openEPDA interconnects

Introduction

In addition to standard building blocks (see openEPDA uPDK ™ Blocks), an electronic-photonic circuit typically contains interconnects, which are arbitrary-shape connectors between the pins of standard building blocks. A simple example of an interconnect is straight or bended optical waveguide.

The interconnects are mostly defined for optical and electrical waveguides, but also can refer to other connections, such as DC metal connections.

This specification defines the standard openEPDA interconnect conventions. Each interconnect type (i.e. line, arc, …) is specified independently of the cross-section. When using an interconnect, a designer should specify the cross-section in which the interconnect has to be drawn.

If not all interconnects make sense for all cross-sections, there could be a list of permitted cross sections for each interconnect type. However, it is not expected that this will be a problem.

Interconnect types

line

The line interconnect has two parameters: the length and the width. For the width, the specification is the logical width. That means that the actual width of the interconnect will be also defined by the type of cross section. For example a different etch type could demand a different trench width for the same logical width of the interconnect.

arc

The arc interconnect has four parameters: the width, radius, angle and offset. The radius refers to the center of the waveguide. The offset parameter defines a lateral shift when connecting a waveguide interconnect. This is needed for a low-loss and a low mode-conversion coupling between e.g. a straight and curved waveguide. The image shows how this is done. It means that the input and output pins of the arc (a0 and b0) are not in the center, but rather are shifted outward by an amount equal to the offset.

linear taper

The linear taper connects two waveguides of different width with a linearly tapering section. It has three parameters: the length and two widths.

parabolic taper

The parabolic taper connects two waveguides of different width with a parabolic tapering section. It has three parameters: the length and two widths. The larger taper angle is at the smaller width side and thus the wide waveguide side tapers more slowly. The function describing the width from $$z=0$$ to $$z=\ell$$, where $$w_1 \le w_2$$, is given by:

$w(z) = \sqrt{\frac{z}{\ell}\cdot (w_2^2-w_1^2) + w_1^2}$

sine bend

The sine bend is a kind of S-bend which has zero curvature at the start and end of the structure. It takes three parameters, width, distance ($$d$$) and shift ($$s$$). The transverse coordinate ($$x$$) of the center line versus the longitudinal coordinate ($$0\le z \le d$$) is given by:

$x(z) = \frac{sz}{d} - \frac{s}{2\pi}\sin\left(\frac{2\pi z}{d}\right)$

Interconnect specification

A draft of interconnect specification (version 0.2) in YAML format.