# Use laser to send morse code?



## emil.s (Jul 24, 2011)

Hello!

I'm thinking about using a laser to send morsecode a few kilometers as an experiment...

However, my experiance with lasers is mostly by pointing at the projector screen in powerpoint presentations.

So some questions.
What power is needed if it should be just possible to see when the laser is turned on and off, at ~5km (3 miles) away at night. (Would be perfect if you couldn't see it on a sunny day, but it should not be any problem at night)

And of cource without ANY risk of damaging your eyes. (IR filter, any more that should be done?)

I guess i'll be fine with some 1mw one? But I have never used lasers over these distances, so I really don't know.

And how many times can you turn a laser on and off before it breaks? (typically).

Or maybe some LED setup would be more suitable?
All input to this project is appreciated!


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## sandnabba (Aug 16, 2011)

Hello! No one who has input on this?


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## Steve K (Aug 16, 2011)

do a search on "free space laser", I think.... (edit: "free space optics" is better) this is the general term for shining a laser through the atmosphere and modulating the laser to carry information. It can be done, but the atmosphere itself is the main limitation. Getting rid of the atmosphere is what allows satellites to use lasers to communicate over many hundreds of miles, 

One way to reduce the potential for eye injuries is to reduce the energy density of the beam. This can be done by expanding the beam so it covers a wider area (perhaps 10's of square mm instead of 1 square mm). This is done with basic optics, but the trick is to keep the beam from diverging too much. 

There's also the trick of keeping the transmitter and receiver pointed at each other. If the receiver has an area of 10mm x 10mm, then at a range of 1km, you'll need to point the transmitter with an accuracy of 0.0005 degrees! If you mount the transmitter on a tripod, can you adjust it by that small of an angle, and will it be rigid enough to not flex or shift that much?

Anyway... there are a number of technical challenges to overcome in order to make something like this work. Do some reading, run some experiments at short distances, etc. I'm sure you'll learn a lot!

oh... no, you won't break a laser by turning it on and off. The main limitation is usually just how long it runs.


regards,
Steve K.

(still have a desire to modulate a laser myself, and use it to send audio, just for fun.  )


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## UberLumens (Aug 16, 2011)

You will need a clear line of sight on top of the other issues above.

It can be done, but you may find other solutions more appropriate. (a nice set of walkie talkies maybe)


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## Steve K (Aug 16, 2011)

there's something to be said for working out a lot of the technical issues by using led(s) instead of the laser. There's a strong urge to look down the line-of-sight that connects the receiver and transmitter, especially when getting the receiver aimed at the transmitter. Eye damage will take a lot of fun out of the project!

Steve K.


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## StarHalo (Aug 16, 2011)

The tripod would be the tricky part there, because you'd have to be able to turn the laser on and off without physically touching it. Also, atmospheric conditions will greatly effect how far your beam is legible; an unusually clear night will give you literally twice the distance over an unusually poor night, with no other visible difference between the two (both nights will look the same until you try using the laser.)


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## UberLumens (Aug 16, 2011)

> you'd have to be able to turn the laser on and off without physically touching it


This can be very simple, just block the beam with something, your hand, a piece of paper etc.

You can even have pre-made messages, ala the original animation technique, a zoetrope (slightly modified)


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## sandnabba (Aug 17, 2011)

Steve:
Thanks for tip about "free space optics". Found some intresting projects. 

However, one idéa with my project is that it should be possible to receive the transmission by hand. (Writing down the morse code in the old style way  )

And since that will be prety hard with a modulated laser, I think I'll stay with turning it on and off. (Which will be done with a computercontrolled relay).

Best for me, would be if the beam was something like 1x1m in 1km (1 milliradian), and it should be about as bright as the moon at ~5km. 

So I don't really know what's best. To build a focused LED setup, or defocus a laser?

And what's the typical runtime on a cheap china laser?


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## CKOD (Aug 17, 2011)

sandnabba said:


> Steve:
> Thanks for tip about "free space optics". Found some intresting projects.
> 
> However, one idéa with my project is that it should be possible to receive the transmission by hand. (Writing down the morse code in the old style way  )
> ...


 
Definitely the laser... 1 miliradian isnt really that defocused at all. Most $x00.00 laser modules I see online are in the <0.5mrad for divergence, I would imagine a cheap pointer would definitely be much worse. Also, with "reciving by hand" do you mean person just looking for the blinky light? or have a reciever that provides a light/buzzer for a person to record the morse like a telegraph would have? 

You'll definitely want some sort of modulation as 5mW spread over 1m^2 at the eyes peak sensitivity (555 nm green) is ~3.5 lux, and its going to be a lot lower for red, and a ton lower if your divergence of your cheap pointer is higher. 1 lux is 1.464 mW/m2 for 555mn green. 

Modulate it at a few khz, and have a reciever with a photodetector with a filter if possible, and bandbass filter for your modulated frequency, then add more gain and you could probably drive an earpiece directly, or use a 555 for turning the X khz square wave into a TTL signal for an indicator.


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## Steve K (Aug 17, 2011)

All in all, this is an interesting sort of project that contains most of the standard issues in setting up a communications link. One of these issues is increasing the signal to noise ratio ("S/N"). Obviously, you'll want to increase the signal as much as possible by increasing the transmitted power and increasing the antenna gain (which in this case will be the proper use of optics). Using a good antenna at both the transmitter and receiver will help a lot. Maybe a cheap telescope? Binoculars? Lots of options. With binoculars, you could easily set up duplex communications (i.e. be able to transmit and receive at the same time). 

Reducing noise can be done by limiting the bandwidth of the received signal. This can be done at the optical stage by using a filter that passes only the wavelength of interest. The photodetector should be sensitive to the wavelength you are transmitting at (i.e. 800nm? 900nm?). 

Modulating the transmitted signal, and then demodulating it at the receiver is another way to pick out a tiny signal out of the noise. Synchronous demodulation is the best, but even having a narrow bandpass filter will help. 

Lots of fun technical challenges! 
I really need to find time to play with this sort of stuff myself. I bought a laser diode about 10 years ago with plans to modulate it with audio, and haven't had time to touch it. I've built a few other gadgets, but I seem to come up with more ideas for projects than I have time for.

Steve K.


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## Steve K (Aug 17, 2011)

sandnabba said:


> And since that will be prety hard with a modulated laser, I think I'll stay with turning it on and off. (Which will be done with a computercontrolled relay).


 
you'll still be turning the laser on and off, but when it's on, it'll actually be a modulated (i.e. just pulsed) beam. This will let the receiver ignore any steady light sources, since you'll equip the receiver with a narrow band pass filter centered at the modulation frequency. Actually, there's a wiki page on modulation that says that this is the standard method for morse code using RF "The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation."


Steve K.


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## Cockatiel (Aug 21, 2011)

Hi,

there are some HAM Radio Amateurs worldwide they using Lasers for Communication. Just Google and type in "Laser Communication" and you'll find tons of Information about.

Greetings


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## Jorad (Aug 24, 2011)

I know a bit about lasers and that but where I can really help you is the electronics side. Using an Arduino it would be simple to modulate a laser beam to send data via morse code. Heck you could even use an arduino to decode and display the info in plain text on an LCD. Let me know


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## Steve K (Aug 26, 2011)

there's another thread on light communications over in the flashlight subforum... and the laser comes out looking like a poorer choice.

Steve K.


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## bshanahan14rulz (Aug 26, 2011)

for information modulation, yes, but seems to me that for something as simple as morse code, laser should beat LED, right? Because with morse code, who cares if the light becomes incoherent or if the wavefront becomes irregularly distorted? You just have to see if it's on or off.


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## SemiMan (Sep 2, 2011)

Talking off the top of my head, so I may be wrong.

Green should have the least attenuation through the atmosphere. It is also the most visible color. That said, if this is daytime? ... then there may be a high green component to compete with.

1M * 1M at 1KM very be a very difficult aim. I would increase the beam size a good factor of 10, but then user binoculars (which will help for aiming the receiver -- your eye), but also greatly increase light gathering.

And last, you could start with a simple wratten filter, but a quality glass filter will be better. Get something that is just going to pass the green from the laser. The narrower the bandpass, the better the signal to noise and the easier to make out the laser. Cheap semi lasers do drift a bit as they warm up, but perhaps if you could get a 5 or 10 nm filter, you would greatly reduce external light ... oh, and an eye cup to keep out stray light.

Semiman


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## Ceecy (Feb 7, 2014)

sandnabba said:


> Steve:
> sandnabba rene idéa with my project is that it should be possible to receive the transmission by hand. (Writing down the morse code in the old style way  )
> 
> i really like this idea. It makes the activity accessible by a wide range of people, with minimal planning and setup. I'm imagining a trial of sending a message 300 km in 3 mile (or horizon to horizon) relays. Could i help you try it?


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