An integrated fluorescence array as a platform for lab-on-a-chip technology using multimode interference splitters

We present the design and fabrication of 1-to-N multimode interference (MMI) splitters, suitable for use in integrated optical fluorescence array sensing, with particular applications in lab-on-a-chip (micro-TAS) technologies. Electron beam irradiation of germanium-doped flame hydrolysis deposited silica was used to define the MMI waveguide regions. The splitters were integrated with microfluidic channels to form direct-excitation fluorescence sensor chips for use at visible wavelengths. Characterization of the waveguides shows that predictable splitting ratios can be achieved. Two devices are presented: a 1/spl times/2 splitter integrated with one analytical chamber and a 1/spl times/4 array device for multipoint excitation. A photomultiplier tube was used to assess the analytical performance of the chip, in response to standard aliquots of fluorophore (31 nM to 1.25 /spl mu/M).     

A New Approach for Optimization of Wavelength Multiplexers with Phased Waveguide Arrays by Use of the Beam Propagation Method

An approach to the optimization of wavelength multiplexers with phased waveguide arrays is presented. It is based on a simulation of the structure by the beam propagation method. The approach takes into account not only the symmetry of the structure but also the perturbations due to both emitted radiation and coupling between the waveguides at the ends of the phased waveguide array. Low-loss cosine-shaped bends are used. Both the design principles and the optimization of real wavelength multiplexers are described. The optimized multiplexers, based on silica waveguides on a silicon substrate, have sizes of about 50 mm 2 30 mm, fibre-to-fibre losses of 1–2 dB and cross-talk values between m 40 and m 29 dB.     

Polymer‐Based Optical Waveguides: Materials,Processing, and Devices

Polymer optical waveguide devices will play a key role in several rapidly developing areas of broadband communications, such as optical networking, metropolitan/access communications, and computing systems due to their easier processibility and integration over inorganic counterparts. The combined advantages also makes them an ideal integration platform where foreign material systems such as YIG (yttrium iron garnet) and lithium niobate, and semiconductor devices such as lasers, detectors, amplifiers, and logic circuits can be inserted into an etched groove in a planar lightwave circuit to enable full amplifier modules or optical add/drop multiplexers on a single substrate. Moreover, the combination of flexibility and toughness in optical polymers makes it suitable for vertical integration to realize 3D and even all‐polymer integrated optics. In this review, a survey of suitable optical polymer systems, their processing techniques, and the integrated optical waveguide components and circuits derived from these materials is summarized. The first part is focused on discussing the characteristics of several important classes of optical polymers, such as their refractive index, optical loss, processibility/mechanical properties, and environmental performance. Then, the emphasis is placed on the discussion of several novel passive and active (electro‐optic and thermo‐optic) polymer systems and versatile processing techniques commonly used for fabricating component devices, such as photoresist‐based patterning, direct lithographic patterning, and soft lithography. At the end, a series of compelling polymer optical waveguide devices including optical interconnects, directional couplers, array waveguide grating (AWG) multi/demultiplexers, switches, tunable filters, variable optical attenuators (VOAs), and amplifiers are reviewed. Several integrated planar lightwave circuits, such as tunable optical add/drop multiplexers (OADMs), photonic crystal superprism waveguides, digital optical switches (DOSs) integrated with VOAs, traveling‐wave heterojunction phototransistors, and three‐dimensionally (3D) integrated optical devices are also highlighted.     

A multiplexer/demultiplexer for coarse wavelength-division multiplexing

Increasing the transmission capacity of existing and newly developed optical communication lines and networks (data exchange systems) implies the use of wavelength-division multiplexing (WDM). Optical fiber multiplexers and demultiplexers are the WDM devices necessary for combining several carriers in a common optical tract and performing the inverse function. We propose a multiplexer/demultiplexer device based on fused single-mode fiber couplers.     

Modal analysis and device considerations of thin high index dielectric overlay slab waveguides

The effect of adding a thin high index dielectric overlay layer onto a 3-layer slab waveguide demonstrates several interesting features that can be exploited in integrated optical device configurations. A simple modal analysis is employed to examine the behavior of guided light launched from a 3-layer waveguide structure then coupled and propagated in the 4-layer overlay region. Modal properties typically overlooked in conventional slab waveguides are made use of in the design and theoretical analysis of an MMI device and optical index of refraction sensor. The optical structure presented here can form the backdrop waveguide design for more complex and active devices.     

Novel approach for design of low index contrast multimode interference devices

Self-imaging theory is widely accepted as a good method in designing multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. We use this approach in the optimal design of a 1?×?4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures.     

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.     

Characteristics of polymeric optical passive single-mode waveguiding devices fabricated by an argon-ion laser

The fabrication of polymeric single-mode Gaussian profile optical waveguides is described. We used poly(methyl methacrylate) and a mixture of two intermiscible monomers for the cladding and the core, respectively, of the waveguides. The cores of the waveguides were fabricated by with an argon-ion laser beam. The waveguides had single-mode Gaussian refractive-index profiles. By using such waveguides, we fabricated directional couplers for power coupling to the adjacent waveguides and also parallel waveguide arrays for preventing power coupling to adjacent waveguides for use in interconnect chips. We analyzed the characteristics of these couplers by using the coupled-mode theory and compared the results with those obtained with the beam propagation method. Experimental results showed good correlation with the theoretical values. We designed optical bus arrays for interconnect chips, considering the variation of normalized frequency V, the power penalty, and the dimensions of the waveguides and the separation between them. The number of waveguides in the bus array increased with increasing V. For a known value of V, a waveguide's density increases with a decrease of its dimensions. The theoretical maximum number of waveguides is ~1490/cm and ~846/cm for 2 mum x 2 mum and 4 mum x 4 mum single-mode waveguides, respectively, to satisfy a 1-dB power penalty criterion at bit-error rate of 10(-9). We fabricated interconnect bus arrays with fifteen 4 mum x 4 mum waveguides for a 3-cm coupling length, and the experimental results were verified to be in good agreement with the theoretical values.     

Assembly of silica nanowires on silica aerogels for microphotonic devices

We report on the assembly of low-loss silica nanowires into functional microphotonics devices on a low-index nondissipative silica aerogel substrate. Using this all-silica technique, we fabricated linear waveguides, waveguide bends, and branch couplers. The devices are significantly smaller than existing comparable devices and have low optical loss, indicating that the all-silica technique presented here has great potential for future applications in optical communication, optical sensing, and high-density optical integration.     

Synthesis of Hadamard transformers by use of multimode interference optical waveguides

We propose a synthesis method of optical Hadamard transformer using multimode interference (MMI) couplers. By using the signal transfer matrix of 2 x 2, 4 x 4, and 8 x 8 MMI couplers, we show that sum and difference units of input signals can be synthesized. An interchange unit of two signals can also be synthesized. One synthesis method of Hadamard transformers is a combination of only 2 x 2 units, and the other is a combination of N x N(N > or = 4) units as well as 2 x 2 units. The design examples of operation units are shown, and the size and the output power of Hadamard transformers are estimated.     

Design of polarization-independent optical couplers composed of three parallel slot waveguides

The characteristics of directional couplers and power splitters based on three-guide optical couplers in slot waveguide structures are analyzed in detail by a three-dimensional full-vectorial beam propagation method. The numerical results show the achievement of a compact three-guide directional coupler operating as polarization independent with a length of 58.0 microm and having almost evenly spaced propagation constants of the three lowest order modes for quasi-TE and quasi-TM modes. Thus, a high coupling efficiency from one outside waveguide to the other outside waveguide is demonstrated and is over 99.5% for both polarization states. For a three-guide power splitter, multiple sets of waveguide parameters for achieving polarization-independent operation are presented. Tolerances to operating wavelength and structural parameters are also analyzed, and the evolution of the injected field along the propagation distance through the proposed devices is demonstrated.     

A procedure to obtain the complete electromagnetic scattering properties at discontinuities between integrated optical waveguides

Abstract

A method to obtain the complete electromagnetic scattering properties of discontinuities between arbitrary integrated optical waveguides is presented. The method involves a new generalized scattering matrix concept, together with the generalized telegraphist equations formulism and the modal matching technique. Radiation losses, as well as reflection and transmission coefficients between proper modes, can be obtained. Single and multiple discontinuities in planar and channel optical waveguides have been analysed. Numerical results of complex scattering coefficients are given. The possibilities of the method for analysing waveguide photonic crystals, as well as optical devices in waveguide periodic waveguide structures, are demonstrated.     

Modeling and experimental observation of an on-chip two-dimensional far-field interference pattern

In this paper, we model and experimentally observe the far-field radiation produced by interfering beams propagating in two-dimensional (2D) slab waveguides. Using a transmission-line analogy, we compare the 2D propagation with standard three-dimensional (3D) far-field representations and derive the 2D conditions for using standard far-field approximations. Then we test our theoretical results by experimentally observing the 2D far-field pattern produced by a 1×3 multimode interference (MMI) coupler on a silicon nanomembrane. The MMI outputs are connected to a slab silicon waveguide, and the far field is observed at the edge of the silicon slab. This represents the observation of 2D far-field pattern produced by an array of on-chip radiators.     

A 2×2 multimode interference coupler with exponentially tapered waveguide

An exponentially tapered structure is introduced into multimode interference (MMI) devices. Compared with a parabolically tapered structure, which has been successfully used in MMI devices, this structure can further reduce the length of these devices. The performances of the 2×2 MMI coupler with exponentially tapered structure, such as the optical transmission, the splitting ratio, the wavelength response and the fabrication tolerance, are investigated by the 2D finite difference beam propagation method. Results show that the exponentially tapered MMI coupler exhibits a similar property to that with a parabolically tapered structure except for the splitting ratio. The exponentially tapered structure can offer a possible application in MMI couplers with a free choice of the splitting ratio.     

Four-channel coarse-wavelength division multiplexing demultiplexer with a modified Mach-Zehnder interferometer configuration on a silicon-on-insulator waveguide

A coarse wavelength division multiplexer is designed on a silicon-on-insulator waveguide using the Mach-Zehnder interferometers with novel multimode interface-periodically segmented waveguide couplers and segmented waveguide arms. It is viable for metro and access applications, since it can be inexpensive and provide easy fabrication, compact size, and good output performance. As a design example, the channel spacing of the demultiplexer is chosen to be 24.5 nm for applications to the 10-Gigabit Ethernet. The simulation results show that the wide-passband demultiplexer can have insertion loss less than 2.3 dB and crosstalk better than 18 dB.     

Design of an ultrashort Si-nanowaveguide-based multimode interference coupler of arbitrary shape

A design procedure for an arbitrarily tapered multimode interference (MMI) coupler based on Si nanowaveguides is presented. First a series of the effective indices of the zeroth and first eigenmodes in multimode waveguides are obtained as the core width increases by using a full-vectorial finite-difference method. Two polynomial functions are used to fit the two relations between the effective indices and the core width. The phase difference Delta phi between the zeroth and first eigenmodes can then be easily calculated when the light goes through any given arbitrarily tapered MMI section. By making the phase difference Delta phi equal to a certain value Delta phi 0 required for an N-fold self-imaging, the length of a MMI coupler is determined. With the present design procedure, an ultrashort 2 x 2 parabolic MMI coupler is designed as an example. The size of the designed ultrashort MMI section is only approximately 1.4 microm x 4.7 microm.     

Dispensed polymer waveguides and laser-fabricated couplers for optical interconnects on printed circuit boards

Optical interconnects can provide chip-to-chip data communication with much needed bandwidth as processor speed and density keep growing. Optical waveguides and couplers are essential components for implementing optical interconnections. Techniques for directly dispensing polymer waveguides in laser-ablated trenches on printed circuit boards and for fabricating optical couplers are presented for quick prototype of optical interconnects. High-quality UV curable polymer waveguides were routinely fabricated. High-efficiency couplers, blazed grating couplers on sloped waveguides, sloped facet metal film couplers, and reflective-undercut facet couplers can be fabricated by using excimer laser ablation.     

Advanced Code-Division Multiplexers for Superconducting Detector Arrays

Multiplexers based on the modulation of superconducting quantum interference devices are now regularly used in multi-kilopixel arrays of superconducting detectors for astrophysics, cosmology, and materials analysis. Over the next decade, much larger arrays will be needed. These larger arrays require new modulation techniques and compact multiplexer elements that fit within each pixel. We present a new in-focal-plane code-division multiplexer that provides multiplexing elements with the required scalability. This code-division multiplexer uses compact lithographic modulation elements that simultaneously multiplex both signal outputs and superconducting transition-edge sensor (TES) detector bias voltages. It eliminates the shunt resistor used to voltage bias TES detectors, greatly reduces power dissipation, allows different dc bias voltages for each TES, and makes all elements sufficiently compact to fit inside the detector pixel area. These in-focal plane code-division multiplexers can be combined with multi-GHz readout based on superconducting microresonators to scale to even larger arrays.     

Microchannel-Based Refractive Index Sensors Monolithically Integrated With Silica Waveguides: Structures and Sensitivities

Refractive index sensors using self-forming microchannels embedded in borophosphosilicate glass and monolithically integrated with silica waveguides are presented. Fabricated devices presented include solid-core and liquid-core directional couplers, liquid-core modal interferometers, Mach-Zehnder interferometers, segmented waveguides, and microchannel grating devices. Sensitivities of these devices are calculated and compared with each other and to other well-known devices. Experimental characterizations show that the performance of fabricated devices agrees well with calculations.