Optical Waveguide Coupler Research Articles

Micro patterning of fused silica by ArF- and F2-laser ablation

Surface-relief gratings on optical materials are required for various telecommunication applications, as light couplers for planar optical waveguides, Bragg reflectors, or alignment grooves for liquid crystals. Vacuum-UV-laser ablation enables precise machining of fused silica. A comparison of ablation results obtained at 157 nm and 193 nm wavelength leads to the following conclusion: at 193 nm the surface properties (e.g. mainly roughness) have strong influence on the ablation behaviour. At 157 nm, the volume properties (e.g. mainly bulk absorption) are dominating instead. Though there are severe constraints concerning deep hole drilling at 193 nm, because irregular cracking can hardly be excluded, the generation of high resolution surface patterns is possible at this laser wavelength. As an example, sub-half-micron surface-relief gratings were fabricated on fused silica by laser ablation with nanosecond (ns) pulses at 193 nm from an ArF excimer laser. The grating relief was generated by imaging a transmission amplitude grating with a Schwarzschild objective of 25x demagnification. To provide high resolution in combination with high fluence, which is required for ablating fused silica, an off-axis mask projection scheme utilizing superposition of the zero order beam with the +1st diffraction order was applied.

Deep proton writing: a rapid prototyping polymer micro-fabrication tool for micro-optical modules

One of the important challenges to deploying the emerging breed of nanotechnology components is interfacing them with the external world, preferably accomplished with low-cost micro-optical devices. In our labs at the Vrije Universiteit Brussel (VUB), we are therefore focusing on the continuous development of a rapid prototyping technology for the fabrication of micro-optical modules. In this technology, which we call deep proton writing (DPW), we bombard polymer samples with swift protons, which will result after chemical processing steps in high quality micro-optical components. The strength of the DPW micro-machining technology is the ability to fabricate monolithic building blocks that include micro-optical and mechanical functionalities which can be precisely integrated into more complex photonic systems. The DPW technology is furthermore compatible with low-cost mass-replication techniques such as micro-injection moulding and hot embossing. In this paper we give an overview of the process steps of the technology and the characteristic qualities we can expect from the components made by DPW. The general overview of the technology is followed by three case studies of different micro-optical components that were fabricated at our labs: (i) two-dimensional fibre connectors, (ii) out-of-plane couplers for optical waveguides embedded in printed circuit boards (PCBs), (iii) intra multi-chip-module (MCM) level optical interconnection via free space optical modules.

Quasi-vectorial analysis of the optical characteristics of rib waveguides and directional couplers based on InGaAs/InAlAs multiple quantum wells

A new three-dimensional quasi-vectorial beam propagation method based on the series expansion (SE-QV-BPM) is proposed for simulating the optical rib waveguide and directional coupler based on InGaAs/InAlAs multiple quantum wells. For the same etched-depth, the confinement of TM mode in horizontal direction is shown to be stronger than that of TE mode, and the singular behaviors for TM modes at the corners are clearly observed. For the same gap width, the coupling length of TM mode for directional coupler is larger than that of TE mode. The effective indices and modal profiles of the quasi-vectorial fundamental TE and TM modes for shallow- and deep-etched rib waveguides and directional couplers are obtained. Besides, the evolution of the injected field with propagation distance through the directional coupler is demonstrated. The calculation has good efficiency due to the small size of the resulting matrix, and the rib structures can be conveniently dealt with since the sinusoidal functions are selected as the basis functions. Moreover, the variable transformation is introduced to avoid the boundary truncation.

Analysis and design of long-period waveguide-grating couplers

By means of coupled-mode theory, an optical waveguide coupler consisting of two parallel identical waveguides, each of which contains a uniform long-period grating was analyzed. The authors show that light can be coupled effectively between the two waveguides through the gratings at specific wavelengths (the resonance wavelengths) in the absence of evanescent-field coupling. The conditions for achieving 100% coupling efficiency at the resonance wavelength under various situations are highlighted and the effects of the grating parameters on the transmission spectra of the coupler are illustrated with examples. Practical waveguide designs for the realization of the coupler are also presented. The long-period waveguide-grating (LPWG) coupler is a versatile waveguide structure and should find potential applications in optical filtering, add/drop multiplexing, and optical switching.

Coupling Characteristics of an Asymmetric Rectangular Core Optical Wave-guide Coupler

An analytical method based on the boundary value technique is developed to study the systematic of symmetric as well as asymmetric wave-guide couplers. We have tested our model for symmetric and asymmetric couplers and our results are in good agreement with those of Takagi et al. Our technique is simple, straightforward with few mathematical simplifications and approximations. We obtain unique self-consistent single solution for the given guide geometry, therefore implying the mono-mode character of the waveguide coupler. Our formulism can predict the performance and the field characteristics of the coupler from the knowledge of the coupler geometry. Thus it may help in the design of diffused channel single mode rectangular core optical wave-guide couplers when adequate data is available.

Coherent destruction of tunneling in waveguide directional couplers

It is theoretically shown that the phenomenon of coherent destruction of tunneling in bistable quantum systems induced by an external driving field can be realized in a periodically curved optical waveguide coupler, the periodic bending of the coupler simulating the effect of the external field in the quantum system.

Fast and Accurate Analysis of 3-D Curved Optical Waveguide Couplers

The structure-related (SR) finite-difference beam-propagation method is applied to integrated optical couplers employing curved waveguide sections. A general nonorthogonal coordinate system is introduced that matches the local structure geometry, yielding a more accurate analysis with coarser meshes than standard schemes, thus providing significant computational resource savings. Furthermore, the algorithm is easily interfaced with conventional schemes. Examples of curved two-dimensional and three-dimensional directional waveguide couplers are examined to show the advantages and generality of SR schemes as well as to explore the role of the extra degrees of freedom offered by nonorthogonal meshing.

Finite size effects on one dimensional coupled cavity optical waveguides

The properties of an infinite periodic coupled cavity optical waveguide are completely determined by the tight binding parameter κ. One dimensional finite size coupled cavity optical waveguide (FSCCOW) structures, which are specified by the cavity number, substrate and covering layers, are investigated using transfer matrix method in this paper. Our results show that the envelope functions of the local minima and maxima in the transmission spectrum are invariant with respect to the cavity number, and the envelopes of the group velocity spectrum do not converge to each other with the increasing of cavity number. These results allow us better understand the asymptotic behaviors of FSCCOWs on increasing cavity number. We also study the impact of the substrate and covering layers on the shape of the envelope functions, and find that the quasiflat ranges shift sensitively from the band center to the band edges when the thickness of the content layers in the substrate layers and covering layers are changed slightly.

Large Bandwidth Optical Limiting by Kerr Medium DopedOne-Dimensional Coupled Cavity Optical Waveguides

We present a new method to realize large bandwidth opticallimiting using Kerr nonlinearity doped one-dimensional coupledcavity optical waveguides (CCOWs). When the incident intensity isweak, the structure is a perfect CCOW that supports a hightransmission band. When the intensity is increased, the Kerreffects change the refractive index of the Kerr cavity whichresults in a large decrease of transmission coefficients in thewhole band. We numerically analyse the limiter using the nonlineartransfer matrix method and nonlinear finite difference in timedomain method. Optical limiting effects are observed inthe whole transmission band, and optimization methods are alsodiscussed.

Extended optical fiber line testing system using new eight-channel L/U-band crossed optical waveguide coupler for L-band WDM transmission

This paper describes the system design for an extended optical fiber line testing system that uses a new L/U-band crossed optical waveguide coupler and a fiber Bragg grating filter for L-band wavelength-division multiplexing transmission. We describe the reflection characteristic required for optical filters located in central offices in order to suppress the ghost signal caused by multireflection in the optical time-domain reflectometry (OTDR) trace. We design and evaluate an eight-channel crossed optical waveguide coupler with a new thin dielectric film filter that separates a 1650-nm test light from the L-band communication light, and confirm that there was no degradation caused by multireflections in the OTDR trace. We also demonstrate the in-service line monitoring of a 10-Gb/s L-band transmission with no degradation in the transmission quality.

Optimized design of polarization-independent and temperature-insensitive broadband optical waveguide coupler by use of fluorinated polyimide.

A statistically optimized design method suitable for a polariation-independent and temperature-insensitive broadband waveguide coupler is proposed. By use of this method, a fluorinated polyimide waveguide 3-dB waveguide coupler for 1490 to approximately 1610 nm application is designed by optimizing polarization and temperature fluctuation. The validity of the design is verified through simulation based on the three-dimensional beam propagation method (3D-BPM), which revealed a coupling ratio of 50 +/- 0.8% in a 120-nm bandwidth in the temperature range -10 to 40 degrees C for both orthogonal polarizations.

Nonlinear optical waveguide coupling with planar solitonic field profile

The determination of field distribution of an electromagnetic wave propagating in an optically nonlinear planar waveguide with uniform Kerr-like medium is considered in principle as a two-dimensional problem in the transverse plane. The associated two-dimensional Helmholtz equation is nevertheless reduced to a one-dimensional problems by means of effective index method, resulting in soliton like spatial distribution of the electric field along the lateral direction parallel to the waveguide plane. Taking into account this nonuniform field distribution in terms of the effective index of the guide, we have studied semi-analytically the nonlinear coupling behaviors of a symmetric dual waveguide system. The result shows excellent agreement with the numerically result obtained by FD-BPM method for planar structure, exhibiting the desired optically controlled logic functions.

Optimized design of temperature-insensitive optical waveguide coupler with 120-nm bandwidth using fluorinated polyimide.

Method for designing optimized temperature-insensitive optical waveguide couplers by use of fluorinated polyimide is presented. Based on measured temperature and dispersion characteristics of fluorinated polyimide, a 3-dB waveguide coupler with a 120-nm bandwidth with minimal temperature variance is designed and verified through simulation based on three-dimensional beam propagation. The coupling ratio of the theoretical device is 50 +/- 0.7% in the waveband 1490 to approximately 1610 nm and the temperature range -10 to approximately 40 degrees C.

Critical coupling and its control in optical waveguide-ring resonator systems

The coupling of optical waveguides to ring resonators holds the promise of a new generation of switches (modulators) which employ orders of magnitude smaller switching (modulation) voltages (or control intensities). This requires a means for voltage (or intensity) control of the coupling between the waveguide and the resonator. Schemes for achieving such control are discussed.

A micromachined connector for the coupling of optical waveguides and ribbon optical fibers

This paper describes in details the fabrication and tests of a micromechanical connector, which is used for the precise optical self-alignment of multi-waveguide optical integrated circuits (OIC) to ribbon optical fibers, without injecting light in the fiber. Nickel alignment pins are electrodeposited on the OIC using a photolithographic process, and these pins are inserted into suitable openings made on a silicon micromachined platform, on which optical fibers are accurately positioned using V-grooves. A simultaneous fabrication of several microstructures which are used as an assistance for the assembly of the fibers and the waveguides is presented for the first time. Design and fabrication issues are reported, as well as preliminary experimental results which show that excess optical losses on the order of 3 dB per coupling facet can be obtained.

Vertical antiresonant reflecting optical waveguide coupler for three-dimensional optical interconnects: optimum design for large tolerance, high coupling efficiency, and short coupling length

A compact antiresonant-reflecting-optical-waveguide-(ARROW-) type vertical coupler for three-dimensional optical interconnects was demonstrated. The coupler consists of stacked ARROW's channeled by the stripe lateral confinement structure, and each waveguide is completely separated by a thin metal film in the separation region. In the coupling region the intermediate cladding of a previous coupler was made of the same material as that of the first cladding or the core. However, we had to overcome the problem that both the high coupling efficiency and the large fabrication tolerance cannot be achieved simultaneously. Thus we incorporated an intermediate cladding made of a material different from that of the core and the first cladding. The refractive index and the thickness of the intermediate cladding were optimally designed to achieve large fabrication tolerance and a short coupling length with a high coupling efficiency. The coupling length was reduced from 4.1 to 0.8 mm, and a high coupling efficiency of 96% was experimentally demonstrated.

Nonlinear antiresonant reflecting optical waveguide coupler with an optically induced grating

We study externally controlled optical switching in an asymmetric antiresonant reflecting optical waveguide directional coupler. The power exchange between two channels with different propagation constants is obtained by means of an optically induced grating in the spacing region. Compared with a conventional coupler with an optically induced grating, the antiresonant reflecting optical waveguide coupler exhibits reduced switching power and permits remote coupling.

Photosensor and optical waveguide coupling in silicon technology

ARROW-type optical waveguides are designed for implementation on silicon using the materials (silicon dioxide and silicon nitride) and techniques (CVD, RIE) of CMOS integrated-circuit technology. Light is detected by a photodiode buried in the silicon substrate, which is made following the same process. The steps of this process are described and their influence on the optical properties of the guides is analysed. The optical signal attenuation in the waveguide-photodiode coupling region and the cut-off frequency of the system are measured in test devices. The advantages of the technological compatibility with CMOS circuits are discussed.

Power Exchange in Coupled Optical Waveguides

Analytical expressions for power exchange in two waveguide directional couplers have been obtained using the modified scalar coupled mode approach that takes into account the non-orthogonality of the fields and also the perturbation correction terms to the effective indices of individual waveguides which were ignored in the old Coupled Mode Theory. Our results show considerable difference when compared with old Coupled Mode Theory, specially for non-identical waveguides. However, they are consistent with results obtained by the exact modal analysis.

Matrix Lie Group-Theoretic Design of Coupled Linear Optical Waveguide Devices

The linear mapping effected by a coupled linear waveguide system on its input electromagnetic field can be defined by the system’s transfer matrix and this paper addresses the systematic design of the system’s internal dimensions to achieve any desired transfer matrix within a broad class. Manufacturing constraints limit the set of all realisable couplers and their transfer matrices can be used to define a Lie group, whose Lie algebra can be used to study the limitations imposed by such constraints. After having defined the group, the paper presents a design procedure that is guaranteed to realise any transfer matrix in the group by a finite concatenation of basic building-block coupler sections. This guarantee is afforded by a simple manipulation of canonical coordinate systems for suitable neighbourhoods of the group’s identity. The design procedure is illustrated for the group $\mathfrak{G}\mathfrak{U}( 2 )$, where the homomorphism with $\mathfrak{G}\mathfrak{D}( 3 )$ yields geometrical insight in terms of rigid rotations of the Poincaré sphere. A further example uses the procedure to realise a fully planar implementation of a $3 \times 3$ coupler of importance in optical sensing and signal processing.