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Related Concept Videos

Standing Waves in a Cavity01:28

Standing Waves in a Cavity

A household microwave and lasers are examples of standing electromagnetic waves in a cavity. When two conducting metal plates are placed parallel at the nodal planes, it creates a cavity where standing waves are formed. The cavity between the two planes is analogous to a stretched string held at the points x = 0 and x = L. Here, the distance 'L' between the two planes must be an integer multiple of half of the wavelength. The wavelengths that satisfy this condition are given by:
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
Double Resonance Techniques: Overview01:12

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved in...
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.

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Related Experiment Video

Updated: Jun 22, 2026

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities
11:08

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Published on: November 30, 2012

Noise in cavity ring-down spectroscopy caused by transverse mode coupling.

Haifeng Huang, Kevin Lehmann

    Optics Express
    |June 24, 2009
    PubMed
    Summary

    Mode coupling in continuous wave cavity ring-down spectroscopy (CW-CRDS) causes instabilities. Suppressing higher-order transverse modes with an aperture greatly improves system stability and sensitivity.

    Area of Science:

    • Physics
    • Spectroscopy
    • Optical Engineering

    Background:

    • Continuous wave cavity ring-down spectroscopy (CW-CRDS) is a sensitive technique for measuring light absorption.
    • Instabilities in CW-CRDS systems, observed as drops in decay time constant, reduce measurement sensitivity.
    • These instabilities are linked to the excitation of higher-order transverse modes in the optical cavity.

    Purpose of the Study:

    • To investigate the cause of instabilities in CW-CRDS experiments.
    • To understand the coupling mechanism between the fundamental (TEM00) mode and higher-order transverse modes.
    • To identify methods for improving CW-CRDS system stability and sensitivity.

    Main Methods:

    • Utilized continuous wave cavity ring-down spectroscopy (CW-CRDS) experiments.

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  • Analyzed cavity decay time constants at varying cavity lengths and intercavity pressures.
  • Employed a two-mode beating model to fit non-single exponential decay signals.
  • Introduced an intra-cavity aperture to suppress higher-order modes.
  • Recorded images of excited higher-order transverse modes using an Indium Gallium Arsenide (InGaAs) camera.

    • Main Results:

    • Observed significant drops in decay time constant at specific cavity lengths and pressures, indicating instabilities.
    • Identified coupling between the TEM00 mode and higher-order transverse modes as the cause of instabilities and non-exponential decays.
    • Demonstrated that an intra-cavity aperture significantly suppresses higher-order modes, improving system stability.
    • Showed that mirror surface scattering can be a coupling mechanism, with scattering intensities as low as ~10⁻¹² affecting decay rates.
    • Observed a tuning rate between modes during cavity pressure scans that exceeded theoretical predictions.

    Conclusions:

    • Mode coupling is a primary source of instability in CW-CRDS, degrading system sensitivity.
    • Intra-cavity apertures are effective in mitigating instabilities by suppressing higher-order modes.
    • Scattering from mirror surfaces plays a role in mode coupling, but an unexplained discrepancy exists in the predicted tuning rates during pressure scans.