Lab course: Science with electronics – DVD laser scanner

The basic idea of the one week lab course is to rip a CD or DVD player apart and use it as a laser scanning microscope.

Laser scanning microscopy

A laser scanning microscope uses a lens and a beam splitter to focus a laser spot on the surface of the sample. The reflected (or fluourescent) light from the sample spot passes again the lens and is focused on a pinhole detector. This lightpass is called confocal. An image is created by either scanning the laser with respect to the sample.

Working principle of a DVD read head

A CD player read head is essentially a laser scanning microscope using the following components:
a) A laser diode (typically 780nm) is used as light source.
b) The movement of the sample with respect to the laserspot in done in two ways. First, a translation stage can move the whole head several mm (i.e. radius of the written part of the DVD). Second, the lens can be moved by tracking coils for ca 1 mm for fine tuning. Another coil is used for focussing of the lens.
c) Detection of the reflected beam is done by photodiodes. The confocal geometry requires in principle only one central photodiode. However, additional diodes allow to adjust the lens focus and track the sample.

Taking control of a DVD readhead

In order to take control of the DVD readhead, you need to identify the central elements and wire them up appropriately:
a) Laser diodes come usually with an additional reference photodiode that allows to measure the light flux from the laser diode. This reference can be used to stabilize the output power of the laser against e.g. thermal drift. For laser safety reasons, we want to limit the output power less than 300 micro Watt. You can identify the pins of the photodiode by shining light on the head and measuring of the photo voltage. Furthermore, the two diodes (laser- & photo- ) usually share one pin, often ground. You need to assign the three pins of the laser diode of the laser head you are using correctly, in order to operate it safely. For this purpose you will get a laser diode driver module.
b) The large translation is usually done by a DC motor or by a stepping motor. Both can be operated from the Adafruit motor shield for the Arduino. The tracking and focus coils have typically 10 Ohm resistance. They need a variable voltage or current source for tuning. We start by playing with the laboratory power supply.
c) The readout of the photodiodes is tricky. If you can use the build in photodiodes, you can try to focus by maximizing the central diode. If the wiring is too difficult, you need to remove the photodiode and place a simpler one.
d) Once you have calibrated the separate components, you can use the Arduino to control the scanning, i.e. scan a test pattern with stripes to find out how good your resolution is.