Mach-Zehnder interferometer with linearized output and method for Mach-Zehnder interferometer

Abstract

A Mach-Zehnder interferometer (MZI) includes: a pair of waveguides; a light input configured to separate light received at the light input between the pair of waveguides; and a respective ring resonator modulator on the pair of waveguides, the respective ring resonator modulator including a respective pn junction having a respective capacitance connected in series. The MZI further includes a common driver configured to receive a differential electrical signal having data encoded therein; and using the differential electrical signals to control respective n-sides of the respective pn junctions to control the respective ring resonator modulators to modulate light into respective optical signals, respectively. The MZI also includes a common voltage bias configured to provide a common voltage to a respective p-side of the respective pn junction. The MZI further includes an optical output configured to receive the respective optical signals from the pair of waveguides; and combining the respective optical signals.

Description

[0001] Cross-references to related applications

[0002] This specification claims priority to U.S. Provisional Patent Application No. 63 / 647677, filed May 15, 2024, which is incorporated herein by reference. Technical Field

[0003] This invention relates to Mach-Zehnder interferometers for optical communication systems, and particularly to Mach-Zehnder interferometers with linearized outputs and methods for using Mach-Zehnder interferometers. Background Technology

[0004] Optical transmitters based on Mach-Zern interferometers (MZIs) present various technical challenges, including those related to the junction behavior of phase modulators on one or more of the corresponding arms (e.g., waveguides) of the MZI. For example, capacitance decreases as the reverse voltage across the junction increases. Furthermore, as junction efficiency increases (e.g., due to more refractive index modulation), junction capacitance increases and also becomes more nonlinear. In other words, junction capacitance typically exhibits nonlinear behavior as a function of the applied voltage. This nonlinearity intensifies with increasing junction efficiency, leading to optical nonlinear modulation. This can manifest as tailed or skewed PAM4 eye diagrams and / or unequally spaced PAM4 levels, significantly impacting end-to-end signal integrity. Additionally, such Mach-Zern interferometers may exhibit different behaviors due to optical self-heating dependent on input power, and may also have power consumption-related issues. Summary of the Invention

[0005] In one aspect of the invention, a Mach-Zernd interferometer (MZI) is provided, comprising: a pair of waveguides; an optical input configured to split light received at the optical input between the pair of waveguides; respective ring resonator modulators located on the pair of waveguides, the respective ring resonator modulators including respective pn junctions having respective capacitors connected in series; a common driver configured to: receive a differential electrical signal in which data is encoded; and use the differential electrical signal to control a respective n-side of the respective pn junction to control the respective ring resonator modulator to modulate the light into respective optical signals; a common voltage bias configured to provide a common voltage to a respective p-side of the respective pn junction; and an optical output configured to: receive the respective optical signals from the pair of waveguides; and combine the respective optical signals.

[0006] In another aspect of the application, a method is provided, comprising determining, via a controller, an optical power at a Mach-Zehnder interferometer (MZI); controlling, via the controller, the MZI in a single-side tuning mode when the optical power is above a threshold optical power; and controlling, via the controller, the MZI in a differential-side tuning mode when the optical power is below the threshold optical power, wherein in the single-side tuning mode, a first ring modulator and a second ring modulator of the MZI are both tuned to a blue side of a resonance frequency, and wherein in the differential-side tuning mode, the first ring modulator is tuned to the blue side of the resonance frequency and the second ring modulator is tuned to a red side of the resonance frequency.

[0007] In yet another aspect of the application, a method is provided, comprising: controlling, via a controller, at least one of a plurality of heaters to a plurality of temperatures, the one or more respective heaters along a pair of waveguides of a Mach-Zehnder interferometer (MZI), the respective heaters configured to heat respective portions of the pair of waveguides; monitoring, via the controller, a change in an output power of the MZI while controlling the at least one heater; determining, via the controller, an optical transfer curve of the MZI using the change in the output power; determining, via the controller, a respective operating condition of the respective heaters to make the MZI quadrature when one of the respective heaters is off; selecting, via the controller, the respective operating condition for a first of the respective heaters that minimizes power of the respective heaters while a second of the respective heaters is off; and controlling, via the controller, the first heater according to the selected respective operating condition while the second heater is off. BRIEF DESCRIPTION OF DRAWINGS

[0008] For a better understanding of the various implementations described herein and to show how they can be implemented, reference will now be made, by way of example only, to the drawings in which:

[0009] Figure 1 A schematic diagram of an optical transmitter according to the prior art is depicted.

[0010] Figure 2A A chart showing the non-linear capacitance behavior across different voltage levels used to encode data in a phase modulator in an optical transmitter according to the prior art is depicted.

[0011] Figure 2B A PAM4 chart showing signal integrity issues arising from a phase modulator in an optical transmitter according to the prior art, such as skew and uneven eye diagrams due to non-linear capacitance as depicted in Figure 2A

[0012] ​Figure 3 An optical transmitter is depicted in accordance with non-limiting examples.

[0013] Figure 4 An optical transmitter is depicted in accordance with non-limiting examples. Figure 3 An electrical schematic of the optical transmitter.

[0014] Figure 5 Example capacitance curves are compared between the optical transmitter and prior art optical transmitters. Figure 3

[0015] A variation of capacitance over time is depicted in accordance with non-limiting examples for different applied voltages of the optical transmitter. Figure 6A Figure 3 A more aligned and open PAM4 eye diagram is depicted in accordance with non-limiting examples for the optical transmitter.

[0016] Figure 6B Figure 3 An optical transmitter is depicted in accordance with non-limiting examples that can be controlled based on input power and / or application type and / or to mitigate power consumption.

[0017] Figure 7 A method for controlling an optical transmitter based on input power or application type is depicted in accordance with non-limiting examples.

[0018] Figure 8 An optical transmitter is depicted in accordance with non-limiting examples that can be controlled based on input power and / or application type and / or to mitigate power consumption.

[0019] Figure 9A An optical transmitter is depicted in accordance with non-limiting examples that can be controlled based on input power and / or application type and / or to mitigate power consumption. Figure 7

[0020] A shift in asymmetric transmission of the optical transmitter due to low frequency stress signals and / or patterns is depicted in accordance with non-limiting examples. Figure 9B Figure 9A Transmission of the optical transmitter as a function of wavelength of the ring resonator modulator when operating with differential side tuning at lower optical power is depicted in accordance with non-limiting examples.

[0021] Figure 10A Figure 7 Phase of the optical transmitter as a function of wavelength of the ring resonator modulator is depicted in accordance with non-limiting examples.

[0022] Figure 10B Transmission of the optical transmitter as a function of wavelength of the ring resonator modulator is depicted in accordance with non-limiting examples. Figure 7

[0023] A shift in asymmetric transmission of the optical transmitter due to low frequency stress signals and / or patterns is depicted in accordance with non-limiting examples. Figure 11 Figure 7 ​​​​​the transmission of the ring resonator modulator of the optical transmitter shifts with increasing temperature.

[0024] Figure 12 is depicted according to non-limiting examples when using differential side tuning, Figure 7 the transmission of the ring resonator modulator of the optical transmitter shifts with decreasing temperature.

[0025] Figure 13A is depicted according to non-limiting examples when using single side tuning, the asymmetric transmission of the ring resonator modulator shifts with increasing temperature.

[0026] Figure 13B is depicted according to non-limiting examples Figure 3 the transmission of the optical transmitter shifts with the operating point moving from the quadrature point towards the zero point.

[0027] Figure 14 is depicted according to non-limiting examples a method for controlling an optical transmitter to be in quadrature while minimizing power usage.

[0028] Figure 15 is depicted according to non-limiting examples Figure 14 an example of the method. DETAILED DESCRIPTION

[0029] Reference is made to Figure 1 , depicts a prior art optical transmitter 100, which includes a pair of waveguides 101, 102 (e.g., arms), which can hereinafter be interchangeably referred to as a first waveguide 101 and a second waveguide 102. The waveguides 101, 102 can have different lengths, as shown by the additional length 103 at the second waveguide 102.

[0030] The waveguides 101, 102 are joined at opposite ends by an optical input 104 and an optical output 106. The optical input 104 can include a beam splitter that splits incoming light (not depicted), e.g., from a laser, between the pair of waveguides 101, 102; in particular, the optical input 104 can include a 50 / 50 beam splitter. Similarly, the optical output 106 can include a beam splitter that receives light from the pair of waveguides 101, 102, which can be modulated as respective optical signals having data encoded therein, and combines the light and / or respective optical signals; the optical output 106 can be optically coupled to an optical waveguide / fiber (not depicted) onto which the combined optical signals are transmitted, e.g., to an optical receiver that decodes data from the combined optical signals.

[0031] In particular, the optical transmitter 100 can include respective phase modulators 111, 112 (e.g., a first phase modulator 111 and a second phase modulator 112) on the waveguides 101, 102, respectively, which can be controlled via a driver 114 to introduce respective phase shifts φ1and φ2into light on the waveguides 101, 102, respectively. Such phase shifts are understood to encode data into the light, e.g., based on a differential electrical signal (e.g., in the form of voltages V inPos , V inNeg received at positive and negative inputs 116, 118 of the driver 114).

[0032] Furthermore, different lengths of the waveguides 101, 102 can be selected so that the respective optical signals combine at or near respective quadrature points.

[0033] For the sake of clarity, optical connections (such as waveguides, etc.) are depicted herein with solid lines, while electrical connections between electrical components (such as wiring, etc.) are depicted with dashed lines, unless otherwise noted.

[0034] Thus, the prior art optical transmitter 100 is understood to include a Mach-Zehnder interferometer (MZI) for encoding data into light from a laser. The optical transmitter 100 suffers from various technical problems.

[0035] For example, the phase modulators 111, 112 can be based on any suitable technology, including but not limited to a resonant ring modulator (RRM). Regardless, the phase modulators 111, 112 are understood to have respective capacitances that are generally non-linear. For example, the phase modulators 111, 112 can include an electrical junction where a device that can be positioned for modulating a phase, and driven by an electrical signal across the junction. As the respective capacitances of the phase modulators 111, 112 are non-linear, various problems arise.

[0036] Furthermore, due to optical self-heating, the optical transmitter 100 can exhibit different behavior depending on input power.

[0037] Furthermore, as described, the optical transmitter 100 can include heaters 161, 162 along the pair of waveguides 101, 102, and in addition to the lengths 103, the heaters 161, 162 can be controlled to heat respective portions of at least one of the pair of waveguides 101, 102 to further control a phase difference between the respective optical signals on the waveguides 101, 102. However, the heaters 161, 162 can consume excessive power, which can need to be controlled.

[0038] With respect to the respective capacitances of the phase modulators 111, 112 being non-linear, it is next noted that Figure 2Awhich depicts an example of the non-linear capacitance of the phase modulators 111, 112, and how the capacitance varies with four different applied voltages (e.g., for the four available amplitude levels used to encode data into light on the waveguides 101, 102), and Figure 2B which depicts the resulting PAM4 (phase-amplitude modulation with 4 levels) diagram for this variation in capacitance.

[0039] For example, Figure 2A A graph 200 is depicted showing a plot 201 of how the capacitance of a single junction of the phase modulators 111, 112 varies with applied voltage, and includes a voltage plot 202 showing how the voltage applied to a single junction of the phase modulators 111, 112 can vary over time between four different voltage levels (e.g., 0, 1, 2, 3), e.g., when encoding data into light on the waveguides 101, 102. The resulting variation in capacitance over time is depicted in graph 204.

[0040] Figure 2B The impact of this variation in capacitance in the PAM4 diagram 206 is shown. For example, the PAM4 diagram 206 includes four eyes 208, which ideally should be aligned, e.g., along line 210, and “open” (e.g., a uniform gap between them). However, the eyes 208 are clearly not aligned along line 210, and in addition the gap between them is clearly not uniform and / or small.

[0041] To address this problem, next attention is drawn to Figure 3 , Figure 3 A light transmitter 300 is depicted, which generally includes an MZI that is used to encode data into light from a laser, as described next. The light transmitter 300 can be a component of a photonic integrated circuit (PIC) that is deployed in an optical communication system.

[0042] The light transmitter 300 includes a pair of waveguides 301, 302 (e.g., a first optical waveguide 301 and a second optical waveguide 302, and which can have different lengths, as indicated by the additional length 303 of the second optical waveguide 302, although the length 303 can be optional), an optical input 304 that is configured to split light received at the optical input between the pair of waveguides 301, 302, and an optical output 306 that is configured to receive respective optical signals from the pair of waveguides 301, 302 and combine the respective optical signals, e.g., at a quadrature point.

[0043] The light input 304 can include a beam splitter that splits incident light, e.g., light provided by a laser 307 at the light input 304, between a pair of waveguides 301, 302. The laser 307 can or can not be a component of the light transmitter 300. In particular, the light input 304 can include a 50 / 50 beam splitter such that light from the laser 307 is divided approximately equally between the light waveguides 301, 302. As described herein, phase modulators on the pair of waveguides 301, 302 can modulate light on the waveguides 301, 302 as respective optical signals that are encoded with data.

[0044] Similarly, the light output 306 can include a beam splitter that receives (e.g., modulated) light from the pair of waveguides 301, 302 and combines the light and / or respective optical signals. The light output 306 can be optically coupled to a light waveguide (not depicted) onto which the combined optical signals are transmitted, e.g., to a light receiver that decodes data from the combined optical signals.

[0045] The light transmitter 300 also includes respective phase modulators in the form of RRM 311, 312 (e.g., a first ring resonator modulator 311 and a second ring resonator modulator 312) on the pair of waveguides 301, 302, the respective RRM 311, 312 including a respective pn-junction with a respective capacitance connected in series. The pn-junction and the respective capacitance are described in further detail below.

[0046] The light transmitter 300 also includes a common driver 314 configured to receive a differential electrical signal in which data is encoded and to use the differential electrical signal to control respective p-sides of the respective pn-junctions to control the respective RRM 311, 312 to modulate light as respective optical signals (e.g., combined by the light output 306). For example, such a differential electrical signal can be provided, e.g., in the form of a voltage V inPos , V inNeg received at positive and negative inputs 316, 318 of the driver 314. Although not depicted, it will be appreciated that the inputs 316, 318 are electrically coupled to a device that provides the differential electrical signal.

[0047] Ring resonator modulators (RRMs) 311, 312 are described next.

[0048] For example, the RRM 311, 312 is understood to include, respectively, the respective optical ring 321, 322 located between respective pn junctions formed by respective p-side 331, 332 (e.g., a region of p-doped material, such as a p-doped material extending at least partially along an outer side of the optical ring 321, 322) and respective n-side 341, 342 (e.g., a region of n-doped material, such as an n-doped material extending at least partially along an inner side of the optical ring 321, 322). The p-side 331, 332 can alternatively be referred to as an outer contact, and the n-side 341, 342 can alternatively be referred to as an inner contact. In practice, it will be appreciated that, in order to control the respective position of the respective resonance wavelength and / or free spectral range (FSR) of the optical ring 321, 322, a respective electric field can be placed across the optical ring 321, 322 by applying a voltage to the respective n-side 341, 342 via the driver 314. Moreover, such respective position of the respective FSR, for example in relation to the wavelength of light input to the waveguide 301, 302 (e.g., laser light from a laser), can be controlled by a respective heater (not depicted for simplicity) of the RRM 311, 312.

[0049] As depicted, and unlike the prior art optical transmitter 100, the optical transmitter 300 further comprises a common voltage bias 350 configured to provide a common voltage to the respective p-side 331, 332 of the respective pn junction; for example, as depicted, the respective p-side 331, 332 is connected via an electrical connection 351, and the common voltage bias 350 is connected to the electrical connection 351. Thus, although not depicted, the optical transmitter 300 can comprise or be connected to one or more voltage sources for providing the voltage of the common voltage bias 350.

[0050] With regard to Figure 4 The electrical properties of the RRM 311, 312 are described in more detail.

[0051] However, before describing the electrical properties of the RRM 311, 312, the following describes Figure 3 the remainder of the disclosure.

[0052] For example, although optional, the optical transmitter 300 can comprise an inductor 352 between the common voltage bias 350 and the electrical connection 351 and / or the respective p-side 331, 332, for example to block any high frequency voltage that can appear at the common voltage bias 350, and which can or be set to a constant value and / or output a constant value.

[0053] Further, as depicted, the optical transmitter 300 can include at least one heater 361, 362 along the pair of waveguides 301, 302. For example, as depicted, the optical transmitter 300 includes a first heater 361 positioned along the first optical waveguide 301 between the first RRM 311 and the optical output 306, and includes a second heater 362 positioned along the second optical waveguide 302 between the second RRM 312 and the optical output 306. The heaters 361, 362 are connected to a voltage supply 364 (e.g., V Heat ), which can be controlled by a controller 379 (e.g., at least one processor) to provide a voltage from the voltage supply 364 to heat one heater 361, 362 or both heaters 361, 362. Thus, although not depicted, the optical transmitter 300 can include or be connected to one or more voltage sources to provide the voltage of the voltage supply 364. For example, the at least one heater 361, 362 is generally configured to heat a respective portion of at least one of the pair of waveguides 301, 302 to control a phase difference between respective optical signals on the waveguides 301, 302. Although the length 303 can introduce such a phase difference, for example, to place the optical signals on the waveguides 301, 302 in quadrature (e.g., a 90° phase difference), one or more of the heaters 361, 362 can be controlled to maintain quadrature of the optical signals on the waveguides 301, 302. Further, the controller 379 can or can not be a component of the optical transmitter 300.

[0054] Thus, the optical transmitter 300 can include or be connected to a controller 379, etc., which implements the control functions described herein, for example, to control the various heaters and / or inputs 316, 318, etc., described herein. Although the controller 379 is not depicted as communicatively coupled to other components of the optical transmitter 300 for simplicity, the controller 379 is still understood to be communicatively coupled to any suitable components.

[0055] As depicted, the optical transmitter 300 can include one or more sensors 380, 381, 382, 383, 384, 385 at the light input 304, before and after the first RRM 311 on the first optical waveguide 301, before and after the second RRM 312 on the second optical waveguide 302, and at the light output 306. The sensors 380, 381, 382, 383, 384, 385 can include respective photodetectors or the like that sample light at the light input 304, the light output 306, and on the waveguides 301, 302, and provide respective outputs (e.g., signals) indicative of the sampled light power and / or the sampled light intensity to the controller 379. Although not depicted, the sensors 380, 381, 382, 383, 384, 385 are understood to be communicatively coupled to the controller 379.

[0056] By comparing the respective outputs from the sensors 380, 381 and / or the respective outputs from the sensors 382, 380, the extent to which light is split by the beamsplitter of the light input 304 can be determined.

[0057] Similarly, by comparing the respective ratios of the respective outputs from the sensors 381, 383, the insertion loss of the first RRM 311 can be determined. Similarly, by comparing the respective ratios of the respective outputs from the sensors 382, 384, the insertion loss of the second RRM 312 can be determined. In general, the insertion losses of the RRMs 311, 312 should be the same; as such, the controller 379 can receive the outputs from the pair of sensors 381, 383 and the pair of sensors 382, 384, determine the insertion loss of each of the RRMs 311, 312, and when the respective insertion losses of the RRMs 311, 312 are not about the same, the controller 379 can control one or more of the heaters of the RRMs 311, 312, respectively, until the respective insertion losses are about the same. Alternatively or additionally, one or more of the heaters of the RRMs 311, 312 can be controlled until the intersection of the transmissions of the RRMs 311, 312 is at about the wavelength of the laser 307 (e.g., see Figure 10A .

[0058] Similarly, by the controller 379 comparing the outputs from the sensors 380, 385, the relative phase of the RRMs 311, 312 and / or whether the light signals on the waveguides 301, 302 are in quadrature or not can be determined. When not in quadrature, the controller 379 can control one or more of the heaters 361, 362 to make the light signals on the waveguides 301, 302 be in quadrature (e.g., as described with respect to Figure 14 to Figure 15 .

[0059] Therefore, for example, by detecting the light intensity and / or light power of one or more of the optical signals on waveguides 301, 302 and / or at optical input 304 and optical output 306, the relative phase of the optical signals on waveguides 301, 302 can be detected and made orthogonal, and / or can be kept orthogonal, for example in a feedback loop having RRMs 311, 312 and / or heaters 361, 362. In particular, sensors 380, 381, 382, ​​383, 384, 385 can be communicatively coupled to a controller 379 that controls heaters 361, 362, and the controller 379 can control heaters 361, 362 in a feedback loop having one or more sensors to maintain the orthogonality of the optical signals on waveguides 301, 302.

[0060] Next, pay attention Figure 4 It depicts a simplified electrical diagram 400 of the light emitter 300. (And...) Figure 3 Unlike other diagrams, for simplicity, the electrical connections between electrical components in Figure 400 are depicted with solid lines (e.g., because...). Figure 4 (Optical connections are not depicted). Although inductor 352 is not depicted for simplicity, inductor 352 may still be present.

[0061] Specifically, the electrical components of the drivers 314, inputs 316, 318, and common voltage bias 350, as well as the pn junctions 411, 412 of the RRMs 311, 312, are shown. In particular, the pn junctions 411, 412 include corresponding capacitors 421, 422 and corresponding resistors 431, 432. In a particular example (e.g., for a resonant ring modulator used in a PIC), capacitors 421, 422 may range from about 50 fF to about 80 fF, and resistors 431, 432 may be about 40 ohms, but any suitable values ​​for capacitors 421, 422 and resistors 431, 432 are within the scope of this specification.

[0062] Furthermore, for the pn junction 411 of the first RRM 311, driver 314 (e.g., in sequence) is connected to capacitor 421, resistor 431, and common voltage bias 350. Similarly, for the pn junction 412 of the second RRM 312, driver 314 (e.g., in sequence) is connected to capacitor 422, resistor 432, and common voltage bias 350. Thus, it should be understood that pn junction 412 shares common voltage bias 350, and furthermore, capacitors 421 and 422 are connected in series. It should also be understood that driver 314 drives RRMs 311 and 312 in reverse phase, and therefore drives pn junctions 411 and 412 in reverse phase, such that when a “high” voltage is applied to the n-side 341 of pn junction 411, a corresponding “low” voltage is applied to the n-side 342 of pn junction 412.

[0063] In practice, since the capacitors 421, 422 are connected in series, whichever capacitor 421, 422 is lower and / or smaller (e.g., due to its non-linearity, as well as the difference in drive voltage of the pn-junctions 411, 412), it dominates the total capacitance of the optical transmitter 300 (e.g., as 1 / C total =∑(1 / C1+1 / C2), where C total is the total capacitance, C1is the value of the capacitor 421, and C2is the value of the capacitor 422).

[0064] Further details of the optical transmitter 300 are next described.

[0065] It will be appreciated that the RRM 311, 312 generally modulate the phase of the optical signal at each arm (e.g., waveguide 301, 302) of the MZI of the optical transmitter 300.

[0066] It will be appreciated that the RRM 311, 312 are differentially monolithically driven, and this enables the use of high-efficiency junctions with high non-linear capacitance, while ensuring linear optical modulation.

[0067] It will also be appreciated that the RRM 311, 312 are direct current (DC) biased by driving the n-side 341, 342 through the common driver 314. For example, as Figure 4 depicted, the capacitors 421, 422 in series with the resistors 431, 432 are understood to create a high impedance in the DC circuit, and as such, the common driver 314 output operating point is not modified. Such an arrangement generally avoids the need for large alternating current (AC) coupling capacitors, regions of anger, and / or complex circuitry needed to solve for the correct bias operating point and additional parasitics required for the common driver 314 to drive.

[0068] In other words, the RRM 311, 312 are both understood to share a common regulator source, specifically the common voltage bias 350, to adjust the bias point of the RRM 311, which generally implements the same bias voltage across the pn-junctions 411, 412. Although not depicted, the voltage Vcom of the common voltage bias 350 can be generated from a buffer that replicates a tunable voltage generated from a VDAC (voltage digital-to-analog converter), and provides the driving capability to absorb the leakage current of the pn-junctions 411, 412. Reference is made to Figure 3 and Figure 4Both, when Vcom is less than the voltage output by driver 314 to n-side 341, 342, pn-junctions 411, 412 are reverse biased, otherwise pn-junctions 411, 412 are forward biased. In this way, the parasitic capacitance driven from driver 314 can be traded off against the electro-optical efficiency of pn-junctions 411, 412 of RRM 311, 312 (e.g., the extinction ratio (ER) of RRM 311, 312, and thus also the optical modulation amplitude (OMA) of RRM 311, 312).

[0069] It should also be appreciated that with reference to Figure 5 , while the more efficient junction increases the non-linearity of the capacitance curve, optical transmitter 300 can have a linearized optical modulation output, which mitigates the effects of such non-linearity. This can typically be due to the two pn-junctions 411, 412 of optical transmitter 300 being connected in series.

[0070] For example, Figure 5 Graph 500 depicts a curve 201 showing the junction of optical transmitter 100, while curve 501 depicts a similar curve for junctions 411, 412 of optical transmitter 300. It is clear from curves 201, 501 that optical transmitter 300 has a higher total capacitance 421, 422, which is also more non-linear.

[0071] However, at optical transmitter 300, the two junction capacitances 421, 422 are placed in series, thereby reducing the total static capacitance to half from the electrical perspective of driver 314. In particular, since the voltage is applied to junction capacitances 421, 422 in opposite directions, and since junction capacitances 421, 422 are connected in series, the dynamic capacitance is mainly determined by the junction capacitance 421, 422 with the smaller value, which is typically more in reverse bias compared to the capacitance 421, 422 with the large value. This arrangement results in better linearization of the optical modulation even when the junction efficiency is high. For example, as the reverse bias voltage decreases, the non-linearity of capacitances 421, 422 as a function of the applied voltage increases, thus the optical transmitter 300 provided herein typically results in an improvement in the linearization of the response of optical transmitter 300 compared to the prior art optical transmitter 100.

[0072] For example, Figure 6AA graph 600 is depicted showing a plot 601 of how the capacitances 421, 422 of the pn junctions 411, 412 vary with applied voltage, and includes a voltage plot 602 showing how the voltage applied to the pn junctions 411, 412 and / or RRM 311, 312 can vary between four different voltage levels (e.g., 0, 1, 2, 3) over time, e.g., when encoding data into light on the waveguide 301, 302. Example values for the different voltage levels are indicated by the lines extending from the voltage plot 602 to the voltage axis of the graph 600. Thus, it will be appreciated that as the voltage level increases from 0 to 1 to 2 to 3, the corresponding voltage decreases (or absolute value increases).

[0073] In the voltage plot 602, the solid line shows the voltage at the pn junction 411, while the dashed line shows the voltage at the pn junction 412. The resulting variation in capacitances 421, 422 over time is depicted in graph 604, with the solid line showing the capacitance 421 and the dashed line showing the capacitance 422.

[0074] However, it will be appreciated that the pn junctions 411, 412 and / or RRM 311, 312 are driven with a reverse bias, such that when the pn junction 411 is at voltage level “0”, the pn junction 412 is at voltage level “3”, and vice versa. Similarly, when the pn junction 411 is at voltage level “1”, the pn junction 412 is at voltage level “2”, and vice versa. Thus, the pn junctions 411, 412 can be driven into four different states: [(0, 3), (3, 0), (1, 2), (2, 1)].

[0075] Furthermore, since the capacitances 421, 422 are connected in series, the lowest capacitance 421, 422 is understood to dominate the total capacitance. For example, the line 499 at graph 604 shows a crossing point at which the capacitances 421, 422 are at about the same value, and it will be appreciated that, e.g., due to the data scheme described above, when one capacitance 421, 422 is high, the other is low. Thus, the total capacitance (not depicted) tends to remain in a relatively narrow range compared to the capacitances of graph 204.

[0076] Indeed, it is also indicated at graph 600 that at time tl, the total capacitance is dominated by the second capacitance 422 of the second pn junction 412; thus, even though the first capacitance 421 of the first pn junction 411 is much higher at time tl, the total capacitance remains relatively low.

[0077] Similarly, at time t2, the total capacitance is dominated by the first capacitance 421 of the first pn junction 411; thus, even though the second capacitance 422 of the second pn junction 412 is much higher at time t2, the total capacitance remains relatively low.

[0078] In other words, from an electrical perspective of the driver 314, the total dynamic capacitance is dominated by the smaller capacitance of the capacitances 421, 422, as the capacitances 421, 422 are provided in series.

[0079] Figure 6B The impact of this capacitance variation in the PAM4 diagram 606 of the optical transmitter 300 is shown, which can be compared to the PAM4 diagram 206 of the optical transmitter 100. For example, similar to the PAM4 diagram 206, the PAM4 diagram 606 includes four eyes 608, which ideally should be aligned, e.g., along the line 610, and “open” (e.g., a consistent gap between them). In fact, comparing the PAM4 diagrams 206, 606, it can be clearly understood that the eyes of the PAM4 diagram 606 are more aligned than the eyes of the PAM4 diagram 206, and moreover, the eyes of the PAM4 diagram 606 are more “open” than the eyes of the PAM4 diagram 206. Thus, the benefits and / or advantages of the optical transmitter 300 over the optical transmitter 100 are clearly demonstrated.

[0080] In fact, such benefits and / or advantages can be due to the use of a common regulator source (e.g., the common voltage bias 350) to bias both pn junctions 411, 412, which can ensure that both pn junctions 411, 412 maintain the same bias point, thereby eliminating and / or reducing any reverse bias offset between the RRMs 311, 312. Such uniformity can enable the RRMs 311, 312 to produce the same level of phase modulation in both waveguides 301, 302, thereby effectively preventing nonlinear modulation.

[0081] Moreover, any offset in the phase modulation caused by the RRMs 311, 312 of the waveguides 301, 302 can result in nonlinear amplitude modulation and unequally spaced levels in the PAM4 eye diagram. However, the use of a common regulator source (e.g., the common voltage bias 350) can eliminate the use of AC coupling in high-speed routing, which in turn can significantly reduce the complexity, size, and additional parasitic capacitance of the circuitry used for AC coupling.

[0082] Next, some specialized operations of an optical transmitter as provided herein are described, and these operations can be used with the optical transmitter 300 and / or the prior art optical transmitter.

[0083] As such, next attention is drawn to Figure 7 which depicts an optical transmitter 700 that is substantially similar to the optical transmitter 300, where like components have like reference numerals, but in the “700” series rather than in the “300” series.

[0084] For example, as depicted, the optical transmitter 700 includes a pair of waveguides 701, 702, an additional length 703 of the second waveguide 702, a light input 704 configured to receive light from a laser 707 (which can or can not be a component of the optical transmitter 700), a light output 706, ring resonator modulators 711, 712, a common driver 714, a positive input 716 and a negative input 718 of the common driver 714, the ring resonator modulators 711, 712 include respective optical rings 721, 722 positioned between respective pn junctions formed by respective p-sides 731, 732 (e.g., regions of p-doped material, such as p-doped material extending at least partially outside of the optical rings 721, 722) and respective n-sides 741, 742. The optical transmitter 700 also includes a common voltage bias 750, an electrical connection 751 between the respective p-sides 731, 732, an optional inductor 752 between the common voltage bias 750 and the respective p-sides 731, 732, at least one heater 761, 762 connected to a voltage supply 764 along the pair of waveguides 701, 702, a controller 779 (which can or can not be a component of the optical transmitter 700), and one or more sensors 780, 781, 782, 783, 784, 785.

[0085] Further, as depicted, components 799 of the optical transmitter 700 can be optional, including the common voltage bias 750 and the electrical connection 751 between the respective p-sides 731, 732 (e.g., and the optional inductor 752). It should be understood that when the components 799 are present, the optical transmitter 700 includes the optical transmitter 300, and when the components 799 are not present, the optical transmitter 700 is similar to prior art optical transmitters. Regardless, the method for controlling the optical transmitter 700 between single-side tuning or differential-side tuning can be implemented with or without the components 799.

[0086] Attention is now directed to Figure 8 , Figure 8 a flowchart representing a method 800 for controlling an optical transmitter (and more specifically, an MZI) based on optical power and / or application type is depicted. The operations of the method 800 correspond to machine-readable instructions executed by the controller 779. In the illustrated example, the instructions represented by the blocks of the method 800 can be stored at a memory in communication with the controller 779. The method 800 of FIG. 10 is one way in which the optical transmitter 700 can be configured. Further, the following discussion of the method 800 will lead to a further understanding of the optical transmitter 700 and its various components.

[0087] The method 800 need not be performed in the precise order shown, and similarly, individual blocks can be performed in parallel rather than sequentially. Thus, the elements of the method 800 are referred to herein as “blocks” rather than “steps.” The method 800 can also be implemented on variations of the optical transmitter 700.

[0088] At block 802, the controller 779 determines the optical power, for example, by communicating with the sensor 780 at the optical input 704 and / or one or more of the sensors 781, 782, etc.

[0089] At block 804, the controller 779 compares the optical power to a threshold optical power to determine whether the optical power is above or below the threshold optical power. The threshold optical power can depend on whether the optical power determined at block 802 is determined from the sensor 780 or from one or more of the sensors 781, 782, as described herein.

[0090] When the optical power is above the threshold optical power (e.g., a “yes” determination at block 804), at block 806, the controller 779 controls the MZI of the optical transmitter 700 to a single-side tuning mode. In the single-side tuning mode, both RRM 711, 712 are tuned to the blue side of the resonance frequency and / or the wavelength of the light input to the optical transmitter 700 is on the blue side, as described in greater detail herein.

[0091] However, when the optical power is below the threshold optical power (e.g., a “no” determination at block 804), at block 808, the controller 779 controls the MZI of the optical transmitter 700 to a differential-side tuning mode. In the differential-side tuning, one of the RRM 711, 712 is tuned to the blue side of the resonance frequency and / or the wavelength of the light input to the optical transmitter 700 is on the blue side, while the other of the RRM 711, 712 is tuned to the red side of the resonance frequency and / or the wavelength of the light input to the optical transmitter 700 is on the red side.

[0092] In particular, the RRM 711, 712 can be tuned to the blue side or the red side using their respective heaters.

[0093] The method 800 is described in greater detail next with reference to FIGS. 9-13.

[0094] In particular, some applications can require a high optical power (optical link budget) that is greater than the threshold optical power. In these examples, both RRM 711, 712 can be tuned to the blue side of the resonance frequency, which is referred to as single-side tuning.

[0095] However, other applications can require a nominal optical power at the input 704 that is less than the threshold optical power. In these applications, a differential side of the RRM 711, 712 can occur. For example, assuming that both RRM 711, 712 can have the same optical properties, and thus the same nominal resonance frequency, the first RRM 711 can be tuned on the blue side of the resonance frequency (e.g., using the respective heater of the first RRM 711), and the second RRM 712 can be tuned on the red side of the resonance frequency (e.g., using the respective heater of the second RRM 712), or vice versa. This differential side tuning can enhance the modulation performance of the RRM 711, 712, and compensate for the effects of optical nonlinearities introduced by the two optical rings 721, 722, e.g., due to optical self-heating effects, thereby stabilizing the performance of the optical transmitter 700.

[0096] Thus, in these examples, the method 800 further includes the controller 779 selecting the single-side tuning or the differential-side tuning based on the received application type at the controller 779. For example, an indication of the application type can be input (e.g., manually) to the controller 779, and the controller 779 can control the RRM 711, 712 (e.g., their respective heaters) accordingly. In particular, the controller 779 can store (e.g., at its memory) at least two indications of the application type: a single-side tuning indicator and a differential-side tuning indicator; and, depending on whether the received indicator matches the single-side tuning indicator or the differential-side tuning indicator, the controller 779 can control the RRM 711, 712 accordingly.

[0097] Indeed, in some examples, the controlling of the RRM 711, 712 to the single-side tuning or the differential-side tuning can be made based on the received indication of the application type, and thereafter the optical power can be monitored at block 802 to change the tuning of the RRM 711, 712 from the single-side tuning to the differential-side tuning, or from the differential-side tuning to the single-side tuning, depending on the optical power. In other words, the RRM 711, 712 can initially be put in one tuning mode, and later changed to the other tuning mode depending on the optical power.

[0098] Reference is next made to Figure 9A and Figure 9B describing the single-side tuning.

[0099] While the RRM 711, 712 is generally efficient, due to optical self-heating effects, the RRM 711, 712 can exhibit bistable and / or non-linear operation at high optical powers (e.g., above a threshold optical power), which can result in a performance penalty. In some MZIs, such self-heating can become significant at optical input powers of about 5 dbm, as measured at the sensor 780 of the input 704; in such examples, the threshold optical power is about 5 dbm.

[0100] However, in some examples, optical power can be measured at one or more of the sensors 781, 782, such that the optical power input to each waveguide 301, 302 is measured; in such examples, the threshold optical power is about 2 dbm for any of the waveguides 301, 302, which corresponds to an optical input power of about 5 dbm measured at the sensor 780 (e.g., assuming some loss). Thus, the threshold optical power can depend on which optical power is measured (e.g., via the sensor 780, or one or more of the sensors 781, 782).

[0101] As Figure 9A indicated at the graph 900, such non-linear behavior can result in an asymmetric resonance 901 (e.g., FSR) of the RRM 711, 712. In particular, the asymmetric resonance 901 is indicated by the asymmetric transmission of the RRM 711, 712 as a function of wavelength. The position of the wavelength 902 of the laser 707 used to generate the light input to the optical input 704 is also indicated at the graph 900. In practice, the position of the wavelength 902 of such a laser is described to show that, in this case, the RRM 711, 712 can be tuned to the blue side of the resonance (e.g., FSR) for operation as Figure 9A depicted at Figure 9A In practice, the position of the wavelength 902 of such a laser is described to show that, in this case, the RRM 711, 712 can be tuned to the blue side of the resonance (e.g., FSR) for operation as

[0102] In other words, the resonance 901 is asymmetric, where the red side of the resonance has a steep step-like behavior with respect to wavelength; if any of the RRM 711, 712 is tuned (e.g., via its respective heater) such that the laser wavelength 902 is at the step 903 and / or on the red side of the step 903, the modulation of the light of the laser wavelength 902 will be challenging, if not impossible. As such, the RRM 711, 712 is tuned (e.g., via its respective heater) such that the laser wavelength 902 is on the blue side of the step 903 and in the region of the blue side of the wavelength 902, and in particular at the frequency difference Af from the step 903.

[0103] This self-heating problem can become more severe when low frequency stress signals and / or patterns pass through the electro-optical channel, thereby heating the RRM 711, 712, and causing the resonance 901 to gradually shift to the right (red-shift) on the graph 900, such that the resonance 901 is shifted in the red direction to the position represented by the resonance 904, as best seen at graph 905 of Figure 9B which also shows the resonance 901. As depicted by the graph 905, the resonance 904 includes a shifted step 906, thereby also increasing the average optical power. Generally, this phenomenon can cause a variation in the average optical power to drift in an end-to-end link (e.g., between the optical transmitter 700 and a receiver in an optical communication system), resulting in a forward error correction (FEC) pre-penalty and a bit error rate (BER) post-FEC penalty.

[0104] Accordingly, to avoid operation of the RRM 711, 712 in the unstable “red” side of the laser wavelength 902, both of the RRM 711, 712 can be operated and / or tuned to the “blue” side.

[0105] Reference is next made to Figure 10A and Figure 10B describe differential side tuning, Figure 10A and Figure 10B graphs 1000, 1002, respectively, depicting other aspects showing the optical transmitter 700. In particular, the graphs 1000, 1002 show differential side tuning of the RRM 711, 712, where differential side tuning can be understood to mean that one of the RRM 711, 712 is tuned to the blue side of the resonance, while the other of the RRM 711, 712 is tuned to the red side of the resonance.

[0106] In particular, the graph 1000 depicts respective resonances 1011, 1012 of the RRM 711, 712 with respect to the laser wavelength 902 when the self-heating effect is at a minimum and / or when the input power is below a threshold optical power. The graph 1002 depicts curves 1021, 1022 showing the resonances 1011, 1012, respectively, as a function of wavelength. Figure 10Athe phase of the RRM 711, 712. The curves 1021, 1022 show that the phase response of the RRM 711, 712 varies linearly around the wavelength 902 of the laser 707, and thus can improve the overall phase modulation at the output 706 (e.g., when the RRM 711, 712 is modulated around the wavelength 902 in a single-side tuning mode or a differential side tuning mode).

[0107] From the resonances 1011, 1012, it can be appreciated that in this configuration, the first RRM 711 can operate on the blue side, and the second RRM 712 can operate on the red side, or vice versa. The difference between the minima of the resonances 1011, 1012 and the laser wavelength 902 is indicated by -Af and +Af, respectively. Again, for a wavelength 902 of about 1310 nm, Af can be about 10 GHz, although any suitable wavelength and / or Af are within the scope of the present specification.

[0108] Indeed, the resonances 1011, 1012 indicate that in this configuration, both RRMs 711, 712 are provided (e.g., manufactured and / or designed) with a low quality factor (broadening the resonances 1011, 1012), which can further reduce the optical self-heating effect.

[0109] Such a configuration can be used in applications where the optical power of the laser input to the optical transmitter 700 is not significantly high and / or less than a threshold optical power, such that differential side tuning is used at the optical transmitter 700. This configuration enhances the modulation efficiency and allows the RRMs 711, 712 to cancel out the thermal variation effects of each other, thus the average power of the waveguides 301, 302 remains constant in case of any temperature variation.

[0110] For clarity, in some examples, an increase in the optical power input to the RRMs 711, 712 (e.g., still below the threshold optical power at which self-heating occurs) should not result in a sharp, steep slope on the red side of the resonances. Further, it should be appreciated that the temperature increase in these examples shifts the resonances 1011, 1012 to the right on the graph 1000 (higher wavelength and / or red shift), and the temperature decrease shifts the resonances 1011, 1012 to the left on the graph 1000 (lower wavelength and / or blue shift).

[0111] For example, next attention is drawn to Figure 11 and Figure 12 which respectively depict the shifting of the resonances 1011, 1012 with temperature increase and decrease.

[0112] As in Figure 11As clearly seen in Figure 1100, as the temperature increases, resonators 1011 and 1012 shift to the right, or undergo a redshift, relative to the laser wavelength 902, to the positions represented by resonators 1111 and 1112. Specifically, while the shift of resonator 1011 to the position represented by resonator 1111 results in an increase in the transmission of the laser wavelength 902, the shift of resonator 1012 to the position represented by resonator 1112 results in a decrease in the transmission of the laser wavelength 902 by approximately the same amount as the increase in resonator 1011. Therefore, the average optical power output by the combination of RRMs 711 and 712 remains approximately constant.

[0113] Similarly, as in Figure 12 As clearly seen in Figure 1200, as the temperature decreases, resonators 1011 and 1012 shift to the left or undergo a blue shift relative to the laser wavelength 902, to the positions represented by resonators 1211 and 1212. Specifically, while the shift of resonator 1011 to the position represented by resonator 1211 results in a decrease in the transmission of the laser wavelength 902, the shift of resonator 1012 to the position represented by resonator 1212 results in an increase in the transmission of the laser wavelength 902 by approximately the same amount. Therefore, the average optical power output by the combination of RRMs 711 and 712 remains approximately constant again.

[0114] Next, pay attention Figure 13A and Figure 13B They depicted charts 1300 and 1302 respectively.

[0115] First, pay attention Figure 13A Figure 1300 is similar to Figure 905, with the same reference numerals for the same components, and illustrates the RRM transmission as a function of wavelength for the nonlinear behavior during self-heating of RRMs 711 and 712. For example, Figure 1300 shows resonances 901 and 904, and the effect of a temperature rise that leads to increased light transmission when resonance 901 is moved to the right to the position of resonance 904, which is caused by the aforementioned low-frequency stress signal and / or mode. Figure 13A It is also shown that the red side of the resonances 901 and 904 is sharply steep, with corresponding steps 903 and 906, and therefore the RRMs 711 and 712 with the resonances 901 and 904 are understood to be tuned to the blue side.

[0116] In other words, as already explained, when an application requires very high optical power at the RRM input, in order to enhance the optical link budget, for example, the slope of the red side of resonators 901 and 904 will become sharply steep, and both RRMs 711 and 712 will be tuned to the same side of the resonance, and especially the blue side.

[0117] Further, in this configuration, both RRM 711, 712 will move in the same direction in response to a temperature change (e.g., unlike the configuration shown in Figure 10A , Figure 11 and Figure 12 ), such that the optical power output by optical transmitter 700 can increase as represented by arrow 1304, which indicates an increase in average optical power. In some cases, the optical power output by optical transmitter 700 can increase and become “too high” such that the optical power output by optical transmitter 700 can exceed the operating optical power of an optical communication system of which optical transmitter 700 can be a component. For example, when the operating optical output power is “too high,” the optical output power can exceed the capabilities of an optical fiber through which an optical signal having that optical output power is output, and / or the optical power output by optical transmitter 700 can exceed the capabilities of a receiver that receives the optical signal. Thus, to compensate for the change in optical power caused by the temperature increase, the operating point of optical transmitter 700 can be moved from the quadrature point toward the zero point to reduce the optical power output by optical transmitter 700.

[0118] For example, note Figure 13B , which depicts graph 1302, which shows the total optical transmission of optical transmitter 700 as a function of wavelength, as well as laser wavelength 902.

[0119] Resonance 1311 is understood to be operating at quadrature point 1390, where in this example, the transmission of optical transmitter 700 is about 50% at laser wavelength 902. However, by adjusting the heaters of RRM 711, 712 through controller 779, the transmission of optical transmitter 700 can be shifted to the left and / or blue direction to a position represented by resonance 1312, which moves zero point 1392 of resonance 1311 toward laser wavelength 902, thereby reducing the optical output power. For completeness, the “full on” and / or maximum point 1392 of resonance 1311 is also depicted. Thus, the transmission of optical transmitter 700 is reduced from about 50% to less than 50%, and to a power range that can be more acceptable to an optical communication system of which optical transmitter 700 can be a component. In particular, graph 1302 depicts arrow 1399, which indicates the reduction in optical output power when resonance 1311 of optical transmitter 700 is changed from quadrature operation to operating closer to zero point 1392.

[0120] In other words, the increase in temperature shifts the resonance of RRM 711, 712 toward higher wavelengths. Thus, for Figure 13AIn the configuration of FIG. 7, the average optical power of the RRM 711, 712 also increases with temperature at a constant laser wavelength 902. By adjusting the operating point of the optical transmitter 700, for example, by shifting the operating point from the quadrature point toward the zero point via control of one or more of the heaters 761, 762, the increase in optical power is mitigated. Such control can occur in a feedback loop by receiving an output from the sensor 785 and controlling one or more of the heaters 761, 762 to reduce the optical output power to a target optical output power, for example, at or below a threshold optical output power, which can include the operating optical power of an optical communication system of which the optical transmitter 700 can be a component.

[0121] Thus, the method 800 can further include the controller 779: determining the optical output power; and when the optical output power is above a threshold optical output power, controlling one or more of the heaters 761, 762 to reduce the optical output power to less than or equal to the threshold optical output.

[0122] Thus, while the optical transmitter 700 can generally be configured to operate at the quadrature point for PAM4 signaling, which can desirably require a 90 degree phase difference between the optical signals in the different waveguides 701, 702, the optical transmitter 700 can be operated non-quadrature to reduce the optical output power to a power that is compatible with an optical communication system of which the optical transmitter 700 can be a component.

[0123] Further, as already described, a length 703 can be introduced into the second waveguide 702 to provide a mismatch in the lengths of the waveguides 701, 702 such that the phase difference between the two arms becomes 90 degrees.

[0124] However, due to manufacturing errors, etc., the operation of the optical transmitter 700 can not be precisely at the quadrature point by using only the length mismatch. Thus, one or more of the heaters 761, 762 can be provided at the waveguides 701, 702, and by using the driver / switch 766, the operation of the heaters 761, 762 can be controlled between dynamic switching. This configuration can enable selective activation of the appropriate heater 761, 762 (e.g., by the controller 779) based on the initial phase state.

[0125] For example, the heater 762 on the second waveguide 702 can be activated to achieve the quadrature point from near the zero point using minimal power (e.g., see Figure 13B ), while the heater 761 on the first waveguide 701 can be activated when the initial state is closer to the “all-pass” point (e.g., see Figure 13B ).

[0126] Dynamic operation of the heaters 761, 762 is advantageous not only for initial calibration or commissioning (e.g., initial parameter setting), but also for ongoing operation of the light emitter 700. For example, when an unwanted phase shift occurs during operation of the light emitter 700, the heaters 761, 762 can be used to facilitate effective recovery of the quadrature point, thereby ensuring minimal power consumption.

[0127] In other words, by monitoring the light output power using the sensor 785, the controller 779 can control one or more of the heaters 761, 762 to maintain a constant output power and / or a target output power, which desirably includes the output power at the quadrature point, but which can be lower when the target output power exceeds a threshold output power.

[0128] Furthermore, static power consumption of the light emitter 700 can be optimized by dynamically switching the heater voltage V Heat Optimizing static power consumption in the light emitter 700 can better enable the light emitter 300 to operate at the quadrature point.

[0129] Furthermore, static power consumption of the light emitter 700 can be optimized by determining a minimum power of the heaters 761, 762 to bring the light emitter 700 into quadrature.

[0130] For example, attention is now directed to Figure 14 which depicts a flowchart representing a method 1400 for controlling a light emitter into quadrature while minimizing power usage. Operations of the method 1400 correspond to machine-readable instructions executed by the controller 779. In the illustrated example, instructions represented by blocks of the method 1400 can be stored at a memory in communication with the controller 779. The method 1400 of FIG. 10 is one way in which the light emitter 700 can be configured. Furthermore, the following discussion of the method 1400 will lead to a further understanding of the light emitter 700 and its various components.

[0131] The method 1400 need not be performed in the precise order shown, and similarly, individual blocks can be performed in parallel rather than sequentially. Accordingly, the elements of the method 1400 are referred to herein as “blocks” rather than “steps.” The method 1400 can also be implemented on variations of the light emitter 700.

[0132] At block 1402, the controller 779 controls at least one of the respective heaters 761, 762 to a plurality of operating conditions, the one or more respective heaters 761, 762 being positioned along a pair of waveguides 701, 702 of a Mach-Zehnder interferometer (MZI) (e.g., the optical transmitter 700), the respective heaters 761, 762 being configured to heat respective portions of the pair of waveguides 701, 702. The plurality of operating conditions can include different heater voltages to which the heaters 761, 762 are controlled.

[0133] At block 1404, the controller 779 monitors changes in output power of the MZI as the at least one heater 761, 762 is controlled. Such monitoring can occur, for example, via the sensor 785.

[0134] At block 1406, the controller 779 determines an optical transfer curve of the MZI using the changes in output power. As Figure 15 depicted, the optical transfer curve can include the output power as a function of heater voltage determined at block 1404.

[0135] At block 1408, the controller 779 determines a respective operating condition of the respective heaters 761, 762 to cause the MZI to be quadrature when one of the respective heaters 761, 762 is on and the other of the respective heaters 761, 762 is off.

[0136] At block 1410, the controller 779 selects a respective operating condition for a first of the respective heaters 761, 762 that minimizes power of the respective heaters 761, 762 while a second of the respective heaters 761, 762 is off.

[0137] At block 1412, the controller 779 controls the first of the respective heaters 761, 762 according to the selected respective operating condition while the second of the respective heaters 761, 762 is off.

[0138] Next, examples of the method 1400 are described with respect to Figure 15 FIG. 15.

[0139] For example, note that Figure 15 , Figure 15A graph 1500 is depicted showing curves 1501, 1502 of light output power as a function of heater voltage for the heaters 761, 762, respectively. Further, a zero voltage point (e.g.,“0”) is indicated for each of the heaters 761, 762, such that when each of the heaters 761, 762 is off, the light output power is at a value 1504, which can depend on the manufacturing of the light emitter 700. In other words, while the light emitter 700 can be nominally designed to be in quadrature when both of the heaters 761, 762 are off, manufacturing tolerances can cause the light emitter 700 not to be in quadrature (e.g., about 50% of the maximum light output power 1506) when both of the heaters 761, 762 are off.

[0140] Further, for the curve 1501, the heater voltage of the first heater 761 is understood to increase from the zero voltage point to the left, and for the curve 1502, the heater voltage of the second heater 762 is understood to increase from the zero voltage point to the right. Further, it is understood that the curve 1501 is taken by controlling the first heater 761 from the zero voltage point to increasing heater voltages with the second heater 762 off, and the curve 1502 is taken by controlling the second heater 762 from the zero voltage point to increasing heater voltages with the first heater 761 off.

[0141] The curves 1501, 1502 are understood to collectively have a predefined shape (e.g., a sinusoidal curve), and it is further understood that the curves 1501, 1502 are continuous with each other along the predefined shape from the zero voltage point. As such, the controller 779 can control only one of the heaters 761, 762, and determine only one of the curves 1501, 1502 to derive the other of the curves 1501, 1502 (e.g., at blocks 1402, 1404, 1406 of the method 1400). In particular, the controller 779 can cause one of the heaters 761, 762 to sweep a voltage range to at least determine the maximum light output power 1506 and the minimum light output power 1508, and the shape of the curves 1501, 1502 from the zero voltage point to the maximum light output power 1506 and the minimum light output power 1508. The voltage range can include a predetermined range that has been heuristically determined to include the maximum light output power and the minimum light output power of previously manufactured light emitters, etc.

[0142] Assuming the quadrature point of the light emitter 700 is at about 50% of the maximum light output power 1506, from Figure 15It can be appreciated that the controller 779 can determine (e.g., at block 1408 of the method 1400) that the light emitter 700 can be orthogonally operated in at least two conditions: when the first heater 761 is at the first voltage 1511 while the second heater 762 is off; or when the second heater 761 is at the second voltage 1512 while the first heater 761 is off.

[0143] The controller 779 can select (e.g., at block 1410 of the method 1400) the lesser of the first voltage 1511 and the second voltage 1512. For example, as depicted, the first voltage 1511 is less than the second voltage 1512, and thus, in the depicted example, the controller 779 selects the first voltage 1511. The controller 779 can make the light emitter 700 orthogonal by controlling (e.g., at block 1412 of the method 1400) the first heater 761 to the first voltage 1511 while the second heater 762 is off. Thus, the minimum voltage 1511, 1512 is used to make the light emitter 700 orthogonal to minimize power usage of the light emitter 700.

[0144] However, in other examples, the second voltage 1512 can be less than the first voltage 1511 due to the zero voltage point varying between different light emitters 700. In these examples, the controller 779 can select (e.g., at block 1410 of the method 1400) the second voltage 1512 that is higher than the first voltage 1511, and make the light emitter 700 orthogonal by controlling (e.g., at block 1412 of the method 1400) the second heater 761 to the second voltage 1512 while the first heater 761 is off.

[0145] Regardless, the method 1400 can be implemented each time the light emitter 700 is turned on, and the operating conditions of the heaters 761, 762 that minimize power usage to make the light emitter 700 orthogonal can be stored at a memory of the controller 779 and used later when the light emitter 700 is later turned off and turned back on. Alternatively or additionally, the method 1400 can be implemented each time the light emitter 700 is turned on, and / or periodically while the light emitter 700 is on in the event that the operating conditions of the heaters 761, 762 to make the light emitter 700 orthogonal drift.

[0146] A first aspect of the present specification provides a Mach-Zehnder interferometer (MZI), comprising: a pair of waveguides; an optical input configured to split light received at the optical input between the pair of waveguides; respective ring resonator modulators on the pair of waveguides, the respective ring resonator modulators comprising respective pn junctions with respective capacitances connected in series; a common driver configured to: receive a differential electrical signal in which data is encoded; and use the differential electrical signal to control respective n-sides of the respective pn junctions to control the respective ring resonator modulators to respectively modulate light into respective optical signals; a common voltage bias configured to provide a common voltage to respective p-sides of the respective pn junctions; and an optical output configured to: receive the respective optical signals from the pair of waveguides; and combine the respective optical signals.

[0147] At the MZI of the first aspect, the respective capacitances of the respective pn junctions are connected in series such that a total capacitance of the respective capacitances is dominated by a smaller capacitance of the respective capacitances.

[0148] At the MZI of the first aspect, the respective capacitances of the respective pn junctions are connected in series and vary over time based on respective voltages applied to the respective pn junctions, and wherein a total capacitance of the respective capacitances is dominated by a smaller capacitance of the respective capacitances at any given point in time.

[0149] At the MZI of the first aspect, the respective pn junctions can be driven in antiphase.

[0150] At the MZI of the first aspect, the common voltage bias can provide a common bias point to the respective pn junctions.

[0151] The MZI of the first aspect can further comprise an electrical connection between the respective p-sides, and wherein the common voltage bias is electrically connected to the electrical connection.

[0152] The MZI of the first aspect can further comprise an inductor between the common voltage bias and the respective p-sides, the inductor configured to block high frequency voltages at the common voltage bias from the respective p-sides.

[0153] At the MZI of the first aspect, the pair of waveguides can have different lengths to control a phase difference between the respective optical signals.

[0154] The MZI of the first aspect can further comprise at least one heater along the pair of waveguides, the at least one heater configured to heat a respective portion of at least one waveguide of the pair of waveguides to control a phase difference between the respective optical signals.

[0155] At the MZI of the first aspect, the respective optical signals can be combined at a quadrature point.

[0156] The MZI of the first aspect can further comprise: at least one heater along the pair of waveguides, the at least one heater configured to heat a respective portion of at least one waveguide of the pair of waveguides; and a controller configured to control the at least one heater to control a phase difference between the respective optical signals to a quadrature point at the optical output.

[0157] The MZI of the first aspect can further comprise: a plurality of sensors configured to detect a power of the light at the optical input, at the optical output, and at one or more respective locations along the pair of waveguides; respective heaters along the pair of waveguides, the respective heaters configured to heat a respective portion of at least one waveguide of the pair of waveguides; and a controller communicatively coupled to the plurality of sensors and the respective heaters, the controller configured to control at least one of the respective heaters based on respective outputs from two or more of the plurality of sensors to control a phase difference between the respective optical signals to a quadrature point at the optical output.

[0158] A second aspect of the present specification provides a method comprising: determining, via a controller, an optical power at a Mach-Zehnder interferometer (MZI); controlling, via the controller, the MZI in a single-side tuning mode when the optical power is above a threshold optical power; and controlling, via the controller, the MZI in a differential-side tuning mode when the optical power is below the threshold optical power, wherein, in the single-side tuning mode, a first ring modulator and a second ring modulator of the MZI are both tuned to a blue side of a resonance frequency, and wherein, in the differential-side tuning mode, the first ring modulator is tuned to the blue side of the resonance frequency and the second ring modulator is tuned to a red side of the resonance frequency.

[0159] In the method of the second aspect, determining the optical power can comprise: determining an optical input power at an input of the MZI; or determining respective optical input powers on one or more waveguides of the MZI, and wherein the threshold optical power depends on a location at which the optical power is determined.

[0160] In the method of the second aspect, controlling the MZI in the single-side tuning mode or the differential-side tuning mode can comprise: controlling respective heaters of the first ring modulator and the second ring modulator.

[0161] The method of the second aspect can further include: determining an application type of the MZI; and selecting the single-side tuning mode or the differential-side tuning mode based on the application type prior to comparing the optical power to the threshold optical power.

[0162] A second aspect of the present specification provides a method comprising: controlling, via a controller, at least one of a respective heater to a plurality of temperatures, the one or more respective heaters along a pair of waveguides of a Mach-Zehnder interferometer (MZI), the respective heaters configured to heat respective portions of the pair of waveguides; monitoring, via the controller, a change in an output power of the MZI while controlling the at least one heater; determining, via the controller, an optical transfer curve of the MZI using the change in the output power; determining, via the controller, a respective operating condition of the respective heaters to make the MZI quadrature when one of the respective heaters is off; selecting, via the controller, the respective operating condition for a first heater of the respective heaters that minimizes power of the respective heaters while a second heater of the respective heaters is off; and controlling, via the controller, the first heater according to the selected respective operating condition while the second heater is off.

[0163] In the method of the third aspect, monitoring the change in the output power can include sampling light intensity using a sensor optically coupled to an output of the MZI.

[0164] In the method of the third aspect, controlling the at least one heater can include: sweeping, independently, a heater voltage applied to at least one of the respective heaters within a predetermined range while the other of the respective heaters is off.

[0165] In the method of the third aspect, determining the optical transfer curve can include: sweeping, independently, a heater voltage applied to one of the respective heaters within a predetermined range while the other of the respective heaters is off; and using a portion of the optical transfer curve determined for the one heater to determine a remaining portion of the optical transfer curve for the other heater.

[0166] Many of the advantages of the embodiments will be apparent from the description, and it is intended to cover all such features and advantages. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the embodiments to the exact construction and operation described and shown, and accordingly, all suitable modifications and equivalents should be included within the scope of the claims.

[0167] It will also be appreciated that examples of the term“configured to”, such as“a computing device configured to”,“a processor configured to”,“a controller configured to”, and the like, can be understood to include a feature of a computer-readable storage medium having stored thereon program instructions that, when executed by a computing device and / or processor and / or controller, and / or the like, can cause the computing device and / or processor and / or controller to perform a set of operations that can include features that the computing device and / or processor and / or controller is configured to implement. Thus, the term“configured to” is understood to be broad enough to encompass a device plus functionality interpretation as well as a device plus state interpretation, among others.

[0168] Further, a description of a processor and / or controller and / or device and / or engine, and the like configured to perform certain functions is understood to include, but not be limited to, more than one processor and / or more than one controller and / or more than one device and / or more than one engine, and the like performing the functions.

[0169] It will be appreciated that, for purposes of this specification, the language“at least one of X, Y, and Z” and“one or more of X, Y, and Z” can be interpreted to include only X, only Y, only Z, or any combination of two or more items X, Y, and Z (e.g., XYZ, XY, YZ, XZ, and the like). Similar logic can apply to any occurrence of the language“at least one” and“one or more” in any of the claims.

[0170] The terms“about”,“substantially”,“essentially”,“approximately”, and the like, are defined as“nearby”, e.g., as understood by one of ordinary skill in the art. In some examples, the term is understood to be“within 10%”, in other examples“within 5%”, in further examples“within 1%”, and in yet further examples“within 0.5%”.

[0171] Those skilled in the art will understand that there are yet further possible alternative examples and modifications, and that the foregoing examples are only one or more examples. Therefore, the scope is only limited by the appended claims.

Claims

1. A Mach-Zern interferometer (MZI), comprising: A pair of waveguides; An optical input, the optical input being configured to separate light received at the optical input between the pair of waveguides; A corresponding ring resonator modulator located on the pair of waveguides, the corresponding ring resonator modulator comprising a corresponding pn junction having a corresponding capacitor connected in series; A common driver configured to receive a differential electrical signal in which data is encoded; And the differential electrical signal is used to control the corresponding n-side of the corresponding pn junction to control the corresponding ring resonator modulator to modulate the light into the corresponding optical signal respectively; A common voltage bias, configured to provide a common voltage to the corresponding p-side of the respective pn junction; and Optical output, the optical output being configured to: receive the corresponding optical signal from the pair of waveguides; and combine the corresponding optical signal.

2. The MZI according to claim 1, wherein, The corresponding capacitors of the corresponding pn junctions are connected in series, such that the total capacitance of the corresponding capacitors is dominated by the smaller capacitance among the corresponding capacitors.

3. The MZI according to claim 1, wherein, The respective capacitances of the respective pn junctions are connected in series and change over time based on the respective voltage applied to the respective pn junctions. In this context, the total capacitance of the respective capacitors is dominated by the smaller capacitance among the respective capacitors at any given point in time.

4. The MZI according to claim 1, wherein, The corresponding pn junction is driven in reverse phase.

5. The MZI according to claim 1, wherein, The common voltage bias provides a common bias point to the corresponding pn junction.

6. The MZI according to claim 1, further comprising an electrical connection between the respective p-sides, and wherein, The common voltage bias is electrically connected to the electrical connection.

7. The MZI of claim 1 further includes an inductor between the common voltage bias and the corresponding p-side, the inductor being configured to block high-frequency voltages from the common voltage bias at the corresponding p-side.

8. The MZI according to claim 1, wherein, The pair of waveguides have different lengths to control the phase difference between the corresponding optical signals.

9. The MZI of claim 1, further comprising at least one heater along the pair of waveguides, the at least one heater being configured to heat a corresponding portion of at least one of the pair of waveguides to control the phase difference between the corresponding optical signals.

10. The MZI according to claim 1, wherein, The corresponding optical signals are combined at the intersection point.

11. The MZI according to claim 1, further comprising: At least one heater along the pair of waveguides, the at least one heater being configured to heat a corresponding portion of at least one of the pair of waveguides; as well as A controller configured to control the at least one heater to control the phase difference between the respective optical signals to an orthogonal point at the optical output.

12. The MZI according to claim 1, further comprising: Multiple sensors are configured to detect the power of the light at the light input, the light output, and at one or more corresponding locations along the pair of waveguides; A corresponding heater along the pair of waveguides, the corresponding heater being configured to heat a corresponding portion of at least one of the pair of waveguides; as well as A controller communicatively coupled to the plurality of sensors and the respective heaters, the controller being configured to control at least one of the respective heaters based on the respective outputs from two or more of the plurality of sensors to control the phase difference between the respective optical signals at the optical output to an orthogonal point.

13. A method comprising: The optical power at the Mach-Zern interferometer (MZI) is determined via a controller; When the optical power is higher than the threshold optical power, the MZI is controlled to a single-sided tuning mode via the controller; as well as When the optical power is lower than the threshold optical power, the MZI is controlled to differential side tuning mode via the controller. In the single-sided tuning mode, both the first and second ring modulators of the MZI are tuned to the blue side of the resonant frequency. In the differential tuning mode, the first ring modulator is tuned to the blue side of the resonant frequency, and the second ring modulator is tuned to the red side of the resonant frequency.

14. The method according to claim 13, wherein, Determining the optical power includes: Determine the optical input power at the input of the MZI; or Determine the corresponding optical input power on one or more waveguides of the MZI, and The threshold optical power depends on the location where the optical power is determined.

15. The method according to claim 13, wherein, Controlling the MZI to the single-sided tuning mode or the differential-sided tuning mode includes: Control the corresponding heaters of the first ring modulator and the second ring modulator.

16. The method of claim 13, further comprising: Determine the application type of the MZI; as well as Before comparing the optical power with the threshold optical power, the single-sided tuning mode or the differential-sided tuning mode is selected based on the application type.

17. A method comprising: At least one of the respective heaters is controlled to multiple temperatures via a controller, the one or more respective heaters being along a pair of waveguides of a Mach-Zernd interferometer (MZI), the respective heaters being configured to heat a respective portion of the pair of waveguides; When controlling the at least one heater, the output power of the MZI is monitored via the controller; The optical transmission curve of the MZI is determined by the controller using the change in the output power. The controller determines the corresponding operating conditions of the respective heaters so that the MZI becomes orthogonal when one of the respective heaters is turned off; Via the controller, while the second heater in the respective heater is turned off, the corresponding operating condition that minimizes the power of the first heater in the respective heater is selected; and While the second heater is turned off, the first heater is controlled via the controller according to the selected corresponding operating conditions.

18. The method according to claim 17, wherein, Monitoring changes in the output power includes sampling the light intensity using a sensor optically coupled to the output of the MZI.

19. The method of claim 17, wherein, Controlling the at least one heater includes: While one of the respective heaters is turned off, the heater voltage applied to at least one of the respective heaters is swept independently within a predetermined range.

20. The method of claim 17, wherein, Determining the optical transmission curve includes: When the other heater in the respective heaters is turned off, the heater voltage applied to one of the respective heaters is swept independently within a predetermined range; and The remaining portion of the light transmission curve for the other heater is determined using a portion of the light transmission curve determined for the one heater.

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