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.     

Direct integration of nanoscale conventional and slot waveguides

Slot waveguides can provide high optical confinement in a nanoscale low-index layer. While a conventional waveguide has a Gaussian-like Eigenmode profile, the Eigenmode profile of a slot waveguide is quite non-Gaussian type, due to the large discontinuity of refractive indices and thus the transverse electric field component between the high and low index layers of a slot waveguide. Although the field profiles of the two types of waveguides seem different, here we show that direct integration of conventional and slot waveguides yields efficient coupling of light into and out of slot waveguides using the rigorous finite-difference time domain method. The proposed direct coupling method has comparable performance to recently proposed taper based coupling methods, while having advantages in easier integration with conventional waveguide optics and higher integration density. We also show that coupling efficiency is not sensitive to the symmetricity of the slot waveguide, resulting in good manufacturing tolerance. The proposed direct coupler may have a number of applications in lightwave interconnects, sensing and data storage.     

Tunable electro-optical 2 × 2 photonic crystal beam splitter

A tunable electro-optical 2?×?2 beam splitter based on two-dimensional rod-type photonic crystals is presented. The beam splitter consists of two orthogonally crossed linear waveguides and a single center rod in square lattice photonic crystals. In order to create a linear waveguide, the radius of a line of rods is reduced. A single center rod is positioned at the intersection of the linear waveguides to divide the input lightwave into output ports. The switching mechanism is a change in the conductance of the waveguide region and hence modulating the guided modes. The tunable beam splitter can be applied to photonic integrated circuits.     

Integrated liquid-crystal switch for both TE and TM modes: proposal and design

An integrated optical switch is proposed and designed based on a weak-anchoring liquid-crystal (LC) cell with a substrate integrating planar lightwave circuits. It consists of a polarization splitter, two switchable polarization converters and a polarization combiner. The polarization splitter/combiner is a directional coupler with an etched slot and filled-in LC covering layer. The switchable polarization converters are straight waveguides with a designed length and covering LC. The proposed configuration is superior to the existing integrated LC switches, as it works for both the TE and TM modes.     

Characteristic analysis of nanosilicon rectangular waveguides for planar light-wave circuits of high integration

When a full-vectorial finite-difference method is used, rectangular Si waveguides can be characterized for planar light-wave circuits of high integration. The single-mode condition for a rectangular Si waveguide is obtained first. The birefringence, which can be adjusted by modifying the thickness of the cladding layer, is also studied. For a nano-Si rectangular waveguide the pure bending loss is very small even for an ultrasmall bending radius (e.g., a few micrometers), and the transition loss becomes dominant. The width and height are optimized to minimize the bending radius for the requirement that the bending loss is smaller than 0.1 dB. Finally the coupling between two parallel straight waveguides is analyzed, and it is shown that there is an optimal width for the maximal coupling length.     

Plasmonic/metallic passive waveguides and waveguide components for photonic dense integrated circuits: a feasibility study based on microwave engineering

To investigate the potential for dense integration of photonic components, we analyse passive plasmonic/ metallic waveguides and waveguide components at optical frequencies by using mostly microwave engineering approaches. Four figures of performance are formulated that are utilised to compare the characteristics of four different slab waveguides with zero frequency cut-off modes. Three of these are metallic based whereas the fourth one, which also serves as a reference, is dielectric based with high index-contrast. It is found that all figures of performance cannot be optimised independently; in particular there is a trade-off between the waveguide Q-value and the transversal field confinement. Microwave methods are used to design several photonic transmission line components. The small Q-value of the metallic waveguides is the main disadvantage when using materials and telecom frequencies of today. Hence plasmonic waveguides do not offer full functionality for some important integrated components, being severe for frequency-selective applications. To achieve a dense integration, it is concluded that new materials are needed that offer Q-values several orders of magnitude higher than metals.     

Three-dimensional analysis of a hybrid photonic crystal-conventional waveguide 90 degree bend

We present a three-dimensional (3D) analysis of a hybrid photonic crystal-conventional waveguide 90 degree bend proposed previously [Opt. Express 10, 1334 (2002)] as an ultracompact component for large-scale planar lightwave circuit integration. Both rigorous 3D finite-difference time-domain modeling and a simple perfect mirror model analysis were carried out for different Si post heights in the photonic crystal region. Results show that the bend efficiency increases rapidly with Si post height. For a post height of 6.5 microm, this structure yields a bend efficiency of 97.3% at a wavelength of 1.55 microm for 90 degree bends in 2 microm x 2 microm square channel conventional waveguides with a refractive index contrast of 3.55%, which is very close to the bend efficiency of 98.2% for the corresponding two-dimensional problem. Our 3D analysis permits the examination of issues such as out-of-plane scattering loss and the effects of finite Si post height that are not considered in two dimensions.     

Modelling of anisotropic superconducting waveguide as equivalent transmission line

This article presents a method of linking full-wave field analysis to distributed-circuit analysis by modelling anisotropic superconducting waveguides, consisting of two or more parallel superconductors, as equivalent transmission lines. As an application of this method, a full-wave solution is first found for an anisotropic superconducting planar waveguide by solving Maxwell's equations and London's equations modified to account for the effect of both anisotropy and normal electron conduction in anisotropic superconductors. Based on the full-wave solution, a complete set of equivalent transmission line parameters is then derived analytically to model the anisotropic superconducting planar waveguide. Numerical results are given to show quantitatively how the effect of anisotropy influences the transmission properties of the anisotropic superconducting planar waveguide at high frequencies. The results are compared with those for an isotropic superconducting planar waveguide as well as with some well-known results from a quasi-static approximation. It is shown that the full set of equivalent transmission line parameters based on the full-wave analysis is desired to model properly some strongly anisotropic superconducting waveguides at millimeter-wave frequencies.     

Two-dimensional array self-assembled quantum dot sub-diffraction waveguides with low loss and low crosstalk

We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantum dot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductor nanoparticles and introducing a signal light, energy coupling of stimulated photons from the QDs enables light transmission along the waveguide. Monte Carlo simulation with randomized inter-dot separation reveals that the optical gain necessary for unity transfer is 3.1 × 10(7)?m(-1) for a 2D (2?μm length by 500?nm width) array compared to 11.6 × 10(7)?m(-1) for a 1D (2?μm length) given 8?nm diameter quantum dots. The theoretical results are borne out in experiments on 2D arrays by measurement of negligible crosstalk component with as little as 200?nm waveguide separation and is indicative of near-field optical coupling behavior. The transmission loss for 500?nm wide structures is determined to be close to 3?dB/4?μm, whereas that for 100?nm width is 3?dB/2.3?μm. Accordingly, higher pump power and gain would be necessary on the narrower device to create similar throughput. Considering existing nanoscale propagation methods, which commonly use negative dielectric materials, our waveguide shows an improved loss characteristic with comparable or smaller dimensions. Thus, the application of QDs to nanophotonic waveguiding represents a promising path towards ultra-high density photonic integrated circuits.     

Synthesis of symmetric and asymmetric planar optical waveguides

A novel and accurate refractive index profile synthesis method for planar optical waveguides is presented and demonstrated using the transmitted near-electric-field-data. This method is based on the inverse transmission-line (TL) technique. From Maxwell's equations, a TL equivalent circuit (electric T-circuit) for the refractive index profile of a planar optical waveguide is derived. The authors demonstrate how to use this model to carry out the inverse problem and synthesise the exact refractive index profile numerically from near-field-data. The TL method can reconstruct arbitrary refractive index profiles for planar optical waveguides that support singlemode or multi- modes. The cases of both symmetric and asymmetric arbitrary refractive index profile planar waveguides are discussed. The accuracy of the reconstructed waveguides is examined numerically.     

Ion-exchanged glass waveguides with low birefringence for a broad range of waveguide widths

Optical communications networks require integrated photonic components with negligible polarization dependence, which typically means that the waveguides must feature very low birefringence. Recent studies have shown that waveguides with low birefringence can be obtained, e.g., by use of silica-on-silicon waveguides or buried ion-exchanged glass waveguides. However, many integrated photonic circuits consist of waveguides with varying widths. Therefore low birefringence is consequently required for waveguides having different widths. This is a difficult task for most waveguide fabrication technologies. We present experimental results on waveguide birefringence for buried silver-sodium ion-exchanged glass waveguides. We show that the waveguide birefringence of the order of 10(-6) for waveguide mask opening widths ranging from 2 to 10 microm can be obtained by postprocessing the sample through annealing at an elevated temperature. The measured values are in agreement with the values calculated with our modeling software for ion-exchanged glass waveguides. This unique feature of ion-exchanged waveguides may be of significant importance in a wide variety of integrated photonic circuits requiring polarization-independent operation.     

AgI-coated silver-clad stainless steel hollow waveguides for infrared lightwave transmission and their applications

We fabricated silver iodide (AgI)-coated silver hollow waveguides to transmit a wide range of infrared (IR) light. Silver-clad stainless steel pipes were used as a supporting pipe. Since this type of metallic hollow waveguide has high mechanical strength and heat resistance, it is suitable as a rigid lightwave probe for various applications such as dental or medical laser treatment, IR spectroscopy, thermal radiometry, and laser processing. Considering these applications, we estimated the hollow waveguides with different thicknesses of the AgI layer. By optimizing the AgI layer thickness according to the wavelength of propagating light, we succeeded in efficiently transmitting Er-YAG and CO(2) laser light. We also studied the optical characteristics of a wide range of incoherent light for IR spectroscopy and radiometry applications using these metallic hollow waveguides as lightwave probes.     

Accurate method for determining the refractive-index profiles of planar waveguides in uniaxial media with the optical axis normal to the surface

A new method for characterization of uniaxial planar waveguides from their measured effective mode indices is presented. The theory is outlined and expressions for efficient computer analysis are given. Uniaxial waveguide samples have been made in c-cut LiNbO(3) by proton exchange with and without post annealing in order to test the method on both steplike and graded-index profiles. The resulting characterization of the samples is discussed in relation to the inverse WKB method. Finally, the importance of incorporating the effects of material birefringence in the characterization of these kinds of waveguides is investigated.     

Subdiffraction photon guidance by quantum-dot cascades

We report on a waveguide composed of a cascade of gain-enabled quantum dots with subwavelength dimensions. Fabrication is demonstrated through DNA-mediated self-assembly and a two-layer molecular self-assembly process that enables rapid prototyping. The device, which is identified with fluorescence microscopy and tested by optical near field detection, allows optical signal transfer at a well-defined wavelength in flexible routing geometry, such as straight paths and 90 degrees bends. The structure serves as a critical building block for nanophotonic systems with high integration density.     

Organo-lanthanide complexes as luminescent dopants in polymer waveguides fabricated by hot embossing

Lanthanide complexes, Eu(dbm)₃(Phen) and Er(dbm)₃(Phen), are employed as luminescent dopants within polymer channel waveguide devices fabricated by hot embossing. Spectroscopic properties of the complexes as dopants in the waveguide core polymer are investigated in detail. Judd–Ofelt parameters are calculated for the europium chelate and radiative properties are determined viz. potential for optical amplification. Channel waveguides fabricated by single level embossing are shown to be capable of guiding visible and infrared light emitted following optical excitation of the dopants. Multi-level polymer micro-optical benches incorporating doped channel waveguides and passive locational features for self-alignment and integration of optical fibres are fabricated in a multi-level single-step embossing process and are shown to successfully out-couple the waveguided dopant emission.     

Planar waveguide photonic crystals as an alternative for refractive index optical sensors design: theoretical evaluation

In this paper, the possibilities of designing refraction index optical sensors in planar waveguide photonic crystals are demonstrated for the first time. Photonic crystals obtained by connecting in cascade planar optical waveguides with high index contrast are analyzed. Photonic band gaps (PBGs) and photonic windows (PWs) were obtained. If a local defect is introduced in the PBG structure, the optical path length is modified and on states can be created in the gap. Besides, the on states wavelengths can be tuned if the optical path of the defect is modified: changing the physical length and/or the refraction index of the defect. In this way, planar waveguide photonic crystals could be used for sensing applications when a specimen modifies the refraction index lattice site. Sensing properties of planar waveguide photonic crystals, with one, two and three sensing channels, are demonstrated.     

Design of a high-efficiency grating coupler based on a silicon nitride overlay for silicon-on-insulator waveguides

A waveguide grating coupler based on a silicon nitride overlay at 1.55 μm for TE polarization is designed with no experimental demonstration. Its coupling efficiency for a fiber is 76%, the 1 dB bandwidth is 75 nm, and the coupling angle is 10°. The effects of different device parameters on the coupling performance for the grating coupler are discussed. The coupling efficiency of our grating coupler is almost equal, yet the 1 dB spectral bandwidth is around 25 nm broader, as compared with the grating coupler design based on a poly-silicon overlay. The coupling performance of our coupling device could still be further improved. The grating coupler presented in this paper is applicable to the optical coupling in nanophotonic integrated circuits.     

Optical masks prepared by using a liquid-crystal light valve for light-induced photorefractive waveguides

The optical mask prepared by using a LCLV (liquid-crystal light valve) are first proposed for light-induced photoreactive waveguides in photorefractive materials. Employing this technique, various waveguide structures can be fabricated, e.g., Y- or multiple-branches waveguides, fiber-like waveguides, and Mach-Zehnder-like switches, and even whole optical circuits may be formed. A Y-branches waveguide and a fiber-like waveguide were demonstrated in a LiNbO3:Fe crystal. Several technical problems, such as intensities, resolutions, writing speed, and so on, were also discussed in detail. Using a LCLV with a fast response and a writing beam with a high intensity, the waveguide structures may be changed in real time.     

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.     

Design of pitch conversion component for formation of multibeam optical recording head

We describe a design of a planar lightwave circuit for parallel information processing using visible light. The circuit serves as a pitch conversion component (PCC) that can align multiple beams close together and easily composes a compact optical system that can project optical spots at a narrow pitch on a certain small plane. From the viewpoint of its application to optical recording, a PCC is designed to have over 50 waveguides according to the fabrication of waveguides for a blue-violet beam. It is analytically confirmed that a PCC contributes to the formation of a multibeam optical recording head with numerous beams.