Lecture 4

Pulse Propagation in Fibers Problem: Inject an optical pulse of width  0 into the fiber at z = 0. What is the speed of propagation and what is  (z)? Given

Pulse Propagation in Fibers

Propagation delay

Example Suppose N = at λ = 1200 nm and N = at λ = 1400 nm. Calculate and .

Example Assume Δ = 1 GHz, λ = 1300 nm, ΔT = 100 ps. What is the maximum L?

Pulse Brodening LEDMultimode LD Single-mode LD – direct modulator Single-mode LD – external modulator Δλ (nm) 503 z (km)0.122

Total dispersion Total dispersion = material dispersion + waveguide dispersion (+ modal dispersion + polarization dispersion). Waveguide dispersion: n eff changes with v j with λ. Commercial multimode fiber: GRIN fiber: modal dispersion = 0.3 – 1 ns/km. SI fiber: modal dispersion = 50 ns/km.

Solitons Pulses that can operate fiber with   0 with no pulse broadening ( ΔT = 0). It could be done by ‘non-linear effects’. Still work to be done before solitons are practical.

Solitons

Rayleigh Backscattering

Example Most of the attenuation is due to Rayleigh scatter. This form of scattering happens to be isotropic, so that some is scattered back toward the transmitter. If you have a fiber with an NA of 0.1 for which all of its 0.5 dB/km attenuation is due to backscatter, and you send a single light pulse of duration T = 1 ns into it, how many dB down will be the peak of the Rayleigh backscatter waveform? Assume that the core index = 1.45.

Biconical tapered couplers

Example Design a single mode fused biconical coupler that accepts at one input a mixture of light at 1300 nm and 1530 nm and deliver 100% of one to one output and 100% of the other to the other output. Assume that throughout the coupling region, each fiber can be represented as having a 30 micron effective core diameter.

Example For the 16x16 star coupler shown in previous slides, what is the total loss and the excess loss in dB assuming each of the couplers has r = 1 with an excess loss of 1 dB?

Power limit by eye safety