Proc. SPIE. 10891, Nanoscale Imaging, Sensing, and Actuation for Biomedical Applications XVI
KEYWORDS: Diffraction, Real time imaging, Imaging systems, Spatial frequencies, Image resolution, Computer programming, Fluorescent markers, Super resolution microscopy, In vivo imaging, Spatial resolution
Almost all known nanoscopy methods rely upon the contrast created by fluorescent labels attached to the object of interest. This causes limitations on their applicability to in vivo imaging.
A new label-free spectral encoding of spatial frequency (SESF) approach to nanoscale probing of three-dimensional structures has been developed. It has been demonstrated that spatial frequencies, encoded with optical wavelengths, can be passed though the optical system independent of the resolution of the imaging system. As a result information about small size structures can be detected even using a low resolution imaging system.
Different versions of the SESF imaging have been published [1-7], including a novel contrast mechanism for high resolution imaging , real time nano-sensitive imaging , reconstruction the axial (along depth) spatial frequency profiles for each point with nano-sensitivity to structural changes , and the adaptation of the SESF approach to depth resolving imaging [4,5]. Recently the SESF approach has been applied to break the diffraction limit and dramatically improve resolution [6,7].
Here we present further development of the SESF approach including correlation mapping SESF imaging. Both results of numerical simulation and preliminary experimental results, including biological objects, will be presented.
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The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased
the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the
laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and
time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material
and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the
removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal
rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical
feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties
(AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber
amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with
the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a
resolution of 0.41 μm by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector
In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In
particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of
stainless steel is investigated to stably reproduce a fixed penetration depth using both CO<sub>2</sub> and Nd:YAG lasers. Plasma
electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of
emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron
temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration
depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set
penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point
corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set
point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with
the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.
Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by
developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have
been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ
monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The
displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced
modulation of the interferometric signal out of the detector system.