The Fabrication And Performance Characterization Of Micro-Optical Components Based On Femtosecond Laser Direct Writing

Micro optics features for their small size, light weight and low cost, they canachieve function of micro, arrays, integrated and wavefront transform that theordinary optical components hard to realize, they have important and wide applicationin fiber communication, information processing, aerospace, biomedical, lasertechnology, optical computing and other fields. In recent years, new micro opticalelements combing of new principles, new concepts and new technology are emergingand demonstrating its potential applications in microelectronics, micro-mechanical,microfluidic and MOEMS, meanwhile the stuctures and the properties of the microoptical elements ask for a higher demand. The preparation of micro optical elementsexit two mainstream technology currently------lithography and single point diamondchips. The two methods can not achieve high precision hybrid optical elementsfabrication; can not realize complex three dimensional structures’ prepatation such asasymmetric, hollow etc. Femtosecond laser direct writing micro-nanofabricationtechnology features for simple process, high accuracy, high compatibility ofprocessing materials, and high tolerance to complexity of geometry, which provide ahuge space for the development of micro optical elements. Therefore, we develop aset of high speed and high precision micro optical elements femtosecond laserfabrication system according to the structural characteristics of micro-opticalcomponents. And using the system, we prepare the asymmetric multi-level zone plateswhich can be used for beam shaping，multi-layer diffractive optical elements whichcan be used in optofluidics and the hybrid optical elements. This thesis consists of thefollowing four parts:1. Optimization of femtosecond laser direct writing micro-nanofabricationsystem. The hardware stability and the controlling software flexibility of the femtosecond laser direct writing micro-nanofabrication system can directly determinethe quality of the processing devices. In this thesis, we built a micro-nano fabricationsystem based on large range three dimension air-bearing stage which driven by linearmotor, design and optimize its hardware and software system separately.On the hardware, large range air-bearing stage features for fast moving speed andlarge range of movement, but when it processing the small size structures the inertiaof the stage will influence the processing flexibility and cause relatively largeprocessing errors. The galvanometer features for processing flexibility, highpositioning accuracy when it’s processing the small size structures, but the processingrange is limited and no more than100μm. Therefore, by considering the advantagesand disadvantages of the galvanometer and the air-bearing stage, we combine thefabrication system based on the air-bearing stage and galvanometer into one system.With the optical seamless that achieves rapid prototyping of any size and highprecision.On the software, we optimize the controlling software by aimed at theasynchronous response of the air-bearing stage and the shutter, the overexposure ofthe endpoint caused by inertial of the air-bearing stage. The controlling software isoptimized by the multi-threading technology. The software position synchronizedoutput is first proposed, it combines with the dynamic data processing will greatlyimproving the precision of the system. Meanwhile, the conventional data modelformat is point cloud data, the amount of data model will greatly increase when thestructures size is increasing and will influence the data transmission speed. So it isnecessary to improve the format of the data model. We take the start point and endpoint to describe the segments and the tangent are to composite the curve, this willreduce the amount of the data, then the surface is described by the curve line and thestructures is composited of the surface. The shell scanning is proposed and willgreatly reduced the processing time and improve the processing efficiency.2. Using the diffractive optical elements for the beam shaping of theedge-emitting semiconductor laser diode. Semiconductor laser features small volume,compact structures, light weight, high electro-optical conversion efficiency, ease of pumping and easy modulation etc. But the output beam have large divergence angle,the spot size are asymmetric parallel and perpendicular to the junction plane, the lightintensity is Gaussian distribution and uneven which limit the application of thesemiconductor laser. So it is necessary to do the beam shaping of the semiconductorlaser. The beam shaping methods such as the Lloyd’s mirror interference, step mirror,two-plane parallel mirrors, double cylinder mirror, lens arrays and so on have beenreported domestic and abroad. The above mentioned method is complex, difficult toprocess, the difficult to align. The beam quality will reduce by repeatedly wavefrontdivision and re-aggregation, the energy will loss a lot by the spreading among themultiple devices.In this thesis, a high level asymmetric zone plate is designed and fabricatedaccording to the characteristics of the semiconductor laser output beam for the beamshaping. Firstly, the eight-level phase zone plate will great improving the diffractionefficiency, the tested diffraction efficiency are higher than90%on average, can highup to92.9%, very close to the theoretical value of the diffraction efficiency of95.1%.Secondly, the theoretical simulation and experimental validation of the opticalproperties of asymmetric multi-order zone plate have a good consistency. Lastly,Shaping by the zone plates, laser diodes divergence angles are simultaneously reduced,from65°(1134mrad, fast axis) and24°(418mrad, slow axis) to7.7mrad and136.5mrad, respectively, with excellent beam shaping properties.3. Using the multi-layer diffractive optical elements (ML-DOE) for theachromatic design. The diffractive elements are wavelength dependent elements. Forthe single layer diffractive elements, the100%maximum efficiency only can beobtained at the designed wavelength, and it will slowly decay across the visiblespectrum, where it is partly diffracted into the other orders. The diffraction efficiencyis an important indicator to evaluate the diffractive optical element, the diffractionefficiency level of the diffractive optical element will influence its applications.In the thesis, we design and fabricate a ML-DOE composed of photosensitiveresin SU-8and ethylene glycol, which is processed by the femtosecond laser directwriting in one procession. Comparing with the ML-DOE based on the air gap, the new designed ML-DOE can effectively reduce the reflection at the interface, increasethe transmittance of the system and improve the energy efficiency. The new designedML-DOE can avoid the diffraction efficiency decline caused by the lateralmisalignment, cascading tilt error of each layer and the inaccuracy of the saw teethmicro structures. As tested in the visible band, the ML-DOE can achieve diffractionefficiency average higher than85%. The design can improve the diffraction efficiencyover the entire designed band and can avoid stray light caused by the non-designdiffraction order. The design will extend the application of ML-DOE in the widespectral imaging.4. Using the hybrid refractive/diffractive optical element for the achromaticdesign. The unique dispersion characteristics and thermal characteristics make it canbe combined with the conventional refractive element, using the refractive elements toprovide most of the surface focus function, and then using the surface reliefstructure to correct various aberrations. The hybrid system can achieve many specialfeatures that traditional optical system can not be achieved.In this thesis, the focusing of different curve surface lens are comparedrespectively, the strong focusing hyperboloid lens have tiny spherical aberration bythe theoretical simulation. Therefore, we take the strong focusing lens as the mainrefractive part of the hybrid system, the eight-level high diffraction efficiency zoneplates as the main diffractive part of the hybrid system for the design. By testing thehybrid system can basically achieved achromatic function. The design has a highflexibility and can realize the any surface, any relief structure’s design. Meanwhile,the hybrid system can fabricate in one process and will greatly reduce the processingtime and cost.