Optical fiber exhibiting low guided acoustic Brillouin scattering (GAWBS) noise and measurement thereof
Aspects of the present disclosure describe systems, methods. and structures in which guided acoustic Brillouin (GAWBS) noise is measured using a homodyne measurement technique and demonstrated using a number of optical fibers, such fibers being commonly used in contemporary optical communications systems. The measurements are made with single spans and determined to be consistent with separate multi-span long-distance measurements. Additionally, a technique for preparing an optical fiber exhibiting superior GAWBS noise characteristics by reducing coherence length of the optical fiber by spinning the fiber at a high rate during the drawing process such that birefringence coherence length is reduced.
1. An arrangement for measuring guided acoustic Brillouin scattering (GAWBS) noise comprising:
a first laser configured to emit first laser light and a second laser configured to emit second laser light;
a first length of optical fiber that optically connects the first laser light to a first port of a coherent receiver;
a second length of optical fiber that optically connects the second laser light to a second port of the coherent receiver; and
a third length of optical fiber that optically connects the first length of optical fiber to the second length optical fiber such that a portion of the first laser light is redirected to the second length of optical fiber wherein it is mixed with the second laser light resulting in mixed laser light;
a fiber under test (FUT) that receives the mixed laser light from a section of the second length of optical fiber such that it is directed to the second port of the coherent receiver via another portion of the second optical fiber; and
the coherent receiver that generates 4 beating signals from light signals received at the first and second ports;
wherein the arrangement is configured to suppress a large carrier using band-pass filters and further configured such that the second laser light emitted from the second laser is outside of a GUIDED ACOUSTIC BRILLOUIN SCATTERING (GAWBS) window and both polarization components of GAWBS are simultaneously measured.
2. The arrangement of claim 1 further comprising:
an optical amplifier interposed between the FUT and the second port of the coherent receiver for amplifying the mixed laser light after it traverses the FUT.
3. The arrangement of claim 2 further comprising:
an optical band pass filter interposed between the optical amplifier and the second port of the coherent receiver.
4. The arrangement of claim 3 further comprising a polarization controller interposed between the band pass filter and the second port of the coherent receiver.
5. The arrangement of claim 3 wherein the first port of the coherent receiver is a local oscillator port.
6. The arrangement of claim 5 wherein the second port of the coherent receiver is an optical signal port.
7. The arrangement of claim 6 wherein the coherent receiver comprises a hybrid that receives as input local oscillator light and signal port light and generates 4 quadrature signals.
8. The arrangement of claim 7 further comprising 4 balanced photodiodes for respectively detecting the 4 quadrature signals.
9. The arrangement of claim 8 further comprising an analog to digital converter configured to sample the output of the 4 balanced photodiodes and digitize same.
10. The arrangement of claim 9 further comprising 4 band pass filters each one respectively interposed between a respective photodiode and an input port of the analog to digital converter.
11. The arrangement of claim 9 further comprising a polarization controller (PC) interposed between the bandpass filter and the coherent receiver along the length of the second length of optical fiber.
12. The arrangement of claim 1 wherein the first length of optical fiber is a polarization maintaining (PM) fiber.