Abstract:
One embodiment of the present invention provides a spatial light phase modulator, which can perform piecewise linear phase modulation of a light beam. This spatial light phase modulator includes an array of movable micromirrors and an array of actuators. Each actuator of the array of actuators is movably coupled to one micromirror of the array of movable micromirrors and can move the micromirror both vertically and rotationally. Additionally, the present invention provides an optical function generator that is a femtosecond pulse shaper. This optical function generator includes a diffraction grating that disperses an input pulse into a dispersed spectrum, a lens assembly to focus the dispersed spectrum onto a micromirror array, and the micromirror array to provide spatial filtering to the dispersed spectrum to provide the filtered spectrum.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the fabrication of microscopic electro-mechanical devices. More specifically, the present invention relates to the fabrication of a phased micromirror array to facilitate programmable shaping of ultra-short optical pulses.  
           [0003]    2. Related Art  
           [0004]    “Femtosecond pulse shaping” has been developed to generate complicated, ultrafast optical pulses according to user specifications. The key component required to shape a femtosecond pulse is a versatile, high resolution, programmable spatial light phase modulator (SLPM). Recent attention has been focused on developing computer controlled programmable one-dimensional SLPMs for this pulse shaping. Existing linear SLPMs fall into two categories: deformable continuous thin membrane mirrors and pixilated arrays of liquid crystal phase shifters.  
           [0005]    Deformation of a continuous membrane produces smooth spectral phase variations and a corresponding high quality optical pulse. Membranes, however, are not capable of producing localized abrupt spectral phase changes. On the other hand, a pixilated array of phase modulators can produce rapid phase changes between pixels. The phase shift for each pixel is constant, however, and the resulting stepwise approximation to smooth phase variations results in undesired energy in the pulse wings.  
           [0006]    What is needed is an SLPM that facilitates programmable shaping of ultra-short optical pulses, which does not exhibit the drawbacks described above.  
         SUMMARY  
         [0007]    One embodiment of the present invention provides a spatial light phase modulator, which can perform piecewise linear phase modulation of a light beam. This spatial light phase modulator includes an array of movable micromirrors and an array of actuators. Each actuator of the array of actuators is movably coupled to one micromirror of the array of movable micromirrors and can move the micromirror both vertically and rotationally. These actuators are dual-mode actuators that can expand or contract to cause vertical motion and can tilt to cause rotational motion.  
           [0008]    In one embodiment of the present invention, control signals applied to the array of actuators can cause the linear array of movable micromirrors to act in concert to perform piecewise linear phase modulation of the light beam.  
           [0009]    In one embodiment of the present invention, vertical forces applied by the actuator can cause the micromirror to have a vertical range of motion of at least pi radians at a specified wavelength.  
           [0010]    In one embodiment of the present invention, the actuator includes a thermo-expansion actuator, a piezoelectric actuator, a magnoelectric actuator, or a capacitive actuator.  
           [0011]    One embodiment of the present invention provides an optical function generator that is a femtosecond pulse shaper. This optical function generator includes a diffraction grating that disperses an input pulse into a dispersed spectrum, a lens assembly to focus the dispersed spectrum onto a micromirror array, and the micromirror array to provide spatial filtering to the dispersed spectrum to provide a filtered spectrum.  
           [0012]    In one embodiment of the present invention, the lens assembly focuses the filtered spectrum on the diffraction grating. This diffraction grating then combines the filtered spectrum into an output pulse.  
           [0013]    In one embodiment of the present invention, the optical function generator includes a plurality of actuators movably coupled to the micromirror array. Each actuator of the plurality of actuators can move one mirror of the micromirror array in both elevation and tilt.  
           [0014]    One embodiment of the present invention provides a two-dimensional coherent mirror array that includes a two-dimensional micromirror array and a plurality of actuators movably coupled to this two-dimensional micromirror array. Each micromirror of the two-dimensional micromirror array is movably coupled to a triad of actuators positioned such that the micromirror can be elevated and tilted in any direction.  
           [0015]    In one embodiment of the present invention, vertical forces applied by the triad of actuators can cause the micromirror to have a vertical range of motion of at least pi radians at a specified wavelength.  
           [0016]    In one embodiment of the present invention, an actuator of the plurality of actuators includes a thermo-expansion actuator, a piezoelectric actuator, a magnoelectric actuator, or a capacitive actuator.  
           [0017]    One embodiment of the present invention provides a programmable micromirror array, including multiple movable micromirrors. A first movable comb is fixed to one edge of each of the movable micromirrors, and a second movable comb is fixed to the opposite edge of each movable micromirror. These movable combs form a movable portion of an interdigitated actuator coupled to each movable micromirror. This movable portion of the interdigitated actuator can apply vertical and rotational motions to the movable micromirrors.  
           [0018]    In one embodiment of the present invention, the programmable micromirror array includes a first folded spring coupled to the distal end of the first movable comb, and a second folded spring coupled to the distal end of the second movable comb. These folded springs provide restoring forces to the movable portion of the interdigitated actuator.  
           [0019]    In one embodiment of the present invention, the interdigitated actuator includes a fixed lower actuator and a fixed upper actuator. These fixed actuators act in concert to apply vertical and rotational forces to the movable portion of the interdigitated actuator.  
           [0020]    In one embodiment of the present invention, the fixed lower actuator includes planar capacitive drives.  
           [0021]    In one embodiment of the present invention, the fixed upper actuator includes vertical comb drives.  
           [0022]    In one embodiment of the present invention, the vertical forces applied by the interdigitated actuator can cause the movable micromirror to have a vertical range of motion of at least pi radians at a specified wavelength.  
           [0023]    One embodiment of the present invention provides a two-dimensional programmable micromirror array including multiple hexagonally shaped, movable micromirrors. Each micromirror includes a first movable actuator fixed to a first edge of the micromirror, a second movable actuator fixed to a second edge of the micromirror, and a third movable actuator fixed to a third edge of the micromirror. These movable actuators are fixed to alternating edges of the micromirror, and act in concert to apply vertical and two-dimensional rotational motions to the micromirror. Note that each actuator applies vertical motions to one edge of the micromirror, and three actuators acting in concert apply rotational motions to the micromirror.  
           [0024]    In one embodiment of the present invention, the movable micromirror includes a first folded spring coupled to the distal end of the first movable actuator, a second folded spring coupled to the distal end of the second movable actuator, and a third folded spring coupled to the distal end of the third movable actuator. These folded springs provides restoring forces to the movable micromirror.  
           [0025]    In one embodiment of the present invention, the movable micromirror includes three actuators. Each actuator includes a movable actuator selected from the first movable actuator, the second movable actuator or the third movable actuator. Each actuator also includes a fixed lower actuator and a fixed upper actuator. These fixed lower and upper actuators can apply vertical and rotational forces to the movable micromirror through the movable actuator.  
           [0026]    In one embodiment of the present invention, the fixed lower actuator includes planar capacitive drives.  
           [0027]    In one embodiment of the present invention, the fixed upper actuator includes vertical comb drives.  
           [0028]    In one embodiment of the present invention, vertical forces applied by the interdigitated actuator can cause the micromirror to have a vertical range of motion of at least pi radians at a specified wavelength. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0029]    [0029]FIG. 1A illustrates an optical function generator in accordance with an embodiment of the present invention.  
         [0030]    [0030]FIG. 1B illustrates micromirror array  108  in accordance with an embodiment of the present invention.  
         [0031]    [0031]FIG. 2A illustrates construction of a movable actuator in accordance with an embodiment of the present invention.  
         [0032]    [0032]FIG. 2B illustrates a cross-sectional view of a movable actuator in accordance with an embodiment of the present invention.  
         [0033]    [0033]FIG. 3A is an exploded view of a movable micromirror assembly in accordance with an embodiment of the present invention.  
         [0034]    [0034]FIG. 3B is a composite view of a movable micromirror assembly in accordance with an embodiment of the present invention.  
         [0035]    [0035]FIG. 4 illustrates a programmable micromirror array in accordance with an embodiment of the present invention.  
         [0036]    [0036]FIG. 5A illustrates a movable comb in a rest position in accordance with an embodiment of the present invention.  
         [0037]    [0037]FIG. 5B illustrates a movable comb in a raised position in accordance with an embodiment of the present invention.  
         [0038]    [0038]FIG. 5C illustrates a movable comb in a lowered position in accordance with an embodiment of the present invention.  
         [0039]    [0039]FIG. 5D illustrates a movable comb rotated in a counter-clockwise position in accordance with an embodiment of the present invention.  
         [0040]    [0040]FIG. 5E illustrates a movable comb rotated in a clockwise position in accordance with an embodiment of the present invention.  
         [0041]    [0041]FIG. 6 illustrates a two-dimensional micromirror assembly in accordance with an embodiment of the present invention.  
         [0042]    [0042]FIG. 7 illustrates an array of two-dimensional micromirror assemblies in accordance with an embodiment of the present invention.  
         [0043]    [0043]FIG. 8 is a flowchart illustrating the process of creating a programmable micromirror array in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0044]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.  
         [0045]    The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet.  
         [0046]    Optical Function Generator  
         [0047]    [0047]FIG. 1A illustrates an optical function generator in accordance with an embodiment of the present invention. The optical function generator includes grating  104 , lens  106 , and micromirror array  108 . Grating  104  receives input light pulse  102  and creates spatially dispersed spectrum  112 . Lens  106  focuses spatially dispersed spectrum  112  on micromirror array  108 .  
         [0048]    Micromirror array  108  filters spatially dispersed spectrum  112  and reflects the filtered spectrum toward lens  106 . Lens  106  focuses the filtered spectrum on grating  104 , which combines the filtered spectrum into output pulse  110 .  
         [0049]    [0049]FIG. 1B illustrates micromirror array  108  in accordance with an embodiment of the present invention. Actuators associated with each micromirror of micromirror array  108  adjust the elevation and tilt of each micromirror within micromirror array  108  to filter spatially dispersed spectrum  112 . The elevation of each micromirror can be adjusted through a range of at least pi radians at a specified wavelength.  
         [0050]    Creating an Actuator  
         [0051]    [0051]FIG. 2A illustrates construction of a movable actuator in accordance with an embodiment of the present invention. The movable actuator and micromirror are constructed on substrate  202 . Substrate  202  can be any suitable material such as silicon. Next, insulating layer  204  is applied to substrate  202 . Insulating layer  204  can be any suitable material such as silicon nitride.  
         [0052]    Lower actuators  206  are then deposited on insulating layer  204 . Lower actuators  206  can be any suitable material such as polysilicon. Note that the steps involved in depositing insulating layer  204  include applying a masking pattern on insulating layer  204  and then etching the pattern so that lower actuators  206  are deposited in the correct location. These steps related to masking are well known in the art and will not be described further herein. Several other layers involved in this process also require the application, etching, and removal of masking layers.  
         [0053]    After depositing lower actuators  206 , first sacrificial layer  208  is applied over lower actuators  206  and over the exposed portions of insulating layer  204 . First sacrificial layer  208  can be any suitable material such as silicon oxide. Projection  209  is then formed to provide a device to prevents movable comb teeth  210  from contacting and sticking to the surface of lower actuators  206 . Next, the mirror assembly including movable comb teeth  210  is deposited on first sacrificial layer  208 . This mirror assembly is described in more detail in conjunction with FIGS. 3A and 3B below. Movable comb teeth  210  and the associated mirror assembly can be any suitable material such as polysilicon.  
         [0054]    Next, second sacrificial layer  212  is applied over movable comb teeth  210  and the mirror assembly. Second sacrificial layer  212  can be any suitable material such as silicon oxide. Vias are then created through sacrificial layers  208  and  212  down to insulating layer  204 . Upper actuators  214  are then deposited in the vias and across the surface of second sacrificial layer  212 . Upper actuators  214  can be any suitable material such as polysilicon.  
         [0055]    [0055]FIG. 2B illustrates a cross-section view of a movable actuator in accordance with an embodiment of the present invention. After depositing upper actuators  214 , sacrificial layers  208  and  212  are selectively etched away to leave the structure illustrated in FIG. 2B. A reflective material such as gold can be deposited on the mirror area of the mirror assembly.  
         [0056]    Micromirror Assembly  
         [0057]    [0057]FIG. 3A is an exploded view of a movable micromirror assembly in accordance with an embodiment of the present invention. Upper actuators  302  and lower actuators  304  operate in conjunction with movable combs  308  to apply vertical and rotational forces to movable combs  308  and, in turn, to movable mirror  310 . Springs  306  provide support and restoring forces to movable combs  308 . Anchors  312  fix the opposite ends of springs  306  to substrate  202 .  
         [0058]    [0058]FIG. 3B is an unexploded view of a movable micromirror assembly in accordance with an embodiment of the present invention. Note that movable combs  308  and upper actuators  302  form an interdigitated actuator for movable mirror  310 .  
         [0059]    Micromirror Array  
         [0060]    [0060]FIG. 4 illustrates a programmable micromirror array in accordance with an embodiment of the present invention. The programmable micromirror array includes micromirror assemblies  402 . Micromirror assemblies  402  are described above in conjunction with FIGS. 3 and 4. Note that there can be more or less micromirror assemblies than the number shown in FIG. 4.  
         [0061]    Micromirror assemblies  402  include movable mirrors  404 ,  406 ,  408 , and  410 . Movable mirrors  404 ,  406 ,  408 , and  410  can be individually moved in a vertical direction and can be individually rotated as described below in conjunction with FIGS. 5A through 5E. Movable mirrors  404 ,  406 ,  408 , and  410  can be controlled, possibly by a computer, to allow femtosecond pulse shaping to generate complicated, ultrafast optical pulses according to user specifications. Fixed mirrors  412  and  414  are available for a fixed reference if necessary.  
         [0062]    Controlling the Mirrors  
         [0063]    [0063]FIG. 5A illustrates a movable comb in a rest position in accordance with an embodiment of the present invention. Springs  306  provide a restoring force to movable comb  506  so that movable comb  506  is at center  508  between upper actuators  502  and lower actuators  504 .  
         [0064]    [0064]FIG. 5B illustrates a movable comb in a raised position in accordance with an embodiment of the present invention. A positive electrical charge is placed on upper actuators  502  causing movable comb  506  to rise above center  508 .  
         [0065]    [0065]FIG. 5C illustrates a movable comb in a lowered position in accordance with an embodiment of the present invention. A positive electrical charge is placed on lower actuators  504  causing movable comb  506  to move below center  508 . Note that the full vertical range of motion of movable comb  506  is at least pi radians at a specified frequency. This range of motion allows precise adjustment of the phase of an optical pulse.  
         [0066]    [0066]FIG. 5D illustrates a movable comb rotated in a counter-clockwise position in accordance with an embodiment of the present invention. A positive electrical charge is placed on left lower actuator  504  and right upper actuator  502  causing movable comb  506  to rotate counter-clockwise.  
         [0067]    [0067]FIG. 5E illustrates a movable comb rotated in a clockwise position in accordance with an embodiment of the present invention. A positive electrical charge is placed on left upper actuator  502  and right lower actuator  504  causing movable comb  506  to rotate clockwise.  
         [0068]    Two-Dimensional Micromirror Assembly  
         [0069]    [0069]FIG. 6 illustrates a two-dimensional micromirror assembly in accordance with an embodiment of the present invention. Two-dimensional micromirror  608  includes three actuators  606 . Actuators  606  can be the actuators described in detail in conjunction with FIGS. 3A and 3B above or may be any suitable actuator which will impart vertical motion to two-dimensional micromirror array  608 . Springs  306  on actuators  606  are fixed to anchors  604 . Anchors  604  are fixed to insulating layer  204 , while supports  602  are fixed to a hexagonal mirror surface. Actuators  606  work in concert to apply vertical and two-dimensional rotational motions to the hexagonal mirror surface. Note that the vertical motion is at least pi radians at a specified frequency. Note also that each actuator individually supplies only vertical motion to two-dimensional micromirror array  608  and, in concert, apply two-dimensional rotational motions to two-dimensional micromirror array  608 .  
         [0070]    Array of Two-Dimensional Micromirror Assemblies  
         [0071]    [0071]FIG. 7 illustrates an array of two-dimensional micromirror assemblies in accordance with an embodiment of the present invention. This array includes multiple two-dimensional micromirrors  608 . Two-dimensional micromirror  608  includes a hexagonal shape to allow efficient packing of the array. Note that the array can include more two-dimensional micromirrors  608  than shown in FIG. 7. This array can be controlled, possibly by a computer, to provide a digital diffractive optic device according to user specifications.  
         [0072]    Creating Micromirror Assemblies  
         [0073]    [0073]FIG. 8 is a flowchart illustrating the process of creating a programmable micromirror array in accordance with an embodiment of the present invention. The process starts when the system receives substrate  202  (step  802 ). Next, the system applies insulating layer  204  on the substrate (step  804 ). After applying insulating layer  204 , the system deposits lower actuators  206  on insulating layer  204  (step  806 ).  
         [0074]    Next, the system applies first sacrificial layer  208  over lower actuators  206  and the exposed portions of insulating layer  204  (step  808 ). The system then deposits the mirror assembly, including movable comb teeth  210 , on the first sacrificial layer (step  810 ). Second sacrificial layer  212  is then applied over the mirror assembly and the exposed portions of first sacrificial layer  208  (step  812 ).  
         [0075]    The system next creates vias through the sacrificial layers for upper actuators  214  (step  814 ). Next, upper actuators  214  are deposited within these vias and across portions of second sacrificial layer  212  (step  816 ). After depositing upper actuators  214 , sacrificial layers  208  and  212  are removed by selective etching (step  818 ). Finally, a reflective coating is applied to the mirror assemblies (step  820 ).  
         [0076]    The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.