Fourier Transform Photoacoustic Spectroscopy with Broadband Lasers

Kaasujen havainnointi on merkittävässä roolissa kestävän, terveellisen ja turvallisen yhteiskunnan luomisessa. Lukuisten eri sovelluskohteiden vaatimukset kaasujen havainnointiin vaihtelevat valtavasti, ja siksi useita erityyppisiä sensoreita tarvitaan. Tämä väitöskirja edistää tätä tavoitetta esittelemällä uuden optisen kaasujen havainnointitekniikan, valoakustisen Fourier-muunnosspektroskopian laajakaistaisella keski-infrapunalaserilla ja herkällä läppämikrofonilla. Tekniikan merkittävimmät hyödyt ovat laajan, ainoastaan valonlähteen rajoittaman spektrisen kaistan nopea mittaus sekä herkkyyden tehokas parantaminen valonlähteen tehoa kasvattamalla. Tässä väitöskirjassa käytettyjen valonlähteiden, superjatkumon ja optisen taajuuskamman hyöty suuren optisen tehotiheyden lisäksi on korkea paikkakoherenssi, joka mahdollistaa erinomaisen spektrisen resoluution sekä tehokkaan kytkennän moniläpäisykammioon. Tekniikan suorituskyky on erinomainen vaadittavan näytetilavuuden ollessa alle kymmenen millilitraa. Metaanin havaintoraja viiden sekunnin mittausajalla on 90 miljardisosaa, jota voidaan parantaa merkittävästi keskiarvoistamalla. Korkein demonstroitu spektrinen resoluutio on 0.013 cm−1, joka ei heikennä systeemin herkkyyttä ja jota rajoittaa paineleveneminen. Kaksi kertaluokkaa huonompi spektrinen resoluutio kuitenkin mahdollistaa jo hyvän selektiivisyyden monen kaasun samanaikaiseen havainnointiin. Hyvän suorituskyvyn ja pienen näytetilavuuden ainutlaatuisen yhdistelmän ansiosta tekniikka on potentiaalinen esimerkiksi haihtuvien yhdisteiden ja saatavuudeltaan rajoitettujen näytteiden analysoimiseen. Eräs potentiaalinen ja tässä väitöskirjassa demonstroitu sovelluskohde on kemiallisten taisteluaineiden havaitseminen rikostutkinnassa ja jatkuvatoimisissa varoitusjärjestelmissä. Lähitulevaisuudessa sekä tekniikan suorityskyvyn että sovellettavuuden odotetaan kehittyvän merkittävästi, mikä vahvistaa tekniikan asemaa vaihtoehtona teollisiin, lääketieteellisiin ja turvallisuussovelluksiin.Gas sensing plays a key role in the progress towards a healthier, safer and more sustainable society. The amount of gas sensing applications are immense with varying requirements, and thus numerous sensors with different characteristics are needed. This thesis contributes to the task by introducing a new optical gas sensing technique, Fourier transform photoacoustic spectroscopy (FT-PAS) implemented with a spectrally broadband mid-infrared laser and a sensitive cantilever microphone. The main benefits of FT-PAS are the fast acquisition of a wide spectral range that is only limited by the light source, and the effective enhancement of sensitivity with a high-power light source. Two light sources are demonstrated in this thesis, namely an incoherent fiber-based supercontinuum and a frequency down-converted mode-locked optical frequency comb. Besides high power spectral density, the advantage of broadband lasers stems from their high spatial coherence enabling high spectral resolution and efficient coupling to multipass cells. The performance of cantilever-enhanced FT-PAS is excellent while requiring a sample volume of less than ten milliliters. The detection limit for methane is 90 parts per billion in five seconds, which can be significantly lowered through longer averaging. The highest demonstrated spectral resolution is 0.013 cm−1 with no compromise in the detection sensitivity and limited by pressure broadening. However, two orders of magnitude worse spectral resolution already provides sufficient selectivity for complex multi-species detection. The unique combination of high performance and low gas consumption makes the technique attractive for the analysis of volatile substances and samples with limited availability. The detection of chemical warfare agents for forensic crime scene investigation and online warning systems is one potential application of the technique demonstrated in this thesis. In the near future, both the performance and the applicability of FT-PAS are expected to remarkably improve, establishing the technique as a notable alternative in many industrial, medical and security applications

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Light Propagation and Gas Absorption Studies in Turbid Media Using Tunable Diode Llaser Techniques

Optical absorption spectroscopy is a widely used analytical tool for constituent analysis in many applications. According to the Beer-Lambert law, the transmitted light intensity through a homogeneous medium is an exponential function of the product of the concentration, the total pathlength, and the absorption cross-section of the absorbing substance. By studying the intensity loss at the unique absorption band of the absorbing substance, its concentration can be retrieved. However, this method will encounter some difficulties if the light is not only absorbed but also strongly scattered in the material, e.g., in a turbid medium (biological tissues, porous ceramics, wood), which results in an unknown absorption pathlength. Such a problem can be solved by studying light propagation with different theoretical models, and the scattering and absorption properties are then retrieved. One aim of the present thesis work is to develop a new experimental approach to study light propagation in turbid media – frequency-modulated light scattering interferometry (FMLSI), originating from the well-known frequency-modulated continuous-wave technique in telecommunication field. This method provides new possibilities to study optical properties and Brownian motion simultaneously, which is particularly useful in biomedical applications, food science, and for colloidal suspensions in general. Another important application of absorption spectroscopy is to monitor gas concentration in turbid media, where the gas absorption pathlength is a priori unknown due to heavy light scattering in the porous medium. The technique is referred to as gas in scattering media absorption spectroscopy (GASMAS), and is based on the principle that the absorption spectrum of gases is much narrower than that for the solid- or liquid-phase host materials. By linearly scanning the wavelength of the light source across an absorption line of the gas and examining the absorption imprint superimposed on the transmitted light signal, the very weak intensity loss due to the gas of interest can be measured for gas concentration assessment. In order to obtain the absolute gas concentration, a focus in the present thesis work is to determine the gas absorption pathlength in turbid media. The FMLSI technique is proposed to obtain the mean optical pathlength – the total pathlength through both the pores and the matrix material. The combined method of FMLSI and GASMAS techniques is then developed to study porous media, where an average gas concentration in the porous media can be obtained. A conventional method for pathlength or optical properties determination – frequency domain photon migration – is also combined with the GASMAS technique to study the total gas absorption pathlength and the porosities of ceramics, which, as a result, also contributes to further understanding of light propagation in porous media. Another method is also proposed to get the absolute gas concentration without knowing the optical pathlength. It is based on absorption line shape analysis – relying on the fact that the line shape depends upon the concentration of the buffer gas. This method is found to be very useful for, e.g., gas concentration monitoring in food packaging

Use of diffuse reflections in tunable diode laser spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is an optical gas sensing technique in which the emission frequency of a laser diode is tuned over a gas absorption line of interest. A fraction of the radiation is absorbed by the sample gas and this can be determined from measurements of initial intensity and the intensity transmitted through the sample. The amount of light absorbed is related to the gas concentration. Additional modulation techniques combined with phase sensitive detection allow detection of very low gas concentrations (several parts per million). The advantages of using TDLAS for trace gas sensing include; fast response times, high sensitivity and high target gas selectivity. However, the sensitivity of many practical TDLAS systems is limited by the formation of unintentional Fabry-Perot interference fringes in the optical path between the source and detector. The spacing between the maxima of these fringes, in particular those generated in gas cells, can be in the same wavelength range as Doppler and pressure-broadened molecular line widths. This can lead to (1) interference fringe signals being mistaken for gas absorption lines leading to false concentration measurements or (2) distortion or complete obscuring of the shape and strength of the absorption line, such that the sensitivity of the instrument is ultimately limited by the fringes. The interference fringe signals are sensitive to thermal and mechanical instabilities and therefore can not be removed by simple subtraction techniques. Methods that have been proposed by previous workers to reduce the effects of interference fringes include careful alignment of optical components and/or mechanically jittering the offending components. In general the alignment of the optical components is critical. This often leads to complex and fragile designs with tight tolerances on optical component alignment, and can therefore be difficult and expensive to maintain in field instruments. This thesis presents an alternative approach based on the deliberate use of diffusely scattering surfaces in gas cells as a means of eliminating spurious signals due to Fabry-Perot etalons. However, their use introduced laser speckle that contributed an intensity uncertainty to gas detection measurements. A methodology for investigating the laser speckle related intensity uncertainty has been developed and confirmed. The intensity uncertainty has been quantified for the different gas cell geometries employing diffusely scattering surfaces including integrating spheres. Methods for reducing the speckle related intensity uncertainty were also investigated and are presented. It has been shown that under the right circumstances robust gas cell designs that do not suffer from Fabry-Perot etalon effects and are relatively easy to align can be realised. The performance was found to be comparable to a conventional cell design (e.g. 3ppm detection limit for a 10cm standard cell and 11ppm for a 10cm diffusive cell). The technique could potentially simplify instrument design, thereby aiding the transfer of technology to industry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Aspects of laser absorption spectroscopy in the mid-infrared and visible

Laser absorption spectroscopy can be used to identify and quantify gas analytes. The sponsor company’s present systems operate in the mid-infrared using room temperature pulsed quantum cascade lasers (pulsed-QCL’s). These systems use the noise reduction / sensitivity enhancing technique of sweep integration (SI). In this work, an extension of measurement capabilities is sought in two ways. Firstly, sensitivity enhancement is pursued. The noise reduction technique of wavelength modulation spectroscopy (WMS) is applied using a room temperature continuous wave (cw) QCL spectrometer. Secondly, molecular oxygen is added to the list of measurable analytes. This molecule’s near-infrared and visible transitions are addressed with a wavenumber prototype semiconductor diode laser. The sensitivities of the SI and WMS techniques are compared for the cw-QCL spectrometer, and compared to the SI sensitivity of a typical company pulsed-QCL system. New analysis and modeling software was written to facilitate the thesis work and to carry it forward. A thorough analysis of a pulsed-QCL CT3000 analyzer is undertaken to minimize a reduction in capability - should an oxygen measuring laser replace one of its pulsed-QCL’s. The experimental work was constrained by time and budget - particularly with regard to the cw-QCL spectrometer’s AC-coupled detection. Using AC-coupled detection had cost and integration advantages, but posed a number of problems - including electronic incompatibility issues. Nevertheless, the outlook is positive, and a modest sensitivity improvement was found for WMS over sweep integration (0.017 absorbance units (a.u.) in 102.4s compared to 0.080 a.u. in 51ms). Both sensitivities are some way behind the present sweep integration performance of the company’s pulsed spectrometers (0.004 a.u. in 10ms). However, the sensitivities are comparable to earlier stages of development. In the case of oxygen spectroscopy, the prototype diode laser’s thermal stability was an issue, but several spectral regions were found to be suitable for single or multimode spectroscopy.Laser absorption spectroscopy can be used to identify and quantify gas analytes. The sponsor company’s present systems operate in the mid-infrared using room temperature pulsed quantum cascade lasers (pulsed-QCL’s). These systems use the noise reduction / sensitivity enhancing technique of sweep integration (SI). In this work, an extension of measurement capabilities is sought in two ways. Firstly, sensitivity enhancement is pursued. The noise reduction technique of wavelength modulation spectroscopy (WMS) is applied using a room temperature continuous wave (cw) QCL spectrometer. Secondly, molecular oxygen is added to the list of measurable analytes. This molecule’s near-infrared and visible transitions are addressed with a wavenumber prototype semiconductor diode laser. The sensitivities of the SI and WMS techniques are compared for the cw-QCL spectrometer, and compared to the SI sensitivity of a typical company pulsed-QCL system. New analysis and modeling software was written to facilitate the thesis work and to carry it forward. A thorough analysis of a pulsed-QCL CT3000 analyzer is undertaken to minimize a reduction in capability - should an oxygen measuring laser replace one of its pulsed-QCL’s. The experimental work was constrained by time and budget - particularly with regard to the cw-QCL spectrometer’s AC-coupled detection. Using AC-coupled detection had cost and integration advantages, but posed a number of problems - including electronic incompatibility issues. Nevertheless, the outlook is positive, and a modest sensitivity improvement was found for WMS over sweep integration (0.017 absorbance units (a.u.) in 102.4s compared to 0.080 a.u. in 51ms). Both sensitivities are some way behind the present sweep integration performance of the company’s pulsed spectrometers (0.004 a.u. in 10ms). However, the sensitivities are comparable to earlier stages of development. In the case of oxygen spectroscopy, the prototype diode laser’s thermal stability was an issue, but several spectral regions were found to be suitable for single or multimode spectroscopy

Photonic Technology for Precision Metrology

Photonics has had a decisive influence on recent scientific and technological achievements. It includes aspects of photon generation and photon–matter interaction. Although it finds many applications in the whole optical range of the wavelengths, most solutions operate in the visible and infrared range. Since the invention of the laser, a source of highly coherent optical radiation, optical measurements have become the perfect tool for highly precise and accurate measurements. Such measurements have the additional advantages of requiring no contact and a fast rate suitable for in-process metrology. However, their extreme precision is ultimately limited by, e.g., the noise of both lasers and photodetectors. The Special Issue of the Applied Science is devoted to the cutting-edge uses of optical sources, detectors, and optoelectronics systems in numerous fields of science and technology (e.g., industry, environment, healthcare, telecommunication, security, and space). The aim is to provide detail on state-of-the-art photonic technology for precision metrology and identify future developmental directions. This issue focuses on metrology principles and measurement instrumentation in optical technology to solve challenging engineering problems

Virtual Histology with Photoacoustic Remote Sensing

Histopathology plays a central role in cancer screening, surgical margin analysis, cancer classification, and understanding disease progression. The vast majority of biopsies or surgical excisions are examined via transmission-mode bright-field microscopy. However, bright-field microscopy requires thin stained tissue samples as it is unable to visualize contrast on thick tissues. Consequently, biopsies and surgically excised specimens undergo extensive tissue processing to prepare histology slides. This tissue processing can take up to two weeks for complex cases before a diagnosis can be presented, potentially resulting in poorer patient outcomes. Surgical margins are commonly analyzed intraoperatively using frozen sectional analysis. While this technique has improved patient outcomes, the quality of frozen sections is often lower than post-operative histologic analysis. This lower quality leads to significant variability in diagnosis. Ultimately, both frozen section analysis and standard histologic analysis are limiting because of the need to process tissues to cater to bright-field microscopy. It would be desirable to forego creating thin tissue sections and instead visualize tissue morphology directly on biopsies and surgical specimens or even directly on the patient’s body (in-situ). Photoacoustic remote sensing (PARSTM) is an emerging non-contact imaging technique. PARS microscopy is an all-optical photoacoustic imaging modality that takes advantage of endogenous optical absorption present within tissues to provide contrast to enable non- contact label-free imaging. PARS has demonstrated excellent resolution and contrast in various applications, such as in-vivo imaging, functional imaging, and deep imaging, while operating in a reflection-mode architecture. This non-contact label-free reflection-mode design lends itself well to imaging unprocessed tissue specimens or in-situ morphological assessment. Using PARS microscopy, this thesis takes preliminary steps towards an in-situ surgical microscope. These steps take the form of developing a PARS system that can recover contrast from DNA and visualize the resulting nuclear morphology in real-time and on arbitrarily sized specimens. Later, this system was expanded to image additional contrasts from hemoglobin to approach the diagnostic information provided by standard histopathology. This research imaged a variety of human tissue types, including breast, gastrointestinal, and skin. These specimens were in the form of thin unstained slides and thick tissue blocks. The tissue blocks serve as an analog to visualization of contrast fresh tissues and in-situ imaging. Adjacent sections of each tissue type were prepared using standard histopathology and compared against the PARS images for experimental validation. These results represent the first reports of imaging human tissues with a non-contact label-free reflection-mode modality. The author believes this research takes vital steps towards an imaging technique that may one day reveal cancer in-situ

Holographic Fourier domain diffuse correlation spectroscopy

Diffuse correlation spectroscopy (DCS) is a non-invasive optical modality which can be used to measure cerebral blood flow (CBF) in real-time. It has important potential applications in clinical monitoring, as well as in neuroscience and the development of a non-invasive brain-computer interface. However, a trade-off exists between the signal-to-noise ratio (SNR) and imaging depth, and thus CBF sensitivity, of this technique. Additionally, as DCS is a diffuse optical technique, it is limited by a lack of inherent depth discrimination within the illuminated region of each source-detector pair, and the CBF signal is therefore also prone to contamination by the extracerebral tissues which the light traverses. Placing a particular emphasis on scalability, affordability, and robustness to ambient light, in this work I demonstrate a novel approach which fuses the fields of digital holography and DCS: holographic Fourier domain DCS (FD-DCS). The mathematical formalism of FD-DCS is derived and validated, followed by the construction and validation (for both in vitro and in vivo experiments) of a holographic FD-DCS instrument. By undertaking a systematic SNR performance assessment and developing a novel multispeckle denoising algorithm, I demonstrate the highest SNR gain reported in the DCS literature to date, achieved using scalable and low-cost camera-based detection. With a view to generating a forward model for holographic FD-DCS, in this thesis I propose a novel framework to simulate statistically accurate time-integrated dynamic speckle patterns in biomedical optics. The solution that I propose to this previously unsolved problem is based on the Karhunen-Loève expansion of the electric field, and I validate this technique against novel expressions for speckle contrast for different forms of homogeneous field. I also show that this method can readily be extended to cases with spatially varying sample properties, and that it can also be used to model optical and acoustic parameters

Novel Nonlinear Optics and Quantum Optics Approaches for Ultrasound-Modulated Optical Tomography in Soft Biological Tissue

Optical imaging of soft biological tissue is highly desirable since it is nonionizing and provides sensitive contrast information which enables the detection of physiological functions and abnormalities, including potentially early cancer detection. However, due to the diffusive nature of light in soft biological tissue, it is difficult to achieve simultaneously good spatial resolution and good imaging depth with pure optical imaging modalities. This work focuses on the ultrasound-modulated optical tomography (UOT): a hybrid technique which combines the advantages of ultrasonic resolution and optical contrast. In this technique, focused ultrasound and optical radiation of high temporal coherence are simultaneously applied to soft biological tissue. The intensity of the sideband, or ultrasound ‗tagged‘ photons depends on the optical absorption in the region of interest where the ultrasound is focused. Demodulation of the optical speckle pattern yields the intensity of tagged photons for each location of the ultrasonic focal spot. Thus UOT yields an image with spatial resolution of the focused ultrasound — typically submillimeter — whose contrast is related to local optical absorption and the diffusive properties of light in the organ. Thus it extends all the advantages of optical imaging deep into highly scattering tissue. However lack of efficient tagged light detection techniques has so far prevented ultrasound-modulated optical tomography from achieving maturity. The signal-to-noise ratio (SNR) and imaging speed are two of the most important figures of merit and need further improvement for UOT to become widely applicable. In the first part of this work, nonlinear optics detection methods have been implemented to demodulate the ―tagged‖ photons. The most common of these is photorefractive (PR) two wave mixing (TWM) interferometry, which is a time-domain filtering technique. When used for UOT, it is found that this approach extracts not only optical properties but also mechanical properties for the area of interest. To improve on TWM, PR four wave mixing (FWM) experiments were performed to read out only the modulated light and at the same time strongly suppressing the ‗untagged‘ light. Spectral-hole burning (SHB) in a rare-earth-ion-doped crystal has been developed for UOT more recently. Experiments in Tm3 :Y3Al5O12 (Tm:YAG) show the outstanding features of SHB: large angle acceptance (etendue), light speckle processing in parallel (insensitive to the diffusive light nature) and real-time signal collection (immune to light speckle decorrelation). With the help of advanced laser stabilization techniques, two orders of magnitude improvement of SNR have been achieved in a persistent SHB material (Pr^3 :Y2SiO5) compared to Tm:YAG. Also slow light with PSHB further reduces noise in Pr:YSO UOT that is caused by polarization leakage by performing time-domain filtering

Pathlength calibration of integrating sphere based gas cells

Integrating sphere based multipass cells, unlike typical multipass cells, have an optically rough reflective surface, which produces multiple diffuse reflections of varying lengths. This has significant advantages, including negating scattering effects in turbid samples, removing periodicity of waves (often the cause of etalon fringes), and simple cell alignment. However, the achievable pathlength is heavily dependent on the sphere wall reflectivity. This presents a challenge for ongoing in-situ measurements as potential sphere wall contamination will cause a reduction in mean reflectivity and thus a deviation from the calibrated pathlength. With this in mind, two techniques for pathlength calibration of an integrating sphere were investigated. In both techniques contamination was simulated by creating low reflectivity tabs e.g. ≈5x7mm, that could be introduced into the sphere (and removed) in a repeatable manner. For the first technique, a four beam configuration, adapted from a turbidity method used in the water industry, was created using a 5cm diameter sphere with an effective pathlength of 1m. Detection of methane gas was carried out at 1650nm. A mathematical model was derived that corrected for pathlength change due to sphere wall contamination in situ, thus enabling gas measurements to continue to be made. For example, for a concentration of 1500ppm of methane where 1.2% of the sphere wall was contaminated with a low reflectivity material, the absorption measurement error was reduced from 41% to 2% when the model was used. However some scenarios introduced errors into the correction, including contamination of the cell windows which introduced errors of, for example, up to 70% if the particulate contamination size was on the order of millimetres. The second technique used high frequency intensity modulation with phase detection to achieve pathlength calibration. Two types of modulation were tested i.e. sinusoidal modulation and pulsed modulation. The technique was implemented using an integrated circuit board which allowed for generation of modulation signals up to 150MHz with synchronous signal processing. Pathlength calibration was achieved by comparison of iii the phase shift for a known length with the measured phase shift for the integrating sphere with unknown pathlength over a range of frequencies. The results for both modulation schemes showed that, over the range of frequencies detected, 3-48MHz, the resultant phase shift varied as an arctangent function for an integrating sphere. This differed from traditional single passes where frequency and phase have a linear relationship