## SCMLab Open Science

We will be sharing some of our experimental methods on this page!

#### Optical Chopper

Optical chopper is a device used for modulating the laser intensity in a periodic way at a certain frequency. This chopping frequency then can be fed in to a phase sensitive detection equipment such as lock-in amplifier to extract the signal below the noise level. In our case as an instance, we use an optical chopper in scanning photocurrent microscopy to extract the laser induced signal. The detection sensitivity is better than a picoampere.

Here is a link to a “proper” chopper. As you can see the problem is the price 🙂 It is almost around $1300. It is not crazy expensive but for a “fan” that rotates at a certain speed, I think they are a little overpriced. Can we make something that works for cheap? Yes. Here is the list of the parts you need: 1. A brushless dc motor. An old HDD motor would do the job just fine. (~$25)
2. A brushless dc motor control board. Speed control is required. (~$10) 3. IR emitter-reader sensor (~$1)
4. A circuit to convert sine wave to TTL   – You can easily make this circuit on a bread board with an opamp (~$1) 5. Chopper wheel – a 3D printed wheel works fine (If you have a printer, almost for free) 6. Power supply (~$10)

A casing and a motor holder is also needed to make it look fancy. Total of ~\$50 is the cost. Now, what is missing here? There is no phase locked loop in this home-brew optical chopper.  An LM565 PLL chip  can be incorporated to the circuitry and the voltage can be provided over the PLL circuitry, however in our experience the frequency is stable enough to provide a locking of the lock-in amplifier.