B. R. Campbell

Drilling of steel and HgCdTe with the femtosecond pulses produced by a commercial laser system

The results of interaction of single and multiple 200 fs laser pulses with thick stainless steel and HgCdTe samples are reported. The threshold laser energy density required to produce surface melting is measured. The melt dynamics and evolution of surface morphology are observed for different pulse energies and number of laser pulses. It is observed that, as with a long laser pulse interaction, a layer of melt can be produced at the sample surface. Melt ejection in the radial direction towards the periphery of the interaction zone is observed when the pulse energy is increased. The observed melt dynamics resembles the evaporation recoil melt removal typical of the laser interactions in the range from nanoseconds to continuous wave (CW). The observed melt ejection is attributed to a nanosecond component of the laser pulse with an estimated energy of approximately 25% of the total laser pulse energy. The measured melting threshold energy density for stainless steel is comparable with the published theoretically predicted threshold for nickel computed using a two-temperature model.

On the possible effect of pedestal pulse on material removal by ultrahigh intensity laser pulses

The results of the interaction of 200?fs and 4?ns laser pulses produced by a Ti?:?Sapphire commercial laser system with various materials are reported. The drilling rates were measured as a function of laser pulse energy and material thickness. Saturation of the material removal rate was observed for high laser pulse energy. The dependence of the material removal rate on the sample thickness was measured. The results of numerical simulation of laser drilling with femtosecond and nanosecond laser pulses using a theoretical model which includes phase transition at the critical temperature are reported. The computed drilling rates are in agreement with the data of measurements. The effect of a suspected nanosecond pedestal pulse containing approximately 30% of total pulse energy on the rate of drilling with femtosecond laser pulses is numerically simulated and the computed results are discussed.

Ultrashort pulse laser micromachining performance enhancements

Lasers have long been promised to be a revolutionary technology for manufacturing, but have not yet achieved the anticipated market penetration. Major hurdles still exist in bringing new technologies, such as ultrashort pulse lasers, to the manufacturing floor, the first being easily repeatable, high quality laser micromachining. We explored methods to improve the spatial and temporal beam quality of a femtosecond laser system in an effort to reduce melt and heat affected zone using percussion and trepanning drilling techniques in the semiconductor HgCdTe. Drilling results will be presented of experiments conducted using spatial filters, beam homogenizers, and second harmonic generation at various laser pulse energies. We report the process parameters that produced the cleanest holes with minimal melt and ejected material.Lasers have long been promised to be a revolutionary technology for manufacturing, but have not yet achieved the anticipated market penetration. Major hurdles still exist in bringing new technologies, such as ultrashort pulse lasers, to the manufacturing floor, the first being easily repeatable, high quality laser micromachining. We explored methods to improve the spatial and temporal beam quality of a femtosecond laser system in an effort to reduce melt and heat affected zone using percussion and trepanning drilling techniques in the semiconductor HgCdTe. Drilling results will be presented of experiments conducted using spatial filters, beam homogenizers, and second harmonic generation at various laser pulse energies. We report the process parameters that produced the cleanest holes with minimal melt and ejected material.

Novel applications of sub-surface laser machining

Lasers can uniquely be used to create physical changes inside a bulk material. Traditional manufacturing processes are limited to surface modifications, but a laser can be focused at any location inside a material transparent to that wavelength. Using sub surface machining methods with ultrashort pulse lasers two practical applications are demonstrated. First, a laser is used to sever short-circuited wires embedded deep inside a thick piece of glass, effectively repairing a defective wire network. Second, subsurface bar-coding was shown to produce readable markings. Surface laser markings were shown to weaken the glass, but subsurface marking had virtually no effect on strength.

A study of material removal rates for shallow drilling with an ultrashort pulse laser

Using a commercial laser system operating at a 532 nm wavelength with 10 ps pulses, experiments were conducted on polished metal samples to study material removal characteristics from a low number of laser pulse exposures. The samples were analyzed with a scanning electron microscope and white light interferometer to gather data on surface deformation and material removal. The effects of energy and various double pulse machining methods were examined. The results from changing the pulse separation for double pulse drilling are compared to prior work with picosecond and nanosecond pulse lasers.

Shallow hole drilling with ultrashort pulse lasers

Using a picosecond laser system that can operate at 1064, 532, 355, and 266nm wavelengths, experiments were conducted with polished metal samples to study material removal from a low number of laser pulse exposures. The samples were analyzed with a scanning electron microscope and white light interferometer to gather data on surface deformation and material removal. The effects of wavelength, energy and a double pulse exposure method were examined. Results were compared with simulations that model the material removal rates from ultrashort pulse drilling.

Precision laser bending of thin precious metal alloys

Lasers are capable of delivering energy to a metal to induce stresses from the thermal gradient through the material. Under the right conditions these stresses can cause the metal to bend. Experiments were conducted to produce bending in the metal, Neyoro® G, and samples with a titanium coating on one side. In the experiments, both upward and downward bending was observed. The titanium coated samples showed potential to be more controllable than the uncoated samples.

An analysis of ultrashort pulse laser micromachining parameters for the optimization of shallow hole drilling

Using a factorial design of experiments approach with ANOVA, laser drilling experiments were performed on the semiconductor mercury-cadmium-telluride (HgCdTe). A commercial CPA femtosecond laser system operating at 775nm was used for the experiments. The test variables include laser parameters such as pulse length, fluence, beam shaping using apertures, assist gas, vacuum, and others. The response variable examined for optimization include hole size, hole depth, and melt effects. The analysis yielded an empirical formula for predicting laser drilling effects.

Comparison of drilling rates and material removal dynamics for nanosecond and femtosecond laser pulses

The results of the interaction of the first harmonic of a 200 femtosecond laser pulse produced by a Ti:Sapphire commercial laser system and the third harmonic of a 40 ns laser pulse produced by a DPSS Nd:YVO4 laser with various materials are reported. The drilling rates were measured as a function of laser pulse energy and material thickness. Differences in material removal rates were observed between the low and high pulse energy. The dependence of the material removal rate on the sample thickness was measured. The observed dependencies of the drilling rate of a femtosecond laser on the laser pulse energy and material thickness are similar to a nanosecond laser drilling. This supports previously suggested hypothesis that a femtosecond laser system produces pulse containing a nanosecond pedestal with estimated energy comparable to the energy of the femtosecond component.

Melting threshold and melt removal dynamics during laser interaction with steel and HgCdTe in femtosecond regime

The results of a study of a single 200 femtosecond laser pulse interaction with thick stainless steel and HgCdTe samples are reported. The threshold pulse energies required to produce sample surface melting are measured. The melt dynamics, material removal rate and evolution of surface morphology were observed for different pulse energies and number of laser pulses. It was observed that, similarly to long laser pulse interaction, a layer of melt can be produced at the sample surface. Increase of laser pulse energy results in melt ejection in the radial direction toward the periphery of the interaction zone resembling evaporation recoil melt removal typical for laser interaction in range from nanosecond to cw. The removal of material from stainless steel sample was observed to be highly nonuniform. The columnar structures were observed on the surface of stainless steel samples. The period of these structures is dependent on laser pulse energy and number of pulses. The observed melting threshold is compared with the theoretical prediction obtained using two-temperature model.