Mismatch resistant thermal control loop for cascaded optical ring resonators

Abstract

An integrated circuit (IC) device includes an optoelectronic circuitry having a first heater and a second heater, and a controller circuitry having an input coupled to a photodiode of the optoelectronic circuitry and an output coupled to the first heater and the second heater of the optoelectronic circuitry, the controller circuitry configured to determine an offset from a baseline heater control signal code based on a transimpedance (TIA) control signal code of an input signal received from the photodiode, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

Description

MISMATCH RESISTANT THERMAL CONTROL LOOP FOR CASCADED OPTICAL RING RESONATORSGOVERNMENT LICENSE RIGHTS

[0001] This invention was made with United States (U.S.) government support under Contract No. HR0011-19-3-0004, awarded by the U.S. Defense Advanced Research Projects Agency (DARPA). The U.S. government has certain rights in the invention.TECHNICAL FIELD

[0002] Examples of the present disclosure generally relate to a temperature- locked loop for optical elements having a temperature-dependent response. BACKGROUND

[0003] Optical devices have been used for communications, such as over a fiber optic channel. Optical communications can implement low loss physical channels and high speeds. Like electrical devices, some optical devices can be used to process or filter signals, albeit optical signals. However, some optical devices can have features that are not present or not as significant in an electrical device. Further growth and implementation of optical devices in future technologies may require these features to be addressed.SUMMARY

[0004] In one or more examples, an integrated circuit (IC) device includes an optoelectronic circuitry having a first heater and a second heater, and a controller circuitry having an input coupled to a photodiode of the optoelectronic circuitry and an output coupled to the first heater and the second heater of the optoelectronic circuitry, the controller circuitry configured to determine an offset from a baseline heater control signal code based on a transimpedance (TIA) control signal code of an input signal received from the photodiode, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

[0005] In one or more examples, a controller circuitry includes a transimpedance (TIA) circuitry having an input coupled to a photodiode of an optoelectronic circuitry and an output coupled to a control code circuitry, the TIA circuitry configured generate a voltage signal based on an input signal received from the photodiode,the control code circuitry including a TIA control circuitry having an input connected to the output of the TIA circuitry; and a heater circuitry having an input coupled to an output of the TIA control circuitry and a first output coupled to a first heater and a second output coupled to a second heater of the optoelectronic circuitry, the heater circuitry configured to determine an offset from a baseline heater control signal code of the optoelectronic circuitry based on the voltage signal received from the TIA circuitry, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

[0006] In one or more examples, a method for operating an integrated circuit device includes determining, by a controller circuitry having an input coupled to a photodiode of an optoelectronic circuitry, a first output coupled to a first heater, and a second output coupled to a second heater of the optoelectronic circuitry, a shape of an input signal received from the photodiode, and determining, by the controller circuitry, an offset from a baseline heater control signal code of the optoelectronic circuitry based on the shape of the input signal, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.BRIEF DESCRIPTION OF DRAWINGS

[0007] So that the manner in which the above recited features can be understood in detail, a more particular description, briefly summarized above, may be had by reference to example implementations, some of which are illustrated in the appended drawings, it is to be noted, however, that the appended drawings illustrate only typical example implementations and are therefore not to be considered limiting of its scope.

[0008] FIG. 1 illustrates an integrated circuit (IC) device according to one or more examples.

[0009] FIG. 2 is a more detailed schematic of the schematic of FIG. 1 according to one or more examples

[0010] FIG. 3 illustrates a flow diagram illustrating a method for calibrating an input signal from an optoelectronic circuit according to one or more examples.

[0011] FIG. 4 illustrates a graph of input signals according to one or more examples.

[0012] FIG. 5 illustrates a flow diagram illustrating a method for a calibrating an optical element having a convex frequency response according to one or more examples.

[0013] FIG. 6 illustrates a graph of an example input signal having a convex shape according to one or more examples.

[0014] FIG. 7 illustrates a flow diagram illustrating a method for calibrating an optical element having a flat shaped frequency response according to one or more examples.

[0015] FIG. 8 illustrates a graph of an example input signal having a flat shape according to one or more examples.

[0016] FIG. 9 illustrates a flow diagram illustrating a method for calibrating an optical element having a concave shaped frequency response according to one or more examples.

[0017] FIG. 10 illustrates a graph of an example input signal having a concave shape according to one or more examples.

[0018] FIG. 11 illustrates a flow diagram illustrating a method for determining an offset heater control signal code teat minimizes insertion loss according to one or more examples.DETAILED DESCRIPTION

[0019] Examples described herein generally relate to devices and methods for a temperature-locked loop for optical elements having a temperature-dependent response. Generally, devices according to some examples indude an electrical integrated circuit (IC) that includes an optoelectronic circuit and controller circuitry. The optoelectronic circuit indudes optical channels, an optical element, a photodiode, and heaters. The optical element is configured to filter an optical signal into its corresponding receiving path, and the optical element has a temperature-dependent optical response. Example optical elements indude a ring modulator, a double ring filter, or a Mech-Zehnder interferometer (MZI), among others. The photodiode is disposed relative to the optical element such that at least some of the optical signal passed through the optical element is incident on the photodiode. The heaters are disposed proximate the optical element. The photodiode is connected to an input of the controller circuitry. An output of the controller circuitry is connected to an input of the heaters. Generally, the controllercircuitry is configured to control a temperature of the heaters, and thereby the optical element, such that the optical element may have a center frequency or wavelength aligned with its input signal and provide a high-quality optical signal to the photodiode. In one or more examples, the optoelectronic circuit is a wavelength divisions multiplexed (WDM) optical transceiver. The WDM optical transceiver is configured to filter out the correct wavelength corresponding to each channel. Double-ring filters, such as cascaded ring resonators (CRRs) may be used because they provide a sharper roll-off and a better channel to channel isolation. However, because they are a resonant based device, the double-ring filters need to be stabilized in a closed-loop fashion to be used across a range of temperatures. Also, process variation and mismatch between the two rings could lead to additional insertion loss and yield loss. The mismatch causes a change in resonance leading to an increases insertion loss. Embodiments herein relate to an optoelectronic circuit that includes individual heaters that can independently tune each ring of the double-ring filter to reduce insertion loss.

[0020] FIG. 1 illustrates an integrated circuit (IC) device 100 according to one or more examples. In one example, the IC device 100 includes an optoelectronic circuitry 102 and a controller circuitry 114. The optoelectronic circuitry 102 indudes an optical source 104, an optical element 106, an optical channel 108, a photodiode 110, and a ring heater 112. The optical element 106 is any optical element configured to filter an optical signal and has a temperature-dependent optical response, such as a ring modulator, a double ring filter, or a Mech-Zehnder interferometer (MZI), among others. In some examples, the optical element 106 is a double-ring filter, such as a cascaded ring resonator (CRR). In one example, the ring heater 112 is located within the optical element 106 (as shown in FIG. 2) or external to the optical element 106 (as shown in FIG. 1). For example, the optical element 106 is a double-ring filter, and the ring heater 112 indudes a first heater 112a external to or within a first ring of the double-ring filter and a second heater 112b located external to or within a second ring of the double-ring filter (FIG. 2). In one example, the first heater 112a and the second heater 112b are resistive heaters.

[0021] Generally, the controller drcuitry 114 generates output signals that are communicated to the optoelectronic circuitry 102. In one example, the output signals are heater signals) that respectively change the temperature of the ringheater 112. The optical source 104 generates an optical signal, and transmits the optical signal via the optical channel 108. The optical element 106 is coupled to the optical channel 108, and the optical channel 108 filters the optical signal within its passband from the optical channel 108 to its output. The photodiode 110 is disposed in the optoelectronic circuitry 102 such that a portion of the optical signal output from the optical element 106 is incident on the photodiode 110. The photodiode 110 generates an input signal in response to the optical signal incident on the photodiode 110. In one example, the output signal of the photodiode 110 is an electrical current The output of the photodiode 110 is coupled to the controller circuitry 114, and the controller circuitry 114 receives the input signal from the photodiode 110. The controller circuitry 114, in response to the input signal, outputs the heater signal(s) to the ring heater 112 based on an offset that reduces insertion loss, a proper center frequency of the input signal, and the shape of the input signal. In response to the heater signal(s), the ring heater 112 can control a temperature of the optical element 106. The optical element 106 has an optical response that varies based on a temperature of the optical element 106. Stated differently, the center frequency of the optical elements 106 may become mismatched due to process variations and / or temperature drift. The controller circuitry 114, via the heater signals), is able to control the temperature of the ring heater 112 to lock the center frequency of the input signal to the proper center frequency.

[0022] FIG. 2 is a more detailed schematic of the schematic of FIG. 1 according to one or more examples. The confroller circuitry 114 includes a digital-to-analog converter (DAC) 201 , a transimpedance (TIA) circuitry 202, a slicer circuitry 204, a control code circuitry 207, a DAC 210a, a DAC 210b, and a DAC 212. In one or more examples, the TIA circuitry 202 includes a current source, a resistor, a TIA amplifier, and a controllable current source (not shown). In one example, the optical element 106 is a double-ring filter.

[0023] A cathode of the photodiode 110 is electrically connected to a first power supply node (e.g., a VDD node), and an anode of the photodiode 110 is coupled to the TIA circuitry 202, which can further be an input of the controller circuitry 114. In one example, the current source of the TIA circuitry 202 is electrically connected between the first power supply node (e.g., the VDD node) and the input of the TIA circuitry 202. The current source can be or indude a current mirror biased by astatic current such that the current source is configured to provide a static or constant current. The controllable current source of the TIA circuitry 202 is connected between the input of the TIA circuitry 202 and a second power supply node (e.g., a ground node).

[0024] A first input of the slicer circuitry 204 is connected to the output of the TIA circuitry 202. A second input of the slicer circuitry 204 is connected to an output of a DAC 201. An output of the slicer circuitry 204 is connected to an input of the control code circuitry 207. In some examples, the slicer circuitry 204 is a comparator. An input of the DAC 201 can be electrically coupled to, e.g., a memory element (such as electrical fuses (eFuses)) that stores a digital value corresponding to a reference voltage (Vref). The reference voltage Vref can be, in some examples, VDD / 2.

[0025] In one example, the control code circuitry 207 includes a TIA control circuitry 206 and a heater circuitry 208. In one example, the output of the TIA control circuitry 206 is connected to an input of the heater circuitry 208 and the DAC 212. A first heater control signal code is provided by the heater circuitry 208 to an input of the DAC 210a. A second heater control signal code is provided by the heater circuitry 208 to an input of the DAC 210b. "The temperature of the ring heater 112 is a combination of the temperature provided by the first heater 112a and the second heater 112b. The TIA control circuitry 206 outputs a TIA control signal to the DAC 212 and the heater circuitry 208. The heater circuitry 208 outputs the first and second heater control signal codes to the DAC 210a and to the DAC 210b, respectively, based on the TIA control signal. In one example, the control code circuitry 207 indudes at least one output connected the TIA circuitry 202. In one example, the control code drcuitry 207 is connected to the controllable current source. In another example, an output of the DAC 212 is connected to a bias voltage node of the controllable current source.

[0026] An output of the DAC 210a is connected to an input of the first heater 112a. An output of the DAC 210b is connected to an input of the second heater 112b. The DAC 210a provides a first heater control signal to the first heater 112a based on the first heater control signal code and the DAC 210b provides a second heater control signal to the second heater 112b based on the second heater control signal code. The temperature provided by the first heater 112a is based on the firstheater control signal. The temperature provided by the second heater 112b is based on the second heater control signal.

[0027] In operation, the photodiode 110 generates a current in response to the optical signal incident on the photodiode 110. " The current generated by the photodiode 110 is received by the controller circuitry 114 at the TIA circuitry 202 as an input signal. The TIA circuitry 202 generates a voltage signal at an output of the TIA circuitry 202 stage based on the input signal. The slicer circuitry 204 generates a logical “1” or a logical “0” based cm whether the voltage signal is greater than a reference voltage output by the DAC 201. The DAC 201 generates the reference voltage based cm, e.g., the digital value stored in a memory dement (not shown) communicatively coupled to the input node of the DAC 201. The reference voltage can be a static voltage, and hence, in some examples, the digital value stored in the memory dement can be static. The TIA control circuitry 206 samples the logical "1"s and logical *0"s generated by the dicer circuitry 204 and outputs a TIA control signal code based on the logical “1"s and logical “0”s.

[0028] The heater circuitry 208 generates the first and second heater control signal codes based on the TIA control signal code. The heater circuitry 208 increases and / or decreases the first and second heater control signal codes based on the TIA control signal code. Stated differently, the heater circuitry 208 increases or decreases the first heater control signal codes transmitted to the DAC 210a and increases or decreases the second heater control signal code transmitted to the DAC 210b. The changes in the first heater control signal code cause the DAC 210a to change the first heater signal to increase or decrease the temperature provided by the first heater 112a. The changes in the second heater control signal code cause the DAC 210b to change the second heater signal to increase or decrease the temperature provided by the second heater 112b. The changes in the first and second heater signal control codes are used to reduce insertion loss and maintain the center frequency of the input signal within a calibration threshold.

[0029] The TIA control signal code responsively adjusts the DC component of the input signal provided to the DAC 212 such that the output of the slicer circuitry 204 has almost an equal amount of Ts and 0's over an observing window. The DC component of the input signal includes the static current of the photodiode 110 and an extra DC current caused by the optical signal from the optical element 106. The optical signal strength and the extra DC current caused by the optical signal fromthe optical element 106 depends on the frequency response and the input signal frequency of the optical element 106. Therefore, the DC current strength reflects the shape of the optical element 106 frequency response and the frequency mismatch between center frequency and input frequency of the optical element 106. Embodiments herein, are related to the control code circuitry 207 changing the first and / or second heater control signal codes to reduce insertion loss and maintain the alignment between the center frequency of optical element 106 and the optical signal provided from optical source 104 based on a shape on the DC input signal from the photodiode 110, a maximum or minimum of the input signal (depending on the shape of the input signal), and a center frequency of the input signal.

[0030] FIG. 3 illustrates a flow diagram illustrating a method 300 for calibrating an input signal from an optoelectronic circuitry 102 according to one or more examples. FIG. 3 is described with respect to FIG. 2.

[0031] At operation 302, the control code circuitry 207 determines a search window. In one example, the search is a frequency search range such as 200 GHz. The search window may have a lower frequency bound and an upper frequency bound. The search window is determined by the heater circuitry 208 by comparing the combined output of the first heater 112a and the second heater 112b to the TIA control signal code to a search window threshold at a lower bound of the search window. If the TIA control code at the lower bound of the search window is above the search window threshold, the heater circuitry 208 will decrease the lower the upper bounds of the search window until the TIA control code at the lower bound of the search window is below the search window threshold, if the TIA control code is below the search range threshold when the search window is initialized, the search window is valid and does not have to be adjusted.

[0032] At operation 304, the control code circuitry 207 initializes the optical element 106. The heater circuitry 208 sweeps the first heater control signal code and the second heater control signal code from the lower bound frequency of the search window to the upper bound frequency of the search window. In one example, the first heater control signal code and the second heater control signal code are equal and are swept equally and during at least partially overlapping periods of time. While the first heater control signal code and the second heatercontrol signal codes are swept, the heater circuitry 208 tracks how the TIA control signal code changes based on the combined output of the first heater 112a and the second heater 112b. Specifically, the heater circuitry 208 sweeps the first heater control signal code supplied to DAC 210a and the second heater control signal code supplied to DAC 210b from the lower bound of the frequency window to the upper bound of the frequency window. The heater circuitry 208 tracks the TIA control signal code output by the TIA control circuitry 206. The TIA control circuitry 206 outputs the TIA control signal code based on the number of logical “1 ’s” and logical “0's” captured from the slicer circuitry 204. This allows the heater circuitry 208 to track how the TIA control signal changes as the heater control signal codes are swept. The heater circuitry 208 visualizes the input signal as a graph, as shown in FIG 4. Because the first and second heater control signal codes are swept at a same rate in a same direction, they can be visualized as a combined heater control signal code (i.e., as a single heater). The input signal is graphed from the lower bound of the search window 401 to the upper bound of the search window 403. The heater circuitry 208 visualizes the input signal as a graph 400 with a combined heater control signal code as the x-axis and the TIA control signal code as the y-axis. If the TIA control signal code never increases (is never greater than) a minimum TIA control signal code threshold 407, the heater circuitry 208 outputs a missing channel flag and the method returns to operation 304. If the control code circuitry 207 determines that the TIA control signal code exceeds the minimum TIA control signal code threshold 407, the heater circuitry 208 determines the heater control signal code(s) that correspond to the maximum TIA control signal code(s) within the search window. The heater circuitry 208 may determine one or more maximum TIA control signal codes within the search window.

[0033] At operation 306, the control code circuitry 207 determines the shape of the input signal. The input signal corresponds to the frequency response of optical element 106. The shape of the input signal may be determined, at least in part, based on the quantity of maximum(s) in the TIA signal control code in the search window TIA signal control code that surround the maximumsfs). As noted above, the first heater control signal code and the second heater control signal code are changed in a same direction at a same rate. Therefore, the heater circuitry 208 evaluates the first heater control signal code and the second heater control signalcode as a combined heater control signal code when determining the shape of the input signal.

[0034] As illustrated in FIG. 4, if the heater circuitry 208 determines that there is one maximum TIA control signal code, the heater circuitry 208 determines that the shape of the input signal is convex, as illustrated by input signal 402, or that the shape of the input signal is flat as illustrated by input signal 404. On the other hand in the heater circuitry 208 determines there are two maximum TIA control signal codes within the search window, the heater circuitry 208 determines that the input signal has a concave shape, as illustrated by input signal 406. If the heater circuitry 208 determines the input signal has a concave shape, the heater circuitry 208 also determines a minimum TIA control signal code that is positioned between the two maximums. Based on the shape of the input signal, the heater circuitry 208 stores the combined heater control signal code and TIA control signal code corresponding to the maximum(s), and the minimum (if necessary). The combined heater control signal code and TIA control signal code corresponding to the one maximum TIA control signal code (for convex and flat) or the combined heater control signal code and TIA control signal code corresponding to the one minimum TIA control signa! code (for concave) are also stored as the center frequency of the input signal.

[0035] At operation 307, the control code circuitry 207 determines an offset heater control signal code that minimizes insertion loss. In one example, as described above, if the optical element 106 includes a double ring filter, the optoelectronic circuitry is more susceptible to insertion loss. Insertion loss causes the cunrent transmitted to the TIA circuitry 202 to decrease, and thus, reduces the power output of the optoelectronic circuity . In one example, the power loss due to insertion loss can be is based on a value of a minimum of a TIA control signal code (for a concave shape) or a maximum (for convex or flat shape). Stated differently the lower the value of the TIA control signal of a minimum of a TIA control signal code (for a concave shape) or a local maximum (for convex or flat shape) the greater the insertion loss. In one example, to reduce insertion loss, the first heater 112a and the second heater 112b can be individually tuned to find an offset that reduces insertion loss. The offset heater control signal code (also referred to as the "offset) is an offset heater control signal code from a baseline heater control signal code when the optoelectronic circuitry 102 is initialized. Stated differently theoffset heater control signal code is a first heater control signal code and / or a second heater control signal code that is (are) offset from a baseline first heater control signal code and / or a baseline second heater control signed code when the optoelectronic circuit is initialized.

[0036] FIG. 11 illustrates a flow diagram illustrating a method 1100 for determining an offset heater control signal code that minimizes insertion loss. FIG. 11 is described with respect to FIG. 2.

[0037] At operation 1102, the heater circuitry 208 increases the first heater control signal code provided to the first heater 112a.

[0038] At operation 1104, the heater circuitry 208 determines whether the minimum or maximum TIA control signal code (depending on the shape) of the input signal increases as the first heater control signal code is increased. Stated differently, based on changes in the TIA control signal code supplied by the TIA control circuitry 206, the heater circuitry 208 determines if file minimum or maximum TIA control signal code increases as the first heater control signal code increases. If the minimum or maximum TIA control signal code increases, the method proceeds to operation 1106, and the heater circuitry 208 increases the first heater control signal code until the minimum or maximum TIA control code stops increasing. The first heater control signal code provided to the first heater 112a when the minimum or maximum TIA control code to stops increasing is saved as the offset.

[0039] On the other hand, at operation 1104 if changing the first heater control signal code does not cause the minimum or maximum TIA control signal code to increase, file method proceeds to operation 1108 and the heater circuitry 208 increases the second heater control signal code provided to the second heater 112b until the minimum or maximum TIA control signed code stops increasing. The second heater control signal code provided to the second heater 112b when file minimum or maximum TIA control signal code to stops increasing is saved as file offset. In one example, the minimum or maximum TIA control signal code corresponding to the offset is also the proper center frequency of the input signal. Stated differently, the offset corresponds to a first and / or second heater control signal code that results in the input signal having the highest maximum or minimum TIA control signal (based on the shape of the input signal). In other examples, the heater circuitry 208 can increase the second heater control signal code first, andthen if necessary, increase the first heater control signal code. In another example, if the minimum or maximum TIA control signal code is above an insertion loss threshold, an offset may not be determined. In another example, if increasing the first and second heater control signal codes does not increase file minimum or maximum TIA control signal code, then it may be determined that insertion loss is negligible and an offset is not saved.

[0040] In another example, the heater control signal codes are changed differentially. Stated differently , while the first heater control signal code is increased, the second heater control signal code is decreased at the same rate (and vice versa) until the offset is determined. For example, the offset indudes both a first heater control signal code and a second heater control signal code that are differential to one another.

[0041] Returning to the flow diagram of FIG. 3, at operation 308, the optical element 106 is set to the offset. The heater circuitry 208 outputs the stored first heater control signal code and / or the second heater control signal code that corresponds to the offset. In one example, the first heater control signal code and / or the second heater control signal code corresponds to the minimum or maximum TIA control signal code that reduces insertion loss and also corresponds to the proper center frequency of the optical element 106.

[0042] At operation 310, the control code circuitry 207 determines that the optical element 106 center wavelength is not calibrated to the frequency of the optical signal generated by the optical source 104. After the optical element 106 is set to the offset, the first and second heater control signal codes correspond to the proper center frequency and are changed (if necessary) in a same direction and at a same rate. In one example, when determining teat the optical element 106 center wavelength is not calibrated the first and second heater control signals are visualized as a single (combined) heater control signal code. Stated differently, the control code circuitry 207 determines that the center wavelength of the optical element is not calibrated based on the combined output of the ring heater 112. The heater circuitry 208 compares the current TIA control signal code with a predetermined calibration threshold 408. For flat and convex cases, if the cunent TIA control signal code is less than the TIA control signal code corresponding to the proper center frequency minus the calibration threshold 408, the optical element 106 is not calibrated and the method continues to operation 310. For concavecases, if the TIA control signal code is less than the TIA control signal code corresponding to the proper center frequency minus the calibration threshold 408, or greater than the TIA control signal code corresponding to the proper center frequency plus the calibration threshold 408, the method continues to operation 312. The calibration threshold 408 may be set to 0 for continuous tuning, or set to a value greater 0 for trigger-typing tuning. The method does not continue to operation 310 unless the heater circuitry 208 determines that the optical element 106 needs calibration.

[0043] At operation 312, the control code circuitry 207 calibrates the optical element 106. Stated differently, the heater circuitry 208 changes the first and the second heater control signal codes based on the shape of the input signal, the current value of the TIA control signal code, and the calibration threshold 408. In one example, the first and second heater control signal codes are changed in the same direction and at the same rate, and as noted above, can be visualized as a single graph and treated as a combined heater control signal code. Advantageously, if the TIA control signal code changes based on a change in ambient temperature, the heater circuitry 208 updates the combined heater control signal code (i.e, the first and second heater control signal codes), which changes the heat output of the first heater 112a and the second heater 112b to maintain the center frequency such that the TIA control signal code variation is within (i.e. above) the calibration threshold 408. During calibration, the first heater control signal code and the second heater control signal code are changed in a same direction at a same rate. Therefore, the heater circuitry 208 evaluates the first heater control signal code and the second heater control signal code as a combined heater control signal code during calibration. Stated differently, the heater circuitry 208 changes the values of the first and second heater control signal codes equal to the changes in values of the combined heater control signal code.

[0044] FIG. 5 illustrates a flow diagram illustrating a method 500 for a calibrating optical element 106 having a convex frequency response according to one or more examples. The DC component of input signal received by the confroller circuity 114 has a corresponding convex response to that of optical element 106. FIG. 6 illustrates a graph 600 of an example input signal 602 having a convex shape according to one or more examples. As noted above, with the exception of determining the offset, the first and second heater control signal codesare changed in the same direction and at the same rate, and therefore, are visualized and described as a combined heater control signal code. Stated differently during operations 502-508 and 512-514 the heater circuitry 208 visualizes the first heater control signal code and the second heater control signal code as a combined heater control signal code.

[0045] At operation 502, the control code circuitry 207 determines that the input signal 602 has a convex shape. In one example determining, by the control code circuitry 207 that the input signal has a convex shape includes operations 504-508.

[0046] At operation 504, the heater circuitry 208 sweeps the combined heater control signal code. As noted above the first and second heater control signal codes are swept at a same rate in a same direction and can be visualized as a single heater control signal code (i.e., as a single heater). Operation 504 is similar to operation 304. The heater circuitry 208 determines the TIA control signal code includes one maximum TIA control signal code 604 within the search window. The heater circuitry 208 stores the maximum TIA control signal code 604 and the corresponding combined heater control signal code.

[0047] At operation 506, the control code circuitry 207 determines a first TIA control signal code 608 and a second TIA control signal code 610 based on the calibration threshold 408. In one example, the heater circuitry 208 determines the first TIA control signal code 608 by changing the combined heater control signal code in a first direction, such as the decreasing direction (or the increasing direction). As described above, changing the combined heater control signal includes changing the first heater control signal code and the second heater control signal code in a same direction at a same rate. The first TIA control signal code 608 is equal to the maximum TIA control signal code 604 minus the calibration threshold 408. The second TIA control signal code 610 is also equal to the maximum TIA control signal code 604 minus the calibration threshold 408. The heater circuitry 208 determines a first combined heater control signal code corresponding to the first TIA control signal code 608 by changing the combined heater control signal code until the TIA control signal code reaches the first TIA control signal code 608. In the illustrated example, the heater circuitry 208 decreases the combined heater control signal code until the first TIA control signal code 608 is reached. In another example, the heater circuitry may increase the combined heater control signal code until the first TIA control signal code 608 isreadied. The heater circuitry 208 determines a second combined heater control signal code corresponding to the second TIA control signal code 610 by changing the combined heater control signal code in the opposite direction used to determine the first TIA control signal code 608 until tie TIA control signal code reaches second TIA control signal code 610.

[0048] At operation 508, the control code circuitry 207 (the heater circuitry 208) determines that the difference between the second combined heater control signal code and the first combined heater control signal code is less than a flat range threshold 612. If the difference between the second combined heater control signal code and the first combined heater control signal code is less than the flat range threshold 612, the input signal 602 has a convex shape. If the difference between the second combined heater control signal code and the first combined heater control signal code is greater than the calibration threshold 606, the input signal has a flat shape (FIG. 8).

[0049] At operation 509, the control code circuitry 207 determines an offset heater control signal code that minimizes insertion loss. Operation 509 is similar to operation 307, and therefore, can be described with reference to method 1100 of FIG. 11. In one example, at operation 1102 the heater circuitry 208 increases file first heater control signal code. At operation 1104, the heater circuitry 208 determines if the maximum TIA control signal code 604 increases as file first heater control signal code increases. If the maximum TIA control signal code 604 increases as the first heater control signal increases, the process proceeds to operation 1106, and the heater circuitry 208 increases the first heater control signal code until the maximum TIA control signal code 604 stops increasing, and stores the first heater control signal when the maximum TIA control signal axle 604 stops increasing as the offset. On the other hand, at operation 1104, if the maximum TIA control signal code 604 does not increase, the process proceeds to operation 1108, and the heater circuitry 208 increases the second heater control signal code until the maximum TIA control signal code 604 stops increasing and stores the second heater control signal when the maximum TIA control signal code 604 stops increasing as the offset.

[0050] At operation 510, the optical element 106 is set to the offset. Operation 510 is similar to operation 308. The heater circuitry 208 sets the first and / or second heater control signal codes to the stored offset. Stated differently, the opticalelement 106 is initially calibrated to the center frequency of the optical signal generated by the optical source 104 with a reduced insertion loss.

[0051] At operation 512, the control code circuitry 207 determines the optical element 106 center wavelength is not calibrated to the frequency of the optical signal generated by the optical source 104. Operation 512 is similar to operation 310.

[0052] At operation 514, the control code circuitry 207 calibrates the optical element 106. The heater circuitry 208 finds the maximum TIA control signal code 604 by changing (increasing or decreasing) the combined heater control signal code (ie., changing the first and second heater control signal codes in a same direction and at a same rate) in the direction that the TIA control signal code increases until the TIA control signal code returns to the maximum TIA control signal code 604. In one example, the heater circuitry 208 updates the value of the first and second heater control signal code equal to the value the combined heater control signal code is changed to find the maximum TIA control signal code (i.e., during calibration). Once the TIA control signal returns to the maximum TIA control signal code 604, the method returns to operation 512.

[0053] FIG. 7 illustrates a flow diagram illustrating a method 700 for calibrating an optical element 106 having a flat shaped frequency response according to one or more examples. In this case, the optical element 106 and the input signal received by the controller circuitry 114 have corresponding flat frequency responses. FIG. 8 illustrates a graph 800 of an example input signal 802 having a flat shape according to one or more examples. As noted above, with the exception of determining the offset, the first and second heater control signal codes are changed in the same direction and at the same rate, and therefore, visualized and described as a combined heater control signal code.

[0054] At operation 702, the control code circuitry 207 determines teat the input signal 802 has a flat shape. In one example determining, by the control code circuitry 207, that the input signal 207 has a flat shape indudes operations 704- 708.

[0055] At operation 704, the heater circuitry 208 sweeps the combined heater control signal (i.e., sweeps the first and second heater control signal codes at a same rate in the same direction). Operation 704 is similar to operation 304. The heater circuitry 208 determines the TIA control code includes one maximum TIAcontrol signal code 804 within the search window. The heater circuitry 208 stores the maximum TIA control signal code 804 and the maximum combined heater control signal code corresponding to the maximum TIA control signal code 804.

[0056] At operation 706, the control code circuitry 207 determines a third TIA control signal code 808 and a fourth TIA control signal code 810 based on the calibration threshold 408. In one example, the heater circuitry 208 determines the third TIA control signal code 808 by changing the combined heater control signal code in a first direction, such as the decreasing direction (or the increasing direction). The first TIA control signal code 608 is equal to the maximum TIA control signal code 804 minus the calibration threshold 408. The fourth TIA control signal code 810 is also equal to the maximum TIA control signal code 804 minus the calibration threshold 408. The heater circuitry 208 determines a combined foird heater control signal code corresponding to the third TIA control signal code 808 by changing the combined heater control signal code until the TIA control signal code readies the tfiird TIA control signal code 808. In the illustrated example, the heater circuitry 208 decreases the combined heater control signal code until the third TIA control signal code 808 is reached. In another example, the heater circuitry 208 may increase the TIA control signal code until the third TIA control signal code 808 is readied. The heater circuitry 208 determines a fourth combined heater control signal code corresponding to the fourth TIA control signal code 810 by changing the combined heater control signal code in the opposite direction used to determine the foird TIA control signal code 808 until the TIA control signal code reaches fourth TIA control signal code 810.

[0057] At operation 708, the control code circuitry 207 (the heater circuitry 208) determines that the difference between the fourth combined heater control signal code and the third combined heater control signal code is greater than a flat range threshold 612. If the difference between the fourth combined heater control signal code and the third combined heater control signal code is greater than the flat range threshold 612, the input signal 802 has a flat shape. If the difference between the fourth combined heater control signal code and the third combined heater control signal code is less than the calibration threshold 606, the input signal has a convex shape (FIG. 6).

[0058] At operation 709, the control code drcuitry 207 determines an offset heater control signal code that minimizes insertion loss. Operation 709 is similar tooperation 307, and therefore, is described with reference to method 1100 of FIG. 11. In one example, at operation 1102, the heater circuitry 208 increases the first heater control signal code. At operation 1104, the heater circuitry 208 determines whether the maximum Tl A control signal code 804 increases as the first heater control signal code increases. If the maximum TIA control signal code 804 increases as the first heater control signal increases, the process continues to operation 1106, and the heater circuitry 208 increases the first heater control signal code until the maximum TIA control signal code 804 stops increasing and stores the first heater control signal code as the offset. On the other hand, at operation 1104, if the maximum TIA control signal code 804 does not increase, the process continues to operation 1108, and the heater circuitry 208 increases the second heater control signal code until the maximum TIA control signal code 804 stops increasing and stores the second heater control signal as the offset.

[0059] At operation 710, the optical element 106 is set to the offset. Operation 710 is similar to operation 308. The heater circuitry 208 sets the first and / or second heater control signal codes to the stored offset. Stated differently, the optical element 106 is initially calibrated to the center frequency of the optical signal generated by the optical source 104 with a reduced insertion loss.

[0060] At operation 712, the control code circuitry 207 determines the optical element 106 center wavelength is not calibrated to the frequency of the optical signal generated by the optical source 104. Operation 712 is similar to operation 310. Once the control code circuitry 207 determines calibration is needed, the method moves onto operation 714.

[0061] At operation 714, the control code circuitry 207 calibrates the optical element. The control code circuitry 207 finds the maximum TIA control signal code 804. The heater circuitry 208 finds the maximum TIA control signal code 804 by changing (increasing or decreasing) the combined heater control signal code (i.e., the first and second heater control signals) in the direction that the TIA control signal code increases until the TIA control signal code reaches either the third TIA control signal code 808 or the fourth TIA control signal code 810. Then once the third TIA control signal code 808 or the fourth TIA control signal code 810 is readied, the heater drcuitry 208 adds or subtracts half of the third combined heater control signal code plus the fourth combined heater control signal code to the current combined heater controi signal code. In one example, the heater drcuitry208 updates the value of the first and second heater control signal code equal to the value the combined heater control signal code is changed during calibration. For example, if the heater circuitry 208 finds the third TIA control signal code 808, the heater circuitry 208 will add half of third combined heater control signal code plus the fourth combined heater control signal code to the toird combined heater control signal code to return to the maximum TIA control signal code 804. On the other hand, if the heater circuitry 208 finds the fourth TIA control signal code 810, the heater circuitry 208 will add half of toird combined heater control signal code plus the fourth combined heater control signal code to the fourth combined heater control signal code to return to the maximum TIA control signal code 804. Once the TIA control signal returns to the maximum TIA control signal code 804, the method returns to operation 712.

[0062] FIG. 9 illustrates a flow diagram illustrating a method 900 for calibrating an optical element 106 having a concave shaped frequency response according to one or more examples, in this case, the optical element 106 and the input signal received by the controller circuitry 114 have corresponding concave frequency responses. FIG. 10 illustrates a graph 1000 of an example input signal 1002 according to one or more examples. As noted above, with the exception of determining the offset, the first and second heater control signal codes are changed in the same direction and at the same rate, and therefore, visualized and described as a combined heater control signal code.

[0063] At operation 902, the control code circuitry 207 determines that the input signal 1002 has a concave shape. The heater circuitry 208 determines that the input signal 1002 has a concave shape by determining two maximums, a first maximum TIA control signal code 1004a and a second maximum TIA control signal code 1004b. After the heater circuitry 208 determines the two maximums, the heater circuitry determines a minimum TIA control signal code 1006 that corresponds to the center frequency. The heater circuitry 208 stores the minimum TIA control signal code 1006 and the corresponding minimum heater control signal code.

[0064] At operation 903, the control code circuitry 207 determines an offset heater control signed code that minimizes insertion loss. Operation 903 is similar to operation 307, and therefore, is described with reference to method 1100 of FIG. 11. In one example, at operation 1102, the heater circuitry 208 increases the firstheater control signed code. At operation 1104, the heater circuitry 208 determines whether the minimum TIA control signal code 1006 increases as the first heater control signal code is increased. If the minimum TIA control signal code 1006 increases as the first heater control signal increases, the process continues to operation 1106, and the heater circuitry 208 increases the first heater control signal code until the minimum TIA control signal code 1006 stops increasing and stores the first heater control signal as the offset. On the other hand, at operation 1104, if the minimum TIA control signal code 1006 does not increase, the process continues to operation 1108, and the heater circuitry 208 increases the second heater control signal code until the minimum TIA control signal code 1006 stops increasing and stores the second heater control signal as the offset.

[0065] At operation 904, the optical element 106 is set to the offset (operation 308). The heater circuitry 208 sets the first and / or second heater control signal codes to the stored offset. Stated differently, the optical element 106 is initially calibrated to the center wavelength of the optical signal generated by the optical source 104 with a reduced insertion loss.

[0066] At operation 906, the control code circuitry 207 determines the optical element 106 center wavelength is not calibrated to the frequency of the optical signal generated by the optical source 104. Operation 906 is similar to operation 310. Once the control code circuitry 207 determines calibration is need, the method moves onto operation 908.

[0067] At operation 908, the heater circuitry 208 determines a direction in which the TIA control signal code begins to decrease. For example, if the TIA control signal code is to the left of the minimum TIA control signal code 1006. the decreasing direction would be to the right. Thus, the heater circuitry 208 would determine the TIA control signal code increases in the inareasing direction of the combined heater control signal code.

[0068] At operation 910, the heater circuitry 208 determines whether the TIA control signal code begins to increase before the TIA control signal code tolls below a concave threshold 1008. The concave threshold 1008 is a pre-determined value. The concave threshold may be equal to or different from the calibration threshold 408. The concave threshold may be greater than or less than the calibration threshold 408. During operation 910 the heater circuitry 208 is changing the combined heater signal control code in direction in which the TIA control signalcode is decreasing. If the TIA control signal code begins to increase prior to reaching the concave threshold 1008 (Le., the TIA control signal code was located between the two maximums), the method proceeds to operation 918 and the heater circuitry 208 determines that the optical element 106 center wavelength is calibrated. Then the method returns to operation 910.

[0069] On the other hand, if the heater circuitry 208 determines that the TIA control signal code decreases past the concave threshold 1008, the method proceeds to operation 912, and the heater circuitry 208 will change the combined heater control signal code in the opposite direction. For example, if the TIA control signal code is to the left of the first maximum TIA control signal code 1004a (outside of file valley), file heater circuitry 208 will decrease file combined heater control signal code. However, the combined heater control signal code will fall below the concave threshold 1008 and would not increase again. Once the heater control signal falls below the concave threshold 1008, the heater circuitry 208 will change the combined heater control signal code in file opposite direction (increase the combined heater control signal code).

[0070] At operation 914, the heater circuitry 208 determines file TIA control signal code begins the decrease. The heater circuitry 208 will continue to change the combined heater control signal code in the same direction after determining the TIA control signal code deweases until file TIA control signal code stops decreasing (i.e., reaches the valley).

[0071] At operation 916 the heater circuitry 208 determines the TIA control signal code begins to increase again. Here, the method proceeds to operation 918, and the heater circuitry 208 determines that the optical element 106 center wavelength is calibrated, and file heater circuitry 208 updates the value of the first and second heater control signal code equal to file value the combined heater control signal code is changed during calibration. At operation 918, after determining the optical element 106 is calibrated and the method returns to operation 910.

[0072] The disclosed technology may be expressed through one or more of file following non-limiting examples.

[0073] Example 1. An integrated circuit (IC) device including: an optoelectronic circuitry having a first heater and a second heater; and a controller circuitry having an input coupled to a photodiode of the optoelectronic circuitry andan output coupled to the first heater and the second heater of the optoelectronic circuitry, the controller circuitry configured to determine an offset from a baseline heater control signal code based on a transimpedance (TIA) control signal code of an input signal received from the photodiode, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

[0074] Example 2. The IC device of Example 1 , wherein the confroller circuitry further includes: a TIA circuitry having a first input coupled to the photodiode and an output coupled to an input of a slicer circuitry; and a control code circuitry having an input coupled to an output of the slicer circuitry, the control code circuitry configured to output a TIA control signal to an input of a first digital- to-analog converter (DAC), a first heater control signal code to a second DAC, and a second heater control signal code to a third DAC, the TIA circuitry configured to cancel out a direct current (DC) component of the input signal from the photodiode based on the TIA control signal, the second DAC configured to output the first heater control signal to the first heater based on the first heater control signal code, and the third DAC configured to output the second heater control signal to the second heater based on the second heater control signal code.

[0075] Example 3. The IC device of Example 2. wherein the control code circuitry is configured to determine that the input signal indudes a convex shape or a fiat shape based on the input signal having one maximum TIA control signal code.

[0076] Example 4. The IC device of Example 3, wherein the control code circuitry is configured to increase the first heater control signal code, and store the first heater control signal code based on the one maximum TIA control signal code ceasing to increase as the offset.

[0077] Example 5. The IC device of Example 4, wherein the control code circuitry is configured to: increase the first heater control signal code; and increase the second heater control signal code, and store the second heater control signal code based on the one maximum TIA control signal code ceasing to increase as the offset.

[0078] Example 6. The IC device of Example 2, wherein the control code circuitry is configured to determine the input signal has a concave shape based on the input signal having two maximum TIA control signal codes and determine oneminimum TIA control signal code located between the two maximum TIA control signal codes.

[0079] Example 7. The IC device of Example 6, wherein the control code circuitry is configured to increase the first heater control signal code, and store the first heater control signal code based on file one minimum TIA control signal code ceasing to increase as the offset.

[0080] Example 8. The IC device of Example 6, wherein the control code circuitry is configured to: increase the first heater control signal code; and increase the second heater control signal code, and store the second heater control signal code based on the one minimum TIA control signal code ceasing to increase as the offset.

[0081] Example 9. The IC device of Example 2, wherein the first heater control signal and the second heater control signal are provided differentially to one another.

[0082] Example 10. A controller circuitry including: a transimpedance (TIA) circuitry having an input coupled to a photodiode of an optoelectronic circuitry and an output coupled to a control code circuitry, the TIA circuitry configured to generate a voltage signal based on an input signal received from the photodiode, the control code circuitry including: a TIA control circuitry having an input connected to the output of the TIA circuitry; and a heater circuitry having an input coupled to an output of the TIA control circuitry and a first output coupled to a first heater and a second output coupled to a second heater of the optoelectronic circuitry, the heater circuitry configured to determine an offset from a baseline heater control signal code of the optoelectronic circuitry based on the voltage signal received from the TIA circuitry, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

[0083] Example 11. The controller circuitry of Example 10, wherein the control code circuitry is configured to adjust the first heater control signal and the second heater control signal to determine the offset

[0084] Example 12. The controller circuitry of Example 11 , wherein the heater circuitry is configured to determine that the input signal includes a concave shape, a convex shape, or a flat shape based on the voltage signal.

[0085] Example 13. A method for operating an integrated circuit device including: determining, by a controller circuitry having an input coupled to a photodiode of an opthelectronic circuitry, a first output coupled to a first heater, and a second output coupled to a second heater of the optoelectronic circuitry, a shape of an input signal received from the photodiode; and determining, by the controller circuitry, an offset from a baseline heater control signal code of the optoelectronic circuitry based on the shape of the input signal, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

[0086] Example 14. The method of Example 13, further including determining that the input signal indudes a convex shape or a flat shape based on the input signal having one maximum TIA control signal code.

[0087] Example 15. The method of Example 14, further including increasing the first heater control signal, and storing the first heater control signal when the one maximum TIA control signal code ceases to increase as the offset.

[0088] Example 16. The method of Example 14, further induding: increasing the first heater control signal; and increasing the second heater control signal, and storing the second heater control signal when the one maximum TIA control signal code ceases to increase as the offset

[0089] Example 17. The method of Example 13, further including: determining the input signal has a concave shape based on the input signal having two maximum TIA control signal codes; and determining one minimum TIA control signal code located between the two maximum TIA control signal codes.

[0090] Example 18. The method of Example 17, further induding increasing the first heater control signal, and storing the first heater control signal when the one minimum TIA control signal code ceases to increase as the offset.

[0091] Example 19. The method of Example 17, further induding: increasing the first heater control signal code; and increasing the second heater control signal code, and storing the second heater control signal code when the one minimum TIA control signal code ceases to increase as the offset.

[0092] 20. The method of Example 13, the first heater control signal and the second heater control signal are provided differentially to one another.

[0093] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departingfrom the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

CLAIMSWhat is claimed is:

1. An integrated circuit (IC) device comprising: an optoelectronic circuitry having a first heater and a second heater; and a controller circuitry having an input coupled to a photodiode of the optoelectronic circuitry and an output coupled to the first heater and the second heater of the optoelectronic circuitry, the controller circuitry configured to determine an offset from a baseline heater control signal code based on a transimpedance (TIA) control signal code of an input signal received from the photodiode, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

2. The IC device of claim 1 , wherein the controller circuitry further comprises: a TIA circuitry having a first input coupled to the photodiode and an output coupled to an input of a slicer circuitry; and a control code circuitry having an input coupled to an output of the slicer circuitry, the control code circuitry configured to output a TIA control signal to an input of a first digital-to-analog converter (DAC), a first heater control signal code to a second DAC, and a second heater control signal code to a third DAC, the TIA circuitry configured to cancel out a direct current (DC) component of the input signal from the photodiode based on the TIA control signal, the second DAC configured to output the first heater control signal to the first heater based on the first heater control signal code, and the third DAC configured to output the second heater control signal to the second heater based on the second heater control signal code.

3. The IC device of claim 2, wherein the control code circuitry is configured to determine that the input signal comprises a convex shape or a flat shape based on the input signal having one maximum TIA control signal code.

4. The IC device of claim 3, wherein the control code circuitry is configured to increase the first heater control signal code, and store the first heater control signalcode based on the one maximum TIA control signal code ceasing to increase as the offset.

5. The IC device of claim 4, wherein the control code circuitry is configured to: increase the first heater control signal code; and increase the second heater control signal code, and store the second heater control signal code based on the one maximum TIA control signal code ceasing to increase as the offset.

6. The IC device of claim 2, wherein the control code circuitry is configured to determine the input signal has a concave shape based on the input signal having two maximum TIA control signal codes and determine one minimum TIA control signal code located between the two maximum TIA control signal codes.

7. The IC device of claim 6, wherein the control code circuitry is configured to increase the first heater control signal code, and store the first heater control signal code based on the one minimum TIA control signal code ceasing to increase as the offset.

8. The IC device of claim 6, wherein the control code circuitry is configured to: increase the first heater control signal code; and increase the second heater control signal code, and store the second heater control signal code based on the one minimum TIA control signal code ceasing to increase as the offset.

9. The IC device of claim 2, wherein the first heater control signal and the second heater control signal are provided differentially to one another.

10. A controller circuitry comprising: a transimpedance (TIA) circuitry having an input coupled to a photodiode of an optoelectronic circuitry and an output coupled to a control code circuitry, the TIA circuitry configured to generate a voltage signal based on an input signal received from the photodiode, the control code circuitry comprising:a TIA control circuitry having an input connected to the output of the TIA circuitry; and a heater circuitry having an input coupled to an output of the TIA control circuitry and a first output coupled to a first heater and a second output coupled to a second heater of the optoelectronic circuitry, the heater circuitry configured to determine an offset from a baseline heater control signal code of the optoelectronic circuitry based on the voltage signal received from the TIA circuitry, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

11. The controller circuitry of claim 10, wherein the control code circuitry is configured to adjust the first heater control signal and the second heater control signal to determine the offset.

12. The controller circuitry of claim 11 , wherein the heater circuitry is configured to determine that the input signal comprises a concave shape, a convex shape, or a flat shape based on the voltage signal.

13. A method for operating an integrated circuit device comprising: determining, by a controller circuitry having an input coupled to a photodiode of an optoelectronic circuitry, a first output coupled to a first heater, and a second output coupled to a second heater of the optoelectronic circuitry, a shape of an input signal received from the photodiode; and determining, by the controller circuitry, an offset from a baseline heater control signal code of the optoelectronic circuitry based on the shape of the input signal, and provide a first heater control signal to the first heater and a second heater control signal to the second heater based on the offset of the optoelectronic circuitry.

14. The method of claim 13, further comprising determining that the input signal comprises a convex shape or a flat shape based on the input signal having one maximum TIA control signal code.

15. The method of claim 14, further comprising at least one or more selected from the group consisting of:(A) increasing the first heater control signal, and storing the first heater control signal when the one maximum TIA control signal code ceases to increase as the offset;(B) increasing the first heater control signal; and increasing the second heater control signal, and storing the second heater control signal when the one maximum TIA control signal code ceases to increase as the offset:(C) determining the input signal has a concave shape based on the input signal having two maximum TIA control signal codes; and determining one minimum TIA control signal code located between the two maximum TIA control signal codes;(D) determining the input signal has a concave shape based on the input signal having two maximum TIA control signal codes; determining one minimum TIA control signal code located between the two maximum TIA control signal codes; and increasing the first heater control signal, and storing the first heater control signal when the one minimum TIA control signal code ceases to increase as the offset; and(E) determining the input signal has a concave shape based on the input signal having two maximum TIA control signal codes; determining one minimum TIA control signal code located between the two maximum TIA control signal codes; increasing the first heater control signal code; and increasing the second heater control signal code, and storing the second heater control signal code when the one minimum TIA control signal code ceases to increase as the offset.

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