Fiber optic network system with low crosstalk using code-division multiplexing

Abstract

A code-division multiplexed system comprising an optical source, a code generator for developing and applying a pseudo-random bit sequence code to the optical source to cause the optical source to develop a unipolar coded optical signal, a means for dividing the unipolar coded optical signal into N optical paths separated from each other by associated successive integer multiples of a bit period T of delay, N information signal sources responsive to the unipolar coded optical signals for selectively producing N differently-delayed optical information signals, a means for combining the N differently-delayed optical information signals into an output optical signal comprised of an intensity sum of the overlapping N differently-delayed optical unipolar information signals, a photodetector responsive to the output optical signal for developing an input electrical signal corresponding to the sum of N overlapping unipolar optical signals, an adjustable delay circuit responsive to the pseudo-random bit sequence code for producing a desired bipolar reference code corresponding to the delay associated with an associated desired one of the N differently-delayed optical information signals, and a correlation circuit responsive to the input electrical signal and to the desired bipolar reference code for synchronously correlating the desired bipolar reference code with the input electrical signal to extract a desired electrical signal corresponding to the desired one of the N differently-delayed optical information signals and to substantially suppress any other electrical signal that is asynchronously aligned with the desired bipolar reference code.

What is claimed and desired to be secured by Letters Patent of the United States is: 
     
       1. A code-division multiplexed system comprising: an optical source;   first means for developing and applying a pseudo-random bit sequence code to said optical source to cause said optical source to develop a unipolar coded optical signal;   means for dividing said unipolar coded optical signal into N optical paths separated from each other by associated successive integer multiples of a bit period T of delay;   N information signal sources for selectively encoding the unipolar coded optical signals in the respective N optical paths for producing N differently-delayed optical information signals;   means for combining said N differently-delayed optical information signals into an output optical signal comprised of an intensity sum of overlapping N differently-delayed unipolar optical information signals;   photodetection means responsive to said output optical signal for developing an input electrical signal corresponding to said sum of N overlapping unipolar optical information signals;   adjustable means responsive to said pseudo-random bit sequence code for producing at least one essentially bipolar reference code corresponding to the delay associated with an associated desired one of said N differently-delayed optical information signals; and   correlation means responsive to said input electrical signal and to said at least one essentially bipolar reference code for synchronously correlating said at least one essentially bipolar reference code with said input electrical signal to extract a desired electrical signal corresponding to the desired one of said N differently-delayed optical information signals and to substantially suppress any other electrical signal that is asynchronously aligned with said at least one essentially bipolar reference code.   
     
     
       2. The system of claim 1 wherein: said first means is a pseudo-random bit, maximal-length sequence code generator.   
     
     
       3. The system of claim 1 wherein said optical source includes: a laser for emitting light; and   a modulator responsive to light from said laser and to the pseudo-random bit sequence code for producing the unipolar coded optical signal.   
     
     
       4. The system of claim 1 wherein said correlation means includes: a difference amplifier having inverting and non-inverting input terminals and an output terminal;   an electronic gate having a signal input for receiving said input electrical signal, first and second output terminals respectively coupled to said inverting and noninverting input terminals, and a control input for receiving said bipolar reference code, (said electonic gate being responsive to each positive state of said bipolar reference code for passing the unipolar electrical signal to said noninverting input terminal and to each negative state of said bipolar reference code for passing the unipolar electrical signal to said inverting input terminal,) said electronic gate operating to pass those portions of the unipolar electrical signal that are synchronously correlated with corresponding portions of said bipolar reference code to said noninverting input terminal and to pass all other portions of the unipolar electrical signal that are asynchronously correlated with corresponding portions of said bipolar reference code; and   a low pass filter for filtering said output terminal of said difference amplifier for substantially filtering out any unwanted other electrical signal that is asynchronously aligned with said at least one essentially bipolar reference code.   
     
     
       5. The system of claim 1 wherein said correlation means includes: a DC coupled analog multiplier for multiplying the input electrical signal with the desired reference code to produce a product signal; and   a low pass filter responsive to the product signal for passing the desired electrical signal corresponding to the desired one of said N differently-delayed optical information signals and substantially suppressing all other unwanted signals contained in the product signal.   
     
     
       6. The system of claim 1 wherein: said adjustable means includes a shift register with N tapped outputs respectively representative of said N optical paths, said shift register responsive to said pseudo-random bit sequence code for producing at associated ones of said N tapped outputs N differently-delayed bipolar reference codes corresponding to the delays associated with said N differently-delayed optical information signals.   
     
     
       7. A code-division multiplexed intensity based sensor array system comprising: an optical source;   first means for developing and applying a pseudo-random bit sequence code to said optical source to cause said optical source to produce a unipolar coded optical signal;   an intensity based sensor array comprised of N sensors respectively disposed in N optical paths and being separated from each other by associated successive integer multiples of a bit period T of delay, said sensor array being responsive to said unipolar coded optical signal for producing N differently-delayed output optical signals respectively indicative of any changes in the physical parameters to which said N sensors are respectively responsive and for combining said N differently-delayed output optical signals into a unipolar output signal comprised of an intensity sum of overlapping N differently-delayed output optical signals;   photodetection means responsive to said unipolar output signal for developing a unipolar electrical signal corresponding to said unipolar output signal;   adjustable means responsive to said pseudo-random bit sequence code for producing at least one essentially bipolar reference code corresponding to the delay associated with an associated desired one of said N differently-delayed output optical signals; and   correlation means responsive to said unipolar electrical signal and to said at least one essentially bipolar reference code for synchronously correlating said at least one essentially bipolar reference code with said unipolar electrical signal to extract a desird electical signal corresponding to the desired one of said N differently-delayed output optical signals and to substantially suppress any other electrical signal that is asynchronously aligned with said at least one essentially bipolar reference code.   
     
     
       8. A code-division multiplexed interferometric sensor array system comprising: an optical source;   first means for developing and applying a pseudo-random bit sequence code to said optical source to cause said optical source to produce a unipolar coded optical signal;   an interferometric sensor array comprised of N sensors respectively disposed in N optical paths and being separated from each other by associated successive integer multiples of a bit period T of delay, said sensor array being responsive to said unipolar coded optical signal for producing N differently-delayed interferometric optical signals respectively indicative of any changes in the physical parameters to which said N sensors are respectively responsive and for combining said N differently-delayed interferometric optical signals into a unipolar output signal comprised of an intensity sum of overlapping N differently-delayed interferometric optical signals;   photodetection means responsive to said unipolar output signal for developing a unipolar electrical signal corresponding to said unipolar output signal;   adjustable means responsive to said pseudo-random bit sequence code for producing at least one essentially bipolar reference code corresponding to the delay associated with an associated desired one of said N differently-delayed interferometric optical signals; and   correlation means responsive to said unipolar electrical signal and to said at least one essentially bipolar reference code for synchronously correlating said at least one essentially bipolar reference code with said unipolar electrical signal to extract a desird electical signal corresponding to the desired one of said N differently-delayed interferometric optical signals and to substantially suppress any other electrical signal that is asynchronously aligned with said at least one essentially bipolar reference code.