Sciencemadness Discussion Board

Red Laser Pointer Reveals a Dirty Green Secret?

m1tanker78 - 2-2-2015 at 20:48

I bought a cheap red laser pointer to practice assembling optical trains, aligning mirrors and so forth. I observed on several occasions what I believed were orange/yellow/green emissions after passing the beam through a dichroic filter set to maximum attenuation of the laser light and observing the band(s) by eye with a diffraction grating. I should mention that the laser only emits ~5mW and the filter attenuates the vast majority of the red light. I wouldn't dare look directly at any laser source -- not even low power ones.

At first, I dismissed those observations as being impossible, even from a cheap laser diode. I also had trouble reliably reproducing the effect. Frankly, I expected to observe a few spectral lines right beside the main laser line since I didn't use a monochromator nor any 'cleanup filter' at the output of the laser. Surprisingly, the laser line is fairly clean as far as my protected eye could detect.

I exploited the fact that diode lasers emit polarized light. (Mansuripur et al.) I placed a polarizing filter oriented cross-planar to the laser source. The polarization filter greatly attenuates the laser beam. The dichroic filter was used to further attenuate the main laser line. The beam was then passed through a diffraction grating or prism. The green component immediately became clear and after a bit of optimizing, I'm now able to reliably reproduce this effect. Depending on the orientation of the laser and optics, the green and red bands could be maximally dispersed to clearly distinguish them or can be made to overlap in order to observe the green-yellow-orange-red pattern. Based on that, it became clear that the yellow and orange light I previously observed was simply the red and green lines overlapping with differing proportion of intensities.

What's the most likely cause of this anomaly from a supposed monochromatic light source?

-Impurity in the gain medium?
-Physical deformity of the cavity that causes out-of-band resonance?
-Fluorescence?
-Anti-Stokes peak of the focusing lens (probably epoxy)? Seems unlikely but...?
-I should just slap a narrow passband filter in front of the source and forget about this green co-emission business?

I was able to take some photographs but had a difficult time trying to align and focus the camera in the dark. I'd appreciate a reference or any insight in this matter. I searched around and came up empty. Maybe I'm using the wrong search terms or something..

Dispersing:
<a href="https://picasaweb.google.com/lh/photo/zCyu-QLDTdH9KM0uu_XTwU-4EyEZuJxi0PQuPcSp7hM?feat=embedwebsite"><img src="https://lh4.googleusercontent.com/--mR9x_hNcH4/VNAUoRtqs4I/AAAAAAAAAzk/Oix7g5IlMJY/s800/20150128_144548_cropped.jpg" height="388" width="548" /></a>

Overlapping:
<a href="https://picasaweb.google.com/lh/photo/GjZopcaWr2fVh23hjpqHNU-4EyEZuJxi0PQuPcSp7hM?feat=embedwebsite"><img src="https://lh3.googleusercontent.com/-yYRxlOi7_5s/VNAUobbd7yI/AAAAAAAAAzY/s16kgtFEENY/s800/20150128_152131_cropped.jpg" height="580" width="492" /></a>



Zombie - 2-2-2015 at 21:56

You must have a frequency leak or bleed over. Most likely is a bad timing crystal. It's the only logical explanation I can think of. Basically the same as your first assumption.
Honestly with the cheap junk shipping out of China it is anyone guess as to which component is at fault.

I've made multiband lasers in the past, and timing, and pulse width were used to create "shots" that were measured with a timing light matching the frequency. This was also 30 years ago.

I think you have a cute anomaly, and it had me thinking for a moment. In the end it is not surprising considering the source.

jock88 - 3-2-2015 at 14:20


I have read that some of the high power lasers coming from china are emmiting far more power than stated. The power declared is only in the colour-that-you-have-purchased band. The rest in the IR. Will demolish your eyes etc in a far shorter time than you would think.

The Volatile Chemist - 3-2-2015 at 14:53

Interesting. I've had a laser pointed at my eyes by a teacher for 'being stupid' (When he left some space on the board, I told him he had to be liberal, for he couldn't be a conservative-ist :P), who stated it wouldn't be able to do any harm for a while.
Regardless, could the timing circuit, etc. be exploited for the production of other useful laser light?

Zombie - 3-2-2015 at 15:44

That is what creates the color. The frequency at which the diode is run. It gets pretty complex depending on the pump type. Some lasers can be tuned to run at different frequencies or in different spectrum's but essentially they are purpose built.
If you fiddled w/ yours to change frequency, what you would find is something very similar to your results in capturing the out of band emissions you posted. There would be a VERY narrow range of variation where the light could be captured or in this case created.

The entire circuit you have to work with had a pre-determined frequency. You could change the pump, and the timing, but then you have a different laser.
Soooo to get to brass tacks, you could tune to create a different color but the focus, and amplitude would be so far off that the result would be negligible, and difficult to capture.

It was more fun (for me) to build a stepped pulse beam containing two colors. Using the correct timing the beam could be seen in either spectrum, and as individual colors stopped in space. Little pieces of floating thread is what the effect was. Similar to a strobe timing light on a rotating object. The object appears to stop in space.
Like I said, it's been a bunch of years so I found some links that are more in depth.

This is a good write up on the basics.

http://www.fas.org/man/dod-101/navy/docs/laser/fundamentals....

This is pretty cool too. It explains multi spectrum lasers, and has a few sentences that explain the phenomenon you experienced.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3270856/

[Edited on 3-2-2015 by Zombie]

gdflp - 3-2-2015 at 15:53

Quote: Originally posted by The Volatile Chemist  
Interesting. I've had a laser pointed at my eyes by a teacher for 'being stupid' (When he left some space on the board, I told him he had to be liberal, for he couldn't be a conservative-ist :P), who stated it wouldn't be able to do any harm for a while.


That seems very dangerous . Especially if it was a cheap laser pointer, those don't have the best quality control. In addition, the actual diode is much stronger in many of them and the power is limited by a resistor(I believe).

Zombie - 3-2-2015 at 16:26

They tend to use whatever diode is available in bulk (cheap), and yes the power output is modulated via the control circuit. Laser diodes need to run on the ragged edge of self destruction anyway so these cheapo pointers can be all over the place as far as specifications are concerned.

Most of the companies manufacturing them have done NO development nor do they understand the circuit they are building. They simply reverse engineer, and recreate the circuit. Hell the same factory runs watches, lasers, ignition modules, and coffee pots at the same time.
Their R&D departments consist of de-soldering stations, and cameras.

This should instill confidence...

http://www.alibaba.com/showroom/cardiac-pacemakers.html

m1tanker78 - 5-2-2015 at 18:13

The package the laser came in states a wavelength range of 630 to 650nm. I went ahead and measured the wavelengths of the red and green lines with a diffraction grating.

lambda=(d*sin(Φ)/n

lambda(red) = 646.35nm
lambda(green) = 549.45nm

I'm going to record input power vs temperature vs wavelength just as an exercise to see just how much the wavelength on one of these cheapies varies with temperature and supply current. The higher power laser will dissipate much more heat and I'm sure I'll need to provide active cooling in order to maintain stability.

Zombie:

I didn't modify anything in the circuit except to do away with batteries and use a buck converter. The green light appears even with batteries and seems to be directly proportional in intensity to the red emission. Thanks for the links, the second one looks interesting!

Zombie - 5-2-2015 at 20:09

Upping the current won't achieve anything other than stress test the components. It's the timing circuit that you have to change to directly see results.
You might release more anomalies as the circuit stresses but that is a speculative matter, and I would think mostly non repeatable.

30 years ago we used timing crystals to control the timing or clock IC's. I couldn't remember a number if my life depended on it but I believe now the IC is self regulated so looking up the control circuits specs you will have a base line to work from.
I was brainstorming as I was typing, and it came to me...

http://ichaus.de/LaserDrivers

Here's what I think you want...

http://www.arimalasers.com/attach/product/20120518095535_doc...

That's the start line.

Maybe I didn't forget as much as I forgot that I forgot.

Keep updating this. I'm R E M e m b e r i n g

[Edited on 6-2-2015 by Zombie]

m1tanker78 - 8-2-2015 at 07:06

Quote: Originally posted by Zombie  
Upping the current won't achieve anything other than stress test the components. It's the timing circuit that you have to change to directly see results.


I think you misunderstood my experimental conditions. What I want to do is measure the wavelength(s) of the laser emission while modulating the current from, say, 5mA to the diode's max current rating. Set temperature in steps of ~2 deg. C within a range falling between 0 - 40 deg. C and repeat the current stepping. In other words, sweep the current and measure laser line wavelength at each of a discrete set of temperatures. I hesitate to complicate this since I won't be using the laser pointer for my final project.

EDIT:

I bought a 2nd laser pointer just to rule out a specific manufacturing defect on the first one. Same result with the green line. I removed the lens from one of them and ruled out possibility of a frequency shift due to the lens material. The green emission is intrinsic to the laser chip.

[Edited on 2-8-2015 by m1tanker78]

Zombie - 8-2-2015 at 16:45

I think that does back to cheap parts bought in bulk.

It's cool that you narrowed it down to that point tho.