Holographyforum.org / holowiki.org

I posted a few results on an earlier thread:

https://holographyforum.org/forum/viewtopic.php?f=5&t=9950

With regards to resolving modes, I think it would depend on the mode spacing. It's difficult to say anything based on my limited testing, but I checked a laser diode which [i]seemed[/i] to have a mode spacing of more than 1nm, and the real-time curve on the Theremino software showed a double-spiked curve, with the two spikes going up and down (alternately). It kind of looked like it was see-sawing back and forth. I took this for two modes which kept switching back and forth, but I can't say for sure. The HeNe curve looked much different, and didn't jump around like that.

Thanks for the links, there has been some forum discussion in the past regarding them. The thing I like about the diffraction-grating spectrometer is that it's simple, and can be used for other purposes, like checking white LEDs, etc. Based on the discussions I've seen, they appear to be limited to a resolution of about 1nm. For greater resolution, other methods (like in the links you provided) would be needed. But, I have to say that the diffraction grating/webcam arrangement does a very good job for many purposes, and the Theremino software is free and works well.

Thank you for those interesting links.
Did you find that grating-based spectrometer had enough finesse to check longitudinal modes of lasers? There’s an interesting blog on the topic and demonstrating SFPI:

http://hololaser.kwaoo.me/laser/SFPI.html

and more details/theory here:

http://www.repairfaq.org/sam/laserlia.htm#liasfpi

I've found that an inexpensive spectrometer (diffraction-grating based) can be useful for checking the spectra of illuminating light (such as LEDs) for holograms, especially color holograms. I have a $70 one from an eBay seller in France, which works reasonably well. I've also used it to check the wavelength and mode-hopping tendencies of laser diodes:

https://www.ebay.com/itm/Mini-USB-Spectrometer-Spectrometre-Spektrometre-ThunderOptics-New-Product/283451821824?hash=item41ff0b8f00:g:RpkAAOSwtBZbyHQA

I wasn't entirely happy with the quality of the build, so I'm working on a homemade spectrometer based on:

https://www.theremino.com/wp-content/uploads/files/Theremino_Spectrometer_Construction_ENG.pdf

The free software "Theremino Spectrometer" can be downloaded from:

http://physicsopenlab.org/resources-downloads/

Some other links:

http://physicsopenlab.org/wp-content/uploads/2015/11/Spettrometro_ENG.pdf

https://publiclab.org/notes/cfastie/2-5-2013/grating-angle

http://renaud.schleck.free.fr/spectrometre.php

Clever and cheap spectrometer setup here (although probably more fun than useful re. accuracy):

http://blog.svenbrauch.de/2017/04/05/poor-mans-optics-50-fourier-transform-spectrometer/