Modulation efficiency

[weartronics]

I think your results are probably accurate, and not really surprising. This is why PWM AOM is used for DPSS modulation in high quality projectors. I'm surprised that more people don't use it in their own made projectors.

[danielbriggs]

Could be wrong on this one, but:
Do the TEC's on the KTP of the laserwave modules "heat and cool" or just cool?
The graph would suggest only cool.
My thought process is if the KTP is only cooled below set temp, then for all intents and purposes the KTP will be at room temp (of which I will assume is less than 25.6 deg C).
The "heating" will therefore come from the waste pump power unconverted. I assume this will cause the KTP to heat up past 25.6 deg C and then the TEC's will kick in to drag down the temp to a stable value. That is the only reason I can think of as to why there will be a few mins of lag before the KTP warms up, and matches the 'pre-warmed' curve.

Would it be possible to change the TEC driver to stabilse the KTP? (assuming it is not already)

Tocket, what was the room temp?

Dan

[-bart-]

I'm showing the relative power instead, which is simply the momentary power divided by the full power of the laser

Tocket, very interesting! could you also show the modulation signal in the diagrams.

Do you have a way of sampling (and averaging) the color-modulation signals ?

Showing the REAL relative power instead, which is simply the momentary power divided by the dictated power of the dac.

You want to remove the "software part" out of the equation.

How much of the available power is used by the show.
In other words what's the efficiency of the show, if one only looks at the modulation outputs.

[ElektroFreak]

Could be wrong on this one, but:
Do the TEC's on the KTP of the laserwave modules "heat and cool" or just cool?
The graph would suggest only cool.
My thought process is if the KTP is only cooled below set temp, then for all intents and purposes the KTP will be at room temp (of which I will assume is less than 25.6 deg C).
The "heating" will therefore come from the waste pump power unconverted. I assume this will cause the KTP to heat up past 25.6 deg C and then the TEC's will kick in to drag down the temp to a stable value. That is the only reason I can think of as to why there will be a few mins of lag before the KTP warms up, and matches the 'pre-warmed' curve.

Would it be possible to change the TEC driver to stabilse the KTP? (assuming it is not already)

Tocket, what was the room temp?

Dan

I've always been under the impression that KTP likes to be warm for the best stability. In larger systems, a KTP oven is employed, but I don't
know what the best temperature range should be.

As has been stated previously, an AOM or PCAOM is the only way to completely negate these effects that I'm aware of after extensive research on this subject in the past. DPSS technology is just too unstable in general.

[DZ]

As has been stated previously, an AOM or PCAOM is the only way to completely negate these effects that I'm aware of after extensive research on this subject in the past. DPSS technology is just too unstable in general.

When using an AOM or PCAOM, I still see one additional problem. Let say you let your blue laser run for a while and it stabilizes around 150mW, so you set your color balance. What is the chance that tomorrow your blue laser settles around 150mW. Or even when running, it still jumps around in brightness, from 150mW to 180mW or down to 120mW and back up again. It seems to best thing to do is have an optical feed back loop tied to the modulation input, even then, the best thing to do would be to set the highest output to the lowest level the blue laser would drop to under normal operating conditions. Unfortunately, the feedback loop so far has proven to be quite difficult and the idea of already throwing away a portion of what the laser is capable isn't cool. Really, the best solution seems to be using just diodes and not dpss at all.

[ElektroFreak]

Really, the best solution seems to be using just diodes and not dpss at all.

That's the real ticket, but even then you can only get so much power (and it ain't much really) without the beam quality going to shit.

One major advantage of DPSS is that they can provide an ion laser-like beam while consuming a mere fraction of the power. That's a valuable asset even in the face of inherent instabilities. It seems though that everywhere one looks for an economical alternative to an expensive, maintenance intensive ion whitelight system, there are serious caveats. For full color depth and near total stability, ion lasers combined with a PCAOM are still the way to go for high-performance combined with cost effectiveness. A whitelight system with similar advantages can be built using top-quality DPSS modules (coherent, melles. etc..) but the cost invariably exceeds an ion system.

[Zoof]

With this kind of diagrams we can at least identify the real problem with DPSS. The just cooling or heating and cooling issue could be the key to better performance, maybe acceptable for our use. Hopefully, someone like Bridge is willing to push to that higher level of performance with his products!
I think a lot can still be gained but the end users have to ask for it and probably pay for it a little more. However, if we don;t ask no-one will make a more expensive product without a clear edge over competitors.

Bridge, what do you say?

[ElektroFreak]

I don't know about that. If Coherent and JDSU don't recommend direct modulation of their DPSS systems (they don't even have the option built in on most of them), which are the finest cavity designs in the world, I have serious reservations as to whether the Chinese could do better. I think the instability of DPSS is an unavoidable consequence of the physics involved.

I think the Chinese could improve the modulation response of their systems to a point, but to eliminate instability altogether? Nah.

The root cause of most of the power instability we're discussing here comes from the rapid temperature changes in the crystal medium and doubler as the pump source pulses it. The incoming 808nm light causes thermal lensing in the crystal medium which causes the shape of the crystal to be modulated as well (albeit at a tiny scale), thereby affecting power output. Precise control of a TEC or oven can stabilize CW output no problem, but it cannot react anywhere nearly fast enough to cope with these sub-millisecond fluctuations. I don't know of a single option currently available that can.

[Zoof]

I have no experience with high end European or American DPSS lasers but I feel that even if the chinese manage to fall just short of their performance, it will satisfy 99% of us here..... but won;t be perfect.

Remember what kind of reputation Coherent has to keep up.

After working with CNI on my green for a while, sending back and forth problem descriptions and performance graphs I now have a green DPSS that does a good job at keeping up with my red diode. Not perfect but good enough. They said it had turned into a higher model. Since I didn;t see any changes to the circuit boards or head I assume it went through a time consuming tuning process - adding value to the unit. The max output power has dropped after these adjustments a bit but is still above spec.

Only when the manus figure we don't want just a max output but prefer good modulation they can better design their products and tuning procedures.

Next time I buy DPSS, I will be paying a lot of attention to the modulation curve and be prepared to pay more for a better model with the same output spec.

[ElektroFreak]

I'm not saying that an improvement is impossible, but I think we're already seeing modulation response at its finest today from the better Chinese manufacturers. Slightly more improvement might come, but never perfection, not from DPSS technology. Tuning can help any laser, and Chinese manus do rank their laser models according to how well they are tuned, but the modulation response is still not close to perfect. Ion laser/PCAOM stability from directly modulated DPSS is out of the question ever, regardless of the manufacturer, country of origin, or rank among other DPSS lasers. I don't believe it to be possible, but I've been wrong before.

Another major issue with DPSS is that even two identical lasers from the same line with sequential serial numbers will always perform slightly differently. They're kind of like snowflakes, with no two being exactly the same. This would make it impossible to publish anything but a general spec for modulation performance, with a fairly large margin for error.

There are some VERY interesting advancements being made currently with bonded hybrid crystals being sandwiched together in stacks that *might* be valuable in improving DPSS modulation response in the future, but the technology is in its infancy, and if the same physical principles apply to these stacked crystal mediums as apply to other crystal mediums (and most of them do) modulation will still produce undesirable consequences. It's not a money issue, it's the fact that everything on earth is bound by physical principles which cannot be changed with any amount of currency.