Optical temperature measurement in photonic circuits

Abstract

Optical temperature measurement in photonic circuits. Temperature measurements of photonic circuit components can be measured optically using the temperature-dependent spectral characteristics of the photonic device itself to be monitored, or the temperature-dependent spectral characteristics of a separate optical temperature sensor placed in its vicinity. Such optical temperature measurements can provide more accurate temperature information and help improve thermal design by supporting the measurement of the temperature of individual photonic devices rather than just the temperature of the overall photonic circuit.

Description

Technical Field

[0001] The present disclosure relates to methods and sensors for temperature measurement in photonic integrated circuits (PICs). Background Art

[0002] Many photonic circuit components (including, for example, lasers and optical modulators) are highly temperature sensitive in their performance. Therefore, an important aspect of the design of photonic circuits is the thermal design of the PIC, which specifies the layout of heat sources (including heat generating photonic devices) as well as heat sinks and heat sinks, with the goal of optimizing the range of ambient temperatures over which the PIC will operate reliably. Thermal design and verification rely on accurate temperature measurements. However, because temperatures can vary greatly across the PIC, temperature measurements of the PIC as a whole are often inappropriate. On the other hand, while temperature measurements of individual PIC elements are desirable for thermal design purposes, temperature measurements of individual PIC elements are difficult because placement of thermal sensors in the vicinity of the associated photonic components is often hampered by space limitations and / or the possibility of electrical interference of the thermal sensors (e.g., resistance temperature sensors (RTDs)) with the photonic components. Summary of the invention

[0003] A photonic integrated circuit (PIC) includes: a first optical waveguide in a device layer of a substrate; an active photonic device coupled to the first optical waveguide, the active photonic device including a diode junction; and a second optical waveguide in the device layer of the substrate, the second optical waveguide including an optical temperature sensor within sensing proximity of the diode junction.

[0004] A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling light into the photonic device via a first optical waveguide; optically interrogating an optical temperature sensor via a second optical waveguide to determine a spectral characteristic of the optical temperature sensor, the optical temperature sensor being placed within a sensing proximity of the photonic device; and computationally converting the determined spectral characteristic to a measured temperature based on a calibrated temperature dependency of the spectral characteristic.

[0005] A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling a first optical signal into the photonic device in a first propagation direction; optically interrogating the photonic device using a second optical signal to determine a spectral characteristic of the photonic device, the second optical signal being coupled into the photonic device in a second propagation direction, the second propagation direction being opposite to the first propagation direction; and computationally converting the determined spectral characteristic to a measured temperature based on a calibrated temperature dependency of the spectral characteristic. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a schematic block diagram of an example photonic transceiver configured to enable optical temperature measurement using an integrated optical filter in accordance with various embodiments.

[0007] Figure 2 is a schematic block diagram of an example photonic transceiver configured to enable optical temperature measurement using an integrated multiplexer in accordance with various embodiments.

[0008] Figure 3A-3C It shows the various embodiments Figure 2 Graph of example spectral responses of a multiplexer of a transceiver operated in a low power mode, the spectral responses being measured over two spectral ranges at receiver case temperatures of 35 degrees Celsius, 70 degrees Celsius, and 80 degrees Celsius, respectively.

[0009] Figure 3D It shows the various embodiments Figure 2 A graph of an example spectral response of a multiplexer of a transceiver operated in a high power mode, the spectral response being measured at a receiver case temperature of 70 degrees Celsius.

[0010] Figure 4 is a flow chart of a method of calibrating and operating a photonic device having separate mission mode functionality for optical temperature measurement according to various embodiments.

[0011] Figure 5 is a schematic block diagram of an example photonic transceiver equipped with an additional optical temperature sensor near the laser and modulator according to various embodiments.

[0012] Fig. 6A and Figure 6B 2. A top view and a cross-sectional side view, respectively, of a Bragg grating-based optical temperature sensor placed adjacent to a compound semiconductor laser of a PIC according to various embodiments.

[0013] Figure 7 is a flow chart of a method for calibrating and using a dedicated optical temperature measurement sensor without mission mode functionality for temperature measurement of a photonic device in accordance with various embodiments. DETAILED DESCRIPTION

[0014] Various methods disclosed herein are used to optically measure the temperature of photonic circuit components, rather than using RTDs or other conventional electronic temperature sensors. In some embodiments, photonic devices that implement various mission modes (e.g., multiplexing / demultiplexing or modulating light) functions are calibrated during use and selectively operated to double as temperature sensors. For example, temperature-dependent spectral shifts in optical filters (such as Mach-Zehnder interferometers (MZIs) or asymmetric MZIs (AMZIs), arrayed waveguide gratings (AWGs), or electro-absorption modulators (EAMs)) can be exploited to measure the temperature of these components. In other embodiments, PICs are enhanced with optical components that do not have mission mode functions, but are used only to determine temperature based on temperature-dependent changes in their optical (e.g., spectral) characteristics. Advantageously, such optical temperature sensors can be placed in close proximity to diode junctions in lasers, modulators, photodetectors, etc., without electrical interference. Furthermore, in hybrid platforms that use compound semiconductor (e.g., III-V) active devices on top of a silicon device layer containing waveguides and other optical passive components, optical temperature sensors can be implemented in the silicon device layer below the active device whose temperature is to be measured, circumventing the spatial limitations present in the compound semiconductor layer.

[0015] To understand the benefits provided by measuring the temperature of individual photonic components rather than the temperature of the overall photonic circuit, consider, for example, an optical transceiver comprising four lasers. Assume that the temperature varies twenty degrees across the lasers at maximum operating current, for example, due to thermal crosstalk from other nearby heat sources (such as optical components within the PIC or electronic components within an adjacent electronic integrated circuit (EIC)). The hottest laser among the lasers may have a significantly (e.g., three times) shorter operating lifetime than the coldest laser among the lasers, and may also suffer from instabilities (e.g., causing mode hopping) and output lower optical power. However, laser instabilities and reduced output power are not necessarily indicative of hot spots, but may also be due to laser damage caused during PIC manufacturing or assembly, or due to defects in the optical design. Therefore, it is important to distinguish temperature effects from laser damage and / or optical design issues. Using optical temperature measurement as described herein, the temperature at the junctions of all four lasers can be measured separately and more accurately than using electronic temperature measurement, which enables identification of hot spots and then modification of the thermal design (including, for example, the heat sink, the spacing of components within the PIC die and adjacent to the PIC die, etc.) to reduce the hot spot temperature.

[0016] Various example embodiments of devices, systems, and methods for optical temperature measurement in a PIC will be described below with reference to the accompanying drawings.

[0017] Figure 1 is a schematic block diagram of an example photonic transceiver 100 configured to enable optical temperature measurement using an integrated optical filter 102, according to various embodiments. Along a transmitter path 104, the photonic transceiver 100 includes a laser that generates an optical carrier signal, an optical modulator 108 that imparts data to the optical carrier signal via amplitude modulation, and the optical filter 102 that acts as a wavelength reference for the laser 106. In addition, the photonic transceiver 100 includes a photodetector 110 that functions as an optical receiver and an associated receiver path 111. In addition, to support temperature measurement using the optical filter 102, the transceiver 100 includes optical taps 112, 114 that bracket the optical filter 102, each optical tap separating a fixed component of light (e.g., 5%) and having two outputs that direct the separated light to two corresponding monitoring photodiodes 116, 117 and monitoring photodiodes 118, 119, the monitoring photodiodes 116, 117 and monitoring photodiodes 118, 119 allowing measurement of optical power entering and leaving the optical filter 102 in either direction of light propagation in the transmitter path 104. For example, the photonic transceiver 100 may be implemented in a hybrid material platform including a compound semiconductor (e.g., III-V) active device component of a laser 106, a modulator 108, a receiver photodetector 110, and monitor photodiodes 116, 117, 118, 119 coupled to waveguides (e.g., implementing a transmitter path 104 and a receiver path 111) and other passive devices or device components formed in a silicon device layer (e.g., of a silicon-on-insulator (SOI) substrate).

[0018] For example, the optical filter 102 may be implemented as an AMZI characterized by the optical path length difference between the two interferometer arms, which produces an interference signal whose intensity varies periodically with the frequency of the light. The period of this variation is the free spectral range (FSR) of the filter 102. The AMZI (or other optical filter 102) is calibrated to achieve a specified transmittance at a specified reference wavelength, with the transmittance measured as the ratio of the output optical power to the input optical power. When the wavelength of the laser 106 deviates from the reference wavelength, the deviation will be reflected in a change in the transmittance of the optical filter 102 (as determined from the input optical power and the output optical power measured by the photodiodes 118, 116, respectively), providing feedback to tune the laser 106 to the reference wavelength. However, the transmittance spectrum of the optical filter 102 is typically temperature dependent, and the temperature of the optical filter 102 may vary during mission mode use (e.g., due to heat dissipated by the laser 106). It is desirable to design the photonic circuit to minimize such heating. Although the temperature of filter 102 is monitored during use and differences from the temperature at which the filter is calibrated to a wavelength reference are actively compensated (eg, using an integrated heater or thermoelectric cooler), it is desirable to minimize the power requirements of such active thermal adjustment.

[0019] According to various embodiments, during thermal design of the transceiver 100, the optical filter 102 is used as its own temperature sensor. For this purpose, an external laser 120 couples light into the optical filter 102 (which is used to transmit a modulated optical signal in mission mode), for example, via a transmitter output port 122, and the transmittance and / or output optical power (in the propagation direction of the light from the external laser 120) of the filter 102 is measured. In some embodiments, the external laser 120 is a tunable laser source that is stepped over a wavelength range to identify the wavelength at which the transmittance reaches a peak (maximum) or a null (minimum) via a measurement of the relative optical output power at a monitoring photodiode 119 after the filter 102, the wavelength range for example including one or more FSRs of the filter 102. Based on the known temperature dependence of the peak or null wavelength as determined by calibration, the measured peak wavelength or null wavelength can then be converted into a measured temperature of the optical filter 102. In an alternative embodiment, the external laser 120 is operated at a single specified wavelength, and the transmittance of the optical filter 102 at that wavelength is measured using monitoring photodiodes 117, 119 before and after the filter 102. Based on the known temperature dependence of the filter transmittance at the selected wavelength of the external laser 120, the temperature of the filter 102 can then be calculated from the measured transmittance.

[0020] To study the effects of other transceiver components on the temperature of the optical filter 102, the thermal design involves operating the transceiver 100 in mission mode with the transceiver laser 106 at full mission mode power. Thus, when the optical filter 102 is optically interrogated by an optical signal from the external laser 120, it simultaneously receives an optical signal from the transceiver laser 106. The two optical signals propagate in opposite directions so that the measurement of the transmitted portion of the external laser signal is not affected by the signal from the transceiver laser 106. The external laser 120 may include an optical isolator that prevents damage caused by light received from the transceiver laser 106. Conversely, light from the external laser 120 that reaches the transceiver laser 106 may somewhat degrade the optical performance of the transceiver laser 106 during the measurement of the filter temperature, but that is generally not a problem at the design stage. Note that with appropriate enhancements to the transceiver 100 (e.g., isolators to protect the modulator 108 and laser 106 from light injected into the transmitter path 104 by the external laser 120), or by using an external laser wavelength that is far detuned from the transceiver operating wavelength, it is also possible in principle to use the optical filter 102 to monitor its own temperature during normal mission mode throughout the lifetime of the transceiver.

[0021] although Figure 1 Only a single channel of the photonic transceiver 100 is shown, but it will be apparent to one of ordinary skill in the art that a multi-channel transceiver can be implemented by duplicating the transmitter and receiver components. The temperature of the filter 102 in each channel can then be measured in the same manner as described above, with the external laser 120 coupled sequentially (or multiple external lasers simultaneously) into the various transmitter paths.

[0022] To provide another example of temperature measurement of a photonic device with separate mission mode functionality, Figure 22 is a schematic block diagram of an example photonic transceiver 200 configured to enable optical temperature measurement using an integrated multiplexer according to various embodiments. The transceiver 200 includes four channels, each channel including a laser 200 and an associated EAM 204 to impart data onto an optical carrier signal. The laser 202 emits light at four different wavelengths λ0, λ1, λ2, and λ3, and the modulated optical signals of the four wavelengths are multiplexed into a combined optical transceiver output signal in two stages. The laser wavelengths are evenly spaced, for example, λ0≈1295nm, λ1≈1300nm, λ2≈1305nm, and λ3≈1310nm. In this example, the component whose temperature is to be measured is a multiplexer 206 in the first stage, which multiplexes λ0, λ1 and λ2, λ3, respectively, into two partially multiplexed signals provided as inputs to a second stage multiplexer 208. For example, multiplexers 206 (and 208) may be implemented as AMZIs, each having two input ports 209 and one common output port (in the direction of propagation of light from transceiver laser 202).

[0023] To measure the temperature of the first stage multiplexers 206, they are interrogated using an external tunable laser (not shown) that is coupled into the transceiver via the transmitter output port and the second stage multiplexer 208 (where the output port of the multiplexer may also constitute the transmitter output port) and propagates in a direction opposite to the direction of the optical signal emitted by the transceiver laser 202. The external laser is swept over a range of wavelengths to determine the spectral transmittance peaks of the light output at one or both input ports 209 of the multiplexer 206 (corresponding to the output ports in the direction of propagation of the external laser signal). If the multiplexers 26 are at their target operating temperatures, these transmittance peaks will appear at λ0, λ1, λ2, and λ3. Any deviation from the target operating temperature will generally cause a shift of the peaks to shorter or longer wavelengths (depending on whether the temperature is above or below the target operating temperature). Note that although transceiver 200 includes taps 210 and associated monitor photodiodes 212 at various inputs and outputs between transceiver laser 202, EAM 204, and multiplexers 206, 208, those taps 210 are configured to measure only light in the direction from transceiver laser 202 to the transmitter output port, and are therefore not used to measure the transmitted portion of the external laser signal at multiplexer 206. Instead of configuring taps 210 and adding monitor photodiodes 212 to measure light from an external laser in the circuit, as shown in FIG. Figure 1As done in transceiver 100 and in principle can also be implemented in transceiver 200, the depicted embodiment uses EAM 204 to measure the optical power of an external laser signal transmitted by multiplexer 206. EAM 204 is operated with a high voltage bias, which causes high photon absorption, and the photon current generated by the EAM is proportional to the optical power of the signal being measured.

[0024] Figure 3A-3C It shows the various embodiments Figure 2 Graphs of example spectral responses of the multiplexer of transceiver 200 operated in low power mode ("LPM"), the spectral responses being measured over two spectral ranges at receiver case temperatures of 35 degrees Celsius, 70 degrees Celsius, and 80 degrees Celsius, respectively. In low power mode, power is sufficient to operate and read the EAM 204, but the transceiver laser 202 and other components are turned off. It is assumed that self-heating of the transceiver 200 is negligible in low power mode, and that the transceiver 200 has time to reach equilibrium so that the multiplexer temperature approaches the receiver case temperature. The four graphs in each figure show the photon current ("IPh") at the EAM 204, which is caused by measurements of the optical power output at the input port 209 of the multiplexer 206. It can be seen that the transmittance peaks and nulls of the four optical signals, as reflected in the photon current, are approximately evenly distributed over the FSR (mirroring the even spacing of the laser wavelengths of the transceiver laser 202, as expected). Compared with 35 degrees Celsius ( Figure 3A ), 70 degrees Celsius ( Figure 3B ) and 80 degrees Celsius ( Figure 3C ), a shift of the spectral response toward longer wavelengths can be observed. For example, the transmission minimum associated with λ2 measured at the multiplexer input port 209 shifts from about 1297.5 nm at 35 degrees Celsius to about 1300 nm at 70 degrees Celsius, and almost 1301 nanometers at 80 degrees Celsius.

[0025] Figure 3D It shows the various embodiments Figure 2 2 is a graph of an example spectral response of the multiplexer of transceiver 200 operated in high power mode ("HPM"), the spectral response being measured at a receiver case temperature of 70 degrees Celsius. In high power mode, the transceiver laser is turned on and tends to heat other transceiver components including multiplexer 206. Therefore, it is expected that the local temperature of multiplexer 206 is higher than the ambient temperature. Indeed, the signal associated with λ2 measured at multiplexer input port 209 in high power mode is compared to the signal associated with λ2 measured at multiplexer input port 209 in low power mode at the same receiver case temperature of 70 degrees Celsius ( Figure 3B), it can be seen that the transmittance minimum has shifted to almost 1301 nanometers, indicating that the temperature of the multiplexer 206 is about 80 degrees Celsius.

[0026] The described methods for optical temperature measurement of photonic devices, illustrated above with wavelength references and splitters as examples, using those same photonic devices as sensors, are generally applicable to any photonic device having and suitable for measuring certain temperature-related spectral characteristics. For example, such characteristics may include spectral features such as the wavelength or amplitude of the peak or zero of an optical transmission, reflection, or absorption spectrum, and the transmission, reflection, or absorption level at a given wavelength. Photonic devices suitable for optical interrogation for temperature measurement include, for example, devices based on symmetric or asymmetric MZIs, AWGs, or EAMs, as they can implement mission mode functions such as optical filters, multiplexers / demultiplexers, or modulators.

[0027] Various modifications may be made to the above method. For example, while interrogation of a photonic device of interest would typically utilize a laser external to the PIC in practice, in principle an internal laser could also be used (e.g., where its output signal is routed to propagate in a direction opposite to the direction of light used for the mission mode function of the photonic device). An internal laser suitable for temperature monitoring may be low power to prevent self-heating, and may be calibrated at two or more temperatures using an external wavelength monitor such as an optical spectrum analyzer (OSA). Alternatively, the internal laser may be placed in a relatively temperature stable portion of the PIC (e.g., a low power density region) while it monitors a hot spot on the other side of the PIC (e.g., in a high power density region). Furthermore, while the example embodiments described above use a photodetector integrated into the PIC, the optical signal measured for temperature determination purposes may also be routed off-chip for detection by an external photodetector. For example, an external photodetector may be used when the spectral characteristics of the photonic device are measured in reflection mode rather than transmission mode.

[0028] Figure 44 is a flow chart of a method 400 of calibrating and operating a photonic device having a separate mission mode functionality for optical temperature measurement according to various embodiments. During a calibration phase 402, a PIC (e.g., a photonic transceiver PIC) including a photonic device is placed in a controlled temperature environment (e.g., an oven) set to an initial temperature and allowed to thermally equilibrate, for example, for about an hour (act 404). After sufficient time has passed, it can be assumed that the entire PIC (including the photonic device of interest) is substantially at the set temperature (e.g., within a few degrees Kelvin, or typically within some acceptable margin of deviation in a given application, which affects the equilibration time). If an integrated photodetector is to be used for calibration, the PIC is turned on and operated in a low power mode sufficient to use the integrated photodetector but avoid other power consumption to minimize heating (act 406). In this state, the photonic device is optically interrogated to determine the spectral characteristics of the photonic device (act 408). Typically (although not necessarily), light for this purpose is coupled into the photonic device from an external laser source. For example, to measure the transmission spectrum of an optical modulator, multiplexer, or wavelength reference in a transmitter path of a photonic transceiver, an optical signal from an external laser can be coupled into the photonic device via the transmitter path in a direction or propagation opposite to the propagation direction of the mission mode optical signal from the PIC laser. The wavelength of the external laser can be stepped over a wavelength range (typically including one or more FSRs of the photonic device), and the wavelengths at the zeros and / or peaks of the measured spectral response are recorded accordingly.

[0029] The controlled temperature environment is then changed to a second temperature (act 410), the PIC is allowed to thermally equilibrate at the second temperature (act 404), and the interrogation of the photonic device for measuring spectral characteristics during the low power mode is repeated at the second temperature (act 408). Optionally, the spectral characteristics are measured at one or more additional temperatures, depending on the desired accuracy of the thermal characterization, as well as cost considerations and time considerations. For example, in some embodiments, calibration (stage 402) is performed only once on one part and is then applied to many parts; in this case, measuring at several or dozens of temperatures may be feasible. On the other hand, if calibration is performed per part, it may rely on only two measurements, for example, taken at two temperatures that cause a relative shift in the spectral characteristics that is less than the FSR.

[0030] The values ​​of the determined spectral characteristic at two or more temperatures may then be used to determine a temperature dependency of the spectral characteristic over a continuous range of temperatures, which may involve interpolating between the temperatures at which measurements were performed and / or exceeding the temperatures at which measurements were performed (action 412). For example, based on measurements of the transmittance peak wavelength or zero-point wavelengths λ1 and λ2 at two respective temperatures T1 and T2, a temperature-dependent wavelength shift may be calculated as dλ / dT=(λ2-λ1) / (T2-T1). The determined temperature dependency is stored in a memory for later use. In some embodiments, the temperature dependency is stored in a memory of an EIC associated with a PIC, for example, an EIC providing control signals and data readout for a transceiver PIC, where the temperature dependency of a transceiver component of interest may be stored together with other calibration data, such as target deviations and modulation settings, etc. However, storage of calibrated temperature dependencies of spectral characteristics is also possible.

[0031] Once the calibration stage 402 is completed, the calibration data can be used to measure the temperature of the photonic device as part of characterizing the mission mode thermal performance of the PIC in stage 414. The PIC is now operated in mission mode, that is, powered on (which is typically a high power mode) and with a mission mode optical signal, e.g., from an internal PIC laser (such as a transceiver laser), coupled into the optical device in one direction (act 416). In order to measure the spectral characteristics of the photonic device, the device is interrogated using an interrogation signal propagating in the other direction (e.g., coupled into the PIC by an external laser) (act 418). Using the stored calibrated temperature dependence of the spectral characteristics, the measured spectral characteristics can then be converted to the temperature of the device (act 420). Continuing with the example of transmission wavelength measurement above, the transmittance peak wavelength or transmission zero wavelength measured during mission mode can be converted to the temperature of the photonic device according to: T=T1+(λ-λ1) / (dλ / dT).

[0032] The foregoing examples all illustrate embodiments in which the photonic device whose temperature is to be measured doubles as its own temperature sensor. In various alternative embodiments, the PIC instead includes one or more dedicated optical temperature sensors, i.e., additional components that do not implement any independent mission mode functions. Each such dedicated temperature sensor is placed in "sensing proximity" to the photonic device whose temperature is to be monitored, meaning that the temperature sensor is physically close enough to a location in the photonic device where the temperature is substantially the same temperature as the temperature of interest (e.g., within a few degrees Kelvin). In various embodiments, the photonic device to be monitored is an optically active device that includes a diode junction, and the temperature at the junction is to be measured. In some embodiments, the temperature sensor is within sensing proximity to the location of interest (such as a diode junction in an active photonic device), if it is less than 10m, preferably less than 3m from the location.

[0033] Figure 5 5 is a schematic block diagram of an example photonic transceiver 500, according to various embodiments, which is equipped with additional optical temperature sensors 502, 504 near a laser 506 and a modulator 508. Here, the laser 506 and the modulator 508 are coupled to a first optical waveguide 510 implementing a transmission path, while the optical temperature sensors 502, 504 are placed in, or coupled to, respective second and third optical waveguides 512, 514 separated from the first waveguide 510. The photodetector 516 of the receiver is coupled to a fourth optical waveguide 518, which (as usual) implements the receiver path. All four waveguides 510, 512, 514, 518 have their own respective input / output ports.

[0034] As in Figure 1 As in the transceiver 100 of FIG. 1 , the laser 506 and modulator 508 of the transmitter, and the photodetector 516 of the receiver can be implemented in a composite semiconductor hybrid platform on silicon, where waveguides in the silicon device layer are coupled to optically active regions formed in a composite semiconductor (e.g., III-V) layer thereon. For example, the active photonic device can be or include a III-V pin diode structure with electrical connections to a p-type layer and an n-type layer to apply a voltage across the diode structure to form a diode junction in the intrinsic, active layer, as is known in the art. The temperature of the diode junction significantly affects the optical performance of the corresponding device and is therefore important to monitor. With electronic temperature sensor implementations, placing the sensor too close to the diode junction poses a risk of electrical interference. On the other hand, the optical temperature sensors 502, 504 do not have this problem and can therefore be placed much closer, which supports more accurate temperature measurements. For example, in some embodiments, the optical sensors 502, 504 are close to the diode junction to about 1 m.

[0035] Similar to the photonic device having mission mode functionality that doubles as a temperature sensor, the optical temperature sensors 502, 504 typically utilize temperature-related spectral characteristics (such as transmittance or reflectance peaks or zeros, or transmission or reflection levels at specified wavelengths) for temperature measurement. Examples of photonic devices used as temperature sensors 502, 504 include Bragg gratings, ring resonators, and AMZIs, all of which can be implemented as passive optical components without any electrical connections. The optical temperature sensors 502, 504 can be interrogated using an external laser 520 and an external photodetector 522 or an integrated photodetector. For example, as shown, the interrogation signal generated by the external laser 520 can be coupled into the optical waveguide 512 at the input / output port 521 of the optical waveguide 512 to propagate to the optical temperature sensor 504 associated with the modulator 508 of the transceiver 500, and the light reflected by the temperature sensor 504 can be returned to the input / output port 521 of the waveguide 512 and coupled to the external photodetector 522. The three-port optical circulator 524 can be used to direct light received at the first circulator port 526 into the waveguide 512 via the second circulator port 527, and to direct reflected light received at the second circulator port 527 to the photodetector 522 via the third circulator port 528. As will be readily appreciated, the interrogation device collectively formed by the external laser 520, the circulator 524, and the external photodetector 522 can be moved to the input / output port 530 of the optical waveguide 514 associated with the optical temperature sensor 502 at the transceiver laser 506, and the temperature sensor 502 can then be similarly interrogated.

[0036] It is also possible to measure the transmitted portion instead of the reflected portion of the interrogation signal. In this case, the transceiver 500 would be modified to extend the waveguide 512 (or 514) through the temperature sensor 504 (or 502) to an integrated photodetector or a second input / output port of the waveguide 512 (or 514) for measuring the light using an external photodetector. Whether to use an external photodetector or an integrated photodetector will generally depend on practical considerations in a specific application. The benefit of using an integrated photodetector is that it can reduce the interrogation device to a simple external laser and support simplified temperature measurement by integrating the readout of the temperature sensor in the EIC associated with the PIC, which may also include a memory for storing calibration data for the sensor. However, these benefits come at the expense of increased manufacturing complexity and chip area associated with the temperature sensor.

[0037] Fig. 6A and Figure 6B6 and 7 are cross-sectional top and side views, respectively, of a Bragg grating-based optical temperature sensor 600 placed adjacent to an active composite semiconductor photonic device 602 of a PIC according to various embodiments. For example, the active device 602 may be a laser, a modulator, or a photodetector (e.g., on a transceiver PIC). It includes a diode structure of a III-V (or other composite semiconductor) material formed above and optically coupled to a silicon waveguide 604 formed in a silicon device layer 606 of an SOI substrate 608. As shown in FIG. Figure 6B As shown, the diode structure may include, for example, an n-type bottom layer 610, and a diode mesa disposed above the n-type bottom layer 610, the diode mesa including an intrinsic or active layer 612 and a p-type top layer 614. In use, light guided in the silicon waveguide 604 is vertically coupled into the active layer 612 (as shown by optical modes 616, 617). In order to be able to apply voltage or current to the diode structure, the device 602 includes electrical connections to the n-type bottom layer 610 and the p-type top layer 614. The electrical connections include a contact metal layer 618 disposed atop the n-type bottom layer 610 (e.g., on both sides or around the diode mesa), a p-type contact metal layer 619 disposed atop the p-type top layer 614, and vertical vias 620, 622 that electrically connect the n-type contact metal layer 618 and the p-type contact metal layer 619 to electrical terminals and drive circuitry. The diode structure is encapsulated in an electrolyte coating 624.

[0038] The temperature sensor 600 is a Bragg grating formed by a periodic refractive index variation in the second silicon waveguide 630. Light propagating in the second silicon waveguide 630 experiences strong reflection at a wavelength that is twice the grating period multiplied by the effective refractive index. Because the grating period varies slightly with temperature due to thermal expansion or contraction, the wavelength at which the reflection peaks is a good indicator of the sensor temperature. Fig. 6A As shown, the second silicon waveguide 630 can be parallel to the first silicon waveguide (604, which is coupled to the active photonic device 602) and below the n-type bottom layer 610 of the diode structure of the active device 602. Therefore, the light guided in the second silicon waveguide 630 and reflected in the Bragg grating is very close to the active region 612 of the photonic device, as indicated by the optical mode 632.

[0039] Figure 7 is a flow chart of a method 700 for calibrating and using a dedicated optical temperature sensor without mission mode functionality for temperature measurement of a photonic device according to various embodiments. For example, the temperature sensor and the photonic device may correspond to Figure 5506, or sensor 504 adjacent to optical modulator 508 of photonic transceiver 500. During calibration phase 702, the PIC (e.g., implementing photonic transceiver 500) is placed in a controlled temperature environment (such as, for example, an oven) set to an initial temperature and allowed to substantially thermally equilibrate (act 704) such that both the temperature sensor(s) and the photonic device(s) reach a set temperature (within an acceptable range). If an external laser (e.g., 510) and photodetector (e.g., 528) are used for calibration, the power of the PIC is set to zero (act 706). (Otherwise, if an integrated photodetector is used, the PIC is operated in a low power mode.) For example, the optical temperature sensor is interrogated (act 708) by coupling light from an external laser into an optical waveguide associated with the sensor and measuring the reflected or transmitted portion of the light using an external (or integrated) photodetector. In some embodiments, the wavelength of the external laser is stepped over a wavelength range (e.g., including the FSR of one or more optical temperature sensors) and the wavelength at the zero (e.g., for an AMZI or all-pass ring implementing a temperature sensor) and / or the peak (e.g., for a Bragg reflector or add-drop ring) of the measured spectral response is recorded.

[0040] The controlled temperature environment is then changed to a second temperature (act 710), the PIC is allowed to thermally equilibrate at the second temperature (act 704), and the interrogation of the optical temperature sensor for measuring the spectral characteristic is repeated at the second temperature (act 708). Optionally, the spectral characteristic is measured at one or more additional temperatures, depending on the desired accuracy of the thermal characterization, as well as cost and time considerations. The values ​​of the spectral characteristic determined at two or more temperatures are then used to determine the temperature dependence of the spectral characteristic over a continuous temperature range, which may involve interpolating between the temperatures at which the measurements were performed and / or exceeding the temperatures at which the measurements were performed (act 712). For example, based on measurements of the reflectivity peak wavelengths λ1 and λ2 at two respective temperatures T1 and T2, the temperature-dependent wavelength shift can be calculated as dλ / dT=(λ2-λ1) / (T2-T1). The determined temperature dependence is stored in a memory (e.g., a memory of an EIC associated with the PIC or an external memory) for later use.

[0041] When the temperature sensor is calibrated, the sensor can be used together with the calibration data to measure the temperature of the adjacent photonic device as part of characterizing the mission mode thermal performance of the PIC in stage 714. The PIC is now operated in mission mode, that is, it is powered on in mission mode (which is typically a high power mode) and a mission mode optical signal, for example from an internal PIC laser (such as a transceiver laser), is coupled into the optical device via an associated first waveguide (action 716). At the same time, the temperature sensor is interrogated via its associated waveguide (which is separate from the waveguide of the photonic device to be monitored) using an interrogation signal coupled (for example by an external laser) into the PIC, for example by an external laser (action 718). Using the stored calibrated temperature dependence of the spectral characteristic, the measured spectral characteristic can then be converted to the temperature of the temperature sensor and, by extension, the temperature of the photonic device due to its proximity (action 720). For example, the measured reflectivity peak wavelength can be converted to the temperature of the temperature sensor and, by extension, the temperature of the photonic device, according to: T=T1+(λ-λ1) / (dλ / dT).

[0042] The following numbered examples further define various illustrative embodiments.

[0043] 1. A photonic integrated circuit (PIC), comprising: a first optical waveguide in a device layer of a substrate; an active photonic device coupled to the first optical waveguide, the active photonic device comprising a diode junction; and a second optical waveguide in the device layer of the substrate, the second optical waveguide comprising an optical temperature sensor, the optical temperature sensor being within sensing proximity of the diode junction.

[0044] 2. The photonic integrated circuit of example 1, wherein the optical temperature sensor comprises at least one of a Bragg grating, a ring resonator, or an asymmetric Mach-Zehnder interferometer.

[0045] 3. The photonic integrated circuit of example 1 or example 2, wherein the active photonic device comprises a composite semiconductor diode structure disposed over the first optical waveguide and comprising a diode junction; and an electrical connection for applying a voltage across the diode structure.

[0046] 4. The photonic integrated circuit of any of Examples 1-3, wherein the active photonic device is one of a laser, an optical modulator, a wavelength reference optical filter, or an optical multiplexer / demultiplexer.

[0047] 5. The photonic integrated circuit of any of Examples 1-4, wherein the active photonic device comprises at least one of a symmetric or asymmetric Mach-Zehnder interferometer, an arrayed waveguide grating, or an electro-absorption modulator.

[0048] 6. A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling light into the photonic device via a first optical waveguide; optically interrogating an optical temperature sensor via a second optical waveguide to determine a spectral characteristic of the optical temperature sensor, the optical temperature sensor being placed within a sensing proximity of the photonic device; and computationally converting the determined spectral characteristic to a measured temperature based on a calibrated temperature dependency of the spectral characteristic.

[0049] 7. The method of example 6, wherein optically interrogating the optical temperature sensor comprises: coupling light into the second optical waveguide using a laser external to the PIC, and measuring at least one of the following using an external photodetector: light reflected in the optical temperature sensor or light transmitted by the optical temperature sensor.

[0050] 8. The method of example 7, wherein optically interrogating the optical temperature sensor comprises tuning a wavelength of an external laser over a range of wavelengths, and measuring reflected light or transmitted light over the range of wavelengths.

[0051] 9. The method of any of Examples 6-8, further comprising calibrating the temperature dependency of the spectral characteristic by: placing the PIC in a controlled temperature environment at a first temperature; after substantial thermal equilibrium at the first temperature, optically interrogating the optical temperature sensor via a second optical waveguide to determine a first value of the spectral characteristic; placing the PIC in a controlled temperature environment at a second temperature; after substantial thermal equilibrium at the second temperature, optically interrogating the optical temperature sensor via a second optical waveguide to determine a second value of the spectral characteristic; and determining the temperature dependency of the spectral characteristic based at least in part on the first value and the second value and the first temperature and the second temperature.

[0052] 10. The method of example 9, wherein determining the temperature dependency of the spectral characteristic comprises at least one of: interpolating the spectral characteristic between the first temperature and the second temperature, or extrapolating the spectral characteristic beyond the first temperature or the second temperature.

[0053] 11. The method of Example 9 or Example 10, wherein calibrating the temperature dependency further comprises: placing the PIC in a controlled temperature environment at a third temperature; and after substantial thermal equilibrium at the third temperature, optically interrogating the optical temperature sensor via a second optical waveguide to determine a third value of the spectral characteristic, wherein the temperature dependency is also based on the third value and the third temperature.

[0054] 12. The method of any of Examples 9-11, wherein the photonic device is an active photonic device comprising a diode structure and an electrical connection for applying a voltage across the diode structure, wherein power to the diode structure is turned off during calibration of the temperature dependency and power to the diode structure is turned on when the temperature of the photonic device is measured based on the calibrated temperature dependency.

[0055] 13. A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling a first optical signal into the photonic device in a first propagation direction; optically interrogating the photonic device using a second optical signal to determine a spectral characteristic of the photonic device, the second optical signal being coupled into the photonic device in a second propagation direction, the second propagation direction being opposite to the first propagation direction; and computationally converting the determined spectral characteristic to a measured temperature based on a calibrated temperature dependence of the spectral characteristic.

[0056] 14. The method of example 13, wherein the first optical signal is generated by a laser of the PIC and the second optical signal is generated by a laser external to the PIC.

[0057] 15. The method of example 13 or example 14, wherein optically interrogating the photonic device comprises measuring an optical power of a transmitted portion of the second optical signal or an optical power of a reflected portion of the second optical signal.

[0058] 16. The method of example 15, wherein optically interrogating the photonic device comprises measuring an optical power of a transmitted portion of the second optical signal using an optical tap at an exit of the photonic device in the second propagation direction and a monitoring photodiode.

[0059] 17. The method of Example 15, wherein optically interrogating the photonic device comprises: measuring the optical power of a transmitted portion of a second optical signal using a second photonic device of the PIC at an exit of the photonic device in a second propagation direction, the second photonic device being operable as a photodetector and having a separate mission mode functionality.

[0060] 18. The method of example 17, wherein the photonic device is a multiplexer / demultiplexer and the second photonic device is an electro-absorption modulator.

[0061] 19. The method of any of Examples 15-18, wherein the second optical signal is tuned over a range of wavelengths and an optical power of a reflected portion of the second optical signal or an optical power of a transmitted portion of the second optical signal is measured over the range of wavelengths.

[0062] 20. The method of example 19, wherein the photonic device is an optical filter, and wherein the wavelength range includes at least one free spectral range of the optical filter.

[0063] 21. The method of example 20, wherein the spectral characteristics include nulls or peaks of the optical filter.

[0064] 22. The method of any of Examples 13-21, further comprising calibrating the temperature dependence of the spectral characteristic by: placing the PIC in a controlled temperature environment at a first temperature; after substantial thermal equilibrium at the first temperature, optically interrogating the photonic device using an optical signal coupled into the photonic device in a second propagation direction to determine a first value of the spectral characteristic of the photonic device; placing the PIC in a controlled temperature environment at a second temperature; after substantial thermal equilibrium at the second temperature, optically interrogating the photonic device using an optical signal coupled into the photonic device in a second propagation direction to determine a second value of the spectral characteristic of the photonic device; and determining the temperature dependence of the spectral characteristic based at least in part on the first value and the second value and the first temperature and the second temperature.

[0065] 23. The method of Example 22, wherein the photonic device is an optical filter, wherein the photonic device is interrogated over a wavelength range that includes at least one free spectral range of the optical filter, and wherein a spectral shift associated with the spectral characteristic between a first temperature and a second temperature is less than the free spectral range.

[0066] 24. The method of example 22 or example 23, wherein determining the temperature dependency of the spectral characteristic comprises at least one of: interpolating the spectral characteristic between the first temperature and the second temperature, or extrapolating the spectral characteristic beyond the first temperature or the second temperature.

[0067] 25. The method of any of Examples 22-24, wherein calibrating the temperature dependency further comprises: placing the PIC in a controlled temperature environment at a third temperature; after substantial thermal equilibrium at the third temperature, optically interrogating the photonic device using an optical signal coupled into the photonic device in a second propagation direction to determine a third value of a spectral characteristic of the photonic device; and wherein the temperature dependency is also based on the third value and the third temperature.

[0068] 26. The method of any of Examples 22-25, further comprising storing the temperature dependency of the spectral characteristic in a memory associated with the PIC.

[0069] 27. The method of any of Examples 22-26, wherein the PIC is operated in a low power mode during calibration.

[0070] Although the inventive subject matter has been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader scope of the inventive subject matter. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A photonic integrated circuit (PIC), comprising: a first optical waveguide in a device layer of a substrate; an active photonic device coupled to the first optical waveguide, the active photonic device comprising a diode junction; as well as A second optical waveguide in the device layer of the substrate, the second optical waveguide comprising an optical temperature sensor within a sensing proximity of the diode junction. 2 . The photonic integrated circuit of claim 1 , wherein the optical temperature sensor comprises at least one of a Bragg grating, a ring resonator, or an asymmetric Mach-Zehnder interferometer.

3. The photonic integrated circuit of claim 1, wherein the active photonic device is one of a laser, an optical modulator, a wavelength reference optical filter, or an optical multiplexer / demultiplexer.

4. The photonic integrated circuit of claim 1, wherein the active photonic device comprises at least one of a symmetric or asymmetric Mach-Zehnder interferometer, an arrayed waveguide grating, or an electro-absorption modulator.

5. A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling light into the photonic device via a first optical waveguide; optically interrogating an optical temperature sensor via a second optical waveguide to determine a spectral characteristic of the optical temperature sensor, the optical temperature sensor being positioned within a sensing proximity of the photonic device; as well as The determined spectral characteristic is computationally converted to the measured temperature based on a calibrated temperature dependency of the spectral characteristic.

6. The method of claim 5, wherein optically interrogating the optical temperature sensor comprises: A laser external to the PIC is used to couple light into the second optical waveguide, and an external photodetector is used to measure at least one of: light reflected in or transmitted by the optical temperature sensor.

7. The method of claim 6, wherein optically interrogating the optical temperature sensor comprises: The wavelength of the external laser is tuned over a range of wavelengths, and the reflected light or the transmitted light is measured over the range of wavelengths.

8. The method of claim 7, further comprising calibrating the temperature dependency of the spectral characteristic by: placing the PIC in a controlled temperature environment at a first temperature; optically interrogating the optical temperature sensor via the second optical waveguide to determine a first value of the spectral characteristic after substantial thermal equilibrium at the first temperature; placing the PIC in a controlled temperature environment at a second temperature; optically interrogating the optical temperature sensor via the second optical waveguide to determine a second value of the spectral characteristic after substantial thermal equilibrium at the second temperature; as well as The temperature dependency of the spectral characteristic is determined based at least in part on the first value and the second value and the first temperature and the second temperature.

9. The method of claim 6, wherein the photonic device is an active photonic device comprising a diode structure and an electrical connection for applying a voltage across the diode structure, wherein power to the diode structure is turned off during calibration of the temperature correlation, and power to the diode structure is turned on when measuring the temperature of the photonic device based on the calibrated temperature correlation.

10. A method for measuring the temperature of a photonic device in a photonic integrated circuit (PIC), the method comprising: coupling a first optical signal into the photonic device in a first propagation direction; optically interrogate the photonic device using a second optical signal to determine a spectral characteristic of the photonic device, the second optical signal being coupled into the photonic device in a second propagation direction, the second propagation direction being opposite to the first propagation direction; and The determined spectral characteristic is computationally converted into the measured temperature based on a calibrated temperature dependency of the spectral characteristic.

11. The method of claim 10, wherein the first optical signal is generated by a laser of the PIC and the second optical signal is generated by a laser external to the PIC.

12. The method of claim 10, wherein optically interrogating the photonic device comprises: An optical power of the transmitted portion of the second optical signal or an optical power of the reflected portion of the second optical signal is measured.

13. The method of claim 12, wherein optically interrogating the photonic device comprises: The optical power of the transmitted portion of the second optical signal is measured using an optical tap and a monitoring photodiode at an exit of the photonic device in the second propagation direction.

14. The method of claim 12, wherein optically interrogating the photonic device comprises: The optical power of the transmitted portion of the second optical signal is measured using a second photonic device of the PIC at an outlet of the photonic device in the second propagation direction, the second photonic device being operable as a photodetector and having a separate mission mode functionality.

15. The method of claim 12, wherein the second optical signal is tuned over a wavelength range and the optical power of the reflected portion of the second optical signal or the optical power of the transmitted portion of the second optical signal is measured over the wavelength range.

16. The method of claim 15, wherein the photonic device is an optical filter, and wherein the wavelength range comprises at least one free spectral range of the optical filter.

17. The method of claim 10, further comprising calibrating the temperature dependency of the spectral characteristic by: placing the PIC in a controlled temperature environment at a first temperature; optically interrogate the photonic device after substantial thermal equilibrium at the first temperature using an optical signal coupled into the photonic device in the second propagation direction to determine a first value of the spectral characteristic of the photonic device; placing the PIC in a controlled temperature environment at a second temperature; optically interrogating the photonic device after substantial thermal equilibrium at the second temperature using an optical signal coupled into the photonic device in the second propagation direction to determine a second value of the spectral characteristic of the photonic device; as well as The temperature dependency of the spectral characteristic is determined based at least in part on the first value and the second value and the first temperature and the second temperature.

18. The method of claim 17, wherein the photonic device is an optical filter, wherein the photonic device is interrogated over a wavelength range that includes at least one free spectral range of the optical filter, and wherein a spectral shift associated with the spectral characteristic between the first temperature and the second temperature is less than the free spectral range.

19. The method of claim 17, further comprising storing the temperature dependency of the spectral characteristic in a memory associated with the PIC.

20. The method of claim 17, wherein the PIC is operated in a low power mode during calibration.

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