2022 Student Challenge – Optical Profilometry

*Information given below may be subjected to change

The goal of the ASPE 2022 Student Challenge is to measure surface topography of a sample of 13 mm diameter. Two main components that of each team will be supplied are an Air-bearing XY translation stage capable of 10 mm travel in each axis and a Blu-ray Optical Pick-up Unit (OPU) with an UV laser, focusing lens, flexure guided voice coil lens actuator, and position sensing detector.

Surface scanning can be achieved using the built-in photodetector of the OPU by

  1. tracking the focal point by moving its wire flexure guided voice coil and measuring the current supplied to the coil to track
  2. measuring height variation of up to 0.3 um by monitoring the change in intensity.

To calibrate displacement of the OPU lens as it relates to current supplied to the coil, teams will be supplied with a capacitance probe that can be used with a mechanism assembled using solid blocks and flexure leafs provided. Following are two possible methods for calibration of the OPU:

A. Voice coil actuated flexure mechanism can be used to servo control the OPU to keep it in focus on the sample surface, while using the displacement feedback as a means of measuring the surface height variation.

B. Two-step process:

  1. The lens of the OPU is translated up in z to move the focal spot away from the mirror attached to the flexure mechanism. Using the OPU intensity signal as feedback, voice coil actuated flexure mechanism brings the light back to focus by moving up, and its motion is measured by the capacitance probe. Doing this, one can related the current driven to the OPU coil to move the lens and its displacement.
  2. The surface is scanned where the current driven to the OPU coil to keep the focus on the surface can be converted to surface height variation
    Reference surface topography measurements will be performed using a commercial optical surface metrology instrument and will be used to determine the fidelity of the developed measurement process.

Koster, ╬╝flow – A compact fluorescence detection system that uses DVD pickup for the optical detection of beads, masters thesis, University of Twente, 2011

Hwu and Boisen, Hacking CD/DVD/Blu-ray for biosensing, ACS Sensors, 3(7), p.1222-1232, 2018

Sekhar and Ajoy, Synchronous detection to reduce offsets in focus error of an optical pickup unit, IEEE transactions on instrumentation and measurement, vol. 70, 2021

Information about registering for the 2022 Student Challenge is forthcoming. To learn more about how the Student Challenge works, visit the 2021 Student Challenge page.