PULSE-DENSITY MODULATION FOR TUNING A THERMALLY CONTROLLED, RESONANT OPTICAL COMPONENT

One some embodiments, a method for tuning optical components includes receiving an optical signal in a waveguide in a photonic-integrated circuit (PIC) and detecting optical outputs of the optical components. The method further includes determining pulse signals for the optical components designed to cause the optical components to each have a peak-resonance wavelength that matches a corresponding wavelength of the optical signal. The method further includes tuning the optical components by sending the pulse signals to the optical components.

BACKGROUND

Applications like machine learning (ML), deep learning (DL), and natural language processing (NLP) require specialized computing systems capable of handling the massive amounts of data and computations needed to carry out the task. Some implementations include computing environments with massive parallel processing power and large amounts of memory available for the data. Hybrid photonic/electronic computing environments have also been designed for these applications, which can take advantage of a photonic layer to process and/or move data in the photonic domain. Utilizing the photonic layer has the benefits of high-speed and low-power and contributes to the feasibility of computing systems that can handle ML, DL, and NLP tasks.

A drawback of using a computing environment that has a photonic layer is that some optical components in the photonic layer can be narrow-band devices whose peak resonance wavelength varies with thermal conditions. In a chip-to-chip configuration the electrical layer and the photonic layer are positioned with spacings that can be smaller than 50 microns. The electrical layer generates a variable amount of heat which is distributed to the photonic layer inconsistently. When the thermal conditions in the photonic layer change, the behavior of narrow-band optical components also changes. A hybrid photonic/electronic computing environment must account for the changes in thermal conditions in the photonic layer, so that the optical components are tuned sufficiently to process data in the photonic domain.

SUMMARY

In some embodiments, a method of tuning temperature controlled resonant optical components (TCROCs) of a photonic integrated circuit (PIC), includes, at several consecutive iterations of a time cycle, detecting a plurality of optical outputs generated by the plurality of TCROCs, wherein the plurality of TCROCs generate the plurality of optical outputs based on receiving an optical signal from a light source. The method further includes, at each time cycle, determining a pulse signal for each of the plurality of TCROC configured to shift a peak resonance wavelength of an associated TCROC to substantially match the wavelength of the light source. The method further includes, at each time cycle, applying, with a thermal tuner driver, the associated pulse signal to each of the plurality of TCROCs, wherein each of the pulse signals is applied during a non-overlapping segment of the time cycle.

In some embodiments, a method of tuning temperature controlled resonant optical components (TCROCs) of a photonic integrated circuit (PIC) includes detecting a first optical output of a first TCROC and a second optical output of a second TCROC. The method further includes determining a first pulse signal for the first TCROC and a second pulse signal for the second TCROC each designed to shift a peak resonance wavelength of the associated TCROC to substantially match a target wavelength of a light source. The method further includes, with a thermal tuner driver, applying the first pulse signal to the first TCROC to heat the first TCROC to a first target temperature associated with the first TCROC operating at the target wavelength. The method further includes, with the thermal tuner driver, while allowing the first TCROC to cool below the first target temperature, applying the second pulse signal to the second TCROC to heat the second TCROC to a second target temperature associated with the second TCROC operating at the target wavelength. The method further includes, with the thermal tuner driver, while allowing the second TCROC to cool below the second target temperature, reapplying the first pulse signal to the first TCROC to heat the first TCROC back to the first target temperature, wherein the first pulse signal is reapplied to the first TCROC within a minimum repetition period that is derived from a thermal time constant of the first TCROC.

In some embodiments, a device for tuning optical components of a photonic integrated circuit (PIC) includes a first thermally controlled resonant optical component (TCROC) configured to receive light from a light source and generate a first optical output, the first TCROC having a first peak resonance wavelength that varies with a first temperature of the first TCROC. The device further includes a second TCROC configured to receive light from the light source and generate a second optical output, the second optical output having a second peak resonance wavelength that varies with a second temperature of the second TCROC. The device further includes at least one detector operatively coupled to the first and second TCROC for detecting the first and second optical outputs. The device further includes a control module configured to determine, based on the first and second optical outputs detected by the at least one detector, a first pulse signal and a second pulse signal. The first pulse signal is designed to change the first temperature and cause the first peak resonance wavelength to substantially match a wavelength of the light source. The second pulse signal is designed to change the second temperature and cause the second peak resonance wavelength to substantially match the wavelength of the light source. The device further includes a thermal tuner driver configured to apply the first pulse signal to the first TCROC and apply the second pulse signal to the second TCROC.

This summary is provided to introduce a selection of concepts that are further described in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Additional features and aspects of embodiments of the disclosure will be set fourth herein, and in part will be obvious from the description, or may be learned by the practice of such embodiments.

DETAILED DESCRIPTION

The present application discloses a method, apparatus, and system for tuning of optical components using pulse-density modulation (PDM) and pulse-width modulation (PWM). Various embodiments are particularly suited for tuning optical components that modulate or demodulate a frequency. Such optical components are referred to generally as thermally controlled resonant optical components (TCROCs). Specific examples include, but are not limited to, a ring, a ring resonator, a single ring resonator, a multiplexer (MUX), a demultiplexer (DEMUX), a ring switch, a Mach-Zehnder Interferometer (MZI) based switch, or any suitable thermal-optical component that is a narrow-band device whose peak resonance wavelength varies depending on its thermal conditions.

FIG.1illustrates the behavior of a prior art system that includes a TCROC. Waveform190describes a variability in a peak wavelength of a TCROC.FIG.1illustrates the difference between a laser wavelength198and a peak resonance wavelength196in a system that includes at least one TCROC. Laser wavelength198is shown here at 1550 nanometers (nm) where the peak resonance wavelength196in the waveform190is in the range of 1547 nm. Thus, a delta194of 3 nm may exist in the system shown inFIG.1. As the thermal conditions change with respect to the TCROC described by waveform190, the peak wavelength may change such that the delta194may increase or decrease as the circuit operates. This may create an TCROC that is never fine-tuned to the current conditions and may always have some delta component that may diminish its performance. As a result, various embodiments described herein may thermally tune one or more TCROCs in a system to reduce or even eliminate the delta194shown inFIG.1for each TCROC. This may be accomplished by a system that adds at least a monitor component in a photonic-integrated circuit (PIC) for every TCROC in a computing environment. The output of the monitor component may provide input to a closed-loop control circuit to tune the resonance of the TCROC to the input laser wavelengths198(e.g., move the peak resonance wavelength196in waveform190to 1550 nm by creating thermal conditions/temperature needed to accomplish this task). Thereafter, the system may track the wavelengths over time through changes in temperature and may continue to adjust the TCROC resonance for each TCROC in a loop. Each TCROC may operate at or near its optimal operating thermal condition at all times according to various embodiments, whenever the computing environment is operating.

FIG.2shows elements and features of a ML processor102, according to at least one embodiment of the present disclosure. The ML processor102may provide a suitable environment that can benefit from various embodiments that use PDM to tune optical components. Various portions, elements, and features of ML processor102are not explicitly shown inFIG.2. As shown inFIG.2, the ML processor102may include sixteen nodes104athrough104p, arranged in a 4×4, two-dimensional grid. It will be understood by someone having ordinary skill in the art that the 4×4 grid ofFIG.2is for purposes of example only. The number, arrangement, and dimensionality of the nodes104athrough104pmay vary in different implementations of the ML processor102. In some embodiments, a photonic integrated circuit (PIC)192is stacked on top of a mixed signal, application-specific integrated-circuit (ASIC)188. In other embodiments, other arrangements and configurations are possible, such as stacking the PIC192on top of the ASIC188or accessing the PIC192and ASIC188separately using an interposer.

In some embodiments, the ML processor102includes a laser light source that sends an optical signal (not shown). The laser light source may be implemented either in the ML processor102or externally. For example, an interposer containing several lasers may be co-packaged and edge coupled with the PIC192. In another example, the lasers may be integrated directly into the PIC192using heterogenous or homogenous integration. A homogenous integration may allow lasers to be directly implemented in the silicon photonic substrate and may facilitate lasers of different materials such as Indium Phosphide, and architectures such as quantum-dot lasers. A heterogenous assembly of lasers on the PIC192may allow for group III-V semiconductors or other materials to be precision attached onto the PIC192and coupled into a waveguide implemented on the PIC192. As mentioned, the laser light source may be implemented externally, such as by a connection to the ML processor102. For example, the laser light source may be connected by one or more of a grating coupler, a fiber, and an edge coupler. Further, although not explicitly shown inFIG.2, the ML processor102may include optical links and connections for one or more fiber connections. Fiber connections may be made by several means, such as fiber array units located over grating couplers (FAUs132aand132bshown inFIG.3), edge couplers, or any other suitable connection. As discussed herein, the ML processor102may function as a network-on-chip (NoC), and more particularly, a hybrid electro-photonic network-on-chip (EP-NoC).

FIG.3depicts a cross-sectional view of the ML processor102, according to at least one embodiment of the present disclosure. The ML processor102may be a system-in-package that includes a chip-to-chip connection between the ASIC188and the PIC192. The ML processor may include grating couplers and fiber array units (FAUs) for connections at the edge of the chip. In some embodiments, the ML processor102shown inFIG.3may be the ML processor102shown inFIG.2and/or may include any of the features of the ML processor102shown and described inFIG.2.

The PIC192may provide a photonic network for interconnecting, among other things, a portion of the electronic elements on the ASIC188(while another portion other electronic elements on the ASIC188may be interconnected electrically). According to the present application, both electrical and photonic signal routings may be employed, and the signal routing tasks may be apportioned between electrical (or electronic) paths and photonic paths. Several ML processors (e.g., several instances of the ML processor102) may be interconnected, (i.e., interconnected chip-to-chip or SIP-to-SIP), to result in a single system referred to as an ML accelerator or a multi-processor computing environment. The interconnection of the several ML processors to form the ML accelerator may be interconnected by a photonic fabric. For example, the photonic networks of the several ML processors, along with optical connections, laser light sources, passive optical components, and external optical fibers on a printed circuit board (PCB)135, which may be utilized in various combinations and configurations along with other photonics elements, may form the photonic fabric. More specific details are described in the '694 Application referenced above, which has been incorporated by reference herein in its entirety.

In the exemplary implementation shown inFIGS.2and3, each of the nodes104athrough104pin the ML processor102may include processor blocks108. The processor blocks108can be one or more processors, such as deep neural network (DNN) processors, tensor engines, hardware math units such as dot-product units or convolution engines, field-programmable gate arrays (FPGAs), central processing units (CPUs), graphics processing units (GPUs), or any other processing component consistent with that described herein, and combinations thereof. In one embodiment, the processor blocks108are implemented in electronic form and reside within the ASIC188. The processor blocks108may be used, for example, as AI accelerators to carry out processing of neural networks. As further shown inFIGS.2and3, in one implementation, each of the nodes104athrough104pincludes message routers110, and memories112(which in practice can be multiple memory units including an L1SRAM and an L2SRAM, for example). The L1SRAM can serve as scratchpad memory for each of the nodes104athrough104p, while the L2SRAM can function as the primary memory for each of the nodes104athrough104p, for example to store the weights of a machine learning model in close physical proximity to the processor blocks108. The L2SRAM may store intermediate results during the execution of a machine learning model. The weights may be used in each layer of a neural network within each ML processor102. This may include, for example, making inference calculations. Each layer of the neural network may be implemented by several of the nodes104athrough104pin the ML processor102, where each of the nodes104athrough104pcomprise one or more neural nodes or neurons.

As further shown inFIG.2, the ML processor102may include one or more of a bus interface122, a CPU124, a GPU126, a memory controller128, and a TCROC control module200. The bus interface122may be any suitable bus standard, such as a peripheral component interconnect express (PCIE) interface. The CPU124and GPU126may be an advanced RISC machine (ARM) core, image processor, or other processing element(s). The external memory controller128may support DRAM, NVRAM, SRAM or other type of memory. The bus interface122may generally enable electrical interconnections between ML processor102and an external component. In particular, weights stored in the memories112may be received over the bus interface122from an external component, such as a dynamic random-access memory (DRAM). The CPU124and/or GPU126may interface with a memory device (not shown) which may be external to ML processor102and may process image data or perform other computing tasks. The memory controller128may communicate with a high bandwidth memory, such as the HBM189shown inFIG.3, which may be external to ML processor102or may be integrated into the ML processor102. Other forms of memory such as non-volatile memory may be attached in a similar manner using a corresponding memory controller in block128.

TCROC control module200may be one or more control circuits having an electrical connection to the PIC192, for example, to take one or more actions and/or send one or more signals to the PIC192. This may be, for example, in response to a thermal characteristic and/or a thermal change in a TCROC in the PIC192that may alter its peak resonant wavelength. A closed-loop control circuit may be implemented by the TCROC control module200which may carry out functionality for a set-up mode205and a peak finding mode210. As discussed herein, a peak finding mode210may be used to continually sample the optical signal on the bus waveguides in the PIC192, such as to determine how to alter the operating characteristics of one or more of the TCROCs300aand300bin the PIC192and/or to counteract the thermal change that may occur in the PIC192. This may typically happen when there is a change in the thermal characteristic in the PIC192. The change in the thermal characteristic may require some action to continue to operate the TCROCs300aand/or300bin an efficient and/or enhanced manner in order to minimize or eliminate the delta194shown inFIG.1. In this way, each TCROC300aor300bmay have a peak resonance frequency that may be tuned precisely with the input laser wavelength.

Referring again toFIG.3, ASIC188is shown as being situated over or stacked on PIC192. Exemplary nodes104a,104b, and104din ASIC188are shown with routers110a,110b, and110d. In some embodiments, a portion of routers110a,110b, and110dreside in the ASIC188while another portion resides in the PIC192. As shown, the routers110aand110dmay have TCROCs300aand300bin the photonic portion of the routers, which may be components of the chip-to chip hardware set-up used to enable chip-to-chip communication. For example, multiple wavelengths may be combined together into a single channel when transmitted from the edge of one chip to another using a TCROC in the form of a MUX. Similarly, when the signal is received by another chip a TCROC in the form of a DEMUX may receive the multiple signals and break them back into their constituent components.

In some embodiments, modulator drivers are situated respectively in the portions of the routers110a,110b, and110dthat reside in the ASIC188and are used to transmit data from the ASIC188to the PIC192. Transimpedance amplifiers (TIAs) may be situated respectively in the portions of the routers110a,110b, and110dthat reside in the ASIC188and may be used to receive data from the PIC192to the ASIC188. In some embodiments, optical modulators are situated directly below respective modulator drivers in the portion of the routers110a,110b, and110dthat reside in the PIC192. Photodetectors (PDs) may be situated directly below respective TIAs in the portion of the routers110a,110b, and110dthat reside in the PIC192. Optical links, such as waveguide343may provide optical paths in the PIC192that may be part of the photonic network for intra-chip communication between the ASIC nodes104a,104b, and104dwithin ML processor102. As shown inFIG.3, the ML processor102may include optical coupling(s) used to make connections between nodes. For example, optical couplings may be implemented using FAUs132aand132band the optical fiber133situated over the PIC192and providing optical input to grating coupler340in PIC192. In another example, optical couplings may be implemented using edge coupling or any other form of coupling. The optical fiber133may be connected to an off-chip laser light source and/or to another processor's FAU that may provide optical input to PIC192. In some implementations, the laser light source may be on-chip as described herein, (i.e., the laser light may not be provided through FAU132a). Optical links, such as the waveguide343, (and/or additional waveguides not shown), may supply the light received by grating coupler340to the routers110a,110b, and110dsituated on PIC192.

The ASIC188may be electrically coupled to the PIC192. The ML processor102may include A TCROC control module200that may be communicative coupled with detectors305. The detectors305may have photonic links to the TCROCs300aand300b, for example by an optical coupling to the waveguide343. The detectors305may sense the optical energy on the waveguide343provided by the light engine412B and may provide input to the TCROC control module200. In turn, the TCROC control module200(for example, when using peak finding mode210) may apply a voltage to the TCROCs300aand/or300bto alter their peak resonant wavelength. The TCROC control module200altering the peak resonant wavelength may cause the waveform of the TCROCs300aand/or300bto have a peak voltage that matches the wavelength of the light engine412B. This TCROC peak wavelength shift may be proportional to the average electrical power delivered to an associated tuner (e.g., thermal tuner497as described herein in association withFIG.4). The average electrical power delivered to the tuner may be determined by the duty cycle of the pulse-train for that TCROC. The average power delivered to a TCROC300aand/or300bmay be determined by the following formula:

Pave=V2R·tT,, whereV=Height of PulseR=Resistance of Element of Tuner(s) That Heats WaveguideT=Width of Pulset=Thermal Time Constant of Tuner(s)
The resistance R of the tuner may be a resistance of a resistive element that heats the waveguide343. As described herein, the height V of the pulse may be determined by and/or associated with a voltage sent to the TCROC, and the width T of the pulse may be determined and/or associated with a time over which the pulse occurs.

In some embodiments, one or more TCROCs (e.g.,300aor300b) may be at least partially thermally insulated from one or more (e.g., surrounding) components and/or from the PIC generally. For example, the PIC may be manufactured with a thermally insulating layer, component, or feature which may reduce the thermal conductivity from the TCROC to the PIC. In some embodiments, the PIC may include a cavity or air gap such that at least a portion of the TCROC does not make physical contact with the PIC in order to reduce the flow of heat from the TCROC. As discussed herein in detail, the reduction in thermal conductivity of the TCROC may contribute to a longer thermal time constant of the TCROC, which may facilitate features and functionalities discussed herein.

As discussed herein, multiple ML processors may be connected to form a ML accelerator. In some embodiments, wavelength division multiplexing (WDM) may be implemented to for optical connections between the multiple ML processors (e.g., inter-chip or chip-to-chip optical communications) in order to reduce the number of fiber connection between the different chips.FIG.4illustrates an example of ML processors102A and102B connected in a multi-chip configuration, according to at least one embodiment of the present disclosure.FIG.4may include any of the features of the ML processor102discussed above in connection withFIGS.2and/or3.FIG.4may show an example of WDM scheme. For example, a first chip such as the ML processor102A may be connected to a second chip such as ML processor102B. The chips may be connected through one or more of a grating coupler440in ML processor102A, a fiber441, a connector460, a fiber443, and a grating coupler450in ML processor102B. A light engine412B, which can be an on-chip or off-chip laser light source, may provide light with between 2 and 16 wavelengths to a splitter tree416B. In some embodiments, the light engine412B provides four wavelengths of light, λb1, λb2, λb3, and λb4 to splitter tree416B. A demultiplexer (DEMUX)420may provide each wavelength λb1, λb2, λb3, and λb4 to a WDM multiplexer (MUX)430on optical links462a,464a,466a, and468a. A MUX430may comprise at least TCROCs472,474,476, and478. TCROCs472,474,476, and478may selectively provide the modulated light with respective wavelengths λb1, λb2, λb3, and λb4 to ML Processor102B (e.g., via the transmit unit). Modulated light with respective wavelengths λb1, λb2, λb3, and λb4 ma also be routed via optical links to monitor photodiodes (MPDs)432,434,436, and438. It should be noted that in the transmit unit, the TCROCs may have respective monitor photodiodes (MPD)s432,434,436, and438. In contrast, in the receive unit, TCROCs482,484,486, and488may be capable of performing both the functionality of the receive photodiodes (RPDs) and also the functionality of receiving data with TCROCs482,484,486, and488, so additional circuitry may not be needed.

An output of the WDM multiplexer430may be provided on a single waveguide to grating coupler440. For example, the output may contain four data streams each using a separate wavelength λb1, λb2, λb3, and λb4, and the output may be provided to fiber441, connector460, fiber443, and grating coupler450in ML processor102B. In some embodiments, edge coupled fibers are used in lieu of or in addition to FAUs and grating couplers. In the ML processor102B, grating coupler450may receive the four data streams from a single fiber443. The DEMUX470may then demultiplex the optical signal provided from a single waveguide connected to grating coupler450and may provide the four data streams to, respectively, TCROCs482,484,486, and488. Although the implementation discussed above is directed to a channel showing four optical links and a WDM DEMUX470receiving four different wavelengths, it should be understood that any number of optical links may be used, and the WDM DEMUX470may accordingly receive and output any number of different wavelengths.

A TCROC control module200may typically be included as a component in the ASIC188. The TCROC control module200may include a pulse density generator490that may construct a pulse signal for each of the TCROCs472,474,476,478,482,484,486, and488, for example, in order to tune them back to their respective light source wavelengths λb1, λb2, λb3, and λb4. The pulse density generator490may be constructed in hardware and may be configured to provide the needed pulse width and height for each TCROC in the system once the optical output of the TCROC is sampled and the needed pulse signal is determined. To this end, each TCROC may be capable of sending an optical output via an electrical connection to the ASIC where the optical output may be received and converted to digital form by an analog to digital converter495in a thermal tuner driver496. In some embodiments, sub-nanosecond pulse control is used by the pulse density generator490. In some embodiments counters on two closely spaced lower frequency clocks491and492are implemented in the timing module493. The Vernier effect may be leveraged between clocks491and492to get extremely fine resolution. For example, the output values of the two clocks491and492may be compared to pre-set thresholds, and the outputs of those two comparators may feed set and reset inputs of a flip-flop. A time resolution that may be achieved may be expressed as (1/frequency of clock491)−(1/frequency of clock492). For example, a frequency of the first and the second clocks491and492may be around 30 MHz with a frequency difference of 1 MHz, a time resolution of 1 nanosecond may be achieved. In some embodiments, the resolution of the wavelength control may be 0.05 nanometers in order to locate the resonance accurately. In the PDM context the minimum wavelength step may be determined by the minimum duty cycle increment that may be achieved in the system, which in turn may depend on the smallest achievable step in pulse-width (t). As described above, the incremental power delivered to the TCROC per step may be determined based on the following formula:

Thus, for a given voltage level of each pulse signal, a value for t may be determined in order to achieve a desired wavelength resolution (e.g., such as 0.05 nm) and a corresponding resolution in tuning power.

FIG.5illustrates an example pulse density signal generated by a pulse density generator, according to at least one embodiment of the present disclosure. A pulse signal may include both a height (V)505and a width (T)510. The height505may represent the voltage sent to a TCROC (e.g., a larger voltage value will increase the height505). The width510may be based on a time T over which the pulse signal occurs. An area515under the waveform may represent the amount of energy that is fed into the TCROC that the pulse density generator490is coupled to. The energy represented by area515may be the energy needed to alter the thermal characteristics of the TCROC such that the TCROC peak resonance wavelength may be tuned back to the wavelength of the laser in one or more (e.g., subsequent) time-cycles, thereby eliminating the inefficiencies previously described inFIG.1.

In some embodiments, a variety of combinations of height (V)505and a width (T)510may generate an equivalent area515. In some embodiments the pulse density generator is optimized for power, such as by constructing the pulse signal and/or the area515such that width (T)510is maximized and height (V)505is minimized. For example, as mentioned above, the height (V) may be determined by or representative of the voltage of the pulse (e.g., input voltage), and the width (T) may be based on the time (e.g., duty cycle) over which the voltage is applied. Therefore, a minimal amount of voltage may be applied to achieve a required area515, so long as there is sufficient time to apply the pulse signal to the TCROC within the bounds of its applicable control scheme. Delta components525and526may create additional area530in a second waveform. This may represent a subsequent pulse signal constructed by the pulse density generator490. For example, the height (V) and/or width (T) may be incremented, decremented, or otherwise modified (e.g., by an input voltage delta and/or duty cycle delta) in order to modify the area of a second waveform (e.g., by a modified input voltage and/or modified duty cycle) in order to provide the requisite energy delivery for tuning the TCROC(s). This may occur within a range of fractions of nanoseconds, where a signal from the pulse density generator490may be sent by a thermal tuner driver496that may individually select and control any of the TCROCs. In some embodiments, the thermal tuner driver496accesses the TCROCs in a 2-dimensional grid by activating a thermal tuner497for any given row or column of the grid, which can heat a waveguide coupled to the TCROC.

The thermal tuner driver496may drive the pulse density generator490using a controller494. The controller may include a data structure such as a table, for example, that may pair the potential optical output of a detector with a duty signal associated with the pulse density generator490. The controller494may be implemented in software and may operate on the megahertz or kilohertz range. Using this scheme, the pulse density generator490may have two degrees of freedom with which to alter the thermal characteristic of the TCROC. Altering the thermal characteristics within two degrees of freedom in this way may provide the advantage of visiting each TCROC less often while still achieving the required thermal alterations/tuning over time. In this manner, the timing module492may ensure, for example, that one pulse signal may be sent to each TCROC in a given time cycle (or multiple cycles) before cycling back and sending new pulses to the TCROC. The timing module492may coordinate the energy needed to send pulse density signals such that a larger pulse of heat than needed in a current cycle may be sent to a TCROC, but the signal may be sent less frequently.

As an illustrative example, a TCROC may require a certain amount of energy for a given time-cycle in order to heat the TCROC in accordance with the techniques described herein. However, in some embodiments, a pulse density signal may be designed to provide double the energy needed for the TCROC for the current time-cycle, for example, by the thermal tuner driver driving the pulse density generator490to construct a waveform with twice the necessary area. The timing module493may accordingly send a shorter burst to each TCROC that delivers twice the energy needed for the given cycle, but the signal may be sent only once every two cycles. This may be facilitated by leveraging the fact that the thermal response by the TCROC may be relatively slow and highly inertial. Thus, a given TCROC may heat at a much slower rate than the pulse density generator490can cycle, and the tuning bias may not need to be constantly applied. A fast train of voltage may accordingly be employed to deliver the required electrical power to the thermal tuners497based on an average. While this example has been described with respect to doubling the area of the pulse/energy delivered and accordingly sending the signal every two cycles, in some embodiments, other ratios may be implemented. For example, a pulse signal may be delivered of 3 times the size every 3 cycles, 4 times the size every 4 cycles and so on, consistent with that described herein.

In some embodiments, each of the TCROCS and/or thermal tuners may have an associated thermal time constant, t. The thermal time constant t may refer to how quickly the temperature of the component adjusts in response to a change in environmental conditions, change in external stimuli, etc. For example, as described herein, the thermal tuner driver496may deliver pulse signals to one or more TCROCs in order to adjust (e.g., raise) a temperature of the TCROC to a target temperature associated with an optimal or desirable peak resonant wavelength of the TCROC. After the pulse signal is no longer being delivered, the temperature of the TCROC may begin to fall, and accordingly the peak resonance wavelength may begin to change sub optimally, degrading performance of the TCROC. The amount of time it takes the temperature to fall to a lower, equilibrium, or threshold temperature may be defined based on the thermal time constant. In some embodiments, the thermal time constant may be the time it takes for the temperature to fall to a threshold temperature, such as an equilibrium temperature at which the TCROC may operate without being heated by the thermal tuner driver. In some embodiments, the thermal time constant may be a time it takes for the temperature to fall or change by a certain percentage from the target temperature, such the time it takes for the temperature to change 63.2% of the difference between the target temperature and the equilibrium temperature. The thermal time constant may be based on any other relevant threshold for the temperature of the TCROCs.

A variety of factors may influence the thermal time constant. For example, the thermal time constant t may be affected by the dynamics, properties, characteristics, implementations, etc., of the TCROCs and/or of the system generally. In some embodiments, the thermal time constant t is based on a thermal conductivity or resistivity of the TCROCs and/or of the system, and how heat is transferred between (e.g., from) the TCROCs to or through any surrounding components. For example, a higher thermal conductivity of the TCROCs may correspond with a shorter thermal time constant, as heat may dissipate quicker from the TCROCs to surrounding components. In a similar manner, a lower thermal conductivity of the TCROCs may correspond with a longer thermal time constant, as the TCROCs may generally hold heat for a longer period of time. In some embodiments, the TCROCs may be implemented in the PIC with a thermally insulating layer in order to reduce the thermal conductivity from the TCROCs to the PIC. For example, the PIC may include a silicon substrate, and the TCROCs may be implemented on the silicon substrate with an air pocket or gap at least partially between each TCROC and the silicon substrate to reduce heat transfer from the TCROCs to the silicon substrate. This may facilitate the implementation of the thermal tuner driver496and/or pulse density generator490with multiple TCROCs in accordance with the techniques described herein.

For example, as described herein, the TCROC control module200may determine and generate a pulse signal for applying to a TCROC in order to adjust the temperature and tune the peak resonance wavelength of the TCROC. The TCROC control module200may leverage the thermal time constant of the TCROC and may apply an electrical power to the TCROC by pulsing the TCROC (e.g., in contrast to applying a constant electrical power) in order to facilitate applying pulse signals to several TCROCs in succession.

For example, the TCROC control module200may apply a first pulse signal to a first TCROC to raise the TCROC to a target temperature associated with an optimal peak resonance wavelength for the first TCROC (e.g., the wavelength of a laser light source). After the first pulse signal is no longer applied, the temperature of the TCROC may begin to fall below the target temperature and accordingly the peak resonance wavelength of the first TCROC may being to change away from the optimal value. However, the temperature may fall according to the thermal time constant of the first TCROC, which may allow the TCROC control module200to address several additional TCROCs by applying pulse signals successively to these additional TCROCs and revisiting the first TCROC before the temperature falls below some threshold value. In this way the TCROC control module200may intentionally allow the temperature of each TCROC to fall to some extent below a target temperature, but may revisit each TCROC and may reapply (or apply a new/modified pulse signal) to each TCROC periodically to again bring the temperature back to the target temperature before each temperature falls below a threshold value. In this way, the TCROC control module200may be implemented to tune the peak resonance wavelength of a plurality of TCROCS, for example, in contrast to implementing a thermal tuner driver496and pulse density generator490for each TCROC.

In some embodiments, a minimum repetition rate per TCROC (T), or the rate or period at which the thermal tuner driver496and/or pulse density generator490revisits each TCROC, can be determined by the thermal time constant, t. In some embodiments, the minimum repetition rate T is significantly smaller than the thermal time constant t. For example, the minimum repetition rate T may be 100 times smaller (or faster) than the thermal time constant t. The minimum repetition rate T may be as little at 10 times, or as much as 1000 times faster than the thermal time constant t. For example, in some embodiments, the thermal time constant t has been experimentally found to be about 100 microseconds (μs) for the TCROCs, and the minimum repetition rate T may be 100 times smaller than the thermal time constant t, or 1 μs.

The minimum repetition rate T may be significantly smaller than the thermal time constant t in this way in order that the TCROC control module may leverage the comparatively larger thermal time constant t of the TCROCs and tune a plurality of TCROCs without allowing any TCROC to fall significantly below the target temperature. For example, by implementing a time cycle for the TCROC control module defined by the minimum repetition rate T, and pulsing each TCROC at least once during each time cycle, the TCROCs may be maintained substantially at or near the target temperature. Different minimum repetitions rates T may be implemented to achieve different purposes of the system. For example, as may be evident based on that described herein, the faster the minimum repetition rate t, the closer the TCROCs may remain to the target temperature, while slower reptations rates may permit the TCROCs to fall farther from the target temperature before being pulsed back to the target temperature. Thus, in some implementations it may be desirable to implement a shorter minimum repetition rate t, such as 1000 times faster than the thermal time constant t, in order to achieve improved performance of the TCROCs. This increase in performance may come at a cost, however, as faster operation of components may correspond with increased energy usage, increased component wear, increased operational costs, etc. In some implementations, it may be desirable to implement a longer minimum repetitions rate T, such as 10 times faster than the thermal time constant t. While such an implementation may result in reduced performance increases, the resulting energy usage, component wear, operational costs, etc., may also be reduced. These two examples may illustrate a spectrum of use-cases, and a specific implementation of the techniques described herein may apply any of a variety of minimum repetition rates T in order to achieve the goals of the specific implementation.

The TCROC control module200may operate in accordance with the minimum repetition rate T in this way in order to facilitate pulsing a plurality of TCROCs. For example, the minimum repetition rate T may define a time cycle for operation of the TCROC control module200. Each time cycle, the TCROC control module200may pulse each TCROC which it controls/with which it is associated. The time cycle may be segmented into non-overlapping portions or segments such that each TCROC is associated with a distinct segment of the time cycle, and the TCROC control module200may visit or address each TCROC during its associated segment. In some embodiments, the time cycle is divided into equal segments such that each TCROC is associated with a segment of equal duration. In some embodiments, one or more of the segments may be longer or shorter than another segment. The TCROC control module may pulse and/or control any number of TCROCs and may accordingly segment the time cycle into any number of segments in order to pulse each TCROC at a distinct and non-overlapping segment of the time cycle. For example, the TCROC control module200may be associated with n TCROCs, and may accordingly address and/or pulse each TCROC at segments of T/n in length.

As mentioned above, the thermal tuner driver496and/or the pulse density generator490may apply a pulse signal to each TCROC in order to raise the temperature of the TCROC to a target temperature associated with an optical peak resonance wavelength for the TCROC. Techniques for determining the pulse signal (e.g., voltage and/or duration of the pulse signal) to apply to a given TCROC are described herein. In some embodiments, the TCROC control module200modifies or updates the pulse signal to apply to a given (or each) TCROC one or more times. For example, the TCROC control module200may update the pulse signal each time cycle in order to fine tune the peak resonance wavelength shift. In another example, the TCROC control module200may update the pulse signal at some other interval of time cycles.

Referring back toFIG.4, the controller494may be a software module that may control the thermal tuner driver496and/or the pulse density generator490to supply the thermal tuner497with the appropriate voltage for the current thermal condition of the TCROC. The controller494may be communicatively coupled to one or more of the MPDs432,434,436,438and the TCROCs482,484,486, and488. The DEMUX470has been shown and described with the functionality of a receive unit, and the TCROCs482,484,486, and488may accordingly be photodiodes implemented in the PIC to receive an optical signal and provide it to the ASIC in electrical form. It should be understood, however, that in some embodiments the DEMUX470, and/or the TCROCs482,484,486, and488may also be implemented as input(s) to the TCROC control module200. Unlike the receive unit in such an embodiment, the transmit unit including the MUX430may not have photodiodes, and MPDs432,434,436,438may be coupled to the MUX430to serve the purpose of providing input to the TCROC control module200.

The output of the MPDs432,434,436,438and TCROCs482,484,486, and488may comprise the light sampled along the waveguide in the PIC192that is on the same fiber as the TCROC. The detected light by the MPDs432,434,436,438and TCROCs482,484,486, and488may be input to the controller494using an electrical coupling between the ASIC and the PIC. For example, the electrical coupling may be a through-substrate-via such as a copper pillar structure, bump attachment unit, or any connection capable of being made between a PIC192and an ASIC188. In some embodiments, such as where less frequent but larger bursts of energy are used, the controller494may sample the MPDs432,434,436,438and TCROCs482,484,486, and488in a megahertz or a kilohertz range using timing module493. The controller494may cause the thermal tuner driver496to drive the TCROCs using the pulse density generator490, whereby the pulse density generator490may be instructed to generate pulse signals with waveforms characterized by multiples of the heat needed for any TCROC in a given time-cycle. In some embodiments, slow and fast clock signals are obtained by utilizing the Vernier effect from leveraging the first clock signal491and the second clock signal492. Typically, a fast signal may be at least an order of magnitude faster than the first and second clocks491and492. However, at a minimum the leveraged fast clock signal may have twice the frequency of the first and second clock signals491and492. In some embodiments, the higher-speed signal may be used to drive two or more TCROCs each time a time-cycle repeats in the timing module493. In some embodiments, hundreds or thousands of TCROCs are controlled where the higher-speed signal is a much smaller fraction of the duration of a clock-cycle from the timing module493.

FIG.6shows an example thermal tuner driver496, according to at least one embodiment of the present disclosure. As described herein, the thermal tuner driver496may be used for tuning TCROCs in an ML accelerator. The thermal tuner driver496may be coupled to a multi-dimensional grid of TCROCs670. The grid may include four (8×8) clusters of TCROCs675, (256 total TCROCs), which may be addressed individually using a multi-dimensional addressing interface694. As shown inFIG.6,64pins680may provide input to the multi-dimensional addressing interface694which may enable 64 clusters of four TCROCs to be addressed using row and column format, for example. The size, arrangement, and dimensionality of the TCROC grid may vary in different embodiments. An analog-to-digital converter495may receive the optical output from detectors associated with the TCROC clusters675(in the case of a transmit unit) or the optical output of the TCROC itself in the case of a receive unit. This may include, for example, a direct coupling and/or a chip-to-chip connection (not shown) between the PIC192and the thermal tuner driver496. For instance, the thermal tuner driver496may be stacked on top of the PIC192such that electrical communication is enabled between the RPDs and/or MPDs and the ADC495. In some embodiments, the optical output of the detectors may be transmitted via an electrical connection, such as a through-silicon-via, a copper pillar, a bump attachment unit, or any suitable electrical connection.

The photo-current readback from the detectors may be implemented using the ADC495and a current comparator. The comparator may give rail-to-rail swing based on the polarity of the input current, which in turn may be the difference between the photo-current and the output of the ADC495. To make a current measurement, the ADC495may be swept monotonically or may implement a binary search. The controller494may use the digital output of the ADC495to enable the pulse density generator490to construct the appropriate signal for any given TCROC. The signal may be transmitted via pins685and may become input on pins680after being sent via a switch610, which may include FETs, for example, between the output pins685and the input pins680.

FIGS.7A and7Billustrate various architectures by which the MPDs may sample light in the waveguides of a PIC192and provide the sampled light to closed-loop control circuit(s) implemented by the TCROC control module200, according to at least one embodiment of the present disclosure. InFIG.7A, an architecture is shown which may include one MPD792,794,796, and798in each through-port of each channel708,710,712, and714. In some embodiments, TCROCs700,702,704, and706are placed in channels708through714. MPDs792through798may each be placed at a through-port associated with the TCROC with which it is paired. The MPDs792,794,796, and798may receive optical power from one of the TCROCs700,702,704, and706on a bus/waveguide740as the optical signal travels to a grating coupler760. The MPDs792through798may reside in the PIC192and may be positioned below and/or adjacent to associated control circuitry in the ASIC188associated with the TCROC control module200. This may enable maximum fidelity in the electrical connections between the output of the MPDs792through798and the input to the TCROC control module200as signals are routed out of the PIC192.

InFIG.7B, an architecture is shown which may include a single MPD780tapped off a bus/waveguide750for each group of 4 channels. In the current example, TCROCs730,732,734, and736reside in channels720,722,724, and726. In some embodiments, a tap790connects the bus/waveguide750with an MPD780while the remainder of the light travels from the bus/waveguide750to a grating coupler770. The tap may utilize 5% of the optical signal on the bus/waveguide750, or another suitable amount. As with the scheme ofFIG.7A, the MPD780may be positioned below and/or adjacent to associated control circuitry in the ASIC188associated with the TCROC control module200to reduce latency.

FIG.8illustrates an architecture by which RPDs sample light in the waveguides of the PIC192and provide the sample light to closed-loop control circuit implemented by the TCROC control module200, according to at least one embodiment of the present disclosure. The architecture may include one RPD870,872,874, and876in each through-port of each channel840,842,844, and846. In some embodiments, TCROCs800,810,820, and830are placed in channels840through846. RPDs870through876may each be placed at a through-port associated with the TCROC it is paired with. The RPDs870,872,874, and876may receive optical power from one of the TCROCs800,810,820, and830on a bus/waveguide850as the light travels from a grating coupler860. In some embodiments, the RPDs870through876use a DC component of the optical signal incident on receiver photodetectors (e.g., already present in the portion of the receive units that reside in the PIC192). The presence of DC offset compensation at the input of a TIA (e.g., the portion of the receive unit that resides in the ASIC188) may give a readily usable current mirror to measure the DC photocurrent. The RPDs870through876may reside in the PIC192and may be positioned below and/or adjacent to associated control circuitry in the ASIC188associated with the TCROC control module200. This may enable maximum fidelity in the electrical connections between the output of the RPDs870through876and the input to the TCROC control module200as signals are routed out of the PIC192.

FIG.9illustrates an operation of a TCROC control module200controlling a system having two TCROCs, according to at least one embodiment of the present disclosure. The PDM scheme for TCROC control ofFIG.9may involve sending voltage pulses to each TCROC900and902in order to tune the resonance(s) to a laser wavelength in that band. As mentioned earlier, the TCROC peak wavelength shift may be proportional to the average electrical power delivered to its tuner, which in turn may be determined by the duty cycle of the pulse-train for that ring. TCROC900in time-cycle T (904) may have a duty cycle with a width of t (908) and a voltage v (910) during the time slice or duration represented by t (908). The remaining time slice in the time cycle T (904) may have a pulse train applied to TCROC902in an analogous manner. Thereafter, the process may repeat at time-cycle T (906) which may also include time slices for TCROCs900and902. It should be noted that in a two TCROC configuration the time slices may be less than or equal to half the frequency of the first time-cycle. In implementations with more than two TCROCs, the frequency of the time slices in each time-cycle may be smaller in duration. It should also be noted that in systems that optimize for power, the pulse train for each TCROC900and902may be configured to minimize the magnitude of voltage v (910) and maximize the duration of width t (908), so long as there is sufficient time to deliver all of the needed pulses in any given time-cycle, as described herein. This may allow the system to use the least amount of power possible, while obtaining the correct signals needed by the TCROCs to operate at the desired peak resonance wavelengths.

FIG.10is a flowchart showing the operation of a startup procedure for an embodiment of a system that uses MUXes in a PIC and/or for a transmit unit that uses TCROCs in a photonic portion, according to at least one embodiment of the present disclosure. At operation1000, the system may be initialized. This may include, for example, ensuring that the thermal tuning power supplied to each TCROC is greater than a minimum value, for example, to ensure that any subsequent increases in PIC temperature can be handled by reducing the tuning power. For instance, to provide a margin that is 35° C. above startup temperature, an accommodation may be made to blue-shift the peak wavelengths by up to 2.5 nanometers at operation1000. At operation1005, the light source may be turned on. At operation1010, the MPDs for each TCROC (MUX) may be read back and a first result stored in memory. At operation1015, the PDM duty cycle may be incremented for each TCROC (MUX). Thereafter, at operation1020, the MUX TCROCs may be read back again and a second result stored in memory. At operation1025, the first and the second results may be compared. When the second result is larger than the first result, the process may repeat at operation1010. The process may continue until the second result no longer exceeds the first result at operation1025. This may occur for example, when the PDM duty cycle increases causing the peak resonance of the TCROC to move past the peak wavelength. When this occurs, the system may use the first result as the peak. To that end, a peak flag may be set to true at operation1030. The light source may be initialized with the first result at operation1035and the light source may be turned off at operation1040.

FIG.11is a flowchart showing the operation of a startup procedure for an embodiment of a system that uses DEMUXes in a PIC and/or for a receive unit that has photodetectors in a photonic portion of its message router, according to at least one embodiment of the present disclosure. At operation1100, the system may be initialized, for example, by ensuring that the thermal tuning power supplied to each TCROC is greater than a minimum value to ensure that any subsequent increases in PIC temperature can be handled by reducing the tuning power. At operation1105, the light source may be turned on. At operation1110, the RPDs for each TCROC (DEMUX) may be read back and a first result stored in memory. At operation1115, the PDM duty cycle may be incremented for each TCROC (DEMUX). Thereafter, at operation1120, the DEMUX TCROCs may be read back again and a second result stored in memory. At operation1125, the first and the second results may be compared. When the second result is larger than the first result, the process may repeat at operation1110. The process may continue until the second result no longer exceeds the first result at operation1125. Thereafter, a peak flag may be set to true at operation1130, the light source may be initialized with the first result at operation1135, and the light source may be turned off at operation1140.

FIG.12is a flowchart showing the operation of a peak-tracking procedure for an embodiment of a system that uses MUXes in the PIC and/or for a transmit unit that uses TCROCs in a photonic portion, according to at least one embodiment of the present disclosure. At operation1200, the system may be initialized. This may include, for example, setting the timing wherein a refresh rate for first and second clock systems is set to tracking. This may enable various embodiments to create and send pulses to the TCROCs at high speeds, while implementing a software control circuit at a much lower speed. This may also include setting a DC minimum voltage that may be provided to a control module and/or driver that may create and send pulse-density signals to TCROCs. At operation,1205a pulse-density modulation duty cycle delta may be set. This may be used to represent the minimum step that the PDM duty cycle may be modulated whenever the duty cycle is altered.

At operation1210, the MPD's for each TCROC (MUX) may be read back and a first result stored in memory. At operation1215, the delta may be added to the PDM duty cycle drivers for each MUX. Thereafter, at operation1220, the MUX TCROCs may be read back again and a second result stored in memory. At operation1225, the first and the second results may be compared. When the second result is larger than the first result, the PDM duty cycle may be incremented using the delta at operation1235and the process may repeat at operation1210. Otherwise, the PDM duty cycle may be decremented using the delta at operation1230and the process may repeat at operation1210. The process may repeat in a closed-loop whenever peak tracking mode is enabled in the controller and/or the system is operating.

FIG.13is a flowchart showing the operation of a peak-tracking procedure for an embodiment of a system that uses DEMUXes in the PIC and/or for a receive unit that has photodetectors in a photonic portion of its message router, according to at least one embodiment of the present disclosure. At operation1300, the system may be initialized. At operation,1305a pulse-density modulation duty cycle delta may be set. This may be used to represent the minimum step that the PDM duty cycle may be modulated whenever the duty cycle is altered. At operation1310, the RPDs for each TCROC (DEMUX) may be read back and a first result stored in memory. At operation1315, the delta may be added to the PDM duty cycle drivers for each DEMUX. Thereafter, at operation1320, the DEMUX TCROCs may be read back again and a second result stored in memory. At operation1325, the first and the second results may be compared. When the second result is larger than the first result, the PDM duty cycle may be incremented using the delta at operation1335and the process may repeat at operation1310. Otherwise, the PDM duty cycle may be decremented using the delta at operation1330and the process may repeat at operation1310. The process may repeat in a closed-loop whenever peak tracking mode is enabled in the controller and/or the system is operating.

FIG.14is a flow diagram for a method1400or a series of acts for tuning TCROCs of a PIC as described herein, according to at least one embodiment of the present disclosure. WhileFIG.14illustrates acts according to one embodiment, alternative embodiments may add to, omit, reorder, and/or modify any of the acts ofFIG.14.

In some embodiments, the method1400includes an act1410of performing one or more sub acts during a time cycle. For example, the time cycle may be recurring, and the sub acts may be performed during each time cycle for several consecutive iterations of the time cycle. As shown, the act1410and the included sub acts may be performed or looped two or more times. In some embodiments, the time cycle is determined based on a thermal time constant of the plurality of TCROCS.

In some embodiments, the method includes an act1420(e.g., a sub act) of detecting a plurality of optical outputs generated by the plurality of TCROCs. The plurality of TCROCs may generate the plurality of optical outputs based on receiving an optical signal from a light source. The plurality of TCROCs may include one or more of a multiplexer and a demultiplexer.

In some embodiments, the method includes an act1430(e.g., a sub act) of determining a pulse signal for each of the plurality of TCROCs configured to shift a peak resonance wavelength of an associated TCROC to substantially match the wavelength of the light source. The pulse signals may each be determined based on an electrical power to apply to each TCROC, wherein the electrical power is determined based on a voltage and a duration of the pulse signal. The pulse signals may each be configured to change a temperature associated with a corresponding TCROC, and the shift in the peak resonance of the TCROC may be based on the associated change in temperature.

In some embodiments, the method includes an act1440(e.g., a sub act) of applying, with a thermal tuner driver, the associated pulse signal to each of the plurality of TCROCs, wherein each of the pulse signals is applied during a non-overlapping segment of the time cycle. The thermal tuner driver may be connected to each of the TCROCs via a switchable circuit such that each TCROC is selectively connectable to the thermal tuner driver. For example, the plurality of TCROCs may include “n” TCROCs and the time cycle may be divided into n segments that each have a maximum duration of:

Time Cycle/n

In some embodiments, the consecutive iterations of the time cycle include a first time cycle and a second time cycle of equal length. During the first time cycle, a first plurality of pulse signals may be determined and applied to the plurality of TCROCs, and during a second time cycle, a second plurality of pulse signals may be determined and applied to the plurality of TCROCs. At least one TCROC may have a pulse signal that is different for the first time cycle and the second time cycle.

FIG.15is a flow diagram for a method1500or a series of acts for tuning TCROCs of a PIC as described herein, according to at least one embodiment of the present disclosure. WhileFIG.15illustrates acts according to one embodiment, alternative embodiments may add to, omit, reorder, or modify any of the acts ofFIG.15.

In some embodiments, the method1500includes and act1510of detecting a first optical output of a first TCROC and a second optical output of a second TCROC.

In some embodiments, the method1500includes an act of1520of determining a first pulse signal for the first TCROC and a second pulse signal for the second TCROC each designed to shift a peak resonance wavelength of the associated TCROC to substantially match a target wavelength of a light source.

In some embodiments, the method1500includes an act1530of performing one or more sub acts with a thermal tuner driver. For example, the thermal tuner driver may perform any of the acts1540,1550, or1560described below.

In some embodiments, the method1500includes an act1540(e.g., a sub act) of applying the first pulse signal to the first TCROC to heat the first TCROC to a first target temperature associated with the first TCROC operating at the target wavelength.

In some embodiments, the method1500includes an act1550(e.g., a sub act) of, while allowing the first TCROC to cool below the first target temperature, applying the second pulse signal to the second TCROC to heat the second TCROC to a second target temperature associated with the second TCROC operating at the target wavelength.

In some embodiments, the method1500includes an act1560(e.g., a sub act) of, while allowing the second TCROC to cool below the second target temperature, reapplying the first pulse signal to the first TCROC to heat the first TCROC back to the first target temperature, wherein the first pulse signal is reapplied to the first TCROC within a minimum repetition period that is derived from a thermal time constant of the first TCROC. The thermal time constant may be based on a thermal conductivity of the first TCROC. For example, the thermal time constant may be a time period that the TCROC takes to fall from the first target temperature to an equilibrium temperature after the first pulse signal is not applied. In some embodiments, the minimum repetition period is at least ten times shorter than the thermal time constant. The minimum repetition period may be a recurring time cycle, and the first and second pulse signals may be alternatingly applied to the first and second TCROCs each time cycle. In some embodiments, the first pulse signal may be modified and reapplied to the first TCROC as a modified first pulse signal. For example, the first pulse signal may be modified to tune the first TCROC based on a response of the first TCROC from the first signal being applied.

In summary, according to various implementations of the present inventive concepts, the present EP-NoC includes a novel scheme for tuning optical components in the EP-NoC. The presently disclosed EP-NoC may result in a significant improvement in the performance of TCROCs. From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described herein, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.