Werner Vandermeiren

Time dependence of CO2 laser pulses recorded in the mixed detector regime of the photon drag and Seebeck effects in n-doped GaAs

In this paper, we present experimental work on the temporal profiling of transversly excited atmosphere (TEA) CO2 laser pulses by using a particular kind of semiconductor photodetector. The detector exploits the combined effects of two fast sensing mechanisms, namely, the electron temperature related Seebeck and photon drag effects, and one slow mechanism, the phonon related Seebeck effect. The design of the photodetector is such that the typical emitted TEA CO2 laser pulses (100ns peak pulses in combination with a microsecond long tail) induce a transit response between the fast and the slow sensing mechanisms. In the fast regime, the output voltage is proportional to the temporal evolution of the pulse intensity. Starting from the falling edge of the pulses, this proportionality changes its characteristics gradually such that the output voltage becomes proportional to the time dependence of the laser pulse energy. All experimental results are backed by a theoretical model and numerical simulations.

GaAs-based grating system for Q-switching on the basis of IMOS structure

GSolver software is used to optimize the parameters of a GaAs-based layers structure for modulating the reflectivity of light. This structure can be used in an Integrated Mirror Optical Switch (IMOS) for the Q-switching technology. A system of low doped GaAs and highly doped AlGaAs structure is built on a binary diffraction grating composed of germanium and gold. The diffraction efficiency is determined with and without the existence of free carriers in the highly doped layer. The impact of the sheet charge density at the interface of the heterostructure is considered in determining of the diffraction efficiency. At the end of the study, the structure parameters and thicknesses are determined for a high sensitive device.

Infrared Thermo-Electric Photodetectors

During the last decade, pulsed laser radiation has gained interest by the material processing industry and the medical sector. For a growing set of applications (laser drilling, laser marking, laser surgery, semiconductor doping profiling, micro-structuring, layer deposition, etc.) it is advantageous to use pulsed laser radiation instead of continuous wave (CW) illumination as time limited exposure often results in reduced collateral damage and more precise processing (Phipps, 2007). In laser ablation for example, one aims to put an intense laser pulse on the surface of a target material in an as short time as possible. This short exposure time, which limits thermal diffusion inside the material, together with a carefully selected wavelength with a minimal absorption depth is required to ensure energy deposition in a small volume of the target material. Hence, for laser pulses which meet the ablation requirements, one can evaporate material in a very controlled fashion. Different methods exists as Q-switching, mode-locking and cavity dumping for achieving the required pulse characteristics for laser ablation. Laser pulses span an enormously large parameter space in terms of wavelength, repetition rate, pulse duration and pulse energy, further referenced as pulse-parameters. Each of these pulse-parameters can be optimized for a given application and target material. Besides its dependence on the temporal characteristics of pulsed laser radiation, for some applications, the processing quality is also strongly dependent on the transverse laser beam profile. Consequently, there is a growing interest in detecting the spatio-temporal behavior of laser pulses. In this chapter we briefly describe the most common infrared detector principles for measuring laser pulses and point out their respective advantages and disadvantages with respect to different pulse-parameters. Next, we show that Seebeck-effect based thermo-electric photodetectors can be designed to cover a relatively broad range of pulse-parameters (Stiens, 2006). Further, we discuss the working principle and operation regimes of the thermo-electric photo detector and explain the corresponding theoretical background in detail. Experimental results concerning short laser pulse induced thermo-voltages in n-GaAs are presented. This chapter is also concerned with the possibility of using the thermo-electric effect to measure the spatio-temporal behavior of laser pulses by means of linear focal plane arrays (LFPA). Certain related issues will be highlighted such as

Fourier processing of the speckle structure of the optical field formed in the process of multiphonon Bragg diffraction

The quantitative characteristics of the speckle structure of an optical field are obtained by Fourier processing the optical signal, using a transparency that differentiates the speckle pattern. It is shown in terms of the Gauss-Schell model that the maximum values of the differential of the processed pattern are proportional to the degree of spatial coherence of the corresponding section of the optical field. The results are found to be in good agreement with the data obtained when other methods are used (in particular, a method based on the angular selectivity of multiphonon acoustooptic Bragg scattering).