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Photonic circuits find applications in biomedicine, manufacturing, quantum computing and communications. Photonic waveguides are crucial components, typically having cross-section orders of magnitude inferior when compared with other photonic components (e.g., optical fibers, light sources and photodetectors). Several light-coupling methods exist, consisting of either on-plane (e.g., adiabatic and end-fire coupling) or off-plane methods (e.g., grating and vertical couplers). The grating coupler is a versatile light-transference technique which can be tested at wafer level, not requiring specific fiber terminations or additional optical components, like lenses, polarizers or prisms. This study focuses on fully-etched grating couplers without a bottom reflector, made from hydrogenated amorphous silicon (a-Si:H), deposited over a silica substrate. Different coupler designs were tested, and of these we highlight two: the superimposition of two lithographic masks with different periods and an offset between them to create a random distribution and a technique based on the quadratic refractive-index variation along the device's length. Results were obtained by 2D-FDTD simulation. The designed grating couplers achieve coupling efficiencies for the TE-like mode over −8 dB (mask overlap) and −3 dB (quadratic variation), at a wavelength of 1550 nm. The coupling scheme considers a 220 nm a-Si:H waveguide and an SMF-28 optical fiber. [ABSTRACT FROM AUTHOR]

Recent advancements in quantum photonic circuits have significantly influenced the field of quantum information processing. The pursuit of an integrated quantum photonic circuit that offers an active, stable platform for large-scale integration and high processing efficiency remains a key objective. The grating coupler, as a crucial element for an efficient transformation output interface in the integrated quantum photonic circuits, presents significant potential for practical applications. Here, we demonstrate the integration block of a highly efficient shallow-etched focusing apodized grating coupler with indium arsenide (InAs) quantum dots (QDs) in gallium arsenide (GaAs) on a SiO2 substrate for active quantum photonic circuits. The designed grating couplers possess a high efficiency over 90% in the broadband (900–930 nm) from the circuit to free space, and a nearly-perfect match with the fiber mode. Experimentally, the efficiency to free space reaches 81.8%, and the match degree with the fiber mode is high up to 92.1%. The proposed integration block offers the potential for large-scale integration of active quantum photonic circuits due to its stable solid substrate and highly performant output for quantum measurements. [ABSTRACT FROM AUTHOR]

Silver-based grating structures offer means for implementing low-cost, efficient grating couplers for use in surface plasmon resonance (SPR) sensors. One-dimensional grating structures with a fixed periodicity are confined to operate effectively within a single planar orientation. However, two-dimensional grating structures as well as grating structures with variable periodicity allow for the plasmon excitation angle to be seamlessly adjusted. This study demonstrates silver-based grating designs that allow for the plasmon excitation angle to be adjusted via rotation or beam position. The flexible angle adjustment opens up the possibility of developing SPR sensor designs with an expanded dynamic range and increased flexibility in sensing applications. The results demonstrate that efficient coupling into two diffraction orders is possible, which ultimately leads to an excitation angle range from 16° to 40° by rotating a single structure. The findings suggest a promising direction for the development of versatile and adaptable SPR sensing platforms with enhanced performance characteristics. [ABSTRACT FROM AUTHOR]

Solid state LiDAR systems traditionally rely on costly active components for efficient beam scanning. In this study, we propose a cost-effective, purely passive steering approach using apodized slanted grating couplers. Through apodization, we achieve a uniform upward emission profile and enhanced upward transmission. Theoretical calculations indicate successful steering of 91. 5 ∘ × 42. 8 ∘ . Experimental results closely match theoretical predictions, validating the capabilities of our passive steering concept. Additionally, the grating couplers, with a length of 3 mm, enable a farfield FWHM of 0.026 ∘ , further enhancing sensing resolution. [ABSTRACT FROM AUTHOR]

Grating couplers are widely used to couple waveguide modes with the far field. Their usefulness is determined not only by energy efficiency but also by additional supported functionality. In this paper, a plasmonic is demonstrated grating on a silicon nitride slab waveguide that couples both transverse electric (TE) and transverse magnetic (TM) waveguide modes with circularly polarized light in the far field. Specifically, it is experimentally confirmed that circularly polarized light excites TE and TM modes propagating in opposite directions, and the direction is controlled by the handedness. The routing efficiency for normally incident light reaches up to 95%. The same structure operates in the outcoupling regime as well, demonstrating up to 97% degree of circular polarization, where the handedness is determined by the polarization and propagation direction of outcoupled modes. The results pave the way for the realization of polarization‐division multiplexers and demultiplexers, integrated circular polarization emitters, as well as detectors of the polarization state of the incident optical field. [ABSTRACT FROM AUTHOR]

In this study, one-dimensional grating coupler on single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) that also served as a polarization splitter was designed. The coupler could separate both orthogonal polarization states into two opposite directions while coupled light from a standard single-mode fiber to a waveguide on LNOI at the same time. Using segmented and apodized designing, the peak coupling efficiencies (CEs) around telecommunication wavelength 1550 nm for fundamental TE and TM modes of −2.82 dB and −2.83 dB, respectively, were achieved. The CEs could be optimized to −1.97 dB and −1.8 dB when a metal layer was added below the silicon dioxide layer. [ABSTRACT FROM AUTHOR]

A grating coupler structure on a X-cut lithium niobate on insulator (LNOI) platform is designed and demonstrated. The utilized inverse design method is based on gradient-based adjoint optimization method, which is time saving and with more optimization degrees of freedom. For TE polarization, the high coupling efficiency of −3.3 dB is experimentally obtained. The maximum coupling efficiency is still −3.9 dB with the broadband 3-dB bandwidth of 90 nm. The grating coupler of high performance is realized by only single step etching and without bottom metal reflection layer.

In this paper, we propose and validate a new approach for increasing the optical bandwidth of waveguide grating couplers by using a center-symmetric grating structure. Our results demonstrate state-of-the-art performance in increasing the 1 dB optical bandwidth to 108 nm and experimentally validated coupling efficiency of −3.0 dB. The fabrication uses standard MPW fabrication service offered by commercial foundry with a minimum feature size above 181 nm. Such a flat wideband and high-efficiency performance is suitable for large-volume manufacturing and will enable the use of grating couplers in wideband applications such as wavelength division multiplexed communications, broadband applications of optical frequency combs and optical sensors.

To fully utilize the advantages of Si3N4 and Silicon-On-Insulator to achieve a high-efficiency wideband grating coupler, we propose and numerically demonstrate a grating coupler based on Si3N4 and a Silicon-On-Insulator heterogeneous integration platform. A two-dimensional model of the coupler was established and a comprehensive finite difference time domain analysis was conducted. Focusing on coupling efficiency as a primary metric, we examined the impact of factors such as grating period, filling factor, etching depth, and the thicknesses of the SiO2 upper cladding, Si3N4, silicon waveguide, and SiO2 buried oxide layers. The calculations yielded an optimized grating coupler with a coupling efficiency of 81.8% (−0.87 dB) at 1550 nm and a 1-dB bandwidth of 540 nm. The grating can be obtained through a single etching step with a low fabrication complexity. Furthermore, the fabrication tolerances of the grating period and etching depth were studied systematically, and the results indicated a high fabrication tolerance. These findings can offer theoretical and parameter guidance for the design and optimization of high-efficiency and broad-bandwidth grating couplers. [ABSTRACT FROM AUTHOR]

We present a high-efficiency silicon grating coupler design based on a left–right mirror-symmetric grating and a metal mirror. The coupler achieves nearly perfect 90-degree vertical coupling. When two SOI chips are placed face to face with a vertical working distance of 50 μm, the chip-to-chip interlayer coupling efficiency reaches as high as 96%. When the vertical working distance ranges from 45 μm to 55 μm, the coupling loss remains below 1 dB. We also verified the effectiveness of our designed vertical coupler through 3D FDTD full-model simulation. The results demonstrate that our proposed vertical coupling structure represents a high-efficiency solution for 3D optical interconnects. The integration of multiple photonic chips in a 3D package with vertical optical and electrical interconnects is also feasible in the foreseeable future. [ABSTRACT FROM AUTHOR]

Photonic circuits find applications in biomedicine, manufacturing, quantum computing and communications. Photonic waveguides are crucial components, typically having cross-section orders of magnitude inferior when compared with other photonic components (e.g., optical fibers, light sources and photodetectors). Several light-coupling methods exist, consisting of either on-plane (e.g., adiabatic and end-fire coupling) or off-plane methods (e.g., grating and vertical couplers). The grating coupler is a versatile light-transference technique which can be tested at wafer level, not requiring specific fiber terminations or additional optical components, like lenses, polarizers or prisms. This study focuses on fully-etched grating couplers without a bottom reflector, made from hydrogenated amorphous silicon (a-Si:H), deposited over a silica substrate. Different coupler designs were tested, and of these we highlight two: the superimposition of two lithographic masks with different periods and an offset between them to create a random distribution and a technique based on the quadratic refractive-index variation along the device’s length. Results were obtained by 2D-FDTD simulation. The designed grating couplers achieve coupling efficiencies for the TE-like mode over −8 dB (mask overlap) and −3 dB (quadratic variation), at a wavelength of 1550 nm. The coupling scheme considers a 220 nm a-Si:H waveguide and an SMF-28 optical fiber.

Silver-based grating structures offer means for implementing low-cost, efficient grating couplers for use in surface plasmon resonance (SPR) sensors. One-dimensional grating structures with a fixed periodicity are confined to operate effectively within a single planar orientation. However, two-dimensional grating structures as well as grating structures with variable periodicity allow for the plasmon excitation angle to be seamlessly adjusted. This study demonstrates silver-based grating designs that allow for the plasmon excitation angle to be adjusted via rotation or beam position. The flexible angle adjustment opens up the possibility of developing SPR sensor designs with an expanded dynamic range and increased flexibility in sensing applications. The results demonstrate that efficient coupling into two diffraction orders is possible, which ultimately leads to an excitation angle range from 16° to 40° by rotating a single structure. The findings suggest a promising direction for the development of versatile and adaptable SPR sensing platforms with enhanced performance characteristics.

The use of surface plasmon resonance sensors allows for the fabrication of highly sensitive, label-free analytical devices. This contribution reports on a grating coupler to enable surface plasmon resonance studies using silver on silicon oxide technology to build long-term stable plasmonic structures for biological molecule sensing. The structural parameters were simulated and the corresponding simulation model was optimized based on the experimental results to improve its reliability. Based on the model, optimized grating nanostructures were fabricated on an oxidized silicon wafer with different structural parameters and characterized using a dedicated optical setup and scanning electron microscopy. The combined theoretical and experimental results show that the most relevant refractive index range for biological samples from 1.32–1.46 may conveniently be covered with a highest sensitivity of 128.85°/RIU. [ABSTRACT FROM AUTHOR]

The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic components in this waveband. Here, an efficient dual-layer grating coupler was designed on a 220 nm thick silicon-on-insulator based on raised polysilicon to address the low directionality issue. For the fiber tilted at an angle of 10°, the grating coupler's simulated coupling efficiency reaches 80.3% (−0.95 dB) at a wavelength of 2002 nm. The 1 dB bandwidth is 66 nm. The structure is completely compatible with the standard silicon photonic fabrication process, making it suitable for large volume fabrication. [ABSTRACT FROM AUTHOR]

An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS2) grating structure. In addition, we confirmed that the monolithic WS2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications. [ABSTRACT FROM AUTHOR]

An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS2) grating structure. In addition, we confirmed that the monolithic WS2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications.

By improving both effective-index (EI) matching and leakage-factor (LF) matching at multiple wavelengths, a method to achieve a wideband-efficient uniform subwavelength grating coupler (SWGC) has been theoretically demonstrated. First, the root-mean-square-error between the grating EI and ideal EI (RMSE-EI) and root-mean-square-error between the grating LF and ideal LF (RMSE-LF) are adopted to weigh EI and LF matching at multiple wavelengths. It is found that when the RMSE-EI decreases and keeping RMSE-LF almost constant, the coupling bandwidth of the uniform SWGC could be substantially increased. Moreover, when RMSE-LF of the uniform SWGC decreases while RMSE-EI remains almost constant, the overall integral coupling efficiency (ICE) within the band from 1525 nm to 1575 nm is enhanced. Furthermore, by simultaneously enhancing both EI matching and LF matching at three wavelengths, a 1-dB bandwidth of 53.7 nm and ICE of 22.31 is obtained in an optimized SOI uniform SWGC. Using the proposed multi-wavelength optimizing methodology, even more efficient wideband couplers could be expected by adopting more optimization accounting for more structural parameters in the future.

We propose and experimentally demonstrate a silicon integrated high-order mode fiber-chip coupler. By utilizing a structure of apodized double-part gratings with mode selective trident, TE1 mode can be coupled with low mode crosstalk. Design details and the corresponding simulation results are presented. The coupler is fabricated on silicon integrated platform. Measured results show the loss is 4.68 dB for TE1-LP11 mode coupling with the mode crosstalk

An efficient optical coupler to transfer the signal between an optical fiber and a silicon waveguide is essential for realizing the applications of silicon photonic integrated circuits such as optical communication and optical sensing. In this paper, we numerically demonstrate a two-dimensional grating coupler based on a silicon-on-insulator platform to obtain completely vertical and polarization-independent couplings, which potentially ease the difficulty of packaging and measurement of photonic integrated circuits. To mitigate the coupling loss induced by the second-order diffraction, two corner mirrors are respectively placed at the two orthogonal ends of the two-dimensional grating coupler to create appropriate interference conditions. Partial single-etch is assumed to form an asymmetric grating to obtain high directionalities without a bottom mirror. The two-dimensional grating coupler is optimized and verified with finite-difference time-domain simulations, achieving a high coupling efficiency of −1.53 dB and a low polarization-dependent loss of 0.015 dB when coupling to a standard single-mode fiber at approximately 1310 nm wavelength. [ABSTRACT FROM AUTHOR]

We propose a perfectly vertical coupling scheme based on metasurface deflectors (meta-deflectors) and grating couplers (GCs). An approach for optimizing the GCs based on the Gaussian-fitting using the genetic algorithm is proposed. An meta-deflector based on amorphous silicon (a-Si) pillars is designed to the optimal coupling angle of the GC to ensure good coupling efficiency (CE). Simulations predict peak vertical CE to be 78% at the wavelength of 2 μm, with 1 dB bandwidth ≥35 nm. The design process of GC and meta-deflector is provided in detail, and the influence of fabrication error on the CE is analyzed. [ABSTRACT FROM AUTHOR]

A high‐performance binary blazed grating coupler with a mixed germanium and zinc selenide structure on a Ge‐on‐Si platform is proposed for a perfectly vertical coupling. To obtain high coupling efficiency, grating parameters are repeatedly and carefully optimized. For the perfectly vertical coupler with transverse‐magnetic polarized incident light, the coupling efficiency is 49.5% at the wavelength of 8 μm with a 1‐dB bandwidth of 110 nm. Thus, the coupling efficiency‐bandwidth product is beyond 54.5 nm. Furthermore, the manufacturing route of the grating is discussed, which adopts the combination of electron‐beam lithography and three‐dimensional nanoprinting. [ABSTRACT FROM AUTHOR]

In this study, one-dimensional grating coupler on single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) that also served as a polarization splitter was designed. The coupler could separate both orthogonal polarization states into two opposite directions while coupled light from a standard single-mode fiber to a waveguide on LNOI at the same time. Using segmented and apodized designing, the peak coupling efficiencies (CEs) around telecommunication wavelength 1550 nm for fundamental TE and TM modes of −2.82 dB and −2.83 dB, respectively, were achieved. The CEs could be optimized to −1.97 dB and −1.8 dB when a metal layer was added below the silicon dioxide layer.

To fully utilize the advantages of Si3N4 and Silicon-On-Insulator to achieve a high-efficiency wideband grating coupler, we propose and numerically demonstrate a grating coupler based on Si3N4 and a Silicon-On-Insulator heterogeneous integration platform. A two-dimensional model of the coupler was established and a comprehensive finite difference time domain analysis was conducted. Focusing on coupling efficiency as a primary metric, we examined the impact of factors such as grating period, filling factor, etching depth, and the thicknesses of the SiO2 upper cladding, Si3N4, silicon waveguide, and SiO2 buried oxide layers. The calculations yielded an optimized grating coupler with a coupling efficiency of 81.8% (−0.87 dB) at 1550 nm and a 1-dB bandwidth of 540 nm. The grating can be obtained through a single etching step with a low fabrication complexity. Furthermore, the fabrication tolerances of the grating period and etching depth were studied systematically, and the results indicated a high fabrication tolerance. These findings can offer theoretical and parameter guidance for the design and optimization of high-efficiency and broad-bandwidth grating couplers.

We present a high-efficiency silicon grating coupler design based on a left–right mirror-symmetric grating and a metal mirror. The coupler achieves nearly perfect 90-degree vertical coupling. When two SOI chips are placed face to face with a vertical working distance of 50 μm, the chip-to-chip interlayer coupling efficiency reaches as high as 96%. When the vertical working distance ranges from 45 μm to 55 μm, the coupling loss remains below 1 dB. We also verified the effectiveness of our designed vertical coupler through 3D FDTD full-model simulation. The results demonstrate that our proposed vertical coupling structure represents a high-efficiency solution for 3D optical interconnects. The integration of multiple photonic chips in a 3D package with vertical optical and electrical interconnects is also feasible in the foreseeable future.

Photonic integrated circuits (PICs) fabricated on silicon nitride (SiN) wafers are witnessing a massive implementation because they can provide low-loss waveguides in a broad wavelength range (from the visible to the near infrared) whilst being fabricated in large volumes using standard silicon fabrication processes. One of the most relevant application fields of this technology is biosensing since SiN PICs enable label-free lab-on-a-chip systems for fast and sensitive detection of minute amounts of different biological entities. To this end, resonant structures such as ring resonators (RR) are highly appropriate, but they need suitable interfaces with optical fiber to be tested on-wafer and used in real applications. The interface that satisfies the two previous requirements is the grating coupler (GC). However, experimental evidence of GCs operating at 1310 nm wavelengths for the transverse magnetic (TM) mode, for which RR-based sensors perform better than for the widely employed transverse electric (TE) mode, has not been reported so far. In this work, the operation of fully-etched focusing GCs at 1310 nm wavelengths for the TM mode in a SiN PIC is demonstrated. Experiments show that the fabricated GCs display an ≈40 nm bandwidth and coupling losses around 13 dB per interface. In addition, we show successful biosensing experiments using bovine serum albumin and anti-bovine serum albumin as biological agents in a microfluidic environment. Further engineering for the interface should lead to lower coupling losses, which would pave the way to practical applications of SiN PICs based on RRs operated for the TM mode and at 1310 nm wavelengths.

Silicon photonics has been studied in various areas by providing small-footprint, high-performance, and mass-producible optoelectronic components integrated on a chip. For the photonic packaging, ultraviolet (UV) curing techniques have been widely utilized. Meanwhile, silica cladding absorption to UV light could alter its refractive index (RI), affecting the optical testing of silicon devices during or after the device packaging. Therefore, it is significant to characterize the dynamics of RI changes induced by UV-light irradiation. However, the in-situ characterization of such devices is seldom explored. Here, we studied the influence of UV-light irradiation on silicon photonic devices by probing resonant wavelength shifts of a racetrack microring resonator. Specifically, we studied the temporary variation of the resonant wavelength and its recovery time under different UV-light exposure durations. Experimental results show that with a UV energy of 21 J/cm2, the resonator had a maximum resonant wavelength shift of 0.31 nm, corresponding to an effective RI change of 0.0009 and recovering time of 95 minutes. Based on the experimental results, we compressively analyzed and compared RI changes induced by plasmon dispersion effect, thermal optical effect and photon-induced silica densification effect. Our study is expected to provide useful guidelines for in-situ silicon photonic testing and packaging.

We propose a silicon photonicgrating coupling scheme for dual-band multiplexing in optical fibers. By utilizing subwavelength grating effective refractive index engineering, a dual-band operating grating coupler and a wavelength-polarization diplexer are designed. Both devices are fabricated on a 220-nm silicon-on-insulator and require only one 220-nm full etch step to facilitate cost-effective production. The subwavelength grating coupler guides S-band and O-band lights from a single-mode fiber into a waveguide as transverse-electric and transverse-magnetic modes, while the subwavelength diplexer separates the two signals into two waveguides. Experimentally, our proposed scheme for coupling and multiplexing can achieve a total insertion loss of –4.26 dB at near 1490 nm and –5.86 dB at near 1310 nm.

We propose and demonstrate an efficient grating coupler for integrated SiGe photonic devices. A bottom metal layer is adopted to enhance the coupling efficiency on the wafer backside. A low coupling loss of −1.34 dB and −0.79 dB can be theoretically obtained with optimal parameters for uniform and apodized grating couplers, respectively. The fabrication process is CMOS compatible without need of wafer bonding. The influence of fabrication errors on the coupling efficiency is analyzed in terms of substrate thickness, grating dimension and material refractive index. The results indicate a large tolerance for the deviations in practical fabrication. The measured coupling loss of the uniform grating is −2.7 dB at approximately 1465 nm with a 3 dB bandwidth of more than 40 nm. The proposed grating coupler provides a promising approach to realize efficient chip-fiber coupling for the SiGe photonic integration. [ABSTRACT FROM AUTHOR]

The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic components in this waveband. Here, an efficient dual-layer grating coupler was designed on a 220 nm thick silicon-on-insulator based on raised polysilicon to address the low directionality issue. For the fiber tilted at an angle of 10°, the grating coupler’s simulated coupling efficiency reaches 80.3% (−0.95 dB) at a wavelength of 2002 nm. The 1 dB bandwidth is 66 nm. The structure is completely compatible with the standard silicon photonic fabrication process, making it suitable for large volume fabrication.

The use of surface plasmon resonance sensors allows for the fabrication of highly sensitive, label-free analytical devices. This contribution reports on a grating coupler to enable surface plasmon resonance studies using silver on silicon oxide technology to build long-term stable plasmonic structures for biological molecule sensing. The structural parameters were simulated and the corresponding simulation model was optimized based on the experimental results to improve its reliability. Based on the model, optimized grating nanostructures were fabricated on an oxidized silicon wafer with different structural parameters and characterized using a dedicated optical setup and scanning electron microscopy. The combined theoretical and experimental results show that the most relevant refractive index range for biological samples from 1.32–1.46 may conveniently be covered with a highest sensitivity of 128.85°/RIU.

A one-dimensional focusing grating coupler array based in silicon nitride (SiN) was proposed for trapped ion qubit manipulation. By applying inverse design optimization with a double-etched grating structure, a directionality of 98% was achieved. A small beam diameter of 2.5 μ m on the target ion with a low crosstalk of −36 dB was attained. Additionally, the impact of fabrication errors was investigated through a Monte Carlo simulation; within the accuracy of an electron beam lithography-based process, the output efficiency was maintained at 93%. [ABSTRACT FROM AUTHOR]

High performance grating coupler array for coupling of a silicon photonic chip to a multi-core fiber mounted perfectly vertical to the chip surface is proposed and demonstrated. Experimental results show a low coupling loss of −0.8 dB with 1-dB bandwidth of 47 nm, a low back reflection of <−15.8 dB in about 40 nm wavelength range, a channel non-uniformity of 0.9 dB, and crosstalk of <−40 dB. These improved results attribute to the design of embedding the diffractive grating in a one-port cavity formed by partially and perfectly reflecting Bragg gratings on the silicon chip and a metal mirror on the chip surface. [ABSTRACT FROM AUTHOR]

Silicon photonics has been studied in various areas by providing small-footprint, high-performance, and mass-producible optoelectronic components integrated on a chip. For the photonic packaging, ultraviolet (UV) curing techniques have been widely utilized. Meanwhile, silica cladding absorption to UV light could alter its refractive index (RI), affecting the optical testing of silicon devices during or after the device packaging. Therefore, it is significant to characterize the dynamics of RI changes induced by UV-light irradiation. However, the in-situ characterization of such devices is seldom explored. Here, we studied the influence of UV-light irradiation on silicon photonic devices by probing resonant wavelength shifts of a racetrack microring resonator. Specifically, we studied the temporary variation of the resonant wavelength and its recovery time under different UV-light exposure durations. Experimental results show that with a UV energy of 21 J/cm2, the resonator had a maximum resonant wavelength shift of 0.31 nm, corresponding to an effective RI change of 0.0009 and recovering time of 95 minutes. Based on the experimental results, we compressively analyzed and compared RI changes induced by plasmon dispersion effect, thermal optical effect and photon-induced silica densification effect. Our study is expected to provide useful guidelines for in-situ silicon photonic testing and packaging. [ABSTRACT FROM AUTHOR]

Atomic magnetometers (AMs) are widely acknowledged as one of the most sensitive kind of instruments for bio-magnetic field measurement. Recently, there has been growing interest in developing chip-scale AMs through nanophotonics and current CMOS-compatible nanofabrication technology, in pursuit of substantial reduction in volume and cost. In this study, an integrated polarization-splitting grating coupler is demonstrated to achieve both efficient coupling and polarization splitting at the D1 transition wavelength of rubidium (795 nm). With this device, linearly polarized probe light that experienced optical rotation due to magnetically induced circular birefringence (of alkali medium) can be coupled and split into individual output ports. This is especially advantageous for emerging chip-scale AMs in that differential detection of ultra-weak magnetic field can be achieved through compact planar optical components. In addition, the device is designed with silicon nitride material on silicon dioxide that is deposited on a silicon substrate, being compatible with the current CMOS nanofabrication industry. Our study paves the way for the development of on-chip AMs that are the foundation for future multi-channel high-spatial resolution bio-magnetic imaging instruments. [ABSTRACT FROM AUTHOR]

An efficient optical coupler to transfer the signal between an optical fiber and a silicon waveguide is essential for realizing the applications of silicon photonic integrated circuits such as optical communication and optical sensing. In this paper, we numerically demonstrate a two-dimensional grating coupler based on a silicon-on-insulator platform to obtain completely vertical and polarization-independent couplings, which potentially ease the difficulty of packaging and measurement of photonic integrated circuits. To mitigate the coupling loss induced by the second-order diffraction, two corner mirrors are respectively placed at the two orthogonal ends of the two-dimensional grating coupler to create appropriate interference conditions. Partial single-etch is assumed to form an asymmetric grating to obtain high directionalities without a bottom mirror. The two-dimensional grating coupler is optimized and verified with finite-difference time-domain simulations, achieving a high coupling efficiency of −1.53 dB and a low polarization-dependent loss of 0.015 dB when coupling to a standard single-mode fiber at approximately 1310 nm wavelength.

We propose a perfectly vertical coupling scheme based on metasurface deflectors (meta-deflectors) and grating couplers (GCs). An approach for optimizing the GCs based on the Gaussian-fitting using the genetic algorithm is proposed. An meta-deflector based on amorphous silicon (a-Si) pillars is designed to the optimal coupling angle of the GC to ensure good coupling efficiency (CE). Simulations predict peak vertical CE to be 78% at the wavelength of 2 μm, with 1 dB bandwidth ≥35 nm. The design process of GC and meta-deflector is provided in detail, and the influence of fabrication error on the CE is analyzed.

We demonstrate a kind of grating coupler that generates a high quality flat-top beam with a small beamwidth from photonic integrated circuits into free-space. The grating coupler is designed on a silicon-on-insulator wafer with a 220-nm-thick silicon layer and consists of a dual-etch grating (DG) and a distributed Bragg reflector (DBR). By adjusting the structural parameters of DG and DBR, a pixel-level (6.6 µm) flat-top beam with a vertical radiation of −0.5 dB and a mode match of 97% at 1550 nm is realized. Furthermore, a series of high-efficiency grating couplers are designed to create a flat-top beam with different scales. [ABSTRACT FROM AUTHOR]

In this paper, a double-layer approach is proposed to design a compact four states polarization-independent grating coupler (GC). The proposed polarization-independent GC is designed to couple a 700 nm polarized light propagated in a 150 nm Gallium Phosphide (GaP) waveguide to a polarization-maintaining fiber. The double-layer approach is based on the deposition of GaP gratings designed to couple the transverse magnetic (TM) light over the GaP gratings designed to couple the transverse electric (TE) light. The two layers are separated by a Hydrogen silsesquioxane (HSQ) with an optimum thickness of 20 nm. The grating periods and the fill factors of the two layers are optimized to achieve maximum possible coupling efficiency (CE). The proposed method resulted in relatively high CEs of 39.2, 31.1, and 23.3% for the TE, TM, and 45°/-45° linearly polarized light, respectively. The polarization-dependent loss (PDL) is 1 dB, 1.26 dB, and 2.26 dB corresponding to TE-TM, TM-45°/-45°, and TE-45°/-45°, respectively. The performance of the double-layer approach is numerically verified by the two-dimensional (2D) finite element algorithm (FEM) using COMSOL software. The proposed method suggests a novel and simple approach to design a compact four states polarization-independent GC that could be used in integrated (on-chip) photonic communication circuits. [ABSTRACT FROM AUTHOR]

The correlation between electrical properties and surface plasmon resonance (SPR) wavelength was discussed using ITO grating coupler. ITO films were prepared on SiO 2 /Si substrate by RF sputtering with thermal annealing, and the grating was directly fabricated on the sample surface by focused ion beam. Electron density and mobility were evaluated by fitting optical reflection spectra according to Drude model, and then SPR wavelength was estimated from the dispersion relation of surface plasmon. Characteristic SPR dip was observed in polarized reflection spectrum, however, measured wavelength was different from theoretically estimated value. The discrepancy was explained by introducing the assumption based on effective medium theory, in which the grating is assumed as a thin film with effective refractive index. [ABSTRACT FROM AUTHOR]

In this paper, we report, for the first time, a theoretical study on passive photonic devices including optical power splitters/combiners and grating couplers (GCs) operating at non-telecom wavelengths above 2 µ m in a monolithic GaSb platform. Passive components were designed to operate, in particular, at around 2.6 µ m for monolithic integration with active photonic devices on the III–V gallium antimonide material platform. The three popular types of splitters/combiners such as directional couplers, multimode interferometer-, and Y-branch-couplers were theoretically investigated. Based on our optimized design and rigorous analysis, fabrication-compatible 1 × 2 optical power splitters with less than 0.12 dB excess losses, large spectral bandwidth, and a 50:50 splitting ratio are achieved. For fiber-to-chip coupling, we also report the design of GCs with an outcoupling efficiency of ∼29% at 2.56 μ m and a 3 dB bandwidth of 80 nm. The results represent a significant step towards developing a complete functional photonic integrated circuits at mid-wave infrared wavelengths.

We propose and demonstrate an efficient grating coupler for integrated SiGe photonic devices. A bottom metal layer is adopted to enhance the coupling efficiency on the wafer backside. A low coupling loss of −1.34 dB and −0.79 dB can be theoretically obtained with optimal parameters for uniform and apodized grating couplers, respectively. The fabrication process is CMOS compatible without need of wafer bonding. The influence of fabrication errors on the coupling efficiency is analyzed in terms of substrate thickness, grating dimension and material refractive index. The results indicate a large tolerance for the deviations in practical fabrication. The measured coupling loss of the uniform grating is −2.7 dB at approximately 1465 nm with a 3 dB bandwidth of more than 40 nm. The proposed grating coupler provides a promising approach to realize efficient chip-fiber coupling for the SiGe photonic integration.

• A kind of "PICs on III-V laser" strategy for integrated lasers was designed. • Gratings, dioxide layers, DBRs were combined to realize efficient integration. • The TE and the polarization-independent coupling efficiency is 72.4 % and 48 %. • We present a new way for the passive-active high efficiency integration. We report here a theoretical investigation on high efficiency grating coupling for surface emitting blue lasers integrated with silicon nitride (Si 3 N 4) waveguide. DBRs (distributed Bragg reflectors) were introduced into the design which located at the upper and one side of the Si 3 N 4 grating coupler region, and oxide buffer layers was also incorporated simultaneously. By optimizing the grating coupler parameters and the DBR structure, the configuration can be adapted for single polarization or TE/TM simultaneous coupling. The coupling efficiency for TE polarization can reach 72.4 %, and the polarization-independent coupling efficiency of 48 % at vertical incidence was also achieved. This work presents a PIC (photonic integrated circuits) on III-V laser strategy for visible light PICs and innovative applications based on blue laser integrated devices. [ABSTRACT FROM AUTHOR]

Atomic magnetometers (AMs) are widely acknowledged as one of the most sensitive kind of instruments for bio-magnetic field measurement. Recently, there has been growing interest in developing chip-scale AMs through nanophotonics and current CMOS-compatible nanofabrication technology, in pursuit of substantial reduction in volume and cost. In this study, an integrated polarization-splitting grating coupler is demonstrated to achieve both efficient coupling and polarization splitting at the D1 transition wavelength of rubidium (795 nm). With this device, linearly polarized probe light that experienced optical rotation due to magnetically induced circular birefringence (of alkali medium) can be coupled and split into individual output ports. This is especially advantageous for emerging chip-scale AMs in that differential detection of ultra-weak magnetic field can be achieved through compact planar optical components. In addition, the device is designed with silicon nitride material on silicon dioxide that is deposited on a silicon substrate, being compatible with the current CMOS nanofabrication industry. Our study paves the way for the development of on-chip AMs that are the foundation for future multi-channel high-spatial resolution bio-magnetic imaging instruments.

We demonstrate a kind of grating coupler that generates a high quality flat-top beam with a small beamwidth from photonic integrated circuits into free-space. The grating coupler is designed on a silicon-on-insulator wafer with a 220-nm-thick silicon layer and consists of a dual-etch grating (DG) and a distributed Bragg reflector (DBR). By adjusting the structural parameters of DG and DBR, a pixel-level (6.6 µm) flat-top beam with a vertical radiation of −0.5 dB and a mode match of 97% at 1550 nm is realized. Furthermore, a series of high-efficiency grating couplers are designed to create a flat-top beam with different scales.

Electrical interconnects are becoming a bottleneck in the way towards meeting future performance requirements of integrated circuits. Moore’s law, which observes the doubling of the number of transistors in integrated circuits every couple of years, can no longer be maintained due to reaching a physical barrier for scaling down the transistor’s size lower than 5 nm. Heading towards multi-core and many-core chips, to mitigate such a barrier and maintain Moore’s law in the future, is the solution being pursued today. However, such distributed nature requires a large interconnect network that is found to consume more than 80% of the microprocessor power. Optical interconnects represent one of the viable future alternatives that can resolve many of the challenges faced by electrical interconnects. However, reaching a maturity level in optical interconnects that would allow for the transition from electrical to optical interconnects for intra-chip and inter-chip communication is still facing several challenges. A review study is required to compare the recent developments in the optical interconnects with the performance requirements needed to reach the required maturity level for the transition to happen. This review paper dissects the optical interconnect system into its components and explains the foundational concepts behind the various passive and active components along with the performance metrics. The performance of different types of on-chip lasers, grating and edge couplers, modulators, and photodetectors are compared. The potential of a slot waveguide is investigated as a new foundation since it allows for guiding and confining light into low index regions of a few tens of nanometers in cross-section. Additionally, it can be tuned to optimize transmissions over 90° bends. Hence, high-density opto-electronic integrated circuits with optical interconnects reaching the dimensions of their electrical counterparts are becoming a possibility. The latest complete optical interconnect systems realized so far are reviewed as well.

In this research, a vertical silicon nitride strip-loaded grating coupler on lithium niobate thin film was proposed, designed, and simulated. In order to improve the coupling efficiency and bandwidth, the parameters such as the SiO2 cladding layer thickness, grating period, duty cycle, fiber position, and fiber angle were optimized and analyzed. The alignment tolerances of the grating coupler parameters were also calculated. The maximum coupling efficiency and the −3 dB bandwidth were optimized to 33.5% and 113 nm, respectively. In addition, the grating coupler exhibited a high alignment tolerance.