Improving OFDR spatial resolution by reducing external clock sampling error
Introduction
optical frequency domain reflectometry (OFDR) has been becoming 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 Brillouin 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], [3], [4], structural health monitoring [5], and optical network 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 TLS 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 interferometer to produce an external sampling clock as data acquisition 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 accurate when the optical length difference of both arms in the auxiliary interferometer is large, but in practical application, having 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 optical 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 preforms better with the increase of the optical frequency tuning rate of the TLS, the spatial resolution 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 jitter with the optical frequency tuning rate of 2500 GHz/s.
Section snippets
Measurement principle
The system diagram in Fig. 1 shows the principle of OFDR interferometer setup, which adopts single-mode optical fiber Michelson configuration to produce the external clock signal.
On the basis of the work [13], the intensity of the beat signal I(t) produced by the interferometer can be expressed as:where E0 is the amplitude of the optical field that is a constant, β is the attenuation coefficient of the fiber at the delay arm, τ is the
Experimental setup
The experimental setup for investigating the influence of the jitter on OFDR system is shown in Fig. 3. In this OFDR system, a TLS is utilized as the optical source of the system, in which the light is divided into two paths by a 5: 95 coupler. The 5% part is used as the optical source of the external clock interferometer, whose structure is the same as the one depicted in Fig. 1. The optical length difference of both arms in the external clock interferometer is 1000 m. The 95% part is taken as
Conclusion
According to the method presented in this paper, the spatial resolution can be improved by reducing external clock sampling error in OFDR system. In our investigation, an external clock sampling error model is built, through which an analytic expression for the relation between the jitter and the spatial resolution is derived and experimentally demonstrated. The result of the experiment shows that the quality of the external clock can be improved by optimizing the optical frequency tuning rate,
Acknowledgments
This work is supported in part by National Basic Research Program of China (973 Program, Grants 2010CB327806, 2010CB327802), National Natural Science Foundation of China(Grant 61475114, 61108070, 61227011, 61378043), National Instrumentation Program(Grant 2013YQ030915), Tianjin Natural Science Foundation (Grant 13JCYBJC16200), Science and Technology Key Project of Chinese Ministry of Education (Grant 313038), Shenzhen Science and Technology Research Project (Grant JCYJ20120831153904083).
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