Curious question about Osram PLTB450B theshold current (?)

[DrMario]

Since I have the Osram PLTB450B 1.6 Watts 450nm blue laser diode, having messed around with it long enough, that raised a curious question; is the lasing threshold current as specified in the datasheet as 200 milliamps or is it merely a speculation?

In my case (and in this picture), it's very close to LED mode but not really a LED at this point (3.3 Volts at 45 milliamps with A123 18650 Lithium : Iron Phosphate battery - the laser I have, Beastly Blue, is merely driven by a modified LED driver for now so it's almost direct driven). As for the laser diode itself, it's a multi-emitter laser, apparently as I compared both the ~1 - 2 mW and 200 mW (330mA at 4.2 Volts) lasing mode with two different Lithium-ion batteries, there are four to five emitters - the lens used is a three-element lens option I got from DTR. (And yes, I did use the laser goggle, it's definitely uncomfortable to look at the ~200mW blue laser, it's so bright.)

 

[paul1598419]

I would love to help with this as it is quite easy to find the threshold current with a good spectrometer and a setup with a constant current/ constant voltage power supply. Unfortunately, I have never bought one of those Osram diodes. I spend time finding the threshold current of many new diodes all the time. Why do you want to know this as you have obviously gotten it to lase. Your photo shows that. Whatever the threshold is, it is below what you are driving this diode with.

 

[DrMario]

Yeah, I was kind of surprised that it's way below what the datasheet specified and I may have seen a few members of LPF here done that so I was wondering if I could see some hard numbers since some Osram laser diodes tend to be, let's say, odd. Nevertheless, in the picture, it was very close to LED threshold - around 1 mW or below, so 38 - 40mA being a hard limit for this particular diode (I may never see that behavior with another diode but if it indeed happens consistently with a few diodes, I'd be colored surprised).

Yet, I feel it is going to vary widely across several blue Gallium Nitride laser diodes, so I am perfectly fine with average threshold current discovered by hand, or lists, which is also fine with me, if anyone know the number they discovered with their diodes. This thread was opened more of curiosity regarding the Osram blue laser diode's quirks. Now I am also curious about Nichia's or Sharp's as I am sure it's different but I won't know because I haven't handled those diodes just yet. I also can re-edit the thread name, if it expands beyond Osram laser diodes.

 

[Lifetime17]

Hi
PLTB450B diode driver set to 1.8A awesome diode. Built plenty at that setting

Rich

 

[DrMario]

1.8 Amps - that's amazing that it can take that much. I guess that goes on to show that datasheet sometimes isn't meant to be trusted religiously. (Of course, I still do check the datasheet however, as I don't want to find the hard limit the hard way.)

 

[GSS]

I have one of Lifetime's build of this PLTB450 at 1.8A that if I remember correct was hitting over 2W's..
It's actually one of my favorites and even with a G2 lens the specs are great, alot better beam profile than the M140 and cheaper on Dtr's sight.
Dr.Mario i'm quite surprised you and others in the past don't know much about these diodes.
Really thinking of getting another before they disappear.

 

[DrMario]

GSS, yeah I am surprised. This is my first build. I am probably used to the hard zener diode knee curve most commonly seen in most singlemode (and many Gallium Arsenide Infrared and red) laser diodes in term of threshold current - when it suddenly switch from LED mode to laser mode. (I have green Osram PL520 50mW laser from DTR being driven hard at 70+mW and it's certainly a lively little singlemode laser diode and it's five years old now.)

Now that I have this blue laser in my hand, I know what to expect out of it when I get around to mess with the eventual diode-pumped dye laser build (because I will be using a few multimode Gallium-Nitride laser diodes of varying wavelength, ideally close to the dye absorption wavelength of the liquid dye I will be using and yes, I will be designing so the laser diode module can be swappable for what's required).

 

[DrMario]

I think I know why I obtained lasing current WAY below threshold current of 200mA on this laser diode: [new link two posts down]

 

[paul1598419]

I think I know why I obtained lasing current WAY below threshold current of 200mA on this laser diode: https://www.osapublishing.org/captcha/?guid=18C7EBF6-BB17-3518-6751-552AB2B07084 (a PDF file of research paper on the blue laser diode threshold current, behind the CAPTCHA wall.)

For some reason your link doesn't work. But, I'll take your word for it. I know from experience that threshold currents are often less than specified.

 

[DrMario]

Odd. I will test the direct PDF link for time sensitivity before I re-edit the previous post for more reliable link. Nevertheless, I am a bit surprised that blue laser diodes don't really care about lasing threshold current - I wonder if the same is true for green laser diode made with the same stuff (Gallium Nitride).

EDITED: Never mind, it's time and location sensitive, apparently so I will just post the link to Google Drive copy (unless someone want me to remove the link and apparently it doesn't say anything about prohibition of redistribution of this paper - I will remove it if so requested): https://drive.google.com/file/d/1-OE5iGRVbxcWmr4Z9Fw-r5qzMKx2Llrr/view?usp=sharing

 

[paul1598419]

Got it. I spent some time going through it and it basically states what I've observed in my testing. So, not a big surprise to me.

 

[DrMario]

Looks like I suppose the research paper and your experience have satisifed my curiosity. Guess I should have known not to expect too much of the zener diode knee curves in Gallium Nitride laser diodes (they can be pretty weird) like I am used to from red Gallium Arsenide laser diodes, in term of threshold current.

When I get ahold of green 520nm laser, I will see what I get from it (505nm Emerald green laser diode will be installed directly into the Convoy S2+ host as I do not want to ruin that diode).

 

[paul1598419]

That 505nm diode is a good color. You can see the difference immediately when compared, side by side, with a 532nm laser. Congratulations on your new laser. :yh:

 

[DrMario]

Emerald Green laser have been on my bucket list so I can now cross that off.

I have a 520nm direct diode green laser, and I knew that 505nm would look a bit blueish (a bit Cyan-ish, actually) compared to the 520nm green light. I would plan on getting 495nm Cyanide green laser diode next if I can still get ahold of those - I am willing to bet it won't be easy as they're selling like hot pancake.

 

[paul1598419]

Yes, I am all out of 495nm diodes. My postal carrier just picked up my packages today that are going out. I have people sending me PMs trying to get their hands on one of these. I'm waiting on a set screw to finish my 495nm build as my host's heat sink came sans one. I think when these become available to the masses, they will sell like the proverbial hot cakes.

 

[DrMario]

I probably may have to ask DTR to see if he plan on stocking those elusive laser diodes. I am hoping he will.

EDITED: Sent him an email, now I wait.