Laser mirror manufacturing technology

At the previous lectures have been noted that RLG dead zone threshold depends on laser mirror quality. The laser mirror quality can be measured by backscatter coefficient defining its surface flatness. So substrate on which mirror layers are deposited should be very flat. Now, the measure of surface flatness, technology and machine tools used to manufacture the required flatness for the substrate and mirror will be considered.

Surface quality parametersSurface processing error (flatness error) characteristics are roughness, waviness and form. The following definition can be done: roughness – is comprising of irregularities that occur due to the mechanism of the material removal at production process; tool geometry, wheel grit, or the electrical discharge machine (EDM) spark; waviness – is component of the surface texture upon which roughness is superimposed, resulting from factors such as; machine or part deflections, vibrations and chatter, material strain and external effects, form – is the overall shape of the surface - ignoring roughness and waviness variations - termed ‘form error’, caused by errors in machine tool slideways.

Form error is regular deviation of the surface from its nominal geometry, for round parts it is ovality or elliptisity in cross section, curvilinearity, lobing, conicity, barrel, convexity, concavity et cet.

Fig.4.1. Regular form error

Roughness is considered to be a real surface condition over small square equal to a few mm2. The example of roughness and waviness is presented in fig. 4.2. Criteria and designations ((ГОСТ 2789-73): If L/Н1 >1000, it is form error;If L/Н1 is in the range of [50...100], it is waviness;If L1/Н <50, it is roughness.Surface roughness parameters:Ra – arithmetic average of profile module value deviation in the limits of basic length L (100 L 0.008 m):

Fig.4.2. Waviness and roughness

(4.1)

Rz - sum of the average of the absolute deviations of the five largest minima Himin

and the five largest peaks Himax within the basic length L (1600 L 0.025 m):

(4.2)

Rmax - maximum height of profile unevenness (1600-0.025 m), Sm - average

spacing of unevenness (12.5-0.002 m), S - average spacing of unevenness peaks (12.5-0.002 m), tp - relative reference profile length, p- profile cross section

level value.

Fig.4.3. To the definition of a surface unevenness.

Surface roughness designation signs are presented fig.4.4.In the places of digits (1, 2, 3, 4) captions are made: (1)Surface roughness parameter according to Std. ГОСТ 2789-73;(2)Surface processing type and other indications; (3)Basic length according to Std. ГОСТ 2789-73;(4)Unevenness direction symbols There are also the following symbols:

Fig.4.4. Roughness designation sign

- Type of processing is not installed by the designer;

- Surface formed by removing the metal layer (turning, grinding, etching, etc.);

- Surface is formed without removing the metal layer (casting, forming, etc.) or surface is not processed by this drawing. Ra is not written in the notation.

Roughness is a basic parameter that influences on the different characteristics of measuring devices like RLG. Roughness is characterized by the height of the unevenness. The height of the unevenness is defined by the:

•Geometric parameters of the tool•Cutting speed and workpiece feeding•Install the tool with respect to the axis of rotation

Standard established 14 classes of surface roughness. The most rough is 1st class - 1, the highest quality - Ra = 0,003 microns. The roughness worse than the 1st class is denoted by a sign which indicates the height of the unevenness in microns. For the 1st class - Rz=300m and Rа=80m; for the 14-th class - Rz=0.05m and Rа=0.01m. Values to estimate roughness is established by the standard “surface finish class”, so surface finish classes from 6 to 12 are defined by the Rа, and from 1 to 5 and 13-14 classes by the Rz. Instruments are allowed to write the surface profile in magnified scale– profilogram. To measure roughness contact measuring instruments– optomechanical, electrodynamical, piezoelectical and inductive profilographs and non-contact measuring instruments – interference, microscopes, microscopes of comparison are used.

Fine mechanical glass processing Grinding

In the process of grinding and polishing distinguish coarse and fine grinding, and then polishing. Rough grinding has to give as quickly as possible the desired flat or a spherical shape. As an abrasive material is used, as a rule, carborundum, emery, or other similar hard materials in powdered form with a different grain's size. To the treated glass surface the so-called the grinding form is pressed that make of glass or cast iron, or brass. Abrasive material, mixed with water until pasty consistency in the proportion of 2 teaspoon per 1 square decimeter surface.

This mixture served between the treated surface and abrasive form. The whole grinding process can be performed both manually and mechanically. What matters is that when grinding, for example, flat surfaces under processing woud be horizontal. Under grinding process it is necessary to move the grinding form back and forth relative to workpiece, leaving a short or long "strip". When grinding it is necessary to wet the grinding form with water from time to time. In addition, the abrasive material should be added from time to time to compensate for its consumption. The pressing force that can be applied to form udner grinding, can only be set in practice, it depends, of course, on the size of the abrasive and workpiece processing surface. From time to time, the surface should be washed and dried. After the end of grinding with a one grain's size the grinding form and the processed detail should be thoroughly washed and cleaned before proceeding with the next grinding abrasive materials.

Polishing

Polishing process makes it possible to more smooth the finely grinding glass surface and thus, eventually, get a surface suitable for optical purposes. To improve the surface smoothness under grinding it has to remove its material, on the contrary in the polishing process, material from the surface is almost not removed.

Instead, the local glass flow along the surface under the pressure of the individual grains of polishing material occur, resulting in filled deepening and smoothed convexities. The surface becomes smooth, and appeared as a result of grinding microscopic cracks disappear. For polishing of glass, abrasive crocus or cerium oxide of varying quality are usually used. To obtain the grain of different sizes crocus roil (mix) the water, then it is given the opportunity to freely settle in tall glass cylinders. In addition to crocus as polishing material, other materials are used, such as tripoli, cerium oxide (Ce2O3), aluminum oxide (Al2O3) chromium oxide (Cr2O3), zinc oxide (ZnO), etc. A more recent polishing technology is Magneto Rheological Figuring (MRF).

Fig.4.5. Multiple parts polishing.

Fig.4.6. Computer numerical control polishing machine.

This polishing process also has a removal function programmed into a computer, but the slurry used contains microscopic magnetic and diamond particles. The slurry runs over a belt on a spinning wheel and the lens is lowered into the slurry from above (Figure 3.7.) The computer can control the hardness of the slurry by applying a magnet beneath the spinning belt.

This effectively allows you to combine many polishing processes into one since you can make your polishing compound harder and softer just by altering the strength of the magnet. Although very precise, MRF is expensive for regular optics since the slurry must be changed every few weeks with regular use. However, it is very cost effective when optics must be polished to λ/10 or λ/20 surface finish.

Fig.4.7. Polishing technology using MRF.

Cerium Oxide (Polirit)

Small, hard, sharp particles used in free or bound form for mechanical processing (e.g. for shaping, hogging, grinding, polishing), a variety of materials and their products (from large steel plates to sheets of plywood, optical glasses and computer chips). There are natural and artificial abrasives. The action of the abrasives results in removal of material from the workpiece surface. The size of abrasive grains are characterized by a scale of 4 (coarse) to 1200 (superfine).

Abrasives

Optical polirit - fine-dispersed crystalline powder mixture of oxides of rare earth elements (Ce, La, Nd). It is used for polishing optical and technical glass, mirrors, electronic equipment, jewelry and ornamental materials. Powder can be

Iron Oxide (Crocus)

Powder, from bright red to dark red color. Crocus's grain has rounded size of about 1.0 micron. It is used for finish polishing of glass and optical materials, as well as brass, bronze, silver, gold and other alloys. Method of application: For high-quality treatment is recommended to pre-classification powder as follows. Get three vessels in one place crocus powder, fill with clean water and mix thoroughly. Give water to settle for 20 - 30 seconds. Liquid with suspended powder gently (not shaking the sediment) to pour in a second vessel. Mix the liquid in the second vessel, and let it settle for 2 minutes, then pour into a third vessel. Thus, the crocus will be divided into large (unsuitable for polishing), medium and small components. After settling the water is drained, and the resulting powder is dried.

GOI PasteSolid bars from green to dark green color. Easy to apply and is well kept on polishing wheels. It is used for grinding and polishing of nonferrous metals, hard metals, plastics, glass and other materials. Coarse paste removes tiny scratches left on the surface after grinding with abrasives.

used in all modes of treatment with either manual or automatic feed of the suspension.

Central paste achieve a smooth gloss polished surface. After polishing by fine and very fine paste, the surface becomes intense luster. Method of application: A small amount of paste applied to the polished round or cloth. In the polishing process to periodically apply the paste to the polishing wheel or cloth. As the material of the polishing wheel or clothe felt, sisal, cotton can be used. The surface of the felt polishing wheels during operation should always be saturated with a paste to avoid scratching. The remains of paste is removed from the treated surface with a clean cloth.

GOI Paste Paste # 1 Paste # 2 Paste # 3 Paste # 4

Parameter Rz after polishing, no more than, m

0.10 0.18 0.80 1.10

Parameter Ra after polishing, no more than, m

0.06 0.07 0.12 1.20

Chromium Oxide

Green fine-dispersion powder insoluble in water, ethyl alcohol, acetone, sparingly soluble in all alkalis and acids. It has a hardness of corundum, scratches quartz.

It is used for the finish polishing of granite, solid semi-precious and other stones, not prone to painting. Method of application: mix with water, put on the felt, polish until the desired purity. Average consumption: 20 g / sq.m.