How to build a simple spectrometer

Building a simple optical spectrometer is quite straightforward and there are many excellent tutorials out there. I recommend taking a look at the respective Wikipedia page: <https://en.wikipedia.org/wiki/Optical_spectrometer> for a starter.

In the following, I share the design that I am currently using.

Simple grating spectrometer assembly. The boxed / shaded area should be placed in a closed box.

In order to get the light into the spectrometer, I use a narrow core multimode fiber which offers both flexibility and ease of use. The fiber also replaces the slit that is normally present in spectrometers and therefore, the core should not be too big. I found 35 um to give good results.

The first lens is used to collimate the output of the fiber. Use a focal length such that you can fully illuminate the grating, 25 mm worked fine for me. If you can get hold of a pigtailed fiber where the collimator is already attached - perfect!

I am using a visible transmission grating from Thorlabs with 0.5 x 0.5” side lengths. Choose the number of grooves per mm depending on your application. Higher groove density gives higher resolution but smaller bandwidth. A value of 300 grooves / mm in combination with the 100 mm cylindrical lens and the Thorlabs DCC1545M CMOS camera gives a spectral coverage of 500 nm (450 to 950 nm), for example.

In order to image the spectrum onto the CMOS chip, I use a cylindrical lens with 100 mm focal length that gets placed exactly in the center between grating and CMOS chip at exactly one focal length distance to each. The cylindrical lens ensures optimal illumination of the entire CMOS chip, while you would choose a spherical lens for a photodiode array.

As a detector, I am currently using the monochrome CMOS camera DCC1545M from Thorlabs without objective lens, which is compatible with the uc480 camera library.