COL 10(4), 040604(2012) CHINESE OPTICS LETTERS April 10, 2012
Performance limit of a multi-frequency probe based
coherent optical time domain reflectometry caused by
nonlinear effects
Lidong L¨u (½½½áááÁÁÁ), Yuejiang Song (yyyôôô), Fan Zhu (ÁÁÁ ~~~), and Xuping Zhang (ÜÜÜRRR°°°)
∗
Institute of Optical Communication Engineering, Nanjing University, Nanjing 210093, China
∗
Corresp onding author: xpzhang@nju.edu.cn
Received August 30, 2011; accept October 25, 2011; posted online January 13, 2012
The nonlinear effects that limit the performance of the multi-frequency probe (MFP) based coherent optical
time domain reflectometry (C-OTDR) are investigated. Based on theoretical analysis and experimental
results, compared with conventional C-OTDR, when the probe pulse has power gradient within the pulse
width, self-phase modulation (SPM) and cross-phase modulation (XPM) are strengthened in the new C-
OTDR scheme. The generation of four-wave mixing (FWM) is dependent on SPM and XPM, and with
mo dulation frequency of phase modulator higher than 40 MHz, the stimulated Brillouin scattering (SBS)
threshold can be enhanced by more than 5 dB, which benefits the maximum dynamic range of the MFP
C-OTDR.
OCIS codes: 060.2370, 120.4825.
doi: 10.3788/COL201210.040604.
Coherent optical time domain reflectometry (C-OTDR)
is a commonly used instrument for fiber characterization
and fault location for long-haul multi-fiber span optical
transmission line monitoring
[1,2]
. At present C-OTDR is
the sole instrument for super long undersea optical trans-
mission line monitoring. Generally, the transmission line
is thousands of kilometers in length with multiple fiber
spans and repeaters of erbium-doped fiber amplifier
(EDFA), and it may take several hours to perform a
successful measurement for such long transmission line.
The monitoring schematic diagram is shown in Fig.
1. Since conventional C-OTDR is based on a single-
frequency probe, it only obtains one intermediate fre-
quency (IF) by coherent detection
[3]
. In this letter, we
use a phase modulator (PM) to convert a single-frequency
probe to multiple frequencies and keep the local oscil-
lator (LO) as the original single frequency. Afterwards,
we can simultaneously detect and process multiple IF
signals generated in the coherent detection process be-
tween the Rayleigh signals of the multi-frequency probe
backscattered in the fiber under test (FUT) and the orig-
inal single-frequency LO. This process speeds up mea-
surement efficiency, compared with conventional single-
frequency probe based C-OTDR
[4,5]
.
The schematic diagram of the multi-frequency probe
(MFP) based C-OTDR is shown in Fig. 2. External
cavity laser diode (ECLD) with a narrow linewidth of
3.7-kHz generates light with a wavelength of 1 561.42 nm.
The laser output from ECLD was split into two paths by
a 90/10 coupler. The one with higher power was used
for probe light, and the other was used as LO. The state
of polarization (SOP) of the probe light was adjusted
to minimize the insertion loss of PM. The modulation
depth of PM was fixed at 1.44, so the output power of
PM was concentrated on three frequencies (0 and ±1
order), all of which had the same light power
[6]
. The
multi-frequency probe light power was adjusted by vari-
able optical attenuator (VOA) to make each of the three
frequencies have the same power level with the condition
of single-frequency probe light interaction in order to con-
duct p erformance comparison of the two methods. The
probe pulses were generated by acousto-optic modulator
(AOM) and polarization scrambled using a p olarization
scrambler (PS). Then, they were launched into the first
port of the circulator and into the FUT through the sec-
ond port of the circulator. The FUT was combined using
two fiber sections. Finally, backscattered Rayleigh light
output from the third port of the circulator was combined
with LO in a 3-dB coupler. Coherent heterodyne gener-
ated many IF signals, which were detected by a balanced
photo detector (BPD); meanwhile, only three IF signals
corresponding to the probe were filtered out for process-
ing using the band pass filters (BPFs) of the correspond-
ing pass channels with a bandwidth of 1 MHz
[7]
. Since
C-OTDR is mainly used in long-range multi-fiber span
undersea optical transmission line monitoring, in this ap-
plication situation, accumulated amplified spontaneous
Fig. 1. Schematic diagram of C-OTDR measurement for long-
haul multi-fiber span undersea optical fiber transmission line.
Fig. 2. Exp erimental system configuration of the MFP C-
OTDR. DDC: digital down conversion; LPF: low pass filter.
1671-7694/2012/040604(4) 040604-1
c
° 2012 Chinese Optics Letters