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    • 专利摘要:
    Integrated filter based on coupled resonators comprising an input waveguide (1), an output waveguide (7), a plurality of Bragg reflectors (5), a plurality of Fabry-Perot cavities (6), two modal converters (2) and a plurality of adapters (4). The two modal converters (2) vary adiabatically between the first width (w1) and the second width (w2), while the plurality of adapters (4) connect the plurality of Bragg reflectors (5) with the plurality of Fabry-Perot cavities (6) and with the two modal converters (2), by means of alternating sections with at least two different effective indices. The second width (w2) of the plurality of Bragg reflectors (5) and their associated forbidden band facilitate the optimization of the device with respect to other solutions based on narrow guides and/or weak networks, improving the built-in filter performance. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2833122A1
    申请号:ES201931104
    申请日:2019-12-11
    公开日:2021-06-14
    发明作者:Fernandez Inigo Molina；Perez Juan Gonzalo Wanguemert；Monux Alejandro Ortega；Robert Halir；Martin Daniel Pereira
    申请人:Universidad de Malaga；
    IPC主号:
    专利说明:

    [0002] Integrated optical filter based on resonators coupled by strong Bragg grating with high lateral confinement
    [0004] Object of the invention
    [0006] The present invention relates to the field of integrated optics, and more specifically to a filter based on resonators coupled through Bragg networks implemented on waveguides. The use of strong Bragg reflectors with high lateral confinement simplifies the design process, increases the free spectral range and reduces radiation losses from the device.
    [0008] Background of the invention
    [0010] Coupled resonator circuits are well known in the microwave field, where they have been applied to various applications such as filters, diplexers, etc. Likewise, design techniques have been developed to adapt the response of said circuits to a certain objective response. Coupled Resonator Optical Waveguides (CROW) optical filters for coupled resonators respond to the same principles of operation and design, but applied to optical frequencies.
    [0012] CROW filters comprise a number N of optical resonators coupled to each other. Each of the resonators is capable of storing energy in its own oscillation mode, at a resonant frequency (f , ). The resonators interconnect with each other, in such a way that they exchange their energy with each other. The rate at which a resonator with a first index (i) and a resonator with a second index (j) exchange energy with each other is measured by the coupling factor (k y ). The resonators are also coupled with the input and / or output guides, thus allowing energy exchange with the rest of the optical circuit. According to what is known in the state of the art, to reduce the losses introduced by CROW filters, it is necessary to have resonators whose internal quality factor (Q) is much higher than the inverse of the fractional bandwidth of the filter, that is that is, the ratio between the central frequency (f c ) and the bandwidth (BW).
    [0013] Note that the techniques for calculating the resonance frequencies (f,) and the coupling factors (kij) to obtain a certain frequency response are well known in the state of the art, and can be applied directly to the present invention, Therefore, they are not detailed in this document, focusing instead on the differential morphological elements of the components of said CROW filters.
    [0015] Typically, coupled resonator optical filters are made by two alternatives: either by ring resonators or by resonators based on Bragg reflectors (also known as "Bragg filters" or "Bragg lattices"). Ring resonators are frequently used in integrated optics but present some problems such as their low tolerance to manufacturing errors and their periodic frequency response, with a relatively small Free Spectral Range (FSR), which limits its applicability. US 7,356,221 B2 presents an example of a CROW filter based on ring waveguides, thermoptically controlled by a set of heaters arranged on the chip.
    [0017] On the other hand, resonators based on Bragg reflectors, in their simplest version, are resonators in which light is trapped by two mirrors formed by periodic structures operating in the Bragg zone. If the periodic structure is implemented by means of a conventional homogeneous guide to which a periodic disturbance is introduced, it can be shown that the range of wavelengths that satisfy the Bragg condition depends on the intensity of the interaction of the electromagnetic field with the disturbance. periodic. This set of wavelengths that satisfy the Bragg condition is called the forbidden band (or more commonly, by its English name “bandgap”). The weaker the interaction of the electromagnetic field with the periodic disturbance, the smaller the width of the forbidden band and the greater the number of periods that the wave penetrates the periodic guide before being fully reflected. On the contrary, if strong disturbances are introduced in the guide, the bandwidth of the forbidden band increases considerably and the penetration depth of the light in the structure is only a few periods. In Bragg filters, it is common to work with the lowest order of resonance, which makes its FSR much higher than that of resonant rings. Another advantage of Bragg filters is their great flexibility to achieve apodizing, that is, for the spectral shaping of a target response.
    [0018] In silicon integrated optics, the Bragg filters known in the state of the art can be divided according to the strength of the Bragg coupling into weak and strong. Although the specific values considered weak or strong may change depending on the particular literature considered, in order for said classification to be more objective, in this text, weak Bragg networks are considered those in which the relationship between the width of the forbidden band and the central wavelength is below 0.1%, intermediate strength Bragg networks those in which the ratio of the band gap width to the central wavelength is between 0.1% and 2.5% and strong networks those in which the ratio between the width of the forbidden band and the central wavelength exceeds 2.5%.
    [0020] Weak Bragg filters have been considered in integrated optics as single section filters, although some proposals based on coupled resonators are known. Strong and intermediate force filters have been proposed in some configurations with coupling between multiple resonators, although the performance obtained still allows considerable room for maneuver. For example, US 5,600,740 A presents a single-stage weak Bragg filter, mentioning waveguides as a possible application, although the proposed geometries are difficult to manufacture.
    [0022] In another example, EP 0,547,859 A1 presents a structure with multiple phases of coupled Bragg reflectors that form a slow light structure. Each Bragg reflector may comprise matching stages to connect said reflector to the input and output waveguides, keeping the width mostly constant in all of these elements (input waveguide, matching stages, Bragg reflectors and waveguide of output), except for small variations to tune the response of each resonator in any of the particular options proposed.
    [0024] In general, filters based on a single weak Bragg network use networks of high length to achieve subnanometric bandwidths, among other functionalities. However, the main problem with weak Bragg networks in waveguides, and particularly in silicon-based technologies, is that it is difficult to control the low level of coupling because the high index contrast of the platform makes it difficult to implement. practice. For example, if a Bragg grating is made on a single-mode silicon guide using corrugations in the width of the guide, to achieve a bandwidth of 0.8nm, a corrugation in the width of the the guides of only 10nm, well below the precision achievable by the current manufacturing techniques. Alternatively, it has been proposed to implement the periodic variation of the refractive index of the Bragg grating by means of double-engraved 'rib' guides or by means of sub-wavelength technologies, improving the manufacturing possibilities, but maintaining limitations in terms of total size of the device, crosstalk, free spectral range, or energy required to technically tune the device.
    [0026] Regarding filters based on multiple weak Bragg networks, some solutions implemented in silicon on insulator are known. Although these solutions have low radiation losses and therefore achieve low insertion losses and high rejection; However, they maintain the problem of making small lateral corrugations on the waveguides, as well as the need to resort to a very high number of periods (which increases the size of the integrated device), and bandwidth limitations, preventing covering an entire communications band with the device.
    [0028] On the other hand, strong Bragg reflectors in integrated optics make it possible to implement associated bandgaps of great width, with the consequent increase in FSR. They are also remarkably compact, as light only needs a few periods to be fully reflected. Furthermore, when used in combination with tuned defects within the reflectors' band gap, they allow the implementation of small Fabry-Perot type resonant cavities. However, this alternative has as its main drawback the difficulty of obtaining a good adaptation between uniform guides and guides with Bragg reflectors, which has significantly limited its practical application up to now. The existence of mismatches between smooth guides and guides with Bragg reflectors causes radiation losses that lower the intrinsic quality factor of the resonators, and make it impossible to design high-quality filters. In the latter case, adaptation is critical in order to implement sub-nanometric transmission windows inside the bandgap.
    [0030] To solve this limitation, some strategies are known in the state of the art to adapt the mode of a uniform guide to the mode of a guide with a Bragg reflector, in such a way as to reduce radiation losses and increase the internal quality factor. resonator. For example, by periodic structures One-dimensional (with rectangular and circular holes) on single-mode silicon waveguides, it is possible to obtain high coupling factors in excess of 5x105, using as transitions between the homogeneous guides and the Bragg reflectors a succession of several dozen quasiperiodic elements. However, the structures required by these solutions are difficult to manufacture, typically with holes of dimensions at the limit or below the minimum range sizes (MFS, English 'Minimum Feature Size') of conventional techniques of manufacture of guides of wave. In addition, the complexity of the response of these structures and their sensitivity to manufacturing errors lead to considerable difficulties when making optimized designs in which multiple stages of Bragg reflectors are combined.
    [0032] Ultimately, there is therefore still a need in the state of the art for filters integrated in waveguides, compact and easy to manufacture, which provide a high quality spectral response in a wide free spectral range.
    [0034] Description of the invention
    [0036] The present invention solves the problems described above by means of an integrated filter based on coupled resonators, in which the high width and associated bandgap of the Bragg reflectors reduces the number of periods required to implement the reflectors, facilitates the optimization of the device with respect to other solutions based on narrow guides and / or weak networks, and improves the performance of the integrated filter resulting in a high free spectral range.
    [0038] Note that throughout this document, the terms "filter", "filter" and other derivatives do not refer exclusively to their meaning of "selecting a spectral region", but to their broad meanings that include said function, but also any other signal processing that can be carried out by this type of device, such as phase equalization or the implementation of delays, among others.
    [0040] The integrated filter comprises the following elements:
    [0042] - An input waveguide and an output waveguide, both singlemode and with a typically equal first width for both. The filter is adapted to receive by the input waveguide an optical signal to be filtered with a central wavelength and a polarization, preferably corresponding to an electrical transverse mode. The input and output waveguides, as well as the rest of the filter waveguide elements, are preferably implemented on a platform selected from silicon on insulator and silicon nitride.
    [0044] - A plurality of Bragg reflectors with a second width, greater than the first width, and whose section presents cross-sections of a rectangular shape without discontinuities. That is, they are Bragg reflectors with a simplified lateral structure, which can include variations in width and / or effective index and / or engraving depth along each reflector, but which do not include structures associated with two-dimensional photonic crystals. . According to both preferred options, the plurality of Bragg reflectors can be implemented by means of homogeneous wave guides or by sub-wavelength structures. That is, when Bragg reflectors are implemented by sub-wavelength structures, said reflectors comprise an alternating arrangement of a plurality of sections of a core material and a plurality of sections of covering material, with a period less than the length wavelength of light guided by said modified birefringence waveguide.
    [0046] Preferably, the second width of the plurality of Bragg reflectors is large enough to exhibit a very high level of lateral confinement. That is, a waveguide with said second width (with the same height and materials used in the plurality of Bragg reflectors, and at the same central wavelength of the optical signal to be filtered), gives rise to an effective index that is difference less than a first threshold from the effective index generated by an equivalent waveguide with an infinite width. That is, the Bragg reflectors act mostly as one-dimensional networks, admitting variations with respect to this ideal behavior defined by said first threshold. Preferably, said first threshold is less than 1%, although it can be lowered, more preferably, below 0.25% to get even closer to said one-dimensional behavior.
    [0048] For example, for the case of C-band filters implemented in silicon on insulator, these conditions on the second width of the plurality of Bragg reflectors typically translate into values of said second width greater than 3.5 microns (threshold below 0.25%), although the particular limits of said values depend on the central wavelength, the height of the layer of material core, cover material, etc. In the same way, for the case of C-band filters implemented in silicon nitride, the conditions on the second width of the plurality of Bragg reflectors typically translate into values of said second width greater than 6 microns (threshold below the 0.25%), although the particular limits of these values depend on the rest of the design factors mentioned.
    [0050] Preferably, the plurality of Bragg reflectors have an associated forbidden band (better known as "bandgap") that represents a greater proportion of a first threshold with respect to the central wavelength of the filter. That is, the ratio between the width of the forbidden band and the central wavelength is greater than said first threshold. Preferably, said first threshold is greater than 1.5%, and more preferably, greater than 2.5%, thus typically covering an entire optical communications band.
    [0052] Note that the associated forbidden band is uniquely determined by the difference in the index periodically implemented on the Bragg reflector, that is, by the difference between the maximum and the minimum effective index observable by the light propagated by said Bragg reflector. That is, given certain core, cover and substrate materials, the ratio between the width of the forbidden band and the central wavelength of a Bragg reflector must be understood as an imposition on the morphology of the reflector, whose particular application depends on the particular way in which said reflector is tuned.
    [0054] That is, each Bragg reflector has a first height and a first length in a plurality of segments that have said maximum effective index and a second height and a second length in the segments that have said minimum effective index, said first height, first length being , second height and second length adapted to generate the aforementioned proportions between the forbidden band and the central wavelength, in accordance with what is generally known in the state of the art for the design of Bragg networks.
    [0055] Depending on the particular embodiment, the plurality of Bragg reflectors can be made by full etching (better known by its English nomenclature "full etch") or by partial or multi-depth etching. That is, each Bragg reflector comprises a plurality of waveguide segments of the second width arranged periodically, so that in the space between two consecutive segments, the waveguide can be recorded up to the substrate, thereby eliminating all the core material in that space; or it may be partially etched, conserving core material with a height less than that of the plurality of segments mentioned. According to both preferred options, the plurality of Bragg reflectors can be implemented by means of homogeneous waveguides or by sub-wavelength structures. That is, when Bragg reflectors are implemented by sub-wavelength structures, said reflectors comprise an alternate arrangement of a plurality of sections of a core material and a plurality of sections of covering material, with a period less than that of wavelength of the light guided by said waveguide.
    [0057] When implemented using sub-wavelength structures, Bragg reflectors have the additional advantage that it is possible to adjust the bandgap width to values close to the first threshold by means of an easy-to-manufacture geometry that only uses one engraving step. .
    [0059] Likewise, four preferred ways of tuning the plurality of Bragg reflectors are contemplated: by varying the engraving depth of each Bragg reflector (that is, the same reflector maintains a constant engraving depth, but which can change between reflectors), by means of variations in the number of periods of each Bragg reflector, by means of variations of the period of each Bragg reflector (that is, the same reflector maintains a constant period, but which can change between reflectors), or by the variation of the effective index synthesized by the sub-wavelength structure.
    [0061] The particular case in which period variations are used to tune the plurality of Bragg reflectors (that is, Bragg reflectors with different center frequencies) allows adjusting the coupling force between Fabry-Perot cavities by de-tuning the Bragg reflectors. This option is preferred for the case of using full recording up to the substrate, since in that case the Bragg reflectors are so strong that a difference of one period can generate significant discretization errors.
    [0063] - A plurality of Fabry-Perot cavities (also called "defects") arranged alternately between the plurality of Bragg reflectors, preferably tuned by a plurality of controllers that tune their effective index, for example by thermal control. According to both preferred options, the plurality of Fabry-Perot cavities can be implemented by means of homogeneous wave guides or by sub-wavelength structures. That is, when Fabry-Perot cavities are implemented by sub-wavelength structures, said cavities comprise an alternating arrangement of a plurality of sections of a core material and a plurality of sections of covering material, with a period less than the wavelength of the light guided by said waveguide.
    [0065] - Two modal converters whose width varies between the first width and the second width. That is, a first modal converter connects (directly or through additional elements) the input waveguide with a first end of the set formed by the plurality of Bragg reflectors, the plurality of Fabry-Perot cavities and a plurality of adapters ( described below), while a second modal converter connects (directly or through additional elements) the output waveguide with a second end of the assembly (opposite the first assembly). The adiabatic transition of the two converters allows that, although the second width of the plurality of Bragg reflectors supports more than one optical mode, the modes of order one or greater are not excited, and therefore, on said plurality Bragg reflectors propagate only the zero-order electrical transverse mode. The particular geometry of the modal converters may vary from one embodiment to another, in accordance with what is generally known in the state of the art, their simplest embodiment being two trapezoids, symmetrical with each other with respect to a plane perpendicular to the direction of optical signal propagation. Modal converters can be implemented by homogeneous waveguides or by sub-wavelength structures. In this case, the use of sub-wavelength structures allows a greater degree of design freedom and a reduction in the effects of manufacturing errors.
    [0067] - The aforementioned plurality of adapters, which connect the plurality of Bragg reflectors with the plurality of Fabry-Perot cavities and with the two modal converters. That is, there is one adapter between each Fabry-Perot cavity and the two adjacent Bragg reflectors, another adapter between the first modal converter and the first Bragg reflector, and a last adapter between the last Bragg reflector and the second modal converter. The use of said plurality of adapters makes it possible to use very strong Bragg reflectors without the radiation losses at the interfaces with said Bragg resonators significantly reducing the intrinsic quality factor (Q) of the circuit.
    [0069] Each adapter of the plurality of adapters comprises alternating sections with at least two different effective indices, typically implemented by the same recording technique (complete or partial) of the plurality of Bragg reflectors. That is, the first of the two different effective indices corresponds to waveguide segments with the same width and height as the waveguide segments of the plurality of Bragg reflectors, while the second of the two different effective indices corresponds to regions that may well be etched down to the substrate, whereby all core material is removed from that space; or they may be partially engraved, conserving core material with a height less than that of the plurality of segments mentioned.
    [0071] Depending on the preferred options, the adapters of the plurality of adapters may well all be the same (except for their symmetry with respect to the Fabry-Perot cavities), which facilitates the design of the device; well have a geometry adapted to each Bragg reflector.
    [0073] Alternatively, in a third preferred option, the segments that form the modal adapters can be implemented by sub-wavelength structures. In this case, the use of sub-wavelength structures allows to adjust the effective index of the adapter segments which allows a greater degree of design freedom and a reduction in the effects of manufacturing errors.
    [0075] Note that the particular design strategy of the plurality of adapters is independent of the present invention, and any design technique existing in the state of the art can be used. For example, an automated optimizer can be used, or strategies known in the state of the art can be used to minimize radiation losses. Some of these loss minimization techniques include optimizing the adapter parameters to make a smooth transition from the guided mode to the Bloch mode of the Bragg resonator, applying radiation loss recycling mechanisms, or applying gap modulations, for example of the parabolic type. .
    [0077] The integrated filter described makes it possible to obtain in a very simple way and adapted to the most common photonic platforms, Bragg reflectors strong enough to have a large free spectral range, being nevertheless easy to adapt with the adapters described to minimize radiation losses.
    [0079] For example, if a filter is implemented as described with partial etching and central wavelength in C band, using an etching depth of 70 nm, standard on multiple platforms, a free spectral range is obtained that completely covers said C band. , being able to obtain intrinsic Q factors above 105 with only two adapters. Furthermore, the coupling values required to make a five-section Butterworth filter with a 25 GHz bandwidth can be easily synthesized by changing the number of network periods between about ten and fifty without noticeable discretization errors occurring. In addition, the transversal geometry is very simple, imposing minimum feature sizes (MFS, from the English ‘minimum feature size’) of the order of 100nm, which allows its standard manufacture.
    [0081] Compared with the solutions existing in the state of the art based on weak Bragg networks (ratio between the band gap width and the central wavelength typically below 0.1%), the proposed filter needs a much lower number of periods in each Bragg reflector, significantly reducing the total size of the filter. Additionally, this reduction in size means a reduction in the energy required to implement the thermal tuning of the device, as well as greater efficiency and a reduction in problems associated with crosstalk. Likewise, the strength of the Bragg reflectors of the proposed filter provides a frequency response curve with a much higher free spectral range, which improves the quality of the filter as it avoids interference with channels of the same band.
    [0083] Compared with existing solutions in the state of the art based on intermediate or strong Bragg networks (that is, with a higher ratio between the bandgap width and the central wavelength), the use of a greater width in the plurality Bragg reflectors greatly facilitates filter design and optimization, reducing the photonic computation problem to a two-dimensional computation. Likewise, it avoids the need to resort to more complex structures such as photonic crystals with circular or rectangular holes, which are notably more difficult to manufacture and design, as well as being significantly more sensitive to manufacturing errors.
    [0085] These and other advantages of the invention will be apparent in light of the detailed description thereof.
    [0087] Description of the figures
    [0089] In order to aid in a better understanding of the characteristics of the invention according to a preferred example of practical embodiment thereof, and to complement this description, the following figures are attached as an integral part thereof, the character of which is illustrative and non-limiting:
    [0091] Figure 1 shows, in a top view, a filter according to a preferred embodiment of the invention, schematically illustrating the main elements that compose it.
    [0093] Figure 2 presents a perspective view of the same preferred embodiment of the invention.
    [0095] Figure 3 is a schematic longitudinal section showing a case of partial recording, according to a preferred embodiment thereof.
    [0096] Figure 4 exemplifies the calculation of the minimum width of the waveguides that make up the Bragg grids of the filter of the invention, according to a preferred embodiment thereof.
    [0098] Figure 5 presents a first example of performance obtainable with different preferred embodiments of the invention for a 50 GHz bandwidth and different number of resonators.
    [0100] Figure 6 presents a second example of performance obtainable with different preferred embodiments of the invention for a 25 GHz bandwidth and different number of resonators.
    [0102] Figure 7 presents a third example of the performance obtainable with a preferred embodiment of the invention, shown in a larger spectral range.
    [0104] Preferred embodiment of the invention
    [0106] In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an exclusive sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined can include more elements, stages, etc.
    [0108] Likewise, in the context of the present invention, the term "approximately" as well as the terms of its family (such as "approximate", etc.) should be understood as indicators of values very close to those that accompany said terms. That is, deviations within reasonable limits with respect to an exact value must be accepted, since a person skilled in the art will understand that such deviations are unavoidable due to measurement inaccuracies, parameter variability, etc. The same applies to the terms "about", "close to" or "substantially".
    [0110] Note that the preferred embodiments of the device object of the invention are preferably implemented in silicon on insulator (SOI) in order to benefit from the high contrast SOI index. However, particular embodiments could be implemented on other different photonic platforms. In other words, all the waveguides of the device are preferably made by means of a silicon core, deposited on an insulating layer such as, for example, silicon dioxide. The material of coverage may vary for different embodiments of the invention, some of the possibilities being silicon dioxide, polymers or air, without this list limiting the use of other possible options.
    [0112] Figure 1 (plan view) and Figure 2 (perspective view) schematically show a preferred embodiment of the integrated filter based on coupled resonators of the invention, for a case of order two (that is, two coupled resonators), although it should be understood as an illustrative example of any number of order (N). In particular, the preferred embodiments of the invention are made with a number of resonators selected from three to nine. With more resonators, it is possible to achieve higher frequency selectivity but increases design complexity, while a small number of resonators reduces overall size and decreases design complexity.
    [0114] The number of resonators is defined by the number of Fabry-Perot cavities (6) or filter defects, while the length and effective index of said Fabry-Perot cavities (6) determine their central operating wavelength of the resonator. In the most common practical cases, all the resonators must have a resonance frequency very close to each other, therefore, to avoid deviations due to manufacturing errors, the filter preferably comprises thermal controllers that act on each resonator. Such drivers can also be used to implement tunable filters by applying frequency shifts simultaneously to all resonators, beyond tradeoffs for manufacturing errors.
    [0116] On each side of the Fabry-Perot cavities (6), the filter comprises two Bragg reflectors (5), the connection between each Fabry-Perot cavity (6) and each Bragg reflector (5) being implemented by means of an adapter (4 ). Note that, for clarity, the number of segments represented for Bragg reflectors (5) is much lower than the number usually required by Bragg grids, said number being determined by conventional coupled resonator filter design techniques, given the morphology and particular materials of each realization. Likewise, although all the adapters (4) have been represented with two waveguide segments, it is only an illustrative example, said adapters (4) being able to be made with a different number of segments, said number also being determined by conventional coupled resonator filter design techniques, given the particular morphology and materials of each embodiment.
    [0118] Note that although the Fabry-Perot cavities (6), the Bragg reflectors (5) and the adapters (4) have been represented as homogeneous segments, they can also be implemented using sub-wavelength structures (SWG). ), that is, periodic arrangements with a period less than the wavelength of the light propagated by them, so that Bragg reflections are avoided, and the periodic arrangement behaves like a homogeneous metamaterial whose effective index depends on the proportion of core material 11 and cover 12 in each period. This allows a greater degree of freedom when designing Fabry-Perot cavities (6), Bragg reflectors (5) and adapters (4) as well as a reduction in the effects of manufacturing errors in said Fabry-Perot cavities. (6), Bragg reflectors (5) and adapters (4).
    [0120] A first Bragg reflector (5), located at a first end of the Bragg reflector assembly (5) is connected to a single-mode input waveguide (1), while a second Bragg reflector (5), located at A second opposite end of the Bragg reflector assembly (5) is connected to a single-mode output waveguide (7). Since the input waveguide (1) and the output waveguide (7) have a first width (wi) smaller than a second width (W 2 ) of the Bragg reflectors (5), the filter comprises both modal converters (2) whose width varies adiabatically between the first width (W 1 ) and the second width (W 2 ). In this way, even though the second width (W 2 ) corresponds to a multimode guide, only the fundamental electrical transverse mode is excited in the Bragg reflectors (5). Note that the particular geometry of the modal converters (2) may vary depending on the particular embodiment, as long as the adiabatic transition condition is maintained, its simplest form being an isosceles trapezoid for each modal converter (2). Likewise, between the modal converters (2) and the adapters (4), the filter can comprise widened segments (3) homogeneous with the second width (W 2 ).
    [0122] Note that the Bragg reflectors (5), the Fabry-Perot cavities (6) and the adapters (4) have the same second width (W 2 ), simplifying the design of the device. Likewise, the high lateral confinement associated with the high value of said second width (W 2 ), allows optimizing the design using traditional techniques considering a two-dimensional approximation of the problem, which reduces computation times and maintains the precision of the frequency response of the filter obtained. Finally, the Bragg reflectors (5), the Fabry-Perot cavities (6) and the adapters (4) have a very simple lateral structure, that is, in a direction perpendicular to the propagation of light and parallel to the interface between the substrate ( 10) and core (11). Said lateral structure has a constant height (for a fixed position along the propagation direction) and is formed by rectangular guide sections without holes, cavities or protrusions. The combination of the high lateral confinement and the simplicity of the lateral structure results in little radiation in the plane of the chip. The radiation that is produced in the transition between homogeneous guides and periodic guides is fundamentally vertical radiation (typically known as 'out of chip'), being possible to minimize it by means of the design of the adapters (4).
    [0124] Figure 3 presents a schematic longitudinal section (that is, cut in a plane parallel to the propagation of light and perpendicular to the interface between substrate 10 and core 11) of a particular embodiment of the filter, for the case in which engraving is used. partial. That is, from a height (H) of the core layer (11), a plurality of sections with a lower effective index are defined by recording a depth (d) less than said height (H), so that said plurality of sections of lower effective index conserves part of the material of the core (11). In the case of Bragg reflectors (5), the length of said sections is constant, while the lengths of the adapter sections (h, h, h, kk l6) are variable, being determined during a design stage for adjust the frequency response of the filter.
    [0126] Note that although an example has been shown with two engraving heights (that is, complete engraving on the sides of the filter guides and depth engraving within said guides), particular embodiments of the filter can be implemented with a greater number of depths. engraving. Also, in the case of using only two engraving heights, the described filter can be used with a depth (d) of 70 nm, standard in many manufacturing platforms.
    [0128] Figure 4 schematically presents a variation of the effective index (neff) of the waveguides of height H of the Bragg reflectors (5), as a function of the second width (W 2 ) of said Bragg reflectors (5) . It can be observed that as said second width (W 2 ) increases, the lateral confinement increases and the The resulting effective index (neff), asymptotically approaching a maximum index (nmax) corresponding to the case of maximum lateral confinement (infinite width). To obtain the advantages described for the filter of the invention, the second width (w 2 ) is greater than a minimum width threshold (wmin) for which the effective index differs from the maximum index less than a first threshold (a).
    [0130] For example, for the case of C-band filters implemented in silicon on insulation, these conditions on the second width (W 2 ) of the plurality of Bragg reflectors (5) are typically translated into values of said second width (W 2 ) greater than 3.5 microns, although the particular limits of these values depend on the central wavelength, the height (H) of the layer of core material, the covering material (12), etc. In the same way, for the case of C-band filters implemented in silicon nitride, the conditions on the second width (W 2 ) of the plurality of Bragg reflectors (5) are typically translated into values of said second width (W 2 ) greater than 6 microns, although the particular limits of these values depend on the rest of the design factors mentioned.
    [0132] Figure 5 presents the frequency response, calculated by photonic simulation software, of three particular embodiments of the invention with the same bandwidth at 3dB of 50 GHz and a central wavelength of 1550 nm: a first embodiment of order two ( 21), a second embodiment of order three (22) and a third embodiment of order five (23). The rejection ratio of the first order two (21) realization is -19 dB, the rejection ratio of the second order three (22) realization is -27 dB and the rejection ratio of the third order realization five (23) is -43 dB.
    [0134] Equivalently, figure 6 presents the frequency response of three particular embodiments of the invention with the same bandwidth at 3dB of 25 GHz. The rejection ratio of the first second-order embodiment (21) is -19 dB, the rejection ratio of the second embodiment of order three (22) is -27 dB and the rejection ratio of the third embodiment of order five (23) is -44 dB; practically identical to the 50 GHz bandwidth case.
    [0136] Finally, figure 7 shows the response of the third embodiment of order five (23) for a wide band of 50 GHz, observing in a much greater range (100 nm). As can be seen, the free spectral range of the filter more than covers the entire band C (1530 nm - 1570 nm).
    [0138] Thanks to the structure and geometry proposed for the filter of the invention, these performances are much higher than those achievable in the state of the art, in terms of quality factor, rejection ratio and free spectral range. Furthermore, these features are obtained with a reduced number of coupled resonators, facilitating their design and reducing the overall size of the device. Likewise, it avoids the use of photonic crystals and other alternative structures that are more sensitive to manufacturing errors and with stricter MFS impositions.
    [0140] In view of this description and figures, the person skilled in the art will be able to understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without departing from the object of the invention such and how it has been claimed.
    权利要求:
    Claims (15)
    [1]
    1. Integrated filter based on coupled resonators comprising:
    - an input waveguide (1) and an output waveguide (7) with a first width (w-i);
    - a plurality of Bragg reflectors (5) with a second constant width (W 2 ) along each reflector of the plurality of Bragg reflectors (5); Y
    - a plurality of Fabry-Perot cavities (6) arranged alternately between the plurality of Bragg reflectors (5);
    characterized in that it also comprises:
    - two modal converters (2) whose width varies between the first width (wi) and the second width (W 2 ); Y
    - a plurality of adapters (4) connecting the plurality of Bragg reflectors (5) with the plurality of Fabry-Perot cavities (6) and with the two modal converters (2), said plurality of adapters (4) comprising alternate sections with at least two different effective indices.
    [2]
    Integrated filter according to claim 1, characterized in that the second width (W 2 ) generates an effective index that differs less than a first threshold (a), less than 1%, with respect to the effective index generated by an infinite width. (nmax).
    [3]
    Integrated filter according to claim 2 characterized in that the first threshold (a) is less than 0.25%.
    [4]
    Integrated filter according to any of the preceding claims characterized in that the plurality of Bragg reflectors (5) have an associated forbidden band that represents a greater proportion of a second threshold (b) with respect to a central wavelength of the filter , said second threshold (b) being greater than 1.5%.
    [5]
    5. Integrated filter according to claim 4 characterized in that the second threshold (b) is greater than 2.5%
    [6]
    Integrated filter according to any of the preceding claims characterized in that the plurality of Bragg reflectors (5) comprise a plurality of waveguide segments separated by sections recorded up to a substrate (10) of the filter.
    [7]
    Integrated filter according to any of claims 1 to 5 characterized in that the plurality of Bragg reflectors (5) comprise a plurality of waveguide segments separated by partially etched sections up to an etching depth (d).
    [8]
    Integrated filter according to claim 7 characterized in that the plurality of Bragg reflectors (5) are tuned by variations of the engraving depth (d) of each Bragg reflector (5).
    [9]
    Integrated filter according to any of claims 1 to 7 characterized in that the plurality of Bragg reflectors (5) are tuned by variations in a number of periods of each Bragg reflector (5).
    [10]
    Integrated filter according to any of claims 1 to 7 characterized in that the plurality of Bragg reflectors (5) are tuned by variations in a period (A) of each Bragg reflector (5).
    [11]
    Integrated filter according to any of the preceding claims, characterized in that the plurality of Fabry-Perot cavities (6) comprise periodic structures with a period less than the wavelength of an optical signal propagated by said periodic structures.
    [12]
    Integrated filter according to any of the preceding claims, characterized in that the plurality of Bragg reflectors (5) comprise periodic structures with a period less than the wavelength of an optical signal propagated by said periodic structures.
    [13]
    Integrated filter according to any of the preceding claims, characterized in that the plurality of adapters (4) comprise periodic structures with a period less than the wavelength of an optical signal propagated by said periodic structures.
    [14]
    Integrated filter according to any of the preceding claims, characterized in that the modal converters (2) comprise periodic structures with a period less than the wavelength of an optical signal propagated by said periodic structures.
    [15]
    Integrated filter according to any of the preceding claims, characterized in that it further comprises widened segments (3) between the modal converters (2) and the adapters (4), the widened segments (3) comprising periodic structures with a period less than the wavelength of an optical signal propagated by said periodic structures.
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    同族专利:
    公开号 | 公开日
    ES2833122B2|2022-02-02|
    WO2021116520A1|2021-06-17|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    EP0547859A1|1991-12-16|1993-06-23|Gec-Marconi Limited|Optical waveguide delay line with coupled resonator grating stucture|
    EP3173754A2|2015-11-26|2017-05-31|Samsung Electronics Co., Ltd.|Bragg grating, and spectroscopy device including the bragg grating|
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    ES201931104A|ES2833122B2|2019-12-11|2019-12-11|Integrated optical filter based on strong Bragg gratings coupled with strong lateral confinement|ES201931104A| ES2833122B2|2019-12-11|2019-12-11|Integrated optical filter based on strong Bragg gratings coupled with strong lateral confinement|
    PCT/ES2020/070776| WO2021116520A1|2019-12-11|2020-12-09|Integrated optical filter based on resonators coupled by means of strong bragg gratings having high lateral confinement|
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