US-12619030-B2 - Optical structure

US12619030B2US 12619030 B2US12619030 B2US 12619030B2US-12619030-B2

Abstract

An optical structure comprising a multimode interference waveguide section, the multimode interference waveguide section comprising an input face, an output face and sidewalls extending therebetween, the sidewalls extending substantially parallel to a length axis; at least one input optical waveguide abutting the input face and spaced apart from the sidewalls; at least one output optical waveguide abutting the output face and spaced apart from the sidewalls; the input face being divided into first and second input face shoulder portions arranged on opposite sides of the at least one input optical waveguide; and the output face being divided into first and second output face shoulder portions arranged on opposite sides of the at least one output optical waveguide; the optical structure further comprising at least one external reflector appendage, the at least one external reflector appendage comprising a reflector portion integrally extending from a shoulder portion and an appendage waveguide integrally extending from the reflector portion along an axis inclined to the length axis, the reflector portion comprising a reflector wall arranged such that light travelling parallel to the length axis which is incident on the reflector wall is reflected into the appendage waveguide.

Inventors

Rob Walker

Assignees

AXENIC LIMITED

Dates

Publication Date: 20260505
Application Date: 20230525
Priority Date: 20220526

Claims (12)

1 . An optical structure comprising: a 1 to 1 multimode interference waveguide section, the multimode interference waveguide section comprising an input face, an output face and sidewalls extending therebetween, the sidewalls extending substantially parallel to a length axis; a single input optical waveguide abutting the input face and spaced apart from the sidewalls; the single input optical waveguide being at least one of inclined to the length axis and arranged closer to one sidewall than the other; a single output optical waveguide abutting the output face and spaced apart from the sidewalls; the input face being divided into first and second input face shoulder portions arranged on opposite sides of the input optical waveguide; and the output face being divided into first and second output face shoulder portions arranged on opposite sides of the output optical waveguide; the optical structure further comprising at least one external reflector appendage, the at least one external reflector appendage comprising a reflector portion integrally extending from a shoulder portion and an appendage waveguide integrally extending from the reflector portion along an axis inclined to the length axis, the reflector portion comprising a reflector wall arranged such that light travelling parallel to the length axis which is incident on the reflector wall is reflected into the appendage waveguide.
2 . The optical structure as claimed in claim 1 , wherein the reflector wall is planar.
3 . The optical structure as claimed in claim 2 , wherein a normal to the reflector wall is at an angle to the length axis such that light travelling parallel to the length axis is totally internally reflected by the reflector wall.
4 . The optical structure as claimed in claim 2 , wherein the normal to the reflector wall is at an angle of between 35 degrees to 75 degrees to the length axis.
5 . The optical structure as claimed in claim 1 , wherein the appendage waveguide extends normal to the length axis.
6 . The optical structure as claimed in claim 1 , comprising a plurality of external reflector appendages.
7 . The optical structure as claimed in claim 6 , wherein for at least one of the input face and output face external reflector appendages extend from both shoulder portions.
8 . The optical structure as claimed in claim 7 , wherein for both external reflector appendages the appendage waveguides also extend integrally from the side walls adjacent to the shoulder portions.
9 . The optical structure as claimed in claim 1 , wherein for at least one of the input face and output face an external reflector appendage extends from one shoulder portion, with the appendage waveguide also extending integrally from the side wall adjacent to the shoulder portion.
10 . The optical structure as claimed in claim 9 wherein the other shoulder portion is a planar reflecting wall, the normal to the reflecting wall being inclined to the length axis such that light travelling parallel to the length axis is reflected by the planar reflecting wall into the appendage waveguide.
11 . The optical structure as claimed in claim 1 further comprising an odd mode filter connected to the input optical waveguide.
12 . The optical structure as claimed in claim 1 , further comprising a photodetector connected to the appendage waveguide.

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to and the benefit of Great Britain Patent Application GB2207785.3 filed 26 May 2022, which is hereby incorporated by reference in its entirety. TECHNICAL FIELD The present invention relates to an optical structure. SUMMARY The present invention relates to an optical structure. More particularly, but not exclusively, the present invention relates to an optical structure comprising a multimode interference waveguide section comprising an input face and an output face, each of which is divided into first and second shoulder portions, the optical structure further comprising at least one external reflector appendage which comprises a reflector portion integrally extending from a shoulder portion and an appendage waveguide integrally extending from the reflector portion, the reflector portion comprising a reflector wall arranged such that light travelling parallel to the length axis which is incident on the reflector wall is reflected into the appendage waveguide. Functionally, a multi-mode interference waveguide (MMI) section is a section of optical waveguide which is made significantly wider than normal in order to support a relatively large number of modes. The term ‘modes’ refers to spatial harmonics of a fundamental waveguide mode, relating to the lateral or ‘width’ dimension. The fundamental mode has a single light-intensity maximum at or near the centre of the lateral profile while modes of higher order exhibit multiple intensity maxima and minima; the mode-order number is, by convention, the same as the number of minima. Modes can be divides into two classes: Even order modes (orders 0, 2, 4 etc.) which have an intensity peak at or near the waveguide centre. The all-important fundamental mode (Order-0) is one such. These modes are symmetric in both phase and amplitude. Odd-order modes (orders 1, 3, 5 etc.) which have an intensity-null at or near the waveguide centre, which is also an optical-field zero-crossing. Though also symmetric in amplitude, odd modes are antisymmetric in phase. Due to this symmetry/anti-symmetry dichotomy, a symmetrically disposed input waveguide mode to an MMI section will generally excite only one type or the other, leading to different re-imaging properties for even and odd mode input. For most purposes, a general waveguide will ideally support only one mode (the fundamental); however, achieving this ideal involves design compromises which may not be acceptable. Here it will be assumed that all waveguides support at least two modes. The harmonic relationship between the form and propagation-velocities of the MMI modes results in useful properties of re-imaging, whereby an input optical profile can be re-created in single or multiple form at the output end-face provided the length of the MMI section is correct. MMI sections are useful for optical splitting and recombination functions. They are also useful for mode filtering as even-order and odd-order modes have different re-imaging lengths. Multimode interference waveguide sections (MMI sections) are well known in the art. Basic MMI sections are rectangular in plan comprising input and output faces with sidewalls extending therebetween, all at least partially reflective to a light wave except where abutted to a waveguide. At least one input optical waveguide is abutted to the input face. At least one output optical waveguide is abutted to the output face. The input and output waveguides are narrower than the MMI section but are otherwise similarly constructed. If approximately centrally aligned to the MMI section, they give rise to shoulder portions at each end face i.e. between the external corner where the end face meets the side walls and the point where the input or output waveguide abuts the end face. Ideally the MMI section is so dimensioned that a fundamental waveguide mode provided to the multimode interference section by the input optical waveguide(s) is identically imaged at the output optical waveguide(s). The provided signal typically comprises multiple modes. It is known that whilst some of these modes (fundamental and even order) will be reimaged at the output waveguides, the others (odd orders) will be displaced and reimaged at the shoulders of the multimode interference section. The shoulder portions retro-reflect at least some of this light back towards the input face where, by reciprocity, it is reimaged back to the launch profile on the input waveguide(s) and may ultimately degrade the optical return loss from the main input port. It may additionally undergo further reflections from other multimode interference sections or imperfectly anti reflection coated facets to create intra-circuit resonances. U.S. Pat. No. 8,649,641B2 discloses a multimode interference section in which a shoulder portion of the end face is cut at an angle so that incident light is reflected out of a port in the side wall into a curved waveguide. This chamfer at on