A new wave of quality control: Shack-Hartmann wavefront sensing
Keely Portway takes a look at the Shack-Hartmann wavefront sensing technique and how it can be used to simply and quickly assess the quality of optical systems
Optical systems can be highly complex, with extensive knowledge and experience required in their design and characterisation. Due to the number of components involved, the skills required must extend to both the system and individual component levels. This includes the evaluation of optical components for quality.
Interferometry has traditionally been used to evaluate optical components and systems, comparing interference fringes of a reference optic to that when the test optic is present. A variety of interferometers and test procedures have been developed over the years, leading to the latest generation of instrumentation and software. But this technique is not without its challenges, and its reliance on the ‘perfect’ reference beam is something that can be tricky to achieve in practice due to vibrations.
The Shack-Hartmann wavefront sensing technique offers an alternative solution. A Shack-Hartmann wavefront sensor consists of a microlens array placed in front of a 2D image sensor. If the sensor is placed at the geometric focal plane of the lens and uniformly illuminated, the integrated gradient of the wavefront across the lens is proportional to the displacement of the centroid. Consequently, any phase aberration can be approximated by a set of discrete tilts.
History of Shack-Hartmann
The technique itself is not new. In fact, the sensor’s design improves upon an array of holes in a mask that had originally been developed in 1904 by German physicist, Johannes Franz Hartmann, as a way of tracing individual rays of light through the optical system of a large telescope. The late 1960s saw American physicist Roland Shack and research student Ben Platt modify the ‘Hartmann screen’ by replacing the apertures in an opaque screen by an array of microlenses. The Shack-Hartmann technique has become more widely adopted in recent times, thanks to its high accuracy, rapidity and reduced sensitivity to vibrations than interferometers.
The technique is even becoming a driving factor in the overall global wavefront sensor market, which a research report by Transparency Market Research (TMR) estimates will grow by about $4bn by 2031. In optical metrology, said the report, Shack-Hartmann wavefront sensors are being used for their versatility and improved performance. This means that vendors operating in the global wavefront sensor market are strengthening their production capabilities in Shack-Hartmann wavefront sensors.
A recent and significant development in the field came from the launch of NASA’s James Webb Space Telescope. To accurately measure the shape of Webb’s mirrors during manufacturing, the team made new improvements to wavefront sensing technology, with a measurement device known as the Scanning Shack-Hartmann Sensor. The improvements have opened up further applications, allowing, for example, eye doctors to get more detailed information about the shape of an eye in seconds rather than hours.
Preetesh Mistry, sales manager – photonics at Pro-Lite Technology, has seen for himself the increase in popularity of the ShackHartmann technique. He said: ‘Over the last 25 years, it’s been picking up, both in terms of technology and also becoming more accurate. The technique is a very versatile solution that can be used in the lab, it can be used in the field, it’s very insensitive to vibration and frequency shifts, and able to measure at the intended wavelength of use of the optics.’
In 2019, Pro-Lite Technology partnered with Imagine Optic for the launch of its ShackHartmann wavefront sensors for laser and lens testing. More recently, the latter introduced the Optical Engineer Companion – an optical metrology system comprising compatible and complementary optical hardware, software and accessories. Mistry said: ‘Imagine Optic is the leader when it comes to specification and in terms of accuracy, reduced errors and more. The products are really top of the range, and while there are cheaper solutions out there, it depends on the application and the degree of accuracy required. For high-end scientific applications, the Imagine Optics kit is incredibly good.’
In terms of applications to which the technique lends itself, Mistry explained that the latest generation of modules allow for numerous market opportunities. ‘We have quite a broad wavelength range these can work in with the various different HASO modules. So, from 350 to 1,100nm for the standard sensors, visible to the near-infrared. There are also the shortwave IR sensors that are used a lot in telecoms and datacoms.’
Applications include free space communication and communication satellites, as well as adaptive optics. Mistry elaborated: ‘One particular application is, if you think of the free space optics going out to a satellite in space, the turbulence in the atmosphere can cause problems. But you can measure that and use the optics to correct the wavefront. So, when it gets through, you’re getting all the information and you’re not losing anything. It’s very much for any kind of optical system, so if it’s an imaging system, a vision system, anything where you’re introducing components, you can then measure them.’
Systems such as those referenced above can also be used for high-powered laser applications, such as those used at the Rutherford Appleton Laboratory (RAL) in Oxfordshire. These are for measuring the wavefront of the petawatt laser when a small amount of the energy is picked off. ‘But with something like this,’ continued Mistry, ‘they could then characterise components and ask, “well, actually, what if there’s a problem?” or if there’s a diagnosis tool, or even if it’s a case of “we’re thinking of putting this optic in from a new manufacturer, let’s see how it works”. It can be a research and development tool, it can be a quality checking tool, so basically anyone working with optics and imaging should be interested in this technique.’
Looking to the future of this technology, Mistry believes the rate at which it is being developed will continue. ‘Things like resolution are always being improved,’ he explained. ‘Each of the sensors in the HASO range has a different number of micro lenses, for example. The more you have, the more points you can measure, so the more of the beam you are sampling. That’s always evolving, as are the algorithms on the software, in terms of increasing the spatial resolution. Ultimately, I think the evolution of the technology is going to be driven by what the markets demand.’
If you'd like to know more about the theory of Shack-Hartmann wavefront sensing and how it can be used to simply and quickly assess the quality of optical systems, download the latest white paper.