an angular shift of the grating over temperature that is typical in the prior art due to use of other materials such as Invar (e.g., 64Fe-36Ni). 
     The grating mount is affixed to a first platform whose angle along a first axis is adjustable relative to a second platform. The first platform is attached to the second platform via a first flexure spring oriented parallel to the first axis and the angle is adjusted using first and second adjustment screws. The screws are threaded through the second platform on opposing sides of the first flexure spring and the tips of the screws make contact with the first platform to adjust the angle. The second platform is attached to a base platform (i.e., the grating assembly mounting base) via a second flexure spring oriented parallel to a second axis, the second axis being substantially orthogonal to the urst axis. The angle between the second and base platforms is adjusted using third and fourth screws. In one or more embodiments, adjustment screws are tipped with a jeweled or other low-friction element and mated to their respective platforms via a ball-and-socket arrangement that prevents incremental slippage of the contact over time and temperature and thus prevents minor changes in the angle between the platforms. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
     FIG. 1 is a cutaway illustration of the functional components of an optical wavelength-division multiplexing (WDM) device, according to one embodiment of the present invention. 
     FIG. 2 is an enlarged view the optical grating subsystem  122  of FIG.  1 . 
     FIG. 3 is a detail view of jeweled contact arrangement  156  of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. 
     Device Construction 
     FIG. 1 is a cutaway illustration of an optical wavelength-division multiplexing (WDM) device  100 , according to one embodiment of the present invention. FIG. 1 shows only those elements of the device that are relevant for the present discussion. Also, although the actual device is a three-dimensional object, the only components illustrated are (1) those that would be visible from a view that is normal to the X-Y plane of the device and (2) those components that can be unambiguously represented using semi-transparency or cutaways. Further, the illustration is not to scale and components are relatively sized to better illustrate their function or functional relationship to other components. 
     Device  100  includes a frame that is preferably implemented in the form of a tube that is open at both ends. The frame may be composed of a single material or multiple materials that are affixed together to achieve a specific composite coefficient of expansion with respect to an operating temperature range of interest. The materials are affixed to each other preferably using welds (e.g., for metal-to-metal bonds) or epoxy (for composite material bonds) though other methods of attachment would also apply such as rivets, screws, slots, and braces as would be understood to one skilled in the art. 
     As illustrated, the frame of optical device  100  is composed of two materials  102  and  104  welded together. Affixed to the left end of the frame is mounting platform  106 . The attachment is such that right surface  108  of mounting platform  106  may move relative to the frame given thermal expansion or contraction of the mounting platform over the operating range of the device. 
     Affixed to mounting platform  106  (via a cut-through) is fiber-optic coupler  110  and affixed to right surface  108  of mounting platform  106  is micro-electro-mechanical system (MEMS) micro-mirror array  112 . Fiber-optic coupler  110  enables input/output fiber  114  to be coupled to device  100 . MEMS micro-mirror array (MMA)  112  includes micro-mirrors  116 ,  118 , and  120 , each of which can be tilted relative to the Y-Z plane (i.e. rotated about Y-axis and/or rotated about the Z-axis independently) under control of an external microcontroller (not illustrated). For clarity of illustration, the MMA of FIG. 1 is depicted as only including three micro-mirrors. However, in practice, devices according to this invention may include MMAs having tens or even hundreds of micro-mirror elements as would be understood to one skilled in the art. 
     Also affixed to the frame are optical diffraction grating subsystem  122  and (via support mounts  124  and  126 ) achromatic compound field lens  128 . 
     Device Operation 
     Device  100  may accomplish a number of different functions including WDM signal equalization, wavelength dropping, and wavelength switching depending on how it is used, although it would be understood that any device that makes use of an optical grating for related purposes and requires an accurate angular mounting of the grating and maintenance over temperature of that mounting would benefit from this invention. 
     As illustrated in FIG. 1, a single fiber  114  carrying a WDM signal composed of three wavelengths (λ 1 ,λ 2 , and λ 3 ) is coupled into device  100  using fiber optic coupler  110 . Light from the fiber diverges at aperture  130  and is incident upon the left-hand side of field lens  128 . Field lens  128  accomplishes, among other things, the task of collimating the incident light which then strikes the surface of optical grating  132 . At the grating, the light is angularly split into its constituent wavelengths and reflected back toward field lens  128 . The light of each constituent wavelength is focused by lens  128  onto a different micro-mirror of MMA  112 . Specifically, the light of constituent wavelength λ 1  is focused onto micro-mirror  120 , the light of constituent wavelength λ 2  is focused onto micro-mirror  118 , and the light of constituent wavelength λ 3  is focused onto micro-mirror  116 . 
     When the device operates as an equalizer, the angular rotation of each micro-mirror about the Y-axis and the Z-axis is nominally controlled such that the surface of the micro-mirror is normal to the light of the constituent wavelength that is incident upon it (this is the micro-mirrors nominal orientation). In this configuration, all of the light of the different wavelengths is reflected back from the micro-mirrors to the field lens where it is collimated in the direction of the grating. At grating  132 , the angular separation of the wavelengths is removed and the resulting multiple-wavelength light signal is reflected back toward the field lens. The light is focused by lens  128  onto aperture  130  of input fiber  114  where it is coupled by a circulator (not shown) to the output path. 
     In the case where the powers of all constituent wavelengths in the input WDM signal are equal, the micro-mirrors are preferably set to their nominal orientations. However, in cases where the powers are not equal, one or more of the micro-mirrors can be rotated (e.g., about the Z-axis) so that a percentage of light of the corresponding constituent wavelengths of the signal is not fully coupled back into the fiber. For example, if the light of wavelength λ 1  is “hot” relative to λ 2  and λ 3  and needs to be attenuated, micro-mirror  120  may be angled slightly away from its nominal orientation to the incident light. As a result, some portion of the λ 1  light that is reflected back from micro-mirror  120  will fail to be coupled back into the aperture at  130  following reflection from grating  132 . The result will be an equalization of the incoming WDM signal. 
     By significantly angling one or more of the mirrors, it is possible to substantially eliminate the corresponding wavelength from the output coupling at aperture  130 . In an embodiment in which an array of optical fibers are mounted on mounting platform  106 , a device similar to device  100  can be used to implement wavelength adding, dropping, multiplexing, and/or demultiplexing functions by appropriately rotating one or more of the micro-mirrors to switch one or more of the wavelengths from one or more incoming fibers to one or more different outgoing fibers. 
     Compensation for Thermal Expansion/Contraction 
     The distance F 1  between the right side  134  of MMA  112  and the center of field lens  128  is selected to provide optimal optical performance (e.g., based on the focal distance of field lens  128 ). If the distance F 1  were to change, then the image of the constituent wavelengths upon each micro-mirror of MMA  112  would be out of alignment. Such misalignment might contribute to signal degradation and cross-talk. To avoid this, the materials used for components  106  and  102  are chosen such that the distance F 1  between the right side of MMA  112  and the center of field lens  128  is maintained essentially constant over temperature. This is done by choosing materials such that the coefficient of expansion of the material of component  102  multiplied by the length (i.e., in the X-dimension) of that component is equal to the coefficient of expansion of the material of component  106  multiplied by the length of that component. In this case we have considered the expansion of the MMA itself to be negligible but that may also considered in implementations where it is of significant scale. Note that the distance between the lens and the grating is less sensitive to minor changes over temperature because of the collimating effect of the field lens on the grating side of the lens. 
     Optical Grating Subsystem 
     Optical grating subsystem  122  is designed to fix and maintain (over the device operating temperature) the angle of grating  132  relative to the incident light from field lens  128  to within an accuracy of +/−0.003 degrees. This requirement is dictated by the properties of the grating (e.g. groove spacing). The angle of the grating is coarsely set by affixing the grating to wedge  136  that has been machined to approximately exhibit the desired relative angle. Wedge  136  is constructed of Super-Invar (63Fe-32Ni-5Co) or a similar material that exhibits a coefficient of expansion ≦0.3 PPM/° C. Wedge  136  is mounted to wedge-mounting platform  138 . Assuming wedge-mounting platform  138  can be adjusted to set the grating to the desired angle and maintain this angle to within the desired accuracy over operating temperature, the angular error caused by the small (0.3 PPM/° C.) coefficient of expansion of the wedge will be maintained within tolerance of the device&#39;s operation. If the coefficient of expansion of the wedge material were greater than 0.3 PPM/° C., then the device might fail to operate within specifications. 
     Wedge-mounting platform  138  is attached to adjustment platform  140  using horizontal flexure spring  142 , and adjustment platform  140  is attached to grating subsystem base  144  using vertical flexure spring  146 . Horizontal flexure spring  142  allows movement of the wedge-mounting platform (and thereby the wedge and the optical grating) substantially in the X-Y plane, while vertical flexure  146  allows movement of the wedge-mounting platform substantially in the X-Z plane. These movements, if small, can be seen at the surface of the grating to approximate minute changes in the angle of the grating. Specifically, flexion of horizontal spring  142  results in angular rotation of the grating about the Z-axis, while flexion of vertical spring  146  results in angular rotation of the grating about the Y-axis. 
     The angle between wedge-mounting platform  138  and adjustment platform  140  is adjusted using upper and lower precision screws  148 - 150 . For example, to achieve a small clockwise rotation (as seen in FIG. 1) of the grating about the Z-axis, lower precision screw  148  is first brought into contact with the lower portion of wedge-mounting platform  138  and then tightened until the desired clockwise rotation is achieved (corresponding to flexion of flexure spring  142 ). The upper precision screw  150  is then brought into contact with the upper portion of wedge-mounting platform  138  to achieve stiffening of the structure (e.g., to provide additional robustness to vibration). 
     The angle between adjustment platform  140  and grating system base  144  is adjusted using foreground precision screw  152  (visible in FIG. 1) and background precision screw  154  (hidden in FIG. 1 behind foreground precision screw  152 ). For example, to achieve a small clockwise rotation (as viewed from the top of FIG. 1 looking along the Y-axis) of the grating about the Y-axis, foreground precision screw  152  is first brought into contact with the foreground portion of adjustment platform  140  and then tightened until the desired clockwise rotation is achieved (corresponding to flexion of flexure spring  146 ). The background precision screw  154  is then brought into contact with the background portion of adjustment platform  140  to achieve stiffening of the structure. 
     Additional elements of this embodiment are now discussed with respect to the enlarged view of grating subsystem  122  provided in FIG.  2 . As previously described, angular adjustment of the grating is achieved using precision screws. These screws are typically made of #416 stainless steel and have greater than 100 threads per inch. Precision screws  152 - 154  are threaded into (at least a portion of) grating subsystem base  144  and may extend through base  144  to make contact with rightmost face  202  of adjustment platform  140 . In particular, foreground screw  152  can be tightened to make contact with the foreground portion of face  202 , and background screw  154  can be tightened to make contact with the background portion of face  202 . Precision screws  148 - 150  pass through grating subsystem base  144 , flexure spring  146 , and the rightmost portion of adjustment platform  140 , and can be threaded into the leftmost portion of adjustment platform  140  to extend through platform  140  to make contact with rightmost face  204  of wedge-mounting platform  138 . In particular, upper screw  150  can be tightened to make contact with the upper portion of face  204 , and lower screw  148  can be tightened to make contact with the lower portion of face  204 . Once the screws are tightened to the achieve the desired orientation of the grating (as discussed previously), locking nuts  206  (shown) and  208  (hidden in this view) and  216  and  218  are tightened to prevent slipping of precision screws  152 - 154  and  148 - 150 , respectively. For generality, two different types of locking nuts are illustrated in FIG. 2, although any locking mechanism that prevents rotational slippage of the precision adjustment screws in this configuration could be used, as would be understood to one skilled in the art. 
     Foreground (illustrated)  206  and background (hidden in this view)  208  locking nuts are conventional nuts that are tightened until they make contact with rightmost surface  214  of base  144 . Note that corresponding elements of the upper and lower precision screws and locking nuts are the same. Upper and lower locking nuts  216  and  218  are sleeve-type nuts whose sleeves  210  and  212 , respectively pass through base  144 , spring  146 , and the rightmost portion of platform  140  and are tightened against the leftmost edges  230  and  232  of sleeve right-of-ways  222  and  224 . Sleeve right-of-ways  222  and  224  provide clearance around screws  148  and  150  and their locking-nut sleeves  210  and  212  to allow the screws and locking-nut sleeves to pass through. As illustrated, the lower locking nut is shown in locked position with the leftmost portion  228  of the sleeve  212  in contact with the leftmost portion of the right-of-way  232 . In an alternative implementation, sleeve right-of-way  222  and  224  do not extend into platform  140 . In this case, the leftmost surfaces  226 , and  228  of the sleeves of the locking nuts will tighten against the right surface  202  of platform  140 . 
     Contact Between Screw and Platform 
     To provide for low-friction contact between the adjustment screws  148 ,  150 ,  152 , and  154  and their respective contact surfaces  204  and  202  both during adjustment and during operation over the device operating temperature (e.g., −5.0° C. to 65° C.), each adjustment screw is tipped with a jeweled or other low-friction element and mated to its contact surface via a low-friction-surfaced ball (e.g., a jewel element such as sapphire) and low-friction socket arrangement  156 . Each ball-and-socket arrangement might be encapsulated in a conformal sleeve or similar container to prevent loss of components when the screw tip is not in direct contact with its target surface or the socket may hold the ball in place. Depending on the implementation, the arrangement may move with the screw tip, may be affixed to the contact surface, or may contain some elements that are affixed to the screw tip, some that are affixed to the contact surface, and some that are held in alignment by the container. Such arrangements prevent incremental slippage of the contact over time and temperature and thus prevent minor angular changes between the platforms. 
     FIG. 3 provides a detail view of one possible implementation of a low-friction ball-and-socket arrangement  156  of FIGS. 1 and 2. As shown, a left, concave jewel-surfaced socket (e.g., sapphire V-cup)  302  is affixed to the contact platform (e.g.,  138 ) and a right, flat jeweled disc (e.g. sapphire disc)  304  is affixed to the end of the screw (e.g., screw  148 ). The disc and the socket may be affixed to their respective surfaces via high temperature cure hard epoxy or similar method. A jeweled sphere (e.g., sapphire ball)  306  is free to move between the socket and disc and accommodate the slight rotation of the contact platform and the corresponding slight deviation of the contact angle away from normal upon tightening of the screws. The conformal sleeve or other suitable container for retaining ball  306  in place between sockets  302  and  304  is not shown in FIG.  3 . As would be understood to one skilled in the art, disc  304  and ball and socket arrangement  302  and  306  may be transposed such that the disc is affixed to platform  138  and the ball and socket arrangement are secured to the tip of screw  148 . Also alternatively in either of the prior cases the ball and socket arrangement might be such that the ball is held in place by the socket. 
     Although the present invention has been described in the context of a structure for mounting and orienting an optical grating in a WDM optical device, the invention is not so limited. In general, the present invention can be used to mount and orient other structures in other types of devices. 
     Although the embodiment described in the text discusses the use of “super-invar” material for the wedge, other materials of similarly low coefficient of expansion could also be used. 
     Although the embodiment of FIG. 2 is described as having two flexure springs allowing independent movement of the mounting platform with respect to two planes that are orthogonal to each other, other implementations are also possible, as long as the allowed planes of movement are not parallel such that the orientation of the mounting platform can be adjusted in three-dimensions. 
     While this invention has been described with reference to illustrative embodiments, this description should not be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims. 
     Although the steps in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.