Estimation of red/IR laser output power based on supply current and temperature - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 09:27:26 am
content: Hello,I've a few laser diodes from dvd-rw teardowns like this:red laser diode:IR laser diode:So, those laser diodes has three terminals (they have builtin photodiode for laser light output feedback as I know)I've tested succesfully eg. red laser diode with supply current ~80mA using modified LM317 based constant current circuit below (added additional protection diodes).There is2.25Vfvoltage drop on red laser (dvd-rw Liteon) diode @80mA, so input diode power is: ~180mW, but what output diode light power could be at lets say perfect 25*C case temperature in this case?I'd like to tune those diodes into safe laser pointers, so I'm looking for way to estimate and limit its output power to levels similar to thosehttps://en.wikipedia.org/wiki/Laser_pointer, but I need this output power to be less than <<5mW.Looking for 1mW output power levels and of course I know that low class laser can be dangereus too, since when low class output beam is concentrated into lower cros scetion area, than such laser can be dangereus to human eye too.Of course no plans to magnify or watch laser beams in optical microscope or directly into its output beams even at 1mW-5mW red lasers, while it could increase laser energy densityI've found a few articles about laser power like this, but they are talking about laser measurement systems, etc.http://www.coherent.com/downloads/AboutMeasuringLaserPowerndEnergyOutputFinal.pdfHowever, they mention that:QuoteShort pulse power of those dvd-rw burning laser diodes, can be a few times higher than its average lets say 200mW or so?QuoteThey defined laser power as:QuoteAny hints howto estimate output power of those dvd-rw lasers based on its supply current, forward voltage drop and case temperature, to design 1mW<<5mW laser pointer with output lightbeam diameternot smaller than1mmand up to 5mm or so?Update: According to Wiki:Quoteso I'm looking for <1mW laser output power, but as I know there is something like threshold current and unsure if those dvd-rw burning lasers can have output power limited to such low levels?Will try lower supply currents then, but without knowledge of those lasers efficiency still difficult estimate its output power...

author: B.B.Bubby
author_rank: Regular Contributor
timestamp: July 09, 2015, 10:17:25 am
content: Could you calibrate against a known <1mW laser based on an LDR reading or some such thing. As long as the wavelengths are similar - should be close enough?

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 10:23:22 am
content: Yep, LDR sensor might help estimate this, but I've found also some efficiency numbersNot sure what LDs models were used in this Liteon dvd-rw burner (I haven,t got its datasheet), but if high-efficiency IR laser diode described therehttp://www.jdsu.com/ProductLiterature/paper_hipower_910_980_laser_diodes.pdfhas overall at 25*C with CW 100um wide laser beam 1.14W/A and as high as ~60% electrical-to-optical conversion efficiency, it leads to <1mA laser diode current to be <1mW laser output power?As i know those dvd-rw laser diodes are similar in output are similar in output power capabilities (IR laser diode slightly stronger),so maybe good estimation could be:electrical supply current mA ~ mW power of laser diode outputNope, even at 10mA without any lenses at daylight hand skin is only slightly red from very short distance, lets say10mm or, so...I've forgot those LDs has much bigger threshold current, so this slope 1.14W/A maybe could be estimate at more linear region

author: oohal
author_rank: Newbie
timestamp: July 09, 2015, 01:18:04 pm
content: Quote from: eneuro on July 09, 2015, 10:23:22 amNot quite, if you want to calculate the output power you need to know two things: The lasing threshold current and the slope efficiency. Below threshold a laser diode just acts like a really shitty LED. You will get some light out, but it's low power and it's not coherent. Once the diode current is above threshold there's a roughly linear relationship between output power and current so: P_out = slope_efficiency * (I - I_thresh). If you measure the output power for a few different currents above threshold you can plot a curve to get the slope efficency and extrapolate to find the actual threshold current. You'll need to build a ghetto optical power meter first out of a photodiode or photocell, but that's the fun part.

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 02:19:13 pm
content: Quote from: oohal on July 09, 2015, 01:18:04 pmI'm the most interested in finding this threshold current, since at 43mA (27R LM317T ) there is 2.1Vf on red laser diode still looks like very low efficiency and probably below this threshold.It was made on white paper in daylight shadow from the window.There is some pattern visible and its light beam is not uniformly distributed as expected on surface-seams to be tricky measure this output radiation and probably some kind of rotating light sensor needed to create averaged value on lets say 5mm in diameter concentrated laser output beamAnyway, probably this LD has 250mW output power at best, so no exact values readings needed in this meter, only we need to be able to see this linear relationship region to calculate slope at current domain and estimate this threshold current, than assume 1W/A slope and find current at expected 1mW output powerBTW: What about using builtin photodiode (spare pin on my photos) in laser module?

author: bktemp
author_rank: Super Contributor
timestamp: July 09, 2015, 02:26:39 pm
content: Quote from: eneuro on July 09, 2015, 02:19:13 pmNo need for any complex light sensor: Just make it big enough to catch all emitted light. That is how laser power meters work:http://electricstuff.co.uk/cohlascheck.html

author: mikerj
author_rank: Super Contributor
timestamp: July 09, 2015, 02:32:36 pm
content: Quote from: eneuro on July 09, 2015, 02:19:13 pmThe internal photodiode samples only a small fraction of the total output power. Unless you know what this fraction is, and the responsivity of the PIN at the wavelength of interest then it's not going to help.Trying to estimate the power output using only forward current and voltage without even any characteristic curves for the diode is little better than guessing. You could try making thiscrude power meterbut accuracy probably won't be great without some standard to calibrate it against.

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 03:24:48 pm
content: Quote from: mikerj on July 09, 2015, 02:32:36 pmEven if it will be the fraction if this PD current will be proportional to output power maybeit could helpI found on this nice laser diode faqInterpreting Laser Diode Specificationsand hopefully they shows for 5mW laser pointer ~0.1 mA typical PD current, so maybe worth to try this approach first and plot how this PD response changes within 100mA+/-50mA supply curent to look for linearity and may be nonlinearity below 50mA,Some laser diodes has typical thershold current as low as 30mA in the case of this laser pointer, but Sony x 16 DVD recorder has this treshold somewhere ~50mA, etc.I have IR laser diode too from this Liteon dvd-rw, so it could be interesting try to use this photodiode pins to see how input current affects its readings.

author: mikerj
author_rank: Super Contributor
timestamp: July 09, 2015, 03:38:06 pm
content: Quote from: eneuro on July 09, 2015, 03:24:48 pmThe photocurrent will be pretty much proportional to output power (you have to take into account dark current, and there may be some coupling differences over the operating current range of the laser). A control circuit varies the laser current to maintain a constant photocurrent and therefore constant output power. However unless you calibrate the monitor diode, it's of no use to measure absolute power, only relative power.Quote from: eneuro on July 09, 2015, 03:24:48 pmThat may be an interesting experiment and you can probably determine the lasing threshold by doing this. However, until you have some kind of standard to calibrate the monitor with it's pretty useless for measuring power.Imagine being handed an analog meter without a scale on it and with no other knowledge of e.g. full scale deflection, and then being asked to measure some voltages with it. You'd be able to tell if one voltage was higher than another, and roughly by what fraction, but you'd have no idea what the actual voltages were.

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 04:40:47 pm
content: Quote from: mikerj on July 09, 2015, 03:38:06 pmI've already did this for this sample SONY x16 DVD recording visual light (red) laser diode (added black line on characteristics for simplicity)However, output power eqaution is (power in , current in  ):Code:Po=a*i+bwhen we calculate a,b for black estimated line than we get: a=1, b= -1/20= -0.050Code:Po(i)= 1*i -0.050 so we get:Code:i(Po)= (20*Po+1)/20 = Po +0.050 While at threshold current point power is low close to zero, we get:Ith= 0.050  = 50 This is what I expect from this Liteon DVD RW red laser burning diode, since so far @ 43mA, it had 2.1Vf and I saw other laser pointer diodes having @ 5mW output 2.2Vf or so.Anyway, probably we can estimate 1 [W/A] slope efficiency-something similar to this SONY x16 DVD shown above.Since we estimate 1 [W/A] after threshold current, than we can try estimate 1mW output power current too now for SONY diode (very close to its threshold current of course):Code:i(0.001): (20*0.001+1)/20= 51mA

author: LaserSteve
author_rank: Super Contributor
timestamp: July 09, 2015, 05:05:34 pm
content: Go to Laserpointerforums.com and ask for help. You'll be overwhelmed...Large Area Silicon Photodiodes are pretty good up to ~ 10 mW when they saturate. If you do not focus the laser down into a spot, a known, large area PD with a responsivity curve on the data sheet is a lot more accurate then trying to guess at laser diode variability, which from my experience is huge. Put a 500 ohm to 2K ohm load on the PD to make sure its pulled down into the linear range.This is in a FAQ I contribute to:http://www.repairfaq.org/sam/laserioi.htm#ioihlpm5 mW is typically too small for a home made Pelter based thermal laser power meter, so a balanced bridge is a good way to go:http://www.vk2zay.net/article/210Steve

author: Kleinstein
author_rank: Super Contributor
timestamp: July 09, 2015, 06:23:41 pm
content: Slope Efficiency is usually quite high with these modern higher power LDs: it needs to be because there would be to much heat to get 200 mW or so the other way. So something like 1 W/A or a little more sounds reasonable. Also no really large variations are expected here.The threshold current and the coupling to the internal monitor diode can vary quite a lot from sample to sample and also with temperature. So stable operation at low power (e.g. 1 mW) may require to use the internal DP for feedback.Be somewhat careful with the current source. The LD s are very sensitive to short pulses of to much power.

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 08:18:54 pm
content: Quote from: LaserSteve on July 09, 2015, 05:05:34 pmThanks, for this link-I've found it already, but when made PDF of this page it is close to .. 200 pages, so satrted to read it from chapter to chapter, but it will take some time, so any hints are welcome to faster find specyfic topicsQuote from: LaserSteve on July 09, 2015, 05:05:34 pmDo you mean external photodiode (PD), not this builtin into those 3 pin can laser diodes modules, I guess?Quote from: Kleinstein on July 09, 2015, 06:23:41 pmThat is why I'd like to monitor this internal PD, but I'm not sure how is it connected inside, since when saw another Liteon red laser diode but 5mW LTLD505T with this crappy old looking datasheethttp://datasheet.octopart.com/LTLD505T-Lite-On-datasheet-13546591.pdfI'm very confused about how internal PD from my DVD RW teardown is connected, since pin 2 in my laser module is connected to can case and pin 1 is LD anode (+), not in a way this LTLD505T pins areSo, yeah probably I will have to read this laser FAQ and learn howto determine connection of this internal PD, pin 3 and how to use this PD in which configuration to sense laser diode output powerI've found quite interesting discrete elementsLaser Head (A2074) Manualschematics for laser diode driver sensing internal PD, but they have different pinout, similar to shown above.So many traps-I've connected red laser diode so far:: pin 1 (+) <-> pin 2 (-) (can case) and not used pin 3 (PD), but pin 2 is anode in : LTLD505T 5mW laser diode-no pinout standards there for can packages of LD?Update: This FAQDetermining the Laser Diode Pinoutcan help, but why on different server?I've can COM on pin (2) so two different choices for PD polarity-coin flip?

author: LaserSteve
author_rank: Super Contributor
timestamp: July 09, 2015, 09:38:00 pm
content: Updated LD pinout thread from LPF:http://laserpointerforums.com/f51/diodes-compilation-all-diodes-data-one-thread-45042.htmlIf you find the right Photodiode or "Silicon" solar cell, say a 2 mm by 2 mm or 4 mm by 4 mm device, you can look at the PD datasheet and find the Amperes Per Watt specification at the diode wavelength. You can then measure the laser power using a voltmeter and a moderate value load resistor.The internal light feedback Photodiode is usually not factory auto-aligned the peak of the back beam inside the LD, so its calibration varies quite a bit.Here is a link to some nice ASCII art, that may make this more understandable:http://www.repairfaq.org/sam/laserdps.htm#dpsto1Steve

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 10:06:35 pm
content: Quote from: LaserSteve on July 09, 2015, 09:38:00 pmThanks for this link, but I can't find Liteon there, are even worse-tried right now connect pin (2) COM and pin (3) PD througth 10k resistor to 5V PSU in two possible polarities and nope, no voltage drop at all between those pins (2) and (3)Tried the same with 1N4007 diode in series with curent limiting resistor and of course I've got 0.5Vf in one case.That does it mean-there is no PD pin in this Liteon DVD RW red laser diode or need to provide higher current than 0.5mA in the case of this internal laser photodiode?

author: eneuro
author_rank: Super Contributor
timestamp: July 09, 2015, 10:22:41 pm
content: There were -In this DVD RW laser optics moving head -also such strange some kind of sensors close to laser diodes inside windows on light paths:Another one from the same module on PCB close to laser diodes:This one I saw somewhere, but can't remember now what it was-disk angle sensor?It could be usable, since it could be possible solder own 4 wires in other project, but what was purpose of using this thing in this laser optics module?I'm not sure how light paths looked like, while there were a few (transparent) mirrors between, so maybe one of them could be used to sense laser output power?I will try examine IR laser diode pins tomorow

author: eneuro
author_rank: Super Contributor
timestamp: July 10, 2015, 05:45:59 pm
content: Tested also IR laser diode and seams to be similar to red laser one-pin (3) X is not connected and it looked like on oryginal flex there were no copper connection too.I've used voltage below 2V first (1.55V to be below LD reverse voltage often 2V or so) and when saw voltage drop to ~1.2V on LD-COM terminals -idetified LD pin, increased to 5V and still saw drop to ~1.4V, so for sure it is LD pinNote: I've used 10k current limiting resistor of course while testing those pins (especially when applied 5V between LD-COM pins) and kept laser diode output pointed somewhere on remote wall without anything that could refract its beam just in the case of mistaken connection, etc.No PD probably inside this IR laser diode-that is fine.I'm ordering photodiodes for a few other projects too, so when I will have them in my hands I'll test those laser diodes threshold currents, but there are high power laser diodes, so this current is not so critical to operate at safe area at higher currents lets say 100mA in burning mode, but for laser pointer some feedback needed, so maybe it could be good idea use one of extracted transparent mirror to reflect part of output laser diode beam to those custom photodiodes to estimate threshold current and let keep laser output power close to this point - operate at as low output power as posible at different laser diode case and ambient temperature which will affect this threshold current