Laser Ethernet Data Link

Hello all,

I have a project that I would like to run past all you laser experts.

I need to add a device to my Ethernet network LAN but its got several problems.
The device is fitted to a crane hook and thus moves about in height and location. The Hook goes down to about 30m underwater. Umbilical's aren't really an option due to them almost definitely getting caught up or snagged as the crane moves about. I only need to communicate with the device in selected location (all underwater).
I've done some tests and I can get between 100 to 150mm between WIFI devices underwater so this may be possible if I can set up a leaky feeder or something.

What's this got to do with Lasers you're thinking. ??

Well I put my 5mw green laser in a sophisticated waterproof housing (two zip lock plastic bags) and taped it up tight and chucked it in the lake. I found that I can clearly see the laser from 30m down though the spot size is rather large. This started me thinking.
Could I get two media converters (Ethernet to fibre optic) and connect a laser on the transmit side and aim it at the receive side. I would still have problems aligning it but I think I may have a solution for that if the communications work.

So question are :-

Has anyone known anything like this before?

What power lasers are normally in these media converters (MC)? Would it be possible to connect a laser pointer instead of the output laser or would it not be able to supply enough power.

Are the lasers in the MC normally red or IR ?

Is there anything special about the MC lasers ? I think the carrier frequency for a 10base network is 2.5Ghz but I'm not even sure of that.

I guess a green laser wouldn't be fast enough as it's normally pumped (I think) but red laser diodes in a laser pointer maybe the same sort of laser in the MC so I'm hoping would work. Of course sods law is the green travels a lot better in the water than the red does but maybe I could always double up on the diodes. I wouldn't really want to go any more powerful due to safety concerns.

Any thoughts ? Thanks in anticipation..

there are direct injection green lasers that have the same modulation characteristics as red lasers.

the osram 520nm diodes here: https://sites.google.com/site/dtrlpf/home/diodes

perfo said:

I guess a green laser wouldn't be fast enough as it's normally pumped (I think) but red laser diodes in a laser pointer maybe the same sort of laser in the MC so I'm hoping would work. Of course sods law is the green travels a lot better in the water than the red does but maybe I could always double up on the diodes. I wouldn't really want to go any more powerful due to safety concerns.

Any thoughts ? Thanks in anticipation..

Click to expand...

Thanks for the link. Any idea if those diodes would be a direct swap for the ones in a media converter ? As this device will be on a crane hook and thus I can't be absolutely sure which direction it'll be pointing I was thinking of trying to keep it below 5mw to reduce the chances of stray beams injuring people , animals or flying things..

Almost all laser communications units use infrared diodes, so no there is no direct swapping possible.

I thought I saw a red dot at the end of a fibre optic cable I once looked at but maybe not, hmm yes it was a good few years ago but a fellow showed me to put a thin bit of paper over a fibre optic and you can see which one is the transmit fibre due to a faint red glow. Mind you that was a good few years ago so maybe they've all swapped to IR.
Do you think no swap possible due to the receiving end not being sensitive enough to a red or green laser or was it something else ?
...I've managed to get myself a few media converters from ebay for £1 each so I'll have a go at hacking them when I get them.

Thanks for your thoughts.

That faint red glow you saw was in fact IR. Your friend has taught you to something which has the
potential to damage your vision. The wavelengths and powers used are in a range where eye damage
could have already occurred by the time you see the light. Some of the 1300nm fiber links are very
dangerous as they use high power lasers which cannot be seen at all, yet will burn your retina. So I
advise you not to do that and instead use a proper IR detector and NEVER look into the end of a fiber,
even if you're sure it is inactive.

Now about your question, you may be able to swap a red diode in place of the IR, but a green one is not
going to work. Green (or greed because they're so expensive) diodes can have a typical voltage drop in
excess of 7V. Even red ones have a higher voltage drop than IR. I can tell you that you don't want to
use IR for water because much of it will be absorbed. The closer you can get to 500nm, the better. I
don't think DPSS will work either because it takes time for energy to build in the cavity.

So you're probably stuck with red or blue if you can get it to work. Actually, blue is the best, but blue
diodes have a voltage drop between red and green. Think about it, what color do you see when you look
through water? You would still need a waterproof lens and housing. The focus would have to be
adjusted underwater because it has a different index of refraction than air.

You're also going to have a problem with the beam moving around and pointing all over the place, so this
is a potential eye hazard. I don't know how you will keep it aimed on your detector.

RF might be the way to go.

Wow thanks for the heads up with the IR damage. I must admit as it had to shine through a bit of paper to get to me I did think I was safe.
I put a good deal of thought in to RF but can't see an easy way of getting any distance with anything that will carry a video signal. No problems for slow stuff but video will need a high frequency carrier and that runs in to the same problems as WIFI. I am also investigating a WIFI option as it appears 2.4ghz WIFI will transmit up to 150mm in water so If I rig up a leaky feeder that runs surface to bottom and have a WIFI transceiver connected to the cam and traversing up and down maintaining a max of 100mm gap to the leaky feeder then this may work.
No problems with blue laser diodes would you have any suggestions as to what type or where I could get some cheap ones for playing with? I agree IR is useless underwater and coming from that end of the spectrum the red would be worse than something from the blue end. The green was used purely as I had a green laser and red laser and wanted to try it. The blue should be the best at travelling but as you’ve indicated I wasn’t sure what colour I’d be able to get for a reasonable price that can be keyed fast enough to replace the lasers in the media converter. I must admit I have totally failed to find out anything about the type of laser used in these things (ie power , wavelength or construction.
My aiming method of the lasers will need a bit of work but I think it is doable. However I can’t be 100% sure the laser wont shine somewhere it shouldn’t that’s why I hoped to keep it to <5mw and if my green 5mw laser did the job then a blue one should (I think) .
Thanks for your comments.

You can get a suitable diode here. https://sites.google.com/site/dtrlpf/home/diodes/450nm-pl450-diodes

You'll also need a waterproof module and lens or a waterproof box with a suitable window. The Ethernet
box may need some modification to drive it since the voltage drop is quite a bit higher than the IR led. I
would try it unmodified first, you never know. I know the ones I have here won't do it because they only
run on 5V. If you do have to modify, since speed is critical, you will most likely be looking at an NPN or
PNP type pull-up or pull-down circuit. I can help with that as well.

You also have to be REALLY careful with static. I cannot stress that enough. A lot of first builds fail by
the ways of ESD. Get a good wrist strap if you do not have one already and work over an ESD mat if
you have one. Same goes for the soldering iron. It's not hard to kill an Ethernet media converter with
ESD either, so that goes x2.

Just be safe. Even 5mW can cause eye damage if it's allowed to shine directly into your eye for long
enough. These PL450B diodes are capable of much more than 5mW, so I would recommend picking
up a pair of safety glasses from Survival Lasers.

Thanks.
Why are those lasers so expensive? Is it due to the construction ? I'm not sure I could stump up well over $100 for a couple of diodes just to kill them in a lake or by soldering. :drool:
The water proof housing is ok, I think I've got that sorted. There will be a bit of loss going through my housing window but the light reaching the target wasn't the problem so I should be ok in that respect. I don't know if the water changes the shape of the wave form at all. It makes the spot size bigger by bouncing off things but If I inject a nice neat square wave will I get a nice neat square wave at the receiver? I'm assuming as you seem to know about these things and how recommended the lasers above the cheap $1 diodes wont work ?

Where are you seeing blue laser diodes for $1? You might be able to get a violet diode a bit cheaper. It
won't work as well, but is still better than a red one. You could also try extracting one from a BluRay
device, or just use an eBay pointer. I think the higher price is because they aren't used in any common
consumer level devices. The red ones are used for reading DVDs and the violet in BluRay.

The water will have detrimental effects on the beam. It will cause scattering, which dirt will make worse.
The surface will also cause refraction and even lensing if there are any waves. It will also alter the
focus, but only if the water is actually touching the lens because it has a different refractive index than
air. None of this will be a problem as long as the laser has the energy to pierce through and reach your
receiver, preferably one with a large surface area.

Sorry it wasn't for blue lasers, I did mean the red chepo pointer types. If the red chepo will work from all other aspects i.e. speed then I could try and do some more test to see if I can eek the 30m out of them. The receiver and transmitter will both be in the water so I'm hoping that will cut out a lot of the problems going from water to air etc and no surface effects. I did think of blue ray as they seem like they may like the high speed etc but I was worried about the increased chance of them damaging someone or something. No one is going to be looking in to the beam deliberately or for any length of time but i can't guarantee I wont get a good reflection off something and hit the crane operator or bystander.

Yes, and the red is easier to get going from an electrical perspective. As long as there is enough power to
punch through, it doesn't really matter what the wavelength is. I also agree with you about the shorter
wavelengths being more dangerous. I don't have any scientific data to back it up, but I think 5mW of violet
is worse for your vision than 5mW of red. I think there was some discussion on the subject and some
agreement in this regard. A pound of feathers and a pound of hammer are both one pound, but the
hammer is going to hurt a lot more if it falls on your foot.

Do you think basically any red laser diode will work (less volt drop and fast enough) or do I need a particular type

For example this one on ebay ?

5mW 650nm 6mm DC 5V Mini Laser Dot Diode Module WL Red Housing Copper Head Tube | eBay

It doesn't give any details other than red and 5mw. Aren't the blue ray ones more like 100 mw as well ?

Thanks

Yes most of the violet pointers will say 5mW but be way overspec, like 100mW. The module in your link
is truly around 5mW. I don't really think it matters what laser diode you get from the perspective of speed.
AFAIK, they're all about the same.

righto ho, thanks for that. I'll await my media converters and start hacking then. With a standard red do you think the volt drop will still be a problem ? Will I need a boost with a transistor ?

The Lightning Stalker said:

Yes, and the red is easier to get going from an electrical perspective. As long as there is enough power to
punch through, it doesn't really matter what the wavelength is. I also agree with you about the shorter
wavelengths being more dangerous. I don't have any scientific data to back it up, but I think 5mW of violet
is worse for your vision than 5mW of red. I think there was some discussion on the subject and some
agreement in this regard. A pound of feathers and a pound of hammer are both one pound, but the
hammer is going to hurt a lot more if it falls on your foot.

Click to expand...

Actually there is some truth to what you've posted.
Red light is readily absorbed by your skin , and in terms of your eyes the frequency of the light is low enough that it passes right through your lens
Without much issue. High output red laser eye damage is mostly
Thermal damage in much the same way IR does.
405nm-visible blue light has a great deal more energy and can cause photochemical injury to the retina by breaking down cells, in much the same way as your skin gets damaged when sun tanning. Only in this case the damage lasts a very long time or is permanent.
Scientists now believe that any source of intense blue light is capable of doing this kind of damage, also now referred to as photoretinitis.
Actually there are a few studies indicating that long term exposure to
Blue light may be responsible for an increasing number of eyesight problems
around the world. Ccfls and fluorescent lights blue spectrum
component is largely believed to be the cause of eye problems.

One such article below, explains the hazard quite well.

http://www.ncbi.nlm.nih.gov/m/pubmed/12061267/