Optical chopper
An optical chopper is a slotted wheel across the beam: open for part of each cycle, shut for the rest. Photodetectors behind it see the chopped light, and its reference output drives a lock-in amplifier.
The model
The chopper is a stop across the beam that is either fully open or fully shut, with sharp edges. It opens for a share \(D\) of each cycle (the duty) at frequency \(f\), starting at a phase \(\phi\) into the cycle. The bench draws it open, and traces it open.
The light through it is a square wave. A photodiode behind it reads its light when open and nothing from it when shut. In the electronics, its power is the Fourier series of that square wave, a DC level \(D\,P\) and harmonics:
\[P(t) = P\left[D + \sum_{n\ge 1} \frac{2\sin(n\pi D)}{n\pi}\cos\!\left(n(2\pi f t - \phi) - n\pi D\right)\right].\]The chopper’s reference output is a TTL square wave at \(f\), in step with the wheel. A lock-in amplifier referenced to it picks out the chopped light’s component at \(f\) and rejects what the chopper does not chop, such as room light or a detector’s offset.
The phase matters only against something else on the same clock, such as another chopper or a generator’s drive. A lock-in referenced to this chopper’s own output sees no change, since its reference shifts with it.
Settings
| Field | Default | Meaning |
|---|---|---|
length | 10 | mm aperture across the beam. |
f | 1000 | Hz, the chopping frequency. |
duty | 50 | % of each cycle open. |
phase | 0 | degrees: when in the cycle it opens, against t = 0 (0: opens at t = 0). |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- The edges are sharp: the wheel’s blades cross the beam instantly, however large the beam.
- Rays and the diffracted field on the bench are drawn with the chopper open. Only the electronics see it chopped.
Related
- Electronics: the lock-in amplifier.
- Photodetector.
- Lock-in amplifier, the block itself.