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![Diffraction grating-based optical coupling with apodized grating.
Schematic diagram of mode overlap difference between (A) the uniform and (B) apodized gratings. Depending on the grating apodization, exponentially decaying or Gaussian-like electric field profiles (red curves, E1′) are propagated from waveguide 1 (grating-based optical couplers) and Gaussian modes (blue curves, ∑E2,k) are the guided modes in waveguide 2 (optical fibers). (C, D) Cross-section and SEM image for the apodized grating couplers based on optimized fill factors. Adapted from Ref. [109]. Copyright (2013) The Optical Society.](publication/328462073/figure/fig10/AS:11431281120823728@1676645392756/Diffraction-grating-based-optical-coupling-with-apodized-grating-Schematic-diagram-of.jpg)
Diffraction grating-based optical coupling with apodized grating. Schematic diagram of mode overlap difference between (A) the uniform and (B) apodized gratings. Depending on the grating apodization, exponentially decaying or Gaussian-like electric field profiles (red curves, E1′) are propagated from waveguide 1 (grating-based optical couplers) and Gaussian modes (blue curves, ∑E2,k) are the guided modes in waveguide 2 (optical fibers). (C, D) Cross-section and SEM image for the apodized grating couplers based on optimized fill factors. Adapted from Ref. [109]. Copyright (2013) The Optical Society.
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Efficient light energy transfer between optical waveguides has been a critical issue in various areas of photonics and optoelectronics. Especially, the light coupling between optical fibers and integrated waveguide structures provides essential input-output interfaces for photonic integrated circuits (PICs) and plays a crucial role in reliable opti...
Citations
... Most edge couplers are designed to entail long adiabatic tapers with lengths exceeding several hundred micrometers, thereby enhancing the mode transfer efficiency. Several techniques make it possible to further improve coupling efficiency, like the 2×2 couplers [27][28][29][30][31], mode multiplexers [31][32][33][34], and polarization splitters/rotators [35][36][37][38][39][40][41][42]. The compact form of the adiabatic taper drastically facilitates its applicability in high-density PICs [15]. ...
Photonic integrated circuits (PICs) play a crucial role in almost every aspect of modern life, such as data storage, telecommunications, medical diagnostics, green energy, autonomous driving, agriculture, and high-performance computing. To fully harness their benefits, an efficient coupling mechanism is required to successfully launch light into waveguides from fibers. This study introduces low-loss coupling strategies and their implementation for a silicon nitride integrated platform. Here we present an overview of coupling technologies, optimized designs, and a tutorial on manufacturing techniques for inverted tapers, which enable effective coupling for both transverse-magnetic and transverse-electric modes. The optimized coupling losses for the UHNA-7 fiber and the inverted taper Si3N4 coupler reached -0.81 dB at 1550 nm per connection for single-mode waveguides with 220x1200 nm cross section. The measured coupling losses in the inverted taper coupler with a standard single-mode fiber were -3.28 dB at 1550 nm per connection for the same platform.
... On the other hand, the coupling loss comes from the fact that we have to couple the free-space light or the on-chip system to the fiber, and direct coupling between them can have some mode mismatch. For the free-space system, with proper alignment, the mode matching can go up very high as 96% [51], while for the integrated system, there are researches on efficiently coupling light between fiber and on-chip components using tapered fiber and evanescent wave [125]. ...
Quantum entanglement is a fundamental resource for various quantum applications and generation of large-scale entanglement is a key quantum technology. In recent years, continuous-variable optical systems have shown promising results in this direction, thanks to deterministic generation and multiplexing via rich degrees of freedom naturally occurring in the optical system. In this paper, we review the generation and applications of multipartite optical quantum entanglement. We begin with a theoretical overview of how to represent and verify multimode continuous-variable quantum states and entanglement. Then, we discuss the multiplexing technique in time, frequency, and spatial domains, and their applications in generation of large-scale entangled states. Afterward, we review the current status and development of the basic technology used in the generation of multipartite quantum entanglement. Finally, we discuss the applications of large-scale quantum entanglement and possible future perspectives.
... For real-world applications, optical interconnections between fibers and silicon photonic chips are crucial as high fiber-tochip coupling loss limits the performance of optical communication systems. Such an issue shall be addressed seriously [18][19][20][21]. ...
Silicon nitride (SiN) plays a critical role in silicon photonics because of its lower refractive index, low waveguide loss, broad operating bandwidth and compatibility with complementary metal oxide semiconductor (CMOS) fabrication process. Here, we propose a polarization-independent sub-wavelength grating (SWG) edge coupler with high efficiency based on SiN-Si dual-stage structure with a length of only 315.8 μm. Such a structure can be fabricated on 8-inch silicon photonics pilot lines and doesn't require special fabrication processes for making it suspended. We simulate the minimum TE/TM light coupling loss from a standard SMF-28 fiber to be 0.61 dB/0.95 dB with polarization dependent loss (PDL) less than 0.4 dB in the whole band between 1500 nm and 1600 nm.
... A major factor contributing to the coupling losses of light between an optical fiber and the waveguide located on a PIC is mode field and polarisation mismatch (elliptical or circular polarisation). Various building blocks such as SSCs, photonic wire bonds, 3D printed lenses, vertically curved and suspended waveguides, out-of-plane and adiabatic couplers are being studied to enable efficient and reliable transfer of optical signals in a variety of applications [6][7][8][9][10][11][12]. Efficient optical coupling requires good alignment as well as inherent matching between the mode fields that are guided within the waveguides. ...
Commercial introduction of emerging integrated photonics technologies requires a long and complex multi-layer product development, industrialization, and qualification cycles at all levels of value chain from initial product design, material sourcing, component-system-module manufacturing, and testing, through marketing and delivery of new products to the market. Scalable assembly and packaging of electronic-photonic integrated modules is important and may take more than a half of the entire product’s costs. In this paper, we will report on some of our industrial processes for scalable photonics packaging, as well as challenges and results obtained from our research and innovation projects
... Diverse coupler designs have been introduced to enhance coupling efficiency by addressing mode shape and impedance mismatches. Notably, Klopfenstein's icant size, featuring a taper length exceeding 20 μm and a taper angle of 25 • or less based on the mode field diameter [3]- [10]. In contrast, Grating Couplers exhibit a limited bandwidth and lower coupling efficiency, often at 90 % or less [11]- [15]. ...
Efficient energy transfer is crucial in electromagnetic communication. Therefore, producing a waveguide coupler that achieves broadband, nonreflective transmission is a challenging task. With the advancement of silicon-based integrated photonic circuits, fiber-to-chip coupling has become increasingly important. Although various couplers have been developed for fiber-to-chip coupling, they often have limitations such as long coupling length, low coupling efficiency, and narrow bandwidth. This is due to the inability to eliminate reflections between the two waveguides. Here, we introduce a method using universal impedance matching theory and transformation optics to eliminate reflections between two waveguides. The coupler, called the universal impedance matching coupler, using this method has the shortest subwavelength coupling length, a 99.9 % coupling efficiency, and a broad bandwidth.
... Another notable example is MNF-assisted optical coupling between standard single-mode fibers and on-chip planar waveguides, which is one of the main challenges in silicon photonics [339] . In 2007, Zhang et al. reported end-fire coupling from a subwavelength-diameter silica MNF to a silicon waveguide, with coupling efficiency higher than 40% over a wide wavelength range of 1300-1700 nm [334] . ...
Optical micro/nanofiber (MNF) is a quasi-one-dimensional free-standing optical waveguide with a diameter close to or less than the vacuum wavelength of the light. Combining the tiny geometry with high refractive-index contrast between the core and the surrounding, the MNF exhibits favorable optical properties such as tight optical confinement, strong evanescent field and large diameter-dependent waveguide dispersion. Meanwhile, as a quasi-one-dimensional structure with extraordinarily high geometric and structural uniformity, the MNF also has low optical loss and high mechanical strength, making it favorable for manipulating light on the micro/nanoscale with high flexibility. Over the past two decades, optical MNFs, typically being operated in single mode, have been emerging as a miniaturized fiber-optic platform for both scientific research and technological applications. This paper aims to provide a comprehensive overview of the representative advances in optical MNFs in recent years. Starting from the basic structures and fabrication techniques of the optical MNFs, we highlight linear and nonlinear optical and mechanical properties of the MNFs. Then, we introduce typical applications of optical MNFs from near-field optics, passive optical components, optical sensors, optomechanics, to fiber lasers and atom optics. Finally, we give a brief summary of the current status of MNF optics and technology, and provide an outlook into future challenges and opportunities.
... Nevertheless, practical applications of LNOI modulators face a considerable challenge: significant optical losses resulting from mode mismatch and refractive index differences between optical fibers and LNOI optical waveguides [10]. To address this issue, two primary solutions have been proposed: grating couplers and edge couplers [15]. The grating coupler represents a periodic structure facilitating light coupling from the fiber into the waveguide through diffraction action at specific angles and polarizations [16,17]. ...
Fiber-chip edge couplers can minimize mode mismatch in integrated lithium niobate (LiNbO3) photonics via facilitating broad optical bandwidth coupling between optical fibers and waveguide circuits. We designed a high-efficiency multi-tip edge coupler utilizing the lithium niobate on insulator (LNOI) platform for achieving superior fiber-to-chip coupling. The device comprises a bilayer LN inversely tapered waveguide, three 3D inversely tapered waveguides, and a silicon oxynitride (SiON) cladding waveguide (CLDWG). Finite difference method (FDM) and eigenmode expansion (EME) simulations were utilized to simulate and optimize the edge coupler structure specifically within the 1550 nm band. This coupler demonstrates a low fiber-chip coupling loss of 0.0682/0.0958 dB/facet for TE/TM mode at 1550 nm when interfaced with a commercially cleaved single-mode fiber (SMF) with a mode field diameter (MFD) of approximately 8.2 μm. Moreover, the 1 dB bandwidth of the coupler is 270 nm for the TE mode and 288 nm for the TM mode. Notably, the coupler exhibits a relatively large tolerance for optical misalignment owing to its large mode spot size of up to 4 μm. Given its ultra-low loss, high-efficiency ultra-broadband capabilities, and substantial tolerance features, this proposed device provides a paradigm for fiber-to-chip edge coupling within lithium niobate photonics.
... In conjunction with strain tuning methods, biexciton and exciton emissions of QD single photon sources can be tuned reversibly without significant deterioration of linewidth and intensity. 22 Beyond the scope of this, QDs are excellent chip-based sources for generating cluster states 40,41 due to their long spin relaxation times and their ease of integration into selectively transition enhancing photonic nanostructures. 42 Especially photonic crystal waveguides are a feasible platform to control the photon emission of embedded QDs and are particularly suitable for scaling up such systems. ...
The on-chip integration of single photon and entangled photon emitters such as epitaxially grown semiconductor quantum dots into photonic frameworks is a rapidly evolving research field. GaAs quantum dots offer high purity and a high degree of entanglement due to, in part, exhibiting very small fine structure splitting along with short radiative lifetimes. Integrating strain-tunable quantum dots into nanostructures enhances the quantum optical fingerprint, i.e., radiative lifetimes and coupling of these sources, and allows for on-chip manipulation and routing of the generated quantum states of light. Efficient out-coupling of photons for off-chip processing and detection requires carefully engineered mesoscopic structures. Here, we present numerical studies of highly efficient grating couplers reaching up to over 90% transmission. A 2D Gaussian mode overlap of 83.39% for enhanced out-coupling of light from within strain-tunable photonic nanostructures for free-space transmission and single-mode fiber coupling is shown. The photon wavelength under consideration is 780 nm, corresponding to the emission from GaAs quantum dots resembling the 87Rb D2 line. The presented numerical study helps implement such sources for applications in complex quantum optical networks.
... Efficient energy exchange between fiber components and TFLN waveguide is essential for applications of TFLN PICs. 27 Grating couplers and edge couplers are two prevalent devices employed for coupling light into and out of on-chip devices. Generally, grating couplers feature large alignment tolerances and are compact and suitable for densely packed photonic circuits. ...
In the past decade, photonic integrated circuits (PICs) based on thin-film lithium niobate (TFLN) have made substantial progress in various fields, including optical communication, nonlinear photonics, and quantum optics. A critical component is an efficient edge coupler facilitating the connection between PICs and light sources or detectors. Here, we propose an innovative edge coupler design with a wedge-shaped TFLN waveguide and a silicon oxynitride cladding. Experimental results show a low coupling loss between the TFLN PIC and a 3-μm mode field diameter (MFD) lensed fiber, measuring at 1.52 dB/facet, with theoretical potential for improvement to 0.43 dB/facet. Additionally, the coupling loss between the edge coupler and a UHNA7 fiber with an MFD of 3.2 μm is reduced to 0.92 dB/facet. This design exhibits robust fabrication and alignment tolerances. Notably, the minimum linewidth of the TFLN waveguide of the coupler (600 nm) can be readily achieved using commercially available i-line stepper lithography. This work benefits the development of TFLN integrated devices, such as on-chip electro-optic modulators, frequency combs, and lasers.
... ILICON photonics [1]- [3] has undergone dramatic progress, moving concepts from academic study to industry; however, Si optical chips confront the challenge of efficient coupling between chip and fiber due to size and mode mismatch [4], [5]. The problem is generally tackled by edge coupling or grating (vertical) coupling [2]. Edge couplers can achieve coupling losses below 0.5 dB and bandwidths greater than 100 nm [6]. ...
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 22.31is 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.
