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To test silicon photonics component performances, a silicon (Si) grating coupler (GC) is used to couple the light from a single-mode fiber (SMF) into the chip. However, silicon nitride (Si3N4) waveguides have recently become more popular for realizing photonic integrated circuits (PICs), which may be attributable to their exceptional characteristics, such as minimal absorption and low back reflection (BR) in the O-band spectrum. Thus, to test the photonic chip, a waveguide converter from Si3N4 to Si needs to be added to the photonic circuit, which can lead to more power losses and BR. To avoid this conversion, we propose in this manuscript a configuration of a GC based on Si3N4 structures, which can be employed to minimize the footprint size and obtain better performance. The achievement of high efficiency was possibly obtained by optimizing the structural properties of the waveguide and the coupling angle from the SMF. The results demonstrated high efficiency within the O-band spectrum by using a wavelength of 1310 nm. Notably, at this specific wavelength, the findings indicated a coupling efficiency of −5.52 db. The proposed design of the GC consists of a uniform grating that offers improvements regarding affordability and simplicity in manufacturing compared to other GC models. For instance, using a reflector or a GC with non-uniform grooved teeth introduces challenges in fabrication and incurs higher costs. Thus, the proposed design can be useful for improving the testing abilities of the Si3N4 photonic chips used in transceiver systems. [ABSTRACT FROM AUTHOR]

A guided-mode-resonance device for both narrow-band retroreflection and waveguide input coupling was designed and fabricated for operation wavelength of 1550 nm. It consists of a focusing grating coupler (FGC) integrated in a waveguide cavity resonator constructed by a pair of distributed Bragg reflectors (DBRs) on a reflection substrate. The size of the FGC was 31 × 20 μm2. The device was used as a laser mirror of an external cavity laser. The oscillation wavelength was fixed to the retroreflection wavelength, and the laser power was coupled to a waveguide. The coupled power depends on the length of one DBR. The optimal length for the maximum coupling was also discussed theoretically and experimentally.

Efficient fiber‐chip couplers operating at distinct wavelength bands are key components to combine or split different optical bands for emerging data transmission and nonlinear applications. Herein, a dual‐band silicon‐integrated grating coupler (GC) operated at 1.55 and 2 μm wavebands is designed and demonstrated. The proposed device can simultaneously couple 1.55 and 2 μm wavebands light into the in‐plane waveguides at the same incident angle. Numerical simulations indicate that coupling efficiencies (CEs) are −2.5 and −3.9 dB for center wavelengths at 1561 and 1979 nm, respectively. The dual‐band GC is experimentally demonstrated on a commercially available 340 nm silicon‐on‐insulator wafer. The fabricated dual‐band GC with center wavelengths of 1559 and 1968 nm obtains CEs of −4.9 and −6.4 dB, with 3 dB bandwidths of 81 and 80.4 nm, respectively. Also, the first proof‐of‐concept demonstration of 10 Gb s−1 wavelength division multiplexing transmission at 1.55 and 2 μm waveband is presented based on the fabricated dual‐band GC.

To test silicon photonics component performances, a silicon (Si) grating coupler (GC) is used to couple the light from a single-mode fiber (SMF) into the chip. However, silicon nitride (Si3N4) waveguides have recently become more popular for realizing photonic integrated circuits (PICs), which may be attributable to their exceptional characteristics, such as minimal absorption and low back reflection (BR) in the O-band spectrum. Thus, to test the photonic chip, a waveguide converter from Si3N4 to Si needs to be added to the photonic circuit, which can lead to more power losses and BR. To avoid this conversion, we propose in this manuscript a configuration of a GC based on Si3N4 structures, which can be employed to minimize the footprint size and obtain better performance. The achievement of high efficiency was possibly obtained by optimizing the structural properties of the waveguide and the coupling angle from the SMF. The results demonstrated high efficiency within the O-band spectrum by using a wavelength of 1310 nm. Notably, at this specific wavelength, the findings indicated a coupling efficiency of −5.52 db. The proposed design of the GC consists of a uniform grating that offers improvements regarding affordability and simplicity in manufacturing compared to other GC models. For instance, using a reflector or a GC with non-uniform grooved teeth introduces challenges in fabrication and incurs higher costs. Thus, the proposed design can be useful for improving the testing abilities of the Si3N4 photonic chips used in transceiver systems.

A cavity-resonator-integrated guided-mode resonance mirror (CRIGM) consisting of a small-aperture grating coupler integrated in a waveguide cavity on a high-reflection substrate reflects an incident free-space wave with reflection-phase variation of 2 π in a resonance condition. CRIGM has been investigated as a key component in surface mount packaging of VCSEL for future optical interconnects but the reflectance of a fabricated CRIGM was not sufficient. Matching of electric field distribution between an incident Gaussian beam and the reflected beam from the CRIGM is discussed for improving the reflectance. A new apodization method of grating-tooth shift is proposed for the field-distribution matching. CRIGMs with the new and a well-known fill-factor apodization methods are characterized by numerical simulation. It is predicted that the previously measured reflectance will be improved by more than 1.2 dB by introducing either of the apodization methods. [ABSTRACT FROM AUTHOR]

Most of the single-mode fiber-coupled grating couplers reported in the literature have been designed for upward directionality to perform wafer-scale testing of photonic-integrated circuits. A coupler designed for downward directionality can help in achieving a face-up electro-optic integration scheme for chip-to-board coupling using 2.5-D/3-D silicon photonics interposers. We demonstrate for the first time the design analysis and coupling performance of TE-polarized O-band through-substrate (backside emitting) grating couplers that have been optimized to produce enhanced directionality in the downward direction into the bulk silicon substrate of a Si photonics chip. The chip substrate was thinned and polished to achieve a bulk silicon thickness of 100 μm. By the use of an optimized reflector and an optimal thickness of anti-reflection coating at the backside of the chip, a -2.3 and -1.7 dB fiber-to-grating coupling efficiency has been measured when coupled through-substrate to a single-mode and multimode fiber, respectively. These gratings with downward directionality can help provide an efficient and alignment-tolerant interface for chip-to-chip or chip-to-board optical interconnects.

We design and experimentally demonstrate highly efficient Silicon Nitride based grating couplers with bottom distributed Bragg reflectors. All the layers were deposited using plasma enhanced chemical vapor deposition processing. We present gratings on two Silicon Nitride thickness (400 nm and 500 nm) platforms. On a 500 nm thick Silicon Nitride, we show a peak coupling efficiency of -2.29 dB/coupler at a wavelength of 1573 nm with a 1 dB bandwidth of 49 nm. On a 400 nm thick platform, we demonstrate a coupling efficiency of -2.58 dB/coupler at 1576 nm with a 1 dB bandwidth of 52 nm. The demonstrated coupling efficiency is the best reported as yet, for both 400 nm and 500 nm thick, plasma deposited Silicon Nitride platforms.

We demonstrate an alignment-tolerant backside coupling interface in the O-band for silicon photonics by generating an optimized through-substrate (downward) directionality beam from a TE-mode grating coupler and hybrid integrating the chip with backside silicon microlenses to achieve expanded beam collimation. The key advantage of using such an expanded beam interface is an increased coupling tolerance to lateral and longitudinal misalignment. A 34 $\mu$ m beam diameter was achieved over a combined substrate thickness of 630 $\mu$ m which was then coupled to a thermally expanded core single-mode fiber to investigate the tolerances. A 1-dB fiber-to-microlens lateral alignment tolerance of 14 $\mu$ m and an angular alignment tolerance of $1^{\circ }$ was measured at a wavelength of 1310 nm. In addition, a large $\pm$ 2.5 $\mu$ m 1-dB backside alignment accuracy was measured for the placement of microlens with respect to the grating. The radius of curvature of Si microlens to achieve a collimated beam was 480 $\mu$ m, and a 1-dB longitudinal alignment tolerance of 700 $\mu$ m was measured for coupling to a single-mode expanded core fiber. The relaxation in alignment tolerances make the demonstrated coupling interface suitable for chip-to-package or chip-to-board coupling [ABSTRACT FROM AUTHOR]

We propose and demonstrate a hybrid waveguide platform using layered amorphous silicon and silicon nitride. The waveguide offers more degrees of freedom to design waveguides with desired confinement, effective index and polarization birefringence. Unlike single core material, the proposed waveguide offers design flexibility, and light confinement in the layers is polarization-dependent. We present a detailed waveguide design and analysis of efficient fiber-chip grating couplers with a coupling efficiency of −3.27 dB and −8 dB for T E and T M polarization, respectively. The couplers offer a 3 dB bandwidth of 100 nm. Furthermore, we demonstrate excitation of TE and TM modes exploiting the polarization-dependent confinement using thermo-optic characteristics of a ring resonator. [ABSTRACT FROM AUTHOR]

A cavity-resonator-integrated guided-mode resonance mirror (CRIGM) consists of a grating coupler integrated in a waveguide cavity formed by a pair of distributed Bragg reflectors (DBRs) on a high-reflectance substrate. It is theoretically predicted that a CRIGM can give interesting characteristics of not only a narrow band-stop reflection spectrum but also flat high reflectance with steep reflection-phase spectrum according to its structure, whereas only a CRIGM with a band-stop reflection spectrum has been confirmed experimentally so far. This time, the cause of undesirable reflectance drop in a CRIGM for flat high reflectance spectrum is investigated theoretically and experimentally. It is theoretically shown that the CRIGM reflectance at the resonance wavelength is affected considerably by a slight reduction of the DBR reflectance. A CRIGM with a rib-structured channel for high lateral confinement is designed and fabricated to keep the DBR reflectance high by suppressing a propagation loss of a guided wave. The improvement of the CRIGM reflection spectrum is confirmed experimentally by measurement of the reflection spectra of the fabricated devices. [ABSTRACT FROM AUTHOR]

We design and experimentally demonstrate highly efficient Silicon Nitride based grating couplers with bottom distributed Bragg reflectors. All the layers were deposited using plasma enhanced chemical vapor deposition processing. We present gratings on two Silicon Nitride thickness (400 nm and 500 nm) platforms. On a 500 nm thick Silicon Nitride, we show a peak coupling efficiency of $-$ 2.29 dB/coupler at a wavelength of 1573 nm with a 1 dB bandwidth of 49 nm. On a 400 nm thick platform, we demonstrate a coupling efficiency of $-$ 2.58 dB/coupler at 1576 nm with a 1 dB bandwidth of 52 nm. The demonstrated coupling efficiency is the best reported as yet, for both 400 nm and 500 nm thick, plasma deposited Silicon Nitride platforms. [ABSTRACT FROM AUTHOR]

We present a gradient-based optimization strategy to design broadband grating couplers. Using this method, we are able to reach, and often surpass, a user-specified target bandwidth during optimization. The designs produced for 220 nm silicon-on-insulator are capable of achieving 3 dB bandwidths exceeding 100 nm while maintaining central coupling efficiencies ranging from $-$ 3.0 to $-$ 5.4 dB, depending on partial-etch fraction. We fabricate a subset of these structures and experimentally demonstrate gratings with 3 dB bandwidths exceeding 120 nm. This inverse design approach provides a flexible design paradigm, allowing for the creation of broadband grating couplers without requiring constraints on grating geometry. [ABSTRACT FROM AUTHOR]

We present an open-source deep artificial neural network (ANN) model for the accelerated design of polarization-insensitive subwavelength grating (SWG) couplers on the silicon-on-insulator platform. Our model can optimize SWG-based grating couplers for a single fundamental-order polarization, or both, by splitting them counter-directionally at the grating level. Alternating, SWG sections are adopted to reduce the reflections (loss) of standard, single-etch devices—further accelerating the design time by eliminating the need to process a second etch. The model of this device is trained by a dense uniform dataset of finite-difference time-domain (FDTD) optical simulations. Our approach requires the FDTD simulations to be made up front, where the resulting ANN model is made openly available for the rapid, software-free design of future standard photonic devices, which may require slightly different design parameters (e.g., fiber angle, center wavelength, and polarization) for their specific application. By transforming the nonlinear input–output relationship of the device into a matrix of learned weights, a set of simple linear algebraic and nonlinear activation calculations can be made to predict the device outputs 1830 times faster than numerical simulations, within 93.2% accuracy of the simulations. [ABSTRACT FROM AUTHOR]

Silicon photonics is an enabling technology that provides integrated photonic devices and systems with low-cost mass manufacturing capability. It has attracted increasing attention in both academia and industry in recent years, not only for its applications in communications, but also in sensing. One important issue of silicon photonics that comes with its high integration density is an interface between its high-performance integrated waveguide devices and optical fibers or free-space optics. Surface grating coupler is a preferred candidate that provides flexibility for circuit design and reduces effort for both fabrication and alignment. In the past decades, considerable research efforts have been made on in-plane grating couplers to address their insufficiency in coupling efficiency, wavelength sensitivity and polarization sensitivity compared with out-of-plane edge-coupling. Apart from improved performances, new functionalities are also on the horizon for grating couplers. In this paper, we review the current research progresses made on grating couplers, starting from their fundamental theories and concepts. Then, we conclude various methods to improve their performance, including coupling efficiency, polarization and wavelength sensitivity. Finally, we discuss some emerging research topics on grating couplers, as well as practical issues such as testing, packaging and promising applications.

An eight‐channel hybrid‐integrated light source with 100 GHz channel spacing fabricated by a passive alignment technology is demonstrated. Side mode suppression ratios of better than 40 dB were obtained via grating couplers under eight‐channel simultaneous operation at a continuous wave current of 600 mA and 25°C. The surface slotted laser array fabricated by standard lithography butt‐joint with silicon photonic devices was demonstrated for the first time, to the best of the authors' knowledge. [ABSTRACT FROM AUTHOR]

Abstract: Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide‐mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto‐optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs. [ABSTRACT FROM AUTHOR]

Optical waveguides and grating couplers based on polymer nanostructures are now considered as promising technologies for integrated biophotonic sensing systems. Commonly, structuration and patterning of polymers at the submicron scale requires the use of time and cost-consuming equipments such as electron beam lithography. Direct patterning of high refractive index polymer devices on CYTOP is now achievable and provides symmetric waveguides with top water-like claddings. In addition, transparency of polymers makes them suitable for operation in the visible range, being of major interest for biophotonic applications. In this paper, we report on the fabrication process of polymer submicronic single-mode waveguides on CYTOP for visible light operation. Sub-wavelength grating couplers with a pitch lower than 300 nm were fabricated to couple the input light into a 350 nm square cross-section waveguide. The whole device is imprinted in a single step using soft stamp lithography with a minimal residual layer. Finally, optical characterizations demonstrate a state-of-the-art transmission efficiency (around 1.5%) at the selected operating wavelength of 507 nm for different coupling angles in good agreement with simulations. [ABSTRACT FROM AUTHOR]

Fiber to chip coupling is a critical aspect of any integrated photonic circuit. In terms of ease of fabrication as well as wafer-scale testability, surface grating couplers are by far the most preferred scheme of the coupling to integrated circuits. In the past decade, considerable effort has been made for designing efficient grating couplers on Silicon-on-Insulator (SOI) and other allied photonic platforms. Highly efficient grating couplers with sub-dB coupling performance have now been demonstrated. In this article, we review the recent advances made to develop grating coupler designs for a variety of applications on SOI platform. We begin with a basic overview of design methodology involving both shallow etched gratings and the emerging field of subwavelength gratings. The feasibility of reducing footprint by way of incorporating compact tapers is also explored. We also discuss novel grating designs like polarization diversity as well as dual band couplers. Lastly, a brief description of various packaging and wafer-scale testing schemes available for fiber-chip couplers is elaborated. [ABSTRACT FROM AUTHOR]

We design and demonstrate the fully-etched apodized grating couplers based on the silicon-on-insulator (SOI) platform using subwavelength structure for both transverse electric (TE) and transverse magnetic (TM) modes operation. The subwavelength grating (SWG) is used to engineer the refractive index using second-order effective medium theory (EMT). The whole designing procedure is given in details, especially a feasible and programmable method is developed to precisely manipulate the coupling strength of each grating cycle. A perfect Gaussian output beam can be synthesized for the TE mode operation, achieving a field overlap up to 98.3% with the Gaussian fiber mode. The simulated peak coupling losses are −4.63 and −2.99 dB for the TE mode and the TM mode, respectively, which are comparable with conventional shallowly etched grating couplers, realizing a fabrication simplification without performance penalty. The measured peak coupling loss is −7.6 dB for the TE mode coupler with a 1 dB bandwidth of 45 nm, and −6.1 dB for the TM mode coupler with a 1 dB bandwidth of 34 nm. [ABSTRACT FROM AUTHOR]

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) is a type of narrowband wavelength filter for an incident free-space beam of small diameter. A CRIGF consists of a grating coupler (GC) and a pair of distributed Bragg reflectors used to construct a waveguide cavity resonator. This time, the dependence of the reflection spectrum on the relative position of the centers of the GC and the waveguide cavity resonator is investigated in detail. The position shift of 3/16 of the GC period reduces the bandwidth from 6 to 1 nm, and the position shift of a quarter of the GC period eliminates the guided-mode resonance effect. [ABSTRACT FROM PUBLISHER]

We present an Nb2O5-based grating coupler that employs multiple Nb2O5 / SiO2 layers to improve the optical input/output power efficiency of a scanning differential laser-Doppler integrated probe for two-dimensional cross-sectional velocity distribution measurements. The multiple Nb2O5 / SiO2 layers are designed for both the transmitting grating coupler and the receiving grating coupler. To demonstrate the effect of the designed multiple layers, the field distribution around the grating coupler is simulated as a feasibility study of the concept. The results indicated that the input/output power at the grating coupler can be improved using multiple Nb2O5 / SiO2 layers over the input/output angle of 0 to 20 deg and the wavelength of 1530 to 1570 nm. The increase of the input/output power and the focusing from the transmitting grating coupler were confirmed by the simulation using the finite-difference time-domain method and beam propagation method. [ABSTRACT FROM AUTHOR]

Fiber to chip coupling is a critical aspect of any integrated photonic circuit. In terms of ease of fabrication as well as wafer-scale testability, surface grating couplers are by far the most preferred scheme of the coupling to integrated circuits. In the past decade, considerable effort has been made for designing efficient grating couplers on Silicon-on-Insulator (SOI) and other allied photonic platforms. Highly efficient grating couplers with sub-dB coupling performance have now been demonstrated. In this article, we review the recent advances made to develop grating coupler designs for a variety of applications on SOI platform. We begin with a basic overview of design methodology involving both shallow etched gratings and the emerging field of subwavelength gratings. The feasibility of reducing footprint by way of incorporating compact tapers is also explored. We also discuss novel grating designs like polarization diversity as well as dual band couplers. Lastly, a brief description of various packaging and wafer-scale testing schemes available for fiber-chip couplers is elaborated.

Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip couplers, spectrometers, high-reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics. [ABSTRACT FROM AUTHOR]

Silicon-on-insulator photonic microring resonators are ideal components for applications that require a high degree of multiplexing because of their small size. However, when very high numbers of microrings are used in parallel, grating couplers and waveguide routing quickly take up more chip space than the actual rings. Therefore, we suggest to combine microring resonators with local weak gratings to read out light directly from the microring while still maintaining high-quality factors of 19 000. We demonstrate simulation results and an experimental analysis of different grating parameters. The resonance spectrum is recovered from the vertical emission of the microring grating, without using a traditional output waveguide. [ABSTRACT FROM PUBLISHER]

A new plasmonic device architecture based on ultrasmooth metallic surfaces with buried plasmonic nanostructures is presented. Using template-stripping techniques, ultrathin gold films with less than 5 Å surface roughness are optically coupled to an arbitrary arrangement of buried metallic gratings, rings, and nanodots. As a prototypical example, linear plasmonic gratings buried under an ultrasmooth 20 nm thick gold surface for biosensing are presented. The optical illumination and collection are completely decoupled from the microfluidic delivery of liquid samples due to the backside, reflection-mode geometry. This allows for sensing with opaque or highly scattering liquids. With the buried nanostructure design, high sensitivity and decoupled backside (reflective) optical access are maintained, as with traditional prism-based surface plasmon resonance (SPR) sensors. In addition, the benefits offered by nanoplasmonic sensors such as spectral tunability and high-resolution, wide-field SPR imaging with normal-incidence epi-illumination that is simple to construct and align are gained as well. Beyond sensing, the buried plasmonic nanostructures with ultrasmooth metallic surfaces can benefit nanophotonic waveguides, surface-enhanced spectroscopy, nanolithography, and optical trapping. [ABSTRACT FROM AUTHOR]

The paper presents the results of numerical analyses of optical structures realized in the form of planar waveguides made of materials with high values of the refractive index n ≈1.85. The analysed structures consist of a waveguide and input-output systems. Input-output couplers are realized in the form of prisms as well as Bragg's grating couplers. Numerical investigations were carried out by applying the finite difference time domain (FDTD) method. [ABSTRACT FROM AUTHOR]

Abstract: State of the art grating couplers for horizontal single and double slot waveguides are presented; in these devices the input signal is transmitted from a single mode optical fiber to silicon on insulator slot waveguide. In the waveguides, atomic layer deposited (ALD) high- k dielectrics form the low refractive index slot. It is demonstrated that a fully etched design combined with precision of ALD result in highly reproducible devices with theoretical efficiency variations less than 1%. Devices have a peak calculated coupling efficiency of 24% at 1.55μm. In order to achieve an optimal design, optical properties of high- k films are studied by spectroscopic ellipsometry. Measured refractive indices show variations from reference values, originated from film variation in densities. Chips with a test slot material are fabricated and the optical efficiency of the couplers is characterized. The maximum measured coupling efficiency of the couplers is 18.5%. [Copyright &y& Elsevier]

Optical beam steering can find applications in several domains such as laser scanning, LiDAR (Light Detection And Ranging), wireless data transfer and optical switches and interconnects. As present beam steering approaches use mechanical motion such as moving mirrors or MEMS (MicroElectroMechanical Systems) or molecular movement using liquid crystals, they are usually limited in speed and/or performance. Therefore we have studied the possibilities of the integrated silicon photonics platform in beam steering applications. In this paper, we have investigated a 16 element one-dimensional optical phased array on silicon-on-insulator with a field-of-view of 23. Using thermo-optic phase tuners, we have shown beam steering over the complete field-of-view. By programming the phase tuners as a lens, we have also shown the focusing capabilities of this one-dimensional optical phased array. The field-of-view can easily be increased by decreasing the width of the waveguides. This clearly shows the potential of silicon photonics in beam steering and scanning applications. [ABSTRACT FROM PUBLISHER]

In this paper, we review our work on efficient interfaces between a silicon-on-insulator photonic IC and a single-mode optical fiber based on grating structures. Several device configurations are presented that provide high efficiency, polarization insensitive, and broadband optical coupling on a small footprint. The high alignment tolerance and the fact that the optical fiber interface is out-of-plane provide opportunities for easy packaging and wafer-scale testing of the photonic IC. Finally, an optical probe based on a grating structure defined on the fiber facet is described. [ABSTRACT FROM PUBLISHER]

Grating couplers with nanoscale periodicity have been fabricated on silicon-on-insulator (SOI) substrates by electron beam lithography and reactive ion etching. A versatile experimental apparatus has been implemented to measure the efficiency of these gratings in coupling free-space radiation into planar waveguides. This coupling efficiency has been measured as a function of grating depth and the angle and wavelength of incident radiation. Coupling efficiencies of at least 5% and as high as 20% are demonstrated for wavelengths in the vicinity of 1550 nm and incident angles around 45deg. [ABSTRACT FROM PUBLISHER]

We present high-efficiency grating couplers for coupling between a single-mode fiber and nanophotonic waveguides, fabricated both in silicon-on-insulator (SOI) and InP membranes using BenzoCycloButene wafer bonding. The coupling efficiency is substantially increased by adding a gold bottom mirror to the structures. The measured coupling efficiency to fiber is 69% for SOI grating couplers and 56% for bonded InP membrane grating couplers [ABSTRACT FROM PUBLISHER]

Interest in silicon photonics is experiencing a dramatic increase due to emerging applications areas and several high profile successes in device and technology development. Despite early work dating back to the mid-1980s, dramatic progress has been made only in the recent years. While many approaches to research have been developed, the striking difference between the work of the early to mid-1990s, and more recent work, is that the latter has been associated with a trend to reduce the cross sectional dimensions of the waveguides that form the devices. The question arises therefore, as to whether one should move to very small strip waveguides (silicon wires) of the order of 250 nm in height and a few hundred nanometres in width for improved device performance but with little hope of polarization independence, or to utilize slightly larger rib waveguides that offer more opportunity to control the polarization dependence of the devices. In this paper, we discuss the devices suitable for one approach or the other, and present the designs associated both with strip and rib waveguides. In particular, we present the designs of polarization-independent ring resonators with free spectral ranges up to 12 nm, we propose modulators for bandwidths in the tens of gigahertz regime, and present grating-based couplers for rib and strip waveguides, and/or for wafer scale testing, as well as a novel means of developing Bragg gratings via ion implantation. [ABSTRACT FROM PUBLISHER]

An experimental characterization of the grating couplers for sub-micrometer silicon-on-insulator (SOI) waveguides is presented. The grating couplers have been designed, realized, and characterized for the +1 diffraction order at an operating wavelength of 1.31 mum for TE polarization. At the resonant angle, a coupling efficiency higher than 55% has been measured. The angular coupling range and the wavelength tolerance have been evaluated to 3deg and 20 nm, respectively. The grating coupler is followed by a taper, and about 50% of the input power at 1.31 mum is coupled into sub-micrometer rib and strip SOI waveguides. The ration between light power decoupled toward the cladding and light power decoupled toward the substrate is about three [ABSTRACT FROM PUBLISHER]

The possibility of designing lossless discontinuities of optical waveguides is investigated by finding solutions that satisfy time reversibility of guided modes. It is found that these junctions are only possible for the TE modes. The use of the design condition for these junctions as a bound allows minimizing radiation loss. The results show that it is possible to minimize radiation loss under strong mode-coupling conditions. Two previously unreported designs are shown to significantly reduce radiation loss in grating couplers. The first design uses two coherently coupled gratings, instead of a single grating, for a 2 × 2 grating-assisted backward coupler. The second design uses an index-modulated buffer layer between the core and substrate of an index-modulation-grating coupler. [ABSTRACT FROM PUBLISHER]

To our knowledge, no blazed grating has been fabricated in silicon (Si) at a pitch of less than half a micron. In this article, we report the fabrication of Si-blazed gratings at the period of 400 nm, using electron beam lithography and ion beam etching techniques. The blazed grating is extremely useful as a grating coupler in integrated optics, operating at the telecommunication wavelength of 1.3 mum, because very high output efficiency of the grating coupler is expected. This will allow coupling to thin film devices in silicon, previously not regarded as promising because coupling to them was very inefficient. [ABSTRACT FROM AUTHOR]