Other than the wavelenght? One looks like a ruby red (635) and the 650 is more of a dark red.
What kind of project? What power do you need out of the diode? What laser are you looking at (brand model)....
A bit more info would help. Also go and introduce yourself in the welcome section, so you look like a polite new user...
In my project, I'll be doing an experiment to observe surface plasmon resonance.
Surface plasmon resonance is oscillating surface electrons on a metal film, excited by the momentum matched incident light. It has been discovered in the 1950s and has many applications especially for sensing. In this project, we would like to setup a prism, a laser source and a thin metal film, to demonstrate surface plasmon resonance.
I am curious about which kind of lasers would be the best for inducing the surface plasmons. Hope this helps, thank you.
I assume you'll be using the Otto setup instead of the Kretschmann configuration then? It is my (very rudimentary) understanding that the laser wavelength has to be very closely related to the frequency of the surface electrons' natural resonant frequency. It would stand to reason that the laser wavelength is then dependent upon the thin film material to be used.
A few searches yield results using NIR and IR wavelengths and not visible ones, but that is not to say that vis cannot be used. 1.7um pops up frequently, but if the electron resonance frequency is related to the atomic spectra of the thin film it would follow that wavelength (and ergo laser) selection should be relatively straightforward.
If you're going to be taking this to the level of SPRI then you might have to consider the selectivity and sensitivity of your imaging equipment in the selection of your excitation source.
For Kretschmann config you need to use PVD or CVD (or some other evaporative deposition method) to adhere the thin metal film directly upon the pristine prism's surface. Are you making your own prism/film or buying a pre-made monolithic piece?
In any event I'm sure there are hard specifics on laser wavelength selection for SPR. How did you come upon the choice between 635 and 650 (which is often 658/660/670)?
Practically speaking the difference between them is the higher wavelength diodes are available in single mode Gaussian TEM00 outputs which is what is generally used for SPRI where as the more powerful 635nm diodes are multimode.