Listening music with acousto optic modulator - Page 1
Forum section: electronics / projects

author: compet17
author_rank: Regular Contributor
timestamp: January 07, 2015, 06:48:21 pm
content: Is there someone with experience of acousto optic modulators (AOM)?At the moment I am listening to music which is transmitted by a laser beam. I played a bit with some junk I had lying around.... a green laser, an AOM a solar cell and a headphone.The AOM is from a huge film exposure unit that was used for printing maps, a nice little toy. It consists basically of a piece of cristal which is shaken by ultrasonic soundwaves at about 20 MHz. These waves create density changes in the cristal that form some sort of optical grid which deflects the laser light.When these soundwaves are modulated with low frequency AC (for ex. music), the laser is modulated by that. Then a small solar cell from a calculator and headphones connected to it picks up the laser beam and you can hear the music.So far so good. It works.Now the question... When the modulator is working, the main beam splits up into 5 beams (in my application). The center beam is the brightest but the least modulated. Then there are two 1st order beams (left and right to the center) and two 2nd order beams even more outside.The strongest signal/modulation is on one of the 1st order beams. But only on one of them.After a few meters, the beams are wide enough apart to choose one by holding the solar cell into it. So I can test which one gives me the strongest signal.Is it normal that the strongest modulation is on the 1st order beam? I wanted to try to send my music over a longer distance but that beam is much weaker than the straight beam.Or am I doing it wrong??

author: LaserSteve
author_rank: Super Contributor
timestamp: January 07, 2015, 10:15:42 pm
content: What model of AO are you using?? What model of Driver? Did you salvage the Bragg Mount from the printer?What laser are you using? Is it linearly polarized?Sounds like your in Raman-Nath alignment, not Bragg mode. First step is to turn on using DC at less then full drive. Peak the first beam to the left or the right of the zeroth order (beam that goes straight thru) at the expense of all other beams by rotating the crystal. You wish to maximize the First Order beam to one side of the Zeroth order. Do not maximize both first order beams.Use your solar cell for this with a DC voltmeter. The human eye is the worse laser power meter ever made by unskilled labor. Load the solar cell with say 470 Ohms to enhance its linearity and high end range. When you hook it back up to the amplifier, make sure its loaded slightly and AC coupled.If you expand the beam with a lens on the wall, when the AO is close to peaked on angle, you should see a thin black stripe appear down the middle of the first order beam, with most types of AOs.Expect 30-50% diffraction efficiency. Anything over that is gravy.Now, AOs have four issues that don't show up in the textbooks.One: They are sensitive to input polarization. Randomly polarized light is barely deflected. The crystal will have a preferred polarization axis at either right angles to the crystal baseplate or parallel to the crystal baseplate.This is the common newbie error. In fact the AO will try to select one polarization if possible.Two, There is a sweet spot in the crystal where the acoustic wave is best for diffraction. You need to the laser scan up or down and left or right across the entrance aperture to find the sweet spot. This takes practice.Three, They do not like to be excited backwards. The laser should go in thru the same hole it went into when installed on the printer. It will work, but you might see some standing wave effects if backwards.Four: Bias. Cheaper modulator drivers often like to have a bit of DC bias on the input to take them above threshold. They also do not like to be overdriven. Overdrive causes square waves to come out of the transducer, which results in Raman-Nath operation and extra orders of beams which zap power. Overdrive will result in echos off the far end of the crystal opposite the transducer. This shows up as extra orders of beams that will not go away. At that point, the Piezo material is possibly close to meltdown.Let me know your answers to my questions. BTW, You probably have a 80 Mhz or higher modulator. 27 Mhz modulators are only used for Q-Switches these days.Friends don't let friends drive the optional FM Bias input of the AO driver with Audio. If this happens, your scanning the beam, not modulating it. Changing carrier frequency changes exit angle, not amplitude.Want Hi-FI? Switch to a reverse biased, small area photodiode and use some of my friend Phil Hobb's circuits:http://electrooptical.net/www/frontends/frontends.pdfSteve

author: compet17
author_rank: Regular Contributor
timestamp: January 08, 2015, 06:42:46 pm
content: Hi SteveI will try to answer your questions... This is my laser. A cheapass chinese diode/KPT green laser with about 50mW @532nm. I don't have infos about polarization.The driver is made by ISOMET, model 232A-1 which has a center frequency (according to the datasheet) of 80MHz and a modulation bandwith if 35MHz.The AOM is probably something custom made. Driver and AOM come from a huge INTERGRAPH Mapsetter film exposer/scanner unit. A highly interesting machine BTW...I don't know if this is a Bragg Mount what I have.... It's a heavy aluminum base which has possibilities to shift and rotate the AOM unit.I marked the light path when I removed the stuff from the printer. And I used it backwards yesterdayBut it worked anyway. I will try it again the right way aroundI really appreciate your tips for adustment. I will try and report the results.I noticed that the negative halfwave of the sound I was playing was clipped at a higher sound level. I assume that I will need a bit of DC bias on the modulator input. Maximum input signal is 1Vpp, so I will try it at 0.5V DC bias... would you agree?But first I have to attach a heat sink on that modulator. It got quite a bit hot yesterday...BTW: Is the wavelenght of the laser light an important factor? The original laser from the printer was an Argon laser with a blue/green 488nm. Now i'm using this 532nm green laser.I also have some nice parts from a Laserscope 800 medical laser. For example the Q-switch, mirrors and a water cooled KPT cristal and a box of other fascinating parts. Unfortunately the xenon lamp was corroded from the cooling water and the whole unit was totally oversized for my hobby workshop (6000W input power and 100W IR / 50W green laser)Maybe you're interested in some parts...?Chris

author: compet17
author_rank: Regular Contributor
timestamp: January 08, 2015, 07:23:53 pm
content: OK.... did some adjustments. Turned the crystal into the right direction, turned it to optimum angle and tried to find a sweet spot which was the most difficult.I adjusted the angle with headphones on the solar cell and music modulation... easy to find the loudest spotNow the only thing left is to give some DC bias to the input. Music sounds not too bad already but the bass lines (biggest amplitude) is very distorted in the lower half wave. DC bias will certainly cure that.With all those adjustments I got a much louder and better signal. I fact I can listen to the music now without an amplifier. I'm surprised of the frequency response of the cell. I expected to hear only the low frequencies.PS: Did you know that a simple LED is a quite good receiver? The voltage is much higher than from the solar cell but of course when a load is attached, the voltage drops to zero. But with an amplifier it should work.... I will try.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 09, 2015, 04:25:24 am
content: OK, ISOMET 232-1 Has a DC bias Pot. Pop the snap in silver cover over the bias pot with a screwdriver, slightly increase it. Beware, this is adding static, un-modulated light to your cell, driving it closer to saturation.232-1 uses a simple crystal based master oscillator, balanced modulator, power amplifier scheme with no FM input.The Modulator is a Doubly Balanced Modulator module, from Mini-Circuits, Typically a SRA-1 or similar.About an SRA-1: This is a RF Schottky diode bridge sandwiched between two RF center tapped transformers. The tap on one transformer is used to introduce the modulation. The tap on the other transformer is grounded through a small load resistor.If I remember correctly:On a 232, two summing resistors go to the high side of the Balanced Modulator. One goes to the DC bias Pot via a resistive RF Pad, the other goes to the input gain pot or a 50 ohm resistor which is connected to the input BNC connector marked VIDEO.Attached is a rusty old memory of what is in that Driver, as a sketch. Don't blow the sensitive diodes in the mixer module. While they are cheap, desoldering it is a Herculean task.Your laser is likely polarized about 400 to 1 if its the modern ND:VANDATE as the host crystal instead of the older ND:YAG. You'll hear some mode beating noise, amplitude modulation from the pump diode, and some white noise from the non-linear process in the KTP crystal.There is no issue using 532 instead of 488, the modulator crystal has a multilayer AR coat that will cover roughly 400-700 nanometers with low loss.And yes, LEDs make good photodiodes for narrow wavelength ranges when the bandgap in the LED lines up with the laser source. There are scientific papers on this, as some LEDs cover wavelength ranges useful with tunable pulsed lasers. In rare cases, if the pulses are strong enough, non-linear effects occur allowing detection of ultra fast pulses by second harmonic generation of the laser light inside the LED.Sorry for the poor quality "Paint" diagram.I don't see a mount with a 80 pitch fine adjust screw for the Bragg angle. You have the loosen, tilt, and clamp and pray, Mount. Those are common in large printers, to save money.I would not ever run a 232-1 without a crystal or 50 Ohm load attached. They are not always unconditionally stable when unloaded and the output transistor will get very hot.I encourage you to open the Driver and trace out the Bias and Input circuit. Its usually quite simple. The Balanced Modulator is the eight legged rectangular can.ISOMET was probably bought by Neos Florida division of Gouch and Housego. Warren the Sales Engineer is usually friendly about AO Data Sheets if that is the case.Steve