# RE: Question on Laser Safety Limits

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Jonathan,
sorry for the delay in the response. I am out of the office at another
standards meeting and not in frequent contact with my email.

The answer you seek, I believe, is found on slide 4 of my March IEEE
presentation on the eye safety changes taking place in IEC. The calculation
on slide 4 shows that for a fiber with an NA of 0.20, 99% of the light will
pass thru the measurement aperture. If the effective NA of the emitted beam
is smaller than the NA of the fiber (i.e.some sort of restricted launch),
the increase in the light passing thru the measurement aperture will be at
most 1% greater. Assuming this is the case (i.e. 100% of the light passing
thru the measurement aperture), then the increase in the eye safety limit
will be reduced by 0.05 dB. The limit will become -1.29 dBm at 840 nm
instead of -1.24 dBm. Since I had previously rounded the result down to
-1.30, this potential change does not affect the proposed transmitter
specifications.

One other observation is that the IEC eye safety standard only cares about
the total power passing thru the measurment aperture, not about the
distribution of power within the aperture. Therefore, there is no need to
build the chart that shows the eye safety limit as a function of restricted
launch condition. With the 0.20 NA of 50 um fiber, we are already at the
portion of the curve that has no slope. Rather uninteresting.

I hope this clears up any issues you may have.

Regards,
Paul Kolesar

> ----------
> From: 	Jonathan
> Thatcher[SMTP:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
> Sent: 	Monday, June 12, 2000 4:04 PM
> To: 	stds-802-3-hssg@xxxxxxxx
> Subject: 	RE: Question on Laser Safety Limits
>
> Shelto (et al),
>
> Yes, I saw the 2.46 dB increase over today's limit.
>
> Now, if we restrict the launch by a factor of two (radius), a grossly
> simplified calculation would indicate that the power would be concentrated
> in an area of 1/4 the size and therefore have 4X the intensity. If we
> assume that this translates to a 6 dB increase into the laser safety
> aperture, and we similarly assume an increase in the safety limit of 2.46
> dB, where did the other 3.54 dB go? Sounds like things got worse instead
> of better.
>
> In Paul's presentation:
> http://grouper.ieee.org/groups/802/3/ae/public/may00/kolesar_1_0500.pdf
> The maximum Tx launch is -1dBm (per charts 24 and 25) which is, I assume,
> kinda like the -1.24 number rounded a bit. In chart 26, there simply isn't
> enough unallocated budget to cover the missing 3.54 dB.
>
> Now, this would not be anywhere near valid if the distribution of the
> launch were not gaussian in both cases. But, you will remember that Paul
> mentioned in a response that in fact the launch was Gaussian in defense of
> the 2r = D question.
>
> So, knowing that the calculations are not as simple as I pretend above, I
> am simply asking the experts to build the chart that shows the effective
> power vs. launch conditions.
>
> So far, no takers....
>
> jonathan
>
>
> 	-----Original Message-----
> 	From: Schelto.Van-Doorn@xxxxxxxxxxxx
> [mailto:Schelto.Van-Doorn@xxxxxxxxxxxx]
> 	Sent: Monday, June 12, 2000 12:22 PM
> 	To: Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx
> 	Cc: stds-802-3-hssg@xxxxxxxx
> 	Subject: RE: Question on Laser Safety Limits
>
>
> 	Hi Jonathan,
>
> 	Paul Kolesar gave a very good presentation at the march plenary
> meeting on this issue. See:
>
>
> http://grouper.ieee.org/groups/802/3/ae/public/mar00/kolesar_1_0300.pdf
>
> 	In this presentation he expects the new levels for 850 nm to be
> -1.24 dBm. A 2.46 dB increase over today's limit .
>
> 	Even more interesting, is the fact that they are anticipating a new
> category "Class 1M" with restrictions similar to Class 3A, that will allow
> levels up to +9.43 dBm.
>
> 	But you bring up an interesting question in regards to the
> restricted launch conditions.
>
> 	What will the test conditions be?
>
> 	With the majority of the light going through the center of the
> exiting light cone, there could be a significant difference between a
> mathematical calculation assuming equal distribution over the cone and a
> practical measurement, measuring the maximum exposure through an aperture.
>
> 	Have fun
> 	Schelto
>
>
>
>
> 	Schelto van Doorn
> 	Engineering Manager
> 	Fiber Optics
> 	Infineon Technologies
> 	1730 North First St.
> 	MS 22303
> 	San Jose, CA 95112
> 	Tel: 408.501.5665 Fax: 408.501.5670
> 	Schelto.vandoorn@xxxxxxxxxxxx
> 	http://www.schelto.com/
>
> 		-----Original Message-----
> 		From: Jonathan Thatcher
> [mailto:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
> 		Sent: Friday, June 09, 2000 11:06 AM
> 		To: HSSG_reflector (E-mail)
> 		Subject: Question on Laser Safety Limits
>
>
> 		Has anyone built a chart that shows what the new laser
> safety limits are in dBm for 850 nm lasers (min lambda = 840?) as a
> function of the restricted launch condition?
>
> 		jonathan
>
> 		Jonathan Thatcher,
> 		Chair, IEEE 802.3ae (10 Gigabit Ethernet)
> 		Principal Engineer, World Wide Packets
> 		PO BOX 141719, Suite B; 12720 E. Nora, Spokane, WA 99214
> 		509-242-9000 X228; Fax 509-242-9001;
> jonathan@xxxxxxxxxxxxxxxxxxxx
>
>
>
>

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