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1
Open a Discussion / Re: MPE
« Last post by jsteinborn on Today at 03:56:29 PM »
It looks like we need some kind of math support for formulas :-)

Matt?
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Open a Discussion / Re: MPE
« Last post by Toshimichi Otsubo on Today at 03:26:06 AM »
Thanks Johann.  Toshi
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Open a Discussion / Re: MPE
« Last post by Johann on Today at 02:19:54 AM »
Toshi

here are the numbers for the MPE (or class 1M) that i found for 1064 nm:

ANSI Z136.1-2007:
27*t^0.75 J/cm2 ~ 1.5e-7 J/cm2
and N^-0.25

ANSI Z136.1-2014:
2e-6 J/cm2
no reduction from repetition rate

IEC 60825-1:2014:
2e-2 J/m2
N^-0.25 for N>600 Hz

for WLRS this leads to:

2007:
~0,6 mJ

2014 (ANSI & 60825):
~8 mJ !!!

it seems that the situation for 1064 nm has relaxed in the latest documents. For all of this numbers the WLRS should be "eyesafe" in the mentioned configuration.

In Wikipedia i found from figure 1: 2e-7 J/cm2
and in figure 3: ~9e-8 J/cm2


However, there is another statement that i found in IEC 60825-1:2014, concerning a Protective Housing:
"Each laser product shall have a protective housing which, when in place, prevents human
access to laser radiation (including errant laser radiation) in excess of the AEL for Class 1,
except when human access is necessary for the performance of the function(s) of the product."

So in principle MPE (class 1M) is not eyesafe. The AEL for class 1 from IEC 60825-1:2014 is:

7.7e-7 J in a 50 mm aperture => ~ 1.2e-7 J/cm2
or ~0.17 mJ for WLRS

There is no clear definition of what needs to be done, when going with lasers through the atmosphere. So it is depending on the corresponding authority, if class 1M is sufficient...

Best regards,
Johann
4
Open a Discussion / Re: MPE
« Last post by Toshimichi Otsubo on October 17, 2017, 04:07:05 PM »

Johann and Jens

Thank you very much.  Amazed to hear that you could track Etalon solidly with such a weak-energy laser.

I am still learning and a bit confused.

I applied the same procedure as Herstmonceux and Stuttgart to the WLRS-IR based on what you wrote.  It does not seem eye-safe in my quick computation.

(W) WLRS:  1064 nm, 0.4 mJ, 400 Hz, 10 ps FWHM, beam diameter 75 cm

The MPE per pulse in the first wikipedia graph is:
  MPE for (W) = 3e-8 J/cm^2 (where real (W) = 9e-8 J/cm^2)
which suggests WLRS-IR is NOT EYE-SAFE.  Am I right?

The MPE per 0.1 s and per 1.0 s in the second wikipedia graph is:
  MPE for (W) = 2e-3 J/cm^2 per 0.1 sec (where real (W) = 4e-6 J/cm^2)
  MPE for (W) = 1e-2 J/cm^2 per 1.0 sec (where real (W) = 4e-5 J/cm^2)
which looks ok.

The N^(-0.25) rule ("Rule 3" in the document from Jens message) will make the MPE simply lower, if I understand correctly.

The PDF document Jens suggested looks useful for visible (400-700 nm) wavelengths - thanks.  I would be glad to see the same thing for 1064 nm.

Toshi

5
Lasers / Re: Start Diodes - What is best?
« Last post by Matt Wilkinson on October 16, 2017, 10:16:51 AM »
We use an AEPX silicon photodiode from Centronic http://www.centronic.co.uk/products/3/high-speed which has a ~0.6ns rise time.

We align the diode, control the intensity with ND, observe the output signal and set the discriminator threshold to give the lowest possible calibration RMS.

Is there something out there that's better?
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Open a Discussion / Re: MPE
« Last post by Johann on October 14, 2017, 01:06:39 AM »
<!--                                                                                                           -->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 400┬ÁJ 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
7
Ideas / Re: Making satellites visible during daylight ranging
« Last post by ZhipengLiang on October 13, 2017, 12:53:55 PM »
We in Changchun also saw Envisat in daylight. Attached is the screenshot when that happened. It was morning. Ignore that system time, it was wrong. The satellite and the beam can both be seen. We used 532nm band pass filter to let in the laser back-scatter, but Envisat was so bright that it is also visible.
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Solutions, Advice and Experience / Changchun station workshop presentation leak
« Last post by ZhipengLiang on October 13, 2017, 12:12:38 PM »
Attached are two presentation slides converted to zipped image folders.  I put them in automation session, but it turned out that mine was actually semi-automation  :o  . Whatever they are here now. Please reply to tell me your thoughts about my work.
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Dr. Kucharsky gave a brilliant presentation about Ajisai optical measurement. It's possible to recover the state of every single mirror on it. Even found a crack on one of the mirrors.
It's awesome! Hope you could see it. :D
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Open a Discussion / Re: Tracking CZ-2C rocket bodies with NORMAL laser
« Last post by ZhipengLiang on October 12, 2017, 03:09:59 AM »
Dear Zhipeng,

what is the per pulse energy of the laser you used for the tracking of the rocket bodies?

Thanks and best

Sven

Hi Sven,

It's our normal SLR laser, with 1mJ pulse energy and 1kHz repetition rate, making 1W laser power. I think other high rate station could do it as well or better.

Regards,
Zhipeng
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