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Open a Discussion / Satellite BLITS-M
« Last post by Sokolov on December 05, 2017, 09:01:56 AM »
BLITS-M is the glass spherical satellite. Diameter of the satellite is 220 mm. The one half of sphere is coated by interference dielectric layers for reflecting the laser beam with wavelength 532 nm. Orbital altitude is 1500 km. The planned time of the launching is May 2018.

The satellite will rotate with speed of 10 turn per minute around the axis perpendicular to the orbital plane.

Is it possible to identify the satellite by a telescope?
 
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Open a Discussion / Re: MPE
« Last post by danielhampf on November 28, 2017, 11:02:06 AM »
Hello everyone,
since our system is mentioned here I thought I should comment. I have just spoken with the laser safety expert in our institute about this. Since I personally know very little about this topic, I will only summarise his comments.
He has checked the above posts and finds many statements which he thinks might be wrong or at least misleading, and therefore dangerous. His main point was: Calculating MPE is a quite difficult business, and almost impossible to do for a general case. Pulse duration, pulse repetition rate, wavelength, beam diameter and divergence are only some of the factors to take into consideration.
We have a detailed analysis of our current system, that defines what precautions are necessary at what point. Indeed, we are currently eye-safe at a distance of a few hundred meters from the transmitter. If we decrease our divergence in the future, this might change. Our procedures will always reflect the specific analysis.
Toshi, since you helped us so much with the data analysis, I suppose we could try to help you with a safety analysis of a proposed system. However, our laser safety officer is reluctant to do public statements in this forum which people might misunderstand, but rely on and possibly do dangerous things with. But if you could send me your specs via PM, I will try to get an analysis from him.
Best regards,
Daniel
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Timing / Pulse collision avoidance
« Last post by danielhampf on November 27, 2017, 08:34:48 AM »
Hello everybody,
I am currently re-working our trigger hard- and software and I was wondering about pulse collision avoidance schemes at high rep rates. Currently, we use either:
1) Standard continuous triggering, ignoring pulse collisions. Works okay up to ~5 kHz in our configuration.
2) Burst mode: One ToF fire, one ToF quiet/receive. Works well at all rep rates, but you always loose half of the time.
I suppose for the really high rep rates, burst mode is about the best we can do. But what about 1 to 10 kHz? Do you have any recommendations of pulse collision avoidance schemes? I suppose we could trigger at, say, 5 kHz and reduce to 4.5 kHz every time the ToF come close to N x LI (laser interval). But is that the best we can do? What do you use? What would be an ideal scheme?
Thanks,
Daniel
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Open a Discussion / Re: MPE
« Last post by jsteinborn on October 18, 2017, 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 October 18, 2017, 03:26:06 AM »
Thanks Johann.  Toshi
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Open a Discussion / Re: MPE
« Last post by Johann on October 18, 2017, 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
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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

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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
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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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