Nano-grinding for fabrication of microlenses on optical fibers endfaces

摘要

This work presents mechanical nano-grinding as an alternative technique for the fabrication of optical fibers endface microlenses. It also presents a novel surface-roughness improvement technique called Loose Abrasive Blasting (LAB). Traditionally, the majority of such microlenses are made using either chemical etching or heating and pulling methods. Despite the success of these methods, they suffer some common drawbacks such as the lack of controllability on the produced lens profile. Consequently the possible variations of the lens profiles that can be made by these methods are also limited. The difficulty to center the lens on the fiber core is another problem associated with heating and pulling method. The exposure to hazardous chemical such as hydrofluoric acid is yet another problem associated with chemical etching.Nano-grinding technique described in this thesis should provide a much better alternative to the traditional optical fabrication techniques. Nano-grinding experiments were conducted on a nano-grinding machine (NGM) specially built for this purpose. The machine incorporates state-of-the-art air-bearing spindles, piezo electric actuators, and capacitive displacement sensors with accuracy down to 2 nm. Such precise motion provided by this system is the key for the success of this technique. With such system, it was possible to produce a multitude variety of lens profiles with high profile accuracy and with surfaces of optical quality without the need for exposure to any kind of hazardous chemicals.In achieving this objective, the research was conducted on many frontiers. First, the possibility of grinding optical fibers without inducing surface and subsurface damages was investigated. Micro-indentation, nano-indentation, and nano-scratch tests were conducted to determine the critical depth of cut that can be achieved before the occurrence of surface and subsurface cracks. Nano-scratch test in particular provided a clear insight to the cracking and the chipping mechanisms that might unfold if the critical depth of cut was exceeded in an actual grinding situation. The knowledge gained from this exercise laid the ground base for the design of the NGM.Using the NGM, further experiments were carried out to determine the optimal grinding parameters for an efficient and successful grinding process. Parameters investigated include the grit size, the cutting speed, and the in-feed rates. The optimum parameters have to ensure the best endface surface quality and the same time maintain a high throughput. This study shows that based on these optimal parameters, it should be possible to produce endface microlenses of optical surface quality free surface and/or subsurface damages in less than 30 seconds with surface roughness (Ra) less than 3 nm.A novel post-grinding surface improvement techniques was also developed. The technique called loose abrasive blasting (LAB) can be used for polishing at and non-flat surfaces. Experiments were conducted on a loose abrasive blasting machine built specially for this purpose. The performance of this technique was compared with other techniques such as slurry polishing and chemical etching used for polishing of brittle materials. The results showed that while chemical etching was found unsuitable for polishing of at optical fiber endfaces, LAB outperformed slurry polishing by significant margin.After the optimal grinding conditions were established, the NGM was used for grinding of different kinds of optical fiber microlens profiles. Among the endface profiles produced were conical lenses, tapered lenses, D-shaped lenses and others. It has also been shown, in case of conical lenses for instance, that there is almost unlimited number of profiles that can be produced by simply changing the contact angle between the fiber endface and the grinding film.The effect of surface roughness on light coupling efficiency between a fiber endface and a laser diode was also investigated. Cleaved fiber endfaces as well as ground endfaces with variant degrees of surface roughness were used in this experiment. The results showed that surface roughness has significant effect on light coupling efficiency. The effect of lens eccentricity on light coupling was also investigated.