Blazed grating couplers on LiNbO3optical channel waveguides and their applications to integrated optical circuits

Blazed grating couplers on LiNbO3optical channel waveguides have been successfully designed, fabricated, and evaluated. A reactive ion beam etching technique was used to shape the groove profile of grating couplers. A large decay factor of 600 dB/cm along optical channel waveguide was obtained. The air coupling intensity ratio of 8:1 between positive and negative light propagation directions for ion beam angle of 55° indicated a strong blaze effect. Highly efficient optical coupling from channel waveguide to Si phototransistor was also demonstrated.     

Wavelength-selective coupling between vertically integratedthin-film waveguides via supermode by a pair of grating couplers

Grating coupling between vertically integrated waveguides via supermode was discussed and demonstrated. Two thin-film waveguides with grating couplers were stacked on a substrate. A guided wave in one waveguide is converted by a grating coupler to a supermode propagating contradirectionally, and then converted by another grating coupler to a guided wave in the other waveguide. The coupling shows high wavelength selectivity, and the wavelength-division add/drop multiplexing function can be combined. A demonstrator was designed and fabricated. Theoretically predicted coupling efficiency was almost 100% with wavelength selectivity of 2 nm, while the experimentally obtained value was 40% in the efficiency and 1 nm in the selectivity     

Flexible optical waveguide film fabrications and optoelectronic devices integration for fully embedded board-level optical interconnects

This paper demonstrates a flexible optical waveguide film with integrated optoelectronic devices (vertical-cavity surface-emitting laser (VCSEL) and p-i-n photodiode arrays) for fully embedded board-level optical interconnects. The optical waveguide circuit with 45/spl deg/ micromirror couplers was fabricated on a thin flexible polymeric substrate by soft molding. The 45/spl deg/ couplers were fabricated by cutting the waveguide with a microtome blade. The waveguide core material was SU-8 photoresist, and the cladding was cycloolefin copolymer. A thin VCSEL and p-i-n photodiode array were directly integrated on the waveguide film. Measured propagation loss of a waveguide was 0.6 dB/cm at 850 nm.     

Optimal design of grating-assisted directional couplers

In this paper, a rigorous leaky mode propagation method has been used to investigate the influence of the grating period and grating index profile on the design of grating-assisted directional couplers (GADC's). A detailed explanation of resonance condition and radiation loss in terms of electromagnetic field contribution in the grating region as a function of the grating period and profile is given. Optimal design parameters hare been found for well-defined structures in order to achieve either minimum coupling length or maximum coupling efficiency. A very fast method to extract the resonance condition in any grating-assisted structure by using a sinusoidal profile is proposed. Numerical results are presented for both moderately and strongly asymmetric structures in terms of normalized propagation constant, mode radiation loss, coupling length and coupling efficiency. Comparisons with grating period and coupling length predictions obtained by other methods are also shown. The rectangular profile with optimized duty cycle has been demonstrated to be the best choice in order to minimize the GADC coupling length     

Grating-assisted coupling of light between semiconductor and glasswaveguides

Floquet-Bloch theory is used to calculate the electromagnetic fields in a leaky-mode grating-assisted directional coupler (LM-GADC) fabricated with semiconductor and glass materials. One waveguide is made from semiconductor materials (refractive index ≈3.2) while the second is made from glass (refractive index ≈1.45). The coupling of light between the two waveguides is assisted by a grating fabricated at the interface of the glass and semiconductor materials. Unlike typical GADC structures where power is exchanged between two waveguides using bound modes, this semiconductor/glass combination couples power between two waveguides using a bound mode (confined to the semiconductor) and a leaky mode (associated with the glass). The characteristics of the LM-GADC are discussed. Such LM-GADC couplers are expected to have numerous applications in areas such as laser-fiber coupling, photonic integrated circuits, and on-chip optical clock distribution. Analyses indicate that simple LM-GADC's can couple over 40% of the optical power from one waveguide to another in distances less than 1.25 mm     

Coupled-mode analysis of highly asymmetric directional couplerswith periodic perturbation

This paper presents an in-depth analysis of highly asymmetric grating assisted directional couplers. The directional coupler consists of a polymer waveguide with dimensions and refractive indices closely matching a single-mode fiber fabricated atop a Ga_0.6Al_0.4As/GaAs/Ga_0.4Al_0.6 As waveguide. The structure is investigated analytically by means of a new orthogonal coupled-mode theory formulated in terms of the Lorentz reciprocity theorem. For the first time, the analysis includes three distinct loss mechanisms, namely, the leakage of power toward the semiconductor substrate, the power lost to radiation modes (mode mismatching), and the grating radiation loss     

New Leaky Surface Waves in Anisotropic Metal-Diffused Optical Waveguides

Propagation of guided waves in anisotropic metal-diffused optical waveguide is investigated. Two-dimensional guide-mode dispersion curves are computed and classified for a metal-diffused waveguide with arbitrary optic-axis orientation in various diffusion depth. It is found that a new leaky surface wave exists in the region where the refractive index is above the cutoff value, not below it. Typical values of decay constant are about 5 dB/cm for the wave propagating along X axis on 128/spl deg/ rotated Y-cut LiNbO/sub 3/, and 35 dB/cm for the wave propagating along the direction making an angle of 70/spl deg/ to X axis on Y-cut plane LiNbO/sub 3/. We were able to observe the leaky surface waves experimentally.     

Analysis of radiation loss in grating-assisted codirectionalcouplers

In this paper, we present a new accurate numerical method, named leaky mode propagation method, to analyze the radiation loss in grating-assisted codirectional couplers. The complex propagation constant and electromagnetic field distribution of the normal modes are evaluated by using a space harmonic field expansion derived from the Floquet theorem. In order to prove the accuracy of our method, a comparison between our results and those calculated by the most used method, i.e., the transfer matrix method, is carried out in terms of power attenuation coefficient and total radiation loss. Quantitative and qualitative discrepancies between the two methods are widely discussed. In particular, an oscillatory variation of radiation loss with grating depth and gap thickness in contrast to the monotonic behavior predicted by the transfer matrix method has been found     

0.15 dB/cm loss in Unibond SOI waveguides

The optical loss of Unibond waveguides is measured and reported for the first time, using grating couplers. At a wavelength of 1.3 μm, a loss of 0.15±0.05 dB/cm is obtained for TE polarisation. This allows good quality low loss integrated optical circuits to be fabricated at low cost     

Helix Leaky Waveguide

The propagation characteristics of leaky waves in a helix waveguide covered with a slitted cylinder are presented by a method of transverse network representation. The main interest is in helix waveguides with small pitch angles, characterized by a hybrid mode consisting of TE/sub 01/, and a small amount of TM/sub 01/ modes. The leaky wave discussed in this paper may then be regarded as a perturbation of the TM/sub 01/ wave by the slitted cylinder outside the helix. The radiation, metal, and dielectric losses are calculated numerically at a frequency of 50 GHz. The relation between the radiation loss and aperture angle of slit is very different from that of an ordinary leaky waveguide composed of a slitted cylinder without helix, especially when the distance between the helix and shield cylinder is about a quarter of the radial wavelength. The metal and dielectric losses are the same order as radiation loss, however the dielectric loss decreases as the power factor /spl epsiv/"/ /spl epsiv/' increases. The measured total attenuation constant averages about 5 dB/km, almost twice the theoretical value.     

ZnS／ZnSe迭层光栅滤波器的光调制特性

本文报导利用真空镀膜技术在ＢＫ７玻璃波导上,制备成ＺｎＳ／ＺｎＳｅ迭层光栅滤波器,在室温下,用Ａｒ＾＝脉冲激光,通过棱镜与波导耦合,实现了光栅滤波器的光控开关调制。     

Characteristics of a hollow-core distributed feedback CO2laser

Using perturbation theory and a plane wave analysis, the scattering amplitudes and coupling coefficients for a planar, hollow-core, double grating distributed feedback (DFB) waveguide laser are derived. Waveguide reflectivity has been compared for a single and double grating waveguide configuration. For a waveguide dimension of 100 μm over a 10 cm length, reflectivity is enhanced from 75 percent for a single grating to 98 percent using a double grating configuration. Included in this analysis is the effect of phase relationship between the two gratings. Using this analysis a means has been devised whereby reflectivity of the DFB waveguide may be optimized during operation of the laser. With the double grating configuration, coupling is enhanced such that a greater plate separation is possible in nearly all cases. Use of a double grating allows the plate separation of the waveguide to be increased from 80 to 100 μm, reducing total waveguide loss from 15.9 to 8.1 dB/m. These results have led to a method of solving heretofore prohibitive characteristics of single grating hollow-core DFB waveguides. Formal relationships for the waveguide parameters where a double grating configuration is used have been presented for designing hollow-core DFB waveguide lasers operating at 10.6 μm.     

Waveguide input grating coupler for wavelength-divisionmultiplexing and wavelength encoding

We demonstrate the wavelength-division multiplexing (WDM) and wavelength-encoding capability of input waveguide grating couplers. The couplers are designed to have a predetermined wavelength response in addition to their conventional function of coupling an incident beam from, e.g., an optical fiber into a planar waveguide. The first example shows the WDM function: separating each of four input wavelengths into a different focus position in the waveguide. The second example shows wavelength encoding: translating a certain wavelength into a desired configuration of focus positions that is different for four different input wavelengths. The couplers were fabricated in an InP waveguide for ~1550-nm wavelength and the separation between the wavelengths was 10 nm. A WDM coupler with a narrower channel separation of 2 nm was also fabricated and successfully demonstrated     

Lithium–Niobate Channel Waveguide for the Realization of Long-Period Gratings

We propose a special lithium-niobate (LiNbO₃) single-mode waveguide for the realization of long-period gratings, which consists of a channel core embedded in a thin slab cladding. We fabricated the waveguide on a z-cut LiNbO₃ substrate with a two-step proton-exchange process and demonstrated its suitability for grating application with a number of removable photoresist long-period gratings deposited on the waveguide surface. The waveguide fabrication process and the LiNbO₃ waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.     

Improved coupled mode analysis of corrugated waveguides and lasers

Corrugated waveguides and lasers in resonant and non-resonant situations are analyzed by an improved coupled mode theory based on a set of the coupled mode equations for guided modes and radiation continuum. The distributed feedback (DFB) coefficient and the radiation loss coefficient are given in closed forms. The formulation can be applicable to arbitrarily shaped gratings and multilayer waveguide structures. The accuracy of the theory is examined by comparing it with Tamir's exact calculation for a nonresonant situation and also with Streifer's one for a DFB structure. Reasonable accuracy is obtained by the proper choice of the unperturbed waveguide parameter. The dependence of the two coefficients on the grating depth, the grating period, the guide layer thickness, and the refractive index difference between core and cladding layers is obtained for all Bragg orders up to the fourth, and for four typical grating shapes, namely, for rectangular, sinusoidal, symmetric triangular, and sawtooth gratings. Both the threshold gain of DFB lasers utilizing higher order Bragg reflection and the output coupling efficiency of grating beam couplers are also calculated for these parameters. A new multilayer structure for controlling the radiation loss is proposed and analyzed. This structure is suitable for the suppression of the radiation loss in DBR reflectors as well as for the improvement of the output coupling efficiency in grating beam couplers.     

Direct writing of planar waveguide power splitters and directionalcouplers using a focused ultraviolet laser beam

Planar waveguide power splitters and directional couplers have been fabricated in silica using a continuous wave UV laser beam. The performance in terms of size, excess loss and spectral response is similar to that obtained with other techniques, while the fabrication time per device is a few minutes     

Quasi-phase-matched second-harmonic generation indiazo-dye-substituted polymer channel waveguides

First-order quasi-phase-matched (QPM) second-harmonic generation (SHG) is demonstrated in a poled diazo-dye-substituted polymer channel waveguide. The channel waveguide with a nonlinear grating was fabricated by the serial grafting technique using conventional photolithography and reactive ion etching. The dependence of the conversion efficiency on both the waveguide parameter and the grating structure was derived theoretically. A normalized internal conversion efficiency of 1.1% W^-1 cm^-2 at 1.586 μm was obtained in the fabricated waveguide with a phase-matched interaction length of 3.4 mm. The experimentally obtained conversion efficiency is compared with the theoretical value, taking into account the effect of mode-mismatching and propagation loss     

Photonic integrated circuits by DUV-induced modification of polymers

The results achieved with polymer Y-splitters, codirectional couplers, and multimode interference couplers, realized by deep ultraviolet lithography are presented. The devices are designed and fabricated for the 1.55-/spl mu/m wavelength region and have a waveguide loss of 1 dB/cm. The waveguide width is 7.5 /spl mu/m. The fiber-chip coupling loss is 0.5 dB per facet. The polarization-dependent loss is <0.15 dB.     

Wafer-fused optoelectronics for switching

Wafer fusion technique for realization of compact waveguide switches and three-dimensional (3-D) photonic integrated circuits is investigated theoretically and experimentally. Calculations based on beam propagation method show that very short vertical directional couplers with coupling lengths from 40 to 220 μm and high extinction ratios from 20 to 32 dB can be realized. These extinction ratios can be further improved using a slight asymmetry in waveguide structure. The optical loss at the fused interface is investigated. Comparison of the transmission loss in InGaAsP-based ridge-loaded waveguide structures with and without a fused layer near the core region, reveals an excess loss of 1.1 dB/cm at 1.55 μm wavelength. Fused straight vertical directional couplers have been fabricated and characterized. Waveguides separated by 0.6 μm gap layer exhibit a coupling length of 62 μm and a switching voltage of about 2.2 V. Implications for GaAs-based fused couplers for 850 nm applications will also be discussed     

Test structures for characterization of electrooptic waveguidemodulators in lithium niobate

A method of determining the critical parameters of waveguide modulators, using a set of test devices fabricated on a single chip, is presented. The five parameters are the depth and lateral Ti diffusion lengths, the peak index change in the waveguides, the electrooptic coefficient, and the buffer layer dielectric constant. The finite element method is used for calculation of optical modes in waveguides with graded refractive index profiles. The integral equation method is used for calculation of the static electric field due to electrodes in a three-layer structure of air, buffer layer, and LiNbO₃. The test set includes a planar waveguide, Mach-Zehnder modulators, symmetrically perturbed directional couplers, and widened X modulators. Several test chips have been fabricated using different fabrication conditions. The parameter values determined using this method are compared with those reported by other authors