Not quite sure how to do this, fiber optic cable help

Hey guys, I'm not quite sure how to accomplish what I'm trying to do; I think it will work but I figured I'd check with some experts.

This isn't exactly what you guys are used to I don't think but here goes:
I have a sensor which appears to have a convex lens over it, the purpose of the sensor is to receiving and read weak pulses from a 904nm laser. The distance between the sensor and laser varies from 2000ft to ~20ft.

The beam of the pulsed laser is very weak, ~90 microwatts. Its beam divergence is 2.5 milliradians which I think equates to a beam slightly less than 36'' in diameter at 1000'.

The problem arises when the especially at closer distances when the beam of the pulsed laser is tighter and the sensor loses sight of the beam. The possible solution to this is to use 2-4 fiber optic cables with lenses to carry the 904nm light from different positions to the sensor, essentially turning one sensor into four.

I know there has to be a way to do this. But I need to figure out the best way. Here are the things I'm trying to determine:
How high a quality fiber optic cable do I need? Cheaper is better and the cable should be around the size of a bic pen.
What kind of lens would be best for the receiving end of the fiber?
How do I "combine" the light from multiple fiber cables and then deliver that light/"information" to the original sensor? I'm assuming just pointing the end of the cables at the sensor isn't enough. Another type of lens at the end? collimator?

Thanks all for any input.

would a beam expander do the trick? lower the divergence and make the beam larger? that way it would be more constant without complicated fiber optics...

What exactly is this for, possibly an audio device for spying? Anyway, like horhy said you want to get a beam expander.

Ramsey_innovations said:

What exactly is this for, possibly an audio device for spying? Anyway, like horhy said you want to get a beam expander.

Click to expand...

Its kind of an obscure application, not spying or anything like that, I don't want all the info posted publicly but I'll be glad to PM anymore more details.

So would the beam expander go on the end of the cable that receives the laser pulses or the end that delivers it to the sensor?

Edit:
I thought about the beam expander just to clarify I don't have access to the laser I'm receiving the pulses from. I can only modify things on my receiving sensor. This will make more sense when I pm you.

the beam expander would go at the laser light source. is it like this?

:laser module: ----------laser beam in open air---------:sensor:

beam expander^ it would go there.

horhay said:

the beam expander would go at the laser light source. is it like this?

:laser module: ----------laser beam in open air---------:sensor:

beam expander^ it would go there.

Click to expand...

yeah I can't do that one, not my laser module sending the pulses, well besides when I'm testing it

Riptide said:

not my laser module sending the pulses

Click to expand...

what exactly is sending the pulses? is it possible to put optics at the source?

this is sounding really sketchy :evil: haha

Fiber optic cables usually aren't that big. You can try something like a TOSLINK fiber optic audio cable and see if you can focus down your beam into it. They're designed for 650nm, but may work with your wavelength.

Sorry for asking ..... 904nm ? ..... have this something to do with laser speeders ?

Anyway, i think you can get better results using more than one sensor in different positions, paralleled on the input stage, than fiber cables ..... is not so easy to focus a lens on a single fiber .....

HIMNL9 said:

Sorry for asking ..... 904nm ? ..... have this something to do with laser speeders ?

Anyway, i think you can get better results using more than one sensor in different positions, paralleled on the input stage, than fiber cables ..... is not so easy to focus a lens on a single fiber .....

Click to expand...

you've got the right idea, yes more than one sensor would work better but I only have a limited number of them

Fibers are usually very small, even multimode fibers have a core around 50um. I don't think that would work. The plastic stuff for decoration is much thicker.

What size is the sensor and how large do you want you detection area to be?

The sensor is the bubble on the left, the diode of the unit itself is located on the right. I want to figure out how to get the receiving portion of it to work before I try to connect the output to fiber.

As far as coverage area goes when the sensor is mounted without any fibers it provides ~18inches radius reception. This obviously changes slightly as the sensor gets closer to the beam its receiving since the beam gets smaller, but 18 inches is a general rule. If I could turn one sensor into 2-4 sensors and get much more coverage area I'd be golden.

I know this is possible because there is a startup company who built test units just like this that worked, with inputs and outputs. But I'm tired of waiting for them to release a product. Its been like 5 months now. If I can figure out how to build it myself before they release it I'll be real happy.

Heres some fiber I was looking at:
Fiber optic cable, fiber optic bundles, theatrical lighting - TPR Enterprises, LTD.
Scroll down to where it says Glass

Get yourself some 1mm or 2mm ESKA plastic fiber optic cable out of a toslink cable or from Edmund Scientific etc.. Buy some say 1" diameter ~30 mm focal length convex lenses. Mount the fiber at or near the focal point of the lens.

1 mm or 2 mm ESKA cuts with a hot knife, Dremel tool, or very sharp nippers and can be polished using 120, 200, and 600 sandpaper in series, or if your very careful, a small flame or fast bench grinder. It comes in a soft black plastic jacket that will inhibit collection of the light you do not want.

Bundle all N fiber ends at the sensor.

Your light collection angle is limited by a physical function known as Numerical Arperture, and you can Google that. The Wiki on it is OK.

Lenses for experiments, at low cost, come from Surplus Shed, Anchor Optics, and other vendors.

This is not the only vendor by any means: Optical Fiber/Cable - Industrial Fiber Optics, Inc.

If you get caught, I don't know you

Steve

Thanks for the info, when I looked at the transmission loss of ESKA cables it seems to be highest at my nm (904) is that not a big deal? Maybe I'll just have to test it out since the prices aren't that bad anyway

LSRFAQ said:

Get yourself some 1mm or 2mm ESKA plastic fiber optic cable out of a toslink cable or from Edmund Scientific etc.. Buy some say 1" diameter ~30 mm focal length convex lenses. Mount the fiber at or near the focal point of the lens.

1 mm or 2 mm ESKA cuts with a hot knife, Dremel tool, or very sharp nippers and can be polished using 120, 200, and 600 sandpaper in series, or if your very careful, a small flame or fast bench grinder. It comes in a soft black plastic jacket that will inhibit collection of the light you do not want.

Bundle all N fiber ends at the sensor.

Your light collection angle is limited by a physical function known as Numerical Arperture, and you can Google that. The Wiki on it is OK.

Lenses for experiments, at low cost, come from Surplus Shed, Anchor Optics, and other vendors.

This is not the only vendor by any means: Optical Fiber/Cable - Industrial Fiber Optics, Inc.

If you get caught, I don't know you

Steve

Click to expand...

Can you give a part number or datasheet of that detector? It looks like it uses a PSD, and if that's true you're trying to solve the wrong problem.

Bluefan said:

Can you give a part number or datasheet of that detector? It looks like it uses a PSD, and if that's true you're trying to solve the wrong problem.

Click to expand...

Part number/data sheet doesn't really exist I'm afraid. Here are some pics taken with an infrared camera of another persons unit. (this is a different kind that has two outputs, mine only has one)





Does that give you a better idea?