Andreas Håkansson

Highly efficient crossing structure for silicon-on-insulator waveguides

A compact waveguide crossing structure with low transmission losses and negligible crosstalk is demonstrated for silicon-on-insulator circuits. The crossing structure is based on a mode expander optimized by means of a genetic algorithm leading to transmission losses lower than 0.2 dB and crosstalk and reflection losses below 40 dB in a broad bandwidth of 20 nm. Furthermore, the resulting crossing structure has a footprint of only 6x6 microm(2) and does not require any additional fabrication steps.

Sound focusing by flat acoustic lenses without negative refraction

We present the experimental realization of two flat acoustic lenses generated by inverse design. The lenses consist of aperiodic lattices of aluminum cylinders confined in a rectangular area. They were obtained by a design tool that combines multiple scattering theory and a genetic algorithm. The cylinders’ positions are optimized by the genetic algorithm in order to produce maximum sound amplification at the focal point. Both the focus and the dimension of the lenses are arbitrarily chosen. Our approach is illustrated by measurements in two lenses fabricated with aluminum cylinders 2m long and having five and nine layers, respectively. Sound amplification up to 6.4dB is obtained at the focus. The excellent agreement found between experimental pressure patterns and theoretical simulations supports our tool of design. It is concluded that flat acoustic lenses made of aperiodic distribution of scatterers can produce sound focusing with no need of negative refraction, a property that has been demonstrated in...

CMOS Compatible Silicon-on-Insulator Polarization Rotator Based on Symmetry Breaking of the Waveguide Cross Section

A polarization rotator in silicon-on-insulator technology based on breaking the symmetry of the waveguide cross section is reported. The 25- μm-long device is designed to be integrated with standard grating couplers without the need for extra fabrication steps. Hence, fabrication is carried out by a 2-etch-step complementary metal-oxide-semiconductor compatible process using 193-nm deep ultraviolet lithography. A polarization conversion efficiency of more than -0.85 & dB with insertion losses ranging from -1 to -2.5 & dB over a wavelength range of 30 nm is demonstrated.

Inverse design of photonic crystal devices

This paper deals with the inverse design in the field of photonic crystal (PC)-based devices. Here, an inverse method containing a fast and accurate simulation method integrated with a competent optimization method is presented. Two designs yielded from this conjunction of multiple scattering theory with a genetic algorithm are analyzed. The potential of this approach is illustrated by designing a lens that has a very low F-number (F=0.47) and a conversion ratio of 11:1. We have also designed a coupler device that introduces the light from an optical fiber into a PC-based waveguide with a predicted coupling efficiency that exceeds 87%.

Inverse designed photonic crystal de-multiplex waveguide coupler

A two-dimensional photonic crystal diplexer integrated with a waveguide coupler is proposed. The design is computer generated through an inverse design process, limited within an area measuring 5 mumx5 mum. The best working device was designed for the optical communication wavelengths, 1.50 mum and 1.55 mum, i.e. a channel spacing of 50 nm. The device exhibits crosstalks suppressed below 40dB and coupling efficiencies close to 80%, for both channels.

High-efficiency defect-based photonic-crystal tapers designed by a genetic algorithm

A method based on a genetic algorithm (GA) is used to design the optimum configuration of defects that when put within a photonic-crystal (PhC) taper improve the coupling efficiency between dielectric and PhC waveguides (WGs). This approach optimizes the whole configuration of defects simultaneously and, therefore, takes into account the correlation among the defects. Transmission efficiencies up to 94% have been predicted for a 3-/spl mu/m-wide dielectric WG into a single-line-defect PhC-WG. This result significantly improves the transmission efficiency of the same PhC taper without defects. The influence on the coupling efficiency of the PhC taper length and geometry are also discussed.

Increased sensitivity through maximizing the extinction ratio of SOI delay-interferometer receiver for 10G DPSK

We present an optimized design for a 10G- differential-phase-shift-keyed (DPSK) receiver based on a silicon-on-insulator (SOI) unbalanced tunable Mach-Zehnder interferometer (MZI) switch in sequence with a Mach-Zehnder delay interferometer (MZDI). The proposed design eliminates the limitation in sensitivity of the device produced by the waveguide propagation losses in the delay line. A 2.3 dB increase in receiver sensitivity at a bit-error-rate (BER) of 10(-9) is experimentally measured over a standard implementation. The enhanced sensitivity is achieved with zero power consumption by tuning the operating wavelength or with less than 5 mW for a fixed wavelength using microheaters. Also the foot-print of the device is minimized to 0.11 mm(2) by the use of compact spirals.

Directional acoustic source by scattering acoustical elements

Highly directional sources are desirables in a variety of fields for many applications. The authors report an inverse designed scattering acoustical element device that transforms an omnidirectional ultrasonic source into one highly directional. This two-dimensional design shows an overall better modeled performance than other previously proposed, including a half-power angular width less than 5°. The experimental demonstration is performed in the ultrasonic range, using a hydrophone as omnidirectional source and an array of alumina rods as building blocks for the scattering acoustical elements. The measured half-power angular width is 6°, a value that supports the high reliability of the designing tool.

Optimal design of microscaled scattering optical elements

A method of inverse design is applied to generate an optical device that acts as a wavelength demultiplexer. The ultracompact device, only 2μm thick, is designed to separate two wavelengths 1.55μm and 1.50μm, respectively, and consists of five layers of 0.4μm×0.4μm square-shaped bars etched in gallium arsenide. The expected cross talk is suppressed below −25dB for both wavelengths. The proposed device is an example of a scattering optical element, a name here introduced to define a class of computer-generated optical devices and whose functionalities are based on the multiple scattering by their individual constituents. For realization of the aforementioned devices, two-dimensional photonic plates can be prepared by only a single integrated circuit processing procedure followed by micromanipulation assembling.

Midinfrared filters based on extraordinary optical transmission through subwavelength structured gold films

An experimental study is made of the enhanced optical transmission of nanostructured gold films in the midinfrared region. Results indicate that the excitation of surface plasmon polaritons due to periodicity plays a fundamental role in producing extraordinary optical transmission. The influence of the surrounding claddings, hole shape, and periodicity on the resonance wavelength and the quality factor is investigated. The aim is to use the subwavelength structures as ultracompact optical filters whose spectral features can be easily tuned and scaled. For filter design purposes, the results show that the main parameters affecting the resonance wavelength are the lattice constant and dielectric cladding. The hole shape and size are found to cause transmission enhancement and there is only a small resonance redshift when the hole area is increased. However, a lower quality factor is achieved when the hole area is increased.