Optical modulation: device architecture and method for optical performance

Abstract

The present disclosure relates to a system for controlling the operation of an optical modulator, the system comprising: a waveguide circuit comprising an input waveguide section, a modulation waveguide section, an output waveguide section and a probe waveguide section, wherein the probe waveguide section is optically coupled to the modulation waveguide section and configured to receive an input probe signal, a control unit and an electric field generator, a photodetector configured to measure the output signal of the probe waveguide section, wherein, based on the measured output signal, the control unit is configured to cause the electric field generator to generate an electric field so as to modulate an optical property of the modulation waveguide section. The present disclosure further relates to a method for monitoring the drift of an optical modulator using a system according to the present disclosure.

Description

[0001] Optical modulation: device architecture and method for optical performance The present disclosure relates to a system for controlling the operation of an optical modulator and a method for monitoring the drift of an optical modulator using a system according to the present disclosure. Background The field of optical modulators is critical to advancing photonic applications across diverse industries, including telecommunications, sensing, metrology, and quantum optics. Optical modulators enable the manipulation of optical signals by altering the refractive index of the material within the modulator, often using mechanisms like electro-optic (EO) or thermo-optic (TO) effects. Devices utilising EO effects, in particular, are highly valued for their rapid modulation capabilities, with speeds exceeding 100 GHz, making them ideal for high-bandwidth applications. EO modulators, which include devices based on lithium niobate and other non- centrosymmetric materials, also offer low chirp, an essential characteristic for long-haul optical communications. In applications that rely on quantum optical states, such as squeezed states or single-photon signals, modulators play a crucial role by providing precise control without compromising the integrity of the quantum state. Despite their advantages, EO modulators are subject to several limitations that affect stability and performance. A significant challenge is an effect known as DC drift, a phenomenon caused by charge migration and trapping within the EO material. When an external voltage is applied, charges within the EO material migrate and can become trapped at specific sites, gradually changing the material’s refractive index and, consequently, the modulator’s performance. This accumulation of trapped charges leads to a drift in the device's characteristics over time, with the rate and extent of drift influenced by factors such as material properties and operating conditions. Crucially, performance drift, such as DC drift, is a reversible effect, meaning that, while the modulator’s properties shift gradually, they can be restored to their optimal state by exposing the material to a controlled external voltage that redistributes or neutralises the trapped charges or, alternatively, by utilising a method different from the electro- optic effect to trim the property of the optical circuit (e.g. thermo-optic effect or strain tuning of the waveguide properties, to name two examples). The reversible nature of DC drift allows for recalibration of the modulator through corrective adjustments, where a controlled external voltage counteracts the drift by mobilising the trapped charges. However, because the drift may occur over time scales from milliseconds to hours, this requires continuous monitoring and correction to maintain stable modulation performance. Without such intervention, the drift progressively degrades the modulator's efficiency and stability, limiting its reliability for high-precision applications and diminishing its long-term effectiveness. To manage DC drift, conventional EO modulator systems implement feedback loops that sample a portion of the modulated output signal, allowing the system to monitor performance and apply corrective measures to restore the device to its intended operating conditions. This approach, while effective for many uses, is incompatible with some applications, such as quantum applications, where even minimal sampling or interference with the modulated output can disrupt the quantum state, thereby compromising system performance. Such interactions with the output signal are particularly detrimental in applications requiring single photons or other sensitive quantum states, where maintaining signal integrity is paramount. Furthermore, feedback mechanisms that rely on sampling introduce practical challenges in implementing high-speed EO modulators on photonic integrated circuits (PICs). These systems typically require fiberised components and extra detectors to gather and analyze output signals, adding complexity and hindering compact, high- speed, on-chip solutions. For applications that demand tight integration and substantial modulation bandwidths, these limitations hinder progress, as current designs cannot fully leverage EO modulation capabilities on a PIC without imposing undesirable trade- offs between stability and speed. Consequently, the application of EO modulators in quantum systems is largely restricted to simpler tasks, such as wavelength trimming and slow modulation applications (<1GHz), rather than exploiting their full bandwidth and modulation capabilities. It is therefore an objective of the present disclosure to provide an improved optical modulation system and method that can maintain stable, high-performance modulation without requiring direct sampling or interaction with the modulated output, thereby overcoming the limitations associated with the reversible drift effect of a modulator and enabling integration in quantum systems and in other applications where the signal to be modulated has such a low signal to noise ratio that it is not possible to sample it (to implement a feedback system and stabilise the modulator) without compromising signal integrity. Summary Considering the prior art described above, it is an objective of the present disclosure to provide a system for controlling the operation of an optical modulator, which allows stable and precise modulation without the need to sample or interfere with the modulated output signal. This solution enables effective modulation control in applications involving quantum states or sensitive optical signals, overcoming the limitations of traditional feedback systems that require direct sampling of a signal and thus compromise signal integrity. The present disclosure therefore relates to a system for controlling the operation of an optical modulator, the system comprising a waveguide circuit comprising an input waveguide section for receiving an input signal to be modulated, a modulation waveguide section for modulating the input signal, an output waveguide section and a probe waveguide section for receiving an input probe signal, wherein the probe waveguide section is optically coupled to the modulation waveguide section and configured to receive an input probe signal. The system may further comprise a control unit for controlling the operation of an electric field generator and can have a photodetector configured to measure the output signal of the probe waveguide section, the output signal being the result of interference between a first part of the input probe signal not modulated by the modulation waveguide section and a second part of the input probe signal modulated by the modulation waveguide section, wherein, based on the measured output signal, the control unit is configured to cause the electric field generator to generate an electric field so as to modulate, such as tune or reconfigure, an optical property of the modulation waveguide section. The present disclosure further relates to a method for monitoring the drift of an optical modulator. The method may comprise the steps of providing any one of the systems according to the present disclosure. The method may also comprise the step of feeding an input signal to be modulated into the input waveguide section of the waveguide circuit and a separate probe signal into the probe waveguide section of the waveguide circuit, a) measuring, using the photodetector, an output signal of the probe waveguide section, the output signal being the result of interference between a first part of the probe signal which has not travelled inside the modulation waveguide section and a second part of the probe signal which has travelled inside the modulation waveguide section due to coupling between a probe waveguide in the probe waveguide section and a modulation waveguide in the modulation waveguide section, b) the control unit instructing, based on the measurement of the output signal, the electric field generator to generate an electric field to adjust a property of the modulation waveguide of the modulation waveguide section related to how an optical signal propagates through the modulation waveguide so as to reduce the drift. With the presently disclosed solution, optical modulators can maintain stable and high- performance operation in applications that require low, minimal or no interference with the signal to modulate and avoid the need for sampling of the modulated output signal. By eliminating the need to directly measure the modulated signal for feedback, the system preserves the signal integrity essential in sensitive applications, such as those involving quantum states, where even minimal disturbance can degrade the quality of the transmitted information. This design can further enhance system reliability by allowing real-time adjustments to optical modulators properties without affecting the output, ensuring consistent performance over extended periods and minimising drift- related degradation of an optical modulator. This approach is particularly beneficial for photonic systems that demand stringent signal preservation, as in quantum optics, where maintaining the purity of quantum states is critical for applications like quantum communication, sensing, and cryptography. By allowing stable modulation control without compromising the output, the system opens the way for optical modulators to be used more broadly in quantum technologies, supporting advanced applications that previously faced limitations due to signal disruption. Description of the drawings The present disclosure will in the following be described in greater detail with reference to the accompanying drawings. Various embodiments are described hereinafter with reference to the drawings. The drawings are examples of embodiments and are intended to illustrate some of the features of the presently disclosed solution, and are not limiting to the presently disclosed system and method. While the present disclosure describes embodiments utilizing electro-optic (EO) materials, the systems and methods are not limited to EO modulators. The same architectures described herein may also be implemented with silicon-based modulators or other non-electro-optic modulators that rely on alternative modulation mechanisms. These implementations remain compatible with the configurations disclosed in this application, ensuring flexibility and broader applicability across different material platforms and operational principles. Similarly, whilst the nomenclature utilised in this disclosure relates to optical devices, in particular optical waveguide and photonic integrated circuits, our invention could find applications in radio frequency (RF) waveguides and, in particular, RF mixers. Fig.1a a schematic view of prior art Mach Zehnder modulator (MZM) implemented on an EO material Fig.1b a schematic view of prior art in which the MZM of fig.1a is used in an optical system where a feedback loop is implemented by sampling the signal at the output of the modulator Fig.1c a schematic view of prior art and is equivalent to the system of Fig.1b. At variance with Fig.1b, in Fig.1c all the components of the optical system have been integrated on one chip Fig.2a a schematic of a MZM coupled to two probe waveguides, as proposed in this disclosure Fig.2b a schematic of the device presented in Fig.2a where the optical path followed by the different optical signals is highlighted Fig.2c a schematic of the cross-section view of the device shown in Fig.2a, for the case of implementation on an EO material (the figure includes: modulation waveguides, probe waveguides, RF coplanar waveguides and DC electrodes) Fig.3 a schematic of the cross-section of a modulation waveguide implemented utilising the EO crystal in another orientation. In this case the electrodes for EO are not on the sides of the modulation waveguide, but rather on top and below the modulation waveguide which has a top cladding Fig.4 a schematic showing the MZM of Fig.2a used in an optical system where a feedback loop is implemented starting from measurements of the probe signals. Fig.5 is a schematic view of prior art Ring Modulator (RM) implemented on an EO material. Fig.6 a schematic view of a Ring Modulator coupled to one probe waveguide, as proposed in this disclosure Fig.7 a schematic view of a particular case of MZM where the modulation waveguides act as the probe waveguides Detailed description of the invention The term 'preferential direction' of an electro-optic material, as used throughout the present disclosure, refers to an orientation of the crystal structure of the electro-optic material. When the applied electric field (such as that generated by the DC electrodes or by the RF electrodes in some embodiments of the present disclosure) is aligned to such preferential direction, the electro-optic response of the material is maximised. This alignment ensures optimal interaction between the electric field and the EO modulator, thereby enabling efficient modulation of optical signals. A preferred embodiment of the present disclosure relates to a system for controlling the operation of an optical modulator, the system comprising a waveguide circuit comprising an input waveguide section for receiving an input signal to be modulated, a modulation waveguide section for modulating the input signal, an output waveguide section and a probe waveguide section for receiving an input probe signal, wherein the probe waveguide section is optically coupled to the modulation waveguide section and configured to receive an input probe signal, a control unit for controlling the operation of an electric field generator, a photodetector configured to measure the output signal of the probe waveguide section, the output signal being the result of interference between a first part of the input probe signal not modulated by the modulation waveguide section and a second part of the input probe signal modulated by the modulation waveguide section, wherein, based on the measured output signal, the control unit is configured to cause the electric field generator to generate an electric field so as to modulate an optical property of the modulation waveguide section. In some embodiments the system is a photonic integrated circuit. In some embodiments, the system comprises a DC tuning section which comprises the electric field generator and, optionally, the electric field generator comprises one or more DC electrodes. In some embodiments the DC electrodes are used to to leverage in a DC (i.e. static or quasi static fashion) effects such as electro-optic effect, thermo-optic effect, strain tuning of the waveguide circuit or another trimming mechanism of the optical length / the optical properties of the waveguide circuit. In some embodiments, the input waveguide section comprises one or more input waveguides for receiving input signals to be modulated. In some embodiments, the input waveguide section is optically coupled to the modulation waveguide section such that the input signal to be modulated is an input signal to the modulated waveguide section. In some embodiments, an input signal to the output waveguide section is a signal modulated by the modulation waveguide section. In some embodiments, the electric field generator comprises one or more direct current (DC) electrodes. In some embodiments, the optical property of the modulation waveguide section modulated by the electric field generator is an optical length of the modulation waveguide section. In some embodiments, the system further comprises an RF modulation section for modulating optical properties of the modulation waveguide section. In some embodiments, the RF modulation section comprises one or more radio frequency (RF) coplanar waveguides. In some embodiments, the control unit is configured to instruct an RF signal generator to generate an RF signal for driving the one or more RF coplanar waveguides so as to cause modulation, such as high speed modulation (> 1GHz, preferably > 10GHz) and / or electric-optic modulation, of the modulation waveguide section, such as one of more modulation waveguides in the modulation waveguide section. In some embodiments, the modulation waveguide section comprises one or more modulation waveguides which each has an optical path length that changes in response to an external electric field. In some embodiments, the modulation waveguide section comprises a plurality of modulation waveguides which are arranged in parallel, series or a combination of parallel and series, creating a mesh configuration of modulation waveguides. In some embodiments, an input waveguide of the input waveguide section is optically coupled to two modulation waveguides via a multi-mode interferometer (MMI). In some embodiments, an input waveguide of the input waveguide section is optically coupled to an arbitrary number of modulation waveguides via an arrayed waveguide grating (AWG). In some embodiments, the modulation waveguides of the modulation waveguide section are connected in such a way to create one or more Mach Zehnder modulators. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section via one or more sidewall coupling regions, such as directional couplers or via a Mach Zehnder interferometer. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section via a first sidewall coupling region and a second sidewall coupling region. In some embodiments, the first sidewall coupling region is configured to split the input probe signal between a first split part which remains within the probe waveguide section and a second split part which enters the modulation waveguide section. In some embodiments, the second sidewall coupling region is configured to split the input probe signal modulated by the modulation waveguide section between a third split part which re-enters the probe waveguide section and a fourth split part which remains within the modulation waveguide section. In some embodiments, the first and second sidewall coupling regions are configured to reduce the degree of optical coupling of the input signal which originated from the input waveguide section to the probe waveguide section. In some embodiments, the first and second sidewall coupling regions are configured to confine the input signal which originated from the input waveguide section within the modulation waveguide section. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section through an optically coupling region constituted by a multi-mode interferometer (MMI). In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section via a first multi-mode interferometer (MMI) region and via a second MMI. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section via a first multi-mode interferometer (MMI) with two input ports and two output ports region and via a second MMI with two input ports and two output ports. In some embodiments the first MMI is such that the first input port of the MMI is connected to an input waveguide; the second input port of the MMI is connected to the part of a probe waveguide section where an input probe signal is received; the first output port of the MMI is connected to a modulation waveguide; the second output port of the MMI is connected the a probe waveguide section. In some embodiments, the second MMI is such that the first output port of the MMI is connected to an output waveguide; the second output port of the MMI is connected to the part of a probe waveguide section where a probe signal is outputted; the first input port of the MMI is connected to a modulation waveguide; the second input port of the MMI is connected the a probe waveguide section. In some embodiments, the first and second sidewall coupling regions are replaced by multi-mode interferometers (MMI) wherein the first MMI is configured to ensure that an input probe signal is split between a modulation waveguide section and a probe waveguide section and ensures that the input signal is not coupled to a probe waveguide section. In some embodiments, the first and second sidewall coupling regions comprise a drop filter, resonant structures, or any other type of photonic filter. In some embodiments, the waveguide circuit is implemented on a chip, such as a photonic integrated circuit. In some embodiments, the waveguide circuit supports a modulation bandwidth of 1 MHz or more. In some embodiments, the waveguide circuit supports high speed modulation, such as modulation bandwidth of 1GHz or, preferably a modulation bandwidth exceeding 10 GHz. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section so as to allow for evanescent coupling. In some embodiments, the probe waveguide section comprises one or more probe waveguides which are each configured to receive an input probe signal. In some embodiments, each probe waveguide is optically coupled to the modulation waveguide section via one or more sidewall coupling regions. In some embodiments, the electric field generator is configured to generate an electric field which is capable of optically modulating the modulation waveguide section so as to change its optical path length. In some embodiments, the probe waveguide section is optically coupled to the modulation waveguide section via a first optical coupling region which allows for at least a part of the input probe signal to enter the modulation waveguide section and a second optical coupling region which allows for the at least a part of the input probe signal to recombine with the remaining part of the input probe signal in the probe waveguide section to form the output signal of the probe waveguide section. In some embodiments, the control unit calculates a drift in an optical property of a device, such as the electro-optical modulator, of the modulation waveguide section from a target operating point of the device based on the measured output signal of the probe waveguide section. In some embodiments, the drift in the optical property is a slowly varying drift, which may be referred to as a performance drift. In some embodiments, the performance drift is a direct current (DC) drift. In some embodiments, the control unit is configured to transmit a control signal to the electric field generator instructing the electric field generator to generate an electric field so as to reduce the drift in the optical property from the target operating point. In some embodiments, the control unit is configured to continuously monitor the optical property of the device and transmit one or more control signals to the electric field generator instructing the electric field generator to generate an electric field so as to reduce the drift in the optical property from the target operating point until the target operating point is reached. In some embodiments, the probe waveguide section comprises one or more probe waveguides. In some embodiments, the modulation waveguide section comprises a closed loop waveguide to form an optical cavity. In some embodiments, the modulation waveguide section comprises a plurality of closed loop waveguides which form a nested cavity configuration. In some embodiments, the waveguide circuit is etched into a layer comprising an electro-optic material and, optionally, the layer is a lithium niobate based layer. In some embodiments, the layer comprising the electro-optic material has a thickness between 100 nm and 100 µm. In some embodiments, the layer of lithium niobate is a thin film having a thickness of less than 2 µm. In some embodiments, the layer comprising an electro-optical material is part of a material stack. In some embodiments, the layer comprising the electro-optic material is capable of supporting a modulation bandwidth of 10 GHz or more. In some embodiments, the material stack comprises another layer comprising a dielectric material and, optionally, the dielectric material acts as an optical insulator. In some embodiments, the material stack comprises another layer having a lower refractive index than the layer comprising the electro-optical material, and wherein the layer comprising the electro-optical material is arranged on top of the another layer. In some embodiments, the another layer comprises silica (SiO2) and / or sapphire (Al2O3). In some embodiments, the another layer has a thickness in the range of 2 µm to 500 µm. In some embodiments, the material stack comprises a further layer configured to provide mechanical stability and the another layer is arranged on top of the further layer. In some embodiments, the further layer comprises a silicon (Si) and / or lithium niobate. In some embodiments, the further layer has a thickness in the range of 200 µm to 500 µm. Advantageously, the further layer is electrically conductive. In some embodiments, the waveguide circuit is configured to split the input signal to be modulated into different waveguides within the modulation waveguide section and recombine the split signals to form an output signal of the modulation waveguide section. In some embodiments, the output signal of the modulation waveguide section is an input signal to the output waveguide section. In some embodiments, the output waveguide section comprises one or more output waveguides for receiving the modulated signals. In some embodiments, the modulation waveguides of the modulation waveguide section recombine in an output waveguide of the output waveguide section via any combination of one or more Y-branches and one or more directional couplers. In some embodiments, an input waveguide of the input waveguide section is optically coupled to a modulation waveguide of the modulation waveguide section, such as via a directional coupler. In some embodiments, an input waveguide of the input waveguide section is optically coupled to two modulation waveguides via a beam splitter, such as a via a Y-branch. In some embodiments, an input waveguide of the input waveguide section is optically coupled to two modulation waveguides via a beam splitter, such as a via a multi-mode interferometer (MMI). In some embodiments, the input signal to be modulated is split by any combination of a Y-branch, a beam splitter and a directional coupler into different waveguides of the modulation waveguide section. In some embodiments, the split signals forming the output signal of the modulation waveguide section are recombined by any combination of a Y-branch, a beam splitter and a directional coupler to form an input signal to the output waveguide section. In some embodiments, the probe waveguides and the modulation waveguides are made from the same material. In some embodiments, the waveguide circuit supports radio frequency waves. In some embodiments, the waveguide circuit supports radio frequency waves and the modulation waveguide section is utilised to implement mixing of a radio frequency signal with another electromagnetic signal. Another preferred embodiment of the present disclosure relates to a method for monitoring the drift of an optical modulator, the method comprising the steps of: providing a system according to any of the preceding embodiments, feeding an input signal to be modulated into the input waveguide section of the waveguide circuit and a separate probe signal into the probe waveguide section of the waveguide circuit; measuring, using the photodetector, an output signal of the probe waveguide section, the output signal being the result of interference between a first part of the probe signal which has not travelled inside the modulation waveguide section and a second part of the probe signal which has travelled inside the modulation waveguide section due to coupling between a probe waveguide in the probe waveguide section and a modulation waveguide in the modulation waveguide section; the control unit instructing, based on the measurement of the output signal, the electric field generator to generate an electric field to adjust a property of the modulation waveguide of the modulation waveguide section related to how an optical signal propagates through the modulation waveguide so as to reduce the drift. In some embodiments, the property of the modulation waveguide section is an optical path length of the modulation waveguide or a refractive index of the modulation waveguide. In some embodiments, the control unit calculates, based on the measurement of the output signal of the probe waveguide section, a drift of the property of the modulation waveguide away from a target operating value of the modulation waveguide. In some embodiments, the target operating value of the modulation waveguide is related to the optical modulation efficiency of the modulation waveguide. In some embodiments, the probe signal couples to a modulation waveguide in the modulation waveguide section via a sidewall coupling region so that a first part of the probe signal remains within the probe waveguide section and a second part of the probe signal enters into the modulation waveguide. In some embodiments, the first part of the probe signal propagates through the modulation waveguide before re-coupling to the probe waveguide section via another sidewall coupling region. In some embodiments, the probe signal couples to a modulation waveguide in the modulation waveguide section via a coupling region constituted by a multi-mode interferometer (MMI) so that a first part of the probe signal remains within the probe waveguide section and a second part of the probe signal enters into the modulation waveguide. In some embodiments, the first part of the probe signal propagates through the modulation waveguide before re-coupling to the probe waveguide section via another MMI. In some embodiments, the control unit instructs the DC tuning section to transmit a feedback signal to the one or more DC electrodes to generate an electric field so as to modulate the optical path length or refractive index of the modulation waveguide to reduce the drift. In some embodiments, the input signal fed into the input waveguide section and the probe signal fed into the probe waveguide section have different wavelengths. In some embodiments, the input signal fed into the input waveguide section and the probe signal fed into the probe waveguide section are orthogonal waveguide modes. In some embodiments, one or more probe signals are fed into the probe waveguide section and the one or more probe signals being orthogonal to one another and / or having different wavelengths. In some embodiments, the input signal to be modulated comprises a squeezed state or single photons. In some embodiments the probe signal and the input signal propagate in the same waveguide in orthogonal modes. In some embodiments the probe waveguide section and the modulation waveguide section are the same waveguide. In some embodiments the probe signal propagates solely in the modulation waveguide section in orthogonal waveguide modes to the mode hosting the input signal and is separated from the input signal after that the modulation waveguides are recombined by any combination of a Y-branch, an MMI, a beam splitter and a directional coupler to form an input signal to the output waveguide section. Advantageously, steps a) and b) are repeated as part of a feedback control loop. In some embodiments, in step b) the a property of the modulation waveguide is adjusted more slowly, such as one order of magnitude more slowly, than the modulation bandwidth of waveguide circuit. ExamplesIn the following, λm refers to the wavelength of the modulated signal, while λp denotesthe wavelength of the probe signal. To achieve modulation of a signal, an RF electronic signal applies an external voltage to the material, thereby modulating the optical properties of the material and, consequently, the optical signal. The optical length of the modulation waveguide may be fine-tuned (slowly) through the thermo-optic (TO) effect as well as the electro-optic (EO) effect or via strain. However, applying an external voltage can induce performance drift—an unwanted, gradual shift in the properties of the modulation waveguide(s), causing the optical length to drift over time. When this occurs, the modulator is no longer operating at its optimal working point. This issue can be addressed with a feedback system that continuously fine-tunes the modulator to maintain the desired working point. The feedback system in this disclosure may operate by implementing an interferometric measurement that continuously evaluates the modulator's working point, enabling the necessary adjustments to counteract performance drift without affecting the modulator output. This approach is achieved by coupling the modulator to a probe structure, which carries a reference signal (the probe signal) for performing interferometric measurements so that the optical length of a selected modulation waveguide within the modulation waveguide section can be determined. The probe waveguide is evanescently coupled to the modulation waveguide, for example, through sidewall coupling in a directional coupler. The evanescent coupling is designed such that both λpand λmtravel through the modulation waveguide, and ideally also so that only the probe wavelength λppropagates in the probe waveguide. Implementing the interferometric measurement therefore relies on the probe structure. The probe structure can be made from the same material as the modulator or a different material. Since the probe structure is not subjected to the modulation, its optical length remains unaffected by DC drift, even if fabricated from the same material as the modulation waveguide. In other words, the probe structure’s optical length remains stable and is uninfluenced by modulation of the signal at λminduced by the RF electrodes. If needed, the optical length of the probe structure can be fine-tuned (for example, quasi-statically) through the TO effect. Calibration of the modulator is conducted to: (A) optimise modulation performance, and, optionally, (B) ensure that when modulation is optimised, the output probe signalis, for example, maximised. During operation, if the outputted probe signal at λpdeviates from its expected value, the modulation waveguide is adjusted (e.g., through TO tuning) until the expected probe signal output is restored, thereby returning the modulator to its optimal working point. As defined above, a modulator may have one or multiple modulation waveguides. For instance, a Ring Modulator has one modulation waveguide, a Mach-Zehnder Modulator (MZM) has two modulation waveguides, and an EO device with a mesh of N MZMs has 2xN modulation waveguides. Generally, the number of probe waveguides required for the disclosed method is equal to the number of modulation waveguides. For example, a Ring Modulator requires one probe waveguide, an MZM requires two, and a mesh of MZMs requires 2xN probe waveguides, with more complex EO devices following the same principle based on their number of modulation waveguides. If multiple probe waveguides are used, there will also be multiple probe signals. Depending on the device architecture, the various probe signals may have the same or different wavelengths. If different probe signals share the same wavelength, they can be coupled to distinct spatial modes to prevent interference and allow multiplexing of the probe signal. If preferred, λpand λmcan share the same waveguide structures (meaning that the probe waveguide section and the modulation waveguide section are physically the same waveguides), propagate on orthogonal waveguide modes and be separated only after that the waveguide structures have recombined in the output waveguide section. Fig.1a shows a schematic view of a prior art Mach-Zehnder modulator (MZM) implemented on an electro-optic (EO) material. The device includes an input waveguide 101 where the optical signal to be modulated is introduced, and an output waveguide 102 from which the modulated optical signal exits. The MZM structure features two Y-branches 106 that split and later recombine the optical signal, allowing for independent phase control in each arm of the modulator. This phase control is essential for achieving effective signal modulation. Within the modulation arms, the modulation waveguides 105 are placed in proximity to RF coplanar waveguides 103, which act as electrodes. These RF electrodes apply an external electric field across the EO material, resulting in a change in the refractive index of the EO material and modulating the phase of the optical signal passing through each arm. The EO material has a preferential direction, as indicated by 100. In addition to the RF electrodes, Fig.1a shows quasi-static DC electrodes 104 positioned near the modulation waveguides. These DC electrodes allow for fine-tuning of the optical path length within each arm. Quasi-static tuning compensates for slow drifts that may occur due to changes in the EO material properties over time, ensuring stable operation. Fig.1b shows a schematic view of a prior art Mach-Zehnder modulator (MZM) deployed within an optical system that incorporates a feedback loop for stabilisation purposes. The modulator includes an input waveguide 101, which receives the optical signal to be modulated, and an output waveguide 102, from which the modulated signal exits. The MZM structure features two Y-branches 106 that split and subsequently recombine the optical signal, allowing for independent phase control in each arm of the modulator. Within the modulation arms, modulation waveguides 105 are placed in proximity to RF coplanar waveguides 103, which apply an external electric field across the electro-optic (EO) material. This electric field modulates the phase of the optical signal passing through each arm by altering the refractive index of the EO material. Additionally, DC electrodes 104 are positioned near the modulation waveguides to allow fine-tuning of the optical path length in each arm. This fine-tuning capability compensates for slow drifts in the EO material properties. In Fig.1b, a beam splitter 111a is positioned at the output of the MZM. The beam splitter divides the modulated signal into two paths. The primary path carries the majority of the modulated power to the output waveguide 102, while a secondary path directs a small portion of the signal to an optical detector 107. The optical detector, in turn, sends a feedback signal to control electronics 128, which monitors the performance of the modulator. Based on the feedback, the control electronics control the DC electrodes 104 to keep the optical modulator at the optimal working point. Fig.1c shows a schematic view of a prior art Mach-Zehnder modulator (MZM) deployed in an optical system similar to that of Fig.1b, but with all components integrated onto a single chip, forming a photonic integrated circuit (PIC). The system incorporates a feedback loop to stabilise the operating point of the modulator. The MZM includes an input waveguide where the optical signal to be modulated is introduced and an output waveguide from which the modulated signal exits. The modulator structure uses two Y-branches to split and subsequently recombine the optical signal, enabling phase control within each arm of the modulator. In this integrated system, a multi-mode interferometre (MMI) 111b replaces the beam splitter of Fig.1b. The MMI divides the modulated signal into two paths: one path carries the majority of the modulated power to the system output waveguide 121, while the second path directs a small portion of the signal to an on-chip photodetector 107. The photodetector generates a feedback signal that is processed by control electronics 128, which then adjust the DC electrodes to maintain the MZM at its optimal operating point. Fig.2a shows a schematic of an example Mach-Zehnder modulator (MZM) implemented on an electro-optic (EO) material, incorporating additional probe structures for monitoring performance without interacting with the modulated output signal. The device includes an input waveguide 201 where the optical signal to be modulated is introduced and an output waveguide 202 from which the modulated signal exits. Two Y-branches 206 split and later recombine the optical signal, allowing for phase modulation within each arm of the device. In other implementations, there may be other splitters having more than two outputs. For example, there may be N modulation waveguides and, in this case, there may be a device that splits the input signal into the N modulation waveguides. The modulation waveguides 205a and 205b, forming the two arms of the MZM, are positioned alongside RF coplanar waveguides 203, which apply an external electric field across the EO material to modulate the phase of the optical signal by adjusting the material’s refractive index. The EO material has a preferential direction 200, which is aligned perpendicularly to the RF coplanar waveguides to maximise modulation efficiency. Additionally, there may be DC electrodes 204 adjacent to the modulation waveguides, enabling fine-tuning of the optical path length within each arm to maintain stable operation by compensating for slow drifts. In this design, there are also probe structures for monitoring the modulator’s performance. Each probe structure includes a probe waveguide 212a, 212b that introduces a probe signal, a directional coupler 209a, 209b that couples a portion of the probe signal to the modulation waveguides 205a, 205b, and a probe waveguide 208a, 208b that guides the coupled probe signal. The probe waveguides terminate at output waveguides 213a and 213b, which carry the monitored signal to a photodetector (not shown). The optical signal and the probe signal may have different wavelengths, allowing for selective coupling and separation, where components such as filters or couplers can interact with the probe signal independently of the optical signal. Alternatively, the optical signal and the probe signal may be at the same wavelength but use orthogonal waveguide modes to prevent interference. There may also be additional directional couplers 219a and 219b at the end of the modulation waveguides, allowing the probe signal to fully couple back to the probe waveguides 208a and 208b. In some implementations, the probe structures are designed so that the modulated signal in the MZM remains unaffected by the probe signal, enabling non-intrusive monitoring. For example, the directional couplers may be constructed to allow only the probe signal to couple between a probe waveguide and a modulation waveguide, while preventing any coupling or interference with the optical signal introduced at the input waveguide. Fig.2b illustrates the same schematic design as Fig.2a, with a focus on highlighting the optical paths followed by the modulated signal and the probe signal within the Mach-Zehnder modulator (MZM) structure. The MZM is implemented on an electro- optic (EO) material, where the modulated signal enters through the input waveguide 201, and after modulation, exits through the output waveguide 202. Two Y-branches 206 are shown, which split and later recombine the modulated signal, enabling phase modulation in each arm of the device. The modulation waveguides 205a and 205b are situated adjacent to RF coplanar waveguides 203, which apply an external electric field across the EO material. This field induces a change in the refractive index along the modulation waveguides, causing phase modulation in the optical signal. Advantageously, the preferential direction of the EO material, indicated by 200, can be aligned to optimise the effect of the applied electric field on the materials refractive index, enhancing the modulation efficiency. However, this is not essential to the working of the invention. In addition to the RF electrodes, DC electrodes 204 are present near the modulation waveguides to allow fine-tuning of the optical path length, ensuring stability by compensating for slow drifts over time. The probe structures, which monitor the MZM’s performance without interacting with the output signal, include probe waveguides 212a and 212b that introduce a probe signal into the device. Each probe waveguide is coupled to the modulation waveguides 205a and 205b via directional couplers 209a and 209b. A portion of the probe signal is directed into the modulation waveguides, allowing for an interferometric measurement of the optical length within the modulation arms. The optical length of each probe waveguide can be tuned, such as quasi-statically tuned, with a DC electrode 214. This coupled probe signal is then guided along probe waveguides 208a and 208b, which terminate at output waveguides 213a and 213b, where the monitored probe signal can be detected, for example, by a photodetector. In this configuration, additional directional couplers 219a and 219b at the end of the modulation waveguides fully couple the probe signal back into the probe waveguides 208a and 208b. This setup allows the modulated signal to remain confined to the modulator output, ensuring that the probe signal is isolated. The modulated signal and probe signal may use different wavelengths or, alternatively, may share the same wavelength but are launched on orthogonal waveguide modes, ensuring they do not interfere or exchange power. Fig.2c shows a cross-sectional view of the Mach-Zehnder modulator (MZM) design illustrated in Fig.2a, highlighting the structural arrangement of the modulation and probe waveguides within the electro-optic (EO) material. This cross-section reveals the layout of the components that enable modulation and probe signal coupling while maintaining signal integrity. In the cross-sectional view, modulation waveguides 205a and 205b are positioned within the EO material, which allows for phase modulation when an external electric field is applied. The EO material is situated on an insulating layer, which may be a buried oxide (BOX) layer 216 that provides optical insulation and serves as a base for the waveguides. Below the BOX layer, there may also be a carrier material 217 that provides mechanical stability to the entire device structure. Additionally, the non-etched part of the optical layer 215 is visible, forming a foundational layer on which the optical waveguides are implemented. This optical layer also comprises the EO material, enabling the refractive index changes required for phase modulation in response to the applied electric field. RF coplanar waveguides 203 are placed on either side of the modulation waveguides 205a and 205b. These RF electrodes apply an external electric field across the EO material in alignment with the preferential direction 200, maximising the EO effect and modulating the phase of the optical signal within each modulation waveguide. Additionally, DC electrodes 204 (not shown in Fig.2c) are positioned near the modulation waveguides. The DC electrodes 204 may allow for quasi-static tuning of the optical path length, which helps to counter slow drifts in the EO material properties over time. Probe waveguides 208a and 208b are also shown in this cross-sectional view, positioned alongside the modulation waveguides. The probe waveguides are coupled to the modulation waveguides via directional couplers, such as 209a and 209b (not visible in this cross-sectional view), which enable a portion of the probe signal to be coupled into the modulation waveguides. This cross-section further reveals that the optical waveguides 205a, 205b, 208a, and 208b may be implemented as rib waveguides, partially etched into the EO material layer to guide the optical signals effectively. The rib structure ensures efficient confinement of the optical modes within the waveguides, supporting stable propagation of both the modulated and probe signals. Although the Figures attached to this disclosure present mostly top-views of schematics where the RF coplanar waveguide used to implement EO modulation lies in the same plane as the photonic integrated circuit (PIC), different orientations of the preferential direction of the EO material are possible. In Fig.3, the preferential direction 300 is oriented perpendicular to the plane of the PIC. Fig.3 shows the arrangements for the electrodes 303 to implement the EO effect relative to the optical waveguide 305, which is implemented in an EO material. In this case, the waveguide is represented as a fully etched (ridged) waveguide. The waveguide includes a top cladding layer 318, which may be the same material as the underlying buried oxide (BOX) layer 316 or another material with a lower refractive index than the EO material to provide sufficient optical confinement. This cross-section also illustrates the carrier layer 317, which provides mechanical support for the entire structure. The carrier layer can be constructed from materials such as silicon or lithium niobate, depending on the design needs of the device. The BOX layer 316 sits between the EO material and the carrier layer, acting as an optical insulator and further contributing to waveguide confinement. Fig.4 shows a schematic view of an optical system that incorporates a Mach-Zehnder modulator (MZM) with a feedback loop for stabilising its operating point. This configuration enables real-time control of the modulator to maintain optimal modulation conditions. The MZM is implemented on an electro-optic (EO) material with a preferential direction 400 that aligns with the applied electric field to maximise modulation efficiency. The modulation waveguides are positioned alongside RF coplanar waveguides 403, which apply an external electric field across the EO material to induce phase modulation by adjusting its refractive index. To ensure stable operation, the system incorporates a feedback loop. The feedback loop utilises photodetectors 407 to monitor the MZM’s performance indirectly. These photodetectors measure a portion of the optical signal associated with the modulator’s operation, providing real-time data on the modulation conditions. Based on this feedback, control electronics 428 adjust the DC electrodes 404 positioned near the modulation waveguides. By fine-tuning the optical path length, the DC electrodes help maintain the MZM at its optimal working point, compensating for slow drifts or changes in the EO material properties over time. The feedback loop in Fig.4 demonstrates an approach to real-time control, where the photodetectors and control electronics work together to stabilise the modulator’s performance without directly interfering with the modulated output signal. This configuration is particularly advantageous for applications where maintaining stable modulation and signal integrity is essential. Fig.5 shows a schematic view of a prior art Ring Modulator (RM) implemented on an electro-optic (EO) material. This device operates by modulating the phase of an optical signal as it travels through a closed-loop waveguide 505, forming a resonant optical cavity, often referred to as a racetrack or ring structure. The Ring Modulator includes an input waveguide 501, where the optical signal to be modulated is introduced, and an output waveguide 502, from which the modulated signal exits. Light couples from the input waveguide into the ring structure through a directional coupler 506, which enables optical coupling of signals between the input waveguide and the ring cavity. RF electrodes 503 are positioned along the ring cavity to apply an external electric field across the EO material, changing its refractive index and thereby modulating the phase of the optical signal circulating within the ring. This RF field enables high-speed modulation of the signal by leveraging the EO effect. Additionally, DC electrodes 504 are located adjacent to the ring cavity. These DC electrodes may provide quasi-static tuning. The DC tuning allows for adjustments of the optical path length within the cavity to maintain optimal operating conditions and compensate for any slow drifts in the EO material properties. The preferential direction of the EO material 500 is oriented to align with the applied electric field from the electrodes, maximising modulation efficiency. Fig.6 shows a schematic view of an example Ring Modulator (RM) implemented on an electro-optic (EO) material, with a probe structures for non-intrusive monitoring of the modulator’s performance. An input waveguide 601, positioned alongside the ring cavity, introduces the optical signal to be modulated. The input waveguide is not directly part of the Ring Modulator 605 but is instead coupled to it through a sidewall coupling region 606, allowing a portion of the optical signal from the input waveguide to enter the ring cavity. Once coupled into the ring, the optical signal circulates within the Ring Modulator 605 and is modulated by it before the signal exits the ring cavity and is directed through an output waveguide 602. RF electrodes 603 are positioned along the ring cavity to apply an external electric field across the EO material, modulating the refractive index and thus the phase of the circulating optical signal. Additionally, DC electrodes 604 are positioned near the ring cavity. These DC electrodes provide DC tuning, allowing for adjustments of the optical path length of the ring cavity to compensate for drifts in the EO material properties over time, ensuring stable modulation performance. The probe structure includes a probe waveguide 612, which introduces a probe signal that is coupled to the ring cavity via a sidewall coupling region, represented by directional coupler 609. An additional directional coupler 619 couples the probe signal back into the probe waveguide 608 where it interferes with a part of the probe signal which has not been modulated by the Ring Modulator 605. The output probe signal may be detected, for example, by a photodetector (not shown). The optical length of the probe waveguide 608 can be tuned, such as quasi-statically tuned, using at least one DC electrode 614, allowing fine adjustments to optimise the interferometric monitoring process. Fig.7 shows a schematic of an alternative implementation of the device disclosed in Fig.2a. In this design, both the modulated signal and the probe signal are introduced through a common input waveguide 701 of the optical modulator. The signals are split by a Y-branch 706, enabling propagation along two modulation waveguides 705a and 705b, which form the two arms of the Mach-Zehnder modulator (MZM). At the output, the Y-branch used in previous configurations is replaced by an output directional coupler 712, which directs the modulated signal to two output waveguides 702a and 702b, providing two outputs for the modulated signal at wavelength λ^. In this implementation, two separate probe structures are present to monitor each arm of the MZM independently. In general, a system according to the present disclosure may provide that the number of probe structures (probe waveguides) matches the number of modulation waveguides. The upper and lower probe structures are sidewall coupled to the top modulation waveguide 705a and bottom modulation waveguide 705b, respectively, through directional couplers 719a and 719b. These directional couplers are engineered to selectively couple only the probe signal out of each modulation waveguide, ensuring that the modulated signal is directed entirely toward the output directional coupler 712 without interference. Each probe signal is guided out through the probe structure output waveguides 713a and 713b, where it can be independently monitored by, for example, photodetectors (not shown) to assess the performance of each arm of the modulator. The probe structures allow for precise interferometric measurements of the optical path length within each modulation waveguide, which may be used by the control unit (not shown) of the system to make adjustments to the refractive index or optical path length of the modulation waveguides to maintain optimal modulation performance. Further details of drawings 100 – preferential direction of the EO material 101 – input waveguide 102 – output waveguide 103 – RF coplanar waveguides 104 – DC electrodes 105 – modulation waveguides 106 – Y-branch 107 – optical detector 111a – beam splitter 111b – multi-mode interferometre (MMI) 121 – system output waveguide 128 – control electronics 200 – preferential direction of the EO material 201 – input waveguide 202 – output waveguide 203 – RF coplanar waveguides 204 – DC electrodes 205a, 205b – modulation waveguides 206 – Y-branch 208a, 208b – probe waveguides 209a, 209b – directional couplers 212a, 212b – probe waveguides 213a, 213b – probe output waveguides 214 – DC electrodes 215 – optical layer 216 – buried oxide (BOX) layer 217 – carrier layer 218 – top cladding layer 300 – preferential direction of the EO material 303 – electrodes for EO effect 305 – fully etched ridge waveguide 316 – buried oxide (BOX) layer 317 – carrier layer 318 – top cladding layer 400 – preferential direction of the EO material 403 – RF coplanar waveguides 404 – DC electrodes 407 – photodetectors 428 – control electronics 500 – preferential direction of the EO material 501 – input waveguide 502 – output waveguide 503 – RF electrodes 504 – DC electrodes 505 – closed-loop waveguide 506 – directional coupler 601 – input waveguide 602 – output waveguide 603 – RF electrodes 604 – DC electrode 605 – ring cavity or racetrack structure 606 – sidewall coupling region 608 – probe waveguide 609 – directional coupler 612 – probe waveguide 613 – probe output waveguide 614 – DC electrode 619 – directional coupler 701 – input waveguide 702a, 702b – output waveguides 705a, 705b – modulation waveguides 706 – Y-branch 712 – output directional coupler 713a, 713b – probe waveguides 719a, 719b – directional couplers

[0002] Items 1. A photonic device for (use in) optic modulation, comprising: ● An optical modulator, comprising: o An input waveguide, where the optical signal to be modulated (“modulated signal”) is launched, o A modulation waveguide section o An output waveguide, from which the modulated optical signal is outputted wherein the input waveguide is coupled to (split or coupled into) a modulation waveguide (a number of modulation waveguides) which is (are) then coupled (coupled or recombined) in the output waveguide. The modulation waveguide section also comprises: o An RF modulation section, where the modulation waveguide(s) is (are) parallel to RF coplanar waveguide(s) (called also RF electrodes) utilised to implement the optical modulation. o A DC tuning section, where the modulation waveguide(s) is (are) in the proximity of DC electrode(s) utilised to trim the optical properties of the modulation waveguide ● At least one probe structure, comprising: o An input waveguide, where the optical signal acting as a probe (“probe signal”) is launched, o A probe waveguide, o An output waveguide, from which the probe signal is outputted. 2. The photonic device according to item 1, wherein the photonic device is on a single chip in the shape of a photonic integrated circuit (PIC). 3. The photonic device according to any of the preceding items, wherein the RF electrodes are in a metal, for example Cr (chromium), Ti (titanium), Au (gold), Al (aluminium), Cu (copper), a combination of these or any other material suited for RF electronics and modulation. 4. The photonic device in 2, wherein the DC electrodes are in a metal, for example Cr (chromium), Ti (titanium), Au (gold), Al (aluminium), Cu (copper), NiCr (Nichrome), a combination of these or any other material suited for implementing micro-heaters or miniaturised capacitors. 5. The photonic device in 2, wherein the optical waveguides of the photonic integrated circuit are configured to support transverse optical modes smaller than 10 μm2.6. The photonic device according to any of the preceding items, wherein the modulation waveguides are implemented on a material suited for optical modulation (in the following called “optical layer” or equivalently “optical material”). 7. The photonic device according to any of the preceding items, wherein the device is implemented on an optical material suited for electro-optical modulation. 8. The photonic device according to any of the preceding item, wherein the device is implemented on an optical material such that the modulation waveguides undergo a slow drift of their optical properties due to the modulation. 9. The photonic device according to any of the preceding item, wherein the device is implemented on an electro-optic material affected by DC-drift effects during electro- optical modulation. 10. The photonic device according to any of the preceding items, wherein the device is implemented on LiNbO3. 11. The photonic device according to any of the preceding items, wherein the optical modulator supports a modulation bandwidth of 10GHz or more. 12. The photonic device according to any of the preceding items, wherein the optical material is a film of thickness between 100 nm and 100 μm. 13. The photonic device according to any of the preceding items, wherein the optical material is in the format of a thin film. 14. The photonic device according to any of the preceding items, wherein the optical material is defined on a dielectric material acting as an optical insulator, such as SiO2or sapphire, referred to as “buried oxide”, BOX. 15. The photonic device according to any of the preceding items, wherein the optical material is defined on a dielectric / optical insulator, such as SiO2, which again is defined on a substrate such as Si or LiNbO3which act as a “carrier material”. 16. The photonic device according to any of the preceding items, wherein the carrier material is mechanically stable. 17. The photonic device according to any of the preceding items, wherein the carrier material comprises a separating layer separating the optical layer and the carrier material. 18. The photonic device according to any of the preceding items, wherein the carrier material is electrically conductive. 19. The photonic device according to any of the preceding items, wherein the input waveguide of the optical modulator is coupled to one modulation waveguide via a directional coupler. 20. The photonic device according to any of the preceding items, wherein the input waveguide is coupled to two modulation waveguides via a directional coupler. 21. The photonic device according to any of the preceding items, wherein the input waveguide is coupled to two modulation waveguides via a splitter (Y-branch). 22. The photonic device according to any of the preceding items, wherein the input waveguide is coupled to two modulation waveguides via a multi-mode- interferometer (MMI). 23. The photonic device according to any of the preceding items, wherein a number of modulation waveguides is arranged in parallel, in series or a combination of parallel and series, creating a mesh of modulation waveguides. 24. The photonic device according to any of the preceding items, wherein the modulation waveguides are connected and arranged in such a way to create a mesh of Mach Zehnder modulators. 25. The photonic device according to any of the preceding items, wherein the modulation waveguides are recombined in at least one output waveguide (serving the optical modulator) via a (series of) Y-branch(es); a (series of) directional coupler(s); or a combination of these. 26. The photonic device according to any of the preceding items, wherein the optical modulator needs calibration prior to use. 27. The photonic device according to any of the preceding items, wherein the optical modulator needs a feedback loop which stabilises the modulation performance. 28. The photonic device according to any of the preceding items, used in a system wherein the signal at the output of the modulator cannot or should not be sampled. 29. The photonic device according to any of the preceding items, wherein the signal at output of the modulator cannot or should not be used in an interferometric measurement meant to implement a feedback system for stabilisation. 30. The photonic device according to any of the preceding items, wherein any observation of the signal at output of the modulator would compromise the quality or the state or the usability of the modulated signal itself. 31. The photonic device according to any of the preceding items, wherein a probe structure and the optical modulator are implemented using the same material. 32. The photonic device according to any of the preceding items, wherein a probe structure and the optical modulator are implemented using the different materials. 33. The photonic device according to any of the preceding items, wherein a probe waveguide and a modulation waveguide are sidewall coupled. 34. The photonic device in the preceding item, wherein a probe waveguide is coupled to a modulation waveguide through a directional coupler obtained by evanescently coupling the modulation waveguide and the probe waveguide. 35. The photonic device in the preceding item, wherein a probe waveguide is coupled to a modulation waveguide through a Mach Zehnder interferometer. 36. The photonic device in the preceding item, wherein the structure used to couple the probe waveguide and the modulation waveguide is reconfigurable. 37. The photonic device according to any of the preceding items, wherein the signal at the input of the optical modulator is never coupled to the probe structure. 38. The photonic device according to any of the preceding items, wherein the signal at the input of the probe structure is coupled to the modulation waveguide. 39. The photonic device according to any of the preceding items, wherein a probe structure (or a part of it) is utilised as a reference in an interferometric measurement of the optical length of a modulation waveguide. 40. The photonic device according to any of the preceding items, wherein a probe structure (or a part of it) is utilised as a reference in an interferometric measurement of the optical length of any part of the optical modulator. 41. The photonic device according to the preceding item, wherein the signal to be modulated and the signal used to perform the interferometric measurement of the modulation waveguide are launched in the same waveguide. 42. The photonic device according to any of the preceding items, wherein the signal to be modulated and the signal used to perform the interferometric measurement of the modulation waveguide have different wavelength. 43. The photonic device according to any of the preceding items, wherein the signal to be modulated and the signal used to perform the interferometric measurement of the modulation waveguide are launched in orthogonal waveguide modes. 44. The photonic device in the preceding item, wherein the signal to be modulated and the signal used to perform the interferometric measurement of the modulation waveguide have the same wavelength. 45. The photonic device in 1, wherein there is only one modulation waveguide. 46. The photonic device in 1, wherein there is more than one modulation waveguide. 47. The photonic device in 1, wherein there is only one probe waveguide. 48. The photonic device in 1, wherein there is more than one probe waveguide. 49. The photonic device in 1, wherein only one signal is coupled to one probe structure. The photonic device in 1, wherein more than a probe signal is coupled to (a section of) the same probe structure, being the different probe signals orthogonal to each other or characterised by different wavelengths. The photonic device according to any of the preceding items, wherein the measurement of the output from a probe structure is utilised in a feedback loop. The photonic device according to any of the preceding items, wherein the measurement of the output from a probe structure is utilised in the feedback loop that compensates for drifts in the properties of the modulator. The photonic device according to any of the preceding items, wherein the measurement of the output from a probe structure is utilised in a feedback loop used to adjusts, through a (quasi-static) trimming, the optical length of a modulation waveguide. The photonic device according to the preceding item, wherein “quasi-static” means one order of magnitude slower than the modulator bandwidth or slower than that. The photonic device according to any of the preceding items, wherein the optical length of a probe waveguide can be actively trimmed. The photonic device according to any of the preceding items, wherein the input of the optical modulator is a squeezed state of light, single photons (or a “few photons” signal or any optical signal with low signal to noise ratio). A method to monitor interferometrically the drift in optical length of a waveguide used for signal modulation comprising the steps of: ● Providing a device according to any of the preceding items, ● Feeding a probe signal to a probe structure which is coupled in at least one point to the waveguide where the modulation of another optical signal is happening, ● Measure the intensity of the probe signal at the output of the device, ● Based on the result of this measurement, actively trim the properties of the waveguide where the modulation takes place, thus compensating for slow drifts in its properties. A method according to the preceding item, wherein the waveguide is used to guide a signal with wavelength different from optical wavelengths. A method according to the preceding item, wherein the waveguide is meant to guide radio frequencies waves. A photonic device for (use in) signal modulation, comprising: ● A modulator, comprising: o An input waveguide, where the signal to be modulated is launched, o A modulation waveguide section o An output waveguide, outputting the modulated signal wherein the input waveguide is coupled to (split or coupled into) a modulation waveguide (a number of modulation waveguides) which is (are) then coupled (coupled or recombined) in the output waveguide. The modulation waveguide section also comprises: o An RF modulation section, where the modulation waveguide(s) is (are) parallel to RF coplanar waveguide(s) (called also RF electrodes) utilised to implement the modulation. o A DC tuning section, where the modulation waveguide(s) is (are) in the proximity of DC electrode(s) utilised to trim the optical properties of the modulation waveguide ● At least one probe structure, comprising: o An input waveguide, where a probe signal is launched, o A probe waveguide, o An output waveguide, from which the probe signal is outputted. The photonic device according to item 60, wherein the device is implemented on a chip in the format of a photonic integrated circuit (PIC) fabricated on a material suited to implement optical modulation with bandwidth above 10GHz, for example an electro-optic material, for example LiNbO3, and wherein the waveguides can support transverse modes smaller than 10 times the signal wavelength. The photonic device according to item 60 or item 61, wherein the modulator and the probe structure(s) are fabricated on a monolithic chip or on different materials bonded together (or otherwise coupled with each other), provided the material where the modulation waveguide section is implemented permits to obtain fast modulation (≥ 10GHz). 63. The photonic device according to any one of items 60 to 62, comprising M probe waveguides and N modulation waveguides and wherein the modulation waveguides are arranged in parallel, in series or a combination of parallel and series, creating a mesh of modulation waveguides. 64. The photonic device according to any of items 60 to 63, wherein each probe waveguide, whose optical length can be trimmed in a quasi-static fashion, is evanescently coupled (for example via a directional coupler or via a reconfigurable Mach Zehnder interferometer) to a modulation waveguide. 65. The photonic device according to any of items 60 to 64, wherein a signal orthogonal (in polarisation, field distribution or any other degree of freedom) or with different wavelength to the one to be modulated is used to interferometrically measure the optical length of (a part of) the optical modulator utilising (a part of) the probe structure as a reference. 66. The photonic device according to any of items 60 to 65, wherein the optical length of (a part of) the optical modulator needs trimming to ensure optimal performance and wherein the feedback system used to control such trimming is implemented starting from the output of one of the probe structures. 67. The photonic device according to any of items 60 to 66 for use in a system wherein the direct observation (by sampling or via interference) of the signal at output of the modulator would compromise the quality or the state or the usability of the modulated signal itself and wherein a signal orthogonal (or with different wavelength) to the modulated one is used to monitor the state of the modulator. 68. A method to monitor interferometrically the drift in optical length of a waveguide used for signal modulation comprising the steps of: ● Providing a device comprising at least one waveguide where optical modulation takes place and at least one probe structure, ● Feeding a probe signal to a probe structure which is coupled in at least one point to the waveguide where another signal is modulated, ● Measure the intensity of the probe signal at the output of the probe structure and, based on the result of such measurement, actively trim the properties of the waveguide where the modulation is implemented. 69. A method according to item 68, wherein the waveguide is meant to guide radio frequencies waves. Statements The follow numbered statements are disclosed. 1. A system for controlling the operation of an optical modulator, the system comprising: a waveguide circuit comprising an input waveguide section for receiving an input signal to be modulated, a modulation waveguide section for modulating the input signal, an output waveguide section and a probe waveguide section for receiving an input probe signal, wherein the probe waveguide section is optically coupled to the modulation waveguide section and configured to receive an input probe signal, a control unit for controlling the operation of an electric field generator, a photodetector configured to measure the output signal of the probe waveguide section, the output signal being the result of interference between a first part of the input probe signal not modulated by the modulation waveguide section and a second part of the input probe signal modulated by the modulation waveguide section, wherein, based on the measured output signal, the control unit is configured to cause the electric field generator to generate an electric field so as to modulate, such as tune or reconfigure, an optical property of the modulation waveguide section. 2. The system according to statement 1, wherein the system comprises a DC tuning section which comprises the electric field generator and, optionally, the electric field generator comprises one or more DC electrodes. 3. The system according to any preceding statement, wherein the input waveguide section comprises one or more input waveguides for receiving input signals to be modulated. 4. The system according to any preceding statement, wherein the input waveguide section is optically coupled to the modulation waveguide section such that the input signal to be modulated is an input signal to the modulated waveguide section. 5. The system according to any preceding statement, wherein an input signal to the output waveguide section is a signal modulated by the modulation waveguide section. 6. The system according to any preceding statement, wherein the electric field generator comprises one or more direct current (DC) electrodes. 7. The system according to any preceding statement, wherein the optical property of the modulation waveguide section modulated by the electric field generator is an optical length of the modulation waveguide section. 8. The system according to any preceding statement, wherein the system further comprises an RF modulation section for modulating optical properties of the modulation waveguide section. 9. The system according to any preceding statement, the RF modulation section comprises one or more radio frequency (RF) coplanar waveguides. 10. The system according to any preceding statement, wherein the control unit is configured to instruct an RF signal generator to generate an RF signal for driving the one or more RF coplanar waveguides so as to cause modulation, such as high speed modulation (>10 GHz) and / or electric-optic modulation, of the modulation waveguide section, such as one of more modulation waveguides in the modulation waveguide section. 11. The system according to any preceding statement, wherein the modulation waveguide section comprises one or more modulation waveguides which each have an optical path length that changes in response to an external electric field. 12. The system according to any preceding statement, wherein the modulation waveguide section comprises a plurality of modulation waveguides which are arranged in parallel, series or a combination of parallel and series, creating a mesh configuration of modulation waveguides. The system according to any preceding statement, wherein an input waveguide of the input waveguide section is optically coupled to two modulation waveguides via a multi-mode interferometre (MMI). The system according to any preceding statement, wherein the modulation waveguides of the modulation waveguide section are connected in such a way to create one or more Mach Zehnder modulators. The system according to any preceding statement, wherein the probe waveguide section is optically coupled to the modulation waveguide section via one or more sidewall coupling regions, such as directional couplers or via a Mach Zehnder interferometer. The system according to any preceding statement, wherein the probe waveguide section is optically coupled to the modulation waveguide section via a first sidewall coupling region and a second sidewall coupling region. The system according to any preceding statement, wherein the first sidewall coupling region is configured to split the input probe signal between a first split part which remains within the probe waveguide section and a second split part which enters the modulation waveguide section. The system according to any preceding statement, wherein the second sidewall coupling region is configured to split the input probe signal modulated by the modulation waveguide section between a third split part which re-enters the probe waveguide section and a fourth split part which remains within the modulation waveguide section. The system according to any preceding statement, wherein the first and second sidewall coupling regions are configured to reduce the degree of optical coupling of the input signal which originated from the input waveguide section to the probe waveguide section. The system according to any preceding statement, wherein the first and second sidewall coupling regions are configured to confine the input signal which originated from the input waveguide section within the modulation waveguide section. The system according to any preceding statement, wherein the first and second sidewall coupling regions comprise a drop filter, resonant structures, or any other type of photonic filter. The system according to any preceding statement, wherein the waveguide circuit is implemented on a chip, such as a photonic integrated circuit. The system according to any preceding statement, wherein the waveguide circuit supports a modulation bandwidth of 10 GHz or more. The system according to any preceding statement, wherein the probe waveguide section is optically coupled to the modulation waveguide section so as to allow for evanescent coupling. The system according to any preceding statement, wherein the probe waveguide section comprises one or more probe waveguides which are each configured to receive an input probe signal. The system according to any preceding statement, wherein each probe waveguide is optically coupled to the modulation waveguide section via one or more sidewall coupling regions. The system according to any preceding statement, wherein the electric field generator is configured to generate an electric field which is capable of optically modulating the modulation waveguide section so as to change its optical path length. The system according to any preceding statement, wherein the probe waveguide section is optically coupled to the modulation waveguide section via a first optical coupling region which allows for at least a part of the input probe signal to enter the modulation waveguide section and a second optical coupling region which allows for the at least a part of the input probe signal to recombine with the remaining part of the input probe signal in the probe waveguide section to form the output signal of the probe waveguide section. The system according to any preceding statement, wherein the control unit calculates a drift in an optical property of a device, such as the electro-optical modulator, of the modulation waveguide section from a target operating point of the device based on the measured output signal of the probe waveguide section. The system according to any preceding statement, wherein the drift in the optical property is a direct current (DC) drift. The system according to any preceding statement, wherein the control unit is configured to transmit a control signal to the electric field generator instructing the electric field generator to generate an electric field so as to reduce the drift in the optical property from the target operating point. The system according to any preceding statement, wherein the control unit is configured to continuously monitor the optical property of the device and transmit one or more control signals to the electric field generator instructing the electric field generator to generate an electric field so as to reduce the drift in the optical property from the target operating point until the target operating point is reached. The system according to any preceding statement, wherein the probe waveguide section comprises one or more probe waveguides. The system according to any preceding statement, wherein the modulation waveguide section comprises a closed loop waveguide to form an optical cavity. The system according to any preceding statement, wherein the modulation waveguide section comprises a plurality of closed loop waveguides which form a nested cavity configuration. The system according to any preceding statement, wherein the waveguide circuit is etched into a layer comprising an electro-optic material and, optionally, the layer is a lithium niobate based layer. 37. The system according to any preceding statement, wherein the layer comprising the electro-optic material has a thickness between 100 nm and 100 µm. 38. The system according to any preceding statement, wherein the layer of lithium niobate is a thin film having a thickness of less than 2 µm. 39. The system according to any preceding statement, wherein the layer comprising an electro-optical material is part of a material stack. 40. The system according to any preceding statement, wherein the layer comprising the electro-optic material is capable of supporting a modulation bandwidth of 10 GHz or more. 41. The system according to any preceding statement, wherein the material stack comprises another layer comprising a dielectric material and, optionally, the dielectric material acts as an optical insulator. 42. The system according to any preceding statement, wherein the material stack comprises another layer having a lower refractive index than the layer comprising the electro-optical material, and wherein the layer comprising the electro-optical material is arranged on top of the another layer. 43. The system according to any preceding statement, wherein the another layer comprises silica (SiO2) and / or sapphire (Al2O3). 44. The system according to any preceding statement, wherein the another layer has a thickness in the range of 2 µm to 500 µm. 45. The system according to any preceding statement, wherein the material stack comprises a further layer configured to provide mechanical stability and the another layer is arranged on top of the further layer. 46. The system according to any preceding statement, wherein the further layer comprises a silicon (Si) and / or lithium niobate. 47. The system according to any preceding statement, wherein the further layer has a thickness in the range of 200 µm to 500 µm. 48. The system according to any preceding statement, wherein the further layer is electrically conductive. 49. The system according to any preceding statement, wherein the waveguide circuit is configured to split the input signal to be modulated into different waveguides within the modulation waveguide section and recombine the split signals to form an output signal of the modulation waveguide section. 50. The system according to any preceding statement, wherein the output signal of the modulation waveguide section is an input signal to the output waveguide section. 51. The system according to any preceding statement, wherein the output waveguide section comprises one or more output waveguides for receiving the modulated signals. 52. The system according to any preceding statement, wherein the modulation waveguides of the modulation waveguide section recombine in an output waveguide of the output waveguide section via any combination of one or more Y-branches and one or more directional couplers. 53. The system according to any preceding statement, wherein an input waveguide of the input waveguide section is optically coupled to a modulation waveguide of the modulation waveguide section, such as via a directional coupler. 54. The system according to any preceding statement, wherein an input waveguide of the input waveguide section is optically coupled to two modulation waveguides via a beam splitter, such as a via a Y-branch. 55. The system according to any preceding statement, wherein the input signal to be modulated is split by any combination of a Y-branch, a beam splitter and a directional coupler into different waveguides of the modulation waveguide section. 56. The system according to any preceding statement, wherein the split signals forming the output signal of the modulation waveguide section are recombined by any combination of a Y-branch, a beam splitter and a directional coupler to form an input signal to the output waveguide section. 57. The system according to any preceding statement, wherein the probe waveguides and the modulation waveguides are made from the same material. 58. The system according to any preceding statement, wherein the waveguide circuit supports radio frequency waves. 59. A method for monitoring the drift of an optical modulator, the method comprising the steps of: providing a system according to any of the preceding claims, feeding an input signal to be modulated into the input waveguide section of the waveguide circuit and a separate probe signal into the probe waveguide section of the waveguide circuit, a) measuring, using the photodetector, an output signal of the probe waveguide section, the output signal being the result of interference between a first part of the probe signal which has not travelled inside the modulation waveguide section and a second part of the probe signal which has travelled inside the modulation waveguide section due to coupling between a probe waveguide in the probe waveguide section and a modulation waveguide in the modulation waveguide section, b) the control unit instructing, based on the measurement of the output signal, the electric field generator to generate an electric field to adjust a property of the modulation waveguide of the modulation waveguide section related to how an optical signal propagates through the modulation waveguide so as to reduce the drift. 60. The method according to statement 59, wherein the property of the modulation waveguide section is an optical path length of the modulation waveguide or a refractive index of the modulation waveguide. 61. The method according to statement 59 or statement 60, wherein the control unit calculates, based on the measurement of the output signal of the probe waveguide section, a drift of the property of the modulation waveguide away from a target operating value of the modulation waveguide. 62. The method according to any preceding statement 58 to 61, wherein the target operating value of the modulation waveguide is related to the optical modulation efficiency of the modulation waveguide. 63. The method according to any one of statement 59 to 62, wherein the probe signal couples to a modulation waveguide in the modulation waveguide section via a sidewall coupling region so that a first part of the probe signal remains within the probe waveguide section and a second part of the probe signal enters into the modulation waveguide. 64. The method according to any one of statement 59 to 63, wherein the first part of the probe signal propagates through the modulation waveguide before re- coupling to the probe waveguide section via another sidewall coupling region. 65. The method according to any one of statement 59 to 64, wherein the control unit instructs the DC tuning section to transmit a feedback signal to the one or more DC electrodes to generate an electric field so as to modulate the optical path length or refractive index of the modulation waveguide to reduce the drift. 66. The method according to any one of statement 59 to 65, wherein the input signal fed into the input waveguide section and the probe signal fed into the probe waveguide section have different wavelengths. 67. The method according to any one of statement 59 to 66, wherein the input signal fed into the input waveguide section and the probe signal fed into the probe waveguide section are orthogonal waveguide modes. 68. The method according to any one of statement 59 to 67, wherein one or more probe signals are fed into the probe waveguide section and the one or more probe signals being orthogonal to one another and / or having different wavelengths. 69. The method according to any one of statement 59 to 68, wherein the input signal to be modulated comprises a squeezed state or single photons. 70. The method according to any one of statement 59 to 69, wherein the steps a) and b) are repeated as part of a feedback control loop. The method according to any one of statement 59 to 70, wherein in step b) the a property of the modulation waveguide is adjusted more slowly, such as one order of magnitude more slowly, than the modulation bandwidth of waveguide circuit.

Claims

Claims 1. A system for controlling the operation of an optical modulator, the system comprising: a waveguide circuit comprising an input waveguide section configured to receive an input signal to be modulated, a modulation waveguide section configured to receive the input signal from the input waveguide and further configured to modulate the input signal, an output waveguide section and a probe waveguide section configured to receive an input probe signal, wherein the probe waveguide section is optically coupled to the modulation waveguide section and configured to receive an input probe signal, a control unit for controlling the operation of an electric field generator, a photodetector configured to measure the output signal of the probe waveguide section, the output signal being the result of interference between a first part of the input probe signal not modulated by the modulation waveguide section and a second part of the input probe signal modulated by the modulation waveguide section, wherein, based on the measured output signal, the control unit is configured to cause the electric field generator to generate an electric field so as to modulate, such as tune or reconfigure, an optical property of the modulation waveguide section.

2. The system according to claim 1, wherein the system comprises a DC tuning section which comprises the electric field generator and, optionally, the electric field generator comprises one or more DC electrodes.

3. The system according to any preceding claim, wherein probe waveguide section and the modulation waveguide section are the same waveguide and wherein, after the modulation waveguide section, only the input signal is coupled to the output waveguide section while the probe signal is directed towards a photodetector configured to measure the output signal of the probe waveguide section.

4. The system according to any preceding claim, wherein the optical property of the modulation waveguide section modulated by the electric field generator is an optical length of the modulation waveguide section and, optionally, wherein themodulation waveguide comprises one or more modulation waveguides which each have an optical path length that changes in response to an external electric field.

5. The system according to any preceding claim, wherein the system further comprises an RF modulation section for modulating optical properties of the modulation waveguide section and the control unit is configured to instruct an RF signal generator to generate an RF signal for driving the one or more RF coplanar waveguides so as to cause modulation, such as high speed modulation and / or electric-optic modulation, of the modulation waveguide section, such as one of more modulation waveguides in the modulation waveguide section.

6. The system according to any preceding claim, wherein the modulation waveguide section comprises a plurality of modulation waveguides which are arranged in parallel, series or a combination of parallel and series, creating a mesh configuration of modulation waveguides, of photonic resonant structures or a combination thereof.

7. The system according to any preceding claim, wherein the modulation waveguides of the modulation waveguide section are connected in such a way to create one or more Mach Zehnder modulators.

8. The system according to any preceding claim, wherein the probe waveguide section is optically coupled to the modulation waveguide section via a first optical coupling region which allows for at least a part of the input probe signal to enter the modulation waveguide section and a second optical coupling region which allows for the at least a part of the input probe signal to recombine with the remaining part of the input probe signal in the probe waveguide section to form the output signal of the probe waveguide section.

9. The system according to any preceding claim, wherein the control unit calculates a drift in an optical property of a device of the modulation waveguide section from a target operating point of the device based on the measured output signal of the probe waveguide section and, optionally, wherein the control unit is configured to continuously monitor the optical property of the device and transmit one or more control signals to the electric field generator instructing the electric field generatorto generate an electric field so as to reduce the drift in the optical property from the target operating point until the target operating point is reached.

10. The system according to any preceding claim, wherein the control unit is configured to transmit a control signal to the electric field generator instructing the electric field generator to generate an electric field so as to reduce the drift in the optical property from the target operating point.

11. The system according to any preceding claim, wherein the waveguide circuit is etched into a layer comprising an electro-optic material and, optionally, the layer is a lithium niobate based layer.

12. The system according to any preceding claim, wherein the layer comprising the electro-optic material has a thickness between 100 nm and 100 µm, preferably between 100 nm and 2 µm.

13. The system according to any preceding claim, wherein the waveguide circuit is configured to split the input signal to be modulated into different waveguides within the modulation waveguide section and recombine the split signals to form an output signal of the modulation waveguide section.

14. The system according to any preceding claim, wherein the modulation waveguides of the modulation waveguide section recombine in an output waveguide of the output waveguide section via any combination of one or more Y-branches and one or more directional couplers. to form an input signal to the output waveguide section.

15. A method for monitoring the drift of an optical modulator, the method comprising the steps of: providing a system according to any of the preceding claims, feeding an input signal to be modulated into the input waveguide section of the waveguide circuit and a separate probe signal into the probe waveguide section of the waveguide circuit, c) measuring, using the photodetector, an output signal of the probe waveguide section, the output signal being the result of interference between a first part of the probe signal which has not travelled inside the modulation waveguide section and a second part of the probesignal which has travelled inside the modulation waveguide section due to coupling between a probe waveguide in the probe waveguide section and a modulation waveguide in the modulation waveguide section, d) the control unit instructing, based on the measurement of the output signal, the electric field generator to generate an electric field to adjust a property of the modulation waveguide of the modulation waveguide section related to how an optical signal propagates through the modulation waveguide so as to reduce the drift.

16. The method according to claim 59 or claim 60, wherein the control unit calculates, based on the measurement of the output signal of the probe waveguide section, a drift of the property of the modulation waveguide away from a target operating value of the modulation waveguide and wherein the control unit is configured to instruct the DC tuning section to transmit a feedback signal to the one or more DC electrodes to generate an electric field so as to modulate the optical path length or refractive index of the modulation waveguide to reduce drift.

17. The method according to any one of claims 59 to 62, wherein the probe signal couples to a modulation waveguide in the modulation waveguide section via a sidewall coupling region so that a first part of the probe signal remains within the probe waveguide section and a second part of the probe signal enters into the modulation waveguide.

18. The method according to any one of claims 59 to 67, wherein one or more probe signals are fed into the probe waveguide section and the one or more probe signals being orthogonal to: both one another and to the signal to be modulated; and / or having different wavelengths.

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