Bias control of optical modulators

Abstract

An optical waveguide modulator with automatic bias control is disclosed. A portion of the modulator light is mixed with reference light and converted to one or more electrical feedback signals. An electrical feedback circuit controls the modulator bias responsive to the feedback signals.

1 - 20 . (canceled) 
     
     
         21 . A modulator device comprising:
 an optical modulator circuit (OMC) configured to modulate signal light to produce modulated output light, the OMC including:
 an input port for receiving the signal light, 
 an output port for transmitting the modulated output light, and 
 a bias control configured to receive an electrical bias signal for controlling a set point of the OMC, and 
   an optoelectronic feedback loop comprising:
 an optical tap port configured to provide tapped light from the OMC indicative of the set point of the OMC; 
 an optical mixer (OM) comprising: a first optical port optically coupled to the optical tap port for receiving the tapped light, and a second optical port for receiving reference light, the optical mixer configured to mix the reference light with the tapped light to produce at least one mixed light signal, each of the at least one mixed light signal combining the reference light and the tapped light; 
 a photodetector (PD) circuit comprising one or more photodetectors (PDs) and configured to convert the at least one mixed light signals into at least one electrical feedback signal responsive to changes in modulator set point; and 
 an electrical feedback circuit (EFC) connected between the PD circuit to the bias control, and configured to generate the electrical bias signal in dependence on the at least one electrical feedback signal. 
   
     
     
         22 . The modulator device according to  claim 21 , further comprising:
 an optical splitter disposed to receive input light and configured to split the input light into the signal light and the reference light, and   a first optical phase tuner configured to tune an optical phase of the reference light in response to a reference control signal;   wherein the electrical feedback circuit is configured to vary the electrical bias signal and the reference control signal in dependence on the at least one electrical feedback signal.   
     
     
         23 . The modulator device according to  claim 21 , wherein the OMC includes a quadrature modulator circuit connected between the input port and the output port and configured to produce an in-phase modulated optical signal and a quadrature modulated optical signal from the signal light, and to combine the in-phase modulated optical signal with the quadrature modulated optical signal to obtain the modulated output light and the tapped light, each of the modulated output light and the tapped light comprising the in-phase modulated optical signal and the quadrature modulated optical signal with an IQ phase shift therebetween; and
 wherein the bias control includes a second optical phase tuner configured to vary the IQ phase shift responsive to the at least one electrical bias signal, wherein the IQ phase shift defines the set point.   
     
     
         24 . The modulator device according to  claim 23 , wherein the optical mixer is configured to output the at least one mixed light signals comprising: a first mixed optical signal combining the tapped light and the reference light with a first optical phase shift therebetween, and a second mixed optical signal combining the tapped light and the reference light with a second optical phase shift therebetween. 
     
     
         25 . The modulator device according to  claim 24 , wherein the PD circuit comprises:
 a first balanced PD receiver configured to produce a first differential PD signal from the first mixed light signal and the second mixed light signal; and   a first radio frequency (RF) rectifier disposed to receive the first differential PD signal and configured to produce the at least one electrical feedback signal comprising a first feedback signal, wherein the first RF rectifier is responsive to data-rate changes in the first differential PD signal.   
     
     
         26 . The modulator device according to  claim 25 , wherein the first RF rectifier comprises an RF squaring detector configured to produce the first feedback signal that is proportional to a square of the first differential PD signal, and 
       wherein the EFC comprises a processor comprising logic configured to monitor changes in the first feedback signal, and to adjust the electrical bias signal so as to reduce the changes in the first feedback signal. 
     
     
         27 . The modulator device according to  claim 25 , wherein:
 the optical mixer comprises a 90° optical hybrid configured to output four mixed optical signals comprising the first optical signal, the second optical signals, and further comprising third and fourth optical mixed signals,   the PD circuit further comprises a second balanced PD receiver disposed to receive the third and fourth mixed optical signals and configured to produce a second differential PD signal therefrom, the PD circuit further comprising a second RF rectifier disposed to receive the second differential PD signal and configured to produce a second feedback signal, wherein the second RF rectifier is responsive to data-rate changes in the second differential PD signal.   
     
     
         28 . The modulator device according to  claim 27 , wherein the EFC comprises a processor comprising logic configured to adjust the electrical bias signal so as to equalize the first feedback signal and the second feedback signal. 
     
     
         29 . The modulator device according to  claim 28 , wherein the logic is configured to obtain a difference signal that is indicative of a difference between the first feedback signal and the second feedback signal, and to monitor changes in the difference signal. 
     
     
         30 . The modulator device according to  claim 21 , wherein the reference light includes an optical power greater than the tapped light for amplifying the at least one feedback signal. 
     
     
         31 . A method of modulate signal light to produce modulated output light using an optical modulator circuit (OMC) comprising an input port for receiving the signal light, an output port for transmitting modulated output light, a bias control configured for receiving an electrical bias signal controlling a set point of the OMC, the method comprising:
 a) tapping light from the OMC to obtain tapped light indicative of the set point of the OMC;   b) mixing the tapped light with reference light of a greater power in an optical mixer to produce one or more mixed light signals, each combining the tapped light with the reference light;   c) converting the one or more mixed light signals into one or more electrical feedback signals comprising information about the set point of the OMC, using a photodetector (PD) circuit; and   d) generating the electric bias signal in dependence on the one or more electric feedback signals using an electric feedback circuit (EFC).   
     
     
         32 . The method according to  claim 31 , further comprising:
 splitting input light into the signal light and the reference light using an optical splitter prior to modulating the signal light by the OMC;   tuning an optical phase of the reference light using an optical phase tuner in response to a reference control signal; and   varying the reference control signal in dependence on the one or more electrical feedback signals.   
     
     
         33 . The method according to  claim 31 , wherein the OMC comprises a quadrature modulator configured to modulate and combine an in-phase modulated optical signal and a quadrature modulated optical signal;
 wherein the bias control comprises a first optical phase shifter, configured to vary an IQ phase shift between the in-phase modulated optical signal and the quadrature modulated optical signal, wherein the IQ phase shift defines the set point of the OMC.   
     
     
         34 . The method according to  claim 33 , wherein the step a) comprises obtaining a first mixed light signal and a second mixed light signal, wherein the tapped light is added to the reference light with a phase shift that differs generally by 180° between the first mixed light signal and the second mixed light signal. 
     
     
         35 . The method according to  claim 34 , wherein the step b) comprises differentially detecting the first mixed light signal and the second mixed light signal to obtain a first differential PD signal, and rectifying the first differential PD signal using an RF rectifier that is responsive to data-rate changes in the first differential PD signal to obtain a first electrical feedback signal of the at least one electrical feedback signals. 
     
     
         36 . The method according to  claim 35 , wherein the step c) comprises detecting an average of the first electrical feedback signal over time. 
     
     
         37 . The method according to  claim 35 , wherein the first RF rectifier comprises an RF squaring detector configured to produce the first feedback signal that is proportional to a square of the first differential PD signal, and 
       wherein the EFC is configured to monitor changes in the first feedback signal that are responsive to the reference control signal, and to adjust the electrical bias signal so as to reduce the changes. 
     
     
         38 . The method according to  claim 35 , wherein the optical mixer comprises a 90° optical hybrid (OH) comprising four output ports;
 wherein the step a) comprises detecting, from the four output ports of the 90° optical hybrid, four mixed optical signals comprising the first mixed optical signal, the second mixed optical signal, a third mixed optical signal, and a fourth mixed optical signal; 
 wherein the step b) further comprises obtaining a second differential PD signal in quadrature with the first differential PD signal from the third mixed optical signal and the fourth mixed optical signal, and rectifying the second differential PD signal to obtain a second electrical feedback signal of the at least one electrical feedback signals; and 
 wherein the step c) comprises using the first optical phase shifter to adjust the IQ phase shift in the quadrature modulator so as to equalize the first electrical feedback signal and the second electrical feedback signal.