COL 13(1), 010401(2015) CHINESE OPTICS LETTERS January 10, 2015
1671-7694/2015/010401(4) 010401-1 © 2015 Chinese Optics Letters
Optical ber acoustic sensing multiplexing system
based on TDM/SFDM
Jiehui Xie (谢杰辉), Fuyin Wang (王付印), Yao Pan (潘 瑶),
Zhengliang Hu (胡正良)
*
, and Yongming Hu (胡永明)
College of Optoelectronic Science and Engineering, National University of Defense
Technology, Changsha 410073, China
*
Corresponding author: zheng_liang_hu@163.com
Received September 10, 2014; accepted November 14, 2014; posted online December 30, 2014
We propose a system of time-division multiplexing (TDM) and spatial frequency-division multiplexing
(SFDM). Extrinsic Fabry–Perot interferometric sensors are applied to detect weak acoustic signals. The
broadband source is employed, the light from it is modulated by a pulse signal sequence and is eiciently
amplied by semiconductor optical ampliers. Experimental results show that the equivalent noise pressure
spectrum level is −97.2 dB re 1 rad/√Hz below 1250 Hz, and the cross talk between two sensors in one TDM
channel is −32.7 dB with a cavity length dierence of 60 μm. The number of sensors in this multiplexing
system can theoretically reach 160.
OCIS codes: 040.1880, 060.2310, 060.4230, 130.6010.
doi: 10.3788/COL201513.010401.
White light interferometers, especially the low-nesse
extrinsic Fabry–Perot interferometric (EFPI) sensors,
have been widely studied for its outstanding proper-
ties such as excellent stability, miniature structure, and
absolute measurement
[1−6]
. An acoustic sensing array
based on the miniaturized EFPI sensors can eiciently
reduce the size of the multiplexing system and realize
the spatial gain of acoustic signals. However, EFPI sen-
sors based on spectra detection have two obvious dis-
advantages: they are intrinsically diicult to multiplex
and they require a broadband source (BBS) with a high
light intensity and a broader spectrum range.
To form a multiplexing system, three methods,
that is, wavelength-division multiplexing (WDM)
[7]
,
frequency-division multiplexing (FDM)
[8,9]
, and time-
division multiplexing (TDM)
[10]
can be separately or
jointly applied. For example, a multiplexing system
with the combination of coarse WDM and spatial FDM
(SFDM) can eiciently take advantage of the light
source
[11]
. However, compared with pure SFDM method,
it cannot increase the number of sensors in theory. To
further increase sensing channels in the multiplexing
system requires another method. Traditionally, TDM
excels in increasing the number of sensing channels
in the multiplexing system, and the fast ber Bragg
grating analyzer (FBGA, BaySpec Inc.) renders sui-
cient sampling rate for spectra detection. Meanwhile,
SFDM is capable of increasing the number of sensors in
a single channel, yet for SFDM with EFPI sensors, the
maximum number is limited by the sensors themselves
and the light intensity of the BBS
[9]
.
In this letter, we propose and demonstrate a method
to increase the number of sensors by combining TDM
and SFDM. Semiconductor optical ampliers (SOAs)
are utilized as the pulse modulators for TDM and the
optical ampliers for SFDM, whereas the fast FBGA is
used to detect the signal. Compared with other EFPI
multiplexing systems based on spectra detection, the
proposed system can theoretically multiplex more sen-
sors and respond faster to dynamic signals. Compared
with the traditional ber-optic coherent sensing system,
the miniaturized EFPI sensor has a lower equivalent
noise pressure spectrum level below 1250 Hz. Experi-
mental results show that our acoustic sensing multi-
plexing system has a low phase noise and a low cross
talk.
The conguration of the multiplexing system is illus-
trated in Fig. 1. The fast FBGA system serves as a
detector array, with a BBS integrated within. This fast
FBGA can scan 5000 frames of reected interference
spectra (RIS) per second, which enables the fast FBGA
with the capacity of detecting RIS with a high scanning
speed. Each spectrum contains 512 pixels in the wave-
length domain, ranging from 1506 to 1593 nm. The
SOAs are modulated by a pulse signal sequence gener-
ated by a function generator. The light from the BBS
passing through SOAs is modulated as pulse signals
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Fig. 1. Schematic diagram of the hybrid TDM/SFDM
multiplexing system.