Hongchen Zhai

Angular multiplexing in pulsed digital holography for aperture synthesis

A new aperture-synthesis approach in a pulsed digital holography system for obtaining superresolution is proposed and experimentally demonstrated in which, with only a single laser pulse divided into two trains of subpulses, angular multiplexing can be applied to both the reference beam for different carrier frequency and the object beam for off-axis and on-axis illuminations. Furthermore, in the recording plane, multiple subholograms covering different spatial frequency ranges diffracted from the object can be recorded in a single frame of a CCD. Our analysis and results show that the resolution of the reconstructed image synthesized upon the reconstructed complex amplitudes of the subholograms can be improved effectively.

Pulsed digital holography system recording ultrafast process of the femtosecond order.

We report, for the first time to our knowledge, a pulsed digital microholographic system with spatial angular multiplexing for recording the ultrafast process of the femtosecond order. The optimized design of the two sets of subpulse-train generators in this system makes it possible to implement a digital holographic recording with spatial angular multiplexing of a frame interval of the femtosecond order, while keeping the incident angle of the object beams unchanged. Three pairs of amplitude and phase images from the same view angle digitally reconstructed by the system demonstrated the ultrafast dynamic process of laser-induced ionization of ambient air at a wavelength of 800 nm, with a time resolution of 50 fs and a frame interval of 300 fs.

Generation of multiple stress waves in silica glass in high fluence femtosecond laser ablation

Shadowgraphs of dynamic processes outside and inside transparent target during the intense femtosecond laser ablation of silica glass are recorded. Two material ejections outside the target and two corresponding stress waves inside the target are observed at different energy fluences. In particular, a third stress wave can be observed at energy fluence as high as 40 J/cm2. The first wave is a thermoelastic wave, while the second and the third may be generated subsequently by the mechanical expansions. In addition, the magnitudes of the three stress waves decrease sequentially based on our analysis.

High-fluence femtosecond laser ablation of silica glass: effects of laser-induced pressure

A pump-probe technique is employed to investigate the dynamic process of femtosecond laser ablation of silica glass, and three stress waves are observed inside the silica glass in the time-resolved shadowgraphs at a fluence of 40 J cm−2. It is believed that the first stress wave is a thermoelastic wave generated by thermal expansion, while the second and the third are generated subsequently by the mechanical expansions as a result of the extremely high pressure induced by the laser. By employing digital holography, the ablation crater is investigated to identify the ablation results, and high-density silica glass is found beneath the ablation crater, which is also the result of the laser-induced pressure. Furthermore, it is found that more stress waves will be generated at a higher fluence, and the thresholds for the generation of the first, second and third stress wave are measured to be about 2 J cm−2, 6 J cm−2 and 31 J cm−2, respectively. The theoretical calculation result shows that the laser-induced pressure increases with the increase in fluence and can be on the scale of TPa, and we believe a higher pressure will induce more mechanical expansions and therefore more stress waves.

Fuzzy color-image retrieval

In this paper, we propose for the first time an application of fuzzy relations in fuzzy set theory to the measure of color identity, where a membership function is used as a measure of the similarity, and a fuzzy-match of two colors is defined by the α-cut fuzzy relation. Besides, a fuzzy-search process of color histograms is developed for the first time to save from massive unnecessary calculations. Experimental results in the application to color-image retrieval confirm the advantages of our approach in both accuracy and efficiency of the color-image retrieval, as predicted in our theoretical analysis.

Improving binary images reconstructed from kinoforms by amplitude adjustment

Abstract A method for amplitude adjustment is proposed in this paper for the first time to improve the quality of binary images obtained from kinoforms by Iterative Fourier Transform Algorithm (IFTA). By setting the amplitude of the “zero” area in a binary image to a low but non-zero amplitude, their phase is involved in the iteration, which results in an increase of the freedom degree in the design of kinoforms, and an improvement of the quality of the reconstructed binary images. Quantitative illustrations and discussions of the improvement effect are given from simulation results.

Comparison of femtosecond laser ablation of aluminum in water and in air by time-resolved optical diagnosis

The dynamic process of material ejection and shock wave evolution during one single femtosecond laser pulse ablation of aluminum target in water and air is experimentally investigated by employing pump-probe technique. Shadowgraphs and digital holograms with high temporal resolution are recorded, which intuitively reveal the characteristics of femtosecond laser ablation in the water-confined environment. The experimental result indicates that the liquid significantly restrict the diffusion of the ejected material, and it has a considerable effect on the attenuation of the shock wave. In addition, the expansion Mach wave generated by the ultrasonic expansion of the shock wave is observed.

Numerical iterative approach for zero-order term elimination in off-axis digital holography

A novel numerical iterative approach is proposed to effectively eliminate the zero-order term and to improve the signal-to-noise ratio of the reconstructed image in off-axis digital holography. The iterations are conducted in the spatial domain, resulting in considerable reduction in the computational time and avoiding the subjectivity involved in selecting a filter window in spectral domain. These advantages promote the application of this approach in real-time detection processes. The feasibility of this approach is confirmed by mathematical deductions and numerical simulations, and the robustness of the proposed approach is tested by means of an experimentally obtained hologram.

Single-exposure approach for expanding the sampled area of a dynamic process by digital holography with combined multiplexing

In this paper, an approach for expanding the sampled area of a dynamic process by digital holography is reported, where angular, polarization and wavelength multiplexing are applied to record different regions of the sample on a series of sub-holograms overlapped in a single frame of the charge-coupled device. This approach based on a single exposure has special potential for the digital holographic recording of dynamic processes.

Spatial angular multiplexing for enlarging the detected area in off-axis digital holography

In off-axis digital holography, e.g., for detecting and imaging ultrafast phenomena, the interference region is confined to a limited region due to the short extent of the light pulse along the propagation axis. Therefore, the detected area of the object wavefront is limited. A recording method for enlarging the detected area in the above case is proposed in this Letter, in which multiple interferences between the object and the reference waves are obtained by spatial angular multiplexing. Experimental results demonstrate the validity of this method.