Coupling Section Research Articles

Continuous Frequency Tuning with near Constant Output Power in Coupled Y-Branched Terahertz Quantum Cascade Lasers with Photonic Lattice

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modeling and the spectral characteristics are modeled using a transfer matrix model. Continuous frequency tuning of ∼19 GHz is demonstrated while maintaining an output power of ∼4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically control...

A single slot micro-ring structure for simultaneous CO2 and CH4 gas sensing

In this paper, we proposed a single silicon-on-insulator micro-ring structure for detecting two different gas components in the same time from one output spectrum. By introducing slot structure, the sensitivity and selectivity of sensor are improved. Specifically, two different sensing mechanisms are synthesized in this structure, thus output spectrum is impacted by varying concentrations of CH4 and CO2 respectively. The resonant wavelength of micro-ring resonator is the absorption peak of CH4, the concentration of CH4 can be measured with the light intensity change. Simultaneously, the combined action of CH4 and CO2 can cause the shift of resonant wavelength, and the total concentration of CH4 and CO2 gas can be obtained through the shift amount. For enhancing the evanescent field fraction in slot area and tuning the resonant wavelength of micro-ring being located on the absorption peak of CH4 (around 3.31 µm), the parameters of slot micro-ring structure, including height of slot in silicon, slot width, radius of micro-ring, waveguide width and the gap distance in coupling section, are well tailored, meanwhile, the quality factor Q of micro-ring is considered for ensuring a satisfied accuracy of sensor. A simulation based on the finite difference time domain method is implemented and the analysis results show that the sensitivity of sensor reaches 2308 nm/refractive index unit.

Wideband microstrip band stop filters realization with interdigital topologies

This article proposes novel wide band stop filter topologies at C-band with simpler implementation in microstrip. The introduction interdigital resonator along with stepped impedance resonator concept is utilized for the placement of transmission zeros to achieve wideband performances. This article demonstrates fractional bandwidth of > 80% with steep skirt rate. This topology is modified further to achieve improved out of band performance. The parallel coupled section for input and output signal coupling results in improved out of band performances. The measured 3-dB bandwidth is from 4 to 10 GHz and transmission loss below 0.55 dB is achieved for both the topologies. Close analogies between simulated and measured results is obtained which are detailed in this article. The presented topologies are simpler and novel achieving the desired performances without vias.

Tunable SIW Cavity-Based Dual-Mode Diplexers With Various Single-Ended and Balanced Ports

This paper presents, for the first time, tunable dual-mode substrate integrated waveguide (SIW) diplexers with various single-ended (SE) or balanced (BAL) ports. Dual-mode diplexing reduces the required volume by half while signal routing is achieved by proper coupling sections. Furthermore, the SIW resonators result in low loss and wide tunability. This paper experimentally demonstrates three designs with SE–SE, SE–BAL, and BAL–BAL port configurations. The SE–SE, SE–BAL, and BAL–BAL diplexers can be tuned starting from 2.07, 2.2, and 2 GHz, respectively, with a tuning range of 45%, 57.2%, and 63.5%, respectively. The average measured insertion loss is 1.32 dB for the SE–SE, 1.95 dB for the SE–BAL, and 2.15 dB for the BAL–BAL. The average size of the diplexer is $55{\times} 55$ mm2. For the proposed SE–BAL and BAL–BAL diplexers, the measured in-band common-mode rejection is better than 40 dB throughout the tuning range.

Reconfigurable Bandwidth Bandpass Filter With Enhanced Out-of-Band Rejection Using $\pi $ -Section-Loaded Ring Resonator

A novel ring resonator bandpass filter with reconfigurable bandwidth with central frequency at 2.4 GHz is demonstrated. Theoretical analysis for computing the resonant frequencies is shown, and the design approach of implementing $\pi $ -section stubs connected to the ring to obtain the bandwidth reconfigurability is explained. The use of reconfigurable $\pi $ -section allows the alteration of the stub’s effective width and therefore the filter’s bandwidth reconfigurability. p-i-n diodes are used as switching elements to achieve the narrowband/wideband response. Coupled line sections are used for suppressing the higher modes by generating out-of-band transmission zeros, resulting in a significantly enhanced out-of-band rejection. Measurements indicate that the 3-dB fractional bandwidth can be switched from 58.5% to 75% at a fixed center frequency of 2.4 GHz with an insertion loss better than 1.1 dB. Moreover, the −20-dB stopband performance is extended to $2.7f_{0}$ .

Achromatic critically coupled racetrack resonators

In this paper, we investigate the spectral response of whispering-gallery-mode (WGM) resonators coupled to their access waveguide with a view to design their constitutive waveguides to promote critical-coupling over a wide spectral range and thereby facilitate their use for high-sensitivity sensing or nonlinear frequency conversion applications. The carried-out theoretical analysis is based on the universal response functions of singlemode and unidirectional devices. A coupled-mode treatment of the coupling region enables to derive two sets of favorable designs. The identified resonator/access waveguide systems exploit waveguides with mismatched propagation constants forming a coupling section exhibiting either an achromatic beat-length or an achromatic power-transfer coefficient. This generic model is followed by a numerical case study of vertically-coupled Si3N4 racetrack resonators. The conventional (quasi-)phase-matched configuration, treated as a reference case, is shown to display a critical-coupling bandwidth of 23 nm at a wavelength of 1550nm, whereas the proposed new designs demonstrate critical bandwidths larger than 330nm, i.e. exhibit bandwidths enhanced by more than one order of magnitude.

Compact Wideband Dual Loop Coupler With High Power Handling Capability for Radar Applications

Couplers are widely used in microwave subsystems, especially for radar applications. They are used to extract a directive sample of the output signal of an RF source. Typically, a coupler consists of a main arm and a secondary arm connected via coupling section. For couplers used in radar applications, power handling is a limiting factor for designers. In addition, the coupling value must be kept a flat with a high directivity over the operating bandwidth of the coupler. In this letter, we provide designs for two compact loop couplers, operating in the frequency range of 1–2 GHz and 2–4 GHz. These designs have flat coupling of −60 dB and achieve matching below −22 dB.

Minimized multi‐layer substrate integrated waveguide 3‐dB small aperture coupler

AbstractA multi‐layer 3‐dB substrate integrated waveguide coupler is presented in this article. As a promising technique, the multi‐layer substrate integrated waveguide has been widely used in microwave and millimeter circuits and systems for its decrease in size. This design combines the Bethe hole coupling theory with the SIW technology to reduce the height and integrate with other planar circuits. More importantly, the strong coupling (3‐dB) coupler presented in this article utilizes the higher frequency band of TE10 mode to get a relatively bigger radius of aperture and in turn reduce the number of coupling sections, which significantly decrease the size of the coupler. The simulated results are in good agreement with the measured data.

Compact size cognitive radio with non‐ and uni‐planar parasitic coupled section for UWB and WLAN/WiMax application

Due to the requirements of increased bandwidth and data transfer rate in wireless communication, cognitive radio (CR) has been proposed as a solution to the spectrum scarcity problem. Regarding the many advantages that ultra-wideband (UWB) antennas offer, they could be used in CR systems. Simultaneously, by designing a band reject filter for avoiding interference with other applications, like wireless local area network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX), safety communication can be provided. In this work, two novel CRs (so that secondary ports are as non-planar and uni-planar parasitic antennas shapes) are presented. Each antenna consists of two ports at the opposite sides; one of them plays as UWB port (first work: 3.1–10.6 GHz and second work: 2.98–11.83 GHz) with filtered WLAN/WiMAX (first work: 4.9–5.8 GHz and second work: 4.9–6.08 GHz) and the other one supports filtered frequency bandwidth (first work: 4.8–6.2 GHz and second work: 5.1–6 GHz). Size reduction occurred in this work with both structures having the same size of 25 × 25 × 0.8 mm3. The designed antennas were measured and results of antennas have agreed well with the simulation analysis.

Broadband Silicon-On-Insulator directional couplers using a combination of straight and curved waveguide sections

Broadband Silicon-On-Insulator (SOI) directional couplers are designed based on a combination of curved and straight coupled waveguide sections. A design methodology based on the transfer matrix method (TMM) is used to determine the required coupler section lengths, radii, and waveguide cross-sections. A 50/50 power splitter with a measured bandwidth of 88 nm is designed and fabricated, with a device footprint of 20 μm × 3 μm. In addition, a balanced Mach-Zehnder interferometer is fabricated showing an extinction ratio of >16 dB over 100 nm of bandwidth.

Theoretical and experimental investigations of efficient light coupling with spatially varied all dielectric striped waveguides

Integrated photonic systems require efficient, compact, and broadband solutions for strong light coupling into and out of optical waveguides. The present work investigates an efficient optical power transferring the problem between optical waveguides having different widths of in/out terminals. We propose a considerably practical and feasible concept to implement and design an optical coupler by introducing gradually index modulation to the coupler section. The index profile of the coupler section is modulated with a Gaussian function by the help of striped waveguides. The effective medium theory is used to replace the original spatially varying index profile with dielectric stripes of a finite length/width having a constant effective refractive index. 2D and 3D finite-difference time-domain analyzes are utilized to investigate the sampling effect of the designed optical coupler and to determine the parameters that play a crucial role in enhancing the optical power transfer performance. Comparing the coupling performance of conventional benchmark adiabatic and butt couplers with the designed striped waveguide coupler, the corresponding coupling efficiency increases from approximately 30% to 95% over a wide frequency interval. In addition, to realize the realistic optical coupler appropriate to integrated photonic applications, the proposed structure is numerically designed on a silicon-on-insulator wafer. The implemented SOI platform based optical coupler operates in the telecom wavelength regime (λ = 1.55 μm), and the dimensions of the striped coupler are kept as 9.77 μm (along the transverse to propagation direction) and 7.69 μm (along the propagation direction) where the unit distance is fixed to be 465 nm. Finally, to demonstrate the operating design principle, the microwave experiments are conducted and the spot size conversion ratio as high as 7.1:1 is measured, whereas a coupling efficiency over 60% in the frequency range of 5.0–16.0 GHz has been also demonstrated.

Optimal interfacing with coupled-cavities slow-light waveguides: mimicking periodic structures with a compact device.

We present a design for optimal interfacing (I/O coupling) with slow-light structures consisting of coupled cavities. The I/O couplers are based on adding a small set of cavities with varying coupling coefficients at edges of the coupled cavities waveguide in order to match its impedance with that of the I/O waveguides. I/O efficiencies exceeding 99.9% are shown to be possible over a bandwidth which is larger than 50% of that of the coupled cavities waveguide. Consequently, the reflections at the edges of the slow-light structure are practically eliminated. We discuss the properties of the perfectly impedance matched slow-light structure as an effective (super-structure) cavity and study the impact of the number of cavities comprising the I/O coupler. We also consider in details the impact of errors and disorder in the I/O coupling sections.

Improved All-Fiber Acousto-Optic Tunable Bandpass Filter

An all-fiber acousto-optic tunable bandpass filter based on a 1.185-mm long coreless core mode blocker is reported. Experimental results demonstrate a minimal insertion loss of 1.2 dB at the optical resonant wavelength of 1527.7 nm with 3-dB optical bandwidth of 0.83 nm. The optimization of the device takes into account the attenuation of the acoustic wave and leads to an asymmetric configuration in which the coupling section is shorter than the recoupling part. Under the effect of a standing flexural wave the device can be operated as a bandpass modulator. The device exhibits a maximum modulation depth of 28%, 4 dB of insertion loss and 0.97 nm of optical bandwidth at 4.774 MHz.

A compact two-way power divider based on five-port structure.

A novel compact five-port waveguide power divider based on the single-ridge waveguide and three-ridge waveguide structure is proposed. It is realized by the coupling of the TE10 and TE30 modes. At the central coupling section, the single-ridge and three-ridge waveguides are used to constitute the conventional rectangular waveguide to reduce the sectional sizes. It consists of one input port, two output ports, and two isolated ports. For validation, a compact five-port power divider is designed, fabricated, and measured. The measured results show that, from 8 to 9 GHz, the return loss of the input port and output ports is higher than 18 dB, the isolation between the output ports is higher than 15 dB, the insertion loss is less than 0.3 dB, and the amplitude and the phase imbalance between the output ports are less than ±0.05 dB and ±1°, respectively. The simulated results basically agree with the simulations. Its sectional sizes are 1.1 λ × 0.4 λ which are more compact than most of the two-way isolated waveguide power dividers.

Novel Generic Asymmetric and Symmetric Equivalent Circuits of 90° Coupled Transmission-Line Sections Applicable to Marchand Baluns

Three different equivalent circuits (asymmetric, symmetric, and mixed) of the 90° coupled transmission-line sections are proposed for Marchand baluns, including the accurate modeling of the effect of the connecting segment. The asymmetric one consists of one set of coupled transmission-line sections and two identical uncoupled transmission-line sections. Due to the asymmetry and a four-port circuit configuration, it seems to be difficult to derive the design formulas for the asymmetric equivalent circuit, thus leading to their indirect derivation by adding two identical transmission-line sections with effectively “negative” electrical lengths to the original 90° coupled transmission-line sections. The symmetric one may be obtained by combining the even- and odd-mode equivalent circuits, while the mixed equivalent circuit is combining the asymmetric and the symmetric ones. As a proof-of-concept, a symmetric and an asymmetric equivalent circuits along with a Marchand balun with a connecting section were implemented at 2 GHz, demonstrating a very good agreement between the simulated and the experimental values.

Nested microring resonator with a doubled free spectral range for sensing application

The microring resonator has received increasing attention in the optical sensing application because of its micro-size, optical property, and high sensitivity. An additional waveguide is commonly used to change the output spectra in the early research on microring resonators. In this study, we proposed a nested microring resonator that doubles the free spectral range (FSR) compared with the conventional single microring. This structure improved the sensing property as the FSR in the filter output spectra could be considered as a measurement range in the microring sensor. Moreover, the parameters including the coupling coefficient of the three coupling sections, length of the U-bend waveguide, and effective index of a waveguide were tested and carefully selected to optimize the sensing properties. The relationship between these parameters and the output spectra was demonstrated. With linear sensitivity, the structure has a good potential in sensing application.

Coupling Ideality of Integrated Planar High-QMicroresonators

Chipscale microresonators with integrated planar optical waveguides are useful building blocks for linear, nonlinear and quantum optical devices. Loss reduction through improving fabrication processes has resulted in several integrated micro resonator platforms attaining quality (Q) factors of several millions. However only few studies have investigated design-dependent losses, especially with regard to the resonator coupling section. Here we investigate design-dependent parasitic losses, described by the coupling ideality, of the commonly employed microresonator design consisting of a microring resonator waveguide side-coupled to a straight bus waveguide. By systematic characterization of multi-mode high-Q silicon nitride microresonator devices, we show that this design can suffer from low coupling ideality. By performing full 3D simulations to numerically investigate the resonator to bus waveguide coupling, we identify the coupling to higher-order bus waveguide modes as the dominant origin of parasitic losses which lead to the low coupling ideality. Using suitably designed bus waveguides, parasitic losses are mitigated, and a nearly unity ideality and strong overcoupling (i.e. a ratio of external coupling to internal resonator loss rate > 9) are demonstrated. Moreover we find that different resonator modes can exchange power through the coupler, which therefore constitutes a mechanism that induces modal coupling, a phenomenon known to distort resonator dispersion properties. Our results demonstrate the potential for significant performance improvements of integrated planar microresonators, achievable by optimized coupler designs.

Enhanced axial confinement in a monolithically integrated self-rolled-up SiNx vertical microring photonic coupler

We report an efficient method to introduce enhanced axial confinement in the self-rolled-up SiNx vertical microtube coupler by depositing a thin layer of high refractive index material strip within the coupling section and effectively forming a vertical microring. Three times wider mode spacing is observed in such a vertical microring coupler monolithically integrated with a silicon nitride ridge waveguide as compared to the one without such axial confinement. More importantly, single mode operation within the telecomm C-band and S-band is achieved.

Hybrid plasmonic waveguide-based ultra-low insertion loss transverse electric-pass polarizer

We have designed and experimentally demonstrated an integrated transverse electric (TE)-pass polarizer on silicon-on-insulator platform. The polarizer consists of an asymmetric coupling section where only the transverse magnetic (TM) mode is coupled to the cross-hybrid plasmonic waveguide and attenuated. The TE mode does not couple and passes through the polarizer. The polarizer was fabricated on the silicon-on-insulator platform. The device is 30 μm long, has a high extinction ratio of more than 28 dB over 150 nm bandwidth, and has a good fabrication tolerance. Most important, the proposed polarizer has an ultra-low insertion loss of less than 0.04 dB for the TE mode.

System Robust Optimization of Ring Resonator-Based Optical Filters

Fabrication variations can have a detrimental effect on the performance of optical filters based on ring resonators. However, by using robust optimization these effects can be minimized and device yield can be significantly improved. This paper presents an efficient robust optimization technique for designing manufacturable optical filters based on serial ring resonators. The serial ring resonator is treated as a system which has computationally expensive (directional coupler section) and cheap components (ring section). Cheap mathematical models are constructed of the directional coupler sections in the resonators. The approximate system response based on the cheap model is then robustly optimized. The robust bandpass filter performance is compared against designs that do not take uncertainties into account. The optimality of the robust solutions is confirmed by simulating it on the expensive physical model as a post-processing step. Results indicate that the employed approach can provide an efficient means for robust optimization of ring resonator-based optical filters.