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nanosecond shutter speeds: optical gating

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hboy007

Sun Sept 16 2012, 10:31AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

in a current experiment, I have to look at luminescence in a small spectral region of 20nm that is located somewhere between 850nm and 1050nm (mainly around 920nm). Since the pump pulse and the initial luminescence within the first 1-2ns has to be rejected, a really fast rise time(~10ns) is required to minimize deadtime.

Would AOMs / AOTFs be an option? I've found this one here

that seems to be fast enough but not explicitly inteded for tuning. Polychromatic AOMs support a larger driving frequency range but require a synthesizer driver which I don't expect to be sufficiently fast (in case a quasi-continuum transmission band is to be created).

There may be other options. I should mention that the detection path is coupled into a multi-mode fiber or a single mode fiber (on demand). I could as well use a polarizing beam splitter in conjunction with a pockels cell before or after the fiber.

Any suggestions?

...

Sun Sept 16 2012, 02:04PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

an AOM will have he optical bandwidth you need, and 10ns is a pretty reasonable rise time as long as you get one with a small active area and high driving frequency. I would look around at the

and

websites and see if anything turns up. They will be pricey, but should get the job done. Also, there is a polarization preference on some of them, something to keep in mind. Any one designed for 1060nm will probably work, the performance may not be specified down to 850nm but it should be decent.

The pockels cell could work, but I suspect you will have a hard time getting good transmission over such a wide wavelength range, since the amount it retards the polarization will depend on wavelength.

Can you filter out the pump using a wavelength filter? You can a really good longpass filter for cheaper than the modulators you are looking at

hboy007

Sun Sept 16 2012, 02:53PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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thanks for the quick reply. Unfortunately it is not only the pump wavelength that matters. Since this is a semiconductor spectroscopy experiment with the excitation pulse situated slightly above the band edge, the immediate response will also include emission from lower localized states. After ns-scale electronic control, a second luminescence pulse should be observed in the same spectral region. Obviously, long pass filtering is not an option once the immediate luminescence is taken into account.
We actually have good "razor edge" type long pass filters. These also modulate the transmission near the edge, prompting further post-processing of the spectra.

Thanks for the links, I'll look into that.

Shrad

Sun Sept 16 2012, 05:53PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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you can tune AOMs to two bands, even if they are not made for it, by multiplexing two frequencies and reducing power drive

I think a peltier cooler stuck on the backplate of the AOM crystal might help in stabilizing the crystal at higher power levels

if you are interested, I have several AOMs for sale (used isomets and AA Optics for visible to near IR, I think 450nm to 1250nm as they are MgF2)

they might need some tuning of the central frequency, or maybe another driver if you want them to be two-channel AOTFs

if you're interested, please send me a PM, the price is right and they are quite in good state

Conundrum

Mon Sept 17 2012, 02:37PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Yeah, Peltiers would be a good idea.
Make sure that they have a decent heatsink or they will cook off and take your AOM with them.

I did look into a variant of the LCD panel, hideously slow but did learn that they switch faster when heated.
A possible fix is to use it at 45 degree and rely on the faster surface switching effect rather than transmission.

This also works with 3-D shutter glasses which are getting ridiculously cheap these days.

Hope this helps, -A
"Bother!" said Pooh, as the Hirogen hung him on the wall as a trophy...

Pinky's Brain

Mon Sept 17 2012, 10:58PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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... wrote ...
The pockels cell could work, but I suspect you will have a hard time getting good transmission over such a wide wavelength range, since the amount it retards the polarization will depend on wavelength.

Assuming there is a spectrograph at the end you could just repeat the experiment for each wavelength with the pockels cell tuned for that wavelength.

For people with a better intuitive grasp of optics a question, assuming you have a semi-collimated beam couldn't you use a two pockels cells like this < with the beam entering from above? (Actually the lower doesn't have to be a complete pockels cell, just have the same refractive index at 0 volt.) You would use the refractive index modulation for a single polarization here, rather than it's change in polarization angle. AFAICS this would let the beam through as is regardless of wavelength at 0 volt, and push it to the side (and chromatically disperse it) when the top pockel cell is driven.

hboy007

Fri Sept 28 2012, 10:49PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

Update: This week we set up a 40 MHz AOM in conjunction with a HeNe laser. The AOM turned out to be really slow (in the 100s of nanoseconds). However, there is also a pretty darn fast AOM that came with the 80 MHz Ti:Sa laser as a pulse picker (not available as a detection path shutter). I'll be playing around with it a little more because I need the lower rep rate for optical excitation.

Here's a little document I found about pulse picker AOMs :

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