Passive on-chip optical longpass filter

Abstract

A passive on-chip optical long-pass filter is used to remove residual pump photons at short wavelengths after nonlinearly generated photons. A thin layer (<100 nm) of amorphous or polycrystalline silicon is deposited on a section of the waveguide to absorb light at wavelengths shorter than the silicon bandgap of about 1.1 μm, while the nonlinearly generated light at wavelengths longer than the silicon bandgap has negligible absorption loss as it propagates in the waveguide. The filter is suitable for nonlinear and quantum photonic chips to achieve more than 120 dB on-chip optical pump photon rejection ratio. The filter is conceptually simple to design, can be fabricated by CMOS processes, and has potential for high wafer-level scalability and low-cost manufacturability. The filter can be implemented in various integrated photonic platforms, including silicon carbide, silicon nitride, lithium niobate, and aluminum nitride.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority and interest in U.S. Provisional Patent Application No. 63 / 513,349, filed July 12, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0003] Abbreviations

[0004] 3C-SiC cubic silicon carbide

[0005] 3C-SiC on 3C-SiCoI insulator

[0006] α-Si amorphous silicon

[0007] ALD (Atomic Layer Deposition)

[0008] AlN aluminum nitride

[0009] AlN on AlNoI insulator

[0010] Al2O3 (alumina)

[0011] c-Si crystalline silicon

[0012] CMOS (Complementary Metal-Oxide-Semiconductor)

[0013] CVD (Chemical Vapor Deposition)

[0014] DRIE (Deep Ion Reactive Etching)

[0015] EME Intrinsic Model Expansion

[0016] ER extinction ratio

[0017] IL insertion loss

[0018] Lithium niobate (LN)

[0019] LNoI on insulator LN

[0020] LPF long-pass filter

[0021] NIR (Near Infrared)

[0022] OSA Spectrometer

[0023] poly-Si polycrystalline silicon

[0024] SEM (Scanning Electron Microscope)

[0025] SiC (Silicon Carbide)

[0026] SiC on SiCoI insulator

[0027] SPD single-photon detector

[0028] SPDC Spontaneous Parametric Conversion

[0029] TE Transverse Radio Wave

[0030] TM transverse magnetic wave

[0031] UMZI Unbalanced Mach-Zehnder Interferometer Technical Field

[0032] This application relates to an optical filter for long-pass filtering of an incident beam, an optical conversion device that uses the optical filter to generate an output beam from an input laser beam, and an optical system that can be implemented on an integrated photonics platform and employ the optical conversion device. Background Technology

[0033] Monolithic nonlinear and quantum photonic chips promise to enable technological applications in a compact footprint without losing photons due to inter-chip coupling losses. Nonlinear chips may enable on-chip wavelength conversion, while quantum chips will enable quantum state preparation and manipulation, including quantum metrology, quantum communication, and quantum computing.

[0034] On-chip nonlinear and quantum light sources have been demonstrated as key integrated components on various material platforms and through different photon generation processes. Single-photon emission from color centers has been demonstrated in integrated SiCoI platforms (the SiC used in the platform is 4H-SiC[1],[2]) and suspended 3C-SiC resonators[3],[4]. These color centers are pumped with shorter wavelengths of laser light and emit photons of NIR wavelength. Nonlinear and photon pair sources based on second-order optical nonlinearity have been demonstrated on integrated 3C-SiCoI platforms[5],[6], integrated AlNoI platforms[7] and LNoI platforms[8]. The photon pair source converts the pump light of 780 nm wavelength to a photon pair of 1550 nm wavelength using SPDC. SPDs that operate at low temperatures using niobate nitride nanowires have been demonstrated on various heterogeneous integrated material platforms[9],

[10] .

[0035] Besides on-chip nonlinear and quantum light sources and SPDs, on-chip pump suppression filters are also fundamental components for integrating quantum photonic circuits, but they have received relatively little attention in the field. Excess on-chip pump photons must be removed before interacting with downstream components and photon detection. For example, a reasonable photon count rate of 1 MHz of generated photons results in a power that is about 120 dB weaker than the power of a 1 mW on-chip pump. Wavelength selective isolation of tens of dB can be achieved by cascading multiple stages of optical microring-based filters

[10] or Mach-Zehnder interferometers

[11] . However, these wavelength-flexible cascaded filters confine single-photon and photon-pair sources to a narrow spectral band. These filters further enforce careful active wavelength alignment between all individual filter stages. Using active thermal and electrical control to align filter wavelengths consumes additional energy and computational resources. Long gratings

[12] , directional coupler-based filters with integrated gratings

[14] , and tapered waveguides

[15] can achieve reasonable ER and relatively wide bandwidths. However, these designs still require cascading carefully designed filter stages, which often increases IL and requires sophisticated manufacturing processes.

[0036] There is a need in the field for an improved design that does not require precision manufacturing processes while achieving a high ER value. Summary of the Invention

[0037] The first aspect of this disclosure is to provide an optical long-pass filter (LPF) for long-pass filtering an incident light beam to produce a filtered beam. The incident light beam includes a desired light component and an undesired light component. The desired light component has one or more first component wavelengths. The undesired light component has one or more second component wavelengths. Each of the one or more first component wavelengths is longer than each of the one or more second component wavelengths.

[0038] The filter comprises a waveguide core and a selective absorption layer. The waveguide core receives an incident light beam, propagates desired and undesired light components within the waveguide core, and outputs a filtered light beam. The selective absorption layer is deposited on the waveguide core. The selective absorption layer is made of an indirect bandgap semiconductor material, which is selected to have a bandgap energy greater than the maximum photon energy associated with one or more first component wavelengths and less than the minimum photon energy associated with one or more second component wavelengths. Thus, when the desired and undesired light components interact with the selective absorption layer during propagation within the waveguide core, the undesired light components are attenuated while the optical power of the desired light components is preserved.

[0039] Note that the filter is formed using a simple process of depositing a selective absorption layer on the waveguide core. This simple filter manufacturing process reduces the need for precision processes during filter fabrication. Furthermore, using an indirect bandgap semiconductor material to absorb unwanted light components while retaining desired light components allows for a higher ER value.

[0040] In some embodiments, the indirect bandgap semiconductor material is selected as α-Si or poly-Si. The thickness of the selective absorption layer can be less than or equal to 100 nm.

[0041] In some embodiments, the waveguide core is implemented in the form of a strip waveguide.

[0042] In some embodiments, the waveguide core is implemented in the form of a ribbed waveguide.

[0043] In some embodiments, the waveguide core is made of 3C-SiC.

[0044] In some embodiments, the waveguide core is made of a material selected from SiC, LN, and AlN.

[0045] In some embodiments, the filter further includes an optical insulating layer on which the waveguide core is located. The optical insulating layer provides a first reflective interface between the waveguide core and the optical insulating layer to reflect the desired light component during propagation within the waveguide core.

[0046] In some embodiments, the optical insulating layer is composed of SiO2.

[0047] In some embodiments, the appropriate materials for forming the optical insulating layer and the waveguide core are selected to allow total internal reflection of the desired light component at the first reflective interface.

[0048] In some embodiments, the filter further includes a cladding deposited on a combination consisting of at least a selective absorption layer and a waveguide core. Specifically, the cladding is an optically insulated cladding that provides a second reflective interface between the waveguide core and the optically insulated cladding to reflect the desired light component during propagation within the waveguide core.

[0049] In some embodiments, the cladding is composed of SiO2.

[0050] In some embodiments, the waveguide core is shaped as a straight line.

[0051] In some embodiments, the waveguide core is shaped as a curved shape.

[0052] In some embodiments, the selective absorption layer is shaped such that the unfiltered segment of the waveguide core abruptly transitions to its filtered segment, wherein the filtered segment is a first longitudinal segment of the waveguide core that is completely covered by the selective absorption layer, and the unfiltered segment is a second longitudinal segment of the waveguide core that is not covered by any selective absorption layer.

[0053] In some embodiments, the longitudinal ends of the selective absorption layer have a sloping profile that covers the transition region between the unfiltered section and the filtered section of the waveguide core, so that the unfiltered section gradually transitions to the filtered section.

[0054] In some embodiments, the longitudinal ends of the selective absorption layer have a first-order tapered profile that covers the transition region, allowing the unfiltered section to gradually transition to the filtered section.

[0055] In some embodiments, the longitudinal ends of the selective absorption layer have a two-stage tapered profile that covers the transition region, allowing the unfiltered section to gradually transition to the filtered section.

[0056] A second aspect of this disclosure is to provide an optical conversion device for generating an output beam from an input laser beam.

[0057] The optical conversion device includes one or more nonlinear quantum light sources configured to perform SPDC on an input laser beam to nonlinearly generate a first beam, such that the first beam includes a desired light component and an undesired light component. The desired light component has one or more first component wavelengths. The undesired light component has one or more second component wavelengths. Each of the one or more first component wavelengths is longer than each of the one or more second component wavelengths. The optical conversion device also includes any embodiment of the optical long-pass filter disclosed above for long-pass filtering the first beam to generate an output beam. The first beam is considered as the incident beam, and the filtered beam is considered as the output beam.

[0058] A third aspect of this disclosure is to provide an optical system.

[0059] The optical system includes any embodiment of the light conversion device disclosed above. Furthermore, the optical system also includes one or more photonic circuits for processing the output beam.

[0060] In some embodiments, the optical system is implemented as an integrated photonic chip.

[0061] Other aspects of this disclosure are disclosed as shown in the embodiments described below. Attached Figure Description

[0062] Figure 1A prototype LPF is depicted, which is implemented as an on-chip LPF based on α-(poly-)Si for integrating the SiCoI quantum photonics platform.

[0063] Figure 2 The top and cross-sectional views of the prototype LPF used in the proof-of-concept experiment are schematically depicted, showing the structure of the prototype LPF.

[0064] Figure 3 The IL results of simulated propagation of beams with wavelengths of 1550 nm and 780 nm using a prototype LPF at a filter length of 1 mm were plotted.

[0065] Figure 4 The simulated mode field amplitude distributions of TE and TM polarization modes of 780nm and 1550nm beams propagating within the first 10μm of the waveguide in the propagation direction inside the prototype filter were plotted under different silicon layer thickness values.

[0066] Figure 5 The manufacturing process for producing the prototype LPF sample is described.

[0067] Figure 6 Two images of the manufactured prototype LPF are shown, in which Figure 6 Subplot (a) shows the fabricated filter images with different filter lengths, and subplot (b) shows the SEM image of the transition region from the waveguide to the filter.

[0068] Figure 7 Lens-to-lens normalized transmission spectra for TE and TM polarization at different filter lengths used in the prototype LPF are shown, where: subfigures (a) and (b) show the corresponding spectra obtained for TE and TM polarization, respectively; subfigures (c) and (d) show the corresponding spectra for TE and TM polarization at a wavelength of 780 nm (765 nm to 795 nm), respectively; subfigures (e) and (f) are near-field images of the output docking coupler obtained from a reference waveguide (with a 0 μm length filter) and a waveguide with a 100 μm length filter; subfigure (g) shows the wavelength-average transmission gain for TE and TM polarization at three wavelength windows (1550 / 1310 / 780 nm) at different filter lengths.

[0069] Figure 8 Output spectra captured by OSA with and without 780 nm pump light coupled to a 100 μm prototype LPF were plotted, showing no re-emission exceeding -90 dB under 1 mW on-chip pump power conditions.

[0070] Figure 9A top view, a longitudinal sectional view, and a side sectional view of an optical LPF according to exemplary embodiments of the present disclosure are depicted.

[0071] Figures 10A to 10D Some embodiments of the disclosed optical LPF are depicted in a side sectional view, wherein:

[0072] Figure 10A As a reference, it is depicted as follows Figure 9 An exemplary embodiment of the optical LPF disclosed herein is a first LPF, wherein an air cladding is used in the first LPF and the waveguide core of the first LPF is a strip waveguide;

[0073] Figure 10B A second LPF obtained from a first LPF is depicted by using a ribbed waveguide instead of a strip waveguide as the waveguide core;

[0074] Figure 10C A third LPF, obtained from the first LPF by further including a cladding deposited on the selective absorption layer and the waveguide core, is depicted; and

[0075] Figure 10D A fourth LPF is depicted, which is formed from a first LPF by using a ribbed waveguide instead of a strip waveguide as the waveguide core and by including a cladding.

[0076] Figure 11A and 11B Two other embodiments of the disclosed optical LPF are described, wherein:

[0077] Figure 11A The fifth LPF with a straight waveguide core is depicted; and

[0078] Figure 11B The sixth LPF with a curved waveguide core is depicted.

[0079] Figures 12A to 12D Some waveguide-to-filter transition designs for the disclosed optical LPF are depicted, wherein:

[0080] Figure 12A This indicates a sudden transition;

[0081] Figure 12B The sloping transition is shown;

[0082] Figure 12C A first-stage tapered transition is shown; and

[0083] Figure 12D A two-stage conical transition is shown.

[0084] Figure 13A schematic diagram illustrating an exemplary embodiment of the disclosed optical system is shown, wherein the optical system includes a light conversion device, the light conversion device including the disclosed optical LPF.

[0085] Those skilled in the art will understand that the components in the figures are shown for simplicity and clarity and are not necessarily drawn to scale. Detailed Implementation

[0086] The present disclosure will now be described more fully with reference to the accompanying drawings. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments described herein.

[0087] This disclosure relates to an optical LPF for long-pass filtering of an incident laser beam, an optical conversion device that uses the optical LPF to generate an output laser beam from an input laser beam, and an optical system that can be implemented on an integrated photonics platform and employs the optical conversion device.

[0088] The inventors developed the disclosed optical LPF based on the following observations. Indirect bandgap semiconductor materials are natural LPFs or are used for short-wavelength absorbers because these materials provide reasonable absorption of photons above their bandgap energy without efficient radiative recombination. For centimeter-scale photonic chips, the intrinsic material absorption of light within the transparent window by these indirect bandgap semiconductor materials is negligible. Among readily available conventional semiconductor materials, α-Si and poly-Si are promising candidates because both materials have bandgap of approximately 1.14 eV and absorb pump light of visible / NIR wavelengths shorter than 1.1 μm, while being largely transparent to light in the O, C, and L bands of telecommunications. α-(poly-)Si materials can be readily deposited on substrates using CVD furnaces in CMOS factories. Compared to c-Si, α-(poly-)Si materials have higher structural disorder and therefore provide a greater density of states than c-Si

[16] . In the visible spectrum, the absorption coefficient of α-Si is about an order of magnitude larger than that of c-Si

[17] . It is known that deposition temperatures above 680°C will cause α-Si to partially crystallize into poly-Si, thereby weakening its absorption capacity

[17] . Since α-Si has better light absorption capacity than poly-Si, and its deposition process on wafers is simple and inexpensive, α-Si has been widely used in solar cells

[18] . α-Si materials are also deposited on silicon nitride photonic platforms as on-chip microheaters because the absorbed light can be converted into heat

[19] .

[0089] The inventors also observed that the disclosed optical LPF can be fabricated using a simple process of depositing an α-(poly-)Si thin film. Advantageously, the fabrication of the disclosed optical filter does not require precision processes.

[0090] In this disclosure, a prototype LPF is first provided as a representative example to illustrate the claimed invention. The prototype LPF is a passive on-chip pump suppression LPF designed to filter out the 780nm pump light component from a multicolor beam with 780nm and 1550nm light components, and is integrated on an integrated photonics platform. The details of the prototype LPF will then be summarized to facilitate the development of the claimed invention.

[0091] A. Prototype LPF

[0092] The prototype LPF utilizes a thin α-(poly-)Si layer (less than 100 nm thick) deposited on the top surface and sidewalls of an integrated waveguide to block pump light in visible / NIR wavelengths shorter than 1.1 μm through intrinsic material absorption along the waveguide propagation direction. Simultaneously, longer NIR wavelengths (signal and idler) light generated nonlinearly within the transparent window of the thin film continue to propagate without significant absorption or mode perturbation. Such waveguide structures with appropriately designed absorption film thickness provide pump suppression ratios exceeding 120 dB within a compact filter length of 1 mm. This prototype LPF is suitable for efficient on-chip pump suppression on integrated quantum photonic chips.

[0093] Figure 1 A schematic diagram of a prototype LPF (labeled 100) is depicted, implemented as an on-chip α-(poly-)Si-based LPF for integration into a SiCoI quantum photonics platform. The prototype LPF 100 is formed by an integrated waveguide structure 110 for guiding and propagating an incident beam 181. The incident beam 181 consists of a signal beam, an idler beam, and a pump beam. The signal beam and idler beam are light components with wavelengths longer than 1100 nm, while the pump beam is another light component with wavelengths shorter than 1100 nm. A thin layer 120 of α-(poly-)Si is deposited on a segment of the integrated waveguide structure 110 to progressively absorb the pump beam without attenuating the signal beam and idler beam, or with only negligible attenuation of the signal beam and idler beam. Therefore, the waveguide structure 110 outputs a filtered beam 182 containing only the signal beam and idler beam. The waveguide structure 110 can be fabricated on various material platforms, including SiC, LN, and AlN. Compared to waveguide materials, the optical insulator layer 130, with its lower refractive index, is used to confine signal and idler light through total internal reflection and can be made from various materials. Beams of different wavelengths propagate in different optical modes within the waveguide structure 110, exhibiting different spatial distributions. Beams with wavelengths shorter than approximately 1.1 μm, overlapping with the absorption region of the α-(poly-)Si layer, are strongly attenuated.

[0094] Note that if the waveguide structure 110 is appropriately designed to have a curved shape, the footprint of the prototype LPF 100 can be made more compact. For example... Figure 1 In the example shown, waveguide structure 110 has a meandering shape to save space. Note that other shapes can also be used to form waveguide structure 110. Straight waveguides can also be used.

[0095] A proof-of-concept experiment was conducted to validate the prototype LPF 100 on the 3C-SiCoI platform.

[0096] A.1. LPF Design

[0097] Figure 2 A top and cross-sectional view of the prototype LPF 100 are schematically depicted to illustrate its structure. In the design process, a 3C-SiC waveguide with a width of 850 nm, a SiC film thickness of 460 nm, and a plate thickness of 100 nm was used to realize the waveguide structure 110. Based on device fabrication results, a waveguide sidewall slope of approximately 80° was also adopted. These design dimensions were adopted based on the requirements of other specific applications and can be varied in practice. In the design process, it was assumed that the α-(poly-)Si layer uniformly covered the top surface and sidewalls of the waveguide. A 30 nm thick Al₂O₃ bonding layer was used beneath the SiC film for bonding to the SiO₂ underlayer. The entire waveguide structure 110 was surrounded by a SiO₂ cladding. In the numerical simulation of the design, standard material parameters for c-Si were used instead of those for α-(poly-)Si.

[0098] Figure 3 Subplots (a) and (b) in the figure show the EME simulation results for optical transmission assuming a waveguide length of 1 mm. Subplots (a) and (b) plot the relationship between the intermodal absorption (IL) and Si cladding thickness for 1550 nm and 780 nm beams in TE / TM polarization modes, respectively, with a filter length of 1 mm. A smaller but increasing IL is observed at 1550 nm due to the redistribution and confinement of waveguide modes in the high-refractive-index Si layer. For the 780 nm beam, absorption depends on the degree of spatial overlap between the mode field and the Si layer 120. Mode overlap does not monotonically increase with Si layer thickness t.

[0099] Figure 4 Subplots (a) to (d) depict the simulated mode field amplitude distribution for TE and TM modes within the first 10 μm of waveguide 110 in the propagation direction at different values ​​of the Si layer 120 thickness t. Figure 2 The horizontal dashed line 290 shown represents the xy-plane where the mode field amplitude distribution is simulated. From Figure 4 The results show that the 780nm waveguide mode exhibits higher-order mode interference when t reaches or exceeds 30nm. This leads to different spatial overlaps, where the 780nm light is periodically coupled into the Si thin film guided mode, such as... Figure 4 The insets of subfigures (a) and (b) show that interference occurs even at 1550 nm wavelengths for TE and TM polarization modes at t = 70 nm. Based on simulation results, a 40 nm thick α-Si thin film was used in device fabrication to achieve a significant target absorption loss of approximately 120 dB / mm under 780 nm light (pump light), while maintaining a negligible IL of less than 1 dB under 1550 nm light (signal light and idler light).

[0100] A.2. Manufacturing

[0101] Prototype LPF through Figure 5 The fabrication process is shown in the diagram. A commercially available 4-inch epitaxial silicon-on-silicon (3C-SiC) wafer was used in this process. The SiC film used to fabricate waveguide 110 was 1.5 μm thick and underwent chemical and mechanical polishing. For wafer-to-wafer bonding, a 4-inch Si substrate with a 3 μm thermal SiO2 layer was used as the carrier wafer. Both wafers were first cleaned with a standard 120°C solution of H2SO4 (98%):H2O2 (30%) = 10:1. The two wafers were then transferred to an Oxford ALD apparatus to simultaneously deposit a 15 nm thick Al2O3 layer, resulting in a total Al2O3 layer thickness of 30 nm after the bonding step. After deposition, the two wafers were brought into contact and pressed to initiate pre-bonding. The pair of wafers was then transferred to a Karl Suss SB6 bonding machine. The pre-bonded wafers were annealed in vacuum at 300°C for 3 hours to achieve permanent bonding. Following the bonding step, a Si polisher was used to remove most of the Si substrate from the SiC side. The remaining Si substrate was removed using a tetramethylammonium hydroxide solution heated to 80°C. After substrate removal, less than 50% of the original SiC film remained on the wafer. The remaining portion was cut into small pieces, each measuring 1.2 cm × 1.2 cm.

[0102] The exposed silicon carbide surface corresponds to the first SiC layer epitaxially grown on the Si substrate. Due to the lattice mismatch between 3C-SiC and Si, the crystal quality of the first SiC layer is poor. Based on our numerical simulation results, we recommend using a DRIE process containing SF6 and O2 to thin the film to a target thickness of 460 nm. A 500 nm thick SiO2 layer fabricated by PECVD was deposited on the 3C-SiC film as a hard mask. The SiO2 hard mask was patterned using electron beam lithography and DRIE. The SiC film was then patterned using the same SF6 / O2 DRIE process. The selectivity of SiC for SiO2 was 1.45. The remaining SiO2 hard mask was then removed using a buffered oxide etchant. The sample was transferred to a CVD furnace to deposit a 40 nm thick α-Si layer. The deposition temperature was maintained at 550 °C to avoid partial crystallization of the α-Si film. After α-Si deposition, photoresist was applied to the desired filter areas using an i-line lithography process. The α-Si layer in other areas was completely removed by wet etching (Freckle etching solution). A 1000 nm thick PECVD SiO2 cladding layer was then deposited to protect the entire sample. Finally, the wafer sample was diced into columnar shapes for optical docking coupling. Docking coupling allows for convenient input / output coupling at various visible / NIR wavelengths.

[0103] Figure 6 Two images of the manufactured prototype LPF are shown. Figure 6 Subfigure (a) shows images of the fabricated filters with different filter lengths, ranging from 0 to 300 μm in 100 μm increments. Subfigure (b) shows a SEM image of the waveguide-to-filter transition region. The transition region is designed to transition in a sloping manner to gradually adjust the impedance mismatch between the waveguide and filter regions. However, it is important to note that this sloping structure was not accurately transferred during wet etching, resulting in a rough sloping transition.

[0104] A.3. Experimental Results

[0105] In the prototype LPF, a pair of long working distance objectives (numerical aperture of 0.42) are used for input / output docking. Lens-to-lens normalized transmission spectra with TE and TM polarization at different filter lengths were obtained experimentally. The corresponding experimental results are shown below. Figure 7 As shown in the image.

[0106] Figure 7Subplots (a) and (b) show the normalized transmission spectra of the fabricated filters with different lengths in the O, C, and L bands in TE and TM polarization, respectively. The results show that IL does not systematically increase with increasing filter length in 100 μm steps, indicating that passive waveguide loss in the transparent window is negligible over the considered length range. However, a systematic difference was observed between the reference waveguide (i.e., the waveguide completely uncovered by α-Si) and the three filters.

[0107] Figure 7 Subplots (c) and (d) show the normalized transmission spectra of TE and TM polarizations at a wavelength of 780 nm (765–795 nm), respectively. Due to the use of a straight waveguide in the filter, the background power caused by direct scattering of the input light is high, approximately 40 dB. It was observed that even for the shortest filter with a length of 100 μm, the transmitted light is at the background level of the scattered light. Near-field images of the output docking coupler captured by a charge-coupled device camera from the reference waveguide (specifically the 100 μm long filter) further confirm this observation. The near-field images are shown in... Figure 7 The subgraphs (e) and (f) are shown. For subgraphs (e) and (f), the exposure times for obtaining the near-field image are 1 ms and 200 ms, respectively, corresponding to a 23 dB enhancement for the longer exposure. After absorption, the transmitted light from the docking coupler is at the background level.

[0108] Figure 7 Subplot (g) shows the wavelength-average transmission gain for TE and TM polarization across three wavelength windows (1550 / 1310 / 780 nm) at different filter lengths. (Note that negative transmission gain implies power loss during transmission). Compared to the reference waveguide (i.e., the waveguide with a filter length of 0 μm), the observed ILs for TE and TM polarized waveguide modes in the O band are 0.8 ± 1.0 dB and 1.9 ± 0.5 dB, respectively, while the ILs for TE and TM polarized waveguide modes in the C and L bands are 1.3 ± 1.2 dB and 1.5 ± 0.9 dB, respectively. For transmission at a wavelength of 780 nm, based on the characteristics of a 100 μm length filter and limited by scattering of the input light, the estimated lower limit ILs for TE and TM polarized waveguide modes are 230 ± 8 dB / mm and 248 ± 12 dB / mm, respectively. The extracted / estimated IL value is significantly larger than the simulated value. This is mainly because the material model used in the numerical simulation is based on standard c-Si, and it is known that the absorption coefficient of c-Si is smaller than the absorption coefficient of α-Si used in the prototype LPF.

[0109] Figure 8The spectra obtained using OSA are shown with 780 nm light coupled to a 100 μm long filter and without 780 nm pump light coupled to the same filter. When 780 nm light is coupled to the filter, a pump power of 1 mW is used to obtain the measured spectrum. No significant optical power was observed from radiative recombination above or below the bandgap energy equivalent to approximately 1100 nm to 1550 nm photon energies. The measured spectrum between approximately 800 nm and 1550 nm is at the OSA noise floor. The results indicate that there is no significant light re-emission above -90 dB after absorbing photons from 780 nm light. This finding ensures that on-chip LPFs using indirect bandgap semiconductor absorbers do not act as secondary light sources. This finding is crucial for the application of on-chip LPFs in quantum photonic circuits.

[0110] B. Details of the embodiments of this disclosure

[0111] The embodiments disclosed herein are developed based on the details, examples, applications, experimental results, etc. of the prototype LPF disclosed primarily in part A above, and are generalized and extended where possible.

[0112] A first aspect of this disclosure is to provide an optical LPF for long-pass filtering an incident light beam to produce a filtered beam. The incident light beam includes a desired light component and an undesired light component. The desired light component has one or more first component wavelengths. The undesired light component has one or more second component wavelengths. Each of the one or more first component wavelengths is longer than each of the one or more second component wavelengths.

[0113] Reference Figure 1 As an illustrative example, the incident beam 181 received by the prototype LPF 100 has a desired light component consisting of a signal light and an idler light, and a non-desired pump light component. The wavelengths of the signal light and the idler light are respectively longer than 1100 nm, while the wavelength of the pump light is less than 1100 nm.

[0114] For example, the following uses Figure 9 Describe the disclosed optical LPF, Figure 9 A top view, a longitudinal sectional view (A-A'), and a side sectional view (B-B') of an exemplary optical LPF 200 are shown.

[0115] like Figure 9 As shown, a three-dimensional rectangular coordinate system 80 is defined herein. In coordinate system 80, the z-direction is interpreted as a reference to the vertical direction. In this specification and the appended claims, positional and directional terms such as "above," "below," "higher," "upper part," "lower part," "top," "bottom," and "horizontal" are interpreted with reference to the z-direction.

[0116] LPF 200 is arranged to receive an incident light beam 281 and output a filtered light beam 282. The incident light beam 281 includes a desired light component 291 and an undesired light component 292. Each wavelength of the desired light component 291 is longer than each wavelength of the undesired light component 292.

[0117] LPF 200 includes a waveguide core 210 and a selective absorption layer 220.

[0118] Waveguide core 210 corresponds to waveguide structure 110 of prototype LPF 100. Waveguide core 210 is used to receive incident light beam 281, propagate desired light component 291 and undesired light component 292 inside waveguide core 210, and output filtered light beam 282.

[0119] A selective absorption layer 220, corresponding to the α-(poly-)Si layer 120 in the prototype LPF 100, is deposited on the waveguide core 210. The selective absorption layer 220 is used to selectively absorb unwanted light components 292 without attenuating desired light components 291, or, when desired light components 291 and unwanted light components 292 interact with the selective absorption layer 220 during propagation within the waveguide core 210, the attenuation of desired light components 291 by the selective absorption layer 220 is negligible compared to the absorption of unwanted light components 292. Specifically, the selective absorption layer 220 is composed of an indirect bandgap semiconductor material, wherein the bandgap energy of the indirect bandgap semiconductor material is selected to be greater than the maximum photon energy associated with one or more first component wavelengths and less than the minimum photon energy associated with one or more second component wavelengths. This results in the selective absorption layer 220 dissipating energy from unwanted light components 292 without dissipating energy from desired light components 291. Therefore, when the desired light component 291 and the undesired light component 292 interact with the selective absorption layer 220 during propagation within the waveguide core 210, the undesired light component 292 attenuates, while the optical power of the desired light component 291 remains constant. Note that the maximum photon energy associated with one or more first component wavelengths is calculated by: (1) calculating one or more photon energies corresponding to one or more first component wavelengths; and (2) selecting the maximum value among the calculated one or more photon energies. The minimum photon energy associated with one or more second component wavelengths is obtained by a similar method.

[0120] Indirect bandgap semiconductor materials can be selected from α-Si or poly-Si. Generally, α-Si is superior to poly-Si as an indirect bandgap semiconductor material because α-Si usually has better light absorption than poly-Si and can be deposited onto wafers at low cost through a simple deposition process.

[0121] In some embodiments, the thickness of the selective absorption layer 220 is less than or equal to 100 nm. As shown in the simulation results of the prototype LPF 100 in Part A, a thickness of 40 nm is sufficient to enable the α-Si layer 120 to achieve a high absorption loss of about 120 dB / mm for 780 nm light, while maintaining a negligible IL of less than 1 dB for 1550 nm light.

[0122] In some embodiments, the waveguide core 210 is composed of SiC. The SiC material used as the waveguide core 210 may be 3C-SiC. Other materials that can be used to form the waveguide core 210 include LN and AlN.

[0123] Typically, the LPF 200 also includes an optical insulating layer 230, on which the waveguide core 210 is located. The optical insulating layer 230 provides a first reflective interface 211 between the waveguide core 210 and the optical insulating layer 230 to reflect the desired light component 291 during its propagation within the waveguide core 210. Note that the first reflective interface 211 is only required to reflect the desired light component 291 and is not specifically designed to reflect the undesired light component 292, because the LPF 200 needs to ensure that the desired light component 291 is output from the LPF 200.

[0124] In some embodiments, the first reflective interface 211 is formed due to total internal reflection of the desired light component 291. Therefore, the corresponding materials for forming the optical insulator layer 230 and the waveguide core 210 are selected to allow total internal reflection of the desired light component 291 at the first reflective interface 211. Consequently, the first refractive index of the waveguide core 210 is higher than the second refractive index of the optical insulator layer 230, wherein the first and second refractive indices are measured at each wavelength of one or more first component wavelengths. In some embodiments, the optical insulator layer 230 is composed of SiO2.

[0125] Note that in the fabrication of the prototype LPF 100 detailed above, a 30 nm thick Al2O3 bonding layer is placed beneath the SiC film (i.e., waveguide 110) for bonding with the SiO2 underlayer. Since the desired light component of interest to the prototype LPF 100 has a composition wavelength of approximately 1550 nm, the thickness of the bonding layer (30 nm) is significantly shorter than the wavelength of each light component. The SiO2 underlayer plays a crucial role in achieving total internal reflection of the desired light component 291. Therefore, the SiO2 underlayer is considered as an optical insulating layer 230 forming the first reflective interface 211 with the waveguide core 210.

[0126] In addition to relying on total internal reflection, a first reflective interface 211 can also be created by directly using a reflective material (such as gold) to form an optical insulating layer 230, wherein the reflective material is reflective at each wavelength of one or more first constituent wavelengths.

[0127] exist Figure 9 In the LPF 200 shown, the exposed areas of the waveguide core 210 and the selective absorption layer 220 are surrounded by air. Since the refractive index of air is almost 1, and lower than the corresponding refractive indices of the waveguide core 210 and the selective absorption layer 220, a second reflective interface 212, similar to the first reflective interface 211, is created to reflect the desired light component 291 back to the waveguide core 210 via total internal reflection. Note that in most cases, the thickness of the selective absorption layer 220 is much shorter than each component wavelength of the desired light component 291. For example, the prototype LPF 100 was designed with an α-Si layer of 40 nm thickness, while the wavelength of the desired light component of interest in the prototype LPF 100 is approximately 1550 nm. Therefore, the waveguide core 210 and the surrounding air play a crucial role in establishing the total internal reflection mechanism and generating the second reflective interface 212.

[0128] Figures 10A to 10D Some embodiments of the LPF 200 are depicted in a side sectional view.

[0129] For reference only. Figure 10A An exemplary embodiment of the first LPF 300a, namely the LPF 200 as described above, i.e., the disclosed optical LPF, is depicted. Note that an air cladding, i.e., a cladding formed of air, is used in the first LPF 300a. Furthermore, the waveguide core 210 is a strip waveguide. A strip waveguide is essentially a strip confined between the cladding layers. A rectangular waveguide is an example of a strip waveguide.

[0130] Figure 10B The second LPF 300b is shown, which is a variant of the first LPF 300a, using a ribbed waveguide instead of a strip waveguide as the waveguide core 210. A ribbed waveguide is a type of waveguide in which the guiding layer is essentially a slab, on which one (or more) strips are superimposed.

[0131] Figure 10CA third LPF 300c is shown, wherein the first LPF 300a further includes a cladding 240 deposited on an assembly 270 consisting of at least a selective absorption layer 220 and a waveguide core 210. In addition to forming a protective layer for protecting the assembly 270, the cladding 240 is also an optically insulating cladding that provides a second reflective interface 212 between the waveguide core 210 and the cladding 240 to reflect the desired light component 291 during propagation within the waveguide core 210. In some embodiments, the cladding 240 is composed of SiO2.

[0132] Figure 10D A fourth LPF 300d is shown, which is formed from the first LPF 300a by: (1) using a ribbed waveguide instead of a strip waveguide as the waveguide core 210; and (2) including a cladding 240, which is deposited on a combination 270 consisting of at least a selective absorption layer 220 and a waveguide core 210.

[0133] Figure 11A and 11B A fifth LPF 400a and a sixth LPF 400b are shown as two further embodiments of the disclosed optical LPF. The difference between the fifth LPF 400a and the sixth LPF 400b is that the fifth LPF 400a has a straight waveguide core 210s, while the sixth LPF 400b has a curved waveguide core 210u. Therefore, the disclosed optical LPF offers the flexibility to shape the waveguide core 210 into a straight or curved form. As shown in the prototype LPF 100, this flexibility allows for a compact footprint.

[0134] Figures 12A to 12D Various waveguide-to-filter transition designs for the disclosed optical LPF are depicted. In particular, Figure 12A This indicates a sudden transition; Figure 12B The sloping transition is shown; Figure 12C A first-stage tapered transition is shown; Figure 12D A two-stage tapered transition is shown. Before detailing the various transitions, it is necessary to define several regions of the waveguide core.

[0135] The filtering section 520 of waveguide core 210 is the first longitudinal section of waveguide core 210, which is completely covered by selective absorption layers 220a / b / c / d. The unfiltered section 510 of waveguide core 210 is the second longitudinal section of waveguide core 210, which is not covered by any selective absorption layers 220a / b / c / d. The transition region 515 of waveguide core 210 is the third longitudinal section of waveguide core 210, which is located between the unfiltered section 510 and the filtering section 520.

[0136] exist Figure 12A In the abrupt transition shown, there is no transition region. The shape of the selective absorption layer 220a causes the unfiltered section 510 to transition abruptly to the filtered section 520. Specifically, the longitudinal end of the selective absorption layer 220a has a profile 521a aligned with the cross-section of the waveguide core 210.

[0137] exist Figure 12B In the tilted transition shown, the longitudinal end of the selective absorption layer 220b has a tilted profile 521b covering the transition region 515, so that the unfiltered section 510 gradually transitions into the filtered section 520.

[0138] In such Figure 12C In the first-stage conical transition shown, the longitudinal end of the selective adsorption layer 220c has a first-stage conical profile 521c covering the transition region 515, so that the unfiltered section 510 gradually transitions into the filtered section 520.

[0139] exist Figure 12D In the two-stage conical transition shown, the longitudinal end of the selective adsorption layer 220d has a two-stage conical profile 521d covering the transition region 515, so that the unfiltered section 510 gradually transitions into the filtered section 520.

[0140] and Figure 12A Compared to the abrupt transition shown, Figures 12B to 12D The non-sudden transition shown has the advantage of reducing impedance mismatch between the unfiltered section 510 and the filtered section 520.

[0141] A second aspect of this disclosure is to provide an optical conversion device for generating an output beam from an input laser beam, wherein the optical conversion device includes any embodiment of the disclosed optical LPF.

[0142] A third aspect of this disclosure is to provide an optical system including any embodiment of the disclosed light conversion device. The disclosed optical system can be implemented as an integrated photonic chip. Furthermore, the optical system disclosed herein can be applied to fields such as quantum communication and the Internet, quantum computing, quantum metrology, and sensing.

[0143] With the help of Figure 13 The optical system and light conversion device disclosed herein are illustrated by way of example. Figure 13 A schematic diagram illustrating an exemplary embodiment of the disclosed optical system is shown. (As shown) Figure 13 As shown, the optical system 1310 includes a light conversion device 1320.

[0144] Optical conversion device 1320 is used to generate an output beam 1373 from an input laser beam 1370. Optical conversion device 1320 includes one or more nonlinear quantum light sources 1321 and an optical LPF 1322. The one or more nonlinear quantum light sources 1321 are collectively configured to perform SPDC on the input laser beam 1370 to nonlinearly generate a first beam 1372, such that the first beam 1372 includes a desired light component and an undesired light component. The desired light component has one or more first component wavelengths. The undesired light component has one or more second component wavelengths. Each of the one or more first component wavelengths is longer than each of the one or more second component wavelengths. The optical LPF 1322 is implemented as one embodiment of the disclosed optical LPF. The optical LPF 1322 is used to perform long-pass filtering on the first beam 1372 to produce the output beam 1373. The first beam 1372 defined for optical conversion device 1320 is considered as the incident beam 281 defined for LPF 200. Similarly, the filtered beam 282 is considered as the output beam 1373.

[0145] The optical system 1310 includes a light conversion device 1320 and one or more photonic circuits 1330. The one or more photonic circuits 1330 receive an output beam 1373 from the light conversion device 1320 and process the output beam 1373 to generate one or more optical signals 1375.

[0146] One or more optical signals 1375 generated can be detected by single-photon detection 1380.

[0147] In fact, the optical system 1310 can be implemented as an integrated photonic chip. Similarly, the optical conversion device 1320 can also be implemented on an integrated photonic chip.

[0148] C. Comparison with existing technologies

[0149] The following is a commentary on the improvements of this disclosure relative to the prior art. Before the commentary, let us review the prior art disclosures under consideration.

[0150] Reference

[11] presents a wavelength filter with high ER based on cascaded microring resonators. In

[11] , a device for TE polarization was designed, and each microring element was aligned to the desired wavelength using on-chip microheaters. The maximum filter bandwidth achieved was 125 GHz. However, the technique in

[11] requires precise control of ten microheaters to tune the microring elements, thus increasing power consumption. The filter bandwidth is limited to 125 GHz around the target wavelength. This is not convenient for applications requiring flexibility in pump light wavelength (e.g., SPDC-based photon sources). This technique is only suitable for designed TE polarization propagation. TM polarization filtering requires a different design considering the polarization-dependent microring resonators.

[0151] Reference

[12] demonstrates a standard reflector based on a Bragg grating for a target pump wavelength with a target ER of 100 dB. The technique described in

[12] requires a grating period of 320 nm and a length of 2.576 mm. However, the filter bandwidth shown in

[12] is very narrow (<0.01 nm) and fixed at the target wavelength. This technique cannot be applied to applications requiring adjustment of the pump wavelength (e.g., SPDC-based photonic sources). This technique requires precise fabrication of the grating.

[0152] Reference

[13] demonstrates an ER of 56 dB using a 4-stage cascaded UMZI. The ER can be further improved by cascading more than 4 stages. The demonstrated filter bandwidth exceeds 1 nm. This technique requires precise fabrication of the cascaded UMZI to achieve the required spectral overlap between the single-stage filter bands without involving active wavelength alignment. The filter bandwidth of this technique remains limited for applications requiring wide bandwidth to suppress short-wavelength (pump) light (e.g., SPDC-based photonic sources).

[0153] Reference

[14] demonstrates a filter based on a cascaded 16-stage grating-assisted reverse coupler. This technique achieves an ER of 68.5 dB and a filtering bandwidth of 3 nm. The technique combines grating and directional coupler filtering. The grating is characterized by a period of 244 nm and a duty cycle of 50%. The total length of the device is 2 mm. However, the filtering bandwidth of this technique is still limited to the center wavelength, thus it is not convenient for applications that require flexible selection of pump light wavelength (e.g., SPDC-based and color center-based quantum light sources). The 244 nm grating period used in this technique requires advanced device fabrication equipment.

[0154] Reference

[15] achieved a 70dB attenuation at wavelengths near 775nm and a 3dB IL at 1550nm by using a filter based on a cascaded tapered directional coupler. This technique has a filter bandwidth of 16nm near 775nm and a passband bandwidth of 50nm at 1550nm. However, the filter bandwidth used by this technique is still limited. The IL increases with the number of cascaded filter stages. It can be expected that a total ER of more than 100dB will cause the IL at 1550nm wavelength to exceed 3dB, which will result in a photon pair count reduction of more than 6dB.

[0155] Compared to the techniques disclosed in

[11] -

[15] mentioned above, this disclosure, based on the intrinsic absorption of deposited thin-film silicon, achieves a significantly wider filter band for short-wavelength pump light and a passband for long-wavelength generated light. The suppression band naturally covers the typical pump light wavelengths of 532, 650, 780, and 900 nm selected for various on-chip photon sources in the literature, while the passband spans the telecommunications bands of approximately 1.31 μm and 1.55 μm.

[0156] Compared to all the technologies mentioned above, the optical LPF concept disclosed in this paper is simple and does not require advanced manufacturing equipment. The key design parameter for the LPF 200 is the thickness of the absorption film layer.

[0157] Compared to all the technologies mentioned above, the optical LPF disclosed in this paper occupies a much smaller space on the chip because the total length of the filter structure is less than 1 mm, and the waveguide filter can be bent into a compact shape.

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Claims

1. An optical long-pass filter for long-pass filtering an incident beam to generate a filtered beam, the incident beam comprising a desired light component and a non-desired light component, the desired light component having one or more first component wavelengths, the non-desired light component having one or more second component wavelengths, each of the one or more first component wavelengths being longer than each of the one or more second component wavelengths, the filter comprising: A waveguide core, wherein the waveguide core is used to receive the incident light beam, propagate the desired light component and the undesired light component within the waveguide core, and output the filtered light beam, wherein the waveguide core is made of a material selected from silicon carbide (SiC), lithium niobate (LN), or aluminum nitride (AlN); and A selective absorption layer is deposited on the waveguide core. The selective absorption layer is composed of an indirect bandgap semiconductor material selected to have a bandgap energy greater than the maximum photon energy associated with the one or more first component wavelengths and less than the minimum photon energy associated with the one or more second component wavelengths. As a result, when the desired light component and the undesired light component interact with the selective absorption layer during propagation within the waveguide core, the undesired light component is attenuated while the optical power of the desired light component is preserved.

2. The filter according to claim 1, wherein, The indirect bandgap semiconductor material is selected as amorphous silicon (α-Si) or polycrystalline silicon (poly-Si).

3. The filter according to claim 2, wherein, The thickness of the selective absorption layer is less than or equal to 100 nm.

4. The filter according to claim 1, wherein, The waveguide core is implemented in the form of a strip waveguide.

5. The filter according to claim 1, wherein, The waveguide core is implemented in the form of a ribbed waveguide.

6. The filter according to claim 1, wherein, The waveguide core is made of cubic silicon carbide (3C-SiC).

7. The filter according to claim 1, further comprising: An optical insulating layer, wherein the waveguide core is located on the optical insulating layer, wherein the optical insulating layer provides a first reflective interface between the waveguide core and the optical insulating layer to reflect the desired light component during its propagation within the waveguide core.

8. The filter according to claim 7, wherein, The optical insulating layer is composed of silicon dioxide (SiO2).

9. The filter according to claim 7, wherein: The appropriate materials for forming the optical insulating layer and the waveguide core are selected to allow total internal reflection of the desired light component at the first reflective interface.

10. The filter according to claim 7, further comprising: A cladding layer is deposited on an assembly consisting of at least the selective absorption layer and the waveguide core, wherein the cladding layer is an optical insulator cladding layer, and a second reflective interface is provided between the waveguide core and the cladding layer to reflect the desired light component during its propagation within the waveguide core.

11. The filter according to claim 10, wherein, The cladding is composed of silicon dioxide (SiO2).

12. The filter according to claim 1, wherein, The waveguide core is shaped as a straight line.

13. The filter according to claim 1, wherein, The waveguide core is shaped as a bend.

14. The filter according to claim 1, wherein: The selective absorption layer is shaped to cause the non-filtered section of the waveguide core to abruptly transition to its filtered section. The filtering section is the first longitudinal section of the waveguide core, and the first longitudinal section is completely covered by the selective absorption layer; and The unfiltered section is the second longitudinal section of the waveguide core, which is completely uncovered by any selective absorption layer.

15. The filter according to claim 1, wherein: The longitudinal end of the selective absorption layer has an inclined profile, which covers the transition region between the unfiltered section and the filtered section of the waveguide core, so that the unfiltered section gradually transitions to the filtered section. The filtering section is the first longitudinal section of the waveguide core, and the first longitudinal section is completely covered by the selective absorption layer; as well as The unfiltered section is the second longitudinal section of the waveguide core, which is completely uncovered by any selective absorption layer.

16. The filter according to claim 1, wherein: The longitudinal end of the selective absorption layer has a first-order tapered profile, which covers the transition region between the unfiltered section and the filtered section of the waveguide core, so that the unfiltered section gradually transitions to the filtered section. The filtering section is the first longitudinal section of the waveguide core, and the first longitudinal section is completely covered by the selective absorption layer; as well as The unfiltered section is the second longitudinal section of the waveguide core, which is completely uncovered by any selective absorption layer.

17. The filter according to claim 1, wherein: The longitudinal end of the selective absorption layer has a two-stage tapered profile, which covers the transition region between the unfiltered section and the filtered section of the waveguide core, so that the unfiltered section gradually transitions to the filtered section. The filtering section is the first longitudinal section of the waveguide core, and the first longitudinal section is completely covered by the selective absorption layer; as well as The unfiltered section is the second longitudinal section of the waveguide core, which is completely uncovered by any selective absorption layer.

18. An optical conversion device for generating an output beam from an input laser beam, comprising: One or more nonlinear quantum light sources are configured to perform spontaneous parametric down-conversion on the input laser beam to nonlinearly generate a first beam, such that the first beam includes a desired light component and an undesired light component, the desired light component having one or more first component wavelengths, the undesired light component having one or more second component wavelengths, each of the one or more first component wavelengths being longer than each of the one or more second component wavelengths. as well as According to claim 1, the optical long-pass filter is used to perform long-pass filtering on the first beam to generate an output beam, wherein the first beam is regarded as the incident beam and the filtered beam is regarded as the output beam.

19. An optical system comprising: The optical conversion device according to claim 18; as well as One or more photonic circuits for processing the output beam.

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