Coupling Loss Research Articles

Hybrid-integrated photodetector array receiving module with power pre-equalization

A hybrid integrated photodetector array receiving module with multiple optical chips is demonstrated, which can be used for a multi-channel high uniformity optical communication system. The receiving module includes a 100 GHz PLC type arrayed waveguide grating, a variable optical attenuator chip with 8-channel, a photodetector chip array, power regulation, an impedance matching circuit, etc. With the hybrid-integration technology and high-precision coupling process, the experimental results show that the coupling loss of the module is 3.40 dB. The crosstalk between the adjacent channels is -30 dB. And the adjustable range of the electric power is up to 20 dB.

Buried 3D spot-size converters for silicon photonics

In this article, an efficient spot-size converter (SSC) for low-loss optical mode transition between large and small waveguides based upon a buried three-dimensional (3D) taper is demonstrated. The SCC can pave the way for scalable, low-loss coupling between on-chip waveguides of different sizes and with external components such as optical fibers and III-V active components, and it can be a key element in solving the challenges surrounding the economic high volume packaging and assembly of photonic integrated circuits. Through the use of a bespoke fabrication process, continual tapering of the waveguide dimensions both in width and height is achieved, offering minimal perturbance of the optical mode throughout the structure. The SSC exploits the space of the buried oxide (BOX) on a standard silicon-on-insulator wafer, leaving a planar top wafer surface, meaning that, crucially, further processing of the wafer is not inhibited in any way. Fabricated proof-of-concept devices demonstrate coupling between standard single-mode 220 nm thick silicon waveguides and large-core waveguides with dimensions about 3 µm wide and 1.5 µm height with BOX thickness of 2 µm. Coupling losses as low as 0.56 dB are achieved, limited mostly by the material loss of the polysilicon used. Substantial improvements can be yielded by simply changing the infill material and through optimization of the fabrication process and design. The demonstrated SSC approach can further be applied to other photonic platforms such as silicon nitride on insulator and so on.

Highly-efficient thin film LiNbO3 surface couplers connected by ridge-waveguide subwavelength gratings

The ridge waveguide integrated grating couplers (GCs) in lithium niobate on insulator (LiNbO3, LNOI) were designed, fabricated and characterized. Two ends of the tapered GCs were connected by the subwavelength gratings (SWG) waveguide of a sub-micrometric-diameter, the photonic-wire SWG structure was featured with the profile of side-walls corrugations, and the effect of geometrical dimensions on the output optical response was investigated. All the devices structure patterns for the integrated LNOI GCs could be simultaneously defined by one step of electron-beam lithography, and then easily fabricated by the optimized dry-etching processes, followed by samples surface cleaning. After the fabrication, a low coupling loss of − 5.1 dB/coupler at the telecommunication wavelength of 1561 nm was measured in the best thin-film LiNbO3 (TFLN) surface grating coupler for quasi-transverse-electric (quasi-TE) polarized signals, and a broad 3-dB optical bandwidth of wider than 95 nm was also obtained. The compact components exhibited magnificent performance, and might show the potential functionalities for the TFLN-based integrated optical waveguide devices.

Subwavelength-Structured High-Efficiency Nanophotonic Coupler for Air Top-Cladded Silicon Waveguide

Efficient coupling between an optical fiber and silicon waveguide has been a major practical challenge for silicon photonics since the first day due to the huge mode size mismatch between them. In this paper, we propose a subwavelength structured edge coupler for efficient coupling light into/out from air top-cladded silicon waveguides. Through refractive index engineering, we demonstrate a single-step patterned coupler with a coupling loss of ~2.5 dB for the wavelength range from 1.5 μm to 1.65 μm.

Effect of junction type on the vibroacoustic response of a system of plates

Modelling of junctions is one of the most challenging tasks in vibroacoustics, especially for Statistical Energy Analysis (SEA), where the results heavily depend on the damping (DLF) and coupling loss factors (CLF). Also, it is an interesting question to determine that to what extent does the DLF or CLF contribute to the overall vibroacoustic characteristics of a structure? The aim of this paper is to investigate via measurements and SEA simulations the effect of the ratio of DLF and CLF on the response of a system for various junctions, such as riveting, bolting, line and point welding, between two steel plates. Loss matrices are determined experimentally by the Power Injection Method in the 200-1600 Hz frequency range. The simulation was performed in the ESI VA One software by using its analytical CLF formulations and compared to experimental data. For the reference case, a bended plate structure was considered, representing an ideal junction between two subsystems. This was equipped with damping foils to ensure the same weight and then compared to the results from other joints. Results showed that increasing the CLF could be more effective than focusing on increasing the DLF.

Single-step etched grating couplers for silicon nitride loaded lithium niobate on insulator platform

Dielectrically loaded thin-film lithium niobate (LiNbO3) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO3 thin film. Silicon nitride (Si3N4) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO3 and can be deposited and patterned by mature fabrication processes. The patterning of Si3N4 opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si3N4-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO3 crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si3N4 loaded LNOI platform.

Conversion efficiency of soliton Kerr combs.

We investigate the conversion efficiency (CE) of soliton modelocked Kerr frequency combs. Our analysis reveals three distinct scaling regimes of CE with the cavity free spectral range (FSR), which depends on the relative contributions of the coupling and propagation loss to the total cavity loss. Our measurements, for the case of critical coupling, verify our theoretical prediction over a range of FSRs and pump powers. Our numerical simulations also indicate that mode crossings have an adverse effect on the achievable CE. Our results indicate that microresonator combs operating with spacings in the electronically detectable regime are highly inefficient, which could have implications for integrated Kerr comb devices.

Design of monolithic distributed Bragg reflector-integrated photodiode using a tapered waveguide with InP and polymer cladding layer

We propose a monolithic distributed Bragg reflector (DBR) integrated photodiode using a tapered waveguide with InP and polymer cladding layers for achieving a high efficiency bandwidth product. In the proposed photodiode, a DBR mirror is formed at the end of the waveguide, thereby increasing the effective absorption length of the device. Furthermore, to reduce the coupling loss, the InP and polymer cladding layers are formed under and on top of the core layer, respectively. Consequently, the mutual tradeoff between the bandwidth and quantum efficiency of the photodetector could be alleviated.

Frequency-comb-calibrated swept-wavelength interferometry.

Lasers are often used to characterize samples in a non-destructive manner and retrieve sensing information transduced in changes in amplitude and phase. In swept wavelength interferometry, a wavelength-tunable laser is used to measure the complex response (i.e. in amplitude and phase) of an optical sample. This technique leverages continuous advances in rapidly tunable lasers and is widely used for sensing, bioimaging and testing of photonic integrated components. However, the tunable laser requires an additional calibration step because, in practice, it does not tune at a constant rate. In this work, we use a self-referenced frequency comb as an optical ruler to calibrate the laser used in swept-wavelength interferometry and optical frequency domain reflectometry. This allows for realizing high-resolution complex spectroscopy over a bandwidth exceeding 10 THz. We apply the technique to the characterization of low-loss integrated photonic devices and demonstrate that the phase information can disentangle intrinsic from coupling losses in the characterization of high-Q microresonators. We also demonstrate the technique in reflection mode, where it can resolve attenuation and dispersion characteristics in integrated long spiral waveguides.

Power Injection Method-based evaluation of the effect of binding technique on the Coupling Loss Factors and Damping Loss Factors in Statistical Energy Anal-ysis simulations

Measurement results on the Damping Loss Factor (DLF) and Coupling Loss Factor (CLF) between two steel plates is presented for 19 different junction types. The junctions involve joining technologies as line welding, point welding, bolting, riveting, gluing or their combinations, with parameters, such as spac-ing between the junction points or the angle between the plates varying. From the measurement results the DLF and CLF values were calculated by the Power Injection Method for the purposes of being ap-plied in Statistical Energy Analysis simulations. Four excitations were applied at each subsystem by impact hammer, while the response was recorded at sixteen response points at each subsystem. The measured CLF values were compared to each other from various aspects. Data were compared to the results obtained from SEA simulations by using the built-in analytical formulas. In general, good com-parison was observed, although the results appeared to be somewhat dependent on the frequency band. Finally, it was examined whether replacing the DLF values with data obtained for an uncoupled flat plate as well the CLF values with data from analytical formulas leads to acceptable accuracy of the re-sults.

Analysis of Dielectric Waveguide Grating and Fabry-Perot Modes in Elastic Grating in Optical Detection of Ultrasound.

In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures.

Evaluation of adjacent channel interference from land-earth station in motion to 5G radio access network in the Ka-frequency band

As the fifth-generation (5G) mobile communication service is expected to operate in one of the 11 candidate frequency bands of 24.25–27.5 GHz, compatibility and coexistence with other adjacent wireless applications must be evaluated. In this paper, the adjacent channel interference (ACI) between Land-Earth Station in Motion (L-ESIM), 5G base station (BS), and user equipment (UE) operating in the adjacent frequency bands is assessed. The minimum coupling loss (MCL) method is used by considering the worst-case scenario to evaluate the effect of geostationary orbit-fixed satellite service's (GSO-FSS) frequency band of 27.5–29.5 GHz on the 5G radio access network (BS and UE) from L-ESIM. From the numerical simulations, minimum separation distances of 35 km and 12 km were recorded for the BS and UE to meet the maximum acceptable interference of -147 dBW/MHz. The obtained results will protect the 5G RAN from harmful interference by ensuring adjacent channel compatibility and coexistence with L-ESIM in their future deployment.

Power coupling losses for misaligned and mode-mismatched higher-order Hermite-Gauss modes.

This paper analytically and numerically investigates misalignment and mode-mismatch-induced power coupling coefficients and losses as a function of Hermite-Gauss (HG) mode order. We show that higher-order HG modes are more susceptible to beam perturbations when, for example, coupling into optical cavities: the misalignment and mode-mismatch-induced power coupling losses scale linearly and quadratically with respect to the mode indices, respectively. As a result, the mode-mismatch tolerance for the ${{\rm HG}_{3,3}}$ mode is reduced to a factor of 0.28 relative to the currently used ${{\rm HG}_{0,0}}$ mode. This is a potential hurdle to using higher-order modes to reduce thermal noise in future gravitational-wave detectors.

Measurement-Based Investigations of the NB-IoT Downlink Performance in Fading Channels

The letter presents experimental results of investigations on the narrowband Internet of Things (NB-IoT) performance under two predefined multipath channel models, related to the reflections-free scenario of an unobstructed signal path. The NB-IoT downlink channel performance was systematically tested for resilience to the multipath propagation phenomenon (naturally occurring in real-life environments of IoT devices deployment), with the most robust modulation-coding scheme enforced, i.e., quadrature phase shift keying (QPSK) modulation without repetitions. Each of these three radio fading channels was artificially generated in electromagnetically isolated laboratory facilities. The NB-IoT equipment setup, the fading simulator for emulating the fading channels as well as the infrastructure that allowed obtaining desirable maximum coupling loss (MCL) values, were described along with the presentation of the measurement scenarios. Analysis of results led to the formulation of practical takeaways regarding the reduction in the maximum pathloss of a fading-free channel, in response to fadings incurred on the NB-IoT downlink signal by Extended Pedestrian-A (EPA) and Extended Typical Urban (ETU) channels.

Finesse Limits in Hollow Core Fiber based Fabry-Perot interferometers

Due to their low sensitivity to changes to the external environment, low optical nonlinearity, low chromatic dispersion, and compatibility with fiber systems, hollow-core optical fibers (HCFs) represent an ideal medium for fiber Fabry-Perot interferometers (FPs). Many applications can benefit from the availability of FPs with high finesse or high finesse-length product. However, the mechanisms that limit the performance of HCF-based FPs are yet to be fully elucidated to the best of our knowledge. In this paper, we present a comprehensive analysis of several factors that impact HCF-FP performance and limit their finesse, e. g., mirror tilt, the distance between the HCF end-face and mirror, HCF cleave angle and HCF attenuation. In a sequence of experiments, we built and characterized five HCF-FPs with lengths ranging from 0.65 m to 9.25 m. By fitting the experimental data with derived analytical expressions, we found the mirror-assisted coupling loss to be below -0.0028 dB (corresponding to a coupling efficiency of 99.94%), which should allow finesse values greater than 5000 to be achieved. Experimentally, we demonstrate here a value of 2400, limited by the parameters of the mirrors available to us presently. We then show that the low coupling loss and high repeatability of mirror alignment and HCF cleave quality allows the effective use of such high-finesse FP for reliable measurements of HCF attenuation, even with short fiber length samples (10 m in our demonstration).

Potential of commercial SiN MPW platforms for developing mid/high-resolution integrated photonic spectrographs for astronomy.

Integrated photonic spectrographs offer an avenue to extreme miniaturization of astronomical instruments, which would greatly benefit extremely large telescopes and future space missions. These devices first require optimization for astronomical applications, which includes design, fabrication, and field testing. Given the high costs of photonic fabrication, multi-project wafer (MPW) silicon nitride (SiN) offerings, where a user purchases a portion of a wafer, provide a convenient and affordable avenue to develop this technology. In this work, we study the potential of two commonly used SiN waveguide geometries by MPW foundries, i.e., square and rectangular profiles, to determine how they affect the performance of mid/high-resolution arrayed waveguide grating (AWG) spectrometers around 1.5 µm. Specifically, we present results from detailed simulations on the mode sizes, shapes, and polarization properties, and on the impact of phase errors on the throughput and cross talk as well as some laboratory results of coupling and propagation losses. From the MPW run tolerances and our phase-error study, we estimate that an AWG with R ∼10,000 can be developed with the MPW runs, and even greater resolving power is achievable with more reliable, dedicated fabrication runs. Depending on the fabrication and design optimizations, it is possible to achieve throughputs ∼60% using the SiN platform. Thus, we show that SiN MPW offerings are highly promising and will play a key role in integrated photonic spectrograph developments for astronomy.

Spherical concave micro-mirror fabricated using gray-tone optical lithography for vertical coupling.

Based on gray-tone optical lithography technology combined with the overlay alignment method, a spherical concave micro-mirror is fabricated at the end of a rectangular optical waveguide (ROW) for low vertical coupling loss. The optimal structures of the spherical concave micro-mirrors were designed through ray-tracing simulation. The results indicate that the minimal vertical coupling loss is only 1.02 dB for the ROW core size of 20 μm × 20 μm. The surface roughness of the micro-mirror is considered, and it should be less than 106 nm to ensure that the vertical coupling loss is less than 1.5 dB. The radius of the fabricated spherical concave micro-mirror was measured as 263.3 μm and the surface roughness of the micro-mirror is 29.19 nm. The vertical coupling loss induced by the micro-mirror was measured as 1.39 dB. 1-dB tolerances in the direction of x-, y-, and z-axes are calculated to be ± 6.9 μm, ± 6.3 μm, and 46.2 μm, respectively.

Particle swarm optimization of silicon photonic crystal waveguide transition.

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540-1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

Thermal-hydraulic analysis of the DEMO CS coil designed by CEA

The superconducting Central Solenoid (CS) of the EU-DEMO tokamak coil will comprise five modules, labeled CSU3, CSU2, CS1, CSL2 and CSL3, situated vertically one above the other. The central CS1 module will be operated under the most severe conditions, i.e. the largest mechanical loads and magnetic field. Two alternative designs of the CS1 module are being developed by the CEA and EPFL-SPC teams. The CS1 module designed by CEA is double-pancake wound using a Nb3Sn Wind&React Cable-in-Conduit Conductor with a central cooling channel. Our work presents the thermal-hydraulic study of the CEA design of the CS1 module based on the EU-DEMO 2018 baseline. The first part of our analysis includes simulations of the CS1 behavior at normal operating conditions, aimed at the estimation of the minimum temperature margin. This part consists of two stages, namely: (i) simulations of the whole simplified “4 points” CS current scenario (not including the breakdown), and (ii) preliminary simulations of the breakdown phase. The second part is focused on quench simulations aimed at the assessment of the maximum hot-spot temperature. The analysis is performed using the THEA code by CryoSoft. We consider the realistic magnetic field and strain profiles along the conductor, heat transfer between neighboring turns and heat loads due to the AC coupling losses. Power of AC coupling losses in all performed simulations is computed based on the effective values of magnetic field and three trial values of the parameter nτ (100 ms, 200 ms and 400 ms). We assume that quench is initiated at the beginning of pre-magnetization phase and we take into account heat generation due to AC losses during the current dump. Minimum temperature margin during the whole simplified current scenario is larger than the 1.5 K acceptance criterion for all considered nτ values. Preliminary simulations of the breakdown phase indicate that the minimum ΔTmarg may drop below 1.5 K, even for the lowest considered nτ value, which calls for attention and requires further more detailed studies. The calculated hot-spot temperatures in all considered cases were safely below the 150 K acceptable limit.

Study on Reducing Magnetization Loss in CORC Cables By Laser Cutting Technology

Conductor on round core (CORC) cables with the features of flexibility and a high engineering current density have been a potential candidate for high magnetic field magnets and accelerator magnets. In these applications, reducing the magnetization loss generated in superconductors is essential in the process of design. CORC cables fabricated by multifilament-coated conductor (CC) tapes have been proved to have a low magnetization loss. However, the degradation of the critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and additional coupling losses significantly affects the performance of CORC cables. In this article, ultraviolet laser cutting technology is used to scribe both the superconducting layer and copper stabilization layers into narrow filaments. First, the magnetization loss of a single tape with different numbers of filaments was measured by a calibration-free method. Then, based on the testing results of a single tape, two typical CORC cables are wound by 10-mm-wide and 4-mm-wide CC tapes. The magnetization loss of cables made of striated tapes was discussed by both experimental and numerical methods. It is found that the magnetization loss significantly decreases with an increasing number of filaments under a high background magnetic field. The magnetization losses decrease in CORC cables made of striated CC tape. Due to the reduced coupling loss and attenuated shielding effect, there is not much difference between the two values of the magnetization loss reduction rates in CORC cables and single tapes. The conclusions obtained in this article will be instructive for the future design of CORC cables.