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Old 03-17-2013, 07:05 AM #1
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Default Driver rise time useful for a laser projector

Hi everyone. I am new to this forum, and I am new to lasers as well. So, sorry if my question is very basic.

I am trying to assemble a simple portable laser projector similar to the one described here: TinyProjector. Basically, lasers are shined at a rotating hexagonal cylinder with mirrors on each of the 6 faces. Then an Arduino pulses them at specified intervals to project separate pixels.

I initially used a lower power 5mW red lasers (each consuming <40mA), which worked fine. However, I wanted to make the projection brighter, so I obtained 5mW green lasers. However, I found that they don't turn on fast enough to project individual pixels. Basically, it takes some time for the laser to become visible. I require the lasers to switch at approximately 500us, which doesn't seem very extreme and I was hoping that the lasers would be able to handle it.

I suspected the driver being the problem, so I built a simple current limiter based on LM317 to be used instead of the driver. However, this did not make any difference.

As a remedy, I tried overdriving the lasers with almost twice as much voltage as they require (5.4V instead of 3V). This helped to make the individual projected pixels a little more legible. I also tried to not power down the lasers to 0V between pixels, and instead keep them constantly at ~2V (such that they don't produce any output, but do turn on faster when applied the full voltage). I feel that these are really naive solutions. I hope that someone will be able to point me in the right direction.

Thanks!


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Old 03-17-2013, 04:46 PM #2
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Default Re: Driver rise time useful for a laser projector

How are you switching them? There is a slight delay for green because the pump needs time to excite the Nd, but this should be much less than 500Ás.

Is the problem rise time (fuzzy dots that resemble lines instead) or just delay?

Leaving the laser right below threshold is actually the best way to improve response time, so that's a good place to start. Keep in mind though, these are current devices. You're trying to feed them varying voltage, and the driver (There is an attached driver, right?) will attempt to source the appropriate current regardless of input voltage.
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Old 03-17-2013, 05:02 PM #3
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Default Re: Driver rise time useful for a laser projector

Thanks for the reply!

I am not sure if the problem is exactly the rise time. Basically, the lasers don't achieve their full brightness immediately, it can be even observed by just turning them on manually. There seems to be a slight delay before the lasers turn on, and also it takes some time for them to achieve full brightness. Does this sound like a driver problem? (yes, the lasers came with drivers attached)

Leaving the lasers just below the threshold helps, but it also degrades the brightness over time. I am guessing that lasers overheat, so they need to be periodically turned off to restore brightness.
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Old 03-17-2013, 06:00 PM #4
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Default Re: Driver rise time useful for a laser projector

If you're talking about green lasers, yes they take some time to warm up and stabilize especially if they don't have any output power feedback. Usually if you're modulating green you have a special analog driver for it, not something attached to the driver of the laser module, so you might have some problems in regulating the green in the same manner as a regular diode laser.
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Old 03-17-2013, 11:13 PM #5
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Default Re: Driver rise time useful for a laser projector

The driver is probably the main reason for the delay. If you're using a regulated power supply, you can get away with throwing the driver out and using a resistor to limit the current (I assume you know how to calculate this value) and then have the drive transistor in series with the whole thing.
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