help with pulse length and power stability?

[albuddah]

sorry if these questions are answered elsewere, but I'm new and couldn't find answers with search

if I use a dpss laser (such as lasercentury 532 nm), what sort of power stability can I expect if using 5 ms pulses at 10-20 Hz through TTL? Also, will the shape of the pulse be close to a square wave at such short pulse lengths, or will the rise and decay times be quite obvious?

[buffo]

As a general rule, DPSS lasers are not known for having good modulation characteristics. And LaserCentury is a Chinese manufacturer that is more well known for selling inexpensive lasers for hobbyists than for an exacting, scientific application.

At the speed you're talking about (5 milliseconds), a *well-designed* DPSS laser should be able to deliver decent pulses, but the rising edge will still be sloped and ragged. This isn't such a big deal for light show use, but it sounds like it will be a problem for you. If your application requires absolutely perfect, pristine square-wave output pulses, then you'll probably need to use an Acousto-Optic Modulator (AOM) to modulate the beam externally.

You may want to get in touch with Madoin123 here on PhotonLexicon and ask him about his experiences. He did a lot of testing with a laser-based strobe concept at frequencies up to 600 Hz, and in the end he decided that direct modulation wasn't feasible for the DPSS green and blue lasers he used. (Search for "DPSS jellybean" if you want to learn more about why DPSS lasers suffer from this problem. This thread has a good picture of the phenomenon.)

Adam

[albuddah]

thanks for the help. i notice on this forum, most people tend towards aom's for modulation rather than shutters - is this because of a size issue (I'm working on a lab bench so space doesn't matter for me), or are there performance or price differences as well?

btw i tried looking for madoin123 but i guess my search skills need great improvement as i couldn't find him or that many threads discussing dpss modulation performance (specifically the power stability).

[buffo]

Well, actually a lot of us simply modulate the pump diode of the DPSS rig and hope for the best. But yeah, an AOM is the preferred method.

Using a mechanical shutter would only give you accuracy to a few 10's of milliseconds. Maybe not even that good. So that's out. (Most of us only use the shutter as a safety mechanism.)

As for the PL member, I had a brain-fart! His user ID is Mados123, not Madoin123. Sorry about that. Here's his profile page. (He only logs in once in a great while, so it might be a while before you hear from him.)

Adam

[albuddah]

hrmm. i've seen shutters from uniblitz that get down to sub-millisecond, but then the shutter+driver runs close to $900. with aom's, will I have to worry about collimation at all, or can I just align so that the zero-order output goes where the output past the shutter would've gone? I'm assuming the beam coming from the laser is roughly collimated, but after the modulating device, I still need it to be collimated so I can focus into a fiber. sorry for the basic questions, and thanks for the great help!

I should also add that the 5 ms pulses will be in trains ~5-10 seconds long, at 5-50 Hz. Would this make a difference in choosing between shutters and aom's?

[mixedgas]

How sharp do your rise and fall times have to be?

galvos can do this, Ao cells can do this, EO cells can do this, and specialized stuff such as this http://www.nmlaser.com./ can do this.

I need to know more to help you.

A AO does not change the collimation.

Steve

[ElektroFreak]

Every 473nm laser I've ever seen has amplitude noise at high frequency. When you move the beam very quickly, it looks like TV static. I'm pretty sure it's normal, since CNI specs the amplitude noise in their MBL series at ~30% which is a lot.

Greens also suffer from power instability, mainly during modulation, but it doesn't work quite the same as 473nm. It tends to be much lower in frequency and the fluctuations usually aren't quite as large.

To my knowledge, no DPSS laser cen be directly modulated (i.e. modulating the pump diode) without some sort of instability. The main reson for this is that when you directly modulate it also modulates the temperature of the crystals. Most non-linear crystals are very temperature dependent, so any temperature fluctuation always results in a change in performance.

[albuddah]

I guess I would like to see rise-decay times of less than half a millisecond. I took another look at my crystalaser 473 and it appears the amplitude noise really is quite low, although the rise and fall times appeared to edge towards over a millsecond each, and the total power was lower than during cw operation.

i guess ttl is not so bad after all if the amplitude noise is low, but from what I've seen so far, the price for a laser with low noise is much greater than the price for a laser and a shutter or aom.

I looked into isomet aoms, but they haven't gotten back to me with a quote. does anyone know if aoms that reach sub-millisecond rise and fall times will cost under 900 usd?

thanks again for all the help

[mixedgas]

Usually suitable AOs can be had off ebay for about 100$ US.

Steve

[mados123]

Hey guys.

On the technical side of the laser info being exchanged here, I really don't have much to contribute as most of the stuff is over my head. I do have a interesting video I shot regarding laser light strobing/freeze framing and it might be useful for proof of concept. I concluded that for my application, by the time I got the pulse width down to an acceptable level of image sharpness, the lighting was too dim. Check it out if you like.

RGB laser light, freeze frame (top right video)
http://www.thinktank-tech.com/projects.html

Spinning disc, freeze frame