Synchronous Amplitude Modulation for Improved Performance of Optical Transmission Systems

Abstract

A method and apparatus is provided that yields improved performance of both single channel and WDM long-distance optical transmission systems by synchronously modulating of the transmitted signal's amplitude. An amplitude modulator receives an optical signal onto which data has been modulated at a predetermined frequency. The modulator re-modulates the amplitude of the optical signal in a continues fashion with a waveform that is periodic, whose fundamental frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. The resulting signal (which is neither a pure NRZ or RZ signal) is more tolerant to the distortions usually found in lightwave transmission systems, thus giving superior transmission performance.

1 - 44 . (canceled) 
     
     
         45 . A method of generating a data carrying optical signal in a communication system, said method comprising:
 providing an optical signal; and   modulating a plurality of digital ones and zeros onto said optical signal with a value of said optical signal between consecutive occurrences of said digital ones being less than a value representing a digital one and greater than a value representing a digital zero.   
     
     
         46 . The method of  claim 45 , wherein said consecutive occurrences of said digital ones are modulated in a predetermined bit period, and wherein the value of said optical signal representing said consecutive digital ones has a maximum associated with each of said consecutive digital ones in consecutive bit periods and exhibits a sinusoidal variation between said maxima. 
     
     
         47 . The method  claim 46  further comprising selectively adjusting a depth of said sinusoidal variation between said maxima. 
     
     
         48 . The method of  claim 46 , wherein said sinusoidal variation between said maxima is imparted at a modulation depth greater than or equal to 20%. 
     
     
         49 . The method of  claim 46 , wherein said sinusoidal variation between said maxima is imparted at a modulation depth greater than or equal to 40%. 
     
     
         50 . The method of  claim 46 , wherein said sinusoidal variation between said maxima is imparted at a modulation depth greater than or equal to 60%. 
     
     
         51 . The method of  claim 46 , wherein said sinusoidal variation between said maxima is imparted at a modulation depth greater than or equal to 80%. 
     
     
         52 . The method  claim 46  further comprising selectively adjusting a phase of said sinusoidal variation between said maxima. 
     
     
         53 . The method of  claim 46 , wherein said plurality of digital ones and zeros are modulated onto said optical signal at a data modulation frequency, and wherein said sinusoidal variation is imparted at an amplitude modulation frequency phase locked to said data modulation frequency. 
     
     
         54 . The method of  claim 46 , wherein said plurality of digital ones and zeros are modulated onto said optical signal at a data modulation frequency, and wherein said sinusoidal variation is imparted at an amplitude modulation frequency equal to said data modulation frequency. 
     
     
         55 . The method of  claim 45 , wherein said modulating comprises modulating the optical phase of said optical signal. 
     
     
         56 . The method of  claim 55 , wherein said plurality of digital ones and zeros are modulated onto said optical signal at a data modulation frequency, and wherein said modulating the optical phase of said optical signal comprises modulating the optical phase of said optical signal at a phase modulation frequency phase locked to said data modulation frequency. 
     
     
         57 . The method of  claim 55 , wherein said plurality of digital ones and zeros are modulated onto said optical signal at a data modulation frequency, and wherein said modulating the optical phase of said optical signal comprises modulating the optical phase of said optical signal at a phase modulation frequency equal to said data modulation frequency. 
     
     
         58 . The method of  claim 55 , said method further comprising selectively adjusting a level of said modulation of the optical phase of said optical signal. 
     
     
         59 . The method of  claim 45 , wherein said modulating comprises modulating the polarization of said optical signal. 
     
     
         60 . The apparatus of  claim 59  wherein said modulating the polarization of said optical signal comprises modulating the state of polarization of said optical signal such that an average value of the state of polarization over a modulation cycle is substantially equal to zero. 
     
     
         61 . The method of  claim 59 , wherein said plurality of digital ones and zeros are modulated onto said optical signal at a data modulation frequency, and wherein said modulating the state of polarization of said optical signal comprises modulating the state of polarization at a polarization modulation frequency at said data modulation frequency with a prescribed phase. 
     
     
         62 . The method of  claim 61 , said method further comprising selectively varying the prescribed phase.