Alejandro Peuriot

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.

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.

Local Realistic Models Tested by the EPRB Experiment with Random Variable Analyzers

We carry out an analysis of the output data files of the Einstein-Podolsky-Rosen-Bohm experiment realized with random variable analyzers (Innsbruck, 1998). That experiment was devised to disprove Local Realistic alternatives (to Quantum Mechanics) based on the predictability of the analyzers’ positions. The disproval is not immediate, for the quantum mechanical results can be reproduced even if the probability of guessing the analyzers’ position is small. Our analysis is aimed to determine, in quantitative terms, whether this experiment was able to disprove Local Realism. We conclude that the experiment did disprove Local Realism. In addition, the experiment provides time-resolved data files which allow us to look for privileged frequencies in the spectrum of the time fluctuations. Such privileged frequencies, if existed, could be interpreted as a sign of a non-ergodic behavior. To our knowledge, this is the first time such a study is proposed and performed.

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.

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.

Photoacoustic detection by means of a differential double resonator cell applied to security and defence

The purpose of this article is to present a sensitive optical system for immediate detection of traces of ammonia by means of photoacoustic spectroscopy and study some properties with both a pulsed CO2 laser (TEA) and a CW CO2 laser. The laser beam is aimed to an innovative dual resonator differential cell, which lowest resonant frequency is the first longitudinal mode at 1205 Hz, filled with a flowing NH3 and N2 mixture. The chosen cell’s material is polypropylene, suitable for reducing the effects of adsorption. As a result of physical adsorption-desorption studies, based on a pulsed CO2 laser, 5 % PA signal decay from an enclosed sample of 248 ppmV of NH3 in N2 is recorded within 1 hour. The setup for CW CO2 laser excitation takes advantage of a differential microphone connected to both resonators by picking up out of phase signals. For this purpose, the beam is modulated at the cell’s resonance by means of a chopper with a special blade which allows both reflection and transmission of the laser beam; the direct and the reflected beam are alternatively aimed to one resonator and the other. The measurements show that for the double resonator configuration a signal increase is achieved, as expected from the study of the sensitivity of both resonators separately, which have been previously characterized. Measurements with this system indicate a limit of detection of 13ppbV at the 10P(32) laser line, deduced from one standard deviation of the PA signal from pure N2.

Photoacoustic systems for NO2 traces detection

Vast research work on visible-laser-based photoacoustic spectroscopy of NO2 in air has been performed at the CEILAP. The study of the mechanisms of acoustical energy losses in a cavity is carried out exciting with a pulsed visible laser. For precise measurements a processing method is developed which allows obtaining high resolution of the photoacoustic signal spectrum when the acquisition window is limited. Devices are shown, where the resonant excitation is obtained with either a mechanically amplitude-modulated or an acousto-optically Q-switched green laser. A simpler, more compact and cheaper acquisition system is obtained through digitizing the pre-amplified signal by the sound card of a PC. The detection limit of different setups and an application to the measurement of the NO2 content in car exhausts are presented. Moreover, a one-dimensional model for cell design and otimization of S/N ratio is developed. It is verified by measurements on NO2 mixtures in a specially built acoustic cavity with a detached Helmholtz resonator.

Extension of the self-filtering unstable resonator to a ring cavity

We present a numerical and experimental study of a confocal unstable ring resonator that performs similarly to a self-filtering unstable resonator. The resonator presents two apertures: one of them is the output coupler and the other provides spatial filtering. A simple rule for the choice of the aperture size enables us to expand the output beam diameter without increasing the magnification or the total length. The resonator is tested in a TEA CO2 laser in which the output beam diameter, the output energy, and the effective reflectivity are measured. The energy ratio of the forward and reverse waves scales approximately as the square of the magnification. The results show good agreement with the theoretical values.