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RE: Question on Laser Safety Limits


Thanks for the response, I do not have a copy of the draft standard so I
don't know what assumptions are being used to base these calculations.

Let me try this with what little information I have available (from you).

1. I do not see how the NA is relevant in the calculation. Since we must
assume the least, not the greatest divergence angle out of the aperture of
the optical device (no fiber in the system), we would have a calculation
that puts 100% of the light through the aperture (agreed, not much

2. Assuming the assumptions and data in the power calculation are correct,
the total max optical output is in fact as you say: 0.743 mW = -1.29 dBm

I'll admit that I don't quite understand how optical intensity avoids coming
into the equation, but hey, this is laser safety.


>-----Original Message-----
>From: Kolesar, Paul F (Paul) [mailto:pkolesar@xxxxxxxxxx]
>Sent: Tuesday, June 13, 2000 1:46 PM
>To: stds-802-3-hssg@xxxxxxxx
>Subject: RE: Question on Laser Safety Limits
>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
>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.
>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:
>> 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:
>> 	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 
>> 		-----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