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     

Optical mode conversion using chirped gratings

A design is presented for a 3 dB TE0-TE1 optical waveguide mode convertor using a chirped grating. Devices were fabricated in Ti in-diffused lithium niobate by ion beam etching the grating pattern into the waveguide surface. TE0-TE1 mode coupling was observed experimentally with an insertion loss of 1.5 dB.     

Thermooptic interferometric switches fabricated by electron beamirradiation of silica-on-silicon

The thermal stability of channel optical waveguide devices fabricated by electron beam irradiation of plasma-enhanced chemical vapor deposition (PECVD) silica-on-silicon is investigated. The degree of stability is dependent on the starting material and on the use of thermal annealing prior to irradiation. High-temperature postprocessing is shown to reduce modal confinement, increasing losses in waveguide bends and the coupling coefficient in directional couplers. A low-temperature cladding process based on a thick MgF₂ layer is described, and low-loss thermooptic Mach-Zehnder interferometric switches are demonstrated     

Low-loss high-silica single-mode channel waveguides

Low-loss high-silica single-mode channel optical waveguides of 0.1 dB/cm have been fabricated on crystal silicon substrates with fibre coupling losses of less than 0.05 dB by flame hydrolysis deposition and reactive ion etching. Directional couplers have also been fabricated with excess loss of 0.5 dB by this waveguide fabrication technique.     

Optical waveguide directional coupler measurements using amicrocomputer-assisted TV camera system

Characterization of optical waveguide directional couplers using a microcomputer-assisted TV camera system is reported. A fluorescent technique is employed to visualize wave propagation in the optical waveguides, which exhibit small scattering. The method is simple and reproducible, and can be used to estimate waveguide losses and evaluate various optical devices such as waveguide lenses and waveguide interferometers on substrates. As an example of its application, parallel and nonparallel dual-chamber directional couplers and parallel triple-channel directional couplers are measured. Power transfer between the coupled channel waveguides is observed clearly, and coupling coefficients of the couplers are obtained nondestructively. The measured performance of the couplers is in good agreement with that predicted by coupled-mode analysis     

3-Dimensionally Integrated Planar Optics for 100 Gb/s Optical Packet Address Detection

We propose a novel planar optical interconnection scheme for 100 Gb/s optical packet address detection, which consists of waveguide grating couplers and a diffractive microlens integrated on a glass substrate 3-dimensionally. Length and duty cycle of the grating couplers have been determined on the bases of the ray-optic propagation-mode analysis in a slab waveguide and of the rigorous coupled-wave diffraction analysis for out-coupled radiation-modes. The 3-dimensionally integrated planar optics makes it possible to connect each address bit-signals of TE₀-waveguide mode to the detector with a power uniformity of 6.4 % and a total coupling efficiency of 72.3 %.     

n-p-n silicon lateral phototransistors for hybrid integrated optical circuits

An etched mesa silicon lateral phototransistor (EMS-LPT) suitable for detecting the light signal from optical channel waveguides has been designed and fabricated. In this paper both n⁺-p-n⁺uniform base and n⁺-p-p^--n⁺double-diffused EMS-LPT's are reported. The photoactive region of the EMS-LPT is highly localized and can be easily coupled either via an evanescent field or to a grating coupler on a channel waveguide. Light coupling, gain, speed, and signal-to-noise ratio of the device are thereby greatly improved. The fabrication techniques of the EMS-LPT's are compatible with those of MOSFET's, permitting integration of multiple EMS-LPT's and MOSFET load transistors to form optically addressed inverters on the same silicon chip. By flip-chip bonding LiNbO3and silicon substrates and coupling LiNbO3channel waveguides to EMS-LPT's via grating couplers, we produce electrooptic switches with optical input and output.     

聚合物波导光栅耦合器的衍射场仿真

为了实现横截面尺寸为50 m50 m的聚硅氧烷聚合物光波导的耦合转向问题,设计了一种表面覆盖高折射率包层的多层蚀刻光栅耦合器。首先,分析了影响聚合物波导光栅耦合器耦合效率的结构因素;然后,采用在光栅表面蚀刻高折射率层的方法,提高了聚合物波导光栅耦合器的耦合效率;接着,对不同的周期（范围:100~4 000 nm）和不同的蚀刻深度（范围:0~50 000 nm）进行排列组合,形成不同的光栅结构,基于时域有限差分法编写程序,遍历所有情况,得到不同光栅结构下的光场情况以及其耦合效率,找到使耦合效率最大的周期以及蚀刻深度。最后,设计了多层蚀刻的光栅耦合器,进一步提高耦合效率。当蚀刻深度为5 000 nm,光栅周期为2 600 nm时,带高折射率层的聚硅氧烷聚合物光波导均匀光栅耦合器的耦合效率达到最大,为17.2%。采用多层蚀刻的方式,对结构进行优化,其耦合效率能达到37.4%。为聚硅氧烷聚合物光波导在光互连中的实际应用提供了理论依据。     

Polylithic Integration of Electrical and Optical Interconnect Technologies for Gigascale Fiber-to-the-Chip Communication

Polylithic integration of electrical and optical interconnect technologies is presented as a solution for merging silicon CMOS and compound semiconductor optoelectronics. In contrast to monolithic and hybrid integration technologies, polylithic integration allows for the elimination of optoelectronic and integrated optic device-related processing from silicon CMOS manufacturing. Printed wiring board-level and compound semiconductor chip-level waveguides terminated with volume grating couplers facilitate bidirectional optical communication, where fiber-to-board and board-to-chip optical coupling occurs through a two-grating (or grating-to-grating) coupling path. A 27% increase in the electrical signal I/O projected by and 33% increase in the number of substrate-level electrical signal interconnect layers implied by the International Technology Roadmap for Semiconductors (ITRS) projections for the 32-nm technology generation are required to facilitate 10 Tb/s aggregate bidirectional fiber-to-the-chip communication. Buried air-gap channels provide for the routing of chip or board-level encapsulated air-clad waveguides for minimum crosstalk and maximum interconnect density. Optical signals routed on-board communicate with on-chip volume grating couplers embedded as part of a wafer-level batch package technology exhibiting compatible electrical and optical input/output interconnects. Measurements of grating-to-grating coupling reveal 31% coupling efficiency between two slab, nonoptimized, nonfocusing volume grating couplers.     

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     

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.     

Design of grating-assisted waveguide couplers with weightedcoupling

The design of grating-assisted, channel waveguide codirectional couplers is demonstrated using the Gel'fand-Levitan-Marchenko inverse scattering method. Weighted coupling coefficients are computed from rational expression for the desired wavelength response. Approximate formulas that relate the waveguide and grating geometries to the computed coupling coefficients are derived. The technique is illustrated by designing a directional coupler with a third-order Butterworth filter characteristic and a 130 Å full-width-half-maximum bandwidth     

A broad-band waveguide grating coupler with a subwavelength grating mirror

A new diffractive device for light coupling between a planar optical waveguide and free space is proposed. The device utilizes a second-order waveguide grating to diffract the fundamental waveguide mode into two free propagating beams and a subwavelength grating (SWG) mirror to combine the two free propagating beams into a single beam. The finite-difference time-domain (FDTD) simulations show that the SWG mirror improves the coupling efficiency of the waveguide fundamental mode into the single out-coupled beam from about 30% to 92%. A high efficiency (>90%) is predicted for a broad wavelength range of 1520-1580nm. The proposed device is compact (/spl sim/80 /spl mu/m in length) and it eliminates the need for blazing the waveguide grating.     

硅基波导光栅耦合器与波导传输损耗的研究

对硅(Si)基波导光栅耦合器的设计与耦合性能进 行了研究。采用本征模展开法对光栅耦合器进行设计与 优化,通过实验测量了光栅的耦合性能,并对均匀光栅、自聚焦光栅和反射光栅等3种光栅 耦合器的耦合 性能进行了比较,耦合效率分别达到了达到47.86、56. 36和48.98%,自聚焦光栅可以有效改善光纤到 光纤的传输效果,耦合效率提高了8.5%。通过实验测量了基于耦合光 栅技术的Si基条形波导和槽型波导的 传输损耗,结果显示,条形波导和槽型波导的传输损耗分别为2.34d B/cm和6.31dB/mm。     

Polarization-independent all-optical switching in a nonlinearGaInAsP-InP highmesa waveguide with a vertically etched Bragg reflector

We have theoretically designed and experimentally demonstrated polarization-independent all-optical switching in a nonlinear GaInAsP-InP highmesa distributed feedback (DFB) waveguide. The device, which is composed of a highmesa waveguide stripe and a vertically etched Bragg reflector, can be simply fabricated using one-step electron beam lithography and a reactive ion etching process. The device is suitable for integration with other photonic devices such as semiconductor optical amplifiers and wavelength converters. The structural birefringence of the device has a dependence on the waveguide parameters such as the refractive index and thickness of core and cladding. The structural birefringence was successfully eliminated by adjusting the width of the highmesa waveguide. The nonlinear vertical-groove DFB highmesa waveguide is attractive for a polarization-independent all-optical switch from the viewpoint of a large grating coupling coefficient, as compared with a grating-loaded DFB highmesa waveguide. The polarization dependence of the grating coupling coefficient has also been investigated experimentally. It is possible to obtain the polarization-independent grating coupling coefficient by adjusting the grating depth in the vertical-groove DFB highmesa waveguide, together with structural zero-birefringence of the device. Polarization-independent all-optical thresholding and bistable switching operations have been successfully demonstrated in the nonlinear vertical-groove DFB highmesa waveguide     

High-Speed Optical Interconnect Coupler Based on Soft Lithography Ribbons

In this paper, high-speed optical ribbon couplers for card-to-backplane interconnect applications are presented. The ribbon couplers are based on evanescent coupling between flexible multimode waveguide arrays. A soft lithographic technique is utilized to fabricate the ribbons. A flexible nonterminating optical data bus has been developed. Using BeamPROP software, we simulated the evanescent light coupling between two closely spaced ribbon waveguides to study the effects of waveguides separation, interaction length, and misalignment on coupling efficiency. Further experimental analysis and tests have been performed to quantify these effects. To investigate data transmission performance, a 12-channel optical interconnect link has been assembled. Experimental results demonstrated successful evanescent coupling; facilitating auto alignment coupling between card and backplane ribbon waveguides at data speeds as high as 10 Gb/s per channel. The evident high-speed interconnect performance and rapid ribbon prototyping approach can result in overall lower cost coupler fabrication for prospective optical interconnect applications.      

Performance and limits of nonlinear optical waveguide couplers onthe basis of CdS1-xSex-doped glasses

In this work methodical and experimental investigations have been carried out in order to realize optical nonlinear waveguide couplers. On the basis of our experiences in the field of ion exchange in glass as substrate material semiconductor doped glasses have been used. Doped with CdS_1-xSe_x these glasses seem to have sufficiently high nonlinear coefficients. Ion exchange has been carried out by substitution of K- by Cs- ions and for waveguide patterning aluminium has been used as mask material. The produced stripe and bent waveguide couplers show qualitatively the expected optical nonlinear behavior in agreement with results obtained by solving the coupling equations. Possibilities in order to optimize this structures with respect to the nonlinear performance are discussed briefly     

Novel Grating Design for Out-of-Plane Coupling With Nonuniform Duty Cycle

We propose a waveguide grating coupler with nonuniform duty cycle for the out-of-plane coupling of light from a planar waveguide to an optical fiber, and evaluate the optical performances based on Bloch-wave analysis. The mode profile and focusing performance of the diffracted output beam are investigated in terms of the duty cycle to improve optical coupling efficiency.     

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     

Single mode grating coupling between thin-film and fiber optical waveguides

Optical directional coupling by means of a periodic perturbation is described between a clad, single mode, cylindrical fiber, and a thin-film planar waveguide. Codirectional coupling was achieved to a clad fiber, with 0.4 percent efficiency; allowing for the geometric mismatch, this is effectively 30 percent coupling. The fiber guided mode is accessed by reducing the fiber diameter by heating and pulling. Mode phase matching is achieved with a periodic grating sputter etched into the film. Unwanted grating induced interactions are minimized by restricting the guide parameters or by modifying the coupling mechanism. A perturbation analysis is used to calculate coupling coefficients and the design procedure for an optimum structure is explained. Measurements on grating couplers are described and the film/fiber codirectional coupling is shown to be effected by the grating.