Optical Coupling Efficiency Research Articles

Hybrid Integration With Efficient Ball Lens-Based Optical Coupling for Compact WDM Transmitters

A photonic integration platform is presented that allows easy manipulation and alignment of a ball lens doublet for efficient coupling between different optical components. The alignment and fixing of the lenses are automated, accurate, and highly scalable for compact parallel optical assemblies. To demonstrate this approach, we built a $4\times25$ -Gb/s compact wavelength-division multiplexed transmitter with low coupling losses of $\sim 2$ dB between lasers and a planar lightwave circuit multiplexing filter.

Micro-pulse upconversion Doppler lidar for wind and visibility detection in the atmospheric boundary layer.

For the first time, to the best of our knowledge, a compact, eye-safe, and versatile direct detection Doppler lidar is developed using an upconversion single-photon detection method at 1.5 μm. An all-fiber and polarization maintaining architecture is realized to guarantee the high optical coupling efficiency and the robust stability. Using integrated-optic components, the conservation of etendue of the optical receiver is achieved by manufacturing a fiber-coupled periodically poled lithium niobate waveguide and an all-fiber Fabry-Perot interferometer (FPI). The double-edge technique is implemented by using a convert single-channel FPI and a single upconversion detector, incorporating a time-division multiplexing method. The backscatter photons at 1548.1 nm are converted into 863 nm via mixing with a pump laser at 1950 nm. The relative error of the system is less than 0.1% over nine weeks. In experiments, atmospheric wind and visibility over 48 h are detected in the boundary layer. The lidar shows good agreement with the ultrasonic wind sensor, with a standard deviation of 1.04 m/s in speed and 12.3° in direction.

Design and Analysis of 2-μm InGaSb/GaSb Quantum Well Lasers Integrated Onto Silicon-on-Insulator (SOI) Waveguide Circuits Through an Al2O3 Bonding Layer

GaSb-based quantum well (QW) laser diode, with emission wavelength ∼2 μm, integrated onto a silicon-on-insulator (SOI) waveguide circuit through a high-thermal-conductivity Al2O3 bonding layer has been designed and analyzed. Prior to bonding, the fabricated Fabry–Perot GaSb QW laser worked under continuous wave operation at room temperature, with a low threshold current of 37 mA at the emission wavelength of 2019 nm, demonstrating high material quality. A tapered structure has been used for evanescent coupling of light from the GaSb laser to the underlying Si waveguide. Instead of using SiO2 for direct bonding or Benzocyclobutene for adhesive bonding, the use of Al2O3 to directly bond GaSb lasers onto SOI wafers is proposed. The optical mode distribution simulations by a beam propagation method software show that light can be coupled efficiently to the underlying Si waveguide through the tapered structure without compromise in optical coupling efficiency. Furthermore, there is a significant reduction (∼70%) in the total thermal resistance compared with the same structure using a SiO2 bonding layer. Our results suggest that the Al2O 3 bonding layer could be a promising candidate for III–V lasers integrated on SOI circuits, where thermal dissipation is very critical.

Plasmonic crystals with sharp optical transmission behaviors

In this work, we for the first time propose and demonstrate a feasible strategy for multi-band ultra-narrow optical transmission of the plasmonic crystals by introducing a high-index dielectric (HID) cavity, which can be acted as an efficient optical field coupling and hybridization component. Five sharp transmission bands with the maximal bandwidth of 6.5nm are achieved. High spectral Q-factor of 327 with the corresponding bandwidth of 2.8nm is observed in the optical region. These features pave a simple and universal way for multispectral narrowband optical transmission based on the typical plasmonic crystals.

Broadband and Polarization-Independent Efficient Vertical Optical Coupling With 45° Mirror for Optical I/O of Si Photonics

Efficient, broadband, and polarization-independent optical input/output (I/O) coupling of silicon photonics chips is significant for its practical application, and single-mode vertical optical I/O coupling will be necessary for high-density optical I/O connections required in future high-performance computing systems. We demonstrated such optical coupling using a 45° mirror, which was integrated by a cost-effective dicing technique with a polishing process. The 45° mirrors were integrated into Si-rich silica (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) waveguides on a Si substrate. To evaluate the light beam profile, which was important factor for the single-mode optical coupling, we measured near field patterns and far field patterns of light beams reflected by the 45° mirror. Owing to the polishing process, high-quality single-mode beams were successfully obtained as the vertical optical output. Using the fabricated 45° mirrors, the efficient single-mode vertical optical coupling with loss penalty of around 0.3 dB was demonstrated. The wavelength-dependent loss was less than 0.6 and 0.2 dB for TE and TM polarization, respectively. The polarization-dependent loss was also very small and it was less than 0.2 dB in 1500–1600 nm.

Direct and Efficient Optical Coupling Into Plasmonic Integrated Circuits From Optical Fibers

We demonstrate direct and efficient optical coupling from an optical fiber into plasmonic integrated circuits (PICs) at 1.55- $\mu \text{m}$ wavelength using ultra-compact plasmonic dipole nanoantennas. The PICs consist of slot waveguides with single-chip integrated Yagi–Uda antennas. To improve the optical coupling efficiency from optical fibers, we add 500-nm-thick Su-8 film with a high refractive index as the top cladding. We experimentally achieve 8.6% couple-in efficiency from a high numerical aperture fiber to the plasmonic slot waveguide and 46% total couple-out efficiency from the plasmonic slot waveguide. We also quantitatively characterize the dependence of the couple-in efficiency to the spot size of the incident light.

An efficient and ultra-broadband unidirectional optical coupler for wide incidence angles

An efficient and unidirectional optical coupling scheme for metal-dielectric-metal (MDM) waveguides with wide-angle and ultra-broadband efficiency is proposed and demonstrated. Based on joint effects of the plasmonic critical angle phenomenon (PCA), the zero-order grating resonance, and the first-order Fabry–Perot (FP) resonance of the MDM waveguide, the illuminated light could be efficiently coupled into MDM plasmonic waveguide with angular-full-width-half-maximum (AFWHM) widen to 50° over a broad wavelength range 750–1000nm. The operation principle and performance of the structure are clarified and theoretically illustrated.

Wearable Heart Rate Sensor Systems for Wireless Canine Health Monitoring

There is an increasing interest from dog handlers and veterinarians in an ability to continuously monitor dogs' vital signs (heart rate, heart rate variability, and respiratory rate) outside laboratory environments, with the aim of identifying physiological correlations to stress, distress, excitement, and other emotional states. We present a non-invasive wearable sensor system combining electrocardiogram (ECG), photoplethysmogram (PPG), and inertial measurement units (IMU) to remotely and continuously monitor the vital signs of dogs. To overcome the limitations imposed by the efficiently insulated skin and dense hair layers of dogs, we investigated the use of various styles of ECG electrodes and the enhancements of these by conductive polymer coatings. We also studied the incorporation of light guides and optical fibers for an efficient optical coupling of PPG sensors to the skin. Combined with our parallel efforts to use IMUs to identify dog behaviors, these physiological sensors will contribute to a canine-body area network to wirelessly and continuously collect data during canine activities with a long-term goal of effectively capturing and interpreting dogs' behavioral responses to environmental stimuli that may yield measurable benefits to handlers' interactions with their dogs.

Compact Hybrid-Integrated 100-Gb/s TOSA Using EADFB Laser Array and AWG Multiplexer

We have developed a compact hybrid-integrated four-lane $\times 25.8$ Gb/s transmitter optical sub-assembly (TOSA) for a 100 Gb/s transceiver for 40-km transmission. The TOSA has a simple configuration in which a four-channel electroabsorption modulator integrated distributed feedback (EADFB) laser array is directly attached to the input waveguide end-faces of a silica-based arrayed waveguide grating (AWG) multiplexer. We integrated a lateral tapered spot-size converter for the EADFB laser to achieve a high optical butt coupling efficiency between the EADFB lasers array and the AWG multiplexer. We confirmed that the TOSA, which is $19.9~\text {mm} \,\times \, 6~\text {mm} \,\times \, 5.8$ mm, achieved error-free operation for a 40-km transmission with a high average optical output power of over 0.3 dBm at an operating temperature of 55 °C.

Grating design for interlayer optical interconnection of in-plane waveguides.

Interlayer grating-to-grating optical interconnect coupling efficiency is simulated and optimized using rigorous coupled-wave analysis (RCWA) for the case of binary rectangular-groove gratings. The "equivalent index slab (EIS)" concept is proposed to alleviate the numerical sensitivity problem inherent in the RCWA-leaky-wave approach, making the method applicable to any multilayer structure that has an arbitrary grating profile, large refractive-index differences, and a limited grating length. The method is easy to implement and computationally efficient and can provide optimal designs based on the system designer's need. To determine the viability of the RCWA-EIS approach, results are compared to those obtained using the finite-difference time-domain method, and an excellent agreement is found.

Coherent tunneling by adiabatic process in a four-waveguide optical coupler

We numerically simulate Schrödinger-like paraxial wave equation of a four-waveguide system. The coherent tunneling by adiabatic passage in a four-waveguide optical coupler is analyzed by borrowing the dressed state theory of coherent atom system. We discuss the optical coupling mechanism and coupling efficiency of light energy in both intuitive and counterintuitive tunneling schemes and analyze the threshold condition from adiabatic to non-adiabatic regimes in intuitive scheme. The results show that this coupler can be used as power splitter under certain conditions.

Broadband and Polarization-Independent Efficient Vertical Optical Coupling With 45° Mirror for Optical I/O of Si Photonics

Efficient, broadband, and polarization-independent optical input/output (I/O) coupling of silicon photonics chips is significant for its practical application, and single-mode vertical optical I/O coupling will be necessary for high-density optical I/O connections required in future high-performance computing systems. We demonstrated such optical coupling using a 45° mirror, which was integrated by a cost-effective dicing technique with a polishing process. The 45° mirrors were integrated into Si-rich silica (SiOx) waveguides on a Si substrate. To evaluate the light beam profile, which was important factor for the single-mode optical coupling, we measured near field patterns and far field patterns of light beams reflected by the 45° mirror. Owing to the polishing process, high-quality single-mode beams were successfully obtained as the vertical optical output. Using the fabricated 45° mirrors, the efficient single-mode vertical optical coupling with loss penalty of around 0.3 dB was demonstrated. The wavelength-dependent loss was less than 0.6 and 0.2 dB for TE and TM polarization, respectively. The polarization-dependent loss was also very small and it was less than 0.2 dB in 1500–1600 nm.

Side coupling of multiple optical channels by spiral patterned zinc oxide coatings on large core plastic optical fibers

Improved optical side coupling efficiency was demonstrated for spiral patterned zinc oxide (ZnO) nanorods coated large core plastic optic fibers (POFs) as opposed to unpatterned continuous coatings. ZnO nanorods were grown by the hydrothermal method directly onto POF surfaces. Nanorods coating enhanced coupling inside the fiber by scattering light but were also capable of causing leakage. Structuring the growth to specific regions allows scattering from different segments along the fiber to contribute to the total coupled power. ZnO nanorods growth time of 12 h and temperature of 90 °C provided the best coupling voltage. Side coupling was measured to be a factor of 2.2 times better for spiral patterned coatings as opposed to unpatterned coatings. The formation of multiple segments was as well used for multiple-wavelength channels excitation where different bands were side coupled from different segments.

Compact Hybrid Integrated 100-Gb/s Transmitter Optical Sub-Assembly Using Optical Butt-Coupling Between EADFB Lasers and Silica-Based AWG Multiplexer

A compact hybrid-integrated 100 Gb/s (4 lane × 25.78125 Gb/s) transmitter optical sub-assembly (TOSA) has been developed for a 100 Gb/s transceiver for 40-km transmission over a single-mode fiber. The TOSA has a simple configuration in which four electro-absorption modulator-integrated distributed feedback (EADFB) lasers are directly attached to the input waveguide end-face of a silica-based arrayed waveguide grating (AWG) multiplexer without bulk lenses. To achieve a high optical butt coupling efficiency between the EADFB lasers and the AWG multiplexer, we integrated a laterally tapered spot-size converter (SSC) for the EADFB laser and employed a waveguide with a high refractive index difference of 2.0% for the AWG multiplexer. By optimizing the laterally tapered SSC structure, we achieved a butt-coupling loss of less than 3 dB, which is an improvement of around 2 dB compared with a laser without an SSC structure. We also developed an ultracompact AWG multiplexer, which was 6.7 mm × 3.5 mm in size with an insertion loss of less than 1.9 dB. We achieved this by using a Mach–Zehnder interferometer-synchronized configuration to obtain a low loss and wide flat-top transmission filter spectra. The TOSA body size was 19.9 mm (L) × 6.0 mm (W) × 5.8 mm (H). Error-free operation was demonstrated for a 40-km transmission when all the lanes were driven simultaneously with a low EA modulator driving voltage of 1.5 V at an operating temperature of 55 °C.

Waveguide-loaded silica fibers for coupling to high-index micro-resonators

Tapered silica fibers are often used to rapidly probe the optical properties of micro-resonators. However, their low refractive index precludes phase-matching when coupling to high-index micro-resonators, reducing efficiency. Here, we demonstrate efficient optical coupling from tapered fibers to high-index micro-resonators by loading the fibers with an ancillary adiabatic waveguide-coupler fabricated via angled-etching. We demonstrate greatly enhanced coupling to a silicon multimode micro-resonator when compared to coupling via the bare fiber only. Signatures of resonator optical bistability are observed at high powers. This scheme can be applied to resonators of any size and material, increasing the functional scope of fiber coupling.

Optofluidic Photonic Crystal Fiber Coupler for Measuring the Refractive Index of Liquids

We report a novel optical sensing device capable of measuring the refractive index of liquids based on evanescent field coupling between a photonic crystal fiber (PCF) that is side-polished and coupled to a side-polished single-mode fiber (SMF). The PCF is used for liquid analyte transport, while the SMF is used for light ingress and egress. The optical coupling efficiency in this device is sensitive to the refractive index of the PCF which varies with the presence of analytes in the holes of the PCF. The fabrication process of the coupler and its performance as a refractive-index sensor are presented. Due to its compact size and features for interactions between the light and the analyte, the sensor can operate as a highly responsive optofluidic device. The results show that this kind of sensing device provides a promising platform for refractive index measurements with an estimated accuracy of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> refractive index unit.

Fabrication of polymer microlenses on single mode optical fibers for light coupling

In this paper, we present a technique for producing fibers optics micro-collimators composed of polydimethylsiloxane PDMS microlenses of different radii of curvature. The waist and working distance values obtained enable the optimization of optical coupling between optical fibers, fibers and optical sources, and fibers and detectors. The principal is based on the injection of polydimethylsiloxane (PDMS) into a conical micro-cavity chemically etched at the end of optical fibers. A spherical microlens is then formed that is self-centered with respect to the axis of the fiber. Typically, an optimal radius of curvature of 10.08μm is obtained. This optimized micro-collimator is characterized by a working distance of 19.27μm and a waist equal to 2.28μm for an SMF 9/125μm fiber. The simulation and experimental results reveal an optical coupling efficiency that can reach a value of 99.75%.

Multicolor Electroluminescence from Intermediate Band Solar Cell Structures

Intermediate band solar cells are a new generation of photovoltaics that allow for better utilization of the solar spectrum. The key and most challenging requirement for these cells is an efficient optical coupling between the intermediate band and the charge conducting bands. GaNAs based intermediate band solar cells have been used to generate electroluminescence. Two electroluminescence peaks are generated in the structure with electrically blocked intermediate band. The peaks are observed for both forward and reverse bias configuration and are attributed to optical transitions from the conduction to the intermediate band, and from the intermediate band to the valence band. The origin of the electroluminescence is confirmed by the temperature dependence of the electroluminescence peak energies that is consistent with the band anticrossing model of the intermediate band formation in dilute nitride alloys. This is the first direct observation of the optical transitions required for the operation of intermediate band solar cells. The results also demonstrate that properly modified intermediate band solar cell structures could be used as multicolor light emitters.

Heterogeneous Integrated III–V Laser on Thin SOI With Single-Stage Adiabatic Coupler: Device Realization and Performance Analysis

A III-V on silicon heterogeneous integrated laser with highly efficient single-stage adiabatic coupler is presented in this paper. The structure consists of an electrically pumped III-V ridge waveguide gain section on silicon, III-V/Si optical adiabatic coupler, and silicon-on-insulator (SOI) nanophotonic waveguide. The adiabatic coupler is 50-μm long and is formed by tapering the III-V ridge and the underneath thin SOI waveguide along the same direction for efficient coupling of light between III-V ridge and silicon waveguide. Fabrication details and characterizations of this heterogeneous III-V/Si Fabry-Pérot (FP) laser are presented. Experimental data show that such structure has a low taper end reflection of ~-37 dB, and a high optical coupling efficiency of ~85%. The fabricated FP laser shows a high differential quantum efficiency of 23.78% under pulse operation at room temperature. The maximal single facet emitting power is about 7.5 mW, and the side-mode suppression ratio is ~30 dB. Thermal characterization shows a length normalized thermal independence of 20.02 °C·mm/W. Since this new heterogeneously integrated III-V/Si laser structure is realized directly on thin SOI, it offers a potential solution for developing more complex, efficient, and scalable integrated on-chip subsystems for various applications.

Glass‐based Manufacturing and Prototyping Platform PhotPack

AbstractThere has been considerable progress in utilizing fully automated machines for the assembly of micro‐optical systems in recent years. Such systems integrate laser sources, passive optical elements, electro‐optical components and detectors into tight packages, and efficiently couple light to waveguides in photonic integrated circuits (PIC), optical backplanes, free space or optical fibers. The required electro‐optical and optical components are placed and aligned passively and actively, depending on optical and mechanical component tolerances. For one, the relative placing of all components is controlled actively by camera systems, and, more importantly, some or all active components are actually operated, and actively aligned with live feedback from internal and external power and beam image detectors. High precision (and also fast) active optical alignment machines nowadays exist, with more than adequate degrees of freedom and accuracies, so that best optical beam qualities and coupling efficiencies can be achieved.