Laser Dispersion Spectroscopy

Laser Dispersion Spectroscopy2019-01-10T11:22:36+00:00

Laser Dispersion Spectroscopy (LDS) takes a completely new approach to tuneable semiconductor laser-based gas sensing. Laser absorption spectroscopy (LAS) systems rely on probing the absorption signal characteristics of a gas sample and are fundamentally limited to the measurement of small signal changes (absorption) on top of a large background (total light intensity arriving at the detector). This limits the the dynamic range of the acquisition system, impacting the detectable concentration range, precision and sensitivity of the measurement.  MIRICO’s patented LDS technique measures molecular dispersion, which can be thought of as changes in the phase of light as opposed to intensity, and uses a differential approach which eliminates the background, overcoming a key limitation of standard tuneable diode laser spectroscopy techniques. Furthermore, as the signal is encoded in the phase of the light as opposed to intensity, the measurement is resilient to “dirty” environments that affect light intensity such as soot, rain, water vapour or fog.

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Our Approach in Brief

  • Zero background measurement for enhanced sensitivity, selectivity, dynamic range and accuracy
  • Linear response over a broad dynamic range, measuring over 5 orders of magnitude of concentration
  • Resilient to “dirty” environments such as fog, rain, soot and water vapour

Molecules possess strongest fundamental ro-vibration bands, and the region exhibits two wide spectral windows, which enables:

  • Ultra high sensitivity at trace levels (parts per billion/ parts per trillion depending on gas)
  • Minimal interference in spectral windows, typically arising from water vapour
  • Chemical sensing of a broad range of gases

Versus Conventional TDLAS

Laser Absorption Spectroscopy

In conventional tuneable diode laser spectroscopy, light intensity absorbed by molecular transitions of the specific gas being analysed is measured to derive concentration. As such the signal is encoded in the intensity of the light.

Laser Dispersion Spectroscopy

A frequency shifter forms a secondary, slightly frequency offset, beam. Each of the two colour beams experiences a slightly different change in refractive index (or phase shift) as they pass through the sample, induced by the molecular transition. Frequency demodulation of the two beams is used to derive concentration.

The mid-IR

CO2 Absorption Spectrum

Quantum Cascade Lasers

The advent of semiconductor lasers capable of emitting light in the mid-infrared (mid-IR) region have significantly improved the accuracy and precision of molecular gas sensing. MIRICO’s Laser Dispersion Spectroscopy technique takes full advantage of such lasers, including (but not limited to) Quantum Cascade Lasers (QCLs) and Interband Cascade Lasers (ICLs).

Fundamental Bands

The mid-IR spectral region is where most molecules posses their strongest fundamental ro-vibrational bands, which allows MIRICO to target a broad range of molecules and achieve high precision, sensitivity and selectivity (avoiding interferences form other species).

Atmospheric Window

In addition to the mid-IR offers two wide spectral windows, enabling molecular sensing with minimal spectral interference (primarily from water vapour). For these reasons, the mid-IR is optimum for gas-phase chemical detection at trace (ultra high sensitivity and selectivity) levels.

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