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MPE

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Toshimichi Otsubo:
Dear laser ranging colleagues,

On Thursday's session in Riga, we heard a few talks and comments on eye safety issues.  I knew almost nothing about MPE (Maximum permissible exposure) and want to learn a bit.

I was wondering how difficult/realistic it will be to make the energy density low enough (as intended in SLR2000).  But I have a question: in what time window(s) should we worry about our eyes?

I found it hard to understand IEC 60825 even in the Japanese translation, but some of the charts in this wikipedia page is partly understandable (but not sure if trustable).
  https://en.wikipedia.org/wiki/Laser_safety

Let me try to understand by assuming two lasers:
(H) Herstmonceux's:  532 nm, 0.5 mJ, 2 kHz, 10 ps FWHM, beam diameter 3 cm
(S) Stuttgart's:  1064 nm, 0.3 mJ, 10 kHz, 3 ns FWHM, beam diameter 7 cm

(1) Energy per pulse
I guess this chart
  https://en.wikipedia.org/wiki/Laser_safety#/media/File:IEC60825_MPE_J_nm.png
  caption: "MPE as energy density versus wavelength for various exposure times (pulse durations)"
is the one to look at.  We can read the MPEs at:
  MPE for (H) = 1e-8 J/cm^2 (where real (H) = 7e-5 J/cm^2)
  MPE for (S) = 2e-6 J/cm^2 (where real (S) = 8e-6 J/cm^2)
so Herstmonceux's pulse is far from eye safety but Stuttgart's is close to eye safety considering the fibre loss.

(2) Energy per longer time span (but how long?)
John Degnan commented that the IR could be dangerous because we cannot see it and the eyes can be exposed for a longer time. 
I am not sure but is this chart
  https://en.wikipedia.org/wiki/Laser_safety#/media/File:IEC60825_MPE_J_s.png
  caption: "Maximum permissible exposure (MPE) at the cornea for a collimated laser beam according to IEC 60825, as energy density versus exposure time for various wavelengths"
useful for this?  Then, we can read at 1 sec (correct?):
  MPE for (H) = 2e-3 J/cm^2 (where real (H) = 0.14e J/cm^2)
  MPE for (S) = 1e-2 J/cm^2 (where real (S) = 8e-2 J/cm^2)
again a similar story as above.

Is my understanding correct?  Can Stuttgart SLR be declared as an eye-safe system by reducing the energy a bit and/or by making the beam aperture large?

Or, if you know the best MPE expert in this community, I would like to contact him/her.

Toshi

jsteinborn:
Hi Toshi,

I think what John was referring to:

In general, the human eye will blink within 0.25 second at visible wavelength.
For IR there is no corneal reflex and the exposure time can be much longer.

Jens

Toshimichi Otsubo:
Thanks Jens, I understand that, but I have not understood what graph/equation/number we should refer.  Toshi

jsteinborn:
This document looks most complete:
http://prod.sandia.gov/techlib/access-control.cgi/2004/041111.pdf

Johann:
<!--                                                                                                           -->Hi Toshi,

to my understanding you are right in all of your points. However, i think there is a bit more than MPE.
Lasers are usually categorized in laser classes. The MPE is similar to laser class 1M, which is safe to the naked eye.
However, if you want an eyesafe laser you have to fullfill the requirements of class 1 (532: max .3,8e-8J in 50 mm Aperture; 1064: max. 3,8e-7J in 50 mm Aperture).
I think the difference between class 1 and 1M is that a class 1 laser is even safe, when using optics with an aperture of up to 50 mm.
ANSI Z 136.1 and i think also IEC 60825 define the requirement for the specific laser classes (enclosures, warning devices, ...).
Finally you also have to take into account the repetition rate of the laser system, which reduces the MPE by a factor of N^-0,25
if the repetition rate is above 600 Hz (IEC 60825-1:2014 p.28).

Concerning the numbers in (A):
Of course it is difficult (impossible) to get eyesafe, or class 1, with a bistatic (small transmitt telescope) SLR-system using picosecond pulse-width.
What our colleagues in Stuttgart do is to use nanosecond pulses to reach class 1M.

To point (B):
I think John meant the "eye-closure reflex", which "should" occure when bright light is seen.
It is assumed that the human eye is closed after a timespan of 0,25 seconds in that case.
Obviously only lasers emitting in the visible spectrum can be categorized in class 2.
I think this laser class is important for lasers with emission duration of more than 0,25 seconds only, not for our ps-lasers.

In Wettzell we are currently working on increasing the repetition rate of the WLRS to 400 Hz. We want to use the whole telescope aperture for
laser beam transmission and we want to switch to 1064 nm. By doing so we are class 1M up to a single pulse energy of 400µJ with our 10 ps laser.
Recently, we received first light from ETALON1 with an echo rate of about 5%. The approach seems promissing at the moment, we will see whats happening ...


Hopefully i am right with all this stuff, i am slowly getting confused ... ;-)

Johann

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