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Modulation of nonpolarized laser beam - how?

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LAN_403

plazmatron

Mon Nov 09 2009, 10:18PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

It is theoretically possible to polarize an unpolarized laser by placing magnets along the bore. This causes Zeeman splitting, of the laser line, and generates an elliptically polarized output.

c4r0

Tue Nov 10 2009, 03:45PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

As far as I know that splits spectral line for two very close to each other and both polarized circularly, one of them CW and the second CCW. I don't think that would help me.

Whats going on here on the forum? Today morning five post from this topic was gone, I wrote another post then. Now these five posts are back but my last post is gone

I got an idea with modulating the tube current. I could blank the laser by limiting the current to about 1 amp, so that the output power was almost nothing but the gas was kept ionized. But could it work fast enough for laser show and wouldn't be harmul for the laser? I assume that i it was so simple people would not use AOMs.

Meatball

Thu Nov 12 2009, 04:43AM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
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Posts: 74

How many wavelengths are in your laser's output? Is it just the normal 488nm? Or do you have a multi-line Argon?

Steve Conner

Thu Nov 12 2009, 10:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Chris transferred the forum to a new server a few days ago, and posts that were made in between the backup and the restore were lost. He e-mailed everyone to warn about this.

Karol, why don't you just pass your argon laser beam through a polarizer and take the 3dB hit in power? Your eyes are log detectors anyway, so you'll probably not even notice.

Proud Mary

Wed Nov 18 2009, 12:00AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It seems silly to suggest the obvious, but if I wanted amplitude and phase modulation I should use a Pockels' cell or similar permuations, after the fact, rather than muck about with the primary laser frequency.

Love,

Stella X

Wed Nov 18 2009, 06:43AM

Registered Member #2372 Joined:
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Posts: 62

what do you mean by phase modulation? You must have a more stable laser than I, as I am unfamiliar with any laser that has any type of phase stability on even the 1 second level, unless you are talking about an ultrafast modulator of some kind.

Proud Mary

Wed Nov 18 2009, 06:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

dugg wrote ...

what do you mean by phase modulation? You must have a more stable laser than I, as I am unfamiliar with any laser that has any type of phase stability on even the 1 second level, unless you are talking about an ultrafast modulator of some kind.

If you would care to Google "laser phase modulation" you will find all manner of papers on the subject. A patent library search will also reward you.

Wed Nov 18 2009, 08:03PM

Registered Member #2372 Joined:
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Posts: 62

Proud Mary wrote ...

If you would care to Google "laser phase modulation" you will find all manner of papers on the subject. A patent library search will also reward you.

I understand that one can modulate the phase of something, I am wondering what YOU mean by phase modulation, what are you going to use it for? I mention stability because phase modulation of a laser seems like something that has to happen faster than most home experimenters have equipment to measure.

...

Thu Nov 19 2009, 03:50AM

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

A phase modulator won't help c4r0, all a phase modulator does in introduce a delay on the order of a few femtoseconds (depending on how much phase change is needed, a 2pi change at 400nm translates to 1.3fs), which can be used in connection with interference to create an amplitude modulation by using a 'Mach-Zehnder interferometer'. These work by interfering the beam with a phase shifted version of itself in a 50/50 beam splitter, such that one can force all of the beam out one port of the beam splitter (constructive interference 0 phase shift), all out of the other port (destructive interference, n*pi phase shift), or any mix in between (n(0-pi) phase shift).

Unfortunately, for this to work, as noted above you need a very stable laser/optical system (keep in mind that a pi phase shift is 200nm, so even a shift of 10nm of the length of the beam paths will give you a noticeable modulation of the output!), as well as a laser that is a single frequency (which contrary to popular belief, very few lasers are actually single frequency, even the trusty HeNe laser will lase at 2-10 frequencies separated by a few tens of GHz apart). Although to the best of my knowledge, you could get around the issue of polarization.

Proud Mary

Thu Nov 19 2009, 09:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Something different:

Pass the beam through an optical cell fitted with an ultrasonic transducer.

The beam will be modulated by the changing refractive index of the liquid in the cell.

Something similar could be attempted with crossed polarizers i.e. the beam is first passed through an X polarizer, and thence into a Y polarizing crystal (tourmaline?) bonded to an ultrasonic transducer.

Again, passing the beam through a transparent piezoelectric crystal (f.eks. quartz) driven by a suitable oscillator, could also create useful modulation as the refractive index of the crystal changes under cyclic compression and expansion at fo.

A silver mirror deposited on an oscillating piezo-electric crystal might also be expected to modulate a beam reflected thereupon.

Caution: I know almost nothing about lasers, & have not tried any of this - these are just a few ideas that came to mind that could be helpful - and there may be good reasons why these thought-sketches will not work.

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