Programmable phase-change material based contra-directional add-drop filter

Abstract

An optical coupler for controlling the propagation and managing light beams within photonic circuits provided with a main waveguide, configured with a section of the main waveguide between an input port and a through port through which an optical beam, an auxiliary or secondary waveguide, configured with a section of the auxiliary waveguide between an add port and a drop port through which an optical beam, wherein the main waveguide is optically coupled with the auxiliary waveguide in one or more portions of said main waveguide and the auxiliary waveguide, one or more elements for modulating the refractive index within at least one portion of at least one of said waveguides thereby modifying one or more propagation characteristics of said at least one waveguide depending on the non-volatile physical state assumed by said modulation elements, wherein the physical state of said one or more elements for modulating the refractive index is modifiable, by means of control means, between a first modulation state and at least a second modulation state, allowing for persistent refractive index control without continuous power.

Description

P8027PC00Programmable Phase-Change Material Based Contra-Directional Add-Drop Filter★ ★ ★Field of Invention

[0001] The present invention relates to optical communication systems, specifically to integrated optical add-drop filter based on phase-change materials (PCM) for elastic and nonvolatile switching and / or multiplexing / demultiplexing in Wavelength Division Multiplexing (WDM) systems.Background

[0002] Known existing optical switches for WDM systems lack non-volatile control and require continuous power consumption to maintain their configuration. Other drawbacks of traditional technologies such as Liquid Crystal on Silicon (LCOS) and Micro-Electro Mechanical Systems (MEMS) is that they suffer from environmental sensitivity and mechanical wear, respectively. Other available technologies such as Integrated photonic devices like Mach-Zehnder Interferometers and MicroRing Resonators require control mechanisms which are volatile as well and lack the programmability and scalability required for modern applications. By the term volatile it is meant a device who needs to be energized to set and maintain a specific configuration which is different from a default configuration and which returns to said default configuration once de-energized.

[0003] By "volatile, " it is meant a device that must be energized to set and maintain a specific configuration different from a default configuration, and which returns to that default configuration once de-energized. Without continuousP8027PC00power applied, these "volatile" device returns to their original design state, thus requiring active components to provide the needed power, resulting in power consumption, heat generation, bigger devices and reduced scalability.

[0004] Other devices are known for different types of optical switching, such as Four-Wave Mixing (FWM), devices based on Photonic Crystals and Non-linear Optical Mirrors, who however present difficulties in production, complex calibrations, and are generally reserved for applications other than WDM systems which the proposed technology aims at.

[0005] Additionally, to the state of the art there are no easily reconfigurable solutions, as devices generally operate on predetermined signal windows ( frequencies and / or bandwidth), which is determined in the design stage and not tunable after the device has been produced and known devices able to be reconfigured result in complex switching systems obtained through the combination of multiple discrete elements for the specific application.Summary of the Invention:

[0006] This invention relates to a non-volatile and programmable switching devices based on integrated photonic technologies, comprising but not limited to silicon photonics, for WDM systems involving phase-change materials (PCM). Leveraging on the PCM characteristics, the device allows for precise control over the optical properties of waveguides through PCM, enabling elastic and programmable filtering capabilities in terms of frequency, bandwidth and optical response. In a specific embodiment, the devices are contradirectional couplers (CDC); however, the benefit of PCM in optical fiber to obtain controllable index variation of the waveguide are not restricted to CDC.P8027PC00

[0007] The device structure is compatible with existing manufacturing processes, offering scalability and reduced production costs. This technology combines the strengths of many alternative solutions through an integrated photonic structure with a non-volatile and highly flexible control system. This allows for the reduction of production costs due to compatibility with modern production processes, enabling large-scale production, integration with other systems and devices within the same technology, and reduced control power compared to non-persistent solutions.

[0008] Different advantages are achieved by the present invention such as:Integrated photonics allows for the miniaturization of devices, reducing both production costs (economy of scale) and installation and calibration costs due to the lack of mechanical or hybrid components.- Non-volatile control through PCM reduces the control power needed, enabling scalability traditionally not possible with other integrated technologies (thermal control / carriers ).- The range of programmable response that can be obtained with the proposed device also replaces and expands the capabilities of integrated multi-device structures that traditionally used several integrated components.Integration into commercial production platforms, such as Silicon Photonics or other integrated photonic technologies, allows for the reduction of production costs on a large scale due to the technological maturity of the production system. Compatibility with production processes traditionally used for electronic components theoretically allows for large-scale production.P8027PC00- The non-volatili ty and programmability of the device simultaneously reduce operational and management costs of routing and switching nodes.- No mechanical parts to implement the switching functionalities are involved, thus simplifying the production and maintenance and improving reliability.Technical Description of the Invention

[0009] The specific device is a multi-port optical add-drop element that allows filtering or adding a programmable frequency response. This device is based on a class of components called Contra-Directional Couplers (CDC). CDC components consist of at least two parallel waveguides placed at a predetermined distance to have weak direct signal coupling between the waveguides, thus facilitating selective wavelength coupling.

[0010] In a typical CDC setup, there are two waveguides:- Main Guide: This guide carries the input signal which needs to be coupled. It provides for an input port and an outport also known as "through" port. Light propagation in this guide from input port to through port is referred as "forward mode".- Auxiliary Guide: This guide is disposed parallel to the main guide along a predetermined path, which can follow any arbitrary shape, such as straight or curved geometries. It receives the signal from the main guide and such transferred signal in the auxiliary guide moves in the opposite direction to that in the main guide. It provides for an add port and for a drop port; the first allowing to introduce (or "add" ) a new wavelength or channel into the main waveguide and the latter allowing to extract (or "drop" ) a specific wavelength or channelP8027PC00from the main waveguide of the fiber optic line. Light propagation in this direction is referred as "backward mode".

[0011] The present invention will be described, if otherwise specified, with regard to a configuration as above but it is still applicable in case of devices with more than one auxiliary guide coupled to the main waveguide as part of the same device.

[0012] Through periodic perturbation of the physical parameters of the guides (such as width or height) in the direction of electromagnetic field propagation, a specific alteration of the local effective refractive index can be controlled and introduced, obtaining a structure known as a Bragg Grating.

[0013] Despite the weak direct coupling of the signal in the intrinsic guides, the presence of the two gratings allows the coupling of the forward mode of one guide with the backward mode of the second guide. This coupling occurs only for a specific frequency range that depends on the geometric structure of the guides and the index perturbation introduced ( for example, by varying the width or height of the guide dimensions of the grating period). This relationship can be expressed in terms of phase matching conditions through the refractive index curves of the guides and the grating period.

[0014] Sp ecifically, the phase matching condition for the Bragg grating can be expressed asnavg=^- where:navgrepresents the average effective refractive index of the waveguides (assuming the two waveguides have n1and n2as respective indexes);X represents the wavelength of the guided light beam carriedP8027PC00by the waveguide;A represents the period of the Bragg Grating.This relationship is used to describe the phase matching point in grating devices, such as contra-directional couplers (CDC).

[0015] To the phase matching condition follows that that efficient coupling between the forward-propagating mode in one waveguide and the backward-propagating mode in the other waveguide occurs when the average refractive index of the waveguides equals half the ratio of the wavelength of the guided light to the grating period.

[0016] This condition ensures that the lightwaves constructively interfere and efficiently transfer energy between the waveguides.

[0017] In contra-directional couplers, this phase matching condition allows for the coupling of light from the input port to the drop port or from the add port to the output port at specific wavelengths, determined by the grating period and the average refractive index.

[0018] By ensuring that this phase matching condition is met, the CDC devices can effectively filter and route specific wavelength channels, which is essential for applications in wavelength-division multiplexing (WDM) systems.

[0019] At the phase match point, there is high field coupling from the input port to the drop port as well as from the add port to the output port: this add-drop element behavior allows filtering of a band and / or adding a signal at the same node.

[0020] The coupling coefficient, as well as internal reflections and co-directional coupling, are dictated by the guidesP8027PC00geometry, in particular the spacing and the grating itself. It should be noted that in the device, there will also be intra-guide reflection phenomena: instead of coupling to the secondary guide, part of the signal can also be reflected back from the input port due to the grating itself. A typical solution to reduce this parasitic effect consists of phase shifting the guide perturbations by 180 degrees from one side to the other of the structure, obtaining destructive interference of the unwanted reflection.

[0021] Another parasitic effect consists of large ripples in transmission at the drop band edges: this effect is mainly due to the rapid transition between the uncoupled region of the guides and the contra-directional coupling region. To mitigate this effect, a technique called apodization is applied, where a gradual transition between the intrinsic guide profile and the perturbed guide profile is made. Multiple profiles have been investigated and proposed in the literature with different trade-offs between attenuation and reduction of the coupling coefficient.

[0022] Starting from the phase matching condition for the Bragg grating introduced in

[0014] , options to change the central frequency of the coupled band between the waveguides is to change the grating period or the average index of the waveguides: this allows control of the frequency for which phase coupling occurs, defining the center of the drop and add band in the CDC device.

[0023] Regarding bandwidth, there are various solutions that allow static design as well as dynamic control with different degrees of effectiveness but based on a similar operational principle. Changing one of the two conditions (grating period or effective index) along the longitudinal direction of the device results in concatenation of the effects of theP8027PC00individual phase coupling conditions. In the design of wideband devices, this technique is called pitch chirping and involves varying the grating period as a function of the longitudinal direction. Through this technique, CDC bands can be extended and designed in specific regions, allowing the creation of highly specific band filters.

[0024] However, the chirping of the period cannot be dynamically controlled since it is a geometrical parameter fixed at design time. Dynamic control is therefore carried out by modulating the effective index of the guides providing the guides with the capability to offer different refraction indexes along its the longitudinal direction, for one or both of the coupled guides.

[0025] So both the frequency and bandwidth of the coupling condition can be controlled by controlling the reflection indexes of the waveguide (s). Traditionally, the two control techniques implemented are through microheaters or free carrier inj ection: using these methods, the effective index can be modulated, resulting in a shift of the transmission bands.

[0026] These methods, however, represent volatile solutions, as continuous power consumption is necessary to maintain the required state. Additionally, programming the device to obtain arbitrary frequency responses is particularly problematic and limited due to the intrinsic characteristics of the materials used, making this control mechanism more suitable for calibration and fine-tuning. So far, due to these limitations, no highly programmable solutions like the proposed one have been devised, which effectively reduces an entire optical switch to a single CDC.

[0027] This invention introduces a new device and control method based on Phase-Change Materials (PCM) to control theP8027PC00reflective index of the waveguides. The crystalline structure of PCMs can be controlled through various techniques such as temperature or electromagnetic field pulses, allowing the stable alteration or programming of the material behaviour: this structural characteristic is persistent and non-volatile, not requiring further external stimuli once programmed to maintain the programmed state, differently from the know devices.

[0028] Given the variation in effective index between the amorphous state of the material and the fully crystallized state, a device adapted for non-volatile control of the guide index is provided: by depositing one or more PCM layer (s) over at least one guide of the device, so that the index can be controlled without significantly altering the modal behavior of the system: the CDC design and various optimization techniques remain fundamentally unchanged.

[0029] Device behaviour control through changing PCM layer crystallization also presents other advantages besides nonvolatility, particularly concerning the magnitude of the index variations. The index excursion introduced by PCMs is not achievable through known thermo-optic control: in the case of silicon guides (high thermo-optic coefficient), the equivalent thermal excursion to be applied on a thermal controlled device based on standard materials would be on the order of thousands of degrees to theoretically reach the same result.

[0030] This achieved extremely high index modulation range allows the creation of devices with bandwidth control capabilities beyond previous limits set by thermo-optic control. By itself alone, the present invention allows the drop window to be shifted to an extended wavelength region.P8027PC00

[0031] Moreover, there could also be a need to allow adjustment of the bandwidth itself. To obtain elastic control of the 3 dB bandwidth or control of multiple channels, a design logic similar to the previously illustrated chirping concept is necessary. Instead of using a single PCM layer controlled uniformly, the material is controlled through a multi-section logic, programming multiple adjacent or non-adjacent drop windows.

[0032] To allow tunability in both directions, it is preferable to consider a 50% crystallization rate when designing the central frequency of the CDC, allowing symmetric control in both directions. Various multi-channel control configurations with respective PCM section states, considering a simulated CDC on the Si / SiO2 platform with Sb2Se3PCM, are shown. Various control configurations are illustrated, showing both multi-channel states with constant bands, multichannel states with variable bands, and single-channel configurations. The device can be adapted to multiple applications following traditional methodologies used for the design of gratings and structures, varying the application band, fundamental channel band dimensions, and the number of independent material control sections.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present disclosure and, together with the description, serve to explain the principles of the embodiments. In the drawings:

[0034] FIG. 1 illustrates a schematic implementation of a CDC devices with two waveguides and details of two coupled Bragg gratings.P8027PC00

[0035] FIG. 2 illustrate a generic CDC with Bragg grating on both waveguides and references to the communication ports and signal path.

[0036] FIG. 3 illustrates a standard Bragg grating opposed to a 180° Phase Shift for Reflection Mitigation.

[0037] FIG. 4 illustrates an example of Apodization Profile in the Grating.

[0038] FIGS. 5a-5d illustrate details of possible implementation of CDC in figure 1, with different PCM structure comprising thickness and crystallization ratios.

[0039] FIG. 6 illustrates a phase coupling condition of a CDC in a 2D diagram showing two refraction indexes on a dispersion curve graph.

[0040] FIGS. 7a and 7b illustrate an example of Band Shift Effect Due to Effective Index or Grating Period Control.

[0041] FIG. 8 illustrates various Control Configurations with Constant Band Multi-Channel, Variable Band Multi-Channel, and Single-Channel Configurations.

[0042] FIG. 9 schematically illustrates a CDC with different PCMs distributed along a longitudinal development of the waveguides.

[0043] FIG. 10 schematically illustrates an embodiment of the CDC in previous figure where numerical dimensions are indicated.DETAILED DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 illustrates a schematic layout of a Contra-Directional Coupler (CDC), a well-known and key component in optical communication systems. The figure shows two parallel optical waveguides: the main waveguide and the auxiliary (orP8027PC00secondary) waveguide. The main waveguide is used for the transmission of optical signals from an input port to a through port. The auxiliary waveguide is aligned parallel to the main waveguide for a part of the path and is used to add or drop specific wavelengths through an add port and a drop port. In the figures the main and auxiliary guides are adjacent in a rectilinear path, but in other embodiments the path can be developed with other geometry, such as curvilinear in one or more segments also with different radius in different segments.

[0045] Al so with reference to figure 2, the coupler works by allowing selective wavelength transfer between the two waveguides based on the presence of Bragg gratings. The top waveguide is the main path for the optical signal. Light enters at the input port and exits through the through port, as shown by the Long dashed line ( Forward Transmission) indicating forward transmission.

[0046] The bottom waveguide is the auxiliary or secondary waveguide, which handles the addition and extraction of specific wavelengths. The drop port extracts specific wavelength channels from the main path, while the add port introduces new ones.

[0047] The dashed-dotted line (Contra-Directional Coupling) indicates the contra-directional coupling mechanism, where light from the main waveguide couples to the auxiliary waveguide in the opposite direction. This occurs at specific wavelengths determined by the Bragg gratings in the waveguides.

[0048] The solid line represents backward reflection, a phenomenon where some portion of the light is reflected back into the input port of the main waveguide. This reflectionP8027PC00can interfere with signal transmission and is typically minimized through design techniques such as phase shifts.

[0049] The dotted line (Co-Directional Coupling) represents co-directional coupling, where light couples from the auxiliary waveguide to the main waveguide in the same direction. This is typically less desirable in contra-directional couplers but can occur depending on the design and alignment of the waveguides.

[0050] Additionally, an add-path (Forward Co-Directional Coupling) is generally provided in CDCs: the signal enters the system through the add port in the auxiliary waveguide and is co-directionally coupled into the main waveguide, exiting via the through port. The path of the added signal travels in the same direction as the existing signal in the main waveguide. This mechanism allows the introduction of a new wavelength channel into the main waveguide, merging the added signal with the existing signal at the through port.

[0051] The primary function of the CDC, as depicted in this figure, is thus to filter and route specific wavelengths. For example, a specific wavelength can be dropped from the main waveguide into the auxiliary waveguide (drop port), or a new wavelength can be added from the auxiliary waveguide into the main one (add port).

[0052] The contra-directional coupling enables selective wavelength routing, making it highly suitable for Wavelength Division Multiplexing (WDM) systems, where precise control over wavelength channels is necessary.

[0053] Optical coupling between the two waveguides occurs in a specific region where Bragg gratings are implemented on both waveguides. These gratings enable contra-directional coupling, meaning that the signal in the main waveguideP8027PC00propagates in the forward direction ( from input port to Through port), while the signal in the auxiliary waveguide propagates in the opposite direction (backward mode: to Drop port or from Add port). This allows selective coupling of light at specific wavelengths, enabling the device to filter, add, or drop specific wavelength channels in Wavelength Division Multiplexing (WDM) systems.

[0054] The Bragg grating is crucial for wavelength selectivity, where only certain wavelengths satisfy the phase-matching condition for coupling. The grating periodicity (Λ) and the average refractive index (navg) of the waveguides determine the central wavelength at which coupling occurs. As discussed above, the phase matching condition is given by:navg= λ / (2Λ)where navgis the average refractive index of the waveguides, λ is the wavelength of the signal, and Λ is the grating period.

[0055] In the drawing, wl is the width of the main waveguide, w2 is the width of the secondary waveguide; AW1 is the variation in the width of the main waveguide and AW2 is the width variation of the secondary waveguide. Al and A2 is the periodicity of the main and secondary Bragg grating and G or " Gap" is the distance or gap between the waveguides in the parallel section.

[0056] Light in the main waveguide travels in the forward mode, while in the auxiliary waveguide, it travels in the backward mode. This contra-directional propagation ensures efficient wavelength-selective coupling, making it possible to either drop a specific wavelength or add a new one at the same node.P8027PC00

[0057] These parameters determine the efficiency of the optical power transfer between the waveguides: the efficient coupling between the two waveguides happens only for a narrow band of wavelengths determined by the physical properties of the gratings and waveguides. This allows precise filtering of optical signals, making the CDC ideal for WDM systems where multiple optical channels must be handled simultaneously.

[0058] With reference to figure 3, it is shown a comparison between a standard Bragg grating and a modified a Bragg grating with a 180-degree phase shift. This technique modifies the phase of the perturbation within the grating, creating a discontinuity. As introduced in

[0020] the phase shift reduces backward reflections that would otherwise propagate back towards the input port of the device. This improvement ensures more efficient coupling and reduces optical losses, enhancing signal purity.

[0059] In figure 4 it is shown a different technique, the grating apodization: the upper part of the second figure represents an apodized Bragg grating, where the intensity of the modulation in the grating varies gradually along its length. This creates a smoother grating profile, reducing the edge effects between coupled and uncoupled regions.

[0060] As anticipated in

[0021] , apodization aims to reduce ripple in the transmission spectrum and minimize reflections at the edges of the coupling band. This leads to smoother transitions and greater device efficiency, particularly useful in applications requiring precise wavelength filtering.

[0061] Figures 5 illustrates a possible implementation of the CDC device utilizing one of the possible configurations of Phase-Change Materials (PCM). These materials are key toP8027PC00enabling non-volatile control of the refractive index within the optical waveguides, allowing for programmable and elastic filtering capabilities. The figure shows how different compositions and configurations of PCM can be applied to the waveguides to modulate the coupling and optical response of the device.

[0062] Figure 5b, which presents a particular view of figure 5a, presents an embodiment where a layer of PCM 20 is deposited on the secondary waveguide 22. There, a Sb2Se3layer having a thickness hpcM is deposited upon a Silicon substrate; other materials, such as GST (Germanium-Antimony-Tellurium) can be used as PCM.

[0063] The PCM can be in either its amorphous or crystalline state, and its refractive index can be modulated via external stimuli, such as thermal or electromagnetic pulses. This allows for non-volatile tuning of the refractive index in the waveguide, thus controlling the filtering of specific wavelength channels. Technologies suitable for PCM control are, amongst other, thermal control optionally using electrodes to transfer heat to the material by means of Joule effect and / or optic control optionally involving a laser source emitting focused light rays to promote the PCM phase change.

[0064] A map of the correlation amongst crystallization state, thickness and resulting channel shift is shown in figure 5d.

[0065] Figure 5c presents the different reflexive indexes in amorphous and crystalline states for Sb2Se3and GST. It can be noted that reflective index changes of about 2.8 (GST) and 0.78 (Sb2Se3) while the PCM changes from amorphous to crystalline and vice-versa.P8027PC00

[0066] A further optional embodiment, not shown, provides for a multi-section design, where the PCM is divided into multiple sections along the length of the waveguides. Each section can be independently controlled, providing multi-channel control for different wavelength bands. This allows for elastic control over multiple wavelength windows, enabling the CDC to filter, add, or drop multiple channels simultaneously, based on the state of each PCM section.

[0067] For example, Sb2Se3and GST, with distinct optical properties, can be used in different sections so that the CDC can achieve highly specific and programmable wavelength filtering. This enables more complex, multi-channel filtering and switching capabilities, as each material provides different refractive index modulation ranges and response times.

[0068] In any case, these implementations demonstrate the flexibility of the CDC design when combined with PCM technology. The ability to modify the refractive index of the waveguides in a programmable, non-volatile manner allows for precise control over the phase matching conditions and the optical response of the device. This results in improved performance for Wavelength Division Multiplexing (WDM) systems, where the ability to dynamically adjust the filtering and routing of optical channels is crucial.

[0069] The use of PCM allows for persistent tuning of the waveguide (s) optical properties without the need for continuous power. By applying external stimuli, the crystallization level of the PCM can be altered, leading to a significant change in the refractive index, which in turn controls the coupling coefficient and bandwidth of the device. This multi-section and multi-material design offers greater flex-P8027PC00ibility and scalability for optical filtering and multiplexing applications.

[0070] Figure 6 is a plot showing the effective index as a function of wavelength (X) for different elements in a contra-directional coupler (CDC) system. The x-axis represents the wavelength, while the y-axis shows the corresponding effective refractive index. Four curves are displayed, each representing different values relevant to the coupler ' s optical behavior.

[0071] Specifically, nl represents the effective refractive index (nl ) of a first waveguide (e. g. the main waveguide of a CDC) as a function of wavelength. The index decreases as the wavelength increases, indicating a typical dispersion behavior where longer wavelengths correspond to lower refractive indices.

[0072] Similarly, n2 shows the effective refractive index (n2 ) of a second waveguide (e. g. the auxiliary waveguide of a CDC). The effective index decreases more gradually compared to nl. This difference between nl and n2 is important for controlling the coupling between the two waveguides.

[0073] Conversely, the same result can be achieved in a configuration with swapped guides, i. e. nl working as auxiliary waveguide and n2 as main waveguide.

[0074] The average refractive index between the two waveguides (navg) is depicted by the dotted line. It represents the mean of nl and n2. This value is critical in determining the phase-matching condition for efficient coupling at specific wavelengths. The average index influences the coupling efficiency and the center wavelength for filtering.

[0075] The purple dashed line represents the value of the expression λ / (2Λ1), where λ is the wavelength and Λ1 is theP8027PC00grating period of the Bragg grating. The intersection of this line with the navgcurve (black dotted line) indicates the phase-matching point, where effective coupling occurs between the forward mode in the main waveguide and the backward mode in the auxiliary waveguide.

[0076] In the phase-matching condition the contra-directional coupling is most efficient, meaning that light from the main waveguide can transfer as effectively as possible to the auxiliary waveguide (or vice versa) at a specific wavelength.

[0077] The refore, the relationship between nl, n2, and navgdictates how the coupling and filtering behavior of the CDC works. Efficient filtering happens when these indices meet the conditions imposed by the Bragg grating, defined by the λ / (2Λ1) curve. By controlling the grating period and refractive indices of the waveguides, the device can be designed to operate at specific wavelengths, making it a key component for Wavelength Division Multiplexing (WDM) systems.

[0078] As a consequence of the refractive index (es) it is possible to move the actual effective bandwidth of a CDC and different scenarios are described by figures 7a and 7b.

[0079] These two figures represent a simulation of the filter band shifting in a contra-directional coupler (CDC), showing how the central wavelength of the coupling region changes as the effective refractive indices (navg) are varied. Each figure shows three different sets of conditions where the index of refraction is altered, causing the corresponding filter band to shift. The figures build on the previous analysis of phase matching and demonstrate how the coupling wavelengths can be tuned.P8027PC00

[0080] In figure 7a, three different average effective refractive indices are shown (dotted lines). Each corresponds to a different grating period (Λ1, Λ2, Λ3), represented by the corresponding dashed lines for λ / (2Λ1), λ / (2Λ2), and λ / (2Λ3), respectively.

[0081] The phase-matching condition occurs where each dotted navgline intersects its corresponding dashed line (λ / (2Λ)). These intersections represent the center wavelength where the coupling is most efficient for each configuration.

[0082] The arrows point to the phase-matching points on the x-axis (wavelength), indicating the specific wavelengths where the coupling occurs for each configuration, respectively below 1.55 μm, around 1.55 μm and above 1.55 μm.

[0083] The shaded regions represent the filter bandwidth for each case. This shows how the filter can target different wavelengths depending on the average refractive index and grating period.

[0084] Figure 7b shows the same general structure as the first one but simulates a band shift. Here, the average effective refractive indices navg) are adjusted to show different positions of the filter band to the left and to the right (towards shorter and longer wavelengths).

[0085] In comparison with figure 7a, the intersections between the dotted navglines and the dashed λ / (2Λ) lines (i.e. the center wavelengths) are further to the opposite directions, indicating that the coupling wavelengths have shifted to lower or higher values, namely closer to 1.545 μm and closer to 1.555 μm.

[0086] The shaded regions also show that the filter bandwidths have shifted in position compared to the first figure, highlighting how the band can be moved across differentP8027PC00wavelengths. It shall be noted from the comparison of figure 7a and 7b that navgl and navg3 are swapped, meaning that in the change from fig. 7a configuration to 7b configuration the PCM state has been controlled and modified in such way to increase navg3 and decrease navgl.

[0087] Advantageously, thanks to the capability of adjusting the refractive index achieved by the invention, it is possible to control the phase-matching condition, thus moving the filter band to different regions of the wavelength spectrum. This aspect reflects the flexibility in designing CDC devices. By choosing appropriate values for navgand A, the device can be tailored to target specific wavelength bands, offering precise control over the optical response and still with a non-volatile and simple solution.

[0088] The following figures ( 8a, 8b, 8c, and 8d) illustrate the relationship between the crystallization ratio in a phase-change material based device and the transmission power at various frequency bands. It is assumed that the PCM is deployed on the totality of the input waveguide, e. g. along a longitudinal path of the rectilinear input waveguide. Other embodiments can provide that PCM, eventually different types of PCMs, is / are deployed upon the secondary or upon both waveguides of a CDC or upon an arbitrary number of guides for devices with more than one auxiliary waveguide, in any combination.

[0089] The left-hand side of each figure shows how the crystallization ratio varies along the longitudinal position of the device, while the remaining parts shows the corresponding through and drop port power spectra in decibels (dB) over a frequency range. The crystallization ratio (expressed in percentile) represents the degree to which the PCM isP8027PC00crystallized along the waveguide longitudinal position (normalized in a 0 to 1 range). The degree of crystallization directly influences the refractive index of the PCM, which in turn affects the optical properties of the waveguide, particularly how wavelengths are filtered or dropped.

[0090] The through (center figure) and drop (right-hand figure) power spectra show how the signal is routed through the device. The drop port extracts specific wavelength channels, while the through port transmits the remaining signal. The crystallization ratio increases stepwise along the longitudinal position of the device, starting from a lower value to a higher value at the end of the longitudinal position.

[0091] On the center and right-hand side of each figure, the power spectrum is depicted, showing different behaviour in terms of attenuation of the signal detectable in the through port and in the drop port.

[0092] The different figures can represent different embodiments of the invention designed in order to have a specific filtering effect. Specifically, selected values are as follows:Figure 8a:Longitudinal Position Crystallization Ratio (%)0.00 to 0.08 00.08 to 0.17 00.17 to 0.25 00.25 to 0.33 00.33 to 0.42 50.42 to 0.50 200.50 to 0.58 200.58 to 0.67 340.67 to 0.75 390.75 to 0.83 580.83 to 0.92 620.92 to 1.00 80P8027PC00Figure 8b:Longitudinal Position Crystallization Ratio (%) 0.00 to 0.08 100.08 to 0.17 190.17 to 0.25 240.25 to 0.33 290.33 to 0.42 480.42 to 0.50 580.50 to 0.58 670.58 to 0.67 720.67 to 0.75 760.75 to 0.83 860.83 to 0.92 950.92 to 1.00 100Figure 8c:Longitudinal Position Crystallization Ratio (%) 0.00 to 0.08 00.08 to 0.17 00.17 to 0.25 00.25 to 0.33 00.33 to 0.42 00.42 to 0.50 00.50 to 0.58 00.58 to 0.67 00.67 to 0.75 00.75 to 0.83 330.83 to 0.92 570.92 to 1.00 96Figure 8d:Longitudinal Position Crystallization Ratio (%) 0.00 to 0.08 00.08 to 0.17 00.17 to 0.25 00.25 to 0.33 00.33 to 0.42 00.42 to 0.50 00.50 to 0.58 00.58 to 0.67 00.67 to 0.75 00.75 to 0.83 81P8027PC000.83 to 0.92 860.92 to 1.00 90

[0093] Considering the average effective index curve of the waveguide system, the phase-match condition is located at the intersection between said curve (neff) and the curve de- 2^fined by the grating pitch / (yl) = —.

[0094] The PCM crystallization introduce a shift in the offset of the index curve (neff) as such the channel wavelength can be defined analytically from the intersection point.

[0095] Assuming a linearized neff profile (which is reasonable in the telecom multi-band application range) an index curve can be fitted asneff(λ) = mneffλ + qneff+ ΔneffPCMwhere:• mneffis the slope of the effective index profile with respect to the wavelength.• qneffis a constant offset of the index curve.• ΔneffPCMis an index shift introduced by PCM crystallization.The channel wavelength (λch) can be found by the equation™mneffλ + qneff+ ΔneffPCM= λch / (2Λ)Then, the channel wavelength that meets the phasematching condition corresponds toλch= 2Λ(qneff+ ΔneffPCM) / (1 — 2Λmneff)P8027PC00

[0096] This equation provides an analytical method for finding the channel wavelength based on material properties, grating characteristics, and index shifts due to PCM.

[0097] The strength of the coupling, or the peak of this transmission window is dependent on the length of the region contributing to the phase-match condition, which cannot be easily generalized for an arbitrary waveguide geometry, but can be fitted on the geometry under analysis.

[0098] Based on the fitted curve and the granularity of the heating structure (which in the previous cased was divided into 12 regions along the longitudinal direction, as to ensure ON / OFF channel toggling) the spectral response can be programmed, by overlapping the different phase-match regions.

[0099] Figure 9 illustrates another embodiment of the invention with regards to the structure of a Contra-Directional Coupler, which can be combined with other variants in scope of the invention. In this case, the waveguides are divided into several sections, each labelled as PCM 1, PCM 2, PCM n, indicating different segments of phase-change materials along the longitudinal direction of linear waveguides belonging to the CDC.

[0100] The crystallization state of these PCM sections can be dynamically controlled through external stimuli (e. g., electrical pulses, heat, optical power), allowing the refractive index of the waveguides to be tuned independently in each section. This modulation alters the coupling between the waveguides, thereby controlling which wavelengths are dropped or added by modulation of the crystallization state of one or more PCMs. This control system enables the tuningP8027PC00of the optical properties of the device without the need for continuous power, as PCMs exhibit non-volatile behavior. Once programmed, they maintain their state, unlike conventional thermal or carrier-based methods.

[0101] The segmentation provides additional flexibility by allowing each section to operate creating custom wavelength responses in each segment. By adjusting the crystallization level in individual sections, users can achieve fine-tuned tailored filtering or routing capability.

[0102] Furthermore, each segment's PCM can be selected to have different physical and chemical properties, allowing the user to apply a distinct refractive index modulation range or response time for each channel. This configuration provides unparalleled flexibility in optical networks, as it enables dynamic, programmable control of each channel without requiring separate physical components.

[0103] The widths of the waveguides (W1 and W2 ), along with the periodic structure of the gratings (Al, A2 ) are shown, indicating that these dimensions are crucial for controlling the phase-matching condition, which defines the wavelength bands that can be filtered or routed.

[0104] Another embodiment is shown in figure 10. The figure illustrates the structure and design parameters of possible configuration of a CDC according to the invention, where the two parallel waveguides are dimensioned as follows:- Main waveguide, featuring a modified Bragg grating with a 180-degree phase shift through which the input signal enters and can exit through the through port, with a minimum transversal size of 600nm (Wl ) and a width variation of 50 nm (AW1 );- Auxiliary waveguide, featuring a modified Bragg gratingP8027PC00with a 180-degree phase shift symmetrical to the main waveguide responsible for coupling specific wavelengths from the main waveguide and routing them to the drop port or accepting signals through the add port, with a minimum transversal size of 400nm (W2 ) and a width variation of 30 nm (AW1 );- multiple sections of Bragg gratings along both waveguides, with different periodicities (Al, A2,... An) in different sections (Tl, T2,... Tm) of the coupler;- separation (G) between the two waveguides of 200 nm.

[0105] In one embodiment, the invention includes a complete optical system designed to manage, control, and modulate light signals within photonic circuits, e. g. Wavelength Division Multiplexing (WDM) applications. This system leverages the integrated optical coupler to control light propagation, using dedicated light sources and detection devices coupled, e. g. directly or by intermediate optical fibers, to specific ports on the coupler.

[0106] The optical system comprises:- At least a first light source, coupled to the input port of the main waveguide of the optical coupler e. g. directly or by intermediate optical fibers. This light source (s) provides optical signals at one or more predetermined wavelengths, which propagate through the main waveguide to the through port. This source (s) can be a laser or other narrowband emitter, optimized to emit within a frequency range suitable for WDM applications. More than one of said first light sources, coupled to the input port of the main waveguide of the optical coupler, can be provided.- Optionally, a second light source, coupled to the addP8027PC00port of the auxiliary waveguide e. g. directly or by intermediate optical fibers, arranged to provide additional optical signals to the optical coupler. This source introduces a new wavelength or channel into the auxiliary waveguide, which can be selectively coupled to the main waveguide for routing through the system as required by the application. More than one of said second light sources, coupled to the add port of the auxiliary waveguide of the optical coupler, can be provided.- Detection devices coupled e. g. directly or by intermediate optical fibers to the through and drop ports of the waveguides to monitor and measure the optical signals. The first detection device at the through port receives the propagated signal from the main waveguide, while the second detection device at the drop port captures the extracted or filtered wavelengths from the auxiliary waveguide.

[0107] The second light source is optional because it is involved when the device is used to add new signal (s) into the main waveguide but, since the device can also be used to extract signals guided through the main waveguide, the second light sources (s) are not essential to make the device working.

[0108] The system also includes a control element to manage the modulation state of the phase-change material (PCM) layers in the optical coupler. This control element adjusts the crystallization state of PCM to selectively modify the refractive index in specific sections of the waveguides, enabling programmable control over which wavelengths are added, dropped, or transmitted through the coupler. The control element can provide thermal or optical energy to alter theP8027PC00PCM state, ensuring precise modulation without continuous power.

[0109] In operation, the first light source introduces a signal at a predetermined wavelength range into the main waveguide, where it propagates toward the through port. When phase-matching conditions are met, this signal can couple with the auxiliary waveguide at specified wavelengths, allowing the signal to be either added to or dropped from the main channel.

[0110] The second light source, coupled to the add port of the auxiliary waveguide, enables selective wavelength addition. This functionality is particularly advantageous for WDM systems, as it allows for flexible channel configuration based on the needs of the network. The detection devices at the through and drop ports monitor signal strength, wavelength accuracy, and overall system performance, ensuring reliable operation of the optical routing.

[0111] The control element within the optical system provides the necessary modulation of PCM layers within the coupler. By adjusting the refractive index of specific waveguide sections through PCM crystallization, the control element ensures that wavelength channel can be managed as desired. Additionally, in the case of partitioned waveguide the control element can selectively adjust one segment's PCM to enable, for instance, a drop function while maintaining through-transmission in another segment, providing high flexibility for multi-channel WDM applications.

[0112] The production of the optical system involves integrating the light sources, detection devices, and control mechanisms with the coupler. Light sources are coupled at the input and add ports, preferably and usually by means ofP8027PC00intermediate optical fibers acting connected to the light source (s) and the device, and detectors are coupled with the through and drop ports preferably and usually by means of intermediate optical fibers acting connected to the light source (s) and the device, to ensure reliable monitoring and performance. The control element, configured to apply electrical or optical energy, is positioned for efficient interaction with PCM layers, enabling consistent modulation and effective optical management within the system.

[0113] The invention includes a method for switching and mixing Wavelength Division Multiplexing (WDM) optical signals within an optical system configured with an optical coupler, PCM-modulated waveguides, light sources, detection devices, interconnecting fibers and a control element. This method leverages PCM modulation to dynamically control signal routing, allowing for flexible wavelength addition, filtering, and removal across WDM channels. The steps of the method are as follows:Step 1: Providing the Optical System

[0114] The method begins by providing an optical system that includes an optical coupler with main and auxiliary waveguides, light sources coupled, directly or via transport fibers, to the input and add ports, detection devices at the through and drop ports, connected directly or via transport fibers, and a control element configured to modulate PCM layers on the waveguides. This setup ensures the system is capable of performing switching and mixing operations across multiple WDM channels.Step 2: Applying Energy for PCM Modulation

[0115] To control the optical paths within the coupler, a specific amount of energy, either electrical or optical, isP8027PC00applied to the PCM sections along the waveguides to adjust their crystallization state, thereby altering the refractive index of each section. This modulation allows selective phase matching, enabling certain wavelengths to couple or decouple between the main and auxiliary waveguides.

[0116] The energy applied to each PCM segment may vary in intensity or duration, allowing precise tuning of the refractive index in response to WDM signal requirements. Electrical modulation can be achieved using resistive heating effect of a dedicated heating device and / or of the PCMs itself, while optical modulation can use laser pulses tuned to the PCM' s absorption characteristics.Step 3: Providing Optical Input Signals

[0117] After PCM modulation is applied, a first optical signal from the primary light source is introduced at the input port of the main waveguide. This signal propagates through the main waveguide and can be selectively filtered or transmitted based on the refractive index modulation achieved by the PCM layers.

[0118] Simultaneously, a second optical signal can be introduced at the add port of the auxiliary waveguide. This allows for adding new wavelengths or channels to the main waveguide, depending on the coupling conditions created by the PCM modulation.Step 4: Detecting Output Signals at Through and Drop Ports

[0119] The method concludes by detecting the modulated optical signals at the through and drop ports. The detection device at the through port, connected to such port directly or via transport fibers, receives the remaining signals propagating along the main waveguide, while the detectionP8027PC00device at the drop port captures selectively extracted wavelengths based on phase-matching conditions.

[0120] These detection devices validate the switching and mixing outcomes, ensuring the intended wavelengths have been successfully added, dropped, or transmitted. The monitored signals can also be used to assess optical power levels and signal integrity, providing feedback for possible re-adjust-ment of PCM modulation if necessary.

[0121] A further embodiment operates when the waveguide is segmented into multiple longitudinal sections with each segment incorporating an independently controlled PCM layer. This configuration allows for independent refractive index modulation in each segment, enabling precise filtering, addition, or drop functionality for specific wavelength channels within each segment. The enhanced steps are as follows: Step 1: Segmenting the Waveguide and Modulating Each PCM Section

[0122] In this enhanced method, the main or auxiliary waveguide is divided into multiple segments, each of which contains a layer of PCM that can be independently modulated. This segmentation allows for individual control over each segment's refractive index, achieved by applying targeted energy pulses (electrical or optical) to each PCM layer.

[0123] The control element directs localized energy to each segment based on the specific wavelength requirements of the WDM system. By adjusting the crystallization ratio of the PCM in each segment, the refractive index can be modulated to tailor the filtering or switching response for each channel independently.Step 2: Customizing Wavelength Filtering and AdditionP8027PC00

[0124] Each waveguide segment operates as an independent optical filter or switch, creating custom wavelength responses across the waveguide. For example, one segment can be tuned to drop a particular wavelength which was allowed to pass by a preceding segment, or another segment can add a different wavelength to the main channel. This flexibility supports multi-channel functionality, allowing different wavelengths to be selectively managed across the coupler based on the WDM application needs.Step 3: Monitoring Multi-Channel Outputs

[0125] In more advanced applications, the control element may be configured to adaptively modulate each PCM segment based on real-time feedback from the detection devices. This allows for dynamic reconfiguration of wavelength channels and bandwidth in response to network demands, offering on-the-fly adjustments that optimize the system's overall performance.

[0126] It should be clearly understood that the various arrangements and processes broadly described and illustrated with respect to the figures, and / or one or more individual components or elements of such arrangements and / or one or more process operations associated of such processes, can be employed independently from or together with one or more other components, elements and / or process operations described and illustrated herein. Accordingly, while various arrangements and processes are broadly contemplated, described and illustrated herein, it should be understood that they are provided merely in illustrative and non-restrictive fashion, and furthermore can be regarded as but mere examples of possible working environments in which one or more arrangements or processes may function or operate.P8027PC00

[0127] The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.

[0128] Other variations are within the spirit of the present disclosure. Thus, while the disclosed techniques are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention, as defined in the appended claims.

[0129] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising, " "having, " "including" and "containing" are to be construed as open-ended terms (i. e., meaning "including, but not limited to, ") unless otherwise noted. The term "connected, " when unmodified and referring to physical connections, is to be construed as partly or wholly contained within, attached to or joined together, even if there is something intervening. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each sepa-P8027PC00rate value falling within the range, unless otherwise indicated herein and each separate value is incorporated into the specification as if it were individually recited herein. The use of the term "set" (e. g., "a set of items") or "subset" unless otherwise noted or contradicted by context, is to be construed as a nonempty collection comprising one or more members. Further, unless otherwise noted or contradicted by context, the term "subset" of a corresponding set does not necessarily denote a proper subset of the corresponding set, but the subset and the corresponding set may be equal.

[0130] Preferred embodiments of this disclosure are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate and the inventors intend for embodiments of the present disclosure to be practiced otherwise than as specifically described herein. Accordingly, the scope of the present disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the scope of the present disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

P8027PC00CLAIMS1. An optical coupler for controlling the propagation and managing light beams within photonic circuits, which comprises:A main waveguide, configured with a section of the main waveguide between an input port and a through port through which an optical beam, composed of one or more waves having predetermined wavelengths within a known wavelength range, can propagate in a direction having at least one longitudinal component coinciding with the longitudinal direction of the waveguide;An auxiliary or secondary waveguide, configured with a section of the auxiliary waveguide between an add port and a drop port through which an optical beam, composed of one or more waves with predetermined wavelengths within a known wavelength range, can propagate in a direction having at least one longitudinal component coinciding with the longitudinal direction of the waveguide;wherein said waveguides are arranged in a predetermined spatial position relative to each other;wherein the main waveguide is optically coupled with the auxiliary waveguide in one or more portions of said section of the main waveguide and said section of the auxiliary waveguide;wherein the optical coupler comprises one or more elements for modulating the refractive index within at least one portion of at least one of said waveguides thereby modifying one or more propagation characteristics of said at least one waveguide depending on the non-volatile physical state assumed by said modulationP8027PC00elements;wherein the physical state of said one or more elements for modulating the refractive index is modifiable, by means of control means, between a first modulation state and at least a second modulation state, allowing for persistent refractive index control without continuous power.

2. The optical coupler according to claim 1, wherein said one or more elements for modulating the refractive index comprise phase change materials (PCM) having known and predefined chemical-physical composition and characteristics and arranged so as to controllably alter the refractive index of the waveguide in the portion of the waveguide that incorporates such materials, wherein there are provided control means configured for adjusting the level of crystallization of at least part of said phase change materials by providing electrical and / or optical energy to said phase change materials.

3. The optical coupler according to claim 1 or 2, characterized by being configured to operate as a counterdirectional optical coupler, wherein:The two waveguides run substantially parallel for a predetermined length of said waveguides;The main waveguide and / or the auxiliary waveguide are provided with Bragg gratings in an optical coupling zone along a longitudinal portion of the respective waveguide sections;Said main waveguide is optically coupled with said auxiliary waveguide in one or more portions of said section of the main waveguide and said section of the auxiliaryP8027PC00waveguide in the portion or portions where the Bragg grating is provided.

4. The optical coupler according to one or more of the previous claims 2 or 3, wherein at least one of the said waveguides is segmented into two or more longitudinal sections provided with one or more phase-change materials (PCM) having known and predefined chemicalphysical composition and characteristics and configured to allow individual refractive index modulation by independently adjusting the crystallization ratio of the PCM in each section5. The optical coupler according to claim 5, wherein the phase-change material (PCM) within two or more longitudinal sections of the waveguide are selected to have a distinct chemical composition or configuration, such that each section has unique refractive index modulation characteristics.

6. The optical coupler according to one or more of the previous claims 2 to 5, wherein said phase change materials are selected from the group comprising:Antimony Selenide (Sb2Se3);Antimony Sulfide (Sb2S3);Germanium-Antimony-Tellurium (GST);Germanium-Antimony-Stibium-Tellurium (GSST), Wherein said materials are used in said optical coupler individually or in any combination or sub-combination.

7. The optical coupler according to one or more of the previous claims 2 to 6, characterized by comprising: A base layer comprising an insulation layer, preferablyP8027PC00silicon dioxide-based;Said main waveguide, preferably based on silicon and / or its compounds and located on top of said insulation layer;Said secondary waveguide, preferably based on silicon and / or its compounds, deposited on said insulation layer and located at least partially adjacent to said main waveguide at a known distance;An enrichment layer based on phase change materials (PCM) deposited on at least one portion of at least one of said waveguides.

8. The optical coupler according to one or more of the previous claims, wherein the main waveguide is provided with a main Bragg grating and the secondary waveguide is provided with a secondary Bragg grating.

9. The optical coupler according to claim 8, wherein the main Bragg grating has one or more geometric dimensions different from the corresponding geometric dimensions of the secondary Bragg grating.

10. The optical coupler according to claim 9 configured to operate in optical C band, wherein:The width (W1) of the main waveguide is in the range between 400 nm and 600 nm, preferably 570 nm;The width (W2) of the secondary waveguide is in the range between 400 nm and 600 nm, preferably 430 nm; The variation (ΔW1) in the width of the main waveguide is in the range between 20 and 150 nm, preferably 60 nm;P8027PC00The variation (ΔW2) in the width of the secondary waveguide is in the range between 20 and 150 nm, preferably 100 nm;The height (h) of the waveguides is in the range between 200 and 250 nm, preferably 220 nm;The periodicity (Λ1) of the main Bragg grating is in the range between 250 and 350 nm, preferably 300 nm; The periodicity (Λ2) of the secondary Bragg grating is in the range between 250 and 350 nm, preferably 300 nm; The height (hpcm) of the phase change material layer is in the range between 50 and 100 nm, preferably 60 nm; The distance or gap (G) between the waveguides in the parallel section is in the range between 150 and 300 nm, preferably 200 nm;Preferably, the width and length of said phase-change material layer cover the entire width of one or both of said waveguides over a predetermined length of the same.

11. The optical coupler according to one or more of the previous claims, comprising two or more auxiliary waveguides optically coupled with the main waveguide.

12. An optical system comprising:An optical coupler according to one or more of the previous claims;A first light source coupled to the input port of the main waveguide of the optical coupler and arranged to provide optical signals to the optical coupler;A first detection device coupled to the through port of the main waveguide of the optical coupler;P8027PC00A second light source coupled to the add port of the auxiliary waveguide of the optical coupler and arranged to provide optical signals to the optical coupler; A second detection device coupled to the drop port of the auxiliary waveguide of the optical coupler;A control element for controlling the modulation state of said one or more elements for modulating the refractive index.

13. A control method for switching and / or mixing WDM optical signals comprising the steps of:Providing an optical system according to the previous claim;Providing a known and predefined amount of energy, preferably in the form of electrical or optical energy suitable to be converted in thermal energy respectively by photothermal effect or resistive heating, to said one or more elements for modulating the refractive index by adjusting the crystallization ratio of the PCM as a consequence of the thermal energy provided to said one or more elements and consequently alter the effective propagation index of the main and / or auxiliary optical waveguide of the optical coupler;Providing a first and second optical input signal respectively to the main and auxiliary waveguide of the optical coupler;Detecting a first and second optical input signal respectively at the through port and drop port, respectively, of the main and auxiliary waveguide of the optical coupler.P8027PC0014. The method according to claim 13, wherein the waveguide is segmented as defined in claim 4 or 5, further comprising the step of independently adjusting the crystallization ratio of the phase-change material (PCM) in each longitudinal section of the segmented waveguide, to achieve specific refractive index modulation in each section, thereby enabling customized wavelength filtering, addition, or drop functionalities across different sections of the waveguide.

15. A method for producing an optical coupler by positioning at least two waveguides on a silicon oxide layer and depositing above or beside said waveguides and in contact with said waveguides at least one phase-change material layer, the layer having known and predefined dimensions, and the PCM material having known and predefined chemical-physical composition and characteristics.

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