Planar Waveguide Circuits Research Articles

Simulation Study on a Planar Quasi-Optical Waveguide Circuit for a W-Band Gyro-TWT With Stability Improvement

In this article, a planar quasi-optical waveguide (PQOW) circuit is proposed for vacuum electron devices (VEDs) for the first time. The circuit is made of a series of metamaterial pillars attached to a pair of parallel plates for phase compensation. It can propagate quasi-optical modes as a pair of concave mirrors does, while the lower order HE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{0}\textit{n}}$</tex-math> </inline-formula> modes experience much heavier circuit loss compared to that of convention confocal waveguide circuits, which can improve the backward oscillation (BWO) stability of gyro-amplifiers. As an example, a W-band gyro-amplifier with PQOW circuits to replace conventional confocal circuits is designed. The 3-D particle-in-cell simulation demonstrated that the circuit is capable of producing stable high-power radiation within 96–108 GHz, with an electron efficiency of about 15% over the entire bandwidth.

Femtosecond Laser Modification of Silica Optical Waveguides for Potential Bragg Gratings Sensing.

The optimum femtosecond laser direct writing of Bragg gratings on silica optical waveguides has been investigated. The silica waveguide has a 6.5 × 6.5 µm2 cross-sectional profile with a 20-µm-thick silicon dioxide cladding layer. Compared with conventional grating inscribed on fiber platforms, the silica planar waveguide circuit can realize a stable performance as well as a high-efficiency coupling with the fiber. A thin waveguide cladding layer also facilitates laser focusing with an improved spherical aberration. Different from the circular fiber core matching with the Gaussian beam profile, a 1030-nm, 400-fs, and 190-nJ laser is optimized to focus on the top surface of the square silica waveguide, and the 3rd-order Bragg gratings are inscribed successfully. A 1.5-mm long uniform Bragg gratings structure with a reflectivity of 90% at a 1548.36-nm wavelength can be obtained. Cascaded Bragg gratings with different periods are also inscribed in the planar waveguide. Different reflection wavelengths can be realized, which shows great potential for wavelength multiplexing-related applications such as optical communications or sensing.

Circulator‐Free Brillouin Photonic Planar Circuit

AbstractOn‐chip stimulated Brillouin scattering (SBS) that results from enhanced light–sound interactions, has recently demonstrated great applicability and versatility for signal generation and manipulation. Progress toward realizing fully integrated Brillouin photonic circuits is promising; however, the control and routing of on‐chip optical waves requires non‐reciprocal elements such as circulators, which is challenging and adds complexity. Here, a circulator‐free Brillouin photonic planar waveguide circuit which eliminates the need for separate non‐reciprocal elements is demonstrated. Backward inter‐modal Brillouin scattering (BIBS) is used in a planar photonic integrated circuit, for the first time, to provide Brillouin processing capability in a multi‐modal waveguide, conveniently allowing for the separation of counter‐propagating pump and signal using passive integrated mode‐selective filters. Using an As2S3–Si hybrid multi‐modal photonic circuit, inter‐optical‐mode energy transfer with a Brillouin gain coefficient of 280 m−1W−1 is experimentally demonstrated, representing two orders of magnitude enhancement compared to conventional optical fibers. This on‐chip BIBS circuit allows for independent routing of pump and signal waves, showing resilience to the cross talk. The experimental demonstrations provide an important basis for achieving fully integrated Brillouin photonic circuits without requiring on‐chip circulators and fine spectral filtering, opening a new dimension for studying spatial‐dependent photon–phonon nonlinear dynamics.

Quasiperiodic Dielectric Gratings for Multiband Fiber-To-Chip Couplers

Dielectric gratings that couple optical fibers and planar waveguide circuits are key for optical-to-electronic (electronic-to-optical) signal conversion, but their applicability to platforms that require broader bandwidths and higher capacity is limited by their single-wavelength response. Herein, we present the design of a quasi-periodic grating coupler with multiband fiber-to-waveguide (waveguide-to-fiber) coupling response, where the grating consists of a periodic repetition of unit cells made of alternating silicon and air grooves according to the Fibonacci sequence. Through finite-difference time-domain (FDTD) calculations, we show that this new device could be used for coupling multi-wavelength fiber modes in a single grating structure. The results were obtained for fibers operating in the wavelength range from 1000 nm to 2000 nm, but the concept can be readily extended to other frequency ranges. Moreover, the allowed modes in the grating are almost insensitive to fiber misalignments and small fabrication errors for high Fibonacci steps, which is useful when alignment of optical components is impractical. It is hoped that properly designed gratings overlapping multiple modes may lead to ultra-broadband fiber-waveguide couplers that can cope with the growing demand for higher capacity and bandwidth in optical communications.

Low Loss, Large Bandwidth Fiber-Chip Edge Couplers Based on Silicon-on-Insulator Platform

Fiber-chip edge couplers are extensively used in integrated silicon photonic for the coupling of light between optical fibers and planar silicon waveguide circuits. Here, we experimentally demonstrate three kinds of edge couplers with eased fabrication process, two fork shape, and one dual-trident SWG shape, based on Silicon-on-Insulator platform. A commercial lensed fiber with mode field diameter of 6 μm is used. The coupling performance and fabrication tolerance are theoretically analyzed and verified by 3D-FDTD simulation. The experimental results show that these edge couplers pose low coupling losses and large bandwidths simultaneously. At the wavelength of 1.55 μm, the coupling losses are 1.25 dB/facet, 1.49 dB/facet, 1.82 dB/facet for fork1, fork2, and dual-trident SWG couplers, respectively. The measured wavelength bandwidths in which the loss below 2 dB are 114 nm, 102 nm, 92 nm, respectively.

Hybrid Plasmonics Slot THz Waveguide for Subwavelength Field Confinement and Crosstalk Between Two Waveguides

The slot waveguide has attracted considerable attention because of its ability to confine and guide electromagnetic energy at the subwavelength scale beyond the diffraction limit. We propose a novel terahertz slot waveguide structure to achieve a better tradeoff between propagation length and field confinement capacity, the novel waveguide consisting of a two slot structure. The performances of terahertz waveguides were investigated using the finite-element method. The results demonstrated that the hybrid slot waveguide (HSW) provides significantly enhanced field confinement in low index slot regions: more than five times that of traditional low index slot waveguides (LISWs). An optimized HSW structure was achieved by tuning the tradeoff between mode confinement and propagation length. We also showed that its integration in conventional planar waveguide circuits was greatly improved compared with the LISWs, by comparing their crosstalk. The proposed new HSW structure has great potential to enable THz production of compact integration and could lead to true semiconductor-basedTHz applications with high performance.

Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides.

Fiber-chip edge couplers are extensively used in integrated optics for coupling of light between planar waveguide circuits and optical fibers. In this work, we report on a new fiber-chip edge coupler concept with large mode size for silicon photonic wire waveguides. The coupler allows direct coupling with conventional cleaved optical fibers with large mode size while circumventing the need for lensed fibers. The coupler is designed for 220 nm silicon-on-insulator (SOI) platform. It exhibits an overall coupling efficiency exceeding 90%, as independently confirmed by 3D Finite-Difference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) calculations. We present two specific coupler designs, namely for a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD) and a standard SMF-28 fiber with 10.4 µm MFD. An important advantage of our coupler concept is the ability to expand the mode at the chip edge without leading to high substrate leakage losses through buried oxide (BOX), which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO2 upper cladding thin high-index Si3N4 layers. The Si3N4 layers increase the effective refractive index of the upper cladding near the facet. The index is controlled along the taper by subwavelength refractive index engineering to facilitate adiabatic mode transformation to the silicon wire waveguide while the Si-wire waveguide is inversely tapered along the coupler. The mode overlap optimization at the chip facet is carried out with a full vectorial mode solver. The mode transformation along the coupler is studied using 3D-FDTD simulations and with fully-vectorial 3D-EME calculations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm.

Monolithic of SOI wafer waveguide and InP-laser with DVS-BCB coating and bonding

Si photonic is an optical information processing technology, including lasers, optical modulators, waveguide and a photodetector, and the light signal is performed by a basic photonic systems. In silicon microelectronics world, hundreds of millions of pieces of single components are integrated into a single platform to “parallel manufacturing” approach at the same time, the current optical transmission systems, the main technology is based on an independent element “series mode” manufacturing. Bonding technology is becoming realized comprising laser, an optical modulator, a waveguide and a photodetector system integration. Several conventional bonding methods have been developed a practical method. Wherein, the silicon substrate and the III–V group bonding, does not need to atomically smooth surface engagement, and the conventional bonding method is comparison with great flexibility, allowing the binding material or structure of the highest quality. Silicon has a high thermal conductivity and low light absorption, and the properties of these two substances silicon photonic application is very advantageous. Relatively low cost and high quality SOI wafers, making them an ideal platform to create a CMOS-compatible planar waveguide circuits. In this research, we focus on BCB coating, bonding, and adhesion observation for monolithic SOI wafer waveguide and InP-laser. The IV (Current–voltage) curve measured under the needle was found operating under current of approximately 8mA at 1.5V, LI (Optical power-Current) measurement results found at 36mA operating maximum optical power of about 1.2mW. This method owns the advantages of simple fabrication process, great performance and high adhesion efficient between BCB layer and Si waveguide.

High‐efficiency single etch step apodized surface grating coupler using subwavelength structure

AbstractGrating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier‐eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is −2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low‐cost and high‐volume fabrication using 193 nm deep‐ultraviolet lithography. It is also shown by simulations that a coupling loss as low as −0.42 dB is predicted for a modified coupler structure with bottom mirror.

Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler.

We propose a fabrication tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform based on the mode-coupling mechanism. The PSR consists of a silicon wire waveguide coupled to a taper-etched waveguide. Compared to previously reported PSRs based on directional couplers which are sensitive to fabrication variations, the partially etched taper structure can compensate for fabrication inaccuracies. In addition, the taper-etched geometry breaks both the horizontal and vertical symmetries of the waveguide, introducing an additional degree of design freedom to accommodate different upper cladding layers. The proposed PSR can be readily integrated in a planar waveguide circuit using e.g. SiO(2) cladding, making it compatible with typical metal back-end-of-line processes. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.09 dB in the C-band (or a conversion efficiency of 98%). A low TE-to-TE through insertion loss (-0.07 dB) and a very low polarization crosstalk (-30 dB) over a wide wavelength range exceeding 160 nm with a large fabrication tolerance (>50 nm) are numerically demonstrated.

Measurement of modal birefringence in optical waveguides based on the Mach-Zehnder interferometer.

A method for measuring the birefringence in planar waveguide circuits is theoretically proposed and validated. The method is based on the Mach-Zehnder interference and by measuring the spectral shift due to orthogonal polarization states. The birefringence of a silica waveguide is measured to be 2.33 × 10(-4) at nearby 1550 nm. In addition, the birefringence variations with the wavelength and its dependence on the external stress are investigated with the proposed method experimentally. The results in measuring birefringence demonstrate a high accuracy with the order of 10(-5), a wide dynamic range from 10(-5) to 10(-3), and a characteristic of multi-wavelength evaluation.

Single etch grating couplers for mass fabrication with DUV lithography

Surface grating couplers enable efficient coupling of light between optical fibers and planar waveguide circuits. While traditional grating designs require two etch steps for efficient coupling to silicon-on-insulator waveguides, recently proposed subwavelength structured gratings can achieve the same coupling efficiencies with a single etch step, thereby significantly reducing fabrication complexity. Here we demonstrate that such couplers can be fabricated on a large scale with ultra-violet lithography, achieving a 5 dB coupling efficiency at 1,550 nm. Through both simulations and experiments we give physical insight on how pattern fidelity impacts the performance of these couplers, and propose strategies to deal with inevitable process variations.

Finite-difference time-domain analysis of refractive index grating on planar light waveguide circuit with optically trapped gold particles

The super-resolution capability of scanning near-field optical microscopy (SNOM) with a gold particle is studied by the two-dimensional finite-difference time-domain (2D FDTD) method. We obtain SNOM signals by integrating the far field within the numerical aperture of an objective lens for a refractive index grating by scanning optically trapped gold particles with different diameters illuminated by focused laser light at the wavelength of 515 nm. The signal is strong at a high refractive index of the grating and exhibits similar behavior to that obtained in the experiment with the grating fabricated on a planar light waveguide circuit with a period of 1060 nm. Furthermore, the signal modulation increases as the gold particle diameter decreases and reaches 0.82 at a diameter of 50 nm.

Design Principles for Higher (Prime) Order Composite Directional Couplers

The theoretical construction of ideal (lossless) 3 times 3, 4 times 4, and 5 times 5 dimensional directional couplers from 2 times 2 directional couplers is demonstrated. Such devices could be fabricated as planar waveguide circuit elements.

Ultra low-loss 1×2 multiplexer using thin-film filters on polymer integration platform

Compact 1×2 wavelength multiplexer modules employing thin-film filters integrated into polymer planar waveguide circuits are presented. The modules also feature on-chip deep-etched U-grooves for precise fibre-chip coupling. Insertion loss &lt;1 dB, crosstalk &lt;−30 dB, and return loss &lt;−50 dB have been achieved by various technological optimisations.

Nanoimprint technology for nano-structured optical devices

Abstract We present the fabrication method for various nano-structured optical devices such as a nanowire grid polarizer, a Bragg grating filter, and an optical ring resonator to demonstrate feasibility of nanoimprint technology for manufacturing nanoscale optical devices. A 50 nm half-pitch nanowire grid polarizer was successfully fabricated by using nanoimprint lithography (NIL), of which extinction ratio is twice higher than that of the commercially available 70 nm half-pitch one. Generation of the uniform and seamless 50 nm half-pitch metal gratings over the large area is impossible or impractical with lithographic techniques other than NIL. The Bragg grating filter and optical ring resonator were fabricated in low-loss optical polymers simply by means of a single imprint process which does not require conventional photolithography and reactive ion etching processes. This work demonstrates the possibility of a low cost production of planar waveguide circuits for optical communications.

Integrated thin film heater and sensor with planar lightwave circuits

This paper describes the use of integrated heaters and closed-loop applications for temperature control of planar waveguide circuits. The history of arrayed waveguide gratings (AWG) temperature control is discussed along with the difficulties in applying this control. Modeling and measurement results for external and integrated heaters will be presented. Fundamental considerations as well as modeling and measurement of the thermal characteristics of various physical constructions are discussed, and the rationale for applying these disciplines to a given problem is highlighted. In this work, we will present the evolution of these unique integrated thin film heaters and review thermal-optical modeling to experimental demonstration of the AWG. The goal of the work is to provide a solution path for integration of multiple optical devices on a single substrate from a thermomechanical as well as optical discipline point of view.

High-power multi-mode laser to single-mode pump conversion

A novel pump-sharing module for multi-channel amplifiers based on integration of high-power multimode semiconductor lasers and planar waveguide circuits is proposed. The feedback-free modules are designed for optimum coupling, throughput and power uniformity on silicon using BPM method and fabricated via the sol–gel process of photo patternable precursors. Modal properties of the multimode lasers are considered by studying spectrum and spatial distribution of laser output power at different temperatures. Employing an 1500mW 980nm multimode laser (TE polarized), via lens and Butt coupling up to 1250mW of laser output is coupled into the multimode channel waveguides, which are tapered and branched to four single mode waveguides. The measured pump power from splitter outputs, randomly polarized, exceeds 600mW. At constant temperatures ranging from 22°C to 30°C, the pump-sharing modules exhibit good stability with a power uniformity of <0.5dB over hours.

Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically-Trapped Gold Particle

A refractive index grating fabricated on a planar light waveguide circuit (PLC) and an optical disk tracking groove profile are observed by scanning an optically-trapped 100-nm diameter gold particle.

Trimming phase and birefringence errors in planar lightwave circuits with deep ultraviolet femtosecond laser

A deep ultraviolet femtosecond laser was employed to trim phase and birefringence errors in silica planar lightwave circuits. A permanent refractive index change of ∼3.8×10−4 and a birefringence change of 1.0×10−4 were induced in hydrogen-free Mach-Zehnder planar waveguide circuits. The ultrafast laser enhances the ultraviolet photosensitivity response in silica waveguides by two orders of magnitude greater than that of a nanosecond 248 nm KrF excimer laser.