Guillermo Santiago

Blue light-emitting diode-based, enhanced resonant excitation of longitudinal acoustic modes in a closed pipe with application to NO2

We present a new, very compact resonant photoacoustic system based on a simple closed pipe, transversally illuminated by an array of blue light-emitting diodes uniformly distributed along the tube to produce an acoustic signal from NO2–N2 samples. The illumination is modulated in a particular way as to produce amplification of the acoustic resonance corresponding to the second longitudinal mode. The linearity of the system for NO2 trace detection is studied. This arrangement provides a simple, compact, and cheap setup, useful for both measuring emissions from diesel engines and teaching photoacoustic spectroscopy in gases.

Modeling thin-film piezoelectric polymer ultrasonic sensors.

This paper presents a model suitable to design and characterize broadband thin film sensors based on piezoelectric polymers. The aim is to describe adequately the sensor behavior, with a reasonable number of parameters and based on well-known physical equations. The mechanical variables are described by an acoustic transmission line. The electrical behavior is described by the quasi-static approximation, given the large difference between the velocities of propagation of the electrical and mechanical disturbances. The line parameters include the effects of the elastic and electrical properties of the material. The model was validated with measurements of a poly(vinylidene flouride) sensor designed for short-pulse detection. The model variables were calculated from the properties of the polymer at frequencies between 100 Hz and 30 MHz and at temperatures between 283 K and 313 K, a relevant range for applications in biology and medicine. The simulations agree very well with the experimental data, predicting satisfactorily the influence of temperature and the dielectric properties of the polymer on the behavior of the sensor. Conversely, the model allowed the calculation of the material dielectric properties from the measured response of the sensor, with good agreement with the published values.

Numerical and experimental study of stable resonators with diffractive output coupling

We present a characterization of the dot-mirror output coupler in the case of low losses. The numerical procedure computes the eigenvalue and field distribution through the Fox-Li method applied to a bare laser cavity. The field associated with the lowest-order mode closely follows the Airy pattern. This fact allows estimating the effective reflectivity and the beam diameter in a simple way. The computed values are compared with the experimental data obtained in a transversely-excited-atmosphere CO 2 laser. The measured reflectivity approximates the expected figure for the lowest-order mode and thus suggests that this cavity discriminates against the higher-order modes.

Photoacoustic beam profiling of pulsed lasers

We introduce a beam profiler of pulsed lasers based on the photoacoustic technique. The method assumes that the initial pressure distribution inside the acoustic cell follows the laser intensity pattern if absorbed energy relaxes rapidly. This initial pressure condition can be described as a superposition of acoustic modes of different amplitudes and phases. We analyze how to reconstruct the intensity profile of the laser beam from the recorded acoustic signals. Finally, we present preliminary results obtained with a frequency doubled Nd:YAG laser that excites NO(2)-CF(2)Cl(2) mixtures.

Reducing the capacitance of piezoelectric film sensors

We present a novel design for large area, wideband, polymer piezoelectric sensor with low capacitance. The large area allows better spatial resolution in applications such as photoacoustic tomography and the reduced capacitance eases the design of fast transimpedance amplifiers. The metalized piezoelectric polymer thin film is segmented into N sections, electrically connected in series. In this way, the total capacitance is reduced by a factor 1/N(2), whereas the mechanical response and the active area of the sensor are not modified. We show the construction details for a two-section sensor, together with the impedance spectroscopy and impulse response experimental results that validate the design.

Novel optical method for background reduction in resonant photoacoustics.

We report a new way of reducing the background originated by window absorption in resonant photoacoustics. The technique employs a secondary light source that is absorbed by the window but not by the gas sample. This auxiliary source is modulated 180 degrees apart from the one used to probe the gas. This way the window is heated almost uniformly during each cycle, thus lessening the associated background signal. We tested the scheme on a photoacoustic setup, conceived to measure NO(2), which is excited by an array of blue light-emitting diodes. Another array of near-infrared, light-emitting diodes served as the secondary light source. With the addition of this cancelling method, the detection limit was decreased to 4% of the previous reached without it.

Recovery of noisy pyroelectric-detector signals through neural-network processing

We introduce a neural-network-based filter devised to extend the dynamic range of pyroelectric detectors which otherwise would only be useful for medium-to-high energy measurements. To accomplish this task, we trained a multilayer perceptron through the back-propagation method using the theoretical signal derived from the detector equivalent electric circuit. We tested the performance of the neural-network filter both numerically and experimentally. In the former case we recovered theoretical signals corrupted with white and impulse noise and compared the results with those obtained through the use of standard filtering methods. In the latter case, we applied the filter to measure pulses from a Nd:YAG laser whose energy was below the detector noise-equivalent energy. With this processing technique in a standard PC we have been able to accurately measure laser energy values as low as one-tenth the detector’s noise-equivalent energy and at 10–20Hz repetition rate.

Acoustic characteristics and Distortion of an Ionic Loudspeaker

This paper presents some acoustic characteristics of a positive-corona-discharge loudspeaker, based on a parallel array of wires (corona electrode) and bars (collector electrode). The acoustic pressure is a combination of a monopolar and a dipolar term. The former arises from heat exchange in the wire surroundings and the latter stems from elastic collisions of drifting ions and neutral molecules in the gap between the electrodes. The relative contribution of both terms is studied. A differential circuit is devised to measure the AC ionic current in real time. This allows to relate it to the acoustic pressure and to compute the loudspeaker efficiency. Measurements on directivity and frequency response evidence that, even though the loudspeaker is a sum of discrete sources, it behaves as a rectangular diaphragm. This reduces the acoustical short-circuit and improves the transducer efficiency. Finally this work proposes a simple method to decrease the sound pressure harmonic distortion by applying a simple linearizing algorithm (based on the I-V discharge relationship) to the input signal.

Applications of a Visible‐LED‐Based Resonant Photoacoustic Device

The analysis of the NO2 content in the exhaust gases of diesel engines is performed by a new simple resonant photoacoustic system. It consists of a special design of transversal illumination from LEDs attached to a cylindrical glass cell. LEDs emitting at 464 and 400 nm allow carrying out differential measurements, to take into account possible interferences due to other absorbing species. The detection was accomplished through a microphone placed at the cell end and a personal computer that performs synchronic detection through a standard sound card. The calibration constant of the setup was obtained for NO2‐air concentrations between 5 and 400 ppmV. During this calibration procedure, the NO2 molecule density was found to change with time due to adsorption. In order to quantify this phenomenon, a theoretical model of the adsorption process was developed under the hypothesis of uniform surface adsorption. To verify its validity, the evolution of the photoacoustic signal amplitude from a low concentration ...

Digitally processed, phase-reading fibre gyroscope

Abstract We present a simple digital processing method that provides a significant improvement for the performance of a phase-reading open-loop fibre-optic gyroscope. The phase-reading gyroscope measures the angular speed in terms of a phase difference between the two counter-propagating beams in the Sagnac interferometer. Instead of electronic phase meters we use a very simple PC-based digital process which computes the Sagnac phase. With this method we measure rotation rates down to 100°h−1. The errors of this scheme are studied for different signal-to-noise ratios and the number of bits of the analog-to-digital converter.