Abstract:
An optomechanical platform that provides both precise alignment and ready manufacture of optical and optomechanical components. A simplified embodiment of the invention includes a substantially symmetric cylindrical tube with an optical device at one end of the tube, a grating at another, opposite end of the tube and optics disposed between. optomechanical/optoelectronic device that is particularly well suited to WDM and imaging applications. Included is an integrated optomechanical/optoelctronic device particularly well suited to imaging and Wavelength-Division-Multiplexed (WDM) applications.

Description:
TECHNICAL FIELD 
     This invention relates generally to the field of optical communications and in particular to an opto-mechanical platform suitable for Wavelength-Division-Multiplexed optical communications applications. 
     BACKGROUND OF THE INVENTION 
     Wavelength division multiplexing (WDM) has been shown as a promising approach for increasing the capacity of existing fiber optic networks. A communications system employing WDM uses plural optical signal channels, each channel being assigned a particular channel wavelength. In such a WDM system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver. 
     The significance WDM communications systems and networks is immense. Consequently, a continuing need exists for apparatus and techniques that facilitate its continued implementation and adoption. 
     SUMMARY OF THE INVENTION 
     An advance is made over the prior art in accordance with the principles of the present invention which is directed to an opto-mechanical platform for WDM components. 
     Viewed from one aspect, the present invention is directed to an optomechanical platform that provides both precise alignment and ready manufacture of optical and optomechanical components. Specifically, a simplified embodiment of the invention includes a substantially symmetric cylindrical tube with an optical device at one end of the tube, a grating at another, opposite end of the tube and optics disposed between. 
     Viewed from another aspect, the present invention is directed to a method for fabricating the optomechanical platform. The method advantageously provides ready manufacture and ease of alignment of the of the components. 
     Viewed from yet another aspect, the present invention is directed to an integrated optomechanical/optoelectronic device that is particularly well suited to WDM and imaging applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawing, in which: 
     FIG. 1 shows an optomechanical platform according to the present invention; 
     FIG. 2 shows an additional embodiment of an optomechanical platform according to the invention; 
     FIG. 3 shows an additional embodiment of the optomechanical platform according to the invention including optical translation element; 
     FIG. 4 shows a prism and movable mirror as applied to the present invention; 
     FIG. 5 shows an integrated device including optical and electro-optical elements according to the present invention; 
     FIG.  6 ( a ) shows a ray-trace diagram of an optomechanical platform incorporating the prism of FIG. 4 according to the present invention; 
     FIG.  6 ( b ) shows the relation of the prism of FIG.  6 ( a ) to a surface normal device according to the present invention; 
     FIG. 7 shows a preliminary assembly of an optomechanical platform according to the present invention; 
     FIG. 8 shows an optical/optoelectronic subassembly for incorporation into the optomechanical platform of FIG. 7; 
     FIG. 9 is an alternative integrated device of FIG. 8; 
     FIG. 10 shows an optomechanical imaging WDM platform according to the present invention; 
     FIG. 11 shows an alternative optomechanical WDM platform according to the present invention; and 
     FIG. 12 is a photograph of a prototype optomechanical platform according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the invention will now be described while referring to the figures, several of which may be simultaneously referred to during the course of the following description. 
     With initial reference now to FIG. 1, there it shows a basic structure for an optomechanical platform which is the subject of the present invention. Specifically, optomechanical platform includes structural support  102  which is shown illustratively as having a substantially cylindrical shape. Consistent with this illustration, the support  102  has a near end  104  and a far end  106  in which optical and/or other components are positioned. In particular, and as shown in FIG. 1, far end  106  of structural support  102  houses a diffraction grating  114  which may serve so separate light impinging thereon into its component wavelengths. 
     At the near end  104  of the structural support  102  is shown an optical transmission system, which may include one or more fiber optic arrays  118  for conveying optical signals into and/or out from the optomechanical platform. A “surface-normal” device  108 , such as an array of photodetectors having suitable electrical connections  120  is positioned at the near end  104  and additional optical elements, such as a lens  116  is positioned between the surface-normal device  108  and the grating  114  within the support  102 . 
     Advantageously, one or more of the elements, i.e., the optical elements  116  or the grating  114  may be permanently affixed to the support  102  in a suitably-aligned configuration. In this exemplary embodiment, the surface-normal device  108  and the fiber optic arrays  118  are adjustably mounted so that they may be positioned relative to the other optical components such as the lens  116  and the grating  114 . Of course, those skilled in the art will readily appreciate that the surface-normal device  108  and the fiber optic arrays  118  may be configured to adjust as a unit or independently of one another. That is, if they adjust as a unit, then their relative alignment to one another is fixed, while their alignment with the other optical components within the support  102  is advantageously adjustable. 
     At this point it should be clear to one skilled in the art how a device or system constructed according to the invention will operate. In particular, light traversing one or more of the fibers included in the fiber array  118  will enter the support  102  and proceed along an optical path that includes optical components within the support  102  such as the lens  116 , the grating  114 , through the lens  116  again and onto surface normal device  108 . Signals resulting from the light impinging upon the surface normal device  108  propagate through electrical connections  112  to control electronics (not shown). 
     Using this “tube” geometry makes blind adjustment relatively straightforward. In fact, there are only a small number of orthogonal adjustments required: rotation of the grating orientation with respect to the device; tip and tilt of the grating to locate a central wavelength on the device; and focus of the lens (z-axis translation to adjust the gap between lens and device array). 
     An important characteristic of my inventive optomechanical platform is that it permits light from an input optical fiber to be imaged onto a device, i.e., the surface-normal device, then be re-imaged onto the original input fiber or an alternative output fiber after reflection from the device. Advantageously, this imaging automatically compensates for any small misalignments of the components, especially if the device aperature is made larger than an illuminating optical spot. Such an examplary structure is shown in FIG. 2, where light entering via input fiber  201  is imaged onto surface normal device  214  after traversing lens, ¼ wave plate, grating  212 . Reflected light likewise is re-imaged onto fiber  216  and separated through the action of circulator  204  such that it may be directed into output fiber  218 . Also shown in this Figure is a tip/tilt mount  206  which alternatively allows the grating  212  to be adjusted relative to the whole platform  200 . 
     With reference now to FIG. 3, there is shown a further improvement to the optomechanical platform of the present invention. Specifically, a prism-like device  304  is introduced between the surface normal device  301  and other optical elements such as lens  306 . Through the use of the prism  304 , light input to the device is refracted (or partially defracted) by an input side prism before being reflected by surface normal device  310 . The reflected light then passes through another refractive (or diffractive) beam deflector (which may be the same physical device such as the prism  304 ). The effect of this prism is that the reflected light is laterally shifted through its action. Consequently, an apparent source location of the imaged input light is laterally shifted. As can be now appreciated by those skilled in the art, if this lateral shift is appropriately designed, then reflected output light may be coupled into an output fiber  314  separate and distinct from an input fiber  302 . Advantageously, such an improvement eliminates the need for the circulator shown in FIG.  2 . 
     For the particular application of a WDM add/drop, FIG. 4 shows how the prism generally shown in FIG. 3 is used to illuminate a tilt mirror, which can direct the output to either of two facets, depending upon the orientation. Specifically, light  406  entering the prism through facet  404  is refracted to a point on the mirror  407  and then reflected out another facet  402  as exit light  408 . Note that a translation of the light, i.e., the distance between the input facet  404  and output facet  408  occurs in this example. Alternatively, the tilt mirror  407  may be positioned such that the entering light  406  may enter and exit the same facet  404 . Finally, note that the prism can be asymmetric with a different deflection angle for the two facets. 
     In describing prism and device fabrication and alignment, optical components may be aligned and fixed to the surface of the device using any of a number of standard techniques for assembly including, but not limited to, flip-chip bonding of the device to a diffractive optical element as shown schematically in FIG.  5 . In this inventive manner, alignment features are “manufactured” into both the device and the optical components. 
     With quick reference now to FIG. 5, there is shown a device  502  with optical elements, i.e., diffraction gratings  504  overlying the device and affixed thereto in a movable or adjustable manner. In this way, the compound device  500 , which includes both optoelectronic components and optical components, are conveniently coupled yet variably adjustable relative to one another. 
     Those skilled in the art will of course recognize that widely known techniques, such as injection-molding, may be utilized to form a optical component such as a diffraction element having one or more diffractive prisms formed on a surface. This component may then coupled to an underlying active device. Advantageously then, the optical components serves as both an optical element and a “cover” that seals the active device from its environment. 
     FIG.  6 ( a ) shows schematically an optical path of my inventive device. In particular, input light  601  is optically treated by lens  602 , ¼ wave plate  603  and grating  604  where it is reflected back through ¼ wave plate  603 , lens  602  to input facet  605  of prism  610 . The light input to the prism strikes device  606  and is reflected back out through prism  610  through output facet  607 . As described previously, the translational difference between the light entering the prism input facet and the light exiting the prism output facet should be readily apparent. Upon exiting the output facet  607  of prism  610 , the light traverses a reverse optical path through lens  602 , ¼ wave plate  603 , grating  604 , ¼ wave plate  603 , before exiting as light  608  which may be advantageously coupled into an output fiber (not shown). FIG.  6 ( b ) shows an exploded view of prism  610  and device  606 . 
     At this point it should be understood that my optomechanical platform and components provide a convenient, and readily manufacturable structure, particularly well suited for WDM applications. A key characteristic of the simplified structure shown in FIG. 1 is that the optical elements (lens, waveplate, grating, etc) are preferably fixed into the structural support which may advantageously be something as simple as a glass tube. As a result, alignment of the overall platform is performed by adjusting the device contained therein. This permits the use of a support tube having a minimal diameter, since no optomechanical a assembly (i.e., tilt/adjustment) is required. Of course, the tube may be made from material having a low thermal expansion such as Invar, glass or ceramic or it may be otherwise “athermalized” by using an appropriate combination of materials of differing thermal expansion. 
     The construction/alignment process may now be described. With initial reference to FIG. 7, starting with a support tube  701  which may preferably be made from metal, ceramic or glass, optical components such as a lens  702 , ¼ wave plate  703  or grating  704  are inserted therein. The positioning of these optical components may be passive, that is, the placement may be determined by features formed into the tube, or spacer rings or through the use of an external jig. Of course, these elements may be fixed in place by epoxy, sintering or other bonding means or a mechanical apparatus such as a ring or strap around the outside of the tube. 
     Note that in FIG. 7 no dimensions are shown and that the lens may be positioned closer to the grating (so that the grating is not in the lens back focal plane). This may reduce maximum device coupling, since it is no longer exactly telecentric, while increasing alignment stability. 
     Next, and with reference now to FIG. 8, there it shows a device mount  801  including one or more active devices  802 , optical elements  803  overlying the active devices  802 , aperture  804 , for optical fibers or fiber array and input/output electrical connections  805 . Generally, the optical fiber is not fixed. It is left free to move. If desired, the optical elements  803 , such as refractive or diffractive prisms, may be mounted on the active device  802  before it is mounted. For example, the optical elements  803  may be integrated into the active device by etching it into the device substrate or by flip chip bonding of a separate optical element onto the device. 
     It is also possible to place optical elements  803  onto the devices after they have been mounted, for example, by forming suitable alignment features into the device, the mount or both. 
     The device mount is inserted into the support assembly of FIG.  7 . Note that the relative rotational alignment of the device arrays with the grating orientation may, advantageously, be relatively loose, in which case a passive alignment to a fixed feature in either the device mount or support or some external jig will suffice. Similarly the focus (separation between the device and lens) may be amenable to passive alignment depending upon lens focal length and device specifics. 
     Final alignment is active, and uses movement of the fiber/fiber arrays within the device. Specifically, a fiber element is positioned using an external, high resolution translation/rotation stage while actively monitoring fiber throughput and/or device operation. After the fiber/fiber arrays is initially positioned, it is moved to a position where optimum performance is obtained. Then, it is fixed in place by any of a variety of methods, including, but not limited to, epoxy, laser welding, sintering, solder hardening, mechanical clamp, etc. A principle benefit to my inventive approach is that expensive optomechanical elements are external to the package, i.e., translation equipment, electrical control equipment, light source, light detectors, etc., and may be re-used for other, subsequent assemblies. 
     Additionally, and as can be readily appreciated by those skilled in the art, the alignment may be over multiple-axis with optical fiber held in a suitable mount—such as a silicon v-groove array thereby limiting the degrees of freedom (such as rotation, yaw and pitch) which may advantageously controlled without active alignment. For example, a component such as a WDM equalizer component or WDM monitor component (or both) where a pitch of the detectors/modulators is small compared to the pitch of the communications wavelengths. 
     With reference now to FIG. 9, there it shows a WDM equalizer/monitor structure  900  constructed according to the present invention. Specifically, device mount  901  has attached thereto one of a variety of detectors (optoelectronic chip)  902 , an equalizer  904  having an optical element  906  affixed thereto. As previously described, the optical element  906  may advantageously include one or more prisms or arrays of prisms  908  which function as previously described. This detector  902  and equalizer  904  assembly are electrically and logically connected to one or more control elements (not shown) by electrical connections  918 . 
     With continued reference to FIG. 9, a fiber aperature  912  is shown which is positioned a fiber array  910 . Advantageously, the fiber array  910  contains both input and output fibers and the array  910  is preferably movable in both lateral and axial directions. As will be readily appreciated by those skilled in the art, the fiber array  910  may have v-shaped groves which act to secure optical fibers therein. 
     In the case of the example structure of FIG. 9, an input fiber (within the fiber array  910 ) carries a multiwavelength signal which is distributed over the optomechanical equalizer chip  904  after passing through a prism  908 . The signal is reflected (transmitted through an adjacent prism window) and collected into a separate output fiber (contained within the fiber array  910 ). The output signal is externally tapped by, for example, a fused fiber 1% coupler) and then fed back into the device  900  by yet another fiber (contained within the fiber array  910 ) and subsequently distributed over a multichannel optoelectronic power meter. 
     Advantageously, the alignment of such a component is eased by fabricating modulator and detector devices with oversize apertures. Most of the alignment is already done when the components is assembled. As described herein, a final alignment only requires as few as two adjustments—a lateral translation of the fiber array to alin the wavelength dispersed columns onto the corresponding device arrays and an axial focus to provide suitable coupling efficiency. If rotation of the device relative to the grating is needed, then this may be performed by rotating the entire subassembly  900  before fixing it into the support of FIG.  1 . 
     With reference now to FIG. 10, there is shown how my invention may advantageously be used in a WDM system. Specifically, light carried over input optical fiber  1006  and affixed within optical fiber array  1010  exits the input optical fiber  1006  as beam  1020  having multiple wavelengths. The light then propagates through lens  1012  where it becomes focused beam  1022  that impinges upon grating  1014 . The action of the grating is such that the light is split into component wavelengths  1024  and then focused by lens  1012  into focused component wavelengths  1026 . Advantageously, and in a preferred embodiment, each one of the individual wavelengths will strike an individual detector  1004  of device array  1002 . As should be apparent, and in this preferred embodiment, light from a single, particular input fiber may be coupled to a particular row of detectors which are part of the device  1002 . Electrical signals resulting therefrom may be conducted via ribbon conductor  1028  for action by external control electronics. 
     FIG. 11 shows a further embodiment of my invention, specifically a WDM system that incorporates my inventive prism described previously. The configuration is similar to that shown in FIG. 10, with the addition of an optical element  1136 , including array of prisms or other translation elements, overlying devices  1130 , and  1132  which may optionally include detectors  1134 , equalizers, modulators or combinations thereof. As should be apparent from previous discussions, the optical element  1136  may serve translate light reflected from a surface normal device such that it may be later directed to another device or another element of the same device. 
     While the invention has been shown and described in detail in the context of a preferred embodiment, it will be apparent to those skilled in the art that variations and modifications are possible without departing from the broad principles and spirit of the invention which should be limited solely by the scope of the claims appended hereto.