Performance of a range gated system is strongly affected by the laser, sensor, target, and atmospheric parameters. This paper performs a theoretical analysis to investigate the influence of multiple factors on range gated reconstruction. The effects of several factors are discussed based on the operating principle of range gated reconstruction, fundamental of radiant energy, signal to noise ratio (SNR), and bidirectional reflection distribution function (BRDF) models. The presented findings establish a comprehensive understanding of the influence factors in range gated reconstruction which are of interest to various applications and future improvement works to perform accurate range correction and compensation.
Recently, multi-layer surface profiling and inspection has been considered an emerging topic that can be used to solve various manufacturing inspection problems, such as graded index lenses, TSV (Thru-Silicon Via), and optical coating. In our study, we proposed a gated wavefront sensing approach to estimate the multi-layer surface profile. In this paper, we set up an experimental platform to validate our theoretical models and methods. Our test bed consists of pulse laser, collimator, prism, well-defined focusing lens, testing specimen, and gated wavefront sensing assembly (e.g., lenslet and gated camera). Typical wavefront measurement steps are carried out for the gated system, except the reflectance is timed against its time of flight as well as its intensity profile. By synchronizing the laser pulses to the camera gate time, it is possible to discriminate a multi-layer wavefront from its neighbouring discrete layer reflections.
General precursors and growth model of Laser Induced Damage (LID) have been the focus of research in fused silica material, such as polishing residues, fractures, and contaminations. Assuming the absorption due to trapped material and mechanical strength is the same across the surfaces, various studies have shown that the LID could be minimized by reducing the light field intensification of the layers upon the laser strikes. By revisiting the definition of non-ionising radiation damage, this paper presents the modelling work and simulation of light intensification of laser induced damage condition. Our contribution is to predict the LID growth that take into various factors, specifically on the light intensification problem. The light intensification problem is a function of the inter-layer or intra-layer micro-optical properties, such as transmittance and absorption coefficient of the material at micro- or sub-micro-meter range. The proposed model will first estimate the light propagation that convoluted with the multiply scattering light and subsequently the field intensification within the nodule dimension. This will allow us to evaluate the geometrical factor of the nodule effect over the intensification. The result show that the light intensification is higher whenever the backscattering and multiple scattering components are higher due to its interference with the incoming wave within its coherency.
Range gated imaging is a remote sensing acquisition which involves the emission of a laser pulse and an intensified
camera to gate the reflected laser pulse. Range accuracy has always been an issue especially when a highly accurate
reconstructed model is expected as the final outcome. The reflected pulse profile and pulse instability are among the
issues that affect the range accuracy when a general solution such as constant offset is not applicable. In this paper, a
study to estimate a more accurate model for the reflected pulse profile has been investigated through experiments. T
Location-Scale model has been proposed to replace the Gaussian model as the general assumption for range-gated image
formation model. The improvement on range accuracy which is around 0.3% has been verified through simulation based
on the acquired samples. The series of range-gated images can be reconstructed into a three-dimensional depth map
through range calculation. This can be used in the subsequent range reconstruction works.
Recently, semiconductor manufacturers have been striving for high speed, large scale multi-layer wafer surface measurement. In this paper, we propose a novel technique in multi-layer wave-front sensing. The measurement uses a gated camera in pico second shutter that can be synchronized to a pico second laser pulse, up to μm accuracy. Subsequently, we propose a compensation technique using time-of-flight wave-front sensing to reconstruct the multilayer surfaces using our proposed gated imaging technique.