PCAOM question

[illuzion]

I have been out of this for approx 6-7 years or so, I am fairly new to the idea of solid state RGB, but I wanted to put a projector together anyways.

A few questions, first off I am looking for high speed color control for smoothest and fastest color changes, I plan on making this a 60k pps system for now. I wanted to use a solid state RGB system, which means straight diode red at either 635 or 650nm (not sure if 100mW@635 or 400mW@650 is better), DPSS green and DPSS blue. Now most of these diode type lasers seem to be straight TTL modulation, and most of the drivers I am looking at do not support analog modulation, and from what I hear it is slow. My plan was to use a PCAOM.

Now here is the question, is a PCAOM compatible with solid state RGB lasers? Or is the polarization screwy and it wont work properly??

Next question, is there any benefit to an 8ch pcaom, if this were an ArKr system I would say yes, so you could salvage every available line from the lasers, you could probably pull 4-5 from the argon and 2-3 from the krypton. But from what I understand the solid state lasers really only have one line per laser module (someone told me if you put a diffraction grating in front of one, there is only one beam output), nothing else worth subtracting.

So is a 8ch driver a waste?? should I stick with a 4ch? Or will a pcaom not work with a solid state laser?

[Hugo Bunk]

Hi Illuzion,

Better save all the trouble and money, just look for an analoge modulated dpss laser for each color. Plenty available.

[Peter Broerse]

HI Illuzion,

IN general you can use a PCAOM to filter the colors from the RGB but in that case you would need to pump the RGB at all levels fully and set it in such a way that the output would be white.

The type and model and quality of both the lasers and the PCAOM would determine the effectiveness of the PCAOM for filtering the colors.

INdeed a combined RGB gives you 3 lines at max as all colors consist of a single wavelength opposed to gas lasers where you can find several lines in the output.

So in general the effect you seek can indeed ( as Hugo states ) be acchieved with good analog controlled lasers. Thusfar there are in general 15 khz switching analog lasers available and some manufacturers are now working on 30 Khz.

Cheers,

Peter

[Peter Mayer]

A single dpss laser (e.g. green 532nm) with an AOM modulator
would be a good high end solution as the Blitz defines an example
of modefree hopping phasematch stable running CW dpss laser with
ultrahigh speed modulation above 100KHz, however for a RGB combined
dpss laser system, I think it would be better in system efficiency to
rely upon analog driver modulation features. As best the driver
is as best is the threshold, phase-time delay and modulation speed
compatible with high speed scanning needs. The laser software
gives in most cases opportunitiy to adjust threshold and maximum
power to optimized full whitelight balance at all fading levels.
If in particular the Red laser consists of combined LD's using
folding mirrors and/or polarization power combiners, the polarization
is random anyway, where no PCAOM can be used at all !

Choose the best, as you select the best components
in your projecting system, Peter Mayer

[Pangolin]

Hi illuzion,

So far, what everyone has written here is correct, but allow me to add some supplemental information.

Solid state lasers can, in most cases, be directly modulated. This means that the computer can be connected directly to the laser power supply (also called the driver) and control the brightness and blanking of the laser. This is desirable since it is the simplest, and most often a cost effective solution.

However, when this technique is used, the modulation rate is certainly not as fast as a PCAOM (but it is certainly fast enough, even for 60K uses).

But there is another problem. When you modulate DPSS lasers, you are not only modulating the light. You are also incidentally modulating the heat inside the laser. The output power of DPSS lasers absolutely depends on a certain, very narrow set of internal temperatures, so when you modulate the heat, the output power is generally degraded. This is one reason why a 500mW DPSS laser will give far less than 500mW when you project laser imagery. Also, this "peak power" and "average power" will change, depending on the "content" of the laser imagery (how often, and when the beam is turned on and off).

This "degraded output power due to temperature modulation" can be abated by keeping the laser turned on all the time, and using an external AOM or PCAOM as you propose. However, of course, this has drawbacks as well. The system is more complex, and requires optical alignment. There are separate power supplies involved, etc. Also, of course, the monetary cost is higher. And last but not least, there will always be some loss involved with the PCAOM, and this loss depends on the optical alignment and also the brand name (manufacturer) of the PCAOM. In general, the more expensive the PCAOM, the better the throughput you will attain...

To answer your specific question, when using solid state lasers, you will only need as many channels as you have individual lasers. However, the best PCAOMs typically come in a configuration that has 8 channels. So, although it is possible to purchase a driver with fewer channels, this might take longer and, in the end, be more expensive.

A few more points. The phenomenon I described above, about "temperature modulation" is particularly problematic in lasers produced by some manufacturers, and particularly absent in others. I am very impressed with what I see in lasers made by JenLas and also with Melles Griot, but particularly disappointed in others -- even other expensive brands. So you will need to find a buddy, and take a close look at his lasers, and see if you think the output is good enough for you or not. If not, find another buddy...

Also, one last thing that is related to this discussion, but not yet brought up. The rise-time and fall-time of lasers can be faster or slower than other lasers. For example, if you have a laser-diode-based red source, and DPSS-based green and blue source, the red source may possibly modulate faster than the green and blue, depending on the implementation of the driver. Also, even the green and blue may turn on faster than it turns off, or turn off faster than it turns on. One reason why I like 671nm red lasers is that they are DPSS lasers. When they are combined with green and blue DPSS lasers, you get inherently similar modulation characteristics, and also inherently similar beam diameters. Some people shy away from 671nm red, because the CIE Chromaticity Diagram tells a story that the visual efficacy is not nearly as high as that of 650 or 635nm sources. But unfortunately, the CIE Chromaticity Diagram does not tell the whole story for lasers, because it does not take into account the effects of Raleigh scattering or Forward scattering... Bottom line, for graphics applications, 671nm is not a bad choice when you consider the beam diameter, beam divergence and similar modulation characteristics.

But let me get back to my bigger point. When doing direct modulation of lasers, the modulation rates of each laser may be "different". Some people have proposed a software solution for this, but software can never provide a complete solution. (It would take me at least as long as this post to tell you why -- but take it from me -- a software guy with a reputation of solving difficult problems -- software can't offer a complete solution. Moreover software SHOULDN'T be the solution to this problem since this problem is equivalent to having one scanner going twice as fast as the other, which nobody would claim should be solved in software...) We have prescribed a simple, cost-effective hardware solution to this problem. It involves a single op-amp, a single diode, and a few capacitors and resistors. You can read about the solution (and other historical information) here:
http://www.pangolin.com/resguide03d.htm

This write-up was done in May of 2004 (or was it May of 2005, I can't remember...), after helping a local company overcome the problem on their recently-acquired RGB laser. I never really completed it to the normal degree of polish found on other Pangolin web pages, so I never linked it in from the rest of the Pangolin pages. Since it was created, I never really had a chance to return to this page and finalize it, due to being busy with other things (people close to Pangolin know how busy we are...).

Note that there are several circuits prescribed, but you only need the simplest one. The "everything but the kitchen sink" one is definitely not needed -- the simpler one with one diode, one op-amp and two pots is definitely the way to go.

That circuit (or circuits like it) has been incorporated into the laser projectors of several RGB laser manufacturers. And we find that there is an additional benefit to using that circuit. The filtering action tends to soften the "heat modulation", thus improving laser power and reducing other undesirable artifacts. To be perfect, a non-linear circuit should be implemented AFTER this filter, to linearize the response of all lasers. Send me a private email for more information on the non-linear circuit if you are interested.

Best regards,

William Benner