In this log, we'll show you how to demonstrate the fundamental principles of a grating spectrometer using a fun, visual setup: combining three lasers and then splitting them apart.

Laser Wavelength Calibration

One of the easiest and most precise ways to calibrate a grating spectrometer is by using lasers with known wavelengths. By shining a laser beam onto the diffraction grating, we can identify the exact pixel location on our sensor where that specific wavelength lands. We can then use these known points to interpolate and create a complete wavelength scale for the entire spectrum. 

For our visible spectrometer, we're using three lasers: red, green, and blue. These represent the extremes and middle of the visible spectrum. When we shine this combined beam onto the grating, the grating acts like a prism, splitting the light into its colour components. The exact position where each color appears depends on the angle of incidence of the light and the groove spacing of the grating.

Our Setup: The Beam Combiner

To merge our three lasers into a single beam, we're using a two-stage setup with dichroic beam combiners. These specialized optical filters are designed to efficiently combine light of different wavelengths.

1. A red and a green laser are combined by the first beam combiner.
2. The resulting red-green beam is then merged with a blue laser by a second beam combiner.

The final, combined beam is a single white-ish beam. When this beam hits the diffraction grating, you can clearly see the separated red, green, and blue spots on a screen.

You'll also notice three distinct spots:

• The zero-order spot is the central, undiffracted combined beam.
• The +1 and -1 order spots are the diffracted spectra on either side of the central spot, showing the separated red, green, and blue components.

Check out our video demonstration here

Why This Matters

While our laser demonstration is a great visual aid for understanding how a spectrometer works, in a real-world scientific setting, we use special calibration lamps. Lamps filled with gases like neon or argon have precisely defined emission peaks that are NIST-traceable, providing a highly accurate standard for calibration. This allows for reliable and reproducible measurements.

In our next log, we'll dive into the grating equation 

We'll show you how we can use this equation to predict the exact wavelength positions for our spectrometer setup, given a known angle of incidence and groove spacing. Stay tuned!