Femtosecond laser-induced damage threshold (LIDT) measurements for different optical components are well studied for a set of laser pulse repetition rates spanning the range between 1 Hz and 1 kHz. Recent years saw the advent of high-repetition-rate femtosecond systems with relatively high pulse energy. Therefore investigation of LIDT in the MHz region is essential. We performed several comparative femtosecond LIDT measurements on typically used ultrafast optical elements with different Ti:Sapphire laser systems having substantially different pulse repetition rates (a 1 kHz regenerative amplifier and a 4.3 MHz long-cavity oscillator) and found a substantially lower MHz LIDT threshhold.
Laser Doppler anemometry offers a non-intrusive in-situ flow measurement method both for scientific and industrial
environments, especially in extreme conditions. Compared to the commercial LDA systems Nano-LDA device was
developed for simultaneous flow measurement, particle counting and sizing down to the nanometer size range. High
detection sensitivity was reached by applying single photon avalanche diodes with photon correlation technique and
special techniques for particle counting (burst selecting) and individual burst signal processing.
In this paper verifying measurements are performed with Palas 2.0 iP aerosol generator and differential mobility analyzer
down to 75nm paraffin particles, which is in accord with the lower size limit of the generator. In case of individual
particle velocity estimations the low SNR signal requires special prepare of the autocorrelation function such as
unfolding, zero padding and windowing. A detailed discussion is shown for the role of the different techniques in
The amplitude technique in the particle sizing requires a calibration process to determine the intensity loss of the system.
The model-based algorithm supports the calibration by the complete simulation of the measurement process and light
scattering. By this way a single calibration measurement for one kind of monodisperse particles can be enough. The
model-based algorithm is tested by measurements with monodisperse particles of different sizes set by the DMA.
A particle sizing algorithm is developed for nanoparticle sizing with laser Doppler anemometry/velocimetry systems. A model-based signal processing method is used to estimate the particle size from the autocorrelation curve corresponding to a single particle transit. Two kinds of figures of merit functions are combined to improve the sensitivity. The optimal setup parameters (refractive index, wavelength, and observation direction) and trajectory error are investigated in independent simulation studies. At 514-nm illumination wavelength, the most sensitive size region is found below 300 nm down to the sizing limit (20 photons on average from a single particle transit). The required laser power is searched for by the lower sizing limit based on the calibration measurement. The size estimation of a polystyrene sphere particle of 50 nm diameter requires at least 123-kW/cm2 laser power density at 350 nm, while 587 kW/cm2 at 514 nm is used in the studied system.
Signal processing method was developed for simultaneous particle counting, sizing and flow velocity measurements with
photon correlation laser Doppler anemometers. The high sensitivity of avalanche photodiodes in photon counting mode
assures the ability to catch the individual particles in the submicron/nanometer size range. A detailed discussion is given
about the optimal set up parameters (wavelength, detector position and refractive index) and calibration measurements
are shown to determine system parameters in a particular arrangement and simulations to estimate the lower size limit of
the particle characterization. Estimations are given for the required laser power to detect 20 photon counts in average for
a single particle transit as a lower limit of the particle sizing procedure. In the calibration measurement the most sensitive
size region was below 300nm down to the sizing limit (20 photons in average) at 514nm illumination wavelength. As a
numerical example we conclude that the size estimation of a polystyrene sphere particle of 50nm diameter requires at
least 123 kW/cm<sup>2</sup> laser power density at 350nm while 587 kW/cm<sup>2</sup> at 514nm in the studied system.
The photon correlation Laser Doppler Anemometers were developed to measure the flow velocity also in the nanometer
particle range. An LDA signal processing method has been developed for dividing the raw data line of photon
correlation LDA into shorter parts corresponding to single particle transit (burst). The commonly used Lee filter was
applied with some modification and an intelligent burst finding algorithm was developed. By this way the LDA system
was adapted for single particle counting. The complete simulation algorithm gives an opportunity for discussing the
burst selecting and so the particle counting efficiency as a function of the SNR. Size estimation from the burst size was
discussed and compared to the model-based signal processing technique. The minimum detectable particle size was
The lower limit of size measurement with PDA is about 150nm particle diameter, because of the small scattering efficiencies under this size. A model-based signal processing method is developed to determine the particle size from the autocorrelation function constructed in photon counting LDA systems by simulating the whole scattering and detecting process. An optimization algorithm is proposed to fit the computed autocorrelation function to the measured one. The errors of this signal processing method are discussed.
A planar phase Doppler system and a photon correlation LDA system was simulated regarding to particle size measurement below 150nm size range. Light scattering on Gaussian beams was simulated by Lorenz-Mie theory (LMT) and superimposed on the detectors' surface. The planar PDA system was found unable to size measurement in this range. Particle size measurement from photon correlated LDA signal is discussed particularly. Adaptability of method developed for visibility measuring was investigated. We established that the total number of photon counts from passing particles is a good estimator of particle size in this size range, because scattered intensity does not oscillate with the size changes. The two particle sizing methods were compared. The photon counting method was found simpler and more sensitive. Dependence on particle trajectory is investigated and found it is comparable for the two methods.