Confirmed cheap multi-line 980nm/490nm pen

NOTE: this is a WIP, I intend to add more to this thread eventually - I wasn't going to post it today but better be incomplete than risk forgetting about it eventually :beer:


There has been some discussion regarding 980nm pens sold on ebay that were producing (what was supposed to be) a very, very weak cyan second harmonic at 490nm. Previous discussion of the (DANGEROUS) experiments can be found on these topics:
http://laserpointerforums.com/f40/slightly-crazy-980-nm-laser-experiment-56198.html
http://laserpointerforums.com/f38/405nm-vs-scotopic-vision-76212.html

After reading zyxwv99's report I went ahead and ordered this one:
980P 100 BL 980nm 5mW IR Infra Red Laser Pointer Pen | eBay

As with most NewWish-style pens it takes 2xAAA and is case-positive.

It arrived with a sort-of-defective button, I have to press very hard for it to work. Not sure how long it'll last. But at that price it's still a very good deal.

First test was on a IR-detector card. The card doesn't seem to pick my weak 780nm, but works fine with the high power 808nm (it has a weak response) and the 980nm pen (very strong response). It's quite strange how fluorescence can work "backwards".

The 980nm dot also shows on my phone camera. Here's the camera looking down the aperture (no IR filter):


Now for the coolest part... it does project a dot which is visible in a totally dark room (on a nearby white wall, not across the room), but the dot isn't IR (I verified with an IR filter). I can't quite make out the color of the dot (looks like indistinct grey "scotopic color" to me), but I'm willing to bet it's the second harmonic at 490nm. It's very unstable and I can't even make sure it's a coherent line.

I was quite surprised when the pen metered at 81mW. The IR is completely invisible to me. So if you are thinking about viewing one directly or even shining on brushed metal - DON'T!

Afterwards I metered it with a (good) IR filter and the Ophir doesn't even pick it up. Some IR does escape (the camera could see it on my first test, yesterday it couldn't) but it's certainly far less than 1mW. My meter can detect a 1mW laser just fine.

After I made absolutely sure it was <1mW I tried to look into it... and there it was - a shiny blueish green light! Not quite cyan to my eyes, more like what I expected from 500nm or so (tried to reproduce it as best as I could on my sig). Bright like when you look straight into a high brightness 5mm LED. I'm not sure it's monochromatic, but it doesn't spread much on a grating and is surely narrow-band enough not to be LED emission.

DON'T TRY THIS AT HOME UNLESS YOU'VE METERED THE LASER WITH THE EXACT SAME IR FILTER YOU'RE USING, AND YOU KNOW FOR SURE YOUR METER PICKS <1mW!!!!!

I hadn't been able to photograph the phenomenon until yesterday. I'm not sure if it's due to the colder weather or to the almost discharged batts but yesterday the cyan line seemed stronger and I managed some pictures.

This one is the camera looking straight at the aperture (with IR filter) - it has to be aligned perfectly or it won't show up. The color looks more cyan to me, but it has some green.


Same thing with lights on. It wasn't perfectly aligned, which is why the dot got "skewed".


Here I placed a diffraction grating a couple cm above the IR filter and took a shot of the aperture (left) and 1st order diffraction (right). it's very hard to align and get a shot of the 1st order diffraction. It's not like with an LED that it's always there, the camera has to be on the exact angle.


Same thing but this is just the 1st order. This is closer to the color I actually saw.


And here's the same thing with lights on.


I'm quite convinced this a monochromatic line around 490nm. Looks more like what I'd expect from 495-500nm, but I've never seen those and could be wrong. It could also be normal diode variation - if this is 990nm instead of 980nm the second harmonic would be 495nm.

I'd really appreciate if someone with a spectrometer would confirm this.

It might also be worth trying this with other IR WLs, maybe this is a general phenomenon and every laser diode has a weak second harmonic - we just never see them because they're washed out by the main line and the LED emission.

I'd also like to perform some experiement to determine if this line is coherent, but I'm out of ideas.


EDIT: The_LED_Museum confirmed this is a multi-line laser: http://laserpointerforums.com/f52/possible-cheap-2-line-980nm-490nm-pen-90391-5.html#post1329369

Re: Possible cheap 2-line 980nm/490nm pen

Interested in reading more about this. Subscribed.

Re: Possible cheap 2-line 980nm/490nm pen

It's not abnormal for diodes to have a second weak emission point, my 520 has a ~600nm emission, strong enough to be seen without much issue and was around all the time (provided the green was filtered out)

Though, for an IR diode... cyan is not anywhere close to what I expected to be possible.

Re: Possible cheap 2-line 980nm/490nm pen

Relevant post by Lazeerer on PL-link. Although, like Raffle said, something ~500nm away in the spectrum seems pretty extreme.

Re: Possible cheap 2-line 980nm/490nm pen

Interesting, have you looked at yours with a grating to see if it's monochromatic?

My 520nm has a lot of stray emission as well - enough to project a yellowish-green dot through laser goggles - but it's broadband and not coherent.

Re: Possible cheap 2-line 980nm/490nm pen

Ordered. Will spectro.

Re: Possible cheap 2-line 980nm/490nm pen

Regarding my old 520nm (long since sold, but I did have it checked out)



Edit:

RHD, can you run a 462 at threshold and show what it's spitting out? I can confirm that both of my diodes do have quite an assortment of colors though I have no way of measuring. curiosity

(It's weird how the 472nm diode is moreso staying around 480nm and the 462 is closer to 500nm being driven the same at threshold)

Re: Possible cheap 2-line 980nm/490nm pen

Very interesting! I look forward to the results from RHD.

Could be a weak NLO effect from the mirror coatings or a second transition with relatively high gain in the lasing medium (single pass emission).

Re: Possible cheap 2-line 980nm/490nm pen

Very interesting .. I am about to purchase some of these pens, and a diode.

Looking at a 500mW TO-5, or a 1W c-mount. Wondering if the more power, the better? Or if only canned diodes will work?

Thoughts?

EDIT: I had a violet diode that LEDed, and I tried removing it from a host, cracking the window and severely messing it up. It's output then was all multicolored from white, yellow, violet, and deep purple. It was wild....

Re: Possible cheap 2-line 980nm/490nm pen

@rhd: much obliged!

Hadn't seen the link to Lazeerer's post when I posted my last (loaded page). I also get that emission on my 462nm when it's below threshold and even when it's just barely lasing. Here's a picture of the first order diffraction with it barely lasing. It's LED-like and goes from violet to green.


My 520nm also has the same kind of emission when shone through my Eagle Pair glasses - tried to get a picture but the first order is too weak to show up - it's goes from green to yellow and doesn't seem to have that orange/red peak.. but that might be due to the goggles. BTW what's the liquid in that bottle?

IMHO this phenomenon is completely different. The emission is much narrower.

@bloom: I think a higher power is worth a shot. If you can get a visible-on-the-lit-wall dot it might be worth sealing it with an IR filter to use as a cyan pointer!

Re: Possible cheap 2-line 980nm/490nm pen

It's water and food dye -- just to say the same thing happened through goggles. I just "discovered" it through the fluid.

Re: Possible cheap 2-line 980nm/490nm pen

interesting. subscribing.

Re: Possible cheap 2-line 980nm/490nm pen

Lil bump here- RHD, have you spectro'd these yet?
Anyone else ordered some and gotten the second line?

Re: Possible cheap 2-line 980nm/490nm pen

Lil bump here- RHD, have you spectro'd these yet?
Anyone else ordered some and gotten the second line?

Re: Possible cheap 2-line 980nm/490nm pen

Nope. Don't have it yet.

Re: Possible cheap 2-line 980nm/490nm pen

Just ordered an ebay 980 pen similar to the one shown. Gonna test this out for myself. Would simply using a pair of IR safety glasses function as an IR filter? If not, where can I get one?

Be interesting to see (if I get a pen that produces the cyan color) if temperature and/or voltage affects it. Might have to strap it to a peltier cooler I have laying around and see what chilling it to freezing does.