Frustum Illumination for Photoconductive Switches

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Solution Overview

Problem

Existing photoconductive switches are inefficient in utilizing incident light and prone to electrical breakdown due to high-intensity irradiance, which limits their switching accuracy and high electric field capabilities in applications like microwave and millimeter wave generation, pulsed power systems, and particle accelerators.

Innovation Solution

The use of a frustum device that spreads high-intensity irradiance from an optical fiber across a larger surface area of the photoconductive switch using axially symmetric regions and total internal reflection, reducing the irradiance to a level below the damage threshold and enhancing light coupling efficiency while maintaining high stand-off voltage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If high-intensity light is used to illuminate the photoconductive switch, then switching speed is improved, but electrical breakdown occurs due to excessive irradiance

Engineering Contradiction:

Improveswitching speedVSAvoidelectrical breakdown

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The frustum structure serves as an intermediary optical element between the light source and the photoconductive switch. It transforms the high-intensity focused light into a distributed illumination pattern, mediating the interaction between light and the photoconductive material to achieve fast switching without electrical breakdown.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention transitions from point-focused illumination to area-based illumination by using the frustum to distribute light across a larger surface area of the photoconductive switch. This dimensional change from 0D point focus to 2D area illumination reduces peak irradiance while maintaining total light energy.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Use of energy by moving object

If light is concentrated to a small area to improve coupling efficiency, then light coupling efficiency is improved, but irradiance exceeds the damage threshold causing electrical breakdown

Engineering Contradiction:

Improvelight coupling efficiencyVSAvoidirradiance damage

Core Design Contradiction:

Use of energy by moving objectVSObject-affected harmful factors

Solution Approach 1:

The frustum structure segments the concentrated light path into multiple reflection paths, distributing the light energy across different spatial locations. This segmentation of the optical path allows efficient light coupling while preventing concentration of energy at any single point that would cause damage.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The frustum changes the spatial distribution parameter of the light by transforming a concentrated beam into a distributed pattern. The geometric parameters of the frustum (angles, dimensions) are optimized to achieve the desired balance between coupling efficiency and irradiance reduction.

Inventive Principle:

Principle #35Parameter changes

3Object-affected harmful factors

If the illumination area is increased to reduce irradiance, then electrical breakdown is prevented, but light coupling efficiency decreases due to light escape

Engineering Contradiction:

Improveelectrical breakdown preventionVSAvoidlight coupling efficiency

Core Design Contradiction:

Object-affected harmful factorsVSUse of energy by moving object

Solution Approach 1:

The frustum performs preliminary light redistribution before the light reaches the photoconductive switch surface. By pre-distributing the light in controlled reflection paths, it ensures that the light arrives at the switch area-optimized for both coupling efficiency and safe irradiance levels, preventing the need for post-adjustment.

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

This approach allows for efficient light utilization, minimizing light escape and increasing switching accuracy and high electric field capabilities, ensuring reliable operation in demanding applications without electrical breakdown.

Implementation Method 1

the truncated cone and the cylinder are configured to propagate the light from the upper surface of the truncated cone to the lower surface of the cylinder using total internal reflection

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Implementation Method 2

illuminate a photoconductive (or semiconductor) switch

Methodology Applied

Scientific EffectPhotoconductivity: Photoconductivity

Data Source

PatentUS11555965B1Illumination frustums for photoconductive switches

Publication Date: 2023.01.17 LAWRENCE LIVERMORE NAT SECURITY LLC

AI summary

Methods and devices for illuminating a photoconductive switch consisting of an optically actuated photoconductive material situated between two electrodes are described. Light from a light source is coupled to an optical fiber, which is attached to a frustum, the other side of which is proximate to the photoconductive switch. Light from the optical fiber enters the frustum, spreads out, and enters the photoconductive switch via the top-side electrode. Some of the light is absorbed, while the remaining light reflects off the bottom-side electrode, travels back through the photoconductive switch, and any unabsorbed light reenters the frustum. The geometry of the frustum is configured such that most of the light reflects back into the switch itself with only a negligible fraction escaping from the optical fiber, which advantageously results in near total utilization of the light.