Frequency stabilizing a green laser with a DCG hologram

Dichromated Gelatin.
profdc9

Frequency stabilizing a green laser with a DCG hologram

Post by profdc9 »

I just thought I would put this idea out there in case anyone was interested in commenting or trying it. I have been trying to get into the DCG holography but the long coherence green or blue lasers needed can be difficult to come by.

There are companies like Ondax (www.ondax.com) that sell volume holographic gratings for frequency stabilizing laser diodes. I think mostly their stuff is used at 405, 635, 650, 800 nm, etc. typical diode laser wavelengths. You place them at the output facet of the laser diode, or in the collimated beam of the laser diode to reflect narrowband filtered light back into the diode cavity and enhances a narrow linewidth.

I wonder if this idea would work to some limited extent with a cheap 532 DPSS source. When I tested the coherence length of a DPSS laser pointer it was not too good (as compared to say, a red laser diode at a stabilized temperature). I was thinking that perhaps a weak dichromated gelatin hologram could act as a volume holographic grating.

The idea would be to create a very thick emulsion (250 to 500 microns thick) of gelatin with a low concentration of dichromate in it. The optical density of the dichromate would be low to have weak volume scattering and not absorb the green light too much. The emulsion could be placed between two thin glass plates.

Then one could place the emulsion in front of a green DPSS laser. The facets of the glass from the emulsion would not reflect the beam back in the diode. A cube corner and a neutral density filter would be placed after the emulsion to temporarily reflect some light back into the diode comparable to what the feedback would be when the grating is developed.

The interference between the light returning from the neutral density filter/cube corner and the light passing forwards through the emulsion would form a reflection grating. This reflection grating would become self-reinforcing after its diffraction efficiency became strong enough because the backscattered light would itself interfere with the illumination beam, and the corner cube and neutral density filter could be removed.

This hologram would not be developed using alcohol as it would be important to prevent shrinkage of the hologram and shifting of the wavelength. Perhaps it could be fixed with heat after the dichromate ions were given a chance to cross-link in the dark.

Anyways, this might be a way to use DCG to help stabilize the lasers to make a laser better for DCG use that is accessible to DCG hobbyists.

Dan
holorefugee

Frequency stabilizing a green laser with a DCG hologram

Post by holorefugee »

Read this site:

http://redlum.xohp.pagesperso-orange.fr/argonlaser.html

And pay attention to the ECDL information. It can be done. It is cheaper to buy a C315.
a_k
Posts: 190
Joined: Thu Jan 15, 2015 10:52 pm

Frequency stabilizing a green laser with a DCG hologram

Post by a_k »

The feedback from the grating is supposed to get the laser diode (LD) to emit light of the wavelength which is selected by the grating. In the case of a DPSS the wavelength of the feedback light is in a completely different range than the light which is produced by the LD. It would not have the desired effect of frequency stabilising the LD.
profdc9

Frequency stabilizing a green laser with a DCG hologram

Post by profdc9 »

I was thinking that the 532 nm feedback might seed the second harmonic generation process, narrowing the bandwidth of it. From my (admittedly cursory) understanding of quantum electrodynamics, the presence of a seed photon through the creation operator can improve the probability of the process, as it does in non-parametric stimulated emission. For example, I worked with a second-harmonic-generation optical parametric amplifier in which a white light continuum (generated by self-phase-modulation in a sapphire disc) seeded the down-conversion of an ultraviolet doubled Ti-sapphire photon into two lower energy photons. I was wondering if such a process could happen in reverse for SHG. Perhaps the SHG that is fed back would be too weak to make a difference.

Also, maybe grating stabilization would work on one of the 445 nm Nichia multimode diodes, and it might be possible to use the feedback to clean up the mode of the diode to boot. You would think Ondax would already be doing this with a multimode diode if this were possible, as it would be a great way to get high power. Or maybe the feedback will just toast the diode.

Best,

Dan
holorefugee

Frequency stabilizing a green laser with a DCG hologram

Post by holorefugee »

The longer the cavity is the better the temperature stability of the mount needs to be. It starts to get really hard for even 10mm cavities.
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