Study of Rayleigh-backscattering induced coherence collapse in an asymmetric DFB FL sensor
Citation: Wen LIU, Lina MA, Zhengliang HU, Ying FENG, and Huayong YANG, “Study of Rayleigh-Backscattering Induced
Coherence Collapse in an Asymmetric DFB FL Sensor,” Photonic Sensors
Study of Rayleigh-Backscattering Induced Coherence Collapse in an Asymmetric DFB FL Sensor
Wen LIU 1
Lina MA 0
Zhengliang HU 0
Ying FENG 1
Huayong YANG 1
Corresponding author: Wen LIU
0 Academy of Ocean Science and Engineering, National University of Defense Technology , Changsha, 410073 , China
1 College of Optoelectronic Science and Engineering, National University of Defense Technology , Changsha, 410073 , China
Rayleigh-back scattering induced coherence collapse of an asymmetric distributed feedback fiber laser (DFB FL) sensor is investigated using a composite cavity model. The coherence collapse threshold condition of the asymmetric DFB FL sensor is measured. The DFB FL sensor shows different dynamic behaviors in different pump configurations. According to the asymmetric behavior to the external optical feedback, a novel method to find the actual phase shift position of the asymmetric DFB FL sensor is presented.
Asymmetric DFB fiber laser sensor; phase shift; coherence collapse; Rayleigh backscattering
Distributed feedback fiber lasers (DFB FLs)
have attracted general attention since 1990s, because
of the impact structure, low noise figure, and the
inherent wavelength-encoded multiplexing
capability [1‒4]. The laser frequency of a DFB FL is
determined by the resonance condition of the laser
cavity, which is sensitive to the physical parameters
that could disturb the oscillation process. By using
the dependence of the laser frequency on the
physical parameters, DFB FL can be employed as a
sensor . DFB FL sensor arrays are normally
arranged through remote pumping and interrogation
configuration, which means that a long lead fiber is
required to feed the pump light into DFB FLs . In
the lead fiber, Rayleigh backscattering may cause
excess frequency noise or even unstable output of
the DFB FLs, namely, coherence collapse , and
substantially affects the performance of DFB FL
sensors. The maximum multiplexing capacity of
DFB FL array remains to be 16 until now , and
coherence collapse induced by Rayleigh
backscattering is considered as one of the main
reasons for the limited array scale . To improve
the performance of the DFB FL sensor array, it is
necessary to study the effect of coherence collapse
induced by Rayleigh scattering of the lead fiber on
the operation status of the DFB FL. The tolerable
length of lead fiber was found to be 135 m‒200 m for
symmetric DFB FL , corresponding to a typical
Rayleigh backscattering of ‒72 dB/m, while the
dynamic behavior of asymmetric DFB FL with
external optical feedback was rarely mentioned.
In this paper, we study the tolerance of
asymmetric DFB FL to Rayleigh backscattering of a
long lead fiber. We start to analyze the coherence
collapse threshold condition of asymmetric DFB FL
in Section 2. The dynamic behaviors of an
asymmetric DFB FL sensor to Rayleigh
backscattering is evaluated in different pump
configurations in Section 3. In Section 4, a novel
method to determine the phase shift position of
asymmetric DFB FL is proposed based on the
coherence-collapse threshold length measurement.
2. Composite cavity model
DFB FL consists of a phase-shift Bragg grating
in a short piece of rare earth doped fiber. The
symmetric grating structure is designed for DFB FL
to obtain the single polarization laser output and
reduce the pump threshold , but many
asymmetric DFB FLs are made on purpose. In a
sensor array, DFB FLs with asymmetrical outputs
can modify the pump power distribution and
improve the multiplexing capacity with fixed pump
power budget [11, 12].
In asymmetric DFB FL, each grating segment on
either side of the phase shift can be considered as a
separate reflector. By simplifying the distributed
feedback cavity to be a Fabry-Perot (FP) structure
and considering the external optical feedback as the
external cavity of the FL , a composite cavity
model can be established as shown in Fig. 1. r1 and
r2 are the reflection coefficients of the FP cavity, rext
is the reflection coefficient of the external reflection
facet, L is the effective length of the FP cavity, and d
is the external path length.
Fig. 1 Schematic diagram of the composite cavity model.
The composite cavity can be equivalent to an FP
cavity, and the reflection coefficients of the
equivalent FP cavity can be written as
rR r2 1 T2rext cos(2 e ) iT2rext sin(2 e )
where T2 is the transmissivity of right piece of
uniform grating segment, and τe is the external round
The steady-state laser oscillation condition for
the equivalent FP cavity can be given as
rL rR exp[(gc )L] 1 (
0 L [2 e ( 0 )
T e 1 2 rext sin(2 0 e 2 e )]
where 0 is the laser frequency without optical
feedback, τL is the roundtrip time of the solitary
laser, α is the linewidth enhancement factor, and θα
is the associated phase. The frequency shift Δ due
to the optical feedback and the feedback parameter
C can be expressed as
e C sin( e 2 0 e ) (
C T e 1 2 rext . (
For very weak feedback with C<1, (
) has only
one solution. When C≥1, there are more than one
solutions for (
), indicating that the external optical
feedback splits the single mode operation into
multiple external cavity modes, accompanying with
dramatically increased noise level and intensity
fluctuation, resulting in coherence collapse. As a
result, C=1 may be defined as the coherence
collapse threshold condition for DFB FL.
For an asymmetric DFB FL, the reflection
coefficient and the transmissivity of each grating
segment at the Bragg wavelength are given by 
ri tanh( li ) (
Ti 4 exp( li ) (
where li with i=1, 2 is the length of the grating
segment. The effective cavity length of DFB FL can
be written as
L (r1 r2 ) / 2 .
By substituting (
) and (
) into (
), the feedback
parameter of asymmetric DFB FL can be rewritten
C 8 1 a2 rext d exp(2 l2 ) . (
r2 (r1 r2 )
For a DFB FL sensor system, the external optical
feedback mainly comes from external reflecting
facet, splice points, and Rayleigh backscattering
. The influence of former two sources can be
eliminated to the negligible extent, but the
detrimental effect of the last one is never possible to
be entirely ignored, which becomes the main factor
to limit the performance of the DFB FL sensor. Then
the coherence collapse threshold length of lead fiber
in the DFB FL system can be given by
d r2 (r1 r2 ) 3 .
8 1 a2 rR exp(2 l2 )
When l1= l2, (
) can be used to calculate the
threshold condition for symmetric DFB FL, which
coincides with the conclusion got by E. Rønnekleiv
et al. .
3. Experiments and results
A high resolution frequency shift demodulation
system is constructed to evaluate the influence of
external optical feedback on the asymmetric DFB
FL sensor. The schematic diagram is shown in Fig. 2.
The DFB FL sensor is a fiber laser hydrophone with
ultrahigh pressure sensitivity . The total length
of the laser cavity is 40 mm, and the phase shift
section is 4 mm long. The DFB FL sensor is pumped
by a 975-nm semiconductor laser at 63 mW through
a 980/1550 WDM (wavelength division multiplexer).
The output signal of the FL sensor is guided to an
unbalanced Michelson interferometer through an
isolator. The optical path difference of the
interferometer is 15 m. A sinusoidal signal at the
frequency of 12.5 kHz is loaded to the piezoelectric
ceramic transducer (PZT) in one arm of the
interferometer. The interference signal is converted
into electric voltage and then acquired by an
acquisition card. A phase generated carrier (PGC)
scheme is adopted to achieve high resolution
interrogation. The whole optical setup is packaged
to isolate the environment fluctuation, and the end
facet of the pigtail is immersed in the refractive
index matched gel to eliminate the back reflection.
Semiconductor laser WDM DFB FL
Index matched gel
Faraday rotator mirror
Faraday rotator mirror
PD A/D PC
Fig. 2 Schematic diagram of the frequency shift
The distance between the WDM and DFB FL
sensor is set to 1 m, while the lead fiber on the other
end of the DFB FL sensor is increased by 10 m each
time until coherence collapse occurs. The two ends
of the asymmetric DFB FL sensor are labeled as
Port 1 and Port 2, and the DFB FL sensor is pumped
through Port 1 at first. The phase noise floor of the
DFB FL sensor with different lengths of lead fiber is
shown in Fig. 3, and the demodulated interference
signals are shown in Fig. 4.
/2 ) 20
1 000 2 000 3 000 4 000 5 000
Fig. 3 Phase noise level of DFB FL sensor due to an increase
in the length of lead fiber.
It is illustrated that the phase noise level of the
DFB FL sensor rises with an increase in the length
of the lead fiber. When no extra lead fiber is added
to the pigtail, the phase noise of the DFB FL sensor
is about –89 dB re. rad/Hz1/2 @1kHz. When the lead
fiber is 30 m long, the phase noise increases by 5 dB,
and the interference signal is still stable as shown in
Fig. 4. However, when another 10-m lead fiber
increases, the phase noise increases by over 10 dB,
and there is self-pulsing in the interference signal,
hinting that coherence collapse occurs.
It is indicated that the asymmetric DFB FL
sensor has distinctive threshold lengths of the lead
fiber in different pump configurations, and the
difference of the threshold length exceeds 50 m.
When the DFB FL sensor system is arranged
through a remote-pumping and interrogation
configuration, the DFB FL sensor should be pumped
from Port 2 to extend the total length of the sensing
4. Phase shift assessment method based on coherence collapse threshold length
Based on (
) and (
), the coherence collapse
threshold length ratio of the asymmetric DFB FL in
different pump configurations satisfies
d1 tanh( l2 ) exp(2 l1 2 l2 )2/3 . (
d2 tanh( l1 )
For an asymmetric DFB FL with 40-mm cavity
length and 4-mm phase shift section, the threshold
length ratio to the grating segment length near Port 1
is shown in Fig. 5, where the estimated grating
coupling coefficient is κ=150 m–1.
0.015 0.018 0.021 0.024
Grating segment length (mm)
Fig. 5 Phase noise level of the DFB FL sensor due to an
increase in the external path length.
As the threshold length ration of the DFB FL
sensor is 10/3, the lengths of grating segments
beside of the phase shift section can be deduced as
15 mm and 21 mm, respectively.
In the field test, the performance of the DFB FL
sensor always differs from the initial DFB FL before
mounting, as non-uniform extension or twisting of
the laser cavity is introduced by the mounting
structure. The method proposed here can be used to
understand the real physical state of the FL cavity,
and help to optimize the sensor performance.
The stability of the asymmetric DFB FL sensor
with Rayleigh scattering of the lead fiber is studied.
The coherence collapse threshold length of an
asymmetric DFB FL sensor is measured based on a
high resolution frequency shift demodulation system.
The difference in the threshold length of the same
FL sensor in different pump configurations suggests
a novel method to attain the phase shift position,
which is significant to enlarge the total length of the
DFB FL sensor system. More details need to be
revealed for the asymmetric DFB FL with different
phase shifts at different positions, and the coherence
collapse characteristics of the asymmetric DFB FL
sensor array will be studied in further work.
was supported by the
Science Foundation of China (NSFC 11304388).
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