In-phase (I) / quadrature (Q) correlation dither for modulator quadrature control
Aspects of the subject disclosure may include, for example, obtaining two inputs x I and x Q based on a digital input signal, and causing a modulator to create two substantially orthogonal output dimensions I and Q based on the two inputs x I and x Q , by performing controlled introduction of a correlation between the two inputs x I and x Q for the modulator, and detecting a resulting output power of the modulator to facilitate operation of the modulator. Other embodiments are disclosed.
1 . A device, comprising:
a modulator configured to output an optical signal; and
a control system configured to cause the modulator to create two orthogonal output dimensions, in-phase (I) and quadrature (Q), based on two inputs x I and x Q derived from a digital input signal, by:
performing controlled introduction of a correlation between the two inputs x I and x Q for the modulator, wherein the controlled introduction corresponds to use of a controlled correlated term from the input x I to the input x Q , or from the input x Q to the input x I , that is filtered out via a digital filter, and wherein the controlled correlated term enables detection of any frequency-dependent non-orthogonality within dimension I radio frequency (RF) drive signals and within dimension Q RF drive signals or within I and Q arms of the modulator; and
detecting a resulting output power of the modulator to facilitate operation of the modulator.
2 . The device of claim 1 , wherein the modulator comprises an electro-optical modulator, and wherein the two inputs x I and x Q comprise electrical signals.
3 . The device of claim 1 , wherein the modulator comprises a nested Mach-Zehnder modulator.
4 . The device of claim 1 , wherein the two inputs xi and x Q are produced by digital-to-analog converters (DACs), and wherein the controlled introduction occurs in a digital domain.
5 . The device of claim 1 , wherein the controlled introduction corresponds to addition of a specific scaled portion of the input x I to the input x Q or addition of a specific scaled portion of the input x Q to the input x I .
6 . The device of claim 1 , wherein the control system is further configured to cause the modulator to create two orthogonal output dimensions YI and YQ for a Y polarization based on two inputs y I and y Q by performing a second controlled introduction of a second correlation between the two inputs y I and y Q .
7 . The device of claim 1 , wherein the controlled introduction comprises an insertion of known symbols into a symbol stream.
8 . The device of claim 1 , wherein the detecting the resulting output power comprises detecting a variation of output power.
9 . The device of claim 8 , wherein the correlation comprises a square-wave dither pattern; and the detecting the variation of output power comprises a correlation of the output power with the square-wave dither pattern, or
wherein the correlation comprises a pseudo-random dither pattern and the detecting the variation of output power comprises a correlation of the output power with the pseudo-random dither pattern.
10 . The device of claim 8 , wherein a sign of the variation of output power is detected, and wherein a magnitude of the variation of output power is detected.
11 . The device of claim 1 , wherein the correlation comprises a value that is less than a threshold value, and wherein the two output dimensions are orthogonal such that a quadrature error (QE) between the two output dimensions is less than a threshold error.
12 . The device of claim 1 , wherein the dimension I represents an in-phase dimension of optical signals created by the modulator and the dimension Q represents a quadrature dimension of optical signals created by the modulator, or
wherein the dimension I represents a dimension within one polarization of optical signals created by the modulator and the Q represents a dimension within an orthogonal polarization.
13 . The device of claim 1 , wherein the control system is further configured to add a dither to a bias control voltage.
14 . The device of claim 1 , wherein the detecting is performed using a detection circuit comprising a photodiode, and wherein the detection circuit has a bandwidth that is less than 0.001 of a bandwidth of the input x I .
15 . The device of claim 1 , wherein the two orthogonal output dimensions I and Q comprise dimensions XI and XQ for an X polarization, wherein the control system is further configured to cause the modulator to create two orthogonal output dimensions YI and YQ for a Y polarization based on two inputs y I and y Q by performing a second controlled introduction of a second correlation between the two inputs y I and y Q , wherein the output dimension YI is also orthogonal to the output dimensions XI and XQ, and wherein the controlled introduction of the correlation between the two inputs x I and x Q and the second controlled introduction of the second correlation between the two inputs y I and y Q are performed simultaneously.
16 . The device of claim 1 , further comprising a Transpose filter, wherein the detecting provides a measure of I/Q nonquadrature, and wherein the operation of the modulator is effected by performing digital pre-compensation of an equal and opposite amount in the Transpose filter based on the measure of the I/Q nonquadrature.
17 . A device, comprising:
a modulator configured to output an optical signal; and
a control system configured to cause the modulator to create two orthogonal output dimensions, in-phase (I) and quadrature (Q), based on two inputs x I and x Q derived from a digital input signal, by:
performing controlled introduction of a correlation between the two inputs x I and x Q for the modulator; and
detecting a resulting output power of the modulator to facilitate operation of the modulator, wherein the detecting is performed using a detection circuit comprising a photodiode, and wherein the detection circuit has a bandwidth that is less than 0.001 of a bandwidth of the input x I .
18 . A device, comprising:
a modulator configured to output an optical signal; and
a control system configured to cause the modulator to create two orthogonal output dimensions, in-phase (I) and quadrature (Q), based on two inputs x I and x Q derived from a digital input signal, by:
performing controlled introduction of a correlation between the two inputs x I and x Q for the modulator, wherein the two orthogonal output dimensions I and Q comprise dimensions XI and XQ for an X polarization;
performing a second controlled introduction of a second correlation between two inputs y I and y Q , thereby creating two orthogonal output dimensions YI and YQ for a Y polarization based on the two inputs y I and y Q , wherein the output dimension YI is orthogonal to the output dimensions XI and XQ, and wherein the controlled introduction of the correlation between the two inputs x I and x Q and the second controlled introduction of the second correlation between the two inputs y I and y Q are performed simultaneously; and
detecting a resulting output power of the modulator to facilitate operation of the modulator.
19 . A device, comprising:
a modulator configured to output an optical signal;
a control system configured to cause the modulator to create two orthogonal output dimensions, in-phase (I) and quadrature (Q), based on two inputs x I and x Q derived from a digital input signal, by:
performing controlled introduction of a correlation between the two inputs x and x Q for the modulator; and
detecting a resulting output power of the modulator to facilitate operation of the modulator; and
a transpose filter, wherein the detecting provides a measure of I/Q nonquadrature, and wherein the operation of the modulator is effected by performing digital pre-compensation of an equal and opposite amount in the transpose filter based on the measure of the I/Q nonquadrature.
20 . The device of claim 19 , wherein the modulator comprises a nested Mach-Zehnder modulator.
21 . The device of claim 19 , wherein the two inputs x I and x Q are produced by digital-to-analog converters (DACs), and wherein the controlled introduction occurs in a digital domain.