532nm modulation problem

[buffo]

It's called "jellybeaning", and it's a common problem with DPSS lasers. Do a search here on PhotonLexicon and you will find lots of posts on the topic.

You can try to adjust the TEC circuit, as thermal instabilities often make the problem worse. Since the problem goes away at full power (when everything in the cavity would normally be at maximum temperature), I would suggest you first try to increase the temperature of the KTP. If your laser doesn't have separate TECs for heating the KTP and cooling the diode, then try backing off on the TEC drive that cools the diode just a bit to increase the temperature that way.

Adam

[epyn]

Thanks for reply!
So this problem is not by PSU.
My green has separated TEM cooling both for KTP Xtal and for IR diode+Vanadate.
The peltier cells driven by a simplified PID controlled circuit.
Not bipolar so only cooling.Works with -+1C° accuracy.
At modulation(when i operate the whole system e.g. in a club or at home) The diode+vanadate is 22-23C°and the KTP's temperature is 30C° (its brass mount) this is the temperature that i tuned.
I'll try to re-tune the KTP's temperature.

[buffo]

My green has separated TEM cooling both for KTP Xtal and for IR diode+Vanadate.

This is an ideal setup. It allows you to keep the diode cool (which is good for long life) while keeping the KTP crystal hot, which increases doubling efficiency.

Not bipolar so only cooling.

So long as you are getting enough heat into the KTP from the diode pump, it should be OK. Remember that the KTP wants to be warm, not cold.

My guess is that if you increase the temperature of the KTP, you will be able to improve your jelly-beaning problem. Since you don't have a bi-polar controller on that TEC, your only option is to reduce the amount of cooling and hope that it warms up enough from the heat from the pump diode. Worse case, you might want to change that TEC controller to one that supports both heating and cooling.

Please post your results here!

Adam

[-bart-]

This is an ideal setup. It allows you to keep the diode cool (which is good for long life) while keeping the KTP crystal hot, which increases doubling efficiency.

Main reason for tec-ing the pump diode is tuning the wavelength to the nd:yag absorption peak, secondary advantage of stable temp is increased lifetime.

I'm not sure if the doubling efficiency is proportional with the shg crystal temp, it sure is proportional to the optical power density.
The reason why the shg temp is kept relatively hot is because it makes it less dependent of the optical power input.

[norty303]

I don't think that what you are seeing is jellybeaning.
It is simply the nature of DPSS lasers, in that they only turn on at a certain threshold. This is usually higher than for pure diode based lasers.

So your DPSS might not turn on until 20% (1V) modulation.

If you are running at 40% overall brightness (2V), then your green won't turn on until at 50%

If you are using Beyond, you can tune the individual brightness curves and thresholds by going into projector settings-->colours-->advanced.
There is a specific test frame for setting up even colour across the brightness range. It looks like a pair of parallel horizontal lines of circles that gradually fade from right to left.
The idea is to get the colour consistent across the whole fade.

[buffo]

I don't think that what you are seeing is jellybeaning.

Actually, it's an almost textbook example of jelly-beaning. The term was coined by Bill Benner (and others) because the resulting lines look like strings of jellybeans laid end-to-end instead of a solid line. Have another look at the pictures he posted and you'll see what I mean.

It is simply the nature of DPSS lasers, in that they only turn on at a certain threshold.

Except that the test patterns he was using don't have any color blending. The laser should be on at 100% power. The fact that it's pulsing even on a test pattern is the very definition of the jellybeaning phenomenon.

Adam

[norty303]

You're right. I suppose I should actually click the pics to make them bigger in future...

However, I reserve my right to comment incorrectly on threads posted in the 'Help' section that aren't about problems with the forum I blame Badger for starting it in the first place....

[epyn]

So your DPSS might not turn on until 20% (1V) modulation.

If you are running at 40% overall brightness (2V), then your green won't turn on until at 50%

You're right.
My green doesn't want to work at low power there is no balanced white light.As you can see on the pictures.
She feels well over 50% of overall power/brightness and there is no 'jellybeaning' over 50-60%
I am using QS with linear scale modulation and the green's fader is at +5%.
My next problem is the Y axis' noise.If you look closer the picture ,you can see it.Partial resonances on the horizontal lines.
It doesn't vanished with pot adjustments so i think it is bearing fail or the mirror's weak gluing.I try to fix these problems on this Weekend,before Xmas.
Thank all of you to tried to help me!
I apologize for my noobish english knowledge i hope you've got the idea
Ferenc

[buffo]

I reserve my right to comment incorrectly on threads posted in the 'Help' section that aren't about problems with the forum

HAHA! Good point - it is labeled as "This is not the 'my laser is broken' forum", after all.

I blame Badger for starting it in the first place....

Agreed. Guess Badger needs to take one for the team this time.

My green doesn't want to work at low power there is no balanced white light.

Adjusting the temperature of the KTP may help with this as well, but again, DPSS lasers are notoriously bad when it comes to modulation performance.

I am using QS with linear scale modulation and the green's fader is at +5%.

You may want to try adjusting the modulation curve again once you've played with the TEC for the doubling crystal.

I apologize for my noobish english knowledge i hope you've got the idea

Your English is quite good! I didn't have any problems understanding you.

Adam

[epyn]

What is the optimal temperature for these little (2*2*5mm) SHG (KTP)crystals?
30C° 35C° or warmer?