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ArcticMyst Security by Avery

Maximum modulation frequency?

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Sep 22, 2007
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So I'm building a simple POV laser projector with a spinning array of mirrors similar to this:


I purchased this laser from Aixiz: 532nm 15mw TTL power adj. laser with DC power supply, AixiZ

The video review on youtube stated that the TTL interface can do between 5-10KHz. Ideally I'd like to modulate it as fast as possible. My question is: what sets the maximum modulation frequency? Is it a property of the diode itself or the TTL? Can I pulse it faster (the arduino can accurately send pulses up to around 200KHz) by bypassing the TTL or is this too complicated and/or not worth it? Also, what will happen if it is pulsed too fast (i.e. no light or kill the diode)?

Sorry for the barrage of questions. Wasn't sure whether to post this in experiments, but figured since it deals with modulation y'all might know better. Thanks for the help.

p.s. last time I was on here was like 3-4 years ago. glad to see this forum is still kickin'
 





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you could go faster.
But there is a problem, green lasers dont modulate as fast as others due to they are not diode lasers like in dvd burners or video projectors.
They use a IR laser diode as a pump for a set of crystals to make the green.
So they do not always respond very well to very high modulation.
If you went with a diode laser you could go very very high.
 

Benm

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Looking at red lasers, like the ones extracted from dvd players: Obviously they can be modulated at great speed, just consider the data rate when writing a dvd: single speed means 10.5 mbit, plus extra parity bits for error correction. Most writers are 22x or so now, so thats easily 300 million pulses per second when operating at top speed.

The driver is another issue however: Most 'standard' drivers are intended to work continously, or be modulated at very limited speed (10-50kHz). It is, however, no problem to design a driver that will work much faster - up to a few MHz with commonly available parts. If the arduino is limited to 200 kHz modulation there shouldn't be any problem getting the driver part to work.
 
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26FPS with each character 6x6 and it looks like about 12 characters per frame. That looks like about 11Khz. Most (all?) TTL laser drivers will handle that just fine.
 
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Benm

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I suppose so, but that project would look a lot better if it had more lines and horizontal resolution to fit.

You'd probably have to come up with a better way of doing the vertical positioting too then - just slanting mirrors at a slightly different angle isnt all that accurate.
 
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Here's the gist of it: while you can modulate the power source of a green laser (i.e. the TTL portion) as fast as you want, the crystals in the DPSS lasers don't respond quite as fast. What results is inconsistent, and highly dependent on your crystals, their temperatures, how fast you modulate, etc. I doubt even the 10khz is guaranteed, and the results will look muddied.

What can you do?

1) Find a different type of laser. Non-DPSS
2) Find alternative modulation methods. These can be expensive.
3) Live with it and adapt your other hardware to the limitations of your laser.
 
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Thanks for the info. There are trade offs to using green vs. red. Although I could pulse the red faster (more fps), the green is brighter and thus doesn't need as many fps. idk, we'll see how it turns out.
Right now my plan is to use 8 mirrors with adjustment screws for fine-tuning the angles. I'm going to use 6x8 pixel characters. So with spaces and 14 characters, I should get a 8x98 display at 12 fps. Thus, pulsing a little under 10kHz. I might try boosting up the frame rate if the laser seems stable there.
 
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The worst case scenario would be a checkerboard pattern, which would cause the highest modulation frequency. You'll probably be able to do okay with that.

Also, the high the power the DPSS, the worse the modulation. So with lower powered lasers you should be able to do better.
 
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honestly go with a 445nm diode.
They are very bright and not that expensive ad can be modulated very fast.
 
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You can also combine 445nm with other colors, such as 635nm, and get a nice exotic color. At close ranges, the shape of the beams shouldn't pose problems.
 

Benm

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Here's the gist of it: while you can modulate the power source of a green laser (i.e. the TTL portion) as fast as you want, the crystals in the DPSS lasers don't respond quite as fast. What results is inconsistent, and highly dependent on your crystals, their temperatures, how fast you modulate, etc. I doubt even the 10khz is guaranteed, and the results will look muddied.

This is a problem with dpss lasers indeed, and already noticable at moderate speeds like 10 kHz. Running up to 30 kHz or so with laser projection, the delay is pretty pronounced when you project a test pattern.

I don't think its avoidable either, the nature of dpss requires some time before population inversion is reached in the solid state laser, and also causes lasing to continue for a short amout of time when the pump light is turned off.

The real downside is that these delays arent -that- predicable, so even when you shift timing for the green (on a projector that combines it with diode red/blue) the results arent consistent.
 

Things

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The fllexmod driver is capable of up to 6Mhz I believe, but as others have said, you'll need to use a diode laser. DPSS lasers have trouble modulating at fast speeds.
 

LSRFAQ

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If your dead set on green, you can use a Acousto Optic Modulator for the green. You'll have a 7 to 10 Mhz bandwidth in the modulator, and with work, you can also have gray scale.

A normal small 50 to 100 mW red diode is good to almost a gigahertz. Around a Gigahertz, some wired problems with carrier lifetime in the diode start to occur.

Steve
 

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Hmm, I swear it was Mhz at some stage! Maybe he realized no one ever needs modulation that fast ;)
 
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Well a better option would be raster scanning but with that your are making it harder.
A good way might be to use 2 stepper motors and drive one coil constant in each motor and use the other to vibrate it.
they should be fast enough to do raster scanning.
Or 2 spining disk mirrors from laser scanners.
 




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