FEATURE
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Smoothness needs stiffness

As delicate as optics are, the processes used to smooth their surfaces are
rough. Rob Coppinger examines how computer control and special fluids
are making the difference

The use of magnets and fluids that become rigid in the presence of electromagnetic effects is perhaps the strangest of the technologies now being applied to optics finishing and polishing after centuries of little change.

‘People have been polishing for hundreds of years, from Galileo’s time, and not a lot has changed in how you polish a precision optic,’ says Paul Dumas, applications engineering manager for optics specialist QED Technologies. ‘It’s pretty amazing. If you walk into optics shops today you find equipment not unlike what it has been for 50 to 100 years, because it works for a wide range of optics.’

Optics makers were artisans – craftsmen that could make very precise surfaces using the tried and tested process of pitch polishing. ‘These very skilled opticians aren’t being trained; they are a dying breed,’ says Dumas. The industry faces a challenge of seeing precision levels and complexity levels of optical surfaces increasing while those artisan skills are being lost.

However, one aspect Dumas says the industry does not appreciate is that lead times are unpredictable. He says: ‘We see the need for faster turnaround; shorter lead times where the traditional optic polishing techniques were used. In the last 20 years there has been quite an advance in technology for that.’

One of those technologies is magneto rheological fluids (MRF). This is a viscous fluid that becomes more viscous when exposed to a magnetic field; so viscous it is effectively rigid. It can hold in place any shape and hold it in place even when the shape or workpiece, is experiencing significant forces from a tool. ‘You need to be able to grind, especially for glass optics. You’re talking about grinding, you’re talking about pre-polishing, smoothing for the grinding damage and then you’re talking about finishing and MRF falls into the finishing stage. The middle stage where you need to smooth out grinding damage is also a challenge,’ says Dumas. ‘With the vision and complexity levels increasing, that is really where MRF technology fits in.’

What he is referring to is that customers now want asphere and freeform optics. The traditional technology of pitch polishing relies on a very stiff tool the opposite shape of the workpiece. By rubbing these two together, a smooth and precise optical surface is created. The problem is, as Dumas says: ‘As soon as you move away from a perfect sphere and flat shape, you can’t do that.’ So MRF is used to hold the optic in place instead, because it can conform to any shape.

‘You can’t have a tool that is the opposite shape of that freeform optic shape; you need a flexible tool that can conform to these very complex and varying curvatures around a complex shape, and you need to go to a very different tool technology and MRF,’ says Dumas. The challenge of freeform optics is that, while aspheres are generally rotational symmetric shapes, freeform shapes have no symmetry. ‘It’s like an optic inside an airplane wing or optic in a heads-up display that will image something on your dashboard; they are literally freeform shapes. Those are very difficult to make,’ says Dumas.

In order to polish and correct these complicated shapes, manufacturers need to be able to measure them. Along with the necessary polishing technology, there is also the commensurate metrology equipment to detect the surface errors on freeform optics. ‘QED, 15 years ago, started commercialising this MRF tech, and within about five years we realised that our customers have this capability in their shop now, but they don’t have good metrology to measure these aspheres, to measure large optics, to feed this polishing technology the right information,’ explains Dumas.

QED then set about developing a new product line for the full aperture metrology of complex shapes. ‘Having full aperture metrology is a critical step. We are continuing to develop metrology products that characterise these complex shapes. The finishing technology is there and we can do a lot, but it is kind of being held back by the metrology sides,’ says Dumas.

At Laser Components, in-process metrology is used for its optics manufacturing processes. Kimberly Loft, technical sales engineer, says: ‘The optics are ground down using a multi-step grinding process with a variety of different stages; CNC [computer numerical control] machines ensure the highest possible quality. In-process metrology allows for the measurement and correction of geometry.’ Laser Components has the capability to make optics from start to finish. The company can take the raw substance for the topic and manufacture the lens substrate in-house. ‘We are able to grind, polish and finish and coat in-house,’ adds Loft. ‘The benefit of having this equipment in-house is that we can turn around optics in a matter of days. It means we can solely rely on the capabilities of Laser Components to ensure the standard of our optics.’

Using the CNC machine to control the process means, according to Loft: ‘We can ensure stability throughout the process and can apply it to any shape that is mathematically defined.’ The CNC machine has a touch screen and easy step-by-step instructions for loading the program.

The polishing process also uses CNC machines. The polishing process results in a high-precision surface quality. ‘It’s an expensive tool but, as it is faster, we can produce more. We can do it with any mathematically defined shape. We can make them very uniform in thickness,’ says Loft. Finally, an MRF polishing machine is used to correct errors. Loft describes magneto rheological finishing as ‘placing the workpiece or optic at a fixed distance from a moving spherical wheel.

An electromagnet just below the wheel generates a magnetic field between the wheel and the optic. A magnetically doped fluid is then delivered to the wheel. ‘When the magnetic field is applied, the consistency of the fluid changes from a honey-like substance to something more viscous, like clay.’

While MRF can be used with freeform and aspheric optics, Laser Components doesn’t manufacture such products. ‘We focus on the spherical shape; we tend not to manufacture aspheric shapes at all,’ says Loft.

While fundamentals of the optics manufacturing process have not changed in generations, slowly but surely new technologies from computer control to magnetically altered fluids are making all the difference.

For those freeform optics that Dumas views as being so widely adopted, the way ahead can be seen through the new capabilities of advanced technology that make their manufacture possible.