Optical Fourier Transform Device

Abstract

The Fourier transform waveguide comprises input ports arranged in a first array, output ports arranged in a second array, an interference region having a perimeter defined by the first and second arrays and by an imaginary plane extending between the ends of the first and second arrays, and a stray light region surrounding or adjacent the perimeter of the interference region, the stray light region comprising a redirecting portion arranged to receive stray light directly from the input ports and redirect the stray light away from the interference region, and / or an optical sink arranged to absorb or attenuate stray light received directly or indirectly from the input ports.

Description

[Technical Field]

[0001] The present disclosure relates to optical Fourier transform devices, and in particular to Fourier transform waveguides. [Background technology]

[0002] Optical and photonic computing techniques promise to perform mathematical operations such as Fourier transforms faster and with significantly less energy consumption than performing similar operations using electronic circuits.

[0003] Optical Fourier transforms (OFT) are traditionally performed using free-space optics and, more recently, integrated photonics. Free-space OFT systems are well known and comprise a Fourier transform lens, arranged in either one or two dimensions, illuminated with a beam of coherent light. At the focal plane of the lens resides the OFT of the input beam. Integrated photonics includes in-silicon (in-wafer) designs, where waveguides and other optical components are etched or stamped / carved / engraved into a block or slab of semiconductor, glass, or other solid material. Integrated photonics can be relief or embossed structures, or a combination of both. OFT devices in this category are sometimes referred to as Fourier transform waveguides.

[0004] The use of integrated photonics devices to perform OFT offers the advantage of built-in, high-tolerance alignment of optical components during manufacture, which improves the accuracy, efficiency, and / or reliability of OFT performed using the devices. In turn, the accuracy and efficiency of such OFT devices can be improved. Summary of the Invention

[0005] Provided is a Fourier transform waveguide comprising: input ports arranged in a first array, output ports arranged in a second array, an interference region having a periphery defined by the first and second arrays and by an imaginary plane extending between the ends of the first and second arrays, and a stray light region surrounding or adjacent to the periphery of the interference region, wherein the stray light region comprises a redirecting portion arranged to receive stray light directly from the input ports and redirect the stray light away from the interference region, and / or an optical sink arranged to absorb or attenuate stray light received directly or indirectly from the input ports.

[0006] Optionally, the stray light region has a surface integral of the light flux passing through the periphery other than through the input port that is less than or equal to 1 / e of the surface integral of the light flux exiting the interference region other than through the output port, such as 1 / e 2 The amount is set to be 5% or less, 1% or less, or 0.1% or less.

[0007] Optionally, the planar integrals of the light flux passing through the periphery other than through the input port include only planar integrals of the light flux that enter the interference region and travel directly towards the input port or the output port.

[0008] Optionally, the Fourier transform waveguide is a slab waveguide or other waveguide based on guided modes.

[0009] Optionally, the redirector is arranged to redirect stray light towards other than the input or output port.

[0010] Optionally, the redirecting portion is a boundary with a step change in refractive index or a boundary region with a gradual change in refractive index.

[0011] Optionally, the redirector is positioned to redirect stray light towards a part of the Fourier plane other than the part containing the output port.

[0012] Optionally, the stray light region comprises a first stray light region adjacent to a first side of the interference region and a second stray light region adjacent to a second side opposite the first side of the interference region.

[0013] Optionally, the redirecting portion comprises a first redirecting portion in the first stray light region and a second redirecting portion in the second stray light region.

[0014] Optionally, the distance between the first redirector and said second redirector increases in a direction from the first array to the second array.

[0015] Optionally, the distance between the first redirector and the second redirector increases over at least one-quarter or one-half the distance between the center of the first array and the center of the second array.

[0016] Optionally, either the first stray light region or the second stray light region comprises a light sink, the light sink being arranged to receive light redirected by the first or second redirector.

[0017] Optionally, the light sink is arranged to attenuate or absorb light received directly from the input port and / or light redirected by the redirector.

[0018] Optionally, the light sink is positioned adjacent to the second array at or parallel to the Fourier plane.

[0019] Optionally, the Fourier transform waveguide further comprises additional output ports arranged in a third array, wherein a spacing between adjacent ends of the second and third arrays is greater than the spacing between the output ports and greater than the spacing between the additional output ports, and an additional interference region having a periphery defined by the first and third arrays and by an imaginary plane extending between the respective ends of the first and third arrays. An optical sink may be disposed between the interference region and the additional interference region at or parallel to the Fourier plane.

[0020] Optionally, the light sink comprises at least one of an absorber, an anti-reflective surface, an anti-reflective structure, a light extractor, or a beam dump.

[0021] Optionally, the light extractor is a waveguide other than an output port for light extracted by the light sink other than to detect or calculate the result of an optical Fourier transform.

[0022] Optionally, the stray light region reduces the area integral of the light flux passing through any continuous portion of the periphery other than through the input port to less than or equal to 1 / e of the area integral of the light flux exiting the interference region through the same continuous portion of the periphery, such as 1 / e 2 The amount is set to be 5% or less, 1% or less, or 0.1% or less.

[0023] Optionally, the stray light region bounds any part of the periphery of the interference region other than where the input or output ports are located. [Brief explanation of the drawings]

[0024] [Figure 1] 1 shows a known OFT device. [Figure 2A] 2 illustrates the OFT device of FIG. 1 with an example of stray light exiting and re-entering an interference region between the array of input ports and the array of output ports. [Figure 2B] 2B shows a plot of intensity versus position in the Fourier plane of the OFT device of FIG. 2A. [Figure 3] 1 shows a schematic diagram of an OFT device according to an embodiment, including an example in which stray light is treated to prevent at least a portion of the stray light from re-entering the interference region. [Figure 4] FIG. 1 shows a schematic diagram of an OFT device with graded boundaries and a light sink. [Figure 5] An embodiment is shown in Figure 4 illustrating the concept of including a tapered absorber. [Figure 6A] 10 shows a further embodiment including a tapered absorbent body. [Figure 6B] 10 shows a further embodiment including a tapered absorbent body. DETAILED DESCRIPTION OF THE INVENTION

[0025] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which:

[0026] In the drawings, like reference numerals refer to like parts, features or components.

[0027] Summary and detailed description of embodiments 1D OFT devices (such as integrated photonics OFT devices) can be realized as two-dimensional structures, where light is confined in one dimension and freely diffracts in the other two dimensions. In an example of such an OFT device, the input and output of the OFT device are arrays of input and output ports that function as exits or entrances to waveguides or optical channels. Light from each input port is diffracted into a wavefront wide enough to cover all output ports. At each output port, if the optical power at all input ports is the same, the contribution from each input port will have the same intensity. The angle of the wavefront at the output array is determined by the angle of the input ports, which sets the phase delay at each output port.

[0028] One way to do this is to place the input and output arrays on arcs of different circles, on which the center of the other circle lies. The Fourier transform of the input array is formed on a line of the second circle. For a given circle radius and number of input ports, the angular spacing of the input and output arrays can be calculated so that the output ports sample a single-order Fourier transform of the input data.

[0029] The device will have some tolerance on how close the input and output ports need to be to the arc by small variations in radial or lateral distance (but not angle) that can be compensated for. If the radius is long enough, the arc can be approximated by a straight line. "Long enough" means that the distance between the arc and its tangent at the edge of the zeroth-order FT is much smaller than the wavelength.

[0030] The inventors have recognized that OFT devices can be inefficient or, in some cases, imprecise. These imperfections can be particularly amplified when using single-mode waveguides that make up the input and output ports in the array, such that a small fraction of the input light is extracted at the output port.

[0031] FIG. 1 illustrates a well-known 1D OFT device (i.e., a Fourier transform waveguide) 100 of this type. Schematically, FIG. 1 illustrates a Fourier transform slab waveguide 100 in which light is confined in one (Cartesian) dimension (z) and freely diffracts in the other two dimensions (x and y). The OFT device includes a first array 110 of input ports 111 and a second array 120 of output ports 121. The second array 120 of output ports is positioned along the Fourier plane of the Fourier transform waveguide. As will be appreciated by those skilled in the art, the Fourier plane can be a plane or curved surface on which the Fourier transform of the light at the input ports is formed. An interference region 130 is defined in the space or volume between the first and second arrays and can include an input coupling region connecting the first input array and the diffraction region and an output coupling region connecting the diffraction region and the second output array. The interference region 130 is bounded by a boundary 141 in the x and y planes. The boundary 141 may be directly adjacent to the interference region 130, or there may be a space between the boundary 141 and at least a part of the edge (periphery) of the interference region 130.

[0032] The interference region and any surrounding volume through which light from the input port propagates can be formed by etching or machining (e.g., as a relief therein or as a separate block or embossed structure) a block, slab, or wafer of material that forms the interference region and any surrounding volumetric space within boundary 141. Thus, boundary 141 surrounding or adjacent to interference region 130 includes sides (as facets of a three-dimensional slab). The interference region is limited in the z-direction (as the thickness direction of the slab) by faces (as major faces of the slab). The faces of the boundary surrounding or adjacent to interference region 130 are exposed to light emitted from the input port.

[0033] A slab waveguide is any photonic waveguide and can be realized using any of the following techniques or a combination thereof: Silicon-on-insulator (SOI) slab waveguides Photonic crystals (PhCs) with or without periodic defects or subwavelength holes Silicon nitride (SiN) slab waveguide Waveguides in composite photonic technologies (III-V or II-VI) Arbitrary plasmonic waveguides Metasurface or other photonic technologies

[0034] The interference region may contain a void, or alternatively a (e.g., homogeneous) medium and / or a medium with or engineered to have a uniform refractive index.

[0035] The first array 110 is disposed on (or along) a first arc 110a of a first circle 110c, and the second array 120 is disposed on (or along) a second arc 120a of a second circle 120c that is offset from the first circle. Preferably, the first circle 110c has a center that lies on the second arc 120a, and the second circle 120c has a center that lies on the first arc 110a. Preferably, the center of the first circle 110c is at or near the center of the second array 120, and the center of the second circle 120c is at or near the center of the first array 110.

[0036] The first arc 110a and the second arc 120a define a first portion 131a and a second portion 131b, respectively, of a periphery 131 of the interference region 130. The third portion 131c and the fourth portion 131d of the periphery 131 join the ends (or edges) of the first portion 131a and the second portion 131b, respectively. That is, the third portion 131c is an imaginary surface extending in a plane between the first end of the first portion 131a and the first end of the second portion 131b, and the fourth portion 131d is an imaginary surface extending in a straight line (or plane) between the second end of the first portion 131a and the second end of the second portion 131b. The first to fourth portions 131a to 131d constitute the entire periphery 131 within the xy plane (the plane on which the input and output ports are arranged).

[0037] The input port 111 is the exit (e.g., exit pupil or exit aperture) of the input waveguide 101 or coupling structure, i.e., the input port can be connected to any or a combination of the following: Waveguides fabricated using the same techniques, methods and / or materials as the free-space Fourier transform slab waveguide region Coupling structures such as grating couplers or edge couplers that couple light from an external light source Coupling structures such as tapered couplers that couple light between vertically offset waveguides (i.e., waveguides in the silicon layer that couple light into a SiN Fourier transform slab waveguide region) Impedance matching structures that reduce reflections between a free-space Fourier transform slab waveguide and any of the above

[0038] Embodiments can be used with existing photonic technology. For example, both the phase and amplitude of the light in the input waveguide 101 connected to the input port 111 can be controlled or modulated using any or a combination of the following: Mach-Zehnder Modulator PN modulator Ring Modulator Thermal modulator Different PICs with different photonic functions Waveguides fabricated using the same techniques, methods and / or materials as the free-space Fourier transform slab waveguide region Coupling structures such as grating couplers or edge couplers that couple light into external waveguides or detectors Coupling structures such as tapered couplers that couple light into longitudinally displaced waveguides (i.e., waveguides in the silicon layer that couple light into a SiN Fourier transform slab waveguide region) Impedance matching structures that reduce reflections between a free-space Fourier transform slab waveguide and any of the above Photodetector Other photonic integrated circuits for further processing or analysis

[0039] The output port 121 may be arranged to sample one or more orders of the Fourier transform of the input data, such as a Fast Fourier Transform or otherwise. Typically, the output port is positioned to capture or sample the zeroth order Fourier transform, but in some embodiments the output port may alternatively or additionally be positioned to capture or sample higher order Fourier transform modes, such as first or second order.

[0040] Embodiments may be directed to sampling a Fast Fourier Transform. When sampling a Fast Fourier Transform, the number of output ports used to extract data must be the same as the number of input ports.

[0041] In operation, the Fourier transform slab waveguide is illuminated with modulated or unmodulated light at one or more of the input ports 111. The injected modulated or unmodulated light undergoes diffraction within the interference region 130. The output port 121 is positioned at the Fourier plane, i.e., where the diffracted light forms the optical Fourier transform of the light pattern from the input port 111.

[0042] Input ports 111 introduce light into interference region 130. The light at input ports 111 has information encoded in the field phase and amplitude. The size and shape of the optical field contained within each input port 111 to the free-space Fourier transform slab waveguide defines the envelope function of the Fourier transform, which obeys the convolution theorem.

[0043] The output port 121 is located in the Fourier plane (which lies along the second arc 120a). Light arrives at the output port 121 as a set of waves over a range of angles that matches the relative positions of the input ports 111. The waves from all the input ports 111 combine (coherently) to form an analog Fourier transform. The Fourier transform at the Fourier (or output) plane (which lies along the second arc 120a) contains the full Fourier transform of the light pattern at the input plane (which lies along the first arc 110a), including the input mode shape and the input envelope function.

[0044] The FT is sampled for detection by output port 121, and a detection method then determines whether the solution to the Fourier transform is analog or digital. The light collected by output port 121 is detected using a photodiode. This light is detected on a single photodiode per channel, or the signal is mixed with a reference beam for homodyne detection. The output port used to extract the data for detection must preserve the amplitude of the Fourier transform, and the phase if phase is being detected.

[0045] In addition to the first arc 110a and the second arc 120a, the waveguide also includes boundaries adjacent to and outside a third portion 131c and a fourth portion 131d of the periphery 131 of the interference region 130. In particular, the waveguide of FIG. 1 includes a first waveguide boundary 141a and a second waveguide boundary 141b. The first waveguide boundary 141a is a planar surface extending between a first end of the first arc 110a and a first end of the second arc 120a. The second waveguide boundary 141b is a planar surface extending between a second end of the first arc 110a and a second end of the second arc 120a.

[0046] The inventors have recognized that in a Fourier transform waveguide, such as that illustrated in FIG. 1 , reflections from waveguide boundaries (e.g., edges or sides) of the Fourier transform waveguide can interfere with the operation of the device, for example, by generating unwanted optical interference patterns in the Fourier plane (e.g., at the output port) or by interfering with light emitted through the input port (e.g., by back-reflecting back into the input port), both of which can adversely affect the operation of the OFT device by reducing the accuracy of the output data. In this case, an unwanted optical interference pattern is one that does not accurately represent the desired optical interference pattern generated by pure interference involving only light received directly from the input port. In other words, ideally, no other light should be present at the output port other than the light traveling directly from the input port to the output port, or the accuracy of the optical Fourier transform operation would be adversely affected.

[0047] The interference region is positioned between the input port (i.e., the input plane of the OFT device) and the output port (i.e., the output or Fourier plane of the OFT device). In the interference region, light from the individual input ports interferes to produce a diffraction pattern. The output port is positioned to capture the diffraction pattern at the Fourier plane (even though the diffraction pattern will be present throughout the interference region).

[0048] Light emerging from the input port and traveling in a direct line from the input port to the output port is considered useful light in the sense that it is this light that forms the desired interference (or diffraction) pattern formed in the Fourier plane. Light emerging from the input port and traveling in a direction other than directly to the output port may be considered stray light.

[0049] Reflection of stray light (e.g., from boundaries or other regions or portions surrounding or adjacent to the interference region) can cause some of the stray light to be (back)reflected back into the interference region. The reflected stray light can cause interference at the output port or upstream of the input waveguide (if back-reflected through the input port).

[0050] Figure 2A shows three examples of this phenomenon occurring within the Fourier transform waveguide of Figure 1. Like numbers refer to like parts.

[0051] 2A, a first example shows a first incident ray (or wavefront propagation vector) 191i directed from input port 111 through interference region 130 toward first waveguide boundary 141a. A first reflected ray 191r is shown directed back into interference region 130 upon reflection from first waveguide boundary 141a.

[0052] In the second example, a second incident ray 192i is shown traveling from input port 111 through interference region 130 toward first waveguide boundary 141a. A second reflected ray 192r is shown traveling back into interference region 130 toward output port 121 after reflecting off first waveguide boundary 141a.

[0053] In the third example, a third incident ray 193i is shown traveling from input port 111 through interference region 130 toward second arc 120a at a location spaced apart from output port 121. A third reflected ray 193r is shown traveling toward interference region 130 and input port 111 upon reflection from second arc 120a.

[0054] In each of the three example light paths in FIG. 2A , light originates from the input port, passes through and exits the interference region 130, and is reflected from the waveguide boundary to re-enter the interference region 130. In the second and third examples, the light is reflected directly toward the output port 121 and the input port 111, respectively. Thus, in the second example, the reflected light may adversely affect the interference pattern at the output port, resulting in erroneous measurements of the Fourier transform. In the third example, the reflected light may interfere with the light originating from the input port, for example, by interfering with a further upstream component (e.g., an encoder or modulator). In the first example, the light is reflected toward the interference region but does not travel directly to the input or output port. However, this may still adversely affect the interference pattern at the Fourier plane. For example, if the first reflected ray 191r is re-reflected from the second waveguide boundary 141b (and subsequently again from the first waveguide boundary 141a), the reflected light may still travel toward the output port 121.

[0055] Thus, in all three cases, the reflected light rays can alter the diffraction pattern that would have been formed at the Fourier plane in the absence of interference from the reflection. In these embodiments, stray light reflections back into the interference region and directly or indirectly toward the input or output ports can adversely affect the operation of the OFT device.

[0056] The present disclosure is directed to OFT devices that are arranged to reduce or minimize the extent to which stray light (reflections) interfere with Fourier transform operations. This is achieved by reducing or avoiding reflections of stray light (back into) the interference region and / or towards the input or output ports.

[0057] As illustrated in Figure 2A and its description, reflection of stray light can be caused by a change in refractive index between media at a boundary surrounding or adjacent to the interference region. On the other hand, the change in refractive index that causes reflection can be gradual or abrupt, such as a change from air to glass or silicon (or vice versa), or from one type or structure of crystal to another. Alternatively, a boundary or region surrounding or adjacent to the interference region can cause reflection in other ways, for example, if there is an abrupt change in medium from transmissive to non-transmissive, i.e., a film, coating, or other layer, that causes reflection.

[0058] FIG. 2B shows a plot of intensity versus position in the Fourier plane for the OFT device of FIG. 2A. The horizontal axis 202 represents arbitrary position in the Fourier plane, with 0 representing the center of the Fourier plane and −1 representing the position of the first waveguide boundary 141a, represented by the dotted line. The vertical axis 201 represents arbitrary intensity, with 0 representing zero intensity and 1.0 representing the maximum intensity in the Fourier plane. The solid line 210 represents a plot of intensity versus position in the Fourier plane ignoring reflections. The dashed line represents the light intensity in the Fourier plane considering only reflections from the first waveguide boundary 141a. As will be appreciated by those skilled in the art, FIG. 2 shows that reflected light has a significant effect on the interference pattern in the Fourier plane, especially toward the outer edges of the interference region.

[0059] A schematic diagram of some of the aspects that can prevent at least a portion of the stray light from re-entering the interference region is shown in Figure 3. Like reference numerals refer to like parts. Figure 3 shows a schematic diagram of a Fourier transform waveguide 200 similar to that shown in Figure 1, but differing in the following respects: The Fourier transform waveguide 200 of Figure 3 does not include the first and second waveguide boundaries 141a and 141b adjacent to the interference region 130 shown in Figure 1.

[0060] 3, the first stray light region 140a is located outside the interference region 130 and adjacent to a third portion 131c of the periphery 131 of the interference region 130. The second stray light region 140b is located outside the interference region 130 and adjacent to a fourth portion 131d of the periphery 131 of the interference region 130.

[0061] In some embodiments, boundaries or regions surrounding or adjacent to an interference region of an OFT device are positioned to direct reflections of stray light away from the interference region, and more preferably away from the input or output ports. This is shown in the first example of the schematic diagram in FIG. 3 , where the redirecting surface 143 of the first stray light region 140a is positioned and tilted to receive light traveling in the direction of a fourth incident ray 196i (or wavefront propagation vector). The fourth incident ray 196i is along a direction from the input port 111 through the interference region 130 into the first stray light region 140a and toward the redirecting surface 143. A fourth reflected ray 196r is shown reflecting off the redirecting surface 143 in a direction away from the interference region 130 and the input and output ports 111 and 121.

[0062] Alternatively or additionally, the relative dimensions of the OFT device in the region surrounding the interference region are such that reflections from the sides are reduced or minimized. For example, the relative dimensions are such that the zeroth-order mode or the first- or second-order modes of the diffraction pattern do not reach the boundaries (the lateral edges or faces of the device extending between the input plane (or surface) and the output plane (or surface)). In this case, at least one, or an array of, optical sinks may be positioned at or parallel to either side of the Fourier plane, e.g., the output port, to prevent light from being reflected back into the interference region.

[0063] Alternatively or additionally, a region adjacent to and outside the interference region can be configured with an extinction coefficient that attenuates the amplitude or intensity of stray light. The extinction coefficient is a volumetric coefficient and can be defined per unit depth of light penetration. This is illustrated in a second example in the schematic diagram of FIG. 3, which shows a light sink in the form of an attenuator 145 in the second stray light region 140b positioned to receive light traveling in the direction of a fifth incident light ray 197i (or wavefront propagation vector). The fifth incident light ray 197i travels from the input port 111 through the interference region 130 into the second stray light region 140b and through the attenuator 145. In this case, the light may subsequently be reflected back into the interference region 130, but because it has been attenuated by the attenuator 145, the amplitude of this light will be reduced compared to its amplitude when it originally exited the interference region 130. Attenuator 145 is configured to attenuate light re-entering interference region 130 at no more than 1 / e of its value when it first exits interference region 130, e 2 It is arranged to have an intensity of 5% or less, 1% or less, or even 0.1% or less.

[0064] Alternatively or additionally, a light sink in the form of an absorber is used to reduce or minimize reflection by decreasing the reflection coefficient (or increasing the absorption coefficient) at one or more of the boundaries of the OFT device. The reflection coefficient can be the reflection coefficient of the surface or, for example, in the case of textured, structured, or composite surfaces, the average or effective reflection coefficient per unit area of ​​the boundary. This is shown in the third example in the schematic diagram of FIG. 3, which shows a light sink in the form of an absorber 146 in the second stray light region 140b. The absorber 146 is positioned and tilted to receive a sixth incident light ray 198i (or wavefront propagation vector). The sixth incident light ray 198i passes from the input port 111 through the interference region 130 into the first stray light region 140a and toward the absorber 146. In this case, the light may subsequently be reflected back into the interference region 130, but because it has been absorbed by the absorber 146, the amplitude of the light is reduced compared to its amplitude when it originally exited the interference region 130.

[0065] The optical sink may be, for example, an anti-reflection layer (or stack), a photonic structure (e.g., a grating structure or a photonic crystal), or other technological device, surface, or structure arranged to suppress reflections. Other examples of suitable optical sinks include tapered absorbers, beam dump arrays, or photonic absorbers, such as electro-absorbent materials positioned or deposited at specific locations. In a detailed example, the optical sink may include a tapered or V-shaped boundary arranged to receive light at or near its wide end and guide reflections from that side toward its narrow end. At the narrow end, the width of the tapered absorber is so narrow that no modes are guided, thereby causing absorption in the surrounding material. Such tapered or V-shaped boundaries may be multiplexed in an array along a boundary or region surrounding or adjacent to the interference region. In this way, more stray light exiting the interference region may be prevented from re-entering the interference region at multiple locations along the boundary.

[0066] Those skilled in the art will understand that the reflection coefficient at the boundary surrounding the interference region is determined by the medium of the interference region (i.e., the medium inside the boundary) and the medium surrounding the interference region (i.e., that outside the boundary), as well as the angle of incidence of light at that boundary. For reflection coefficients less than 1, it is natural that a portion of the light exiting the interference region will be absorbed by the boundary surrounding the interference region, even in well-known Fourier transform waveguides such as those shown in FIGS. 1 and 2. For example, when the Fourier transform waveguides of FIGS. 1 and 2 are fabricated by etching a waveguide including the interference region as a relief in a silicon wafer, the reflection coefficient at the first and second waveguide boundaries 141 a, 141 b for a wavelength of, for example, 1000 nm is approximately 0.32 for light at normal incidence (air-silicon boundary when the interference region is an air gap). On the other hand, because much of the light from the input port in the OFT device of Figures 1 and 2 strikes the first and second waveguide boundaries 141a and 141b at angles greater than the critical angle, total internal reflection occurs along most of the boundaries, and therefore most of the light exiting the interference region is reflected back into the interference region, with a significant portion of that light being directed towards the output port.

[0067] The optical sinks described herein may be arranged to have an effective reflection coefficient per unit area of ​​light at normal incidence that is lower than the reflection coefficient per unit area at the boundary between the medium of the interference region and the surrounding medium (e.g., the medium that provides light confinement in the direction out of the plane containing the input and output ports).

[0068] Alternatively or additionally, optical sinks described herein (including but not limited to attenuators 145 and absorbers 146 described with reference to FIG. 3 ) may be configured to ensure that any light that re-enters the interference region after interacting with the optical sink has an intensity or amplitude that is less than or equal to 1 / e of the value of the intensity or amplitude when it initially exits the interference region. 2 or less, arranged to have an intensity or amplitude of 5% or less, 1% or less, or even 0.1% or less.

[0069] Any of the light sinks described herein may be positioned along the boundary between the input plane and the Fourier plane or in the Fourier plane on either side of an output port. Alternatively or additionally, the present disclosure encompasses variations on the illustrated Fourier transform devices by having groups of output ports separated by gaps in the Fourier plane. The gaps between groups are wider than the gaps between output ports of the same group. In this case, the interference region referred to in the present disclosure may be defined as the space between the input port and one of the groups of output ports or a combination of multiple such spaces (one space for each group of output ports). In the case of two groups of output ports, a third stray light region is defined in a V-shape between two intersecting such spaces. Thus, a light sink between groups of output ports resides in a stray light region at or parallel to the Fourier plane and can reduce light re-entering the stray light region in a similar manner and to the same extent as described for any of the other light sinks described herein.

[0070] More generally, considering, for example, all three examples of Figure 3, it can be seen that the stray light region according to the present disclosure can prevent a significant proportion of stray light from re-entering the interference region, particularly in a direction towards the input or output ports. A significant proportion can be understood to mean that the light that originates from the input port and exits the interference region via a port other than the output port is greater than the light that (re-)enters the interference region via a port other than the input port. The surface integral of the (re-)entering light flux is less than or equal to 1 / e of the surface integral of the exiting light flux, and is generally less than or equal to 1 / e 2 It may be less than 5%, less than 1%, or even less than 0.1%. Another measure of a significant proportion is that the light that originates from the input port and leaves the interference region through any continuous portion of the periphery of the interference region (other than that of the input or output port) is greater than the light that (re)enters the interference region through the same continuous portion of the periphery. The surface integral of the (re)entering luminous flux is less than or equal to 1 / e of the surface integral of the exiting luminous flux, 1 / e 2 It may be 5% or less, 1% or less, or even 0.1% or less.

[0071] It should be understood that the stray light region described herein is not a region that includes an input port or an output port. In other words, the stray light region bounds portions of the periphery of the interference region other than where any input or output port is located. That is, the stray light region may exist in any or all areas surrounding the periphery of the interference region other than the output port or input port.

[0072] Alternatively or additionally, the light that originates from the input port and exits the interference region via a port other than the output port is greater than the light that (re)enters the interference region in a direction toward the input port or the output port (via a port other than the input port). The surface integral of the light flux that (re)enters in a direction toward the input port or the output port is less than or equal to 1 / e of the surface integral of the exiting light flux. 2 It may be 5% or less, 1% or less, or even 0.1% or less. Those skilled in the art will be able to identify whether this is accomplished using a single optical module or a computer simulation to simulate the path of light through the Fourier transform waveguide and the changes in various light intensities as the light interacts with various surfaces and volumes. Such models can be easily constructed by those skilled in the art, taking into account reflection, attenuation, and absorption at any boundaries or regions of the waveguide depending on the materials and surfaces used to construct the waveguide and, in particular, the stray light region. The planar integrals of the light fluxes pass through the boundary surfaces of the interference region extending between the respective ends of the array of input and output ports, and exclude any light fluxes in the limiting direction (e.g., the z-direction) through the major surface of the slab waveguide. That is, the planar integrals of the light fluxes described herein are obtained, for example, through the periphery 131 of the interference region 130. As noted above, the planar integrals of the light fluxes exiting the interference region through the output ports and the planar integrals of the light fluxes (re-)entering the interference region through the input ports are ignored in these calculations.

[0073] When the term "boundary or area surrounding or adjacent to an interference area" (or similar terminology) is used herein, it may be understood to mean partially surrounding or adjacent to an interference area on either side of the interference area between an input port and an output port, or in an area surrounding or adjacent to the interference area other than where the (array of) input or output ports are located. Furthermore, the term may be understood to mean directly surrounding or adjacent to the interference area, or surrounding or adjacent to it at a particular distance from the interference area (so as to create a spatial margin between the interference area and the boundary or area surrounding or adjacent to the interference area).

[0074] A synergistic effect can be created by using a boundary or region for controlling the propagation direction of reflected stray light in combination with a boundary or region configured as a light sink. That is, a first portion of the boundary or region surrounding or adjacent to the interference region can be positioned to direct the reflection of stray light away from the interference region and / or away from the input and output ports. Meanwhile, a second portion other than the first portion includes a light sink for absorbing or attenuating light received directly from the input port and / or light reflected from the first portion. This is illustrated as an extension of the first example shown in FIG. 3. In this extension, the fourth reflected light ray 196r continues within the first stray light region until it hits the first light sink 144. The first light sink 144 can be an attenuator or absorber, such as the attenuator 145 or absorber 146 shown in (and described with respect to) the second stray light region 140b of FIG. 3.

[0075] The inventors recognized that it is possible to reduce back reflections from the Fourier (output) plane, where the plane is illuminated by the input port but light is not consequently extracted by the output port. This can be achieved by using an optical sink at or along a portion of the Fourier plane. For example, an OFT device can be formed so that the portion of the Fourier plane directly illuminated by the input port is wider than the array of output ports. This allows boundaries or regions surrounding or adjacent to the interference region extending between (i.e., joining) the edge of the array of input ports and the Fourier output plane to be farther apart at the Fourier plane than at the input plane. This effectively tilts these boundaries or regions away from the input port, thereby redirecting (e.g., reflecting) stray light from the input port in a direction other than the output port, mostly toward the Fourier plane on either side of the output port. Optical sinks at the Fourier plane on either side of the output port then prevent or reduce back reflection of stray light from the input port (or stray light reflected / redirected by the side boundaries or regions) back toward the input port.

[0076] In one such configuration, the shape and / or relative dimensions of a first portion of the boundary surrounding the interference region are configured to direct reflections of stray light in a direction other than toward the output port. While this can ensure that light from the input port travels to the output port without (or with reduced) impact from reflections of stray light, it may be the case that a second portion of the boundary other than the first portion is tilted such that light received directly from the input port and / or stray light reflected by the second portion is reflected back toward the input port. In this case, including an optical sink as described herein in or along the second portion can reduce or minimize such back reflections.

[0077] This is illustrated in the schematic diagram of Figure 4, which shows another embodiment of a Fourier transform waveguide described herein. In Figure 4, the Fourier transform waveguide includes a first angled boundary 441a extending from a first end of the arc where the input port 411 resides to a Fourier plane where the output port 421 resides. The Fourier transform waveguide also includes a second angled boundary 441b extending from a second end of the arc where the input port 411 resides to the Fourier plane. Each of the first and second angled boundaries 441a, 441b shown in Figure 4 operates similarly to the redirecting surface 143 described with reference to Figure 3. The Fourier transform waveguide of Figure 4 also includes a first optical sink 445a positioned to one side of the output port 421 in the Fourier plane and a second optical sink 445b positioned to the other side of the output port 421 in the Fourier plane.

[0078] In operation, light from input port 411 passes through interference region 430 into first stray light region 440a and second stray light region 440b and is reflected from first slanted boundary 441a and second slanted boundary 441b towards first optical sink 445a and second optical sink 445b, respectively, thereby preventing stray light from re-entering interference region 430 and adversely affecting the interference pattern at the output port.

[0079] In other embodiments, the sloped boundaries are sloped so that a large portion of stray light from the input ports does not reach them but instead travels directly to optical sinks in the Fourier plane. This is exemplified in the Fourier transform waveguide shown in FIG. 5. In this embodiment, there are first and second optical sink arrays 545a and 545b at the Fourier plane positioned on either side of the output port 521. The first and second optical sink arrays 545a and 545b each comprise a series of tapered absorbers 5045, each having a wide end positioned on the Fourier plane and a narrow end positioned further away from the input port 511. The tapered absorbers 5045 are sloped so that light from the input port 511 enters the wide end and is guided to the narrow end by multiple reflections, undergoing absorption at each reflection so that the light is not reflected back toward the input port 511. In other embodiments, the tapered absorbers may be replaced with a waveguide or other equivalent light extractor following the absorber (other than the detector).

[0080] Figure 5 further shows the light intensity throughout the input waveguide 501, output waveguide 502, and tapered absorber 5045, as well as the light intensity in the interference region 530 and stray light regions 540a and 540b. The interference pattern in the Fourier plane is such that part of the output port 521 is positioned at a maximum point in the interference pattern, and part of the output port is positioned at a minimum point. Part of the higher-order maximum point is incident on the tapered absorber 5045. In Figure 5, the black outline indicates the edge of the silicon structure in a silicon-on-insulator device. The simulation was performed using Lumerical FDTD, feeding light of the same amplitude and phase into two of the four input ports. The grayscale data represents the light intensity at a single frequency for a steady-state input. The equivalent numerical result is [1.0, 0.0, 1.0, 0.0]. The diffraction pattern formed shows exactly three iterations of the Fourier transform, with the central pattern extracted by output port 521 and the other maxima absorbed in silicon by tapered absorber 5045.

[0081] Figure 6A shows another embodiment in which tapered absorbers 6045a are positioned in a semi-elliptical pattern around the interference region rather than along the Fourier plane. Positioning the absorbers to surround the interference region in this manner also reduces any stray light re-entry into the interference region 630a due to the optical sink occupying a greater percentage of the boundary or area surrounding or adjacent to the interference region. Figure 6B shows yet another embodiment in which tapered absorbers 6045b are positioned closer to the interference region 630b. This increases the percentage of the boundary occupied by the optical sink while requiring a smaller footprint per Fourier transform waveguide, allowing for an increased density of such waveguides, for example, within an optical chip.

[0082] Advantages of the embodiments described herein include: Accurate Fourier transform without interference from reflected light from the edges of the device, and / or Reduction of back reflections that interfere with upstream components, e.g., those associated with the input waveguide It could be.

[0083] Those skilled in the art will understand that Fourier transform waveguides can be designed and manufactured for specific wavelengths or wavelength ranges, with the input and output ports positioned such that an optical Fourier transform is formed at the output port. Those skilled in the art will know how to design the relative and / or absolute dimensions of the Fourier transform waveguides described herein to accommodate various wavelengths of input light. For example, for a given wavelength, the distance between the array of input ports and the array of output ports, as well as the spacing between the individual ports themselves, can be determined by calculation. Furthermore, those skilled in the art will know how to design or optimize various forms of light sinks or light redirecting surfaces or regions (i.e., those that perform any of the functions described herein) when the wavelength of the input light (and therefore stray light) is known.

[0084] The described embodiments are provided for illustrative purposes and are not intended to be limiting. Various modifications to the embodiments may be made as will be appreciated by those skilled in the art. The present invention is defined by the following claims.

Claims

1. A Fourier transform waveguide, Input ports located in the first array, Output ports arranged in a second array along the Fourier plane of the Fourier transform waveguide, An interference region having a periphery defined by the first array and the second array and by a virtual plane extending between the respective ends of the first array and the second array, A stray light region that surrounds or is adjacent to the periphery of the interference region, A redirection unit, which is arranged to directly receive stray light from the input port and redirect the stray light toward a region other than the interference region, and / or An optical sink is arranged to absorb or attenuate stray light received directly or indirectly from the aforementioned input port. A stray light region comprising, A Fourier transform waveguide equipped with [the necessary components].

2. The stray light region is such that the area portion of the light beam passing through the periphery via a port other than the input port is 1 / e or less of the area portion of the light beam emitting from the interference region via a port other than the output port, or 1 / e ２ The Fourier transform waveguide according to claim 1, wherein the following components are arranged to be 5% or less, 1% or less, or 0.1% or less.

3. The Fourier transform waveguide according to claim 1, wherein the area fraction of the light beam passing through the periphery via a port other than the input port includes only the area fraction of the light beam incident on the interference region and traveling directly toward the input port or output port.

4. The Fourier transform waveguide according to claim 1, which is a slab waveguide or another waveguide based on guide modes.

5. The Fourier transform waveguide according to claim 1, wherein the redirection unit is arranged to redirect the stray light toward a port other than the input port or output port.

6. The Fourier transform waveguide according to claim 1, wherein the redirection portion is a boundary with a stepwise change in refractive index or a boundary region with a gradual change in refractive index.

7. The Fourier transform waveguide according to claim 1, wherein the redirection unit is configured to redirect the stray light toward a portion of the Fourier plane other than the portion including the output port.

8. The Fourier transform waveguide according to claim 1, wherein the stray light region comprises a first stray light region adjacent to the first side of the interference region, and a second stray light region adjacent to the second side facing the first side of the interference region.

9. The Fourier transform waveguide according to claim 8, wherein the redirection section comprises a first redirection section in the first stray light region and a second redirection section in the second stray light region.

10. The Fourier transform waveguide according to claim 9, wherein the distance between the first redirection section and the second redirection section increases in the direction from the first array to the second array.

11. The Fourier transform waveguide according to claim 9, wherein the distance between the first redirection section and the second redirection section increases by at least one-quarter or one-half of the distance between the center of the first array and the center of the second array.

12. The Fourier transform waveguide according to claim 8, wherein either the first stray light region or the second stray light region comprises the optical sink, and the optical sink is arranged to receive light redirected by the first or second redirection unit.

13. The aforementioned optical sink is Light received directly from the aforementioned input port, and / or Light redirected by the aforementioned redirection unit The Fourier transform waveguide according to claim 1, which is arranged to attenuate or absorb.

14. The Fourier transform waveguide according to claim 1, wherein the optical sink is positioned adjacent to the second array in the Fourier plane or parallel to the Fourier plane.

15. An additional output port located in a third array, wherein the distance between adjacent ends of the second and third arrays is greater than the distance between the output ports and greater than the distance between the additional output ports, An additional interference region having a periphery defined by the first array and the third array and by a virtual plane extending between the respective ends of the first array and the third array, Furthermore, The Fourier transform waveguide according to claim 1, wherein the optical sink is arranged between the interference region and the additional interference region in the Fourier plane or parallel to the Fourier plane.

16. The aforementioned optical sink is absorbent material, anti-reflective surface, anti-reflection structure, Light extractor, or Beam Dump A Fourier transform waveguide according to claim 1, comprising at least one of the following.

17. The Fourier transform waveguide according to claim 1, wherein the optical sink is a waveguide other than an output port for purposes other than detecting or calculating the results of the optical Fourier transform of the light extracted by the optical sink.

18. The stray light region is such that the area fraction of the light beam incident on any continuous portion of the periphery via a means other than the input port is less than or equal to 1 / e of the area fraction of the light beam exiting the interference region via the same continuous portion of the periphery, or 1 / e ２ The Fourier transform waveguide according to claim 1, wherein the following components are arranged to be 5% or less, 1% or less, or 0.1% or less.

19. The Fourier transform waveguide according to any one of claims 1 to 18, wherein the stray light region defines any portion of the periphery of the interference region other than the location of the input port or output port.

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