Glass has been the material of choice for making optical elements, in large part due to its intrinsic properties: a temperature-dependent viscosity, which enables shaping the material into a broad variety of functional and artistic glassware. Silica glass (SiO2) in particular, provides exceptional properties for optical applications in general, in particular at high temperature or in the ultraviolet spectrum. However, its elevated glass transition temperature makes molding operation extremely challenging. At the small scales, scaling laws make the production of sub-millimeter-sized micro-optics and freeform surfaces challenging when approached by conventional ultra-precision milling, in part due to the surface finishing requirements. While the production of complex surface shapes is possible by laser, and notably by femtosecond exposure combined with wet etching, the required complexity of the laser-scanning paths and the limited surface quality achievable remain challenges to overcome.
Toward the goal of fabricating freeform surfaces in silica glass, we propose a method that combines the fabrication of easy-to-produce primitive shapes by femtosecond laser followed by a continuous topological transformation driven by surface tension forces activated by mid-Infrared continuous wave laser heating.
The first step of this work consisted of designing an experimental laser setup for reflowing preforms, with the implementation of an in situ two-color pyrometer-based temperature measurements used for stabilizing the process. Additionally, a goal-oriented control method is proposed, which differs from other forms of control loop in that, rather than using a process parameter to close the loop, the final optical function is itself monitored and used to terminate the reflow. Starting from generic cylindrical preforms, we produce arrays of spherical micro-lenses by laser reflow, which focal lengths are adjusted from one iteration to the next.We applied this process to silica glass and expanded its applicability to ultra-low-expansion glass, known under the trade name 'ULE®7972'.
A modeling framework was developed to predict the final shape of the component. To assess its accuracy, the simulated 2D rotationally symmetric profiles are compared with profiles extracted from optical shadow images of the preform, while the volume is undergoing simultaneous shaping and surface smoothing. We demonstrate that finite-element simulation of the laser-induced reflow process is in good agreement with our observations. The shape of the micro-lenses produced is essential spherical, and evolve with the duration of the reflow. The produced focal length can be tuned by changing the set point of the monitoring apparatus.
Finally, we dedicated a detailed account of the design and the fabrication of a miniature monolithic free-space optical fiber coupler device in ULE®. By the combination of ultrafast laser inscription, wet etching and laser induced reflow of pre-positioned preforms, we demonstrated the feasibility of such a device and achieved a dimensional accuracy within ten micrometers. The design entails an adjustment-free five-point kinematic mount holding each fiber in position with respect to collimating and focusing ball-lenses, two millimeters. The ball lenses were transformed in place by laser reflow from cuboid preforms. This is particularly relevant for applications such as precision instruments, fine mechanics and optics in general.