A wavefront sensor is a device for measuring an optical wavefront and any aberrations within it. The wavefront is a region in which all the points sampled in the wave have the same phase at a given instant.
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Characterization of the wavefront of a pulse is important as optical components are used to modify the characteristics of the wavefront e.g. a lens can be used to change the curvature of the wavefront, resulting in a focused beam.
There are many different designs of wavefront sensor available. The right choice of sensor depends on the light's characteristics, such as the wavelength, power and pulse duration.1 For high-power laser systems, wavefront sensors are often an essential part of an optical set-up, as the high thermal load can induce deformation in optical components, creating new aberrations in the beam.2
Many high-power systems for fusion experiments also use optical components that are meters in size. Therefore, it is challenging to avoid manufacturing defects over the full area of the optical component, which can, in turn, be characterized through the use of a wavefront sensor.
Wavefront sensors can either measure the wavefront aberrations through direct or indirect methods. One of the most common types of wavefront sensor design is the Shack-Hartmann wavefront sensor.
Design Types
A Shack-Hartmann wavefront sensor consists of an array of microlenses and an image sensor. Each of the individual lenses focuses the incoming radiation onto a spot on the sensor to create a 2D distribution of the focused light.
An algorithm is then used to calculate any deviations of the measured spot distribution from what would be expected of an ideal, aberration-free wavefront. Measurement of the intensity of the spots in the distribution can also be used to calculate the M2 value of the laser beam, which is a measure of beam quality.
Since their inception, Shack-Hartmann sensors have found use in astronomy, ophthalmic applications and for measurements of high-power laser beams.3 There are now a number of commercial designs and instruments available that make use of this configuration.
Another type of wavefront sensor design is a pyramid wavefront sensor. A pyramid wavefront sensor uses a pyramidal prism to split the focused incident beam into multiple parts. The split beam then passes through a second lens and onto an imaging detector. The prism is then normally dynamically modulated in the incident beam path, though modulation-free designs exist.4
There are a number of different modeling approaches for the reconstruction of the incident beam from the imaging data. Most methods, though, will recover typical wavefront aberrations such as wavefront tilts and curvature.
Pyramid wavefront sensors have become particularly popular in the field of astronomical adaptive optics. Many telescopes consist of very large area optics and often require adaptive designs as it is very difficult to manufacture large-scale optics with the required surface uniform.5
The high spatial resolutions achievable with pyramid wavefront sensors and their relative robustness with regard to vibrations make them ideal for such designs.
Applications
Adaptive optics has become one of the main areas in which wavefront sensors are used. The optical system can be continuously realigned by creating a feedback loop between the wavefront sensor and the adaptive optic, such as a deformable mirror, and the beamshape adapted as necessary to correct any aberrations.
Wavefront sensors and adaptive optics have become commonly used in X-ray free-electron lasers (FELs) for both alignment and stability of the FEL beam and also for the characterization of the X-ray beams at the final sample position where the spot size may be too small to be measured accurately with direct imaging methods.6 The high peak intensities of the FEL beam are also problematic in terms of the wavefront sensor design as this, combined with the short wavelengths, means that damage to the sensor components is a serious issue.
Grating interferometry and speckle tracking are commonly used methods as these techniques, unlike the aforementioned Shack-Hartmann and pyramid sensors, do not need to operate in the beam focus, reducing the risk of damage. Such techniques can also provide single-shot pulse characterization, which is important for many of the experimental techniques that use FELs where beam aberrations need to be corrected on a shot-by-shot basis.
Advances in multilayer mirror fabrication and the spatial resolution of wavefront sensors have now made it possible to generate and measure sub-10 nm X-ray foci with wavefront sensors.7 Such tight foci are desirable for many machining or imaging applications as well as non-linear optics experiments, where very intense laser fields are required to drive the process of interest.
New methodologies for wavefront sensing and image reconstruction are continually being developed. Wavefront sensors are now a well-established technology and they are found in most ground-based telescopes. Still, the proliferation of adaptive optics in many photonics applications, including quantum communications systems, means there is still growing demand for new technologies in this area.
References and Further Reading
Geary, J. M. (1995). Introduction to wavefront sensors (Vol. 18). Spie Press. doi.org/10.1117/3.179559
Wang, H., Liu, C., He, X., Pan, X., Zhou, S., Wu, R., & Zhu, J. (2014). Wavefront measurement techniques used in high power lasers. High Power Laser Science and Engineering, 2, pp. 1–12. doi.org/10.1017/hpl.2014.28
Schwiegerling, J., & Neal, D. R. (2005). Historical development of the Shack-Hartmann wavefront sensor. Robert Shannon and Roland Shack: Legends in Applied Optics, edited by JE Harvey and RB Hooker—SPIE, Bellingham, WA, pp. 132–139.
Ragazzoni, R., Diolaiti, E., & Vernet, E. (2002). A pyramid wavefront sensor with no dynamic modulation. Optics Communications, 208(1–3), pp. 51–60. doi.org/10.1016/S0030-4018(02)01580-8
Shatokhina, I., Hutterer, V., & Ramlau, R. (2020). Review on methods for wavefront reconstruction from pyramid wavefront sensor data. Journal of Astronomical Telescopes, Instruments, and Systems, 6(01), p. 1. doi.org/10.1117/1.jatis.6.1.010901
Seaberg, M., Cojocaru, R., Berujon, S., Ziegler, E., Jaggi, A., Krempasky, J., Seiboth, F., Aquila, A., Liu, Y., Sakdinawat, A., Lee, H. J., Flechsig, U., Patthey, L., Koch, F., Seniutinas, G., David, C., Zhu, D., Mikeš, L., Makita, M., … Vagovič, P. (2019). Wavefront sensing at X-ray free-electron lasers. Journal of Synchrotron Radiation, 26(January), pp. 1115–1126. doi.org/10.1107/S1600577519005721
Matsuyama, S., Inoue, T., Yamada, J., Kim, J., Yumoto, H., Inubushi, Y., Osaka, T., Inoue, I., Koyama, T., Tono, K., Ohashi, H., Yabashi, M., Ishikawa, T., & Yamauchi, K. (2018). Nanofocusing of X-ray free-electron laser using wavefront-corrected multilayer focusing mirrors. Scientific Reports, 8(1), pp. 1–10. doi.org/10.1038/s41598-018-35611-0
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