Modulation efficiency

The classical specifications of lasers gives us useful data like CW power and beam characteristics. Most of this data is collected under static conditions, i.e. operation at full power after warm-up. For most of us, the lasers in our projectors are typically not operated under static conditions - they are modulated. This modulation can significantly change the characteristics of the system, especially when DPSS lasers are involved.

Now that I finally have a nice power meter at work I'm able to do some measurements on my lasers. Here I present some preliminary results that I hope we can discuss.

First I will show what a typical result using the method described later looks like. Because the lasers used are of different different powers I'm showing the relative power instead, which is simply the momentary power divided by the full power of the laser.
http://blue.orgchem.kth.se/pics/modeff/RGB.png
Not surprisingly the diode laser handles the modulation best. The two DPSS lasers are similar, but both have rather low power compared to the diode laser.

To better be able to compare the results you can look at the area under the graph, which is simply the emitted energy (as energy = power*time). It is easily obtained by integration.

140 mW red diode: 22.1 J
430 mW green laserwave: 40.3 J
130 mW blue viasho: 9.16 J

We see that for example the blue viasho, which at full power reaches nearly the same power as the diode laser manages to squeeze out less than half of the energy of the diode. If respect is taken to the the lasers' respective full power output and assume that the diode is ideal (100% efficient) it is possible to calculate a "modulation efficiency". For the green laser that efficiency is 59% and the blue is merely 45% efficient. Interesting!

What practical implications does it have though? Well, I don't want to jump to conclusions, because there are some more measurements I'd like to do. However, two immediate thoughts that occur to me are:

• The ~20% loss in PCAOMs might not be so bad after all
• An explanation to why people get away with using less red (and 405 nm) power from diode lasers than predicted by color models

Looking forward to some comments now...


Equipment used:
Ophir Nova II power meter with a 3A-P-FS thermopile head
Pangolin Flashback 3 SE
Laserwave 532nm 400mW DPSS laser
Viasho 473nm 100mW DPSS laser
Opnext HL6385DG 642nm direct injection diode with FlexMod2 driver
DZ's color correction board

Method:
Rising and falling edge delay were set to 0 and gain to maximum on the color correction board. The offset was then adjusted to just below the lasing threshold. Each laser was allowed to warm up at full power and the output power recorded. After the warm-up was completed the LAStudio show "Show must go on" was played by the FB3 at default settings (single color) through DZ's color correction board and the power logged.

The (radiant) energy Q is given by integrating the radiant flux Φ over time. The modulation efficiency is calculated like:
Q*Φ[ref,CW]/(Q[ref]*Φ[CW])
Where Φ[ref,CW] is the full power radiant flux under and Q[ref] the radiant energy of the reference diode and Φ[CW] the full power radiant flux of the tested laser.

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The ~20% loss in PCAOMs might not be so bad after all

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I completely agree, I wonder though, would having the solid state lasers on full 100% of the time severely reduce the life of the laser.

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Pangolin Flashback 3 SE

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Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words :) ) going on thats causing lower power than you might see with a QM or other DAC?

I was just discussing this very same thing with Buffo in another thread recently. Everyone knows that a good mix of RGB with the proper depth can provide up to 16.7 million color hues. To my knowledge the only way to actually realize anywhere near the full color spread possible is with a PCAOM.. The lousy response of DPSS and many diode lasers to direct modulation makes it impossible to get that kind of color spread in the real world. Truthfully, anything other than a purely linear and predictable laser response to direct modulation would ruin any hopes of millions of colors. Thousands of colors might be possible with very fine modulation response and good color correction, but really I cant recall seeing more than perhaps a few hundred hues from even the best solid state projectors I've ever seen, both in videos and in real life.

@DZ: to the best of my knowledge, the lifetime rating on any laser is based on 100% duty cycle unless some other duty cycle is directly specified. For almost all show lasers, that means CW at full power.

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Originally Posted by DZ

I completely agree, I wonder though, would having the solid state lasers on full 100% of the time severely reduce the life of the laser.

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Actually this would be ideal living for a DPSS unit. Diode structures have improved and the rated MTTF is just that...operation at desired power...below upper limits. The stress of power input flexing the thermal limits of sub-par crystal cuts and diode modulation problems would almost be unheard of. After the laser comes to operating normal temps....it is very stable at that point with regards to the point of the design in the laser. I think a valid test is in order.

Hmm, I also think a test is in order. Any manufacturer out there willing to put up a pair of DPSS's for this experiment? 100% duty cycle on one and 50% duty cycle on the other. My money's on the 50% duty cycle laser lasting the longest.

^No doubt. In the past there's been concern with facet damage from temperature changes and rapidly changing photon flux caused by direct modulation. I don't believe it's an issue, personally, especially with today's diode designs, but a head to head test could be interesting. There is always a good amount of potential for premature death, though, and this possibility might complicate results. Since laser manufacturers typically rate their system's MTTF as CW operating time, as long as the systems exceed 10,000 hours CW they meet spec.

Something which is apparently relevant to this test is how the laser was operating before the test. I chose to bring the lasers from full power to playing the show in the previous test, but what happens if your show calls for a short break from the lasers? Here is the blue viasho that has been idling at the lasing threshold for 2 minutes after playing the last show compared to taking it straight from full power.

http://blue.orgchem.kth.se/pics/modeff/blue.png

We can see some small differences at the beginning, but after that they are a very close match. Running the numbers shows that the efficiency has dropped from 45% to 43%, not enough to cause a significant shift in color.

Doing the same for the laserwave green definitely gives some interesting results. In this test the pause was only 1 minute, but that was enough to give a very significant change.

http://blue.orgchem.kth.se/pics/modeff/green.png

I think that this kind of behavior is problematic. How could I ever calibrate the colors for a system containing this laser, when I'll get different results depending on what the projector was displaying 5 minutes ago? Such a large change in output power will easily shift a perfect yellow to an orange color.

Looking at the numbers, the modulation efficiency drops from 59% to 40%. The effect is even more pronounced at the beginning of the show, where the numbers are 68% and 33% respectively.

I'm interested in hearing fixes to this problem I have, but perhaps that's the subject of another thread.

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Originally Posted by norty303

Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words :) ) going on thats causing lower power than you might see with a QM or other DAC?

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I don't know, should I? These lasers are receiving the same signal and I'm comparing the DPSS lasers to the diode laser. Even if the FB3 does something to lower the power, that effect should be the same for all lasers unless there's a problem with them, which is kind of what I'm looking for.

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Originally Posted by norty303 http://www.photonlexicon.com/forums/...s/viewpost.gif Do you perhaps also need to factor in the issue of the FB3 having 'excessive' something (almost Bill's own words :) ) going on thats causing lower power than you might see with a QM or other DAC?
I don't know, should I? These lasers are receiving the same signal and I'm comparing the DPSS lasers to the diode laser. Even if the FB3 does something to lower the power, that effect should be the same for all lasers unless there's a problem with them, which is kind of what I'm looking for

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I can only say I don't know because the 'something' is currently an unknown. However, if that 'something' is excessive or increased modulation, then the FB3 test is going to show the problem up to be bigger than it neccessarily is. E.g. the relative power will be lower overall for those lasers that modulate badly (or at least have slow rise times)

Tocket, very interesting! could you also show the modulation signal in the diagrams.
The warm-up issue can also play part at a very small time scale, i.e., the length of a frame. It is the same problem but I previously characterized that as a duty-cycle dependency problem.
As you point out the main issue is color: with a low green duty cycle* in a frame my yellow would indeed come out orange.
I didn;t and don't have access to such a nice meter so I used the eye's sensitivity to color changes to eyeball the quality of modulation.
(the eye is much more sensitive to color changes that brightness changes)

*the green laser would be operating colder, reducing the optical output for the same modulation input.

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.

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

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

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

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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.

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Originally Posted by 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

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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.

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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.

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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.

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Originally Posted by DZ

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

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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.

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?

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.

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.

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.

Well, I discovered the first problem with my method... I'm not using DZ's color correction board correctly (RTFM fail I guess). I forgot to set the gain so that 5V in gives 5V out for each offset... :o

I'll update the results soon...

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Originally Posted by Zoof

Tocket, very interesting! could you also show the modulation signal in the diagrams.
The warm-up issue can also play part at a very small time scale, i.e., the length of a frame. It is the same problem but I previously characterized that as a duty-cycle dependency problem.
As you point out the main issue is color: with a low green duty cycle* in a frame my yellow would indeed come out orange.
I didn;t and don't have access to such a nice meter so I used the eye's sensitivity to color changes to eyeball the quality of modulation.
(the eye is much more sensitive to color changes that brightness changes)

*the green laser would be operating colder, reducing the optical output for the same modulation input.

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I have been trying to figure out how to acquire the modulation signal. I managed to find one of these here:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14124
I think that would be ideal, but it is used in a rather important system, so I'm trying to find another solution. Don't want to stir up a shit storm by breaking it. :p

If anyone else here has a DAQ and FB3 I'd be really interested in obtaining the modulation signal.

Alternatively, I have one of these:
http://labjack.com/u12/specs
That I can use freely. I suspect that it is too slow (1200 samples/s) to capture the signal unless I can average it somehow. I have no clue about how to do that though.

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Originally Posted by 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

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I think they can only cool, at least that's how I understood it from my conversations with bridge.

The ambient temperature was 21C when I did these measurements. Personally I think that's a very comfortable temperature. Apparently chinese lasers do not.

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Originally Posted by -bart-

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.

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Yes, I'm working on obtaining the modulation signal. However, if we assume that the diode is ideal with respect to modulation (My gut feeling tells me it's actually >90%, but I'm hoping drlava will give his view on this) then the measured power from the diode can be viewed as the time-averaged modulation signal.

Unlike lower powered single-mode diodes, multimode diodes (such as the one used in your tests) do not respond in an "ideal" fashion to modulation either. As current increases, more and more emission regions in the die become active. As each emission region becomes active, the power spikes upward, thereby departing from an ideal linear response curve.

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Originally Posted by ElektroFreak

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.

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What we need is a photon capacitor! >:)

Yes we do! That would be a cure-all for LOTS of laser related issues.

It would be interesting to see the modulation step response in the millisecond time scale using a fast photodiode such as in previous blue dpss reviews. Then there would be no guessing what the important timescale is in terms of modulation response to the signal. What you are seeing is pretty common, it shows the benefits that can be gained from optical feedback modulation control or better lasers!

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It would be interesting to see the modulation step response in the millisecond time scale using a fast photodiode such as in previous blue dpss reviews.

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This pic was from quite a while ago, but it shows the modulation input (bottom) and what the optical sensor sees from a blue laser (top). Bit of a mess! If I get a chance, I'll post a new pic from all 3 lasers going through the sensor. Red looks good, green look ok and blue is a disaster!

Optical feedback is a good way to improve modulation response, and gives the best results available currently. It seems that Viasho has integrated it into their latest line of high-powered green modules.

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Originally Posted by ElektroFreak

Viasho has integrated it into their latest line of high-powered green modules.

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They did? That's a good start! Did you read that somewhere?

One thing about the pic above, to make the light output absolutely stable, you'd have to set the max power output the lowest point on the top trace.

According to Laser Ben's review of their fresh new 4W design, optical feedback is available. Here's the thread: http://photonlexicon.com/forums/show...253#post126253

Look in the "improvements" section.

Optical feedback would require the laser to run at a maximum of the lowest power indicated on the scope. That's not really too much of a problem, though.. many people here would love to see a slightly less powerful, but more stable laser.

unfortunately that review leaves out the very tests that I wanted to do on that laser when he posted about it, covering issues like this.

Interesting. I agree drlava, one of the most important tests is not even part of that review.

Hi dazebtwn. Welcome to the discussion. I think it's very nice to see results from more users, but your pictures are not working right now, which is really a shame since I'd really like to see them.

On another note I tried the DAQ and collected the modulation signal for the entire show at 100 kS/s, now I just need to figure out how to import this 800MB text file into MATLAB...

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If you guys don't mind I would like to join in this discussion.

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Hi dazebtwn, you are absolutely encouraged to join this thread. The more folks looking at this issue the better the chances are of an improvement being made.

Unfortunately, the images you added are not showing up for me.

^same here.. no images.

Very nice! I like the animation. That really shows a strong case for running them full power all the time through an AOM!

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Originally Posted by dazebtwn

Greetings,

It is easy to see why a laser that is sluggish to respond to blanking would appear to "weak" when compared to CW operation. With the tec's setup for CW they appear to over cool in a pulsed system.

http://www.photonlexicon.com/forums/...pictureid=2331

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There is a laser that has to be better than most "bulk" lasers made. I have an RGB international laser that pre-heats the BiBo and presets a general field temp for the diode. So the temps should be very close to what it needs when it is blanked or left idle for a period of time while powered up....unmodulated. Fuzcub has working lasers from this manu....he may be able to perform a couple of tests....

I don't have the fancy test eq. some of you have but I'm throwing my results back into the mix again. Also looking at scope graphs might lead people to strive for perfection while it is not needed for a decent output.

Here is the case where it was a problem for me: http://picasaweb.google.com/DrZoof/R...76997930531858

I've posted it before, sorry to repeat, but it illustrates well what the effect was of a low green duty cycle: yellow turns orange. I found it unacceptable and I got it fixed under warranty. Probably the curves don't look perfect but it is as good as not visible.

To help get this worked out with manufacturers we need to quantify what is an acceptable worst behavior, so what is acceptable for a show and what does that look like on a scope?

Along with a constant power loop, I think some kind of feed-forward would be an improvement. I.e. more power demanded -> TEC's start cooling more

Agreed. That would be a great improvement.

The Chinese have been accustomed to making lasers that are very much cheaper than their American and European counterparts. Obviously this involves using simple cavities and only the most basic stability features, unless you're talking about some of their high-end scientific lasers, but those cost good $$$$$ even from China.

The reason so many people use Chinese lasers is because of the cost-effectiveness. If too many features are added, even the Chinese systems could become cost-prohibitive, so I also believe the current designs should be kept around for those who can't afford the high-end stuff (like me)..

Again, just a thought:
I know AOM's have been mentioned.
However even taking into account the ~10-20% loss through it, would one PCAOM after all 3+ lasers run CW still average more "modulated power" than not having modulation and letting the power suffer from modulation?
But if you're spending that sort of £££ on a setup like that, must make sense to buy 445nm?

Another thought: Would TTL modulated lasers be affected more than analogue lasers? Me thinks so... just wanted to check.
*IF* one was running a TTL setup, wouldn't it be more prudent to run all the lasers fully on CW, and implement some sort of galvo, Z-blanking on each beam path?
Again... just ideas...
Still scratching my head for a good analog solution. :cool:

445 is only an option if you don't mind sloppy beams. If you want good beam quality with minimal divergence, a TEM00 beam is necessary. To me, buying a PCAOM and running your DPSS lasers CW is the best option. The losses might not be much more than if you limited the DPSS output to a lower power and used optical feedback.

Honestly, beam quality is not overly important with laser shows, so if you're not concerned with it you could go with diodes for both blue and red, but you'll still have green DPSS to contend with, and they can be just as noisy as 473nm DPSS under modulation in many cases.

The good news is that there are a bunch of new direct injection diode designs that give a flat-top profile. It's not TEM00, but it is fairly even power distribution across a circular profile. TEM00 is more desirable to people with a desire for high-quality beams, but a flat top profile is MUCH cleaner than the rectangular multimode diode output many of us are used to.

TTL modulation can and in my experience does cause more power fluctuations than analog. Allowing the current to ramp up rather than suddenly pop on seems to smooth the fluctuations. They don't disappear by any means, but they do seem less noticeable. Galvo blanking or an AOM does do TTL nicely and gets rid of these issues, but in my case I just use direct modulation since my TTL systems are only one color.

So, if this is essentially a thermal issue, which part of the laser is having the problem? The pump? Could the pump be changing wavelength while it's heating up? If that's the case, then I wonder what would happen if you added a heater around the laser head, forcing it to remain "hot" while not modulating.

What we're discussing is caused by thermal lensing in the crystals. When the extremely powerful 808nm pump beam is focused into the crystals, it causes localized heating, which causes expansion, which in turn causes power fluctuations since the alignment of the cavity has changed minutely. When you modulate, this happens over and over with each pulse. When the laser is cold, the amount of thermal lensing that occurs with each pulse is large, but as it warms up it gradually reaches sort of a thermal equilibrium (and is kept there by a TEC) and the amount of lensing per pulse is reduced, but never eliminated. This is why each successive pulse in the scope shots above become larger, and the fluctuations remain no matter what the temperature. If there is a pause in the modulation, even for a second, it allows the crystals to cool slightly which in turn resets the whole process slightly. I hope that makes sense.. In essence, the crystal (and consequently the entire cavity) are oscillating physically as the laser is modulated. If the very shape of a laser cavity is physically vibrating, even at nano scale, expect fluctuations.

Your idea of using a heater is already employed in many high-end frequency-doubled lasers. Doubling crystals (such as KTP in the case of green DPSS lasers) prefer to be kept at a precise temperature for optimum stability. In the case of KTP, it likes to be warm, so a KTP oven is used to heat the crystal to improve stability.

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Your idea of using a heater is already employed in many high-end frequency-doubled lasers. Doubling crystals (such as KTP in the case of green DPSS lasers) prefer to be kept at a precise temperature for optimum stability. In the case of KTP, it likes to be warm, so a KTP oven is used to heat the crystal to improve stability.

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Ah, that's right, forgot about that. And in that case the heat is applied very specifically whereas the idea I came up with would be for the entire unit, in which case it's only effect might have been to reduce the life of the laser.

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Originally Posted by ElektroFreak

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.

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*Thinking out loud* :)

Am I correct in assuming the instability is because the TEC doesn't know what will happen next as to regards the power output of the diode? Well what if the TEC(s) knew ahead of time, before the diode was modulated to that state? :)

For a pre-programmed fixed show, what if it were possible to somehow feed the same colour modulation signals to the TEC's at a time constant ahead of modulation of the diode?
No good for say use with LivePro or on the go stuff, but surely the TEC's would be able to "predict" what's about to hit it with more or less absolute precision? It may be possible to even apply this to a "live" output if the time offset required was small, by buffering the colour modulation for a millisecond or so?
Kind of analogous to a prefetcher if you will...

However I appreciate this would be hard to implement and take a lot of time and effort to research and perfect in practice, as well as some hefty hardware modifications of DAC and laser driver...

What do you reckon?

Again, just a thought experiment :D

@DZ: Depending on how hot you're talking, you would be reducing the life of the laser, yes.

Also, I really should add that you're partially correct about the diode influencing this process as well. As I said in a previous post, multimode diodes do not respond linearly to a change in current, but they are much better than DPSS as a whole. Since pump diodes are almost always multimode, their own nonlinearity is transferred to the crystals which adds to the overall potential for instability. As you can see, DPSS technology is quite hairy due to the complexity of the physics involved. That in a nutshell is why the really good cavities cost $$$$$, and it's also why I personally believe it is impossible to directly modulate a DPSS laser with 100% stability. Some good ingenuity will lead to improvements over time I have no doubt, but 100% stability under direct modulation is out of the question.


Regarding TECs, they respond very slowly to a change in current, so they could never keep up with the fluctuations. The only good method that exists presently is optical feedback, but I think it needs to be implemented such that it reduces the pump diode output when the green output spikes, rather than increase the pump output when the green output sags. Increasing the current rapidly is what is responsible for the instability in the first place, and decreasing the current is less stressful to the diode. Even then, however, there will still be fluctuations that cannot be controlled.

Just for clarification, earlier it was mentioned that the viasho had a photodiode integrated, that actually isn't true. Ben added it to the post as a way to improve the product.

Ahh.. that's a big clarification! Thanks for that..

Quote:

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Originally Posted by ElektroFreak

Regarding TECs, they respond very slowly to a change in current, so they could never keep up with the fluctuations.

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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?

:rolleyes:

I can't say I've ever heard that one before. Interesting idea.

My thoughts are that you'll probably run into a host of new problems implementing such a system. A big one would be that doubling crystals double more than one wavelength, so you'd need to find a wavelength that isn't doubled at all by the crystal, otherwise it would throw off your green output. 1064nm into KTP gives us 532nm. 1065 would give us 532.5nm, 1100 would give us 550nm and so on. KTP is only efficient above ~1000nm pump wavelength, and I'm not sure off the top of my head how high it will go, but you can see why you couldn't use just any diode. It would need to be carefully chosen depending on the type of doubling crystal.

And that's just the beginning..

Now I'm not saying it wouldn't work, but it would be a real bitch to figure out.