Exploiting spontaneous Raman scattering in hollow core anti-resonant and photonic bandgap fibres for simultaneous and multi-species quantitative gas sensing

Abstract

The aim of the work described in this thesis is the development of a gas sensor, capable of high sensitivity and selectivity. Particularly, the requirement was for an instrument that could detect and distinguish gases in a mixture, even the ones with low concentration (e.g. 380ppm of CO2 in ambient air). This work was carried out as part of a collaborative project with IS-instruments, a company whose expertise are the design, development and manufacturing of spectrometers for remote and compact sensor applications. A key application area identified by IS-Instruments is in gas pipelines where the pressure of the gas mixture to be tested can reach 90 bar, so a further requirement on the sensor design was the potential for high pressure operation. One of the most studied and exploited spectroscopic techniques is Raman spectroscopy, based on Raman scattering. When applied to gas sensing, it can achieve real-time and simultaneous detection of single gases in a mixture. Several research groups have tested this techinque for a wide range of applications, from the industrial field, like the one Is-instruments is interested to develop, to the medical or environmental fields, like breath analysis, or pollution control (some examples can be found in references [1-7]).

Raman scattering can be produced when a monochromatic beam hits matter, like a molecule in a gas. The scattered radiation might have the same frequency of the excitation radiation (Rayleigh scattering) or a shifted frequency (inelastic scattering) that may be caused by some change in the vibrational or rotational state of the molecule; when this happens, the scattered light is called Raman scattering. The energies required to produce such changes are fixed for the type of molecule under observation, meaning that every species needs a defined amount of energy to produce the vibrational or the rotational state change. The so-called Stokes and anti-Stokes Raman lines, shifted towards lower and higher frequency respectively from the excitation one, are the results of those energy changes and are species-specific. Thanks to this property, Raman spectroscopy can be used to distinguish one gas from another so it meets the requirement of specificity for the gas sensor we wanted to develop. However, Raman scattering depends on the scattering cross section of the molecule involved, which has an extremely low value for gases. For instance, considering Nitrogen at room temperature and pressure contained in a transparent cell in which a laser source of ~100 mW at 785 nm is focused (free-space setup), the order of magnitude of the Raman Stokes power is ~ 10-14 W, as the scattering cross section of N2 at 785nm is equal to 7.467 × 10-32 cm2/sr∙molecules [8]. A key limiting factor in the free-space experiment is the interaction length between the gas sample and the laser beam, which is just ~ twice the Rayleigh length of the focused beam [9].

A promising solution to increase this interaction length is based on the use of hollow core photonic crystal fibres (HC-PCFs). The periodic cladding structure of these type of fibres allows the guidance in the lower refractive index hollow core. The possibility to introduce the gas under analysis into the guiding core of a HC-PCF, enables long interaction lengths between the light and the sample and a much more compact device than in a free space arrangement. The Raman signal strength (Stokes power) obtainable using this technique can be hundreds of times higher than what can be obtained in free-space [1]. Furthermore, the fibres can act like 9 probes for remote gas detection, an important requisite when it comes to environments in which it is necessary to control and detect dangerous or explosive gases, like the gas pipelines. In this thesis, we investigate the use of HC-PCFs for Raman based gas sensing and develop a sensing scheme which has the potential to meet the necessary requirements of high sensitivity, high selectivity and high-pressure operation. A key part of this work is the comparison of the performance of two different types of HC-PCFs in our sensing scheme.

We will expose the results of experiments based in the use of the so-called photonic bandgap fibres (PBGFs), in which the photonic bandgap created by the periodic cladding is used to guide the light [10, 11]. Furthermore, for the first time we report the use of the so-called anti-resonant (or Kagome) HC-PCF, a type of hollow core fibre never used before in spontaneous Raman spectroscopy [12]. The motivation behind this comparison is that these two fibres types have different physical and optical properties which may or may not provide advantages for Raman gas sensing. For example, the guidance mechanism in anti-resonant fibres, allows for wider transmission windows than the ones achievable in PBGFs which has potential advantages in enabling simultaneous detection of gas species with widely different Raman shifts. In addition, there is more flexibility in the choice of the core size in an anti-resonant fibre and as larger cores can lead to faster loading of samples this can be important in the final application. Being part of a research group in which the hollow core fibres are constantly designed and fabricated, we have been able to employ custom-made fibres to well suit the requirements of the project.

The report is divided in three sections plus the conclusion. In chapter 1, the fundamental theory behind Raman Effect and the hollow core fibres will be discussed. Moreover, the chapter will present a comparison between different spectroscopy techniques for gas sensing and the advantages of Raman spectroscopy based on hollow core fibres. In chapter 2, the setup used for the experiments and the reasons behind the choice of every component and parameter will be explained. Finally, the last chapter (chapter 3) will expose and analyse the results of the experiments that have been completed so far.

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Identifiers

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

ORCID for Natalie Wheeler: orcid.org/0000-0002-1265-9510

ORCID for Marco Petrovich: orcid.org/0000-0002-3905-5901

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