AOM's and PCAOM's

[gogu]

Got me reading over and over too

Possibly a mistake, but he does refer to the AA aom as a aotf/pcaom a number times

For the AOM / Q-Switch part of the lab:
Crystal Technology 3080-120 AO Modulator

For the PCAOM part of the lab:
AA Optoelectronics AA.MTS.110/A3-VIS AOM used as a PCAOM in this experiment

PCAOMs:

This part of the lab examines the use of an AOM element as a tunable filter (called an AOTF or a PCAOM: PolyChromatic AOM) - this device can be used to select a single laser line from a multi-wavelength laser.

For the AA Optoelectronics AOM (used as an AOTF), f is variable (set by the VCO) and v is different (it is in shear mode). Of course the wavelengths are different as well.

Another reason i "think" that it can be used as an Aotf, haven't actually tested it. (Was hoping Steve would find some time to take a look at one. ) is the Kvant bazzar section has 2 listings for the same model AA aom listed as aotf.
http://www.lasershow.sk/bazzar

[shrad]

I have bought a book recently, "understand and use high frequency electronics" from publitronic/elektor (Jon B. Hagen) which, I think, contains everything needed for learning the basis

it contains everything you need to know about filters, amplifiers, couplers, modulation and so on, and I was planning to read and experiment as well

it is true that a 2 channel AOM would be awesome, you'd only need 2 to have a pristine linear modulation on a RGBV laser projector...

simply the fact that an acoustic wave is standing in that crystal and diffracting the beam makes me dream...

now, if I follow another idea, if I was to produce a slight deviation of the beam with an AOM, would it be possible with that said acoustic wave to vary that deflection? I was willing for a time now to try mounting two AO crystals orthogonally and make a beam traverse them to get a small X/Y motion, but have never investigated further (though I know there are AO deflectors based on that principle)

[tocket]

AOM noob question: Is it the 1st or 0th order beam you use as output?

I had always assumed it was the 0th, but now I'm starting to think that the 0th order beam probably can't be completely extinguished.

I'm seriously considering investing in an AOM to use with my green laserwave, because modulation is simply too poor on it now. It only works well at full power CW. Would it be hard to use one of gogu's AOMs for this?

[shrad]

You have to use the first order beam if I'm not wrong

I have one of gogu's AOMs and it behaves really well under 532nm light

I'll try to separately modulate my 532 and 642 lasers before combining, in order to have a colourful demonstration, as I have two AOMs

I already made some videos of the modulation process but they are really crude and the modulation input was only 3V instead of 5V, so the beam is not entirely modulated (also, the mounting and setup was very crude)

part 1:
http://www.youtube.com/watch?v=sczuM...eature=channel

part 2:
http://www.youtube.com/watch?v=fQmp-...eature=channel

[mixedgas]

AOM noob question: Is it the 1st or 0th order beam you use as output?

I had always assumed it was the 0th, but now I'm starting to think that the 0th order beam probably can't be completely extinguished.

I'm seriously considering investing in an AOM to use with my green laserwave, because modulation is simply too poor on it now. It only works well at full power CW. Would it be hard to use one of gogu's AOMs for this?

Yes, check the polarization. If linear polarized, things are awesome with a modern isomet 1205. If linear, you can hit 90% according to the data sheet. If random polarized, and your laser has orthogonal polarizations like a hene, You'll get roughly 80% of one and 40-60% of the other.

I'm running that experiment on a sample Gogu sent me, sometime this weekend.

If the laser were all one polarization, data sheet spec on that Isomet is 90% power into the first order beam with a TEM00 mode laser. Mind you, the power you get is determined by the quality of your mounting scheme. Tilts in the plane of the baseplate can come back to haunt you. For various reasons, I'd be happy at the traditional 82% with my polarized system, but it looks like isomet has improved things.
I'm testing with a 80 pitch screw on my angle adjust.

One thing that is worth it with AOM, no matter what, having your reds and greens turn on within a few microseconds of each other, with stable power. You WILL see the difference in the graphics.

Steve

[mixedgas]

Before this spirals out of control, lets define a few things

AOM
AOBD
AOTF
PCAOM

AOM accousto-optic modulator. Modulates by deflecting the beam off its normal path. Can if specially designed, be used as a scanner, called a AOBD, ie acousto-optic beam deflector. AOBDs are very fast, but are expensive and scan very small angles

AOTF, accousto-opticly tuned filter. There are 3 or 4 variations, a PCAOM is one. NOT all AOTFs act exactly like a pcaom.

A PCAOM passes all diffracted beams in a colinear beam using a slow shear wave crystal. A AOM uses a compression wave, not shear. Shearing the crystal vs compressing it allows the designer to use polarization changes in the crystal to deflect the beam. AOMs are cut from the crystal along its INACTIVE optical axis, it might as well be water for all polarization purposes. PCAOMS and MOST AOTFs are cut along a birefringent axis and are thus active with regards to polarization. If you hold a polarizer up to a AOM and look through it, all polarizations go through the crystal when the driver is OFF. If you hold a polarizer up to a inactive PCAOM, it acts as a waveplate.

A PCAOM has a broad band transducer array of often more then one transducer that can handle high power, because you can have up to 125 mW of power per beam times 8 beams in a neos, or 1 watt. This by definition means a PCAOM is about 8 times more efficient in mW of laser transmitted per milliwatt of RF, because a normal AO needs a quarter watt to one watt for ONEwavelength. This is the downfall of using a normal AO as a polychromatic modulator, your power limited so you dont blow the driver off the crystal and the power per mW is less. Not to mention you are not creating two gratings, when you modulate AOM with two RF carriers, your creating a grating with spacing that changes periodically. If you took the 88-109 mhz FM radio band and amplified it and launched it into a 110 mhZ AOM, you'd see a spot for each radio station, the width of the spot is the bandwidth of the station and the brightness of the spot is the signal strength. So a AOM with a linear CCD detector can be, and is, used for RF spectrum analysis.

You can select a wavelength with the AOM, when it is energized. When it is not driven, a whitelight beam goes straight through, unchanged. When the AOM is energized, its a grating. when its not driven, its a block of glass. If its not overdriven and its driven with a sine wave, its a special case of grating where most power goes into the first order, ie Bragg diffraction. If its overdriven, the amplifier is clipping, or if its driven with a sqaure wave, or if its misaligned, you get RAMAN-NATH diffrraction, multiple orders, and most of the power does NOT go into the first order. In that case its the same as using a unblazed diffraction grating.

If you aim a whitelight beam into a PCAOM or MOST AOTFS, you get a spectrum out at a angle of 5-20 degrees from the entry beam path when the CRYSTAL IS NOT DRIVEN. If you apply a carrier to the PCAOM it will deflect ONE wavelength per carrier back into the entry beam path, colinear to the entrance beam. IE a selected beam goes straight through.

SO when DR CESELE tells his students to use the device, the questions are, 1. does it deflect when NOT energized. 2. Does it normally deflect a white beam off into a spectrum when NOT energized. If the answer to both questions is NO, its a AOM.

Since he tells them to align the red beam from the hecad at 68 mhz to a diffraction, my guess is NOT a AO. He may be misusing or interchanging the term PCAOM with the term AOTF.
A AO can act as a poor to moderate quality AOTF, but a real AOTF can select a beam with MUCH greater resolution then a AOM.

The reason this gets confusing is most scientists only would want to deflect one wavelength at a time. That one wavelength is interest to them, hence the term tunable filter. We display folks use more then one wavelength at any time.

Most older AOTFs are the same slow shear wave design developed at Brevard College in Florida for the PCAOM. So the question I have never had answered is: Which came first, the AOTF or the PCAOM. My bet is BOTH at the same time.

Steve

[mixedgas]

[QUOTE=tocket;131375]AOM noob question: Is it the 1st or 0th order beam you use as output?

I had always assumed it was the 0th, but now I'm starting to think that the 0th order beam probably can't be completely extinguished.
end quote

oth doesnt go 100% out. for active HIGH high deflection, ie RF on means unblanked laser show projector, you use the first order. If you want to do something like Qswitch, which means you prevent the laser from lasing(blank it) by deflecting the beam out of the cavity, you use the oth order path.

Steve

[robotarmy]

yes, RF is a useful skill, at VHF its fairly easy, and worth learning. I will help you.
This can be done cheaply.

Steve

Thanks a lot for all your rad info, i'll dig around and do some reading about RF circuitry. I studied the math behind AM and FM synthesis in university, but we never got our fingers dirty with any hands-on RF board layouts or circuits...

Do you think it'd be worth getting in touch with the prof that wrote that lab and see what the deal is?

[mixedgas]

Thanks a lot for all your rad info, i'll dig around and do some reading about RF circuitry. I studied the math behind AM and FM synthesis in university, but we never got our fingers dirty with any hands-on RF board layouts or circuits...

Do you think it'd be worth getting in touch with the prof that wrote that lab and see what the deal is?

I think a better exercise is to look at the part numbers he gives and go to the AA and Isomet/CT websites and look up what he is using.

Start also with Wenzel techlib scroll down to RF. Look at measuring RF voltages with a V.O.M and the 1 watt class c RF amplifier pages. Dr Wenzel was kind enough to write the RF voltage measurement stuff many years ago when I was a student and asked him for help, as I could not afford a RF millivoltmeter back then. www.techlib.com

Steve