IEEE 802.3z Standard

(Figures A1-A3)

Figure A1. – Optical Transmission Characteristics.

Figure A2 – Optical Link Budget Parameters.

Figure A3. – Beam Divergence vs. Numerical Aperture Requirement.

Beam Divergence

The IEEE standard 802.3z requires that the beam divergence of an optical source be less than the angle of numerical aperture of the fiber in order to ensure signal propagation down the optical fiber. The calculations below indicate that the angle of numerical aperture for a multi-mode fiber (the difference between graded and step index fibers is negligible) is almost 12 degrees.

n1 ≈ 1.456 ∆ ≈ 1% NA = .205909 sin-1(NA) = 11.88◦ [IEEE]

The worst possible beam divergence angle given by the unconnectorized 4080-321 VCSEL is 20 degrees at full width-half maximum. Since the numerical aperture angle was calculated at half width, the corresponding divergence angle from the VCSEL is 10 degrees. Therefore, since the beam divergence is less than the numerical aperture, signal propagation is assured.