  Fiber-fed etalons are widely employed in advanced interferometric instruments
such as gravitational-wave detectors, ultrastable lasers and calibration
reference for high-precision spectrographs. We demonstrate that variation in
near-field distribution of the feeding fiber would deteriorate the spectrum
precision of the fiber-fed Fabry-Perot etalon, especially when precision at the
order of 3 * 10-10 or higher is required. The octagonal fiber reinforced with
double scrambler could greatly improve the steadiness and uniformness of the
near-field distribution. When building wavelength calibrators for sub-m s-1
precision radial-velocity instruments, the double scrambler should be
considered meticulously.
