Optic Rotator

Highlights

• Polarization rotation by 120° in 5ms
• Continuous 360° motion range
• Standard ø0.5″ optic compatibility

Background

Various imaging modalities employ adjustable polarizers in a number of different capacities. In polarization microscopy, their purpose is to enhance image contrast and highlight features not detectable with brightfield illumination. In structured illumination superresolution microscopy, they are employed to improve the modulation contrast of the illumination pattern. To effectively complement fast sCMOS cameras, polarization change must to keep up with the 200 frame-per-second or faster imaging rate. Very fast polarization switching is achievable with electro-optic devices and liquid crystal polarizers, but it is optically advantageous to vary polarization by physically rotating a polarization optic.

Approach

The rate of intermittent mechanical movement is fundamentally limited by the inertia of the payload. In order to achieve adjustment steps on a time scale of a few milliseconds, it is essential to optimize all aspects of the design. Firstly, the moment of inertia of the moving parts is minimized by properly sizing the optic to complement the rear aperture of a microscope objective. The rotating element is held in a light-weight cylinder supported on a miniature thin section bearing and optimally coupled for maximum power transfer to a high-power-density low-inertia coreless brushless servo motor. A drive circuit matching the motor and its target operating point interfaces to a control system optimized for minimum power profile of the strenuous duty cycle and robust operation over a wide temperature range.

The repeatability of the closed loop step response is captured in the series of plots below. Beginning with temperature just below thermal shutdown at 80°C, 200 responses are collected as the system gradually returns to ambient temperature. The bottom-left heat map plot highlights the deviation from the nominal trajectory over all 200 responses.

 

Specifications

Optic Size	ø12.5mm  |  ø0.5″  |  1-3mm thick
Orientation Change	20° in 3ms  |  60° in 4ms  |  120° in 5ms
Interface	Digital trigger  |  RS-232
Size	127mm × 102mm × 25mm
Weight	1kg