Optical Coupling Efficiency Research Articles

Ribbon plastic optical fiber linked optical transmitter and receiver modules featuring a high alignment tolerance

Ribbon plastic optical fiber (POF) linked four-channel optical transmitter (Tx) and receiver (Rx) modules have been proposed and realized featuring an excellent alignment tolerance. The two modules share a common configuration involving an optical sub-assembly (OSA) with vertical cavity surface emitting lasers (VCSELs)/photodetectors (PDs), and their driver ICs, which are integrated onto a single printed circuit board (PCB) substrate. The OSA includes an alignment structure, a beam router and a fiber guide, which were produced by using plastic injection molding. We have accomplished a fully passive alignment between the VCSELs/PDs and the ribbon POF by taking advantage of the alignment structure that serves as a reference during the alignment of the constituent parts of the OSA. The electrical link, which largely determines the operation speed, has been remarkably shortened, due to a direct wire-bonding between the VCSELs/PDs and the driver circuits. The light sources and the detectors can be individually positioned, thereby overcoming the pitch limitations of the ribbon POF, which is made up of perfluorinated graded-index (GI) POF with a 62.5 μm core diameter. The overall alignment tolerance was first assessed by observing the optical coupling efficiency in terms of VCSEL/PD misalignment. The horizontal and vertical 3-dB alignment tolerances were about 20 μm and 150 μm for the Tx and 50 μm and over 200 μm for the Rx, respectively. The VCSEL-to-POF coupling loss for the Tx and the POF-to-PD loss for the Rx were 3.25 dB and 1.35 dB at a wavelength of 850 nm, respectively. Subsequently, a high-speed signal at 3.2 Gb/s was satisfactorily delivered via the Tx and Rx modules over a temperature range of -30 to 70°C with no significant errors; the channel crosstalk was below -30 dB. Finally, the performance of the prepared modules was verified by transmitting a 1080p HDMI video supplied by a Bluelay player to an LCD TV.

Vertical-coupling optical interface for on-chip optical interconnection

We present a vertical-coupling optical interface with a grating coupler for transmitting and receiving optical signals between single-mode optical fibers and microphotonic waveguides with a view to realize on-chip optical interconnection. The optical interface consisting of a simple grating structure with a reflective mirror and an optical power combiner exhibits high optical coupling efficiency and wide tolerance range for the misalignment of optical fibers. The optical interface exhibits high coupling efficiency even if the optical input is almost vertical to the chip surface.

Integrated Optical Carrier for Optical/Electrical Interconnect

A simple and novel design, integrating discrete commercial micro-lens and vertical illuminated optoelectronic component in a substrate with high accuracy, is presented here. Without affecting the optical performances, this integrated optical carrier also allows high-frequency radio-frequency interconnects. This feature is critical for high-speed operation. The high accuracy integrated optical carrier improves the optical coupling efficiency and helps to relax the tight circuit assembly tolerance requirement. The integrated optical carrier can be used for various photonic applications which employ vertical illuminated optoelectronic components. An integrated optical carrier prototype is designed here for the optical electrical interconnect printed circuit board (OECB). For this OECB design, the simulated results show that the integrated optical carrier helps to give an assembly misalignment tolerance of more than ±20 μm with an increase of 1 dB coupling loss. In addition, the simulated optical insertion loss from the transmitter to the receiver is less than 1.4 dB. The optical performance of the prototype integrated optical carrier is measured and compared with the simulation results to ascertain the design concept. For different OECB waveguide designs, dimensions and positions, this integrated optical carrier design is amended to give a better performance and misalignment tolerance.

Near-Zero Shift Attachment for Optoelectronic Components

High performance laser-based optoelectronic devices commonly feature the use of free-space optical coupling between the laser diode and optical elements such as filters, secondary harmonic generators and optical fibers. A critical challenge in the assembly of such components is maintaining the required optical alignment precision during attachment of the optical subcomponents to a common platform. In the case of devices based on single mode waveguides, the post-attach shift must often be held to less than a few hundred nanometers to achieve the desired optical coupling efficiency. Historically, these tight tolerances have required the use of costly post-work operations such as laser hammering or re-bend to achieve performance objectives. Over the course of designing several such optoelectronic components, we have used and developed a variety of design concepts and assembly processes which have allowed us to achieve these demanding tolerances, often without the use of post-work. UV-curable structural adhesives and Nd:YAG laser spot welding have been used, individually and in combination, to perform the required sub-micron optomechanical attachments. Several approaches which have been successfully used will be described and their relative merits will be compared. In addition, key design and process elements which can impact post-attach shift will be discussed.

Optimization of broadband (11–15 μm) optical coupling in Quantum Well Infrared Photodetectors for space applications

Large format detectors in the long wavelength region of the thermal infrared (TIR) spectrum ( λ ∼ 11–15 μm) with very high radiometric and imaging performances are of increasing scientific and operational interest for, e.g. meteorology and atmospheric chemistry in missions such as Meteosat Third Generation (MTG) or Planetary Science missions. A 30 months ESA project started in 2008, whose overall purpose is to expand and assess the performance of broadband 11–15 μm Quantum Well Infrared Photodetectors (QWIP) for spectro-imaging applications: Dispersive Spectrometers (DS) and Fourier Transform Spectrometers (FTS). The primary objective is the development of an optimized detection layer focusing on its technology and the necessary performance levels. In this paper, we present a theoretical and experimental study of the optical coupling inside the pixels of a QWIP focal plane array. The broadband 11–15 μm absorption spectrum is achieved by the use of a quantum well stack containing several kinds of quantum wells. The natural narrowband optical coupling of classical diffraction gratings doesn’t allow an efficient and uniform broadband optical coupling of the IR incident light over the whole 11–15 μm spectral band. Specific aperiodic or quasiperiodic optical coupling schemes have to be used. In this work we demonstrate the ability of obtaining a good spectral matching with various original broadband diffractive structures. An extensive experimental study is presented. Comparison with simulated spectra is performed.

Passively Aligned Transmit Optical Subassembly Module Based on a WDM Incorporating VCSELs

A short-reach transmit optical subassembly module enabled by a passive alignment has been proposed and manufactured which takes advantage of a filter free coarse wavelength-division multiplexer (WDM). A four-channel data transmission was accomplished by employing only two plastic optical fibers and four vertical-cavity surface-emitting lasers (VCSELs). A multiaxis beam combination was introduced to overcome the use of WDM filters. The proposed module was constructed via a passive alignment; the overall alignment tolerance was examined by assessing the optical coupling efficiency in terms of the VCSEL displacement. The achieved tolerance was ~20 and ~150 μ m in the horizontal and vertical direction respectively, while the demonstrated peak coupling was ~41%. Finally, a high-speed 2.5-Gb/s signal was successfully delivered via the demonstrated module.

Wideband group velocity independent coupling into slow light silicon photonic crystal waveguide

We experimentally demonstrate efficient optical coupling into a slow light photonic crystal waveguide (PCW) that is independent of the group velocity of the guided mode. With a group index taper to match the group velocity between a strip waveguide and a PCW, the optical coupling efficiency is nearly constant throughout the spectrum of the defect-mode, including the slow light region near the band edge. Compared to strip-PCW butt-coupling without a group index taper, our measurement results show a 20 dB enhancement in coupling efficiency with 5 dB less Fabry–Perot fluctuations. The measurements show excellent agreement with two-dimensional finite-difference time domain simulations.

Compact bidirectional optical subassembly vertically stacked with 1.25-Gbps transmitter and 10-Gbps receiver for passive optical networks

A compact bidirectional optical subassembly (BOSA) for a 1.25/10-Gbps passive optical network is developed. A vertically stacked 1.25-Gbps transmitter based on a silicon optical bench, and a 10-Gbps receiver based on a low temperature cofired ceramic are implemented to realize low-cost manufacturing and miniaturization for single package application. The proposed BOSA delivers an extinction ratio more than 10 dB at 1.25-Gbps modulation, optical output coupling efficiency is more than 60%, rise and fall time is under 300 ps, and the side mode suppression ratio is more than 35 dB for the transmitter part. For the receiver part, responsivity is more than 0.6 A/W, and sensitivity is lower than -17 dBm at a 10-Gbps bit error rate 10-12 and -21 dBm at BER 10-3 without forward error correction. The cross talk between receiver and transmitter is less than -53 dB up to 10 GHz, and optical isolation is 33 dB.

Power scaling of quantum cascade lasers via multiemitter beam combining

Different approaches to power scaling of 4.5- to 5-µm emitting quantum cascade (QC) lasers by multiemitter beam combining are investigated. Spectral beam combining of linear arrays of QC lasers consisting of several individual emitters located side by side is demonstrated as a first variant, using an external cavity equipped with a diffraction grating and a partially transmitting output mirror providing wavelength-selective feedback to each emitter. In this way, spectral beam combining of up to eight individual QC lasers is achieved with an optical coupling efficiency of 60% for an array of six emitters. The resulting beam quality (M2 < 2 for both fast and slow axes) is close to that observed for single emitters. As a second approach, a linear array of QC lasers is coupled to a custom-made array of silicon microlenses positioned in front of the output facets of the QC lasers. This technique produces a set of closely spaced parallel output beams, strongly overlapping in the far field, without introducing any coupling losses. The resulting beam divergence is given by the aperture size of the microlenses, which is limited by the center-to-center spacing of the QC lasers (500 µm in our case).

180$^{\circ}$ Light Path Conversion Device With Tapered Self-Written Waveguide for Optical Interconnection

A 180° light path conversion device with a tapered self-written waveguide (SWW) is proposed for optical interconnection. Larger tolerance width and improved light-coupling efficiency are expected when coupling this device to optical devices, because the size of the end faces at both the light-emitting and light-receiving devices can be optimized, owing to the tapered shape of the SWW. A ray-tracing simulation indicates that positional tolerance width of 1 dB down can be increased more than two fold by fabricating a tapered core. We constructed a prototype device with a tapered SWW and achieved optical coupling efficiency of -3.6 dB.

Optoelectronic Mixer Based on Composite Transparent Gate InAlAs–InGaAs Metamorphic HEMTs

In this study, sputtered indium-tin-oxide (ITO) formed ITO/Au/ITO was used to form composite transparent gate InAlAs-InGaAs metamorphic HEMTs (CTG-MHEMT), with an optoelectronic mixer significantly markedly improved front-side optical coupling efficiency. The proposed CTG-MHEMT exhibits a high responsivity (λ = 1310 nm) of 1.71 A/W under optimal bias conditions. A -3 dB electrical bandwidth of 400 MHz is produced by the photovoltaic effect and dominated by the long lifetime of the excess holes. The -3 dB electrical bandwidth associated with the photoconductive effect is 2.3 GHz, and is determined mainly by the short electron life time. A power gain cut-off frequency (fmax) of CTG-MHEMT of 18.2 GHz was achieved. This value, is much larger than that of TG-MHEMT (14.6 GHz) because Au nano particles improved the gate resistance. The optoelectronic mixing efficiency was enhanced by tuning the gate bias conditions. The CTG-MHEMT optoelectronic mixer is a cost-effective device, and based on the optical and electrical characteristics, is a promising candidate for simplifying the system architecture in fiber-optic microwave transmission applications.

Efficient Directional Coupling between Silicon and Copper Plasmonic Nanoslot Waveguides: toward Metal−Oxide−Silicon Nanophotonics

Coupling plasmonics and silicon photonics is the best way to bridge the size gap between macroscopic optics and nanodevices in general and especially nanoelectronic devices. We report on the realization of key blocks for future plasmonic planar integrated optics, nano-optical couplers, and nanoslot waveguides that are compatible both with the silicon photonics and the CMOS microelectronics. Copper-based devices provide for very efficient optical coupling, unexpectedly low propagation losses and a broadband sub-50 nm optical confinement. The fabrication in a standard frontline microelectronic facilities hints broad possibilities of hybrid opto-electronic very large scale integration.

Ultracompact Photonic-Waveguide Circuits in Si-Pillar Photonic-Crystal Structures for Integrated Nanophotonic Switches

Highly integrated optical device technology based on square-lattice Si-pillar photonic-crystal-(PC) waveguides is described. The Si-pillar PC waveguides are now ready to use, since efficient optical coupling structures to Si-wire waveguides have been devised. Nanophotonic switches using the Si-pillar-PC waveguides were experimentally demonstrated. The nanophotonic switches make use of two of the features of Si-pillar photonic crystal waveguides. One is the property of slow-light and the other is the usability of zero-radius 90 degrees bends, both of which enable waveguide-based optical devices to be greatly miniaturized. Even apart from the cut-off wavelength, the group index of the pillar-PC waveguides was about 7.8, which was about twice that of a Si-wire waveguide for the entire C-band of telecommunications wavelengths. The 3-dB couplers we fabricated were only 3.2-microm long thanks to the 90 degrees sharp bends, and they operated throughout the entire C-band. Waveguide-cross operation was also demonstrated in the entire C-band. Asymmetric Mach-Zehnder interferometers (MZIs) were configured by using the 3-dB couplers in an area of 13.2 x 37.2 microm. An MZI with a Si-wire heater successfully operated with an extinction ratio of about 20 dB at a heating power of 17 mW. It is strongly suggested that Si-pillar PC photonic-waveguide technology should help us to achieve densely integrated optical-matrix switches demanded for future photonic-telecommunication systems.

효율적인 라인램프를 위한 발광다이오드-광섬유 결합기의 설계 및 해석

Line lamps, which utilize leaky optical fibers based on periodic bending of plastic optical fibers, are proposed. The LED-optical fiber coupler, the key component of the line lamp, is designed and analyzed. The analysis by the Monte Carlo photon simulation method has shown that the optical coupling efficiency is affected rather sensitively by 

Design and Fabrication of 10-/40-Gb/s, Uncooled Electroabsorption Modulator Integrated DFB Laser With Butt-Joint Structure

This paper describes the wide temperature range operation of an electroabsorption modulator (EAM) integrated with DFB laser diodes (LDs) (EML) designed to reduce the power consumption and size of optical transmitters. We optimized the multiquantum wells (MQWs) for LD and EAM separately to realize uncooled operation. We employed a conduction band offset (¿ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ec</i> ) of around 250 meV for the LD and 150 meV for the EAM. The number of well layers was set at 6 for the LD and 12 for the EAM, respectively. We fabricated the EML using a butt-joint (BJ) process to allow us to design the LD and the EAM independently. We introduced a ridge waveguide structure for the LD and EAM waveguides, and designed the width of the LD and EAM mesa to achieve a high optical coupling efficiency between the LD and the EAM. We then used the 200-¿ m-long EAM for 10-Gb/s operation and the 150- ¿m-long EAM for 40-Gb/s operation, and thus obtained a dynamic extinction ratio of over 9 dB at 10 Gb/s from -25 °C to 100 °C and of 8.2 dB at 40 Gb/s from -15 to 80°C. We achieved a power penalty of less than 2 dB after an 80-km single-mode fiber (SMF) transmission at 10 Gb/s and a 2-km SMF transmission at 40 Gb/s over a wide temperature range. These results confirm the suitability of this EML with a BJ structure for use as a 10-Gb/s or 40-Gb/s uncooled light source.

Compactly packaged superconducting nanowire single-photon detector with an optical cavity for multichannel system

We developed superconducting nanowire single-photon detectors with an optical cavity (OC-SNSPDs) for multichannel systems. For efficient coupling, the devices were installed in compact fiber-coupled packages after their substrate thickness was reduced from 400 to 45 microm. The measured detection efficiency (DE) measurement at different substrate thicknesses and the estimation of optical coupling efficiency indicated that approximately 98% of the input light beam could be radiated on a 15 x 15 microm2 nanowire area from behind the substrate. The DEs of a NbN OC-SNSPD system were observed to be 9.5% and 25% at 1550 nm and 1310 nm, respectively (dark-count rate: 100 c/s).

The optical coupling improvement of THz quantum well infrared photodetectors based on the plasmonic induced near-field effect

Near-field effect has been recognized to play an important role in optical coupling of multiple quantum well infrared photodetectors (QWIP). Both the optical near-field effect and the plasmonic effect in the quantum well active region of QWIP device have been studied by the finite difference time-domain (FDTD) method at the optical frequency of 4.2 THz. Our numerical results indicate that both the near-field effect and the plasmonic effect can induce an evanescent wave with the electrical component being parallel to the growth direction of quantum well. The induced electrical component in the QWIP growth direction is 2.5 times larger than that of the incident optical fields. The induced polarization can increase the optical coupling efficiency of QWIP and the detectivity of QWIP can be improved at 4.2 THz.

InGaAlAs-InGaAsP Heteromaterial Monolithic Integration for Advanced Long-Wavelength Optoelectronic Devices

High-coupling-efficiency high-reliability hetero-material integration of InGaAlAs-based and InGaAsP-based optical components on a single InP substrate was achieved. A butt-jointing process with in situ cleaning was used to integrate an InGaAlAs-based component and an InGaAsP-based component. Optical-coupling efficiency at the butt-jointed interface of a novel multiple-butt-jointed laser was quantitatively estimated to be more than 97%. An InGaAlAs laser integrated with an InGaAsP-based component and an InGaAsP laser integrated with an InGaAlAs-based component were fabricated by the butt-jointing process. The fabricated 1.3- mum InGaAlAs laser integrated with an InGaAsP distributed Bragg reflector exhibited 100degC, 10-Gbps direct modulation at a low drive current of 14-mA peak-to-peak. Furthermore, the fabricated 1.55-mum InGaAsP distributed feedback laser integrated with an InGaAlAs electroabsorption modulator exhibited the first uncooled 10-Gbps 40-km transmission (with a 1-dB power penalty) from 0 to 85degC. Aging tests on both types of lasers showed no significant degradation in their driving current for more than one thousand hours.

Optimal laser welding assembly sequences for butterfly laser module packages

We present an optimal laser welding assembly sequence for butterfly laser packages: 1) initial shift, 2) front welding, 3) rear welding, 4) joint gripper releasing, 5) mechanical fine tuning of horizontal misalignment. This sequence has been optimized significantly by modeling the initial shift and experimental investigations of three assembly sequences. Our results show that misalignment from the Post-Weld-Shift (PWS) can be compensated by accurately estimating the initial shift in the vertical direction. Furthermore, the laser hammering procedure, to compensate for misalignment of the vertical direction, can be eliminated by proper package design. Using only final mechanical tuning for horizontal misalignments, optical coupling efficiencies of 73-99% have been achieved for lasers packaged in butterfly modules.

Analysis and comparison of n-Al xGa 1−xAs/GaAs QWIPs with different device structures and optical coupling

This paper discusses optical coupling for n-GaAs/AlGaAs multiple quantum well infrared photodetectors (MQWIPs). The optical responsivity has been compared with different grating structures fabricated by reactive ion etching (RIE), device form, and incidence mode. The optical coupling efficiencies are further analyzed by the modal expansion model (MEM), including optical field distributions in different size photosensitive element and interrelated influences with scattering matrix method based on plane-wave expansion (PWE). Some extra coupling parameters have been obtained in designing and optimizing QWIPs FPA.