Abstract:
An analog optical adder system that achieves high precision results. The system uses an analog optical carry function to provide a result having a precision higher than the precision of the individual elements of the system. The optical carry function is created by optical carry determinators that are configured to add an optical carry, if any, to an optical signal associated with a next adjacent byte of the digital signals being added. The use of optical carry enables greater overall addition precision.

Description:
FIELD 
     This disclosure relates to an analog optical adder. 
     BACKGROUND 
     The use of analog optical processing elements to perform analog optical addition is known. The processing elements used typically have low precision. One way in which the low precision of the processing elements has been addressed is by writing specific algorithms in an attempt to make use of the lower precision functions. Another way in which the low precision has been addressed is by using external digital logic to correct or improve precision. 
     SUMMARY 
     An analog optical adder system and method are described that achieves high precision results. The system uses individual lower precision elements together with an analog optical carry function to provide a result having a precision higher than the precision of the individual elements. 
     The system uses optical carry determinators that provide the carry function to add an optical carry, if any, to an optical signal of a next adjacent byte of the digital signals being added. The use of optical carry enables greater overall addition precision. 
     In one disclosed embodiment, an optical adding system is described for adding first and second digital data signals. Each data signal has a plurality of bytes including a most significant byte and a least significant byte. For each byte, a digital to optical converter converts each byte of the first and second digital data signals into a respective optical signal. Optical summers are connected to the digital to optical converters of common byte positions, with the optical summers being configured to sum the optical signals associated with the common byte positions to create a plurality of summed optical signals equal in number to the number of bytes in each of the first and second digital data signals. Optical carry determinators are connected to the optical summers of the common byte positions, except for the optical summer of the most significant byte position. The optical carry determinators determine whether an optical carry should be added to the summed optical signal of a next adjacent byte position. An optical to digital converter for each summed optical signal converts the summed optical signals, plus any added carry, to digital data signals and assembles the summed digital data signals into byte order to create a summed digital data signal representing a sum of the first and second digital data signals. 
     A method of adding first and second digital data signals is also described, where each data signal has a plurality of bytes including a most significant byte and a least significant byte. Each byte of the first and second digital data signals is converted into a respective optical signal. The optical signals of common byte positions of the first and second digital data signals are summed to create a plurality of summed optical signals equal in number to the number of bytes in each of the first and second digital data signals. For the summed optical signal associated with the least significant bytes, it is determined whether the power of the summed optical signal is above a reference power threshold, and a carry is created if the power of the summed optical signal of the least significant bytes is above the reference power threshold and the carry is added to the summed optical signal associated with the next byte position of the first and second digital data signals. Thereafter, the summed optical signals, plus any carry, are converted to summed digital data signals and the summed digital data signals are assembled into byte order to create a summed data signal representing a sum of the first and second digital data signals. 
    
    
     
       DRAWINGS 
       The drawing depicts the exemplary analog optical adder system described in the detailed description. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary analog optical adder system  10  is illustrated in the drawing. The system  10  is designed to add first and second digital data signals  12 ,  14 . In the illustrated example, the data signals  12 ,  14  are 32 bit data signals, with each data signal having a plurality of bytes of 8 bits each. The bytes are arranged in a LSB  0  numbering scheme including a most significant byte  16  at byte position  3 , followed by byte  18  at byte position  2 , followed by byte  20  at byte position  1 , followed by a least significant byte  22  at byte position  0 . Although four bytes are illustrated, the number of bytes and bits of each signal described herein is exemplary only, it being understood that digital signals containing other numbers of bytes and bits, including, but not limited to, 4 bits, 16 bits, 64 bits, or other number of bits, could be used. In addition, although the system  10  is described as adding two data signals, the concepts described herein can be applied to an adder that adds more than two data signals together. 
     The system  10  adds the data signals  12 ,  14  and produces a summed data signal  24  representing a sum of the first and second digital data signals. The summed data signal  24  is a 32 bit digital signal arranged in the original LSB  0  numbering scheme of the original signals  12 ,  14 . The elements used in the system  10  can have 9-bit precision so the summed data signal  24  can have a carry over bit  26 . 
     A digital to optical converter  30  is assigned to each byte  16 ,  18 ,  20 ,  22  that converts each byte into a respective optical signal  32 . For example, for the least significant bytes  22  and byte positions  0 , the converters  30  at those two byte positions convert the 8 bits of each byte into optical signals  32   a ,  32   b . Any devices suitable for converting digital signals into optical signals can be used. In the illustrated example, the converters  30  include a digital-to-analog converter  34  for converting the digital signals to analog signals  36 , with the analog signals  36  then being input into laser diodes  38  that convert the analog signals into the optical signals  32 . 
     The optical signals  32  from each pair of common byte positions are input into an optical summer  40 . Each summer  40  sum the two optical signals  32  to create a plurality of summed optical signals  42  equal in number to the number of bytes, in this example four, in the first and second digital data signals. 
     Optical carry determinators  50  are connected to the optical summers  40  of byte positions  0 ,  1  and  2 . The optical carry determinators  50  are configured to be able to sense the summed optical signals  42  of byte positions  0 ,  1 ,  2 , determine whether the power of the summed optical signal is above a reference power threshold, create an optical carry if the power of the summed optical signal is above the reference power threshold and add the carry to the summed optical signal associated with the next left byte position. 
     Any devices that are able to perform these functions of the determinators  50  can be used. In the illustrated system  10 , each determinator  50  comprises a photo diode  52  that senses the summed signal  42  and outputs an electrical signal representative of the power level of the summed optical signal. The electrical signal is fed to a logic device  54  that also receives or has programmed therein a reference power threshold  56 . The logic device  54  compares the signal from the photo diode  52  with the threshold  56  and if the signal from the photo diode  52  is greater than the threshold  56 , then the logic device  54  sends a signal  58  to an optical modulator  60  which adds an optical carry  62  (i.e. an increment of optical power) to the summed optical signal  42  of the next left byte position. If the signal from the photo diode  52  is less than the threshold  56 , then an optical carry is not added to the next left byte position. The logic device  54  can be any device that can perform these described functions of the logic device, such as a comparator or a thresholder. 
     In one example, the threshold  56  for each of the determinators  50  is the same. However, in some circumstances, the thresholds  56  of one or more of the determinators  50  could be different from the thresholds  56  of other determinators. In fact, due to changes in the optical power levels, the same function could have different threshold levels. Also, in some applications, multiple thresholds could be used, for example in more complex arithmetic operations. Any threshold(s) can be used that provide the ability to make a determination at one stage that can control an optical output of another stage(s). 
     In the illustrated example, the optical carry  62  is the difference between the power level of the summed optical signal  42  and the reference power threshold  56 . That difference in the power levels is the amount of incremental optical power that is to be added to the summed optical signal  42  of the next left byte position. In another example, the optical carry is a fixed amount of incremental optical power regardless of how much the optical power exceeds the threshold. 
     For example, for the least significant bytes  22 , the signals  32   a ,  32   b  are summed resulting in the summed signal  42 . The photodiode  52  senses the summed signal  42  and outputs an electrical signal that is fed to the logic device  54 . The logic device compares the power level of the summed signal  42  with the reference power threshold  56 . If the power of the summed optical signal is above the threshold  56 , the difference between the two is determined. The logic device  54  then sends the signal  58  to the optical modulator  60  instructing the modulator to optically send an increment of optical power (i.e. the optical carry  62 ) representing the difference to the summed optical signal  42  of byte position  1 . That carry  62  is then added to the summed optical signal  42  of byte position  1 . A similar process occurs in the optical carry determinators  50  for byte position  1  and byte position  2 . 
     Each of the summed optical signals  42 , plus any added carry in the case of byte positions  1 ,  2  and  3 , is fed to an optical to digital converter  70  that converts the summed optical signals into the summed digital signal  24 . Any devices suitable for doing this conversion can be used. In the illustrated example, the converters  70  include photodiodes  72  for converting the optical signals to analog signals  74 , with the analog signals  74  then being input into analog-to-digital converters  76  that convert the analog signals into digital signals. The converters  76  are arranged so that the individual digital signals are assembled in byte order to create the summed signal  24  representing a sum of the first and second digital data signals. 
     The examples disclosed in this application are to be considered in all respects as illustrative and not limitative. The scope of the invention is indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.