dB Coupling Research Articles

Technique of microball lens formation for efficient optical coupling

A batch-fabricated microball lens array not only provides accurate coupling distances but also replaces traditional components such as aspheric lenses and expensive graded-index lenses without sacrificing performance and reduces assembly cost. The results of extensive experiments show a critical aspect ratio of approximately 0.3. That is, when the aspect ratio is larger than 0.3, the final shape of a reflowed lens changes to that of a microball rather than of a mushroom. Using a laser with a 633-nm wavelength yields an optimum coupling distance of approximately 8 microm with an insertion loss below -1.3 dB (coupling efficiency, approximately 73%).

1.55 µm spot-size converter integrated polarization-insensitive quantum-well semiconductor optical amplifier with tensile-strained barriers

A 1.55 µm polarization-insensitive lateral tapered spot-size converter integrated semiconductor optical amplifier (SSC-SOA) with tensile-strained barriers was investigated. The optical amplifier structure used a conventional ridge guide for the active layers and a second larger ridge for the passive waveguide. Low beam divergence of 12° × 15° results in about 3.1 dB coupling losses with −1 dB positional tolerances of ±2.3 µm and ±1.6 µm in horizontal and vertical directions using an anti-reflection coated flat-ended single-mode fibre. The active layer of SSC-SOA consisted of a tensile-strained barrier multiple-quantum-well structure. The SSC-SOA exhibited a signal gain of 25.5 dB and a saturation output power of 11.2 dB m with excellent polarization insensitivity (less than 0.5 dB) at 200 mA.

Multi-wavelength superfluorescent fibre source based on a Mach–Zehnder interferometer

A multi-wavelength fibre source (MWFS) based on a spectrum pre-slicing configuration is demonstrated. A fibre double pass Mach–Zehnder interferometer (MZI), which only consists of three 3 dB couplers, yet has an extinction ratio of larger than 27 dB, has been employed as a reflected comb filter. About 40 channels around 1550 nm with wavelength spacing of nm has been achieved. The extinction ratio and power variation between channels is larger than 16 dB (limited by the resolution bandwidth of the optical spectrum analyser) and less than 0.5 dB, respectively. Each of the 40 channels has an integral power of mW without any amplification. Through adjustment of the MZI arm length difference, a multi-wavelength output with other wavelength spacing has also been obtained. This kind of MWFS can be easy fabricated with small volume and should be useful in wavelength-division multiplexing (WDM) systems and integrated optics.

Wideband HNRD guide directional coupler based on multi-hole coupling structure for V-band

A new design of a V-band hyper-non-radiative dielectric (HNRD) guide directional coupler using a multi-hole structure is described and measured results are given. The coupling structure using multi-hole between the two HNRD guides is employed to obtain wideband coupling. Measured results for the fabricated 3 and 10 dB coupler showed that return loss is greater than 16 dB, isolation is better than 26 dB, and the average coupler loss is 1.1 dB, over a percentage bandwidth of 10% at 60 GHz.

Backward directional coupler for weak coupling of double-sided microstrip lines using longitudinal double-slots

A backward directional coupler for double-sided microstrip lines is presented. Aligned microstrip lines are coupled via two parallel slots in the common ground plane. This design works well for weak coupling smaller than 20 dB and has some advantages over the single-slot coupler. Geometrical dimensions of the proposed coupler design are presented for coupling values of 20, 25 and 30 dB, together with measurements for a 30 dB coupler.

ICRF heating for Wendelstein 7-X

The stellarator Wendelstein 7-X is being built in Greifswald, Germany, by the Max-Planck-Institut für Plasmaphysik. Initially, the heating system will consist of 10 MW ECRH and 4 MW ICRF heating (both cw) and 5 MW NBI (for 15 s every 3 min). ECRH is the principal heating method at a magnetic field of B=2.5 T. At other magnetic fields or densities higher than the ECRH cutoff, ICRF heating will sustain the plasmas. The ICRF system is planned to consist of two radially movable antennas powered by two generators with an output power of 2 MW each. The antenna plasma load will be matched in real time to the generator impedance using ferrite loaded transmission line elements. In addition, 3 dB couplers will passively protect the generators from RF power reflections during rapid changes of the antenna plasma load that are too fast for the ferrite matching system.

Novel coupled-line conductor-backed coplanar and microstrip directional couplers for PCB and LTCC applications

Novel coupled-line conductor-backed coplanar and microstrip directional couplers, convenient for manufacturing in standard printed circuit board (PCB) or low-temperature co-fired ceramic (LTCC) technology, are proposed. The coplanar coupler, consisting of a layered structure with four strips and covering a -10 to -2.6 dB coupling coefficient, is theoretically ideally impedance matched and perfectly isolated at all frequencies under the assumption of validity of quasi-static approximation and two-mode propagation. The microstrip coupler, a modification of the coplanar one, can be ideally compensated only in an area of strong coupling coefficients. The novel couplers are not sensitive to lateral misalignment of conductive layers, and not sensitive to thickness and dielectric permittivity tolerances of applied dielectric substrates. Preliminary experimental results for 2.7-, 3-, and 8.34-dB couplers, designed both in PCB, as well as LTCC technology are presented.

Compact NRD-Guide Switch by Using Schottky Barrier Diodes

Based on a miniaturized NRD-guide coupling structure, a new switch circuit is proposed. The 3dB coupler and two schottky barrier diodes with improved diode mounts and matching circuits are used. It features with wideband performance and compact size. The high isolation performance can also be realized using the same circuit by adjusting the bias currents of the diodes. An experimental prototype is implemented and the tested characteristics are supplied.

All-SOP all-fibre depolariser

A single mode fibre depolariser is demonstrated, based on a 3 dB coupler in association with polarisation combiners. Wide spectral band operation and low degree of polarisation (5%) are obtained. The linear design allows high stability and ease of integration.

A novel coplanar-waveguide directional coupler with finite-extent backed conductor

A novel edge-coupled coplanar-waveguide (CPW) directional coupler with a floating backed conductor is proposed. For design purposes, a theoretical transmission-line model for the coupler is established, which is based on the characteristic parameters of the related coupled CPW structures. To this end, the effective dielectric constants and characteristic impedances of the conductor-backed edge-coupled CPW structures are analyzed using the full-wave analysis based on the spectral-domain approach. Specifically, the coupling coefficient of this new coupler can be controlled by adjusting the width of the backed conductor. In this paper, two conductor-backed edge-coupled CPW directional couplers with moderate (-6dB) and tight (-3dB) coupling coefficients are designed and implemented based on the theoretical transmission-line model. The theoretical results are compared with the measured ones, and the agreement among them confirms the usefulness of the coupler design model, as well as the full-wave approach for the related coupled CPW structures.

Halogenation of ORMOSIL: A Promising Rout for Tailoring Optical Absorptions

AbstractFluorinated and photo-imageable precursors are synthesized through a Barbier-Grignard reaction for 1550-nm window. The precursors are used for the sol-gel process of integrated optic components for silica-on-silicon technology. Material compositions and process parameters are optimized to achieve internal absorptions >0.1 dB/cm and propagation losses of about 0.5 dB/cm at 1550 nm. Compact 1×16 Beam splitters are designed and fabricated which exhibit >0.3 dB power uniformity, >0.1 dB PDL and 1.5 dB coupling loss. By hybrid integration of the passive splitters and in-house fiber amplifiers, amplifying splitters are demonstrated at various signal intensities.

Reflection characteristics and cascadability of a multi-mode interference 3 dB coupler

Reflection characteristics and cascadability of multi-mode interference (MMI) 3 dB couplers are investigated. Quantitative reflection characteristics of MMI couplers were investigated using a low-coherence interferometric reflectometer. The MMI couplers were fabricated in an InP/InGaAsP/InP double-hetero structure. The reflectivity from the interface between the MMI region and output waveguide is shown to be less than −30 dB. The cascadability of MMI couplers is also investigated by comparing a branching ratio of an MMI coupler and a Y branch. It is shown that the mode-filtering effect of MMI couplers enables the cascade connection with stable blanching ratio.

A 3-d broadband dual-layer multiaperture microstrip directional coupler

A three-dimensional (3-D) broadband dual-layer multiaperature microstrip directional coupler is presented in this paper. The dual-layer, multiaperature directional coupler consists of two-layer back-to-back substrates with 15 small coupling apertures on the center ground plane. A novel rectangular multiaperature coupling structure with very small width is used to greatly extend the bandwidth and operating frequency of the coupler. The /spl plusmn/2.4 dB coupling variation bandwidth of the couplers is more than 1.5 octaves from 9.79 to 29.55 GHz. The insertion losses, return losses at all ports and directivities, are better than 1.7 dB, 10.26 dB, and 10.3 dB over the bandwidth, respectively. The measurements agree well with the simulations.

Design and testing of ECE radiometer for Sinp-Tokamak

A multi channel ECE Radiometer, in the frequency range of 75 - 110 GHz, has been designed, constructed and tested for the SINP-Tokamak to diagnose second harmonic X-mode electron cyclotron emission (ECE). This frequency range covers approximately 80% of the plasma cross section in case of a magnetic eld of B = 1.5 T. For the maximum field of the SINP-Tokamak B = 2.0 T, about 50% is covered. The ECE radiation is received by a single horn antenna with a gain of approximately 30 dB. To increase the spatial resolution perpendicular to the line of sight, a focusing lens is employed. The incoming radiation power is split into two branches by a 3dB coupler. Before down conversion of the input spectrum, the upper and the lower side band are selected respectively by means of a high pass and a low pass filter. Behind the filters, identical components are employed. The local oscillator frequency is 92 GHz for both mixers. The IF frequency range after down conversion is 2 to 18 GHz. The IF spectrum is amplified by two amplifiers with a total gain of approximately 60 dB. The outputs of these amplifiers are split into 6 branches. The frequency selection is made by band pass filters and the received power is detected by Schottky diode detectors followed by video amplification and ltering. The data is recorded by an ADC with 1 MHz sampling rate. The radiometer has been tested and calibrated with a high temperature blackbody radiator. The effective input noise temperature for the radiometer channels is found to be in the order of 0.1 eV. The linear regime of operation extends up to an input power which corresponds to an electron temperature of approximately 200 eV. Above this temperature the 1 dB compression point of the Schottky diode detectors is exceeded and the IF gain should be reduced. This can be easily done by incorporating appropriate attenuator behind the IF amplifiers.

Millimetre-wave wideband reflection-type CPW MMIC phase shifter

A reflection-type coplanar waveguide (CPW) phase shifter fabricated using a standard monolithic microwave integrated circuit (MMIC) process is presented. Air-gap overlay CPW couplers were employed for wideband 3 dB coupling and low loss at millimetre wave. The two-stage cascaded analogue phase shifter showed insertion losses of 6.9±1.6 dB, return losses >10 dB, and maximum rms phase error of ±5.5° for the relative phase shift from −20° to 135°, over a wideband 27 to 47 GHz.

An S-Band Reflection-Type Phase Shifter - A Design Example Using Ferroelectrics

AbstractOne of the challenges faced in using ferroelectrics in high frequency devices is how to effectively use the material in a circuit design. A compact reflection-type phase shifter fabricated on sapphire substrates coated with ferroelectric barium strontium titanate (BST) thin-films has been built which shows the promise of using BST thin films in the design of tunable microwave devices. The phase shifter, fabricated as one monolithic assembly, consists of a 3dB coupler, meandered line inductors and tunable interdigital capacitors. A continuously variable phase shift range of more than 100° using the branch-line coupler was obtained at a center frequency of 2.95 GHz, and more than 90° phase shift over 200 MHz bandwidth with a bias voltage range from 0 V to 175 V. The phase shifter using the Lange coupler has over 700 MHz bandwidth centered at 2.2 GHz with a phase shift of more than 90° and an insertion loss less than 2 dB and return loss of greater than 14 dB, over a bias voltage range from 0 V to 160 V. The loss of the BST phase shifter presented in this work is on the order of other commercially available RF front-end components, such as bandpass filters and RF switches. This holds promise for the practical realization of smart antenna systems in cellular handsets and wireless LAN cards.

A 2 x 2 crosspoint switch fabricated on the passive active resonant coupler (PARC) platform

We demonstrate the fabrication of a 2×2 crosspoint switch monolithically fabricated on the passive active resonant coupler (PARC) platform by utilizing vertical resonant coupling over a taper between an active and a passive waveguide. The coupling taper was 100 μm long with less than 0.15 dB coupling loss. By pushing the mode up and down as and when required, we are able to integrate passive waveguides and electroabsorption modulators on one chip. The static performance of the switch has been tested, and a modulation depth of 30 dB has been achieved at the wavelength of 1.57 μm for an applied bias of 2.5 V.

Analysis of SiO 2/TiO 2–SiO 2/SiO 2 coupled parallel waveguide structures using computer aided design techniques

The analysis of two coupled parallel rib waveguides of SiO 2/TiO 2–SiO 2/SiO 2 on silicon substrate is presented. The indigenously developed beam propagation method (BPM) simulation tools are applied to observe the power coupling between two identical parallel rib waveguides and to study the dependence of the coupling length on various parameters of the rib waveguides including the refractive indices, rib height, guide thickness, clad thickness and separation between them. The designs for the 3 dB coupler and demultiplexer for DWDM, based on the symmetric directional coupler, are optimized for minimum dimensions using the techniques, for the first time. The optimum device length for 3 dB coupler was calculated to be 650 μm with 0.016 dB losses. The 1.3 μm and 1.304 μm demultiplexer has been designed with the optimized device length of 1.02 mm and the losses less than 0.40 dB at both the wavelengths.

Design of 10 dB 90° branch line coupler using microstrip line with defected ground structure

The authors present a 10 dB 90° branch line coupler operating at 1.8 GHz and having a defected ground structure (DGS), a kind of periodic structure realised by etching on the ground plane under the microstrip line. Owing to the additional effective inductance of the DGS, the characteristic impedance of the microstrip line is increased for the same width of conductor. The microstrip line of 150 Ω of characteristic impedance with 1 mm of conductor width is realised by adding the DGS, while 1 mm corresponds to 82 Ω of conventional microstrip line on the RT/Duroid 5880 substrate with 2.2 of dielectric constant and 31 mils of thickness. It is shown that a 90° branch-line 10 dB coupler has been fabricated using the 150 Ω line with DGS. Its measured performances are in good agreement with the predicted results.

A Novel Linearization Method of Multiple Quantum Well (MQW) Electroabsorption Analog Modulator

A linearization method for a multiple quantum well (MQW) electroabsorption analog modulator is studied. We analyze the nonlinear characteristics of the light-voltage (L-V) transfer curve in a MQW electroabsorption modulator and estimate its intermodulation distortion (IMD). We propose a novel linearization method, the integration of twin MQW modulators with optical 3 dB couplers. This method can reduce IMD2 and IMD3 simultaneously by only adjusting the modulation depth and the bias voltage of each modulator. Results of simulations show that the method can suppress IMD2 and IMD3 to below -65 dBc at a modulation depth of 10% without any serious loss of optical signal power or increase in chirp.