# Re: [10GMMF] the minutes of task 4 Oct. 4th call

Yu,
Thanks for the minutes.  Reading them reminded me of an issue that has been troubling me.  I'm hoping someone can clarify.

I have been wondering about the use of a 7um FWHM singlemode beam in the simulations.  From my calculations I find this beam size to correspond to a fiber with an 8.4 um mode field diameter, which is below the normal range of 8.8 to 9.5 um.  This can lead to differences between the simulated and measured DMD and other mode-coupling dependent calculations.  My calculations follow.  I'd appreciate a review and any comments regarding the effects of using a beam that appears to be on the small side.  For example, such a beam will have a larger divergence angle and might couple into more mode groups than an actual singlemode fiber used in DMD measurements or actual TOSA launch optics.

The intensity profile of a step index singlemode fiber is given by the following equation that describes the shape of a Gaussian distribution:

I = Io e^(-2r^2/wo^2)
where Io is the intensity at the center (r=0),
wo = mode field radius (the radius where the intensity drops to 1/e^2 = 0.135Io).

The electric field is the square root of the intensity, and for Io normalized to 1:

E = e^(-r^2/wo^2)

At the half max level

E = 0.5 = e^(-r^2/wo^2)
r = -(ln0.5)^1/2 wo = 0.833wo

Usual MFD = 2wo = 8.8 to 9.5 um depending on fiber design, so
usual FWHM Efield= (0.833)(MFD) = 7.3 to 7.9 um.

Regards,
Paul Kolesar
SYSTIMAX® SOLUTIONS
1300 East Lookout Drive
Richardson, TX 75082
Phone:  972.792.3155
Fax:      972.792.3111
eMail:   pkolesar@systimax.com

yusun@IEEE.ORG
Sent by: owner-stds-802-3-10gmmf@IEEE.ORG

10/06/2004 11:04 AM
 Please respond to "IEEE P802.3aq 10GBASE-LRM"

 To STDS-802-3-10GMMF@listserv.ieee.org cc Subject [10GMMF] the minitues of task 4 Oct. 4th call

Dear colleagues,

I attached the minutes of tele-conference of task 4 group on Oct. 4th. I would appreciate your comments.
The proposed next task 4 meeting is
Oct 11th, 2004 (Monday), 11am east coast, 8 am west coast and 4 pm UK.

Best regards,

Yu