Patent Description:
Many photonic circuit components, including, e.g., lasers and optical modulators, are highly temperature-sensitive in their performance. An important aspect of photonic-circuit design is, therefore, the thermal design of the PIC, which specifies the layout of heat sources (including heat-generating photonic devices) and heat spreaders and sinks, with the goal to optimize the range of ambient temperatures at which the PIC will operate reliably. Thermal design and validation rely on accurate temperature measurements. Since the temperature can vary widely across the PIC, however, temperature measurements of the PIC as a whole are often inadequate. On the other hand, temperature measurements of the individual PIC elements, while desirable for purposes of thermal design, can be difficult because thermal sensor placement near the relevant photonic components is often hindered by spatial constraints and/or the potential for electrical interference of the thermal sensor (such as, e.g., a resistance temperature sensor (RTD) with the photonic component.

<CIT> refers to a protonic integrated circuit incorporating a bandgap temperature sensor. <CIT> refers to systems and methods for providing a stable wavelength reference in an integrated protonic circuit. <CIT> refers to a fiber optic device for measuring a parameter of interest. Neither of these documents disclose, at least, a second optical waveguide in a device layer of a substrate, a second optical waveguide comprising an optical temperature sensor in sensing proximity to a diode junction.

Disclosed herein are various approaches to measuring the temperature of photonic circuit components optically, rather than using RTDs or other conventional, electronic temperature sensors. In some embodiments, photonic devices that fulfill various mission-mode function, such as, e.g., multiplexing/demultiplexing or modulating light, are calibrated and selectively operated during use to double as temperature sensors. For example, the temperature-dependent spectral shift in optical filters such as Mach-Zehnder interferometers (MZIs) or asymmetric MZIs (AMZIs), arrayed waveguide gratings (AWGs), or in electro-absorption modulators (EAMs), can be exploited to measure the temperature of these components. In other embodiments, the PIC is enhanced with optical components that have no mission-mode function, but serve solely to determine the temperature based on a temperature-dependent change in their optical (e.g., spectral) properties. Beneficially, such optical temperature sensors can be placed in very close proximity to diode junctions in lasers, modulators, photodetectors, etc., without interfering electrically. Furthermore, in hybrid platforms that use compound semiconductor (e.g., III-V) active devices on a silicon device layer including waveguides and other optically passive components, optical temperature sensors can be implemented in the silicon device layer underneath the active devices whose temperature is to be measured, circumventing spatial constraints that exist in the compound semiconductor layer.

To appreciate the benefits afforded by measuring the temperature of individual photonic components, rather than of the photonic circuit at large, consider, for example, an optical transceiver including four lasers. Assume that the temperature across the lasers varies by twenty degrees at the maximum operating current, e.g., due to thermal cross-talk from other, nearby heat sources, such as optical components within the PIC or electronic components within an adjacent electronic integrated circuit (EIC). The hottest of the lasers may have a significantly (e.g., three times) shorter operating lifetime than the coolest of the lasers, and may further suffer from instability (e.g., leading to mode-hopping) and output lower optical power. Laser instability and reduced output power do, however, not necessarily indicate a hotspot, but may also be due to laser damage caused during PIC fabrication or assembly or due to deficiencies in the optical design. It is important, therefore, to separate temperature effects from laser damage and/or optical design issues. Using optical temperature measurements as described herein, the temperatures at the junctions of all four lasers can be measured individually, and more accurately than using electronic temperature measurements, which facilitates identifying the hotspot, and then modifying the thermal design (including, e.g., the heat spreader, spacing of components within and adjacent the PIC die, etc.) to reduce the hotspot temperature.

Various example embodiments of devices, systems, and methods for optical temperature measurements in PICs will in the following be described with reference to the accompanying drawings.

<FIG> is a schematic block diagram of an example photonic transceiver <NUM> configured to enable optical temperature measurements with an integrated optical filter <NUM>, in accordance with various embodiments. The photonic transceiver <NUM> includes, along a transmitter path <NUM>, a laser <NUM> to generate an optical carrier signal, an optical modulator <NUM> to impart data onto the optical carrier signal via amplitude modulation, and the optical filter <NUM>, which functions as a wavelength reference for the laser <NUM>. Further, the photonic transceiver <NUM> includes a photodetector <NUM> and associated receiver path <NUM> serving as an optical receiver. In addition, to facilitate temperature measurements with the optical filter <NUM>, the transceiver <NUM> includes optical taps <NUM>, <NUM> bracketing the optical filter <NUM>, each splitting off a fixed fraction of the light (e.g., <NUM>%) and having two outputs directing the split-off light to two respective monitor photodiodes <NUM>, <NUM> and <NUM>, <NUM> that allow measuring the optical power entering and exiting the optical filter <NUM> in either direction of light propagation in the transmitter path <NUM>. The photonic transceiver <NUM> may be implemented, e.g., in a hybrid material platform comprising compound-semiconductor (e.g., III-V) active device components of the laser <NUM>, modulator <NUM>, receiver photodetector <NUM>, and monitor photodiodes <NUM>, <NUM>, <NUM>, <NUM> coupled to waveguides (implementing, e.g., the transmitter path <NUM> and receiver path <NUM>) and other passive devices or device components formed in a silicon device layer (e.g., of a silicon-on-insulator (SOI) substrate).

The optical filter <NUM> may be implemented, e.g., as an AMZI, which is characterized by an optical path-length difference between two interferometer arms that produces an interference signal whose intensity varies periodically with the frequency of the light. The period of that variation is the free spectral range (FSR) of the filter <NUM>. The AMZI (or other optical filter <NUM>) is calibrated to achieve a specified transmission, measured as the ratio of output optical power to input optical power, at a specified reference wavelength. When the wavelength of the laser <NUM> deviates from the reference wavelength, this deviation will manifest itself in a change of the transmission of the filter <NUM> (as determined from the input and output optical powers measured by photodiodes <NUM>, <NUM>, respectively), providing for feedback to tune the laser <NUM> to the reference wavelength. However, the transmission spectrum of the filter <NUM> is generally temperature-dependent, and the temperature of the filter <NUM> can vary in mission-mode use, e.g., due to heat emanating from the laser <NUM>. It is desirable to design the photonic circuit to minimize any such heating. Even if the temperature of the filter <NUM> is monitored during use and discrepancies from the temperature at the time of calibrating the filter as a wavelength reference are actively compensated for, e.g., using an integrated heater or thermo-electric cooler, it is desirable to minimize the power requirements of such active heat adjustments.

In accordance with various embodiments, the optical filter <NUM> is used as its own temperature sensor during thermal design of the transceiver <NUM>. For this purpose, an external laser <NUM> couples light into the optical filter <NUM>, e.g., via a transmitter output port <NUM> (which serves, in mission mode, to transmit the modulated optical signal), and the transmission and/or output optical power of the filter <NUM> (in a direction of propagation of the light coming from the external laser <NUM>) is measured. In some embodiments, the external laser <NUM> is a tunable laser source that is stepped over a range of wavelengths, e.g., including one or more FSRs of the filter <NUM>, to identify the wavelength at which the transmission reaches a peak (maximum) or null (minimum) via measurements of the relative optical output power at the monitor photodiode <NUM> following the filter <NUM>. Based on the known temperature dependence of the peak or null wavelength as determined by calibration, the measured peak or null wavelength can then be converted to the measured temperature of the optical filter <NUM>. In alternative embodiments, the external laser <NUM> is operated at a single specified wavelength, and the transmission of the optical filter <NUM> at that wavelength is measured using the monitor photodiodes <NUM>, <NUM> preceding and following the filter <NUM>. Based on the known temperature dependence of the filter transmission at the selected wavelength of the external laser <NUM>, the temperature of the filter <NUM> can then be calculated from the measured transmission.

To study effect of other transceiver components on the temperature of the optical filter <NUM>, thermal design involves operating the transceiver <NUM> in mission mode, with the transceiver laser <NUM> at full mission-mode power. Thus, as the optical filter <NUM> is optically interrogated by an optical signal from the external laser <NUM>, it simultaneously receives an optical signal from the transceiver laser <NUM>. The two optical signals propagate in opposite directions, such that measurement of the transmitted portion of the external laser signal is unaffected by the signal from the transceiver laser <NUM>. The external laser <NUM> may include an optical isolator that prevents damage due to light received from the transceiver laser <NUM>. Conversely, light from the external laser <NUM> reaching the transceiver laser <NUM> can somewhat degrade the optical performance of the latter during the measurement of the filter temperature, but that is generally not an issue during the design phase. Note that, with suitable enhancements to the transceiver <NUM> (e.g., an isolator protecting the modulator <NUM> and laser <NUM> from light injected into the transmitter path <NUM> by the external laser <NUM>), or by using an external laser wavelength far detuned from the transceiver operating wavelength, it is in principle also possible to use the optical filter <NUM> to monitor its own temperature during ordinary mission-mode through-out the lifetime of the transceiver.

While <FIG> shows only a single lane of the photonic transceiver <NUM>, it will be readily apparent to those of ordinary skill in the art that a multiple-lane transceiver can be implemented by replicating the transmitter and receiver components. The temperature of the optical filter <NUM> in each lane can then be measured in the same manner as described above, coupling the external laser <NUM> sequentially (or multiple external lasers simultaneously) into the various transmitter paths.

To provide another example of temperature measurements with photonic devices that possess a separate mission-mode function, <FIG> is a schematic block diagram of an example photonic transceiver <NUM> configured to enable optical temperature measurements with an integrated multiplexer, in accordance with various embodiments. The transceiver <NUM> includes four lanes, each including a laser <NUM> and associated EAMs <NUM> to impart data onto the optical carrier signal. The lasers <NUM> emit light at four different wavelengths λ<NUM>, λ<NUM>, λ<NUM>, and λ<NUM>, and the modulated optical signals at the four wavelengths are multiplexed in two stages into a combined optical transceiver output signal. The laser wavelengths may be evenly spaced, e.g., at λ<NUM> ≈ <NUM>, λ<NUM> ≈ <NUM>, λ<NUM> ≈ <NUM>, and λ<NUM> ≈ <NUM>. The components whose temperatures are to be measured are, in this example, the multiplexers <NUM> at the first stage, which multiplex λ<NUM>, λ<NUM> and λ<NUM>, λ<NUM>, respectively, into two partially multiplexed signals provided as input to the second-stage multiplexer <NUM>. The multiplexers <NUM> (as well as <NUM>) may be implemented, for example, as AMZIs each with two input ports <NUM> and one common output port (in the direction of propagation of light from the transceiver lasers <NUM>).

To measure the temperature of the first-stage multiplexers <NUM>, they are interrogated with an external tunable laser (not shown) that is coupled into the transceiver via the transmitter output port and the second-stage multiplexer <NUM> (where the output port of the multiplexer may also constitute the transmitter output port) and propagates in a direction opposite to that of the optical signals emitted by the transceiver lasers <NUM>. The external laser is swept over a wavelength range to determine the spectral transmission peak of light output at one or both input ports <NUM> (corresponding to output ports in the direction of propagation of the external laser signal) of the multiplexers <NUM>. If the multiplexers <NUM> are at their target operating temperatures, these transmission peaks will occur at λ<NUM>, λ<NUM>, λ<NUM>, and λ<NUM>. 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 the transceiver <NUM> includes taps <NUM> and associated monitor photodiodes <NUM> at various inputs and outputs between the transceiver lasers <NUM>, EAMs <NUM>, and multiplexers <NUM>, <NUM>, those taps <NUM> are configured to only measure light going in the direction from the transceiver lasers <NUM> to the transmitter output port, and can therefore not be used to measure the transmitted portions, at the multiplexers <NUM>, of the external laser signal. Instead of configuring the taps <NUM> and adding monitor photodiodes <NUM> to measure the light from the external laser in the circuit, as is done in the transceiver <NUM> of <FIG> and can in principle be implemented in transceiver <NUM> as well, the depicted embodiment uses the EAMs <NUM> to measure the optical power of the external laser signal transmitted by the multiplexers <NUM>. The EAMs <NUM> are operated at a high voltage bias, which causes high photo-absorption, and the photocurrent generated by the EAMs is proportional to the optical power of the measured signal.

<FIG> show graphs of example spectral responses of the multiplexers of transceiver <NUM> of <FIG>, operated in low-power mode ("LPM"), as measured over two spectral ranges at receiver case temperatures of <NUM>, <NUM>, and <NUM>, respectively, in accordance with various embodiments. In low-power mode, power is sufficient to operate and read the EAMs <NUM>, but the transceiver lasers <NUM> and other components are turned off. It is assumed that the self-heating of the transceiver <NUM> in low-power mode is negligible, and that the transceiver <NUM> had time to equilibrate, such that the multiplexer temperature is close to the receiver case temperature. The four graphs in each figure show the photocurrents ("IPh") at the EAMs <NUM> resulting from measurement of the optical power output at the input ports <NUM> of the multiplexers <NUM>. As can be seen, the transmission peaks and nulls of the four optical signals, as reflected in the photocurrents, are approximately evenly distributed over an FSR (mirroring, as is to be expected, the uniform spacing of the laser wavelengths of the transceiver lasers <NUM>). Comparing the graphs at <NUM> (<FIG>), <NUM> (<FIG>), and <NUM> (<FIG>), a shift of the spectral responses towards longer wavelengths can be observed. For example, the transmission minimum of the signal measured at the multiplexer input port <NUM> associated with λ<NUM> shifts from about <NUM> at <NUM> to about <NUM> at <NUM> and almost <NUM> at <NUM>.

<FIG> shows graphs of example spectral responses of the multiplexers of transceiver <NUM> of <FIG>, operated in high-power mode ("HPM"), as measured at a receiver case temperature of <NUM>, in accordance with various embodiments. In high-power mode, the transceiver lasers <NUM> are turned on, and tend to heat up other transceiver components, including the multiplexers <NUM>. It is to be expected, therefore, that the local temperature of the multiplexers <NUM> is higher than the ambient temperature. Indeed, comparing the signal measured at the multiplexer input port <NUM> associated with λ<NUM> in high-power mode with that measured in low-power mode at the same receiver case temperature of <NUM> (<FIG>), it can be seen that the transmission minimum has shifted to almost <NUM>, indicating that the temperature of the multiplexer <NUM> is about <NUM>.

The described approach to optical temperature measurements of photonic devices using those same device as sensors, illustrated above with the examples of a wavelength reference and demultiplexers, is generally applicable to any photonic device that features, and is amenable to measurement of, some temperature-dependent spectral property. Such properties may include, for example, spectral features, like the wavelengths or amplitudes of peaks and nulls, of optical transmission, reflection, or absorption spectra, as well as transmission, reflection, or absorption levels at a given wavelength. Photonic devices suited to optical interrogation for temperature measurements include devices based on, for instance, symmetric or asymmetric MZIs, AWGs, or EAMs, as may fulfil mission-mode functions of, e.g., optical filters, multiplexers/demultiplexers, or modulators.

Various modifications to the above approach may be made. For example, while the interrogation of the phonic device of interest will in practice often utilize a laser external to the PIC, it is in principle also possible to use an internal laser (e.g., with its output signal routed to propagate in an opposite direction to the light used for the mission-mode function of the photonic device). The internal laser, to be 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 PIC (e.g., in a high-power-density region). Further, while the above-described example embodiments use photodetectors integrated into the PIC, it is also possible to route the optical signal that is to be measured for temperature-determination purposes off-chip for detection by external photodetectors. External photodetectors may be used, e.g., when the spectral property of the photonic device is measured in reflection rather than transmission mode.

<FIG> is a flow chart of a method <NUM> of calibrating and operating a photonic device with separate mission-mode function for optical temperature measurements, in accordance with various embodiments. During a calibration phase <NUM>, the PIC including the photonic device (e.g., a photonic transceiver PIC) is placed into a controlled-temperature environment (such as, e.g., an oven) set to an initial temperature, and allowed to thermally equilibrate, e.g., for about an hour (act <NUM>). 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 generally some acceptable margin of deviation for the given application, which will affect the equilibration time). If integrated photodetectors are to be used for the calibration, the PIC is then turned on and operated in a low-power mode, sufficient to use the integrated photodetectors, but avoiding other power consumption to minimize heating (act <NUM>). In this state, the photonic device is optically interrogated to determine a spectral property of the photonic device (act <NUM>). Usually (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 the transmitter path of a photonic transceiver, an optical signal from an external laser may be coupled into the photonic device via the transmitter path, in a direction or propagation that is opposite to the direction of propagation of the mission-mode optical signal from the PIC laser. The wavelength of the external laser may be stepped over a wavelength range including, typically, one or more FSRs of the photonic device, and the wavelength at the null and/or peak of the thus measured spectral response are recorded.

The controlled-temperature environment is then changed to a second temperature (act <NUM>), the PIC is allowed to thermally equilibrate at the second temperature (act <NUM>), and the interrogation of the photonic device to measure the spectral property during low-power mode is repeated at the second temperature (act <NUM>). Optionally, the spectral property is measured at one or more additional temperatures, depending on the desired accuracy of the thermal characterization, as well as cost and time considerations. For example, in some embodiments, the calibration (phase <NUM>) is performed only once on one part and thereafter applied to many parts; in this case, it may be feasible to measure at several or tens of temperatures. On the other hand, if the calibration is performed per part, it may rely on merely two measurements, e.g., taken at two temperatures that cause a relative shift in the spectral property of less than the FSR.

The determined values of the spectral property at two or more temperatures are then used to determine the temperature dependence of the spectral property over a continuous range of temperatures, which may involve interpolating between and/or extrapolating beyond the temperatures at which the measurements were performed (act <NUM>). For example, based on measurements of the transmission peak or null wavelengths λ<NUM> and λ<NUM> at two respective temperatures T<NUM> and T<NUM>, the temperature-dependent wavelength shift can be calculated as dAldT = (λ<NUM> - λ<NUM>)/(T<NUM> - T<NUM>). The determined temperature dependence is stored in memory for later use. In some embodiments, the temperature dependence is stored in memory of an EIC associated with the PIC, e.g., the EIC providing control signals and data readout for a transceiver PIC, where the temperature dependence of the transceiver component of interest may be stored along with other calibration data, such as target bias and modulation settings, etc. However, storage of the calibrated temperature dependence of the spectral property is also possible.

Once the calibration phase <NUM> has been 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 phase <NUM>. The PIC is now operated in mission mode, that is, powered on as in mission mode (which is generally 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 photonic device in one direction (act <NUM>). To measure the spectral property of the photonic device, the device is interrogated with an interrogation signal propagating in the other direction, e.g., coupled into the PIC by an external laser (act <NUM>). Using the stored, calibrated temperature dependence of the spectral property, the measured spectral property can then be converted to a temperature of the device (act <NUM>). Continuing the above example of transmission wavelength measurements, a transmission peak or null wavelength of l measured during mission mode may be converted into the temperature of the photonic device according to: T = T<NUM> + (λ - λ<NUM>)/(dλ/dT).

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, that is, added components that do not fulfill any independent mission-mode function. Each such dedicated temperature sensor is placed in "sensing proximity" to the photonic devices whose temperature is to monitored, meaning that the temperature sensor is physically close enough to the location in the photonic device where the temperature is of interest to be at substantially the same temperature (e.g., within a few degrees K). In various embodiments, the photonic device to be monitored is an optically active device including a diode junction, and the temperature at that junction is to be measured. In some embodiments, the temperature sensor is within sensing proximity from the location of interest, such as the diode junction in the active photonic device, if it is less than <NUM>, preferably less than <NUM> away from that location.

<FIG> is a schematic block diagram of an example photonic transceiver <NUM> equipped with added optical temperature sensors <NUM>, <NUM> near the laser <NUM> and modulator <NUM>, in accordance with various embodiments. Here, the laser <NUM> and modulator <NUM> are coupled to a first optical waveguide <NUM>, which implements the transmission path, whereas the optical temperature sensors <NUM>, <NUM> are placed within, or coupled to, respective second and third optical waveguides <NUM>, <NUM> that are separate from the first waveguide <NUM>. The photodetector <NUM> of the receive is coupled to a fourth optical waveguide <NUM> , which implements the receiver path (as usual). All four waveguides <NUM>, <NUM>, <NUM>, <NUM> have their own respective input/output ports.

As in transceiver <NUM> of <FIG>, the laser <NUM> and modulator <NUM> of the transmitter, and the photodetector <NUM> of the receiver, may be implemented in a compound-semiconductor-on-silicon hybrid platform, with waveguides in the silicon device layer coupled to optically active regions formed in a the compound semiconductor (e.g., III-V) layer thereabove. The active photonic devices may, for instance, be or include a III-V p-i-n diode structure with electrical connections to the p-type and n-type layers to apply a voltage across the diode structure, forming a diode junction in the intrinsic, active layer, as is known in the art. The temperature of this diode junction significantly affects the optical performance of the respective device, and is therefore important to monitor. With electronic temperature sensor implementations, placing the sensor too close to the diode junction risks causing electrical interference. The optical temperature sensors <NUM>, <NUM>, on the other hand, do not pose this problem, and can, thus, be placed much closer, facilitating more accurate temperature measurements. For example, in some embodiments, the optical sensors <NUM>, <NUM> are as close as about <NUM> to the diode junction.

Similarly to photonic devices with mission-mode function doubling as temperature sensors, the optical temperature sensors <NUM>, <NUM> generally exploit a temperature-dependent spectral property, such as a transmission or reflection peak or null, or a transmission or reflection level at a specified wavelength, for temperature measurements. Examples of photonic devices useful as temperature sensors <NUM>, <NUM> include Bragg gratings, ring resonators, and AMZIs, all of which can be implemented as passive optical components, without any need for electrical connections. The optical temperature sensors <NUM>, <NUM> may be interrogated using an external laser <NUM> and an external photodetector <NUM> or integrated photodetector. For example, as shown, an interrogation signal generated by the external laser <NUM> may be coupled at the input/output port <NUM> of the optical waveguide <NUM> into the optical waveguide <NUM> to propagate to the optical temperature sensor <NUM> associated with the modulator <NUM> of the transceiver <NUM>, and light reflected by the temperature sensor <NUM> may return to the input/output port <NUM> of the waveguide <NUM> and be coupled to an external photodetector <NUM>. A three-port optical circulator <NUM> may serve to direct light received at a first circulator port <NUM> via a second circulator port <NUM> into the waveguide <NUM>, and to directed reflected light received at the second circulator port <NUM> via a third circulator port <NUM> to the photodetector <NUM>. As will be readily appreciated, the interrogation apparatus formed collectively by the external laser <NUM>, circulator <NUM>, and external photodetector <NUM> can be moved to the input/output port <NUM> of the optical waveguide <NUM> associated the optical temperature sensor <NUM> at the transceiver laser <NUM>, and that temperature sensor <NUM> can then be interrogated analogously.

Instead of measuring the reflected portion of the interrogation signal, its is also possible to measure the transmitted portion. The transceiver <NUM> would, in this case, be modified to extend the waveguide <NUM> (or <NUM>) past the temperature sensor <NUM> (or <NUM>) to an integrated photodetector or to a second input/output port of the waveguide <NUM> (or <NUM>) for measuring the light with an external photodetector. Whether external or integrated photodetectors are used will generally depend on practical considerations in the particular application. The use of integrated photodetectors is beneficial in that it reduces the interrogation apparatus to simply the external laser, and facilitates streamlining temperature measurements by integrating read-out of the temperature sensor in the EIC associated with the PIC, which may also include memory storing the calibration data for the sensor. These benefits, however, come at the cost of increased manufacturing complexity and chip area associated with the temperature sensor.

<FIG> and <FIG> are cross-sectional top and side views, respectively, of a Bragg-grating-based optical temperature sensor <NUM> placed adjacent an active compound semiconductor photonic device <NUM> of a PIC, in accordance with various embodiments. The active device <NUM> may be, for instance, a laser, modulator, or photodetector (e.g., on a transceiver PIC). It includes a diode structure of III-V (or other compound semiconductor) material formed above, and optically coupled to, a silicon waveguide <NUM> formed in the silicon device layer <NUM> of an SOI substrate <NUM>. As shown in <FIG>, the diode structure may include, e.g., an n-type bottom layer <NUM> and, disposed above the n-type bottom layer <NUM>, a diode mesa including an intrinsic, or active, layer <NUM> and a p-type top layer <NUM>. In use, light guided in the silicon waveguide <NUM> couples vertically into the active layer <NUM> (as indicated by optical modes <NUM>, <NUM>). To enable application of a voltage or current to the diode structure, the device <NUM> includes electrical connections to the n-type bottom and p-type top layers <NUM>, <NUM>. The electrical connections include a contact metal layer <NUM> disposed on top of the n-type bottom layer <NUM> (e.g., to both sides or surrounding the diode mesa), a p-type contact metal layer <NUM> is disposed on top of the p-type top layer <NUM>, and vertical vias <NUM>, <NUM> electrically connecting the n-type and p-type contact metal layers <NUM>, <NUM> to electrical terminals of driver circuitry. The diode structure is enclosed in a dielectric cladding <NUM>.

The temperature sensor <NUM> is a Bragg grating formed by periodic refractive-index variations in a second silicon waveguide <NUM>. Light propagating in that second silicon waveguide <NUM> experiences a strong reflection at a wavelength twice the grating period multiplied by the effective refractive index. Since 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. As shown in <FIG>, the second silicon waveguide <NUM> may run parallel to the first silicon waveguide (<NUM>, which is coupled to the active photonic device <NUM>), and underneath the n-type bottom layer <NUM> of the diode structure of the active device <NUM>. Thus, light guided in the second silicon waveguide <NUM> and reflected in the Bragg grating, indicated by optical mode <NUM>, comes very close to the active region <NUM> of the photonic device.

<FIG> is a flow chart of a method <NUM> of calibrating and using a dedicated optical temperature sensor without mission-mode function for temperature measurements of a photonic device, in accordance with various embodiments. The temperature sensor and photonic device may, for instance, correspond to the sensor <NUM> adjacent laser <NUM>, or to the sensor <NUM> adjacent optical modulator <NUM>, of the photonic transceiver <NUM> of <FIG>. During a calibration phase <NUM>, the PIC (e.g., implementing photonic transceiver <NUM>) is placed into a controlled-temperature environment (such as, e.g., an oven) set to an initial temperature, and allowed to substantially thermally equilibrate (act <NUM>), such that the temperature sensor(s) and photonic device(s) all reach the set temperature (within acceptable margins). If an external laser (e.g., <NUM>) and photodetector (e.g., <NUM>) are used for the calibration, the power to the PIC is set to zero, (act <NUM>). (Otherwise, if an integrated photodetector is being used, the PIC is operated in low-power mode. ) The optical temperature sensor is interrogated (act <NUM>), e.g., by coupling light from the external laser into the optical waveguide associated with the sensor and measuring the reflected or transmitted portion of the light with the external (or integrated) photodetector. In some embodiments, the wavelength of the external laser is stepped over a wavelength range, e.g., including one or more FSRs of the optical temperature sensor, and the wavelength at the null (e.g., for an AMZI or all-pass ring implementing the temperature sensor) and/or peak (e.g., for a Bragg reflector or add-drop ring) of the measured spectral response are recorded.

The controlled-temperature environment is then changed to a second temperature (act <NUM>), the PIC is allowed to thermally equilibrate at the second temperature (act <NUM>), and the interrogation of the optical temperature sensor to measure the spectral property is repeated at the second temperature (act <NUM>). Optionally, the spectral property 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 determined values of the spectral property at two or more temperatures are then used to determine the temperature dependence of the spectral property over a continuous range of temperatures, which may involve interpolating between and/or extrapolating beyond the temperatures at which the measurements were performed (act <NUM>). For example, based on measurements of the reflection peak wavelengths λ<NUM> and λ<NUM> at two respective temperatures T<NUM> and T<NUM>, the temperature-dependent wavelength shift can be calculated as dAldT = (λ<NUM> - λ<NUM>)/(T<NUM> - T<NUM>). The determined temperature dependence is stored in memory (e.g., of an EIC associated with the PIC or an external memory) for later use.

Upon calibration of the temperature sensor, the sensor can be used, in conjunction 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 phase <NUM>. The PIC is now operated in mission mode, that is, powered on as in mission mode (which is generally 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 photonic device via an associated first waveguide (act <NUM>). Simultaneously, the temperature sensor is interrogated via its associated waveguide (which is separate from the waveguide of the phonic device to be monitored) with an interrogation signal coupled into the PIC, e.g., by an external laser (act <NUM>). Using the stored, calibrated temperature dependence of the spectral property of the optical temperature sensor, the measured spectral property can then be converted to a temperature of the temperature sensor and, by extension due to its proximity, the photonic device (act <NUM>). For example, a measured reflection peak wavelength can be converted into the temperature of the temperature sensor and photonic device according to: <MAT>.

Claim 1:
A photonic integrated circuit (PIC) comprising:
a first optical waveguide (<NUM>, <NUM>) in a device layer (<NUM>) of a substrate (<NUM>);
an active photonic device (<NUM>, <NUM>, <NUM>) coupled to the first optical waveguide, the active photonic device comprising a diode junction; and
a second optical waveguide (<NUM>, <NUM>, <NUM>) in the device layer of the substrate, the second optical waveguide comprising an optical temperature sensor (<NUM>, <NUM>, <NUM>) in sensing proximity to the diode junction.