Cavity Ring-Down Spectroscopy Backward Mode Noise Reduction

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Solution Overview

Problem

In cavity ring-down spectroscopy using a traveling wave cavity, the excitation of the backward mode introduces noise and variability in ring-down time measurements due to mode coupling, leading to biased results and frequency-dependent interference effects when only the forward mode is monitored.

Innovation Solution

Monitoring and summing both forward and backward mode intensities during ring-down measurements, either using separate detectors or a single detector with optical interferometry, to perform an exponential fit on the sum of intensities, reducing noise and variability, and optionally using an optical circulator to separate source radiation from the backward mode signal.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If only the forward mode is monitored during ring-down measurements, then the measurement system is simpler, but noise and variability in ring-down time measurements increase due to mode coupling effects

Engineering Contradiction:

Improvemeasurement system complexityVSAvoidring-down time measurement precision

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent combines the monitoring of both forward and backward modes into a single measurement system. By summing the intensities of both modes, the system achieves more precise ring-down time measurements while managing the complexity through integrated detection architecture.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent introduces an optical circulator as an intermediary component to separate and direct the backward mode signal to the detector. This mediator enables the inclusion of backward mode information without fundamentally complicating the core measurement system, as the circulator cleanly isolates and routes signals.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Device complexity

If the backward mode is neglected in conventional CRDS operation, then the analysis is simpler, but biased results and frequency-dependent interference effects occur

Engineering Contradiction:

Improvedata analysis complexityVSAvoidcavity loss measurement accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent converts the previously harmful backward mode into a beneficial component of the measurement. By intentionally monitoring and summing the backward mode intensity with the forward mode, the patent eliminates bias and frequency-dependent interference effects, transforming a source of error into a source of improved accuracy.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Ease of operation

If mode coupling effects are not accounted for, then the measurement process is simpler, but noise and variability in successive ring-down time measurements increase

Engineering Contradiction:

Improvemeasurement process simplicityVSAvoidmeasurement consistency

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent implements a feedback mechanism where the backward mode signal is continuously monitored and summed with the forward mode signal. This feedback loop ensures that mode coupling effects are continuously compensated for, maintaining consistent and reliable measurements across successive ring-down time measurements.

Inventive Principle:

Principle #23Feedback

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 significantly reduces noise and variability in ring-down time measurements, providing more accurate cavity loss spectra by accounting for mode coupling effects, resulting in improved precision and reduced frequency-dependent noise.

Implementation Method 1

The decay rate of this emitted radiation is related to the loss in the cavity (lower loss leads to slower decay). Typically, an exponential decay is fitted to the measured radiation intensity to determine the ring-down time.

Methodology Applied

Scientific EffectExponential decay:

Implementation Method 2

the (monochromatic) excitation has been turned off

Methodology Applied

Scientific EffectMonochromatic radiation emission:

Implementation Method 3

an interferometer is provided which combines the forward and backward mode optical signals to produce an interferogram

Methodology Applied

Scientific EffectOptical interference: Interference

Implementation Method 4

an optical detection unit that receives forward mode and backward mode optical signals from the cavity and provides one or more electrical detector signals

Methodology Applied

Scientific EffectPhotoelectric detection: Photoelectric Effect

Data Source

PatentUS9116042B1Cavity ring down spectroscopy using measured backward mode data

Publication Date: 2015.08.25 PICARRO INC

AI summary

In cavity ring-down spectroscopy (CRDS), scattering into the backward mode of a traveling wave ring-down cavity can degrade conventional CRDS performance. We have found that this performance degradation can be alleviated by measuring the backward mode signal emitted from the ring-down cavity, and using this signal to improve the processing for extracting ring-down times from the measured data. For example, fitting an exponential to the sum of the intensities of the forward and backward signals often provides substantially better results for the ring-down time than fitting an exponential to the forward signal alone. Other possibilities include extracting cavity eigenmode signals from the forward and backward signals and performing separate exponential fits to the eigenmode signals. An optical circulator can be used to facilitate measurement of the backward mode signal.