Low-loss chalcogenide waveguides on lithium niobate for the mid-infrared

We demonstrate low-loss chalcogenide (As(2)S(3)) waveguides on a LiNbO(3) substrate for the mid-IR wavelength (4.8 μm). Designed for single-mode propagation, they are fabricated through photolithography and dry-etching technology and characterized on a mid-IR measurement setup with a quantum cascade laser. For straight waveguides, propagation loss as low as 0.33 dB/cm is measured and low-loss bends on the order of 100 μm are simulated, with measurement results showing <3 dB for a 250 μm bend radius. The coupling efficiency is estimated to be 81%. In addition, the influences of variations in width and bend radius are also investigated.     

Silver halide single-mode strip waveguides for the mid-infrared

We report the demonstration of single-mode AgCl(x)Br(1-x) strip waveguides for the mid-infrared (MIR). The waveguides were made by the deposition of AgCl(x)Br(1-x) layers on top of a Si-SiO(2) substrate, followed by photolithographic and lift-off processing. The propagation losses of 20 dB/cm were measured at λ=10.6 μm using the cut-back method. The development of these waveguides is a crucial step toward realizing AgCl(x)Br(1-x) MIR integrated optical circuits, which would be used for applications such as chemical sensing and stellar interferometry.     

Trapped modes in finite quantum waveguides

The eigenstates of an electron in an infinite quantum waveguide (e.g., a bent strip or a twisted tube) are often trapped or localized in a bounded region that prohibits the electron transmission through the waveguide at the corresponding energies. We revisit this statement for resonators with long but finite branches that we call ??finite waveguides??. Although the Laplace operator in bounded domains has no continuous spectrum and all eigenfunctions have finite L ₂ norm, the trapping of an eigenfunction can be understood as its exponential decay inside the branches. We describe a general variational formalism for detecting trapped modes in such resonators. For finite waveguides with general cylindrical branches, we obtain a sufficient condition which determines the minimal length of branches for getting a trapped eigenmode. Varying the branch lengths may switch certain eigenmodes from non-trapped to trapped or, equivalently, the waveguide state from conducting to insulating. These concepts are illustrated for several typical waveguides (L-shape, bent strip, crossing of two strips, etc.). We conclude that the well-established theory of trapping in infinite waveguides may be incomplete and require further development for applications to finite-size microscopic quantum devices.     

Large-scale photonic crystals with inserted defects and their optical properties

Deliberately introducing defects into photonic crystals is an important way to functionalize the photonic crystals. We prepare a special large-scale three-dimensional(3D) photonic crystal(PC) with designed defects by an easy and low-cost method. The defect layer consists of photoresist strips or air-core strips. Field emission scanning electron microscopy(FESEM) shows that the 3D PC is of good quality and the defect layer is uniform. Different defect states shown in the ultraviolet-visible spectra are induced by the photoresist strip layer and air-core strip layer. The special large-scale 3D PC can be tested for integrated optical circuits, and the defects can act as optical waveguides.     

Radiatively coupled waveguide polarization splitter simulated by wave-matching-based coupled mode theory

Coupled mode theory is applied to an arrangement of three raised strip waveguides with a multimode central strip. We use semivectorial numerically computed modes of the three single isolated waveguides as a basis for propagating supermode analysis of the entire structure. The pronounced polarization dependence of the raised strip guides allows for the design of a conveniently short polarization splitter. We discuss design guidelines and estimate the fabrication tolerances. The accuracy of the coupled mode approach is assessed by comparison with rigorously computed supermodes for comparable two waveguide couplers. Both types of structures indicate the limits in the applicability of the coupled mode model.     

Subwavelength grating Fourier‐transform interferometer array in silicon‐on‐insulator

A planar waveguide Fourier‐transform spectrometer with densely arrayed Mach‐Zehnder interferometers is demonstrated. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach‐Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free‐spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides, loss imbalance is minimized and high interferometic extinction ratio of −25 to −30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements.     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.     

Fabrication of ultralow-loss Si/SiO(2) waveguides by roughness reduction

We demonstrate 0.8-dB/cm transmission loss for a single-mode, strip Si/SiO(2) waveguide with submicrometer cross-sectional dimensions. We compare the conventional waveguide-fabrication method with two smoothing technologies that we have developed, oxidation smoothing and anisotropic etching. We observe significant reduction of sidewall roughness with our smoothing technologies, which directly results in reduced scattering losses. The rapid increase in the scattering losses as the waveguide dimension is miniaturized, as seen in conventionally fabricated waveguides, is effectively suppressed in the waveguides made with our smoothing technologies. In the oxidation smoothing case, the loss is reduced from 32 dB/cm for the conventional fabrication method to 0.8 dB/cm for the single-mode waveguide width of 0.5 microm . This is to our knowledge the smallest reported loss for a high-index-difference system such as a Si/SiO(2) strip waveguide.     

Planar waveguide-coupled, high-index-contrast, high-Q resonators in chalcogenide glass for sensing

High-index-contrast compact microdisk resonators in thermally evaporated As2S3 and Ge17Sb12S71 chalcogenide glass films are designed and fabricated using standard UV lithography and characterized. Our pulley coupler configuration demonstrates coupling of the resonators to monolithically integrated photonic wire waveguides without resorting to demanding fine-line lithography. Microdisk resonators in As2S3 support whispering-gallery-mode with cavity quality factors (Q) exceeding 2 x 10(5), the highest Q value reported in resonator structures in chalcogenide glasses to the best of our knowledge. We have successfully demonstrated a lab-on-a-chip prototype sensor device with the integration of our resonator with planar microfluidic systems. The sensor shows a refractive index sensitivity of 182 nm/RIU (refractive index unit) and a wavelength resolution of 0.1 pm through a resonant peak fit. This corresponds to a refractive index detection limit of 8 x 10(-7) RIU at 1550 nm in wavelength, which could be further improved by shifting the operating wavelength to a region where water absorption is reduced.     

Photosensitivity of As2S3 chalcogenide thin films at 1.5 microm

Chalcogenide glasses are promising candidates for all-optical switching and various nonlinear applications. However, we show that As2S3 thin films are photosensitive at wavelengths in the 1.5-microm telecommunication window. This sensitivity is evidenced by the formation of self-written waveguides in slabs, where channels as narrow as 1 microm are created. We also show the detrimental effects of such photosensitivity in ridge waveguides. This photosensitivity seems to occur only in thin-film form and not in bulk samples or fibers.     

可调谐集成光学Ti:LiNbO3光波导定向耦合滤波器

波分复用技术是当今光纤通信系统中的关键技术之一。具有增加光纤通信系统容量的巨大潜力,集成光学复用技术对于单模光纤通信系统特别重要本文给出了一种集成光学复用器件的设计和制作,这种集成光学滤波器是在之切LiNbO₃衬底上用非对称钛扩散条波导定向耦合器构成,它适于1.32μm和1.55μm波长耦合滤波。为了使单模光纤和条波导有效耦合,我们研制出了硅V型槽,并给出了一种光纤与波导的连接结构。提供了一种切实可行的途径。     

Fabrication and characterization of on-chip optical nonlinear chalcogenide nanofiber devices

Chalcogenide (As(2)S(3)) nanofibers as narrow as 200 nm in diameter are drawn by the fiber pulling method, are successfully embedded in SU8 polymer, and form on-chip waveguides and high-Q microknot resonators (Q = 3.9 × 10(4)) with smooth cleaved end faces. Resonance tuning of resonators is realized by localized laser irradiation. Strong supercontinuum generation with a bandwidth of 500 nm is achieved in a 7-cm-long on-chip chalcogenide waveguide. Our result provides a method for the development of compact, high-optical-quality, and robust photonic devices.     

Spontaneous Raman scattering in ultrasmall silicon waveguides

We report spontaneous Raman scattering at 1550 nm in ultrasmall silicon-on-insulator (SOI) strip waveguides of 0.098-microm2 cross-sectional area. The submicrometer-scale dimensions provide tight optical confinement and, hence, highly efficient Raman scattering with milliwatt-level cw pump powers. The prospect of Raman amplification in such a deeply scaled-down waveguide device in the presence of various loss mechanisms, particularly free-carrier loss that arises from two-photon absorption, is discussed, and the feasibility of high-gain SOI-based fully integrated optical amplifiers is shown.     

A simple variational analysis of diffused channel waveguides

The variational analysis of Ti:LiNbO₃ strip waveguides is resolved into two related one-dimension problems, which provides a new iterative approach for the calculation of the mode parameters and the propagation constant. The analytical expressions of the local effective index profiles can be obtained from the variational analysis. The parameters of four trial solutions for the fundamental mode in a diffused-channel waveguide have been determined by using the present method. Comparison with results using the effective method shows that these four approximate expressions are accurate enough. It is also shown that this method converges very quickly and is suitable for both diffused channel waveguides and optical waveguides with rectangular cross-section.     

Characterization and application of a channel- planar composite waveguide

A new structural waveguide, which is referred to as a channel-planar composite optical waveguide (COWG), has been fabricated by sputtering of a titanium dioxide (TiO>(2)) film onto a glass substrate with potassium ion-exchanged channel waveguides. By use of a mask during deposition, the TiO(2) film was formed into a 27-nm-thick, 5-mm-wide strip with two 1-mm-long tapered ends perpendicular to the channel waveguides. Adiabatic transition of the TE(00) mode and the TE(00)- TM(00) mode separation inside such a channel-planar COWG were demonstrated by combination of theoretical analysis and measurement of the experimental attenuation that arises from scattering loss and evanescent-field dye absorption. Changing the superstrate index in the region of the TiO(2) film in the channel-planar COWG yielded polarimetric interference patterns. This new technique can be applied to integrated optical chemical and biological sensors to produce enhanced sensitivity.     

Analysis of rectangular-core waveguide structures and directional couplers by an iterated moment method

Using an iterated moment method (weighted index method), formulae are derived for the accurate calculation of the dispersion characteristics and field distributions of guided modes in strip dielectric waveguides and strip-loaded waveguides, and for the calculation of the coupling length of rectangular-core dual-channel directional couplers. The formulae are simple and the results obtained by the present analysis agree well with computer-aided exact numerical analysis. This method can be readily extended to the analysis of other dielectric waveguide structures.     

Ion implantation into fused quartz for integrated optical circuits

Results obtained in the fabrication of slab and strip waveguides by ion implantation into fused quartz are discussed. Using a step-index waveguide model the increase in refractive index is calculated. The optical loss is smaller than 1 dB/cm at λ = 568 nm without annealing. The properties of strip waveguides fabricated by ion implantation through photoresist masks of thicknesses from 0.4 μm to 0.8 μm are described. A bright fluorescence is observed with emission at 530 nm and 640 nm and its dependence on ion fluence and ion energy is measured.     

Proton-exchanged optical waveguides fabricated by glutaric acid

In this paper, we first report that a new proton source, glutaric acid, has been used to fabricate optical waveguides in Z-cut lithium niobate crystals. The relationship was experimentally established between proton-exchanged (PE) waveguide parameters and fabrication conditions. It is shown that this new organic acid can be used to obtain deep PE waveguides in fast diffusion speed (0.275 μm²/h at 221°C) and with low loss (0.2 dB/cm). It provides an alternative approach for fabricating PE waveguides in lithium niobate substrate.     

Efficient laser emissions at 1.06 μm of swift heavy ion irradiated Nd:YCOB waveguides

We report on the fabrication of Nd:YCOB (Nd:YCa(4)O(BO(3))(3)) optical waveguides by using 170 MeVAr(8+) ion irradiation at an ultralow fluence of 2×10(12) ions/cm(2). The confocal microphotoluminescence investigation on the produced waveguides has shown the well-preserved fluorescence features within the guiding layer with respect to the bulks. Under the optical pump at wavelength of 810 nm, continuous wave waveguide lasers at 1061.2 nm have been generated at room temperature with a high slope efficiency of ~67.9%.     

矩形芯介质波导色散关系及定向耦合器耦合长度的加权余量法计算

本文用加权余量法导出了用以计算各种条形介质波导色散关系,矩形芯双沟道耦合器耦合长度以及条载型波导色散关系的数学表达式,其精确度较其它近似方法高,且简便易行。