balancing and modulation loss - help needed

[Pangolin]

Hi Zoof,

What you have there is a very low frequency version of what I call "jelly beaning". The term "Jelly beaning" is a phrase that I coin to describe when you are using a DPSS laser to do laser graphics projection, and instead of seeing continuous lines, you see dashed lines, as if you lined a bunch of jelly beans up, end to end. You can also get this effect out of NEON, if you use a high frequency transformer. Some transformers are designed to produce the "Jelly bean NEON" effect...

Anyway, what you have there is a very low frequency version of that. What's happening is that when you turn the laser on, it generates heat. That "heat impulse" takes a while to settle. While that's happening, the TEC might kick on to compensate.

The vast majority of DPSS lasers are only designed (if they are even designed at all...) to operate at CW. That is, to produce an unmodulated beam. Also, "in the lab", they are usually only tested while producing a full power CW beam. So then the manufacturer (if we can call some of them that) puts the laser into the hands of a lightshow guy who modulates the power all over the place.

Since this "heat impulse" aspect takes a certain amount of time to occur, you will find that the power level of your laser will seem to change, depending on the graphic images (and thus the ratio and time of on/off intervals) you are displaying.

The the circuit that was described earlier may help you for a few reasons. First, the primary purpose of that circuit is to be give you the ability to adjust the delay of each of your lasers, so that they (hopefully) can appear to be similar to other lasers you are using. For example, a regular diode laser will turn on and off much faster than a DPSS laser. So normally you use one of these circuits with a diode laser to slow-down the diode laser, to be the same speed as the DPSS laser. Also, DPSS lasers may require a different amount of time to turn on, when compared to the time it takes to turn off. So that's why the diode is there -- to allow you to adjust the turn-on time separate from the turn-off time. Even though this circuit will find primary use in the path from the computer to the regular diode laser, we have found that it may help reduce the "jelly beaning" effect, by "softening" the rise time and thus, helping to "soften" the heat impulse effect.

In your case, this is a pretty low frequency effect, so I doubt that the circuit will mitigate what you are seeing. But I guess it's worth a try... In any case, the circuit finds particular use in helping to avoid "colored tails" that appear in graphic images.

Best regards,

William Benner

[marconi]

Thanks Bill for reminding me of your delay circuit.
I knew Id seen it somewhere before.

I am giving that much more thought into incorporating that into my drivers.
Something I have been noticing lately when viewing other RGB systems using Solid State is the fact that using my red lasers I see that Green and Blue lag behind.
So I need to make my reds adjustable delay to match them.

Altho, I would strongly recommend that everyone add adjustable delay for each of their diode laser drivers.
I still have yet to build one to test this thoery out altho sound as thats one of the reasons that I do use an additional op-amp in my designs.
That is to take advantage of the additional delay I get by adding another device in line. besides for protection.

Would you mind if I built-up some 1,2 or even 3 channel boards for the guys here on PL .
Of course you would get the credit for its design.

[allthatwhichis]

Something I have been noticing lately when viewing other RGB systems using Solid State is the fact that using my red lasers I see that Green and Blue lag behind...

I have noticed this also... I thought it was a problem with the green and blue.

[marconi]

Yep,

I guess if they were all DPSS lasers it probably wouldnt be noticed.

Its just that the diode lasers are a bit faster since they dont have to
excite any crystals...

[Zoof]

Thanks for the replies. I've been reading this tread for a couple of times now to get the most out of it. There are some important points made. After some more testing of the phenomenom I find that the laser simply is too slow. So the lagging of blue and green you have been talking about is on a much smaller time scale than what I've been seeing with my green. It is true that my blue also has some lagging and needs more time to rise than the red diode but not nearly as bad as the green. CNI has assured that they consider it a warrenty issue so I guess it will be back on its way to China.

[Zoof]

I've added an video to my gallery to show the effect I'm talking about.
view it here
The top left rabbit is defined to turn yellow but as you can see it is orange with a low green duty cycle and turns green yellow for a high green duty cycle.

The bottom right rabbit shows how the white balance changes. However the camera was saturated but to the eye the white rabbit turns from pink to bright white.

[dave]

Just out of interest, did you get a 2k analogue, or a 10k analogue from CNI?

[Zoof]

good question: it is the 15kHz analog, that is why it has been bugging me.

I'm trying to get a feel for how much of these effects to expect, are normal, and should be accepted.

What do you guys in the field experience?

[marconi]

I wish I could help you out there..
I usually only get to see the problem units...
But I expect each unit to be a little bit different from the next..
All depends on who assembled and tuned it and components.
We're dependant on the "luck of the draw".
problem is, not too many people have the luxury of comparing several units side-by-side.
But then again, If its that obvious, I would suspect something was up.

[Zoof]

Just got my green back from CNI and.... problem solved!
They really payed attention to the problem and listened well. They showed me their test reports: input and output plots at various duty cycles before sending the laser back in my direction. Just finished testing it and all seems to work very well - as much as I had hoped for. Very happy that I did not accept the laser as it first was, it would always have been a visible issue.
In the process I think I helped them understand better what customers expect and that the duty cycle dependency is a problem in RGB projectors while it isn't as much in green only projectors where color balance does not suffer and duty cycle is generally always high.

thanks for the input in this thread.