Time-resolved Digital Holography Research Articles

Time-resolved holographic study of laser-induced fatigue in bulk of sapphire and fused silica

In this work, the fatigue effect caused by multi-pulse laser irradiation in bulk of transparent optical medium for ultrashort pulses is investigated. Time-resolved digital holography is used as a sensitive tool for quantifying changes in material response below single-shot damage threshold. In case of two investigated fused quartz samples the survivable dose is limited by increase in the yield of free electrons and self-trapped exciton densities with number of expositions prior to damage formation. Meanwhile, no change was detected in free electron plasma for sapphire before multi-pulse damage which lead to nondeterministic damage. It was estimated that optical damage could form bellow critical plasma density for all samples. The data on low energy irradiation and damage morphology suggest a strong contribution from defects to the initiation of multi-pulse optical damage in bulk media.

(INVITED) Determination of non-linear refractive index of laser crystals and ceramics via different optical techniques

The exact knowledge of optical material parameters is crucial for laser systems design. Therefore, the work reported herein was dedicated to the determination of an important parameter that is typically not known or only known with insufficient precision: the Kerr coefficient determined by the third order non-linearity, also called the n2-parameter. The optical Kerr effect is responsible for the accumulated nonlinear phase (the B-integral) in high energy laser amplifiers, which often represents a serious limitation. Therefore, the knowledge of n2 is especially required for new types of laser materials. In this paper we report measurements carried out on the widely used optical material Ytterbium-doped Yttrium Aluminium Garnet (Yb:YAG) ceramics. Furthermore, the new Neodymium-doped Calcium Fluoride (Nd:CaF2) crystal was investigated. Specifically, three different approaches have been employed to determine experimentally the nonlinear refractive index of these materials. Thus classical Z-scan technique (at two different wavelengths), the degenerated four waves mixing and the time-resolved digital holography techniques, were compared. These different approaches have permitted the precise measurements of these parameters as well as their dispersion estimations.

Persistent thin water films encapsulate oil droplets crossing an oil-water interface

Understanding of oil-water interfacial phenomena is essential for predicting mixing and/or phase-separation in environmental and industrial systems. Time-resolved digital holography and planar laser-induced fluorescence are used for examining processes occurring after ascending buoyant oil droplets of varying viscosity cross a stratified oil-water interface. Previous studies have focused on the crossing process, and the current belief is that once this droplet becomes immersed in the oil, its content can mix with the bulk fluid. In contrast, we show that the droplets remain encapsulated by a stable continuous submicron thin water film, which prevent them from mixing. This film forms even in pure oil and water with minimal surfactant concentration and persists for periods that are three to four orders of magnitude longer than those of the crossing process. Observations following the film evolution reveal that segments located close to the interface appear to be attracted to the bulk water, causing the entire droplet to flatten slowly. The resulting reduction in the peripheral radius of curvature eventually breaks up the film into suspended submicron droplets. The morphology of this flattening process varies with oil viscosity, and its duration increases from seconds to nearly one hour as the oil viscosity increases from one to fifty cSt. In processes involving multiple oil droplets crossing the interface, they form a separate persistent long-lasting layer containing a complex thin-film structure that does not mix with the bulk oil. In contrast, thin oil films do not form around a descending water droplet after it crosses the interface.

Quantitative assessment of nonlinearly absorbed energy in fused silica via time-resolved digital holography.

A fraction of incident optical energy nonlinearly absorbed by a solid medium is considered to be the main quantitative parameter of damage-inducing light-matter interaction. However, its reliable experimental evaluation is a non-trivial task. We have addressed this problem using time-resolved digital holography. This well-proven technique enables recording of time-dependent single-shot induced thermal lens in fused silica excited at fluence levels above the damage threshold and constructing a detailed picture of the dissipation of nonlinearly absorbed optical energy. In addition, we explored the dependence between the absorbed laser pulse energy and incident energy. We found that material modification started to occur when the sample absorbed more than 10% of incident energy, while the absorbance above 15% resulted in catastrophic damage. The proposed approach is expected to become a convenient tool for future studies of light-matter interaction in transparent solids.

Nonlinear refractive index measurements using time-resolved digital holography.

In this Letter, a novel method to evaluate nonlinear refractive index using time-resolved digital holographic microscopy is introduced. To demonstrate the viability of the method, cross-correlative nonlinear refractive index values for sapphire are measured experimentally: 2.75·10-20 m2/W at 1030nm and 4.10·10-20 m2/W 515nm wavelengths. The obtained results for sapphire are compared to those available in literature obtained by other methods.

Time-Resolved Digital Holography in the Investigation of Ablation and Micro Fabrication by Femtosecond Pulses

During the last years it was shown that time-resolved digital holography (TRDH), combining the conventional offaxis digital holography and ultrashort probing laser pulses results in versatile tool suitable for ultra-fast phenomena studies. The temporal resolution of this method depends on the duration of the used ultrashort probing pulses and measurements could be performed in temporal range from ~20 fs to ~10 ns. We review features of TRDH with femtosecond pulses and its applications for the investigation of such processes occurred or used in laser micro fabrication as femtosecond pulse filamentation in transparent solids, thin film damage by femtosecond pulses and one and double pulse femtosecond laser ablation.