High Power Phenomena

High Power Phenomena

There are basically two factors that limit the maximum power that can be transmitted with a single-mode or polarization-maintaning fiber.
  • Bulk effect: Stimulated Brillouin Scattering
  • Fiber end-face effect: Scorching of the fiber end-face or Photocontamination (this effect can be limited by fiber end caps) 

Stimulated Brillouin Scattering/Brillouin Threshold

Stimulated Brillouin Scattering (SBS) is an effect that limits the maximum power that can be transmitted by the fiber. Unlike photo-contamination and direct scorching of the fiber end-face, which limit the power that can be transmitted at the fiber end-face, stimulated Brillouin scattering is a bulk medium effect.
The electro-magnetic wave propagating within the optical fiber is scattered by acoustical phonons that are caused by electrostriction. The acoustical phonons induce periodic changes in refractive index (elasto-optical effect) that serve as a Bragg grating, reflecting the incoming radiation. The wavelength of the reflected radiation (Stokes photon) is shifted towards higher wavelengths.
If the input power exceeds the Brillouin threshold Pcr , almost the entire radiation is reflected. The threshold is defined as:

{!{!{P_{cr}=\frac{21\cdot A_{eff}}{g_B\cdot L_{eff}}}!}!}

Aeff ∼ (MFD)2 = effective core diameter
Leff = effective cable length, dependent on fiber losses
gB = gain coefficient of the Brillouin spectrum

The critical power is wavelength-dependent and influenced by other fiber parameters. The amount of power that can be transmitted by a particular fiber needs to be determined for each fiber individually.

Fiber with end cap

Preventing detrimental effects on the fiber end- face: end caps

The described detrimental effects can be obviated using a fiber end cap, in which a short length of fiber (<300 µm) without a core is spliced onto the polarization-maintaining ­fiber .
Without a fiber core to confine the beam, the mode field diameter of the beam already starts to diverge within the fiber end cap and the resulting beam area at the end-face is significantly ­larger. More details are found here.