Spot Size Converter with Low Index Cladding for Quantum Interconnects

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Solution Overview

Problem

Optical quantum computing systems face high coupling losses at interfaces between optical fibers and photonic integrated circuits due to mismatched mode sizes, which hinder efficient transport and manipulation of qubits and entangled photons.

Innovation Solution

A spot size converter with a tapered waveguide core and a cladding layer of varying refractive index is used to match the mode sizes of optical fibers and waveguides, reducing coupling losses by up to 30 mdB/facet, and enabling polarization-independent coupling.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If direct coupling between optical fibers and waveguides is used, then device complexity is reduced, but coupling losses increase due to mode size mismatch

Engineering Contradiction:

Improvecoupling structure complexityVSAvoidcoupling loss

Core Design Contradiction:

Device complexityVSLoss of energy

Solution Approach 1:

A spot size converter is introduced as an intermediary component between the optical fiber and the waveguide. This converter includes a tapered core region that gradually transforms the mode size from the fiber mode to the waveguide mode, enabling efficient coupling while maintaining relatively simple device architecture. The spot size converter acts as a bridge that resolves the mode mismatch problem without requiring complex alignment mechanisms or multiple coupling elements.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The spot size converter utilizes gradual parameter changes in its tapered core region, where the core width or height varies continuously along the propagation direction. This gradual parameter transformation allows the optical mode to adapt smoothly from the larger fiber mode size to the smaller waveguide mode size, minimizing coupling losses while keeping the overall device structure simple and manufacturable.

Inventive Principle:

Principle #35Parameter changes

2Loss of energy

If mode size matching is achieved through conventional methods, then coupling efficiency improves, but polarization dependence increases

Engineering Contradiction:

Improvecoupling lossVSAvoidpolarization independence

Core Design Contradiction:

Loss of energyVSAdaptability or versatility

Solution Approach 1:

The spot size converter employs local quality changes through its tapered geometry, where the gradual dimensional change creates a symmetric transformation region that treats both polarization states equally. The tapered structure modifies the local optical properties along the propagation direction, enabling polarization-independent mode transformation while achieving efficient coupling between fiber and waveguide modes.

Inventive Principle:

Principle #3Local quality

3Manufacturing precision

If precise alignment structures are implemented, then coupling precision improves, but manufacturing complexity increases

Engineering Contradiction:

Improvealignment precisionVSAvoidfabrication difficulty

Core Design Contradiction:

Manufacturing precisionVSEase of manufacture

Solution Approach 1:

The spot size converter is designed with built-in alignment features such as V-grooves or reference marks that are fabricated simultaneously with the converter structure itself. These preliminary alignment structures guide the positioning of optical fibers during assembly, achieving precise alignment without requiring separate, complex alignment mechanisms or multiple fabrication steps. The alignment features are integrated into the converter design, simplifying the overall manufacturing process.

Inventive Principle:

Principle #10Preliminary action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution significantly reduces coupling losses, enhances coupling efficiency to over 95% for both transverse electric and magnetic modes, and maintains mechanical robustness, suitable for cryogenic temperatures, thus supporting low-loss interconnects in quantum computing systems.

Implementation Method 1

a cladding layer surrounding the oxide layer, the cladding layer having a second refractive index lower than the first refractive index of the oxide layer

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Implementation Method 2

at least a portion of the oxide layer is tapered such that a first width of the oxide layer at the first end of the spot size converter is smaller than a second width of the oxide layer at the second end of the spot size converter

Methodology Applied

Scientific EffectAdiabatic expansion:

Data Source

PatentUS12259579B1Single photon interconnect employing low index cladding spot size converter

Publication Date: 2025.03.25 PSIQUANTUM CORP

AI summary

A device includes a spot size converter on a substrate. The substrate includes an optical fiber alignment structure formed thereon. The spot size converter is aligned with the optical fiber alignment structure. The spot size converter includes an oxide layer that has a first refractive index and includes a tapered section such that a first end of the spot size converter is smaller than a second end of the spot size converter. The spot size converter also includes a waveguide core in the oxide layer. The waveguide core is tapered and is smaller at the first end than at the second end of the spot size converter. The spot size converter further includes a cladding layer surrounding the oxide layer, the cladding layer having a second refractive index lower than the first refractive index of the oxide layer.