Analysing the impact of non-parallelism in Fabry-Perot etalons through optical modelling

Fabry-Perot (FP) etalons, composed of two parallel mirrors, are used widely as optical filters and sensors. In certain applications, however, such as when FP etalons with polymer cavities are used to detect ultrasound, the mirrors may not be perfectly parallel due to manufacturing limitations. As little is known about how the mirrors being non-parallel impacts upon FP etalon performance, it is challenging to optimize the design of such devices. To address this challenge, we developed a model of light propagation in non-parallel FP etalons. The model is valid for arbitrary monochromatic beams and calculates both the reflected and transmitted beams, assuming full-wave description of light. Wavelength resolved transmissivity simulations were computed to predict the effect that non-parallel mirrors have on the sensitivity, spectral bandwidth and peak transmissivity of FP etalons. Theoretical predictions show that the impact of the non-parallel mirrors increases with both mirror reflectivity and incident Gaussian beam waist. Guidelines regarding the maximum angle allowed between FP mirrors whilst maintaining the sensitivity and peak transmissivity of a parallel mirror FP etalon are provided as a function of mirror reflectivity, cavity thickness and Gaussian beam waist. This information, and the model, could be useful for guiding the design of FP etalons suffering a known degree of non-parallelism, for example, to optimize the sensitivity of polymer based FP ultrasound sensors

The visible and near-infrared optical absorption coefficient spectrum of Parylene C measured by transmitting light through thin films in liquid filled cuvettes

Parylene C (PPXC) is a polymer deposited from the gas phase to form optically clear thin films used in devices including waveguides and sensors. The performance of these devices depends on the visible and near infrared absorption coefficient of PPXC. However, the absorption coefficient is difficult to measure. This is because PPXC films are typically too thin to exhibit detectable absorption in conventional transmittance measurements. To address this challenge, a method involving measuring the transmittance of multiple films immersed together in a liquid filled cuvette was devised. This increased the sensitivity to absorption by increasing the path length in PPXC, while also minimizing reflections and surface losses. Using 200-500 µm thick films, this method was applied to measure the absorption coefficient of PPXC at wavelengths in the range 330-3300 nm. The coefficient was found to vary spectrally by more than two orders of magnitude from 0.025 mm-1 at 1562 nm to 7.7 mm-1 at 3262 nm. These absorption measurements could aid the design of PPXC based sensors and waveguides. The method could be useful for measuring the absorption coefficient of other thin, low-loss materials, particularly those for which it is challenging to obtain thick samples such as other polymers deposited from the gas phase in a similar manner to PPXC

Interrogating Fabry-Perot ultrasound sensors with Bessel beams for photoacoustic imaging

Photoacoustic Tomography (PAT) systems based on Fabry-Perot (FP) sensors provide high-resolution images limited by the system’s sensitivity. The sensitivity is limited by the optical Q-factor of the FP cavity (i.e., the optical confinement of the interrogation laser beam in the FP cavity). In existing systems, a focussed Gaussian beam is used to interrogate the sensor. While providing a small acoustic element required for high-resolution imaging, this interrogation beam naturally diverges inside the FP cavity, leading to the current sensitivity limit. To break this limit, a new approach of interrogating the FP sensor using a Bessel beam is investigated. The Noise Equivalent Pressure (NEP) and both axial and lateral PAT resolutions using Bessel beam interrogation were quantified. Bessel beam interrogation provided lower NEP, similar axial resolution, but lower lateral resolution. Thus, Bessel beam might be an alternative interrogation scheme for deep PAT imaging as high sensitivity is needed and the lateral resolution is limited by the aperture of the PAT system

Probing the optical readout characteristics of Fabry-Perot ultrasound sensors through realistic modelling

The Fabry-Perot interferometer (FPI) is widely used in photoacoustic imaging (PAI) as an ultrasound (US) sensor due to its high sensitivity to weak US waves. Such high sensitivity is important as it allows for increasing the depth in tissue at which PAI can access, thus strongly influencing its clinical applicability. FPI sensitivity is impacted by many factors including the FPI mirror reflectivity, focussed beam spot size, FPI cavity thickness and aberrations introduced by the optical readout system. Improving FPI sensitivity requires a mathematical model of its optical response which takes all of these factors into account. Previous attempts to construct such a model have been critically limited by unrealistic assumptions. In this work we have developed a general model of FPI optical readout which based upon electromagnetic theory. By making very few assumptions, the model is able to replicate experimental results and allows insight to be gained into the operating principles of the sensor

The Gut Microbiota and Their Metabolites in Human Arterial Stiffness

Aim: Gut microbiota-derived metabolites, such as short-chain fatty acids (SCFAs) have vasodilator properties in animal and human ex vivo arteries. However, the role of the gut microbiota and SCFAs in arterial stiffness in humans is still unclear. Here we aimed to determine associations between the gut microbiome, SCFA and their G-protein coupled sensing receptors (GPCRs) in relation to human arterial stiffness. / Methods: Ambulatory arterial stiffness index (AASI) was determined from ambulatory blood pressure (BP) monitoring in 69 participants from regional and metropolitan regions in Australia (55.1% women; mean, 59.8± SD, 7.26 years of age). The gut microbiome was determined by 16S rRNA sequencing, SCFA levels by gas chromatography, and GPCR expression in circulating immune cells by real-time PCR. / Results: There was no association between metrics of bacterial α and β diversity and AASI or AASI quartiles in men and women. We identified two main bacteria taxa that were associated with AASI quartiles: Lactobacillus spp. was only present in the lowest quartile, while Clostridium spp. was present in all quartiles but the lowest. AASI was positively associated with higher levels of plasma, but not faecal, butyrate. Finally, we identified that the expression of GPR43 (FFAR2) and GPR41 (FFAR3) in circulating immune cells were negatively associated with AASI. / Conclusions: Our results suggest that arterial stiffness is associated with lower levels of the metabolite-sensing receptors GPR41/GPR43 in humans, blunting its response to BP-lowering metabolites such as butyrate. The role of Lactobacillus spp. and Clostridium spp., as well as butyrate-sensing receptors GPR41/GPR43, in human arterial stiffness needs to be determined

Essential Hypertension Is Associated With Changes in Gut Microbial Metabolic Pathways A Multisite Analysis of Ambulatory Blood Pressure

Recent evidence supports a role for the gut microbiota in hypertension, but whether ambulatory blood pressure is associated with gut microbiota and their metabolites remains unclear. We characterized the function of the gut microbiota, their metabolites and receptors in untreated human hypertensive participants in Australian metropolitan and regional areas. Ambulatory blood pressure, fecal microbiome predicted from 16S rRNA gene sequencing, plasma and fecal metabolites called short-chain fatty acid, and expression of their receptors were analyzed in 70 untreated and otherwise healthy participants from metropolitan and regional communities. Most normotensives were female (66%) compared with hypertensives (35%, P<0.01), but there was no difference in age between the groups (59.2±7.7 versus 60.3±6.6 years old). Based on machine learning multivariate covariance analyses of de-noised amplicon sequence variant prevalence data, we determined that there were no significant differences in predicted gut microbiome α- and β-diversity metrics between normotensives versus essential or masked hypertensives. However, select taxa were specific to these groups, notably Acidaminococcus spp., Eubacterium fissicatena, and Muribaculaceae were higher, while Ruminococcus and Eubacterium eligens were lower in hypertensives. Importantly, normotensive and essential hypertensive cohorts could be differentiated based on gut microbiome gene pathways and metabolites. Specifically, hypertensive participants exhibited higher plasma acetate and butyrate, but their immune cells expressed reduced levels of short-chain fatty acid-activated GPR43 (G-protein coupled receptor 43). In conclusion, gut microbial diversity did not change in essential hypertension, but we observed a significant shift in microbial gene pathways. Hypertensive subjects had lower levels of GPR43, putatively blunting their response to blood pressure-lowering metabolites

Superpartner spectrum of minimal gaugino-gauge mediation

We evaluate the sparticle mass spectrum in the minimal four-dimensional construction that interpolates between gaugino and ordinary gauge mediation at the weak scale. We find that even in the hybrid case -- when the messenger scale is comparable to the mass of the additional gauge particles -- both the right-handed as well as the left-handed sleptons are lighter than the bino in the low-scale mediation regime. This implies a chain of lepton production and, consequently, striking signatures that may be probed at the LHC already in the near future.Comment: 8 pages, 3 figures; V2: refs and a few comments added; V3 title change

On the intuitive understanding of interrogating Fabry-Perot etalon with a focused beam

Polymer film Fabry-Perot (FP) sensors are commonly used to detect ultrasound for Photoacoustic (PA) imaging providing high resolution 3D images. Such high image quality is possible due to their low Noise Equivalent Pressure (NEP) because of their broadband response and small acoustic element size. The acoustic element size is small (<100 μm) as defined, to first approximation, by the spot size of the focused interrogation beam. However, it has been difficult until now to gain an accurate intuitive understanding of the working principle of FP sensors interrogated with a focused beam. To overcome this limitation a highly realistic rigorous model of the FP sensor's optical response has used to establish a new intuitive understanding. The origin of fringe depth reduction and asymmetry associated with the FP sensors optical response is explained using the model developed

Design of Resonant Optical Cavities for Ultrasound Detection using Rigorous Electromagnetic Modelling

Detection of very weak ultrasound waves with a Fabry-Perot interferometer has enabled a new range of biomedical applications such as photoacoustic imaging. We propose a realistic model of the optical readout of this device valid for arbitrary focussed readout beams and optical fibre based detection