The Influence of the Processing Parameters on the Laser-Ablation of Stainless Steel and Brass during the Engraving by Nanosecond Fiber Laser

In this paper, we investigate the influence of the following parameters: pulse duration, pulse repetition rate, line-to-line and pulse-to-pulse overlaps, and scanning strategy on the ablation of AISI 316L steel and CuZn37 brass with a nanosecond, 1064-nm, Yb fiber laser. The results show that the material removal rate (MRR) increases monotonically with pulse duration up to the characteristic repetition rate (f0) where pulse energy and average power are maximal. The maximum MRR is reached at a repetition rate that is equal or slightly higher as f0. The exact value depends on the correlation between the fluence of the laser pulses and the pulse repetition rate, as well as on the material properties of the sample. The results show that shielding of the laser beam by plasma and ejected material plays an important role in reducing the MRR. The surface roughness is mainly influenced by the line-to-line and the pulse-to-pulse overlaps, where larger overlap leads to lower roughness. Process optimization indicates that while operating with laser processing parameters resulting in the highest MRR, the best ratio between the MRR and surface roughness appears at ~50% overlap of the laser pulses, regardless of the material being processed.

In vitro study of cataract extraction by bursts of microsecond 1.54-mm laser pulses

Laser extraction of a model porcine eye cataract has been performed for the first time in an in vitro experiment using a 1.54-μm Yb,Er : glass laser generating bursts of microsecond pulses. We used effective pulse repetition rates from 36 to 75 Hz and average laser output powers from 3.9 to 5.25 W. The results demonstrate for the first time that, at an effective pulse repetition rate of 45 Hz, burst repetition rate of 15 Hz, three microsecond pulses per burst, and a burst energy from 260 to 265 mJ, the laser step duration in cataract extraction is 130 plusmn; 10 s, which is comparable to the ultrasonic phacoemulsification and laser extraction time in the case of a Nd : YAG laser emitting at 1.44 μm. Acoustometry and high speed video recording of hydroacoustic processes accompanying interaction of water with 1.54-μm radiation from the Yb, Er : glass laser generating bursts of microsecond pulses have made it possible for the first time to detect overlap of hydroacoustic processes at the pulse spacing in bursts reduced to under 700 μs. In the case of overlap of hydroacoustic processes, despite the increase in average power and effective pulse repetition rate, acoustic wave generation is ineffective because pulses propagate through bubbles formed in the water. Laser cataract extraction is shown to be most effective at a lower average power, lower effective pulse repetition rate, and burst pulse spacing of 850 ± 10 μs.

Impact of second-trip echoes for space-borne high-pulse-repetition-frequency nadir-looking W-band cloud radars

The appearance of second-trip echoes generated by mirror images over the ocean and by multiple scattering tails in correspondence with deep convective cores has been investigated for space-borne nadir-looking W-band cloud radar observations. Examples extracted from the CloudSat radar are used to demonstrate the mechanisms of formation and to validate the modelling of such returns. A statistical analysis shows that, for CloudSat observations, second-trip echoes are rare and appear only above 20 km (thus easy to remove). CloudSat climatology is then used to estimate the occurrence of second-trip echoes in the different configurations envisaged for the operations of the EarthCARE radar, which will adopt pulse repetition frequencies much higher than the one used by the CloudSat radar in order to improve its Doppler capabilities. Our findings predict that the presence of such echoes in EarthCARE observations cannot be neglected: in particular, over the ocean, mirror images will tend to populate the EarthCARE sampling window with a maximum frequency at its upper boundary. This will create an additional fake cloud cover in the upper troposphere (of the order of 3 % at the top of the sampling window and steadily decreasing moving downwards), and, in much less frequent instances, it will cause an amplification of signals in areas where clouds are already present. Multiple scattering tails will also produce second-trip echoes but with much lower frequencies: less than 1 profile out of 1000 in the tropics and practically no effects at high latitudes. At the moment, level-2 algorithms of the EarthCARE radar do not account for such occurrences. We recommend to properly remove these second-trip echoes and to correct for reflectivity enhancements, where needed. More generally this work is relevant for the design of future space-borne Doppler W-band radar missions.

Formation of volume discharges in dense gases at pulse repetition rates up to 10 kHz

Abstact It is shown that for the formation of stable volume discharges at pulse repetition frequencies up to 10 kHz and more, it is recommended to use sectioning of the discharge gap with subsequent excitation of discharges in each section from an autonomous pulse generator. The role of an autonomous pulse generator can also be performed by an auxiliary circuit that directs the necessary part of the energy stored in the main energy storage of a pulse generator with only a single switch to an individual discharge gaps. The description and results of the study of one of these variants of a partitioned discharge gap and a pulse generator are given. Pumping energy densities of 100–150 mJ-cm−3 at pulse repetition frequencies up to 8 kHz in CO2 laser mixtures and 80–100 mJ-cm−3 in N2-Ne and Xe–Ne mixtures at pulse repetition frequencies up to 10 kHz were achieved.

Influence of the cathode region preionization on the operating parameters of the eptron

Investigations of the operating parameters of a plasma-cathode switch based on a capillary discharge in helium and neon in the burst mode are presented. An increase in the efficiency of the switch is demonstrated when an additional preionization pulse is applied at a low pulse repetition frequency (5 kHz). The compression ratio of voltage pulses more than 300 is achieved at a pulse repetition frequencies less than 40 kHz.

Design and Processing Method for Doppler-Tolerant Stepped-Frequency Waveform Using Staggered PRF

Stepped-frequency waveform may be used to synthesize a wideband signal with several narrow-band pulses and achieve a high-resolution range profile without increasing the instantaneous bandwidth. Nevertheless, the conventional stepped-frequency waveform is Doppler sensitive, which greatly limits its application to moving targets. For this reason, this paper proposes a waveform design method using a staggered pulse repetition frequency to improve the Doppler tolerance effectively. First, a generalized echo model of the stepped-frequency waveform is constructed in order to analyze the Doppler sensitivity. Then, waveform design is carried out in the stepped-frequency waveform by using a staggered pulse repetition frequency so as to eliminate the high-order phase component that is caused by the target’s velocity. Further, the waveform design method is extended to the sparse stepped-frequency waveform, and we also propose corresponding methods for high-resolution range profile synthesis and motion compensation. Finally, experiments with electromagnetic data verify the high Doppler tolerance of the proposed waveform.

Parameter Optimization of Laser Direct-Write Patterning on Indium Tin Oxide/Polycarbonate Thin Films Using Multi-Performance Characteristics Analysis

Indium tin oxide (ITO) thin films on polycarbonate (PC) substrates were patterned using the laser direct-write (LDW) technique to form an isolation line. The effect of the LDW parameters (power, pulse repetition rate, and defocusing distance) on the isolation line width, depth and roughness of the PC within the line was investigated. Additionally, the Taguchi method of experimental design was applied to determine the optimal parameters of LDW. Results showed that increasing the power led to an increase in the isolation line width and decrease in the surface roughness of the PC within the line. The increase in the pulse repetition rate and defocusing distance caused a decrease in the isolation line width. The optimal parameters were found to be A2B3C3, consisting of power of 5 W, pulse repetition rate of 100 kHz, and defocusing distance of +3 mm. Under these parameters, we obtained an isolation line width of 48.4 μm, and a surface roughness of Ra 38 nm of the PC within the isolation line. We confirmed that the ITO films separated by the isolation lines attained electrical isolation.