Using fibre optic cable to direct a laser

[cheeseslice]

Hey,

I started a new thread a couple of days ago for extending an IR remote and it was suggested by a few smart people that using fibre would be the best idea.

It got me thinking about if I could use this in a project that I am about to start (once my parts arrive from china) so I just wanted to check to see if I'm going along the right lines.

Basically what I have is a long tube 60 inches or so that needs to be aligned exactly to a fixed reference point about 50ft away. I have made a smaller tube/casing that will fit snugly inside this tube and I have ordered a 5mw red laser which i am going to place inside the casing.

The problem I had was ensuring that the laser was going to be dead centre,even a small bit off and it would mess things up over that sort of distance. So what I had thought of doing was putting a cover over the other end of the 60inch tube with a hole dead centre and some sort of lens to focus the laser.

Would it be a better idea to run fibre from the laser to the end and have it glued to a cap in the centre, that way I wouldnt have to build the laser inside such a small casing. the tube is only 14mm in diameter.

Would the laser lose any of its intensity going through the cable? The reason I'm worried about this is that this alignment might need to be done outdoors up to 100m, I don't even know if this laser I have bought will work to that distance in daylight, but I'm hoping it will.

Another part of this project is using a 433 mhz transmitter and receiver to turn it on and off which would mean that I could just fit everything in the one box and build some sort of interface to attach the fibre cable to the laser.

 

[Peter M]

I'm not sure why you want the laser dead centre, but any deviation of the fibre starts the laser beam bouncing around inside the fibre, so it wont be coming strait out the end anyway, even if it did manage not to be scattered as well.
Think of optic fibre as a pipe completely surronded by a curved mirror

As to allignment of the laser in the first place, being I assume cheap chinese as well, the beam probably wont launch at a tangent to the face anyway.
Even most cheap laser levels have a screw arrangement like a tripod to align them.

If your tube is 60" long why cant you just look down it to align it?
Ive used this method to align a laser.
Look through the tube, see what you can see, then align the laser dot on the centre of what was seen!

Only guessing here as we dont know what your trying to do?

 

[cheeseslice]

A friend asked me if it would be possible to make this for him and I thought that seeing as I would get to play with a laser I would give it a go.

http://www.midwayusa.com/viewproduct/?productnumber=486659

I think the proper term is a boresighter.

So when I say 60inch tube, I mean the barrel of a hunting rifle!

I have looked at a few of the designs that can be bought and a lot of them opt for having the laser coming out of the end of the barrel, that way they can use it on different calibres with different sized rubber clamps put in the barrel to hold it in place. However I thought that it would be easier to build the laser into an empty casing that he gave me, that way I would know that it was going to be following the same path as the bullet. He is only using it for one type of gun.

The reason it has to be centre is because they calibrate their scopes at around 100m/100yards. If it was off even slightly this would be a massive difference by the time it got to the target. Some of the websites for these products only work to 25ft indoors but they use <1mw lasers which is why I bought the 5mw from China. It was only £3.50 anyway so I don't mind if it doesn't work, I can keep it and use it for something myself.

As far as the laser goes, are you saying that it probably won't come out straight?

Is there an easy way to focus this to a point?

That's why I had thought about putting a cap over the end with a whole in the middle and a lens of some sort.

As far as the remote side of it goes, I think it is so that the gun can be put in a clamp and the laser can be turned on and off when he is at the other end of the range, at the target.

Do you think I'm over simplifying this and there is a lot more to it?

 

[Dippy]

Only guessing too..

Umm, do you really think you will have perfect optics from a £3.50 cheap laser? They're probably only being sold off cheap as they are excess stock or 'seconds'.

Focussing may be a problem. Your average big fat lens may not help much. I think you will have to try a few lenses and find out for yourself. Sorry.
Certainly at close distance before much divergence a large lens has little effect as the the beam width is so small . You will need a smallish lens so that the beam width : lens diamter ratio is significant.

I completely agree with Peter.The fibre optic will 'ruin' your laser beam.

Trouble is people think a laser beam is 'magic'.
As alluded to by Peter you will have to have precision adjustable mounting of the module in a carrier.

A lump of pretty plastic with a laser module glued to it and plonked into/onto a tube will be useless, unless you are fortunate.
All this alignment stuff requires precise engineering, adjustable mounting and calibration.

Compare it with telescopic sights on a rifle. If they lined up first time from out-of-the-box then you'd be the luckiest sniper in the world.

PS. Have you ever wondered why laser alignment stuff is expensive?

Will the laser work in daylight?
- yeah, it'll work. Whether you can see it is the question I think.
You can improve visibility by getting some good quality transparent acrylic sheet of the same-ish colour as the laser.
When you look through it it will enhance the laser to background ratio.
Obviously it will depend on the 'target' (and your eyesight) as to whether you'll be able see it at 50 feet without binoculars.
Fire it at a bike reflector or maybe a car number plate and you'll see it easily. Fire it at your teacher's red jumper and you might not.
Have a go. Have a try. Experiment Dear Boy!

Good luck. I'm sure you can squeeze in a PICAXE to make this a PICAXE Forum question

 

[Peter M]

The most acurate way of aligning a barrel to a site is to actually use it for what its intended for, as these high speed projectiles, lets call em picaxes, (just to keep Dippy smilin') are all slightly different anyway

But must agree with Dippy you'll pay for a well aligned low divergance laser

Check this out
http://www.laserex.net/detail.LDM-2.X-Y.Adjustment.htm

.

 

[cheeseslice]

Good luck. I'm sure you can squeeze in a PICAXE to make this a PICAXE Forum question

I'll get the picaxe in when I put in the 433mhz receiver! He will be paying for it to be remote controlled whether he likes it or not.

Assuming of course I can work out the laser part of it

 

[cheeseslice]

The most acurate way of aligning a barrel to a site is to actually use it for what its intended for, as these high speed projectiles, lets call em picaxes, (just to keep Dippy smilin') are all slightly different anyway

But must agree with Dippy you'll pay for a well aligned low divergance laser

Check this out
http://www.laserex.net/detail.LDM-2.X-Y.Adjustment.htm

.

So basically the laser I am getting is going to be cheap and nasty and I've got a bit ahead of myself, typical.

Do you think it would even do enough to save him a few boxes of ammo and get him onto the target so that he can fine tune it the old fashioned way?

 

[Peter M]

typically the ones i've seen vary quite a lot in launch angle, you may be lucky.
But if not, with a bit of mechanical addition, if you have enough room in the breech you should be able to mount your laser module on the back of your empty round (needs the primer removed or the end drilled out).

possability 1.....make a platform, like a washer with three set screws supporting a second platform which the laser goes on seperated by a spring.
Next if you have access to a lathe, put the assembly in the chuck, shell case first, turn it slowly and see if the laser dot stays still on the wall (or whatever) next to the head of the lathe if not adjust the set screws till it does.

possability 2.....Who knows if your laser module is close to the size of the round you may even be able to align it then solder it in (hobby shops have varying size brass tube that may work as well!)

If its not dead centre in the barrel, the max error if its parallel with the barrel is half the width of a bullet. so if its aligned well, it should work well.

In use you could maybe modulate the laser and have a picaxe driven target so it tells you when your on target, save the hassle of whether or not you can see it easily or not in day time.
.

 

[Dippy]

Maybe cheap'n'nasty.
Definitely unknown. But at that price its disposable if not OK.
It'll probably be fine as 50 feet isn't very far.

Many of us have had good results with cheap lasers.
I've used £15 modules which have been perfectly OK for 100m, BUT as you can't guarantee perpendicularity then I too had to mount the module on an adjustable base.
In fact very similar to that described by Peter except I had springs on the screws (machine screws like bolts) with locknuts.

So, even if a little wonky, your laser module may be fine but YOU will have to put some effort into the mounting if you want good results.
Obviously your mounting and support will have to be rock-steady too.

Experiment, have a go and have fun.

 

[cheeseslice]

possability 2.....Who knows if your laser module is close to the size of the round you may even be able to align it then solder it in (hobby shops have varying size brass tube that may work as well!)

If its not dead centre in the barrel, the max error if its parallel with the barrel is half the width of a bullet. so if its aligned well, it should work well.

According to the specs the laser is 12mm, I think I will have 11.8mm to work with so I'll be doing a big of sanding, hopefully I can squeeze them together.

In use you could maybe modulate the laser and have a picaxe driven target so it tells you when your on target, save the hassle of whether or not you can see it easily or not in day time.
.

I never thought of that, I think it's a brilliant idea. I'll have a picaxe run target on the bullseye 100m away, have it send back a signal when the laser is on it and then line up the scope to that target.

So the next question would be what is the best way to detect the laser. What would the purpose of modulating it be and how would it be done?

Or I could just ignore it sending back a signal and have it light up a number of bright leds on the actual target itself, that would mean he could see everything through the scope.

Thanks again

 

[Dippy]

"Or I could just ignore it sending back a signal and have it light up a number of bright leds on the actual target itself, that would mean he could see everything through the scope."

- sounds great. Try it. You don't need us to hold your hand


Sensing?
This will cause arguments.
I would use a shrouded photodiode.
Photodiodes have a greater dynamic range than phototransistors and are far better when there is a lot of ambinent light.

The problem for you will be huge amounts of ambient.
Fortunately a laser will give you plenty of signal.

Modulation?
Other than power saving and lower heat at the transmision end, modulation makes detection easier. You can notice 'flashing' more easily than a constant light.
Similalrly, a ph-diode plus AC coupled op-amp with high-pass or band-pass filtering wil be much, much more sensitive than just looking for changes in levels.
(This may not be so significant with a laser but is CRUCIAL when using wimpy IR LEDs and photo diodes.)

Maybe you could hack one of those IR 38kHz modules?

In addition to that, modultion will help because your signal:noise at visible wavelengths will be worse than at IR wavelengths.
Optically good coloured filter plastic will help too and shrouding with baffles will help hugely. (Same sort of technique used in refracting telescopes).

One of the best chips I've seen or filtering and amplifying signals from a Photo-diode is the TBA2800. Still available apparently.

There are numerous ways to detect modulations (i.e. a frequency). I'll leave that to others.

I have used a similar method in the past because I was using IR lasers, and it is fiddly. Remember, a modulated laser will look dimmer than a constant one. If you can't see the spot, you will waving your target around for ages trying to find the spot.
So, good luck and eat some carrots.

 

[Peter M]

not sure how fast you can modulate a laser module (as you are powering it up and down, depends on how the current/power limiting circuitry works on your module), but I was thinking serout to drive the laser with a qualifier
i.e. serout 1, T600, ("ABC",b0)
and simular on the target with serin

maybe its been done or someone else has tried it. Try a search

And yes set the target to light up or even change colour so its easy to see all the time?

looks like Dippy's said most of it!

 

[cheeseslice]

Right, I'll stop asking questions now until I get the parts in and start building it. I'll probably be back then to torture people with specific questions.

Thanks for the help.