Technology
Principle of Operation & Technical Documents
Cavity Ring-Down Spectroscopy (CRDS)
Detection at the Speed of Light
Based on absorption spectroscopy, Cavity Ring-Down Spectroscopy (CRDS) works by attuning light rays to the unique molecular fingerprint of the sample species. By measuring the time it takes the light to fade or "ring-down", you receive an accurate molecular count in milliseconds. The time of light decay, in essence, provides an exact, non-invasive, and rapid means to detect contaminants in the air, in gases, and even in the breath.
A breakthrough discovery by Professor Kevin Lehmann, Ph.D., of Princeton University made the commercialization of this technique possible. He proved that compact, relatively inexpensive, and widely available Continuous Wave (CW) lasers can substitute for the costly, cumbersome pulsed lasers previously used in CRDS-based research. He thereby made the requisite power of light affordable and practical for commercial use.
How It Works:
- A Continuous Wave (CW) diode laser emits a directed beam of light energy through an ultra-high reflective mirror into the absorption cell (cavity).
- The light reflects back and forth between two ultra-high reflective mirrors multiple times, up to a total path length of 100 kilometers.
- Once the photodiode detector “sees” a preset level of light energy, the light source is shuttered or diverted from the cavity.
- On each successive pass, a small amount of light or ring-down signal emits through the second mirror and is sensed by the light detector.
- Once the light "rings down", the detector achieves a point of zero light energy in milliseconds, and the measurement is complete.
CRDS Schematic
The key components illustrated are as follows:
- Diode laser: Emits light energy
- Isolator: Prevents light energy feed-back from interfering with the laser Acousto-Optical
- Modulator: Shuttering device for the light source
- Absorption Cell: With mirrors, creates measure-ment cavity
- Photodiode: Monitors the light energy from the absorption cell
- Trigger: Works in concert with the photo-diode and sends signal to the AOM to activate the ring-down cycle.
Principle of CRDS
- The computer-controlled system tunes the laser off the absorption peak for the sample species to determine the τ empty value, equivalent to a zero baseline correction.
- It tunes back to the absorption peak to determine the τ value, dependent on the sample species concentration.
- The concentration of the sample species is directly calculated using the above equations.
- Based on Beer’s Law, this value constitutes an absolute measurement and is unaffected by losses outside the ring-down cavity.
Ring Down/ Exponential Decay
This graph depicts the concept of ring-down decay within the cavity after the laser source is shuttered. As the laser light bounces back and forth between the ultra-high reflective mirrors, the sample species absorbs the light energy until it’s all gone.
Cavity Ring-Down Spectroscopy (CRDS)
Detection at the Speed of Light
Based on absorption spectroscopy, Cavity Ring-Down Spectroscopy (CRDS) works by attuning light rays to the unique molecular fingerprint of the sample species. By measuring the time it takes the light to fade or "ring-down", you receive an accurate molecular count in milliseconds. The time of light decay, in essence, provides an exact, non-invasive, and rapid means to detect contaminants in the air, in gases, and even in the breath.
A breakthrough discovery by Professor Kevin Lehmann, Ph.D., of Princeton University made the commercialization of this technique possible. He proved that compact, relatively inexpensive, and widely available Continuous Wave (CW) lasers can substitute for the costly, cumbersome pulsed lasers previously used in CRDS-based research. He thereby made the requisite power of light affordable and practical for commercial use.
How It Works:
- A Continuous Wave (CW) diode laser emits a directed beam of light energy through an ultra-high reflective mirror into the absorption cell (cavity).
- The light reflects back and forth between two ultra-high reflective mirrors multiple times, up to a total path length of 100 kilometers.
- Once the photodiode detector “sees” a preset level of light energy, the light source is shuttered or diverted from the cavity.
- On each successive pass, a small amount of light or ring-down signal emits through the second mirror and is sensed by the light detector.
- Once the light "rings down", the detector achieves a point of zero light energy in milliseconds, and the measurement is complete.
CRDS Schematic
The key components illustrated are as follows:
- Diode laser: Emits light energy
- Isolator: Prevents light energy feed-back from interfering with the laser Acousto-Optical
- Modulator: Shuttering device for the light source
- Absorption Cell: With mirrors, creates measure-ment cavity
- Photodiode: Monitors the light energy from the absorption cell
- Trigger: Works in concert with the photo-diode and sends signal to the AOM to activate the ring-down cycle.
Principle of CRDS
- The computer-controlled system tunes the laser off the absorption peak for the sample species to determine the τ empty value, equivalent to a zero baseline correction.
- It tunes back to the absorption peak to determine the τ value, dependent on the sample species concentration.
- The concentration of the sample species is directly calculated using the above equations.
- Based on Beer’s Law, this value constitutes an absolute measurement and is unaffected by losses outside the ring-down cavity.
Ring Down/ Exponential Decay
This graph depicts the concept of ring-down decay within the cavity after the laser source is shuttered. As the laser light bounces back and forth between the ultra-high reflective mirrors, the sample species absorbs the light energy until it’s all gone.
| Title / Description | |
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Ammonia Takes the Stage |
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Ammonia the Building Block |
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Applied Spectroscopy H2O in PH3 by CRDS |
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Clean Fuel Starts with Clean Gas |
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Consistent HCl Purity Delivery through the use of a Built-in Cylinder Purifier (MegaBIP® HCl) |
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CRDS Becomes Method of Choice for SEMI F112 |
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Detection of Trace Water Vapor in High-Purity Phosphine Using Cavity Ring-down Spectroscopy |
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Development of reliable technique for evaluating the properties of water vapor barriers |
Open external link |
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EURAMET Compares Standards in Moisture Measurement at Leading Institutes |
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Gas Analyzers, HALO and Prismatic |
Open external link |
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High-brightness LEDs: a rising tide lifts many boats |
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Hydrogen Bromide Cylinder Gas ppb-level Water Vapor Measurement |
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Hydrogen Fuel--Product Specification |
Open external link |
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Improvement of Flow and Pressure Controls in Diffusion-Tube Humidity Generator: Performance Evaluation of Trace-Moisture Generation Using CRDS |
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Laboratory Measurements and Theoretical Calculations of O2 A Band Electric Quadrupole Transitions |
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Laser-Based Analyzers - Shining New Stars |
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Lotus Consulting Presents: Hydrogen Fuel Analyzer |
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Meeting the needs of EUV lithography |
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Methods for the analysis of trace-level impurities in hydrogen for fuel cell applications |
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Parts-Per-Trillion Moisture Measurement Using Cavity Ring-Down Spectroscopy |
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Performance Evaluation of a Trace-Moisture Analyzer Based on CRDS_Feb 2011 |
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Qualitative Comparison of Cavity Ring-Down vs. Direct Measurement Absorption Spectroscopy for Determining ppb Moisture Levels in UHP Gases |
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Response of a Ring-Down Cavity to an Arbitrary Excitation |
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Spectroscopic Line Parameters of Water Vapor for Rotation-vibration Transitions Near 7180 cm-1 |
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Spectroscopy Exposes Trace-Water Contamination in Process Gases |
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Standard Test Method for Hydrogen Purity Analysis Using a Continuous Wave Cavity Ring-Down Spectroscopy Analyzer |
Open external link |
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The Hard Truth About Moisture in Electronic Grade Oxygen |
Open external link |
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The Superposition Principle and Cavity Ring-Down Spectroscopy |
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Tiger Optics Analyzers for Semi (Handout) |
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Tiger Optics Hails Section of CRDS for New Industry Standard |
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Timed to Perfection |
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White Paper: Monitoring of HCl |
Open external link |
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Analyzer Specifications Technical Bulletin |
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Calibration of CW CRDS analyzers |
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Electrostatic Discharge (PDF) |
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Laser Locking and Optimization (PDF) |
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Laser Safety |
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Moisture Measurement Best Practices Review |
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Stability of Zero Calibration |
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Tiger Optics Takes on the Servomex DF-749 |
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DFB Diode Laser Based Sensor for Isotope Ratio Detection of Methane using Continuous Wave CRDS |
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High Performance Gas Analysis with Continuous-Wave Cavity Ring-Down Spectroscopy on a New Generation Low-Cost Platform |
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Trace Detection of Carbon Monoxide in Hydrocarbon Feedstock Processing Using Continuous-Wave Cavity Ring-Down Spectroscopy (CW-CRDS) |
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Trace Water Vapor Analysis Using Cavity Ring Down Spectroscopy, Oscillator Quartz Crystal and Impedance Sensor Technologies |
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