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:

  1. A Continuous Wave (CW) diode laser emits a directed beam of light energy through an ultra-high reflective mirror into the absorption cell (cavity).
  2. The light reflects back and forth between two ultra-high reflective mirrors multiple times, up to a total path length of 100 kilometers.
  3. Once the photodiode detector “sees” a preset level of light energy, the light source is shuttered or diverted from the cavity.
  4. 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.
  5. 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:

  1. Diode laser: Emits light energy
  2. Isolator: Prevents light energy feed-back from interfering with the laser Acousto-Optical
  3. Modulator: Shuttering device for the light source
  4. Absorption Cell: With mirrors, creates measure-ment cavity
  5. Photodiode: Monitors the light energy from the absorption cell
  6. Trigger: Works in concert with the photo-diode and sends signal to the AOM to activate the ring-down cycle.

Principle of CRDS
  1. 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.
  2. It tunes back to the absorption peak to determine the τ value, dependent on the sample species concentration.
  3. The concentration of the sample species is directly calculated using the above equations.
  4. 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

"Big Physics"

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CH4 Emissions Monitoring

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CO-rekt: Hydrogen Purity Analysis in Hydrogen-Carbon Monoxide Plants

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Cost-Effective Purity Analysis in the Cryogenic Air Separation Process

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CRDS Becomes Method of Choice for F112

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Fuel Cell Hydrogen Purity Analysis

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HCl Continuous Emissions Monitoring

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HF Monitoring for Aluminum Smelters

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Hydrogen Purity Analysis in Hydrogen-Carbon Monoxide Plants

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Moisture Measurement in the Industrial Gas Industry

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Moisture Measurement in the Packaged Gas - Quality Control Laboratory

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Moisture Monitoring in Low-Temperature Silicon-Germanium Epitaxy

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NH3 Emissions Monitoring

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Selective Catalytic Reduction-Ammonia Slip

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Use of Tiger Optics’ HALO for Accurate Online Analysis of Tube Trailer Filling

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Title / Description

Calibration of CW CRDS analyzers

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Electrostatic Discharge

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Laser Locking and Optimization

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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 Analyzer Specifications Technical Bulletin

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Tiger Optics Takes on the Servomex DF-749

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Title / Description

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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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:

  1. A Continuous Wave (CW) diode laser emits a directed beam of light energy through an ultra-high reflective mirror into the absorption cell (cavity).
  2. The light reflects back and forth between two ultra-high reflective mirrors multiple times, up to a total path length of 100 kilometers.
  3. Once the photodiode detector “sees” a preset level of light energy, the light source is shuttered or diverted from the cavity.
  4. 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.
  5. 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:

  1. Diode laser: Emits light energy
  2. Isolator: Prevents light energy feed-back from interfering with the laser Acousto-Optical
  3. Modulator: Shuttering device for the light source
  4. Absorption Cell: With mirrors, creates measure-ment cavity
  5. Photodiode: Monitors the light energy from the absorption cell
  6. Trigger: Works in concert with the photo-diode and sends signal to the AOM to activate the ring-down cycle.

Principle of CRDS
  1. 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.
  2. It tunes back to the absorption peak to determine the τ value, dependent on the sample species concentration.
  3. The concentration of the sample species is directly calculated using the above equations.
  4. 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

"Big Physics"
HALO 3, LaserTrace 3, Prismatic, VROOM

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CH4 Emissions Monitoring
Tiger-i 2000 CH4

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CRDS Becomes Method of Choice for F112
HALO 3 H2O, HALO KA H2O, LaserTrace 2.5 H2O, LaserTrace 3 H2O

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Ensuring Purity in the Cryogenic Air Separation Process
HALO 3 CO2

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Fuel Cell Hydrogen Purity Analysis
HALO 3, LaserTrace 3, Prismatic, VROOM

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HCl Continuous Emissions Monitoring
Tiger-i 2000

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HF Monitoring for Aluminum Smelters
Tiger-i 2000

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Hydrogen Purity Analysis in Hydrogen-Carbon Monoxide Plants
CO-rekt

Download file

Moisture Measurement in the Industrial Gas Industry
HALO-500-H2O

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Moisture Measurement in the Packaged Gas - Quality Control Laboratory
HALO-500-H2O

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NH3 Emissions Monitoring
Tiger-i 2000 NH3

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Selective Catalytic Reduction-Ammonia Slip
Tiger-i 2000

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Use of Tiger Optics’ HALO for Accurate Online Analysis of Tube Trailer Filling
HALO

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Title / Description

Ammonia Takes the Stage
Specialty Gas Report

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Ammonia the Building Block
December 2010 (PDF) ~ Gasworld

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An Evaluation of Performance of Trace Moisture Measurement Methods
NPL, BOC Edwards

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Applied Spectroscopy H2O in PH3 by CRDS
Applied Spectroscopy, Volume 61, 2007

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CH4 Emissions Monitoring
Tiger Optics, July, 2013

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Clean Fuel Starts with Clean Gas
June 6th, 2014 ~ Tiger Optics

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Consistent HCl Purity Delivery through the use of a Built-in Cylinder Purifier (MegaBIP® HCl)
Air Products

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Detection of Trace Water Vapor in High-Purity Phosphine Using Cavity Ring-down Spectroscopy
Applied Spectroscopy

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Development of reliable technique for evaluating the properties of water vapor barriers
AIP Advances, November, 2015

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EURAMET Compares Standards in Moisture Measurement at Leading Institutes
NPL

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Fuel Cell Hydrogen Purity Analysis
July 30th, 2013 ~ Tiger Optics HALO 3, LaserTrace 3, Prismatic, VROOM | PDF

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Gas Analyzers, HALO and Prismatic
R&D Magazine

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High-brightness LEDs: a rising tide lifts many boats
Global LEDs/OLEDs

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Hydrogen Bromide Cylinder Gas ppb-level Water Vapor Measurement
Solid State Technology

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Hydrogen Fuel--Product Specification
ISO/TS

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Improvement of Flow and Pressure Controls in Diffusion-Tube Humidity Generator: Performance Evaluation of Trace-Moisture Generation Using CRDS
ScienceDirect

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Laboratory Measurements and Theoretical Calculations of O2 A Band Electric Quadrupole Transitions
Physical Review

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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
Cleanroom Technology, August 2012

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Methods for the analysis of trace-level impurities in hydrogen for fuel cell applications
NPL

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NH3 Emissions Monitoring
Tiger Optics, July 2013

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Parts-Per-Trillion Moisture Measurement Using Cavity Ring-Down Spectroscopy
Gases & Technology

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Performance Evaluation of a Trace-Moisture Analyzer Based on CRDS_Feb 2011
Science Direct

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Qualitative Comparison of Cavity Ring-Down vs. Direct Measurement Absorption Spectroscopy for Determining ppb Moisture Levels in UHP Gases
Gases & Technology

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Response of a Ring-Down Cavity to an Arbitrary Excitation
Journal of Chemical Physics

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SEMI F112 Application Note
Tiger Optics, August, 2013

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Spectroscopic Line Parameters of Water Vapor for Rotation-vibration Transitions Near 7180 cm-1
Physical Review

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Spectroscopy Exposes Trace-Water Contamination in Process Gases
October, 2007, Compound Semiconductor

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Standard Test Method for Hydrogen Purity Analysis Using a Continuous Wave Cavity Ring-Down Spectroscopy Analyzer
ASTM International D7941

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The Hard Truth About Moisture in Electronic Grade Oxygen
Gasworld

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The Superposition Principle and Cavity Ring-Down Spectroscopy
Journal of Chemical Physics

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Tiger Optics Analyzers for Semi (Handout)
Tiger Optics, July, 2014

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Tiger Optics Hails Section of CRDS for New Industry Standard

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Timed to Perfection
Specialty Gas Report

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White Paper: Monitoring of HCl
Institute of Clean Air Companies 2013

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Title / Description

Analyzer Specifications

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Calibration of CW CRDS analyzers

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Electrostatic Discharge (PDF)

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HALO KA H2O Specifications Technical Bulletin

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Laser Locking and Optimization (PDF)

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Laser Safety

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Moisture Measurement Best Practices Review

Download file

Stability of Zero Calibration

Download file

Tiger Optics Takes on the Servomex DF-749
Following tests done by the National Metrology Institute of Japan, we compare the DF-749 to two analyzers from Tiger Optics LLC that excel in the analysis of ultra-high purity gases.

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Title / Description

DFB Diode Laser Based Sensor for Isotope Ratio Detection of Methane using Continuous Wave CRDS
Shaoyue L. Yang-2/2013

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High Performance Gas Analysis with Continuous-Wave Cavity Ring-Down Spectroscopy on a New Generation Low-Cost Platform
Yu Chen and Erika Coyne, 2015, Tiger Optics, LLC

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Trace Detection of Carbon Monoxide in Hydrocarbon Feedstock Processing Using Continuous-Wave Cavity Ring-Down Spectroscopy (CW-CRDS)
Yu Chen and Erika Coyne, 2015, Tiger Optics, LLC

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Trace Water Vapor Analysis Using Cavity Ring Down Spectroscopy, Oscillator Quartz Crystal and Impedance Sensor Technologies
March, 2009, Matheson Tri-Gas Inc.

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