Optical Coupling Efficiency Research Articles

Pixel-to-Pixel Fiber-Coupled Emissive Micro-Light-Emitting Diode Arrays

We report on an integrated fiber-coupled bi-linear micro-light-emitting diode array, serving as a portable microdisplay system. The fiber bundle transforms the bi-linearly arranged optical signals from the emissive array into a 6-by-8 pixel microdisplay, offering a crisp and clear optical output. The pixel-to-pixel coupling arrangement ensures optical coupling efficiency. Due to the narrow acceptance cones of optical fibers, individual pixels can be well resolved with minimal crosstalk. The performance and functionality of this optical system is fully evaluated. A model to determine the fiber-coupling efficiency was constructed; it was found that the simulated results compare well with the measured data.

Evaluation of Silicon Detectors With Integrated JFET for Biomedical Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents initial results from electrical, spectroscopic and ion beam induced charge (IBIC) characterisation of a novel silicon PIN detector, featuring an on-chip <formula formulatype="inline"><tex Notation="TeX">$n$</tex> </formula>-channel JFET and matched feedback capacitor integrated on its <formula formulatype="inline"><tex Notation="TeX">$p$</tex></formula>-side (frontside). This structure reduces electronic noise by minimising stray capacitance and enables highly efficient optical coupling between the detector back-side and scintillator, providing a fill factor of close to 100%. The detector is specifically designed for use in high resolution gamma cameras, where a pixellated scintillator crystal is directly coupled to an array of silicon photodetectors. The on-chip JFET is matched with the photodiode capacitance and forms the input stage of an external charge sensitive preamplifier (CSA). The integrated monolithic feedback capacitor eliminates the need for an external feedback capacitor in the external electronic readout circuit, improving the system performance by eliminating uncontrolled parasitic capacitances. An optimised noise figure of 152 electrons RMS was obtained with a shaping time of 2 <formula formulatype="inline"> <tex Notation="TeX">$\mu$</tex></formula>s and a total detector capacitance of 2pF. The energy resolution obtained at room temperature (21<formula formulatype="inline"> <tex Notation="TeX">$^{\circ}$</tex></formula>C) at 27 keV (direct interaction of I-125 gamma rays) was 5.09%, measured at full width at half maximum (FWHM). The effectiveness of the guard ring in minimising the detector leakage current and its influence on the total charge collection volume is clearly demonstrated by the IBIC images. </para>

Laser guiding of cold molecules in a hollow photonic bandgap fiber

We propose a promising scheme to guide buffer-gas cooled arbitrary neutral molecules in a hollow-core photonic bandgap (HC-PBG) fiber by using a red-detuned Gaussian mode and calculate the optical coupling efficiency of the Gaussian mode in the HC-PBG and its optical potential for I2 molecules. Also, we calculate both the straight guiding efficiency and bend guiding one by using a classical model and the dynamic process of the laser guiding of cold molecules in the HC-PBG by Monte Carlo simulation, by which we obtain the transverse and longitudinal velocity distributions of the output guided molecular beam. Our study shows that when the input laser power is 2 kW, the straight guiding efficiency is ~24.6% and when the input laser power is 200 W and R=0.2 cm, the bent guiding efficiency is ~1.2%. We also find from the simulated results that our laser bent guiding scheme is a simple and desirable method to generate a CW cold arbitrary molecular beam.

Optoelectronic characterization of a fiber-coupled NbN superconducting nanowire single photon detector

This paper reports our experimental setups to characterize the superconducting nanowire single photon detector (SNSPD) that is packaged in-house. Our packaging method ensures not only a satisfied optical coupling efficiency but also a wide band electrical connection. The packaging approach has been proved to meet the needs of cryogenic operations. The basic optoelectronic characterization results including quantum efficiency and dark count rate indicate a good agreement with the expected values.

Full C-Band Tunable DFB Laser Array Copackaged With InP Mach–Zehnder Modulator for DWDM Optical Communication Systems

We fabricated a compact 10-Gb/s-wavelength tunable laser module by integrating a tunable DFB laser array and an InP-based n-i-n Mach-Zehnder modulator in one package. We developed a multichip assembly technology to realize high optical coupling efficiency and achieved a continuous-wave output power of +6.5 dBm. We demonstrated a full C-band 10-Gb/s optical duobinary transmission with a constant modulation voltage of 2.4 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> . And we achieved a 200-km single-mode-fiber transmission with a power penalty of less than 1.0 dB over the entire C-band range.

Contribution to Research on Micro-Lensed Fibers for Modes Coupling

In this paper we present a review of different micro-lenses we have developed at the end of single-mode fibers for improving optical coupling efficiency in communication systems. These micro-lenses find their applications in laser-diodes or integrated waveguides to single mode fibers, fiber to detector coupling, or fiber-to-fiber interconnecting. More generally, the applications concern active or passive components to single mode fiber coupling. Three kinds of micro-lenses are considered whose fabrication process has been designed to be very simple and low cost. Some of them are produced by companies in Lannion such as IDIL Fibers Optiques.

An Integrated 2-D Active Optical Fiber Manipulator With Microfluidic Channel for Optical Trapping and Manipulation

We report a new two-axis active optical fiber manipulator for on-chip optical manipulation and detection in microfluidic environment. The system comprising of air chambers, fiber channels, controllable moving walls, and membrane structures were fabricated by using microelectromechanical systems technology. By adjusting air pressures to control the deflection of the pneumatic chambers placed orthogonal to and underneath the fiber channels, accurate alignment of a pair of approximately coaxial optical fibers, which was indicated by maximizing fiber-to-fiber optical-coupling measured in real time, has been achieved. A maximum displacement of a buried fiber as large as 13 mum at an applied pressure of 40 lb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for one air chamber has been demonstrated. It was sufficient to accurately align two approximately coaxial optical fibers to maximize the optical coupling efficiency. The maximum coupling efficiency for two single-mode optical fibers facing each other at a distance of 200 mum was measured to be 4.1%. The following features have been successfully demonstrated with this system: 1) stable optical trapping and stretching of a single red blood cell; 2) stable optical trapping of multiple microparticles; 3) optically driven controlled motion of single and multiple microparticles; and 4) integration of a counterpropagating dual-beam trap with single-beam optical tweezers. In addition to optical trapping and manipulation, the proposed device is promising for applications requiring coaxial input/output fibers for in-line optical analysis. Furthermore, it can be easily integrated with other microfluidic devices such as microcapillary electrophoresis channels or microflow cytometers for DNA, protein, and cell analysis.

Novel Grating Design for Out-of-Plane Coupling With Nonuniform Duty Cycle

We propose a waveguide grating coupler with nonuniform duty cycle for the out-of-plane coupling of light from a planar waveguide to an optical fiber, and evaluate the optical performances based on Bloch-wave analysis. The mode profile and focusing performance of the diffracted output beam are investigated in terms of the duty cycle to improve optical coupling efficiency.

Waveguide coupled terahertz photoconductive antennas: Toward integrated photonic terahertz devices

We demonstrate the operation of a low temperature GaAs photoconductive antenna pumped via a monolitically integrated AlGaAs based dielectric waveguide using 100fs pulses at 860nm. This technology demonstrates the feasibility of monolithically integrating dielectric waveguides and fast photoconductivity terahertz materials. We also demonstrate convenient pumping normal to the surface of the waveguide device via a surface grating coupler fabriacted using a gallium focused ion beam. Our initial results show a ∼10% optical coupling efficiency into the photomixer element. We attribute this low coupling efficiency to unoptimized processing techniques.

Ultra-strong coupling effects with quantum metamaterials

We study a semiconductor-based quantum metamaterial which has the optical characteristics of a metal in two directions, but behaves like a collection of artificial atoms, whose properties can be designed-in using quantum theory, in the third. We find that it supports a type of guided collective plasma resonance (CPR) mode which exhibits efficient optical coupling and long propagation distances. Furthermore, the coupling of the CPR mode with the ‘artificial atom’ transition leads to a case of “Ultra-Strong-Coupling”, demonstrated by a record vacuum Rabi splitting of 65 meV, a sizable fraction (42%), of the bare intersubband energy.

A new scheme of fiber end-face fabrication employing a variable torque technique

A new scheme of optical fibers with different types of end-faces by applying different types of periodical variable torques to control the polishing pressure is proposed and fabricated. Two types of mechanisms, mechanical torque and electrical torque controls, are developed to generate different types of periodical variable torques. The mechanical torque control type is mainly designed for fabricating an elliptical-cone-shaped fiber end-face that can be used in efficient coupling between a laser diode and fiber with a coupling efficiency up to 83% (Lu et al 2007 Opt. Express 15 1434). The electrical torque control type is even more versatile for fabricating optical fibers or micro-probes with polygon pyramid end-faces that may have other potential applications, such as various kinds of micro-indenters. Both types of fiber end-faces can deliver satisfactory results with single-step fabrication, a high fabrication yield and sufficient optical coupling efficiency.

Efficient optical coupling into metal-insulator-metal plasmon modes with subwavelength diffraction gratings

We demonstrate efficient optical coupling into metal-insulator-metal (MIM) plasmon modes. Subwavelength grating couplers are used to optically excite the MIM plasmon mode, which is observed with reflection spectroscopy. Coupling efficiencies of up to 28% are measured for insulator thicknesses of 12nm. It is found that the MIM resonance has a significant shift in energy as a function of grating depth. This shift is much larger than that seen from traditional surface plasmon modes. MIM plasmons are promising tools for probing molecular junctions due to strong field confinement and high field intensities within the insulator.

Evaluation of pixellated, back-sided planar photodetectors for high-resolution imaging instrumentation

Future generations of solid-state high-energy gamma imaging cameras require pixellated semiconductor photodetectors with the greatest possible detection efficiency. In this paper, a new type of planar silicon PIN photodiode is presented, in which both p+ and n+ electrodes are located on the same side of the device. The design offers highly efficient optical coupling between the backside and scintillator, providing a fill factor of close to 100%. The performance of this detector is modelled using the ISE-TCAD simulator and its electrical and spectroscopic characteristics are experimentally investigated for the case of direct interaction of ionising radiation and also with an attached CsI(Tl) scintillator. The energy resolutions obtained at room temperature (21∘C) at 662keV (scintillated) and 27keV (direct) were 6.3% and 7.0% respectively, measured at full width at half maximum (FWHM). The new detector can be easily manufactured into arrays, for a variety of imaging (instrumentation) applications.

Control of duty ratio in waveguide gratings using a near-field holographic lithography system with a variable aperture

A waveguide grating coupler has been developed for optical coupling between a planar waveguide and an optical fiber by some researchers. In a waveguide grating coupler, the shape of diffracted output beam profiles and their focusing performance are dependent on the variations of duty ratios. Therefore, a control of duty ratio in waveguide grating is important for more efficient optical coupling. However, the patterning of a non-uniform duty ratio in waveguide grating is generally very difficult to realize. In our experiments, we propose a new simple fabrication method, which is a variable aperture system incorporating to near-field holographic (NFH) lithography system, to realize a non-uniform duty ratio in waveguide gratings.

Fabrication of high fill factor optical film using two-layer photoresists

A new process to fabricate gapless triangular micro-lens array (GTMA) optical film is realized in this study. Two-layer photoresists are used to define a triangular column array template. The upper and lower layer photoresists are AZ 4620 and AZ 9260, respectively. The two-layer photoresists form a higher aspect ratio than the individual photoresist does. This process includes a two-layer ultraviolet (UV) lithography, photoresist reflow process, Ni–Co electroplating and the hot embossing technique. After a triangular column array of two layers of photoresist is defined by UV lithography, the reflow technique is applied to melt the triangular column array into the shape of a triangular micro-lens array. With this reflowed triangular micro-lens array, Ni–Co alloy is electroplated and covered uniformly on the triangular micro-lens array to form the GTMA mould. After this electroplating process, a mould of GTMA is obtained, which serves as the primary mould. Next, with the passivation technique applied on this primary mould's surface, a secondary mould is obtained by applying the electroplating process again. This secondary mould serves as a master for the subsequent hot embossing process to replicate the GTMA pattern onto a polymethyl methacrylate (PMMA) sheet. The Ni–Co mould with a hardness over 650 Hardness of Vicker (Hv) is obtained. The stiffness and hardness of the mould play important roles in the GTMA hot embossing process. In addition, this PMMA-based GTMA film used as optical film offers a 100% fill factor and high optical coupling efficiency to improve the luminance. The optical measurement shows that this optical film with GTMA pattern increases 18.39% of luminance for a backlight module (BLM) of a liquid crystal display (LCD).

Optical and Thermal Analysis of Nano-Patterned Medium Structure for Near-Field Assisted Recording Using FDTD Method

Simulation of near-field assisted recording using the finite-difference time-domain (FDTD) method and thermal analysis was carried out. Improvement of optical coupling efficiency between nano-patterned medium and optical near-field probe is the most important issue to design and optimize heat assisted near-field magnetic recording system. We propose the new nano-patterned medium structure that constructs the multi layer structure of silver and magnetic recording material. Local plasmon enhancement of silver layer is effective in order to improve optical near-field coupling efficiency of nano-patterned medium. The generated heat at silver layer conducts into magnetic recording material layer.

Mechanically Flexible Chip-to-Substrate Optical Interconnections Using Optical Pillars

We experimentally characterize the benefits of using surface-normal mechanically flexible optical waveguides, or optical pillars, for chip-to-substrate optical interconnection. In order to benchmark the performance of the optical pillars, the optical coupling efficiency from a light source to an optical aperture with and without an optical pillar is measured. For a light source with 12deg beam divergence, a 50times150 mum optical pillar improves the coupling efficiency by 2-4 dB compared to pillar-free (free-space) optical coupling. A 30times150 m optical pillar improves the coupling efficiency by 3-4.5 dB. This demonstrates the importance of using optical pillars when small photodetectors (PDs) and dense optical input/outputs (I/Os) are needed. The optical excess losses of 50times150 mum optical pillars are measured to be less than 0.2 dB. Due to the high mechanical flexibility of the pillars, we also demonstrate that optical pillars enhance the optical coupling efficiency between the chip and substrate when they are misaligned in the lateral direction. This is especially important since the coefficient of thermal expansion of the chip and substrate are often mismatched, and preserving optical alignment and interconnection between them is critical during thermal excursions. The lateral mechanical compliance of the optical pillars is also measured and can be as great as 30 mum/mN. The optical pillars are also shown to be compliant under a compressive force thus allowing the optical I/Os to be assembled on nonplanar surfaces such as low-cost organic substrates.

High optical coupling efficiency using 45°-ended fibre for low-height and low-cost optical interconnect modules

High optical coupling efficiency of more than 80% between an optical fibre and a vertical-cavity surface-emitting laser (VCSEL) has been achieved without use of a lens. A 45°-ended fine-drawn optical fibre can be positioned closer to a VCSEL than a conventional multimode fibre. 12.5 Gbit/s/ch optical signal transmissions using a low-height optical coupling structure have been demonstrated with optical output power as high as +2.0 dBm without use of a lens.

Beam steering and tilt coupling in tunable hollow waveguide

We present the new function of tunable hollow waveguides with a variable air core. We show a possibility of wide-angle beam steering from the hollow waveguide and efficient tilt-coupling in narrow air core hollow waveguides. Wide-angle beam steering was demonstrated by calculation and preliminary experiment. In addition, a possibility of an efficient optical coupling with a narrow air core was presented by the tilt-coupling scheme. Input and output coupling losses are 2.5dB and 1dB for 1µm air core, respectively. Wide tunability in sub-wavelength air cores enables us to realize new functions of tunable hollow waveguides and circuits.

Hybrid Integration of End-to-End Optical Interconnects on Printed Circuit Boards

This paper discusses the integration of an end-to-end optical interconnect testbed on printed circuit boards using inexpensive off-the-shelf, bare die, optoelectronic components. We developed a process for efficient and simultaneous in-plane optical coupling between edge emitting laser and waveguides, and between photodetector and waveguide. We demonstrated an optically smooth buffer layer separating the printed circuit layer from the optical transport layer. The demonstrated radically new optical interconnect technology, which we refer to as interface optical coupling, is able to efficiently and simultaneously form optical interfaces between waveguides, lasers and photodetectors by photolithographic technique, thereby eliminating the need for micro-lenses and manual alignment. The measured laser to waveguide coupling efficiency is 45% and measured waveguide to photodetector coupling is 35%. The optical link is demonstrated to operate at 10 Gbps.