Molded high curvature core-aligned micro-lenses for single-mode fibers.

A polymer-based fiber micro-lens molding fabrication technique with, to our knowledge, unprecedented performances is presented along with its advantages and applications. This technique is a fast and affordable tool to achieve a wide variety of possible spherical and aspherical micro-lens sizes and curvatures. The alignment of the micro-lens mold with the fiber core receiving the lens is done optically, which allows high precision. Using the proposed technique, different micro-lenses are fabricated. Then the output beams of two different micro-lenses on single-mode fibers are characterized. Fiber micro-lenses with curvature as small as 5 µm are achieved. This shows how low curvature micro-lenses can achieve collimation and how high curvature ones on single-mode fibers lead to high focusing (FWHM=λ) that is much smaller than what most conventional commercial techniques can reach. Given that this technique imposes no stress and causes no damage to the fiber receiving the micro-lens, it presents a significant potential for compatibility with non-silica-based and micro-structured fibers such as photonic crystal and quasi-crystal fibers.

Direct imaging of a photonic jet at shaped fiber tips.

This Letter presents, to our knowledge, the first direct measurement of the three-dimensional distribution of photonic jets (PJs) generated by shaped-tip multimode optical fibers. A PJ at the distal end of optical fibers makes it easier to scan a sample, for lithography or optical analysis, for example, with a spot smaller than the diffraction limit. The backscattered light can also be easily collected. In this study, the volume of the PJ has been reconstructed using a stack of image planes and compared to numerical simulations. For the first time, the power distribution of the non-fundamental mode around the PJ has been observed, giving a better understanding of PJ-based laser etching using multimode optical fibers. An original 50/125 fiber with a microlens fitting just on its core has made it possible to strongly reduce the power spread compared to the thermoformed 100/140 fibers used in our previous works.

Long focal length high repetition rate femtosecond laser glass welding.

A long focal length focusing device is proposed for the process of glass welding by femtosecond laser pulses at high repetition rate and we report on the significant advantages. The study is performed using a 100 mm focusing length F-theta lens. The results are compared to those obtained with a high numerical aperture microscope objective. The long focal length with the associated Rayleigh length method allows a robust high process speed: welding at 1000 mm/s has been achieved, several orders of magnitude larger compared to what was reported until now. Moreover, for one pulse, the same energy on a larger laser spot leads to a lower thermic gradient increase. As a result, with the proposed parameters, the heat accumulation process reduces the residual stress in the welding seams, preventing the formation of fractures inside the seams: mechanical resistance at 30 MPa has been measured.

Large-mode-area optical fiber for photonic nanojet generation.

The photonic nanojet (PNJ) generated by a shaped optical fiber tip is an attractive technology for laser micro-machining. The working distance has the same order of size as the fiber core diameter; therefore, multimode (MM) fibers are generally preferred. However, the PNJ is due to the fundamental mode and, therefore, the energy coupled on the high-order modes does not contribute to the process. We demonstrate the benefit of a large-mode-area (LMA) optical fiber in the generation of the PNJ. A homemade 40 µm mode field diameter LMA fiber is compared with a 100/140 MM-shaped fiber tip. Similar micro-peaks are obtained, and an energy gain is demonstrated. The coupled energy required was eight times less intense with the LMA fiber, which may open new possibilities for laser micro- and nano-processing.