DPSS Laser Pointers, question regarding why they are unstable....Diode WL, YVO4, KTP?

[Alaskan]

Curious what the main problem is which causes our 300-800 mw 532 nm laser pointers to have varying output power (well, for that matter, just about any power DPSS laser pointer). Is it the diode going too far away from the 806-812 nm acceptance wavelength window for the YVO4 crystal to produce 1064 nm output to drive the NL crystal, or the non-linear/KTP crystals sweet spot temperature drifting too far away from optimum? My guess is the KTP crystal temperature drifting, going too high in temperature is the main problem, correct?

The reason I ask, I have a wavelength locked 808.4 nm laser diode and if there is a problem with diodes going off wavelength too far, that diode would fix that problem, but if the problem is mostly the KTP temperature, I'm thinking it probably would be a waste to use a wavelength locked diode.

Anyone?

Darn it, I just saw I posted this in the wrong section, since I'm writing about a green 532 nm pointer.
Moved to "Green Lasers" sub-forum -E.P.

 

[Radim]

I guess all laser components play role. However excluding pump diode effect (output change under different temperatures), the main influence will be solid state laser crystal - 1064 output will change a lot I guess - especially under heavy load.

It is not discussed for KTP and not even exactly about that but for context:
https://hal.archives-ouvertes.fr/hal-00575917/document

I read just intro and conclusion and went briefly over it.

Edit:

In this study there is a nice chart - Fig. 3 b.

http://www.orc.soton.ac.uk/publications/63xx/6366.pdf

Edit2: Hopefully it is good to read - on mobile - sorry for image resolution inconvenience.



Edit3: Pump diode study (short):
http://www.ias.ac.in/article/fulltext/pram/076/01/0119-0125

Edit4: KTP:
https://www.osapublishing.org/col/abstract.cfm?uri=col-3-2-85

https://www.google.cz/url?sa=t&rct=...07.pdf&usg=AFQjCNFRd5t_BvAEFt8NFRBqD4z5H9Lu5w

 

[Alaskan]

Thank you great info, I have found information like that in the past and what you found added more info to help, but I just can't find anything which shows what causes our laser pointers to be so unstable, except perhaps the NL crystal temperatures varying. I don't know how far the wavelength of our small laser diodes drift up or down from 808nm, the YVO4 has a fairly wide WL acceptance, so I don't think that is the problem, probably the diode and NL crystal, but which has the most problem? My guess is the NL crystal.

 

[Radim]

Let's see... Maybe somebody is aware of study comparing that. I considered that KTP is quite stable in common temperatures region, but in this region absorption coeficient decrese quite a lot and laser therefore might lose population inversion faster due various (thermal) effects.

I imagine another effect of power loss as the metastabile level of electron might be disturbed more likely under higher temperature (more oscilations in form of internal heat), and when not initiated by other photon it is basically not stimulated emission. Therefore more instabilities and lower power occur.

I recently observed also fast mode hopping with my Evo - looking like strobe mode. This might also reduce power since there is more mess in the laser IMO.
Here: http://laserpointerforums.com/f48/radim-s-laser-painting-99824-8.html#post1478645

Edit: Wanted to rep you for an interesting question you posted, but I need to spread - will come as soon as I can. So far thank yor the great question - made me look for some studies.

Edit2: Did you notice slight absorption wavelength increase with temperature on fig 3b I posted? Not shown, but if you look at maxima around 808 nm it goes against pump diode if not locked and even if locked to 808.4 it goes quite far considering normal wavelength/power distribution of even locked pump diode.

 

[Alaskan]

Thanks, I've considered LBO for the NL crystal, but those are used for high peak power pulses from YAG's, KTP is the only NL crystal I will probably use for CW pumping. Although I do have a collection of LBO and KTP crystals of various sizes to work with already, some wonderful YVO4 crystals I found on ebay recently too, 4x4x4, 5x5x5 mm and much longer, up to 12mm long, but those are probably big enough for what I want

 

[diachi]

Thank you great info, I have found information like that in the past and what you found added more info to help, but I just can't find anything which shows what causes our laser pointers to be so unstable, except perhaps the NL crystal temperatures varying. I don't know how far the wavelength of our small laser diodes drift up or down from 808nm, the YVO4 has a fairly wide WL acceptance, so I don't think that is the problem, probably the diode and NL crystal, but which has the most problem? My guess is the NL crystal.

NL crystal temperature sensitivity, diode wavelength shift, possibly thermal lensing in the Vanadate, simple cavity design with little thermal stability (temperature changes causing physical changes in cavity size/alignment), cheap components and designs that are only intended for fairly low stability and short duty cycles in the first place. Those are the reasons that come to mind for me.

 

[Radim]

NL crystal temperature sensitivity, diode wavelength shift, possibly thermal lensing in the Vanadate, simple cavity design with little thermal stability (temperature changes causing physical changes in cavity size/alignment), cheap components and designs that are only intended for fairly low stability and short duty cycles in the first place. Those are the reasons that come to mind for me.

Exactly those I meaned by "various" (in terms of solid state laser). Thanks for your opinion.

Thanks, I've considered LBO for the NL crystal, but those are used for high peak power pulses from YAG's, KTP is the only NL crystal I will probably use for CW pumping. Although I do have a collection of LBO and KTP crystals of various sizes to work with already, some wonderful YVO4 crystals I found on ebay recently too, 4x4x4, 5x5x5 mm and much longer, up to 12mm long, but those are probably big enough for what I want

Good luck and do not forget to show us the result. I'm looking forward to receive RPL-II and to see how it behaves. We might try to compare it somehow with your design.

 

[Alaskan]

Mine will be a lot bigger than conventional pointers, if I ever get out of Iraq and start on it, that is. Thanks.

 

[paul1598419]

Interesting thread, Chris. Sorry I haven't been here to contribute. With the way pointers are put together, they are naturally ill designed to minimized the effects of mode hopping. If you have one that is mode hopping quite a bit, it is clarifying to run it through a diffraction grating. You should be able to see the spots hopping for one place to another while it is occurring. Temperature regulation of not only the crystals but the pump diodes as well are necessary to keep a stable output from each part of the laser. Unfortunately, pointers are not designed to do this.

 

[Radim]

Mine will be a lot bigger than conventional pointers, if I ever get out of Iraq and start on it, that is. Thanks.

Hopefully it will be soon. Anyway I'm still interested in your project.

Interesting thread, Chris. Sorry I haven't been here to contribute. With the way pointers are put together, they are naturally ill designed to minimized the effects of mode hopping. If you have one that is mode hopping quite a bit, it is clarifying to run it through a diffraction grating. You should be able to see the spots hopping for one place to another while it is occurring. Temperature regulation of not only the crystals but the pump diodes as well are necessary to keep a stable output from each part of the laser. Unfortunately, pointers are not designed to do this.

For mode hopping examination I use larger distance or some lens to spread the dot into blob. I like more larger distance as you can clearly observe multiple beams as well in case of common DPSS lasers.

Still in the link with my experiment I posted above the Evo was very far from its optimal temperature and the aim was to induce instabilities to learn more about them. I loved seeing TEM03 stabile for a few seconds. Also the strobe-like mode hopping was impressive. It was of stabile frequency. When I imagined (according to theory of DPSS lasers I learned from some study materials) what is happening inside the cavity and gain medium it gave me much closer idea how it works and how complex it is. All was induced by higher temperature and specific pump diode power (Evo has variable power switch). The stability of strobing mode hopping (strobe function was not used of course ) was clearly showing periodic behaviour of mode switching. Maybe I'll search for some study describing it exactly to get better insight and discuss my idea gained with it.

 

[RedCowboy]

I just saw the question and thought HEAT, but interesting answers.

 

[Alaskan]

Sure would like to see a study of different pointers though to know what component causes the most problem. Could be a lot of reasons, expanding cavity due to heating as Diachi said, and other reasons, but I believe all heat related as RCB, Paul & Radim said too. I was hoping it was mostly the temp. of the NL crystal which I could control alone much easier than all of the other components.

So far we have the following reasons:

1. Diode wavelength drifting below or above 806-812 nm.
2. Lensing of the YVO4 as it is heated.
3. Cavity length and alignment being affected by heat.
4. The KTP or non-linear crystal temperature too far from optimum.

 

[Radim]

Alaskan, add also pump diode current - more current, more power to crystal but at certain level it is saturated and produces no more power at 1064 nm, just more heat is generated and I read the power might even decrease (likely due to temperature rise).

With my Evo the current/pump power also influences mode hopping but only TEM01 (min power) to TEM00 (max power) at normal temperature (however sometimes it keeps TEM00 even at low - slightly above min - power under some certain temperature).

But the temperature is the key factor IMO.

 

[paul1598419]

Sure would like to see a study of different pointers though to know what component causes the most problem. Could be a lot of reasons, expanding cavity due to heating as Diachi said, and other reasons, but I believe all heat related as RCB, Paul & Radim said too. I was hoping it was mostly the temp. of the NL crystal which I could control alone much easier than all of the other components.

So far we have the following reasons:

1. Diode wavelength drifting below or above 806-812 nm.
2. Lensing of the YVO4 as it is heated.
3. Cavity length and alignment being affected by heat.
4. The KTP or non-linear crystal temperature too far from optimum.

I doubt very seriously that anyone has done a study to conclude what is the most problematic issue with DPSS laser pointers that causes mode hopping. So, that question may never have an adequate answer.

 

[Radim]

I doubt very seriously that anyone has done a study to conclude what is the most problematic issue with DPSS laser pointers that causes mode hopping. So, that question may never have an adequate answer.

Not only mode hopping but even power instability, Paul. I would say temperature is the main reason - affects all laser components. Still I think it would be a nice project for university students to examine it in depth. Of course not with portable but with individual laser components fitted to thermal control units on optical bench. So if someone here is looking for a thesis topic - here you are.

Regarding transverse mode hopping entire solid state laser part has a huge influence. For example in pointers/portables curved mirrors are used as flat mirrors are harder to adjust. With curved mirrors however it is easier for laser to switch mode - more options for stabile resonance within the cavity as at curved mirrors design allow to fit standing waves to more places within the cavity and they might do output in different angles (producing spatial modes - like there are "more lasers in one cavity"). Especially when temperature changes, the cavity enlarges or contracts, so varying distance between mirror allows to switch from TEM00. If the mirrors are flat, there is not that much options (assuming not perfectly flat real mirrors). Also the wavelength might change as the distance changes.

But for longitudial (temporal) modes the quality of components and laser design nees to keep laser stabile even more (narrower spectrum of output). It is matter of coherence in time - the more stable the resonance in time the longer distance light might travel as being coherent (temporal coherence length). It is not issue for portables, but it is issue for depth of field of holograms made with that laser for example as offset between phases might ruin the interference pattern being captured on very fine grain medium. In portables I calculated temporal coherence length and for my Sky Lasers greenie it is about 3 mm. For great SLM DPSS labbies it might be even 300 m (but I'm not aware of any at powers like 1 W - mostly they seem to be like 200 mW max). But these use more complex cavity designs and especially larger cavity to reach narrow spectrum.

Sorry for inaccuracies it is morning here and I've not had my coffee yet.