We propose a new method of image encryption using Fourier computer-generated hologram (CGH) in the encryption system of multiple Fresnel diffraction transforms with phase masks. The digital image to be encrypted is modulated by a series of three random-phase masks in Fresnel diffraction system and finally is transformed into a complex-amplitude image which is stationary white noise (in which the information is like stationary-white-noise). Because the complex-amplitude information is not easy to be directly saved, the binary real value Fourier CGH is applied to record it. Compared with the traditional double random-phase image encryption technology, this method adds new keys which enhance the image encryption security and the Fourier CGH greatly improves the antinoise performance.
Up to now, a variety of methods have been developed for the single-shot THz detection, which include spectral encoding
technique [2-3], optical streak camera , non-collinear geometry spatial encoding , non-collinear cross correlation
technique , retrieval algorithm based on in-line spectral interferometry , two-dimensional electro-optic imaging
with dual echelons , tilted front collinear geometry , etc.
For a single-shot characterization of THz pulse, all of the schemes mentioned above can be, however, only employed to
measure the electric field of a single-shot THz either in its spatial or temporal domain, respectively, in real time.
In this paper, we describe a method for a single-shot recording of the full spatiotemporal electric field, E(x, y, t), of
freely propagating terahertz pulse based on the electro-optic (E-O) sampling technique and the pulsed digital holographic
approach. From a series of sub-holograms recorded digitally, the complete THz electric field E(x, y, t) can be recovered
by the following digital reconstruction algorithm. The spatial and temporal resolutions are limited by the wavelength of
terahertz pulse and the probe pulse duration, respectively. Our new method will open a possibility of a full
characterization of the three-dimensional THz field E(x, y, t) in a single-shot mode.
Shadowgraphs of dynamic processes outside and inside the target during the intense femtosecond laser ablation of silica
glass at different energy fluences are recorded. Two material ejections outside the target and two corresponding stress
waves inside the target are observed. In particular, a third stress wave can be observed at energy fluence as high as 40
J/cm<sup>2</sup>. The pressure, the temperature, the free electron density, and the ionic components at the laser pulse end are
estimated, based on which the mechanical reaction of the laser heated material is investigated. According to our analysis,
the first wave is a thermoelastic wave, while the second and the third may be generated subsequently by the mechanical
expansions. Besides, the velocities of the stress waves are deduced from the time-resolved shadowgraphs, and it is found
that the first stress wave propagates with a velocity greater than the sound velocity, while the second stress wave
propagates with a velocity less than the sound velocity. Therefore, the first wave is a supersonic shockwave with a high
stress magnitude, while the second may be the plastic stress wave or subsonic shockwave with a lower stress magnitude.
Further more, the temporal evolution the second stress wave is investigated, and its velocity is found to increases
gradually at large delay times. According to the extrapolation curve, however, it is speculated that the velocity decreases
from a high value initially, which could be due to the interaction between the first and second stress waves at small delay
times. These results can provide a further support to the theory of highpressure shock phenomena in femtosecond laser
We report on approaches of pulsed digital holographic recording and its relevant digital reconstruction based on Fourier
optics for measurements of high resolution in time domain or in space domain, in which angular division multiplexing
system is respectively employed to record multiple images in a single frame of a CCD. This new approach can be used to
record either a series of images in an ultra-fast process with high resolution in time domain, or to record a transparent
object with super-resolution in space domain. In the digital reconstruction process, Fourier transformation and frequency
filtering in the Fourier plane will be employed to separate the spatial spectra of the multiple recordings, and each of the
reconstructed images can be displayed individually with a time resolution of the femto-second order or fused into an
image of the object with a synthetic aperture.
We report on pulsed digital micro holographic systems recording ultra-fast process of the femto-second order, by
spatially angular division multiplexing (SADM) and wavelength division multiplexing (WDM), respectively. Both
intensity and phase images of the digitally reconstructed images are obtained through Fourier transformation and digital
filtering, which show clearly the plasma forming and propagating dynamic process of laser induced ionization of
ambient air at the wavelength of 800 nm, with a time resolution of 50 fs and frame intervals of 300 to 550 fs.
In this paper, we report, for the first time, a specially designed optical gating system of spatially angular multiplexing for ultra-short pulsed digital holography, which can be employed to record ultra-fast processes of the order of femto-second. The sampling resolution of the dynamic process ranges from 50 fs to 10 ps. By adjusting the time delays and the angles included between the sub-pulses, the system sampling interval ranging from 67 fs to 170 ps is obtained.
The laser induced ionization of ambient air is studied experimentally with laser pulses whose durations range from 50 fs up to 10 ps at 800 nm. It is found that the minimum pulse energy for detectable air ionization follows the scaling law of ε<i><sub>th</sub></i> varies direct as <i>t<sub>p</sub><sup>x</sup></i>, with 0.23 < x < 0.5, and x tends to rise for longer pulses within the range of 50 fs - 500 fs. For laser pulses from 0.7 ps to 10 ps, however, x is approximately equal to 0.8. The dependence of the critical intensity for air ionization on the beam spot size is also examined with a variety of focused laser beam spot sizes in the experiments.