Burst-mode transmitter stabilization

Abstract

This disclosure presents a photonic integrated chip (PIC) for an optical network unit (ONU) of a passive optical network (PON). The PIC comprises an optical transmitter for burst-mode transmission, configured to transition between an off-state (TXOFF) and on-state (TXON) and transmitting a data burst during TXON. The PIC comprises a heat-producing component and an associated heater. Heat dissipation of the heat-producing component changes with the transition(s) between TXOFF and TXON. A controller is configured to controls a heat dissipation of the heater to increase with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component decreases with the transition, and to decrease with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component increases with the transition.

Description

[0001] BURST-MODE TRANSMITTER STABILIZATION

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to optical communication systems. The disclosure presents a photonic integrated chip or circuit (PIC) for an optical network unit (ONU) of a passive optical network (PON). The PIC comprises an optical transmitter for burst-mode transmission, and the disclosure is concerned with a stabilization of the transmitter using the burst-mode.

[0004] BACKGROUND

[0005] A PON is a point to multi-point network, wherein a central optical line terminal (OLT) is connected optically to multiple optical network units (ONUs), for example, via passive splitters. Most PON standards utilize a single wavelength for upstream and downstream traffic, respectively. While all ONUs receive the continuous downstream traffic in parallel, time division multiplexing (TDM) is used to manage the upstream traffic from multiple ONUs.

[0006] Each ONU thus only transmits upstream traffic during an allocated time slot, and is shuttered during all other time slots, in order to avoid interference with the other ONUs. An on-state of the optical transmission of the ONU is referred to as TXON, while an off-state of the optical transmission of the ONU is referred to as TXOFF. Since PONs with up to 128 ONUs may become common, even low amounts of emitted power during TXOFF can sum up and degrade signal to noise ratio (SNR) and link performance.

[0007] Consequently, high TXON / TXOFF ratios above 50dB for the ONUs are desired. Such high TXON / TXOFF ratios can be achieved by switching off a directly-modulated laser completely, when the ONU is not transmitting. Additionally, a laser driver can be powered-off during TXOFF, in order to lower the power consumption of the ONU when it is not transmitting. Limitations of this approach, however, become apparent as the symbol rate of the optical transmitter of the ONU steadily increases from PON generation to PON generation. Therefore, new approaches are needed for achieving higher symbol rates.

[0008] SUMMARY

[0009] This disclosure therefore aims for a different approach, which is based on the use of a PIC. The solutions of this disclosure for this approach are based on the following further considerations.

[0010] Next generation PONs (e.g., very high speed PON, VHSP) may require ONUs that no longer use directly modulated lasers, because a fundamental chirp associated with these lasers will result in large CD-related penalties for symbol rates exceeding lOOGBd. ONUs for VHSP are thus expected to utilize PICs, which either employ intensity modulation with externally modulated laser, e.g. using an electro-absorption modulator (EAM) or a Mach-Zehnder-Modulators (MZM) on the PIC, or employ complex (phase-encoded) modulation formats that require in-phase and quadrature (IQ) modulators (IQMs) or MZMs on the PIC. The latter is usually referred to as being suitable for a coherent PON (CPON).

[0011] Transmitters of ONUs that utilize modulators like an MZM or IQM may require continuous control loops to assure the operation of the modulators at certain required operating points. Such control loops are typically relatively slow, with iteration cycles in the order of ~100ms to ~ls, while ONU burst durations are in the order of ~1 its to ~lms. Therefore, lasers employed by such ONUs may be required to be always on, in order to allow for convergence of the control loops during TXOFF. Consequently, a shutter may have to be integrated (e.g., using a variable optical attenuator, VOA, or a semiconductor optical amplifier, SOA) to comply with the above-mentioned requirement for a large TXON / TXOFF ratio. However, ideal bias voltages for the modulator may differ from TXOFF to TXON, for instance, due to crosstalk between the modulator and thermally coupled heat-producing components on the PIC, which are toggled when transitioning from TXOFF to TXON. Examples for such heat-producing components are, e.g., a driver amplifier that is electrically connected (e.g., with short traces and / or bonding wires) to the modulator, the modulator termination resistors, which transform an RF data signal into heat at the end of the modulator, or integrated photonic shutters or amplifiers (like PIN-diode VOAs or III / V SOAs), or others. These heat-producing components can change the thermal gradient on the PIC when switching. Consequently, the interference condition of, for example, any interferometric structure of the PIC or of a resonator on the PIC can be affected. As a numeric example, a change of an average temperature by 0.1 °C in the two arms of an MZM can correspond to a phase error of 12.9° on a silicon PIC. In case a laser is also integrated, or closely coupled to the PIC, also the laser’s resonance and emitted frequency could be affected by the temperature change or gradient.

[0012] A time constant of thermal crosstalk between, for example, a modulator or laser and a locally switched heat-producing component depends on the physical distance between these components, and may range from a few its to few ms, which corresponds to the expected range of ONU burst durations. Neither control loops of the modulator nor of the laser would be fast enough to compensate for these changes in thermal gradients. Therefore, non-ideal operating points for integrated modulators and lasers may be expected, especially, during burst-mode transmission.

[0013] In view of this, an objective of this disclosure is to provide a solution for fast changes of temperature or thermal gradients on a PIC during burst-mode operation of an optical transmitter of the PIC. A consequent objective is to reduce or avoid interferometric phase errors in resonant structures, for instance, modulators or lasers of the PIC. To this end, an objective is also to minimize temperature fluctuations on the PIC, which may minimize the interferometric phase errors during TXON. Another objective is to reduce a power consumption of the PIC, especially during TXOFF.

[0014] These and other objectives are achieved by the solutions of this disclosure as described in the independent claims. Advantageous implementations are further described in the dependent claims.

[0015] A first aspect of this disclosure provides a PIC for an ONU of a PON, wherein the PIC comprises: an optical transmitter configured to transition between an off-state (TXOFF), and an on-state (TXON), and to transmit a data burst during TXON; a heat-producing component, wherein a heat dissipation of the heat-producing component changes with the transition between TXOFF and TXON; a heater associated with the heat-producing component; and a controller configured to control a heat dissipation of the heater to increase with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component decreases with the transition, and to decrease with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component increases with the transition.

[0016] The disclosure accordingly proposes to use a combination of one or more distributed and controllable heaters on the PIC, in order to stabilize a temperature or thermal gradient caused by one or more heat-producing components on the PIC . F or example, the stabilization is beneficial in the proximity of one or more interferometric structures and / or resonators of the PIC. A goal is to make certain required operating points of, for example, a laser or modulator, as similar as possible during TXOFF and TXON. In this way, burst-mode related transmission penalties of the optical transmitter, which arise due to non-ideal operating points, may be avoided or reduced.

[0017] In the PIC of the first aspect, for example, the heater (may be one of multiple heaters of the PIC) is used to compensate fast changes of thermal gradients during a burst-mode operation of the optical transmitter, wherein the thermal gradient changes are caused by the heat-producing component associated with said heater. As a consequence, reduced interferometric phase errors in resonant structures, for instance, in modulators or lasers of the PIC may be expected, if they are affected by these thermal gradients. The controller may reduce or minimize temperature fluctuations on the PIC between TXOFF and TXON by controlling the heater.

[0018] In an implementation form of the first aspect, at least one sub-area of the PIC is sensitive to a temperature change or temperature-gradient change. For example, the temperature-sensitive sub-area can include an interferometric structure or resonant structure, which may be affected by the temperature or temperature-gradient change, and are in this sense “sensitive” to the temperature or temperature-gradient change.

[0019] In an implementation form of the first aspect, the controller is configured to control the heat dissipation of the heater during TXON to correspond to a heat spread of the heat-producing component to the at least one sub-area during TXOFF; and / or control the heat dissipation of the heater during TXOFF to correspond to the heat spread of the heat-producing component to at least one sub-area during TXON. In this way, temperature or temperature-gradient changes affecting the sensitive sub-area can be reduced or avoided entirely. This may reduce errors in, for example, interferometric structure and / or resonant structures in the sub-area.

[0020] In an implementation form of the first aspect, the controller is configured to control the heat dissipation of the heater based on a computed or simulated heat dissipation of the heat-producing component and based on the relative position of heater and heat-producing component on the PIC and / or based on the shape of the heat-producing component and of the heater. Thus, an optimized heat compensation to compensate temperature and / or temperature gradient changes caused by associated heatproducing components, wherein the heat compensation is provided by the heater through means of the controller, can be calculated and provided in advance.

[0021] In an implementation form of the first aspect, the PIC further comprises one or more temperature sensors configured to measure a temperature and / or a temperature gradient on the PIC and / or of the heat-producing component during TXON and / or during TXOFF; wherein the controller is configured to control the heat dissipation of the heater based on at least one temperature measurement of the one or more temperature sensors. In this way, the control of one or more heaters on the PIC by the controller may be more accurate - and particularly may be adjusted dynamically - to compensate temperature and / or temperature gradient changes caused by associated heat-producing components.

[0022] In an implementation form of the first aspect, the controller is configured to control the heat dissipation of the heater to increase or decrease with a delay to or in advance of a time point of the transition between TXOFF and TXON. This allows optimizing the desired heat compensation by the heater, for instance, depending on the location of the heater. For example, the relative location of the heater and the associated heat-producing component to a temperature-sensitive sub-area of the PIC may be taken into account.

[0023] In an implementation form of the first aspect, the controller is configured to control the heat dissipation of the heater by supplying a static or ramped current to the heater. This allows the controller to produce an optimized heat compensation with the heater.

[0024] In an implementation form of the first aspect, the heat-producing component is a transmitter component of the optical transmitter. Such transmitter component will typically be active during TXON, and may show increased heat dissipation when active.

[0025] In an implementation form of the first aspect, the heat-producing component is a modulator driver, or an optical amplifier, or an optical shutter, or a radio frequency termination, or a variable optical attenuator, or a connection of the PIC to a DC power supply. Such heat-producing components are switched with a transition between TXON and TXOFF (note that a transition between these two states could be from TXON to TXOFF or vice versa from TXOFF to TXON in this disclosure), and thus produced a change of temperature or temperature gradient on the PIC.

[0026] In an implementation form of the first aspect, the heater is located in the vicinity of the associated heat-producing component and / or is thermally coupled to the associated heat-producing component. In this way, the heat compensation with the heater may be implemented effectively.

[0027] In an implementation form of the first aspect, a shape and / or location of the heater on the PIC is selected based on one or more of: a low power dissipation or consumption; a minimum temperature change in at least one sub-area of the PIC; a constant temperature profile in proximity of an optical interferometric structures of the PIC. This optimization allows reducing the overall power consumption of the PIC and improved performance, especially during burst-mode transmission.

[0028] In an implementation form of the first aspect, the heater comprises metal and / or semiconductor material.

[0029] In an implementation form of the first aspect, the semiconductor material of the heater comprises a PN junction, a PIN junction, or a heterojunction.

[0030] In an implementation form of the first aspect, the heater is a resistive heater or an optically disconnected copy of the heatproducing component.

[0031] In an implementation form of the first aspect, the PIC further comprises an optical interferometric structure, which is thermally coupled to the heat-producing component and the heater. The optical interferometric structure may be impacted by the toggling of the heat-producing component, which can be compensated by controlling the heater.

[0032] In an implementation form of the first aspect, the optical interferometric structure is part of an optical modulator.

[0033] In an implementation form of the first aspect, the optical modulator comprises a Mach-Zehnder modulator, or an in-phase quadrature modulator, or a ring resonator.

[0034] In an implementation form of the first aspect, the heater is provided on or is integrated with a contact pad of the Mach-Zehnder modulator.

[0035] In an implementation form of the first aspect, the PIC further comprises one or more on-chip lasers, or an optical coupler configured to couple to an external laser or to a waveguide.

[0036] In an implementation form of the first aspect, the optical interferometric structure is part of a laser.

[0037] In an implementation form of the first aspect, the laser comprises Mach-Zehnder interferometers, gratings, or ring resonators.

[0038] In an implementation form of the first aspect, the PIC comprises an optical receiver. The optical receiver may be one of the at least one sub-area of the PIC, which is sensitive to a temperature change or temperature-gradient change.

[0039] In an implementation form of the first aspect, the optical interferometric structure is part of a receiver circuit. The receiver circuit may be part of the optical receiver of the PIC. In an implementation form of the first aspect, the receiver circuit comprises at least one optical filter.

[0040] In an implementation form of the first aspect, the at least one optical filter of the receiver circuit comprises one or more Mach- Zehnder interferometers, and / or gratings, and / or ring resonators.

[0041] In an implementation form of the first aspect, the receiver circuit comprises a laser.

[0042] In an implementation form of the first aspect, the laser of the receiver circuit comprises one or more Mach-Zehnder interferometers, and / or gratings, and / or or ring resonators.

[0043] In an implementation form of the first aspect, the heat-producing component is a driver amplifier, which is configured to be always on, or is configured to be respectively activated and deactivated with the transition between TXOFF and TXON.

[0044] In an implementation form of the first aspect, the PIC comprises multiple heat-producing components, wherein a respective heat dissipation of each heat-producing component changes with the transition between TXOFF and TXON; multiple heaters, wherein each heater is associated with one or more of the heat-producing components; and wherein the controller is configured to individually control the heat dissipation of each heater to change with the transition between TXOFF and TXON. One or more heaters can be controlled for compensating one heat-producing component. One heater can, however, also be used for compensating several heat-producing components in its vicinity.

[0045] A second aspect of this disclosure provides a method for controlling a thermal distribution on a PIC for an ONU of a PON during an off-state, TXOFF, and an on-state, TXON of an optical transmitter of the PIC, which transmits a data burst during TXON, wherein the method comprises: controlling a heat dissipation of one or more heaters to increase with the transition between TXOFF and TXON, if a heat dissipation of a heat-producing component, which is associated with the heater, decreases with the transition between TXOFF and TXON; and controlling a heat dissipation of one or more heaters to decrease with the transition between TXOFF and TXON, if the heat dissipation of the associated heat-producing component increases with the transition between TXOFF and TXON.

[0046] In an implementation form of the second aspect, at least one sub-area of the PIC is sensitive to a temperature change or temperature-gradient change.

[0047] In an implementation form of the second aspect, the method further comprises controlling the heat dissipation of the heater during TXON to correspond to a heat spread of the heat-producing component to the at least one sub-area during TXOFF; and / or controlling the heat dissipation of the heater during TXOFF to correspond to the heat spread of the heat-producing component to the at least one sub-area during TXON.

[0048] In an implementation form of the second aspect, the method further comprises controlling the heat dissipation of the heater based on a computed or simulated heat dissipation of the heat-producing component and based on the relative position of heater and heat-producing component on the PIC and / or based on the shape of the heat-producing component and of the heater.

[0049] In an implementation form of the second aspect, the PIC further comprises one or more temperature sensors configured to measure a temperature and / or a temperature gradient on the PIC and / or of the heat-producing component during TXON and / or during TXOFF; wherein the method further comprises controlling the heat dissipation of the heater based on at least one temperature measurement of the one or more temperature sensors. In an implementation form of the second aspect, the method comprises controlling the heat dissipation of the heater to increase or decrease with a delay to or in advance of a time point of the transition between TXOFF and TXON.

[0050] In an implementation form of the second aspect, the method comprises controlling the heat dissipation of the heater by supplying a static or ramped current to the heater.

[0051] In an implementation form of the second aspect, the heat-producing component is a transmitter component of the optical transmitter.

[0052] In an implementation form of the second aspect, the heat-producing component is a modulator driver, or an optical amplifier, or an optical shutter, or a radio frequency termination, or a variable optical attenuator, or a connection of the PIC to a DC power supply.

[0053] In an implementation form of the second aspect, the heater is located in the vicinity of the associated heat-producing component and / or is thermally coupled to the associated heat-producing component.

[0054] In an implementation form of the second aspect, a shape and / or location of the heater on the PIC is based on one or more of: a low power dissipation or consumption; a minimum temperature change in at least one sub-area of the PIC; a constant temperature profile in proximity of an optical interferometric structures of the PIC.

[0055] In an implementation form of the second aspect, the heater comprises metal and / or semiconductor material.

[0056] In an implementation form of the second aspect, the semiconductor material of the heater comprises a PN junction, a PIN junction, or a heterojunction.

[0057] In an implementation form of the second aspect, the heater is a resistive heater or an optically disconnected copy of the heatproducing component.

[0058] In an implementation form of the second aspect, the PIC further comprises an optical interferometric structure, which is thermally coupled to the heat-producing component and the heater.

[0059] In an implementation form of the second aspect, the optical interferometric structure is part of an optical modulator.

[0060] In an implementation form of the second aspect, the optical modulator comprises a Mach-Zehnder modulator, or an in-phase quadrature modulator, or a ring resonator.

[0061] In an implementation form of the second aspect, the heater is provided on or is integrated with a contact pad of the Mach- Zehnder modulator.

[0062] In an implementation form of the second aspect, the PIC further comprises one or more on-chip lasers, or an optical coupler configured to couple to an external laser or to a waveguide.

[0063] In an implementation form of the second aspect, the optical interferometric structure is part of a laser.

[0064] In an implementation form of the second aspect, the laser comprises Mach-Zehnder interferometers, gratings, or ring resonators. In an implementation form of the second aspect, the PIC comprises an optical receiver.

[0065] In an implementation form of the second aspect, the optical interferometric structure is part of a receiver circuit.

[0066] In an implementation form of the second aspect, the receiver circuit comprises at least one optical filter.

[0067] In an implementation form of the second aspect, the at least one optical filter of the receiver circuit comprises one or more Mach-Zehnder interferometers, and / or gratings, and / or ring resonators.

[0068] In an implementation form of the second aspect, the receiver circuit comprises a laser.

[0069] In an implementation form of the second aspect, the laser of the receiver circuit comprises one or more Mach-Zehnder interferometers, and / or gratings, and / or or ring resonators.

[0070] In an implementation form of the second aspect, the heat-producing component is a driver amplifier, which is configured to be always on, or is configured to be respectively activated and deactivated with the transition between TXOFF and TXON.

[0071] In an implementation form of the second aspect, the PIC comprises multiple heat-producing components, wherein a respective heat dissipation of each heat-producing component changes with the transition between TXOFF and TXON; multiple heaters, wherein each heater is associated with one or more of the heat-producing components; and wherein the method comprises individually controlling the heat dissipation of each heater to change with the transition between TXOFF and TXON.

[0072] A third aspect of this disclosure provides a computer program comprising instructions which, when the computer program is executed by a processor of a controller, causes the controller to perform the method steps of the method of the second aspect or any implementation form thereof.

[0073] In summary of the above aspects and implementation forms, this disclosure proposes integrating one or more so-called local compensation heaters on the PIC. The one or more heaters serve the purpose of minimizing thermal fluctuations, especially in critical regions of the PIC, when the module is transitioning from TXOFF to TXON and vice versa. Critical regions may be temperature or temperature gradient sensitive sub-areas, like interferometric or resonant structures. The one or more heaters may be positioned close to one or more heat-producing components of the PIC (e.g., driver amplifier connection, optical shutter / amplifier, RF termination resistor). The one or more heaters may be switched from active to inactive when transitioning from TXOFF to TXON, if their associated heat-producing components) increase(s) their power consumption when transitioning to TXON, or may be switched from inactive to active when transitioning from TXOFF to TXON, if their associated heat-producing component(s) decrease(s) their power consumption when transitioning to TXON.

[0074] The power dissipation of the one or more heaters may be set such (by the controller) that it emulates the dissipated heat of the associated heat-producing components). In this way, thermal gradients (on the critical regions or sensitive sub-areas) of the PIC can be minimized. Consequently, a bias and / or operating voltage of, for instance, modulators and lasers of the PIC, which may be set and optimized by control loops and that may have converged during TXOFF, may also be at least close to the ideal during TXON.

[0075] It has to be noted that some entities, elements, units and means described in the present application could be implemented by software or hardware elements or any kind of combination thereof. Some steps performed by the various entities described in the present application, as well as the functionalities described to be performed by the various entities, are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity, which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented by respective software or hardware elements, or any kind of combination thereof.

[0076] BRIEF DESCRIPTION OF DRAWINGS

[0077] The above described aspects and implementation forms are explained in the following description in relation to the enclosed drawings, in which:

[0078] FIG. 1 shows a PIC according to this disclosure.

[0079] FIG. 2 shows a first exemplary PIC according to this disclosure.

[0080] FIG. 3 shows a second exemplary PIC according to this disclosure.

[0081] FIG. 4 shows an example of a shape and location optimized heater for PIC including an MZM.

[0082] FIG. 5 shows a fourth exemplary PIC according to this disclosure.

[0083] FIG. 6 shows a fifth exemplary PIC according to this disclosure.

[0084] FIG. 7 shows a sixth exemplary PIC according to this disclosure.

[0085] FIG. 8 shows a method for controlling a thermal distribution on a PIC, according to this disclosure.

[0086] FIG. 9 shows an exemplary method for burst-mode data transmission of a PIC, according to this disclosure.

[0087] FIG. 10 shows a seventh exemplary PIC according to this disclosure, which includes an optical receiver.

[0088] DETAILED DESCRIPTION OF EMBODIMENTS

[0089] FIG. 1 shows a PIC 100 according to this disclosure. The PIC 100 can be used in an ONU of a PON. The PON may be a point to multi-point network, including a central optical line terminal (OLT) connected optically to multiple ONUs, for example, via passive splitters. One or more or all ONUs may comprise a PIC 100. Also, the OLT may comprise a PIC 100 as described.

[0090] The PIC 100 comprises an optical transmitter 101, which is configured to transition between an off-state (TXOFF) and an on- state (TXON), and to transmit a data burst 102 during TXON. The optical transmitter 101 may also be able to transmit data normally, e.g., not as data burst 102, however, the solutions of this disclosure are especially relevant for stabilizing the optical transmitter 101 in burst-mode transmission. The optical transmitter may comprise typical optical transmitter components used in a conventional ONU. The PIC 100 further comprises a heat-producing component 103. The heat-producing component 103 may belong to the optical transmitter 101 (as illustrated), but can also be outside the optical transmitter 101, e.g., a component in addition to the optical transmitter 101 on the PIC 100. The heat-producing component 103 may be a heat source, which switches with TXOFF / TXON. Specifically, a heat dissipation of the heat-producing component 103 changes with the transition between TXOFF and TXON (either direction). The heat-producing component 103 may, for example, be a modulator driver, or an optical amplifier, or an optical shutter, or a radio frequency termination, or a variable optical attenuator, or a similar component on a PIC. The PIC 100 may comprise more than one heat-producing component 103, for instance, multiple heat-producing components 103 of the same type and / or multiple heat-producing components 103 of different types.

[0091] The PIC 100 further comprises a heater 104, which is associated with the heat-producing component 103. Associated means that the heater 104 will be used to at least partly compensate changes in the heat dissipation of the heat-producing component 103, and may mean that the heater 104 is in the vicinity of the heat-producing component 103. The PIC 100 may comprise more than one heater 104 associated with the heat-producing component 103. If the PIC 100 comprises more than one heatproducing component 103, the PIC 104 may also comprise multiple heaters 104 associated respectively (at least one) with the multiple heat-producing components 103. Each heater 104 may be implemented as a resistive heater, or as an optically disconnected copy of the heat-producing component 103 it is associated with. An "optically disconnected component" may refer to a component of the PIC 100 that is not part of the active optical pathway within the PIC 100. This may mean that it does not interact with or influence optical signals being processed or transmitted through the optical waveguides or other optical elements on the PIC 100.

[0092] The PIC 100 further comprises a controller 105. The controller 105 may comprise a processor (not shown) or processing circuitry configured to perform, conduct or initiate various operations of the controller 105 described herein. The processing circuitry may comprise hardware and / or the processing circuitry may be controlled by software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The controller 105 may respectively further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor or by the processing circuitry, in particular under control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor or the processing circuitry, causes the various operations of the controller 105 to be performed. In one embodiment, the processing circuitry comprises one or more processors and a non-transitory memory connected to the one or more processors. The non-transitory memory may carry executable program code which, when executed by the one or more processors, causes the controller 105 to perform, conduct or initiate the operations or methods described herein.

[0093] The memory may also be configured to store heater control parameters, for example, in the form of EEPROM entries. The heater control parameters may comprise one or more of a target heater current, a current gradient, or a tum-on time delay, etc.

[0094] Specifically, the controller 105 is configured to control the heater 104 (or more than one heater 104). The controller 105 is configured to control a heat dissipation of the heater 104. The controller 105 can control the heat dissipation to increase with the transition between TXOFF and TXON, if the heat dissipation of the associated heat-producing component 103 decreases with the transition, and can control the heat dissipation to decrease with the transition between TXOFF and TXON if the heat dissipation of the associated heat-producing component 103 increases with the transition. For example, the controller 105 may be configured to control the heat dissipation of the heater 104 during TXON to correspond to a heat spread of the heat-producing component 103 - e.g. to at least one temperature or temperature-gradient sensitive sub-area - during TXOFF. Additionally, or alternatively, the controller 105 may be configured to control the heat dissipation of the heater 104 during TXOFF to correspond to the heat spread of the heat-producing component 103 - e.g. to the at least one sub-area - during TXON. The controller 105 may control the heater 104 based on a simulated and / or computed heat dissipation of the associated heat-producing component 103, and / or based on temperature measurements conducted on the PIC 100 during operation.

[0095] FIG. 2 and FIG. 3 show examples of a PIC 100 according to this disclosure, which is based on the PIC 100 of FIG. 1. Same elements in FIG. 1 and FIGs 2 and 3, respectively, are labelled with the same reference signs and may be implemented likewise.

[0096] The exemplary PIC 100 has at least one integrated laser 202 and one modulator. The modulator may be an MZM as depicted in FIG. 2, or may be a dual-polarization MZM (DP-MZM), or may be an IQ modulator, or may be a dual-polarization IQ (DP- IQ) modulator as depicted in FIG. 3. One or more integrated heaters 104 (referred to as “compensation heaters” in this disclosure and, for instance, implemented as heating elements), e.g. resistive metal heaters, are positioned on the PIC 100. Typically, but not necessarily, each heater 104 is located in proximity of at least one heat-producing component 103 (which is a heat source that switches with TXON / TXOFF), and may be used to emulate the heat generation of the at least one heatproducing component 103 during TXOFF. Various examples of such heat-producing components 103 are depicted in FIG. 2 and FIG. 3, respectively, and comprise one or more of an optical shutter and / or an optical amplifier, RF terminations that convert RF power into heat, a thermal coupling of driver amplifier channels to modulator RF electrodes, and a DC power supply connected to a RF termination resistor (in case the RF driver, e.g. an open-collector driver, is at least partially powered through an MZM). This list of possible heat-producing components 103 is to be understood as exemplary, and not exhaustive. Further heat-producing components 103 or heat sources may be identified and compensated by the approach of this disclosure as well.

[0097] Optionally, one or more temperature sensors 210 can be arranged and / or distributed on the PIC 100, and may be used to measure temperature gradients during TXOFF and TXON, respectively. For instance, during calibration of the PIC 100 to identify suitable currents for the compensation heaters 104, or during operation of the PIC 100. However, the currents for the compensation heaters 104 could also be set according to calculations and / or simulations of experimentally determined systematic settings. In an example, the one or more temperature sensors 201 are configured to measure a temperature and / or a temperature gradient on the PIC 100 and / or of at least one heat-producing component 103, during TXON and / or during TXOFF. In this case, the controller 105 is configured to control the heat dissipation of the heater 104 based on at least one temperature measurement of the one or more temperature sensors 201. In another example, the controller 105 is configured to control the heat dissipation of the heater 104 based on a computed and / or simulated heat dissipation of at least one heat-producing component 103, and based on a relative position of at least one heater 104 and the at least one heat-producing component 103 on the PIC 100. The examples may be combined, that is, the controller 105 may use both simulations / calculations and measurements, based on which to control one or more heaters 104.

[0098] The controller 105 may additionally or alternatively be configured to control the heat dissipation of the heater 104 based on the shape of the heat-producing component 103 and / or of the heater 104, and / or based on their respective or relative location on the PIC 100. The shape and location of each heater 104 may be optimized to minimize thermal changes on the PIC 100, while simultaneously minimizing power consumption and dynamic behavior. For example, FIG. 4 shows a heater 104 with an optimized geometry and location, wherein the heater 104 is associated with a shutter / amplifier as the heat-producing component 103, and may be used to compensate the heat dissipation changes towards an MZM 401, which may be a temperature or temperature-gradient sensitive sub-area of the PIC 100. Compensation heater currents could be set statically or in ramped manner, and / or could be set by a control loop. Further, they could be delayed or set in advance of the onset of the transmission data burst 102, in order to minimize thermal variations in interferometric structures in a modulator or laser section, for example. There could also be receiver circuitry co-integrated on the PIC 100. That is, the PIC 100 may further comprise and optical receiver. The optical receiver may comprise an optical filter, which is an interferometric structure that is temperature sensitive. At least one heater 104 that is arranged on or at the optical transmitter 101, may also be controlled to help minimizing temperature changes in the optical filter of the optical receiver. For example, the controller 105 may be configured to control the heat dissipation of the heater 104 to increase or decrease with a delay to or in advance of a time point of the transition between TXOFF and TXON.

[0099] Advantageously, the addition of the one or more compensation heaters 104 may lead to a minimization of thermal changes in, for instance, integrated interferometric structures on the PIC 100 (like the MZM or DP-IQ modulator of FIG. 2 and FIG. 3, respectively). In this way, the burst mode performance of the optical transmitter 101 of the PIC 100 can be improved, ideally optimized. The addition of, for instance, one or more MZM pad compensation heaters 104, e.g., one or more heaters 104 provided on or integrated with a contact pad of the MZM, may allow turning off the driver during TXOFF, which reduces power consumption of the PIC 100. The optimization of the location and / or shape of the heater 104 may also reduce the power consumption. For example, placing the compensation heater(s) 104 closer to the interferometric structures may reduce the required power to achieve the same amount of heating of the interferometric structure, e.g., may reduce the power consumption of the heater(s) 104.

[0100] Further exemplary PICs 100, which are based on the PICs 100 of FIG. 2 and FIG. 3, respectively, are shown in FIG. 5 and FIG. 6. The PICs 100 are the same as in FIGs 2 and 3, but without integrated laser (source) 202. Rather, an external laser source is coupled to the PIC 100, either through a fiber or via free space optics. The PIC 100 may have a suitable optical coupler. Since the laser source is off-chip in this case, for example, is arranged on another chip than the PIC 100, the thermal coupling between the laser source and the PIC 100 are minimized. The compensation heaters 104 on the PIC 100 will thus primarily improve the performance of structures on the PIC 100, for instance, a modulator and / or a potential receiver filter, but not the laser.

[0101] FIG. 7 shows another example of a PIC 100, which builds on the PICs shown in FIG. 2. Same elements are labelled with the same reference signs and may be implemented likewise. The PIC 100 shown in FIG. 7 is the same as in FIG. 2, but with a driver amplifier that is configured to be always on. In that case, the MZM pad compensation heaters 104 can be omitted. A similar example is possible based on the PIC 100 shown in FIG. 3, e.g., also with omitted MZM pad heaters 104. The advantages thereof are that no thermal fluctuations arise due to driver amplifier power toggling. However, this can come at the price of larger power consumption during TXOFF.

[0102] In all examples of the PIC 100 according to this disclosure, any heater 104 on the PIC 100 could be realized in various ways, namely as: metal heater; doped semiconductor; PN-junction and hetero-junction; PIN-junction; hetero-junction; SOA.

[0103] Fundamentally, it would be possible to realize any heater 104 as an unconnected copy of the heat-producing component 103 it is associated with (e.g., a second SOA 104 associated with a first SOA 103, wherein an optical input and output of the second SOA 104 are not connected to the optical circuit). When transitioning between TXON / TXOFF, a toggling between functional and copied component may be performed.

[0104] FIG. 8 shows a method 800 for controlling a thermal distribution on a PIC 100 according to this disclosure. The PIC 100 may be any PIC 100 shown in FIGs 1-7, and comprises the optical transmitter 101, which transmits a data burst 102 during TXON.

[0105] The method 800 comprises a step 801 of controlling a heat dissipation of one or more first heaters 104 of the PIC 100 to increase with the transition between TXOFF and TXON, if a heat dissipation of one or more first heat-producing components 103, which are associated with the one or more first heaters 104, decreases with the transition between TXOFF and TXON, and / or comprises a step 802 of controlling a heat dissipation of one or more second heaters 104 to decrease with the transition between TXOFF and TXON, if the heat dissipation of one or more second heat-producing components 103 associated with the one or more second heaters 104 increases with the transition between TXOFF and TXON. FIG. 9 illustrates a response of the PIC 100 according to this disclosure to a burst transmission request. In this example, the PIC 100 is included in an ONU of a PON.

[0106] The ONU receives the request from the OLT of the PON, to send an upstream data burst 102 at a specific time tb and for a specific duration At, to assure that there are no TDM collisions with other ONUs. For transmitting the data, the ONU is transitioning from TXOFF to TXON for the duration of the data burst 102. During the data burst 102, the ONU may receive RF data signals to its electrical driver amplifiers, which provide an amplified drive signal to the modulator. The driver amplifier may or may not be toggled between its high power and low power state, when transitioning from TXON to TXOFF.

[0107] During TXON the power consumption of the PIC 100 typically increases due to activation of on-chip components, such optical amplifiers, etc. The resulting change in heat dissipation on the PIC 100 changes the thermal gradients on the PIC 100, which may lead to undesired offsets from the ideal values of the set bias voltages of, for instance, interferometric structures. To minimize these changes in the thermal gradients, the one or more compensation heaters 104 are added on the PIC 100, typically, but not necessarily, in proximity of the heat-producing components 103, which include the optical shutters, optical amplifiers etc. The heaters 104 may be controlled to emulate the heat generation of the heat-producing component(s) 103 during TXOFF. In this way, the state of the interferometric structure, for example, may only marginally change when transitioning from TXOFF to TXON, or vice versa. An illustration of using the toggling compensation heaters 104 during burst operation is provided by FIG. 9.

[0108] In particular, as shown in block 901 of FIG. 9, the optical transmitter 101 of the PIC 100 is initially in TXOFF. As shown in block 902, the ONU including the PIC 100 then receives the burst transmission request. As shown in block 904, this causes one or more switchable transmitter components of the optical transmitter 101 of the PIC 100 to be activated at the transition from TXOFF to TXON (or slightly before). As further shown in block 903, the one or more compensation heaters 104 are turned off or on at the transition at time tb between TXOFF and TXON (or slightly before or after), depending e.g. on the location of the heater 104. The optical transmitter 101 of the PIC 100 is now in TXON, as shown in block 905, and may transmit a data burst 102. As shown in block 907, the switchable transmitter components of the optical transmitter 101 of the PIC 100 are then deactivated after the data burst 102 at time tb+ At (or slightly after). As shown in block 906, the compensation heaters 104 are turned on or off at the transition between TXON and TXOFF, e.g. at the end of the data burst 102 at time tb + At (or slightly before or after), e.g. depending on the location of the heater 104. Then, the optical transmitter 101 of the PIC 100 is again in TXOFF, as in block 901. The procedure of FIG. 9 may repeat with the same or next burst request.

[0109] FIG. 10 shows another example of a PIC 100 according to this disclosure, which is based on the PIC 100 of FIG. 1. Same elements in FIG. 1 and FIG. 10 are labelled with the same reference signs and may be implemented likewise.

[0110] In addition to the PIC components shown in FIG. 1 , the PIC 100 of FIG. 10 also comprises an optical receiver 1001. The optical receiver 1001 is configured to receive data 1002, for instance, continuous data or a data burst.

[0111] The optical receiver 1001 may be thermally coupled to the optical transmitter 101, and may be thermally coupled to at least one heat-producing component 103. This may be a heat-producing component 103 of the optical transmitter 101 (as illustrated). Additionally, or alternatively, this may be another heat-producing component 103, for instance, of the optical receiver 1001 or located near it (as also illustrated).

[0112] The optical receiver 1001 may be a thermally sensitive sub-area of the PIC 100, e.g., may be sensitive to a temperature change or temperature-gradient change. For example, the optical receiver 1001 may comprise and optical filter (not shown), which may be implemented as interferometric structure or resonant structure, and / or may comprise a receiver laser for coherent reception. All these components are temperature sensitive, and could be affected during a transition between TXON and TXOFF, for the same reasons as explained above.

[0113] The controller 105 may control the at least one heater 104 - or a dedicated receiver heater 104 -with the purpose of stabilizing the optical transmitter 1001. For example, the controller 105 may control the heat dissipation of the at least one heater 104 during TXON or TXOFF, to correspond to a heat spread of at least one heat-producing component 103 to the optical receiver 1001 during TXOFF or TXON, respectively.

[0114] Any one of the exemplary PICs 100 shown in FIG. 2, 3, 5, 6, or 7 may comprise an optical receiver 1001.

[0115] In summary, the solutions of this disclosure, which are based on controlling at least one heater 104 associated with at least one heat-producing component 103 of the PIC 100, provide several advantages. For example, modulator and laser operating points may be closer to the ideal ones, which may lead to a reduction of receiver (RX) sensitivity penalty during upstream burst(s). For example, a reduction of power consumption may be achieved, because a driver can be switched off. For example, and optimized location and resistance profile of the compensation heaters 104 may also lower the power consumption.

[0116] The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

CLAIMS1. A photonic integrated chip, PIC, (100) for an optical network unit, ONU, of a passive optical network, PON, wherein the PIC (100) comprises: an optical transmitter (101) configured to transition between an off-state, TXOFF, and an on-state, TXON, and to transmit a data burst (102) during TXON; a heat-producing component (103), wherein a heat dissipation of the heat-producing component (103) changes with the transition between TXOFF and TXON; a heater (104) associated with the heat-producing component (103); and a controller (105) configured to control a heat dissipation of the heater (104) to increase with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component (103) decreases with the transition, and to decrease with the transition between TXOFF and TXON if the heat dissipation of the heat-producing component (103) increases with the transition.

2. The PIC (100) according to claim 1, wherein at least one sub-area of the PIC (100) is sensitive to a temperature change or temperature-gradient change.

3. The PIC (100) according to claim 2, wherein the controller (105) is configured to: control the heat dissipation of the heater (104) during TXON to correspond to a heat spread of the heat-producing component (103) to the at least one sub-area during TXOFF; and / or control the heat dissipation of the heater (104) during TXOFF to correspond to the heat spread of the heat-producing component (103) to the at least one sub-area during TXON.

4. The PIC (100) according to one of the claims 1 to 3, wherein the controller (105) is configured to control the heat dissipation of the heater (104) based on a computed or simulated heat dissipation of the heat-producing component (103) and based on the relative position of heater (104) and heatproducing component (103) on the PIC (100) and / or based on the shape of the heat-producing component (103) and of the heater (104).

5. The PIC (100) according to one of the claims 1 to 4, further comprising: one or more temperature sensors (201) configured to measure a temperature and / or a temperature gradient on the PIC and / or of the heat-producing component during TXON and / or during TXOFF; wherein the controller (105) is configured to control the heat dissipation of the heater (104) based on at least one temperature measurement of the one or more temperature sensors (201 ).

6. The PIC (100) according to one of the claims 1 to 5, wherein the controller (105) is configured to control the heat dissipation of the heater (104) to increase or decrease with a delay to or in advance of a time point of the transition between TXOFF and TXON.

7. The PIC (100) according to one of the claims 1 to 6, wherein the controller (105) is configured to control the heat dissipation of the heater (104) by supplying a static or ramped current to the heater (104).

8. The PIC (100) according to one of the claims 1 to 7, wherein the heat-producing component (103) is a transmitter component of the optical transmitter (101).

9. The PIC (100) according to one of the claims 1 to 8, wherein the heat-producing component (103) is a modulator driver, or an optical amplifier, or an optical shutter, or a radio frequency termination, or a variable optical attenuator, or a connection of the PIC to a DC power supply.

10. The PIC (100) according to one of the claims 1 to 9, wherein the heater (104) is located in the vicinity of the associated heat-producing component (103) and / or is thermally coupled to the associated heat-producing component (103).

11. The PIC (100) according to one of the claims 1 to 10, wherein a shape and / or location of the heater (104) on the PIC (100) is selected based on one or more of: a low power dissipation or consumption; a minimum temperature change in at least one sub-area of the PIC (100); a constant temperature profile in proximity of an optical interferometric structures of the PIC (100).

12. The PIC (100) according to one of the claims 1 to 11, wherein the heater (104) comprises metal and / or semiconductor material.

13. The PIC (100) according to one of the claims 1 to 12, wherein the semiconductor material of the heater (104) comprises a PN junction, a PIN junction, or a heterojunction.

14. The PIC (100) according to one of the claims 1 to 13, wherein the heater (104) is a resistive heater or an optically disconnected copy of the heat-producing component (103).

15. The PIC (100) according to one of the claims 1 to 14, further comprising an optical interferometric structure, which is thermally coupled to the heat-producing component (103) and the heater (104).

16. The PIC (100) according to claim 15, wherein the optical interferometric structure is part of an optical modulator.

17. The PIC (100) according to claim 16, wherein the optical modulator comprises a Mach-Zehnder modulator (401), or an in-phase quadrature modulator, or a ring resonator.

18. The PIC (100) according to claim 16 or 17, wherein the heater (104) is provided on or is integrated with a contact pad of the Mach-Zehnder modulator (401 ).

19. The PIC (100) according to one of the claims 1 to 18, further comprising one or more on-chip lasers (202), or an optical coupler configured to couple to an external laser or to a waveguide.

20. The PIC (100) according to one of the claims 15 to 18, wherein the optical interferometric structure is part of a laser (202).

21. The PIC (100) according to claim 20, wherein the laser comprises Mach-Zehnder interferometers, gratings, or ring resonators.

22. The PIC (100) according to one of the claims 1 to 21, wherein the heat-producing component (103) is a driver amplifier, which is configured to be always on, or is configured to be respectively activated and deactivated with the transition between TXOFF and TXON.

23. The PIC (100) according to one of the claims 1 to 22, comprising: multiple heat-producing components (103), wherein a respective heat dissipation of each heat-producing component (103) changes with the transition between TXOFF and TXON; multiple heaters (104), wherein each heater (104) is associated with one or more of the heat-producing components (103); and wherein the controller (105) is configured to individually control the heat dissipation of each heater (104) to change with the transition between TXOFF and TXON.

24. A method (800) for controlling a thermal distribution on a photonic integrated chip, PIC, (100) for an optical network unit, ONU, of a passive optical network, PON, during an off-state, TXOFF, and an on-state, TXON of an optical transmitter (101) of the PIC (100), which transmits a data burst (102) during TXON, wherein the method (800) comprises: controlling (801) a heat dissipation of one or more heaters (104) to increase with the transition between TXOFF and TXON, if a heat dissipation of a heat-producing component (103), which is associated with the heater (104), decreases with the transition between TXOFF and TXON; and controlling (802) a heat dissipation of one or more heaters (104) to decrease with the transition between TXOFF and TXON, if the heat dissipation of the associated heat-producing component (103) increases with the transition between TXOFF and TXON.

25. A computer program comprising instructions which, when the computer program is executed by a processor of a controller (105), causes the controller (105) to perform the steps of the method (800) of claim 24.16

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