Method and apparatus for transmission of two modulated signals via an optical channel

Abstract

A transmitter with two optical sources generates two optical carrier signals having different frequencies. The optical carrier signals are combined and divided in a first coupler and fed to carrier signal inputs of two modulators. The mixed carrier signals are separately modulated by two modulation signals (a(t)) and (b(t)) and the modulated signals are combined in a first combiner and emitted as transmission signal. Only one demodulator is necessary to regain the modulation signals (a(t) and b(t)).

1 . A method for receiving a transmission signal composed of two modulated optical signals, the method which comprises:
 receiving and demodulating a transmission signal and converting the transmission signal into an electrical transmission signal;   low pass filtering the electrical transmission signal to generate a first filter output signal;   modulating and filtering the electrical transmission signal to generate a second filter output signal; and   adding the first and second filter output signals and subtracting the filter output signals from each other to retrieve a first modulation signal and a second modulation signal.   
     
     
         2 . The method according to  claim 1 , which comprises applying a modulation selected from the group consisting of intensity modulation, phase modulation, and phase difference phase modulation. 
     
     
         3 . The method according to  claim 2 , which comprises generating the mixed carrier signals by modulating a constant wave signal. 
     
     
         4 . The method according to  claim 1 , which comprises converting a received intensity modulated transmission signal into an demodulated electrical signal by an optical-electrical converter. 
     
     
         5 . The method according to  claim 1 , which comprises processing a received difference phase modulated transmission signal in a single delay interferometer and converting the output signals in a balanced optical-electrical converter. 
     
     
         6 . A receiver of an assembly for transmitting two modulated signals via an optical channel, the receiver comprising:
 a demodulator unit configured for converting a received transmission signal into a squared electrical signal, said demodulator unit having an output;   a signal separation circuit configured for retrieving a first and a second modulation signal with a first low pass filter connected to said output of said demodulator unit, and a modulator and a second low path filter connected in series and to said output of said demodulator unit, said first low pass filter having an output and said second low path filter having an output;   a first adder and a second adder, having first inputs connected to said output of said first low path filter, having second inputs connected to said output of said second low pass filter, and having outputs emitting the modulation signals.   
     
     
         7 . The receiver according to  claim 6 , wherein said demodulator unit for demodulating an intensity modulated received transmission signal is a photo diode. 
     
     
         8 . The receiver according to  claim 6 , wherein said demodulator unit for demodulating a DPSK modulated transmission signal includes a delay interferometer and a pair of optical-electrical converters receiving optical signals from outputs of said delay interferometer.