Weird PMT Signal: Help Solve Distortion

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Discussion Overview

The discussion revolves around the distortion observed in the signal from a photomultiplier tube (PMT) setup when using a 532 nm ND:YAG laser with a monochromator. Participants explore potential causes of the distortion, including saturation and the characteristics of the PMT and its amplification system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the observed waveform distortion when the laser is used, noting a significant dip in the signal and the disappearance of noise.
  • Another participant suggests that saturation of the PMT may be occurring due to the high power of the laser and the narrow wavelength selection of the monochromator.
  • Concerns are raised about the PMT's automatic gain control (AGC) potentially affecting the signal modulation.
  • A participant points out that most PMTs produce a negative output and emphasizes the implications of using a monochromator with a high-power laser.
  • Calculations regarding the maximum input wattage for the PMT are discussed, indicating that the current setup may exceed the PMT's capacity.
  • Suggestions are made to reduce the laser power using a beam splitter or lens to spread the beam before it enters the monochromator.
  • One participant questions the effectiveness of chopping the beam, suggesting it may not reduce the overall power but only the duration of exposure.

Areas of Agreement / Disagreement

Participants express differing views on the cause of the distortion, particularly regarding the role of saturation and the PMT's characteristics. There is no consensus on the best approach to resolve the issue, with multiple competing suggestions being offered.

Contextual Notes

Limitations include potential assumptions about the PMT's behavior under high power and the specific characteristics of the monochromator setup. The discussion does not resolve the mathematical steps involved in determining the maximum input wattage for the PMT.

CasualDays
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Right now I am working with:

PMT: Hamamatsu R928/0115/0381
PMT Power Supply: McPherson PMT Power Supply Model # 7640
O-Scope: Tektronix TDS 5104 Oscilloscope
PMT Housing: PhotoCool Housing
PMT Housing Power Supply: PhotoCool Power Supply
Input: 200 mW 532 nm ND:YAG laser

I have pictures uploaded but I cannot post them until I have 15 posts. Sigh.

Here is the problem:

We are selecting the wavelength through a monochromator. We have the wavelength of the monochromator set to 533NM. We have been able to see some signal from this wavelength setting on the monochromator in the past. So, with a shutter, I took a picture of the waveform with no laser and with laser. Both tests were performed at 935 volts. The first picture shows a normal amount of noise. The second picture however, with the ND:YAG, has a huge dip and then levels off. The noise disappears in this second waveform. This distortion is repeatable, but has been intermitent.

Saturation has been suggested. It does not seem entirely possibly but right now because of the monochromator, but the current plan is to ND filter the laser and see if any noticeable difference occurs. I also have a chopper lined up to attempt to properly generate a useable signal.

Anyone have any other suggestions on what may be causing distortion with this PMT setup?
 
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CasualDays said:
I have pictures uploaded but I cannot post them until I have 15 posts. Sigh.

Send me the link via PM, and I'll post it.
 
CasualDays said:
Here is the problem:

We are selecting the wavelength through a monochromator. We have the wavelength of the monochromator set to 533NM. We have been able to see some signal from this wavelength setting on the monochromator in the past. So, with a shutter, I took a picture of the waveform with no laser and with laser. Both tests were performed at 935 volts. The first picture shows a normal amount of noise. The second picture however, with the ND:YAG, has a huge dip and then levels off. The noise disappears in this second waveform. This distortion is repeatable, but has been intermitent.

Saturation has been suggested. It does not seem entirely possibly but right now because of the monochromator, but the current plan is to ND filter the laser and see if any noticeable difference occurs. I also have a chopper lined up to attempt to properly generate a useable signal.

Anyone have any other suggestions on what may be causing distortion with this PMT setup?

What about the PMT amp itself? Does it maybe have automatic gain control (AGC) that is modulating with the strength of the receive signal?
 
For a start, most pmt's will give a negative output.
secondly, you are using a monochromator to view the light signal...so in effect you are allowing nearly all of your laser through unto the pmt...
the rated wattage of your laser is 200mW and its seeing nearly all of it with the exception of the small sidebands you get with lasers and how well they are tuned...
your pmt at that power is absolutely bound to saturate in my opinion...

consider that if you had a small light source running through your monochromator, then the power would be distributed amongst the differing wavelengths...here you are confining the power to less than a few nanometers...

looking at the datasheet for the type...
http://209.73.52.252/assets/pdf/parts_R/R928.pdf

i'd say that you have exceeded the max power per lumen that the anode can deliver which would cause saturation...

The quantum efficiency of the wavelength would make it less because of its peak at 400nm...the chart reveals somewhere around 55%, really its less as at 400nm is not quite a 100% so it would be best to do the ratio properly but I'm just doing it roughly...

from here we can see how to calculate the maximum input wattage.
http://books.google.co.uk/books?id=...=9TSE4i6tZsK3Cip1-JFTWuC-c24&hl=en#PPA286,M1"

which is the anode current divided by the maximum anode sensitivity (radiant)
= .1/(roughly 500000) = 0.2uW

and you are using 200mW...

chopping is not going to reduce the power...just the time spent...

i'd consider trying to either reduce the beam power by using a beam splitter or going for a lens and spreading the beam somewhat in front of the chromator.
To tell the truth, a beam splitter might only bring it down to 5% which is still much higher...
maybe by scatter would be a decent solution??

hope this helps.
 
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