Comparative analysis of pressure wave propagation, following laser-induced ablation of the human cornea, was performed. Evolution of the pressure field as well as temporal and spatial dependencies of ...the transient pressure amplitudes created at various input laser parameters, corresponding to different typical procedures of laser eye surgery, is presented comprehensively. The computations were performed with the next generation of the acoustic eye model, previously validated against existing measurements. The analysis allows the assessment of potentially problematic regions within the eye where the resultant positive and/or negative pressure may exceed values which are considered safe for the patient.
We present an optical detection technique, called the beam deflection probe, to accurately measure contact times of normal-incidence impacts of steel and sapphire spheres with a transparent glass ...block. It exploits the deformation of the area that is in a mechanical touch during the rebound. The deformation of the surface acts as a dynamic contact mirror. When illuminated by a laser beam, total internal reflection takes place at the contact mirror, the beam is deflected from it, and its angular deflection history is monitored by a quadrant photodiode. A simple threshold-level data processing of the photodiode signals is used to determine the impact duration. It is shown that the shape of the signal is highly dependent on the location of the impact relative to the center of the laser-beam illuminated area while the determination of the contact time does not depend on the impact position. Using an automated ball release mechanism, the contact time of low-velocity impacts was measured for various ball diameters and approach velocities conforming to the Hertz contact theory. The proposed optical detection of contact times supplements the existing measurement techniques and represents the only alternative to the piezoelectric detection when contact times are to be measured on the microsecond scale.
The design and development of the acoustic eye model (AEM) is reported. The model consists of a computer-based simulation that describes the propagation of mechanical disturbance inside a simplified ...model of a human eye. The capabilities of the model are illustrated with examples, using different laser-induced initial loading conditions in different geometrical configurations typically occurring in ophthalmic medical procedures. The potential of the AEM is to predict the mechanical response of the treated eye tissue in advance, thus complementing other preliminary procedures preceding medical treatments.
A distributed feedback (DFB) diode, gain switched to produce pulses from 60ps at high peak power of over 0.5W, is used in burst mode to seed a fiber amplifier chain. High seed power, spectral ...filtering between amplifier stages and pulsed pumping are used to mitigate amplified spontaneous emission (ASE). The effect of pulse pumping synchronized with the seed on the ASE is explored for the power amplifier at low repetition. Different input and output energies at different burst repetition rates are examined and up to 85% reduction in ASE is achieved compared to continuous pumping. Finally, a numerical model is used to predict further reduction of ASE.
•A DFB diode driven to produce pulses in 10s of ps with peak powers over 0.5W.•Preshaped bursts amplification at low repetition rate.•A theoretical and experimental study of the effect of pulsed pumping on the ASE.•ASE reduction using synchronous pulsed pumping up to 85%.
For quality control in high volume manufacturing of thin layers and for tracking of physical and chemical processes, ellipsometry is a common measurement technology. For such kinds of applications we ...present a novel approach of fast ellipsometric measurements. Instead of a conventional setup that uses a standard photo-elastic modulator, we use a 92 kHz Single Crystal Photo-Elastic Modulator (SCPEM), which is a LiTaO3 crystal with a size of 28 × 9 × 4 mm. This small, simple, and cost-effective solution also offers the advantage of direct control of the retardation via the current amplitude, which is important for repeatability of the measurements. Instead of a Lock-In Amplifier, an automated digital processing based on a fast analog to digital converter controlled by a highly flexible Field Programmable Gate Array is used. This and the extremely compact and efficient polarization modulation allow fast ellipsometric testing where the upper limit of measurement rates is mainly limited by the desired accuracy and repeatability of the measurements. The standard deviation that is related to the repeatability +/-0.002° for dielectric layers can be easily reached.
A study of the effects of a cavitation bubble on a thin elastic membrane is presented. A cavitation bubble was produced by focusing a high-energy laser beam near an elastic membrane submerged in ...water, which corresponds to conditions often encountered in ophthalmology. The bubble effects on the membrane were studied as a function of distance between the bubble and membrane and laser pulse energy. Recordings of bubble dynamics with an ultra-high-speed camera were performed on both sides of the membrane, providing better insight into the mechanisms of membrane rupture. Observations have revealed distinct bubble behavior regimes with respect to coefficient γ. Cavitation bubbles centered on the membrane produce less damage than bubbles shifted slightly away. Significant damages were observed at intermediate distances between the bubble and membrane after bubble collapse, while weaker interactions were recorded at larger distances with less or no damaging effect to the membrane.
•Observation of bubble dynamics near an elastic membrane with a high-speed camera.•Membrane rupturing caused by a laser-induced bubble.•Observation of different regimes of bubble behavior with respect to parameter γ.•Comparison of bubble effects at different laser pulse energies and values of γ.
Recently, we developed an optodynamic experimental technique that makes it possible to measure the linear momentum obtained by a metal target sample in the shape of a rod during a nanosecond laser ...pulse interaction in the ablative regime. The height of the rod’s rear end axial step-like displacement, caused by the first reflection of the laser-generated ultrasonic wave, is proportional to the linear momentum acquired by the rod. In comparison with commonly used ballistic methods, we can determine the acquired momentum on a much shorter time scale corresponding to the wave transition time, from the front to the rear end of the rod. Using this method we investigated the ambient air pressure dependence on the formation of linear momentum over a laser intensity range, from the ablation threshold to values about ten times higher. Steel rods of various diameters were used to demonstrate the effect of an expanding blast wave which delivers additional momentum to the target, when the laser beam on the target surface is smaller than the target itself. The typical value of the acquired target momentum is on the order of μN s and 10 μN s/J for the momentum coupling coefficient.
A micro-explosion occurring after pulsed laser-induced breakdown in water can be effectively described as an optodynamic phenomenon. It is a source of shock waves that propagate into the surrounding ...medium. We present a measurement of the source size based on far field detection of the propagating waves using a beam deflection probe technique. By taking into consideration the moving breakdown model the relation between the size of the breakdown region and the apparent size of the optodynamic source is shown. An energy dependent enlargement parameter d is introduced which describes the relation.
The retardation-, current-, and displacement-amplitudes of a Single Crystal Photo-Elastic Modulator (SCPEM) made of a LiTaO 3 -crystal with a resonance frequency of 127.16kHz is measured with ...interferometry and the underlying theory verified. It is found that the retardation amplitude and the current amplitude are exactly linearly correlated. The relation is independent from the crystal damping and from the excitation voltage. A control of the current is therefore sufficient to control the crystal retardation.
A practical optodynamic study was performed to determine the usability of different lengths of laser pulses for the generation of ultrasonic transients in a solid material. The aim of the study was ...to evaluate the possibility of a dual use for a laser pulse—for laser material processing, on the one hand, and for the ultrasonic wave generation on the other—with both processes being combined on the same production line. The propagation of the laser-generated ultrasonic waves is evaluated by detecting and measuring with a PID-controlled stabilized interferometer. Thus, both systems provided the basic tools, the generation and detection of ultrasonic waves, for an ultrasonic, laser-based, non-destructive material evaluation. The ultrasonic transients generated by ‘classical’ nanosecond laser pulses were compared with the transients generated by industrial laser pulses with a duration of a few tenths of a microsecond. The experimental results are compared with the results of a time-of-flight analysis that also involved part of a mode-conversion analysis for both regimes in a layered material structure. The differences between the two waveforms were assessed in terms of their visibility, wavelength and resolution. The limit values were calculated and estimated for the laser-pulse parameters, when such pulses are intended for use in an ultrasonic, laser-based, non-destructive evaluation. The possibility of using an industrial marking laser for laser ultrasound generation is thus demonstrated.
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