Abstract:
A digitally controlled optical delay apparatus providing optical signal delays electrically selectable in the picosecond to nanosecond range by way of selectable signal path lengths. Path lengths are incremented in physical length and path delay time according to digital ratios. The delay element includes micro-miniature path changing mirrors controlled in path length selecting positioning by input signals of logic level magnitude. Fiber optic coupling of signals to and from the delay element and a combination of fixed position and movable mirror included optical signal path lengths are included.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     In the generation of test and simulated images for a laser radar apparatus, for example, there is need for delaying optical signals for selectable time intervals in the sub nanosecond time range. Similarly, in the performance of signal processing for other light-based optical systems it is also desirable to introduce small digitally selected signal delays for scanner-free imaging, diffraction/holographic imaging, optical mixing, and delay-measurements. 
     Although such generation of optical signal delays has doubtless been heretofore accomplished in the signal processing art, it is notable that the use of digitally incremented signal delays, i.e., digital delay devices, appears to have been omitted from this art. The present invention addresses this omission. 
     SUMMARY OF THE INVENTION 
     The present invention provides a digitally incremented optical signal delay generation device. 
     It is therefore an object of the present invention to provide an optical signal delay apparatus useful in achieving selectable picosecond to nanosecond optical energy signal delay intervals. 
     It is another object of the invention to provide a fiber optic coupled optical signal delay apparatus that may be conveniently received in high component density locations of an optical signal processing apparatus. 
     It is another object of the invention to provide an electrically controllable optical signal delay apparatus. 
     It is another object of the invention to provide an optical signal delay apparatus that may be expanded or contracted in delay interval with respect to a disclosed typical embodiment of the apparatus. 
     It is another object of the invention to provide an optical signal delay apparatus that may be fabricated with use of electrostatic comb-drive actuators or MEMS devices or other electromechanical movable elements. 
     It is another object of the invention to provide an optical signal delay apparatus in which folded optical signal delay paths enable a small overall physical size of the apparatus. 
     It is another object of the invention to provide an optical signal delay apparatus realizable with use of conventional micromachining fabrication techniques. 
     It is another object of the invention to provide an optical signal delay apparatus that can make use of semiconductor materials in the fabrication of included control elements. 
     It is another object of the invention to provide an optical signal delay apparatus in which transition between selected optical signal delay times may be accomplished during intervals within the microsecond time range. 
     These and other objects of the invention will become apparent as the description of the representative embodiments proceeds. 
     These and other objects of the invention are achieved by a digital optical signal delay apparatus comprising the combination of: 
     a first optical signal communicating path of unit length and unit optical signal propagation delay time; 
     a second optical signal communicating path of twice unit length and twice unit optical signal propagation delay time disposable in serial with said first optical signal communicating path; 
     a plurality of subsequent optical signal communicating paths, each of twice length and twice optical signal propagation delay time of an immediately preceding optical signal communicating path, and each disposed in ordered increment with a preceding optical signal communicating path; 
     a plurality of electrically controlled optical switching elements, each located intermediate a serial pair of said optical signal communicating paths, and each selectable between a signal delay inclusive of elected of said optical signal communicating paths and a faster, optical signal communicating path delay-bypassing, path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  shows a five bit embodiment of the optical signal delay apparatus invention in schematic form. 
         FIG. 2  includes the views of  FIG. 2   a ,  FIG. 2   b  and shows additional two state details of one bit in the  FIG. 1  optical signal delay apparatus. 
         FIG. 3  shows the  FIG. 1  and  FIG. 2  optical signal delay apparatus in a maximum delay state. 
         FIG. 4  shows the  FIG. 1  and  FIG. 2  optical signal delay apparatus in a minimum delay state. 
         FIG. 5  shows the  FIG. 1  and  FIG. 2  optical signal delay apparatus in a specific one bit active delay state. 
         FIG. 6  shows the  FIG. 1  and  FIG. 2  optical signal delay apparatus in a specific two bit active delay state. 
         FIG. 7  shows the  FIG. 1  and  FIG. 2  optical signal delay apparatus in one possible physical embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  in the drawings shows a top view schematic of a 5-bit embodiment of the present invention optical signal delay apparatus. As may be observed in this  FIG. 1  view the present invention delay apparatus operates by use of the principle of optical signal delay occurring along the controlled length of a signal communication path and with electrically movable optical signal deflecting mirrors being used to determine the differing possible lengths of this path. In the  FIG. 1  embodiment of the invention, an array of five sets of electrostatically positionable mirrors  100 - 102 ,  104 - 106 ,  108 - 110 ,  112 - 114  and  116 - 118  cooperate with an array of five sets of fixed position mirrors  142 - 144 ,  146 - 148 ,  150 - 152 ,  154 - 156 ,  158 - 160  to determine the path length for an optical signal, and thus the incurred signal delay in traveling from left to right through the  FIG. 1  apparatus. The  FIG. 1  drawing also shows that an additional array of mirror-resembling elements  120 - 122 ,  124 - 126 ,  128 - 130 ,  132 - 134  and  136 - 138  appear in the  FIG. 1  drawing as parts of the electrically movable optical signal deflecting mirrors apparatus. These mirror appearing elements are actually fixed portions of the electrostatic actuators for the electrostatically positionable mirrors  100 - 102 ,  104 - 106 ,  108 - 110 ,  112 - 114  and  116 - 118  determining the path of a delayed signal in each bit of signal delay of the  FIG. 1  apparatus. 
     The  FIG. 1  drawing shows several additional details of an optical signal digital delay device according to the present invention. These include the dashed line paths  162 ,  164 ,  166 ,  168  and  170  used by an optical signal traversing the  FIG. 1  apparatus, i.e., paths inclusive of the described fixed position and movable position mirrors. These details also include the additional set of fixed position mirrors  180 ,  182 ,  184 ,  186  and  188  used during the “OFF” or inactive or non delayed or minimum delay status of the mirrors  100 - 102 ,  104 - 106 ,  108 - 110 ,  112 - 114  and  116 - 118  in each respective bit of the  FIG. 1  apparatus. Additionally shown in the  FIG. 1  drawing are an overall apparatus length dimension  190 , the electrodes  194 ,  195 ,  196 ,  197  and  198  for receipt of input signals controlling the electrostatically positionable mirrors  100 - 102 ,  104 - 106 ,  108 - 110 ,  112 - 114  and  116 - 118  and an actual delay time interval  192  for a delay of one unit length. The identification of each unit or multi-unit delay represented in the  FIG. 1  drawing is provided by the symbols at  199  in  FIG. 1 . 
       FIG. 2  in the drawings shows additional details of one optical signal delay element of a digital delay device or one bit of the  FIG. 1  five-bit delay apparatus. In the  FIG. 2  drawings, several labels are provided in order to name the elements shown and indicate the function provided by each element. As indicated by the numbering of various structural elements in the  FIG. 2  drawing, this one bit or one delay element is selected as the D1 delay element, and is shown to include the movable mirrors  104  and  106  and other identified elements from the  FIG. 1  drawing. The  FIG. 2  drawing includes the views of  FIG. 2   a  and  FIG. 2   b , views showing the active or delay “ON” and delay “OFF” states of this D1 element, and especially the different optical paths and differing mirror arrangements provided for each state. The convention of light movement from left to right used in  FIG. 1  is maintained in the  FIG. 2  drawing. 
     The effective delay path geometries are depicted as dashed lines in the  FIG. 1  drawing and these paths are not drawn to scale in  FIG. 1 . For example, the effective distance between the electrostatically actuated micromirrors and the pair of stationary reflecting micromirrors  116  and  118  of delay path D 4  in  FIG. 1  may be on the order of 16 centimeters. Little effort is, however, made herein to accurately scale the drawings according to these distances. Moreover, the effective distance in a carefully arranged or large quantity manufactured delay device is preferably achieved by using multiple sets of stationary reflecting micromirrors in the D 4  path to reduce the actual length of the overall delay device to, for example, no more than 4 centimeters. The effective distances of delay paths D 3 , D 2 , D 1 , and D 0  are in the order of 8 centimeters, 4 centimeters, 2 centimeters, and 1 centimeter, respectively. The actual delay achieved for path D 0  in its inactive state is δ=32.3 picoseconds as shown in  FIG. 1   
     The  FIG. 2  drawings include additional details of the electrostatic comb-drive actuators contemplated as one arrangement usable to rotate the movable mirror elements  104  and  106  of the D1 delay element. Comb-drive actuators of this nature are known in the electromechanical art, and are also disclosed in a plurality of U.S. Patents, including, for example, U.S. Pat. No. 5,631,514, a document hereby incorporated by reference herein. A representative illustration of the two positions and the degree of mechanical movement contemplated for the comb-drive actuator  204  in the  FIG. 2  drawings appears at  206  in the  FIG. 2   b  drawing. This ten degrees of mechanical motion is found to be sufficient to direct the optical beam  200  between the positions shown in  FIG. 2   a  and  FIG. 2   b  where the different fixed mirrors  146 - 148  and  182  are used to achieve signal delayed and signal non-delayed conditions, respectively. Alternate devices such as a MEMS rotatable mirror actuator may be used in lieu of the comb-drive actuators specified above in embodying the invention. 
     In operation of the  FIG. 2  delay, an optical signal (light) is delayed by digitally diverting the light down the select path, or delay line, by means of movable micromirrors  104  and  106 . These mirrors, two per delay line head, are controlled by an electrostatic comb-drive actuator  204  and have two distinct positions. In the non-actuated position, or digital “1” condition, no voltage is applied to the comb-drive actuator so that the mirrors at the delay line head stay in such orientation as to force the light to deflect at a 90° angle down the longest available path to a fixed mirror  148  disposed a designated distance from the delay line head. The mirror  148  at the end of the delay line forces the light to again deflect at a 90° angle toward another fixed mirror  146 , which is in an inverted orientation to the first mirror at the end of the delay line so that the light is directed back up to the delay line head to the second mirror  104  on the delay line head. This second mirror is attached to the same comb-drive as the first mirror, is also in the non actuated digital “1” position, forcing the light to travel at a 90° angle toward the next delay line head, which is represented by the next bit or delay path. 
     In the actuated position, or digital “0” condition, a control signal of, for example, 5-volt magnitude is applied to the  FIG. 2  comb drive actuator causing both mirrors on the delay line head to be deflected by 10 degrees, thus guiding the incoming light to travel at an angle sufficient to direct it toward a fixed pass-through mirror  182  located directly between and just below the two mirrors on the delay line head as is shown in the  FIG. 2   b  drawing. This mirror  182  then guides the light back toward the second mirror on the delay line head and on to the next delay line head. When control voltage is removed, mechanical springs located at each mirror pivot axis cause the deflected mirrors to return to their original positions. 
     With reference to the  FIG. 1  arrangement of the invention, the first delay line head  103  corresponds to the longest delay line (D 4 ) and is represented by the most significant digital bit being a logic “1”, with an open or no voltage signal applied to the comb-drive  140 . Alternately a logic “0”, with a closed or active voltage signal such as 5 volts applied to the comb-drive can be used to achieve the desired mirror position if a differing arrangement of the delay line head  103  and the electrostatic comb-drive actuator  140  is selected. 
     A digital delay device with all delay line heads in the “1” position corresponds to the longest available delay time for the  FIG. 1  showing of the invention, a delay of 1 nanosecond. This arrangement of the invention is depicted in the  FIG. 3  drawing. A digital delay device with all delay line heads in the “0” position corresponds the shortest available delay of δ=32.3 picoseconds, and is depicted in the drawing of  FIG. 4 . The drawings of  FIGS. 5 and 6  represent intermediate delay times of 0.26 and 0.32 nanoseconds, respectively. 
     The effective lengths of the delay lines are multiples of the length of the least significant bit&#39;s delay, i.e., 1 centimeter or δ=32.3 picoseconds. Representing the shortest effective length as 1, each delay line is thus 1*2 N  in length, where D N  is the corresponding bit position of the delay line from N=0 to M−1 for M number of delay lines. The digital delay device of the invention can, of course, be extended to more than 5 bits for additional combinations of longer delays. 
     The digital delay device of the present invention can be fabricated through use of a commercially available micromachining process such as the Metal MUMPs® process offered by MEMSCAP of 4021 Stirrup Creek Drive, Suite 120, Durham, N.C. 27709-9352, phone: 919-314-2200, fax: 919-314-220. 
     The Metal MUMPs process offers a 25 micrometer-thick electroplated nickel layer whose sidewalls can be coated with an evaporated gold film. This nickel layer can be utilized to realize the micromirrors, comb-drive actuators, wiring, and bonding pads. Additional surface micromachined films of Silicon Nitride, Si 3 N 4 , and polycrystalline silicon can be utilized to realize additional electrical connections, structural supports, return springs, and pivot axes. 
     A group of microstructure embodiments of the invention may be fabricated on a 500 μm thick crystalline silicon substrate, for example. 
       FIG. 7  in the drawings shows a physical arrangement usable to contain the described embodiment of the present invention. In the  FIG. 7  drawing there appears a rectangular tube  700  in which the delay paths shown in  FIG. 1 , for example, may be realized in the above recited additional mirror and folded delay path fashion. The  FIG. 7  drawing also shows the input and output fiber optic signal conductors  702  and  704  that may be used to convey optical signals to and from the delay device  700 . These input and output conductors may be parts of a continuous optical path to which the delay device  700  has been added. An overall length dimension of 4 centimeters is shown for the delay device  700 . An array of electrical contacts usable to connect the  FIG. 7  apparatus to an electrically energizing source of digital control signals appears at  706  in the  FIG. 7  drawing. The leftmost of these contacts may, for example, be the common electrical ground connection to which one terminal of each electrostatic comb-drive actuator is connected. These contacts are, of course, the contacts indicated at  194  etc. in the  FIG. 1  drawing. Typically incoming signal to a  FIG. 7  type of device is launched into the delay path(s) using a well cleaved fiber, mirror, or lens. Such use of fiber is preferred, since the incoming data/signal is thusly launched as a precise ray as required. 
     The illustrated and discussed arrangement of the digital delay device is by no means optimized for size, and can be scaled down to realize a smaller, more compact arrangement. The maximum switching speed of the digital delay device is limited only by the switching speed of the electronic drive circuitry and the switching speed of the electrostatically actuated comb-drive mirrors. The switching speed of the mirrors is determined by the mass of a mirror, stiffness of the return spring, and damping. The damping of the mirrors can be controlled by the geometry of the comb-drive and the mirror, and by the packaging conditions such as the vacuum or positive pressure atmosphere of air or another fluid used. 
     The foregoing description of the preferred embodiment has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.