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Faraday Rotator for optical communications

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LAN_403

Signification

Sun Apr 12 2015, 03:12PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I have been collecting parts, as they became available, for years to build a 'better' Faraday rotator which is a device that uses the property that a strong magnetic field (such as that inside a solenoid) can rotate the polarization angle of light--usually a polarized laser beam that is passed through the center of a solenoid and emerges through an analyzer (another Polaroid). Significant rotation is somewhat difficult to obtain. You need either a huge current flowing through the solenoid coil (I want to vary the current linearly with an audio signal), or a modulating material with a large "Verdet" constant--or both!

I have recently found a quantity of nitrobenzene on ebay and plan to use that as my active ingredient. There is a Scientific American article (11/1970) that describes this, but I don't like their coil of 300 turns of magnet wire for up to 20A IIRC. The modulating circuit is constructed from vacuum tubes and radio transformers ("bearskins and stone knives"). I do plan on using the nitrobenzene in the optically transparent solenoid ends. I think I have found the magic solution for modulation with music...Here is the plan: To get the absolute HUGE current, I use a single layer (about 12") of 1/4" x 1/4" giant magnet wire from a welder. I will modulate it with the output of a 1600 Watt, 2 Ohm capable car audio amplifier.

To modulate the current through the solenoid, I use two 320 Watt soldering guns with their 120V power cords in series, after removing the huge soldering tips, I connected the thick copper solenoid coil to the two gun outputs, in a series configuration, which seems to be a great match for the available 2 to 8 Ohm amplifier output. Of course, the setup will be adjustable for any combination of series/parallel connections to the 120V input /~0.75V output of the soldering guns.

In the "dry-run" I did with a 200W amp and a smaller gun things seemed to work fine, with one exception--the soldering gun played music!

BTW, I did find a low power Faraday setup at a place called "TeachSpin". They suggested purchasing a Lock-In amp to improve the somewhat 'weak' output (not audio). Total price $7825.00 !!! Without power supply!

I chose optical communication since I have been fascinated with this subject most of my life. Does anyone here have experience with Faraday Rotators?

...

Sun Apr 12 2015, 07:44PM

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

You are correct that the Faraday effect is incredibly small in most all optically clear materials. This is why everyone uses electro-optic, acousto-optic, LCD, etc based modulators

That said, as long as you are planning to hook your modulator up to a 1600watt amplifier I don't see any reason why it shouldn't work, however I am confused as to why you decided to use such an odd coil geometry. Why not simply wind a 2-ohm coil and directly connect it to your amplifier?

You may also find that optimizing the length of your coil helps, despite the fact that going to a longer coil does not fundamentally help you (the Faraday effect is linear in terms of applied magnetic field generating a polarization shift at these field strengths, so using a liquid cell twice as long with a half as strong magnetic field does not gain you anything). At the very least issues relating to heat dissipation are minimized with a long single layer coil made out of moderate wire (500ft of 16awg wire would be well matched to your amplifier). Also, don't forget that for a DC electromagnet (which is more or less the case for this modulator), for a given input power (your 1600w amplifier in this case) the generated magnetic field is roughly proportional to the weight of the copper wire in your magnet (regardless of any matching transformers that you use).

Signification

Mon Apr 13 2015, 09:10AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Here is the reason I initially did it like this: I had several if these thick welding-coils wound with a special (borrowed a few years back) machine that could only wind 1" diameter coils. I used the others for quarter shrinkers and really wanted to make another use of the rest of it. I had experimented with ultra-high current, low voltage before--mainly MOTS and soldering guns. So, in my state of mind, this design took shape. But, I really should eliminate the soldering guns and hook the hi-current, low-V to a large winding of 2-Ohm 16-AWG coil. My mind, unorganized in this scheme, took a fork in the road, and didn't look back. You know, nobody else suggested this! --Thanks-- I should do it this way. The soldering guns are an unnecessary step--darn--they looked (and sounded) so cool too. But are simply not needed here.

BTW my receiver is a PMT in a powered socket that runs on a simple battery--Much better than conventional. I use a 405nm diode laser with a narrow band-pass filter. The data sheet says the PMT sensitivity peaks here. I think these are STILL the most sensitive optical receivers--and now portable and battery-operated.

klugesmith

Mon Apr 13 2015, 07:21PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Good to see people doing physics instead of just talking about it.

Those pictures of atomic bomb fireballs in the first milliseconds were taken with Faraday-effect optical shutters. I think the word to search for is Rapatronic. HV capacitor discharge into coils like we have lately discussed for magneforming.

I thought I had replied hours ago, but it somehow got lost. It included my usual point, which you already understand. Musical soldering guns are cool. But your coil volume could be rewound with thinner wire to suit the un-transformed amplifier output. With same ID, OD, and length, the copper mass and electrical power requirement are invariant (aside from differences in fill-factor).
I see 3 paths to significantly increase power efficiency:
- use a conductor with lower resistivity than warm copper.
- increase coil volume and copper mass
- change coil aspect ratio to one more efficient for the task.

Any idea what your actual current and/or magnetic field strength are? The latter could be measured with a helical sense coil in place of the optical cell.

Sulaiman

Mon Apr 13 2015, 07:27PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I suspect that what you want to modulate is the coil current
I think that reactance (2.pi.f.L) will be greater than resistance at the higher audio frequencies
You need current feedback rather than voltage feedback.
e.g. 0.1 Ohm would give 2V feedback signal at 20 A
take voltage feedback signal from the 0.1 Ohm instead of the output
and reduce the feedback by a factor of ten

P.S. if this is for long range communications,
I vaguely remember reading somewhere that the atmosphere randomly (unpredictably) rotates the angle of polarization,
may (or may not) be significant for your project.

radiotech

Thu Jun 25 2015, 09:23AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Nitro Benzene, or Oil of Mirbane is a dangerous neurotoxin.

Its nasty stuff. A coroner had a bottle of it in his collection of
things causing accidental death to show us as part of training long ago.

. Some persons interested in rocketry had added to the fuel mix they were heating in a closed room.

The Pokel cells and Kerr cells use electric field modulation. This would seem
to be easier than magnetic.

Bored Chemist

Thu Jun 25 2015, 07:10PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

radiotech wrote ...

A coroner had a bottle of it in his collection of things causing accidental death to show us as part of training long ago.

Can I quote that the next time I want to explain the importance of punctuation?
" A coroner had a bottle of it in his collection of things, causing accidental death to show us as part of training long ago."
isn't the same as
" A coroner had a bottle of it in his collection of things causing accidental death, to show us as part of training long ago."

hen918

Fri Jun 26 2015, 05:18PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Bored Chemist wrote ...

radiotech wrote ...

A coroner had a bottle of it in his collection of things causing accidental death to show us as part of training long ago.

Yeah, I did think the coroner killed himself with it accidentally, after all; the panda eats, shoots, and leaves!

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