Abstract:
A MEMS-based adjustable mirror module allows faster, lower cost, and easier alignment of optical fibers in substrates. Movable mirrors formed on the substrate allow adjustment of the light path after the optical fiber is attached, after which the mirrors are affixed in place to prevent misalignment.

Description:
BACKGROUND OF THE INVENTION 
     Optical switches and other optoelectronic devices have advanced rapidly with developments in manufacturing technologies over the years. With the advent of Micro Electro Mechanical Systems (MEMS) technology, such devices could be made smaller, but problems arose when trying to align a light beam emitting from an optical fiber transporting light between a light source and transmission/conversion chips. These conversion chips generally provide the function of optical switching or conversion to/from electrical signals. For single-mode optical fibers, the tolerance of alignment between fibers and the targeted area is usually about 0.1 μm. Multi-mode optical fibers have a slightly wider alignment tolerance, however this is usually still below 5 μm. Such high-precision alignments are currently performed manually and are expensive. 
     An additional disadvantage of current Micro Optical Electrical Mechanical Systems (MOEMS) is that in order to tolerate misalignment of optical fibers, the active area of the photodiode is generally enlarged to cover all areas on which light can project. A larger active area yields a larger p-n junction, resulting in a large junction capacitance that can lower the switching speed of the MOEMS system. Manual alignment is generally needed in the aforementioned system to achieve higher conversion efficiency. 
     For these reasons, development of a low cost, high-precision alignment mechanism for fiber-chip connections is important to, for example, reduce the cost of hardware of optical fiber communication systems and also reduce the costs of many optical systems that require optical fibers as media for guiding light signals. 
     SUMMARY OF THE INVENTION 
     The principles of the present invention provide for a Micro Optical Electro Mechanical System (MOEMS) including a MEMS mirror module for high-precision alignment between optical fibers and MOEMS chips. Instead of aligning chips and optical fibers under a microscope, the present invention uses an easier method: adjusting the path of a light beam emerging from an optical fiber with MEMS mirrors such that the light beam projects on a targeted area. The beam divergence problem introduced when the light waves travel through free space between mirrors can be solved by passing the beam through a curved optical element, such as a spherically curved mirror or a lens in the mirror module, to converge and/or collimate the light. Through experiments, it was found that the efficiency of, for example, a five-mirror module is on the order of approximately 62.4% when the MEMS mirrors are coated with gold, which is high enough for most applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This disclosure includes the attached Figures, which Figures are summarized as follows: 
     FIG. 1 shows a schematic cross section of a portion of embodiments of the invention. 
     FIG. 2 shows a schematic cross section of a portion of embodiments of the invention with a device positioned atop the substrate. 
     FIG. 3 is a schematic elevational view of a portion of embodiments of the invention with additional mirrors and an optical device atop the substrate. 
     FIG. 4 is a schematic cross sectional view of the view of FIG.  4 . 
     FIG. 5 shows a comprehensive schematic elevational view of a preferred implementation of embodiments of the invention including movable and fixed mirrors and an optical device atop the substrate. 
     FIG. 6 is a top view of the view of FIG.  5 . 
     FIG. 7 is a cross sectional side view of the groove shown in FIGS. 5 and 6. 
     FIG. 8 is a chart illustrating the relative performances of two different types of mirrors that can be used with the invention. 
    
    
     DETAILED DESCRIPTION 
     MEMS technology provides a solution to the problems described above, particularly to the costly manual alignment of optical fibers. Instead of moving chips and optical fibers under a microscope, a user adjusts the path of the light beam with MEMS mirrors that project the light beam on the prescribed spot. Before describing the subject approach designed to align an optical fiber on a MOEMS chip, an exemplary machine that might include the system will be described. 
     As illustrated in the FIGS., and particularly in FIGS. 1 and 2, device  100  includes a substrate  101  in which a groove  110  is formed. An optical fiber  10  lies in the groove  110  with its end facing a reflective inclined end surface  113  of the groove. The inclination angle  115  of the groove end surface  113  is less than 90 degrees relative to an imaginary extension of the bottom surface of the groove  110 , as shown, for example, in FIGS. 4 and 7, so that light  11  incident upon the reflective end surface  113  reflects out of the groove  110 , as represented by arrow  12 . In embodiments, the angle of the end surface is between about 45 and about 65 degrees as measured from the bottom of the groove; an angle of about 54.7 degrees is beneficial in some embodiments. In one exemplary implementation of the device  100 , the light can shine upon an optical device. For example, the optical device can be a photodetector, spectrophotometric grid, interferometer, diffraction grating, or another optical or optoelectronic element, such as the flip-chip bonded optical device  13  shown in FIG.  2 . To enhance performance of the reflective end surface  113 , a coating  114  of a reflection enhancing material, such as gold or silver, can be included. 
     As indicated in FIGS. 3 and 4, the integration of optical components into a MOEMS is permitted. For example, an optical device  124 , such as a photodiode array, can be placed on the substrate  101  and can receive light  11  from the fiber  10  via mirrors formed on the substrate  101 . Also, for example, one mirror  120  can be placed above the reflective end surface  113  of the groove  110  so that it reflects the light toward another mirror  121  that reflects the light onto the optical device  124 . The mirrors  120 ,  121  can be held on the substrate with hinges  123 ,  124 , and are preferably formed from polysilicon, single crystal silicon, or another suitable material. When desired, the mirrors  120 ,  121  can be coated in similar fashion to the end surface  113  to enhance their reflectivity. Thus, the mirrors  120 ,  121  and the reflective end surface  113  form a light path between the end of the optical fiber  10  and the optical device  124 , and can send light from one to the other, vice versa, or both. As seen particularly in FIG. 4, the first mirror  120  is positioned to reflect the light parallel to the surface of the substrate  101 . 
     FIG. 5 illustrates one specific implementation of a MOEMS. In this example, an anisotropic wet etch, in which potassium hydroxide (KOH) or the like is used to etch or erode the substrate surface with techniques known in the art, defines a V- or trapezoidal-shaped trench or groove  110 ,  110 ′ into the substrate  101 . Fibers requiring, for example, a 200 μm-deep groove have been used, but it should be readily apparent to those skilled in the art that the size of the trench  110  will vary widely depending upon the particular dimensions of the fibers used and the particular desired module characteristics. The trench  110  is oriented so that the surface  113  at the end of the trench  110  can be used to reflect the light  11  upward to a mirror  131  similar to that shown in FIG.  3 . Preferably, as with the mirrors of FIG. 3, the mirror  131  is formed from polysilicon or single-crystal silicon (SCS). The diameter of many single-mode optical fibers is approximately 100-125 μm and can fit well into a 200 μm-deep groove with misalignment in the x and y directions of less than 1.0 μm, as shown in FIG.  1 . The etched surfaces are smooth enough to function as efficient optical mirrors as demonstrated in literatures. As mentioned above, the surface can be coated with gold or aluminum to increase the reflectivity of the mirror. As shown in FIG. 3, when a mirror  120  is added on top of the trench  110  and is oriented at about 35.3° relative to the chip surface in various embodiments, the light reflected from the mirror will be substantially parallel to the chip surface. With the addition of another MEMS mirror  121 , or of another optical device, such as a grating plate, the light from an optical fiber can be guided to project on an on-chip optical device  122 , such as a photodiode array as shown in FIG. 3 for spectroscopy application. 
     When an optical fiber  10  is put into this V-shaped groove or trench  110 , misalignments in the x direction, the y direction, or both, can occur, as shown in FIG.  1 . Any misalignment in the z-direction can change the coupling efficiency from the fiber  10  to the chip  101  but not the projection position on the targeted optical device  122 . As shown in FIG. 3, taking the MEMS spectrophotometer as an example, any misalignment in the x-detraction can be resolved by extending the width of the active region of photo diodes in the optical device  122 . For example, when the optical fiber  10  is misaligned 10 μm in the x-direction, the reflected light will be shifted 10 μm laterally on the grating plate. However, because of the extended width of each photodiode pixel, the light dissolved from the grating plate  121  will still fall on the active region of photo-diodes. When the fiber  10  is misaligned in the y-direction (perpendicular to the wafer surface), the light output will shift along the photodiode array. For example, when the original design the spectral components should fall on photo diodes number  101  to  612  in the array, because of misalignment the optical signals may be shifted to falling on photo diodes  218  to  729 . In this case, the output signals from the photo diode array have to be calibrated to compensate the offset. Applying a reference light source to identify its projection address on the photo diode array can achieve this. This is usually a one-time calibration and can be performed after the fiber is assembled on the chip. 
     Particular Description of a Five-Mirror Alignment Module 
     With the addition of comb drive actuators  139 ,  140  and additional mirrors  132 ,  133 , the misalignment in x and y direction can be corrected by applying an electrical signal on the actuators  139 ,  140  to adjust the position of the MEMS mirrors  131 ,  133 , as shown in FIG.  5 . FIG. 5 shows a 5-mirror module for fiber-chip connection. The optical fiber  10  is fitted into a trapezoidal/triangular groove  110  etched into the silicon substrate  101 . The depth and width of this trapezoidal groove  110  is designed to accommodate an optical fiber  10  such that the light  11  can be guided to hit on the surface  113  at the end of the trapezoidal groove  110 , and be reflected upward along a path as designed. The surface  113  can be coated with gold or another suitable material  114  to increase its reflectivity. As the etch of this trapezoidal groove  110  can be accurately controlled to within ±1 μm, the misalignment on positioning optical fibers  10  into the trapezoidal groove  110  can be minimized, and this small deviation can be fixed by adjusting the position of the guiding mirrors. After the optical fiber  10  is put on its final position in the trapezoidal groove  110 , it can be glued on in this position. This does not require a high-precision alignment because the relative position between the fiber and the chip is largely controlled by the photolithography step and the wet etch used to define the trapezoidal groove  110 . 
     After being reflected by the reflecting end surface  113  in the trapezoidal groove  110 , the light  11  is guided to hit a movable mirror  131 , as shown in FIGS. 5 and 6. This mirror  131  sits on an movable platform  136 , as shown in FIG. 7, and its position can be adjusted by applying an voltage on the electrostatic comb drive  139  which is attached to the platform  136 . With the adjustment of position of the first mirror  121 , the height (perpendicular to the wafer surface) of the outgoing light beam can be controlled. This latitude of control is converted into the adjustment of x-position of the light beam after it reaches the final mirror  134 , which is shown as being fixed in this exemplary implementation. The light signals are next guided to impinge on a fixed mirror  132 , then a second movable mirror  133 . The movement of the second movable mirror  133  provides the latitude of controlling the y-position of its final falling spot on the optical device  135 , such as a photo-diode/laser diode, as shown in FIGS. 5 and 6. 
     Another factor is the divergence of the light beam after it leaves a fiber. The increase in the beam size as a function of the free space propagation distance can be calculated according to the Gaussian beam theory. The light beam with a wavelength λ, after it propagates in free space for a distance z away from the origin, where the light beam has the smallest radius r 0 , has a beam radius:          r        (   z   )       =         r   0          [     1   +       (       z                 λ         r   0   2        π       )     2       ]         1   2                              
     The length of free space light path in this system is preferably in the range of from about 600 μm to about 800 μm and will introduce a beam divergence problem. 
     To compensate for beam divergence associated with the long light path introduced by this 5-mirror system, the first movable mirror  131  can, for example, be made spherically curved to converge the light beam. 
     Efficiency of a 5-Mirror Light Guiding System 
     One concern about such a 5-mirror light guiding system is the efficiency of the light signal after multiple reflection. The efficiency of this light guiding system is 
     
       
         
           E=R 
           1 
           ·R 
           2 
           ·R 
           3 
           ·R 
           4 
           ·R 
           5 
         
       
     
     where R 1 , R 2 , R 3 , R 4 , R 5  are the reflectivities of MEMS mirrors (includes the silicon surface mirror in the groove), respectively. Now when the mirror used is single-crystal silicon surface, the reflectivity is shown in FIG.  8 . The wavelength of the light source used in this measurement was 1.55 μm. Without gold coating, the reflectivity ranges from 32% to 37%, depending on the incident angles of light. In this case, the efficiency of this 5-mirror system is 
       E= 0.33·0.36·0.37·0.37·0.37=0.0056≈0.5% 
     The reflectivity of the MEMS mirror increases to about 91% when the mirrors are coated with gold, and the overall system efficiency is 
     
       
           H= 0.91·0.91·0.91·0.91·0.91=0.624≈62.4% 
       
     
     This efficiency value is adequate for most applications. 
     It is known in the art that a polysilicon mirror after chemical mechanical polish (CMP) has a reflectivity similar to that of a SCS mirror. As a result, a polysilicon mirror module would provide an overall reflectivity close to that of SCS mirrors. After the mirrors are moved to their final positions, the platform supporting these mirrors will be glued to these positions and the voltages on the comb drives will be turned off. 
     The preceding description of the invention is exemplary in nature as it pertains to particular embodiments disclosed and no limitation as to the scope of the claims is intended by the particular choices of embodiments disclosed. 
     Other modifications of the present invention may occur to those skilled in the art subsequent to a review of the present application, and these modifications, including equivalents thereof, are intended to be included within the scope of the present invention.