User:Pranav Rathi/Notebook/OT/2011/08/15/AOM oscillation problem

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!--now realigning the laser so we can continue the experiments--

This document is to help in understanding and investigating the problem with AOM. It contains some information about the investigation we did and our conversation with NEOS tech (Warren Seale) regarding the problem.

The major and only problem with the AOM is having oscillations in the first order beam when the intensity is instantaneously modulated in front of it OR input-RF signal to AOM is changed. As can be seen through the picture above; sum (lower part) is the overall amplitude and in upper part there are two curves; the red one is y-signal and white is x-signal from QPD. X-signal is also in the transverse direction to beam propagation and parallel to the ground in the AOM crystal plane. When we change the RF voltage from low to high we get this oscillation which seems to be periodic in x-signal and also in y-signal to some extent. This oscillation is also seen when only the laser intensity in front of the AOM is instantaneously changed keeping the RF constant. This oscillation damps pretty quickly, but its a problem for our experiments. This started happening sometime in November last year. I did a full investigation of this issue and reached the following:

1) Is it the setup? To investigate that I did put laser-AOM-some ND filters and the detector (QPD). No other object was in the setup besides driver, AOM and QPD (with some ND filters). The oscillation is still there. I also moved the beam across at the input plane of the crystal, just to see it we its distance dependent from the transducer. But the oscillation is still generated when either the RF OR the intensity is changed.

2) Is it the detector? To investigate that I removed the AOM and changed the intensity if front of the detector as I did earlier, but no oscillation, so the detector is fine.

3) Is it the driver? To investigate that; I put driver RF-output to 30db attenuator and then the oscilloscope. The RF output from the driver is terminated into 30ohm. As a result; 2-sets of 3-pictures are attached. First set shows the RF-output in a form of (some what) sine-wave at the oscilloscope at 1, 2, and 5 (pictures are labeled exactly) volts of analog input for RF. As it can be seen clearly that RF-output is not an exact sine wave which I think it suppose to be. Now in next set we used a low pass filter with 140MHz upper limit in between the attenuator and the oscilloscope. The wave-form is tremendously improved. A comparison is shown below:

So it looks like that the driver is fine.

4) Any AM on the carrier that is at the same frequency as the oscillation? To investigate that: I repeatedly changed the input voltage for RF input and watched the corresponding amplitude on the oscilloscope, but I did not see any AM. There were some fluctuations in the amplitude over time but they were little. There was nothing instantaneous (in amplitude) as we change the input RF voltage. So I tried something else to check the AOM: I put a variable attenuator (6db to 30db) in between the RF out of the diver and AOM. I put the diver at 5 volts all the time and modulate the input RF power for AOM through the attenuator. The result shows the similar oscillations when you vary the attenuation (the input RF power for AOM). So it means that the driver is fine, because I never changed the input voltage for RF at the driver, all I did was, changed the RF-power input to AOM.

5) Investigation of input analog signal to driver: Analog RF-input signal for driver, form the computer is DC and we change it in steps and it looks perfectly fine.

6) Does oscillation depend on the rate of RF power to AOM; I notices one thing: oscillations depends on the rate of change of RF-power to AOM. I put the driver in CW mode. I had the attenuator in between FR-power out and AOM. So I modulated the intensity at several rates. And I noticed at slower rate the oscillation was less prominent and peaks were farther apart. At higher rate oscillations were more prominent and peaks were closer. But oscillations never go away while modulating the RF-power to AOM regardless the rate.

7) What if the RF output to AOM is kept constant and the intensity is changed instantaneously in front of the AOM? To investigate that: I kept the AOM on CW mode and block/unblock the laser intensity in front of the AOM. I see the same oscillation when the intensity is changed from high to low or low to high. I do not understand this because in this experiment no part of AOM or driver is involved, because the intensity is changed by an external object (filter or a shutter) and I still see the oscillation.

8) Could it be the cables? No, I used the cables (BNC) of different type and length and no difference.


My email conversation with Warren Seale is attached below: Hi Pranav,

We're not sure if replacing the AOM will fix the problem. We've not seen this problem before with any other application. If you buy a new AOM we cannot guarantee that the problem will go away.

If you want us to test the AOM/driver to see if we can observe the problem then maybe we can identify the cause. There will be a $75 evaluation fee to do this.

Let us know what you would like to do.


Regards, Warren


________________________________________ From: PRANAV RATHI [1] Sent: Tuesday, August 09, 2011 3:10 PM To: Warren Seale Subject: Re: AOM Quote Hey Warren, I think you might have the detail of our conversation about the issue. If not than you can get it bellow (I am attaching the email-talk we had). In brief it is not the setup, driver or anything else. The oscillations are generated when the laser power is changed in front of the AOM and keep everything same. The oscillations are last only for few seconds. And I have done this when we had Laser AOM and QPD and nothing else in the setup. So it is not the Laser OR QPD because I checked them. It is not the driver because I saw its signal on oscilloscope and it looks fine. Thanks Warren



Hi Pranav,

What were the various rates of change that you tried? What is the period or frequency of the oscillation when you do this?

What happens to the oscillation frequency when you change cable lengths between the RF output of the driver and the AOM?

Regards, Warren


________________________________________ From: PRANAV RATHI [2] Sent: Wednesday, January 26, 2011 12:42 PM To: Warren Seale Subject: RE: AOM oscillation Thanks Warren. Oscillation comes no matter what.

  • (second question) Analog RF-input signal form the computer is DC and we change it in steps and it looks perfectly fine.
  • (first question) I notice one thing: oscillations depend on the rate of change of RF-power to AOM. I put the driver in CW mode. I had the attenuator in between FR-power out and AOM. So I modulated the intensity at several rates. And I noticed at slower rate the oscillation was less prominent and peaks were farther apart. At higher rate oscillations were more prominent and peaks were closer. But oscillations never go away while modulating the RF-power to AOM regardless the rate.

PR --- On Wed, 1/26/11, Warren Seale <WSeale@goochandhousego.com> wrote:

From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Scott Soden" <ssoden@goochandhousego.com>, "Jim Dooley" <jdooley@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Wednesday, January 26, 2011, 5:14 AM Hi Pranav,

Does this behavior still exist if you change the rate at which the amplitude is changed?

What does the analog modulation signal look like that is coming from your electronics?

Regards, Warren


________________________________________ From: PRANAV RATHI [3] Sent: Tuesday, January 25, 2011 7:55 PM To: Warren Seale Subject: RE: AOM oscillation Thanks Warren. I have been doing the same; all the pictures are taken that way only. In the setup I have just the driver-AOM-and the detector. So there is no other active or passive device in the setup. Pranav

--- On Tue, 1/25/11, Warren Seale <WSeale@goochandhousego.com> wrote:

From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Scott Soden" <ssoden@goochandhousego.com>, "Jim Dooley" <jdooley@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Tuesday, January 25, 2011, 2:14 PM Hi Pranav,

The AOM itself would not manifest this type of behavior. However, if you want to check it you can remove the AOM and driver from the system and look at the diffracted light with your detector by itself. Operate the driver in the same manner.

Regards, Warren


________________________________________ From: PRANAV RATHI [4] Sent: Tuesday, January 25, 2011 2:26 PM To: Warren Seale Subject: RE: AOM oscillation Thanks Warren. I tried what you said. I repeatedly changed the input voltage for RF input and watched the corresponding amplitude on the oscilloscope, but i did not see any AM. There were some fluctuations in the amplitude over time but they were little. There was nothing instantaneous (in amplitude) as we change the input RF voltage. So i tried something else to check the AOM: I put a variable attenuator(6db to 30db) in between the RF out of the diver and AOM. I put the diver at 5 volts all the time and modulate the input RF power for AOM through the attenuator. The result (attached) shows the similar oscillations when you vary the attenuation ( the input RF power for AOM). So it means that the driver is not the problem, there is something wrong with the AOM, because i never changed the input voltage for RF at the driver, all i did was, changed the RF-power input on AOM. What do you say? Pranav

--- On Mon, 1/24/11, Warren Seale <WSeale@goochandhousego.com> wrote:

From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Jim Dooley" <jdooley@goochandhousego.com>, "Scott Soden" <ssoden@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Monday, January 24, 2011, 5:16 AM

Hi Pranav,

What you are seeing are higher harmonics of the 80 MHz RF signal. This is normal and not a problem because the AOM will only respond to the 80 MHz part of the carrier content. What we need to see is if there is any AM on the carrier that is at the same frequency as the oscillation you have been observing.

Regards, Warren


________________________________________ From: PRANAV RATHI [5] Sent: Friday, January 21, 2011 6:51 PM To: Warren Seale Subject: RE: AOM oscillation Hi Warren,

I did the exact; I put driver RF-output to 30db attenuator and than the oscilloscope. As a result; 2-sets of 3-pictures are attached. First set shows the RF-output in a form of (some what) sine-wave at the oscilloscope at 1, 2, and 5 (pictures are labeled exactly) volts of analog input for RF. As it can be seen clearly that RF-output is not a exact sine wave which i think it suppose to be. Now in next set we used a low pass filter with 140MHz upper limit in between the attenuator and the oscilloscope. The wave-form is tremendously improved.

So does it mean that there are higher frequency components present with 80 MHz driving frequency? because it looks like. And are we getting the oscillation because of that; there might be other frequencies beating inside the crystal? OR its just the AOM the bad boy? (the pictures are labeled accordingly to guide that at what input voltage with/without filter it is taken) Thanks, Pranav

--- On Thu, 1/20/11, Warren Seale <WSeale@goochandhousego.com> wrote:


From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Scott Soden" <ssoden@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Thursday, January 20, 2011, 2:03 PM Hi Pranav,

It would be safe to use a 30 dB RF attenuator between the driver and the scope. The driver generates 2W.

Regards, Warren


________________________________________ From: PRANAV RATHI [6] Sent: Thursday, January 20, 2011 3:48 PM To: Warren Seale Subject: RE: AOM oscillation thanks Warren, can you please suggest that how much attenuation do i need, i have regular tektronix oscilloscope. Or what is the RF output power by the driver so i can figure out the attenuation. PR

--- On Thu, 1/20/11, Warren Seale <WSeale@goochandhousego.com> wrote:

From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Jim Dooley" <jdooley@goochandhousego.com>, "Scott Soden" <ssoden@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Thursday, January 20, 2011, 1:19 PM Hi Pranav,

The next step is to look at the RF signal out of the driver to see if there is any amplitude modulation on the RF carrier. When you do this be sure that the RF output is terminated into 50 ohms and that the scope can handle the RF power level.

Regards, Warren


________________________________________ From: PRANAV RATHI [7] Sent: Thursday, January 20, 2011 3:08 PM To: Warren Seale Subject: RE: AOM oscillation Dear Warren, I did exactly; i put laser-AOM-some ND filters and the detector. The oscillation is still there (see the results). I also moved the beam across at the input plane of the crystal, just to see it we its distance dependent from the transducer. But the oscillation is still there. What do you suggest? PR

--- On Thu, 1/20/11, Warren Seale <WSeale@goochandhousego.com> wrote:

From: Warren Seale <WSeale@goochandhousego.com> Subject: RE: AOM oscillation To: "PRANAV RATHI" <pranav_rathi@yahoo.com>, "Scott Soden" <ssoden@goochandhousego.com>, "Jim Dooley" <jdooley@goochandhousego.com>, "GHFlorida" <GHFlorida@goochandhousego.com> Date: Thursday, January 20, 2011, 5:18 AM Hi Pranav,

We recommend checking the AOM and driver by itself with no other parts of your system involved. Look at the diffracted light with your detector as the modulation input of the driver is varied. Let us know what you find.

Regards,

Warren Seale Acousto-Optic Product Manager for the Americas Gooch & Housego 4005 Opportunity Drive, Melbourne, FL 32934, USA Tel: 321-242-7818 ext 17 Fax: 321-242-1019 wseale@goochandhousego.com www.goochandhousego.com www.neostech.com

"CONFIDENTIALITY NOTICE: This e-mail transmission and any attachments are hereby labelled confidential and is subject to the terms of a Nondisclosure Agreement where a Nondisclosure Agreement is in effect. The information is only for the use of the individuals or entity to whom it is intended even if addressed incorrectly. If you have received this e-mail in error, please immediately notify the sender by e-mail at the address shown. Please delete it from your files if you are not the intended recipient."



________________________________________ From: PRANAV RATHI [8] Sent: Wednesday, January 19, 2011 5:47 PM To: NEOS Subject: AOM oscillation Dear NEOS Tech. We bought following items: AOM item # R12D2404; R23080-2-1.06-LTD, serial # 138252 ADS item # 12D2752; 12080-2AS, serial # 149474 on 01/11/10 with customer po TB1352. We are having a problem: when we change the RF-input voltage from somewhere low to high. We get this oscillation in the first-order diffracted beam. We are using the AOM for power modulation and a QPD as a detectorfor IR 1064 2W laser. As you can see in the picture (attached) sum (lower part) is the overall amplitude and in upper part there are two curves; the red one is y-signal and white is x-signal from QPD. X-signal is also in the transverse direction to beam propagation and parallel to the ground in the AOM crystal plane. When we change the RF voltage from low to high we get this oscillation which seems to be periodic in x-signal and also in y-signal to some extent. This oscillation damps pretty quickly, but its a problem for our experiments. This oscillation just started coming in for few months. Can you please shed some light on it? And how do we get rid off it OR Is some thing wrong with AOM or the driver? I appreciate you help and thank you in advance for your time. Sincerely, Pranav Rathi University of New-Mexico-CHTM 505-453-6203





________________________________________ From: Warren Seale <WSeale@goochandhousego.com> To: PRANAV RATHI <pranav_rathi@yahoo.com>; Jim Dooley <jdooley@goochandhousego.com>; GHFlorida <GHFlorida@goochandhousego.com> Sent: Tuesday, August 9, 2011 12:49 PM Subject: RE: AOM Quote Hi Pranav,

Were there any changes made to your application when the oscillation appeared? Was it working OK then suddenly started to do this?

Regards,

Warren Seale Acousto-Optic Product Manager for the Americas Gooch & Housego 4005 Opportunity Drive, Melbourne, FL 32934, USA Tel: 321-242-7818 ext 17 Fax: 321-242-1019 wseale@goochandhousego.com www.goochandhousego.com www.neostech.com

"CONFIDENTIALITY NOTICE: This e-mail transmission and any attachments are hereby labelled confidential and is subject to the terms of a Nondisclosure Agreement where a Nondisclosure Agreement is in effect. The information is only for the use of the individuals or entity to whom it is intended even if addressed incorrectly. If you have received this e-mail in error, please immediately notify the sender by e-mail at the address shown. Please delete it from your files if you are not the intended recipient."


________________________________________ From: PRANAV RATHI [9] Sent: Tuesday, August 09, 2011 2:01 PM To: NEOS Subject: AOM Quote Dear NEOS Tech. We bought following items: AOM item # R12D2404; R23080-2-1.06-LTD, serial # 138252 ADS item # 12D2752; 12080-2AS, serial # 149474 on 01/11/10 with customer po TB1352.

The AOM has started having weird oscillation problem. We would like to get it fixed or buy a new AOM for the same driver. Can you please Quote me on fixing it and buying it new? Thank you, Pranav CHTM University of New mexico

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