Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Dr. David Kirkby (Kirkby Microwave Ltd)
On 1 May 2016 15:02, "Bruce Griffiths"  wrote:
>
> The solution with high power is to use a beam expander so that the
unaided eye cannot collect a power greater than the safe limit. Using near
IR beams also helps.
> Bruce

It is a long time since I worked with lasers, so my knowledge is both out
of date and my memory not perfect. But I was a long while ago regularly
using and sometimes aligning a picosecond pulsed laser at about 800 nm. The
average output power was 1 W, so a very dangerous class IV laser. It
certainly hurt if one had the beam on ones skin, but it was not
sufficiently powerful to noticeably burn the skin if one did the obvious
thing and moves ones hand away.

But I believe people need to be particularly careful using IR lasers. The
lens in the eye will not focus Iinfra red on the retina,  so that is
probably why you say IR is safer. However one of the protective methods the
eye has is a "blink response". One blinks if one perceives a light source
as bright. Blinking offers some protection to the eye. But since one does
not see an IR laser source, one  does not blink, so IR lasers disable one
of the eyes protective mechanisms.

Hence there are various complications that arrive when discussing laser
safety issues.  I don't have the knowledge to advise on what is or not
safe,  but will warn there are several effects which are important,  and
many people don't realise this.

Laser safety is a nontrivial subject.

Dave.
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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Ilia Platone

Thank you, I personally were talking about 1400nm 1mw lasers, however.

Supplying just above the threshold current is not a problem.

does the raising time can be reduced if using lower current/voltage 
raises or falls? I mean: how's calculated the raise time, full-scale 
pulse or for a mW/mA or so amount?


Ilia.


Il 01/05/2016 18:48, Mark Sims ha scritto:

There are "eye-safe" wavelengths that some laser diodes can operate at 
(generally greater than 1300 nm).  These are much less prone to damage eyes.  Basically 
your eyeball juice blocks the wavelength.  Still, there is some potential for cornea and 
lens damage at higher powers.
http://www.laserfocusworld.com/articles/2008/03/photonic-frontiers-eye-safe-lasers-retina-safe-wavelengths-benefit-open-air-applications.html


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Cell +39 349 1075999

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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread jimlux

On 5/1/16 9:48 AM, Mark Sims wrote:

There are "eye-safe" wavelengths that some laser diodes can operate at 
(generally greater than 1300 nm).  These are much less prone to damage eyes.  Basically 
your eyeball juice blocks the wavelength.  Still, there is some potential for cornea and 
lens damage at higher powers.
http://www.laserfocusworld.com/articles/2008/03/photonic-frontiers-eye-safe-lasers-retina-safe-wavelengths-benefit-open-air-applications.html

If they eye blocks (as in absorbs) the power, fine, you don't get a 
retina burn, but you get cataracts or other damage instead (more like 
"looking into the waveguide with your remaining good eye").



It's all about energy deposition and there's copious literature on what 
safe levels are (on Wikipedia even)



This kind of thing is fairly easy to do safely, you just have to work 
through the scenarios and recognize that doing it outside isn't like 
doing it the relatively controlled environment of a lab.


With reference to my previous comment about binoculars and telescopes, I 
don't think you have to worry about someone at the intended receiver 
site looking back at the transmitter (any beam divergence at all will 
make the energy density so low at the receiver, it's probably not an issue).


It's someone who's a lot closer than you expected turning around and 
looking back at the transmitter or an unexpected specular reflection off 
something.



BTW those geophysics measurements with red visible lasers across 
California were mostly done with HeNe lasers, because that's what was 
available back then.  They also have a nice long cavity (The 1mW 
SpectraPhysics 155 that I had back in 1978 had a 30cm-ish cavity), so 
the spectral purity is quite good and the beam divergence is low.  The 
HeNe are also a lot brighter visually than the red diode laser pointers 
for the same optical power.


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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread jimlux

On 5/1/16 3:22 AM, Bruce Griffiths wrote:

The solution with high power is to use a beam expander so that the unaided eye 
cannot collect a power greater than the safe limit. Using near IR beams also 
helps.
Bruce



IR is a problem for eye safety, because IR doesn't trigger the blink 
reflex, so you can inadvertently "stare into the laser with the 
remaining good eye".



If you want to stay below, say, 1 mW/square cm, and you're running a 20 
mW laser, you'd want 20 square cm of aperture.  That's about 5cm diameter.


You should bear in mind that if you're doing "mountain top to mountain 
top" type applications there might be someone looking at you with 
binoculars or a telescope, which makes their "light gathering aperture" 
much larger, and increases the risk of injury.



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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Michael Wouters
Dear Ilia

On Sun, May 1, 2016 at 6:40 PM, Ilia Platone  wrote:
> The problem would be modulating a 10GBASE-T signals into a single laser
> beam, and demodulating it using (I think) an APD.
>

The White Rabbit cards use SFP (small form-factor pluggable) lasers
that plug into the card. These incorporate both the laser(s) (there
can be an uplink and downlink laser) and APD. Modulation in these
devices is simply by varying the current. This puts a frequency chirp
on the laser which exacerbates dispersive effects. You can do better
with an electro-optic device like a Mach-Zehnder intensity modulator -
but this isn't necessary.

> except the one depending on light travel, that shouldn't be a problem if
> using White Rabbit, there could be some problem with the modulating and
> transmitter/receiver delay response times.

I remember reading that the SFPs are a source of jitter but
nonetheless, sub-ns timing is achievable.

> I mean that lasers offer 100ps rise time, and the APDs I found offer 5ps
> rise time, these must be multiplied by all the wires that GigE needs, which
> should be 4 pairs if I remember correctly, and some are bi-directional, plus
> the modulation process.
>
> Ilia.
>

Cheers
Michael
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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Bruce Griffiths
The solution with high power is to use a beam expander so that the unaided eye 
cannot collect a power greater than the safe limit. Using near IR beams also 
helps.
Bruce
 

On Sunday, 1 May 2016 9:00 PM, Dr. David Kirkby (Kirkby Microwave Ltd) 
 wrote:
 

 > Hi,
> Several (many?) years ago National Geographic magazine show a picture
taken here in southern California of the state government sending red laser
signals between different mountain tops to keep track what was going on
near fault lines
> There were no technical details on what was taking place.  So it can be
done.
> At a hamfest a few years ago I bought both a red and green 35 mW laser
pen for about $15 each.  They do shine a long, long way.

35 mW is certainly unsafe to the eyes, so be very careful.  There maybe
legal issues about doing this.

> Whether these are powerful enough, or can be properly modulated for what
is needed, I have no idea.

You can pretty much modulate any laser diode. There are two important
currents to know about

* Threshold current I_th - below which it will not lase.
* Maximum operating current I_max - above which the device will be
destroyed.

You can AM modulate them by applying a DC current

I_th + (I_max - I_th)/2.

Then superimpose the modulation which has a peak value of (I_max - I_th)/2.

Those currents ensure that the laser is always lasing,  and gets you
theoretically 100% modulation.  For best lifetime,  run at lower levels of
peak modulation current.

Watch out for transient currents - lasers make transistors look like
antisurge fuses!

For point to point contact you want a beam which diverges as little as
possible.  IIRC the divergence is something like inversely proportion to
the cavity length.  For this reason a diode laser with its short cavity is
not optimal. But of course they are cheap.

A veey long time ago I used to know quite a lot about lasers,  but not
using them for years I realise that I have forgotten an awful lot!

FWIW, at university we had a 10 W argon ion laser. I think it took about 50
kW to produce those 10 W. When it was disposed of, it was sold to a company
that put on light shows.

Dave
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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Ilia Platone
Threshold current should not be a problem because if there's no data the 
laser could go into "power saving mode".


As am modulation a simple buffer/r2r network DAC should do the job. The 
signals to transmit are three: Tx, and two bidirectional.


Ilia.


Il 01/05/2016 10:27, Dr. David Kirkby (Kirkby Microwave Ltd) ha scritto:

Hi,
Several (many?) years ago National Geographic magazine show a picture

taken here in southern California of the state government sending red laser
signals between different mountain tops to keep track what was going on
near fault lines

There were no technical details on what was taking place.  So it can be

done.

At a hamfest a few years ago I bought both a red and green 35 mW laser

pen for about $15 each.  They do shine a long, long way.

35 mW is certainly unsafe to the eyes, so be very careful.  There maybe
legal issues about doing this.


Whether these are powerful enough, or can be properly modulated for what

is needed, I have no idea.

You can pretty much modulate any laser diode. There are two important
currents to know about

* Threshold current I_th - below which it will not lase.
* Maximum operating current I_max - above which the device will be
destroyed.

You can AM modulate them by applying a DC current

I_th + (I_max - I_th)/2.

Then superimpose the modulation which has a peak value of (I_max - I_th)/2.

Those currents ensure that the laser is always lasing,  and gets you
theoretically 100% modulation.  For best lifetime,  run at lower levels of
peak modulation current.

Watch out for transient currents - lasers make transistors look like
antisurge fuses!

For point to point contact you want a beam which diverges as little as
possible.  IIRC the divergence is something like inversely proportion to
the cavity length.  For this reason a diode laser with its short cavity is
not optimal. But of course they are cheap.

A veey long time ago I used to know quite a lot about lasers,  but not
using them for years I realise that I have forgotten an awful lot!

FWIW, at university we had a 10 W argon ion laser. I think it took about 50
kW to produce those 10 W. When it was disposed of, it was sold to a company
that put on light shows.

Dave
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--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999

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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Dr. David Kirkby (Kirkby Microwave Ltd)
> Hi,
> Several (many?) years ago National Geographic magazine show a picture
taken here in southern California of the state government sending red laser
signals between different mountain tops to keep track what was going on
near fault lines
> There were no technical details on what was taking place.  So it can be
done.
> At a hamfest a few years ago I bought both a red and green 35 mW laser
pen for about $15 each.  They do shine a long, long way.

35 mW is certainly unsafe to the eyes, so be very careful.  There maybe
legal issues about doing this.

> Whether these are powerful enough, or can be properly modulated for what
is needed, I have no idea.

You can pretty much modulate any laser diode. There are two important
currents to know about

* Threshold current I_th - below which it will not lase.
* Maximum operating current I_max - above which the device will be
destroyed.

You can AM modulate them by applying a DC current

I_th + (I_max - I_th)/2.

Then superimpose the modulation which has a peak value of (I_max - I_th)/2.

Those currents ensure that the laser is always lasing,  and gets you
theoretically 100% modulation.  For best lifetime,  run at lower levels of
peak modulation current.

Watch out for transient currents - lasers make transistors look like
antisurge fuses!

For point to point contact you want a beam which diverges as little as
possible.  IIRC the divergence is something like inversely proportion to
the cavity length.  For this reason a diode laser with its short cavity is
not optimal. But of course they are cheap.

A veey long time ago I used to know quite a lot about lasers,  but not
using them for years I realise that I have forgotten an awful lot!

FWIW, at university we had a 10 W argon ion laser. I think it took about 50
kW to produce those 10 W. When it was disposed of, it was sold to a company
that put on light shows.

Dave
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Re: [time-nuts] Using lasers for data transmission

2016-05-01 Thread Ilia Platone
The problem would be modulating a 10GBASE-T signals into a single laser 
beam, and demodulating it using (I think) an APD.


except the one depending on light travel, that shouldn't be a problem if 
using White Rabbit, there could be some problem with the modulating and 
transmitter/receiver delay response times.


I mean that lasers offer 100ps rise time, and the APDs I found offer 5ps 
rise time, these must be multiplied by all the wires that GigE needs, 
which should be 4 pairs if I remember correctly, and some are 
bi-directional, plus the modulation process.


Ilia.


Il 01/05/2016 07:13, Perry Sandeen via time-nuts ha scritto:

Hi,
Several (many?) years ago National Geographic magazine show a picture taken 
here in southern California of the state government sending red laser signals 
between different mountain tops to keep track what was going on near fault 
lines.
There were no technical details on what was taking place.  So it can be done.
At a hamfest a few years ago I bought both a red and green 35 mW laser pen for 
about $15 each.  They do shine a long, long way.  Whether these are powerful 
enough, or can be properly modulated for what is needed, I have no idea.
On line I saw at least one China site that had much larger outputs available.  
The prices were modest. FWIW.
Regards,
Perrier
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--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999

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