How to address laser spectrum analysis and signal processing in a practical way with undergraduate students?
In order to answer this question, we asked Dr. Guillermo Martin, professor at Grenoble Alpes University, which practical cases he set up to address these topics with his students.
Hello Guillermo. Could you please introduce yourself and describe your professional background?
I did a joint Ph.D between ENS-Cachan and the University of Madrid on the study of nonlinear optics in organic material for frequency conversion. In 2006 I joined the IPAG (Institut de Planétologie et d’Astrophysique de Grenoble) laboratory where I still work on guided optics, more particularly on building compact devices for characterization of unknown light sources.
In addition to this, what are you teaching activities?
At the moment I teach many different classes such as guided-wave optics, optoelectronics, basic ray optics, mathematics at the IUT(1) of Grenoble Alpes University. I also teach in the optics and radiofrequency master offered by Grenoble Institute of Technology.
How do you address the laser spectrum analysis with your students?
With my IUT students, I only address the basics of laser physics: what a longitudinal mode is, the differences between open and closed cavities, mostly through practical cases.
To do so, I set up a practical work on VCSEL characterization. What I want is the students to understand that a light source emits a spectrum that has its central wavelength and an intensity that are changing with the temperature and the current set. Following this, my main goal is to explain how to decompose an optical signal in frequencies to illustrate the Fourier transform.
Can you please describe the practical work you set up?
To begin with, I use a spectrometer to look at the broad fluorescence peaks. Then we close the cavity to observe a Lorentzian line shape, and thanks to the MICRO Spectra educational kit [editor’s note: MICRO Spectra is a Laser Spectrum Analyzer based on SWIFTSTMtechnology], we retrieve the accurate spectrum and the frequencies corresponding to the source wavelengths. We also access to the interferogram to observe the interferences fringes on a single-, bi- or multi-mode signal.
The SWIFTS practical case developed by RESOLUTION Spectra Systems is very well done because it enables to illustrate the Fourier transform very easily.
Additionally, this practical work can be used to illustrate the notion of data sampling, optical resolution, non-uniform Fourier transform, aliasing windows. There is a lot to do and to learn from it!
What is interesting when using the MICRO Spectra is the SWIFTS technology on which it is based: it is neither based on an optical grating diffraction element, nor on a prism. It is really innovative; we simply make an interference with a stationary wave, a mirror and we add some light diffusion to it.
We could also set up other practical works focused on cross talk, light diffusion… There are many possibilities.
Do you have other ideas to improve the MICRO Spectra educational kit?
Using very multimode VCSEL would be interesting to create a single-, then bi- then multi-mode signal to see how the interfering fringes become more and more complex. Even though the interferogram becomes really “weird” and complex, by mean of a mere Fourier transform, the signal makes sense and we are able to identify the peaks.
It would also be very interesting to add a raw Fourier transform to the software, to show the difference between uniform and non-uniform Fourier transform, and the notion of data sampling and calibration.
Eventually, choosing the sampled data (by activating/deactivating part of the diffusing nanodots) would be interesting to illustrate the concepts of FWHM (Full Width Half Maximum) and resolution, since these features are proportional to the size of the sampled data and distance between sampling points. (editor’s note: this function has been added to the latest version of the software).
(1) Equivalent to the first two years of bachelor.