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

MEMS Technology for Biomedical Imaging Applications

Biomedical imaging is the key technique and process to create informative images of the human body or other organic structures for clinical purposes or medical science. Micro-electro-mechanical systems (MEMS) technology has demonstrated enormous potential in biomedical imaging applications due to its outstanding advantages of, for instance, miniaturization, high speed, higher resolution, and convenience of batch fabrication. There are many advancements and breakthroughs developing in the academic community, and there are a few challenges raised accordingly upon the designs, structures, fabrication, integration, and applications of MEMS for all kinds of biomedical imaging. This Special Issue aims to collate and showcase research papers, short commutations, perspectives, and insightful review articles from esteemed colleagues that demonstrate: (1) original works on the topic of MEMS components or devices based on various kinds of mechanisms for biomedical imaging; and (2) new developments and potentials of applying MEMS technology of any kind in biomedical imaging. The objective of this special session is to provide insightful information regarding the technological advancements for the researchers in the community

Miniaturized photoacoustic trace gas sensing using a raman fiber amplifier

This paper presents the development of a Raman fiber amplifier optical source with a maximum output power of 1.1 W centered around 1651 nm, and its application in miniaturized 3D printed photoacoustic spectroscopy (PAS) trace gas sensing of methane. The Raman amplifier has been constructed using 4.5 km of dispersion shifted fiber, a 1651 nm DFB seed laser and a commercial 4W EDFA pump. The suppression of stimulated Brillouin scattering (SBS) using a high frequency modulation of the seed laser is investigated for a range of frequencies, leading to an increase in optical output power of the amplifier and reduction of its noise content. The amplifier output was used as the source for a miniature PAS sensor by applying a second modulation to the seed laser at the resonant frequency of 15.2 kHz of the miniature 3D printed gas cell. For the targeted methane absorption line at 6057 cm-1 the sensor system performance and influence of the SBS suppression is characterized, leading to a detection limit (1σ) of 17 ppb methane for a signal acquisition time of 130 s, with a normalized noise equivalent absorption coefficient of 4.1•10-9 cm-1 W Hz-1/2 for the system

Ferrule-top micromachined devices: A universal platform for optomechanical sensing

Iannuzzi, D. [Promotor

Development, characterization and miniaturization of a trace gas detection system for NO₂ in air based on photoacoustic spectroscopy

This thesis provides a detailed theoretical discussion about common absorption spectroscopy (AS) and, in particular, about photoacoustic spectroscopy (PAS). The physical concepts of signal generation are illustrated in view of amplitude modulation (AM) and wavelength modulation (WM). Furthermore the advantages and disadvantages of the techniques are presented. As a result, PAS was identified to outclass AS, thus it turned out to be the method of choice in view of developing a miniaturized trace gas sensing application. The theoretical part of this work further outlines various approaches of signal enhancement, e.g. by acoustic and/or mechanical resonance amplification. Besides, several phenomena of signal attenuation are addressed, e.g. acoustic detuning, vibrational-translational (VT) relaxation and vibrational-vibrational (VV) energy transfer processes, which have to be considered with regard to the individual measuring conditions. Simulation and experimental chapters illustrate the pre-development and the practical implementation of a laboratory photoacoustic setup, a portable trace gas monitoring device and various photoacoustic cell (PAC) designs. These include a conventional bulky design, an optimized low-cost 3D printed PAC, a miniaturized quartz enhanced photoacoustic spectroscopic (QEPAS) scheme and a further integrated microelectromechanical system (MEMS) based sensor chip, respectively. Although several parts of this thesis also provide preparatory work for multi-component analysis, nitrogen dioxide (NO2) was used as primary analyte in order to characterize the above mentioned photoacoustic cell designs. This involves acoustic resonance and noise analysis, determination of optimal operating parameters (e.g. gas flow rate and lock-in time constant), performance evaluation (e.g. response behavior, optical performance, calibration characteristics and long-term signal stability) as well as interference studies towards oxygen (O2), carbon dioxide (CO2), humidity (H2O) and acoustic noise. In conclusion, NO2 detection by means of the low-cost 3D printed PAC and the QEPAS configuration even revealed two world record detection limits (1sigma) of 33 pptV and 600 pptV, respectively

MEMS Cantilever Sensor for THz Photoacoustic Chemical Sensing and Spectroscopy

Sensitive Microelectromechanical System (MEMS) cantilever designs were modeled, fabricated, and tested to measure the photoacoustic (PA) response of gasses to terahertz (THz) radiation. Surface and bulk micromachining technologies were employed to create the extremely sensitive devices that could detect very small changes in pressure. Fabricated devices were then tested in a custom made THz PA vacuum test chamber where the cantilever deflections caused by the photoacoustic effect were measured with a laser interferometer and iris beam clipped methods. The sensitive cantilever designs achieved a normalized noise equivalent absorption coefficient of 2.83x10-10 cm-1 W Hz-1/2 using a 25 µW radiation source power and a 1 s sampling time. Traditional gas phase molecular spectroscopy absorption cells are large and bulky. The outcome of this research resulted was a photoacoustic detection method that was virtually independent of the absorption path-length, which allowed the chamber dimensions to be greatly reduced, leading to the possibility of a compact, portable chemical detection and spectroscopy system

Use of the Optical Cantilever Microphone in Photoacoustic Spectroscopy

A novel cantilever pressure sensor was developed in the Department of Physics at the University of Turku in order to solve the sensitivity problems which are encountered when condenser microphones are used in photoacoustic spectroscopy. The cantilever pressure sensor, combined with a laser interferometer for the measurement of the cantilever movements, proved to be highly sensitive. The original aim of this work was to integrate the sensor in a photoacoustic gas detector working in a differential measurement scheme. The integration was made successfully into three prototypes. In addition, the cantilever was also integrated in the photoacoustic FTIR measurement schemes of gas-, liquid-, and solid-phase samples. A theoretical model for the signal generation in each measurement scheme was created and the optimal celldesign discussed. The sensitivity and selectivity of the differential method were evaluated when a blackbody radiator and a mechanical chopper were used with CO2, CH4, CO, and C2H4 gases. The detection limits were in the sub-ppm level for all four gases with only a 1.3 second integration time and the cross interference was well below one percent for all gas combinations other than those between hydrocarbons. Sensitivity with other infrared sources was compared using ethylene as an example gas. In the comparison of sensitivity with different infrared sources the electrically modulated blackbody radiator gave a 35 times higher and the CO2-laser a 100 times lower detection limit than the blackbody radiator with a mechanical chopper. As a conclusion, the differential system is well suited to rapid single gas measurements. Gas-phase photoacoustic FTIR spectroscopy gives the best performance, when several components have to be analyzed simultaneously from multicomponent samples. Multicomponent measurements were demonstrated with a sample that contained different concentrations of CO2, H2O, CO, and four different hydrocarbons. It required an approximately 10 times longer measurement time to achieve the same detection limit for a single gas as with the differential system. The properties of the photoacoustic FTIR spectroscopy were also compared to conventional transmission FTIR spectroscopy by simulations. Solid- and liquid-phase photoacoustic FTIR spectroscopy has several advantages compared to other techniques and therefore it also has a great variety of applications. A comparison of the signal-to-noise ratio between photoacoustic cells with a cantilever microphone and a condenser microphone was done with standard carbon black, polyethene, and sunflower oil samples. The cell with the cantilever microphone proved to have a 5-10 times higher signal-to-noise ratio than the reference detector, depending on the sample. Cantilever enhanced photoacoustics will be an effective tool for gas detection and analysis of solid- and liquid-phase samples. The preliminary prototypes gave good results in all three measurement schemes that were studied. According to simulations, there are possibilities for further enhancement of the sensitivity, as well as other properties, of each system.Siirretty Doriast

Interferometrian ja valoakustisen spektroskopian soveltaminen taustavapaaseen hivenkaasuanalyysiin

A trace amount of a specific gas in air, breath, or an industrial process can profoundly affect the chemistry and properties of the medium. Therefore, an accurate measurement of the concentration of the trace gas can provide invaluable information. This thesis focuses on the development of trace gas detection methods based on background-free laser absorption spectroscopic techniques. Background-free techniques possess characteristics that greatly benefit the detection of minuscule amounts of gases. These include, for example, scalability with optical power and diminished sensitivity to optical power fluctuations. The thesis deals with two spectroscopic approaches: a novel interferometric method for broadband optical background suppression in absorption spectroscopy, and cantilever-enhanced photo-acoustic spectroscopy. We performed the spectroscopy mainly in the two atmospheric windows of 2000 to 3000 cm^(−1) and 800 to 1200 cm^(−1) found in the mid-infrared region. The employed light sources encompass various broadband and single mode laser devices, including optical parametric oscillators, optical frequency combs, and a quantum cascade laser. The presented results include a demonstration of the interferometric background suppression with a state-of-the-art mid-infrared dual-comb spectrometer. We used the setup to compare the signal-to-noise ratio in direct absorption spectroscopy with and without the background suppression technique. The novel method was found to improve the signal-to-noise ratio by approximately a factor of five. The improvement was limited by the available optical power, and is expected to increase considerably with high power laser light sources. In the cantilever-enhanced photo-acoustic experiments, we investigated the use of high optical power in improving the trace gas detection performance. Using a high power mid-infrared optical parametric oscillator as a laser light source, we reached a record level noise equivalent concentration of 2.5 ppt in 15 s measurement time for hydrogen fluoride. In another work, we reached a record normalised noise equivalent absorption of 1.75×10^(−12) W cm^(−1) Hz^(−1/2) by using an optical build-up cavity to enhance the optical power in the photo-acoustic cell. Lastly, we presented results on hyphenation of the cantilever-enhanced photo-acoustic detector and a gas chromatograph. With the hyphenation, we demonstrated the capability of quantitatively analysing a complex mixture of small to large molecular weight compounds, at a detection sensitivity far better than what can be obtained with a conventional Fourier-transform based infrared detector used in gas chromatography. Quantitative analysis of the sample would have been difficult for laser absorption spectroscopy without the chromatographic separation. The results show a great potential for laser absorption spectroscopy to be used as a detector for gas chromatography in the development of a field deployable multigas analyser.Hivenkaasuilla tarkoitetaan aineita, joita on vain hyvin vähän väliaineessa, kuten ilmassa. Pienistä pitoisuuksista huolimatta hivenkaasuilla voi olla merkittävä vaikutus kaasuseoksen kemiallisiin ominaisuuksiin. Siksi on tärkeää määrittää hivenkaasujen laatu ja pitoisuudet tarkasti. Väitöskirja keskittyy taustavapaiden laserabsorptiospektroskopiaan perustuvien hivenkaasumittausmenetelmien kehittämiseen. Taustavapaalla tarkoitan sitä, että muusta kuin mittauskohteesta tuleva signaali pyritään poistamaan, koska se häiritsee mittausta. Kyseisillä mittausmenetelmillä on erityispiirteitä, jotka sopivat hyvin erittäin pienten pitoisuuksien mittaamiseen. Näihin lukeutuu esimerkiksi havaittavan signaalin voimistuminen mittalaitteen optisen tehon suhteen sekä vähäisempi herkkyys optisen tehon vaihtelulle. Väitöskirjatyössäni hyödynsin kahta spektroskooppista mittausperiaatetta: uudenlaista interferometrista menetelmää laajakaistaisen taustavapaan absorptiospektrin mittaamiseksi sekä läppävahvisteista valoakustista spektroskopiaa. Suoritin mittaukset keski-infrapuna-alueella käyttäen useita erilaisia laservalon lähteitä, kuten optisia parametrivärähtelijöitä, optisia taajuuskampoja sekä kvanttikaskadilasereita. Tutkimustuloksiini lukeutuu kehittämäni uuden interferometrisen taustavapaan mittausmenetelmän havainnollistaminen tämän hetkistä huipputasoa edustavan keski-infrapunakaksoiskampaspektrometrin avulla. Mittauksilla osoitin, että uusi menetelmä parantaa absorptiomittauksen signaali-kohinasuhdetta noin kertoimella viisi verrattuna tavalliseen suoraan absorptiospektroskopiaan. Saavutettua etua rajoitti käytettyjen laserien pieni optinen teho, ja signaali-kohinasuhdetta onkin mahdollista parantaa tulevaisuudessa suurteholasereiden avulla. Tutkimuksissani läppävahvisteisen valoakustisen spektroskopian alalla saavutin ennätyksellisiä havaintoherkkyyksiä hyödyntämällä suuria lasertehoja. Ensimmäisessä tutkimuksessa mittauskohinaa vastaava pitoisuus erittäin haitalliselle fluorivedylle oli 2.5 ppt (2.5 biljoonasosaa) 15 s mittausajalla, kun käytin suuritehoista optista parametrivärähtelijää valonlähteenä. Vastaavasti toisessa tutkimuksessa lasertehoa vahvistavan optisen resonaattorin avulla saavutin 1.75×10^(-12) Wcm^(-1) Hz^(-1/2) suuruisen ennätyksellisen alhaisen normalisoitua mittauskohinaa vastaavan absorption. Erityisen herkkien mittauksien lisäksi kehitin uudenlaisen kaasukromatografian ja läppävahvisteisen valoakustisen spektroskopian yhdistävän menetelmän. Uuden menetelmän avulla on mahdollista analysoida aiempaa luotettavammin monimutkaisia kaasuseoksia, jotka sisältävät sekä pienen että suuren molekyylimassan yhdisteitä. Menetelmä osoittautui jo alustavissa tutkimustuloksissa selvästi herkemmäksi kuin verrattavissa oleva pitkään käytössä ollut kaasukromatografian ja Fourier-muunnos infrapunaspektrometrian yhdistelmä. Tulokset havainnollistavat laserabsorptiospektroskopian soveltuvuuden kehittyneeksi kaasukromatografian ilmaisimeksi etenkin kenttäsovelluksissa, joissa laserien pienestä koosta ja huoltovapaudesta on etua

Novel miniaturised and highly versatile biomechatronic platforms for the characterisation of melanoma cancer cells

There has been an increasing demand to acquire highly sensitive devices that are able to detect and characterize cancer at a single cell level. Despite the moderate progress in this field, the majority of approaches failed to reach cell characterization with optimal sensitivity and specificity. Accordingly, in this study highly sensitive, miniaturized-biomechatronic platforms have been modeled, designed, optimized, microfabricated, and characterized, which can be used to detect and differentiate various stages of melanoma cancer cells. The melanoma cell has been chosen as a legitimate cancer model, where electrophysiological and analytical expression of cell-membrane potential have been derived, and cellular contractile force has been obtained through a correlation with micromechanical deflections of a miniaturized cantilever beam. The main objectives of this study are in fourfold: (1) to quantify cell-membrane potential, (2) correlate cellular biophysics to respective contractile force of a cell in association with various stages of the melanoma disease, (3) examine the morphology of each stage of melanoma, and (4) arrive at a relation that would interrelate stage of the disease, cellular contractile force, and cellular electrophysiology based on conducted in vitro experimental findings. Various well-characterized melanoma cancer cell lines, with varying degrees of genetic complexities have been utilized. In this study, two-miniaturized-versatile-biomechatronic platforms have been developed to extract the electrophysiology of cells, and cellular mechanics (mechanobiology). The former platform consists of a microfluidic module, and stimulating and recording array of electrodes patterned on a glass substrate, forming multi-electrode arrays (MEAs), whereas the latter system consists of a microcantilever-based biosensor with an embedded Wheatstone bridge, and a microfluidic module. Furthermore, in support of this work main objectives, dedicated microelectronics together with customized software have been attained to functionalize, and empower the two-biomechatronic platforms. The bio-mechatronic system performance has been tested throughout a sufficient number of in vitro experiments.Open Acces