Methods to depolarize narrow and broad spectrum light

In many optics applications, it is important to use well-polarized light. However, there are situations in which randomly polarized light has distinct advantages. We demonstrate two approaches by which a polarized light beam can be totally depolarized, each using a simple setup and inexpensive components. The first method, designed for narrow spectrum light, works by combining the horizontal polarization component of the beam with the delayed vertical component. The second method, which is most suitable for broad spectrum light, uses birefringent quartz plates. In both approaches, the polarization state is characterized by Stokes parameters measured using a rotating quarter-wave plate and fixed polarizer. We measure the coherence function of the electric fields and determine the minimum delay or quartz plate thickness required for decoherence. Coherences are modelled by Gaussian or Lorentzian functions and compared with the spectral properties of the light sources.</p

Electrical source of pseudothermal light

We describe a simple and compact electrical version of a pseudothermal light source. The source is based on electrical white noise whose spectral properties are tailored by analog filters. This signal is used to drive a light-emitting diode. The type of second-order coherence of the output light can be either Gaussian or Lorentzian, and the intensity distribution can be either Gaussian or non-Gaussian. The output light field is similar in all viewing angles, and thus, there is no need for a small aperture or optical fiber in temporal coherence analysis. (C) 2018 American Association of Physics Teachers

Single-pixel camera

A single-pixel camera is an interesting alternative to modern digital cameras featuring millions of pixels. A single-pixel camera is a method that produces images by exploring the object features with a series of spatially resolved patterns of light field while measuring the correlated intensity on a single detector. Nowadays, single-pixel cameras are used on those applications where multi-pixel detectors are not available because the wavelength is not in visible range or light intensity is extremely low. The spatial light modulator is an essential part of any single-pixel camera systems. They are, unfortunately, very expensive. We describe a low-cost version of single pixel camera that can be used in undergraduate physics laboratories. We show that with this camera setup students can easily demonstrate basic characteristics of computational ghost imaging and traditional raster and basis scan. Finally, we explain how to perform compressive sampling of images where the number of measurements is well below the actual pixel number. Compressive sampling is a rapidly expanding method to perform image or signal reconstructions in many field of research

Data transmission in a multimode optical fiber using a neural network

In digital data transmission, single mode optical fibers are commonly used since they can carry very short optical pulses without any significant distortions. In contrast, multimode fibers support many propagation modes that travel with different speeds; thus, they cannot maintain the shape of a light pulse. This feature of multiple propagation modes can be a benefit since it makes possible the transmission of data through several channels simultaneously. We demonstrate how multimode fibers can be used to transmit images. Because of the different propagation constants of the modes, the transmitted image is scrambled to apparently random speckle patterns. A simple neural network can be used to model the transmission through the multimode fiber. We show how the neural network can be trained to recognize a set of patterns with high accuracy. (C) 2022 Published under anexclusive license by American Association of Physics Teachers

Fine structure of the low-frequency spectra of heart rate and blood pressure

BACKGROUND: The aim of this study was to explore the principal frequency components of the heart rate and blood pressure variability in the low frequency (LF) and very low frequency (VLF) band. The spectral composition of the R–R interval (RRI) and systolic arterial blood pressure (SAP) in the frequency range below 0.15 Hz were carefully analyzed using three different spectral methods: Fast Fourier transform (FFT), Wigner-Ville distribution (WVD), and autoregression (AR). All spectral methods were used to create time–frequency plots to uncover the principal spectral components that are least dependent on time. The accurate frequencies of these components were calculated from the pole decomposition of the AR spectral density after determining the optimal model order – the most crucial factor when using this method – with the help of FFT and WVD methods. RESULTS: Spectral analysis of the RRI and SAP of 12 healthy subjects revealed that there are always at least three spectral components below 0.15 Hz. The three principal frequency components are 0.026 ± 0.003 (mean ± SD) Hz, 0.076 ± 0.012 Hz, and 0.117 ± 0.016 Hz. These principal components vary only slightly over time. FFT-based coherence and phase-function analysis suggests that the second and third components are related to the baroreflex control of blood pressure, since the phase difference between SAP and RRI was negative and almost constant, whereas the origin of the first component is different since no clear SAP–RRI phase relationship was found. CONCLUSION: The above data indicate that spontaneous fluctuations in heart rate and blood pressure within the standard low-frequency range of 0.04–0.15 Hz typically occur at two frequency components rather than only at one as widely believed, and these components are not harmonically related. This new observation in humans can help explain divergent results in the literature concerning spontaneous low-frequency oscillations. It also raises methodological and computational questions regarding the usability and validity of the low-frequency spectral band when estimating sympathetic activity and baroreflex gain

A stochastic model for heart rate fluctuations

Normal human heart rate shows complex fluctuations in time, which is natural, since heart rate is controlled by a large number of different feedback control loops. These unpredictable fluctuations have been shown to display fractal dynamics, long-term correlations, and 1/f noise. These characterizations are statistical and they have been widely studied and used, but much less is known about the detailed time evolution (dynamics) of the heart rate control mechanism. Here we show that a simple one-dimensional Langevin-type stochastic difference equation can accurately model the heart rate fluctuations in a time scale from minutes to hours. The model consists of a deterministic nonlinear part and a stochastic part typical to Gaussian noise, and both parts can be directly determined from the measured heart rate data. Studies of 27 healthy subjects reveal that in most cases the deterministic part has a form typically seen in bistable systems: there are two stable fixed points and one unstable one.Comment: 8 pages in PDF, Revtex style. Added more dat

Muscle sympathetic nerve activation during the Valsalva maneuver: Interpretive and analytical caveats

Introduction: We investigated the relationship between arterial pressure and muscle sympathetic nerve activity (MSNA) to test the hypothesis that the Valsalva maneuver may be used to estimate magnitudes of sympathetic baroreflex activation. Methods: We recorded the ECG, beat-by-beat arterial pressure, and MSNA in 33 subjects (25 men and 8 women, aged 18–25 yr) who performed three Valsalva maneuvers at 40 mmHg expiratory pressure for 15 s. Valsalva phases were identified and the magnitude of pressure changes were correlated with MSNA. Arterial pressure-MSNA relations were probed further with beat-by-beat linear regression analysis after subjects had been separated into responders (n = 20) and non-responders (n = 13) (\u3e or \u3c 10 mmHg decrease in diastolic pressure, respectively). Results: We detected no significant correlations among the magnitudes of either systolic or diastolic pressure reductions and total MSNA. Slopes relating MSNA to beat-by-beat diastolic pressure decreases were greater (p = 0.01) for responders (−3.4 bursts · min−1 · mmHg−1) than non-responders (0.8 bursts · min−1 · mmHg−1), but total MSNA during straining was not different between the two groups. With both groups combined, total MSNA during phase II and III was positively correlated to both systolic (r = 0.41) and diastolic (r = 0.57) pressure during phase IV. Conclusions: Sympathetic activation during the Valsalva maneuver does not necessarily reflect arterial baroreflex mechanisms alone. Phase IV increases of arterial pressure correlate positively to MSNA during phase II and III, and therefore gross estimations of sympathetic neural activation are possible through examination of terminal arterial pressure elevations after release from strain