Nanoimprint lithography fabrication of waveguide-integrated optical gratings with inexpensive stamps

We demonstrate that a replica grating can be effectively used as an inexpensive stamp for nanoimprint lithography to pattern diffractive optical couplers integrated with planar optical waveguides. Imprinted grating patterns were integrated with silicon oxynitride waveguide films to be used as an evanescent wave sensor in the input grating-coupler configuration. An anti-adhesion layer using an inexpensive, two-step chemical functionalization was developed for the stamps. The stamps were able to withstand imprint temperatures ranging from 140 to 190 °C and high fidelity imprints were obtained. The groove pattern was integrated in waveguide films by etch transfer and light-coupling properties of gratings with 1.2 μm pitch were tested using a λ = 1.55 μm laser. Compared to etched silicon masters, replica optical gratings provide uniform pattern density over their entire surface with no unstructured regions, are inexpensive, and readily available for R&D use.     

Silicon thin‐film solar cells with rectangular‐shaped grating couplers

As an alternative to randomly textured transparent conductive oxides as front contact for thin‐film silicon solar cells the application of transparent grating couplers was studied. The grating couplers were prepared by sputtering of aluminium‐doped zinc oxide (ZnO) on glass substrate, a photolithography and a lift‐off process and were used as periodically textured substrates. The period size and groove depth of these transparent gratings were tuned independently from each other and varied between 1 and 4 μm and 100–600 nm. The optical properties of rectangular‐shaped gratings and the opto‐electronic behaviour of amorphous and microcrystalline silicon solar cells with integrated grating couplers as a function of the grating parameters (period size P and groove depth h_g) are presented. The optical properties of the gratings are discussed with respect to randomly textured substrates and the achieved solar cell results are compared with the opto‐electronic properties of solar cells deposited on untextured (flat) and randomly textured substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

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     

Fabrication of Butt‐Coupled SGDBR Laser Integrated with Semiconductor Optical Amplifier Having a Lateral Tapered Waveguide

We have demonstrated a high‐power widely tunable sampled grating distributed Bragg reflector (SGDBR) laser integrated monolithically with a semiconductor optical amplifier (SOA) having a lateral tapered waveguide, which is the first to emit a fiber‐coupled output power of more than 10 dBm using a planar buried heterostructure (PBH). The output facet reflectivity of the integrated SOA using a lateral tapered waveguide and two‐layer AR coating of TiO₂ and SiO₂ was lower than 3 × 10^?4 over a wide bandwidth of 85 nm. The spectra of 40 channels spaced by 50 GHz within the tuning range of 33 nm were obtained by a precise control of SG and phase control currents. A side‐mode suppression ratio of more than 35 dB was obtained in the whole tuning range. Fiber‐coupled output power of more than 11 dBm and an output power variation of less than 1 dB were obtained for the whole tuning range.     

Waveguide polarization-independent optical circulator

We fabricated a new type of waveguide polarization-independent optical circulator which does not need a polarization-beam splitter. The circulator is based on a non-reciprocal Mach-Zehnder interferometer which consists of two waveguide Faraday rotators, two thin-film half-waveplates and two planar lightwave circuit-type 3-dB couplers. The fabricated circulator provides a 14.0-23.7-dB isolation and a 3.0-3.3 dB insertion loss at λ=1.55 μm. This circulator presents a new possibility for developing non-reciprocal devices in the field of integrated optics     

采用光波导调制器进行的光学双稳态和多稳态的研究

采用平面光波导电光调制器及简单的双光束干涉装置成功地进行了光学双稳态和多稳态的实验研究。光波导调制器是在自制的钛扩散铌酸埋平面光波导上蒸镀铝电极制成的。采取激光由调制器端面直接输入,由锗酸铋棱镜耦合输出的方式。实验中消除了锗酸铋晶体旋光性的影响。本文除了对光学双稳性进行了分析讨论外,还着重对不同形态的多稳实验回线进行了探讨。     

A WDM receiver photonic integrated circuit with net on-chip gain

We demonstrate a photonic circuit with an optical preamplifier, a WDM filter and a fast photodetector integrated on the same chip. The passive integrated filter is formed by an aspheric waveguide lens and a planar Bragg grating. This arrangements yields narrow filter response (8 Å width at -3 dB is demonstrated) and high rejection ratio of 24 dB. The optical preamplifier consisting of strained multi-quantum well layers provides sufficient amplification to overcome the passive losses. Response curves demonstrate net on-chip gain with low ripple     

Study of optical bistability and multistability with optical waveguide modulator

Using a planar optical waveguide electro-optic modulator and simple double beam interfering device, the experimental study of optical bistability and multistability is successfully performed. The waveguide modulator is made by depositing aluminium as electrodes on the Ti-diffused LiNbO₂ planar optical waveguide which is fabricated by ourselves. Laser beam is coupled directly into the end of optical waveguide modulator and out of the Bi₁₂GeO₂₀ prism. Optical rotation arising from Bi₁₂GeO₂₀ crystal is overcome in experiment. In addition to analyzing and discussing optical bistability, the multistabilitical hysteresis curves of different shapes obtained in experiments are particularly investigated.     

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.     

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     

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。     

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.     

Influence des effets de contrainte induits par la température sur la conversion TE-TM dans des guides planaires de YIG substitué

We have demonstrated the tuning of linear birefringence in magnetooptic planar thin film waveguides by temperature adjustment. Using this method, phase-matching between coupled te₀and tm₀ modes, a necessary condition for the integrated optical isolator based on mode conversion, has been obtained at λ = 1.52 µm in bilayer gadolinium gallium substituted yig films. The waveguide temperature was 85°C. Moreover, phase-matching temperature can be modified by a slight variation of gadolinium and gallium substitutions as shown by a phase-matching observed in a waveguide at room temperature.     

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.     

Tunable waveguide transmission gratings based on active gain control

Mode loss or gain can have a significant effect on the operation of waveguide transmission gratings as optical filters. Taking for example a long-period grating (LPG), we present a computer simulation that accounts for these effects. We show that cladding loss or gain result in qualitatively different LPG spectral behavior that can be used as a tuning mechanism in guiding structures where cladding mode properties can be controlled. One possible control mechanism is to have pump-driven gain in a doped cladding, which can perform gradual loss compensation and eventually gain excitement. Based on simulation results, we propose new approaches to tuneable amplifying filter designs in special fibers, as well as grating assisted co-directional couplers for integrated optics platforms.     

Low-Dimensional Waveguide Grating Fabrication in GaN with Use of SiCl<Subscript>4</Subscript>/Cl<Subscript>2</Subscript>/Ar-Based Inductively Coupled Plasma Dry Etching

Optimized fabrication of submicron-sized features in gallium nitride (GaN) with the use of inductively coupled plasma (ICP) dry etching, based on SiCl₄/Cl₂/Ar gas mixture, is presented. Dense periodic patterns, i.e., 400-nm-period gratings, were transferred into a gallium nitride waveguide under different etching conditions. ICP power, radiofrequency (RF) power, chamber pressure, and Ar/Cl₂ gas mixing ratio were altered during the experiment. Depths of fabricated grating couplers up to 670 nm were achieved. The most suitable etching conditions are discussed with the assessment based on etching selectivity, scanning electron microscopy (SEM) observation of grating tooth slope, hard-mask erosion process, and etched surface morphology.     

Monolithically integrated optical channel monitor for DWDM transmission systems

The design, fabrication, and performance of an InP-based monolithically integrated optical power monitor are presented. It contains 44 wavelength channels separated by 100 GHz and demonstrates record small footprint size. The device's major components, which include the echelle diffractive grating demultiplexer, passive waveguide circuitry and single-mode vertically integrated waveguide PIN photodetectors, are characterized and discussed as generic building blocks of InP-based planar lightwave technology for DWDM.     

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