Modulation efficiency

[Jem]

We will soon ( after the store reopens ) have a few options for 457 at more or less the price that Mccarrot quoted.

There will be options of lab or oem head ( oem head requires a heatsink ) with lab or oem style power supply

Looking forward to seeing them Dave. I hope they are pretty small as the inside of my case is fairly 'snug' to say the least

Cheers

Jem

[-bart-]

Ok, now I've analyzed the data with respect to pulse width. It seems 200 µs is the most common width (23% of the pulses). 77% of the pulses are shorter than 1 ms and 99% shorter than 5 ms.

Here's the pulse width distribution:

Great analysis !!
But don't you think your data might be polluted by the show you used.
If a particular frame is heavily used, this will weigh also heavily on your statistics. So to be truly useful, huge heaps of beam and graphic shows should be analyzed.

I think you might be jumping to conclusions, (no offense !)
A Dpps laser showed to be dependent of a synthetic duty-cycle test.
That does not automatically mean the laser is pulse-width dependent.
We should not jump to the conclusion that we should tune our modules to the most common pulse width.

In fact, think that it is not the duty cycle or pulse-width which affects the output. I really think that the average (or maybe rms) value, affects the output power.
Therefore I think it is interesting to analyze all kinds of shows mathematically, for average power r,g and b, this will take the analog levels into account, which pulse width analysis does not.

Ideally, in the end we should come up with a synthetic signal that's easy to reproduce, that manufacturers could optimize their modules to, and will yield provable-better real-life modulation results.

[ElektroFreak]

My question is: How would the manufacturers tune modulation characteristics? The way a crystal reacts to pump energy isn't governed solely by alignment or temperature, so once those are optimized, the system will still exhibit direct modulation instability. The only practical way to smooth the output is with optical feedback, but as said earlier the only way to safely implement it is to have it retard the current to the diode whenever the main output power spikes. In many cases this would involve taking a laser that is capable of 100mW average and running it at 25-30mW average with greater stability. Not everyone will be OK with the substantial price increase that would result.

[mixedgas]

My question is: How would the manufacturers tune modulation characteristics? The way a crystal reacts to pump energy isn't governed solely by alignment or temperature, so once those are optimized, the system will still exhibit direct modulation instability. The only practical way to smooth the output is with optical feedback, but as said earlier the only way to safely implement it is to have it retard the current to the diode whenever the main output power spikes. In many cases this would involve taking a laser that is capable of 100mW average and running it at 25-30mW average with greater stability. Not everyone will be OK with the substantial price increase that would result.

This is getting interesting. I was just looking at a blue in a planetarium that was probably only turning on in the last tenth of a volt and thus was cold and not ready to lase when it did turn on.

I hit the manual over ride for a while, and suddenly we had RGB graphics for a good 5 minutes. No, it was not a dirty switch or a wiring issue, it was a thermal control issue.

Steve

[tocket]

Naturally the pulse width distribution is show dependent. This is just one example. Unfortunately it takes some time to analyze a show manually, something I don't have a lot of right now. If I was any good at LabVIEW it would be a lot easier, but I'm not.

I don't intend to jump to conclusions, as I said in the first post, these are preliminary results of a work in progress. I have not come to any conclusions yet.

I do have some thoughts though. We know that the transient behaviour of DPSS lasers can be quite nasty, at least looking at the oscilloscope. This problem would manifest itself in a show as for example a yellow line appearing as orange at the start.

We also know that the fluctuations can happen over much larger timespans. In the case of my laserwave - several minutes. With such slow (but very significant) fluctuations it is probably not that interesting to look at the signal at a microsecond level.

I don't agree that it's only the average value that matters. The kinetics of a DPSS laser are more complicated than that - there are several processes that take place at vastly different rates. That is why you need to take time into consideration.

[ElektroFreak]

^We also know what causes the fluctuations. What we lack is a method of countering them that is effective and doesn't kill the laser's output potential.

The slow fluctuations are typically dealt with by tying the diode and crystal TEC signals to a photodiode sensing the output for long-term stability. A TEC can easily keep up with these slow fluctuations, but it can't come close to keeping up with the rapid fluctuations caused by direct modulation. Since there is no optical feedback TEC loop for long-term stability built in to Chinese systems, we have these long-term fluctuations as well. They are absent in most high-end lasers from leading manufacturers.

[Zoof]

I agree that optical feedback is good for the slow dynamics, but will not work for the fast dynamics.
The idea quoted below needs more attention IMO. Not sure about the exact implementation but the idea is to heat the crystals at the same place and in equal amount during all the time, modulated or not, to keep there temps more constant. Especially keeping constant the temp distributions and gradients in the crystals. The remaining smaller and (hopefully) slower dynamics can be controlled through temp feedback to the TECs.
The elegance of this proposed solution is that it attempts solving the problem where it starts. This is in my experience more successful than adding outer (optical) control loops to tame inner (thermal) dynamics.

Again, I'm just thinking of alternatives for the sake of it...
What if you were not to use TEC's then to heat the crystal. What if you were to use another IR diode (say pointing at the crystal 90 deg to surface) (and say of a different wavelength that does not interfere with the 532 lasing) then pump the crystal with the difference of the modulation input. i.e. if the 808nm diode is instantaneously modulated to 40%, the other IR diode would be modulated to 60%, i.e. the sum of the two diodes power would always equal effectively keeping the crystal at ideal temperature.
Thus the KTP always receives a constant "100%" pumping power, just the ratio of lasing and non-lasing wavelengths differ as per the modulation input?
On paper this looks like it would solve the modulation problem, while keeping the cavity at effective 100% CW operation?

[ElektroFreak]

Using optical feedback tied to diode current is a method which would easily keep up with high-speed fluctuation fluctuations. Using a second laser diode as a heat source would introduce so many new issues that it stands an extremely good likelihood of of being impractical..

A couple initial problems would be as follows: The direction of the second diode's output is a factor, since thermal lensing on the side of the crystal would not necessarily negate the effect of lensing on the face. It could easily add to the instability. Also, the wavelength of the second diode would need to be one which doesn't have any impact on the doubling process, whether it be to cause interference or some other NLO function like summing.

The issue is localized heating at the surface of the crystal, not heating of the crystal as a whole. That's why it's such a tough issue to solve. We're talking about very tiny physical changes in shape in a very tiny part of the crystal brought on by localized heating and expansion. If the whole crystal was expanding evenly over it's full volume, a method meant to bring thermal equilibrium to the whole crystal might work better. Using a diode to counter localized heating with more localized heating is a tough scenario to realize, IMO.

[Zoof]

Here is a sample of the optical output relative to the duty cycle.

532nm 200mW CNI
Input 1mS pulse 0-5v
Duty cycle variable from 6% to 55% (I'll need to modify my function generator to get a higher percentage...)

The maximum power output is 3 div or 1.5v out of the detector.



A 50% or higher duty cycle appears somewhat acceptable which raises the question, what is the average duty cycle in a show?

Tocket, can you extract the average pulse width and duty cycle form the DAQ data?

If you have time, could you so the exact same experiment but with the pulse only 30% of total output power? I wonder if the deficiencies scale down and are still present, or if there will be better performance....

[Zoof]

The issue is localized heating at the surface of the crystal, not heating of the crystal as a whole. That's why it's such a tough issue to solve. We're talking about very tiny physical changes in shape in a very tiny part of the crystal brought on by localized heating and expansion. If the whole crystal was expanding evenly over it's full volume, a method meant to bring thermal equilibrium to the whole crystal might work better. Using a diode to conter localized heating with more localized heating is a tough scenario to realize, IMO.

That is exactly why danielbriggs said what he said: heat the crystals from inside out....
Also: not to _counter_ heating but to _complement_ heating when the pump diode is modulated to low ore zero power, resulting in a more constant heat flow through the crystal.

I agree it is a long shot but it does not have to be perfect - and it will never be, but can bring us closer to what is acceptable for our use. Anyway, I found it worth it to quote this idea and draw a bit more attention to it.

From my point of view, taking 70% margin on your max diode power to compensate for thermal issues does not make more sense anyway.