brillouin_threshold

photonforge.fiber.brillouin_threshold(length_km, *, effective_area=.0, gain_coefficient=5e-11, attenuation=0.2, source_linewidth=0.0, brillouin_linewidth=20e6, polarization_scrambled=True)[source]

Critical power for stimulated Brillouin scattering.

\[P_{\rm th} \simeq \frac{21\,A_{\rm eff}}{g_B L_{\rm eff}} \left(1 + \frac{\Delta\nu_{\rm source}}{\Delta\nu_B}\right)\]

The 21 is the standard Smith result for the gain needed to bring spontaneous Brillouin scattering up to the launched power.

The linewidth factor is the whole reason SBS is manageable in practice. The Brillouin gain sits in a 20 MHz window, so a source broader than that only puts a fraction of its power inside the gain band and the threshold rises in proportion. A 10 Gb/s modulated signal is a few GHz wide and lifts the threshold by more than two orders of magnitude; an unmodulated narrow-line laser has none of that protection and hits SBS at a few mW.

Parameters:

  • length_km (Annotated[float, exclusiveMinimum=0]) – Span length, in km.

  • effective_area (Annotated[float, exclusiveMinimum=0, units='μm²']) – Nonlinear effective area, in μm².

  • gain_coefficient (Annotated[float, exclusiveMinimum=0, units='m/W']) – \(g_B\), in m/W. About 5e-11 for silica.

  • attenuation (Annotated[float, minimum=0, units='dB/km']) – Fibre loss, in dB/km.

  • source_linewidth (Annotated[float, minimum=0, units='Hz']) – Source linewidth, in Hz. Zero for a pure tone.

  • brillouin_linewidth (Annotated[float, exclusiveMinimum=0, units='Hz']) – \(\Delta\nu_B\), in Hz. 20 MHz at 1550 nm.

  • polarization_scrambled (bool) – Halve the effective gain, appropriate for a fibre with no polarization maintenance, which is the usual case.

Returns:

Threshold power in W.

Return type:

float