Author Options:

If i hook up a laser pointer to a fiber-optic cable, will the laser still exit the cable while lighting it up? Answered

i want to make a power-glove style party glove. I want to hook up laser pointers to each finger, but i figured it would be cooler if i could get the glove to light up. So to save myself some soldering, would i be able to shine the lasers through fiber optic cables to light up the glove itself, while still having the laser beams exit the ends of the fiber optic cables?

9 Replies

CrLzBest Answer (author)2010-09-25

I can't picture precisely what you are hoping to do. However, assuming you want the fiber-cable to light-up / "glow" AND you want laser beams shooting out of the fiber ends / fingertips :

Somewhat possible, not efficient, Dangerous! (see NOTE)

To make the cable glow, you need to get light traveling along the outside layer, the cladding.  On cheap optical cable, the cladding is a clear layer surrounding the core, with a different refraction index ("optical density").

To have laser beams shooting out of the fingertips, you need to send the laser down the core.

If you purposely mis-align the laser, you can split the beam between the cladding and the core.  This happens as you try to align a beam.  Right before you get the correct alignment, the cladding starts glowing in a dramatic way.

However, this is not an efficient way to get both effects.  To get a better glowing cable, using a second, LED light source would produce a much more dramatic result.

Gloves with lasers shooting out the fingertip would be unsafe at a dance party.  LED party gloves would be safer.  Only effect you'd reduce is the beam of light showing up in a smoky path.  But non-coherent light can do this, albeit less spectacularly.  High-power LED sources would make "beams" that show up in a smoky dance party.  And would be much safer. 

Besides, blinding someone to get a date is pain cheating....

Select as Best AnswerUndo Best Answer

steveastrouk (author)2010-09-26

Clever, but I doubt you'll see a beam, because the fibre has an intrinsic property which will cause divergence as the beam leaves the fibre.

That divergence will be the same as the acceptance cone.

Have you any cite that proves that light is propagated in the cladding layer ? My understanding it that it HAS to happen at the boundary between the two different refractive indices. Steve

Select as Best AnswerUndo Best Answer

CrLz (author)steveastrouk2010-09-26

Hi steveastrouk,

was that a question for me?

Select as Best AnswerUndo Best Answer

steveastrouk (author)CrLz2010-09-26

Yes, I kind of see what you are trying to do, and I can't definitely say it won't work, because I haven't done it....but I have me doubts...

Select as Best AnswerUndo Best Answer

CrLz (author)steveastrouk2010-09-27

First off, I totally agree with karnuvap and you, trying to use a laser to glow the cladding AND still propagate out the far end is really not efficient (per my original post). As we all agree, it would be better to create a TRON+laser pointer glove in other ways.

As for the propagation question, only experience has shown me that you can do it. I pursued a MS in Physics in grad school and took an optics lab as one of my credits. Lots of fiber optic projects, lots of laser work.

I suspect our differences come from me not being specific enough with what type of fiber is being used. You are probably talking about single-mode fiber and I suspect I was using multimode fiber in the laboratory(single mode being too expensive for a lab class).

In single mode fiber the laser needs to be treated as a EM-wave, not a geometric line. The core is too small for geometric optics equations to correctly model the propagation. In that case, the wave travels down both parts actually, as an evanescent wave, which could be thought of as a waves in both mediums bound at the boundary.

In multimode fiber, you are correct about needing to align within the acceptance cone of the fiber for propagation down the light guide. However, this is essentially rotational alignments (think yaw, pitch and roll angular displacements). "Proper" alignment also requires the correct linear alignments (think x, y, z "altitude" displacements). Ultimately, proper alignment is both hitting the bulls-eye (linear) and being ~orthogonal to the target face (angular, within the acceptance cone).

It is important to realize that both the laser beam and the multimode fiber have cross sectional area. Neither is an infinitely fine line. Fairly obvious with mulitmode fiber. Perhaps less obvious with the laser, but definitely true, particularly for chemical lasers. So, when you go to align a laser, you get a problem similar to the concentricity error in the splicing error pic below (taken from a discussion of fiber optic splicing technique). Just before you hit the bulls-eye, part of the laser beam will traverse the cladding layer.

The cladding is a transparent medium, very similar actually, to the core material, just a different index of refraction. Light can propagate down this layer also and it has it's own "acceptance cone".  It will bleed out, however, this making the cladding "glow".

Hope I redeemed myself a bit with this explanation.  In the end, I still agree a single laser is NOT an efficient way to go. And still dangerous.

Select as Best AnswerUndo Best Answer

steveastrouk (author)CrLz2010-09-28

What a superb answer. Thanks for posting that. VERY informative. Telescope optics are my interest.


Select as Best AnswerUndo Best Answer

CrLz (author)steveastrouk2010-09-28

Whew! Glad it helped.

Tough answer for me, but good exercise. I confess, professionally I've drifted into numerical methods / data statistics and away from the more hands on contemporary physics of optoelectronics.  I hope I did Dr. Martini proud...

Select as Best AnswerUndo Best Answer

karnuvap (author)2010-09-26

You cannot really have both. If the laser light is 'leaking' out of the fibre on its journey to the tip then there won't be much of it left to exit the end.

My advice is to keep the laser going down the fibre for the effect you desire and buy some e-l wire for the glowing glove effect and sew it on to the outside.

To encourage the light to leak out of the fibre - if that it really what you want - then you should roughen to outer coating of the fibre with very fine grade sand paper.

Select as Best AnswerUndo Best Answer

kamikaze33 (author)karnuvap2010-09-27

ok sweet thanks! thanks everyone else for the input too! for the sake of time constraints i think im going to go with some e-l wire. karnuvap how pliable is that stuff? i notice it comes in more or less "single" connected lengths; if i want five lines going up my arm down to my hand, would i be able to bend the cable at a sharp enough angle (hiding the bends in a glove)?
or would i be able to just trim the cable as is and power it on its own?

Select as Best AnswerUndo Best Answer