Question

Determine the threshold optical power in a long single-mode fiber with 0.5 dB km^-1 for stimulated Brillouin scattering within the fiber at a wavelength of 1.3 nm. The fiber core diameter is 6 μm and laser source bandwidth is 600 MHz.
1. 1.5 W
2. 1.38 W
3. 80.3 mW
4. 65.2 mW

Answer 1

Correct Answer - Option 3 : 80.3 mW

Stimulated Brillouin scattering (SBS):

• Stimulated Brillouin scattering (SBS) is a nonlinear process that can occur in optical fibers that limits the output power of a fiber laser.
• This nonlinear scattering causes the optical power from one mode to be transferred in either the forward or backward direction to the same, or other modes, at a different frequency.
• It occurs through the generation of a backward-propagating Stokes wave that carries most of the input power, once the Brillouin threshold is reached.
• It is for this reason that SBS limits the channel power in optical communication systems.

 

The threshold power is given by:

P_B = (4.4 × 10^-3) d² λ² α_dB B Watts

PB = Threshold power

αdB = Fiber attenuation in decibels per kilometer

d = Core diameter in μm.

λ = Operating wavelength in nm

B = Source bandwidth in GHz

Calculation:

With αdB = 0.5, d = 6 μm, λ = 1.3 nm, and B = 600 MHz = 0.6 GHz, the threshold power will be:

PB = (4.4 × 10-3) (6)2 (1300)2 (0.5) (0.6) Watts

PB = 80.30 mW