Frequency Spectrum Of Laser Research Articles

On using the DFOS for damage identification in civil engineering structures

AbstractThe application of distributed fiber optic sensing (DFOS) technologies to the monitoring of civil engineering structures has been widely developed in recent years. In particular, fiber optic sensing (FOS) using the Rayleigh backscattering phenomenon of the laser frequency spectrum has been used to continuously measure strain along the fiber optic sensor with high spatial resolution. However, the practical implementation of fiber optic sensorsin the field is still a challenge. These include, but are not limited to, the bonding between fiber sensor cables and the surrounding medium, the practical way in which fiber optic sensors can be installed in concrete structures, and the analysis of crack width using the strain distribution along the fiber. Compared to classical discrete sensors, the DFOS has a great potential to detect, localize and quantify crack formation even in the early stages of microcracking. This paper discusses the potential and challenges of using distributed fiber optic sensors for crack detection in concrete structures. In this sense, different types of mechanical tests on concrete structures using different fiber optic sensors, and different bonding techniques are presented to explore the possibilities of crack detection in concrete structures and its potential for practical structural health monitoring (SHM) systems of civil structures.

Time-dependent ionization models designed for intense and short laser pulse propagation in dielectric materials

When an intense and short laser pulse propagates in a dielectric material, significant production of conduction electrons through multiphoton absorption (MPA) may occur. In addition to the laser intensity, the MPA process depends mainly on the laser frequency spectrum which may evolve significantly during the course of laser propagation in the material. Simple models for MPA accounting for possible time-dependent evolution of the laser frequency spectrum (as harmonic generation, chirping or broadening) are addressed. The first model is based on Bloch-Volkov states whereas the second approach relies on the density matrix formalism which has been adapted for the present study. Both models are well adapted for their introduction in a propagation code and are shown to correctly account for the MPA process whatever the characteristics of the laser frequency spectrum. The reliability of these approaches has been studied in two cases of practical interest. First, in the case where a second harmonic is present within the fundamental pulse, calculations show that the ionization rate may be significantly enhanced. Second, in the case of a chirped pulse, models are shown to correctly account for possible change in the multiphoton order during the course of interaction.

Extended Rate Equations Model for Describing the Dynamics of the Laser Frequency Spectrum

Laser rate equations are written and solved in the frequency domain for homogeneous as well as for inhomogeneous gain. The resulting laser model is capable of describing the ne structure as well as the dynamics of the laser frequency spectrum. The calculation shows that laser lines have a Lorentzian-like lineshape. The linewidth is found to be close to the spectral width of the cavity in case of inhomogeneous gain, whereas it is proved to approach the quantum limit for homogeneous gain.

Optical heterodyne measurement of pulsed lasers: Toward high-precision pulsed spectroscopy

In this paper we explore theoretically and experimentally the effect of fluctuations in the instantaneous frequency of a pulsed laser on the shape and position of two-photon transition spectra. The usual procedure of characterizing a pulsed laser by its frequency energy spectrum is insufficient for precision measurements. The nonlinear nature of the two-photon transition produces a systematic shift of the atomic spectrum with respect to the laser frequency spectrum that is dependent on the phase evolution of the laser pulse. In fact, any nonlinear process (e.g., second-harmonic generation) may result in displaced or distorted spectra. We also find that Fabry-P\'erot filtering a laser pulse can result in large frequency chirps. We use an optical heterodyne technique to measure the instantaneous frequency of our excimer-pumped dye-laser system to an uncertainty of 1.3 MHz and determine the effect of the inherent frequency chirps of this system on a two-photon transition. We conclude that with this technique, precision nonlinear spectroscopy to the level of 1 MHz may be achieved with pulsed lasers.

Influence of optical feedback on laser frequency spectrum and threshold conditions

The steady state behavior of the external cavity operated laser has been analyzed, taking into account multiple reflections. The effect of optical feedback is included in the phase- and gain-conditions by a factor which is shown to have a simple geometrical representation. From this representation it is easily seen how the laser frequency spectrum and the threshold gain depend on external parameters such as distance to the reflection point and the amount of optical feedback. Furthermore, by inserting a variable attenuator in the external cavity and measuring the threshold current versus transmittance we have simultaneously determined the photon lifetime and the absolute amount of optical feedback. For the laser considered we found the photon lifetime \tau_{p} = 1.55 ps.

Frequency Spectrum of Laser Echoes from Atmospheric Constituents and Determination of the Aerosol Content of Air

Atmospheric constituents laser echoes spectrum analysis and determination of air aerosol, considering wind motion, temperature and composition