Machining techniques for the production of optics for a laser system laser system

During the manufacturing processes, such as grinding and polishing, of high quality fused silica optical surfaces, surface imperfections and contaminants are inevitably left.

It is believed that the morphology of LID on fused silica surfaces could vary with choice of polishing method. This can be proved by Figure 2.7. Therefore the manufacturing methods of optical surfaces do make sense in the research on LID of fused silica optics.

To prepare fused silica optics from blank materials, the common processes include rough grinding, fine grinding, rough polishing and final polishing. Hence this section

41

will review some ultra-precision polishing techniques. Ultra-precision polishing techniques have developed rapidly over recent years and a lot of deterministic optical polishing methods have been established. These methods include abrasive jet polishing [25, 26], ion beam-based finishing, lapping [27], elastic emission machining (EEM) [28, 29], and magnetorheological finishing (MRF).

2.4.1 Abrasive jet polishing

To meet the challenge to shape and finish optical surfaces with steep and concave sections in brittle materials such as glass, Fahnle and Brug developed Fluid Jet Polishing (FJP) using the idea that a stream of prepared fluid is ejected at a high speed and guided by a nozzle onto the optical surface under pressure [30]. Usually in the FJP process, abrasive fluid is composed of water and a polishing compound such as Cerium Oxide (CeO2). Therefore the FJP method is called water jet polishing sometimes. Figure 2.11 shows a schematic diagram of FJP, of which the T, G, P and N are fluid container, work-piece, pump and nozzle respectively.

Horiuchia, et al. [31] developed a method of ultraprecision abrasion machining named

“Nano-abrasion machining”, which is similar to FJP and uses machining liquid

Therefore it is a challenge in conventional abrasive jet polishing to ensure that material removal is deterministic and stable. To avoid the disadvantages of instability, Tricard, et al. [25] developed a method of jet stabilization, named magnetorheolgical (MR) jet

42

Figure 2.11 Schematic diagram of Fluid Jet Polishing [31]

43

finishing. In MR jet finishing, the abrasive fluid is not a typical abrasive fluid. An MR fluid consists of water, carbonyl iron particles and abrasives. The jet of magnetorheological (MR) fluid can be concentrated and forms a stable and slender jet because it is magnetised in a magnetic field when it is ejected out of the nozzle. This stable jet may keep its structure when travelling a long distance (200 nozzle diameters, i.e. 0.4 metre for 2 mm nozzle diameter) [25]. Therefore, MR jet polishing is a very good method for machining steep concave surfaces and cavities stably.

2.4.2 Ion beam-based finishing

Ion beam-based finishing, also called ‘Ion Beam Etching (IBE)’,‘Ion Beam Figuring (IBF)’,

‘ion beam polishing’, or ‘ion beam sputtering’, is a form of highly deterministic polishing method which has been developed rapidly in some companies and research institutions, such as the Eastman Kodak Company [32], Cannon, Nikon [33], and Centre Spatial de Liege (CSL). Ion beam-based finishing technologies are developed for correcting and figuring of high precision and large scale optical components [34].

Unlike other polishing methods, ion beam-based finishing is a non-contact technique that avoids problems such as edge effect, tool wear, and force loading of the work-piece. In an ion beam-based finishing process, material is removed from the optical surface via a sputtering phenomenon. A typical sputtering event is a knocking out phenomenon which usually begins with energetic ion particles bombarding surface atoms or molecules. It is based on an ion beam etching system and utilises a beam of noble ions, such as argon (Ar), krypton (Kr), and xenon (Xe) ions, which is generated and accelerated in a discharge chamber, to remove material from the surface selectively [35].

Figure 2.12 shows the mechanism of the ion beam-based finishing process. Ions with high energy hit the surface of the work-piece, of which some atoms get enough energy

44

Figure 2.12 Mechanism of the ion beam-based finishing process[36]. Some surface or near-surface atoms, namely sputtered atoms, obtain enough energy from

incident (Ar) ions and move away from substrate surface.

45

from the ions and move away from work-piece. The surface or near-surface atoms of the substrate material, of which if some atoms obtain enough energy from ions and move away from substrate surface, they are named sputtered atoms. Sputtered atoms occur when the actual energy transferred exceeds the usual binding energy of 5-10 eV [37].

2.4.3 Lapping/Chemical Mechanical Polishing (CMP)

Lapping may be one of the oldest manufacturing processes for optical components. A uniform load is applied to the polishing pad which usually is made of pitch or polyurethane which are much softer than the abrasives and work-piece surface [38, 39].

Lapping utilises abrasive slurry which is sandwiched between a lapping pad and the surface of the work-piece. Abrasives are fixed on a lapping pad and motion between the work-piece surface and the lapping pad provides the polishing process. Usually the abrasive slurry is an aqueous suspension of colloidal abrasive particles with specific chemical properties depending on the needs. Consequently, lapping is also known as chemical mechanical polishing (CMP). The schematic setup of the lapping process is illustrated in Figure 2.13.

In a traditional lapping processes, expert opticians perform most of work manually with a precisely shaped rigid lap [38]. However, because of well-developed computing technology, aspherical optical surfaces can also be manufactured by computer controlled polishing with sub-aperture pads. Figure 2.14 (a) and (b) present a photograph and motion schematic view of a computer controlled lapping tool.

2.4.4 Elastic Emission Machining (EEM)

Elastic emission machining (EEM), was developed by Y. Mori, K. Yamauchi and K. Endo [28, 29]. It is an ultraprecision machining method that utilizes the chemical reaction

46

Figure 2.13 Schematic diagram of a lapping process, after [38]

Figure 2.14 Computer controlled polishing with a lapping pad [40]

47

between two solid surfaces as the machining principle.

In the EEM process, two solid materials touch each other and make chemical bonds at their interface; one of the solids may bring away the atoms of the other solid surface when they are separated. Thus, this process is a chemical reaction between reactive solid surfaces and the material removal occurs at the atomic level. The removal mechanism of EEM is shown in Figure 2.15.

2.4.5 Magnetorheological Finishing (MRF)