Delay Calibrations

Here is a quick rundown on how the delay stages are calibrated to relate the rotation angle to the delay time of the laser pulse.

An interferometer is built by inserting a mirror/beamsplitter combination after the delays to interfere pairs of the four beams. A Helium-Neon (HeNe) laser is used as the light source, as the atomic emission at 632.8 nm is very well defined — a requirement to make a nice interference pattern and for use in the calibration fit equation.

Delay Calibration - HeNe Laser
A HeNe laser is used for calibration of the delays.
Delay Calibration - Vertical Pair
Three of four HeNe laser spots used for delay calibration.

Here we show the vertical configuration of the beamsplitter/mirror combination, which interferes the top and bottom beams, directing them downwards towards the table. The beam is directed to a fibre-optic which feeds the spectrograph and camera.

Overview of back end of interferometer used for delay calibration, in the vertical configuration.

I think this is the first look here at some of the spectrometer software! The intensity at the 632.8 nm laser line is collected as a function of one of the rotational stage angles. The rotational stage is swept between 0° (glass perpendicular to beam) to around 45°.  An interference pattern is generated.

Delay calibration interference pattern and fit.

The grey trace is the raw intensity data as a function of rotational stage angle. The yellow trace is with a Savitzky–Golay noise filter applied. The red trace, and the yellow curve in the bottom panel show the theoretical angle-to-delay relationship. As the data is collected, or when the “Fit” button pressed, the fit is performed and the three fitting parameters (α, β, γ) are determined. The fit can also be adjusted in realtime as the parameters are changed manually.

2D spectrometer software - delay calibration
Zoom into part of the collected delay calibration interferogram.

The theoretical model of the glass angle vs delay match the data very well across the entire range of rotation angles. That’s good, as it’s pretty fundamental to the whole spectrometer design!

Leave a Reply

Your email address will not be published. Required fields are marked *