Temperature insensitive filter

Abstract

An integrated wavelength selective filter device (10; 20; 30; 40) comprising a first optical element (14) for directing received radiation in a direction defined by a first angle (a) and a second optical element (15) which is a diffractive element configured to diffract the directed radiation with a second angle such that, for a single reference wavelength, the diffracted radiation is directed into a propagation medium to travel therein to a predetermined location of the first optical element so as to filter radiation having a wavelength substantially matching the reference wavelength from radiation having a substantially different wavelength. The propagation medium is formed of a material different from any material of the substrates of the first and second optical elements.

Description

[0001] This application is a divisional application of the application filed on December 31, 2018, with application number 201880084427.0, entitled "Temperature Insensitive Filter". Technical Field

[0002] This invention generally relates to wavelength-selective optical filters and optical systems including such filters, and more particularly to integrated thermal wavelength-selective optical filters and integrated optical systems including such filters. Background Technology

[0003] Optical wavelength filters in integrated photonic platforms often drift with temperature, leading to erroneous wavelength readings. This is caused by two effects: changes in the material's refractive index due to the thermo-optic (TO) effect and thermal expansion of the device. Typically, the thermo-optic effect dominates, but thermal expansion cannot be ignored for accurate measurements. For spectroscopic applications or other applications requiring absolute measurement of optical wavelengths, active temperature tracking or the use of a fixed reference wavelength is necessary.

[0004] In the past, various attempts have been made to provide a solution that takes into account thermo-optical effects and / or thermal expansion. One solution uses a thermo-optical waveguide. The optical waveguide used on the chip is coated with a material whose thermo-optical coefficient has the opposite sign to that of the waveguide, thus canceling out this effect. However, these approaches often suffer from chemical instability, higher waveguide losses, and CMOS incompatibility.

[0005] In another solution, a thermally self-compensating filter is used. Therefore, the Mach-Zehnder interferometer is designed to resist temperature fluctuations by using different polarization states or waveguide widths in the two arms. However, this method still suffers from drift due to thermal expansion. It is also highly susceptible to manufacturing defects.

[0006] Another solution utilizes active thermal compensation. The temperature of the photonic integrated circuit can be controlled and stabilized using a Peltier element, which allows for both cooling and heating of the chip. The main drawbacks of this approach are high power consumption and the need for a complex control system. Similarly, it is difficult to achieve sufficient stability for spectral applications.

[0007] The use of external reference optical filters (e.g., gas cells) was also explored. Gas cells remain expensive, and the need for external filters offsets the advantages of photonic integration.

[0008] Therefore, there is still room for improvement. Summary of the Invention

[0009] The purpose of the various embodiments of the present invention is to provide good thermally inert wavelength-selective optical filters and integrated optical systems including these filters.

[0010] The advantages of the embodiments of the present invention are that they provide a wavelength filter that is substantially insensitive to temperature and is purely passive without requiring active control. The advantages of the embodiments of the present invention are that they at least compensate for the thermo-optical effects that cause changes in refractive index.

[0011] The advantage of the various embodiments of the present invention is that radiation that does not substantially match the reference wavelength will not be coupled back to the first optical element or further optical elements, and therefore will not pass through the wavelength selective filter.

[0012] The advantage of various embodiments of the present invention is that it provides a wavelength filter that is substantially insensitive to temperature and acts as a passband filter, the output of which can be used as a reference wavelength.

[0013] The advantage of each embodiment is that it provides a filter whose design can tolerate manufacturing defects.

[0014] The advantage of the various embodiments of the present invention is that it does not require active temperature tracking.

[0015] The advantage of the various embodiments of the present invention is that they are provided as integrated solutions, for example, integrated on photonic integrated circuits, thereby avoiding the need for external components.

[0016] The above objectives are achieved by the method and apparatus according to the present invention.

[0017] This invention relates to an integrated wavelength selective filter device, comprising:

[0018] A first optical element, patterned on or in a substrate, is configured to receive radiation incident thereon and to direct at least partially the received radiation in a direction defined by a first angle.

[0019] A second optical element, which is patterned on or in a substrate and is a diffractive element extending longitudinally and arranged away from the first optical element, is configured to receive the directional radiation at an incident angle and adapted to diffract the directional radiation at a second angle, the second angle depending on the wavelength.

[0020] The first optical element directs the received radiation into a propagation medium, through which the directed radiation propagates until it is received by the second optical element. The propagation medium is formed of a material different from the substrate of the first and second optical elements.

[0021] The second optical element is configured such that, for a single reference wavelength, the diffracted radiation is directed into the propagation medium to travel to a predetermined position on the first or another optical element in order to filter out radiation with a wavelength substantially matching the reference wavelength from radiation with significantly different wavelengths, thereby uniquely determining the reference wavelength for the entire device.

[0022] Different materials of the propagation medium can make its thermo-optical refractive index coefficient much smaller than the corresponding refractive index of any material of the substrate of the first and second optical elements, for example, at least one order of magnitude or at least two orders of magnitude smaller. Alternatively or additionally, different materials of the propagation medium can make its thermo-optical refractive index coefficient much smaller than the corresponding refractive index of any material of the substrate of the first and second optical elements, for example, at least 10%, at least 20%, or at least 50% smaller.

[0023] The first angle can be the central angle of the central ray of the directional radiation beam, such as the central angle of a diverging beam. The diverging beam has an associated angular span, so the second angle (diffraction angle) may be different for different angular spectral components of the directional radiation incident on the second optical element.

[0024] The first optical element and / or the second optical element can be a grating or a grating coupler. Then, the first angle can depend on the wavelength.

[0025] A waveguide can be used to provide radiation to a first optical element. This waveguide can be a single-mode waveguide or a multimode waveguide.

[0026] Therefore, the first optical element and another optical element (if present) can be connected to the waveguide for transmitting optical input and output signals into and out of the filter device.

[0027] The first optical element can be a corrugated grating coupler. An advantage of the various embodiments of the invention is that the corrugated grating can have a large angular dispersion, which reduces the length / height of the deflection path, thereby allowing for a compact filter design.

[0028] The second optical element can be a corrugated grating. This grating can be a blazed grating. An advantage of the embodiments of the invention is that the grooves of the grating can be well defined, thus resulting in less ghosting. Another advantage of the embodiments of the invention is that integration with the first optical element (e.g., a first grating coupler) minimizes the positioning error of the optical elements relative to each other. The corrugated grating can be a chirped grating whose reference wavelength satisfies the Littrow condition.

[0029] The advantages of the various embodiments of the present invention are that the chirp of the grating can be well controlled based on refractive index modulation. Compared with, for example, doped gratings that involve more processing steps and are more expensive, the embodiments have the advantage of easy processing. One advantage of the Littoral grating is that the Littoral condition ensures that the diffraction angle is independent of the refractive index of the grating material, and therefore also independent of its temperature change.

[0030] The first optical element, the second optical element, and, if present, the third optical element, may be patterned or integrated on or on the same substrate.

[0031] In some embodiments, the second optical element may be adapted to reflect the directional radiation back into the propagation medium so as to travel at the same elevation angle relative to a flat surface of a substrate on or therein patterned with the second optical element, but with a propagation direction opposite to that of the radiation incident on the second optical element.

[0032] When a waveguide is used to provide radiation to a first optical element, the radiation can be coupled out of the first optical element through the same waveguide after interacting with a second optical element. Therefore, filtering can be achieved by the radiation being incident on the first optical element again at a first angle after interacting with the second optical element. The radiation can thus be reflected back to the same waveguide used to couple the radiation in the wavelength-selective filter. Optical components for separating the output from the input, such as, for example, circulators or 3dB couplers, can be used.

[0033] In some embodiments, the second optical element may be configured off-axis to diffract radiation incident on the second optical element in a different azimuth direction than the azimuth direction in which the incident radiation is incident on the second optical element. The azimuth direction may be defined relative to a flat surface of a substrate on or therein where the second optical element is patterned. The filter device then further includes another optical element having similar characteristics to the first optical element and configured to couple out the filtered radiation.

[0034] Therefore, after interacting with the second optical element, filtering can also be achieved by incidenting radiation onto a different optical element, wherein the second optical element is operated in a slightly off-axis configuration. The selected radiation is then coupled out via a different waveguide than the input waveguide.

[0035] The filter device may include at least one material having a coefficient of thermal expansion or designed such that, for a change in at least the second angle (diffraction angle) caused by a temperature change in the device, the length of the propagation path of the directional radiation from the first optical element to the second optical element in the propagation medium is automatically adapted by the thermal expansion of the device due to the temperature change, such that the diffracted radiation is again directed to a predetermined position for a reference wavelength. For a grating serving as the first optical element, a change in the first angle (deflection angle) may also be considered. An advantage of the embodiments of the invention is that the adaptation of the path length is a completely passive process caused purely by temperature change. An advantage of the embodiments of the invention is that it compensates not only for thermo-optical effects but also for thermal expansion effects. An advantage of the embodiments is that this is achieved passively.

[0036] The filter device may further include a reflective element such that the radiation is reflected by the reflective surface of the reflective element along the propagation path of the directional radiation from the first optical element to the second optical element. By reducing the height, the device can be made more compact. The reflective surface may be a reflective Fresnel lens. Therefore, the diffraction cone of the light can be reduced by collimation or refocusing.

[0037] Both the first optical element and the second optical element are formed on a flat surface of the device, and the reflective element is located above or below the flat surface.

[0038] The device may further include at least one support for supporting a reflective element located above or below the flat surface. The support may, for example, have a rectangular shape. An advantage of the various embodiments of the invention is that this solution allows for parallel alignment of the mirror surface / reflective element surface relative to the surface of the optical element and reduces errors in the second angle of incidence due to manufacturing tolerances; good height control can be achieved through uniform growth control.

[0039] The path length can be adapted by the overall thermal expansion of the support. Compensation for the thermal expansion effect can be achieved through appropriate selection of materials and support dimensions.

[0040] The support material can be selected such that its total coefficient of thermal expansion is approximately three times that of the substrate material on which the first and second optical elements are formed, for example, two to four times greater, or 2.5 to 3.5 times greater (such as three times greater). This condition may be optimal for a first angle between 40 and 50 degrees, such as, for example, for a first angle of 45 degrees. An advantage of the various embodiments of the invention is that optimal temperature insensitivity is achieved. The support, or a portion thereof, may be the substrate of the first or second optical element. A support at least partially formed from a substrate can advantageously simplify the alignment of the mirror surface / reflective element with the substrate on which the first and second optical elements are formed.

[0041] In silicon platforms, temperature insensitivity can be less than 1 pm / K over a temperature range of 100 K. This is approximately a hundredfold reduction compared to conventional uncompensated silicon wavelength filters.

[0042] The first and second optical elements can be positioned above each other, and the propagation path of at least partially diffracted radiation through the propagation medium can be straight. In various embodiments of the invention, the propagation path of the at least partially diffracted radiation propagating from the first optical element to the second optical element through the propagation medium can be a path through a gas or vacuum (e.g., a path through air). Advantageously, this path passes through a propagation medium having a refractive index that is substantially independent of temperature, such as, for example, a gas, such as air or a vacuum. An advantage of the various embodiments of the invention is that no light absorption loss occurs along the path. Another advantage of the various embodiments of the invention is that no additional manufacturing steps are required.

[0043] The first and second optical elements can be formed in the semiconductor or dielectric substrate of the chip. An advantage of the embodiments of the present invention is that a compact system can be obtained. An advantage of the embodiments of the present invention is that production can be based on mass production and low-cost manufacturing technologies. An advantage of the embodiments of the present invention is that densely integrated devices can be obtained.

[0044] The first and second optical elements can be fabricated in a semiconductor-on-insulator platform, such as a silicon-on-insulator platform. In some embodiments, the substrate can be a semiconductor substrate; in other embodiments, the substrate can be a dielectric substrate.

[0045] The present invention also relates to an integrated system for providing a stable reference wavelength, the system comprising:

[0046] The integrated wavelength selective filter device described above

[0047] Integrated broadband light source, and

[0048] At least one light guide is coupled to a light source and to a first dispersive element of a wavelength selective filter device for providing broadband light to a first optical element and for extracting radiation with a reference wavelength from the first optical element.

[0049] The light guide can also be used to extract radiation with a reference wavelength from the first optical element. Alternatively, additional optical elements and additional light guides can be used to extract radiation from the reference wavelength.

[0050] The advantage of the various embodiments of the present invention is that, for measurement systems that require a stable reference, such as a spectrum, a compact, mass-producible, and low-cost stable reference wavelength can be obtained.

[0051] The system may further include multiple integrated detector elements, wherein an integrated wavelength selective filter is adapted to diffract radiation of different wavelengths to different predetermined locations in order to direct radiation of different wavelengths to different integrated detector elements.

[0052] The present invention further relates to an integrated spectrometer system comprising: a plurality of integrated wavelength-selective filter devices as described above, a plurality of light-guiding circuits, and a plurality of integrated detectors, wherein each of the plurality of wavelength-selective filter devices is configured to have a different reference wavelength, thereby defining a set of reference wavelengths, and wherein each of the plurality of light-guiding circuits is adapted to receive a small portion of an externally applied radiation signal and transmit it to one of the plurality of wavelength-selective filter devices for obtaining radiation at a specific reference wavelength and for directing the obtained radiation to one of the plurality of detectors. The advantages of the embodiments of the present invention are that a compact, manufacturable, low-cost, and stable spectrometer can be obtained.

[0053] Specific and preferred aspects of the invention are set forth in the appended independent and dependent claims. Features from the dependent claims may be combined, where appropriate, with features of the independent and other dependent claims, and are not merely explicitly set forth in those claims.

[0054] For the purpose of summarizing the invention and the advantages achieved beyond those of the prior art, certain objects and advantages of the invention have been described above. It should be understood, of course, that not all such objects or advantages can be achieved according to any particular embodiment of the invention. Therefore, for example, those skilled in the art will recognize that the invention can be embodied or practiced in a manner that achieves or optimizes one or more advantages as taught herein, without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0055] The above and other aspects of the invention will be apparent from the embodiments described herein(s) ...)(s)(s)(s)(s))(s)(s)(s) Attached Figure Description

[0056] The invention will now be further described by way of example with reference to the accompanying drawings, in which:

[0057] Figure 1 This is a schematic cross-sectional view of an integrated wavelength selective filter device according to a first embodiment of the present invention, including two integrated gratings as first and second optical elements arranged on a flat surface and an additional reflective surface located above the flat surface.

[0058] Figure 2This is a schematic cross-sectional view of an integrated wavelength selective filter device according to a second embodiment of the present invention, including two integrated gratings as first and second optical elements respectively arranged on a lower first flat surface and an upper second flat surface, and without any additional reflective surfaces.

[0059] Figure 3 This is a schematic top view of an integrated wavelength selective filter device according to a first embodiment of the present invention, including two integrated gratings as first and second optical elements and an additional reflective surface.

[0060] Figure 4 This is a schematic top view of an off-axis integrated wavelength selective filter device according to an embodiment of the present invention, the horizontal position of which is at the height of a flat surface included by a second substrate.

[0061] Figure 5 This is a schematic perspective view of an off-axis integrated wavelength selective filter device according to an embodiment of the present invention.

[0062] Figure 6 This is a schematic cross-sectional view of an integrated wavelength selective filter device according to an alternative embodiment of the present invention, including two integrated gratings as first and second optical elements arranged on a flat surface and an additional reflective surface located below the flat surface.

[0063] Figure 7 This is a schematic cross-sectional view of an integrated wavelength selective filter device according to an alternative embodiment of the present invention, including two integrated gratings as first and second optical elements respectively arranged on an upper first flat surface and a lower second flat surface, and without any additional reflective surfaces.

[0064] These accompanying drawings are illustrative only and not restrictive. In the drawings, some elements may be enlarged and not drawn to scale for illustrative purposes. Scale and relative scale do not necessarily correspond to an actual simplification of the practice of this invention.

[0065] Any reference numerals in the claims should not be construed as limiting the scope.

[0066] In different accompanying drawings, the same reference numerals refer to the same or similar elements. Detailed Implementation

[0067] The invention will be described with reference to specific embodiments and particular drawings, but the invention is not limited thereto but is defined only by the claims.

[0068] The terms "first," "second," etc., used in the specification and claims are used to distinguish between similar elements and are not necessarily used to describe a temporal, spatial, hierarchical, or any other order. It should be understood that the terms thus used are interchangeable where appropriate, and the various embodiments of the invention described herein can be operated in a different order than that described or illustrated herein.

[0069] Furthermore, directional terms such as top, bottom, front, rear, leading edge, trailing edge, below, and above, used in the specification and claims are for descriptive purposes of orientation with reference to the figures being described, and not necessarily for describing relative positions. Because components of various embodiments of the invention can be positioned in several different orientations, directional terms are used for illustrative purposes only and are not intended to be limiting unless otherwise stated. Therefore, it should be understood that these terms, as used herein, are interchangeable where appropriate, and that the embodiments of the invention described herein can operate in orientations other than those described or illustrated herein.

[0070] It should be noted that the term "comprising" as used in the claims should not be construed as limiting oneself to the means listed thereafter; it does not exclude other elements or steps. Therefore, the term should be interpreted as specifying the presence of the features, integers, steps, or components stated as mentioned, but does not exclude the presence or addition of one or more other features, integers, steps, or components, or groups thereof. Thus, the scope of the statement "an apparatus comprising means A and B" should not be limited to an apparatus consisting solely of components A and B. This means that for the purposes of this invention, the only relevant components of the apparatus are A and B.

[0071] Throughout this specification, the reference to "an embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Therefore, the phrase "in an embodiment" or "in an embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, but may refer to the same embodiment. Furthermore, in one or more embodiments, as will be obvious to those skilled in the art according to this disclosure, particular features, structures, or characteristics can be combined in any suitable manner.

[0072] Similarly, it should be understood that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, drawing, or description for the purpose of simplification and aiding in the understanding of one or more of the various inventive aspects. However, this method of disclosure should not be construed as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as reflected in the appended claims, inventive aspects lie in fewer features than all the features of a single foregoing disclosed embodiment. Therefore, the claims appended to the detailed description are thus explicitly incorporated into that detailed description, wherein each claim itself represents a separate embodiment of the invention.

[0073] Furthermore, while some embodiments described herein include features that are included in other embodiments but not others, it will be understood by those skilled in the art that combinations of features from different embodiments are intended to fall within the scope of the invention and form different embodiments. For example, any embodiment of the claimed embodiments in the appended claims can be used in any combination.

[0074] It should be noted that the use of specific terms in describing certain features or aspects of the invention should not be construed as implying that the term is redefined herein to be limited to any particular characteristic of the invention that includes the term and is associated with it.

[0075] Numerous specific details are set forth in the description provided herein. However, it should be understood that various embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0076] Whenever radiation is mentioned in the instruction manual, it refers to radiation in the form of light, which is part of the electromagnetic spectrum. This specifically includes light in the ultraviolet, visible, near-infrared, mid-infrared, and far-infrared spectra.

[0077] A thermal optical filter or filtering device is intended to represent an optical filter that includes one or more technical means to significantly reduce its temperature sensitivity or compensate for temperature drift relative to the center wavelength of the passband, the absence of which would negatively affect the performance of the filter or filtering device.

[0078] In the context of this invention, an "integrated" wavelength selective filter device refers to a device comprising at least first and second optical elements adapted to orient or redirect received radiation to a spatial direction, which can be characterized by an angle depending on a specific wavelength of the received radiation. The first and second optical elements are each patterned on or within a substrate. The first and second optical elements may be grating structures or diffraction structures patterned on or within the substrate, respectively. An "integrated" wavelength selective filter device may include additional optical elements, a radiation source, or a detector. An "integrated" wavelength selective filter device is well-suited for further integration onto a single chip, for example, including chips that cooperate with the wavelength selective filter device to form a single compact, functional device of control electronics or readout electronics.

[0079] "Substrate" refers to a material or combination of materials that forms a physical support base, used for incorporating, including, attaching, or depositing additional materials on or within the substrate material, and for excluding, removing, or modifying materials already present in the substrate. More than one substrate can be combined into a new substrate, for example, by bonding or stacking. In various embodiments of the invention, the substrate may be a photonic circuit die or a planar optical wave circuit substrate; the substrate may include silicon or silicon-on-insulator. A photonic circuit die may be obtained as part of a larger wafer that has been diced, sliced, diced, or otherwise divided into smaller pieces. In some embodiments, "substrate" may also refer only to a portion of the physical support base—perhaps because its strength is insufficient to act as a physical support without an additional material layer; therefore, it will be understood that in such cases, further material, such as, for example, a thick silicon layer, is typically present in the device to provide a mechanical carrier.

[0080] In the various embodiments of the invention, "support" refers to one or more structural support elements constituting an integrated wavelength selective filter device and having a wall-like, columnar, or spherical structure. The "support" acts as a spacer, ensuring structural support and controlled distance spacing between combined or assembled elements that form part of the integrated wavelength selective filter device (e.g., between two flat surfaces of different but combined substrates facing each other). In the embodiments of the invention that provide "supports," their presence must not affect the propagation of radiation between the first and second optical elements. This means that the "support," as a support structure, is arranged in such a way that directional or diffracted radiation within the operating filter device does not propagate through the "support" material as a propagation medium. This can be achieved by placing the "support" around the first and second optical elements. In one embodiment of the invention, the support may utilize part or all of the substrate material itself, in which case the substrate material needs to be partially removed to prevent radiation from propagating through it.

[0081] A well-known fact in analytic and differential geometry is that for any smooth two-dimensional surface embedded in three-dimensional Euclidean space, a local surface normal can be constructed and established at a point on that surface. The direction in three-dimensional Euclidean space at that point on the surface can then be used as a direction vector, parameterized relative to the surface normal by two direction angles. Common choices of direction angles include elevation and azimuth. The elevation angle is the angle formed by the surface normal and the direction vector on the plane subtended by the surface normal and the direction vector, while the azimuth angle is the angle formed between the projection of the direction vector onto the tangent (perpendicular to the surface normal) at that point on the surface and a reference direction / line in the same tangent. There is no preferred orientation for such a reference direction / line in the tangent, but for various embodiments of the invention, a natural selection of the reference direction / line in the tangent can be adopted by choosing the local radius of curvature of the ridge or groove that forms part of the corrugated surface as the reference direction / line. The tangent then corresponds to the corrugated surface. This also... Figure 5 The explanation is in the middle.

[0082] In a first aspect, embodiments of the present invention relate to an integrated wavelength selective filter device. The integrated wavelength selective filter device includes a first optical element patterned on or in a substrate and configured to receive radiation incident thereon and to direct said received radiation at least partially toward a direction defined by a first angle (α). The first optical element may be, for example, a mirror or a grating. The first angle (α) is generally wavelength-dependent. The filter device also includes a second optical element patterned in or on the substrate and extending longitudinally and arranged away from the first element. The second optical element is a diffraction element. The second optical element is configured to receive directed radiation at an incident angle and is adapted to diffract the directed radiation at a second diffraction angle. Both the incident angle and the second diffraction angle are wavelength-dependent. The first optical element directs the received radiation into a propagation medium through which the directed received radiation propagates until it is received at the second optical element. The propagation medium is formed of a material different from any substrate material and having a smaller refractive index than any substrate material. Furthermore, the second optical element is configured such that the second diffraction angle is such that, for a single reference wavelength, the diffracted radiation is directed into the propagation medium to travel to a predetermined position on the first or another optical element in order to filter out radiation with a wavelength substantially matching the reference wavelength from radiation with significantly different wavelengths, thereby uniquely determining the reference wavelength for the entire device.

[0083] In some embodiments, the second optical element is configured to back-reflect directional radiation into a propagation medium so that it travels toward the first optical element at the same elevation direction relative to a flat surface of a substrate on which or therein is patterned, but has a propagation direction opposite to that of the radiation incident on the second optical element for coupling out filtered radiation. In other embodiments, the second optical element is configured off-axis to back-diffuse the radiation incident on the second optical element in a different azimuth direction (e.g., a slightly different azimuth direction) compared to the azimuth direction in which the incident radiation is incident on the second optical element. The azimuth direction may be defined relative to a flat surface of a substrate on which or therein is patterned. The filter device may then further include another optical element having similar characteristics to the first optical element and configured to couple out filtered radiation.

[0084] The standard and optional features of various embodiments of the present invention will now be described in more detail with reference to different types of embodiments.

[0085] Integrated optical wavelength-selective filters according to various embodiments of the present invention typically include an input port, which in some embodiments also functions as an output port. The input port is typically configured to receive incident polychromatic radiation and to pass filtered (i.e., monochromatic) radiation. Thus, in some embodiments, the optical wavelength-selective filter further includes a first optical element and a second optical element, the first optical element typically connected to the input port to disperse the polychromatic radiation incident thereon, optionally connected to one or more reflective surfaces to fold or redirect the path of the radiated beam, while the second optical element is used in a retroreflective structure. The input port is preferably waveguide-type, such as an integrated waveguide with a ridge, rib, or groove shape on or embedded in a substrate, for example, an integrated waveguide patterned in a sapphire or polymer substrate, in silicon-on-insulator (SOI), indium phosphide, gallium arsenide, or other semiconductor or dielectric platform, but not limited thereto. Even more preferably, a waveguide-type input port results in the incident polychromatic radiation being confined to a single-guide mode (e.g., the fundamental mode of the waveguide), whereby the single-guide mode / fundamental mode is parameterized by the wavelength. Other types of input ports can be the aperture or exit pupil of an integrated optical element, such as the facet of a microprism, the pupil of a lens, or the pupil of a lens system (e.g., the pupil of a miniature spherical lens bonded to an integrated optical circuit substrate). In such cases, the incident radiation is preferably in an unrestricted fundamental mode, such as a zero-order collimated Gaussian beam. Fundamental modes are advantageous because they provide spatially more localized incident radiation, which allows for a reduction in the size of light guides, light buckling, or light dispersing structures (e.g., narrower ridge waveguides, shorter gratings, etc.). This enables more compact and cost-effective designs for integrated optical filters and integrated optical systems including such filters.

[0086] The first optical element is preferably provided as an integrated grating, such as a shallow or deep etched grating in an SOI platform. For grating-type first optical elements, it is further preferred to excite only a few diffraction orders, and most preferably only a single diffraction order. This has the advantage of avoiding potential sources of stray light and background signals, and of improving the diffraction efficiency of one or more orders. This increases the overall optical filter throughput in the passband by reducing insertion loss and thus increasing the stopband attenuation factor. If desired in some embodiments of the invention, the top cladding of the integrated waveguide and the first optical element (e.g., an integrated grating) is preferably air, although the embodiments are not limited thereto. In fact, any other material with a significantly weaker temperature-dependent refractive index (i.e., thermo-optic coefficient) compared to the effective refractive index of the integrated waveguide or the first integrated grating is also suitable. Alternatively, the first optical element may be a bonded ruled or holographic grating, a bonded microprism, an AWG-type arrayed phase grating, a cleaved waveguide facet, a mirror, etc.

[0087] The second optical element is also preferably provided as an integrated grating, such as a shallow or deep etched grating in an SOI platform. Even more preferably, the second optical element is a blazed grating to maximize diffraction efficiency. Alternatively, the second optical element may also be a bonded ruled or holographic grating. Both the first and second optical elements may have one or more surfaces exposed to the radiated signal coated with a thin conformal coating to improve their reflection or transmission characteristics within the desired filter passband. When the first optical element operates in a transmission scheme, the conformal coating may be a thin conformal antireflective coating. For the second optical element operating in a reflection scheme, a thin conformal metallic film or a thin conformal reflective coating may be applied.

[0088] When polychromatic radiation is incident on the first optical element at a first incident angle, it is not or partially dispersed into different angular directions, i.e., different deflection angles depending on the wavelength. Dispersion into different angular directions may also be a result of the redirected received radiation propagating freely within the propagation medium. The second optical element is arranged so far away from the first optical element that the at least partially deflected polychromatic radiation reaches the second optical element at a second incident angle without significant obstruction. That is, a substantially lossless radiation propagation path is established between the first and second optical elements. In some embodiments, the second optical element is configured to back-reflect radiation in a narrow band region around the center wavelength of the integrated optical filter in such a way that it proceeds substantially along the same radiation propagation path between the first and second optical elements but in the opposite direction, i.e., for these wavelengths, the second incident angle and the diffraction angle associated with the second optical element coincide. The radiation wavelengths in the filter stopband are diffracted to different diffraction angles, such that the return path of the radiation signals of these wavelengths is different from the initial path. As a result, the radiation signals of those wavelengths may encounter significant obstruction, such as being absorbed, missing the first optical element, or not being directed back to the input / output port. Regardless, they are effectively suppressed. This filtering effect is amplified by extending the initial path using one or more reflective surfaces, such as by total internal reflection on a surface, metal, or dielectric mirror (e.g., a thin metal or dielectric film coating applied to a surface), so that even small differences in diffraction angles result in increasingly larger spatial spacing along the return path. As an alternative to the described configuration, the second optical element can be configured off-axis and radiation can be coupled out through a third optical element that differs from the first optical element but includes the same or substantially similar properties. Thus, the angle of the diffracted wave at the second optical element differs from the angle of incidence at the second optical element. For example, this can be a significantly different angle, or in some embodiments, it can simply be a difference in the azimuth component of the relevant angle. In some embodiments, the difference can be small, for example, sufficient to direct the radiation back to the third optical element, which is located at a different position but close to the first optical element.

[0089] By way of explanation, the embodiments of the present invention are not limited thereto. Some specific examples of the embodiments will be further described to explain the standards and optional features.

[0090] Figure 1 This is a schematic cross-sectional view of an integrated photonic chip that realizes a thermal optical wavelength selective filter 10. The integrated photonic chip is not limited to... Figure 1 The portion shown may also include additional components not shown, such as curved input grating couplers, tapered wide waveguides or bonded prism couplers for coupling light (e.g., light emitted from a nearby fiber end facet or a focused beam incident on the integrated photonic chip) to the integrated photonic chip, as well as routing, splitting and combining devices (e.g., curved waveguides, broadband directional couplers, etc.).

[0091] The input facet or cross-section of the integrated waveguide 131 can serve as the input port 13 of the integrated optical wavelength selective filter 10. The integrated waveguide 131 can be a single-mode silicon waveguide formed in a silicon layer 12 on top of a buried silicon oxide substrate 11. The substrate 11 can be made of different materials, such as sapphire, GaAs, InP, polymers, etc., and can also include more than one material layer. For example, the substrate 11 can be a buried silicon oxide layer on top of a silicon wafer layer. Furthermore, the silicon layer 12 (hereinafter referred to as waveguide layer 12) is not limited to silicon, and other material layers may also be selected, such as InP, Ge, SiGe, SiN, SiN, SiON, general glass, and chalcogenides, especially tetravalent materials in the III-V platform (such as InGaAsP in the InP platform). The specific selection depends on the target center wavelength, temperature range, design rules, etc., of the filter.

[0092] An integrated waveguide 131 is connected to a first optical element, which in this example is an integrated grating 14 formed within the same layer 12 as the integrated waveguide 131, and incident radiation is directed from the input port 13 (e.g., a cross-section or facet of the integrated waveguide 131) to the first integrated grating 14. According to this embodiment, the first integrated grating 14 is selected as the first optical element. A grating with rectangular grating teeth can be obtained by selectively etching the waveguide layer 12. However, different shapes of the grating 14 are also possible, such as triangular, wedge-shaped, sawtooth-shaped, or sinusoidal. The first integrated grating 14 is preferably optimized for the center wavelength of the optical filter 10, for example by selecting a specific grating period so that the light of the center wavelength is diffracted to a suitable angular range at varying temperatures, or by optimizing the grating profile to achieve an efficient diffraction scheme for the center wavelength. For example, if the grating is a rectangular grating shallowly etched in a silicon waveguide, a grating pitch of approximately 810 nm will diffract a wavelength of 1.55 μm to an angle of 45 degrees at room temperature; the invention is not limited thereto. Introducing chirp into the grating period of the first integrated grating may also be advantageous, as it increases the associated diffraction bandwidth and reduces undesirable back reflections at the first integrated grating 14 or its end facets. Alternatively, off-axis gratings or grating couplers known to have particularly low reflection can be provided. Furthermore, the first integrated grating 14 is preferably configured to diffract the incident light into a single order, e.g., order m = -1. This is advantageous because it avoids overlapping diffraction orders, which reduces the complexity of design considerations. It also avoids spreading signal energy across multiple orders, which could result in additional losses and a weaker output signal, e.g., a very low stopband attenuation factor. However, if design rules rule out grating periods that are too small to be properly manufactured, it may be necessary to design a higher-order first integrated grating 14, e.g., with order |m| ≥ 2. Furthermore, it may be beneficial, for example, to coat the surface profile of the first integrated grating by applying a thin anti-reflective coating in order to minimize reflection loss in the passband wavelength range when filtered light in this range re-enters the first integrated grating 14 before being directed back to the input port 13, which also serves as an output port.

[0093] In this example, the second optical element 15, serving as the integrated grating 15, is patterned within the same silicon layer 12 as both the integrated waveguide 131 and the first integrated grating 14, but is not directly connected to the first integrated grating 14 via this very similar silicon layer 12, for example in... Figure 1In the cross-sectional view, a gap region exists between the first and second integrated gratings 14, 15. In this embodiment, the first and second integrated gratings 14, 15, formed so far apart, can be separated by a spacing preferably between 0.5 mm and 10 mm, and most preferably between 1 mm and 5 mm. The second integrated grating 15 is typically designed as a blazed grating. This generally results in less reflection loss, especially when the second integrated grating 15 is in its most preferred configuration: the Littoral configuration. The Littoral configuration is a special case in which, for a given diffraction order of interest, the blaze angle is equal to the incident angle and also equal to the diffraction angle. Here, the diffraction angle is determined relative to the grating surface normal and the orientation opposite to the orientation of the incident angle. Under this configuration, the grating efficiency is typically greatly improved at a given diffraction order. To further improve the efficiency of the second integrated grating 15, its surface profile can be coated with a thin reflective film (e.g., a thin layer of aluminum, gold, or silver) or a stack of thin reflective dielectric layers. The blazed grating 15 can be fabricated by anisotropic wet or dry etching, wherein the etchant, etchant concentration, etchant selectivity relative to the crystal surface, crystal / wafer orientation, and crystal / wafer dicing angle can be selected to obtain at least one specific blazed angle. For example, if the waveguide layer 12 is a

[100] oriented crystalline silicon wafer layer, the wet etchant can be KOH or TMAH, resulting in an etch stop at an angle of 54.7° on the {111} plane. More complex blazed grating fabrication can tune the blazed angle to account for longitudinal variations in the angle of light incident on the second integrated grating 15, and also to account for the curvature of the grating lines in a plane parallel to the substrate 11. For example, a more complex blazed grating 15 can be fabricated using a focused ion beam etching process. However, the second integrated grating 15 can also be designed to exhibit rectangular, sinusoidal, stepped, etc., surface profiles. The grating lines 151 are typically not just straight lines (as seen in a plane parallel to the substrate 11, in...) Figure 1 (Not visible in the center), but can adapt to various line types, for example, following circular or elliptical line types in a plane parallel to the substrate 11. This is shown in the top view of this embodiment. Figure 3 The following explanation is provided. Its advantage lies in the fact that, if conical diffraction is required, for example, if the size of the first integrated grating 14 of the present invention is much smaller than the second integrated grating 15 or the distance between them, then the first integrated grating 14 can reasonably be approximated as a directional point emitter with its emission angle correlated with wavelength. The incident angle is then uniform along the grating line 151 of the second integrated grating 15.

[0094] One or more supports 18 (e.g.) Figure 1 The two support pillars 18 or a single rectangular path can be arranged on both sides of the optical integrated filter 10, adjacent to the first and second integrated gratings 14, 15. One or more supports 18 can be made of alumina, sapphire, cermet, talc, etc., and the invention is not limited thereto.

[0095] The support 18 can be selectively grown, sputtered, or deposited using evaporation techniques, or more preferably, directly bonded to the substrate 11 (where it is exposed) or the waveguide layer 12 (where it is not removed). The support 18 can extend from 0.1 mm to 5 mm in the vertical direction, preferably from 0.1 mm to 2 mm, and in this embodiment, this vertical direction coincides with both the substrate normal and the grating normal 121.

[0096] A second substrate 16 (which may or may not include additional material layers) is suspended at a height h over at least one region of a substantially planar waveguide layer 12, which includes the first and second integrated gratings 14, 15 and the gap region between them, wherein the height h corresponds to the height of the support 18 that structurally supports the second substrate 16. Such a second substrate may be made, for example, of fused silica. A surface portion 161 of the second substrate 16 facing the waveguide layer 12 may additionally include a thin metal film (e.g., a thin film of gold or silver) or a thin reflective coating to further increase the reflectivity and reflectivity of the surface portion 161. This is particularly suitable for substrates 16 that are substantially transmissive to light incident on their surface. The surface portion 161 is positioned such that it at least faces the gap region between the first and second integrated gratings 14, 15 arranged remotely, and more preferably extends laterally, such that light rays 19 of the center wavelength emitted by the first integrated grating 14 or deflected back by the second integrated grating 15 strike and reflect from it across the entire target temperature range of the filter 10. From an alignment tolerance perspective, selecting a larger surface portion 161 is also advantageous because during alignment of the surface portion 161 relative to the first and second integrated gratings 14, 15, if the surface portion 161 has additional margin, then small offsets will not significantly affect its reflectivity. The space 17, which is at least partially confined, is defined by the boundary walls of the support 18 and the suspended substrate 16, and is preferably filled with air. That is, at least a portion of the integrated waveguide 131 and the entire first and second integrated gratings 14, 15 both have an air-top cladding. Although air is the preferred cladding medium for this embodiment due to the natural presence of air and the fact that no special steps for removing air are required, alternative embodiments of the invention may provide different cladding media. If space 17 is a fully defined, hermetically sealed space, the cladding medium may, for example, be an inert gas or a vacuum. In other embodiments of the invention, space 17 may be filled with a solid dielectric material, preferably having a low thermo-optic coefficient and a coefficient of thermal expansion matching the coefficients of thermal expansion of the two substrates, so as not to excessively deform the substrate 16 and locally alter the reflectivity of its surface portion 161. Although a planar substrate 16 is preferred in this embodiment for ease of manufacture and integration, a curved or other surface profile substrate 16 may be provided, such that the surface portion 161 also has, for example, a focusing device for refocusing a diverging beam generated by the small numerical aperture of the first integrated grating 14.

[0097] In operation, the integrated optical filter 10 receives a multicolor light signal at its input port 13. An integrated waveguide 131 directs this multicolor signal to a first integrated grating 14 in a substantially lossless manner, in which the multicolor signal is coupled into an air-filled space 17 with an order m = -1. Due to the waveguide effect, the incident multicolor light signal propagates at a first incident angle of substantially 90 degrees relative to the grating normal 121. The deflection center angle α (the first angle) is obtained from the grating formula given in Equation 1, where n a This refers to the refractive index, n of the cladding medium (e.g., air) in space 17. eff Let m be the real part of the effective refractive index of the corrugated leaky waveguide defined by the first integrated grating 14, m be the diffraction order, λ be the wavelength component of the polychromatic signal, and Λ be the grating period of the first integrated grating 14.

[0098]

[0099] As can be seen from Equation 1, the deflection angle α is wavelength-dependent. A ray 19, representing a specific wavelength component λ (e.g., the center wavelength) of the polychromatic light signal, travels through the medium of space 17 at a deflection angle α relative to the grating normal 121 until it strikes a reflective surface portion 161 (e.g., a reflective surface, such as a thin gold film deposited on substrate 16), from where it is reflected and travels further through the medium of space 17. Finally, this ray 19 is incident on the corrugated surface of the second integrated grating 15 at a second incident angle, which is exactly the same angle α. This is a result of the mirror effect of the surface portion 161, which simply folds the path of the ray 19 and the arrangement of the first and second integrated gratings onto the same plane (e.g., the surface of substrate 11), such that their normals 121 are collinear. At the second integrated grating 15, the multicolor light signal is diffracted again; the diffraction angle β is calculated according to the grating formula given in Equation 2, where k indicates the diffraction order, λ indicates a specific wavelength component of the multicolor light signal, and D indicates the local grating period at the position where the ray 19 encounters the surface of the second integrated grating 15. Due to the wavelength-related deflection angle, each wavelength component of the multicolor light signal has a slightly different incident position on the second integrated grating 15.

[0100]

[0101] For the center wavelength λ of optical filter 10 cThe grating period D of the second integrated grating 15 is set, at least locally, in the portion of the grating 15 illuminated by a pencil beam or a diverging beam corresponding to the center wavelength, to a value that enables the second integrated grating 15 to operate under the Littoral condition (α = β or the second incident angle equals the diffraction angle). Therefore, for a first-order Littoral configuration with k = 1, the period D is set as expressed in Equation 3, where the second equivalence is a direct result of Equation 1 with m = -1.

[0102]

[0103] Equation 3 applies only to the direction of the central angle of the beam exiting the first optical element. Due to free-space diffraction, this beam diverges and is therefore composed of multiple directions. These different directions fall on different locations of the second optical element. Therefore, to ensure that the second optical element satisfies the Litterow condition at all locations for the same wavelength, it needs to be chirped. In this embodiment, the landing position s, representing the distance from a given location on the second optical element to the center of the first optical element, is determined by the filter geometry and is given in Equation 4. This introduces a chirp, or position-dependent grating period D, for the second integrated grating 15.

[0104] s=2h tanα (4)

[0105] According to the definition of s, if the first optical element essentially acts as a point source emitter, then the grating of the second optical element will have a circularly curved grating groove. This ensures that the second optical element converts the two-dimensional divergent incident beam into a two-dimensional convergent diffracted beam, which will be refocused onto the first optical element for the reference / center wavelength.

[0106] Combining Equations 3 and 4, the position-dependent (i.e. locally varying) grating period D(s) of the second integrated grating 15 can be derived and recorded in Equation 5.

[0107]

[0108] Now, this chirp of the second integrated grating 15, or the spatially varying grating period D(s), ensures that the center wavelength satisfies the Litterow condition, regardless of its respective deflection angle α (which is equal to the second incident angle). As a result, light signals characterized by wavelengths substantially different from the center wavelength are diffracted to an angular direction β significantly different from the second incident angle α. Therefore, only light in a narrow wavelength range close to the center wavelength (filter passband) is reflected back substantially along the same propagation path to the second integrated grating 15, and thus effectively coupled back to the first integrated grating 14 before being directed back to the input port (which also serves as the filter output port) 13. However, due to the relatively small numerical aperture of the first integrated grating 14, light signals characterized by wavelengths outside the filter passband (i.e., within the filter stopband) are not coupled back to the input port 13. In fact, light signals within the filter stopband are rejected due to their spatial deviation from the position of the first integrated grating 14 on its return path. This spatial separation effect is further amplified by the folded propagation path of the light ray 19 (e.g., by the reflective surface portion 161). Due to the refractive index n eff and n a The temperature dependence (e.g., due to its thermo-optic coefficient) is the same for any temperature change that affects the deflection angle α of the first integrated grating 14.

[0109] In an alternative embodiment, the second optical element is placed off-axis and coupled out of the filter after the radiation interacts at the second optical element by using another optical element 14a and optionally also a separate external coupling waveguide. Figure 4 And still Figure 5 The following explains this principle. As indicated above, the second optical element 15 is therefore placed off-axis, and then the azimuth angle of the diffracted wave at the second optical element 15... azimuth angle of incidence on the second optical element 15 The difference lies in the fact that, for example, the second optical element 15, configured off-axis, acts as a specular reflector in the azimuth direction, while the elevation angle "θ" of the diffracted wave at the second optical element 15 is substantially equal to the elevation angle "θ" of the incident wave on the second optical element 15 (e.g., ignoring variations due to manufacturing defects). For example, the second optical element 15, configured off-axis, still acts as an ideal conformal reflector in the elevation direction. However, the complete incident angle α, including both azimuth and elevation components, differs from the diffraction angle β of the output diffracted wave. This difference between the incident angle α and the diffraction angle β can be, for example, significantly different angles (e.g., the azimuth and elevation components of α and β are significantly different), or in some embodiments, it can be merely a difference in the azimuth component of the relevant angle. In some embodiments, the difference can be small, for example, sufficient to direct radiation back to a third optical element located at a different position but close to the first optical element. Figure 5In an exemplary embodiment, only the azimuth component Due to the off-axis arrangement of the second optical element 15, the elevation component θ is preserved. More specifically, the azimuth component... Reflection occurs around a local radius of curvature in the plane of the flat substrate 11, meaning that the second optical element 15, in an off-axis configuration, acts similarly to a mirror in this plane. For the off-axis configuration, it is also typical that the local radius of curvature forms a non-zero angle with respect to the line connecting the centers of the first and second optical elements 14, 15 in this plane.

[0110] For embodiments of the invention that rely on the reflection quality of the surface portion 161 of the second substrate 16, the tilt angle (defined relative to the flat surface of the first substrate 11 or the waveguide layer 12 (patterned within or on the first substrate 11 it faces)) should be carefully controlled to ensure proper operation of the integrated wavelength selective filter device. Indeed, a tilt angle close to zero degrees is preferred, as shown in the following sensitivity analysis of possible tilt angles. A substantially zero tilt angle corresponds to the case where the two flat surfaces of the first and second substrates facing each other are parallel. Brief Reference Figure 5 and the xyz coordinate system defined therein, with two possible tilt angles ω x and ω y These represent the rotation of the mirrored surface portion 161 or the second substrate 16 on which the surface portion 161 is formed about the x-axis and y-axis, respectively. Note that rotation about the x-axis causes displacement of the light rays propagating between the first integrated grating 14 and the second integrated grating 15 only in the yz plane. However, rotation about the y-axis will cause displacement of the light rays propagating between the first integrated grating 14 and the second integrated grating 15 only in the xz plane.

[0111] Regarding the tilt angle ω about the x-axis x In the embodiment, the following displacement Δy of the light ray along the y-direction is observed, wherein the second optical element 15 diffracts the incident light ray back into the propagation medium and diffracts it towards the direction in which the light ray was initially emitted into the propagation medium (e.g., Figure 3 The position of the first optical element 14 in the embodiment mentioned above. The reverse diffracted rays deviate beyond the critical distance Δy. crit The result could be that the back-propagating light misses the aperture of the first optical element 14 and therefore does not couple out of the wavelength-selective filter device as originally intended. That is, in operation, the tilt angle ω... x It is precisely controlled or adjusted so that Δy < Δy crit Geometry teaches that displacement Δy can be expressed as...

[0112]

[0113] Here, h is a reference distance in the z-direction at which the reflective surface portion 161 (e.g., a mirror) is positioned relative to the flat surface of the first substrate 11 (e.g., the waveguide layer 12 formed thereon). For small tilt angles ω x The following first-order approximation holds:

[0114] Δy≈8ω x h.

[0115] For example, assuming that the critical displacement Δy is permissible at a distance h = 1 mm. crit =3μm, which means that the tilt angle ω of the mirror surface portion 161 (mirror) relative to the flat surface of the first substrate 11 (e.g., waveguide layer 12) is 3μm. x The tilt angle ω is precisely controlled during the attachment and positioning of the mirror surface portion 161, or precisely adjusted during equipment operation, to ensure that the tilt angle ω is within acceptable limits. x Not exceeding 0.02 degrees (radians). Continue with respect to the tilt angle ω about the y-axis. y Based on the above sensitivity analysis, the following displacement δx of the light ray along the x-direction is observed on the forward path from the first optical element 14 emitting the light ray toward the second optical element 15:

[0116]

[0117] For small tilt angles ω y A first-order approximation is made, where h is a reference distance in the z-direction at the first optical element 14, at which the mirror surface portion 161 (e.g., a mirror) is positioned relative to the flat surface of the first substrate 11 (e.g., the waveguide layer 12 formed thereon). Angle α refers to the reference deflection angle (first angle) of the first optical element 14 and also corresponds to the incident angle of the second optical element 15; this same angle α in Figure 1 The second optical element 15 (e.g., a chirped Littoral grating with a chirped period D(s = x)) is designed to reflect the reference wavelength λ back if the incident angle is equal to α at the reference distance x = x0 = 2h tan(α) without the tilt of the mirror surface portion 161. c The incident ray at point α. With a tilt angle ωy about the y-axis, the second incident angle shifts to α+2ω. y Furthermore, the corresponding diffraction angle β of the second optical element 15 no longer satisfies the Littorh condition α = β at the shift distance x = x0 + δx, but deviates slightly.

[0118]

[0119] Thus, after another reflection at the inclined surface portion 161, when returning to the first optical element 14 (e.g., x = 0), the back-diffracted ray is displaced at the first optical element 14 (e.g., x = 0) relative to its (emission) deflection point. Then, analytical geometry teaches that the final displacement Δx at the first optical element 14 (e.g., where the center of its aperture is at x = 0) relative to the deflection point is given by the following equation:

[0120]

[0121] Δx≈4ω y h.

[0122] Here, a small tilt angle ω is used again. y The first-order approximation. Therefore, the similarity critical displacement Δx of the mirror surface portion 161 at the first optical element 14 (e.g., at x = 0 mm) is... crit =3μm and height distance h=1mm, for tilt angle ω y The control requirements are not so stringent; for example, a critical tilt angle ω can be tolerated. xcrit Twice the value, for example, 0.04 degrees.

[0123] The tilt sensitivity analysis described above is particularly useful when determining the alignment error tolerance of a reflective element 161 (e.g., a mirror) positioned above or below the substrate supporting the first and second optical elements and aligned with it.

[0124] In an embodiment where another optical element 14a is used in combination with the second optical element 15 in an off-axis arrangement to couple radiation out of the filter device, for example in Figure 3 or Figure 5 In the embodiments shown, tilt sensitivity analysis can be repeated.

[0125] Figure 2This is a schematic cross-sectional view of an optical integrated filter 20 according to a second specific embodiment of the first aspect of the invention. Unlike the first embodiment, there is no reflective surface 161 for folding the propagation path of the light ray 19, and the second integrated grating 15 is formed in different waveguide layers 162 on different substrates 163. The substrates 163 are bonded or otherwise attached to the support 18 such that the contour surface of the second integrated grating 15 faces the waveguide layer 12. This can be achieved by dicing and flip-chip mounting a similar photonic integrated chip including the second integrated grating 15 onto a main photonic integrated chip including the input port 13, waveguide 131, and the first integrated grating 13. Similar photonic integrated chips from the same wafer (e.g., a silicon-on-insulator wafer carrying the photonic chip) have the advantage of highly uniform manufacturing results, such as highly uniform etch depth, waveguide linewidth, etc. This increases the good matching of the material or geometric properties of the first and second integrated gratings 14, 15 relative to their different chips. However, it may also be advantageous to manufacture the second integrated grating 15 on a different platform, thus providing more options for materials and design parameters.

[0126] In order to increase the spatial separation of optical signals with wavelengths different from the center wavelength of filter 20, the height h of support 18 can be greater than the height h of the first embodiment.

[0127] Preferably, the space 17 defined at least partially by the support 18 and the second substrate 163 is filled with air, but other transparent dielectric materials are also possible.

[0128] By way of explanation, the embodiments of the present invention are not limited thereto, and can be interpreted based on the following considerations. Figure 1 The example operation of the temperature-insensitive reflective passband filter of the exemplary system shown is illustrated.

[0129] exist Figure 1 In the example, the incident radiation is first diffracted by the grating coupler 14 and then reflected in the back by the chirped Littoral grating 15.

[0130] The coupling at the grating coupler can be expressed as Equation 1, but it is assumed that the refractive index is n. a Air or vacuum with a density of 1 is the medium through which light propagates between the first optical element 14 and the second optical element 15, thus yielding the following equation:

[0131]

[0132] Where α is the central angle (deflection angle) of the radiation diffracted by the grating coupler 14 relative to the normal of the grating surface, and n eff Λ is the effective refractive index and Λ is the grating period of the grating coupler.

[0133] The coupling at the chirped Littoral grating can be expressed by the following equation, still assuming a refractive index of n. a =1 air or vacuum is the medium for light propagation between the first optical element 14 and the second optical element 15:

[0134]

[0135] or

[0136]

[0137] Where K Lg It is the local grating K vector of the Littoral grating, and D is its local grating period or pitch.

[0138] A ray of light leaving the grating coupler at a general angle α (not limited to the central angle) will fall on the Littoral grating at a distance s as defined by the following formula:

[0139] s = 2h tanα

[0140] At this position, the pitch D must be Therefore, the chirp pitch is described by the following formula.

[0141] in

[0142]

[0143]

[0144] Therefore, a given chirp rate will only occur for one value (h). However, the total D scaling will simply scale the passband λ.

[0145] Temperature insensitivity is affected by both factors. Temperature difference leads to thermo-optical effects (changes in effective refractive index) and thermal expansion.

[0146] The change in effective refractive index has no effect on the spectral operation of the filter because the thermo-optical effect of the propagation medium filling the confined space 17 is several orders of magnitude smaller than the thermo-optical effect associated with the effective refractive index of the first optical element 14. The change in effective refractive index n eff The resulting change in angle α has no effect on the operation. If the propagation medium is a gas, such as air or a vacuum (as a rarefied gas), where dn a / dT<<dn eff / dT (e.g., dn) a / dT≈10 -6 <<10 -4 ≈dn eff If / dT), then this is usually correct.

[0147] Regarding thermal expansion, the following factors need to be considered: In the current example, when the chip is considered a silicon-based chip, all dimensions of the chip will be measured by the coefficient of thermal expansion. Increase. As a result, the new position s of a specific grating tooth. new It is given by the following formula:

[0148]

[0149] At the same time, the grating period itself will also increase.

[0150]

[0151] The support has also been enlarged. Assume the support material has a coefficient of thermal expansion. This results in a support distance of h. new

[0152]

[0153] The corresponding new wavelength λ caused by thermal expansion new (relative to the old wavelength λ) old It can be determined as follows:

[0154] A ray with a deflection angle of α (equal to the second incident angle) will fall on the grating at the following positions.

[0155] s new =2h new tanα

[0156] At this location 's', you will find the original location 's'. new The pitch at -Δs. Therefore, this pitch is

[0157]

[0158] However, the pitch itself has also increased. Therefore, the pitch is given by the following formula.

[0159]

[0160] in

[0161]

[0162] The new wavelength λ diffracted by Littrow at this location new Given by the following formula

[0163]

[0164] When using a first-order expansion

[0165]

[0166] For the case where α = 45 degrees, this will lead to The situation where the expansion rate of the support should be three times that of silicon can be addressed by modifying the above conditions to resolve the thermo-optical effect caused by the propagation medium extending along the light path between the first optical element 14 and the second optical element 15. In this case, the effect arises from the change in the refractive index of the propagation medium with temperature (e.g., dn). a Another first-order correction term of / dT) will cause a new chirp pitch change of (1-ΔT d(ln n)). a The amount of ) / dT) leads to the following situation:

[0167]

[0168] Regarding the foregoing derivation, the coefficient of thermal expansion of silicon has been used for silicon-based chips, such as silicon-on-insulator (SOI) chips having a silicon waveguide layer 12 formed on or within a substrate 11 (e.g., a substrate including a buried oxide layer). After carefully considering the residual tensile stress and thickness values ​​of the upper waveguide layer and the substrate layer (or layers in a stack) directly contacting the waveguide layer from below, those skilled in the art will know which coefficient of thermal expansion is relevant. For example, those skilled in the art will know that a sufficiently thick silicon waveguide layer will mitigate thermally induced stresses caused by the substrate material (e.g., stresses caused by strain applied by the buried oxide layer), and that the coefficient of thermal expansion of silicon dominates the expansion effect in the waveguide layer. However, similar to a bimetallic strip of a thermometer, the thermal expansion of a thin silicon waveguide layer will be limited by the substrate layer directly contacting it below, and will build up internal stresses. In such cases, those skilled in the art will recognize that the coefficient of thermal expansion of the substrate layer (e.g., the buried oxide layer) is more relevant and substitute it into the foregoing derivation.

[0169] Furthermore, the sensitivity to errors in the support height is discussed through explanation. The design is assumed to be based on given h = h0, given λ = λ0, and a given angle α = α0 (i.e., the central direction of the beam from the first optical element). Considering...

[0170]

[0171] This equation describes the spectral shift caused by a small error (denoted as Δh) in height h, given a deflection angle α. For a deflection angle α = 90 degrees, this expression is first-order invariant. Therefore, in advantageous embodiments, a large deflection angle α is used.

[0172] In a second aspect, the present invention relates to an integrated system for providing a stable reference wavelength. The system includes an integrated wavelength-selective filter device according to embodiments of the first aspect, an integrated broadband light source, and at least one light guide coupled to the light source and to a first optical element of the wavelength-selective filter device for providing broadband light to the first optical element and for extracting radiation having a reference wavelength from the first optical element. In some embodiments, the system further includes a plurality of integrated detector elements. The integrated wavelength-selective filter is then adapted to diffract radiation of different wavelengths to different predetermined locations to direct radiation of different wavelengths to different integrated detector elements. The integrated system can then be an integrated spectrometer. Thus, a single off-axis oriented second optical element is used to select different wavelengths for different spectral channels of the spectrometer. Further features and advantages may correspond to those described with respect to embodiments of the first aspect.

[0173] In a third aspect, the present invention relates to an integrated spectrometer system comprising a plurality of integrated wavelength-selective filter devices, a plurality of light-guiding circuits, and a plurality of integrated detectors according to embodiments of the invention. Each of the plurality of wavelength-selective filter devices is configured to have a different reference wavelength, thereby defining a set of reference wavelengths, and each of the plurality of light-guiding circuits is adapted to receive a small portion of an externally applied radiation signal and transmit it to one of the plurality of wavelength-selective filter devices for obtaining radiation at a specific reference wavelength and for directing the obtained radiation to one of the plurality of detectors. Thus, in these embodiments, the plurality of integrated wavelength-selective filter devices are used to provide a specific wavelength for a specific spectral channel of the spectrometer. Further features and advantages may correspond to the features and advantages of the embodiments of the first aspect.

[0174] Although the invention has been described and illustrated in detail in the accompanying drawings and the foregoing description, such description and illustration are to be considered illustrative or exemplary, and not restrictive. The foregoing description specifically illustrates certain embodiments of the invention. However, it should be understood that the invention can be implemented in many ways, regardless of how detailed the foregoing appears in the text. The invention is not limited to the disclosed embodiments.

[0175] For example, refer to Figure 6Alternative embodiments of the wavelength selective filter device are described. A schematic cross-sectional view of an integrated wavelength selective filter device 30 is shown. The integrated wavelength selective filter device 30 includes two integrated gratings as first and second optical elements 14, 15, which are arranged on a flat surface of the device, for example, patterned onto the same top surface of a substrate including substrate layers 11, 12a-b. A non-limiting example of substrate layers 11, 12a-b may be a buried oxide layer 11 and a silicon waveguide layer 12a at least partially covered by an oxide cladding layer 12b. The integrated wavelength selective filter device 30 also includes a reflective surface portion 161 that forms part of a second substrate 16 located below the plane on which the first and second optical elements 14, 15 are arranged. A support 18 also included in the integrated wavelength selective filter device 30 is provided as a columnar or wall-like stack of materials, comprising, for example, four different material layers 11, 18a, 18b, and 18c. The support partially defines a space 17 filled with a propagation medium (e.g., an air-filled space 17) such that directional received light propagates from the first optical element 14 through the space 17 filled with the propagation medium to the second optical element 15, and returns after being back-diffracted by the second optical element 15. In this particular embodiment, the support 18 is partially formed from a material layer 11 of the first substrate, for example by etching trenches or cavities or complete channel openings through the substrate layer 11, such as by deep reactive ion etching. Thus, it is advantageous that the support can be formed simultaneously with the defined space of the propagation medium. Three stacked material layers 18a, 18b, and 18c are optional and can be provided for attaching the second substrate 16 and / or for precisely controlling the height (h) of the support 18. Additionally, the thickness and material composition of each of the three stacked material layers 18a, 18b, and 18c can be selected separately. This is advantageous because the relationship between the coefficients of thermal expansion of the support material and the substrate material 12 to which the first and second optical elements 14, 15 are patterned can be obtained as an average across all the material layers included in the support 18. For example, the coefficient of thermal expansion of the support material is approximately three times that of silicon in the substrate material 12, which may be difficult to obtain or control in a very precise manner. In this case, providing a stack of material layers as the support material can alleviate this problem, because the composition and thermal expansion properties of each individual material layer 18a, 18b, and 18c included in the support 18 can be designed individually.

[0176] The efficiency of the grating coupler serving as the first optical element 14 can be further improved by locally depositing a thin layer of reflective metal (e.g., gold) on top of the grating structure of the first optical element.

[0177] In this variant of the embodiment, the three additional material layers 18a-c may not be present within the support 18. Therefore, the second substrate 16, including the reflective surface portion 161, can be directly attached to the bottom surface of the substrate (material) layer 11. This variant can be particularly advantageous if precise positioning of the reflective surface portion 161 relative to a flat surface on which the first and second optical elements 14, 15 are disposed, is desired, since the back surface of the first substrate is typically very smooth and flat, such as the back surface of a wafer-processing photonic chip substrate. Therefore, only a small tilt error is expected for the reflective surface portion 161.

[0178] Furthermore, refer to Figure 7 Alternative embodiments of the wavelength selective filter device are described. A schematic cross-sectional view of an integrated wavelength selective filter device 40 is shown. The integrated wavelength selective filter device 40 includes two integrated gratings as first and second optical elements 14, 15, arranged on two different but parallel flat surfaces of the device, for example, patterned on the top surfaces of first and second substrates, wherein the first substrate may include substrate layers 11, 12a-b, and the second substrate may include substrate layers 162, 163. A non-limiting example of substrate layers 11, 12a-b may be a buried oxide layer 11 and a silicon waveguide layer 12a at least partially covered by an oxide cladding layer 12b. Similarly, substrate layers 162, 163 may comprise silicon and buried oxide, respectively. A first opening may be etched into or through the first substrate to provide a partially defined space 17, which includes a propagation medium (e.g., air) and at least a portion of a support 18. For example, the remaining substrate material of substrate layer 11 may provide part or all of the support 18. Optionally, the support 18 may further include further material layers 18a, 18b, which may facilitate adjustment of the height h of the support 18 and / or facilitate the design of the average / combined coefficient of thermal expansion of the support 18. The second substrate, including the second optical element 15, can be directly attached to the back side of the first substrate (e.g., to the bottom surface of substrate layer 11) or to the surface of one of the additional material layers 18a, 18b (which may optionally be included in the support 8). No additional reflective surface is required in this embodiment.

[0179] In carrying out the claimed invention, other variations of the disclosed embodiments may be understood and implemented by those skilled in the art from a study of the drawings, this disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a / an" does not exclude a plural. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that combinations of these measures cannot be advantageously used. Any reference numerals in the claims should not be construed as limiting the scope.

Claims

1. A wavelength selective filter device (10; 20; 30; 40) integrated on a photonic chip, the wavelength selective filter device comprising: A first optical element (14) is patterned on or within a substrate (11, 12), the first optical element (14) being configured to receive radiation incident thereon and to direct said received radiation at least partially into a direction defined by a first angle (α) to form directional radiation. A second optical element (15) is patterned on or within a substrate (11, 12, 16). The second optical element (15) is a diffractive element and extends longitudinally and is arranged away from the first optical element (14). The second optical element (15) is configured to receive the directional radiation at an incident angle and is adapted to diffract the directional radiation at a second angle (β) to form diffracted radiation, the second angle depending on the wavelength. The first optical element (14) directs the received radiation into a propagation medium, which propagates purely through the propagation medium until it is received at the second optical element (15). The propagation medium is formed of a material different from that of the substrate of the first and second optical elements. The second optical element (15) is configured such that the second angle is as follows: for a single reference wavelength, the diffracted radiation is directed into the propagation medium to travel in the propagation medium toward a predetermined position on the first optical element (14) in order to filter out radiation with a wavelength substantially matching the reference wavelength from radiation with significantly different wavelengths, thereby uniquely determining the reference wavelength for the entire device, and The second optical element (15) is adapted to reflect the directional radiation back into the propagation medium to travel in the same elevation direction (θ) in the propagation medium, but with a propagation direction opposite to that of the radiation incident on the second optical element (15), the elevation direction (θ) being defined relative to the flat surface of the substrate of the second optical element (15).

2. The wavelength selective filter device (10; 20; 30; 40) as described in claim 1, characterized in that, The propagation medium is formed of a material whose thermo-optical refractive index coefficient is at least one order of magnitude smaller than that of any material of the substrate of the first optical element and the second optical element (14, 15).

3. The wavelength selective filter device (10; 20; 30; 40) as described in claim 1, characterized in that, The filter device includes a support material having a coefficient of thermal expansion, such that, for at least a change in the second angle caused by a temperature change in the device, the length of the propagation path of the directional radiation from the first optical element to the second optical element in the propagation medium is automatically adapted by the thermal expansion of the support material due to the temperature change, such that, for the reference wavelength, the diffracted radiation is again directed to the predetermined position.

4. The wavelength selective filter device (10; 30) as described in any one of claims 1 to 3, characterized in that, The first optical element and the second optical element are patterned in or on the same substrate, or the first optical element and the second optical element are formed in the semiconductor or dielectric substrate of the photonic chip.

5. The wavelength selective filter device (10; 30) as described in any one of claims 1 to 3, characterized in that, The filter device includes a reflective element (161) such that the directional radiation is reflected by the reflective surface of the reflective element (161) along the path of propagation of the directional radiation from the first optical element to the second optical element in the propagation medium.

6. The wavelength selective filter device (10; 30) as described in claim 5, characterized in that, Both the first optical element (14) and the second optical element (15) are formed on the same flat surface of the device, and the reflective element (161) is located above or below the flat surface.

7. The wavelength selective filter device (10; 30) as described in claim 6, characterized in that, The device further includes at least one support (18) for supporting the reflective element (161) located above or below the flat surface.

8. The wavelength selective filter device (10; 30) as described in claim 7, characterized in that, The adaptation to the path length is achieved through the thermal expansion of the at least one support (18), and The first optical element and the second optical element are patterned in or on the same substrate, or the first optical element and the second optical element are formed in a semiconductor or dielectric substrate of the chip.

9. The wavelength selective filter device (10; 30) as described in claim 8, characterized in that, Both the first optical element (14) and the second optical element (15) are patterned in or on the same substrate material, and the support comprises at least one material selected such that its total coefficient of thermal expansion is two to four times that of the substrate material of the first optical element and the second optical element.

10. The wavelength selective filter device (10; 30) as described in claim 7, characterized in that, By partially removing a portion of the material of the substrate, the at least one support (18) is formed at least partially of the material of the substrate of the first and second optical elements (14, 15).

11. The wavelength selective filter device (10; 30) as described in claim 9, characterized in that, It exhibits temperature insensitivity of less than 1 pm / K over a temperature range of 100K.

12. The wavelength selective filter device (10; 20; 30; 40) as described in any one of claims 1 to 3, characterized in that, The first optical element (14) is located above the second optical element (15) or the second optical element (15) is located above the first optical element (14), and the diffraction radiation propagates along a straight path through the propagation medium.

13. The wavelength selective filter device (10; 20; 30; 40) as described in any one of claims 1 to 3, characterized in that, The first optical element (14) is connected to the waveguide for transmitting optical input and output signals into and out of the filter device.

14. An integrated system for providing a stable reference wavelength, the system comprising: -The wavelength selective filter device as described in claim 1 -Integrated broadband light source, and - At least one light guide, said at least one light guide being coupled to the light source and coupled to the first optical element of the wavelength selective filter device for providing broadband light to the first optical element.

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