Greg Blackman on the intricacies of characterising optics with wavefront sensors
The gold standard in optical metrology is still interferometry, as it’s the technique with the highest accuracies. However, interferometers are typically large, expensive and restricted to a lab as they need to be isolated from vibration. To make inline characterisation of optics during production, other analysis criteria are used such as modulation transfer function (MTF) or, if more information is required, data from a wavefront sensor.
A common type of wavefront sensor is Shack-Hartmann, originally developed by German physicist Johannes Hartmann around 1900 and later modified by Roland Shack and Ben Platt in the 1970s. Hartmann’s original design consisted of an array of apertures, which was later updated by Shack and Platt by incorporating a lenslet array. The device works by calculating the shift in position of points of light falling through an aperture array compared to where they would fall for a perfect wavefront. The light passing through the lenslets is measured by a CCD sensor, with a shift in position of the points of light attributed to aberrations in the optic.
‘Modulation transfer function is a basic test used widely for optics characterisation, telling the user whether the optic complies with set criteria – it’s a pass/fail test,’ comments Xavier Levecq, CTO at Imagine Optic, a company providing Shack-Hartmann sensors. ‘A wavefront sensor, on the other hand, will tell the user why an optic has failed, providing information on the type of aberration.’
Wavefront sensors can also be used to align and characterise very complex lens objectives, such as those for film cameras or complex lenses for ophthalmology. From the types of aberration, the user can backtrack along an optical path and locate which lens in an objective is faulty.
‘A wavefront sensor is good for characterising an entire optical system, but it’s also very useful for characterising the individual optics that make up the system,’ explains Mark Zacharria of Imagine Optic. ‘Individual elements in the optical chain can incorporate aberrations in and of themselves and so knowing that a component has an aberration allows you to compensate for it along the optical path.’
Imagine Optic has a client in Belgium that manufactures thermal sensors. The company receives large blocks of infrared lenses from third-party distributors, which are then tested on an optical bench designed by Imagine Optic. A wavefront sensor, explains Zacharria, identifies not only whether the lenses pass or fail testing, but what the problem is if they do fail, so that the company can go back to the vendor with evidence of the fault.
Testing contact lenses
A big market for wavefront sensors is analysing contact lenses and intraocular lenses (IOLs), an area that’s traditionally relied upon interferometers for batch testing in the lab or profilometers to make contact measurements. According to Gael Launay, sales and marketing manager at wavefront analysis company, PhaseView, a wavefront sensor is faster and higher resolution than profilometers, and isn’t affected by vibration unlike an interferometer, which makes it ideal for integration into a production line. PhaseView is working with major contact lens manufacturers for lens characterisation. Its wavefront analysis technology is based on curvature sensing, in that it measures the curvature of the wavefront rather than the gradient, which is what a Shack-Hartmann sensor measures.
PhaseView’s technology relies on the variation of the electromagnetic wave’s intensity in the optical axis direction to extract a wavefront measurement. Instead of using micro-lenses like in Shack-Hartmann sensors, two image planes along the beam propagation axis are acquired in real time for measuring the curvature.
‘Our technology can provide very high lateral x-y resolution, identical to a CCD camera, and dynamic range flexibility as it’s not limited by micro-lenses,’ states Launay. As the intensity image is not sampled, surface defects like scratches or digs can be detected at the same time as measuring the power, aberrations, modulation transfer function and other features like radius of curvature and lens surface shape.
‘The trend is moving towards fabricating freeform IOLs and contact lenses, which are more complex optics with widely differing shapes from one lens to the next,’ says Launay. The high lateral resolution of PhaseView’s wavefront sensor allows precise measurement of very small surface variations. The sensor can measure either lenses in transmission or lens moulds using reflection.
While an interferometer provides high resolution, the dynamic range is lower than a wavefront sensor. To characterise aspheres by interferometry typically requires a computer generated hologram (CGH) to cover the surface of the lens. However, CGHs are expensive and are specific to each optical element. According to Aurélie Azam, marketing manager at Phasics, wavefront sensing technology allows the field surface of the lens to be analysed in one acquisition without CGHs.
Phasics’ wavefront analysis technology is based on quadri-wave lateral shearing interferometry. Its systems use a modified Hartmann mask integrated with a pi-shift plate phase checker board. The grating, also called a Modified Hartmann Mask (MHM), diffracts the incoming beam into four replicas that then interfere. The generated interferogram pattern is analysed based on Fourier theory to retrieve both the intensity and the phase information.
The technology provides high resolution (300 x 300 measurement points) compared to Shack-Hartmann sensors, which are typically limited to approximately 100 x 100 measurement points. Phasics’ systems also provide a high dynamic range (up to 100µm), which, combined with the high resolution, to use them for asphere analysis.
‘Since our technology is based on a diffraction grating and not micro-lenses, it can measure either a converging or diverging beams,’ explains Azam. Consequently, it is a very good solution for characterising high numerical aperture optics or aligning a lens group directly without having to collimate the beam. Phasics has developed a specific module for toric lenses.
‘In reflection mode, our technology can find and measure both the exact minimum and maximum radii of curvature of toric lenses and the orientation between the related axes,’ explains Azam, adding that this is not possible with an interferometer or classic profilometer, (a profilometer measures the radii only at the point of contact, whereas Phasics’ technology generates a surface map from which the minimum and maximum radii are found).
Working with extreme ultraviolet (EUV) or X-ray radiation presents unique challenges for wavefront sensors. The micro-lenses used in a Shack-Hartmann sensor are ineffective with EUV radiation and Imagine Optic supplies a modified Hartmann system to characterise EUV optics.
‘The EUV wavefront sensors are simply Hartmann systems comprised of a micro-hole array without the micro-lenses,’ Levecq says. He adds that the distance between the array and the detector is also further with EUV radiation (in the range of a few hundred millimetres) than for visible light (a few millimetres), improving the sensitivity.
Levecq states that the Shack-Hartmann sensor is seen as a complementary technique rather than an alternative to interferometry. The Shack-Hartmann sensor, for instance, has the advantage of being able to characterise a complete optical system utilising the system’s light source, which would not be practical to do with interferometry.
Zacharria adds that there is an increased demand for metrology as a service rather than directly purchasing a wavefront sensor. ‘As optics and photonics move into different fields – scientists want to know exactly what kinds of aberrations are in their microscope objective, for instance, so they can correct for them – we’re seeing a lot of demand for metrology as a service. A wavefront sensor is a big investment, costing between €10,000 and €40,000 for a piece of equipment that’s only going to be used once or twice,’ he says. Imagine Optic’s wavefront sensors are also used by companies running metrology as a service, which offer customers not only the equipment but also the metrology expertise.