Improving OFDR spatial resolution by reducing external clock sampling
error
Bowen Feng, Kun Liu
n
, Tiegen Liu, Junfeng Jiang, Yang Du
Institute of Optical Fiber Sensing of Tianjin University, Key Laboratory of Opto-Electronics Information Technology, College of Precision Instrument & Opto-
Electronics Engineering, Tianjin University, Tianjin 300072, China
article info
Article history:
Received 5 September 2015
Received in revised form
26 October 2015
Accepted 31 October 2015
Keywords:
Optical frequency domain reflectometry
Tuning rate
Jitter
Resolution
Sensing location accuracy
abstract
Utilizing an auxiliary interferometer to produce external clock signals as the data acquirement clock is
widely used to compensate the nonlinearity of the tunable laser source (TLS) in optical frequency domain
reflectometry (OFDR). However, this method is not always accurate because of the large optical length
difference of both arms in the auxiliary interferometer. To investigate the deviation, we study the source
and influence of the external clock sampling error in OFDR system. Based on the model, we find that the
sampling error declines with the increase of the TLS's optical frequency tuning rate. The spatial re-
solution can be as high as 4.8 cm and the strain sensing location accuracy can be up to 0.15 m at the
measurement length of 310 m under the minimum sampling error with the optical frequency tuning rate
of 2500 GHz/s. Hence, the spatial resolution can be improved by reducing external clock sampling error
in OFDR system.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
optical frequency domain reflectometry (OFDR) has been be-
coming more and more attractive in the field of distributed optical
fiber sensors. Compared with other methods in this field, such as
the Raman optical time domain reflectometry (OTDR), the Bril-
louin OTDR and the fiber Bragg gratings (FBGs), OFDR has various
advantages [1], including high spatial resolution, high sensitivity
and large measurement range. Based on these advantages, OFDR is
extensively applied for distributed acoustic, temperature or strain
sensing [2–4], structural health monitoring [5], and optical net-
work diagnosis [6] et cetera.
In OFDR system, the key point is that the tunable laser source
(TLS) must supply a large tunable optical frequency range while
maintaining fast tuning rate and linear tuning. In addition, OFDR
relies on the Fast Fourier Transform (FFT) to convert the frequency
domain signal to the spatial domain signal, which requires
the beat signal generated by OFDR system sampled at an equal
interval of optical frequency. Nevertheless, all of the TLSs have the
defect of tuning nonlinearity, which indicates that the optical
frequency change of the TL S at the same interval is not always
identical. In this case, the output light from the TLS will contain
phase noise that can lead to the degradation of the spatial
resolution of the system [7]. To tackle this issue, many approaches
have been proposed. One of them is to change the tuning principle
of the TLS to reduce the nonlinearity, guaranteeing that the optical
frequency of the output light is a linear curve in time, such as
single side band suppressed carrier (SSB-SC) [8] or double side
band suppressed carrier (DSB-SC) [9] modulation technology.
Another approach is to employ a relevant algorithm to compensate
the signal that contains the phase noise, such as the concatenately
generated phase (CGP) compensation [10], the spline interpolation
[11] and the concatenative reference method (CRM) [12]. However,
the most widely employed method is to utilize an auxiliary in-
terferometer to produce an external sampling clock as data ac-
quisition trigger. This method can achieve a standard equal optical
frequency interval sampling theoretically. Nevertheless according
to the research of Eric D.Moore et al. [13], this method is not ac-
curate when the optical length difference of both arms in the
auxiliary interferometer is large, but in practical application, hav-
ing large optical length difference is especially necessary for large
range and high resolution measurement. Therefore, the condition
above must be considered.
In this paper, we further research the source of the external
clock sampling error based on the work [13]. As mentioned above,
the phase information of the external clock signal generated by
OFDR interferometer system is analyzed and the theoretical op-
tical frequency interval error of two neighboring sampling triggers
(self-defined jitter) is calculated. The result of the experiment
unveils that the jitter becomes weaker while the OFDR system
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/optcom
Optics Communications
http://dx.doi.org/10.1016/j.optcom.2015.10.065
0030-4018/& 2015 Elsevier B.V. All rights reserved.
n
Corresponding author.
E-mail address: beiyangkl@tju.edu.cn (K. Liu).
Optics Communications 363 (2016) 74–79