Air Trenches Research Articles

Novel multimode interference devices for low index contrast materials systems featuring deeply etched air trenches

A novel design of multimode interference (MMI) device is proposed for photonic integrated circuits fabricated in low index contrast materials. In this design, deeply etched air trenches are used to define the boundaries of the multimode section to achieve strong lateral confinement. This results in superior imaging quality and therefore lower loss and imbalance, which can only be achieved in strongly guiding MMIs. However, the access waveguides remain weakly guiding, which is essential for low loss fibre connection. We demonstrate the merit of the design in 1 × 4 MMI power splitters. By introducing air trenches, loss decreases from 0.45 to 0.053 dB and imbalance decreases from 0.15 to 0.058 dB for TE polarization. For TM polarization, loss decreases from 0.46 to 0.003 dB and imbalance decreases from 0.16 to 0.067 dB.

Small-area bends and beamsplitters for lowindex-contrast waveguides

We explore the use of air trenches to achieve compact high efficiency 90 degrees waveguide bends and beamsplitters for waveguide material systems that have low refractive index and low refractive index contrast between the core and clad materials. For a single air interface, simulation results show that the optical efficiency of a waveguide bend can be increased from 78.4% to 99.2% by simply decreasing the bend angle from 90 degrees to 60 degrees . This can be explained by the angular spectrum of the waveguide mode optical field. For 90 degrees bends we use a micro-genetic algorithm (GA) with a 2-D finite difference time domain (FDTD) method to rigorously design high efficiency waveguide bends composed of multiple air trenches. Simulation results show an optical efficiency of 97.2% for an optimized bend composed of three air trenches. Similarly, a single air trench can be designed to function as a 90 degrees beamsplitter with 98.5% total efficiency.

A Passband-Flattened EDG Demultiplexer with an Air-Trenched Taper

A novel passband flattening method for a planar waveguide demultiplexer based on an etched diffraction grating is presented. A strongly confined taper is formed by etching air trenches at both sides of the input waveguide. The geometric parameters of the taper are optimized by considering the figure of merit, ripple, and loss. Numerical simulation for a design example is given to illustrate the passband flattening method.

Air trenches for sharp silica waveguide bends

Air trench structures for reduced-size bends in low-index contrast waveguides are proposed. To minimize junction loss, the structures are designed to provide adiabatic mode shaping between low- and high-index contrast regions, which is achieved by the introduction of "cladding tapers." Drastic reduction in effective bend radius is predicted. We present two-dimensional (2-D) finite-difference time-domain/effective index method simulations of bends in representative silica index contrasts. We also argue that substrate loss, while present, can be controlled with such air trenches and reduced to arbitrarily low levels limited only by fabrication capabilities. The required trench depth, given an acceptable substrate loss, is calculated in three dimensions using an approximate equivalent current sheet method and also by a numerical solver for full-vector leaky modes. A simple, compact waveguide T-splitter using air trench bends is presented.

Polarization conversion in the reflectivity properties of photonic crystal waveguides

Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.

Diffraction of cylindrical Bragg reflectors surrounding an in-plane semiconductor microcavity

In-plane microresonators consisting of an ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure waveguide and deep etched cylindrical trenches give both out-of-plane and lateral-light confinement. The air trenches, acting as a Bragg reflector also allow diffraction into air, so that the far-field pattern reveals interesting information on the resonant cavity modes. By the use of a two-dimensional cylindrical model and a transfer-matrix method based on Hankel functions, the energy and angular dependences of the diffracted field are calculated and successfully compared to measurements.

Shallow Trenches and Propagation of Surface Waves

The effect of the placement of shallow trenches of semi-circular or rectangular shape in the path of propagation of surface waves is studied. The ground is represented as a linear elastic halfplane, the vehicle as a point load vibrating in a wide range of frequencies, and an open air trench is interposed between the vehicle and the receiver. A two-dimensional direct boundary element method approach defined in the frequency domain is used to numerically investiate this problem. The results obtained indicate that trenches are beneficial in reducing the amplitude of vibration only in a narrow band around the mid-frequency range and in all other cases a large amplification of the dynamic signal is obtained. The actual shape of the trench is of minor importance.