I spent a year on the road as a Pulsed Laser FSE and Installer.
Our company policy was to not fly on airlines with our own oscilloscopes. We had enough to carry with the toolkit, laptop, and luggage. Thus I used over 60 different customer supplied oscilloscopes. When installing a Nanosecond system, meaning our output pulse width was 10 nSec or less, we specified a customer supplied 100 Mhz scope as the minimum. A few customers supplied us with 3 Ghz or 10 Ghz scopes. With few exceptions, using our typical issue Thorlabs DET10A 1 Nanosecond photodiodes, a 100 Mhz Scope was enough for these sorts of timing issues.
The major exception was when the user was at a accelerator ring or a large Marx generator and had very critical timing issues. We would ship in our own scopes for that...
Most academic laser users supplied a standard four channel delay generator box to control their experiments. Usually something like a SRS DG535, or a Berkely Nucleonics model 575.. You should have one during product development. Our staff liked units from Quantum Composers and Highland Technology.
Four lessons learned:
A. The quality of the timing generator controlling the experiment is very critical with the scope setup. Many commercial timing generators have double terminated output stages which can cause issues with commercial laser and oscilloscope trigger inputs from many vendors. Most customers purchased off the shelf four channel nanosecond or sub-nano second resolution timing and delay boxes. We had a list of the ones that did not develop their full rated amplitude into a 50 ohm 5 volt input. That list was long. The back termination resistor, which is 50 ohms in series with the output, meant that many of these devices developed a reduced output when ran into a standard 49.9 Ohm load resistor that was the "front end" on all our laser trigger inputs. Yet the back termination was often omitted in the user manual... More then once I had to tell the Customer their TG did not meet it's own specifications.
B. The triggering scheme and trigger input on the oscilloscope determined if I was going to have an easy day, or a day of struggling with system timing setups. If the oscilloscope had a poor trigger section or if I had to use a coaxial attenuator or 50 Ohm termination on a BNC "TEE" to match a standard 1 Meg 20 pF input to the timing signal, then I would often have severe jitter or signal level issues. Make sure the scope has a proper 50 Ohm input for the trigger, or can also be switched to "X" Meg, 20 PF...
Your usually working on a single shot, non repetitive, event with laboratory laser systems. That means the scope's trigger input is where the rubber meets the road.
What I'm trying to say is that if the Timebase section has versatile holdoff and delay controls, an easy and accurate, high resolution way, to set the trigger level, and if it can select rising or falling edge, life is good. Life is better if the trigger system bandwidth remotely matched the A and B input bandwidths. MANY scopes require you to pull a zoom knob to get that last short sweep range, and usually if that is the case, the trigger input will probably not match the scope's bandwidth.
A surprising number of small 100 Mhz and up low end scopes, no matter what vendor, had lousy trigger hardware and even worse trigger software. Some of them had issues with missing the international standard 1 microsecond, 50 ohm, 5V, pulse from the trigger or timing generator. And not just with our internal trigger and sync outputs. Often it did not matter what brand, how much bandwidth was available, or what price was paid for the scope. In many cases the scope designer went cheap in the trigger section.
Especially if the trigger is pulled from the A or B channel and the scope has a BNC for triggering only as an afterthought.
A hint, scopes that let you view the trigger signal input usually have good trigger bandwidth.
C: Make sure your project's timing generator can give you a pre-event trigger signal well in advance of laser firing. It is difficult to trigger solely off your diagnostic photodiode, as the required trigger level can change drastically with output levels that shift all over the place as the PDs output level changes with input intensity. Depending on signal strength at the PD, you can get some very interesting falling edge slopes or pulse stretching or saturation from the PD output... It is a PITA when the customer's system design requires you to trigger off the scattered laser pulse without a pre-trigger, and that is where many scopes made my life difficult.
A few of our installs required me to trigger off the flashlamp plasma or scattered laser light, and inevitably the PD output would be "dirty" in this case. Don't assume the laser's built in diagnostic PD is "Awesome" in bandwidth either.
D. Buy a 50 Ohm 3dB coaxial attenuator and keep it in your toolkit when working on this project.
Moral of the story, get the scope on evaluation loan before you buy, and make damn sure your happy with the triggering section when working with actual laser pulses. A fast ND: YAG laser for this project might be 7 nanoseconds or less, or even in the 100-300 picosecond FWHM range if it has optical pulse compression or is mode locked. Sixty percent of that laser's energy will be in the first 1-2 nanoseconds of the pulse. A nitrogen laser pulse for LIBS is usually 900 picoseconds to 3 microseconds long...
Don't assume repetitive sampling and averaging of ten Hz Laser repetition rates will save you when digging into laser timing issues. Thus storage depth is important. An older 1990s 450 Mhz scope that only has 32 or 64 K of storage depth will not help you in this situation. The ability to view data that is"pre-trigger" is a bonus.
I don't want to pick on one brand, but if I found I was using a 100 or 200 Mhz "Digital Phosphor Oscilloscope" commonly sold at a major discount to academic users, I was having a VERY bad day.
Hope this helps..
Steve
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