Alexander V. Smirnov

Bimodal temporal distribution of photons in ultrashort laser pulse passed through a turbid medium

Abstract The criteria were determined for simultaneous observation of scattered and unscattered (ballistic) peaks in temporal distribution of a narrow pulsed laser beam passed through a strongly scattering medium. Conditions were found for such an observation on the basis of non-stationary two-flux model for radiation transport. Calculation results permitted the definition of a compromise between the parameters of the initial laser pulse and the properties of a scattering medium. This provided the separate observation of various types of photons. The theoretical calculations were confirmed by experiments on recording pulse profiles of passed radiation of fs-laser in a water–milk solution.

200-femtosecond streak camera: development and dynamic measurements

On the basis of the recently developed PV-FS-type femtosecond streak tubes supplied with the original photocathode-accelerating mesh assembly (dedicated for pulse operation) and ultra-sensitive high frequency deflection system, the experimental prototypes of a 200 fs streak camera was designed, manufactured and tested. The camera experimental time resolution > 200 femtosecond, its maximum spatial resolution along the slit direction is better than 30 line pairs/mm in dynamic mode. The range of the streak duration along the 25 mm output screen is from 50 ps to 5 ns, which corresponds to the streak speeds of 5 × 1010 - 5 × 108 cm/s. The camera triggering delay is 50 ns with ± 20 ps jitter at triggering signal of 5 Volts/50Ohm/1ns rise time with the maximum repetition rate of 100 Hz. At the maximum time resolution the dynamic range is about 10. The spectral range of the recorded optical events may cover 115 - 1550 nm. The typical optical radiation intensity at 800 nm needed to illuminate the input photocathode in order to get maximum time resolution is 500 Watts/cm2.

Single-shot autocorrelator

The single shot autocorrelator designed to measure the AF of the ultrashort laser pulses in the temporal range of 30 - 200 fs and in spectral range 730 - 820 nm is described.

Picosecond streak camera application in experiments on optical tomography of biological media

There are some difficulties in the use of a well-known X-ray computer tomography technique due to the strong light scattering in biological tissues. We propose to analyze the whole light scattering picture for the development of novel processing algorithms in tomographical imaging. Time-resolved experiments on the forward scattering of IR-light from a model of biological media illuminated by a picosecond YAG-laser with 10 ps pulse duration have been performed. The scattering radiation was recorded by a streak camera having 1.5 ps time resolution. From these results and theoretical considerations the analytic expression for intensity against time distribution of the laser radiation passed through the media has been obtained. The expression can be used as the basis for tomographical reconstruction.

Photon type selection in contrasted scattering media

The selection of ballistic photons from a radiation, passed through a biological medium, is necessary for obtaining qualitative images in optical tomography. Within the framework of a non-stationary two-flux model of radiation transport in high scattering medium there is found the fraction of ballistic photons as function on macroscopic characteristics of a medium. With the purpose of separation of photons the application of bisphthalocianine dyes of rare-earth and transitional elements is considered. The experiments on deriving contrasted images in margarine and water solution of milk are carried out.

Description of bimodal temporal distribution of photons in laser pulse passed through a turbid medium

The criteria were determined for simultaneous observation of scattered and unscattered (ballistic) peaks in temporal distribution of a narrow pulsed laser beam passed through a strongly scattering medium. Conditions were found for such an observation on the basis of non-stationary two-flux model for radiation transport. Calculation results permitted to define a compromise between parameters of the initial laser pulse and properties of a scattering medium. This has provided the separate observation of various types of photons. The theoretical calculations were confirmed by experiments on recording pulse profiles of passed radiation of fs-laser in a water-milk solution.

Attenuation and scattering of short-duration laser pulses in a strongly scattering medium

The dependence of the optical density of a model strongly scattering medium-aqueous milk solution on its layer thickness was investigated. IR lasers generating pulses of various short (from nanosecond to femtosecond) duration were used as the radiation sources. There were determined the dependences of the attenuation coefficients of such pulses on the solution concentration in the areas of low and more high optical densities of the solution layer for different values of radiation detector angular aperture. A modification of the two-flux Kubelka-Munk model was used to derive an expression describing the dependence of the transmission of a solution layer on its parameters when radiation detectors with a finite angular aperture are used. The absorption and scattering coefficients of the medium were obtained. A comparison of the calculations and experiments revealed a forward scattering anisotropy of short-duration laser radiation in an aqueous milk solution characteristic for the Mies scattering.

Optical system development for image converter tube photocathode illumination by femtosecond laser pulse

The paper discusses potential causes for femtosecond laser pulse (FP) broadening by an optics system. Design options are suggested for optics systems with minimized distortion in the FP space-time pattern.

Limiting time resolution of streak cameras versus the recorded wavelength

The goal of the present work is to determinate the limiting time resolution value for streak camera versus the recorded wavelength and to detect the photoelectron energy dispersion by processing the obtained data. In the experiments the femtosecond laser system operating in spectral range from 210 nm to 1500 nm and the Imacon-500 streak cameras equipped with the PV-001/S1 were used. Based on the received data the dependence of energy photoelectrons dispersion versus the recorded wavelength was obtained which is in a good agreement with the published data.

Research and development on femtosecond cameras and diffractometers

Current GPI status on femtosecond cameras and diffractometers research and development is overviewed. Discussed are the key components of the femtosecond diffractometer which is now under design. The first experimental prototype of 20 - 40 kV, femtosecond photoelectron diffractometer is computer designed, manufactured and tested.