Possibly impossible?

[WizardG]

All the high power blue (and now green) diodes are multi-mode. Would it be possible, even in theory, to use an external cavity to tame one of these diodes and select a single output mode? Even at the cost of 90% of the power obtainable from multi-mode operation I'd like to have a single mode output.

Thanks

 

[Alaskan]

WARNING: LPF knowledge base exceeded....

 

[swamidog]

if you're going to sacrifice 90% of your power, why not just use a single mode blue or green diode.

All the high power blue (and now green) diodes are multi-mode. Would it be possible, even in theory, to use an external cavity to tame one of these diodes and select a single output mode? Even at the cost of 90% of the power obtainable from multi-mode operation I'd like to have a single mode output.

Thanks

 

[Singlemode Laser]

,

do you mean single transverse mode or single longitudinal mode?

In the first case just use the low power versions of the diodes you are talking about. Almost all of them emit a single transverse mode.

In the second case you will need an external cavity as you mentioned. A multimode diode is usually not a good choice for this. At least you have already built some ECDL's in the past.

Singlemode

 

[WizardG]

Well, I haven't built one on my own (yet) but I tinker with them from time to time @ work. The x140 diodes are cheap enough now that maybe I ought to give it a try. I'm fortunate to work in a place where I have access to the tools and facilities needed to remove the laser diode die from its' can and play with it.

 

[Singlemode Laser]

Well, I haven't built one on my own (yet) but I tinker with them from time to time @ work. The x140 diodes are cheap enough now that maybe I ought to give it a try. I'm fortunate to work in a place where I have access to the tools and facilities needed to remove the laser diode die from its' can and play with it.

I still don't understand if you plan to achieve single transverse mode (nearly Gaussian beam), single longitudinal beam (only one frequency mode is operating) or both at the same time.

The first one can be achieved by a simple spatial filter (a focusing lens combined with a pinhole), or a single mode fibre. You will lose the power in the other modes but that is straight forward.

The second one usually needs a diffraction grating in littrow configuration. There is a ton a ton of good free resources and publications out there and I can list some later.

The last one is very tricky and requires a combination of a grating with a position dependent feedback. Depending on the laser diode up to 50% of the nominal power can be achieved. This is a relatively new approach, but some publication are out there. Blue multi-mode diodes show also a spontaneous phase-locking of their multiple stripes, leading to single mode (both, transverse and long) operation.

I have tried all three techniques (the second is my specialty thought) and can provide some useful tips, if you can say more about your goal and the equipment you can use at your work place.

Singlemode

PS: I would not recommend to work with opened diode cans for now. The provide a hermetic sealing that helps a ton to increase temperature stability and reduces changes of the optical path due to pressure changes (important for single frequency operation). If you still decide to do so (there are some cases where it make sense) a good ecdl housing that reduces air circulation would be good.

 

[CurtisOliver]

All the high power blue (and now green) diodes are multi-mode. Would it be possible, even in theory, to use an external cavity to tame one of these diodes and select a single output mode? Even at the cost of 90% of the power obtainable from multi-mode operation I'd like to have a single mode output.

Thanks

The answer would almost definitely be a no, and for that amount of power loss I have absolutely no idea why you would want to do that. You can just opt for a single mode anyway. :thinking: Most wavelengths I've come across that have multimode diodes, also have lower power single modes.

 

[diachi]

Spatial Filtering:

Littrow Configuration:

 

[CurtisOliver]

Totally forgot about spatial filtering. But either way, getting a single mode probably will still be better IMO.

 

[paul1598419]

Not trying to knock planters as I have enjoyed his presentations many times, but this is the worst case of spatial filtering I've seen. A professional spatial filter does this sooo much better it is no contest. Rather that placing your pinhole past the focus of the first lens, you always place it directly at the focal point. Using high quality optics the pinhole can be much smaller effectively filtering out more noise than this setup. If we were using a long coherence length laser, we could temporarily eliminate the second lens and look at the Airy disc. It will look like an unfocused beam with no noise, just a pure color across all the field. That is where the spatial filter should be set, and the second lens could then be added. I have done this many times in the past and it works very well for cleaning up noise in your beam.

 

[WizardG]

Sorry about the delay in responding. What I'm after is a top quality holography laser. My old SP needs to be re-gassed again but in its day was awesome. Just add a spatial filter and I had more coherence length than I knew what to do with.

I thought about a spatial filter in front of a diode but I wanted more coherence length. That's what got me thinking about an external cavity. The goal was getting a single longitudinal mode to narrow the line width. If I could achieve both a SLM and a nice clean TEM00 at the same time, well, that would sort of be the holy grail.

As to the resources @ work. I work for a semiconductor manufacturer. I'm in the manufacturing R&D department. We have the tools to do........anything you can think of short of sci-fi level nanotechnology. I'm friendly enough with those in the labs that if I asked real nice I could have someone rework the facets on a laser die with one of our SIMs

 

[CurtisOliver]

Well, again there is no point using a multimode to begin with. Holography doesn’t require much power anyway. Some single mode diodes have a good line width and a good Gaussian beam profile. But you won’t beat the line width of a HeNe. HeNe and Argon have always been the preferred choice for making holograms for a good reason.

 

[paul1598419]

You can get a single line out with diffraction tuning. I would also stabilize it with TEC temperature control. You could get 20 mW out of a decent single mode diode with a very long coherence length. It might not be cheap, though. A polarized HeNe is still the cheapest route for holography.

 

[Singlemode Laser]

I successfully operated the 502nm and the 450nm Osrams (single transverse mode) in an ECDL.

Linewidth achieved was way lower than 1Mhz and at wavelenghts a HeNe can't cover. Here is a great page to start with:

Red diode lasers for holography

If you have questions or s.e. ask and I will try to help.

Singlemode

 

[Singlemode Laser]

Here a two pictures of my very simple ECDL design for a fast first look at the
tuning range and other things. The grating and compensation mirror are glued on a mirror mount:

Now I am almost finished with a hermetic sealed design machined from one aluminium block to increase the mechanical stability.

Singlemode

 

[diachi]

Here a two pictures of my very simple ECDL design for a fast first look at the
tuning range and other things. The grating and compensation mirror are glued on a mirror mount:





Now I am almost finished with a hermetic sealed design machined from one aluminium block to increase the mechanical stability.

Singlemode

Very nice, makes me wish I had an optical breadboard!

WizardG, have you looked at the cheap Coherent Compass units that come up on eBay? They are SLM and good for holography.

Although, experimenting with diodes would be more interesting. :beer: