Alain Cournoyer

Material micromachining using a pulsed fiber laser platform with fine temporal nanosecond pulse shaping capability

We report on recent advances in laser material processing using a novel pulsed fiber laser platform providing pulse shape agility at the nanosecond time scale and at high repetition rates. The pulse shapes can be programmed with a time resolution of 2.5 ns and with an amplitude resolution of 10 bits. Depending on the desired laser performances, the pulses are generated either by directly modulating the drive current of a seed laser diode or by modulating the output of a seed laser diode operated in CW with electro-optic modulators. The pulses are amplified in an amplifier chain in a MOPA configuration. Advanced polarization maintaining LMA fiber designs enable output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2< 1.1) and narrow line widths suitable for efficient frequency conversion. Micro-milling experiments were carried out with stainless steel, in which processing microstructures of a few tens of microns in size usually represents a challenge, and aluminum, whose thermal conductivity is about 20 times higher than stainless steel. The results obtained with two metals having very different thermal properties using different pulse shapes with durations varying between 3 ns and 80 ns demonstrate the benefits of using lasers offering flexible pulse durations and controllable pulse intensity profiles for rapidly optimizing a process in different applications while using the same laser with respect to conventional methods based on pulsed laser with fixed pulse shapes. Numerous applications are envisioned in a near future, like the micromachining of multi-layered structures, in particular when working with the harmonics of the laser.

Micro-milling process improvement using an agile pulse-shaping fiber laser

We demonstrate the usefulness of INOs pulse-shaping fiber laser platform to rapidly develop complex laser micromachining processes. The versatility of such laser sources allows for straightforward control of the emitting energy envelop on the nanosecond timescale to create multi-amplitude level pulses and/or multi-pulse regimes. The pulses are amplified in an amplifier chain in a MOPA configuration that delivers output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2 < 1.1) and narrow line widths suitable for efficient frequency conversion. Also, their pulse-on-demand and pulse-to-pulse shape selection capability at high repetition rates makes those agile laser sources suitable for the implementation of high-throughput complex laser processing. Micro-milling experiments were carried out on two metals, aluminum and stainless steel, having very different thermal properties. For aluminum, our results show that the material removal efficiency depends strongly on the pulse shape, especially near the ablation threshold, and can be maximized to develop efficient laser micro-milling processes. But, the material removal efficiency is not always correlated with a good surface quality. However, the roughness of the milled surface can be improved by removing a few layers of material using another type of pulse shape. The agility of INOs fiber laser enables the implementation of a fast laser process including two steps employing different pulse characteristics for maximizing the material removal rate and obtaining a good surface quality at the same time. A comparison of material removal efficiency with stainless steel, well known to be difficult to mill on the micron scale, is also presented.

Material micromachining using bursts of high repetition rate picosecond pulses from a fiber laser source

In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape. The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz, grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 40 ns. The laser output central wavelength was at 1064 nm and the output beam M2 value was below 1.15. Micro-milling experiments were performed on silicon for two levels of energy per burst and with different burst amplitude profiles. We show that the maximum material removal efficiency and the surface quality can be increased by more than 25% when using bursts of picosecond pulses with respect to nanosecond pulses with similar energy per pulse. Effect of shaping the burst envelope of the picosecond laser on the maximum material removal efficiency is also presented.

Laser drilling and routing in optical fibers and tapered micropipettes using excimer, femtosecond, and CO2 lasers

We used an excimer laser (193 nm), a femtosecond laser (775 nm) and a CO2 laser (10.6 µm) to drill cylindrical holes in fused silica optical fibers and in glass micro-pipettes. CO2 laser-drilling using tip processing results in tapered holes with larger diameters than the holes drilled with the excimer and the femtosecond laser. Although routing of holes of various shapes results in sharper edges and a higher aspect ratio when the femtosecond laser is used, the CO2 laser could still be used to route rectangular holes in fused silica optical fibers. Albeit hole dimensions and details are smaller when micromachined with the excimer and the femtosecond lasers, the optical fibers are very brittle at the end of the process. CO2 lasers offer the advantage of producing higher fused silica ablation rates with much better polished surfaces and a better mechanical integrity, which are usually more suitable in some applications.

Maximizing laser ablation efficiency of silicon through optimization of the temporal pulse shape

The commercial availability of fiber lasers based on MOPA architectures with arbitrary temporal pulse shaping capabilities offers completely new possibilities for laser material processing. In this study, based on numerical modeling results in the nanosecond regime for the case of silicon at 1064 nm wavelength, we show that not only the single pulse laser ablation efficiency depends on the temporal pulse shape but, we also demonstrate how a stochastic approach can be applied in order to reach an optimized pulse shape maximizing the material vaporization rate for given laser pulse energy and duration. Experimental results are compared to the numerical modeling results, and the discrepancies are discussed in terms of the role played by plasma shielding effects and melt ejection at high intensity.

Influence of different laser operation regimes on the specific energy required for rock removal in oil and gas well drilling applications

Although many practical hurdles remain to be addressed in the future, laser oil and gas well drilling has potential advantages over the conventional rotary drilling approach, such as a smaller footprint of the drilling rig, higher rates of penetration, reduction of downtime due to dull bits, reduction of waste caused by drilling mud, creation of a natural casing while drilling, and ability to drill in hard rock formations. One of the most promising applications is downhole laser perforation for well completion as an alternative to explosive technologies currently in use. In order to establish both the technical and economic feasibility of using lasers in oil and gas drilling operations, one can measure the laser energy required to remove a unit volume of rock. The resulting specific energy is a measure of the efficiency of the laser drilling process and depends on the rock type and the laser operation regime that determines the laser-rock interaction mechanism. In the present feasibility study, we compare the results of laser drilling tests conducted in two types of reservoir rocks, namely limestone and sandstone, at different laser wavelengths and for different laser operation regimes (continuous wave and pulsed regimes, different repetition rates and duty cycles) in terms of specific energy. We also discuss preliminary results on the influence of the temporal shape of the laser pulses in the nanosecond regime on the rock removal process as obtained with INO pulse-shaping fiber laser platform, with the objective to take advantage of the flexibility and the agility of such a laser source for drilling operations in different rock types.

Combined fiber optic sensor for Colour and refractive Index (CI) monitoring

Two fiber optic probes are combined for the simultaneous monitoring of colour and refractive index of a liquid. It is shown how, through modelling, the design can be adapted to cover different ranges in refractive index and light absorption.

Sub-spot-size CO2 laser micromachining of features in fused silica by V-groove etching

Ablation of fused silica using the Gaussian irradiance profile of the TEM00 mode of a CO2 laser is a very efficient way for micromachining features up to ten times smaller than the beam diameter. A series of laser-etched V-grooves sequentially shifted in a given fashion can be used to micromachine simple or structured patterns on the surface of fused silica substrates. Surface gratings with a periodicity of 12 μm were produced using a CO2 laser beam of 100 µm (1/e2) in diameter. Rectangular wells 50 μm wide and 50 μm deep were also micromachined using the same technique with a radius of curvature of roughly 8 µm at the bottom edges. Although the resolution of the periodic pattern is not fully understood, it appears to be partly governed by the amount of material removal by the top portion of the Gaussian beam (tip processing), as well as a carefully controlled shifting of the etched V-grooves on the fused silica substrate. Physical mechanisms that could be at the origin of the V shape of the grooves are also discussed.

CO2 laser processing of optical fibers

Fused silica exhibits attractive thermophysical properties for CO2 laser processing and the micromachining of optical fibers can benefit from this peculiar laser-matter interaction. Applications like laser cleaving of specialty optical fibers and laser end shaping of single mode and multimode fibers are addressed. The CO2 laser processing of an optical fiber device for the measurement of the refractive index of liquids is also considered. Emphasis is put on sub-spot size micromachined features in fibers.Fused silica exhibits attractive thermophysical properties for CO2 laser processing and the micromachining of optical fibers can benefit from this peculiar laser-matter interaction. Applications like laser cleaving of specialty optical fibers and laser end shaping of single mode and multimode fibers are addressed. The CO2 laser processing of an optical fiber device for the measurement of the refractive index of liquids is also considered. Emphasis is put on sub-spot size micromachined features in fibers.