Written by Berndt Zingrebe 8 December 2014

The main application for aspheric lenses is focusing laser beams in a production environment. For many material processing applications often a laser spot on the work piece is needed with highest possible energy in a smallest possible spot. One possibility for achieving this is to use a laser with higher output power, but this option comes with lots of limitations and drawback.

Adapting focusing optics offers another option, but also with limitations. Classical lenses all have surfaces with spherical shapes. They can be easily manufactured on a tool that rotates along the center of the sphere making the shape of the lens surface. However, when a collimated beam of light passes such a lens and is focused to a focal plane , rays passing through the edge arrive at  the (central) optical axis short from rays that passed close to the optical axis. The result is that the focus spot is rather a blur. The larger the ratio between the beam diameter and lens focal length, the worse the blur. However, for getting small laser spots on your work piece, the ratio of beam diameter to focal length should be as large as possible.

The classic spherical manufacturing method with multi-element objectives was the only marketplace solution. This method uses using two or three lens elements in a laser focusing objective, each made from a different optical glass material, the radii of all the surfaces can be matched so that the focus spot becomes nicely small.

The next challenge, however, is that high-power las er beams generate heat in lenses and optical glass made lenses have problems in conducting heat. The heat can cause the lenses to expand, thus changing the radii of its surfaces and its focal length. What users notice is that the focal spot is wandering away or toward the lens when the power knob on the laser is turned. The result is that over the work piece, the engraved, drilled or cut spots or lines have different widths. This is called thermal lensing.

To prevent thermal lensing it is possible to use synthetic quartz (fused silica) instead of optical glass. This material conducts heat about 10x better than optical glass. However, it only exists by itself, so that aberration compensation by use of several elements from different materials is not possible.

It is possible to design the curvature of a lens surfac e so that all rays arrive it the same focal spot. The spherical curve of the surface must be flatter toward the edges, so that there the focusing power becomes a bit less and the rays arrive at the optical axis a bit further away from the lens. Designing such surfaces can be done with just one lens element, rather than 2 or 3 as needed in the classical laser focusing lenses.

Manufacturing aspheric surfaces requires CNC (computer numerical control) production techniques. The CNC techniques were developed for metal production rather than fused silica, so using them for aspheric production was requiring solutions for many unexplored process and materials related challenges.

Sill Optics began production of precise asphere lens-elements in 2008 and can use all glass types including fused silica. To  start the production the progress department will order round disks either for prototypes and smaller numbers, or molded blanks for larger quantities and mass production.

Aspheric lens production at Sill Optics

The organisation of the production and the arrangement of all types of machines can handle all sizes of asphere-lenses from 10mm to 120mm in diameter and 25mm concave to +10mm convex radius.

The asphere-lens-elements are manufactured (grinded and polished) as spherical elements on the first side in a first step.  The surfaces are pre-polished on CNC-machines and the result is tested and inspected. Additional processes for the asphere-surfaces follow with grinding, pre-polishing and testing on special asphere-machines and equipments. The final polishing is done with the MRF (magnetorheological finishing)-technology according to the figures (terms) calculated by the testing equipment and fed back to the MRF in order to receive the required curve. (QED Technologies, specialists in MRF finishing, offers details here on how MRF works.)  

MRF aspheric lens surface polishing technology. Image courtesy of QED

Each process and surface is tested with special tactile equipment and optical measurement. The inspection of the asphere-surface is done with an optical sensor (Mahr MFU 200).

Surface map of aspheric lens from tactile measuring after polishing. Chart shows number of surface profiles.

The sensor can work up to an angle of 55 degrees. Centering with a CNC-machine and applying an anti-reflection coating follow as final operations. 

The final inspection of aspheres is done with an interferometer. The test procedure works as follows: first, the laser beam in the interferometer is focused, then the aspheric lens is placed in the path to re-collimate the beam. After this, the beam shows an interferogram. The final calculation is recognising the double pass of the beam. All aspheres manufactured from any glass types can be tested with this instrument, as long as the wavefront error is not larger than 4 fringes. The advantage of this method is that both surfaces of the asphereic lens, are tested with the same transmitted beam path as the customer is using in his application. The advantage of wavefront testing is the flexibility to measure many different lenses instead of needing a CGH (Computer Generated Hologram) for every single aspheric lens surface, as previously was the only option for testing.

3-dimensional interferometer image of the accuracy of the surface of an aspheric lens. Image courtesy of Zygo.

The production line currently includes 18 special CNC-machines and test equipment that is interconnected by software written for this purpose. To streamline workflow, at least 2 machines for each different job are implemented in the line. New test equipment allows for measurement of the aspheric curves during production, as well as for the final quality specs. Laser users benefit from the new manufacturing processes, which enable optics suitable for higher power laser beams.

Written by Berndt Zingrebe, Managing Director of Sill Optics GmbH & Co. KG in Wendelstein,Germany

Labels: aspheric lens,thermal lensing,MRF,magnetorheological finishing,interferometer,CNC,Sill Optics,Mahr,QED,Zygo

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