Precision Microscopy: Detector for Changes in Direction of a Light Beam

• Angle measurement accuracy of up to pico-rad
• Cheap and simple to implement
• Reliable
• Different camera types usable
• Possible sensitivity in two directions
• Extendible for absolute spot position monitoring

• Applicable to all types of high precision microscopes

In order to reach a position accuracy of 1 nm in a microscope, the angle of the light beam must be set with a precision of 1μrad, which makes the detection of deviations essential.
Current solutions for this problem are often based on using a segmented photosensor to monitor relative changes on each segment. As its measurements also depend on the beam intensity and shape, the setup needs careful calibration, and its accuracy is strictly limited by the detector resolution.

A new method based on interferometry has been developed to solve the problem of accurately monitoring the beam angle in microscopes.
The Invention can be exemplarily implemented, as shown in figure 3, into a laser-scanning microscope (12), where the light beam (2, 14) is provided by a laser (13) and focused by the objective (15). The focal point can be moved across the sample (17) by the scanner (16). To monitor the beam angle, the invention (1) investigates the light provided by a beam splitter (21) and focusses it onto a shear-interferometer (18) with a parallel glass plate (19). As the focal plane (22) is located behind the interferometer, the front (23) and back (24) of the plate are reflecting the light, generating parallel beams (4, 5) and forming separate focal points (25, 26). From these points, spherical waves propagate towards the photodetector (7) of the camera (8) and interfere with each other, resulting in a periodic pattern (6). The image data (10) are evaluated by a computer (11).
A variation of the angle of the initial laser beam (14) directly results in a shift of the interreference pattern (6) perpendicular to the propagation direction (9). The distance of the shift is proportional to the change of the beam angle and can be calibrated. As the initial beam (2) in the apparatus (1) is collimated and focused internally, the interference pattern is insensitive to lateral movements of the beam, while being highly sensitive to its direction.
The presented method can be easily implemented on various ways into different types of microscopes as well as developed around other types of interferometry. It can be adapted for two dimensions as well as absolute spot position measurements.

PCT (WO2020083744A1), EPO (EP3871021A1), USPTO (US20210239452A1)