Laser diode basics and efficency  - Page 1
Forum section: electronics / beginners

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 15, 2023, 05:31:59 pm
content: Assuming a large 5W laser light this (using something large for effect).LINK to example 5W:https://dammedia.osram.info/media/resource/hires/osram-dam-16220713/PLPT9%20450LB_E_EN.pdfHere is what I think I know (feel free to correct!)- Laser diodes like this need a constant current source.- This will get very hot very quickly. Heatsinking laser diodes seems like "a whole thing".- Assuming I want any more than few uS pulse, I want to operate in a CW / continuous mode. Even if my pulse is 1-5mS, that's still CW mode.- UV range is more efficient than IR range. So if I'm just trying to signal from one thing to another and the wavelength doesn't matter, then I should just get the UV range. As I understand it these are easier to make and cheaper too.- At least considering CW mode, I can never, even for a nanosecond extend over the absolute maximum of the datasheet. Whatever the max for the diode is, they mean it. So if I need ~1W of output, I'll need a 1W+ part. I won't be able to overdrive a 500mW part.And now the things I know I need help with...A.How do I figure out how fast this will heat up and cool down? I want to know how hot this will get after 1mS of full power. Or 1mS of 2W output.B.If I just want to deliver light into a 100% absorbing black target, then like just sending to a sensor, the wavelength shouldn't matter?C.I think people use and grow crystals to change the wave length. I haven't seen any diodes in the 300nm-400nm range. I assume you would need some laser and magic crystals to get there?D.I feel like fiber optics should melt when shot with a strong laser, but they are "conductive to light" so I guess I see why they don't, but given extremes they would? So what is the rating for this? If I start looking into fiber optic cables will I see "rated for 1W"? What about overtime, would the ends start to melt and conduct less efficiently?E.I assume collimating and beam splitting lenses will have some efficiency loss or absorption. Is this relevant at less than 5W scale? What would this factor be called in a datasheet?That's probably enough to start.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 02:46:53 pm
content: Is this topic too advanced for the beginner section? I had thought these questions would have fairly easy replies, but the silence is making me question that.

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2023, 03:45:43 pm
content: Bonjour cher MonsieurBravo pour votre questions intéressant !1, Laser diodes above 5 mW are in classes restricted by law and regulations for use, carry or import in USA, EU, CA. check out the laws and regulations that pertaining to your region2. visible, IR and UV laser beams can cause eyesight damage, retina burns and blindness even for a,momentary reflection or from.a.diffuesed,beam.The exposure is,proportionally to power, time and inverse to beamwidthSpecial lab precautions, expensive laser filter,goggles, and laboratoire facilités are required by professional optical and,laser,engineering.3. Laser diodes required special power supplies, constant current and highly transients protect..4. Chinese and Taiwan firms can supply laser modules, power suppliers and diodesMany of these lasers,and diodes are illegally imported to the USA and Canada.5. misuse of laser pointers eg harassment of airplane, helicopter, police, lead to prison terms,and,terrorism . charges.6. FOR YOUR OWN SAFETY AND SECURITY PLEASE DROP THIS PROJECT IDEA.contact your insurances ( health, liability) and lawyers for advise.CordialementJon

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 04:40:54 pm
content: JonPaul, I am aware of the laws, safety issues, and this project can not in any way make it to someone's eyes. It's all enclosed and for research. As to why you would suggest I might shine it at an aircraft (or maratime craft) I have literally no idea. I'm a pilot so I'm pretty sure I would not do that.I'm pretty sure the Mouser.com diode that I linked to is not illegally imported.No, I will not stop this project idea because it isn't my idea, my company has tasked me with learning enough about lasers to make a determination as to the direction of a project.If you do not have any helpful answers to my questions, thanks for your effort so far. I hope someone has something relevant to the topic though.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2023, 05:04:56 pm
content: For how fast the part will heat up there are things like a transient thermal impedance curves. Kind of a time / frequency dependent heat resistance.If UV/blue / IR lasers are easier than IIR / red ones - I don't know the current status. Before about 2000 blue and let alone UV laser diodes were a rather new thing. So it was tricky in the beginning, but once they got started they already got quite high power. I remember seeing one the first blue laser pointers in around 1998, though still at the early sample stage they got slightly more than 100 mW power.Just for the normal eye the beam was relatively dim, but with a flourecent paper is got scary bright.At least for the low power range the red / NIR ones are relatively easy and were to the first ones around, especially used in CD players.Even if you can bye the parts from Mouser this does not mean you can legally operate them without the proper training / permits / safety interlocks. The details depend on the country and maybe the wavelegths and maybe product the laser is used in (e.g. enclosure with safety interlocks).

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2023, 05:25:50 pm
content: confusing post as you start with 5 W laser but the Mouser part is 20 mW.What exactly do you want?j

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 05:30:00 pm
content: Quote from: Kleinstein on March 16, 2023, 05:04:56 pmThank you. It will be a USA device, and the specific state has no laser laws. So as far as interlocks go, I'm not too terribly worried. The actual distance this will shine is about 2mm, it just might go through a lot of fiber first. This wouldn't get away from reflections in theory, but anyway.So on that datasheet I linked (edit, previously linked the wrong one, fixed), there really is just no way to tell what it's heating/cooling profile would be. According the (correct) sheet, that 0.6amp gets you to ~1W but also brings it up above 80ºC. There is no way to see what will happen, it seems this requires testing. I mean, there is a realization though, that a 5Wand a 1.6W might make 1W of output with 0.6amps, but that 5W will be ~80º and the 1.6W will have hit that at 0.15amps. Makes sense mostly, but good to confirm.Still gives me no idea how much these will heat up over a few milliseconds.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 05:30:59 pm
content: Quote from: jonpaul on March 16, 2023, 05:25:50 pmYea, linked the wrong one initally, fixed it. We're pretty sure we need 1W for <3ms. The 5W was an example part that would have a better thermal profile compared to a part rated for closer to 1W.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 16, 2023, 05:37:26 pm
content: Based on your questions and if you really talk about 5Watt and not 5 milliWatt, you have a lot of studying to do.Start with a small laser and work your way up, is my advise.And you can not be careful enough, take the warnings seriously!

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 06:49:03 pm
content: This is the first step in studying. I don't think it is prudent to burn 10 work hours trying to figure out how hot a diode will heat up based on a 5 mW sample - which might not even be remotely correct and scale - whenmaybesomeone could just help answer the most pressing questions.I'm noticing no comments at all on fiber, lenses, or crystals. I think I'm probably in the wrong forum for this info.

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2023, 07:09:51 pm
content: Hello againI don't imply that YOU would abuse a laser, just to mention there's cases of,abuse hat led,to the laws,and restrictions. For instance in Switzerland ALL laser pointers are,illegal !The laser diodes , heatsink, PSU are,often available as a packaged,module called a " physics package"The visible and UV use nonlinear crystals, normally joined optically to the laser.An IR cut filter is,employed, after the crystal.The fibers are a whole separate issue.Best laser, fiber, lenses, optical bench supplies are from Edmond Scientific in Barrington NJ, also Milles,Groiut, Rolyn, etc.For optical fibers info see Corning GlassFinally check the many books, application notes on use, mount, coupling care,and feeding of laser diodes.Bon chanceJonPS In France we have a traditional politeness. Perhaps less so in the States?

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2023, 08:18:17 pm
content: The OP's flag is set to Canada, so as an American I bear no responsibility for his politeness.Common opinion in the US has Canadian courtesy at a reasonable level, but I have found courtesy to be a function of the individual.

author: m98
author_rank: Frequent Contributor
timestamp: March 16, 2023, 08:47:25 pm
content: Quote from: NaDobraNich on March 15, 2023, 05:31:59 pm

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 08:55:42 pm
content: Quote from: NaDobraNich on March 16, 2023, 05:30:00 pmThe CDRH is a federal agency so there are absolutely laser laws and you do have to worry about interlocks. It doesn't matter if the state has additional laser laws, the federal laws trump everything else. If you really mean 5 watts and not 5 mW then that is a class IV laser and it is extremely dangerous, 5W is enough to cause instantaneous eye damage, even reflecting off surfaces.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2023, 09:14:09 pm
content: Quote from: james_s on March 16, 2023, 08:55:42 pmIn the US, states are allowed to have regulations more restrictive than CDRH (a division of the FDA), but not less so.Similarly, the OSHA regulations for laser safety allows states to be more restrictive: seehttps://www.osha.gov/stateplansSee section 1.4 ofhttps://aip.scitation.org/doi/10.1063/9780735422957_001for the differences between regulations and standards for laser safety.

author: Infraviolet
author_rank: Super Contributor
timestamp: March 16, 2023, 09:19:59 pm
content: 5 Watts can be terrifyingly dangerous, even the slightest diffuse reflection is blinding, but to move the topic away from unimportant things like laws towards relevant things which the poster was probably trying to ask about, like how the hell such monster diodes really operate and could be used safely...I have often noticed, mostly in pondering about much lower powered use, that on many component websites laser diodes often quote huge amounts of maximum power then can handle (RS for example has, among laser diodes in stock, virtually everything being above the already very scary 20mW ). Can any such diodes be used like a LED with a sufficicently large series resistor to decrease power output to safe levels via current limiting to give weaker laser beams? Or are laser diodes all really fiddly thngs, I hear rumours that hey need a whole fancy feedback loop and only operate within +/- a few percent of their rated current and power? How do laser pointers then actually work to give low power beams, if all the diodes need high power to operate in actual lasing mode?

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 16, 2023, 09:31:40 pm
content: Quote from: Infraviolet on March 16, 2023, 09:19:59 pmDon't take my word for it because I'm here asking the same questions. But as I understand it, you can absolutely feed it a series resistor worth of power. However, it's not clear what wavelength or stability in you are going to end up with. It might be a mess. I think part of that problem is the forward voltage, draw and heat all work together to change constantly. So thus a constant current feed is what you want.

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2023, 09:47:43 pm
content: For a working LED or laser diode, the wavelength is determined by its material and construction.The voltage across the junction for a given device is a function of both the current and the junction temperature.The junction temperature is a function of the power (voltage x current) and the case temperature, as in other semiconductor devices mounted on heat sinks.For pulsed operation, you need to take the various thermal capacities of the device and heat sink into account.

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2023, 09:56:22 pm
content: Laser diodes like all others have a non linearity and temperature sensitive voltage drop.they have a threshold minimum current below which the laser action ceases.Current Compensation for temperature, voltage drop is, in some lasers, aided by a built in feedback photo diode.Generally, a series resistance cannot properly drive or " dim" them.Special laser diode driver ICS take care of all of this, and can be adapted to a wide choice of laser diodes.Consult the diodes manufacturer spec sheet, driver reccomended and applications notes.bon chancejon

author: jmelson
author_rank: Super Contributor
timestamp: March 16, 2023, 09:57:00 pm
content: Quote from: NaDobraNich on March 16, 2023, 09:31:40 pmYes, but be aware that the beam SHAPE will change as the power is reduced. At higher power, the laser amplification effect makes the beam narrower as well as more sharp in wavelength.Jon

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2023, 10:14:53 pm
content: https://www.osram.us/os/applications/application-notes/application_notes_lasers.jsp

author: m98
author_rank: Frequent Contributor
timestamp: March 16, 2023, 10:36:49 pm
content: Quote from: Infraviolet on March 16, 2023, 09:19:59 pmLaser diodes have a lasing threshold current, above which they actually become more than a glorified LED, and where the output power will start to linearly increase with current. As long as you're above the lasing threshold and below the maximum current, while having enough cooling as not to exceed the the maximum junction temperature, you're good. The fancy control loops come in if your application requires constant output power, low noise, a stable frequency or narrow line width. Packages will then usually either contain feedback photodiodes, thermistors, or might even have the laser diode mounted on a TEC with thermistor and photodiode. Sometimes you might even use a spectrometer or an interferometer in the control loop.High power laser diodes are pretty tolerant, but the closer you get to the limits, the more careful you need to be not to exceed them. There are also a lot of fakes and factory rejects on the market, those might have all kind of issues and can suffer from rapid aging or randomly break for no good reason.

author: james_s
author_rank: Super Contributor
timestamp: March 16, 2023, 10:43:10 pm
content: Quote from: Infraviolet on March 16, 2023, 09:19:59 pmYes I have a 1 watt visible laser and it is incredibly bright. It has a tendency to set dark colored materials on fire, even across the room.

author: DavidKo
author_rank: Frequent Contributor
timestamp: March 17, 2023, 08:23:10 am
content: Do you have laser safety training? If not than study it at first. Lasers of such a power are dangerous a lot. That is the reason for interlocks, but at first you need to know all risks what it can cause to correctly design the device. They are not devices to experiment with.It is the same like working with high voltage, but with different end results. Laser will mostly do not kill you, but 5W a laser can do from slight burns on you skin to burn whole house due to the misuse.By the way you can buy complete laser modules from China likehttps://www.aliexpress.com/item/1005003100566327.html. I know that it is the China Watts, but still the optical output will be behind all the limits.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 17, 2023, 06:40:03 pm
content: Quote from: m98 on March 16, 2023, 10:36:49 pmThanks for your post.On that topic... if you didn't care about wavelength, and just needed a signal through an optically isolated but thick medium for 2-3ms (completely sealed), I do kind of wonder if you could get away with a really basic power supply and heat mitigation?I wish I could find someone that has actually done this. I just need to know if you can overheat to the point of device failure in the millisecond range. I mean, obviously you can, but at regular/rated values. Do you have 3ms?

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2023, 06:52:19 pm
content: It might help if you could explain a bit more about what you're trying to do.There are two main considerations for laser diode power handling. One is thermal and the other is catastrophic optical damage. For thermal, small pulses are handled easily by the thermal mass of the laser diode and housing and that works just like any device, you can hit it with significantly larger pulses of short duration without overheating. Catastrophic optical damage is another matter, at some point the intra-cavity power reaches a point at which the mirror surfaces in the die are damaged and this occurs instantaneously, even a very brief excursion into the power level that will do this will damage or destroy the laser.In early laser diodes the current at which catastrophic damage would occur was very close to the lasing threshold at which it starts acting like a laser and emitting coherent light. This is why a feedback photodiode was always integrated and closed loop control was needed. Many modern laser diodes are far more robust and can operate over a wide range of power.

author: jonpaul
author_rank: Super Contributor
timestamp: March 17, 2023, 06:56:13 pm
content: see the app notes and links sent already, with specific info on laser diode pulsing and thermal management.j

author: NaDobraNich
author_rank: Regular Contributor
timestamp: March 18, 2023, 05:43:52 am
content: Quote from: james_s on March 17, 2023, 06:52:19 pmI need upto 3ms of 700-850nm laser shined into liquid that is inches away, everything in a opt-isolated environment.jonpaul, The best I could find in those links what an interesting note that the majority of the getting the diode up to temperature will happen within the first 100ms. Which to me means that 2 or 3 ms will not instantly overheat the diode. Which thinking about it makes sense because your heatsink needs time to work. The package can dissipate 8W, so, I don't think I'm going to find a way to see the temp after 2-3ms, but it also seems like it can't be too high to the point I would need heat transfer at all - if I decided to do this - which I have not. Just researching.

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2023, 06:30:01 am
content: For 3ms you're not going to need a heatsink at all if it's low duty cycle, for heatsinking with short pulses what matters is the average power.