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Two questions about the dynamics of a laser diode

Shellback

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I am using a laser diode to count aerosol particles flowing through a gas stream. The refracted light is being funneled to a photo-multiplier via fiber optics. When there is no gas flowing (and therefore no particles) we are seeing some strange results from the output signal of the photomultiplier. After troubleshooting this problem for more than a year, I am left to believe that there is something going on with the laser diode that I am not able to capture. Is it possible that the light being emitted from the diode is shifting around and causing slight changes in the focal point? What I'm seeing, electrically, is an occasional burst of very small square waves coming from the PMT (photomultiplier). Sometimes it'll last for less than a minute, sometimes it'll last for more than an hour. It starts as occasional spikes (either positive or negative) which get wider and more frequent until the square wave develops and then fades out to more spikes inverse to what it started as. This is my first foray into working with lasers after 20 years with electronics. A more optically experienced hand would be very appreciated.

I would also like to know if the Iop of a diode is posted at a given mA, is it advisable to use it at that current, or should it be operated somewhere between the posted operating current and the threshold current?
 

MarioMaster

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I see no reason why a laser would emit random square waves, maybe you should check out the power supply for the laser diode?

A thermally stable laser diode really shouldn't change in output unless the mirror facets are on the way out. (IE, optical damage from being overdriven)

You can drive a laser diode anywhere from the threshold to the max normal drive current, although it will be more unstable if you're right at the threshold.
 

Eudaimonium

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Laser diode will increase in wavelenght as the temperature of the dye rises, as you probably know. However, that should cause the change to be pretty linear, instead of lasting few minutes to an hour.

If your diode is being cooled by some sort of setup with a feedback, make sure that the theshold of the feedback is lower, and it should kick in earler, before the weird readouts start.

If the diode is cooled without any feedback loops, then... Iono, check your cooling setup to make sure it's stable all the way? No other ideas come to mind.
 

Shellback

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I see no reason why a laser would emit random square waves, maybe you should check out the power supply for the laser diode?

A thermally stable laser diode really shouldn't change in output unless the mirror facets are on the way out. (IE, optical damage from being overdriven)

You can drive a laser diode anywhere from the threshold to the max normal drive current, although it will be more unstable if you're right at the threshold.
Thanks for your reply

I've monitored the input and output power of the driver board, various power supplies, bias voltage for the PMT. I've monitored at low speed over a 24 hour period, and at extremely high speeds (too fast to reasonably log the data) during some of these episodes. I have gone over everything electrical in this set-up. The thing is, I'm looking at counting electrical spikes caused by sub-micron aerosol particles. This only produces a mV change in voltage. Through troubleshooting this problem, I've managed to get my noise contamination down to microvolts. We have also seen this with numerous diodes over the course of several years. With the main beam being aimed at the edge of a mask, just the slightest change in focus would show up in the signal from the PMT. That is why I'm wondering about the diode. I know the beam being emitted is very consistent in brightness, but is it possible that the point on the side of the diode where the photons are actually leaving the substrate could be moving slightly. Or, perhaps I don't have a substantial enough heat sink. Would that cause the wave forms as described?
 

Eudaimonium

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Well, if the temperature of the diode changes at all, then yes it's possible it could produce it.

Question is, how much of a temperature change would cause the readout error you are having - and that I cannot give the answer to.

So, it would be best if you first make sure that the diode's temperature is perfectly stable. See if we can't work from there.
 

MarioMaster

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Thanks for your reply

I've monitored the input and output power of the driver board, various power supplies, bias voltage for the PMT. I've monitored at low speed over a 24 hour period, and at extremely high speeds (too fast to reasonably log the data) during some of these episodes. I have gone over everything electrical in this set-up. The thing is, I'm looking at counting electrical spikes caused by sub-micron aerosol particles. This only produces a mV change in voltage. Through troubleshooting this problem, I've managed to get my noise contamination down to microvolts. We have also seen this with numerous diodes over the course of several years. With the main beam being aimed at the edge of a mask, just the slightest change in focus would show up in the signal from the PMT. That is why I'm wondering about the diode. I know the beam being emitted is very consistent in brightness, but is it possible that the point on the side of the diode where the photons are actually leaving the substrate could be moving slightly. Or, perhaps I don't have a substantial enough heat sink. Would that cause the wave forms as described?
It's possible you may have some kind of oscillation in the non-lasing parts of the diode die, but I'd mark that up to just some kind of unusual manufacturing phenomenon if it does. As long as you've got a decent heatsink and everything reaches thermal equilibrium, thermal issues shouldn't be a problem. I'd also try a rigidly fixed glass lens if you're currently using a plastic one.
 
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Shellback,

If you are confident in your diode supply and temperature regulation, then perhaps you are experiencing mechanical vibrations. Is it possible that mechanical vibrations are moving your focal point relative to the mask?

What kind of structure is your experiment operating on? Are you on a vibration-isolating table? In the basement of your building? Are there trucks passing by on the road outside? Footsteps in the hallway?

Cheers
 

Shellback

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Laser diode will increase in wavelenght as the temperature of the dye rises, as you probably know. However, that should cause the change to be pretty linear, instead of lasting few minutes to an hour.

If your diode is being cooled by some sort of setup with a feedback, make sure that the theshold of the feedback is lower, and it should kick in earler, before the weird readouts start.

If the diode is cooled without any feedback loops, then... Iono, check your cooling setup to make sure it's stable all the way? No other ideas come to mind.
Unfortunately, we have no options (that we're aware of) for actively cooling the diode. We have the diode mounted to a pigtail between an aluminum collar and a steel,threaded containing ring that screws into the lens housing and holds everything in compression and is very stable. I have also added a couple of extra threaded rings to act as jam-nuts to keep everything in place.

The problem never actually goes away, but does get better or worse when we change diodes and that's what I keep coming back to. There has to be some characteristic to the diode itself that is causing this.
 
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How are your fibers mounted? Can they create any kind of a feedback to the laser and cause instabilities? Did you try a different detector like a photodiode. Did you try to place a beamsplitter with a photodiode to monitor the laser output on the oscilloscope?

Any pictures of the whole system would be helpful too.
 

Shellback

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How are your fibers mounted? Can they create any kind of a feedback to the laser and cause instabilities? Did you try a different detector like a photodiode. Did you try to place a beamsplitter with a photodiode to monitor the laser output on the oscilloscope?

Any pictures of the whole system would be helpful too.
I have very limited space available for added equipment. I have suggested a beam splitter as you mentioned, but was shot down because of available space. This is all going into a rather small explosion proof enclosure. We went with a PMT because of the required sensitivity. A photodiode will not give the kind of signal to noise ration that I need. We are coming from the diode and going through a small micron orifice then through an ND Filter, then through a quartz tube then through two lenses (one to collect the scattered light and the other to focus that to the fiber), then through the fiber to the PMT. The laser is mounted such that it floats and even when sitting on a table next to my desk (away from all heavy equipment in the field) we will get these wave forms. It very nearly has us stumped. My next step is examining the frequency spectrum to see if there's something hidden in the noise that can help me isolate the problem, but I still have a little equipment gathering to do.
 

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Several more questions:
Is a laser diode power controlled by any feedback? Can you switch it to a plain current driven circuit? May you remove a fiber and place a PMT directly into the beam? What is a fiber size? Maybe a coupling is not good. Do you have any oscilloscope waveforms to show with a noisy behavior?

Depending on a laser diode type you may have a photodiode in the can already. Any way to put a wire there and to connect to a scope?
 

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Umm... I know you have gone in detail about eliminating noise, but I have to ask: Have you considered cell phones? The pattern you are describing sounds very much like the pattern of interference caused by cell phones.


I have another idea along another track. The photo-multiplyer you are using to detect: Does it have any sort of delimiter or check sum in the data stream? Are you sure the baud rate isn't getting slightly off? It might be something as simple as a slightly slower clock.

I've worked with a lot of sensor data streams and they all have their bugs, you have to error check each and every one of them.
 

Shellback

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Several more questions:
Is a laser diode power controlled by any feedback? Can you switch it to a plain current driven circuit? May you remove a fiber and place a PMT directly into the beam? What is a fiber size? Maybe a coupling is not good. Do you have any oscilloscope waveforms to show with a noisy behavior?

Depending on a laser diode type you may have a photodiode in the can already. Any way to put a wire there and to connect to a scope?
1) Is a laser diode power controlled by any feedback?
The laser driver board has a current monitor built in to the circuitry. We set it per laser diode that it powers. Each diode comes in a package that is labeled with the values for that particular diode.

2) Can you switch it to a plain current driven circuit?
The driver board we are using has two options. One option is for a power controlled output (APC) the other is for current controlled output (ACC). Currently, we are using ACC. However, as I'm typing this, I'm wondering why not power control. I know the voltage output should be steady at 2.5V, and I also know what the current is for the diode, it is only simple math to find the desired power. Maybe, if I control the power, I can use one value to compensate for any fine adjustments the other needs to make. But anyway, this is just me thinking aloud.

3) May you remove a fiber and place a PMT directly into the beam?
We have thought about doing that, but the guy we have who designed this says that the fiber is actually serving more of a purpose than just moving the light to the PMT. It is manipulating the signal in some way. I would like to cut out the fiber all together, but that is not an option right now.

4)What is a fiber size?
Core diameter is 600um. The charts for the fiber we're using show that it has an attenuation rate of 3dB/km at the wavelength we're using (and we're using maybe half a meter so the attenuation of the fiber is almost nothing). I have noticed that if the fiber connections are loose, there is a terrible signal, but the wave-form looks nothing like the problem I'm working on.

5) Do you have any oscilloscope waveforms to show with a noisy behavior?
Attached. The image with spikes is how the waveform starts and ends, the one with the square waves is how it looks at it's peak.

6)Depending on a laser diode type you may have a photodiode in the can already. Any way to put a wire there and to connect to a scope?
This LD does not have a MPD built in. I really wish it did, that would certainly help with the issues. The laser was built around the purpose. The wavelength is optimal for this application given the size constraints. The power of the laser is also optimal for the application. These two things are what have driven the size of the apparatus and the LD that was chosen. Measuring sub-micron aerosol particles in a high pressure gas stream at 3 L/min is not as easy as one might think.
 

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Please do me a favour and shake or move a fiber with your hand. Do you get a same or similar noise?

The noise looks like a "green noise" for a DPSS green laser. However, you say it is a laser diode :undecided:

As Ablaze wrote, check all kind of electrical interference, noises, grounds etc. What is a PMT voltage? Can it mess up with a laser diode circuit?
 
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Shellback

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Umm... I know you have gone in detail about eliminating noise, but I have to ask: Have you considered cell phones? The pattern you are describing sounds very much like the pattern of interference caused by cell phones.


I have another idea along another track. The photo-multiplyer you are using to detect: Does it have any sort of delimiter or check sum in the data stream? Are you sure the baud rate isn't getting slightly off? It might be something as simple as a slightly slower clock.

I've worked with a lot of sensor data streams and they all have their bugs, you have to error check each and every one of them.
1) Have you considered cell phones?
Yes, we have considered this. Unfortunately, I have recorded these issues over night when no one has been here. Also, we barely get any kind of reception here at all from the tower, so I don't think that is the culprit. Also, the problem stays the same (not better or worse) regardless of location. We have also looked into possible interference from the AC compressors, the industrial air compressor on the other side of the wall from me. A combination of any and all. I've even looked into tracking the brightness of the overhead lighting. All to no avail.

2) Does it have any sort of delimiter or check sum in the data stream?
The PMT we are using has a basic power supply and a vacuum tube in it. It is a simple analog device that takes +/-15VDC and a very low bias voltage (0.3VDC is what we're using) and converts very low light levels (on the order of nW) to an analog signal from 0.002V - 10V. It is this extreme sensitivity that allows us to see the photons refracted from a 600nm particle. It is also the extreme sensitivity that is why I think very minuscule changes in the laser would prompt such issues for me. We are wiring the analog signal to one of the AI channels on the sbRIO-9631 and sampling that signal every 8 microseconds.
 

Shellback

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Please do me a favour and shake or move a fiber with your hand. Do you get a same or similar noise?

The noise looks like a "green noise" for a DPSS green laser. However, you say it is a laser diode :undecided:

As Ablaze wrote, check all kind of electrical interference, noises, grounds etc. What is a PMT voltage? Can it mess up with a laser diode circuit?
1) Do you get a same or similar noise?
I have attached a sample of the signal that is generated when bumping the fiber at the connections. What you are looking (and what almost has an impact on what I'm doing) is the vibration from this impact. My sample rate is too high to consider movement of the fiber to be an issue. The samples I'm showing you are being taken at 50,000 samples/second. The unit that I'm working on is taking 125,000 samples/second. We are taking these samples in 1000 sample blocks and analyzing them. Taking the 1000 sample block, we are finding the baseline value in the noise and setting our first threshold a calibrated value above that. Any peak rising above that calibrated value is counted. A 60Hz interference signal is not going to happen at a high enough rate to impact my process, but these square waves are.

We are using a near IR laser diode and the PMT is designed with a peak amplification value of 10nm above our current wavelength.
 

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