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

done.

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.

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

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:

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

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) <drkirkby@kirkbymicrowave.co.uk> wrote:

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

done.

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.

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

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

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:

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

Dear Ilia

On Sun, May 1, 2016 at 6:40 PM, Ilia Platone info@iliaplatone.com wrote:

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.

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

Cheers
Michael

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

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.

On 1 May 2016 15:02, "Bruce Griffiths" bruce.griffiths@xtra.co.nz wrote:

unaided eye cannot collect a power greater than the safe limit. Using near
IR beams also helps.

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.