Acousto-Optical / Liquid-Crystal Tunable Filters

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J_Rogers
Posts: 87
Joined: Mon Aug 09, 2021 2:29 pm
Location: North Carolina, USA

Acousto-Optical / Liquid-Crystal Tunable Filters

Post by J_Rogers »

Foremost, does anyone know the specifics on how these tunable bandpass filters are driven and controlled? Information behind the electronics to control them are virtually non-existent. I know both roughly are modulated via changes in frequency, but when you look at the connectors of the various units for sale, clearly more information than just that is needed to use them.

I pose the aforementioned question first, because I would love to buy a second hand one and then "experiment" until I can happily say I managed to get it working, but I am afraid there is a proverbial mountain to climb before these things will work. I do not care to spend all that money just to hook up a signal generator and suddenly the optics shatter. So I am making this post in hopes to pool information, as well as any possible alternatives that are more budget friendly.

Current sources for new units (that I know of) are:
Spectrolight FWS-Poly - $34,000
Thorlabs LCTF - $9,000-13,000
AA Opto-Electronic - $unknown
Nova Photonics - $unknown

The closest solution that I have found is Semrock's VersaChrome edge tunable filters. However, I see they have been discontinued. Incredibly cool technology that for whatever reason didn't last long. Even at a price of $1600 per filter & needing 2 of them, they are still cheaper than every other thing out there.
smk-versachrome-wp-2.jpg
Linear variable filters exist and are cheap, but I think that opens a different can of worms when dealing with precision tuning an the optics needed to use them so I am leaving them out.

Given the jaw dropping prices on all of these, has anyone successfully found a workaround? Or managed to get a surplus unit working without the help of the manufacturer controller?

houstontx
Posts: 395
Joined: Fri Jul 31, 2015 1:07 pm

Re: Acousto-Optical / Liquid-Crystal Tunable Filters

Post by houstontx »

I follow someone on twitter who recently "hacked" an AA-optoelectronics from a Leica AOTF confocals I think TCSP5 possibly TCSP2 (there is a aotf and non aotf versions)

Not sure of all the details but this might help...

http://www.metronixlaser.de/AA-PCAOM-Anleitung.pdf


J_Rogers
Posts: 87
Joined: Mon Aug 09, 2021 2:29 pm
Location: North Carolina, USA

Re: Acousto-Optical / Liquid-Crystal Tunable Filters

Post by J_Rogers »

Thanks to the link houstontx posted, I have been down a rabbit hole trying to figure these things out.

I made the following diagram with what I believe would work in order to modulate the bandwidth of a broad spectrum source. I'm still not 100% certain, as the whole field dealing with RF is too much to to simply skim through researching. It is evident now, that the new commercial units are likely using some variation of 8 channel DDS waveform generators coupled with board mount VCO circuits. This is something that without the software to configure the waveform frequencies - used multi channel AOTF drivers are almost out of the question. In theory to mitigate this, one needs to either manually change the drive frequency via RF signal generator or either modulating or sweeping the main drive frequency through a tunable range via a frequency synthesizer & phase locked loop. Again, there are likely nuances such as settling time and phase noise etc.

Someone with more RF knowledge can correct me on the diagram. There is a possibility that the input dc voltage is not actually supplied via the bias tee but somewhere in the PLL control. At which point there will likely need to be even more components to be able to both monitor and adjust said voltage.
BLOCK-DIAGRAM-AOTF-DRIVER_export.jpg
Driving an AOTF for a single linewidth at a time (their FWHM is less than 5nm) now seems to be the easy part. DC input -> into a VCO -> coupled to a low noise RF amplifier -> sent to the AOTF. But the moment you try to adjust that FHWM or have software change the drive frequency on the fly, is the moment the costs start to rapidly pile up. I may play around with it some more; funds permitting.

-I got a quote for one of these things https://www.taborelec.com/p1288b and just about had an aneurism.

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