Many industrial laser processes, such as surface cleaning, require the removal of small thicknesses of matter, often on large samples. An experimental study has been performed in order to characterize and enhance the ablation of materials by means of the interaction between a pulsed laser beam and matter using common industrial laser sources, particularly at 248, 308, 532, and 1064 nm. Ablation was achieved on a static sample with one or several successive pulses and for different energy densities. These parameters enabled us to control the depth of the ablation in the micrometer range. Experiments have been carried out in ambient air, under gaseous cover, and then under a flowing water film on the material surface at atmospheric pressure. The material was a stainless-steel alloy. The experiments made it possible to determine the ablation fluence threshold for each wavelength and the alteration of the surface by the successive laser pulses. In order to avoid oxidation during the process, an argon gaseous medium was used, but peripheral oxide deposits always appeared and the efficiency of the ablation did not seem to increase. However, when the water flow on the surface was employed, the efficiency of the ablation increased by a factor of 2–15 for a certain incident power density. Peripheral oxide redeposition was absent using this method. Experiments have demonstrated that the confinement of the plasma within a transparent layer such as water at the given laser wavelength is an effective method for increasing ablation yield. This technique prevents the expansion of the plasma upstream of the target and the mechanical impulse communicated to the material increases. Calculation of ablated mass confirms that ablation yield could be increased by 15 for higher power densities. Similar results have been found using other materials such as alumina or silica. This means that this ablation technique can be employed in various types of industrial laser applications, for example, for cleaning oxidized surfaces or removing paint coatings.

Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers-Kronig relations. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications.

Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515 nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850 nm and propagation loss of <0.37 dB∕cm are reported for a 33 mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn ~2.5 × 10 −3 ) up to 1400°C.Since its first demonstration by Davis et al. in 1996 [1], femtosecond laser direct inscription has evolved into an established technology and found a wide range of scientific and industrial applications. The technique has been used to develop novel photonic components such as quantum photonic circuits [2], lab-on-a-fiber [3], and on-surface refractometric sensors for liquids [4]. Lately there has been a strong incentive for the fabrication of photoinscribed devices that operate in the near and mid-infrared (IR) wavelength ranges. The mid-IR range (3-10 μm), also called the molecular fingerprint region, is of particular interest because of the presence of strong absorption from basic molecular bonds such as O-H, C-H, and C-O. As such, tremendous efforts were deployed to find optical materials that could act as adequate hosts for mid-IR photoinscribed devices. For that purpose, the two prerequisites are high transmission at the wavelength of operation and sufficient photosensitivity. Up to now, very few materials have proven to possess both. Optical waveguides that operate in the mid-IR were successfully inscribed in amorphous materials such as chalcogenide [5,6], fluoride [7], and germanate [8] glasses among others. In most cases however, one of the two requirements is partly or simply not met. An important challenge arises from the presence of hydroxyl ion impurities in glass that significantly affect the optical transmission near the water absorption bands, thus preventing or limiting a large number of sensing applications.

The ultrafast laser writing of optical waveguides and devices is increasingly ubiquitous among the photonics community, mostly for its flexibility and three-dimensional fabrication capability. The well-known astigmatic beam technique is the simplest method to inscribe near-circular cross-section waveguides. In this paper, we report on a significant enhancement to the widely used astigmatic beam technique that makes it more flexible and yields a more circular waveguide cross section. By simply superposing a long-focus lens before the laser inscription objective lens, we demonstrate that the normalized squared radial deviation from a perfectly circular waveguide cross section can be reduced to <2022