Estimating mode field distribution in optical fibers from guided acoustic wave brillouin scattering
Aspects of the present disclosure describe a method for estimating mode field distribution in optical fibers from guided acoustic-wave Brillouin scattering wherein light for which the optical mode-field distribution is determined remains in the optical fibers and the distribution is made for light inside the fiber, and not at a fiber/air interface or other perturbation points to the fiber resulting in a more accurate representation of the optical mode-field distribution in the fiber. Since light is always in the fiber during the determination, no complicated fiber preparation steps or procedures are required and the mode-field distribution is determined as an average distribution along the length of the fiber under test.
1. A method for estimating mode field distribution in optical fibers from guided acoustic wave Brillouin scattering, said method comprising:
operating an arrangement providing for the accurate measurement for guided acoustic-wave Brillouin scattering (GAWBS), to determine the guided-acoustic-wave Brillouin scattering exhibited by an optical fiber under test (FUT);
said arrangement including:
a coherent optical receiver having a local oscillator input port and a signal port;
a length of polarization maintaining (PM) optical fiber in optical communication with the coherent receiver LO port;
the length of an optical fiber under test (FUT) in optical communication with the coherent receiver signal port;
a continuous-wave (CW) laser in optical communication with the PM optical fiber and the FUT; and
a calibrating laser (Laser-cal) in optical communication with the FUT;
said arrangement configured such that
light is output from the Laser-cal and light is output from the CW laser;
the light output from the CW laser is split into two beams, one beam directed into the PM optical fiber and the other beam directed into the FUT;
the light output from the Laser-cal is combined with the CW laser beam directed into the FUT;
the coherent receiver receives light from the CW laser at its LO port and the combined Laser-cal output light and CW light at its signal port;
the light received at the LO port and combined light received at the signal port is detected by a plurality of photodetectors and photodetector output signals resulting therefrom are subsequently filtered by respective bandpass filters;
the filtered output signals are digitized through the effect of an analog to digital converter (ADC); and
the GAWBS output spectrum is determined from digitized output signals of the ADC; and
estimating the optical mode-field distribution from the GAWBS from the GAWBS spectrum.
2. The method of claim 1 wherein the light for which the optical mode-field distribution is determined is light that always remains within the FUT.
3. The method of claim 2 wherein the optical mode-field distribution estimated is an average of mode-field distribution along the length of the FUT.
4. The method of claim 3 further comprising measuring and determining the GAWBS spectrum in both polarizations.
5. The method of claim 4 further comprising isolating GAWBS peaks in the GAWBS spectrum from ROm modes.
6. The method of claim 5 further comprising determining strain functions for the ROm modes.
7. The method of claim 6 further comprising determining a mode-field square value from an inverse transform.
8. The method of claim 7 further comprising estimating the mode-field from the mode field square.