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Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range: demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9  ×  10−14 cm−1
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2015 (English)In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 32, no 10, 2104-2114 p.Article in journal (Refereed) Published
Abstract [en]

Expressions for the in-coupling of white noise and etalons into fitted signal strengths are derived. These show that the amount of noise picked up is affected by the scanning range. A methodology for finding the optimum scanning range from a single set of measurements has been developed. This was used to estimate the optimum conditions of a noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup. The methodology was validated by measurements. This resulted in a spectral noise equivalent absorption per unit length of 2.6 × 10−13 cm−1 Hz−1∕2 and a minimum Allan deviation of 9 × 10−14 cm−1 at 30 s, which are, to our knowledge, the lowest reported for Doppler-broadened NICE-OHMS

Place, publisher, year, edition, pages
Optical Society of America, 2015. Vol. 32, no 10, 2104-2114 p.
Keyword [en]
Absorption, Spectroscopy - heterodyne, Spectroscopy - modulation, Spectroscopy - molecular
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:umu:diva-110081DOI: 10.1364/JOSAB.32.002104ISI: 000367235700012OAI: oai:DiVA.org:umu-110081DiVA: diva2:861080
Available from: 2015-10-15 Created: 2015-10-15 Last updated: 2016-01-25Bibliographically approved
In thesis
1. Cavity enhanced optical sensing
Open this publication in new window or tab >>Cavity enhanced optical sensing
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Kavitetsförstärkt optisk detektion
Abstract [en]

An optical cavity comprises a set of mirrors between which light can be reflected a number of times. The selectivity and stability of optical cavities make them extremely useful as frequency references or discri­mi­nators. With light coupled into the cavity, a sample placed inside a cavity will experience a significantly increased interaction length. Hence, they can be used also as amplifiers for sensing purposes. In the field of laser spectroscopy, some of the most sensitive techniques are therefore built upon optical cavities. In this work optical cavities are used to measure properties of gas samples, i.e. absorption, dispersion, and refractivity, with unprecedented precision.

The most sensitive detection technique of all, Doppler-broadened noise-immune cavity enhanced optical heterodyne molecular spectrometry (Db NICE-OHMS), has in this work been developed to an ultra-sensitive spectroscopic technique with unprecedented detection sensitivity. By identifying limiting factors, realizing new experimental setups, and deter­mining optimal detection conditions, the sensitivity of the technique has been improved several orders of magnitude, from 8 × 10-11 to 9 × 10-14 cm-1. The pressure interval in which NICE-OHMS can be applied has been extended by deri­vation and verification of dispersions equations for so-called Dicke narrowing and speed dependent broadening effects. The theoretical description of NICE-OHMS has been expanded through the development of a formalism that can be applied to the situations when the cavity absorption cannot be considered to be small, which has expanded the dynamic range of the technique. In order to enable analysis of a large number of molecules at their most sensitive transitions (mainly their funda­mental CH vibrational transitions) NICE-OHMS instrumentation has also been developed for measurements in the mid-infrared (MIR) region. While it has been difficult to realize this in the past due to a lack of optical modulators in the MIR range, the system has been based on an optical para­metric oscillator, which can be modulated in the near-infrared (NIR) range.

As the index of refraction can be related to density, it is possible to retrieve gas density from measurements of the index of refraction. Two such instru­men­tations have been realized. The first one is based on a laser locked to a measure­ment cavity whose frequency is measured by compassion with an optical frequency comb. The second one is based on two lasers locked to a dual-cavity (i.e. one reference and one measurement cavity). By these methods changes in gas density down to 1 × 10-9 kg/m3 can be detected.

All instrumentations presented in this work have pushed forward the limits of what previously has been considered measurable. The knowledge acquired will be of great use for future ultrasensitive cavity-based detection methods.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2015. 124 p.
Keyword
Optical resonators, Fiber Laser, Parametric oscillators, Optical frequency comb, Infrared, Spectroscopy heterodyne, Spectroscopy molecular, Absorption, Dispersion, Lineshapes, Optical standards and testing, Refractivety measurements
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:umu:diva-110278 (URN)978-91-7601-338-0 (ISBN)
Public defence
2015-11-13, KBC-huset, KB3A9 (lilla hörsalen i KBC-huset), Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilThe Kempe Foundations
Available from: 2015-10-23 Created: 2015-10-19 Last updated: 2015-10-29Bibliographically approved

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Silander, IsakHausmaninger, ThomasAxner, Ove
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