Tel: +81-3-5841-6783, Fax: +81-3-5841-6025, e-mail: {fan, ka, hotate}@sagnac.t.u-toAyo.ac.jp. Abstract: The first experimental observation of dynamic grating in ...
OTuL2.pdf
A novel strain- and temperature-sensing mechanism based on dynamic grating in polarization-maintaining erbium-doped fiber Xinyu Fan, Zuyuan He, and Kazuo Hotate
Department ofElectronic Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Tel: +81-3-5841-6783, Fax: +81-3-5841-6025, e-mail: {fan, ka, hotate}@sagnac.t.u-toAyo.ac.jp
Abstract: The first experimental observation of dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) is reported and a novel fiber-optic strain- and temperature-sensing mechanism based on the dynamic grating in PM-EDF is demonstrated experimentally. © 2005 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (060.2410) Fibers, erbium; (060.2420) Fibers, polarization-maintaining
1. Introduction Distributed fiber-optic strain and temperature sensors have become an important technology for monitoring the condition of structures and structural materials because of their spatially continuous measurement capabilities. A typical technique is based on the stimulated Brillouin scattering (SBS) process, such as Brillouin optical time-domain analysis (BOTDA) [1] and Brillouin optical coherence-domain analysis (BOCDA) [2]. We have proposed a novel scheme based on a dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) [3]. In this scheme, the dynamic grating is localized and swept along the fiber by the technique of synthesis of optical coherence function (SOCF) [4] to realize fully distributed sensing. It is predicted by simulation that the performance of strain-sensitivity and temperature-sensitivity comparable or even better than SBS-based technology can be expected with this scheme [3]. On the other hand, the dynamic grating in erbium-doped fiber (EDF) has attracted attentions for its potential in applications in optical communication [5-8] and sensing [3]. Up to date, however, the published experimental observations about dynamic grating are all based on single mode EDF. In this presentation, we report, for the first time to the best of our knowledge, an experimental observation of the dynamic grating in PM-EDF and demonstrate a novel fiber-optic strain- and temperature-sensing mechanism based on the dynamic grating in PM-EDF. 2. Principle
When two counter-propagating coherent light beams (referred to as writing beams hereafter) are launched into a pumped erbium-doped fiber (EDF), they interfere to each other and form stationary interference fringes in the EDF. The interference fringes then create a periodical gain structure per the phenomenon of gain saturation and hence produce a dynamic grating in the EDF [5-8]. The period of the grating is a half of the writing beam's wavelength in the fiber. A third beam launched into the fiber (referred to as reading beam hereafter) is reflected by the dynamic grating when its optical frequency (wavelength) is the same as the writing beams'. In other words, a reading beam is reflected from the dynamic grating when it satisfies the Bragg condition. If we write the dynamic grating in PM-EDF along one primary polarization axis and read the grating along the other primary polarization axis, the detected Bragg reflection frequency is different from the frequency of the writing beams due to the birefringence along the two primary polarization axes: fBraggj=x(nx/ny),
(1)
wherefBragg andfx denote the Bragg frequency and the writing beams' frequency, and nx and ny the refractive indices of the two primary polarization axes, respectively. When a strain is applied to the fiber or temperature around the fiber changes, the refractive indices change because of photo-elastic effect, which results in the change of birefringence. As a result, the detected Bragg reflection frequency shifts in proportion to the birefringence change, which is proportional to strain or temperature: where A\f and AB denotes the shift of Bragg frequency and the birefringence change caused by strain or temperature variation. Therefore, we can know the magnitude of strain and temperature change by measuring the shift of Bragg reflection frequency A\f
OTuL2.pdf (ny A\)/ ( afx), T = (nyA\l/( ffx),
(3) (4)
£=
where £ and JT denote the strain and temperature change, and a and ,6 the strain coefficient and temperature coefficient of the birefringence. Our simulation result shows a strain sensitivity of 0.426 MHz/4t under a conservative assumption in the parameters [3]. 3. Experimental setup The experimental setup is shown in Fig. 1. Two light beams from LD 1, a distributed feedback laser diode (DFB-LD), are used to write the dynamic grating in polarization direction X into the PM-EDF (50 cm Nufem PM-ESF-7/125 high-doped PM-EDF), which is pumped by the 980-nm pump laser diode. We use a reading beam from LD2, a wavelength-tunable DFB-LD, to read the grating in polarization direction Y. Three tunable attenuators are used to obtain optimum writing intensities and reading intensity. Three polarizers (Pol) and a polarization controller (PC) are used to enhance the polarization extinction ratio and control the polarization direction. A polarization beam splitter/combiner (PBS/PBC) is used to introduce the reading beam to the PM-EDF and to output the reflectoion at the dynamic grating in polarization direction Y. Because polarization maintaining components have only a limited extinction ratio, the light beam in one polarization direction couples to the other polarization direction. Therefore, when we measure the reflection at the dynamic grating in polarization direction Y, not only the reflection of the reading beam but also the coupled writing beam are received. To distinguish them, we chop the reading beam and use the synchronous detection. The chopping frequency is selected in our experiment as high as 37 MHz to avoid the gain modulation in EDF [6]. Here, the reading beam is chopped by using a LiNbO3 electro-optic intensity modulator (IM) for the synchrounous detection with a lock-in amplifier LIA 1 to distinguish the reflected reading beam from the leakage of the writing beam. Even after the synchronous detection by LIA 1, there are still spurious noises observed, which are modulated at the same frequency as the chopping frequency. Some of them are related to the reading beam, such as amplified Rayleigh scattering of the reading beam and amplified reflection of the reading beam at the splicing points or connection points. Another kind of noise comes from the beating of the writing beam related light and the reading beam related light. All these noises make it very difficult to detect the weak reflection from the dynamic grating. In order to get rid of these noises, we design a scheme to switch the dynamic grating ON and OFF. In the ON-state, the dynamic grating is formed, and in the OFF-state is not. Then the difference between the two states is purely related to the reflection at the dynamic grating; all other reflections and beatings, which are common between the both states, are canceled out. The OFF-state is realized here by modulating the light frequency of LD 1 in sine wave at 47 kHz with FG1 to make the two writing beams not interfere inside the PM-EDF. The ON-OFF is switched with FG2 at 57 Hz in the experiment, which is slow enough for the dynamic grating being set up in the PM-EDF. In fact, all these modulation frequencies are selected to be primitive numbers in order to avoid any correlation relationship. The diffemece between ON-OFF states is obtained with LIA2. All the data are recorded by the computer via GPIB. 980nm pump
