Spectral-domain measurement of beat length in polarization-maintaining fibers
Xinwei Chen
a,b,c,
⇑
, Tiegen Liu
a,b
, Hongxia Zhang
a,b
, Wei Lu
c
, Lichang Huang
c
, Liang Cheng
c
,
Yimo Zhang
a,b
a
College of Precision Instrument & Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China
b
Key Laboratory of Optoelectronics Information Technical Science, EMC, Tianjin 300072, China
c
NR Electric Co., Ltd., Nanjing 211102, China
article info
Article history:
Received 23 May 2012
Revised 5 August 2012
Available online 25 September 2012
Keywords:
Spectral interferometry
Beat length
Fiber measurement
Polarization mode coupling
abstract
A spectral white-light interferometric (WLI) method for measuring the beat length in high birefringence
polarization-maintaining fibers (PMFs) is presented. The approach is based on the stress-induced polar-
ization mode coupling between the two polarization eigenmodes and utilizes their spectral interfero-
grams. By recording the spectral interferograms before and after the displacement of the force, the
beat length can be obtained by the displacement of the force and the relative phase variation retrieved
from the two spectral interferograms. According to the principle that the phase difference between adja-
cent fringes peaks is 2
p
, the phase can be retrieved from the interferograms simply. The factors affecting
the measurement precision of this method and the main source of errors are investigated in detail.
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
High birefringence polarization-maintaining fibers (PMFs) can
maintain the polarization state of the light propagating in the fibers,
which have been widely employed in interferometric fiber-optic
sensors and polarization sensitive optical devices. The polarization
maintaining ability along the PMFs is greatly governed by the modal
birefringence whose magnitude is expressed in beat length. Because
the beat length is a function of the optical wavelength, strong cou-
pling will occur within a limited optical bandwidth. Moreover,
investigating the power spectrum of birefringent perturbation is
useful in some application; therefore, the wavelength dependence
of the beat length is of considerable interest [1,2]. The beat length
is the distance over which the two orthogonally polarized modes
differ in the phase by 2
p
.
There are several techniques that have been developed to mea-
sure this parameter, including cutback method [3], twist method
[4], optical polarization [5,6], Rayleigh scattering [7], Brillouin scat-
tering frequency-domain analysis method [8], polarization shuttle
pulse method [9], electro-optic, magneto-optic [10], acousto-optical
modulation method [11], wavelength sweeping [12,13]. White-
light interferometric (WLI) technology had also been used to mea-
sure the beat length. The temporal WLI technology by lateral force
and a scanning interferometer can achieve a measurement accuracy
of 19
l
m [14]. The spectral WLI technology using a windowed
Fourier transform is applied to reconstruct the phase to obtain the
beat length with a measurement precision of 74
l
m [15].
In this paper, a spectral WLI technique based on stress-induced
polarization mode coupling to measure the beat length in PMFs is
presented. A point-like force was applied on the PMFs to cause a
coupling point; a scanning Michelson interferometer was used to
find the coupling point and an optical spectrum analyzer (OSA)
was used to record the interferogram. By using a very simple phase
retrieval method, the beat length can be obtained by only two
spectral interferograms with simple operation, rapid measurement
time and high signal-to-noise ratio. Adopting a scanning Michelson
interferometer, we achieved long PMFs measurement. The beat
length of any specific section along a fiber from the point-like force
position to the output end of the fiber can be measured.
2. Theory
The principle of the beat length measurement system is based
on the stress-induced polarization mode coupling in PMFs shown
in Fig. 1.
Linear polarized light injects into the tested PMFs with only one
polarization mode being excited and propagating on the slow axis.
If there is a polarization coupling point along the PMFs, a coupling
mode will be excited at the polarization coupling point and prop-
agates on the fast axis. At the output end of the fiber, there is an
optical path difference (OPD) between the two modes due to the
modal birefringence
D
n
b
ðkÞ of the fiber. The spectral interference
fringes can be obtained by an OSA using a scanning Michelson
interferometer to compensate the OPD. The frequency dependent
1068-5200/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.yofte.2012.09.001
⇑
Corresponding author at: College of Precision Instrument & Opto-Electronics
Engineering, Tianjin University, Tianjin 300072, China.
E-mail address: cxw553@tju.edu.cn (X. Chen).
Optical Fiber Technology 18 (2012) 527–531
Contents lists available at SciVerse ScienceDirect
Optical Fiber Technology
www.elsevier.com/locate/yofte
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