Patent Description:
Narrowband radiation sources are needed for various applications, e.g., optical sensors in the field of optical absorption sensor applications. For optical sensing, the mid-infrared region from about <NUM> to <NUM> is, for example, interesting as many environmental gasses have a unique fingerprint in this wavelength region, which allows the development of a selective absorption sensor and minimize cross sensitivities.

<CIT> relates to the formation of an optical waveguide type diffraction grating in an optical waveguide portion, such as an optical fiber or a thin-film waveguide. In a first stage, an optical fiber is irradiated with luminous fluxes for irradiation via an exposure mask and an optical system. This optical fiber has a core added with Ge. The refractive index of the core part increases when the core part is irradiated with light near a wavelength of <NUM>. A refractive index change is induced in the core part by irradiating the core part with the UV rays of such wavelength as the luminous fluxes for irradiation. The optical system is a cylindrical lens. In a second stage, the core part of the optical fiber in which the refractive index change is induced in the first stage is irradiated with the luminous fluxes for irradiation via a phase mask, by which a diffraction grating forming a part is formed.

<CIT> relates to the manufacturing method of a transmissive fiber grating filter which implements a bandpass filter only by the grating of an optical fiber. A phase shift mask having mask patterns of a linearly changing pitch is fixed to an optical fiber in tight contact therewith. Only the sections P of the pitch corresponding to the prescribed wavelength of these mask patterns are covered with a shielding plate. The optical fiber fixed with the phase shift mask is irradiated with a UV laser beam at a uniform velocity along its longitudinal direction, by which chirp gratings lacking in the parts corresponding to the prescribed wavelength, i.e., the wavelength to be transmitted.

<CIT> relates to a single-element optical wavelength bandpass filter formed in an optical fiber or optical waveguide and to a method of manufacturing thereof. The optical wavelength filter having one or more bands of transmission wavelengths of light, within a reflection spectrum of the filter, is formed using a section of an optical fiber as an optical fiber grating. The optical fiber core is configured with a periodically varying diffraction coefficient structure formed such that the grating pitch continuously varies in a fixed direction, with that structure containing one or more regions of interruption of the continuous variation of pitch, whereby the bands of transmission wavelengths are respectively defined by these interruption regions.

<NPL>) relates to a wide-stopband chirped fiber moiré grating transmission filter.

<NPL>," relates to integrated band-pass transmission filters based on Sagnac loops incorporating Bragg gratings in SOI. It demonstrates integrated bandpass transmission filters based on incorporating uniform and chirped Bragg gratings in Sagnac loops in SOI. The latter is used as an optical spectral shaper to generate chirped microwave waveforms. The chirped Bragg grating is based on tapering the waveguide width.

However, currently used radiation sources need to be tailored to the specific application, e.g., for mid-infrared emitter applications. Therefore, there is a need in the art for an approach to implement a bandpass transmission filter (= bandpass filter) offering a combination of a narrow transmission band and a spectrally broad stop-band, and providing a relatively low complexity of structural filter design resulting in an inexpensive filter fabrication and radiation source fabrication.

Such a need can be solved by the bandpass transmission filter according to independent claim <NUM> and the narrowband radiation source according to claim <NUM>.

The present invention provides for bandpass transmission filter according to claim <NUM>. Further developments of the invention are defined in the dependent claims. Any embodiments and examples of the description not falling within the scope of the claims do not form part of the invention and are provided for illustrative purposes only.

Using the bandpass transmission filter having a spectrally broad stop-band with a narrow transmission band at the center wavelength λ<NUM> of transmission in combination with a broad-band emitter, e.g., a thermal emitter, allows the creation of a narrowband radiation source. The filtered radiation which is transmitted through the bandpass transmission filter can be coupled, for example, into any kind of waveguides, e.g., a slab waveguide , a strip waveguide, a photonic crystal waveguide, etc., which can be used as sensor element for sensing the presence of a target gas in the environment of the waveguide.

Embodiments of the present bandpass transmission filter and the narrowband radiation source are described herein making reference to the appended drawings and figures.

Before discussing the present embodiments in further detail using the drawings, it is pointed out that in the figures and the specification identical elements and elements having the same functionality and/or the same technical or physical effect are usually provided with the same reference numbers or are identified with the same name, so that the description of these elements and of the functionality thereof as illustrated in the different embodiments are mutually exchangeable or may be applied to one another in the different embodiments.

In the following description, embodiments are discussed in detail, however, it should be appreciated that the embodiments provide many applicable concepts that can be embodied in a wide variety of semiconductor devices. The specific embodiments discussed are merely illustrative of specific ways to make and use the present concept, and do not limit the scope of the embodiments. In the following description of embodiments, the same or similar elements having the same function have associated therewith the same reference signs or the same name, and a description of such elements will not be repeated for every embodiment. Moreover, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

It is understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or intermediate elements may be present. Conversely, when an element is referred to as being "directly" connected or coupled to another element, there are no intermediate elements. Other terms used to describe the relationship between elements should be construed in a similar fashion (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", and "on" versus "directly on", etc.).

For facilitating the description of the different embodiments, the figures comprise a Cartesian coordinate system x, y, z, wherein the x-y-plane corresponds, i.e. is parallel, to the first main surface region of a substrate, and wherein the depth direction vertical to the first main surface region and into the substrate corresponds to the "z" direction, i.e. is parallel to the z direction. In the following description, the term "lateral" means a direction parallel to the x-direction, wherein the term "vertical" means a direction parallel to the z-direction.

<FIG> shows a schematic cross-sectional view of bandpass transmission filter <NUM> having a center wavelength λ<NUM> of transmission according to an embodiment.

As shown in <FIG>, the bandpass transmission filter (= bandpass filter) <NUM> having a center wavelength λ<NUM> of transmission comprises a waveguide structure <NUM>, a grating structure <NUM> in the waveguide structure <NUM>, and a radiation absorbing structure <NUM>. The grating structure <NUM> has changing grating pitch values Λi for diffracting a radiation "R" propagating in the waveguide structure <NUM> having a wavelength λ<NUM> which is lower than the center wavelength λ<NUM>, with λ<NUM> < λ<NUM>, and for reflecting, e.g., by means of a Bragg reflection, a radiation R in the waveguide structure <NUM> having a further wavelength λ<NUM> which is higher than the center wavelengths λ<NUM>, with λ<NUM> > λ<NUM>.

As shown in <FIG>, the grating structure <NUM> may be formed in a first main surface region (or front face) <NUM>-A of the waveguide structure <NUM>. According to an embodiment, the grating structure <NUM> is formed in at least one of the faces <NUM>-A - <NUM>-D of the waveguide structure <NUM>, wherein the top and bottom faces <NUM>-A and <NUM>-B of the waveguide structure <NUM> are vertically opposing to each other (see <FIG>), and wherein the side faces (= sidewalls) <NUM>-C and <NUM>-D of the waveguide structure <NUM> are laterally opposing to each other (see <FIG>, for example).

The radiation absorbing structure <NUM> may be an integrated part of the waveguide structure <NUM> or is formed as a layer, e.g., a dielectric layer, arranged adjacent to the waveguide structure <NUM>, for absorbing a radiation R guided by the waveguide structure <NUM> having a wavelength λ<NUM> higher than the wavelength λ<NUM>, with λ<NUM> < λ<NUM> < λ<NUM>.

According to an embodiment, the waveguide structure <NUM> is arranged on a substrate <NUM>, e.g., a semiconductor substrate, such as a silicon substrate, or a glass substrate, as exemplarily shown in <FIG>.

According to an embodiment, the change in grating pitch values Λi depends on the index i according to at least one of a linear function, an exponential function, a polynomial function or a combination thereof. For example, the changing grating pitch values Λi may be defined as a combination, e.g. a sum or a product, of a basis pitch value Λ<NUM> and a variable pitch value ΔΛi, wherein the variable pitch value ΔΛi is at least one of linearly changing, exponentially changing and polynomially changing with "i" or a combination thereof, with Λi=Λ(i).

According to an embodiment, the grating structure <NUM> may be formed as a "chirped" grating structure having monotonically changing (e.g. increasing or decreasing) grating pitch values Λi.

According to the claimed invention, the radiation absorbing structure <NUM> at least partially covers the bottom face <NUM>-B of the waveguide structure <NUM>. As shown in the embodiment of <FIG>, the radiation absorbing structure <NUM> covers the second main surface region (= bottom face) <NUM>-B of the waveguide structure <NUM>.

With respect to <FIG>, the following description provides a summary of the physical characteristics for designing the bandpass transmission filter <NUM> having the center wavelength λo of transmission. To be more specific, <FIG> shows a schematic exemplary representation of the diffraction regime, the Bragg reflection regime and sub-wavelength regime. It is to be noted that the regimes are depicted in <FIG> for a constant periodicity of the grating structure <NUM> (top) and for a constant wavelength (bottom).

In general, depending on the center wavelength λo of transmission of the bandpass transmission filter <NUM> and the pitch Λi of the grating structure <NUM>, the radiation R can either be diffracted out of the waveguide <NUM> (= radiation regime), reflected back within the waveguide (= Bragg reflection regime) or not be influenced at all, e.g., influenced only in terms of the propagation constant (sub-wavelength regime), as shown in <FIG>. The transmission band is formed in the transmission window between the diffraction and Bragg reflection regime.

As a Bragg grating would lead to a narrowband rejection filter at the design wavelength λo (center wavelength), the bandpass transmission filter <NUM> comprises the waveguide structure <NUM> with the grating structure <NUM> with changing grating pitch values Λi in the waveguide structure <NUM>, e.g., a chirped grating structure, in order to widen the diffraction regime and the reflection regime, i.e., in order to broaden the stop band of the resulting bandpass transmission filter <NUM>. At the same time the transmission window between the diffraction and Bragg reflection regime is narrowed, leading to a narrow transmission band. However, this approach on its own is limited, because the parts of the grating structure <NUM> with large grating constants will cause short wavelengths λ<NUM> of the guided radiation R to be diffracted out. At a certain point, this would also lead to a diffraction of the desired transmission wavelength λ<NUM>. Thus, the Bragg reflection regime cannot extend over the whole wavelength spectral range. Therefore, the combination of a grating structure having changing grating pitch values Λi, e.g., a chirped grating structure, with a material absorption provided by the radiation absorbing structure <NUM> is proposed according to an embodiment. The resulting filter structure allows the suppression of long wavelengths by tuning the waveguide dimensions and by choosing materials with optical parameters that meet the needs of the design. In total, the bandpass transmission filter <NUM> combines three physical effects in the form of diffraction, Bragg reflection and absorption.

To summarize, the bandpass transmission filter <NUM> uses diffraction to filter small wavelengths λ<NUM> of the guided radiation R, i.e. with λ<NUM> < λ<NUM>, uses Bragg reflection to filter the wavelength λ<NUM> of the propagating radiation R longer than the desired wavelength (= center wavelength λo) with λ<NUM> > λ<NUM>. As the Bragg reflection regime cannot be arbitrarily extended, since at a certain point, it would lead to a diffraction of the desired wavelength λ<NUM>, an absorption (= absorbing layer <NUM>) is used for filtering long wavelengths λ<NUM>, with λ<NUM> < λ<NUM> < λ<NUM>, in the sub-wavelength regime.

<FIG> shows a schematic plot of a combination of different Bragg gratings for providing a narrow transmission window between the diffraction and Bragg reflection regime. The illustration shows how the narrow transmission band can be created with a chirped grating. For this illustration three different Bragg gratings I, II, III are combined, in order to narrow down the transmission window between the diffraction and Bragg reflection regime. The combination of all three gratings yields the combined characteristics shown in course IV with a thermal emitter, for example.

<FIG> shows a schematic cross-sectional view (parallel to the x-z-plane) of the grating structure <NUM> on top of a considered waveguide platform, i.e. the waveguide structure <NUM>, for illustrating a diffraction of short wavelengths (left figure) and a reflection of long wavelengths (right figure) according to an embodiment.

Referring to <FIG>, two physical effects in the form of the Bragg reflections and the diffraction induced by the grating structure <NUM> are described in the context of guided modes within the dielectric waveguide <NUM>. Thereafter, the grating structure <NUM> having the changing grating pitch values Λi is described in detail together with simulation plots showing different filter characteristics of the bandpass transmission filter <NUM> for specific implementations.

Diffraction/Bragg reflection at the grating structure: The grating structure is based on a modulation of the refractive index, e.g., based on a periodical modulation, an aperiodic modulation and/or monotonically varying (increasing or decreasing) modulation of the refractive index n<NUM>. Such a modulation of the refractive index n<NUM> of the grating structure <NUM> can be achieved, for example, by modifying the optical material properties of the waveguide structure <NUM> or by modifying the geometry of the waveguide structure <NUM>, e.g., by a modification of the height and/or the width of the waveguide structure <NUM>, which also leads to a modification, e.g., a periodic or aperiodic modification, of the effective mode index of the propagating wave neff(<NUM>) and the resulting grating characteristics of the grating structure <NUM> in the waveguide structure <NUM>.

<FIG> depicts the wave vectors of a mode propagating in a waveguide <NUM> with a grating structure <NUM> on top, for small wavelengths (a) and for the Bragg-wavelength (b) of the radiation R. Small wavelengths are diffracted, while the specific wavelength, where the Bragg condition holds , is reflected. Longer wavelengths ( <MAT>) ideally just see an effect on their propagation constants, but are not diffracted or reflected at all from the grating.

Diffraction: The grating equation for a periodic structure can be written as: <MAT>.

Where θ<NUM>, is the incoming angle, θ<NUM>m the diffracted angle for the order m, λ is the wavelength in the material and Λ is the pitch of the grating.

In the considered case, the grating is formed on a material interface (nAir and n<NUM> = nSi).

For convenience, here ni is assumed to be purely real. Therefore the different propagation velocities have to be accounted for by adding the refractive indices of the materials to Equation <NUM>. For diffraction into air (= environment atmosphere), SiO<NUM> (= further dielectric layer <NUM>) and Si<NUM>N<NUM> (= dielectric layer <NUM>), respectively, this leads to: <MAT>.

The angle of the wave vector of a waveguide mode can be calculated using <MAT>.

Where neff is the effective index of the waveguide mode. Inserting (<NUM>) into (<NUM>) leads to the angle at which the radiation R is reflected out of the waveguide <NUM>: <MAT>.

Bragg reflection: A special case is the Bragg reflection, where the radiation R is reflected from the periodic structure <NUM> (see also <FIG> - right). Starting again from Equation <NUM>, this reflection occurs if sin θ<NUM> = -sin θ<NUM>. Inserting this condition into Equation <NUM> and using m=<NUM>, leads to <MAT> for the waveguide grating <NUM>. The result is the condition for Bragg reflection in the grating <NUM>.

Changing (Chirped) Grating Filter: The two effects described above are used to form a narrow transmission band. The Bragg reflection is a narrow band effect, i.e. a grating with a single pitch would only reflect a narrow band. In order to make this band wider, the grating pitch is changed, e.g. continuously increased (chirped), which leads to a broadening of the reflection band, which can therefore be used to narrow the transmission window between the diffraction and Bragg reflection regime.

For simplicity, in this section the chirped grating filter <NUM> realization is described for a slab waveguide <NUM>. Analogously, the bandpass transmission filter <NUM> can also be realized on different kinds of waveguides. <FIG> shows a schematic representation of the waveguide structure <NUM> with the chirped grating <NUM>. The grating structure <NUM> can be realized on a silicon or on insulator (dielectric) platform.

The changing grating pitch values Λi depend on at least one of a linear function, an exponential function and a polynomial function, wherein the changing grating pitch values Λi may comprise a combination of a basis pitch value "Λ<NUM>" and a variable pitch value ΔΛi, wherein the variable pitch value is linearly changing, exponentially changing and/or polynomially changing with "i". According to an embodiment, the grating structure <NUM> may be a chirped grating structure having monotonically changing, i.e. increasing or decreasing, grating pitch values Λi.

The changing grating pitch values Λi may be calculated using <MAT> wherein the changing grating pitch values Λi may be defined as a combination, e.g. a sum or a product, of a basis pitch value Λ<NUM> and a variable pitch value ΔΛi, wherein the variable pitch value ΔΛi is at least one of linearly changing, exponentially changing and polynomially changing with "I" or a combination thereof.

The first or basis pitch of the grating structure <NUM> may be therefore defined as Λ<NUM> = Λcenter + ΛGap. The center pitch Λcenter is calculated using a rearranged form of the Bragg condition (<NUM>), where Λ<NUM> is the vacuum wavelength at which high transmission is desired (center wavelength λ<NUM>) and neff the effective mode index. In order to avoid Bragg reflections at the center wavelength λ<NUM>, the pitch of Λ<NUM> is increased by a gap = ΛGap. The pitch of the higher number periods is calculated based on the variable pitch value ΔΛi.

To summarize, the basis pitch value "Λ<NUM>" comprises a center pitch value "ΔCenter" and an additional gap value "ΛGap", to avoid Bragg reflection at the center wavelength λ<NUM>, wherein the term "ΛCenter" depends on the function: ΛCenter = λ<NUM>ml<NUM>neff, wherein λ<NUM> is the center wave-length of transmission, neff is the effective mode index of the mode that propagates in the waveguide structure <NUM>, and m is the diffraction order.

In order to comply with industrial design rules (e.g. <NUM> grid), the continuous grating period change ΔΛi can be discretized to a minimum of e.g. <NUM> for lines and spaces (depending on the design rules). This can raise the need to repeat each individual grating period for a few times, before increasing the pitch Λi, in order to achieve a sufficient stop-characteristic. Thus, the i-th grating pitch value Λi may be repeated N-times with N = <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>,. or at least <NUM> times with N ≥ <NUM>.

According to an embodiment, the changing grating pitch values Λi may depend on a linear (e.g. increasing) function with Λi = Λ<NUM>, + ΔΛ × i, wherein "Λ<NUM>" is the basis pitch value and "ΔΛ × i" is the variable pitch value. The center pitch "ΛCenter" is <NUM> or between <NUM> - <NUM>, for example, the gap "ΛGap" is <NUM> or between <NUM> - <NUM>, for example, the increment or variation for the pitches ΔΛ is <NUM> or between <NUM> - <NUM>, for example, and the number of periods i is <NUM> or between <NUM> - <NUM>, for example for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example.

According to an embodiment, the changing grating pitch values Λi may depend on a linear (decreasing) function with Λi = Λ<NUM>, + ΔΛ × (N - i), wherein "Λ<NUM>" is the basis pitch value and "ΔΛ × (N - i)" is the variable pitch value. The center pitch "ΛCenter" is <NUM> or between <NUM> - <NUM>, for example, the gap "ΛGap" is <NUM> or between <NUM> - <NUM>, for example, the increment or variation for the pitches ΔΛ is <NUM> or between <NUM>-<NUM>, for example, and the number of periods i is <NUM> or between <NUM> - <NUM>, for example, for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example.

According to an embodiment, the changing grating pitch values Λi may depend on an exponential function with Λi = Λ<NUM> + (F<NUM>)i, wherein "Λ<NUM>" is the basis pitch value and "F<NUM> " is the variable pitch value. The center pitch "ΛCenter" is <NUM> or between <NUM> - <NUM>, for example, the gap "ΛGap" is <NUM> or between <NUM> - <NUM>, for example, and the variation for the pitches F<NUM> = <NUM> or between <NUM> and <NUM>, for example, and the number of periods i is <NUM> or between <NUM> - <NUM>, for example, for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example.

According to an embodiment, the changing grating pitch values Λi may depend on an exponential function with Λi = Λ<NUM> × (F<NUM>)i, wherein "Λ<NUM>" is the basis pitch value and "F<NUM>" is the variable pitch value. The center pitch "ΔCenter" is <NUM> or between <NUM> - <NUM>, for example, the gap "ΛGap" is <NUM> or between <NUM> - <NUM>, for example, and the variation for the pitches F<NUM> = <NUM> or between <NUM> and <NUM>, for example, and the number of periods i is <NUM> or between <NUM> - <NUM>, for example, for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example.

According to an embodiment, the changing grating pitch values Λi may depend on a polynomial function with <MAT>, wherein "Λ<NUM>" is the basis pitch value and " <MAT>" is the variable pitch value. Thus, an exemplary polynomial function third order, with n=<NUM>, would result in: <MAT>.

According to a further exemplary embodiment, the constants A, B, C may comprise the values A = <NUM>, B = <NUM>, C = <NUM>, e.g. within a tolerance range of the associated Λi of ± <NUM>% (or ± <NUM>%), for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example. The center pitch "ΛCenter" is <NUM> or between <NUM> - <NUM>, for example, the gap "ΛGap" is <NUM> or between <NUM> - <NUM>, for example, and the number of periods i is <NUM> or between <NUM> - <NUM>, for example, for achieving an exemplary center wavelength λ<NUM> of transmission with λ<NUM> = <NUM> or between <NUM> and <NUM>, for example.

According to the claimed invention, the radiation absorbing structure <NUM> at least partially covers a bottom face of the waveguide structure <NUM> having the grating structure <NUM>.

According to the claimed invention, the waveguide structure <NUM> is arranged on a substrate <NUM>, wherein a dielectric layer <NUM>, which forms the radiation absorbing structure <NUM>, and a further dielectric layer <NUM> are arranged between the waveguide structure <NUM> and the substrate <NUM>. The further dielectric layer <NUM> is formed between the dielectric layer <NUM> and the substrate <NUM>.

According to the claimed invention, the further dielectric layer <NUM> is formed between the waveguide structure <NUM> and the substrate <NUM>, wherein the further dielectric layer <NUM> has a material thickness for suppressing a coupling of the waveguide mode from the waveguide structure <NUM> into the substrate <NUM>.

According to the claimed invention, the dielectric layer (= absorbing layer) <NUM> comprises an Si<NUM>N<NUM> material. The further dielectric layer (= lower cladding) <NUM> comprises an SiO<NUM> material. The waveguide structure <NUM> comprises one of a (poly) silicon (Poly-Si), Si, Ge, AIN and As<NUM>Se<NUM> material. The substrate <NUM> may comprise at least one of Si and glass material.

In a specific example for such a platform, which was also used for the simulations below, a Si0<NUM> layer <NUM> (≥ <NUM>) may be deposited on a Si substrate <NUM> in order to suppress coupling of the waveguide mode into the substrate <NUM>. On top of the Si0<NUM> layer <NUM>, a thin Si<NUM>N<NUM> (e.g. <NUM>) layer <NUM> is used as absorbing layer for long wavelengths. The waveguide <NUM> on top may be made of silicon with a height of e.g. <NUM>. Eventually the grating <NUM> is etched into the top of the silicon waveguide <NUM>. <FIG> shows a schematic representation of the described platform.

According to a further embodiment, the waveguide structure <NUM> may comprise a waveguide extension structure <NUM>-<NUM>, wherein the radiation absorbing layer <NUM> at least partially covers a face of the waveguide extension structure <NUM>-<NUM> for reducing a sideband transmission of the bandpass transmission filter <NUM>.

The bandpass transmission filter <NUM> may be used in combination with a broadband thermal emitter for providing a narrowband radiation source, which can be employed for various applications, e.g. optical sensors.

The bandpass transmission filter (= narrow-band filter) <NUM> can be adapted to a specific application (e.g. a CO<NUM> absorption band <NUM> and a bandwidth of e.g. <NUM>). The bandpass transmission filter <NUM> provides a spectrally broad stop-band with a narrow transmission band. The combination of this bandpass transmission filter <NUM> with a broadband source (e.g. thermal emitter) leads to a narrow band radiation source.

The bandpass transmission filter <NUM> can be fabricated with low costs. When using a waveguide platform, for some embodiments, the bandpass transmission filter <NUM> can be fabricated without additional processing steps, which means no extra efforts and costs. To be more specific, the fabrication of such a bandpass transmission filter <NUM> requires no extra fabrication steps when carried out as sidewall grating <NUM>, or one extra litho/etching process when carried out on top of the waveguide <NUM>. This gives the possibility to easily create a low cost narrow band radiation source using a thermal emitter and the described bandpass transmission filter <NUM>.

A possible application of the bandpass transmission filter <NUM> may be an optical IR sensor. The bandpass transmission filter <NUM> can be combined with a thermal emitter in order to create a narrow band radiation source. The filtered radiation which is present at the end of the bandpass transmission filter <NUM> can be coupled into different kinds of waveguides (e.g. slab, strip, photonic crystals, etc.) which may act as sensor elements.

To summarize, the bandpass transmission filter <NUM> comprises a waveguide <NUM> with a changing or chirped grating <NUM>, which is a grating <NUM> with a variable grating constant. Tailoring this grating and the interaction with the used materials allows creating a narrow transmission band, adapted to the desired wavelength.

The bandpass transmission filter <NUM> provides a filter concept, which is well suited for optical integrated waveguides. The modulation is most favorably introduced by geometrical variations of the waveguide dimensions and the waveguide design can be tuned to the material characteristics.

According to the present filter concept, a changing or chirped grating <NUM> is engineered in order to create the transmission band filter <NUM>, which can be used e.g. to create narrow-band IR sources. According to the present filter concept, three physical effects are combined, diffraction, Bragg reflection and absorption, wherein a tailored waveguide structure <NUM> allows realizing the narrow-band transmission filter <NUM> with a wide spectral rejection range. The filter <NUM> can be adapted for various application and fabrication tolerances. This narrow-band transmission filter <NUM> can be implemented to most waveguide designs, without significant additional fabrication costs. According to the present filter concept Bragg gratings, and also chirped Bragg gratings, may be applied for an effective limitation of a very broad spectral rejection band. According to the present filter concept, a waveguide design and a Bragg-grating design are combined.

<FIG> shows a schematic 3D top view of different schematic representation of changing or chirped gratings <NUM> on slab and strip waveguides <NUM> as different design options according to an embodiment. As shown in <FIG> (left), the waveguide structure <NUM> may be implemented as a slab waveguide having a top face grating <NUM>. As shown in <FIG> (middle), the waveguide structure <NUM> may comprises a strip waveguide having a top face grating <NUM>. As shown in <FIG> (right), the waveguide structure <NUM> may comprise a strip waveguide having a side face grating <NUM>.

According to a further embodiment, the waveguide structure may comprise a strip waveguide having a side face grating or having a top face grating (not shown in <FIG>). According to a further embodiment, the waveguide structure may comprise a slot waveguide having a side face grating or having a top face grating (not shown in <FIG>). According to a further embodiment, the waveguide structure may comprise a rib waveguide having a side face grating or having a top face grating (not shown in <FIG>).

<FIG> shows a schematic 3D top view of the bandpass transmission filter <NUM> with a strip waveguide <NUM> having the changing or "chirped" grating structure <NUM> integrated in the sidewalls <NUM>-C, <NUM>-D of the strip waveguide <NUM> and an absorbing layer for wavelengths > λ<NUM> on top of the waveguide.

As shown in <FIG>, the waveguide structure <NUM> comprises the waveguide extension structure <NUM>-<NUM>, wherein the radiation absorbing layer <NUM> (at least partially) covers the first main surface region or top face <NUM>-A in <FIG> of the waveguide extension structure <NUM>-<NUM> for reducing a sideband transmission of the filter <NUM>. As further shown in <FIG>, (at least) the part of the waveguide structure <NUM> having the grating structure <NUM> and the waveguide extension structure <NUM>-<NUM> is sandwiched between the radiation absorbing layer <NUM> (= long wavelength absorbing material) and the further dielectric layer <NUM> (= lower cladding material).

According to the bandpass transmission filter <NUM> as shown in <FIG> the long wavelength absorber <NUM> is placed on top of the waveguide structure <NUM>, which may facilitate the fabrication process.

To summarize, <FIG> shows the bandpass transmission filter <NUM> using a strip waveguide <NUM>, which can be used in order to filter a broadband source, e.g. thermal emitter (not shown in <FIG>). Here, the grating structure <NUM> is realized on the sidewalls <NUM>-C, <NUM>-D of the waveguide structure <NUM>. Besides varying the grating period Λi for the depicted embodiment, it is also possible to arbitrarily vary the depth d<NUM> of the grating structure <NUM> in order to optimize the filter performance. In this realization, the dielectric layer <NUM> of a long wavelength absorbing material is attached on the top (first main surface region) <NUM>-A of the waveguide structure <NUM>. The length L of this layer can be designed in order to sufficiently suppress the side band transmission at long wavelengths A<NUM>. Afterwards the waveguide structure <NUM> is continued without the absorbing layer <NUM> on top in order to reduce the intrinsic losses.

In the following, an exemplary numerical investigation of such a grating structure <NUM> is presented.

According to an embodiment, the bandpass transmission filter <NUM> was designed for a center wavelength λ<NUM> = <NUM> (transmission wavelength λ<NUM>) and for TE-polarized radiation. The center pitch ΛCenter was <NUM> (calculated using Equation <NUM>), the Gap (ΛGap) was <NUM> and the variation ΔΛi was <NUM>. This leads to a variation of <NUM> for lines and spaces. The grating pitch Λi was increased <NUM> times (I = <NUM>), leading to a maximum pitch of <NUM>. The height h<NUM> of the waveguide was <NUM> and the depth d<NUM> of the grating was <NUM>. The thickness t<NUM> of the dielectric layer <NUM>, e.g. a Si<NUM>N<NUM> layer, was <NUM>, and below the Si<NUM>N<NUM> layer <NUM> the further dielectric layer <NUM>, e.g. a Si0<NUM> layer, was considered to be infinite. Due to the <NUM> grid it was necessary to repeat each pitch <NUM> times, N = <NUM>, in order to achieve the band-pass performance as illustrated below. On both ends of the bandpass transmission filter <NUM>, the waveguide structure <NUM> was elongated (with the waveguide extension structure <NUM>-<NUM>) for <NUM> pm, leading to a total filter length of <NUM>.

<FIG> shows a schematic plot of FEM (FEM = finite element method) simulation results of the transmittance T (curve I), reflection R (curve II) and the transmittance plus reflection T+R (curve III) without taking into account material absorption.

<FIG> shows a schematic plot of FEM (FEM = finite element method) simulation results of the transmittance T (curve I), reflection R (curve II) and the transmittance plus reflection T+R (curve III) taking into account material absorption.

<FIG> shows a schematic plot of FEM (FEM = finite element method) simulation results of the transmittances T through the grating filter <NUM> with material absorption (curve I) and without material absorption (curve II) and the transmittance (curve III) through the waveguide structure <NUM> in form of a slab waveguide with a length of <NUM> and taking into account the material absorption according to an embodiment.

To summarize, <FIG> show FEM simulation results with (a) and without (b) taking into account material absorption, depicting the transmittance and the reflectance as well as the sum of both. Furthermore <FIG> shows a comparison of the transmittance through the grating filter with and without material absorption, and the transmittance through a plain slab waveguide with a length of <NUM> and taking into account the material absorption.

As shown in <FIG> (curve I), the filter shows a narrow transmission band at the design wavelength (λ<NUM> = <NUM>) while there is almost no transmission for shorter wavelengths due to diffraction. The stop-band from wavelengths ~<NUM> to ~<NUM> is due to Bragg reflections in the grating structure <NUM>. Above wavelengths ~<NUM> the grating structure <NUM> has no influence on the propagation of the mode in the waveguide structure <NUM>. In the region where the sum of Reflectance and Transmittance R+T (curve III) is smaller than <NUM>, a diffraction occurs.

When taking into account also the intrinsic losses due to material absorption, the transmission at longer wavelengths drastically decreases leading to virtually no transmission at wavelengths ><NUM> (see <FIG>). Nevertheless, there is still a significant side band visible between the wavelengths <NUM> and <NUM>.

<FIG> shows comparisons of the transmittance T through the waveguide structure with a chirped grating structure <NUM> without (curve I - n"=<NUM>, from <FIG>) taking into account material absorption and with (curve II - n"= f(λ), from <FIG>) taking into account material absorption, as well as the simulated transmittance of a waveguide without grating and a length of <NUM> (curve III - taking into account material absorption). For the sake of completeness, without taking into account the material absorption, the waveguide without grating transmits up to <NUM> before strong damping is induced due to leaking of the mode.

<FIG> show schematic plots of the "diffraction angles" as a function of the wavelength, the grating pitch and the cladding material, and of the "grating pitch", which satisfies the condition for Bragg reflection as a function of the wavelength.

<FIG> depicts the diffraction angles calculated according to Equation <NUM> for the minimum and maximum pitch Λi, for the two dominant cladding materials in the form of the environmental atmosphere, e.g. air, and the further dielectric layer <NUM>, i.e. the SiO<NUM> layer. For the maximum pitch the diffraction into the further dielectric layer <NUM>, i.e. the SiO<NUM> layer, already reaches the desired transmission wavelength λ<NUM>, which is also visible in <FIG>, since the transmittance plus reflection R+T does not reach a value of <NUM>. As there are very limited periods with a high pitch, only a small fraction of the radiation R with a wavelength around <NUM> is diffracted. Nevertheless, this effect leads to limitations for the stopband of the Bragg reflection.

<FIG> shows the grating pitch which satisfies the condition for Bragg reflection as a function of the wavelength. The dashed lines mark the range for which Bragg condition is fulfilled with the used grating pitches (~<NUM> to <NUM>). As shown in <FIG>, although the Bragg condition is only satisfied up to <NUM> the grating influences the transmission up to ~<NUM>.

<FIG> show schematic plots of the filtered spectra of the bandpass transmission filter according to an embodiment. Finally, <FIG> shows the filtered spectra when the length of the waveguide structure <NUM>, on which the chirped grating structure <NUM> is placed, is extended by <NUM>. The transmittance is calculated using <MAT> using TGrating,n"=f(λ) and TWG<NUM>mm,n"=f(λ) based on the data shown in <FIG>. The result shows that the side-band transmission can be reduced by increasing the length of the waveguide structure <NUM>, e.g. by means of the waveguide extension structure <NUM>-<NUM>, as the waveguide structure <NUM> has high losses for long wavelengths due to the used materials.

<FIG> depicts a zoom of the spectra, shown that the transmission band is centered around the wavelength λ<NUM> = <NUM> and covers the whole CO<NUM> absorption band (αCO2).

For designing a bandpass transmission filter <NUM> for a different wavelength λ<NUM>, the changing or chirped grating structure may be accordingly redesigned on the basis of the above equations. Moreover, the interaction between waveguide <NUM> characteristics and material properties may be considered in order to achieve the bandpass transmission filter performance.

<FIG> shows an exemplary block diagram of a narrowband radiation source <NUM> having the bandpass transmission filter <NUM> and a broadband radiation source <NUM>, e.g. a thermal emitter, according to an embodiment.

The narrowband radiation source <NUM> comprises the bandpass transmission filter <NUM> as described in <FIG>, <FIG> and <FIG> above. The narrowband radiation source <NUM> further comprises a broadband radiation source <NUM>, e.g. a thermal emitter. The bandpass transmission filter <NUM> is arranged in a radiation R emission direction of the broadband radiation source <NUM> downstream to the broadband radiation source <NUM>.

The broadband radiation source <NUM> may be implemented as a thermal IR (IR = infrared) emitter. The IR emitter may comprise a conductive strip, e.g. a semiconductor strip or a highly doped polysilicon material, having a main emission surface region vertical to the bandpass transmission filter <NUM> for emitting a broadband IR radiation R<NUM> in a main radiation emission direction to the bandpass transmission filter <NUM>. The IR emitter may comprise a metallic cover layer which at least partially covers the main emission surface region of the conductive strip. The IR emitter may form a black body radiator and is configured to have in an actuated condition an operating temperature in a range between <NUM> and <NUM>, and wherein the IR emitter is connected to a power source for providing the electrical energy to bring the IR emitter in the actuated condition.

The bandpass transmission filter <NUM> is arranged in an illumination direction downstream of the broadband radiation source <NUM> and is configured to filter the broadband IR radiation R<NUM> emitted by the IR emitter and to provide a filtered or narrowband IR radiation R(λ<NUM>) having a center wavelength λ<NUM>.

Although some aspects have been described as features in the context of an apparatus it is clear that such a description may also be regarded as a description of corresponding features of a method. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be regarded as a description of corresponding features concerning the functionality of an apparatus.

In the foregoing Detailed Description, it can be seen that various features are grouped together in examples for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed examples require more features than are expressly recited in each claim. Rather, as the following claims reflect, subject matter may lie in fewer than all features of a single disclosed example. Thus the following claims are hereby incorporated into the Detailed Description, where each claim may stand on its own as a separate example. While each claim may stand on its own as a separate example, it is to be noted that, although a dependent claim may refer in the claims to a specific combination with one or more other claims, other examples may also include a combination of the dependent claim with the subject matter of each other dependent claim or a combination of each feature with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended.

Claim 1:
Bandpass transmission filter (<NUM>) having a center wavelength λ<NUM> of transmission, comprising:
A waveguide structure (<NUM>) arranged on a substrate (<NUM>),
a grating structure (<NUM>) in the waveguide structure (<NUM>) having a side face grating or a top face grating, the grating structure (<NUM>) having changing grating pitch values Λi for diffracting out a guided radiation in the waveguide structure (<NUM>) having wavelengths λ<NUM> which are lower than the center wavelength λ<NUM>, and for reflecting back a guided radiation in the waveguide structure having a wavelength λ<NUM> which is higher than the center wavelength λ<NUM>, and
a radiation absorbing structure (<NUM>), which is formed as a dielectric layer arranged adjacent to the waveguide structure (<NUM>), for absorbing a radiation guided by the waveguide structure (<NUM>) having a wavelength λ<NUM> higher than the wavelength λ<NUM>, with λ<NUM> < λ<NUM> < λ<NUM>, wherein the radiation absorbing structure (<NUM>) at least partially covers a bottom face of the waveguide structure (<NUM>) with the grating structure (<NUM>), and wherein a further dielectric layer (<NUM>) is formed between the waveguide structure (<NUM>) and the substrate (<NUM>), wherein the dielectric layer (<NUM>) and the further dielectric layer (<NUM>) are arranged between the waveguide structure (<NUM>) and the substrate (<NUM>), wherein the further dielectric layer (<NUM>) is formed between the dielectric layer (<NUM>) and the substrate (<NUM>), and wherein the further dielectric layer (<NUM>) has a material thickness for suppressing a coupling of the waveguide mode from the waveguide structure (<NUM>) into the substrate (<NUM>);
wherein the waveguide structure (<NUM>) comprises a silicon, polysilicon, Si, Ge, AIN or As<NUM>Se<NUM> material, wherein the dielectric layer (<NUM>) comprises a Si<NUM>N<NUM> material, and wherein the further dielectric layer (<NUM>) comprises a SiO<NUM> material.