The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Continuous wave (cw) diode lasers are increasingly being used as light sources in the visible and near-IR regions of the spectrum for cavity ring-down spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS); the latter technique is also widely known as integrated cavity output spectroscopy (ICOS). The very high sensitivities to weak absorptions that are possible with cw CRDS and CEAS, coupled with the quantitative nature of the absorption measurements, are enabling a rapidly expanding range of applications. We review the benefits and practical implementation of these techniques; methods of data analysis for extraction of quantitative absorption data; the sensitivities of cw CRDS and CEAS, and how they might be optimised; and applications of cw CRDS and CEAS in molecular spectroscopy, atmospheric chemistry, plasma and flame chemistry, analytical science, and medical diagnosis via breath analysis. The development of CRDS and CEAS techniques exploiting cw diode lasers and, very recently, high luminosity light-emitting diodes, has stimulated a wealth of high-sensitivity measurements. Highlights include quantitative measurement of various ultra-trace gases such as: NO3, NO2 and ethene in ambient air samples; CO2 isotopologues, ethane and other organic compounds in human breath samples; and excited electronic states of N2 and O2 in plasmas and discharges. Exciting developments include wavelength extension into the mid-IR and UV regions, and use of novel locked-cavity techniques to increase data acquisition rates and sensitivities.

4 Cavity ring-down and cavity enhanced spectroscopy using diode lasers

A simplified method for measuring the effective photon lifetime in an optical resonator was developed. The technique requires the passage of a modulated continuous-wave laser beam through the resonator and the measurement of the resultant shift in the phase of the transmitted intensity. The method not only permits a quick and precise measurement of the mirror reflectances, but also permits these measurements to be in situ. Such an 'on the spot' evaluation capability should be extremely useful in applications ranging from the investigation of new laser systems to the development of improved optical coatings. The method is also sensitive to the effects of absorption, scattering, and transmission from elements in the cavity. Cavity losses smaller than 100 ppm were detected.

Sensitive measurement of photon lifetime and true reflectances in an optical cavity by a phase shift method. Technical report

An historical overview of laser-based, spectroscopic methods that employ high-finesse optical resonators is presented. The overview begins with the early work in atomic absorption (1962) and optica...

An historical overview of cavity-enhanced methods

Retinal ganglion cells that respond selectively to a dark spot on a brighter background (OFF cells) have smaller dendritic fields than their ON counterparts and are more numerous. OFF cells also branch more densely, and thus collect more synapses per visual angle. That the retina devotes more resources to processing dark contrasts predicts that natural images contain more dark information. We confirm this across a range of spatial scales and trace the origin of this phenomenon to the statistical structure of natural scenes. We show that the optimal mosaics for encoding natural images are also asymmetric, with OFF elements smaller and more numerous, matching retinal structure. Finally, the concentration of synapses within a dendritic field matches the information content, suggesting a simple principle to connect a concrete fact of neuroanatomy with the abstract concept of information: equal synapses for equal bits.

https://www.pnas.org/content/pnas/107/40/17368.full.pdf

Retina is structured to process an excess of darkness in natural scenes

A novel measurement principle for fiber-optic sensing is presented. Use of a cavity-ring-down scheme enables measurements of minute optical losses in high-finesse fiber-optic cavities. The loss may be induced by evanescent-field absorption, fiber bending, fiber degradation, Bragg gratings, or any other effect that might change the fiber transmission or cavity reflector properties. The principle is proved to be rather insensitive to ambient perturbations such as temperature changes. A high-sensitivity measurement of loss due to bending is presented as a proof-of-principle. With a cavity finesse of 627 a sensitivity for induced loss of 108 ppm (4.68 × 10-4 dB) is achieved. Preliminary measurements of evanescent-field absorption are also discussed.

Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing.

The invention relates to a method and to an optical device in all-optical signal processing It provides a novel way of taking into use the potential bandpass filtering capabilities of an optical complex one-pole resonator by, 1) excitating with at least part of the input signal an optical resonator arrangement that comprises two substantially parallel, independent, complex one-pole resonators arranged in a manner that one of said resonators is matched, and the other one is non-matched with the input signal, and 2) based on polarization separating further from the output of said optical resonator arrangement at least one optical output signal so that both said matched and non-matched resonators contribute to the formation of said output signal The invention can be utilized, for example, for optical signal analysis, optical clock recovery, or for producing high-frequency outputs from lower frequency input signals, like eg optical microwave generation

All-optical signal processing method and device

Time constant extraction from noisy cavity ring-down signals

We present a new method for quantification of minute birefringence in high-finesse resonators. The method is based on observing the homodyne polarization mode beat at the output of the resonator. We show that the mode beat is generated by a phase mismatch of a polarization mode in the cavity and that the magnitude of the birefringence is proportional to the beat frequency. We demonstrate the sensitivity of the technique by measuring polarization properties of a twisted 0.275 m long single-mode fiber cavity. Maximum beat length of the fiber was found to be 10.6 m, which is almost 40 times longer than the length of the studied fiber.

Resonator based measurement technique for quantification of minute birefringence.

Light from a light source (1) is fed into a resonator comprising an resonator fiber (3) and highly reflective couplers (4, 5). Light coupled out of the resonator fiber (3) is detected by a light sensor (7). The resonator is built such that its losses depend on a physical parameter to be measured. The light fed to the light source is switched on and off in step-like manner and the corresponding build-up or decay of the light detector signal is used to determine the time constant of the resonator and therefrom the physical parameter. It is found that, even when light of a comparatively broad bandwidth is used, accurate measurements of the time constant are possible.

Tuomo von Lerber

Papers

Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing.

All-optical signal processing method and device

Time constant extraction from noisy cavity ring-down signals

Resonator based measurement technique for quantification of minute birefringence.

Fiber optic sensor with an optical resonator