Oscilloscope for LIBS - Page 1
Forum section: electronics / projects

author: ehilarioc
author_rank: Contributor
timestamp: October 05, 2015, 10:35:00 pm
content: Hello everyone,At work my boss told me we need to buy an oscilloscope as a tool to develop Laser Induced Breakdown Spectroscopy. So far I've done some research and the fastest stuff I need to measure are optical pulses in the range of 1-20ns (I will sense them with an avalanche photodiode).A Rigol DS4054 will be good for the proyect? Or is a bit of an overkill. I was planning to tell my boss to buy the DS4014 and unlock them by steps if needed. However if you people can suggest a better suited equipment, for less than $2500, that would be great.Edwin

author: ConKbot
author_rank: Super Contributor
timestamp: October 06, 2015, 12:05:27 am
content: Few thingsA) what's the rise time of the pulse, and how accurate does it need to be? A 1 nanosecond pulse with 100ps edges, is gonna need a good chunk of bandwidth. 500ps edges won't be as bad.B) is the event repeating? Do you have a stable trigger? If you have a stable trigger, and the jitter of the detector and the event is small enough, you can use equivalent time sampling (sometimes just "sampling", but don't mix it up with "real time sampling") which drops your scope requirements a lot.

author: ehilarioc
author_rank: Contributor
timestamp: October 06, 2015, 05:58:58 pm
content: Well, LIBS says that the generated optical pulses are between 1-20ns, however I doubt that the setup I'll use will be capable to generate a 1ns optical pulse. That being said, and after looking at:I see the practical formula trise=0.4/BW, with a 500Mhz Oscilloscope will give me 800ps of rise time. What is more, in the video is shown that the 500Mhz oscilloscope actually had 769Mhz Bandwidth.Also, I saw that high speed 1064nm photodetectors have 1ns rise times. I think I made a good choice in selecting the DS4014, if the optical pulse I manage to get is wide enough, maybe I only need to upgrade it up to 350Mhz and get good measurements.I don't have a stable trigger so equivalent time sampling is discardedDoes the MrKrabs upgrade file still works? you know, to unlock the 500Mhz bandwidth.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 07, 2015, 12:28:28 am
content: 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

author: liquibyte
author_rank: Frequent Contributor
timestamp: October 07, 2015, 02:24:58 am
content: See, this is why I love coming here. Some of you guys have absolutely fascinating jobs, makes me wish I could start over again.

author: tggzzz
author_rank: Super Contributor
timestamp: October 07, 2015, 08:37:08 am
content: Quote from: LaserSteve on October 07, 2015, 12:28:28 amThank you for enumerating those excellent points. I've linked to your post as it is a good set of examples about why triggering is often more important than equipment's headline specifications.I wish that everybody recommemnding or reviewing oscilloscopes would understand and address those points before putting pen to paper (oops, fingers to keyboard).

author: LaserSteve
author_rank: Super Contributor
timestamp: October 07, 2015, 02:57:44 pm
content: TGGZZZ,Thank you for the complement.I rarely worry about rated bandwidth or impulse response when I read a scope review. I'm far more concerned about trigger sensitivity, trigger jitter, and trigger response at bandwidth extremes. I also check and see the trigger properly fires on the first pulse and correct slope when two sharp pulses are close together. Then I try to set the triggering to ignore the first pulse and properly display the second.One day I'll draft a post on how to really evaluate triggering. I need to think of some low cost ways to do that.Building two of the Jim Williams Avalanche Circuits, and using their adjustable trigger input delay might be a way to accomplish that task.Old Berkeley 7010 delay generators show up on Ebay for as little as 20$. All Scope evaluators should have one...Steve

author: dan3460
author_rank: Frequent Contributor
timestamp: October 07, 2015, 05:18:31 pm
content: Quote from: liquibyte on October 07, 2015, 02:24:58 amI wanted to go into electronics when I was a boy in my native country, life got in the way. So, now I make the magic smoke leave most of my components.

author: liquibyte
author_rank: Frequent Contributor
timestamp: October 07, 2015, 05:57:17 pm
content: Quote from: dan3460 on October 07, 2015, 05:18:31 pmYou and I are in the same boat. I tend not to blow things up these days but I cheat by simulating everything I can first.

author: ehilarioc
author_rank: Contributor
timestamp: October 09, 2015, 05:07:56 am
content: Quote from: LaserSteve on October 07, 2015, 12:28:28 amWow, those are great lessons man. Thanks a lot!!!. the Rigol DS4054 has enough bandwidth and 140Mpts of memory, I think that'll do the job.I actually have to design the control (temperature, current and pump trigger) and delay circuits for the whole laser head to trigger the spectrometer in the best time possible. I even have to design the optical system. The idea I have is to use a passive Q-switched, side pumped, ND:YAG crystal to generate the high power the optical pulse at 1064nm. Have you ever seen something like that?Thanks a lot again.Edwin

author: LaserSteve
author_rank: Super Contributor
timestamp: October 09, 2015, 02:03:38 pm
content: I'm a academic laser technician with several years on the road as a laser service engineer and laser show guy as well. Passive Q-switches are good up to a certain level of power, maybe 7-10 millijoules. If you can live with the timing jitter and with the fact that under certain sparse drive conditions they multi-pulse. If your building your own electronics you can "learn" where in the pump diode or flashlamp curve the laser will pulse in software.Then you find active EO Q-switches are more fun to use, not that expensive over the life of an instrument, are compact, and can produce really good results.Are you planning on flashlamp pumping or laser diode pumping? I've used both side and end pumped lasers, I have a preference for end pumping when using diode pumped systems for short pulses. This is due to thermal lensing and polarization sensitive pump adsorption in certain crystals. Again, it all depends on the targeted pulse rate and design energy.For low rep rates and low to medium energies I actually prefer flashlamps to diodes unless its a microchip laser or very low energies. Flashlamps these days are a mature technology and are very good, they have a bad reputation from the olden days. The only reason I mention this is a cost consideration, you often have to TEC chill the pump diode to keep its wavelength on the vandate or YAG adsorption peak.I still have good relations with my former employer and we made lots of small crystal and optics parts for production stuff like this at reasonable prices. They have a very good line of CR:YAG Q-Switches as well as OEM EO Q-switches. Their parts are made in Eastern Europe, and have far better quality control then the low cost Asian parts that are so "tempting" cost wise. Send me a PM and I'll link you up.CR:YAG is very good for making short pulse widths.How many Millijoules are you looking for? 1064 or 532 wavelength ?Steve