Weird PMT Signal: Help Solve Distortion

In summary, the individual is working with a PMT (Hamamatsu R928/0115/0381), PMT power supply (McPherson PMT Power Supply Model #7640), O-scope (Tektronix TDS 5104 Oscilloscope), PMT housing (PhotoCool Housing), and PMT housing power supply (PhotoCool Power Supply). They are using a 200 mW 532 nm ND:YAG laser and a monochromator set to 533nm. The waveform with no laser has normal noise, but when the laser is introduced, there is a large dip and then the noise disappears. This distortion is repeatable but intermittent. Suggestions have been made to use an ND
  • #1
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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  • #2
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.
 
  • #4
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?
 
  • #5
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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1. What causes a weird PMT signal?

There are several factors that could contribute to a weird PMT (photomultiplier tube) signal, such as electrical noise, faulty equipment, or improper calibration. It is important to troubleshoot and eliminate these possibilities before concluding that the signal is truly abnormal.

2. How can I fix distortion in a PMT signal?

The solution to fixing distortion in a PMT signal will vary depending on the specific cause. Some possible solutions include adjusting the gain and offset settings, replacing faulty components, or using shielding to reduce electrical noise. It is important to identify the root cause of the distortion before attempting to fix it.

3. Can the PMT signal be affected by external factors?

Yes, the PMT signal can be affected by external factors such as temperature changes, magnetic fields, and ambient light. These external factors can introduce noise and distortions in the signal, so it is important to minimize their impact by properly shielding the equipment and conducting experiments in a controlled environment.

4. Is there a way to prevent weird PMT signals?

While it is impossible to completely prevent weird PMT signals from occurring, there are steps that can be taken to minimize the chances of it happening. These include properly calibrating and maintaining equipment, using proper shielding, and conducting experiments in a controlled environment.

5. Are there any specific troubleshooting steps for a weird PMT signal?

Yes, there are specific troubleshooting steps that can be taken to identify and address a weird PMT signal. These steps include checking for loose connections, calibrating the equipment, and ruling out external factors. It may also be helpful to consult with experts in the field for further assistance.

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