Optical N-level quadrature amplitude modulation (NQAM) tuned by dithering associated heaters

Abstract

A technique for tuning a silicon photonics (SiP) based nested (parent/child) Mach-Zehnder modulator (MZM). The technique includes a sequence of applying dither tones on individual arms of the child MZMs, observing changes in the output of the MZM, and adjusting the MZM until reaching the null points for the child MZMs and the quad point for the parent MZM.

Claims

1. A controller for a nested Mach-Zehnder modulator (MZM), the controller comprising: an optical monitor; and a processor, wherein the nested MZM comprises: an input waveguide; an output waveguide; a first child MZM comprising an input waveguide, an output waveguide, two arms, and a first heater on one arm, wherein the first child MZM provides the in-phase (I) modulation; a second child MZM comprising an input waveguide, an output waveguide, two arms, and a second heater on one arm, wherein the second child MZM provides the quadrature phase (Q) modulation; and a third heater on one of the output waveguide of the first child MZM and the output waveguide of the second child MZM, wherein the optical monitor is coupled with the output waveguide of the nested MZM, wherein the processor is coupled with the first heater, the second heater, the third heater, and the optical monitor, wherein the optical monitor is configured to measure an optical signal on the output waveguide of the nested MZM, and wherein the processor is configured to: determine a P phase quad point; determine an I phase null point; and determine a Q phase null point.

2. The controller of claim 1, wherein to determine the P phase quad point, the processor is configured to: cause the first heater to apply a first dither tone; scan the second heater; record a first curve according to the optical signal measured by the optical monitor; cause the second heater to apply a second dither tone; scan the second heater; record a second curve according to the optical signal measured by the optical monitor; adjust the temperature of the third heater when the phase difference between the first curve and second curve is not equal to ninety degrees; and determine the P phase quad point when the phase difference between the first curve and second curve is equal to ninety degrees.

3. The controller of claim 2, wherein the optical monitor measures an output intensity of the optical signal, the phase of the first dither tone, and the phase of the second dither tone, wherein the first curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the first dither tone, and wherein the second curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the second dither tone.

4. The controller of claim 2, wherein an output intensity I.sub.out of the optical signal is described by the relationship: $I_{\mathrm{out}}=4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HI}}+\Delta {\phi }_I}{2}\right]+4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right]+4.Math.\cos \left[\frac{{\phi }_{\mathrm{HI}}+\Delta {\phi }_I}{2}\right].Math.\cos \left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right].Math.2.Math.\cos \left[\frac{{\phi }_{M+}{\phi }_{\mathrm{HI}}+2.Math.{\phi }_P}{2}-\frac{{\phi }_M+{\phi }_{\mathrm{HQ}}}{2}\right]$ wherein φ.sub.HI, φ.sub.HQ, and φ.sub.P are the phase shifts associated with the first, second, and third heaters, wherein Δφ.sub.I is the difference in phase between the two arms of the first child MZM, wherein Δφ.sub.Q is the difference in phase between the two arms of the second child MZM, and wherein φ.sub.M is the sum of the phases at the two arms of the first child MZM and the two arms of the second child MZM.

5. The controller of claim 4, wherein a first order differential of the output intensity dI.sub.out relates to a first order differential of the phase of the first dither tone dφ.sub.HI according to the relationship: $\frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HI}}}=-8\sin \left[{\phi }_{\mathrm{HI}}\right]\cos \left[.Math.{\mathrm{\Delta \phi }}_I.Math.\right]-8.Math.\left\{\sin \left[{\phi }_{P0}\right]+\sin \left[{\phi }_{P0}+{\phi }_{\mathrm{HQ}}\right]\right\}\cos \left[\frac{.Math.\Delta {\phi }_I.Math.}{2}\right]\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_Q.Math.}{2}\right]$ wherein φ.sub.P0 is φ.sub.P+φ.sub.HI−φ.sub.HQ.

6. The controller of claim 2, wherein to determine the I phase null point, the processor is further configured to: cause the first heater to apply a third dither tone; scan the second heater; record a third curve according to the optical signal measured by the optical monitor; adjust the second heater to the point where the third curve reaches a maximum value; cause the first heater to apply a fourth dither tone; scan the first heater; record a fourth curve according to the optical signal measured by the optical monitor; and determine the I phase null point as a point where the fourth curve crosses zero.

7. The controller of claim 6, wherein the optical monitor measures an output intensity of the optical signal, the phase of the third dither tone, and the phase of the fourth dither tone, wherein the third curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the third dither tone, and wherein the fourth curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the fourth dither tone.

8. The controller of claim 6, wherein the processor is further configured to adjust the third heater in step with adjusting the first heater.

9. The controller of claim 6, wherein to determine the Q phase null point, the processor is further configured to: adjust the first heater to the I phase null point; cause the second heater to apply a fifth dither tone; scan the second heater; record a fifth curve according to the optical signal measured by the optical monitor; and determine a point where the fifth curve crosses zero as the Q phase null point.

10. The controller of claim 9, wherein the optical monitor measures an output intensity of the optical signal and the phase of the fifth dither tone and wherein the fifth curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the fifth dither tone.

11. The controller of claim 9, wherein the processor is further configured to adjust the third heater in step with adjusting the second heater.

12. A method to tune a nested Mach-Zehnder modulator (MZM), the method comprising: applying a first dither tone by a first heater, wherein the first heater is on one arm of a first child MZM and wherein the first child MZM provides the in-phase (I) modulation for the nested MZM; scanning a second heater, wherein the second heater is on one arm of a second child MZM and wherein the second child MZM provides the quadrature phase (Q) modulation for the nested MZM; recording a first curve according to an optical signal output by the nested MZM; applying a second dither tone by the second heater; scanning the second heater; recording a second curve according to the optical signal output by the nested MZM; adjusting the temperature of a third heater when the phase difference between the first curve and the second curve is not equal to ninety degrees, wherein the third heater is on an output waveguide of one of the first child MZM and the second child MZM; and determining the parent phase (P) quad point when the phase difference between the first curve and the second curve is equal to ninety degrees.

13. The method of claim 12, wherein recording the first curve and the second curve further comprises measuring an output intensity of the optical signal, wherein the first curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the first dither tone, and wherein the second curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the second dither tone.

14. The method of claim 13, wherein the output intensity I.sub.out of the optical signal is described by the relationship: $I_{\mathrm{out}}=4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HI}}+\Delta {\phi }_I}{2}\right]+4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right]+4.Math.\cos \left[\frac{{\phi }_{\mathrm{HI}}+\Delta {\phi }_I}{2}\right].Math.\cos \left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right].Math.2.Math.\cos \left[\frac{{\phi }_M+{\phi }_{\mathrm{HI}}+2.Math.{\phi }_P}{2}-\frac{{\phi }_M+{\phi }_{\mathrm{HQ}}}{2}\right]$ wherein φ.sub.HI, φ.sub.HQ, and φ.sub.P are the phase shifts associated with the first, second, and third heaters, wherein Δφ.sub.I is the difference in phase between the two arms of the first child MZM, wherein Δφ.sub.Q is the difference in phase between the two arms of the second child MZM, and wherein Δφ.sub.Q is the sum of the phases at the two arms of the first child MZM and the two arms of the second child MZM.

15. The method of claim 14, wherein the first order differential of the output intensity dI.sub.out relates to the first order differential of the phase of the first dither tone dφ.sub.HI according to the relationship: $\frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HI}}}=-8\sin \left[{\phi }_{\mathrm{HI}}\right]\cos \left[.Math.{\mathrm{\Delta \phi }}_I.Math.\right]-8.Math.\left\{\sin \left[{\phi }_{P0}\right]+\sin \left[{\phi }_{P0}+{\phi }_{\mathrm{HQ}}\right]\right\}\cos \left[\frac{.Math.\Delta {\phi }_I.Math.}{2}\right]\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_Q.Math.}{2}\right]$ wherein φ.sub.P0 is φ.sub.P+φ.sub.HI−φ.sub.HQ.

16. The method of claim 12, further comprising: applying a third dither tone by the first heater; scanning the second heater; recording a third curve according to the optical signal output by the nested MZM; adjusting the second heater to a point where the third curve reaches a maximum value; applying a fourth dither tone by the first heater; scanning the first heater; recording a fourth curve according to the optical signal output by the nested MZM; and determining the I phase null point as a point where the fourth curve crosses zero.

17. The method of claim 16, wherein recording the third curve and the fourth curve further comprises measuring an output intensity of the optical signal, wherein the third curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the third dither tone, and wherein the fourth curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the fourth dither tone.

18. The method of claim 16, further comprising adjusting the third heater in step with adjusting the first heater or the second heater.

19. The method of claim 16, further comprising: adjusting the first heater to the I phase null point; applying a fifth dither tone by the second heater; scanning the second heater; recording a fifth curve according to the optical signal measured by the optical monitor; determining a point where the fifth curve crosses zero as the Q phase null point.

20. The method of claim 19, wherein recording the fifth curve according to the optical signal further comprises measuring an output intensity of the optical signal and wherein the fifth curve is a plot of the first order differential of the output intensity to the first order differential of the phase of the fifth dither tone.

21. A controller for tuning a nested Mach-Zehnder modulator (MZM), the controller comprising: a non-transitory memory storage comprising instructions; and one or more processors in communication with the memory, wherein the one or more processors execute the instructions to: apply a first dither tone by a first heater, wherein the first heater is on one arm of a first child MZM and wherein the first child MZM provides the in-phase (I) modulation for the nested MZM; scan a second heater, wherein the second heater is on one arm of a second child MZM and wherein the second child MZM provides the quadrature phase (Q) modulation for the nested MZM; record a first curve according to an optical signal output by the nested MZM; apply a second dither tone by the second heater; scan the second heater; record a second curve according to the optical signal output by the nested MZM; adjust the temperature of a third heater when the phase difference between the first curve and the second curve is not equal to ninety degrees, wherein the third heater is on an output waveguide of one of the first child MZM and the second child MZM; and determine a parent phase (P) quad point when the phase difference between the first curve and the second curve is equal to ninety degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

(2) FIG. 1 is a conventional Mach-Zehnder modulator.

(3) FIG. 2 is a conventional quadrature phase shift keying (QPSK) modulator.

(4) FIG. 3 is a SiP-based Mach-Zehnder modulator according to the present disclosure.

(5) FIG. 4 is a SiP-based IQ Mach-Zehnder modulator according to the present disclosure.

(6) FIGS. 5A-5C disclose a flowchart for tuning an IQ Mach-Zehnder modulator according to an embodiment of the present disclosure.

(7) FIG. 6-9 are exemplary plots of curves recorded during the course of tuning an IQ Mach-Zehnder modulator according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

(8) It should be understood at the outset that, although illustrative implementations of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

(9) In an embodiment of the present inventions, first consider a SiP-based Mach-Zehnder modulator 3 shown in FIG. 3 (some implementation details have been omitted for simplicity). Coherent light 300 is generated from light source 302, and is split to two silicon waveguides. Upper arm phase shifter 312 and lower arm phase shifter 322 of the silicon waveguides have been doped to form a pn junction across the waveguide so that when the doped segment is exposed to an electric field, the refractive index of the segment changes in proportion to the electric field. Thus, as coherent light passes through the doped segment, the phase of the light changes according to the length of the segment and the electric field. In addition, heater 316 of one of the silicon waveguide has a doped structure near the waveguide so that when this segment is heated through the application of external voltage, the refractive index of the segment changes quadratically with respect to the heating voltage. The heat can be provided by a resistive metallic heater or a doped semiconductor layer adjacent to segment or by other heating means known to one of ordinary skill. There is a phase shift φ.sub.I1 associated with the upper arm phase shifter 312, a phase shift φ.sub.I2 associated with the lower arm phase shifter 322, and a phase shift φ.sub.HI associated with the heater 316. The electrical field strength E.sub.I of the output at 330 is described as:

(10) $\begin{array}{cc}{E}_I=e^{i\left({\phi }_{I1}+{\phi }_{\mathrm{HI}}\right)}+e^{i\left({\phi }_{I2}\right)}=2.Math.\cos \frac{{\phi }_{I1}+{\phi }_{\mathrm{HI}}-{\phi }_{I2}}{2}.Math.e^{i\left(\frac{{\phi }_{I1}+{\phi }_{I2}+{\phi }_{\mathrm{HI}}}{2}\right)}& \left(\mathrm{eq}.1\right)\end{array}$

(11) where e is Euler's number and i the imaginary unit. As can be seen by this equation, the exiting amplitude is the function of the delta of the upper arm phase shifter 312 and the lower arm phase shifter 322 of the modulator and the exiting phase is the average phase of the upper and lower arms of the modulator. For push-pull type modulators, φ.sub.I1+φ.sub.I2 is constant and φ.sub.H−φ.sub.I2 varies as the modulation changes. A change in φ.sub.HI will necessarily result in a change not only in the electrical field but also the amplitude.

(12) The electrical field strength for a nested Mach-Zehnder modulator, as might be used in a QPSK application, is similar. FIG. 4 shows a nested Mach-Zehnder modulator having child modulators 4a and 4b forming a parent modulator 4c (again, some of the implementation details have been omitted). Like the modulator of FIG. 3, there are phase shifts φ.sub.I1 and φ.sub.Q1 associated with the upper arm phase shifters 412.sub.I and 412.sub.Q, phase shifts φ.sub.I2 and φ.sub.Q2 associated with the lower arm phase shifters 422.sub.I and 422.sub.Q, phase shift φ.sub.HI and φ.sub.HQ associated with heaters 416.sub.I and 416.sub.Q, and phase shift φ.sub.P associated with heater 426.sub.I. An optical monitor, tapped photodiode 440, has been added to monitor the throughput power at 430. The electric field strength E is described as:

(13) $\begin{array}{cc}\begin{array}{c}E=E_I{e}^{i\left({\phi }_p\right)}+E_Q\\ =2.Math.\cos \frac{{\phi }_{I1}+{\phi }_{\mathrm{HI}}-{\phi }_{I2}}{2}.Math.e^{i\left(\frac{{\phi }_{I1}+{\phi }_{I2}+{\phi }_{\mathrm{HI}}+2{\phi }_P}{2}\right)}+\\ 2.Math.\cos \frac{{\phi }_{I1}+{\phi }_{\mathrm{HQ}}-{\phi }_{Q2}}{2}.Math.e^{i\left(\frac{{\phi }_{Q1}+{\phi }_{Q2}+{\phi }_{\mathrm{HQ}}}{2}\right)}\end{array}& \left(\mathrm{eq}.2\right)\end{array}$

(14) Then the output intensity I.sub.out is described as:

(15) $\begin{array}{cc}{I}_{\mathrm{out}}=4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HI}}+\Delta {\phi }_I}{2}\right]+4.Math.{\cos }^2\left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right]+4.Math.\cos \left[\frac{{\phi }_{\mathrm{HI}}+{\mathrm{\Delta \phi }}_I}{2}\right].Math.\cos \left[\frac{{\phi }_{\mathrm{HQ}}+\Delta {\phi }_Q}{2}\right].Math.2.Math.\cos \left[\frac{{\phi }_M+{\phi }_{\mathrm{HI}}+2.Math.{\phi }_p}{2}-\frac{{\phi }_M+{\phi }_{\mathrm{HQ}}}{2}\right]& \left(\mathrm{eq}.3\right)\end{array}$

(16) where Δφ.sub.I=Δφ.sub.I1−Δφ.sub.I2, Δφ.sub.Q φ.sub.Q1−φ.sub.Q2, and φ.sub.M=φ.sub.I1+φ.sub.I2=φ.sub.Q1+φ.sub.Q2.

(17) The foregoing equations for E and I.sub.out (eq. 2 and eq. 3) are applicable for both high-speed (several dozen gigahertz range) and low-speed (kilohertz range) electro-optical signal transformation.

(18) By applying low frequency dither tones and monitoring the power response, the working point condition can be located. Following eq. 3, this leads to:

(19) $\begin{array}{cc}\frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HI}}}=-2.Math.\sin \left[{\phi }_{\mathrm{HI}}+\Delta {\phi }_I\right]-2.Math.\left[{\phi }_{\mathrm{HI}}+{\phi }_P-{\phi }_{\mathrm{HQ}}+\frac{\Delta {\phi }_I-\Delta {\phi }_Q}{2}\right]-\sin \left[{\phi }_{\mathrm{HI}}+{\phi }_P+\frac{\Delta {\phi }_I+\Delta {\phi }_Q}{2}\right]& \left(\mathrm{eq}.4\right)\end{array}$

(20) Because the frequency of the RF modulation is a few orders of magnitude higher than the frequency of the dither tone, a quadrature phase average can be applied to the response signal to yield:

(21) $\begin{array}{cc}\frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HI}}}=-8\sin \left[{\phi }_{\mathrm{HI}}\right]\cos \left[.Math.\Delta {\phi }_I.Math.\right]\pm 16.Math.\sin \left[{\phi }_{\mathrm{HI}}+{\phi }_P+\frac{{\phi }_{\mathrm{HQ}}}{2}\right].Math.\cos \left[\frac{{\phi }_{\mathrm{HQ}}}{2}\right]\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_I.Math.}{2}\right]\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_Q.Math.}{2}\right]& \left(\mathrm{eq}.5\right)\end{array}$

(22) By setting the working point of φ.sub.HI and φ.sub.HQ equal to π and φ.sub.HP equal to π/2 into eq. 5, the first order term for output (dI.sub.out) versus φ.sub.HI phase dither (dφ.sub.HI) is then zero. Eq. 5 can be re-written similarly for φ.sub.HQ phase dither (dφ.sub.HQ). Note that the “±” in eq. 5 depends on the phase arm location and the electrical wirings.

(23) The second order differential on P with respect to dither tones on I and Q leads to:

(24) $\begin{array}{cc}\frac{d^2{I}_{\mathrm{out}}}{d{\phi }_{HI}d{\phi }_{\mathrm{HQ}}}=8.Math.\cos \left[{\phi }_{\mathrm{HI}}+{\phi }_P-{\phi }_{\mathrm{HQ}}\right].Math.\cos \left[\frac{.Math.\Delta {\phi }_I.Math.}{2}\right].Math.\cos \left[\frac{.Math.\Delta {\phi }_Q.Math.}{2}\right]& \left(\mathrm{eq}.6\right)\end{array}$

(25) With this, the second order differential to the I and Q dither is zero when φ.sub.HI+φ.sub.P−φ.sub.HQ=π/2. Note in eq. 6 the strong phase coupling of φ.sub.P, φ.sub.HQ, φ.sub.HI. When tuning either of the child modulators 4a and 4b, the parent modulator 4c sees the impact of the changes to the child modulator heaters. Because of this, when adjusting the heaters 416.sub.I and 416.sub.Q on the child modulators 4a and 4b during tuning of the child modulators, the heater 426.sub.I for the parent modulator 4c must be adjusted as well so that eq. 6 is kept at zero.

(26) When φ.sub.P=φ.sub.P0+φ.sub.HQ−φ.sub.HI, eq. 5 can be rewritten for φ.sub.HQ and φ.sub.HI as follows:

(27) $\begin{array}{cc}\frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HI}}}=-8\sin \left[{\phi }_{\mathrm{HI}}\right]\cos \left[.Math.\Delta {\phi }_I.Math.\right]-8.Math.\left\{\sin \left[{\phi }_{P0}\right]+\sin \left[{\phi }_{P0}+{\phi }_{\mathrm{HQ}}\right]\right\}\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_I.Math.}{2}\right]\cos \left[\frac{.Math.\Delta {\phi }_Q.Math.}{2}\right]& \left(\mathrm{eq}.7\right)\\ \frac{d{I}_{\mathrm{out}}}{d{\phi }_{\mathrm{HQ}}}=-8\sin \left[{\phi }_{\mathrm{HQ}}\right]\cos \left[.Math.\Delta {\phi }_Q.Math.\right]-8.Math.\left\{\sin \left[{\phi }_{P0}\right]+\sin \left[{\phi }_{P0}+{\phi }_{\mathrm{HI}}\right]\right\}\cos \left[\frac{.Math.{\mathrm{\Delta \phi }}_I.Math.}{2}\right]\cos \left[\frac{.Math.\Delta {\phi }_Q.Math.}{2}\right]& \left(\mathrm{eq}.8\right)\end{array}$

(28) At the working point, φ.sub.HI and φ.sub.HQ equal π and φ.sub.P equals π/2, and both eq. 7 and eq. 8 are zero. φ.sub.HI equals π and φ.sub.P equals π/2, so when “scanning” φ.sub.HQ for 360 degrees, eq. 7 contains a cosine term and eq. 8 contain a sine term on φ.sub.HQ. (“Scanning” in this context, refers to sequentially increasing voltages or currents, typically in linear equal spaced increments, and retrieving the response.) Therefore eq. 7 and eq. 8 have a 90 degree phase shift. The first order differential to φ.sub.HQ cross zero, and the first order differential to φ.sub.HI has a maximum at φ.sub.HQ of 180 degrees, which is the null working point for φ.sub.IQ. When φ.sub.HI is not equal to π but φ.sub.P equals π/2, when scanning φ.sub.IQ for 360 degrees, the first order differential to φ.sub.HQ does not cross zero, but the first order differential to φ.sub.HI has a maximum at φ.sub.HQ of 180 degrees which, again, is the null working point for φ.sub.IQ.

(29) Once P is at the quad point (π/2), φ.sub.HQ can be scanned to determine the maximum value to get a “course scan” where the phase of φ.sub.IQ is π. After setting φ.sub.HQ, φ.sub.HI can be scanned to get a “fine scan” where the phase of φ.sub.HI is π.

(30) An embodiment of the foregoing is shown in FIGS. 4-9. In FIG. 4, control circuit 450 is electrically coupled with the other components shown in FIG. 4, including light source 402 producing coherent light signal 400, phase shifters 412.sub.I, 412.sub.Q, 422.sub.I, and 422.sub.Q, heaters 416.sub.I and 416.sub.Q, and 426.sub.I, and tapped photodiode 440. Control circuit 450 can be a digital signal processor, a microcontroller, an application specific integrated circuit, or any other component (or combination of components) known to one of ordinary skill for receiving and sending electrical signals and performing calculations. The electrical coupling between control circuit 450 and the other components in FIG. 4 (suggested by the plurality of dashed lines 451) can be direct or indirect and can use any form of connectivity known to one of ordinary skill. In this embodiment, modulator 4c of FIG. 4 is tuned such that φ.sub.HI and φ.sub.HQ are at null (0°) when φ.sub.P is at π/2 (90°).

(31) FIGS. 5A through 5C shows a flowchart of the steps for tuning in this embodiment. First, control circuit 450 tunes the P phase according to steps 5.1 through 5.5. In step 5.1, a dither tone is applied by heater 416.sub.I, heater 416.sub.Q is scanned, and the modulator output at photodiode 440 is recorded. Applying eq. 8, this should result in a curve like curve 601 as shown in FIG. 6. In step 5.2, a dither tone is applied by heater 416.sub.Q, heater 416.sub.Q is scanned, and the modulator output at photodiode 440 is recorded. Applying eq. 7, this should result in a curve like curve 602 as shown in FIG. 6. In Step 5.3, the phase difference between the two curves is measured. If the phase difference is 90 degrees, then P is tuned and the process moves to step 5.5. If the phase difference is not 90 degrees, then in step 5.4 heater 426.sub.I is adjusted and steps 5.1 through 5.3 are repeated until the phase difference is 90 degrees. At step 5.5, once the phase difference is 90 degrees, then P is at the quad point. Note that alternatively, instead of scanning 416.sub.Q in steps 5.1 and 5.2, 416.sub.I could be scanned.

(32) Next, control circuit 450 tunes the I phase according to steps 5.6 through 5.10. In step 5.6, a dither tone is applied by heater 416.sub.I, heater 416.sub.Q is scanned, and the modulator output at photodiode 440 is recorded. Applying eq. 7, this should result in curve 701 in FIG. 7. In step 5.7, the value of Q is determined where curve 701 reaches its maximum value. This value of Q is the “course null value” for Q. In step 5.8, the course null value determined in step 5.7 is set on heater 416.sub.Q. In step 5.9, a dither tone is applied by heater 416.sub.I, heater 416.sub.I is scanned, and the modulator output at photodiode 440 is recorded. Applying eq. 7, this should result in curve like curve 801 in FIG. 8. In step 5.10, the value of I is determined where curve 801 crosses zero. This value is the null point for I.

(33) Finally, control circuit 450 tunes the Q phase according to steps 5.11 through 5.13. First, in step 5.11, I is set to the I phase null point as determined in step 5.10. In step 5.12, a dither tone is applied by heater 416.sub.Q, heater 416.sub.Q is scanned, and the modulator output at photodiode 440 is recorded. Applying eq. 8, this should result in a curve like curve 901 in FIG. 9. In step 5.13, the value of Q is determined where curve 901 crosses zero. This value is the null point for Q.

(34) Note that when adjusting the I and Q phase on heaters 416.sub.I and 416.sub.Q, the phase of P on heater 4261 may be impacted by the phase leak from the child modulator phase tune in. To compensate, the P phase of heater 426.sub.I can be adjusted so that the parent modulator quad point remains locked. Typically a 1:1 degree compensation will keep the parent modulator quad point locked.

(35) The foregoing embodiment can also be adopted for other modulator configurations. By way of example and not limitation, this embodiment could be applied to tune a dual-polarization IQ modulator, in which case the steps disclosed in FIG. 5 could be performed on the X-polarization signal multiplexing with identical steps on the Y-polarization signal. Further, by way of example and not limitation, this embodiment could be applied to tune the modulator in a wavelength division multiplexing system, in which case the steps disclosed in FIG. 5 could be performed on the output of each of the modulators prior to multiplexing of the signals.

(36) Disclosed herein is a controller for a nested Mach-Zehnder modulator (MZM) comprising a means for monitoring the optical signal output by the MZM, a means for tuning determining a P phase quad point, a means for determining an I phase null point, and a means for determine a Q phase null point.

(37) Further disclosed herein is a method to tune a nested Mach-Zehnder modulator (MZM) comprising a means for applying a first dither tone by a first heater, a means for scanning a second heater, a means for recording a first curve according to an optical signal output by the nested MZM, a means for applying a second dither tone by the second heater, a means for scanning the second heater, a means for recording a second curve according to the optical signal output by the nested MZM, a means for adjusting the temperature of a third heater when the phase difference between the first curve and the second curve is not equal to ninety degrees, wherein the third heater is on an output waveguide of one of the first child MZM and the second child MZM, and a means for determining the parent phase (P) quad point when the phase difference between the first curve and the second curve is equal to ninety degrees.

(38) Further disclosed herein is a controller for tuning a nested Mach-Zehnder modulator (MZM) controller comprising a means for storing instructions, a means for executing instructions, a means for applying a first dither tone by a first heater, a means for scanning a second heater, a means for recording a first curve according to an optical signal output by the nested MZM, a means for applying a second dither tone by the second heater, a means for scanning the second heater, a means for recording a second curve according to the optical signal output by the nested MZM, a means for adjusting the temperature of a third heater, and a means for determining a parent phase (P) quad point.

(39) While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.

(40) In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.