Sensing Bandwidth Enlargement with Ten Orthogonal Codes in

Quasi-distributed Acoustic Sensing System

Ziwen Deng

, Ruobing Xu

, Yuyao Wang

, Jialin Jiang

and Zinan Wang

Key Lab of Optical Fiber Sensing & Communications, University of Electronic Science and Technology of China,

Chengdu, China

Keywords: Optical Fiber Sensing, Acoustic Sensing, Orthogonal Codes, Sensing Bandwidth, Real-time.

Abstract: In recent years, quasi-distributed acoustic sensing (QDAS) has attracted lots of attentions and shows great

advantages in various fields such as structure health monitoring, intrusion sensing and so on. Sensing

bandwidth is one of the essential indexes in QDAS, and there is a trade-off between sensing bandwidth and

sensing distance. To break the limitation, the multiple-input multiple-output (MIMO) technology and ten

orthogonal codes in the same frequency (OCSF) are utilized as an innovative technology to multiplex the

sensing channel in a real-time long-distance QDAS system. In this paper, the sensing bandwidth of QDAS is

enlarged to 5 kHz on 99.4 km sensing fiber, which is ten times of that in the conventional QDAS without

channel multiplexing; a 4.9 kHz sinusoidal signal is retrieved in real time successfully, with 10 m spatial

resolution and 43.8 𝑝ɛ/

√

𝐻𝑧

noise level.

1 INTRODUCTION

With the increasing demand for high-sensitivity

acoustic sensing in the fields like railway traffic

system supervision, border security, oil and gas

monitoring and so on, distributed optical fiber sensing

(DOFS) technology is playing an important role in

these scenarios. With various advantages, DOFS is a

research hot area in recent decades and the further

value of it is gradually being explored.

As an important branch of DOFS, phase sensitive

optical time domain reflectometry (Ф-OTDR) uses

high coherent narrow-linewidth laser as the light

source to sense the fluctuation of the external

environment through the phase of the Rayleigh

backscattering (RBS) light. Ф-OTDR is based on

optical fiber to perceive acoustic signal, and it has the

merit of high sensitivity, high temperature resistance,

anti-corrosion and being immune to electromagnetic

interference, so it can be used for tracking the trains

and cars, monitoring earthquakes and other important

fields.

https://orcid.org/0000-0003-3198-5423

https://orcid.org/0000-0001-8510-6135

https://orcid.org/0000-0002-6846-9089

https://orcid.org/0000-0001-5354-4157

https://orcid.org/0000-0002-6924-3428

Ф-OTDR can be divided to distributed acoustic

sensing (DAS) and quasi-distributed acoustic sensing

(QDAS) according to the different sensing media.

DAS is based on common single mode fiber (SMF)

and QDAS is based on SMF inscribed with fiber

Bragg gratings (FBGs) or scattering enhanced points

(SEPs) array.

DAS has the advantages of low cost and flexible

spatial resolution. However, the weak and random

Rayleigh scattering may cause the fading

phenomenon, which need more resources to deal

with. While in QDAS, the FBG can offer controllable

and stable reflection. Thus, QDAS has higher

sensitivity and signal-to-noise ratio (SNR), without

fading problem. Due to these advantages, QDAS has

developed rapidly and has been a preferred choice in

some applications.

In QDAS, the sensing bandwidth is an important

parameter and need to be enlarged effectively. By

applying Vernier effect in QDAS, the sensing

bandwidth can be enlarged to dozens of times, but the

target acoustic signal is presupposed to be narrow

152

Deng, Z., Xu, R., Wang, Y., Jiang, J. and Wang, Z.

Sensing Bandwidth Enlargement with Ten Orthogonal Codes in Quasi-distributed Acoustic Sensing System.

DOI: 10.5220/0010972100003121

In Proceedings of the 10th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2022), pages 152-157

ISBN: 978-989-758-554-8; ISSN: 2184-4364

band only. Therefore, to avoid limiting the frequency

feature of signal, multiplexing the sensing bandwidth

is the only way to achieve it. Besides, there is a trade-

off between sensing bandwidth and sensing distance,

because in QDAS, the sensing signals from different

FBGs are distinguished by time of flight. To break the

limitations, frequency-division-multiplexing (FDM)

is an efficient method, which can multiplex the

sensing channel. However, FDM requires extra

frequency-domain resources to realize channel-

multiplexing, but the frequency-domain resources are

precious and supposed to be utilized efficiently.

Recently, our group proposed a QDAS based on

multiple-input multiple-output (MIMO) coding

technology, which utilized the orthogonal codes in

the same frequency (OCSF) to multiplex the sensing

channel. The number of the multiplexed sensing

channels depend on the number of the groups of the

codes used. We utilized sequential quadratic

programming (SQP) algorithm to obtain five groups

of OCSF with high auto-correlation and cross-

correlation suppression ratio, indicating the sensing

channels were multiplexed five times and the sensing

bandwidth was five times of that in the conventional

QDAS.

In this paper, to further enlarge the number of the

multiplexed channels, ten codes with about 15.4 dB

peak-to-sidelobe ratio of auto-correlation function

and cross-correlation are generated, which are the

most codes utilized in QDAS. The spatial resolution

is determined by the interval of FBGs, i.e. 10 m. By

utilizing the OCSF and the MIMO technology, the

sensing bandwidth is enlarged to ten times of that in

the conventional QDAS without channel-

multiplexing. Furthermore, a 1455 nm Raman pump

is used for distributed amplification to achieve the

long-distance fiber sensing. Moreover, by utilizing

GPU parallel processing, the disturbance can be

demodulated in real time. As a result, a 5 kHz sensing

bandwidth is achieved on a 99.4 km fiber, and the

external disturbance, can be retrieved real-time

successfully with about 20.5 dB SNR and the strain

noise level of 43.8 𝑝ɛ/

√

𝐻𝑧

2 PRINCIPLES

2.1 The Sensing Principle of QDAS

In QDAS, the sensing fiber is an SMF with a series of

enhanced points, which are consist of FBGs/SEPs,

whose reflectivity is much higher than the Rayleigh

scattering (RS).

The light source is modulated into probe pulses

and then injected into the sensing fiber. The

lightwaves will be reflected by the enhanced points,

and the speckled trace is obtained. When a

perturbation, usually acoustic wave, is imposed on the

sensing fiber, the optical path experienced by the

probe lightwaves will be changed. Therefore, the

perturbation can be retrieved and its position can be

located by calculating the phase difference of the

adjacent signals and the time of flight.

To introduce the MIMO-QDAS, the sensing

probe pulse

()Pt is coded by M groups of OCSF

each with N bits length. The amplitudes of OCSF are

the same but the phases are different. The reflected

lightwave

()Et can be expressed as:

() () ( / )

Et ht P t iT M

=∗ −



(1)

() () exp( 2 )

tpt jftP

=⋅

(2)

where

()ht is the impulse response of the sensing

fiber;

T is the repeat period of the lightwave; ()

is the

i modulation function of the probe pulse; M

is the number of dimensions of OCSF and

is the

frequency of optical carrier.

After detecting by coherent detection, the decoded

sensing signal can be expressed as:

1,..., ,

'(,) () ()

() ( / ) ()

jMji

Eti Et pt

ht p t iT M p t

ht p t jT M p t

=≠

=⊗

=∗− ⊗

+∗−⊗



(3)

where

∗ denotes the convolution operation and

⊗

means the correlation operation.

Assuming the codes have high peak-to-sidelobe

ratio of auto-correlation function and cross-

correlation function, then

'( , )Etican be approximated

as:

/'( , ) ( ) ( )

main

Eti ht P t iT≈∗ −

(4)

where

main

P denotes the main lobe of the auto-

correlation function of

()

pt.

Eq. (4) shows that the sensing channel probed by

the pulse

main

P with a repetition period of

extracted. Therefore, the disturbance sampling rate is

increased by

M times and the OCSF need no extra

frequency band for multiplexing. The acoustic signal

can be demodulated by demodulating the phase

information, demonstrating that the sensing channel

Sensing Bandwidth Enlargement with Ten Orthogonal Codes in Quasi-distributed Acoustic Sensing System

153

has been multiplexed M times and the sensing

bandwidth is enhanced

M times of that in the

conventional QDAS without channel-multiplexing.

2.2 the Generation of Codes

According to Eq. (3), the OCSF are supposed to be

with high peak-to-sidelobe ratio of auto-correlation

function and cross-correlation function, so that the

equation can be approximated as Eq. (4).

To obtain the required codes is an optimization

problem, and the objective function and constraint

condition are given by:

min

.. ( ) 0,

1, 1 1,0;

()0,

,1 , 1 1, ;

01;

02;

st A m t

mMkNNk

Cmn t

mn M k N N m n

ϕπ

−≤

==−+−≠

−≤

==−+−≠

≤≤



A,C, ,t

(5)

where

N is the code length; ,()kAm is the

k value of

the auto-correlation function of the

m code;

,,()nkCm is the

k value of the cross-correlation

function of the

m code and the

n code;

represents the amplitudes of the codes, which is a

(2 1)MN×− matrix and

means the phases of the

codes, which is a

N×

matrix.

The amplitudes of the OCSF are the same, but the

phases of them are different, which range from 0 to

. The optimization problem above can be solved

by sequential quadratic programming (SQP)

Figure 1: The auto-correlation (a) and cross-correlation (b)

functions of the 10 generated codes.

algorithm, which can be realized by the mathematical

software in practice. With iteration process, the

minimized objective function t can be obtained,

demonstrating the minimized auto-correlation

functions and cross- correlation functions are realized.

Through the method above, ten groups of codes

are successfully generated, and all of them have about

15.4 dB auto-correlation and cross-correlation

suppression ratio, shown in Figure 1 (a) and (b)

respectively.

Figure 2: The experimental setup. I/Q: I/Q electro-optical modulator; EDFA: erbium-doped fiber amplifier; DWDM: dense

wavelength division multiplexing modulator; VOA: variable optical attenuator; PC: polarization controller.

-40 -20 0 20 40

Dela

[bit

]

-20

-15

-10

-5

Intensity [dB]

code-1

code-2

code-3

15.4 dB

(a)

Intensity [dB]

PHOTOPTICS 2022 - 10th International Conference on Photonics, Optics and Laser Technology

154

3 EXPERIMENTAL RESULTS

The experimental setup is shown in Figure 2. The

lightwave is emitted from the laser (NKT X15) and

the continuous lightwave from laser source is split

into two branches equally by a polarization

maintaining 1:1 coupler. The upper branch is the

sensing arm, and the lower one is used as the local

oscillator (LO).

The lightwaves in the sensing arm are modulated

by the I/Q electro-optical modulator according to the

codes generated above. All of the ten codes have a

400 MHz frequency shift, so that the heterodyne

detection can be applied and every code is with 400

ns width. The modulated probe codes are amplified

by the erbium-doped fiber amplifier (EDFA) and be

filtered out by a tunable filter.

Then the probe signal is guided into the sensing

section. The Raman shift in SMF is 13.2 THz,

indicating that a pump light with a wavelength of

1455 nm has a large Raman gain at 1550 nm. Thus,

to achieve the long distance fiber sensing, a 1455 nm

Raman pump is used for distributed amplification.

The probe signal and the pump signal are combined

by the dense wavelength division multiplexing

(DWDM) modulator to achieve amplification. Then

the signals are guided into the sensing fiber, which is

composed of a 99.4 km SMF and a pair of FBGs. The

reflectivity of the FBGs is about -10 dB, and the

interval between the two FBGs is 10 m. A variable

optical attenuator (VOA) is added to the fiber line to

compensate for the difference in fiber loss between

SMF and FBGs. Besides, a piezoelectric ceramics

transducer (PZT) is applied between two FBGs at the

end of the fiber to simulate the external vibration.

The reflected signal from sensing fiber will be

collected by the circulator and filtered out by a

tunable filter. Then the signal is beat with the LO in

an optical hybrid. The mixed lightwaves are detected

by two balanced photodetectors (BPDs). Finally, the

two-channel photoelectric-signals are sampled by a

high speed analog to digital converter (ADC) with 3.2

GSa/s acquisition rate. By utilizing GPU parallel

processing technology, the speed of computation can

be greatly improved and the signal can be

demodulated in real time.

In the conventional QDAS, the sensing bandwidth

is limited by the pulse round-trip time in sensing

fiber. The total length of the sensing distance is 99.4

km, representing 972.9 µs round-trip time. The total

repetition period is supposed to be a little longer than

the round-trip time of the pulse. In practice, the

interval between adjacent codes is 100 µs, indicating

that the total repetition period is 1 ms. The repetition

Figure 3: The intensity curves of the sensing signal.

Figure 4: The measured 0.8 kHz sinusoidal signal.

period represents a 1 kHz scan-rate in the

conventional QDAS system and according to the

Nyquist sampling theorem, the sensing bandwidth of

it is 500 Hz. By introducing the MIMO technology in

QDAS, the scan-rate of the system is as high as 10

kHz, which is ten times of the maximum scan-rate in

traditional single-pule scheme, bounded by the 1 ms

repetition period, and the sensing bandwidth is

accordingly enlarged to 5 kHz.

A 800 Hz sinusoidal signal is applied on the PZT

first to verify the sensing ability of MIMO-QDAS.

The demodulated traces are plotted in Figure 3, and

the reflected signals at each FBG are separated

clearly, indicating the spatial resolution of 10 m is

realized. The measured signal in real time is shown in

Figure 4, the black dots are experimental results and

the fitted curve of the measured signal is also plotted

in blue, which is approximately a sinusoidal wave

model, demonstrating the good sensing performance

of the real-time MIMO-QDAS.

In order to verify the enlargement of the sensing

bandwidth, sinusoidal signals of different frequencies

are added on the PZT. The frequencies of the three

signals are 1.2 kHz, 3.1 kHz and 4.9 kHz respectively.

Intensity [dB]

Sensing Bandwidth Enlargement with Ten Orthogonal Codes in Quasi-distributed Acoustic Sensing System

155

Figure 5: The PSD of the measured acoustic signals in

different frequencies.

The power spectrum density (PSD) of the real-

time demodulated signals are illustrated in Figure 5.

The lowest SNR of the acoustic signals is 20.5 dB,

which indicates that the real-time acoustic signals can

be required with high quality clearly. The maximum

noise level around the 4.9 kHz disturbance frequency

is about -48.1 dB, and the strain noise level is

43.8 𝑝ɛ/

√

𝐻𝑧

4 CONCLUSIONS

In this paper, a real-time MIMO-QDAS with high

performance is demonstrated by utilizing the OCSF.

The generation of the codes is elaborated and ten

groups of codes with about 15.4 dB auto-correlation

and cross-correlation suppression ratio are obtained.

As a result, the sensing bandwidth is enlarged to 5

kHz on 99.4 km sensing distance, which is ten times

of that in the conventional QDAS without

multiplexing and the spatial resolution is 10 m. The

multiplexed channels are doubled compared with the

previous MIMO-QDAS system that our group

achieved, and real-time signal demodulation is

demonstrated. Particularly, a 4.9 kHz sinusoidal

strain signal is retrieved in real time, with 43.8 𝑝ɛ/

√

𝐻𝑧 noise level.

ACKNOWLEDGEMENTS

National Natural Science Foundation of China

(62075030); Sichuan Provincial Project for

Outstanding Young Scholars in Science and

Technology (2020JDJQ0024).

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