Laser Diode Control - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: February 21, 2016, 02:31:37 am
content: I've been working on a problem on the side for a while. It works but it's not quite as good as I would have liked.I'm working with a low-power laser diode (120mW max) and I'm attempting to make the power output as stable as possible. What tends to happen is that the output power can fluctuate a couple mW over about a week. This is with the LD on 24/7.I've been working with an ic-Haus driver (http://www.ichaus.de/product/iC-NZP) that's specifically designed for driving laser diodes. But I'm starting to wonder if this is actually the best approach. To my understanding, most of these types of lasers and drivers are geared at high speeds for use in communication. I'm using the laser in a different application that requires a 100% duty cycle for up to a month at a time.Would a discrete precision current source be a better solution for something like this?

author: mazurov
author_rank: Frequent Contributor
timestamp: February 21, 2016, 02:49:40 am
content: Temperature variation could be a factor as well as quality of the current source. Have a look at Linear Tech application notes 89,90 for ideas:http://cds.linear.com/docs/en/application-note/an89.pdfhttp://cds.linear.com/docs/en/application-note/an90f.pdf

author: Psi
author_rank: Super Contributor
timestamp: February 21, 2016, 03:25:12 am
content: Yep, i also suspect temperature.A lot of quality laser modules use a peltier block to regulate the temp.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2016, 03:58:27 am
content: AFAIK, much better to servo current based on a measurement of the emission itself. It's not a co-pack with a photodiode, is it?Tim

author: ajb
author_rank: Super Contributor
timestamp: February 21, 2016, 04:07:37 am
content: Are you using photodiode feedback, or just constant current? Temperature of the diode is important for sure, especially if you're not using optical feedback. Also note that the reference in the IC you're using has a drift of about 0.5%/C, so its temperature matters as well. And of course, there's the question of drift and thermal coefficient (and consistency of technique) of whatever you're using to measure the output power. A relatively low percentage of laser applications really care about a variations of ~1%, so I wouldn't expect to find an integrated solution intended to achieve that. Building a driver for un-modulated CW operation wouldn't be too difficult, but will take a bit of work to achieve the stability you're after. Depending on whether you have more time or money to throw at the problem it may be worth looking at scientific laser diode drivers. Some will incorporate TEC drivers for thermal control as well.

author: rs20
author_rank: Super Contributor
timestamp: February 21, 2016, 04:11:03 am
content: Quote from: Pack34 on February 21, 2016, 02:31:37 amHow are you measuring this? If you're able to make this measurement, surely you can just hook that measurement into an op-amp driving a FET driving the laser and control the laser accordingly (servo current as mentioned by T3sl4co1l.)

author: mathias
author_rank: Regular Contributor
timestamp: February 21, 2016, 08:06:04 am
content: What kind of laser are you using? FP, ECDL, Bragg-grating, ...? (I am now assuming a run of the mill red FP LD)To get the power stability you want, take care of the following:1) Limit the temperature, small heatsink (with or without a peltier) should suffice. (Absolute) temperature control of 0.1 deg C is irrelevant, unless you want to prevent mode-hopping etc. Since I assume a FP LD, you have many modes already.2) Measure the output optical power, use the in-package photodiode (PD) or sample the beam with a beamsplitter and place a PD there. The latter has the advantage of being insensitive to the temperature of the LD, the former suffer from that effect.3) Adjust the current through the LD with the signal coming from the PD.As a side note, always always always protect your LD from ESD by a Schottky and a Zener. Always always always.

author: Pack34
author_rank: Frequent Contributor
timestamp: February 21, 2016, 02:21:23 pm
content: Right now I'm working with some PDLD and IPS lasers. These do not have a co-packed photodiode. The more I look into laser diodes the more it seems that anything to provide some feedback would make this much more reliable.QuoteI'm using a laser power meter. It relys on the laser hitting a target some distance away.QuoteThe diode is heat sinked with a TEC and there isn't an in-package PD.Tomorrow I'll setup the test again with some thermocouple to see if the issue is temperature drift.

author: chris_leyson
author_rank: Super Contributor
timestamp: February 21, 2016, 02:26:44 pm
content: Most certainly temerature drift due to self heating. If you drive the laser with constant current expect the output power to drop as the laser warms up.

author: Pack34
author_rank: Frequent Contributor
timestamp: March 01, 2016, 09:13:01 pm
content: Just to chime back on this, it definitely seems directly correlated to the temperature of the laser diode. So I've been looking into how to stabilize temperature as much as possible.I did notice something after I started re-working the heat sinking. The driver for the TEC was directly on the opposite side of the chassis from the hot side of the TEC. I'm wondering if that was the cause of some of the issues I've seen with it. The TEC driver failing when it got too hot.I've reworked the thermals and TEC feedback, now to let it sit for 10 days to see how stable it is.

author: Eng_hassan85
author_rank: Newbie
timestamp: March 01, 2016, 10:14:41 pm
content: Great Topic .. Good Luck

author: jeremy
author_rank: Super Contributor
timestamp: March 01, 2016, 10:21:03 pm
content: Is the laser located in an air-conditioned environment? One thing I noticed once was that because the air-conditioning in the particular room I was using automatically switched off overnight is that there was a drift event right as the air conditioning turned on in the morning. Depending on exactly what power meter interface you are using (fiber coupled, etc), make sure that the hot air from your instruments is not passing over any of your lens holders/mirror stands, etc as the thermal expansion of the metal can misalign your system.

author: collingtech
author_rank: Contributor
timestamp: March 02, 2016, 02:10:04 am
content: any draft for the circuit ? and how the status?

author: _Wim_
author_rank: Super Contributor
timestamp: March 19, 2016, 08:35:09 pm
content: Interested in an update also. Thinking about trying something comparable, so and tips would be more than welcome...

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 19, 2016, 09:53:45 pm
content: @pack34I have used ICHaus before. Good stuff. Unfortnately for my applications they never seem to be sufficient in one respect or another. If not restricted, can you provide the laser you are using and some details(schematic) of your drive circuit. It seems from your description that you may have a thermal issue. This is quite common. Below are some generic comments, not necessarily applicable to you.Both APC and/or thermal control via a peltier and(TEC) controller are often necessary to keep laser power and wavelength stable. But not always. It seems you have quite a variation in power and many things can contribute to that. Constant current drive without some sort of thermal control will lead to power driftIf you are using the internal back facet monitor diode in the laser package as the feedback element in your control loop, keep in mind that it is only monitoring the laser light in a spatially optimal location, with some possible error in tracking light vs photodiode current as result of material drift or CTEAlso, look for error or hystersis in your control loop and possible circuit tempco errors(unlikely)The laser will have a negative tempco power vs temperature. An increase in temperature decreases output power. The wavelength also shifts but in the opposite direction vs temp. If the change in output power is slow it is probably thermal. Use the time constant as a clue in a thermal inertia assumption.Thermally, keep the self heating and ambient/case temperature stable. A Peltier cooler and control circuit might make things worse if not implemented correctly. Beware.External light monitoring is fraught with all kinds of problems as well since there are tons of places for drift and error to occur. Trust me on this one.Validate your test gear and/or test setup.Also, don't trust your laser vendor. Specs and the actual device can vary. Many lasers are hand fabricated. Lasers are not 2N3904s.Good luck. I have spend 32 years mucking around with lasers, so this is an easy one for me.

author: _Wim_
author_rank: Super Contributor
timestamp: March 20, 2016, 06:37:59 am
content: @grouchobyteWhich laser vendor would you recommend? I would need a low power (< 50 mW) near infra-red laser source that is really stable in temperature. At the moment I am looking at Thorlabs products (https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=4737), and would prefer a package with collimation optics already included (at least for the proof of concept) like a pigtail of butterfly style and of course internally mounted photo diode. But I find prices quite high for these kind of packages, especially so because this will be my first laser project, and this could cause some casualties during testing...The power should be stable preferably within 0.5%. Wavelength stability is less important. The laser will be used in a turbidity (transmittance/absorption setup) measurement of a liquid. Detector will be a photodiode. Connection between the process and my laser/detector I would prefer through fiber optic cables (hence the reason for the butterfly/pigtail style of package).From the explanation of the OP so far, I am guessing he is working on something similar. If not, sorry for high jacking this thread and I will start my own thread if desired…

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 20, 2016, 12:51:58 pm
content: Quote from: _Wim_ on March 20, 2016, 06:37:59 am

author: _Wim_
author_rank: Super Contributor
timestamp: March 20, 2016, 01:27:15 pm
content: Hi, thanks for your reply.-Near IR covers a pretty wide range....specifically what wavelength?=> 830nm-Multimode or single mode fiber pigtailed?=> I was under the impression that for my application this would not matter much (short distance, no datatransfer), but was going to buy single mode-True, hopefully laser safety precautions are being taken=> that is the goal, but as a beginner is this area, something can always go wrong. I was thing zener diode/series of schotkey diode and an RC network.-Ge, InGaAs or Si? Active area?=> Si (based on cost), and the larger the active area the better I would think (again limited by cost). I do not need high speed (at all). Are there any benefits in noise/dynamic range/stability to opt for InGaAs or Ge?-Recovering after breaking my shoulder=> wow, sounds painful, hope you recover soon. Explanation was not cryptic at all, very clear.

author: _Wim_
author_rank: Super Contributor
timestamp: March 20, 2016, 01:30:42 pm
content: Forgot to answer one question:Termination of far end: I was planning to use something line this (grin lens):http://www.thorlabs.de/newgrouppage9.cfm?objectgroup_ID=1340

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 20, 2016, 02:47:57 pm
content: Okay, great.1. 830 nm ....lots of laser vendors but not the cheaper telecom ones.....check out Roithner. You are in the EU, I presume. Your flag suggests that.http://www.roithner-laser.com2. Multimode would be better simply because launching 830nm light into a single mode fiber is challenging from a optics standpoint and so the cost may be higher3. If you are talking about damage to the laser diode itself, a fast schottky diode across the pn junction of the LD will suffice in preventing reverse spikes. Zeners? i haven't used a zener since 1979. Giggling makes my shoulder hurt.4 Ge covers a wide wavelenth range but is noisy. InGaAs wont cover you at 830nm (well barely with extended InGaAs). Si seems like the best material for the detector. I agree.BestGrouchobyte

author: _Wim_
author_rank: Super Contributor
timestamp: March 20, 2016, 03:19:51 pm
content: Quote from: grouchobyte on March 20, 2016, 02:47:57 pmI have, and I was only born that year...Thanks for the excellent feedback.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 20, 2016, 04:32:23 pm
content: Quote from: _Wim_ on March 20, 2016, 03:19:51 pmWhen you mentioned "casualities" I had imagined real people getting hurt. Sorry, it must be the drugs I am onYes, lasers are very easily zapped by circuits and the stupid people who use them and design them. As a previous poster had earlier warned.....YES, use every means possible to prevent laser damage including the use of zeners, snubbers, limiters and fuses (just joking on this one). Always, Always Always...... Better safe than sorry with lasers at potentially KiloEuros each, ouch

author: LaserSteve
author_rank: Super Contributor
timestamp: March 21, 2016, 11:18:02 pm
content: For the spikes, Google "Lasorb" by Pangolin. I'm a big fan of this device and I've been working with diode lasers for 12 years. The red model will work just fine with IR lasers. It's a ultrafast shunt device based on a mosfet and transistor on the same die.STEVE

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 22, 2016, 01:30:20 am
content: Quote from: LaserSteve on March 21, 2016, 11:18:02 pmThanks Steve! I learned something today from an expert. Finally after all these years of rolling my own(so to speak)@grouchobyte

author: LaserSteve
author_rank: Super Contributor
timestamp: March 22, 2016, 03:10:13 pm
content: Grouchobyte.. I know the inventor personally. I've seen the hand-held Tesla/Oudin coil demo, that you probably saw in the video. That thing switches in less then 800 picoseconds, limited only by lead inductance.. Worth every penny, and shipping inside the package of many telecom lasers...Glad you like it. Bill solved a serious problem with that one...Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: March 22, 2016, 03:21:00 pm
content: For the OP, take a look at two papers:Instrumentation for the Stable Operation of Laser DiodesC. C. Bradley, J. Chen and Randall G. HuletReview of Scientific InstrumentsRev. Sci. Instrum. 61, 2097 (1990)It has a simple TC controller that can hold the diode body to +/- 0.3 Milli Kelvin for periods greater then three hours... Most people can't even measure that small a change... I know my gear can not... At that point, once the current is stable and the temperature is stable, your next enemy for stability is tiny amounts of back reflection, and that takes a great deal of expertise to deal with, especially with a fiber coupled diode.. Your seeing changes that may be fiber flex noise.Bryn mawr has that one up for free:http://www.brynmawr.edu/physics/courses/305/RSI_Bradley.pdfSee also:A Low Noise High Speed Diode Laser Current ControllerK. G. Libbrech and J. L. HallRev. Sci. Instrum. 64, 2133 (1993)CALTECH has it on line for free:http://authors.library.caltech.edu/29621/Several groups have published low noise, thermally stabilized circuits for spectroscopy on line:A typical Google search would be "stabilized diode laser rubidium cesium hyperfine spectroscopy pdf schematic" or some combination of those words..Or "amplitude stabilized laser diode" These days you can have a Bragg Grating spliced into the fiber for a bit of wavelength stability, which helps with the amplitude stability... Or just use Log Ratio detection and cancel out the need for the ultra stable diode..See the very clever circuit work of P.C.D Hobbs, here:http://electrooptical.net/www/canceller/noisecan.pdfhttp://electrooptical.net/www/canceller/iodine.pdfhttp://electrooptical.net/www/canceller/shotnoise_opn.pdfhttp://electrooptical.net/www/canceller/snspie.pdfThen read this one twice...http://electrooptical.net/www/frontends/frontends.pdfBook:http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470402296.htmlHobbs book takes you step by step thru some interesting problems in using lasers for measurement, including back reflection, thermal stability, and interference effects.. I like the part where he tells you that some times you need to crush the window out of photodiode packages, if they are not anti-reflection coated. That step alone saved me on one diode laser project...Hobb's book is worth it if you've never designed systems that use electrons and photons at the same time...When Grouchobyte says this can get difficult, he means it, see this one, then read the reference list... Always read the references and citations on papers...They lead you further down the road, even if your going backwards to get forwards..http://steck.us/papers/laser.pdfhttp://atomoptics-nas.uoregon.edu/unilaser/The above design is the sort of stuff I deal with on a daily basis in a university lab...Usually if a Professor posts a link to a paper, you can break down the directory structure on his web page, and if it doesn't 404, you can find more related papers.I'm told it greatly angers smart people when post a bunch of links and tell people to read them, without explaining the content I link too. SO I'll stop now... I'll make the counter-argument that if you spend a few hours per week re-reading the papers, you can learn it... Too many people like "instant Gratification Success" from data sheet application notes and sample circuits these days...Steve

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 22, 2016, 03:38:53 pm
content: @lasersteve800 ps, nice! Wow that's faster than a pulse generator I did for an electroAcoustic modulator in a Brillioun scattering system (920ps FWHM). I designed that back in 2008.Also, thanks much for the quality refrence material.As you most certainly know, this stuff comes in handy and is nice to be aware of before a design is commenced.BTW Phil Hobbs is brilliant. He has written some stuff on TIAs and photodiode front ends and his insights are remarkably informative. I think he did some time at IBM, if memory serves me right...Feel free to PM me anytime.@grouchobyte

author: LaserSteve
author_rank: Super Contributor
timestamp: March 22, 2016, 03:59:49 pm
content: Grouchobyte.. Where'[s Home, Toronto per chance?Steve

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 22, 2016, 04:13:24 pm
content: Quote from: LaserSteve on March 22, 2016, 03:59:49 pmNopeOrignally from Montreal. I PM'ed you.