K. Iiyama et al., Phase-decorrelated FMCW reflectometry for long optical fiber characterization by using a laser diode with modulated external-cavity, IEICE TR EL, E83C(3), 2000, pp. 428-434
We describe FMCW reflectometry for characterization of long optical fibers
by using an external-cavity laser diode as a light source. Since the optica
l path difference between the reference beam and the reflected beam from th
e optical fiber under test is much longer than the coherence length of the
light source, the reference and the reflected beams are phase-decorrelated.
As a result, the beat spectrum between the reference and the reflected bea
ms is measured. In the phase-decorrelated FMCW reflectomety, the spatial re
solution is enhanced by narrowing the spectral linewidth of the light sourc
e and increasing the repetition frequency of the optical frequency sweep as
well as increasing the chirping range of the optical frequency sweep. In t
he experiments, an external-cavity DFB laser is used as a narrow linewidth
light source, and the optical frequency is swept by minute modulation of th
e external cavity length. Long single mode optical fibers are characterized
, and the maximum measurement range of 80 km is achieved, and the spatial r
esolutions of 46 m, 100 m and 2 km are achieved at 5 km, 11 km and 80 km di
stant, respectively. The Rayleigh backscattering is clearly measured and th
e propagation loss of optical fiber is also measured. The optical gain of a
n erbium-doped optical fiber amplifier (EDFA) is also estimated from the ch
ange in the Rayleigh backscattering level in the optical fiber followed aft
er the EDFA.