- Email: [email protected]
Optics manufacturing by laser radiation
Optics and Lasers in Engineering 59 (2014) 34–40
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Optics manufacturing by laser radiation Sebastian Heidrich a,n, Annika Richmann b, Patrick Schmitz c, Edgar Willenborg a, Konrad Wissenbach a, Peter Loosen b, Reinhart Poprawe a,d a
Fraunhofer Institute for Laser Technology ILT, Steinbachstr. 15, 52074 Aachen, Germany Chair for the Technology of Optical Systems TOS, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany c RWTH Aachen University, Templergraben 55, 52062 Aachen, Germany d Chair for Laser Technology LLT, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2013 Received in revised form 4 March 2014 Accepted 7 March 2014 Available online 29 March 2014
Current results of the development of a process chain for optics manufacturing with laser radiation are presented. The process chain consists of three process steps: High Speed Laser Ablation creates the surface geometry by material ablation, Laser Polishing reduces the surface roughness by material remelting and High Precision Laser Ablation applies a form correction by removing redundant material. Compared to conventional optics manufacturing methods, this process chain benefits from its high flexibility concerning the optics geometry and its processing speed, which is independent from the processed geometry. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Optical design and fabrication Aspherics Laser materials processing Polishing Silica
1. Introduction Optics with aspherical or even freeform surfaces enable high degrees of freedom as well as lightweight design since they can take the place of several spherical lenses. Therefore, the demand for non-spherical components is increasing in the fast-growing market of optical components at a very high rate [1]. Conventional methods for manufacturing aspherical and freeform glass optics enable the fabrication of optics either with slight aspherical shape in small production lots with grinding and polishing, or optics with complex surface geometry in very large production lots with glass (precision) molding. However, the production of single pieces or small series of non-spherical optics is very expensive and seldom economically feasible. In order to improve the manufacturing process of glass optics with complex surface geometry, laser radiation can be used instead of conventional tools due to its flexibility and its decoupling of tool and work piece geometry. Several research institutes have been investigating glass treatment by laser radiation. This research covers the production of micro-optics [2,3] as well as single process steps like polishing [4] or form creation [5] with laser radiation. At the Fraunhofer Institute for Laser Technology ILT, a completely laser-based process chain for manufacturing
n
Corresponding author. Tel.: þ 49 241 8906 645; Fax: þ 49 241 8906 121. E-mail address: [email protected] (S. Heidrich).
http://dx.doi.org/10.1016/j.optlaseng.2014.03.001 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
aspherical and freeform optics is in development [6]. The aim of this process chain is to create an economical production method for manufacturing single pieces or small series of optical components with variable and complex geometries by using laser radiation for the whole manufacturing process. This paper presents current results of the development of laser-based process chain and its single process steps.
2. Description of the process chain and its single process steps The process chain is shown in Fig. 1. Three laser-based process steps are used to manufacture the desired optics, which generally begin by starting a simple process step to manufacture glass preforms such as a spherical lens or an ingot with a ground surface. Within the first process step, High Speed Laser Ablation, the optics geometry is created by locally removing glass material from the surface. The roughness of resulting surface after the first process step is too high for optical applications. For reducing this surface roughness, Laser Polishing as second step of the process chain is applied. Here, the surface is remelted and the surface roughness is reduced due to surface tension. During this step, no material is removed from the glass surface and its geometry is preserved. The third step, High Precision Laser Ablation, is used to apply a final form correction. Prior to this process step, the surface has to be measured so that deviations from the desired shape can be
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
35
Fig. 1. Laser-based process chain for optics manufacturing.
localized. Based on these measurement results, High Precision Laser Ablation removes smallest amounts of redundant material locally at a high spatial resolution. If needed, another Laser Polishing step can be added afterwards. Compared to conventional manufacturing methods of optical components, this process chain enables a shorter processing time for complex non-spherical geometries [6]. Due to the decoupling of tool and work piece geometry and the possibility to control the laser radiation very rapidly, the processing time remains nearly constant regardless of the geometry to be processed, so that every geometry can be processed in the same time and with the same tool. In particular, producing smaller lot sizes down to single optics is economically possible. Since CO2-laser radiation is absorbed at the glass surface at a rate of about 80% in a few tenths of a micron, it is used for the Laser Polishing step. For economic reasons, this laser source is favored for two ablation processes as well, although they can also be carried out with ultra-short pulsed laser radiation. Within this paper, the results shown have been achieved with CO2-laser radiation. The experimental setup is described in Section 3. In the following, the single process steps are described in more detail.
2.1. High Speed Laser Ablation The aim of High Speed Laser Ablation is to create the optics geometry in a short time with moderate surface roughness. During this process step, CO2-laser radiation is used to heat up the glass material to evaporation temperature and, therefore, ablate it from the preform. With laser powers of up to PL ¼ 1.2 kW, short processing times are realized. Based on the achieved ablation depth z, the ablation rate which describes the amount of ablated material per time is calculated according to the following equation: zdy vs V_ ¼ n
ð1Þ
A scan strategy, which describes the pattern in which the laser beam moves across the glass surface, is of great importance because it determines the form accuracy as well as the resulting surface roughness. An example of the scan strategy in combination with the relevant process parameters is shown in Fig. 2. Here, the focused laser radiation with the laser power PL and the focus diameter ds is moved across the glass surface at the scan speed vs in a meandering scan strategy. The track pitch dy is set to values smaller than ds so that contiguous areas are ablated. If necessary, the number of exposure layers n can be increased for a higher ablation depth. In addition to the scan strategy shown in Fig. 2, a unidirectional scan strategy is possible and will also be discussed in this paper.
dy
z Fig. 2. Procedural principle of High Speed Laser Ablation.
2.2. Laser Polishing The aim of Laser Polishing is to reduce the roughness of glass surface. As for High Speed Laser Ablation, CO2-laser radiation is used to heat up the glass material, but only to values below its vaporization temperature. In order to prevent the glass material from being destroyed due to thermal tensions occurring during Laser Polishing, it is preheated to a temperature TV which is usually as high as the glass transition temperature. The procedural principle for flat surfaces is shown in Fig. 3 (left). The defocused laser spot is moved at a high scan speed vs in the y-direction to create a quasi-line with length lLine and width bLine. This line is then moved in the x-direction at the feed speed vfeed. If a non-perpendicular incidence with the angle β of laser beam and the normal of surface takes place, an elliptical laser spot is formed on the surface of sample, as shown in Fig. 3 (right). The angle γ is the angle between the feed speed vfeed and the longest principal axis of the ellipsoid. In this case the width of the focal line bLine is given as bLine cos ðβÞ= cos ðγÞ. The resulting temperature on the glass surface depends on the process parameters and can be measured and controlled with a pyrometer. The pyrometer measures the process temperature in the middle of the focal line and controls the laser power in order to achieve constant temperatures on the surface. With this closed loop control, the process temperature can be controlled within 715 K during Laser Polishing [8]. On the one hand, the process temperature has to be kept below evaporation temperature to avoid material ablation, which would result in dents in the surface. On the other hand, the viscosity of glass material decreases with increasing process temperature, which leads to a lower surface roughness. For best results, the process temperature has to be set to values just below the evaporation temperature of the glass material. The biggest influence on the surface roughness achievable with Laser Polishing is the interaction time between laser radiation and glass material. As interaction time increases, the surface
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S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
Laser beam
y PL
bLine
x
vS vfeed
dbeam dbeam/cos(β)
vs
vfeed
Reversal point
β
γ
Lens vfeed
lLine
Fig. 3. (left) Procedural principle of Laser Polishing (right) adjustments for laser polishing a non-perpendicular surface [7].
Flowbox Beam path X-Axis Z-Axis Scanner Pyrometer
Extraction system Crossjet Heatingplate Fig. 4. Experimental setup: (left) isometric view, (right) front view.
roughness, especially with long wavelengths, can be reduced (cf. Fig. 8). Aside from the interaction time, the process temperature has the biggest influence on the achievable roughness. Both values can be influenced by the process parameters, especially laser power and scan speed [8]. 2.3. High Precision Laser Ablation The aim of High Precision Laser Ablation is the final form correction after the Laser Polishing step. Therefore, material has to be ablated with high vertical (o30 nm) as well as lateral (o100 mm) resolution. The basic principle of this process step is the same as the one from High Speed Laser Ablation shown in Fig. 2, but with PL o50 W and, therefore, smaller ablation depths. The regions to be processed are identified by measuring the surface with a suitable measurement method. For flat surfaces, white light interferometry is used. As shown in Fig. 1, another Laser Polishing step can be added after High Precision Laser Ablation if the surface roughness has increased.
3. Experimental setup and analysis procedure The experimental setup is shown in Fig. 4. The laser radiation of the CO2-laser source with a maximum output power PL,max ¼ 1.5 kW is guided through a scanner system which is mounted on an x–z-axis system. This enables the laser beam on the glass sample to be positioned and moved. A ZnSe f-theta lens with a focal length of f¼ 450 mm is used to focus the laser beam to a diameter of ds ¼450 mm.
The glass material can be placed on a heating plate which preheats the samples up to Tmax ¼600 1C. Furthermore, it is also possible to process the glass material in an oven to even higher preheating temperatures. An extraction system in combination with a cross-jet is used to remove the ablated glass material from the processing area during the ablation processes. The pyrometer is used for process control during laser polishing. It continuously measures the temperature within the polishing spot and adjusts the laser power accordingly. The whole setup is integrated in a flowbox to prevent dust particles on the glass. For roughness and ablation depth measurements, white light interferometry (WLI) is used. An example of measurement of a 10 10 mm² ablation field is shown in Fig. 5. Here, the ablated area can be identified by its difference in height relative to the initial surface. The ablation depth as well as the surface roughness is measured in a 2 1 mm² field in the middle of ablated area. This reduces the measurement time without influencing the precision of the measurement results. In order to identify the roughness in dependence on the wavelength for Laser Polishing, a Fourier transformation is conducted on measurements with different magnifications taken by white light interferometry as well as atomic force microscopy (AFM). The measurement parameters are listed in Table 1.
4. Results Due to the modular design of process chain, the single process steps have been developed separately. For each process step, experiments were conducted in order to study the process characteristics and to determine suitable process parameters.
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
Initial surface 0 Ablated area Measurement field (z andRa) for High Speed Laser Ablation μm
Reversal points -0.5 10 mm Fig. 5. White light interferometry (WLI) measurement for measuring ablation depth and surface roughness.
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for each line to maintain constant distances and, thus, constant times for each return path and to avoid uncontrolled heating outside the ablation field. A comparison of the surface roughness in dependence on the ablation depth for the meandering and the unidirectional scan strategy is shown in Fig. 7 (right). For different ablation depths, different combinations of scan speed as well as number of exposure layers have been used. For each parameter combination, three to five test fields have been ablated. With increasing ablation depth, the surface roughness increases as well. For ablation depths z4 600 mm, both a lower surface roughness and a smaller deviation can be obtained with the unidirectional scan strategy. The reason for this is the more homogeneous energy distribution throughout the processing time due to the omitted reversal points. While using the unidirectional scan strategy, a surface roughness Ra¼ 5.7 mm is measured for an ablation depth of z ¼1800 mm. Moreover, based on the error bars shown in Fig. 7, the reproducibility has risen up to 70%. 4.2. Laser Polishing
Table 1 List of the measurement parameters for Laser Polishing [8]. Wavelength (lm)
Measured area (mm2)
Resolution (lm/pixel)
Measuring instrument
320–2500 20–320 5–20 1–5 0.05–1
14.2 10.6 1.4 1.0 0.35 0.26 0.073 0.055 0.004 0.004
20.6 2.2 0.55 0.11 0.008
WLI WLI WLI WLI AFM
In this section, current results of each process step are presented. The experiments were carried out using fused silica as sample material. 4.1. High Speed Laser Ablation For identifying process parameters suitable for High Speed Laser Ablation, test fields with an edge length of 10 mm were ablated with constant parameters, and the roughness as well as the ablation depth z was measured with white light interferometry, as described in Section 3. For short processing times, the maximum laser power of PL ¼ 1.2 kW on the sample surface was used in combination with a scan speed 1000 mm/s rvs r 10,000 mm/s, a track pitch 0.1 mm rdy r0.25 mm and a number of exposure layers 1 rn r64. Exemplary results of the achieved roughness and ablation depth of single test fields in dependence on the scan speed for different track pitches are shown in Fig. 6. With increasing scan speed and track pitch, the ablation depth as well as the resulting surface roughness are reduced. To obtain a locally variable ablation depth with high ablation rate, the laser power was kept constant and the scan speed altered. This led to an ablation rate 4 20 mm³/s and, moreover, avoided process influences caused by transient effects of laser power. Due to thermal heating, the meander scan strategy, which is shown in Fig. 2, results in a higher ablation depth at reversal points at the edges of processed area. Therefore, a unidirectional scan strategy was investigated. The basic principle of this scan strategy is shown in Fig. 7 (left). In this example, the area to be ablated is marked in blue and is processed from left to right. To avoid turning the laser off and on again, which would lead to undesired fluctuations in the ablation process, the laser beam is guided around the ablation field after each line in order to realize this unidirectional scan strategy. A different return path is needed
The results of Laser Polishing and conventional polishing of a flat grinded fused silica surface are shown in Fig. 8 (left). Here, the surface roughness depending on the spatial wavelength of the roughness is shown [8,9]. With Laser Polishing, the initial roughness of a conventionally ground surface can be significantly reduced. For spatial wavelengths λo 100 mm, the laser-polished surface has an equal or smaller roughness than the conventionally polished surfaces. Compared to the initial grinded state, the roughness for spatial wavelengths λ4100 mm can be reduced with Laser Polishing, but not to values which are obtained by conventional polishing. One reason for this is the interaction time, which is too small for a much efficient reduction of longer wavelength roughness. A longer interaction time realized by smaller feed speeds, on the other hand, would lead to an increased form deviation due to the higher energy input and is, thus, not favored. Measurements of a conventionally polished as well as a laserpolished surface in different magnifications are shown in Fig. 8 (right). When comparing both measurements of the highest magnification taken by AFM, it can be seen that the laserpolished surface exhibits virtually no micro-defects, which result in a lower roughness of Rms ¼0.22 nm. Towards smaller magnifications and larger areas, the spatial roughness of the laserpolished surface increases and reaches Rms ¼0.84 nm on a field of 1 1 mm² with a filter, which removes a fourth order geometry. As can be seen in the smallest magnification in Fig. 8 (right), the glass surface is deformed when processed with Laser Polishing. This is on account of high thermal stress within the glass material, which can, however, be prevented by using a preheating temperature in the region of glass transition temperature. Alternatively, the sample can be polished from both sides by annealing after each polishing steps. Due to very high preheating temperatures of T¼ 1000 1C, which are required for fused silica, the second processing strategy is preferred. For other glass types, a preheating temperature is essential because the samples will otherwise be destroyed during Laser Polishing due to the thermal stresses. When non-planar surfaces are processed with Laser Polishing, the scan speed in the area of non-perpendicular incident of the beam is reduced in order to achieve a constant processing temperature on all points of the surface. Measurements of laserpolished spherical surfaces are shown in Fig. 9. On the left side, a ground surface of a diameter of 28 mm and a radius of curvature of 13.8 mm exhibits a form deviation of Rms o200 nm after Laser Polishing, with the deviation depending mainly on the form of the ground surface. In order to show that the geometry is only slightly
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24 22 20 18 16 14 12 10 8 6 4 2 0
1000
track pitch dy
Roughness Ra [μm]
Ablation depth z [μm]
1200
0.10 0.15 0.20 0.25
800 600 400 200 0 0
2000
4000
6000
8000
track pitch dy 0.10 0.15 0.20 0.25
0
10000
2000
Scan speed vs [mm/s]
4000
6000
8000
10000
Scan speed vs [mm/s]
Fig. 6. (left) Ablation depth in dependence on the scan speed for different track pitches. (right) Surface roughness in dependence on the scan speed for different track pitches. Parameters: PL ¼ 1,2 kW, n¼ 8, meander scan strategy. 16 meander scan strategy unidirectional scan strategy
Surface roughness Ra [μm]
14 12 10 8 6 4 2
0
200 400 600 800 1000 1200 1400 1600 1800 2000 Ablation depth z [μm]
Fig. 7. (left) Schematic drawing of the unidirectional scan strategy (one exposure layer). (right) Roughness in dependence on ablation depth for different scan strategies. Parameters: PL ¼ 1,2 kW. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Conventionally polished λ/20
+2 nm
AFM without filter
100
-2 nm
10
WLI remove 4th order
Rms = 0.51 nm
Roughness Ra [nm]
Laser polished
Initial state Conv. polished, λ/10 Conv. polished, λ/20 Laser polished
1
0.1
Rms = 0.22 nm +3 nm
200 μm
200 μm
Rms = 0.35 nm
Rms = 0.84 nm
-3 nm
Waviness
0.01
1
10
100
1000
Spatial wavelength λ [μm]
WLI without filter
+500 nm Roughness
5 mm
Rms = 7.9 nm
5 mm
-500 nm
Rms = 100.1 nm
Fig. 8. (left) Roughness in dependence on spatial wavelength for Laser Polishing and conventional polishing [8]. (right) Measurements of the polished surfaces by AFM and WLI at different magnifications. Parameters: field dimension ¼20 20 mm², PL ¼controlled for constant process temperature, interaction time ¼ 2,7 s, vfeed ¼ 2,5 mm/s, and area rate¼ 50 mm²/s.
modified by Laser Polishing, conventional polished lenses are laserpolished, which is shown in Fig. 9 (right). The form deviation was Rms ¼10 nm before and Rms ¼75 nm after Laser Polishing and the radius of curvature was only slightly changed.
The results show that the roughness and waviness achieved on spherical surfaces are equivalent to those on planar surfaces. Moreover, aspherical and freeform surfaces have also been laserpolished with nearly the same results. This is made possible
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
Ground + Laser polished
39
Conventionally polished
Conventionally polished
+ Laser polished +500 nm
5 mm
5 mm
5 mm
Rms = 197.1 nm
Rms = 9.8 nm
-500 nm
Rms = 74.8 nm
Fig. 9. Laser interferometer measurements of laser-polished spherical lenses with ground (left) and conventional polished (right) initial state.
Ablation depth z [nm]
10000
25 mm
+ 10
1000
nm 100
-25
10
Ablated surface
1 0
200
400
600
800
1000
1200
Scan speed vs [mm/s]
Initial surface
Fig. 11. Fused silica surface with a selective local material removal with different ablation depths using High Precision Laser Ablation (measured with white light interferometry). Processing time tP E 25 s.
Fig. 10. Ablation depth z in dependence on scan speed vs for High Precision Laser Ablation. Parameter: PL ¼ 16 W, dy ¼ 0.04 mm, n¼ 4.
by adapted parameters and the pyrometer, which controls the temperature even on aspherical surfaces by moving parallel to the laser affected zone. Independent from the geometry, an area rate of 1 cm²/s can be reached with 1.5 kW CO2-laser source when processing fused silica. With this area rate, the processing time for laser polishing an optics with 2.5 in diameter is below 60 s regardless of its geometry. 4.3. High Precision Laser Ablation The aim of High Precision Laser Ablation is to reduce the waviness and the form deviation remaining after the Laser Polishing process. As described in Section 4.2, the main amount of remaining roughness is located in spatial wavelengths λ 4100 mm. Moreover, the amplitude of these wavelengths exhibits values in a region o10 nm (see Fig. 8 right). For removing this roughness, a selective material removal with spatial resolution o100 mm and a vertical resolution o10 nm has to be achieved. To remove material with varying ablation depths, the dependence between ablation depth and scan speed is used (see Fig. 10). By increasing the scan speed, the ablation depth is reduced. The reason for this is the decreasing inserted energy per area. By increasing the scan speed above a certain value, no material is ablated due to the insufficient amount of energy. On account of this dependence, a selective ablation of glass material with ablation depths of some nanometers can be achieved. One example is shown in Fig. 11. The entire conventionally polished initial surface was processed with laser radiation, but only ablated in selected regions by locally reducing the scan speed. As with High Speed Laser Ablation, short processing times can be achieved with High Precision Laser Ablation. The area shown in Fig. 11 was processed in tp E25 s.
10 mm Fig. 12. Results of geometries generated by High Speed Laser Ablation. Processing time tP E30 s. Parameters: PL ¼ 1.2 kW, dy ¼ 0.1 mm.
The smallest lateral resolution reached in the example shown in Fig. 11 is about 250 mm. For a smaller lateral resolution, the beam diameter has to be reduced.
4.4. Demonstrators 4.4.1. High Speed Laser Ablation Results of High Speed Laser Ablation with different ablation depths are shown in Fig. 12. For the ablation process, parameters described in Section 4.1 have been used. Each geometry shown has been processed in a time tp E30 s. The geometries shown in Fig. 12 all feature a freeform surface to demonstrate the flexibility of High Speed Laser Ablation as well as its processing time, the latter of which is only dependent on the amount of material to be removed. The roughness achieved on the
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S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
the single process steps of the process chain can also be used alone or in combination with conventional optics manufacturing methods (e.g. conventional grinding and Laser Polishing). With High Speed Laser Ablation, ablation rates Z20 mm³/s have been achieved independent of the geometry to be generated. With the Laser Polishing step, Rms o1 nm for wavelengths o100 mm can be achieved with an area rate of up to 1 cm²/s, whereas the roughness for wavelengths 4100 mm increases and laser-polished surfaces are, thus, not sufficient for imaging optics at the moment. With High Precision Laser Ablation, an ablation depth o 10 nm can be realized with a lateral resolution of 250 mm, which has to be reduced to values o100 mm. The next steps will include the optimization of single process steps as well as the optimized combination of process steps in order to increase the surface quality and form accuracy of the processed glass optics. Fig. 13. Result of Laser Polishing a conventionally grinded aspherical geometry with a diameter of 64 mm. Processing time tP E38 s. Parameters: PL ¼controlled for constant process temperature, area rate ¼ 1 cm²/s.
ablated surface is nearly the same as the values shown in Fig. 7 (right). 4.4.2. Laser Polishing An example of a laser-polished aspherical surface is shown in Fig. 13. In this example, Laser Polishing reduces the roughness of initially grind surface significantly down to the values shown in Fig. 8, which are already sufficient for illumination optics. More information about the adaption of process parameters towards non-planar surfaces can be found in [8]. 4.4.3. High Precision Laser Ablation An example of a conventionally polished glass surface processed with selective High Precision Laser Ablation is shown in Fig. 11. The ability to apply a selective form correction of a laser polished glass surface with High Precision Laser Ablation will be shown in the near future. 5. Conclusion Current results of a laser-based process chain for manufacturing (freeform) optics have been presented. This process chain consists of three process steps: High Speed Laser Ablation for form generation, Laser Polishing for reducing the surface roughness and High Precision Laser Ablation for form correction. Compared to current optics manufacturing methods, this process chain features many advantages, such as high flexibility combined with short processing times independent of the optics surface geometry, all of which pave the way towards a faster and more efficient method of manufacturing non-spherical optics in small lot sizes. Moreover,
Acknowledgments Parts of this work have been conducted within the FoPoLas project which was supported by the Federal Ministry of Education and Research (BMBF, 13N11174) and the project executing organization VDI-TZ. The polishing step is based on the developments within the PoliLas project which was supported by the Federal Ministry of Economics and Technology (BMWi, 16IN0558) and the project executing organization VDI/VDE-IT. The above results were acquired using facilities and devices funded by the Federal State of North-Rhine Westphalia within the center for nanophotonics under Grant number 290047022. References [1] Jain A, Experimental study and numerical analysis of compression molding process for manufacturing precision aspherical glass lenses [Dissertation]. Ohio State University; 2006. [2] Nowak KM, Rapid prototyping of micro-optics for brightness restoration of diode lasers [Dissertation]. Heriot-Watt University; 2003. [3] Nowak KM, Baker HJ, Hall DR. Efficient laser polishing of silica micro-optic components. Appl Opt 2006;45(1). [4] Hecht K, Entwicklung eines Laserstrahlpolierverfahrens für Quarzglasoberflächen [Dissertation]. TU Ilmenau; 2012. [5] Schindler C,et al.Controlled USP laser ablation strategies for shaping optics. In: Proceedings of SPIE – the international society for optical engineering; 2012. p. 8428. [6] Heidrich S, Richmann A, Willenborg E, Development of a laser based process chain for manufacturing free form optics. In: Proceedings of SPIE – the international society for optical engineering; 2012. p. 8433 [7] Richmann, A, Willenborg, E and Wissenbach, K Laser polishing of lenses of fused silica and BK7. In: Proceedings of the optical fabrication & testing (OF&T) conference, Technical Digest (CD) Optical Society of America; 2012. Paper OM4D.3. [8] Richmann A,Polieren von Gläsern und Kunststoffen mit CO2-Laserstrahlung [Dissertation]. RWTH Aachen University; 2013. [9] Willenborg E,Polieren von Werkzeugstählen mit Laserstrahlung [Dissertation]. RWTH Aachen University; 2006.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Optics manufacturing by laser radiation Sebastian Heidrich a,n, Annika Richmann b, Patrick Schmitz c, Edgar Willenborg a, Konrad Wissenbach a, Peter Loosen b, Reinhart Poprawe a,d a
Fraunhofer Institute for Laser Technology ILT, Steinbachstr. 15, 52074 Aachen, Germany Chair for the Technology of Optical Systems TOS, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany c RWTH Aachen University, Templergraben 55, 52062 Aachen, Germany d Chair for Laser Technology LLT, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2013 Received in revised form 4 March 2014 Accepted 7 March 2014 Available online 29 March 2014
Current results of the development of a process chain for optics manufacturing with laser radiation are presented. The process chain consists of three process steps: High Speed Laser Ablation creates the surface geometry by material ablation, Laser Polishing reduces the surface roughness by material remelting and High Precision Laser Ablation applies a form correction by removing redundant material. Compared to conventional optics manufacturing methods, this process chain benefits from its high flexibility concerning the optics geometry and its processing speed, which is independent from the processed geometry. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Optical design and fabrication Aspherics Laser materials processing Polishing Silica
1. Introduction Optics with aspherical or even freeform surfaces enable high degrees of freedom as well as lightweight design since they can take the place of several spherical lenses. Therefore, the demand for non-spherical components is increasing in the fast-growing market of optical components at a very high rate [1]. Conventional methods for manufacturing aspherical and freeform glass optics enable the fabrication of optics either with slight aspherical shape in small production lots with grinding and polishing, or optics with complex surface geometry in very large production lots with glass (precision) molding. However, the production of single pieces or small series of non-spherical optics is very expensive and seldom economically feasible. In order to improve the manufacturing process of glass optics with complex surface geometry, laser radiation can be used instead of conventional tools due to its flexibility and its decoupling of tool and work piece geometry. Several research institutes have been investigating glass treatment by laser radiation. This research covers the production of micro-optics [2,3] as well as single process steps like polishing [4] or form creation [5] with laser radiation. At the Fraunhofer Institute for Laser Technology ILT, a completely laser-based process chain for manufacturing
n
Corresponding author. Tel.: þ 49 241 8906 645; Fax: þ 49 241 8906 121. E-mail address: [email protected] (S. Heidrich).
http://dx.doi.org/10.1016/j.optlaseng.2014.03.001 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
aspherical and freeform optics is in development [6]. The aim of this process chain is to create an economical production method for manufacturing single pieces or small series of optical components with variable and complex geometries by using laser radiation for the whole manufacturing process. This paper presents current results of the development of laser-based process chain and its single process steps.
2. Description of the process chain and its single process steps The process chain is shown in Fig. 1. Three laser-based process steps are used to manufacture the desired optics, which generally begin by starting a simple process step to manufacture glass preforms such as a spherical lens or an ingot with a ground surface. Within the first process step, High Speed Laser Ablation, the optics geometry is created by locally removing glass material from the surface. The roughness of resulting surface after the first process step is too high for optical applications. For reducing this surface roughness, Laser Polishing as second step of the process chain is applied. Here, the surface is remelted and the surface roughness is reduced due to surface tension. During this step, no material is removed from the glass surface and its geometry is preserved. The third step, High Precision Laser Ablation, is used to apply a final form correction. Prior to this process step, the surface has to be measured so that deviations from the desired shape can be
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
35
Fig. 1. Laser-based process chain for optics manufacturing.
localized. Based on these measurement results, High Precision Laser Ablation removes smallest amounts of redundant material locally at a high spatial resolution. If needed, another Laser Polishing step can be added afterwards. Compared to conventional manufacturing methods of optical components, this process chain enables a shorter processing time for complex non-spherical geometries [6]. Due to the decoupling of tool and work piece geometry and the possibility to control the laser radiation very rapidly, the processing time remains nearly constant regardless of the geometry to be processed, so that every geometry can be processed in the same time and with the same tool. In particular, producing smaller lot sizes down to single optics is economically possible. Since CO2-laser radiation is absorbed at the glass surface at a rate of about 80% in a few tenths of a micron, it is used for the Laser Polishing step. For economic reasons, this laser source is favored for two ablation processes as well, although they can also be carried out with ultra-short pulsed laser radiation. Within this paper, the results shown have been achieved with CO2-laser radiation. The experimental setup is described in Section 3. In the following, the single process steps are described in more detail.
2.1. High Speed Laser Ablation The aim of High Speed Laser Ablation is to create the optics geometry in a short time with moderate surface roughness. During this process step, CO2-laser radiation is used to heat up the glass material to evaporation temperature and, therefore, ablate it from the preform. With laser powers of up to PL ¼ 1.2 kW, short processing times are realized. Based on the achieved ablation depth z, the ablation rate which describes the amount of ablated material per time is calculated according to the following equation: zdy vs V_ ¼ n
ð1Þ
A scan strategy, which describes the pattern in which the laser beam moves across the glass surface, is of great importance because it determines the form accuracy as well as the resulting surface roughness. An example of the scan strategy in combination with the relevant process parameters is shown in Fig. 2. Here, the focused laser radiation with the laser power PL and the focus diameter ds is moved across the glass surface at the scan speed vs in a meandering scan strategy. The track pitch dy is set to values smaller than ds so that contiguous areas are ablated. If necessary, the number of exposure layers n can be increased for a higher ablation depth. In addition to the scan strategy shown in Fig. 2, a unidirectional scan strategy is possible and will also be discussed in this paper.
dy
z Fig. 2. Procedural principle of High Speed Laser Ablation.
2.2. Laser Polishing The aim of Laser Polishing is to reduce the roughness of glass surface. As for High Speed Laser Ablation, CO2-laser radiation is used to heat up the glass material, but only to values below its vaporization temperature. In order to prevent the glass material from being destroyed due to thermal tensions occurring during Laser Polishing, it is preheated to a temperature TV which is usually as high as the glass transition temperature. The procedural principle for flat surfaces is shown in Fig. 3 (left). The defocused laser spot is moved at a high scan speed vs in the y-direction to create a quasi-line with length lLine and width bLine. This line is then moved in the x-direction at the feed speed vfeed. If a non-perpendicular incidence with the angle β of laser beam and the normal of surface takes place, an elliptical laser spot is formed on the surface of sample, as shown in Fig. 3 (right). The angle γ is the angle between the feed speed vfeed and the longest principal axis of the ellipsoid. In this case the width of the focal line bLine is given as bLine cos ðβÞ= cos ðγÞ. The resulting temperature on the glass surface depends on the process parameters and can be measured and controlled with a pyrometer. The pyrometer measures the process temperature in the middle of the focal line and controls the laser power in order to achieve constant temperatures on the surface. With this closed loop control, the process temperature can be controlled within 715 K during Laser Polishing [8]. On the one hand, the process temperature has to be kept below evaporation temperature to avoid material ablation, which would result in dents in the surface. On the other hand, the viscosity of glass material decreases with increasing process temperature, which leads to a lower surface roughness. For best results, the process temperature has to be set to values just below the evaporation temperature of the glass material. The biggest influence on the surface roughness achievable with Laser Polishing is the interaction time between laser radiation and glass material. As interaction time increases, the surface
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Laser beam
y PL
bLine
x
vS vfeed
dbeam dbeam/cos(β)
vs
vfeed
Reversal point
β
γ
Lens vfeed
lLine
Fig. 3. (left) Procedural principle of Laser Polishing (right) adjustments for laser polishing a non-perpendicular surface [7].
Flowbox Beam path X-Axis Z-Axis Scanner Pyrometer
Extraction system Crossjet Heatingplate Fig. 4. Experimental setup: (left) isometric view, (right) front view.
roughness, especially with long wavelengths, can be reduced (cf. Fig. 8). Aside from the interaction time, the process temperature has the biggest influence on the achievable roughness. Both values can be influenced by the process parameters, especially laser power and scan speed [8]. 2.3. High Precision Laser Ablation The aim of High Precision Laser Ablation is the final form correction after the Laser Polishing step. Therefore, material has to be ablated with high vertical (o30 nm) as well as lateral (o100 mm) resolution. The basic principle of this process step is the same as the one from High Speed Laser Ablation shown in Fig. 2, but with PL o50 W and, therefore, smaller ablation depths. The regions to be processed are identified by measuring the surface with a suitable measurement method. For flat surfaces, white light interferometry is used. As shown in Fig. 1, another Laser Polishing step can be added after High Precision Laser Ablation if the surface roughness has increased.
3. Experimental setup and analysis procedure The experimental setup is shown in Fig. 4. The laser radiation of the CO2-laser source with a maximum output power PL,max ¼ 1.5 kW is guided through a scanner system which is mounted on an x–z-axis system. This enables the laser beam on the glass sample to be positioned and moved. A ZnSe f-theta lens with a focal length of f¼ 450 mm is used to focus the laser beam to a diameter of ds ¼450 mm.
The glass material can be placed on a heating plate which preheats the samples up to Tmax ¼600 1C. Furthermore, it is also possible to process the glass material in an oven to even higher preheating temperatures. An extraction system in combination with a cross-jet is used to remove the ablated glass material from the processing area during the ablation processes. The pyrometer is used for process control during laser polishing. It continuously measures the temperature within the polishing spot and adjusts the laser power accordingly. The whole setup is integrated in a flowbox to prevent dust particles on the glass. For roughness and ablation depth measurements, white light interferometry (WLI) is used. An example of measurement of a 10 10 mm² ablation field is shown in Fig. 5. Here, the ablated area can be identified by its difference in height relative to the initial surface. The ablation depth as well as the surface roughness is measured in a 2 1 mm² field in the middle of ablated area. This reduces the measurement time without influencing the precision of the measurement results. In order to identify the roughness in dependence on the wavelength for Laser Polishing, a Fourier transformation is conducted on measurements with different magnifications taken by white light interferometry as well as atomic force microscopy (AFM). The measurement parameters are listed in Table 1.
4. Results Due to the modular design of process chain, the single process steps have been developed separately. For each process step, experiments were conducted in order to study the process characteristics and to determine suitable process parameters.
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
Initial surface 0 Ablated area Measurement field (z andRa) for High Speed Laser Ablation μm
Reversal points -0.5 10 mm Fig. 5. White light interferometry (WLI) measurement for measuring ablation depth and surface roughness.
37
for each line to maintain constant distances and, thus, constant times for each return path and to avoid uncontrolled heating outside the ablation field. A comparison of the surface roughness in dependence on the ablation depth for the meandering and the unidirectional scan strategy is shown in Fig. 7 (right). For different ablation depths, different combinations of scan speed as well as number of exposure layers have been used. For each parameter combination, three to five test fields have been ablated. With increasing ablation depth, the surface roughness increases as well. For ablation depths z4 600 mm, both a lower surface roughness and a smaller deviation can be obtained with the unidirectional scan strategy. The reason for this is the more homogeneous energy distribution throughout the processing time due to the omitted reversal points. While using the unidirectional scan strategy, a surface roughness Ra¼ 5.7 mm is measured for an ablation depth of z ¼1800 mm. Moreover, based on the error bars shown in Fig. 7, the reproducibility has risen up to 70%. 4.2. Laser Polishing
Table 1 List of the measurement parameters for Laser Polishing [8]. Wavelength (lm)
Measured area (mm2)
Resolution (lm/pixel)
Measuring instrument
320–2500 20–320 5–20 1–5 0.05–1
14.2 10.6 1.4 1.0 0.35 0.26 0.073 0.055 0.004 0.004
20.6 2.2 0.55 0.11 0.008
WLI WLI WLI WLI AFM
In this section, current results of each process step are presented. The experiments were carried out using fused silica as sample material. 4.1. High Speed Laser Ablation For identifying process parameters suitable for High Speed Laser Ablation, test fields with an edge length of 10 mm were ablated with constant parameters, and the roughness as well as the ablation depth z was measured with white light interferometry, as described in Section 3. For short processing times, the maximum laser power of PL ¼ 1.2 kW on the sample surface was used in combination with a scan speed 1000 mm/s rvs r 10,000 mm/s, a track pitch 0.1 mm rdy r0.25 mm and a number of exposure layers 1 rn r64. Exemplary results of the achieved roughness and ablation depth of single test fields in dependence on the scan speed for different track pitches are shown in Fig. 6. With increasing scan speed and track pitch, the ablation depth as well as the resulting surface roughness are reduced. To obtain a locally variable ablation depth with high ablation rate, the laser power was kept constant and the scan speed altered. This led to an ablation rate 4 20 mm³/s and, moreover, avoided process influences caused by transient effects of laser power. Due to thermal heating, the meander scan strategy, which is shown in Fig. 2, results in a higher ablation depth at reversal points at the edges of processed area. Therefore, a unidirectional scan strategy was investigated. The basic principle of this scan strategy is shown in Fig. 7 (left). In this example, the area to be ablated is marked in blue and is processed from left to right. To avoid turning the laser off and on again, which would lead to undesired fluctuations in the ablation process, the laser beam is guided around the ablation field after each line in order to realize this unidirectional scan strategy. A different return path is needed
The results of Laser Polishing and conventional polishing of a flat grinded fused silica surface are shown in Fig. 8 (left). Here, the surface roughness depending on the spatial wavelength of the roughness is shown [8,9]. With Laser Polishing, the initial roughness of a conventionally ground surface can be significantly reduced. For spatial wavelengths λo 100 mm, the laser-polished surface has an equal or smaller roughness than the conventionally polished surfaces. Compared to the initial grinded state, the roughness for spatial wavelengths λ4100 mm can be reduced with Laser Polishing, but not to values which are obtained by conventional polishing. One reason for this is the interaction time, which is too small for a much efficient reduction of longer wavelength roughness. A longer interaction time realized by smaller feed speeds, on the other hand, would lead to an increased form deviation due to the higher energy input and is, thus, not favored. Measurements of a conventionally polished as well as a laserpolished surface in different magnifications are shown in Fig. 8 (right). When comparing both measurements of the highest magnification taken by AFM, it can be seen that the laserpolished surface exhibits virtually no micro-defects, which result in a lower roughness of Rms ¼0.22 nm. Towards smaller magnifications and larger areas, the spatial roughness of the laserpolished surface increases and reaches Rms ¼0.84 nm on a field of 1 1 mm² with a filter, which removes a fourth order geometry. As can be seen in the smallest magnification in Fig. 8 (right), the glass surface is deformed when processed with Laser Polishing. This is on account of high thermal stress within the glass material, which can, however, be prevented by using a preheating temperature in the region of glass transition temperature. Alternatively, the sample can be polished from both sides by annealing after each polishing steps. Due to very high preheating temperatures of T¼ 1000 1C, which are required for fused silica, the second processing strategy is preferred. For other glass types, a preheating temperature is essential because the samples will otherwise be destroyed during Laser Polishing due to the thermal stresses. When non-planar surfaces are processed with Laser Polishing, the scan speed in the area of non-perpendicular incident of the beam is reduced in order to achieve a constant processing temperature on all points of the surface. Measurements of laserpolished spherical surfaces are shown in Fig. 9. On the left side, a ground surface of a diameter of 28 mm and a radius of curvature of 13.8 mm exhibits a form deviation of Rms o200 nm after Laser Polishing, with the deviation depending mainly on the form of the ground surface. In order to show that the geometry is only slightly
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24 22 20 18 16 14 12 10 8 6 4 2 0
1000
track pitch dy
Roughness Ra [μm]
Ablation depth z [μm]
1200
0.10 0.15 0.20 0.25
800 600 400 200 0 0
2000
4000
6000
8000
track pitch dy 0.10 0.15 0.20 0.25
0
10000
2000
Scan speed vs [mm/s]
4000
6000
8000
10000
Scan speed vs [mm/s]
Fig. 6. (left) Ablation depth in dependence on the scan speed for different track pitches. (right) Surface roughness in dependence on the scan speed for different track pitches. Parameters: PL ¼ 1,2 kW, n¼ 8, meander scan strategy. 16 meander scan strategy unidirectional scan strategy
Surface roughness Ra [μm]
14 12 10 8 6 4 2
0
200 400 600 800 1000 1200 1400 1600 1800 2000 Ablation depth z [μm]
Fig. 7. (left) Schematic drawing of the unidirectional scan strategy (one exposure layer). (right) Roughness in dependence on ablation depth for different scan strategies. Parameters: PL ¼ 1,2 kW. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Conventionally polished λ/20
+2 nm
AFM without filter
100
-2 nm
10
WLI remove 4th order
Rms = 0.51 nm
Roughness Ra [nm]
Laser polished
Initial state Conv. polished, λ/10 Conv. polished, λ/20 Laser polished
1
0.1
Rms = 0.22 nm +3 nm
200 μm
200 μm
Rms = 0.35 nm
Rms = 0.84 nm
-3 nm
Waviness
0.01
1
10
100
1000
Spatial wavelength λ [μm]
WLI without filter
+500 nm Roughness
5 mm
Rms = 7.9 nm
5 mm
-500 nm
Rms = 100.1 nm
Fig. 8. (left) Roughness in dependence on spatial wavelength for Laser Polishing and conventional polishing [8]. (right) Measurements of the polished surfaces by AFM and WLI at different magnifications. Parameters: field dimension ¼20 20 mm², PL ¼controlled for constant process temperature, interaction time ¼ 2,7 s, vfeed ¼ 2,5 mm/s, and area rate¼ 50 mm²/s.
modified by Laser Polishing, conventional polished lenses are laserpolished, which is shown in Fig. 9 (right). The form deviation was Rms ¼10 nm before and Rms ¼75 nm after Laser Polishing and the radius of curvature was only slightly changed.
The results show that the roughness and waviness achieved on spherical surfaces are equivalent to those on planar surfaces. Moreover, aspherical and freeform surfaces have also been laserpolished with nearly the same results. This is made possible
S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
Ground + Laser polished
39
Conventionally polished
Conventionally polished
+ Laser polished +500 nm
5 mm
5 mm
5 mm
Rms = 197.1 nm
Rms = 9.8 nm
-500 nm
Rms = 74.8 nm
Fig. 9. Laser interferometer measurements of laser-polished spherical lenses with ground (left) and conventional polished (right) initial state.
Ablation depth z [nm]
10000
25 mm
+ 10
1000
nm 100
-25
10
Ablated surface
1 0
200
400
600
800
1000
1200
Scan speed vs [mm/s]
Initial surface
Fig. 11. Fused silica surface with a selective local material removal with different ablation depths using High Precision Laser Ablation (measured with white light interferometry). Processing time tP E 25 s.
Fig. 10. Ablation depth z in dependence on scan speed vs for High Precision Laser Ablation. Parameter: PL ¼ 16 W, dy ¼ 0.04 mm, n¼ 4.
by adapted parameters and the pyrometer, which controls the temperature even on aspherical surfaces by moving parallel to the laser affected zone. Independent from the geometry, an area rate of 1 cm²/s can be reached with 1.5 kW CO2-laser source when processing fused silica. With this area rate, the processing time for laser polishing an optics with 2.5 in diameter is below 60 s regardless of its geometry. 4.3. High Precision Laser Ablation The aim of High Precision Laser Ablation is to reduce the waviness and the form deviation remaining after the Laser Polishing process. As described in Section 4.2, the main amount of remaining roughness is located in spatial wavelengths λ 4100 mm. Moreover, the amplitude of these wavelengths exhibits values in a region o10 nm (see Fig. 8 right). For removing this roughness, a selective material removal with spatial resolution o100 mm and a vertical resolution o10 nm has to be achieved. To remove material with varying ablation depths, the dependence between ablation depth and scan speed is used (see Fig. 10). By increasing the scan speed, the ablation depth is reduced. The reason for this is the decreasing inserted energy per area. By increasing the scan speed above a certain value, no material is ablated due to the insufficient amount of energy. On account of this dependence, a selective ablation of glass material with ablation depths of some nanometers can be achieved. One example is shown in Fig. 11. The entire conventionally polished initial surface was processed with laser radiation, but only ablated in selected regions by locally reducing the scan speed. As with High Speed Laser Ablation, short processing times can be achieved with High Precision Laser Ablation. The area shown in Fig. 11 was processed in tp E25 s.
10 mm Fig. 12. Results of geometries generated by High Speed Laser Ablation. Processing time tP E30 s. Parameters: PL ¼ 1.2 kW, dy ¼ 0.1 mm.
The smallest lateral resolution reached in the example shown in Fig. 11 is about 250 mm. For a smaller lateral resolution, the beam diameter has to be reduced.
4.4. Demonstrators 4.4.1. High Speed Laser Ablation Results of High Speed Laser Ablation with different ablation depths are shown in Fig. 12. For the ablation process, parameters described in Section 4.1 have been used. Each geometry shown has been processed in a time tp E30 s. The geometries shown in Fig. 12 all feature a freeform surface to demonstrate the flexibility of High Speed Laser Ablation as well as its processing time, the latter of which is only dependent on the amount of material to be removed. The roughness achieved on the
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S. Heidrich et al. / Optics and Lasers in Engineering 59 (2014) 34–40
the single process steps of the process chain can also be used alone or in combination with conventional optics manufacturing methods (e.g. conventional grinding and Laser Polishing). With High Speed Laser Ablation, ablation rates Z20 mm³/s have been achieved independent of the geometry to be generated. With the Laser Polishing step, Rms o1 nm for wavelengths o100 mm can be achieved with an area rate of up to 1 cm²/s, whereas the roughness for wavelengths 4100 mm increases and laser-polished surfaces are, thus, not sufficient for imaging optics at the moment. With High Precision Laser Ablation, an ablation depth o 10 nm can be realized with a lateral resolution of 250 mm, which has to be reduced to values o100 mm. The next steps will include the optimization of single process steps as well as the optimized combination of process steps in order to increase the surface quality and form accuracy of the processed glass optics. Fig. 13. Result of Laser Polishing a conventionally grinded aspherical geometry with a diameter of 64 mm. Processing time tP E38 s. Parameters: PL ¼controlled for constant process temperature, area rate ¼ 1 cm²/s.
ablated surface is nearly the same as the values shown in Fig. 7 (right). 4.4.2. Laser Polishing An example of a laser-polished aspherical surface is shown in Fig. 13. In this example, Laser Polishing reduces the roughness of initially grind surface significantly down to the values shown in Fig. 8, which are already sufficient for illumination optics. More information about the adaption of process parameters towards non-planar surfaces can be found in [8]. 4.4.3. High Precision Laser Ablation An example of a conventionally polished glass surface processed with selective High Precision Laser Ablation is shown in Fig. 11. The ability to apply a selective form correction of a laser polished glass surface with High Precision Laser Ablation will be shown in the near future. 5. Conclusion Current results of a laser-based process chain for manufacturing (freeform) optics have been presented. This process chain consists of three process steps: High Speed Laser Ablation for form generation, Laser Polishing for reducing the surface roughness and High Precision Laser Ablation for form correction. Compared to current optics manufacturing methods, this process chain features many advantages, such as high flexibility combined with short processing times independent of the optics surface geometry, all of which pave the way towards a faster and more efficient method of manufacturing non-spherical optics in small lot sizes. Moreover,
Acknowledgments Parts of this work have been conducted within the FoPoLas project which was supported by the Federal Ministry of Education and Research (BMBF, 13N11174) and the project executing organization VDI-TZ. The polishing step is based on the developments within the PoliLas project which was supported by the Federal Ministry of Economics and Technology (BMWi, 16IN0558) and the project executing organization VDI/VDE-IT. The above results were acquired using facilities and devices funded by the Federal State of North-Rhine Westphalia within the center for nanophotonics under Grant number 290047022. References [1] Jain A, Experimental study and numerical analysis of compression molding process for manufacturing precision aspherical glass lenses [Dissertation]. Ohio State University; 2006. [2] Nowak KM, Rapid prototyping of micro-optics for brightness restoration of diode lasers [Dissertation]. Heriot-Watt University; 2003. [3] Nowak KM, Baker HJ, Hall DR. Efficient laser polishing of silica micro-optic components. Appl Opt 2006;45(1). [4] Hecht K, Entwicklung eines Laserstrahlpolierverfahrens für Quarzglasoberflächen [Dissertation]. TU Ilmenau; 2012. [5] Schindler C,et al.Controlled USP laser ablation strategies for shaping optics. In: Proceedings of SPIE – the international society for optical engineering; 2012. p. 8428. [6] Heidrich S, Richmann A, Willenborg E, Development of a laser based process chain for manufacturing free form optics. In: Proceedings of SPIE – the international society for optical engineering; 2012. p. 8433 [7] Richmann, A, Willenborg, E and Wissenbach, K Laser polishing of lenses of fused silica and BK7. In: Proceedings of the optical fabrication & testing (OF&T) conference, Technical Digest (CD) Optical Society of America; 2012. Paper OM4D.3. [8] Richmann A,Polieren von Gläsern und Kunststoffen mit CO2-Laserstrahlung [Dissertation]. RWTH Aachen University; 2013. [9] Willenborg E,Polieren von Werkzeugstählen mit Laserstrahlung [Dissertation]. RWTH Aachen University; 2006.