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Characterizing and calibrating optical wavefront sensors

A measurement system is currently being set up at PTB to calibrate wavefront sensors with flat and spherical wavefronts in a traceable way, so that services in this area can later be offered. Wavefront sensors such as Shack-Hartmann sensors are used in various fields (e.g. in interferometry, ophthalmology or in adaptive lens systems). Shack-Hartmann sensors consist of a microlens array and of an imaging sensor that is located behind the microlens array.

Systematic sensor errors may occur due to imperfections in the microlens array or to alignment errors between the microlens array and the imaging sensor. These errors can be determined within the scope of a calibration. For this purpose, both flat and spherical test wavefronts are used. The flat wavefronts are used to calibrate the zero point of the spot positions, and the curved/spherical wavefronts are used to calibrate the deflections of the spot positions.

To characterize wavefront sensors in a traceable way, it is necessary to know the deviations of the test wavefronts used from the ideal form. In the procedures we employed, the currently available wavefronts are calibrated themselves during the measurements, which allows highly accurate calibrations of wavefront sensors. Both procedures (using flat and/or spherical test wavefronts) will be presented briefly below.

Calibration by means of flat wavefronts

Figure 1 shows the measurement principle: An almost flat wavefront is generated by means of a collimator and a single light source (laser or LED). The diameter of the collimated wavefront is at least twice that of the surface of the wavefront sensor to be calibrated. The wavefront sensor is displaced across the bundled beam generated by the collimator. Positions within the scanning range are approached following the microlens spacing. At each position, the wavefront gradients indicated by the microlenses of the wavefront sensors are recorded. In addition, the tilt of the wavefront sensor (which occurs after each translation motion and is small but inevitable) is measured by means of an optical autocollimator. Based on the traceable multiple sensor procedure (TMS) [Wie09], the input wavefront and the systematic sensor errors are determined from the measurement data. The decisive aspect here is that no prior knowledge of the test wavefront is necessary. The only prerequisite is that the wavefront may not change during the scanning process.


Figure 1: Principle sketch of calibration with plane wavefronts.


The current setup allows the reference spot position of each microlens of the wavefront sensor to be determined with a measurement uncertainty of approx. 2 µrad ( k = 2) [Bau18].


Calibration by means of spherical wavefronts


An almost point-shaped light source is used here. This light source is realized by the fiber output of an optical waveguide. Either lasers or LEDs can also be used here. The corresponding setup is depicted in Fig. 2 [Bau19]. The point-shaped light source is displaced vertically to the optical axis. Here too, the scanning range corresponds to twice the sensor size. Scanning takes place in a raster according to the pitch of the microlenses, and data of the wavefront sensors are recorded at each position.


Figure 2: Principle sketch of calibration with spherical wavefronts.


This procedure also allows the test wavefront and the microlens-specific reference positions to be determined simultaneously [Bau20]. With the current setup, estimations of the measurement uncertainty of the reference spot positions yield a value of approx 4 µrad (k = 2) [Bau20].

To obtain a more compact and stable setup and to ensure more accurate position determination, we are planning on modifying the setup (Fig. 3). By using two linear interferometers (LI-1 and LI-2, each with 3 beams), the positions and tilts of the light source will be determined more accurately, and the displacements will be traced to the meter. This setup is expected to achieve a smaller measurement uncertainty.


Figure 3: Design layout of the measurement setup for the characterization of wavefront sensors with spherical wavefronts.


Publications


[Bau20] J. Bautsch, M. Schake, G. Ehret, U. Berg, L. Wagner, J. Pfund, R. Tutsch, „Traceable calibration of Shack-Hartmann wavefront sensors employing spherical wavefronts,“ Optical Engineering, Optical Engineering, 59(8), 084104 (2020). doi.org/10.1117/1.OE.59.8.084104

[Ehr20] G. Ehret, J. Bautsch, M. Schulz: „Kalibrierung von Wellenfrontsensoren mit ebenen und sphärischen Wellenfronten.“ DGaO Proceedings (2020). www.dgao-proceedings.de/download/121/121_p6.pdf

 [Bau19] J. Bautsch, G. Ehret, U. Berg, L. Wagner, J. Pfund, „Charakterisierung von Wellenfrontsensoren mittels sphärischer Wellenfronten,“ (2019). www.dgao-proceedings.de/download/120/120_b12.pdf

[Bau18] J. Bautsch, G. Ehret, U. Berg, L. Wagner, J. Pfund, „Hochgenaue und rückgeführte Charakterisierung von Wellenfrontsensoren,“ (2018). www.dgao-proceedings.de/download/119/119_p34.pdf

[Wie09] A. Wiegmann: „Multiple Sensorsysteme zur Topographiebestimmung optischer Oberflächen“, Dissertation (2009). opus.kobv.de/tuberlin/volltexte/2009/2394/pdf/wiegmann_axel.pdf


Contact

Dr.-Ing. Gerd Ehret
Phone: +49 531 592-4220
E-Mail: gerd.ehret(at)ptb.de