Quadrature modulator with feedback control and optical communications system using the same

Abstract

The method and system are disclosed for automatic feedback control of integrated optical quadrature modulator for generation of optical quaternary phase-shift-keyed signal in coherent optical communications. The method comprises the steps of detecting at least a part of an output optical signal from the QPSK modulator, extracting of a particular portion of the output signal in frequency domain, and processing the signal in frequency domain to optimize the transmission of an optical link. The system and method of optical communications in fiber or free space are disclosed that implement the quadrature data modulator with automatic feedback control.

1 . The method for automatic feedback control of an integrated optical quadrature modulator comprising the steps of:
 data encoding of an input optical signal in the quadrature modulator,   detecting at least a first part of an output optical signal from the quadrature modulator, extracting of a particular portion of the output signal in RF frequency domain and minimizing the portion of the output signal in RF frequency domain by tuning a voltage applied to a phase shifter connected to I and Q components of the quadrature modulator, the phase shifter providing a 90 degrees phase shift between I and Q components of the optical signal;   achieving convergence to an optimal operating point of the quadrature modulator by the voltage tuning; and   outputting a second part of the output optical signal with 90-degrees phase shift between I and Q signal components, wherein   the second part of the output optical signal is PSK modulated.   
     
     
         2 . The method of  claim 1 , wherein the tuning consists of dithering a control voltage applied to the phase shifter. 
     
     
         3 . The method of  claim 1 , wherein the tuning is performed using a steepest descent algorithm. 
     
     
         4 . The method of  claim 1 , wherein the tuning is performed continuously during the quadrature modulator operation. 
     
     
         5 . The method of  claim 1 , further comprising discontinuing tuning when the optimal operating point is achieved. 
     
     
         6 . The method of  claim 1 , wherein I component of the output signal is produced a first Mach-Zehnder (MZM) modulator and Q component of the output signal is produced a second Mach-Zehnder modulator, two MZMs forming a MZ interferometer, which serves as the quadrature modulator; and wherein
 each MZM is driven to produce binary PSK.   
     
     
         7 . The method of  claim 6 , wherein the optimal operating point corresponds to each of the MZMs is biased at their null transmission point and the MZ interferometer is biased at the quadrature phase. 
     
     
         8 . The method of  claim 1 , wherein the second output optical signal is QPSK modulated. 
     
     
         9 . The method of  claim 1 , wherein the input signal to the quadrature modulator is pulsed signal. 
     
     
         10 . The method of  claim 1 , further comprising:
 detecting a power of the output optical signal,   tuning a first voltage applied to a first bias of the quadrature modulator and a second voltage applied to a second bias, and   minimizing the power of the output optical signal.   
     
     
         11 . The method of  claim 1 , further comprising:
 detecting a power of the output optical signal,   tuning a first voltage applied to a first bias of the quadrature modulator and a second voltage applied to a second bias, and   maximizing the power of the output optical signal.   
     
     
         12 . A quadrature modulator, comprising:
 a first modulator producing I component of an optical beam;   a second modulator producing Q component of the optical beam; the second modulator positioned in parallel to the first modulator forming a MZ interferometer;   a phase shifter introducing a 90-degrees phase shift between I and Q components of the optical beam;   a combiner combining I and Q components of the optical beam and outputting a quadrature modulated optical signal; and   a feedback loop connected to the combiner and the phase shifter; the feedback loop optimizing operation of the phase shifter to achieve the modulator operation at an optimal operating point.   
     
     
         13 . The modulator of  claim 12 , wherein the first and the second modulators are Mach-Zehnder modulators. 
     
     
         14 . The modulator of  claim 13 , wherein the feedback loop further includes branches providing optimization of MZMs operation. 
     
     
         15 . The modulator of  claim 13 , wherein the optimal operation point corresponds to each of the MZMs is biased at their null transmission point and the MZ interferometer is biased at a π/2 phase. 
     
     
         16 . The modulator of  claim 12 , wherein the feedback loop comprises a waveguide connected to the quadrature modulator output, extracting of a particular portion of the output signal, converting in RF frequency domain and minimizing the portion of the output signal in RF frequency domain by tuning a voltage applied to a phase shifter of the MZ interferometer. 
     
     
         17 . The modulator of  claim 16 , further comprising a DSP unit controlling the feedback loop. 
     
     
         18 . An optical communications system, comprising:
 a transmitter for creating and sending a quadrature modulated signal, the transmitter including a quadrature modulator producing QPSK data modulation of an optical signal; the quadrature modulator comprising a first feedback loop connected to an output of the quadrature modulator and controlling a phase shifter of the quadrature modulator to achieve its operation at 90 degrees point;   a receiver, receiving the quadrature modulated signal and recovering QPSK modulated data.   
     
     
         19 . The system of  claim 18 , wherein the first feedback loop provides an improved system performance allowing to achieve higher BER than without the feedback loop. 
     
     
         20 . The system of  claim 18 , further comprising a second and a thirds feedback loop connected to an output of the quadrature modulator and controlling a first and a second Mach-Zehnder modulators, which produce I and Q components of the QPSK signal, respectively; and all three feedback loops improving performance of the system allowing to achieve higher BER than without the feedback loops.