Patent ID: 8849129

Claim:
A DP-QPSK optical transmitter comprising: a) an outer Mach-Zehnder modulator comprising: i. an optical input that receives an optical beam from a laser source; ii. a first arm comprising a first parent Mach-Zehnder modulator comprising: 1. a first child Mach-Zehnder modulator of the first parent Mach-Zehnder modulator optically coupled into a first arm of the first parent Mach-Zehnder modulator; and 2. a second child Mach-Zehnder modulator of the first parent Mach-Zehnder modulator optically coupled to a π/2 optical phase shifter and optically coupled into a second arm of the first parent Mach-Zehnder modulator, the first parent Mach-Zehnder modulator modulating a QPSK signal with a first polarization; iii. a second arm comprising a second parent Mach-Zehnder modulator comprising: 1. a first child Mach-Zehnder modulator of the second parent Mach-Zehnder modulator optically coupled into a first arm of the second parent Mach-Zehnder modulator; and 2. a second child Mach-Zehnder modulator of the second parent Mach-Zehnder modulator optically coupled to a π/2 optical phase shifter and optically coupled into a second arm of the second parent Mach-Zehnder modulator; and iv. an optical polarization rotator optically coupled to the second arm, the second parent Mach-Zehnder modulator modulating a QPSK signal with a second polarization, wherein the outer Mach-Zehnder modulator multiplexes the first and second polarization embedded into a dual-polarization QPSK signal generation; b) a first optical detector optical coupled to an output of the first parent Mach-Zehnder modulator, the first optical detector detecting the QPSK signal generated by the first parent Mach-Zehnder modulator with the first polarization; c) a second optical detector optical coupled to an output of the second parent Mach-Zehnder modulator, the second optical detector detecting the QPSK signal generated by the second parent Mach-Zehnder modulator with the second polarization; and d) a bias control circuit having a first and second input electrically connected to respective outputs of the first and second optical detector, and at least one output that is electrically coupled to at least one of a respective bias input of the first and second child Mach-Zehnder modulator of the first parent Mach-Zehnder modulator, a phase bias input of the first parent Mach-Zehnder modulator, respective bias inputs of the first and second child Mach-Zehnder modulators of the second parent Mach-Zehnder modulator, and a phase bias input of the second parent Mach-Zehnder modulator, the bias control circuit generating bias signals on at least one output in response to both DC and AC signals detected by the first and second optical detectors that stabilizes the DP-QPSK signal.