Coupling Loss Research Articles

Densely packed 1.1 μm band vertical cavity surface emitting laser array for co-packaged optics

We demonstrated the 1.1 μm band 16-channel vertical cavity surface emitting laser (VCSEL) array for multi-core fiber (MCF) transmission towards co-packaged optics. Single-mode 16-ch top and bottom emitting VCSEL arrays were fabricated by either 3 inch or 6 inch wafer foundry process. The thermal crosstalk is estimated by wavelength shift. When the distance between adjacent VCSELs is 40 μm, the total thermal crosstalk is as low as 7 K. Bottom emitting metal-aperture VCSEL array were also fabricated via a full 3 inch wafer process. It shows low resistance and good single mode characteristic with an oxidation aperture of 7 μm thanks to an intra-cavity metal aperture. A large mode field diameter of 6 μm enables low coupling loss between bottom VCSEL array and MCF through a GaAs substrate for a flip-chip bonding process.

Evaluation of AC losses in toroidal field magnets in the China Fusion Engineering Test Reactor

Toroidal field (TF) superconducting magnets are the key components of the China Fusion Engineering Test Reactor (CFETR). While the TF magnets operate in a steady-state, they are located in a time-varying poloidal field environment, which may lead to AC losses. In this paper, the AC losses of a CFETR TF magnet during excitation and plasma disruption were analyzed. According to the experimental data, the relationship between the critical current density and the magnetic field (Jc–B) was obtained, allowing the hysteresis loss of a TF magnet to be calculated. The results show that the hysteresis loss during excitation is high, at about 1.26 MJ. The coupling loss of a TF magnet can be evaluated when the time constant of a conductor is obtained theoretically. The AC losses of TF magnet joints were analyzed by the same method, which showed that they are much less than those of a TF magnet.

High-Efficiency Sky-Blue Perovskite Light-Emitting Diodes via the Trade-Off between the Electron–Phonon Coupling Loss and Defect Passivation

High-Efficiency Sky-Blue Perovskite Light-Emitting Diodes via the Trade-Off between the Electron–Phonon Coupling Loss and Defect Passivation

Dual-parameter detection with an open-loop dual-core plasmonic optical fiber sensor

The present article proposes an open-loop dual-core plasmonic optical fiber sensor for dual-parameter detection. For the first time, a graphene-TiO2-gold composite structure was used in an open-loop channel of D-type, and a two-parameter detection was produced using a PDMS-Au temperature sensing channel. The sensing mechanism is based on the surface plasmon resonance (SPR) interaction between the photonic crystal fiber core and the metal layer. The analytical approach is complete vector finite element analysis. The coupling loss, resonant peak, sensitivity, and other performance factors are analyzed. According to analogy and simulation analysis, the sensor has a maximum wavelength sensitivity of 27794.37 nm / RIU when the refractive index of the measured area is 1.31∼1.42, and maximum temperature sensitivity of 10.50 nm /°C in the range of 10 °C to 40 °C.

Third order nonlinearity of a PFCB polymer core rib waveguide

PFCB is a semi-fluorinated polymer suitable for integrated optics due to its low absorption at the telecommunication wavelength range, compatibility with standard fabrication tools and processes, and thermomechanical robustness. In this work we report the fabrication and characterization of waveguides optimized for Kerr nonlinearity enhancement, comprised of PFCB core and optical-grade Cytop cladding having a 10% index contrast and supporting single-mode operation with an effective mode area of 4 µm2. Propagation loss of 0.9 dB/cm and 1 dB/facet coupling loss using lensed optical fibers having 2.5 µm spot size were obtained, leading to an effective length of Leff =0.82 cm. A nonlinear coefficient of γ=0.18 ± 0.03 (W·m)-1 was measured for both TE and TM polarizations using a four-wave mixing arrangement about 1.55 µm, with no observed nonlinear absorption. A relatively high Kerr coefficient of |n2|=1.9×10−15 cm2/W is derived for PFCB in the telecom wavelength range for the first time.

Thermo-optic tunable optical filters with GHz-bandwidth and flat-top passband on thin film lithium niobate platform.

Lithium niobate on insulator (LNOI) is a new photonic integrated platform that provides high optical confinement and retains the inherent excellent properties of lithium niobate (LN). Tunable filters are one of the indispensable devices for integrated optics. Here we design and fabricate a thermo-optic (TO) tunable optical filter using two cascaded racetrack microring resonators (MRRs) based on LNOI. The filter shows a narrow and flat top passband with intra band ripple less than 0.3 dB, 3 dB bandwidth of 4.8 GHz and out-of-band rejection of about 35 dB. The insertion loss of the filter is about -14 dB, including grating coupling loss about -6.5 dB and on-chip loss less than -1 dB. The heating power for center wavelength shift of the filter is about 89.4 mW per free spectral range (FSR). Relevant applications of such filters include optical information processing and microwave photonics.

Tapered self-written waveguide for a silicon photonic chip I/O.

An optical coupling method with high alignment tolerance by self-written waveguide (SWW) formation is a promising candidate for co-packaged optics (CPO) by silicon photonics (SiPh). However, conventional SWWs cannot be used with Si waveguides because visible light for SWW formation cannot radiate from the waveguide facet. Here, we devised a new, to the best of our knowledge, optical circuit with SiOxNy waveguides for SWW formation from an SiPh chip. With our circuit, we achieved optical coupling between an SiPh chip and a standard single-mode fiber (SSMF) with a tapered SWW (TSWW). The lowest excess coupling loss compared to butt coupling with a high-numerical aperture (NA) fiber is approximately 0.6 dB over the C-band with the TSWW. In addition, our coupling method has higher alignment tolerances than butt coupling with a high-NA fiber (HNF).

AC loss and contact resistance in highly flexible rebco cable for fusion applications

High Temperature Superconductor (HTS) materials can operate at higher magnetic fields up to 20 T with high critical current and higher operating temperature, compared to low temperature superconductors (LTS). A Highly Flexible REBCO Cable (HFRC) is introduced at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP); a cabling method that is suitable for REBCO HTS tape having anisotropic material properties in its thin REBCO layer. This type of HTS superconducting cable shows high potential for applications in nuclear fusion. The alternating currents and magnetic fields in tokamak type of fusion magnets, cause AC power losses in such cables, which can provoke instability of the conductor by induced currents and increase the temperature. As a first step in characterizing the electromagnetic (EM) performance of an HFRC cable, the AC loss and contact resistance of the HFRC prototype cable were measured at the University of Twente. The measurements were done in liquid helium (4.2 K) with AC magnetic fields, applied perpendicular to the cable's long axis. The AC loss was measured simultaneously by a calibrated gas flow calorimeter utilizing the helium boil-off method, and by the magnetization method using pick-up coils. For the applied test conditions, no coupling loss could be distinguished as a part of the overall AC loss. It is suggested that this might be explained by the shielding of the conductor interior from the applied magnetic field by the outer tape layer due to the high critical current density of the REBCO tape, leading to a high penetration field.

Small-core hollow-core nested antiresonant nodeless fiber with semi-circular tubes.

Hollow-core nested anti-resonant nodeless fibers (HC-NANFs) exhibit great performance in low loss and large bandwidth. Large core sizes are usually used to reduce confinement losses, but meanwhile, bring side effects such as high bending and coupling losses. This study proposes a small-core HC-NANF with a relatively low confinement loss. Semi-circular tubes (SCTs) are added to constitute the core boundary and reduce the fiber-core radius (R). Double NANFs tubes and single-ring tubes are added inside the SCTs to reduce loss. Simulation results show that the optimized structure with R of 5 µm has confinement loss and total loss of 0.687 dB/km and 4.27 dB/km at 1.55 µm, respectively. The bending loss is less than 10 dB/km at 1.4 ∼ 1.6 µm with a bending radius of 10 mm. The direct coupling loss with standard single mode fiber is greatly reduced to ∼ 0.125 dB compared to other HC-NANFs. The modified structure of HC-NANFs also shows a large bandwidth, effective single-mode operation, potentially high birefringence performance, and remarkable robustness of the optimized structure parameters, making it suitable for short-haul applications in laser-based gas sensing, miniaturized fiber sensing, etc.

Achieving extensive lossless coupling of photovoltaic and thermoelectric devices through parallel connection

Photovoltaic-thermoelectric hybrid systems have received broad attention in recent years owing to the capability of achieving efficient utilization of full-spectrum solar energy. This paper investigates the lossless coupling method of photovoltaic and thermoelectric devices to address the issue of large electrical coupling losses. The properties of the photovoltaic-thermoelectric hybrid systems with three connection modes, which are electrically separated, series, and parallel, are experimentally compared under different incident power densities. The electrical-thermal interaction characteristics of the photovoltaic cell and the thermoelectric device under different connection modes are further investigated with the theoretical method. The mechanism of achieving extensive lossless coupling of photovoltaic and thermoelectric devices under parallel connection is illustrated. The results demonstrate that when the incident power density is less than 15Wcm−2, the photovoltaic and thermoelectric devices can be losslessly coupled by the parallel connection. Both electrically separated and parallel coupling systems achieve the output power of 0.85 W, while the series coupling system power is only 0.41 W under the same incident power density of 15Wcm−2. Compared with the series mode, the parallel lossless coupling method can be achieved over a broader range of operating conditions and is more stable because of the small variation of the photovoltaic maximum power voltage under the practical varying irradiation and the effect of the weakened Peltier effect.

T-shaped double-strip spoof surface plasmon polariton transmission lines and application to microwave resonators

A microwave double-strip spoof surface plasmon polariton (DS SSPP) is proposed for high-speed interconnects and high-performance microwave circuits. Based on the dispersion analysis, a T-shaped double-strip structure is designed to provide strong surface- and slow-wave properties from very low to very high frequencies (~ 40 GHz). It allows the tight field confinement and greatly reduces the electromagnetic wave leakage. It exhibits broadband performance with reduced ripples in the insertion loss. It also shows more constant group delay and impedance than counterpart single-strip SSPP. The compact coaxial-to-microstrip-to-DS SSPP transition are designed using gradient grooves. The measurement shows that the DS SSPP lines can exhibit the lower coupling and lower insertion loss than the microstrip lines, so that the former is well-suited for the densely packed high-speed interconnects. The designed DS SSPP is utilized for high quality (Q)-factor microwave ring resonator. The measured unloaded Q-factor is 107.9 at the resonant frequency of 8.7 GHz, which is 1.3 times higher than the microstrip ring resonator. It is found to be caused by the reduction of the radiation loss, according to the loss analysis. The size is also reduced due to the short wavelength, occupying 56.8% of that of the microstrip ring resonator. Therefore, the proposed T-shaped DS SSPP can be also applied for high-performance miniaturized microwave circuits.

Optimising microring resonator based optical frequency comb distillation for optical communications systems.

Microring resonators (MRR) can be used as devices for filtering out broadband noise on optical frequency combs, in cases where significant amplification of a generated comb is required. While comb distillation has been demonstrated experimentally for optical communication systems, approaches to optimise device and sub-system parameters have not been explored. Here, we investigate how the performance of comb distillation through micro-ring filtering depends on device parameters. We also explore device parameter dependent performance when the comb and MRR are misaligned in line spacing. For the device platform we investigate, we find that the required optical signal-to-noise ratio (OSNR) of a comb line can be reduced by 16 dB, independent of modulation format, using a MRR with a resonance bandwidth of 100 MHz and coupling loss of 3 dB.

High-efficiency grating coupler for an ultralow-loss Si3N4-based platform.

Integrated silicon nitride waveguides of 100 nm height can achieve ultralow propagation losses below 0.1 dB/cm at the 1550 nm wavelength band but lack the scattering strength to form efficient grating couplers. An enhanced grating coupler design based on an amorphous silicon layer on top of silicon nitride is proposed and demonstrated to improve the directionality of the coupler. The fabrication process is optimized for a self-alignment process between the amorphous silicon and silicon nitride layers without increasing waveguide losses. Experimental coupling losses of 5 dB and a 3 dB bandwidth of 75 nm are achieved with both regular and focusing designs.

Case study: Simulating and measuring the vibration and radiated sound of a large industrial chiller

This paper presents a case study for measuring and simulating the vibration and radiated sound for a large complex industrial chiller. We describe a simulation framework based on statistical energy analysis (SEA), which is populated using a hybrid mix of analytic, numerical, and measurement techniques. This hybrid approach was motivated by preliminary measurements of vibration and sound from the operating chiller, as well as a series of vibroacoustic transfer functions measured during quiescent non-operating conditions. Some components, like the chiller condenser shell, are modally dense and well suited to statistical estimates of SEA parameters like modal density and coupling between the shell modes and internal and external acoustic spaces. Other components, like the discharge pipe between the compressor and condenser shell, have only a few well separated modes with low damping. This component is better modeled using mobilities either measured or calculated using finite element analysis. We estimate other parameters like internal and coupling loss factors using a mix of analytics and measurements where appropriate. We validate the general model by comparing simulated and measured transfer functions between the discharge pipe and condenser shell — the components that radiated the most sound. We estimate structure-borne and fluid-borne input powers from the compressor using inference techniques based on transfer function measurements at quiescent conditions and averaged surface vibrations measured at operating conditions. This inference approach allows for estimating input powers over any chiller operating condition. Simulated vibrations and radiated sound are generally within 3 dB of measurements for several operating conditions. This case study provides a useful general methodology for modeling and measuring the vibroacoustics of chillers and other complex machinery

Hybrid Integration of VCSEL and 3-μm Silicon Waveguide Based on a Monolithic Lens System

We design an optical interface system for vertical coupling between vertical-cavity surface-emitting lasers (VCSELs) and 3-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> silicon waveguides. The system includes a photoresist lens and a total internal reflection mirror, which allows for passive alignment and flip-chip bonding (FCB) of VCSELs. The simulated coupling loss is 0.7 dB, in optimal conditions, which increases to 1 dB when 3- and 20-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> alignment tolerances are considered in lateral and longitudinal directions, respectively. The system is built by post-processing lens and metallic wiring on the backside of a 3-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> silicon photonic platform. After FCB and embedding of the VCSELs, the system performance is characterized, and VCSEL-to-waveguide coupling efficiency of &#x2212;7.5 dB is obtained.

Influence of twist pitch on hysteretic losses and transport J c in overpressure processed high J c Bi-2212 round wires

Bi-2212 is the only high field, high-temperature superconductor (HTS) available in the macroscopically isotropic, multifilament high J c round wire (RW) form capable of generating high uniformity fields with minimum-screening current errors. However, the heat treatment that enables impressively high J c (4.2 K, 30 T) values that can attain ∼5000 A mm−2 also produces significant filament bonding (bridging). Filament bridging appears to significantly enhance hysteretic losses of the filaments themselves by coupling neighboring, nominally independent filaments, enabling shielding currents to flow across multiple filaments as though they were one filament of much larger diameter. Wire twisting can be employed to reduce filament-to-filament eddy current coupling losses due to induced currents flowing across the matrix, but twisting is less effective in reducing increased losses from bridging. Here, we compare the twist-pitch dependence of the losses of overpressure processed (OP) high J c Bi-2212 RWs with partially bridged filaments to those found in OP Bi-2212 RWs with discrete, not-bridged filaments. We show that filament sub-bundles in standard, partially-bridged wires that have some superconducting connections between filaments can exhibit significant coupling (much larger effective filament diameter), but twisting still reduces their hysteretic losses to values close to or below the ITER Nb3Sn wire loss specification, even though Bi-2212 wires have significantly larger J c values. Although it has been reported that twisting can reduce wire J c by damaging filaments, we found no reduction in transport J c , even for nominal twist pitches of 12 mm in 0.8 mm diameter wires. Evaluation of more-recent, higher J c Engi-Mat powder wires showed that their reduced filament bridging and improved longitudinal connectivity significantly improved transport J c and reduced the J c normalized losses, signaling that J c can be further improved without commensurate increase in losses. This important result strengthens the argument for production of high field, low loss HTS magnets made with Bi-2212 RWs.

Research on mode-coupled multi-mode Fiber mode-division in multiplexing transmission system

The mode coupling and random loss between different modes have important effects on the reliability of the multimode fiber-based mode division multiplexing transmission system. Using the VPI optical communication simulation platform, a short-distance analog-division multiplexing transmission system based on multimode fiber was designed and built. By setting the relevant parameters of mode coupling and random loss caused by bending, the quality of the eye diagram of the received signal is analyzed by the photoelectric signal analyzer at the receiving end. The experimental results show that the mode coupling has great influence on the anti-noise performance and anti-inter-symbol interference performance of the multimode fiber-based transmission system, and the random loss has great influence on the anti-noise performance of the system, this effect is more obvious in high-order linear polarization mode fiber.

Analysis and Design of Plasmonic-Organic Hybrid Electro-Optic Modulators Based on Directional Couplers

We present a detailed simulation study on plasmonic-organic hybrid electro-optic modulators based on coupled symmetric or asymmetric plasmonic slots. An electro-optic polymer is exploited as an active material, and the device is compatible with a silicon photonics platform. The proposed device operates at 1550 nm wavelength, typical of data center or long-haul telecommunication systems. The device performance in terms of area, plasmonic losses, optical bandwidth, intrinsic modulation bandwidth and energy dissipation are comparable to already proposed Mach-Zehnder solutions, but with potentially better extinction ratio, coupling losses due to photonic-plasmonic transitions, and flexibility in exploiting, without any performance penalty, asymmetric slots to shift the ON and OFF states bias. Finally, the bias dependence of the modulation chirp is investigated, comparing through and cross-coupling configurations.

Optimizing two-dimensional polarization-diversity metagrating couplers for silicon photonics

Polarization-diversity couplers are promising industrially scalable optical devices that can couple optical signals with unknown polarization states into silicon photonic chips. Here we propose a novel, to the best of our knowledge, two-dimensional (2D) metagrating coupler (MGC) for the polarization-diversity system, which improves its coupling efficiency for both polarizations. Compared with the previously reported 2D grating couplers (GCs) with identical grating cells, the proposed apodized design gradually changes the aspect ratio and orientation angles of the elliptical patterns simultaneously. This 2D array of the varied grating cells modulates the diffracted mode field patterns and phases locally, and achieves a better overlap with the Gaussian fiber modes for both polarizations. The peak coupling loss (PCL) for the S-polarized and P-polarized light are − 1.6 d B and − 2.7 d B , respectively. The calculated polarization dependent loss is lower than 0.2 dB from 1522 nm to 1540 nm. The device is robust to the fabrication tolerance and fiber misalignment. The simulated fabrication tolerance analysis show that the etch depth error of ±20 nm results in less than 0.3 dB and 0.1 dB PCL drop, while the grating cell feature size error of ± 40 n m results in less than 0.6 dB and 0.2 dB PCL drop for both types of polarization light, respectively. The fiber misalignment should be within the range from − 2 µ m to + 2 µ m in two perpendicular directions, and the coupling angle deviation should be within the range from − 3 ∘ to + 3 ∘ , in order to assure 0.5 dB penalty loss for both polarizations. The performance of the design is insensitive to the mask misalignments. To assure 0.5 dB penalty loss for both polarizations, Δ θ is suggested to be within the range from − 3 ∘ to + 3 ∘ .

Low-Noise Phase-sensitive Parametric Amplifiers Based on Integrated Silicon-Nitride-Waveguides for Optical Signal Processing

Low-noise optical amplification has been a long-lasting research topic in various fields including communication, metrology and quantum optics. Here we report optical signal processing at different wavelengths enabled by a low-noise optical phase-sensitive amplifier (PSA) using a single compact integrated silicon nitride photonic waveguide. For 10 Gbit/s non-return-to-zero optical signals with central wavelengths tuned in the telecommunication band, the waveguide-based PSA exhibits a gain of more than 10 dB and a noise figure of about 1.2 dB when the coupling losses are neglected. For a bit-error-rate level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> , penalties of less than 0.8 dB for all channels are observed after the signals propagating through the chip-based PSA, which may be due to the distortions in the few-mode waveguide. These experimental results generally agree with theoretical prediction and indicate that silicon-nitride-based PSAs are promising for the next generation of broadband optical signal processing and communication systems.